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POLYMER SCIENCE AND TECHNOLOGY
ENCYCLOPEDIA OF POLYMER RESEARCH (2 VOLUME SET)
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POLYMER SCIENCE AND TECHNOLOGY
ENCYCLOPEDIA OF POLYMER RESEARCH (2 VOLUME SET)
CAROLINE E. JONES EDITOR
Nova Science Publishers, Inc. New York
Copyright © 2011 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. Additional color graphics may be available in the e-book version of this book. LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA Encyclopedia of polymer research / Editor, Caroline E. Jones. p. cm. ISBN 978-1-61122-209-8 (eBook) 1. Polymers--Encyclopedias. I. Jones, Caroline E. TP1110.E46 20 668.9--dc22 2010037853
Published by Nova Science Publishers, Inc. † New York
CONTENTS Volume 1 xi
Preface Chapter 1
Chapter 2
Chapter 3
Chapter 4
Chapter 5
Research and Review and Studies Induced Self-Assembly of Diblock Copolymers Eri Yoshida
1
A Novel Thermosensitive Composite Hydrogel Based on Poly(Ethylene Glycol)-Poly(Ε-Caprolactone)-Poly(Ethylene Glycol) (PECE) Copolymer and Pluronic F127 ChangYang Gong, Shuai Shi, PengWei Dong, MaLing Gou, XingYi Li, YuQuan Wei and ZhiYong Qian
29
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer for Catalytic Enantioselective Reactions Christine Saluzzo and Stéphane Guillarme
45
Small Molecule Stabilization: A Novel Concept for the Stabilization of Small Inorganic Nanoparticles Georg Garnweitner
173
Molecular Implications in the Solubilization of the Antibacterial Agent Triclocarban by Means of Branched Poly (Ethylene Oxide)Poly (Propylene Oxide) Polymeric Micelles Diego A. Chiappetta, José Degrossi, Ruth A. Lizarazo, Deisy L. Salinas, Fleming Martínez and Alejandro Sosnik
197
213
Chapter 6
Siloxane-Containing Compounds as Polymer Stabilizers Carmen Racles, Thierry Hamaide and Etienne Fleury
Chapter 7
Amphiphilic Block Copolymers: Potent Efflux Pump Inhibitors for Drug Delivery and Cancer Therapy Martin Werle and Hirofumi Takeuchi
235
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers Z. Yang
243
Chapter 8
vi
Contents
Chapter 9
Current Developments in Double Hydrophilic Block Copolymers G. Mountrichas and S. Pispas
Chapter 10
Thermo-Oxidation Stability of Poly(Butylene Terephthalate) and Catalyst Composition Antonio Massa, Valeria Bugatti, Arrigo Scettri and Socrate Contessa
Chapter 11
Chapter 12
Chapter 13
Chapter 14
Chapter 15
Chapter 16
Chapter 17
Chapter 18
Chapter 19
Chapter 20
Hindered Amine Stabilizers as Sources of Markers of the Heterogeneous Photooxidation / Photostabilization of Carbon Chain Polymers J. Pilař and J. Pospíšil A Review of the Hazardous Effects of Polycyclic Aromatic Hydrocarbons on Living Organisms Maria Aparecida Marin-Morales; Daniela Morais Leme and Dânia Elisa Christofoletti Mazzeo
291
327
343
359
Polycyclic Aromatic Hydrocarbons Toxicity in Animals and Humans Pierina Visciano and Monia Perugini
407
Polycyclic Aromatic Hydrocarbons and Carcinogenic Risk Assessment in Developing Countries Mahmoud A. Hassanien
423
Adipose Tissue as a key Contributor to Polycyclic Aromatic Hydrocarbon-Induced Carcinogenesis P. Irigaray and D. Belpomme
453
Chlorinate Polycyclic Aromatic Hydrocarbons: A New Class of Environmental Toxin? Takeshi Ohura and Hiroyuki Sakakibara
463
Recent Trends in Analytical Methods to Chemically Monitor PAH Pollution Verónica Pino
469
New Tendencies in Liquid Chromatography for Controlling PAHs Pollution R. Herráez-Hernández, P. Campíns-Falcó, J. Verdú-Andrés, A. Sevillano-Cabeza and L. Ferrando-Climent
557
Pahs in Sediments Associated with Coal and Coal-Derived Particles - Occurrence, Mobility and Risk Assessment Yi Yang and Thilo Hofmann
577
Analysis of Polycyclic Aromatic Hydrocarbons Adsorbed on Soot Surface by Laser Desorption, Laser Ionization and Mass Spectrometry Michael Ziskind, Alessandro Faccinetto, Pascale Desgroux, Eric Therssen and Cristian Focsa
587
Contents
vii 605
Chapter 21
Toxicity and Toxicology of Benzo[a]pyrene M. S. Yang and W. S. To
Chapter 22
Chromatographic Analysis of Polycyclic Aromatic Hydrocarbons by Π-Electron Containing Polymeric Organic Phase-Grafted on Silica M. Mizanur Rahman, Makoto Takafuji, Ashequl A. Rana, Abul K. Mallik, Usha G. Gautam and Hirotaka Ihara
621
Determination of Polycyclic Aromatic Hydrocarbons in SolidMatrix Foods Using a Non-Destructive Method Diana Martin and Jorge Ruiz
651
Chapter 23
Chapter 24
Chapter 25
PAH Removal by Two Native Tropical Plants Cultured on Model Contaminated Soil E. Escalante-Espinosa, L. Rodríguez-García and M. Gutiérrez-Rojas In Vitro Phytoremediation Mechanisms of PAH Removal by Two Plant Species L. Reynoso-Cuevas, F. Cruz-Sosa and M. Gutiérrez-Rojas
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677 691
Chapter 26
Fibers Spun from Polysaccharides Lingyan Kong, Gregory R. Ziegler and Rajesh Bhosale
Chapter 27
Development and Characterization of Starch Based Nano-Composites Xiaozhi Tang and Sajid Alavi
735
Characterization and Mechanism of Chitosan Hydrolysis by Non-specific Enzymes Wenshui Xia and Ping Liu
775
Volume 2
Chapter 28
Chapter 29
Use of Cyclodextrin Polymers in Separation of Organic Species Cezary A. Kozlowski and Wanda Sliwa
Chapter 30
Properties, Applications and Current Development of Edible Polysaccharide Films and Coatings Pau Talens, María José Fabra and Amparo Chiralt
Chapter 31
Carbohydrate Binding Modules: Functions and Applications Susana Moreira and Miguel Gama
Chapter 32
Biodegradation of Cellulose Fibers and its Inhibition by Chemical Modification Barbara Simončič, Brigita Tomšič, Boris Orel and Ivan Jerman
817
857 891
927
viii Chapter 33
Contents Pathways to Bioactive Oligosaccharides: Biological Functions and Potential Applications Ellen C. Giese, Aneli M. Barbosa and Robert F. H. Dekker
969
Chapter 34
Amidation of Cellulose Materials Nadege Follain
1001
Chapter 35
D/L-Hybrid Galactans from Red Seaweeds: Do they Really Exist? Marina Ciancia and Alberto S. Cerezo
1045
Chapter 36
Development of Polysaccharide Networks for L-(+)-Ascorbic Acid Stabilization into Antioxidant/Nutritional Supplier-Edible Films Ana M. Rojas, Paula G. León, Silvia K. Flores, Carolina D. Pérez and María D. De’Nobili
Chapter 37
Chapter 38
Chapter 39
Chapter 40
Chapter 41
Chapter 42
Employing Polymer-Polymer Incompatibility for Oil Microencapsulation by Coacervation Jaroslav M. Katona, Verica J. Sovilj, Lidija B. Petrović and Jadranka L. Milanović Main Characteristics of Some Gelling and Thickening Polysaccharides: Relation between the Molecular Structure, the Physical Properties and the Applications Marguerite Rinaudo
1083
1109
1145
Development and Applications of Nutraceuticals from Marine Carbohydrates Y. Dominic Ravichandran and Se-Kwon Kim
1169
Carbohydrate Polymers as Versatile Materials in Controlled Release Technologies Anamika Roy, J. Bajpai and A.K. Bajpai
1195
Structural Organization and Biological Activity of Chitin Nanofibrils Anyarat Watthanaphanit and Ratana Rujiravanit
1225
Development of Carbohydrate Polymer Based New Hydrogel Materials Derived from Seaweed Polysaccharides A. K. Siddhanta, Ramavatar Meena, Gayatri Prasad, Mahesh U Chhatbar, Gaurav K Mehta, Mihir D Oza, Sanjay Kumar and Kamalesh Prasad
1245
Chapter 43
Chitosans: New Vectors for Gene Therapy Riccardo A.A. Muzzarelli
1273
Chapter 44
Extremophiles as Sources of Exopolysaccharides Hande Kazak, Ebru Toksoy Öner and Robert F. H. Dekker
1295
Chapter 45
Alginate: A Promising Polysaccharide for Delivery of Essential Oils Pranee Lertsutthiwong, Ian S. Haworth and Pornchai Rojsitthisak
1311
Contents Chapter 46
Chapter 47
Obtention of Products Enriched in Carbohydrate Polymers from the Food Industry Leftovers Lía N.Gerschenson, Ana M. Rojas, Marina de Escalada Pla, Eliana Fissore and María E. Latorre Recent Developments in the Preparation and Applications of Chitosan-Based Microspheres Dongfeng Wang, Haiyan Li, Binjie Liu and Xingyu Wang
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1329
1345
Chapter 48
Chemoenzymatic Synthesis of Amylose-Grafted Polymers Yoshiro Kaneko and Jun-ichi Kadokawa
1361
Chapter 49
Polysaccharides and their Derivatives for Biomedical Applications Liqun Yang and Li-Ming Zhang
1383
Chapter 50
Feruloylated Arabinoxylans Recovered from Low-Value Maize by-Products Guillermo Niño-Medina, Elizabeth Carvajal-Millan, Jaime Lizardi, Agustín Rascón-Chu, Alfonso A. Gardea
Chapter 51
Chapter 52
Chapter 53
1401
Effect of Visible Light on Physical and Enzymatic Transformations of Polysaccharides Maciej Fiedorowicz and Piotr Tomasik
1417
The Topology Morphology Comparison of Nano-structural Units between Heating and High-pressure Gelatinization of Starch Granules Liu Zhongdong, Guo Yanyan, Gao li, Liu Peng, Gao Xinlou, Chen Zhaotan, Dong Caiwen and Liu Boxiang
1431
Partial Characterization of a New Acidic Heteropolysaccharide Produced by a Native Isolate of Lactobacillus sp. CFR-2180 R. Mohan and S. V. N. Vijayendra
1447
Chapter 54
“Polysaccharide-based Intelligent Delivery” Calls Cancer to Stop Hong Wu, Li Fan and Fei Li
1457
Chapter 55
Study on the Physical Properties of Waxy Wheat Starch Chen Jianhui, Liu Zhongdong, Liu Peng, Zhang Zesheng, Lv Xiaoling, Sun Ping and Liu Boxiang
1469
Chapter 56
Studies on the Morphology of Waxy Maize Zein Film Liu Zhongdong, An Hongjie, Sun Youning, Guo Yunchang, Liu Peng and Li Junchao
1483
Chapter 57
Carbohydrate Polymers from Mushrooms: Recent Development and Applications as Functional Foods Peter C. K. Cheung
1489
x Chapter 58
Index
Contents Application of Macrocyclic Polymers for Metal Ions Separation Cezary Kozlowski, Wladyslaw Walkowiak, Joanna Konczyk and Anna Nowik-Zajac
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PREFACE This book presents important research advances in the study of polymer research. Topics discussed herein include small molecule stabilization; amphiphilic block copolymers; effects of polycyclic aromatic hydrocarbons in living organisms; PAHs pollution; cyclodextrin polymers; polymer-polymer incompatability; carbohydrate polymers in controlled release technologies; and macrocyclic polymers for metal ions separation. Chapter 1 - The molecular self-assembly is induced by variation in the surroundings, such as temperature, pressure, pH, salt formation, and noncovalent bond cross-linking. The block copolymers are molecularly converted in situ from the nonamphiphilic copolymers completely dissolved in a solvent to amphiphilic copolymers due to these stimuli. Therefore, the association and dissociation of the isolated copolymers are reversibly controlled by such stimuli. The induced self-assembly has advantages over direct self-assembly of amphiphilic copolymers in molecular designing. There is no dependence on the balance of solvophilic and solvophobic moieties when designing the copolymers. Thus, a better selection of the driving force can be provided. The advantages also include the fact that a variety of amphiphilic copolymers can be created from one nonamphiphilic copolymer in situ by selecting the stimuli. Chapter 2 - A novel kind of biodegradable thermosensitive composite hydrogel was successfully prepared in this work, which was a flowing sol at ambient temperature and became a non-flowing gel at body temperature. The composite hydrogel was composed of poly(ethylene glycol)-poly(ε-caprolactone)-poly(ethylene glycol) (PEG-PCL-PEG, PECE) and Pluronic F127 copolymer. By varying the composition of above two copolymers, in vivo degradation rate and in vitro drug release behavior could be controlled. Histopathological study of tissue at injection site showed no significant inflammatory reaction and toxicity, which means that the composite hydrogel might serve as a safe candidate as in situ gelforming controlled drug delivery system. Chapter 3 - This paper reviews the recent progress made in insoluble polymer supported amino alcohols, amino thiols, oxazolines, salens, sulphonamides, oxazaborolidines and diamines ligands. This paper deals also with various approaches of stabilization of the catalytic system by immobilization of the chiral catalyst onto the polymer by the way of immobilization of the chiral ligand. Different types of ligand immobilization are presented: pendant ligands anchored on a polymer prepared by a polymer reaction, ligands on the backbone prepared by copolymerization and molecular imprinting technique. Examples of their use, performance and recyclability in a variety of enantioselective reactions such as alkylation and reductions of C=O bonds (hydrogenation, hydrogen transfer reduction)
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reduction of C=N bonds, C-O bond formations (epoxidation, dihydroxylation), C-C bond formations (Diels Alder, cyclopropanation, aldolisation, allylic substitution) and oxidation … are presented. Chapter 4 - In the last 20 years, the synthesis of nanoparticles with defined size and shape has been studied with strongly growing interest, leading to a multitude of synthetic approaches and strategies. Whereas the synthesis of the nanocrystals has been studied in great detail, far less effort has been directed towards the stabilization of the obtained materials against agglomeration. This is surprising as the stabilization determines their dispersibility in various solvents, which is a crucial parameter for most applications. For conventional colloids, the classical theories of electrostatic, steric and electrosteric stabilization are well established, but application of these theories to the stabilization of small nanomaterials leads to some peculiarities and at the same time has some limitations, which is known from experimental experience but has not been studied in a systematic fashion yet. One important conclusion from the theories is that short organic molecules sufficiently serve to provide steric stabilization of nanoparticles less than about 50 nm in size, without a need for long-chain polymeric stabilizers. This concept has been successfully applied using commercial metal oxide nanoparticles in the 50 nm size range, and it is even possible to tailor nanoparticle dispersions with respect to their rheological properties by adjustment of the stabilizer size. Through proper choice of the stabilizer, nanoparticle slurries with high solids content but at the same time low viscosity can be realized, which is highly advantageous for applications especially in the field of ceramic processing. For ultrasmall nanoparticles in the sub-10-nm regime, the picture is somewhat different. On the one hand, the dispersions of such particles in a stabilized state show very special properties on the verge to molecular solutions, rendering them highly relevant for applications and thus their preparation highly important. On the other hand, due to the lack of suitable model materials, the fundamentals of interaction and stabilization of such small nanoparticles remains largely in the dark. Only a small number of reports were specifically directed to adress these problems and systematically investigate the effects of stabilizer chemistry and structure as well as solvent influence. A brief overview of these studies is provided to show that first concepts have been presented, but the general applicability of these concepts still remains to be seen, and to demonstrate the substantial need for further research in this field in order to develop concepts for the rational stabilization and preparation of dispersions with tailored nanoparticle interactions and thus tailored properties. Chapter 5 - Aiming to gain further insight into the complexity of drug/polymeric micelle interaction phenomena, the present chapter investigated the incorporation of the poorly watersoluble topical antibacterial agent triclocarban (TCC) into polymeric micelles of the branched pH/temperature-responsive poly(ethylene oxide)-poly(propylene oxide) block copolymers Tetronic® 1107 (MW = 15 kDa, 70 wt% PEO) and 1307 (MW = 15 kDa, 70 wt% PEO). Solubility extents showed a sharp increase of up to 4 orders of magnitude. Due to the pHdependent character of both the carrier and the drug, studies were performed under different pH conditions. Due to a more efficient poloxamine aggregation at higher pH-values, a clear increase in the solubilization capacity was apparent under these conditions. However, ionization of TCC at pH 12.7 constrained the formation of hydrogen bonds between the urea moieties and the polyether chain, leading to a decrease in solubility above this pH. The size and size distribution of drug-loaded micelles was evaluated by Dynamic Light Scattering (DLS). Findings indicated the increase in the size of the aggregates with the incorporation of
