Alkene Polymerization Reactions with Transition Metal Catalysts

To Anna, James, Samuel, and Gabriel

PREFACE THAT SHOULD BE READ During the past 30 years, the field of alkene polymerization with transitionmetal catalysts has undergone three major changes: 1. The most visible change, of course, was the vast expansion of research devoted to metallocene and other soluble transition-metal catalysts. The discovery of MAOcocatalyzed metallocene systems for alkene polymerization in 1975 and the subsequent establishment of the metallocenium mechanism of these reactions transformed the field of metallocene polymerization catalysis from an obscure academic research area into a vigorously pursued subject, both in academia and in industry. Currently, the focus of this research has shifted further, toward the studies of numerous multidentate complexes of early and late transition metals. 2. Most of the earlier researchers, starting with Natta himself, realized that solid polymerization catalysts have active centers of different types in terms of their isospecificity. However, relatively recent developments in several analytical fields, in particular, gel permeation chromatography, analytical temperature-rising fractionation, and crystallization fractionation, provided the first reliable information about differences between various active centers in terms of the molecular weights of alkene polymers they produce, the stereoregularity of homopolymers, compositions of copolymers, etc. These developments brought one unexpected setback. They showed that various types of active centers are formed and decay at significantly different rates. These findings made a large part of earlier kinetic studies of solid and supported Ziegler-Natta catalysts (the studies that essentially pre-supposed that the catalysts have one type of active center) suspect or obsolete. 3. The rapid development of high-resolution 13C NMR spectroscopy resulted in a greatly expanded understanding of chemical and steric features of polyolefins and alkene copolymers, such as the detailed structures of chemical and steric defects in polymer chains, the structure of chain ends, etc. The NMR data were complemented by GC studies of alkene oligomers, the fractions of polymers with the lowest molecular weights. These oligomer studies gave for the first time an opportunity to examine each polymer molecule (albeit only a very short polymer molecule), in every detail, including the structure of both its chain ends, the chemical and the steric structure of the chain, etc. The main outcome of this research was realization that deviations from regular chain initiation, growth, and transfer patterns, although often minor, can profoundly affect polymerization kinetics and polymer properties. These three developments call for a completely new review of all aspects of alkene polymerization reactions over transition-metal catalysts, both solid and soluble. Such a review should also be structured in a radically different way. In the vii

viii

Preface

traditional approach, the exposition usually starts with the preparation methods and compositions of solid catalysts or transition-metal complexes, then moves on to polymerization reactions of particular alkenes (first, ethylene; then propylene, etc.), and, finally, proceeds to the description of the polymerization kinetics, stereoregularity, etc. This review structure has lost its rationale due to a much better understanding of the true nature of active centers in the catalysts. Another complication arises from the sheer volume of information. The number of publications describing various aspects of catalytic alkene polymerization reactions adds up to several thousands. In addition to research articles, the literature includes several books and compilations of symposium proceedings [1-22]. Therefore, a new structure is adopted in this book. Chapter 1 is introductory; it describes, in condensed form, the basics of polymerization reactions with transition-metal catalysts: the types of catalysts and cocatalysts, the phenomenon of stereoregularity, copolymerization reactions, etc. It also contains technical information about such aspects of alkene polymerization reactions as compositions of the most popular catalyst systems, structures of metallocene complexes, reactivity of various alkenes, the helix types of isotactic and syndiotactic polyolefins, etc. All researchers who are new the field of Ziegler-Natta catalysis should read this chapter, while specialists may easily skip it. Chapter 2 addresses one of the principal issues of alkene polymerization catalysis: the existence of catalyst systems with only one type of active center (mostly, hydrocarbon-soluble catalysts) and catalyst systems with several types of active centers (all solid and supported catalysts). This chapter briefly describes the modern analytical techniques of polymer characterization that revolutionized our understanding of polymerization catalysis. It also presents both the experimental manifestations of the single- and multi-center catalysis and their implications for the studies of polymerization kinetics, polymer stereoregularity, copolymer composition distribution, etc. Chapter 3 is devoted to the chemistry and stereochemistry of polymerization reactions. This field of research greatly benefited from the introduction of highresolution 13C NMR spectroscopy. Twenty five years ago such areas of research as the chemistry and stereochemistry of chain initiation, growth and transfer reactions were in a rudimentary state, but at the present time they provide a very detailed picture of all aspects of polymerization reactions. Chapter 4 describes three subjects: (a) the formal chemical composition of various commercial polymerization catalysts (mostly based on the patent literature), (b) the true chemical composition of the catalysts (mostly based on spectroscopic data), and, (c) the basic chemistry of the reactions between the catalysts and cocatalysts. Chapter 5 describes the polymerization kinetics. As far as our understanding of alkene polymerization reactions with solid and supported Ziegler-Natta is concerned, this research lost some of its value over the last two decades. The main reason for this has been the development of modern analytical techniques for polymer characterization. The application of these techniques has shown that all the solid and supported polymerization catalysts (as well as a number of soluble catalysts) have several types of active centers which significantly differ in their kinetic and

Preface

ix

stereochemical parameters. Therefore, the field of polymerization kinetics has shifted its focus from attempts at describing reaction kinetics in terms of simple kinetic schemes borrowed from the fields of radical and anionic polymerization reactions to less over-reaching but more realistic tasks. They include the effects of the catalyst type and the reaction parameters on the distribution of active centers, main kinetic features of the most abundant active centers, etc. Chapter 6 is devoted to the mechanistic aspects of polymerization reactions, including relevant experimental data and theoretical analysis. The texts of all the chapters contain numerous cross-references so that the information about particular catalyst systems and reactions that is spread over several chapters can be collected and examined separately. Yury V. Kissin Rutgers – The State University of New Jersey USA

ABBREVIATIONS

AND

DEFINITIONS

Alkyl groups: CH3 ¼ Me; C2H5 ¼ Et; n-C3H7 ¼ n-Pr; iso-C3H7 ¼ i-Pr; nC4H9 ¼ n-Bu; iso-C4H9 ¼ i-Bu; tert-C4H9 ¼ t-Bu; n-C6H13 ¼ n-Hex; nC8H17 ¼ n-Oct; Cyclopentyl ¼ Cpy; Cyclohexyl ¼ Cy. Aryl groups: Phenyl (C6H5) ¼ Ph; Benzyl (C6H5
CH2
) ¼ Bz. Z5 Ligands in metallocene complexes: Cyclopentadienyl (Z5-C5H5) ¼ Cp; Pentamethyl cyclopentadienyl (Z5-C5Me5) ¼ Cp; Indenyl (Z5-9-C6H4C5H3) ¼ Ind; 1,2,3,4-Tetrahydroindenyl (Z5-9-C6H8C5H3) ¼ Ind-H4; Z5-9-Fluorenyl ¼ Flu; Z5-1,2,3,4-Tetrahydro-9-fluorenyl ¼ Flu-H4; Z5-Octahydro-fluorenyl ¼ Flu-H8; Z5-Benz[e]indenyl ¼ Benz[e]Ind. Monodentate and bidentate ligands: Tetrahydrofuran ¼ THF; Pyridine ¼ Py, 2,4pentanedionato ligand (derived from acetyl acetone) ¼ acac. Bridge atoms or groups between two metal atoms: (m-Cl), (m-OR), (m-Me), etc. For example, the dimer of AlEt3 ¼ Et2Al(m-Et)2AlEt2. Silicon compounds: The nomenclature of silicon compounds in the literature on Ziegler–Natta catalysts is different from the common nomenclature: the symbols of alkyl/aryl substituents are placed before the Si symbol. For example, diphenyldimethoxy silane is depicted as Ph2Si(OMe)2 rather than SiPh2(OMe)2. Organic compounds added to components of heterogeneous catalyst systems: Modern supported catalyst systems contain, in addition to Ti (or V) and Mg species, two groups of organic compounds that are added to catalyst components with the main goal of improving the yield of the crystalline fraction of the polymers. In the literature, these organic compounds are usually called ‘‘electron donors,’’ they supposedly modify the structure of solid catalysts and cocatalysts. Two types of donor compounds are often used. The chemical compound added to a solid catalyst component is named ‘‘an internal donor’’ and a chemical compound added to a cocatalyst is named ‘‘an external donor.’’ Numerous experiments show that when components of these catalyst systems are produced, by reacting the solid catalyst and the cocatalysts, these donor compounds are usually converted into completely different compounds, and the latter compounds, in turn, play different roles in the catalysts, which only rarely depend on their electron– donating properties. For this reason, the organic ‘‘donor’’ compounds are called here organic modifiers: An organic compound used in the preparation of a solid catalyst component, ‘‘internal electron donor’’ ¼ Modifier I. An organic compound added to a cocatalyst, ‘‘external electron donor’’ ¼ Modifier II. Active centers: For uniformity, the same symbols for active centers are used throughout the book: Active centers in heterogeneous catalysts: WTi–R, WV–R, WCr–R, etc. Active centers in metallocene catalysts: Cp2Ti+–R, Cp2Zr+–R, Cp2Hf +–R, etc. Active centers in non-metallocene homogeneous catalysts: (L)Ti–R, (L)Ni–R, (L)Fe–R, etc. xi

CHAPTER 1

The Beginner’s Course: General Description of Transition Metal Catalysts and Catalytic Polymerization Reactions

Contents 1.1. Classifications of Transition Metal Catalysts 1.1.1. Components of transition metal catalysts 1.1.2. Catalyst classification based on solubility 1.2. Composition and Structure of Ziegler–Natta Catalysts 1.2.1. Organoaluminum cocatalysts 1.2.2. Transition metal catalyst components of Ziegler–Natta catalysts 1.2.3. Examples of Ziegler–Natta catalysts 1.3. Metallocene Catalysts 1.4. Homogeneous Catalysts Containing Non-Metallocene Complexes of Early- and Late-Period Transition Metals 1.5. Chromium Oxide Catalysts 1.6. Main Features of Alkene Polymerization Reactions 1.6.1. Basic principles of polymerization kinetics 1.6.2. Copolymerization reactions of alkenes 1.6.3. Auto-copolymerization reactions and formation of polymer chains with long-chain branches 1.6.4. Oligomerization reactions 1.6.5. Stereospecific alkene polymerization and stereoregular polyolefins 1.6.6. Nonuniformity of active centers in transition metal catalysts 1.7. Classes of Polymers Produced with Transition Metal Catalysts 1.7.1. Linear polyethylene and semi-crystalline ethylene copolymers 1.7.2. Ethylene/propylene elastomers 1.7.3. Poly(olefins)

2 3 4 6 6 6 7 11 14 15 17 18 20 21 22 22 27 28 29 31 32

The main purpose of this introductory chapter is to provide a concise description of a large family of transition metal catalysts used for the polymerization of various alkenes. The chapter is intended for scientists and engineers who are just beginning their work in the field of alkene polymerization catalysis. Two sets of definitions related to alkene polymerization reactions are used interchangeably in academia and in industry:
The historic name for alkenes, olefins, is nearly universally used in industry whereas researchers in academia usually prefer the chemically correct term, 1

2

Alkene Polymerization Reactions with Transition Metal Catalysts

alkenes. In this book, the term ‘‘alkene’’ is used to define a hydrocarbon with one double bond. The double bond is nearly always the vinyl bond, CH2QCH–, and the alkene is denoted as CH2QCH–R, where RQH for ethylene, CH3 for propylene, cyclo-C6H11 for vinylcyclohexane, etc. Three other classes of unsaturated hydrocarbons are mentioned in the book, (a) dienes, both conjugated, such as butadiene and isoprene, and non-conjugated, such as 1,4-pentadiene, 1,9-decadiene, etc., (b) cycloalkenes such as cyclopentene and norbornene, and (c) styrenes.
Alkene polymers are nearly always called polyolefins. Terms ‘‘polymers’’ or ‘‘copolymers’’ are used for the description of these materials in academia (e.g., propylene polymers, ethylene/1-alkene copolymers) whereas the same materials are called ‘‘resins’’ in industry: polypropylene resins, LLDPE resins, etc.

1.1. Classifications of Transition Metal Catalysts In the past, the term ‘‘Ziegler–Natta catalysts’’ was used as a generic expression that describes a variety of catalysts based on transition metal compounds and capable of polymerizing and copolymerizing alkenes and dienes. The products of these polymerization reactions, poly(alkenes), alkene copolymers, poly(dienes), and poly(cycloalkenes), are manufactured commercially in a very large volume and have numerous applications as general-purpose and engineering plastics, elastomers, and synthetic rubbers. Polymers produced with Ziegler–Natta catalysts include many widely known commercial materials: high-density polyethylene; linear low-density polyethylene; ethylene-based plastomers; crystalline isotactic polyolefins such as polypropylene, poly(1-butene), and poly(4-methyl-1-pentene); crystalline syndiotactic polypropylene and polystyrene; ethylene–propylene elastomers; ethylene– cycloalkene engineering plastics, and synthetic rubbers based on polybutadiene and polyisoprene. However, the development of numerous new catalysts for alkene polymerization in the last 20 years called for separation of all transition metal-based polymerization catalysts into several groups. The following general terminology is commonly adopted. The first group, which includes mostly titanium and vanadium-based catalysts, has retained the name ‘‘Ziegler–Natta catalysts.’’ These catalysts are named after Karl Ziegler (Germany) and Giulio Natta (Italy). In the early 1950s, these chemists discovered the first catalytically active compositions for alkene polymerization, determined principles of their action, and investigated the structures and properties of polymers produced with the catalysts [1,23,24]. The monumental contributions of Ziegler and Natta received universal recognition and these scientists were jointly awarded the Nobel Prize in chemistry in 1963. Ziegler–Natta catalysts have been used in the commercial manufacture of various polymeric materials since 1956. Today, the total volume of plastics, elastomers, and rubbers produced from alkenes with these catalysts worldwide exceeds 100 million metric tons. Together, these

Transition Metal Catalysts and Catalytic Polymerization Reactions

3

polymers represent the largest-volume commodity plastics as well as the largestvolume commodity chemicals in the world. The members of the second catalyst group are commonly called ‘‘metallocene polymerization catalysts.’’ D. Breslow (USA) and G. Natta discovered first metallocene catalysts for alkene polymerization soon after the original discovery of the Ziegler–Natta catalysts [25,26]. The early metallocene catalysts had relatively low activity and were regarded as most suitable for academic research. However, German scientists W. Kaminsky and H. Sinn in 1976 discovered a new class of metallocene catalyst systems that exhibit extremely high activity [27–29]. Nowadays, two types of metallocene complexes are widely used as components of catalyst systems (see Scheme 1.1). The first type of the metallocene complex contains two cyclopentadienyl rings attached to a transition metal atom (usually Zr, Ti, or Hf) and the second type contains one cyclopentadienyl ring. Both types of metallocene complexes were the subjects of an enormous volume of research, both in academia and in industry. These catalysts and their subsequent modifications presently compete with Ziegler–Natta catalysts for many applications. The third group includes polymerization catalysts based on hydrocarbon-soluble non-metallocene transition metal complexes. M. Brookhart (USA) in 1995 discovered the first catalysts of this type [30]. In the past several years this field underwent a rapid development and now encompasses well-defined complexes of many early-period and late-period transition metals in the Periodic Table (Schemes 1.2 and 1.3). Some of these catalysts are relatively stable in a polar environment. They also provide the best route to the synthesis of alkene copolymers with polar vinyl compounds. The fourth group constitutes chromium-based catalysts. Historically, chromium oxide catalysts were the first transition metal catalysts used for alkene polymerization; J. P. Hogan and R. L. Banks (USA) discovered them in the early 1950s [31,32]. Phillips Petroleum Company extensively used these catalysts for the polymerization of ethylene to high molecular, highly crystalline ethylene homopolymers. Later, researchers at Phillips Petroleum Company have found that the same type of catalyst, after modification, is suitable for the polymerization of other alkenes and for alkene copolymerization reactions [33].

1.1.1. Components of transition metal catalysts The majority of transition metal catalyst systems, except for chromium oxide catalysts, consist of two components. The first component is a derivative of a transition metal, such as titanium, vanadium, zirconium, nickel, palladium, iron, cobalt, etc. For example, typical transition metal compounds that were used in the early Ziegler–Natta catalysts and which are still universally used for the manufacture of modern catalysts are TiCl4, TiCl3, VCl4, and VOCl3; the majority of metallocene catalysts are based on complexes of zirconium and titanium, etc. The second components of the catalyst systems, which are called cocatalysts, are organometallic compounds, mostly organoaluminum compounds. Typical organoaluminum cocatalysts are Al(CH3)3, Al(C2H5)3, Al(i-C4H9)3, Al(C2H5)2Cl, Al(i-C4H9)2Cl, Al2(C2H5)3Cl3, etc.; and methylalumoxane, [Al(CH3)O]n. Some

4

Alkene Polymerization Reactions with Transition Metal Catalysts

metallocene and late-period transition metal catalysts use second components of a completely different type, perfluorinated boronaromatic compounds. Although these catalyst components are employed for different reasons than organoaluminum cocatalysts, they are also usually called ‘‘cocatalysts.’’ Neither of these two catalyst components, if used alone, can polymerize alkenes. However, when the two components of the catalyst systems are mixed, a series of chemical reactions takes place and some of the products of these reactions, called active centers, readily polymerize alkenes and dienes. Although transition metal catalysts have been known for more than 50 years, the exact chemical structure of active centers in Ziegler–Natta and chromium oxide catalysts is still unknown. On the other hand, a much higher level of understanding of the true structure of active centers was achieved for metallocene and non-metallocene soluble catalysts. Chapter 6 discusses possible structures of the true catalytic species, mostly based on detailed spectroscopic studies and kinetic analysis. In addition to the two principal catalyst components, many modern commercial transition metal catalysts also contain three other components: supports, inert carriers, and modifiers. Catalyst supports, although inactive by themselves, have a significant influence on the catalyst performance; they increase catalyst activity or change properties of polymers produced with the catalysts. In terms of their effect on active centers, supports can be viewed as ligands in homogeneous catalysis. The most widely used supports include MgCl2 and silica [4,6,34,35]. In contrast, carriers do not affect catalyst performance to any noticeable degree and their use is warranted by various technological requirements. For instance, carriers dilute very active solid catalysts, make them more easily transportable, and agglomerate catalyst species into particles of a specific desirable shape, usually spheres. Often, the same material, e.g., MgCl2 and silica, serves both purposes; it acts as a true support and, simultaneously, forms catalyst particles of a required shape. Soluble metallocene and non-metallocene transition metal catalysts are also often produced in the supported form or packaged inside carrier particles. In addition, nearly all modern titanium- and vanadium-based Ziegler–Natta catalyst systems include one or several organic components which are, intermittingly, caller catalyst modifiers, catalyst activators, external or internal organic donors, etc. Their true role in the catalysis is discussed in detail in Chapter 4.

1.1.2. Catalyst classification based on solubility Until the 1970s, nearly all polymerization reactions with transition metal catalysts were carried out in inert hydrocarbon solvents such as hexane, heptane, or toluene. These solvents readily dissolve all alkenes and all above-listed organoaluminum cocatalysts. Historically, the catalyst systems were classified based on solubility of their transition metal components in a polymerization medium. This classification, although initially arbitrary, turned out to be sufficiently meaningful in practice and is still widely used (see examples in Table 1.1). Homogeneous transition metal catalysts: The catalyst systems in which both the starting transition metal compounds and the products of their interaction with organometallic cocatalysts, including active centers of polymerization reactions, are

Transition Metal Catalysts and Catalytic Polymerization Reactions

Table 1.1

5

Classification of Ziegler–Natta catalysts

Catalysts for ethylene polymerization

Catalysts for polymerization and copolymerization of 1-alkenes

Homogeneous catalyst systems V(acetylacetonate)3-AlEt2Cl (at low temperatures) VCl4-AlEt2Cl

VCl4-AlEt2Cl VOCl3-Al2Et3Cl3

Pseudo-homogeneous catalyst systems TiCl4-AlEt2Cl VCl4-AlEt2Cl VOCl3-Al2Et3Cl3

TiCl4-AlEt3 VOCl3-AlEt3

Heterogeneous catalyst systems d-TiCl3-AlEt3 TiCl4/silica-AlEt3 TiCl4/MgCl2/silica-AlEt3 VOCl3/MgCl2/silica-AlEt3

a-TiCl3-AlEt3 d-TiCl3-AlEt3 VCl3-Ali-Bu3 TiCl4/MgCl2/modifier – AlEt3/modifier

soluble in the reaction medium. Table 1.1 lists some homogeneous Ziegler–Natta catalysts that found wide laboratory applications. Several catalysts of this type containing vanadium compounds, VCl4 and VOCl3, have industrial significance in the manufacture of elastomeric ethylene–propylene copolymers and cross-linkable ethylene-propylene–diene terpolymers. All unsupported catalyst systems based on metallocene complexes (Section 1.3) and soluble non-metallocene transition metal complexes (Section 1.4) also belong to the group of homogeneous polymerization catalysts, although in this case the implied solvent is not aliphatic but aromatic, usually toluene. Pseudo-homogeneous transition metal catalysts: The group of catalyst systems in which the starting transition metal compound is also soluble in the hydrocarbons. However, when its solution is combined with solution of an organoaluminum cocatalyst, a rapid reaction ensues with the formation of solid products (Table 1.1). The most important example of such a transition metal compound is TiCl4. The first industrial catalysts for ethylene polymerization discovered by Ziegler in 1953, TiCl4-Al(C2H5)2Cl and TiCl4-Al(C2H5)3 systems, belong to this class [23]. A number of pseudo-homogeneous titanium-based Ziegler–Natta systems are still important in commercial polymerization reactions of ethylene and its copolymerization with alkenes. Heterogeneous transition metal catalysts: Catalyst systems in which both the starting catalyst containing a transition metal component and the products of its interaction with an organometallic cocatalyst are insoluble in the polymerization medium. This large class includes the catalysts currently widely used in industry for polymerization and copolymerization of alkenes. All supported Ziegler–Natta catalysts, supported metallocene and non-metallocene transition metal catalysts, as well as chromium oxide catalysts belong to this class.

6

Alkene Polymerization Reactions with Transition Metal Catalysts

1.2. Composition and Structure of Ziegler–Natta Catalysts 1.2.1. Organoaluminum cocatalysts All organometallic compounds used as cocatalysts in titanium- and vanadium-based Ziegler–Natta catalyst systems are liquids with high boiling points. They readily dissolve in saturated and aromatic hydrocarbons and in liquid alkenes. These compounds usually exist as dimers or trimers [36]. The dimerization occurs via the formation of three-atom bridges Al    C–Al and Al    Cl–Al between two or three organoaluminum molecules. Typical dimeric compounds of this type are [Al(C2H5)3]2 (this molecule contains two Al    C–Al bridges) and [Al(C2H5)2Cl]2 (this molecule contains two Al    Cl–Al bridges). Only a few organometallic cocatalysts, e.g., Al(i-C4H9)3 and Zn(C2H5)2, are monomers. It is customary to abbreviate structures of alkyl groups in organometallic compounds in the following manner (consistently with abbreviations in organometallic chemistry in general): CH3 ¼ Me, C2H5 ¼ Et, n-C3H7 ¼ n-Pr, isoC3H7 ¼ i-Pr, n-C4H9 ¼ n-Bu, iso-C4H9 ¼ i-Bu, n-C6H13 ¼ n-Hex, C6H5 ¼ Ph, C6H5–CH2 (benzyl) ¼ Bz, cyclohexyl ¼ Cy, etc. Thus, Al(CH3)3 is depicted as AlMe3, Al(C2H5)3 as AlEt3, Al(C2H5)2Cl as AlEt2Cl, Al(iso-C4H9)3 as Ali-Bu3, etc. These abbreviations are used throughout this book (see also page xi). All the organometallic compounds are highly reactive in contact with the majority of organic and inorganic compounds, with a few exceptions including nitrogen, noble gases, saturated and aromatic hydrocarbons, and alkenes [6,36]. All organoaluminum cocatalysts violently react with water and alcohols and burn when exposed to air. For this reason, they are always handled in an inert atmosphere and with extreme caution.

1.2.2. Transition metal catalyst components of Ziegler–Natta catalysts The list of transition metal compounds used as components of homogeneous and pseudo-homogeneous Ziegler–Natta catalysts which are soluble in hydrocarbon solvents is short, TiCl4, VCl4, and VOCl3. All these compounds are dense liquids. The same compounds, especially TiCl4, are also used for the synthesis of modern supported Ziegler–Natta catalysts (Sections 4.2 and 4.3). The list of solid transition metal compounds commonly used as components of Ziegler–Natta catalysts is also short; overwhelmingly, they are TiCl3 and VCl3. Both these compounds are crystalline; they are completely insoluble in aliphatic or aromatic hydrocarbons but are soluble in polar solvents such as alcohols, ethers, and esters. TiCl3 is also widely used for the preparation of supported Ziegler–Natta catalysts. TiCl3 was employed as a component of early heterogeneous Ziegler–Natta catalysts. This compound exists in several crystalline modifications. Three of them, the a-form, the g-form, and the d-form, as well as other similar halogen derivatives of transition metals, such as TiBr3, VCl3, and CrCl3, have the same elementary crystal pattern shown in Figure 4.1, a three-layered flat sheet [37]. Each elemental

Transition Metal Catalysts and Catalytic Polymerization Reactions

7

sheet contains a layer of transition metal cations sandwiched between two layers of chlorine anions. The three TiCl3 modifications differ only in the relative stacking arrangement of these three-layered sheets. The d-form of TiCl3 is still used as a commercial catalyst component. It is prepared by reacting TiCl4 with metallic aluminum and contains Al atoms; its chemical composition is usually TiCl3  0.33 AlCl3.

1.2.3. Examples of Ziegler–Natta catalysts 1.2.3.1. Early and modern Ziegler–Natta catalysts During the first 15 years after the discovery of Ziegler–Natta catalysts, four transition metal compounds, TiCl4, TiCl3, VOCl3, and VCl4, were used as components of all commercial catalyst systems. Only a few catalysts were used in industry at that time, the three most important being the TiCl4–AlEt2Cl system for polyethylene production, the d-TiCl3  0.33AlCl3-AlEt2Cl system for the production of crystalline polypropylene and other polyolefins, and the VOCl3-Al2Et3Cl3 and VCl4-Al2Et3Cl3 systems for the production of elastomeric ethylene-propylene copolymers. However, over time, continuous efforts in the development of new catalytic compositions produced a variety of new catalysts and brought about drastic improvements in their performance. These efforts were directed toward two main goals, an increase in the catalyst productivity and an improvement in polymer quality. To achieve these goals, a large variety of catalytic modifiers were evaluated and special supports for titanium and vanadium compounds were developed. Together, these improvements increased the catalyst activity greatly, over 100–200 times. Currently, the overwhelming majority of commercial Ziegler–Natta catalysts for the synthesis of polyolefins are fine-tuned for the synthesis of particular polymers. Catalysts for the synthesis of polyethylene, polypropylene, and ethylene/alkene copolymers are very different in terms of catalyst preparation procedures. The catalysts are also fine-tuned for particular polymerization processes (gas-phase, slurry, or solution) and for the manufacture of polyolefins with particular combinations of properties and morphology. In the case of gas-phase and slurry polymerization processes, nearly all the catalysts are supported on silica or MgCl2 [4,6,34,38,39]. Commercial companies that manufacture polyolefins have developed two strategies with respect to Ziegler–Natta catalysts. Some companies either license recipes from a few specialized chemical companies excelled in the development of such catalysts or just buy ready-to-use catalysts. Other polyolefin-producing companies have developed their own proprietary catalytic compositions for alkene polymerization [39]. Several catalysts of different types are described in detail in Chapter 4. Many academic researchers often follow industry trends and use in their research universally accepted commercial catalysts. The following examples give the gist of the approaches used for the design of the catalysts. A reader should be aware that each successful catalyst composition is an outcome of many years of intense research, most of it empirical. Therefore, a

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Alkene Polymerization Reactions with Transition Metal Catalysts

rationale for using one or another component during the catalyst preparation or for particular synthesis conditions described in patents is often not clearly understood or it may be deliberately made obscure. All these catalysts should be judged strictly on the basis of their performance and the quality of polymers they produce. Another point to be made is that after a catalyst is designed (and, possibly, successfully used in industry), efforts to understand the chemistry behind its success may continue for many years and may produce the understanding of the underlying chemistry very different from the original hypothesis which led to practical success. In very general terms, every supported Ti-based polymerization catalyst contains three functionally different components: 1. An active ingredient, nearly always TiCl4. 2. A support, usually MgCl2. The principal role of the support is to increase the productivity of the active ingredient. 3. A carrier, an inert material that gives a desirable shape, preferably spherical, to catalyst particles. The most popular carrier is spherical porous amorphous silica. In many cases, the support material itself is shaped into catalyst particles of a suitable morphology, such as spherical MgCl2 particles. In these cases, a special carrier is not needed.

1.2.3.2. Examples of catalysts for polymerization of ethylene and for copolymerization of ethylene with higher 1-alkenes Catalyst containing complex of TiIV and Mg chlorides in silica carrier [40–43]: The active ingredient is the ionic crystalline Ti–Mg complex [Mg2Cl3  (THF)6]+ [TiCl5  THF] formed in a reaction between TiCl4 and MgCl2 in tetrahydrofuran solution [40,41,44]. The complex is also soluble in tetrahydrofuran. The carrier for the complex is prepared from porous silica [40]. To achieve maximum catalyst activity, hydroxyl groups are removed from the silica surface, first by dehydrating it at 500–6001C and then by treating remaining OH groups with AlEt3. Solution of the Ti–Mg complex is mixed with the OH group-free silica and then tetrahydrofuran is removed by evaporation at 50–601C causing the ionic complex to crystallize within silica pores. This solid product, when combined with AlEt3, is a very active polymerization catalyst. It was found beneficial, from the catalyst performance viewpoint, to remove a part of tetrahydrofuran from the crystalline Ti–Mg complex by pre-treating the catalyst with organoaluminum compounds [42,43]. Catalyst utilizing MgCl2 both as a carrier and a support [45–48]: The catalyst is prepared in several steps: (a) synthesis of a carrier/support, (b) introduction of a transition metal compound into the support, and (c) optionally, a prepolymerization reaction. Although crystalline anhydrous MgCl2 is readily available commercially, the size and the shape of its particles are not suitable for the synthesis of high-quality polymerization catalysts. For this reason, MgCl2 is synthesized from a dialkylmagnesium compound, MgR2. First, hydrocarbon solution of MgR2 is mixed with di-i-amyl ether at an [ether]:[MgR2] ratio of 0.4. The ether is an electron donor and it forms a complex with MgR2. tert-Butyl chloride, a source of Cl atoms, is slowly

Transition Metal Catalysts and Catalytic Polymerization Reactions

9

added to the solution at 40–501C at a [tert-C4H9Cl]:[MgR2] ratio of B1.9. The reaction produces MgCl2 in a highly dispersed form: MgR2 þ 2 tert-C4 H9 Cl ! MgCl2 þ 2 tert-C4 H9 2R

(1.1)

The precipitate still contains some unreacted Mg–C bonds. The size of MgCl2 particles produced in Reaction (1.1) is controlled by the amount of di-i-amyl ether, reaction temperature, and the rate at which tert-butyl chloride is added. Under best conditions, MgCl2 forms small spherical particles, 25–50 mm in diameter. The particles are porous; their specific surface area is about 40 m2/g. They serve both as a support and as a catalyst carrier. In the second step, a catalyst is prepared by reacting TiCl4 with an organoaluminum compound, usually AlR2Cl, in the presence of preformed spherical MgCl2 particles. The reaction is carried out at 50–801C at a [Ti]:[Al]:[Mg] molar ratio of 1:1.2:3.5. The principal chemical reaction is: TiCl4 þ AlR2 Cl ! TiCl3 þ AlRCl2 þ R2R

(1.2)

The final catalyst is a complex mixture of inorganic and organometallic compounds; its approximate composition (per gram atom of Ti) is: TiIII (0.94), TiIV (0.04), Al (0.2), Mg (3.9), Cl (10.0), ether (0.1). 1.2.3.3. Examples of catalysts for polymerization of propylene and higher alkenes Modern catalysts developed for the polymerization of propylene and other 1-alkenes usually use MgCl2 both as a support and a carrier [49,50]. The catalysts are differentiated depending on the procedure used for fashioning catalyst particles from MgCl2 and depending on the type of organic modifiers employed during catalyst preparation and use. These organic modifiers are traditionally called ‘‘internal donors’’ and ‘‘external donors,’’ although the real roles of these organic compounds often have nothing to do with their electron-donor properties. Two most important technological characteristics of all these catalysts are high productivity and a high fraction of the crystalline isotactic polymer they produce at 70–801C under standard polymerization conditions. The latter parameter is determined by fractionation with boiling n-heptane or with xylene, as described in Section 2.3.2.1. There are three different commonly used techniques for the manufacture of suitable MgCl2 particles: (a) grinding commercially available anhydrous MgCl2, (b) precipitation of MgCl2 from solutions in alcohols, and (c) synthesis of MgCl2 from dialkylmagnesium compounds or Grignard reagents. Catalysts containing ground MgCl2 and aromatic acid esters: These catalysts are prepared by co-grinding anhydrous MgCl2 and TiCl4 in the presence of aromatic acid esters [51]. The catalysts are employed with cocatalyst mixtures containing AlEt3 or Ali-Bu3 and a similar aromatic acid ester, usually ethyl benzoate or ethyl anisate [52–54]. One particular example of such a catalyst is prepared by prolonged co-milling MgCl2 and TiCl4 in a ball mill in the presence of ethyl benzoate [52]. When this catalyst is activated with AlEt3 alone, it is very active but produces polypropylene containing a very high fraction, up to 30–40%, of an amorphous

10

Alkene Polymerization Reactions with Transition Metal Catalysts

atactic material (called the atactic fraction), as well as a material with a low degree of crystallinity (usually called the stereoblock fraction). However, when the same catalyst is activated with a mixture of AlEt3 and ethyl anisate in a B3:1 ratio, polypropylene with a very highly content of crystalline material is produced. Catalysts containing MgCl2 and esters of aromatic diacids: These supported catalysts are produced by employing esters of aromatic diacids, usually dialkyl phthalates, instead of esters of aromatic monoacids. When these catalyst systems employ mixtures of AlEt3 and arylalkoxysilanes RxSi(ORu)4x as cocatalysts at an [Al]:[Si] ratio of 10:1 to 20:1 [55–60], they produce alkene polymers that contain 95–99% of highly crystalline material. A further development of the catalysts of this type resulted in replacement of the esters of aromatic diacids with 2,2-dialkyl-substituted 1,3-diethers (see Section 4.3.2). The latter catalysts usually employ pure AlEt3 as a cocatalyst. Table 1.2 gives several examples of the relative activity of different TiCl4/MgCl2 catalysts in propylene polymerization reactions and the content of crystalline isotactic fractions in the polymers, and Table 1.3 describes the effects of organic modifiers in TiCl4/MgCl2 catalysts and in AlEt3-based cocatalysts on the fraction of crystalline polypropylene. Table 1.2

a

Activity of various TiCl4/MgCl2 catalysts and yields of crystalline polypropylene [61]

Catalyst

Cocatalyst

Productivity, kg/g catalysta

Relative yield of crystalline polymer

Isotactic indexb (%)

TiCl4/MgCl2 TiCl4/MgCl2/di-i-butyl phthalate TiCl4/MgCl2/ 1,3-dietherc

AlEt3 AlEt3/ PhSi(OEt)3 AlEt3

24 40

(1) 3.9

42 97

80

7.8

98

Polymerization reactions at 701C. Fraction of crystalline polymer insoluble in xylene at 251C. 2,2-Di-i-butyl-1,3-dimethoxypropane.

b c

Table 1.3 [49,50]

Yield of crystalline polypropylene produced with different TiCl4/MgCl2 catalysts

Catalyst

Cocatalyst

Isotactic index (%)

TiCl4/MgCl2 TiCl4/MgCl2/ester of aromatic acid TiCl4/MgCl2/ester of aromatic acid

AlEt3 AlEt3 AlEt3/ester of aromatic acid AlEt3/alkoxysilane

B40 B60 B95 97–99

AlEt3

97–99

TiCl4/MgCl2/ester of aromatic diacid TiCl4/MgCl2/1,3-diether

Transition Metal Catalysts and Catalytic Polymerization Reactions

11

1.2.3.4. Catalysts for copolymerization of ethylene and propylene Ethylene-propylene copolymerization reactions are usually carried out with homogeneous vanadium-based catalysts, e.g., VOCl3-Ali-Bu2Cl or VOCl3-Al2Et3Cl3 [1,4,62]. The catalysts are modified with chlorinated organic compounds. The same catalysts are used for terpolymerization reactions of ethylene, propylene, and nonconjugated dienes. The most frequently used dienes are ethylidene norbornene and dicyclopentadiene. The latter polymer products contain double bonds in their chains and can be vulcanized with sulfur and other cross-linking agents [63]. These vanadium-based catalyst systems are widely used in commercial practice.

1.3. Metallocene Catalysts Two major types of metallocene catalysts for alkene polymerization were developed. The catalysts of the first type are bis-metallocene complexes of three transition metals, titanium, zirconium, and hafnium. The catalysts of the second group are based on complexes of the same transition metals containing one cyclopentadienyl group (see Scheme 1.1). Early metallocene catalysts: The original metallocene catalyst systems contained (C5H5)2TiCl2 (usually depicted as Cp2TiCl2) and a dialkylaluminum chloride cocatalyst, such as AlMe2Cl, AlEt2Cl, etc. [25,26]. These catalyst systems are soluble in aromatic solvents (benzene, toluene) and in halogenated alkanes. The titanocene catalysts were the first to produce ethylene copolymers with propylene and 1-butene that possess a high degree of compositional uniformity [64]. Unfortunately, these catalysts have only moderate activity, and the molecular weight of copolymers they produce rapidly decreases with an increase of the 1-alkene content [24]. This circumstance greatly reduced the practical usefulness of early metallocene catalysts. Kaminsky–Sinn catalysts: In 1976, Kaminsky and Sinn discovered several new catalyst systems based on metallocene complexes of early-period transition metals that are highly active in ethylene polymerization reactions [27,29,65]. Several types of metallocene complexes can be used for the synthesis of polyolefins and alkene copolymers. Original catalyst systems of this type contain two components (their list is given in Table 1.4), a metallocene complex of zirconium, titanium, or hafnium with two cyclopentadienyl rings, and a cocatalyst, methylalumoxane (commonly abbreviated as MAO) [27,29,65,66]. MAO is usually synthesized by controlled hydrolysis of AlMe3 [27,67,68]. Ten to thirty percent of Al atoms in commercially manufactured MAO belong to residual AlMe3, the starting material for MAO synthesis. Pure MAO is a white amorphous pyrophoric solid soluble in aromatic solvents [69]. From the structural viewpoint, MAO is a mixture of oligomeric organoaluminum compounds [Al(Me)O]n with n ¼ 4–25 [68,70]. The exact structure of MAO is not yet precisely known. Nuclear magnetic resonance (NMR) data showed that virtually all Al atoms in MAO are four-coordinated and all O atoms are three-coordinated [68,71]. MAO is not the only alkylalumoxane compound capable of activating metallocene complexes. Other compounds of a

12

Table 1.4

Alkene Polymerization Reactions with Transition Metal Catalysts

Classification of metallocene polymerization catalysts

Catalysts for ethylene polymerization

Catalysts for polymerization of 1-alkenes

Early metallocene catalyst systems Cp2TiCl2-AlEt2Cl Kaminsky–Sinn catalyst systems Cp2ZrCl2-MAO (R-p)2ZrCl2-MAO Cp2ZrMe2-MAO Ind2ZrCl2-MAO Ionic metallocene catalyst systems [Cp2ZrMe]+ [B(C6F5)4] [(RCp2)ZrMe]+ [MeB(C6F5)3]

rac-C2H4(Ind)2ZrCl2 – MAO (isospecific catalyst) rac-C2H4(Ind-H4)2ZrCl2-MAO (isospecific catalyst) Me2Si(Cp)(Flu)ZrCl2-MAO (syndiospecific catalyst) CpTiCl3-MAO (syndiospecific catalyst for styrene polymerization) [rac-C2H4(Ind)2ZrMe]+ [B(C6F5)4] (isospecific catalyst) [rac-C2H4(Ind-H4)2ZrMe]+ [B(C6F5)4](isospecific catalyst)

Monocyclopentadienyl (constrained-geometry) catalyst systems [C5H4-SiMe2-(tert-Bu)N]TiCl2-MAO [C5Me4-SiMe2-(tert-Bu)N]TiCl2-MAO {[C5H4-SiMe2-(tert-Bu)N]TiMe}+ [B(C6F5)4]

similar structure, e.g., i-butylalumoxane, can also be used as cocatalysts, although their efficiency is substantially lower. A large number of metallocene complexes can be used in Kaminsky–Sinn catalysts [29,72]. They usually belong to the following classes (Table 1.4):
Bis(cyclopentadienyl) complexes Cp2MX2, where M is Ti, Zr, or Hf, and X are halogen atoms, H, or small alkyl groups. The cyclopentadienyl groups can carry various alkyl substituents.
Bis-metallocene complexes in which one or both ligands are indenyl groups, C9H7, tetrahydroindenyl groups, C9H11, or fluorenyl groups, C13H9.
Bridged metallocene complexes with cyclopentadienyl, indenyl, tetrahydroindenyl, and fluorenyl ligands. The bridges connect two cyclopentadienyl rings. The most frequently used bridges are –CH2–CH2–, WSiMe2, WCMe2, WSiPh2, and –CHPh–CHPh–. The structures of these metallocene complexes are shown in Scheme 1.1. Among metallocene complexes of this type, zirconium complexes are usually preferred because of their high activity. When polymerization reactions with Kaminsky–Sinn catalysts are carried out in solution, either in an aromatic or in an aliphatic solvent, the [MAO]:[Zr] ratio is usually high, 1,000–2,000, although much

13

Transition Metal Catalysts and Catalytic Polymerization Reactions

R X

X

X

M

M

M X

X

X R

I

III

II

X X

X

M

M

M

E

X

X

X

VI

V

IV

X M X3

X1

VII

X

N

X3

X1

X2

M

E

M X2

R′

VIII

IX

R2 E

MX2

E

E

MX2

MX2

R1

R1

rac-X

meso-X

XI

R

R R

R2 E

MX2

E

MX2

R 3

4 5

R1

XII

R1

XIII

2 6 E Subsitution in indenyl group

E

MX2

MX2

E

R′ R′′ XIV

Scheme 1.1

XV

Metallocene complexes used in polymerization catalysis.

14

Alkene Polymerization Reactions with Transition Metal Catalysts

lower ratios are employed under commercial conditions. Kaminsky–Sinn catalysts, when used for ethylene polymerization in toluene solutions at sufficiently high [Al]:[Zr] ratios, exhibit exceptionally high activity, B5  106 g polymer/g Zr  h  bar ethylene [29,66]. Many commercial polymerization processes require preparation of supported Kaminsky–Sinn catalysts. Several techniques were developed to synthesize supported metallocene catalysts (see Section 4.8). As a rule, highly porous supports with particles of a spherical shape are used, silica or alumina. Ionic metallocene catalysts: A large volume of research was carried out with a goal of finding alternative cocatalysts for metallocene complexes to replace MAO. The most efficient among these catalyst systems are prepared from alkylated metallocene complexes Cp2MR2, where R is usually CH3. The active species in these catalyst systems are ionic pairs [Cp2MR]+ A containing metallocenium cations [Cp2MR]+ and bulky non-coordinative anions A with a broadly distributed negative charge. The preferred anions are usually perfluorinated phenylboron moieties, e.g., [B(C6F5)4]. Several reactions were invented to produce the ionic pairs. They are described in detail in Chapters 4 and 6. In theory, ionic metallocene catalysts do not need any organoaluminum cocatalysts. However, monomers of very high purity are required in these reactions to prevent catalyst poisoning. To avoid the very expensive monomer purification, a small quantity of a trialkylaluminum compound, AlMe3 or Ali-Bu3, is usually added to the catalysts as an impurity scavenger. Monometallocene catalysts: These catalyst systems, which are often called ‘‘constrained-geometry metallocene catalysts,’’ contain monocyclopentadienyl complexes of transition metals [73,74]. One of the carbon atoms in the cyclopentadienyl ring and the metal atom are linked by a bridge, –SiMe2–(tert-Bu)N– or –CH2–(tert-Bu)N–, with the nitrogen atom in the bridge attached to the metal atom, e.g., [C5Me4SiMe2(tert-Bu)N]TiCl2 (see complex IX in Scheme 1.1). These complexes are converted to polymerization catalysts by reacting them with MAO or by forming ionic pairs with non-coordinative anions such as [B(C6F5)4]. Catalysts of this type are very effective in copolymerization reactions of ethylene with various 1-alkenes, cycloalkenes, and styrene. They are stable at temperatures up to 1601C and produce high molecular weight polymers.

1.4. Homogeneous Catalysts Containing Non-Metallocene Complexes of Early- and Late-Period Transition Metals At the present time, the development of homogeneous catalyst systems based on non-metallocene complexes of various transition metals is the leading research area in the field of alkene polymerization catalysis. A large variety of multidentate complexes of early- and late-period transition metals are being explored (Schemes 1.2 and 1.3). Some of the most promising catalysts of this type are described in Section 4.7. The first catalysts with late-period transition metals suitable for alkene polymerization were cationic complexes of PdII and NiII with bulky a-diimine ligands [(Ar–NQC(R)C(R)QN–Ar)M(CH3)(OEt2)]+ [BAru4], where Ar and

15

Transition Metal Catalysts and Catalytic Polymerization Reactions

Complexes with diphenoxy ligands

O

O

R R

O

R

O

Ti

Ti

R

O

R Ti

O

R

Complexes with diamide ligands R X

N

R = 2,6-iso-Pr2-C6H3 , 2,6-Me2-C6H3

Ti X = Cl, Me, CH2Ph

X

N R

Complexes with phenoxy-imine ligands R

R

M = Ti, Zr O

R′

Ar N R′

....

M

R = Me, t-Bu, halogen atoms

N Ar

Cl

Cl O

R

....

Ar = Ph, C 6F5, 3,5-F2-Ph

R′ = H (aldimine complexes), R′ = alkyl group (ketimine complexes)

R

Scheme 1.2 Non-metallocene complexes of early-period transition metals used in polymerization catalysis.

Aru are the substituted aryl groups, and R is H or a methyl group. The complexes are soluble in toluene and methylene chloride. Neutral a-diimine Ni complexes of this type combined with MAO are extremely active as catalysts for ethylene polymerization at temperatures from 0 to 251C; their productivity approaches 200 kg/g Ni  h. A unique feature of the complexes is their ability to polymerize ethylene with the formation of branched polymers.

1.5. Chromium Oxide Catalysts All chromium oxide catalysts are based on CrVI and are supported on inert porous substrates [75–79]. The best support is silica. Others supports include alumosilicates with a low alumina content, silica-titania, and aluminum phosphate.

16

Alkene Polymerization Reactions with Transition Metal Catalysts

Complexes with α-diimine ligands M = Ni, Pd X = Cl, Br, Me, NCS, NCSe R

R

R′ = H, Me, Et

N

R′

X R′+R′ = α,α-Dinaphthyl, cyclohexyl,

M X R′

cyclohexenyl

N R

R = Me, i-Pr, CF3, Ph, C6F5

R

Complexes with bis(imino)pyridyl ligands M = Co, Fe, V X1 and X2 = Me, i-Pr, t-Bu, R

R

N N

X3

X3 = H, Me, Et, i-Pr

N

M

R = H, Me

Cl

Cl

Cl, Br X1

X1

X2

X2

X3

Ni ylide complex Ph

Ph P

Ph Ni

Ph

O

PPh3

Scheme 1.3 Non-metallocene complexes of late-period transition metals used in polymerization catalysis.

Preferred supports have a specific surface area greater than 300 m2/g and a large pore volume, W1 cm3/g. The catalyst synthesis consists of two main steps. In the first step, a particulate support is impregnated with aqueous solution of chromic acid or organic solution of a chromium salt. In the second step, the solvent is removed from the catalyst by evaporation at elevated temperatures, after which the dry catalyst is activated by calcination at 500–850 1C in a dry oxidizing environment. At these temperatures, remaining water and most of hydroxyl groups are removed from the catalyst surface and precursors of active sites, silyl chromate groups containing CrVI species, are formed [77,80]. A typical chromium content in the finished catalysts ranges from 0.1 to 5 wt.%. In some cases, activated catalysts are subsequently reduced with CO or H2 at 300–5001C. Numerous modifications of chromium-based catalysts were

Transition Metal Catalysts and Catalytic Polymerization Reactions

17

prepared through the introduction of various additives. The most effective additives are titanium alkoxides [81]. Chromium-based catalysts are usually activated in the beginning of a polymerization reaction through their exposure to ethylene at high temperatures. The activation step can be accelerated with carbon monoxide. Chromium oxide catalysts operate at 85–1101C [81,82]. After the catalysts are exposed to ethylene, they are initially virtually inactive, then the polymerization rate starts gradually to increase and, finally, the catalyst has a period of nearly constant activity. The catalysts exhibit very high productivity, from 3 to 10 kg polyethylene/gram of catalyst, corresponding to 300–1,000 kg polymer/g Cr.

1.6. Main Features of Alkene Polymerization Reactions Transition metal polymerization catalysts operate is a wide temperature range, from 0 to 100–1501C, and at monomer pressures from 0.1 to 2 MPa (1–20 atm, or 15–300 psi). The polymerization reactions are carried out in an inert liquid medium (e.g., hexane, isobutane, or a high-boiling alkane mixture), in liquid monomer, or in the gas phase. The molecular weight of polyolefins obtained with these catalysts without any additional control is often quite high and make the polymers unsuitable for commercial processing. To lower the molecular weight, molecular hydrogen is widely employed as a chain transfer agent when the polymerization reactions are carried out with Ziegler–Natta or metallocene catalysts. Chromium oxide catalysts are not sensitive to hydrogen; a higher polymerization temperature is needed to reduce the molecular weight of the polymers. Three features of the transition metal catalysts are important, both from the academic and the practical viewpoint: 1. Nearly all these catalysts can copolymerize various alkenes. 2. Some of the catalysts produce polymer chains with a particular regular spatial arrangement of neighboring monomer units. This feature of the catalysts is called stereospecificity (see Section 1.6.5). 3. All solid, supported, and pseudo-homogeneous transition metal catalysts, as well as some homogeneous catalysts, irrespective of the type of transition metal, contain active centers of several types. The centers differ one from others in many respects. A large part of research in the field of transition metal catalysis is devoted to the study of these three aspects of alkene polymerization reactions. The principal terminology is described in this section. It provides the basis for a more detailed description of the polymerization reactions in subsequent chapters of the book. The mechanism of all these polymerization reactions has one common feature, which was exhaustively confirmed in numerous experiments described in Chapters 3, 5, and 6. The growth of a polymer chain with any transition metal catalyst occurs via the insertion reaction of the CQC bond of an alkene molecule CH2QCH–R

18

Table 1.5 [6,83,84]

Alkene Polymerization Reactions with Transition Metal Catalysts

Reactivities of alkenes in polymerization reactions with Ziegler–Natta catalysts

Alkene

Formula

Relative reactivity

Ethylene Propylene 1-Butene 1-Pentene 1-Hexene 1-Decene 1-Octadecene 3-Methyl-1-butene 3-Methyl-1-pentene 4-Methyl-1-pentene 4-Methyl-1-hexene Vinylcyclohexane Styrene Vinylnaphthalene

CH2QCH2 CH2QCH–CH3 CH2QCH–C2H5 CH2QCH–C3H7 CH2QCH–C4H9 CH2QCH–C8H17 CH2QCH–C16H33 CH2QCH–CH(CH3)2 CH2QCH–CH–C(CH3)–C2H5 CH2QCH–CH2–CH(CH3)2 CH2QCH–CH2–CH(CH3)–C2H5 CH2QCH–cyclo-C6H11 CH2QCH–C6H5 CH2QCH–C10H7

100 (reference) B5–10 2.0 1.8 1.3 0.8 0.6 0.2 0.2 0.8 0.6 0.05 B1 B0.5

into the transition metalcarbon bond of an active center, MC. M2CH2 2Polymer þ CH2 QCH2R ! M2CH2 2CHR2CH2 2Polymer

(1.3)

In the majority of cases, the alkene molecule inserts into the MC bond in such a way that its CH2 group becomes attached to the metal atom, as shown in Reaction (1.3), although the opposite orientation of alkene molecules can also take place, as described in Chapter 3. The reactivity of various alkenes in Reaction (1.3) strongly depends on the size and the spatial shape of the alkyl substituent R. This dependence can be approximately represented by relative reactivities of different alkenes in polymerization reactions with Ti-based Ziegler–Natta catalysts shown in Table 1.5.

1.6.1. Basic principles of polymerization kinetics The simplest polymerization reaction scheme consists of four reactions. The active centers in the catalysts are formed in reactions between M–X bonds in catalyst precursors (X is usually Cl, M is Ti or V in Ziegler–Natta catalysts, Zr, Ti, or Hf in metallocene catalysts, etc.) and the AlRu bond in a cocatalyst. The reaction produces the MRu group. ½M2Cl þ AlR03 ! ½M  R0 þ AlR02 Cl

(1.4)

Some of these MRu groups become active centers in polymerization reactions; they insert double bonds of alkene molecules into their M–C bonds (Reaction (1.3)) with the formation of a growing polymer chain attached to the metal atom. This reaction is called the chain growth reaction (chain propagation reaction). kp

½M2R0 þ nCH2 QCH2R ! ½M2ðCH2 2CHRÞn 2R0

(1.5)

19

Transition Metal Catalysts and Catalytic Polymerization Reactions

The chain growth reaction usually occurs many thousands times before any other reaction intervenes. Very infrequently, an alkene molecule reacts with a growing polymer chain in a different way compared to that in Reaction (1.3). kM t

½M2ðCH2 2CHRÞn 2R0 þ CH2 QCH2R ! ½M2CH2 2CH2 2R þ CH2 QCR2ðCH2 2CHRÞn1 2R0

(1:6Þ

This reaction is called the chain transfer reaction to a monomer. Reaction (1.6) results in disengagement of the polymer chain from the active center, but the active center remains intact; it contains a short alkyl group CH2–CH2–R attached to the transition metal atom and it retains ability to insert double bonds of alkene molecules in Reaction (1.3) and to grow a new polymer chain. Because Reaction (1.6) occurs rarely, the polymer chains contain, on average, a large number of monomer units, the n value can reach from 10,000 to 50,000. Polymers with such a high polymerization degree (a very high molecular weight) are difficult to process and have limited commercial value. A special agent (nearly always, hydrogen) is usually added to polymerization reactions with Ziegler–Natta catalysts. It hydrogenates the MC bond in the growing polymer chain. kH t

½M2ðCH2 2CHRÞn 2R þ H2 ! ½M2H

(1:7Þ

þ CH3 2CHR2ðCH2 2CHRÞn1 2R Reaction (1.7) is called the chain transfer reaction to hydrogen. The center with the M–H bond remains active; it also can insert double bonds of alkene molecules, similarly to Reaction (1.3). Reaction (1.7) proceeds with a much higher rate than Reaction (1.6), and the molecular weight of polymers produced in the presence of hydrogen is always much lower. Polymerization reactions of alkenes with transition metal catalysts are nearly always carried out at a constant monomer concentration. The polymerization rate, the rate of monomer consumption, is traditionally expressed as Rpol ¼ kp  C   C M 

(1.8)

where C is the concentration of active centers in the catalyst and CM the monomer concentration. The experimental evaluation of the C value is very difficult (see Chapters 5 and 6); therefore, the product of two values in Equation (1.8), kp and C, which is called the effective rate constant, keff,  is often used as a single parameter that characterizes the catalyst activity. Different catalysts differ greatly in the keff value, in some cases, over a thousand times. Some transition metal catalysts exhibit a stable level of activity (their C value does not change in time) but the majority of the catalysts are unstable, their C value initially increases and then decreases. There are numerous studies of polymerization kinetics which are primarily concerned with the effects of reaction conditions on the C value (Chapter 5). The average polymerization degree n of a polymer chain formed in Reactions (1.5)–(1.7) is equal to the ratio between the probability (or the reaction rate) of the chain growth reaction (Reaction (1.5)) and the combined probability of two chain

20

Alkene Polymerization Reactions with Transition Metal Catalysts

transfer reactions, Reactions (1.6) and (1.7). n ¼ Rate of chain growth reaction=Sum of rates of chain transfer reactions  H   kp  C   C mon =ðkM t  C  C M þ kt  C  C H Þ

¼

kp  C mon =ðkM t

 C M þ kH t

ð1:9Þ

 CHÞ

H Here CH is the hydrogen concentration and kp, kM t , and kt are the rate constants of Reactions (1.5), (1.6), and (1.7), respectively. The average molecular weight of a polymer is the product of the n value and the molecular weight of the respective alkene molecule.

1.6.2. Copolymerization reactions of alkenes Transition metal catalysts easily copolymerize various alkenes [1,4,6,62,83,84]. In a copolymerization reaction, two different 1-alkene molecules, CH2QCH–Ru and CH2QCH–Rv, are competitively inserted into the M–C bond of an active center. In the simplest case, four chain growth reactions should be considered instead of a single chain growth reaction in a homopolymerization reaction (Reaction (1.5)). M2CH2 2CHR0 2Polymer þ CH2 QCH2R0 ðk11 Þ

! M2CH2 2CHR0 2CH2 2CHR0 2Polymer

(1:10Þ

M2CH2 2CHR0 2Polymer þ CH2 QCH2R00 ðk12 Þ

! M  CH2 2CHR00 2CH2 2CHR0 2Polymer

(1:11Þ

M2CH2 2CHR00 2Polymer þ CH2 QCH2R00 ðk22 Þ

! M  CH2 2CHR00 2CH2 2CHR00 2Polymer

(1:12Þ

M2CH2 2CHR00 2Polymer þ CH2 QCH2R0 ðk21 Þ

! M  CH2 2CHR0 2CH2 2CHR00 2Polymer

(1:13Þ

Rate constants in the above set of reactions, kij, are marked in the following way: the first number, i, identifies the 1-alkene molecule that inserts into the M–C bond, 1 for the first comonomer, CH2QCH–Ru, and 2 for the second comonomer, CH2QCH–Rv. The second number, j, identifies the alkene molecule which produced the last monomer unit in the copolymer chain, the unit that is attached to the metal atom. Two of the above reactions represent homopolymerization growth reactions, Reaction (1.10) corresponds to homopolymerization of CH2QCH–Ru and Reaction (1.12) homopolymerization of CH2QCH–Rv. Two other reactions represent chain cross-growth reactions. The values of each of four rate constants in this reaction scheme are different, which means that the rate of the insertion reaction of a particular alkene into the M–C bond depends not only on the structure of the alkene itself but also on the structure of the last monomer unit attached to the transition metal atom, CH2–CHRu or CH2–CHRv.

Transition Metal Catalysts and Catalytic Polymerization Reactions

21

It is customary to combine these rate constants into two ratios, called the reactivity ratios, r1 ¼ k11/k12 and r2 ¼ k22/k21 [4,6,84]. The evaluation of r1 and r2 values usually requires carrying out a series of copolymerization experiments at different comonomer concentrations and measuring compositions of the produced copolymers. The standard equation for the copolymer composition gives the following relationship between the ratio of monomer concentrations in the reaction medium, F ¼ (CMu/CMv)comonomer, and the ratio of comonomer contents in the respective copolymer, f ¼ (CMu/CMv)copolymer. f ¼ F  ðr 1 F þ 1Þ=ðr 2 þ FÞ or f ¼ r 1 F  ðr 1 F þ 1Þ=ðr 1 r 2 þ r 1 FÞ

(1.14)

In many practically important cases, first of all, manufacture of resins called linear low-density polyethylenes, LLDPE (see Section 1.7.1), the main goal is the synthesis of copolymers of ethylene with various 1-alkenes containing only a few mole percents of the alkenes, usually from 2 to 5 mol.%. For such copolymers, the f value is quite high, from B100 to B20, and Equation (1.14) can be approximated as f Er1F. The values of the reactivity ratios for different 1-alkene pairs are reported in numerous articles; they vary in a very wide range. Table 3.51 lists r1 and r2 values for copolymerization reactions of ethylene with various 1-alkenes, and Tables 3.52 and 3.53 list r1 and r2 values for pairs of different 1-alkenes. Alkene copolymerization reactions have great commercial significance. The reactions provide the basis for the manufacture of linear low-density polyethylene, copolymers of ethylene containing from 3 to 5 mol.% of 1-butene, 1-hexene, 1-octene, or 4-methyl-1-pentene. Other commercial products produced in the copolymerization reactions are impact-resistant polypropylene (mixtures of polypropylene and ethylene/ propylene copolymers); commercial poly(4-methyl-1-pentene) (a copolymer of 4-methyl-1-pentene containing 3–5% of a higher linear 1-alkene); ethylene-propylene elastomers; and ethylene-propylene-diene synthetic rubbers. Copolymers of alkenes produced with transition metal catalysts belong to the class of random copolymers. Four chain growth steps represented by Reactions (1.10– 1.13) occur in a purely statistical manner. During the growth period of an average copolymer molecule (this time is very short, usually from a fraction of a second to several seconds), many thousands of chain growth reactions occur at each active center. If concentrations of both comonomers are kept constant, a given active center produces, in a period of one to two hours, many thousands of copolymer molecules with approximately the same average composition described by Equation (1.14). Within each copolymer chain, monomer units derived from both comonomers, the CH2–CHRu unit and the CH2–CHRv unit, are positioned randomly. The required statistical description of copolymer chains is presented in Sections 3.8.2 and 3.8.3.

1.6.3. Auto-copolymerization reactions and formation of polymer chains with long-chain branches When polymerization reactions with transition metal catalysts are carried out in the absence of specially introduced chain transfer agents (first of all, hydrogen), polymer molecules contain double bonds at one end of nearly each polymer chain formed in Reaction (1.6). If the last monomer unit in a polymer chain happens to be an

22

Alkene Polymerization Reactions with Transition Metal Catalysts

ethylene unit (in ethylene homopolymerization reactions and ethylene/alkene copolymerization reactions), this polymer molecule has the structure CH2QCHPolymer. From the chemical viewpoint, this is a high molecular weight 1-alkene which itself can be polymerized with the same catalyst, although its reactivity in the polymerization reaction may be quite low. The polymer molecules with vinyl double bonds are usually called macromonomers (or macromers) to distinguish them from ‘‘comonomer molecules,’’ 1-alkene molecules that are deliberately added to the reaction medium. Some metallocene catalysts, in particular, catalysts containing complexes with one cyclopentadienyl ring described above, are quite active in polymerization reactions of 1-alkene molecules with large alkyl groups. Therefore, they can copolymerize ethylene with just produced polyethylene macromers containing vinyl double bonds. The result of such an auto-copolymerization reaction is the formation of a significant fraction of polyethylene molecules with one or several long-chain side-groups (usually called long-chain branches) due to the macromer units. Although the content of branches in copolymers with ‘‘in situ’’ formed 1-alkene macromers is usually very low, their presence in polymer chains strongly affects the rheological behavior of such polymers.

1.6.4. Oligomerization reactions By definition, oligomers are a subclass of polymer molecules with very low molecular weights. Whereas polymers of alkenes are either crystalline solid materials or elastomers, oligomers of the same alkenes are either liquids or waxes with low melting points. Conceptually, the difference between polymerization and oligomerization reactions of alkenes is small. In both cases, Reaction (1.5) represents the chain growth reactions and the only difference between the two reactions is the frequency of the chain transfer reaction (Reaction (1.6)) in which a polymer chain is separated from the transition metal atom in an active center. Ethylene oligomers are important commercial products. They are linear molecules with a vinyl double bond at one end, CH2QCH–(CH2–CH2)nH, where n is a small number, from 1 (1-butene) to 30–40. They are used as comonomers in copolymerization reactions with other alkenes (e.g., 1-butene, 1-hexene, and 1-octene with ethylene, higher linear alkenes with 4-methyl-1pentene). Ethylene oligomers are also raw materials for the manufacture of synthetic lubricating oils, synthetic detergents, plasticizers, etc. Ethylene oligomers are produced with several types of transition metal catalysts. The most successful of them was developed by W. Keim (Germany), it is a special nickel complex (see Section 4.7.2). This complex is used as a single-component catalyst in a commercial ethylene oligomerization process called Shell Higher Olefin Process (SHOP) [85–87].

1.6.5. Stereospecific alkene polymerization and stereoregular polyolefins G. Natta and his coworkers discovered in 1954 that polymers of 1-alkenes produced in reactions catalyzed by heterogeneous titanium- and vanadium-based catalysts are crystalline [1,24]. This discovery had very important consequences. Crystalline

23

Transition Metal Catalysts and Catalytic Polymerization Reactions

polymers of 1-lkenes have very attractive mechanical properties. Some of them, including polypropylene, poly(1-butene), and poly(4-methyl-1-pentene), became widely used as general-purpose and engineering materials. Crystallographic studies of the polymers revealed that their crystallinity derives from a specific feature of their molecular structure called stereoregularity [1,4,6,24]. Stereoregularity is the existence of a special order in the configuration of a polymer chain. This spatial regularity is twofold. First (and most important for the subject of this book), monomer units of 1-alkenes are linked during the polymer synthesis in a particular regular manner that depends on the structure of a catalyst and polymerization conditions. Second, these regularly linked polymer chains spontaneously arrange themselves in a spatially regular form, usually in a helix form. The regularity of the first type (it is called the tacticity of a polymer chain) arises in the course of a polymerization reaction. It cannot be altered or destroyed in any subsequent physical transformation of a polymer, such as melting or stretching, although it can be altered in a chemical reaction called epimerization. A chain of any alkene polymer, except for polyethylene, has a backbone with the chemical structure.

R

R

R

R

R

R

Here R is a side-group attached to the CH group, CH3 in polypropylene, C2H5 in poly(1-butene), etc. If one draws the polymer backbone in the form of a planar zigzag (Figure 1.1), several regular patterns of side-group positions can be easily visualized [1,4,6,66]. 1. All side-groups R lie on one side of the zigzag plane (above or below it), as in Figure 1.1A. Natta named these polymers isotactic. All crystalline polymers of 1-alkenes produced with heterogeneous Ziegler–Natta catalysts are isotactic. Some metallocene, chromium oxide, and homogeneous non-metallocene catalysts also produce isotactic polyolefins. Only a few catalysts can produce perfectly isotactic polymers of 1-alkenes. In many cases, occasional errors in the steric structure of a polymer chain occur. Depending on the nature and frequency of the steric errors, partially isotactic polymers can be qualitatively classified either as iso-block (Figure 1.1B) or stereo-block (Figure 1.1C) polymers. 2. Side-groups R alternatively occupy positions above and below the backbone plane (Figure 1.1E). These polymers were named syndiotactic. Several metallocene and homogeneous non-metallocene catalysts afford the synthesis of syndiotactic crystalline polyolefins and polystyrene. Again, only a few catalysts can produce perfectly syndiotactic polyolefins; usually, occasional errors in the steric structure of polymer chains occur. 3. A special group of stereoregular polyolefins is called hemi-isotactic (Figure 1.1D). In this case, every second monomer unit occupies a nearly perfectly isotactic position whereas alternate monomer units can be, with a nearly equal probability,

24

Alkene Polymerization Reactions with Transition Metal Catalysts

R

R

R

R

R

R

R

R

A

R

R

R

R

R

R

B R R

R

R

R

R

C R R

(R)

(R)

R

(R)

(R)

(R)

R

(R)

R

R

(R)

(R)

(R) R

R

R

R

R

D

R

E R R

R R

R

R R

R

R

R F

R

R

Figure 1.1 Representation of polyole¢n chains of di¡erent stereoregularity in the £at zigzag form: A, isotactic polymer; B, iso-block polymer; C, stereoblock polymer; D, hemi-isotactic polymer; E, syndiotactic polymer; F-atactic polymer.

either in the isotactic or in the syndiotactic arrangement with respect to the neighboring groups. The synthesis of several polyolefins of this type became possible with special metallocene catalysts. 4. If no regular pattern can be found in the arrangement of side-groups, the polymer is said to be atactic, or stereo-irregular (Figure 1.1F). Atactic polyolefins are usually produced as by-products in the synthesis of isotactic polymers with Ziegler–Natta catalysts, with special supported Ti-based catalysts, as well as with many non-bridged metallocene catalysts. Atactic polyolefins are amorphous. The regularity of the second type in the polyolefin structure refers to the actual spatial structure of polymer chains. Flat structures of isotactic and syndiotactic polyolefins shown in Figures 1.1A and 1.1E are purely imaginary. In reality, the chains are always coiled in helixes [1,6]. The form of the helix depends primarily on

Transition Metal Catalysts and Catalytic Polymerization Reactions

25

the size and the shape of its side-groups R. Usually, only one helical type is possible for a given R. Isotactic polyolefins form three main types of helices, as shown in Figure 1.2. All polyolefin macromolecules in the helix form easily aggregate and produce three-dimensional regular structures, crystals. Sometimes the same helixes can, depending on crystallization conditions, form crystals of different types, but usually only one of the crystal structures is thermodynamically stable. Over several decades,

Figure 1.2 Chain helices of isotactic polyole¢ns. Large open circles are side-alkyl groups; helices are characterized by two numbers which give the number of monomer units per one repeating element, e.g., 72 means seven monomer units per two turns of a helix.

26

Alkene Polymerization Reactions with Transition Metal Catalysts

great efforts were undertaken to study the types of the helixes and crystal structures of numerous polyolefins [1,4,6,88]. The growth of a predominantly isotactic polyolefin chain in Ziegler–Natta catalysis is governed by a special steric control mechanism which is discussed in detail in Section 3.1.3.1. 1-Alkene molecules are predominantly inserted into the M–C bond in Reaction (1.3) in the same spatial manner, which is determined by the structure of an active center. As a result, adjacent monomer units become attached to each other in the isotactic manner. Occasionally, a steric error occurs, one alkene molecule is inserted into the M–C bond with its alkyl group aimed in the opposite direction. If the catalyst is highly isospecific, it usually corrects this error during the insertion of the next alkene molecule. The stereocontrol mechanism in polymerization reactions with isospecific metallocene catalysts is more complex but, superficially, it produces the same type of a dominant steric mistake. In the past 20 years, the 13C NMR technique became the dominant method for the evaluation of polymer stereoregularity. As a consequence, the stereochemical nomenclature particular to the NMR method has been universally accepted for the description of polymer stereoregularity in general. In a 13C NMR spectrum of any polymer, peak positions of carbon atoms in a given monomer unit in the chain are determined by their steric relationship with neighboring units. If any two neighboring units have the same steric arrangement, i.e., if two monomer units form the isotactic link, the combination of the two units is defined as the meso diad, m. If the two neighboring units have opposite steric arrangements, i.e., if two monomer units form the syndiotactic link, the combination is defined as the racemic diad, r. Statistics of imperfectly isotactic polymers: The below example shows a part of an imperfectly isotactic polymer chain with a single steric error and the corresponding NMR definition of the chain.

m m m m m m m r

r

m m m m m m

The steric error in this chain (one monomer unit in the inverted position) contains two adjacent r links. A set of statistical equations called the enantiomorphic statistical model for a predominantly isotactic polymer was developed to describe the chain statistics for this stereocontrol mechanism [51]. Statistical equations for this model are listed in Table 3.1. The equations have only one parameter, the probability of an errorless continuation of the isotactic chain, piso (the probability of the steric error is 1 – piso). For the perfectly isotactic chain, piso ¼ 1. If piso is slightly lower than 1, the polymer is predominantly isotactic, and if piso is close to 0.5 the polymer is atactic. For example, the average piso value for the propylene chain growth reaction on a highly isospecific active center is 0.97–0.99 [51,89]. Statistics of imperfectly syndiotactic polymers: The examples below show parts of mostly syndiotactic polymer chains with two types of steric errors and corresponding NMR definitions of such chains. In catalytic polymerization reactions, the reversal of the unit orientation in a syndiotactic chain usually occurs at every step, thus

Transition Metal Catalysts and Catalytic Polymerization Reactions

27

leading to rrrr sequences. However, on two occasions in this example, m m r r r r r r r r r r r r r r m m r r r

the orientation of the monomer unit was not reversed. The predominant steric error in this chain contains three adjacent monomer units in the mm arrangement, i.e., it contains one predominant steric error, the mm triad flanked by two mr triads. A set of statistical equations called the enantiomorphic stereocontrol model for a predominantly syndiotactic polymer was developed to describe the chain statistics (see Section 3.1.3.3). Another type of stereo-control mechanism in syndiospecific polymerization reactions, called the chain-end mechanism, leads to a different type of steric error. r m r r r r r r r r r r r r r r m r r r

The predominant steric error in this chain is the m diad flanked by two mr triads. Statistical equations for a predominantly syndiotactic polymer of this type are given in Section 3.1.3.4.

1.6.6. Nonuniformity of active centers in transition metal catalysts All polymerization products of 1-alkenes obtained with heterogeneous Ziegler– Natta catalysts, chromium oxide catalysts, and with some metallocene catalysts are polymer mixtures containing macromolecules of different stereoregularity [1,6]. These polymer mixtures can be separated by several methods, the simplest being the solvent fractionation. Amorphous atactic polymers easily dissolve at room temperature in many solvents, including n-heptane or diethyl ether, whereas highly crystalline isotactic polymers have very low solubility or are completely insoluble in these solvents even at high temperatures. Polypropylene represents the best-studied case. Any propylene polymer produced with a titanium-based Ziegler–Natta catalyst contains several fractions. They usually include a highly crystalline isotactic fraction insoluble in boiling n-heptane, an amorphous atactic fraction soluble in n-heptane at room temperature (the same fraction can be separated with boiling diethyl ether), and an intermediate fraction of reduced crystallinity soluble in boiling heptane (it was named the stereoblock polymer) [1]. Crystalline isotactic polymers have a much higher commercial value, and Ziegler–Natta catalysts are specially designed or modified to maximize the yield of crystalline isotactic material. For example, modern supported catalysts for propylene polymerization (Section 1.2.3.3) produce polypropylene resins with the yields of the crystalline fraction exceeding 95–97%. These fractionation data indicate that active centers in Ziegler–Natta catalysts are not identical with respect to isospecificity, the ability to produce isotactic polymers. The catalysts always contain active centers of several types, ranging from highly isospecific to completely stereo-aspecific [6]. This feature of the catalysts is usually called ‘‘nonuniformity of active centers.’’ When various organic modifiers are added to these catalysts, they usually have little effect on the isospecific centers but they

28

Alkene Polymerization Reactions with Transition Metal Catalysts

poison or alter the aspecific centers and in this way greatly improve the quality of the polymers [6]. The situation is opposite for homogeneous polymerization catalysts based on metallocene complexes. These catalysts usually have only one type of active center and they produce sterically uniform products. For example, the homogeneous Cp2ZrCl2-MAO system polymerizes propylene to an atactic material that does not contain any crystalline fraction [65]. On the other hand, the homogeneous catalyst system containing the bridged C2H4(Ind)2ZrCl2 complex and MAO polymerizes propylene to a nearly completely isotactic material [28]. The existence of active centers of different types in heterogeneous Ziegler– Natta catalysts has another important manifestation. Alkene copolymers produced with heterogeneous catalysts always contain copolymer molecules of different compositions. This happens because active centers of different types have different reactivities in the copolymerization reactions (i.e., different values of reactivity ratios r1 and r2, see Equation (1.14)). On the other hand, most homogeneous metallocene catalysts produce compositionally uniform copolymers, all macromolecules in them have approximately the same composition. The uniformity of the copolymer composition distribution is very important in the case ethylene/1-alkene copolymers containing small amounts of 1-alkenes. The subject of nonuniformity of active centers is discussed in detail in Chapters 2 and 3.

1.7. Classes of Polymers Produced with Transition Metal Catalysts It is customary to apply a generic name ‘‘polyolefin resins’’ to polymer materials belonging to a family of homopolymers and copolymers derived from various alkenes. Polyolefin resins have the general formula (CH2–CHR)n where R ¼ H in polyethylene, CH3 in polypropylene, C2H5 in poly(1-butene), CH2–CH(CH3)2 in poly(4-methyl-1-pentene), etc. Molecular weights of polyolefin resins vary from very small, 300–600, in alkene oligomers and waxes to very large, W3,000,000, in polyolefin resins with an ultrahigh molecular weight. Commercial resins manufactured with transition metal catalysts are divided into six classes: 1. 2. 3. 4. 5. 6.

Ethylene polymers and copolymers. Propylene polymers and copolymers. Ethylene/propylene elastomers and synthetic rubbers. Polymers and copolymers of higher 1-alkenes. Polymers and copolymers of cycloalkenes and cyclodienes. Crystalline syndiotactic polystyrene.

Polyolefin resins are produced worldwide in a very large volume, B120  106 metric tons/year in 2006, which corresponds to over 50% of the total production of all plastic materials. Ethylene polymers and copolymers account for 60–65% of the

Transition Metal Catalysts and Catalytic Polymerization Reactions

29

total polyolefin production and propylene polymers for B35%; other types of polyolefin resins are manufactured in much lower volumes.

1.7.1. Linear polyethylene and semi-crystalline ethylene copolymers All catalytically produced semi-crystalline polymers and copolymers derived mostly from ethylene and used as commodity plastics are called polyethylene resins. Most polyethylene molecules contain branches in their chains. In very general terms, the molecular structure of a polyethylene resin can be represented by a formula –(CH2–CH2)x–Branch1–(CH2CH2)y–Branch2–(CH2–CH2)z–Branch3–, where the –CH2–CH2– units come from ethylene, and x, y, and z values can vary from 4–5 to a very large number. The branches are alkyl substituents in 1-alkenes, they are introduced into polyethylene resins when ethylene is copolymerized with 1-alkenes (Section 1.2.3.2). Some polyethylene resins are produced in ethylene homopolymerization reactions; they are called ‘‘linear polyethylene’’ and contain no branches at all. The commercial name of these materials is ‘‘high-density polyethylene resins’’ (HDPE). The content of 1-alkenes in commercially manufactured ethylene/1-alkene copolymers varies in a very large range, up to 25–30 wt.%. These copolymers, depending on the content of a 1-alkene, are called medium-density polyethylene resins (MDPE), linear low-density polyethylene resins (LLDPE), or very low-density polyethylene resins (VLDPE). Polymerization processes utilizing metallocene catalysts can also introduce long-chain branches in polyethylene chains (see Sections 1.6.3 and 3.8.4). Some metallocene catalysts can copolymerize ethylene with cycloalkenes, such as cyclopentene, cyclooctene, norbornene, etc. In this case, the branches are either small cycles consisting from 5 to 10 carbon atoms, or two fused cycles. These materials are called cycloalkene copolymers (COC) [91]. Some polyethylene resins have uniform compositional distributions, any polymer molecule contains about the same fraction of branches as any other polymer molecule. The majority of LLDPE and VLDPE resins produced with metallocene catalysts belong to this group. In contrast, LLDPE and VLDPE resins produced with heterogeneous Ziegler–Natta catalysts and with chromium oxide catalysts are mixtures of polymer molecules with very different contents of 1-alkenes. Some macromolecules in these mixtures contain a very small number of 1-alkene units while other copolymer molecules contain a relatively large number of the units. The commercial classification of polyethylene resins is given in Table 1.6. It is based on two easily measured parameters, density (this parameter is in a reciprocal dependence with the average content of a 1-alkene in a copolymer) and its melt index, a rheological parameter inversely correlated with the molecular weight of a polymer. This classification provides a simple means for basic differentiation of commercial polyethylene resins. 1.7.1.1. Catalysts and technologies of manufacture of polyethylene resins Three technological processes are used for the manufacture of polyethylene resins in catalytic reactions.

30

Table 1.6

Alkene Polymerization Reactions with Transition Metal Catalysts

Commercial classification of polyethylenes

Name

Abbreviation

Density (g/cm3)

Crystallinity (%)

High-density polyethylene Polyethylene of ultrahigh molecular weight, linear polymer with molecular weight W3  106 Medium-density polyethylene Linear low-density polyethylene Low-density polyethylene, produced in high-pressure processes Very low-density polyethylene

HDPE UHMW PE

Z0.941 0.935–0.930

60–70 40–50

MDPE LLDPE LDPE

0.926–0.940 0.915–0.925 0.910–0.940

45–55 35–45

VLDPE

0.915–0.880

10–20

Polymerization in slurry: Catalytic polymerization reactions in hydrocarbon slurries, usually in light saturated hydrocarbons (isobutane, hexane, heptane), were historically the first commercial ethylene polymerization processes using chromium oxide and Ziegler–Natta catalysts. These processes still enjoy high popularity due to their versatility and convenience of operation [19,88]. Polymerization in the gas phase: Many polymerization catalysts, both Ziegler– Natta catalysts and metallocene catalysts supported on inert carriers, can be adopted for the use in the gas phase [20,88,92]. A gas-phase reactor contains a large bed of polymer particles. The particles are agitated either by a mechanical stirrer or by employing the fluidized-bed technique. A fresh catalyst is continuously added to the bed and polymer particles are continuously removed from it. These processes are very economical due to the absence of solvent recovery stages. Polymerization in solution: Many hydrocarbons dissolve polyethylene at 120–1501C. A variety of Ziegler–Natta and metallocene catalysts survive high temperatures and can be used in solution processes [88]. These processes require, as their last step, the removal of the solvent from the polymer.

1.7.1.2. Control of polyethylene properties and its commercial uses The density and the crystallinity level of catalytically produced polyethylenes are primarily determined by the amount of 1-alkenes in ethylene/1-alkene copolymers; it is achieved by varying relative amounts of ethylene and the 1-alkenes in polymerization reactions. The molecular weight of polyethylene resins is controlled by two means: (a) reaction temperature, the principal control method in polymerization processes with chromium oxide catalysts and (b) the use of a special chemical agent for chain transfer, hydrogen, with Ziegler–Natta catalysts. Catalytically produced polyethylene resins enjoy a broad range of applications, both as commodity resins, engineering resins, and in some cases as specialty polymers. The most important application is thin film that is made from HDPE and

Transition Metal Catalysts and Catalytic Polymerization Reactions

31

LLDPE. The film is mostly used for bags and packaging. Polyethylene resins are also used to manufacture household and industrial containers, bottles of different sizes and shapes for water, food products, detergents, liquid fuels, etc.; for coating of paper, metal, and glass; for wire and cable insulation; toys; pipe and tubing of various types. Because of its versatility, polyethylene resins, in total, have become the largest commercially manufactured polymers in the world.

1.7.2. Ethylene/propylene elastomers Two types of amorphous elastomeric ethylene/propylene copolymers are manufactured commercially. The first one is a random copolymer of ethylene and propylene containing from 40 to 60 wt.% of ethylene. This material has the industrial name ‘‘ethylene/propylene (EP) elastomer.’’ The second type is a random ternary polymer of ethylene, propylene, and a cyclodiene, such as ethylidene norbornene or dicyclopentadiene. These copolymers are called ‘‘ethylene/propylene/diene (EPDM) elastomers.’’ They contain 55–60 wt.% of ethylene, 35–40 wt.% of propylene, and 5–10% of the cyclodiene. The presence of cyclodiene units in EPDM elastomers affords their vulcanization with sulfur and cross-linking with peroxides and other agents. The early copolymerization catalysts developed in the 1960s are still widely used for EP and EPDM production [8]. They are based on VCl4 or VOCl3 as transition metal catalysts, AlEt2Cl or AlEt3 as cocatalysts, and various modifiers, mostly chlorinated aliphatic hydrocarbons. The second type of catalyst used in the commercial manufacture of EP and EPDM elastomers is based on metallocene complexes. Both types of catalysts perform under commercially accepted conditions as single-center catalysts; they produce copolymers in which the composition of all macromolecules is approximately the same independently on their molecular weight. EP and EPDM elastomers are soluble in aliphatic and aromatic hydrocarbons. A large part of EP and EPDM production is based on continuous stirred-tank solution processes under moderate reaction conditions, at 20–501C, using light hydrocarbons as solvents. The second technology of EP and EPDM manufacture utilizes a continuous stirred-tank slurry process in which propylene is used as the reaction medium. The EP/propylene slurry has relatively low viscosity and the reactant conversion up to 30% is possible before the slurry becomes excessively thick. The third technology for the manufacture of the elastomers is a fluidized-bed gas-phase process similar to those used for the manufacture of HDPE, LLDPE, and polypropylene. The manufacture of sticky amorphous ethylene/propylene elastomers in a gas phase-reactor is difficult. To avoid agglomeration of polymer particles, the addition of a significant quantity of an inorganic filler powder to the reactor, mostly carbon black, is necessary. EP and EPDM elastomers are amorphous materials with a glass point of –50 to –601C. The elastomers are usually used in the cross-linked form. Three types of cross-linking are practiced, radiation (best suitable for EP resins), with organic peroxides, and with sulfur-based agents similar to those used for vulcanization of natural and synthetic rubber. Two properties of the elastomers make them attractive

32

Alkene Polymerization Reactions with Transition Metal Catalysts

for industrial use. The first one is their exceptionally high chemical stability to a variety of corrosive inorganic compounds, to heat, light, and to oxidation, particularly by ozone. Due to the complete absence of double bonds in the polymer structure in EP rubbers and their low content in EPDM rubbers, the ozone resistance of the elastomers is significantly higher that that of natural rubber, butyl rubber or poly(chloroprene). The second distinctive feature of the elastomers is their ability to accommodate very large quantities of oils and inorganic fillers with the retention of elasticity and physical strength. EP and EPDM rubbers are widely used in the automotive industry for the manufacture of profiled articles, radiator hoses, and seals. Other major applications of the elastomers are binders in roofing materials, cable and wire insulation and jacketing, and in a variety of molded articles.

1.7.3. Poly(olefins) 1.7.3.1. Propylene polymers and copolymers Four types of propylene polymers are produced by industry: 1. Isotactic polypropylene, a tough semi-crystalline material widely used as a commodity and an engineering plastic. 2. Impact resistant polypropylene, a mixture of isotactic polypropylene and an amorphous ethylene/propylene copolymer. 3. Syndiotactic polypropylene, a semi-crystalline material used as an engineering plastic. 4. Atactic polypropylene, an amorphous material used as a binder for a variety of plastics and inorganic materials, especially in the construction industry. Two types of catalysts are used for the synthesis of isotactic polypropylene, supported catalysts based on TiCl4 and MgCl2, and metallocene catalysts [19,21,22,93]. Propylene polymers synthesized with TiCl4/MgCl2 catalysts dominate the word production, 93–95%. Syndiotactic polypropylene is produced exclusively with metallocene catalysts. Two different slurry processes are used in industry for the manufacture of polypropylenes [21]. The first one uses liquid propylene as the reaction medium; it is called a ‘‘bulk’’ (or liquid-pool) polymerization process. This is the most important technology for the manufacture of polypropylene, it accounts for nearly 60% of its worldwide production. Another type of the slurry process uses a hydrocarbon diluent, B15% of the world production [21]. The second most important technology of polypropylene manufacture is the fluidized-bed gas-phase technology, B25% of the world production [21,92]. Both Ziegler–Natta and supported metallocene catalysts can be used in gas-phase reactors. Demand for isotactic polypropylene with improved impact resistance at low temperatures resulted in the development and commercial production of impactresistant polypropylene, a mixture of isotactic polypropylene and an amorphous ethylene/propylene copolymer. These resins account for B25% of the total world production of polypropylene. They are manufactured in two linked reactors [94,95]. Highly crystalline isotactic polypropylene is produced in the first reactor and then

Transition Metal Catalysts and Catalytic Polymerization Reactions

33

the resin particles with a still active catalyst are transferred into the second reactor where an ethylene/propylene copolymerization reaction takes place. Isotactic polypropylene is a rigid semi-crystalline plastic [8,19–22,93]. It has density of B0.903 g/cm3, its melting point is B1651C, and its glass point is B–101C. The resin has high mechanical strength, its tensile strength is 35 MPa and the flexural modulus is 1.2–1.7 GPa. Because of these properties, isotactic polypropylene is used as an engineering thermoplastic. When isotactic polypropylene is mixed with 10–15% of ethylene/propylene copolymer (impact-resistant polypropylene), its glass point decreases even further and these mixed materials can be used at below-freezing temperatures. Isotactic polypropylene prepared with metallocene catalysts has a small fraction of chemical defects in its polymer chains and, as a consequence, it usually has a lower melting point, 155–1601C. This resin is also very tough and is used as a general-purpose and an engineering plastic. Syndiotactic polypropylene is a rigid semi-crystalline plastic with density of B0.88 g/cm3. It has a lower melting point than isotactic polypropylene, B1301C. Its crystallinity, tensile strength (B15 MPa), and the modulus (480–500 MPa) are also lower than for isotactic polypropylene. However, this polymer has exceptional softness, much higher clarity and gloss, and a good scratch resistance [19,22,93]. Atactic polypropylene is an amorphous material with a glass point of about – 20 1C. This material is always present in small quantities (1–5%) in commercial polypropylene resins produced with Ziegler–Natta catalysts. The presence of a small amount of the atactic fraction modifies properties of commercial polypropylene resins; it improves their low-temperature performance, processability, and optical properties, but decreases stiffness and long-term aging properties [22]. Over 50% of isotactic polypropylene is converted by injection molding into a variety of articles with complex profiles [19,20,22]. Important markets for injectionmolded articles are automotive; household appliances; toys; rigid packaging; and medical products. Melt-spun fibers and nonwoven fibers represent the second most significant use of isotactic polypropylene, 25–30% [20]. The biggest fraction of the fiber is used for the manufacture of rugs and wall-to-wall carpeting. Other applications of isotactic polypropylene include biaxially oriented film and cast film (10–13%), extruded sheet of various thickness, and pipes (3–4%). Syndiotactic polypropylene is produced in a relatively small volume, B2% of the overall polypropylene production. It is mostly used for the manufacture of sheet, film, and for injection molding. Other markets for syndiotactic polypropylene include blends with other polymers, sterilizable medical products, elastomers, and plastomers. 1.7.3.2. Commercial polymers of higher 1-alkenes All alkenes CH2QCH-R with R other than H and CH3 are called in industry higher 1-alkenes. Two types of polymers of higher 1-alkenes are produced commercially, isotactic poly(1-butene) and isotactic poly(4-methyl-1-pentene). A variety of catalysts and technological processes are used for the manufacture of poly(1-alkenes). Isotactic polymers of higher 1-alkenes are produced with supported

34

Alkene Polymerization Reactions with Transition Metal Catalysts

Ti-based Ziegler–Natta catalysts of the same type that is used for the manufacture of isotactic polypropylene. These polymerization reactions are usually carried out at 50–801C in slurry in pure monomers or in their mixtures with hydrocarbon solvents [96–98]. Isotactic poly(1-butene) is a semi-crystalline plastic. It has very strong resistance to creep at moderate temperatures and is used to manufacture pipe and tubing, as well as film. Isotactic poly(4-methyl-1-pentene) is also a semi-crystalline plastic. Its applications usually capitalize on high optical transparency of the resin, its excellent dielectric characteristics, high thermal stability, and chemical resistance. The uses include medical equipment, such as hypodermic syringes, blood collection, and transfusion equipment, etc., as well as chemical and biomedical laboratory equipment. Higher 1-alkenes can be also polymerized with metallocene catalysts with the formation of amorphous atactic polymers or, when the polymerization reactions are carried out at 70–1001C, low molecular weight oligomers which can be used as components of synthetic lubricating oils [8,99]. 1.7.3.3. Poly(cycloalkenes) and cycloalkene copolymers Cycloalkenes are polymerized by means of two different mechanisms. Catalysts based on tungsten, ruthenium, and molybdenum compounds (ROMP catalysts) induce the metathesis reaction, ring-opening polymerization of monocycloalkenes with the formation of elastomers containing regularly spaced double bonds in polymer chains [100,101]. The polymerization of cycloalkenes with metallocene catalysts produces polymers and copolymers with a completely different structure. The reaction proceeds via the double bond opening in cycloalkene molecules and the formation of C–C bonds between adjacent rings [101,102]. Copolymers of cycloalkenes and ethylene synthesized with metallocene catalysts are tough transparent amorphous materials with high glass points, from 70 to 1801C, depending on the copolymer composition [91,103]. They are used in many optical applications (lenses, prisms, etc.), and for the manufacture of transparent containers and film. 1.7.3.4. Syndiotactic polystyrene Researchers at Idemitsu Company in Japan synthesized syndiotactic polystyrene in 1986 [104,106]. The catalysts for its manufacture include metallocene complexes in combination with MAO [104–106]. The most suitable among them are monometallocene complexes (complexes VII and VIII in Scheme 1.1), CpTiCl3, CpTi(OR)3, IndTiCl3, etc. [104–108]. Syndiotactic polystyrene is a highly crystalline plastic with a very high melting point, B2751C, and a high crystallization rate. Its thermal stability and mechanical properties are in the range typical for engineering plastics [106,109].

CHAPTER 2

Single-Center and Multi-Center Polymerization Catalysis

Contents 2.1. Definition of Single Type of Active Center 2.2. Molecular Weight Distribution of Polymers Produced with Single-Center Catalysts 2.2.1. Molecular weight distribution, theory 2.2.2. Experimental techniques for analysis of molecular weight distribution, gel permeation chromatography 2.2.3. Experimental techniques for the measurement of molecular weight distribution used in industry 2.2.4. Experimental techniques for the analysis of molecular weight distribution, gas chromatography 2.3. Structural Uniformity of Polymers and Copolymers Produced with SingleCenter Catalysts 2.3.1. Structural uniformity of polymers and copolymers, theory 2.3.2. Experimental techniques for the analysis of steric structure of alkene homopolymers and compositional distribution of copolymers 2.4. Examples of Polymers and Copolymers Produced with Single-Center Catalysts 2.4.1. Molecular weight distribution of polymers produced with single-center catalysts 2.4.2. Structural uniformity of alkene polymers produced with single-center catalysts 2.5. Examples of Polymers and Copolymers Produced with Multi-Center Catalysts 2.5.1. Molecular weight distribution of polymers produced with multi-center catalysts 2.5.2. Steric structure of alkene homopolymers, different definitions of stereoregularity 2.5.3. Steric structure of alkene homopolymers produced with multi-center catalysts 2.5.4. Compositional distribution of copolymers produced with multi-center catalysts

36 37 37 40 43 45 46 47 48 63 63 64 65 66 74 76 79

It can be stated with confidence that the overwhelming majority of solid and supported transition metal catalysts used for alkene polymerization, as well as many soluble catalysts, contain several types of active centers. The level of this confidence

35

36

Alkene Polymerization Reactions with Transition Metal Catalysts

becomes clear if one compares alkene polymerization catalysts with heterogeneous catalysts used for other transformations of organic compounds. One such an example is zeolite catalysts for cracking of alkanes and isoalkanes. A single high molecular weight isoalkane, 2-methyltetradecane, was cracked in a model reaction over a fresh commercial zeolite catalyst at 3501C, which are very mild conditions compared to commercial conditions for the use of such catalysts. The reaction generated, with widely different yields, approximately 200 hydrocarbons with molecular weights lower than C15, as well as B30 products in the C15 range and a number of products with the carbon atom numbers higher than C15 [110]. It is virtually impossible to determine whether all these compounds are formed on the same centers at a different time or whether several types of centers coexist in the catalyst, each producing a specific set of products. For example, one type of center may isomerize the feed molecule into other C15 isoalkanes, the second type may crack the molecule to lighter alkanes and alkenes, the third type of center may produce aromatic compounds, etc. Another such example is Fisher-Tropsch catalysis, conversion of a mixture or CO and H2 into complex mixtures containing linear alkanes, isoalkanes, alkenes, alcohols, aldehydes, ketones, esters, and carboxylic acids [111]. Again, it is very difficult to determine whether these compounds are formed on the same centers or whether several types of centers coexist in the catalysts, one producing alkanes, another polar organic molecules, still another alkenes, etc. Polymerization catalysts are much more amendable to this type of investigation because every polymer molecule represents a record of thousands consecutive reactions starting with a single chain initiation step, followed by many chain growth steps and ending with a single chain transfer step. Chemical and stereochemical details of each of the steps are accessible to modern analytical techniques, primarily 13 C NMR; they are described in detail in Chapter 3. Even a simple solvent fractionation of any 1-alkene polymer prepared with a heterogeneous Ziegler–Natta catalyst shows that it is a mixture of different types of macromolecules (Section 2.3.2.1). Some of the centers produce nearly perfectly isotactic polymer molecules with rare steric mistakes, other types of the centers generate these mistakes (or slightly different mistakes) much more frequently (and, therefore, the mistakes are repeated many times over the growth time of a single polymer chain), still other types of centers do not exhibit any appreciable stereospecificity at all. Similarly, fractionation of any alkene copolymer prepared with such a catalyst under constant comonomer concentrations immediately shows that different copolymer chains have different compositions, some chains contain monomer units from both alkenes randomly positioned in the chains whereas other copolymer molecules contains monomer units from one alkene in great excess.

2.1. Definition of Single Type of Active Center The definition of a single type of active center in a catalytic polymerization reaction can be formulated on the basis of its kinetic and stereochemical

37

Single-Center and Multi-Center Polymerization Catalysis

properties. Active centers of a single type have the following common characteristics: 1. The value of the propagation rate constant in a polymerization reaction of a particular alkene, as well as the rate constants of all chain transfer reactions, is the same for all the centers. These kinetic features lead to a particular type of the molecular weight distribution of any polymer produced with uniform active centers; it is described in Section 2.2.1. 2. The stereospecificity of all the centers of a given type (represented, e.g., by the probability of steric errors in homopolymer chains) is the same. This characteristics leads to a very narrow stereoregularity distribution of alkene homopolymers; it is described in Section 2.3.1. 3. In copolymerization reactions of two alkenes, relative reactivities of the alkenes (represented by reactivity ratios r1 and r2) are the same for all the centers of a given type. This characteristics leads to a narrow compositional distribution of alkene copolymers (see Section 2.3.1). All processes of chain growth and transfer in polymerization reactions are statistical in nature. The above three conditions of center uniformity do not translate into complete identity of all polymer chains even when the polymerization reactions are carried out under stable, carefully controlled conditions. For purely statistical reasons, different polymer chains will still have different molecular weights and slightly different isotacticity or syndiotacticity, and different copolymer molecules will have slightly different compositions. These purely statistical deviations from average values can be strictly defined in mathematical terms.

2.2. Molecular Weight Distribution of Polymers Produced with Single-Center Catalysts 2.2.1. Molecular weight distribution, theory The simplest polymerization scheme consists of two reactions. The chain growth reaction is the insertion reaction of the double bond of an alkene molecule CH2QCH
R into the transition metal
carbon bond M–C; it proceeds with the rate constant kp: ðkp Þ

M2ðCH2 2CHRÞn2H þ CH2QCH2R
! M
ðCH2 2CHRÞnþ12H

(2.1)

On occasion, the chain growth is interrupted by a reaction with one of several chain transfer agents X1–X2 (their nature is not essential in this formal analysis); it proceeds with the rate constant kit ðkit Þ

M2CH2 2CHR2Polymer þ X1
X2
! M2X1 þ X2 2Polymer

(2.2)

The theory of the molecular weight distribution describes the simplest molecular weight distribution applicable to the kinetic scheme consisting of

38

Alkene Polymerization Reactions with Transition Metal Catalysts

Reactions (2.1) and (2.2). It is usually referred to as the Flory-Schulz distribution function [112–115]. This function applies to polymerization processes in which 1. All chain-growing centers (M–C bonds in Reactions (2.1) and (2.2) have the same relative probability of chain growth. Formally, this condition signifies that the ratios of the two rate constants, kp =kit , should be the same for a given type of active center. 2. The kp =kit ratio does not depend on the length of a polymer chain attached to the active center. 3. Some transition metal polymerization catalysts are unstable; their activity rapidly deteriorates over a period of time that can be as short as 5–10 minutes. This phenomenon requires an additional condition, that the concentration of propagating centers should remain approximately constant over an average growth time of a single polymer chain, or, in other words, that the half-life of active centers should be much higher than the average growth time of an average polymer chain. The average time of chain growth in catalytic polymerization reactions of alkenes is of the order of several seconds or, at most, several minutes (Chapter 5); therefore, this condition is usually easily met. 4. Polymerization reactions should be carried out at constant monomer concentrations as well as at constant concentrations of all X1–X2 agents affecting the molecular weight of polymer chains. Kinetic confirmation: The experimental validity of the second condition is very difficult to evaluate in polymerization reactions. However, it was partially verified in oligomerization reactions, when every ‘‘polymer’’ molecule is so short that its relative number can be determined by the gas chromatographic method. One such verification was carried out for ethylene/1-alkene co-oligomerization reactions with a single-center homogeneous catalyst based on sulfonated Ni ylide (Scheme 1.3) [116,117]. The data show that the length of an alkyl group attached to the transition metal atom does not influence relative reactivities of different alkenes both in chain growth reactions and in chain transfer reactions. Molecular weight distribution functions: The theory of the molecular weight distribution states that if a polymerization reaction produces macromolecules of a high molecular weight, the instantaneous distribution function of growing gr polymer chains, F number ðnÞ, is given by [112–115] gr

F number ðnÞ ¼ mgr  expð
n=m gr Þ

(2.3)

gr F number ðnÞ

where is the normalized fraction of growing polymer chains containing n monomer units and mgr the average polymerization degree of growing polymer chains. The expression for mgr is mgr ¼ Rp =SRit ¼ kp  C   C M =Sðkit  C   C iagent Þ (2.4) P i where Rp is the chain propagation rate (Reaction (2.1)), Rt is the sum of chain transfer rates (Reactions (2.2)), and C, CM, and C iagent are the concentrations of the active centers, the monomer, and all the chain transfer agents X1–X2 in Reactions (2.1) and (2.2), respectively. The Flory-Schulz distribution for growing polymer chains signifies that a given polymerization reaction is characterized by a single mgr

Single-Center and Multi-Center Polymerization Catalysis

39

value, the mgr value does not depend on n, and it is constant in the course of a given polymerization reaction. Flory distribution function: When each growing chain produces one ‘‘dead’’ polymer chain in any of possible Reactions (2.2), the number distribution function gr for the dead chains, F number ðnÞ, is the same as the F number ðnÞ function in Equation (2.3), and the number-average polymerization degree of dead polymer molecules mn is equal to mgr. For brevity, this molecular weight distribution function is called the Flory function (the most probable or the random distribution function), and any polymer product with chains distributed with respect to the polymerization degree n according to Equation (2.3) is called a Flory component. The weight distribution function of a Flory-distributed polymer, F weight ðnÞ, represents the normalized weight fraction of polymer chains consisting of n monomer units. F weight ðnÞ ¼ n  F number ðnÞ=½n  F number ðnÞ  dn ¼ ðn=mn Þ  expð
n=mn Þ

(2:5Þ

The weight-average polymerization degree mw of such a polymer is equal to 2mn and the width of the molecular weight distribution, which is defined as the mw/mn ratio, is equal to 2. Poisson distribution function: In many alkene polymerization reactions al low temperatures and in short-duration reactions at moderate temperatures (in stoppedflow experiments discussed in Chapter 5), the majority of growing polymer chains remains attached to active centers. In these situations, if the chain initiation reactions are relatively rapid, all macromolecules continue to grow in the course of a given polymerization experiment. Examples of these reactions include propylene polymerization with V-based catalysts at low temperatures [118–122], polymerization of 1-hexene with a metallocene catalyst at low temperatures [123], and ethylene/norbornene copolymerization reactions with metallocene catalysts [124], as well as several polymerization reactions with non-metallocene homogeneous catalysts described in Chapter 4. Theoretically, if active centers in a polymerization reaction without any chain transfer or termination reactions (such reactions are usually called living polymerization reactions) are formed rapidly, all the polymer chains start growing nearly simultaneously and continue growing until the polymerization reaction is stopped, when either the monomer becomes unavailable or the catalyst is destroyed. The average polymerization degree of a polymer produced in such experiments after a reaction time t is mn ¼ kp  CM  t. The molecular weight distribution of these polymers is described by the Poisson distribution function. F 0number ðnÞ ¼ ðmnn =n!Þ  expð
m n Þ (2.6) The mw/mn ratio corresponding to this distribution function is equal to 1+1/mn, and for any polymer of a sufficiently high molecular weight, mw/mnE1. If the chain initiation reaction in these reactions is relatively slow, the molecular weight distribution broadens because different macromolecules begin growing after different time delays with respect to the start of the polymerization reaction. The distribution curve for these polymer molecules develops a pronounced low molecular weight tail and the mw/mn ratio increases from B1 to the limiting value of 1.33.

40

Alkene Polymerization Reactions with Transition Metal Catalysts

2.2.2. Experimental techniques for analysis of molecular weight distribution, gel permeation chromatography Preparative methods: The first technique for the fractionation of polymers and copolymers of 1-alkenes by molecular weight is called the solvent-gradient elution fractionation. It consists of two steps [125,126]. First, a large quantity of polymer, up to 30–50 g, is precipitated from solution into an inert nonporous carrier by slowly cooling the solution from B110–130 to 30–401C at a rate ranging from 0.5 to 0.251C/hour. The carrier with the polymer is placed into a glass column and then the polymer is redissolved at a high temperature, B1201C, with a series of binary solvent mixtures; a good solvent, xylene, ortho-dichlorobenzene or 1,2,4trichlorobenzene; and a poor solvent (a precipitant), usually ethyl or n-butyl cellosolve. The ratio between the solvent and the precipitant is slowly varied from B0.5–0.7 to 2.2–2.5. Eluted polymer samples are collected and analyzed for their molecular weight, steric structure, or copolymer composition. The same technique can be used for the fractionation of amorphous polymers, such as poly(1-octene) [127]. In this case, THF is used as the solvent, 2-butanone as the precipitant, and the fractionation is carried out at 301C. The second fractionation method is based on differences in solubility of macromolecules of different molecular weights in supercritical organic solvents [128,129]. This technique fractionates alkene polymers prepared with single-center metallocene catalysts mostly with respect to the molecular weight. Analytical methods: Currently, the universally used technique for the measurement of both average molecular weights and the molecular weight distribution of polyolefins and alkene copolymers is the gel permeation chromatographic (GPC) method (another name of this technique is size-exclusion chromatography, SEC) [131]. A dilute solution of a polymer in a good solvent (typically, orthodichlorobenzene or 1,2,4-trichlorobenzene) passes at a high temperature (110– 1201C) through a set of columns filled with cross-linked polystyrene gel. Due to differences in diffusion rates of polymer molecules of different molecular weights through layers of the swelled gel, the average residence time of a particular polymer molecule in the columns is in a reciprocal dependence to its molecular weight. The concentration of polymer molecules exiting the last column is measured with a highly sensitive detector. Flory distribution function in GPC coordinates: In order to determine the shape of the GPC curve of polymers with the molecular weight distribution described by Equation (2.5), one has to take into account a peculiar shape of a GPC calibration curve. A nearly perfect linear correlation exists between the retention time t of a peak of a monodispersed polymer (mw/mn ¼ 1) and the logarithm of its molecular weight M (or its polymerization degree n), t ¼ k1
k2  log(n). Taking this relationship into account gives the expression for the Flory distribution function in the GPC coordinates as [132,133]
1 m
2 n  expf2  lnð10Þ  logðnÞ
m n  exp½lnð10Þ  logðnÞg vs: logðnÞ

(2.7)

Equation (2.7) represents the same weight-distribution function as the Fweight(n) function in Equation (2.5) but with a different abscissa, log(n) instead of n. The plot

41

Single-Center and Multi-Center Polymerization Catalysis

of this function vs. log(n) is an asymmetric curve with a sharp maximum. Similarly to the Fweight(n) function, Equation (2.7) contains only one parameter, the numberaverage polymerization degree mn, it does not have any adjustable parameters for the width of a GPC peak. The maximum of the Flory function in the GPC coordinates is positioned at log(n)max ¼ log(2mn), i.e., nmax ¼ mw . Figures 2.1 and 2.2 give two examples of the applicability of Equation (2.7) for the description of the molecular weight distribution of polymers produced with single-center catalysts. The GPC technique has limited resolution. Because of this, GPC curves of polymers are somewhat broadened compared to Equation (2.7). Several computer procedures were developed to account for the peak broadening [132]. If one neglects this experimental peak broadening, the measured Mw/Mn values for perfectly monodispersed polymers are slightly higher than 1.0, usually in the 1.05–1.1 range, depending on the molecular weight-resolving properties of GPC equipment. Equation (2.7) can be readily applied to a variety of catalytic polymerization reactions under stationary conditions. In the past, several other MWD distribution functions have been proposed for more complex polymerization models, including the generalized kinetic model of multi-center polymerization catalysis [134] and quasi-living polymerization reactions [135,136]. However, the simplest model of the molecular weight distribution of a single Flory component in the GPC coordinates represented by Equation (2.7) is sufficient for the description of the molecular weight distribution of alkene polymers and copolymers produced in the majority of catalytic polymerization reactions. Virtually all GPC curves of alkene

d(W)/d(logMW)

1.2

0.8

0.4

0.0 103

104

105

106

Molecular weight

Figure 2.1 GPC curve of ethylene-hexene copolymer (CHex ¼ 2.0 mol.%) prepared with Cp2ZrCl2 -MAO system at [Al]:[Zr] ¼ 9,200 (dots). Solid line represents single Flory component (Equation (2.7)).

42

Alkene Polymerization Reactions with Transition Metal Catalysts

1.2

dW/d(logMW)

0.8

0.4

0.0 103

104

105

106

107

Molecular weight

Figure 2.2 GPC curve of ethylene-propylene copolymer prepared with homogeneous VCl4 -based catalyst [306] (dots). Solid line represents single Flory component (Equation (2.7)).

polymers, however complex, can be represented in a satisfactory manner as combinations of several Flory curves, each described by Equation (2.7). One complication can be encountered when the separation of GPC curves into their constituent Flory components is carried out. It is related to the fact that linear and branched macromolecules of the same molecular weight have different hydrodynamic radii in solution and, therefore, their retention times in the GPC analysis are different. When the branches are few and they are relatively short, e.g., when linear polyethylene molecules and ethylene/1-alkene copolymers containing a few percents of 1-alkenes are compared, copolymer molecules of the same molecular weight will elute from GPC columns slightly later and, therefore, will be assigned a slightly lower molecular weight. This difference is not very large and will not affect most aspects of kinetic analysis. However, this complication becomes very significant if some of the polymer molecules have long-chain branches (the mechanism of their formation is discussed in Section 3.8.4). In this case, the standard interpretation of GPC data becomes quite unreliable and should be complimented by the use of more advanced analytical methods in conjunction with GPC, e.g., independent viscosimetric measurements of molecular weight. Poisson distribution function in GPC coordinates: As described above, if active centers in a living polymerization reaction are formed rapidly, all the polymer chains start growing nearly simultaneously and continue growing until the polymerization reaction is stopped. The molecular weight distribution of such polymers is described by the Poisson function, Equation (2.6). GPC curves of such polymers are very narrow, slightly asymmetric peaks corresponding to the effective Mw/Mn value of 1.05–1.2 [118–124,137,138]. When the formation of active centers is relatively

Single-Center and Multi-Center Polymerization Catalysis

43

slow, the theoretical Mw/Mn ratio increases and can reach 1.33. GPC peaks of such polymers are still quite narrow, much narrower than the peaks of the Flory components, and they have a pronounced low molecular weight tail [139]. Another deviation from the living-chain conditions includes polymerization reactions when the living-chain conditions exist only in the beginning of the reactions but the chain transfer reactions become gradually evident. In this situations, GPC curves of the polymers are initially narrow (Mw/MnB1.1–1.2) and the molecular weight grows nearly linearly with time, but over time the GPC peaks become broader and asymmetric (as Equation (2.7) predicts), and the molecular weight does not increase with time anymore [124]. Log-normal distribution function: An alternative approach to modeling molecular weight distributions in the GPC coordinates uses a log-normal distribution function for the dependence between the weight fraction of polymer chains, F(w), and the logarithm of their polymerization degree n [140,141]. Although the log-normal distribution function is purely empirical and does not have a particular theoretical meaning [142], it is a useful function for an approximate description of relatively narrow molecular weight distributions corresponding to Mw/Mn ratios of o3. The log-normal equation is a moderately broad symmetrical function with a maximum at Blog(Mw) [140]. The application of this function for the analysis of complex broad GPC curves is mostly useful for the identification of families of active centers producing polymer fractions with widely different Mw values [140]. Most advanced variants of the GPC method include a second analysis of polymer solutions leaving GPC columns, in addition to measuring the polymer content in them. One technique was developed for the analysis of ethylene/1alkene copolymers [143–145] and alkene/styrene copolymers [146,147,150]. It measures the composition of eluted copolymer molecules. Another doubledetector method is used to analyze the molecular weight distribution of polymers containing long-chain branches [148]. In this case, the second detector is an on-line viscometer. The detection is based on the principle that the mean-square radius os2W of a macromolecule containing long-chain branches (and its intrinsic viscosity [Z]) are lower than the same parameters of a linear macromolecule of the same molecular weight [149]. For example, the [Z] value of high molecular weight polyethylene containing B1.5 long-chain branches per 1,000 C atoms (B0.3 mol.%) decreases by B1/3, and the viscosity of polymers with a higher content of long chain branches can be decreased by a factor of four [148].

2.2.3. Experimental techniques for the measurement of molecular weight distribution used in industry The molecular weight and the molecular weight distribution of ethylene homopolymers and ethylene/1-alkene copolymers (LLDPE resins) are characterized in industry by their rheological parameters called melt indices (ASTM D-1238). Melt indices are measured using an apparatus called a melt index tester or an extrusion plastometer. A small amount of a polymer, B5–6 g, is placed in a heated cylindrical barrel with a round capillary opening (2.095 mm diameter, 8 mm in length) at its bottom. The resin is kept at 1901C for 6 minutes to achieve complete

44

Alkene Polymerization Reactions with Transition Metal Catalysts

melting and then the melt is pressurized by loading a particular weight on a plunger inserted into the barrel. The pressure forces the melt through the capillary, and the extruded melted rod is collected. By definition, the melt index is the weight of polymer melt discharged through the capillary opening over a period of 10 minutes. Three different standard weights are usually used to measure melt indices, 2.16, 10.16, and 21.6 kg. The melt indices are respectively designated as I2, I10, and I21. If no indication of a weight is given, the I2 value is traditionally reported. In very approximate terms, the correlation between the I2 value for polyethylene and its average molecular weight is log(I2) ¼ A–B  log(Mw) where parameters A and B depend on the molecular weight distribution of the polymer [88]. When polyethylene is prepared with metallocene catalysts and has a narrow molecular weight distribution, AE17.5 and BE3.45. The molecular weight distribution of ethylene polymers is usually characterized by one of two ratios, either I21/I2 (often called the melt flow ratio, MFR) or I10/I2. Both these ratios approximately correlate with the Mw/Mn ratio of the polymer. The range of possible I21/I2 values is very large. Ethylene polymers with a narrow molecular weight distribution (Mw/MnB2) have the I21/I2 value of 14–16, whereas ethylene polymers with a broad molecular weight distribution prepared with chromium oxide catalysts can have I21/I2 values of over 100. Melt index ratios are also quite sensitive to the presence of long-chain branching in ethylene polymers. For example, if compositionally uniform ethylene/1-alkene copolymers with a 1-alkene content of 3–5 mol.% produced with metallocene catalysts have a small number of long-chain branches, their I10/I2 ratios increase from 5–6 to 8–12 [151,152]. A correlation between the I21/I2 ratio for polyethylene prepared with multi-center catalysts and its Mw/Mn ratio can be approximated as Mw/MnE0.24  (I21/I2)
2.4 [153]. The molecular weight of commercial polypropylene resins is estimated in industry by measuring their rheological parameter called the melt flow rate (MFR, easy to confuse with the definition of the MFR ratio of polyethylene described above). The measurement is carried out in the same apparatus as the one used for the measurement of polyethylene melt indices and at the same load, 2.16 kg, but at a higher temperature, 2301C, according to the ASTM standard D-1238, condition L (European standard ISO 1133). MFR values for different grades of polypropylene range from B0.5 to over 50. A correlation between the MFR value of polypropylene and its average molecular weight usually has a form log(MFR) ¼ A–B  log(Mw) where parameters A and B depend on the molecular weight distribution of the polymer. When isotactic polypropylene is prepared with metallocene catalysts and has a narrow molecular weight distribution AE19.0 and BE3.4; in the case of propylene polymers prepared with typical Ti-based catalysts AE18.9 and BE3.3. The parameter used in industry for the estimation of the molecular weight distribution of polypropylene is called the polydispersity index (PI). This is also a rheological parameter; it is determined by measuring the creep resistance of a molten polymer at 2001C. The creep resistance parameter, called modulus separation at a low modulus value (500 Pa), is measured with a parallel-plates

45

Single-Center and Multi-Center Polymerization Catalysis

rheometer that operates at an oscillation frequency increasing from 0.1 rad/s to 100 rad/s. The value of the modulus separation is the ratio of two frequencies, one at the storage modulus Gu equal to 500 Pa, and another at the loss modulus Gv equal to 500 Pa. The definition of the polydispersity index is PI ¼ 54:6  ðmodulus separationÞ
1:76

(2.8)

Polypropylene resins prepared with common Ti-based supported catalysts have PI values from 3.5 to 4.5 whereas the resins with a broad molecular weight distribution have PI values between 5.5 and 6.2.

2.2.4. Experimental techniques for the analysis of molecular weight distribution, gas chromatography Oligomerization and polymerization reactions of alkenes, from the kinetics’ point of view, are the same reactions. They often occur with the same catalysts and with the same alkenes. A relatively small change in reaction temperature or the type of an alkene can cause a shift in the physical appearance of polymerization products from solid materials (polymers) to liquids (oligomers). The only real difference between oligomerization and polymerization reactions is the ratio between the rate of chain transfer and chain growth reactions (see detailed description of these reactions in Chapter 3), this ratio is quite low in polymerization reactions and it is relatively high, of the order of 0.1–0.5, in oligomerization reactions. The molecular weight distribution of alkene oligomers is measured experimentally using the gas chromatographic (GC) method by employing relatively long capillary columns and performing the analysis at temperatures increasing from 20–40 to B3001C at a 2–51C/minute rate. The use of modern capillary columns capable of sustaining high temperatures affords the identification of individual oligomer molecules with polymerization numbers approaching 50–70. The number distribution function, Fweight(n) of an oligomer molecule prepared with a single-center catalyst represents the normalized molecular fraction of oligomer chains consisting of n monomer units [112,114,115,154]. F number ðnÞ ¼ A  gn

(2.9)

where A is a normalization parameter and g is the probability of chain growth. g ¼ Rp =ðRp þ SRit Þ ¼ kp  C   C M =½kp  C   C M þ Sðkit  C   C iagent Þ

(2.10)

In polymerization reactions, kp  C M  Sðkit  C iagent Þ, the g values are close to 1, and Equation (2.9) is equivalent to Equation (2.3). Areas under GC peaks are proportional to the weight contents of particular oligomers, Peak area  F weight ðnÞ. GC data on oligomer distributions are conveniently linearized in the coordinates log½F weight ðnÞ=n  log½ðPeak areaÞ=n ¼ logðAÞ þ n  logðgÞ

(2.11)

46

Alkene Polymerization Reactions with Transition Metal Catalysts

Relative yield/n

103

102

101 3

4

5

6

7

8 9 10 11 12 Oligomerization number

13

14

15

16

Figure 2.3 Oligomer distributions vs. polymerization degree in coordinates of Equation (2.11). Oligomer products in ethylene/1-pentene copolymerization reactions with T|Cl4/MgCl2/ SiO2 -AlEt3 system at 851C [386]: (E)n K, (E)n
1-Pentene J, Pentene-(E)n
1 ’, Pentene(E)n
2 -Pentene &.

with the slope of log(g). The literature provides numerous examples of oligomer distributions described by Equation (2.18) [116,154–158]. Four of them are shown in Figure 2.3. Oligomerization kinetics becomes more complex when any parameter in the expression for g becomes dependent on the oligomerization degree n. Apparently, this dependence was observed in the case of oligomerization of propylene and 1-hexene with sterically crowded metallocene complexes (t-Bux-Cp)2ZrCl2 (x ¼ 1 and 2) activated with MAO [159]. In these examples, a strong decrease in the kp value from n ¼ 1 to nZ2 is explained by a high steric demand on the insertion of 1-alkene molecules into the Zr–C bond. These deviations in the kinetic behavior result in an increase of relative yields of alkene dimers compared to predictions from Equation (2.9) and in a reduction of the Mw/Mn ratio for the oligomers from 2.0 to 1.3–1.4 [159].

2.3. Structural Uniformity of Polymers and Copolymers Produced with Single-Center Catalysts The second and the third characteristics of uniform active centers in polymerization reactions (see Section 2.1) are: (a) the stereospecificity of each center is the same and (b) relative reactivities of alkenes in copolymerization reactions are the same for each center. In the simplest case, the stereospecificity

47

Single-Center and Multi-Center Polymerization Catalysis

of a polymerization center can be kinetically described in terms of two stereochemically different chain growths reactions. M

Polymer

Polymer M

+ M

Polymer

The first reaction leads to the formation of the isotactic link between two adjacent monomer units CH2
CHR (its rate constant is kiso) and the second reaction leads to the formation of the syndiotactic link; its rate constant is ksyndio. (One should keep in mind that the stereocontrol in alkene polymerization reactions is usually much more complex, it is discussed in detail in Section 3.1.3.) The ratio of the two rate constants, kiso/ksyndio, determines the stereospecificity of the center: if kiso/ksyndioc1, the center is isospecific, if kiso/ksyndio{1, the center is syndiospecific, and if kiso/ksyndio ¼ 1, the center is aspecific. The steric uniformity of active centers implies that the kiso/ksyndio ratio is the same for all the centers.

2.3.1. Structural uniformity of polymers and copolymers, theory Theoretical aspects of structural uniformity are well developed for copolymers. Stockmayer proposed a special function for the description of the compositional distribution of binary of Mu/Mvcopolymers produced with single-center catalysts [160]. The function (it is called the bivariate distribution function) describes both the molecular weight distribution and the compositional distribution of a copolymer. p
1 F weight ðn; dcÞ  dn  dðdcÞ ¼ n  m
2 n  expð
n=m n Þ  dn  ½ ð2p  b=nÞ  exp½
n  dc 2 =2  b dðdcÞ

ð2:12Þ

where n is the polymerization degree of a particular copolymer macromolecule; m is the number-average polymerization degree, the same parameter as in Equation (2.3); and dc is a deviation from the average mole fraction of comonomer Mu in the copolymer macromolecule, C(Mu). The parameter b in Equation (2.12) is b ¼ CðM0 Þ  ½1
CðM0 Þ  f1 þ 4  CðM0 Þ  ½1
CðM0 Þ  ðr 1  r 2
1Þg0:5

(2.13)

This parameter is a measure of randomness of a copolymer in terms of the distribution of monomer units in copolymer chains. When a copolymer is completely random, r1  r2 is equal to 1, and b ¼ C(Mu)  [1
C(Mu)]; when

48

Alkene Polymerization Reactions with Transition Metal Catalysts

r1  r2 ¼ 0 (a copolymer in which monomer units of the comonomer of a lower reactivity are isolated [88]), b ¼ C(Mu)  [1
C(Mu)]  [1
2  C(Mu)], etc. The component of the bivariate equation describing deviations in copolymer composition from the average value, C(Mu), is the Gaussian function with a variance s2 ¼ b/n. Two conclusions follow from analysis of Equations (2.12) and (2.13) [160,161]: 1. The distribution of copolymer chains with respect to their composition (the deviation from the average copolymer composition) is broader for short chains (smaller n values). 2. For a copolymer chain of a given length, the compositional distribution is broader when the r1  r2 product is higher. Equation (2.12) can be integrated for all chain lengths to give the component of the bivariate equation that describes only the deviation in copolymer composition from the average value C(Mu) independently of molecular weight [160,161]. Z p F weight ðdcÞ ¼ F weight ðn; dcÞ  dn ¼ 0:75=f ð2  b=m n Þ  ½1 þ dc 2 =ð2  b=mn Þ5=2 g (2.14) In the case of 1-alkene copolymers produced with single-center catalysts (e.g., with metallocene catalysts), the r1  r2 value is usually from 0.3 to 0.6 [88,97,98]. Several estimations of the deviation in copolymer composition from the average values were given in [161]. They showed that compositional distributions predicted by Equation (2.14) are quite narrow, within 71 mol.%. As an example, the halfwidth of the compositional distribution in an ethylene/1-octene copolymer produced with a single-center constrained-geometry catalyst with the ethylene content of 86.2 mol.% is merely B70.9 mol.%. This variation is beyond the sensitivity of modern techniques for the measurement of compositional distributions in copolymers, as described in the following sections.

2.3.2. Experimental techniques for the analysis of steric structure of alkene homopolymers and compositional distribution of copolymers 2.3.2.1. Early fractionation methods Solvent fractionation: Historically, first polymers and copolymers of 1-alkenes were produced with multi-center catalysts. Therefore, analytical techniques for the structural analysis of the polymers were mainly aimed at identification of the multicenter nature of the catalysts. The earliest technique for the analysis of polypropylene and poly(1-butene) prepared with transition metal catalysts was based on differences in polymer solubility in cold and boiling alkanes, usually n-heptane. A Soxhlet apparatus is usually used for these fractionation procedures and several hydrocarbon solvents with increased boiling points are employed. The efficiency of these simple procedures can be best demonstrated by fractionation of polymers produced with single-center catalysts. For example, polypropylene prepared with the isospecific rac-Me2Si(Ind)2ZrCl2-[CPh3]+

49

Single-Center and Multi-Center Polymerization Catalysis

[B(C6F5)4]
-Ali-Bu3 system was fractionated with boiling n-hexane into two fractions, an insoluble isotactic polymer (B94%, [mmmm] ¼ 0.98) and a soluble atactic polymer. On the other hand, a propylene homopolymer prepared with the aspecific C2H4(Flu)2ZrCl2-[CPh3]+ [B(C6F5)4]
-Ali-Bu3 system is completely soluble in boiling n-hexane [162]. When this separation procedure was applied to a product of propylene polymerization prepared with a supported catalyst containing both metallocene catalysts, the polymer was cleanly separated into an n-hexane-soluble atactic fraction and an n-hexane-insoluble isotactic fraction [162]. Table 2.1 gives two examples of polypropylene fractionation with a series of solvents. These results clearly demonstrate that polymers prepared with the early Ziegler–Natta system, TiCl3-AlEt3, are mixtures of macromolecules of different stereoregularity, as evidenced from differences in the crystallinity level and melting points of the fractions. The polymer prepared with the first commercially viable catalyst system, d-TiCl3-AlEt2Cl, has a significantly higher fraction of the crystalline material insoluble in boiling n-heptane. This polypropylene fractionation technique with cold and boiling n-heptane is in continuous use for over 50 years, it is adequate for many studies of polymer physical properties [163]. Polymers of 1-alkenes prepared with stereospecific metallocene catalysts are usually regarded as sterically uniform, but solvent fractionation of such polymers shows that several types of active centers coexist in some of these catalysts. Although the centers have similar stereo-controlling parameters, they are not fully identical [166,177]. Table 2.2 presents solvent fractionation data for a propylene polymer prepared with the hemi-isospecific Ph2C(3-Me3Si-Cp)(Flu)ZrCl2 complex Table 2.1

Solvent fractionation of polypropylene produced with TiCl3-based catalysts Crystallinity (%)

Tm (1C)

Polymerization with a-TiCl3-AlEt3 system, 801C [164,165] Diethyl ether 35 8.0 n-Pentane 36 1.4 n-Hexane 69 2.6 n-Heptane 98 3.3 3-Ethylhexane 120 18.0 n-Octane 126 16.2 Residue 126 50.5

0 27 36 52 62 64 68

– 114 130 159 170 174 174

Polymerization with d-TiCl3-AlEt2Cl system, 701C [136] Acetone 20 0.3 Diethyl ether 20 1.8 Diethyl ether 35 0.3 n-Pentane 36 0.4 n-Hexane 69 0.5 n-Heptane 98 2.1 n-Octane 126 21.0 Residue 126 73.6

0 0 10 28 39 47 67 73

– – – 95–102 – 135–141 163 165

Solvent

Extraction temperature (1C)

Fraction (%)

50

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 2.2 Solvent fractionation of polypropylene produced with a hemi-isotactic metallocene catalysta [168]

a

Solvent

Fraction (%)

[mmmm]

[rrrr]

Mw

Total polymer n-Hexane n-Heptane n-Octane

100 27 34 39

0.338 0.697 0.212 0.234

0.465 0.051 0.657 0.637

3.0  105 1.8  105 3.2  105 3.4  105

Catalyst system Ph2C(3-Me3Si-Cp)(Flu)ZrCl2-[Me2PhNH]+ [B(C6F5)4]
-Ali-Bu3, 401C.

Table 2.3

Solvent fractionation of alkene copolymers prepared with TiCl4/MgCl2 catalysts

Solvent

Temperature (1C)

Fraction (%)

CHex (mol.%)

r1

Ethylene/1-hexene copolymer, TiCl4/MgCl2/silica – AlEt3 system at 701C [153] Total polymer 100 3.6 n-Hexane (cold) 20 7.6 B25 3.5 n-Hexane 69 4.0 9.6 10 n-Heptane 98 4.6 9.1 11 n-Octane 126 12.2 5.4 19 Residue 126 71.5 1.2 90 Temperature (1C)

Fraction (%)

CHex (mol.%)

r1

Propylene/1-hexene copolymer, TiCl4/MgCl2 – AlEt3 system at 601C, fractionation n-octane [174] Total polymer 100 5.6 22 43.8 10.6 3.5 45 5.7 5.2 10 65 7.1 3.9 13 80 8.1 2.6 16 90 8.6 2.2 19 100 8.8 1.3 39 110 17.9 0.4 52

[mm]

with

0.56 0.87 0.90 0.93 0.96 0.98 0.99

activated with an ion-forming cocatalyst [168]. Although the polymer has a narrow molecular weight distribution, Mw/Mn ¼ 2.1 (suggesting the single-center nature of the catalyst), is in effect a mixture of two polymers with opposite types of stereoregularity, a moderately isotactic polymer with a melting point of B1151C (the fraction soluble in boiling hexane) and a moderately syndiotactic polymer with a melting point of B1351C. Alkene copolymers prepared with many Ziegler–Natta catalysts are partially crystalline and can also be solvent-separated into fractions of widely different compositions [64,153,169–174]. Table 2.3 gives two examples of such fractionation

51

Single-Center and Multi-Center Polymerization Catalysis

experiments. On the other hand, solvent fractionation of ethylene/propylene copolymers prepared with two homogeneous metallocene systems, Cp2TiCl2AlEt2Cl at 201C [64], and rac-C2H4(Ind-H4)ZrCl2-MAO at 501C [175], produced fractions of approximately the same compositions.

2.3.2.2. Preparative fractionation methods Preparative dissolution and crystallization fractionation: The simple solvent-extraction techniques described in the previous section were recognized as inadequate since 1960s due to an arbitrary choice of solvents and extraction temperatures. Several more advanced separation techniques were developed at that time including isothermal crystallization of polyolefins at successively lower temperatures and isothermal dissolution of polymers at progressively rising temperatures. Isothermal crystallization from solution at steadily decreasing temperatures separates polymers mostly with respect to stereoregularity [176,177]. Table 2.4 gives one example of polypropylene fractionation by this method. Industry employs a simplified variant of this technique for fractionation of polypropylene, single-point crystallization with o-xylene. The polymer is completely dissolved in the solvent at 1351C at a [solvent]:[polymer] volume ratio of B100, then the solution is cooled to 251C and the precipitated crystalline material is filtered, dried and weighed [163]. The content of the crystalline material measured by this method is called ‘‘the isotacticity index’’ and is usually abbreviated as II. The technique of isothermal dissolution at a series of increasing temperatures is called preparative temperature-rising elution fractionation (the preparative Tref method). It is widely used for characterization of alkene polymers and copolymers [61,64,126,129,178–196]. The preparative Tref method requires fractionation of a large quantity of a polymer, usually from 0.5 to B10 g, first by precipitating the polymer from its xylene solution into an inert support at a slowly decreasing temperature and then re-eluting the crystallized polymer with xylene at an increasing temperature. Numerous fractions are collected, weighed, and analyzed by NMR or IR. Potentially, this technique produces a very detailed set of data on the structural uniformity of alkene polymers, especially when combined with fractionation by molecular weight [126,182]. Table 2.4 Isothermal crystallization of polypropylene from solution at several temperaturesa [177]

a

Temperature (1C)

[mmm]

Mvb

Tm (1C)

93 88 72.5 50.5 10

0.90 0.85 0.84 0.80 0.70

847,000 896,000 727,000 647,000 681,000

167 163 158 148 127

Polymer prepared with b-TiCl3-AlEt2I system at 151C. Viscosity-average molecular weight, MvEMw.

b

52

Alkene Polymerization Reactions with Transition Metal Catalysts

Preparative Tref analysis of isotactic polypropylene produced with the racC2H4(Ind)2ZrCl2-MAO system [182], and syndiotactic polypropylene prepared with the Ph2C(Cp)(Flu)ZrCl2-MAO system [193] provided convincing demonstrations of the single-center nature of the catalysts in terms of their stereo-controlling ability. Nearly 80% of the total polymer samples were eluted in narrow temperature ranges, between 75 and 851C in the first example and between 70 and 851C in the second example. All fractions of the syndiotactic polymer had the same level of syndiotacticity as the unfractionated polymer, [rr] B0.930. Supporting the syndiospecific catalyst on silica did not result in any principal change in the nature of the active species, the catalyst remained predominantly single-center, although the distribution of polymer fractions with respect to their [rr] value became slightly broader, ranging from 0.83 to 0.95 [193]. In contrast, preparative Tref fractionation of polypropylene prepared with the d-TiCl3-AlEt2Cl system produced seven fractions in comparable amounts eluting in a broad temperature range from 40 and 1251C [182]. Differential scanning calorimetry (DSC) and 13C NMR analysis of several fractions showed that the fractionation is controlled solely by the stereoregularity of polymer chains. One of the most thorough dual analyses was applied to isotactic polypropylene samples produced with a Ti-based catalyst [197]. The Mw/Mn ratio for the unfractionated polymers ranges from 5 to 11, whereas the Mw/Mn ratios of many Tref fractions are close to 2 indicating that these fractions are produced by single types of active centers. An example of the application of the preparative Tref method for the analysis of polypropylene is shown in Table 2.5. More advanced automated analytical fractionation techniques combine the Tref separation method and the GPC method for the measurement of molecular weight of each polymer fraction. It is called cross-fractionation chromatography (CFC) [199], and it was successfully used for the analysis of both homopolymers and copolymers of alkenes and styrene [146,147,194,200,201]. In this technique, a polymer is dissolved in an appropriate solvent and deposited in a column with an inert carrier by slow cooling of the solution. Then the highly dispersed solid Table 2.5

a

Preparative Tref fractionation of polypropylenea [61]

Elution temperature (1C)

Fraction (%)

[mmmm]

25 26–99 100–105 106–108 109 110 111 112

2.5 18.0 4.8 8.7 8.5 16.9 16.3 12.7

B0.22 (atactic fraction) 0.863 0.938 0.945 0.967 0.970 0.975 0.985

Mwb

Tm (1C)

6.95  104

157

1.16  105

161

2.22  105

163

4.10  105

164

Polymer prepared with supported TiCl4/MgCl2/2,2-di-i-butyl-1,3-dimethoxypropane-AlEt3 system at 701C. Average polymer characteristics: Mw ¼ 2.43  105, [mmmm] ¼ 0.92. b Estimated from intrinsic viscosity [198].

Single-Center and Multi-Center Polymerization Catalysis

53

material is extracted with the same solvent in numerous batches at step-wise increasing temperatures, and every batch is analyzed by GPC. The use of CFC for the analysis of ethylene/1-octene polymers prepared with a single-center constrained-geometry catalysts combined with NMR analysis of several fractions demonstrated great utility of this technique [194]. Each Tref fraction has a narrow molecular weight distribution; the Mw/Mn ratios for the fractions vary from 1.3 to 1.6. Because the copolymers are compositionally uniform, the fractionation range is very narrow. For example, an ethylene/1-octene copolymer with a narrow molecular weight distribution (Mw/Mn ¼ 1.95) can be separated into 17 fractions eluting from 25 to 1301C. However, the bulk of the copolymer, B87%, is eluted in a narrow temperature range, from 69 to 851C. The fractions differ mostly in molecular weight whereas their compositions are practically the same and are equal to the average composition of the copolymer [194]. The CFC method is very sensitive to even minor variations in copolymer composition. Ethylene/norbornene copolymers prepared with bridged metallocene complexes and different cocatalysts showed this subtle phenomenon. Different copolymer fractions eluted at temperatures from 0 to 1001C have approximately the same molecular weight but they noticeably differ in copolymer composition, from B14 mol.% of norbornene in the fraction eluted at 01C to B2–3 mol.% in fractions eluted at 90–1001C [201]. The same compositional differences were discovered in propylene/norbornene copolymers prepared with the same catalysts whereas they are absent in alkene/cyclopentene copolymers prepared with these metallocene catalysts, which are compositionally uniform [202,203]. In contrast, the preparative Tref analysis of an ethylene/1-octene copolymer produced in a high-temperature reaction with the TiCl4/MgCl2-AlEt3 system produced six copolymer fractions ranging in 1-octene content from B1.1 to W8.5% [182]. Several other preparative methods can be used for fractionation of alkene polymers and copolymers. The simplest of these methods involves dissolution of a polymer or a copolymer in a large volume of solvent followed by stepwise addition of aliquots of an alcohol at a constant temperature [204]. Fractions precipitated after each alcohol addition are collected and analyzed. Other preparative methods are described in Section 2.3.2.1. One is based on differences in solubility of different macromolecules in supercritical solvents at a constant temperature [128,129]. Another, the solvent-gradient fractionation, involves precipitation of a polymer from solution into an inert carrier at a slowly decreasing temperature, from 110–130 to 30–401C, followed by dissolving the precipitated polymer in mixtures of a good and a poor solvent. Alkene polymers and copolymers prepared with single-center metallocene catalysts are also mostly fractionated with this method with respect to molecular weight [125], whereas polymers produced with Ziegler–Natta catalysts are fractionated both with respect to molecular weight and stereoregularity [129,205] (as discussed in Chapter 3, these parameters are usually interconnected). 2.3.2.3. Automated methods, analytical Tref and Crystaf methods The preparative Tref technique, as well as all other preparative fractionation techniques described in Section 2.3.2.2, have two main disadvantages: they are

54

Alkene Polymerization Reactions with Transition Metal Catalysts

labor-intensive and slow [62,126,183–185,191,197,204,206]. At the present time, two complimentary modern fractionation techniques, analytical temperature-rising elution fractionation (analytical Tref ) [176,182,197,207–214] and crystallization fractionation (Crystaf ) [215–219], provide the basis of most detailed investigations of the steric structure of homopolymers and the composition distribution of alkene copolymers produced with single-center and multi-center catalysts. Both techniques exploit the same principle: polymer molecules of different stereoregularity, or semi-crystalline copolymer molecules of different compositions, crystallize from solution at different temperatures. The first step in both methods is dissolution of a small polymer sample in a suitable solvent at a high temperature. Two most often used solvents are ortho-dichlorobenzene and 1,2,4-trichlorobenzene, and the dissolution temperature is 130–1401C. The solution is slowly cooled, at a rate of several degrees per hour, resulting in slow crystallization of the polymer. Monrabal developed the Crystaf method in 1980s [215,217,220]. In this technique, the concentration of the polymer remaining in solution is monitored with an IR detector as a function of temperature. In the analytical Tref method, the polymer is completely crystallized on some inert inorganic carrier, then it is slowly re-dissolved with a fresh solvent at a gradually increasing temperature and the concentration of the re-dissolved polymer is monitored as a function of temperature. The analytical Tref method was successfully used for detailed characterization of the isotacticity distribution in polypropylene samples prepared with different Ti-based catalysts [61,126,182,186,197,209,210,213,221–225] and with metallocene catalysts [168]. The results of Tref analysis are mainly dependent on stereoregularity and almost independent of molecular weight. The Crystaf method also was used for the analysis of compositionally homogeneous and inhomogeneous semi-crystalline polymers. The process is fully automated, and commercial equipment for Crystaf analysis is currently available [215,217,220]. Both Crystaf and analytical Tref techniques are also effective tools in discriminating mixtures of polymers from block-copolymers. Both methods cleanly separate polymer blends into their components; and positions of respective peaks (see below) provide information about the structure of each component in the blend. On the other hand, true block-copolymers are eluted (or crystallize) as single components at intermediate temperatures between peak temperatures of the constituent blocks. For example, a physical mixture of two ethylene/1-octene copolymers of different compositions (prepared in a solution polymerization reaction at 1201C) was cleanly separated with the Crystaf method into two fractions. A fraction containing B2.8 mol.% of 1-octene crystallizes at 781C and an amorphous fraction containing B6.5 mol.% of 1-octene remains in solution even at 301C [227]. However, a block-copolymer containing copolymer segments of the same two compositions crystallizes at B401C as a single peak. Analytical Tref analysis, peak position and shape: Figure 2.4 shows Tref curves of three copolymers prepared with single-center catalysts. The position of the peak maximum of a single Tref component depends on the structural uniformity of the material, stereoregularity of a polymer or the composition of a copolymer. Polymers with a high degree of stereoregularity and copolymers with a low content of one of the comonomers are highly crystalline and they elute at higher

55

Single-Center and Multi-Center Polymerization Catalysis

dH/dT

Temperature, C Figure 2.4 Analytical Tref curves of three ethylene/1-hexene copolymers of di¡erent compositions prepared with single-center catalyst [207].

temperatures. The exact elution temperature depends on the nature of the polymer and on analysis conditions, as an example of one polypropylene analysis shows [227]. [mmmm] Elution temperature (1C)

0.96 B115

0.94 109

0.92 100

0.82 92

0.78 89

0.64 B70

Another example is Tref fractionation of an ethylene/1-butene copolymer [211]. Elution temperatures of copolymer components with very low 1-butene contents, 0.2–0.5 mol.%, are between 91 and 941C, only slightly lower than for the ethylene homopolymer, 951C. On the other hand, copolymer fractions with high 1-butene contents, B9 mol.%, elute at a much lower temperature, B501C. 1-Butene content (mol.%) Elution temperature (1C)

B0.2 94.0

1.0 89.3

1.6 86.0

4.1 72.5

5.7 64.5

9.0 B48

Calibration curves for the analytical Tref method were published both for polypropylene [61,183,197,228] and for ethylene/1-alkene copolymers [130,206,207,229–231]. The latter usually have a linear shape of the type Calkene (mol.%) ¼ A
B  Tmax. Peaks of individual Tref components are quite narrow and nearly symmetrical [206], their shape can be represented by the Lorentz function [193,211]. HðT Þ ¼ H max  DT 2 =½DT 2 þ ðT
T max Þ2 

(2.15)

where Hmax is the peak height at its maximum, Tmax (1C) the temperature of the peak at maximum, and DT (1C) the half-width of the peak. The latter value usually increases as the Tmax value decreases. Figure 2.5 gives one example of resolution of an experimental Tref curve of a crystalline polypropylene fraction prepared with a

56

Alkene Polymerization Reactions with Transition Metal Catalysts

120

100

dH /dT

80

60

40

20

0 95

100

105

110 115 Temperature, °C

120

125

Figure 2.5 Analytical Tref curve of highly crystalline fraction of polypropylene prepared with a supported catalyst and its resolution into elemental Tref components [322]. Catalyst T|Cl4/MgCl2/di-i-butyl phthalate; cocatalyst AlEt3/(Cpy)2Si(OMe)2, [Al]:[silane] ¼ 20.

solid TiCl4/MgCl2/di-i-butyl phthalate catalyst and an AlEt3/(Cpy)2Si(OMe)2 cocatalyst into elemental Tref components described by Equation (2.15). Crystaf analysis, peak position and shape: Figure 2.6 shows Crystaf curves of two semi-crystalline ethylene/1-hexene copolymers prepared with a single-center metallocene catalyst. Similarly to the analytical Tref method, the position of the peak maximum of a single Crystaf component depends on the structural uniformity of the polymer. One example of this correlation for ethylene/1-hexene copolymers produced with the (n-Bu-Cp)2ZrCl2-MAO [232] system is CHex (mol.%) Elution temperature (1C)

B0.25 84.9

0.45 80.6

1.0 77.4

1.6 72.5

2.75 60.7

3.8 54.8

4.2 B49.5

A similar correlation was also determined for ethylene/styrene copolymers produced with two different types of metallocene catalysts [233]. Theoretically, the dependence between the Crystaf peak maximum (Tmax) for a (co)polymer and the peak position of a perfectly stereoregular homopolymer (T omax ) can be presented as [234,235] T max ¼ T omax
½R  ðT omax Þ2 =DH u   C d

(2.16)

where R is the gas constant, DHu the crystallization heat of the polymer, and Cd the concentration of defects (steric errors in a homopolymer, the content of the second

57

Single-Center and Multi-Center Polymerization Catalysis

30

25

dW/dT

20

15

10

5

0 30

40

50

60 Temperature, °C

70

80

90

Figure 2.6 Crystaf curves of two semi-crystalline ethylene/1-hexene copolymers prepared with single-center metallocene catalyst.

monomer in a copolymer). In practice, these correlations can be approximated by an empirical linear dependence, Calkene (mol.%) ¼ A
B  Tmax [161,232–234,236]. The shape of an individual Crystaf peak (the rate of polymer precipitation, dWpeak/dT, as a function of temperature T ) can be represented by the Gaussian function with respect to temperature T. dW peak =dT ¼ ½ð1=ðs  2pÞ  Exp½
ðT max
T Þ2 =2  s2Gauss 

(2.17)

where Tmax is the temperature of the peak maximum for a compositionally uniform component and s is the width parameter of the peakU In practice, experimentally observed peaks of uniform polymers are skewed toward low temperatures [161,232,237]. A number of asymmetric broadening functions for Gaussian functions are published [232,238]. Figure 2.7 gives two examples of such broadened Crystaf peaks.

2.3.2.4. Melting point measurement, differential scanning calorimetry The measurement of the melting point provides a convenient rapid method for a semi-quantitative estimation of chemical and steric regularity of polymer chains. The measurement is usually carried out with the DSC equipment. This technique can be applied to the analysis of any semi-crystalline homopolymer or an alkene copolymer, including ethylene/1-alkene copolymers with low 1-alkene contents (LLDPE resins). An introduction of any type of irregularity in a polymer chain, either a steric defect in a homopolymer chain or a monomer unit of a different type

58

Alkene Polymerization Reactions with Transition Metal Catalysts

Figure 2.7 Crystaf curves of two narrow fractions of ethylene/1-hexene copolymers (points) and their modeling as skewed Gauss curves [232].

in a copolymer chain, causes depression of the melting point. Two alternative theoretical models are used for the description of melting point depression. Flory model of melting point depression: The Flory model is thermodynamic in nature [239]. It correlates the equilibrium melting point of a semi-crystalline polymer, Tm, and the probability parameter of chain regularity, p (the probability of a given monomer unit, taken at random, to be followed by a monomer unit with the same chemical type and the same steric configuration). 1=T m
1=T om ¼
ðR=DH u Þ  lnðpÞ

(2.18)

where T om ðKÞ is the equilibrium melting point of the perfectly regular polymer of the same nature, R the gas constant, and DHu the fusion heat of the polymer per crystallizable monomer unit. DSC melting points of pre-annealed polymer samples recorded at relatively low heating rates are usually used as Tm values. The statistical meaning of the p value depends on the type of polymer. For ethylene/1-alkene copolymers, p ¼ (r1F )/(1 + r1F ), where F is the monomer concentration ratio in a copolymerization reaction and r1 is the reactivity ratio. For predominantly isotactic polymers with the polymer chain statistics described by the enantiomorphic stereocontrol mechanism (Section 3.1.3.1), p ¼ [(kiso/ksyndio)2 + 1]/(kiso/ksyndio + 1)2 [51], where kiso/ksyndio is the ratio of two rate constants of chain propagation reactions leading to the isotactic and the syndiotactic linkage of adjacent monomer units. For predominantly isotactic polymers with the polymer chain statistics described by the chain-end stereocontrol mechanism (Section 3.1.3.2), p ¼ kiso/ksyndio/ (kiso/ksyndio + 1). For predominantly syndiotactic polymers with the polymer chain statistics described by the chain-end stereocontrol mechanism (Section 3.1.3.4), p ¼ ksyndio/ kiso/(ksyndio/kiso + 1). Apparent heats of fusion and T om values depend on DSC measurement conditions. Both values published in the literature vary in relatively broad ranges,

59

Single-Center and Multi-Center Polymerization Catalysis

which makes the estimation of statistical parameters derived from the p values somewhat unreliable. Polyethylene: DHu from 0.96 to 1.12 kcal/mol, T om from 135.5 to 1411C [240–242]. Isotactic polypropylene: DHu from 1.90 to 2.60 kcal/mol, T om from 170 to 1851C [242–244]. Syndiotactic polypropylene: DHu ¼ 0.404 kcal/mol, T om B2141C [245]. Isotactic polystyrene: DHu ¼ 2.07 kcal/mol, T om from 213 to 2281C [242, 246,247]. Syndiotactic polystyrene: DHu ¼ 2.07 kcal/mol, T om B2811C [248,249]. Thompson-Gibbs model of melting point depression: The second theoretical approach to interpreting the melting point depression is the thermodynamic theory of polymer crystallization. The Thompson-Gibbs relationship for the melting point Tm vs. the thickness of crystalline lamella L is [250] T m ¼ T om  ð1
L  =LÞ

(2.19)

where L is the maximum attainable lamella thickness, twice the ratio between the specific surface-cohesion enthalpy and the DHu value. The use of Equation (2.19) for practical applications, such as the calculation of the crystallinity degree and melting points of semi-crystalline polymers, requires the derivation of equations for lamella thickness distribution [248]. It is a complicated procedure but in some cases it provides estimations of the number of steric defects in polymer chains quite close to NMR data. Apart from these two theoretical approaches to the description of melting point depression for imperfectly regular polymers, the measurement of melting points provides an easy qualitative estimation of the steric purity of polyolefins and compositions of alkene copolymers. Melting point depression in alkene homopolymers: Figure 2.8 shows the dependence between melting points of imperfectly isotactic propylene polymers produced with metallocene catalysts and their 13C NMR [mm] values. A sharp decrease of the melting point with the decrease in steric purity of the polymers is obvious. A similar dependence was found for polypropylene produced with other metallocene systems [251–253]. The correlation can be represented by several empirical linear dependences which are valid in the [mmmm] range from 0.4 to 1.0: Tm (1C) ¼ 151  [mmmm]+11.4 [252]; Tm (1C) ¼ 168.1  [mmmm]
1.2 [253]; Tm (1C) ¼ 247.3  [mm]
82.65 [253]. Melting points of polypropylene were used in several studies as the principal measure of stereoregularity [166,254]. The following data were produced to characterize the temperature effect on the isospecificity of the rac-C2H4(Ind)2ZrMe2[Ph3C]+ [B(C6F5)4]
system [254]: Reaction temperature (1C) Tm (1C)


55 160.8


20 153.8

0 147.2

20 141.2

60

Alkene Polymerization Reactions with Transition Metal Catalysts

170 160 150

Melting point, C

140 130 120 110 100 90 80 1.00

0.95

0.90

0.85

0.80

0.75

0.70

0.65

[mm]

Figure 2.8 Ref. [387].

Melting points of polypropylenes of di¡erent degree of isotacticity. Data from

The data show that (a) polypropylene prepared with this metallocene catalyst even at
551C is not perfectly isotactic (the Tm value for highly isotactic polypropylene is B1701C) and (b) the isospecificity of the catalyst sharply decreases with temperature. In the case of isotactic polypropylene prepared with Ti-based heterogeneous catalysts, a decrease in the content of mmmm pentads of crystalline fractions from 0.987 to 0.943 is also accompanied by a Tm decrease from 168.0 to 164.31C [214]. Melting points of syndiotactic polypropylene and syndiotactic polystyrene [248] are also very sensitive indicators of stereoregularity. For some polymers, a measurement of the melting point is the only reliable option for the evaluation of steric purity. Polymers of 3-methyl-1-butene are highly crystalline and are insoluble in any solvent at o1501C; therefore, their solution NMR spectra cannot be recorded [255] and recording the spectra in the molten state requires a very high temperature [256]. However, the melting point of poly(3-methyl-1-butene) can be used as a measure of its isotacticity. The Tm value of the highly isotactic polymer prepared with Ti-based Ziegler–Natta catalysts is B3051C [257] whereas isotactic polymers produced with bridged bis-indenyl zirconocene complexes melt at B280–3001C due to the presence of regio- and steric mistakes in their chains [255,256]. The melting point measurement was also successfully used to characterize mixtures of isotactic and syndiotactic polypropylenes prepared with mixtures of metallocene catalysts [258]. Solvent fractionation of these polymer products is difficult (they have a similar crystallinity degree) but their DSC curves clearly show melting peaks of different polymer fractions. The melting point measurement is a sensitive indicator of compositional uniformity of alkene homopolymers prepared with multi-center catalysts. These

61

Single-Center and Multi-Center Polymerization Catalysis

materials are complex mixtures containing macromolecules with different levels of stereoregularity (Tables 2.1, 2.6 and 2.7). Melting of these mixtures is dominated by their highly crystalline components with the most regular structure. As a result, melting points of structurally heterogeneous polymers are not very sensitive to their average stereo-characteristics and are close to melting points of their most regular components. For example, melting points of artificial mixtures of two polypropylene fractions, one highly crystalline and another amorphous, depend only slightly on the composition of the mixtures [165]. Content of amorphous fraction Tm (1C)

0 176.0

10% 174.5

20% 173.2

30% 172.2

Propylene polymers prepared with single-center catalysts usually have depressed melting points. All these materials are polymers of low stereoregularity rather than mixtures of highly isotactic and atactic fractions [251,259]. Melting point depression in alkene copolymers: Any transition from an ethylene homopolymer to an ethylene/1-alkene copolymer prepared with a single-center catalyst is accompanied by strong depression of the melting point [260–266]. Table 2.6 gives several examples of this effect for ethylene/1-alkene copolymers prepared with homogeneous bis-metallocene catalysts. The strong depression of the melting point was observed also for copolymers of propylene and various 1-alkenes prepared with stereospecific metallocene catalysts [237,265,267–272]. Two examples illustrate this effect. 1. Copolymers of propylene and 1-butene produced with the syndiospecific Me2C(Cp)(Flu)ZrCl2-MAO system [267]: Propylene content (mol.%) Tm (1C)

100 147.1

97.9 139.4

93.4 124.7

88.5 112.4

80.9 94.5

2. Terpolymers of propylene, ethylene, and linear 1-alkenes produced with the isospecific rac-C2H4(Ind)2ZrCl2-MAO system [271]: Propylene content (mol.%) Tm (1C) Table 2.6

100 134.5

96.6 116.0

95.2 113.7

94.9 111.0

88.7 110.1

75.6 70.5

Melting points of ethylene/1-alkene copolymers vs. 1-alkene content

Ethylene/propylene copolymers [261] 0 1.4 CPr (mol.%) 138.4 129.9 Tm (1C)

3.8 112.8

5.9 102.5

8.7 89.6

14.4 65.4

Ethylene/1-butene copolymers [262] 0 0.9 CB (mol.%) B136 128.2 Tm (1C)

2.0 117.9

3.0 111.7

4.4 105.6

5.2 101.7

Ethylene/1-octene copolymers [263] 0 0.7 COct (mol.%) 136.6 128.9 Tm (1C)

1.9 120.9

3.5 111.1

5.3 100.7

6.2 94.9

62

Alkene Polymerization Reactions with Transition Metal Catalysts

The value of the melting point is a sensitive indicator of the compositional uniformity of alkene copolymers prepared with multi-center catalysts. For example, melting points of ethylene/1-alkene copolymers produced with heterogeneous Ti-based catalysts are always in the range from B120 to B1301C and they only weakly depend on the average copolymer composition [273–278], in contrast to melting points of the same copolymers produced with metallocene catalysts (Table 2.6). The following data for ethylene/1-butene copolymers prepared with a supported Ti-based catalyst illustrate this trend [263]. 1-Butene content (mol.%) Tm (1C)

0 136.5

0.6 130.4

1.3 126.4

1.8 125.3

2.6 122.4

6.1 121.2

These melting points reflect the melting of copolymer fractions with a very low content of 1-butene, B0.2–0.5 mol.%. The same trend was found in copolymerization of propylene and vinylcyclohexane with the heterogeneous d-TiCl3-AlEt2Cl system [279]. Propylene content (mol.%) Tm (1C)

100 169.0

90 156.0

81 155.5

77 153.5

70 152.0

55 152.0

A comparison of these melting points and Tm estimations for propylene copolymers (Equation (2.18)) shows that experimentally measured Tm values reflect the melting of copolymer fractions with the vinylcyclohexane content of B10–15 mol.%, whereas the rest of the products are amorphous. The melting point depression is especially strong in ethylene/styrene copolymers prepared with monometallocene systems such as CpTiCl3-MAO. When these catalysts are used for copolymerization of ethylene and styrene, they produce mixtures of three polymers, linear polyethylene, syndiotactic polystyrene, and true ethylene/styrene copolymers [280–284]. All styrene units in the copolymers are isolated (the r2 value is zero [281,284]), and the disruption of ethylene block lengths by single styrene units is very pronounced [285]. Styrene content (mol.%) Tm (1C)

0 B140

12 122

15 110

33 amorph.

Potentially, if a measurement of melting curves is performed at low heating rates, it can be viewed as a crude variant of the inversed Tref method, the polymers fractions of the lowest regularity melt at lower temperatures, and the fractions of the highest regularity melt at significantly higher temperatures. The use of this potentially useful indicator of polymer homogeneity is complicated by co-crystallization of macromolecules with different degrees of regularity. It can be successfully applied only in the simplest situations, e.g., when a polymer consists of two fractions of a different regularity which do not cocrystallize and melt at different temperatures [286].

Single-Center and Multi-Center Polymerization Catalysis

63

2.4. Examples of Polymers and Copolymers Produced with Single-Center Catalysts Several examples of molecular weight distribution, stereochemical distribution of alkene homopolymers, and compositional distribution of alkene copolymers presented in the following sections were chosen to demonstrate the utility of modern techniques for the analysis of alkene polymers produced with different types of transition metal catalysts. A combination of these data and the data on polymerization kinetics provides the best means to understand the working of these catalysts.

2.4.1. Molecular weight distribution of polymers produced with singlecenter catalysts Alkene polymers prepared with homogeneous transition metal catalysts are especially suitable for testing the applicability of the Flory distribution function in the GPC coordinates, Equation (2.7). Polymerization reactions of gaseous alkenes, ethylene and propylene, are usually carried out isothermally and at constant monomer concentrations. If liquid monomers are used in copolymerization reactions, their reactivities are usually quite low compared to those of the lighter comonomers and their concentrations in reactions also remain approximately constant. Some soluble catalysts contain a single type of active center and have a single value of each reaction constant. In these cases, all reaction rates and all m values are constant. Figure 2.1 shows the GPC curve of an ethylene/1-hexene copolymer prepared with the Cp2ZrCl2-MAO system [28,287,288]. When such polymers are produced at high [MAO]:[Zr] ratios, their molecular weight distributions are quite narrow, Mw/MnB1.9–2.2 [28,71,289–291]. The experimental GPC curve in Figure 2.1 conforms very well to the Flory distribution function calculated with Equation (2.7). The match is not merely limited to the peak width but also reflects the specific asymmetric shape of the curve. Most ethylene-based polymers produced with metallocene catalysts at [MAO]:[Zr] ratios over 2,000–5,000 exhibit such matches between experimental GPC curves and the Flory distribution function [294–295]. Several examples of these polymers are given in Section 2.5.1.2. The first metallocene catalyst system, Cp2TiCl2-AlEt2Cl, also produces polyethylene with the GPC curve described by a single Flory distribution function [296]. However, the use of metallocene complexes does not automatically lead to the formation of single-center catalysts. When catalysts of the Cp2ZrCl2-MAO type are used at lower [MAO]:[Zr] ratios, they often contain two or several types of active centers [132,297,298]. The molecular weights of the polymer components produced by different centers are usually relatively close. If one takes into account the presence of inherent errors in the calculation of average molecular weights from GPC data, it is obvious that the measurement of Mw/Mn ratios is not by itself sufficient to claim the single-center nature of a particular catalyst. The analysis

64

Alkene Polymerization Reactions with Transition Metal Catalysts

of the shape of such GPC curves is a more reliable test [132,288,298–306] (see Section 2.5.1.2). Figure 2.2 shows the GPC curve of an ethylene-propylene copolymer prepared with a soluble VCl4-based catalyst [306]. This catalyst also has only one type of active center [204,307]; it produces copolymers with a narrow molecular weight distribution, Mw/MnB2. The calculated GPC curve in the figure represents a single Flory MWD function with m ¼ 2,300. Different homogeneous V-based catalysts have different numbers of active centers. Some, like the VCl4-Al2Et3Cl3 system in
10 to +401C range, contain only one type of center; they produce compositionally uniform ethylene/propylene copolymers with a narrow molecular weight distribution [204]. Other homogeneous V-based systems, such as VOCl3AlEt2Cl, contain several types of centers and produce ethylene-propylene copolymers with broad multi-component molecular weight distributions (Mw/Mn ratios from 8 to W20) although different centers still produce copolymer molecules of approximately the same composition [308].