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the drug. The morphology of the nanostructures was visualized by transmission electron microscopy (TEM). The stability of the systems over time was also evaluated. Finally, the antibacterial activity of different TCC/poloxamine complexes was assayed on different bacteria collections. For example, while a poloxamine-free TCC aqueous solution (pH 7.4) was not effective on Staphylococcus aureus, a 10% drug-containing T1307 system inhibited the bacterial growth to some extent. These results supported the release of the drug from the polymeric reservoir. However, as opposed to previous reports, overall findings indicated the limited intrinsic activity of TCC against the investigated pathogens. Chapter 6 – Generally, surfactants are used as stabilizers of interfaces or particles and their applications are very wide, from foams or adhesion modifiers to the orientation of chemical reactions. Siloxane surfactants are known for their ability to decrease the surface tension of liquids in such extent that is comparable only with some fluorinated compounds, which are thought to exhibit potential toxicological problems. On the other hand, polysiloxanes are unique by their set of properties, like for example low glass transition temperature, hydrophobic behavior, transparency to visible and UV light, high permeability to various gases (especially oxygen), physiological inertness, excellent blood compatibility (low interaction with plasma proteins). In addition, their chemistry is very versatile, and as a result, a very broad range of siloxane-organic compounds can be synthesized, including amphiphilic macromers or polymers. The most commonly known siloxane surfactants are the so called „silicone polyethers”, but other nonionic, as well as ionic surface active agents have been prepared and used over the years in cosmetics, textile conditioning, foam stabilization, coatings or agriculture. Recent developments in this research field and especially our experimental results on the synthesis, properties and applications of siloxane-containing surfactants will be reviewed. Our main interest is to propose new surfactants or alternative synthetic procedures, and new stabilization systems for polymeric nanoparticles. Carbohydrate modified (poly)siloxanes with different architectures have particularily been studied and tested, due to their biocompatibility and bioavailability. Chapter 7 - The ability of amphiphilic block copolymers to modulate multi drug resistance related processes has been demonstrated the first time more than 10 years ago. Nowadays, the efflux pump inhibitory activity of amphiphilic block copolymers is used in two main areas. First, to improve the transport of efflux pump substrates across the blood brain barrier (BBB) and second, in cancer therapy. It has been shown that in the presence of amphiphilic block copolymers higher concentrations of certain anticancer drugs, which are known as efflux pump substrates, can be found in the brain. Within the current chapter, recent developments in the field of amphiphilic block copolymer mediated efflux pump inhibition are discussed. Besides presenting data from in vitro and vivo studies, also the mechanisms involved in efflux pump inhibition are addressed. In addition, the influence of hydrophilicity/lipophilicity of various amphiphilic block copolymers as well as factors such as micelle formation on the efflux pump inhibitory activity are explained. Chapter 8 - The effects of physical aging near the surface region of glassy polymers are studied via the relaxations of (1) surface topographic features created by rubbing, and (2) the rubbing induced birefringence (RIB). Extensive experimental results are presented to show that physical aging processes that would have drastic effects on the relaxations of bulk polymers have little effects on the relaxations of rubbed surfaces. The authors also found that
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surface topographic features, such as ditches and ridges created by rubbing, relax at temperatures at about 20 °C below the bulk glass transition temperature of the polystyrene for the molecular weight of 442 kg/mol, even though the Laplace Pressure driving the relaxation is 1/500 of the yield limit. The relaxation of RIB in polystyrene (PS), its derivatives with modified side group, and polycarbonate (PC), involves only the length scale of the order of an individual segment. A phenomenological model based on individual birefringence elements is proposed for the RIB relaxation. The relaxation times (RT’s) of the elements are found to be independent of the thermal or stress history of the samples, either before or after the formation of the birefringence. The RT’s are also independent of the molecular weight, rubbing conditions, and film thickness, while the RT’s distribution function does depend on the molecular weight and rubbing conditions. The model provides quantitative interpretations that agree very well with all the reported experimental results, and sheds important light to the novel behaviors of the RIB relaxation. The absence of physical aging effects is probably due to the combined effects of small length scale of the RIB relaxation, and the accelerated aging speed in the near surface region. This is consistent with the mobility enhancement in the surface layer previously reported in the literature. Chapter 9 - Double hydrophilic block copolymers (DHBCs) constitute a novel class of water-soluble macromolecules with potential utilization in a wide range of applications. The exceptional combination of features, coming from their block copolymer structure and their ability to be stimuli responsive, establishes this class of copolymers as a core of intense research interest, aiming at elucidating aspects regarding their targeted synthesis, solution behavior and application possibilities. In this chapter, the current developments in the field of double hydrophilic block copolymers are discussed. In particular, synthetic strategies leading to the preparation of DHBCs are described. Moreover, their aqueous solution behavior is examined in respect to their ability to self assemble, due to changes in the solution temperature, and/or pH, as well as due to complexation. Additionally, the potential applications of DHBCs in mineralization processes, nanomedicine, nanotechnology and so on are mentioned. Finally, future perspectives in the field of DHBCs regarding general polymer science and nanotechnology issues, as well as open scientific questions, on synthesis and solution behavior of this class of materials, are also discussed. Chapter 10 - Polyesters are heterochain macromolecular substances characterized by the presence of carboxylate ester groups in the repeating units of their chains. Predominant in terms of volume and products value are those based on poly(ethylene terephthalate) (PET), long established as basis of fibers, films, molding plastics and containers for liquids, and poly(butylene terephthalate) (PBT) largely used to produce fibers as well as for special applications in motor and electric industry. Chapter 11 - Application of Hindered Amine Stabilizers (HAS) is the state-of-the-art approach to protection of carbon-chain polymers such as polyolefins and polystyrene or blends containing these against weathering. During outdoor exposure, the polymers loose their material properties due to solar radiation-triggered photooxidation. The complex mechanism of the stabilization involving cyclic oxidation-triggered transformation of HAS is outlined. Monitoring of the formation of the HAS-developed key transformation products, HAS-related nitroxides, responsible within the regenerative mechanism for the effective stabilization was used to confirm the heterogeneous character of photooxidation of two carbon-chain polymers, polypropylene and a specific polyethylene copolymer. Depth profiles of nitroxides were monitored in a long-term photooxidation regime using Electron Spin
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Resonance Imaging (ESRI) technique. The shape of concentration profiles of the nitroxides accumulated in the equilibrium state upon filtered Xenon lamp-equipped Weather-Ometer exposure was interpreted in terms of the oxygen diffusion limited oxidation and radiation penetration in oxidation-stressed polymer surfaces. The data indicate differences in the character of the heterogeneous process in dependence on the polymer matrix and on the used stabilizer system based on secondary HAS and O-alkylhydroxylamine HAS and/or HAS combination with UV absorbers. Imaging of nitroxides is a precise tool for marking heterogeneous oxidation of polyolefins. Chapter 12 - Polycyclic aromatic hydrocarbons (PAHs) are one of the most important, abundant and dangerous class of persistent organic contaminants found in air, water, food and waste sites. The great concern with this class of pollutants is due to the fact that these chemicals cause various deleterious effects, which could jeopardize the genome of exposed organisms or even lead them to death. Many studies have been conducted in several laboratories distributed around the world and with different organisms to quantify the impact of those pollutants and to diagnose their action mechanisms. Thus, there is a real necessity to compile the data already described in the literature, obtained from tests in situ, in vivo and in vitro, derived from simulation experiments, occupational exposure or environmental disasters, in order to produce a report with the main effects already described for that class of pollutant. This review article intends to address the following issues: toxicity, cytotoxicity, genotoxicity, mutagenicity, teratogenicity and carcinogenicity of PAHs, besides information about estrogenicity and metabolism of these substances. In this context, it will be also presented the test systems that better responded to the assessments of the PAHs effects, in an attempt to show the most sensitive and efficient organisms to detect the contamination of these agents. Chapter 13 - Polycyclic aromatic hydrocarbons (PAHs) are a group of about 100 different chemicals that are formed during incomplete combustion or heat-induced decomposition of organic matter. Humans can be exposed to PAHs through different routes. For non-smokers, the major route of exposure is from food with a minor contribution from inhaled air. Food can be contaminated from environmental sources, industrial food processing and from home food preparation. The European regulator has found that there is a low risk to consumers from PAHs in foods based on average dietary exposure but risk management action may be required for high dietary intake. Several PAHs have been shown to be genotoxic carcinogens. In 2002, the Scientific Committee on Food (SCF) reviewed PAH toxicity. For 15 compounds it concluded that there was clear evidence for their toxicity. In 2005, the Joint FAO/WHO Expert Committee on Food Additives (JECFA) performed a risk assessment on PAHs, and basically agreed with the SCF selection. Also the US Environmental Protection Agency has classified some PAHs as priority-pollutants based on toxicity, potential for human exposure, frequency of occurrence and the extent of information available. Seven of these PAHs (benzo(a)anthracene, chrysene, benzo(a)pyrene, benzo(b)fluoranthene, benzo(k)fluoranthene, dibenzo(a,h)anthracene and indeno(1,2,3-cd)pyrene) are considered as probable human carcinogens, but benzo(a)pyrene is the most studied and used as a marker for PAHs in ambient air and food. PAHs are themselves chemically inert but undergo metabolic activation in mammalian cells to diol-epoxides that bind covalently to cellular macromolecules, including DNA, causing errors in DNA replication and mutations that initiate the carcinogenic process. Other reactive intermediates can be generated by one-electron oxidation process that can result in chemically unstable alkylation of DNA, which leads to depurination,