2.4.2. Structural uniformity of alkene polymers produced with singlecenter catalysts When stereoregular polypropylene is prepared with homogeneous metallocene catalysts, the molecular weight distribution of the polymers is usually narrow indicating the single-center nature of the catalysts. Solvent fractionation of the polymers mostly separates them with respect to molecular weight [259]. The stereoregularity of different fractions is very close judging by the 13C NMR data and melting points of the fractions, as the results for isotactic polypropylene prepared with the rac-C2H4(Ind)2ZrCl2-MAO system at
101C show [259] Fraction [mmmm] Tm (1C)

Soluble in n-heptane (84%) 0.952 146.6

Insoluble in n-heptane (16%) 0.946 147.8

Polypropylene produced with the syndiospecific R2C(Cp)(Flu)ZrCl2-MAO system at 801C exhibits the same level of compositional uniformity as evidenced both from the preparative Tref analysis [193] and from solvent fractionation data [245]. Fraction soluble in n-heptane at [rrrr] Tm (1C)

821C (7.2%) 0.915 166

881C (82.2%) 0.914 164

981C (3.7%) 0.915 170

One of the most thorough analyses of alkene copolymers produced with metallocene catalysts was carried out using a combination of two preparative techniques described in Section 2.3.2.2, solvent-gradient fractionation and preparative Tref [125]. First, an ethylene/1-hexene copolymer containing 8.1 mol.% of 1-hexene was fractionated into 31 narrow fractions with Mw values increasing from B1.0  104 to W4.7  105. This fractionation showed that over 99% of macromolecules in the copolymer had very similar compositions, from 6.4 to 8.6 mol.% of 1-hexene. Next, one of the narrow fractions containing 8.1 mol.% of

Single-Center and Multi-Center Polymerization Catalysis

65

1-hexene was additionally fractionated using the preparative Tref method. Again, the majority of the 14 produced fractions had very similar copolymer compositions ranging from 8 to 9 mol.% of 1-hexene. This exhaustive analysis is the best available proof of a high degree of compositional uniformity of these copolymers, as well as the variation in their composition predicted by the Stockmayer equation (Equation (2.12)). A similar Tref analysis of an ethylene/1-hexene copolymer prepared with the Me2Si(Ind)2ZrCl2-MAO system at 251C also clearly demonstrated the single-center nature of the catalyst [309]. Nine fractions of the copolymer had molecular weights differing by a factor of 10 but their compositions were essentially the same, the CHex value ranged from 43.8 to 45.9 mol.%. Ethylene/styrene copolymers produced with the constrained-geometry Me2Si[(Cp)(t-Bu-N)]TiCl2-MAO system also have a narrow compositional distribution [311]. Fractionation of one such copolymer with an average styrene content of 13.8 mol.% by the preparative Tref method accompanied by the analysis of narrow fractions by a variety of analytical techniques showed that compositions of different fractions are all in a narrow range, 11.3 to 14.5 mol.% of styrene [312]. As discussed in the previous section, not all metallocene catalysts have the singlecenter nature. Soares presented an example for the need of an experimental proof of single-site catalysis [219]. The C2H4(Ind)2ZrCl2-MAO system at 601C produces ethylene/1-hexene copolymers with a narrow molecular weight distribution (Mw/Mn ¼ 2.5) and with a very narrow compositional distribution, as evidenced from their Crystaf analysis. On the other hand, a similar catalyst system, Cp2HfCl2MAO, produces a copolymer with a much broader molecular weight distribution (Mw/Mn ¼ 5.2), and this material contains three distinct fractions of different compositions. A similar difference was observed between two constrainedgeometry catalysts. Combined preparative Tref/GPC/NMR analysis of ethylene/ 1-octene copolymers prepared with the [Me2Si(Me4Cp)(t-Bu-N)]TiMe2-MMAOB(C6F5)3 system showed that they have a narrow compositional distribution typical for single-center catalysis [194]. In contrast, a similar constrained-geometry system, [C2H4(Cp)(Et-N)]Ti(NEt2)2-MAO, contains two types of active centers which differ both in the molecular weight of polyethylene components they produce (the Mw/Mn value for the combined polymer is B3) and in the composition of ethylene/propylene copolymers [310].

2.5. Examples of Polymers and Copolymers Produced with Multi-Center Catalysts All solid Ti- and V-based Ziegler–Natta catalysts contain several types of active centers. The same is true for many soluble catalysts, even those usually regarded as ‘‘single-site’’ catalysts. Manifestations of the multi-center nature of a polymerization catalyst are several. 1. Alkene homopolymers and copolymers produced with such catalysts have broad molecular weight distributions [132,287,288,313,314], in contrast to polymers prepared with single-center catalysts, which have a narrow molecular weight

66

2. 3. 4.

5.

Alkene Polymerization Reactions with Transition Metal Catalysts

distribution described by a single Flory function (Section 2.2.1). Polymers produced with any heterogeneous Ti-based catalyst consist of at least four or five Flory components. A broad molecular weight distribution is also typical for the polymers that are completely soluble in a reaction medium, such as poly(1-hexene) and poly(1-octene); the observation that excludes possible diffusion effects in the monomer transport to active centers as the reason for the broad molecular weight distribution. When a 1-alkene is homopolymerized with multi-center catalysts, different types of active centers often produce macromolecules with a different degree of stereoregularity. When 1-alkene and ethylene, or two different 1-alkenes, are copolymerized with multi-center catalysts, different types of active centers usually produce copolymer molecules of a different composition. Active centers of different types are often formed and decay at different rates. As a consequence, structural properties of polymers produced with multi-center catalysts (their molecular weight distribution, stereo-composition, copolymer composition, etc.) may vary with reaction time [315–318]. Chemical poisons affect active centers of different types to a different degree. This variation in reactivity is widely used for the synthesis of 1-alkene polymers with a high degree of fractional isotacticity and for the synthesis of alkene copolymers with a desired level of compositional uniformity [58,132, 213,225,226,315–320].

2.5.1. Molecular weight distribution of polymers produced with multicenter catalysts 2.5.1.1. Heterogeneous Ziegler–Natta catalysts Heterogeneous Ti-based catalysts are the workhorse of the modern polyolefin industry. A large number of different catalyst formulations are described in Chapter 4. Modern supported catalyst systems for polymerization of 1-alkenes usually contain two groups of organic compounds (called Modifiers I and Modifiers II in this book). These compounds are added to the catalysts with a goal of increasing their activity and improving the yield of the crystalline polymer fraction or, alternatively, eliminating this material if the manufacture of atactic polymers is the goal. Polymers of 1-alkenes prepared with different Ti-based catalysts have widely varying molecular weight distributions depending on the type of catalyst and the chemical nature of Modifiers I and II. Table 2.7 gives several examples for propylene polymers prepared with different catalyst systems. All these catalysts produce very high fractions of the crystalline material and, therefore, these data mostly represent polymer molecules produced by isospecific active centers. Different catalyst modifiers afford the synthesis of polymers with a widely varying width of molecular weight distribution, from quite narrow, Mw/Mn ¼ 3.4, to very broad, Z10. From the mechanistic standpoint, the use of any single parameter to describe the molecular weight distribution of a polymer produced with a multi-center catalyst,

67

Single-Center and Multi-Center Polymerization Catalysis

such as the Mw/Mn value, is inadequate. Any alkene homopolymer or a copolymer produced with a multi-center catalyst is mixture of several fractions (Flory components), each produced by a single type of active center. All alkene polymers prepared with heterogeneous Ti-based catalysts consist of at least four or five Flory components. As an illustration, Figure 2.9 shows the GPC curve of isotactic polypropylene produced with a supported Ti-based catalyst [322]. This molecular weight distribution can be represented by five Flory components with widely different average molecular weights, from B1.3  104 to B1.3  106 (Table 2.8). One should keep in mind that the GPC peak resolution procedure is to a significant

Table 2.7 Molecular weight distribution of polypropylene produced with TiCl4/MgCl2/ Modifier I-AlEt3/Modifier II systemsa [321]

a

Modi¢er I

Modi¢er II

I.Ib (%)

Mw 105

Mw/Mn

Ethyl benzoate Di-i-butyl phthalate Succinate 1,3-dietherc

Ethyl p-EtO-benzoate (Cy)(Me)Si(OMe)2 (Cy)(Me)Si(OMe)2 –

94.5 97.5 97.3 98.3

2.64 2.81 3.24 2.54

4.8 7.0 9.8 3.4

Polymerization in liquid monomer at 701C in the presence of H2. Fraction insoluble in cold xylene [187]. 2,2-Di-i-butyl-1,3-dimethoxypropane (see Scheme 4.2).

b c

150

d(W)/d(logMW)

100

50

0 103

104

105

106

107

Molecular weight

Figure 2.9 GPC curve of highly isotactic polypropylene prepared at 701C with supported T|-based catalyst [322] and its resolution into Flory components. Catalyst T|Cl4/MgCl2/di-ibutyl phthalate, cocatalyst AlEt3/Cpy2Si(OMe)2.

68

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 2.8 Flory components in polypropylene produced with supported Ti-based catalyst systema [322]

a

Flory component

Mw

Fraction (%)

I II III IV V

1.27  104 5.60  104 1.53  105 4.58  105 1.28  106

5.2 17.4 36.5 30.8 10.1

Catalyst TiCl4/MgCl2/di-i-butyl phthalate, cocatalyst AlEt3/Cpy2Si(OMe)2, polymerization at 701C in the presence of H2.

degree formal, it merely gives the minimum number of distinct active centers in terms of average molecular weight. The active centers in multi-center catalysts also differ in stereospecificity and in copolymerization ability, the differences that can be identified by the Tref or the Crystaf method (Section 2.3.2.3). A comparison of GPC data and analytical Tref or Crystaf data often shows that some of the Flory components (such as those in Figure 2.9 and Table 2.8) are mixtures of macromolecules of different tacticity (or copolymer molecules of different compositions), which are produced by different types of active centers. These dissimilar active centers can give rise to single Flory components simply because the respective polymer fractions happen to have similar average molecular weights and cannot be separated with the GPC method [211,221,232]. Copolymers of ethylene and different 1-alkenes prepared with heterogeneous Ziegler–Natta catalysts also always have broad multi-Flory molecular weight distributions. One characteristic example is shown in Figure 2.10. In some cases, supported catalysts even produce polymers with a distinct bimodal molecular weight distribution [323]. The existence of several discrete types of active centers was also found in polymers collected after a very short reaction time in propylene polymerization reactions with supported TiCl4/MgCl2 catalysts [222–224] and in ethylene/propylene copolymerization reactions with similar catalysts [278]. For example, three distinct Flory components are present in polypropylene produced with the TiCl4/MgCl2-AlEt3 system even after a 0.15-second reaction, the Mw/Mn ratio of the polymer is 3.1 [222,223]. 2.5.1.2. Metallocene catalysts Metallocene catalysts are often viewed as essentially single-center catalysts. However, numerous examples show that some metallocene catalysts may have two or several active centers that differ in their kinetic parameters and produce polymer fractions of a different molecular weight. GPC curves of such polymers have expressed multimodality [324–326]. Several sources of the multi-center nature of metallocene catalysts are discussed in the literature. Naturally occurring mixtures of metallocene complexes: Some clear examples of the multi-center metallocene catalysis were encountered when metallocene complexes exist in two isomeric forms and when the isomers are difficult to separate. As an

69

Single-Center and Multi-Center Polymerization Catalysis

1.0

dW/d(logMW)

0.8

0.6

0.4

0.2

0.0 103

104

105 Molecular weight

106

107

Figure 2.10 GPC curve of ethylene-1-hexene copolymer (CHex ¼ 2.8 mol.%) prepared a heterogeneous T|-based catalyst (example 2E in Section 4.2.1.2) and its resolution into Flory components.

example, ethylene/1-octene copolymers prepared with the meso-isomer of Me2Si(2-Me-Ind)2ZrCl2 (98.5% purity) at 01C have a broad molecular weight distribution (Mw/Mn B7–8) and their GPC curves show the presence of two polymer fractions with widely different molecular weights, B2,500 and 6  105 [327]. The high molecular weight fraction, a random copolymer, is produced by a small impurity in the metallocene complex, the racemic metallocene isomer (it makes a very active catalyst), whereas the low molecular weight fraction, an alternating copolymer, is produced by the dominant meso-isomer. Polymerization of propylene with a mixture of meso- and racemic isomers of Me2Si(2-Me-Ind)2ZrCl2 activated with MAO exhibits the same feature [328]. These polymers are mixtures of two materials, an isotactic fraction of a high molecular weight produced by active species derived from the racemic isomer of the metallocene complex and an atactic fraction with a six times lower molecular weight produced by active species derived from the meso-isomer. Polymerization reactions of 1-butene and 1-hexene with this mixture of metallocene complexes showed the same pattern [328]. Supporting metallocene systems on inert carriers can produce either essentially single-center catalysts [329,330] or can lead to the formation of multi-center catalysts [296,331,332] depending on the type of metallocene complex and the procedure of catalyst synthesis (see Section 4.8). Coexistence of active centers with different kinetic parameters: When the Cp2ZrCl2MAO system was studied in ethylene polymerization reactions, GPC analysis revealed the presence of two Flory components in the polymers [299]. The respective active centers have very similar kinetic characteristics when prepared with AlMe3-free MAO and, therefore, are difficult to distinguish. However,

70

Alkene Polymerization Reactions with Transition Metal Catalysts

the centers have different reactivities in chain transfer reactions to AlMe3 (see Section 3.3.1.2.1), and the Flory fractions produced by the two centers become clearly separated in GPC curves of the polymers produced in the presence of high concentrations of AlMe3. When the rac-Me2C(Ind)2ZrCl2-MAO system was used in polymerization reactions of ethylene and propylene at 401C, the results for the two alkenes were different. Judging by GPC analysis, the propylene polymers were produced with a single type of active center; their Mw/Mn ratio was close to 2 [300]. However, ethylene polymers prepared with the same catalyst and under the same conditions were obviously produced by two clearly distinguishable types of centers. The centers have different reactivities in chain transfer reactions to AlMe3 and, as a result, the Mw/Mn ratio of combined polymers increased to 3.5–4. Effects of reaction parameters: In many cases, the presence of several types of active centers in soluble catalyst systems utilizing a single metallocene complex is revealed when the polymerization conditions are changed. At 501C, the rac-C2H4(Ind)2HfCl2-MAO system is essentially a single-center catalyst in propylene polymerization reactions, the GPC curve of the polymer is well represented by a single Flory component with MwB3.8  105 [251]. However, as the polymerization temperature decreases, a second Flory component with MwB7  104 appears in the GPC curve. Both polymer components are present in equal amounts at 01C and the low molecular weight component dominates at
301C. Obviously, two active centers co-exist in this catalyst, they have similar isospecificity (the [mm] value of the combined polymer remains approximately constant (see Table 3.32), but they have different kinetic parameters and different temperature windows of stability [251]. Similar manifestations of multi-center catalysis were also found in polymerization reactions of ethylene and propylene with monometallocene catalysts systems of the CpTi(OR)3-MAO type [333,334] and with non-bridged analogs of constrainedgeometry catalysts, Cp(R2N)TiCl2 [335]. When the rac-Me2C(Ind-H4)2ZrCl2MAO system was used to polymerize propylene at
101C, the GPC analysis of the polymer also showed the presence of two isotactic fractions produced by different types of active centers [302]. The fractions had different solubility in heptane, they slightly differed in isotacticity ([mm] values were 0.986 and 0.961 for the insoluble and the soluble fraction, respectively), and had different Mw values, B2.2  105 and 5  104. Coexistence of several types of active centers in the catalysts was also discovered in polymerization reactions of 1-hexene with metallocene catalysts. Two types of active centers are formed in Cp2ZrCl2-MAO and C2H4(Ind)2ZrCl2-MAO systems. The first center dominates at temperatures from
78 to
501C and the second center is more active from 30 to 801C, and both centers coexist in the intermediate temperature range resulting in broadening of the GPC curves and in an increase of the Mw/Mn ratio [336]. When a polymerization reaction of 1-hexene with the Me2Si(Ind)2ZrCl2-MAO system at 601C is carried out at low metallocene concentrations and a high [Al]:[Zr] ratio, B5,000, the polymer exhibits the characteristics of a material prepared with a single-center catalyst. However, when the [Al]:[Zr] ratio is decreased to B150, the molecular weight distribution of the polymer becomes trimodal [326].

Single-Center and Multi-Center Polymerization Catalysis

71

Differences between single- and multi-center catalysis are especially obvious when polymerization reactions are carried out in the living-chain mode. The constrained-geometry [Me2Si(Me2Flu)(t-Bu-N)]TiMe2 complex activated with AlMe3-free MAO produces at 201C random amorphous ethylene/norbornene copolymers of a very narrow molecular weight distribution (Mw/Mn ¼ B1.2) typical for living-chain reactions. However, the same complex activated with the ion-forming cocatalyst [Ph3C]+ [B(C6F5)4]
-Aln-Oct3 produces a multi-center catalyst, the copolymers prepared with it have the Mw/Mn ratio of W3 [368]. Early metallocene catalysts: The first discovered metallocene system, Cp2TiCl2AlEt2Cl, is essentially a single-center catalyst when used in ethylene polymerization reactions in aromatic solvents at moderate temperatures. However, when ethyl chloride is used as the solvent, both the reaction kinetics and the structure of ethylene polymers and copolymers undergo significant changes [303,304]. Two copolymer fractions with different average molecular weights are formed with comparable yields in ethylene/1-butene copolymerization reactions, one with Mw of B2.0  104 and another B1.0  105 [303]. The results produced in ethylene/ propylene copolymerization reactions with the same catalyst indicate that the formation of these two fractions is separated in time [304]. Ethylene polymers containing multiple Flory components were also formed in polymerization reactions catalyzed by Cp2Ti(Me)Cl and Cp2Ti(Et)Cl activated with chloroalkyl alumoxanes produced from AlEt2Cl, AlMe2Cl, etc. [305]. 2.5.1.3. Non-metallocene homogeneous catalysts Homogeneous non-metallocene catalysts provide many examples of both the single- and the multi-center polymerization catalysis. Phenoxy-imine ligands (Scheme 1.2) are bidentate ligands coordinated to a transition metal atom through one PhO– bond and one N atom. Ti and Zr complexes containing these ligands can potentially exist in several isomeric forms with respect to the arrangement of the coordinating heteroatoms [337–339]. Most zirconium complexes exist in solution predominantly as single isomers [339,340]; catalyst systems prepared from these complexes usually contain one type of active center and produce linear ethylene polymers with Mw/MnB2 [338,339]. However, some zirconium complexes are mixtures of several isomers and each isomer can form an active center. Ethylene polymers produced with such systems have broad multimodal molecular weight distributions [338,339]. The relative activity of different centers in the latter catalysts depends mostly on polymerization temperature. Only one type of center is active at 01C, it produces a single-Flory polymer of a low molecular weight. As the temperature increases, two other types of centers become active. These centers produce Flory components with B10 and B100 times higher molecular weights, respectively [338]. Combinations of phenoxy-imine Zr complexes and organoaluminum compounds AlR3 (R ¼ Me, Et, Hex) also produce multi-center soluble catalysts [341]. Ethylene homopolymers prepared with these systems have broad multi-component molecular weight distributions that vary with reaction time. Each catalyst contains two populations of active centers. The centers of the first type are very active in the

72

Alkene Polymerization Reactions with Transition Metal Catalysts

beginning of the polymerization reactions, the first 5–10 minutes, and produce polymers with a low molecular weight, MwB2.0  104. The centers of the second type emerge after B30 minutes, they produce several Flory components of a high molecular weight, with Mw from B2  105 to 2  106 [341]. In contrast, combinations of similar TiIV complexes and MAO have only one type of active center and they polymerize alkenes to materials with a very narrow molecular weight distribution [342–345]. Homogeneous catalysts prepared from 2,6-bis(imino)pyridyl complexes of Fe and Co (Scheme 1.3) and MAO exhibit a similar behavior. The Co complexes form essentially single-center active species (Section 5.5.4) whereas the Fe complexes generate multi-center systems and polymerize ethylene to products with a very broad multi-component molecular weight distribution (see, e.g., Figure 2.11). The ability to produce several types of active centers is especially pronounced in supported catalysts of non-metallocene complexes. A homogeneous catalyst prepared with a Ni diimine complex (Scheme 1.3) is a single-center catalyst, it generates highly branched polyethylene of a high molecular weight with Mw/MnB1.6 and TmB901C [346]. When the same complex is supported on montmorilonite, it contains two distinctly different types of active centers [346]. The first type is similar to the center in the homogeneous catalyst whereas the second type of center produces a much less branched polymer (TmB1181C) of a low molecular weight.

60

50

d(W)/d(logMW)

40

30

20

10

0 102

103

104 105 Molecular weight

106

107

Figure 2.11 Molecular weight distribution of polyethylene produced with homogeneous system containing Fe 2,6-bis(imino)pyridyl complex (Scheme 1.3) and MMAO [388].

Single-Center and Multi-Center Polymerization Catalysis

73

2.5.1.4. Chromium-based and multi-component catalysts All chromium-based catalysts for ethylene polymerization contain several types of active centers that produce Flory components with widely different molecular weights. Figure 2.12 shows the GPC curve of an ethylene homopolymer prepared with a chromium oxide catalyst at 901C. At least six Flory components with molecular weights ranging from B3  103 to W1.2  106 are required for a satisfactory description of this molecular weight distribution. Catalysts prepared from organochromium compounds also have several types of active centers producing Flory components with Mw values from 1.2  103 to B1  106 [347]. Different organochromium catalysts produce essentially the same Flory components but in greatly varying proportions depending on the type and the chemical nature of the support. Bicomponent catalysts of two types, metallocene/metallocene and metallocene/ Ziegler–Natta, are specially designed to produce polymer mixtures with broad, multi-component molecular weight distributions (Section 4.9). Figure 2.13 presents the GPC curve of one such polyethylene resin. Polypropylene resins produced with a combination of a supported TiCl4/MgCl2 catalyst and a single-center metallocene catalyst based on the rac-C2H4(Ind)2ZrCl2 complex also have bimodal molecular weight distributions [301]. GPC curves of these propylene polymers are similar to that shown in Figure 2.13; they consist of two components, a single Flory component produced by the metallocene catalyst and six Flory components produced by the Ziegler–Natta component of the catalyst.

Figure 2.12 GPC curve of ethylene homopolymer prepared with chromium oxide catalyst at 901C. Molecular weights of Flory components B3 103, 1.4 104, 4.6 104, 1.36 105, 4.07 105, and 1.21 106.

74

Alkene Polymerization Reactions with Transition Metal Catalysts

5

dW/d(logMW)

4

3

2

1

0 102

103

104 105 Molecular weight

106

107

Figure 2.13 GPC curve of polyethylene resin prepared with a bicomponent metallocene/ Ziegler^Natta catalyst (dots). Solid lines represent GPC curves of polymer components prepared with metallocene and Ziegler^Natta catalysts.

2.5.2. Steric structure of alkene homopolymers, different definitions of stereoregularity Fractional stereoregularity: Historically, two parameters were used to describe the stereoregularity of polypropylene produced in polymerization reactions with multicenter catalysts. The first parameter is the fractional stereoregularity, the content of a polymer fraction insoluble in boiling n-heptane or in an aromatic solvent, hot toluene or cold xylene. At the present time, two procedures are commonly used to measure the fractional stereoregularity. In the first procedure (Section 2.3.2.2), a polymer sample is dissolved in boiling xylene (B1–2 g/100 cc of solvent), the solution is cooled to 20–251C, and the crystallized polymer is collected and weighed. This simple fractionation cleanly separates any polypropylene sample into two fractions, a highly crystalline isotactic polymer and an amorphous material of a complex microstructure [136,348]. The content of the insoluble fraction is usually called ‘‘the isotacticity index,’’ or II. It is more reasonable to define this value as the fractional isotacticity and to call the respective fractions, however conditionally, as the crystalline fraction and the amorphous fraction. One should also take into account that the crystallinity level of the ‘‘crystalline’’ fraction may vary depending on the catalyst and reaction conditions and that some components of the ‘‘amorphous’’ fraction may have a low level of crystallinity. The second procedure for the measurement of fractional isotacticity uses the solid-state NMR method. It is based on differences in the relaxation time of the polymer components, it is much

75

Single-Center and Multi-Center Polymerization Catalysis

higher for the crystalline fraction than for the amorphous fraction. This method is very fast and can be used to analyze polypropylene both in the granular form and in pellets. Spectroscopic measurement of stereoregularity: The second measure commonly used to determine the stereoregularity of 1-alkene polymers is based on spectroscopic analysis, either NMR or IR [349–351]. At the present time, the 13C NMR technique has emerged as a method of preference. This technique is very sensitive to the steric structure of polypropylene and readily affords the measurement of sterically different heptads and nonads [348,352]. When the spectroscopic measurements are applied to unfractionated polymers, their results, such as the content of mmmm pentads or different IR spectroscopic parameters, usually correlate well with the fractional isotacticity and do not provide any principally new information. However, when 13C NMR measurements are used for the characterization of the crystalline fractions, they acquire a new significance, they describe the ‘‘quality’’ of the fractions, the average degree of deviation from perfect isotacticity [348,349,352–358] and the nature of occasional steric defects in predominantly isotactic chains [348,352–362]. Table 2.9 compares fractionation data and NMR stereoregularity data for crystalline fractions of propylene polymers produced with several Ti-based supported catalysts. These results show that the solvent fractionation of polypropylene is not a reliable measure of its stereoregularity: (a) none of the n-heptane-insoluble fractions is perfectly isotactic (this observation equally applies to xylene-insoluble crystalline fractions) and (b) isotacticity of the crystalline fractions (the content of mmmm pentads) noticeably differs from catalyst to catalyst. Although both methods for measuring polypropylene ‘‘isotacticity’’ are widely used for the comparison and characterization of different Ti-based catalysts, neither of the measures is sufficient for the mechanistic understanding of alkene polymerization reactions.

Table 2.9 Fractional isotacticity of polypropylene prepared with several TiCl4/MgCl2 catalysts and 13C NMR isotacticity of crystalline fractions IIa (%)

[mmmm]b

Polymerization temperature 411C [363] AlEt3 TiCl4/MgCl2/ethyl benzoate AlEt2Cl TiCl4/MgCl2/ethyl benzoate AlEt3 TiCl4/MgCl2/methyl acetate

72 53 57

90 88 87

Polymerization temperature 501C [364] TiCl4/MgCl2 TiCl4/MgCl2 TiCl4/MgCl2/di-i-octyl phthalate TiCl4/MgCl2/di-i-octyl phthalate TiCl4/MgCl2/di-i-octyl phthalate

35 85 75 94 96

84 92 89 90 91

Solid catalyst

a

Fraction insoluble in boiling n-heptane. NMR data for crystalline fractions.

b

Cocatalyst

AlEt3 AlEt3/PhSi(OEt)3 AlEt3 AlEt3/Me4-piperidine AlEt3/PhSi(OEt)3

76

Alkene Polymerization Reactions with Transition Metal Catalysts

The application of modern fractionation techniques, analytical Tref and Crystaf, provides more detailed information on the stereoregularity of alkene polymers.

2.5.3. Steric structure of alkene homopolymers produced with multicenter catalysts Even a simple solvent fractionation procedure (see Table 2.1) demonstrates that propylene polymers produced with solid Ti-based Ziegler–Natta catalysts consist of several components of different stereoregularity and that relative contents of these components differ depending on the catalyst. Preparative and automated fractionation techniques support this conclusion and provide more structural details [61,126,129,178–190,213,225]. Figure 2.5 shows the analytical Tref curve of a highly crystalline polypropylene fraction prepared with a supported Ti catalyst and its resolution into elemental Tref components, each described by Equation (2.15) [322]. The resolution procedure provides a means of subdividing each crystalline fraction into several components with respect to isotacticity. The level of detail produced by the technique varies depending on the fractionation method. As a minimum, it separates the crystalline material into three components, the highly isotactic fraction, the fraction of reduced isotacticity, and the fraction of low isotacticity (see, e.g., Table 2.10). However, Figure 2.5 demonstrates that the potential of the analytical Tref method can be much higher [213,322]. Table 2.11 lists the temperature of each elemental Tref peak, the estimation of NMR isotacticity of the respective Table 2.10

a

Resolution of Tref curves of crystalline fraction of polypropylenea [221]

Component

[mmmm]b

Fraction (%)

Highly isotactic Moderately isotactic Low isotacticity

B0.95 (av.) B0.92 B0.78

B80 B9 B11

Polymer prepared with TiCl4/MgCl2/dibutyl phthalate-Ali-Bu3 system at 901C. Approximate evaluation based on calibration in [228].

b

Table 2.11 Resolution of analytical Tref curve of crystalline polypropylene fraction prepared Ti-based supported catalysta [322]

a

Tref component

Temperature (1C)

[mmmm]

Fraction (%)

A B C E F G

117.1 115.1 112.2 108.7 104.8 102.0

0.993 0.986 0.976 0.963 0.950 0.940

59.8 26.5 3.1 3.4 6.3 0.9

Catalyst TiCl4/MgCl2/di-i-butyl phthalate, cocatalyst AlEt3/Cpy2Si(OMe)2.

77

Single-Center and Multi-Center Polymerization Catalysis

polypropylene component, and its content in the crystalline polypropylene fraction. These results confirm that polypropylene fractions insoluble in cold o-xylene (‘‘isotactic’’ polymers by definition) are by no means structurally uniform. Some of their Tref components, usually those eluting at the highest temperatures, are dominant in the mixtures; other components are minor. The highly isotactic component of the crystalline material consists of several distinct components. Two of them, components A and B in Table 2.11, have exceptionally high [mmmm] values. The probability of the isotactic chain propagation for the active centers producing these Tref components is between 0.997 and 0.999. The crystalline material also contains several components of slightly lower isotacticity, components C and E. Their [mmmm] values are also quite high, 0.960–0.970; corresponding to the probability of isotactic chain propagation of B0.995–0.996. These two Tref components are the components of the highest isotacticity in the crystalline fractions produced by the majority of Ti-based Ziegler–Natta catalysts [322]. The crystalline material also contains small amounts of two components of lower stereoregularity, components F and G, each less than 10%. Their [mmmm] values are 0.955–0.935 and the probability of isotactic chain propagation for these centers is B0.987–0.990. Crystaf analysis of polypropylene provides complimentary information about their structure. Figure 2.14 shows the Crystaf curve of the crystalline fraction of polypropylene prepared with a TiCl4/MgCl2/diether catalyst activated with AlEt3 and the resolution of the curve into Gaussian components (Equation (2.17)). The fraction of the highly isotactic material constitutes B91% of the polymer. 12

10

B

dW/dT

8

6

4

2

0 30

40

50

60

70

80

90

Temperature, °C

Figure 2.14 Crystaf curve of crystalline polypropylene fraction prepared with a T|Cl4/ MgCl2/1,3-diether catalyst activated with AlEt3 and its resolution into Gaussian components (Equation (2.17)) [232].

78

Alkene Polymerization Reactions with Transition Metal Catalysts

This fraction consists of two closely spaced components with Tmax values 81.8 and 78.31C in a B2:1 ratio. The polymer also contains three small fractions of lower isotacticity which crystallize at 69, B60 and B501C. This product also contains B3% of amorphous material soluble in 1,2,4-trichlorobenzene at 301C. Several discrete types of polymerization centers in terms of stereospecificity can be distinguished even at the earliest stages of propylene polymerization reactions with supported TiCl4/MgCl2 catalysts, e.g., after a fraction of a second [191,224]. If the solid catalyst does not contain Modifier I, two highly isospecific centers are rapidly formed, they produce polypropylene fractions with [mmmm] values of B0.95 and 0.93 in a B1:3 ratio [224]. The third type of center produces a small quantity of a stereo-irregular polymer. In general, the formation and decay rates of different highly isospecific centers are quite similar, and only stopped-flow experiments at low temperatures and at very short reaction times (B0.15 seconds) reveal small differences in their kinetic characteristics [191,365]. Numerous Tref analysis results for propylene polymers prepared with various Ti-based Ziegler–Natta catalysts [61,126,129,178–185,187,189,190,213,221,225, 232] support the conclusion followed from the data in Figure 2.5 and Table 2.11 that each crystalline fraction consists of several components of different stereoregularity. These results make understandable the meaning of average [mmmm] values for the crystalline polypropylene fractions listed in Table 2.9. The variation in the average [mmmm] content in the crystalline fractions, as well as the distribution of steric mistakes in these fractions [364,366], is mostly explained by the variations in the relative contents of polymer components of different isotacticity [186,221]. Several examples of these variations are described in Chapter 5. Some metallocene catalyst systems also contain active centers of different stereospecificity and the polymer products prepared with them can be separated into fractions produced by each center. One example is shown in Table 2.2. A propylene polymer prepared with the hemi-isospecific Ph2C(3-Me3Si-Cp)(Flu)ZrCl2 complex activated with the ion-forming cocatalyst [Me2PhNH]+ [B(C6F5)4]
-Ali-Bu3, consists of two unrelated polymers, a moderately isotactic fraction soluble in hot heptane (the same material is produced when this metallocene complex is activated with MAO) and a moderately syndiotactic polymer [168]. Both components are crystalline, and the Tref analysis shows that each component has a narrow stereodistribution. Because molecular weights of the two polymer components are relatively close (Table 2.2), the combined product has a narrow molecular weight distribution (Mw/Mn ¼ 2.1), which masks the fact that this is a mixture of two sterically different polymers produced by different types of active centers. Two independent types of active centers coexist also in catalyst systems based on another complex of C1 symmetry, Ph2C(3-PhCH2-Cp)(Flu)ZrCl2, activated either with MAO or with an ion-forming cocatalyst [Me2PhNH]+ [B(C6F5)4]
-Ali-Bu3 [168]. Both catalysts produce propylene polymers of a very high molecular weight and each polymer can be separated into two fractions, an amorphous atactic polymer soluble in hexane (6% and 33%, respectively), and a hexane-insoluble, crystalline syndiotactic polymer with [rrrr] ¼ 0.89–0.94. The presence of different types of active centers in some metallocene catalysts can be traced to isomerization of

79

Single-Center and Multi-Center Polymerization Catalysis

racemic metallocene complexes producing isotactic polymers to meso-isomers producing atactic polymers [367].

2.5.4. Compositional distribution of copolymers produced with multicenter catalysts Copolymers produced with Ziegler–Natta catalysts: Even simple solvent-fractionation experiments clearly demonstrate that any ethylene/1-alkene copolymer prepared with a solid Ti- or V-based Ziegler–Natta catalyst under stationary reaction conditions is a complex mixture containing copolymer molecules of different compositions. Table 2.3 gives one example for an ethylene/1-hexene copolymer produced with a supported TiCl4/MgCl2-AlEt3 system. Different active centers in such catalysts are characterized by different values of the reactivity ratios. The centers producing copolymer fractions soluble in cold n-hexane are nearly 30 times more reactive toward 1-hexene compared to the centers responsible for the formation of the fraction insoluble in boiling n-octane [153]. Broad compositional distributions are typical for most copolymers produced with Ti-based catalysts. As an example, Table 2.12 shows solvent fractionation data for a styrene/(S)-4-methyl-1-hexene copolymer produced with the TiCl4-Ali-Bu3 system at 201C. The average content of styrene in the copolymer is 41 mol.%. However, this reaction product is in reality a mixture of copolymer molecules with a wide range of compositions, from 1 to over 70 mol.% of styrene, and with different molecular weights. In addition, the polymer product contains B9% of chloroform-soluble polystyrene which is probably formed in a radical or a cationic side-reaction. The fact that different copolymer fractions have different optical rotation (due to the use of optically rotating (S )-4-methyl-1-hexene) confirms a high degree of compositional nonuniformity of the combined polymer product. Analysis of numerous alkene copolymers with such fractionation techniques as solvent-gradient elution fractionation, the Tref method, or the Crystaf method, supports the conclusion that the copolymers consist of several discrete components that differ widely in composition [126,189,206,211,215,228,230,231,371–375]. Table 2.13 gives two examples of ethylene/1-hexene copolymers prepared with TiCl4/MgCl2/THF-AlEt3 catalysts using the preparative Tref technique [189]. Some of active centers in these catalysts produce essentially ethylene homopolymers Table 2.12

a

Solvent fractionation of styrene/(S)-4-methyl-1-hexene copolymera [369,370]

Solvent

Fraction (%)

CStyrene (mol.%)

[g]b (dl/g)

[aD]25 (1)

Acetone Ethyl acetate Diethyl ether Cyclohexane Chloroform

4.8 30.5 25.4 30.6 8.7

72.3 38.5 3.4 1.0 W95

0.13 1.7 4.6 9.4 3.1

+43 +120 +129 +191

Copolymer produced with TiCl4-Ali-Bu3 system. Intrinsic viscosity, measured in tetralin at 1201C.

b

80

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 2.13 Preparative Tref fractionation of ethylene/1-hexene copolymers produced with TiCl4/MgCl2/THF – AlEt3 systems at 701C [190] Elution temperature (1C)

Fraction (%)

CHex (mol. %)

Catalyst: Unfractionated 40 60 70 75 80 85 90 95 100 110

TiCl4/MgCl2/THF 100 2.4 2.4 7.0 12.5 5.2 13.6 3.6 8.7 2.9 9.9 2.2 10.7 1.5 8.6 1.2 21.0 B0.15 10.4 B0 0.4 B0

Fraction (%)

CHex (mol.%)

‘‘–’’, heated at 801C, 60 min 100 2.9 2.0 7.6 10.9 5.2 12.2 3.6 7.9 2.6 9.2 2.3 10.7 1.4 9.2 1.0 24.1 B0.15 11.4 B0 0.4 B0

rather than the copolymers. Not only these centers can be identified by various fractionation techniques, in some cases they can be also observed when the molecular weight distribution of the copolymers is analyzed. For example, ethylene/propylene copolymers produced with the supported TiCl4/(Al2O3SiO2)-AlEt3 system contain, in addition to copolymer macromolecules of different compositions with average molecular weights ranging from 1  104 to 3  104 (depending on copolymer composition), a distinct fraction with a very low propylene content and a high molecular weight, MwB7.4  105 [374]. Figure 2.15 shows the Crystaf curve of an ethylene/1-hexene copolymer with the average 1-hexene content of 4.1 mol.% produced with the supported TiCl4/ MgCl2/SiO2-AlEt3 system. This example represents one of the most complex cases; eight elemental Crystaf components are required to represent the data in an adequate manner [232]. The composition of the components varies in a broad range. Crystallization temperature (1C) 84.0 81.6 78.8 72.2 63.5 54.5 B44 B33 Fraction (%) 6.9 17.4 9.9 11.6 9.5 10.3 9.2 4.8 CHex (mol.%) B0.2 0.5 0.8 1.6 2.6 3.7 4.8 6.2

In addition, B20% of the material, a completely amorphous fraction, remains dissolved at the end of the analysis and is not counted in the figure. The compositional distribution of ethylene/1-alkene copolymers becomes especially broad when they are produced with supported catalysts of a TiCl4/MgCl2/Modifier I type activated with cocatalyst mixtures containing ZnEt2 and Ali-Bu3 [375]. The addition of ZnEt2 brings about two changes, the molecular weight distribution greatly broadens (the Mw/Mn ratio increases from B5 to B21) and the copolymer mixture now contains an increased fraction of a material with a high content of 1-hexene. An apparent cause for these changes is a chemical interaction between the cocatalyst components leading to the formation of a new type of active center [375].

81

Single-Center and Multi-Center Polymerization Catalysis

6

5

dW/dT

4

3

2

1

0 30

40

50

60

70

80

90

Temperature, °C

Figure 2.15 Crystaf curve of ethylene/1-hexene copolymer produced with supported T|Cl4/ MgCl2/SiO2 -AlEt3 system and its resolution into elemental components [232]. Cav Hex ¼ 4:1 mol:%.

Differences in reactivities of active centers in copolymerization reactions can be observed even at very low concentrations of one of the monomers. For example, an ethylene/propylene copolymer prepared with the isospecific d-TiCl3-AlEt2Cl system at 701C and containing a very low fraction of ethylene units, 0.41 mol.% (13C-labeled ethylene was used to measure the composition) still contains copolymer fractions of a different composition. The ethylene content in the fractions ranges from 0.23 mol.% in the material prepared with active centers of the highest stereospecificity ([mmmm] B0.98) to B1.3 mol.% in the material prepared with active centers of the lowest stereospecificity ([mmmm]B0.55) [228]. Copolymers of propylene and 1-butene prepared with multi-center Ti-based catalysts have a much narrower composition distribution [195,376,377] compared to ethylene/1-alkene copolymers. The data for one such copolymer with a high 1-butene content in are shown in Table 2.14. A similar compositional variation was found in copolymers prepared from propylene and a small amount of 13C-labeled 1-butene [228]. Overall, a clear trend is evident in these data: the higher is the isospecificity of the active centers the lower is their reactivity toward 1-butene in these copolymerization reactions [195,376]. Copolymers produced with metallocene catalysts: The compositional distribution of alkene copolymers prepared with different metallocene catalysts varies greatly depending on the type of metallocene complex and reaction conditions. Some copolymers have very narrow compositional distributions expected for singlecenter catalysis [219,233,378]. However, several metallocene catalysts were found to have two or several active centers that differ in their copolymerization ability.

82

Table 2.14

a

Alkene Polymerization Reactions with Transition Metal Catalysts

Preparative Tref fractionation of propylene/1-butene copolymera [377]

Elution temperature (1C)

Fraction (%)

CB (mol.%)

Crystallinity (%)

Unfractionated 5 20 30 40 50 60 70

100 2.7 1.9 4.1 12.3 6.3 11.4 9.9

25.9 47.1 43.0 39.1 33.6 25.4 20.4 15.7

49 0 39 51 50 52 51 53

Polymerization with TiCl4/MgCl2/ethyl benzoate – Ali-Bu3/ethyl benzoate system at 701C.

Copolymers produced with these systems have expressed compositional bimodality or multi-modality [145,192,219,378,379]. As an example, an ethylene/1-hexene copolymer prepared with the C2H4(Ind)2ZrCl2-[Me2N(Ph)H]+ [B(C6F5)4]
-AliBu3 system at 401C has a very uniform compositional distribution judging by its Tref and DSC features [378]. However, a copolymer prepared under the same conditions with the Hf analog of the metallocene complex consists of two copolymer fractions, one (B30%) with B3.5 mol.% of 1-hexene (it melts at 1011C), and another with a much higher 1-hexene content melting at B271C. Often, one of the copolymer fractions is formed for a short period of time early in a copolymerization reaction and another fraction is formed later in the same reaction [379]. In one extreme example, ethylene/1-pentene copolymers prepared with the homogeneous Cp2Zr(Cl)X-MAO system (where the ligand X is a carbene moiety –O–C(CH3)QW(CO)5) exhibited all the features typical for Ti-based Ziegler–Natta catalysts and even for bimetallic catalysts (see Section 4.9). These features include a very broad multi-modal weight distribution (Mw/Mn ratios can vary from 10 to 50) and a very broad compositional distribution, with low molecular weight components containing B10 times more 1-pentene than high molecular weight components [145]. Supported metallocene catalysts exhibit a very complex behavior in ethylene/ 1-alkene copolymerization reactions. Soares carried out the detailed analysis of molecular weight distribution and compositional distribution of ethylene/1-hexene and ethylene/1-octene copolymers prepared with several silica-supported metallocene catalysts and with catalyst mixtures [219,314,380–383]. These copolymers have a molecular weight distribution that ranges from a narrow distribution typical for the unsupported catalysts of this type [380] to a quite broad distribution containing up to seven Flory components, or even a multi-modal distribution [314]. Their compositional distribution (measured by Crystaf or Tref) also varies depending on the structure of the metallocene complex. Supported catalysts prepared from rac-C2H4-Ind2ZrCl2 produce copolymers with a relatively narrow (but still not completely uniform) compositional distribution, similarly to the unsupported catalyst [219,381,384] whereas supported catalysts based on Cp2HfCl2, Me2Si(2-Me-Benz[e]Ind)2ZrCl2, and on constrained-geometry monotitanocene

Single-Center and Multi-Center Polymerization Catalysis

83

complexes produce ethylene/1-alkene copolymers with a bimodal or a trimodal compositional distribution. This compositional complexity indicates the existence of several types of active centers with different values of reactivity ratios [219,314,380–384]. A strong effect of the carrier was found in ethylene/1-alkene copolymerization reactions with supported catalysts based on the (n-Bu-Cp)2ZrCl2 complex [385]. Deposition of the complex on modified silica calcined at 2001C resulted in the formation of a single-center catalyst that produced compositionally homogeneous ethylene/1-hexene copolymers. However, deposition of the same complex on the MgCl2  (THF)2 support pretreated with Ali-Bu3 generated a catalyst containing three distinct types of active centers, one (dominant) the same as in the silicasupported catalyst and the other two with noticeably different abilities to copolymerize 1-hexene with ethylene [385]. The presence of several types of active centers is especially obvious in ethylene/ styrene copolymerization reactions with a tri-component system containing CpTi(OPh)3, MAO, and AlMe3 [281]. The polymer material produced from a 1:1 molar mixture of ethylene and styrene consists of three components, a true ethylene/styrene copolymer (amorphous polymer soluble in THF) and two homopolymers, polyethylene and syndiotactic polystyrene. The relative contents of the components depend primarily on the ratio between AlMe3 and MAO in the cocatalyst mixture.

CHAPTER 3

Chemistry and Stereochemistry of Polymerization and Copolymerization Reactions with Transition Metal Catalysts

Contents 3.1. Chemistry and Stereochemistry of Polymerization Reactions 86 3.1.1. Definition of regioselectivity 87 3.1.2. Stereospecificity in alkene polymerization reactions 88 3.1.3. Statistics of predominantly stereoregular polymers 88 3.2. Heterogeneous Titanium- and Vanadium-Based Ziegler–Natta Catalysts 98 3.2.1. Chemistry of chain initiation, propagation, and transfer reactions 98 3.2.2. Reactivities of alkenes in polymerization reactions 109 3.2.3. Stereospecificity of titanium-based polymerization catalysts 114 3.3. Metallocene Catalysts 124 3.3.1. Chemistry of chain initiation, propagation, and transfer reactions 125 3.3.2. Stereochemistry of chain growth reactions 148 3.3.3. Polymerization and copolymerization reactions of styrene 164 3.4. Homogeneous Catalysts Based on Early-Period Transition Metals 166 3.4.1. Complexes with monodentate ligands 166 3.4.2. Complexes with bidentate, tridentate, and tetradentate ligands 169 3.4.3. Chain insertion reactions in polymerization of alkenes with internal double bonds 174 3.4.4. Styrene polymerization and copolymerization reactions 175 3.5. Homogeneous Catalysts Based on Late-Period Transition Metals 176 3.5.1. Regiochemistry of chain initiation and chain growth reactions 176 3.5.2. Stereochemistry of chain growth reactions 178 3.5.3. Chain isomerization reactions 179 3.6. Chromium-Based Catalysts 183 3.6.1. Chromium oxide catalysts 183 3.6.2. Organochromium catalysts 183 3.7. Stereoselective and Stereoelective Polymerization Reactions of Branched 1-Alkenes 184 3.7.1. Stereoselective polymerization reactions with Ziegler–Natta catalysts 186 3.7.2. Stereoelective polymerization reactions with Ziegler–Natta and metallocene catalysts 188 3.8. Copolymerization Reactions of Alkenes 190 3.8.1. Copolymerization reactions, reactivity ratios for various alkene pairs 190 3.8.2. Statistical description of copolymer structure in terms of block length 200

85

86

Alkene Polymerization Reactions with Transition Metal Catalysts

3.8.3. Statistical description of copolymer structure suitable for NMR analysis 3.8.4. Auto-copolymerization reactions and long-chain branching in alkene polymers

201 203

3.1. Chemistry and Stereochemistry of Polymerization Reactions As late as 20–25 years ago, the subject of the chemistry of alkene polymerization reactions with transition metal catalysts could be easily covered on 3–6 pages; the chemistry was simple, the polymerization reactions appeared to be very regular, and the number of chemical errors in the polymer chains was small. Two experimental advances changed this situation, discoveries of metallocene and postmetallocene catalysts (which produce a much greater repertoire of chemical and stereochemical errors) and a steady improvement in analytical techniques, primarily high-resolution 13C NMR spectroscopy. Over time, a very big volume of information was accumulated about the chemistry and the stereochemistry of each reaction stage that determine the structure of individual polymer molecules; chain initiation, chain growth, and chain transfer reactions. Historically, the terms ‘‘chain initiation reactions,’’ ‘‘chain growth reactions’’ and ‘‘chain termination reactions,’’ when applied to alkene polymerization reactions with transition metal catalysts, were borrowed from the arsenal of terms developed in the studies of radical polymerization reactions. In the latter reactions, each term has a specific chemical meaning. The chain initiation reaction is a reaction between a free radical derived from an initiator and a monomer molecule (often, a relatively slow reaction), the chain growth reactions are addition reactions of monomer molecules to the polymer radical, and chain termination reactions are the reactions between two polymer radicals resulting in their mutual destruction. A strict application of this terminology to alkene polymerization reactions with transition metal catalysts can be misleading, mostly because the chemistry of the respective stages is quite different. Kinetic studies of catalytic polymerization reactions showed that each active center usually produces, over a period of a single polymerization reaction, from several hundreds to several thousands of polymer molecules, whereas a single radical usually produces one polymer molecule. Although transition metal catalysts are often not very stable, their average lifetime usually varies from several minutes to many hours, much longer than a typical growth time of a single polymer molecule (several seconds or less). Therefore, one can safely assume that true termination reactions (irreversible destruction of active centers) play only a minor role during the growth time of a single polymer molecule. Mechanistic studies of alkene polymerization reactions showed that reactions of polymer chain formation involve only three groups of participants. The first participant is the bond between the transition metal atom and a carbon atom, M
C (in some cases, this is the M
H bond). The second participant is the CQC bond

Chemistry and Stereochemistry of Polymerization and Copolymerization

87

in the monomer, an alkene molecule. The third participants in the polymerization reactions (called chain transfer agents) sever the M
C bond between the transition metal atom and the last monomer unit in the polymer chain. They are either the monomer molecules themselves or special chemical compounds that compete with the alkene molecules in reactions with the M
C bond. The latter compounds can be present in the reaction system as a part of the catalyst composition (e.g., a cocatalyst) or they are deliberately added to the reaction. The following formal terminology is well suited to describe polymer chains and their formation reactions. Each polymer molecule consists of three structurally different parts, two chain ends, the starting chain end and the last chain end, and the body of the macromolecule containing many (or just a few) monomer units: Last chain end - (Internal monomer unit)n
Starting chain end. The difference between the starting and the last chain end is both chronological and structural. Every polymer molecule is born in a chain initiation reaction, the insertion of the first monomer molecule into the M
C or the M
H bond. The chain grows in a number of chain growth steps, insertion reactions of CQC bonds into the M
C bond. The formation of the last chain end marks two simultaneous events, the polymer chain becomes detached from the active center, and the next starting chain end is formed. Therefore, the structures of starting and last chain-ending groups are inter-related, and the reactions leading to their formation are discussed in parallel. Unfortunately, this discussion often has a circular character, one has to refer to the last step of polymer chain formation (one of the chain transfer reactions) in order to explain the chemistry of the chain initiation reaction that follows it.

3.1.1. Definition of regioselectivity In chemical terms, all chain initiation and chain growth reactions of 1-alkenes of the general formula CH2QCHR are insertion reactions of their CQC bonds into two types of bonds in active centers, M
C or M
H. The insertion reaction can proceed with two different orientations of the CH2QCHR molecule with respect to the M
C or the M
H bond. Primary insertion (1,2-insertion): The insertion step after which the CH2 group of the alkene molecule becomes attached to the metal atom in the active center: 4M2C  þ CH2 QCHR ! 4M2CH2 2CHR2C 

(3.1)

Secondary insertion (2,1-insertion): The insertion step after which the CR group of the alkene molecule becomes attached to the metal atom in the active center: 4M2C  þ CHRQCH2 ! 4M2CHR2CH2 2C 

(3.2)

The insertion of a 1-alkene molecule into the M–H bond can proceed with the same two orientations as the insertion into the M–C bond. Numerous experiments showed that both CQC bond orientations, the primary and the secondary, are possible. Their relative frequency depends mostly on the polarity of the M–C and M–H bonds but also on the steric requirements at the center. In the majority of

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Alkene Polymerization Reactions with Transition Metal Catalysts

catalytic polymerization reactions, one of the insertion modes, either primary or secondary, is strongly favored compared to the opposite mode. Regioselectivity is defined as the frequency of a deviation from the preferred insertion orientation of 1-alkene molecules, the lower the frequency the higher is the regioselectivity. Some active centers insert 1-alkenes nearly exclusively in one orientation, either primary or secondary. They are completely regioselective and produce exclusively ‘‘head-totail’’ homopolymers and copolymers. In this case, the insertion reaction with the opposite orientation of a monomer molecule is regarded as a regio-mistake. The formation of a regio-mistake is usually followed either by its correction in the next chain growth step or by a chain transfer reaction. Other active centers do not exhibit a strict control over 1-alkene orientation and produce regio-irregular polymers with a significant number of ‘‘head-to-head’’ and ‘‘tail-to-tail’’-connected monomer units. The term ‘‘regioselectivity’’ does not apply, of course, to insertion reactions of ethylene and cycloalkenes with internal CQC bonds.

3.1.2. Stereospecificity in alkene polymerization reactions The most prominent feature of many transition metal catalysts is their ability to produce stereoregular polymers of 1-alkenes. This feature adds a new dimension to the characterization of transition metal catalysts, the analysis of catalyst features and polymerization parameters influencing the type and the level of polymer stereoregularity. Main principles of polymer stereoregularity (which have no intrinsic relationship to transition metal catalysis as such) were developed in classical works of Natta, Corradini, and their collaborators [1]. They are briefly described in Chapter 1. In general, polymer stereoregularity can be defined as the existence of a certain order in the spatial structure of a polymer chain. The order is determined by two circumstances. First, during the polymer synthesis the monomer units are linked with each other in a certain regular manner (Figure 1.1), which depends on the structure of active centers. This regularity becomes ‘‘frozen’’ during the polymer chain formation and it cannot be later altered or changed, except for some rear cases of post-polymerization epimerization reactions (usually radical or cationic reactions). This aspect of polymer stereoregularity is one of the principal subjects of this book. Second, regularly linked polymer chains spontaneously arrange themselves into particular helical forms shown in Figure 1.2. These helical structures easily form three-dimensional regular agglomerates, polymer crystals. Many physical properties of alkene polymers, including those affecting analytical techniques described in Chapter 2 (preparative and analytical Tref methods, Crystaf method, melting point measurements) are determined by the level of polymer crystallinity.

3.1.3. Statistics of predominantly stereoregular polymers Generally speaking, statistics of stereocontrol in chain growth reactions of 1-alkenes is a very involved subject. Several models of various complexities have been developed to describe this phenomenon. They cover many possibilities of chain stereocontrol mechanisms as well as interactions of two statistical processes, those

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Chemistry and Stereochemistry of Polymerization and Copolymerization

resulting in regioselectivity and stereospecificity [51,90,389–391]. The following four sections provide the simplest statistical equations that allow a chemist to visualize most common microstructures of predominantly isotactic and syndiotactic polyolefin chains. 3.1.3.1. Isospecific catalysis, site-control (enantiomorphic) mechanism According to the simplest statistical model proposed by Furukawa [392,393], the growth of a predominantly isotactic polymer chain produced with a heterogeneous Ziegler–Natta catalyst is governed by the stereochemical site-control (or enantiomorphic) mechanism [51,361,362,394–398]. Two types of active centers exist in these catalysts in equal numbers (their possible structures are discussed in Chapter 6). When a 1-alkene molecule CH2QCHR inserts into the metal
carbon bond in the primary orientation, the carbon atom in its CHR group is formally chiral: 4M2Polymer þ CH2 QCHR ! 4M2CH2 2C HR2Polymer

(3.3)

The four different substituents at the b-C atom are CH2–M, CH2–CHR
Polymer, R, and H. One type of the center produces prevailingly the R enantiomer and another the S enantiomer of the WM–CH2–CHR–CH2– group. As the chain growth reaction continues and a given monomer unit moves away from the metal atom in the active center, the chirality of its carbon atom rapidly disappears because two of the substituents at the CHR group become virtually identical, –CH2–CHR– Polymer. What matters in such polymer chains is the relative configuration of any two neighboring monomer units. If two adjacent monomer units have the same absolute configuration, either R or S, they form the meso link, m, and when the monomer units have opposite configurations, the link is racemic, r [399]. The meso-connected monomer units form an isotactic sequence. Occasionally, a stereochemical error occurs in the course of the chain growth, one monomer unit is inserted into the M–C bond with its alkyl group in the orientation opposite to the (prevailing) orientation of the previous monomer unit and a racemic link is formed. If the active center is highly isospecific, it corrects this steric error during the insertion of the next 1-alkene molecule. The following example shows in a graphical form a part of a mostly isotactic polymer chain with a single stereochemical error. m

m

m

m

m

m

r

r′

m

m

m

m

m

Because the predominant stereochemical error in this chain is one monomer unit in the inverted position, the chain contains two adjacent r links, although the probability of the second racemic link (ru, the stereo-corrective step) is much higher than the formation probability of the mistake, the r link. According to this simple stereocontrol mechanism, two inverted monomer units also can form the meso link, the mu link, m

m

m

m

m

m

r′

m′ r

m

m

m

m

m

90

Alkene Polymerization Reactions with Transition Metal Catalysts

although the probability of the mu link is much lower that the probability of the m link. However, steric sequences of this type were not found experimentally [400]. This makes this model of stereocontrol merely an approximation that is acceptable only for the description of highly isotactic polymers where the expected fraction of mu links is very low anyway. In the simplest case of the site-control mechanism, formation probabilities of different diads are: for the m diad, piso, for the mu diad, 1– piso, for the r diad, 1– piso, for the ru diad, piso. The transition probability matrix for this defective isotactic chain is

m mu r ru

m

mu

r

ru

piso 0 0 piso

0 1 – piso 1 – piso 0

1 – piso 0 0 1 – piso

0 piso piso 0

The matrix shows that from the probabilistic point of view this simple stereocontrol model corresponds to the Markoff chain of the first order with four primitive events, m, mu, r, and ru. Currently, the 13C NMR technique is the dominant method for the evaluation of polymer stereoregularity, and the stereochemical nomenclature of the NMR method has been universally accepted for the description of polymer stereoregularity in general. 13C NMR peak positions of carbon atoms in a given monomer unit are determined by their stereochemical relationship with the closest neighboring units. As an example, if a segment of a polymer chain contains a single stereo-error, , the polymer chain should contain the mrrm pentad in the center of the segment flanked by two mmrr and two mmmr pentads. A set of equations for the chain statistics of this stereocontrol mechanism [51,90] is listed in Table 3.1. All the equations in the table have only one variable. Two choices of the variable are used. 1. The ratio of two rate constants, one for the isospecific and another for the syndiospecific chain growth reaction, Riso ¼ kiso/ksyndio. The average Riso value for highly isospecific active centers in Ti-based catalysts is usually very high, B30–50 [51,89,221]. 2. The conditional probability of the isotactic placement of two neighboring monomer units, piso ¼ kiso/(kiso+ksyndio) ¼ Riso/(1+Riso). The equations in Table 3.1 are suitable for the statistical description of any highly isotactic polymer chain when steric errors are overwhelmingly single. However, they are not applicable for the statistical description of polymers of low isotacticity or atactic polymers. The latter polymers are usually complex mixtures of macromolecules produced by active centers governed by different stereocontrol mechanisms and, therefore, having different stereochemical structures. They should not be described by any single statistical scheme, however appealing a fit between a statistical model and their 13C NMR data could look.

in terms of Riso=kiso/ksyndio

in terms of piso=kiso/(kiso+ksyndio)

Diads Isotactic: [m] Syndiotactic: [r]

ð1 þ R2iso Þ=ð1 þ Riso Þ2 2Riso =ð1 þ Riso Þ2

p2iso þ ð1
piso Þ2 2piso  ð1
piso Þ

Triads Isotactic: [mm] Heterotactic: [mr]+[rm] Syndiotactic: [rr]

ð1 þ R3iso Þ=ð1 þ Riso Þ3 2Riso =ð1 þ Riso Þ2 Riso =ð1 þ Riso Þ2

p3iso þ ð1
piso Þ3 2piso  ð1
piso Þ piso  ð1
piso Þ

Tetrads [mmm] [mmr]+[rmm]=[mrr]+[rrm] [mrm]=[rmr] [rrr]

ð1 þ R4iso Þ=ð1 þ Riso Þ4 2ðRiso þ R3iso Þ=ð1 þ Riso Þ4 2R2iso =ð1 þ Riso Þ4 2R2iso =ð1 þ Riso Þ4

p4iso þ ð1
piso Þ4 2½p3iso  ð1
piso Þ þ piso  ð1
piso Þ3  2p2iso  ð1
piso Þ2 2p2iso  ð1
piso Þ2

Pentads [mmmm] [mmmr]+[rmmm]=[mmrr]+[rrmm] [mrrm] [rmmr] [mrrr]+[rrrm]=[rmrr]+[rrmr]=[rmrm]+[mrmr]=[mmrm]+[mrmm] [rrrr]

ð1 þ R5iso Þ=ð1 þ Riso Þ5 2ðRiso þ R4iso Þ=ð1 þ Riso Þ5 ðRiso þ R4iso Þ=ð1 þ Riso Þ5 ðR2iso þ R3iso Þ=ð1 þ Riso Þ5 2ðR2iso þ R3iso Þ=ð1 þ Riso Þ5 ðR2iso þ R3iso Þ=ð1 þ Riso Þ5

p5iso þ ð1
piso Þ5 2½p4iso  ð1
piso Þ þ piso  ð1
piso Þ4  p4iso  ð1
piso Þ þ piso  ð1
piso Þ4 p2iso  ð1
piso Þ2 2p2iso  ð1
piso Þ2 p2iso  ð1
piso Þ2

91

Stereo-sequences

Chemistry and Stereochemistry of Polymerization and Copolymerization

Table 3.1 Statistical equations for the simplest model of active-site (enantiomorphic) stereocontrol mechanism of isotactic polymer chains [51,90,401]

92

Table 3.1 (Continued ) in terms of Riso=kiso/ksyndio

in terms of piso=kiso/(kiso+ksyndio)

Heptads [m6] [m5r]+[rm5]=[m4rr]+[rrm4] [mmrrmm] [mmrrmr] [rmrrmr]

ð1 þ R7iso Þ=ð1 þ Riso Þ7 2ðR6iso þ Riso Þ=ð1 þ Riso Þ7 ðR6iso þ Riso Þ=ð1 þ Riso Þ7 2ðR5iso þ R2iso Þ=ð1 þ Riso Þ7 R3iso =ð1 þ Riso Þ6

p7iso þ ð1
piso Þ7 2½p6iso  ð1
piso Þ þ piso  ð1
piso Þ6  p6iso  ð1
piso Þ þ piso  ð1
piso Þ6 2½p5iso  ð1
piso Þ2 þ p2iso  ð1
piso Þ5  p3iso  ð1
piso Þ3

Nonads [m8] [m7r]+[rm7]=[m6rr]+[rrm6] [mmmrrmmm] [mmmrrmmr] [rmmrrmmr]

ð1 þ R9iso Þ=ð1 þ Riso Þ9 2ðR8iso þ Riso Þ=ð1 þ Riso Þ9 ðR8iso þ Riso Þ=ð1 þ Riso Þ8 2ðR7iso þ R2iso Þ=ð1 þ Riso Þ9 ðR6iso þ R3iso Þ=ð1 þ Riso Þ9

p9iso þ ð1
piso Þ9 2½p8iso  ð1
piso Þ þ piso  ð1
piso Þ8  p8iso  ð1
piso Þ þ piso  ð1
piso Þ8 2½p7iso  ð1
piso Þ2 þ p2iso  ð1
piso Þ7  p6iso  ð1
piso Þ3 þ p3iso  ð1
piso Þ6

Alkene Polymerization Reactions with Transition Metal Catalysts

Stereo-sequences

Chemistry and Stereochemistry of Polymerization and Copolymerization

93

Many physical and mechanical properties of alkene polymers, including their crystallinity level and melting points, as well as relative absorbances of ‘‘isotacticity’’ bands and ‘‘crystallinity’’ bands in their IR spectra [350,351,402–405], are determined by the presence of long isotactic sequences that can form polymer crystals. An isotactic block can be defined as a block of meso-linked units flanked by two racemically linked units. For example, the sequence contains 10 monomer units in an isotactic block. The following equation gives the fraction of monomer units in the sum of all long isotactic blocks starting with n units [51,350,403]: X 2 n
2 ðisoÞn ¼ ½Rnþ1 iso  ðR iso þ n þ 1Þ þ n  R iso  ð1 þ R iso Þ (3.4) þ Riso  ð1 þ nÞ þ 1=ð1 þ Riso Þnþ2

3.1.3.2. Isospecific catalysis, chain-end stereocontrol mechanism The second type of stereocontrol, the chain-end control, is determined exclusively by the steric structure of the last monomer unit in the growing end of a polymer chain. Two possibilities of such a stereocontrol exist, one resulting in the growth of a predominantly isotactic polymer and another leading to the formation of a predominantly syndiotactic polymer (Section 3.1.3.4). In the case of the isospecific chain-end stereocontrol, the steric structure of a predominantly isotactic polymer chain with a single steric error can be schematically presented as [406,407] m

m m m m m r m m m m m m

The predominant type of linking of any two neighboring monomer units in the chain is the meso-linking, and the main stereochemical error is the inversion of the steric attachment of two monomer units resulting in the formation of a single racemic link. Such a chain segment contains two mrmm pentads in the center of the sequence flanked by mmmr pentads. In the simplest case of this stereocontrol mechanism, called Bernoullian statistics, the structure of a predominantly isotactic chain is governed by only one parameter, the ratio of two propagation rate constants, one leading to the formation of the isotactic link, kiso, and another leading to the formation of the syndiotactic link (a steric mistake), ksyndio [51,90,408]. Under Bernoullian statistics, both the kiso value and the ksyndio value do not depend on the steric structure of the penultimate monomer link in the chain. For example, the kiso value is assumed to be the same in a 1-alkene insertion reaction into the M bond in these two alternative cases, M
Polymer and M
Polymer. As before, two single statistical variables can be used: 1. The ratio of rate constants of the two chain growth reactions, R0iso ¼ kiso =ksyndio : 2. The conditional probability of isotactic linking of two monomer units, p0iso ¼ kiso =ðkiso þ ksyndio Þ ¼ R0iso =ð1 þ R0iso Þ:

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Alkene Polymerization Reactions with Transition Metal Catalysts

Table 3.2 Statistical equations for chain-end stereocontrol mechanism (Bernoullian statistics) for isotactic polymer chains [51,401] Stereo-sequences

Diads Isotactic: [m] Syndiotactic: [r] Triads Isotactic: [mm] Syndiotactic: [rr] Heterotactic: [mr]+[rm] Tetrads Isotactic: [mmm] Syndiotactic: [rrr] [mmr]+[rmm] [mrm] [mrr]+[rrm] [rmr] Pentads Isotactic: [mmmm] Syndiotactic: [rrrr] [mmmr]+[rmmm]=[mmrm]+[mrmm] [mmrr]+[rrmm]=[mrmr]+[rmrm] [rmmr]=[mrrm] [rmrr]+[rrmr]=[mrrr]+[rrrm]

in terms of R0iso

in terms of P 0iso

R0iso =ð1 þ R0iso Þ 1=ð1 þ R0iso Þ

p0iso 1
p0iso 0

0

2 piso ð1
p0iso Þ2 2p0iso  ð1
p0iso Þ

0

3 piso ð1
p0iso Þ3 0 2 2piso  ð1
p0iso Þ 0 2 piso  ð1
p0iso Þ 0 2 2piso  ð1
p0iso Þ2 0 piso  ð1
p0iso Þ2

0

4 piso ð1
p0iso Þ4 0 3 2piso  ð1
p0iso Þ 0 3 2piso  ð1
p0iso Þ2 0 2 piso  ð1
p0iso Þ2 p0iso  ð1
p0iso Þ3

2 =ð1 þ R0iso Þ2 Riso 1=ð1 þ R0iso Þ2 2R0iso =ð1 þ R0iso Þ2 3 =ð1 þ Riso Þ3 Riso 1=ð1 þ R0iso Þ3 0 2 =ð1 þ R0iso Þ3 2Riso 0 2 Riso =ð1 þ R0iso Þ3 0 2 =ð1 þ R0iso Þ3 2Riso 0 Riso =ð1 þ R0iso Þ3 4 =ð1 þ R0iso Þ4 Riso 1=ð1 þ R0iso Þ4 0 3 =ð1 þ R0iso Þ4 2Riso 0 2 2Riso =ð1 þ R0iso Þ4 0 2 =ð1 þ R0iso Þ4 Riso 2R0iso =ð1 þ R0iso Þ4

0

0

When R0iso  1 and p0iso  1, the polymer is predominantly isotactic, when  1 and p0iso  0, the polymer is predominantly syndiotactic, and when R0iso is close to 1 and p0iso  0:5, the polymer is, by definition, atactic. Statistical equations for this stereocontrol model are listed in Table 3.2; they are suitable for an approximate statistical description of any polymer chain with a high to moderate degree of isotacticity produced by the chain-end mechanism. According to this stereocontrol model (when R0iso  1), a fraction of monomer units in the sum of all long isotactic blocks starting with n units is 0 n X Riso  ðR0iso þ n þ 1Þ ðisoÞn ¼ (3.5) ð1 þ R0iso Þnþ1

R0iso

3.1.3.3. Syndiospecific catalysis, site-control (enantiomorphic) mechanism The same stereocontrol mechanisms, the site-control and the chain-end control, are used as the basis for the statistical description of predominantly syndiotactic polymers

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Chemistry and Stereochemistry of Polymerization and Copolymerization

[51,354,408–414]. In the case of the site-control (enantiomorphic) mechanism, an occasional steric error (marked by the asterisk) is . This steric mistake produces two jointed meso links, the rmmr pentad flanked by two rrmm pentads and two rrrm pentads. The simplest approach to the statistical description of such a chain is to use a single probability parameter, psyndio, describing the predominant racemic attachment of a given monomer unit in the chain. Then the 1–psyndio value gives the probability of the dominant steric mistake. Statistical equations corresponding to this statistical model are listed in Table 3.3. The statistical function that reflects physical and mechanical properties of imperfectly syndiotactic polymers gives the fraction of monomer units in the sum of all long syndiotactic sequences starting with n monomer units: X ðsyndioÞn ¼ pnsyndion  ½psyndio þ ðn þ 1Þð1
psyndio Þ 2 þ n  p2syndio  ½ð1
psyndio Þn þ pn
2 syndio  ð1
psyndio Þ 

þ ðn þ 1Þpsyndio ð1
psyndio Þ

nþ1

ð3:6Þ

nþ2

þ ð1
psyndio Þ

This function correlates with such polymer properties as the crystallinity level of syndiotactic polymers, their melting points, etc. Table 3.3 Statistical equations for the simplest model of active-site (enantiomorphic) stereocontrol mechanism for predominantly syndiotactic polymer chain [401]

Diads Syndiotactic: [r] Isotactic: [m] Triads Syndiotactic: [rr] Heterotactic: [mr]+[rm] Isotactic: [mm] Tetrads [rrr] [mrr]+[rrm]=[mmr]+[rmm] [mrm]=[rmr] [mmm] Pentads [rrrr] [mrrr]+[rrrm]=[mmrr]+[rrmm] [rmmr] [rmrm]+[mrmr]=[mmrm]+[mrmm] =[mmmr]+[rmmm]=[mmrr]+[rrmm] [mmmm]=[mrrm]

p2syndio þ ð1
psyndio Þ2 2psyndio  ð1
psyndio Þ p3syndio þ ð1
psyndio Þ3 2psyndio  ð1
psyndio Þ psyndio  ð1
psyndio Þ p4syndio þ ð1
psyndio Þ4 2½p3syndio  ð1
psyndio Þ þ psyndio  ð1
psyndio Þ3  2p2syndio  ð1
psyndio Þ2 2p2syndio  ð1
psyndio Þ2 2p5syndio þ ð1
psyndio Þ5 2½p4syndio  ð1
psyndio Þ þ psyndio  ð1
psyndio Þ4  p4syndio  ð1
psyndio Þ þ psyndio  ð1
psyndio Þ4  2p2syndio  ð1
psyndio Þ2 p2syndio  ð1
psyndio Þ2

96

Alkene Polymerization Reactions with Transition Metal Catalysts

Numerous experimental NMR data show that syndiospecific metallocene catalysts, while formally operating under the site-control mechanism, can introduce the second type of steric mistakes in predominantly syndiotactic chains (Section 3.3.2.3). They arise when a growing polymer chain spontaneously changes its position in the coordination sphere of a transition metal atom without monomer participation. This shift (it is often called the epimerization reaction of a growing polymer chain or a skipped insertion step) produces a different type of steric mistake, . Such isolated steric mistakes correspond to rrmr pentads flanked by rrrm pentads. The mechanism of this reaction is discussed in Sections 6.1.3. The generation of two different types of steric mistakes makes the complete statistical description of these polymerization reactions quite complex [408,415]. The statistics includes two probability parameters, one, psyndio, is the same as in Table 3.3, and another is the probability of chain epimerization, pepi, [412,414]. The matrix for the propagation state for this two-mistakes scheme is [414].

R S R S

R

S

R

S

psyndio  pepi psyndio  pepi psyndio  (1 – pepi) psyndio  (1 – pepi)

(1 – psyndio)  pepi (1 – psyndio)  pepi (1 – psyndio)  (1 – pepi) (1 – psyndio)  (1 – pepi)

(1 – psyndio)  (1 – pepi) (1 – psyndio)  (1 – pepi) (1 – psyndio)  pepi (1 – psyndio)  pepi

psyndio  (1 – pepi) psyndio  (1 – pepi) psyndio  pepi psyndio  pepi

3.1.3.4. Syndiospecific catalysis, chain-end stereocontrol mechanism The most common isolated steric error in the case of the chain-end stereocontrol mechanism is [354,359,360,366,409]. If the stereocontrol is weak, a variety of short stereo-irregular sequences can be present in the polymers, two consecutive steric errors, and steric errors due to two closely positioned steric errors. In the simplest case (the Bernoullian statistics) a single statistical parameter, either the R0syndio ¼ ksyndio =kiso ratio or the conditional probability of syndiotactic linking, p0syndio , can be used for the statistical description of such polymer chains. The statistical equations that can be used for the analysis of NMR spectra are given in Table 3.4. As with the predominantly isotactic polymers, many physical and mechanical properties of syndiotactic polyolefins are determined by the presence of long crystallizable syndiotactic sequences in their chains. For these polymers, the stereoregular block can be defined as a syndiotactic sequence flanked by two isotactic links; e.g., the sequence contains 12 monomer units in a syndiotactic block. The following equation gives the fraction of monomer units in the sum of all long syndiotactic blocks starting with n units [51]: 0

0

n
1 2 X Rsyndio  ½Rsyndio þ R0syndio  ðn þ 1Þ þ n ðsyndioÞn ¼ ð1 þ R0syndio Þnþ1

(3.7)

97

Chemistry and Stereochemistry of Polymerization and Copolymerization

Table 3.4 Statistical equations for chain-end stereocontrol mechanism (Bernoullian statistics) for predominantly syndiotactic polymer chains [51,399,416] Stereo-sequences

Diads Syndiotactic: [r] Isotactic: [m] Triads Syndiotactic: [rr] Isotactic: [mm] Heterotactic: [mr]+[rm] Tetrads Syndiotactic: [rrr] Isotactic: [mmm] [mrr]+[rrm] [rmr] [mmr]+[rmm] [mrm] Pentads Syndiotactic: [rrrr] Isotactic: [mmmm] [rrrm]+[mrrr]=[rrmr]+[rmrr] [mmrr]+[rrmm]=[mrmr]+[rmrm] [mrrm]=[rmmr] [mrmm]+[mmrm]=[mmmr]+[rmmm]

in terms of Rusyndio

in terms of pusyndio

R0syndio =ð1 þ R0syndio Þ 1=ð1 þ R0syndio Þ

p0syndio 1
p0syndio 0

0

2 psyndio ð1
p0syndio Þ2 2p0syndio  ð1
p0syndio Þ

0

3 psyndio ð1
p0syndio Þ3 0 2 2psyndio  ð1
p0syndio Þ 0 2 psyndio  ð1
p0syndio Þ 0 2psyndio  ð1
p0syndio Þ2 p0syndio  ð1
p0syndio Þ2

0

4 psyndio ð1
p0syndio Þ4 0 3 2psyndio  ð1
p0syndio Þ 0 2 2psyndio  ð1
p0syndio Þ2 0 2 psyndio  ð1
p0syndio Þ2 0 psyndio  ð1
p0syndio Þ3

2 =ð1 þ R0syndio Þ2 Rsyndio 1=ð1 þ R0syndio Þ2 2R0syndio =ð1 þ R0syndio Þ2 3 =ð1 þ R0syndio Þ3 Rsyndio 1=ð1 þ R0syndio Þ3 1=ð1 þ R0syndio Þ3 0 2 =ð1 þ R0syndio Þ3 2Rsyndio 0 2 Rsyndio =ð1 þ R0syndio Þ3 2R0syndio =ð1 þ R0syndio Þ3 R0syndio =ð1 þ R0syndio Þ3 4 =ð1 þ R0syndio Þ4 Rsyndio 1=ð1 þ R0syndio Þ4 0 3 =ð1 þ R0syndio Þ4 2Rsyndio 0 2 2Rsyndio =ð1 þ R0syndio Þ4 0 2 =ð1 þ R0syndio Þ4 Rsyndio 0 2Rsyndio =ð1 þ R0syndio Þ4

0

0

3.1.3.5. Mixed statistical schemes in stereospecific polymerization reactions All heterogeneous Ziegler–Natta catalysts and many metallocene catalysts contain several types of active centers, as examples in Section 2.4.2 illustrate. Polymers produced with such catalysts can be viewed as thoroughly blended mixtures of several polymer fractions of different stereoregularity or several copolymer fractions of different compositions. Obviously, statistical descriptions of such polymer mixtures are more complex than those outlined in previous sections. In the simplest case, the polymers contain two fractions. Each fraction is produced by a single type of center and each governed either by the same stereocontrol statistics (site-control or chain-end control) or by two different stereocontrol mechanisms. Several statistical models were developed to describe such polymer mixtures: 1. The Coleman-Fox scheme [417,418] and the Bovey scheme [419], a mixture of two homopolymers, each following the chain-end (Bernoullian) statistics. 2. A mixture of two copolymers, each following the Bernoullian statistics in the description of the copolymer microstructure (see details in Sections 3.8.1–3.8.2) [420–422].

98

Alkene Polymerization Reactions with Transition Metal Catalysts

3. The Chuˆjoˆ scheme [423,424] and the Zambelli scheme [425], a mixture of two homopolymers, one following the site-control statistics and another the chainend statistics. 4. The Cheng scheme [420], a mixture of polymers produced by three different centers, two enantiomorphic (with different piso values) and one governed by the chain-end stereocontrol mechanism. 5. The models of a fluxional active center, the Busico model [355,358,366,413], the Randall model [205,426], and the Cheng model [401]. These statistical models regard each polymer chain as a composite macromolecule containing several (at least two) connected segments of different stereoregularity. Each macromolecule is produced with by same single-center catalyst but the centers change their stereocontrol mechanism in the course of the growth time of a single polymer chain. Each of the segments is governed either by the same stereocontrol mechanism (site-control or chain-end) or by two different stereocontrol mechanisms.

3.2. Heterogeneous Titanium- and Vanadium-Based Ziegler–Natta Catalysts From the chemical standpoint, alkene polymerization reactions with heterogeneous Ti- and V-based Ziegler–Natta catalysts represent the simplest case. Vanadium-based catalysts account for a small fraction of these catalysts and, for simplicity, the WTi symbol is used below to represent any active center in these catalysts.

3.2.1. Chemistry of chain initiation, propagation, and transfer reactions 3.2.1.1. Chain growth reactions 3.2.1.1.1. Standard chain growth reactions. Chain growth reactions are insertion reactions of 1-alkene molecules CH2QCHR into Ti
C bonds. 13C NMR studies of alkene polymers prepared with the isospecific d-TiCl3-AlMe3 system and with various supported catalysts of the TiCl4/MgCl2 type [90,427–433] showed that all 1-alkenes, as well as styrene [150,433], are predominantly inserted into the Ti
C bond in the primary orientation: 4Ti2CH2 2CHR2Polymer þ CH2 QCHR ! 4Ti2CH2 2CHR2CH2 2CHR2Polymer

(3.8)

If ethylene is added in a small amount to a 1-alkene polymerization reaction with a Ti-based catalyst, insertion of a single ethylene unit into a polymer chain, does not affect the regiochemistry of the next 1-alkene insertion, it remains primary [90,362,431,434]: 4Ti2CH2 2CHR2Polymer þ CH2 QCH2 ! 4Ti2CH2 2CH2 2CH2 2CHR2Polymer

(3.9)

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Chemistry and Stereochemistry of Polymerization and Copolymerization

4Ti2CH2 2CH2 2CH2 2CHR2Polymer þ CH2 QCH2R

(3.10) ! 4Ti2CH2 2CHR2CH2 2CH2 2CH2 2CHR2Polymer On rear occasions, alkene molecules insert into the growing chain in the secondary position: 4Ti2CH2 2CHR2Polymer þ R2CHQCH2 ! 4Ti2CHR2CH2 2CH2 2CHR2Polymer

(3.11)

Reaction (3.11) produces a slowly reacting (sleeping) active center. Further insertion reactions of 1-alkene molecules into the Ti–CHR bond are difficult, both in the primary and in the secondary orientation. For example, 13C NMR measurements showed that the fraction of monomer units inserted in the secondary orientation in propylene polymerization reactions with the d-TiCl3-AlEt3 system at 401C (Reaction (3.11) followed by Reaction (3.8)) is o0.1% [435]. 13C NMR data for several MCl3-AlEt2Cl systems are given in Table 3.5. The only unambiguous example of a head-to-head linkage was found in stereo-irregular fractions of polypropylene prepared with some Ti-based supported catalysts [190]. However, an ethylene molecule can insert in the Ti
CHR bond more easily [321,431,437]: 4Ti2CHR2CH2 2Polymer þ CH2 QCH2 ! 4Ti2CH2 2CH2 2CHR2CH2 2Polymer

(3.12)

After that, insertion reactions of 1-alkene molecules in the primary orientation can continue. Reaction (3.12) was thoroughly investigated when a small quantity of 13Clabeled ethylene was used in propylene polymerization reactions [358,437]. It gives a more dependable estimation of the probability of Reaction (3.11) vs. (3.8) (see Tables 3.6 and 3.7 [321,437]). These precise estimations are in a qualitative agreement with the earlier estimations shown in Table 3.5, the centers of higher isospecificity produce macromolecules with a nearly 10 times lower content of regio-errors. Table 3.5 Content of monomer units in secondary orientation in propylene polymers prepared with MClx-AlEt2Cl systems at 151C [398] Catalyst

Fraction

Amount (%)

(CH2)2 (mol.%)a

d-TiCl3

n-C7-soluble n-C7-insoluble n-C5-soluble n-C7-soluble n-C7-insoluble n-C5-soluble n-C7-soluble n-C8-insoluble n-C7-insoluble

1.5 88.5 31.0 9.0 47.5 33.5 5.5 57.5 98.5

1.1 B0 2.1 0.8 B0 1.6 0.8 B0 B0

b-TiCl3 VCl3 CrCl3 a

IR data [436].

100

Alkene Polymerization Reactions with Transition Metal Catalysts

Probability of secondary propylene insertiona (Reaction (3.11) vs. Reaction (3.8)) [358]

Table 3.6

a

Fraction

Content (%)

Secondary insertion (%)

Total polymer Insoluble in hot n-hexaneb Soluble in hot n-hexane

100 43 57

0.72 0.19 1.52

Polymerization with TiCl4/MgCl2-Ali-Bu3 system at 501C [mmmm]=B0.86.

b

Table 3.7 [321,437]

Probability of secondary propylene insertiona (Reaction (3.11) vs. Reaction (3.8))

Modi¢er I

Modi¢er II

Di-i-butyl phthalate (CH)(Me)Si(OEt)2 – 1,3-Dietherb a

Secondary insertion (%) Total polymer

Heptane-insoluble fraction

0.19 0.26

0.08 0.15

Polymerization with TiCl4/MgCl2/Modifier I-AlEt3/Modifier II systems in liquid propylene at 701C. 2,2-Di-i-butyl-1,3-dimethoxypropane (Scheme 4.2).

b

The data in Table 3.5 show that the secondary insertion of propylene molecules and the formation of tail-to-tail added monomer units occur more frequently if VCl3 is used as the solid catalyst component. However, when VCl4 is supported on MgCl2, propylene polymerization reactions become highly regiospecific [438]. The existence of Reaction (3.11) was also revealed when chain transfer reactions following this reaction were investigated, as described in Section 3.2.1.2.3. 3.2.1.1.2. Unconventional chain growth reactions. Polymethylene sequences in poly(1-alkene) chains: Propylene polymers prepared with some TiCl4/MgCl2/ 1,3-diether-AlEt3 systems at very high temperatures, 100–1201C, contain small quantities of –CH2–(CH2–CH2)n– units, B0.05% [439,440]. The source of these groups is in situ formed ethylene generated in decomposition reactions of AlEt3 assisted by the diethers [440]. The formed ethylene mostly concentrates in stereoirregular fractions of the polymers, whereas the isotactic fractions contain only isolated ethylene units. If Ali-Bu3 is used as the cocatalyst instead of AlEt3, the generated isobutene does not copolymerize with propylene [439]. Chain insertion reactions in polymerization of a,o-dienes: Polymerization of nonconjugated a,o-dienes is, in the chemical sense, equivalent to copolymerization of two 1-alkenes linked through their alkyl groups. The insertion of the first CH2QCH– bond proceeds predominantly in the primary mode:

4Ti2CH2 2Polymer þ CH2 QCH2ðCH2 Þn 2CHQCH2 ! 4Ti2CH2 2CH½2ðCH2 Þn 2CHQCH2 2CH2 2Polymer

(3.13)

101

Chemistry and Stereochemistry of Polymerization and Copolymerization

In the case of 1,5-hexadiene and 1,6-heptadiene, the second vinyl bond can also participate in the CQC bond insertion reaction at the same active center. This reaction results in the formation of polymers with cycloalkyl rings in the main chain [191,441,442]: >T i

Polymer

> Ti

Polymer

(3.14)

H2C

(CH2)3 CH

The polymerization of 1,5-hexadiene produces 1,3-enchained cyclopentyl rings and the polymerization of 1,6-heptadiene produces 1,3-enchained cyclohexyl rings. 1,2-Divinylcyclohexane and 1,2,4-trivinylcyclohexane can be viewed as substituted 1,5-hexadiene molecules. When 1,2,4-trivinylcyclohexane is copolymerized with propylene using the TiCl4/MgCl2-AlEt3 system, the two neighboring vinyl groups in each triene molecule participate in the polymerization reaction [443]. Because the starting monomer molecule already contains the cyclohexane ring, these copolymers contain 1,3-enchained hexahydroindane units in the main chain and pendant vinyl groups. The stereocontrol in polymerization reactions of a,o-dienes is a complicated subject. The tacticity of the polymers is determined by the enantio-selectivity of the insertion reaction (Reaction (3.13)) while the diastereoselectivity of the cyclization step (Reaction (3.14)) determines whether cis- or trans-rings are formed [444,445]. Detailed stopped-flow experiments with the TiCl4/MgCl2/ethyl benzoate-AlEt3 system accompanied by Tref and 13C NMR analysis showed that practically all monomer units from 1,5-hexadiene participating in Reaction (3.13) form cycles in Reaction (3.14) [191,365]. At least three different types of active centers were found in the catalyst. The centers differ in the molecular weight of the polymer fractions they produce, in the fraction of cis-1,3-cyclopentyl rings in the polymer chains (70–75% in two crystalline fractions, B50% in the amorphous fraction), and in the stereo-arrangement of neighboring cis–cis and trans–trans diads [191]. The fraction of the centers of the highest stereospecificity gradually decreases with time. The discovery of several types of active centers in these reactions is in agreement with the presence of several types of highly isospecific centers in propylene polymerization reactions with the same catalysts (Section 3.2.3.2).

3.2.1.2. Chain transfer and chain initiation reactions When heterogeneous Ti- and V-based catalysts are activated with an organometallic cocatalyst MR0x , the chain initiation reaction is defined as the insertion of the first alkene molecule CH2QCHR into one of the three Ti
C bonds, Ti
Ru, Ti–CH2, and Ti–CHR, or into the Ti–H bond. All these bonds are formed in particular chain transfer reactions (described below), which produce different starting end-groups in polymer chains. Each of the starting end-groups has distinct NMR characteristics and each was identified in polymers of 1-alkenes [187,427,428]. One prerequisite for the identification of starting end-groups is

102

Alkene Polymerization Reactions with Transition Metal Catalysts

their sufficiently high concentration in polymer chains or their enhanced ‘‘NMR visibility.’’ It is usually achieved by several means: 1. High concentrations of hydrogen in a polymerization reaction. 2. The use of organozinc compounds, ZnMe2 or ZnEt2 (both very effective chaintransfer agents) as cocatalyst components. 3. The use of 13C-labeled cocatalysts. 4. The use of selectively 13C-labeled 1-alkenes. 5. Carrying out polymerization reactions at very high temperatures to increase the frequency of chain transfer reactions (Section 3.2.1.2.1). 6. Carrying out polymerization reactions for a very short reaction time (in stoppedflow experiments, (see Section 5.3.3.2)) to increase the relative abundance of starting end-groups. 3.2.1.2.1. Chain transfer reactions after primary insertion of the last monomer unit and the following chain initiation reactions. Each chain transfer reaction has two names, the kinetic name (chain transfer to a monomer, to hydrogen, spontaneous chain transfer, etc.), and the chemical name (b-H atom transfer to a monomer, hydrogenolysis, b-H atom transfer to a transition metal atom, etc.). Chain transfer to a monomer: In chemical terms, the chain transfer reaction to a monomer is the transfer of the b-H atom in a growing chain to a coordinated monomer molecule:

4Ti2CH2 2CHR2Polymer þ CH2 QCHR ! 4Ti2CH2 2CH2 R þ CH2 QCR2Polymer

(3.15)

Reaction (3.15) results in the formation of a polymer molecule with the double bond as the last chain end. This double bond is the vinyl bond CH2QCH
in ethylene polymerization reactions (R ¼ H) and the vinylidene bond CH2QCR– in polymerization reactions of all 1-alkenes. These end-groups were identified in polypropylene prepared with the d-TiCl3-AlEt3 system [435], TiCl4/MgCl2/ diester-AlEt3 systems [432,433,446], the TiCl4/MgCl2/diether-AlEt3 system [213,225,319,447], and with TiII-based catalysts [448]. These bonds are also formed in polymerization reactions of higher 1-alkenes [449]. When ethylene is polymerized with Ti-based catalysts, Reaction (3.15) produces high molecular weight 1-alkenes CH2QCH–(CH2–CH2)n
1–H with even carbon atom numbers. Ethylene oligomers of this type were identified by gas chromatography (GC) [386]. When propylene is polymerized, Reaction (3.15) produces CH2QC(CH3)–[CH2–CH(CH3)]n
1–H molecules. These oligomers (n ¼ 3 and 4) were also identified by GC [213]. Reaction (3.15) is the principal chain transfer reaction in all polymerization reactions of 1-alkenes with Ziegler–Natta catalysts in the absence of hydrogen [213,225,319,435,449,450]. The WTi–CH2–CH2R moiety formed in Reaction (3.15) is the starting chain end of the next polymer molecule: 4Ti2CH2 2CH2 R þ CH2 QCHR ! 4Ti2CH2 2CHR2CH2 2CH2 R

(3.16)

Chemistry and Stereochemistry of Polymerization and Copolymerization

103

When a polymerization reaction is carried out in the absence hydrogen, the content of the starting end-groups WTi–CH2–CH2R and the content of the last end-groups CH2QCR–, both formed in Reaction (3.15), are equal [435]. Reaction (3.16) is kinetically indistinguishable from the chain growth reaction, Reaction (3.8). It is highly regioselective. Reaction (3.15) pre-supposes the coordination of a 1-alkene molecule at the active center in the primary orientation, the same orientation as in the standard chain growth reaction. If the 1-alkene molecule coordinates to the transition metal atom in the secondary orientation, the last chain end in the leaving polymer molecule is the same as in Reaction (3.15) but the active center has a different starting chain end: 4Ti2CH2 2CHR2Polymer þ CHRQCH2 (3.17) ! 4Ti2CHðCH3 ÞR þ CH2 QCR2Polymer Steric considerations suggest that the insertion of the next 1-alkene molecule into the Ti–C bond in the WTi–CH(CH3)R species is difficult, and Reaction (3.17) may convert some active centers into ‘‘temporarily sleeping’’ centers until the onset of other chain transfer reactions listed below, those to hydrogen, a cocatalyst or the spontaneous chain transfer. Chain transfer to hydrogen and subsequent chain initiation reactions: Hydrogen is the most important chain transfer agent in alkene polymerization reactions with Ziegler–Natta catalysts. In chemical terms, the chain transfer reaction to hydrogen is hydrogenolysis of the Ti–C bond in the active center: 4Ti2CH2 2CHR2CH2 2CHR2Polymer þ H2 (3.18) ! 4Ti2H þ CH3 2CHR2CH2 2CHR2Polymer When the polymerization reactions are carried out in the presence of hydrogen, Reaction (3.18) becomes the principal chain transfer reaction and the WTi–H species produced in it becomes the principal chain initiation species. The starting chain end of the next polymer molecule is generated after a 1-alkene molecule is inserted into the Ti–H bond. This step is less regioselective than the 1-alkene insertion into any Ti–C bond: 4Ti2H þ CH2 QCHR ! 4Ti2CH2 2CH2 2R (3.19) 4Ti2H þ RCHQCH2 ! 4Ti2CHR2CH3

(3.20)

Relative probabilities of Reactions (3.19) and (3.20) in propylene polymerization reactions are listed in Table 3.8. When the monomer insertion is primary (Reaction (3.19)), the next monomer insertion step is also overwhelmingly primary: 4Ti2CH2 2CH2 2R þ CH2 QCHR (3.21) ! 4Ti2CH2 2CHR2CH2 2CH2 2R The sequence of Reactions (3.19) and (3.21) in propylene polymerization reactions produces the n-propyl group as a starting chain end. This group was observed in polymerization reactions with many Ti-based catalysts in the presence of hydrogen [173,187,213,427,428,448,451].

104

Table 3.8

Alkene Polymerization Reactions with Transition Metal Catalysts

Estimation of propylene insertion regioselectivity into Ti–H bond [452,453]

Catalyst system

Temp. (1C)

pprimary/psecondary

d-TiCl3-AlEt3 ‘‘-’’ TiCl4/MgCl2/dibutyl phthalate-AlEt3 ‘‘-’’

85 105 85 100

o2.6–3.2 o3.7–4.0 o3.5–4.0 o2.2–3.0

In terms of the structure of starting end-groups, a combination of Reactions (3.19) and (3.21) is equivalent to Reaction (3.16). Molecular weights of all alkene polymers prepared with Ti-based catalysts are much lower in the presence of hydrogen and the concentration of starting end-groups formed in Reactions (3.19) and (3.21) is much higher. The combination of Reactions (3.19) and (3.21) in ethylene polymerization reactions results in the formation of saturated polymer molecules H–(CH2–CH2)n–H, high molecular weight n-alkanes with even carbon atom numbers. These products with the lowest n values are soluble in the reaction medium and are easily identified by GC [386]. In propylene polymerization reactions, Reaction (3.21) produces saturated polymer molecules with isobutyl groups as last chain ends, CH3–CH(CH3)–CH2–Polymer [187,427,428,448]. Similar chain ends are formed in ethylene/1-alkene copolymerization reactions when the last monomer unit in a growing polymer chain is derived from the 1-alkene molecule, CH3–CHR–CH2–Copolymer. The shortest copolymer molecules with this structure were indeed identified by GC in copolymerization products of ethylene and 1-hexene [386]. When a 1-alkene inserts into the Ti–H bond in the secondary orientation (Reaction (3.20)), the insertion of the next 1-alkene molecule into the Ti–CH(R)CH3 bond is slow. It produces the starting 2,3-dimethylbutyl end-group in propylene polymerization reactions [213,225,319]: 4Ti2CHðCH3 Þ2 þ CH2 QCH2CH3 ! 4Ti2CH2 2CHðCH3 Þ2CHðCH3 Þ2CH3

(3.22)

Reaction (3.20) is easily observed in ethylene/1-alkene copolymerization reactions because an ethylene molecule inserts into the Ti–CH(R)CH3 bond more easily compared to any 1-alkene molecule and the chain growth reaction continues virtually unimpeded: 4Ti2CHR2CH3 þ CH2 QCH2 ! 4Ti2CH2 2CH2 2CHR2CH3

(3.23)

The starting chain ends formed in Reaction (3.23) were indeed found in low molecular weight (oligomeric) fractions of ethylene/propylene, ethylene/1-butene, and ethylene/4-methyl-1pentene copolymers produced with TiCl4/MgCl2 catalysts [452,453]. Detailed analysis of the oligomer distributions yielded an approximate regioselectivity estimation of the WTi–H species in chain initiation reactions with propylene, the probability of Reaction (3.19) vs. Reaction (3.20). It is given in Table 3.8.

Chemistry and Stereochemistry of Polymerization and Copolymerization

105

Chain transfer to a cocatalyst and subsequent chain initiation reactions: In chemical terms, the chain transfer reaction to a cocatalyst MR0x is an exchange reaction of alkyl groups between the Ti–C bond and the M–Ru bond: 4Ti2CH2 2CHR2Polymer þ MR0x ! 4Ti2R0 þ R0x
1 M2CH2 2CHR2Polymer

(3.24)

Reaction (3.24) produces a ‘‘dead’’ polymer molecule with an MRux
1 group at the end. Chain ends formed due to Reaction (3.24) are confidently observed when organozinc compounds, ZnMe2 or ZnEt2 (both very reactive chain-transfer agents) are used as cocatalysts. Reaction (3.24) affects the molecular weight of polymers. GPC data show that the efficiency of AlR3 compounds as chain-transfer agents increases in the order Ali-Bu3oAlEt3oAlEt2H [455]. MR0x
1
C products formed in Reaction (3.24) are not stable. Depending on the conditions of polymer post-treatment, they are converted into different stable last groups. If the freshly produced polymer is exposed to moisture or if the polymerization reaction is terminated by addition of an alcohol, a saturated chain-end (isobutyl group in propylene polymerization) is formed: MR0x
1 2CH2 2CHR2Polymer þ ROH ! MR0x
1 2OR þ CH3 2CHR2Polymer

(3.25)

If the polymer mixture formed in a propylene polymerization reaction is treated with molecular oxygen for a significant period of time, the MR0x
1
CH2
CHR
species generated in Reaction (3.24) is converted to the hydroxymethylene group HO–CH2–CHR–. This group is indeed observable by 1H NMR in polypropylene produced with TiCl4/MgCl2/diester catalysts activated with AlEt3 [446] or with Ali-Bu3 [450]. If the polymer mixtures are thermally post-treated, the MR0x
1
CH2
CHR
species can be dehydrometallated to MR0x
1
H and to a polymer molecule ending with the CH2QCR– bond [446]. The chain initiation reaction that follows Reaction (3.24) is the 1-alkene insertion into the Ti
Ru bond: 4Ti2R0 þ CH2 QCHR ! 4Ti2CH2 2CHR2R0

(3.26)

The chemistry of this step was thoroughly studied in propylene polymerization reactions. 13C NMR studies of polypropylene prepared at 201C with d-TiCl3 activated with two mixtures of 13C-labeled cocatalysts, AlMe3+ZnMe2 and AlEt3+ZnEt2, showed that the propylene insertion mode in Reaction (3.26) is exclusively primary [173,395,428,429,456–458]. Similarly, when ZnPh2 is used as the cocatalyst in propylene polymerization reactions, the WTi–Ph species formed in Reaction (3.24) produces the WTi–CH2–CH(CH3)–Ph species [89]. Under typical reaction conditions, if the concentration of a cocatalyst AlR3 is relatively low and ZnR2 is not used, starting chain ends formed in Reaction (3.26), –CH2–CHR–Ru, are difficult to observe [435]. When AlMe3 is used as a cocatalyst, this starting chain end was observed only at very high AlMe3 concentrations and at a very short reaction time, B5 seconds [454]. However, these chain ends become

106

Alkene Polymerization Reactions with Transition Metal Catalysts

noticeable in polymers produced at temperatures above 1001C [432,433]. GC analysis of low molecular weight fractions of polyethylene prepared with a V-based supported catalyst at 901C in the presence of hydrogen unambiguously identified oligomer molecules H–(CH2–CH2)n–Ru initiated in Reaction (3.26) and terminated in a reaction with hydrogen. When Aln-Pr3 is used as a cocatalyst, Ru ¼ n-Pr, and when Ali-Bu3 is used, Ru ¼ i-Bu [386]. Spontaneous chain transfer and subsequent chain initiation reactions: Potentially, all Ti–CH2 bonds can participate in the b-H transfer reaction to the transition metal atom: 4Ti2CH2 2CHR2Polymer ! 4Ti2H þ CH2 QCR2Polymer (3.27) This reaction produces the same WTi–H species as that formed in Reaction (3.18) but in the absence of hydrogen. The last chain ends in the polymer molecules formed in Reaction (3.27) have the same double bonds as those produced in Reaction (3.15). Reaction (3.27) can be observed only in polymerization reactions at very high temperatures. Chain initiation reactions that follow Reaction (3.27) are the same as after the chain transfer to hydrogen (Reactions (3.19) and (3.20)). 3.2.1.2.2. ‘‘Initial’’ chain initiation reactions. The first Ti–C bond suitable for alkene insertion at the earliest stages of the polymerization reactions is generated in an exchange reaction between the Ti–Cl bond of the Ti species in the catalysts and the M–C bond in cocatalysts MRux. The Ti–Ru bonds are rapidly consumed in chain growth reactions and the existence of these ‘‘initial’’ chain initiation reactions can be proved only by analyzing polymer products formed after very short reaction times, a fraction of a second, before the first chain transfer reaction takes place [459]. For example, if Ali-Bu3 is used as the cocatalyst for Ti-based catalysts, alkylation of the Ti species in the catalyst produces the isobutyl group as the initial Ti
Ru group. The first 1-alkene insertion step into this group is equivalent to Reaction (3.26) and produces the –CH2–CHR–CH2–CH(CH3)2 chain end. Such starting end-groups were indeed observed in the products of ethylene and propylene polymerization reactions with supported Ti-based catalysts activated with Ali-Bu3 [459,460]. 3.2.1.2.3. Chain transfer reactions after secondary insertion of the last monomer unit and the following chain initiation reactions. Secondary insertion reactions of 1-alkene molecules into growing polymer chains are rare. They produce slowly reacting (sleeping) active centers WTi–CHR–CH2–Polymer. Such growing chains usually terminate in two reactions, chain transfer to hydrogen (if it is present in the reaction) or to a cocatalyst. Chain transfer to hydrogen:

4Ti2CHR2CH2 2CH2 2CHR2Polymer þ H2 ! 4Ti2H þ RCH2 2CH2 2CH2 2CHR2Polymer

(3.28)

In propylene polymerization reactions, this step results in the formation of saturated polymer molecules with n-butyl groups as last chain ends [187,213,225, 427,428,448,461]. When the hydrogen concentration in a polymerization reaction

107

Chemistry and Stereochemistry of Polymerization and Copolymerization

is high, Reaction (3.28) follows nearly all secondary propylene insertion steps whereas the frequency of the chain transfer step to hydrogen after the primary propylene insertion step in Reaction (3.18) continues to increase [213,225]. As a result, the ratio between two principal last chain ends in polypropylene molecules, the n-butyl group formed in Reaction (3.28) after the secondary propylene insertion and the isobutyl group formed in Reaction (3.18) after the primary propylene insertion decreases with an increase of the hydrogen concentration [213,225] as shown in Table 3.9. A comparison of the data in Table 3.9 and the data on the relative probability of secondary vs. primary insertion in crystalline fractions of polypropylene produced with the same two catalysts (Table 3.7) shows an important feature: the secondary insertion of propylene, while relatively insignificant in the framework of polymer synthesis as such, is present prominently as far as chain transfer reactions to hydrogen are concerned. The chain transfer reactions to hydrogen after the secondary insertion of 1-alkene molecules were unambiguously observed in oligomerization reactions of propylene, 1-pentene, and 1-hexene with Ti- and V-based supported Ziegler–Natta catalysts in the presence of hydrogen [213,386]. When all chain growth steps in oligomerization reactions of propylene over supported TiCl4/MgCl2 catalysts are primary, the reactions produce branched oligomer molecules CH3–CH(CH3)–[CH2– CH(CH3)]n–CH2–CH2–CH3. However, if the last propylene insertion step before the chain transfer to hydrogen is secondary, a different type of oligomer molecules is formed, CH3–CH2–CH2–[CH2–CH(CH3)]n–CH2–CH2–CH3 [213]. GC analysis easily distinguishes these two types of oligomers. Similarly, when an oligomerization reaction of 1-pentene and 1-hexene over a V-based catalyst proceeds exclusively via the primary monomer orientation, it produces oligomer molecules CH3–CHR– (CH2–CHR)n–CH2–CH2R where R ¼ n-C3H7 in the 1-pentene reaction and n-C4H9 in the 1-hexene reaction. The smallest of the oligomer molecules are dimers, CH3–CH(n-C3H7)–CH2–CH2–n-C3H7 (4-methylnonane) from 1-pentene and CH3–CH(n-C4H9)–CH2–CH2–n-C4H9 (4-methylundecane) from 1-hexene. However, if the last monomer insertion step is secondary, the dimers are linear alkanes, n-decane from 1-pentene and n-dodecane from 1-hexene [386]. These compounds were indeed detected in the oligomer mixtures. Their relative yields give an approximate estimation of the relative probability of primary vs. secondary Table 3.9 [213,221]

Population ratio of two last end-groups in crystalline fractionsa of polypropyleneb

Modi¢er I

Modi¢er II

PH (bar)

[mmmm]

[i-Bu]:[n-Bu]

Di-i-butyl Phthalate

(Cy)(Me)Si(OEt)3

2.5 8.0 1.0 4.0

0.975 0.980 0.974 0.975

1.17 3.16 0.54 1.22

1,3-dietherc a

Fractions insoluble in cold xylene. Produced with TiCl4/MgCl2-AlEt3 systems in liquid propylene at 701C in the presence of H2. 2,2-Di-i-butyl-1,3-dimethoxypropane (Scheme 4.2).

b c

–

108

Alkene Polymerization Reactions with Transition Metal Catalysts

insertion of a CH2QCHR molecule into the V–C bond of the WV–CH2–CH2R species, B5.5 for 1-pentene and B7.5 for 1-hexene. A particular ‘‘chain transfer’’ event takes place after a propylene molecule inserts into the Ti–H bond in the secondary orientation (Reaction (3.20)). The WTi– CH(CH3)2 species formed after this step has low reactivity in the primary propylene insertion reaction but it readily reacts with hydrogen: 4Ti2CHðCH3 Þ2 þ H2 ! 4Ti2H þ CH3 2CH2 2CH3

(3.29)

Reaction (3.29) explains the formation of propane in propylene polymerization reactions with heterogeneous Ziegler–Natta catalysts in the presence of hydrogen, the phenomenon known both in industry and in laboratory practice [462–464]. Chain transfer to cocatalyst MRux: 4Ti2CHR2CH2 2Polymer þ MR0x ! 4Ti2R0 þ MR0x
1 2CHR2CH2 2Polymer

(3.30)

This reaction is similar to Reaction (3.24); it results in the formation of a polymer molecule with a MR0x
1 group at the end. A subsequent exposure to moisture or termination of the polymerization reaction with an alcohol produces a saturated chain-end, the n-butyl group in propylene polymerization [432,433,448]. Kinetic considerations suggest that Reaction (3.30) is not an important chain transfer reaction in Ziegler–Natta catalysis. If this reaction were to play a significant role, higher quantities of MR0x in a polymerization reaction should produce higher reaction rates due to consumption of sleeping centers WTi–CHR–CH2– and WTi–CHR–CH3, the same effect as hydrogen exerts in these polymerization reactions due to Reaction (3.28) (see discussion in Section 5.7.2.2). This effect was not found in experiment. The only apparent exception is AlEt2H; it indeed accelerates propylene polymerization reactions with TiCl4/MgCl2-AlEt3 systems [455]. Two other minor chain transfer reactions are possible after Reaction (3.11). The first one is the chain transfer to a monomer. It is similar to Reaction (3.15) but produces a polymer molecule with an internal double bond, R–CHQCH–CH2– [432,433,449]. The second minor reaction is the spontaneous chain transfer reaction, b-H elimination; it produces a polymer molecule with the same internal double bond. Unconventional chain transfer reactions: Homopolymerization reactions of ethylene with supported V-based catalysts produce macromolecules with the same chain ends as in polyethylene produced with Ti-based Ziegler–Natta catalysts. For example, if a polymerization reaction with V-based catalysts is carried out in the presence of hydrogen, the main polymer products are n-alkanes with even carbon atom numbers, H–(CH2–CH2)n–H [386,465]. However, both the 13C NMR analysis of these homopolymers and the GC analysis of their lightest polymerization products (oligomers) revealed the presence of unexpected branched units in the polymer chains [386,465,466]. The type of these units depends on the type of an aliphatic hydrocarbon used as the reaction medium and it can be best determined by the analysis of the oligomers. Polymerization reactions in n-pentane produce n-alkanes with odd carbon atom numbers and 4-methyl-substitued alkanes [465],

Chemistry and Stereochemistry of Polymerization and Copolymerization

109

reactions in n-hexane produce 5-methyl-substitued alkanes [386], reactions in n-heptane produce n-alkanes with odd carbon atom numbers and 6-methylsubstitued alkanes [465], and reactions in cyclohexane produce n-alkyl-substituted cyclohexanes [465]. The most probable of these products is a peculiar chain transfer reaction, s-bond metathesis of a solvent molecule (italized below to follow its transformations) catalyzed by the V–C bond in the growing polymer chain. These reactions are well known for electron-deficient centers containing early-period transition metal atoms; they proceed via a four-atom transition state involving the M–C bond and a C–H bond [467]. Reactions involving the methyl group in a solvent molecule lead to the formation of n-alkanes: V2CH2 2CH2 2CH2 2Polymer þ H2CH2 2CH2 2ðCH2 Þx 2CH3 ðx ¼ 2; 3; 4Þ ! V2CH2 2CH2 2ðCH2 Þx 2CH3 þ H2CH2 2CH2 2CH2 2Polymer V2CH2 2CH2 2ðCH2 Þx 2CH3 þ n CH2 QCH2 ! V2ðCH2 2CH2 Þn 2CH2 2CH2 2ðCH2 Þx 2CH3 V2ðCH2 2CH2 Þn 2CH 2 2CH 2 2ðCH 2 Þx 2CH 3 þ H2 ! V2H þ CH3 2CH2 2ðCH2 2CH2 Þn
1 2CH 2 2CH 2 2ðCH 2 Þx 2CH 3

(3.31)

(3.32)

(3.33)

These reactions produce linear alkanes with odd numbers of carbon atoms when the solvents are n-pentane or n-heptane and linear alkanes with even numbers of carbon atoms (indistinguishable from standard polymerization products) when the solvent is n-hexane. Similar s-bond metathesis reactions involving the a-CH2 group in a solvent molecule form methyl-branched alkanes of the general formula CH3–CH2–(CH2–CH2)n
1–CH(CH3)–(CH2)x–CH3. The position of the methyl group in these products depends on the x value. s-Bond metathesis reactions involving the b-CH2 group in solvent molecules produce 4-ethyl-branched macromolecules [386]. Reactions similar to Reactions (3.21–3.23) can also involve polyethylene chains; they result in the formation of long-chain branches in ethylene homopolymers prepared with V-based catalysts of this type. The presence of a small number of such branches, B0.2 mol.%, is observable by 13C NMR [386,465,466] and it was verified by analyzing rheological properties of the polymers [465].

3.2.2. Reactivities of alkenes in polymerization reactions 3.2.2.1. Reactivities of alkenes in chain growth reactions The reactivity of 1-alkenes CH2QCH
R depends on electronic and steric parameters of their substituents R. Table 3.10 gives the range of alkene reactivity in polymerization reactions with the same catalyst and under the same conditions. The range of reactivities is much higher for supported catalysts, as shown in Table 3.11. Such comparisons of alkene reactivity contain one uncertainty. Both the

110

Table 3.10

Alkene Polymerization Reactions with Transition Metal Catalysts

Polymerization reactivities of 1-alkenesa [83,468,469]

1-Alkene

Rate (M/[Ti]  h)

Propylene 1-Butene 5-Methyl-1-heptene 4-Methyl-1-pentene 4-Methyl-1-hexene 4-Phenyl-1-butene Styrene 3-Methyl-1-butene 3-Methyl-1-pentene 3-Ethyl-1-pentene Vinylcyclohexane 3-Phenyl-1-butene 3,5,5-Trimethyl-1-hexene 4-Vinyl-1-cyclohexene Vinylnaphthalene 5-Vinyl-2-norbornene a

Relative reactivityb

1 (reference) 0.71 0.50 0.15 0.17

175 95 7.2 1.7

0.30–0.05 0.13–0.06 0.09–0.05 0.018 0.012

0.63 0.15 0.14 0.08

0.10 0.03

Catalyst d-TiCl3-AlEt3, [Al]:[Ti] ¼ 2.0, [monomer] ¼ 3 M, 801C [468]. Average reactivities, comparison of data for Ti-based systems under similar conditions [83,469].

b

Table 3.11 [470,471]

Reactivity of various alkenes in polymerization reactions with supported catalysts

Alkene

TiCl4/MgO-AlEt3 system, 701C [470] Ethylene Propylene 1-Butene 1-Hexene TiCl4/MgCl2-AlEt3 system, 201C [471] Propylene 4-Methyl-1-hexene 3,7-Dimethyl-1-octene

Rate (M [Ti]
1 s
1)

952 1.6 1.55 0.91

kp (M
1 s
1)

2,440 4.8 4.6 2.5

7,850 821 285

productivity data (often used to compare alkene reactivities) and the values of polymerization rates (Table 3.10) depend not only on the true reactivity of a given 1-alkene but on the number of active centers in the catalyst as well. Although the same catalyst system may be used in the experiments, detailed kinetic data discussed in Chapter 5 show that alkenes participate in the formation of active centers and, therefore, different alkenes can activate different numbers of the centers. This uncertainty can be avoided if the reactivity of different alkenes in copolymerization reactions is compared [83,472].

Chemistry and Stereochemistry of Polymerization and Copolymerization

111

Reactivity of linear 1-alkenes: Relative reactivities of 1-alkenes with linear alkyl substituents were estimated in a single polymerization reaction, copolymerization of ethylene and a specially produced (in an ethylene oligomerization reaction) mixture of linear 1-alkenes. The reciprocal of the r1 value in these reactions, 1/r1=kM(E)/kE(E), is the measure of reactivity of a 1-alkene M in its insertion into the Ti
CH2 bond of an active center with the last ethylene unit in a growing polymer chain. Figure 3.1A shows the dependence of the 1/r1 value for linear 1-alkenes in copolymerization reactions with the TiCl4/MgCl2-AlEt3 system at 901C vs. the carbon atom number of the alkenes [472]. The reactivity of any 1-alkene is always much lower than that of ethylene; it initially decreases with an increase of the size of the alkyl group but it reaches a constant value of 1/r1 B0.0016–0.0018 starting with nB6. The results for ethylene/1-alkene copolymerization reactions with the d-TiCl3-AlEt3 system are similar [472]. Figure 3.1B gives the plot of the modified steric factor E cs for linear 1-alkenes. This factor takes into account the hyperconjugation effect of H atoms in their a-CH2groups (calculated according to [473]). A comparison of Figures 3.1 and 3.2 shows that the reactivity of 1-alkenes in copolymerization reactions with ethylene closely parallels the steric factor that characterizes the size of alkyl groups in 1-alkenes. Reactivity of 1-alkenes with branched alkyl groups: To emphasize the steric effect of various alkyl substituents (both linear and branched) on the reactivity of 1-alkenes, Figure 3.2 uses the reciprocal value of another reactivity ratio, 1/r1 ¼ kM(Pr)/kPr(Pr). This parameter compares reactivities of various 1-alkenes M and the reactivity of propylene (Pr) in their copolymerization reactions with TiCl3-based solid catalysts. The data are plotted in the coordinates of the Taft equation [474] as a function of the same steric parameter of alkyl groups, E cs [83]: log ðreactivityÞ ¼ ds  Ecs

(3.34)

Equation (3.34) (also plotted in Figure 3.2) represents well most of the experimental data. The slope of the plot, ds ¼
0.65, indicates substantial sensitivity of 1-alkene reactivity to the bulkiness of their alkyl groups. Electronic characteristics of the last monomer unit in the growing polymer chain (the ethylene unit in Figure 3.1, the propylene unit in Figure 3.2) apparently do not influence the reactivity of the active centers, and the electronic properties of the alkyl groups in 1-alkene molecules are mostly masked by more pronounced differences in their steric properties [83]. The reactivities of 1-alkenes with a-branched alkyl groups, 3-metyl-1-butene and vinylcyclohexane, are lower than those expected from their E cs values, indicating a special sensitivity of the CQC insertion step to the substituent volume in a close vicinity to the double bond. 3,3-Dimethyl-1-butene is virtually inert in these polymerization reactions. Electronic effects: Electronic effects on the reactivity of 1-alkenes are difficult to assess because the range of electronic parameters for different alkyl groups is relatively small. For example, the range of the Hammett s parameter for alkyl groups is from
0.1 to
0.2. To avoid an interference with the much larger steric effect, the electronic effect was evaluated in polymerization reactions of styrenes

112

Alkene Polymerization Reactions with Transition Metal Catalysts

0.012 A

1-Alkene reactivity, 1/r1

0.01

0.008

0.006

0.004

0.002

0 5

10

15 25 20 Carbon atom number, 2n+2

30

35

-0.9 B

Steric factor Esc

-0.95

-1

-1.05

-1.1 5

10

15

20

25

30

35

Carbon atom number, 2n+2

Figure 3.1 A, Reactivity of linear 1-alkenes (represented by 1/r1 value) in ethylene/1-alkene copolymerization reactions with supported T|Cl4/MgCl2 -AlEt3 catalyst at 901C [472]; B, Modi¢ed steric factor Ecs for 1-alkenes CH2QCH
CnH2n+1 [473].

with para- and meta-positioned halogen atoms in the coordinates of the Hammett equation [475]: logðreactivityÞ ¼ r  s

(3.35)

Electronic parameters s of substituents in the benzene ring can be varied in a significant range. The estimated r value, the sensitivity of styrene polymerization

113

Chemistry and Stereochemistry of Polymerization and Copolymerization

101

log(k(Pr-M)/k(Pr-Pr))

E

100 5M1H

1Hx

4M1H

10-1

B

1Hp

4M1P

Pr

1D 1OD

3M1B 3M1P

VCH

-2

10

-2.0

-1.5

-1.0

-0.5

0.0

0.5

Steric factor Esc

Figure 3.2 Reactivity of 1-alkenes (represented by log(kM(Pr)/kPr(Pr)) value) vs. steric parameter of alkyl groups Ecs in coordinates of Taft equation (Equation (3.34), solid line) [83].

reactions to electronic properties of substituents in the benzene ring, is
0.95. The negative sign of the r value signifies that electron-donating substituents increase the reactivity of the styrene molecule. This electronic effect is opposite to that observed in anionic polymerization reactions of the same substituted styrene molecules, where r ¼ +5 [476]. This difference in the sign of the r value implies a difference in the nature of the insertion reaction of a 1-alkene molecule into the Ti–C bond. The limiting factor in this step in an anionic polymerization reaction is an electronic interaction between the negatively charged end-CH2 group in the growing polymer chain and the CQC bond whereas the limiting factor in catalytic insertion reactions is an interaction between the CQC bond and the electropositive metal atom in the active center [475]. 3.2.2.2. Reactivities of alkenes in chain initiation reactions Large differences in reactivities of various alkenes in the insertion reaction into the Ti
C bond are especially obvious when it is the Ti
CH3 bond. The below example compares reactivities of several alkenes in chain initiation reactions with the d-TiCl3-AlMe3-ZnMe2 system at 501C [469]. Alkene

Ethylene

Propylene

1-Butene

Relative reactivity

1

0.2

0.14

3-Methyl1-butene 0.028

3-Methyl1-pentene 0.018

3-Ethyl1-pentene 0.0036

114

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 3.12 Estimations of rate constant ratio, insertion of ethylene vs. propylene into Ti–C bonda [477]

a

Bond

kethylene/kpropylene

Ti–CH3 Ti–CH2–CH2–CH3 Ti–CH2–CH(CH3)2 Ti–CH2–CH(CH3)–CH2–CH(CH3)2

3.5–4.9 21–35 16–25 16–24

Copolymerization reaction with d-TiCl3-AlMe3/ZnMe2 system at 501C.

Careful analysis of chain-end populations in several ethylene/propylene copolymers of a different composition prepared with the 13C-labeled d-TiCl3AlMe3/ZnMe2 system at 501C afforded a very detailed information on alkene insertion reactions into different Ti
C bonds, as shown in Table 3.12. These results show that relative reactivities of ethylene and propylene in chain initiation reactions depend both on the structure of the alkyl group attached to the Ti atom, WTi–CH2–CH2 vs. WTi–CH2–CH(CH3), and on the size of the alkyl group. The only exception from this rule is the insertion reaction into the Ti–CH3 bond. These data show also that after the first monomer unit is inserted into the original Ti–C bond, insertion of next units (chain growth steps) have practically the same kinetic characteristics; the same conclusion as that from the analysis of oligomerization kinetics [116,117].

3.2.3. Stereospecificity of titanium-based polymerization catalysts The evaluation of stereospecificity of Ti-based catalysts in chain growth reactions is greatly complicated by the well-established fact that the catalysts contain different types of active centers in varying proportions. The research of some seemingly straightforward subjects, such the temperature effect or the effect of Modifiers II on the stereospecificity of a particular catalyst (see Section 5.7.2.2), has to address a complex subject that affects interpretation of the experimental data. Any change of polymer stereoregularity can be viewed either as the direct effect of a given parameter on the performance of individual centers or, alternatively, as the effect of the same parameter on the relative populations of centers of different stereospecificity. The most appropriate approach to this research is a combination of the best available analytical fractionation techniques (Tref, Crystaf) and the best analytical tool, the 13C NMR spectroscopy. 3.2.3.1. Two alternative models of predominantly isotactic polymer chains Many propylene polymers prepared with Ti-based Ziegler–Natta catalysts have a peculiar structural feature easily observable by 13C NMR: their crystalline fractions, in addition to isotactic sequences and occasional steric errors characteristic for imperfectly isotactic chains (mostly mmrr and mrrm pentads), contain a measurable fraction of long syndiotactic sequences, rrrr pentads, rrrrrr heptads, etc. One

115

Chemistry and Stereochemistry of Polymerization and Copolymerization

example, the 13C NMR spectrum of the crystalline fraction of polypropylene produced with the TiCl4/MgCl2/dibutyl phthalate-Ali-Bu3 system, is shown in Figure 3.3. The existence of long syndiotactic sequences in polypropylene samples prepared with Ti-based Ziegler–Natta catalysts has been known since 1960s [478]; it has been extensively studied by NMR [90,177,205,221,348,354,355,366,398, 400,479,480,481–484]. The content of the sequences depends to a significant degree on the type of organic modifiers and cocatalyst. Some catalyst systems produce nearly 30% of long syndiotactic blocks [485] while most polymers contain merely 1–2% of these sequences, still readily identifiable by 13C NMR. The main manifestation of syndiotactic sequences is the presence of the signals of rrrrrr, rrrrrm, and mrrrrm heptads in the 13C NMR spectra (Figure 3.3). The presence of rmrrrr and rrmrrr resonances [185,221] and a virtual absence of the rrmmrr resonance suggest that the formation of these syndiotactic sequences can be approximately described by the chain-end stereocontrol mechanism described in Section 3.1.3.4. Two alternative approaches were proposed to explain coexistence of long isotactic and syndiotactic sequences in crystalline polypropylene, a multicenter model and a fluxional model. Multi-center model: The earlier model, proposed by Zambelli [425], Chuˆjoˆ [423,424], and Doi [90], postulates a two-center polymerization mechanism. The first type of center produces predominantly isotactic macromolecules (with the fraction o), which are formed according to the site-stereocontrol mechanism (Section 3.1.3.1). The second type of center produces a mostly amorphous fraction containing partially syndiotactic or atactic macromolecules (their fraction equal to 1 – o) generated according to the chain-end stereocontrol mechanism (Section 3.1.3.4). This simple model is quite formal in its approach to the characterization of complex mixtures of different macromolecules prepared with

mmmr

mmrr

mmmm rrrrrr

rrrrrm

mmrrm

mmrrmr

mrrrrm

22

21.5

21 20.5 Chemical shift, ppm

20

0 19.5

Figure 3.3 CH3 range in 13C NMR spectrum of crystalline polypropylene fraction produced with T|Cl4/MgCl2/dibutyl phthalate-Ali-Bu3 catalyst at 701C.

116

Alkene Polymerization Reactions with Transition Metal Catalysts

multi-center Ziegler–Natta catalysts. However, it provides a good quantitative interpretation of high-resolution 13C NMR data. For example, polypropylene prepared with the TiCl4/MgCl2-AlEt3 system at 401C can be formally described as a polymer mixture containing 77% of highly isotactic polypropylene produced with enantiomorphic active centers (piso ¼ 0.966) and 23% of moderately syndiotactic polypropylene produced according to the chain-end stereocontrol mechanism ðp0syndio  0:73Þ [435]. Another example, crystalline polypropylene produced with the TiCl4/MgCl2/dibutyl phthalate catalyst activated with a mixture of AlEt3 and a silane [424]. The polymer can be viewed as a mixture of two fractions, 98.5% of it is produced by highly isospecific enantiomorphic centers with piso ¼ 0.994 and the rest by highly syndiospecific centers with ðp0syndio  0:952Þ. Table 3.13 gives several estimations of o, piso, and p0syndio values for fractions of polypropylene prepared with the d-TiCl3-AlEt2Cl system at 651C. The two-site model is useful for many practical studies of Ti-based Ziegler– Natta catalysis. The study of a silane effect in the TiCl4/MgCl2/di-i-butyl phthalate catalyst activated with mixtures of AlEt3 and 15 different silanes demonstrated an excellent correlation between the o value derived from 13C NMR data and the content of the polypropylene fraction insoluble in boiling n-heptane [486]. Cheng modified the two-center model by proposing the existence of three types of centers, two enantiomorphic centers with different piso values, B0.97–0.99 and 0.83–0.85, and one governed by the chain-end stereocontrol mechanism, with p0syndio of 0.76–0.83 [420]. Although this approach is less reliable from the statistical point of view, its results also match well one-point polypropylene fractionation data with boiling n-heptane [487]. An extension of this model to crystalline polypropylene fractions explains the structural diversity of steric errors in polypropylene chains as a manifestation of co-existence of two different groups of stereospecific active centers, both producing crystalline materials. The majority of the centers is highly isospecific and produces predominantly isotactic polymer molecules with a few steric defects predicted by the simple site-stereocontrol model (Section 3.1.3.1). Other centers, usually a small minority, produce a crystalline syndiotactic polymer. Indeed, several examples were reported when the syndiotactic fraction could be relatively cleanly separated from the bulk of crystalline isotactic material, either in multi-stage fractionation procedures [185,226,478,481,488,489] or chromatographically [478]. However, Table 3.13 [173]

a

Two-center stereocontrol model, solvent fractionation of isotactic polypropylenea

Fraction soluble in

x, content of polymer produced by enantiomorphic center

piso

p0syndio

n-Hexane n-Heptane n-Octane Residue

0.56 0.68 0.94 0.99

0.826 0.924 0.989 0.993

0.779 0.769 0.742 0.735

Polymer produced with d-TiCl3-AlEt2Cl catalyst at 651C.

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Chemistry and Stereochemistry of Polymerization and Copolymerization

such separation is usually difficult to achieve [163]. Moreover, because separation of crystalline and amorphous materials is never perfect, polymer fractions of intermediate crystallinity often contain atactic polymer components in addition to the isotactic and the syndiotactic material [163]. The amount of the syndiotactic polymer depends on the nature of the catalyst. It is present in quite high amounts in polypropylene produced with the TiCl3GaEt3 system [482] and in some fractions of polypropylene produced with the TiCl4/MgCl2-AlEt3 system modified with methyl-substituted piperidines [354–366,400]. On the other hand, the syndiotactic material is virtually absent in polymers produced with the early catalyst systems, such as a-TiCl3-AlEt3 [482], and in polymers prepared with supported TiCl4/MgCl2/diether catalysts [61]. Crystallization processes often induce a strong differentiation of isotactic and syndiotactic polypropylene fractions. This separation sometimes leads to a paradoxical phenomenon, a particular crystalline fraction with less than 5% of rrrr pentads may still contain a small fraction of crystalline syndiotactic polypropylene [163]. Chapter 4 describes one type of Modifiers II, ortho-dimethoxybenzenes, that transform some supported Ti-based catalysts, the content of the isotactic crystalline polypropylene fraction is reduced to less than 5%, and the polymers become mostly amorphous [484,490]. Although these modifiers exert a very strong effect on isospecific active centers, 13C NMR data show that they do not change the structure of the syndiotactic blocks; another argument supporting the hypothesis that the syndiotactic material is produced by separate active centers [221]. Fluxional model: Busico and Randall proposed another model explaining coexistence of isotactic and syndiotactic blocks in crystalline polypropylene fractions [205,355,358,366,413,426]. Their model assumes that only one type of a highly stereospecific active center exists in the catalysts but the center can undergo several changes in the stereocontrol mechanism during the lifetime of a single macromolecule. This hypothetical center produces, in succession, long isotactic blocks, syndiotactic blocks, and atactic blocks, all connected to each others. A junction of isotactic and syndiotactic blocks in such a polymer chain can be schematically presented as m m m m m m m m m m m m m r r r r r r r r r

r

These junctions should produce, in addition to steric mistakes typical for imperfectly isotactic sequences, mmrr and mrrm, and imperfectly syndiotactic sequences, rmmr, several specific NMR-discernible heptads, including mmmmrr, mmmrrr, and mmrrrr. Indeed, very small signals of these sequences were observed in the 150 MHz 13C NMR spectrum of the n-hexane soluble/n-pentane insoluble fraction of polypropylene prepared with a TiCl4/MgCl2 catalyst activated with a mixture of AlEt3 and 2,2,6,6-tetramethylpiperidine [366]. Statistical analysis of the spectrum showed that the probability of switching from the predominantly isotactic sequence to the predominantly syndiotactic sequence in this polymer fraction is quite low, B0.03. As a result, isotactic and syndiotactic blocks coexist in the same polymer molecules and they form separate crystals [366]. This model of the steric structure proposes that macromolecules present in crystalline (boiling n-heptane- and

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cold xylene-insoluble) fractions of polypropylene prepared with Ti-based catalysts are not perfectly isotactic in two senses [364,366]. First, they contain small fractions of NMR-observable steric mistakes predicted by the simple enantiomorphic statistical model described in Section 3.1.3.1, the mrrm pentad flanked by two mmrr pentads and two mmmr pentads. This material dominates polypropylene resins produced with modern supported Ziegler–Natta catalysts and it controls the crystallization behavior of the polymers [129]. In addition, these fractions contain small amounts of relatively short syndio-blocks in the same macromolecules, 10–20 units in length. The formation probability of syndiotactic blocks during the lifetime of a single polymer molecule is determined by the reversible formation of complexes between the active centers in the catalysts and organic ingredients in them (Lewis bases). The switching probability in the case of highly crystalline fractions is low, B0.001–0.002 [364], and their predominantly isotactic sequences are long, 200–1,000 units on average [205]. 3.2.3.2. Stereospecificity in chain growth reactions It is impossible to make any general statement about the stereospecificity of active centers in Ti-based catalysts. The most reasonable approach is to discuss separately the behavior of four types of active centers, centers of the highest isospecificity, centers of moderate isospecificity, syndiospecific centers, and aspecific centers. Centers of the highest isospecificity: In propylene polymerization reactions, the material produced by these centers constitutes the largest part of crystalline fractions, usually from 80 to 95% (the remainder of the crystalline fractions is material produced by centers of a moderate isospecificity (see discussion below)). Detail investigations by Tref, Crystaf, 13C NMR, and GPC methods provided important insights into the performance of these centers [61,183,191,197,221,322]. Figure 3.4 shows analytical Tref curves of crystalline polypropylene fractions prepared with two supported catalysts, a catalyst system of the 3rd generation, TiCl4/MgCl2/ethyl benzoate-AlEt3/p-ethoxy-ethyl benzoate, and a catalyst system of the 4th generation, TiCl4/MgCl2/di-i-butyl phthalate-AlEt3/(Cy)(Me)Si(OMe)2. Each Tref curve is resolved into individual components (see Section 2.3.2.3) and Table 3.14 gives the results of the resolution. These data show distinct differences between relative amounts of individual components. Components with [mmmm] W0.960 (components B–D in Table 3.14) are dominant in the crystalline fractions; they are produced by the active centers of the highest isospecificity. These centers operate according to the enantiomorphic stereocontrol mechanism described in Section 3.1.3.1, they form macromolecules with the simplest steric structure, nearly perfectly isotactic chains with rare isolated steric mistakes BmmmmmrrmmmmmB [183,427,448,493]. The probability of isotactic monomer linking for these centers is high, piso values range from B0.995 to B0.999. Every Ti-based catalyst has from three to four types of the centers of the highest isospecificity with slightly different piso values. When the same solid catalyst is used, the number of highly isospecific centers and their relative productivity depend on the type of Modifier II in the cocatalyst mixture. One example of such dependence is presented in Table 3.15.

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Chemistry and Stereochemistry of Polymerization and Copolymerization

A 120

100

80

60

40

20

0 95

100

105

110

115

120

125

115

120

125

Temperature, C B 120

100

80

60

40

20

0 95

100

105

110 Temperature, C

Figure 3.4 Analytical Tref curves of crystalline polypropylene fractions prepared with two supported catalysts [322]. A, Catalyst system of the 3rd generation, catalyst T|Cl4/MgCl2/ethyl benzoate, cocatalyst AlEt3/p-ethoxy ethyl benzoate, [AlEt3]:[ester]Q1.8; B, Catalyst system of the 4th generation, catalyst T|Cl4/MgCl2/di-i-butyl phthalate, cocatalyst AlEt3/(Me)(Cy)Si(OMe)2, [AlEt3]:[silane] ¼ 20.

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Alkene Polymerization Reactions with Transition Metal Catalysts

Table 3.14 Tref resolution data for crystalline fractions of propylene polymers prepared Ti-based supported catalysts of three generations [322] System of Catalysta

a

3rd generation (TiCl4/ MgCl2/EB)

4th generation TiCl4/ MgCl2/DIBP

5th generation TiCl4/ MgCl2/1,3-diether

Cocatalyst/ AlEt3/p-EtO-EB Modi¢er II

AlEt3/ (Cy)(Me)Si(OMe)2

AlEt3

Tref [mmmm] component

Fraction (%)

[mmmm]

Fraction (%)

[mmmm]

Fraction (%)

B C D E F G

7.6 71.2 28.3 6.2 7.6 7.3

0.977 0.972 0.966 0.954 0.939

32.3 48.0 8.6 6.9 4.2

0.979 0.975 0.963 0.950 0.935

32.7 47.5 5.9 11.3 2.6

0.986 0.978 0.969 0.961 0.954 0.941

Modifiers I: EB=ethyl benzoate, DIBP=di-i-butyl phthalate, 1,3-diether=2-i-propyl-2-i-pentyl-1,3-dimethoxypropane.

Table 3.15 Stereospecificitya of isospecific active centers in Ti-based supported catalyst systems of 4th generationb [322] Modi¢er II

(3,3,3-F3 -Pr)(Me)Si(OMe)2

Component [mmmm]

A B C D E F G a

0.969 0.965 0.948 0.935

Fraction (%)

28.3 51.7 12.0 8.0

(Cy)(Me)Si(OMe)2

Cpy2Si(OMe)2

[mmmm] Fraction (%) [mmmm] Fraction (%)

0.977 0.972 0.966 0.954 0.939

32.3 48.0 8.6 6.9 4.2

0.993 0.986 0.976

59.8 26.5 3.1

0.963 0.951 0.940

3.4 6.3 0.9

Evaluation from Tref data. Catalyst TiCl4/MgCl2/di-i-butyl phthalate, [AlEt3]:[Modifier II]=20 [494].

b

A comparison of the data in Tables 3.15 and 3.17 invites a suggestion that Tref components of different polymers that elute at the same temperature and have the same stereospecificity are produced by active centers of the same structure and that the catalysts differ mostly in the proportions between the centers [322]. For example, the main Tref component in Figure 3.4A (the peak at 112.91C, component C in Table 3.14) is practically the same material as the Tref component with the peak maximum at 112.61C in Figure 3.4B (the first of the two largest peaks). The data in Table 3.15 suggest that an assortment of highly isospecific

Chemistry and Stereochemistry of Polymerization and Copolymerization

121

centers in a given catalyst depends primarily on the nature of Modifier II. The use of Cpy2Si(OMe)2 leads to the formation of active centers of the highest isospecificity (they produce Tref components A and B). (Cy)(Me)Si(OMe)2 produces centers of slightly lower isospecificity, whereas the ‘‘best’’ centers formed with (3,3,3-F3-Pr)(Me)Si(OMe)2 are noticeably inferior, although they still produce a highly crystalline material with [mmmm]av W0.96. The differences in the properties of highly isospecific centers were also found in early Ziegler–Natta catalysts. For example, the centers of the highest stereospecificity in the d-TiCl3-AlEt2Cl system produce the material with [mmmm]av ¼ 0.975 whereas the centers in the b-TiCl3-AlEt2Cl system produce under the same conditions a material with [mmmm] B0.945 [183]. A high degree of isospecific stereocontrol is not limited to Ti-based polymerization catalysts. As an example, a catalyst prepared by supporting CrIII(acac)3 on MgCl2 and modified with ethyl benzoate is highly isospecific [456], the yield of the crystalline fraction can reach 98–100%, this fraction has a high degree of steric purity [mmmm] ¼ 0.96–0.97, and the pattern of steric errors in it is in agreement with the enantiomorphic stereocontrol mechanism. Centers of moderate isospecificity: In addition to highly isospecific centers, all Ti-based Ziegler–Natta catalysts contain several types of active centers of moderate isospecificity [197,221,495]. They produce Tref and Crystaf components with a broad range of [mmmm] values, from 0.935–0.955 (components F and G in Tables 3.15 and 3.17) to B0.7–0.8. The piso values for the respective centers vary from B0.99 to o0.93. The content of these polymer fractions strongly depends on the type of catalyst and the nature of Modifier II. For example, the relative amount of this material in polypropylene produced with catalysts of the 4th generation is quite low, 5–10%, when (Cy)(Me)Si(OMe)2 or Cpy2Si(OMe)2 are used as Modifiers II but it can exceed 20% when (3,3,3-F3-Pr)(Me)Si(OMe)2 is used (Table 3.15). The polymer material produced by active centers of moderate isospecificity, depending on its molecular weight and conditions of solvent fractionation, either remains insoluble and is counted as a minor part of the crystalline fraction (see Figure 3.4) or it is separated from the highly isotactic polymer into a separate fraction called the stereoblock fraction [24,51,197]. Because steric mistakes in these materials are more abundant and, therefore, are much easier to observe by 13C NMR, a large part of research on the stereocontrol mechanism in Ziegler–Natta catalysis is based on detailed spectroscopic analysis of the stereoblock fractions [205,355,358,366,413]. Syndiospecific active centers: As discussed in Section 3.2.3.1, the multi-center catalyst model proposes that Ti-based heterogeneous catalysts have a small number of separate syndiospecific centers. Most of the polymer material produced by these centers is usually soluble under the same conditions as the polymers of decreased isotacticity and ends up in the stereoblock fraction. However, a nearly clean separation of syndiotactic polypropylene is possible with the preparative Tref technique [185,481] or chromatographically [478]. The content of syndiotactic polypropylene in polypropylene prepared with supported TiCl4/MgCl2 catalysts can be very small, 0.1–0.2%, but this material is still easily observed in crystalline

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Alkene Polymerization Reactions with Transition Metal Catalysts

and stereoblock fractions by 13C NMR and sometimes even by the X-ray method. Judging by 13C NMR data, the syndiospecific centers are highly regioselective (they insert 1-alkene molecules in the primary orientation [481]) and they operate according to the chain-end stereocontrol mechanism (Section 3.1.3.4) [185,481]. The principal steric mistake in their chains is rrmrrr, and the probability of syndiotactic linking varies depending on catalyst and polymerization temperature [185,221,481], as discussed in Section 5.7.2.2. Aspecific active centers: The material produced by these centers is usually called ‘‘the atactic fraction.’’ It is nearly completely amorphous and cannot be analyzed by Tref or Crystaf methods. GPC analysis of several atactic fractions showed that they have broad molecular weight distributions (Mw/Mn ratios from 6 to B14) and consist of at least four or five Flory components with molecular weights ranging from 1  104 to over 1  106 [221,497]. Relative contents of different Flory components in these fractions strongly depend on the [AlEt3]:[Modifier II] ratio in cocatalyst mixtures [497]. One can expect that stereo-regulating properties of each of these centers, although all very low, can differ one from another. 13C NMR studies of these fractions revealed their very complex structure, which was the subject of numerous modeling studies with complex statistical schemes [51]. It appears however that any attempt to treat these polymers as structurally uniform and investigate their average properties, such as average Mw values or average 13C NMR characteristics [498], as a function of catalyst composition or parameters of polymerization reactions are not productive. Some supported TiCl4/MgCl2 and Ti(On-Bu)4/MgCl2 catalysts, when they are activated with AlEt2Cl instead of AlEt3, behave similarly to homogeneous Ti-based catalysts described in Section 3.4.2. Their active centers are mostly aspecific and produce propylene polymers with the [mm] content of 0.35–0.45 [496]. These active centers are not highly regioselective either; from 4 to 8% of monomer units in the polymers they produce are regio-inverted. Atactic polystyrene produced with heterogeneous Ti-based catalysts represents a special case. This material consists of two products of a different origin. One is the atactic polymer produced by aspecific Ti-based active centers, the same centers that are responsible for the formation of atactic polypropylene. The second type of atactic material, which is usually formed early in the polymerization reactions, is the product of a side-reaction, cationic styrene polymerization with acidic species in the catalysts [491,492]. 3.2.3.3. Stereochemistry of chain initiation reactions Zambelli carried out detailed stereochemical analysis of chain initiation reactions in a special case when a 1-alkene molecule inserts into the Ti–CH3 bond. The basis of this analysis is a particular feature of 13C NMR spectra of branched alkanes that model the chain-end structure of polypropylene and poly(1-butene) [429,430,499]. The simplest of the alkanes is 2,4,6-trimetylheptane. This compound has no chiral carbon atoms and is not diastereomeric. However, the CH3 signal of its isopropyl groups is split into two equal components due to the effect called ‘‘distereotopicity’’ [500]: the end-CH3 groups are not completely equivalent sterically with respect to

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Chemistry and Stereochemistry of Polymerization and Copolymerization

their relationship to the central CH3 group. Two projections of this molecule, similarly to the representation of a polypropylene chain, underline this difference: 2,4,6-trimethylheptane as:

a segment of isotactic chain

a segment of syndiotactic chain

Positions of the end-CH3 groups are sensitive to the steric arrangement of both the closest structural unit and the next one. These model data provided the basis for the stereochemical analysis of end-groups in polypropylene and poly(1-butene) [429,430,499]. One important feature of chain initiation reactions is that the catalyst stereospecificity in the insertion step involving the Ti–R bond with R ¼ Me and Et is noticeably lower than in chain growth reactions and, therefore, are easier to use as a tool [356,499,501]. The differences in stereoselection probabilities of two reaction steps, initiation and propagation, are shown in Table 3.16. Several conclusions follow from the Table 3.16 Isospecificity of chain growth vs. chain initiation steps in propylene polymerization reactions [356,458,501–504] Catalyst system

d-TiCl3-AlMe3/ZnMe2, 201Ce d-TiCl3-AlMe2Cl, 201Ce d-TiCl3-AlMe2I, 751Ce d-TiCl3-AlEt3/ZnEt2, 201C a-TiCl3-AlEt3, 301Ce TiI3-AlEt3, 301Ce a-TiCl3-AlEt2I, 301Ce d-TiCl3-AlEt2I, 901Cg d-TiCl3-AlEt3, 851Ce d-TiCl3-ZnEt2, 201C TiCl4/MgCl2/EBd-AlEt3/ZnEt2, 201Cf TiCl4/MgCl2-AlEt3, 601Cg TiCl4/MgCl2-AlEt3, 201Ce TiCl4/MgCl2/1,3-dietherd-AlEt3/ZnEt2, 601Cf TiCl4/MgCl2/DIBPd-AlEt3/MeSi(OEt)3, 601Cg TiCl4/MgCl2/DIBPd-AlEt3/Ph2Si(OEt)2, 601Cg a

Chain growth

Chain initiation

[mm]

pisoa

[erythro]b

p0iso c

0.79 0.76 0.82 0.91 0.98 0.97 0.98 0.90 0.96 0.93 0.89 0.95 0.94 0.97 0.98 0.97

0.92 0.91 0.94 0.97 0.99 0.99 0.99 0.96 0.99 0.98 0.96 0.98 0.98 0.99 0.99 0.99

0.52 0.50 0.75 0.79 0.77 0.89 0.84 0.84 0.86 0.69 0.80 0.67 0.69 0.83 0.82 0.87

0.52 0.50 0.78 0.88 0.87 0.97 0.91 0.91 0.92 0.80 0.89 0.79 0.81 0.91 0.90 0.93

Probability of isotactic propagation, calculated from [mm] value (Table 3.1). Measured from 13C NMR spectra of polymers prepared with 13C-labeled cocatalyst mixtures. Probability of isotactic initiation calculated from [erythro] values [356,502]. d Modifiers II: EB=ethyl benzoate, 1,3-diether=2,2-di-i-butyl-1,3-dimethoxypropane, DIBP=di-i-butyl phthalate. e Fraction soluble in boiling n-heptane. f Fraction soluble in boiling n-octane. g Fraction insoluble in boiling n-heptane. b c

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Alkene Polymerization Reactions with Transition Metal Catalysts

Table 3.17 1-Alkene

Propylene 1-Butene 1-Pentene a

Isospecificity of chain growth vs. chain initiation steps for three 1-alkenesa [396] Chain growth

Chain initiation

[mm]

piso

[erythro]b

p0iso c

X0.86 X0.95 X0.87

X0.95 X0.98 X0.96

0.77 0.69 0.66

0.87 0.80 0.78

Polymerization with d-TiCl3-AlEt3 system, data for crystalline fractions. From 13C NMR data for polymers prepared with 13C-labeled cocatalyst. Probability of isotactic initiation calculated from [erythro] values [356,502].

b c

results in Table 3.16 and from numerous similar data [356,394,396,428,501, 505,506]. 1. When the R group in the active center is methyl (the WTi
CH3 species), the chain initiation step is not stereospecific unless the active center is asymmetric due to the exchange of its Cl atoms for the I atom from AlMe2I. 2. The minimum size of the alkyl group in the WTi
R center required for the chain initiation step to become isospecific is the ethyl group, but the stereocontrol in this step is significantly lower than in chain growth reactions. 3. NMR data for polypropylene fractions of reduced stereoregularity show that the overall isospecificity of active centers responsible for the formation of a given fraction and the stereoselection ability of the same centers in propylene insertion reactions into the Ti
C2H5 bond vary in parallel. 4. Starting with the Ti
n-Pr bond, the stereospecificity of the chain initiation step is approximately as high as that in the chain growth reaction. 5. The stereospecificity of the initiation step is slightly lower for higher 1-alkenes [394,396], as shown in Table 3.17. When a 1-alkene molecule contains a chiral carbon atom (3-methyl-1-pentene), a more subtle steric effect comes into play in the chain initiation reaction because of a steric interaction between active centers containing the Ti
CH3 bond and the alkyl substituent in the monomer molecule [507–509]. The active centers containing Ti atoms are inherently asymmetric and prefer one of the two stereoisomers in the racemic mixture of R- and S-3-methyl-1-pentene. In the preferred arrangement, two chiral carbon atoms, one in the alkyl group of the monomer unit, –CH2–CH(CH3)–C2H5, and another formed after the initiation step, Ti–CH2–CH(i-C4H9)–CH3, have the same absolute configuration.

3.3. Metallocene Catalysts Although the majority of metallocene catalyst systems are single-center catalysts, polymerization reactions of 1-alkenes with these catalysts are more complex both in the chemical and in the stereochemical sense in comparison to

Chemistry and Stereochemistry of Polymerization and Copolymerization

125

polymerization reactions of the same alkenes with heterogeneous Ti- and V-based catalysts. Both the regioselectivity and the stereocontrol of active centers in metallocene catalysts are lower resulting in numerous chemical and steric irregularities in the polymer chains. For simplicity, active centers in metallocene catalysts are designated below as Cp2M, e.g., Cp2Zr, Cp2Ti, or CpM in monometallocene complexes, to distinguish them from active centers in Ziegler–Natta catalysts.

3.3.1. Chemistry of chain initiation, propagation, and transfer reactions 3.3.1.1. Chain growth reactions 3.3.1.1.1. Standard chain growth reactions. Numerous 13C NMR studies of propylene polymers prepared with different metallocene catalysts showed that two chain growth reactions, insertion reactions of the CQC bond of a 1-alkene molecule into the M
C bond in a metallocenium ion [Cp2M+
Polymer] take place. These insertion reactions are mostly primary, Cp2 Mþ 2CH2 2CHR2Polymer þ CH2 QCH2R ! Cp2 Mþ 2CH2 2CHR2CH2 2CHR2Polymer

(3.36)

but occasional secondary 1-alkene insertions are also observed, Cp2 Mþ 2CH2 2CHR2Polymer þ R2CHQCH2 ! Cp2 Mþ 2CHR2CH2 2CH2 2CHR2Polymer

(3.37)

The chain next growth step (the step following Reaction (3.37)) is predominantly primary [510–514], i.e., the regio-error generated in Reaction (3.37) does not propagate: Cp2 Mþ 2CHR2CH2 2Polymer þ CH2 QCH2R ! Cp2 Mþ 2CH2 2CHR2CHR2CH2 2Polymer

(3.38)

Because of Reactions (3.37) and (3.38), polymers of 1-alkenes produced with metallocene catalysts have a measurable number of isolated head-to-head and tail-to-tail attached monomer pairs [251,259,302,397,510,511,515]. However, Reaction (3.37) is practically absent when the 1-alkene molecule has a bulky alkyl group, e.g., in polymerization of 3-methyl-1-butene [255,323]. Reactivity estimations of a sterically crowded Cp2M+
CHR bond formed in Reaction (3.37) vary in a broad range depending on the measurement method. According to some evaluations, the rate constant in Reaction (3.38) is from 100 to 1,000 times lower than the rate constant in Reaction (3.36) [511,512,516–519]. If this estimation is correct, a large fraction of the active centers, from 40 to B95%, should be relatively unreactive Cp2M+–CHR–CH2–Polymer species [517,520]. However, direct kinetic measurements of propylene insertion reactions at
801C into model cationic metallocene catalysts that imitate Reactions (3.36) and (3.38) showed that the rate constant of Reaction (3.38) is only B30% lower than in Reaction (3.36) [521]. These measurements are supported by NMR analysis of growing polymer chains in propylene polymerization reactions, no accumulation of the Cp2Zr+–CH(CH3)–CH2–Polymer chains was observed [521,522].

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Alkene Polymerization Reactions with Transition Metal Catalysts

The probability of the primary insertion in Reaction (3.36) vs. the secondary insertion in Reaction (3.37) depends on several factors. The principal factor is the nature and the number of substituents in cyclopentadienyl rings of the metallocene complexes. For example, active centers formed from racemic complexes, rac-C2H4(3-t-Bu-Ind)2ZrCl2 and rac-Me2C(3-t-Bu-Ind)2ZrCl2 produce strictly head-to-head polypropylene macromolecules in Reaction (3.36) [523,524]. However, active centers generated from complexes of the same symmetry, racMe2Si(2-Me-4-Aryl-Ind)2ZrCl2, rac-Me2Si(Benz[e]Ind)2ZrCl2, and rac-Me2Si(2-MeBenz[e]Ind)2ZrCl2, produce polypropylene with 0.4–1.0% of head-to-head propylene pairs [511,515,525], and a combination of C2H4(4,7-Me2-Ind-H4)2ZrCl2 and MAO produces polypropylene with nearly 20% of regio-mistakes [719]. Syndiospecific metallocene catalysts based on metallocene complexes of Cs symmetry produce essentially head-to-tail polypropylene [193,526,527]. The size of the alkyl group attached to the active center also affects the probability of primary vs. secondary insertion. A GC study of the early stages of propylene polymerization with the Cp2TiMe2-MAO system at –101C showed that when the ‘‘polymer chain’’ is only one monomer unit long, Cp2Ti+–CH2–CH(CH3)2, the rate constant ratio for Reactions (3.36) and (3.37) is B8:1. However, the ratio increases to B30 after two propylene units are placed in the chain, and all subsequent propylene insertion steps are nearly entirely primary [528]. Table 3.18 lists frequencies of regio-errors in propylene polymers prepared with different bridged bis-zirconocene complexes activated with MAO, and Tables 3.23 and 3.24 give examples of effects of reaction parameters on the regioregularity of several metallocene systems.

Table 3.18 Content of regio-inversions in polypropylene prepared with isospecific bridged bis-zirconocene complexes activated with MAO Complex

Secondary insertion (%)

Reference

Polymerization reactions at 301C rac-C2H4(Ind)2ZrCl2 rac-C2H4(Ind-H4)2ZrCl2 rac-Me2Si(Ind)2ZrCl2 rac-Me2Si(2-Me-Ind)2ZrCl2 rac-Me2Si(Benz[e]Ind)2ZrCl2 rac-Me2Si(2-Me-Benz[e]Ind)2ZrCl2

0.51 B0 0.41 0.19 0.74 0.32

[514] ‘‘-’’ ‘‘-’’ ‘‘-’’ ‘‘-’’ ‘‘-’’

Polymerization reactions at 501C rac-Me2Si(Ind)2ZrCl2 rac-Me2Si(2-Me-Ind)2ZrCl2 rac-Me2Si(4,7-Me2-Ind)2ZrCl2 rac-Me2Si(3-t-Bu-Cp)2ZrCl2 rac-Me2Si(2-Me-4-t-Bu-Cp)2ZrCl2 rac-C2H4(Ind-H4)2ZrCl2 rac-C2H4(4,7-Me2-Ind)2ZrCl2

0.48 0.33 1.84 1.5 0.4 0.97 18.9

[529] [526] ‘‘-’’ [530] ‘‘-’’ [531] ‘‘-’’

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Chemistry and Stereochemistry of Polymerization and Copolymerization

The probability of secondary insertion in polymerization reactions of 1-hexene also varies in a broad range [139,532]. When the polymerization reactions are carried out with ionic metallocene systems at low temperatures, the secondary insertion is B800 times less frequent and it immediately leads to chain termination (Reaction (3.70)). However, polymers of 1-hexene produced with the rac-Me2Si(Ind)2ZrCl2-MAO system under ambient conditions have a measurable content of inverted monomer units, and, as expected, their fraction increases with temperature [532]. Temperature (1C) Secondary insertion (%)

0 0.32

5 1.0
1.6

80 3.9

100 4.1

When ethylene is copolymerized with 1-alkenes, insertion of an ethylene molecule into a sterically crowded Cp2M+
CHR bond proceeds relatively easily Cp2 Mþ 2CHR2CH2 2Polymer þ CH2 QCH2 ! Cp2 Mþ 2CH2 2CH2 2CHR2CH2 2Polymer

(3.39)

Kinetic data on ethylene/propylene copolymerization reactions [534] and model experiments with Cp2Zr+
CH(CH3)
CH2
R species [521] both confirm high reactivity of ethylene in Reaction (3.39). If the ethylene concentration in the ethylene/propylene mixture is low, Reaction (3.39) is usually followed by the primary insertion of the propylene molecule and produces a characteristic chain segment, the
CH(CH3)
group flanked by two
CH2
CH2
groups [259,533]. If 13C-labeled ethylene is added in a small amount to a propylene polymerization reaction with a metallocene catalyst, the presence of an easily identifiable sequence
CH2
CH(CH3)
13CH2
13CH2
CH(CH3)
CH2
is the most dependable technique for the evaluation of a low-frequency secondary propylene insertion reaction into the Cp2M+
C bond [520,534]. For example, the probability of secondary propylene insertion (the rate constant ratio in Reaction (3.37) vs. Reaction (3.36)) estimated with this method in polymerization reactions with three MAO-activated metallocene complexes at –151C was found to be quite low [534]: Complex k2,1/k1,2

Cp2TiCl2 2.2  10
4

Cp2ZrCl2 3.1  10
3

Me2Si(Cp)2ZrCl2 2.1  10
3

The studies of monometallocene constrained-geometry catalysts (Section 4.6.1) do not produce a coherent picture in respect to their regioselectivity in polymerization reactions of 1-alkenes. Some reports state that constrained-geometry catalysts exhibit quite poor regioselectivity at ambient and elevated temperatures [537], whereas other studies show that polymerization reactions of propylene and 1-hexene with these catalysts are highly regioregular both at +401C [538] and at
501C [539]. The low regioselectivity is especially prominent in the

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Alkene Polymerization Reactions with Transition Metal Catalysts

case of CpTiCl3 [540]. When this complex is supported on SiO2, it can be activated both with MAO and with trialkylaluminum compounds [540,541]. The following probabilities of chain growth reactions were determined by the 13C NMR method in propylene polymerization with this catalyst at 401C [540]. Insertion step Cp2Ti+
Polymer Cp2Ti+
Polymer Cp2Ti+
Polymer Cp2Ti+
Polymer

+ + + +

-Cp2Ti+ -Cp2Ti+ -Cp2Ti+ -Cp2Ti+

-Polymer -Polymer -Polymer -Polymer

Probability 0.88 0.12 0.41 0.59

Bis-metallocene catalysts do not homopolymerize 1-alkenes with vinylidene double bonds (isobutene, 2-methyl-1-pentene). However, some monometallocene catalysts and constrained-geometry catalysts can copolymerize these alkenes with ethylene [266,535]. In these reactions, vinylidene monomers insert into the CpM+
C bond strictly in the primary orientation and only when the last monomer unit in the chain is the ethylene unit [266,536]: CpMþ 2CH2 2CH2 2Polymer þ CH2 QCðCH3 ÞR ! CpMþ 2CH2 2CðCH3 ÞR2CH2 2CH2 2Polymer

(3.40)

3.3.1.1.2. Chain insertion/isomerization reactions. Some chain growth reactions of 1-alkenes are accompanied by isomerization of monomer units. 3,1-enchainment of propylene units [251,302,397,517,518,542–544]: This isomerization step occurs after a propylene molecule is inserted into the Cp2M+
C bond in the secondary orientation (Reaction (3.37)):

Cp2 Mþ 2CHðCH3 Þ2CH2 2CH2 2CHðCH3 Þ2Polymer ! Cp2 Mþ 2CH2 2CH2 2CH2 2CH2 2CHðCH3 Þ2Polymer

(3.41)

The mechanism of Reaction (3.41) is discussed in Section 6.1.2.2. The existence of this reaction was confirmed in model experiments with zirconocenium cations at low temperatures, slow isomerization of the secondary alkyl group into the primary group [521]: Cp2 Zrþ 2CHðCH3 Þ2CH2 2C3 H7 ! Cp2 Zrþ 2CH2 2CH2 2CH2 2C3 H7

(3.42)

This is a monomolecular reaction, its t1/2 is B5 hours at
801C and B20 minutes at –401C.