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a potentially mutagenic event. In this chapter the most important effects of PAHs on animals and humans health are reported and discussed to define new possible preventive and regulatory programs for these pollutants. Chapter 14 - Polycyclic aromatic hydrocarbons (PAHs) are a group of organic compounds composed of fused aromatic rings. Some high molecular weight PAHs are considered to be carcinogenic, mutagenic, teratogenic even at low concentrations and have received more environmental concern. PAHs are resistant to biodegradation and can bioaccumulate in the environment due to their hydrophobic nature and low solubility. PAHs are ubiquitous environmental contaminants found in air, soil and water. The main sources of PAHs in the environment and for general population exposure are natural and intentional combustion of organic matter and petroleum manipulation. Atmospheric emissions will arise from the processing of coal and petroleum products and incomplete combustion during industrial processes. In the developing countries, present information concerning the sources and inputs of PAHs in these countries is rather limited. PAHs concentrations have been reported in a number of urban areas as well as in remote places and are consistently higher in winter than in summer, and in urban than in rural areas. The higher values are found in particular places like stack gases, tunnels of motorways, open burning sites, etc. There are no ambient air quality standards and no specific guideline value can be recommended for PAHs in air. PAHs in air are attached to particles, which may also play a role in their carcinogenicity. Although food is thought to be the major source of human exposure to PAHs, part of this contamination may arise from air pollution with PAHs. Exposure to PAHs mainly occurs from atmospheric pollution through inhalation, ingestion and dermal contact. PAHs half-lives in humans are in fact in the range of days/hours, and metabolism is responsible of the formation of carcinogenic metabolites. Risk assessments and potency assessments of various individual and complex mixtures of PAHs have been attempted. BaP is the only PAH for which a database is available, allowing a quantitative risk assessment. PAH potencies are used to determine quantitative health risks posed by PAH exposure. The risks posed by a mixture of PAHs are based on an assumption of additively of the individual risks posed by the PAHs. The objectives of this chapter are to: 1) report PAH concentrations in the environmental media in developing countries; 2) collate published and unpublished PAH information from environmental agencies and write a report on the state of knowledge of PAHs; 3) characterize the priority PAHs, carcinogenic risk assessment and potential health impacts. Chapter 15 – The authors have previously shown that benzo[a]pyrene used at extremely low dose can cause weight gain in mice and that the increase in adipose tissue mass is due to inhibition of β-adrenergic stimulation of lipolysis. Moreover they have suggested that adipose tissue is a reservoir for many carcinogens including Polycyclic Aromatic Hydrocarbons (PAHs).In this chapter, the authors demonstrate that the two C4 PAHs pyrene and phenanthrene can accumulate into adipocytes, but at the difference of benzo[a]pyrene have no impact on lipolysis.Since benzo[a]pyrene has been proved to be mutagenic while C4 PAHs are tumor promoters, they consider that a mixture of the three PAHs may act as a complete carcinogen. Furthermore, because adipose tissue can be induced by benzo[a]pyrene, is a reservoir for many PAHs and other carcinogens and can stimulate proliferation of normal or cancer cells, through the secretion of adipokines by adipocytes, they therefore propose that
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adipose tissue is a key contributor to chemical carcinogenesis by acting not only upon initiation and promotion but also upon progression. Chapter 16 - A review has provided the environmental sources and behavior of chlorinated 3- to 5-ring polycyclic aromatic hydrocarbons (ClPAHs). ClPAHs have been known to be ubiquitous contaminants, which can be found in urban air, vehicle exhaust gas, snow, tap water and sediments. The concentrations in the each environment are generally higher than those of chlorinated dioxins, although which are significantly lower than the concentrations of parent compounds, PAHs. Based on the environmental data and emission sources analysis for the ClPAHs, the dominant production process is by incineration of organic materials including chlorine like as the case of dioxins. On the other hand, treatment of PAHs with chlorine in water also produced ClPAHs. Indeed, such secondary reaction for ClPAHs generation could occur in the gaseous phase. For the biological effects, the toxicities such as mutagenicity and aryl hydrocarbon receptor activity of certain ClPAHs are emphasized than those of parent PAH, in which the toxicities of exposure to ClPAHs based on AhR activity were estimated to be approximately 30-50 times higher than that of dioxins. To investigate the sources and environmental behavior of ClPAHs is of great importance to assess human health risks. Chapter 17 - Polycyclic aromatic hydrocarbons (PAHs) are a large group of organic compounds with two or more fused aromatic rings. They have received increased attention in recent years in pollution studies because some of these compounds are highly carcinogenic or mutagenic. Some, while not carcinogenic, may act as synergists. These compounds are widespread in the environment. Their occurrence is largely a result of anthropogenic and natural emissions such as fossil fuel burning, motor vehicles, waste incinerators, oil refining, coke and asphalt production, aluminum production, etc. PAHs have relatively low solubility in water but are highly lipophilic. Most of the PAHs with low vapor pressure in the air are adsorbed on particles. When dissolved in water or adsorbed on particulate matter, PAHs can undergo photodecomposition when exposed to UV light. In the atmosphere, PAHs can react with many other pollutants such as ozone, nitrogen oxides and sulfur dioxide, yielding diones, nitro- and dinitro-PAHs, and sulfonic acids, respectively. PAHs may also be degraded by some microorganisms in soils. PAHs in soils/sediments can be taken up by biota, and some bio-magnification of these toxic compounds may occur as they pass through the food chain. The US Environmental Protection Agency (EPA) has identified 16 un-substituted PAHs as priority pollutants. The European Food Security Authority (EFSA) has classified 15 PAHs by their food toxicity. Thus, potential exposure to PAHs must be assessed by testing contaminated soils, sediments, air, water and foods, and estimating the degree of intake of each of these media into the human body, as well as the overall toxicity.There are several main problems associated with the determination of PAHs in environmental and food samples: their low concentrations, the complexity of the samples, and the necessity for fast and reliable analytical methods. To achieve successful results, many extraction-separation techniques have been developed depending on the nature of the sample. An in-depth description of the most commonly-used techniques, together with recent trends in analytical methods to monitor PAHs levels in different kind of samples, will be the main focus of this chapter. Among those that will be cited are Soxhlet extraction, liquid-liquid extraction (LLE), microwave-assisted extraction (MAE), ultrasound-assisted extraction (UAE), pressurized fluid extraction (PFE), supercritical fluid extraction (SFE), solid-phase extraction (SPE), and solid-phase microextraction (SPME), mainly in combination with chromatographic
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techniques (HPLC and GC). Moreover, the utilization of active or passive samplers in pollution monitoring, as well as the increased utilization of semi-permeable membrane passive samplers (SPMDs), will be described. Chapter 18 - Different configurations were studied for the screening analysis of common polycyclic aromatic hydrocarbons (PAHs) in environmental waters by liquid chromatography (LC). Analyte enrichment on a traping microcolumn connected by column switching to the analytical column, and in-valve in-tube solid-phase microextraction (IT-SPME) were tested as alternatives to conventional solid-phase extraction (SPE) with disposable cartridges. For separation conventional LC, capillary LC and LC with a monolithic column were tested and compared, whereas UV-diode array detection (DAD) or fluorescence detection were used for signal monitoring. The compounds assayed were naphthalene, anthracene, fluoranthene, benzo[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, dibenzo[a,h]anthracene and benzo[ghi]perylene. On the basis of the results obtained, new methods are presented for the on-line screening analysis of PAHs in several water matrices. The main advantages of the proposed procedures are: (i) the minimum off-line sample manipulation as the samples only need to be filtered; (ii) the rapidity, as the total analysis time can be reduced to less than 20 min, (iii) the improved detection limits, especially for the in-valve IT-SPME configurations, as they are compatible with the injection of very large sample volumes (up to 4.0 mL), and (iv) the total elimination of organic solvents in the sample treatment. Therefore, the proposed methods are environmentally friendly and costeffective alternatives for the routine monitoring of PAHs in waters. The occurrence of the tested PAHs in waters of The Comunidad Valenciana (Spain) was also tested. Chapter 19 - Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous hydrophobic organic contaminants in the environment. They tend to be associated with particles and are widely transported by flooding and atmospheric pathways, resulting in elevated concentrations in sediments/soils. Coal and coal-derived particles in natural sediments/soils can act not only as strong sinks for the PAHs, but also as very important sources of PAHs in sediments/soils. The understanding of the mobility of these contaminants from the sediments/soils, especially the sequestration of PAHs by coal and coal-derived particles is very important, because they can control the transportation, bioavailability, degradation and hence the potential risk of these contaminants in the environment. Chapter 20 - Polycyclic aromatic hydrocarbons (PAHs) are notably formed and emitted by incomplete combustion of coal, oil, petrol, wood, tobacco. In addition, they participate to soot inception. The remaining gaseous PAHs are then adsorbed onto the porous soot matrix so formed, conditioning physicochemical properties of these particles and consequently their impact on human health and environment. This has motivated various studies. However, an extensive analysis of the adsorbed PAHs onto soot particles is still challenging. The authors are developing an experimental set-up, based on the combined use of 3 techniques - laser desorption, multi-photon ionization and time-of-flight mass spectrometry, dedicated to this study with particular focus on the dependence of the adsorbed species on the stage of combustion and the nature of the fuel. The complexity of the first obtained mass spectra led us to successively investigate the laser desorption of pure PAHs then "synthetic" soot samples- obtained by adsorbing PAHs on various carbonaous substrates- to characterize the processes involved in our method, and finally combustion soot collected from various diffusion flames. They present an overview of these studies together with their first practical implications, obtained on soot collected from Diesel and Diesel surrogate flames.
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Chapter 21 - Benzo[a]pyrene (BaP) is one of the most well studied PAHs. It is a product of incomplete combustion of fossil fuel. BaP can be deposited into food, soil and water. The major route of human exposure to BaP is via food consumption. BaP can interact with DNA to form bulky adducts, or biotransformed to produce free radicals. Both processes may lead to mutation and cancer. Despite the clarity of its actions, it is still difficult to establish a clear dose-response relation between BaP and cancer in human. Currently, BaP is considered class 2A carcinogen by the International Agency for Research in Cancer (IARC), a chemical with well documented evidence as an animal carcinogen, but limited evidence as human carcinogen. It is believed that only by understanding the basic difference between animal and human biochemistry will it be possible to address the toxicity of BaP in the various biological systems. In the present chapter, experiments pertaining to studying the response to BaP in human and animal cells are discussed. Chapter 22 - Polycyclic aromatic hydrocarbons (PAHs) are well-known organic compounds derived from incomplete combustion of organic matters. PAHs have been widely studied in a broad field such as material science, industrial technologies and pharmaceutical sciences, as well known environmental pollutants, and an increasing particular concern has been paid to their adverse harmful effects to human due to the carcinogenic and mutagenic properties of many PAH species. Clearly, the concerns over adverse health effects of PAHs in human and wildlife provide the impetus for research on their sources, environmental fate and food chain transfer. High performance liquid chromatography (HPLC) is useful and conventional method to monitor PAHs. More than 60% of all HPLC separations are carried out under reversed-phase (RP) conditions and alkylated-silicas were successfully employed for the separation of mixtures of PAHs. In RP-HPLC chromatography, not only long alkyl chains but also aromatic compounds such as benzene, pyrene, and hetero aromatic selector such as vinylpyridine have been investigated for organic phase of stationary phase so far. However, development of new stationary phases has been required for separation of similar compounds such as structural isomers, positional isomers and optical isomers. In this chapter, the authors focus on the chromatographic analysis of PAHs by π-electron containing ordered and disordered polymeric organic phase-grafted silica. Polymeric phase-grafted silica stationary phases exhibits unexpected enhancement of separation for PAHs by a multipleinteraction mechanism, even if the polymer does not form any specific secondary structure. Poly(octadecylacrylate) and poly(4-vinylpyridine)-grafted mesoporous silica particles (SilODAn and Sil-VPn, respectively) will be described as typical examples of such stationary phases. Effects of grafting method and grafting-density of polymeric phases on the separation of PAHs in HPLC will be also discussed. Extensive chromatographic studies showed that selectivity for PAHs can be enhanced by multiple π-π interactions between the π-electron containing polymeric phases and the guest PAHs molecules. The presence of aligned carbonyl groups in a crystalline state of poly(octadecylacrylate) on silica significantly enhanced the selectivity for PAHs by π-π interaction mechanism. However, Sil-VPn recognizes molecular shape (planar and disk-like) of PAHs through π-π interactions between pyridyl groups of grafted-polymer and PAHs, along with electrostatic interactions involving a inductive effects and /or a quadrupolar effect in the normal phase HPLC. Comparison of the HPLC separations for PAHs with conventional octadecylsilylated silica (ODS) demonstrates the efficiencies and effectiveness of these π-electron containing polymeric organic phases grafted silica particles.