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Chemistry and Stereochemistry of Polymerization and Copolymerization

Reaction (3.41) is usually followed by a primary propylene insertion step: Cp2 Mþ 2CH2 2CH2 2CH2 2CH2 2CHðCH3 Þ2Polymer þ CH2 QCH2CH3 ! Cp2 Mþ 2CH2 2CHðCH3 Þ2CH2 2CH2 2CH2 2CH2 2CHðCH3 Þ2Polymer (3.43)

Reaction (3.43) results in the formation of a sequence containing four CH2 units in polypropylene chains [251,259,292,302,397,510,514,543,544]. The same type of 3,1-propylene enchainment was unambiguously observed in propylene oligomerization reactions [545]. It is especially frequent after the preceding 2,1-insertion step. In the case of a propylene trimer, the resulting structure (with the 3,1-inserted unit italized) is CH2QC(CH3)
CH2
CH2
CH2
CH(CH3)2. Polymerization reactions of 1-butene with metallocene catalysts can lead to 4,1-enchainment of monomer units, it produces polymer chains with five CH2 units in a row, BCH2
CH(C2H5)
CH2
CH2
CH2
CH2
CH2
CH(C2H5)B [518,546]. The content of 4,1-inserted units in polymers of 1-butene prepared with the racC2H4(Ind)2ZrCl2-MAO system and its analogs can reach B1% [518]. (Table 3.19) The combined frequency of both chemical errors in propylene polymerization reactions, Reactions (3.37) and (3.41), increases with temperature, as shown in Table 3.15. A relative significance of Reaction (3.41) decreases at higher monomer concentrations because the rate of the chain isomerization step leading to 3,1-enchainment is independent of the monomer concentration whereas the rate Table 3.19

Chemical errors in polypropylene prepared with bridged bis-metallocene systems

Temperature (1C)

Secondary insertion (%)

3,1-insertion (%)

rac-C2H4(Ind)2ZrCl2-MAO [259]
30
10 10 50

0.09 0.32 0.47 0.60

o0.02 o0.02 0.05 0.10

rac-Me2Si(Ind)2ZrCl2-MAO [517] 30 60 80

0.5 0.5 0.3

B0 0.2 0.5

rac-C2H4(Ind)2HfCl2-MAO [251]
30 0 20 50

0.4 0.5 0.6 0.7

B0 B0 B0 0.2

Me2Si[2-Me-4-a-naphthyl-Flu]2ZrCl2-MAO/SiO2 [547] 60 0.7 75 0.4 90 0.8

B0 0.2 0.2

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Alkene Polymerization Reactions with Transition Metal Catalysts

Table 3.20 Chemical errors in polypropylene prepared with two isospecific bridged biszirconocene complexes activated with MAO: effect of reaction conditions [513] [MAO]:[Zr]

PPr (atm)

Secondary insertion (%)

3,1-insertion (%)

Propylene concentration effect, rac-C2H4(Ind)2ZrCl2, 401C 3,000 1.0 0.33 ‘‘-’’ 1.5 0.38 ‘‘-’’ 2.5 0.54 ‘‘-’’ 3.5 0.57 ‘‘-’’ 4.0 0.64

0.155 0.112 0.099 0.044 0.041

[MAO]:[Zr] effect, rac-C2H4(Ind)2ZrCl2, 401C t 3,000 1.0 0.33 6,000 0.62 10,000 0.46 18,000 0.49

0.155 0.069 0.057 0.063

Propylene concentration effect, rac-C2H4(4,7-Me2-Ind)2ZrCl2, 401C 1,000 1.5 1.25 ‘‘-’’ 2.5 1.38 ‘‘-’’ 3.5 1.26

0.148 0.076 0.068

[MAO]:[Zr] effect, rac-C2H4(4,7-Me2-Ind)2ZrCl2, 401C 1,000 1.5 1.25 8,000 1.37 15,000 1.55

0.148 0.133 0.126

of 2,1-insertion (Reaction (3.37)) is proportional to it. This effect is illustrated in Table 3.20. The combined frequency of both propylene misinsertion reactions is also subtly affected by the monomer concentration and by the [MAO]:[Zr] ratio in the catalyst system, as shown in Table 3.20. Both these effects can be apparently explained by spatial restrictions in the transition state of the secondary propylene insertion and Reaction (3.41), as discussed in Chapter 6. A similar type of monomer isomerization was also observed in polymerization reactions of 3-methyl-1-butene catalyzed by (Me)(Ph)C(Cp)(Flu)ZrCl2 [255] and Ph2C(Cp)(Flu)ZrCl2 [323]. The isomerization step occurs quite often after the alkene molecule is inserted in the secondary orientation. It involves one of the methyl groups in the isopropyl side-group and proceeds similarly to Reaction (3.41): Cp2 Zrþ 2CH½CHðCH3 Þ2 2CH2 2Polymer ! Cp2 Zrþ 2CH2 2CHðCH3 Þ2CH2 2CH2 2Polymer

(3.44)

Formation of ethyl branches: Oliva and Marks discovered another type of chain isomerization in ethylene homopolymerization reactions with racC2H4(Ind)2ZrCl2-MAO and rac-Me2Si(Ind)2ZrCl2-MAO systems and with the constrained-geometry [Me2Si(3-Et-Ind)(t-Bu-N)]ZrMe2-[CPh3]+ [B(C6F5)4]
system, the formation of ethyl-branched polyethylene [548–552]. The proposed

Chemistry and Stereochemistry of Polymerization and Copolymerization

131

mechanism involves chain transfer to ethylene (see Reaction (3.50)) immediately followed by re-insertion of the CQC bond of the produced macromolecule, CH2QCH
Polymer, into the Cp2Zr+
CH2CH3 bond: Cp2 Zrþ 2CH2 2CH2
Polymer þ CH2 QCH2 ! Cp2 Zrþ 2CH2 CH3 þ CH2 QCH2Polymer ! Cp2 Zrþ 2CH2 2CHðPolymerÞ2CH2 CH3 þ n CH2 QCH2

(3.45)

þ

! Cp2 Zr 2ðCH2
CH2 Þn 2CH2 2CHðPolymerÞ2CH2 CH3 This reaction occurs rarely (the number of ethyl branches ranges from 1 to 1.5%) and only when unsubstituted zirconocene complexes are used. A replacement of Zr with Ti or Hf or the introduction of alkyl substituents into the indenyl rings results in the formation of strictly linear polyethylene [550]. A similar re-insertion reaction of the vinylidene bond, CH2QC(CH3)
Polymer, into the Cp2Zr+
CH2CH3 center was proposed as an explanation of the formation of a small number of quaternary
CH2
C(CH3)(C2H5)
CH2
groups in ethylene/propylene copolymers prepared with some metallocene catalysts [553]. 3.3.1.1.3. Chain insertion reactions in polymerization of a,o-dienes. The synthesis of alkene polymers with double bonds in pendant groups attracts a considerable interest. These double bonds can be converted into different functional groups for post-polymerization modification of polymers or they can be used for cross-linking. Such polymers can be produced in copolymerization reactions of light alkenes (ethylene, propylene) and nonconjugated dienes. Metallocene catalysts are much more suitable for polymerization and copolymerization reactions of nonconjugated dienes compared to heterogeneous Ti-based catalysts due to their enhanced ability to copolymerize various alkenes (see Section 3.8.1). A number of nonconjugated dienes with two different types of double bonds were tested in these copolymerization reactions, including 1,4-hexadiene [554], vinylcyclohexene [555,556], 6-phenyl-1,5-hexadiene, 7-methyl-1,6-octadiene, 5,7-dimethyl-1,6octadiene [557], norbornadiene [558], 5-ethylidene-2-norbornene [559], and dicyclopentadiene [560]. The chemistry of chain growth reactions in polymerization reactions of nonconjugated a,o-dienes with two vinyl bonds represents a special interest. When these dienes are polymerized with metallocene catalysts, they behave as two 1-alkene molecules linked through their alkyl groups (compare to Section 3.2.1.1.2). The insertion of the first CH2QCH
bond proceeds predominantly in the primary mode, as characteristic for 1-alkenes in general: Cp2 Zrþ 2CH2 2Polymer þ CH2 QCH2ðCH2 Þn 2CHQCH2 ! Cp2 Zrþ 2CH2 2CH2½2ðCH2 Þn 2CHQCH2 2CH2 2Polymer

(3.46)

The behavior of the second vinyl bond depends on the length of the methylene chain between the two vinyl bonds. When the link contains two CH2 groups (in 1,5-hexadiene), three CH2 groups (in 1,6-heptadiene), or four CH2 groups (in 1,7-octadiene), the second vinyl bond also participates in the CQC bond

132

Alkene Polymerization Reactions with Transition Metal Catalysts

insertion reaction and also in the primary orientation. This step results in the formation of polymers with cycloalkyl rings in the main chain [343–445,561–570], as an example with 1,6-heptadiene shows [Cp]Zr

Polymer

CH2

(CH2)3

[Cp]Zr

Polymer

(3.47)

CH

The type of the formed ring is determined by the number of CH2 groups between the two vinyl bonds, polymerization of 1,5-hexadiene produces the 1,3enchained cyclopentyl ring, 1,6-heptadiene forms the 1,3-enchained cyclohexyl ring, and 1,7-octadiene forms 1,3-enchained cycloheptane ring. However, the cyclization reaction is not observed in polymerization reactions of dienes with a higher number of CH2 groups such as 1,9-decadiene [570]. Cyclization efficiency of metallocene catalysts depends on the structure of metallocene complexes. Bis-metallocene complexes, both nonbridged, like Cp2ZrCl2, and bridged, Me2Si(Ind)2ZrCl2, usually produce active centers that nearly completely cyclopolymerize 1,5-hexadiene whereas the center produced from the CpTiCl2(O-2,6-iPr2-C6H3) complex leaves B30% of second vinyl bonds in the monomer units intact [571,572]. An excellent model reaction for the cyclization step in Reaction (3.47) is the transformation of bis-titanocene complexes with alkyl substituents ending with the vinyl double bond, Cp2Ti(Cl)
CH2
CH2
(CHR)3
CHQCH2 [573]. Several complexes of this type were synthesized and converted to active centers in a reaction with EtAlCl2 (see Section 6.1.1.3). The vinyl double bond at the end of the alkyl groups inserts into the Cp2Ti+
C bond in the active centers and new Cp2Ti+
CH2
C(cyclohexyl) species are formed. Their decomposition with HCl produces substituted cyclohexanes, methylcyclohexane is formed when R ¼ H and dimethylcyclohexanes when any of the R substituents is the methyl group [573]. When nonconjugated a,o-dienes are copolymerized with ethylene or propylene, their monomer units also engage in intramolecular cyclization [562,564,570,572, 574,575]. However, the light alkenes compete with the second vinyl bond of the diene for the insertion into the Cp2M+
CH2-Polymer bond formed in Reaction (3.46). As a consequence, cyclization selectivity in these copolymerization reactions depends on the number of CH2 groups between the two vinyl bonds, it is still very high for 1,5-hexadiene whereas copolymers of 1,7-octadiene with ethylene or propylene contain a significant number of unreacted vinyl bonds. Because of this competition, an increase in the concentration of the light alkene decreases the degree of cyclization [564,572,574]. The unreacted vinyl bonds can participate in copolymerization reactions with other growing polymer chains; this reaction leads to the formation of insoluble cross-linked polymers [562,564, 572,574–576]. A combination of the constrained-geometry Me2Si(Me4-Cp)(t-Bu-N)TiCl2 complex and MAO is also a very effective catalyst for polymerization and copolymerization reactions of nonconjugated a,o-dienes. When this catalyst was

133

Chemistry and Stereochemistry of Polymerization and Copolymerization

used for copolymerization of ethylene and 1,7-octadiene, two cyclic structures were identified in the polymer chains [577]. The first structure, the 1,3-enchained cycloheptane ring, is expected in Reaction (3.47). The second type of cycle is formed when the insertion of the first CQC bond of the 1,7-octadiene molecule into the CpTi+–C bond is immediately followed by a single ethylene insertion step and produces the CpTi+
CH2
CH2
CH2
CH
[
(CH2)4
CHQCH2]
Polymer species. At this moment, the CQC bond of the penultimate 1,7octadiene unit inserts into the CpTi+
CH2 bond. This reaction leads to the formation of 1,5-enchained cyclononane rings in the polymer chains [577]. The stereocontrol mechanism in homopolymerization reactions of a,o-dienes is a complicated issue. The tacticity of the polymers is determined by the enantioselectivity of the alkene insertion reaction (Reaction (3.46)) while the diastereoselectivity of the cyclization step (Reaction (3.47)) determines whether a cis- or a trans-ring is formed [444,445,564,567,570,573]. For example, cyclopolymer of 1,5-hexadiene (formally, this is poly(methylene-1,3-cyclopentane)) can have four sterically different structures: 1. Two structures containing the trans-cyclopentane ring, rac-diisotactic and racdisyndiotactic. 2. Two structures containing the cis-cyclopentane ring, meso-diisotactic and mesodisyndiotactic. The rac-diisotactic polymer was indeed synthesized with a chiral metallocene catalyst; it has significant optical rotation in solution [567]. Copolymerization reactions of 1,3-butadiene and ethylene with metallocene catalysts produce unusually rich chemistry of insertion reactions [578,579]. The expected polymerization pattern of 1,3-butadiene includes its 1,2-insertion reactions leading to the –CHQCH2 side-groups in the monomer units and 1,4-insertion reactions leading to –CH2–CHQCH–CH2– units in the main chain. Instead, the copolymers were found to contain four different types of monomer units derived from the 1,3-butadiene molecule.

1,2-cyclopropane unit

1,2-cyclopentane unit

1,1-butadiene unit

1,3-butadiene unit

The ratio between different types of the units depends on the type of metallocene complex and on the [butadiene]:[ethylene] ratio in the copolymerization reactions [579]. Chain isomerization in polymerization reactions of cycloalkenes: Bridged bismetallocene complexes polymerize cyclopentene without ring opening and copolymerize cyclopentene with ethylene [580] and with propylene [581]. Detailed NMR analysis of polymers and hydrooligomers of cyclopentene (trimers and tetramers) showed that the chain growth reaction is accompanied by isomerization

134

Alkene Polymerization Reactions with Transition Metal Catalysts

of each monomer unit, the linking of adjacent units occurs in the 1,3-position instead of the expected 1,2-position [582,583].

The proposed chain-isomerization mechanism involves several steps [582]. First, the CQC bond of a cyclopentene molecule is inserted into the Cp2Zr+
C bond, the same chain growth step as for all alkenes: [Cp]Zr-Polymer

[Cp]Zr

+

Polymer

(3.48)

Then the b-H elimination step from the nearest CH2 group in the ring takes place to give a metal hydride–alkene complex, and the coordinated alkene species reinserts into the Cp2Zr+
H bond: Polymer

[Cp]Zr ...

[Cp]Zr

Polymer

(3.49)

H

When the polymerization reaction is catalyzed by the rac-C2H4(Ind)2ZrCl2MAO or the rac-Me2Si(Ind)2ZrCl2-MAO system, the monomer units are incorporated in the chain exclusively in the cis-1,3-manner and the stereochemical relationship between the neighboring cyclopentene units is isotactic [582,584]. This polymer is highly crystalline and has a very high melting point, B395–4001C [585]. When the polymerization reaction is catalyzed by the rac-C2H4(Ind-H4)2ZrCl2MAO system, cyclopentene units are also 1,3-incorporated but both in the cis- and the trans-arrangement [583,586], whereas the Cp2ZrCl2-MAO and the Ph2C(Cp)(Flu)ZrCl2-MAO systems have very low stereoselectivity [584]. When cyclopentene with ethylene are copolymerized with metallocene catalysts, the steric strain causing the 1,3-enchainment of cyclopentene units is absent and the monomer units are nearly exclusively 1,2-disubstitited cyclopentane rings [202,587,588].

Finally, copolymerization reactions of cyclopentene with propylene provide examples of both types of enchainment of cyclopentene units, 1,2- and 1,3- [581,588]. Constrained-geometry catalysts are also very effective in copolymerizing ethylene and various cycloalkenes and cycloalkadienes [589]. The copolymerization proceeds through the 1,2-insertion of the internal CQC bond into the CpTi+–C bond. The

Chemistry and Stereochemistry of Polymerization and Copolymerization

135

reactivity of different cycloalkenes is mostly determined by steric strain in the cycle: norbornene Wcycloheptene Wcyclooctene Wcyclopentene. However, constrained-geometry catalysts do not copolymerize ethylene with cyclohexene and cyclododecene; only linear polyethylene is formed in both reactions. The same catalysts copolymerize ethylene with cycloalkadienes, including 1,5-cyclooctadiene, 2,5-norbornadiene, 1,3-cyclopentadiene, and dicyclopentadiene, but not 1,4cyclohexadiene. The reactivity of cycloalkadienes in copolymerization reactions with ethylene is usually higher than that of cycloalkenes. The ability of metallocene catalysts to copolymerize light alkenes (ethylene, propylene) and cycloalkenes with internal double bonds such as norbornadiene [558], 5-ethylidene-2-norbornene [148,559], and dicyclopentadiene [560] is important for the synthesis of cross-linkable elastomers. When norbornadiene is copolymerized with ethylene, the copolymerization reaction occurs exclusively through one of its two double bonds and leaves the second bond available for polymer postmodification [558]. Copolymerization reactions of this cyclodiene with 1-alkenes proceeds similarly [590], but they produce copolymers of a low molecular weight. 3.3.1.2. Chain transfer and chain initiation reactions Every chain transfer reaction in polymerization reactions catalyzed by metallocene catalysts is immediately followed by a chain initiation step. Consequently, the chemistry of chain initiation reactions is determined by the structure of active centers left after separation of polymer molecules from them. The only exception from this circular sequence of reactions is the ‘‘initial’’ chain initiation reaction, which involves original Cp2M+
C bonds from the catalysts themselves. These initiation reactions are noticeable only in the first few moments of the polymerization reactions and their chemistry is not different from standard chain initiation reactions. The principal tool for the identification of various chain transfer and chain initiation reactions is 13C NMR analysis of chain ends in propylene homopolymers and in ethylene/1-alkene copolymers of a low molecular weight [389,397,510, 591–593]. 3.3.1.2.1. Chain transfer reactions after primary insertion of the last monomer unit and subsequent chain initiation reactions. Chain transfer reactions in alkene polymerization reactions with metallocene catalysts were studied in great detail [416,592,594]. Each chain transfer reaction results in dissociation of the chemical bond between the metal atom in a metallocene active center and the last monomer unit in the growing polymer chain. Chain transfer to a monomer CH2QCHR: This reaction is the b-H transfer reaction from the growing polymer chain to a monomer molecule coordinated at the transition metal atom:

Cp2 Mþ 2CH2 2CHR2Polymer þ CH2 QCHR ! Cp2 Mþ 2CH2 2CH2 R þ CH2 QCR2Polymer

(3.50)

Reaction (3.50) results in the formation of polymer molecules with the vinyl double bond in ethylene polymerization reactions (R ¼ H), and polymer

136

Alkene Polymerization Reactions with Transition Metal Catalysts

molecules with vinylidene double bond in polymerization and copolymerization reactions of 1-alkenes [265,295,303,304,513,520,525,532,553,590–613]. 13C NMR analysis of propylene homopolymers of a low molecular weight prepared with metallocene catalysts of different stereospecificity [389] and polymers of deuterated propylenes [600] showed that Reaction (3.50) is the dominant chain transfer reaction. Both the aspecific Cp2ZrMe2-MAO system and the isospecific racC2H4(Ind)2ZrCl2-MAO system at increased temperatures and in the absence of excess AlR3 mostly produce only one type of the last end-group, the vinylidene double bond, which is readily observable both by NMR and IR [397,510,590,614]. Oligomer molecules with the vinylidene end-group were also identified in low molecular weight polypropylene by 1H NMR, 13C NMR [159], IR [615], and GC [528,616], including the dimer CH2QC(CH3)
CH2
CH2
CH3 and the trimer CH2QC(CH3)
CH2
CH(CH3)
CH2
CH2
CH3 [545,616]. Dimers and trimers with the same structure were also identified in oligomerization products of 1-butene [545] and 1-hexene [159]. The rate constant of Reaction (3.50) is usually lower in ethylene polymerization reactions in comparison with polymerization reactions of all 1-alkenes, especially when nonbridged metallocene complexes are employed. As a result, molecular weights of ethylene homopolymers prepared with metallocene catalysts are usually much higher than molecular weights of any other alkene polymers. In ethylene/ 1-alkene copolymerization reactions, both types of double bonds, the vinyl and the vinylidene, are formed in parallel. As the concentration of a 1-alkene in the reaction mixture increases (and, correspondingly, the content of 1-alkene units in the copolymer increases), the probability of the vinyl bond formation does not change whereas the formation probability of the vinylidene bond significantly increases [597,617]. The relative probability of the chain transfer reaction to a monomer (Reaction (3.50)) in comparison with the chain growth reaction (Reaction (3.36)) depends on the type of 1-alkene and metallocene complex. Several examples are given in Table 3.21. These data demonstrate that the relative probability of the chain transfer to a monomer increases with an increase of the size of the alkyl group R in a 1-alkene molecule CH2QCHR. The relative significance of Reaction (3.50) in comparison with other chain transfer reactions (discussed below) is estimated by measuring the effect of the monomer concentration on the molecular weight of the produced polymers. Both the chain growth reaction (Reaction (3.36)) and the chain transfer reaction to a monomer (Reaction (3.50)) are first-order reactions with respect to the monomer concentration, and if the molecular weight of a polymer does not depend on the monomer concentration, it signifies that Reaction (3.50) is the dominant chain transfer reaction. For example, Reaction (3.50) is the principal chain transfer reaction in propylene polymerization reactions with the rac-Me2Si(Benz[e]Ind)2ZrCl2-MAO system but it is relatively insignificant for the rac-Me2Si(2-Me-Benz[e]Ind)2ZrCl2-MAO system [605]. Isomerization of end-CQC bonds: When ethylene polymerization reactions with metallocene catalysts are carried out at high temperatures, the vinyl double bond in polymer molecules formed in Reaction (3.50) is catalytically isomerized into the trans-CHQCH bond [203,378,603,607,612,618–620]. Similarly, polymers and

137

Chemistry and Stereochemistry of Polymerization and Copolymerization

Table 3.21 Probability of chain transfer to monomer vs. chain growth in 1-alkene polymerization reactions with metallocene catalysts [294] 1-Alkene

kM t =kp at 301C

rac-C2H4(Ind)2ZrCl2-MAO Propylene 1-Butene 1-Pentene 1-Hexene 1-Heptene 1-Octene 4-Methyl-1-pentene

0.0012 0.0015 0.0016 0.0023 0.0026 0.0049 0.0026

rac-Me2Si(2-Me-Benz[e]Ind)2ZrCl2-MAO Propylene 1-Butene 1-Pentene 1-Hexene 1-Heptene 1-Octene 4-Methyl-1-pentene

0.000082 0.00037 0.00029 0.00036 0.00049 0.00053 0.00076

copolymers of 1-alkenes often contain tri-substituted CQC bonds formed in the isomerization reaction of the vinylidene bond [513,545,593,602,605,609,618, 621,622,630]: CH2 QCR2CH2 2CHR2Polymer ! CH3 2CRQCH2CHR2Polymer

(3.51)

NMR analysis of poly(1-butene) prepared with the rac-Me2Si(Ind-H4)2ZrCl2MAO system at 1001C showed that the H atom undertaking the shift in Reaction (3.51) could come with a nearly equal probability either from the g-CH2 group in the main chain or from the CH2 group in the R group [593]. The isomerization reaction of the vinylidene double bond is a common phenomenon, it can occur in the course of polymerization reactions [609,630], during post-treatment of the polymers at high temperatures, or even during recording of their NMR spectra [608]. For example, Sacchi showed that Reaction (3.51) in polypropylene produced with the rac-C2H4(Ind)2ZrCl2-MAO system is a postpolymerization event [608], probably promoted by catalyst decomposition products [621]. Chain initiation after chain transfer to a monomer: Reaction (3.50) is immediately followed by the chain-initiation step: Cp2 Mþ 2CH2 2CH2 R þ CH2 QCHR ! Cp2 Mþ 2CH2 2CHR2CH2 2CH2 R

(3.52)

The alkyl group
CH2
CH2R formed in Reaction (3.50) in propylene polymerization reactions is the n-propyl group. Its formation is usually followed by the

138

Alkene Polymerization Reactions with Transition Metal Catalysts

primary propylene insertion (Reaction (3.52)) and produces the Cp2Zr+
CH2
CH(CH3)
CH2
CH2
CH3 species. Indeed, both aspecific systems (Cp2TiMe2MAO and Cp2ZrMe2-MAO) and isospecific systems (C2H4(Ind)2ZrCl2-MAO at 701C and C2H4(Ind-H4)2ZrCl2-MAO at 251C) produce this type of the starting chain end. Based on the data on ethylene oligomerization reactions with the Cp2ZrMe2MAO system at 251C, Siedle proposed another type of a chain transfer reaction to a monomer, s-bond metathesis of the Cp2Zr+
C bond and ethylene [623]: Cp2 Mþ 2CH2 2CH2 2Polymer þ CH2 QCH2 ! Cp2 Mþ 2CHQCH2 þ CH3 2CH2 2Polymer

(3.53)

Re-initiation of a polymerization reaction by the Cp2M+
CHQCH2 species yields polymer molecules with the vinyl group as the starting chain end. The significance of Reaction (3.53) in comparison with other chain transfer reactions greatly decreases at higher temperatures and at high ethylene concentrations [623]. Spontaneous chain transfer reactions, b-H transfer: Two spontaneous chain transfer reactions are observed in polymerization reactions with metallocene catalysts. The first reaction, the b-H elimination in the Cp2M+
CH2
CHR
group (or b-H transfer to a transition metal atom), is the same as a similar rare reaction in polymerization reactions catalyzed by Ti-based catalysts, Reaction (3.27) [592]: Cp2 Mþ 2CH2 2CHR2Polymer ! Cp2 Mþ 2H þ CH2 QCR2Polymer

(3.54)

Reaction (3.54) produces polymer molecules with the same type of double bond as Reaction (3.50). To discriminate between the two reactions, the monomer concentration effect on the molecular weight of the polymers should be analyzed (see discussion in Section 5.3.1). In several simplest examples, discrimination between Reactions (3.50) and (3.54) is straightforward. Rytter carried out a detailed study of ethylene homopolymerization reactions with the Cp2ZrCl2-MAO and the (1,2,4-Me3-Cp)2ZrCl2-MAO systems at 801C [612,613]. The molecular weight of both polymers is independent on the ethylene concentration in a range from 0.04 to 0.95 M, and most polymer chains have the vinyl bond as the last chain end. These data clearly indicate that Reaction (3.50) is the principal chain transfer step. The preference for a chain transfer in Reaction (3.50) vs. Reaction (3.54) is often dictated by spatial conditions in the vicinity of the transition metal atom in the active center [624–626]. The b-H transfer step in Reaction (3.50) is preceded by coordination of a 1-alkene molecule at the metal atom and it is favored when the active center is more open sterically. For example, Reaction (3.50) is the preferred chain transfer reaction in propylene polymerization with the metallocene catalyst based on the meso-isomer of Me2Si(3-Me-Ind)(Ind)ZrCl2 (one of its lateral sides is open) and it occurs quite frequently, the polymerization degree is merely B20 at 01C and 15 at 251C [624]. On the other hand, the active center based on the racemic isomer of the same complex has both its coordination positions sterically crowded. The spontaneous chain transfer in this case is two times more preferred

Chemistry and Stereochemistry of Polymerization and Copolymerization

139

compared to the chain transfer to a monomer, and the polymerization degree is much higher, B1,200 at 01C and 470–670 at 251C. The probability of both chain transfer reactions (Reactions (3.50) and (3.54)) with respect to the chain growth reaction strongly depends on the bulkiness of alkyl substituents in 1-alkene molecules [627,628]. Polymerization reactions of such 1-alkenes as 3,7-dimethyl-1-octene, vinylcyclopentane, 3-ethyl-1-pentene, and vinylcyclohexane with the Me2Si(Cp)2ZrCl2-MAO system mostly produce dimers [628]. Spontaneous chain transfer reactions, b-CH3 transfer: The second type of a spontaneous chain transfer reaction in propylene polymerization reactions is a transfer of the b-CH3-group in the Cp2M+
CH2
CH(CH3)
moiety to the transition metal atom: Cp2 Mþ 2CH2 2CHðCH3 Þ2Polymer ! Cp2 Mþ 2CH3 þ CH2 QCH2Polymer

(3.55)

Reaction (3.55) produces polypropylene molecules with vinyl (allyl) double bonds [416,513,592,608,629–633]. When propylene polymerization reactions  are carried out with Cp 2 ZrCl2- and Cp2 HfCl2-based catalysts, the selectivity of Reaction (3.55) in comparison with other chain transfer reactions is quite high, 92–98% [629,632]. This reaction also produces propylene oligomers, the dimer CH2QCH
CH2
CH2
CH3, the trimer CH2QCH
[CH2
CH(CH3)]
CH2
CH2
CH3, etc. [616]. Polymer molecules with the vinyl bond formed in Reaction (3.55) can copolymerize with propylene and form propylene polymers with long chain branches (Section 3.8.4). The second example of the b-CH3-transfer reaction was observed in ethylene/ isobutene copolymerization reactions with constrained-geometry catalysts [536]. It follows Reaction (3.40) and produces the same active center as in Reaction (3.55), CpM+
CH3, and polymer molecules with the vinylidene double bond, CH2QC(CH3)
CH2
CH2
Polymer. Chain transfer to hydrogen: Metallocene catalysts are usually very sensitive to the presence of hydrogen, they easily undergo hydrogenolysis of the Cp2M+
C bond [332,520,605,633–636]: Cp2 Mþ 2CH2 2CHR2Polymer þ H2 ! Cp2 Mþ 2H þ CH3 2CHR2Polymer

(3.56)

In propylene polymerization reactions, Reaction (3.56) results in the formation of saturated polymer molecules with the isobutyl group as the last chain end, (CH3)2CH
CH2
. This group is readily identified by 13C NMR. Table 3.22 gives two examples of the hydrogen pressure effect on the molecular weight of polypropylene. Hydrogenolysis of Cp2M+
C bonds occurs even when alkenes do not participate in polymerization reactions as such, 2-substituted 1-alkenes, alkenes with internal CQC bonds, and styrene [637]: Cp2 Mþ 2H þ CH2 QCR2 Ð Cp2 Mþ 2CH2 2CR2 H 2ðH2 Þ! Cp2 Mþ 2H þ CH3 2CR2 H

(3.57)

140

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 3.22 Hydrogen effect on molecular weight of polypropylene prepared with isospecific metallocene catalystsa [517] Catalyst system

PH (bar)

Mn

rac-H4C2(Ind)2ZrCl2-MAO

0 10 20 40 0 10 20 40

7.14  103 2.14  103 1.51  103 1.13  103 9.24  103 2.01  103 1.34  103 1.13  103

rac-Me2Si(Ind)2ZrCl2-MAO

a

Polymerization at 601C, CPr=1.2 M.

Re-initiation of a polymer chain after the chain transfer to hydrogen occurs easily. If deuterium is used instead of hydrogen in the ethylene polymerization reactions, the insertion of the first ethylene molecule in the Cp2M+
D bond produces the starting
CH2
CH2D moiety [638], Cp2 Mþ 2D þ CH2 QCH2 ! Cp2 Mþ 2CH2 2CH2 2D

(3.58)

and the combined effect of Reactions (3.56) and (3.58) is the formation of alkanes D
(CH2
CH2)n
D [639]. Regioselectivity of insertion reactions of 1-alkenes CH2QCH
R into the Cp2M+
H bond formed in Reaction (3.56) depends on the size of the alkyl group R. In propylene polymerization reactions, the chain re-initiation is not as regioselective as the propylene insertion into any Cp2M+
C bond. Two chain initiation reactions are possible Cp2 Mþ 2H þ CH2 QCH2CH3 ! Cp2 Mþ 2CH2 2CH2 2CH3

(3.59)

Cp2 Mþ 2H þ CH3 2CHQCH2 ! Cp2 Mþ 2CHðCH3 Þ2

(3.60)

When the propylene insertion is primary (Reaction (3.59)), the starting chain end is the n-propyl group, the same as in Reaction (3.52) [464,528,633]. When the propylene insertion is secondary (Reaction (3.60)) and the next propylene molecule inserts into the formed Cp2M+
CH(CH3)2 bond in the primary orientation (a slow reaction), it produces the 2,3-dimethylbutyl group Cp2M+
CH2
CH(CH3)
CH(CH3)2 [464,545,633]. Reaction (3.60) is readily observed in ethylene/propylene copolymerization reactions because ethylene inserts into the M
CH(CH3)2 bond more easily than propylene. Chain transfer to a cocatalyst: All cocatalysts for metallocene complexes, in addition to anion-forming components (MAO, boron compounds, etc.), usually contain organoaluminum compounds. In the case of MAO it is free AlMe3 and in the case of MMAO a mixture of AlMe3 and Ali-Bu3. 13C NMR studies showed that methyl groups in AlMe3 can exchange with methyl groups in Cp2ZrMe2, Cp 2 ZrMe2, Ind2ZrCl2, etc. [640]. The same exchange reaction of alkyl groups

Chemistry and Stereochemistry of Polymerization and Copolymerization

141

takes place between AlR03 and the active centers; the chain transfer reaction to a cocatalyst: Cp2 Mþ 2CH2 2CHR2Polymer þ AlR03 ! Cp2 Mþ 2R0 þ R02 Al2CH2 2CHR2Polymer

(3.61)

This reaction produces a polymer molecule with an AlRu2 group at one end. Re-initiation reactions with the Cp2M+
Ru species produce polymer molecules with Ru groups as starting chain ends [607,614,623]. Exposure of the AlR02
CH2
CHR
Polymer species formed in Reaction (3.61) to moisture or quenching the polymerization reactions with alcohols produces saturated chain ends, CH3
CHR
CH2
CHR
Polymer (compare to Reaction (3.25)) [595], and their reactions with oxygen followed by hydrolysis produce OH-capped polymer chains [596]. Reaction (3.61) is relatively insignificant in ethylene polymerization reactions with metallocene catalysts [613], including polymerization with the rac-C2H4(IndH4)2ZrCl2-MAO system at 601C in a wide range of [AlMe3][Zr] ratios, from 0 to 2,700 [649]. However, this reaction plays an important role in polymerization reactions of propylene and other 1-alkenes with the same catalysts, especially in the presence of excess AlMe3 in MAO [292,299,596,607,614,619,641–645]. Reaction (3.61) is the principal chain transfer reaction in polymerization reactions of 1-alkenes with branched alkyl groups, e.g., in homopolymerization reactions of 3-methyl-1-butene [255,323], cyclopolymerization of 1,5-hexadiene, and copolymerization of ethylene and allylbenzene [646–648,652]. This chain transfer reaction is observed most easily at low temperatures when the chain transfer to a monomer is suppressed, e.g., in propylene polymerization with the constrained-geometry Me2Si(Flu)(t-Bu-N)TiMe2-B(C6F5)3 system [538,539,651]. If the AlR03 compound in Reaction (3.61) is AlMe3, it produces the Cp2Zr+
CH3 bond. The formation of this initiation species was observed in ethylene polymerization with the Cp2ZrCl2-MAO system at 701C in the presence of D2. In addition to linear alkanes D
(CH2
CH2)n
D formed in Reactions (3.56) and (3.58), this reaction produces small quantities of alkanes with odd numbers of carbon atoms, D
(CH2
CH2)n
CH3, which are initiated by ethylene insertion into the Cp2Zr+
CH3 bond [639]. The chain re-initiation step with the Cp2M+
CH3 species in propylene polymerization reactions generates the isobutyl group as the starting chain end: Cp2 Mþ 2CH3 þ CH2 QCH2CH3 ! Cp2 Mþ 2CH2 2CHðCH3 Þ2

(3.62)

Chain initiation reactions similar to Reaction (3.62) are also dominant in polymerization reactions of 3-methyl-1-butene with the isospecific racC2H4(Ind)2ZrCl2-MAO system at 601C in the presence of excess AlMe3 [255], as well as in polymerization reactions of the same monomer with hemistereospecific metallocene catalysts based on Me2C(3-Me-Cp)(Flu)ZrCl2 [323]. These reactions produce the starting 2,3-dimethyl-butyl end-group, Cp2Zr+
CH2
CH(CH3)
CH(CH3)2.

142

Alkene Polymerization Reactions with Transition Metal Catalysts

If the AlR03 compound in Reaction (3.61) is AlEt3, it produces the Cp2Zr+
C2H5 bond, and chain re-initiation with this species in propylene polymerization reactions generates the isopentyl group, Cp2M+
CH2
CH(CH3)(C2H5) [645]. If the AlRu3 compound in Reaction (3.61) is Ali-Bu3, this reaction produces the Cp2Zr+
i-C4H9 species. Chain re-initiation with it in propylene polymerization reactions generates the isobutyl chain end, which is indistinguishable from the chain end formed in Reaction (3.62). Reaction (3.61) is reversible in a sense that the alkylaluminum compound carrying a polymer chain as one of its alkyl groups can react with any other active center and transfer the polymer chain to it. This reaction is untraceable if only one type of metallocene complex is used as a catalyst component but it becomes observable if two metallocene catalysts with different properties operate in parallel. Chien and Brintzinger found an indication of these chain-swapping reactions in propylene polymerization reactions with combinations of two different metallocenium cation, one isospecific and another either aspecific or syndiospecific [162,642,650]: 0 þ 0 0 Cp2 Mþ iso 2ðiso-PolymerÞ þ AlR3 ! Cp2 Miso 2R þ AlR2 2ðiso-PolymerÞ (3.63) 0 0 Cp2 Mþ syndio 2R þ AlR2 2ðiso-PolymerÞ 0 ! Cp2 Mþ syndio 2ðiso-PolymerÞ þ AlR3

Cp2 Mþ syndio 2ðiso-PolymerÞ þ monomer ! Cp2 Mþ syndio 2ðsyndio-PolymerÞ2ðiso-PolymerÞ

(3.64)

(3.65)

Analysis of cocatalyst effects in ethylene/norbornene copolymerization reactions with several metallocene catalysts showed that chain transfer reactions similar to Reaction (3.61) are especially pronounced when dialkylzinc compounds, either ZnMe2 or ZnEt2, are added to the reactions [653]. However, addition of extraneous organometallic compounds to metallocene catalysts also produces significant changes in the nature of the active species, such as the formation of heterodinuclear complexes (Section 6.1.1.1). This change in the nature of the active species can lead to unexpected consequences. For example, when Ali-Bu3 is added to ethylene/ norbornene copolymerization reactions, the molecular weight of the copolymers increases rather than decreases because Ali-Bu3 forms a stronger complex with the active center and suppresses Reaction (3.61) with AlMe3 [653]. Chain transfer to silanes: Marks discovered that organosilane compounds containing the Si–H bond, such as n-HexSiH3, EtSiH3, and PhSiH3, are effective chain transfer agents in polymerization reactions with homogeneous [654,655] and silicasupported [656] metallocene catalysts: Cp2 Mþ 2CH2 2CHR2Polymer þ R3 Si2H ! Cp2 Mþ 2H þ R3 Si2CH2 2CHR2Polymer

(3.66)

Chemistry and Stereochemistry of Polymerization and Copolymerization

143

Primary alkyl silanes RSiH3 are more efficient in this reaction compared to secondary alkyl silanes. The re-initiation reaction following Reaction (3.66) is the same as after the chain transfer reaction to hydrogen (Reaction (3.58)). Reaction (3.66) produces silyl-capped polymer molecules, which can be used for chemical post-modification of polyolefins. A side-reaction, dehydrogenative silane coupling, complicates the use of silanes in Reaction (3.66): 2R3 Si2H 2ðCp2 Mþ 2HÞ! R3 Si2SiR3 þ H2

(3.67)

The generated H2 is an effective chain transfer agent (Reaction (3.56)). Because of Reaction (3.67), polymer products formed in the presence of silanes are mixtures of polymer molecules with silyl groups as the last chain ends and with saturated last chain ends. The ratio between the molar yields of these products is mostly determined by the efficiency of silanes as chain transfer agents in Reaction (3.66) and by the rate of hydrogen formation in Reaction (3.67): 3.3.1.2.2. ‘‘Initial’’ chain initiation reactions. The first Cp2M+
C bonds available for alkene insertion at the earliest stages of polymerization reactions with metallocene catalysts come from two sources:

1. Some metallocene complexes already contain this bond, e.g., Cp2TiMe2 or Cp2ZrMe2. 2. The ‘‘initial’’ Cp2M+
C bond is generated in an exchange reaction between a metallocene complex and a cocatalyst, MAO or MRux (Section 6.1.1.4). Original Cp2M+
C bonds are rapidly consumed in chain initiation reactions and the existence of ‘‘initial’’ Cp2M+
C bonds can be proved only by analyzing polymer products formed either at very low temperatures or after a very short reaction times, before the first chain transfer reaction. Several reports describe unambiguous manifestations of these chain initiation reactions: 1. NMR analysis of growing polymer chains produced in low-temperature polymerization reactions of 1-hexene with a preformed rac-C2H4(Ind)2Zr+
CD3 cation identified the insertion of a 1-hexene molecule into the Cp2Zr+
CD3 bond with the formation of Cp2Zr+
CH2
CH(C4H9)
CD3 [139]. 2. When propylene polymerization reactions with the Cp2TiMe2-MAO system were carried out at –101C for short periods of times, the earliest light polymerization products (analyzed by GC) contained 3n+1 numbers of carbon atoms, including the carbon atom derived from the methyl group in the original titanocene complex [528,545]: Cp2 Tiþ 2CH3 þ CH2 QCH2CH3 ! Cp2 Tiþ 2CH2
CHðCH3 Þ2CH3

(3.68)

3. Relative yields of these products decrease rapidly with time due to the onset of chain transfer reactions (Reactions (3.50) and (3.54)) which both produce oligomers containing the 3n numbers of carbon atoms [528].

144

Alkene Polymerization Reactions with Transition Metal Catalysts

4. In ethylene oligomerization reactions, the consequence of Reaction (3.68) is the generation of linear oligomers with odd numbers of carbon atoms. These oligomers were observed in ethylene oligomerization reaction with the Cp2ZrMe2-MAO system at 01C [657] and 251C [623]. NMR analysis of ethylene oligomers prepared with the Cp2Zr(13CH3)2-MAO system confirmed that Reaction (3.68) takes place at the earliest stages of this polymerization reaction [623]. The second technique for the identification of these ‘‘initial’’ oligomers involves quenching a polymerization reaction with a small quantity of a trapping agent that reacts exclusively with metallocenium ions followed by mass-spectroscopic analysis of ethylene oligomers with attached trapping species [657]. This analysis also showed the presence of growing polymer chains Cp2Zr+–(CH2–CH2)n–CH3 with n ranging from 1 to B25. 5. Polymerization of 1-hexene with the CpTiMe3-B(C6F5)3 system at –781C (a slow reaction) provided one more indication of the initial chain initiation reaction involving the Cp(Me)Ti+
CH3 species [599]. 3.3.1.2.3. Chain transfer after secondary insertion of the last monomer unit and the following chain initiation reactions. Secondary insertion of 1-alkene molecules into Cp2M+
C bonds of growing polymer chains (Reaction (3.37)) is quite frequent in metallocene catalysis, as described in Section 3.3.1.1.1. This reaction produces growing chains Cp2M+
CHR
CH2
Polymer, which have lower reactivity in chain growth reactions compared to growing chains of the Cp2M+
CH2
CHR
Polymer type. The former species usually either isomerizes (Reaction (3.41)) or terminates in three chain transfer reactions, those to a monomer, hydrogen, or in a spontaneous chain transfer reaction [592]. Chain transfer to a monomer and spontaneous chain transfer: The chain transfer reaction to a monomer is the b-H transfer reaction to a coordinated 1-alkene molecule (the same as Reaction (3.50)) but it produces polymer molecules with the internal
CHQCH
bond:

Cp2 Mþ 2CHR2CH2 2Polymer þ CH2 QCHR ! Cp2 Mþ 2CH2 2CHR þ R2CHQCH2Polymer

(3.69)

The spontaneous chain transfer reaction is the b-H elimination reaction in the Cp2M+
CHR
CH2
group (the b-H transfer reaction to the transition metal atom); it produces polymer molecules with the same type of internal double bond: Cp2 Mþ 2CHR2CH2 2Polymer ! Cp2 Mþ 2H þ R2CHQCH2Polymer

(3.70)

Reactions (3.69) and (3.70) are observed in homopolymerization reactions of 1-alkenes both at moderate and high temperatures [139,599,610,611,618], as well as in ethylene/1-alkene copolymerization reactions [613,617]. Formally, Reactions (3.69) and (3.70) differ in their reaction order with respect to the monomer concentration, the first order in Reaction (3.69) and the zero order in Reaction (3.70). However, detailed kinetic analysis of 1-hexene polymerization reactions

145

Chemistry and Stereochemistry of Polymerization and Copolymerization

with isospecific ionic metallocene catalysts indicated a more complex picture [139]. When both chain propagation reactions, the primary (Reaction (3.36)) and the secondary (Reaction (3.37)), occur in competition, and when each secondary insertion step is immediately followed by a chain transfer, the rate of Reaction (3.70) also becomes proportional to the monomer concentration, i.e., Reactions (3.69) and (3.70) become indistinguishable. The main source of the H atom in Reactions (3.69) and (3.70) in propylene polymerization reactions is the b-CH2 group, and both reactions produce the cis-2-butenyl group [520,590,592,602,605,608,610,611,658]. In polymerization reactions of 1-butene and higher linear 1-alkenes, the H atom comes with a comparable probability from two b-CH2 groups, one in the main chain and another in the side-group [139,593], and the
CHQCH
bond can be both cis and trans [139]. Polymerization of vinylcyclohexane yields only the –CHQCH– bond expected in Reaction (3.69) [265]. The ratio between two types of unsaturated last chain ends in polymers of 1-alkenes, the vinylidene bond formed in Reaction (3.50) and the internal double bond formed in Reactions (3.69) and (3.70), depends on an interplay of several kinetic factors. They include the probability of the primary vs. the secondary insertion of 1-alkene molecules in chain growth reactions and the probabilities of the respective chain transfer reactions. For example, nearly all double bonds in low molecular weight poly(1-hexene) produced with highly regioselective Cp2ZrCl2MAO, (n-Bu-Cp)2ZrCl2-MAO, and Me2C(Cp)2ZrCl2-MAO systems at 501C are vinylidene bonds whereas internal CQC-bonds dominate in the polymers prepared with a poorly regioselective system, rac-C2H4(Ind)2ZrCl2-MAO, under the same conditions [590]. On the other hand, polymers of vinylcyclohexane produced with the same rac-C2H4(Ind)2ZrCl2-MAO system contain mostly vinylidene bonds [265]. The isospecific rac-Me2Si(Ind-H4)2ZrCl2-MAO system has moderate regioselectivity in polymerization reactions of 1-hexene and the type of last end-groups in the polymers produced with it strongly depends on temperature, the products of Reactions (3.69) and (3.70) dominate at low temperatures and the products of Reaction (3.50) at temperatures over 801C [532]. Sacchi demonstrated that another step could occur in propylene polymerization reactions following Reaction (3.37), migration of the Zr atom to the methyl group of the second monomer unit [608]. [Cp]Z

[Cp]Z -H +

[Cp]Zr Polymer

Polymer

(3.71) Polymer

If a chain transfer reaction occurs immediately after the migration, a polymer chain with the 4-butenyl end-group is formed [608,610], and if chain growth reactions continue, a small number of n-butyl side-groups appear in the polymer. Another possible route to 4-butenyl groups in polypropylene is catalytic conversion of 2-butenyl end-groups (formed in Reaction (3.50)) by zirconocene hydride complexes [611].

146

Alkene Polymerization Reactions with Transition Metal Catalysts

Chain transfer to hydrogen: Cp2 Mþ 2CHR2CH2 2Polymer þ H2 ! Cp2 Mþ 2H þ RCH2 2CH2 2Polymer

(3.72)

In propylene polymerization reactions, Reaction (3.72) produces saturated polymer molecules with n-butyl groups. Model low-temperature experiments showed that the rate constant of Reaction (3.72) is very high, at least 100 times higher than the rate constant of the chain transfer to hydrogen following the primary propylene insertion (Reaction (3.56)) [521]. All chain transfer reactions after the secondary insertion of 1-alkene molecules into Cp2M+
C bonds (Reactions (3.69), (3.70), and (3.72)) occur much faster than chain transfer reactions after the primary insertion of 1-alkene molecules (Reactions (3.50–3.55)). A metallocene catalyst may produce very infrequent regio-errors in a polymer chain, but these errors are usually immediately followed by chain transfer reactions and, as a result, most of the last chain ends in the polymers may arise after Reactions (3.69), (3.70), or (3.72). Hydrogen does not affect the regioselectivity of metallocene catalysts as such but because chain transfer reactions to hydrogen proceed at a higher rate when the last propylene unit is in the secondary orientation, the effective concentration of regio-errors decreases in the presence of hydrogen because potential regio-errors become last chain ends [520]. Reactions (3.70) and (3.72) both produce the catalytically active Cp2M+
H species, the same as in Reactions (3.56) and (3.54). Respective chain re-initiation reactions are also the same (Reactions (3.59) and (3.60)). Because the growing polymer chain with the last monomer unit in the secondary orientation has low reactivity in chain growth reactions, the introduction of hydrogen usually increases the overall rate of polymerization reactions of 1-alkenes with metallocene catalysts by replacing an unreactive Cp2M+
CHR bond with a highly reactive Cp2M+
H bond. As an example, the introduction of hydrogen into a propylene polymerization reaction with the rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2-MAO system at 801C results in an increase of the catalyst productivity from B5  105 to B3  107 g/mol Zr  h  CPr. Of course, the molecular weight of the polymer is strongly reduced in the presence of hydrogen [659]. One exception from this rule is a catalyst produced from the dinuclear Cp(Me2)Ti(m-Me)B(C6F5)3 complex, the reaction product CpTiMe2 and B(C6F5)3 (Reaction (6.8) in Chapter 6) [636]. The hydrogen addition decreases the molecular weight of polypropylene with this catalyst as well but the reactivity of the catalyst is not significantly increased because the secondary insertion of propylene into the CpTi
C bond in the active species does not noticeably reduce the rate of the next (primary) monomer insertion step. The variety of above-described chain growth and transfer reactions in metallocene polymerization catalysis produces polymer molecules with several types of chemical chain defects (in addition to steric defects discussed in Section 3.3.2) and several types of unsaturated end-groups. Sacchi gave a good representative picture of this variability using as an example propylene polymerization reactions with the rac-C2H4(Ind)2ZrCl2-MAO system [608]. The principal results are shown in Table 3.23. A gradual decrease in the number of unsaturated end-groups with the

147

Chemistry and Stereochemistry of Polymerization and Copolymerization

Table 3.23 Chemical chain defects and end-groups in polypropylene produced with metallocene catalysts [608] PPr (bar)

Temp. (1C)

Regio-irregular unitsa 2,1-insertion

a

Chain endsa

3,1-insertion CH2QC(CH3)--

CH3 --CHQCH--

rac-C2H4(Ind)2ZrCl2-MAO 0.15 30 18 0.4 30 55 0.8 30 58 1.1 30 69

36 15 7 5

33 22 15 9

3 11 13 7

rac-Me2Si(Ind)2ZrCl2-MAO 0.4 30 41 1.1 30 53

3 3

9 9

7 4

rac-Me2Si(Benz[e]Ind)2ZrCl2-MAO 1.1 30 91 B0 1.1 60 64 12 1.1 100 15 85

6 36 94

10 15 13

Contents of all units are per 10,000 monomer units.

monomer concentration in the first part of the table, which is paralleled by a decrease in the number of starting n-propyl groups (Reaction (3.52)), shows the role of the spontaneous chain transfer reaction (Reaction (3.54)). The frequency of this reaction does not depend on the propylene concentration. The increase of the frequency of the 2,1-insertion reaction and the chain transfer reaction to the monomer with temperature are self-explanatory. An increase of 2,1-insertions with the monomer concentration reflects an increase in the relative probability of the slow correction step (Reaction (3.38)) in comparison with chain termination after the 2,1-insertion. A decrease in the number of 3,1-isomerized monomer units with the monomer concentration indicates that the correction step (the first-order reaction with respect to the monomer concentration) competes with chain-end isomerization (Reaction (3.41)), which does not depend on the monomer presence. 3.3.1.2.4. Generation of molecular hydrogen by metallocene catalysts. Alkene polymerization reactions with many metallocene catalysts are accompanied by generation of H2. This reaction is especially pronounced at high temperatures and in copolymerization reactions of ethylene and 1-alkenes [609,617,618]. Some of the hydrogen is consumed in chain transfer reactions to hydrogen (Reactions (3.56) and (3.72)). Several chemical routes leading to the hydrogen generation were proposed. One is s-bond metathesis of the vinylic C–H bond in a polymer chain [618]:

Cp2 Zrþ 2H þ CH2 QCH2Polymer ! Cp2 Zrþ ðHÞ  H2CHQCH2Polymer þ

! H2 þ Cp2 Zr 2CHQCH2Polymer

(3.73)

148

Alkene Polymerization Reactions with Transition Metal Catalysts

Chain re-initiation on the Cp2Zr+
CHQCH
Polymer species (similar to Reaction (3.73)) should result in the formation of polymer molecules with internal double bonds, Polymer
CHQCH
Polymer. Another possibility is the abstraction of the second H atom from the b-CH2 group and the formation of the p-allyl (Z3) intermediate [416,609,617]: Cp2M

Polymer

Polymer

+ H2

(3.74)

Cp2M

A switch from the Z3 coordination of the transition metal atom back to the Z1 coordination at different carbon atoms in the allyl group should lead either to the formation of internal double bonds in polyethylene chains, Cp2M+
CH2
CHQ CH
CH2
Polymer, or to the formation of vinyl branches, Cp2M+
CH(CHQ CH2)
CH2
Polymer. If the p-allyl intermediate is formed at a point when the last monomer unit in the polymer chain is derived from a 1-alkene molecule, the same Z3-Z1 coordination shift would form vinylidene side-groups or three-substituted internal bonds in the main chain. All these structural features were indeed identified in polyethylene and ethylene/1-octene copolymers prepared with metallocene catalysts at very high temperatures [609] and in polypropylene [610,661].

3.3.2. Stereochemistry of chain growth reactions Stereochemistry of chain growth reactions in polymerization reactions of 1-alkenes with metallocene catalysts is a function of the steric structure of metallocene complexes used for catalyst preparation. It is convenient to separate all these catalysts into four large groups, catalysts based on nonbridged bis-metallocene complexes, isospecific catalysts based on bridged bis-metallocene complexes, syndiospecific catalysts based on bridged bis-metallocene complexes, and hemi-isospecific metallocene catalysts. 3.3.2.1. Catalysts based on nonbridged bis-metallocene and monometallocene complexes Bis-metallocene catalysts: The stereochemical behavior of polymerization catalysts produced from nonbridged bis-metallocene complexes strongly depends on reaction temperature. When propylene polymerization reactions with typical catalyst systems of this type, such as Cp2TiCl2-MAO, Cp2TiMe2-MAO, Cp2ZrCl2MAO, Cp2ZrPh2-MAO, and Cp2ZrMe2-MAO, are carried out at 0 to 501C, the polymers are practically atactic [27,262,397,406,528,595,602,662–666]. However, Ewen discovered that when the same reactions are performed at
60 to
781C, the polymers are predominantly isotactic [595]. Main steric mistakes in these isotactic polymers are mmmr and mmrm pentads in a B1:1 ratio [40,595,663–666]. These are the steric mistakes expected for the chain-end stereocontrol mechanism described in Section 3.1.3.2. The probability of the isotactic monomer linking, p0iso at
781C is 0.84 for Cp2TiPh2 [595], 0.85 for Cp2TiCl2 [662,664], and 0.64 for Cp2ZrCl2 [662].

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Chemistry and Stereochemistry of Polymerization and Copolymerization

The results of systematic investigations of these catalysts are shown in Table 3.24. In general, the stereo-regulating ability of the catalysts produced from nonbridged bis-metallocene complexes is poor. The stereo-selection energy DEact(syndio/iso) is merely 0.7–1.2 kcal/mol (3–5 kJ/mol). The introduction of alkyl substituents into cyclopentadienyl or indenyl ligands of the complexes results in a very small increase in the stereo-regulating power of the catalysts, and bridging two cyclopentadienyl ligands makes the catalyst completely aspecific, mostly due to the increase of the aperture angle between two Z5 ligands in the active center. The titanocene produces a catalyst with the highest tendency to form isotactic sequences due to a smaller radius of the Ti atom. The stereospecificity of these catalyst systems does not depend on the type of cocatalyst [663]. For example, the [mm] value for polypropylene produced with the Cp2TiMe2-MAO system at
501C is 0.69, and it ranges from 0.64 to 0.69 if different ion-forming cocatalysts are used instead of MAO. A replacement of the methyl group in Cp2ZrMeCl with a chiral sec-butyl group failed to change the stereo-regulating ability of the metallocene catalysts to any significant degree [667]. When both Z5 ligands in Cp2TiCl2 contain one isopropyl substituent, the stereoregularity trend with temperature is similar to that apparent from Table 3.24, the catalyst is modestly isospecific at
501C ðp0iso ¼ 0:80Þ, aspecific at
101C ðp0iso ¼ 0:50Þ, and even slightly syndiospecific at +101C ðp0syndio  0:60Þ [668]. Table 3.24 Temperature effect on stereoregularity of polypropylene prepared with bismetallocene complexes activated with MMAO [665] Stereoregularity parameter [m]a Temperature (1C)


20

Effect of substituents in (R-Cp)2ZrCl2 Cp2ZrCl2 0.607 0.619 (Me-Cp)2ZrCl2 0.638 (n-Bu-Cp)2ZrCl2 0.641 (i-Bu-Cp)2ZrCl2 0.619 (Cpy-Cp)2ZrCl2 0.690 (Bz-Cp)2ZrCl2

DEact(syndio/iso)

[mmmm] 0

40


20

0

40

0.537 0.599 0.606 0.624 0.596 0.669

0.491 0.525 0.540 0.562 0.538 0.573

0.102 0.126 0.140 0.151 0.115 0.214

0.078 0.106 0.113 0.132 0.110 0.194

0.051 0.066 0.079 0.093 0.071 0.103

(kJ/mol (kcal/mol)) 4.98 3.31 4.48 3.73 3.68 5.65

(1.19) (1.03) (1.07) (0.89) (0.88) (1.35)

Effect of substituents in (R-Ind)2ZrCl2 Ind2ZrCl2 0.581 0.555 0.548 0.107 0.084 0.090 0.609 0.582 0.572 0.144 0.085 0.113 (Me-Ind)2ZrCl2 Effect of transition metal in Cp2MCl2 Cp2TiCl2 0.769 0.687 0.461 0.344 0.241 0.039 15.07 (3.60) 0.607 0.537 0.491 0.102 0.078 0.051 4.98 (1.19) Cp2ZrCl2 0.698 0.691 0.627 0.208 0.193 0.130 3.64 (0.87) Cp2HfCl2 Effect of bridging in Cp2ZrCl2 Cp2ZrCl2 Me2Si(Cp)2ZrCl2 a

0.607 0.537 0.491 0.102 0.078 0.051 0.500 0.483 0.434 0.059 0.053 0.040

[m] value in Bernoulli statistical model is equal to p0iso (see Table 3.2).

4.98 (1.19) 3.01 (0.72)

150

Alkene Polymerization Reactions with Transition Metal Catalysts

On the other hand, similar nonbridged bis-metallocene complexes of Zr and Hf containing Cp ligands have a tendency to produce moderately syndiospecific catalysts [665,669,670]. For example, polypropylene prepared with the Cp 2 ZrCl2MAO system at 401C has the [rr] content of 0.39, which corresponds to p0syndio of B0.62; and the polymer of 1-butene prepared with the Cp 2 HfCl2-MAO system at
201C has the [rr] content of 0.77 ðp0syndio  0:88Þ. Bis-metallocene catalysts with bulky substituents: There are several exceptions from the rule that metallocene catalysts prepared from nonbridged complexes produce mostly atactic polymers at moderate temperatures. If bulky alkyl substituents are placed in the 3rd positions of nonbridged bis-indenyl complexes, they force the Z5 ligands to rotate until they acquire a stable conformation which imitates bridged meso-bis-indenyl complexes (see the next section). For example, placing a-cholestanyl substituents in the 3rd positions of each indenyl ligand in Ind2ZrCl2 transforms it into a catalyst precursor of significant isospecificity. When used at –301C, it produces polypropylene with [mmmm] B0.8, which corresponds to the p0iso value of W0.955, very similar to that for the bridged rac-C2H4(Ind)2ZrCl2 complex at the same temperature [671]. If the Z5 ligands themselves are very bulky, e.g., two fluorenyl ligands (complex IV in Scheme 1.1), then even placing two methyl groups in the 1st positions of their phenyl rings is sufficient to fix the relative orientation of the ligands and transform the metallocene complex into a rigid and chiral molecule of C2 symmetry [673]. Activation of this complex with MAO produces a highly isospecific catalyst system, the [mmmm] value for polypropylene prepared with this system at 601C is B0.83, higher than for polypropylene produced with the nominally isospecific bridged Ph(H)C(Flu)2ZrCl2-MAO system [mmmm] ¼ 0.64 at 301C and 0.31 at 701C [674]. The third exception is nonbridged bis-indenyl metallocene complexes with bulky aromatic substituents in the second position of the cyclopentadienyl ring, e.g. (2-Ph-Ind)2ZrCl2 (complexes XIII in Scheme 1.1). These catalysts were designed by Waymouth and Coates [381,675]; and they too differ from unsubstituted nonbridged complexes by a relatively low rotation rate of the Z5 ligands. These complexes exist for significant periods of time (with respect to average insertion times of alkene molecules into the Cp2M+–C bond) as either chiral anti-rotamers, which are sterically equivalent to racemic bridged bis-metallocene complexes of C2 symmetry (Section 3.3.2.2), or achiral syn-rotamers sterically equivalent to bridged meso-bis-metallocene complexes. The two conformations of the complexes have approximately the same energy and the rate of transition from one to another rotamer depends on temperature and on the presence of substituents at the phenyl rings [381,676–681]. The anti-rotamer produces moderately isotactic polypropylene, as if it has a bridge between two Z5 ligands whereas the syn-rotamer produces atactic polypropylene [381,682]. The catalysts of this type are also very effective for copolymerization of ethylene with 1-alkenes [685] (see Table 3.51). The behavior of catalysts produced with these complexes depends on reaction temperature (which determines the rate of Z5-ligand rotation) and the monomer concentration, which determines the length of the polymer segment formed between two rotation events. When these complexes are combined with MAO and used at moderate temperatures, they produce true stereoblock polymers of low

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Chemistry and Stereochemistry of Polymerization and Copolymerization

average stereoregularity [mmmm]av B0.2–0.6. However, these polymers have quite high melting points, up to 1501C, indicating that they contain isotactic segments of a significant length connected to atactic segments [675,679,680,682–684]. As expected, when polymerization reactions with these catalysts are carried at a high propylene concentration (in liquid propylene), the average isotacticity of the polymer is greatly increased [684]. Increasing the bulk of phenyl substituents in (2-Ph-Ind)2ZrCl2 by introducing two t-Bu groups in the 3rd and the 5th position of both rings increases the stability of the anti-rotamer and, therefore, increases the average isotacticity of the block-copolymers [679]. Studies of propylene polymerization reactions with (2-Ph-Ind)2ZrCl2 activated with MAO [681], MMAO [683], and an ion-forming cocatalyst [678] showed that these polymers are mixtures of macromolecules of different stereoregularity and can be separated by extraction with different solvents. As the data in Table 3.25 show, high melting points of these polymers are due to the presence of polypropylene fractions (from 5 to 20%) of relatively high isotacticity, which are formed when the active centers are in the anti-orientation. Melting points of the polymers prepared at temperatures from 20 to 401C are similar and the differences in the average isotacticity of the polymers (see [mmmm]av values in Table 3.25) are mostly due to the differences in rotation speed between the two different rotamers rather than due Table 3.25 Average stereoregularity and fractional composition of stereoblock polypropylene prepared with nonbridged (2-Ph-Ind)2ZrCl2 complex activated with MAO Temperature e¡ect [683] Reaction temperature (1C)

20 30 40

Average parameters

[mmmm] values of fractions

[mmmm]av

Tm (1C)

Fraction 1a

Fraction 2a

Fraction 3a

0.409 0.399 0.298

142.7 146.1 142.3

0.250

0.624

0.798

Monomer concentration e¡ect, reaction at 231C [684] Propylene concentration (M)

a

Average parameters

[mmmm], (Tm values) of fractions

[mmmm]

Tm (1C)

Fraction 1b

Fraction 2b

Fraction 3b

B10

0.32

138.1

B2

0.17

131.8

0.18 – 0.14

0.33 (79.71C) 0.30 –

0.51 (141.61C) – (127.91C)

Fractionation with xylene. Fractionation with boiling ether (Fraction 1), and boiling n-heptane (Fraction 2 soluble, Fraction 3 insoluble).

b

152

Alkene Polymerization Reactions with Transition Metal Catalysts

to their intrinsic stereospecificity. A detailed 13C NMR study of fractions of one of the polymers produced with a catalyst based on (2-Ph-Ind)2ZrCl2 determined several features of the catalyst [678]. 1. The chiral anti-rotamer produces the active center of very high stereospecificity; piso W0.99. 2. The achiral syn-rotamer produces a nearly stereo-aspecific active center, piso B0.6. 3. NMR data afford the observation of chemical junctions between nearly perfectly isotactic and atactic stereoblocks, such as the mmmrmr heptad. The lengths of the stereoblocks and, hence, melting points of the polymers are determined by the speed of inter-conversion between the two rotamers. Sacchi developed an alternative to (2-Ph-Ind)2ZrCl2 in which the benzene ring in each phenyl group is replaced with a cycloalkyl group
(CH2)n
with n from 4 to 6 [686]. When these complexes are activated with MAO and used in propylene polymerization reactions at low temperatures, they produce stereoblock polymers similar in structure to those prepared with (2-Ph-Ind)2ZrCl2. Monometallocene catalysts: Catalysts derived from monocyclopentadienyl Ti complexes produce mostly atactic polypropylene with a significant content of regio-errors. Table 3.26 compares the steric structure of polypropylene prepared with Cp2TiCl2-, CpTiCl3-, and CpTiMe3-based catalysts. The CpTiX3-based systems have a tendency to produce polymers with a slight tendency to form syndiotactic links between monomer units. Polymerization of 1-butene with similar monocyclopentadienyl systems, CpTi(OBz)3-MAO [687] and CpTi(O
CH2
CHQCH
C6H5)3-MAO [688], also produces atactic polymers. For example, the polymer prepared with the last catalyst at 301C is a regio-irregular polymer consisting of relatively long blocks of primary- and secondary-inserted monomer units in a 2:1 ratio. The polymer is virtually atactic [mmmm] ¼ 0.03, [rrrr] ¼ 0.10 [688]. Constrained-geometry catalysts (complexes IX in Scheme 1.1): Polypropylene prepared with the constrained-geometry Me2Si(Me4-Cp)(t-Bu-N)TiMe2-MAO system is predominantly syndiotactic [rrr] ¼ 0.72, p0syndio  0:9 [672]. However, polymerization of 4-methyl-1-pentene with the same cocatalysts produces either atactic or borderline isotactic polymers, as shown in Table 3.27. Table 3.26 Stereoregularity of polypropylene prepared with catalysts based on Cp2TiCl2, CpTiCl3, and CpTiMe3a [665]

a

Complex

[mmmm] [mmmr] [mmrr] [rmrr] [rmrm] [rrrr] [rrrm] [mrrm] 2,1-Insertion (%)

Cp2TiCl2 CpTiCl3 CpTiMe3 CpTiMe3b

0.55 0.05 0.05 0.005

0.18 0.11 0.10 0.03

0.03 0.11 0.10 0.08

0.18 0.29 0.25 0.151

0.03 0.10 0.13 0.30

Cocatalyst MAO at [MAO]:[Ti] B600–1,000, polymerization at –60 1C. Cocatalyst B(C6F5)3.

b

B0 0.05 0.06 0.13

B0.0 0.18 0.14 0.18

0.01 0.06 0.12 0.07

o1 7 10 B2

153

Chemistry and Stereochemistry of Polymerization and Copolymerization

Table 3.27 Isospecificity of catalysts based on constrained-geometry complexes [Me2Si(Cp ligand)(t-Bu-N)]MCl2a [689]

a

M

g5 Ligand

Productivity (kg/mol M  h)

Mw

[mmmm]

2,1-Units (%)

Ti Ti Ti Zr Ti Ti Ti

Me4Cp Ind Flu Flu Benz[e]Ind 2-Me-Benz[e]Ind 2,3-Me2-Benz[e]Ind

1,250 750 295 170 1,420 2,510 2,760

1.45  105 7.80  104 4.57  105 4.60  105 9.80  104 1.64  105 2.18  105

0.055 0.174 0.305 0.123 0.245 0.328 0.385

3.4 4.5 0 0 2.5 1.6 0.8

Polymerization reactions of 4-methyl-1-pentene at 451C, cocatalyst MAO, [Al]:[M] ¼ 2,000.

3.3.2.2. Isospecific catalysts based on bridged bis-metallocene complexes Two types of bridged bis-metallocene complexes (ansa-metallocene complexes) produce homogeneous isospecific catalysts (Section 4.6.4), racemic isomers of bisindenyl and bis-tetrahydroindenyl complexes of C2 symmetry (complexes X in Scheme 1.1) [28,407,514,515,520,523,524,637,690–693,704,705] and asymmetric bis-metallocene complexes with a particular pattern of substitution in each cyclopentadienyl ring (complexes XI in Scheme 1.1) [168,253,606,691,693–703]. 3.3.2.2.1. Bis-metallocene complexes of C2 symmetry. Complexes of C2 symmetry, effect of structure: Structural factors affecting the performance of catalysts produced from metallocene complexes of C2 symmetry were thoroughly investigated [515,519,693,702,708,709]. Both the substitution in any of the rings in the indenyl ligands and a change in the type of the bridging group between the two ligands produce significant changes in the regioselectivity and the stereoselectivity of the catalysts. The direction and the magnitude of these changes do not lend themselves to a unified straightforward interpretation, although some of them can be explained as a manifestation of electronic effects of the substituents [515]. Stereochemical analysis of propylene polymerization reactions with two most commonly used systems of this type, rac-C2H4(Ind)2ZrCl2-MAO and racC2H4(Ind)2ZrMe2-MAO, at temperatures from
30 to +501C showed that they both produce highly isotactic polymers with rare steric errors accounted for by the enantiomorphic model described in Section 3.1.3.1 [259,397,510]. In addition to steric defects, the chains have two types of chemical defects, isolated propylene units in the secondary orientation and 3,1-inserted units [259,302,397]. However, the isospecific nature of active centers is preserved despite of the 3,1-monomer addition [302,397,510]. Polymerization of 1-hexene with a similar system, racC2H4(Ind)2ZrMe2-B(C6F5)3, at temperatures from
10 to +501C is both nearly perfectly regioselective and nearly perfectly isospecific, [mmmm] B0.99 [139,718]. In general, the stereo-regulating ability of these catalysts is a function of several parameters, the type of the Z5 ligand, the type of the transition metal in the complex, and the type of the bridge between the cyclopentadienyl rings. Several

154

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 3.28 Isospecificity of catalysts based on bridged bis-metallocene complexes of C2 symmetry in propylene polymerization reactions Metallocene complex

Mw

[mmmm]

2,1-Units (%)

Me2Si-bridged complexes, polymerization reactions at 301C 5.50  104 rac-Me2Si(Ind)2ZrCl2 2.43  105 rac-Me2Si(2-Me-Ind)2ZrCl2 3.36  104 rac-Me2Si(Benz[e]Ind)2ZrCl2 2.42  105 rac-Me2Si(2-Me-Benz[e]Ind)2ZrCl2

[514,520] 0.927 0.944 0.933 0.963

0.41 0.19 0.74 0.32

Me2Si-bridged complexes, polymerization reactions at 701C 3.50  104 rac-Me2Si(Ind)2ZrCl2 1.90  105 rac-Me2Si(2-Me-Ind)2ZrCl2 4.20  104 rac-Me2Si(4-Ph-Ind)2ZrCl2 7.40  105 rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2 rac-Me2Si(2-Me-4-Naphthyl-Ind)2ZrCl2 4.80  105

[515,693] 0.901a 0.926a 0.935a 0.989a 0.990a

0.4–0.6 0.5 1.7 0.3 0.1

136 145 148 159 160

R2C-bridged complexes, polymerization reactions at 501C [523,524,704,705] 6.5  103 0.807 0.4–0.6 rac-Me2C(Ind)2ZrCl2 3.1  103 0.714 ‘‘-’’ rac-H2C(Ind)2ZrCl2 1.11  104 0.948 ‘‘-’’ rac-Me2C(3-t-Bu-Ind)2ZrCl2 2.37  104 0.970 ‘‘-’’ rac-H2C(3-t-Bu-Ind)2ZrCl2 2.8  104 0.904 ‘‘-’’ rac-H2C(2-Me-3-i-Pr-Ind)2ZrCl2 1.64  105 0.156 ‘‘-’’ rac-Me2C(3-i-Pr-Ind)2ZrCl2 1.01  105 0.255 ‘‘-’’ rac-H2C(3-i-Pr-Ind)2ZrCl2 a

Tm (1C)

127 110 152 162 145 amorph. amorph.

[mm] values.

Table 3.29 Isospecificity of catalysts based on bridged bis-metallocene complexes of C2 symmetry in polymerization reactions of 4-methyl-1-pentenea [689] Metallocene complex

rac-C2H4(Ind)2ZrCl2 rac-Me2Si(Ind)2ZrCl2 rac-Me2Si(2-Me-Ind)2ZrCl2 rac-Me2Si(Benz[e]Ind)2ZrCl2 rac-Me2Si(2-Me-Benz[e]Ind)2ZrCl2 rac-Me2Si(2-Me-4-Naphthyl-Ind)2ZrCl2 a

Mw 4

7.5  10 8.5  104 1.35  105 9.5  104 9.5  104 1.69  105

[mmmm]

2,1-Units (%)

0.856 0.885 0.900 0.925 0.925 0.941

1.5 1.2 0.8 0.8 0.8 0.5

Polymerization in toluene at 451C, cocatalyst MAO, [Al]:[M] ¼ 2,000.

examples of these effects are shown in Table 3.28 for propylene polymerization reactions and Table 3.29 lists the same data for polymerization reactions of 4-methyl-1-pentene. The introduction of bulky substituents in the 2nd and the 4th positions of the indenyl rings always increase the catalyst isospecificity. However, catalytic properties of complexes substituted in the 3rd positions strongly depend on the type of the substituent. The complexes with the t-Bu group in this position produce highly isospecific catalysts whereas catalysts based on

155

Chemistry and Stereochemistry of Polymerization and Copolymerization

isopropyl-substituted complexes yield completely atactic polymers of a high molecular weight [704]. The presence of ring moieties attached to the cyclopentadienyl rings, as in racC2H4(Ind)2ZrCl2 and C2H4(Ind-H4)2ZrCl2, is not necessary for the formation of isospecific catalysts. Brintzinger demonstrated that bridged bis-cyclopentadienyl zirconocene complexes of C2 symmetry with bulky alkyl substituents, such as t-Bu, also produce highly isospecific catalysts with the piso value of B0.995 [530,621]. Two stereoisomers of bridged bis-indenyl and bis-tetrahydroindenyl complexes, the racemic and the meso-isomer (X in Scheme 1.1), are usually synthesized and used together [545]. The racemic isomer produces crystalline isotactic polymers of 1-alkenes and the meso-isomer produces amorphous, practically atactic polymers [328,595,706]. Some synthetic routes afford separate synthesis of the two isomers. Table 3.30 compares the stereospecificity of catalyst systems produced from two individual isomers. In some instances, racemic isomers of bridged bis-indenyl complexes can photo-isomerize in solution under moderate conditions [367,707]. The isomerization leads to the formation of two polymer fractions, one highly isotactic produced by the racemic complex and another atactic produced by the meso-complex [367]. The relative activity of the two isomers was evaluated by comparing the contents of the two complexes in the mixture (by NMR) and the fractions of polypropylene each isomer produced (by GPC) [328]. rac-Me2Si(2-MeInd)2ZrCl2 activated with MAO is B8 times more active in propylene polymerization reactions at 401C compared to its meso-isomer and it produces the polymer with a B6 times higher molecular weight. The effects of the type of transition metal on the stereospecificity of bridged isospecific bis-metallocene catalysts are different for different types of Z5 ligands. When the complexes of the rac-C2H4(Ind)2MCl2 are used, their isospecificity decreases in the order: Zr W Hf W Ti [251,528]. (Propylene polymers produced with the C2H4(Ind)2TiCl2-MAO system contain two fractions, isotactic and atactic [528].) However, when complexes of the rac-Me2Si(2-Me-4-Ph-Ind)2MCl2 type are used, the order of stereospecificity changes, as shown in Table 3.31 [659]. The Ti and Hf complexes produce catalysts of nearly equal and very high isospecificity. When this Ti complex was used in propylene polymerization reactions at 501C at a high propylene concentration, it produced a polymer virtually free of any regio- or steric defects with a melting point of 1651C [659]. However, the Zr-based catalyst, while extremely active, is inferior in this respect. Complexes of C2 symmetry, effects of reaction parameters: The stereospecificity of catalyst systems based on rac-C2H4(Ind)2ZrCl2 always decreases with temperature [167,259,605,608,620,710,711]. Some data for propylene polymerization reactions Table 3.30

Catalysts based on two isomers of Me2Si(2-Ph-Ind)2ZrCl2a [381]

Metallocene complex

Productivity (kg/mmol Zr  h) Mw

rac-Me2Si(2-Ph-Ind)2ZrCl2 2.5 meso-Me2Si(2-Ph-Ind)2ZrCl2 4.0 a

Propylene polymerization reactions at 201C, cocatalyst MAO, [Al]:[Zr] ¼ 33.

[mmmm]

1.3–1.7  105 0.86–0.88 0.6–0.9  105 0.06–0.07

156

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 3.31 Performance of rac-Me2Si(2-Me-4-Ph-Ind)2MCl2-MAO systems in propylene polymerization reactionsa [659]

a

Complex

Productivity (kg/mol Zr  h  CPr)

[mmmm]

rac-Me2Si(2-Me-4-Ph-Ind)2TiCl2 rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2 rac-Me2Si(2-Me-4-Ph-Ind)2HfCl2

3.2  102 3.3  104 2.7  103

0.97 0.88 0.95

piso

0.994 0.975 0.990

Tm (1C)

158 156

Polymerization at 801C, [Al]:[Zr] ¼ 5,000, CPr B4 M.

Table 3.32 Temperature effects on isospecificity of bridged racemic bis-metallocene complexes in propylene polymerization reactions

rac-C2H4(Ind)2ZrCl2-MAO [259] Temperature (1C)
30 [mmmm] 0.954


10 0.952

10 0.934

50 0.886

rac-C2H4(Ind)2Zr(NMe2)2-[CPh3]+ [B(C6F5)4]
-Ali-Bu3 [710] Temperature (1C) 20 30 50 [mmmm] 0.891 0.827 0.732

60 0.688

70 0.566

rac-Me2Si(2-Me-4-t-Bu-Cp)2Zr(NMe2)2-MAO [167] Temperature (1C)
78 10 30 [mmmm] W0.99 0.980 0.972

50 0.870

60 0.836

are shown in Table 3.32. The same trend was observed in polymerization reactions of 3-methyl-1-butene [256] and 1-hexene [532]. Brintzinger and Resconi discovered, and many researchers later confirmed, that the stereospecificity of metallocene catalysts based on racemic ansa-metallocene complexes of C2 symmetry increases with the monomer concentration (monomer pressure) [518,600,608,643,658,659,712–716]. Several examples of this effect in propylene polymerization reactions are shown in Figure 3.5. The effect levels off at high propylene concentrations. The same leveling-off change was observed even for a highly isospecific catalyst rac-Me2Si(2-Me-Benz[e]Ind)2ZrCl2-MAO and it is accompanied by an increase in the melting point of the polymers [605,717]. A similar effect was also observed in polymerization reactions of 1-hexene with the rac-Me2Si(Ind-H4)2ZrCl2-MAO system at 501 [532]. This change was explained as an outcome of a special chain isomerization reaction, a relatively slow epimerization of the last monomer unit in the growing polymer chain that competes with the isospecific chain growth. The proposed mechanisms are discussed in Section 6.1.2.2. The epimerization reaction produces the same type of steric error in isotactic polymer chains as random steric errors during the isospecific chain growth, mmrr pentads and mrrm pentads (Section 3.1.3.1). A definite discrimination between these two sources of steric errors is possible only in syndiospecific polymerization reactions with metallocene catalysts described in Section 3.3.2.3.

157

Chemistry and Stereochemistry of Polymerization and Copolymerization

1.0

[meso diads]

0.9

0.8

0.7

0.6

0.5 0

1

2 3 Propylene concentration, M

4

5

Figure 3.5 E¡ect of propylene concentration on stereospeci¢city of three metallocene catalysts. rac-C2H4(Ind)2ZrCl2 -MAO, 501C [658]; J rac-Me2Si(2-Me-Ind)2ZrCl2 -MAO, 801C [715]; ’ rac-C2H4(Ind-H4)2ZrCl2 -MAO, 501C [712].



The isospecificity of some bridged racemic bis-indenyl catalysts is also affected by the [MAO]:[Zr] ratio in the catalyst [513]. The effect is not large and its direction is different for different metallocene complexes. The stereospecificity of these catalysts does not change in the presence of hydrogen [514]. Copolymerization reactions of 1-alkenes provide an additional insight into the stereocontrol mechanism of the metallocene catalysts. One type of behavior is exhibited by the rac-C2H4(Ind)2ZrCl2-MAO system. It is moderately isospecific in homopolymerization reactions of propylene ([mmmm] ¼ 0.88 at 301C) and 1-pentene ([mmmm] ¼ 0.92), and this property remains mostly unaffected in copolymerization reactions of propylene with higher 1-alkenes [293,294] and with ethylene [720]. In contrast, rac-Me2Si(2-Me-Benz[e]Ind)2ZrCl2 produces a highly isospecific active species in propylene homopolymerization ([mmmm] ¼ 0.95) but the isospecific control greatly suffers in copolymerization reactions with higher 1-alkenes (1-butene, 1-pentene, 4-methyl-1-pentene) [293,294]. Stereochemistry of chain initiation reactions: The stereospecificity of chain initiation steps is usually noticeably lower than the stereospecificity of chain growth reactions with the same catalysts. This is true both for heterogeneous Ziegler–Natta catalysts (Section 3.2.3.3) and for metallocene catalysts [690]. Table 3.33 compares the stereospecificity of chain growth and initiation reactions in propylene polymerization reactions with three metallocene catalysts. The results show that the stereospecificity of these catalysts at the stage of chain initiation, propylene

158

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 3.33 Stereospecificity of chain growth vs. Chain initiation reactions in propylene polymerization reactions with metallocene catalystsa [690] Complex

Me2Si(Ind)2ZrCl2 Me2Si(Benz[e]Ind)2ZrCl2 Me2Si(2-Me-Benz[e]Ind)2ZrCl2

Chain growth

Chain initiation

[mmmm]

pisob

[erythro]c

p0iso d

0.78 0.93 0.93

0.952 0.986 0.986

0.60 0.54 0.54

0.72 0.65 0.65

a

Cocatalyst MAO, polymerization at 501C. Probability of isotactic propagation, calculated from [mmmm] value (Table 3.1). c From 13C NMR spectra of polymers prepared with 13C-labeled cocatalyst mixtures. d Probability of isotactic initiation, calculated from [erythro] values. b

insertion into the Cp2Zr+–Me bond, is very low in general and it is always significantly lower than at the stage of chain growth. When a 1-alkene molecule contains a chiral carbon atom, i.e., 3-methyl-1pentene, two subtle steric effects were detected in polymerization reactions of racemic monomer mixtures with bis-metallocene catalysts of C2 symmetry [721]. Even the first insertion step of a 3-methyl-1-pentene molecule into the Cp2Zr+– Me bond is slightly stereoselective due to an inherent asymmetry of the complexes, in contrast to heterogeneous Ziegler–Natta catalysts [507–509]. This selectivity (enantioselectivity) is not high (B0.6) and is most probably caused by steric interactions between the coordinated monomer molecule and the ligand framework in the metallocene active center. The second effect, the diastereoselectivity of the next insertion step of a 3-methyl-1-pentene molecule, relates to absolute configurations of two chiral carbon atoms. The first of them is in the alkyl group of the monomer unit,
CH2
CH(CH3)
C2H5, and the second atom is in the main chain, Cp2Zr+
CH2
CH(i-C4H9)
CH3. This steric interaction produces a small preference, B2:1, for the same absolute configuration of the two chiral carbon atoms [721]. 3.3.2.2.2. Asymmetric bis-metallocene complexes. The isospecificity of catalysts based on asymmetric bis-metallocene complexes (XII in Scheme 1.1, complexes of C1 symmetry with two different Z5 ligands) depends of the type of Z5 ligands. The complexes with two bulky Z5 ligands of a similar nature usually produce highly isospecific catalysts [661], as the data in Table 3.34 show. In general, combinations of these complexes and MAO form active centers of approximately the same regioselectivity but higher isospecificity compared to catalysts prepared from complexes of C2 symmetry (see Table 3.28), which is also reflected in higher melting points of polypropylene. The types of steric errors in propylene polymers produced with these catalysts are also described by the enantiomorphic stereocontrol mechanism. On the other hand, asymmetric bridged bis-metallocene complexes of the Me2Si(Cp)(Ind)MCl2 and Me2C(Cp)(Ind)MCl2 type (M ¼ Ti, Zr, Hf) and Me2Si(3-Me-Ind)(Ind)ZrCl2 produce catalysts of moderate-to-low isospecificity [606,624,722]. In contrast to symmetric complexes of the

159

Chemistry and Stereochemistry of Polymerization and Copolymerization

Table 3.34 Performance of asymmetric bis-indenyl complexes in propylene polymerization reactions [661,664] Complex

Mn

[mmmm]

Polymerization reactions at 701C rac-Me2Si(2-Me-Ind)(Ind) ZrCl2 rac-Me2Si(2-Me-4-Ph-Ind)(2-Me-Ind)ZrCl2

1.28  105 5.30  105

0.958 0.960

151 155

Polymerization reactions at 401C rac-Me2Si(3-Me-Ind)(Ind)ZrCl2 meso-Me2Si(3-Me-Ind)(Ind)ZrCl2

2.65  104 3.20  103

0.419 0.421

B44 B54

Tm (1C)

R2X(Ind)2ZrCl2 type, the stereospecificity of the catalysts based on two stereoisomers of Me2Si(3-Me-Ind)(Ind)ZrCl2 is similar [624], as shown in Table 3.34. The effects of temperature and monomer concentration on the asymmetric catalysts vary depending on the structure of the metallocene complexes. For example, the isospecificity of the catalyst produced from rac-Me2C(Cp)(Ind)HfCl2 decreases with temperature [606]; the same effect as for the catalysts produced from complexes of C2 symmetry [606,624,702,722,724]. However, the isospecificity of the catalyst prepared from rac-Me2Si(3-Me-Ind)(Ind)ZrCl2 increases with temperature [624], the [mmmm] value for polypropylene produced at 01C is B0.30 but in increases to B0.49 at 401C. The isospecificity of catalysts based on asymmetric bridged complexes usually decreases with the monomer concentration [606,624], in contrast to the isospecificity trend for catalysts produced from bis-metallocene complexes of C2 symmetry (see Table 3.32). One such example is propylene polymerization with the rac-Me2C(Cp)(Ind)HfCl2-MAO system 251C [606]. [C3H6] (atm) [mm]

1.1 0.59

3.1 0.53

4.1 0.52

B10 0.46

The difference in monomer concentration effects on catalyst isospecificity is the result of different mechanisms of stereo-mistake formation. They are described in Section 6.1.3.2.3. Monometallocene complexes imitating bridged bis-metallocene complexes: Longo and Zambelli described 1-alkene polymerization reactions with monometallocene complexes CpMCl3 (M ¼ Ti, Zr), which contain aromatic substituents attached via a bridging group to the cyclopentadienyl moiety [725,726], e.g., with Cp ¼ Ind– Me2C–Naphthyl [726]. When these complexes are used in combination with MAO in propylene polymerization reactions at low temperatures, the naphthyl ring form a p-complex with the transition metal atom and the active species becomes an imitation of bridged bis-metallocene complexes of the Me2C(Ind)2ZrCl2 type. Similarly to the latter complexes, the bridged hapto-Naphthyl-Zr-Ind species can

160

Alkene Polymerization Reactions with Transition Metal Catalysts

exist either in the racemic or in the meso form, and the racemic isomer polymerizes propylene to isotactic polymers. However, an inter-conversion between the pseudo-racemic and the pseudo-meso form of the active center is quite rapid and this catalyst produces mixtures of propylene macromolecules of different stereoregularity. The pseudo-racemic form produces a crystalline polymer with [mmmm] ¼ 0.72–0.76 and a melting point of 141–1451C [726]. The nature of steric mistakes in the fraction indicates the enantiomorphic type of stereo-control, the same as for catalysts based on true rac-Me2C(Ind)2ZrCl2. The pseudo-meso form produces an essentially atactic polymer, and, if inter-conversion between the two forms occurs in the course of the growth of a single macromolecule, isotactic/ atactic stereoblock polymers are formed. 3.3.2.3. Syndiospecific catalysts based on bridged bis-metallocene complexes Syndiospecific metallocene catalysts are described in Section 4.6.4. Four types of complexes are used for the synthesis of syndiotactic polypropylene: 1. Complexes of Cs symmetry with one cyclopentadienyl and one fluorenyl ligand (complex XIV in Scheme 1.1) [166,411,595,727]. 2. Asymmetric single-bridged complexes of C1 symmetry with the structure imitating that of metallocene complexes of Cs symmetry [728]. 3. Double-bridged complexes of Cs symmetry [411,693,702]. 4. Constrained-geometry complexes with fluorenyl ligands [414,729–731]. The syndiospecificity of single-bridged bis-zirconocene complexes (bridge) (Cp)(Flu)ZrCl2 and (bridge)(Cp)(Flu)ZrMe2 strongly depend on the type of the bridge between the two Z5 ligands. Complexes with the R2Co bridge have true bilateral (Cs) symmetry, and the active species derived from them have the S,Renantiomeric nature [595,698]. These catalysts contain only one type of active center (Mw/Mn ratio of polypropylene is B1.8), they are highly regioselective [193,526,735], and highly syndiospecific [rrrr] ¼ B0.85 at 301C and B0.93 at 101C [733]. Complexes of the C2H4(Cp)(Flu)ZrX2 type do not exhibit exact Cs symmetry. They exist in the crystalline state as two distinct conformers with substantially different interatomic distances and angles, although the interconversion between the two conformers in solution is fast on the NMR timescale [699,732]. When tested under identical conditions as Me2C(Cp)(Flu)ZrMe2, the catalyst prepared from C2H4(Cp)(Flu)ZrMe2 and MAO contains at least two types of active species (Mw/Mn ¼ B3.2) and it is less syndiospecific, [rrrr] ¼ 0.53 [733]. The catalyst prepared from a similar complex containing the Me2Sio bridge produces practically atactic polypropylene [734]. This effect of the bridge type on catalyst stereospecificity differs sharply from the behavior of bridged isospecific complexes described in Section 3.3.2.2. Marks determined principal effects of polymerization parameters in propylene polymerization reactions on the stereospecificity of the archetypal Me2C(Cp) (Flu)ZrMe2-MAO system and two ionic catalysts utilizing the same metallocene complex. The stereoregularity of polymers prepared with them decreases with reaction temperature and noticeably increases with the monomer concentration,

161

Chemistry and Stereochemistry of Polymerization and Copolymerization

Table 3.35 Stereoregularity of catalysts based on Me2C(Cp)(Flu)ZrMe2 and different cocatalysts [735] Cocatalyst

MAO

Temp. (1C)

[rrrr]

[Ph3C]+[B(C6F5)4]


B(C6F5)3 Tm (1C)

[rrrr]

Tm (1C)

[rrrr]

Tm (1C)

Temperature effect at PPr=1
101C 0.953 01C 0.941 +101C 0.936 +251C 0.887 +401C 0.831 +601C 0.638

atm 156.0 151.0 148.9 141.5 129.5 –

0.886 0.852 0.818 0.690 0.531 0.346

147.0 140.4 127.3 101.4 69.0 –

0.929 0.908 0.885 0.827 0.724 0.495

150.9 147.4 143.2 130.3 108.2 –

Monomer concentration effect 0.36M 0.627 0.76M 0.729 1.18M 0.752 1.61M 0.802 2.05M 0.822

at 60 1C – 113.8 122.2 125.4 131.6

0.336 0.439 0.495 0.545 0.571

– – – – –

0.489 0.602 0.646 0.705 0.735

– – 99.0 106.0 109.6

as shown in Table 3.35. Polymerization reactions of 1-butene, other linear 1-alkenes, and 4-methyl-1-pentene with the Me2C(Cp)(Flu)ZrCl2-MAO system and its analogs also produce syndiotactic polymers [166,411,703,727,736,737,740] whereas vinylcyclohexane yields a virtually atactic polymer [727]. The stereocontrol in these polymerization reactions is governed by two mechanisms (see Section 6.1.3.3.1). The first one is the enantiomorphic mechanism of stereocontrol that is exerted on the coordination stage of a 1-alkene molecule at the transition metal atom in the active center (Section 3.1.3.3). This mechanism is responsible for occasional rmmr errors in the syndiotactic chains. The frequency of this steric error does not depend on the monomer concentration and it increases with temperature [728,738]; see Table 3.35. The second source of steric mistakes in these polymers is an isomerization reaction of the active center, the back-skip reaction. This reaction results in a different type of steric error, rmrr [738,739]. Syndiotactic propylene polymers produced with bridged complexes of the R2C(Cp)(Flu)ZrCl2 type usually contain both these steric errors in comparable numbers [193,245,723]. Double-Me2Si-bridged bis-metallocene complexes of Cs symmetry are more rigid than single-bridged complexes. When used in propylene polymerization reactions at
601C, they exhibit an exceptionally high level of syndiospecificity corresponding to psyndio of nearly 0.99 and, respectively, the polymers have very high melting points, B1701C [411,693]. However, the syndiospecificity of these catalysts rapidly deteriorates with temperature. When the Zr atom in the doublebridged (Me2Si)2(4-i-Pr-Cp)[3,5-(i-Pr)2Cp]MCl2 complex is replaced with Hf atom, the syndiospecificity of the catalyst is much lower in spite of virtually identical

162

Alkene Polymerization Reactions with Transition Metal Catalysts

atomic radii of the Zr and the Hf atoms. Propylene polymerization reactions with the Ti analog of the same complex produce essentially atactic polymers. Asymmetric single-bridged complexes (complexes XV in Scheme 1.1) with the structure imitating that of their symmetric analogs (complexes XIV), when activated with MAO, produce propylene polymers with a medium degree of syndiotacticity [382,728,741]. Table 3.36 gives several examples of catalyst systems based on these complexes. Polymers with the highest [rrrr] value, B0.75, were obtained when the cyclopentadienyl ring in the indenyl ligand contained two bulky CH2SiMe2 substituents in the 2nd and the 3rd positions [382]. Similarly to catalysts based on metallocene complexes of Cs symmetry, the stereospecificity of these catalysts decreases with temperature and increases with the monomer concentration [728]. Constrained-geometry catalysts based on Ti complexes with fluorenyl ligands, such as Me2Si(Flu)(t-Bu-N)TiMe2, produce moderately syndiotactic polypropylene [414,538,729–731]. Several examples are shown in Table 3.37. The application of the statistical model of syndiospecific chain growth provided an estimation of two parameters in these polymerization reactions, the probability of syndiospecific monomer insertion, psyndio, and the probability of chain epimerization, pepi (Section 3.1.3.3) [412,414]. Apart from the expected temperature effect on the psyndio value, the syndiospecificity of this catalyst improves at high monomer concentrations. This effect is a clear indication of the chain epimerization effect on stereospecificity, the pepi value is inversely related to the monomer concentration. If a different cocatalyst, Table 3.36 Stereospecificity of catalysts based on bridged complexes Me2C(Cp)(2-Ru-3-RvInd)MCl2 of Cs symmetrya [728] M

Zr Zr Zr Hf Hf a

Ru

H H Me H Me

Rv

Mw

Et CH2SiMe3 CH2SiMe3 CH2SiMe3 CH2SiMe3

3

4.2  10 1.2  104 3.1  104 1.1  105 3.3  105

[rrrr]

Tm (1C)

0.489 0.656 0.738 B0.43 B0.58

– 110 125 B70 110

Polymerization in liquid propylene at 201C.

Table 3.37 Stereospecificity of constrained-geometry (Me2Si)2(3,6-t-Bu2-Flu)[N-t-Bu]TiCl2MAO system [414] Temperature (1C)

CPr (M)

[rrrr]

psyndio

pepi

30 30 30 50 50 50

1.3 2.4 5.2 1.0 1.7 3.5

0.629 0.727 0.789 0.390 0.472 0.634

0.979 0.980 0.980 0.970 0.972 0.979

0.049 0.029 0.016 0.115 0.088 0.047

163

Chemistry and Stereochemistry of Polymerization and Copolymerization

a combination of [Me2N(Ph)H]+ [B(C6F5)4]
and Ali-Bu3, is used with the same metallocene complex, the overall syndiotacticity of polypropylene decreases dramatically, to [rrrr] of B0.1–0.3. However, detailed NMR analysis of these polymers shows that the psyndio value for this catalyst remains quite high, B0.90– 0.95, and the main reason for the large decrease in the average syndiospecificity is attributed to an increase of the probability of chain isomerization, the pepi value increases to 0.12–0.24 [414]. Catalytic properties of Zr analogs of the same constrained-geometry catalysts also strongly depend on the type of activator. When MAO is used to produce the active species, propylene polymerization reactions yield either syndiotactic polypropylene of a low molecular weight or propylene oligomers, depending on polymerization conditions [615,729]. When a combination of [Ph3C]+ [B(C6F5)
4 and Ali-Bu3 is used as a cocatalyst, multi-center catalysts are formed and the polymer products become mixtures containing syndiotactic polypropylene with [rrrr] B0.80, propylene oligomers, and small amounts of isotactic polypropylene [615,729]. 3.3.2.4. Hemi-isospecific metallocene catalysts In general, properties of metallocene catalysts based on bridged Me2C(3-RCp)(Flu)ZrCl2 complexes of C1 symmetry strongly depend on the type of the substituent R in the cyclopentadienyl ring. Several examples in Table 3.38 show that the stereospecificity of the catalysts varies from highly syndiospecific for symmetric complexes (R ¼ H) to hemi-isospecific when R ¼ t-Bu [695,699, 703,723,742]. The Me2C(3-Me-Cp)(Flu)ZrCl2-MAO system [723,742], and Ph2C(3-Me-Cp)(Flu)ZrCl2, and Ph2C(3-i-Pr-Cp)(Flu)ZrCl2 activated with the ionic cocatalyst [Me2PhNH]+ [B(C6F5)4]
-Ali-Bu3 [168] also produce hemiisotactic polymers. Superficially, the pentad distribution in the hemi-isotactic polymers is similar to those in atactic polypropylene. However, these polymers are mildly crystalline and melt at B1351C [168]. The type of cocatalyst does not affect the performance of these catalysts. Discussion in Section 6.1.3.2.5 shows that the ability of metallocene complexes of this type to form stereoregular links between monomer units is determined by a special isomerization reaction in the active center that proceeds without the participation of a monomer and is called the back-skip of a polymer chain. The rate of this isomerization step, which exposes the faces of the active centers of different Table 3.38

a

Stereospecificity of hemi-isospecific Ph2C(3-R-Cp)(Flu)ZrCl2-MAO systemsa [723]

Substituent R

[rrrr]

[mmmm]

H Me Et i-Pr t-Bu

0.931 0.216 0.262 0.190 0.0

0.0 0.134 0.113 0.154 0.835

Propylene polymerization reactions at 10 1C.

164

Alkene Polymerization Reactions with Transition Metal Catalysts

stereospecificity to monomer coordination, depends on temperature. As a result, the temperature effect on the stereospecificity of hemi-isotactic catalysts is an interplay of several factors and does not yield to a simple straightforward explanation. The stereo-regulating power of highly stereospecific metallocene catalysts, both isospecific and syndiospecific, decreases with temperature whereas the [mmmm] value of polypropylene prepared with a typical hemi-isotactic system, Ph2C(3-t-BuCp)(Flu)ZrCl2-MAO, slightly increases with temperature, from 0.835 at 101C to 0.878 at 501C [723].

3.3.3. Polymerization and copolymerization reactions of styrene Homogeneous catalysts that polymerize styrene with the formation of isotactic and syndiotactic polymers are described in Section 4.10. Syndiospecific polymerization reactions of styrene: Two types of metallocene catalysts are used for the synthesis of syndiotactic polystyrene. The first type includes monocyclopentadienyl Ti complexes (both TiIII and TiIV) in combination with MAO. The Cp complexes have the highest activity among various CpTiClx complexes, they produce polymers with the highest molecular weight and the highest stereoregularity (see Table 3.39. Another type of monometallocene complexes that produce syndiospecific catalysts for styrene polymerization are non-bridged analogs of constrained-geometry complexes, Cp[N(Me)(R)]TiCl2 and (1,3-Me2-Cp)[N(Me)(R)]TiCl2, with R ¼ Me, Et, and Cy [335]. Judging by Mw/Mn values of the polymers, all these catalysts contain only one type of active center. CpTiCl3 produces perfectly regioregular and nearly perfectly syndiotactic polystyrene [248] whereas polymers prepared with CpTiCl3 have very small fractions of isolated meso-errors, rrrrm, rrrmr, and rrmrr, in the 2:2:1 ratio [248,744]. The presence of the cyclopentadienyl ring in the transition metal complex is not essential for the synthesis of highly syndiotactic polystyrene. Many derivatives of TiIV combined with MAO or its analogs form equally effective catalyst systems [745]. The syndiospecific polymerization of styrene represents a rare example of regioselectivity in chain growth reactions, the insertion of styrene molecules into the CpTi+
C bond in the active centers occurs exclusively in the secondary Table 3.39 [248]

a

Syndiospecific polymerization reactions of styrene with metallocene complexesa

Complex

Temp. (1C)

Mw

Mw/Mn

[rrrrr]

CpTiCl3 ‘‘-’’ ‘‘-’’ Cp*TiCl3 (Ind)TiCl3 (Me3Si-Ind)TiCl3

40 60 80 60 60 60

3.58  105 1.60  105 6.3  104 3.00  105 2.50  105 7.4  104

2.4 2.1 1.8 1.8 2.0 2.2

0.985 0.979 0.977 W0.99 0.995 0.989

Cocatalyst MAO.

Chemistry and Stereochemistry of Polymerization and Copolymerization

165

orientation [746–749]: CpTiþ 2CHPh2CH2 2Polymer þ CHPhQCH2 ! CpTiþ 2CHPh2CH2 2CHPh2CH2 2Polymer

(3.75)

The secondary insertion mode also prevails in styrene insertion into the CpTi+
H bond [748], the CpTi+
CH3 bond [750], and the CpTi+
CH2CH3 bond [751]. The stereocontrol in Reaction (3.75) is governed by the chain-end mechanism (Section 3.1.3.4), the polymer chains contain rmr tetrads but no rmm tetrads [753]. The stereospecificity in the chain growth step can be very high, the [rrrr] value of syndiotactic polystyrene can reach B0.98, which corresponds to the p0syndio value of B0.995 [753]. Three principal chain-transfer reactions were identified, the b-H transfer to styrene [748,752] CpTiþ 2CHPh2CH2 2Polymer þ CHPhQCH2 ! CpTiþ 2CHPh2CH3 þ CHPhQCH2Polymer

(3.76)

the spontaneous chain transfer (b-H elimination reaction) CpTiþ 2CHPh2CH2 2Polymer ! CpTiþ 2H þ CHPh Q CH2Polymer

(3.77)

and the chain transfer reaction to hydrogen: CpTiþ 2CHPh2CH2 2Polymer þ H2 ! CpTiþ 2H þ CH2 Ph2CH2 2Polymer

(3.78)

In the absence of hydrogen in the reaction medium, Reaction (3.76) is the dominant chain transfer reaction [754]. The chain initiation species formed in Reactions (3.77) and (3.78) is CpTi+
H and the main chain initiation reaction is [748] CpTiþ 2H þ CHPhQCH2 ! CpTiþ 2CHPh2CH3

(3.79)

The ‘‘initial’’ chain initiation reaction (the first styrene insertion step after the formation of active centers) in the catalyst systems activated by mixtures of MAO and AlMe3 is also the secondary insertion of a styrene molecule into the CpTi+
CH3 bond [750,752]: CpTiþ 2CH3 þ CHPhQCH2 ! CpTiþ 2CHPh2CH2 2CH3

(3.80)

If Reaction (3.79) is restricted to insertion of a single styrene molecule and then interrupted by alcoholysis of the catalyst, the main reaction product is ethyl benzene, and if the same treatment is carried out after Reaction (3.80), the main reaction product is n-propyl benzene [752]. Isospecific polymerization reactions of styrene: The catalyst produced from Me2C(3-tBu-Ind)2ZrCl2 and MAO homopolymerizes styrene to a crystalline isotactic polymer [755]. (This catalyst also produces highly isotactic polypropylene [524].) In terms of reaction regiochemistry, this polymerization reaction is completely

166

Alkene Polymerization Reactions with Transition Metal Catalysts

different from syndiospecific polymerization reactions of styrene (Reactions (3.75– 3.80)) and similar of polymerization reactions of 1-alkenes with metallocene catalysts. The dominant insertion mode of a styrene molecule into the Cp2Zr+–C bond is primary, both in the chain growth step (as in Reaction (3.36)) and in the chain initiation step, the insertion into the Cp2Zr+–CH3 bond (as in Reaction (3.68)). Two main chain transfer reactions, either to a monomer or spontaneous, leave the vinylidene double bond CH2QCPh
as the last monomer unit in the polystyrene chains. An occasional secondary insertion of a styrene molecule into the Cp2Zr+–C bond is immediately followed by the b-H transfer reaction and the formation of the Ph–CHQCH–CH2– chain end (compare to Reaction (3.70)).

3.4. Homogeneous Catalysts Based on Early-Period Transition Metals This group of catalysts encompasses a large variety of systems based on complexes of transition metals with monodentate, bidentate, and tridentate ligands. It includes well-defined complexes of the general types MX4, (L)MX3, (L)MX2, (L)2MX2, (L)MX, etc., where X are halogen atoms or various RO groups, and transition metals are Ti, V, Cr, and Zr. These complexes are transformed into polymerization catalysts by the same activators as metallocene catalysts (MAO, ionforming cocatalysts). In terms of polymer stereochemistry, these complexes produce the same classes of 1-alkene polymers as metallocene catalysts, atactic, isotactic, and syndiotactic. The classification of homogeneous non-metallocene catalysts in this section is based on the type of ligand in the original complex.

3.4.1. Complexes with monodentate ligands Ziegler discovered the first homogeneous transition metal catalysts, combinations of Ti(OR)4 and organoaluminum chlorides, AlEt2Cl or Al2Et2Cl3. Although these catalysts are formally similar to common Ziegler–Natta catalysts in terms of chemical components used for their preparation, their performance in alkene polymerization reactions is quite different. Chain propagation reactions with these catalysts have much lower regioselectivity than in the case of TiCl4-based catalysts. The low regioselectivity was observed, e.g., in 1-butene polymerization reactions with a soluble Ti(OBu)4/EtOH complex activated with Al2Et2Cl3 at 201C [764]. Although a 1-butene molecule inserts into the Ti
C bond in this catalyst mostly in the primary mode and forms predominantly isotactic links between monomer units, a frequent inversion of the monomer orientation takes place. Table 3.40 lists probabilities of 1-butene insertion in different regio-orientations and the effect of orientation of the last monomer unit on the regioselectivity of the next insertion step (13C NMR data). Homogeneous V-based catalysts VCl4-AlR2Cl, VCl4-AlR2Cl-anisole (R ¼ Me, Et, i-Bu) polymerize propylene at low temperatures to prevailingly

167

Chemistry and Stereochemistry of Polymerization and Copolymerization

Table 3.40 Probabilities of chain growth reactions in 1-butene polymerization with regiononselective homogeneous Ti(OBu)4/EtOH-Al2Et2Cl3 system [764] Reaction

Probability

Ti–CH2–CH(C2H5)–Pol.+CH2QCH–C2H5-Ti–CH2–CH(C2H5)–CH2–CH(C2H5)–Pol. Ti–CH2–CH(C2H5)–Pol.+CH2QCH–C2H5-Ti–CH(C2H5)–CH2–CH2–CH(C2H5)–Pol. Ti–CH(C2H5)–CH2–Pol.+CH2QCH–C2H5-Ti–CH2–CH(C2H5)–CH(C2H5)–CH2–Pol. Ti–CH(C2H5)–CH2–Pol.+CH2QCH–C2H5-Ti–CH(C2H5)–CH2
CH(C2H5)–CH2–Pol.

0.95 0.05 0.75 0.25

syndiotactic polymers [118–122,430,765–769]. V(acac)3-AlR2Cl systems belong to the same type of catalysts. 13C NMR studies of polymers prepared at
781C [121,429,430,767] and the studies of ethylene/propylene copolymers prepared with the same catalysts [457,591,768] showed that the regioselectivity of active centers in these catalysts is relatively poor and is mostly determined by the orientation of the last monomer unit in the growing chain. When the last monomer unit is in the primary orientation, the next propylene molecule is also preferably inserted in the primary orientation: ðLÞV2CH2 2CHðCH3 Þ2Polymer þ CH2 QCH2CH3 ! ðLÞV2CH2 2CHðCH3 Þ2CH2 2CHðCH3 Þ2Polymer

(3.81)

When the last unit is in the secondary orientation, the next monomer unit is also preferably inserted in the secondary orientation: ðLÞV2CHðCH3 Þ2CH2 2Polymer þ CH3 2CHQCH2 ! ðLÞV2CHðCH3 Þ2CH2 2CHðCH3 Þ2CH2 2Polymer

(3.82)

The secondary insertion mode dominates at low temperatures [120,121,436, 766,767,769,770]. This reaction mode was confirmed by NMR analysis of last chain ends in living-chain polymerization reactions with V(acac)3-AlR2Cl systems terminated with iodine [771], I
CH(CH3)
CH2
Polymer molecules. This mechanism of regioselection leads to a significant number of head-to-head- and tail–to-tail-linked propylene units which are easily observable by IR and 13C NMR [436,767,772]. For example, polypropylene produced with the V-based catalysts activated with AlEt2Cl at temperatures from
50 to 01C contains 13–15% of inverted monomer units [120,496]. At lower temperatures, the frequency of the secondary-to-primary switch in monomer orientation is much lower, propylene polymers prepared with soluble V-based catalysts at
781C contain only 1–2% of monomer units formed in primary insertion reactions [120]. These polymers consist of long syndiotactic sequences of monomer units inserted in the secondary position, B100 units on average, and very short blocks of primary-inserted molecules, 3–4 units [120,773]. Table 3.41 gives the probabilities of different chain growth steps in propylene polymerization reactions with the VCl4-AlMe2Cl system at
781C [774]. If a small amount of ethylene is added to a propylene polymerization reaction with the syndiospecific VCl4-AlEt2Cl system, it perturbs the orientation pattern of

168

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 3.41 Probabilities of four types of chain growth reactions in propylene polymerization with VCl4-AlMe2Cl system at
781C [774] Reaction

Probability

V–CH2–CH(CH3)–Pol.+CH2QCH–CH3-V–CH2–CH(CH3)–CH2–CH(CH3)–Pol. V–CH2–CH(CH3)–Pol.+CH2QCH–CH3-V–CH(CH3)–CH2–CH2–CH(CH3)–Pol. V–CH(CH3)–CH2–Pol.+CH2QCH–CH3-V–CH2–CH(CH3)–CH(CH3)–CH2–Pol. V–CH(CH3)–CH2–Pol.+CH2QCH–CH3-V–CH(CH3)–CH2–CH(CH3)–CH2–Pol.

0.86 0.14 0.02 0.98

propylene units. After a single ethylene unit is inserted into a growing syndiotactic chain, the (L)V
CH2
CH2
CH(CH3)
CH2
Polymer species is formed and the next propylene molecule can insert into the V
CH2 bond both in the secondary and in the primary orientation with nearly equal probabilities [362,776]. The most frequent chain transfer reaction in these reactions under ambient conditions occurs when the last monomer unit in a growing chain is in the primary orientation (after Reaction (3.81) [591]): ðLÞV2CH2 2CHðCH3 Þ2Polymer þ CH2 QCH2CH3 ! ðLÞV2CH2 2CH2 2CH3 þ CH2 QCðCH3 Þ2Polymer

(3.83)

When these reactions are carried out at
781C in the living-chain mode, the dominant orientation of monomer units is secondary. If these reactions are terminated by addition of an alcohol, the structure of the last chain end, the n-propyl group, reflects the secondary orientation of the last monomer unit [774]: ðLÞV2CHðCH3 Þ2CH2 2CHðCH3 Þ2CH2 2Polymer þ ROH ! ðLÞV2OR þ CH3 2CH2 2CH2 2CHðCH3 Þ2CH2 2Polymer

(3.84)

When a propylene molecule is inserted into the (L)V
C bond in the secondary orientation, the probability of its syndio-linking is moderate, p0syndio  0:85 [767], and the difference in the activation energy for the formation of the syndio-diad vs. the iso-diad is merely B4.2 kJ/mol (B1 kcal/mol) [775]. The preference for syndiolinking is explained by the asymmetric configuration of the last monomer unit in the growing polymer chain; the chain-end mechanism of steric control described in Section 3.1.3.4 [361,409,776]. The syndiospecificity of the catalysts depends on the type of the V compound [767], VCl4-AlR2Cl-anisole W VCl4-AlR2Cl W V(acac)3AlR2Cl, and on the type of cocatalyst [778], AlEt2Cl W Aln-Pr2Cl WAli-Bu2Cl. The regioselectivity of the V-based active centers in chain initiation reactions depends on the type of 1-alkene. It is predominantly primary in propylene polymerization reactions (in contrast to chain growth reactions) and produces the (L)V
CH2
CH(CH3)
R center [429,430,767]. However, the chain initiation steps exhibit no regioselectivity in polymerization reactions of 1-pentene [803]. Propylene polymerization reaction with the VOCl3-Ali-Bi2Cl system represents one of the most complex examples in terms of the pattern of chain growth reactions [389]. The dominant propylene insertion orientation in this reaction at 301C is still secondary but its probability is only B0.75, and the change of monomer

Chemistry and Stereochemistry of Polymerization and Copolymerization

169

orientation (resulting in head-to-head- and the tail-to-tail- monomer linking) occurs with a probability of B0.04. The regioselectivity of this catalyst in chain initiation reactions is also very poor. If one takes into account that the stereospecificity of the catalyst is very poor irrespective of the regio-mode of monomer insertion, the complexity of its NMR spectrum and the complexity of statistical models required for the description of the polymer microstructure becomes obvious [389]. When VCl4 is supported on MgCl2 and activated with AlEt3 or with its mixtures with methyl p-toluate, the behavior of the active V species undergoes a profound change. Instead of the predominantly secondary insertion pattern characteristic for homogeneous V-based catalysts, the supported catalyst inserts propylene molecules into the V
C bond exclusively in the primary orientation [438]. All chain transfer reactions in these reactions are depressed; the molecular weight of polypropylene prepared at 501C exceeds 2  106. The supported VCl4based catalyst is highly isospecific, the content of the crystalline (n-heptane insoluble) fraction in the polymer is W97%, and it is nearly perfectly isotactic [438]. Overall, the performance of such VCl4-based catalysts is similar to the performance of supported TiCl4/MgCl2 catalysts [438,777]. Catalyst systems based on TiCl4 and AlR2Cl, although formally belonging to the class of pseudo-homogeneous Ziegler–Natta catalysts, are much closer to Ti- and V-based homogeneous catalysts in polymerization reactions of 1-alkenes in terms of their performance. When the TiCl4-Ali-Bu2Cl system was employed in propylene polymerization reactions from 60 to 1001C at low [Al]:[Ti] ratios, the reactions produced propylene oligomers of a low molecular weight, 300–1,700 [804]. Each polymer molecule contained one double bond. The majority of them are vinylidene bonds, the product of the chain transfer reaction similar to Reaction (3.83). This catalyst is not highly regioselective; an occasional secondary insertion of propylene molecule leads to the formation of vinyl and internal double bonds in chain transfer reactions [804]. Pellecchia described a different class of homogeneous catalysts based on Ti and Zr compounds with monodentate ligands, combinations of TiBz4 or ZrBz4 and MAO [779]. These systems, as well as a similar system, Ti(OBu)4-MAO, contain two types of active centers; they polymerize propylene under moderate conditions to mixtures of isotactic and atactic polymers. The content of the crystalline isotactic fraction in these polymers ranges from 30 to 60%. The isotactic polymer chains are mostly produced via the primary insertion of propylene molecules (although some regio-irregular sequences are present), and the nature of steric errors in these fractions, mmrr and mrrm pentads, is consistent with the enantiomorphic stereocontrol mechanism. Similar polymers were also produced when ZrBz4 was combined with an ion-forming activator [Me2N(C6H5)H]+ [B(C6F5)4]
[780].

3.4.2. Complexes with bidentate, tridentate, and tetradentate ligands Homogeneous catalysts containing bidentate and tridentate ligands are usually described by the terms ‘‘non-metallocene homogeneous catalysts’’ or ‘‘postmetallocene homogeneous catalysts.’’ Currently, these catalysts are the subjects of

170

Alkene Polymerization Reactions with Transition Metal Catalysts

intensive research. Regio- and stereochemistry of several homogeneous catalyst systems based on these complexes was examined in great detail. Complexes with phenoxy-imine ligands: Bis(phenoxy-imine) complexes of TiIV and IV Zr contain two bidentate ligands (Scheme 1.2), each coordinated to the transition metal atom through the PhO– bond and the nitrogen atom. If phenoxy-imine ligands in complexes contain alkyl substituents in the 2nd and the 4th positions of their phenyl rings, they produce moderately syndiospecific catalysts [342,343, 644,781–786]. Detailed investigations of the Ti and Zr complexes with the same ligands showed a surprising difference in the chemistry of polymerization reactions. Catalyst systems based on bis(phenoxy-imine) complexes of TiIV and MAO are effective catalysts for polymerization of ethylene and propylene [342–344,783,805– 807]. Detailed 13C NMR analysis of end-groups in polypropylene prepared with several catalysts of this type showed unusual chemical and regio-effects [342,343]. The chain propagation reaction with this catalyst at 0–251C proceeds predominantly in the secondary orientation, the same as in Reaction (3.82) [343,783,784]. This orientation mode is governed exclusively by steric interactions between two methyl groups, one in the inserting propylene molecule and another in the last monomer unit in the growing chain, rather than by any steric or electronic interactions with the active center itself. The primary propylene insertion in this reaction occurs rarely; the content of head-to-head units is only B2–4% [783,784]. NMR analysis of propylene/ethylene copolymers containing a few mol.% of 13 C-labeled ethylene confirmed this unusual regiochemical pattern [343,783,785]. After insertion of a single ethylene unit, ðLÞTi2CHðCH3 Þ2CH2 2Polymer þ

13

CH2 Q13 CH2

! ðLÞTi213 CH2 213 CH2 2CHðCH3 Þ2CH2 2Polymer

(3.85)

the following propylene insertion step is predominantly primary, the same as in similar ethylene/propylene copolymerization reactions with the VCl4-AlEt2Cl system described in the previous section [362,776]. Several more primary insertion steps often follow this step before the active center reverts to the secondary insertion pattern [783]. The principal chain transfer step in these reactions is the b-H elimination reaction from the methyl group of the last propylene unit leading to the formation of the allyl group: ðLÞTi2CHðCH3 Þ2CH2 2Polymer ! ðLÞTi2H þ CH2 QCH2CH2 2Polymer

(3.86)

The chain initiation reaction involving the generated (L)Ti
H bond proceeds exclusively in the primary monomer orientation and produces the (L)Ti
CH2
CH2
CH3 species. However, the next propylene insertion step is not regioselective; primary and the secondary insertion reactions occur with a nearly equal frequency and produce n-propyl and n-butyl groups as the starting chain ends, respectively. All subsequent propylene insertion reactions proceed nearly exclusively in the secondary orientation [343,783]. On the other hand, when the

171

Chemistry and Stereochemistry of Polymerization and Copolymerization

same bis(phenoxy-imine) complexes of TiIV are activated with ion-forming cocatalysts instead of MAO, the active species lose regiospecificity in polymerization reactions of higher 1-alkenes, 1-hexene, 1-octene, 1-decene, and 4-methyl-1pentene [788]. The secondary chain growth step in propylene polymerization reactions is syndiospecific [342–345,783,784,789,807]. Table 3.42 gives several examples of substituent effects on the [rr] content. Active centers of the highest stereospecificity are produced when the aryl group attached to the nitrogen atom in the phenoxyimine ligand is C6F5 (Scheme 1.2) and the phenyl group attached to the oxygen atom carries bulky t-Bu or SiMe3 substituents in the ortho-position to the C–O bond. The [rrrr] content in polypropylene produced with these complexes at 0–201C can reach 0.95–0.96 corresponding to a very high probability of syndiotactic linking, B0.99 [342,343,644,786,807]. However, a replacement of these substituents with two bromine or two I atoms changes the regiocontrol to primary and the stereo-control to moderately isospecific, the [mm] values of polypropylene prepared at 01C increase from 0.17 for dialkyl-substituted complexes to 0.73 for diiodo-substituted complexes [790]. Steric mistakes in the highly syndiotactic chains are of the rmrr type, consistent with the chain-end stereocontrol mechanism (Section 3.1.3.4). Overall, this regiochemical and stereochemical behavior is very similar to that discovered for homogeneous VCl4-based catalysts described in the previous section, and it probably reflects a similar chemical structure of the active centers. Table 3.42 [783,784]

Stereospecificity of catalysts produced from Ti bis(phenoxy-imine) complexesa

Substituents in ligandb

R1

a

[rr]

p0syndio c

Ar

Data from [783] t-Bu t-Bu H t-Bu H t-Bu

Ph Ph C6F5

Data from [784] H t-Bu t-Bu t-Bu Me t-Bu H SiMe3 H SiEt3 H H H Me H i-Pr

C6F5 C6F5 C6F5 C6F5 C6F5 C6F5 C6F5 C6F5

0.30 0.21 1.2 1.9 4.4 2.3 4.8 3.5 30.8 69.2 31.3

Propylene polymerization reactions at 20–251C, cocatalyst MAO. Scheme 1.2. Probability of syndiotactic linking of monomer units (see Table 3.4).

b c

R2

Productivity (kg/mol Ti  h  atm)

0.85 0.81 0.87–0.94

0.92 0.90 0.93–0.97

0.87 0.86 – 0.93 – 0.43 0.50 0.75

0.93 0.93 0.96 0.66 0.71 0.87

172

Alkene Polymerization Reactions with Transition Metal Catalysts

Analysis of the effects of reaction parameters on the performance of two bis(phenoxy-imine) Ti complexes demonstrated one more time a pitfall inherent in the assumption that homogeneous polymerization catalysts contain only one type of active center [786]. Simple solvent fractionation experiments showed that at least two types of active centers are present in each catalyst. When the original complex contains the Ph–N bond (Scheme 1.2), one of the centers is syndiospecific ([rr] B0.7) and the second one is isospecific ([mmmm] ¼ 0.62). When the complex contains the C6F5–N bond, both centers are syndiospecific and produce polymer fractions with [rr] values of B0.5 and 0.92, respectively. In addition, the stereospecificity of each type of active center depends on the type of cocatalyst and the polarity of the reaction medium [786]. Catalysts produced from bis(phenoxy-imine) TiIV complexes retain their syndiospecificity when they are supported [787]. If the complexes are supported on MgCl2 =AlR0x ðORÞ3
x carriers, they do not require MAO as a cocatalyst. The stereospecificity of the supported catalysts also depends on the type of substituents in the ligand. When the SiMe3 group is placed at the ortho-position to the C–O bond and the C6F5 group is attached to the nitrogen atom, the catalysts are even more syndiospecific than their homogeneous analogs, the [rr] content in the polypropylene prepared at 251C is 0.97 [787]. Bis(phenoxy-imine) complexes of Zr produce active centers with completely different regiochemical characteristics [785]. Both the chain initiation and the chain growth reactions proceed via the primary orientation of monomer units: ðLÞZr2CH2 2CHðCH3 Þ2Polymer þ CH2 QCH2CH3 ! ðLÞZr2CH2 2CHðCH3 Þ2CH2 2CHðCH3 Þ2Polymer

(3.87)

Two observed chain transfer reactions also occur when the last monomer unit in the growing chain is in the primary orientation. The main reaction is the chain transfer to AlMe3 and a minor chain transfer reaction is the b-H transfer to the Zr atom. NMR analysis of propylene/ethylene copolymers containing a few mol.% of 13 C-labeled ethylene confirmed the primary regiochemical insertion mode. Although the regiochemistry of chain growth steps in propylene polymerization reactions with bis(phenoxy-imine) complexes of Ti and Zr is opposite, the mechanism of stereocontrol is the same, the chain-end mechanism, and the syndiospecificity of both catalysts is very similar [785]. The types of chain transfer reactions in ethylene polymerization reactions with bis(phenoxy-imine) Zr complexes strongly depends on the type of substituent at the nitrogen atom in the ligand (Scheme 1.2). If the substituent is the phenyl group, the main chain transfer reaction is the b-H transfer step (similar to Reaction (3.86)) leading to the formation of the vinyl bond [808,809]. However, if the substituent is a bulkier 2-isopropyl-phenyl group, this reaction is completely suppressed, and the only remaining chain transfer reaction is that to AlMe3 present in MAO [809]. This step occurs rarely if small amounts of commercial MAO with a low AlMe3 content are used in the polymerization reactions (and it results in polyethylene with a molecular weight of W7  105) but adding AlMe3 to the reaction medium increases its frequency.

173

Chemistry and Stereochemistry of Polymerization and Copolymerization

Complexes with diamide ligands: Diamide complexes of Ti (Scheme 1.2) are precursors of highly active catalysts for polymerization of various 1-alkenes [792– 794]. These complexes are activated with the same cocatalysts as metallocene complexes, either with MAO at an [Al]:[Ti] ratio of 250–1,000, or with ionforming activators, such as B(C6F5)3 (when X ¼ Me) at a [B]:[Ti] ratio of 1 [792,793,795,796] or [CPh3]+ [B(C6F5)4]
-AlR3 [796]. Although these catalysts are soluble in aromatic solvents, they behave as typical heterogeneous multi-center catalysts. Propylene polymerization reactions with the catalysts produce mixtures of atactic and moderately isotactic crystalline products [795,796]. The relative content of the crystalline fraction depends on several factors (see Table 3.43), including the type of aryl groups Aru and Arv attached to both nitrogen atoms in the ligand, the type of cocatalyst, and the propylene concentration. 13C NMR analysis of the crystalline fractions shows that the primary regioselectivity of these catalysts is very high and that the stereoselection mechanism is enantiomorphic [795]. The isospecificity of the catalysts is low, as the [mmmm] values and melting points of polypropylene in Table 3.43 testify. When the diamide Ti complexes are activated with AlMe3-free MAO or with silica-supported MAO (to avoid chain transfer reactions to AlMe3), propylene polymerization at 01C proceeds in the living-chain mode [797]. Polymerization of 1-hexene with these complexes activated with MAO at B701C produces high molecular weight atactic poly(1-hexene) (Mw from 5  104 to 2.4  105) with a narrow molecular weight distribution, the Mw/Mn ratio is in the range of 1.6–1.8. A peculiar property of the Ti diamide catalysts is a nearly complete absence of b-H elimination reactions, either to a monomer or to the catalyst species. Polymers of 1-hexene prepared with these catalysts do not contain any unsaturated chain ends characteristic for metallocene and Ziegler–Natta catalysis [793]. NMR analysis of poly(1-hexene) produced with the diamide Table 3.43 Isospecificity of catalysts based Ti on diamide complexes and different cocatalystsa [796] Ligandb

Cocatalyst

NAr
(CH2)3
NAr NAr
(CH2)3
NAr NAr
(CH2)3
NAr NAr–(CH2)3–NAr NAr–(CH2)3–NAr NAr–(CH2)3
NAr NAr–(CH2)3–NAr

MAO [CPh3]+ [B(C6F5)4]
-AlMe3 [CPh3]+ [B(C6F5)4]
-AlEt3 [CPh3]+ [B(C6F5)4]
-Ali-Bu3 [CPh3]+ [B(C6F5)4]
-AlHex3 [CPh3]+ [B(C6F5)4]
-AlOct3 [PhMe2NH]+ [B(C6F5)4]
Ali-Bu3 MAOd NAru–(CH2)3–NAru [CPh3]+ [B(C6F5)4]
-Ali-Bu3 NAru–(CH2)3–NAru NAru
CH2PhCH2
NAru MAOd a

Polymerization reactions in liquid propylene at 401C. Scheme (1.3), Ar ¼ 2,6-i-Pr2-C6H3, Aru ¼ 2,6-Me2-C6H3. Fractions soluble in boiling hexane. d [MAO]:[Ti] ¼ 500–1,000. b c

Productivity (kg/mol Ti  h)

II (%) [mmmm]c Tm (1C)

270 211 26 175 42 99 32

49 B0 B0 79 51 27 20

216 32 64

B0 B0 14

–

128.0

0.784 0.833 – 0.807

129.8 131.4 131.7 131.7

–

135.5

174

Alkene Polymerization Reactions with Transition Metal Catalysts

complexes containing the (L)Ti–13CH3 bond shows that the 1-alkene molecules are inserted into the Ti–C bond in the primary orientation. Complexes with multidentate ligands: Homogeneous catalysts based on zirconium complexes LZrBz2 with a tetradentate ligand L ¼
O
Ar
CH2
NR
C2H4
NR
CH2
Ar
O
are isospecific [798–800]. The ligand has two phenoxy groups attached to the hexa-coordinated Zr atom in the trans-arrangement and two nitrogen atoms in the cis-arrangement. These complexes already have (L)Zr–C bonds (Zr–CH2Ph); they can be activated with B(C6F5)3 [798], with combinations of [PhMe2NH]+ [B(C6F5)4]
and AlR3 [799–801], or with MAO [801]. These catalysts polymerize 1-alkenes at room temperature in the quasi-living chain manner (see Section 5.5.1) and produce polymers with Mw/Mn values from B1.1 to 1.5 depending on the type of 1-alkene and reaction conditions. The stereospecificity of these systems depends on the type of alkyl substituents in the ortho-position to the C
O bond in both aryl groups [800,801]. Each of these groups is a phenyl ring containing four substituents, the C
O bond, the CH2
NR group in the ortho-position to the C
O bond, and two alkyl substituents, one in the ortho- and another in the para-position with respect to the C
O bond. Table 3.44 gives several examples of the substituent effects. The presence of a bulky alkyl group in the ortho-position to the C
O bond is essential for the formation of highly isospecific homogeneous catalysts. Poly(1-hexene) produced with the t-Busubstituted catalyst is also isotactic, the [mm] value is B0.95 [798].

3.4.3. Chain insertion reactions in polymerization of alkenes with internal double bonds Alkenes with internal CQC bonds do not form homopolymers in the presence of any Ziegler–Natta catalyst. However, V-based homogeneous catalysts insert 1-alkene molecules into the (L)V
C bond in the secondary orientation (Section 3.4.1.1), which explains a possibility of copolymerization reactions of ethylene and 2-alkenes. Natta described the first example of such reactions, copolymerization of Table 3.44 Isospecificity of catalysts based on complexes LZrBz2 with tetradentate ligands L=
O
Ar
CH2
NR
C2H4
NR
CH2
Ar
O
a [801] Substituents in aryl group

a

ortho-position to C
O bond

para-position to C
O bond

Productivity (kg/mol Zr  CPr  h)

[mmmm]

Tm (1C)

Me t-Bu 1-Adamantyl 9-Antracenyl Cumyl

Me t-Bu Me Me Me

0.9 1.6 4.1 32 70

0.02 0.80 0.985 0.02 0.89

amorph. 123 151 amorph. 136

Cocatalyst MAO, propylene polymerization at 401C, CPr ¼ 1.36 M.

175

Chemistry and Stereochemistry of Polymerization and Copolymerization

2-butene and ethylene in the presence of the VCl4-AlHex3 and the V(acac)3AlEt2Cl systems [802]. The copolymers do not have linked units of 2-butene; the insertion of 2-butene is only possible after ethylene insertion: ðLÞV2CH2 2CH2 2Polymer þ CH3 2CHQCH2CH3 ! ðLÞV2CHðCH3 Þ2CHðCH3 Þ2CH2 2CH2 2Polymer

(3.88)

When these copolymerization reactions are carried out at low temperatures and at a low [ethylene]:[2-butene] molar ratio, the copolymers have a predominantly alternating structure: 2CHðCH3 Þ2CHðCH3 Þ2CH2 2CH2 2CHðCH3 Þ2CHðCH3 Þ2 2CH2 2CH2 2CHðCH3 Þ2CHðCH3 Þ2CH2 2CH2 2 These copolymers can be viewed as a perfect model of ‘‘head-to-head,’’ ‘‘tail-totail’’ polypropylene. A similar insertion of an internal CQC bond into the M–C bond was observed in polymerization of 1,5-heptadiene with the TiCl4-AlEt3 system at very low [Al]:[Ti] ratios [563]. 1,5-Heptadiene has one vinyl and one trans-2-vinylene bond. The polymerization reaction occurs predominantly with the participation of the vinyl bond. However, the second CQC bond also can participate in the reaction by inserting into the Ti
C bond and forming a cyclopentane unit in the main chain: >Ti

Polymer

+

>Ti

Polymer >Ti

Polymer

(3.89)

Only about one third of the internal double bonds take part in the second stage of Reaction (3.89) [563]. The polymer chain after Reaction (3.89) is usually separated from the active center with the formation of either the vinyl bond or a trisubstituted double bond.

3.4.4. Styrene polymerization and copolymerization reactions Several non-metallocene Ti complexes, Ti(OR)4 and (L)TiCl2 and (L)Ti(OR)2 complexes with the bidentate ligand L derived from 2,2u-thio-bis(6-t-Bu-4-Mephenol) [791], when activated with MAO, form very effective catalysts for the syndiospecific polymerization of styrene. Styrene polymers produced by these catalysts are highly regioregular and highly syndiospecific; [rrrr] values are higher than 0.98, which translates into the probability of the syndio-monomer addition p0syndio 40:995. Analysis of ethylene/styrene copolymerization reactions with these catalysts underlines their complexity. Judging by the molecular weight distribution of styrene homopolymers (Mw/Mn ratios from 2.1 to 2.8 [791]), these are mostly single-center catalysts. The ethylene/styrene copolymer prepared with the (L)Ti(OR)2-MAO system at 801C also has a narrow molecular weight distribution, Mw/Mn B1.9 [791]. However, the copolymer consist of two fractions, the syndiotactic homopolymer of styrene and an alternating 1:1 ethylene/styrene copolymer in which the styrene units are in a strictly isotactic arrangement [791].

176

Alkene Polymerization Reactions with Transition Metal Catalysts

Homogeneous non-metallocene catalysts based on Ti complexes (L)TiCl2 with the
O
Ar
CH2
S
C2H4
S
CH2
Ar
O
ligand [761] activated with MAO are effective isospecific catalysts for polymerization of styrene [762]. Active centers in these catalysts insert styrene molecules into the Ti–C bonds strictly in the secondary orientation [762,763] although the same active species insert propylene molecules predominantly in the primary orientation [763]. When styrene and propylene are copolymerized with this catalyst, the product is a peculiar styrene/ propylene block-copolymer containing relatively long isotactic crystallizable polystyrene blocks and relatively short polypropylene blocks [763]. Because the regiochemistry of chain growth reactions for propylene and styrene is opposite, the blocks are connected by head-to-head propylene-styrene links (–CHMe–CHPh–) and tail-to-tail styrene-propylene links (–CH2–CH2–), ð2CHPh2CH2 Þn 2CHPh2CH2 2CH2 2CHMe2 ðCH2 2CHMeÞm ð2CHPh2CH2 Þn 2

3.5. Homogeneous Catalysts Based on Late-Period Transition Metals This group of catalysts includes the systems based on complexes of three lateperiod transition metals (Ni, Fe, and Pd) with bidentate and tridentate ligands. These catalysts either do not need any activators (cocatalysts) or employ the same activators as metallocene catalysts (MAO, ion-forming cocatalysts). The chemistry, the regiochemistry, and the stereochemistry of alkene polymerization reactions with these catalysts vary greatly depending on the nature of the metal and the type of the ligand.

3.5.1. Regiochemistry of chain initiation and chain growth reactions Ethylene oligomerization reactions with Ni ylide catalysts: Keim described same of the earliest examples of alkene polymerization reactions catalyzed by soluble derivatives of late-period transition metals, ethylene oligomerization reactions with squareplane Ni complexes containing bidentate ylide ligands (Scheme 1.3) [87,810]. The initial catalytic species in these catalysts contain the (L)Ni
H bond [810] (Section 6.2.2). GC analysis of the oligomers affords identification of each short ‘‘polymer’’ molecule and the measurement of its relative yield. Two reactions are sufficient to describe the reaction scheme [158,811], the chain growth reaction ðLÞNi2ðCH2 2CH2 Þn 2H þ CH2 QCH2 ! ðLÞNi2ðCH2 2CH2 Þnþ1 2H

(3.90)

and the chain transfer reaction to ethylene ðLÞNi2ðCH2 2CH2 Þn 2H þ CH2 QCH2 ! ðLÞNi2CH2 2CH3 þ CH2 QCH2ðCH2 2CH2 Þn
1 2H

(3.91)

177

Chemistry and Stereochemistry of Polymerization and Copolymerization

The expected products of the oligomerization reactions are linear 1-alkenes with even carbon-atom numbers and with vinyl double bonds as last chain ends [158,811]. The distribution of the oligomers with respect to the number of monomer units in their chains provides an estimation of the ratio of rate constants in Reactions (3.90) and (3.91), which ranges from B4 at 501C to B9 at 901C. Ni ylide complexes do not polymerize or oligomerize any alkene except for ethylene. However, they co-oligomerize ethylene with many 1-alkenes [116,117]. The regioselectivity of the chain initiation step with the participation of a 1-alkene molecule, its insertion into the (L)Ni
H bond, is quite poor [116,158], both the primary insertion producing the (L)Ni
CH2
CH2
R centers and the secondary insertion producing the (L)Ni
CH(CH3)R centers were identified. However, chain growth steps of 1-alkenes, insertion into the (L)Ni
CH2 bond, mostly proceed in the primary mode and if the secondary insertion occurs it is invariably followed by the chain transfer reaction [116,117]. Propylene oligomerization reactions with Ni diimine catalysts: a-Diimine complexes of NiII (Scheme 1.3) activated with MAO polymerize propylene at low temperatures to syndiotactic polymers [767,812–815]. When 13C-labeled AlMe3 was added to MAO, the chemistry of the chain initiation step became observable [813]. Propylene insertion into the (L)Ni
CH3 bond at
451C proceeds predominantly in the primary mode: ðLÞNi213 CH3 þ CH2 QCH2CH3 ! ðLÞNi2CH2 2CHðCH3 Þ213 CH3

(3.92)

It is followed by primary chain growth steps. Both the regiochemical and the stereochemical regularity of polypropylene prepared with these catalysts are quite poor even at –451C (see Table 3.45), it deteriorates further at 01C [813], and at +501C regio-irregular and stereo-irregular polymers are formed [814]. The observation of last end-groups in these polymers became possible when polypropylene of a low molecular weight was prepared at
451C at a low monomer concentration [814]. A small content of vinylidene bonds in the polymer indicates Table 3.45 Structure of polypropylene produced with catalysts based on complexes (L)NiBr2 with a-diimine ligands La (Scheme 1.3) [815]

a

Ortho-substituents in aryl rings

Mw/Mn

Regio-inverted units (%)

3,1-Inserted units (%)

[rr]

Me, Me i-Pr, i-Pr Me, i-Pr (racemic complex) Me, t-Bu (racemic complex) Me, t-Bu (meso-complex)

2.3 1.7 2.5

23 7 24

o1 o1 3

0.61 0.75 0.25

1.8

8

14

0.33

2.2

10

20

0.66

Cocatalyst MAO, propylene polymerization in liquid monomer at
451C.

178

Alkene Polymerization Reactions with Transition Metal Catalysts

the b-H transfer reaction to the catalyst (similar to Reactions (3.15), (3.27), and (3.83)). However, most of polymer chains remains attached to the active species at this temperature and can be decomposed with alcohols with the formation of the isobutyl group typical for the primary orientation of propylene units. Polymerization reactions with Co and Fe bis(imino)pyridyl catalysts: Other thoroughly examined catalyst systems containing late-period transition metals include Fe and Co complexes with bis(imino)pyridyl ligands (Scheme 1.3). These catalysts produce linear ethylene polymers of a very low molecular weight, CH2QCH–(CH2– CH2)n–C2H5 [816–818]. Kinetic data show (Section 5.5.4) that the principal chain transfer reaction in these polymerization reactions is that to ethylene. If Ali-Bu3 is a part of the cocatalyst, the isobutyl chain end is formed in the chain transfer reaction to cocatalyst [816]. Propylene polymerization reactions with bis(imino)pyridyl complexes of Fe at 0 to
201C produce head-to-tail enchained polymers with a narrow molecular weight distribution, Mw/Mn B1.8–2.4 [819,820]. Detailed NMR analysis of chain ends in these polymers provided information about each stage of the polymerization reactions [819,820]. The principal chain growth reaction is standard, secondary insertion of a propylene molecule into the (L)Fe–C bond. If a propylene molecule is coordinated at the Fe atom in the primary orientation, a chain transfer reaction takes place, the b-H transfer from the methyl group of the last monomer unit to the coordinated propylene molecule: ðLÞFe2CHðCH3 Þ2CH2 2Polymer þ CH2 QCH2CH3 ! ðLÞFe2CH2 2CH2 2CH3 þ CH2 QCH2CH2 2Polymer

(3.93)

The next propylene molecule inserts into the (L)Fe
CH2 bond formed in Reaction (3.93) in the secondary position, as in all chain growth steps: ðLÞFe2CH2 2CH2 2CH3 þ CH3 2CHQCH2 ! ðLÞFe2CHðCH3 Þ2CH2 2CH2 2CH2 2CH3

(3.94)

The polymer molecules usually contain the n-butyl group as the starting chain end formed in Reaction (3.94) and the allyl (1-propenyl) group as the last chain end formed in Reaction (3.93). Chain transfer reactions to AlR3 lead to the second type of the starting chain end, (L)Fe
CH(CH3)
CH2
R, the n-propyl group in the case of AlEt3 or the 2-methylbutyl group in the case of Ali-Bu3 [820].

3.5.2. Stereochemistry of chain growth reactions When a-diimine complexes of NiII (Scheme 1.3) activated with MAO are used in propylene polymerization reactions at low temperatures, they produce crystalline syndiotactic polymers [rr] values are B0.8 at
781C, 0.72–0.74 at
451C, and B0.65 at 01C [812–814]. In terms of the Bernoullian statistics of imperfectly syndiotactic chains (Table 3.4), these [rr] values correspond to the probability of syndiotactic monomer linking p0syndio from 0.89 to 0.80. Pellecchia found that both the mechanism and the level of stereocontrol in these polymerization reactions depend on the structure of the a-diimine ligand and on the

179

Chemistry and Stereochemistry of Polymerization and Copolymerization

overall symmetry of the complex [815]. The results are shown in Table 3.45. When both ortho-positions in each phenyl group contain identical substituents (the first two examples in the table), the complexes have C2v symmetry. The chain-end stereocontrol mechanism is responsible for the preferred syndiotactic linking of propylene units in these catalysts and it can be very effective, the p0syndio value for the polymer produced with the complex containing four isopropyl groups is B0.87. This stereocontrol mechanism is also confirmed by molecular modeling analysis [821]. If a small quantity of ethylene is added to the propylene polymerization reactions, isolated ethylene units at the ends of prevailingly syndiotactic blocks of propylene units do not disrupt the stereo-regulating ability of the growing polymer chain [767]. When ortho-positions in each phenyl group contain different substituents (the last three examples in Table 3.45), the complexes are asymmetric and can exist in two isomeric forms. The racemic isomers produce atactic polypropylene whereas the meso-isomers produce moderately syndiotactic polymers. Propylene polymerization reactions with Fe bis(imino)pyridyl complexes proceed via the secondary insertion of propylene molecules into the (L)Fe–C bonds (similar to Reaction (3.94)) [819,820] and are moderately isospecific, a unique example among propylene polymerization reactions with homogeneous transition metal catalysts of this type. Table 3.46 gives several examples of the catalyst stereospecificity. NMR analysis of steric mistakes in the polymer chains ([mmmr] ¼ [mmrm]) shows that the chain-end stereocontrol mechanism operates in these reactions [819,820]. The data in Table 3.46 show that the level of stereocontrol (represented by the probability of isotactic linking p0iso ) decreases with temperature, it is only slightly affected by the steric bulk of ortho-substituents in both phenyl groups in the complexes, and it is not affected by the symmetry type of the ligands.

3.5.3. Chain isomerization reactions In general, homogeneous catalysts based on late-period transition metals have a pronounced tendency to form isomerized polymer chains. In its simplest form, this Table 3.46 Isospecificity of catalysts based on Fe bis(imino)pyridyl complexes (Scheme 1.3) in propylene polymerization reactionsa [819,820]

a

ortho-Substituents X1 and X2 in aryl rings

Temp. (1C)

i-Pr, i-Pr; i-Pr, i-Pr ‘‘-’’ ‘‘-’’ ‘‘-’’ Me, t-Bu; Me, i-Pr i-Pr, i-Pr; Me, Me i-Pr, Me; i-Pr, Me i-Pr, i-Pr; i-Pr, Me

50 20 0
20
20
20
20
20

Cocatalysts MAO and MMAO, PPr ¼ 1 atm. Definition of P 0iso is in Table 3.2.

b

Mw/Mn

[mm]

[mmmm]

P 0iso b

Reference

0.55 0.59 0.56 0.67 0.59

0.77 0.81 0.83 0.86 0.88 0.87 0.91 0.88

[819] ‘‘-’’ ‘‘-’’ [820] ‘‘-’’ ‘‘-’’ ‘‘-’’ ‘‘-’’

0.59 0.66 0.69 2.1 2.2 2.2 1.9 1.8

180

Alkene Polymerization Reactions with Transition Metal Catalysts

reaction is observed in ethylene polymerization reactions with catalysts based on a-diimine complexes of Ni and Pd [30,822–825]. When the reaction catalyzed by a combination of a Ni a-diimine complex and AlEtCl2 is carried out at 01C and at PE ¼ 15 atm, an essentially linear polymer with a melting point of 1331C is formed. However, when the same reaction is carried out at 201C and at PE ¼ 1 atm, the polymer is highly branched and its melting point is decreased to 931C [825]. Main branches in the ethylene polymers are methyl: ðLÞMþ 2R þ n CH2 QCH2 (3.95) ! ðLÞMþ 2ðCH2 2CH2 Þx 2CHðCH3 Þ2ðCH2 2CH2 Þy 2Polymer Their content is B4.5 mol.% at 501C and W7 mol.% at 701C. Other branches are linear, from ethyl to n-hexyl; they are present in the amount from 0.1 to B1 mol.% [823,826,827]. Brookhart explained the formation of methyl branches by a series of b-H elimination/alkene reinsertion steps, the process called ‘‘chain walking:’’ ðLÞMþ 2CH2 2CH2 2CH2 2CH2 2Polymer (3.96) ! ðLÞðHÞMþ    CH2 QCH2CH2 2CH2 2Polymer ðLÞðHÞMþ    CH2 QCH2CH2 2CH2 2Polymer ! ðLÞMþ 2CHðCH3 Þ2CH2 2CH2 2Polymer ðLÞMþ 2CHðCH3 Þ2CH2 2CH2 2Polymer þ CH2 QCH2 ! ðLÞMþ 2CH2 2CH2 2CHðCH3 Þ2CH2 2CH2 2Polymer

(3.97)

(3.98)

The mechanism of these reactions is discussed in Section 6.2.4. Active centers in catalysts derived from a-diimine complexes of Ni and Pd preferably insert coordinated 1-alkene molecules into the (L)M
H bond in the secondary orientation (Reaction (3.97)); the step that explains the generation of branches in the polyethylene chains. If the b-H elimination step occurs after Reaction (3.97), it usually involves the b-CH2 group, and the subsequent reinsertion step produces the ethyl branch, etc: ðLÞMþ 2CHðCH3 Þ2CH2 2CH2 2Polymer (3.99) ! ðLÞMþ 2H    CH3 2CHQCH2CH2 2Polymer ðLÞMþ 2H    CH3 2CHQCH2CH2 2Polymer ! ðLÞMþ 2CHðCH2 CH3 Þ2CH2 2Polymer

(3.100)

A competition of chain walking steps (Reactions (3.96) and (3.37)) and standard ethylene insertion steps in ethylene polymerization reactions catalyzed by Ni and Pd a-diimine complexes [828–830] and Ni complexes of a similar type [831] produces polymer chains with a complex branching pattern. When the polymerization reactions are carried out at high ethylene partial pressures, most of the branches are isolated methyl branches although isolated ethyl and longer linear branches are also formed. In addition, several types of branches separated by two CH2 groups

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Chemistry and Stereochemistry of Polymerization and Copolymerization

Table 3.47 Types of branches in ethylene homopolymers produced with homogeneous catalyst based on Pd a-diimine complexa at 351C [828]

a

PE (atm) Branches (mol. %)

0.1

1.0

7

34

Methyl Ethyl n-Propyl n-Butyl n-Pentyl n-Hexyl+ Total

7.8 5.8 0.6 2.4 0.7 7.3 24.4

7.9 5.9 0.6 2.4 0.5 7.3 24.6

8.0 5.4 0.6 2.4 0.6 6.8 23.6

8.3 4.6 0.9 1.9 0.9 6.4 22.8

Scheme 1.3, i-Pr substituents in all ortho-positions.

are present,
CH(CH3)
CH2
CH2
CH(R)
, where R is either CH3 or a linear alkyl group [830]. Their most probable formation mechanism is a chainwalking step followed by two ethylene insertion steps and by another chain walking step. Ethylene polymers prepared with Pd a-diimine complexes have a much higher degree of branching [828,829], as shown in Table 3.47. At the limit, these polymers are completely amorphous and soluble in common organic solvents under ambient conditions [828]. Detailed rheological and NMR analysis of these polymers showed that the ethylene concentration does not affect much the branching frequency as such (see Table 3.47) but it strongly influences the architecture of the branched chains [828,829]. When the polymerization reactions are carried out at an ethylene partial pressure of 10 kPa (B0.1 atm), branch-on-branch structures are formed, which makes macromolecules more compact and greatly affects their rheological properties [828,829]. Stochastic simulations of chain growth/branching reactions in these reactions showed that the topology of the branched polyethylene chains is quite complex [832]. A variety of chain structures can be formed depending on reaction conditions, from predominantly linear macromolecules with occasional short branches of different lengths (see Table 3.47) to exceptionally highly branched structures resembling star-shaped macromolecules. Polymerization reactions of propylene and 1-butene with a-diimine complexes of Ni and Pd under ambient conditions can produce polymers of a radically different nature, mostly depending on the type of substituents in the ortho-positions of phenyl groups attached to the nitrogen atoms and the type of ligand X (Scheme 1.3). The polymerization products vary from mostly regioregular, moderately syndiotactic polymers to highly branched macromolecules with chain structures having no resemblance to ‘‘normal’’ polypropylene or poly(1-butene). These polymers contain short
(CH2)n
sequences, adjacent methyl side-groups,
CH(CH3)
CH(CH3)
, isobutyl side-groups, and other n-alkyl and isoalkyl side-groups of different sizes [346,833–835]. The same catalysts polymerize 1-alkenes with linear alkyl groups (1-butene, 1-pentene, 1-hexene, 1-nonene, etc.) in unusual 1,o- and 2,o-linking patterns

182

Alkene Polymerization Reactions with Transition Metal Catalysts

called chain-straightening reactions [834–838] (the carbon atom in the methyl group is marked to emphasize the isomerization pattern): CH2 QCH2ðCH2 Þn 2C H3 2ðCatalystÞ! ½2CH2 2CH2 2ðCH2 Þn 2C H2 2x CH2 QCH2ðCH2 Þn 2C H3 2ðCatalystÞ! ½2CHðCH3 Þ2ðCH2 Þn 2C H2 2x

(3.101)

(3.102)

In both these reactions, the o-methyl group in the 1-alkene molecule is converted into a CH2 group and some of the CH2 groups in the vinyl bonds are converted into methyl groups attached to linear polymer chains. The explanation of the chain-straightening reactions is based on the mechanisms of chain growth steps typical for polymerization reactions of other alkenes with these catalysts [836]. The first step in the overall Reaction (3.101) is the secondary insertion of a 1-alkene into the (L)M+
C bond [836]: ðLÞMþ 2CH2 2Polymer þ CH2 QCH2ðCH2 Þn 2C H3 ! ðLÞMþ 2CH½2ðCH2 Þn 2C H3 2CH2 2CH2 2Polymer

(3.103)

The active center (L)M+
CHRuRv with the a-branched polymer chain formed in Reaction (3.103) has very poor reactivity in secondary insertion reactions of 1alkene molecules. However, this species readily undergoes chain migration through the side-group of the last monomer unit. Each migration step to the next CH2 group recreates a center with the a-branched polymer chain until the metal atom reaches the methyl group: ðLÞMþ 2CH½2ðCH2 Þn 2C H3 2CH2 2CH2 2Polymer ! ðLÞMþ 2C H2 2ðCH2 Þn 2CH2 2CH2 2CH2 2Polymer

(3.104)

The active center formed in Reaction (3.104) imposes no restriction on the next secondary insertion step of a 1-alkene molecule, and the cycle represented by Reactions (3.103) and (3.104) continues. The formation of methyl side-groups in these polymer chains (Reaction (3.102)) is explained by a similar sequence of steps, which begin with the primary 1-alkene insertion into the (L)M
CH2 bond instead of the secondary insertion in Reaction (3.103). Final polymer products formed in Reactions (3.101) and (3.102) resemble ethylene/propylene copolymers [30,836]. The length of ‘‘polyethylene’’ blocks in these polymer chains increases with the length of the alkyl substituent in the monomer. When linear 1-alkenes are polymerized at low temperatures, these ‘‘polyethylene’’ blocks are quite long and the polymers become partially crystalline, with melting points between 40 and 601C [836]. The same chain walking mechanism explains a peculiar structure of polymers prepared from cis- and trans-2-butene with the catalysts based on Ni a-diimine complexes [839,840]. Polymerization reactions at 01C and 251C produce high molecular weight polymers with a narrow molecular weight distribution. The

Chemistry and Stereochemistry of Polymerization and Copolymerization

183

polymers contain B250 methyl branches per 1,000 carbon atoms and their NMR analysis indicates that the principal structure of monomer units in these polymers is –CH2–CH2–CH(CH3)–. Each chain growth step starts with insertion of a 2-butene molecule into the (L)Ni–C bond: ðLÞNiþ 2C þ CH3 2CHQCH2CH3 (3.105) ! ðLÞNiþ 2CHðCH3 Þ2CHðCH3 Þ2C No further monomer insertion is possible at this point due to steric reasons. However, the migration of the Ni atom to the closest methyl group relieves the congestion ðLÞNiþ 2CHðCH3 Þ2CHðCH3 Þ2C (3.106) ! ðLÞNiþ 2CH2 2CH2 2CHðCH3 Þ2C and makes the next insertion step possible. An occasional migration of the Ni atom to the g-methyl group produces a small number of ethyl branches [839,840].

3.6. Chromium-Based Catalysts 3.6.1. Chromium oxide catalysts Chromium oxide catalysts differ little from Ti-based Ziegler–Natta catalysts in terms of the chemistry of alkene polymerization reactions, as IR, 13C NMR, and oligomer GC data demonstrate. Ethylene homopolymers prepared with chromium oxide catalysts are strictly linear. Low molecular weight fractions of ethylene homopolymers have a simple structure similar to that of ethylene homopolymers produced with Ziegler–Natta catalysts, CH2QCH
(CH2
CH2)n
CH2
CH3. However, as the discussion of the polymerization mechanism in Section 6.4.2 shows, no definitive conclusion is reached yet which of the two chain ends in these polymer molecules is the starting chain end and which is the last chain end. IR and 13C NMR analysis of ethylene/1-alkene copolymers produced with chromium oxide catalysts shows a lack of strict regioselectivity in 1-alkene insertion reactions into the Cr
C bond. Both modes of 1-alkene insertion are possible, primary and secondary. As a consequence, when ethylene/1-alkene copolymerization reactions with chromium oxide catalysts are carried out at low 1-alkene concentrations, chain transfer reactions to ethylene leave two types of double bonds (in addition to the vinyl bond) in the copolymer molecules, CH2QCR
Polymer and R
CHQ CH
Polymer [841,842].

3.6.2. Organochromium catalysts The chemistry of alkene polymerization reactions with supported organochromium catalysts varies depending on the type of organochromium compound used for the catalyst synthesis (Section 4.2.3.2). Catalysts produced by reacting Cp2Cr with silica do not undergo noticeable chain transfer reactions to a monomer or b-H elimination reactions, which are so prominent in Ziegler–Natta and

184

Alkene Polymerization Reactions with Transition Metal Catalysts

metallocene catalysis. However, these catalysts show high reactivity toward hydrogen [843–846]: ðLÞCr2CH2 2CH2 2Polymer þ H2 ! ðLÞCr2H þ CH3 2CH2 2Polymer

(3.107)

These catalysts have relatively poor copolymerization ability. Low molecular components of ethylene/1-hexene copolymers (co-oligomers) produced with these catalysts in the presence of hydrogen consist mainly of n-alkanes with even numbers of carbon atoms, H
(CH2
CH2)n
H (the products of Reaction (3.107)). Small amounts of isoalkanes H
CH2
CH(C4H9)
(CH2
CH2)n
1
H in these oligomers are formed in a series of chain growth reactions of ethylene molecules culminating with the primary insertion of a 1-hexene molecule into the Cr
C bond and followed by the hydrogenation reaction similar to Reaction (3.107) [842]. However, if Cp2Cr in these organochromium catalysts is replaced with its openchain analog containing two 2,4-dimethylpentadienyl ligands, the b-H elimination reaction becomes the dominant chain transfer reaction ðLÞCr2CH2 2CH2 2Polymer ! ðLÞCr2H þ CH2 QCH2Polymer

(3.108)

although the chain transfer to hydrogen, Reaction 3.107, is still quite effective [846]. Two types of supported organochromium catalysts, Cp2Cr/silica [841,842,846] and silyl chromate catalysts [847,848], exhibit very infrequent chain-walking reactions of active centers in ethylene homopolymerization reactions, the same as Reaction 3.95. They produce a small number of methyl and n-butyl branches in the polymer chains. The probability of chain walking increases with temperature, polyethylene produced with the Cp2Cr/SiO2 catalyst at 901C contains a small number of branches, 1.7 CH3/1,000C, but at 1401C their number increases to B16 CH3/1,000C.

3.7. Stereoselective and Stereoelective Polymerization Reactions of Branched 1-Alkenes A significant body of literature in the field of Ziegler–Natta and metallocene catalysis is devoted to polymerization reactions of branched 1-alkenes with chiral carbon atoms in their alkyl substituents. These polymers have no special practical significance and the interest in these reactions is driven by their ability to reveal an important feature of Ziegler–Natta catalysts, ability of their active centers to distinguish the type of the chiral carbon atom in a 1-alkene molecule [849]. The three most studied monomers of this type are

3-methyl-1-pentene

4-methyl-1-hexene and 3,7-dimethyl-1-octene

Chemistry and Stereochemistry of Polymerization and Copolymerization

185

These 1-alkenes have much lower reactivity than propylene (Section 3.2.2.1). The content of crystalline fractions in their polymers is similar to that in polypropylene [471]. When 1-alkenes with chiral carbon atoms are produced by standard synthetic routes, they always exist as mixtures of two optically active enantiomers (usually designated as CH2QCH
R and CH2QCH
S) in equal amounts. However, special synthetic routes afford the synthesis of some pure enantiomers. (When chiral monomers are polymerized with stereospecific Ziegler–Natta or metallocene catalysts, the nomenclature of the respective stereoregular polymers becomes a complicated subject [850].) If an optically inactive equimolar mixture of two enantiomers is polymerized with a particular catalyst, three different limiting outcomes can be envisaged with respect to the distribution of stereomer units in polymer chains, as shown in Scheme 3.1. 1. An active center cannot distinguish between CH2QCH
R and CH2QCH
S enantiomers. Consequently, each polymer molecule is a statistical copolymer of
CH2
CHR
and
CH2
CHS
units. The main chain of such a polymer can be either isotactic, or syndiotactic, or atactic (Equation (3.109)). 2. An active center perfectly distinguishes between two enantiomers and produces two types of stereoregular polymer molecules. One type contains only
CH2
CHR
units and another only
CH2
CHS
units, as shown in Equation (3.110). In principle, polymer mixtures described by Equation (3.110) can be separated into two fractions with the same physical characteristics but with opposite signs of optical rotation in solution. The polymerization mechanism that leads to these polymer mixtures is called stereospecific stereoselective. It should be noticed that no straightforward correlation exists between the sign of optical rotation of a 1-alkene with a chiral carbon atom and the monomer unit produced from it. For example, (S)-4-methyl-1-hexene has the (
) rotation and produces the isotactic polymer with the (+) rotation. On the other hand, 1. The absence of stereoselection: random “copolymer” –CH2–CHR*–CH2–CHR*–CH2–CHS*–CH2–CHR*–CH2–CHS*–CH2–CHS*–CH2–CHR*––

(3.109)

2. Stereospecific stereoselective polymerization: mixture of two “homopolymers” –CH2–CHR*–CH2–CHR*–CH2–CHR*–CH2–CHR*–CH2–CHR*–CH2–CHR*–CH2–CHR*– and –CH2–CHS*–CH2–CHS*–CH2–CHS*–CH2–CHS*–CH2–CHS*–CH2–CHS*–CH2–CHS*–

(3.110)

3. Stereospecific stereoelective polymerization: “homopolymer” and unreacted enantiomer –CH2–CHR*–CH2–CHR*–CH2–CHR*–CH2–CHR*–CH2–CHR*–CH2–CHR*–CH2–CHR*– + unreacted CH 2=CH-S*

Scheme 3.1 CH
S.

(3.111)

Polymerization reactions in monomer mixture CH2QCH
R+CH2Q

186

Alkene Polymerization Reactions with Transition Metal Catalysts

(S)-3,7-dimethyl-1-octene has the (+) rotation and also produces the isotactic polymer with the (+) rotation [851]. 3. An active center perfectly distinguishes between the enantiomers; it polymerizes only one of them and leaves an unreacted monomer mixture enriched with the second enantiomer. At its limit, this polymerization reaction can be represented by Equation (3.111). This polymerization mechanism is called stereospecific stereoelective. All three cases of the polymerization reactions shown in Scheme 3.1 were observed in experiment.

3.7.1. Stereoselective polymerization reactions with Ziegler–Natta catalysts An ability to determine whether the stereoselective mechanism operates in a particular polymerization reaction relies on a possibility of separation of two polymer fractions represented by Equation (3.110) in Scheme 3.1. Two separation processes were developed, solvent extraction of a polymer supported on a special substrate and the elution chromatography of adsorbed polymers. Both techniques utilize a highly crystalline, optically active polymer, poly(S-3-methyl-1-pentene) [852,853]. Neither of the separation methods is perfect [849]; in each case, the separation efficiency is at best between 20 and 40%. Nevertheless, these separation methods provided the first unambiguous proof that some degree of stereoselection is common in heterogeneous and in pseudo-homogeneous Ziegler–Natta catalysis. Table 3.48 gives several examples of stereoselection. The stereoselection ratios in the table give the probability ratio of perfect stereo-selection leading to the formation of structures shown in Equation (3.110) in Scheme 3.1 vs. a random mixture of two stereomers in a single polymer chain (Equation (3.109)). In many respects, the stereoselection ratio is similar to the reactivity ratio in copolymerization reactions (Section 3.8.1). A perfect separation of 1-alkene enantiomers into two polymer molecules would correspond to the infinite stereoselection ratio while Table 3.48 Stereoselection in polymerization of racemic alkene mixtures with Ziegler–Natta catalysts [849,851–854] Monomer

Catalyst

Stereoselection ratio

Effect of 1-alkene structure 3,7-Dimethyl-1-octene 4-Methyl-1-hexene 5-Methyl-1-heptene

TiCl4-Ali-Bu3 TiCl4-Ali-Bu3 TiCl4-Ali-Bu3

W20 1.5 1

Effect of catalyst 4-Methyl-1-hexene ‘‘-’’ ‘‘-’’ ‘‘-’’

TiCl4-Ali-Bu3 a-TiCl3-Ali-Bu3 TiCl4/MgCl2-AlEt3 TiBz4-AlBz3

1.50 1.38 1.31 1.04

Chemistry and Stereochemistry of Polymerization and Copolymerization

187

a complete lack of stereoselection corresponds to the stereoselection ratio of 1. The data in Table 3.48 show that the stereoselection efficiency strongly depends on the proximity of the chiral carbon atom in the side-group of a 1-alkene molecule to its double bond. When the branching point is in the a-position to the double bond, in 3,7-dimethyl-1-octene or in 3-methyl-1-pentene, the stereoselection degree is quite high [849,855]. If the chiral carbon atom is separated by one CH2 group from the double bond, in 4-methyl-1-hexene, the degree of stereoselection is low. These polymers can be viewed as ‘‘copolymers’’ containing more of one 1-alkene stereomer than the other. Finally, if two or three CH2 groups separate the chiral carbon atom and the double bond, in 5-methyl-1-heptene, no stereoselection was observed [854], corresponding to the structure in Equation (3.109). The type of catalyst also influences the stereoselection level. Solid and pseudohomogeneous Ti-based catalysts, irrespective of their exact composition, exhibit approximately the same degree of stereoselection (Table 3.48) whereas the homogeneous TiBz4-AlBz3 system shows virtually no ability for stereoselection. Homogeneous metallocene catalysts based on C2H4(Ind)2ZrCl2 and Me2Si(Ind)2ZrCl2 do not possess any stereoselection ability in polymerization reactions of racemic 3-methyl-1-hexene at 501C [867] but exhibit a significant level of stereoselectivity in polymerization of racemic 4-methyl-1-hexene at 01C [868,869]. In general, two different effects can explain the phenomenon of stereoselection. The first one is a steric interaction between the last monomer unit in the polymer chain and a coordinated 1-alkene molecule. In this case, the last monomer unit in the chain, e.g., Ti
CH2
CHR
Polymer, determines the preference for CH2QCH
R insertion vs. CH2QCH
S insertion, and vice versa. The second possible source of stereoselection is the active center itself. In this case, centers of two types are present in the catalyst in equal numbers, one preferentially polymerizes the CH2QCH
R alkene and another preferentially polymerizes the CH2QCH
S alkene, irrespective of the structure of the last monomer unit in polymerization chains attached to the centers. One way to choose between these two alternative mechanisms is to carry out a copolymerization reaction of a branched 1-alkene with ethylene and to determine whether the stereoselection ability is decreased when the active center and the last chiral monomer unit in the chain are separated by one ethylene unit, WTi
CH2
CH2
CH2
CHR
Polymer. An experimental investigation of these possible mechanisms, by copolymerizing ethylene with 3,7-dimethyl-1-octene and with 4-methyl-1-hexene, showed that the introduction of 20–40% of ethylene in the polymer molecules does not decrease the degree of stereoselection [856]. These findings support the hypothesis that the active centers themselves rather than the polymer chains attached to them guide the stereoselection process. Additional support for the mechanism assigning stereoselection to the active centers came from a 13C NMR study of end-groups in polymers of 3-methyl-1pentene [857,858]. The precisely measured stereoselection ratio for 3-methyl-1pentene was B2.5, less than for 3,7-dimethyl-1-octene (see Table 3.48) but higher than for 4-methyl-1-hexene. NMR results also showed that the degree of

188

Alkene Polymerization Reactions with Transition Metal Catalysts

stereoselection does not depend on the length of the polymer chain attached to the active center [857]. It appears that the stereoselectivity of heterogeneous Ziegler– Natta catalysts is derived from a specific steric interaction between their active centers and alkyl groups of approaching alkene molecules. In terms of the difference in the activation energy, the stereoselection preference is small, B0.4 kcal/mol, and all these polymers are in reality ‘‘copolymers’’ of CH2QCH
R and CH2QCH
S alkenes with two to three times higher contents of one enantiomer vs. the other.

3.7.2. Stereoelective polymerization reactions with Ziegler–Natta and metallocene catalysts As the results in the preceding section show, Ziegler–Natta catalysts contain two types of active centers in equal proportions, one type of center preferentially polymerizes CH2QCH
R enantiomers and the other CH2QCH
S enantiomers. As a consequence, the stereoselection process produces equimolar mixtures of polymer molecules with a preferential incorporation of one of the two enantiomers. However, if the reactivity of one type of the center can be suppressed, or the reactivity of another type of active center enhanced, one can expect to carry out stereoelective polymerization, one of the alkene enantiomers will be preferentially polymerized and another enantiomer left as a monomer. The first definite proof that this modification of the catalysts is indeed possible came from studies of supported catalysts [471,859]. When a supported TiCl4/MgCl2 catalyst was activated with a combination of Ali-Bu3 and an optically active Modifier II, (
)-menthyl anisate, instead of usually used ethyl benzoate, the catalyst system became modestly stereoelective [471]. Two examples of these polymerization reactions are given in Table 3.49. The stereoelection effect is small, much lower that stereoselection effects with the same monomers in Table 3.48. A similar stereoelection effect was observed in polymerization reactions of 3,5,5-trimethyl-1-hexene, which also has a chiral carbon atom at the a-position to the double bond. When this monomer was polymerized with the a-TiCl3-Ali-Bu3 system modified with (
)-L-menthol, the unreacted alkene acquired positive optical rotation [a]D22 ¼ +3.241 [860]. Aspecific centers do not exhibit the stereoelection effect and polymerize both stereomers randomly. Table 3.49 Stereoelective polymerization of racemic alkene mixtures with Ziegler–Natta catalyst containing chiral Modifier IIa [471]

a

Recovered monomer

Crystalline polymer

Monomer

Chirality

Optical purity (%)

Chirality

Optical purity (%)

3,7-Dimethyl-1-octene 4-Methyl-1-hexene

S S

1.34 2.70

R R

3.58 1.67

Catalyst system TiCl4/MgCl2-Ali-Bu3/(
)-menthyl anisate.

Chemistry and Stereochemistry of Polymerization and Copolymerization

189

A much higher level of stereoelection in polymerization reactions of 3,7dimethyl-1-octene, although still modest on the absolute scale, was achieved when Modifier I, (
)-menthyl anisate, rather than Modifier II contained a chiral carbon atom [861]. The highest level of stereoelection, a 6.6% enrichment of the unreacted monomer, was accomplished when two chiral esters were used, (
)-menthyl anisate as Modifier I and (
)-dimenthyl terephthalate as Modifier II. A comparison of stereoelective effects for two Modifiers I, derivatives of (S)-2-phenylbutanoic acid and (R)-3,7-dimethyl-6-octanoic acid, showed that the level of stereoelectivity is determined primarily by the proximity of the chiral carbon atom in the modifier to the Ti atom in an active center [862]. The second experimental approach to inducing the stereoelection is based on a hypothesis that this effect can be caused by a steric interaction between the last monomer unit in a growing polymer chain and the inserting 1-alkene molecule. This hypothesis was tested in copolymerization reactions of two 1-alkenes containing chiral carbon atoms, one a single enantiomer CH2QCH
R 1 and  another a racemic mixture of CH2QCH
R 2 and CH2QCH
S2 . In the ultimate case of perfect stereoelection, one of the formed polymers in such monomer mixtures would be a random copolymer of CH2QCH
R 1 and  CH2QCH
R 2 and another product the homopolymer of CH2QCH
S2 [863– 866]. Two series of experiments with such mixtures were carried out. In one series, mixtures of a pure enantiomer, (R)-3,7-dimethyl-1-octene, and a racemic mixture of (R)(S)-3-methyl-1-pentene were copolymerized with the TiCl3-Ali-Bu3 system [863]. In another series of experiments, mixtures of racemic (R)(S)-3,7-dimethyl-1octene and pure (S)-3-methyl-1-pentene were copolymerized with a highly stereospecific catalyst of the TiCl4/Modifier I/MgCl2 type activated with combinations of Ali-Bu3 and chiral aromatic esters [864]. Both experiments unambiguously confirmed the stereoelection effect; each reaction produced two polymer fractions with opposite signs of optical rotation. However, the stereoelection level was quite low: two populations of copolymer molecules were formed, one enriched with monomer units of the same chirality and another enriched with the remaining antipode [864–866]. Homogeneous isospecific metallocene catalysts produced from bridged complexes with chiral ligands also exhibit some degree of stereoelectivity [870]. When a metallocene complex C2H4(Ind)2Zr[O-acetyl-(R)-mandelate]2 activated with MAO was used in polymerization reactions of racemic 4-methyl-1-hexene at 25 and 501C, the unreacted monomer became enriched with the R enantiomer and the polymer with the S enantiomer. These results do not submit to an easy interpretation. According to the generally accepted mechanism of alkene polymerization reactions with metallocene catalysts (discussed in Chapter 6), s-bonded ligands in metallocene complexes, like acetyl-(R)-mandelate in this example, are rapidly detached from the complexes in a reaction with cocatalysts and, therefore, they cannot directly affect the active centers. Moreover, when the polymers produced with this metallocene catalyst were fractionated using polar solvents, different fractions had different signs of optical rotation ranging from
131 for the acetone-soluble fractions to +311 for the ether-soluble fraction [870].

190

Alkene Polymerization Reactions with Transition Metal Catalysts

3.8. Copolymerization Reactions of Alkenes 3.8.1. Copolymerization reactions, reactivity ratios for various alkene pairs The simplest kinetic model of a copolymerization reaction involving two alkene monomers, Mu and Mv, includes four chain growth reactions; it is sufficient for most practical purposes related to the description of the structure and physical properties of alkene copolymers: C 2M0 2Polymer þ M0 2kM0 ðM0 Þ ! C 2M0 2M0 2Polymer (3.112) C 2M0 2Polymer þ M00 2kM00 ðM0 Þ ! C 2M00 2M0 2Polymer 

00

0



0

00

(3.113)

C 2M 2Polymer þ M 2kM0 ðM00 Þ ! C 2M 2M 2Polymer

(3.114)

C 2M00 2Polymer þ M00 2kM00 ðM00 Þ ! C 2M00 2M00 2Polymer

(3.115)

Here, symbols Mu and Mv stand for both the monomers and for the respective monomer units in copolymer chains. The values of rate constants of chain propagation steps in Reactions (3.112–3.115) depend both on the type of the inserting monomer and on the structure of the last monomer unit in the growing polymer chain, kM0 ðM0 Þ akM0 ðM00 Þ and kM00 ðM00 Þ akM00 ðM0 Þ : Table 3.50 represents a rare example where all rate constants in Reactions (3.112–3.115) were estimated. The four rate constants in Reactions (3.112–3.115) are conventionally combined into two ratios called the reactivity ratios, r 1 ¼ kM0 ðM0 Þ =kM00 ðM0 Þ and r 2 ¼ kM00 ðM00 Þ =kM0 ðM00 Þ : The reactivity ratios are indicators of the relative reactivity of different alkenes in copolymerization reactions. For example, ethylene is much more reactive in copolymerization reactions compared to any other alkene; therefore, r1W1 and r2o1 for any ethylene/1-alkene pair; propylene is more reactive than any higher 1-alkene, etc. Tables 3.51–3.54 list the values of reactivity ratios for different monomer pairs and for different catalysts. The values of the reactivity ratios determine the relationship between the monomer concentration ratio in a copolymerization reaction, F ¼ ðC M0 =C M00 Þcomonomer and the ratio between the molar contents of the two alkenes in the copolymer, f ¼ ðC M0 =C M00 Þcopolymer f ¼ F  ðr 1 F þ 1Þ=ðr 2 þ FÞ or f ¼ r 1 F  ðr 1 F þ 1Þ=ðr 1 r 2 þ r 1 FÞ (3.116)

Table 3.50 Individual rate constants (M
1s
1) in ethylene/1-butene copolymerization reactions with Cp2ZrMe2-MAO system [262] Temperature (1C)

kE(E)

kE(B)

kB(E)

kB(B)

40 60 80

1,430 1,950 5,270

45 100 130

26 30 62

0.9 1.0 1.3

191

Chemistry and Stereochemistry of Polymerization and Copolymerization

Table 3.51 1-alkene

Reactivity ratios in copolymerization reactions of ethylene and 1-alkenes Catalyst system

r1

r2

Reference

d-TiCl3  0.33AlCl3-AlEt3, 401C d-TiCl3  0.33AlCl3-AlEt2Cl, 701C d-TiCl3  (Solvay cat.)-AlEt2Cl, 701C d-TiCl3  0.33AlCl3-AlEt3, 901C TiCl4/MgCl2-AlEt3, 701C TiCl4/MgCl2-AlEt3, 701C TiCl4/MgCl2-AlEt3, 401C TiCl4/MgCl2-AlEt3 TiCl4/MgCl2-AlEt3 TiCl4/MgCl2-AlEt3, 901C VCl3-Aln-Hex3, 251C VOCl3-Aln-Hex3, 251C VCl4/MgCl2-AlEt3, 701C CrO3/SiO2 Cp2Cr/SiO2 Cp2TiCl2-AlEt2Cl, 201C Cp2TiCl2-MAO, 301C Cp2TiCl2-MAO, –151C Cp2TiPh2-MAO, 501C Cp2Ti=CH2-MAO, 501C Cp2ZrCl2-MAO, 251C Cp2ZrCl2-MAO, 301C Cp2ZrCl2-MAO, 501C Cp2ZrMe2-MAO, 201C Cp2ZrCl2-MAO, –151C Cp2HfCl2-MAO, 251C Cp2HfCl2-MAO, 301C (Me-Cp)2ZrCl2-MAO, 501C (n-Bu-Cp)2ZrCl2-MAO (n-Bu-Cp)2ZrCl2-MAO, 801C Cp 2 ZrCl2-MAO, 501C Ind2ZrCl2-MAO Me2Si(Cp)2ZrCl2-MAO, 501C Me2Si(Cp)2ZrCl2-MAO, –151C rac-C2H4(Ind)2ZrCl2-MAO, 201C rac-C2H4(Ind)2ZrCl2-MAO, 11C rac-C2H4(Ind)2ZrCl2-MAO, 251C rac-C2H4(Ind)2ZrCl2-MAO, 251C rac-C2H4(Ind)2HfCl2-MAO, 251C rac-C2H4(Ind)2ZrCl2-MAO, 301C rac-C2H4(Ind)2ZrCl2-MAO, 1401C rac-Me2Si(Ind)2ZrCl2-MAO, 501C rac-Me2Si(Ind)2ZrCl2-MAO, 251C rac-Me2Si(Ind)2HfCl2-MAO, 251C rac-C2H4(Ind-H4)2ZrCl2-MAO, 301C rac-C2H4(Ind-H4)2ZrCl2-MAO, 501C rac-C2H4(Ind-H4)2ZrCl2-MAO, 901C

7.3 11.6 4.3 13 5.5 16 9.0 13 9.4 6.1 5.6 18 3.4

0.76 0.35 0.19 0.18 0.36 0.031 0.21

[434] [871] [872] [157] [871] [872] [873] [874] [875] [157] [876] ‘‘-’’ [872] [874] ‘‘-’’ [64] [877] [534] [40] ‘‘-’’ [878] [877] [40] [290] [534] [878] [877] [40] [879] [617] [40] [879] [40] [534] [554] [291] [878] [880] [878] [877] [630] [881] [878] ‘‘-’’ [877] [175] [882]

Propylene

24 8.7 15.7 21 19.5 24 9.3 16 48 32 7.3 13.3 21 60 27 19 250 20 24 2.1 2.8 5.4 4.4 6.3 4.0 2.6 36 1.3 2.9 3.0 2.9 10.4 18.5

0.39 0.15 0.15 0.07 0.056 4.4 0.009 0.020 0.02 0.0085 0.11 0.025 0.015 0.005 0.036 0.22 0.074

0.005 0.002 0.029 0.19 0.3 0.09 0.22 0.11 0.32 0.39 0.26 0.36 0.19 0.20 0.28 0.05 0.05

192

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 3.51 (Continued ) 1-alkene

Catalyst system

r1

r2

Reference

rac-H2C(3-t-Bu-Ind)2ZrCl2-MAO, 501C rac-H2C(3-t-Bu-Ind)2ZrCl2-MAO, 01C Me2C(Cp)(Flu)ZrCl2-MAO, 251C Me2C(Cp)(Flu)ZrCl2-IFCb, 1501C Ph2C(Cp)(Flu)ZrCl2-MAO, 801C C2H4(Flu)2ZrCl2-MAO, 501C (2-Ph-Ind)2ZrCl2-MAO, 201C rac-C2H4(Ind-H4)2ZrCl2/SiO2-Ali-Bu3, 401C rac-C2H4(Ind-H4)2ZrCl2/MgCl2-AlMe3, 401C rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2-MAO (Me2Si)2(4-i-Pr-Cp)(3,5 i-Pr2-Cp)TiCl2-MAO (Me2Si)2(4-i-Pr-Cp)(3,5-i-Pr2-Cp)ZrCl2-MAO (Me2Si)2(4-i-Pr-Cp)(3,5 i-Pr2-Cp)HfCl2-MAO Me2Si(Me4Cp)(t-Bu-N)TiCl2a-MAO, 501C Me2Si(Me4Cp)(t-Bu-N)TiCl2a-MAO, 901C Me2Si(Me4Cp)(t-Bu-N)TiMe2a-MMAO, 1401C (Me2Si)2(Ind)(t-Bu-N)TiCl2a-MAO, 501C

17.4 29 1.3 5.5 3.8 1.0 5.4 7.0 10.5 B5 B3 B6 B10 1.4 3.8 4.3 2.4

0.10 0.30 0.2 0.09 0.1 o0.01 0.24 0.15 0.077 B0.35 B0.2 B0.2 0.4 0.82 0.38 0.38 0.48

‘‘-’’ ‘‘-’’ [883] [622] [617] [881] [884] [885] ‘‘-’’ [292] [886] ‘‘-’’ ‘‘-’’ [887] [882] [630] [888]

d-TiCl3  0.33AlCl3-AlEt3, 901C d-TiCl3  0.33AlCl3-AlEt3, 701C a-TiCl3-AlR3 d-TiCl3  0.33AlCl3-AlEt2Cl TiCl4/MgCl2-AlEt3 d-TiCl3-MeTiCl3 TiCl4/MgCl2-AlEt3 TiCl4/MgCl2/THF-AlEt3, 701C TiCl4/MgCl2/THF/SiO2-AlEt3,701C TiCl4/MgH2-AlEt3, 201C TiCl4/MgCl2-AlEt3, 701C TiCl4/MgCl2-AlEt3, 801C TiCl4/MgCl2-AlEt3, 851C TiCl4/MgCl2-Ali-Bu3, 701C TiCl4/TiCl3-Cp2TiMe2, 401C TiCl3/MgCl2-DIBAO, 1501C VCl3-Aln-Hex3, 251C VCl4-Aln-Hex3, 251C VCl4-Ali-Bu2Cl VCl4/MgCl2-AlEt3 VOCl3-Al2Et3Cl3 VOCl3/MgCl2-AlEt3 V(acac)3-Ali-Bu2Cl CrO3/SiO2 CrO3/TiO2/SiO2 CrO2/SiO2-AlEt2OEt Cp2TiCl2-AlEt2Cl, 201C Cp2ZrCl2-MAO, 801C (n-Bu-Cp)2ZrCl2-MAO, 801C (Ind-H4)2ZrCl2-MAO, 1801C

40 23 60 85 29 3.6 25 26 31 55 29 45 67 23 69 23 30 30 32 13 12.5 13 26 13 13 43 18.4 85 56 53

0.02 0.029 0.025 0.015

[889] [40] [890] [891] [892] [893] [875] [40] ‘‘-’’ [894,895] [896] [897] [898] [48] [899] [900] [901] ‘‘-’’ [902] [896,903] [274] [892] [902] [874] ‘‘-’’ ‘‘-’’ [303] [262] [617] [904]

1-Butene

0.16 0.10 0.055 0.027 0.02 0.16 0.08 0.091 0.058 0.043 0.019 0.019 0.030 0.022

0 0.01 0.005 0.02

193

Chemistry and Stereochemistry of Polymerization and Copolymerization

Table 3.51 (Continued ) 1-alkene

Catalyst system

r1

r2

Reference

rac-C2H4(Ind)2ZrCl2-MAO, 701C rac-C2H4(Ind-H4)2ZrCl2-MAO, 901C rac-C2H4(Ind)2HfCl2-MAO, 701C Me2C(Cp)(Flu)ZrCl2-IFCb, 1501C Ph2C(Cp)(Flu)ZrCl2-MAO, 801C

29 30 6.8 16 5.9

0.04 0.034 0.21 0.022 0.041

[904] [905] [904] [622] [617]

TiCl4/MgH2-AlEt3, 201C VCl4-Ali-Bu2Cl V(acac)3-Ali-Bu2Cl rac-Me2Si(Ind)2ZrCl2-MAO, 701C

50 42 32 9.5

0.02 0.015 0.014 0.031

[894,895] [902] ‘‘-’’ [602]

0.078 0.033

0.03

[873] [899] [892,896,903] [897] [894,895] [906] [892,896,903] [907] [908] [909] [910] [911] [912] [879] [913] [756] [879] [273,910,908] [911] [273] [914] [911] ‘‘-’’ [685] [908] [273,910] [915] [908] [911] [910] [602] [909] [273] [885]

0.019 0.023

‘‘-’’ ‘‘-’’

1-Pentene

1-Hexene TiCl4/MgCl2-AlEt3, 401C 22 TiCl4/TiCl3-AlEt3, 401C 69 TiCl4/MgCl2-AlEt3, 701C 56 TiCl4/MgCl2-AlEt3, 801C 130 TiCl4/MgH2-AlEt3, 201C 47 TiCl4/MgCl2-Ali-Bu2Cl, 1601C 29 VCl4/MgCl2-AlEt3, 701C 38 V(acac)3-Ali-Bu2Cl 41 Cp2ZrCl2-MAO, 601C 62 Cp2ZrCl2-MAO, 601C 57 Cp2ZrCl2-MAO, 601C 62 Cp2ZrCl2-MAO, 47 Cp2ZrMe2-MAO, 201C 55 n-Bu-Cp2ZrCl2-MAO 19 CpTiCl3/MAO/SiO2, 401C 23–25 CpTiCl2(O–2,6-i-Pr2-C6H3)-MAO 2.7 Ind2ZrCl2-MAO 30 Ind2ZrCl2-MAO, 601C 88 Ind2ZrCl2-MAO 150 Ind2ZrCl2-MAO, 1801C 88 (Ind-H4)2ZrCl2-MAO, 1801C 63 (2-Me-Ind)2ZrCl2-MAO 19 (2-Ph-Ind)2ZrCl2-MAO 8 (2-Ph-Ind)(1-Me-2-Ph-Ind)ZrCl2-MAO 13 Me2C(Cp)2ZrCl2-MAO, 601C 33 rac-C2H4(Ind)2ZrCl2-MAO, 601C 31 rac-C2H4(Ind)2ZrCl2-MAO, 601C 32 rac-C2H4(Ind)2ZrCl2-MAO, 601C 31 rac-C2H4(Ind)2ZrCl2-MAO 14 Me2Si(Cp)2ZrCl2-MMAO, 601C 33 rac-Me2Si(Ind)2ZrCl2-MAO, 701C 9.3–11 rac-C2H4(Ind-H4)2ZrCl2-MAO, 601C 59 rac-C2H4(Ind-H4)2ZrCl2-MAO, 601C 31–36 rac-C2H4(Ind-H4)2ZrCl2/MAO/SiO2-Ali-Bu3, 22 401C rac-C2H4(Ind-H4)2ZrCl2/SiO2- Ali-Bu3, 401C 20 rac-C2H4(Ind-H4)2ZrCl2/MgCl2- AlMe3, 401C 45

0.01 0.02 0.1 0.036 0.003 0.012 0.003 0.003 0.005 B0.05 0.10 0.4 0.005 0.004 0.02 0.048 0.09 0.091 0.027 0.013 0.015 0.013 0.027 0.027 0.038 0.012

194

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 3.51 (Continued ) 1-alkene

Catalyst system

r1

r2

Reference

rac-C2H4(Ind-H4)2ZrCl2/MAO/SiO2-Ali-Bu3, 401C rac-C2H4(Ind-H4)2ZrCl2/ Al2O3-Ali-Bu3,401C rac-C2H4(Ind-H4)2ZrCl2/Al2O3-Ali-Bu3,401C rac-C2H4(Ind-H4)2ZrCl2/MgCl2-Ali-Bu3 Zr(Bz)4/SiO2, 801C CrO3/(Al2O3-SiO2), 1401C (Me2Si)2(Ind)(t-Bu-N)TiCl 2 -MAO, 501C

36

0.015

[916]

21 36 36 14 1.0 5.8

0.03 0.014 0.015 0.044 0.16

[885] [916] ‘‘-’’ [917] [918] [888]

TiCl4/MgH2-AlEt3, 201C

75

0.013

[894,895]

0.01

‘‘-’’ [897] [274] [295,991] [908,910] ‘‘-’’ [915] ‘‘-’’ [295,991] [602] [295,991] ‘‘-’’ ‘‘-’’ [919] [295,991] [537]

1-Heptene 1-Octene TiCl4/MgH2-AlEt3, 201C 90 TiCl4/MgCl2-AlEt3, 801C 170 VOCl3-Al2Et3Cl3 16 Cp2ZrCl2-MAO, 401C 33 Cp2ZrCl2-MAO, 601C 62 Me2Si(Cp)2ZrCl2-MAO, 601C 42 rac-C2H4(Ind)2ZrCl2-MAO, 601C 32 rac-C2H4(Ind)2ZrCl2-MAO, 601C 59 rac-Me2Si(Ind)2ZrCl2-MAO, 401C 19 rac-Me2Si(Ind)2ZrCl2-MAO, 701C 8.1 rac-Me2Si(2-Me-Ind)2ZrCl2-MAO, 401C 20 rac-Me2Si(Benz[e]Ind)2ZrCl2-MAO, 401C 11 rac-Me2Si(2-Me-Benz[e]Ind)2ZrCl2-MAO, 401C10 Me2Si(Me4Cp)(t-Bu-N)TiMe 7.9 2 -MMAO Me2Si(Me4Cp)(t-Bu-N)TiCl2-MAO, 401C 4.1 Me2Si(2-Me-Benz[e]Ind)(t-Bu-N)TiCl 2 - MAO 2.2

0.005 0.06 0.030 0.035 0.05 0.004 0.014 0.043 0.013 0.076 0.12 0.099 0.20 0.55

1-Decene d-TiCl3  0.33AlCl3-AlEt3, 901C TiCl4/MgH2-AlEt3, 201C TiCl4/MgCl2-AlEt3, 801C TiCl4/MgCl2-AlEt3, 801C VOCl3-Al2Et3Cl3 (Ind-H4)2ZrCl2-MAO, 1801C rac-C2H4(Ind)2ZrCl2-MAO, 601C rac-Me2Si(Ind)2ZrCl2-MAO, 701C

230 550 272 70 20 80 190 11

0.03 0.004 0.11 0.05 0.01 0.008 0.037

[889] [894,895] [897] [920] [274] [914] [915] [602]

360 1,500 12 9.5 7.2

0.03 0.004 0.036 0.043 0.057

[889] [895] [921] [602] [921]

13

0.031

[602]

74 28 420

0.04 0.05 0.03

[922] ‘‘-’’ [889]

1-Dodecene d-TiCl3  0.33AlCl3-AlEt3, 901C TiCl4/MgH2-AlEt3, 201C rac-Me2Si(Ind)2ZrCl2-MAO, 251C rac-Me2Si(Ind)2ZrCl2-MAO, 701C Me2C(Cp)(Flu)ZrCl2-MAO, 251C 1-Tetradecene rac-Me2Si(Ind)2ZrCl2-MAO, 701C 1-Hexadecene TiCl4/MgCl2-AlEt3, 401C Cp2ZrCl2/SiO2-MAO, 401C TiCl3  0.33AlCl3-AlEt3, 901C

195

Chemistry and Stereochemistry of Polymerization and Copolymerization

Table 3.51 (Continued ) 1-alkene

Catalyst system

TiCl4/MgH2-AlEt3, 201C 3-Methyl-1-butene TiCl4/MgCl2-Ali-Bu3, 701C VOCl3/MgCl2-Ali-Bu3, 701C VOCl3/MgCl2-Ali-Bu3, 701C V(acac)3-Ali-Bu2Cl 3-Methyl-1-pentene VOCl3/MgCl2-Ali-Bu3, 701C 4-Methyl-1-pentene d-TiCl3-AlEt2Cl TiCl4/TiCl3-AlEt3, 401C V(acac)3-Ali-Bu2Cl Cp2ZrCl2-MAO, 301C rac-C2H4(Ind)2ZrCl2-MAO, 301C rac-Me2Si(Ind)2ZrCl2-MAO, 451C rac-Me2Si(Benz[e]Ind)2ZrCl2-MAO, 451C Me2Si(Me4Cp)(t-Bu-N)TiCl2a-MAO, 451C Me2Si(Ind)(t-Bu-N)TiCl2a-MAO, 451C Me2Si(Flu)(t-Bu-N)ZrCl2a-MAO, 451C Me2Si(Flu)(t-Bu-N)TiCl2a-MAO, 451C 3-Ethyl-1-pentene VOCl3/MgCl2-Ali-Bu3, 701C Vinylcyclohexane VOCl3/MgCl2-Ali-Bu3, 701C t-Bu-Cp2ZrCl2-MAO, 501C Styrene CpTiCl3-MAO, 201C Me2Si(Me4Cp)(t-Bu-N)TiCl2a-MAO Me2Si(Me4Cp)(t-Bu-N)TiCl2a-MAO (4-Me-6-t-Bu-C6H2O)2Ti(Oi-Pr)2-MAO Complex LTiCl2 with tetradentate ligand Lc p-Methylstyrene rac-C2H4(Ind)2ZrCl2-MAO, 201C Me2Si(Me4Cp)(t-Bu-N)TiCl2a-MAO, 201C Norbornene Cp2ZrCl2-MAO, 401C Cp 2 ZrCl2-MAO, 501C rac-C2H4(Ind)2ZrCl2-MAO, 401C rac-C2H4(Ind)2ZrCl2-MAO, 301C Me(H)C(Cp)2ZrCl2-MAO, 701C Me2C(Cp)(Ind)ZrCl2-MAO, 701C Me2C(Cp)(Flu)ZrCl2-MAO, 301C Me2C(Cp)(Flu)ZrCl2-MAO, 701C rac-Me2Si(Ind)2ZrCl2-MAO, 301C Me2Si(Me4Cp)(t-Bu-N)TiCl2a-MAO a

r2

Reference

1,050

0.0008

[895]

0.001

[275] ‘‘-’’ [923] [924]

500 132 130 243 530 195 150 47 103 50 29 13 3.8 4 4.6 1.2

[923] 0.028 0.034 0.015 0.004 0.042 0.095 0.085 0.087 0.024 0.014

[84] [873,899] [925] [926] ‘‘-’’ [689] ‘‘-’’ ‘‘-’’ ‘‘-’’ ‘‘-’’ ‘‘-’’

430

[923]

235 65

[923] [927]

5.8 B9 23 111 1.2

0.04 0.015 0.055 0.031

[928] [929] [311] [930] [931]

89 20

B0 0.04

[759] ‘‘-’’

4.0 27 1.9 2.6 0.83 0.88 B1.1 1.3 2.6 3.8

0.03 B0 0.03 0.028 0.29 0.05 0.04 0.03 0.052 0.0

[932] [933] [934] [935] [936] ‘‘-’’ [937] [936] [935] [148]

Constrained-geometry catalysts (complexes IX in Scheme 1.1). IFC ¼ ion-forming cocatalyst [CPh3]+ [B(C6F5)4]
or [Me2N(Ph)H]+ [B(C6F5)4]
. L ¼
O
Ar
CH2
S
C2H4
S
CH2
Ar
O
(see Section 4.7.1).

b c

r1

196

Table 3.52

Alkene Polymerization Reactions with Transition Metal Catalysts

Reactivity ratios in copolymerization reactions of propylene and 1-alkenes r1

r2

Reference

TiCl4-AlEt3, 601C a-TiCl3-AlEt2Cl, 231C d-TiCl3-AlEt2Cl, 301C d-TiCl3  0.33AlCl3-AlEt3, 901C TiCl4/MgCl2/diester-AlEt3, 151C TiCl4/MgCl2-AlEt3 VCl4-AlHex3 VCl3-AlHex3, 801C rac-C2H4(Ind)2HfCl2-MAO, 301C

2.4 4.5 3.3 2.5 B7 3.3 4.4 4.0 1.0

0.5 0.2 0.45 0.62 0.2 0.31 0.23 0.25 0.69

[938] [939] [940] [157] [376] [875] [876] ‘‘-’’ [941]

TiCl3-AlEt2Cl TiCl4/MgCl2-AlEt3 V(acac)3-AlEt2Cl rac-C2H4(Ind)2ZrCl2-MAO rac-C2H4(Ind)2Zr(NMe2)2-MAO rac-Me2Si(Ind)2ZrCl2-MAO, 301C rac-C2H4(Ind-H4)2ZrCl2-MAO rac-C2H4(Ind)2HfCl2-MAO rac-Me2Si(Ind-H4)2ZrCl2-MAO rac-C2H4(Ind-H4)2ZrCl2/MAO/ SiO2-AlEt3 rac-C2H4(Ind-H4)2ZrCl2/SiO2- AliBu3, 401C rac-C2H4(Ind-H4)2ZrMe2-MAO, 301C Me2C(Cp)(Flu)ZrCl2-MAO, Me2C(Cp)(Flu)ZrCl2-MAO, 301C Me2C(Cp)(Flu)ZrMe2-MAO, 301C Me2C(Cp)(Flu)ZrMe2-MAO, 301C

4.2 13.6 2.5 1.5–2.5 B10 6.8 2.4 1.8 10.8 2.9

0.16 0.82 0.88 0.55–0.75 0.12 – 0.4 0.59 0.088 0.3

[943] [873] [942] [944] [945] [268] [946] [941] [947] [885]

2.8

0.47

‘‘-’’

5.5–6.6

0.18–0.26

[733]

31 3.0 2.1–3.6 2.2–3.2

0.032 – 0.22–0.55 0.24–0.46

[237] [268] [733] ‘‘-’’

TiCl4/MgCl2-AlEt3 C2H4(Ind)2HfCl2-MAO rac-Me2Si(Ind-H4)2ZrCl2- MAO

6.5 2.1 10

0.20 0.55 0.17

[873] [941] [947]

7.7 10.5

0.15

[873] [947]

9.9

0.13

[873]

2.4 8.6

0.47 0.072

[941] [947]

13

0.10

[873]

1-Alkene Catalyst system

1-Butene

1-Hexene

1-Octene

1-Decene TiCl4/MgCl2-AlEt3 rac-Me2Si(Ind-H4)2ZrCl2- MAO 1-Dodecene TiCl4/MgCl2-AlEt3 1-Dodecene C2H4(Ind)2HfCl2-MAO rac-Me2Si(Ind-H4)2ZrCl2- MAO 1-Tetradecene TiCl4/MgCl2-AlEt3

197

Chemistry and Stereochemistry of Polymerization and Copolymerization

Table 3.52 (Continued ) 1-Alkene Catalyst system

1-Hexadecene TiCl4/MgCl2-AlEt3 1-Hexadecene rac-C2H4(Ind)2HfCl2-MAO 1-Octadecene Me2C(Cp)(Flu)ZrCl2-MAO, 4-Methyl-1-pentene d-TiCl3-AlEt2Cl Vinylcyclohexane d-TiCl3-AlEt2Cl Cyclopentene rac-Me2Si(Ind)2ZrCl2- MAO, 601C Styrene TiCl3-AlEt3 TiCl3-AlEt3 TiCl3-Ali-Bu3 VCl3-AlEt3 TiCl4/TiCl3-Cp2TiMe2 401C

r1

r2

Reference

14

0.08

‘‘-’’

2.8

0.42

[941]

93

0.010

[237]

6.4

0.31

[948,949]

80

0.049

[279]

40

0.001

[581]

B20 20 20 7.2 130

0.3 0.3 0.20 0.16 0.18

[950] [951] ‘‘-’’ ‘‘-’’ [952]

Equation (3.116) is a quadratic equation in terms of the r1F value, its solution with respect to r1F is r 1 F ¼ 0:5  fðf
1Þ þ ½ðf
1Þ2 þ 4r 1 r 2  f 0:5 g

(3.117)

Relative reactivities of different alkenes in chain growth reactions, the penultimate effect: Detailed 13C NMR analysis of several compositionally uniform ethylene/1-butene and ethylene/1-hexene copolymers prepared with metallocene catalysts showed that a more complex statistical model is required for the precise description of the copolymer chain structure [309,882,980]. This model, called the penultimate-effect model, assumes that the structure of two last monomer units in a growing polymer chain (instead of a single monomer unit in Reactions (3.112–3.115)) is needed to account for some subtle kinetic effects in copolymerization reactions. For example, the ethylene insertion step in Reaction 3.112 can be presented by the following two reactions: Cat2E2E2Polymer þ E 2kEðE;EÞ ! Cat2E2E2E2Polymer

(3.118)

Cat2E2M2Polymer þ E 2kEðE;MÞ ! Cat2E2E2M2Polymer

(3.119)

Table 3.55 gives estimates of rate constant ratios in Reactions 3.118 and 3.119 for three metallocene catalysts [309,921]. Formally, each rate constant ratio in the table is equivalent to the r1 value and the differences between the two columns characterize the effect of the penultimate monomer unit in a growing polymer chain on the ethylene reactivity. The data show that when the last monomer unit in

198

Table 3.53

Alkene Polymerization Reactions with Transition Metal Catalysts

Reactivity ratios in copolymerization reactions of various 1-alkenes

Monomer pair 1-Butene/1-pentene 1-Butene/1-hexene 1-Butene/1-heptene 1-Butene/1-octene 1-Butene/1-decene 1-Butene/3-methyl-1-butene

Catalyst system

r1

VCl3-AlEt3 0.30 0.13 VCl3-AlEt3, 801C 1.9 a-TiCl3-AlEt3, 801C 0.74 VCl3-AlEt3, 801C 1.5 d-TiCl3-AlEt2Cl, 231C 6.22 TiCl4-AlEt3, 451C 6.7 a-TiCl3-AlEt3, 301C 4.0 a-TiCl3-Ali-Bu3, 801C 0.40 1-Butene/4-methyl-1-pentene d-TiCl3-Ali-Bu3, 801C 240 1-Butene/3,5,5-trimethyl-1-hexene a-TiCl3-AlEt3, 301C 170 a-TiCl3-Ali-Bu3, 301C 7.0 a-TiCl3-Ali-Bu3, 801C 60 1-Butene/vinylcyclohexane a-TiCl3-AlEt3, 301C 3.7 1-Butene/allylcyclohexane a-TiCl3-AlEt3, 401C 8.6 1-Butene/5-vinyl-2-norbornene a-TiCl3-Ali-Bu3, 301C 5.4 a-TiCl3-Ali-Bu3, 801C 3.9 1-Butene/styrene a-TiCl3-AlEt3, 801C 29 TiCl4/MgCl2-Ali-Bu3, 501C 8.3 1-Pentene/3-methyl-1-butene TiCl3-AlEt3 5.4 1-Hexene/4-methyl-1-pentene TiCl3-AlEt2Cl 6.0 d-TiCl3-AlEt2Cl 1.3 1-Hexene/1-decene TiCl3-AlEt2Cl 0.74 1-Hexene/norbornadiene Cp2ZrCl2-MAO, 501C 0.75 1-Heptene/4-phenyl-1-butene TiCl3-AlEt3 35 1-Decene/3-methyl-1-butene TiCl4-AlEt3 0.11 3-Methyl-1-butene/4-methyl-1-pentene TiCl4-AlEt3 1.0 3-Methyl-1-butene/vinylcyclohexane TiCl3-Ali-Bu3 6.2 4-Methyl-1-pentene/3-methyl-1-pentene d-TiCl3-AlEt3 3.17 4-Methyl-1-pentene/vinylcyclohexane d-TiCl3-AlEt2Cl 3.67 4-Methyl-1-pentene/styrene TiCl3-AlEt3 3.17 TiCl3-Ali-Bu3 0.70 4-Methyl-1-pentene/4-phenyl-1-butene TiCl3-AlEt3 0.18 Vinylcyclohexane/styrene TiCl3-Ali-Bu3, 601C 0.38 TiCl3-AlEt3 0.2 Vinylcyclohexane/4-vinyl-1-cyclohexene TiCl3-Ali-Bu3, 601C

r2

Reference

0.74 0.90 2.05 0.77 0.7 0.33 0.34 0.2 0.05 0.03 0.001 0.041 0.07 0.2 0.062 0.025 0.02 0.47 0.28 0.62 0.37 0.9 1.55 0.55 0.05 9.0 0.98 0.1 0.38 0.89 0.38 0.40 2.12 3.17 3.8

[953] [954] [468] [955] [939] [956] [468] [957] [958] [468] [959] ‘‘-’’ [468] [960] [961] ‘‘-’’ [962] [963] [964] [965] [966] [967] [950] [968] [964] [969] [970] [971] [972] [973] [974] [975] [976] [972] [15]

the growing chain is ethylene, the penultimate effect is either relatively small, 1.5–2, or does not exist al all, and Reactions (3.112–3.115) represent the copolymerization reactions in a satisfactory manner. However, the penultimate effect becomes significant, of the order of 2–4, if the last monomer unit in the chain is a 1-alkene unit with a bulky alkyl group [175,309,921] or a cycloalkene [148]. Temperature effect on reactivity ratios: The values of reactivity ratios depend on temperature, although the dependence in not very strong, in contrast to the temperature dependence of individual rate constants (Table 3.50). Table 3.56 gives two examples of the temperature effect in copolymerization reactions of ethylene and 1-butene. Copolymerization reactions of alkenes with internal CQC bonds: Several examples of alkene copolymerization reactions were described when one of the comonomers

199

Chemistry and Stereochemistry of Polymerization and Copolymerization

Table 3.54

a

Reactivity ratios in copolymerization reactions of styrenes (styrene=monomer 1)

Comonomer

r1

r2

Catalyst systems TiCl3-AlR3, 40 and 601C [475] p-Methylstyrene m-Methylstyrene o-Methylstyrene 2,4-Dimethylstyrene p-Ethylstyrene p-Cyclohexylstyrene p-Fluorostyrene m-Fluorostyrene o-Fluorostyrene p-Chlorostyrene m-Chlorostyrene p-Bromostyrene p-Methoxystyrene

0.82 2.0 – – 1.0 – 1.5 – – 2.2 – 1.7 –

1.15 0.5 0.13 0.14 1.0 0.58 0.7 0.47 0.18 0.5 0.43 0.48 1.6

Catalyst system Ti(O-Mentyl)4-MAO, 271C [977] p-Methylstyrene p-Chlorostyrene m-Chlorostyrene p-Bromostyrene

0.40 1.49 1.96 1.26

1.47 0.19 0.23 0.79

Metallocene catalysts, 30–601C [978] CpTiCl3-MAO CpTiCl3-MAO (Flu-H8)Ti(OMe)3-MAO

0.69 0.68 0.52

2.28 1.13 1.43

Complex (L)TiCl2 with tetradentate ligand La-MAO, 251C [979] p-Methylstyrene p-Chlorostyrene p-Bromostyrene

1.50 1.74 1.50

1.14 0.31 0.44

L=
O
Ar
CH2
S
C2H4
S
CH2
Ar
O
(Section 4.7.1).

cannot form M
M links. The earliest example of this type is copolymerization reactions of ethylene and 2-butenes with VCl4-AlHex3 and V(acac)3-AlEt2Cl systems [802]. 2-Butene does not form homopolymers with these catalysts and the r2 value in the copolymerization reaction is equal to zero, the insertion of a 2-butene molecule into a growing polymer chain is possible only after an ethylene insertion (Reaction (3.88)). When these copolymerization reactions are carried out at low [ethylene]:[2-butene] molar ratios, the copolymers have the alternating structure. Another example of this type is a copolymerization reaction of ethylene and norbornene with sterically demanding asymmetric metallocene catalysts, combinations of (3-R-Cp)(Flu)ZrCl2 (R ¼ Me and i-Pr) ands MAO [982,983], or with constrained-geometry catalysts [148]. In this case r2 ¼ 0 as well, and the copolymers prepared at low ethylene concentrations have a predominantly alternating structure. Moreover, the catalyst produced from (3-i-Pr-Cp)(Flu)ZrCl2 has a lower insertion rate for the norbornene molecule even when the previous

200

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 3.55 Penultimate effects in ethylene/1-alkene copolymerization reactions with metallocene catalysts [175,309,921,935,981]

a

1-Alkene (M)

kE(E,E)/kM(E,E)

kE(E,M)/kM(E,M)

C2H4(Ind-H4)2ZrCl2, cocatalysts MAO and IOAOa Propylene (MAO) Propylene (in hexane, IOAOa) Propylene (in toluene, IOAOa)

11 14 15

10 11 15

Me2Si(Ind)2ZrCl2-MAO Propylene 1-Butene 1-Hexene 1-Dodecene 1-Octadecene Norbornene

6.3–7.1 7.9 10 12 15 3.0

5.5–5.6 6.8 6.4 5.7 8.4 2.5

Catalyst system Me2C(Cp)(Flu)ZrCl2-MAO 1-Butene 1-Hexene 1-Dodecene

3.6 3.2 7.2

2.9 2.6 5.0

Tetra-i-octylalumoxane [558].

Table 3.56 reactions

Temperature effects on reactivity ratios in ethylene/1-butene copolymerization

Temperature, 1C

r1

r2

TiCl4/MgCl2-AlEt3 system [898]. 50 65 85 120

130725 110720 6778 5175

0.170.05 0.170.05 0.0870.04 0.0470.02

Cp2ZrMe2-MAO system [262]. 40 60 80

55 65 85

0.02 0.01 0.01

norbornene monomer unit is separated from the Zr atom in the active center by one ethylene unit. Using the nomenclature of Reactions (3.118) and (3.119), this kinetic feature signifies a strong penultimate effect, kMðE;MÞ  kMðE;EÞ [982].

3.8.2. Statistical description of copolymer structure in terms of block length Many physical, thermal, and mechanical properties of alkene copolymers depend on the presence of long monomer blocks, blocks of Mu units, Mv
(Mu)n
Mv, and

Chemistry and Stereochemistry of Polymerization and Copolymerization

201

blocks of Mv units, Mu
(Mv)n
Mu. The number of monomer units in such a block, n, can vary from one (an isolated monomer unit; sequences Mv
Mu
Mv or Mu
Mv
Mu) to a very large number. The following expressions for the distribution of monomer units in blocks of different lengths are used for the statistical description of copolymers. The fraction of Mu units in blocks containing n monomer units normalized to the total molar content of Mu units in the copolymer is dðM0 Þn ¼ n  ðr 1 FÞn
1 =ð1 þ r 1 FÞnþ1 (3.120) The fraction of Mu units in the sum of all long blocks starting with n monomer units, i.e., the sum of blocks Mv
(Mu)n-Mv, Mv
(Mu)n+1
Mv, Mv
(Mu)n+2
Mv, etc., normalized to the total molar content of Mu units in the copolymer is X ðM0 Þn ¼ ðn þ r 1 FÞ  ðr 1 FÞn
1 =ð1 þ r 1 FÞn (3.121) The same equations for Mv units are produced by replacing the r1F term in Equations (3.120) and (3.121) with r1r2/r1F. Equations (3.120) and (3.121) are sufficient for the statistical description of many basic properties of alkene copolymers. It is especially instructive to use these equations in combination with Equation (3.117). This analysis shows that distributions of both types of monomer units in blocks of different lengths are the functions of only two parameters, the copolymer composition (the f value in Equation (3.117), this value is usually measured experimentally, either by IR or by NMR methods) and the product of reactivity ratios, r1r2. The latter value characterizes the degree of randomness in positions of monomer units in copolymer chains. When the r1r2 value is equal to zero (because either r1 ¼ 0 or r2 ¼ 0), at least one of the comonomers cannot form M
M links and is always positioned in copolymer chains as isolated units. These copolymers are called alternating. For example, ethylene/propylene copolymers with a monomer unit distribution close to alternating can be produced with asymmetric bridged bis-metallocene complexes containing one substituted cyclopentadienyl or indenyl ligand and one fluorenyl ligand [553,984–988]. Another example of alkene copolymers with an alternating structure are ethylene/ norbornene copolymers synthesized with bis-metallocene and constrainedgeometry catalysts under mild reaction conditions at high [norbornene]/[ethylene] feed ratios [148,982,983]. If the r1r2 value is equal to 1, Equation (3.117) is reduced to r1F ¼ f, i.e., the copolymer statistics is determined by a single parameter, the copolymer composition ratio f. These copolymers are called random.

3.8.3. Statistical description of copolymer structure suitable for NMR analysis 13

C NMR spectroscopy is the most powerful tool for the structural analysis of alkene copolymers. To compare NMR data with copolymer statistics, the statistical equations should be written in the form acceptable for the NMR spectroscopic method. The equations should give contents of short NMR-observable sequences in copolymer chains, such as isolated Mu units, Mv
Mu
Mv, diads Mv
Mu
Mu
Mv, triads Mv
Mu
Mu
Mu
Mv, etc. These equations are given in Table 3.57. As discussed in the previous section, if the reaction scheme for a

202

Table 3.57

Alkene Polymerization Reactions with Transition Metal Catalysts

Statistical representation of copolymer structure in terms of n-ads [6]

Common intermediate parameter: U=(r1F)2+2r1F+r1r2 Diads [Mu-Mu]=(r1F)2/U [Mv-Mv]=r1r2/U [Mv-Mu]=[Mu-Mv]=r1F/U Triads [Mu-Mu-Mu]=(r1F)3/[(1+r1F)  U] [Mv-Mv-Mv]=(r1r2)2/[(r1r2+r1F)  U] [Mv-Mu-Mu]=[Mu-Mu-Mv]=(r1F)2/[(1+r1F)  U] [Mu-Mv-Mu]=(r1F)2/[(r1r2+r1F)  U] [Mv-Mu-Mv]=r1F/[(1+r1F)  U] [Mv-Mv-Mu]=[Mu-Mv-Mv]=r1r2  (r1F)/[(r1r2+r1F)  U] Tetrads [Mu-Mu-Mu-Mu]=(r1F)4/[(1+r1F)2  U] [Mu-Mu-Mu-Mv]=[Mv-Mu-Mu-Mu]=(r1F)3/[(1+r1F)2  U] [Mu-Mu-Mv-Mu]=[Mu-Mv-Mu-Mu]=(r1F)3/[(1+r1F)  (r1r2+r1F)  U] [Mu-Mu-Mv-Mv]=[Mv-Mv-Mu-Mu]=r1r2  (r1F)2/[(1+r1F)  (r1r2+r1F)  U] [Mu-Mv-Mu-Mv]=[Mv-Mu-Mv-Mu]=(r1F)2/[(1+r1F)  (r1r2+r1F)  U] [Mv-Mu-Mu-Mv]=(r1F)2/[(1+r1F)2  U] [Mu-Mv-Mv-Mu]=r1r2  (r1F)2/[(r1r2+r1F)2  U] [Mu-Mv-Mv-Mv]=[Mv-Mv-Mv-Mu]=(r1r2)2  r1F/[(r1r2+r1F)2  U] Selected pentads [Mu-Mu-Mu-Mu-Mu]=(r1F)5/[(1+r1F)3  U] [Mu-Mu-Mu-Mu-Mv]=[Mv-Mu-Mu-Mu-Mu]=(r1F)4/[(1+r1F)3  U] [Mv-Mu-Mu-Mu-Mv]=(r1F)3/[(1+r1F)3  U]

copolymerization reaction is described by four chain growth reactions, Equations (3.112–3.115), then all expressions describing relative contents of various NMRdetectable sequences in copolymer chains are the functions of only two parameters, the copolymer composition ratio f and the product of reactivity ratios, r1r2. Statistical equations in Table 3.57 correspond to the simplest kinetic model of a copolymerization reaction (Equations (3.112–3.115)). They are useful to a chemist as a means to visualize the chain structure of any alkene copolymer. More complex statistical schemes of copolymer chains are also described in the literature. They define the steric structure of monomer blocks (Mu)n and (Mv)n [51], penultimate effects in copolymerization reactions [175,309,981,989,990], and effects of the presence of several types of active centers in catalysts [989]. 13 C NMR analysis of alkene copolymers provides the most dependable methods for the evaluation of both the reactivity ratio values and their products, through the use of equations in Table 3.57 and the equation for the copolymer composition (Equation (3.117)). Individual reactivity ratios and r1r2 values can vary in a broad range even for single-center metallocene catalysts depending on the type of a comonomer pair and the catalyst structure. Table 3.58 gives several examples of this variability for ethylene/1-octene copolymerization reactions with single-center

203

Chemistry and Stereochemistry of Polymerization and Copolymerization

Table 3.58 Reactivity ratios in ethylene/1-octene copolymerization reactions with metallocene catalystsa [295,991]

a

Complex

r1

r2

r1  r2

Cp2ZrCl2 rac-Me2Si(Ind)2ZrCl2 rac-Me2Si(2-Me-Ind)2ZrCl2 rac-Me2Si(Benz(e)Ind)2ZrCl2 Me2Si(2-Me-Benz(e)Ind)2ZrCl2 [Me2Si(Me4-Cp)(t-Bu-N)]TiCl2

32.8 18.9 19.5 10.7 10.1 4.1

0.05 0.014 0.013 0.076 0.118 0.29

0.17 0.27 0.25 0.81 1.20 1.19

Copolymerization reactions at 401C, cocatalyst MAO.

catalysts containing the Cp2ZrCl2-MAO catalyst, four bridged bis-zirconocene complexes, and a constrained-geometry catalyst.

3.8.4. Auto-copolymerization reactions and long-chain branching in alkene polymers By definition, an auto-copolymerization reaction is a copolymerization reaction of a given monomer and polymer molecules with vinyl double bonds at the ends of their chains, which are generated in the course of the polymerization reaction. Typical auto-copolymerization reactions occur in ethylene homopolymerization reactions catalyzed by homogeneous metallocene or non-metallocene catalysts. Both the chain transfer reaction to ethylene (Reaction (3.50)) and the spontaneous chain transfer reaction (Reaction (3.54)) in ethylene polymerization reactions produce molecules with the vinyl double bond, CH2QCH
Polymer. These molecules are usually called macromers. The molecular weight of the macromers can vary in a wide range, from small (1-hexene, 1-octene, etc.) to very high. Potentially, all macromers, including high molecular weight products if they have a sufficient migratory ability, can copolymerize with ethylene, e.g., when the polymers are dissolved in a reaction medium at high temperatures. Products of auto-copolymerization reactions usually contain isolated branches derived from macromers,
(CH2
CH2)x
CH2
CH(Polymer)
(CH2
CH2)y
[824,992, 993]. Molar concentrations of macromers in reaction mixtures are always low, and an obvious prerequisite for the formation of auto-copolymers is the ability of a catalyst to copolymerize 1-alkenes with ethylene. Auto-copolymerization reactions were observed even in ethylene polymerization reactions with standard metallocene catalyst systems (e.g., a combination of Cp2ZrMe2 and B(C6F5)3) at 201C when the reactions were carried out at low ethylene concentrations [994] and with chromium oxide catalysts [1013]. The presence of a small number of long-chain branches was also proved by NMR in polyethylene and ethylene/1-hexene copolymers prepared with metallocene catalysts rac-C2H4(Ind)2ZrCl2-MAO and C2H4(Ind-H4)2ZrCl2-MAO [572]. Constrained-geometry catalysts (complexes IX in Scheme 1.1) are well suited for auto-copolymerization reactions of ethylene. These catalysts have high

204

Alkene Polymerization Reactions with Transition Metal Catalysts

temperature stability and ability to copolymerize 1-alkenes with ethylene (low r1 values in ethylene/1-alkene copolymerization reactions (see Table 3.51) [73,148, 604,993,995]. For example, the r1 value for the ethylene/1-octene pair in copolymerization reactions with constrained-geometry catalysts can be as low as 2.6 at 851C [598] and B4 at 1301C [992]. As a result, when constrained-geometry catalysts are employed in solution polymerization reactions at very high temperatures, they readily copolymerize the macromers with ethylene [151,993,996–999]. 13 C NMR analysis of these polymers showed the presence of 0.02 to B0.2 mol.% of alkyl branches. Relative reactivities of macromers in copolymerization reactions with ethylene can be characterized by the reactivity ratio r 1M ¼ kEðEÞ =kMacroðEÞ, which is similar to the r1 ratio describing copolymerization reactions of ethylene with 1-alkenes. The r1M value in ethylene polymerization reactions with constrained-geometry catalysts at 140–1901C was estimated as B7–12 [610,630]. When constrained-geometry catalysts are used in copolymerization reactions of ethylene and 1-alkenes (propylene, 1-hexene, 1-octene, etc.), auto-copolymerization reactions produce terpolymers that contain both short branches from 1-alkene units and long-chain branches. The latter branches are also present in polyethylene and in ethylene/1-butene copolymers prepared with silica- and alumina-supported metallocene catalysts [1000]. The same type of long-chain branching was discovered in high-temperature ethylene polymerization reactions with non-metallocene homogeneous catalysts containing combinations of tris(N,N-di-n-Oct-amino)titanium chloride and MgR2/AlEt3 cocatalyst mixtures [1001]. The presence of long-chain branches in ethylene polymers prepared with constrained-geometry catalysts strongly affects the rheological behavior of the polymers, their crystallinity degree, the melting point, etc. [73,148,630,993, 999,1000,1002–1005]. Some of these changes are very advantageous from the commercial standpoint. For example, ethylene polymers and copolymers containing long-chain branches have significantly better processability in the melt [151,152, 993,1006]. All these effects can be quantified if instead of studying polymers with naturally formed long chain branches they are modeled by copolymerizing ethylene with high molecular weight 1-alkenes, e.g., 1-eicosene [1005]. In several cases, auto-copolymerization reactions of ethylene and macromolecules with vinyl double bonds were observed even when heterogeneous TiCl4/ MgCl2-type catalysts were used at high temperatures, in spite of poor copolymerization ability of the catalysts [1007]. The content of macromer units in auto-copolymers can be directly observed by 13C NMR [1008]. The autocopolymerization reaction results in a gradual increase of the molecular weight of polyethylene [1007]. Dual-center route to synthesis of polymers with long-chain branches: An alternative route to the synthesis of branched polyethylene involves the use of two catalysts or a single catalyst with two types of active centers. In the simplest case, two metallocene catalysts are employed [598]. One of the catalysts, a bis-metallocene or a constrained-geometry catalyst, produces ethylene macromers and the second catalyst copolymerizes the macromers with ethylene [598]. A chromium-based catalyst, Cr(II)/SiO2 modified by t-BuLi, contains active centers of two different types [1009]. One center oligomerizes ethylene to linear

Chemistry and Stereochemistry of Polymerization and Copolymerization

205

1-alkenes with even numbers of carbon atoms, 1-butene, 1-hexene, 1-octene, etc. The center produces polyethylene of a high molecular weight and a broad molecular weight distribution. NMR analysis of the polyethylene showed that it is a copolymer of ethylene and the in situ generated 1-alkenes, B4 mol.% of 1-butene, B2.5 mol.% of 1-hexene, etc. [1009]. The CpTiCl3-B(C6F5)3-AlMe3 system, when used under moderate conditions, also contains two types of active species [1010]. One of them (probably, the [CpTiII]+ cation) selectively trimerizes ethylene to 1-hexene via the metallocycle mechanism and the second center containing TiIV species copolymerizes generated 1-hexene with ethylene. The formation of long-chain branches in propylene polymerization reactions is also possible [610,631,807,1004,1011]. Chain transfer steps in propylene homopolymerization reactions usually produce polymer molecules with vinylidene double bonds, (Reactions (3.15) and (3.54)), which are not reactive in copolymerization reactions activated by most transition metal catalysts. However, some metallocene catalysts, such as the Cp 2 ZrCl2-MAO system, produce polypropylene molecules with the vinyl bond (Reaction (3.55)) [416,592,610, 629,631,632,709]. Low molecular weight polypropylene with vinyl (allyl) endgroups can also be produced with bis(phenoxy-imine) complexes of Ti, Reaction (3.86) [807]. These macromers can be copolymerized with propylene using a second catalyst system. The catalyst produced from rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2 and MAO performs both these functions [631,1004], it produces polypropylene macromers terminated with the vinyl double bond and copolymerizes them with propylene. The incorporation level of macromers in the final product depends on the propylene concentration and can reach B1 mol.%. The presence of long-chain branches strongly affects the rheological behavior of the polymer melt [151,152, 1003,1004,1006]. Synthesis of polypropylene containing polyethylene long-chain branches was carried out in two-step processes [632,1011]. First, ethylene macromers were synthesized with a metallocene catalyst [631,632,1012] and then they are copolymerized with propylene using catalysts based on rac-Me2Si(2-Me-4-PhInd)2ZrCl2, rac-Me2Si(2-Me-Ind)2ZrCl2,or rac-Me2Si(2-Me-Benz[e]Ind)2ZrCl2. Depending on polymerization conditions, from 4 to 10 wt.% of polyethylene macromers can be incorporated into propylene polymers [1011].

CHAPTER 4

Synthesis, Chemical Composition, and Structure of Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

Contents 4.1. Early Solid Catalysts 4.2. Supported Catalysts for Homopolymerization and Copolymerization of Ethylene 4.2.1. Titanium-based Ziegler–Natta catalysts 4.2.2. Vanadium-based Ziegler–Natta catalysts 4.2.3. Chromium-based catalysts 4.3. Supported Ziegler–Natta Catalysts for Polymerization of Propylene and Higher 1-Alkenes 4.3.1. Catalyst based on d-TiCl3 and TiCl4 4.3.2. Catalysts utilizing MgCl2 as a support 4.4. Chemical Composition of Solid Components and Cocatalyst Mixtures of Ti-Based Ziegler–Natta Catalyst Systems 4.4.1. Supported TiCl4/MgCl2 catalysts, catalyst models 4.4.2. Supported TiCl4/MgCl2 catalysts, structure of solid components 4.4.3. Cocatalyst compositions, reactions of AlR3 and Modifiers II 4.5. Reactions Leading to Formation of Active Centers in Ziegler–Natta Catalysts 4.5.1. Early catalyst compositions, reactions between MCl3 and AlR3 4.5.2. Supported catalyst compositions, reactions between catalysts and cocatalysts 4.6. Metallocene Catalysts 4.6.1. Types of metallocene complexes used in polymerization catalysts 4.6.2. Cocatalysts for metallocene complexes 4.6.3. Activity of metallocene catalysts 4.6.4. Stereospecific metallocene catalysts 4.6.5. Reactions leading to active centers in metallocene catalysts 4.7. Non-Metallocene Homogeneous Catalysts 4.7.1. Complexes of early-period transition metals 4.7.2. Complexes of late-period transition metals 4.8. Supported Homogeneous Catalysts 4.9. Bicomponent Catalysts 4.9.1. Catalysts for polymers with a broad molecular weight distribution 4.9.2. Catalysts for synthesis of block-copolymers and branched polymers 4.9.3. Binary Ziegler–Natta/metallocene systems

209 211 212 220 221 224 227 228 236 237 238 242 243 244 245 253 253 255 262 267 269 270 271 275 277 284 284 285 287

207

208

Alkene Polymerization Reactions with Transition Metal Catalysts

4.10. Catalysts for Stereospecific Polymerization of Styrenes 4.10.1. Isospecific catalysts 4.10.2. Syndiospecific catalysts

287 287 288

This chapter reviews diverse subjects: (a) synthesis techniques and compositions of supported Ziegler–Natta catalysts, (b) structures of transition metal compounds serving as precursors in homogeneous polymerization catalysts, (c) the chemistry of reactions between various transition metal catalysts and cocatalysts, and (d) basic information about catalyst activity. In the case of metallocene catalyst systems and many non-metallocene homogeneous catalysts, the second subject is straightforward; the structures of the initial complexes are known, mostly from X-ray and NMR data. The chemistry of principal reactions between the soluble complexes of transition metals and organoaluminum cocatalysts, including the reactions leading to the formation of active centers in homogeneous catalysts, is also well researched. On the other hand, nearly all commercial Ti-based Ziegler–Natta catalysts are very complex multi-component mixtures. This chapter contains information on both components of these catalysts, Ti-containing catalysts and cocatalyst mixtures. Sections 4.2 and 4.3 present recipes of several important types of commercial supported catalysts. These recipes, mostly from the patent literature, describe procedures in which the solid components are prepared: chemical ingredients, the order of their interactions, the physical structure of final solid materials, etc. These sections also provide general information about the performance of the catalysts, which is usually found in patents and in articles describing the synthetic procedures. These general data usually include (a) catalyst productivity, (b) copolymerization ability of catalysts designed for ethylene copolymerization reactions, (c) fractional isospecificity of catalysts designed for propylene polymerization reactions, and (d) molecular weight distribution of polymers. The data on molecular weights of polymers produced with different solid and supported catalysts are not essential in this description; the molecular weight is always precisely controlled, either with hydrogen in Ziegler–Natta catalysis or with reaction temperature in chromium oxide catalysis. Patents rarely discuss the underlying chemistry of reaction steps leading to final solid catalysts. Section 4.4 provides information on the chemistry of the synthetic steps and the real chemical composition of the catalysts. Because the cocatalysts in modern supported Ziegler–Natta systems are also combinations of several components, Section 4.4 also describes chemical reactions that occur when these cocatalyst mixtures are prepared and used. The active centers of the catalysts are formed when catalyst components containing transition metals and cocatalysts are combined. Under conditions typical for alkene polymerization reactions, these components react vigorously and produce multiple identifiable products. The understanding of these reactions, which are described in Sections 4.5 and 4.6.5, is an essential prerequisite for the discussion of the structure of active centers in the catalysts in Chapter 6.

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

209

4.1. Early Solid Catalysts The first reports on alkene polymerization described two types of catalysts: (a) heterogeneous catalysts, combinations of solid transition metal compounds, mostly TiCl3, and organoaluminum cocatalysts and (b) pseudo-homogeneous catalysts, combinations of transition metal compounds soluble in aliphatic hydrocarbons, usually TiCl4, and organoaluminum compounds, which produce solid products when mixed. The first catalysts that afforded the synthesis of polypropylene with a relatively high content of the crystalline fraction were based on different crystal modifications of TiCl3. Three of the modifications, the a-form, the g-form, and the d-form, belong to the same crystal class, the FeCl3- or the CdCl2-type. The crystals consist of stacks of elementary sandwich-like sheets. Each sheet contains two layers of hexagonally packed Cl atoms and a layer of Ti atoms placed between the Cl layers [37,1014]. Figure 4.1 shows the elementary motif of the MX3 crystals (M ¼ Ti, V, Cr; X ¼ Cl, Br). Crystal cell parameters of TiCl3, VCl3, and CrCl3 are given in Table 4.1. The coordination number of the transition metal atoms is 6 and that of the Cl atoms is 2. The difference between the electronegativities of the transition metal atoms (1.5–1.6) and halogen atoms (2.8–3.0) is relatively small, and the ionic character of M–Cl bonds in the lattices is B30–40%. Extended X-ray absorption fine structure (EXAFS) analysis of g-TiCl3 confirmed this structure; all Ti
Cl ˚ ) and are much shorter than the sum of ionic radii of Ti distances are equal (2.21 A and Cl atoms indicating a predominantly covalent nature of the Ti–Cl bonding [1015]. The three TiCl3 modifications, a, g, and d, differ only in the relative stacking arrangement of the three-layered structures, as depicted in Figure 4.2. The differences are related to stacking of Cl atoms, the hexagonal packing in the a-form, the cubic packing in the g-form, and a random stacking in the d-form. The b-form of TiCl3 has a completely different structure. It consists of linear polymeric TiCl3

Figure 4.1 Elementary pattern of basal plane of MX3 crystals (M ¼ T|, V, Cr; X ¼ Cl, Br). Open circles, halogen atoms; solid circles, transition metal atoms. Top layer of halogen atoms is removed to expose arrangement of transition metal atoms.

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Alkene Polymerization Reactions with Transition Metal Catalysts

Table 4.1

Crystal parameters of MClx (hexagonal system) [37,1016]

MCl3

a0

b0

Ion radius of M (A
)

TiCl3 VCl3 CrCl3 MgCl2

6.12 6.01 5.93 3.63

17.50 17.32 17.44 17.79

0.68 0.64 0.62 0.72

A

A

A

A

B

B

B

C

A

C

C

A

B

A

A

C

A

B

B

A

B

C

C

C

A

A

A

A

B

B

B

C










Figure 4.2 Lateral faces of four modi¢cations of T|Cl3. Layer B in a-, g-, and d-modi¢cations corresponds to layer of chlorine anions in Figure 4.1.

rods, –Cl3Ti(m-Cl3)Ti(m-Cl3)Ti(m-Cl3)Ti–. The rods are stuck together resulting in the closest hexagonal packing of the Cl atoms. All Ti–Cl distances in this crystal ˚ , the Ti–Ti distances along the chain are 2.91 A ˚ , and the Cl–Cl form are 2.45 A ˚ distances within the chain are 3.40 and 3.51 A [37]. The first widely commercially used solid catalyst was the d-form of TiCl3 [4,6,49,50,1014]. It is produced by reducing TiCl4 with metallic aluminum at increased temperatures. This type of TiCl3 contains Al2Cl6 species in its crystals (its chemical composition is usually TiCl3  0.33 AlCl3) and it has a specific surface area of B40–50 m2/g. A more active form of d-TiCl3 containing 0.4–0.6 mol AlCl3/mol TiCl3 is produced by reducing TiCl4 with Al2Et3Cl3 below 01C. The third technique leading to the production of d-TiCl3 is prolonged grinding of a-TiCl3 or g-TiCl3 [1014]. The ground material contains two disordered crystalline forms that differ in the fraction of the cubic and the hexagonal packing of threelayered flat Cl-Ti-Cl sheets. The average size of crystallites in the ground material is B70 A˚ [1014]. The d-modification of TiCl3 is much more reactive as a catalyst than other crystalline forms of TiCl3 and it is still used in industry for the synthesis of isotactic polypropylene. The yield of the crystalline fraction strongly depends both on the crystal structure of TiCl3 and on the type of cocatalyst [1,4,6,49,50,214,1019], as shown in Table 4.2. The first successful commercial catalyst system for the

211

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

Table 4.2

a

Isotacticity of polypropylene produced with Ti-based Ziegler–Natta catalysts [214]

Catalyst

Cocatalyst

Temperature (1C)

Crystalline fraction (%)a

[mmmm]

d-TiCl3 d-TiCl3 d-TiCl3 d-TiCl3 TiCl4/d-TiCl3b TiCl4/d-TiCl3b TiCl4/d-TiCl3b

AlEt3 AlEt2Cl AlEt2Br AlEt2I AlEt3 AlEt2Cl AlEt2I

60 60 60 60 70 60 50

60 90 88 93 59 97 95

0.964 0.967 0.965 0.975 0.958 0.973 0.983

Fraction insoluble in boiling n-heptane. Solvay catalyst (Section 4.3.1).

b

manufacture of isotactic polypropylene, a combination of d-TiCl3 and AlEt2Cl, uses relatively inexpensive ingredients and produces polypropylene containing B95% of the crystalline fraction [4,6,49,50]. Commercial manufacture of isotactic poly(1-butene) initially employed the same catalyst modified by addition of iodine [1017,1019–1021]. A reaction between AlEt2Cl and I2 produces AlEt2I, a cocatalyst leading to a higher yield of the crystalline fraction (Table 4.2).

4.2. Supported Catalysts for Homopolymerization and Copolymerization of Ethylene Soon after the discovery of first heterogeneous Ziegler–Natta catalysts, the development of new catalyst systems became driven by requirements imposed by commercial manufacturers of polyolefins. These requirements are mostly determined by the following factors: 1. A particular manufacturing process where the respective catalyst is employed (slurry, gas phase, etc.). 2. Performance of the catalyst in terms of polymer properties. Below, these requirements are spelled out for most types of alkene polymers and commercial processes for their manufacture. 3. ‘‘Originality’’ of the catalyst. Many manufacturers require a ‘‘novel’’ catalyst composition protected by a patent. As a result, the patent literature contains several hundreds of patents describing numerous catalytic systems for homopolymerization and copolymerization of alkenes. Many of the patents have no practical significance; some were deliberately filed to confuse competitors or to hide one really useful catalyst recipe among dozens of less effective compositions. All modern commercially useful Ti-based catalysts are variations on combinations of a few simple chemical compounds. The differences between various catalyst recipes (often, a diligently guarded information) is mostly related to several

212

Alkene Polymerization Reactions with Transition Metal Catalysts

alternative techniques for combining these compounds into solid materials with a particular set of catalytic properties and physical characteristics. All modern supported Ti-based catalysts (and some V-based catalysts) for alkene polymerization can be roughly subdivided into two classes: (a) catalysts suitable for homopolymerization of ethylene and for ethylene/1-alkene copolymerization reactions leading to copolymers with a low 1-alkene content, 2–4 mol.% (LLDPE resins) and (b) catalysts suitable for the synthesis of isotactic 1-alkenes. The overlap between these two subclasses is relatively small because the requirements to the respective catalysts differ widely.

4.2.1. Titanium-based Ziegler–Natta catalysts 4.2.1.1. General features of catalysts for ethylene/1-alkene copolymerization The selection of a particular solid Ti-based catalytic system for the use in ethylene homopolymerization and ethylene/1-alkene copolymerization reactions under commercial conditions (usually in gas-phase reactions) is based on several technical criteria. These criteria are well defined for ethylene/1-alkene copolymers with a low content of 1-alkenes, from 2.5 to 4.0 mol.%. These copolymers are called in industry linear low density polyethylene (LLDPE) resins. Three 1-alkenes are used for the manufacture of these copolymers in gas-phase reactors, 1-butene, 1-hexene, and 4-methyl-1-pentene [88]. Four groups of criteria for the catalysts have to be taken into account: 1. End-use properties of the copolymers. Most of them are related to the properties of blown film: the dart impact strength and the tear strength of film, the content of an extractable material in it (a parameter important for food packaging applications), and cling properties (for film wrap). 2. Properties determining the behavior of a catalyst in the plant. These properties are very important for manufacturers of LLDPE resins in gas-phase fluidized-bed reactors. They include (a) the shape of catalyst particles (the spherical shape is preferred), (b) the absence of very small catalyst particles (fines), (c) moderate activity, (d) high reactivity toward 1-alkenes, (e) sufficiently high bulk density of particulate resins, and (f) low electrostatic charging. Catalysts of very high activity are avoided to prevent overheating of catalyst particles under the gas-phase environment. The formation of polymer particles with high bulk density is preferable; fluidized-bed reactors are difficult to operate if the bulk density of resins is too low, and the resins require more space in storage silos. Low electrostatic charging ability is required to avoid fouling of gas-phase reactors. 3. Resin processability, important parameters for manufacturers of LLDPE film, a high melt flow rate at high melts pressures (to maintain a high throughput of the resin-processing equipment), bubble stability of molten film, etc. 4. Catalyst cost. The structure of most solid catalysts used in gas-phase polymerization processes can be represented by a single scheme: Active Ti ingredient/Support/Carrier.

213

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

Table 4.3

Concentration of silanol groups in calcined silica

Calcination temperature (1C)

300

W1.6 [SiOH] (mmol/g SiO2) Fraction of isolated Si-OH

400

500

600

700

800

B1.3

B1.1

0.7–0.8

0.5–0.6

0.4–0.5

B0.4

B0.6

B0.7

B0.8

Nearly all catalyst recipes use TiCl4 or TiCl3 as active Ti ingredients. The support in the majority of the catalysts is MgCl2. Only one more crystalline support, catalytically prepared MgH2, is suitable for the synthesis of highly active TiCl4based catalysts [1022,1023]. Amorphous polymers with functional groups (hydroxyl, ester, etc.) capable of chelating TiCl4 can also be used as supports in ethylene polymerization catalysts [1024]. A carrier is a catalyst component that determines the size and the shape of catalyst particles [1025]. The preferred carrier is microporous spherical particles of amorphous silica with a pore volume of 1–3 mL/g and a specific surface area of 250–400 m2/g. The preferred diameter of the silica particles is 30–40 mm. The silica is typically calcined at temperatures from 500 to 7001C to reduce the concentration of silanol (Si–OH) groups, as shown in Table 4.3. The calcination leaves two types of silanol groups on the silica surface, isolated Si–OH groups and vicinal Si–OH groups, two OH groups bound to adjacent Si atoms. The amount of silanol groups is determined by reacting them with Grignard reagents or with ZnEt2 [1026,1027]. The fractions of the isolated and the vicinal silanol groups depend on the calcination temperature [1026] (see Table 4.3). Treating calcined silica with organometallic compounds (usually with AlEt3) further reduces the number of the silanol groups: RSi2OH þ AlEt3 ! RSi2O2AlEt2 þ C2 H6 O

Si

OH

Si

OH

+ AlEt3

O

Si

O

Si

O

Al

Et

+ 2C2H6

(4.1) (4.2)

Another reaction often used to remove silanol groups from silica is its treatment with TiCl4 [1027–1029] RSi2OH þ TiCl4 ! RSi2O2TiCl3 þ HCl Si Si

Si

OH

O OH

+ TiCl4

O

O

Cl + 2HCl

Ti Si

O

(4.3) (4.4)

Cl

When silica is treated with highly concentrated TiCl4 solution of with neat TiCl4, the silanol groups completely disappear from its surface [1028]. g-Al2O3 is also used as the carrier for ethylene polymerization catalysts. It contains a small number of OH groups on the surface, which can be removed by treating the carrier with AlEt3 or with TiCl4. In some catalyst compositions, spherical MgCl2 particles serve both as a support and a carrier.

214

Alkene Polymerization Reactions with Transition Metal Catalysts

All procedures of catalyst synthesis can be subdivided in two classes depending on the method in which MgCl2 is introduced into a carrier: 1. Catalysts that use commercially available anhydrous MgCl2. The main difficulty in preparation of these catalysts is to make MgCl2 soluble in organic solvents, a step necessary for impregnation of carriers. 2. Catalysts in which MgCl2 is synthesized during catalyst preparation. The sources of magnesium are organomagnesium compounds, Mg(OR)2, MgR2, mixtures of MgR2 and AlR3, or Grignard reagents. The chlorine sources needed to convert the organomagnesium compounds to MgCl2 are TiCl4, HCl, or chloro-organic compounds, t-C4H9Cl, n-C4H9Cl, or CCl4.

4.2.1.2. Catalysts produced from soluble MgCl2 complexes Example 1E. Catalyst recipes based on TiCl4, MgCl2, and THF: The active ingredient in these catalysts is a Ti-Mg complex formed in a reaction between TiCl4 and MgCl2 in THF (tetrahydrofuran) solution [41,44,1030–1035]. Each of the two starting compounds forms THF-soluble complexes, MgCl2  (THF)2 and TiCl4  (THF)2. Mixing the two solutions produces bimetallic complexes of welldefined structures [41,1033,1036,1037]. The [Ti]:[Mg] ratio in the complexes ranges from 1:1 to 1:5 depending of the ratio between TiCl4 and MgCl2 in solution [1033]. Most of the complexes are ionic [41,44,1031–1033], e.g: TiCl4  ðTHFÞ2 þ 2 MgCl2  ðTHFÞ2 2ðTHFÞ ! ½Mg2 ðm-ClÞ3  ðTHFÞ6 þ ½TiCl5  THF


(4.5)

These complexes are active polymerization catalysts by themselves [41,1033,1034]. However, they are usually placed into silica pores [1031,1038]. Hydroxyl groups are removed from the silica surface, first by calcining it at 6001C and then by treating remaining silanol groups with AlEt3 (Reactions (4.1) and (4.2)). Solution of TiCl4 and MgCl2 in THF is mixed with the treated silica and then THF is removed by evaporation at 50–601C causing the ionic Ti-Mg complex to precipitate within silica pores. These solid products, when combined with AlEt3, form very active polymerization catalysts. However, they have several deficiencies, including relatively poor reactivity toward 1-alkenes. This deficiency was alleviated by pre-treating the solid catalysts with mixtures of trialkylaluminum compounds and AlEt2Cl, a step resulting in the removal of a part of THF from the crystalline complexes [1039–1041]. Example 2E. Catalyst recipes based on TiCl3, MgCl2, and THF: A similar catalyst is prepared from the d-modification of TiCl3 (TiCl3  0.33AlCl3) [1034,1039,1040, 1042]. Mixtures of two solids, MgCl2 and TiCl3  0.33AlCl3, readily dissolve in THF. The solution contains the bimetallic MgAlCl5  (THF)6 complex [41,1035]. When a pretreated silica carrier is impregnated with this solution, it produces a highly active catalyst. It is also treated with mixtures of trialkylaluminum compounds and AlEt2Cl to produce high-quality catalysts [320,1042].

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

215

A modification of this catalyst produces catalyst particles consisting of two components. The kernel of each particle is a small nonporous spherical particle of fumed silica. Each particle is ‘‘sugar-coated’’ with a thick layer of a solid TiCl3  0.33AlCl3/MgCl2/THF mixture deposited from solution in THF using the spray-drying method [1040]. Because each particle is essentially a pure catalyst ingredient, its productivity is several times higher than that of the catalyst with the same ingredients produced by impregnating porous silica. Example 3E. Catalysts based on MgCl2/alcohol complexes: MgCl2 forms numerous complexes with alcohols. The [alcohol]:[MgCl2] ratio in the complexes depends on the type of alcohol and the temperature at which the complexes are formed [1043]. The complexes usually melt at 80–1001C, much lower temperatures than MgCl2 itself, and they are readily soluble in an excess of alcohol. Both these features are useful for the preparation of Ti-based MgCl2-supported catalysts. One recipe of a catalyst of this type consists of two stages, the synthesis of MgCl2 support and its impregnation with TiCl4 [1043–1046]. To prepare the support/ carrier particles, MgCl2 is mixed with ethanol and a dispersant, paraffin oil. After the reaction mass is heated to 1201C, the melt of the MgCl2  xEtOH complex (x B2.8–3.0) is formed [1043]. It is thoroughly mixed with the dispersant until emulsion of the molten complex is formed. The emulsion is rapidly dispensed into a large volume of cold n-heptane and the MgCl2  xEtOH complex crystallizes into spherical solid particles 60–100 mm in diameter. A variation of this technique involves the use of a 1:1 mixture of paraffin oil and silicon oil for dispersion of the MgCl2  xEtOH complex [1047]. Two procedures are used to prepare Ti-based catalysts with this support. In the first procedure, TiCl4 is used to remove ethanol from the solid complex [1044]. Spherical particles of the complex are twice treated with neat TiCl4, first by heating the slurry in n-heptane from 20 to 1001C and the second time at 1301C [35]. This treatment produces the final TiCl4/MgCl2 catalyst containing B7.5 wt.% of Ti. In an alternative procedure, ethanol is removed from the MgCl2  xEtOH complex by heating and by the treatment with an organoaluminum compound [1045]. First, the spherical particles are heated from 50 to 1501C until the [alcohol]:[MgCl2] ratio in the complex decreases from B2.8 to 1.1. Then the support is suspended in hexane and remaining ethanol is removed at 601C by repeated treatment with solution of AlEt3. Finally, the support is treated with Ti(OBu)4 and SiCl4, first at room temperature and then at 651C. The composition of the final catalyst: Ti B9 wt.%, Mg B10.6 wt.%, Cl B44 wt.%. Both these catalysts have high activity in ethylene polymerization reactions, B100 kg/g Ti, and produce large spherical polymer particles. Similar catalysts are used for the preparation of mixtures of different alkene polymers within a single polymer particle (polyolefin alloys) [35,1045,1046]. 4.2.1.3. Catalysts produced by synthesis of MgCl2 A large variety of these catalysts are described in the literature. Several examples below are intended to provide a gist of procedures usually employed to produce the catalysts.

216

Alkene Polymerization Reactions with Transition Metal Catalysts

Example 4E: A catalyst uses MgCl2 both as a carrier and a support [1048,1049]. The MgCl2 particles are prepared from a dialkyl magnesium compound. Solution of MgR2 is mixed with di-i-amyl ether at an [ether]:[MgR2] ratio of B0.4 and forms an MgR2/ether complex. t-Butyl chloride is slowly added to the vigorously stirred solution at 501C at a [t-C4H9Cl]:[MgR2] ratio of B1.9–2.0: MgR2 þ 2 t-C4 H9 Cl ! MgCl2 þ 2 t-C4 H9 2R

(4.6)

MgCl2 produced in Reaction (4.6) precipitates in a form of porous spherical particles 25–50 mm in diameter with a specific surface area of B40 m2/g. The size of the particles is controlled by the amount of the ether, reaction temperature, stirring speed, and the rate at which t-butyl chloride is added. In the second step, the catalyst is prepared by reacting TiCl4 with AlR2Cl in the presence of the MgCl2 support. The reaction is carried out in hexane-ether slurry at 50–801C at a [Ti]:[Al]:[Mg] molar ratio of B1:1.2:3.5. The principal chemical reaction is exhaustive reduction of TiIV species to TiIII: TiCl4 þ AlR2 Cl ! TiCl3 þ AlRCl2 þ R2R

(4.7)

The final catalyst has the following composition: [TiIII]:[Mg] ¼ 0.24, [Cl]:[Mg] ¼ 2.56, [Al]:[Mg] ¼ 0.05. This catalyst is designed for the use in gas-phase fluidized-bed polymerization processes. A significant technological advantage was found in its prepolymerization [1048]. This stage is carried out in two steps. First, the catalyst is combined with a trialkylaluminum cocatalyst and reacted with a small amount of ethylene to produce a 2:1–2.5:1 polyethylene/catalyst mixture. The second prepolymerization step is carried out with an ethylene/1-alkene monomer mixture, and the polymer/catalyst ratio is increased to 50–100. Spherical catalyst particles based on MgCl2 produce spherical prepolymerized catalyst particles of a larger diameter, which are easily handled in gas-phase reactors. Example 5E: A different group of catalysts uses Mg compounds as supports and silica as a carrier [1050,1051]. Spherical amorphous silica is calcined at 6001C, slurried in an alkane medium and treated with Mgn-Bu2 at 50–601C. The organomagnesium compound migrates into silica pores and partially reacts with silanol groups on the silica surface, similarly to Reactions (4.1) and (4.2). In the next step, an organic or an organometallic compound (a modifier) is added to the slurry. If the modifier is CCl4 or CCl3CH3, it converts Mgn-Bu2 into MgCl2, and if the modifier is an alcohol or Si(OEt)4 it converts Mgn-Bu2 into Mg(OR)2. Finally, the mixture is reacted with TiCl4 at 50–601C. The main advantage of this catalyst preparation scheme is versatility, catalysts with different reactivity toward 1-alkenes are produced by changing the nature of the modifying organic compound or by varying its amount. These catalysts can be used with two different types of cocatalysts, AlR3 or AlR2Cl [1050,1051]. Several examples in Table 4.4 demonstrate inter-dependencies between the catalyst recipes (the modifying compound and the type of cocatalyst), the productivity of the catalysts, and the molecular weight distribution of ethylene/1-hexene copolymers (LLDPE resins) they produce.

217

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

Table 4.4 Performance of catalysts containing synthesized MgCl2 in ethylene/1-hexene copolymerization reactionsa at 801C [1050,1051]

a

Modifying organic compound

Cocatalyst

Productivity (g/g cat h)

Mw/Mn

none n-C4H9OH CCl4 CCl4 CCl3CH3 Si(OEt)4 Si(OEt)4 Si(OEt)4

AlEt3 AlEt3 AlEt3 AlMe2Cl AlEt3 AlEt3 AlMe3 AlMe2Cl

1,250 2,130 3,400–4,660 2,610 3,080 3,900 3,460 1,570

B10 5.5 5.8–6.2 6.9 6.7 5.0 3.6 5.6

Copolymerization conditions are selected to produce copolymers containing 1.5–1.7 mol.% of 1-hexene.

Example 6E: This group of catalysts has an origin in a family of pseudohomogeneous Ti-based catalysts [1052,1053], which are used in solution polymerization processes at high temperatures [1054,1055]. All pseudo-homogeneous catalysts employ hydrocarbon-soluble starting ingredients, and solid catalyst particles are formed directly in a reactor when the dissolved transition metal compounds are combined with cocatalysts. Several examples of these pseudo-homogeneous catalysts are described in Section 4.2.1.5. Supported catalysts based on pseudohomogeneous catalysts are prepared in several steps [1056,1057]. The carrier is amorphous silica; it is dried at 150 and 2001C and then calcined at 6001C. The carrier is slurried in hexane and treated with AlEt3 at 201C to remove residual silanol groups from its surface. In the next step, the silica particles are treated with TiCl4 at 01C in hydrocarbon slurry at a [TiCl4]:[silica] ratio of 0.15 mmol/g. The last step generates a support, the silica/TiCl4 product is reacted in hydrocarbon slurry with a mixture of MgBu2 and AlEt3 at a [MgBu2]:[silica] ratio of B1.2 mmol/g. Then MgBu2 is reacted with an excess of t-C4H9Cl at a B2:1 molar ratio. This reaction produces finely dispersed MgCl2 particles inside silica pores. The order of these steps can be changed. For example, the calcined silica can be reacted first with MgBu2, then with AlEt3 or Aln-Oct3 [1057], then with t-C4H9Cl, and, finally, with TiCl4. This catalyst formulation can also include organic modifiers, an ether or an ester, at a [modifier]:[Ti] ratio of 3:1–10:1 [1056,1057]. 4.2.1.4. Specialized Ti-based catalysts for ethylene polymerization Several types of Ti-based Ziegler–Natta catalysts are used for the manufacture of a special grade of polyethylene called ‘‘bimodal polyethylene resins.’’ These resins are B1:1 mixtures of two finely dispersed polymer components, one with a very high molecular weight and another with a very low molecular weight. The mixtures are produced by polymerizing ethylene, in succession, in two reactors, first at a very high hydrogen concentration to produce the component of a low molecular weight (Mw 1.5–2.0  104) and then at a very low hydrogen concentration to produce the high molecular weight component (4–6  105).

218

Alkene Polymerization Reactions with Transition Metal Catalysts

The telltale ‘‘signature’’ of these polymers is their GPC curves; they consist of two clearly identifiable broad components. The bimodal resins are widely used for the manufacture of polyethylene film and pipes. The main requirement to catalysts suitable for the synthesis of these bicomponent mixtures is based on their kinetic behavior. In order to produce a homopolymer component with a very low molecular weight, the first step of the polymer synthesis is carried out at a very high hydrogen concentration. The activity of Ti-based catalysts in ethylene polymerization reactions is strongly depressed in the presence of high hydrogen concentrations (see Section 5.7.1.1.2). Therefore, the main requirement for catalysts employed in these polymerization reactions is very high activity. These catalysts are usually very similar to those used for isospecific polymerization of propylene and other 1-alkenes (Section 4.3.2). Another type of specialized Ziegler–Natta catalysts is developed for the synthesis of ethylene/1-alkene copolymers from a single monomer, ethylene. Economics and supply problems resulting from a lack of 1-butene sometimes make the manufacture of ethylene/1-butene copolymers impractical. Several specialized systems with two catalytic components were developed to allow the synthesis of ethylene/1-butene copolymers from a single feed. The first catalyst component oligomerizes ethylene to a mixture of light 1-alkenes (with 1-butene as the main component), and another catalyst component copolymerizes 1-butene with ethylene. The polymerization component has the same nature as Ziegler–Natta, chromium oxide, or metallocene catalysts. The ethylene dimerization/oligomerization components are based on several catalyst systems: 1. Ti(OR)4-based ethylene dimerization catalysts. Ziegler discovered in 1955 that combinations of Ti(OR)4 and AlR3 cleanly dimerize ethylene to 1-butene [1058]. The mechanism of this reaction is different from the mechanism of alkene polymerization reactions; it involves the formation of metallocycle intermediates [1059–1061]. 2. Immobilized Ni- and Zr-based catalysts of the general formula RMClx/ Ali-Bu2Cl, where M is Ni or Zr and R is an oligomer of butadiene, isoprene, or allene [1062]. These catalysts are produced in a reaction between NiCl2 or ZrCl4 and Ali-Bu3 in the presence of the diene. 3. (Imino)pyridyl complexes of CoCl2 (Section 4.7.2.1) [1063]. The complexes are converted to active species in a reaction with MAO, they oligomerize ethylene to mixtures of light alkenes, B80% of 1-butene and B15% of 1-hexene. These bifunctional catalysts are successfully employed both in slurry and in gasphase commercial processes [1064]. When the oligomerization/polymerization reactions are carried out in a continuous mode, the concentration of the 1-alkene rapidly reaches a stationary level when the rate of its formation by the oligomerization component becomes equal to the rate of its consumption in the copolymerization reaction with ethylene. The composition of ethylene/1-butene copolymers produced in these processes is determined by relative productivities of oligomerization and polymerization components in the catalysts.

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

219

4.2.1.5. Pseudo-homogeneous Ti-based catalysts for ethylene polymerization Ziegler developed the first commercially successful catalyst for the synthesis of linear polyethylene in 1953, a combination of TiCl4 and AlEt2Cl [23]. This catalyst system has several advantages: its components are cheap, both are soluble in aliphatic hydrocarbons and are easily metered into polymerization reactors, and the polymers produced with the catalyst are immediately suitable for many applications. This catalyst belongs to the class of pseudo-homogeneous catalyst systems. When the two catalyst components are combined, they vigorously react with the formation of catalyst species, a highly dispersed solid with a specific surface area of B30 m2/g after 15 minutes and B90 m2/g after 1 hour. In spite of the fact that this catalyst was in commercial use for several decades and was thoroughly studied in academia, the structure of the solid component and the chemical nature of active centers in the catalyst remain mostly unknown. Formally, the solid material formed after mixing the catalyst components under ambient conditions (the moment when the catalyst reaches the highest activity) has an empirical formula [TiCl3Et] [1066] but it contains mostly TiIII species [1066,1067]. The composition of the solid particles remaining in contact with the liquid phase at typical polymerization temperatures continuously changes and after 20–30 minutes it can be approximately represented as TiAl2Cl5, B10% of it is TiIII species [1065]. The liquid over the solid component also contains Ti components, mostly TiCl4 and some unknown TiIII species [1066]. When these solid materials and the liquid phase are separated, neither of them is catalytically active, but the polymerization activity is restored when the solid and the liquid components are recombined. The situation is further complicated by an observation that the separated solid material combined with fresh AlEt2Cl, behaves like b-TiCl3 and is not as active as the initial mixture of TiCl4 and AlEt2Cl [1066]. Because the nature and the concentrations of the catalyst components change with time, the polymerization activity of the catalyst mixture also varies depending on the time it is pre-reacted before the introduction of ethylene. For example, when the TiCl4-AlEt2Cl system is prepared at an [Al]:[Ti] ratio of 10 and kept at 301C before ethylene introduction, the catalyst productivity increases with the prereaction time and reaches a flat maximum after 60–120 minutes whereas a further pre-contact results in gradual deterioration of the system [1068]. Because of this kinetic effect, commercial processes for the manufacture of polyethylene with the TiCl4-AlEt2Cl system included a pre-contact chamber where the mixed catalyst components were kept for a certain period of time in the absence of a monomer. Catalyst systems of the TiCl4-AlR2Cl type can be used for polymerization of propylene as well. However, they produce polymers with a very low content of the crystalline fraction, o50%, and present no commercial interest in this respect. Pseudo-homogeneous TiCl4-AlR2Cl systems not only polymerize ethylene and propylene but they can react with aromatic hydrocarbons as well, e.g. [1065]: 4Ti2CH2 2CHR2Polymer þ C6 H5 2CH3 ! 4Ti2C6 H4 2CH3 þ CH3 2CHR2Polymer

(4.8)

220

Alkene Polymerization Reactions with Transition Metal Catalysts

This reaction can be viewed as a chain transfer reaction to the H–Carom bond. The Ti– C6H4–CH3 species generated in Reaction (4.8) acts as a chain initiation center and produces propylene oligomers with C6H4–CH3 groups as the starting chain end [1065]. Pseudo-homogeneous catalysts are widely used in industry in solution polymerization processes for the synthesis of ethylene/1-octene copolymers. These polymerization reactions are performed at 130–2001C in heavy hydrocarbons (C8– C10 mixtures) at reactor pressures of 3–20 MPa [88,151]. The catalysts of the highest activity are formed directly in a reactor [1052,1053]. Combining an organomagnesium compound, e.g., a mixture of MgBu2 and AlEt3, and a source of Cl atoms, t-butyl chloride under these conditions produces a finely dispersed MgCl2 support. The active ingredient in the catalysts is usually TiCl4 and the cocatalyst is AlEt2OEt [1052,1053]. These polymerization processes are very flexible and can accommodate several streams of soluble catalyst components [1053].

4.2.2. Vanadium-based Ziegler–Natta catalysts Vanadium based Ziegler–Natta catalysts for homopolymerization and copolymerization of ethylene are usually supported on specially prepared MgCl2 particles in processes similar to those employed for the manufacture of Ti-based catalysts [41,896,1069–1073]. The support can be commercially available MgCl2 or it is synthesized from metallic Mg or organomagnesium compounds [1069]. VCl4 or VOCl3 are supported on MgCl2 particles from solution in CCl4. The catalysts contain from 1 to 2.5 wt.% of V [1069]. V-based catalysts are also prepared by co-milling VCl4 or VOCl3 and the MgCl2  THF2 complex followed by activation with AlEt2Cl [1074]. All V-based catalysts are very active in ethylene polymerizations reactions, their productivity is B50 kg/g V atm  h. Depending on synthesis conditions, they produce polyethylene with a molecular weight distribution which is either broader than that of the polymers produced with Ti-based catalysts [1069,1073] or similar to it [1075]. The catalysts have a very high response to hydrogen, about one order of magnitude higher than Ti-based supported catalysts of the same type. They afford the synthesis of polyethylene with a molecular weight ranging from 2–3  106 to very low, B4  104 [41,1073,1075,1076]. Another advantage of V-based catalysts is their high reactivity toward 1-alkenes in ethylene/1-alkene copolymerization reactions [896,1073,1076,1077]. Several examples comparing Ti- and V-based supported catalysts are given in Table 4.5. Some V-based supported catalysts suitable for homopolymerization of ethylene and its copolymerization with 1-alkenes do not use MgCl2 as a support. Instead, TiCl3 (produced within silica pores by reacting TiCl4 with AlEt3) is used as the support and VOCl3 is used as the active ingredient [1078]. Two potential sources of active centers, Ti and V species, are present in these catalysts. However, their performance in ethylene/1-alkene copolymerization reactions indicates that the active species contain V atoms, a need for a chloro-containing activator, a broad molecular weight distribution of the copolymers (Mw/Mn B8–10), high reactivity toward 1-alkenes, and high sensitivity to hydrogen.

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

221

Table 4.5 Reactivity ratios in ethylene/1-alkene copolymerization reactions with Ti- and V-based supported catalystsa [1076,1077] 1-Alkene

Propylene 1-Butene 1-Hexene 1-Octene 4-Methyl-1-pentene a

r1 valuesb TiCl4/MgCl2

VCl4/MgCl2

13 29 100 200 140

4 13 23 60 31

Polymerization reactions at 701C, cocatalyst Ali-Bu3. r1 value is the reciprocal measure of copolymerization ability.

b

4.2.3. Chromium-based catalysts Several chromium-based catalysts are used in ethylene polymerization reactions. Two of them have great commercial importance, chromium oxide catalysts and supported organochromium catalysts. These catalysts account for nearly 40% of all the polyethylene resins (mostly ethylene homopolymers) manufactured throughout the world. 4.2.3.1. Chromium oxide catalysts Phillips Petroleum Company originally developed chromium oxide catalysts for the manufacture of ethylene homopolymers (HDPE resins). The catalysts convert ethylene to high molecular weight resins with a broad molecular weight distribution. These catalysts also copolymerize ethylene and 1-alkenes. Chromium oxide catalysts are supported on inert porous substrates, silica, silica-alumina, and silica-titania. The average particle size of the supports is 180–250 m and they have a very high specific surface area, W300 m2/g [31,33,38,75–79,1079–1083]. Amorphous alumophosphates, including AlPO4, are also used as the supports [347,1084,1085]. The synthesis procedure of most chromium oxide catalysts consists of two main steps. First, the support is impregnated with a source of Cr, aqueous solution of chromic acid or chromium nitrate, or alcoholic solution of chromium acetate. The solvent is removed at elevated temperatures, then the catalyst is dehydrated at B2001C and finally activated by calcination in a fluidizedbed reactor at 500–8501C in a dry oxidizing environment. The calcination step results in anchoring the Cr species to the surface of the support and their oxidation to CrVI [75]. The Cr content in the catalysts is usually low, from 0.5 to 1 wt.%, which corresponds to 2–4 Cr atoms per 10 nm2 area, and a further increase in the Cr loading does not increase the catalyst productivity [1086]. Depending on the activation conditions, the catalyst contains from 50 to B100% of its CrVI species in a form of silyl monochromates (they dominate at low Cr loading) and bichromates

222

Alkene Polymerization Reactions with Transition Metal Catalysts

[75,80,82,1082,1087], e.g. O O 2 SiOH + CrO3

Cr

Si

O Si

(4.9)

O

The main prerequisite for the synthesis of highly active catalysts is high porosity and relatively low mechanical strength of the support [1088]. These two conditions are required for rapid disintegration of the catalyst particles during polymerization reactions from B200 to B8–10 m [75,1087,1089]. The formation of chromates on the silica surface is practically complete when the calcination temperature reaches 5001C. However, the activity of the catalysts continues to increase with calcination temperature due to a gradual elimination of remaining silanol groups. The maximum activity is observed when the catalysts are calcined at 9251C [1080,1090]. When these catalysts are used in ethylene polymerization reactions, the chromate species are reduced to CrII species [75]: ðRSi2OÞ2 CrO2 þ C2 H4 ! ðRSi2OÞ2 Cr þ 2CH2 O

(4.10)

The catalysts can be also activated with carbon monoxide at 300–3501C [75,80, 82,1082,1091,1092] or with organomagnesium compounds [1093]. Both treatments reduce the CrVI species to CrII and CrIII. Several modifications of chromium oxide catalysts were introduced. The most successful include the treatment of the pre-catalysts with Ti(OR)4 or with (NH4)2SiF6 before calcination [81,1093]. Ti(OR)4 reduces silyl chromates to titanates of CrIII, which are later oxidized by hot air to titanyl chromates. Ti(OR)4modified chromium oxide catalysts produce polyethylene resins with a broader molecular weight distribution, their Mw/Mn ratio increases from 8–11 to 12–15; a change that makes the resins easier to process [75,81]. The productivity of chromium oxide catalysts under typical polymerization conditions at B80–901C is very high, B500 kg/mol Cr  atm  h [1094]. The molecular weight of the polyethylene is controlled mostly by reaction temperature. The main difference between chromium oxide catalysts and Ti-based Ziegler–Natta catalysts is complete insensitivity of the former to the presence of hydrogen. Ethylene homopolymers produced with chromium oxide catalysts have a high molecular weight and a broad molecular weight distribution, they are best suitable for blow molding applications, manufacture of plastic bottles and containers. However, these catalysts were also modified for ethylene copolymerization reactions with 1-alkenes [79,1095,1096]. One of the recipes for the copolymerization catalysts consists of the following steps: 1. Instead of silica, silica-titania is used as the support. It usually contains 5–8 wt.% of Ti and it has a very high specific surface area, W500 m2/g. 2. Chromium species are added to the support in the usual way, by forming a co-precipitated gel of silica, titania, and chromium oxide or by impregnating

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

223

silica-titania with CrIII salts. The chromium content in the catalyst ranges from 1 to 2 wt.%. 3. The CrIII species are oxidized with an oxygen/air mixture at 600–8001C. This process converts nearly all Cr species to CrVI [1097,1098]. Studies of model chromium oxide catalysts prepared on a flat surface of amorphous silica demonstrated that the active centers are derived exclusively from surfacebound silyl monochromates, which are stable at high temperatures [1099, 1100]. 4. Finally, the Cr species are reduced to CrII with carbon monoxide at 350–4501C. Another type of a chromium oxide catalyst uses silica as a support and a combination of Ti(Oi-Pr)4 and (NH4)2SiF6 as a modifier [841,1095]. Some chromium oxide catalysts used for ethylene copolymerization with 1-alkenes consist of the solid catalysts and a trialkylboron cocatalyst [347]. 4.2.3.2. Supported organochromium catalysts Several supported organochromium catalysts are commercially significant [843, 1101,1102]. The first of them is prepared by reacting bis(triphenylsilyl) chromate and calcined silica in a hydrocarbon medium [846,1103]. The chromate migrates into silica pores and forms a complex with isolated silanol group on the silica surface, (Ph3Si–O)Cr(O2)–OPh2–Ph    H–O–SiR [1104]. These Cr species are reduced with Et2AlOEt and produce covalently bonded CrVI species on the silica surface [1104] ðPh3 Si2OÞCrðO2 Þ2OPh2 2Ph    H2O2SiR þ Et2 AlOEt ! Lx Cr2O2AlðEtÞ2O2SiR

(4.11)

where L can be OQ or Ph3Si–O–. This catalyst is mainly used for homopolymerization of ethylene. This organochromium catalyst and chromium oxide catalysts are interconvertible. When an organochromium catalyst produced in Reaction (4.11) is calcined under vacuum or in a stream of oxygen, it is converted into a chromium oxide catalyst [1104]. Alternatively, a chromium oxide catalyst can be converted into a silyl chromate catalyst by reacting it with Ph3SiOH under moderate conditions [1101]: ðRSi2OÞ2 CrO2 þ Ph3 SiOH ! RSi2O2CrO2 2O2SiPh3 þ RSi2OH

(4.12)

XPS analysis of similar catalysts prepared with a chiral analog of Ph3SiOH, (Me)(Ph)(naphthyl)SiOH, showed that conversion in Reaction (4.12) varies from B50 to B100% depending on the [silanol]:[Cr] ratio [848], and that B95% of Cr atoms is the catalysts remain in the CrVI state [848]. These catalysts are activated with AlEt3; they polymerize ethylene at 601C to high molecular weight products with a very broad or a bimodal molecular weight distribution with Mw/Mn ratios from 40 to 100.

224

Alkene Polymerization Reactions with Transition Metal Catalysts

The second type of organochromium catalyst is produced by reacting chromocene with silanol groups on the surface of silica [843–845,1105] or alumina [1106]: Cp2 Cr þ H2O2SiR ! CpCr2O2SiR þ cyclopentadiene

(4.13)

The support is calcined at 6001C, then it is combined with chromocene solution in toluene at a Cr loading of 1.5–2.0 wt.%, and the solvent is removed by drying the catalyst at a moderate temperature. Supported chromocene catalysts are used in ethylene polymerization reactions at 90–1101C, they are highly active (productivity of 5–7 kg PE/g cat  h at 801C) and produce polymers with the molecular weight distribution of a medium width, Mw/Mn from B4–5 to W8. Molecular weights of the resins are controlled with hydrogen. These catalysts are predominantly used for the synthesis of low molecular weight ethylene homopolymers suitable for injection molding processing. Supporting another organochromium compound, CrIII[CH(SiMe3)2]3, on calcined silica also yields a highly active ethylene polymerization catalyst [1102]. This catalyst exhibits many characteristics of chromium oxide catalysts, it produces ethylene polymers of high molecular weight and with a broad molecular weight distribution, and the average molecular weight of the polymers decreases with temperature. These polymers contain a small number of long-chain branches, B0.02 mol.%. The activity of this catalyst with respect to its Cr content is 6–7 times higher than the activity of chromium oxide catalysts prepared with the same silica.

4.3. Supported Ziegler–Natta Catalysts for Polymerization of Propylene and Higher 1-Alkenes All modern supported Ziegler–Natta catalysts designed for polymerization of propylene and higher 1-alkenes are prepared with the use of organic modifiers. Historically, these organic compounds were called internal electron donors and external electron donors (or, simply, internal donors and external donors) because they form donor–acceptor complexes with various inorganic and organometallic components in the catalysts. However, the later research determined that the formation of donor–acceptor complexes is merely the first step. The modifiers also chemically react with inorganic ingredients of the solid catalysts during their synthesis and they usually react with organoaluminum cocatalysts. The products of these reactions, which often do not possess any special electron donating properties, perform different functions in the catalysts, some of which are still poorly understood. To avoid the confusion, the following definitions are used in this book:  Organic compounds used in the preparation of solid catalyst components, ‘‘internal electron donors’’ ¼ Modifiers I.  Organic compounds added to cocatalysts, ‘‘external electron donors’’ ¼ Modifiers II. From the historic perspective, Ti-based catalysts employed for the manufacture of isotactic polypropylene and polymers of higher 1-alkenes can be divided into several categories (or generations) [49,50].

225

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

Catalysts of 1st generation: Aluminum-activated TiCl3 (AA-TiCl3 or d-TiCl3) [4,6,49,50]. This catalyst is described in Section 4.1. In industry, d-TiCl3 is usually used in combination with AlEt2Cl as a cocatalyst. Catalysts of 2nd generation: Catalysts based on highly dispersed d-TiCl3 with a low AlCl3 content prepared in a special synthetic procedure from TiCl4 and organoaluminum compounds at temperatures below 01C [1107–1109]. Catalysts of 3rd generation: Catalysts utilizing esters of organic acids as Modifiers I [53,1110]. The most common modifier of this type is ethyl benzoate; however, esters of other aromatic and aliphatic acids can be used for this purpose as well [1110,1111]. Catalysts of the 3rd generation are nearly always used in combination with binary cocatalyst systems containing AlEt3 and Modifiers II. The latter are usually also esters of aromatic acids; they are employed at a molar [Al]:[ester] ratio of B3. Catalysts of 4th generation: Catalysts utilizing esters of organic diacids, such as phthalates or succinates, as Modifiers I [21,1112,1113] (see Scheme 4.1). These catalysts are also used in combination with binary cocatalysts containing AlEt3 and Modifiers II. The latter can be silane compounds RxSi(ORu)4
x (Scheme 4.1), sterically hindered piperidines, or acetals RuRvC(OMe)2 [21,1112–1114]. The molar [Al]:[Si] ratio in the cocatalyst mixtures varies from 10:1 to 20:1, and the molar [Al]:[Ti] ratio in the final catalyst systems is B250 [55,57–60,1115,1116]. These catalysts produce polypropylene that is 97–99% crystalline. Catalysts of 5th generation: Catalysts utilizing 1,3-diethers or diketones as single modifiers [49,50,1117,1119–1121]. The structures of some of the modifiers are shown in Scheme 4.2. Only 1,3-diethers with two bulky alkyl substituents in the 2nd position produce commercially acceptable catalysts [1119]. An obvious reason for the presence of these bulky alkyl groups is to secure a molecular conformation that makes the diether molecules effective bidentate ligands for transition metal atoms. 9,9u-Bis(methoxymethyl)fuorene (Scheme 4.2) is especially efficient as a modifier of this type [1120,1121]. In principle, catalysts of the 5th generation do

O

R′

O R

R

R′

O

O

Modifiers I: O

O R

R

R′′

O

R′

O

Phthalates

O

R′

Succinates

R

Si

Modifiers II: R′′

O

R′

R

R′

Si O

R

R

O

O

R

O

R

C O

R

R′′

Scheme 4.1 Organic components in supported Ziegler^Natta catalysts of 4th generation.

226

Alkene Polymerization Reactions with Transition Metal Catalysts

O

R′

R′′

R O

R

R′ R

O

O

O

R

R R

R′′ O

1,3-Diethers

Bis(alkoxymethyl)fluorenes

Diketones

Scheme 4.2 Organic components in supported Ziegler^Natta catalysts of 5th generation.

not require the use of Modifiers II as components of cocatalyst mixtures; simple AlR3 compounds are sufficient for the purpose. However, the use of Modifiers II further improves the performance of the catalysts. For example, although the content of the crystalline fraction in polypropylene prepared with a catalyst based on the fluorene-derived diether and AlEt3 is high, 95–96%, addition of Cpy2Si(OMe)2 (a typical Modifier II of the 4th generation) to AlEt3 increases the I.I. value to 99% [1121]. The 1,3-diethers can be also used as Modifiers II in catalyst compositions of the 4th generation [1117,1118]. For a period of over 50 years since the discovery of isospecific polymerization catalysts, researchers engaged in the synthesis of these catalysts pursued two overriding goals, to increase the catalyst productivity and to increase the fraction of the crystalline isotactic polymer. The definition of the ‘‘isotactic polypropylene fraction’’ has changed over time. Initially, the fraction insoluble in boiling n-heptane was universally assumed to be the isotactic fraction but currently a crystallization procedure with xylene is adopted as a measure of the content of crystalline polymer (Section 2.3.2.2). Similar procedures are used to evaluate the fractional isospecificity of the catalysts in polymerization reactions of other 1-alkenes and styrene. As the discussion in Chapters 2 and 3 shows, the content of the crystalline fraction is a very imprecise measure of catalyst isospecificity. It does not take into account the fact that all these catalysts contain several types of imperfectly isospecific active centers. Nevertheless, this single measure of the catalyst performance, fractional isotacticity, is extremely important from the practical standpoint. The content of the crystalline fraction is called ‘‘Isotacticity Index’’ or, in the abbreviated form, I.I. The I.I. number is usually accompanied by mentioning the type of the solvent used for polymer fractionation. This definition is used throughout this chapter when the merits of different catalysts and the methods for improving them are discussed. The molecular weight distribution of polypropylene produced with modern supported catalysts varies in a significant range depending on the type of catalyst. Table 4.6 gives several examples of the range. The choice of the catalyst depends on a particular application of the polymer. Polypropylene resins with a narrow molecular weight distribution are preferred for the manufacture of fiber and those with a broad molecular weight distribution for the manufacture of biaxially oriented film.

227

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

Table 4.6 Molecular weight distribution of polypropylene produced with TiCl4/MgCl2 catalystsa [321]

a

Modi¢er I

Modi¢er II

I.I.b (%)

Mw 105

Mw/Mn

Ethyl benzoate Di-i-butyl phthalate Dimethyl succinate 1,3-dietherc

Ethyl p-EtO-benzoate (Cy)(Me)Si(OMe)2 (Cy)(Me)Si(OMe)2 –

94.5 97.5 97.3 98.3

2.64 2.81 3.24 2.54

4.8 7.0 9.8 3.4

Cocatalyst AlEt3, polymerization in liquid monomer at 701C. Fraction insoluble in cold xylene. 2,2-Di-i-butyl-1,3-dimethoxypropane.

b c

4.3.1. Catalyst based on d-TiCl3 and TiCl4 The d-form of TiCl3 (Section 4.1) was used as the principal commercial catalyst for polymerization of propylene for several decades starting with late 1950s. Eventually, significant improvements in the performance of this catalyst were achieved, mostly by applying procedures similar to those used for the synthesis of MgCl2-supported catalysts described below. The first catalyst composition of this type was developed by Solvay Company and is often called ‘‘the Solvay catalyst’’ in the literature [1107,1109]. Example 1P: The first stage in the synthesis of the catalyst is the formation of the b-crystalline form of TiCl3. It is produced by reacting TiCl4 with EtAlCl2 or with Al2Et3Cl3 in hydrocarbon solution at 01C for several hours followed by heating to 601C. The reaction produces slurry of dense uniform spherical particles with a diameter of 15–30 mm consisting of a mixture of b-TiCl3 and AlCl3 in a 1:0.3 molar ratio [1107,1109]. This slurry is treated at 30–501C with an aliphatic ether, preferably with di-i-amyl ether. The treatment removes most of AlCl3 from the solid and incorporates a small amount of the ether into the b-TiCl3 particles. In the last step, slurry of the b-TiCl3 particles is treated with an excess of TiCl4 at 60–801C. This treatment drastically changes the nature of the solid material. The b-form of TiCl3 is converted into d-TiCl3 with a very high specific surface area, B150–200 m2/g. In addition, TiCl4 extracts all the ether from the solid particles and forms epitaxial complexes on the surface of d-TiCl3 [1109]. The final catalyst has a complex morphology; it consists of spherical particles 15–50 mm in diameter ˚. composed of very small primary crystals, 50–100 A Example 2P: An alternative recipe for the catalyst of this type starts with the formation of the TiCl4/dibutyl ether complex, by contacting the reactants at –651C at a molar [ether]:[TiCl4] ratio of B0.7 [1108,1122]. Then the TiCl4/ether complex is reduced with AlEt2Cl to b-TiCl3 at –651C, and the slurry of b-TiCl3 particles is converted to d-TiCl3 by treating it with an excess of TiCl4 at 701C. Similarly to Example 1P, several reactions occur during the last stage, aluminum compounds are extracted from the surface of the TiCl3 crystals and the latter develop a porous structure with a high specific surface area. The activity of this catalyst can be increased, and the content of the crystalline polypropylene fraction it produces can be improved, if Modifier II, usually an ester of an aromatic acid or a diacid, is added to the b-TiCl3 pre-catalyst before it is converted to d-TiCl3 [1122].

228

Table 4.7

a

Alkene Polymerization Reactions with Transition Metal Catalysts

Performance of TiCl4/d-TiCl3 Solvay catalyst vs. TiCl4/MgCl2 catalysta [1123]

Catalyst

Productivity (kg/g cat h)

I.I.b (%)

[mmmm]c

TiCl4/d-TiCl3 TiCl4/MgCl2

70 70

95 43

0.83 0.47

Cocatalyst AlEt2Cl, polymerization at 601C, PPr ¼ 2.5 bar. Fraction insoluble in boiling n-heptane. Measured by 13C NMR for unfractionated polymer.

b c

A comparison of preparation routes of these catalysts with those of MgCl2supported catalysts described in Section 4.3.2 shows many similarities. They include the formation of support particles of a required morphology, their treatment with an oxygen-containing organic modifier, and the final treatment with an excess of TiCl4 at a high temperature. These similarities allow one to describe these catalysts as TiCl4-based supported catalysts in which the support is d-TiCl3 rather than MgCl2. On the other hand, these catalysts exhibit many features typical for original d-TiCl3 catalysts; their fractional isospecificity is much higher when they are used with AlEt2Cl as a cocatalyst instead of AlEt3 [1107,1123]. Table 4.7 compares the performance of the Solvay TiCl4/d-TiCl3 catalyst and a TiCl4/MgCl2 catalyst [1123]. The TiCl4/MgCl2 catalyst prepared without organic modifiers has very poor fractional isospecificity whereas the TiCl4/d-TiCl3 catalyst produces commercially acceptable, highly crystalline polymers. Aluminum alkoxides AlR2(OR), where OR is sterically hindered phenoxy group, such as 4-Me-2,5-t-Bu2-C6H2O–, can be used as alternative cocatalysts with the TiCl4/ d-TiCl3 catalyst instead of AlEt2Cl [1108]. These cocatalysts are easily produced from AlR3 and hindered phenols. Combinations of AlR2(OR) and AlEt2Cl produce more active catalysts systems than AlEt2Cl alone and increase the content of the crystalline fraction to B98% [1108].

4.3.2. Catalysts utilizing MgCl2 as a support At the present time, the principal support for Ti-based catalysts designed for isospecific polymerization of propylene and other 1-alkenes is MgCl2 in a microcrystalline form [53]. Several ingenious techniques were developed for the manufacture of the support, either by a physical or a chemical treatment of commercially available anhydrous MgCl2 or through its chemical synthesis. The following techniques are the most common: 1. Milling anhydrous MgCl2 in the pure state or in mixtures with other catalyst ingredients, TiCl4 and Modifiers I (esters of aromatic acids and diacids, diethers, etc. (see Schemes 4.1 and 4.2)). 2. Crystallization of MgCl2 complexes with polar organic compounds into particles of a desired size and shape followed by the removal of the polar compounds from the complexes, either by heating or by reacting them with TiCl4, SiCl4, or AlR3. 3. Synthesis of MgCl2 from MgR2 and chloro-containing inorganic compounds [1124].

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

229

Most modern supported catalysts except for the catalysts of the 5th generation are activated by combinations AlEt3 and different Modifiers II, the molar [AlEt3]:[Ti] ratio in the catalyst systems varies from 100 to B250. All catalysts of the TiClx/MgCl2 type contain several types of active centers. They produce polymers with relatively broad molecular weight distributions and with different contents of crystalline polymer fractions (Table 4.6). Both parameters strongly depend on the type and the amounts of Modifier I and Modifier II. If TiCl4/MgCl2 catalysts of the 3rd or the 4th generation are prepared without Modifiers I and tested under typical reaction conditions, 50–701C and with pure AlEt3 as a cocatalyst, they produce polypropylene with the content of the crystalline fraction of merely 30–50% [484,497,1125–1128]. When the same catalysts are prepared with Modifiers I, I.I. values increase to 80–85% (still unacceptable commercially), but when Modifiers I and Modifiers II are both applied, thee I.I. value increases to B93–95% [484,497,1125–1128]. Organic diethers exert a strong modifying effect on the catalysts of the TiCl4/ MgCl2 type. The best catalysts of the 5th generation are prepared with aliphatic 1,3diethers, b-diketones, or b-diesters with bulky substituents attached to the carbon atom separating the two functional groups (Scheme 4.2). Typical examples of such modifiers are 2,2-cyclohexyl-1,3-dimethoxypropane, 2,2,4,6,6-pentamethyl-3,5heptanedione, and diethyl 2-n-butylmalonate [1119]. All these catalysts produce polypropylene with the content of the crystalline fraction in excess of 95%. Some aromatic diethers exert the opposite effect on the Ti-based catalysts, their addition results in a drastic decrease in the content of the crystalline polypropylene fraction and the formation of amorphous atactic polypropylene [484,490] (see Section 4.3.2.5). The third type of organic modifiers does not affect the steric composition of alkene polymers in any noticeable degree but significantly increases the overall activity of the catalysts of the 4th generation. One example of such a modifier is t-butyl methyl ether [459,1129]. When this compound is used as Modifier II for the TiCl4/MgCl2/dibutyl phthalate-Ali-Bu3 system, it doubles the productivity of the catalyst [459].

4.3.2.1. Catalysts produced by milling MgCl2 First MgCl2-supported TiCl4-based catalysts of the 3rd generation were produced in several alternative procedures, each involving intensive milling of anhydrous MgCl2 or co-milling MgCl2 with Modifier I or with a combination of the modifier and TiCl4. Example 3P [1130]: The support is prepared by grinding anhydrous MgCl2 in a ball mill. The ground material is treated with neat TiCl4 at 801C and excess TiCl4 is removed with a high-boiling aliphatic solvent (decane). These catalysts, when combined with AlEt3 as a cocatalyst, are very active in propylene polymerization reactions, their productivity exceeds the productivity of the d-TiCl3-AlEt3 system by a factor of B40. However, the fraction of crystalline polypropylene in these reaction products is very low, o30–35% [1130]. The addition of ethyl benzoate to the cocatalyst produces a significant increase in the content of the crystalline

230

Alkene Polymerization Reactions with Transition Metal Catalysts

fraction, it increases to B80% at a molar [ester]:[AlEt3] ratio of 0.3 and to 90% at [ester]:[AlEt3] B1. Examples 4P–6P [490,1131]: Three catalysts were prepared in multi-step processes involving co-milling anhydrous MgCl2 and other catalyst ingredients. The first catalyst was prepared by co-milling MgCl2 and the crystalline TiCl4/ethyl benzoate complex (produced by reacting equimolar quantities of the ester and TiCl4 in heptane solution at 401C). The synthesis of the second catalyst starts with co-milling MgCl2 and ethyl benzoate. The finely dispersed material is reacted with an excess of neat TiCl4 at 901C, and unreacted TiCl4 is removed with heptane. The third supported catalyst of this type is prepared by co-grinding MgCl2 and dibutyl phthalate at a 25:1 ratio followed by a reaction with TiCl4 at 1201C [490]. When these catalysts are used with combinations of AlEt3 and esters or aromatic acids as Modifiers II, they exhibit the behavior typical for all TiCl4/MgCl2 catalysts, their productivity significantly decreases with an increase of the amount of Modifiers II but the fraction of the crystalline polymer they produce increases. This effect is similar in polymerization reactions of propylene [1131] and such higher 1-alkenes as 4-methyl-1-hexene and 3,7-dimethyl-1-octene [471]. Example 7P [438,777]: A supported V-based catalyst designed for polymerization of propylene was prepared by ball-milling anhydrous MgCl2 with ethyl benzoate and by treating the finely dispersed product with solution of VCl4. This catalyst, in combination with a mixture of AlEt3 and methyl p-toluate at an [Al]:[ester] ratio of B3, is highly active, its productivity at 501C is B30 kg/mol V  h  mol C3H6, and the content of the crystalline fraction exceeds 97% [438]. The crystalline polypropylene fraction is nearly perfectly isotactic. In general, polymerization properties of this V-based catalyst are very similar to those of similarly prepared Ti-based catalysts. 4.3.2.2. Catalysts produced from soluble MgCl2 complexes MgCl2 dissolves in aliphatic alcohols at increased temperatures with the formation of MgCl2  xROH complexes. The composition of the complexes depends on the type of alcohol, the [alcohol]:[MgCl2] molar ratio in solution, and temperature. When the solutions are cooled, the complexes usually crystallize in a form of spherical particles with a diameter from 20 to 50 mm. Several techniques are used to remove alcohol molecules from the crystallized complexes and to convert them into particles of microcrystalline MgCl2: (a) treatment with an excess of TiCl4, (b) treatment with organoaluminum compounds, and (c) thermal treatment. It should be taken into account that when these catalysts are prepared from MgCl2  xROH complexes and the synthesis temperature exceeds 1101C (Examples 8P, 10P, and 11P below), a trans-esterification reaction catalyzed by TiCl4 takes place; the original ester Ph(COORu)2 is converted to mixed esters Ph(COOR)(COORu) and to Ph(COOR)2 [1132]. As a result, structural features and the performance of the catalysts do not depend on the size of the Ru group. Examples 8P–9P [1133,1134]: The solid catalyst is produced in four steps. First, anhydrous MgCl2 is dissolved at 1201C in a mixture of 2-ethylhexanol and n-decane at an [alcohol]:[MgCl2] ratio of B20. This step converts MgCl2 into the

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

231

1:1 MgCl2/alcohol complex soluble in excess alcohol. A small quantity of ethyl benzoate is added to the solution at the same temperature. Then the solution is cooled to B01C and mixed with an excess of TiCl4 and with ethyl benzoate. Initially, all the ingredients of the mixture remain in solution, but when it is slowly heated to 601C TiCl4 starts reacting with the alcohol with the liberation of HCl and the formation of particles containing MgCl2 [1134]: ½MgCl2  x ROH þ TiCl4 ! ½MgCl2  þ ROTiCl3 þ HCl

(4.14)

The presence of ethyl benzoate is essential at this stage, it replaces ROTiCl3 in the pre-catalyst particles. Finally, the particles are treated in two stages, first with an aliquot of fresh ethyl benzoate at 801C and then with an excess of TiCl4 in the presence of a small quantity of ethyl benzoate. The finished catalyst is activated with a 10:1:1 mixture of AlEt3, ethyl benzoate, and 2,2,6,6-tetramethyl piperidine. Its productivity in propylene polymerization reactions exceeds 7 kg per gram of catalyst in 30 minutes, and the fraction of crystalline polypropylene in the product is B92% [1134]. A similar procedure is used for the synthesis of a 4th generation catalyst containing precipitated MgCl2 and di-i-butyl phthalate [1116,1137]. Anhydrous MgCl2 and 2-ethylhexyl alcohol are mixed at a B3:1 [alcohol]:[Mg] molar ratio in n-decane. Heating the mixture at 1301C for 2 hours results in dissolution of MgCl2 as the MgCl2  xROH complex. Phthalic anhydride is added to the solution at 1301C in an amount of 0.15 mmol/mmol MgCl2, then the solution is cooled to 201C and slowly added to a large excess of neat TiCl4 at
201C. After addition of di-i-butyl phthalate, the solution is heated to 1101C, which results in precipitation of spherical porous pre-catalyst particles. The size of the particles and their porosity are determined by the temperature at this stage, the amount of TiCl4, dilution with n-decane, and the stirring speed. Example 10P [1043–1046]: Anhydrous MgCl2 is mixed with a dispersant (paraffin oil) and reacted with ethanol at 1201C. The reaction produces a melt of the MgCl2  xEtOH complex with x B2.8–3.0 [1043]. The molten complex is vigorously mixed with the dispersant until emulsion is formed. A variant of this recipe uses a two-step dispersion technique. First, MgCl2 is dissolved at 70–801C in ethanol at a 2.5–4.0 molar ratio and then the solution is dispersed in a 1:1 mixture of paraffin oil and silicon oil at 1201C [1047]. In both procedures, emulsion of the MgCl2  xEtOH complex is rapidly dispensed into a large volume of a cold aliphatic hydrocarbon, hexane or heptane, and the complex rapidly crystallizes into spherical porous particles with a diameter of 60–100 mm. To convert the MgCl2  xEtOH complex into the catalyst support, the particles are suspended in heptane at 0–51C, solution of AlEt3 is slowly added to the suspension, and the mixture is heated to 801C. AlEt3 extracts ethanol molecules from the MgCl2  xEtOH complex and the generated AlEt2OEt is removed from the support by washing. The produced spherical particular MgCl2 support has a specific surface area of W800 m2/g; its porosity is B0.6 cc/g. To convert this support to a catalyst of the 3rd generation, it is suspended in heptane, heptane solution of ethyl benzoate is added to it at 801C, and, finally, the support is twice treated with neat TiCl4 at 1101C. The finished washed catalyst contains B1.6 wt.% Ti and 7.3 wt.% of ethyl benzoate. Its specific

232

Alkene Polymerization Reactions with Transition Metal Catalysts

˚ , and the catalyst porosity surface area is B400 m2/g, the mean pore radius is B20 A is B0.4 cc/g. Example 11P [1047,1135]: Catalysts of the 4th generation containing large spherical particles of MgCl2 and esters of aromatic diacids are prepared using the same particulate MgCl2 support as that described in Example 10P. Particles of the MgCl2  xEtOH complex are suspended in a high-boiling alkane at 0–101C and are treated with an excess of TiCl4 at a molar [Ti]:[Mg] ratio of B20. The goal of this treatment is the removal of ethanol from the MgCl2 complex in Reaction (4.14) [1047,1135,1136]. EtOTiCl3 formed in this reaction is soluble in excess TiCl4; it is not an efficient catalytic Ti ingredient [1132]. Next, an ester of an aromatic diacid, di-i-butyl phthalate or di-i-octyl phthalate, is added to the mixture, and it is heated to 1201C resulting in incorporation of a significant amount of the ester into the solid product. After that, the solid material is again treated with an excess of TiCl4 at 1201C. Two important functions of Modifier I are (a) to protect MgCl2 particles from disintegration and recrystallization and (b) to control fixation of TiCl4 on the MgCl2 surface [1135]. In spite of large quantities of TiCl4 involved in the synthesis of the catalyst, the content of Ti species in the final catalyst composition is relatively small, the [Ti]:[Mg] ratio ranges from 0.08 to 0.1 [1135]. The particles on this catalyst are large, B80 mm in diameter, and have a specific surface area of B170 m2/g [1135]. A variant of the same preparation procedure involves mixing the solid MgCl2  xEtOH complex at
201C with neat TiCl4 at a molar [Ti]:[Mg] molar ratio of B50 followed by heating the slurry to 601C, adding the ester, and the second treatment with neat TiCl4 at 1201C [1047]. Example 12P: Some catalysts of the 4th generation include 2,3-disubstituted succinates as Modifiers I (Scheme 4.1). A large number of these esters of aliphatic diacids with R ¼ Me, Et, i-Pr, i-Bu, etc., and Ru and Rv alkyl or cycloalkyl groups are suitable for the synthesis of highly active catalysts [196,1138,1139]. Polymers prepared with these catalysts usually have a significantly broader molecular weight distribution, a prerequisite for their use in several important commercial applications, such as the manufacture of biaxially oriented polypropylene film. In a typical synthesis procedure, microspherical particles of the MgCl2  2.8EtOH complex prepared as described in Example 11P are contacted at 01C with an excess of neat TiCl4 and diethyl 2,3-di-i-propyl succinate. The temperature of the slurry is increased to B1001C and the mixture is kept under these conditions for 2 hours to allow for a complete removal of ethanol from the solid. Porous microparticles of MgCl2 containing the succinate are filtered and again treated with a large excess of neat TiCl4 at 1201C [1138]. These catalysts typically contain from 10 to 25 wt.% of the ester and 3–5 wt.% of Ti. The performance of succinate-based catalysts strongly depends on the type of Ru and Rv substituents at the two chiral carbon atoms in the succinate molecule (Scheme 4.1). The absence of alkyl substituents at these two carbon atoms or the presence of a small alkyl group at only one of them result in the formation of catalyst compositions of inferior activity and isospecificity. However, when two bulky alkyl substituents are introduced, for example, in 2,3-isopropyl succinates, the catalyst productivity can reach 70 kg/g Ti and the I.I. value approaches 99% [1138].

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

233

Catalysts prepared with aliphatic oxygen-containing modifiers: Catalysts of the 5th generation use aliphatic 1,3-diethers, 1,3-diketones, or 1,3-diesters as single modifiers [1117,1140,1141]. The structures of these modifiers are shown in Scheme 4.2. Preparation procedures of these catalysts are similar to those for catalysts containing precipitated MgCl2 and esters of aromatic diacids. As a rule, the catalysts of the 5th generation are used in combination with common cocatalysts, AlEt3 or Ali-Bu3, without any other modifiers. The main advantages of catalysts containing 1,3-diethers are very high activity, W100 kg/g catalyst, a very high content of the crystalline fraction, usually W97%, and a relatively narrow molecular weight distribution [61,1121,1140,1141]. However, the performance of these catalysts can be further improved when Modifiers II, the same as those used in catalysts of the 4th generation, are added to the cocatalysts. For example, a catalyst of the 5th generation synthesized with 9,9u-bis(methoxymethyl)fluorene (Scheme 4.2) as Modifier I produces polypropylene containing 95–96% of the crystalline fraction. The addition of Cpy2Si(OMe)2 as Modifier II to AlEt3 further increases the content of the crystalline fraction to 99% at the expense of a B20% loss in productivity [1121]. 4.3.2.3. Catalysts produced by synthesis of MgCl2 High-quality MgCl2 supports can be synthesized from metallic magnesium, Mg(OR)2, or dialkyl magnesium compounds. The chemicals are converted to MgCl2 by reacting them with chloro-containing compounds, TiCl4, SiCl4, CCl4, alkyl chlorides, or CCl3CH2OH. Example 13P [1142]: A highly active catalyst is prepared from MgBu2, CCl3CH2OH, di-n-butyl phthalate, and TiCl4. Silica with a high specific surface area is employed as a carrier to accommodate all catalyst ingredients within its pores. The spherical shape of silica particles aids in the formation of a stable, easily manageable concentrated slurry of polymer particles. Another source of magnesium for the production of MgCl2 in these catalysts is Mg(OEt)2. It reacts with an excess of TiCl4 in the presence of dialkyl phthalates and forms a very active catalyst for the synthesis of crystalline polyolefins. Example 14P [1143]: The support is synthesized by reacting metallic magnesium with ethanol in an 8:1 molar ratio in heptane slurry at 401C in the presence of a small quantity of I2 (B0.6 mol.%). The produced particulate Mg(OEt)2 is treated in octane slurry at 401C with a 20 molar excess of SiCl4 to replace OEt groups in Mg(OEt)2 with Cl atoms. The generated spherical porous MgCl2 particles are treated at 801C with di-n-butyl phthalate (0.013 mol/mol Mg) and then with TiCl4 (0.56 mol/mol Mg) at 1251C. Finally, the catalyst is washed and again treated with TiCl4 at a 1:1 mol/mol Mg ratio at 1251C. Propylene polymerization reactions with this catalyst are carried out at 801C using as cocatalysts mixtures of AlEt3 and Cpy2Si(OMe)2 in an 8:1 molar ratio. The productivity of the catalyst is B11–14 kg/g cat  h, and the content of the crystalline material is W98%. Example 15P [1144]: The support is synthesized by reacting powder of metallic magnesium with n-butyl chloride at a 1:3 molar ratio in heptane slurry at 1001C.

234

Alkene Polymerization Reactions with Transition Metal Catalysts

Produced particulate MgCl2 contains B10% of an organic residue and has a specific surface area of B90 m2/g. An ethyl benzoate-containing catalyst is prepared with this support by reacting it at 1051C with concentrated solution of TiCl4 in chlorobenzene at [TiCl4]:[MgCl2] ¼ 10 in the presence ethyl benzoate ([EB]:[MgCl2] ¼ 0.1) followed by two treatments with the same TiCl4/chlorobenzene solution. The final catalyst contains 0.16 mmol/g of Ti and 0.95 mmol/g of ethyl benzoate. A catalyst containing dibutyl phthalate is prepared in a similar way at 1151C and at a [DBP]:[MgCl2] ratio of 0.07. 4.3.2.4. Effects of Modifiers I and II on catalyst performance The effects of various organic modifiers on the performance of TiCl4/MgCl2-type catalysts in propylene polymerization reactions were thoroughly investigated [13,17,21,49,50,213,322,495,503,1111,1145,1146]. The data in Table 4.8 show some trends in the performance of a 4th-generation catalyst employing di-i-butyl phthalate as Modifier I and its prototype prepared without any modifier. In general terms, the following parameters determine the fractional isospecificity of the catalysts of this type: 1. Both organic modifiers are effective poisons of aspecific polymerization centers. Their presence in the catalysts and cocatalysts decreases the activity of the catalyst systems and increases the content of the crystalline fraction whereas the productivity of isospecific centers remains generally unchanged. 2. A combination of both organic modifiers, I and II, is essential for the formation of catalysts containing mostly isospecific centers. 3. GPC data show that the application of Modifier II increases the fraction of the polymer material with the highest molecular weight, Mw B1  106 [503]. This Table 4.8

Performance of TiCl4/MgCl2-type catalysts of 4th generationa [503,1111]

Modi¢er I

None

Di-i-butyl phthalate

a

Modi¢er II

None MeSi(OEt)3 PhSi(OEt)3 Ph2Si(OMe)2 Ph2Si(OEt)2 None MeSi(OEt)3 PhSi(OEt)3 Ph2Si(OMe)2 Ph2Si(OEt)2

Total productivity (g/g cat h)

Crystalline polymer fractionb Content (%)

Yield (g/g cat h) Mw/Mn

5,150 2,760 3,410 3,090 3,820 3,300 2,070 2,970 3,580 3,070

44 78 74 81 72 63 93 96 97 94

2,270 2,160 2,520 2,500 2,750 2,080 1,930 2,853 3,470 2,890

Propylene polymerization reactions at 601C, PPr ¼ 1 atm, cocatalyst AlEt3 [AlEt3]/[Modifier II] ¼ 10:1. Fraction insoluble in boiling n-heptane.

b

6.0 5.9 5.6 5.7 5.9 5.2 7.1 5.5 6.5 5.4

235

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

fraction has the highest stereoregularity. This shift in the molecular weight distribution is the principal factor determining an increase of the average molecular weight of the crystalline fraction prepared with these catalysts in the presence of Modifiers II. Silanes R2Si(ORu)2 are common Modifiers II in catalyst systems of the 4th generation. An increase in bulkiness of the alkyl group R in their molecules, Meon-Buoi-Bu ¼ Phoi-ProCy, is accompanied by a significant increase of the crystalline fraction in polypropylene produced with these catalysts. On the other hand, an increase in the bulkiness of ORu groups in R2Si(ORu)2 molecules has an opposite effect. These trends explain a frequent use of Ph2Si(OMe)2, (Me)(Ph)Si(OMe)2, Cy2Si(OMe)2, (Me)(Cy)Si(OMe)2, and Cpy2Si(OMe)2 as Modifiers II in industry. When piperidines are used as Modifiers II, the fraction of crystalline polypropylene increases with an increase of the number of methyl substituents in the a-position to the N atom, from B75% for 2,6-dimethyl piperidine to 95% for 2,2,6,6-tetramethyl piperidine, whereas tertiary piperidines are ineffective [495]. Preferential combinations of modifiers: The data in Table 4.9 illustrate one effect of modifiers in Ti-based polymerization catalysts that is well known in practice but poorly understood, preferential efficiency of particular combinations of Modifiers I and II in catalysts of the 3rd and the 4th generation. The first part of the table shows that although all three types of Modifiers II, esters, silanes, and piperidines, can be used with catalysts of the 3rd generation, only the ester produces catalyst systems of high fractional isospecificity. Similarly, 4th-generation catalysts form the most efficient catalyst systems when proper Modifiers II, silanes or piperidines, are used, whereas the use of ethyl benzoate results in catalysts of low activity and Table 4.9 Productivity of TiCl4/MgCl2/Modifier I catalysts activated with mixtures of AlEt3 and Modifier IIa [495]

a

Modi¢er I

Modi¢er II

Productivity (g/g cat h)

I.I.b (%)

Catalyst of 3rd generation Ethyl benzoate ‘‘-’’ ‘‘-’’ ‘‘-’’

None Ethyl benzoatec PhSi(OMe)3d 2,2,6,6-Me4-piperidined

1,170 500 750 710

44 98 80 83

Catalyst of 4th generation Di-i-butyl phthalate ‘‘-’’ ‘‘-’’ ‘‘-’’

None Ph2Si(OMe)2d 2,2,6,6-Me4-piperidined Ethyl benzoatec

1,000 740 820 57

71 95 95 79

Propylene polymerization reactions at 501C in the absence of hydrogen. Content of fraction insoluble in boiling n-heptane. c 3:1 Mixtures of AlEt3 and Modifier II. d 10:1 Mixtures of AlEt3 and Modifier II. b

236

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 4.10

a

Catalyst systems for synthesis of atactic polypropylenea [484]

Modi¢er II

Productivity (kg/g Ti)

I.I.b (%)

None o-Dimethoxybenzene 3,4-Dimethoxytoluene 1-Allyl-3,4-dimethoxybenzene

276 352 327 410

57 19 17 19

Catalyst TiCl4/MgCl2/dibutyl phthalate [AlEt3]:[Modifier II] ¼ 0.1, polymerization at 701C for 2 hours. Fraction insoluble in boiling n-heptane.

b

poor fractional isotacticity. This preferential effect is not universal. For example, 1,3-diethers are effective as Modifiers II in cocatalyst systems of the 3rd and the 4th generation [1120,1121]. 4.3.2.5. Catalysts for synthesis of atactic polypropylene Atactic polypropylene of a high molecular weight is an important commercial product and several Ti-based catalysts were designed with a goal of maximizing its yield. Two different approaches were developed. The first approach is based on common TiCl4/MgCl2 catalysts and cocatalyst compositions containing special Modifiers II, aromatic diethers with ortho-positioned methoxy groups [484,490]. Several examples of these Modifiers II are shown in Table 4.10. The most potent Modifier II of this type is 1-allyl-3,4-dimethoxybenzene. When a TiCl4/MgCl2/ dibutyl phthalate catalyst is activated with its mixture with AlEt3, the yield of amorphous polypropylene increases from B55% at 501C to 95% at 80–901C [490]. The polymers have a high molecular weight, Mw B2–3  105, and a narrow molecular weight distribution, Mw/Mn B3. The second approach is based on the synthesis of special supported catalysts that use AlEt3 as a cocatalyst and polymerize propylene to essentially amorphous materials [492,1147]. An efficient catalyst of this type was produced by reacting highly dispersed MgCl2 with AlEt3 and then with the solid TiCl3  (Py)3 complex. AlEt3 decomposes the complex; no pyridine is present in the final catalyst [492].

4.4. Chemical Composition of Solid Components and Cocatalyst Mixtures of Ti-Based Ziegler–Natta Catalyst Systems A large volume of research is devoted to detailed structural studies of supported catalysts for polymerization of ethylene and propylene, mostly by various spectroscopic techniques. In spite of a very thorough manner of this research, a side-by-side comparison of the results points at one underlying problem, different spectroscopic techniques lead to different, sometimes opposite, conclusions about the structure of the catalysts and the nature of active species in them.

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

237

4.4.1. Supported TiCl4/MgCl2 catalysts, catalyst models A large volume of literature is devoted to spectroscopic studies of TiCl4/MgCl2 catalysts. Most of them describe catalysts that contain either esters of aromatic acids (the catalysts of the 3rd generation) or esters of aromatic diacids (the catalysts of the 4th generation). Potentially, all these esters can form complexes both with anhydrous MgCl2 and with TiCl4, and significant efforts were undertaken to study these binary complexes. A cautionary note must be added at this point: the structure of the complexes, although important in itself, may not be related to the structure of final catalyst systems. Reactions between solid catalyst components and cocatalyst always result in a drastic rearrangement and decomposition of these original complexes, as discussed in Section 4.5. Complexes of MgCl2 and esters of aromatic acids and diacids: The motif of anhydrous a-MgCl2 with the hexagonal structure is shown in Figure 4.3. The lattice parameters are a ¼ 0.3640 nm, c ¼ 1.7673 nm, the distance between Mg atoms and ˚ . Finely dispersed MgCl2 and esters of organic acids form Cl atoms 2.101 A complexes, both in the bulk and on the surface of MgCl2 crystals. These complexes can be easily produced by co-milling MgCl2 and the esters. The complexes were thoroughly studied by IR [1128,1131,1148–1157]. The formation of the complexes results in the shift of the n(CQO) band from 1,725 to 1,690–1,680 cm
1. The formation of complexes between MgCl2 and esters of aromatic diacids (di-n-butyl phthalate, di-i-butyl phthalate, di-i-octyl phthalate) produces the same changes in IR spectra, the n(CQO) band shifts from B1,730 to 1,584 cm
1 [1115,1158] or to B1645–1650 cm
1, depending on the composition of the complexes [1115,1158–1160]. This change indicates that all the esters are bound to MgCl2 via their carbonyl groups [1152]. The position of the coordinated ester molecules with respect to MgCl2 crystal planes is not definitely determined. The n(CQO) band in the spectra of MgCl2/ester complexes is broad and asymmetric indicating the formation of several types of surface complexes. For example, several complexes

A

B

Figure 4.3 Motif of anhydrous a-MgCl2 with hexagonal structure. A, view from basal face; B, view from lateral face.

238

Alkene Polymerization Reactions with Transition Metal Catalysts

of ethyl benzoate can exist on the MgCl2 surface depending on the ratio between MgCl2 and the ester [1161]. [ethyl benzoate]:[MgCl2] n(CQO) (cm
1)

0.3 1,620

1.6 1,660

15 1,685

31 1,685

These complexes are relatively stable, their thermal decomposition starts at 230–2321C [1131,1148,1150]. Complexes and reaction products of TiCl4 and esters of aromatic acids: TiCl4 and esters of aromatic acids form strong complexes insoluble in aliphatic hydrocarbons. The nature of the complexes depends on the [ester]:[Ti] ratio, both 1:1 and 2:1 complexes were reported [1131,1149,1150,1152,1153,1156,1157,1160,1162]. An X-ray examination of crystalline TiCl4/ethyl benzoate complexes showed that the 2:1 complex has the Ti atom in the hexa-coordinated state with two CQO groups coordinated to it in the cis-arrangement [1157,1163]. The 1:1 TiCl4/ethyl benzoate complex is in reality the 2:2 complex, two TiCl4 molecules form a double bridge involving two Cl atoms, and the ester molecules are coordinated via their CQO bonds to each Ti atom. Ester of aromatic diacids form 1:1 complexes with TiCl4; both carbonyl groups of the ester molecules are coordinated to the same Ti atom [1160]. The complexes are easily identified by IR [1131,1149,1150,1152,1153, 1156,1157,1162]; most of them have two strong IR bands, one at 1,600– 1,590 cm
1 and another at 1,570–1,560 cm
1. When esters of aromatic acids are contacted with a large excess of TiCl4 at elevated temperatures, the conditions typical for the catalyst synthesis, they vigorously react [1115,1159,1164]: C6 H5 -CðQOÞOR þ TiCl4 ! C6 H5 -CðQOÞCl þ TiCl3 ðORÞ

(4.15)

C6 H4 ½CðQOÞOR2 þ TiCl4 ! C6 H4 ½CðQOÞCl2 þ TiCl3 ðORÞ

(4.16)

Acid chlorides produced in these reactions form complexes both with TiCl4 and MgCl2. As the temperature of the catalyst synthesis increases, Reactions (4.15) and (4.16) become more prominent and the fraction of MgCl2 complexes with acid chlorides increases at the expense of MgCl2/ester complexes [1159,1164]. The formation of acid chlorides is not detrimental to the performance of the catalysts; these compounds are used as Modifiers I in some catalyst recipes instead of esters [1165].

4.4.2. Supported TiCl4/MgCl2 catalysts, structure of solid components Solid TiCl4/MgCl2/ester catalysts are prepared by thoroughly mixing their three components using different techniques described in Sections 4.2 and 4.3. The nature of the products depends on the ratios between various catalyst ingredients, [MgCl2]:[ester], [TiCl4]:[MgCl2], etc., and on the method of catalyst preparation. Some physical characteristics of the catalysts should be taken into account. The shape of the catalyst particles depends on their preparation method. Milling techniques often produce particles of irregular shape whereas particles prepared by precipitation from MgCl2 solutions in polar solvents and by chemical synthesis

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

239

usually have the spherical shape [1043,1135,1166]. The preparation technique also affects the distribution of Ti species throughout MgCl2 particles. Catalyst recipes based on chemical synthesis and precipitation methods usually produce particles with a uniform distribution of Ti species both in the bulk of porous particles and on their external surfaces [1164,1166]. 4.4.2.1. Structure of MgCl2 support Anhydrous MgCl2 has a singular crystal motif, double chlorine layers with interstitial Mg ions in the sixfold coordination, the same packing motif as in the crystals of TiCl3 shown in Figure 4.1. This motif crystallizes in two forms. The common form has the cubic close packing (ABCABC stacking sequence) and a thermodynamically less stable form has the hexagonal close packing (ABAB stacking sequence) [49,50,53,1167], see Figure 4.3. Physical working of the crystals (milling, etc.) produces a rotationally and translationally disordered microcrystalline form resulting from 7601 rotation of Cl-Mg-Cl layers. X-ray analysis of TiCl4/MgCl2/Modifier I catalysts [49,50,1034,1111,1131, 1136,1148,1150,1151,1167–1172] showed that the same disordered form of MgCl2 is present in all these catalysts irrespectively of the technique used for their preparation. This disordered form is characterized by a very broad peak at 2y B30– 351 in the X-ray diffraction spectrum [49,50,1167,1172]. The spacing of the broad ˚ , is intermediate between spacings of the cubic and the hexagonal peak, 2.65A form [1167,1173]. The crystal mixture contains 22–25% of the hexagonal form, B30% of the cubic form, and B50% of the disordered form [1171]. Electronmicroscopic observations of milled MgCl2 also show that the material consists of small crystallites, 3–5 nm in length, embedded in the amorphous matrix [1173]. Simple ion-packing considerations show that Mg ions located at the lateral surfaces and crystal edges in each MgCl2 modification should be coordinatively unsaturated [1174,1175]. However, exposed metal atoms are always thermodynamically unstable. XPS data (see Section 6.3.1) indicate that they can form coordination bonds with a variety of molecules, including organic donors and TiCl4. The most probable cleavage faces of MgCl2 crystals, (100), (110), and (101), contain Mg ions with four and five chlorine anions instead of six chlorine anions in the bulk of the crystals [1167,1172]. These Mg atoms have chemical characteristics of Lewis acids. Their properties were explored with adsorption techniques widely used in the studies of heterogeneous acidic catalysts in general, such as adsorption of CO [1176,1177]. When TiCl4 is used as a probe and contacted with highly dispersed MgCl2, it mostly interacts with these acidic centers [1176]. Raman spectroscopic analysis of milled TiCl4-MgCl2 mixtures suggests that only one stable Ti species is present in the products (apart from weakly coordinated TiCl4 molecules), a complex of monomeric TiCl4 at (110) MgCl2 surfaces [1178,1179]. 4.4.2.2. Esters in catalysts Infrared spectroscopy is the most popular technique for the study of TiCl4/MgCl2/ ester catalysts. Although the catalysts can be prepared by a variety of techniques, IR data show that the outcomes of all these manipulations (co-milling, precipitation of

240

Alkene Polymerization Reactions with Transition Metal Catalysts

MgCl2/alcohol complexes, chemical synthesis of MgCl2) are practically the same, the final products are always the complexes of the esters with MgCl2 characterized by the n(CQO) band at 1,675–1,985 cm
1. In the most obvious example, when a catalyst is produced by co-milling anhydrous MgCl2 and the TiCl4/ethyl benzoate complex [1157], the n(CQO) doublet of at 1,636 and 1,577 cm
1 characterizing the initial TiCl4/ester complex gradually disappears and is replaced with a strong asymmetric band at 1,680 cm
1 typical for MgCl2/ester complexes. Supported TiCl4/MgCl2 catalysts of the 4th generation containing esters of aromatic diacids have the same structural features. The formation of complexes between the esters of organic diacids and MgCl2 gives raise to several n(CQO) bands in the 1,700–1,685 cm
1 range [1115,1158,1159]. Figure 4.4 shows a part of the IR spectrum of a typical supported TiCl4/MgCl2/dibutyl phthalate catalyst for propylene polymerization [1164,1180]. Three types of carbonyl species in the solid catalyst are identified [1164]. The most prominent among them are complexes of the ester and MgCl2, three IR bands at B1,700, 1,692, and B1,720 cm
1. Positions of these bands are very close to the positions of respective bands in the spectra of model MgCl2/di-i-butyl phthalate and MgCl2/ethyl benzoate complexes suggesting that all these complexes have a similar nature, one carbonyl group coordinated to one Mg ion. The second IR-identifiable species in the catalyst belong to phthalyl chloride formed in Reaction (4.16). The bands at 1,861 and B1,835 cm
1 belong to physically adsorbed phthalyl chloride on the MgCl2 surface, and the band and 1,761 cm
1 represents a MgCl2/phthalyl chloride complex. Conversion of dialkyl phthalates to phthalyl chloride during the synthesis of such catalysts usually varies 2 1.8 1.6

Absorbance

1.4 1.2 1 0.8 0.6 0.4 0.2 0 1900

1850

1800

1750

1700

Wavenumber,

Figure 4.4 catalyst.

1650

1600

1550

cm-1

n(CQO) range of IR spectrum of supported T|Cl4/MgCl2/dibutyl phthalate

241

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

Table 4.11 Performance of TiCl4/MgCl2/dibutyl phthalate catalysta after 1-hour extraction with toluene [459]

a

Extraction temperature (1C)

[Ti] (mmol/g)

Productivity (kg/g cat 2 h)

IIb (%)

Mn

Mw/Mn

No extraction 70 110

0.33 0.19 0.11

530 1,200 1,260

95 97 98

3.3  104 4.6  104 7.4  104

6.1 5.8 4.6

Propylene polymerization at 701C in the presence of H2, cocatalyst AlEt3/Ph2Si(OMe)2, PPr=7.4 bar. Fraction insoluble in boiling n-heptane.

b

from 15–20 to B50% depending on the reaction conditions during the contact of the esters and TiCl4. Because esters of aromatic diacids are present in the catalysts both in the bulk of MgCl2 crystals and on their surfaces, it is reasonable to assume that practically all ester molecules on the surface of the catalyst particles are converted to acid chlorides. Over 70% of generated acid chlorides form complexes with MgCl2, the rest are physisorbed. Identification of the third type of carbonyl species in the catalysts is much less certain. Judging by the position of their IR band, B1,670 cm
1 (Figure 4.4) this may be a complex of the phthalate molecule and TiCl4 in which the ester serves as a bidentate ligand, or it may be a strong MgCl2/di-i-butyl phthalate complex. These species accounts for B20% of the ester molecules in the catalyst. Some of the phthalate species and the TiCl4-derived species in the catalysts are weakly attached to the surface of MgCl2 crystals and can be relatively easily removed by extracting them with aromatic solvents. Their removal does not hurt the catalyst performance; in fact, it produces more isospecific and more active catalysts, as shown in Table 4.11. 4.4.2.3. Ti species in catalysts The extended x-ray absorption fine structure (EXAFS) technique affords the direct observation of Ti species in supported TiCl4/MgCl2-type catalysts [1181–1183]. In the simplest case, TiCl4 deposited on the surface of highly dispersed MgCl2, the analysis at the Ti K-edge and curve fitting for different models of Ti species suggest that TiCl4 is present in the catalyst in a form of the dimer attached at the (100) face of MgCl2 crystals. Each of the Ti atoms in the dimer is coordinated to a single Mg atom ˚ and the Ti-Ti distance in the [1183]. The Ti-Mg distance in this structure is B3.6 A ˚ dimer is 3.95 A. The TiCl4/MgCl2/ethyl benzoate catalyst apparently contains surface structures containing two different Ti species, a 1:1 complex of TiCl4 and the ˚ ) and Mg ion bridged by two Cl atoms (the Ti-Cl distance in the bridge is B2.5 A the ethyl benzoate molecule coordinated to the Ti atom through its CQO group [1181]. When TiCl4 is deposited on MgH2 instead of MgCl2, the distance between ˚ , suggesting that the two atoms are the Ti and the Mg atoms is very short, B2.9 A bridged by H atoms rather than by Cl atoms [1182]. XPS analysis of several TiCl4/MgCl2/ester catalysts did not find any evidence of complexes between TiCl4 and the esters [1184–1186]. When the catalysts are

242

Alkene Polymerization Reactions with Transition Metal Catalysts

prepared from TiCl4-esters complexes and MgCl2, the narrow peak of the TiCl4 complex at B459.2 eV disappears from the XPS spectra and a broad peak appears at 458.2–458.8 eV, the position typical for TiCl4 in TiCl4/MgCl2 catalysts produced in the absence of esters. All these spectroscopic data lead to one conclusion: whatever a procedure for the synthesis of TiCl4/MgCl2/ester catalysts is employed, the final catalysts nearly always have the same structure, finely dispersed MgCl2 crystallites with ester and TiCl4 molecules coordinated to the surface of MgCl2 microcrystals.