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Chapter 23 - The use of a direct extraction device (DED) for coupling to solid phase microextraction (SPME) is described. Such a device enables the introduction of the SPME fibre in the core of solid matrixes, allowing the analysis of volatile compounds, such as polycyclic aromatic hydrocarbons (PAHs), with no or little deterioration of the solid product. The feasibility of using SPME-DED for extraction of PAHs from solid matrixes mimicking solid foodstuffs (gelatine alone, gelatine containing fat and salt, or cooked chicken patties) and from commercial smoked meat products (smoked tenderloin, smoked jowl, smoked dry cured sausage and smoked potato sausage) was tested. Moreover, the diffusion of PAHs in solid matrixes by SPME-DED was also evaluated. SPME-DED appeared as a rapid, nondestructive technique for primary screening of low molecular weight (MW) PAHs in all solid matrixes assayed, as well as for the study of the diffusion process of PAHs within solid systems, without sampling and any laboratorial manipulation. Chapter 24 - Cyperus laxus and Cyperus ligularis are native plants growing at swamp highly contaminated with hydrocarbons. Anthracene and pyrene phytoremediation from a hydrocarbon mixture (hexadecane 50%; pristane 14%; anthracene 18% and pyrene 18%) by both species were evaluated. The aim of this chapter was to demonstrate the ability of two native plants to remove PAH from contaminated model soils under greenhouse controlled conditions. Experiments were carried out in a model system using perlite to simulate soil conditions. Initial hydrocarbon mixture concentration was 4 000 mg kg-1 of dry perlite for C. laxus and 1 000 mg kg-1 of dry perlite for C. ligularis. Plants were inoculated with hydrocarbon-degrader microorganisms. Root dry biomass, total microbial viable counts, hydrocarbon removal and hydrocarbon sorption by roots were determined during 150 days of culture. Root dry biomass from inoculated plants was 1.3 times higher than those from non inoculated plants. At the end of culture, microbial viable counts for all treatments were around 109 CFU g-1 of dry perlite for bacteria and 107 CFU g-1 of dry perlite for fungi. PAH and aliphatics were completely removed by C. ligularis during first 60 days of culture. C. laxus removed pyrene totally until 150 days of culture while anthracene only 75%. Hydrocarbons from C. ligularis roots were negligible. PAH sorption by roots was detected for C. laxus since 75 days of culture, 125 and 215 mg kg-1 of dry roots for pyrene and anthracene, respectively. At the end of culture, pyrene sorption was reduced (60 mg kg-1 of dry roots) possibly due to plant metabolism. For species, hydrocarbon removal and accumulation in roots, at 150 days of culture, was independent on the microbial inoculum added. C. laxus and C. ligularis were capable to remove efficiently anthracene and pyrene, these plants can be useful for phytoremediation of PAH contaminated soils. Chapter 25 - Polycyclic aromatic hydrocarbons (PAH) are widespread environmental pollutants of natural and anthropogenic origins. Despite their poor water solubility, they can be taken up and bioaccumulated by plants. The effect of PAH on selected plant (Festuca arundinacea and Bouteloua curtipendula) at early grown stage was evaluated under in vitro conditions. Plant species were cultured on Murashige and Skoog (MS) medium with three different PAH: phenanthrene (PHE), pyrene (PYR), and benzo[a]pyrene (BaP). PAH plant accumulation and distribution were evaluated with GC and HPLC. To date, number phytoremediation mechanisms of pollutants removal are known, including: (i) degradation (phyto- and rhizodegradation), (ii) removal (phytoextraction, phytovolatilization) and (iii) immobilization (phytoestabilization). The objective of this study was to identify the phytoremediation mechanisms of selected plants use to control and/or remove PAH from MS medium. To accomplish the task, plant samples, after 90 d culture, were collected and
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analyzed periodically to determine the residual PAH content in MS medium, roots and plants stems. F. arundinacea was able to transform 40.40% of initial PHE, accumulated 6.99% in stems and almost three times in roots (20.66%). With B. curtipendula, total removal was 66.14% and PHE was found in roots (14.38%) and stems (6.45%). Both species were able to remove and transform PYR, higher concentrations were found in roots than in stems. BaP outlined in a different pattern: both species were able to remove BaP from MS medium, B. curtipendula was more efficient than F. arundinacea 57.41% and 38.93%, respectively; bioaccumulation of BaP was the higher while transformation the lower. The dominant mechanisms observed for PHE and PYR in both plant species were phytodegradation and phytoextraction. B. curtipendula employed phytoextraction for BaP; it was able to bioaccumulate 45.05% in roots, 1.8 times more than F. arundinacea (24.99%). Several studies investigated the effect of plant-microbe interactions on the degradation of organic contaminants. In our study, plants were able to remove and transform PAH under sterile in vitro conditions, i.e. without microorganisms. Chapter 26 - Polysaccharides are complex carbohydrates made of chains of monosaccharide units linked by glycosidic bonds. Some polysaccharides, e.g. cellulose and chitin, are naturally occurring fibrous materials developed during cellular growth, while other polysaccharides are not capable of forming elongated structures in vivo because of their molecular structure or physicochemical environments. A variety of polysaccharides have been artificially spun into fibers, for instance, cellulose and its derivatives, chitin and chitosan, alginate, hyaluronic acid, pullulan, and dextran, as well as polysaccharide blends or blends with other materials. Ideal substitutes for synthetic fibers derived from fossil fuels, polysaccharides are renewable and exhibit biodegradability and biocompatibility, and often preferred to protein fibers (silks) in terms of their abundance and economics. This chapter reviews the literature on fibers spun from several polysaccharides and fiber spinning techniques, including both conventional fiber spinning methods and electrospinning. Resulting fiber properties and prospective applications are discussed. Chapter 27 - Starch-based biodegradable materials are considered one of the most promising candidates to replace certain types of conventional plastics. Starch is relatively cheap and available from a broad range of plants. However, the use of starch for packaging materials is restricted because of its sensitivity to water and inferior mechanical properties. The unique properties of nano-structured substances have opened windows of opportunity for the creation of high performance materials with a critical impact on starch-based materials. This chapter presents a review of ongoing research and development activities in our lab and in the wider scientific community related to starch-clay nanocomposites. As a valuable way to further enhance properties of these nanocomposites, blends involving biodegradable polymers such as poly (vinyl alcohol) are also discussed. Melt intercalation, using extrusion processing, has proved to be an efficient way for preparation of starch-based nanocomposites with improved thermal, mechanical, and barrier properties. The chapter also elaborates the mechanisms of clay exfoliation and dispersion in the starch matrix, which are based on both chemistry and processing effects. Improved interfacial compatibility of the multiphase system (starch, biodegradable polymer, plasticizers, nanofiller, etc.), as well optimized processing parameters, can lead to the design of nanocomposites with enhanced properties. A special emphasis is also given to an overview of mathematic models for some critical nanocomposite properties, such as barrier and mechanical properties. The establishment of a mathematic
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model provides guidelines for tailoring clay modification, clay dispersion and processing parameters for fabricating starch-based nanocomposites with unique properties. Chapter 28 - Chitosan, the only alkaline polysaccharide of β-1,4 linked N-acetylglucosamine and glucosamine,could be hydrolyzed by many non-specific enzymes such as cellulase, protease, and lipase, especially cellulase, which show high activity on chitosan. The hydrolytic mechanisms of these non-specific enzymes have been received growing attentions. The focus of this chapter was the characterizations and hydrolyzing mechanism of the non-specific enzymes toward chitosan choosing the three typical non- specific enzymes: cellulase, lipase and papain as objects. The enzymatic characteristics, purification, product analysis, glycoside bond cleavage, active sites and gene cloning of these enzymes to expatiate their non-specific hydrolysis mechanism were studied. From these, the authors obtained two bifunctional enzymes with chitosanolytic activity from commercial cellulase and lipase,respectively,and one chitosanase from papain.The three purified enzymes were the main reasons for the non-specific chitosanolytic hydrolysis of cellulase,lipase and papain, respectively. Moreover, It is identified that the bifunctional enzyme with chitosanolytic and cellulolytic activity(CCBE) from cellulase (T.viride) is identified as a cellobiohydrolase I with exo-β-D-glucosaminidase activity belonging to glycosyl hydrolysase 7 family.The enzmye with chitinase and chitosanase activity (CNBE)from lipase (A. oryzae) is the exo-β-D -glucosaminidase with N-acetyl-chitobiosidase activity belonging to glycosyl hydrolysase 18 family. Both of the two enzymes are novel and first reported in chitosanase families. Besides, the active sites and gene expression analysis of CCBE indicated that their dual activities originated from two distinct catalytic domains; while the two active sites overlapped partially. Chapter 29 - In the first part the preparation of CD polymers is described and examples of CD polymers useful as drug carriers are shown. It is pointed out that CD polymer nanoparticles are promising for application in various biomedical and biotechnological applications, e.g. in drug delivery or enzyme immobilization. A special attention was paid to removal of organic dyes from industrial wastewaters by β-CD polymers, having in view the wide use of dyes in textile and cosmetic plants. The preparation of β-CD copolymers with carbon nanotubes is described in the aspect of their application for removal of p-nitrophenol and trichloroethylene from water. It was pointed out that ceramic membranes impregnated with crosslinked silylated CD polymers are promising for removal of polycyclic aromatic hydrocarbons from water. In recent years the importance of imprinted CD polymers is growing; as an example of their application the recognition of oligopeptide structures is presented. Membranes based on CD polymers are often used today in separation processes; an increasing attention is paid now especially to chiral separation processes. In this aspect the use of chitosan/CD composite membranes in chiral separation of tryptophan was described. In the final part the selected examples of supramolecular architectures based on CD polymers are presented having in view the rapid development of supramolecular chemistry. Chapter 30 - Moisture, oxygen, carbon dioxide, lipid, flavor and/or aroma transfer between food components or between foods and their surrounding environment can provoke deterioration of food texture, flavour, color, aroma or nutritional values which results in food quality loss. Regulating the mass transfer in food systems by edible films and coatings can increase food-product shelf life and food quality. Besides their barrier properties, edible films and coatings can act as carriers for functional food additives, antioxidants, antimicrobial
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agents and nutrients; and due to their biodegradability nature, could have an impact on overall packaging requirements. Edible films and coatings are produced from edible biopolymers and food-grade additives. Film-forming biopolymers can be proteins, polysaccharides (carbohydrates and gums) or lipids. Plasticizers and other additives are combined with the film-forming biopolymers to modify the physical properties or functionality of films. The composition of the film must be chosen according to specific food applications, the type of food products and the major mechanisms of quality deterioration. Polysaccharide films and coatings are used to extend the shelf life of fruits, vegetables, seafood, meats and confectionary products by preventing dehydration, oxidation rancidity, surface browning and oil diffusion; and in some specific cases can improve the physicochemical, nutritional and sensorial properties of the products. The common poly saccharides used for edible films are: starches and their derivatives; cellulose and its derivatives; seaweed extracts; gums; pectins and chitosan. The objectives of this chapter are to (a) review research on polysaccharide film-formation and characteristics, (b) analyze mechanical and barrier properties (water vapour permeability, gas permeabilities and volatile permeability) of polysaccharide-based films, (c) summarize applications of polysaccharide films in food products, and (d) make conclusions as to the status of polysaccharide films and their future developmental direction. Chapter 31 - Carbohydrate-active enzymes (CAZymes) are associated to the synthesis and breakdown of complex carbohydrates and glycoconjugates. CAZymes, beside the catalytic domain (CD), usually present a substrate-binding module named carbohydratebinding module (CBM), which has independent fold and function. Nearly 7% of the CAZymes contain at least one CBM module. Presently, 58 CBMs families are described in the CAZy database (http://www.cazy.org/), presenting considerable heterogeneity in binding specificity, towards crystalline, amorphous and soluble polysaccharides, both between and within the families. CBMs are known to potentiate the activity of many enzymes, by targeting and promoting a prolonged interaction with the substrate. Since CBMs are functional and structurally independent of the other protein modules, several applications have been described using CBMs obtained by enzyme proteolysis or by DNA recombinant technology. The present revision focus on recent developments on CBMs applications in the biomedical, biological and biotechnological fields. Chapter 32 - Natural cellulose fibers are high-molecular polymers that are strongly susceptible to microbial degradation. Fungal and bacterial attack on the fibers cause depolymerization of cellulose macromolecules, which is reflected in decreased molecular weight and strength, increased solubility and a changed crystallinity. Although biodegradable textile fibers are classified as environmentally-friendly materials, the biodegradation process that occurs when the textile product is still in use could cause serious functional, aesthetic and hygienic problems because of textile deterioration, staining, discoloration and odor. To protect the cellulose against biodegradation, chemical modification is of great importance. In this research, therefore, fibers were chemically modified by the use of an antimicrobial finish on the basis of AgCl (AG) in combination with a reactive organic-inorganic binder (RB), water and oil repellent finishes on the basis of fluoroalkylfunctional siloxane (FAS), perfluorooctyltriethoxysilane (PFOTES) in combination with di-ureapropyltriethoxysilane [bis(aminopropyl)terminated-polydimethylsiloxane (1000) (PDMSU) and aminopropylperfluoroisooctyl polyhedral oligomeric silsesquioxane (AP2PF2IO4 POSS), as well as with an
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easy-care and durable press finish on the basis of imidasolidinone (DMeDHEU). Moreover, it was expected that these finishes would provide the active or passive antimicrobial properties of the modified fibers. Biodegradation of the finished cellulose fibers was carried out by the soil burial test according to SIST EN ISO 11721-2:2003. The chemical and functional properties of the fibers were determined by means of DP, SEM, AFM, FT-IR, XPS, ICP-MS, appropriate microbiological tests and static contact angle measurements of different liquids on the fiber surface. It can be seen from the study that AgCl embedded in RB imparts active antibacterial and antifungal properties to the coated fibers resulting in excellent microbial reduction and, consequently, strong inhibition of biodegradation. The presence of FAS, PFOTES-PDMSU and AP2PF2IO4 POSS coatings, which cause an increase in fiber hydrophobicity, as well as a high drop in surface free energy, results in decreased adhesion of microorganisms and impaired conditions of their growth. In the modification process of cellulose fibers by DMeDHEU, the formation of covalent bonds between the finish and the cellulose macromolecules strengthens the less ordered amorphous regions, resulting in a decrease of fiber swelling. This inhibits the penetration of microorganisms into the fibers, where biodegradation takes place. By applying a combination of FAS and DMeDHEU finishes, a synergistic action of the components is obtained in the coating. In this case, the protective properties of the coating against biodegradation are much higher than those obtained by single component finishing. Chapter 33 - Oligosaccharides present specific physicochemical and biological properties that can be exploited for specific applications in foods and pharmacology. They can be produced through a number of different physical, chemical and enzymatic catalysed reactions from their parent polysaccharides as well as through transglycosylation reactions. This chapter examines the pathways leading to the production of bioactive oligosaccharides that have biotechnological applications. These carbohydrate oligomers constitute a nutritional type of “fiber” that benefits the growth of bifidobacteria and lactobacilli in the colon promoting human health and well-being. The use of oligosaccharides to modify biological responses was recently reported, and this has included their effects as anti-inflammatory and anti-cholesterolaemic stimulating compounds. An overview of nutraceutical and biological functions of these carbohydrate fragments mainly for human health is also reported. Chapter 34 - Native cellulose is a structural material that is biosynthesized as microfibrils by a number of living organisms ranging from higher and lower plants, to some ameobae, sea animals, bacteria and fungi. Depending on their origin, individual cellulose microfibrils have diameters from 2 to 20 nm, while their length can reach several tens of microns. The chemical modification of cellulose microfibrils is investigated for preparing new bio-based materials with end-use properties in the fields of adhesion, textile, detergent, paint, cosmetic, medicine, food, etc. Among the possible chemical modifications, the selective oxidation of the primary alcohol group of polysaccharides has been studied for more than a half-century. Recently, a method for selectively oxidizing primary alcohol groups of polysaccharides has been described in literature without degradation of products. The technique is based on a reaction catalyzed by 2,2,6,6-tetramethyl-1-piperidine oxoammonium radical (TEMPO) in presence of NaOBr, generated in situ by NaOCl and NaBr, the catalyst being regenerated during the reaction. The chemical modification is a way to modify and introduce specific functionalities leading to the development of new biopolymers in macromolecular prodrug carrier, bio-based composites, nanocomposites, for example. The polymer must be biodegradable and / or biocompatible and must contain appropriate functional sites for chemical conjugation.
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Despites on large interest on natural and synthetic biodegradable polymers investigated, important efforts are continuing to search for new systems, notably on cellulose. In this report, the amidation of cellulose materials previously modified by carboxylation reaction is realized from the selection of some amines (cyclic and linear structures). The carboxylation is resulted from the TEMPO mediated oxidation of cellulose, leading to partially or totally oxidized cellulose, in presence of carbodiimide which is known to increase the reactivity of carboxyl groups toward amidation but used rarely for polysaccharides holding carboxyl moieties. Few reports were found in the literature on the use of carbodiimide in the preparation of cellulose conjugates with amines in order to develop new modified cellulose materials. The goals of this report are to develop ways of preparing the cellulose conjugates which can be water-soluble materials or water-insoluble materials, to identify linkage with carboxylated cellulose materials through amide bonds and to understand the obtained results following by FT-IR, NMR spectroscopies (at liquid and solid states) and electron paramagnetic resonance spectroscopy. The carboxyl groups content of oxidized cellulose materials after carboxylation and after amidation reactions are equally determined by titration curves of conductimetry and elemental analysis. Chapter 35 - Several points concerning the presence in red seaweeds of galactans with cooccurrence of carrageenan and agaran structures (DL-hybrid galactans) are addressed: a.
Analysis of D- and L-galactose enantiomers and their methylated and cyclic derivatives. Spectroscopic, enzymatic and chemical methodologies. b. Systems of galactans, in which the presence of D- and L-galactose has been determined (DL-galactan systems). c. The possible deviant structures of carrageenans and agarans in DL-hybrid galactans. d. The isolation of “pure” carrageenans and agarans from DL-galactan systems. e. The formation of molecular complexes between carrageenans, agarans and carrageenans/agarans. f. DL-hybrid galactans or molecular complexes: Evidences in favor of each hypothesis.