4.4.3. Cocatalyst compositions, reactions of AlR3 and Modifiers II The traditional name for chemical compounds used as Modifiers II in cocatalyst mixtures is ‘‘external electron donors.’’ As the research described in this section demonstrates, this is a misleading term. In reality, most of these organic compounds extensively react with organoaluminum cocatalysts. 4.4.3.1. Reactions of AlR3 and esters of aromatic acids Supported TiCl4/MgCl2/ester catalysts of the 3rd generation are activated with mixtures of AlR3 and esters of aromatic acids, usually ethyl benzoate or ethyl anisate, at a molar [AlR3]:[ester] ratio of B3. Reactions between AlR3 and the esters are well studied. They proceed in two stages, the formation of AlR3/ester complexes and reduction of the esters [36,1154,1155,1187–1190]. Complexes between AlR3 and esters form very rapidly; they have a distinctive yellow color. Several complexes of different compositions can form in AlEt3-ethyl benzoate mixtures depending on the [AlR3]:[ester] ratio [1149,1154,1161,1188,1189,1191]. When the ratio is close to 1, the 1:1 AlEt3/ethyl benzoate complex and the free ester are present in the mixture [1191] whereas both the 1:1 and the 1:2 complexes are formed at [AlR3]:[ester] B2. Similar complexes are formed in mixtures of AlEt3 and methyl p-toluate [1128] and in mixtures of AlEt3 and esters of aromatic diacids [1164]. These AlR3/ester complexes are stable only at an ambient temperature and only at [AlR3]:[ester] ratios of o1. At higher [AlR3]:[ester] ratios (standard ratios for the cocatalyst mixtures), the esters react with an excess of AlR3 with the formation of aluminum alkoxides [36,1155,1161,1164,1187,1188,1191]: Ph2CðQOÞ
OR0 þ 2 AlR3 ! R2 Al2OR0 þ R2 Al2O2CR2 Ph

(4.17)

For example, Reaction (4.17) between ethyl benzoate and AlEt3 produces two main alkoxides, Et2Al–OEt and Et2Al–OCEt2Ph. Both alkoxides, in turn, can form complexes with the esters (if the latter still remain in the system) and can participate in Reaction (4.17) instead of AlEt3 [1188]. Reaction (4.17) is relatively slow at room temperature but it greatly accelerates at temperatures typically encountered in polymerization reactions, 70–901C [1161,1188]. Acid chlorides (the products of reactions between esters and TiCl4, Reactions (4.15) and (4.16)) also vigorously react with organoaluminum compounds: Ph2CðQOÞ2Cl þ 2 AlR3 ! R2 Al2O2CR2 Ph þ AlR2 Cl

(4.18)

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

243

2,2,6,6-Tetra-substituted piperidines are used as Modifiers II in some catalysts of the 4th generation as well as in some earlier catalyst systems [1193]. Their reaction with an excess of organoaluminum compounds is similar to reactions of other secondary amines [36,1194]: RN2H þ AlR3 ! RN2AlR2 þ RH

(4.19)

4.4.3.2. Reactions of AlR3 with alkylalkoxy silanes and diethers TiCl4/MgCl2 catalysts of the 4th generation prepared with esters of aromatic diacids are usually activated with mixtures of AlR3 and alkoxysilanes at [AlR3]:[silane] molar ratios of 10:1 to 20:1. Organoaluminum compounds interact with the organosilane compounds. Detailed 13C NMR, 29Si NMR, and IR studies of PhxSi(OMe)4
x-AlEt3 and RxSi(OMe)4
x-AlEt3 mixtures (x ¼ 0–3) showed complexity of these interactions [1146,1164,1195,1196]. For example, reactions between PhSi(OMe)3 and AlEt3 can be described by the following scheme: PhSiðOMeÞ3 þ AlEt3 ! PhðMeOÞ2 Si2OMe    AlEt3

(4.20)

PhEtðMeOÞSi2OMe þ AlEt3 ! PhðMeOÞ2 Si2Et þ AlEt2 OMe

(4.21)

PhðMeOÞ2 Si2OMe    AlEt3 ! AlEt2 OMe þ PhEtðMeOÞSi2Et

(4.22)

PhEt2 Si2OMe þ AlEt3 ! PhEt2 Si2OMe    AlEt3

(4.23)

Relative yields of various complexes and products in Reactions (4.20–4.23) depend on the [Si]:[Al] ratio and reaction temperature. However, these reactions are not very fast under typical polymerization conditions [1161]. Monoalkoxysilanes Ph3Si-OMe and PhEt2Si-OMe are chemically stable in contact with AlEt3 and form complexes with them, similarly to Reaction (4.20) [497]. Diethers used as modifiers of supported catalysts of the 5th generation are bound to solid catalyst components quite strongly irrespectively of the manner in which they are introduced into the catalysts, either during the synthesis of the solid catalyst components or immediately before a polymerization reaction, together with cocatalysts [502]. AlEt3 does not extract them from the catalysts at temperatures from 20 to 701C [502,1197], and even at higher temperatures, provided that its concentration is low [440]. However, some diethers react with an excess of AlEt3 at 100–1201C with the release of ethylene [440].

4.5. Reactions Leading to Formation of Active Centers in Ziegler–Natta Catalysts The most demanding part of the research of Ti-based Ziegler–Natta catalysis is the studies of real catalyst systems, mixtures of solid catalyst components and cocatalysts. These mixtures are always slurries and a separate investigation of their solid and liquid components is required.

244

Alkene Polymerization Reactions with Transition Metal Catalysts

4.5.1. Early catalyst compositions, reactions between MCl3 and AlR3 Solid chlorides of ScIII, TiIII, and VIII rapidly and extensively react with organometallic compounds even under relatively mild conditions [1198–1201]. When the transition metal cannot be reduced, the principal reaction is its alkylation on the surface of MCl3 crystals, e.g: 4ScIII 2Cl þ ZnEt2 ! 4ScIII 2CH2 2CH3 þ ZnðClÞEt

(4.24)

A reaction between a-TiCl3 and AlMe3 vapor produces AlMe2Cl: ½TiCl3  þ AlMe3 ! ½TiCl2 2CH3  þ AlMe2 Cl

(4.25)

Even under moderate conditions, nearly one half of all Ti atoms on the surface of a-TiCl3 crystals participate in Reaction (4.25). Cocatalysts also readily coordinate with the Ti species. This phenomenon is common for all heterogeneous catalysts, it is easily detected by the elemental analysis of the reaction products or in labeling experiments [1198,1199,1202]: ½MCl3  þ M0 Rn ! ½MCl3     ðM0 Rn Þads

(4.26)

If the transition metal is reducible, Reaction (4.25) is followed by a reduction step: ½TiCl2 2R ! ½TiCl2  þ alkanes þ alkenes

(4.27)

The chemistry of the reduction reactions is very complex and their rate depends on the nature of AlR3 [1198,1199,1203]. Reactions (4.25) and (4.27) readily occur in hydrocarbon solvents, especially at elevated temperatures [1204]. For example, when d-TiCl3 is contacted with Ali-Bu3 at 301C, the yield of generated isobutene corresponds to alkylation of every Cl atom on the d-TiCl3 surface [1200,1203]: ½d-TiCl3  þ Ali-Bu3 ! ½TiCl2  þ Ali-Bu2 Cl þ i-C4 H10 þ i-C4 H8 (4.28) When TiCl3 is contacted with AlEt3, ethylene produced in Reaction (4.27) is partially consumed in a polymerization reaction [1199]. An interaction of VCl3 and Ali-Bu3 at 30 1C also produces isobutane and isobutene in equal amounts, possibly due to reduction of adjacent alkylated VIII species [1200,1203]: 4VIII 2Cl þ Ali-Bu3 ! 4VIII 2CH2 2CHðCH3 Þ2 þ Ali-Bu2 Cl 2 4VIII 2CH2 2CHðCH3 Þ2 ! 2 4VII þ CH2 QCðCH3 Þ2 þ CH3 2CHðCH3 Þ2

(4.29) (4.30)

A small fraction of M–R bonds generated in Reactions (4.25) and (4.29) survive Reactions (4.27) and (4.30). The presence of these M–C bonds was revealed when a-methyl styrene was added to aged TiCl3-Ali-Bu3 or VCl3-Ali-Bu3 systems: 4VIII 2CH2 2CHðCH3 Þ2 þ CH2 QCðCH3 ÞPh ! 4VIII 2CH2 2CHðCH3 ÞPh þ CH2 QCðCH3 Þ2

(4.31)

When the WVIII–CH2–CH(CH3)Ph species are decomposed with CH3O3H, they produce 3H-labeled cumene, 3HCH2–CH(CH3)Ph [1200]. The amount of isobutene released in Reaction (4.31) and the amount of the labeled cumene are

245

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

approximately equal and correspond to the number of active centers in the aged catalysts (see Section 5.7.4.2.6).

4.5.2. Supported catalyst compositions, reactions between catalysts and cocatalysts 4.5.2.1. Reactions in model catalyst systems Results of several model experiments with well-defined crystalline binuclear complexes explain many features of a complex set of chemical reactions that take place when supported catalysts of the TiCl4/MgCl2 type are contacted with organoaluminum cocatalysts. One of the model complexes is the crystalline (ethyl acetate)4(Cl)Mg(m-Cl)2TiCl3 complex containing four ethyl acetate ligands at the Mg atom and a double Cl bridge between the Mg and the TiIV atoms [1205,1206]. (Combinations of this complex and AlR3 readily polymerize ethylene.) When the complex is reacted with Ali-Bu3 under ambient conditions, several changes occur in the solid, as shown in Table 4.12. 1. Chemical analysis and IR data show that the ester is gradually removed from the solid in a reaction with Ali-Bu3, similarly to reactions between AlR3 and esters of aromatic acids described in Section 4.4.3.1. This reaction is relatively slow due to the low reaction temperature. 2. The Ti–Cl bonds of the original complex gradually disappear and IR features characteristic of highly dispersed MgCl2 appear. 3. The TiIV species in the original complex are reduced to TiIII species. 4. Some Ti species become solubilized resulting in a steady decrease of the molar [Ti]:[Mg] ratio. 5. The specific surface area of the solid greatly increases. This change is accompanied by the disappearance of the X-ray pattern of the original crystalline complex and the development of X-ray features typical for microcrystalline MgCl2. These chemical changes were compared with the polymerization activity of the solids. The initial complex becomes active in ethylene polymerization reactions at 901C only after significant induction periods, and the maximum polymerization rates are reached only after B90 minutes. The complexes pretreated with Ali-Bu3 for 30 and 60 minutes before the introduction of ethylene also exhibit the Table 4.12 Composition of solid products formed in reaction between (ethyl acetate)4 (Cl)Mg(m-Cl)2TiCl3 complex and Ali-Bu3a [1206]

a

Time (min)

Ethyl acetate (wt.%)

[Ti]:[Mg] (molar)

TiIII/S[Ti]

So (m2/g)

0 30 60 90

55.3 36.3 30.1 5.2

0.96 0.91 0.69 0.60b

0 0.22 0.70 0.93

o1 B3 45 84

Reaction at 201C at [Al]:[Ti] ¼ 250. Solid contains B6 wt.% of Al.

b

246

Alkene Polymerization Reactions with Transition Metal Catalysts

acceleration-type kinetics whereas the complex reacted for 90 minutes has the highest activity immediately after the start of polymerization reactions. The second example of well defined model crystalline complexes that can be used as polymerization catalysts are Ti-Mg complexes formed in a reaction between TiCl4 and MgCl2 in THF solution [41,1033,1036,1037] (see Example 1E in Section 4.2.1.2). Mixing THF solutions of MgCl2 and TiCl4 produces several binuclear crystalline complexes with the [Ti]:[Mg] ratio ranging from 1:1 to 1:5 depending on the [TiCl4]:[MgCl2] ratio in solution [1033]. All these complexes have distinct X-ray characteristics. When any of them is combined with AlEt3, it forms a highly active polymerization catalyst [41,1033]. However, the activity of these systems in ethylene homopolymerization and copolymerization reactions calculated per unit of Ti is practically independent on the composition or the crystal structure of a particular complex [41,1033]. Moreover, very similar catalysts are formed when TiCl4 in the TiCl4-MgCl2 compositions is replaced with TiCl3 [1039,1042]. Two principal conclusions can be made from these model experiments, First, true active ingredients in the majority of the catalysts of this type are some Ti species (most probably, TiIII species) on the surface of highly dispersed MgCl2 crystallites. Second, the structure of original complexes and the valence state of Ti atoms in them has no relevance to the structure of active centers in the catalysts. The research of reactions between solid catalyst components and cocatalysts serves an important purpose, to determine the chemical structure of the reaction products, which, potentially, can be active centers in alkene polymerization reactions. However, the active centers constitute a small fraction, usually no more than 2–5%, of all the Ti atoms in the supported catalysts (see Section 5.7.4). Therefore, a big gap exists between our understanding of the gross chemical composition of the catalysts, which can be examined experimentally by a variety of chemical and spectroscopic methods, and the structure of active centers themselves. The active centers are usually impervious to direct spectroscopic investigation and their nature can be revealed only through chemical investigation, by employing chemical reactions specific to the polymerization centers rather than reactions of many abundant but catalytically inactive ingredients in the catalysts. 4.5.2.2. Reactions between cocatalysts and Modifiers I Reactions between organoaluminum cocatalysts and esters of organic acids are described in Section 4.4.3.1. These reactions also take place when solid catalysts of the 3rd and 4th generations containing these esters as Modifiers I are brought into contact with the cocatalysts. The effects of these reactions for a model catalyst containing ethyl acetate as Modifier I are described in the previous section. The same changes were found in all catalysts of this type, a contact of ester-containing solid catalysts with cocatalysts results in a partial removal of the esters and the products of their reactions with TiCl4 (acid chlorides) from the solids. Reactions (4.17) and (4.18) represent these processes in a simplified form [196,1164, 1188,1192].

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

247

The speed of ester leaching from solid catalysts depends on several factors. Esters of aromatic monoacids are relatively easily removed from the catalysts with pure AlEt3 [497,1192]. For example, aging of a TiCl4/MgCl2/ethyl benzoate catalyst in the presence of AlEt3 at 501C reduces the amount of the ester in the solid catalyst by B90% after 10 minutes and by 95% after 30 minutes [1192]. However, this effect is masked in catalysts of the 3rd generation, when the cocatalysts are prepared by reacting AlEt3 and the same esters [1192]. The removal of esters and acid chlorides from catalysts of the 4th generation can be observed more clearly when Modifiers II do not obscure it [503,1164]. One example is shown in Figure 4.5, and some quantitative data are given in Table 4.13. They show that when a catalyst of the 4th generation reacts with AlEt3 or with 1.2 1

1.0

Absorbance

0.8 2

0.6 0.4 0.2

3

0.0 1860 1840 1820 1800 1780 1760 1740 1720 1700 1680 1660 1640 Wavenumber, cm-1

Figure 4.5 Removal of esters and acid chlorides from solid catalyst of 4th generation, T|Cl4/ MgCl2/di-i-butyl phthalate: original catalyst (1), after 60-minute contact with 0.1 M solution of AlEt3 (2) and 1.0 M (3) at 601C.

Table 4.13 Complex

Reaction between TiCl4/MgCl2/di-i-butyl phthalate catalyst and cocatalysta [1164] Total [CQO]b MgCl2 -PhClc (%) (%)

Initial catalyst 100 Mild reaction conditions B35 ([Al]:[Ti] ¼ 30, 201C) Standard polymerization B20 conditions ([Al]:[Ti] ¼ 180, 801C) a

Cocatalyst AlEt3/(Cy)(Me)Si(OMe)2, [Al]:[Si] ¼ 10. Amounts of carbonyl complexes remaining in solid products. DIBP ¼ di-i-butyl phthalate, PhCl ¼ phthalyl chloride.

b c

MgCl2 -DIBPc (%)

TiCl4 -DIBPc (%)

30 B4

53 B25

17 B4

B3

B15

B3

248

Alkene Polymerization Reactions with Transition Metal Catalysts

AlEt3/(Cy)(Me)Si(OMe)2 cocatalyst mixtures under conditions typical for propylene polymerization reactions, only B20% of the initial carbonyl species remain in the solid. A comparison with an assortment of different carbonyl species in the original solid catalyst (Figure 4.4) gives their reactivity order in reactions with AlEt3 [1164]: physisorbed acid chloride W MgCl2/acid chloride complexes W W MgCl2/ester complexes E TiCl4/ester complexes. Electron-microscopic studies show that most of ester molecules in catalysts of the 4th generation are buried inside catalyst crystallites [1180]. The data in Table 4.13 suggest that the surface of these catalysts is essentially free of carbonyl complexes after a contact with cocatalysts. NMR and IR analysis of soluble products in these reactions confirm that a fraction of reaction products between the esters and AlR3 at high temperatures are transferred to solution and that they are the same aluminum alkoxides that are formed in Reaction (4.17) in mixtures of AlEt3 and esters of aromatic acids. The removal of Modifiers I from the surface of TiCl4/MgCl2/ester catalysts by AlEt3 also manifests itself when components of the catalyst systems are precontacted in the absence of a monomer (a common procedure in industry). The pre-activation step produces a rapid change in the distribution of active centers that can be best investigated in polymerization reactions by the stopped-flow method [223,365]. Even a very short pre-activation at 301C results in a noticeable change both in the molecular weight distribution and in the stereo-distribution of polypropylene produced immediately after the pretreatment step due to the formation of a new family of highly isospecific active centers. Supported catalysts of the 5th generation contain aliphatic 1,3-diethers as single modifiers. The diethers are bidentate ligands (Scheme 4.2); they form strong 1:1 complexes with TiCl4 [1197]. The treatment of TiCl4/MgCl2/diether catalysts with AlEt3 does not remove the diethers from the catalysts [447]. The same diethers can be used as Modifiers II (instead of alkoxysilanes) with catalysts of the 4th generation. When solid TiCl4/MgCl2/ester catalysts are contacted with combinations of AlEt3 and the diethers, the esters are mostly removed from the catalysts in reactions with AlEt3 and the diethers coordinate with metal atoms of the catalysts. In this respect, the behavior of diethers is similar to the behavior of alkoxysilanes when the latter are used as cocatalyst components. 4.5.2.3. Complexes of MgCl2 and solid catalysts with silanes Supported catalysts of the 4th generation usually employ alkoxysilanes as Modifiers II. The silanes form complexes with organoaluminum cocatalysts (Section 4.4.3.2). When these complexes are contacted with MgCl2 at increased temperatures, the silanes form complexes with MgCl2 [1161,1164,1207]. Similar complexes are formed when the silanes are contacted with TiCl4/MgCl2 catalysts. As an example, when a MgCl2/TiCl4/di-i-butyl phthalate catalyst was reacted with a 10:1 mixture of AlEt3 and PhSi(OEt)3 at 751C for 15 minutes, two chemical transformations took place. Over 90% of the ester molecules were extracted from the solid catalyst, and were substituted with the same amount of PhSi(OEt)3 and its reaction product with AlEt3, (Ph)(Et)Si(OEt)2 [1161]. Several examples of accumulation of silane compounds in a

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

249

TiCl4/MgCl2/di-i-butyl phthalate catalyst treated at 601C with mixtures of AlEt3 and various silanes at an [AlEt3]:[silane] ratio of 10:1 are given in [503]. Silane MeSi(OEt)3 PhSi(OEt)3 Ph2Si(OMe)2 Ph2Si(OEt)2 Amount in catalyst (mmol/g) 0.35 0.30 0.30 0.16

The nature of complexes between alkylalkoxy silanes RSi(OMe)3 and various components of TiCl4/MgCl2 catalysts was studied by solid state cross-polarization 13 C NMR and 27Al NMR [1154,1208]. NMR data indicate that the coordination between RSi(OMe)3 and MgCl2 is quite weak, the silane compounds can be considered as inter-crystalline fluids in the pores of MgCl2 microcrystals. The treatment of the RSi(OMe)3/MgCl2 products with TiCl4 apparently results in the formation of TiCl4/RSi(OMe)3 complexes. A subsequent treatment with AlEt3 produces a small amount of AlEt3/RSi(OMe)3 complexes in solution (Section 4.4.3.2). The preformed TiCl4/MgCl2-RSi(OMe)3 complexes are quite stable. When contacted with AlEt3, they are initially inactive in polymerization reactions [1209,1210], whereas a preliminary contact of AlEt3/silane mixtures (and ensuing Reactions (4.20–4.23)) rapidly activates TiCl4/MgCl2 catalysts and produces systems with increased fractional isospecificity [1210]. 4.5.2.4. Valence state of titanium atoms Titanium-based catalysts for alkene polymerization usually use either TiCl4 or TiCl3 as initial transition metal components (Sections 4.2.1 and 4.3). However, reactions between these species and organoaluminum cocatalysts invariably result in a significant degree of Ti reduction. Chemical analysis: The reduction of TiIV species to TiIII species takes place when any supported catalyst TiCl4/MgCl2/Modifier I is reacted with a cocatalyst, either with pure AlR3 or with reaction products of AlR3 and Modifier II. Table 4.14 gives the distribution of Ti valence states in catalyst systems of the 3rd and the 4th generation. Obviously, the reduction level depends on the type of the cocatalyst, the [Al]:[Ti] ratio, temperature, and the presence of cocatalyst modifiers [777,1128,1211–1217]. The reduction reaction of Ti species in the catalysts occurs rapidly both in the absence of monomers and in their presence (within 5–10 minutes at 501C) and the distribution of the valence states remains mostly unchanged in the course of polymerization reactions [1211,1212]. Spectroscopic methods, XPS: The most potentially useful method for evaluating the valence state of Ti atoms on the surface of solid materials could be X-ray photoelectron spectroscopy (XPS). The energy of the largest Ti peak in XPS spectra, Ti 2p3/2, is sufficiently sensitive to the valence of Ti atoms. TiIV (uncoordinated)
458.1
458.7 eV [1218–1220]. TiIV (coordinated to organic ligands)
459.0
459.6 eV [1024,1166]. TiIII
457.0
457.8 eV [1166,1180,1221,1222]. TiII
456.0
456.1 eV [1180,1220].

250

Table 4.14

Alkene Polymerization Reactions with Transition Metal Catalysts

Reduction of Ti species in catalyst-cocatalyst systems.

Catalysta

Cocatalystb

TiCl4/MgCl2/ EB ‘‘-’’c ‘‘-’’ ‘‘-’’ ‘‘-’’c ‘‘-’’ ‘‘-’’c ‘‘-’’ ‘‘-’’ ‘‘-’’ ‘‘-’’ TiCl4/MgCl2/ DNBP ‘‘-’’ ‘‘-’’

none

Temperature Time (1C) (min)

6

TiIV (%)

TiIII (%)

TiII (%)

Ref.

90

B6

B4

[1211]

AlEt3 25 25 AlEt3 50 AlEt3 AlEt3/MPT 25 AlEt3/MPT a 25 AlEt3 50 AlEt3/MPT 50 60 AlEt3 60 AlEt3 AlEt3/EBa 60 AlEt3 50

5 10 10 5 10 5 10 120 2 2 60

B0 6 8 36 36 32 B0 B0 B0 B0 B28

71 65 74 25 25 24 B85 B20 B100 B100 39

29 29 18 39 39 44 B15 B80 B33

‘‘-’’ [1213] [777] [1211] [1213] [1212] [1214] [1215] [1216] ‘‘-’’ ‘‘-’’

50 AlEt3 AlEt3/silane 50

60 60

B7 B28

74 39

B19 B33

[1217] ‘‘-’’

a

Modifiers: EB ¼ ethyl benzoate, DNBP ¼ di-n-butyl phthalate, MPT ¼ methyl p-toluate. [Al]:[Ti] ¼ 50. c Measured in polymerization reactions. b

However, several attempts to apply the XPS method to the analysis of Ti-based catalysts produced contradictory results. The only observable Ti species on the surface of solid Ti-based catalysts are TiIV atoms [1024,1166,1180,1184,1185,1221– 1223]. A treatment of the catalysts with organometallic cocatalysts either leaves the TiIV species on the catalyst surface intact [1166,1184,1185] or reduces them to TiII species [1024,1184,1185], but TiIII species were never observed although their formation was clearly determined by chemical analysis. (The failure to observe any TiIII species cannot be attributed to insufficient sensitivity of the XPS technique. XPS analysis of TiCl4  (THF)2 and TiCl4  MgCl2  (THF)x complexes subjected to thermal decomposition at 4501C clearly shows the formation of TiIII species [1222,1224].) Moreover, a large fraction of Ti atoms on the surface of solid TiCl3 samples are the TiIV species as well [1166,1180,1185]. This observation is usually explained as a result of surface oxidation of TiIII atoms with impurities; a common feature in XPS analysis, which is sensitive only to the surface composition in solids. This inability to observe TiIII species in Ti-based catalysts reduces the role of XPS analysis to the studies of a few catalysts prepared by special vapor-deposition methods under extremely clean conditions [1186,1223]. The surface structure of these model catalysts and their reaction products with organoaluminum cocatalysts are discussed in Section 6.3.1. Spectroscopic methods, esr: The electron spin resonance (esr) technique provides a means of classification of TiIII species in polymerization catalysts into two categories, isolated TiIII atoms (which give a readily identifiable narrow signal in esr

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

251

spectra) and ‘‘esr-invisible’’ multinuclear associates of TiIII atoms [2,1042,1212, 1214,1216,1225–1227]. Several examples of these two Ti types were observed experimentally. The first example is a solid TiCl3/MgCl2/THF catalyst [1034,1039,1042]. All TiIII atoms in it are isolated and observable; the esr spectrum of the catalyst has a strong isotropic signal. The d-form of TiCl3 represents the opposite example. Less than 1% of TiIII atoms in its crystals are isolated and can be detected by esr [1226] whereas the majority of the Ti atoms are quadrupolarcoupled and produce a weak anisotropic esr signal [1214,1226]. Some Ti-based MgCl2-supported catalysts are prepared with organomagnesium compounds (Sections 4.2.1.3 and 4.3.2.3), and a partial reduction of TiCl4 to TiIII species occurs in the course of the catalyst synthesis. The level of reduction depends on the molar [Mg-R]:[TiCl4] ratio [1228,1229] although practically no TiII species are formed [1229]. However, even when the TiIV species in the catalysts are completely reduced, only B20% of the TiIII atoms are observable by esr. According to some analysis [1228,1229], even the esr-observable species are not monomeric but rather the same clusters in which the TiIII–TiIII bonding is broken because TiCl4 molecules coordinate to the TiIII species. Ti species in catalyst/cocatalyst systems: The solid TiCl3/MgCl2/THF catalyst provides the clearest illustration of the effects of catalyst/cocatalyst interactions because all TiIII atoms in the original crystalline material are observable by esr [1034,1039,1074]. When this solid is treated with AlEt2Cl (a part of the reaction sequence aimed at preparing a commercially acceptable catalyst), TiIII atoms are not reduced but THF molecules coordinated to the TiIII atoms are gradually removed from the solid, and this process leads to the formation of esr-silent, exchange-coupled associates of TiIII. When TiCl4/MgCl2 and TiCl4/MgCl2/ester catalysts are reacted with pure AlR3 at room temperature, the TiIV species are mostly reduced to TiIII species. More than 70% of the TiIII species form esr-invisible multinuclear associates of TiIII atoms [777,1072,1212,1214,1216,1217]. However, the presence of TiIII atoms in the reduced catalysts becomes obvious if the products are treated with pyridine; pyridine forms complexes with the TiIII atoms and they become esr-observable [1226]. A reaction of ethylene polymerization catalysts, TiCl4/SiO2 and TiCl4/ MgO, with AlEt3 or with AlEt2Cl in the absence of monomers also results in the reduction of their TiIV atoms and the formation of TiIII species (60–70%) and TiII species (10–25%) [1230,1231]. Esr spectra of reduced catalysts are quite complex and the signal assignment in them is controversial [1217]. Several types of esr-observable TiIII species were identified in the catalysts [2,1214,1216,1225,1226]: 1. Isolated TiIII atoms in trigonally distorted octahedral Cl environment. This is the principal isolated TiIII species in TiCl4/MgCl2/ester-AlEt3 systems [1214]. 2. Exchange-bound TiIII atoms in the Cl environment. 3. Isolated TiIII atoms in the tetrahedral Cl environment. 4. TiIII alkyl or TiIII hydride species in the rhombic environment. This is the principal esr-observable TiIII species in TiCl4/MgCl2/ester-AlEt3/ester systems. It is not stable at increased temperatures and rapidly converts to TiIII species with axial symmetry [1214].

252

Alkene Polymerization Reactions with Transition Metal Catalysts

None of the observed esr-observable isolated TiIII species can be attributed to active centers. Most probably, the active centers are either some constituents of esr-invisible small clusters of TiIII species [1216,1226,1229] or they are derived from unreduced TiIV species [1229]. 4.5.2.5. Aluminum species in solid catalysts Reactions between crystalline transition metal compounds (TiCl3, VCl3) and organoaluminum cocatalysts result in coordination of significant amounts of Al species on the surface of the solids (Reaction (4.26)). The same phenomenon was observed in reactions between organoaluminum cocatalysts and supported polymerization catalysts. Solid-state 27Al NMR spectra of TiCl4/MgCl2 catalysts prepared in the absence of esters are especially suitable for the investigation of these interactions [1232]. A reaction between AlEt3 and a TiCl4/MgCl2 catalyst leaves several Al species on the MgCl2 surface, hexa-coordinated species with two ethyl groups and four Cl atoms (a signal at 10 ppm), hexa-coordinated species with three ethyl groups (35 ppm), and penta-coordinated species with two ethyl groups (70–80 ppm) [1232]. 27 Al NMR analysis of TiCl4/MgCl2 catalysts containing Modifiers I (di-i-butyl phthalate and 2,2-di-i-propyl-1,3-dimethoxypropane) produced similar results, hexa- and penta-coordinated Al species were observed. They were assigned either to AlEt2Cl adsorbed on TiIII species [1233] or to adsorbed AlEt3 [1166]. These Al species are formed in the catalyst systems after 1–2 minutes and subsequent propylene polymerization reactions do not affect them [1166]. 4.5.2.6. Reactions in vanadium-based catalysts Homogeneous catalyst systems utilizing two V compounds, VCl4 and VOCl3, organoaluminum cocatalysts AlEt2Cl or Al2Et3Cl3, and chlorinated aliphatic hydrocarbons are widely used for the synthesis of compositionally uniform ethylene/propylene copolymers [8]. Organoaluminum compounds rapidly reduce both VCl4 and VOCl3. For example, a reaction between VCl4 and AlEt2Cl at 251C at an [Al]:[V] ratio of 1 results in a complete conversion of VCl4 to VIII species, and an increase of the [Al]:[V] ratio to 2 causes a further reduction of most VIII species to VII [1074]. According to the current consensus, the active centers in all these catalyst systems contain VIII species [1074,1234,1235]. A rapid loss of their activity with time, which occurs even at low temperatures, is attributed to reduction of the VIII centers to VII species. Halogen-containing organic compounds oxidize the VII products and prevent catalyst deactivation [41,1074,1234,1235]. Supported V-based catalysts can be subdivided into two groups, silica-supported catalysts for ethylene polymerization and catalysts for propylene polymerization employing MgCl2 as a support. The supported catalysts for ethylene polymerization prepared with VOCl3 have one advantage from the mechanistic point of view, one can examine their solid-state 51V NMR spectra and magnetic properties and determine the nature of V species in them [1236–1238]. 51V NMR data indicate that when silica is calcined at 6001C and reacted with VOCl3, the most probable

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

253

structure of VV species on the silica surface is WSi–O–V(Cl2)QO. If the silica is dehydrated at 2001C, the majority of its silanol groups are vicinal, and dominant VV species in these catalysts form bridges between two adjacent Si atoms, WSi–O– V(Cl)(QO)–O–Sio. However, when both these catalysts are activated with AlEt3 and used in ethylene polymerization reactions under the same conditions, they have practically the same activity and produce polymers with the same molecular weight distribution [1237]. Supported VCl4-based catalysts for polymerization of propylene usually use MgCl2 as a support. They are prepared similarly to Ti-based catalysts of the same type [438]. The esr spectrum of a solid V-based catalyst contains a signal characteristic of VIV species, an axially symmetric V(3d1) atom [438]; its intensity corresponds to B16% of all V atoms in the catalyst [438]. Another esr study of several VCl4/MgCl2 catalysts identified two adsorbed V species, one monomeric ˚ 2 of the MgCl2 surface) and another an esr(one VCl4 molecule per B400 A invisible, quadrupolar-coupled associate. Its presence was revealed when the product was treated with pyridine and the VCl4 associates were destroyed [1072]. Supporting VOCl3 on highly dispersed MgCl2 produces VV atoms in a distorted pentahedral environment; they are bound to the support either through two Cl atoms or through one Cl atom and one O atom [1238]. When either VCl4 or VOCl3 are supported on MgCl2 and reacted with AlEt2Cl, they are both reduced to VIII species but the subsequent reduction to VII is suppressed and the VIII species dominate in these catalysts even at [Al]:[V] W2,000.

4.6. Metallocene Catalysts 4.6.1. Types of metallocene complexes used in polymerization catalysts A large number of metallocene complexes are used as components of homogeneous polymerization catalysts. Their synthesis is a subject of synthetic organometallic chemistry; it is outside the scope of this book. References [29,71,72,252,262,273, 333,515,601,693,704,910,1239,1240,1241,1246–1250] describe main synthetic routes to the preparation of the complexes. Bis-metallocene complexes: Scheme 1.1 shows the structures of bis-metallocene complexes Cp2MX2 used in polymerization catalysis: 1. 2. 3. 4. 5. 6.

Bis(cyclopentadienyl) complexes (Rx-Cp)2MX2 (complexes I). Bis(indenyl) complexes (Rx-Ind)2MX2 (complexes II). Bis(1,2,3,4-tetrahydroindenyl) complexes (Rx-Ind-H4)2MX2 (complexes III). Bis(9-fluorenyl) complexes (Rx-Flu)2MX2 (complexes IV). Complexes with two different Z5 ligands, e.g., (Rx-Cp)(Ry-Flu)MX2 (complexes V). Bridged bis-metallocene complexes of the general formula (bridge E)(Z5 ligand1)(Z5 ligand2)MX2 (complexes VI, X–XV). These complexes are often called ansa-metallocene complexes; ansa means ‘‘a handle’’ in Latin. The most often used bridges E (either one or two) connecting two Z5 ligands are

254

Alkene Polymerization Reactions with Transition Metal Catalysts

–CH2–CH2–, –CHPh–CHPh–, WCMe2, WCPh2, WSiMe2, and WSiPh2. A variety of other bridge groups are also described in the literature; they include siloxy bridges –SiMe2–O–SiMe2–, cycloalkane moieties CnH2n
2 (n ¼ 4–6) with one of the carbon atoms performing the same role as in WCMe2 [1251], a norbornane moiety [233], a cyclo-C4H8So moiety [1252], a boron-based bridge WB–NR2 [1253], etc. Two bridges of the WSiR2 type can be additionally linked by an organic or a silo-organic group to produce dinuclear metallocene complexes connected by a second bridge –SiMe2–, –SiMe2–O– SiMe2–, etc. [702,1254–1256]. 7. Analogs of metallocene complexes containing thienocyclopentadienyl ligands, five-atom aromatic rings in which one CH group is replaced with the S atom [252,253,1266,1267]. These complexes mimic the performance of bismetallocene complexes both in terms of activity and stereospecificity. In all these metallocene complexes, the metal atoms M are TiIV, ZrIV, or Hf IV; and the X ligands are halogen atoms (usually Cl), H, alkyl groups (usually methyl or benzyl), phenyl groups, or NR2 groups. The substituents R in the cyclopentadienyl rings and in the aromatic rings fused with them (in indenyl and fluorenyl ligands) are alkyl groups (both linear and branched) [391,416,519,1268], aryl groups [1269], perfluorinated aryl groups [1269], or alkylsiloxy groups R3Si–O– [124]. Bis-metallocene complexes of lanthanoid metals also produce homogeneous  polymerization catalysts. They include LnIII complexes, such as Cp 2 LnX, Cp2 LnH,  and Cp2 LnR, and bridged complexes of the Me2Si(Cp)2LnX type (Ln ¼ La, Nd; X ¼ halogen atom; R ¼ an alkyl group) [660,1272–1274]. Monometallocene complexes: Several types of monometallocene complexes are used for the synthesis of homogeneous polymerization catalysts. 1. Monocyclopentadienyl complexes of the CpMX1X2X3 type (complexes VII and VIII) where Cp ¼ Cp, Rx-Cp, Cp, Ind, etc., and X1, X2, and X3 are monodentate ligands including F, Cl, OR, Me, Bz, NR2, SR, etc. [335,760,1257–1262]. These monometallocene complexes produce effective syndiospecific catalysts for styrene polymerization and for copolymerization of ethylene and styrene (Section 4.10.2) [104–108,752,1261,1262]. When CpMX3 complexes are used in alkene polymerization reactions, they do not show any particular advantages compared to Cp2MX2 complexes. However, Nomura discovered that when one of the X ligands is an alkoxy group with two bulky substituents in ortho-positions to the C–O bond, the complexes produce very efficient single-center catalysts which are suitable for polymerization of ethylene, higher 1-alkenes, styrene, as well as for copolymerization of ethylene with 1-alkenes and styrene [146,147,333,334,535,743,756,757,1263,1264]. 2. Bidentate monometallocene complexes of the general formula [bridge(Z5 ligand)(RxA)]MX2 (complexes IX). The single Z5 ligand in these complexes can be either an unsubstituted or a substituted Cp, Ind, or Flu group, and the RxA ligand is attached to the transition metal atom through a heteroatom A ¼ N, O, or P. Preferred substituents at the heteroatom are bulky alkyl groups, i-Pr or t-Bu. The bridge between the Z5 ligand and the heteroatom can be WSiMe2 or –(CH2)n– [73,151,537,995,997,998,1006,1239,1265,1286,1287].

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

255

This type of bridged monocyclopentadienyl complexes is traditionally called in the literature constrained-geometry complexes. The commonly used complex with this structure is [Me2Si(Me4-Cp)(t-Bu-N)]TiCl2. Homogeneous polymerization catalysts prepared from these complexes exhibit significant stability at high temperatures, above 120–1301C, and they are preferred for alkene polymerization reactions in high-temperature solution processes. Two constrainedgeometry complexes can be bound by the –CH2– or the –CH2–CH2– bridge between their cyclopentadienyl rings [552]. 3. Monometallocene complexes of CrIII produce ethylene polymerization catalysts of very high activity [1270,1271]. These complexes have the general formula (R-Cp)CrIIICl2 with the substituent R containing a heteroatom at the end of an alkyl chain, R ¼ –(CH2)2–ZRu2, where Z ¼ N, P or As, and Ru is an alkyl, a cycloalkyl or an aryl group. The length of the alkyl link is sufficient for the formation of a strong bond between the heteroatom and the CrIII atom. These complexes can be viewed as analogs of constrained-geometry catalysts but with two differences, the transition metal atom is in the 3+ valence state and it is connected with the heteroatom through a donor–acceptor bond instead of a s bond.

4.6.2. Cocatalysts for metallocene complexes 4.6.2.1. Cocatalysts in early metallocene catalysts Breslow and Natta discovered the first metallocene catalysts in mid-1950s. They contain Cp2TiCl2 and dialkylaluminum chlorides, AlMe2Cl or AlEt2Cl [26,1242]. A combination of AlMe3 and AlR2F is also an efficient cocatalyst of this type [1243,1244]. All these catalyst systems are soluble in aromatic compounds and in halogenated alkanes. The solubility allowed detailed investigations of reactions that occur between the catalyst components (Section 6.1.1.3). Homogeneous catalysts based on titanocene complexes and AlR2Cl were the first polymerization catalysts that afforded the synthesis of compositionally uniform ethylene copolymers with propylene [64] and 1-butene [303]. Unfortunately, these catalysts are only moderately active and produce polymers of a relatively low molecular weight [64,303]. This circumstance greatly reduced the practical usefulness of these catalysts. Zirconocene complexes cannot be activated with AlR2Cl and they have very low polymerization activity when combined with AlR3 compounds. 4.6.2.2. Alkylalumoxanes The fortunes of metallocene polymerization catalysts have greatly improved when Kaminsky and Sinn discovered that catalyst systems employing methylalumoxane (MAO) as a cocatalyst have very high activity [27,1245]. The first clues to the existence of this cocatalyst came from the studies of beneficial effects of small quantities of water on the polymerization activity of Cp2Ti(R)Cl-AlR2Cl systems [65,1296]. MAO is a member of a special class of oligomeric organoaluminum

256

Alkene Polymerization Reactions with Transition Metal Catalysts

compounds with the alumoxane backbone [Al(R)–O]n (R ¼ Me, Et, i-Bu, t-Bu, etc.). Attempts to use alkylalumoxanes as cocatalysts for Ti- and V-based Ziegler– Natta catalysts were only moderately successful [1299]. However, MAO is an exceptionally effective cocatalyst for metallocene complexes [27,29,1245,1300]. Synthesis from water: Alkylalumoxanes are formed in reactions between organoaluminum compounds and water [27,68,1297,1298]: AlR3 þ H2 O ! R2 AlOH þ RH (4.32) R2 AlOH þ AlR3 ! R2 Al2O2AlR2 þ RH R2 Al2O2AlR2 þ H2 O þ AlR3 ! R2 Al2O2AlR2O2AlR2 þ 2RH; etc:

(4.33) (4.34)

MAO is conventionally produced by hydrolysis of AlMe3 [27,70,1297–1301]. The most common methods are a reaction of AlMe3 with salt hydrates, CuSO4  5H2O, Al2(SO4)3  15H2O, LiOH  H2O, [71,1297,1301,1302], and a reaction of AlMe3 and ice [1298]. The [CH3]:[Al] ratio in most products with the MAO structure is substantially higher than one due to the presence of Me2AlO
groups. Additionally, free AlMe3 is usually coordinated to MAO molecules. Its concentration affects the oligomerization degree of MAO [1303]. Alternative synthetic routes: Several non-hydrolytic routes to the synthesis of MAO have been developed. One of them is a reaction between AlMe3 and organotin compounds containing Sn
O bonds [745,1304,1305], e.g., a reaction with (R3Sn)2O: (4.35) AlMe3 þ ðR3 SnÞ2 O ! R3 SnMe þ Me2 Al2O2SnR3 Me2 Al2O2SnR3 þ ðR3 SnÞ2 O ! R3 SnMe þ R3 Sn2O2AlðMeÞ2O2SnR3 2 Me2 Al2O2SnR3 ! R3 SnMe þ Me2 Al2O2AlðMeÞ2O2SnR3 Me2 Al2O2AlðMeÞ2O2SnR3 þ AlMe3 ! R3 SnMe þ Me2 Al2O2AlðMeÞ2O2AlMe2 ; etc:

(4.36) (4.37) (4.38)

In these reactions (R3Sn)2O molecules can be viewed as analogs of H2O with R3Sn groups instead of H atoms. Similar products are formed in reactions between AlMe3 and R3SnOH or R2SnO [745]. Thermal decomposition of dimethylaluminum alkoxides Me2AlOR with sterically crowded alkyl groups R such as CPh3 also produces MAO. The reaction is catalyzed by AlMe3 or Lewis acids [1306]: (4.39) Me2 Al2O2CPh3 2ðAlMe3 Þ ! ½AlðMeÞ2On þ Ph3 CMe MAO is also produced in reactions of AlMe3 with compounds containing carbonyl and carboxyl groups, including benzophenone, benzoic acid, and CO2 [1306,1310]. For example, a reaction between benzoic acid and an excess of AlMe3 under ambient conditions produces the MAO dimer [1310]: (4.40) Ph2CðQOÞ2OH þ AlMe3 ! Ph2CðQOÞ2O2AlMe2 þ CH4

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

Ph2CðQOÞ2O2AlMe2 þ 2 AlMe3 ! Ph2CMe2 2O2AlMe2 þ Me2 Al2O2AlMe2

257

(4.41)

Heating the products of Reaction (4.41) results in condensation of the dimer into oligomeric MAO [1310]. Structure of MAO: Formally, MAO is an oligomeric organoaluminum compound [O–Al(CH3)]n with n ¼ 4–20 [70,1297,1302,1311,1312]. NMR data show that virtually all Al atoms in MAO are four-coordinated (although threecoordinated Al atoms become NMR-observable at high temperatures [623]) and all O atoms are three-coordinated [623,1297,1307,1313,1314]. These data indicate a very high degree of inter- and intramolecular association in MAO molecules. Both commercial and laboratory-prepared samples of MAO often contain significant quantities of AlMe3, which is coordinated to MAO molecules and serves as an effective scavenger of impurities in polymerization reactions [644,1315–1319]. Moreover, a significant part of MAO in metallocene catalysts can be replaced with AlMe3 without any detriment to the cocatalyst performance [1301]. Dry MAO is a white pyrophoric solid. Its heating is accompanied by a release of a significant amount of associated AlMe3 and results in a substantial loss of solubility in aromatic solvents and a partial loss of efficiency as a cocatalyst [1316]. Another chemical reaction leading to the loss of MAO efficiency is its selfcondensation with evolution of methane and the formation of Al–CH2–Al bonds [1320]. The most frequently used techniques for the removal of AlMe3 from commercial MAO samples are (a) evaporation of all its volatile constituents [342,781,1318] and (b) reactions with sterically hindered phenols converting AlMe3 into catalytically inert aluminum alkoxides [614,644]. In spite of the presence of excess AlMe3, MAO contains a small quantity of OH groups, remnants of its synthesis from water. The exact nature of the species containing these OH groups is not known but their presence is supported by IR evidence [1321] and by the fact that a reaction of MAO with Cp2ZrCl2 produces HCl [1322]. Barron thoroughly investigated the structure of an MAO analog, t-butylalumoxane [1307,1308]. Two oligomeric states of this compound were identified, a hexamer and a nonamer. They both have a three-dimensional cage structure, their cores consist of Al and O atoms forming ball-like cages whereas the t-butyl groups are positioned at the periphery of the cages. Sinn proposed a similar structure for MAO in which four linear MAO tetramers, Al4O3Me6, form a half-open dodecahedron structure, Al16O12Me24 [1312]. At the present time, most researchers agree that MAO molecules have a cage structure composed of hexagonal and square faces [1303,1307,1311,1312,1317,1323]. However, the size of the oligomers is not definitely determined yet; the proposed structures range from Al12 to Al20 [1303,1311,1312,1324]. Other alkylalumoxanes: Alkylalumoxanes [
Al(R)
O
]n with R ¼ i-Bu, i-Oct and n-Oct are less effective cocatalysts compared to MAO [175,298,558, 1325,1326] and alumoxanes with R ¼ Et and t-Bu are virtually inactive [1307,1308]. For example, the efficiency of different alkylalumoxanes in

258

Alkene Polymerization Reactions with Transition Metal Catalysts

ethylene/propylene copolymerization reactions with C2H4(Ind-H4)2ZrCl2 at 501C decreases as [175] Alkylalumoxane Productivity (kg/g Zr)

[–Al(Me)–O–]n 930

[–Al(i-Oct)–O–]n 660

[–Al(i-Bu)–O–]n 430

t-Butylalumoxane is completely inefficient as a cocatalyst for metallocene complexes [1307,1308]. However, reacting this compound with one equivalent of AlMe3 leads to an exchange of alkyl groups between the Al–Me bond in AlMe3 and the Al–t-Bu bond in the alumoxane. It results in the formation of hybrid methyl, t-butylalumoxanes [Al6(t-Bu)5MeO6] with a cage structure and one Al(t-Bu)2Me molecule coordinated to the cage [1309]. This compound exhibits significant cocatalytic activity in alkene polymerization reactions. When the ratio between AlMe3 and t-butylalumoxane is increased, the efficiency of the reaction products as cocatalysts for zirconocene complexes also progressively increases. Eventually, all Al–t-Bu bonds can be replaced with Al–Me bonds, i.e., t-butylalumoxane can be transformed into AlMe3-free MAO [1309]. Modified MAO (MMAO): MAO is readily soluble in aromatic solvents (benzene, toluene) but it has limited solubility in aliphatic hydrocarbons. The latter can be significantly increased if a fraction of methyl groups in MAO is replaced with isobutyl groups. This product is called ‘‘modified MAO’’ (or MMAO); it is prepared by controlled hydrolysis of mixtures of AlMe3 and Ali-Bu3. The efficiency of MAO and MMAO as cocatalysts for metallocene complexes is similar [1325,1326], and solubility of MMAO in aliphatic hydrocarbons makes this material very attractive. Several independent studies demonstrated that the efficiency of MAO as a cocatalyst is significantly increased when Ali-Bu3 is added to it [268,653,1281, 1324,1327] whereas additional amounts of other organometallic compounds either have no effect (AlMe3) or reduce the MAO efficiency (AlEt3, ZnR2) [653]. Three reasons for the special advantage of Ali-Bu3 are cited [1324]: (a) the replacement of Al–Me bonds in MAO molecules with Al–i-Bu bonds leads to the formation of MMAO, (b) Ali-Bu3 modifies Lewis-acidic centers in MAO molecules (see Section 6.1.1.4), and (c) the introduction of Ali-Bu3 prevents metallocenium cations from forming complexes with AlMe3 present in MAO. The replacement of a part of methyl groups in MAO with C6F5 groups also produces active cocatalysts for metallocene complexes [1331]. Mixed (ethyl)(i-butyl)alumoxanes, which are produced similarly to MMAO by controlled hydrolysis of AlEt3-Ali-Bu3 mixtures of different compositions, are much more effective as cocatalysts in metallocene systems than either ethyl alumoxane or i-butyl alumoxane [1328,1329]. When these mixed alkylalumoxanes are used in combination with sterically crowded metallocene complexes, they can be even more effective than MAO [1330]. MAO as a cocatalyst: MAO and its analogs (see the next section) are extremely efficient cocatalysts for zirconocene complexes. The productivity of these catalyst systems strongly depends on the [MAO]:[Zr] ratio: the higher the ratio, the higher the productivity per mole of a metallocene complex [72,262,1301,1302,

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

259

1332,1333]. The following example illustrates the significance of the [MAO]:[Zr] ratio in an ethylene/1-hexene copolymerization reaction with the Cp2ZrCl2-MAO system at 401C (5-minute reaction in toluene) [192]. [MAO]:[Zr] Yield (g/mmol Zr)

100 6

200 64

500 454

1,000 459

2,000 680

When polymerization reactions with metallocene catalysts are carried out in solution, the [MAO]:[Zr] ratio is usually maintained at a 1,000–2,000 level, although much lower ratios are employed under commercial conditions. A significant fraction of MAO in these catalysts serves merely as an impurity scavenger and can be replaced with AlMe3 without any loss of activity [1301,1332]. Kinetic effects associated with the variation of the [MAO]:[Zr] ratio are discussed in Section 5.4.3.2.2. Under favorable conditions (polymerization reactions in a toluene medium under 1 bar of ethylene at very high [MAO]:[zirconocene] ratios), polymerization rates with these catalysts can exceed 5  106 g polymer per gram of Zr in 1 hour [72,262,1299]. In one particular case, the efficiency of MAO as a cocatalyst for Cp2TiCl2 was directly compared to the efficiency of AlEt2Cl, which was used for activation of Cp2TiCl2 prior to the discovery of MAO [1334]. The productivity of the Cp2TiCl2-MAO system in a homopolymerization reaction of ethylene at 251C calculated per mole of the Ti complex is over 80 times higher than the productivity of the Cp2TiCl2-AlEt2Cl system. However, this comparison also reveals the principal drawback of MAO-activated metallocene catalysts. To achieve such a high activity, the Cp2TiCl2-MAO system was used at an [Al]:[Ti] molar ratio of 3,000, whereas the Cp2TiCl2-MAO catalyst is fully active at an [Al]:[Ti] molar ratio of merely 2.5 [1334]. 4.6.2.3. Analogs of alkylalumoxanes Several oligomeric compounds with the alumoxane backbone structure can serve as efficient cocatalysts in metallocene catalysts [1335–1337]. They all contain AlO3 moieties with an organic group attached to one of the O atoms, as shown in Scheme 4.3. 1. Sterically hindered phenoxy alumoxanes (1), oligomeric compounds containing [Al(OPh)–O] units, where Ph is a phenyl group substituted in the second and the 6th positions with bulky alkyl or aryl groups [1335–1337]. 2. Phenoxy alumoxanes with C6F5 moieties (2) [683,1338]. 3. Sterically hindered alkoxy alumoxanes (3) containing [Al(OR)–O] units where R is a sterically hindered alkyl group [1335–1337,1339]. All these compounds are produced via two alternative routes. The first one is a reaction between MAO and a phenol PhOH or an alcohol ROH: ½2AlðMeÞ2O2n þ PhOH ! ½2AlðOPhÞ2O2n þ CH4

(4.42)

½2AlðMeÞ2O2n þ ROH ! ½2AlðORÞ2O2n þ CH4

(4.43)

260

Alkene Polymerization Reactions with Transition Metal Catalysts

R2

F R1 = tert-Bu, Ph R2 = H, Me, Aryl

R1

R1

Me

F O

Al

Scheme 4.3

O

Me

O

Al O

1

C

F

O

O

Me

F

F

Al O

O

O

2

3

Analogs of MAO used as cocatalysts for metallocene complexes.

Both reactions produce mixed methyl/phenoxy or methyl/alkoxy alumoxanes, respectively. Their compositions are determined by the ratios between MAO and the OH-carrying reagents. The second route involves two consecutive reactions. The first reaction is between AlR3 and the phenol or the alcohol: AlR3 þ PhOH ! AlR2 OPh þ AlRðOPhÞ2 þ RH

(4.44)

AlR3 þ ROH ! AlR2 OR þ AlRðORÞ2 þ RH

(4.45)

The ratio between the yields of AlR2OPh and AlR(OPh)2 or between the yields of AlR2OR and AlR(OR)2 in the products of Reactions (4.44) and (4.45) depends mostly on the ratios between the alcohols and AlR3. In the second stage, the products of Reactions (4.44) and (4.45) are contacted with water: AlR2 OPh þ H2 O ! ½2AlðOPhÞ2O2n þ 2 RH

(4.46)

AlR2 OR þ H2 O ! ½2AlðORÞ2O2n þ 2 RH

(4.47)

This stage is carried out under moderate conditions at room temperature in contrast to the syntheses of alkylalumoxanes from AlR3 and water (Reactions (4.32–4.34)), which have to be carried out at low temperatures under carefully controlled conditions to avoid overheating and the formation of alumina. From the standpoint of cocatalyst efficiency, the most effective substituents Ph in phenoxy alumoxanes (1) are phenyl groups with two ortho-positioned t-butyl or phenyl groups, and the most effective substituent R in alkoxy alumoxanes (3) is CPh3 [1337] or bulky alkyl groups derived from adamantanes [1339]. When alumoxanes (1)–(3) are used as cocatalysts for metallocene complexes, a trialkylaluminum compound, preferably AlMe3, should be added to catalyst mixtures. Its main function, in addition to that of the impurity scavenger, is alkylation of metallocene complexes (see Section 6.1.1.4). Reaction products of AlMe3 and three types of minerals, gibbsite [Al(OH)3], boehmite [AlO(OH)], and multi-layered inorganic clays, are also efficient cocatalysts for metallocene complexes [1340,1341]. AlMe3 reacts with hydroxyl

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

261

groups of the surfaces of the mineral particles and produces O–AlMe and O–AlMe2 groups, which serve as cocatalyst moieties. 4.6.2.4. Ion-forming cocatalysts Several types of ion-forming cocatalysts for metallocene complexes were discovered over the past 20 years. They are used as activators of Cp2MR2 [254,604,703,1349– 1361], CpMR3 [749,1257], and constrained-geometry complexes [604,1357]. The synthesis of alkylated metallocene complexes for these catalyst systems can be avoided if AlMe3 or Ali-Bu3 is added to catalyst compositions containing Cp2MCl2 and ion-forming cocatalysts [1372–1374]. The most important among the ion-forming activators are organoborane compounds: 1. Perfluorinated organoboranes with strong Lewis acidity, B(C6F5)3 and its analogs, perfluorobiphenylborane and tris(b-perfluoronaphthyl)borane [735,1355,1362– 1364]. 2. Organoborate salts A+ [B(C6F5)4]
and [B(C6F4Sii-Pr2)4]
, where A+ ¼ [R3NH]+, [CPh3]+, [Me2N(Ph)H]+, etc. [1365]. 3. Salts containing the B(C6F5)3 motif, A+ [(C6F5)3B–CRN–B(C6F5)3]
, A+ [(C6F5)3B–N(H2)–B(C6F5)3]
, A+ M[–CRN–B(C6F5)3]
(M ¼ Ni, Pd) 4 [1366,1367]. Aluminum compounds analogous to the organoborane activators are also used as activators of alkylated metallocene complexes. The list of the Al compounds includes Al(C6F5)3, Al(C6F4-C6F5)3, and [Ph3C]+ [Al(C6F5)4]
[139,254,735,1354, 1363,1368–1370]. Another group of perfluorinated organometallic compounds suitable as activators for alkylated metallocene complexes includes metal alkoxides [Ph3C]+ [Al(OC6F5)4]
, [Ph3C]+ [Nb(OC6F5)4]
, and [Ph3C]+ [Ta(C6F5)4]
[741,1371]. Molar quantities of ion-forming cocatalysts in metallocene systems are usually of the same order of magnitude as those of the metallocene complexes, in contrast to MAO-activated catalysts, which require a large excess of MAO (Section 4.6.2). Comparisons of MAO and perfluorinated organoborane compounds as cocatalysts for metallocene complexes are usually uncertain due to the necessity to use alkylated metallocene complexes and different amounts of impurity scavengers. One such comparison was carried out for Me2Si(Ind)ZrMe2 activated with MAO at a high [Al]:[Zr] ratio and with the [Bu3NH]+ [B(C6F5)4]
salt at [B]:[Zr] ¼ 0.33 [1350]. Both catalyst systems have a single type of active center in propylene polymerization reactions at 251C (Mw/Mn ¼ 2.0–2.1) and both produce isotactic polypropylene with approximately the same molecular weight, the same degree of stereoregularity ([mmmm] ¼ 0.93–0.94), and the same melting point, 150–1521C. All these similarities indicate an equivalence of active centers formed with the two different classes of cocatalysts. Anhydrous crystalline MgCl2, MgF2, MgBr2, and LiCl also can be used as activators of alkylated metallocene complexes. Marks described the first catalyst system of this type, a combination of Cp 2 ThMe2 and MgCl2 [1380]. The best

262

Alkene Polymerization Reactions with Transition Metal Catalysts

sources of these activators are chemically synthesized materials produced from MgR2 or LiR. The sources of halogen atoms in the syntheses are HCl [1380], CX4 (X ¼ Cl or Br) [1305], or AlR2X [1305,1381,1382], e.g. 2 AlMe2 Cl þ Mgðn-BuÞ2 ! MgCl2 þ 2 AlMe2 n-Bu

(4.48)

X-ray analysis shows that MgCl2 crystallites formed in Reaction (4.48) have a very small size and a high specific surface area, B250 m2/g [1380,1382]. Catalysts of this type usually contain three components [1305,1381–1383]: (a) MgCl2 or LiCl, (b) a metallocene complex, and (c) an alkylating agent for the metallocene complex, which is either formed in Reaction (4.48) or added to the catalyst mixture. These catalysts produce polyethylene, ethylene/1-alkene copolymers, and stereoregular polymers of 1-alkenes. The polymers have a relatively broad molecular weight distribution (Mw/Mn B10–15) indicating co-existence of several types of active centers, but ethylene/1-alkene copolymers prepared with the catalysts are compositionally uniform [1382].

4.6.3. Activity of metallocene catalysts The activity of homogeneous catalysts based on various metallocene complexes of the general formula (Rx-Cpu)(Ry-Cpv)MX2 depends on several factors. The most important of them are the type of transition metal atom M, the type of Z5 ligands Cpu and Cpv, the number and the type of alkyl substituents R in them, the type of the bridge between the Z5 ligands (if present), and the type of the ligand X. Several examples below illustrate these dependencies. When the data of different researchers on the activity of metallocene complexes are compared, several circumstances should be taken into account: 1. Most metallocene catalysts are extremely active and their activity is affected by parameters of polymerization reactions. The best approach to a reliable comparison of different catalysts is to carry out the polymerization reactions in a single series of experiments, in one research group, and, desirably, with a single batch of chemicals and solvents. 2. Many metallocene catalysts are unstable, especially if tested at elevated temperatures; their activity can deteriorate significantly over a period of 30–60 minutes (Section 5.4.1). The often-used measure of catalyst activity, the average productivity of a catalyst over a period of 1 hour, can be misleading when effects of relatively small changes in the structure of metallocene complexes are compared. 3. Different metallocene complexes are usually compared in polymerization reactions with MAO as a cocatalyst. These catalyst systems exhibit the high activity usually associated with metallocene catalysis only in the presence of a large excess of MAO corresponding to [Al]:[M] ratios of 2,000–5,000. Effects of transition metal: Titanocene and hafnocene complexes are usually less active in comparison with zirconocene complexes of the same structure. Table 4.15 gives three examples for bridged zirconocene and hafnocene complexes. Hafnocene complexes nearly always produce polymers of a higher molecular weight [1277,1278]. The lower activity of titanocene complexes is mostly the result of

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

263

Table 4.15 Reactivities of metallocene complexes in ethylene polymerization reactions, effect of transition metal Complex

Activity (g/mmol Zr  h  CM)

Mw

Complexes with –C2H4– bridge [1277] C2H4(Ind)2ZrCl2 C2H4(Ind)2HfCl2

41,000 2,900

1.4  105 4.8  105

Complexes with Me2Co bridge [1277] Me2C(Cp)(Flu)ZrCl2 Me2C(Cp)(Flu)HfCl2

2,000 890

5.0  105 5.6  105

Complexes with Me2Sio bridge [1278] Me2Si(Me4-Cp)2ZrCl2 Me2Si(Me4-Cp)2HfCl2

1,350 1,050

1.3  105 3.8  105

their high susceptibility to reduction with AlMe3 and other organoaluminum compounds. If polymerization reactions are carried out at sufficiently low temperatures, the reactivity order is reversed, especially at short reaction times, as the productivity data demonstrate [1279]. Temperature (1C) Cp2Ti(Me)Cl (g/mmol M  10 min) Cp2Zr(Me)Cl (g/mmol M  10 min)

+25 25.1 18.8


15 32.2 0.5

+70 1.3 26.6

Effects of X ligands: The effect of X ligands in metallocene complexes on their activity in alkene polymerization reactions is usually relatively small. When the metallocene complexes are combined with organoaluminum components in cocatalysts, these ligands are rapidly replaced with small alkyl groups (Section 4.6.5), and the latter are also very rapidly transformed into polymer chains attached to the transition metal atoms. Therefore, ligands X affect the polymerization reactions indirectly, through their participation in the structure of cocatalysts. Several examples demonstrate this effect: 1. The effect of polar ligands X on the activity of complexes Cp2ZrX2 in ethylene polymerization reactions [1280]. X Relative activity

Cl 1

F 0.64

I 0.43

NMe2 0.46

O-t-Bu 0.24

OSO2CF3 1.0

OSO2Cme3 0.60

2. The activity of complexes Cp2ZrX2 with organic groups X in ethylene polymerization reactions increases in the order Cp2ZrMe2oCp2ZrPh2oCp2Zr(CH2Ph)2oCp2Zr(CH2SiMe3)2 [71]. 3. The effect of ligands X on the activity of complexes rac-C2H4(Ind)2ZrX2 in polymerization reactions of 1-hexene at 201C [1281]. X Productivity (kg/mol Zr  h)

Cl 1,290

Me 1,190

Bz 970

NMe2 B30

264

Alkene Polymerization Reactions with Transition Metal Catalysts

Effects of substituents in Z5 ligands: Effects of substituents R in nonbridged Z5 ligands R-Cp are quite varied depending on the nature of the substituents and their number [603,1247,1269,1275–1277]. As an illustration, Tables 4.16 and 4.17 compare effects of alkyl substituents R in (R-Cp)2ZrCl2 in ethylene polymerization reactions. Several independent series of experiments showed that all catalysts produced with these complexes and MAO at high [MAO]:[Zr] ratios are singlecenter systems, that their activity is very high, and that within a margin of error expected in testing of such highly active catalysts their reactivity is relatively weakly affected by the size of a single alkyl substituent. The only obvious exception is the reduced activity of the complex carrying a bulky t-butyl group. The introduction of multiple alkyl substituents into the rings usually leads to reduction in catalyst activity. There were several attempts to separate two possible effects of substituents R, electronic and steric, on the activity of metallocene catalysts. However, the list of useful substituents R is short and mostly limited to different alkyl groups, aryl groups, fluoro-substituted organic groups, and SiRu3 groups [1247,1269,1276,1277]. Two examples demonstrate electronic effects of the substituents [1269]: 1. Productivities of catalyst systems based on three complexes, Cp2ZrCl2, (Ph-Cp)2ZrCl2, and (Ph2-Cp)(Cp)ZrCl2, in ethylene/1-hexene copolymerization reactions are similar in spite of the expected retardation for the latter two complexes due to the steric effect of phenyl substituents. Apparently, the steric effect is offset by the electron-donating effect of the phenyl group. 2. The performance of (Ph-Cp)2ZrCl2 and (Ph2-Cp)(Cp)ZrCl2 was compared to that of (C6F5-Cp)2ZrCl2 and [(C6F5)2-Cp](Cp)ZrCl2. The steric bulk of the phenyl group and the C6F5 group is similar. However, the productivity of the catalysts derived from the perfluorinated complexes was lower by a factor of 3–4 due to the electron-withdrawing effect of the C6F5 substituent. Table 4.16 Performance of zirconocene catalysts in ethylene homopolymerization reactions [1275] Complex

Cp2ZrCl2 (Me-Cp)2ZrCl2 (n-Bu-Cp)2ZrCl2 (t-Bu-Cp)2ZrCl2 (1,2-Me2-Cp)2ZrCl2 (1,2,4-Me3-Cp)2ZrCl2 (Me4-Cp)2ZrCl2 Cp 2 ZrCl2 a

Cocatalyst: Productivity (kg/mol Zr  15 min) MAOa

[Ph3C]+ [B(C6F5)4]
-AlMe3b

8,550 7,150 11,700 B1,500 9,500 4,350 5,800 7,050

700 610 1,650 930 600 930 840 1,950

Reactions at 701C, [Zr] ¼ 1  10
5 M, [MAO]:[Zr] ¼ 1,000, PE ¼ 2 atm. Reactions at 501C, [Zr] ¼ 1.4  10
5 M, [B]:[Zr]:[Al] ¼ 1:1:10, PE ¼ 3 atm.

b

265

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

Bridged bis-metallocene complexes, effects of bridge structure: Catalysts prepared from bis-metallocene complexes with bridges connecting two Z5 ligands usually have higher activity compared to their nonbridged analogs, especially in polymerization reactions of 1-alkenes [1727,1728]. When a bridge of the same type is used, the nature of the linking atom and that of its substituents also affect the performance of the catalysts to some degree, as shown in Table 4.18. Bridged bis-metallocene complexes, effects of steric structure: Bridged bis-indenyl complexes exist as mixtures of two isomers, racemic and meso (complexes X in Scheme 1.1). The two isomers are difficult to separate and they are often used in polymerization reactions together. Shiono carried out a comparative evaluation of the efficiency of two isomers of Me2Si(2-Me-Ind)2ZrCl2 activated Table 4.17 Performance of catalysts based on (R-Cp)2ZrCl2 complexes in ethylene homopolymerization reactionsa [603]

a

Substituent R

Productivity (ton/mol Zr  h  CE)

Mn

Mw/Mn

H CH3 C2H5 n-C3H7 i-C3H7 n-C4H9 n-C12H25

460 400 460 510 390 540 410

9.8  104 9.8  104 1.27  105 1.07  105 1.23  105 8.6  104 2.61  105

2.6 2.5 2.3 2.4 2.3 3.1 2.7

Polymerization reactions in toluene at 701C, [MAO]:[Zr] ¼ 2,000, PE ¼ 1 bar.

Table 4.18 Relative activities of metallocene complexes in ethylene homopolymerization reactionsa, effect of bridge structure [1277] Activity (kg/mmol Zr  h  CM)

Mnb

41.0 36.9 20.2 12.2

140,000 260,000 320,000 350,000

(Cp)(Ind) complexes Me2C(Cp)(Ind)ZrCl2 Ph2C(Cp)(Ind)ZrCl2

1.6 3.3

25,000 18,000

(Cp)(Flu) complexes Me2C(Cp)(Flu)ZrCl2 Ph2C(Cp)(Flu)ZrCl2

2.0 2.9

500,000 630,000

Complex

Bis-indenyl complexes C2H4(Ind)2ZrCl2 Me2Si(Ind)2ZrCl2 Ph2Si(Ind)2ZrCl2 Bz2Si(Ind)2ZrCl2

a

Polymerization at 301C and PE ¼ 2.5 bar, cocatalyst MAO, [Al]:[Zr] ¼ 250, [Zr] ¼ 6.25  10
6 M. Viscosity-average molecular weight, Mn E Mw.

b

266

Alkene Polymerization Reactions with Transition Metal Catalysts

with MAO and ion-forming cocatalysts in polymerization reactions of propylene, 1-butene and 1-hexene [328]. The results show that rac-Me2Si(2-Me-Ind)2ZrCl2 (the isomer producing the isospecific catalyst) is significantly more active than the meso-isomer producing the aspecific catalyst. The racemic/meso productivity ratio in polymerization reactions of propylene and 1-butene is 6–8 when MAO is employed as a cocatalyst but it increases to 20–100 for ion-forming cocatalysts. The racemic isomer always produces polymers with higher molecular weights [328]. Reactivity of metallocene catalysts in ethylene/1-alkene copolymerization reactions: Metallocene catalysts differ quite noticeably in their ability to copolymerize 1-alkenes with ethylene [273,910,1283]. The principal features of metallocene complexes that affect this aspect of catalyst reactivity are the same as in homopolymerization reactions, the type of transition metal, the type and number of substituents at the cyclopentadienyl groups, and the presence of bridges between two rings. Three examples for bis-indenyl complexes illustrate this effect in ethylene/ 1-hexene copolymerization reactions at 751C with MAO as a cocatalyst [1283]. Complex Ind2ZrCl2 C2H4(Ind)2ZrCl2 Me2Si(Ind)2ZrCl2 Productivity (kg/mmol Zr  h  PM) 154 421 341

Bridged bis-metallocene complexes are highly efficient in copolymerizing 1-alkenes with ethylene [1284,1285]. This property is represented in Table 4.19 by the reciprocal value of the reactivity ratio, 1/r1, and it is normalized to the reactivity of the unsubstituted zirconocene complex. Hafnocene complexes are less active in ethylene/1-alkene copolymerization reactions than zirconocene complexes of the same type (Table 4.15). However, they produce copolymers with higher 1-alkene contents (they are characterized by lower r1 values) and the copolymers have higher molecular weights. Constrained-geometry catalysts: The reactivity of constrained-geometry catalysts depends on the structure of each component of the original bridged complex, first of all, on the structure of the two ligands attached to the transition metal atom [1286– 1289]. For example, a bulky alkyl group R at the N atom in C2H4(Cp)(R-N)TiX2 is required to produce catalysts of high activity in ethylene polymerization reactions (see Table 4.20) whereas the nature of this group has a very small effect on the productivity of the same catalysts in propylene polymerization reactions. Molecular weights of both polymers are greatly reduced in the order of R: Me W i-Pr W t-Bu. Table 4.19 Relative activity of metallocene systems in copolymerization reactions of 1-hexene with ethylene [1284] Complex

(1/r1)norm

C2H4(Ind)2ZrCl2 Me2C(Cp)(Flu)ZrCl2 Me2Si(Cp)2ZrCl2 (Me-Cp)2ZrCl2 Cp2ZrCl2 Cp 2 ZrCl2

5.0 4.1 2.0 1.38 1 (standard) 0.14

267

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

Table 4.20 Performance of constrained-geometry [C2H4(Cp)(Ru-N)]Ti(NEt2)2-MAO systems in polymerization reactions of ethylene and propylene [1289] Substituent Ru

Productivity (kg/mol Ti  h)

Mw

Mw/Mn

Ethylene polymerization, 501C [MAO]:[Ti] ¼ 520, PE ¼ 0.2 MPa Me 840 7.77  105 i-Pr 1,470 1.23  105 t-Bu 1,850 5.8  104

3.7 2.4 1.9

Propylene polymerization, 301C [MAO]:[Ti] ¼ 520, PPr ¼ 0.47 MPa Me 295 7.20  105 i-Pr 114 1.63  104 t-Bu 224 1.14  104

2.1 1.9 1.8

4.6.4. Stereospecific metallocene catalysts Isospecific metallocene catalysts: Two types of bridged bis-metallocene complexes are used for the synthesis of isotactic polypropylene: 1. Racemic bridged bis-indenyl and bis-tetrahydroindenyl complexes of C2 symmetry (complexes rac-X in Scheme 1.1). 2. Asymmetric bridged bis-metallocene complexes with a particular pattern of substitution R1–R2 in each metallocene ring (complexes XI). Brintzinger [519,1290], Kaminsky [28], and Ewen [595] were the first to use metallocene complexes of C2 symmetry for the synthesis of isotactic polypropylene. The ability of these complexes to form isospecific catalysts is a function of many factors, the type of Z5 ligand, the type of transition metal in the complex, the type of the bridge between two Z5 ligands, and the type of cocatalyst. The bridge E connecting two indenyl ligands is usually –C2H4–, Me2Sio, Me2Co, Ph2Co, etc. Two most common representatives of these complexes are rac-C2H4(Ind)2ZrCl2 and rac-C2H4(Ind-H4)2ZrCl2 [667,694,709,1291] (see Table 4.21). Several strategies have been developed in order to increase the isotacticity of polypropylene prepared with metallocene catalysts. The two most effective approaches is the introduction of substituents in the indenyl ligand of racC2H4(Ind)2ZrCl2 and a change of the bridging group between the two ligands. Placing various alkyl and aryl substituents both into the cyclopentadienyl rings and the phenyl rings of rac-C2H4(Ind)2ZrCl2 significantly affects the performance of the catalysts produced from them [515,523,525]. Several types of metallocene complexes form the catalysts of the highest isospecificity: 1. The Resconi complex, rac-Me2C(3-t-Bu-Ind)2ZrCl2 [387,515,523]. 2. Spaleck complexes, Me2Si(2-Me-4-Ph-Ind)2ZrCl2 and Me2Si[2-Me-4(1-naphthyl)-Ind]2ZrCl2 [515]. 3. Brintzinger complexes, Me2Si(Benz[e]Ind)2ZrCl2 and Me2Si(2-Me-Benz [e]Ind)2ZrCl2 [525].

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Alkene Polymerization Reactions with Transition Metal Catalysts

Table 4.21 Stereospecificity of catalysts based on bridged bis-metallocene complexes in propylene polymerization reactions Complex

[mmmm]

Tm (1C)

Polymerization at 301C, PPr ¼ 2 bar [585] C2H4(Ind)2ZrCl2 C2H4(Ind)2HfCl2 C2H4(Ind-H4)2ZrCl2 Me2Si(Ind)2ZrCl2 Ph2Si(Ind)2ZrCl2 Bz2Si(Ind)2ZrCl2

0.95 0.94 0.98 0.97 0.97 0.95

136 134 140 148 146 144

Polymerization at 401C, PPr ¼ 3 bar [1293] C2H4(2-Me-Cp)2ZrCl2 C2H4(2-i-Pr-Cp)2ZrCl2 C2H4(2-t-Bu-Cp)2ZrCl2 C2H4(Ind)2ZrCl2 C2H4(Ind-H4)2ZrCl2

0.922 0.946 0.976 0.890 0.859

133 136 141 137 129

All three types of catalysts operate at temperatures from
70 to +501C and produce highly isotactic polymers of propylene and other 1-alkenes with [mmmm] W0.99 [659,662,1292]. Marks, Even, Razavi, Miyake, and Spaleck developed another class of bismetallocene complexes producing highly isospecific catalysts (complexes XI in Scheme 1.1) [253,693,696–703]. These complexes do not have any elements of symmetry in terms of substituent positions in each Z5 ligand. When used at moderate temperatures, these catalysts also produce polypropylene with a very high degree of isotacticity corresponding to [mmmm] B0.95–0.98. Syndiospecific metallocene catalysts: Four types of bridged metallocene complexes produce catalyst systems suitable for the synthesis of syndiotactic polypropylene: 1. Single-bridged complexes of Cs symmetry with one cyclopentadienyl and one fluorenyl ligand (complexes XIV in Scheme 1.1) [166,411,595,727,736]. 2. Asymmetric single-bridged complexes with the structure imitating that of metallocene complexes of Cs symmetry (complexes XV) [728]. 3. Monometallocene constrained-geometry complexes with fluorenyl ligands (similar to complexes IX in Scheme 1.1), e.g., Me2Si(Flu)(t-Bu-N)TiMe2 [414,729–731]. 4. Stereo-rigid double-bridged complexes of Cs symmetry [411,693,702,1294, 1295]. Even discovered the first syndiospecific metallocene catalyst based on the bridged bis-zirconocene complex Me2C(Cp)(Flu)ZrCl2 [736]. A combination of this complex and MAO produces at 251C crystalline syndiotactic polypropylene with [rrrr] B0.86. A number of modifications of this basic structure were later introduced, they include the replacement of the Me2Co bridging group with Ph2Co and Me2Sio [738]. Bercaw [702,1294] and Canich [1295] discovered

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Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

another group of syndiospecific catalysts based on di-bridged bis-metallocene complexes. As an example, MAO-activated zirconocene complexes of Cs symmetry with two Me2Si groups bridging a cyclopentadienyl ligand (preferably substituted in the 4th position) and a 3,5-di-i-propyl-substituted cyclopentadienyl ligand produce highly syndiotactic polypropylene with [rrrr] p0.99, which corresponds to an exceptionally high probability of syndiotactic monomer linking, B0.997 [1294]. Di-bridged metallocene catalysts of C2 symmetry also produce syndiotactic polypropylene with [rrrr] from B0.75 to 0.99, depending on ring substitution [1241]. The stereo-regulating ability of all syndiospecific metallocene catalysts depends on the type of transition metal in the complex, the type of both Z5 ligands, the type (length) of the bridge between two Z5 ligands, the type of an alkyl substituent in the 1-alkene and on reaction temperature. Aspecific metallocene catalysts: Metallocene catalysts prepared from non-bridged complexes produce atactic polymers of 1-alkenes at ambient temperature [28,528, 912] and when the polymerization reactions are carried out at 60–801C the main reaction products are low molecular weight oligomers. The stereospecificity of constrained-geometry catalysts is determined by the type of their Z5 ligand. Catalysts based on complexes with an unsubstituted Cp ligand are practically aspecific, the contents of mm and rr triads in propylene polymers they produce are 0.14–0.24 and 0.23–0.37, respectively. The same complexes with bulkier Z5 ligands, such as Me4-Cp or Flu, produce moderately syndiospecific catalysts [672,731], the [rr] value for polypropylene prepared with them at 01C is B0.65.

4.6.5. Reactions leading to active centers in metallocene catalysts Metallocene complexes vigorously react both with MAO and with AlR3 compounds. These reactions were the subject of numerous studies, mostly by NMR and UV methods. This section describes the firmly established aspects of these reactions. The proposed structures of active centers in metallocene catalysts, a more speculative subject, in discussed in Section 6.1.1. The early stages of a reaction between Cp2ZrCl2 and MAO at low temperatures were analyzed by 1H and 13C NMR [1321,1342,1343], by UV-vis methods [1190,1344–1346], and IR [1347]. The principal reaction at a [MAO]:[Zr] molar ratio of 7:1 is mono-alkylation of the zirconocene complex with AlMe3 present in MAO or with MAO itself [1342,1347,1348]: Cp2 ZrCl2 þ AlMe3 ! Cp2 ZrðMeÞCl þ AlMe2 Cl

(4.49)

Alkylated metallocene complexes form stable 1:1 complexes with MAO [1342] and with AlMe3 [1343]. At higher temperatures, Cp2Zr(Me)Cl undergoes the second alkylation [1321]: Cp2 ZrðMeÞCl þ AlMe3 ! Cp2 ZrMe2 þ AlMe2 Cl

(4.50)

Cp2Ti(Me)Cl  AlMe3 is also alkylated with MAO and produces Cp2TiMe2 as the main product [1343].

270

Alkene Polymerization Reactions with Transition Metal Catalysts

Neither Cp2M(Me)Cl nor Cp2MMe2 can polymerize alkenes by themselves. The Cp2Zr(Me)Cl  AlMe3 complex does polymerizes ethylene but at a much lower rate than catalysts containing MAO. All metallocene catalysts produced with the MAO participation are inherently unstable. The chemistry of reactions leading to catalyst deactivation is discussed in Section 6.1.1.5. When homogeneous catalysts are produced from alkylated metallocene complexes Cp2MR2 or CpMR3 and ion-forming cocatalysts, the latter have two functions, (a) abstraction of the alkyl group from the metallocene complex and the formation of a metallocenium ion, Cp2M+Me or CpM+Me2, respectively and (b) the formation of stable, chemically resistant counter-anionic species with a widely distributed negative charge. One example is a reaction between CpTiMe3 and B(C6F5)3 at low temperatures [1257]: Cp TiMe3 þ BðC6 F5 Þ3 ! Cp ðMe2 ÞTiðm-MeÞBðC6 F5 Þ3 Cp ðMe2 ÞTiðm-MeÞBðC6 F5 Þ3 Ð ½Cp Tiþ Me2   ½BMeðC6 F5 Þ3 


(4.51) (4.52)

The dissociation equilibrium in Reaction (4.52) is strongly shifted to the left [749,1257,1375]. Metallocenium ions Cp2M+–Me or Cp(Me)M+–Me form strong complexes with their parent metallocene complexes via bridge methyl groups. The Cp2Zr+– Me ion forms a homo-dinuclear metallocenium ion Cp2(Me)Zr+(m-Me) Zr(Me)Cp2, and the Cp(Me)Ti+–Me ion produces a similar ion CpMe2Ti+ (m-Me)TiMe2Cp [1257]. All these dinuclear ions can exist as contact or solventseparated ion pairs with their counter-anions [1257,1376–1379]. Marks demonstrated that the structure of the counter-anions could be tuned to influence the activity of metallocene catalysts [735,1355,1356,1362–1364]. Several approaches to the modification of the counter-anions were developed. They include replacement of C6F5 groups in B(C6F5)3 or in [B(C6F5)4]
with their perfluorinated biphenyl or naphthyl analogs [735,1355,1362–1364], replacement of one F atom in the C6F5 group with the R3Si group [1356], and an increase of the number of B(C6F5)3 groups in the anion, e.g., the use of the [CPh3]+ [(C6F5)3 B–CRN–B(C6F5)3]
salt [1367].

4.7. Non-Metallocene Homogeneous Catalysts These catalysts are usually described in the literature as ‘‘non-metallocene homogeneous catalysts’’ or ‘‘post-metallocene homogeneous catalysts.’’ The family of non-metallocene homogeneous catalysts utilizes a variety of complexes of various metals, ranging from d0 metals (Sc) to lanthanoid and actinoid metals, and a large variety of monodentate, bidentate, and multidentate ligands containing oxygen, nitrogen, phosphorus, and sulfur as metal-coordinating atoms. The complexes are usually isolated and are well characterized by the X-ray method. They are transformed into polymerization catalysts using the same cocatalysts as those in metallocene catalysis, MAO or ion-forming activators. This field is well reviewed [1318,1384–1386], with the main emphasis on the synthetic routes to the

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

271

complexes and on the data on their activity in ethylene polymerization reactions under mild conditions. The total number of different transition metal complexes that can be transformed into alkene polymerization catalysts is quite large. However, only a few of the catalysts prepared from these complexes were subjected to detailed polymerization-centered studies. For convenience, the catalysts described in this chapter are separated into two classes, catalysts produced from complexes of the early-period transition metals and catalysts produced from complexes of the lateperiod transition metals.

4.7.1. Complexes of early-period transition metals 4.7.1.1. Complexes with monodentate ligands Ziegler discovered the first homogeneous catalysts of this type, Ti(OR)4-AlR3 [1058]. These catalysts dimerize ethylene to 1-butene and produce small quantities of ethylene/1-butene co-dimers as side-products. The mechanism of the ethylene dimerization reaction with these catalysts is principally different from the mechanism of alkene polymerization reactions with transition metal catalysts, it involves the formation of metallocycle intermediates from the Ti atom and the CQC bonds of two ethylene molecules [1059–1061]. Complexes of ZrCl4 with ethers, such as ZrCl4  2THF or ZrCl4  2Et2O, are soluble in aromatic solvents; their combinations with MAO are potent homogeneous polymerization catalysts both for alkenes and for styrene [1387]. All these catalysts contain several types of active centers, Mw/Mn values for polyethylene and polypropylene prepared with them range from B4 to over 10. These catalysts produce mixtures of atactic polypropylene and isotactic polypropylene with [mmmm] ¼ 0.88–0.92. AlMe3 present in MAO removes the ether ligands from the ZrCl4  2(ether) complexes; this reaction leads to the formation of aspecific active centers. When AlMe3 is removed from MAO, the content of the crystalline polypropylene fraction increases from B10 to 70% and the melting point of the polymers increases from B140 to W1631C. Overall, the behavior of these catalysts strongly resembles the behavior of heterogeneous Ziegler–Natta catalysts [1387]. Catalysts for ethylene trimerization: A special class of organochromium catalysts performs a reaction of ethylene trimerization to 1-hexene. The ‘‘chain growth’’ reaction in this case does not involve the insertion of the CQC bond into a transition metal-carbon bond but instead proceeds via the metallocycle mechanism [1403,1404]. Several catalyst compositions for this reaction are described in the literature [1404,1405]; the two most efficient are a quaternary mixture of Cr(Oi-Oct)3, dimethylpyrrole, AlEt3, and AlEt2Cl [1406]; and a complex of CrCl3 and (2-alkylsulfanylethyl)amine activated with MAO [1407]. 4.7.1.2. Complexes with bidentate ligands Complexes with acac ligands: Combinations of V(acac)3 and AlR2Cl were one of the first homogeneous catalysts developed for polymerization of ethylene and its copolymerization with 1-alkenes. When these catalysts are employed under mild conditions, they contain only one type of active center. Table 4.22 gives several

272

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 4.22 Ethylene/propylene copolymerization reactions with catalyst systems containing V(acac)3 complexes and AlEt2Cla [1388]

a

Complex

V(acac)3

V(Cy-acac)3

V(t-Bu-acac)3

V(F6 -acac)3

Activity (kg/mol V) Mw Mw/Mn (mol.%) Ccopol E

994 2.19  105 2.3 62

1168 2.18  105 2.3 59

746 2.90  105 2.2 65

762 2.77  105 2.2 63

Reaction conditions: 221C, Ptotal ¼ 2 bar, CE:CPr ¼ 1:1, reaction time 30 minutes.

examples of ethylene/propylene polymerization reactions with V(acac)3-AlEt2Cl systems. The performance of the catalysts is not affected by the nature of substituents in the acac ligands because the ligands are rapidly removed from the complexes in reactions with cocatalysts (Section 6.2.1). A replacement of AlEt2Cl with Al2Et3Cl3 also does not affect the properties of the copolymers [1388]. Complexes with diphenoxy ligands: Complexes of the general formula (L)Ti(OR)2, where L is a bidentate ligand with the bis-phenol structure derived from 2,2u-biphenol, 1,1u-bi-2-naphthol or 1,1u-CH2-di-2-naphthol (Scheme 1.2), activated with organoaluminum chlorides (e.g., with Al2Et3Cl3) form homogeneous single-center catalysts for ethylene polymerization [1389]. When these reactions are carried out in aromatic solvents above 601C, their principal products are crystalline polyethylene waxes with a molecular weight from B800 to B4,000, depending on the structure of the bis-phenoxy ligand and temperature. The same Ti complexes with diphenoxy ligands can be activated with ionforming cocatalysts based on MgCl2, which is produced in a reaction between MgR2 with AlR2X (X ¼ Cl or F) [1390]. Ethylene/1-hexene copolymers produced with these catalysts have a broad molecular weight distribution and a broad compositional distribution indicating that the catalysts have several types of active centers. These catalysts polymerize propylene and 4-methyl-1-pentene to products of low fractional isotacticity [1390]. Complexes with diamide ligands: McConville found that chelating diamide complexes of the (L)TiX2 type (Scheme 1.2) can be used as precursors of highly active catalysts for polymerization of 1-alkenes [792–794]. Synthesis procedures of these complexes are described in [794,796]. These complexes are converted into active species with the same cocatalysts as metallocene complexes, either with MAO (when X ¼ Cl and Me) at [Al]:[Ti] B250 or with B(C6F5)3 (when X ¼ Me) at [B]:[Ti] B1 [792,793]. Propylene polymerization reactions with these catalysts yield mixtures of atactic and moderately isotactic products [795,796]. The relative contents of the two polymer fractions depend on many factors including the type of aryl groups attached to both N atoms in the ligand, the type of cocatalyst, and the propylene concentration. The highest yield of the crystalline fraction in polypropylene produced at 401C is B50% when MAO is used as the cocatalyst but it can reach 80% when ion-forming activators are employed [796]. The crystalline material is highly regioregular [795], but its NMR isotacticity is low ([mmmm] ¼ 0.78–0.83) and its melting point is also low, 128–1321C [796]. If the

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

273

diamide complexes are used in combination with a solid cocatalyst prepared from MMAO and alumina, propylene polymerization reactions at 01C yield essentially atactic polypropylene of a very high molecular weight, with Mn over 1.5  106 [1392]. Polymerization reactions of 1-hexene with MAO-activated complexes at B701C also produce high molecular weight atactic polymers with a narrow molecular weight distribution, Mw/Mn B1.6–1.8. The same complexes activated with MAO or with [Ph3C]+ [B(C6F5)4]
-AlR3 are suitable for copolymerization of ethylene and 1-alkenes [1391]. Complexes (L)VIVCl2 containing diamide ligands similar to those shown in Scheme 1.2 but with the –SiMe2–CH2–CHMe2–SiMe2– bridge instead of the– (CH2)3– bridge are activated with AlEt2Cl or Al2Et3Cl3 [1388]. These combinations are very effective catalysts for polymerization of ethylene and for ethylene/ propylene copolymerization. When employed at low temperatures, they polymerize propylene to a moderately syndiotactic polymer with a significant content of 2,1-inserted propylene units [1388]. Overall, the performance of these catalysts strongly resembles that of VCl4-based catalysts suggesting the presence of the same type of active species. Complexes with phenoxy-imine ligands: Complexes of early-period transition metals of the general formula (L)2TiCl2 containing two phenoxy-imine ligands (Scheme 1.2) were widely explored as components of alkene polymerization catalysts. These ligands are sometimes defined in the literature as bis(salicylaldimine) ligands (when Ru ¼ H) or as bis(ketimine) ligands (when Ru ¼ alkyl group). The phenoxy-imine ligands are coordinated to the transition metal atom through one PhO– bond and one N atom. All these complexes have the octahedral configuration and can potentially exist in five isomeric forms with respect to the arrangement of the coordinating heteroatoms. According to x-ray data, DFT calculations, and NMR data, the most stable isomer has two PhO– bonds in trans-positions and each of the N and the Cl pairs in cis-positions [340,1393–1396]. The complexes are fluxional in solution through rapid dissociation/association of the Ti    N bonds [337,338]. The most abundant among the isomers in solution is the isomer with identical ligand pairs in cis-positions [340]. Bis(phenoxy-imine) complexes of TiIV and ZrIV combined with MAO or with common organoaluminum compounds form effective catalysts for polymerization of ethylene, 1-alkenes, and for ethylene/1-alkene copolymerization [227,339,342–344,782,783,789,790,805,806,808,1393]. The productivity of catalyst systems containing the Ti complexes and MAO in ethylene polymerization reactions at 25–751C can reach 2,000–4,000 kg/mol Ti  h. Catalyst systems containing bis(phenoxy-imine) complexes of Zr and AlR3 (R ¼ Me, Et, Hex) are also very active in ethylene polymerization reactions but produce polymers with a broad multi-component molecular weight distribution that varies with reaction time and temperature [338,341]. Bis(phenoxy-imine) complexes of TiIV containing C6F5 groups attached to the N atom in each bidentate ligand form catalyst systems with very low chain transfer rates. They copolymerize ethylene with propylene and higher 1-alkenes under living-chain conditions (Mw/Mn ¼ 1.07–1.19) at temperatures as high as 501C [138,342,343,781,784,806,808] and they are suitable for the synthesis of alkene

274

Alkene Polymerization Reactions with Transition Metal Catalysts

block-copolymers [138,342,343,789,808,1397]. Polypropylene produced with combinations of the aldimine Ti complexes and MAO or AlR3-[Ph3C]+ [B(C6F5)4]
as cocatalysts under moderate conditions also has a very narrow molecular weight distribution [342–345,783,784,789]. The stereospecificity of catalyst systems based on bis(phenoxy-imine) complexes depends on the type of substituents in the ligands (substituents R in Scheme 1.2). When R ¼ H or Me, the catalysts produce polypropylene with a predominantly syndiotactic structure. However, complexes of the same type bearing two bromine or two I atoms in the 2nd and the 4th positions of the phenyl groups produce moderately isotactic polypropylene with [mm] from B0.5 to 0.73, depending on reaction temperature, and the dichloro-substituted complex produces atactic polypropylene [790]. Bis(phenoxy-imine) complexes of TiIV activated with ion-forming cocatalysts instead of MAO, such as Ali-Bu3-[CPh3]+ [B(C6F5)4]
are efficient single-center catalysts for polymerization of higher 1-alkenes, 1-hexene, 1-octene, 1-decene, and 4-methyl-1-pentene [788]. The catalysts have very high activity and produce regioand stereo-irregular polymers with very high molecular weights, p1.4  106. 4.7.1.3. Complexes with tetradentate ligands Complexes with phenoxy-amine ligands and their analogs: Catalysts derived from several types of Ti, Zr, and Hf complexes of the general type (L)MX2 with tetradentate ligands L were tested in alkene polymerization reactions. The tetradentate ligand L in all these complexes has the general formula –O–Ar–bridge–Ar–O–. The phenoxy groups are linked by a bridge connecting ortho-positions in their benzene rings. Several types of bridges are used including bis-amine chains, –CH2–N(Me)– CH2–CH2–N(Me)–CH2– and –CH2–N(C2H4NMe2)–CH2–, and their analogs with one or two N atoms replaced by other heteroatoms. In all these complexes, phenoxy groups are attached to hexa-coordinated transition metal atoms in the trans-arrangement and two heteroatoms are in the cis-arrangement. The properties of the catalysts depends on the nature of the ligand L: 1. L ¼ –O–Ar–CH2–N(C2H4NMe2)–CH2–Ar–O– [798,1398–1401]. Complexes (L)ZrBz2 with the ligands of this type can be activated with B(C6F5)3, with combinations of [Ph3C]+ [B(C6F5)4]
and Ali-Bu3, or with MAO. These singlecenter catalysts are suitable for the synthesis of random etylene/1-hexene copolymers [1401]; they also produce highly active but stereo-aspecific systems for polymerization of 1-hexene [1401,1402], 4-methyl-1-pentene, and vinylcyclohexane [1402]. The Hf complexes form catalysts with the same properties but they are less active [798]. 2. L ¼ –O–Ar–CH2–N[C2H4X(Me)]–CH2–Ar–O– with X ¼ O and S [1400]. The activity of the (L)ZrBz2 and (L)HfBz2 complexes with the oxygencontaining ligands in 1-hexene polymerization reactions is even higher than the activity of the diamine complexes. 3. L ¼ –O–Ar–CH2–NR–C2H4–NR–CH2–Ar–O– [798–800,1401]. Zirconium complexes with ligands of this type are also activated with B(C6F5)3 [798], with combinations of [PhMe2NH]+ [B(C6F5)4]
and Ali-Bu3 [799,800], and with

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

275

MAO [1401]. These catalysts polymerize propylene and 1-hexene at room temperature in a quasi living-chain manner and produce polymers with Mw/Mn values ranging from B1.1 to 1.5. The stereospecificity of these systems depends on the type of alkyl substituents in the ortho-position to the C–O bond in both aryl groups Ar. The catalysts are aspecific when this alkyl group is small but they are isospecific when a bulky alkyl group is placed in this position. The [mmmm] value for polypropylene prepared at 251C with the latter catalysts is B0.80 [799], and the [mm] value for poly(1-hexene) is B0.95 [798]. Ti complexes (L)TiCl2 with L ¼ –O–Ar–CH2–S–C2H4–S–CH2–Ar–O– [761] activated with MAO produce atactic polypropylene [763]; they are effective single-center isospecific catalysts for polymerization of styrene [762,979] and its copolymerization with ethylene [931].

4.7.2. Complexes of late-period transition metals 4.7.2.1. Complexes with bidentate ligands Complexes with ylide ligands: Nickel ylide complexes contain a phosphine-enolate (ylide) ligand shown in Scheme 1.3. The ligand is bidentate and is formally negatively charged [87,1408,1409]. Keim discovered that the Ni ylide complex is a highly efficient mono-component catalyst for oligomerization of ethylene into mixtures of linear C4–C20+ 1-alkenes [86,87,810]. This catalyst (usually designated as Shell Higher Olefin Process, or SHOP catalyst) and its later modifications do not require the use of a cocatalyst and exhibits higher tolerance to polar organic compounds compared to complexes of early-period transition metals; they can be used in such polar solvents as ethanol or acetone [158]. The SHOP catalyst represents one of the most important industrial applications of homogeneous catalysis. Very high linearity of ethylene oligomers produced with this complex and very high selectivity toward the formation of 1-alkenes is explained by the presence of sterically bulky phenyl groups at the P atom in the complex. These groups hinder the coordination of the reaction products (1-alkene oligomers) to the Ni atom and prevent auto-co-oligomerization reactions of the alkenes with ethylene. Complexes with a-diimine ligands: Brookhart described a group of (L)NiX2 complexes with bidentate a-diimine ligands (Scheme 1.3), which are used as precursors of alkene-polymerization catalysts. These complexes are soluble in toluene and methylene chloride and are usually activated with MAO. The complexes can be synthesized separately [30,822,824] or they can be produced in situ, e.g., from Ni(acac)2 and the a-diimine ligand [1410]. The performance of these Ni a-diimine complexes in ethylene polymerization reactions strongly depends on the type of substituents in the complexes, especially ortho-positioned substituents in the aryl groups [30,346,812,814,822,824,825,1410]. Table 4.23 gives several examples of the substituent effects. GPC data show that these catalysts usually contain either one or two types of active centers. If the aryl groups have no ortho-substituents [30,1411,1412] or if only one of the ortho-positions is substituted [822,824], the catalysts produce ethylene

276

Table 4.23

Alkene Polymerization Reactions with Transition Metal Catalysts

Ethylene polymerization reactions with Ni a-diimine complexesa [822] Mn

Mw/Mn

Branching (mol.%)

Complexes with X ¼ Br, Ru ¼ Me, Et i-Pr, i-Pr 25

7.66  105

1.7

5.6

96

Complex with X ¼ Br, Ru+Ru ¼ Cy i-Pr, i-Pr 45

8.44  105

1.7

7.8

100

Complexes with X ¼ Br, Ru+Ru ¼ a,a-dinaphthyl 6 1.3  103 H, CH3 45 5.92  104 CH3, CH3 i-Pr, i-Pr 67 8.44  105 31 3.8  103 H, CF3 14 4.00  105 CH3, CF3 18 B1  103 H, C6F5 31 7.37  104 CH3, C6F5

– 2.5 1.7 – 3.3 – 2.6

0.4 2.6 7.8 1.0 5.4 1.0 0.8

Substituents in Ar group

a

Productivity (kg/mmol Ni  h)

Tm (1C)

128 100 125 127 134

Cocatalyst MAO, 351C, PE ¼ 13.5 atm.

oligomers with the vinyl and the internal CH3CHQCH
bonds as the last endgroups. Only the complexes with two bulky ortho-substituents in each aryl group form highly active catalysts and produce polyethylene of a high molecular weight [822]. Ethylene homopolymers prepared with all these complexes are branched [30,822,824], their chains mostly contain isolated methyl side-groups. The level of branching is significantly lower for the Pd complexes compared to the Ni complexes. It also increases with an increase of the bulk of ortho-substituents in the aryl groups. The presence of branches depresses the melting point of the polymers, as shown in Table 4.23 (the Tm value for linear polyethylene with a narrow molecular weight distribution is B136–1381C). The mechanism of chain isomerization leading to branch formation is discussed in Section 6.2.4.2. An increase in the ethylene concentration has no effect on the molecular weight of the polymers (indicating that the principal chain transfer reaction is that to a monomer) but it results in a significant decrease of chain branching [822]. Active centers in these catalysts are stable under ambient conditions but they decay at 601C, t0.5 B40 minutes [822]. As expected, the molecular weight of ethylene homopolymers decreases with temperature whereas the branching degree increases (Chapter 3). One example for a Ni a-diimine complex with X ¼ Br and Ru+Ru ¼ Cy illustrates this trend. Temperature (1C) Branching (mol.%)

35 7.8

60 12

85 B19

a-Diimine complexes of Ni and Pd are relatively insensitive to the presence of oxygen-containing organic compounds and they are widely used for copolymerization of alkenes and polar monomers with CQC bonds [1413,1414].

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

277

Several electro-neutral keto-ilide and phenoxy-imine complexes of NiII containing Ni–Ph and Ni–Me bonds and a phosphine or pyridine ligand coordinated to the Ni atom are relatively stable in polar solvents. They are used as single-component catalysts for ethylene polymerization in polar solvents and even water [834,1415–1417]. The reactions are performed in emulsion in at 50–701C and at a high ethylene pressure, 50 bars. Although the activity of the catalysts in polar solvents is lower than in aromatic solvents, linear and branched ethylene polymers with a molecular weight from several thousands up to B4  105 can be prepared as solid materials or latexes [1415,1417]. Polymerization reactions in water emulsions are also suitable for the synthesis of ethylene copolymers with a variety of 1-alkenes, nonconjugated a,o-dienes, styrene, and 1-alkenes with polar side-groups [1416].

4.7.2.2. Complexes with tridentate ligands Complexes with bis(imino)pyridyl ligands: The structure of these complexes is shown in Scheme 1.3. V, Fe, and Co complexes of this type activated with MAO form very active catalyst systems for polymerization of ethylene [816,817,1418–1423] and propylene [819,820], as well as for the synthesis of ethylene oligomers (linear 1-alkenes) [817,818]. These catalysts can be also supported on silica [1421,1424, 1425], alumina [1426], and MgCl2 [1427]. Representative data on ethylene polymerization reactions with these catalysts are given in Table 4.24. Bis(imino)pyridyl complexes of CoII form single-center catalysts of low activity and usually produce ethylene polymers of a low molecular weight and a narrow molecular weight distribution [388,816,817,1419]. In contrast, bis(imino)pyridyl complexes of VIII, FeII, and FeIII are highly active and produce ethylene polymers with a broad multi-component molecular weight distribution. The width of the distribution depends on several reaction conditions, the [MAO]:[Fe] ratio in the catalyst systems, reaction time and temperature, ethylene concentration, etc. (see Section 5.5.4) [816–818,820,1418,1419,1422, 1424,1426,1428]. Supported catalysts utilizing the same FeII complexes and silica, alumina or MgCl2 as carriers also produce polyethylene with a very broad molecular weight distribution [1426,1427]. However, the same Fe-based systems polymerize propylene as single-center catalysts [819,820]; they produce moderately isotactic polymers with a narrow molecular weight distribution. Catalysts produced from bis(imino)pyridyl FeII complexes have relatively poor temperature stability; however, an introduction of nitro-groups in para- and meta-positions of their phenyl rings affords significantly more stable catalysts [1423].

4.8. Supported Homogeneous Catalysts Homogeneous catalysts based on metallocene complexes and other soluble transition metal complexes, while very attractive for academic research, are usually unsuitable for large-scale commercial polymerization processes. Two technical

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Alkene Polymerization Reactions with Transition Metal Catalysts

Table 4.24 Ethylene polymerization reactions with bis(imino)pyridyl complexes of Fe and Coa [388,816,817]. Complex (Scheme 1.3, X1 ¼ X2) Data from [816]b FeII, R ¼ Me, X1 ¼ i-Pr, X3 ¼ H FeII, R ¼ Me, X1 ¼ Me, X3 ¼ H FeII, R ¼ Me, X1 ¼ Me, X3 ¼ Me FeII, R ¼ H, X1 ¼ i-Pr, X3 ¼ H FeIII, R ¼ Me, X1 ¼ Me, X3 ¼ Me CoII, R ¼ Me, X1 ¼ i-Pr, X3 ¼ H CoII, R ¼ H, X1 ¼ Me, X3 ¼ Me Data from [817]b FeII, R ¼ Me, X1 ¼ Cl, X3 ¼ H FeII, R ¼ Me, X1 ¼ Br, X3 ¼ H FeII, R ¼ Me, X1 ¼ Br, X3 ¼ H CoII, R ¼ Me, X1 ¼ Br, X3 ¼ H CoII, R ¼ Me, X1 ¼ Br, X3 ¼ H CoII, R ¼ Me, X1 ¼ Br, X3 ¼ H a

Temperature PE [Al]:[M] Productivityb,c Mw (1C) (Mpa)

Mw/Mn

50

1.0

1,000

5,340

6.11  105

9.5

50

1.0

1,000

9,340

2.42  105

25.3

50

1.0

1,000

20,600

1.48  105

10.7

35

1.0

200

305

1.32  105

38.9

50

1.0

200

435

1.12  105

70.5

50

1.0

1,000

450

1.4  104

3.3

35

1.0

200

340

1.6  103

2.6

0

0.1

2,500

5.4

1.44  105 104

0

0.1

1,250

8.6

1.01  105

54.3

40

0.1

1,250

6.8

5.23  104

33.3

0

0.1

1,250

0.6

1.22  104

3.4

25

0.4

1,000

3.3

5.1  103

2.5

25

1.2

1,000

6.9a

5.3  103

2.6

Cocatalysts MMAO and MAO. Productivity in [816]: kg/mol M  bar  h. Productivity in [817]: kg/mol M h.

b c

requirements explain this deficiency. The first one is a matter of adaptation to an existing technology. Most modern processes for the manufacture of polyethylene and isotactic polypropylene involve the introduction of catalysts into reactors in a form of dry powder or slurry rather than solution. The second reason for the deficiency of soluble catalysts stems from their physical state. Both polyethylene and stereoregular polypropylene produced with soluble catalysts under standard reaction conditions are crystalline materials completely insoluble in the reaction media that dissolve the catalysts. Soon after the onset of the polymerization reactions, very small polymer crystals are formed, each crystal containing just a few polymer molecules. These tiny crystals tend to adhere to internal surfaces of the reactors and rapidly foul them. To avoid the fouling and to use homogeneous catalysts as ‘‘drop-in’’ catalysts to replace heterogeneous Ziegler–Natta catalysts,

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

279

soluble catalysts should be ‘heterogenized’’ (supported) and fashioned into particles similar in size and shape to particles of Ziegler–Natta catalysts. The synthesis of supported homogeneous catalysts (it is often called ‘‘immobilization’’ of homogeneous catalysts) has many features similar to the synthesis of supported Ti-based Ziegler–Natta catalysts described in Sections 4.2 and 4.3. No common supports are chemically compatible with metallocene complexes in a sense that they can be expected to directly affect the activity of metallocene catalysts. Instead, several strategies were developed to place metallocene catalysts within pores of inorganic and organic carriers. Silica is a carrier of choice in these catalysts although alumina, alumosillicates, zeolites, MgCl2, and porous polymer beads are used as well [1427,1429–1436]. All supported homogeneous catalysts have several common features. In many cases, MAO is either deposited into the carrier or synthesized inside its pores. The physical state of MAO supported on silica was studied by electron microscopy using model carrier surfaces prepared on a flat silicon substrate covered by amorphous silica and impregnated with solutions containing MAO and metallocene complexes [1437]. Dry MAO forms a homogeneous film of the silica surface, and metallocene complexes are uniformly dispersed in the MAO layer. When ethylene or propylene is added to the model catalysts, the polymerization reactions occur in a relatively uniform manner within the MAO/metallocene layer. They result in an expansion of the MAO layer and the formation of uniform polymer film covering the silica surface. When a homogeneous catalyst activated with an ion-forming borate cocatalyst is deposited on this model silica surface, the catalyst forms flat clusters loosely attached to the carrier. A subsequent polymerization reaction with this catalyst produces large polymer beads [1437]. MAO species in carrier pores can be viewed as Lewis acidic sites [1392,1438]. XPS analysis of MMAO absorbed in several carriers showed that the Lewis acidity of the alumoxane increases in the order MMAO/MgO E free MMAOoMMAO/ Al2O3oMMAO/SiO2 [1392]. Two different acidic sites were identified in MAO/ SiO2 products using the CO molecule as a probe [1439,1440], and Cp2ZrMe2 interacts with both sites [1440]. When solution of MAO is used for activation of supported metallocene catalysts, a reaction between the metallocene complexes and MAO often leads to desorption of the complexes from the surface of the carriers and the formation of homogeneous metallocene catalysts [331,1320,1431,1441,1442]. Catalytic properties of such ‘‘supported’’ catalysts and the structure of polymers they produce are indistinguishable from those prepared with respective homogeneous catalysts both in terms of the molecular weight distribution of the polymers and the compositional uniformity of the copolymers [1000,1441,1443]. The leaching of active ingredients from the catalyst particles can be suppressed by prepolymerization of a small quantity of a 1-alkene. Silica-immobilized MAO is a commercially available product. This material can be impregnated with solutions of metallocene complexes and other transition metal complexes, either in a separate catalyst preparation step or directly in a polymerization reactor prior to monomer introduction [219,826]. No additional MAO is needed to activate such supported catalysts provided that AlMe3 or other trialkylaluminum compounds are used as impurity scavengers.

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Alkene Polymerization Reactions with Transition Metal Catalysts

Direct impregnation of carrier with metallocene complex [1240,1430–1448]: Many porous carriers suitable for impregnation with metallocene complexes contain Brønsted centers (acidic hydroxyl groups) or Lewis acidic centers [1318,1430]. When toluene solutions of metallocene complexes are contacted with calcined silica, they can react with both types of the centers. A reaction between silanol groups on the silica surface and metallocene complexes results in their binding to silica: CpMClx þ H2O2SiR ! CpMClx
1 2O2SiR þ HCl

(4.53)

Similar oxo-complexes were proposed as reaction products between metallocene complexes and metal oxides Mu–O with expressed Lewis basic properties [1449]: CpMClx þ M0 2O2M0 2O2M0 ! CpMClx
1 2O2M0 2O2M0 ðClÞ2O2M0

(4.54)

Immobilization of MAO or a metallocene-MAO mixture on a carrier [162,380,638,656, 1000,1240,1430,1431,1435,1439,1442,1446,1448,1450–1457]: This procedure includes two steps, a reaction of a carrier (silica or alumina) with MAO followed by a reaction of immobilized MAO with a metallocene complex. Commercial MAO always contains a significant quantity of AlMe3 that reacts with silanol groups in silica [1439,1458]: AlMe3 þ H2O2SiR ! Me2 Al2O2SiR þ CH4

(4.55)

MAO does not react with the silanol groups directly but it is adsorbed on the AlMe3-modified silica surface [1439]. The quality of silica impregnation with MAO is improved if the impregnation step is carried out at increased temperatures [1448]. Premixed MAO-metallocene compositions can also be used in this procedure. A variant of this technique includes a treatment of silica with a coupling agent such as Me2SiCl2 prior to the reaction with MAO [1459]. These catalysts usually require additional MAO or AlR3 as activators [1446,1451,1452,1460,1461]. If these silica-supported catalysts are prepared with bridged racemic biszirconocene catalysts of C2 symmetry, they produce isotactic polypropylene. The molecular weight distribution of the polymers ranges from narrow [1455,1456,1465,1466] to broad or even bimodal [331]; their stereoregularity is slightly lower than that of polypropylene prepared with unsupported analogs of the same catalysts [1456]. The use of bridged bis-zirconocene complexes of Cs symmetry leads to supported syndiospecific catalysts [1435,1466]. The polymers they produce have a narrow molecular weight distribution; their stereoregularity ([rrrr] ¼ 0.83–0.87) and melting points (140–1461C) do not depend on reaction temperature [1435]. Supporting two metallocene catalysts, one isospecific and another syndiospecific, on the same silica carrier leads to production of polypropylene mixtures [1466]. A variant of this impregnation technique utilizes spherical porous organic supports produced from polymers with polar groups, such as styrene/ 4-vinylpyridine copolymers [1461], 1,4-divinylbenzene/4-vinylpyridine copolymers [1434], styrene/acrylamide copolymers [1462], copolymers of propylene with polar monomers [1463], or styrene/divinylbenzene copolymers [1464].

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

281

These supports are reacted, in sequence, with MAO and with a zirconocene complex. According to IR and XPS data, these procedures result in efficient immobilization of metallocene species and produce solid catalysts of high activity [1461]. Bioorganic polymeric materials with polar groups, such as starch and cyclodextrin, can be also employed for the immobilization of metallocene catalysts [545,1490]. Generation of MAO in a carrier: [1240,1430,1467,1468]. If silica is not calcined at a high temperature, it contains a significant amount of occluded water which can be reacted with AlMe3 with the formation of MAO supported on silica [1468]. The efficiency of these catalysts mostly depends on the ratio between the amounts of water and hydroxyl groups in silica (both depend on calcination temperature (see Table 4.3)) and the amount of added AlMe3. MAO/silica compositions are treated with solutions of metallocene complexes and dried. These supported catalysts polymerize ethylene without an additional cocatalyst. Mineral clays as carriers: [258,1431–1433,1469–1471]. Procedures for the synthesis of clay-supported catalysts are similar to those for silica-supported catalysts. Acidic silanol groups on the surface of an inorganic clay (two-layered kaolin, three-layered montmorilonite, or a zeolite) are treated with AlMe3 or with Ali-Bu3. Reaction (4.55) produces R2Al–O–SiR groups on the clay surface and then solution of a metallocene compound is added to the support to form a final catalyst. Claysupported metallocene catalysts are highly active in polymerization reactions of ethylene and propylene [1432,1441]. They do not require any additional cocatalysts although AlMe3 or Ali-Bu3 is used in the polymerization reactions as an impurity scavenger. MAO-free supported catalysts: Some materials containing strong Lewis acidic centers, such as alumina or MgCl2, can be used for the preparation of supported MAO-free metallocene catalysts. The synthesis usually includes two steps, the calcined support is treated with AlMe3 and then with a zirconocene complex [1430,1444,1472]. The productivity of these AlMe3-activated catalysts is 3–5 times lower than for similar homogeneous catalysts containing MAO, and the molecular weight distribution of polymers produced with them is the same or slightly broader, the Mw/Mn ratio varies from B2 to W4. If bridged metallocene complexes are used in the synthesis of these catalysts, their stereospecificity can be higher than that of their homogeneous analogs [1430]. A variant of this method includes the preparation of special MgCl2/ AlRn(OR)3
n supports by reacting soluble or solid MgCl2  nROH adducts (R ¼ Et, i-Hex; n ¼ 1–3) with aluminum alkyls under mild conditions [787,1473]. A vigorous reaction between the coordinated alcohol molecules and AlR3 produces porous supports. A variety of single-center catalysts can be immobilized on these supports including CpTiCl3, IndTiCl3, Cp2TiCl2, and constrained-geometry complexes. These catalysts are used in combination with AlR3 as cocatalysts/impurity scavengers. They are relatively stable and have high activity in ethylene polymerization reactions, 6–9 ton/mol Ti  bar  h at 501C. Chemical linking of boronaromatic compounds to carriers: When supported metallocene catalysts with boronaromatic cocatalysts are supported, the boronaromatic compounds can be chemically linked to the surface of supports containing

282

Alkene Polymerization Reactions with Transition Metal Catalysts

acidic hydroxyl groups. This approach was pioneered by Hlatky [1474] and Bochmann [1475,1476]; its goal is the generation of the R+ [B(C6F5)3–SiO2]
species on the support surface with R+ ¼ [Ph3C]+ or [HNMe2Ph]+. Reacting these species with dialkylated metallocene complexes Cp2MR2 produces surfaceanchored metallocenium cations Cp2M+
R. Several modifications of this general approach are described in the literature [1446,1477–1479]. Reactions (4.56) and (4.57) are used to immobilize ion-forming cocatalysts on the surface of silica and Reaction (4.58) illustrates the formation of the active species [1479]: RSi2O2H þ BðC6 F5 Þ3 þ HNMe2 ! RSi2O2B
ðC6 F5 Þ3    ½HNMe2 Phþ RSi2O2H þ BðC6 F5 Þ3 þ ClCPh3 ! RSi2O2B
ðC6 F5 Þ3    ½Ph3 Cþ þ HCl RSi2O2B
ðC6 F5 Þ3    ½Ph3 Cþ þ Cp2 ZrR2 ! RSi2O2B
ðC6 F5 Þ3    ½Cp2 Zrþ 2R þ CPh3 R

(4.56)

(4.57)

(4.58)

Chemical linking of a cyclopentadienyl ring to supports: This approach was developed by Soga [885,1450–1452,1480–1482]. Several synthetic routes were introduced for anchoring metallocene complexes to a carrier surface with a spacer; a chemical link between the surface and one of the cyclopentadienyl rings in the complex. One route involves building up both the cyclopentadienyl ligand and the metallocene complex itself at the support surface [1483–1485]: 1. Treatment of calcined silica with a coupling agent bearing a cyclopentadienyl group: RSi2O2H þ ðEtOÞ3 Si2ðCH2 Þ3 2cyclo-C5 H5

(4.59)

! RSi2O2ðEtOÞ2 Si2ðCH2 Þ3 2cyclo-C5 H5 2. Conversion of the cyclopentadienyl group into a cyclopentadienyl anion: RSi2O2ðEtOÞ2 Si2ðCH2 Þ3 2cyclo-C5 H5 þ BuLi ! RSi2O2ðEtOÞ2 Si2ðCH2 Þ3 2Cp
Liþ

(4.60)

3. Formation of a ligand-anchored metallocene complex: RSi2O2ðEtOÞ2 Si2ðCH2 Þ3 2Cp
Liþ þ CpZrCl3 ! RSi2O2ðEtOÞ2 Si2ðCH2 Þ3 2CpðCpÞZrCl2 þ LiCl

(4.61)

Each step of this synthesis is well documented [1483]. This procedure produces supported catalysts, which, after activation with MAO, are highly active in ethylene homopolymerization reactions and produce polymers with a molecular weight distribution similar to that of their unsupported analogs. Catalyst compositions of the (Cp)(Ind)ZrCl2/silica type containing –(CH2)5– and –(SiMe2–O)3– spacers between the silica surface and the metallocene complex also have the single-center nature and exhibit very high polymerization activity [1486].

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

283

The –SiMe2– group is also used as the spacer [260]; it leads to the synthesis of anchored metallocene complexes, RSi–O–SiMe2–Cp(Cp)ZrCl2, RSi–O– SiMe2–Cp(Ind)ZrCl2, and RSi–O–SiMe2–Ind(Ind)ZrCl2. All these catalysts are suitable for homopolymerization of ethylene and its copolymerization with 1-alkenes with the formation of compositionally uniform copolymers [260]. Other procedures for linking cyclopentadienyl groups to the surface of supports include the Diels-Alder reaction of the spacer groups with cross-linked polystyrene [1487] or pre-synthesis of a metallocene complex with a spacer group attached to one of its Z5 ligands. This group is used as an anchor for immobilizing the complex on the support surface [1488,1489]. Chemical linking of cyclopentadienyl rings to polymer chains: Bis-metallocene complexes can be inserted in the main chain of siloxane polymers [1482]. This catalyst recipe starts with the synthesis of special dialkyldichlorosilanes, (Cp)(Flu)SiCl2 or (Me4-Cp)(Flu)SiCl2. Their condensation with an excess of Me2SiCl2 produces polydimethyl siloxanes containing a small number of (Cp)(Flu)Si(O
)2 or (Me4-Cp)(Flu)Si(O
)2 units. Subsequent reactions of these units with n-BuLi and ZrCl4 produce modified siloxane copolymers containing B1.5 mmol/g of anchored equivalents of WSi(Cp)(Flu)ZrCl2 and WSi(Me4-Cp)(Flu)ZrCl2. Propylene polymerization reactions with these polymeric catalysts activated with MAO or with [Ph3C]+ [B(C6F5)4]
-AlEt3 combinations produced syndiotactic polymers with [rrrr] X0.76 and small quantities of atactic and isotactic polymers. Soga used a different type of a polymer molecule to construct a metallocene complex of Cs symmetry attached to a polymer chain, a radical copolymer of styrene and a styrene derivative, 1-vinyl-4-(1-cyclopentadienyl-1-fluorenyl)ethylbenzene. This monomer unit carries both Z5 ligands required to synthesize a supported syndiospecific metallocene catalyst [1481]. Immobilization of homogeneous non-metallocene catalysts: A choice of carriers for the immobilization of homogeneous catalysts based on non-metallocene complexes depends on the nature of the complexes and a need for an organoaluminum cocatalyst. Some of these catalysts do not require MAO as a cocatalyst, which significantly expands both the types of suitable carriers and immobilization techniques. One of the often-used carriers in these catalysts is spherical microcrystalline MgCl2. Several techniques for the preparation of these carriers are described in Sections 4.2.1.3 and 4.3.2.2. Particulate MgCl2 carriers were used for the immobilization of NiII complexes with bidentate a-diimine ligands [1436,1491] and (imino)pyridyl complexes of FeCl2 [1426,1436,1492]. The supported Ni a-diimine complexes have high activity in ethylene polymerization reactions [1436] and exhibit many features typical for unsupported catalyst systems based on the same complexes [1491]. They produce branched polymers of a high molecular weight with a narrow molecular weight distribution. The types of branches are the same as in other ethylene homopolymers prepared with the catalysts of this class (Table 3.66), and their number depends on the type of substitution in the Ni complexes. Solid MgCl2  3ROH complexes were also employed for the immobilization of bis(phenoxy-imine) complexes of TiIV and ZrIV [787]. The stereospecificity of these MAO-free supported catalysts depends on the substituents in the bis(phenoxy-imine) ligand (Table 3.42). When the SiMe3 group is present at the

284

Alkene Polymerization Reactions with Transition Metal Catalysts

ortho-position to the C–O bond and the C6F5 group is attached to the N atom, the supported catalyst is highly syndiospecific, the rr-triad content in the polypropylene prepared at 251C is 0.97, and it has a high melting point, 1551C [787]. Silica and mineral clays are also used as carriers for immobilized nonmetallocene catalysts, including Ni a-diimine complexes [286,346,823,826,1495] and Ni ylide complexes [1493,1494].

4.9. Bicomponent Catalysts 4.9.1. Catalysts for polymers with a broad molecular weight distribution Nearly all catalyst systems based on metallocene complexes are either single-center catalysts or contain several types of active centers producing polymer fractions of a similar molecular weight. The Mw/Mn values for these polymers are relatively low, from 2 to 4. Such polymers have a significant disadvantage when they are converted into different articles using modern polymer-processing equipment. This equipment (extruders, machines for the manufacture of polymer film and containers) makes use of the non-Newtonian rheological behavior of polymer melts: when the pressure applied to a melt flowing through a narrow orifice increases, the viscosity of the melt significantly decreases. This phenomenon is called the shear thinning of polymer melts; it is an important factor that determines the manufacture rate of polymer items. The magnitude of the shear thinning is reciprocal to the Mw/Mn value of a polymer, and, therefore, metallocene-derived polymers with low Mw/Mn values are relatively difficult to process. This handicap can be overcome if two metallocene catalysts are used together, one catalyst producing a polymer with a relatively high molecular weight and another with a relatively low molecular weight. Many combinations of metallocene catalysts (most of them activated with MAO) were proposed for homopolymerization of ethylene and for copolymerization of ethylene with 1-alkenes. Combinations of Cp 2ZrCl2 with Cp2TiPh2 [40,1496], (1,2,4-Me3-Cp)2ZrCl2 [1497], or rac-C2H4(Ind-H4)2ZrCl2 [649]. Combinations of rac-C2H4(Ind)2ZrCl2 with Ind2ZrCl2, Cp2HfCl2, and racC2H4(Ind)2HfCl2 [904,1498–1500]. Combinations of Cp2TiCl2 with rac-C2H4(Ind)2ZrCl2, Cp2ZrCl2, and Cp2HfCl2 [384,1501,1502]. Combinations of rac-C2H4(Ind)2ZrCl2 and constrained-geometry Ti catalysts [384,1503]. Combinations of two constrained-geometry catalysts, a zirconocene catalyst for the production of the low molecular weight polymer and a titanocene catalyst for the high molecular weight component [551]. Supported catalysts with two metallocene complexes, C2H4(Ind-H4)2ZrCl2 and Cp2HfCl2 [219]. The same approach was used for the synthesis of mixtures of isotactic and syndiotactic polypropylene in a single polymerization reaction, e.g., with a mixture

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

285

of the MAO-activated isospecific metallocene complex, rac-Me2Si(Ind)2ZrCl2, and the syndiospecific complex, Ph2C4(Cp)(Flu)ZrCl2, both either as homogeneous catalysts or in the supported form [258,1466,1504]. Mutual effects in binary catalysts: The two-catalyst approach exploits inherent differences in the molecular weights of polymers produced by each catalyst in the mixture as well as differences in the response of the catalysts to polymerization parameters. The use of catalyst combinations is based on the premise that each metallocene complex forms its own active species and that the two types of active centers generate polymers independently. Indeed, analysis of several polymer products prepared with combinations of two metallocene catalysts showed the presence of two thoroughly mixed fractions of different molecular weights and different structures. These polymer products include ethylene homopolymers prepared with mixtures of two constrained-geometry catalysts [551], ethylene/ 1-hexene copolymers produced with a mixed supported catalyst containing racC2H4-Ind2ZrCl2 and a constrained-geometry Ti catalyst [384], and ethylene/ 1-butene copolymers produced with mixtures of rac-C2H4-Ind2ZrCl2 and racC2H4-Ind2HfCl2 [904]. The main difficulty in employing binary metallocene systems is balancing the activities of their components to produce polymer mixtures with a desired proportion between the two polymer components. The activity of individual catalysts can differ by several orders of magnitude, and careful tuning of the relative catalyst amounts is required for each set of reaction conditions. The second difficulty in employing binary metallocene catalysts is the control of their mutual effects. The introduction of a second metallocene complex into a polymerization reaction catalyzed by a single catalyst can significantly alter the activity of the latter catalyst, and vice versa. This mutual influence has several causes. One of them is the mediating action of organoaluminum compounds. Chien, Coates, and Brintzinger showed that when free AlMe3 or Ali-Bu3 are present in polymerization reactions catalyzed by mixtures of metallocene complexes, these organoaluminum compounds play the role of polymer transferring agents between two different active centers due to their participation in chain transfer reactions [162,391,642,650] (see Section 3.3.1.2). This chain transfer between different centers explains often observed deviations from the expected behavior of two independent active species in their binary mixtures, including deviations in the reactivity of each catalyst, in the copolymerization ability of each center, and in molecular weights of polymers produced by each center [1497,1504].

4.9.2. Catalysts for synthesis of block-copolymers and branched polymers Catalyst systems for synthesis of block-copolymers: The existence of chain-transferring reactions between different types of metallocene active centers provides an opportunity for the synthesis of stereoblock copolymers (Section 5.6.2). For example, polypropylene materials containing isotactic blocks and atactic blocks linked together were produced with a catalyst mixture containing racMe2Si(Ind)2ZrCl2, C2H4(Flu)2ZrCl2, and a single ion-forming cocatalyst [CPh3]+

286

Alkene Polymerization Reactions with Transition Metal Catalysts

[B(C6F5)4]
, with Ali-Bu3 as a chain-transfer agent [162]. Solvent fractionation of these polymers showed that they contain several fractions of block-homopolymers containing macromolecules with isotactic and atactic blocks in different proportions, as well as completely atactic material. The cocatalyst-mediating effect also affords the synthesis of propylene polymers containing isotactic and syndiotactic blocks. When the isospecific rac-C2H4(Ind)2 ZrR2-[CPh3]+ [B(C6F5)4]
system and the syndiospecific Ph2C(Cp)(Flu)ZrR2[CPh3]+ [B(C6F5)4]
system are combined in a polymerization reaction, they produce two separate and physically incompatible propylene polymers, isotactic and syndiotactic [650]. However, when the two catalysts are used together with an excess of Ali-Bu3, the total polymer product, in addition to two stereoregular homopolymers, contains a stereoblock copolymer (iso-PP)-(syndio-PP). This blockcopolymer acts as a compatibilization agent for the two homopolymers, it significantly modifies thermo-physical and mechanical properties of the polymer mixtures [650]. The same type of material was produced with silica-supported catalysts containing the isospecific rac-Me2Si(Ind)2ZrCl2-MAO component and the syndiospecific Me2C(Cp)(Flu)ZrCl2-MAO component [1505]. Either Ali-Bu3 or AlEt3 can be employed as chain-transferring agents in these propylene polymerization reactions. Polymerization products prepared with these binary catalysts consist of three components: (a) highly syndiotactic polypropylene with [rrrr] B0.86, (b) highly isotactic polypropylene with [mmmm] B0.91, and (c) a mixture of blockpolymers with [mmmm]/[rrrr] ratios from B4:1 to B1:4. Catalyst systems for synthesis of branched polymers: Combinations of metallocene catalysts can be used to polymerize a single monomer, ethylene, with the formation of branched polymers of a general formula B(CH2–CH2)x–CH2–CH (n-CnH2n+1)–(CH2–CH2)yB containing linear branches with the number of carbon atoms n ranging from 2 to B100–150. One of the components in these binary catalysts produces low molecular weight ethylene oligomers CH2QCH–nCnH2n+1with the vinyl chain end and the second catalyst component copolymerizes the oligomers with ethylene [598,622,1003,1506]. The oligomerization component is produced from a bis-metallocene complex, a constrained-geometry monometallocene complex, or a homogeneous catalyst based on a late-period transition metal. Such binary catalysts are especially effective when both catalyst components share the same cocatalyst [622]. If branches in such polymerization products are sufficiently long, nW30–40, these products are called polyethylene with long-chain branches [1507]. Ethylene polymers containing long-chain branches have significantly better processability in the melt compared to linear ethylene homopolymers with a narrow molecular weight distribution [151,152,1003,1006]. A special class of binary homogeneous catalysts was developed to produce ethylene/1-hexene copolymers from a single monomer source, ethylene [235]. The first catalyst component, a complex of CrCl3 and (2-alkylsulfanylethyl)amine activated with MAO, exclusively trimerizes ethylene to 1-hexene [1407]. The second catalyst component is produced from a metallocene complex, Cp2ZrCl2 or Me2Si(2-Me-Ind)2ZrCl2, or a constrained-geometry [Me2Si(Me4-Cp)(t-Bu-N)] TiCl2 complex. This component copolymerizes ethylene with the in situ generated

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

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1-hexene. The synthesis of these ethylene/1-hexene auto-copolymers is carried out in a single reactor and the relative amounts of the two catalyst components are adjusted to produce ethylene/1-hexene copolymers of a desired composition.

4.9.3. Binary Ziegler–Natta/metallocene systems The synthesis of bicomponent (hybrid) metallocene/Ziegler–Natta catalysts has important commercial significance. Mechanical properties of ethylene homopolymers produced with Ti-based Ziegler–Natta catalysts improve when their molecular weight increases. However, polymers of a high molecular weight are difficult to process into useful articles (e.g., into film) due to high viscosity of their melts. This problem can be overcome if a part of the polymers has a relatively low molecular weight so that the average melt viscosity of the mixture remains within the acceptable range. Unfortunately, mechanical blending of two ethylene homopolymers in the molten state cannot produce such polymer materials because polymer melts of vastly different viscosities do not mix well. An alternative route to the manufacture of such polymer blends is the use of bicomponent metallocene/ Ziegler–Natta catalysts. Although both components in such catalysts polymerize ethylene at the same time and under the same reaction conditions, the sensitivity of respective active centers to hydrogen (the kinetic feature that determines the molecular weight of the polymers) is so different that binary metallocene/Ziegler– Natta catalysts produce polymer products with bimodal molecular weight distributions. Every catalyst particle in these catalysts contains active centers of both types, and the mixing of the two polymer fractions occurs during their synthesis rather than during post-polymerization processing. Combining two types of catalysts, one a Ti-based Ziegler–Natta catalyst and another a metallocene catalyst, has proved to be a difficult task, mostly due to mutual adverse effects of the catalyst components [1508]. Nevertheless, several successful bicomponent catalysts are described in the literature. They usually contain combinations of supported Ti-based catalysts of the TiCl4/MgCl2/silica type and metallocene/MAO catalysts [301,1508–1513]. The skill of a catalyst chemist in manufacturing such bicomponent catalysts is twofold, (a) to choose a combination of catalyst components that produces polymer mixtures of a desired average molecular weight and (b) to develop a technique for the catalyst synthesis that minimizes the mutual interference of the catalyst components.

4.10. Catalysts for Stereospecific Polymerization of Styrenes 4.10.1. Isospecific catalysts Conceptually, the synthesis of isotactic polystyrene can be carried out much more easily than that of isotactic polymers of 1-alkenes, even without the use of transition metal compounds. Such catalysts include Alfin initiators [1514] and combinations of LiOH and organolithium compounds at low temperatures [1515]. Typically, the

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Alkene Polymerization Reactions with Transition Metal Catalysts

isospecific polymerization of styrene and its alkyl-substituted analogs is carried out with the same heterogeneous Ziegler–Natta catalysts, both solid [1516] and supported [492], as those used for the isospecific polymerization of propylene and other 1-alkenes. Polymerization reactions of styrene with supported catalysts of the 3rd generation produce W90% of crystalline isotactic polystyrene [492] and a combination of the Solvay TiC4/TiCl3 catalyst and an organotitanium compound Cp2TiMe2 as a cocatalyst produces a particularly isospecific catalyst, it yields W99% of highly crystalline, nearly perfectly isotactic polystyrene [492]. Polymerization reactions of para-alkyl-substituted styrenes with the d-TiCl3-AlEt3 system also yield highly isotactic polymers with Tm B1301C [1516]. Oliva discovered that a homogeneous catalyst based on Me2C(3-t-BuInd)2ZrCl2 and MAO also produces highly isotactic crystalline polystyrene [755] (this catalyst produces highly isotactic polypropylene as well [524]). This catalyst also copolymerizes ethylene and styrene with the formation of true copolymers that contain long blocks of ethylene units and isotactic blocks of styrene units. Isotactic polystyrene was also prepared (usually in a mixture with atactic cationic polystyrene) using several homogeneous non-metallocene systems, a combination of Ni(acac)3 and MAO [1517], its silica-supported analog [1518], and the Nd phosphonateIBAO system [1519,1520].

4.10.2. Syndiospecific catalysts Homogeneous catalysts: The syndiospecific polymerization of styrene is carried out with a variety of soluble catalysts, both metallocene and non-metallocene. Ishihara described the first synthesis of syndiotactic polystyrene in 1986 [104]. Following this discovery, several families of homogeneous catalysts were developed that are suitable for the manufacture of crystalline syndiotactic polystyrene. The catalysts are based on the following types of transition metal complexes: 1. Monometallocene complexes CpTiX3, CpTiX3, IndTiX3 (X ¼ F, Cl, OR) and their ring-substituted analogs [104–108,746,752,1261,1262,1516,1521,1522]. 2. TiX4 compounds with X ¼ Cl, Br and OR [752,1523,1524]. 3. TiBz4 [748,751,1525,1526]. 4. Bis-metallocene Ti complexes [1527]. 5. Alkylated metallocene complexes, such as CpTiMe3 [1528,1529] and Me2C(Cp2)TiMe2 [752], in combination with B(C6F5)3 and similar ionforming cocatalysts. 6. Constrained-geometry Ti complexes (complexes IX in Scheme 1.1) with Cp or Cp ligands, X ¼ Cl, Me or Bz; R1 and R2 ¼ i-Pr or t-Bu; and the bridge E ¼ WSiMe2 or –(CH2)n–, as well as their indenyl and flourenyl analogs [283,997]. These complexes can be activated with MAO, its analogs, and with ion-forming cocatalysts described in Sections 4.6.2.2, 4.6.2.3, and 4.6.2.4. The molar ratio between MAO and the transition metal compounds in these catalysts ranges from 200–300 to W5,000, and the reaction temperature is usually from 30 to 701C. The CpTiF3-based catalysts have the highest activity, it exceeds the activity of

Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization

289

CpTiCl3-based systems by a factor of 50 [1530]. The CpTiF3-MAO system produces syndiotactic polystyrene of a high molecular weight with the highest melting point, 2771C [1530]. A subclass of CpTiX3 complexes, mixed monometallocene complexes CpTiCl2(OR) and CpTiCl2(OAr), especially when Ar is an ortho-sterically crowded phenyl group, such as CpTiCl2(O–2,6-i-Pr2-C6H3), produce more active catalysts than CpTiCl3 or CpTiCl3 [756,1531]. These monometallocene complexes can be supported on different porous materials, silica [1532–1534], alumina [1533], and MgCl2 [1534]. If combinations of two different metallocene complexes are used, e.g., CpTiCl3 and CpTiCl3, the polymers have a much broader molecular weight distribution, a feature advantageous in polymer processing [1535]. Polymer products prepared with all these catalysts usually contain highly crystalline syndiotactic polystyrene ([rrrr] B0.95–0.98, Tm from 260 to 2751C) and 2–8 wt.% of atactic polystyrene, which can be extracted with 2-butanone or THF [1527]. Table 4.25 gives examples of the productivity of monocyclopentadienyl complexes of Ti with different substituents in the ring. The same catalysts are suitable for polymerization of ring-substituted styrenes carrying both alkyl substituents and halogen atoms [105,752,1536]. The size of an alkyl substituent in the para-position to the CQC bond in a styrene molecule affects both the reactivity of the monomer and the stereospecificity of the catalysts [1516]. For example, the syndiospecificity of the CpTiCl3-MAO system is very high in polymerization reactions of styrene and p-methylstyrene [rr] W0.99, but it is greatly diminished when Et, n-Pr, and n-Bu groups are placed in the para-position, the [rr] value decreases to 0.32–0.44. However, polymerization of para-alkyl-substituted styrenes with long n-alkyl groups, C6–C12, using the CpTi(OMe)3-MAO system also produced syndiotactic polymers [1537]. Heterogeneous and supported catalysts: Supported catalysts for the synthesis of syndiotactic polystyrene were prepared from metallocene complexes of the CpTiX3 type and common supports including silica, alumina, and polar polymers, and using the same procedures as those described in Section 4.8 for the synthesis of other supported metallocene catalysts. Several examples are presented in [1451,1546]. Porri found an unusual pattern in styrene polymerization reactions with solid TiIII compounds activated with MAO [1545]. Interaction of MAO with a-TiCl3 Table 4.25 Relative activity of titanocene systems in syndiospecific polymerization reactions of styrenea [752]

a

Complex

Productivity (kg/g Ti)

CpTi(OMe)3 [(Me3Si)2Cp]Ti(OMe)3 (Me4Cp)Ti(OMe)3 [(Me3Si)(Me4)Cp]Ti(OMe)3 CpTi(OMe)3 (EtMe4Cp)Ti(OMe)3

10 25 130 135 200 210

Reactions at 501C, cocatalyst MAO.

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Alkene Polymerization Reactions with Transition Metal Catalysts

and d-TiCl3 produces two types of active centers. One center, which is insoluble in toluene or styrene, produces isotactic polystyrene, as expected for isospecific TiCl3AlR3 systems in general. Another type of active species is soluble in aromatic hydrocarbons and produces syndiotactic polystyrene. Solid TiCl3  THF3 complexes also generate syndiospecific catalysts after reaction with MAO. Catalysts for copolymerization of styrene and ethylene: When monometallocene catalysts of the CpTiX3-MAO type, as well as bridged bis-metallocene complexes [280–284,929,997,1538,1539] and other homogeneous non-metallocene catalysts [930,1540], were tested in copolymerization reactions of ethylene and styrene, the results were unexpected. The ‘‘copolymerization products’’ contain mixtures of three polymers, linear polyethylene, syndiotactic polystyrene (the component with the highest molecular weight [1541]), and true ethylene/styrene copolymers [281,284,928,1264,1539–1542]. The latter do not contain any styrene–styrene diads; nearly all the styrene units in the polymer chains are single [281,284, 758,1538]. The maximum content of styrene units in these copolymers is B50%; such copolymers have a regular monomer-alternating structure and they are crystalline. The alternating copolymer of ethylene and p-chlorostyrene is also crystalline [1544]. Most other metallocene catalysts also produce alternating ethylene/styrene copolymers: 1. MAO-activated bridged bis-metallocene complexes of C2, Cs, and C1 symmetry [282,283,1538,1543]. 2. Constrained-geometry catalysts [Me2Si(Me4-Cp)(t-Bu-N)]TiCl2-MAO and [Me2Si(Me4-Cp)(Cy-N)]TiCl2-MAO [743,758–760]. Efficient single-center catalysts for true random copolymerization of ethylene and styrene utilize a special subclass of monometallocene complexes, CpTi (O-Ar)Cl2, where the aryl group Ar contains two bulky substituents in orthopositions to the C–O bond, such as O–2,6-i-Pr2-C6H3 [146,147,333,334,743,756– 758,1264]. The monometallocene (1,3-Me2-Cp)Ti(O-2,6-i-Pr2-C6H3)Cl2-MAO system (which is a highly syndiospecific catalyst for styrene homopolymerization) exhibits the highest activity in ethylene/styrene copolymerization reactions [758]. The copolymers are random, their chains contain diads and triads of styrene units attached in the head-to-tail order as well as head-to-head styrene diads. Rigid bridged bis-indenyl titanocene complexes with the norbornane radical as a bridge also produce catalysts capable of the synthesis of random, compositionally homogeneous ethylene/styrene copolymers [233].

CHAPTER 5

Kinetics of Alkene Polymerization Reactions with Transition Metal Catalysts

Contents 5.1. Two Aspects of Polymerization Kinetics 5.2. Role of Diffusion in Alkene Polymerization Reactions 5.3. Formal Kinetic Description of Alkene Polymerization Reactions with Transition Metal Catalysts 5.3.1. Homopolymerization reactions 5.3.2. Copolymerization reactions 5.3.3. Stopped-flow kinetic method and living-chain polymerization reactions 5.4. Polymerization Reactions with Metallocene Catalysts 5.4.1. General kinetic behavior 5.4.2. Detailed kinetic studies 5.4.3. General kinetic studies, effects of reaction parameters 5.5. Polymerization Reactions with Non-Metallocene Homogeneous Catalysts 5.5.1. Living-chain polymerization reactions 5.5.2. Kinetics of oligomerization reactions 5.5.3. Limiting kinetic steps in polymerization reactions 5.5.4. Single- vs. multi-center polymerization catalysis 5.6. Synthesis of Alkene Block-Copolymers 5.6.1. Living-chain polymerization reactions and synthesis of alkene block-copolymers 5.6.2. Synthesis of alkene block-copolymers using chain transfer agents 5.7. Polymerization Reactions with Solid and Supported Ziegler–Natta Catalysts 5.7.1. Ethylene polymerization reactions 5.7.2. Propylene polymerization reactions 5.7.3. Polymerization reactions of higher 1-alkenes and styrene 5.7.4. Estimation of number of active centers in Ziegler–Natta catalysts 5.7.5. General classification of active centers in heterogeneous Ziegler–Natta catalysts 5.7.6. Physical effects in polymerization reactions with heterogeneous Ziegler–Natta catalysts 5.8. Polymerization Reactions with Pseudo-Homogeneous Catalysts 5.9. Polymerization Reactions with Chromium Oxide Catalysts 5.9.1. General kinetic behavior 5.9.2. Effects of reaction parameters

292 295 299 299 305 306 310 310 310 317 334 334 337 339 341 343 343 347 349 351 369 389 391 407 409 412 413 413 415

291

292

Alkene Polymerization Reactions with Transition Metal Catalysts

5.1. Two Aspects of Polymerization Kinetics In classical chemistry, kinetic studies are the main source of information leading to the formulation of a reaction mechanism. This is especially true for noncatalytic homogeneous reactions where main mechanistic principles are established based mostly on kinetic investigation. This approach was also applied with significant success to many reactions catalyzed by organometallic compounds. Unfortunately, such kinetic studies have proved to be less fruitful when applied to catalytic polymerization reactions of alkenes, especially those catalyzed by heterogeneous catalysts. The first complication becomes obvious if one considers that two different chemical processes occur in parallel in every catalytic polymerization reaction: 1. The polymerization reactions. The chemistry of polymerization reactions catalyzed by transition metal compounds is described in Chapter 3. All these reactions are very fast; a typical timeframe for the formation of a single polymer molecule usually ranges from a fraction of a second to less than 1 minute, depending on reaction conditions. 2. Changes in the catalyst. The active centers are formed in the beginning of polymerization reactions, in a reaction between the catalyst and a cocatalyst; sometimes the active centers are pre-formed in advance. The active centers are not stable; the activity of all transition metal catalysts gradually decreases with reaction time. The reactions between catalysts and cocatalysts take place independently of polymerization reactions and are relatively slow. The period when the concentration of active centers increases usually lasts from 3–5 minutes to over an hour. Some active centers are extremely stable, a polymerization reaction can be interrupted (by removing the monomer from the reactor) and resumed 10–20 hours later without any appreciable loss of catalyst activity. However, most active centers are unstable, they decay over a significant period of time, often measured in hours. In the past, numerous attempts were undertaken to explain these slow changes in catalyst activity as a manifestation of polymerization reactions themselves. For example, an increase of the polymerization rate in the beginning of the reactions was attributed to slow chain initiation reactions, and the decline in catalyst activity was attributed to a diffusion retardation of polymerization reactions due to an increase in the thickness of a polymer layer surrounding each active center. At the present time, the trend in interpreting the polymerization kinetics has changed and most of variations in the polymerization rate are usually ascribed to the ‘‘inner life’’ of the active species rather than the polymerization reactions as such. The separation of these two independent kinetic processes is the basis for the kinetic analysis presented in this chapter. Conceptually, the separation of the kinetics of polymerization reactions and the kinetics of active center transformations is not difficult. Because the polymerization reactions are very fast, only two techniques are currently used for their direct studies. The first technique is the stopped-flow method. The duration of stopped-flow experiments is

Kinetics of Alkene Polymerization Reactions

293

usually one second or less and, potentially, they can provide information on the growth of a single macromolecule. Of course, post-reaction analysis of polymer properties (molecular weight and the molecular weight distribution, steric structure of homopolymers, composition of copolymers, etc.) provides most of the kinetic information. The second technique for the kinetic studies of the polymerization reactions involves carrying out the reactions at very low temperatures when the average growth time of a single macromolecule may stretch to several minutes. The kinetic analysis of transition metal polymerization catalysis is also highly developed. Modern experimental techniques for the studies of alkene polymerization kinetics are very sophisticated, especially when the alkenes are gases, ethylene, propylene, or 1-butene. Polymerization reactions of light alkenes under moderate conditions, at 30–801C, are typically carried out at a constant monomer concentration (a constant alkene pressure in a reactor). The main measurable kinetic parameter in these reactions is the rate of polymer formation; it is equal to the rate of monomer addition to the reactor (usually measured with a mass flowmeter) required to maintain a constant pressure in the reactor. Figure 5.1 shows the kinetics of two polymerization reactions, those of ethylene and propylene, with a heterogeneous Ti-based catalyst of the 1st generation. In both cases, polymerization rates initially increase, reach a maximum, and then steadily decline, although the rates of all the changes are very different for the two alkenes. These changes provide information on the kinetic features of active centers, their rate of formation, stability, etc. Modern equipment affords detailed investigations of the effects of several reaction parameters on the polymerization kinetics. The most rigorous approach in these studies is the analysis of changes in reaction conditions in the course of a single polymerization reaction, such as repeated changes of monomer pressure and temperature or the introduction of extraneous compounds (other alkenes, reaction poisons) and the observation of their immediate effects on the active centers. These experimental options afford the level of detail in kinetic studies of alkene polymerization reactions that is rarely achieved in kinetic studies of other polymerization reactions, e.g., reactions involving radical or cationic initiators. Kinetic studies of alkene polymerization reactions are hindered by several circumstances. Many homogeneous catalysts, and all heterogeneous catalysts, have several types of active centers which have different kinetic characteristics and which are formed and decay at different rates. Experimental techniques for distinguishing between true single-center and multi-center catalysts are described in Chapter 2. These analytical techniques provide additional information on the kinetic behavior of individual active centers, the ratios of chain growth and chain transfer constants for particular active centers, their stereospecificity and copolymerization ability, etc. Wide dissemination of these analytical techniques made inadequate any treatment of the polymerization kinetic data under an implicit assumption that they contain only one type of active center, by measuring average molecular weights, average stereoregularity of polymers, average copolymer compositions, etc. The chemical structure of the active centers is still poorly understood, especially for heterogeneous catalysts. The relevant information is presented in Chapter 6. In this respect, kinetic studies of alkene polymerization reactions are at a

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Alkene Polymerization Reactions with Transition Metal Catalysts

Figure 5.1 Kinetics of alkene polymerization reactions with d-T|Cl3 -AlEt3 system at 901C. A, propylene (PPr ¼ 0.62 MPa, CPr ¼ 1.68 M); B, -ethylene (PE ¼ 0.45 MPa, CE ¼ 0.32 M) [453].

disadvantage in comparison with radical polymerization reactions, where the nature of radical initiators is well known and where the rates of their decomposition to free radicals can be measured independently. Another complication in the kinetic studies of alkene polymerization reactions stems from the physical features of these reactions. Short polymer molecules that are formed at the earliest stages of the polymerization reactions (and which are still attached to active centers) remain soluble in the reaction media, aliphatic or aromatic hydrocarbons, for only a short time. As a rule, polymer chains produced

Kinetics of Alkene Polymerization Reactions

295

from light alkenes (polyethylene, isotactic and syndiotactic polypropylene) begin to crystallize after a few minutes of the reaction. In homogeneous polymerization catalysis, the active centers, bulky organometallic complexes, are expelled from polymer crystals, and the polymerization reactions, while remaining homogeneous, become hetero-phaseous. Significant kinetic effects of a poorly understood nature often accompany this change [1547,1548]. These effects can be avoided when the polymers are amorphous and remain soluble in the reaction medium (e.g., atactic polypropylene produced with metallocene catalysts) or have a low molecular weight (in oligomerization reactions). The difficulties multiply when the kinetics of polymerization reactions with heterogeneous catalysts is examined. In the two decades following the discovery of the transition metal polymerization catalysis, numerous attempts were made to investigate the kinetics of polymerization reactions with solid catalysts as if they were homogeneous reactions. The researchers carried out careful measurements of reaction rates vs. time and a molecular weight change vs. time, and, by applying kinetic techniques borrowed from radical and anionic polymerization studies, estimated concentrations of active centers and rate constants for different reaction steps. Unfortunately, as new experimental techniques for the analysis of multicomponent polymer mixtures were being developed (Chapter 2), the futility of this approach became apparent. At the present time, kinetic studies of alkene polymerization reactions with solid and supported catalysts can be divided into two general categories. The kinetic studies of the first type continue to view the polymerization reactions with heterogeneous catalysts as a kind of single-center (or, at best, two- or three-center) homogeneous reactions. This approach has significant merits in developing semi-empirical schemes for modeling commercial polymerization processes. The second approach is based on the application of the full potential of new experimental techniques. Results produced with this approach are usually semi-quantitative but they provide a much more detailed and realistic kinetic description of catalyzed polymerization reactions. The emphasis of this approach is placed on investigating effects of reaction variables on two aspects of the polymerization reactions, (a) reaction kinetics as such (the change of reaction rate vs. time, etc.) and (b) behavior of specific types of active centers. For example, studies of catalytic poisons (the staple of research in general chemical kinetics) are very fruitful when applied to alkene polymerization reactions. In particular, catalyst modifiers used in the synthesis of supported catalysts and their reaction products with organometallic cocatalysts (Sections 4.4.3 and 4.5.2.2), can all be viewed as selective poisons that deactivate certain types of active centers and thus change the distribution of polymers in terms of their molecular weight, stereoregularity, copolymer composition, etc.

5.2. Role of Diffusion in Alkene Polymerization Reactions Before the kinetics of alkene polymerization reaction with transition metal catalysts is discussed, one issue should be resolved first, a possibility that many

296

Alkene Polymerization Reactions with Transition Metal Catalysts

kinetic features of these reactions are governed by diffusion phenomena, in particular, monomer diffusion into relatively large and continuously growing polymer/catalyst particles. Three features of the polymerization kinetics are especially suspicious in this respect: 1. A steady reduction in catalyst activity with reaction time (see, e.g., Figure 5.1). 2. A broad molecular weight distribution (in some cases, a multimodal distribution) of alkene polymers. 3. Activation of ethylene polymerization reactions upon addition of 1-alkenes (propylene, 1-butene, etc.) to the reactions. This effect may be potentially caused by a decrease of the level of polyethylene crystallinity and an acceleration of monomer transport through the polymer layer enveloping active centers. Indeed, many researchers proposed monomer diffusion effects as the main reason for all these kinetic features. However, many of these diffusion-effect explanations were thoroughly analyzed over the past 40 years, and both experimental observations and theoretical considerations were produced which refuted diffusion explanations. The most notable of them are 1. The decay of catalysts in alkene polymerization reactions does not depend on the concentration or even the presence of monomers, i.e., the decrease in activity cannot be attributed to a gradual increase of the thickness of a diffusion barrier both in solid Ziegler–Natta catalysts and in metallocene catalysts [711, 1192,1549–1552]. 2. Catalysts that produce atactic polymers also decrease their activity over time although the atactic polymers remain dissolved in the reaction medium. This observation also applies to the synthesis of isotactic poly(1-butene), which remains in solution during its synthesis in the liquid monomer [1018]. 3. The activation effect of 1-alkenes in ethylene polymerization reactions is reasonably well explained by chemical reasons (see Section 5.7.1.2) rather than by easier ethylene diffusion through ethylene/1-alkene copolymers vs. polyethylene. The introduction of 1-alkenes also leads to an activity increase of Ziegler–Natta catalysts in ethylene polymerization reactions at high temperatures, 160–1801C, when the polymers are completely dissolved in the polymerization medium [906]. 4. The decay rate of metallocene catalysts can be significantly different when they polymerize different alkenes, although the alkenes are converted into highly crystalline polymers in each reaction. For example, some isospecific metallocene systems are quite stable when producing crystalline isotactic polypropylene but decay faster when producing crystalline linear polyethylene [300]. Similarly, a change of a metallocene complex can change the shape of the molecular weight distribution of polyethylene from a bimodal to a narrow, single-Flory distribution although the same highly crystalline polymer is produced in both cases and with similar yields [300]. 5. Polymers with a broad molecular weight distribution are formed over solid and supported Ti-based catalysts even when polymer yields are very low, e.g., at the earliest stages of polymerization reactions [1553] and when the polymers, such

Kinetics of Alkene Polymerization Reactions

6. 7.

8.

9.

10.

11.

297

as poly(1-hexene) and poly(1-octene), are completely soluble in the reaction medium [1554]. From the theoretical standpoint, the fact that the molecular weight distribution of polyolefins is not affected by the monomer concentration or by the hydrogen concentration contradicts the diffusion explanation of the kinetic effects [1555]. Ethylene/1-alkene polymerization reactions can be carried out in high-boiling hydrocarbon solvents at temperatures in excess of 120–1301C. (The processes of this type are practiced on the commercial scale.) Polymer products in these reactions remain in solution until polymer/solvent mixtures are released and the solvent is stripped from them. When heterogeneous Ziegler–Natta catalysts are used in such processes, the copolymers exhibit all the properties typical for the polymers produced with the same catalysts in slurry polymerization reactions at much lower temperatures, including a broad molecular weight distribution and a broad compositional distribution. Polymerization of alkenes with Ti-based Ziegler–Natta catalysts, with supported metallocene catalysts, and with chromium oxide catalysts is accompanied by rapid fragmentation of original catalyst particles (see Sections 5.7.6 and 5.9.2). Polymer particles produced with these catalysts are porous and contain big gaps between small spherical polymer microparticles, less than 1 m in diameter [1124,1556]. The diffusion barrier from such small polymer particles is not high. It should be noted that the fragmentation phenomenon is not universal. Particles of some silica-supported metallocene catalysts are fragmented with difficulty in ethylene polymerization reactions under mild conditions [1557]. The laser-reflection interferometry technique measures the thickness of a polymer film growing on specially prepared supported catalysts at the earliest stages of polymerization reactions [1558]. An investigation of the growth of polyethylene macromolecules with this method showed that the roughness of the polymer surface and polymer porosity develop very early in the polymerization reactions because the chain growth rate exceeds the rate of polymer crystallization. All diffusion-limitation phenomena can be subdivided into three stages, internal diffusion through a porous catalyst particle at the earliest stages of polymerization reactions, diffusion through a polymer layer enveloping active centers, and, in the case of gaseous monomers, monomer diffusion through the gas/liquid interface. The internal diffusion limitation in a catalyst particle is evaluated by calculating the Thiele modulus, the ratio of the characteristic diffusion time to the characteristic reaction time. Several such estimations showed that the effect of internal diffusion is not significant even in the fastest alkene polymerization reactions [711,1029,1559–1561]. Empirical estimations of the magnitude of the second effect, monomer diffusion through a layer of polymer to active centers on the catalyst surface, also showed the absence of this effect [1559]. Finally, vigorous mixing of slurries in polymerization reactors can effectively control the external diffusion. The literature contains numerous examples when reaction conditions (monomer concentration, temperature, addition of catalyst poisons) were changed repeatedly in the course of a single polymerization reaction (Figures 5.2A and B)

298

Alkene Polymerization Reactions with Transition Metal Catalysts

Figure 5.2 E¡ects of variation of reaction parameters in the course of gas-phase ethylene polymerization reactions with T|Cl4/MgCl2/SiO2 -AlEt3 system [1550]. A, variation of ethylene partial pressure at 801C (PH in the last part of experiment are 0.56 and 0.28 MPa, respectively); B, variation of hydrogen partial pressure at 901C.

[605,1167,1550,1562–1564]. The common observation in these experiments is that when the reaction conditions are returned to the original values, the reaction rate also rapidly returns to the original value. This reversibility indicates that the observed kinetic effects are caused by reversible changes in the active center concentration/reactivity rather than by the mass transfer effect because the latter effect would be expected to depend on the previous history of a reaction.