Chapter 36 - In the past half century, synthetic petroleum-based polymers have been widely used in a variety of packaging materials but have become a major source of waste disposal problems due to their poor biodegradability. With the increasing demand by consumers for high-quality foods and concerns about limited natural resources and the environment, the use of renewable resources to produce edible and biodegradable packaging materials that can improve product quality and reduce waste disposal problems are being explored. Biopolymer-based edible films and coatings are intended to function as barriers against moisture, oxygen, flavor, aroma and oil, as well as carriers of additives thereby improving food quality and enhancing the shelf life of food products. Water-soluble polysaccharides are commonly used in food related applications as thickeners due to their increase in viscosity when hydrated. Polysaccharides can also constitute edible films, an important quality since these polymers are of natural origin and come from renewable sources and are biodegradable as well. Though it is recognized that polysaccharide edible films are not good barriers against water vapour, they can find interesting applications as food interfaces, as carriers of active compounds or preservatives, constituting delivery systems with local activity.
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The present chapter deals with the study of edible films based on deacylated and/or acylated forms of gellan gum to support L-(+)-ascorbic acid (AA) in view of natural antioxidant protection of foods, by leveraging its activity as a vitamin in the human metabolism. Kinetics of AA-destruction and subsequent non enzymatic browning development were studied in the films stored at constant temperature (25ºC) and relative humidity (33.3, 57.7 or 75.2%) and their relationship with the microstructure, at the macromolecular and molecular levels, was also analyzed with the purpose of ensuring a better AA retention as well as lower browning rate as a consequence of controlled water mobility in the polymeric networks. Chapter 37 - Microencapsulation by coacervation is a common method for microcapsules production. It can be achieved by employing different methods, where the most common one is formation of an insoluble complex of two oppositely charged polymers and its subsequent deposition at surface of dispersed particles (e.g. emulsified oil droplets). In this way, microcapsules with coacervate shell are formed. Composition and microstructure of the coacervate shell are key to determine properties and application of microcapsules. In this chapter, novel method for microencapsulation by coacervation is presented. The method employs polymer-polymer incompatibility taking place in a ternary system composed of sodium carboxymethyl cellulose (NaCMC), hydroxypropylmethyl cellulose (HPMC), and sodium dodecylsulfate (SDS). In the ternary system, various interactions between HPMCNaCMC, HPMC-SDS and NaCMC-(HPMC-SDS) take place. The interactions were investigated by carrying out detailed conductometric, tensiometric, turbidimetric, viscosimetric, and rheological study. The interactions may result in coacervate formation as a result of incompatibility between NaCMC molecules and HPMC/SDS complex, where the ternary system phase separates in HPMC/SDS complex rich coacervate and NaCMC rich equilibrium solution. By tuning the interactions in the ternary system coacervate of controlled rheological properties was obtained. Thus obtained coacervate was deposited at the surface of dispersed oil droplets in emulsion, and oil-content microcapsules with a coacervate shell of different properties were obtained. Formation mechanism and stability of the coacervate shell, as well as stability of emulsions depend on HPMC-NaCMC-SDS interaction. Emulsions stabilized with coacervate of different properties were spray dried and powder of microcapsules was obtained. Dispersion properties of microcapsules, and microencapsulation efficiency were investigated and found to depend on both properties of deposited coacervate and the encapsulated oil type. Chapter 38 - Natural polysaccharides are produced by animals, plants or microorganisms; a large variety of chemical structures is found and as a consequence a large variety of physical properties is available. This chapter focuses on water soluble polysaccharides from different origins; their chemical structures are given as well as their macromolecular characteristics. The selected polysaccharides presented in the following are: alginates from seaweeds, gellan and xanthan from bacteria, galactomannans from seeds, chitin and chitosan from crustaceous shells. Galactomannans, xanthan and chitosans are thickening linear polysaccharides; the first one is neutral, xanthan is an anionic polysaccharide and chitosan is a cationic polymer in acidic conditions. The main physical properties of these different polysaccharides in dependence with the experimental conditions (temperature, ionic concentrations, nature of counterions…) are described. Considering alginates and gellan, a specific behaviour is
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observed: physical gelation for which the mechanisms of gelation are discussed in relation with thermodynamic conditions. The main applications of these different polysaccharides are described in different domains. At the end, mixtures of two of these polymers are considered: polyelectrolyte complexes formed between alginate and chitosan; cooperative interaction between galactomannans and xanthan. These mixtures open the way to new applications. Chapter 39 - Nutraceuticals are food or food ingredient that provides medical or health benefits. Carbohydrates are one of the most abundant bioactive substances in the marine organisms. Many marine organisms produce carbohydrates with diverse applications due to biofunctional properties. A great deal of research has been conducted in order to assess the possible use of these marine carbohydrates. The marine carbohydrates including glucosamine glycon, chitin, chitosan, fucoidan, carageenan and alginic acid have a host of bioactivities such as antioxidative, antibacterial, antiviral, antitumor, immunostimulatory and anticoagulant. Moreover, these carbohydrates have many beneficial effects and hence could be developed into potential nutraceuticals. This review describes the current progress in the development of marine carbohydrates into nutraceuticals and their applications. Chapter 40 - The present chapter intends to focus mainly on applications of various carbohydrate polymers such as aloe vera, chitosan, guar gum, starch etc. to design controlled delivery formulations for release of a variety of bioactive agents such as low and high molecular weights drugs and pharmaceutical compounds, agrochemicals such as pesticides, insecticides etc. The chapter also presents a critical review of various studies carried out over recent past particularly in the fields of controlled release technologies using carbohydrate polymers as carriers. A mention of the current state-of-the-art of carbohydrate polymers in biomedical, pharmaceutical and agricultural fields has also been presented. Chapter 41 - Chitin, the second most abundant structural biopolymer, has a complicated, multi-level supermolecular construction. All chitins are developed from superfine fibrils having diameters in the nano to micro scales, and each nanofibril contains ordered nanocrystallites inserted into low-ordered non crystalline domains. Acid hydrolysis can be used to dissolve away regions of the marginal domains so that the water-insoluble, highly crystalline residue can be converted into a stable suspension by vigorous mechanical shearing action. Highly crystalline chitin nanofibrils, otherwise called ‘whiskers’, exhibit enormous surface improvement that allows them to impart strength to several materials such as poly(caprolactone), soy proteins, natural rubber, poly(vinyl alcohol), chitosan, and silk fibroin. Apart from their ability to improve mechanical integrity, chitin nanofibrils may interact well with enzymes, platelets, growth factors, and other cell compounds existing in living tissues. Thus, the wound recovery ability and the ability to induce the formation of granulation tissues are obtained so that the applicability of chitin nanofibrils in medical areas is supported. In this chapter, the structural building principles and nanofibril structure of chitin from various sources and the methods of preparation of nanofibrillated chitin are discussed. Biological activities, as well as attempts to modify chitin nanofibril, are subsequently described. Chapter 42 - In the course of the work on value addition of Indian seaweeds in the laboratory, modifications of seaweed polysaccharides by various methods such as grafting of these carbohydrate polymers with different substrates, blending and cross linking were embarked upon. This has resulted in hydrogel materials exhibiting improved properties and
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functions. In this article, our earlier works on modification of seaweeds using grafting as well as cross linking with non-toxic genipin, the naturally occurring cross linker, will be discussed including those recently carried out with the polysaccharides of the seaweed species Chetomorpha antennina, Chamaedoris auriculata, Grateloupia indica and Sargassum wightii of Indian waters. Galactans, arabinogalatan, rhamno-galacto-ribo-arabinan, polyuronates and other heteropolymers as well as polysaccharide blends were cross linked with genipin in a water-based eco-friendly process yielding new polymeric hydrogel materials. Galactanpolyuronic acid blend was grafted with a polymeric substrate to yield a robust hydrogel. It appears that the genipin cross linked polymeric products with improved properties are best prepared with homopolymers, the heteropolymers are comparatively less amicable to this modification strategy, which produces remarkably improved functional effects. Polysaccharide blends are also a good starting material of choice for the modification work. The cross linking reaction brings about a significant makeover in the chiroptical properties of the polysaccharides. The modified products exhibited considerable thermal stability and pHresponsiveness in aqueous media. All these new properties predispose these modified products to potential applications in various domains including ingestible and non-ingestible ones. Chapter 43 - During the last quinquennium, chitosan has suddenly gained importance as a plasmid vector to be used in gene therapy in alternative to unsafe viral vectors. The main characteristics that qualify chitosan for this role are its spontaneous reactivity with DNA and oligonucleotides in general, the protection of DNA against enzymatic destruction by nucleases, the slow release of DNA, the capacity to penetrate into tissues by opening tight junctions of cells, besides its full biocompatibility with human cells and its safe assimilation in the body. The chitosan chemistry has been refined in order to impart to chitosan better solubility in physiological media, by acting on its cationicity and hydrophilicity / hydro phobicity, or combining it with poly(ethyleneimine), polyethylene glycol and derivatives, or conjugating it with cell-specific ligands. The transfection efficacy of said modified chitosans was greatly improved compared to plain chitosan. In order to understand the fate of internalized vector-DNA complexes within cells, fluorophores including the green fluorescent protein have been conjugated with DNA or with chitosan. After the exceptionally good performances of chitosans as functional wound dressing materials and drug carriers, it appears that the involvement of chitins and chitosans in gene therapy will develop into an additional exceptional achievement for this class of polysaccharides. Chapter 44 - Extreme environments, generally characterized by atypical temperatures, pH, pressure, salinity, toxicity and radiation levels, are inhabited by various microorganisms specifically adapted to these particular conditions. These microorganisms, called extre mophiles, are of significant biotechnological importance as their enzymes (extremozymes) and biopolymers possess unique properties that offer insights into their biology and evolution. The enthusiastic search for novel extremophiles has largely been stimulated by the uniqueness of their survival mechanisms. This uniqueness can be transformed into valuable applications ranging from wastewater treatment to the diagnosis of infectious and genetic diseases. One adaptation strategy of particular importance to extremophiles is the production of extracellular polymeric substances (EPSs) that envelop the cell as a barrier protecting them against environmental extremes such as desiccation, temperature, pressure, salinity, acidity, heavy metals, and radiation. Due to their many interesting physicochemical and rheological properties, these biopolymers possess novel functionality that is generally superior to
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petrochemical-derived polymers in aspects that embrace biodegradability, and environmental and human compatibility. Consequently, biopolymers of extremophiles are widely used in foods, cosmetics, pharmaceutical products, textiles, detergents, adhesives, oil-recovery from wells, brewing and waste treatment processes. This chapter presents a brief overview of life under extreme environmental conditions. This is followed by a discussion of extremophilic microorganisms and their adaptation mechanisms, and specifically focuses on the production of EPSs and their ecological and physiological functions. The application areas of industrially important EPSs from various extremophilic producer strains are also mentioned. Chapter 45 - Naturally occurring polysaccharides are of interest in pharmaceutical and cosmetic applications due to their biodegradability, biocompatibility and non-toxicity. Among these polysaccharides, alginate, a linear polysaccharide that is extracted from marine brown algae, is widely used as a particulate carrier for encapsulation, controlled release and delivery of a variety of bioactive compounds to target organs. Due to its bioadhesive, anionic and colloidal properties, alginate can interact with divalent cations or cationic polymers to form hydrogel micro- or nanoparticles that have specificity for mucosal tissues. A particular area of interest is the use of alginate as a shell or matrix material for encapsulation of essential oils. These oils have therapeutic properties that include antibacterial, antifungal, antiviral, antioxidant, and insect-repellent effects, but are insoluble in water, volatile and unstable in certain environments. This has limited development of new formulations and encapsulation of the oil in carriers is required to overcome these problems. Size reduction of the carriers to the nanometer scale greatly improves the efficacy of the delivery system and may allow targeting of the pharmacological activity of the oil to specific organs. This review focuses on an overview of alginate and essential oils, the preparation of alginate matrices from large to nanometer scale, and encapsulation of essential oils in these matrices. The prospects for development of improved alginate nanoparticles as delivery systems for essential oils are discussed at the end of the chapter. Chapter 46 - Wastes from the food processing industry have some common characteristics such as large amounts of organic materials: proteins, carbohydrates and lipids. The great production of processed vegetable products is generating an increase of the amount of wastes which represent a valuable source of by products such as carbohydrate polymers. Utilization of by-products may contribute to the efficiency of the processing and also to the sustainability of the environment. Fruits and vegetables are important sources of dietary fiber although their content is lower than the one of cereals. The cell wall matrix is the major fiber source in this type of foods and these fibers are rich in pectins. The carbohydrate composition, molecular weight and physiological properties of these fibers might be influenced by conditions of extraction as well as sources, location and many other environmental factors. The object of this research was the study of products enriched in carbohydrate polymers and obtained from quince (Cydonia oblonga Miller) or red beet (Beta vulgaris L. var. conditiva). Different methods of obtention which involved dehydration, acid and/or ethanol treatment were assayed and yield, chemical composition, molecular weight, physiological and technological properties of isolated products were stated. Chapter 47 - In recent years, interesting works have been focused on the amino polysaccharide chitosan-based microspheres. The chitosan microspheres have been applied in diversified fields, such as food processing, water treatment and biological fields. To bring new or improved properties, the chemical modification of chitosan-based microspheres is of