Kinetics of Alkene Polymerization Reactions

299

5.3. Formal Kinetic Description of Alkene Polymerization Reactions with Transition Metal Catalysts 5.3.1. Homopolymerization reactions Kinetic analysis of alkene polymerization reactions with transition metal catalysts represents a great challenge. On one hand, studies of polymerization reactions of gaseous alkenes can provide a much more detailed information on polymerization kinetic compared to classical studies of polymerization reactions of liquid monomers. Polymerization reactions of gaseous alkenes are usually carried out at a constant monomer pressure and, if the polymer precipitates in the course of the reaction, at a practically constant monomer concentration in solution. The use of rapid-response mass flowmeters affords a very precise measurement of the instantaneous polymerization rate and a very precise measurement of catalyst responses to any change in reaction conditions in the course of a single reaction, such as the monomer concentration, temperature, addition of catalyst poisons, etc. As an example, Figure 5.2A shows the effects of variation of the ethylene partial pressure in the course of a single gas-phase polymerization reaction with the TiCl4/MgCl2/ SiO2-AlEt3 system at 801C. Figure 5.2B shows effects of the variation of the hydrogen partial pressure in the course of a similar polymerization reaction [1550]. Numerous experiments of this type reported in the literature provide much more precise kinetic data that any data gathered in several separate polymerization experiments. The laser reflection interferometry provides an alternative source of information of the polymerization kinetics, by measuring the thickness of a polyethylene layer growing on the surface of heterogeneous catalysts in gas-phase polymerization reactions [1558,1565]. The measurements confirm generally known kinetic features of polymerization reactions with highly active catalysts, including a high initial reaction rate and a gradual deactivation of the catalysts over a period of B30 minutes [1558]. Scheme 5.1 represents the simplest kinetic scheme of a catalytic polymerization reaction, an alkene monomer M is polymerized with a single-center catalyst at a constant monomer concentration CM. In this scheme, the active centers are formed from transition metal atom species in the catalyst (their initial concentration is Ccat) in two successive steps, Reactions (5.1) and (5.2), with rate constants kf,1 and kf,2, respectively. The chemistry of these reactions is described in Chapter 6. The choice of the two-step formation reaction of the active centers instead of a single reaction is not justified by any particular chemical data. Rather, it comes from empirical experience, a two-stage activation reaction better describes kinetic curves of alkene polymerization reactions at moderate temperatures whereas a single-step reaction is usually adequate when the reactions are carried out at high temperatures. At least one of the two center-formation reactions, Reaction (5.1) or (5.2), involves a cocatalyst. The cocatalyst concentration in catalytic polymerization reactions is usually several orders of magnitude higher in comparison with the concentration of potential active centers, Ccat, and it remains approximately constant in the course of a given polymerization reaction. Therefore both Reaction (5.1) and (5.2)

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Alkene Polymerization Reactions with Transition Metal Catalysts

Two-step formation reaction of active center Cp* from potential center Ccat: Ccat —(kf,1)→ Co Co —(kf,2)→ C*

(5.1) (5.2)

Chain propagation reaction (reaction of polymer formation): C*-Mn + M —(kp)→ C*-Mn+1

(5.3)

Deactivation reaction of active center Cp*: C*-Mn —(kd)→ Polymer(n) + Catalytically inactive species

(5.4)

Chain transfer to monomer: C*-Mn + M —(ktM)→ C*-M + Polymer(n)

(5.5)

Chain initiation reaction after chain transfer to monomer: C*-M + M —(kiM)→ C*-M2

(5.6)

Chain transfer reaction to hydrogen: C*-Mn + H2 —(ktH)→ C*-H + Polymer(n)

(5.7)

Chain initiation reaction after chain transfer to hydrogen: C*-H + M —(kiH)→ C*-M

(5.8)

Chain transfer reaction to organoaluminum cocatalyst: C*-Mn + AlR3 —(ktAl)→ C*-R + AlR2-Polymer(n)

(5.9)

Chain initiation reaction after chain transfer to organoaluminum cocatalyst: C*-R + M —(kiAl)→ C*-M-R

(5.10)

Spontaneous chain transfer reaction (
-H elimination): C*-Mn —(ktsp)→ C*-H + Polymer(n)

(5.11)

Chain initiation reaction after spontaneous chain transfer reaction: C*-H + M —(kisp)→ C*-M-H

(5.12)

Scheme 5.1 Simple kinetic scheme of homopolymerization reaction with single-center catalyst.

can be regarded as monomolecular reactions that involve only potential active centers. ‘‘Working’’ active centers are characterized by their concentration, C (mol/g cat), which varies in time, and by their reactivity in the chain growth reaction (Reaction (5.3)), the propagation rate constant kp (M
1  s
1). As a rule, these two parameters, C and kp, cannot be measured independently unless a special method for the

Kinetics of Alkene Polymerization Reactions

301

measurement of the C value is used (Section 5.7.4). The product of the two values, kp  C, is called the effective rate constant, keff (l/g cat  min); it is used in the majority of the kinetic research. Active centers in alkene polymerization reactions are never completely stable; they slowly decay (Reaction (5.4), rate constant kd) over a period of time that can vary from several minutes to several hours, depending on catalyst and reaction conditions. The chemistry of four chain transfer reactions in Scheme 5.1 (Reactions (5.5), (5.7), (5.9), and (5.11)) as well as the chemistry of chain initiation reactions that follow each chain transfer step (Reactions (5.6), (5.8), (5.10), and (5.12)) is described in Chapter 3 for different types of polymerization catalysts. The relative significance of these chain transfer reactions varies in a very broad range depending on the catalyst. Although the list of reactions in Scheme 5.1 describing the kinetics of a homopolymerization reaction with a single-center catalyst is relatively short, its representation in the standard kinetic form (a change of the reaction rate with time) is quite complex and leads to cumbersome equations [1566]. Therefore, the list of reactions in Scheme 5.1 is usually further shortened by sp H AL assuming that rate constants kM i ; ki ; ki and ki characterizing four chain initiation reactions have approximately the same values as kp. Under this approximation, the life cycle of an active center is formally described by three reactions, two reactions of its formation, Reactions (5.1) and (5.2), and the deactivation reaction (Reaction (5.4)). All other reactions in Scheme 5.1 are accounted for in equations for the average polymerization degree (average molecular weight) given below. Expressions for reaction rate and polymer yield: It is usually assumed that the rate of the chain propagation reaction (Reaction (5.3)), Rpol(t), is proportional to the monomer concentration in solution, CM (mol/l), Rpol(t) ¼ keff  CM. A joint solution of three differential equations describing the formation of active centers in the two-stage reaction and their deactivation, Reactions (5.1), (5.2), and (5.4), and the rate of monomer consumption in Reaction (5.3) produces the following equation for the polymerization rate Rpol as a function of reaction time t [6]:
 Rpol ðtÞ ðM=sÞ ¼ kp  C cat  kf ;1  kf ;2  C M =ðkf ;2
kf ;1 þ kd Þ   ½1
expð
kf ;1  t þ kd tÞ=ðkf ;1
kd Þ (5.13)  þ½1
expð
kf ;2  tÞ=kf ;2 The concentration of precursors of active centers, Ccat, is an unknown value in most catalytic polymerization reactions. In practice, the polymerization rate is usually measured in the units of the effective rate, g polymer/g cat  min: Reff ðtÞ ¼ MWmon  Rpol ðtÞ=Qcat  ðg  polymer=g  cat  minÞ

(5.14)

where MWmon is the molecular weight of the monomer and Qcat is the concentration of the catalyst in a reactor, g/l for heterogeneous catalysts or M for homogeneous catalysts.

302

Alkene Polymerization Reactions with Transition Metal Catalysts

The expression for a polymer yield Qpol(T ) as a function of time T is quite cumbersome: Qpol ðT ÞðMÞ ¼ kp  C cat  C M  ½kf ;1  kf ;2  =ðkf ;2
kf ;1 þ kd Þ  ðf½1
expð
kf ;1  T Þ=kf ;1
½1
expð
kd  T Þ=kd Þg=ðkf ;1
kd Þ

(5.15)

þ f½1
expð
ðkf ;2 þ kd Þ  T Þ=ðkf ;2 þ kd Þ
½1
expð
kd  T Þ=kd Þg=kf ;2 Þ In some cases, the two-stage process of active center formation (Reactions (5.1) and (5.2)) can be replaced with a single reaction with a rate constant kf : C cat }ðkf Þ ! C 

(5.16)

Then Equation (5.13) is simplified as: Rpol ðtÞðM=sÞ ¼ kp  C cat  kf  C M  ½expð
kf  tÞ
expð
kd  tÞ=ðkd
kf Þ (5.17) Finally, if the active centers are stable under given experimental conditions (kd ¼ 0), Equations (5.13) and (5.17) are further reduced to basic kinetic equations for the rates of product formation in two-step and one-step consecutive reactions, respectively: Rpol ðtÞ ¼ ½kp  C cat  kf ;1  kf ;2  C M =ðkf ;2
kf ;1 Þ  f½1
expð
kf ;1  tÞ=kf ;1 þ ½1
expð
kf ;2  tÞ=kf ;2 g Rpol ðtÞ ¼ kp  C cat C M  ½1
expð
kf  tÞ

(5.18) (5.19)

Complex expressions for the polymerization rate and the polymer yield as functions of time represented by Equations (5.13), (5.15), or (5.17) give no possibility to determine the values of rate constants in Scheme 5.1 by applying linearization procedures used in classical chemical kinetics. However, these rate constants can be estimated with the use of various non-linear curve-fitting computer programs. Figure 5.3 gives an example of such curve fitting for propylene polymerization kinetics with the VCl3-Ali-Bu3 system at 601C [6]. It should be stressed that this type of kinetic analysis can be applied only to simplest examples of polymerization reactions with single-center catalysts. The use of Scheme 5.1 and many similar kinetic schemes, usually by applying the first-order or the secondorder kinetic equation, which are abundant in the literature on Ziegler–Natta catalysis, is strictly formal. All these catalysts are definitely not single-center catalysts, which is the underlying assumption in Scheme 5.1 and in other similar kinetic schemes. Each such catalyst contains several types of active centers that are formed and decay at substantially different rates. Therefore, the use of a single set of kinetic parameters in Scheme 5.1 is gross simplification. This approach may be suitable for practical modeling of polymerization reactions required for control of commercial processes but it is definitely inadequate for any mechanistic analysis of the polymerization kinetics. Several examples presented in this chapter demonstrate complexity of the polymerization kinetics and provide a more detailed approach to its study. In general terms, a more realistic approach involves the use of

Kinetics of Alkene Polymerization Reactions

303

Figure 5.3 Kinetics of propylene polymerization reaction with VCl3 -Ali-Bu3 system at 601C. Data are from [1786], kinetic curve is calculated with Equation (5.13).

several sets of reactions in Scheme 5.1 with different values of rate constants of all reactions determining life spans of active centers (values of kf,1, kf,2, and kd) and their reactivity (kp). In this approach, a large volume of kinetic information is required, including measurements of molecular weights, molecular weight distributions, and structural parameters (stereoregularity, copolymer composition) of polymers formed at different reaction times. However, this approach produces a much more nuanced and detailed picture of the kinetic behavior of a catalyst. Accounting for a high reaction order: As a number of studies described in Sections 5.4.1 and 5.7.1.1.2 attests, the experimentally observed reaction order of many polymerization reactions with respect to the monomer concentration CM is significantly higher than the first order assumed in Scheme 5.1. In principle, these observations can be artifacts of complex kinetic phenomena unrelated to propagation reactions as such. Nevertheless, several attempts were made to design a simple formal kinetic scheme that accounts for the high reaction order with respect to CM. The model proposed by Resconi and Corradini suits this purpose well [1567]. The model (Scheme 5.2), which is called a single-center, two-state catalyst model, can be equally applied to single-center metallocene catalysts and to each type of active center in multi-center heterogeneous catalysts. Each active center C  (formed in Equation (5.2)) is assumed to exist in two states, one, C fast , with a high kp;1 value, and another, C slow , with a low (but not negligible) kp;2 value. The two

304

Alkene Polymerization Reactions with Transition Metal Catalysts

C*(fast) kp,1, CM C*(fast)

Scheme 5.2

C*(slow) kp,2, CM C*(fast)

Single-center, two-state kinetic model of alkene polymerization reactions.

states are in equilibrium; k1 and k
1 are the rate constants in the equilibrium. If one assumes that the insertion of one monomer unit into the C slow center transforms it into the C fast center and that C fast þ C slow ¼ C  , this simple scheme results in the following expression for the overall reaction rate under steady-state conditions: Rpol ¼ C   ðA  C M þ kp;1  C 2M Þ=ðB þ C M Þ

(5.20)

Parameters A and B in Equation (5.20) are combinations of rate constants in Scheme 5.2, A ¼ k1 þ kp;1 k
1 =kp;2 and B ¼ ðk1 þ k
1 Þ=kp;2 . Depending on the relative values of the rate constants, Equation (5.20) can be transformed into the first-order dependence on C M when k1  k
1 (most centers exist in the ‘‘fast’’ state) or when kp;2  C M  k1 : Rpol  kp;1  C   C M . Alternatively, the secondorder dependence on CM will be observed when kp;1  C M  k1  kp;2  C M  k
1 : Rpol  kp;1  C   C 2M [1567]. A reaction order intermediate between the first and the second will be observed in situations when kp;1  C M 4k1 4kp;2  C M 4k
1 . Scheme 5.2 puts no restrictions on the nature of the two types of active centers, C fast and C slow , on the nature of transformations between the centers, or on the length of a polymer chain attached to the transition metal atom in each center. Over the years, several mechanisms were proposed to explain the difference between the reactivity of the same active center under different circumstances. For example, the C slow center in stereospecific polymerization reactions may be a center with the last monomer unit in a sterically inverted position (a steric mistake). However, this explanation cannot be used to describe high reaction orders in ethylene polymerization reactions or in 1-alkene polymerization reactions with aspecific catalysts [1568]. Another proposed option is a center with the last monomer unit in the secondary orientation, after which the primary insertion of the next 1-alkene molecule can indeed be much slower. However, a high reaction order with respect to CM was observed in polymerization reactions with metallocene and Ziegler– Natta catalysts when the regio-errors are virtually absent, as well as in ethylene polymerization reactions. Still another possibility is the existence of two states of the same active center, which differ in the type of an agostic interaction between H atoms in the last monomer unit and the transition metal atom [318,1567,1569–1571]. Expressions for molecular weight: In the case of a single-center catalyst, the numberaverage polymerization degree of a polymer n is defined as n ¼ chain growth rate/S (chain transfer rates). The reciprocal value of n is usually used to determine kinetic parameters of various chain transfer reactions: Al H sp 1=n ¼ ðkM t  C M þ kt  C Al þ kt  C H þ kt Þ=ðkp  C M Þ

where

kM t ;

kA1 t ;

kH t ;

and

ksp t

(5.21)

are rate constants of four chain transfer reactions in

Kinetics of Alkene Polymerization Reactions

305

Scheme 5.1, Reactions (5.5), (5.7), (5.9), and (5.11). If one keeps most reaction parameters constant and varies the concentration of one of the reagents at a time (monomer, hydrogen, cocatalyst), one can determine the relative significance of different chain transfer reactions and estimate respective kt/kp values. Two important particular cases are often encountered in the kinetic analysis of alkene polymerization reactions. When these reactions are carried out at a relatively low cocatalyst concentration (when the kA1 t  C A1 term in the numerator of Equation (5.21) is less than other terms) and at relatively mild temperatures, when the ksp t value is low, Equation (5.21) is reduced to H M H 1=n ¼ ðkM t  C M þ kt  C H Þ=ðkp  C M Þ ¼ kt =kp þ ðkt =kp Þ  ðC H =C M Þ

(5.22)

If the reaction is carried out in the absence of hydrogen, this expression is further simplified: kM 1=n ¼ t (5.23) kp Equations (5.22) and (5.23) are widely used in the analysis of polymer molecular weights as a function of reaction parameters. Similarly to the interpretation of the polymerization kinetics, the measurement of average molecular weights of polymers produced with multi-center catalysts and the analysis of their dependence on reaction parameters is meaningless from the mechanistic viewpoint because reaction parameters affect both the kt/kp values for each center and the relative contents of different active centers. A much more detailed information can be derived from the analysis of GPC curves of such polymers, as described in Section 2.2.2.

5.3.2. Copolymerization reactions The kinetic treatment of copolymerization reactions is very cumbersome and is usually restricted to the simplest ideal case, a steady-state copolymerization reaction with a single-center catalyst. This treatment does not take into account kinetic complications caused by chain initiation and chain transfer reactions. As a minimum, four different propagation reactions must be considered (the same as Reactions 3.112–3.115) instead of the single chain propagation reaction (Reaction (5.3)): C  2M0 2Copolymer þ M0 }kM0 ðM0 Þ ! C  2M0 2M0 2Copolymer

(5.24)

C  2M0 2Copolymer þ M00 }kM00 ðM0 Þ ! C  2M00 2M0 2Copolymer

(5.25)

C  2M00 2Copolymer þ M0 }kM0 ðM00 Þ ! C  2M0 2M00 2Copolymer

(5.26)

C  2M00 2Copolymer þ M00 }kM00 ðM00 Þ ! C  2M00 2M00 2Copolymer

(5.27)

The principal goal of such kinetic exercises is usually a comparison of two reaction rates, one, Rhomo, in a homopolymerization reaction of a particular alkene Mu and another, Rcop, in a copolymerization reaction of Mu and Mv. Both rates are measured under conditions when the combined concentration of two active

306

Alkene Polymerization Reactions with Transition Metal Catalysts

centers, C–Mu–Copolymer + C–Mv–Copolymer, does not vary with time. Waymouth proposed the following expression for the ratio of the two reaction rates [884]: Rcop =Rhomo ¼ ðkM0 ðM0 Þ;cop ½M0 cop =kM0 ðM0 Þ;homo ½M0 homo Þ  ½ðr 1 =r 2  F 2 Þ þ 2=ðr 2  FÞ þ 1

(5.28)

Where F is the concentration ratio of comonomers Mu and Mv in the copolymerization reaction, r1 and r2 are the reactivity ratios, and kMu(Mu),cop and kMu(Mu),homo are propagation rate constants of monomer Mu in the copolymerization and the homopolymerization reaction, respectively. If the values of two latter rate constants are equal, Equation (5.28) can be simplified as Rcop =Rhomo ¼ ð½M0 cop =½M0 homo Þ½ðr 1 =r 2  F 2 Þ þ 2=ðr 2  FÞ þ 1

(5.29)

The kinetic analysis of copolymerization reactions can be greatly aided if the kinetics of two polymerization reactions is compared, one a true copolymerization reaction of monomers Mu and Mv and another a consecutive copolymerization reaction, when the monomer Mu is completely replaced with the monomer Mv in the course of a single reaction [949]. The comparison of monomer consumption rates in the latter reaction gives the ratio of two homopolymerization rate constants, kMu(Mu)/kMv(Mv) (Reactions (5.24) and (5.27)) assuming that the concentration of the active centers remains unchanged when one monomer is replaced with another. When the polymerization rate of a given monomer Mu is measured in a consecutive copolymerization reaction (R0homo ) and in a copolymerization reaction with another monomer (Rcop), the ratio of these two rates is given as [949]: Rcop =R0homo ¼ ½1 þ ð1=FÞð1=r 1 Þ=½1 þ ð1=FÞðr 2 =r 1 ÞðkM0 ðM0 Þ =kM00 ðM00 Þ Þ (5.30)

5.3.3. Stopped-flow kinetic method and living-chain polymerization reactions The stopped-flow kinetic method is used for the studies of alkene polymerization reactions at very short reaction times, from a fraction of a second to several seconds. A typical arrangement for a stopped-flow experiment involves the preparation of solutions or slurries containing the catalyst the cocatalyst in two different vessels. Both liquids are saturated with the monomer and brought to equilibrium: the same temperature, both liquid phases have the same monomer concentration. Then the two liquids are rapidly mixed at the entrance of a short thin tube by applying pressure to both vessels with the monomer or an inert gas. The polymerization reaction starts at the point where the two liquids meet in the tube and continues for a short period of time while the mixture flows through the tube and until it is discharged into a large vessel containing a reaction-quenching agent. The reaction time depends on the diameter of the tube, its length, and the pressure over the liquids in the vessels; it is carefully calibrated in blank experiments. Small quantities of polymers collected after each reaction are weighted and are subjected to the same arsenal of analytical measurements as those applied to alkene polymers in general,

Kinetics of Alkene Polymerization Reactions

307

the measurement of the molecular weight and the molecular weight distribution by GPC, stereoregularity by 13C NMR, and distributional characteristics of the polymers by Tref or Crystaf methods. Ideal living-chain reactions: In the simplest case (in terms of reaction kinetics), polymerization reactions in stopped-flow experiments are expected to proceed under living-chain conditions. Three main preconditions for such polymerization reactions are: (a) active centers are formed instantly, (b) active centers remain stable during a given experiment, and (c) virtually no chain transfer or termination takes place. In the simplest case of a single-center catalyst and assuming that the reaction rate Rpol is proportional to the monomer concentration CM, the expression for Rpol is Rpol ¼ kp  C   C M

(5.31)

If the C value is constant (the active centers are stable), the reaction rate is also constant, and the polymer yield (a directly measured parameter) increases proportionally to the reaction time T until depletion of the monomer in the reaction mixture becomes noticeable (stopped-flow experiments do not allow for the introduction of a fresh monomer to compensate for its loss): Qpol ðTÞ ¼ kp  C   C M  T

(5.32)

The second measured kinetic parameter in a living-chain reaction is the polymerization degree n; it also increases proportionally to the reaction time: nðT Þ ¼ kp  C M  T

(5.33)

Equation (5.33) provides an immediate estimation of the propagation rate constant kp, after which the estimation of the number of active centers with Equation (5.32) is straightforward. In some polymerization reactions, the number of active centers is known in advance (or is assumed to be known), e.g., C E [metallocene complex] in polymerization reactions with ionic metallocene catalysts [661]. A direct calculation of the kp value is then possible using Equation (5.32). However, these situations are rare. Living-chain reactions with slow active-center formation: The first of preconditions for ideal living-chain reactions, that all active centers are formed virtually instantly, is usually met only when the catalysts are preactivated. In the majority of cases, the catalyst activity at the earliest stages of the polymerization reactions increases with time for at least several minutes (Figure 5.1), much longer than the duration of typical stopped flow experiments. In the first approximation, Reaction (5.16) can represent this center-formation reaction, and the expression for the polymer yield changes to Qpol ðT Þ ¼ kp  C   C M  T  f1
½1
expð
kf  T Þ=ðkf  T Þg

(5.34)

The molecular weight distribution in such reactions changes from monodisperse (all polymer chains have the same polymerization degree described by Equation (5.33)) to the Poisson distribution (see Section 2.2.1), and the kp value can be approximately estimated (using Equation (5.33)) from the high molecular weight edge of the molecular weight distribution.

308

Alkene Polymerization Reactions with Transition Metal Catalysts

Numerous experimental kinetic curves presented in the literature show that the rates of alkene polymerization reactions always increase in time in the beginning of the reactions. The acceleration period can last from 3 to 5 minutes to over 1 hour (see, e.g., Figure 5.1). On the other hand, when the reaction yields are measured in stopped flow experiments after very short periods of time, the polymer yields increase proportionally to the reaction time [1574–1580], and, hence, the reaction rate (Equation (5.31)) should be constant rather than steadily increasing. On a purely formal ground, these two contradictory observations can be reconciled: polymer yields in stopped-flow experiments are measured at very short time intervals when any slowly increasing reaction rate can be approximated by a constant value. However, when the reaction rates are estimated after 1 or 2 seconds after the beginning of a polymerization reaction, one has to be aware that only a small fraction of eventually available active centers has been already engaged in the chain growth reaction at this moment. In this situation, the C value measured in stopped-flow experiments can be significantly lower than the maximally possible number of active centers, which is reached after the reaction accelerates for several minutes. Polymerization reactions with slow chain transfer: Numerous stopped-flow studies of alkene polymerization reactions under conditions which were expected to lead to the living-chain kinetics showed that the third precondition for the use of Equation (5.33), a complete absence of chain transfer reactions, usually does not hold. The molecular weight of polymers in these experiments increases proportionally to the reaction time only at the earliest stages of the reactions, and then levels off. For example, even stopped-flow polymerization experiments for very short reaction times, 0.1–0.2 seconds, with a typical homogeneous metallocene system, racMe2Si(2-Me-4-Ph-Ind)(Cp)ZrCl2-MAO, produce polymers with Mw/Mn ratios between 1.6 and 2.0, indicating a significant effect of chain transfer reactions [1574]. This complication does not impede the kinetic analysis. If a slow chain transfer reaction (with a constant rate Rtr) indeed occurs, then the increase of the polymerization degree n(T ) becomes slower as the polymerization reaction proceeds, and the dependence between n(T ) and time T can still be used for the measurement of the kp value, nðT Þ ¼ kp  C M  T =ð1 þ Rtr  TÞ

(5.35)

or, in reciprocal coordinates 1=nðT Þ ¼ Rtr =ðkp  C M Þ þ 1=ðkp  C M  TÞ

(5.36)

Non-ideal living-chain reactions: The most realistic examples of the reaction kinetics at the earliest stages of polymerization reactions include situations when all preconditions of living-chain reactions are partially violated, the concentration of the active centers increases with time and chain transfer reactions become noticeable soon after the onset of the reactions. Even under these conditions, which are usually called ‘‘quasi-living chain conditions,’’ the experimental data still can be used for the kinetic analysis. In these situations, the molecular weight of the polymer increases with time T, although not in a linear manner, as in Equation (5.33), but according to Equations (5.35) and (5.36).

Kinetics of Alkene Polymerization Reactions

309

In these (the most common) situations, several experimental parameters should be measured and calculated in a series of experiments under identical experimental conditions: 1. The polymer yield Qpol(T ) as a function of T. 2. The polymerization rate Rpol(T ) as a function of T; e.g., by calculating slopes of the Qpol(T ) vs. T dependency at different times T. 3. The molecular weight Mn(T ) as a function of T. 4. The total number of polymer chains formed in a reaction after time T : N(T ) ¼ Qpol(T )/M(T ). The following relationship between N(T ) and kinetic parameters of the polymerization reaction exists [1127,1214,1572,1573]: N ðT Þ ¼ C  þ ½kt  C  =Rpol ðT ÞQpol ðTÞ

(5.37)

(here kt is the rate constant for the dominant chain transfer reaction, usually Reaction (5.5) or (5.9).) Experimental data for these reactions can be plotted in the coordinates of Equation (5.37); they give the C value as the intercept in the linear dependence between N(T ) and Qpol ðT Þ=Rpol ðT Þ. Of course, an important precondition for the use of Equations (5.33) or (5.37) is the single-center nature of a catalyst, all active centers should have the same kinetic characteristics, first of all, the same kp values. Polymers produced at the earliest stages of these reactions should have a very narrow molecular weight distribution. If the active centers start growing polymer chains immediately, the Mw/Mn value should be close to 1, and, if the reactions of chain formation are relatively slow, the Mw/Mn value can increase to B1.3 (the Poisson molecular weight distribution, Section 2.2.1). Finally, after the onset of chain transfer reactions, the molecular weight distribution broadens but the Mw/Mn value should remain lower than 2.0. These conditions should be regarded as minimal before the kinetic apparatus of quasi-living chain reactions is applied. Fortunately, several clear examples are described in the literature when these conditions are met. They include alkene polymerization reactions with metallocene catalysts at low temperatures (Section 5.4.2) and the polymerization reactions with several types of homogeneous non-metallocene catalysts, which exhibit many features of living-chain and quasi-living chain reactions at temperatures as high as 30–401C (Section 5.5.1). On the other hand, the application of the outlined kinetic approach to alkene polymerization reactions with heterogeneous multi-center catalysts is riddled with difficulties, primarily because different centers in these catalysts are formed at different rates and produce polymer molecules of a different molecular weight. In some cases, GPC and Tref analysis of polypropylene produced with Ziegler–Natta catalysts in stopped flow experiments for a fraction of a second already show the existence of at least two different populations of active centers. The kp value for each type of center can still be estimated using Equations (5.33– 5.36). However, the C measurements become P unreliable because even in the ðkp;i  C i Þ  C M where i is a simplest case Equation (5.31) becomes Rav pol ¼ number of different types of active centers with different kinetic characteristics.

310

Alkene Polymerization Reactions with Transition Metal Catalysts

5.4. Polymerization Reactions with Metallocene Catalysts 5.4.1. General kinetic behavior Aupriori, alkene polymerization reactions with homogeneous catalysts, either metallocene or utilizing other transition metal compounds, should be the easiest to research from the standpoint of reaction kinetics. Each active center in these systems acts independently of all other centers and each remains in solution throughout a given polymerization experiment. However, polymers of light alkenes (polyethylene, isotactic and syndiotactic polypropylene) begin to crystallize soon after the beginning of the reactions. The time elapsing from the start of a polymerization reaction to the onset of polymer crystallization is usually measured in several minutes at temperatures from 0 to 201C or in a few seconds at 50–801C [1296,1581]. When this point is reached, growing polymer chains still carrying active centers at their ends co-crystallize with other live of dead polymer molecules, and the active centers, bulky organometallic complexes, are expelled from the polymer crystals. Significant kinetic effects, which are not yet well understood, accompany this change, the reaction rate either significantly increases or decreases [1547,1548,1582].

5.4.2. Detailed kinetic studies Several kinetic studies of alkene polymerization reactions with ionic metallocene catalysts take advantage of a possibility to pre-form the active species and to determine their structure (usually by NMR) prior to their reactions with alkenes. This option provides an opportunity to disengage two kinetic processes that usually proceed in parallel, transformations of the active centers and polymerization reactions themselves. Immediate precursors of the active centers in such studies are synthesized in advance and then used for polymerization reactions under mild conditions when these precursors are stable. These studies, which represent the most thorough kinetic investigations of alkene polymerization reactions with soluble catalysts, are the subject of this section, and the next section describes (much more numerous) kinetic studies of the general type, when the generation of active centers and the polymerization reactions occur in parallel. 5.4.2.1. Ethylene polymerization reactions Bercaw produced detailed kinetic data on chain initiation and growth reactions in low-temperature polymerization reactions of ethylene with electro-neutral scandocenes Cp 2 Sc–R, which serve as good models of common bis-metallocene complexes [1583]. The rate constant for the ethylene insertion reaction into the Sc–C bond at
801C depends on the type of the R group attached to the Sc atom: R ki (M
1  s
1)

H W1  10
2

Me 8.1  10
3

Et 4.4  10
4

n-Pr 6.1  10
3

Higher Cn B6  10
3

311

Kinetics of Alkene Polymerization Reactions

The lower reactivity of the Sc–Et bond is explained by the ground-state stabilization of Cp 2 Sc–Et due to the b-H agostic interaction of its CH3 group with the Sc atom, whereas the reactivity of the Sc–C bond for linear alkyl groups starting with R ¼ n-Pr is approximately the same. At low temperatures, all these short polyethylene chains remain attached to the Sc atoms. When the R group in the original Sc complex is Me or n-Pr, hydrolysis of the reaction products generates ethylene oligomers with odd carbon atom numbers, and their yields are distributed according to the Poisson law [637] (Section 2.2.1). Reichert described the simplest kinetic example of a chain growth reaction, ethylene oligomerization with the Cp2Ti(Et)Cl-AlEtCl2 system at 101C [1547]. The active species in this catalyst is practically ‘‘pre-formed,’’ the alkylated titanocene complex is synthesized separately, and the active center is formed when this compound forms a complex with AlEtCl2, a very rapid reaction [1584]. Studies of ethylene polymerization reactions by the stopped-flow technique combined with GC analysis demonstrated a steady increase of the average length of very short polyethylene molecules attached to the Ti atom, from C4–C6 products after B0.3 seconds to C10–C12 products after B1.7 seconds [1585,1586]. Nearly all Cp2Ti(Et)Cl is converted into the active centers. The rate constant of the ethylene insertion reaction into the Ti+–C bond at 101C Cp2 Tiþ 2ðCH2 2CH2 Þn 2H þ CH2 ¼ CH2 ! Cp2 Tiþ 2ðCH2 2CH2 Þnþ1 2H

(5.38)

is practically independent on the number of ethylene units in the growing chain starting with n ¼ 3, kp ¼ B50 M
1 s
1 [1586]. Fink carried out 13C NMR studies of a similar system, Cp2Ti(Me)Cl-AlMeCl2, by reacting it with 13C-labeled ethylene at
601C [1587]. These data provided detailed information about the earliest steps of the polymerization reactions: 1. The active species in this system are formed in two equilibrium steps involving the alkylated titanocene complex and the cocatalyst (compare to Scheme 5.1): Cp2 TiðMeÞCl þ AlMeCl2 Ð Intermediate complex ½Cp2 TiðMeÞCl    AlMeCl2  Intermediate complex þ ðAlMeCl2 Þ2 Ð C 

(5.39) (5.40)

The equilibrium in the first reaction is strongly shifted to the right, and the equilibrium in the second reaction is strongly shifted to the left. As a result, only a small fraction of the original titanocene complex is converted to the active center at this low temperature. 2. Ethylene molecules do not form experimentally observed complexes with the active species. 3. An ethylene molecule rapidly inserts into the Cp2Ti+
CH3 bond and produces a distinctly observable Ti
n-Pr group: Cp2 Tiþ 2CH3 þ

13

CH2 ¼ 13 CH2

! Cp2 Tiþ 213 CH2 213 CH2 2CH3

(5.41)

312

Alkene Polymerization Reactions with Transition Metal Catalysts

4. Several other ethylene addition products are also observable: Cp2 Tiþ 213 CH2 213 CH2 2CH3 þ

13

CH2 ¼ 13 CH2

! Cp2 Tiþ 2ð13 CH2 213 CH2 Þn 2CH3

(5.42)

in particular, the Cp2Ti-pentyl group (n ¼ 2) and the Cp2Ti-heptyl group (n ¼ 3). As the polymerization reaction progresses, these groups are present in the reaction mixture in steady-state concentrations. 5. The rate constant of the ethylene insertion into the Ti
C bond of the Cp2Ti+
n-Pr species is B2 times higher than for the Cp2Ti+
n-Pentyl species. Stopped-flow experiments with the Cp2TiCl2–MAO system at 201C provided additional kinetic details of these ethylene polymerization reactions [1588]. From the formal point of view, kinetics of this reactions satisfies main requirements of a living polymerization process, the dependence of the polymer yield vs. time shows a short acceleration period (B0.05 seconds) (Figure 5.4A) and the molecular weight of the polymer increases with the reaction time in a nearly linear manner, as shown in Figure 5.4B. The treatment of these results using the kinetic approach for stopped-flow reactions with a slow active center formation step (see, e.g., Equation (5.34)) shows that every molecule of the metallocene complex after B0.15–0.2 seconds grows a polymer chain and the propagation rate constant is very high (B1.9  105 M
1  s
1). 5.4.2.2. Propylene polymerization reactions Bochmann examined the kinetics of the earliest stages of isospecific propylene polymerization reactions in stopped-flow experiments at 251C with racMe2Si(Ind)2ZrCl2 activated with an ion-forming cocatalyst mixture [Ph3C]+ [CN{B(C6F5)3}2]
-Ali-Bu3, and the same reactions at 401C using MAO as a cocatalyst [661]. These studies underlined difficulties in the application of the living-chain kinetic approach to the analysis of catalytic polymerization reactions. The value of the propagation rate constant for the ionic catalyst can be calculated using Equation (5.34) under the assumption that all zirconocene molecules are activated after B1 second after the beginning of the reaction. The estimation gave a high kp value, 1.3–1.9  103 M
1  s
1 at 251C. However, the experimental kinetic data contradict the assumption that all active centers start the polymerization reaction immediately after mixing of the catalyst components: (a) the kinetic curves have expressed acceleration stages and (b) the molecular weight distribution is relatively broad, the Mw/Mn ratios are B1.7–1.9 at any reaction time from 0.2 to 5 seconds. When molecular weights at the earliest stages of the same experiments are used for the kp calculation (with Equation (5.35)), this value (B1.7  104 M
1  s
1) is 10 times higher than the estimation based on polymer yields. The difference suggests that only B8% of the metallocene complex is involved in the polymerization reaction at any given moment whereas the majority of the active centers are in the dormant state. The kinetic analysis of propylene polymerization with the rac-Me2Si(Ind)2ZrCl2-MAO system at 401C showed a similar relationship, only B8% of the metallocene complex exists as the active center at any given

313

Kinetics of Alkene Polymerization Reactions

6000

Yield, g/mol Ti

A 4000

2000

0 0.00

0.05

0.10

0.15

0.20

Time, s 8000 B

Mn

6000

4000

2000

0 0.00

0.05

0.10

0.15

0.20

Time, s

Figure 5.4 Kinetics of stopped-£ow ethylene homopolymerization reaction with Cp2T|Cl2 MAO system at 201C [Al]:[T|] ¼ 600, CE ¼ 0.13 M; data from [1588]. A, yield vs. time; B, Mn vs. time.

moment. The value of the propagation rate constant for the latter catalyst is merely B50 M
1  s
1 reflecting the effect of the counter-ion on the activity of the zirconocenium ions. Polymerization reactions of propylene were also carried out with similar preformed isospecific catalyst species prepared from rac-Me2Si(Ind)2Zr(Me)CH2SiMe3 and two different ion-forming activators [1589]. The value of the propagation rate constant measured at 201C strongly depends on the type of the anion in the cocatalyst: the kp value is 1,360 M
1  s
1 for [B(C6F5)4]
but only 20 M
1  s
1 for [MeB(C6F5)3]
. The difference was explained as a result of differences in displacement rates of the anions from the coordination site at the active species (Reaction 6.8). Landis carried out a detailed kinetic study of propylene polymerization reactions with a pre-formed active species [C2H4(Ind)2Zr+–[CH2–CH(C4H9)]n
CH3] [MeB(C6F5)3]
[522]. This active center was generated at low temperatures

314

Alkene Polymerization Reactions with Transition Metal Catalysts

in a prepolymerization step between the ion pair [rac-C2H4(Ind)2Zr+
Me] [MeB(C6F5)3]
and several molecules of 1-hexene [1590]. The use of this active species provides an opportunity to avoid a slow chain initiation step, the insertion of the first 1-alkene molecule into the Cp2Zr+
Me bond [139]. The chain growth reaction of propylene with the pre-formed active center is [522]: C2 H4 ðIndÞ2 Zrþ 2½CH2 2CHðC4 H9 Þn 2CH3 þ CH2 ¼ CH2CH3 }ðkp Þ ! C2 H4 ðIndÞ2 Zrþ 2½CH2 2CHðCH3 Þm 2½CH2 2CHðC4 H9 n 2CH3 (5.43) All prepolymerized active centers participate in Reaction (5.43). The reaction has the first order with respect to the concentrations of both participants, the active center and propylene, in a temperature range from
40 to +201C. The propylene insertion step in Reaction (5.43) is exclusively primary, the kp value increases from 2.6 M
1  s
1 at
401C to B10 M
1  s
1 at +201C; the activation parameters of the reaction are DH¼ ¼ 11.7 kJ/mol (3.4 kcal/mol), DS¼ ¼
0.152 kJ/mol  K (
44 cal/mol  K). If the amount of propylene in Reaction (5.43) is small and completely exhausted, the growing polymer chain undergoes a very slow chain transfer reaction via the b-H elimination route: ½C2 H4 ðIndÞ2 Zrþ 2½CH2 2CHðCH3 Þm 2CH3  ½MeBðC6 F5 Þ3 
}ðkt b Þ ! ½C2 H4 ðIndÞ2 Zrþ 2H ½HBðC6 F5 Þ3 
þ CH2 ¼ CðCH3 Þ2½CH2 2CHðCH3 Þm
1 2CH3 (5.44) This reaction is practically irreversible, its activation parameters are DH¼ ¼ 54 kJ/mol (12.9 kcal/mol), DS¼ ¼
0.096 kJ/mol  K (
23 cal/mol  K). Low-temperature NMR research also provides important data on the relative reactivities of two types of active centers typical for metallocene catalysis, centers with the last monomer unit in the primary and the secondary orientation, Cp2Zr+
CH2CH(CH3)
Polymer vs. Cp2Zr+
CH(CH3)CH2
Polymer. When the metallocenium ion has a large aperture angle between its cyclopentadienyl rings [when Cp ¼ rac-C2H4(Ind)2], both active centers have similar reactivities in insertion reactions of both propylene and ethylene [521]. The biggest difference between the reactivities of primary and secondary active centers was observed only in a hydrogenolysis reaction, the Cp2Zr+
CH(CH3) bond reacts with hydrogen at
401C at least 100 times faster compared to the Cp2Zr+
CH2 bond [521]. A detailed kinetic investigation of propylene polymerization reactions with several metallocene catalysts based on the same syndiospecific complex, Me2C(Cp)(Flu)ZrMe2, and several cocatalysts showed that all the combinations produce single-center catalysts, ion pairs Me2C(Cp)(Flu)Zr+
Me A
[735]. The stability of the ion pairs (determined by NMR in the absence of monomer) increases in a series of A
: [B(C6F5)4]
o[MeB(C6F4
C6F5)3]
oo [MeB(C6F5)3]
oo [FAl(C6F4
C6F5)3]
. Only approximate estimations of

315

Kinetics of Alkene Polymerization Reactions

propagation rate constants are possible (from polymer yields at 601C): A


[Me-MAO]
[MeB(C6F5)3]
[B(C6F5)4]
[MeB(C6F4– C6F5)3]
–1
1 kp (M  s ) B400 42 300 320 M
3
3
3 1.1  10 2.4  10 1.1  10 1.4  10
3 kt /kp

[FAl(C6F4– C6F5)3]
34 1.1  10
3

The principal chain transfer reaction for these catalysts is that to propylene (Reaction (3.50)). It has the first-order dependence on the monomer concentration, the same as the chain growth reaction. The kM t /kp ratio only slightly depends on the type of the counter-anion A
, except for [MeB(C6F5)3]
. Constrained-geometry systems derived from monofluorenyl complexes of Ti, [Me2Si(t-Bu2-Flu)(t-Bu-N)]TiMe2, provide a rare opportunity to carry out propylene polymerization reactions in the living-chain mode at temperatures from 0 to 251C for significant periods of time without using the stopped-flow technique [137,1591]. The prerequisite for this unusual kinetic behavior is the presence of two bulky t-butyl substituents in the 2,7-positions or the 3,6-positions of the fluorenyl ligand. The molecular weight distribution of polypropylene prepared with these complexes activated with MMAO (Mw/Mn ¼ 1.3–1.6) is higher than that expected under living-chain conditions because of a slow chain initiation step [137]. Nevertheless, the reactions exhibit two features typical for living-chain reactions, the molecular weight increases proportionally to reaction time, and the formation of alkene block-polymers is possible even at 251C. About 60–65% of the complexes form active centers; they produce moderately syndiotactic polypropylene with [rrrr] o0.86 [1591]. 5.4.2.3. Polymerization reactions of higher 1-alkenes Landis carried out detailed kinetic studies of polymerization reactions of 1-hexene with several preformed ionic metallocene catalysts [139,1369]. A single metallocenium cation rac-C2H4(Ind)2Zr+
Me was used in most experiments; it was prepared from rac-C2H4(Ind)2ZrMe2 and three different ion-forming activators, B(C6F5)3, Al(C6F5)3, and [PhNHMe2]+ [B(C6F5)4]
(Reactions (6.3)–(6.6)). The ion-formation step is a very fast reaction even at low temperatures and it does not interfere with the kinetics of the polymerization reactions [139]. Three distinct stages of the polymerization reactions, chain initiation, growth, and two chain termination reactions, were observed. The chain initiation reaction Cp2 Zrþ 2CH3

A
þ CH2 ¼ CH2C4 H9

! Cp2 Zrþ 2CH2 2CHðC4 H9 Þ2CH3

A


(5.45)

proceeds at a significantly lower rate than all subsequent insertion reactions of 1-hexene (chain growth reactions), and its kinetics can be measured independently. Stopped-flow experiments at low temperatures showed that Reaction (5.45) is a bimolecular reaction: d½C  =dt ¼ ki  ½Zr  C Hex

(5.46)

316

Alkene Polymerization Reactions with Transition Metal Catalysts

The rate constant of this reaction strongly depends on the nature of the counteranion, as estimations at 01C show [1369]: Counter-ion A
ki (M
1  s
1)

[MeB(C6F5)3]
0.026

[MeAl(C6F5)3]
B3.3  10
4

[B(C6F5)4]
0.020

This difference apparently reflects the strength of ion pairs [rac-C2H4(Ind)2Zr+
Me] A
. Activation parameters of the initiation reaction for the [MeB(C6F5)3]
counterion are DH¼ ¼ 48.1 kJ/mol (11.5 kcal/mol), DS¼ ¼
0.1 kJ/mol  K (
24 cal/mol  K). The propagation reaction in these experiments represents the insertion reaction of a 1-hexene molecule into the [rac-C2H4(Ind)2Zr+
R] bond where R is a growing polymer chain [CH2
CH(C4H9)]nCH3. The experimentally determined expression for the instant polymerization rate is Rp ¼ kp  ½C    C Hex

(5.47)

where C is a steadily increasing concentration of propagation centers described by Equation (5.46). The following principal kinetic features of the chain propagation reaction were determined: 1. The rate of polymer formation has the first order with respect to the monomer concentration. 2. About 90% of the initial zirconocene complex is converted into active centers in Reaction (5.45). 3. The reaction rate is not affected by the presence of an excess [MeB(C6F5)3]
counter-ion. 4. The kp value ranges from B0.2 M
1  s
1 at
101C to B17 M
1  s
1 at 501C. Activation parameters of the propagation reaction are DH¼ ¼ 26.8 kJ/mol (6.4 kcal/mol), DS¼ ¼
0.14 kJ/mol  K (
33 cal/mol  K). 5. A significant kinetic difference exists between the chain initiation reaction (Reaction (5.45)) and the chain propagation reaction. The kp/ki ratio is B10 at
101C and it is W30 at 501C. Mechanistic implications of this difference are discussed in Section 6.1.2.1. Two spontaneous chain transfer processes occur in these polymerization reactions. The first of them, the b-H atom transfer from a 1,2-inserted monomer unit to the Zr atom (Reaction (3.54)) has the following activation parameters, DH¼ ¼ 66.8 kJ/mol (16.2 kcal/mol), DS¼ ¼
50 J/mol  K (
12 cal/mol  K). The second chain transfer reaction is similar, the b-H atom transfer from a 2,1-inserted monomer unit to the Zr atom (Reaction (3.70)), its activation parameters are DH¼ ¼ 40.6 kJ/mol (9.7 kcal/mol), DS¼ ¼
146 J/mol  K (
35 cal/mol  K). The ratio of chain ends produced in these two reactions, the vinylidene CH2QCo bond and the internal CHQCH bond, is B1:3. When these polymerization reactions are terminated after a short time, 1–2 minutes, most of the active centers still bear the initial growing polymer chains but a small part of the active centers already underwent the two chain transfer reactions. This situation translates into a complex form of the molecular weight distribution, a

Kinetics of Alkene Polymerization Reactions

317

combination of the Poisson distribution for the growing chains (Equation (2.6)) and the Flory distribution for the separated polymer chains (Equation (2.5)). The combined molecular weight distribution for these polymers remains very narrow, the Mw/Mn value is B1.2 [139]. Polymerization reactions of styrene and 4-methylstyrene at low temperatures with a typical syndiospecific system, CpTiMe3-B(C6F5)3, also proceed in the living-chain mode [1592,1593]. These reactions exhibit all attributes of living-chain reactions: (a) the reaction rates are constant for over 30 minutes, (b) they have the first-order dependence with respect to the monomer concentration, (c) the numbers of the polymer chains are constant, B3–4% of CpTiMe3, (d) the molecular weight of the polymers increases proportionally to conversion, and (e) the Mw/Mn value of the polymers remains 1.15–1.19 for poly(4-methylstyrene) and B1.5 for polystyrene. When styrene polymerization reactions with CpTiMe3 and CpTiBz3 activated with MAO, B(C6F5)3, and CPh3B(C6F5)4 are carried out at a higher temperature, +251C, the reactions retain the first order with respect to the monomer concentration. However, chain transfer reactions become significant under these conditions and the Mw/Mn value of the polymers increases to B2 [754]. The fraction of the active centers, which is approximately estimated from molecular weights of the polymers prepared at very short reaction times and in experiments terminated with CH3O3H, corresponds to 10–30% of the titanocene complexes, depending on cocatalyst. The dominant chain transfer reaction is the b-H transfer to the Ti atom, and the second in significance in the chain transfer to a monomer (Reactions (3.117) and (3.116)). The respective rate constant ratios for the CpTiBz3-B(C6F5)3 system at 501C are: kbt /kp ¼ 5.8  10
3
3 M
1, kM t /kp ¼ 1.3  10 . When a monometallocene complex CpTiCl2X, where X is the ketimide group –N=Ct-Bu2, is activated with MAO, living-chain copolymerization reactions of ethylene and styrene are possible even at 25 and 401C [1264]. In these reactions, the molecular weight of the copolymers increases linearly with polymer yield for a period of time exceeding 30 minutes, and the molecular weight distribution remains narrow, the Mw/Mn ratio is 1.15–1.30.

5.4.3. General kinetic studies, effects of reaction parameters 5.4.3.1. Polymerization reactions with ionic metallocene catalysts In principle, kinetic studies of alkene polymerization reactions with ionic metallocene catalysts containing metallocene complexes Cp2MX2, alkylating agents (usually AlR3), and ion-forming cocatalysts such as [CPh3]+ [B(C6F5)4]
should have been relatively straightforward. Numerous NMR studies of these catalyst systems show that the active species in them, metallocenium ions Cp2M+–R, are formed very rapidly even at low temperatures (Section 6.1.1.1). In practice, the kinetics of these polymerization reactions is quite complex [710]. Even when the active species are preformed and then employed at 501C (e.g. , in ternary rac-C2H4(Ind)2Zr(NMe2)2-[CPh3]+ [B(C6F5)4]
-AlR3 systems), kinetic curves exhibit an acceleration stage for 3–5 minutes and then the catalysts rapidly decay,

318

Alkene Polymerization Reactions with Transition Metal Catalysts

Figure 5.5 Kinetics of propylene polymerization reactions with rac-C2H4(Ind)2Zr(NMe2)2 [B(C6F5)3]-AlMe3 system at 501C [B]:[Zr] ¼ 1, PPr ¼ 1.3 atm [710]. [AlMe3]:[Zr] ratios: 8 (a), 11 (b), 15 (c), 27 (d).

as shown in Figure 5.5. The maximum activity of the catalysts depends both on the type of AlR3 (R ¼ Me, Et, i-Bu) and on the [AlR3]:[Zr] ratio. If AlMe3 is used, the maximum rate is the highest at [AlMe3]:[Zr] B10 but the system is completely inactive when this ratio is B30; and when AlEt3 is used, the maximum rate is the highest at [AlEt3]:[Zr] B30 but the system is inactive when this ratio is B60. Kinetic comparisons of different metallocene catalysts are often complicated by phenomena similar to those encountered in polymerization reactions with heterogeneous catalysts. The most obvious of them is the existence of several types of active centers with different kinetic characteristics. For example, the C2H4(Ind)2ZrCl2-MAO and the C2H4(Ind)2HfCl2-MAO systems (see the next section) prepared at an [MAO][M] ratio of 4,000 both contain only one type of active center at 401C, as follows from the structural analysis of ethylene/1-hexene copolymers prepared with them [378]. When MAO is replaced with an ionforming [Me2N(Ph)H]+ [B(C6F5)4]
-Ali-Bu3 pair, the zirconocene catalyst remains a single-center catalyst and its active center apparently has the same kinetic characteristics as in the MAO-cocatalyzed system. The single-center nature of similar zirconocene catalysts does not change even at very high reaction

Kinetics of Alkene Polymerization Reactions

319

temperatures, 140–1601C [622]. On the other hand, C2H4(Ind)2HfCl2 reacts with the same cocatalyst at 401C with the formation of two types of active centers that differ in the molecular weight and the composition of ethylene/1-hexene copolymer fractions they produce [378]. 5.4.3.2. Polymerization reactions with MAO-activated metallocene catalysts The level of detail presented in the studies of polymerization kinetics with ionic metallocene catalysts at low temperatures (Section 5.4.2) is rarely achieved in the studies of metallocene systems utilizing MAO as a cocatalyst. The biggest challenge in the research of the latter type is to find an explanation for the need of a large excess of MAO in order to produce highly active catalysts. This circumstance hampers even the kinetic studies where the structure of immediate precursors of the active centers is known. The presence of AlMe3 in MAO causes additional complications in the kinetic studies. Reactions between the catalyst precursors, Cp2ZrMe2, and AlMe3 produce analogs of Tebbe’s reagents, Cp2Zr(Me)–CH2– AlMe2 and Cp2Zr(–CH2–AlMe2)2, some of which are ineffective as polymerization catalysts [27,65]. General features of polymerization kinetics: Kim described a characteristic example of polymerization reactions with MAO-activated metallocene complexes, propylene polymerization reactions with a highly isospecific system rac-Me2Si(2-Me-4-tBu-Cp)2Zr(NMe2)2-MAO [167]. When the [Al]:[Zr] ratio in this catalyst exceeds 40, the initial metallocene complex is rapidly transformed into the metallocenium ion rac-Me2Si(2-Me-4-t-Bu-Cp)2Zr+–Me. This reaction is fast even at
781C. However, this system, as well as the same systems prepared at [Al]:[Zr]o250, have negligible activity in polymerization reactions. Only when the [Al]:[Zr] ratio is increased to B5,000 the catalyst develops the high activity typical for MAOactivated metallocene catalysts in general. The low activity at low [Al]:[Zr] ratios is not caused by contamination of the reaction medium; the addition of an ionforming activator [Ph3C]+ [B(C6F5)4]
results in an immediate sharp increase in the polymerization rate. A detailed kinetic study of the earliest stages of this polymerization reaction at different temperatures (Figure 5.6) demonstrates that the reaction has three sages: 1. An induction period which decreases from 2.5 minutes at 101C to B40 seconds at 701C. 2. A period of a gradual increase of the reaction rate. This stage does not depend on reaction temperature and usually lasts 2–2.5 minutes. 3. After the maximum reaction rate is reached, a period of a steady decline in activity begins. Nearly all polymerization reactions of light alkenes with MAO-activated metallocene complexes exhibit the same type of kinetic behavior, the reaction rate starts from zero, rapidly reaches a maximum (usually within B10 minutes or less), and then steadily decreases [514,603,641,711,1594–1596]. Formally, these kinetic curves can be well described by Equation (5.13) or Equation (5.17), although such curve-fitting exercises add very little to real understanding of the underlying kinetic

320

Alkene Polymerization Reactions with Transition Metal Catalysts

Figure 5.6 Kinetics of propylene polymerization reactions with rac-Me2Si(2-Me-4-t-BuCp)2Zr(NMe2)2 -MAO system at [Al]:[Zr] B5,000 and PPr ¼ 1.3 atm [167]. Reaction temperatures: 101C (a), 201C (b), 301C (c), 401C (d), 501C (e), 601C (f), 701C (g).

steps. Even more complex formal kinetic schemes can be developed (and were indeed published) to describe all experimental observations but they still provide no assistance in an explanation for the need of high MAO concentrations in metallocene systems. Superficially, main kinetic features of ethylene homopolymerization reactions with standard metallocene catalysts under moderate conditions yield to a straightforward interpretation. The rate of ethylene polymerization with the Cp2ZrCl2-MAO system and similar catalysts at temperatures below 40–501C, after the initial acceleration period, is either practically constant for over 40 minutes or slowly decreases with time [300,1333,1442,1597,1598]. The aging of these systems at 20–301C in the absence of ethylene for up to 90 minutes does not result in any deactivation [612,1321]. Only when the reaction temperature is increased to 701C the activity decrease with time becomes pronounced [1332,1597–1599]. These data indicate that active centers in these polymerization reactions are potentially very stable. Two factors complicate the kinetic analysis of alkene polymerization reactions with MAO-activated metallocene catalysts. The first factor is the same as that encountered in polymerization reactions with ionic metallocene catalysts (Section

Kinetics of Alkene Polymerization Reactions

321

5.4.3.1), the existence of several types of active centers in some of the systems, especially at low [MAO]:[transition metal] ratios (Section 2.5.1.2). For example, kinetic analysis of ethylene polymerization reactions with four MAO-activated metallocene complexes, Cp2ZrCl2, Cp 2 ZrCl2, rac-Me2C(Ind)2ZrCl2, and racMe2Si(Ind)2ZrCl2, showed that all these systems contain two types of active centers. The centers differ in rate constants of chain transfer reactions to AlMe3 present in MAO and produce two Flory components with average molecular weights differing by a factor of B10 [299,300]. The centers also have different stabilities, the centers producing the low molecular weight component decay significantly faster [300]. The second complicating factor is apparent in the kinetic analysis of 1-alkene polymerization reactions with metallocene systems of this type. 1-Alkene molecules can insert into the Cp2Zr+–C bonds in two orientations, primary and secondary. Although the secondary insertion step has a much lower probability than the primary step, the primary chain growth step that follows it may be kinetically difficult compared to the regular primary insertion step. For example, several NMR estimations give the range of the retardation effect after the secondary insertion of a propylene molecule as 102–103 [512,516–518]. The existence of this rate depression amounts to transformation of highly active centers Cp2Zr+– CH2CH(CH3)R into centers of lower reactivity, Cp2Zr+–CH(CH3)CH2R [461,512,516–519,605]. 5.4.3.2.1. Acceleration period. The majority of kinetic studies of alkene polymerization reactions with MAO-activated metallocene complexes describe the existence of an acceleration period. It can last from B2 to over 10 minutes and was detected in polymerization reactions at a temperature ranging from 30 to 1101C [271,298,547,597,605,610,711,733,1332,1333,1346,1600]. Physical effects: Rieger and Rytter carried out precise kinetic studies of alkene polymerization reactions with several metallocene catalysts, which afforded the measurement of polymerization rates immediately after the onset of the polymerization reactions [603,1332]. These studies identified an additional kinetic stage in these polymerization reactions caused by a transition from a homo-phaseous to a hetero-phaseous reaction. When an ethylene homopolymerization reaction with the Cp2ZrCl2-MAO system is carried out at 1101C [1332], the polymer remains dissolved in toluene for first 3–5 minutes and the polymerization reaction is homo-phaseous. During this time, the reaction rate rapidly decreases because catalyst deactivation is well pronounced at this high temperature. However, as soon as polyethylene precipitates from solution, the reaction rate starts increasing for several minutes, reaches a maximum and again starts to decline. The maximum was formally explained by an equilibrium reaction:

Cp2 ZrCl2 þ MAO Ð ½Cp2 Zrþ 2Polymer þ MAO
homo Ð ½Cp2 Zrþ 2Polymer þ MAO
hetero

(5.48)

The precipitation of polymer molecules with attached active centers shifts the second equilibrium to the right, which may explain the maximum on the kinetic curve. This phase transition occurs much faster at lower reaction temperatures due

322

Alkene Polymerization Reactions with Transition Metal Catalysts

to lower polymer solubility, when the initial stage caused by polymer precipitation is difficult to observe [603]. Instead, the reaction starts at a very low rate and gradually accelerates to a maximum. Nature of acceleration period: Exact reasons for the acceleration period in polymerization reactions with metallocene catalysts remain uncertain. Several experimental attempts were made to find the reaction parameters and the catalyst features that significantly affect its duration. The length of the acceleration period does not depend in any appreciable way on the type of the monomer in the polymerization reactions, and its value in ethylene polymerization reactions is not affected by the addition of 1-alkenes [597]. The first stage of the active center formation in metallocene systems is the alkylation of metallocene complexes Cp2ZrCl2 (see Section 4.6.5). However, the acceleration period had the same length, B10 minutes, when MAO-activated catalysts prepared from Cp2ZrCl2, Cp2Zr(Me)Cl, and Cp2ZrMe2 were tested in ethylene polymerization reactions at 801C [29]. Pre-activation of (R-Cp)2ZrCl2 complexes with MAO or with AlMe3 before polymerization reactions [603,610,711,1346] or during the synthesis of supported catalysts [1442,1480] does not eliminate the activation period and does not affect its duration. Both these features indicate that alkylation of Cp2ZrCl2 complexes with MAO or with AlMe3 are very fast reactions and cannot explain the existence of the activation stage. The only apparent structural feature that affects the length of the acceleration stage is the type of the substituent R in a series of zirconocene complexes (R-Cp)2ZrCl2 activated with MAO, it increases from B1 minute for R=Me to 12 minutes for R ¼ n-Pr [603]. This effect was ascribed to a low rate of the ‘‘initial’’ initiation reaction, the insertion of the first CQC bond into the Cp2Zr+–CH3 bond in the original active center which is stabilized by b- and g-agostic interactions between alkyl groups R in the Z5 ligands and the Zr atom, which are especially significant for the n-propyl group [603]. However, the results produced with model scandocene complexes do not support this assertion [1583] (see Section 5.4.2.1). Rytter [711] and Kim [1596,1601] carried out detailed kinetic studies of activation periods in propylene polymerization reactions with several MAOactivated systems based on the same metallocene complexes in a broad range of temperatures, from 25 to 1201C. In all these cases, kinetic curves exhibit the acceleration period that lasts from 2–3 to 10 minutes. Several observations about the acceleration period were made in these studies: 1. Premixing the catalyst components at low (251C) or high (601C) temperatures for periods of time from 3 to 60 minutes does not eliminate the activation period and does not make it significantly shorter. 2. The time required to reach the maximum polymerization activity increases with the propylene concentration, from B1.5 minutes at PPr ¼ 1 bar to B5 minutes at 2.2 bar. 3. The activation time does not depend on the concentration of zirconocene complexes.

323

Kinetics of Alkene Polymerization Reactions

4. Whereas the total activity of the catalysts greatly increases with the [MAO]:[Zr] ratio (as in practically all polymerization reactions of this type), the time required to reach the maximum activity does not significantly change when the [MAO]:[Zr] ratio is increased from o2,000 to 6,500. In general, all these experimental data suggest that the acceleration stage is chemical in nature rather than related to a change in the physicals state of the produced polymers. Catalysts based on non-bridged metallocene complexes produce atactic polypropylene that does not precipitate from solution. Nevertheless, these reactions also exhibit a pronounced activation stage. The temperature effect on the acceleration stage also suggests its chemical origin: the higher the reaction temperature the faster a catalyst reaches the maximum activity. 5.4.3.2.2. Stationary period, effects of reaction parameters. Kinetic studies of alkene polymerization reactions with metallocene catalysts are mostly devoted to the analysis of the effects of reaction parameters on the stationary (or nearly stationary) polymerization rate. If these reactions are carried out below 70–801C, zirconocene systems are usually quite stable and their kinetic behavior under different conditions can be readily examined. For example, when propylene is polymerized with the isospecific rac-C2H4(Ind)2ZrCl2-MAO system at temperatures from 40 to 751C, the reaction rate reaches the maximum value after B7–10 minutes and then either remains nearly constant for 30–45 minutes (at lower temperatures) or slowly decreases [271,610]. The catalyst contains only one type of active centers (Mw/Mn ¼ 1.9) and the centers are highly isospecific, [mm] B0.9. This ‘‘kinetic simplicity’’ greatly assists the kinetic studies. Stopped-flow ethylene and propylene polymerization experiments at temperatures from 20 to 601C with preformed active centers in the rac-Me2Si(2-Me-4-PhInd)(Cp)ZrCl2-MAO system provided important information on principal kinetic features of the reactions [1574]. The reactions produced polymers with the Mw/Mn ratios of 1.6–2.0 even after very short reaction times, 0.1–0.2 seconds; the manifestation of rapid chain transfers reactions. Because the molecular weight of the polymers formed in these reactions is quite high, the ability to observe chain transfer reactions after very short reaction times signifies that the propagation rate constants are very high. Kinetic Table 5.1 Kinetic parameters of polymerization reactions with rac-Me2Si(2-Me-4-Ph-Ind) (Cp)ZrCl2-MAO system determined in stopped-flow experiments [1574] Temperature (1C)

Ethylene polymerization 20 40 60 Propylene polymerization 40

kp (M
1  s
1)

C (mol/mol Zr)

2.6  105 1.1  106 2.8  106

0.046 0.10 0.23

4.7  103

0.58

324

Alkene Polymerization Reactions with Transition Metal Catalysts

parameters of chain growth reactions are listed in Table 5.1. The activation energy of the chain growth reaction with this catalyst is 41.5 kJ/mol (12 kcal/mol). Effect of metallocene and MAO concentration: The kinetic complexity of alkene polymerization reactions with metallocene catalysts becomes apparent when one examines the concentration effects of the catalyst components on their activity. As an example, Figure 5.7 shows the concentration effect of Cp2ZrCl2 on the ethylene polymerization rate in a single polymerization reaction performed at a constant MAO concentration of 0.05 M. This polymerization reaction was carried out in a toluene medium at 201C under the conditions when the catalyst is stable and when an introduction of a fresh aliquot of catalyst solution produces an immediate effect on the reaction rate. Fushman determined three ranges in the ‘‘activity vs. [Cp2ZrCl2]’’ dependence [1321]. At very low Cp2ZrCl2 concentrations, from 5  10
7 to B8  10
5 M (and, respectively, at high [MAO]:[Zr] ratios, 1  105 to B600), the rate of ethylene consumption is proportional to the concentration of the metallocene complex. This is the range of [MAO]:[Zr] ratios typically examined in kinetic polymerization studies. If the polymerization reactions are carried out within this [MAO]:[Zr] range, the variation in the MAO concentration has only a weak effect on the catalyst activity [1321,1490]. Virtually any metallocene complex used under these conditions is rapidly converted to active centers, as was proved in C measurements using the CO poisoning method [1602]. At intermediate Cp2ZrCl2 concentrations, from 8  10
5 to B3  10
4 M, corresponding to [MAO]:[Zr] ratios from 600 to B150, the polymerization rate does not depend on the Cp2ZrCl2 concentration. If the polymerization reactions are carried out within this [MAO]:[Zr] range, an increase of the MAO

Figure 5.7 Concentration e¡ect of Cp2ZrCl2 on ethylene polymerization rate with Cp2ZrCl2 -MAO system at 201C [MAO] ¼ 0.05 M [1321].

Kinetics of Alkene Polymerization Reactions

325

concentration has a strong effect on catalyst activity [617,1301], also the effect noticed in many kinetic studies. At high Cp2ZrCl2 concentrations, above B3  10
4 M and at [MAO]:[Zr] ratios below 150, the polymerization rate decreases with the Cp2ZrCl2 concentration [1319,1321,1603,1604], and when [Cp2ZrCl2] B0.02 M the catalyst completely loses activity. Only a small fraction of the metallocene complex employed under these conditions is converted into active centers, C{[Zr], as was demonstrated by C measurements with CO [1594]. Similar effects of the metallocene concentration on polymerization rates were found in polymerization reactions of ethylene with a dinuclear complex, Cp2Zr(Cl)–O–Zr(Cl)Cp2 [1321], propylene with rac-C2H4(Ind-H4)2ZrCl2 [1600], and 1-hexene with Me2Si(Ind)2ZrCl2 [326]. These findings defy simple kinetic schemes [749] according to which the active centers are solvent-separated ion pairs containing metallocenium ions and MAOderived counter-anions existing in equilibrium with a dialkylated zirconocene complex Cp2 ZrMe2 þ MAO Ð Active center C ð½Cp2 Zrþ
Me þ ½MAOðMeÞ
Þ (5.49) when the equilibrium is strongly shifted to the left. Instead, Fushman introduced a complex kinetic scheme for the formation of active centers, which emphasizes the role of a small, spectroscopically unobservable ingredient P in MAO (with concentration [P]{[MAO]) that serves as the true activator of alkylated metallocene complexes [1321]. In the simplest case, the metallocene complex and the ingredient P participate in an equilibrium reaction strongly shifted to the right: Cp2 ZrMe2 þ P Ð Active centers C  ðCp2 Zrþ
Me þ ½PðMeÞ
Þ

(5.50)

In this kinetic scheme, the complex kinetic behavior of metallocene catalysts in ethylene polymerization reactions evident from Figure 5.7 is explained by relative amounts of Cp2ZrMe2 and the activator P under particular reaction conditions. A more detailed kinetic scheme explaining the dependence shown in Figure 5.7 involves a series of two equilibrium reactions similar to Reaction (5.50) [641,1321, 1603,1604]. The most probable ingredients P in MAO are rare tri-coordinated Al atoms in the MAO structure [1321]. Following Barron, these Al atoms are latent Lewis centers, they are formed in the heterolytic cleavage of the Al–O bond in MAO molecules due to the ring strain [1605] and have sufficiently strong Lewis acidity to participate in Reaction (5.50). Several different Lewis-acidic centers of this type can be potentially formed in MAO resulting in the formation of several types of active centers with different kinetic characteristics. This possibility is evidenced from the broadening of the molecular weight distribution of ethylene polymers prepared at low [MAO]:[Zr] ratios (Section 2.5.1.2). Concentration effects of metallocene complexes and MAO in isospecific polymerization reactions of propylene are even more difficult to discern. Kinetic profiles of these reactions depend on the concentrations of the catalyst components, the activity rapidly decays over time at low [MAO]:[Zr] ratios but the catalysts are relatively stable at high [MAO]:[Zr] ratios [1606]. This difference shows that the use

326

Alkene Polymerization Reactions with Transition Metal Catalysts

of the average productivity after a particular reaction time as a kinetic parameter cannot be an adequate measure of catalyst performance. The molecular weights of alkene polymers prepared under steady reaction conditions depend mostly on the type of the zirconocene complex and, to a smaller degree, on the type of MAO. Some results for ethylene polymerization reactions are shown in Table 5.2. Effect of organometallic compounds: Commercial MAO samples usually contain free AlMe3. Its effect on the alkene polymerization kinetics is quite complex; it can both activate and deactivate the catalysts, depending on reaction temperature [614]. The deactivation effect of AlMe3 is usually associated with the conversion of metallocenium cations Cp2Zr+
R into catalytically inactive hetero-dinuclear species [Cp2Zr(m-Me)2AlMe2]+y[MAO(Me)]
(Section 6.1.1.4) [614]. Another effect of excess AlMe3 is the reduction of the molecular weight of alkene polymers due to the chain transfer reaction to a cocatalyst. Table 5.3 compares the relative significance of principal chain transfer reactions in propylene polymerization reactions with the syndiospecific Ph2C(Cp)(Flu)ZrCl2-MAO system (Equation (5.21)). These results are mostly expected: as the temperature increases, the probabilities of both the chain transfer reaction to AlMe3 (Reactions (3.61) and (5.9)) and b-H elimination (Reactions (3.54) and (5.12)) significantly increase, in contrast to the chain transfer to a monomer (Reactions (3.50) and (5.5)). The relative significance of two major chain transfer reactions, those to a monomer and AlMe3, is determined by the concentration ratio of these two compounds. If the [monomer]:[AlMe3] ratio is of the order of several hundreds (a typical situation in polymerization reactions of this type), analysis of Equation (5.21) shows that the Table 5.2 Molecular weight of polyethylene produced with different metallocene complexesa [617]

a

Metallocene complex

Cocatalyst: MAO

Cocatalyst: MMAO

(n-Bu-Cp)2ZrCl2 (Ind)2ZrCl2 C2H4(Ind)2ZrCl2 Me2Si(Cp)2ZrCl2 Ph2C(Cp)(Ind)ZrCl2 Me2Si(2-Me-Ind)2ZrCl2

1.9  104–8.0  104 2.7  104–5.7  105 5.4  104–5.6  104 B5  103 1.7  105–2.3  105 1.46  105

5.7  104–18.4  104 16.8  104 6.9  104 B1.2  104 0.6  105–1.8  105

Polymerization at 851C, PE B9 atm.

Table 5.3 Kinetic parameters of propylene polymerization reaction catalyzed by Ph2C(Cp)(Flu)ZrCl2-MAO system [614] Temperature (1C)

kM t =kp

kAl t =kp

ksp t =kp

40 80

5  10
6 4  10
6

2.7  10
4 1.5  10
3

4  10
7 2  10
5

Kinetics of Alkene Polymerization Reactions

327

chain transfer reaction to a monomer dominates, the conclusion that found several experimental confirmations [613,614]. However, an opposite relationship between the probabilities of these two chain transfer reactions is observed when the same reactions are carried out at a low monomer concentration. Addition of other organoaluminum compounds to MAO also significantly affects both the kinetics of alkene polymerization reactions and the molecular weights of the polymers. Superficially, the biggest effect is observed when Ali-Bu3 is added to MAO. The productivity of ternary systems containing zirconocene complexes, MAO and Ali-Bu3 is significantly higher than in the absence of Ali-Bu3 [268,653,1281,1324,1327]. However, a kinetic study of this effect showed that the main advantage of the Ali-Bu3 addition is a decrease of the catalyst decomposition rate whereas initial activities of the systems with and without Ali-Bu3 are very similar [1324]. In contrast, the addition of AlEt3 to MAO doubles the initial activity of the catalysts but the deactivation rate also becomes very high and the catalysts become completely inactive after 1–2 minutes [1324]. The addition of various organometallic compounds to metallocene systems also affects the molecular weight of the polymers they produce. These effects are usually assigned to chain transfer reactions to organometallic compounds similar, e.g., to Reaction (3.61). Indeed, this reaction is strongly pronounced when either ZnMe2 or ZnEt2 is added to the reactions [653]. However, the addition of extraneous organometallic compounds also strongly affects the nature of active species in metallocene catalysis, as discussed in Section 6.1.1.4. This influence can be very significant. For example, when Ali-Bu3 is added to ethylene/norbornene copolymerization reactions with metallocene systems, the molecular weight of the copolymers increases rather than decreases because Ali-Bu3 forms a stronger complex with active centers and suppresses Reaction (3.61) with AlMe3 present in MAO [653]. Effect of monomer concentration: Two effects of the monomer concentration were examined separately, the effect on the reaction rate and the effect on the molecular weight of produced polymers. The literature on alkene polymerization reactions with metallocene catalysts gives many examples of the estimation of the monomer concentration effect on the catalyst activity. Unfortunately, these estimations often rely on a polymer yield after a particular reaction time as a measure of catalyst reactivity. As discussed in the previous sections, the use of this parameter is not justified for alkene polymerization reactions with metallocene catalysts which are often unstable, especially at increased temperatures, and which nearly always have a pronounced activation stage. Two more reliable kinetic measures of the monomer concentration effect are the monomer consumption rate at the stationary stage of polymerization reactions at low temperatures or during step-wise changes of a monomer concentration in the course of a single reaction, as shown in Figure 5.2 for a Ti-based Ziegler–Natta catalyst. When such measurements are indeed carried out, the polymerization reactions usually have the first-order dependence on the monomer concentration. Several examples of the first-order dependencies are described in Section 5.4.2 for lowtemperature polymerization reactions of different alkenes. Other examples of

328

Alkene Polymerization Reactions with Transition Metal Catalysts

dependencies close to the first-order law for reactions at higher temperatures are: 1. Ethylene polymerization with the t-Bu-Cp2ZrCl2-MAO system at 501C [1607] and with the (1,2,4-Me3-Cp)2ZrCl2-MAO system at 801C [613]. 2. MAO-cocatalyzed propylene polymerization with meso-Me2Si(3-Me-Ind)(Ind) ZrCl2 at 251C [624], with rac-Me2Si(Ind)2ZrCl2 and rac-Me2Si(2-MeInd)2ZrCl2 [711], with rac-C2H4(Ind)2ZrCl2 at 50, 65, and 751C [610], and with highly isospecific Me2Si[2-Me-4(a-naphthyl)-Flu]2ZrCl2 at temperatures from 0 to 901C [547]. The first-order dependence was also established in stopped-flow propylene polymerization experiments with the rac-Me2Si(Ind)2ZrCl2-Ali-Bu3[Ph3C]+ [CN{B(C6F5)3}2]
system at 251C [661]. 3. MAO-cocatalyzed 1-hexene polymerization reactions with Cp2ZrCl2 at –20 to 01C [326,336], with Me2Si(Ind)2ZrCl2 in at 0–601C [326,336,532], and with C2H4(Ind)2ZrCl2 [1608,1609]. However, a significantly higher reaction order with respect to the monomer concentration was reported in many publications as well, usually ranging from 1.5 to the second order [525,605,711,738,742,1568,1610]. Several explanations of the high reaction order were proposed. They include (a) complex equilibria involving active species and a monomer [605], (b) competing chain growth reactions and epimerization reactions of growing chain ends in polymerization reactions of 1-alkenes, and (c) a‘‘single-center, two-state catalyst’’ model (Scheme 5.2). In the latter model, an equilibrium exists between two states of any active center, one with a high propagation rate constant and another with a low propagation rate constant [1567,1569,1570]. Varying the concentration of monomers in a wide range usually does not affect the molecular weight of alkene polymers produced with metallocene systems under moderate conditions [203,326,620,1608,1609]. These results indicate that the dominant chain transfer reaction in these reactions is that to a monomer (Reaction (3.50)). Effects of 1-alkenes in ethylene polymerization reactions: Most copolymerization reactions of ethylene and various 1-alkenes with Ti-based Ziegler–Natta catalysts exhibit a pronounced kinetic effect, the addition of 1-alkenes results in a large increase of the ethylene consumption rate; as described in Section 5.7.1.2. This effect was also explored in ethylene/1-alkene copolymerization reactions with several homogeneous metallocene catalysts [309,597,921,1611,1612]. The results were not as straightforward as for the heterogeneous catalysts. In general, the addition effects of 1-alkenes to ethylene polymerization reactions are relatively small and depend on four factors, the type of metallocene complex, reaction temperature, concentration of the 1-alkene, and solubility of the copolymers in the reaction medium. Both polyethylene and ethylene/1-alkene copolymers are better soluble in toluene than in aliphatic hydrocarbons, and the polymer solubility increases with temperature and with an increase of a 1-alkene content in the copolymers. Even taking into account these differences in reaction conditions, the results are often inconsistent, both activation and deactivation effects were reported. The introduction of a 1-alkene into an ethylene polymerization reaction usually brings about a significant decrease in the molecular weight of the polymerization

329

Kinetics of Alkene Polymerization Reactions

products and even reveals the existence of two types of active centers in seemingly single-center systems such as the (1,2,4-Me3-Cp)2ZrCl2-MAO system [613]. The two centers are indistinguishable in ethylene homopolymerization reactions (Mw/Mn B2) but they have a different response to 1-hexene resulting in the appearance of a second Flory component in the GPC curves and an increase of the Mw/Mn ratio to B3.5. Temperature effects: Polymerization reactions of ethylene with metallocene catalysts often take place in two phases, the active centers remain in solution but the polymers precipitate from the reaction medium. The temperature effect on the activity of the catalysts strongly depends on the reaction conditions (see several examples in Tables 5.4 and 5.5). Some researchers found a maximum activity in the 30–801C range [641,1301], others report a progressive increase in activity with temperature [1250,1613], still others found a rapid increase at temperatures above 701C, when the polymers become soluble in the reaction medium [1614]. Because of this uncertainty, estimations of the effective activation energy of the Table 5.4 Temperature effects in ethylene homopolymerization reactions with Cp2ZrCl2-MAO systema [641,1301] Temperature ( 1C)

0 30 50 70 80 90 a

Productivity (kg/mol Zr  h  atm) 2

Mw

3.25  105 2.32  105 7.41  104 1.96  104 1.83  104 B7.3  103

3.8  10 1.6  103 1.5  103 1.7  103 1.5  103

Polymerization reactions in toluene at [Al]:[Zr] ¼ 1,070.

Table 5.5 Temperature effect in propylene homopolymerization reactions with Me2Si[2-Me4(a-naphthyl)-Flu]2ZrCl2MAO/SiO2 systema [547] Mv

[mmmm]

Homogeneous catalyst, CPr ¼ 0.6 M 60 5.9  105 75 1.2  106 90 9.6  105

4.77  105 2.22  105 1.13  105

0.969 0.965 0.965

Silica-supported catalyst, CPr ¼ 1.4 M 0 8.7  104 30 1.6  105 60 5.1  105 75 8.3  105 90 3.3  105

1.69  107 1.38  106 4.99  105 2.65  105 1.80  105

W0.99 W0.99 0.988 0.984 0.972

Temperature ( 1C)

a

Productivity (kg/mol Zr  h)

Polymerization reactions in toluene at [Al]:[Zr] ¼ 3.9  105.

330

Alkene Polymerization Reactions with Transition Metal Catalysts

polymerization reactions from polymer yields or reaction rates at different temperatures produce highly variable results. On the other hand, the temperature effect on the molecular weight of alkene polymers is unambiguous. The molecular weight of both polyethylene and polypropylene always monotonously and sharply decreases with temperature (Tables 5.4 and 5.5). Temperature effects in polymerization reactions of higher 1-alkenes are similar [203,262,325,336,532,620,1600,1614]. When polymerization reactions of higher 1-alkenes are carried out at temperatures from 80 to 1001C, the reaction products are usually liquid oligomers with the average molecular weight of B2,000 [532]. However, temperature effects in such reactions are more complex than one would expect for single-center catalysis. GPC analysis of polymers of 1-hexene prepared with the Cp2ZrCl2-MAO system revealed that at least two different types of active centers operate. One of the centers dominates at low temperatures, from
78 to
501C (it produces polymers of a relatively high molecular weight), the second type of active center is more active from 30 to 801C (it produces a polymer with a lower molecular weight), and both types of the centers coexist in the intermediate temperature range. The same dependence was observed for the C2H4(Ind)2ZrCl2-MAO system. The temperature ranges for the systems derived from Me2Si(Cp)2ZrCl2, Me2Si(Ind)2ZrCl2, and Me2C(Cp)(Ind)ZrCl2 are shifted; single active centers dominate from
80 to +301C but two centers coexist above 501C resulting in an increase of the Mw/Mn ratio to 4–5 [336]. Hydrogen effects: The introduction of hydrogen brings about two kinetic effects in alkene polymerization reactions with metallocene catalysts: 1. Hydrogen strongly reduces the molecular weight of any polymer produced with metallocene catalysts due to the chain transfer reaction to hydrogen, Reaction (5.7) [332,633–635,641,1615]. 2. Hydrogen does not affect much the activity of most metallocene catalysts in ethylene polymerization reactions but it increases the activity of the same catalysts in polymerization reactions of 1-alkenes. Both hydrogen effects are exemplified by the data in Table 5.6. Supported metallocene catalysts also have a high response to hydrogen as an agent of molecular weight control, similarly to unsupported catalysts of the same type [634,635]. The ratios of two rate constants, those of the chain transfer reaction to hydrogen and the chain growth reaction, kH t /kp, are very high, from 0.6–0.8 for the supported Cp2ZrCl2/MAO/SiO2 catalyst to 32–38 for the supported (Cp)(Flu) ZrCl2/MAO/SiO2 catalyst [1615]. The introduction of hydrogen often results in broadening of the molecular weight distribution of the polymers [638] or even the formation of polymers with a bimodal molecular weight distribution [330–332]. The activation effect of hydrogen in propylene polymerization reactions with isospecific metallocene catalysts is practically universal and ranges from B1.5 to B3 in terms of catalyst productivity at moderate temperatures [1616]. The most common explanation of this effect is a relief of steric congestion after the secondary alkene insertion reaction [461,463,516,517,1617], the latter produces the Cp2Zr
CHR bond of lower reactivity in the next alkene insertion step (Reaction (3.50)), and hydrogenolysis of this bond regenerates the highly active Cp2Zr
H

331

Kinetics of Alkene Polymerization Reactions

Table 5.6 Hydrogen effects in polymerization reactions of ethylene and propylene with metallocene catalysts PH (bar)

a

Productivity

Mw

Ethylene polymerization, Cp2ZrCl2-MAO system, 701C [641]. 0 13.9a 0.06 10.4a 0.20 9.5a 0.19 11.4a 0.38 9.1a

3.0  104 2.1  104 1.3  104 7.3  103 5.6  103

Propylene polymerization, rac-(3-t-Bu-Ind)2ZrCl2-MAO system, 501C [633]. 0 44.3b 1 47.5b 2 81.3b 3 61.7b 4 62.8b

8.8  104 5.0  104 4.2  104 1.5  104 B6  103

kg/mmol Zr  h  bar. kg/mmol Zr  h.

b

bond. However, Sacchi and Tritto did not find a clear correlation between the frequency of the secondary insertion in these propylene polymerization reactions and the magnitude of the hydrogen activation effect [1616]. 5.4.3.2.3. Catalyst deactivation. Metallocene catalysts prepared from zirconocene complexes and MAO are relatively stable only at mild temperatures but when the polymerization reactions are carried out at temperatures above 50–601C the activity of the catalysts gradually decreases over time. Rytter carried out detailed analysis of the deactivation stage in propylene polymerization reactions with two catalyst systems, rac-Me2Si(Ind)2ZrCl2-MAO and rac-Me2Si(2-Me-Ind)2ZrCl2-MAO, at temperatures from 25 to 1201C [711]. The kinetic curves in all these reactions exhibit an acceleration period which lasts from 2–3 to 10 minutes and is followed by a long decay period. The following observations about the decay phenomena can be made based on this research and similar results [517,547]:

1. The catalyst decay is not related to polymer accumulation in the reactor and the ensuing diffusion limitation. An interruption of the polymerization reaction with the rac-Me2Si(2-Me-Ind)2ZrCl2-MAO system at 801C for 40 minutes did not change the decay rate. An independence of the decay phenomena on the monomer presence was also observed in polymerization reactions with the racMe2Si(Ind)2ZrCl2-MAO system at 80 and 1201C. 2. Premixing the catalyst components at temperatures from 25 to 601C or for periods of time from 3 to 60 minutes does not affect much the kinetics of the decay stage. 3. Premixing the catalyst components at 1051C greatly deactivates the catalyst. 4. The deactivation rate measured at high [Al]:[Zr] ratios strongly depends on the concentration of the zirconocene complex. When the latter is low, B5  10
6 M,

332

Alkene Polymerization Reactions with Transition Metal Catalysts

the catalyst is very stable but when it is increased B12 times, the decay stage becomes pronounced. 5. The deactivation rate does not significantly change when the [MAO]:[Zr] ratio is increased from o2,000 to 6,500. 6. The deactivation rate increases with temperature. The rac-Me2Si(Ind)2ZrCl2MAO system is practically stable at temperatures up to 601C, it decays at a moderate rate at 80–901C (a B15–20% loss of activity after 1 hour), but it becomes very unstable at higher temperatures and loses all activity after B40 minutes at 1301C. 7. Carrying out polymerization reactions at different propylene partial pressures also does not affect the decay rate. This insensitivity to the monomer presence signifies that the formation of sleeping active centers with the last propylene unit in the secondary orientation cannot be an important reason for the activity decrease. No single explanation or a mechanism responsible for the catalyst decay has emerged yet. In the case of titanocene complexes activated either with organoaluminum chlorides, AlR2Cl or Al2R3Cl3, or with MAO, the main reaction leading to the catalyst deactivation is the reduction of the active center Cp2TiIV–R to a catalytically inactive Cp2TiIII species [1618,1619] (see Section 6.1.1.5). A similar, although a much slower reduction reaction of active centers was also proposed as a reason for the deactivation of zirconocene catalysts in the presence of MAO [1620–1623]. 5.4.3.2.4. Poisoning of active centers in metallocene catalysts. Metallocene catalysts are easily poisoned with CO [1594,1602]. If 14C-labeled CO is used in these reactions, a 14C tag appears in the polymer molecules. Measurements of the 14C content in polyethylene produced with the Cp2ZrCl2-MAO system at 601C showed that the entire original metallocene complex is converted into the active species in a reaction with MAO. The poisoning of the centers is caused by the CO insertion reaction into the (Cp)Zr+
CH2 bond; it is completed within 2–4 minutes [1602]. If the catalyst is kept with the mixture of ethylene and CO for longer periods of time, a slow copolymerization reaction of ethylene and CO occurs resulting in a gradual increase of the 14C content in the polymer [1624–1626]. Stopped-flow experiments of ethylene polymerization reactions followed by the introduction of CO (Section 6.1.2.1.8) provided a satisfactory mechanistic explanation of the reaction chemistry in CO-poisoned polymerization reactions of this type. Metallocene catalysts can be poisoned with CS2 as well [1627]. However, the catalyst activity in ethylene polymerization with the Cp2ZrCl2-MAO system does not cease completely even when the amount of added CS2 exceeds the concentration of Cp2ZrCl2 by a factor of 10. The amount of sulfur incorporated in the polymer chains also exceeds the maximum possible concentration of active centers in the catalyst [1627]. Apparently, the CS2 molecule not only coordinatively poisons the catalyst but inserts into the Zr
C bond, and a slow copolymerization reaction of ethylene and CS2 takes place.

Kinetics of Alkene Polymerization Reactions

333

5.4.3.2.5. Number of active centers in metallocene catalysts. When an ethylene polymerization reaction with the Cp2ZrCl2-MAO system is carried out at 301C for short periods of time (1 to 6 minutes) the molecular weight of the polymer steadily increases with time [291]. Principal kinetic parameters of this reaction were evaluated by using kinetic dependences presented in Section 5.3.1. The fraction of the active centers in the catalyst is high, from B100% of the Zr atoms when the reaction is carried out in toluene to B60% when it is carried out in n-decane, and the propagation rate constants are also high, B1,700 and 300 M
1  s
1, respectively. A commonly used method for the determination of the instant number of growing polymer chains in alkene polymerization reactions involves quenching the reactions with an alcohol, RuOH, where the H atom is either deuterium or tritium [139,641,1369,1372,1594,1595,1628]:

Cp2 Zrþ
CH2 2CHR
Polymer þ R0 OH ! Cp2 ZrðOR0 Þ2 þ H2CH2 2CHR2Polymer

(5.51)

The use of RuO3H ensures high sensitivity of this method at the expense of a detailed knowledge of the underlying chemistry. Landis showed that the use of RuOD instead of RuO3H in 1-hexene polymerization reactions with the racC2H4(Ind)2ZrMe2-B(C6F5)3 system gives an opportunity to determine three features of Reaction (5.51) [1369]: 1. The chemistry of the reaction was established, the DCH2
group is formed as the last unit in the chain. 2. The regiochemistry of the monomer insertion reaction into the Cp2Zr+
C bond prior to Reaction (5.51) is exclusively primary. 3. The concentration of growing polymer chains increases to B70% of the zirconocene molecules over a period of 70 seconds. Active species in metallocene catalysts rapidly react with Br2 [994]. Model tests with two precursors of the active centers, Cp2ZrMe2 and a dinuclear complex [Cp2Zr+(Me)(m-Me)B
(C6F5)3] (Reaction (6.8)), showed that they both cleanly react with Br2 with the formation of Cp2ZrBr2 and CH3Br. When polymerization reactions of ethylene and propylene were catalyzed by a combination of Cp2ZrMe2 and B(C6F5)3 and were quenched with Br2 after a short reaction time, 10 to 60 seconds, the following reactions were observed [994]: ½Cp2 Zrþ
CH2
CHR
Polymer ½MeBðC6 F5 Þ3 
þ Br2 ! Cp2 ZrBr2 þ BðC6 F5 Þ3 þ Br
CH2 2CHR2Polymer

(5.52)

The fraction of the active centers in these reactions was 4–10% of the metallocene complex for the Cp2ZrMe2-B(C6F5)3 system and 30–80% for the isospecific racC2H4(Ind)2ZrMe2-B(C6F5)3 system [994]. Kinetic parameters of ethylene polymerization reactions with several metallocene systems are given in Table 5.7.

334

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 5.7 catalysts

Kinetic parameters of ethylene polymerization reactions with metallocene

Catalyst system

Temperature (1C)

0 Cp2TiEtCl-AlEtCl2 Cp2ZrCl2-MAO 60 v-v 60 v-v 60 v-v 60 v-v 70 (Nme-Cp-2ZrCl2c- 50 MAO 30 rac–C2H4-IndH4-2ZrCl2-MAO rac-Me2Si(2-Me-4-Ph-Ind) (Cp)ZrCl2-MAO 60 Cp2ZrMe2-B(C6F5)3 20

Method

C (mol/mol M)

kp (M
1  s
1)

Reference

RO3Ha RO3Ha 14 CO 14 CO CS2 RO3H RO3H

0.16–0.23 0.87 1.0 0.13

– 760 2,400 – 1,200b B160

[1629] [1630] [1602] [1594] [1630] [1301] [1631]

14

0.65

–

[1595]

S-FMd Br2

0.23 0.04–0.10

2.8  106 –

[1574] [994]

CO

0.4 0.51

a

Reaction 5.51. Assuming C ¼ 1 mol/mol Zr. c Nme ¼ neomenthyl. d Stopped-flow method. b

5.5. Polymerization Reactions with Non-Metallocene Homogeneous Catalysts 5.5.1. Living-chain polymerization reactions Polymerization reactions with V(acac)3-based catalysts: Low-temperature propylene polymerization reactions with V(acac)3 activated with AlEt2Cl or Ali-Bu2Cl and with the V(2-Me-acac)3-AlEt2Cl system give the simplest example of polymerization kinetics with homogeneous non-metallocene catalysts [766,803,1632–1637]. The active centers form rapidly at
781C, and no chain transfer or catalyst-induced deactivation reactions take place under these conditions, all the signs of living-chain reactions. The polymers produced in these reactions have a very narrow molecular weight distribution; the Mw/Mn ratio is B1.05–1.2 [121,122,1632–1635]. However, an addition of hydrogen causes the chain transfer reaction and the Mw/Mn ratio increases to B2 [766,1635]. In the absence of hydrogen, both the polymer yield and the molecular weight of the polymers increase with time in a linear manner. These dependencies give the estimation of the propagation rate constant, kp B4–5  10
3 M
1  s
1. The efficiency of the V(acac)3-AlEt2Cl system is low, the fraction of Vatoms that produce polymerization centers ranges from 3–4 to 10–14% [1633,1635,1636]. On the other hand, nearly all V species form active centers in the V(2-methyl-2,4-butanedionato)3-AlEt2Cl system [1634] and the V(2-methyl-1,3-butanedionato)3-AlEt2Cl

335

Kinetics of Alkene Polymerization Reactions

system [122]. In contrast to the V(acac)3-AlEt2Cl system, ethylene polymerization reactions with the latter two catalysts do not exhibit any features of living-chain polymerization reactions, the molecular weight of the polymers does not depend on reaction time and the Mw/Mn ratio of the polymers is B2 [1636]. The very low value of the propagation rate constant in the case of the V(acac)3AlEt2Cl system gives an opportunity to study the chemistry of the polymerization reaction in more detail, by replacing propylene with a much less reactive 1-pentene [803]. Based on the kinetic analysis of reactions between the catalyst components, the most probable active species is an alkylated VIII complex, V(Et)(Cl)(acac)  Al2Et4Cl2. This complex is formed in ligand exchange/complex formation reactions between V(acac)3 and the AlEt2Cl dimer. The chain initiation reaction is the insertion of the first 1-alkene molecule into the V–C2H5 bond in the initial active center. As typical for many V-based catalysts, this reaction is not particularly regioselective (Section 3.4.1): (L)V-CH2-CHR-C2H5

(5.53)

(L)V-C2H5 + CH 2=CH-R (L)V-CHR-CH2-C2H5

When the 1-alkene in this reaction is 1-pentene, the chain growth reaction does not take place, both organovanadium species produced in Reaction (5.53) are formed rapidly and remain intact for nearly 5 hours. After the reaction system is decomposed with water, the only recovered organic products are 3-methylhexane and n-heptane in a B1:2 ratio. When the 1-alkene in this reaction is propylene, the polymer chains start growing. Kinetic analysis of this reaction showed that each chain growth step, which is mostly primary [429,430], proceeds in two distinct steps, the coordination of a propylene molecule at the V atom and the insertion of the coordinated molecule into the V–C bond, as shown in Scheme 5.3. As discussed below and in Chapter 6, such two-stage chain growth reactions are often assumed in alkene polymerization reactions with transition metal catalysts. However, this is the only example of a kinetic study where the experimental data indeed support the hypothesis of alkene coordination prior to insertion [803]. The most significant manifestation of kinetic steps in Scheme 5.3 is a saturation-type dependence between the polymerization rate and the propylene concentration. The value of the equilibrium constant in the first stage of Scheme 5.3 for the catalysts produced with different AlR2Cl cocatalysts is 0.3–0.4 M
1, and the value of the insertion rate constant at
781C is B0.05–0.09 s
1. When the VCl4-AlEt2Cl system is employed under the same conditions, the equilibrium constant is approximately the same but the insertion rate constant is significantly higher, B1.2 s
1 [1638]: Polymer (L)V

+

Polymer

Polymer CH2=C-CH 3

(L)V

(L)V

CH2=C-CH 3

Scheme 5.3 Kinetic scheme of propylene polymerization reaction with homogeneous V-based catalyst.

336

Alkene Polymerization Reactions with Transition Metal Catalysts

Polymerization reactions with bis(phenoxy-imine)Ti-based catalysts: The second example of alkene polymerization reactions with non-metallocene homogeneous catalysts allow the application of the kinetic arsenal developed for living-chain polymerization reactions even though these reactions are carried out at temperatures from 0 to 501C and last from several minutes to several hours. Two circumstances make this analysis possible: (a) these catalysts contain only one type of active center, and (b) propagation rate constants are significantly lower than for metallocene catalysts. Bis(phenoxy-imine) complexes of TiIV containing two bidentate ligands with C6F5 groups attached to each N atom (Scheme 1.2) form active species that are characterized by very low chain transfer rates and, therefore, polymerize ethylene and propylene under essentially living-chain conditions [342–344,781,783,784,789, 805,806,808]. Kinetic studies of ethylene polymerization reactions [808] and propylene polymerization reactions [784] with these catalysts demonstrated that all features typical for living-chain reactions are present when these reactions are carried out at 251C. The catalyst activity remains constant for over 1 hour in the ethylene polymerization reactions and for 3 hours in the propylene polymerization reactions and it very slowly decreases after that. The molecular weight increases with reaction time in a nearly linear manner in the same time ranges, and the molecular weight distributions of the polymers remain narrow, Mw/Mn ¼ 1.07
1.10. The value of the propagation rate constant for ethylene polymerization reaction at 251C estimated from these data [808] using Equation (5.33) is B1,400–1,900 M
1  s
1, it is comparable to the same value in metallocene catalysis (Table 5.7). The fraction of the original Ti complex converted to active center is close to 100% [808]. The syndiospecific polymerization of propylene with this catalyst also exhibits kinetic features of living-chain polymerization reactions [342–344,805]. Both the polymer yield and the molecular weight of polypropylene produced at 01C increase nearly proportionally to the reaction time, and the molecular weight distribution of the polymer remains narrow (Mw/Mn B1.08–1.11) [342]. Kinetic parameters of the propylene polymerization reactions at 0
251C are: kp ¼ 0.05– 0.06 M
1  s
1, [C]/[Ti] ¼ 0.4–0.6% [342,784]. However, when the same complex contains 3,5-difluoro-substituted benzene rings, chain transfer reactions occur much more readily and the polymer has the typical molecular weight distribution characteristics of a material synthesized with a single-center catalyst, Mw/Mn B2.5 [343]. Polymerization reactions with Ti diamide-based catalysts: Low-temperature propylene polymerization reactions with a Ti diamide complex LTiMe2 (Scheme 1.2) activated with MMAO or with silica- and alumina-supported MMAO give an interesting example of long-term living-chain reactions [1392]. These catalysts are very stable kinetically at 01C and the Mn value of the produced polymers increases with time in a linear manner for over 30 minutes resulting in the formation of atactic polymers of a very high molecular weight, with Mn B1.5  106. The kp value for the homogeneous catalyst system is B5 M
1  s
1 at 01C and when MMAO is supported on silica (this step increases the acidity of Al atoms in MMAO), the kp value increases to B20 M
1  s
1.

337

Kinetics of Alkene Polymerization Reactions

5.5.2. Kinetics of oligomerization reactions Ethylene oligomerization reactions catalyzed by square-plane Ni complexes containing bidentate ylide ligands (Scheme 1.3) represent one of a few examples when a straightforward kinetic analysis is meaningful. There are two reasons for this: (a) the true catalytic species are known, square-plane Ni complexes containing bidentate ylide ligands and the Ni
C bond [87,810] (Section 6.2.2) and (b) the kinetic scheme of the oligomerization reactions under stationary conditions is simple. It can be adequately represented by three reactions [158,811], the chain growth reaction (Reaction (5.54)), and two chain transfer reactions, to a monomer (Reaction (5.55)) and the b-H elimination reaction (Reaction (5.56)): ðLÞNi
ðCH2
CH2 Þn
H þ CH2 ¼ CH2
ðkp Þ !

(5.54)

ðLÞNi
ðCH2
CH2 Þnþ1
H ðLÞNi2ðCH2 2CH2 Þn 2H þ CH2 ¼ CH2 }ðkt E Þ !

(5.55)

ðLÞNi2CH2 2CH3 þ CH2 ¼ CH2ðCH2 2CH2 Þn
1 2H ðLÞNi2ðCH2 2CH2 Þn 2H }ðkt sp Þ !

(5.56)

ðLÞNi2H þ CH2 ¼ CH2ðCH2 2CH2 Þn
1 2H

Equation (2.9) gives the distribution of ethylene oligomers vs. the oligomerization degree n at a given ethylene concentration CE. The probability of chain growth g is g ¼ kp  C E =ðkp  C E þ kEt  C E þ ksp t Þ

(5.57)

Experimental data on the molecular weight distribution of the ethylene oligomers provide the basis for the estimation of g values and contributions of Reactions (5.55) and (5.56) to the overall chain transfer process [811]. They are shown in Table 5.8. The overall probability of chain growth remains approximately constant from 50 to 901C. However, a treatment of 1/g vs. 1/CE according to Equation (5.57) shows that, as one would expect, the relative probability of the b-H elimination reaction increases with temperature. Figure 5.8 shows that the activity of the homogeneous catalyst is the highest in the beginning of the reaction and gradually decreases with time. The deactivation Table 5.8 Kinetic parameters of ethylene oligomerization reactions with sulfonated Ni ylideAlEt2OEt systema [811]

a

Temperature gb (1C)

kEt =kp

50 70 90

B0.19 B0.10 B0.07 B0.24 – –

0.78 0.77 0.76

ksp t =kp ; M

Roc (M/min)

kp (M
1  min
1)

kd (min
1)

1.9  10
2 2.3  10
2 3.9  10
2

4.7  103 6.9  103 13.9  103

0.0056 0.024 0.044

Polymerization reactions at [Ni] ¼ 6.2  10
6 M and [Al]:[Ni] ¼ 200. Probability of chain growth reaction at CE B1 M. c Initial polymerization rate at CE B1 M. b

338

Alkene Polymerization Reactions with Transition Metal Catalysts

Figure 5.8 Consumption of ethylene (K, left) and reaction rate (right) vs. time in ethylene oligomerization reaction with sulfonated Ni ylide complex (Scheme 1.3) activated with AlEt2OEt at 701C [811].

reaction follows the first-order law and, most probably, is the result of the inherent instability of the Ni complex [811]. The deactivation rate constant kd listed in Table 5.8 is expectedly higher at high temperatures. The values of the propagation rate constant kp at different temperatures were calculated from initial polymerization rates assuming that the entire Ni ylide complex is initially catalytically active. The activation energy of the oligomerization reaction is 26.8 kJ/mol (6.4 kcal/mol). Ni ylide complexes do not oligomerize any alkene except for ethylene; however, they co-oligomerize ethylene with linear 1-alkenes [116,158]. Some of the kinetic data for ethylene/1-alkene co-oligomerization reactions are shown in Table 5.9. These data describe the expected trends: 1. The reactivity of 1-alkenes in primary insertion reactions into the Ni
C bond in the (L)Ni
CH2
CH2
Polymer chain is much lower than that of ethylene. These reactivities correspond to reactivity ratios r1 in copolymerization reactions equal to 57, 63, and 73, respectively. The decrease of 1-alkene reactivity with an increase of the length of its alkyl group is relatively small. 2. The reactivity of 1-alkenes in secondary insertion reactions into the Ni
C bond in the (L)Ni
CH2
CH2
Polymer chain is B40% lower than their reactivity in primary insertion reactions into the same bond. Overall, the regioselectivity of these Ni-based catalysts in 1-alkene insertion reactions is very poor. 3. The ethylene insertion reaction into the Ni
C bond in the (L)Ni
CH2
CHR
Polymer chain is 30–50% lower when R is a linear alkyl group compared to the case when R is a hydrogen atom.

339

Kinetics of Alkene Polymerization Reactions

Table 5.9 Kinetic parameters of ethylene/1-alkene co-oligomerization reaction with sulfonated Ni ylide-AlEt2OEt systema [117] 1-Alkene

Insertion reactions, kalkene =kEp p (L)Ni–CH2–CH2–Polymer+CH2QCH–R (L)Ni–CH2–CH2–Polymer+R–CHQCH2 Ethylene insertion reaction, k0p =kp (L)Ni
CH2
CHR
Polymer vs. (L)Ni
CH2
CH2
Polymer Chain transfer reaction, k0t =kt (L)Ni
CH2
CHR
Polymer vs. Ni
CH2
CH2
Polymer a

1-hexene

1-octene

1-decene

0.018 –

0.016 B0.01

0.014 0.009

B0.7

B0.6

B0.55

B2.1

B2.6

B2.5

Polymerization reactions at 901C, [Ni] ¼ 6.2  10
6 M. [Al]:[Ni] ¼ 200.

4. The scission of the Ni
C bond in a chain transfer reaction (either chain transfer to ethylene or the b-H elimination reaction) occurs with a 2–2.5 times higher probability when the last monomer unit in the oligomer chain is derived from an 1-alkene molecule. 5. The size of the alkyl group attached to the Ni atom in the growing polymer chain Ni
(CH2CH2)n
H does not influence relative reactivities of 1-alkenes in chain growth reaction (the kalkene /kEp ratios) or in chain transfer reactions [117]. p

5.5.3. Limiting kinetic steps in polymerization reactions Polymerization reactions with a-diimine complexes of Ni and Pd (Scheme 1.2) were the subject of detailed kinetic investigations [30,1639]. These studies revealed significant differences between the kinetics of the polymerization reactions with complexes of late-period transition metals and with metallocene catalysts. Judging by the width of the molecular weight distribution of polymers prepared with the former catalysts (see, e.g., Tables 3.61, 3.63, and 4.23) and by the shapes of Crystaf curves of the polymers [826], these homogeneous systems are single-center catalysts. When alkene polymerization reactions with these complexes are investigated at very low temperatures,
801C to
1301C, several stages of CQC bond insertion reactions are amenable to NMR analysis. The ‘‘resting state’’ in the chain growth of ethylene polymerization reactions with the complexes of Ni and Pd is the p-complex of the active species, the (L)M+–R cation, and an ethylene molecule, (L)(R)M+yCH2QCH2, the same as the first stage in Scheme 5.3 These complexes were experimentally observed both for the Pd catalysts at
801C [30] and for the Ni catalysts at
1101C [1639]. The rate-limiting step in the overall chain growth reaction sequence is the insertion of the coordinated ethylene molecule into the (L)M+–C bond (Reaction (5.58)); the step that was also observed by NMR. This step is immediately followed by the formation of another p-complex and the development of the agostic bond between the transition metal

340

Alkene Polymerization Reactions with Transition Metal Catalysts

atom and the b-C–H bond in the chain: ðLÞMþ
R    CH2 ¼ CH2 }ðslowÞ ! ðLÞMþ
CH2
CH2
R ðLÞMþ
CH2
CH2
R þ CH2 ¼ CH2 }ðfastÞ !

(5.58) (5.59)

ðLÞMþ
CH2
CH2
R    CH2 ¼ CH2

Reaction (5.58) is a zero-order reaction with respect to the ethylene concentration. Some kinetic data for this reaction are given in Table 5.10. The reactivity of the Ni complexes is much higher that that of the Pd complexes, and kinetic parameters of the first insertion reaction (into the (L)M+
CH3 bond) and of all subsequent insertion reactions are similar. However, it appears that relative rates of Reactions (5.58) and (5.59) change when the polymerization reactions are carried out at higher temperatures. Ethylene polymerization reactions with these catalysts are accompanied by a significant degree of branch formation via the chain-walking mechanism, as discussed in Section 3.5.3 (Reactions (3.96)–(3.98)). These reactions start with the b-H elimination step in the growing polymer chain and the formation of a p-complex, (L)Ni+
HyCH2QCH
Polymer. This step is possible only if the active center does not bear a coordinated ethylene molecule. The data on the content of methyl branches (and longer branches) in polyethylene prepared with a silica-supported Ni a-diimine complex at 301C show that the contents of all branches decrease as the ethylene concentration in the reaction increases [826]: CE (M) 0.21 Methyl branches (mol.%) 5.7

0.97 2.6

1.66 1.5

2.35 1.1

This dependence suggests that the isomerization reaction (after completion of Reaction (5.58)) and chain growth in Reaction (5.59) proceed at comparable rates. The propylene insertion reaction into the (L)Pd+
C bond proceeds in the secondary mode and follows the same kinetic rule [30] (Table 5.10). However, reactions between Ni complexes of the same type and propylene have a different Table 5.10 Kinetic parameters of ethylene polymerization reactions catalyzed by a-diimine complexes of Ni and Pd (Scheme 1.2) activated with MAO [30,1639,1640] Active centers Monomer

(L)Ni+
R (
70–901C) (L)Pd+
R (20–301C) (L)Pd+
R (
301C)

Insertion into (L)M+--CH3 bond

Insertion into (L)M+--CH2 -bond

kins (s
1)

DG¼ (kcal/ mol)

kins (s
1)

DG¼ (kcal/ mol)

Ethylene

3–10  10
4

13.3–13.7

10–13  10
4

13.5–14.0

Ethylene

0.6–1.9  10
3 17.3–18.4

0.9–3.4  10
3 16.9–18.6

Propylene

5.4  10
4

1.2  10
3

17.8

17.4

Kinetics of Alkene Polymerization Reactions

341

resting state, the b-H agostically stabilized complex (L)Ni+
CH2
CH(CH3)
R [1639]. The kinetically limiting stage of the overall insertion reaction in this case is the coordination step of a propylene molecule (the same as Reaction (5.59)), the step that is the first-order reaction with respect to the propylene concentration. The ratio between the free (L)Ni+
Polymer species and their p-complexes with propylene at
401C was estimated as B2,000 [1639]. Ethylene polymerization reactions with Pd a-diimine complexes of at 51C in chlorobenzene proceed in the living-chain manner [1641]. The molecular weight of the polymer increases in the linear manner with time over a period of several hours and can reach B2  105 while the molecular weight distribution remains very narrow, Mw/Mn ¼ 1.05–1.09. Because the resting state in this chain growth reaction is the p-complex (L)Pd+
PolymeryCH2QCH2, the rate of these polymerization reactions does not depend on the ethylene concentration. Livingchain polymerization reactions of propylene and 1-hexene can also be carried out with these catalysts at 01C [835], but the molecular weight of the produced polymers is significantly lower (2–4  104).

5.5.4. Single- vs. multi-center polymerization catalysis Kinetic features of ethylene polymerization reactions with catalysts derived from three types of complexes containing bis(imino)pyridyl ligands, the Co complexes, the V complexes, and the Fe complexes (Scheme 1.3) represent an interesting example of single- vs. multi-center catalysis in chemically similar catalysts [388,1418]. Ethylene homopolymers prepared with the Co catalysts of this type are linear and have a low molecular weight [816–818]. Their chain structure can be depicted as CH2QCH–(CH2–CH2)n–C2H5 [817,1421]. The activity of these catalysts increases nearly proportionally to PE [1421,1642]. GPC analysis of ethylene homopolymers prepared with one such CoII complex showed that 90–95% of their molecular weight distribution can be described by a single Flory component [388]. Its molecular weight, 5.3–5.4  103, is virtually independent on ethylene concentration (reaction at 251C, [Al]:[Co] B1,500). Overall, these results yield to a simple kinetic interpretation: both the chain growth reaction and the dominant chain transfer reaction, the chain transfer to ethylene (Reaction (5.5)), have the first-order dependence on ethylene concentration. The activation energy of the chain transfer reaction to ethylene is higher than that of the chain growth reaction by B3 kcal/mol. As a consequence, the average molecular weight decreases with temperature [388]; the effect that is common for most metallocene polymerization catalysts as well. The kinetic behavior of catalysts based on bis(imino)pyridyl complexes of V and Fe is much more complex; it is in many respects similar to the behavior of heterogeneous Ti-based polymerization catalysts. One example, the GPC curve of an ethylene homopolymer prepared at 01C with the FeII complex (Scheme 4.11, R ¼ Me, X1 ¼ X2 ¼ Br, X3 ¼ H) is shown in Figure 2.11. At least seven Flory components are required to represent this molecular weight distribution. These components can be combined into two groups, a low molecular weight fraction

342

Alkene Polymerization Reactions with Transition Metal Catalysts

containing three Flory components, and a high molecular weight fraction containing four Flory components: Low-MW fraction (B60%): Mw ¼ 7,200 (37%), 2,100 (37%), and B820 (26%) High-MW fraction (B40%): Mw ¼ 7.4  105 (16%), 3.2  105 (25%), 1.0  105 (30%), and 3.2  104 (29%)

High-temperature ethylene homopolymerization and copolymerization reactions with the V bis(imino)pyridyl complex activated with MAO also produce polymers with similar GPC curves containing two distinct polymer fractions, one with Mw of B1  104 and another B6  105 [1418]. The active centers producing two polymer components in Figure 2.11 are differently affected by reaction variables. The low molecular weight fraction is formed in a first few minutes of the polymerization reactions [1420,1424]. The molecular weight of each of the three Flory components constituting this fraction does not depend on the ethylene partial pressure [388,1424] indicating that chain transfer to a monomer (Reaction (5.5)) is the dominant chain transfer reaction. CO rapidly and irreversibly poisons the active centers producing this polymer fraction [1426,1428]; their number corresponds to 2–4% of the introduced Fe complexes [1428] and the propagation rate constant in ethylene polymerization reactions is very high (1–2  104 M
1  s
1) [1428]. The number of active centers in supported catalysts containing these FeII complexes is also B2–3% [1426]. The active centers producing the high molecular weight fraction (see Figure 2.11) are more stable and their reactivity is practically proportional to the ethylene concentration [388]. The molecular weight of every Flory component in this polyethylene fraction is also nearly independent of ethylene concentration, in agreement with the polymerization scheme represented by Reactions (5.3) and (5.5). These four Flory components are produced by kinetically (and, probably, chemically) similar types of active centers with different kinetic characteristics; kM t /kp values for the respective Flory components differ by a factor of 25–30. This difference suggests that the electronic/steric environment provided by various MMAO species coordinated to the Fe-containing active centers is substantially different. Molecular weights of Flory components in this fraction decrease with temperature [388], a change that is traditionally explained by a higher activation energy of Reaction (5.5) compared to that of Reaction (5.3); the DEact value is B5 kcal/mol. Two kinetic explanations are proposed in the literature to account for large differences in the molecular weights of Flory components in two polymer fractions obvious from Figure 2.11. According to one explanation, the low molecular weight fraction is formed at the early stages of the polymerization reactions as a consequence of a very high rate of the chain transfer reaction to free AlR3 compounds present in alkylalumoxanes (Reaction (5.9)) [1420]. Two distinct fractions of a greatly different molecular weight can only be formed if this chain transfer reaction completely dominates in the beginning of the polymerization reactions and, after all free AlR3 is consumed in it, the high molecular fraction starts forming because of a much lower rate constant of Reaction (5.5). An alternative explanation of this complex kinetic behavior proposes the existence of two

Kinetics of Alkene Polymerization Reactions

343

independent types of active centers, one rapidly converting to another [388]. It is presented in Section 6.2.3.

5.6. Synthesis of Alkene Block-Copolymers The formal definition of an alkene block-copolymer is any of macromolecules containing: 1. Two (or more) chemically linked blocks of different monomers ½
M1
M1
M1
M1
M1
M1
M1
M1 
½M2
M2
M2
M2
M2
M2
M2
 2. Chemically linked copolymer blocks of different compositions, e.g., blocks of a homopolymer containing exclusively M1 units and a M1/M2 copolymer ½
M1
M1
M1
M1
M1
M1
M1
M1 
½M2
M1
M2
M2
M1
M1
M2
 3. Linked blocks of two sterically different monomer sequences ½
meso-M1
meso-M1
meso-M1
meso-M1
meso-M1 
½rac-M1
rac-M1
rac-M1
rac-M1
rac-M1
 ðisotactic block 2 syndiotactic blockÞ or ½
meso-M1
meso-M1
meso-M1
meso-M1
meso-M1 
½rac-M1
meso-M1
rac-M1
rac-M1
meso-M1
 ðisotactic block 2 atactic blockÞ: For many years, the synthesis of such alkene block-copolymers remained an elusive goal. Over time, two approaches were developed to produce these polymers. The first technique is based on polymerization reactions with a single catalyst in the course of which the reaction conditions are rapidly changed; e.g., one monomer is replaced with another. The second approach involves polymerization reactions with two independent catalysts and an agent that transfers growing polymer chains from one type of active center to another.

5.6.1. Living-chain polymerization reactions and synthesis of alkene block-copolymers To implement the first technique for the synthesis of alkene block-copolymers, polymerization reactions should be carried out under living-chain conditions. In general terms, living polymerization reactions are reactions in which chain transfer reactions are practically absent, polymer molecules remain attached to active centers

344

Alkene Polymerization Reactions with Transition Metal Catalysts

throughout the duration of a given polymerization run, and the molecular weight of each chain increases with reaction time in a linear manner. Polymers produced under these conditions have a very narrow molecular weight distribution, as described in Section 2.2.1. Several examples illustrate the scope of catalysts potentially suitable for the synthesis of alkene block-copolymers by polymerizing alkenes under living-chain or quasi-living conditions (mostly at low temperatures): 1. V(acac)3-AlEt2Cl, V(2-Me-acac)3-AlEt2Cl, and V(2-Me-1,3-butanedionato)3AlEt2Cl systems; syndiospecific propylene polymerization reactions at
701C [121,122,1632–1635]. 2. Constrained-geometry monometallocene systems [Me2Si(Flu)(t-Bu-N)]TiMe2B(C6F5)3; propylene polymerization reactions at low temperatures, 0 to
501C [539,651,731,1591]. 3. The constrained-geometry [Me2Si(Me4-Cp)(t-Bu-N)]TiCl2-MAO system; ethylene/norbornene copolymerization reactions at B301C [148]. 4. Ti diamide complexes; propylene polymerization reactions at 01C [797]. 5. Combinations of TiIV bis(phenoxy-imine) complexes and MAO; ethylene polymerization reactions and syndiospecific propylene polymerization reactions [138,342–344,789,805,808]. 6. Ni a-diimine complexes; 1-alkene polymerization reactions at low temperatures [30,822,836]. 7. Ti indolide-imine complexes; ethylene polymerization reactions at –10 to 01C [1643]. Synthesis of block-copolymers in short-duration reactions: Several successful syntheses of di-block and tri-block copolymers were carried out at low temperatures, –70 to –781C, with soluble V catalysts [803,1202,1632–1637,1644]. Under these conditions, polymerization centers form rapidly and practically no chain transfer or catalyst deactivation reactions take place. The first block in these copolymers is that of polypropylene (it is predominantly syndiotactic [rr] ¼ 0.79) and the second block is a random ethylene/propylene copolymer [121,122, 1634,1637]. A modification of V(acac)3-based catalysts with anisole produces more syndiospecific catalyst systems and affords the synthesis of tri-block polymers (syndio-PP)-(random-E/P)-(syndio-PP), with a very narrow molecular weight distribution, Mw/Mn ¼ 1.2 [1644]. Similarly built block-copolymers of highly syndiotactic polypropylene and a random ethylene/propylene copolymer were produced with a catalyst consisting of the TiIV bis(phenoxy-imine) complex and MAO [342]. If the polymerization reactions are conducted at higher temperatures, 20–401C, when the average growth time of a polymer molecule is counted in seconds, two experimental techniques were found suitable for the synthesis of block-copolymers. The first one is a modified stopped-flow method [365,1645–1650]. Three solutions are prepared; two of them contain components of a catalyst system (e.g., one a catalyst and another a cocatalyst, both saturated with the same monomer), and the third solution is saturated with another monomer. When the first two solutions are mixed, the catalyst is activated and a homopolymer of the first alkene is formed in a

Kinetics of Alkene Polymerization Reactions

345

quasi-living chain mode. After the third solution is added to the reaction stream, a copolymerization reaction takes place. Although the total residence time in this method is always very short, 0.2–0.3 seconds, the duration of each polymerization step can be independently varied from B0.05 to 0.2 seconds resulting in blockcopolymers of different compositions. This approach produced the first true diblock material with a supported Ti-based catalyst activated with AlEt3, a block of isotactic polypropylene chemically linked to a block of a random ethylene/propylene copolymer [365,1646,1649,1650]. The block-polymer nature of the material was proved by a combination of GPC, analytical Tref, and solvent extraction techniques, as well as the direct observation of the material by atomic force microscopy [1651]. Yields of the block-copolymers can be greatly increased without compromising the nature of the materials if these polymerization reactions are carried out at high monomer concentrations [1646,1650]. Another technique for the synthesis of block-copolymer uses a tubular plugflow reactor with a very short residence time [144,1645]. Conceptually, this technology is very similar to the modified stopped-flow method. A homopolymerization reaction of one alkene proceeds in the first section of the tubular reactor and then the second monomer is added to the flow to produce a copolymer. Potentially, tri-block polymers (homopolymer)-(first copolymer)-(second copolymer), can also be synthesized with this technique [144]. As an example, a block-copolymer of the (PE)-(random E/P) type was prepared at 15–201C using the homogeneous VCl4Al2Et3Cl3 system at a residence time of 6–12 seconds [144]. The copolymers contain from 30 to 50% of the ethylene homopolymer block, which is produced during the first 1.5–2 seconds of the reaction. Even under these conditions, some chain transfer/termination reactions are unavoidable. Only 0.4 macromolecules per V atom in the catalyst are generated, the molecular weight distribution of the blockcopolymers is broader than that expected under living-chain conditions (Mw/ Mn ¼ 2.0–2.4), and from 10 to 20% of the macromolecules do not contain either a polyethylene block or a copolymer block [144]. The requirement to carry out alkene polymerization reactions in the livingchain mode is the main practical limitation to the synthesis of alkene blockcopolymers. Living-chain reaction conditions are possible only at low temperatures and at short reaction times. Both these conditions result in low yields of the polymer materials, one polymer chain per one active center, instead of several thousands polymer chains n common polymerization reactions. Synthesis of block-copolymers with homogeneous non-metallocene catalysts: Several nonmetallocene complexes of early-period transition metals with multidentate ligands exhibit a rare kinetic feature, they form living-chain or quasi-living-chain catalysts under relatively moderate polymerization conditions. Average times of the polymer chain growth with these catalysts range from several minutes to several hours, and technical issues of a rapid replacement of one monomer with another become surmountable. An obvious disadvantage of these catalysts is, of course, the same as for other catalysts used under living-chain conditions, low productivity, one active center in a giver run produces only one polymer chain. Bis(phenoxy-imine) complexes of TiIV and ZrIV containing two bidentate ligands (Scheme 1.2) are especially suitable for the synthesis of alkene block-copolymers

346

Alkene Polymerization Reactions with Transition Metal Catalysts

under moderate conditions. Ti complexes of this type with the C6F5 group attached to the nitrogen atom in each ligand produce MAO-cocatalyzed systems with very low chain transfer rates, they polymerize both ethylene and propylene under living-chain conditions at temperatures as high as 501C [342–344,781,783,784, 789,806,808]. These catalysts were employed for the synthesis of various alkene diblock- and triblock-copolymers [138,342,343,789,808,1397]. Another type of homogeneous non-metallocene catalysts suitable for the synthesis of alkene block-copolymers is produced from (L)ZrBz2 complexes with a tetradentate ligand L ¼
O
Ar
CH2
NR
C2H4
NR
CH2
Ar
O
[798–801]. The active species formed in the interaction of these complexes with ion-forming cocatalysts or MAO is moderately isospecific. Half-times of chain growth in ethylene and propylene polymerization reactions with these catalysts at 251C are B6 and B30 minutes, respectively, providing an ample time for changing monomers in the reactor [800,801]. (Polyethylene)-(isotactic polypropylene) blockcopolymers with nearly equal lengths of the blocks were prepared with the catalysts in sequential reactions at 251C, ethylene homopolymerization for 1.5 minutes followed by propylene homopolymerization for 20 minutes. The materials exhibit structural features expected for true block-copolymers, their molecular weight distribution is very narrow (Mw/Mn ¼ 1.2–1.3), their solubility and thermophysical properties are different from those of homopolymer mixtures, and their 13C NMR spectra show the presence of links between polypropylene and polyethylene blocks [800]. Both copolymers have two melting points, one for the polyethylene segment (B1251C) and another for the polypropylene segment. The latter value depends on the isospecificity of the catalyst, it varies from B120 to B1501C [800,801]. Synthesis of block-copolymers with Ziegler–Natta catalysts: The list of catalysts suitable for the synthesis of block-copolymers does not include heterogeneous Ziegler– Natta catalysts. The principal problem with carrying out living-chain polymerization reactions with these catalysts is a sort average growth time of alkene macromolecules; at most, a few seconds [1645,1646]. Numerous early attempts at the synthesis of alkene block-copolymers with Ziegler–Natta catalysts [6] were essentially a failure, the produced materials were mixtures of two homopolymers, e.g., mixtures of ethylene and propylene homopolymers instead of the desired polyethylene-polypropylene block-copolymers. Only after the researchers realized how high the values of propagation rate constants in these reactions are, rational (but purely academic) approaches to the synthesis of such block-copolymers were developed [365,1646,1649,1650]. However, even when the fastest possible techniques for a rapid change of monomers in the reactions were used (gas-phase reactions at moderate temperatures and at low monomer concentrations, monomer change cycles of 10–20 seconds), attempts at the synthesis of diblock (PE)-(randomE/P) copolymers still mostly produced mixtures of two homopolymers [1652]. Only when the monomer concentration was reduced 10 times and the average chain lifetime was considerably increased, the desired block-copolymers were produced but, of course, with very low yields [1653]. Commercial resins usually called ‘‘impact resistant polypropylene’’ are produced with highly isospecific supported Ziegler–Natta catalysts in two-reactor processes.

Kinetics of Alkene Polymerization Reactions

347

Isotactic polypropylene is generated in the first reactor, then the polymerization mass containing an active catalyst is transferred into the second reactor and an ethylene/propylene copolymer is produced there. Although the combined material is sometimes called a block-copolymer, conditions of its synthesis (70–801C, residence times in each reactor B60 minutes) are unsuitable for the formation of chemical links between polypropylene chains and copolymer chains. These resins can be better described as intimate mixtures of three incompatible materials, crystalline isotactic polypropylene, a random amorphous ethylene/propylene copolymer, and a semi-crystalline ethylene/propylene copolymer with very low propylene content [365,453,1654,1655] (see discussion in Section 5.7.2.3).

5.6.2. Synthesis of alkene block-copolymers using chain transfer agents The second approach to the synthesis of alkene block-copolymers is based on the use of two types of active centers with different polymerization abilities, which function either in parallel or in a sequence. When two types of active centers are employed simultaneously, a special chain transfer agent should be added to the polymerization reaction that transfers growing polymer chains between the two centers. In these cases, the synthesis of bock-copolymers does not require a rapid change of a monomer. Rather, the formation of block-polymers is determined by a difference between two kinetic parameters, the average chain-growth time of a single macromolecule (which can be very short) and the average time a given growing chain is attached to a given active center before it is transferred to another active center with different kinetic/structural parameters. Chien, Coates, and Brintzinger were the first to develop a method for the synthesis of blockhomopolymers relying on a simultaneous use of two metallocene complexes [162,391,642,650]. The chain transfer agents they used were organoaluminum compounds AlR3 (AlMe3 and Ali-Bu3), which are usually present in MAO and MMAO. The impetus for this technique was an observation made during kinetic studies of alkene polymerization reactions with metallocene catalysts, a relatively high rate of chain transfer reactions with these organoaluminum compounds. For example, propylene was polymerized with a mixture of two metallocene catalysts, C2H4(Flu)2ZrCl2 generated active centers producing atactic polypropylene and a bridged zirconocene complex, C2H4(Ind)2ZrCl2 or a Me2Si(Ind)2ZrCl2, generated active centers producing the isotactic polymer. MAO or MMAO were used as cocatalysts for both complexes. When AlMe3 or Ali-Bu3 are present in the cocatalysts in significant amounts, polymer mixtures consisting of three components were formed in these polymerization reactions, an isotactic homopolymer, an atactic homopolymer, and a block-copolymer containing long isotactic and atactic monomer sequences. If one of the metallocene complexes produces isotactic polypropylene and another syndiotactic polypropylene, the polymer mixture consists of a purely isotactic material, a purely syndiotactic material, and a blockcopolymer containing long isotactic and syndiotactic monomer sequences [650,1505]. In all these examples, a polymer chain is transferred from one metallocene active center to AlR3 (to form the AlR2-Polymer species) and then the

348

Alkene Polymerization Reactions with Transition Metal Catalysts

Al-Polymer group exchanges the polymer chain with another metallocene active center. This approach was extended to non-metallocene homogeneous catalysts. The Zr bis(phenoxy-imine) complex was used for the synthesis of an ethylene/1octene copolymer with a low 1-octene content, and a complex of Hf with a tridentate ligand was used for the synthesis of an ethylene/1-octene copolymer with a high 1-octene content. ZnEt2 was selected as a chain transfer agent that carries copolymer segments of different compositions between the two active centers [227]. When these ethylene/1-octene copolymerization reactions are performed at B1201C, the average chain-growth time of a single macromolecule is measured in a fraction of a second. Nevertheless, the high efficiency of ZnEt2 as a chain transfer reagent (Reaction (3.61) affords the synthesis of segmented copolymers containing blocks of ethylene/1-octene copolymers of different compositions [227]. A variation of this approach to the synthesis of alkene block-copolymers relies on catalysts with active centers that convert from one active state to another. One of them is a special group of nonbridged bis-indenyl metallocene complexes with bulky aromatic substituents in the second position of each cyclopentadienyl ring shown in Scheme 5.4 [675]. Metallocenium cations produced in reactions between these metallocene complexes and MAO have a relatively low rate of rotation of their Z5 ligands and, as a result, their structure alternates between the isospecific antirotamer and the essentially aspecific syn-rotamer. The rate of rotation (the transition rate from one rotamer to another) depends on temperature and the presence of substituents in the phenyl rings [676,677]. Waymouth and Coates used these catalysts to produce stereoblock copolymers consisting of monomer sequences of different stereoregularity. The average stereoregularity of the block-copolymers is low [mmmm]av values vary from 0.2 to B0.6. However, these materials have quite high melting points for such seemingly ‘‘nearly atactic’’ polymers, up to 1501C, indicating that the polymers have highly regular isotactic segments of a considerable length capable of crystallization, which are chemically linked to atactic segments [675,682]. Sita developed a similar transformation reaction between the functioning active center, a metallocenium ion Cp2Zr+
Polymer, and the dormant species, Cp2ZrMe2 [123,626]. In this case, the active centers can be switched on and off and there is sufficient time to change the monomer without deactivating the catalyst. When employed at low temperatures, this technique afforded the synthesis of several block-copolymers under quasi-living chain conditions, including [atactic poly(1-hexene)]-[isotactic poly(1-hexene)] and [atactic poly(1-hexene)]-[isotactic poly(1-octene)] materials.

MX2

anti-rotamer

Scheme 5.4 copolymers.

MX2

syn-rotamer

Non-bridged bis-metallocene complexes for synthesis of alkene block-

Kinetics of Alkene Polymerization Reactions

349

5.7. Polymerization Reactions with Solid and Supported Ziegler–Natta Catalysts Kinetic research of alkene polymerization reactions with heterogeneous Ziegler– Natta catalysts is mostly carried out with gaseous monomers, ethylene and propylene. Apart from the great commercial significance of polymers and copolymers prepared from these two monomers, their use has one important experimental advantage, the development of highly sensitive and precise mass flowmeters enables very accurate and reproducible kinetic studies of these reactions directly in the coordinates ‘‘rate of monomer consumption vs. time.’’ Additionally, the ease of handling gaseous monomers affords numerous kinetic manipulations that are virtually impossible with liquid monomers. For example, the concentration of a monomer or the reaction temperature can be repeatedly varied in the course of a single experiment or one gaseous monomer can be replaced with another gaseous monomer or with a monomer mixture. Alternatively, small quantities of gaseous modifiers (H2, see Figure 5.2) or poisons (CO, CO2) can be repeatedly added to the reactor and then removed from it, etc. All solid Ti-based and V-based Ziegler–Natta catalysts, both the early catalysts utilizing TiCl3 or VCl3, and modern supported catalysts, contain several types of active centers. Numerous manifestations of the multi-center nature of the catalysts are described in the literature: 1. Homopolymers and copolymers produced with these catalysts have broad molecular weight distributions (Section 2.5.1.1), in contrast to polymers prepared with homogeneous single-center metallocene and non-metallocene catalysts. Polymers produced with any heterogeneous Ti-based catalyst consist of at least four or five Flory components with different molecular weights. 2. When 1-alkenes are homopolymerized with these catalysts, different types of active centers produce macromolecules of different stereoregularity, as described in Section 3.2.3.2. Some of the centers are highly isospecific whereas other centers produce polymers of medium isotacticity or completely sterically irregular polymers. 3. When 1-alkenes and ethylene, or two different 1-alkenes, are copolymerized with these catalysts, different types of active centers produce copolymer molecules of different compositions, as described in Section 2.5.4. 4. In the course of the polymerization reactions, active centers of different types are often formed and decay at different rates, especially in reactions involving ethylene. Therefore, structural properties of combined polymers, their molecular weight distribution, steric composition, copolymer composition, etc., often vary with reaction time. 5. Active centers of different types can be poisoned by different chemical compounds. This difference in reactivity is widely used for the synthesis of 1-alkene polymers with an improved degree of fractional isotacticity and for the synthesis of copolymers of desired compositional uniformity. The existence of different types of active centers in heterogeneous Ziegler– Natta catalysts invites an important question about the centers that polymerize

350

Alkene Polymerization Reactions with Transition Metal Catalysts

different alkenes: is it possible to determine polymer components produced by the same centers in polymerization reactions of different alkenes (e.g., in homopolymerization reactions of ethylene and propylene)? This difficulty can be illustrated by comparing kinetic features of two homopolymerization reactions, those of ethylene and propylene, with the same d-TiCl3-AlEt3 system and at the same temperature, 801C. Figure 5.1 shows the kinetics of these two reactions. This example illustrates the most complex case of an alkene polymerization reaction. The d-TiCl3-AlEt3 system has low fractional stereospecificity, propylene polymers it produces contain fractions of different stereoregularity in comparable amounts. When used in ethylene/1-alkene copolymerization reactions, the catalyst produces copolymer fractions of different compositions [51]. Figure 5.1B shows that active centers in the ethylene homopolymerization reaction with this catalyst system are obviously formed more slowly and they are more stable than active centers in the propylene polymerization reaction. GPC curves of both homopolymers prepared with the d-TiCl3-AlEt3 system consist of several Flory components (these curves are quite similar to those shown in Figures. 2.9 and 2.10). The molecular weight of each Flory component in the polymers and its content are listed in Table 5.11. The polyethylene has a relatively narrow molecular weight distribution (Mw/Mn ¼ 5.3) that can be represented by five Flory components marked I to V in Table 5.11 in the order of increasing molecular weights. This means that the catalyst has, as a minimum, five different types of active centers. Tref and Crystaf data on ethylene/1-alkene copolymers presented in Section 5.7.1.2 show that the real number of different centers in the catalysts of this type is significantly higher. GPC analysis does not distinguish active centers if they produce Flory components of similar molecular weights even if other properties of the centers are significantly different. Polypropylene prepared with the same catalyst has a much broader molecular weight distribution (Mw/Mn ¼ 14.6). As a minimum, six Flory components are Table 5.11 Flory components in polypropylene and polyethylene produced with d-TiCl3-AlEt3 system at 801C [453]

a

Polymer

Mw

Mw/Mn

Component

Mw

Fraction (%)

Polypropylene

2.74  105

14.6

Polyethylenea

1.52  105

5.3

Iu IIu IIIu IVu Vu VIu I II III IV V

5.3  103 1.7  104 4.8  104 1.3  105 4.5  105 9.9  105 2.6  103 1.7  104 5.9  104 1.5  105 4.7  105

5.9 15.1 21.4 21.7 22.3 13.5 0.5 10.5 40.1 32.4 16.6

H2 was used to reduce molecular weight of polymer and to make it completely soluble in GPC analysis.

Kinetics of Alkene Polymerization Reactions

351

required to represent its GPC curve. In reality, the situation is even more complicated because the crude polymer mixture contains macromolecules with widely different levels of isotacticity. The data in Figures. 5.1 and in Table 5.11 provide the basis for the formulation of difficulties encountered in kinetic studies of heterogeneous Ziegler–Natta catalysts: 1. Is it possible to determine kinetic differences between different types of active centers in a given polymerization reaction, apart from obvious differences between their Skt/kp ratios (the latter are easily estimated from Mw values)? 2. Is it possible to determine any correspondence between active centers producing different Flory components in ethylene polymerization reactions and active centers producing different Flory components in propylene polymerization reactions with the same catalyst? Numerous kinetic studies of polymerization reactions of ethylene and propylene, as well as the studies of the copolymerization kinetics with solid and supported Ziegler–Natta catalysts were designed to answer these questions. They showed that many kinetic features of polymerization reactions of ethylene and 1-alkenes are different. It is therefore useful to examine them separately.

5.7.1. Ethylene polymerization reactions This section is subdivided into two subsections. The first one examines kinetics of ethylene homopolymerization reactions with Ti-based heterogeneous catalysts, and the second subsection discusses kinetic changes caused by addition of 1-alkenes to these reactions. Most experiments of this type are carried out under conditions when the incorporation of 1-alkenes in polyethylene chains is small, 1–5 mol.%, which is the range of the 1-alkene content typical for commercial LLDPE resins. However, kinetic effects of 1-alkenes in the copolymerization reactions are very significant even at this low incorporation level, and the investigation of these effects is an important source of information about differences in the kinetic behavior between different active centers in the catalysts. 5.7.1.1. Ethylene homopolymerization reactions 5.7.1.1.1. General kinetic behavior. Figure 5.1B shows the kinetic curve of an ethylene homopolymerization reaction at 801C with the d-TiCl3-AlEt3 system. Kinetics of ethylene homopolymerization reactions with supported TiCl4/MgCl2type catalysts are similar [453], the catalysts are initially inactive, the reaction rates rapidly increase with time and a nearly stable activity is reached after 10–12 minutes, depending on the catalyst type and reaction temperature. The nature of catalyst activation in the beginning of ethylene polymerization reactions is not yet well understood. Explanations proposed in 1960s–1970s, a low rate of the ‘‘initial’’ chain initiation reaction, the ethylene insertion step into the Ti–C bond formed in the

352

Alkene Polymerization Reactions with Transition Metal Catalysts

interaction between solid catalysts and cocatalysts, did not find an experimental confirmation. Currently, the most often cited explanation for the initial activation effect is a low rate of active center formation (Reactions (5.1) and (5.2) in Scheme 5.1). Indeed, the initial activation step for some catalyst systems can be convincingly explained by the composition of original solid catalysts. As an example, the TiCl4/MgCl2/di-iamyl ether catalyst activated with AlR3 exhibits significant acceleration periods in ethylene polymerization reactions associated with the removal of the ether from the solid catalyst component [1656]. AlEt3 is more efficient in removing the ether than Ali-Bu3 and the acceleration period is shorter. Pre-reaction of this catalyst, as well as pre-reaction of a similar TiCl4/MgCl2 catalyst, with AlEt3 or Ali-Bu3 shortens the acceleration region, and some AlR3-pre-reduced catalysts have very short acceleration periods. The formation rate of the centers is lower at lower temperatures, and acceleration periods are longer (see Figure 5.9). Another generally observed kinetic effect in ethylene homopolymerization reactions is a change in catalyst stability (after reaching the maximum activity) with temperature. As are rule, the reaction rate remains constant for significant periods of time at 30–501C but it steadily decreases at temperatures above 801C [1029,1656,1657]. Change of molecular weight distribution with time: Ethylene homopolymers prepared with the same Ti-based catalyst for different periods of time have different average molecular weights and different molecular weight distributions. Separation of their GPC curves into Flory components explains these variations. For example, GPC curves of ethylene polymers produced with the TiCl3/MgCl2/SiO2-AlEt3 system can be represented by five Flory components. The molecular weight of each Flory component does not depend on the duration of a polymerization run but relative

60

Rate, g/g cat min

50 40 30 20 10 0 0

20

40 60 Time, min

80

100

Figure 5.9 Kinetics of ethylene homopolymerization reactions at 501C (J) and 801C (K) with supported T|Cl3/MgCl2/SiO2 -AlEt3 system (Example 2E in Section 4.2.1.2) at PE ¼ 6.8 atm.

353

Kinetics of Alkene Polymerization Reactions

contents of the components do (the data for polymerization reactions at 801C in the presence of hydrogen [288]): Mw 3 minutes 240 minutes

B4  103 3.5% B0.5%

1.6  104 14% 6%

5.2  104 37.5% 34%

1.6  105 29% 37%

B6.5  105 16% 22.5%

Two Flory components with the lowest molecular weights are formed early in the polymerization reactions and their fractions steadily decline in long-duration experiments because the yields of Flory components with higher molecular weights increase over time. Such changes in relative populations of active centers producing different Flory components are common for many Ti-based supported catalysts [1658]. Its often-observed manifestation is a gradual increase of the average molecular weight of polyethylene with reaction time [1659]. This example demonstrates the complexity of ethylene homopolymerization reactions and underlines the futility of numerous early attempts to approach the polymerization kinetics with heterogeneous Ziegler–Natta catalysts as if the catalysts contained only one type of active center. Currently, this ‘‘single-center’’ approach to describing the polymerization kinetics of heterogeneous catalysts is being gradually replaced with more realistic schemes. The new kinetic approach takes into account the following features of the polymerization reactions. First, two independent kinetic processes occur in parallel with each type of active center, the polymerization reaction itself (which is very rapid; one polymer molecules grows for several seconds at the most) and transformation reactions of the active centers, their formation and gradual decomposition. All stages of the latter process develop much slower; some active centers do not decay at all. Second, kinetic features of the active centers, both those related to their generation and decay and those related to the polymer synthesis, are different for different types of active centers. These differences greatly complicate kinetic studies of ethylene polymerization reactions. At best, one can determine general features of the reactions for each type of active center, mostly by combining the kinetic data with the structural analysis of produced homopolymers and by selective poisoning of some types of active centers (although the selectivity is never perfect) and observing ensuing kinetic changes. 5.7.1.1.2. Effects of reaction parameters. The following reaction parameters are usually investigated in ethylene homopolymerization reactions: the ethylene concentration (partial pressure), temperature, the presence of hydrogen, and the presence of reaction poisons. The fifth variable, the concentration of a cocatalyst, if it is sufficiently high, has a relatively insignificant effect on the reaction kinetics [1660]. Effects of ethylene concentration: Two independent kinetic effects of the ethylene concentration (partial pressure) were examined, the effect on the reaction rate and the effect on the molecular weight. The second effect is unambiguous: when ethylene polymerization reactions are carried out at elevated temperatures in the absence of hydrogen, Flory components of the same average molecular weight are

354

Alkene Polymerization Reactions with Transition Metal Catalysts

present in polymers prepared at different ethylene concentrations [318]. This constancy indicates that the general equation for the polymerization degree of a polymer Equation (5.21) can be reduced to a simple expression applicable to each Flory component, 1=n  kM t =kp (Equation (5.23)). Table 5.12 lists molecular weights of Flory components and kM t /kp values for one supported catalyst. The kinetic effect of the ethylene partial pressure on the reaction rate in these reactions, the n value in the R ¼ keff PnE equation, is a more complex issue. Theoretical analysis of the alkene polymerization mechanism usually predicts the first-order effect of the monomer concentration on the alkene polymerization rate. This prediction is based on the assumption that the complex formation between the Ti atom in the active center and any alkene molecule is the rate-limiting step in the reaction sequence leading to the insertion of the CQC bond into the Ti–C bond (see Section 6.3.5). This prediction indeed holds true for the polymerization of 1-alkenes (Section 5.7.2.2) and styrenes. However, numerous kinetic studies showed that the monomer partial pressure has a much stronger effect on the ethylene polymerization rate. These data are especially convincing if they are produced in multi-stage gas-phase experiments, when the issue of ethylene solubility in a reaction medium can be avoided. Figure 5.2A shows the results of one such experiment with the TiCl4/MgCl2/SiO2-AlEt3 system at 801C. The ethylene concentration effect is very rapid, completely reversible, and large. In this particular case, the reaction order evaluated in several experiments at 80 and 901C is 1.5–1.6 [1550]. Difficulties inherent in the estimation of the reaction order in separate experiments under slurry conditions have led to a broad spread in the estimations, from the first order [1029, 1559,1560] to over 2.5 [1661], but the majority of the estimations centers at n ¼ 1.5–1.7 [1552,1661]. Temperature effect: The effective activation energy of ethylene polymerization reactions with various Ti-based heterogeneous Ziegler–Natta catalysts is usually estimated either in multi-stage experiments or by comparing stationary parts of kinetic curves. Typical DEact estimations range from B47 kJ/mol (B10 kcal/mol) to B54 kJ/mol (13 kcal/mol) [2,4,6,1662,1663]. Hydrogen effects: Similarly to the ethylene concentration effect, two independent kinetic effects of the hydrogen concentration are usually examined, the effect on the Table 5.12 Kinetic parameters of ethylene homopolymerization reactions at 851C with TiCl4/ MgCl2/SiO2-AlEt3 system [318] Flory component

I

II

III

IV

V

Approximate molecular weight in the absence of hydrogen Mw B2  104 5.1–5.5  104 1.4–1.6  105 4.5–4.9  105 1.5–1.6  106 Chain transfer vs. chain growth reactions 0.0027 0.00105 kM t =kp 0.20 0.0489 kH t =kp

0.00039 0.0163

0.000121 0.00634

0.0000347 0.00323

Reaction order with respect to CE n in R ¼ keff  CnE –

1.5–1.7

1.6–1.8

2.1–2.2

–

Kinetics of Alkene Polymerization Reactions

355

reaction rate and the effect on the molecular weight. The first line in Table 5.12 shows that molecular weights of most Flory components in ethylene homopolymers produced with heterogeneous Ti-based catalysts are very high. This phenomenon is common for catalysts of this type independently on the source of Ti compounds used for the catalyst preparation. Ethylene homopolymers prepared without hydrogen have a low commercial value except for a special grade of ultrahigh molecular weight HDPE resins. The use of hydrogen is the universal means of controlling the molecular weight of ethylene homopolymers. This method is both practically simple and very effective. The general expression for the average polymerization degree of each Flory component in alkene homopolymers prepared in the presence of hydrogen is described by Equation (5.22) and, under usually employed conditions when H H kM t  CM { kt  CH, it can be further reduced to 1/n E kt  CH/kp  CM. Equation 5.22 is usually used as the basis for the evaluation of the kH t /kp ratio in polymerization experiments in the presence of hydrogen. The kH t /kp values for different Flory components are listed in Table 5.12. On the molar basis, the chain transfer reaction to hydrogen proceeds at a 50–100 times higher rate than the chain transfer reaction to ethylene. The addition of hydrogen to ethylene polymerization reactions with any Ti-based heterogeneous catalyst has a strong effect on catalyst activity. As an example, Figure 5.2B shows changes of the instantaneous polymerization rate in a gas-phase polymerization reaction with the TiCl4/MgCl2/SiO2-AlEt3 system at 901C at a nearly constant ethylene partial pressure of 60–67 MPa as a function of hydrogen pressure. The addition of hydrogen does not change the kinetic profile of the polymerization reaction as such but it invariably results in a decrease of the reaction rate [1071,1073,1550,1664]. This hydrogen effect is completely reversible [1029,1071, 1073,1550,1665], as shown in Figure 5.2B. The reversibility suggests that the rate decrease is caused by some rapidly changing conditions at the active centers rather than by such possible long-lasting effects as the reduction of Ti atoms in the catalysts by hydrogen. Figure 5.10 shows the rate reduction as a function of hydrogen partial pressure (these results were produced in several multi-stage experiments similar to that in Figure 5.2B). The data demonstrate an obvious saturation effect, the relative activity of the catalysts decreases approximately two times, after which hydrogen does not affect the reactivity of the polymerization centers anymore. All catalysts of the TiCl4/MgCl2-type have several types of active centers that produce ethylene homopolymer fractions of widely different molecular weights. Hydrogen affects each type of active center in a similar way. This finding led to the development of a unified kinetic scheme of ethylene polymerization reactions with Ti-based catalysts presented in Section 5.7.1.3. Hydrogen also strongly and reversibly deactivates supported VCl4/MgCl2-AliBu3 and VOCl3/MgCl2-AlEt2Cl systems in ethylene homopolymerization reactions [1071,1075,1077,1666]. When the hydrogen concentration in an ethylene polymerization reaction with the former catalyst at 701C increases to B10 mM, the reaction rate decreases nearly six times. This effect is stronger than for similar Ti-based catalysts, but a further increase of the hydrogen concentration does not affect the catalyst activity anymore [1077], similarly to the effect on Ti-based

356

Alkene Polymerization Reactions with Transition Metal Catalysts

Figure 5.10 E¡ect of hydrogen partial pressure on relative polymerization rate in three multi-step gas-phase ethylene polymerization reactions with T|Cl4/MgCl2/SiO2 -AlEt3 system at 901C [1550].

catalysts shown in Figure 5.10. The measurement of the number of active centers in the VCl4/MgCl2-Ali-Bu3 system showed that the total number of active centers is not affected by hydrogen, confirming the equilibrium nature of the hydrogencaused deactivation. Effects of reaction poisons: Numerous polar organic and inorganic compounds poison active centers in ethylene polymerization reactions. The poisoning is rarely selective. As a rule, the introduction of a poisonous compound brings about a decrease in the ethylene polymerization rate without affecting the shape of the kinetic curve. One typical example is the effect of PhSi(OEt)3 on the ethylene homopolymerization reaction with the TiCl4/MgCl2/SiO2-AlEt3 system in the absence of hydrogen. The introduction of the silane (which is an effective modifier of propylene polymerization catalysts of the 4th generation (see Section 5.7.2.2)) in an amount of 5 mol.% with respect to the amount of the cocatalyst results in a productivity decrease by B55% [453]. 5.7.1.2. Ethylene/1-alkene copolymerization reactions The kinetics of ethylene/1-alkene copolymerization reactions is quite different from the kinetics of ethylene homopolymerization reactions. Figure 5.11 compares kinetic curves of three reactions with the same catalyst and under the same reaction conditions, ethylene homopolymerization and two ethylene/1-hexene copolymerization reactions [318]. This comparison, as well as the data presented

Kinetics of Alkene Polymerization Reactions

357

Figure 5.11 Kinetics of ethylene homopolymerization reaction and ethylene/1-hexene copolymerization reactions with T|Cl4/MgCl2/SiO2 -AlEt3 system at 851C and CE ¼ 0.55 M [318]. Content of 1-hexene in copolymers (bottom to top): 0, 0.6, and 1.7 mol.%.

below in Section 5.7.2.3, demonstrates why the joint kinetic analysis of three types of polymerization reactions, ethylene homopolymerization, ethylene/1-alkene copolymerization, and homopolymerization of 1-alkenes (usually, propylene), provides the most significant information on differences between active centers in heterogeneous catalysts. Molecular weight distribution, homopolymers vs. copolymers: Table 5.13 compares two molecular weight distributions, those of an ethylene homopolymer and an ethylene/1-hexene copolymer prepared with the TiCl4/MgCl2/SiO2-AlEt3 system. Molecular weights of individual Flory components in the copolymer are only slightly lower than molecular weights of the respective Flory components in the homopolymer (Flory component I is very small in the homopolymer). This is a common situation for copolymerization reactions of ethylene and 1-alkenes with Ti-based catalysts: the same types of active centers that homopolymerize ethylene also copolymerize it with various 1-alkenes and produce Flory components with similar molecular weights. The principal difference between two types of reactions is higher yields of low molecular weight fractions, Flory components I, II, and III, with respect to the yields of high molecular weight fractions, Flory components IV and V (Table 5.13). Copolymerization properties of different active centers: Tref and Crystaf analyses of ethylene/1-alkene copolymers prepared with various heterogeneous Ziegler–Natta catalysts demonstrate that different types of active centers exhibit very large differences in their ability to copolymerize 1-alkenes with ethylene. These differences translate into large differences in the 1-alkene content in different

358

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 5.13 GPC analysis of polyethylene and ethylene/1-hexene copolymer produced with TiCl4/MgCl2/SiO2-AlEt3 system at 851C [318] Copolymer Ccop Hex ¼ 0.9 mol.%

Homopolymer Flory component

Mw

Fraction (%)

Flory component

Mw

Fraction (%)

I II III IV V

– 9.4  104 1.45  105 4.52  105 1.55  106

– 4.8 15.2 45.2 34.8

I II III IV V

1.3  104 4.6  104 1.41  105 4.37  105 1.38  106

4.0 13.2 25.2 33.6 24.0

Table 5.14 Analysis of GPC and Crystaf data for ethylene/1-hexene copolymer produced with TiCl4/MgCl2/dibutyl phthalate system at 851C [453,1655] GPC data

Crystaf data

Flory component

Mw

Fraction (%)

Component

Fraction (%)

CHex (mol.%)

I II III IV V

4.6  103 1.54  104 4.71  104 1.28  105 3.64  106

1.0 12.1 42.0 34.7 10.3

Sol. fraction E+F C+D B A

23.8 24.9 12.5 16.1 22.7

W15 5.0 2.3 0.7 B0.4

copolymer fractions. Two relevant examples are given in Table 2.13. One more example of these differences is shown in Table 5.14. The table presents the data on the molecular weight distribution and the compositional distribution of an ethylene/1-hexene copolymer produced with the TiCl4/MgCl2/dibutyl phthalateAlEt3 system (a typical catalyst for polymerization of propylene and other 1-alkenes). Not only different active centers produce material with widely different molecular weights but, obviously, they have very different abilities to copolymerize 1-alkenes with ethylene. This phenomenon is common for all Ti-based heterogeneous catalysts [153,315–318,1382]. Correlation between molecular weight distribution and compositional distribution: Comparison of GPC data for ethylene/1-alkene copolymers (see, e.g. in Tables 5.8, 5.13, and 5.14 and in Figure 2.10), Crystaf data (Table 5.14, Figure 2.15), and Tref data (Figure 5.12 and Table 2.14) serves as the basis for a correlation between Flory components and the Tref/Crystaf components in a given copolymer. Numerous data for different ethylene/1-alkene copolymers show that Flory component I, the copolymer component with the lowest molecular weight, always

359

Kinetics of Alkene Polymerization Reactions

100 A 80

dH/dT

60

40

20

0 30

40

50

60

70

80

90

100

110

40

50

60

70

80

90

100

110

150 B

dH/dT

100

50

0 30

Temperature °C

Figure 5.12 Analytical Tref curves of two ethylene/1-butene copolymers produced with T|Cl4/MgCl2/SiO2 -AlEt3 system at 801C and their resolution into elemental components [211]. Reaction time 5 minutes (A) and 40 minutes (B).