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rapidly growing interest. After chemical modification, chitosan-based microspheres can be used as functional materials with modified properties, such as adsorption resins, magnetic resins and nanocomposites, for specific end use in food industry. This paper presents a review of the developments in the preparation and applications of chitosan-based microspheres. The main objective of this review is to provide recent information and to show the development trend about chitosan-based microspheres. Chapter 48 – This chapter reviews the synthesis of amylose-grafted polymeric materials with well-defined structures by chemoenzymatic method, which is the combination of following chemical with enzymatic reactions. As a chemical reaction for the present chemoenzymatic method, introduction of a maltooligosaccharide as a primer, which is an initiating point for the amylose-forming polymerization, to polymer backbones or polymerizable groups is carried out to produce maltooligosaccharide-grafted polymeric materials or macromonomers having maltooligosaccharide components. As an enzymatic reaction, formation of amylose is performed by phosphorylase-catalyzed enzymatic poly merization using α-D-glucose-1-phosphate as a monomer. This enzymatic polymerization is initiated from a meltooligosaccharide as a primer and the propagation proceeds by the reversible reaction to produce amylose. The main-chain backbones of the amylose-grafted polymeric materials describing in this chapter are polystyrene and polyacetylene as organic synthetic polymers, polydimethylsiloxane and silica gel as inorganic materials, and chitin, chitosan, cellulose, and polypeptide as biopolymers. Chapter 49 - Polysaccharides are one of essential biomacromolecules in nature, which play important roles in life activities. Some of them exhibit various biological activities or have special functions. Moreover, polysaccharides contain reactive groups, such as hydroxyl, amino and carboxylic acid groups. Thus, they may react with some compounds to produce polysaccharide derivatives. Amphiphilic polysaccharide derivatives can self-assemble into stable micelles in aqueous solution, promising as a new carrier for the controlled release of drugs. In order to overcome various barriers which hinder drug delivery, the drug molecules may be conjugated with polysaccharide chains through chemical bonds to form prodrugs. The parent drug can be released through an enzymatic or chemical transformation during the metabolizing process. This article reviews the investigations of bioactive and functional polysaccharides, amphiphlic polysaccharide derivatives and polysaccharide-based prodrugs for biomedical applications. Chapter 50 - Maize bran and nejayote (a maize processing waste water) generated from tortilla-making industries in Mexico were investigated as source of water soluble feruloylated arabinoxylans. The tortilla industry is important in Mexico since half of the total volume of consumed food is maize. Therefore, maize residues are potential source of these added-value biomolecules as hydrocolloids for the food industry, considering the volume they represent. Both by-products were treated for these polysaccharides extraction. Because of their different nature, the polymers extracted from each byproduct showed different composition, physicochemical and functional properties. Feruloylated arabinoxylans from maize bran (FAXMB) presented an arabinoxylan content of 74% (w/w), a ferulic acid content of 0.34 μg/mg FAXMB, an arabinose to xylose ratio (A/X) of 0.85, an intrinsic viscosity [η] of 208 mL/g and a molecular weight (Mw) of 190 kDa. Gels were obtained from this FAXMB by laccase covalent cross-linking of ferulic acid leading to the formation of diferulic (di-FA) and triferulic (tri-FA) acid. Gels elasticity increased from 9 to 14 Pa as the FAXMB concentration changed from 2.5 to 3.5 % (w/v), while the di-FA and tri-FA contents remained
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constant (0.030 and 0.015 μg/mg FAXMB, respectively). On the other hand, feruloylated arabinoxylans extracted from nejayote (FAXN) showed a ferulic acid content of 0.23 μg/mg, an A/X ratio of 0.65, an [η] of 183 mL/g and a Mw of 60 kDa. Gels were obtained from 4% (w/v) FAXN solution, while no gelation was observed at lower concentrations. Gels elasticity increased from 2 to 4 Pa, as the FAXN concentration increased from 4 to 8 % (w/v), while no change in the di-FA and tri-FA contents was registered (0.020 and 0.010 μg/mg FAXN, respectively). The entrapment of insulin or β-lactoglobulin at 0.1% (w/v) in gels at 3.5 % (w/v) in FAXMB was investigated as well. Insulin and β-lacto-globulin did not modify either the gel elasticity or the cross-links content. Protein release rate from gels was dependent upon their molecular weight. The apparent diffusion coefficient was 0.95 × 10-7 and 0.74 × 10-7 cm2/s for insulin (5 kDa) and β-lactoglobulin (18 kDa), respectively. The results suggest that FAXMB gels can be potential candidates for the controlled release of proteins. Furthermore, recovery of these gums from low-value maize by-products could represent a commercial advantage in comparison to other gums commonly used. Chapter 51 - There are a number of reports on the use of polarized light to cure severe burns, heal wounds, and treat eczema psoriasis. Illumination with polarized light increases immunological response in peripheral blood. Amazing applications of polarized light have been recently extended to technically important transformations of polysaccharides. Visible light, particularly linearly polarized visible light (LPVL), has been found useful in transformations of polysaccharides, starches of various botanical origin, such as xylan, chitin, chitosan, and cellulose. There are two features of the application of that source of energy. Thus, on illumination of aqueous suspension of polysaccharides with LPVL the side chains of branched polysaccharides have initially been split, however, on prolonged illumination repolymerization of the split side chains into linear polysaccharide took place. Response of the polysaccharide to the illumination with LPVL depended on crystallinity of the polysaccharides. The effect of the illumination has been stronger in highly crystalline polysaccharide networks. Illumination of polysaccharides with red and green LPVL showed certain differences in the action of those wavelengths. Green light stimulated depolymerization of the polysaccharide to a higher degree with no effect on repolymerization of split fragments whereas red light was slightly more efficient in repolymerization than in depolymerization. LPVL has been capable of stimulation of hydrolases and cyclases providing acceleration of the enzymatic hydrolysis of amylopectin, chitin, chitosan, xylan and cellulose with relevant enzymes and higher yield of the products of hydrolysis. Depending on polysaccharide and relevant enzyme used, effect of the stimulation with white non-polarized light could be observed but effect of such illumination was always much weaker compared to the effect of LPVL. Illumination of cyclase with LPVL resulted in production α-, β- and γ-cyclodextrins with higher yield and different proportion of three products compared to these available from the process run without stimulation of the enzyme with LPVL. Stimulation of enzymes with LPVL presents an essential potential in practical applications. It is beneficial to perform such 1 -2 hour stimulation in a small reaction vessel followed by the application of the enzyme into bioreactor without any further illumination. Chapter 52 - It is directly observed a variety of morphologies of the cereal starch nanostructural units after the heating gelatinization by using atomic force microscope in the atmosphere. According to the observation results, nano-structural units of starch grains can be
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classified into spherical, ball chain, branched, cyclic, rod-shaped form and so on. On this basis, the authors observed the starch nano-structural units of high-pressure gelatinization, and compared the topology structure differences of cereal starch nano-structural units under different conditions of gelatinization in nano-scaled. The results showed: cereal starch nano-structural units is intermediates which are composed of natural starch granules, and their topology morphology is not the simple "chain" connections of glucose 1-4 and 1-6 glycosidic bond speculated by classic study, they have more complex composition ways, structural level. Cereal starch of High-pressure gelatinization compared with that heating gelatinization, have different nano-structure patterns; and the same germplasm starch also have different nano-structure patterns after different pressure treatment; therefore, it can be made different gelatinization characteristics of starch nano-materials by different gelatinization methods. Chapter 53 - Polysaccharides are being used in food and allied industries as texture improving agents, stabilizers, for the preparation of the edible films etc. Plant and seaweed derived polysaccharides suffer lack of assured supply and variations in quality. Microbial exopolysaccharides (EPS) provide a valid alternative. An EPS producing lactic acid bacterium was isolated from cabbage and identified as Lactobacillus sp. CFR-2180. Production of 22 g/L of EPS in 24 h at 30°C was noticed. The EPS had 65% total carbohydrates, 0.7% protein, 10% uronic acid and 2.0% moisture. Analysis by gas chromatography revealed that the EPS is a heteropolysaccharide with the presence of mannose, galactose and glucose in a ratio of 1:7:5, respectively. Gel permeation chromatography and HPLC analysis of the EPS indicated presence of multiple peaks with molecular weight ranging from 1.8 x 104 to 2.5 x 106 Da., confirming the heterogeneity of the EPS. The results of the preliminary characterization of the EPS indicated that it is a new EPS that has not been reported earlier. Chapter 54 - The article presents an overview of the latest advances in investigations about the development of polysaccharide-based intelligent antitumor drug delivery. Biodegradable, non-toxic and stimuli sensitivity nature of many kinds of polysaccharide encourages its potential use as a carrier for drug delivery system. Besides, it is reported to possess many biological activities, such as antiviral, antitumor, antimicrobial and anticoagulant activities. The innovative idea of incorporating polysaccharide to “intelligent delivery” had motivated the development of nano drug carriers with a triggered release in response to specific external or internal stimuli. Especially, acidic changes are characteristic signals for treating solid-tumors, since the relatively acidic extracellular pH is a distinguishing phenotype of solid tumors from surrounding normal tissues, and more acidic conditions are also encountered in endosomes once the micelle or nanoparticle enters the cells via endocytosis pathways. Base on this idea, quite a few intelligent polysaccharides based polymeric nanocarriers have been exploited to date. This paper summarized innovative ideas polysaccharide-based intelligent antitumor drug delivery, such as chitosan based pH-sensitive micelle systems loaded Paclitaxel, chitosan-alginate multilayer microcapsules as drug delivery vehicle, cholesteryl-bearing pullulans nanogels trapping hydrophobic molecules (antitumor drugs), etc. The paper also contained its developments in the past few decades. Meanwhile, it highlights recent progress of pH sensitive nanosystems developed in our research group mainly about preparation, characterization and biological evaluating of chitosan and dextran based super pH sensitive nanoparticles loading antitumor drugs and introduced previous misdirections and the existing research directions. This article also
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pointed out prior errors and problems facing in the experiments, one step further investigated the trends in this field. Chapter 55 - At present, with the rapid development of starch industries, there is a need to expand the breeds of starch, to look for excellent starch resources to meet application requirements in different fields. The mesoscopic scale and form of starch directly effect food quality, color and biological titer. For example, the amylopectin starch structure of cereal starch determines the nature of the material, and also the strength, flexibility and scope of use of starch environmental protection material. Research on all kinds of topology structures such as the column, chain, and ring of starch nano-structural units, obtain the relationship between cereal food quality and the form of starch nano-structural units, provide micro-scale reference for selecting high-quality breeding; in this indicator, successfully developing specialty corn and wheat germplasm, it was found that high-branched-chain corn and wheat products triggered a sense of unique food, having a quality specificity mechanism such as food rheology and so on. Waxy wheat, a new breed of wheat varieties, is about to realize commercial cultivation. Its starch can be applied to industrial production, and industrial application must understand the physical properties. Compared with traditional wheat, the research on waxy wheat is just in the initial stage. Only part of the nature of waxy wheat starch is studied, but there is no further research on the structure and application of starch. The study, through system and indepth researching on a variety of physical and chemical properties and nano-structure units, comparing the similarities and differences with common wheat starch, waxy corn starch and other commonly used industrial starch, provides a theoretical foundation for waxy wheat starch applications in industry. The results show that in the physical properties, the waxy wheat starch has low gelatinization onset temperature, a large gelatinization range and low gelatinization endothermic enthalpy change; there are a high peak viscosity, a moderate low viscosity and cold plastic viscosity, the larger collapse of the value and high viscosity of the anti-aging properties in gelatinization processing; the gel surface formatted is brittle, easily broken, the smaller internal hardness, the smaller adhesion and good flexibility. If played by external force, it is easier to restore to the original shape. After the gelatinization of its starch, it has higher transparency, water-holding capacity and moderate apparent viscosity, and antifreeze-thaw stability, viscosity shear stability and viscosity stability is the very best. In general, a variety of physical and chemical characteristics of the waxy wheat starch are similar to that of potato starch. The research on Waxy starch is helpful to the huge macromolecular level, providing references for the application and selection of food ingredients and starch materials, guiding the effective application of starch resources, resolving the scientific questions, for example, whether this reflects the starch grain varieties characteristics, genetic characteristics and nutritional characteristics, material characteristics and so on. Chapter 56 - Zein is the major storage protein of maize, which is widely used environmentally for packing-materials in food engineering. In this paper, nano-structures of waxy maize zeins were investigated in virtue of atomic force microscopy (AFM), and morphological properties of nano-particles composing waxy maize zein film were analyzed. Chapter 57 - Mushrooms along with other fungi belong to the Myceteae Kingdom. Mushrooms have a very diverse structural morphology that they exist as aerial fleshy fruiting bodies, vegetative filamentous mycelia and underground nonfleshy sclerotia. The differentiation of all these structures involve very complicated fungal morphogenesis and
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metamorphosis that are related to cell wall synthesis and metabolism (Bartnicki-Garcia, 1999; De Groot et al., 2005). With such unique characteristic, mushrooms become a rich source of carbohydrate polymers that are derived from fungal cell wall formed at different morphological stages (Zhang et al., 2007). Unlike carbohydrate polymers obtained from cell wall of higher plants, mushroom carbohydrate polymers are mainly non-starch polysaccharides with beta-glucan-chitin complex and mannoproteins (Bartnicki-Garcia, 1970). Very large structural variations and content of these mushroom carbohydrate polymers exist among the different developmental stages of the mushrooms (Zhang et al., 2007). Mushroom carbohydrate polymers have been known for their bioactive functions such as immunopotentiating, antitumor, antioxidant activities of which their mechanisms are not fully understood (Wasser, 2002; Cheung, 2008; Ooi, 2008). Recently, there is a growing popularity of developing mushroom carbohydrate polymers as functional foods (Chang, 2008). The success of such application requires active research. Chapter 58 - In this chapter selected examples of metal ion separations with polymeric macrocycles such as crown ethers, calixarenes, resorcinarenes, calixcrowns and cyclodextrins, reported in recent literature, are presented. Particularly, the use of those polymers in separation processes such as ion flotation, solvent extraction as well as transport across liquid and polymer membranes is shown. First, selected examples of crown ether polymers variety cross-linked as metal ion carriers are described, then selectivity species by donor sites bonding and coordination are characterized. Selected calixarene and calixcrown polymers as ligands of metal ions are described showing their possible applications, especially in the aspect of the separation of toxic metals ions. Other macrocyclic compounds such as cyclodextrins (CD) can be also polymerized and then used as ion carriers for transport of metal ions from aqueous solutions. The hydrophobic CD polymers have been used as ion carriers for separation of transition metal cations from dilute aqueous solutions by transport across polymer inclusion membranes. Recent developments of the CD polymers application in sorption processes of heavy metal ion from aqueous solution will be also presented in this chapter. It is also shown that hydrophilic, water soluble CD polymers obtained by cross-linking of CD with anhydrides are useful in the removal of transition metal ions from diluted aqueous solutions by ion flotation process.
In: Encyclopedia of Polymer Research Ed: Caroline E. Jones
ISBN: 978-1-61761-926-7 © 2011 Nova Science Publishers, Inc.
Chapter 1
RESEARCH AND REVIEW AND STUDIES INDUCED SELF-ASSEMBLY OF DIBLOCK COPOLYMERS Eri Yoshida Department of Materials Science, Toyohashi University of Technology, Hibarigaoka, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan
1. INDUCED SELF-ASSEMBLY BY ELECTRON TRANSFER The molecular self-assembly is induced by variation in the surroundings, such as temperature [1-4], pressure [5-9], pH [10-14], salt formation [13-18], and noncovalent bond cross-linking [1921]. The block copolymers are molecularly converted in situ from the nonamphiphilic copolymers completely dissolved in a solvent to amphiphilic copolymers due to these stimuli. Therefore, the association and dissociation of the isolated copolymers are reversibly controlled by such stimuli. The induced self-assembly has advantages over direct self-assembly of amphiphilic copolymers in molecular designing. There is no dependence on the balance of solvophilic and solvophobic moieties when designing the copolymers. Thus, a better selection of the driving force can be provided. The advantages also include the fact that a variety of amphiphilic copolymers can be created from one nonamphiphilic copolymer in situ by selecting the stimuli. Electron transport systems perform important functions concerning respiration and energy metabolism in eucaryotes [22, 23]. The electron transport reactions occur at the mitochondria inner membrane formed by electron transport proteins [24] and the lipid bilayer built up by the self-assembly of phospholipids as vital surfactants [25, 26]. The electron transport proteins include redox catalysts such as nicotinamide, iron [27, 28], and quinones [29].
Figure 1-1-1. Redox system of TEMPO
The electrons produced by these redox reactions transfer through the lipid bilayer. While the relationship between the electron transport mechanisms and the molecular self-assembly
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in vivo has been clarified, control of the self-assembly by electron transport has been applied for an artificial polymeric surfactant.
Figure 1-1-2. The PVTEMPO-b-PSt diblock copolymer.
1.1. Oxidation-Induced Micellization Oxidation-induced micellization of a diblock copolymer was determined for a diblock copolymer containing 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) on the side chains [30]. TEMPO is a stable nitroxyl radical known as a spin trapping reagent [31], a spin label reagent [32], and a mediator for living radical polymerization [33, 34]. TEMPO forms a redox system in which the radical is converted into the oxoaminium cation (OA) by one-electron oxidation and is converted into the aminoxy anion (AA) by one-electron reduction [35] (Figure 1-1-1). The oxidation of TEMPO into the OA is caused by chlorine [36], bromine [37], copper (II), and iron (III) [38], while the reduction into AA is brought about by hydrazobenzene [39], quinones [40], and ascorbic acid [41, 42]. The OA salt serves as a oneelectron oxidizing agent for amines [35, 36], sulfides [35, 43], and organometallic compounds [44] to produce their radical cation salts or radical intermediates. The OA salt also acts as a two-electron oxidizing agent for converting an alcohol into an aldehyde or ketone [45]. The salts such as the OA chloride, nitrate, trifluoroborate, and hexafluroantimonate are easily prepared by disproportionation of TEMPO in ether by the acids [46]. The oxidation-induced micellization was attained using poly(4-vinylbenzyloxy-TEMPO)block-polystyrene (PVTEMPO-b-PSt) diblock copolymer obtained by the reaction of 4hydroxy-TEMPO and PVBC-b-PSt (Figure 1-1-2). The molecular weight of copolymer was Mn(PVTEMPO-b-PSt) = 31,200-b-49,400. The PVTEMPO-b-PSt diblock copolymer showed no self-assembly in carbon tetrachloride, a nonselective solvent. Dynamic light scattering demonstrated that the copolymer self-assembled into micelles when chlorine gas was added to the copolymer solution. An excess of chlorine (1.94 equivalents relative to the TEMPO) was added in order to complete the reaction with the TEMPO when it was taken into consideration that part of the chlorine gas would escape. The hydrodynamic diameter (DH) of the micelles was estimated to be 49.5 nm by cumulant analysis, while that of the isolated copolymer was 15.6 nm. Figure 1-1-3 shows the scattering intensity distribution vs the hydrodynamic diameter of the copolymer before and after the reaction. The scattering intensity distribution was obtained by the Marquadt analysis [47]. The scattering intensity distribution of the micelles completely took the place of the unimer distribution by the reaction with chlorine.
Research and Review and Studies Induced Self-Assembly of Diblock Copolymers
Figure 1-1-3. Scattering intensity distribution of the hydrodynamic diameter for before and after the reaction with the chlorine. [PVTEMPO-b-PSt] = 1.71 X 10-3 g/mL.
Figure 1-1-4. ESR spectra of PVTEMPO-b-PSt in CCl4 before (a) and after (b) the reaction with the chlorine, and after the reaction with TMPD (c), and of Wursters’ blue chloride separately prepared (d).
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Figure 1-1-5. Micellization of PVTEMPO-b-PSt by the chlorine.
ESR studies verified that the radical concentration of the TEMPO in the copolymer decreased due to the reaction with chlorine. Figure 1-1-4 shows the ESR spectra of the copolymer before and after the reaction. Before the reaction with chlorine, a broad signal was observed due to the random orientation, probably caused by the restriction of the mobility of the TEMPO supported on the side chains. They should undergo a strong interaction with each other. After the reaction, the broad signal changed to a characteristic triplet attributed to the isotropy along with a decrease in the signal intensity. The g values of the radicals before and after the reaction were 2.0066 and 2.0064, respectively. This negligible difference in the g values indicates that they are identical radicals originating from the TEMPO. The initial concentration of the TEMPO radical was estimated to be 2.30 mM based on the molar ratio of the VTEMPO unit to the St (VTEMPO/St = 0.186/0.814). The radical concentration after the reaction with 1.94 equivalents of chlorine was estimated to be 6.76×10-2 mM on the basis of the integral curves obtained from the differential curves of the radicals. Ninety-seven percent of the TEMPO was consumed by 1.94 equivalents of the chlorine and only 3% of the TEMPO remained unreacted.
Figure 1-1-6. Variation in the UV absorbance of the PVTEMPO-b-PSt copolymer during the micellization. The chlorine to the VTEMPO unit was 0, 0.13, 0.28, 0.58, 1.11, and 1.94 equivalents from the bottom.
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Figure 1-1-7. The variation in UV absorbance at 360 nm, relative scattering intensity (I/I0),and hydrodynamic diameter (DH) of PVTEMPO-b-PSt vs amount of the chlorine.
UV analysis revealed that as the TEMPO were oxidized into the OA chloride (OAC), the block copolymer became amphiphilic in nature, and hence the polymers underwent micellization (Figure 1-1-5). OA salts are insoluble in carbon tetrachloride; however, in good solvents, such as acetonitrile, the salts show absorption at 360 nm. As can be seen in Figure 1-1-6, the absorption at 360 nm increased as a result of increasing the chlorine. The increase in the absorption at 360 nm indicates an increase in the OAC. Figure 1-1-7 shows the plots of the absorbance at this wavelength, the relative scattering intensity (I/I0), and the hydrodynamic diameter of the copolymer vs the amount of chlorine. The absorbance increased with an increase in the amount of chlorine, while the scattering intensity and hydrodynamic diameter remained almost constant over 1.11 equivalents of chlorine. It was assumed that no reaction except for the oxidation of the TEMPO by chlorine to the OAC occurred, and the degrees of oxidation of the TEMPO to the OAC were estimated at each
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amount of chlorine. The oxidation degrees were determined based on the UV absorbance and the conversion at 1.94 equivalents by the ESR analysis. Figure 1-1-8 shows the variation in the scattering intensity and hydrodynamic diameter of the copolymer vs the oxidation degree. The hydrodynamic diameter rapidly increased at a 16% oxidation degree. Only 16 % of the OAC induced the micellization. The scattering intensity also rapidly increased at the 16% oxidation degree; however, it increased almost proportionally with an increase in the oxidation degree. The continuous increase in the scattering intensity over 16% should be based on increases in the aggregation number or the number of micelles. This consequence was supported by the results for the dependence of the scattering intensity on the copolymer concentration. Figure 1-1-9 shows the plots of the scattering intensity and hydrodynamic diameter of the micelles vs the copolymer concentration. Whereas the micellar size was almost independent of the copolymer concentration, the scattering intensity increased with increasing copolymer concentration. The number of micelles increased as a result of increasing copolymer concentration, causing an increase in the scattering intensity. TEM observations confirmed that the POAC-b-PSt copolymer self-assembled into spherical micelles (Figure 1-1-10). The size of the micelles was almost equal to that estimated by the dynamic light scattering. In common cases, some micelles show a smaller size in the TEM image than in light scattering due to swelling of the micelles in solution. The POAC-b-PSt micelles may have difficulty swelling in carbon tetrachloride, because the micelles have the salts with low affinity for the solvent in the micellar cores, resulting in a slight difference in micellar size between that in light scattering and that in TEM.
Figure 1-1-8. The plots of relative scattering intensity and hydrodynamic diameter of PVTEMPO-b-PSt vs the degree of oxidation.
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The POAC-b-PSt copolymer seemed not to be very thermally stable, because the orange color of the OAC gradually faded out over room temperature, although the micellar structure was maintained even after the color disappeared. However, below 0°C the micellar solution retained the orange color for several hours.
Figure 1-1-9. The plots of the relative scattering intensity and hydrodynamic diameter of PVTEMPO-bPSt vs copolymer concentration.
Figure 1-1-10. A TEM image of the POAC-b-PSt micelles.
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Figure 1-1-11. A 1H NMR spectrum of the POAC-b-PSt micelles after the reaction with benzyl alcohol. Solvent: CCl4 with benzene-d6 as the lock solvent and diethyl ether as the standard to estimate the conversion.
The micelles served as an oxidizing agent for converting benzyl alcohol into benzaldehyde. When 1 equivalent of benzyl alcohol relative to the VTEMPO unit was added to the POAC-b-PSt micellar solution in carbon tetrachloride, the orange solution became colorless. 1H NMR demonstrated the quantitative formation of benzaldehyde. Figure 1-1-11 shows the 1H NMR spectrum of the reaction mixture. Signals originating from benzaldehyde are observed at 7.72, 8.00, and 10.13 ppm. The signals at 1.30 and 3.54 ppm are attributed to diethyl ether added as a standard to estimate the conversion into benzaldehyde. The conversions were determined from the ratio of the signal intensity at 10.13 ppm to that at 3.54 ppm, with the results after 20 and 45 min being 91 and 97%, respectively. The 97% conversion of benzyl alcohol into benzaldehyde confirms that the VTEMPO units were almost quantitatively converted to the OAC by the chlorine. The signals based on the blocks containing the pendant groups were not observed even after the oxidation of benzylalcohol, indicating that the copolymer maintains the micellar structure after the reaction. The light scattering revealed that no changes occurred in the micellar size and in the relative scattering intensity after the reaction. The OAC served as a two-electron oxidizing agent for benzyl alcohol, converting to the insoluble hydroxylamine-hydrochloride salt. Consequently, no dissociation of the micelles occurred due to the oxidation. It can be deduced that the micelles oxidized benzyl alcohol in the cores and released soluble benzaldehyde from the cores maintaining the micellar structure (Figure 1-1-12).
Figure 1-1-12. Oxidation of benzyl alcohol into benzaldehyde by the POAC-b-PSt micelles.
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Figure 1-1-13. The UV spectrum of Wurster’s blue chloride produced through the oxidation of TMPD by the POAC-b-PSt micelles.
The POAC-b-PSt micelles also oxidized N,N,N’,N’-tetramethyl-1,4-phenylenediamine (TMPD) to produce Wurster’s blue chloride. As 1 equivalent of TMPD relative to the VTEMPO unit was added to the micellar solution prepared by 1.94 equivalents of chlorine, the solution with orange colored micelles immediately turned purple. Figure 1-1-13 shows the UV spectrum of the micellar solution after the reaction. The characteristic absorption of Wurster’s blue [48] was confirmed at 536, 574, and 624 nm. It was suggested that the Wurster’s blue chloride was generated in the micellar cores by a one-electron transfer from TMPD to the OAC, because the insoluble Wurster’s blue chloride was dissolved into carbon tetrachloride.
Figure 1-1-14. Scattering intensity distribution, weight exchange distribution, and number exchange distribution of the hydrodynamic diameter of the copolymer after the reaction with TMPD.
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The one-electron transfer mechanism from TMPD to the oxoaminium salt was supported by the ESR analysis. As can be seen in Figure 1-1-4c, the signal intensity of the TEMPO increased due to the reaction with TMPD. The g value of the signal was 2.0063, showing good agreement with that before the reaction (g = 2.0064). In the triplet signal, another sharp signal was discerned. This singlet signal had a g value of 2.0034. We separately prepared Wurster’s blue chloride in carbon tetrachloride by the reaction of TMPD with chlorine. Wurster’s blue chloride was obtained as an insoluble black precipitate by the direct oxidation of TMPD by chlorine in carbon tetrachloride; however, the radical salt was unstable by itself and was rapidly decomposed thus losing its radical nature. Figure 1-1-4d shows the ESR spectrum of Wurster’s blue chloride. Wurster’s blue chloride alone showed a singlet signal with g = 2.0034 in carbon tetrachloride. The identification of g values verified that Wurster’s blue chloride was produced from the reaction of TMPD and the POAC-b-PSt micelles and was solubilized within the micellar cores. The Marquadt analysis also revealed that the POAC-b-PSt micelles were dissociated into the PVTEMPO-b-PSt copolymer by the reaction with TMPD. Figure 1-1-14 shows three different distributions of the hydrodynamic diameter of the copolymer; i.e., the scattering intensity distribution, weight exchange distribution, and number exchange distribution. The scattering intensity distribution showed the formation of huge particles over 500 nm, in addition to particles with a size similar to that of the POAC-b-PSt micelles. The huge particles should be attributed to the insoluble Wurster’s blue dropped from the micelles, because the resulting solution gradually became a white suspension, thus losing the purple color. However, there were not many huge particles, because the distribution of the huge particles was not seen in the weight exchange distribution. On the other hand, the unimer distribution slightly discerned in the scattering intensity distribution was clearly observed in the weight exchange distribution. The number exchange distribution showed only the unimer distribution, suggesting that most of the micelles were dissociated into unimers by the reaction with TMPD. TEM observations showed that the POAC-b-PSt micelles reverted into PVTEMPO-b-PSt unimers. Figure 1-1-15 shows a TEM image of the copolymer after the reaction with TMPD. It is observed that larger particles with cores and smaller particles almost without cores co-exist. The larger particles were expected to originate from the micelles including Wurster’s blue. The larger particles are still bigger than the POAC-b-PSt micelles and have a somewhat distorted shape compared with the micelles. The distortion of the shape should be caused by the copolymer associating through a weak force. This weak association of the copolymer is also reflected in the fact that Wurster’s blue chloride gradually dropped out of the micelles. The many small particles were considered to be the isolated copolymers, because the average size of the particles was 17.0 nm, almost the same size as the unimer determined by light scattering. Furthermore, unimers separating from the large particles were also observed. (Figure 1-1-16). It was deduced that the POAC-b-PSt micelles oxidized TMPD to the Wurster’s blue chloride, reverting into the PVTEMPO-b-PSt copolymers (Figure 1-1-17). Most of the copolymers reverted into the isolated copolymers, while some of them still surrounded the Wurster’s blue particles to solubilize them.