360

Alkene Polymerization Reactions with Transition Metal Catalysts

has a very high 1-alkene content, W10–15 mol.%. This material is amorphous and cannot be analyzed by Crystaf or Tref methods [232,316–318,453]. On the opposite side of the compositional distribution spectrum, Flory components IV and V, the components of the highest molecular weights, always have very low 1-alkene contents, p1 mol.%, and they have clear counterparts in Tref and Crystaf components A and B, respectively. These two particular types of active centers exist in all Ti-based heterogeneous catalysts. The ability of both centers to copolymerize 1-alkenes with ethylene is poor and they produce two distinct high molecular weight copolymer fractions. A comparison of the molecular weight distribution and the compositional distribution in the middle of the distribution range (Table 5.14) is more difficult. It appears that Flory component II corresponds to two Tref/Crystaf components, E and F, and Flory component III corresponds to other two Tref/Crystaf components, C and D [211]. Each of these Flory components is a mixture of macromolecules produced by a pair of active centers. In both cases, the centers differ in terms of their copolymerization ability (as follows from the copolymer compositions) and, therefore, can be identified with Tref or Crystaf methods. However, each of the pairs of active centers gives rise to a single Flory component simply because respective copolymer fractions have similar average molecular weights and cannot be separated by GPC. The reciprocal dependence between the 1-alkene content in individual Flory components and their molecular weight obvious from Table 5.14 is common for all ethylene/1-alkene copolymers produced with Ti-based heterogeneous catalysts, including ethylene/1-butene copolymers [126,211], ethylene/1-hexene copolymers [211,316–318,453], and ethylene/1-octene copolymers [130]. Change of compositional distribution with reaction time: Different types of active centers in ethylene homopolymerization reactions are formed and decay at different rates, as described in Section 5.7.1.1. The same differences in the kinetic behavior were observed in ethylene/1-alkene copolymerization reactions. The centers that produce low molecular weight copolymer fractions with high 1-alkene contents are formed rapidly but are less stable compared to the centers that produce high molecular weight copolymer components containing low amounts of 1-alkenes [211,453]. As an illustration, the following contents of Tref fractions with high and low contents of 1-butene were measured in ethylene/ 1-butene copolymers produced with the TiCl4/MgCl2/SiO2-AlEt3 system at 801C [211]: Reaction time (minutes) Tref components E+F (CB from 5 to 9 mol.%) Tref components A+B (CB from 0.3 to 1 mol.%)

5 30% 29%

40 10.5% 52.5%

Kinetic effects of 1-alkene addition: Figure 5.11 shows that the copolymerization reactions differ from ethylene homopolymerization reactions in three respects: (a) the active centers are formed much faster in the copolymerization reactions, (b) reaction rates are significantly higher in the beginning of the copolymerization reactions (after the maximum rate is reached), and (c) catalysts become less stable. If such

Kinetics of Alkene Polymerization Reactions

361

copolymerization reactions continue for longer periods of time, 2.5–3 hours, the rates in the homopolymerization and the copolymerization reactions become similar. This activation effect of 1-alkenes at the early stages of ethylene polymerization reactions is common for all Ti- and V-based heterogeneous catalysts [291,315–318, 1031,1227,1611,1612,1661,1667–1674]. Even 1-alkenes that have very low activity in copolymerization reactions with ethylene, such as 3-methyl-1-butene, activate the catalysts [275,1675]. On the other hand, ‘‘inert’’ alkenes, including 2-substituted 1-alkenes, alkenes with internal double bonds, and cycloalkenes, as well as 1-alkenes with strongly hindered double bonds, such as 3,3-dimethyl-1-butene, do not affect the catalyst activity or may even slightly reduce it [1612,1676]. The magnitude of the activation effect depends on the moment when a 1-alkene is added to an ethylene polymerization reaction. The activation is the highest when a 1-alkene is present in the reaction from the beginning but it becomes progressively less pronounced when a 1-alkene is added at the late stages of the same reactions [1611, 1661,1669,1673,1674]. A combination of the kinetic data (Figure 5.11) and the data on the molecular weight distribution of the copolymers shows that different types of active centers are activated in the presence of 1-alkenes to a different degree. The centers that copolymerize 1-alkenes with ethylene well (they produce Flory components I–III and Tref/Crystaf components C–E) are significantly activated in the presence of 1-alkenes. On the other hand, the centers that polymerize 1-alkenes with ethylene poorly (they produce Flory components IV and V and Tref/Crystaf components A and B) are not affected by the presence of 1-alkenes [318]. As described above, different types of active centers in Ti-based heterogeneous catalysts are formed and decay at different rates. Differences in stability of different types of centers, which are noticeable even in ethylene homopolymerization reactions, become even more pronounced in ethylene/1-alkene copolymerization reactions. Figure 5.13 shows the separation of the kinetic curve of an ethylene/ 1-hexene copolymerization reaction with the TiCl4/MgCl2/SiO2-AlEt3 system at 851C into kinetic curves of five different types of active centers producing Flory components I–V [211,317,318,453]. Table 5.15 gives estimations of kinetic parameters for each type of center (see Scheme 5.1 for definitions). Both their intrinsic activity (represented by the effective rate of ethylene consumption, keff) and stability (represented by the rate constant of catalyst deactivation, kd) differs very significantly for active centers producing different Flory components. The centers producing Flory components IV and V usually exhibit a very stable kinetic behavior (the respective kd values are small), and their activity is virtually unaffected by the presence of 1-alkenes. The centers producing Flory components I, II and III decay much faster, and their activity markedly increases in the presence of 1-alkenes. This difference in the sensitivity of different centers to 1-alkenes also explains the differences in kinetic profiles of ethylene homopolymerization reactions and ethylene/1-alkene copolymerization reactions shown in Figure 5.11. The main apparent reason for the low stability of the active centers is the presence of tertiary C
H bonds in the b-position to the Ti atom in the active centers, either in 1-alkene homopolymerization reactions or in ethylene/1-alkene copolymerization reactions. Centers

362

Alkene Polymerization Reactions with Transition Metal Catalysts

Figure 5.13 Detailed kinetics of ethylene/1-hexene copolymerization reaction at 851C with T|Cl4/MgCl2/SiO2 -AlEt3 system [318]. Table 5.15 Kinetic parameters of active centers producing different Flory components in ethylene homopolymerization and ethylene/1-hexene copolymerization reactionsa [318,1677] Flory component

Homopolymerization reaction keff (l/g cat  min) Copolymerization reaction keff (l/g cat  min) kd (min
1) Reactivity ratio r1 a

I

II

III

IV

V

0.03

0.08

0.13

0.21

0.16

0.21 B0.07 45–55

0.24 0.018 100–115

0.44 0.013 120–140

0.25 0.004 B800

0.13 B0.001 B1,500

Catalyst system TiCl4/MgCl2/SiO2-AlEt3, 851C.

producing Flory components IV and V are kinetically stable in ethylene/1-alkene copolymerization reactions simply because these centers incorporate 1-alkene molecules into polymer chains very poorly. The differences in stability of different centers also account for the effect of aging of Ti-based catalysts. This effect was clearly observed in stopped-flow ethylene/propylene copolymerization experiments with TiCl4/MgCl2/ester-AlEt3 systems [365,1678]. Not only the activity of the catalysts decreases after aging at 301C for 20 minutes (because the centers producing Flory components I and II are significantly deactivated by that time), but the content of propylene in the copolymers decreases as well, similarly to other ethylene/1-alkene copolymerization reactions. Hydrogen effects: Hydrogen produces two effects in ethylene/1-alkene polymerization reactions. The first effect, a decrease in the molecular weight of the

363

Kinetics of Alkene Polymerization Reactions

copolymers, is similar to that observed in ethylene homopolymerization reactions (Section 5.7.1.1.2). The second effect is also similar, the addition of hydrogen leads to an immediate depression of the polymerization rate [318]. However, this second effect is more nuanced than that in ethylene homopolymerization reactions. Hydrogen depresses the activity of each active center in the homopolymerization reactions to approximately the same degree. However, when ethylene/1-alkene copolymerization reactions are carried out under the same conditions, the activity of the centers capable of copolymerizing ethylene with 1-alkenes well (the centers producing Flory components I, II, and III) is not affected by hydrogen anymore whereas the activity of the centers producing Flory components IV and V is still strongly depressed [318]. Because Flory components IV and V always contain very small amounts of 1-alkenes, the deactivation of active centers producing these copolymer fractions leads to an increase of the average content of 1-alkenes in the copolymers [318]. Temperature effects: Superficially, temperature effects in copolymerization reactions of ethylene with 1-alkenes are well known: as the temperature increases, average molecular weights of the copolymers decrease and the average 1-alkene content in them slightly increases [1677]. The following results for ethylene/1hexene copolymerization reactions with the TiCl4/MgCl2/SiO2-AlEt3 system at the same molar monomer ratio illustrate this trend [232,1677]: Temperature (1C) Mw: cop C Hex ; mol.%

75 1.71  105 1.2

85 1.50  105 1.8

95 1.14  105 2.6

Parallel analysis of Crystaf and GPC data for these copolymers provides a much more detailed information. Table 5.16 gives the results of GPC analysis for two of the copolymers. They show that the main temperature effect on multi-center Ziegler–Natta catalysts is a redistribution of the yields of different copolymer components. The reactivity of centers producing Flory components IV and V (the components with low 1-hexene contents, 0.3–0.6 mol.%) is depressed at the higher temperature whereas molecular weights of all Flory components decrease with Table 5.16 Temperature effect on molecular weight distribution of ethylene/1-hexene copolymers produced with TiCl4/MgCl2/SiO2-AlEt3 system, GPC data [232] Reaction temperature (1C) 75 Flory component Mw

I II III IV V

– 1.83  104 6.26  104 1.60  105 4.12  105

95 Fraction (%)

Mw

Fraction (%)

5.9 31.3 43.0 19.9

6.6  103 1.80  104 5.51  104 1.39  105 3.43  105

1.8 12.1 38.6 35.6 11.8

364

Alkene Polymerization Reactions with Transition Metal Catalysts

temperature only slightly (Table 5.16) and the compositions of elemental Crystaf components remain practically unchanged [232,1677]. This single change in relative yields of different copolymer fractions provides a satisfactory explanation for most experimentally observed temperature effects in the copolymerization reactions. Effects of cocatalysts: The choice of a cocatalyst influences the performance of Tibased Ziegler–Natta catalysts in many respects. The principal effect of cocatalysts (or cocatalyst/catalyst ratios) on the molecular weight of ethylene/1-alkene copolymers is usually attributed to the chain transfer reaction to a cocatalyst (Reaction (3.24)). This reaction is expected to produce a small decrease of the average molecular weight in the presence of high amounts of organoaluminum cocatalysts, as an example of ethylene/1-hexene copolymerization reactions at 851C with the TiCl4/ MgCl2/SiO2-AlEt3 system shows [1677]: [AlEt3] (mmol) Productivity (g/g cat  h) M av w

4.2 ([Al]:[Ti] ¼ 130) 4,400 1.27  105

21 ([Al]:[Ti] ¼ 560) 3,120 1.03  105

Detailed analysis of the molecular weight distribution data confirms this explanation [1677]. First, AlEt3, when used at a high concentration, suppresses the activity of every type of active center approximately to the same degree (this explains the loss of catalyst activity). Second, AlEt3 indeed acts as a chain transfer agent. Estimations of this effect using Equation (5.21) give the following kAl t =kp values: Flory component kAl t /kp

I 0.304

II 0.051

III 0.012

IV 0.006

V 0.003

If one takes into account that cocatalysts are usually used in polymerization reactions at much lower concentrations than monomers, these kAl t =kp values signify that, in practical terms, one can neglect the chain transfer reaction with a cocatalyst in comparison with two major chain transfer reactions, those to hydrogen and to a monomer. The ability of a given active center to participate in the chain transfer reaction to a cocatalyst is apparently related to its ability to copolymerize 1-alkenes with ethylene, the kAl t =kp values parallel the reciprocals of the reactivity ratio r1 for the same centers (Table 5.15). The cocatalyst effects are much larger when two types of cocatalysts are compared, AlR3 and AlR2Cl [1677]. In terms of average effects, three significant differences between AlR3 and AlR2Cl are usually observed, AlR2Cl-activated catalysts have lower productivities; they produce polymers with significantly higher average molecular weights and with broader molecular weight distributions [4,6,1677,1679]. One example for ethylene/1-hexene copolymerization reactions with a TiCl4/MgCl2/SiO2 catalyst at 851C illustrates these changes [1677]: Cocatalyst Productivity (g/g cat  h) Mw (and Mw/Mn)

AlEt3 5,920 1.06  105 (4.1)

AlEt2Cl 2,560 1.52  105 (5.6)

Kinetics of Alkene Polymerization Reactions

365

Analysis of the molecular weight distribution of ethylene/1-alkene copolymers prepared with AlR3- and AlR2Cl-activated catalysts explains many of the observed differences. Figure 5.14 compares GPC curves of two ethylene/1-hexene copolymers prepared under the same conditions and with the same catalyst activated with AlMe3 and AlMe2Cl. In both cases, five Flory components adequately represent the molecular weight distribution of the copolymers. As obvious from Figure 5.14, average molecular weights of Flory components I through IV and their relative contributions are not affected by the cocatalyst replacement. GPC peaks of four of the components are similarly spaced and their combined Mw/Mn value remains practically the same (4.2 vs. 4.9). The main difference between the two cocatalysts is the properties of Flory component V. When AlMe3 is replaced with AlMe2Cl, the Mw value of this component increases Bthree times, from B4  105 to B1.2  106, and its fraction increases 1.7 times. These two changes lead to an increase of the Mw/Mn value for the total copolymer from 5.5 to 10.9. A replacement of AlEt3 and AlEt2Cl produces similar effects in such copolymerization reactions, molecular weights of Flory components I–IV and their relative contents remain approximately the same, but the molecular weight of Flory component V nearly doubles [1677]. It is reasonable to assume that active centers producing Flory component V are the only type of centers directly influenced by the chemical nature of organoaluminum cocatalysts. Activation of supported catalysts of the TiCl4/MgCl2 type with cocatalyst mixtures containing combinations ZnEt2 with AlEt3 or with Ali-Bu3 has a similar effect on the distribution of active centers in the catalysts [375]. ZnEt2 is usually regarded as an effective chain transfer agent in alkene polymerization reactions, similar to hydrogen. However, a combination of GPC and Tref data indicates an additional mechanism of its action. The addition of ZnEt2 to the cocatalysts brings about two changes, the molecular weight distribution of ethylene/1-hexene copolymers greatly broadens (the Mw/Mn ratio increases from B5 to 21–25) and

Figure 5.14 GPC curves of ethylene/1-hexene copolymers prepared with T|Cl4/MgCl2/SiO2 catalyst activated with AlMe3 and AlMe2Cl, and separation of curves into Flory components [1677].

366

Alkene Polymerization Reactions with Transition Metal Catalysts

noticeable changes in the compositional distribution of the copolymers become apparent. These changes are different for different cocatalyst mixtures. A combination of ZnEt2 and Ali-Bu3 generates additional active centers that copolymerize 1-hexene with ethylene well (apparently due to a chemical interaction between the cocatalyst components) whereas a combination of ZnEt2 and AlEt3 produces additional active centers that copolymerize 1-hexene with ethylene very poorly resulting in an increase of the copolymers’ melting point [375]. Selective poisoning of active centers: Nearly all catalyst poisons introduced into ethylene/1-alkene copolymerization reactions exhibit some degree of selectivity. They preferentially poison either the active centers that readily copolymerize 1-alkenes with ethylene (the centers producing Flory components I, II, and III and Tref/Crystaf components C–E) or they preferentially poison the centers that copolymerize 1-alkenes with ethylene poorly, the centers producing Flory components IV and V and Tref/Crystaf components A and B. The poisoning selectivity is never perfect; each type of the center is usually poisoned but to a different degree. Typical poisons of the first type are silanes of the general formula RxSi(ORu)4
x, where x varies from 0 to 3. The active centers that produce Flory components I–III or Crystaf and Tref components C–F have lower stability and produce Flory components with lower molecular weights. The general effect of the RxSi(ORu)4
x poisons is the same, the catalyst activity decreases (all these silane compounds are poisons) but, because the centers producing Flory components I–III are poisoned to a higher degree, the initial polymerization rate decreases, the 1-alkene content in the copolymers decreases, and the average molecular weight of the copolymers increases. Two examples illustrating these effects are shown in Table 5.17. The poisons of the second type, chemical compounds that preferentially poison the centers producing Flory components IV and V (the centers with a poor ability to copolymerize 1-alkenes with ethylene) are rare. A few known examples include conjugated dienes, 1,3-pentadiene and 2,4-hexadiene [315,318]. The studies of these poisoning effects are best carried if the poisons are introduced after significant reaction times, 3–4 hours, when the active centers producing Flory components I–III are strongly deactivated (Figure 5.13). The following effects of conjugated and

Table 5.17 Effects of silanes RxSi(ORu)4
x on performance of TiCl4/MgCl2/SiO2-AlEt3 system in ethylene/1-hexene copolymerization reactionsa [315,453] Silane

Amount

Productivity (kg/g cat  h)

Ccop Hex ¼ 0.9 mol.% Mw

Mw/Mn

Si(OEt)4

0 0.88b 0 0.05c

2.85 1.85 13.1 7.2

B5 2.8 4.3 2.1

B8 6.9 4.5 4.0

PhSi(OEt)3 a

Polymerization reactions at 851C in the presence of hydrogen. mmol/g cat; reaction at CE ¼ 0.49 M and CHex ¼ 1.6 M. mmol/mmol Al; reaction at CE ¼ 0.51 M and CHex ¼ 3.0 M.

b c

0.89  105 1.32  105 1.03  105 1.44  105

367

Kinetics of Alkene Polymerization Reactions

nonconjugated dienes after 3 hours of undisturbed polymerization reactions with a Ti-based catalyst were observed. Diene 1,3-butadiene Amount (vol. %) 0.035 Rate reduction B33%

1,3-butadiene 0.089 B45%

1,4-pentadiene 0.042 B40%

1,4-pentadiene 0.126 B60%

These data suggest that active centers generating Flory components IV and V have two adjacent vacancies for ethylene coordination, which are both blocked by coordination with 1,3- and 1,4-linked diene molecules. On the other hand, acetylene, another poison of the second type, affects the copolymerization reactions only temporarily, the reaction rate decreases by 60% at the acetylene content of 0.009% but the catalyst activity completely recovers after 10–15 minutes due to copolymerization of acetylene and ethylene resulting in gradual incorporation of the poison into the polymer. 5.7.1.3. General kinetic scheme of ethylene polymerization reactions Two dominant factors determine the kinetic behavior of Ti-based heterogeneous Ziegler–Natta catalysts in homopolymerization and copolymerization reactions of ethylene. The first factor is the presence of several types of active centers [316,317, 452,1382,1571,1677,1680]. Different centers produce polymer fractions of a different molecular weight (Flory components), they have different ability to copolymerize 1-alkenes with ethylene (resulting in complex multi-component Tref and Crystaf curves of the copolymers), they have different kinetic stability, and they differently respond to catalyst poisons. The second factor affecting the kinetic behavior of the Ti-based catalysts is related to three peculiar effects in ethylene polymerization reactions, a high reaction order with respect to the ethylene concentration, a reversible rate retardation in the presence of hydrogen (Figure 5.2B), and catalyst activation in the presence of 1-alkenes (Figure 5.11). All these effects were explained by a single kinetic feature of the active centers, a hypothesis that Ti-based active centers with alkyl groups containing methyl groups in the b-position with respect to the Ti atom, the WTi– CH2–CH3 species, are less reactive in ethylene insertion reactions in comparison with the active centers carrying longer alkyl chains [316,317,1571]. This WTi
C2H5 group can be viewed as a growing polymer chain with one ethylene unit, which is stabilized by a stronger agostic interaction between the b-CH3 group and the Ti atom compared to the agostic interaction between the b-CH2 group and the Ti atom. This type of b-agostic interaction is apparently common for many alkylated transition metal compounds; it was experimentally demonstrated for several zirconocene complexes [1583,1681] (see Section 5.4.2.1), and it was proposed in extensive theoretical studies of the Ti(C2H5)(PH3)2Cl2H complex [1682,1683]. Mechanistic grounds for the b-C–H agostic interaction are discussed in Section 6.1.2.1.7. An ethylene polymerization scheme that takes into account stability and lability of the WTi
C2H5 group in ethylene polymerization reactions is shown in

368

Alkene Polymerization Reactions with Transition Metal Catalysts

+H 2

-Polymer -C2H6

+ n C 2 H4

+H 2

>Ti

H

+C 2H4

>Ti CH2 H

>Ti

>Ti (C2H4)n C2H5

C2 H5

CH2

-Polymer +C 2H4

-C2H4

+ 1-alkene

Scheme 5.5 Kinetic scheme of ethylene polymerization reactions with T|-based catalysts.

Scheme 5.5 [316,317,1571]. This mechanism differs from the standard alkene polymerization mechanism in Scheme 5.1 in three main features: 1. The b-agostic interaction between the H atom of the CH3 group of the WTi
C2H5 species and the Ti atom is very labile. 2. The agostically stabilized Ti
C2H5 bond can undergo b-H elimination and produce the Ti
H bond. 3. When any 1-alkene molecule is inserted into the Ti
H bond, the reaction bypasses the stage of the stabilized Ti
C2H5 bond. For example, if the 1-alkene in Scheme 5.5 is 1-hexene, its insertion into the Ti
H bond produces the WTin-C6H13 moiety that can be viewed as a growing polymer chain containing three ethylene units, WTi
(CH2
CH2)3
H. These additions to the standard reaction scheme (Scheme 5.1) provide plausible explanations for most peculiarities of ethylene polymerization reactions with Ti-based catalysts [316,317,1566,1571]. According to Scheme 5.5, the WTi
C2H5 group is formed in an ethylene polymerization reaction each time after the ethylene insertion into the Ti
H bond and after each step of chain transfer to a monomer. Modeling kinetic estimations based on Scheme 5.5 explain the apparent high reaction order with respect to the ethylene concentration in ethylene homopolymerization reactions and indicate that the relative stability of the WTi
C2H5 group keeps a large fraction of active centers in ethylene polymerization reactions, B70–75%, in a ‘‘temporarily sleeping’’ state [1566]. Because the frequency of the

369

Kinetics of Alkene Polymerization Reactions

Ti
H bond formation (followed by the generation of the WTi
C2H5 group) is greatly increased in the presence of hydrogen, the addition of hydrogen always decreases the ethylene polymerization rate [316,317,1382]. Insertion of any 1-alkene molecule into the Ti
H bond in the primary orientation bypasses the stage of the Ti
C2H5 bond. As a result, the reactivity of active centers that copolymerize 1-alkene with ethylene well increases in copolymerization reactions in comparison with ethylene homopolymerization reactions whereas the active centers that copolymerize 1-alkene with ethylene poorly are not affected [316,317,1382].

5.7.2. Propylene polymerization reactions From the kinetic viewpoint, propylene homopolymerization reactions with solid and supported Ti- and V-based Ziegler–Natta catalysts are significantly more complex than ethylene homopolymerization reactions described in the previous sections. The principal reason for the complexity is the existence of a variety of active centers of widely different isospecificity. In addition, each type of center, from highly isospecific to stereo-aspecific, can be subdivided into several classes based on the average molecular weight of polymer molecules it produces. Figure 5.15 shows the GPC curve of a propylene homopolymer prepared with the TiCl4/MgCl2/di-n-butyl phthalate-Ali-Bu3 system and GPC curves of its two polymer components, the crystalline fraction (insoluble in boiling n-heptane) and the amorphous heptane-soluble fraction [221]. Figure 5.16 shows separate GPC curves of both fractions and their resolution into Flory components. Each of the polymer fractions consists of several Flory components of a different molecular weight, four components with Mw values from 1.05  105 to 3.9  106 in the 0.8

dW/dMW

0.6

Atactic fraction 0.4

Crystalline fraction

0.2

0.0 10 3

10 4

10 5 10 6 Molecular weight

10 7

108

Figure 5.15 GPC curve of propylene homopolymer prepared with T|Cl4/MgCl2/di-n-butyl phthalate-Ali-Bu3 system at 801C and GPC curves of its two fractions, atactic and crystalline (insoluble in boiling n-heptane) [221].

370

Alkene Polymerization Reactions with Transition Metal Catalysts

0.8 A

dW/d(MW)

0.6

0.4

0.2

0.0 103

104

105 106 Molecular weight

107

108

0.8 B

dW/d(MW)

0.6

0.5

0.3

0.2

0.0 103

104

105

106

107

108

Molecular weight

Figure 5.16 GPC curves of crystalline polypropylene fraction (A) and amorphous polypropylene fraction (B) and their resolution into Flory components. Catalyst system T|Cl4/MgCl2/din-butyl phthalate-Ali-Bu3, 801C [221].

crystalline fraction, and five components with Mw values from 2.0  104 to 1.1  106 in the amorphous fraction. Molecular weight distributions of two fractions partially overlap (Figure 5.15), and Mw values of some Flory components in each fraction are very close (Figure 5.16). This circumstance makes any attempt to characterize a given unfractionated propylene polymer by the values of its average molecular weight or the width of its molecular weight distribution practically

Kinetics of Alkene Polymerization Reactions

371

meaningless. Furthermore, Tref and Crystaf analyses of crystalline polypropylene fractions produced by various Ti-based supported catalysts (see Figures 2.5, 2.14, and 3.4) show that these fractions are always more complex mixtures in the stereochemical sense than the GPC data alone may suggest. It is reasonable to assume that each Flory component in crystalline polypropylene fractions is a composite; each represents an overlap of several elemental components produced by centers of different isospecificity that generate polymer molecules of a similar molecular weight [221]. Attempts to analyze the kinetic behavior of heterogeneous Ti-based catalysts in polymerization reactions of 1-alkenes by assuming that these catalysts contain only a few types of active centers, e.g., the centers producing ‘‘isotactic,’’ ‘‘stereoblock,’’ and ‘‘atactic’’ polymers, should be regarded as over-simplified. 5.7.2.1. General kinetic behavior The kinetic behavior of the early solid Ziegler–Natta catalysts based on different modifications of TiCl3 and VCl3 and the kinetic behavior of supported Ti-based catalysts (mostly of the TiCl4/MCl2 type) in propylene polymerization reactions has many similar kinetic features. It should be stressed that propylene polymerization reactions with solid catalyst systems of the TiCl3-AlEt3 type or with supported catalysts of the TiCl4/MgCl2/ Modifier I type activated with AlR3 alone (without the use of Modifier II) are inconvenient objects for the kinetic research. These catalysts produce propylene polymers with low fractional isotacticity, from B40 to B70%, and each polymer is a complex mixture of macromolecules with widely varying degrees of isotacticity and molecular weight. This complication can be avoided when the kinetic studies are restricted to propylene polymerization reactions with highly isospecific catalysts which produce polymers with fractional isotacticity higher than B95–97%. In propylene polymerization reactions, the catalysts usually become active relatively rapidly and decay relatively rapidly as well (see, e.g., Figures 5.1A and 5.3). This instability raises an obvious question: are activation/deactivation kinetic parameters for the centers of different types approximately the same or significantly different? As described in Sections 5.7.1.1 and 5.7.1.2, the average molecular weight in ethylene polymerization reactions varies with time, especially at the early stages of the reactions, due to different formation rates of active centers responsible for the formation of different polymer components (Figure 5.13). However, both the average molecular weight of propylene polymers and the width of their molecular weight distribution are mostly independent on reaction time [2,462,1553, 1680,1684]. The fractional isotacticity of Ti-based catalysts also changes little with reaction time. All these data suggest that different types of active centers in propylene polymerization reactions are formed and decay at approximately the same rates. However, the stopped-flow polymerization technique revealed small differences in the formation rates of different types of active canters in propylene polymerization reactions [222,223,365]. After a solid catalyst and a cocatalyst are brought into contact at 301C, the first to form are stereo-aspecific centers and the centers of low isospecificity which produce a polypropylene fraction with [mmmm] B0.82–0.85. Two groups of highly isospecific active centers are generated only

372

Alkene Polymerization Reactions with Transition Metal Catalysts

after B10 seconds of the reaction between the catalyst components; they produce polymer fractions with [mmmm] B0.92 and 0.96 [222]. Detailed kinetic analysis of 1-alkene polymerization reactions with Ti- and V-based Ziegler–Natta catalysts is additionally complicated by the existence of two propagation modes for 1-alkene molecules, primary (Reaction (3.8)) and secondary (Reaction (3.11)) [187,427,1685]. Although the secondary insertion mode has a very low probability compared to that of the primary insertion mode, the subsequent chain growth step, either in the primary or in the secondary monomer orientation, may be kinetically difficult compared to the regular primary insertion step [90,429–433] (Table 3.6). In effect, the secondary insertion step may produce a ‘‘sleeping’’ growing chain WTi
CHR
CH2
CH2
CHR
Polymer. Most of such growing chains are terminated in two chain transfer reactions, those to hydrogen (Reaction (3.28)) or in chain transfer to a cocatalyst MRux (Reaction (3.30)) [187,213,225,427,428,448,461]. 5.7.2.2. Effects of reaction parameters Similarly to kinetic studies of ethylene polymerization reactions described in Section 5.7.1.1.2, the following reaction parameters are usually investigated in propylene homopolymerization reactions, the monomer concentration (partial pressure), temperature, the presence of hydrogen, effects of cocatalysts, and the presence of reaction poisons. The number of experimentally examined effects is always higher in polymerization reactions of 1-alkenes. In addition to purely kinetic parameters, such as reaction rates and molecular weights of elemental components, two other dependencies are usually evaluated, the distribution of active centers with respect to stereospecificity (as a minimum, the measurement of the content of the crystalline fraction) and structural properties of the centers of the highest stereospecificity, by 13C NMR. Effects of propylene concentration: The rate of propylene polymerization reactions over a variety of solid and supported Ti-based catalysts has the first-order dependence with respect to the propylene partial pressure or its concentration. This reaction order was determined both for commercial catalysts of the 1st generation containing d-TiCl3 and AlEt2Cl (both in slurry [1686] and in the gas phase [1687]), and for supported catalysts of the TiCl4/MgCl2/Modifier I-AlEt3/Modifier II type [1551,1552]. Temperature effects: Ti-based catalysts for the synthesis of isotactic polypropylene usually operate at temperatures from 60 and 801C. Estimations of the effective activation energy vary from B35.5 kJ/mol (8.5 kcal/mol) to B58 kJ/mol (14 kcal/ mol) [6,188,949,1551,1686,1688]. The temperature effects on the overall performance of heterogeneous Ziegler– Natta catalysts vary depending on the type of catalyst and cocatalyst. The average activity of the catalysts usually increases as the reaction temperature increases from 40 to 801C, but the catalysts become progressively unstable at higher temperatures, which explains a decrease in the average productivity above 801C. When supported catalyst systems of the 4th generation are used without Modifiers II, both the average molecular weights of polypropylene they produce and the contents of

Kinetics of Alkene Polymerization Reactions

373

crystalline fractions in them decrease with temperature, as the data for the TiCl4/ MgCl2/dibutyl phthalate-Ali-Bu3 system exemplify [221]: Temperature (1C) 40 50 60 70 80 90 5 5 5 5 5 M av 8.92  10 9.05  10 7.85  10 6.25  10 3.28  10 2.67  105 w I.I. (%) 77 67 75 57 26 22

These changes merely reflect the changes in relative populations of different types active centers and carry no significant kinetic information. If effective Modifiers II, silanes or substituted piperidines, are employed, the catalysts have very high fractional isotacticity, 90–97%, and all temperature trends are reversed, the molecular weight of the polymers and the content of their crystalline fraction increase with temperature [1689]. Figure 5.17 shows GPC curves of two crystalline fractions produced at 50 and 901C with the TiCl4/MgCl2/dibutyl phthalate-Ali-Bu3 system [221]. Each GPC curve consists of four major Flory components with widely different molecular weights, Mw values for Flory components with the lowest and the highest molecular weight differ by a factor of 35–40. The figure shows that the molecular weight of each Flory component remains approximately the same at different temperatures. What changes with temperature is the contribution of different Flory components to the make-up of the crystalline fraction. As the temperature increases, the fractions of two components with the highest molecular weights gradually decrease, the content of Flory component IVu decreases from 26% at 401C to 14% at 901C, and the content of Flory component Vu decreases from 15% to 6%. As a result, the fractions of two components of lower molecular weights, IIu and IIIu, increase [221]. This change in the relative contents of different polymer components rather than a temperature effect on the kinetics of chain propagation vs. chain transfer reactions is the primary reason for the decrease of the average molecular weight of the crystalline fraction with temperature. When a propylene polymerization reaction is carried out in the absence of hydrogen, the expression for the average polymerization degree of a polymer produced by a single type of active center is simple Equation (5.23): n E kp/kM t . The observation that molecular weights of major Flory components of isotactic polypropylene in the absence of hydrogen change little with temperature indicates that activation energies for the rate constants of two reactions that determine the M Mw(Flory) values, kp and kM t , are similar. The kt /kp values for the four Flory components of isotactic polypropylene produced with the TiCl4/MgCl2/di-i-butyl phthalate-AlEt3 system at 50–801C are Flory component: Iu IIu IIIu IVu kM 7.6–8.1  10
4 2.0–2.6  10
4 7.6–9.1  10
5 2.0–2.5  10
5 t /kp:

Temperature effects on stereospecificity: The effect of reaction temperature on the stereospecificity of heterogeneous Ziegler–Natta catalysts is a complex subject [221, 349,1689]. The temperature effect on the fractional isotacticity is unambiguous: as the reaction temperature increases, the fraction of the crystalline material in the

374

Alkene Polymerization Reactions with Transition Metal Catalysts

0.8

50°C

dW/d(logMW)

0.6

0.5

0.3

0.2

0.0 10 3

10 4

10 5 10 6 Molecular weight

10 7

10 8

0.8

90°C

dW/d(logMW)

0.6

0.5

0.3

0.2

0.0 10 3

10 4

10 5 10 6 Molecular weight

10 7

10 8

Figure 5.17 GPC curves of two crystalline fractions of polymers produced with T|Cl4/ MgCl2/di-n-butyl phthalate-Ali-Bu3 system at 501C and 901C and their resolution into Flory components [221].

polymer decreases, as the data in the previous section demonstrate. The extent of this decrease depends on the catalyst and the presence of Modifier II in the cocatalyst. The reaction temperature also affects average properties of centers of the highest isospecificity. A common effect is typical for many supported catalyst systems, as the reaction temperature increases, the 13C NMR isotacticity of the combined crystalline fraction increases. One example, for xylene-insoluble fractions of propylene polymers produced with the TiCl4/MgCl2/di-i-butyl phthalate-AlEt3/ Cpy2Si(OMe)2 system, illustrates this trend [1689]: Temperature (1C) ½mmmmav cr

20 0.878

40 0.908

60 0.914

80 0.949

375

Kinetics of Alkene Polymerization Reactions

The same changes were observed when other Modifiers II, 2,2,6,6-tetramethylpiperidine and 1,3-diethers, were employed [1689], as well as for catalyst systems without Modifiers II [221,1689]. As discussed earlier, Tref and Crystaf analyses of crystalline polypropylene (Figures 2.5, 2.14, and 3.4) show that these fractions are complex mixtures of polymer components exhibiting significant variations in stereoregularity. Therefore, the ½mmmmav cr value in the above example is the average value for a broad stereochemical distribution. Analytical Tref data for crystalline fractions of polypropylene prepared with the TiCl4/MgCl2/di-n-butyl phthalate-Ali-Bu3 system at 40 and 901C provided the basis for the explanation of the temperature effect on ½mmmmav cr values. Tref curves of these two crystalline fractions [221] are overlaps of several elemental components of different isotacticity, the same result as for all crystalline fractions of polypropylene produced with heterogeneous catalysts (Sections 2.3.2.3 and 2.5.3). Table 5.18 gives the principal results of the Tref analysis. Each crystalline fraction is subdivided into three components with respect to isotacticity, the highly isotactic fraction (Tref elution peaks at 110–1161C), the fraction of reduced isotacticity (100–1101C) and the fraction of low isotacticity (75–1001C). Judging by the positions of peak maximums, the highly crystalline material produced at 401C has a higher degree of isotacticity (the material elutes at 116
1141C) compared to the material produced at 901C, which elutes at 112
1081C. DSC analysis is especially sensitive to the properties of the polymer material with the highest crystallinity degree (Section 2.3.2.4). It confirms that the isospecificity of the ‘‘best’’ isospecific centers at 401C is slightly higher (Tm ¼ 155.81C) compared to that at 901C (Tm ¼ 155.11C) [221]. However, as the polymerization temperature increases, the content of the highly isotactic component in the crystalline fraction increases from B65 to B80%. The material of low isotacticity is present in both crystalline fractions, and its content decreases by half in this temperature range, from B20 to B10%. This change in the relative contents of the highly isotactic component vs. the component of low isotacticity, rather than a change in the isotacticity level of the highly isotactic component, is the principal reason for the observed increase in the ½mmmmav cr Table 5.18 Resolution of Tref curves of crystalline polypropylene fractions prepared with TiCl4/MgCl2/dibutyl phthalate-Ali-Bu3 system at different temperatures [221] Temperature ( 1C)

Component

Tmax (Tref) (1C)

[mmmm]a

Fraction (%)

40

Highly isotactic Moderately isotactic Low isotacticity (1) Low isotacticity (2) Highly isotactic Moderately isotactic Low isotacticity

116.1 and 114.0 109.0 B92 B70 112.4 and 108.4 99.9 B89

B0.96 (av.) B0.94 B0.82 B0.64 B0.95 (av.) B0.92 B0.78

66 12 7 15 80 9 11

90

a

Approximate evaluation based on calibration in [228].

376

Alkene Polymerization Reactions with Transition Metal Catalysts

value in crystalline polypropylene fractions produced at high temperatures [221,1689]. Hydrogen effects: The use of hydrogen for the reduction of the molecular weight of polypropylene and polymers of other 1-alkenes is the universally practiced method in industry and in laboratory. The amount of hydrogen required to decrease the molecular weight of polypropylene to the commercially acceptable level is usually significantly lower than in ethylene polymerization reactions. Two general effects of hydrogen addition are common for Ti-based heterogeneous catalysts: as the hydrogen concentration in a propylene polymerization reaction increases, the average molecular weight of the polymer strongly decreases and, in parallel, the activity of the catalyst (both the reaction rate and productivity) increases [316,317,452,462,463,1680]. These two effects can be illustrated by the data for propylene polymerization reactions with a TiCl4/MgCl2/ dibutyl phthalate-AlEt3/PhSi(OEt)3 system at 701C performed at a constant propylene partial pressure [453,1680]: PH/PPr Productivity (kg/g Ti h) Mw

0 45 4.13  106

0.052 244 7.60  105

0.130 189 5.95  105

0.184 179 5.89  105

Figure 5.18 gives two examples of GPC curves of propylene polymers prepared with the TiCl4/MgCl2/dibutyl phthalate-AlEt3/PhSi(OEt)3 system at 701C, one without hydrogen and another in the presence of hydrogen, and their resolution into Flory components. Five types of active centers, marked Iu through Vu in the order of increasing molecular weight, describe the molecular weight distribution of these polymers. When hydrogen is present in the reaction, the molecular weight of each Flory component decreases approximately in parallel whereas the contents of the components remain approximately the same independently on the hydrogen concentration. The overall effect of these changes is, of course, a gradual shift of the GPC curve toward lower molecular weights and a decrease of average Mw and Mn values without any significant change in the width of the molecular weight distribution. Equation 5.23 gives the general expression for the average polymerization degree of a single Flory component in propylene homopolymers prepared in the presence of hydrogen. Plotting the 1/n values for individual components vs. the CH/CM ratio should, in principle, allow the evaluation of both the kH t /kp ratio and the kM t /kp ratio in polymerization experiments performed at different hydrogen concentrations. However, the experimental data show [462,1680] that the 1/n vs. CH/CM dependencies for different active centers are not linear, as one would expect from Equation (5.23). In simple terms, the absence of the linear 1/n vs. CH/CM dependence means that when the hydrogen concentration in a propylene polymerization reaction is very high (much higher than that usually encountered in the majority of such experiments), hydrogen does not affect the molecular weight of polypropylene anymore. The reasons for this deviation from the simple kinetic rule in Scheme 5.1 are not clear.

377

Kinetics of Alkene Polymerization Reactions

0.8

PH=0

dW/d(logMW)

0.6

0.5

0.3

0.2

0.0 10 2

10 3

10 4

10 5

10 6

10 7

Molecular weight 0.8

PH=0.41 MPa

dW/d(logMW)

0.6

0.5

0.3

0.2

0.0 10 2

10 3

10 4 10 5 Molecular weight

10 6

10 7

Figure 5.18 GPC curves of polypropylene produced with T|Cl4/MgCl2/dibutyl phthalateAlEt3/PhSi(OEt)3 system at 701C in the absence and in the presence of hydrogen, and their resolution into Flory components [1680].

Hydrogen significantly, by a factor of 2.0–2.5, activates all Ti-based heterogeneous catalysts in propylene polymerization reactions. Giannini reported the activation effect for the first time [1690] and it was later confirmed in numerous publications including testing of catalysts activated with mixtures of AlEt3 and esters of aromatic acids, phthalate-containing catalysts activated with mixtures of AlEt3 and alkoxysilanes, and catalysts containing aliphatic diethers [53,187,188,448,449,462, 463,1161,1212,1617,1664,1685,1687,1690–1697]. This effect is opposite to the ratesuppression hydrogen effect in ethylene polymerization reactions with the same catalysts (Section 5.7.1.1.2). The activation caused by addition of hydrogen is fully reversible, a removal of hydrogen reduces the polymerization rate to its original level, and the addition of hydrogen in the course on an established polymerization reaction results in an immediate activation of the catalysts [225,463,1691,1695,1696]. Possible

378

Alkene Polymerization Reactions with Transition Metal Catalysts

hydrogen activation mechanisms are discussed in Section 6.3.5.1. This effect appears to have the same origin as the hydrogen deactivation effect in ethylene polymerization reactions, the relative kinetic stability of growing polymer chains with a methyl group in the b-position with respect to the Ti atom in the active center, WTi–CH(CH3) –Polymer. This arrangement can occur in two situations, after the secondary insertion of a propylene molecule into the growing polymer chain (it produces the WTi–CH(CH3)–CH2–Polymer species) or after the secondary insertion of a propylene molecule into the Ti–H bond (it produces the WTi–CH(CH3)2 species). Both types of growing chains have low reactivity in primary propylene insertion reactions, and hydrogen, by converting the Ti–C bonds into the Ti–H bond, restores the activity of the center. Hydrogen has one more kinetic effect in propylene polymerization reactions with supported Ziegler–Natta catalysts, their activity decreases faster over time [53,1212,1696,1697]. Although the maximum polymerization rate can be two times higher in the hydrogen presence, the activity decrease is also more rapid, and, as a result, the average productivity of a given catalyst, if measured after a significant reaction time, can be quite similar in the presence and in the absence of hydrogen [1212,1697]. Cocatalyst effects: All modern supported catalysts for propylene polymerization are quite specific in the selection of cocatalysts. Overwhelmingly, the cocatalysts of choice are AlEt3 or Ali-Bu3, which are used either alone (for Ti-based catalysts of the 5th generation) or in combination with different Modifiers II for catalysts of the 3rd and 4th generations, and AlEt2Cl for Solvay-type catalysts (Section 4.3.1). Very little research has been done to compare the cocatalyst effects in detail. Of course, these effects were studied and reviewed for the early solid catalysts, mostly different modifications of TiCl3, but these studies were restricted to simple kinetic comparisons without the benefit of modern polymer characterization methods [1,2,4,6]. As a rule, the effect of the cocatalyst concentration of the activity of different catalysts follows the same pattern. As the cocatalyst concentration increases starting from a very low level, the catalyst activity (measured either as a polymer yield or a polymerization rate) initially increases proportionally to the cocatalyst amount, then it reaches a certain maximum activity and after that it is barely affected by additional amounts of the cocatalyst. This dependence is typical both for solid Ti-based catalysts [2,1204,1698] and for various supported catalysts [1551]. Only very high amounts of cocatalysts cause a significant (and reversible) suppression of the reaction rates [1699]. The overwhelming number of kinetic experiments described in the literature is performed at cocatalyst concentrations in the ‘‘safe’’ range, when doubling or even tripling the amount of an added cocatalyst does not affect the reaction rate. The effect of the cocatalyst concentration on the average molecular weight and the molecular weight distribution of polypropylene prepared with TiCl4/MgCl2-type catalysts in a broad range of [cocatalyst]:[catalyst] ratios is also quite small indicating relative insignificance of the chain transfer reaction to a cocatalyst [1700]. The activity of supported TiCl4/MgCl2/Modifier I catalysts exhibited marked sensitivity to the type of cocatalyst (in the absence of Modifiers II) in stopped-flow propylene polymerization experiments at 301C, as shown in Table 5.19. These

379

Kinetics of Alkene Polymerization Reactions

Table 5.19 Cocatalyst effects in propylene polymerization reactions with TiCl4/MgCl2 catalysta [1578]

a

Cocatalyst

Relative total activity

[mmmm]av

Relative activity of isospeci¢c centers

AlEt3 Aln-Bu3 Aln-Hex3 Aln-Oct3 Ali-Bu3

1 (reference) 0.66 0.44 0.25 0.26

0.928 0.929 0.918 0.893 0.878

1 (reference) 0.93 0.57 0.21 0.21

Stopped-flow propylene polymerization experiments at 301C.

experiments demonstrated also that the average value of the propagation rate constant for isospecific active centers does not depend on the type of the cocatalyst and is equal to B4.7–4.9  103 M
1  s
1 [1578]. Therefore, the differences in the relative activity of isospecific centers in Table 5.19 reflect differences in the number of isospecific centers formed in the presence of different cocatalysts. All crystalline fractions in these experiments have the [mmmm]cr value of p1.0, and the [mmmm]av values in the table are primarily the function of the content of the crystalline fractions. 5.7.2.3. Catalyst modifiers, selective poisoning of active centers A successful use of Ti-based supported catalysts for propylene polymerization heavily relies on the application of catalyst modifiers. Nearly all of them can be viewed as selective poisons. Both Modifiers I and Modifiers II are usually selected with the goal of a drastic reduction of the fraction of amorphous atactic polypropylene. Other types of modifiers can be introduced to the same catalyst systems to achieve an opposite effect, to reduce the amount of the crystalline fraction and to produce amorphous atactic polypropylene, an important commercial product (Section 4.3.2.5). Additionally, some modifiers are selected to affect the molecular weight distribution of isotactic polypropylene. Superficially, selective poisons produce three effects in polymerization reactions of 1-alkenes: 1. They strongly reduce the content of the amorphous atactic fraction in the polymers. 2. They increase the average molecular weight of crystalline fractions. 3. They increase the average stereoregularity of crystalline fractions. All these effects are closely related and can be mostly attributed to the effect of the poisons on the distribution of active centers in the catalysts. One of the simplest examples involves the use of NEt3 for the modification of a catalyst of the 1st generation, d-TiCl3-AlEt3 [1701]. NEt3 is a weak electron donor, it forms a complex with AlEt3 and, apparently, with some active centers in the catalyst. GPC analysis of crystalline polypropylene fractions prepared with the dTiCl3-AlEt3/NEt3 system at 701C shows that NEt3 poisons the active centers of reduced isospecificity (they produce macromolecules of a lower molecular weight),

380

Alkene Polymerization Reactions with Transition Metal Catalysts

and thus increases the fraction of the highly isotactic material of a high molecular weight. As a result, the average molecular weight of the crystalline fraction increases from 5.5  105 to B8.0  105, its IR isotacticity parameter increases from 0.85 to 0.93, and its crystallinity from 68 to 75%. The effects of selective poisoning on supported TiCl4/MgCl2 catalysts are especially obvious when they are prepared without Modifiers I. For example, addition of ethyl benzoate to AlEt3 and the use of this cocatalyst mixture with a TiCl4/MgCl2 catalyst at different [ester]:[AlEt3] ratios have two effects. First, the cocatalyst mixture reduces the yield of amorphous polypropylene by a factor of 3.5 and, second, the molecular weight of the crystalline material prepared in its presence increases from B1  105 to B1  106 [1130]. Similar changes were observed in catalysts of the 3rd generation containing esters of aromatic acids as both Modifiers I and II [1702]. Tref data show that centers of the highest isospecificity formed in TiCl4/MgCl2 catalysts activated with pure AlEt3 produce polypropylene with a relatively low average isotacticity, ½mmmmav cr B0.94–0.95 [183,214]. However, when methyl p-toluate is added to AlEt3, two distinct groups of active centers are generated. The centers of the first group have the same stereospecificity as those in the unmodified catalyst, and the centers of the second group are more stereospecific, they produce polypropylene with [mmmm] of B0.975. Modifiers II have the same two selective poisoning effects on the performance of catalysts of the 4th and the 5th generation. The first effect, poisoning of active centers producing atactic polypropylene and polymer fractions of low isotacticity, is universal for all catalysts of this type; it is widely described in the literature [497, 1136,1703]. Several examples of this effect are given in Chapter 4. The second effect of the modifiers, an increase of the average molecular weight of the crystalline fraction, finds an explanation when the molecular weight distribution of these fractions is analyzed. Table 5.20 gives the results of the molecular weight distribution analysis of crystalline fractions prepared with a supported TiCl4/ MgCl2/dibutyl phthalate catalyst at 801C. One polymer is produced with Ali-Bu3 as a cocatalyst without Modifier II and the second one with the AlEt3Cpy2Si(OMe)2 mixture at [Al]:[Si]molar ¼ 8. The silane significantly reduces the relative productivity of active centers producing Flory component Iu, and, Table 5.20 GPC analysis for crystalline polypropylene fractions prepared at 801C with TiCl4/ MgCl2/dibutyl phthalate catalyst activated with different cocatalysts [221] Cocatalyst

Flory component

Mw

Fraction (%)

Ali-Bu3

Iu IIu IIIu IVu Iu IIu IIIu IVu

1.05  105 3.24  105 9.71  105 3.93  106 B9.0  104 4.31  105 1.17  106 3.71  106

39.8 38.0 15.7 6.4 4.7 47.0 36.3 12.0

AlEt3-Cpy2Si(OMe)2

381

Kinetics of Alkene Polymerization Reactions

correspondingly, increases the contents of Flory components IIIu and IVu in the crystalline fraction. However, the silane does not affect the molecular weight of any Flory component and it does not affect the ratio between the contents of components IIIu and IVu. This change in the contents of different Flory component results in a large increase of the average molecular weight, from 5.7  105 in the absence of Modifier II to B1.1  106 in its presence [221]. The same effect was observed in propylene polymerization reactions with a TiCl4/MgCl2/di-i-butyl phthalate catalyst activated with a mixture of AlEt3 and different silanes [426,1704]. The third effect of selective poisons is subtler. Modifiers II (or, rather, the products of their reactions with cocatalysts (see Section 4.4.3), in addition to poisoning aspecific centers and centers of reduced isospecificity, apparently affect the distribution and, possibly, the structure of highly isospecific active centers as well. This effect was examined in propylene polymerization reactions with TiCl4/ MgCl2/Modifier I-AlEt3/Modifier II systems [61,197,221,426]; it leads to an increase of the average isospecificity of active centers producing the crystalline polypropylene fraction, their ½mmmmav cr values (Table 5.21). This improvement is also reflected in the melting point increase of the material produced by these centers.

Table 5.21 Isotacticity of polypropylene fraction produced by centers of highest isospecificity in TiCl4/MgCl2/Modifier I-AlR3/Modifier II systems at 701C Modi¢er I

Modi¢er II

a [mmmm]cr

Tm (1C)

None

0.95–0.96

158–161

PhSi(OMe)3

0.985

164–165

None PhSi(OMe)3 none

0.981–0.985 0.983 0.985

164–165 164 164

None Cpy2Si(OMe)2

0.892 0.978

None Cpy2Si(OMe)2

0.941 B0.99

Data from [61,197] None Catalyst of 4th generation DIBPb Catalysts of 5th generation 2,2-R1,R2-1,3dimethoxypropane R1 ¼ R2 ¼ i-Bu R1 ¼ R2 ¼ i-Bu R1 ¼ R2 ¼ Cpy Data from [221] Catalyst of 4th generation DIBPb DIBPb Data from [426] Catalyst of 4th generation DIBPb DIBPb a

[mmmm] value for polypropylene fraction of highest isotacticity. DIBP ¼ di-i-butyl phthalate.

b

159.2 164.4–166.3

382

Alkene Polymerization Reactions with Transition Metal Catalysts

Two interpretations of the third effect of Modifiers II are proposed in the literature. The first one is based on parallels between the data on the molecular weight distribution and the 13C NMR estimation of average stereoregularity. Active centers producing different Flory components in crystalline polypropylene fractions have different stereospecificity, Flory components IIIu and IVu are more stereoregular than Flory components Iu and IIu [221]. Preferential poisoning on the centers producing Flory components Iu and IIu can by itself account for the changes in the average ½mmmmav cr value of crystalline fractions prepared in the presence of Modifiers II. Another explanation of this effect suggests a direct participation of Modifiers II in the makeup of active centers of the highest stereospecificity. These modifiers are present in final catalyst compositions (Section 4.5.2.3), and their coordination at transition metal atoms in the isospecific active centers can affect the level of stereocontrol. Different centers of the highest isospecificity have similar kinetic characteristics [221,322]. For example, the formation and the decay rates of isospecific centers of slightly different isospecificity in the TiCl4/MgCl2/ethyl benzoate-AlEt3 system are generally quite similar. Only stopped-flow experiments at a low temperature, 301C, and at a very short reaction time, 0.15 seconds, reveal a small kinetic difference, the centers of the highest isospecificity ([mmmm] B0.98) are formed at a lower rate than the centers of lower isospecificity ([mmmm] B0.95 and B0.80) [191,365].

5.7.2.4. Nonselective catalyst poisons Similarly to ethylene polymerization reactions, nearly all polar organic compounds and many inorganic compounds poison active centers in propylene polymerization reactions. In contrast to selective poisons (catalyst modifiers) described in the previous section, many chemical compounds have a general poisoning effect without any particular selectivity. These compounds can be subdivided into two categories, coordinative poisons and destructive poisons. CO and CO2 are the most known coordinative poisons. CO2 is used in industry to interrupt polymerization reactions without contamination of reactors and other equipment. Both compounds are also used for the measurement of the number of active centers, as described in Section 5.7.4.2. As a rule, the introduction of a coordinative poisonous compound brings about a decrease in the propylene polymerization rate without affecting the shape of the kinetic curve. CS2 represents another example of a poorly selective coordinative poison. When small amounts of CS2 are added to propylene polymerization reactions with the d-TiCl3-AlEt3 system at 701C, the total activity of the catalyst can be reduced 10–15 times without a significant change in the fractional isotacticity of the produced polymers [1705]. Several alkenes and dienes also affect propylene polymerization reactions with Ti-based heterogeneous catalysts; some of them can be qualified as unselective coordinative poisons. The biggest rate-depressing effect is caused by conjugated and nonconjugated dienes, 1,3-butadiene and 1,4-pentadiene [1676], whereas the rate depression after the addition of alkenes with internal CQC bonds, cis-2-butene or cyclopentene, is relatively small.

Kinetics of Alkene Polymerization Reactions

383

The most common destructive poisons in all heterogeneous Ziegler–Natta catalysts are alcohols and water. If applied in large quantities, they completely and irreversibly decompose both the active centers on the catalyst surface and the cocatalysts. However, when used in very small quantities, they exhibit more varied effects [1204]. Effects of water: Polymerization reactions of alkenes with Ti-based catalysts in the presence of minute amounts of water have significant induction periods [1706]. Water, both in the free form and adsorbed on the surface of solid catalyst components, interacts with cocatalysts in two steps with the formation of alkylalumoxanes. When the [AlR3]:[H2O] ratio is high, the main product of these two reactions is dialumoxane: AlR3 þ H2 O ! AlR2 2OH þ R2H

(5.60)

AlR3 þ AlR2 OH ! AlR2 2O2AlR2 þ R2H

(5.61)

The duration of the induction period t can be approximated by the following equation [1204,1706]: t ¼ ðk2  ½AlR3 Þ
1  lnð1 þ k2  ½H2 Oo =k1  ½Cat  So Þ

(5.62)

where [Cat] is the amount of a solid catalyst, So is its specific surface area, and k1 and k2 are effective rate constants of Reactions (5.60) and (5.61) on the catalyst surface and in solution, respectively. If water and AlR3 are allowed to react before the introduction of solid catalysts, the induction period is considerably shortened and if this interaction is long enough the induction period can be avoided completely. Dialumoxane generated in Reaction (5.61) has no appreciable effect on the catalyst activity and if the reaction between water and AlR3 is brought to completion the catalyst activity is the same as in blank experiments in the absence of water. The overall kinetic effect of small quantities of water on the polymerization rate, after the completion of the induction period, is quite complex [1204,1707]. It can be best determined if early solid catalysts of the a-TiCl3 type are used because these catalysts exhibit a very stable polymerization behavior. Two competing water effects can be distinguished. The first product of the water interaction with AlR3, AlR2OH (Reaction (5.60)), is a catalyst poison and it has a detrimental effect on the catalyst activity. The effect can be diminished if an additional amount of fresh AlR3 is introduced after the interaction between water and AlR3 is completed. Additionally, an interaction between a-TiCl3 and water results in the formation of new Ti species. They can be precursors of active centers and thus, potentially, a reaction between a-TiCl3 and water can even increase the catalyst activity. This effect is absent if the d-form of TiCl3 containing AlCl3 is used instead of a-TiCl3. An interplay between these two effects leads to a complex dependence of the effective polymerization rate in the a-TiCl3-AlEt3-water system vs. the amount of added water. At low amounts of added water with respect to TiCl3, o0.5 wt.%, the reaction rate (after completion of the induction period) increases with [H2O]o but it deteriorates at higher water amounts [1204,1707]. Effects of alcohols: Alcohols in large quantities are widely used for termination of alkene polymerization reactions with transition metal-based catalysts. However,

384

Alkene Polymerization Reactions with Transition Metal Catalysts

kinetic effects of very small quantities of alcohols on the propylene polymerization kinetics with Ti-based catalysts are also varied, similarly to the water effects. Reactions between alcohols and organoaluminum cocatalysts are very rapid [36,1194]: AlR3 þ ROH ! AlR2 2OR þ R2H

(5.63)

The rate constant of Reaction (5.63) between AlEt3 and different alcohols at room temperature varies from 2  103 to 2  104 M
1  s
1 [1204]. Alkoxides generated in Reaction (5.63) form stable dimers AlR2(m-OR)AlR2, or trimers if R ¼ Me. These compounds inhibit alkene polymerization reactions with Ti-based heterogeneous catalysts. However, if the [AlR3]:[ROH] molar ratio is high, the main products of Reaction (5.63) are hemi-alkoxides AlR2(m-OR)(m-R)AlR2, which are effective cocatalysts for Ti-based heterogeneous catalysts [1708]. If the order of mixing in the alcohol-modified polymerization reaction is (a-TiCl3+ ROH)+AlEt3, alcohols are adsorbed on the catalyst surface and their interaction with AlR3 proceeds at a much lower rate than in solution. As a result, significant induction periods are observed in these reactions. The duration of these induction periods does not depend on the total amount of an alcohol in the mixture because the alcohol remaining in solution reacts with AlR3 very rapidly and only the adsorbed alcohol molecules react slowly. The duration of the induction period and the acceleration stage that follows it can be very significant, 30–45 minutes at 701C [1204,1708]. However, after these two reaction stages are completed, the stationary polymerization rate can be nearly two times higher than in the absence of an alcohol. In contrast, if d-TiCl3 is used in alcohol-modified polymerization reactions instead of a-TiCl3, an interaction between AlCl3 present in d-TiCl3 and the alcohol sharply reduces the catalyst activity, whereas a preliminary interaction between the same amount of alcohol and AlEt3 produces catalysts of increased activity [1204]. Effects of other destructive poisons: Other studied nonselective destructive poisons for heterogeneous Ziegler–Natta catalyst include HCl [1709], O2 [1698,1710,1711,1712], and several sulfur and selenium compounds [1713]. All these compounds rapidly react with organoaluminum cocatalysts and their kinetic effects are usually similar to those in mixtures of different cocatalysts [1204]. HCl converts AlR3 into AlR2Cl, and propylene polymerization reactions with the a-TiCl3-AlEt3-HCl system closely resemble kinetically the polymerization reactions with ternary a-TiCl3-AlEt3-AlEt2Cl systems [1709]. Until the molar [HCl]:[AlEt3] ratio reaches B1, neither the reaction rates not the content of the crystalline fraction (B75%) are affected by the HCl addition. However, as soon as the [HCl]:[AlEt3] reaches 1, the kinetic behavior of the a-TiCl3-AlEt3-HCl system becomes similar to that of the a-TiCl3-AlEt2Cl system, although the fraction of the crystalline material does not increase to the level typical to the latter catalyst. A further increase of the [HCl]:[AlEt3] ratio converts a part of AlEt2Cl into AlEtCl2, and the catalyst becomes inactive. Molecular oxygen vigorously reacts with AlR3 with the formation of alkoxides AlR2OR [36]; therefore, the kinetic behavior of the TiCl3-AlEt3-O2 system resembles those of TiCl3-AlEt3-AlEt2OR systems or TiCl3-AlEt3-ROH systems [1698,1710–1712]. Until the molar [O2]:[AlEt3] ratio reaches 0.25 (which corresponds to the [AlEt2OEt]:[AlEt3] molar ratio of B1), reaction rates of ethylene and propylene polymerization reactions remain approximately constant

Kinetics of Alkene Polymerization Reactions

385

[1204,1710] but a further increase of the oxygen amount results in rapid deactivation of the catalyst [1710]. If oxygen is contacted with the solid catalyst component before the introduction of the cocatalyst, kinetic consequences depend on the type of the catalyst. When b-TiCl3 is employed, dry oxygen converts a part of it into TiCl4 and the catalyst activity increases. In contrast, if d-TiCl3 is used as the solid catalyst, its performance always deteriorates, especially if AlEt2Cl is used as the cocatalyst [1698,1711,1712]. 5.7.2.5. Other kinetic features of propylene polymerization reactions Catalyst stability: All propylene polymerization catalysts are unstable at temperatures above 401C. In spite of extensive research for several decades, exact reasons for the gradual deactivation of the catalysts remain largely unknown. The only definite observation about this effect is that the deactivation is caused by some chemical reactions rather than by diffusion retardation due to a gradual polymer accumulation. The following experimental observations support this conclusion: 1. Numerous polymerization reactions were carried out with aged catalyst systems, when a solid catalyst and a cocatalyst were brought into contact for a certain period of time in the absence of monomer. The catalyst activity in these experiments, measured immediately after the monomer introduction, is usually the same as the activity of a freshly formed system at this reaction time, i.e., the catalysts lose activity independently on the polymerization reaction [2,1714, 1715]. If a catalyst system is inherently stable, it retains it original activity even after a prolonged pre-contact time. 2. The deactivation rate does not depend on the amount of the formed polymer. When the same catalyst is used in polymerization reactions at different monomer concentrations (and hence produces different amounts of polymer), the rate of its deactivation remains unchanged. Two potential sources of catalyst deactivation are usually considered, interactions between catalysts and cocatalysts, and monomolecular transformations of active centers. When the VCl3-Ali-Bu3 system is used in propylene polymerization reactions, the first reason, gradual VIII-VII reduction, is definitely responsible for rapid deactivation of the catalyst, and the deactivation rate is proportional to the cocatalyst concentration [1204,1715]. The same TiIII-TiII reduction reaction is often proposed as a reason for deactivation of solid and supported Ti-based catalysts [2,777,1204,1215,1226,1551]. However, the deactivation rate of Ti-based catalysts usually does not depend on the cocatalyst concentration (except for very high cocatalyst concentrations [1699]), which makes this hypothesis less plausible. Very little is known experimentally about the nature of monomolecular reactions of active centers that can cause their destruction except for the fact that the deactivation rate always increases with temperature. Kinetic analysis of deactivation reactions adds relatively little to the understanding of the phenomenon because different types of active centers may have different deactivation rates. For example, when a kinetically stable system, a-TiCl3-AlEt3, is aged at 701C for up to 60 minutes and then immediately tested in propylene polymerization reactions, the

386

Alkene Polymerization Reactions with Transition Metal Catalysts

aging leads to a B30% loss of activity, although remaining centers are still very stable kinetically [1716]. The only clue to the nature of the deactivation chemistry is related to the fact that most heterogeneous catalysts are relatively stable in ethylene polymerization reactions but quite rapidly lose activity in polymerization reactions of 1-alkenes or in 1-alkene/ethylene copolymerization reactions (Figure 5.1) [317, 318,1655,1672,1677]. Early stages of propylene polymerization reactions: Studies of alkene polymerization reactions with heterogeneous Ziegler–Natta catalysts by the stopped-flow method were expected to produce detailed information about the kinetic behavior of these complex systems. In some respect, these expectations were fulfilled. Terano produced detailed GPC and Tref data for propylene polymers prepared with several TiCl4/ MgCl2-type catalysts activated with AlEt3 at 301C at very short reaction times, 0.05 to 0.3 seconds [224]. The multi-center nature of the catalysts manifests itself immediately after the polymerization reactions start. Polypropylene produced in such reactions already has a broad molecular weight distribution and Tref fractionation data indicate the presence of at least three discrete types of active centers in the catalysts. Because the molecular weight of the polymers in stopped-flow experiments usually increases proportionally to the polymer yield, these reactions can be considered living-chain polymerization reactions. Their kinetic parameters and concentrations of different active centers can be estimated using the approach described in Section 5.3.3. The results of this research are summarized in Table 5.22. Syndiospecific active centers: Propylene polymers prepared with Ti-based heterogeneous catalysts often contain small fractions of long syndiotactic sequences. Highresolution 13C NMR analysis affords their dependable identification even when their content in a particular polymer is very small, 0.5–1.0%. The formation of these syndiotactic sequences can be approximately described by the chain-end stereocontrol mechanism (Section 3.1.3.4). Two alternative explanations of the presence of these sequences are proposed in the literature (Section 3.2.3.1): (a) the syndiotactic sequences belong to separate macromolecules produced by a few syndiospecific active centers in the catalysts, or (b) these sequences are blocks of syndio-linked monomer units present in predominantly isotactic polymer chains. This difference is not of the outmost significance from the stereo-kinetic point of view. What matters is the effect of reaction parameters on syndiotacticity of these blocks. The statistical parameter defining the stereo-regulating power of syndiospecific active centers according to the chain-end control mechanism, p0syndio ¼ ksyndio =ðksyndio þ kiso Þ, can be estimated from the contents of two most prominent syndio-heptads, and rrrrrm and rrrrrr, as described in Table 3.4. Estimations of the p0syndio value for the syndiotactic component in crystalline fractions of propylene produced with the TiCl4/MgCl2/ dibutyl phthalate-Ali-Bu3 system show that the stereo-regulating power of the syndiospecific centers is quite low and deteriorates with temperature [221]: Temperature (1C) p0syndio

40 0.86

70 0.82

80 0.80

90 0.71

First center kpa (M
1  s
1)

Second center C (%)

[mmmm]

kp (M
1  s
1)

TiCl4/MgCl2-AlEt3 system 23,000 B0.08 – 5,700 TiCl4/MgCl2-AlEt3/ethyl benzoate system 34,000 0.08–0.1 – 9,300 TiCl4/MgCl2/1,3-dietherb-AlEt3/ethyl benzoate system 33,000 0.06–0.08 B0.95 16,000

Third center C (%)

[mmmm]

C (%)

[mmmm]

0.6–0.8

–

900

4.5–1.5

Aspecific

0.3–0.5

–

1,800

1.4–0.6

Aspecific

0.15–0.2

B0.93

3,000

B0.4

Aspecific

kp (M
1  s
1)

Kinetics of Alkene Polymerization Reactions

Table 5.22 Kinetic parameters of active centers in propylene polymerization reactions with TiCl4/MgCl2-AlEt3 systems at 301C [224]

a

Propagation rate constant. b 2-i-propyl,2-i-pentyl-1,3-dimethoxypropane (Scheme 4.2).

387

388

Alkene Polymerization Reactions with Transition Metal Catalysts

The DEact value for rate constants of syndiotactic vs. isotactic monomer addition at these centers is B12.5 kJ/mol (3.0 kcal/mol). 5.7.2.6. Comparison of ethylene and propylene copolymerization kinetics An important subject in kinetic studies of ethylene and 1-alkene polymerization reactions with any heterogeneous Ti-based catalyst can be formulated in the following way: what is a correspondence between active centers producing different components in polymerization reactions of propylene and other 1-alkenes and active centers producing different components in ethylene polymerization reactions? For example, the centers producing Flory components IV and V in ethylene polymerization reactions (the components with the highest molecular weights) polymerize ethylene nearly exclusively and are very stable (Figure 5.13) whereas the centers producing Flory components I–III are true copolymerization centers and they decay much faster. Active centers producing Flory components IV and V in ethylene polymerization reactions are practically inactive in propylene polymerization reactions. On the other hand, Flory components I–III in polyethylene are apparently produced by the same active centers as Flory components Iu–Vu in propylene polymerization reactions. The validity of this argument was evaluated in sequential ethylene/propylene homopolymerization reactions with different heterogeneous catalysts. In these experiments, one of the monomers was polymerized for several hours and then replaced with another monomer [453,1655]. When propylene is polymerized first for long periods of time, the catalysts lose nearly all activity but after the replacement of propylene with ethylene they regain very high activity typical for ethylene homopolymerization reactions (compare Figures. 5.1A and B). In alternative experiments, ethylene was polymerized with the TiCl4/MgCl2/dibutyl phthalate-AlEt3 system at 901C for 3 hours to allow the active centers producing polyethylene Flory components I, II, and III to decay. Then the temperature was decreased to 701C, ethylene was removed from the reactor and propylene was introduced at a seven times higher concentration. Nevertheless, no propylene polymerization activity was detected for over 30 minutes. Finally, the viability of the catalyst was confirmed when ethylene was reintroduced into the reactor and the ethylene homopolymerization reaction has resumed at virtually the same rate as before the replacement with propylene [453,1655]. The existence of active centers that nearly exclusively polymerize ethylene and have very low activity in polymerization of any 1-alkene is also revealed in the structural analysis of ethylene/propylene copolymers prepared with Ti-based heterogeneous catalysts. Such polymerization reactions under step-wise changing conditions are widely used in industry for the synthesis of impact-resistant polypropylene resins. These materials are manufactured in two-reactor processes; highly isotactic polypropylene is produced in the first reactor for 60–80 minutes and then the polymer mixture containing an active catalyst is transferred to another reactor and exposed to an ethylene/propylene mixture for another 60 minutes [1717]. IR analysis of the final polymers [453,1655] and Tref analysis of these products [365,1654] both show that the polymer material prepared in the

Kinetics of Alkene Polymerization Reactions

389

second reactor is not merely random ethylene/propylene copolymer. Rather, it is a mixture of two types of macromolecules, the expected amorphous random ethylene/propylene copolymer and a semi-crystalline ethylene/propylene copolymer with a propylene content of o5 mol.%. The latter material is produced by the active centers responsible for the formation of Flory components IV and V in ethylene polymerization reactions. These active centers remain silent during the polypropylene synthesis stage but start functioning as soon as ethylene is added.

5.7.3. Polymerization reactions of higher 1-alkenes and styrene Polymerization of higher 1-alkenes: Polymerization reactions of higher 1-alkenes with solid and supported Ziegler–Natta catalysts exhibit many of the features observed in propylene polymerization reactions with the same catalysts. The most obvious among them is the multi-center-nature of the catalysts. Figure 5.19 shows GPC curves of four polymers of 1-octene produced at 301C with several Ti-based catalysts, two solid, b-TiCl3 and d-TiCl3  0.3AlCl3, and two supported, the Solvay TiCl4/d-TiCl3 catalyst and a TiCl4/MgCl2 catalyst, all using AlEt3 as a cocatalyst [127,1718]. Superficially, four polymers have different average molecular weights and molecular weight distributions of a different width, the broadest in the case of the TiCl4/MgCl2 catalyst and the narrowest in the case of d-TiCl3  0.3AlCl3. However, resolution of the GPC curves into Flory components shows that all these polymers consist of the same five Flory components with weight-average molecular

Figure 5.19 GPC curves of four polymers of 1-octene produced with T|-based catalysts, two solid, b-T|Cl3 and d-T|Cl3  0.3AlCl3, and two supported,T|Cl4/d-T|Cl3 (Chapter 4, Section 4.3.1) and T|Cl4/MgCl2 [1718]; cocatalyst AlEt3, 301C.

390

Alkene Polymerization Reactions with Transition Metal Catalysts

weights that vary from B1.0–1.2  106 for the component with the highest molecular weight to B6–12  103 for the component with the lowest molecular weight. The catalysts differ mostly in proportions between different Flory components in the polymer mixtures they produce. The contents of different Flory components slightly change with reaction time (monomer conversion). A combination of the kinetic data and the measurement of the number of active centers using acetyl chloride as a destructive poison (Section 5.7.4.2.5) gives the following kinetic parameters of the active centers producing Flory components IIIu, IVu, and Vu in the 1-octene polymerization reaction with the TiCl4/d-TiCl3-AlEt3 catalyst system [127,1718]: Flory components C (% of Ti atoms) kp (M
1  s
1)

IIu 0.01 32

IIIu 0.0067 70

IVu 0.02 240

When AlEt3 is replaced with AlEt2Cl as a cocatalyst for the TiCl4/d-TiCl3 catalyst, the average molecular weight of the 1-octene polymer increases from B4  105 to B8  105 [1718]. However, GPC data show that this change can be attributed mostly to a large increase in the fraction of the Flory component with the highest molecular weight in the polymer mixture (component IVu) rather than to any changes in the nature of active centers. Similar effects of the cocatalyst replacement from AlR3 to AlR2Cl in ethylene/1-alkene copolymerization reactions are described in Section 5.7.1.2. A change in the reaction temperature also mostly affects relative contents of different Flory components rather than their molecular weights. As the temperature increases from 10 to 301C, the fraction of Flory component Vu decreases from 65 to B34% whereas its molecular weight changes very little, from 1.4  106 to 1.1  106 [127,1718]. The effect of hydrogen on the polymerization kinetics of higher 1-alkenes is also very similar to its effect in propylene polymerization reactions described in Section 5.7.2.2. The introduction of hydrogen produces the same two changes: molecular weights of the polymers decrease and the activity of the catalysts noticeably increases. This increase was observed, e.g., in 1-butene polymerization reactions with the d-TiCl3-AlEt2Cl system [1719], in 4-methyl-1-pentene polymerization reactions with the g-TiCl3-AlEt2Cl system [1720], and in 1-decene polymerization reactions with the TiCl4/MgCl2/ethyl benzoate-AlEt3 system [1212]. Polymerization of styrene: Polymerization reactions of styrene with solid and supported Ti-based catalysts exhibit the same level of complexity as the polymerization reactions of 1-alkenes. Styrene polymers prepared with these catalysts are mixtures of macromolecules of different stereoregularity, from a highly crystalline, nearly perfectly isotactic material to completely amorphous atactic fractions of a low molecular weight, B3,000. These atactic fractions consist of two different products. One is the atactic polymer produced with aspecific Ti-based active centers, the same centers that are responsible for the formation of atactic polymers of propylene and other 1-alkenes. The second type of atactic material is the product of a side-reaction, cationic styrene polymerization with acidic species in the catalysts.

Kinetics of Alkene Polymerization Reactions

391

GPC curves of crystalline polystyrene fractions (the material insoluble in boiling methyl ethyl ketone) prepared with Ti-based supported catalysts are usually broad and multimodal indicating the presence of several families of active centers that produce macromolecules with molecular weights ranging from B3  103 to B1  106 [492]. The kinetics of styrene polymerization reaction with the a-TiCl3AlEt3 system exhibits the same features as the kinetics of propylene polymerization reactions with the same catalyst, the reaction rate at 701C increases for B60 minutes and then remains approximately constant for over 8 hours [6]. The average molecular weight of the polymer increases for nearly 2 hours, from B4  105 to B1.5  106, apparently for the same reason as the molecular weight of polyethylene: the centers that produce the material of a lower molecular weight are formed relatively rapidly but they deactivate faster. Active centers of different stereospecificity in TiCl4/MgCl2 catalysts activated with AlEt3 develop at different rates. These differences are usually small in propylene polymerization reactions and they were detected only in stopped-flow experiments. Styrene polymerization reactions with the TiCl4/MgCl2-AlEt3 system represent an example of a different kinetic behavior [491]. Differences in the formation rates of different types of active centers are significant in these reactions and can be observed either by examining analytical fractionation data or the changes in the molecular weight distribution of the polymers. Judging by the GPC data of polymers produced at different reaction times, this catalyst system has two groups of isospecific active centers. The first group produces polymers with an average molecular weight of B1  105. These centers are formed rapidly, within 1 minute after mixing catalyst components. The second group of the centers produces polymers with Mw of B2  106; these centers are formed much slower, after B10 minutes [491]. This example demonstrates one more time the futility of using average molecular weights of multi-component polymers for the estimation of such kinetic parameters as the propagation rate constant or the number of active centers. The average molecular weight of isotactic polystyrene produced in these reactions indeed gradually increases with reaction time. However, this increase is caused by a redistribution of polymer fractions of different molecular weights over time rather than by the living-chain nature of the polymerization reactions.

5.7.4. Estimation of number of active centers in Ziegler–Natta catalysts The issue of the concentration of active centers C in heterogeneous Ziegler–Natta catalysts was a subject of contention for a significant period of time. When early heterogeneous Ti-based catalysts are studied, the goal appears to be simple. These catalysts, after the stage of the activity increase, often display a very stable polymerization behavior, and the polymerization rate in these experiments can be measured quite precisely. Formally, Equation (5.11) represents the expression for the polymerization rate under these conditions. The measurement of one of two values in Equation (5.31), either kp or C, is in principle sufficient to determine both essential kinetic features of the polymerization centers, their concentration and reactivity. However, such measurements have turned out to be tantalizingly difficult to carry out.

392

Alkene Polymerization Reactions with Transition Metal Catalysts

Two approaches to the estimation of the C values were pursued: 1. Estimations of the propagation rate constant kp from kinetic data, by studying dependencies between molecular weights of alkene polymers and reaction time [6,1572,1651], especially in stopped-flow experiments at very short reaction times (Section 5.3.3). 2. Direct estimations of the C value by applying the techniques widely used in the field of heterogeneous catalysis in general, through poisoning the catalysts.

5.7.4.1. Kinetic approaches to estimation of number of active centers C measurements in short-duration experiments: If a significant increase of the molecular weight of a polymer over time is observed, Equations (5.31–5.37) can be used for an approximate estimation of the propagation rate constant kp and then for the calculation of the C value. Equation (5.37) is especially suitable for this purpose because it can be used even in commonly encountered situations, when catalysts are not stable (the C value changes with time) and when chain transfer reactions manifest themselves as slow leveling-off of the molecular weight over time. In these situations, the relationship between the total number of the polymer chains N(T ) formed in a polymerization reaction after the reaction time T and kinetic parameters of the polymerization reaction (all directly measured in experiment) provide the means for the estimation of the C value [1127,1214,1572,1573]. Several measurements of molecular weights of 1-alkene polymers prepared with supported TiCl4/MgCl2/Modifier I catalysts showed that both the average molecular weight of the polymers prepared with these catalysts at 40–601C and their molecular weight distribution change little with reaction time [1573, 1684,1695]. These data alone indicate that the average chain-growth time of a polymer molecule with these catalysts is less than 5 seconds. This rough estimation translates into a high kp value, W450 M
1  s
1, and, correspondingly, into a low C value, 2–3% of Ti atoms in the supported catalysts. Nevertheless, attempts to evaluate kinetic parameters of these polymerization reactions based on changes of the molecular weight at the earliest stages of the reactions initially appeared promising. Potentially, the simplest implementation of this technique is a singlepoint measurement, a polymerization reaction is stopped after a very short period of time, a few seconds at most, the molecular weight of the polymer is measured, and the lower limit of the kp value is estimated using Equation (5.33). Several examples of these estimations are given in Table 5.23. Of course, when these catalysts are used for alkene polymerization at relatively high temperatures and for significant periods of time, at least several minutes, a linear increase of the molecular weight with time predicted by Equation (5.33) is never observed. However, even when the experiments are interrupted after short reaction times, the molecular weight often increases with time only slightly in the beginning of the reactions before becoming approximately constant. In this situation, if the catalysts indeed were true single-center catalysts, the second approach to C and kp measurements based on the use of Equation (5.37) could be applied [6,1572,1651]. These estimations gave following kinetic parameters of the

393

Kinetics of Alkene Polymerization Reactions

Table 5.23 Approximate estimations of concentration of active centers and low limits of propagation rate constant in propylene polymerization reactions by single-point kinetic method

a

Catalyst

Cocatalyst

Temperature (1C)

C (mol.% of kp (M
1 s
1) Ti)

TiCl4/MgBu2 TiCl4/MgCl2/ EBa v-v v-v v-v v-v

– AlEt3

40 20

6.0

v-v v-v v-v AlEt3/EBa

30 40 40 60

6.8 o2.3 2.8

Reference

500–1,000 1,230

[1721] [1580]

2,180 500–1,000 B500 2,700

[1580] [1684] [1722] [1573]

EB ¼ ethyl benzoate.

TiCl4/MgCl2/ethyl benzoate-AlEt3 system, C=B2–6% of Ti atoms in the catalyst, kp ¼ 500–1500 M
1  s
1. Another example, a propylene polymerization reaction with the same catalyst at 501C, gives similar results, C ¼ 2.0% of the Ti atoms, kp ¼ 393 M
1  s
1 [1723]. In practice, this approach has two significant drawbacks: 1. If alkene polymerization reactions with Ziegler–Natta catalysts are performed at short reaction times, reaction rates R(t) usually change little with time but both the number of formed polymer molecules, N(t), and the Yield(t) values (see Equation (5.37)) are relatively large and both change rapidly with time. Because the C value, the intercept in Equation (5.37), is, in relative terms, a very small number compared to any N(t) value, the precision of the C estimation is very low. 2. All Ti-based solid catalysts have several types of active centers, and the observed changes of the molecular weight with time at the early stages of polymerization reactions (the prerequisite for the use of Equation (5.37)) are usually related to changes in relative populations of different centers rather than to a slow chain propagation reaction. C measurements in stopped flow experiments: In stopped-flow polymerization experiments, reactions are studied for significantly shorter periods of time compared to experiments described in the previous section, usually less than 1 second. If propylene polymerization reactions are carried out at 20–301C for 0.2–0.3 seconds, polymer yields increase proportionally to the reaction time [1575–1580], e.g., the reaction rate is practically constant. However, kinetic experiments at much longer reaction times show that the reaction rates steeply increase for several minutes (see Figures. 5.1A and 5.3. The molecular weight of polymers in the stopped-flow experiments also increases practically proportionally to the reaction time (up to a certain point), making the use of Equation (5.33) (or Equation (5.35)) suitable for the evaluation of the kp values and, respectively, Equation (5.31) can be used for the evaluation of C values. Table 5.24 gives several examples of these measurements in

394

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 5.24 Kinetic parameters of TiCl4/MgCl2-AlR3 systems in stopped-flow propylene polymerization experiments at 301C [365,1578,1579] AlR3

Modi¢er I

kp (M
1  s
1)

C/[Ti] (mol.%)

[mmmm]av

AlEt3 Aln-Bu3 Ali-Bu3 Aln-Hex3 Aln-Oct3 AlEt3 AlEt3 v-v, pretreated catalyst

None v-v v-v v-v v-v Ethyl benzoate Dibutyl phthalate

3,900 4,000 2,800 3,700 3,400 830 720 1,000

5.6 3.6 2.0 2.7 1.6 4.0 0.7 1.6

0.93 0.93 0.88 0.92 0.89

stopped-flow experiments lasting 0.2–0.3 seconds [365,1578]. The main conclusions from these results are: 1. The concentration of active centers in the catalysts is quite low, a few percents of the total amount of Ti atoms. This makes a direct observation of active centers by physical/spectroscopic methods very difficult. 2. The value of the propagation rate constant for this family of catalysts is very high. Even at 301C, the average growth time of a polymer chain with Mn of B1.0  105 at a propylene concentration of 1 M is about 1 second. 3. The value of the propagation rate constant is approximately the same for catalysts activated with different cocatalysts. The differences in catalyst activity, at least at the earliest stages of polymerization reactions, are mostly related to differences in the concentration of active centers. 4. Judging by average [mmmm] values of the polymers, the active centers formed initially in the catalysts are mostly isospecific. The [mmmm] values for crystalline fractions of these polymers are all close to 0.99, and the propagation rate constant for highly isospecific centers is B5,000 M
1  s
1. 5. Polypropylene samples prepared with the TiCl4/MgCl2-AlEt3 system even after very short reaction times have broad molecular weight distributions [365,1578,1579]. For example, the Mw/Mn ratio of a polymer produced with the TiCl4/MgCl2/dibutyl phthalate-AlEt3 system at 301C after 0.34 seconds is already 5.0 [1579]. These data show that the catalyst has several types of active centers and that all the centers operate in parallel immediately after the catalyst components are brought into contact. The molecular weight distribution of propylene polymers produced in these experiments can be described by the Flory-type distribution curve even after very short times [224] whereas GPC curves of true living polymers prepared with homogeneous catalysts in similar experiments are very narrow (see Section 5.4.2). The stopped flow technique was also applied to copolymerization reactions of different alkenes. When ethylene/propylene copolymerization experiments with the TiCl4/MgCl2/ethyl benzoate-AlEt3 system were carried out at 201C,

395

Kinetics of Alkene Polymerization Reactions

the Mw/Mn value of the copolymer produced after 0.04 seconds was already 5.5 [278]. Both the propylene content in the copolymers and the distribution of active centers in the catalyst change very little at reaction times from 0.035 to 0.145 seconds. Although the polymer yield increases nearly proportionally to the reaction time (as in many similar stopped-flow experiments), an increase of the molecular weight with time in not linear indicating that chain transfer reactions even at 201C cannot be neglected in such short-duration experiments. The number of active centers in this catalyst system also corresponds to B3% of Ti atoms in the catalyst, and propagation rate constants for two chain growth reactions, those of ethylene and propylene, are 4.7  104 M
1  s
1 and 2.5  103 M
1  s
1, respectively. When stopped-flow polymerization experiments are carried out at higher temperatures, further complications arise [459]. At 701C, the molecular weight of polypropylene produced with the TiCl4/MgCl2/dibutyl phthalate-Ali-Bu3 system reaches the saturation level even after 0.15–0.2 seconds. The average chain growth time at this temperature is B0.2–0.3 seconds and the C value corresponds to 1.6% of Ti atoms in the catalyst. In addition, the fraction of highly isospecific active centers in the catalyst decreases over time (as evidenced from the decrease of the [mmmm]av value) indicating that the results produced at very short reaction times may not be representative of the performance of the same catalysts in ‘‘realistictime’’ experiments [459]: Reaction time [mmmm]av

0.2 sec. 0.911

20 sec. 0.862

10 min. 0.783

120 min. 0.679

5.7.4.2. Poisoning of active centers and estimation of their number The estimation of the number of active centers in catalyst-poisoning experiments is a classic technique in heterogeneous catalysis studies. This method was also applied to Ziegler–Natta catalysts. Virtually any polar organic and inorganic compound poisons active centers in Ti-based catalysts (Sections 5.7.1.2 and 5.7.2.2). A number of different compounds were tested as the poisons. This research has two goals, elucidation of the chemical structure of the centers (discussed in Chapter 6) and the measurement of their concentration. The list of poisons used for the second purpose includes CO, CO2, I2, CS2, SO2, alcohols, allene, trialkylamines, phosphines, organic acid chlorides, etc. [1551,1692,1705,1724–1731]. 5.7.4.2.1. CO and CO2 as poisons, step-poisoning experiments. Carbon monoxide and carbon dioxide are very potent poisons for any Ti-based catalyst. Two types of studies were employed to investigate the poisoning effect of these two compounds, kinetic analysis and the use of 14C- and 13C-labeled CO or CO2. These studies showed great complexity of poisoning reactions. When a small quantity of CO or CO2 is added to a stable polymerization reaction of ethylene or propylene with the d-TiCl3-AlEt3, the TiCl3-AlEt2Cl or the TiCl4/ TiCl3-AlEt2Cl system at 50–701C, the reaction rate is depressed very rapidly, in 1–2 minutes [1167,1562–1564]. Vanadium-based supported catalysts are also efficiently

396

Alkene Polymerization Reactions with Transition Metal Catalysts

poisoned with CO [1071]. If the poison is not removed from the reactor, the activity remains depressed for 10–20 minutes, depending on the temperature, but then it partially recovers over the next 30–40 minutes [1564]. The recovery of polymerization activity in the presence of CO is caused by its gradual consumption. (Some of these CO consumption reactions do not involve a monomer; they are side-reactions with different constituents of the catalysts, most probably, with alkylated Ti species in the catalysts because neither solid catalyst components alone nor cocatalysts react with CO [1564].) If the poison is removed from the reactor, the catalyst activity recovers rapidly and sometimes nearly completely [1167, 1562,1563]. These poisoning data were also used for the C measurement. For example, a plot of the amount of added CO vs. reaction rate, after extrapolation to the CO amount causing the complete poisoning of the catalyst, gives the estimation of the C value as B1.9% of Ti atoms in the catalyst, and a very high value of the propagation rate constant, kp B18,000 M
1 sec
1 [1029]. Two prerequisites for the use of this technique for the C measurement should be met: 1. The reactions should be carried out with gaseous monomers and in the absence of any solvent in order to avoid distribution of CO between the gas and the liquid phase. This condition is easily met in polymerization reactions of ethylene and propylene. 2. The outcome of the C measurement is based on the assumption that all introduced CO coordinates exclusively at polymerization centers, > Ti

Polymer

+

C=O

> Ti

Polymer

(5.64) C=O

and does not react with other transition metal atoms. Formally, these poisoning/ reactivation steps can be described by a simple Langmuir adsorption scheme with a very high equilibrium constant [1732]. The absence of direct information about the distribution of CO molecules in the catalysts makes such poisoning experiments suitable only for the estimation of the maximum possible number of active centers. Several such estimations were carried out by CO poisoning of propylene polymerization reactions in the gas phase at B401C [1551]: Catalyst TiCl3-AlEt3 TiCl3-AlEt2Cl TiCl4/MgCl2/ethyl benzoate-AlEt3 C (mol/mol Ti) 1.5% 0.8% 0.6–1.6%

The value of the propagation rate constant in these reactions varies from B30 M
1  sec
1 for TiCl3-based catalysts to 300–350 M
1  sec
1 for the supported catalyst. A similar CO-titration method was used in gas-phase propylene polymerization reactions at 501C with the Solvay TiCl4/d-TiCl3-AlEt2Cl system, C is equal to B3% of Ti atoms in the catalyst [1733]. However, when the same

397

Kinetics of Alkene Polymerization Reactions

approach was applied to propylene polymerization reactions with several highly active TiCl4/Mg(OEt)2/benzoyl chloride-AlR3 systems at 30–501C, it gave much higher C values, from B10 to 30 mol.% of the Ti atoms whereas respective average kp values were similar, 200–300 M
1 sec
1 [1733,1734]. These measurements of C values at different reaction times showed that a decrease in activity typical for many propylene polymerization catalysts (Section 5.7.2.1) is caused by the decrease in the number of active centers. 5.7.4.2.2. CO and CO2 as poisons, 14C-labeling. Interruption of polymerization reactions by the addition of CO or CO2 offers the second avenue to the determination of the number of active centers, by using 14C-labeled CO or CO2. A large volume of experimental data (IR, NMR, etc.) shows that the CO molecule coordinated at the Ti atom in an active center (Reaction (5.64)) reacts with it. The most significant results clarifying the chemistry of these secondary reactions were produced in studies of model catalyst mixtures [1735] and in stopped-flow polymerization experiments [460]. Stopped-flow ethylene polymerization experiments with two TiCl4/MgCl2/ethyl benzoate-AlR3 systems (R=Me and i-Bu) at 201C for B0.06 seconds produced very short polyethylene chains (the average polymerization degree ranging from B200 to B400), which were still mostly attached to the active centers. A prolonged exposure of this mixture to a small quantity of CO at
781C followed by destruction of the catalysts with acidic ethanol did not result in any significant incorporation of CO moieties in the polymer. However, an exposure to a large quantity of CO generated carbonyl groups in the polymer due to insertion of the coordinated CO molecule into the Ti
C bond: > Ti C=O

Polymer

> Ti Polymer

(5.65)

O

Reaction (5.65) also takes place when a catalyst is contacted with a cocatalyst AlR3 and the system is poisoned with14CO in the absence of any monomer [1738]. In the latter reactions, the coordinated CO molecule inserts into the Ti
R bond formed in the reaction between the Ti
Cl bond in the catalyst and AlR3 and produces hydrocarbon-soluble 14C-labeled products. These results justify the second approach to the measurement of C values, by using 14C-labeled CO and tracing 14C tags in the polymers. CO2 can also be used for the C measurement but it is less suitable for these studies because of its interaction with organoaluminum compounds [36,1194,1735]. Ti species formed in Reaction (5.65) are usually assumed to be unreactive with alkenes and are destroyed with alcohols at the end of polymerization reactions: 4Ti214 Cð¼ OÞ2CH2 2Polymer þ ROH ! 4Ti2OR þ H214 Cð¼ OÞ
CH2
Polymer

(5.66)

Indeed, when 14C-labeled CO and CO2 are used to poison Ti-based catalysts and the catalysts are decomposed with large quantities of alcohols, 14C tags are found in the recovered polymers [1563,1726,1727,1729,1736,1737].

398

Alkene Polymerization Reactions with Transition Metal Catalysts

Chemistry of CO reactions: Two features of Reaction (5.65) remain unknown: (a) how complete, fast, and irreversible this insertion reaction is and (b) how ‘‘dead’’ is the active center produced in the reaction, e.g., whether it is possible that the insertion of an 1-alkene molecule into the Ti
C(¼O) bond, however slow, can still take place: 4Ti214 Cð¼ OÞ
CH2 2Polymer þ CH2 ¼ CHR ! 4Ti2CH2 2CHR214 Cð¼ OÞ2CH2
Polymer

(5.67)

If Reaction (5.67) occurs, another Reaction (5.65) can follow it, resulting in the introduction of several 14C labels into a single polymer molecule. (One has also to take into account an additional complication, ketone species, including those generated in Reactions (5.65) and (5.67), are prone to reduction by organoaluminum cocatalysts, especially at increased temperatures typical for alkene polymerization reactions [36,1194,1731,1739].) These questions were answered in experiments when a monomer was removed from the reaction medium, 14CO was allowed to react with polymerization centers for different periods of time, and, finally, the monomer was reintroduced. Experiments with propylene polymerization with TiCl3-AlR3, TiCl2-AlR3, and TiCl4/MgCl2/ethyl benzoate-AlR3/ methyl p-toluate systems showed the following results [1562,1563,1629,1700]: 1. When the reaction with the TiCl4/MgCl2/ethyl benzoate catalyst at 01C (a stable polymerization reaction) continues for a given period of time, then the monomer is removed and 14CO is introduced for 1 hour, the [14C-label]/[Ti] ratio steadily increases with polymerization time, from B0.8% after 30 minutes to B1.9% after 4 hours. This result is compatible with a low formation rate of active centers in propylene polymerization reactions at mild temperatures. 2. When the same reaction at 01C is followed by the reaction with 14CO for a much longer time, 48 hours, the [14C-label]/[Ti] ratio also steadily increases with polymerization time and the number of 14C labels nearly doubles, up to B3.5% of Ti atoms in the catalyst after a 4-hour reaction. This observation led to the conclusion that different types of active centers in heterogeneous catalysts react with CO at different rates, some very rapidly and other slowly [1572,1602,1740]. 3. When CO is introduced into the reaction system and, then it is removed and propylene is reintroduced, the polymerization reaction continues either at a reduced rate [1562] or at practically the same rate [1563]. The number of 14C tags does not change if the polymerization reaction is restored [1563]. 4. When 14CO is introduced into the reaction system, the polymerization reaction immediately stops but the content of CO units in the polymer continues to increase for a significant period of time [1629]. All these findings indicate that the CQO coordination reaction at a polymerization center (Reaction (5.64)) is reversible and that the CQO insertion reaction, Reaction (5.65), although irreversible, is relatively slow. In addition, IR and 13C NMR data showed that, opposite to expectation, solvolysis of

Kinetics of Alkene Polymerization Reactions

399

CO-poisoned catalysts with alcohols does not produce aldehydes (Reaction (5.66)) [1741] but ketones [460] or alkenes with internal CQC bonds [1735]. Kinetics of CO reactions: The poisoning effect of CO in supported Ti-based Ziegler–Natta catalysts depends on the amount of added CO. The introduction of a significant quantity of CO results in a nearly instantaneous and permanent suppression of polymerization reactions. However, when a small quantity of CO is added to stable polymerization reactions of ethylene [1029] or propylene [1627, 1733,1734,1737] at 50–801C, reaction rates are also depressed very rapidly but they slowly recover by themselves over a period of 40–60 minutes. Several reasons for the self-recovery were proposed. The first one is trivial, the amount of CO in the reactor gradually decreases indicating its consumption, either in side-reactions with different constituents of the catalysts [1564] or in catalysisrelated reactions [1733]. The second explanation of the self-recovery assumes that Reaction (5.67) indeed takes place (a relatively slow step [1029]). It produces a growing polymer chain with a g-positioned carbonyl group, WTi
CH2
CHR
C(QO)
CH2
Polymer, that may form an intramolecular complex with the Ti atom (this also explains the formation of ketone groups in polymers instead of expected aldehyde groups). If both an alkene and CO are present in the reactor, the possibility of Reaction (5.67) signifies a slow copolymerization reaction of the alkene and CO resulting in a steady accumulation of 14C tags in the polymers, an experimentally observed fact [1563,1629,1726,1727,1729,1736, 1737]. According to this explanation, a full recovery of polymerization activity occurs after all added CO is consumed in the copolymerization reaction. To prevent it from occurring (an important prerequisite for measuring the C value), phosphines can be added to the reaction mixture following the CO admission [1742]. The third explanation of the spontaneous catalyst recovery suggests a particular chain transfer reaction with a cocatalyst, an exchange reaction between a sleeping CO-poisoned active center (after Reaction (5.67)) and the cocatalyst [1563,1564, 1733,1734,1743]: 4Ti2CH2 2CHR2Cð¼ OÞ2CH2 2Polymer þ AlR3 ! 4Ti2R þ R2 Al2CH2 2CHR2Cð¼ OÞ2CH2 2Polymer

(5.68)

Solvolysis of organoaluminum compounds formed in Reaction (5.68) with alcohols also leads to polymer chains containing ketone groups. Chemical complexity of CO-catalyst interactions and their kinetic features require a very cautionary approach to C estimations based on the use of 14Clabeled CO. The data in Table 5.25 show that this technique gives very low C levels in the majority of examined catalysts, usually less than 0.1–0.2% of Ti atoms in the catalysts, and, correspondingly, very high propagation rate constants. Both CO and CO2 were also used for the C estimation in ethylene polymerization reactions with supported organotitanium and organozirconium catalysts [843,1742,1745]. According to experiments with 14C-labeled CO, merely B1% of transition metal atoms in the latter catalysts form active centers.

400

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 5.25 Concentration of active centers and propagation rate constant at 70–801C, measurement by poisoning with CO [1699,1727] Temperature (1C)

C (mol.% of M)

kp (M
1 s
1)

Ethylene polymerization reactions – TiCl2 AlEt3 TiCl2 AlEt2Cl TiCl2 AlEt3 a-TiCl3 AlEt3 d-TiCl3 Ali-Bu3 d-TiCl3 Ali-Bu3 VCl3

80 70 80 80 80 80 70

0.011 0.003 0.008 0.05 0.01 0.112 0.01

12,000 12,000 12,000 14,000 12,000 14,000

Propylene polymerization reactions – TiCl2 AlEt3 TiCl2 AlEt2Cl TiCl2 AlEt2Cl a-TiCl3 AlEt2Cl d-TiCl3 Ali-Bu3 d-TiCl3 Ali-Bu3 VCl3

70 70 70 70 70 70 70

0.013 0.005 0.008 0.33 1.7 5.8 0.23

76 120 94 71 90 100 370

Catalyst

Cocatalyst

5.7.4.2.3. Allene and CS2 as poisons. The effect of allene, CH2QCQCH2, on alkene polymerization reactions with Ti-based catalysts is similar to that of CO. An addition of a small quantity of allene to a propylene polymerization reaction catalyzed by the d-TiCl3-AlEt2Cl system brings about a nearly instantaneous decrease of the polymerization rate due to the complex formation between allene molecules and Ti atoms in active centers (similar to Reaction (5.64)). However, the polymerization reaction slowly but completely recovers over a period of 1 hour due to consumption of allene in copolymerization with propylene [1564,1572,1746]. The degree of the initial activity decrease depends on the amount of added allene. This dependence was used for the evaluation of the concentration of active centers in two catalysts, as shown in Table 5.26. CS2 is also a potent poison in alkene polymerization reactions [1627,1692, 1705,1728,1730,1747,1748]. Addition of excess CS2 to an ongoing polymerization reaction brings about an instantaneous decrease of the reaction rate or completely halts the reaction [1627,1705,1730]. However, if CS2 is removed from the reactor, the polymerization reaction can be partially restored [1730,1748]. Propylene polymers produced with the d-TiCl3-AlEt3 system and TiCl4/MgCl2type catalysts and recovered after exposure to CS2 contain small amounts of sulfur corresponding to a [S]:[Ti] ratio of B0.4–0.6%. However, if both propylene and CS2 are present together in poisoned reaction systems, the [S]:[Ti] ratio gradually increases suggesting a slow propylene/CS2 ‘‘copolymerization’’ reaction [1728, 1730,1747,1748]. Investigations of CS2 poisoning effects are complicated by side-reactions between CS2 and cocatalysts, AlEt3 and AlEt2Cl [1747]. Analysis of the fractional composition of propylene polymers produced with the

401

Kinetics of Alkene Polymerization Reactions

Table 5.26

a

Concentration of active centers, measurement by poisoning with allenea [1572]

Catalyst

Cocatalyst

C (mol.% of Ti)

d-TiCl3 TiCl4/d-TiCl3b TiCl4/d-TiCl3b

AlEt2Cl AlEt2Cl AlEt3

1.5 2.2–2.7 10–12

Propylene polymerization reactions at 601C. Solvay catalyst (Section 4.3.1).

b

TiCl4/MgCl2/diester-AlEt3/(EtO)3SiPh system at 701C showed that CS2 nearly equally poisons both isospecific and aspecific active centers [1692,1748]. 5.7.4.2.4. Destructive poisons, alcohols. Alcohols vigorously react with polymerization centers and destroy them [844,1725,1744,1749,1750,1751]. If the alcohol is labeled with tritium, RO3H, the tritium atom is transferred to the polymer chain as one of the H atoms in the last methyl group in the polymer:

4Ti2CH2 2Polymer þ RO3 H ! 4Ti2OR þ Cð3 HÞH2 2Polymer (5.69) The kinetic isotope effect of ROH vs. RO3H in Reaction (5.69) is 1.6–1.7 [1744, 1750]. There is one more polymer-bearing species in any alkene polymerization reaction that reacts with alcohols equally vigorously and produce labeled polymer chains, the products of chain transfer reactions to cocatalysts: R2 Al2CH2 2Polymer þ RO3 H ! R2 Al2OR þ Cð3 HÞH2 2Polymer

(5.70)

The measurement of the combined tritium content in the products of Reactions (5.69) and (5.70) gives the total number of metal–polymer bonds (MPB) in a reaction system. It is obvious that if the polymerization rate is stable or slowly declines and the yield of the polymer increases, the number of the Al–Polymer bonds in the system participating in Reaction (5.70) should steadily increase whereas the number of the Ti–Polymer bonds participating in Reaction (5.69) should remain approximately the same. This increase in the total MPB value was indeed observed many times [1665,1700,1744,1750,1752]. Conceptually, the intercept of the MPB/[Ti] vs. time correlations to zero time (or to zero yield) should give the number of active centers in the catalyst. In practice, the use of 3H-labeled alcohols for the measurement of the concentration of active centers encounters several kinetics-related difficulties. One reason for the difficulties becomes obvious if one compares the MPB number with the number of Ti atoms in a given catalyst. The following examples of ethylene polymerization reactions with the TiCl4/MgCl2-AlEt3 system can serve as an illustration [1665]: Reaction time (minutes) [MPB]/[Ti] at 501C [MPB]/[Ti] at 701C

5 1.5 3.0

10 1.9 4.7

20 3.7 6.8

40 14.0

60 9.8 24.4

80 10.4 21.4

402

Alkene Polymerization Reactions with Transition Metal Catalysts

In both examples, [MPB] values steeply increase with reaction time and experimentally measured [MPB]/[Ti] ratios are always higher than one. Both these facts indicate that Reaction (5.70) is responsible for a large fraction of the [MPB]/ [Ti] value. In these situations, an extrapolation of the [MPB] value to a zero time involves significant uncertainty which makes the estimation of C values semiquantitative. The second source of difficulties becomes clear when the C values are measured in TiCl2-derived catalysts. Pure TiCl2 polymerizes ethylene without any cocatalysts, and Reaction (5.70) is absent. Measurements of the C value in this catalyst with CO (Table 5.25), CO2, and MeO3H give similar numbers, 1  10
5 to 4  10
5 C/Ti, which correspond to B0.1% of Ti atoms on the catalyst surface [1727]. However, when a combination of TiCl2 and AlEt2Cl is used in the polymerization reactions, the measurements with CO and CO2 still give approximately the same number of active centers whereas the number of 3H tags in polymers in alcohol-terminated reactions increases B100 times. Moreover, if the polymerization reaction with the TiCl2-AlEt2Cl system is stopped by poisoning the catalyst with CO or CO2 and then MeO3H is added to the reaction mixture, a large number of 3H tags are still present in the polymers underlying the extent of Reaction (5.70). The use of Reaction (5.69) for the C measurement relies on its strict directionality, the OR group becomes attached to the transition metal atom and the hydrogen atom is attached to the last unit in the growing polymer chain. This directionality is indeed sustained in polymerization reactions catalyzed by Ti-based systems but is does not hold for V-based heterogeneous catalysts [1751] where two types of last polymer units, C(3H)H2
Polymer and RO
CH2
Polymer, are formed in comparable amounts. One more complicating factor in measuring the C value using Reaction (5.69) is a slow isotope exchange between 3H in RO3H and H atoms in polymers [1665,1744,1750,1751,1753]. The latter reaction is completely unrelated to polymerization reactions as such and is possibly acidcatalyzed [1753]; it can lead to overestimation of the C number. The exchange is negligible in propylene polymerization reactions with supported catalysts [1744] and with VCl3 [1750]. However, a significant level of the isotope exchange was found between RO3H and polyethylene [1753]. Table 5.27 gives several examples of the measurement of active centers and the values of propagation rate constants in poisoning experiments of different catalysts with RO3H. In the case of solid polymerization catalysts, g-TiCl3 and d-TiCl3, the results are generally consistent with C estimations by other poisoning methods, the C values are less than 1% of transition metal atoms in the catalysts. However C estimations for supported catalysts sometimes give much higher fractions of transition metal atoms as active centers. This inconsistency calls for great caution in applying this technique. 5.7.4.2.5. Destructive poisons, acid chlorides. Acid chlorides are potent destructive poisons in alkene polymerization reactions. Addition of a small amount of acetyl chloride to an ongoing propylene polymerization reaction with a TiCl4/MgCl2-type catalyst activated with AlR2Cl, corresponding to an

C (mol.%)

kpa (M
1 s
1)

Reference

Ethylene polymerization reactions AlEt2Cl d-TiCl3 AlEt3 TiCl4/MgO v-v v-v AlEt3 TiCl4/(Al2O3-SiO2) – TiCl4/MgBu2 – TiCl4/AlEt3 AlEt3 TiCl4/MgCl2

60 70 50 70 70 70 60

0.7 21 39 24 0.08–0.15 0.01–0.02 8–10

78 2,380 2,440 107 10,000–15,000 10,000–13,000 2,200–3,500

[1754] [1755] [470] [1755] [1721] [1721] [1627]

Propylene polymerization reactions g-TiCl3 AlEt3 AlEt2Cl d-TiCl3 v-v v-v v-v v-v v-v v-v v-v v-v v-v v-v v-v v-v v-v v-v AlEt3 TiCl4/MgO – TiCl4/MgBu2 – TiCl4/AlEt3 AlEt3 TiCl4/MgCl2 v-v v-v AlEt3 TiCl4/MgCl2/EBb v-v v-v v-v v-v AlEt3/EAb TiCl4/MgCl2/EBb

50 60 50 60 60 70 60 50 60 70 70 70 70 70 70 60 70 70

0.4 0.5 0.6 1.0 0.5–1.2 0.06–0.08 1–2 B0.2–0.9 1–2 33 B0.03 B0.01 0.9–1.5 0.5–0.9 0.4–1.2 0.3–0.4 0.3–0.7 2

84 18 8.5 10 13–35 90–100 4–8 6–40

[1741] [1754] [1741] [1572] [1740] [1736] [1756] [1629] [1572] [470] [1721] [1721] [1721] [1722] [1721] [1737] [1722] [1696]

Cocatalyst

4.8 410–560 B100 700–800 740–810 800–1,200 200–250 870–1,250 330

403

Temperature (1C)

Catalyst

Kinetics of Alkene Polymerization Reactions

Table 5.27 Concentration of active centers and propagation rate constants, measurement by poisoning with ROH

404

Table 5.27 (Continued ) Catalyst

Cocatalyst

Temperature (1C)

C (mol.%)

TiCl4/MgCl2/EBb

AlEt3/MPTb v-v v-v AlEt3/PhSi(OEt)3 AlEt3/Cpy2Si(OEt)2 AlEt3 AlEt2Cl AlEt3 AlEt3/MPTb Aln-Bu3

0 25 75 70 50 50 60 60 50 60

0.8 6.6 11 1.4 2.2–3.9 5.3–7.5 0.8–1.3 2.1 0.02 0.3

60 70

0.4 34

7 4.6

[1754] [470]

30

0.02–0.06

B50

[1757]

50

B16

24

[1758]

1-Butene polymerization reactions AlEt2Cl d-TiCl3 AlEt3 TiCl4/MgO 4-Methyl-1-pentene polymerization reactions Ali-Bu3 VCl3 1-Decene polymerization reactions AlEt3/MPTb TiCl4/MgCl2/EBb a

Data for propylene: kp value for isospecific centers. b EB ¼ ethyl benzoate, EA ¼ ethyl anisate, MPT ¼ methyl p-toluate, DIBP ¼ di-i-butyl phthalate. c Catalyst of 5th generation (Section 4.3.2). d Solvay catalyst (Section 4.3.1).

600 270–340 230–320 10–20 48–58 160

Reference

[1700] [1700] [1700] [1696] [1752] [1752] [1572] [1572] [438] [1572]

Alkene Polymerization Reactions with Transition Metal Catalysts

TiCl4/MgCl2/DIBPb TiCl4/MgCl2/DIBPb TiCl4 /MgCl2/dietherc TiCl4/d-TiCl3d TiCl4/d-TiCl3d VCl4/MgCl2/EBb VCl3

kpa (M
1 s
1)

Kinetics of Alkene Polymerization Reactions

405

[acetyl chloride]:[Ti] ratio of B0.08, results in an immediate and complete halt of the reaction [127,1730]: 4Ti2CH2 2Polymer þ R2Cð¼ OÞCl ! 4Ti2Cl þ R2Cð¼ OÞ2CH2 2Polymer

(5.71)

However, when the amount of the quenching agent is lower than the amount of the cocatalyst in the reaction, the active centers gradually recover through alkylation of the Ti
Cl species formed in Reaction (5.71) and the polymerization activity of the catalyst is eventually restored. Only large amounts of acid chlorides irreversibly poison catalysts by completely destroying cocatalysts [1550,1730]: AlR3 þ 2 R2Cð¼ OÞCl ! AlRCl2 þ 2 R2Cð¼ OÞ
R

(5.72)

The main obstacle to the use of Reaction (5.71) for the C measurement stems from an observation made in the studies of ethylene polymerization reactions with a supported TiCl4/MgCl2/SiO2 catalyst activated with AlEt3 at 851C. After acetyl chloride was added to the reaction mixture, recovered polymers contained small amounts of ester species rather than the ketone species expected in Reaction (5.71) [1739]. These data suggest that the ketone group formed in Reaction (5.71) underwent secondary reactions with the participation of excess cocatalyst. An alternative method of the C measurement with acid chlorides is based on chemical details of alkene polymerization reactions described in Section 3.2.1.2.1 [1739]. If a monomer is depleted in the course of a polymerization reaction or removed from the reactor, and the cocatalyst is also removed, growing polymer chains eventually separate from active centers in the b-H elimination reaction (Reaction (3.27)) and leave Ti
H bonds in the catalyst: 4Ti2ðCH2 2CHRÞn 2R0 ! 4Ti2H þ CH2 ¼ CR2ðCH2 2CHRÞn
1 2R0

(5.73)

Under usual polymerization conditions, this reaction is relatively slow in comparison with other chain transfer reactions, the half-conversion time for Reaction (5.73) at 701C is B0.5–2.5 minutes, about 100–200 times higher than the average chain-growth time of a polyolefin macromolecule [6]. If an acid chloride is added to the reaction mixture at this point to quench the active centers, two different reactions of organotitanium species will take place, reactions with Ti
H bonds formed in Reaction (5.73) and reactions with catalytically inactive Ti
R groups that are always present in the catalysts: 4Ti2H þ R2Cð¼ OÞCl ! 4Ti2Cl þ R2Cð¼ OÞ2H

(5.74)

4Ti2R þ R2Cð¼ OÞCl ! 4Ti2Cl þ R2Cð¼ OÞ2R

(5.75)

Both products in Reactions (5.74) and (5.75) have low molecular weights, and their amounts can be measured by GC. A choice of the acid chloride R–C(=O)Cl in Reactions (5.74) and (5.75) is dictated by convenience of its analysis (benzoyl chloride is preferred in GC analysis). The preferred cocatalyst in this method is AlMe3; it assures the absence of undesirable side-reactions which may lead to the formation of catalytically inactive WTi
H species [1739]. An investigation of the

406

Alkene Polymerization Reactions with Transition Metal Catalysts

reaction chemistry associated with interactions of acid chlorides with working heterogeneous cocatalysts demonstrated the complexity of numerous interactions in the systems, in particular, reactions between carbonyl species produced in Reactions (5.74) and (5.75) with an excess of cocatalysts, acid chlorides, and alcohols used for the final quenching of the catalysts [1739]. The results of the C determination for several catalysts are given in Table 5.28. According to this C counting method, from 1.0 to 1.5% of all Ti atoms in the TiCl4/MgCl2/SiO2 catalyst activated with AlMe3 are converted into active centers in 1-hexene polymerization reactions at 851C. When ethylene is polymerized with the same catalyst, either alone or in combination with 1-hexene, the concentration of active centers nearly doubles. The concentration of active centers in propylene polymerization reactions with a typical catalyst of the 4th generation, TiCl4/ MgCl2/dioctyl phthalate, also corresponds to approximately 1.5% of its Ti atoms. Crystalline d-TiCl3 has a specific surface area of 20 m2/g, and only 2.5% of Ti atoms in the solid are situated on its surface. Taking this into consideration, the results for the d-TiCl3-based catalyst show that active centers account for B6% of the surface Ti atoms in ethylene/1-hexene copolymerization reactions and B3% in the 1-hexene polymerization reaction. The discussion in Section 5.7.4.2.2 shows that CO tagging methods for the C measurement in ethylene and propylene polymerization reactions usually give very diverse results, depending on the exact technique and the catalyst, from 0.2–0.4% [470] to 3–5% of the Ti atoms [1737]. The C measurement method based on the use of acid chlorides gives C estimations within this range. Table 5.28 Concentration of active centers and propagation rate constants, measurements by poisoning with acetyl chloride and benzoyl chloride Catalyst

Cocatalyst

Ethylene polymerization reactions [1739] AlMe3 TiCl4/MgCl2/SiO2

a

C (mol.% Ti)

85

3.3

Ethylene/1-hexene copolymerization reactions [1739] d-TiCl3 AlMe3 85 AlMe3 85 TiCl4/MgCl2/SiO2

3.3 0.15

Propylene polymerization reactions [1739] AlMe3 TiCl4/MgCl2/DOPa

80

1.4

1-Hexene polymerization reactions [1739] AlMe3 d-TiCl3 AlMe3 TiCl4/MgCl2/SiO2

85 85

0.08 1.9–1.5

1-Octene polymerization reactions [127] d-TiCl3 AlEt3 AlEt3 TiCl4/d-TiCl3b Ali-Bu3 TiCl4/d-TiCl3b

30 30 30

0.05–0.15 0.37 0.4

DOP = dioctyl phthalate. Solvay catalyst (Section 4.3.1).

b

Temperature (1C)

Kinetics of Alkene Polymerization Reactions

407

5.7.4.2.6. Other C measurement methods. Burfield proposed an original idea for the measurement of the number of active centers in solid catalysts of the MCl3 type. It involves the formation of active centers in a reaction between the catalysts, a cocatalyst (Ali-Bu3) and a-methylstyrene [1200]. The first stage is the alkylation of the transition metal compound, e.g:

4MIII 2Cl þ Ali-Bu3 ! 4MIII 2CH2 2CHðCH3 Þ2 þ Ali-Bu2 Cl

(5.76)

Only a small fraction of the WM–R species generated in Reaction (5.76) are capable of polymerizing alkenes. Their presence is revealed when a-methylstyrene is added to the aged MCl3-Ali-Bu3 products: 4MIII 2CH2 2CHðCH3 Þ2 þ CH2 ¼ CðCH3 ÞPh ! 4MIII 2CH2 2CHðCH3 ÞPh þ CH2 ¼ CðCH3 Þ2

(5.77)

The WMIII–CH2–CH(CH3)Ph species is decomposed with CH3O3H and produces 3H-labeled cumene. Its amount corresponds to the number of active centers in the catalysts, B0.3% of V atoms in the VCl3-Ali-Bu3 system and B3.5% of Ti atoms in the d-TiCl3-Ali-Bu3 system [1200].

5.7.5. General classification of active centers in heterogeneous Ziegler–Natta catalysts Heterogeneous Ti- and V-based polymerization catalysts contain several populations of active centers that differ in many crucial properties: 1. Ability to polymerize 1-alkenes with ethylene. 2. Stereospecificity in 1-alkene polymerization reactions. The centers vary from highly isospecific to practically aspecific to moderately syndiospecific, with many intermediate possibilities. 3. Kinetic features, the rates of formation and stability. 4. Preference for non-destructive (coordinative) chemical poisons/modifiers of different types. Scheme 5.6 gives a general overview of different types of the centers. In spite of obvious simplifications and generality, the scheme provides a convenient means of identifying different types of centers in terms of their reactivity and behavior. The principal means of differentiating between the centers is based on the data provided by distributive polymer characterization techniques, GPC, analytical Tref, and Crystaf. Because there are no generally accepted classification rules for naming the individual components in the analytical data, the following provisional definitions are used. GPC data: Flory components in ethylene homopolymers and ethylene/1-alkene copolymers with low 1-alkene contents are marked I, II, III, IV, and V in the order of increasing molecular weight. Flory components in crystalline fractions of 1-alkene homopolymers are marked Iu, IIu, IIIu, IVu, Vu, etc., also in the order of increasing molecular weight. Flory components in amorphous fractions of 1-alkene homopolymers are similarly marked Iv, IIv, IIIv, IVv, Vv, etc. One should take into account that some catalysts produce very small quantities of low molecular weight

408

Alkene Polymerization Reactions with Transition Metal Catalysts

—————————————————————————————————————————— ← Group of → Behavior in 1-alkene Behavior in ethylene copolymerization reactions active centers polymerization reactions —————————————————————————————————————————— Centers I and II copolymerize ← Center I → Three-five different types of stereo-aspecific centers.b 1-alkenes with ethylene well.a "- "

←

Centers III are combinations ← of several types of active centers producing copolymers of different compositions with similar molecular weights.d

Center II

→

Two or three different types of moderately isospecific centers.c

Center III

→

At least two or three different types of highly isospecific centers producing polymers of 1-alkenes with different molecular weights.†

Centers IV and V copolymerize ← Center IV, → Centers IV and V do not Center V homopolymerize 1-alkenes.††† 1-alkenes with ethylene poorly.†† —————————————————————————————————————————— a Reactivity ratios r1 for ethylene/1-alkene pairs are: Center I ~6-15, Center II from 20-30 to ~40-60 [211,315]. b Flory components I"-V" in atactic polypropylene fraction [222]. c Centers II produce Flory components II 'and III' in crystalline polypropylene fraction; their [mmmm] values are 0.65-0.80 and 0.85-0.90, respectively [221,232,322]. d Average reactivity ratios r1 for ethylene/1-alkene pairs for Center III are from ~40 to ~100-140 [211,318]. † Centers III produce Flory components IV 'and V' in crystalline polypropylene fraction; their [mmmm] values are 0.96-0.98 [221,232,322]. †† Average reactivity ratios r1 for ethylene/1-hexene monomer pairs for Center IV are 800-900 and for Center V ~1000-1400 [211,315,318]. ††† Section 5.7.2.3.

Scheme 5.6 catalysts.

Classi¢cation of active center populations in heterogeneous Ziegler^Natta

components I, Iu, or Iv. In such cases, the respective components are not listed in this chapter. Analytical Tref and Crystaf data: Elemental components in ethylene/1-alkene copolymers are marked A, B, C, D, etc., in the order of decreasing temperature at which they are eluted or crystallize. In the case of stereoregular 1-alkene polymers (mostly polypropylene), the respective components are marked Au, Bu, Cu, Du, etc. (no experimental techniques of this type exist for the analysis of amorphous polymers or copolymers). Several of the identifiable elemental components in 1alkene homopolymers represent materials of the highest isotacticity, components Au and Bu. Components A and B or A, B, and C in ethylene/1-alkene copolymer represent copolymer fractions with the lowest 1-alkene content. Examples of their structure are presented in Sections 2.5.3 and 2.5.4 and in Table 5.18. The classification of active centers is shown in Scheme 5.6. It is based on resolution of GPC data into Flory components. This basis is chosen for several reasons: (a) GPC analysis is by far the most widely used technique for the study of alkene polymers. (b) GPC data are firmly anchored in a sense that molecular weights of the constituent Flory components are established reasonably precisely with standard calibration procedures. (c) GPC analysis gives the smallest number of individual polymer components.

Kinetics of Alkene Polymerization Reactions

409

Scheme 5.6 gives the simplest overview of the distribution of active centers in heterogeneous Ziegler–Natta catalysts (it does not include a small fraction of centers that produce syndiotactic polymers). According to this concept, different heterogeneous Ziegler–Natta catalysts based on the same type of transition metal essentially contain the same sets of active centers, only in significantly different proportions. The skill of a researcher engaged in the catalyst synthesis lies is selection of synthesis conditions that afford different relative contents of different active centers and the selection of catalyst modifiers that either poison some of the centers or moderate their activity. For example, Modifiers II used in catalysts for polymerization of propylene and other 1-alkenes usually perform two separate functions. One of them is suppression of activity of Centers I and II. Many types of Modifiers II are well suited for this purpose. Another, more subtle role of Modifiers II, especially obvious in the choice of silanes in propylene polymerization reactions, involves manipulation of the set of active centers generally described as Centers III (highly isospecific centers). Different silanes suppress the activity of different subclasses of these centers to a noticeably different degree, resulting in the production of polypropylene containing crystalline fractions of different NMR isotacticity (Section 2.5.3). On the other hand, requirements to the catalysts for copolymerization of ethylene are indifferent to stereospecificity but put great emphasis on the uniformity of active centers with respect to their copolymerization ability. Organic modifiers used in the latter catalysts (they are either deliberately introduced or are formed as side-products in the course of catalyst synthesis procedures described in Chapter 4) preferably should produce two effects. They should selectively poison Centers I (the centers that generate copolymer fractions with an excessively high 1-alkene content) and they should increase the relative fraction of Centers III, the centers that produce copolymer fractions in a desirable intermediate composition range. There also exist several proprietary catalyst poisons that selectively depress the activity of Centers IV and V, the effect equivalent to narrowing the compositional distribution of ethylene/1-alkene copolymers.

5.7.6. Physical effects in polymerization reactions with heterogeneous Ziegler–Natta catalysts Catalyst particles of supported Ti-based catalysts, whether they are prepared with silica-type carriers or utilize MgCl2 as both a support and a carrier, are usually relatively large, from 30 to 50 mm in diameter; they can reach B80–100 mm in some catalysts of the 4th generation [1043–1047]. The shape of carrier particles determines the morphology of original catalyst particles. Usually, this is either the spherical shape of silica particles (produced in spray-dry processes) or a nearly spherical shape of specially manufactured MgCl2 particles [1047]. On the macroscopic level, the physical transformation of catalyst particles in the course of a polymerization reaction is straightforward, one catalyst particle produces one polymer particle. If the original catalyst particle has a spherical shape, the growing polymer particle will also have a spherical shape; the phenomenon called ‘‘shape replication’’ [75,1047,1089,1448,1556,1759,1760]. Moreover, the grainy

410

Alkene Polymerization Reactions with Transition Metal Catalysts

surface texture of catalyst particles is also replicated in the grainy surface texture of polymer particles [1448,1759,1761]. On a microscopic level, the growth of polymer particles is a much more complex process. It is mostly determined by the morphology of initial catalyst particles. The best data were produced for relatively large catalyst particles based on MgCl2. These particles have a complex architecture that develops in the course of MgCl2 crystallization during catalyst synthesis. Principal building blocks of the ˚) particles are microparticles (sometimes called catalyst grain), very small (100–400 A crystallites of MgCl2 of a pseudo-hexagonal shape [1759,1762]. These microparticles are usually agglomerated into structural units (sub-particles) several nanometers in size. Finally, the sub-particles are agglomerated into large spherical catalyst particles [1759,1763]. The sub-particles can be sometimes observed under high magnification on the surface of catalyst particles. Both the sub-particles and the catalyst particles themselves are highly porous. This particle architecture rapidly disintegrates in the beginning of polymerization reactions because the inner parts of catalyst particles and sub-particles are exposed to cocatalyst and monomer. Physical changes in catalyst particles at different stages of polymerization reactions were the subject of thorough research [1087,1759,1762–1768]. In general, the morphology of growing polymer particles follows the morphology of catalyst particles [1759,1765,1767–1770]: ˚ ) produce porous polymer microglobules, 500– Catalyst microparticles (100–400 A 1,000 nm in size. Catalyst sub-particles (2–5 nm) produce porous polymer sub-globules 1–2 mm in diameter. Each spherical porous catalyst particle produces one porous polymer particle. The diameter of the latter depends on three parameters, the diameter of the initial catalyst particle, catalyst productivity, and porosity of the polymer particle. Numerous polymer tie molecules hold all morphological components of the polymer particles together. This experimental research was the basis for several models of the particle growth during polymerization reactions, the original multi-grain model [1764, 1771,1772] and its later modification, the double-grain model [1759,1765]. Thorough morphologic/kinetic analysis of propylene polymerization reactions at their early stages were carried both with large-diameter spherical particles (B70 m) of a TiCl4/MgCl2/ethyl benzoate catalyst [1762] and with medium-diameter particles (20–40 m) of a TiCl4/MgCl2/di-i-butyl phthalate catalyst [1556,1766, 1768]. Active centers in both catalysts are positioned on the surfaces of catalyst microparticles, which means that they are nearly uniformly distributed within the volume of catalyst sub-particles. Because of high porosity of catalyst particles, the polymer starts growing uniformly throughout all their structural elements. As a result, the particles undergo several stages of fragmentation. Initially, at very low polymer yields, B2 g/g cat, polymer molecules occupy all the available pore volume inside the particles, each catalyst sub-particle and spaces between them become filled with polymer [1759]. At this stage, the fragmentation is relatively small; a thin polymer layer envelops each intact catalyst particle and all its structural

411

Kinetics of Alkene Polymerization Reactions

elements are contained within the borders of this envelope [1766]. After the polymer yield increases to B7 g/g cat, the original catalyst particle gradually fragments into smaller pieces, less that 0.5–1 mm in length, each containing several original sub-particles. The fragments do not separate, they remain uniformly anchored within a single polymer particle; the phenomenon called ‘‘catalyst encapsulation.’’ The same carrier fragmentation pattern was observed in ethylene polymerization reactions with the TiCl4/MgCl2/silica-AlEt3 system [1087,1124], the average diameter of catalyst particles was reduced from B25 m to B8 m very early in the polymerization reaction. Polymerization of ethylene with chromium oxide catalysts [75,1087,1089] and with silica-supported metallocene catalysts [1773] is also accompanied by rapid fragmentation of catalyst particles and usually includes the same steps as those observed in the fragmentation of TiCl4/MgCl2type catalyst particles (see Section 5.9.2). Disintegration of catalyst particles can be clearly observed if the polymer prepared with them is soluble in the reaction medium, e.g., when poly(1-hexene) is produced instead of polypropylene. The TiCl4/MgCl2/di-i-butyl phthalate-AlEt3 catalyst was prepolymerized with 1-hexene at 201C to a yield of B10 g/g cat and then used in propylene polymerization at 701C [1761]. The polymer of 1-hexene formed during the prepolymerization stage breaks the catalyst particles into several fragments but holds polymer/catalyst particles together as single units. During the propylene polymerization stage, the poly(1-hexene) is quickly dissolved in liquid propylene (the reaction medium) and the disintegrated polymer particles disperse. The measurement of the particle size distribution before and after this two-stage polymerization reactions showed that initial particles, 70–80 mm in diameter, were broken down during the prepolymerization step and produced 1.5–2 mm fragments [1761]. The two models of the growth of polymer particles shown in Figure 5.20 differ in the description of the disintegration process. The multigrain model proposes that each catalyst microparticle grows a thick envelope of a polymer around it while remaining mostly intact [1764,1767,1768,1774]. The double grain model is based on morphological studies of original catalyst particles and is more realistic. It proposes that each catalyst particle has two morphological tiers. As the polymerization reaction proceeds, each part of the particle structure, microparticles and sub-particles, produces its own polymer structure. The catalyst sub-particles produce polymer sub-globules, which are agglomerated into final polymer particles. The sub-globules are not uniform; they consist of many polymer shells Microparticle

Microglobule

Polymer particle

Catalyst particle Multigrain model

Polymer subglobule

Double grain model

Figure 5.20 Schematic representation of two growth models of polymer particles produced with supported Ziegler^Natta catalysts, multigrain model and double grain model [1759].

412

Alkene Polymerization Reactions with Transition Metal Catalysts

(microglobules) around each catalyst microparticle. The catalyst microparticles are further dispersed throughout the microglobules and are pushed to their surfaces, where the polymer growth continues [1759,1764,1765,1769,1770]. It is instructive to compare properties of the active centers that are formed on external surfaces of catalyst particles and the active centers that are formed inside the particles. A sample of a TiCl4/MgCl2 catalyst was ground to reduce its average particle size from 6.3 to 2.7 mm, and then two catalyst batches, the original and the ground, were activated with AlEt3 [1775]. To avoid the effect of fragmentation, propylene polymerization reactions were carried out by the stopped-flow method for a very short time, 0.05–0.2 seconds. Kinetic analysis showed that active centers in both catalysts have the same propagation rate constants, B2,800 M
1  s
1, and that most of them were isospecific, [mmmm]av ¼ 0.92–0.93 [365,1775]. The only difference was a B50% higher concentration of active centers in the ground sample, 7.5 mol.% of the Ti atoms vs. 5.2 mol.%, the result of immediate activation of all polymerization centers, both in the outer regions and the interior of the catalyst particles.

5.8. Polymerization Reactions with PseudoHomogeneous Catalysts Detailed analysis of alkene polymerization kinetics with pseudo-homogeneous Ziegler–Natta catalysts is a very difficult research subject. These catalysts nearly always consist of several types of active centers, some of them in solution and other in the solid products of the reactions between the catalyst components. One example demonstrating this complex behavior is the catalyst system produced by reacting Ti(On-Bu)4 and Al2Et3Cl3 [1776]. At low temperatures, from
20 to +201C, this is a homogeneous and essentially a single-center catalyst. Materials produced in propylene polymerization reactions under these conditions have a narrow molecular weight distribution, Mw/Mw B2. Judging by their GPC curves, two active centers coexist in the homogeneous catalyst. Both produce polymers of a low molecular weight, one with Mw B6  103 (the dominant material) and another B4  104. These polymers have very low isotacticity [mm] B0.5, and contain B10% of head-to-head units. When the same catalyst components are combined at higher temperatures, W301C, solid products start forming in the catalyst mixture. They are also catalytically active and contain a large number of different active centers. This solid catalyst produces polypropylene with a very broad molecular weight distribution, Mw/Mn B18, and with a much higher molecular weight, Mw B1  105. The latter polymers also have a very low level of isotacticity [mm] B0.6, and contain B5% of head-to-head connected monomer units [1776]. Another feature that makes the kinetic research of polymerization reactions with these catalysts so difficult is their high instability. The activity of the catalysts usually first increases and then rapidly declines [4,1068]. The speed of both stages is usually much higher for V-based systems, such as VCl4-AlEt2Cl, VOCl3-AlEt3, or VOCl3AlEt2Cl, than for the typical Ti-based pseudo-homogeneous system, TiCl4AlEt2Cl. With the former systems, the activation period lasts from 2 to 5 minutes

413

Kinetics of Alkene Polymerization Reactions

and the deactivation stage continues for 10–20 minutes [1068]. On the other hand, if the TiCl4-AlEt2Cl system at [Al]:[Ti] B10 is pre-reacted in the absence of ethylene at 301C and then introduced into a polymerization reactor, the maximum catalyst activity is retained for at least 40 minutes [1068]. Because of this effect (caused by very complex and relatively slow chemical processes in the catalyst), the earliest commercial processes for the manufacture of polyethylene utilizing the TiCl4-AlEt2Cl system often included a pre-contact chamber where the catalyst components were mixed in the absence of monomer. The duration of this incubation period, which mostly depended on temperature, could be quite long. The rate of the catalyst decay significantly increases when a monomer is added to the catalyst [1068]. A change in the assortment of active centers in pseudo-homogeneous catalysts due to changes in catalyst composition also manifests itself in the changes of the molecular weight and the molecular weight distribution of polyethylene they produce. One example shows the effect of the [AlEt2Cl]:[TiCl4] ratio in ethylene homopolymerization reactions with TiCl4-AlEt2Cl system at 501C [23]: [Al]:[Ti] Mw

0.67 2.4  104

1.0 5.4  104

1.3 13.2  104

1.5 18.8  104

1.7 26.2  104

2.0 28.8  104

In view of this variability, attempts to characterize these catalysts kinetically over a wide temperature range and at different catalyst compositions by applying a single set of kinetic equations (and, therefore, implicitly assuming that active centers are the same under different reaction conditions) appear futile.

5.9. Polymerization Reactions with Chromium Oxide Catalysts 5.9.1. General kinetic behavior Kinetics of alkene polymerization reactions with chromium oxide catalysts is studied relatively little. Three circumstances make this research difficult: 1. Chromium oxide catalysts always have a large number of different types of active centers, at least six to eight, as illustrated in Figure 2.12. The centers differ greatly in the molecular weight of the polymer material they produce, as one example of the molecular weight range for different Flory components in an ethylene homopolymer (Mw/Mn ¼ 12.3) produced with a typical chromium oxide catalyst at 901C show. Six Flory components with molecular weights ranging from B3,000 (polyethylene wax) to over 1.2  106 are required for a satisfactory description of this molecular weight distribution: Flory I II III IV V VI component Mw 2.1  103 1.4  104 4.6  104 1.37  105 4.07  105 1.21  106 Content (%) 3.8 11.8 30.1 30.8 18.2 6.1

414

Alkene Polymerization Reactions with Transition Metal Catalysts

2. The kinetics of these ethylene homopolymerization reactions is quite complex. The catalysts do not require a cocatalyst and are usually activated with ethylene. Initially, they are completely inactive for a period that can last from several minutes to nearly an hour, depending on reaction conditions. After that, a long stage of catalyst activation takes place and, finally, but not always, a stable polymerization rate is achieved [75,82,1087,1089,1777]. 3. The catalysts are very sensitive to minute amounts of impurities in ethylene, oxygen, alcohols, CO, acetylene, thiols, etc. [1089,1725,1778,1779,1780]. The origin of the induction period is usually attributed to reduction of the initial CrVI species in the catalysts to CrII species [75,1087,1091,1781]. When the reducing agent is the monomer itself, formaldehyde is produced as the main ethylene oxidation product [1782]. The reduction can be also carried out with carbon monoxide [75,1091]. The length of the dormancy stage depends on reaction conditions. If ethylene is used at 80–1201C (a typical range of ethylene polymerization reactions with these catalysts), this stage usually lasts several minutes [75,82,1087] but it is very short at 1501C [1087]. When the catalysts are pre-reduced with CO, the induction period is o10 minutes even at B251C [1025]. The induction period is followed by the stage when the catalyst activity gradually increases over time. This stage can last up to 60 minutes before the catalyst reaches the stable activity [1087,1089,1783]. The activation period is caused by chemical transformations in the catalysts; it is not a consequence of a gradual fragmentation of catalyst particles [1087]. Some reasons for the activation effect are trivial, first of all, slow consumption of impurities in the reaction system, it can be alleviated by adding a small quantity of AlR3 [1089,1162] or other organometallic compounds, MgR2 or LiR [1093]. After the catalyst is fully activated, the reaction rate becomes nearly constant [82,1087,1089,1783]. Kinetics of ethylene polymerization reactions was studied in detail with a chromium oxide catalyst activated with MgR2 at the beginning of polymerization reactions [842]. Figure 5.21 shows the kinetics of ethylene homopolymerization reactions in at 901C at three different ethylene partial pressures. The kinetic curves exhibit all three features common to these catalysts, the induction period (apparent only in the reaction at a low ethylene partial pressure), the acceleration period that lasts B30 minutes at 901C, and the period of the steady reaction. The kinetic behavior of active centers of these reactions at relatively high ethylene partial pressures can be formally described by a simple formal kinetic scheme represented by Scheme 5.1, a two-stage process of active center formation and a single-step deactivation reaction. One has to keeping in mind, however, that this catalyst contains at least six types of active centers that produce polymer molecules with widely different molecular weights. The kinetic behavior of different centers can be different, which necessarily makes this kinetic approach quite approximate and more suitable for modeling polymerization processes rather than the mechanistic analysis of the polymerization reactions.

Kinetics of Alkene Polymerization Reactions

415

Figure 5.21 Kinetics of ethylene homopolymerization with chromium oxide catalyst activated with MgR2 at 901C at three di¡erent ethylene partial pressures.

5.9.2. Effects of reaction parameters Basic kinetic features of ethylene polymerization reactions with chromium oxide catalyst activated with MgR2 were determined in slurry polymerization reactions. Kinetic curves in Figure 5.21 give the effect of the ethylene partial pressure on the stationary polymerization reaction, its rate has the first order with respect to the ethylene partial pressure [1677]. Figure 5.21 also shows that the ethylene partial pressure has a strong effect on the length of the acceleration stage, an observation important for the mechanistic understanding of active center formation described in Section 6.4.1. GPC curves of ethylene homopolymers produced with chromium oxide catalysts at different reaction pressures are very similar. This constancy indicates that the main chain transfer step in these reactions is the chain transfer to a monomer. The molecular weight of each Flory component is independent of the ethylene concentration, n(Flory) E kp/kM t (Section 5.3.1). The effect of reaction temperature on the polymerization kinetics with a chromium oxide catalyst is common for many other heterogeneous polymerization catalysts, as the temperature increases, the activation period becomes shorter, but active centers become less stable. The effective activation energy of the ethylene polymerization reaction is 42.8 kJ/mol (10.2 kcal/mol). One of the main differences between chromium oxide and Ti-based polymerization catalysts is nearly complete insensitivity of the former to the presence of hydrogen. Hydrogen has no effect on the average molecular weight of ethylene polymers prepared with chromium oxide catalysts or on the molecular

416

Alkene Polymerization Reactions with Transition Metal Catalysts

weight of any of their constituent Flory components. The activity of chromium oxide catalysts is also unaffected by hydrogen [842] or in some cases it even increases in its presence [1784]. Low molecular weight polymer components produced with these catalysts (ethylene oligomers (see Section 3.6.1)) have the vinyl double bond as one of their chain ends whether hydrogen is present in the reaction or not. This hydrogen insensitivity indicates that hydrogen does not hydrogenate the Cr–C bond in chromium oxide catalysts. This feature is by no means common for all chromium-based polymerization catalysts. For example, the hydrogen effect in ethylene polymerization reactions with an organochromium catalyst, Cp2Cr/ AlPO4, at 951C is very strong [846]: PH (kPa) Mav w

0 W4  106

69 2.0  105

138 1.1  105

276 7.1  104

Positions of individual Flory components in GPC curves of these ethylene homopolymers shift to lower molecular weights in parallel, similarly to polymers prepared with Ti-based catalysts (Section 5.7.1.1.2), without affecting the width of the molecular weight distribution; Mw/MnB4.2–4.8. As described in Section 5.7.1.2, addition of 1-alkenes to ethylene polymerization reactions with Ti-based heterogeneous catalysts greatly increases the catalyst activity, especially at early stages of the reactions. In the case of chromium oxide catalysts, the effect of 1-alkene addition depends on the moment of the addition [1677,1783]. If 1-hexene is added to an established polymerization reaction at a point when the polymerization rate is constant (see Figure 5.21), the effect is minimal. However, if a mixture of ethylene and a 1-alkene (from propylene to 1heptene) is added to the fresh catalyst, the influence of a 1-alkene can be quite significant [1783]. Several different effects should be distinguished. First, the length of the catalyst activation stage is reduced in the 1-alkene presence [1677,1783]. 1-Alkenes can also influence the catalyst activity at the stage of a steady reaction, although the activity increase reported by different authors varies from quite strong [1783] to relatively insignificant [1677]. Branched 1-alkenes (3- and 4-methyl-1pentene) reduce the activity of chromium oxide catalysts and isobutene makes the catalysts nearly completely inactive [1783]. In terms of their ability to copolymerize 1-alkenes with ethylene, chromium oxide catalysts occupy an intermediate position between metallocene and Ti-based catalysts. The reactivity ratio r1 for the ethylene/1-hexene pair for chromium oxide catalysts is B30, higher than for metallocene catalysts (B20) but lower than for Tibased Ziegler–Natta catalysts (B80–120). The product of reactivity ratios, the r1  r2 value, for the ethylene/1-hexene pair (determined from 13C NMR analysis of the copolymers) is B0.2, i.e., active centers in the catalyst produce random copolymers with a significant tendency to alternate monomer units. Chromium oxide catalysts undergo extensive fragmentation during ethylene polymerization reactions. Each catalyst particle forms a single polymer particle that has approximately the same shape but is 1,000–2,000 times bigger in diameter. This shape-replicating effect is similar to that observed in alkene polymerization reactions with heterogeneous Ziegler–Natta catalysts (Section 5.7.6). McDaniel determined

Kinetics of Alkene Polymerization Reactions

417

that the fragmentation of catalyst particles starts at the earliest stages of polymerization reactions and is usually complete after the first few minutes, much earlier than the moment when the steady polymerization rate is achieved [75,1087,1089]. The fragmentation proceeds in two steps. First, original large particles are dispersed into very small fragments, from 0.1 to 1 m in size and, following this, the fragments form loosely held agglomerates 7–10 m in diameter [75,1087,1089]. Fragmentation of CO-reduced catalysts proceeds in a similar way [1785]. Atomic-force microscopic analysis of the earliest reaction stages shows a gradual covering of the catalyst surface with very thin (B80 nm), sheaf-like aggregates of polymer molecules [1099].

CHAPTER 6

Active Centers in Transition Metal Catalysts and Mechanisms of Polymerization Reactions

Contents 6.1. Catalysts Derived from Metallocene Complexes 6.1.1. Formation and structure of active centers 6.1.2. Mechanism of alkene polymerization reactions, experimental data and theoretical analysis 6.1.3. Stereospecificity of active centers in metallocene catalysts 6.1.4. Mechanism of styrene polymerization 6.2. Non-Metallocene Homogeneous Catalysts 6.2.1. Vanadium-based catalysts 6.2.2. Ni ylide catalysts for ethylene oligomerization 6.2.3. Catalysts derived from complexes with phenoxy-imine ligands 6.2.4. Catalysts derived from complexes with (imino)pyridyl ligands 6.2.5. Catalysts derived from complexes with a-diimine ligands 6.3. Active Centers in Heterogeneous Ziegler–Natta Catalysts 6.3.1. Formation of active centers 6.3.2. Structural features of active centers 6.3.3. Poisoning of active centers 6.3.4. Physical observations, position of active centers on catalyst surface 6.3.5. Mechanism of alkene polymerization reactions with Ziegler–Natta catalysts 6.3.6. Stereospecificity of active centers 6.4. Active Centers in Chromium Oxide Catalysts 6.4.1. Formation and structure of active centers 6.4.2. Mechanism of alkene polymerization

420 421 434 458 474 476 477 479 480 482 483 486 486 489 491 493 496 505 515 515 518

This chapter describes the current understanding of the structure of active centers in various transition metal catalysts and mechanisms of alkene polymerization reactions. In the case of stereospecific catalysts, it is reasonable to separate two mechanistic issues of the polymerization reactions, the chemistry (Sections 6.1.1, 6.1.2, 6.3.1–6.3.5), and the stereospecificity (Sections 6.1.3 and 6.3.6). Two types of information are presented in each section below, one derived from experimental data and another from theoretical analysis. The experimental data required to

419

420

Alkene Polymerization Reactions with Transition Metal Catalysts

produce an adequate model of active centers come from a combination of different sources, some of them discussed in the previous chapters. All alkene polymerization reactions with transition metal catalysts are, in the mechanistic sense, particular cases of one general type of reaction, carbometallation, i.e., insertion of the CQC bond of an alkene molecule into the metal–carbon bond [1586,1787]. The earliest examples of these reactions are growth reactions of alkyl chains in organoaluminum compounds in reactions with ethylene at temperatures above 1101C [1194,1788]: AlR3 þ CH2 QCH2 ! Al½2ðCH2 2CH2 Þx 2R
3

(6.1)

The rate-limiting step of this reaction is ethylene insertion into the Al–C bond via a four-atom transition state. Any organic compound with electron–donor properties retards this reaction indicating that a coordinatively unsaturated monomeric AlEt3 molecule is the true active species. All Al–C bonds in AlR3 compounds have similar reactivities in Reaction (6.1), and each Al–C bond grows a short polymer chain. The chain termination step in carbometallation reactions is also similar to that in alkene polymerization reactions with transition metal catalysts, dissociation of the Al–C bond (in the terminology of polymer chemistry, spontaneous chain termination): R02 Al2CH2 2CHR2Polymer ! R02 Al2H þ CH2 QCR2Polymer (6.2) Catalyst systems derived from metallocene complexes of transition metals provide the best-studied examples of the polymerization mechanism. Our understanding of all aspects of these reactions, the chemical structure of active species, the mechanism of alkene insertion reactions, and the steric control in the reactions, are the fullest, although they are far from complete, as the discussion below demonstrates. For this reason, the discussion of the mechanism of alkene polymerization reactions starts with metallocene catalysts and then proceeds to less confirmed mechanisms of polymerization reactions with non-metallocene homogeneous catalysts and, finally, with heterogeneous catalysts.

6.1. Catalysts Derived from Metallocene Complexes Three distinct types of catalysts based on metallocene complexes are known: 1. The early catalysts, combinations of titanocene complexes and organoaluminum chlorides. 2. Kaminsky-Sinn catalysts, combinations of metallocene complexes and MAO. 3. Catalysts containing alkylated metallocene complexes and ion-forming cocatalysts. Detailed investigations of the three groups of catalysts have shown that the same active species are present in all of them, metallocenium cations containing a transition metal–carbon bond.

Active Centers in Transition Metal Catalysts

421

6.1.1. Formation and structure of active centers 6.1.1.1. Catalysts utilizing ion-forming cocatalysts In principle, the most clear-cut examples of the formation reaction of metallocenium active centers should be generation of ion pairs from dialkylated metallocene complexes and organoborane compounds with strong Lewis acidity, such as B(C6F5)3, or organoborate salts such as [PhMe2NH]+ [B(C6F5)4] or [CPh3]+ [B(C6F5)4]. To simplify the presentation of reactions involving metallocene complexes of different structures, any nonspecificed Z5 ligand in such complexes is represented below by the Cp symbol, e.g., Cp2MR2 complexes, Cp2M+–R metallocenium ions, etc. Several reactions of metallocenium ion formation were thoroughly investigated, and their products, salts of metallocenium cations (in the form of tight ion pairs), have been characterized by NMR and isolated [254,735,1257,1351,1353,1354, 1358,1363,1377,1379,1589,1789,1790]: Cp2 MR2 þ ½Ph3 C
þ ½BðC6 F5 Þ4
 ! ½Cp2 Mþ 2R
 ½BðC6 F5 Þ4
 þ CMePh3 Cp2 MR2 þ ½PhMe2 NH
þ ½BðC6 F5 Þ4
 ! ½Cp2 Mþ 2R
 ½BðC6 F5 Þ4
 þ PhNMe2 þ RH Cp2 MCl2 þ AlR3 þ ½CPh3
þ ½BðC6 F5 Þ4
 ! ½Cp2 Mþ 2R
 ½BðC6 F5 Þ4
 þ AlR2 Cl þ CRPh3 Cp2 MR2 þ BðC6 F5 Þ3 ! ½Cp2 Mþ 2R
 ½BRðC6 F5 Þ3


(6.3)

(6.4)

(6.5) (6.6)

Complexes of metallocenium ions: One should take into consideration that Reactions (6.3)–(6.6) are merely simplifications as far as the structure of their reaction products is concerned. The products of these reactions, 14-electron Cp2M+Me cations, are strongly Lewis-acidic and form complexes with a variety of organic molecules, even with aromatic solvents [1791,1792]. When metallocenium cations are produced by abstraction of R in Reactions (6.3)–(6.6), several such complex-formation agents are available, unreacted Cp2MR2, alkylaluminum compounds used in Reaction (6.5), and even alkyl groups R in [R(C6F5)3] anions in Reaction (6.6) [1793]. For example, when Reaction (6.3) with Cp2ZrMe2 is carried out at relatively low temperatures, the Cp2Zr+–Me cation forms a strong homo-dinuclear complex with Cp2ZrMe2 via the bridging methyl group [382,1363,1368,1376–1379,1794]: 2Cp2 ZrMe2 þ ½Ph3 C
þ ½BðC6 F5 Þ4
 ! ½Cp2 ðMeÞZrðm-MeÞZrðMeÞCp2
þ  ½BðC6 F5 Þ4
 þ CMePh3

(6.7)

Similar dinuclear complexes were produced from Cp(Me)Ti+Me [1257] and C2H4(Cp)(Flu)Zr+–Me cations [735]. If AlMe3 is used in Reaction (6.5), heterodinuclear complexes [Cp2Zr(m-Me)2AlMe2]+ are formed [1377,1596,1601]. Similar heterodinuclear complexes are formed between metallocenium ions and other organometallic compounds, they significantly affect both the reactivity of the catalysts and molecular weights of the polymers they produce [653]. When the

422

Alkene Polymerization Reactions with Transition Metal Catalysts

zirconocene complex is a hydride (it exists as a homodinuclear complex [Cp2Zr(mH)H]2), its reaction with a mixture of [Ph3C]+ [B(C6F5)4] and Ali-Bu2H produces a heterodinuclear cation [Cp2Zr(m-H)2Ali-Bu2]+ [710]. If B(C6F5)3 is used to abstract a methyl group from Cp2ZrMe2 in Reaction (6.6), the interaction between the metallocenium ion Cp2Zr+–Me and the methyl group in [MeB(C6F5)3] is so strong that the product in Reaction (6.6) should be viewed as a heterodinuclear complex [1379,1589]: Cp2 ZrMe2 þ BðC6 F5 Þ3 ! ½Cp2 Zrþ ðMeÞðm-MeÞB ðC6 F5 Þ3


(6.8)

Complexes of the same type are formed when Al(C6F5)3 is used as the activator instead of B(C6F5)3. Moreover, kinetic analysis of propylene polymerization reactions with Me2C(Cp)(Ind)ZrMe2 and Me2C(Cp)(Flu)ZrMe2 activated with B(C6F5)3 strongly suggests that after the first methyl group is abstracted from the complexes by B(C6F5)3 in Reaction (6.8) the second methyl group can also be attacked by B(C6F5)3 [1795]. The reaction produces a hetero-trinuclear complex, Cp2Zr2+[(m-Me)B(C6F5)3]2. One such trinuclear complex was indeed experimentally isolated when B(C6F5)3 was replaced with Al(C6F5)3 [1370]. Theoretical calculations suggest a possibility of another tight ion pair formed in a reaction between Cp2Zr(R)Me (with R ¼ Me or Et) and two molecules of Al(C6F5)3, [Cp2Zr+–R] [(C6F5)3Al(m-Me)Al(C6F5)3] [1796]. Most of these cationic dinuclear and trinuclear ions can be isolated; they can exist as contact or solvent-separated ion pairs with counter-anions produced in Reactions (6.3)–(6.6). X-ray data show that these complexes have very short ˚ [1369] and distances between CH3 groups of their anions and Zr atoms, 2.5–2.60 A can be viewed as very tight ion pairs. Some of these ion pairs are catalytically active in spite of their ionic character, which, expectedly, limits their solubility in aromatic hydrocarbons under typical conditions of alkene polymerization reactions. To convert experimentally observable (albeit only at relatively low temperatures) multinuclear metallocenium cations into active centers in alkene polymerization reactions, two additional chemical transformations should take place. The first transformation is the dissociation of the complexes into tight ion pairs [Cp2Zr+– Me]  A, and the second required transformation is the replacement of the anion A in the coordination sphere of the transition metal atom with a coordinated alkene molecule, i.e., the formation of an alkene-separated ion pair [Cp2(Me)Zr+]    alkene    A. Both transformations were observed experimentally. The situation with dissociation of different binuclear metallocenium complexes becomes somewhat simpler in alkene polymerization reactions. Bochmann showed that most dinuclear cations containing methyl bridges (such as the complexes formed in Reaction (6.7)) slowly dissociate into mononuclear ion pairs [1366]. However, the stability of m-alkyl-bridged complexes rapidly decreases when the size of the alkyl group increases [661], and one can assume that when metallocenium ions Cp2Zr+–R carry large alkyl groups R (i.e., when they are active centers with attached growing polymer chains) the mononuclear species dominate. For example, the first insertion step of a propylene molecule into the Zr+–C bond in the mononuclear ion Me2Si(Ind)2Zr+–Me existing in equilibrium with a bridged homo-dinuclear complex produces the Me2Si(Ind)2Zr+–CH2–CHMe2 ion, and

Active Centers in Transition Metal Catalysts

423

this step is apparently sufficient to shift the equilibrium from the dinuclear to the mononuclear ion [1366]. Geometry of metallocenium cations: NMR spectroscopy provides an insight into the structure of the most realistic models of polymerization centers, solutions of salts [Cp2Zr+–Me]  [MeB(C6F5)3] in aromatic solvents [1318,1358,1359,1797]. These salts are highly crystalline and their dissolution in toluene apparently involves dissociation of a relatively weak chemical bond between the methyl group in the [MeB(C6F5)3] anion and the Zr atom and the formation of a solvent-separated ion pair [Cp2Zr+–Me]  (solvent)  [MeB(C6F5)3]. According to the molecular-orbital description of the Cp2Zr+–Me ion, three metal-centered orbitals are present in the equatorial plane between two Z5 ligands, one central and two lateral. The methyl group occupies one of the lateral positions and the bulky counter-anion the second lateral site. In contrast, a free H2Si(Cp)2Ti+–Me cation has symmetry close to C2v (DFT calculations), the Ti–CH3 bond occupies the central position and lies practically in the plane of the Cp-Ti-Cp space angle [1798]. The reason for this arrangement is a strong a-agostic interaction between one of the H atoms of the methyl group and the Ti atom. This geometry was confirmed in ab initio calculations of similar cations [1799,1800]. Calculations of another free cation, H2Si(Cp)2Zr+–Me, suggest the same preferred geometry. The Zr+–CH3 bond also occupies the central position with respect to the skeleton of the complex so that centers of both cyclopentadienyl rings, the Zr atom, and the methyl group are all in the same plane, although the aagostic interaction is not evident in this structure. This geometry mostly agrees with X-ray data for the salt of a nonbridged metallocenium cation [(1,2-Me2-Cp)2Zr+– Me]  [MeB(C6F5)3] [1351] except for the position of the methyl group, which is moved away from the central position to one of the lateral positions to accommodate the closely positioned counter-anion. Nominally, bis-metallocenium cations Cp2M+–R have the Z5 structure (Z5 hapticity), every carbon atom in the cyclopentadienyl rings is at the same distance from the transition metal atom. However, DFT calculations of propylene insertion reactions into the Zr–C bond in different metallocenium ions showed that some steric interactions in the transition state of these reactions could significantly shift cyclopentadienyl rings from this symmetrical position [1802]. The shift is called a partial Z5-Z3 hapticity change, or the Z5-Z3 ring slippage. It is minimal for the small Cp ligand but it may become significant for bulkier Z5 ligands, Ind and Flu. Shift of alkyl groups in metallocenium ions: Three positions with similar energies exist for alkyl groups in metallocenium ions Cp2M+–R, one central and two lateral. The energetics of the alkyl group shift from one position to another depends on its size and on the bulkiness of Z ligands in the complexes. According to some calculations [1800], the shift of a methyl group from the central to the lateral position in a free metallocenium ion occurs easily. The energy difference between the central and the lateral positions is less than B8.4 kJ/mol (2 kcal/mol), and this shift can be caused by coordination of many ligands, both the counter-anion and the approaching alkene molecule. However, a shift of a methyl group from one lateral position to another (possibly via the ion quadruple mechanism [1797]) is a more demanding step. NMR data indicate that metallocenium ions are relatively rigid in

424

Alkene Polymerization Reactions with Transition Metal Catalysts

solution in the stereochemical sense. The free energy of activation of lateral/lateral methyl shifts in them range from 51.5–69.5 kJ/mol (12.3–16.6 kcal/mol) for nonbridged ions to 61.5–W80 kJ/mol (14.7–W19 kcal/mol) for bridged ions [1358]. This shift in the Cp2Zr+–Me species is an intra-cationic process inside the solvent cage, its DS¼ B0, and it does not involve any significant a-agostic interaction between the H atoms of the methyl group and the Zr atom [1358]. The barrier to the shift decreases with an increase in the dielectric constant of the solvent, for example, from 66.2 kJ/mol (15.8 kcal/mol) in toluene to B33.5 kJ/mol (8 kcal/mol) in dichloromethane and chlorobenzene for the Ind2Zr+–Me ion. Bulky substituents in Z5 ligands tend to lower the barrier of the shift by facilitating the first step of the process, the cation–anion separation. The introduction of a bridge between the Z5 ligands results in a higher barrier for the shift because of a tighter cation–anion interaction in a more open Cp-Zr-Cp space. This lateral/lateral shift of an alkyl group, when a polymer chain is present instead of the methyl group, plays an important role in the stereocontrol efficiency of syndiospecific metallocene catalysts, where it is called polymer chain migration or a back-skip process [1801,1802]. The enthalpy of alkyl group migration in the (1,2-Me2-Cp)2Zr+–R cation greatly declines with the bulkiness of R in the order: Me W CH2-t-Bu W CH2-SiMe2c CH(SiMe2)2 [1803]. Reactivity of metallocenium cations: Mononuclear metallocenium cations are very reactive species. They can be stabilized (and isolated) by coordination with various polar ligands, CH3CN, THF, and even CH2Cl2 [1804,1805]. Some metallocenium cations even react with C–H bonds in aromatic rings of counter-ions and form zwitter-ions [1810,1811]. Such stabilized ions have low activity in alkene polymerization reactions [1805–1807]. The same is true for chromocene ions [CpCrIII–Me    (ligand)n]+ [1808,1809]. Only a few naked metallocenium ions were experimentally observed. Two of them, examined by 1H NMR at low temperatures in THF solution, are Cp2Zr+– CH2Ph [1804] and (Me-Cp)2Zr+–CH2Ph [1805] with BPh 4 as the counter-anion. The reason for the relative stability of these cationic species is the Z2 interaction between the Zr+ atom and p-electrons of the benzene ring. The second of these naked ions rapidly polymerizes ethylene in CH2Cl2 solution. However, naked metallocenium ions bearing growing polymer chains could not be observed [1805]. The third known ion pair containing a naked metallocenium ion, ½Cpn2 Tiþ  Me
[BPh 4 ], is too sterically hindered to act as an ethylene polymerization catalyst [1349]. Free metallocenium cations as active centers: The main difficulty in proposing a plausible mechanism of alkene polymerization reactions with metallocene catalysts becomes apparent when one examines a possibility of forming a free metallocenium ion Cp2Zr+–R or a solvent-separated ion pair [Cp2Zr+–R    (solvent)n] [A] from experimentally observed tight ion pairs described above. Polymerization reactions of alkenes with metallocene catalysts readily occur in non-polar solvents, both aromatic and aliphatic. None of these hydrocarbons can be regarded as a suitable solvating agent for metallocenium ions. Solvent-separated ion pairs were experimentally observed only at low temperatures in bromobenzene solution for several constrained-geometry cationic species, [Me2Si(Me4Cp)(t-Bu-N)]Zr+–Bz

425

Active Centers in Transition Metal Catalysts

and [(CH2)n(Cp)(N-Ru]Zr+–Bz (n ¼ 2 and 3), with the [Bz-B(C6F5)3] counteranion [1265]. To examine a possibility of generating solvent-separated and alkene-separated ion pairs of metallocenium cations, Brintzinger studied several types of models of chain initiation reactions using various Lewis bases as models of alkenes: 1. Displacement of the [MeB(C6F5)3] anion from its tight ion pair with Cp2Zr+– Me by reacting the Cp2ZrMe2-B(C6F5)3 and the Ind2ZrMe2-B(C6F5)3 system with phosphines [1794], anilines, amines, and ethers [1812]. 2. Similar displacement reactions of the [MeB(C6F5)3] ion from its tight ion pairs with several bridged Cp2Zr+–Me ions by anilines, amines, and ethers [1812]. 3. Anion exchange reactions between various tight ion pairs [Cp2Zr+–Me]  [MeB(C6F5)3] and the effect of free [Me-B(C6F5)3] or [B(C6F5)4] ions (with Li+ as the counter-cation) on the anion exchange [1797]. These observations led to the conclusion that the proposed active species at the initiation step in alkene polymerization reactions, metallocenium cations [Cp2Zr+– Me], cannot exist in hydrocarbon solutions without close proximity of a counteranion. These data include thermodynamic and kinetic studies of displacement of the [Me-B(C6F5)3] anion to the outer coordination sphere by several strong and weak Lewis bases, and the kinetics of anion exchange between different tight ion pairs [Cp2Zr+–Me]  [Me-B(C6F5)3] [1794,1812]. Even after the anion is displaced from the inner coordination sphere by a Lewis base, it remains in the form of a loose outer-sphere complex. The formation of separated solvated ions in Reactions (6.3)– (6.6) does not occur to any significant extent [1812]. The ability of counter-anions to coordinate with metallocenium cations can greatly affect the effective reactivity of the cations. This prediction was borne out by the measurement of differences in the activation energy of the propylene insertion step into the Cp2Zr+–Me bond derived from rac-Me2Si(Ind)2ZrMe2 at 251C. These measurements used the least-coordinating anion [(C6F5)3B–CRN–B(C6F5)3] as a benchmark [1366]: Counter- [(C6F5)3B–CRN–B(C6F5)3] anion DDG¼ 0 (reference) (kJ/mol)

 [B(C6F5)4] Ni[–CRNB(C6F5)3] 4 [MeB(C6F5)3]

1.1

4.1

12.7

When metallocene systems activated with ion-forming cocatalysts are tested under very pure conditions at low temperatures, they can function even at [activator]:[metallocene] ratios significantly lover than one [123]. The activator converts a part of Cp2ZrMe2 into metallocenium cations Cp2Zr+–Me, the latter initiate polymerization reactions of 1-alkenes and are transformed into Cp2Zr+– Polymer species. Co-existence of unreactive Cp2ZrMe2 species and the growing polymer chains results in a rapid exchange of their alkyl ligands: Cp2 ZrMe2 þ Cp2 Zrþ 2Polymer Ð Cp2 ZrðMeÞ2Polymer þ Cp2 Zrþ 2Me (6.9)

426

Alkene Polymerization Reactions with Transition Metal Catalysts

This reaction is called ‘‘degenerative chain transfer’’; it leads to a rapid engagement of all zirconocene complexes in the growth reactions. Anhydrous crystalline salts MgCl2, MgF2, MgBr2, and LiCl represent a different type of ion-forming cocatalysts for alkylated metallocene complexes (Section 4.6.2.4). These salts are usually produced by chemical synthesis, e.g., in reactions of MgR2 with HCl [1380], AlR2X (X ¼ Cl or F) [1305,1381,1382], or CX4 (X ¼ Cl or Br) [1305]. Solid-state 13C NMR data show that when Cpn2 ThMe2 is combined with MgCl2, one of the methyl groups in the complex is transferred to the MgCl2 surface with the formation of the ion pair [Cp2 Th+–Me]    [WMg–Me] [1380]. Addition of ethylene to this catalyst at 77 K results in its slow insertion reaction into the Th–C bond and the formation of short polyethylene chains attached to the Th atoms, whereas the anionic [WMg–Me] species remains intact. 6.1.1.2. Catalysts derived from constrained-geometry complexes Monometallocene complexes Sc, Ti, and Zr of the general formula [bridge-(R4Cp)(Ru-A)]MX2] containing one Z5 ligand, C5H4 or C5Me4 (complexes IX in Scheme 1.1) form very active catalysts for alkene polymerization [73,74,151, 995,996,998]. The second ligand in these complexes, Ru-A (A ¼ N or P; Ru ¼ a branched alkyl group), acts as both the s- and the p-ligand. It simulates the cyclopentadienyl group to a significant extent, and constrained-geometry catalysts exhibit many features, both in terms of reactivity and stereochemical behavior, similar to those of catalysts derived from bridged bis-cyclopentadienyl complexes. Their ability to polymerize virtually any 1-alkene and styrene, as well as alkenes with disubstituted double bonds, both R–CHQCH–R and CH2QCR2 [536,1868], can be ascribed to their open coordination sphere, as shown in Scheme 6.1. Transition metal atoms in their metallocenium ions are more approachable than in catalysts derived from bis-metallocene complexes. A detailed NMR study of a model monotitanocene complex of this type with two benzyl groups attached to the Ti atom showed than when this complex reacts with B(C6F5)3 under mild conditions, a stable cationic species is produced and it forms a solvent-separated ion pair with the [B(C6F5)3(CH2Ph)]– counter-ion [74]. The issue of a particular steric strain imposed by the bridge between the two ligands (implied by the name of these complexes, ‘‘constrained-geometry’’) remains debatable [1287]. For example, ‘‘constrained-geometry’’ bridged monometallocene

C6F5 + M

E A

B-

R C6F5

C6F5

C6F5

R′

Scheme 6.1 Model of active center derived from constrained-geometry complex activated with ion-forming cocatalyst.

Active Centers in Transition Metal Catalysts

427

complexes shown in Scheme 1.1 and their nonbridged analogs CpZrX3 perform quite similarly in ethylene/1-octene copolymerization reactions under identical conditions [1287]. Non-bridged analogs of constrained-geometry complexes CpTiCl2[N(Me)(R)] and (1,3-Me2-Cp)TiCl2[N(Me)(R)] with different alkyl substituents at the nitrogen atom, R ¼ Me, Et, and Cy, were synthesized and were converted into effective catalysts for polymerization of ethylene, 1-alkenes, and styrene [335]. The polymerization efficiency of these complexes under moderate conditions also differs little in comparison with that of their bridged analogs. DFT analysis of an ethylene insertion reaction into the Zr+–CH3 bond in two types of metallocenium cations, a constrained-geometry active center CH2QC(Cp) (O)Zr+–CH3 and its ‘‘unconstrained’’ dimer CH2QC(Cp)Cl2Zr(m-O)2Zr+ (–CH3)(Cp)CQCH2, showed virtually no difference in the energy profile. Activation energies for the insertion step of the coordinated ethylene molecule were estimated as 22.3 kJ/mol (5.3 kcal/mol) and 24.6 kJ/mol (5.9 kcal/mol), respectively [1287]. It appears that the chemical nature of ligands in the active center in these catalysts rather than the strain on their coordination imposed by the bridge between them (and the resulting distortion in the coordination sphere around the transition metal atom) affects the activity of the catalysts. 6.1.1.3. Early metallocene catalysts The second group of metallocene polymerization catalysis where the formation of metallocenium ions was experimentally proven is the family of early metallocene catalysts, combinations of titanocene complexes and alkylaluminum chlorides, AlMe2Cl or AlEt2Cl. The active species in the catalysts are derived from complexes of alkylated TiIV compounds [26,1826,1827,1831]: Cp2 TiCl2 þ AlR2 Cl ! Cp2 TiðRÞCl þ AlRCl2 Cp2 TiðRÞCl þ AlRx Cl3x Ð Cp2 TiðRÞCl  AlRx Cl3x

(6.10) (6.11)

Reactions (6.11) can involve various organoaluminum compounds (AlRxCl3x ¼ AlR2Cl, AlRCl2, AlCl3); they are equilibrium reactions strongly shifted to the right [1830–1832]. Complexes of alkylated Ti species formed in Reactions (6.10) and (6.11) catalyze ethylene polymerization reactions [1831] (except for AlRxCl3x ¼ AlCl3 [1832]). Based on kinetic and electroconductivity studies of these catalyst systems, Shilov and Dyachkovsky concluded that the real active centers in these catalysts are not the alkylated TiIV complexes themselves but cationic species (titanocenium ions), which are present in very low equilibrium concentrations [1828]: Cp2 TiðRÞCl  AlRCl2 Ð ½Cp2 Tiþ 2R
 ½AlRCl3


(6.12)

The role of metallocenium cations as active centers in these catalysts was later confirmed in a series of model experiments [1791,1805,1829]. The simplest metallocene system in terms of reactions leading to the formation of active species is a combination of Cp2TiCl2 and AlMeCl2 [1787]. Two species were identified in low-temperature reactions, the Cp2Ti(Cl)–Cl    AlMeCl2

428

Alkene Polymerization Reactions with Transition Metal Catalysts

complex (analog of the product of Reaction (6.11)) and a tight ion pair [Cp2Ti+– Me]  [AlCl4], an analog of the product of Reaction (6.12). The equilibrium in the latter reaction is strongly shifted to the left. The spectroscopic (NMR, UV) confirmation of the formation of this ion pair is lacking [1787,1805] because of the high reactivity of naked Cp2M+–R ions and because of a variety of side-reactions they can participate in [1805]. However, Eisch demonstrated that when an acetylenic compound, Ph–CRC–SiMe3, is used as the model of a 1-alkene molecule, it cleanly reacts with the TiCH3 bond in the metallocenium cation Cp2Ti+–Me (syn-carbotitanation of the CRC bond) [1787]: ½Cp2 Tiþ 2Me
 ½AlCl4
 þ Ph2CRC  SiMe3 ! ½Cp2 Tiþ 2CðSiMe3 ÞQCðPhÞMe
 ½AlCl4


(6.13)

Ion pairs similar to those formed in Reactions (6.12) were also proposed in metallocene catalysts containing Cp2MCl2 complexes and mixed AlMe3+AlMe2F cocatalysts [1243,1611]: 2Cp2 MCl2 þ 3AlMe3 þ AlMe2 F Ð ½Cp2 Mþ 2Me
 ½Al2 Me6 F
 þ 2 AlMe2 Cl (6.14) 6.1.1.4. Metallocene catalysts utilizing MAO as a cocatalyst The active centers in catalyst systems containing metallocene complexes and MAO are metallocenium cations, the same as those in two types of metallocene catalysts described in Sections 6.1.1.1 and 6.1.1.3. The cations are formed in reactions between alkylated metallocene complexes and MAO. The formation of alkylated metallocene complexes Cp2ZrMe2 from Cp2ZrCl2 is firmly established; it is described in Section 4.6.5. This reaction is merely the first step in a complex sequence of steps leading to the formation of the active species. Relatively low [MAO]:[Zr] ratios (10–30) are usually sufficient to convert all initial metallocene complexes into the Cp2ZrMe2 species [1596,1833]. However, much higher [MAO]:[Zr] ratios are needed to produce active catalysts. In this respect, it is telling that when the preformed rac-C2H4(Ind)2ZrMe2-MAO system was used in a propylene polymerization reaction at 301C at a [MAO]:[Zr] ratio of 300, it was completely inactive but the addition of an ion-forming activator, [NHMePh2]+ [B(C6F5)4], resulted in rapid activation of the catalyst [1596]. Formation reactions of metallocenium ions: The principal difficulty in formulating the mechanism of metallocenium-ion formation is a lack of precise knowledge about the structure of MAO. An esr study with a stable nitroxyl radical showed that MAO has two strong Al-centered Lewis-acidic centers [1324]. XPS analysis also identified two types of acidic sites in MMAO adsorbed on several carriers [1392,1439,1440]. The ratio between concentrations of the centers is affected by the presence of extraneous organometallic compounds, both naturally present in MAO (AlMe3) and deliberately added to it [1324]. In general terms, only threecoordinated Al atoms in MAO are regarded to be sufficiently strong acidic centers capable of converting Cp2ZrMe2 into metallocenium cations Cp2M+–Me. Barron

429

Active Centers in Transition Metal Catalysts

suggested that the cocatalyst efficiency of alkylalumoxanes in metallocene catalysis is a consequence of their ‘‘latent’’ Lewis acidity, ability of an alumoxane molecule to undergo a heterolytic Al–O bond cleavage (due to ring strain of Al–O bonds) and generate true Lewis acidic sites [1605]. It is generally accepted that the cocatalytic action of MAO involves the abstraction of an anionic ligand from an alkylated metallocene complex, most probably, the abstraction of Cl from Cp2M(Me)Cl or abstraction of Me from Cp2MMe2 [749,1322,1834–1836]: Cp2 MðMeÞCl þ MAO ! Cp2 Mþ 2Me þ ½MAOðClÞ


(6.15)

Cp2 MMe2 þ MAO ! Cp2 Mþ 2Me þ ½MAOðMeÞ


(6.16)

+

The formation of electro-deficient Cp2Zr –Me ions from Cp2ZrCl2, Cp2ZrMeCl, or Cp2MMe2 upon their reaction with MAO was confirmed by XPS [1322]. After a Cp2Zr+–Me ion is exposed to ethylene (and after the b-H elimination reaction is completed), this ion is converted to the Cp2Zr+–H ion. Kinetic data show that the activity of Cp2ZrMe2-MAO and Cp2ZrCl2-MAO systems in ethylene polymerization reactions is practically the same [1603,1604]. A replacement of a large fraction of MAO in the latter catalyst with a strong alkylating agent, AlMe3, does not diminish the reactivity of the catalyst either [1301]. Both these findings suggest that Cp2ZrMe2 is the most probable precursor of the active centers (Reaction (6.16)). The cationic nature of the active centers was also confirmed in comparative studies of polymerization reactions using MAO-activated metallocene complexes and synthesized ionic pairs containing metallocenium cations (described below) [670,1837]. The anions with a delocalized charge formed in Reactions (6.15) and (6.16) are usually regarded as much weaker Lewis bases compared to mononuclear anions of the [AlRxCl4x] type. Therefore, they are less capable of forming tight ion pairs with metallocenium ions Cp2M+–R, which interfere with coordination of alkene molecules at the transition metal atoms [1318]. Marks, in his studies of Cp2Zr(13CH3)2-MAO interactions at low [MAO]:[Zr] ratios by solid-state 13C NMR, produced the first proof that MAO converts these metallocene complexes into cations [1834]: Cp2 ZrMe2 þ MAO ! ½Cp2 Zrþ 2Me
 ½MAOðMeÞ


(6.17)

91

Zr NMR analysis also confirmed the formation of these ions, which are closely associated with O atoms of MAO [623]. Tritto and Sacchi identified a similar ion pair in reaction products of the Cp2Ti(Me)Cl  AlMe3 complex at low temperatures [1343,1838,1839]: Cp2 TiðMeÞCl  AlMe3 þ MAO ! Cp2 TiMe2 þ ½Cp2 Tiþ 2Me
 ½MAOðMeÞ


(6.18)

A similar reaction, abstraction of Cl by MAO, takes place with Cp2Ti(Me)Cl [1840]. Complexes of metallocenium ions: Metallocenium ions exist in solution either as tight ion pairs or as complexes with various ingredients in the catalysts. Tritto and Zakharov found several products in reactions between Cp2ZrMe2 and MAO. Their

430

Alkene Polymerization Reactions with Transition Metal Catalysts

structure depends on reaction temperature and a [MAO]:[Zr] ratio. When the reaction is carried out at low temperatures and at low [MAO]:[Zr] ratios, two complexes are predominant [1347,1376,1840–1842], a weak neutral Lewis acid adduct, possibly in equilibrium with a tight ion pair [1347], Cp2 ðMeÞZrðm-MeÞAl½MAO
Ð ½Cp2 Zrþ 2Me
 ½MAOðMeÞ


(6.19)

and a homo-dinuclear cation, ½Cp2 ðMeÞZrðm-MeÞZrðMeÞCp2
þ  ½MAOðMeÞ


(6.20)

When the [MAO]:[Zr] ratio increases to several hundreds (the ratio typical for alkene polymerization reactions), two other types of tight ion pairs are observed in reactions of Cp2ZrCl2, (n-Bu-Cp)2ZrCl2, (t-Bu-Cp)2ZrCl2, and (1,2,3-Me3Cp)2ZrCl2 with MAO [1275,1324,1376,1790,1840–1842]. These two new species are mononuclear ion pairs and heterodinuclear ion pairs in equilibrium: ½Cp2 Zrþ 2Me
   ½MAOðMeÞ
 þ AlMe3 Ð ½Cp2 Zrðm-MeÞ2 AlMe2
þ  ½MAOðMeÞ


(6.21)

A thorough comparison of NMR data for these metallocene complexes and the data on their activity in ethylene polymerization reactions led to the conclusion that the dinuclear complex formed in Reaction (6.21) is the direct precursor of the active species in MAO-activated metallocene catalysts [1275,1790]. Theoretical analysis agrees with these experimental findings. Fusco [1279,1843] and Ziegler [1844] carried out DFT calculations of several zirconocene species coordinated to AlMe3 and to different models of MAO molecules. According to the calculations, the formation of neutral complexes between Cp2Zr(Me)Cl [1843] or Cp2ZrMe2 [1844] and different organoaluminum compounds proceeds easily but the transformation of these complexes into ion pairs is an energy-demanding step. The most probable candidate for the immediate precursor of the active species is the dinuclear complex [Cp2(Me)Zr(m-Me)AlMe2]+  [MAO(Me)], it is slightly different from the experimentally observed dinuclear complex formed in Reaction (6.21) and contains only one weak m-CH3-bridge between the Zr and the Al atoms. The methyl group in the bridge of the latter complex and the methyl group attached to the Zr atom exchange rapidly. Breaking the bond between the m-CH3 group and the Zr atom requires 203 kJ/mol (48.4 kcal/mol); it produces the metallocenium active species Cp2Zr+–Me. If the content of AlMe3 in MAO is high, additional complications in the chemistry may arise, the formation of analogs of Tebbe’s reagents Cp2Zr(Me)– CH2–AlMe2 and Cp2Zr(–CH2–AlMe2)2, which are ineffective as polymerization catalysts [27,65]. Metallocene complexes with amido ligands: The formation of ion pairs was also observed by NMR and UV in reactions of two zirconocene complexes with samido ligands, rac-C2H4(Ind)2Zr(NMe2)2, and rac-Me2Si(2-Me-4-Ph-Ind)2Zr (NMe2)2 [167,710,1281,1789,1845]. Under moderate conditions, the reaction proceeds in several steps. First, original metallocene complexes exchange both their NMe2 groups with methyl groups in a reaction either with pure AlMe3 or with AlMe3 present in MAO. The exchange reaction produces dialkylated metallocene

Active Centers in Transition Metal Catalysts

431

complexes, e.g., rac-C2H4(Ind)2ZrMe2. As one would expect, the interaction between this alkylated complex and AlMe3 produces the same heterodinuclear complex as that formed in Reaction (6.21) [1845], and its reaction with MAO gives an ion par identical to that formed in Reaction (6.17) [1789,1845]. The similarity in the chemistry of active center formation between two MAO-activated isospecific systems, one derived from rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2 and another from racMe2Si(2-Me-4-Ph-Ind)2Zr(NMe2)2, is reflected in nearly identical regioregularity and stereoregularity of the catalysts [1281]. Nature of catalytically active species: Two principal assumptions with respect to the nature of active species in MAO-activated metallocene systems are the same as for metallocene catalysts activated with ion-forming cocatalysts: 1. The species formed in reactions (6.17), (6.20), and (6.21) may exist in solution either as contact ion pairs (which are experimentally observed) or as solventseparated ion pairs [1841,1842]. 2. Tight ion pairs and dinuclear complexes (6.20) and (6.21) dissociate at increased temperatures with the formation of mononuclear metallocenium cations Cp2M+–Me solvent separated from their counter-ions [1841]. These solventseparated ion pairs are the proposed active centers in metallocene catalysts. A strong effect of the solvent’s dielectric constant on the rates of propylene polymerization [742,944,1244,1608] and propylene/1-hexene copolymerization reactions [944] supports this hypothesis. Rytter and Ystenes proposed a different pathway to the formation of solventseparated metallocenium ions in catalyst systems containing Cp2ZrMe2 and MAO [1347]. When a molar [MAO]:[Zr] ratio is low, a stable complex (6.19) is formed, and then a tight ion par of the type shown in Reaction (6.19) is generated from it. However, as the [MAO]:[Zr] ratio increases, additional MAO molecules compete with the Cp2Zr+–Me ion for complex formation with [MAO(Me)] by forming methyl-bridged associates with the latter, which provides a more uniform distribution of the negative charge in the counter-anions: Cp2 ðMeÞZrðm-MeÞAl½MAO
þ MAO ! ½Cp2 Zrþ 2Me
þ ½MAOðm-MeÞMAO


(6.22)

The existence of methyl-bridged associates of MAO molecules was proposed independently based on IR data [1846]. DFT calculations showed that the formation enthalpy of a solvent-separated ion pair in Reaction (6.22) is B84 kJ/mol (B20 kcal/mol), lower than in Reaction (6.19) [1347]. The efficiency of MAO-derived anions [MAO(X)] as counter-ions for metallocenium ions depends on the nature of ligand X in the original metallocene complex. A combined UV-vis/polymerization study showed that the [MAO]:[Zr] ratio required for the formation of catalytically active species is very high (several thousands) when X ¼ Cl but it can be merely 50–150 when X ¼ Me or CH2Ph [1282]. The productivity of resulting catalysts also depends on the nature of X indicating that the ability of the anion [MAO(X)] to coordinate with a metallocenium ion and to compete for the coordination site at the transition metal

432

Alkene Polymerization Reactions with Transition Metal Catalysts

atom with the CQC bond of an alkene molecule also depends on the nature of group X. 6.1.1.5. Chemistry and mechanism of catalyst deactivation reactions Catalyst systems prepared with ion-forming cocatalysts: Polymerization centers in metallocene systems activated with ion-forming cocatalysts are not very stable under typical polymerization conditions. Some of the reasons for the catalyst deactivation are trivial, poisoning with impurities in solvents, monomers, etc. Other mechanisms of chemical deactivation, which were studied mostly for constrainedgeometry complexes, can be separated into several categories. The first are reactions of Cp2MR2 and metallocenium ions Cp2M+–R with counter-anions [1258–1260]. They include the transfer of an aryl group, Cp2 Mþ 2R þ ½CH3 BðC6 F5 Þ3
 ! Cp2 MðRÞ2C6 F5 þ RBðC6 F5 Þ2 (6.23) the transfer of an H atom from the counter-ion, Cp2 Mþ 2R þ ½CH3 BðC6 F5 Þ3
 ! Cp2 MðC6 F5 Þ2CH2 2BðC6 F5 Þ2 þ CH4

(6.24)

as well as C–H bond activation and metathesis, Cp2 MMe2 ð þ AÞ ! Cp2 MðCMe2 Þ2 MCp2 þ 2CH4

(6.25)

DFT calculations suggest that Reaction (6.24) is more facile for the Ti catalysts [1824] whereas the transfer of the C6F5 group in Reaction (6.23) proceeds faster for the Zr catalysts [1825]. The steric bulk of Z5 ligands suppresses all these transfer reactions [1824,1825]. The second deactivation mechanism of metallocene catalysts involves an interaction of the active species and alkylaluminum compounds which are added as impurity scavengers. This interaction leads to the formation of the Cp2M(R)–CH2– AlRu2 species similar to Tebbe’s reagents, e.g., Cp2Zr(Me)–CH2–AlMe2, which are ineffective as polymerization catalysts [27,65,710,1099]. Kinetic analysis of propylene polymerization reactions with typical ionic metallocene catalysts, combinations of rac-C2H4(Ind)2Zr(NMe2)2, [(CPh3]+ [B(C6F5)4], and different alkylaluminum compounds, showed that both the activity of the catalysts (after the maximum polymerization rate is reached) and the rate of catalyst deactivation at 501C depend on several factors [710]: 1. The type of the alkylaluminum compound. Catalyst systems employing AlMe3 and AlEt3 have approximately the same maximum activity (the same level of metallocene complex utilization) and they both decay very rapidly, within B10 minutes, most probably due to the formation of heterodinuclear complexes. The use of alkylaluminum compounds with bulkier alkyl groups, Ali-Bu3 and AliBu2H, produces equally active but more stable active centers that can retain activity for B60 minutes. 2. The concentration of the alkylaluminum compound. The racC2H4(Ind)2Zr(NMe2)2-[CPh3]+ [B(C6F5)4]-Ali-Bu3 system, as well as its

Active Centers in Transition Metal Catalysts

433

Ali-Bu2H-containing analog, becomes more stable as the concentration of AliBu3 increases. 3. The concentration of a metallocene complex in the polymerization medium. When propylene polymerization reactions were carried out with the racC2H4(Ind)2Zr(NMe2)2-[(CPh3]+ [B(C6F5)4]-Ali-Bu2H system at a constant ratio between all catalyst components, an increase of the zirconocene concentration brought two effects. The maximum polymerization rate significantly decreased (possibly due to the formation of homodinuclear complexes and other bimolecular reactions [1595]) but the catalyst became more stable, its deactivation half-time increased from 15–17 to B60 minutes. 4. The catalyst deactivation is irreversible. The addition of an excess of [(CPh3]+ [B(C6F5)4] salt does not result in a recovery of polymerization activity. There is no clear kinetic scheme yet to accommodate all these features related to the activity and stability of metallocenium ions generated in the presence of ionforming cocatalysts. In general, however, these catalysts are less stable than combinations of the same metallocene complexes and MAO. Catalyst systems prepared with MAO: Metallocene catalysts prepared from zirconocene complexes and MAO are relatively stable at mild temperatures. Several reasons for the stabilization of metallocenium cations in solutions containing MAO are usually considered. The first is strong delocalization of the negative charge in counter-anions [MAO(X)] and [MAO(m-Me)MAO] (MAO associates), which reduces the significance of catalyst deactivation reactions typical for metallocene catalysts utilizing ion-forming cocatalysts. The second is a possible effect of an alkene in the formation of alkene-separated ion pairs [Cp2M+– Me]    CH2QCH2    [MAO(X)] [519,1279,1843]. Studies of ethylene/norbornene and propylene/norbornene copolymerization reactions with bridged metallocene complexes and MMAO showed that such complexes between metallocenium cations and norbornene are sufficiently stable to be observable by VIS [201]. When polymerization reactions in the presence of MAO are carried out at higher temperatures, the activity gradually decreases with time. Several reactions were considered that could lead to the formation of catalytically inactive derivatives of metallocene complexes. One is reduction of TiIV and ZrIV species to catalytically inactive TiIII and ZrIII species. The reduction of monoalkylated titanocene complexes is the main reason for a rapid deactivation of the early titanocene systems [1618,1619]: Cp2 TiIV ðClÞ2Et  AlEtCl2 ! Cp2 TiIII Cl  AlEtCl2

(6.26)

A similar reduction reaction is responsible for deactivation of titanocene catalysts employing MAO as a cocatalyst [1620–1623]. Zirconocene complexes are much less prone to these reduction reactions, which explains their higher kinetic stability at increased temperatures [1622,1623,1847]. Some of the zirconocene reduction reactions were observed experimentally. The reduction of CpZrCl3 in contact with MAO at an [Al]:[Zr] ratio of 10 at 251C occurs slowly and apparently proceeds through the formation of the CpZrCl2Me

434

Alkene Polymerization Reactions with Transition Metal Catalysts

intermediate [1622]. The structure of the first reduction product, which is formed after B50 hours, was determined from its esr spectrum: a dinuclear ZrIII-Al complex, Cp(Cl)Zr(m-Cl)2Al(Oo)2. If the [Al]:[Zr] ratio is increased to 175, two hydrido-complexes of a similar structure are generated, Cp(H)Zr(m-Cl)2Al(Oo)2 and Cp(H)Zr(m-Cl)(m-Me)Al(Oo)2. As expected, the reduction of the Ti analog, CpTiCl3, by MAO at 251C at [Al]:[Ti] ¼ 185 occurs much faster; it is completed in B30 minutes. In this case, two hydrido-complexes are formed as well, first Cp(H)Ti(m-Cl)2Al(Oo)2 and then Cp(H)Ti(m-Cl)(m-Me)Al(Oo)2 [1622]. Another reaction potentially leading to the formation of catalytically inactive titanocene complexes is the formation of alkylidene species, Cp2TiQCH2    MAO [1840,1848]. The third reaction that can deactivate metallocene-MAO systems is the a-H transfer reaction between metallocenium cations and MAO [1245,1320,1348]: Cp2 Mþ 2Me þ Me2 Al2O2 ! Cp2 Mþ 2CH2 2AlðMeÞ2O2 þ CH4 (6.27) This reaction is suppressed if metallocene-MAO systems are supported on inert carriers, presumably due to loss of mobility of the species participating in Reaction (6.27) [1320].

6.1.2. Mechanism of alkene polymerization reactions, experimental data and theoretical analysis 6.1.2.1. Mechanism of normal chain growth and chain transfer reactions A number of thorough experimental mechanistic studies and theoretical calculations concerning the mechanism of alkene polymerization reactions have been published starting from the 1980s. Two competing chemical mechanisms of chain growth reactions with metallocene catalysts are discussed in the literature. The first mechanism is a variation of the Cossee mechanism proposed earlier for alkene polymerization reactions with Ti-based Ziegler–Natta catalysts [1849], it is shown in Scheme 6.2. The first step of the chain growth reaction is the equilibrium coordination of the CQC bond of a 1-alkene molecule at the transition metal atom in a metallocenium cation Cp2M+–CH2–R. The coordinated CQC bond is parallel to the M+–CH2 bond in the cation. In the second step, the coordinated CQC bond inserts into the M–C bond via the four-center transition state. Rooney and Green proposed an alternative mechanism of the chain growth reaction [1850]. Its later variant developed by Crabtree involves the formation of a metallocarbene species by abstraction of H+ by a base B (e.g., MAO) from the

Scheme 6.2 nium cation.

+

+

+

M R -CH2

A- +

M R -CH2

A−

A−

A− M R -CH2

+

M

CH2-CH2-CH2-R

Two-stage mechanism of CQC bond insertion into M^C bond in metalloce-

435

Active Centers in Transition Metal Catalysts

Polymer [Cp2M]+

Polymer +B -

BH+

Polymer

+ BH+ [Cp2M]

[Cp2M] R

Scheme 6.3

R

-B

[Cp2M]+

Polymer H

R

Modi¢ed carbene mechanism of alkene polymerization reactions [1851].

M+–CH2 bond and insertion of an alkene molecule in a metathesis reaction via the metallocyclobutane transition state, as shown in Scheme 6.3 [1851]. Brookhard and Green proposed a modification of this mechanism [1852]. It assumes a strong agostic interaction between the a-H atom in the growing polymer chain and the transition metal atom in the metallocenium cation as the first step in the CQC bond insertion. In the ultimate case, this agostic interaction is so strong that the metal carbene bond, M+QCH–Polymer, is the true intermediate in the chain growth reaction. The principal validity test of these mechanisms is based on the studies of two chain initiation reactions in a low-temperature polymerization reaction of ethylene with the Cp2Ti(13CH3)Cl-MAO system [1848]. The insertion of an ethylene molecule into the Cp2Ti+–13CH3 bond does not involve any scrambling of the 13 CH3 label, the effect expected in the mechanism in Scheme 6.2 but not anticipated by the metallocarbene mechanism. Currently, the generally accepted structure of the active center in metallocene-catalyzed polymerization reactions is a metallocenium cation with the coordinated counter-anion A positioned in the vicinity of the transition metal atom, Cp2M+–R    A. According to DFT calculations of Cp2Zr+–R ions, if the alkyl group R is bigger than the methyl group, the ion is additionally stabilized by agostic interactions [1853]. Among possible agostic interactions, the one between the b-H atom in the alkyl group R and the transition metal atom is preferred in comparison with the a-H interaction (+46.8 kJ/mol (11.2 kcal/mol)), the g-H interaction (+26.9 kJ/mol (6.4 kcal/mol)), and the d-H interaction (+30.9 kJ/mol (7.4 kcal/mol)). All these agostic interactions shift very rapidly, on a time scale of one picosecond [1854]. The role of the agostic interactions is discussed in Section 6.1.2.1.7. 6.1.2.1.1. The CQC bond coordination stage. As shown in Scheme 6.2, an alkene molecule approaches the transition metal atom from the direction between the M+–R bond and the position of A and coordinates with the metal atom displacing the anion. Theoretical calculations find this direction of the alkene attack the most preferable [1855,1856]. Based on theoretical analysis of aniondisplacement reactions in [Cp2M+–Me]  [A] ion pairs by ethylene and propylene molecules, a plausible mechanism of the coordination stage is a nucleophilic attack of the alkene molecule on the Cp2M+–Me bond. This is an equilibrium reaction strongly shifted to the left:

½Cp2 ðMeÞMþ
 ½A
 þ CH2 QCHR Ð ½Cp2 ðMeÞMþ   CH2 QCHR
 ½A
 (6.28)

436

Alkene Polymerization Reactions with Transition Metal Catalysts

Thermodynamics of Reaction (6.28) cannot be measured experimentally due to a high rate of the next step, the CQC bond insertion into the Cp2M+–CH3 bond. Theoretical calculations of DH values for Reaction (6.28) demonstrate that, as expected, alkenes are much weaker Lewis bases than the compounds for which experimental measurements are possible. The calculations give diverging estimations for the thermodynamics of Reaction (6.28) depending on the type of anion (e.g., [MeB(C6F5)3] vs. [B(C6F5)4]), the type of Z5 ligands in the metallocenium ion, geometry of the coordination complex, and computation method. In a hypothetical case when the counter-anion is absent, these calculations predict that the complex formation in Reaction (6.28) is strongly exothermic [1800]. However, most computational estimations of ‘‘real life’’ scenarios agree that Reaction (6.28) is endothermic. Evaluations of the reaction energy for the displacement of [MeB(C6F5)3] by an ethylene molecule in toluene solution vary from +52 kJ/mol (B12.5 kcal/mol) [1812] to +34 kJ/mol (B8 kcal/mol) [1794]. The displacement of the [B(C6F5)4] anion by an alkene molecule proceeds easier, the required energy is lower by B35–40 J/mol (B8.3–9.5 kcal/mol). Ab initio calculations of complex formation between a free H2Si(Cp)2Zr+–Me cation and an ethylene molecule show that the CQC bond of the coordinated ethylene molecule, the Zr atom, and the methyl group are all positioned in one plane. The methyl group it tilted away from the central position by B431 to allow for ethylene coordination [1800]. According to model studies of reactions between ionic pairs and weak donor molecules discussed in Section 6.1.1.1 [1812], kinetic features of 1-alkene polymerization reactions [139], and theoretical analysis [1822,1823,1857], three features characterize the CQC bond coordination step: 1. The reaction proceeds via the associative SN2-type mechanism through the formation of a penta-coordinated intermediate of the transition metal atom, in agreement with a large negative value of the activation entropy, DS¼ ¼ 0.14 kJ/mol  K (33 cal/mol  K) [139]. 2. Substitution of the counter-anion A is stereospecific, the alkene molecule occupies the coordination site vacated by A. 3. The attack results in the displacement of the counter-anion from the inner coordination sphere of the metallocenium ion to its outer sphere [1794,1812]; completely separated ions do not exist in hydrocarbon solutions. 4. DFT calculations for ethylene reactions with the b-agostically stabilized Cp2Zr+–Et ion show that the coordination step is exothermic, DH ¼ 37.1 kJ/mol (8.9 kcal/mol) [1853]. The displacement of an anion A derived from MAO by an alkene molecule (Reaction (6.28), Scheme 6.2) is also reversible and both its constituent reactions are relatively slow, much slower than the alkene polymerization reactions themselves. The slow forward reaction may indicate the existence of a slow ‘‘initiation’’ stage. In reality, this may be the stage of active center formation masked as a slow insertion reaction of an alkene molecule into the ‘‘initial’’ Cp2M+–R bond. The slow reverse step in Reaction (6.28) suggests a possibility that the chain growth occurs in (significantly long) time intervals after the displacement of the [MAO(Me)] anion and before its return to the inner coordination sphere of the transition metal atom.

437

Active Centers in Transition Metal Catalysts

The CQC bond coordination stage is not observed experimentally in alkene polymerization reactions catalyzed by metallocenium ions but its existence was proved in two series of experiments, with metallocene complexes of yttrium and with metallocenium ions containing anchored CQC bonds. Models of coordination stage, reactions of yttrium complexes: The existence of the alkene coordination stage was demonstrated for electro-neutral yttrium complexes Cpn2 Y  R [1858,1859]. The main advantage of studying the latter complexes is the ability to observe each stage in Scheme 6.2 without an interference of a counter-ion, albeit at very low temperatures (–100 to –1401C). The principle disadvantage, of course, is a lack of a direct relationship between the chemistry and reactivity patterns of Y vs. Zr or Ti metallocene complexes. The mechanism of the CQC bond insertion reaction into the Y–C bond is depicted in Scheme 6.4 [1859]. The equilibrium in the first stage of the reaction is achieved very rapidly on the NMR scale even at very low temperatures. Thermodynamic parameters of this equilibrium, given in Table 6.1, show that the complexes are weak. They are especially weak for the highly branched molecule, 3-methyl-1-butene. The structure of the alkyl group Ru attached to the Y atom has only a minor influence on the coordination ability of a propylene molecule. In contrast, the kinetics of the second stage in Scheme 6.4, the immigration of the alkyl group Ru to the coordinated 1-alkene molecule, which is represented by the DG¼migr value in Table 6.1, strongly depends on the type of Ru. Models of coordination stage, metallocenium ions containing anchored CQC bonds: Another experimental approach to the study of CQC coordination at the early-period transition metal atoms in metallocenium cations involved synthesis of surrogates for coordinated of 1-alkene molecules. These surrogates are metallocene complexes Cp*2 Y-R′ + CH2=C-R

Cp*2 Y-R′

Cp*2 Y-CH2 -CHR-R′

CH2 =CH-R

Scheme 6.4

Mechanism of CQC bond insertion into CpY^C bond [1860].

Table 6.1 Thermodynamics of 1-alkene coordination and kinetics of alkyl group migration in Scheme 6.4[1859] 1-Alkene

Alkyl group Ru

DHo (cal/mol  K)

DSo (kcal/mol)

Propylene 1-Butene 1-Hexene 3-Methyl-1-butene Propylene Propylene Propylene

CH2CH2CH(CH3)2 CH2CH2CH(CH3)2 CH2CH2CH(CH3)2 CH2CH2CH(CH3)2 (CH2)5CH3 CH2CH(CH3)2 Cyclopentyl

4.5 3.7 3.7 o3.2 4.4 4.4 4.3

30 29 26 B30 29 28 29

DG¼migr (kcal/mol)

11.5 11.0 11.7 11.3 13.6 W15.4

438

Alkene Polymerization Reactions with Transition Metal Catalysts

in which the vinyl bond is artificially placed in the vicinity of the transition metal atom by chemically linking it either to the metal atom itself or the cyclopentadienyl ring. Several p-complexes of these types were synthesized and the nature of the interaction between the CQC bond and the metal atom was investigated. These model complexes are Cp2Ti(Cl)(CH2)n–CHQCH2+AlEt2Cl, n ¼ 4 and 5 [1813]; Cpn2 Y–CH2–CH2–CMe2–CHQCH2 [1814,1815]; Cp2Zr+–O–CMe2– CH2–CH2–CHQCH2 [MeB(C6F5)3] [1816]; Cp(PhCH2)Zr+Cp–SiMe2–CH2– CHQCH2 [B(C6F5)4] [1817]; and Cpn2 Zr+–CH2–CH[CH2B(C6F5) 3 ]–CHQCH2 [1818]. All these studies proved, at least in principle, the reality of the universally accepted polymerization mechanism in Scheme 6.2. At low temperatures, the CQC bond in all the complexes is indeed coordinated to transition metal atoms and in some cases it inserts into the M–C bond with the formation of metallocycloalkanes. From the kinetics point of view, the coordination of an alkene molecule at the transition metal in a metallocenium ion is the rate-limiting stage of the overall alkene insertion reaction into the Cp2M+–C bond. As typical for any bimolecular reaction with negative activation entropy, the rate of this step (and, therefore, the rate of the overall polymerization reaction) is significantly increased at very high pressures. The following example related to the activity of the Me2Si(Me4Cp)2ZrCl2-MAO system in polymerization reactions of 1-hexene at 201C at [MAO]:[Zr] ¼ 10,000 confirms this effect [1278]. Pressure (MPa) Productivity (ton/mol Zr  h)

0.1 1.35

100 44.3

250 110.8

500 263.5

Direction of alkene attack: Theoretical analysis of chain growth reactions usually starts with the determination of the attack direction of an approaching alkene molecule with respect to the b-agostically stabilized Cp2Zr+–R ion. Two geometries of this attack are possible, front-side and back-side, where the ‘‘front’’ of the active center is defined as the point at which the agostically coordinated b-H atom is placed, as shown in Scheme 6.5. In the front-side attack, the CQC bond insertion should be preceded by rotation of the growing polymer chain around the Zr–a-C bond, which is required to open the coordination site for the alkene. This rotation is energetically demanding, B47 kJ/mol (B11 kcal/mol) [1860].

H

+

+

H

H

+

Zr

Zr

Zr R front-side attack

R

R

back-side attack

Scheme 6.5 Front-side and back-side coordination of CQC bond at b-agostically stabilized metallocenium ion [613].

Active Centers in Transition Metal Catalysts

439

If the rotation does not occur, the b-H atom in the growing chain and one of the carbon atoms of the approaching CQC bond become very close, which leads to an exchange of this H atom between the alkyl chain and the double bond. This exchange, in kinetic terms, is the chain transfer reaction to a monomer. In the backside attack, no such rearrangement of the b-agostic interaction in the growing chain is necessary. Theoretical analysis shows that two attack directions shown in Scheme 6.5 have comparable energetics, and no clear consensus still emerged concerning the preferred attack direction. Experimental data on the subject are absent and the dependability of the calculations is affected by many particular details of selected computation methods and by a chosen geometry of transition states [612,1853, 1854,1860]. 6.1.2.1.2. The CQC bond insertion step in model systems. Some experimental models of active species in alkene polymerization reactions are electro-neutral, including alkyl-bearing metallocene complexes of scandium [996,998], niobium, tantalum [1861,1862], yttrium, and rare-earth metals [1819,1863,1864]. The use of these model complexes has significant advantages, they usually can be presynthesized, isolated, and thoroughly characterized, and they do not require any cocatalyst provided that the alkenes are pure. The second stage of the alkene insertion reaction into the Cp2M–C bond (Scheme 6.2) was modeled experimentally by using electro-neutral complexes of Cp2 NbH3 with different alkenes [1861,1862]. Alkene molecules displace H2 from this complex and form complexes Cp2 ðHÞNb    CH2 QCHR. The ethylene complex of this type is relatively stable and was isolated; it afforded separate kinetic/ mechanistic analysis of CQC bond insertion into the Cp2 Nb2H bond. Detailed kinetic analysis of this reaction and insertion reactions involving complexes of substituted styrenes showed that the insertion step most probably proceeds via a concerted cyclic four-atom transition state. Electronic effects dominate in this step, the hydrogen atom leaving the Nb atom develops a significant negative charge, and one of the carbon atoms in the coordinated ethylene molecule develops a positive charge [1861,1862]. Therefore, electron-donating substituents attached to this carbon atom accelerate the insertion. Other experimentally examined models of ethylene insertion reactions similar to that in Scheme 6.2 used monometallocene complexes of late-period transition metals, CpRh(L)–R [1820] and CpCo(L)–R [1821], where L ¼ P(OMe)3. In the latter example, the CpCo(L)–Et complex with a b-agostically coordinated alkyl group is relatively stable at low temperatures, and significant concentrations of its p-complex with ethylene was observed by NMR at a high ethylene pressure, B35 atm [1821]. Estimated activation energies of b-migratory insertion of a coordinated ethylene molecule into different M–R bonds are [1820,1821] Cp[P(OMe)3]MR M ¼ Rh, R ¼ H M ¼ Rh, R ¼ Et M ¼ Co, R ¼ H M ¼ Co, R ¼ Et DG¼, kJ/mol (kcal/mol) 50.2 (12.0) 93.4 (22.3) 25–33 (6–8) 59.9 (14.3)

440

Alkene Polymerization Reactions with Transition Metal Catalysts

6.1.2.1.3. CQC bond insertion reactions in metallocenium ions. Systems with AlR2Cl as cocatalyst: In 1981, Fink investigated by NMR the early stages of ethylene polymerization reactions with the preformed Cp2Ti(Cl)Et-AlEt2Cl system at low temperatures [1865,1866]. Combining the two compounds produces the Cp2Ti(Et)Cl    AlEt2Cl complex (Reaction (6.11)). After a small quantity of 13 C-labeled ethylene was added to the mixture at 781C, the intensity of the signal belonging to the CH2 group in the Cp2Ti–CH2–CH3 moiety was reduced and a number of new peaks appeared in the NMR spectrum indicating a procession of ethylene insertion reactions:

Cp2 Ti2CH2 2CH3 þ

13

CH2 Q13 CH2 !

Cp2 Ti213 CH2 213 CH2 2CH2 2CH3 2ð

13

CH2 Q13 CH2 Þ !

Cp2 Ti213 CH2 213 CH2 213 CH2 213 CH2 2CH2 2CH3 ; etc:

(6.29)

(6.30)

These data represented the first direct proof that the ethylene polymerization reaction with metallocene catalysts is indeed the insertion reaction of the CQC bond into the transition metal–carbon bond. No coordination of ethylene molecules at the Ti atoms was observed in this study. A further extension of this research to the Cp2Ti(Cl)Me-AlMe2Cl system at low temperatures confirmed the formation of short growing chains, Cp2Ti–(13CH2–13CH2)n–CH3 [1587,1867]. The shortest of them, with n ¼ 1 and 2, are present in equilibrium amounts. These studies also showed a peculiarity of the first insertion: Cp2 Ti2CH3 þ

13

CH2 Q13 CH2 ! Cp2 Ti213 CH2 213 CH2 2CH3

(6.31)

Formally, Reaction (6.31) proceeds at a much lower rate than all subsequent insertion reactions; it is nearly 100 times slower than the insertion of the next ethylene molecule. As a consequence, only B15% of starting Cp2Ti(Cl)Me is consumed in Reaction (6.31) by the time when most of added ethylene is already converted to longer alkyl groups attached to the Ti atoms. These data suggest that the true active species in these polymerization reactions, metallocenium cations Cp2Ti+–Polymer (which were not experimentally observed) are generated in equilibrium (Reaction (6.12)) with the dinuclear complex Cp2Ti(Polymer)Cl    AlR2Cl. The ability to observe neutral species Cp2Ti(Cl)–(CH2–CH2)n–R with growing polymer chains of different lengths suggests that the equilibrium in Reaction (6.12) is reestablished after each ethylene insertion. An apparent difference in the reactivity of the Cp2Ti–CH3 bond vs. the Cp2Ti–CH2CH2CH3 bond is mostly determined by the values of equilibrium constants in Reaction (6.12) rather than by any differences in intrinsic reactivities of the respective Ti–C bonds. Systems with MAO as cocatalyst: Low-temperature 13C NMR studies also afforded identification of early stages of ethylene polymerization reactions with catalyst systems containing metallocene complexes and MAO. The addition of 13C-labeled ethylene to the Cp2Ti+–13CH3  cation at 781C results in a gradual disappearance of the (Cp2Ti+)13CH3 signal and the appearance of the signal of 13C-labeled polyethylene chains [1840,1841,1848]. In a similar reaction of unlabeled ethylene

441

Active Centers in Transition Metal Catalysts

with the Cp2Ti+–13CH3 cation the chain initiation step, the insertion of an ethylene molecule into the Cp2Ti+–13CH3 bond, proceeds without any scrambling of the label and produces the Cp2Ti+–CH2–CH2–13CH3 ion [1848]. The same reaction with ethylene was observed by 1H NMR in the [Ind2Ti+–CH3]  [BPh4] system at 401C [1807] and by 13C NMR in two zirconocenium ions carrying polymer chains [Cp2Zr+–13CH2–Polymer]  [13CH3-MAO] in the presence of MAO and [Cp2Zr+–13CH2–Polymer]  [13CH3B(C6F5)3] in the presence of B(C6F5)3 [1841]. Systems with ion-forming cocatalysts: Landis produced additional mechanistic details of alkene insertion reactions into Cp2Zr+–C bonds by studying kinetics of 1-hexene polymerization reactions with the [rac-C2H4(Ind)2Zr+–R]  [MeB(C6F5)3] system [139]. The propagation step in these reactions is not inhibited in the presence of excess of the [MeB(C6F5)3] counter-ion. This observation confirms that active centers in these reactions do not exist as completely separated ions produced in dissociation of starting [Cp2M+–R]  [A] ion pairs but are either solvent-separated or tight ion pairs. A significant kinetic difference exists between two insertion reactions of a 1-hexene molecule, into the Cp2Zr+–CH3 bond (chain initiation reaction) and into the Cp2Zr+–CH2CH(C4H9)Polymer bond (chain propagation reaction). The ratio of the respective rate constants is B0.1 at 101C and B0.03 at 501C [139]. Two factors may contribute to the slow rate of initiation relative to propagation: (a) a greater strength of the Cp2Zr+–Me bond relative to the Cp2Zr+– CH2–Polymer bond and (b) weakened ion pairing in the propagation species resulting from its higher steric bulk [1361,1803]. A replacement of 12C with 13C in the CQC bond of a propylene molecule produces a 13C/12C isotope effect in insertion reactions into the [Ind]2Zr+–CH3 bond. This effect was both experimentally measured and calculated. It is very small, B1–2%. A comparison of the experimental measurements and calculations showed that the transition stage in the CQC bond insertion step is not influenced by the nature of the counter-ion, which forms a loose ion pair with the metallocenium ion [1628]. Theoretical analysis indicates also that the commonly assumed reaction sequence, CQC bond coordination followed by insertion, can proceed in a concerted (one-step) mode without an expressed energy minimum corresponding to the coordinated alkene molecule [1822,1823]. Synthesis of metallocenium ions with different growing chains: Successive addition of different alkenes to the ion pair [rac-C2H4(Ind)2Zr+–CH3]  [MeB(C6F5)3] at low temperatures afforded the synthesis of several zirconocenium ions in the state of ‘‘suspended animation’’ [522,1590]. By varying the nature and the amounts of added alkenes, four different types of metallocenium ions were produced: C2 H4 ðIndÞ2 Zrþ 2½CH2 2CHðC4 H9 Þ
n 2CH3

(6.32) +

(insertion product of several 1-hexene molecules into the Cp2Zr –CH3 bond) C2 H4 ðIndÞ2 Zrþ 2CH2 2CH2 2½CH2 2CHðC4 H9 Þ
n 2CH3 (insertion product of one ethylene molecule into center (6.32))

(6.33)

442

Alkene Polymerization Reactions with Transition Metal Catalysts

C2 H4 ðIndÞ2 Zrþ 2CH2 2CHðCH3 Þ2½CH2 2CHðC4 H9 Þ
n 2CH3

(6.34)

(insertion product of one propylene molecule into center (6.32)) C2 H4 ðIndÞ2 Zrþ 2½CH2 2CHðCH3 Þ
m 2½CH2 2CHðC4 H9 Þ
n 2CH3

(6.35)

(insertion product of several propylene molecules into center (6.32)) Detailed NMR analysis determined several structural properties and reactivity features of these active centers in alkene insertion reactions at 401C. Most properties of the metallocenium ions depend on two features, the structure of the alkyl chain in the vicinity of the Zr atom and the nature of a counter-ion: 1. The coordination of alkene molecules to Zr atoms in all these active centers is not detectable; the only observable species are complexes (6.32)–(6.34) and initial [rac-C2H4(Ind)2Zr+–CH3]. 2. The insertion of the first 1-hexene molecule into the Zr+–CH3 bond in the original metallocenium ion proceeds much more slowly than all subsequent insertion steps into the Zr+–CH2 bond. This conclusion was supported by detailed kinetic studies of the same reaction [139]; it suggests that alkene insertion reactions into Cp2Zr+–CH3 bonds are relatively poor models of chain growth reactions. 3. The strength of coordination of the [MeB(C6F5)3] anion to the Zr atom decreases with the bulk of the alkyl group: complex (6.32)ocomplex (6.34)ocomplex (6.33)orac-C2H4(Ind)2Zr+–CH3. 4. The counter-ion forms a loose ion pair with Zr+, its position in a given active center does not change with time (there is no intermittent association/ dissociation phenomena) and it is not affected by the absence of monomer. Addition of a fresh monomer leads to an immediate resumption of insertion steps. Metallocenium ions with different coordination sites: Meso-isomers of bridged bisindenyl complexes provide a good example of active centers with two distinctly different types of active sites, as shown in Scheme 6.6. One of the coordination sites is very crowded and can accommodate only an ethylene molecule whereas the second site is sterically open and can coordinate virtually any 1-alkene molecule. As a result, when an ethylene/1-octene copolymerization reaction is carried out with a combination of meso-Me2Si(2-Me-1-Ind)2ZrCl2 and MAO at 01C at a low

E

M

+

E

R

M

+

R

Scheme 6.6 complex.

Two coordination sites of metallocenium ion from bridged meso-bis-indenyl

Active Centers in Transition Metal Catalysts

443

ethylene/1-octene monomer ratio, the copolymer has the alternating structure [327]. One of the coordination sites incorporates only ethylene molecules and another site reacts mostly with 1-octene molecules due to their much higher concentration. Of course, this type of a chain growth reaction is possible only if the growing polymer chain does not migrate from the more crowded to the less crowded site (the back-skip step (see Section 6.1.3.2.4)). 6.1.2.1.4. The CQC bond insertion step into the Cp2M+–H bond. The insertion of an alkene molecule into the Cp2Zr+–H bond represents an important chain initiation reaction in alkene polymerization reactions with metallocene catalysts. This reaction is energetically much more favorable than chain growth reactions. Due to a very high rate of the former reaction, most studies of alkene insertion reactions into transition metal–hydrogen bonds use various model complexes. Monometallocene complexes of scandium and yttrium provide good neutral models of metallocenium ions with the Cp2Zr+–H bond. Insertion reactions of 1-alkene molecules into the Y–H bond in the homo-binuclear CpY(m-H)2YCp complex [1863,1864] and in other yttrium complexes modeling constrainedgeometry catalysts [335] were observed at moderate temperatures, and the products of the insertion reactions containing the Y–CH2 bond were isolated. Two scandium complexes, Me2Si(C5Me4)(t-Bu-N)]Sc–H and Me2Si(C5Me4)(t-Bu-N)]Sc–nC3H7, were used as the models of alkene insertion reactions into the M–H and the M–C bond [996]. The first of the species is formed in the dissociation reaction of the dimer:

½Me2 SiðC5 Me4 Þðt-Bu-NÞ
Sc  PMe3
2 ðm-HÞ2 Ð Me2 SiðC5 Me4 Þðt-Bu-NÞ
Sc2H þ PMe3

(6.36)

Due to the equilibrium nature of Reaction (6.36), the monomeric species with the Sc–H bond rapidly become active centers after they are exposed to 1-alkenes (propylene, 1-butene, 1-pentene). They slowly polymerize 1-alkenes under mild conditions to atactic polymers of low molecular weight. The molecular weight distribution of propylene oligomers prepared with this complex follows the Poisson distribution function (Section 2.2.1), indicating the absence of chain transfer reactions. The second synthesized Sc complex can be viewed as a product of a single propylene insertion into the Cp2Sc–H bond. It bears the Cp2Sc–n-C3H7 bond and it also exists in the dimeric form in equilibrium with the monomeric active center [996]. Both species are present in solution in comparable amounts. The product of a single propylene insertion into the Sc–C bond of the monomeric complex was also experimentally observed. This product, Me2Si(C5Me4)(t-Bu-N)]Sc–CH2– CH(CH3)–n-C3H7, exists predominantly in the monomeric form due to an increased steric crowding. The relevance of the studies of these Sc and Y complexes as models of polymerization centers in metallocene catalysis involving Ti and Zr species is a disputed subject. Ab initio analysis of a series of Cp2M–R complexes (M ¼ ScIII, TiIII,

444

Alkene Polymerization Reactions with Transition Metal Catalysts

and TiIV; R ¼ H, CH3 and SiH3) showed one main difference in their geometry [1869]. The Cp2Sc–R and the Cp2 TiIII–R complexes are planar, the R group is positioned in the plane including the metal atom and the centers of both Z5 ligands. An additional 8–11 kcal/mol energy is required to transform these complexes to the pyramidal geometry. In contrast, Cp2TiIV–R complexes are pyramidal both according to calculations and experimental NMR data. Hydrides of bis-metallocene complexes of group 5 metals (Nb, Ta) are also convenient models [1862,1870]. These Cp2M–H complexes are electro-neutral, relatively stable, and form complexes with ethylene at low temperatures. The products of ethylene insertion into their M–H bonds, Cp2M–n-C3H7, are immediately converted to d 0 MV hydrides containing metallocyclopropane rings [1870]. Even electro-neutral metallocene complexes containing the Cp2MH bond can insert one alkene molecule. Bercaw investigated one such reaction imitating the chain initiation step, insertion of various alkenes into the Cp2MH bond in Cp2 ZrH2 and Cp2 HfH2 at low temperatures [1871]. The reaction proceeds in a single step. Cp2 MH2 þ CHR1 QCR2 R3 ! Cp2 MðHÞ2CHR1 2CR2 R3 H

(6.37)

A variety of alkenes can participate in this reaction including 1-alkenes (1-pentene, styrene), 2-alkenes (2-butenes, cyclopentene) and isobutene. The reactivity of the alkenes strongly depends on their structure, of course, as the data for Cpn2 HfH2 at 431C demonstrate: Alkene 1-Pentene Styrene cis-2-Butene Cyclopentene trans-2-Butene Isobutene 104 38 1 Relative rate 8.2  107 4.6  106 7.7  103

Thermodynamics analysis of insertion reactions for several para-substituted amethylstyrenes into the Zr–H bond of the Cpn2 ZrH2 molecule indicates that they proceed via the rate-determining H atom transfer to the coordinated alkene molecule. A small positive charge develops on the b-C atom of the inserting alkene molecule [1871]. A metallocenium cation containing the Cp2Zr+–H bond, racC2H4(Ind)2Zr+–H (in the form of its complex with Ali-Bu2H) was synthesized [710]. It readily inserts propylene molecules even at 781C; first it is converted to rac-C2H4(Ind)2Zr+–n-C3H7 and then produces isotactic polypropylene. However, theoretical analysis of two possible propylene insertion reactions into the Cp2Zr+–H bond of another metallocenium ion derived from rac-(3-t-Bu-Ind)2ZrCl2 showed that the secondary insertion with the formation of the Cp2Zr+–CH(CH3)2 species is preferred [633]. This preference is electronic in nature and mostly caused by the b-agostic interaction between the Zr atom and the H atoms in the methyl groups [633,1872]. 6.1.2.1.5. Theoretical analysis of CQC bond insertion step. Rigorous theoretical analysis of alkene polymerization reactions with metallocene catalysts is complicated by the fact that several types of interactions of their active centers (in addition to reactions with alkenes) should be taken into account, interactions between

Active Centers in Transition Metal Catalysts

445

metallocenium ions and their counter-anions and solvating effects of aromatic solvents [1873–1875]. This analysis is not only prohibitively complex for ‘‘real-life’’ anions but it is practically impossible for MAO-cocatalyzed reactions due to the absence of detailed structural information about the counter-anions. To avoid these complications, especially in reactions involving 1-alkenes instead of ethylene, most of the theoretical analysis is carried out using as the starting species ‘‘naked’’ metallocenium ions. The most successful of these computational attempts involve the simplest model reactions, the alkene insertion reaction into the Cp2M+–CH3 bond [1800,1822, 1843,1854,1857,1873,1875]. Morokuma investigated several such ‘‘stripped-down’’ versions, insertion of ethylene and propylene molecules into the Cp2Zr+–CH3 bond [1876] and into the H2Si(Cp)2Zr+–Me bond [1800]. Prior to insertion, the CQC bond of the coordinated ethylene molecule, the Zr atom, and the CH3 group are all positioned in one plane. The four-center transition state is also nearly coplanar. The changes in inter-atomic distances in the transition state are as ˚ to 2.38 A˚, the CQC bond expected, the ZrCH3 bond is lengthened from 2.28 A ˚ ˚ is also lengthened, from 1.33 A to 1.42 A, and two forming bonds are the Zr–CH2 ˚ [1800]. This quantum-chemical bond, 2.28 A˚, and the CH2–CH3 bond, 2.15 A analysis is in agreement with the well-known experimental data, the ethylene insertion reaction has a lower activation barrier, B27 kJ/mol (6.5 kcal/mol) compared to propylene, B50 k/mol (11.9 kcal/mol) [1876]. Another issue related to the mechanism of insertion reactions of 1-alkenes into the Cp2M+–CH3 bond is the regioselectivity of the reactions. Molecular mechanics analysis of a propylene insertion reaction into the H2Si(Cp)2Zr+–Me cation (assuming the same geometry of the transition state as in the ethylene insertion reaction) shows that the preference for the primary insertion mode can be explained by steric effects alone [1800]. The transition state barrier for the secondary insertion, which is accompanied by steric repulsion between the CH3 group in the propylene molecule and one of the cyclopentadienyl rings, is higher by B12.5 kJ/ mol (B3 kcal/mol) compared to the primary insertion. The steric preference for the primary insertion increases even further, to 18–21 kJ/mol (B4.3–5.1 kcal/mol), when the methyl group attached to the Zr atom in a Cp2M+–R cation is replaced with the ethyl group or the 2-methylbutyl group. Agostic interactions between transition metal atoms in the active centers and hydrogen atoms in growing polymer chains play an important role in the mechanism of the insertion reactions, as discussed in Section 6.1.2.1.7. Ziegler and Rytter carried out DFT analysis of the ethylene insertion step into the Zr–C bond of the b-agostically stabilized Cp2Zr+–Et ion [612,1853]. The insertion step of a coordinated CQC bond proceeds in a concerted manner and includes several computationally distinguishable stages. First, the b-agostically bound Cp2Zr+–Et ion is rearranged into the a-agostically bound Cp2Zr+–Et ion via ethyl group rotation around the Zr–C bond. The effective free energy of this rearrangement is very small, +0.34 kJ/mol (0.08 kcal/mol) because the positive entropy and the negative change in the zero-point energy compensate for a significant electronic barrier of the rearrangement. This rearrangement leads to the a-agostically bound Cp2Zr+–Et ion with a coordinated ethylene molecule. After that, the insertion of

446

Alkene Polymerization Reactions with Transition Metal Catalysts

the CQC bond into the Zr–C bond in this complex proceeds easily; the barrier for the insertion is merely 2.0 kJ/mol (0.5 kcal/mol). The insertion product, the Cp2Zr+–n-Bu ion, is initially g-agostically bound but it rapidly rearranges into the more energetically favorable b-agostically stabilized ion. Other semi-empirical calculations of the same reactions are in general agreement with these results [1268]. Latter calculations also showed that if a 1-hexene molecule is inserted into the Zr–C bond of the Cp2Zr+–Me ion, then the subsequent ethylene insertion reaction into the Zr–CH2 bond of the Cp2Zr+CH2CH(C4H9)C3H7 ion proceeds through a noticeably lower barrier (by B3.5 kcal/mol) than in an ethylene polymerization reaction. This difference may explain activation effects of 1-alkenes on the rate of ethylene consumption in metallocene-catalyzed polymerization reactions described in Section 5.4.3.2.2. Busico compared experimental data on the reactivity of several metallocene catalysts based on Cp2MX2 complexes and DFT analysis of propylene insertion reactions into M–C bonds in respective metallocenium ions Cp2M+–R [534,1802]. The comparison showed that theoretical investigations of naked cations, after standard empirical corrections for solvation and for counter-anion effects, are a reliable and useful tool in the analysis of such catalysts. Several conclusions were reached using this approach [534,1802], mostly in agreement with earlier calculations [1800,1876]: 1. Geometries of the alkene coordination stage and the transition stage of insertion reactions for ion pairs containing Cp2M+–C2H5 or Cp2M+–CH(CH3)2 cation and ethylene or propylene as monomers are very similar, the M+–C bond and the CQC bond are nearly coplanar. 2. The insertion barrier for ethylene is lower than for propylene by B8.5–12.5 kJ/ mol (2–3 kcal/mol). 3. Regioselectivity of the propylene insertion reaction into the M+–C2H5 bond, primary W secondary, corresponds to a DE¼ difference of 17.6–20.1 kJ/mol (4.2–4.8 kcal/mol), mostly for steric reasons. This preference does not depend on the chirality (or the absence thereof) of a metallocenium ion. 4. The propylene insertion step into the Cp2M+–CH(CH3)2 bond imitates propylene insertion after a regio-error. The primary insertion is still preferred but the DE¼ difference is very small, 2.9–5.8 kJ/mol (0.7–1.4 kcal/mol). In contrast, ethylene inserts into the Cp2M+–C2H5 bond and Cp2M+–CH(CH3)2 bond with a nearly equal ease. 6.1.2.1.6. Mechanism of chain transfer reactions. Two principal chain transfer reactions in alkene polymerization reactions with metallocene catalysts in the absence of free trialkylaluminum compounds are the chain transfer reaction to a monomer (Reaction (3.50)) and the spontaneous chain transfer reaction (Reaction (3.81)). Both reactions involve the transfer of the b-H atom from a growing polymer chain Cp2M+–CH2–CHRPolymer, either to a coordinated alkene molecule (Reaction (3.50)) or to the transition metal atom in the metallocenium cation in Reaction (3.81). Both reactions produce macromolecules with the same last chain end, CH2QCR–Polymer, and both apparently involve a b-H-agostic

447

Active Centers in Transition Metal Catalysts

H

+

Cp2M

R

+

R

Cp2M

H

β-H transfer

R′

R′

+

H

+

Cp2M R′′

+

β-H elimination R

Cp2M H +

Scheme 6.7 ions [1878].

R′′

R′′

+ H Cp2M

R′

R

R R′

R′

Chain transfer reactions in alkene polymerization reactions with metallocenium

interaction between the last monomer unit in the chain and the transition metal atom in the metallocenium ion, as shown in Scheme 6.7. However, two reactions in Scheme 6.7 obviously have different transition states and different reaction orders with respect to the monomer concentration [1860,1878]. Several examples of model polymerization reactions were described in the literature when preventing these b-H-agostic interactions, mostly by modifying the steric environment at the transition metal atom, led to greatly reduced rates of both chain transfer reactions and, hence, to the ability to carry out living polymerization reactions at relatively high temperatures [1879,1880]. Relative frequencies of two chain transfer reactions in Scheme 6.7 with respect to chain growth reactions strongly depend on two parameters, the electronic and the steric structure of the metallocenium ion, and the branching degree in the 1alkene. The data in Chapter 3 show that polymerization reactions of 1-alkenes with bulky alkyl substituents in the g-position to the CQC bond leads to very frequent chain transfer reactions and, respectively, to high fractional yields of dimers and light oligomers, all with the ‘‘head-to-tail’’ structure, CH2QCHR– CH2–CH2R. Longo rationalized these effects strictly on the basis of experimentally measured steric effects in chain growth and b-H transfer reactions involving Me2Si(Cp)2Zr+–CH2–CHR–C2H5 ions, insertion products of 1-alkene molecules CH2QCHR into the Me2Si(Cp)2Zr+–C2H5 bond [628]. Molecular modeling calculations show that two types of repulsive steric interactions exist in transition states of three competing reactions, a single chain growth event and two chain transfer reactions competing with it (Scheme 6.7). These repulsive interactions develop between the alkyl group in the inserting monomer molecule and the last monomer unit in the growing chain and between both these alkyl groups and Z5 ligands in the metallocenium ion. One example of these estimations is shown in Table 6.2 for polymerization reactions of three 1-alkenes with the Me2Si(Cp)2ZrCl2-MAO system. When the 1-alkenes contain bulky alkyl groups attached to the CQC bond, these steric restrictions make more difficult both reactions requiring the stage of CQC bond coordination, chain growth and chain transfer to a monomer. On the other hand, the ground state in the b-H transfer reaction to the Zr atom (Scheme 6.7) involves a strong agostic interaction between the H atom in the b-C–H bond in the CHR group resulting in a shortened distance between the H and the Zr atom, 2.16–2.23 A˚. This agostic interaction causes a significant repulsion between cyclopentadienyl rings in the

448

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 6.2 Estimation of steric effects in three steps of polymerization reactions of branched 1-alkenes with Me2Si(Cp)2Zr+–CH2–CH2R–C2H5 ion [628] 1-Alkene

Propylene

3-Methyl-1-pentene

Chain growth, E¼-Ecoordination (kcal/mol) Chain transfer to a monomer, E¼-Ecoordination (kcal/mol) Spontaneous chain transfer, E¼-Eground state (kcal/mol)

1.5

3.8

3.2

–

6.7

6.4

Vinylcyclopentane

4.4 11.3 5.8

active center and carbon atoms in both g-positions in the last monomer unit. The combination of these opposite steric effects, a more difficult chain propagation reaction and a less difficult b-H transfer reaction, explain the formation of low molecular weight products in polymerization reactions of heavily branched 1-alkenes. Kinetics and thermodynamics of the b-H elimination reaction was studied also by trapping the generated Cpn2 MH2 molecules with an alkyne. Alkylhydride complexes (Cp1)(Cp2)Zr(H)i-Bu were used to investigate the b-H elimination reaction (it produces (Cp1)(Cp2)ZrH2 and isobutene). The following relative reactivities were measured [1871]: Cp1 Cp2 Relative rate:

Cp CMe3-Cp (CMe3)2-Cp Cp Me4-Cp  Cp Cp Cp Ind-H4 Me4-Cp 8,000 3,200 1,000 400 190

Me4-Cp Cp Cp Cp 5 1

These data show that the probability of b-H elimination greatly decreases for metallocene complexes with highly substituted and sterically crowded Z5 ligands. Both chain transfer reactions in Scheme 6.7 were also subjected to thorough theoretical analysis. This analysis usually uses as a starting point a b-agostically stabilized Cp2Zr+–R ion [612,1853]. The calculated minimum activation barrier for the degenerated reaction Cp2 Zrþ 2C2 H4 2Hn þ CH 2 QCH 2 ! Cp2 Zrþ 2C 2 H 4 2Hn þ CH2 QCH2

(6.38)

is higher than the activation barrier of the competing insertion step of CH2QCH2 into the Cp2Zr+–C2H5 bond only by B14 kJ/mol (3.3 kcal/mol) but it requires a nearly coplanar arrangement of all four carbon atoms and the H atom participating in this ligand exchange [1853]. However, a significant entropy gain resulting from rotation of the methyl group in the Cp2Zr+–C2H5 ligand out of the plane makes this chain transfer reaction much less frequent. According to the calculations, the second chain transfer reaction in Scheme 6.7, the b-H atom transfer to the Zr atom, is strongly endothermic [612,1799,1878]. For example, free Cp2Zr+–C3H7 and Cp2Zr+–C4H9 ions are more stable than their decomposition products, Cp2Zr+– H+CH2QCH–R, by 113–115 kJ/mol (B27 kcal/mol) [612].

Active Centers in Transition Metal Catalysts

449

DFT calculations for Cp2Zr+–C2H5 and Cp2Zr+–n-C4H9 ions showed also that the spontaneous chain transfer step involving the release of an alkene molecule (ethylene from Cp2Zr+–C2H5, 1-butene from Cp2Zr+–n-C4H9) is highly endothermic (DH B80 kJ/mol (B19 kcal/mol)) and has a high activation barrier, DEact B83 kJ/mol (B20 kcal/mol) [1878]. In contrast, the chain transfer step to a monomer requires a much lower activation barrier, DEact B31 kJ/mol (7.5 kcal/ mol). This analysis matches well experimental findings about the dominance of the chain transfer reaction to a monomer in these alkene polymerization reactions at increased temperatures (Section 3.3.1.2.1). The b-H transfer reaction to an ethylene molecule (Scheme 6.7) is relatively free from steric interference. However, molecular mechanic analysis showed that when this reaction involves any 1-alkene CH2QCHR, significant steric repulsion develops between two R groups in the reaction, one belonging to a coordinated alkene molecule and another to the last monomer unit in the growing chain [1878]. Steric obstacles in this reaction increase further when a metallocenium cation has a bridge between its two Z5 ligands and when both cyclopentadienyl groups have an additional methyl substituent next to the bridge, e.g., in rac-Me2C[2-MeBenz(e)Ind]2Zr+–Polymer. The greatest steric repulsion in this case exists between the methyl group in the ligand and the first CH2 group in the growing polymer chain because the growing chain is forced into the conformation when the distance between these two groups is merely B3 A˚. Of course, this steric effect is present both in the chain growth reaction and in the chain transfer reaction to a monomer (both reactions have similar transition-state configurations), but its energy is higher in the latter reaction by B6 kJ/mol (B1.5 kcal/mol) [1878]. This estimation explains why the introduction of a methyl substituent into the second position of the cyclopentadienyl ring (next to the bridge) reduces the probability of the chain transfer reaction and increases the molecular weight of polypropylene produced with these catalysts [525,605]. 6.1.2.1.7. Agostic interactions in active centers. Most research of the alkene polymerization mechanism emphasizes the role of b-agostic interactions in chain growth and chain transfer reactions [629,1348,1583,1802,1860]. This non-bonding interaction between the empty d orbital of the transition metal atom and the b-H atom in the Cp2M+–CH2–CHR moiety (the Cp2M+–CH2–CH2 moiety in ethylene polymerization reactions) stabilizes the growing center and facilitates b-H transfer reactions. Several situations exist when these b-agostic interactions are not possible, e.g., when both the metallocene complex has bulky cyclopentadienyl rings [i.e., rac-C2H4(Ind)2ZrCl2] and the last monomer unit in a growing polymer chain also has a bulky substituent in the b-position, such as the CH2-Ph group in ethylene/allylbenzene copolymerization reactions [596]. In the absence of the bagostic interaction, the growing chain end becomes electronically unstable and prone to reactions with AlR3 present in MAO [596,1603]. An experimental proof of the existence of growing polymer chains with the b-H atom agostically coordinated to the transition metal atom is very difficult. Several model compounds were synthesized which demonstrate that such an interaction indeed exists. Most ion pars in these model systems contain a metallocenium ion

450

Alkene Polymerization Reactions with Transition Metal Catalysts

(Cp)(L)M+–R, where M is Zr and Hf; L is a monodentate or a bidentate ligand (PMe3 or acetamidinate); alkyl groups R are Et, n-Pr, i-Pr, n-Bu, i-Bu, and 2-EtBu, and the counter-anion is [B(C6F5)4] [123,626,1681,1877]. The existence of the b-agostic interaction was proved in X-ray analysis of the [(C5H4Me)Zr+CH2– CH3](PMe3) ion [1681]. The Zr–b-C and the Zr–b-H distances in the ion are ˚ , respectively, and the b-H atom lies in the plane shortened, 2.63 and 2.16 A containing the Zr LUMO orbital, as expected for a three-center, two-electron interaction. NMR data show that this complex, as well as similar zirconocene cations with longer alkyl groups, maintain the b-agostic structures in solution and that an exchange between this b-agostically bound H atom and another (free) b-H atom is rapid [1681]. Other models of this type are zirconocene complexes (Cp)(L)Zr+–CH2–CH(CH3)2 formed after insertion of a single propylene molecule into the Zr+–Me bond [123,1877]. This species are stable at temperatures below 251C. NMR and X-ray data show the presence of a strong agostic interaction ˚. between the tertiary H atom and the Zr atom, the Zr-H distance is merely 2.25 A Several other zirconocene complexes also show NMR signs of the b-agostic interaction [1877]. The (Cp)(L)Zr+–CH2–CH(C2H5)2 ion represents the most interest in this respect; it mimics the insertion product of a single 1-butene molecule into the Zr+–Et bond [626,1877]. b-Agostic interactions stabilize the ground state of the Cp2Zr+–CH2–CHR2 species [1583], which explains thermal stability of these complexes at temperatures when a facile b-H transfer to the Zr atom is expected. In contrast, the model of a polymer chain-bearing metallocenium cation, (Cp)(L)Zr+–[CH2CH(CH2CH3)]10CH3, does not exhibit any NMR features of b-agostic interaction and this species is indeed prone to the b-H transfer to the Zr atom and to chain-walking reactions discussed in Section 6.1.2.2 [626,1877]. b-Agostic interactions apparently play a large role in ethylene polymerization reactions with a particular type of constrained-geometry catalysts, bimetallic species in which two active centers are connected by a –CH2– or a –CH2–CH2– bridge between their cyclopentadienyl rings [552]. In these cases, close proximity between the centers permits an inter-center b-agostic interaction, the b-H atom in the growing chain attached to one Zr atom forms an agostic bond with the second Zr atom. This interaction reduces the propensity of growing polymer chains to participate in the chain transfer reaction to a monomer, which leads to the synthesis of polyethylene with a much higher molecular weight. Agostic interactions apparently play an important role in unusual reactivity patterns in polymerization and copolymerization reactions of cycloalkenes. Metallocene catalysts readily copolymerize ethylene and norbornene, although norbornene has the internal cis-CHQCH–bond and in spite of the apparent large bulk of the norbornene molecule. Moreover, the values of the reactivity ratio r1 for the ethylene/norbornene pair are close to 1 (see Table 3.51). If metallocene complexes with a large aperture angle between the two Z5 ligands are used in these copolymerization reactions, e.g., Me(H)C(Cp)2ZrCl2 or Me2C(Cp)(Ind)ZrCl2, the r1 values are even lower than 1, i.e., norbornene is more reactive in the insertion reaction into the Cp2Zr–CH2 bond than ethylene [936]. Detailed theoretical analysis suggests that, paradoxically, steric restrictions during the approach of the norbornene’s CQC bond to the Zr atom assist in the coordination of the alkene

Active Centers in Transition Metal Catalysts

451

molecule prior to its insertion [1881]. This coordination is not symmetric (as in the ethylene coordination), it brings one of the H atoms in the cis-CHQCHbond closer to the Zr atom resulting in a significant energy gain due to the agostic QC–H    Zr interaction. The viability of the notion of the b-agostic interaction was also confirmed in DFT computational analysis of the relative stability of the Cp2Zr+–CH2–CH2–CH3 species [1882]. Among three possible structures of this ion involving different agostic interactions, the b-agostic structure has the highest stability, it is followed by the g-agostic structure (DG298 ¼ 24.3 kJ/mol (5.8 kcal/mol)), and the a-agostic structure is the least stable (DG298 ¼ 37.6 kJ/mol (8.0 kcal/mol)). 6.1.2.1.8. Poisoning of active centers in metallocene catalysts. Metallocene catalysts are easily poisoned by CO [1594,1602], similarly to Ziegler–Natta catalysts (Section 6.3.3). The study of a model catalyst, the reaction product of Cp2Ti(Me)Cl and AlMeCl2, determined main stages of the CO reaction [1883]. This homogeneous system is stable in the absence of alkenes; it exists as an equilibrium between the Cp2Ti(Me)Cl    AlMeCl2 complex and the [Cp2Ti+– Me] [AlMeCl 3 ] ion pair (Reaction (6.12)) [1883]. A CO molecule coordinates with the Ti atom in the cation and then inserts into the TiMe bond producing a homolitically unstable product Cp2Ti+–C(QO)–Me. Detailed studies of polymer products formed in CO-interrupted ethylene polymerization reactions with a similar homogeneous system, Cp2TiCl2-AlMe2Cl [1884] and stopped-flow experiments with the Cp2TiCl2-MAO system [1588] provided additional data on the chemistry and the mechanism of CO poisoning. CO rapidly terminates polymerization reactions with metallocene catalysts. According to IR and NMR analysis, the only end-groups formed in reactions between the growing polymer chain and 13CO are two ketone groups, CH3–C(QO)–CH2– Polymer (the major product, B90%) and Polymer–CH2–C(QO)–CH2–Polymer. The following reaction scheme can account for these observations. Inhibition of the polymerization reaction is caused by coordination of a CO molecule at the Ti atom followed by CO insertion into the Cp2Ti+–C bond (the same as in the model Cp2Ti+–Me system [1883]):

Cp2 Tiþ 2Polymer þ CQO Ð Cp2 ðPolymerÞTiþ   CQO Ð Cp2 Tiþ 2CðQOÞ2Polymer

(6.39)

The acyl derivative of Ti formed in Reaction (6.39) strongly coordinates with the second CO molecule, Cp2 Tiþ 2CðQOÞ2Polymer þ CQO Ð ½Polymer2CðQOÞ2
Cp2 Tiþ   CQO

(6.40)

and the active center remains inhibited until CO is present in the reaction system. In chemical terms, an outcome of the CO addition depends on the conditions under which the Ti-acyl compound is kept before the catalyst is destroyed with alcohols at the end of CO-poisoned polymerization reactions. If the cocatalyst is

452

Alkene Polymerization Reactions with Transition Metal Catalysts

present at a low concentration and if it is a poor alkylating agent for transition metal compounds, such as AlRxCl3x, Reactions (6.39) and (6.40) are gradually reversed and the polymer chains do not contain carbonyl groups [1883]. If the cocatalyst is present at a high concentration and if it is a good alkylating agent, such as MAOAlMe3 mixtures, methyl alkyl ketone is formed [1588]: Cp2 Tiþ 2CðQOÞ2Polymer þ 4Al2CH3 ! CH3 2CðQOÞ2CH2 2Polymer

(6.41)

If, however, CO is removed and ethylene is reintroduced into the reaction, Reaction (6.40) is immediately reversed, an ethylene molecule is coordinated to the Ti-acyl species produced in Reaction (6.39) and, after repeated ethylene insertions, the second type of ketone group appears in the polymer: Cp2 Tiþ 2CðQOÞ2Polymer þ n CH2 QCH2 ! Cp2 Tiþ 2ðCH2 2CH2 Þn 2CðQOÞ2Polymer

(6.42)

6.1.2.2. Mechanisms of chain isomerization Polymerization reactions catalyzed by metallocene systems are often accompanied by chain isomerization. They are described in Section 3.3.1.1.2. Several reactions were observed: 1. Isomerization of the vinyl double bond in the last monomer unit in polyethylene molecules into the trans-CHQCH bond (Section 3.3.1.2.1). 2. Formation of 3,1-enchained propylene units (Reaction (3.41)). 3. Epimerization of monomer units in isospecific polymerization reactions of propylene with bridged racemic bis-metallocene complexes leading to steric errors in polymer chains (Section 3.3.2.2). Apparently, all these isomerization reactions have a common underlying mechanistic principle. Double-bond isomerization mechanism: The proposed mechanism of CQC-bond isomerization involves several steps [612,619,658,713]. The first one is the spontaneous chain transfer reaction, the transfer of a hydrogen atom from the bCH2 group to the transition metal atom (Reaction (3.81)): Cp2 M2CH2 2CH2 2CH2 2CH2 2Polymer (6.43) ! Cp2 ðHÞM  CH2 QCH2CH2 2CH2 2Polymer The proposed isomerization mechanism hinges on the assumption that the CQC bond formed in Reaction (6.43) remains coordinated to the metal atom, as shown in Scheme 6.8. Two options were proposed. According to the first option, the double bond rotates in the coordination sphere of the metal atom and reinserts into the Cp2M–H bond in the secondary orientation. The new Cp2M–CHRuRv center either immediately dissociates in the b-H elimination reaction (producing the CH3–CHQCH chain end) or continues chain growth [378,612,619,620].

453

Active Centers in Transition Metal Catalysts

Polymer

H

Polymer

H H

+

+ Cp M

H

2

H

Cp2M

H

+

H

Cp M

Polymer

2

H H

rotation

H termination

H H H

Polymer

+ Cp2M H Polymer allylic activation

+ Cp2M H

termination H2

-H2

+

+ Cp2M Polymer

Polymer

Cp2M Polymer

Scheme 6.8 Chain isomerization in ethylene polymerization reactions with metallocene catalysts [621].

According to the second option, the coordinated vinyl bond formed in Reaction (6.43) is converted into the allyl group. The new Cp2M–CH2–CHQCH–CH– Polymer center either terminates or continues chain growth [592,609]. Both mechanisms predict that the probability of CQC-bond isomerization increases at high temperatures and at low ethylene concentrations, when the isomerization reactions in Scheme 6.8, which are independent on the presence of a monomer, become competitive with chain growth reactions, which are first-order order reactions with respect to CE. This change was indeed experimentally observed [203,620]. However, the mechanism in Scheme 6.8 proposes two reaction steps of low probability [166]. The first assumption is that the double bond in the polymer molecule formed in Reaction (6.43) remains coordinated to the metal atom. The second assumption is that this vinyl double bond is inserted into the Cp2M–H bond in the secondary orientation. This reaction should be comparable in incidence to the secondary insertion of 1-alkene molecules into the Cp2M–H bond. The isomerization mechanism similar to that in Scheme 6.8 was indeed experimentally demonstrated in ethylene polymerization reactions with homogeneous catalysts produced from a-diimine complexes of Ni and Pd (Reactions (6.60–6.62)) but there it is based on the independently confirmed stability of coordination complexes between the active centers and alkene molecules, the information lacking for Reaction (6.43). Rytter carried out DFT computational analysis of this chain-end isomerization step [612]. As with all other reaction steps involving metallocenium cations discussed in Section 6.1.2.1, the starting species in the isomerization process is the

454

Alkene Polymerization Reactions with Transition Metal Catalysts

b-agostically stabilized Cp2Zr+–R ion. The rate-determining step in the overall isomerization reaction in the Cp2Zr+–n-C4H9 ion is a partial transfer of the bagostically attached H atom to the Zr atom; its energy barrier is B42 kJ/mol (10 kcal/mol). The next step is the rotation of the CQC bond of the coordinated 1-butene molecule with respect to the newly formed Zr+–H bond; it proceeds relatively easily, DE BB10 kJ/mol (2.4 kcal/mol). The last step, re-insertion of the CQC bond into the Zr+–H bond with the generation of the Cp2Zr+– CH(CH3)C2H5 ion, is an equally easy process. The latter ion is stabilized by two b-agostic interactions, one with the b-CH3 group and another, slightly stronger, with the b-CH2 group [612]. These calculations also indicated that chain isomerization reactions in Scheme 6.8 are not hindered by alkyl substituents in Cp groups, e.g., when the Cp ligand is replaced with the Cp ligand. Small differences in the energetics of the isomerization reactions for different metallocene complexes can explain often observed differences between the type of the last endgroups in polyethylene molecules prepared with different metallocene catalysts. If the chain isomerization reaction is more difficult (in the Cp2ZrCl2-MAO system), most of the last chain ends are the vinyl groups formed in Reaction (6.43). However, if the chain isomerization reaction is slightly more preferred (in the Cp2 ZrCl2 -MAO system), both the vinyl group formed in Reactions (6.43) and the trans-CH3–CHQCH–Polymer bond are present in the chains in approximately equal numbers [612]. Chain end isomerization mechanism: The proposed mechanism for the formation of 3,1-inserted propylene units is mechanistically similar. It consists of three stages [517,542]. The first stage involves the transfer of a b-H atom from the methyl group of a 2,1-inserted propylene unit: Cp2 M2CHðCH3 Þ2CH2 2Polymer ! Cp2 ðHÞM  CH2 QCH2CH2 2Polymer

(6.44)

This reaction is less probable compared to the b-H atom transfer reaction from the CH2 group, which leads to the internal CQC bond. The next step involves rotation of the vinyl bond in the coordination sphere of the transition metal atom and its re-insertion into the Cp2M–H bond: Cp2 ðHÞM  CH2 QCH2CH2 2Polymer ! Cp2 M2CH2 2CH2 2CH2 2Polymer

(6.45)

A similar type of monomer isomerization was observed in polymerization reactions of 1-butene [518,546] and 3-methyl-1-butene catalyzed by (Me)(Ph)C[Cp] (Flu)ZrCl2 [255] and Ph2C[Cp](Flu)ZrCl2 [323]. Chain-walking mechanism: The third chain-end isomerization reaction of a growing polymer molecule in metallocene catalysis involves an intramolecular shift of the M–C bond. This shift is mostly confined to last monomer units in growing chains; it is called ‘‘chain walking.’’ Several independent proofs of this reaction were reported. Studies of polymerization reactions of 1-butene at 101C with a model

455

Active Centers in Transition Metal Catalysts

Polymer

Polymer [CpZr] +

Polymer [CpZr] + [CpZr] +

Polymer [CpZr] +

Scheme 6.9 Mechanism of chain-walking reactions in metallocene catalysis [627].

metallocene catalyst, [(Cp)(L)Zr+–Me] [B(C6F5)4], where L is a bidentate ligand, acetamidinate, showed that low molecular weight polymers contain three types of end-double bonds. The vinylidene double bond CH2QC(C2H5)–Polymer, which is formed in the standard b-H transfer reaction to the Zr atom (Reaction (3.81)), is dominant, but two other bonds, CH3–C(QCH–CH3)–Polymer and CH3–CH(– CHQCH2)–Polymer, are also present [626,1877]. The most plausible mechanism of this chain-end isomerization is shown in Scheme 6.9. A model of this growing chain, the cationic (Cp)(L)Zr+–CH2–CH(CH2– CH3)2 species, was synthesized. When it is kept it solution at 01C, it undergoes a slow b-H transfer to the Zr atom. In addition to the expected isoalkene, CH2QC(CH2–CH3)2, the decomposition products contain all three alkenes predicted by the chain-walking mechanism. The shift to the CH2 group in the ethyl branch produces cis- and trans-alkenes CH3–C(QCH–CH3)–CH2–CH3, and a subsequent shift to the CH3 group in the ethyl branch produces a 1-alkene, CH3– C(–CHQCH2)–CH2–CH3. Brintzinger and Rieger observed chemical effects of chain walking in polymerization of two deuterated propylenes, CHDQCH–CH3 and CH2QCD–CH3. When any of these monomers is polymerized with metallocene catalysts derived from bridged racemic bis-zirconocene complexes, such as the racC2H4(Ind-H4)2ZrCl2, some of the D atoms are shifted to the methyl group and the polymer chains contain small fractions of –CH2–CH(CH2D)– monomer units [600,620,643,714]. These units are present both in isotactic sequences, mmmm, and in steric errors, mrrm, in a B1:1 ratio [600]. A kinetic manifestation of the same chain isomerization processes was discovered in regular studies of monomer concentration effects on the isospecificity of the same racemic bis-indenyl complexes in propylene polymerization reactions (Section 3.3.2.2). The experimental data convincingly show that these metallocene catalysts are highly isospecific at high propylene concentrations but they lose this property when the monomer concentration is decreased to B1 M, and, in the limiting case (high temperatures, low propylene concentrations), can become completely aspecific [28,518,542,713,716]. Landis produced an additional proof of chain-walking/isomerization reactions in detailed 13C and 1H NMR analysis of chain isomerization in polymerization reactions of 1-13C-labeled propylene at 401C with a pre-formed active species C2H4(Ind)2Zr+–Polymer and [MeB(C6F5)3] as a counter-anion [522]. As expected, a single monomer insertion step at low temperatures produces the C2H4(Ind)2Zr+–13CH2–CH(CH3)–Polymer species. The principal chain termination reaction under these conditions also produces the expected product,

456

Alkene Polymerization Reactions with Transition Metal Catalysts

13

CH2QC(CH3)–Polymer. However, the active center C2H4(Ind)2Zr+–13CH2– CH(CH3)–Polymer is slowly converted into several other centers:

1. The absolute configuration at the chiral carbon atom in the last monomer unit in the chain changes. This change leads to epimerization of the last monomer unit in the growing chain: C2 H4 ðIndÞ2 Zrþ 213 CH2 2C HðCH3 Þ2Polymer

(6.46)

Ð C2 H4 ðIndÞ2 Zrþ 213 CH2 2C HðPolymerÞ2CH3

2. The last monomer unit in the growing chain undergoes 1,3-isomerization: C2 H4 ðIndÞ2 Zrþ 213 CH2 2CHðCH3 Þ2Polymer

(6.47)

! C2 H4 ðIndÞ2 Zrþ 2CH2 2CHð13 CH3 Þ2Polymer

The monomer unit formed after the 1,3-isomerization step also epimerizes, similarly to Reaction (6.46), and a chain transfer reaction following Reaction (6.47) produces CH2QC(13CH3)–Polymer species. Kinetic analysis of these reactions at low temperatures showed that the epimerization, the 1,3-isomerization, and both chain transfer reactions occur in parallel and at similar rates [522]. Chien, Busico, and Landis proposed a complex mechanism of chain-walking isomerization reactions shown in Scheme 6.10. The mechanism is conceptually similar to the formation mechanism of trans-CHQCH bonds in polyethylene (Scheme 6.8) [203,620] and the formation of 3,1-inserted units in propylene polymerization reactions (Reactions (6.44) and (6.45)) [518,542,658,712–714]. The mechanism includes a series of steps, starting with the b-H elimination step in a growing polymer chain. Then the vinylidene bond rotates in the coordination sphere of the transition metal atom and re-inserts into the Cp2Zr–H bond, which generates a sterically strained tertiary-alkyl intermediate, the Cp2Zr–C(CH3)2– moiety. To relieve the steric strain, the latter species undergoes a series of reactions involving abstraction of a b-H atom from one of the methyl groups. The final stage of these transformations amounts to regeneration of the same growing polymer chain as the starting chain, but either with the reversed configuration of the last monomer unit (shown in the scheme) or a return to the original configuration.

+

Cp2M H

H CH3

+

Cp2M Polymer -β-H H

+

H

H CH 3

+

Polymer H

rotation

+

2

CH3 H

insertion

insertion

+

Cp2M

CH3

H

rotation H

Scheme 6.10 Mechanism of chain-walking reactions [659].

CH3 Polymer -β-H

H Polymer

Cp2M

Cp M H

CH3

CH3

+

Cp2M

H

Polymer

H

Polymer

H

H

Cp2M

HH H

CH3 Polymer

Active Centers in Transition Metal Catalysts

457

The viability of the reaction sequence involving re-insertion of the vinylidene bond into the Cp2Zr–H bond is the most challenging step in the proposed mechanism. It supposes that the vinylidene bond never leaves the coordination position at the transition metal atom; otherwise, it will compete for the coordination site with the monomer, which is always present in the reaction in a much higher concentration and has a much higher reactivity in the insertion reaction into the Cp2Zr–H bond [1871,1877,1885]. Although the formation of a tertiary alkyl intermediate Cp2Zr–C(CH3)2– proposed in Scheme 6.10 has never been experimentally confirmed, Sita proved several salient features of the mechanism [626,1877]. He synthesized several model compounds imitating different stages of chain-walking reactions in the metallocenium cation Cp2Zr+–R and carried out NMR studies of their stability and isomerization patterns. The following reactions pertinent to the isomerization mechanism of the last unit in a growing polymer chain were experimentally observed: 1. The Cp2Zr+–CH(CH3)2 species isomerizes into the Cp2Zr+–CH2–CH2–CH3 species [1877]. 2. The Cp2Zr+–CH2–CH(CH3)2 species was prepared to imitate the product of a single propylene insertion step into the Cp2Zr+–CH3 bond. When a doublelabeled propylene molecule, 13CH2QCD–CH3, was used for the synthesis of this model, the metallocenium ion rapidly isomerized at 01C into the mixture of Cp2Zr+–CH2–CH(CH3)(13CH2D) and Cp2Zr+–13CHD–CH(CH3)2, thus confirming the plausibility of the isomerization mechanism in Scheme 6.10. 3. Scheme 6.10 assumes the transient formation of a metallocenium cation with the tertiary carbon atom. Although zirconocenium cations containing Cp2Zr– CRuRvRvu groups were not produced, their hafnium analog, Cp2Hf–CMe3, was synthesized and, as expected, it rapidly isomerizes to Cp2Hf–CH2–CH(CH3)2 [1877]. 4. Chain-end epimerization and chain-walking steps in growing polymer chains are not necessarily interconnected. When a growing poly(1-decene) chain attached to the metallocenium cation Cp2Zr+–[CH2CH(C8H19)]15i-Bu was kept in the absence of monomer at 101C for significant periods of time, chain-end epimerization did not take place [1877]. However, chain-walking reactions and the b-H transfer to the Zr atom in these chains occur relatively easily [626,1877]. An additional argument in favor of the epimerization mechanism is the structure of cyclopentene homopolymers shown in Reaction (3.49). This reaction is driven by the steric strain associated with 1,2-linking of cyclopentene monomer units, and it produces 1,3-linked cyclopentene units. When the strain is eliminated, in ethylene/cyclopentene copolymerization reactions, the majority of cyclopentene units in the copolymer chains are 1,2-linked. The problem with accepting this epimerization mechanism stems from the fact that it cannot convincingly explain chain-end epimerization in polymerization reactions of 1-butene [518,1886], and additional steps in the mechanism in Scheme 6.10 were introduced to rectify the difficulty [658, 1887].

458

Alkene Polymerization Reactions with Transition Metal Catalysts

According to theoretical analysis by Brintzinger, both these chain isomerization reactions, the formation of 3,1-enchained propylene units in Reactions (6.44) and (6.45) and the epimerization reaction in Scheme 6.10, have the same mechanism, ‘‘active-site chain walking,’’ or ability of a catalytically active transition metal atom to migrate along the segment of the polymer chain attached to it [1872]. These Zr migration reactions appear to be related to ‘‘chain-walking’’ reactions observed for Ni- and Pd-based polymerization catalysts and discussed in Section 6.2.5.2. Calculations for several Cp2Zr+–R ions with short alkyl groups R showed that the migration of a Cp2Zr+ center between adjacent carbon atoms of the alkyl group R proceeds via the classical reaction route [1872] similar to that shown in Scheme 6.10. The calculated activation barrier for this reaction sequence depends on the type of the alkyl group R:  Degenerative isomerization of Cp2Zr+–CH2–CH3 – 75 kJ/mol (17.9 kcal/mol).  Isomerization of Cp2Zr+–CH2–CH2–CH3 to Cp2Zr+–CH(CH3)2 – 49 kJ/mol (11.7 kcal/mol). The Eact value is decreased in comparison with the previous reaction due to the hyperconjugation effect of the methyl group.  Isomerization of Cp2Zr+–CH(CH3)2 to Cp2Zr+–CH2–CH2–CH3 – 41 kJ/mol (9.8 kcal/mol). This isomerization reaction models the transformation of a propylene unit in the secondary orientation, Cp2Zr+–CH(CH3)–CH2–Polymer, into the (CH2)3 sequence, i.e., into the 3,1-enchained propylene unit (Reactions (6.44) and (6.45)), if the donor of the b-H atom is the methyl group.  Isomerization of Cp2Zr+–CH2–CH(CH3)2 to Cp2Zr+–CH(CH3)3 – 40 kJ/mol (9.6 kcal/mol).  Isomerization of Cp2Zr+–CH(CH3)3 to Cp2Zr+–CH2–CH(CH3)2 – 31 kJ/mol (7.4 kcal/mol). The last two isomerization reactions model the stereo-isomerization step (epimerization), which competes with the isospecific chain growth in bridged racemic bis-metallocene catalysts.

6.1.3. Stereospecificity of active centers in metallocene catalysts 6.1.3.1. Non-bridged metallocene complexes The stereospecificity of metallocenium cations derived from nonbridged metallocene complexes is quite poor, as the data presented in Section 3.3.2.1 show. When propylene polymerization reactions with Cp2TiCl2, Cp2TiMe2, Cp2ZrCl2, Cp2ZrPh2, and Cp2ZrMe2 (all activated with MAO) are carried out at moderate temperatures, 0–501C, the polymers are practically atactic. However, the polymers are moderately isotactic when synthesized at 60 to 781C [27,262,397,406, 528,595,602,662–664]. The nature of main steric mistakes in their chains, mmmr and mmrm pentads in the B1:1 ratio, shows that the stereospecificity of the active centers is governed by the chain-end stereocontrol mechanism [40,595,663–665]. Corradini and Guerra carried out molecular mechanics calculations of steric effects in propylene polymerization reactions with the participation of an achiral active center Cp2Ti+–R [1888]. Geometrical restrictions on coordination and insertion of the monomer molecule into two bonds, Ti–C2H5 and Ti–CH2CH(CH3)C2H5, are

Active Centers in Transition Metal Catalysts

459

similar to those present in isospecific bis-metallocene active centers (see the next section). The calculations show that the routes leading to the isotactic and the syndiotactic linking of an inserting propylene molecule are energetically similar, but the isotactic linking is more preferable due to a small difference in the steric repulsion between the coordinated propylene molecule and the last monomer unit in the growing chain. 6.1.3.2. Isospecific bridged metallocene complexes Isotactic polymers of 1-alkenes are produced with catalysts derived from two types of bis-metallocene complexes (Scheme 1.1): 1. Bridged bis-metallocene complexes of C2 symmetry (racemic isomers of complexes X), e.g., rac-C2H4(Ind)2MCl2 or rac-C2H4(Ind-H4)2MCl2 [28,407, 515,637,690–694]. 2. Bridged bis-metallocene complexes without any elements of symmetry (C1 symmetry, complexes XI) [606,691,693,695–703]. 6.1.3.2.1. Active centers derived from complexes of C2 symmetry. Ewen proposed that the steric features of active centers derived from complexes of C2 symmetry determine the preferred isotactic linking of adjoining monomer units in polymer chains produced by these active centers [595]. This idea became the cornerstone of the modern concept of isospecific metallocene catalysis. The exact origin of isospecificity of metallocene centers derived from bridged complexes of C2 symmetry was determined in a series of experimental studies by Pino, Zambelli, Tritto, and Sacchi using the rac-C2H4(Ind)2ZrCl2-MAO system [407,637,690]. The steric placement of a propylene molecule prior to its insertion step depends on the type of the group attached to the Zr atom in the metallocenium cation. When this is a hydrogen atom, the propylene molecule is oriented in such a way that after it is inserted intro the Cp2Zr–H bond in the primary orientation and after the next propylene unit is inserted into the just formed Cp2Zr–CH2 bond, these two propylene units are mostly in the syndio-arrangement [637]. If the original metallocenium cation contains the Cp2Zr–CH3 bond, the insertion of the first propylene unit is also primary but it is not stereoselective. It means that the first propylene unit and the propylene unit following it can form iso and syndio-links with a nearly equal probability [690] (see Table 3.33). However, when the original metallocenium cation contains the Cp2Zr–CH2–CH3 group, the insertion of the first propylene unit becomes stereoselective, and the first two monomer units in the chain are predominantly in the iso-arrangement [407,690]. The insertion of subsequent monomer units is also predominantly isospecific. Scheme 6.11 shows the mechanism of stereospecific polymerization reactions with metallocene complexes of C2 symmetry. The two possible positions for the growing polymer chain, before and after a single insertion step, and, consequently, the positions available for 1-alkene coordination, are identical. These coordination positions are called homotopic [408,1889,1890].

460

Alkene Polymerization Reactions with Transition Metal Catalysts

CH3 Polymer CH CH2 CH2

CH3 M+

C3H6 Polymer CH CH2 CH2

M+

M

CH2

CH 2 CH

CH3

Polymer

CH3

Scheme 6.11 Mechanism of isospeci¢c polymerization reactions with metallocenium ions derived from complexes of C2 symmetry. Bridge between two g5 ligands in not shown.

6.1.3.2.2. Centers of C2 symmetry, mechanism of isospecific chain growth. Corradini and Guerra carried out conformational analysis of propylene polymerization reactions on active centers of C2 symmetry [1891,1892]. This analysis formulated the origin of isospecificity in metallocene catalysts in general. The first C–C bond in the alkyl chain attached to the transition metal atom (Scheme 6.11), the C(a)–C(b) bond, is oriented in such a way that the steric repulsion between the b-C atom in it and substituted Z5 ligands in the metallocenium cation is at a minimum. This first C–C bond is the CH2–CH3 bond in a metallocenium cation containing the Cp2M+–CH2–CH3 group or the first CH2–CH(CH3)Polymer bond in a metallocenium cation containing a growing polypropylene chain. In the case of the indenyl ligand, this C–C bond is turned away from the benzene ring (Scheme 6.11). Several experimental studies with labeled species indicate that this orientation of the alkyl chain may be further stabilized by an a-agostic interaction between one of the H atoms in the a-CH2 group and the transition metal atom producing a relatively rigid three-atom M’H–C(a)H ring [643,1852,1893]. The coordination position of a propylene molecule at the active center is determined by two factors. First, the CH2QCH bond is parallel to the M–CH2 bond (the mechanistic prerequisite for any CQC bond insertion reaction according to the mechanism in Scheme 6.2), and, second, its CH–CH3 bond is positioned in the opposite (trans) direction with respect to the direction of the first C–C bond in the growing polymer chain. This coordination is shown in Scheme 6.11, it results in the formation of a meso-link between the inserting monomer and the previous monomer unit in the chain. After the insertion of the coordinated propylene molecule into the M–CH2 bond is completed, the position of the M–Polymer moiety is shifted to the site previously occupied by the coordinated alkene molecule and the previous position of the growing polymer chain becomes open for the next CQC bond coordination. Morokuma confirmed this model in rigorous theoretical analysis using a combination of ab initio and molecular mechanics methods [1800,1876]. Isospecific metallocenium ions of C2 symmetry were represented by an H2Si-bridged ion with two methyl groups (one in each cyclopentadienyl ring) in meso-positions, H2Si (3-Me-Cp)(4-Me-Cp)Zr+–R [1800]. The calculations were carried out in several steps. First, ab initio calculations determined the optimum geometry and energetics of an ethylene insertion reaction for R ¼ CH3. Next, the molecular mechanics analysis was used to determine the regiochemistry of a similar propylene insertion

Active Centers in Transition Metal Catalysts

461

reaction. These calculations showed that the primary regioselectivity of propylene insertion into the Cp2Zr+–CH3 bond is steric in nature. Next, molecular mechanics calculations were extended to examine the stereochemistry of propylene insertion reactions into different Zr+–R bonds. When R ¼ CH3, the propylene insertion reaction does not exhibit any steric preference, in agreement with experimental data [690]. However, when R is either C2H5 or CH2–CH(CH3)–C2H5, the coordination of a propylene molecule leading to its subsequent iso-insertion is clearly preferable over the coordination leading to its subsequent syndio-insertion. A detailed comparison of ethylene and propylene insertion steps into two latter active centers confirmed the main principle of the Corradini model [1891]. Placing a methyl group at the 3rd carbon atom in the cyclopentadienyl ring imitates the benzene ring in the indenyl ligand in Scheme 6.11 (this group faces the open half-sphere where the insertion reaction takes place). The introduction of the methyl group forces the growing polymer chain, either polyethylene or polypropylene, to acquire a local conformation where the first C–C bond in the Cp2Zr+–CH2–CHR–Polymer species is directed away from this methyl group. This conformation results in a gain of B25 kJ/mol (6 kcal/mol) compared to other possible conformations of the chain. When a propylene molecule coordinates at the Zr atom, the direct steric effect of the methyl group in the cyclopentadienyl ring is very small by itself and is not sufficient to control the stereochemistry of propylene coordination. However, steric interactions between the coordinated propylene molecule and the first C–C bond in the growing polymer chain are quite significant and they force the propylene molecule to coordinate in the position leading to its isotactic linking to the previous monomer unit. Final ab initio calculations showed that the difference between activation energies of insertion steps following these two coordination positions (one leading to isotactic linking and another to syndiotactic linking) is B31 kJ/mol (B7.5 kcal/mol) even for R ¼ C2H5. The stereochemistry of this model of active centers derived from metallocene complexes of C2 symmetry was additionally analyzed by molecular mechanics methods [534,1802,1873,1874,1894–1896]. These models predict several features of the catalysts, all in agreement with experimental data: 1. The active centers are highly regioselective; insertion steps of 1-alkene molecules proceed in the primary orientation. An occasional regio-error, the secondary insertion, is immediately corrected, mostly because of non-bonded (agostic) interactions between transition metal atoms and hydrogen atoms in the growing polymer chains. 2. Titanocene active centers have higher regioselectivity compared to zirconocene centers [1895]. 3. The active centers are highly enantioselective not only in the course of primary insertion steps but after occasional regio-errors as well [1895]. 4. The energy advantage of the preferred conformation of the polymer chain end shown in Scheme 6.11 is noticeable even for very short ‘‘polymer chains,’’ Et or i-Bu, and it is higher for the isobutyl group (which corresponds to a polymer chain with one propylene unit) than for the ethyl group [534,1802]. For

462

Alkene Polymerization Reactions with Transition Metal Catalysts

example, this conformation translates into the following decrease in the activation energy of propylene insertion into the Cp2Zr+–C bond: Center DE¼ (kJ/mol)

Me2Si(Ind)2Zr+–Et 10.9

Me2Si(Ind)2Zr+–i-Bu 17.2

5. Estimations of the relative significance of two factors determining the overall stereospecificity of metallocene active centers, the ‘‘correct’’ conformation of the chain end and the ‘‘correct’’ coordination of the propylene molecule, showed that both factors play comparable roles in the overall stereocontrol efficiency [534,1802]. 6. The introduction of alkyl substituents into the 2nd and the 3rd position of indenyl rings increase the regioselectivity of the catalysts whereas the introduction of two methyl groups into the 4th and the 7th positions in the rings decrease it [1896]. 6.1.3.2.3. Centers of C2 symmetry, mechanisms of steric errors. Several mechanisms were proposed to account for the types of steric errors in predominantly isotactic polymer chains produced with bridged racemic bismetallocene complexes of C2 symmetry. The first mechanism assumes that an occasional steric error occurs due to the inversed (and, therefore, more energydemanding) coordination of a 1-alkene molecule at the transition metal atom. This error does not affect the outcome of the 1-alkene insertion that follows it; the next 1-alkene molecule still preferably coordinates in such an orientation that, after the insertion, it continues the formation of isotactic links in the polymer chain. This automatic stereo-correction mechanism is the essence of the site-control stereomechanism described in Section 3.1.3.1. This stereo-correction mechanism leads to the steric error of a particular type, two mmrr pentads flanking the mrrm pentad. Brintzinger carried out a thorough NMR investigation of steric error formation based on the use of deuterium-labeled propylenes [600,643]. It revealed several features of this steric error mechanism. As expected, the probability of this type of steric error increases with temperature. For example, the fraction of mrrm pentads in polypropylene produced according to this mechanism with the rac-C2H4(IndH4)2ZrCl2-MAO catalyst increases from B0.013 at 301C to B0.03–0.035 at 501C. The probability of this steric error does not depend on the monomer concentration because both types of 1-alkene coordination, the more preferred and the less preferred, are first-order reactions with respect to 1-alkene concentration. The probability of this steric error depends on the steric bulk of substituents at the cyclopentadienyl rings, as several examples in the second column of Table 6.3 show. This probability depends also on the rigidity of the racemic scaffolding around the transition metal atom in the active centers. Several fluxional distortion mechanisms in bridged bis-Z5 ligands (including indenyl ring slippage and thermally induced twisting) were proposed to explain the temperature effect and substituent effects on the isospecificity of these catalysts [166,621,1846]. The second mechanism of steric error formation has a completely different nature. It is caused by a chemical isomerization reaction at the active center,

463

Active Centers in Transition Metal Catalysts

Table 6.3 Two mechanisms of steric errors in propylene polymerization reactions with isospecific racemic bis-zirconocene complexes at 501C [600] Zirconocene complex

[mrrm], ¢rst mechanism (monomer coordination error)

[mrrm], second mechanism (epimerization reaction)

Me2Si(2-Me-4-t-BuCp)2ZrCl2 Me2Si(2-MeBenz[e]Ind)2ZrCl2 Me2Si(2,4-Me2-Cp)2ZrCl2 Me2Si(Ind-H4)2ZrCl2

o0.005

o0.005

0.01 0.008–0.01 0.027–0.035

B0 0.018 0.069–0.071

epimerization of the last monomer unit in the growing polymer chain. This isomerization can be presented using CHDQCH–CH3 instead of unlabeled propylene to emphasize the nature of the rearrangement: Cp2 Mþ 2CHD2CHðCH3 Þ2Polymer ! Cp2 Mþ 2CH2 2CHðCH2 DÞ2Polymer

(6.48)

A detailed chemical mechanism of this reaction is described in Section 6.1.2.2. This rearrangement is accompanied by the formation of the same steric error as in the previous case, the mrrm pentad flanked by two rrmm pentads. These two stereo-error mechanisms can be distinguished by two means. One is obvious from Reaction (6.48): when deuterated propylenes, CHDQCH–CH3 or CH2QCD– CH3, are used in the polymerization reactions, the central propylene unit in the steric error, the mrrm pentad, should be CH2D instead of CH3. The second means of distinguishing the two mechanisms is the effect of the monomer concentration on the frequency of steric errors. NMR investigations of steric errors due to a chain-end epimerization were based on the use of deuterium-labeled propylenes [600]. They determined the following features of the mechanism: 1. The probability of this steric error also increases with temperature. For example, the fraction of mrrm pentads formed via the second mechanism in polypropylene produced with the rac-C2H4(Ind-H4)2ZrCl2-MAO catalyst increases from 0.022–0.03 at 301C to B0.07 at 501C. 2. The probability of this steric error is inversely related to the monomer concentration because the epimerization rate is independent on it whereas the chain growth reaction (which is mostly isospecific) is the first-order reaction with respect to the monomer concentration. This kinetic effect was the basis for the initial identification of the epimerization reactions [28,518,542,713,716]. 3. The probability of this steric error strongly depends on the steric bulk of substituents at the cyclopentadienyl rings, as shown in the last column in Table 6.3.

464

Alkene Polymerization Reactions with Transition Metal Catalysts

Sacchi and Tritto found an additional peculiar type of steric error in propylene/ 1-pentene copolymerization reactions with the highly isospecific rac-Me2Si(Benz[e]Ind)2ZrCl2-MAO system [1897,1898]. Although this catalyst produces nearly perfectly isotactic polypropylene with [mmmm] ¼ 0.950 and with a small number of steric errors expected for the enantiomorphic stereocontrol mechanism, an introduction of 7–10% of 1-pentene produces unexpected steric defects in long propylene blocks, most of them of the rrrr type. These defects are completely absent from the same copolymers prepared with a less isospecific system, racC2H4(Ind)2ZrCl2-MAO. A possible explanation for these steric defects is a temporary coordination shift of the bulky Benz[e]Ind ligand from the Z5 to the Z3 type under the influence of a bulky monomer unit derived from 1-pentene and persisting during several subsequent insertion steps of propylene molecules. 6.1.3.2.4. Active centers derived from complexes of C1 symmetry. Bismetallocene complexes of the second type producing isospecific catalysts do not have any elements of symmetry in terms of the arrangement of substituents in each of their cyclopentadienyl rings (C1 symmetry) [606,693,696–701,703,1899]. In general, the performance of these catalysts depends on two factors, the size of each of the Z5 ligands and steric differences between them. Two different classes of such complexes produce isospecific polymerization catalysts. Complexes of the first class have two large and asymmetric Z5 ligands; one of the ligands carries additional alkyl or aryl substituents. The examples include rac-Me2Si(2-Me-Ind)(Ind) ZrCl2, rac-Me2Si (2-Me,4-Ph-Ind)(2-Me-Ind)ZrCl2, and similar CH2–CH2-bridged complexes [515, 693,1900]. The probability of isotactic linking of propylene units with some of these catalysts can be very high, piso values can reach 0.990–0.995, and, correspondingly, melting points of these polymers are also high, B1701C [1900]. Metallocenium ions derived from these complexes have two nonequivalent (diastereotopic) coordination sites. The spatial arrangement of Z5 ligands around transition metal atoms in all these metallocenium ions forces the growing end of the polymer chain to acquire a unique preferable position at the least sterically crowded coordination site of the metallocenium ion (Scheme 6.12). This arrangement leads to a unique preferable position of a coordinated alkene molecule at the opposite coordination site. Each insertion step of a coordinated 1-alkene molecule results in an exchange of the position of the M–C bond and the coordination site. However, the insertion step is immediately followed by the return of the M–C bond back to its original (more energetically favorable) position which this bond had before the insertion. This change of the chain end position is called in the literature the back skip of the polymer chain end. In reality, the growing polymer chain end is usually positioned in close proximity to the crystal lamella of the isotactic alkene polymer, and the back-skip step involves a movement of the metallocenium active center rather than a movement of the polymer chain attached to it. The existence of this chain migration process in metallocenium cations carrying small alkyl groups instead of polymer chains was demonstrated experimentally by dynamic NMR spectroscopy [1318,1358,1359,1797,1803]. For example, the methyl group attached to the Zr atom in the ion pair [(R-Cp)2Zr+– CH3]  (solvent)  [MeB(C6F5)3] can shift from one lateral position to another.

465

Active Centers in Transition Metal Catalysts

C 3H 6

Polymer -CH2 M + M1

Polymer -CH2 M +

kp,1 isospecific state

CH3 k-1

aspecific insertion, p′′iso

isospecific insertion, p′iso

k1

M CH3

CH3

CH3

+

C 3 H6 CH2 Polymer CH3

kp,2

M2 + M

CH2 Polymer

CH3 aspecific state

Scheme 6.12 Mechanism of stereospeci¢c polymerization reactions with metallocenium ions derived from complexes of C1 symmetry [625]. Bridge between two g5 ligands in not shown.

When the size of the R group in the (1,2-Me2-Cp)2Zr+–R cation is increased, the migration enthalpy greatly declines with bulkiness in the order: Me W CH2-t-Bu W CH2-SiMe2 c CH(SiMe2)2. When the active centers are produced from metallocene complexes of C2 symmetry, this rearrangement has no effect on the stereochemistry of the following alkene insertion steps. However, this rearrangement is the crucial step in the stereo-mechanism of active centers derived from asymmetric metallocene complexes. The second class of asymmetric metallocene complexes producing isospecific polymerization catalysts has only one asymmetric Z5 ligand while another is symmetric. The symmetric ligand can be quite small, Cp or Cp, [606,722], or large, e.g., Flu [1899,1901]. Examples of such complexes include Me2Si(Cp)(Ind)MCl2, Me2C(Cp)(Ind)MCl2, Me(H)C(Cp)(Ind)MCl2 [606,722], C2H4(Flu)(2,5,7-Me3-Ind)HfCl2, and C2H4(Flu)(2,4,6-Me3-Ind)HfCl2 [1899, 1901]. Catalysts derived from Me2Si(3-Me-Ind)(Ind)ZrCl2 belong to the same class [624]. When these complexes are converted into metallocenium ions, both the growing polymer chain and the coordinated alkene molecule can, potentially, occupy any of the two positions at the transition metal atom. One of the positions of the growing polymer chain leads to the isotactic linking of monomer units and another to their atactic linking. These catalysts produce moderately isotactic polymers of 1-alkenes. 6.1.3.2.5. Centers of C1 symmetry, mechanism of isospecific chain growth. Active centers derived from asymmetric bis-metallocene complexes with large Z5 ligands exhibit several kinetic features that distinguish them from active centers derived from complexes of C2 symmetry. Any 1-alkene molecule can coordinate at, and insert into the Cp2M–C bond, at a unique preferable position. If these catalysts are employed in ethylene/propylene copolymerization reactions at

466

Alkene Polymerization Reactions with Transition Metal Catalysts

low temperatures and at very low CE:CPr ratios, they produce practically alternating ethylene/propylene copolymers [987,1902]. Because propylene coordinates and inserts into the growing polymer chain at a single position, nearly all propylene units in these copolymers are in the isotactic arrangement. These catalysts cannot homopolymerize alkenes with internal CQC bonds, such as 2-butene or norbornene, due to a very high steric demand for their coordination and insertion. However, these catalysts can copolymerize these alkenes with ethylene [982]. In the limiting case, when [ethylene]:[alkene] ratios in the monomer mixtures are very low, these copolymers also have a nearly alternating structure. Because the bulky cycloalkene always coordinates at the same (more open) site, each successive cycloalkene units in these polymers chains, while separated by single ethylene units, are connected in the isotactic manner [982]. Metallocenium cations derived from asymmetric complexes of the second type (one symmetric Z5 ligand and one asymmetric Z5 ligand) also have two coordination sites, one leading to predominantly isotactic linking of monomer units and another practically aspecific. However, due to a high degree of steric congestion at the aspecific coordination site of the cations, the back-skip step plays a significant role in propylene polymerization reactions with the catalysts. The rate of the back skip to the isospecific configuration increases with temperature. Consequently, the isotacticity level of propylene polymers prepared with these catalysts increases with temperature in a nearly linear manner, the [mmmm] value increases from 0.696 at 301C to 0.816 at 601C. This trend is opposite to that observed for isospecific metallocenium ions derived from complexes of C2 symmetry where the back skip is relatively rare. Also, because the rate of propylene insertion at the aspecific site increases with the monomer concentration, the isotacticity level of these polymers decreases, the effect that is also opposite to the effect for isospecific metallocene complexes of C2 symmetry. The type of the main steric error in the former polymers, mrrm, is in agreement with the back-skip mechanism of stereocontrol [1901]. Scheme 6.12 presents the mechanism of stereospecific polymerization reactions with metallocenium ions derived from these complexes. The active center has two sterically nonequivalent positions for monomer coordination, M1 and M2, which differ in the stereoselection ability (represented by two probabilities of mesoenchainment, p0iso and p00iso ). In many cases, one of the positions is highly isoselective (p0iso  1) whereas another position is poorly stereoselective, with p00iso only slightly higher than zero. The growing polymer chain can switch from one position to another via two mechanisms: 1. Migratory insertion of a 1-alkene molecule into the Cp2M–C bond. These are two propagation reactions with the rate constants kp,1 and kp,2; both reaction rates are proportional to the monomer concentration. 2. Migration at the active center from one position to another, the back-skip step. These are two monomolecular reactions with rate constants k1 and k1; their rates are independent of monomer concentration. If these reactions proceed at a much higher rate than chain growth reactions, the back-skip migration can be viewed as an equilibrium process with the constant K ¼ k1/k1 [624].

Active Centers in Transition Metal Catalysts

467

In a general case, the total probability of meso-enchainment of two neighboring monomer units (the parameter calculated from experimentally measured values [mm] or [mmmm], using equations for the enantiomorphic stereocontrol statistics in Table 3.1) can be presented as piso ¼ P M1  p0iso þ ð1  P M1 Þ  p00iso

(6.49)

where PM1 is the probability to find the active center in the M1 position open for monomer coordination. The interchange rate between M1 and M2 positions depends both on the rates of back-skip steps, k1 and k1, and on average propagation rates at both positions, kp,1  CM and kp,2  CM: P M1 ¼ 1  P M1 ¼ ðkp;2  C M þ k1 Þ=½k1  þ k1 þ ðkp;1 þ kp;2 Þ  C M


(6.50)

A combination of Equations (6.49) and (6.50) describes the stereoselectivity of many isospecific metallocene catalysts. Several limiting cases illustrate the utility of this formal approach: 1. If a complex has C2 symmetry, M1 and M2 positions are degenerated (formally, PM1 ¼ PM2 ¼ 0.5), and both positions are equally isoselective ð p0iso ¼ p00iso Þ. Therefore, Equation (6.49) is reduced to piso ¼ p0iso, i.e., only one kinetic parameter, piso, describes the steric structure of a polymer. The back-skip step does not affect the stereospecificity of these catalysts. 2. If the equilibrium between M1 and M2 positions is strongly shifted to the direction of M1 (k1ck1) and if the back-shift reaction is very facile (k1ckp,1  CM, k1{kp,1  CM at any CM), the stereo-kinetic behavior of the catalyst represents the original Arlman-Cossee model of stereocontrol [1201], PM1E1. The stereospecificity of this active center is represented by a single parameter, pisou [1901,1903]. 3. If the back-skip step is forbidden (k1 ¼ k1 ¼ 0), the active center alternates between a highly isospecific mode ðp0iso  1Þ and a poorly isospecific or an aspecific mode ðp00iso  0Þ. This type of catalysis is called hemi-isospecific. Catalysts of this type can be produced from complexes XI and XII in Scheme 1.1. A number of intermediate situations can be envisaged depending on the relative values of four rate constants in Scheme 6.12 and the isospecificity level at each coordination position, as well as on the temperature dependence of each parameter. Several statistical models of varying complexity describe these situations [390,401,408,624,722,1890,1904]. This analysis is very useful for the interpretation of experimental data, in particular, the effect of the type of transition metal in complexes of C1 symmetry and the effect of the monomer concentration on stereospecificity, but it has low predictive power. 6.1.3.3. Syndiospecific bridged metallocene complexes Syndiotactic polymers of propylene and other 1-alkenes are produced by catalysts derived from two types of metallocene complexes, complexes of Cs symmetry (complexes XIV in Scheme 1.1) and some complexes without any elements of symmetry (C1 symmetry).

468

Alkene Polymerization Reactions with Transition Metal Catalysts

CH3 Polymer

CH3

CH3 +

M

C3H6

Polymer M

+

M

CH3

+

Polymer

Scheme 6.13 Mechanism of syndiospeci¢c propylene polymerization with metallocenium ions derived from complexes of Cs symmetry. Bridge between two g5 ligands in not shown.

6.1.3.3.1. Centers of Cs symmetry, mechanism of syndiospecific chain growth. Active centers derived from metallocene complexes of Cs symmetry have two different (enantiotopic) positions for coordination of 1-alkene molecules, as shown in Scheme 6.13. Insertion of monomer units into M–C bonds in these centers is enantioselective and it leads to the formation of predominantly syndiotactic polymers [736,1837,1891,1905]. This mechanism is very similar to the mechanism of isospecific metallocene catalysis described in Section 6.1.3.2, although different opinions exist about the precise nature of steric interactions leading to syndiotactic linking of monomer units. The first C–C bond in the alkyl chain attached to the transition metal atom (the first CH2–CHR bond in a metallocenium cation containing a growing polymer chain) is oriented in such a way that this C–C bond is turned away from the fluorenyl group. This orientation reduces the steric repulsion between the b-C atom in it (the CH group in the growing polymer chain) and the Z5 ligand. This conformation may be additionally stabilized by an agostic interaction between the a-CH2 group and the transition metal atom [1294,1891]. An approaching 1-alkene molecule is coordinated at the transition metal atom in the position when its CH2QCH bond is parallel to the Zr–CH2 bond and its CH–R bond is pointed in the opposite direction with respect to the direction of the first C–C bond in the growing polymer chain and toward the fluorenyl ring [1891]. This coordination, after the completion of the CQC bond insertion reaction, results in the formation of the racemic link between the last two monomer units in the chain. Another reason for the preferred orientation of the coordinated monomer molecule can be a steric interaction of its alkyl substituent and the fluorenyl ligand in the metallocenium ion [736]. After insertion of the coordinated alkene molecule into the Zr–CH2 bond, the position of the Zr–polymer moiety and the open coordination position exchange places, thus producing –rrrrr– monomer sequences. The same mechanism explains the formation of racemically linked norbornene diads and meso-linked norborneneethylene-norbornene triads in ethylene/norbornene copolymers prepared with these catalysts [937]. The stereochemistry of models of metallocene active centers of Cs symmetry was analyzed in great detail with molecular mechanics methods [1800,1891,1906]. Some of these calculations were carried out for transition states in the insertion steps of a propylene molecule into a model of the active center, a metallocenium cation carrying a polymer chain consisting of two propylene units [1906]. The calculations showed that the chain growth reaction is highly regioselective and that the racemic

Active Centers in Transition Metal Catalysts

469

(syndiotactic) linking of the inserting propylene molecule and the last monomer unit in the chain is more favorable compared to its meso (isotactic) linking by B8.8 kJ/mol (2.1 kcal/mol). This energy difference is somewhat affected by the type of the bridge between the Z5 ligands. This estimation is B30% higher than the experimentally estimated preference (from 13C NMR) for syndiotactic enchainment of neighboring propylene units [1906]. Morokuma analyzed the nature of stereocontrol in syndiospecific polymerization of propylene and 4-methyl-1-pentene using two models of syndiospecific metallocenium ions of Cs symmetry, a model of the standard ion, H2Si(Cp)(Flu) Zr+–R, and the H2Si-bridged ion with methyl groups in all available positions of one cyclopentadienyl ring, H2Si(2,3,4,5-Me4-Cp)(Cp)Zr+–R [1800]. The geometry of coordination complexes and transition states in the insertion reactions was pre-calculated based on ab initio estimations for ethylene insertion into the Zr+– CH3 bond. The molecular mechanics calculations for R ¼ CH3 showed that both the regiocontrol and the stereocontrol in the absence of a polymer chain attached to the Zr atom are weak and cannot explain high regioselectivity and high syndiospecificity of these catalysts known from experiment. However, when the R group in the H2Si(Cp)(Flu)Zr+–R ion is changed to C2H5 and to CH2–CH(CH3)–C2H5, the stereocontrol improves. The preference for syndiospecific coordination/ addition of a propylene molecule increases to B7.5 kJ/mol (B1.8 kcal/mol) for R ¼ C2H5 and to B17.5 kJ/mol (B4.2 kcal/mol) for R ¼ CH2–CH(CH3)– C2H5. Similarly to isospecific metallocene catalysis discussed in Section 6.1.3.2.2, the stereocontrol exerted by the Z5 ligands is indirect. The ligands affect the local conformation of the growing polymer chain in the proximity to the transition metal atom, and, in turn, the conformation of the chain determines the preferred coordination orientation of the propylene molecule leading to its syndio-linking to the preceding monomer unit (see Scheme 6.13). If a 4-methyl-1-pentene molecule is used in this analysis instead of a propylene molecule, the calculations predict an even stronger stereocontrol, B20 kJ/mol (B4.8 kcal/mol) for the H2Si(Me4-Cp) (Cp)Zr+–R center and B31.5 kJ/mol (B7.5 kcal/mol) for the H2Si(Cp)(Flu)Zr+– CH2CH(CH3)C2H5. The latter prediction agrees well with the experimental data [1907]. DFT/molecular mechanics analysis of the syndiospecific chain growth in propylene polymerization reactions with a constrained-geometry catalyst derived from a monometallocene complex with a fluorenyl ligand, [Me2Si(3,6-t-Bu2Flu)(t-Bu-N)]TiCl2, showed that the stereocontrol mechanism in this catalyst is similar to that for bis-metallocene complexes of Cs symmetry [414]. The energy difference can be also affected by agostic interactions between the metal atom and hydrogen atoms in the growing polymer chain. The a-agostic interaction enhances the syndio-selectivity whereas the g-agostic interaction may change the stereoselectivity to predominantly isotactic [1294]. These effects may explain puzzling differences in the stereoregularity of polymers of different 1-alkenes prepared with catalysts derived from bis-metallocene complexes of Cs symmetry. Although all these complexes polymerize propylene to the syndiotactic polymer, some of them polymerize 1-alkenes with branched alkyl substituents to mildly isotactic polymers, as shown in Table 6.4.

470

Alkene Polymerization Reactions with Transition Metal Catalysts

Table 6.4 Stereoregularity of 1-alkene polymers produced with metallocene complexes of different symmetry Complex

Monomer

Structure

Reference

Me2C(Cp)(Flu)ZrCl2 Ph2C(Cp)(Flu)ZrCl2 Me2C(Me-Cp)(Flu)ZrCl2 Me2C(Cp)(Flu)ZrCl2 Ph2C(Cp)(Flu)ZrCl2 (Me)(Ph)C(Cp)(Flu)ZrCl2

4-methyl-1-pentene ‘-’ ‘-’ 3-methyl-1-butene ‘-’ ‘-’

Syndiotactic Isotactic Syndiotactic (?) Syndiotactic Isotactic Isotactic

[1907] [1908] ‘-’ [1909] [323] [255]

6.1.3.3.2. Centers of Cs symmetry, mechanism of steric errors. Several mechanisms were proposed to explain the formation of steric errors in syndiospecific metallocene catalysis. The first one is an occasional steric error resulting from ‘‘wrong’’ orientation of a coordinated propylene molecule (enantiofacial misinsertion). This misstep produces the steric error of the rmmr type, . This steric mistake does not affect the outcome of the next insertion step, and its probability should not depend on the monomer concentration but should increase with temperature. This automatic stereo-correction mechanism is typical for the site-control stereoselection mechanism described in Section 3.1.3.3. Molecular-mechanics calculations [1873,1910,1911] and experimental estimations [166] give the following values for the stereocontrol energy difference for meso- vs. racemic linking with syndiospecific catalysts: Active center Me2C(Cp)(Flu)ZrCl2 Ph2C(Cp)(Flu)ZrCl2 from DEo (kJ/mol 8.8 (2.1) 11.3 (2.7) (kcal/mol))

Me3C(H)C(Cp)(Flu)ZrCl2 13.0 (3.1)

The second mechanism of steric error formation operates when a polymer chain attached to the transition metal atom spontaneously exchanges its position with the position of the vacancy open for alkene coordination, the back-skip step (or skipped insertion). This rearrangement, which results in the rotation of the active center with respect to the polymer chain, can be viewed as the migration of the chain from one coordination position to another [412,739,1912]. In syndiospecific catalysts, this step results in a different steric mistake, rrmr, . The third proposed mechanism of steric error formation is caused by a chemical isomerization reaction at the active center, epimerization of the last monomer unit attached to the transition metal atom. This is the same epimerization reaction as in the isospecific metallocene catalysis (Reaction (6.48)). Different types of epimerization reactions can potentially lead to two different types of steric errors in polymer chains, either rmrr or rmmr, a great complication in the interpretation of the experimental data [735]. The epimerization rate does not depend on the presence of a monomer, similarly to the back-skip step of a polymer chain.

471

Active Centers in Transition Metal Catalysts

The fourth proposed mechanism of stereo-defect formation may be caused by a particular type of coordination of a monomer molecule at the transition metal atom (‘‘back-side’’ misinsertion) in which the coordinated alkene molecule and the counter-anion occupy coordination positions at both sides of the Cp2M–CH2 bond [735,1913]. From the experimental point of view, two types of steric mistakes are observed in prevailingly syndiotactic polypropylene, rmrr or rmmr, and in order to sort out exact sources of these mistakes their frequencies should be measured at different monomer concentrations and at different temperatures. Several thorough investigations of this type were carried out. Preparative Tref/NMR analysis of polypropylene produced at 501C with the syndiospecific Ph2C(Cp)(Flu)ZrCl2-MAO system showed that both types of steric mistakes are indeed present in every polymer fraction irrespective of its molecular weight and in an approximately equal proportion, B2:1 [193]. They were attributed to the back-skip step. The same step explains the formation of occasional racemically linked norbornene-ethylene-norbornene triads in ethylene/norbornene copolymers produced with syndiospecific metallocene catalysts [937]. Marks carried out detailed analysis of the effects of several reaction parameters (temperature, monomer concentration, solvent type) on steric errors in syndiotactic polypropylene produced with several well-defined metallocenium ions [Me2C(Cp)(Flu)Zr+–Me]  A and different counter-ions [735]. Principal conclusions about the nature of steric defects are shown in Table 6.5. The first two columns in the table list total fractions of both observable types of steric defects at 601C, when the overall stereospecificity of the catalysts is low. The majority of rmmr defects are caused by the steric misinsertion of the monomer; they are responsible for 60–90% of all defects of this type even at PPr ¼ 1 atm, and this type of steric error dominates at high propylene concentrations. The second type of steric error, Table 6.5 Kinetic parameters of steric defect formation in propylene polymerization reactions with ionic metallocene catalysts [Me2C(Cp)(Flu)Zr+Me]  [A] at 601C [735] Counter-anion A

[Me-MAO] [MeB(C6F5)3] [B(C6F5)4] [MeB(C6F4C6F5)3] [FAl(C6F4C6F5)3] a

0.028 0.043 0.032 0.032

[rmrr]

0.08 0.20 0.13 0.13

0.027–0.0530.024

Probability of:

monomer misinsertiona

chain backskipb

last unit epimerizationc

0.020 0.026 0.025 0.029

0.024 0.044 0.037 0.034

0.0026 0.0057 0.0023 0.0009

0.014

0.0009

Probability of enantiofacial error E 1 – psyndio (Table 3.3 in Chapter 3), does not depend on CPr. Probability of chain migration at PPr ¼ 1 atm (CPr ¼ 0.36 M). Probability of epimerization of last monomer unit at PPr ¼ 1 atm (CPr ¼ 0.36 M).

b c

[rmmr]

472

Alkene Polymerization Reactions with Transition Metal Catalysts

rmrr, is mostly caused by the back skip of the polymer chain. The data in Table 6.5 show that the probability of steric misinsertion does not change much for different counter-ions A, the transition state of the misinsertion step is similar in geometry to the transition state of the ‘‘normal’’ syndiospecific growth step. However, both the back skip and the last-unit epimerization (which involves a temporary b-H shift to the metal atom (Reaction (6.48))) require significant movements of counteranions, and probabilities of the respective steps are strongly influenced by tightness of [Me2C(Cp)(Flu)Zr+–Me]–A pairing [735]. Another manifestation of the significance of cation–anion interactions in these catalysts is the effect of solvent polarity on stereospecificity [735]. A replacement of toluene with octane in propylene polymerization reactions with [Me2C(Cp) (Flu)Zr+–Me]  [A] systems does not affect the [rrrr] value of polymers prepared at 251C. However, a replacement of toluene (e ¼ 2.15) with more polar 1,3dichlorobenzene (e ¼ 5.04) and the resulting weakening of ion pairing produces two effects, the stereospecificity of the catalysts is reduced and it does not depend on the type of counter-anion. Kaminsky examined temperature and monomer concentration effects on the stereo-regulating power of two syndiospecific metallocene catalysts, the homogeneous Me2C(Cp)(Flu)ZrCl2-MAO system and the silica-supported catalyst of the same composition [1442]. The results in Table 6.6 demonstrate that both mechanisms of steric error formation are operative and that the second mechanism, changing positions of the polymer chain and the coordination site, dominates at high temperatures and at low monomer concentrations. Busico found that if a small amount of ethylene is added to a propylene polymerization reaction with the highly syndiospecific (Me)(Ph)C(Cp)(Flu)ZrCl2MAO system at 101C, it produces a noticeable change in the catalyst behavior [739]. The use of 1-13C-labeled ethylene revealed details of the copolymer structure unattainable with unlabeled ethylene. After a single ethylene unit is incorporated into the growing chain and a metallocenium ion Cp2Zr+–CH2–CH2–Polymer is formed, the insertion stereochemistry of the next propylene molecule changes. Although the primary regio-selection remains quite high, the stereoselection is Table 6.6 Effects of reaction parameters on stereo-regulating power of syndiospecific metallocene catalyst Me2C(Cp)(Flu)ZrCl2-MAO in propylene polymerization reactions [1442] Temperature (1C)

[rrrr]

[rmmr]

[rrmr]

0 30 60

0.958 0.917 0.818

0.006 0.012 0.017

0.008 0.018 0.049

CPr (M) 0.06 0.29 1.29 3.29

[rrrr] 0.757 0.875 0.917 0.954

[rmmr] 0.005 0.009 0.012 0.015

[rrmr] 0.086 0.035 0.018 0.008

473

Active Centers in Transition Metal Catalysts

Table 6.7 Two probability parameters determining syndiospecificity of catalysts based on constrained-geometry complex (Me2Si)2(3,6-t-Bu2-Flu)[N-t-Bu]TiCl2 [414] Cocatalyst:

[Me2N(Ph)H]+ [B(C6F5)4]-Ali-Bu3

MAO

Temperature (1C)

CPr (M)

psyndio

pepi

psyndio

pepi

30 30 30 50 50 50

1.3 2.4 5.2 1.0 1.7 3.5

0.979 0.980 0.980 0.970 0.972 0.979

0.049 0.029 0.016 0.115 0.088 0.047

0.948 0.963

0.189 0.123

0.897 0.926 0.955

0.245 0.201 0.170

greatly diminished, the psyndio value decreases from B0.97 to B0.54. In addition, the probability of chain-site isomerization (the back skip) increases from B0.01 to 0.3–0.4. Both these effects are the consequence of the same change in the chain-end structure. The monomer-coordination mechanism in the syndiospecific catalysts described above depends on a relatively rigid orientation of the b-C atom in the growing chain end and on a steric interaction between this atom and an approaching monomer molecule [1891]. The rigidity of this chain-end orientation is mostly determined by the b-agostic interaction between the [W(CH3)C]–H atom and the Zr atom. The replacement of the Cp2Zr+–CH2–CH(CH3)–Polymer species with the Cp2Zr+–CH2–CH2–Polymer species reduces this nonbonding interaction and, as a result, it reduces the stereoselectivity of the center. The same change accounts for an increase in the switching frequency of the chain end between two available coordination positions [739]. Constrained-geometry complexes with a single fluorenyl ligand, such as [Me2Si(Flu)(t-Bu-N)]TiMe2, have the overall symmetry similar to that of Cssymmetric bis-metallocene complexes and they also produce moderately syndiotactic polypropylene [414,729–731]. Busico applied the statistical model for a predominantly syndiospecific chain growth [412,414] outlined in Section 3.1.3.3 and estimated two probability parameters, the probability of syndiospecific monomer linking, psyndio, and the probability of chain epimerization, pepi. The results are shown in Table 6.7. In the case of the MAO-activated catalyst, the psyndio value only weakly depends on temperature and monomer concentration, whereas the pepi value decreases with propylene concentration and increases with temperature, as expected [414]. The use of an ion-forming cocatalyst, the [Me2N(Ph)H]+ [B(C6F5)4] salt, results in a large increase of the pepi value. As a result, the overall syndiotacticity level of polypropylene prepared with the latter system dramatically declines, the [rrrr] value decreases to 0.1–0.3 [414]. 6.1.3.3.3. Centers of C1 symmetry, mechanism of syndiospecific chain growth. A significant deviation from the simple stereoselection mechanism described in Section 3.1.3.3 may develop when a small substituent is placed in the second position

474

Alkene Polymerization Reactions with Transition Metal Catalysts

of the cyclopentadienyl ring of Me2C(Cp)(Flu)MCl2 complexes, e.g., Me2C(3-MeCp)(Flu)ZrCl2 [1912]. The metallocenium cation Me2C(3-Me-Cp)(Flu)M+– Polymer derived from these complexes is diastereomeric, it has two sterically nonequivalent positions for the orientation of the growing polymer chain and, respectively, for alkene coordination. Two consequences of this nonequivalence can be envisaged. The first consequence is that the strict alternation of positions of the growing polymer chain after each monomer insertion step (the principal feature of the chain migratory mechanism in Scheme 6.13) can be interrupted. If the position of the chain is switched after a given insertion step, the next insertion step still produces the syndiotactic link of two monomer units. However, if the chain switches its position but then migrates back to the more sterically preferable position, the following insertion step produces the isotactic link [1912]. Polymer chains of this type have the stereoblock nature and consist of relatively short syndiotactic and isotactic monomer sequences. According to the second proposed explanation of catalyst modification, the alkyl substituent in the cyclopentadienyl group does not interfere with the chain migration as such but it prevents the end of the chain attached to the metal atom from adopting the preferred spatial orientation [703]. When the chain is in the sterically open position and acquires the preferred conformation, an approaching monomer always coordinates and inserts in the same manner, but when the chain is in the sterically hindered quadrant, the monomer orientation is not as highly stereospecific. The outcome of this chain growth mechanism is a hemi-isotactic polymer [703,988,1914,1915]. Both mechanisms predict similar distributions of steric errors in polymer chains; experimental verification of differences between them is difficult [1912], and computational estimations of stereospecificity become very complicated [1801]. A variety of outcomes are theoretically possible [1801] and were indeed experimentally observed [724]. They depend on the rigidity of a bridged metallocenium ion (which, in turn, depends on the length of the bridge, –CH2–CH2– or –CHPh–CH2– vs. Me2Co), the types of Z5 ligands, and the monomer concentration. According to the calculations, various asymmetric metallocene complexes of this type can produce polypropylene with a structure ranging from predominantly syndiotactic for the Me2C(3-Me-Cp)(Flu)ZrCl2derived catalyst (when the back-skip step is possible), to a polymer of moderate isotacticity ([mmmm] B0.35–0.40) for the rac-CHPhCH2(Cp)(Ind)ZrCl2-derived catalyst, when the back-skip step does not take place.

6.1.4. Mechanism of styrene polymerization Original reaction products in metallocene catalyst systems for syndiospecific polymerization of styrene have the structure similar to that of the products found in other metallocene systems. For example, a reaction between CpTiMe3 and B(C6F5)3 produces the expected ion pair, [CpTi+Me2]  [BMe(C6F5)3] [1916,1917] (compare to Reaction (6.6)). A similar ion pair is formed in a reaction between CpTi(CH2SiMe3)3 and B(C6F5)3 [1918]. However, NMR analysis of model systems showed that metallocenium cations in these ion pairs do not polymerize styrene [1916]. Several studies demonstrated that the active species in

Active Centers in Transition Metal Catalysts

475

syndiospecific polymerization reactions of styrene contain TiIII species rather than TiIV species [1536,1919]. The presence of reduced cationic species, [CpTiIII–H]+ and [CpTiIII–CH3]+, was determined by esr in catalysts containing CpTiX3 or CpTiX3 (X ¼ Cl or OR) and MAO [107,1541,1546,1920] and in catalysts containing CpTiMe3 and B(C6F5)3 [1916,1921]. The highest concentration of TiIII species is achieved when the complexes are pre-contacted with mixtures of MAO and Ali-Bu3 at room temperature for several hours [1541]. The proposed route to the [CpTiIII–R]+ species involves alkylation of CpTiX3 with AlMe3 present with MAO or with specially added organoaluminum compounds, reduction of TiIV species to TiIII species, most probably, to CpTiIIIR2 or CpTiIIIXR, and abstraction of R or X by MAO [752]. The same valence state of Ti species, TiIII [107], and the same chemistry of styrene insertion reactions [751,1526] were observed in polymerization reactions catalyzed by a combination of TiBz4 and MAO. Several experimental studies proved that syndiospecific polymerization reactions of styrene with metallocene catalysts occur via the insertion mechanism, the same as in alkene polymerization reactions with transition metal catalysts. Detailed 13C and 1 H NMR analysis of copolymers of deuterated styrenes, PhCDQCD2+cisPhCHQCHD and PhCDQCD2+trans-PhCHQCHD, convincingly proved that the insertion occurs through the cis-opening of the double bond in the styrene molecule, the same as in insertion reactions of any alkene [752,1922]. However, insertion of styrene molecules into the CpTiIII–C bond occurs exclusively in the secondary orientation, in contrast to most polymerization reactions of 1-alkenes [746–749]. In particular, experiments with 13C-labeled catalysts and two monomer molecules, p-chlorostyrene and its a-13C-labeled analog (this monomer has a much lower reactivity in polymerization reactions, which provides an opportunity to observe its earliest stages) demonstrated the nature of both the chain initiation and the chain growth reaction [747,1923]: CpTiIIIþ 2R þ CHðC6 H4 ClÞQCH2 ! CpTiIIIþ 2CHðC6 H4 ClÞ2CH2 2R

(6.51)

13

C NMR characteristics of the polymer products showed that the d1 electron in the TiIII species is partially delocalized on the aromatic ring of the last monomer unit in the growing polystyrene chain, which indicates that the benzene ring in the last monomer unit is p-coordinated to the Ti atom [747,1923]. Based on these mechanistic findings, the principal reaction step in syndiospecific polymerization of styrene with monometallocene catalysts derived from TiIV complexes was postulated as the secondary insertion of a styrene molecule into the T–C bond in the cationic active species containing the TiIII atom [107,747, 752,1919,1923,1924]: CpTiIIIþ 2CHPh2CH2 2Polymer þ CHPhQCH2 ! CpTiIIIþ 2CHPh2CH2 2CHPh2CH2 2Polymer

(6.52)

The stereocontrol in Reaction (6.52) is governed by the chain-end mechanism (Section 3.1.3.4) and the stereospecificity in the chain growth reaction can be very

476

Alkene Polymerization Reactions with Transition Metal Catalysts

high, corresponding to the p0syndio value of B0.995 [752,753]. To explain the high level of stereocontrol, several possibilities for the structure of the polymer chain end were proposed, the Z3-allyl species or the a-agostic interaction between the TiIII cation and the hydrogen atom of the phenyl ring [1919]. Theoretical DFT analysis of the styrene insertion step into the M–C bond was carried out using two models, the species with the benzyl group, [CpTiIII– CH2Ph]+ [1926], and the [CpTiIII–CHPh–CH3]+ species, which is the product of secondary styrene insertion into the CpTiIII–H bond [1925]. The benzene ring in the latter species (it represents the last monomer unit in the polymer chain attached to the TiIII atom) is strongly p-coordinated to the Ti atom, in agreement with the experimental observations [747,1923]. A styrene molecule is coordinated to the TiIII atom in the cis-Z4-manner. The calculations also showed that the most stable transition state in the CQC bond insertion reaction leads to the formation of the syndiotactic link. The preference to syndiotactic linking depends on the type of cyclopentadienyl ligand, it is B6 kJ/mol (B1.4 kcal/mol) for active centers derived from Cp complexes and B16 kJ/mol (B3.8 kcal/mol) for active centers derived from Cp complexes [1925]. Discussion in Section 6.1.2.1.4 shows that the insertion of an alkene molecule into the Cp2Zr+–H bond is an important chain initiation reaction in alkene polymerization reactions with metallocene catalysts. Bercaw investigated a similar model reaction imitating the chain initiation step in styrene polymerization reactions, primary and secondary insertion reactions of several substituted styrenes and a-methylstyrenes CH2QCRPh (R ¼ H or CH3) into the Cp2M–H bond in electro-neutral bis-metallocene complexes Cp2MH2 [1871]. The regioselectivity of styrene insertion into the Cp2Zr–H bond at low temperatures strongly depends on the type of metallocene complex, it is strictly primary for sterically crowded Cp2 ZrH2 and [1,3-(Me3C)2-Cp]2ZrH2, strictly secondary for sterically open (Me3C-Cp)2ZrH2, and regio-nonselective for (Cp)(Ind-H4)ZrH2.

6.2. Non-Metallocene Homogeneous Catalysts A large variety of homogeneous catalysts for alkene polymerization were developed for the past 50 years (see Section 4.7). The examples below describe the best-researched structures of active centers and the mechanisms of alkene polymerization reactions. Complexes of transition metals (both early- and late-period) with various ligands, which produce homogeneous catalysts when combined with such cocatalysts as MAO or ion-forming activators, are usually isolated and well characterized by the X-ray method and NMR. However, mechanistic studies of soluble catalysts based on these complexes are greatly complicated by very rich and still poorly explored chemistry of their interactions with cocatalysts. These interactions include the reduction of transition metal atoms, modification of the ligands and a change of their nature [1418], or even a ligand exchange between the

477

Active Centers in Transition Metal Catalysts

transition metal complexes and cocatalysts [1388]. This section also describes several well-researched mechanistic examples of alkene polymerization reactions with different non-metallocene homogeneous catalysts.

6.2.1. Vanadium-based catalysts The chemistry of early homogeneous and pseudo-homogeneous catalysts containing TiCl4, VCl4, or VOCl3 as transition metal compounds and various organoaluminum compounds is very complex and is relatively poorly studied [1066,1067]. Theoretical analysis of possible structures of active centers in these catalysts is mostly based on their similarities with active centers in heterogeneous Ziegler–Natta catalysts and on the chemistry of chain growth reactions with these catalysts. Catalysts derived from VCl4: VCl4-AlRxCl3x systems are important commercial catalysts which are used for the synthesis of random ethylene/propylene copolymers. Some of these systems are potentially suitable for a detailed analysis of interactions between catalysts and cocatalysts leading to the formation of active centers. One catalyst system of this type is a combination of VCl4 and Al2Et3Cl3. When this system is used between 10 and +401C, it contains only one type of active center [204]. A thorough UV/IR plug-flow investigation provided the basic kinetic information on the formation of the active centers [204]. The first step is a ligand exchange/complex formation reaction: VCl4 þ Al2 Et3 Cl3 ! 1 : 1 complex of VIV  Et species and Al species (6.53) This complex is very unstable (t0.5 B1 minute at –151C) and is not catalytically active. It reacts with excess Al2Et3Cl3 and produces true active centers that are not stable either; they are rapidly reduced to catalytically inactive insoluble VII species. The conversion of VCl4 to the final VII species is complete within B20 seconds even at relatively low [Al]:[V] ratios. Because the active centers are transient components in this reaction sequence, their concentration at any given moment is merely a small fraction of the initial VCl4 concentration, although all initial VCl4 is eventually converted to polymerization centers [204,307]. Zambelli carried out theoretical analysis of possible active species formed in a reaction between VCl4 and AlR2Cl [767]. This catalyst produces syndiotactic polypropylene when used at low temperatures (Section 3.4.1). Experimental information pertaining to chemical reactions in this system is scarce [1927,1928]. Chemical data on the reaction products at increased temperatures and the analysis of the starting and the last chain ends in propylene polymers produced with this catalyst suggest the following features of the active species: 1. The most probable valence state of the active species is VIII. 2. The initial V–C bond in the active center is formed in an exchange reaction between VCl4 and AlR2Cl. 3. The dominant regio-orientation of propylene molecules in chain growth reactions is secondary.

478

Alkene Polymerization Reactions with Transition Metal Catalysts

Cl

V

Al Cl

Scheme 6.14

R

Cl Cl

Al

Structure of active species in homogeneous V-based catalysts [768].

DFT calculations suggested that the core of the active center is the VCl2R species and that it is coordinated with one or two molecules of AlR2Cl via m-Cl bridges, as shown in Scheme 6.14. The calculations cannot provide an unambiguous selection between these active centers although the center containing two AlR2Cl molecules and a penta-coordinated VIII atom with one vacancy has the lowest activation barrier for double bond insertion into the V–C bond, 7.1 kJ/mol (1.7 kcal/mol). Evaluations of regioselectivity of this active center in insertion of a propylene molecule showed that if the R group in the active center models the primary insertion of the previous monomer unit (R ¼ i-Bu), the active center does not exhibit any regioselectivity. However, if the R group models the previous secondary insertion (R ¼ i-Pr), the secondary propylene orientation is preferable [767]. These estimations match reasonably well experimentally observed regio-orientation effects of last monomer units in the polymer chain on the regiochemistry of propylene insertion reactions (see Table 3.41). The model of the active center in Scheme 6.14 also accounts for the syndiospecific stereocontrol of these V-based catalysts [1929] and, overall, can be regarded as the most plausible model of active centers in catalysts of the VCl4-AlR2Cl type. Guerra proposed another type of active center in these catalysts, a VIV atom in an octahedral complex of C2 symmetry. Four of the ligands at the V atom are Cl atoms (forming bridges with two organoaluminum moieties), the fifth ligand in the growing polymer chain, and the remaining coordination site is open for alkene coordination [1930]. As described in Section 3.4.1, the dominant configuration of monomer units in these growing polymer chains is secondary. This arrangement determines both the chirality of the octahedral center and the stereochemistry of monomer coordination. After the CQC bond insertion into the [Cl4]V–C bond is complete, the chirality of the active center is reversed, which explains the predominantly syndiospecific character of these catalysts. Catalysts based on (L)VIVCl2 complexes containing diamide ligands (Scheme 1.2), as well as their nonbridged analogs containing two separate amide ligands exhibit many features characteristic of VCl4-based catalysts, which suggests the same type of active species as that in Scheme 6.14 [1388]. Catalysts derived from V(acac)3: Combinations of V(acac)3 and AlR2Cl were one of the first homogeneous catalysts developed for polymerization of ethylene and its copolymerization with 1-alkenes. When these catalysts are used under mild conditions, they contain only one type of active center (see Table 4.50). Unfortunately, attempts to investigate the chemistry of interactions between the catalyst components have met with significant difficulties [1388]. These catalysts

479

Active Centers in Transition Metal Catalysts

have a very high initial activity at moderate temperatures they but usually deactivate within 10–15 minutes [1637,1931]. The data in Table 4.22 show that the performance of the catalysts in ethylene/propylene copolymerization reactions under ambient conditions is practically insensitive to the type of substituents in the acac ligand or to the type of cocatalyst, AlEt2Cl vs. Al2Et3Cl3. Investigations of reactions between V(acac)3 and AlEt2Cl did not identify the nature of the catalytically active species but they determined several of its important features [1388]. The reaction between V(acac)3 and AlEt2Cl results in a rapid ligand exchange between the two organometallic compounds; the acac moiety nearly completely migrates to the Al atom, which explains insensitivity of the catalyst performance to electronic and steric effects of substituents in the acac ligand. After the ligand exchange, the V species are rapidly reduced to VII and eventually precipitate from solution.

6.2.2. Ni ylide catalysts for ethylene oligomerization One of the earliest examples of a polymerization mechanism with a soluble derivative of a late-period transition metal involves ethylene oligomerization reactions with square-plane Ni complexes containing bidentate ligands [85,87,810]. The active species in one such catalyst, the Ni ylide complex (Scheme 1.3) is formed when an ethylene molecule displaces the PPh3 ligand from the Ni coordination sphere and inserts into the Ni–Ph bond [810]: ðLÞNi2Ph  PPh3 þ CH2 QCH2 ! ðLÞNi2CH2 2CH2 2Ph þ PPh3 (6.54) This reaction is followed by elimination of styrene and the formation of the true catalytic species containing the NiH bond [1932,1933]: ðLÞNi2CH2 2CH2 2Ph ! ðLÞNi2H þ CH2 QCH2Ph

(6.55)

Reaction (6.55) is followed by a chain initiation reaction, insertion of an ethylene molecule [810,1932,1933] or a 1-alkene molecule [116,117] into the Ni–H bond. The chemistry of oligomerization reactions is represented by Reactions (5.64)– (5.66). Scheme 6.15 shows the mechanism of principal reaction steps in ethylene oligomerization reactions using as a model the first chain growth step, the insertion of an ethylene molecule into the Ni–C2H5 bond [1934]. Both the starting and the final structure contains an agostically stabilized Ni–CH2R bond. Two types of these interactions are possible, a and b. Both cause the respective C–H bond to lengthen ˚ . The b-agostic structure is more stable and it represents the by B0.02–0.03 A resting state of the complex prior to ethylene coordination. The asymmetry of the ylide ligand leads to two possible insertion pathways shown in Scheme 6.15 and, hence, to two possible insertion transition states. DFT calculations showed that the complex with the alkyl group in the trans-position to the P atom (the bottom part in the scheme) is less energetically stable compared to its isomer with the alkyl group in the trans-position to the O atom; the energy difference is B20 kJ/mol (B5 kcal/mol). However, the overall ethylene insertion

480

Alkene Polymerization Reactions with Transition Metal Catalysts

CH3

H P

H P

CH Ni

H

+ C2 H4

CH2CH3 Ni CH2 O C H2

Ni CH2

O

O

H P

C 2H 5

H2C

H P Ni O

H CH

H P + C 2H4

Ni O

H2 C

H P

H 2C CH2 C 2H 5

Ni O

CH2 CH2CH3

H P Ni Bu

O

H P

Bu Ni

O

CH3 Ni-Et (α-agostic)

Scheme 6.15 plex [1935].

Ni(alkyl)(C2H4)

Transition State

Ni-Bu,α-agostic

Two routes in ethylene insertion reaction into Ni^CH2 bond in Ni ylide com-

barrier, beginning from the p-complex and culminating in the formation of a new Ni–CH2 bond in the trans-position to the O atom, is energetically preferable, DDG ¼ 39 kJ/mol (11.7 kcal/mol) [1934]. This mechanism implies that each ethylene insertion step results in a change of the position of the alkyl group. Before the next insertion step takes place, the complex with a coordinated ethylene molecule (the second complex in the top part of Scheme 6.15) undergoes an isomerization step, the coordinated ethylene molecule slides in the plane of the ligand while the alkyl group rotates perpendicularly to the ligand plane. This ligand re-coordination is very similar to the back-skip step proposed as an important part of many alkene polymerization reactions with metallocene complexes (Section 6.1.2.2); it is also the vital part of the Cossee polymerization mechanism for heterogeneous Ziegler–Natta catalysts (Section 6.3.6.2). The Ni ylide complex readily reacts with CO [1935]. A CO molecule coordinates with the Ni atom and inserts into the Ni–Ph bond with the formation of the Ni acyl species, (L)Ni–C(QO)–Ph. The same species is also formed if CO is introduced in the course of an ethylene oligomerization reaction, (L)Ni–C(QO)– (CH2–CH2)n–H. This species is stable; ethylene molecules do not insert into the (L)Ni–C(QO) bond. Heterogeneous catalysts containing supported Ni ylide complexes are also easily poisoned by CO [1630]. Addition of CO at a [CO]:[Ni] ratio of B1 is sufficient for very rapid and complete poisoning of the Ni ylide/ MgH2 catalyst at 651C. The use of 14C-labeled CO in this reaction showed that B60% of the Ni complex in the supported catalyst inserts CO into the Ni–C bond in the active centers [1630].

6.2.3. Catalysts derived from complexes with phenoxy-imine ligands Complexes of early-period transition metals (L)2TiCl2 and (L)2ZrCl2 containing two phenoxy-imine ligands L (Scheme 1.2) represent an example when the steric structure of complexes affects their performance as catalyst precursors (see Section 4.7.1.2). Some of the Zr complexes exist in solution predominantly in the form of a

Active Centers in Transition Metal Catalysts

481

single isomer [338–340] and catalysts produced from them have a single type of active center [338,339] whereas other complexes are more fluxional and exist as mixtures of several isomers, each producing a different type of active center [338,339]. When the Ti complexes contain ortho-fluoro-substituted phenyl groups at the nitrogen atom in each bidentate ligand, they produce active centers with a very low chain transfer rate. These systems polymerize ethylene and propylene under living-chain conditions at temperatures as high as 501C [342,343,781, 784,806,808]. MAO-activated bis(phenoxy-imine) complexes of TiIV and ZrIV polymerize propylene to predominantly syndiotactic polymers [342,343,644,781–784, 1936,1937]. 13C NMR analysis of starting end-groups and the main chain structure in the polymers prepared at 0–251C show that the chain propagation reaction, after one or two primary propylene insertion steps, proceeds predominantly in the secondary orientation due to the chain-end regiochemical control [343,783,784]. Three features of active centers generated from bis(phenoxy-imine) complexes require an explanation: (a) insertion of 1-alkene molecules in the secondary orientation, (b) very low rates of chain transfer reactions for complexes carrying the F atom in the ortho-position to the nitrogen atom, and (c) syndiospecificity. DFT calculations of models of expected active centers, (L)2Ti+-n-C3H7 cations, showed that when the F atom is placed in the ortho-position to the nitrogen atom in the ligand, it strongly interacts with the b-H atom of the growing polymer chain attached to the active center. This interaction results in elongation of the respective ˚ and in a shortened F    H distance, B2.3 A˚ [808]. This C–H bond to B1.113 A interaction decreases the reactivity of the b-H atom in the growing chain in both reactions leading to the polymer chain transfer, their shift to the Ti atom (spontaneous chain transfer) or to a coordinated ethylene molecule (chain transfer to a monomer). High syndiospecificity of Ti bis(phenoxy-imine) complexes with the C6F5 group attached to the nitrogen atom was also subjected to theoretical analysis [1938]. Experimental studies of similar neutral complexes demonstrated that they are highly fluxional in solution, probably through rapid dissociation/association of Ti    N bonds in the complexes [337,338]. Active centers derived from the complexes are also expected to be highly fluxional. Therefore, the experimentally observed stereoselectivity of these active centers cannot be explained by a fixed chiral structure of the active center. Neither it can be explained by any steric interactions between the growing polymer chain and a coordinated propylene molecule; the distance between the two moieties is too high and thus can only lead to centers of negligible stereoselectivity. A combination of quantum mechanic and molecular mechanics analysis of chain growth reactions produced the most plausible scenario of stereoselection. The impetus for the stereoselection is the steric repulsion between the ortho-F atom in the C6F5 group of the complexes and the methyl group in the second monomer unit in the growing polypropylene chain. This repulsion forces isomerization of the active center; it acquires one of two possible chiral structures and retains it. In turn, this (temporarily fixed) chiral arrangement of the phenoxy-imine ligands at the Ti

482

Alkene Polymerization Reactions with Transition Metal Catalysts

atom forces the approaching propylene molecule to coordinate in a specific arrangement which, after insertion of its CQC bond into the Ti–C bond, produces a syndiotactic link. The DE value for this coordination type is B3.9 kcal/ mol. As soon as a propylene insertion step is complete, the chirality of the polymer end is reversed, and it causes the reversal of the steric structure of the fluxional active center before the next coordination/insertion cycle begins [1938]. These calculations showed that without this reversal of chirality the active center would be highly isospecific rather than syndiospecific. Experimental NMR analysis showed that steric mistakes in the polypropylene chains are of the rrrm and the rmrr type, which is consistent with the proposed chain-end stereocontrol mechanism through the steric inversion of the active center (see Section 3.1.3.4). Overall, this regiochemical and the stereochemical behavior of Ti bis(phenoxyimine) complexes is very similar to those of homogeneous VCl4-based catalysts described above and it probably reflects a very similar chemical structure of the respective active centers [767]. Catalysts derived from Ti bis(phenoxy-imine) complexes are much more stereospecific, when a propylene polymerization reaction with the (N)-C6F5 substituted complex is carried out at 0 1C, the p0syndio value can reach B0.99 [343,644,1939].

6.2.4. Catalysts derived from complexes with (imino)pyridyl ligands From the kinetic viewpoint, catalysts based on three types of complexes (L)MCl2 containing bis(imino)pyridyl ligands (Scheme 1.3), Co complexes, V complexes, and Fe complexes, are very different. The Co complexes, after activation with MAO, produce essentially single-center catalysts [388]; they polymerize ethylene to linear polymers of low molecular weight [816–818] (Section 5.5.4). NMR data suggest that the initial active species in these catalysts are formed in alkylation reactions of the Co complex with MAO [1421]. Most probably, this reaction is followed by reduction of CoII and the true active center is a mononuclear complex containing the CoI–C bond [1940,1941]. Kinetic data on ethylene polymerization reactions with catalysts derived from these Co complexes show that these active species are very unstable, even at 351C they decompose after 20–30 minutes [1421]. In contrast, reactions between bis(imino)pyridyl complexes of Fe or V and organoaluminum compounds, both AlR3 and MAO, are very complex and can result both in the change of the valence state of the transition metal atom and in a significant change in the nature of the ligand. The first stage of the reaction between (L)FeIICl2 complexes and MAO or with AlMe3 produces two binuclear complexes with two m-methyl bridges, (L)(Cl)FeII(m-Me)2AlMe2 at low [MAO]:[Fe] ratios and (L)(Me)FeII(m-Me)2AlMe2 at high [MAO]:[Fe] ratios [1642]. In the absence of ethylene, th