1.2. Reduction-Induced Micellization While the oxidation-induced micellization was based on the OAC/TEMPO system using chlorine as the oxidizing agent, the reduction-induced was attained through the TEMPO/HA system using phenylhydrazine as the reducing agent [49].
Research and Review and Studies Induced Self-Assembly of Diblock Copolymers
Figure 1-1-15. A TEM image of the POAC-b-PSt copolymer after the reaction with TMPD.
Figure 1-1-16. TEM images of PVTEMPO-b-PSt separating from the micelles.
Figure 1-1-17. The reaction of TMPD by the POAC-b-PSt micelles.
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Figure 1-2-1. Variation in the UV absorbance as phenylhydrazine was added to the copolymer solution in benzene. The PH/TEMPO ratios were 0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 from the top.
Figure 1-2-2. Plots of the UV absorbance at 500 nm (a), relative scattering intensity (b) and hydrodynamic diameter (c) vs the PH/TEMPO ratio.
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TEMPO is red in color and has a UV absorption around 470 nm. This radical is reduced by phenylhydrazine to the colorless hydroxylamine [50]. The PVTEMPO-b-PSt diblock copolymer showed an absorption with a λmax value at 467 nm based on the TEMPO radical. It was found that the red color of the copolymer solution in benzene faded out as phenylhydrazine was added to the solution. Figure 1-2-1 shows the variation in the UV absorption of the copolymer with the addition of the phenylhydrazine. The absorbance based on TEMPO decreased with an increase in the molar ratio of the phenylhydrazine to the VTEMPO unit (PH/TEMPO). The absorbance plotted at 500 nm versus the PH/TEMPO ratio is shown in Figure 1-2-2a. The absorbance continued to decrease up to 0.8 and was almost constant above it. The absorbance did not reach zero even at 1.0 due to the long foot of the large absorption peak observed at 240-390 nm. The formation of the hydroxylamine was also confirmed by the 1H NMR measurement of the copolymer in CDCl3 in the presence of phenylhydrazine. Signals based on the tetramethyl protons of the hydroxylamine derivative of TEMPO were observed at 1.18 and 1.26 ppm. Light scattering studies demonstrated that the scattering intensity of the copolymer solution was inversely correlative to the variability in the UV absorbance. Figure 1-2-2b shows the variation in the relative scattering intensity versus the PH/TEMPO ratio. The scattering intensity increased with an increase in the ratio and became almost constant over 0.8. The increase in the scattering intensity suggests the formation of micelles. The hydrodynamic diameter of the copolymer also increased with the increase in the PH/TEMPO ratio (Figure 1-2-2c). The diameter of the copolymer rapidly increased with the addition of a small amount of phenylhydrazine and became steady over 0.2. Whereas the scattering intensity became constant over the PH/VTEMPO ratio of 0.8, the hydrodynamic diameter was constant over 0.2. This difference in the variability between the hydrodynamic diameter and scattering intensity can be accounted for by the fact that the increase in the hydrodynamic diameter indicates the formation of micelles, while the increase in the scattering intensity exactly means an increase in the number of the micelles. The formation of the micelles by the addition of only a 0.2 ratio of the PH/TEMPO suggests that the micellization occurred before all the TEMPO radicals were converted into the hydroxylamine. The hydrogen bonding among the hydroxyl groups should have effectively caused the micellization in the nonpolar solvent. The Marquadte analysis of the scattering intensity distribution of the copolymer also revealed the micelle formation. The PVTEMPO-b-PSt copolymer showed no self-assembly in benzene, because both the blocks of PVTEMPO and PSt were solvophilic to benzene. Therefore, PVTEMPO-b-PSt existed as isolated copolymers, that is, unimers in the absence of phenylhydrazine. The hydrodynamic diameter of the unimers was estimated to be 19.2 nm based on the Marquadte analysis. Figure 1-2-3 shows the scattering intensity distributions obtained at the PH/VTEMPO ratios of 0.1 and 1.0. The scattering intensity distribution in the absence of phenylhydrazine could not be obtained due to the very low scattering intensity. It is obvious that the distribution of the unimers was shifted to the higher side of the hydrodynamic diameter at the 1.0 ratio, although part of the distributions overlapped. The micellar size was estimated to be 55.6 nm. It has been reported that hydroxylamine is oxidized to TEMPO by oxygen [34]. To the micellar solution containing the hydroxylamine was added oxygen by bubbling after the hydroxylamine was converted into the aminoxy anions by sodium hydride in order to facilitate the oxidation by oxygen. As oxygen was added to the micellar solution, the UV absorbance due
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to the TEMPO radicals increased. The hydroxylamine in the copolymer was converted into TEMPO by the oxygen, although the radicals immediately reverted to the hydroxylamine again due to the presence of phenylhydrazine in the solution. In addition, this experiment was performed at the PH/TEMPO ratio of 0.5 in order to minimize the influence of the hydrazine.
Figure 1-2-3. Scattering intensity distribution of the hydrodynamic diameter of the copolymer at the 0.1 and 1.0 PH/TEMPO ratios.
1.3. Disproportionation-Induced Micellization TEMPO is disproportionated into the OA and the AA by the acids [46]. The disproportionation of TEMPO also promoted the micellization of the PVTEMPO-b-PSt copolymer [51]. The series of micellizations using the TEMPO redox systems indicate that the electron transport becomes a trigger that causes self-assembly of molecules, in addition to external triggers such as temperature, pressure, pH, salt formation, and noncovalent bond cross-linking. PVTEMPO-b-PSt shows no self-assembly in 1,4-dioxane because both blocks are solvophilic to the solvent. Dynamic light scattering studies demonstrated that the copolymer formed micelles in this solvent by the addition of hydrochloric acid. Figure 1-3-1 shows variation in the hydrodynamic diameter and scattering intensity of the copolymer as the hydrochloric acid was added to the copolymer solution. The hydrodynamic diameter rapidly increased at a 0.8 molar ratio of hydrochloric acid to the VTEMPO unit (HCl/VTEMPO). This suggested that the micellization started at 0.8. The hydrodynamic diameter gradually continued to increase over 0.8 and almost became a constant at 1.6. At the complete micellization, the copolymer formed micelles with the hydrodynamic diameter of 53.8 nm, estimated by the cumulant analysis. The scattering intensity also started increasing at 0.8. The disagreement of the variation in the scattering intensity with that of the hydrodinamic diameter was based on the fact that the scattering intensity was attributed to the aggregation number of the micelles. Figure 1-3-2 shows the scattering intensity distribution obtained by
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the Marquadt analysis for the hydrodynamic diameter of the copolymer before and after the addition of the hydrochloric acid. The Marquadt method is much better than the cumulant in analyzing the intensity distribution of the hydrodynamic diameter for polymers with comparatively narrow molecular weight distributions. The distribution of the hydrodynamic diameter for the isolated copolymers, that is unimers, was observed around 30 nm. The distribution of the micelles at the HCl/VTEMPO ratio of 2.0 was observed around ca. 55 nm. The unimer distribution was completely shifted to the micellar distribution, indicating that all the unimers were engaged in forming the micelles.
Figure 1-3-1. Variation in the hydrodynamic diameter and scattering intensity of the PVTEMPO-b-PSt copolymer vs. HCl/VTEMPO. [PVTEMPO-b-PSt] = 1.71 g/L.
Figure 1-3-2. Scattering intensity distribution of the hydrodynamic diameter for PVTEMPO-b-PSt before and after the reaction with HCl at HCl/VTEMPO of 2.0. [PVTEMPO-b-PSt] = 1.71 g/L.
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Figure 1-3-3. Variation in the UV absorbance of the PVTEMPO-b-PSt copolymer during the micellization. HCl/VTEMPO from the bottom = 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, and 2.0. [PVTEMPO-b-PSt] = 1.71 g/L.
Figure 1-3-4. Micellization of PVTEMPO-b-PSt by the disproportionation of TEMPO.
A UV analysis confirmed that the oxoaminium chloride was formed in the micellar cores via the disproportionation using hydrochloric acid. Figure 1-3-3 shows the variability in the absorbance of the copolymer solution during the micellization. The oxoaminium chloride derived from 4-methoxy-TEMPO was insoluble in 1,4-dioxane, however, in a good solvent such as acetonitrile, the oxoaminium chloride showed an absorption at 360 nm. The hydroxylamine is soluble in the solvent, but had no UV absorption. The absorbance at 360 nm increased with an increase in the amount of HCl. This observation implies that the insoluble oxoaminium chloride was dissolved in the micelles by being supported on the core blocks. The increase in the absorbance indicates an increase in the amount of the oxoaminium chloride in the micellar cores. The disproportionation of TEMPO proceeded into the oxoaminium chloride and the hydroxylamine as the amount of the hydrochloric acid increased (Figure 1-3-4). ESR studies demonstrated that the radical concentration of the TEMPO in the copolymer decreased with the increasing HCl. The ESR spectra of the copolymer during the micellization are shown in Figure 1-3-5. The copolymer showed broad signals before the addition of HCl. The broad signals are based on the random orientation caused by the restriction of the mobility of the TEMPO supported on the polymer side chains. The broad signals gradually changed to the typical triplet of TEMPO as the HCl increased. It is considered that the number of TEMPO molecules decreased as the disproportionation proceeded, resulting in the fact that the interaction among the TEMPO molecules was
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reduced and showed the original triplet. In fact, the signal intensity decreased with this change in the signal, although the g values were almost constant throughout the micellization. The radical concentrations were estimated on the basis of the integral curves obtained from these differential curves in the ESR. Figure 1-3-6 shows the variation in the TEMPO concentration during the micellization. The radical concentration decreased as the HCl increased and became constant over a 1.6 HCl/VTEMPO ratio. It was found that 40 mol% of the TEMPO radicals remained unreacted. This result suggests that the micellization hindered the TEMPO disproportionation due to the fact that the remaining TEMPO was isolated in each of the micellar cores. However, the disproportionation continued to proceed unless the micellization was completed, because the TEMPO concentration continued to decrease even over the HCl/TEMPO ratio of 0.8 at which the micellization started. TEM observations demonstrated that spherical micelles were formed by the disproportionation-induced micellization. The TEM image of the micelles is shown in Figure 1-3-7. The micelles did not have completely spherical outlines, suggesting that the micelles were constructed with a weak association force. This weak association should be caused by non-quantitative disproportionation and by the presence of the soluble hydroxylamine in the cores. The diameter of the micelles was ca. 72.7 nm in the TEM image. This micellar size was larger than that estimated by the dynamic light scattering. This difference may be accounted for by the fact that the micelles expanded by an increase in the repulsion among the OA cations when the solvent was removed.
Figure 1-3-5. ESR spectra of PVTEMPO-b-PSt. [PVTEMPO-b-PSt] = 1.71 g/L.
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Figure 1-3-6. Plots of the radical concentration of the TEMPO vs. HCl/VTEMPO.
Figure 1-3-7. A TEM image of the micelles obtained by the disproportionation.
REFERENCES [1] [2] [3] [4] [5] [6]
Alexandre A., Reynafarje B., Lehninger AL (1978). Proc Natl Acad Sci, 75, 5296. Arotcarena M., Heise B., Ishaya S., Laschewsky A (2002). J Am Chem Soc, 124, 3787. Bar RS., Deamer DW., Cornwall DG (1966). Science, 153, 1010. Brik ME (1990). Synth Commun, 20, 3283. Brontein LM., Sidorov SN., Valetsky PM (1999). Langmuir, 15, 6256. Buhler E., Dobrynin AV., DeSimone JM., Rubinstein M (1998). Macromolecules, 31, 7347. [7] Capaldi RA (1982). Biochim Biophys Acta, 695, 291. [8] Celso L., Triolo A., Triolo F., Donato DI., Steinhart M., Kriechbaum M., Amenitsch H., Triolo R (2002). Eur Phys J Soft Matter, 8, 311. [9] Chan SI., Li PM (1990). Biochemistry, 29, 1. [10] Erecinska M., Wilson DF (1982). J Membr Biol, 70, 1.
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[11] Georges MK., Veregin RPN., Kazmaier PM., Hamer GK (1993). Macromolecules, 26, 2987. [12] Gohy JF., Lohmeijer GG., Varshney SK., Decamps B., Leroy E., Boileau S., Schubert US (2002). Macromolecules, 35, 9748. [13] Gohy JF., Varshney SK., Jerome R (2001). Macromolecules, 34, 3361. [14] Golubev VA., Rozantsev EG., Neiman MB (1965). Bull Acad Sci USSR, 1898. [15] Golubev VA., Rozantsev EG., Neiman MB (1965). Izv Akad Nauk SSSR Ser Khim, 1927. [16] Griffiths DE (1965). Essays Biochem, 1, 120. [17] Guranova VV., Kovarsksaya BM., Krinitzkaya LA., Neiman MB., Rozantsev EG (1965). Vysokomol Soedin, 7, 1515. [18] Hageman HJ., Overeem T (1981). Makromol Chem Rapid Commun, 2, 719. [19] Hu Y., Kramer MC., Boudreaux CJ., McCormick CL (1995). Macromolecules, 28, 7100. [20] Hubbel WL., Metcalfe JC., Metcalfe SM., McConnell HM (1970). Biochim Biophys Acta., 219, 415. [21] Koga T., Zhou S., Chu B (2001). Appl Optics, 40, 4170. [22] Lee AS., Butun V., Vamvakaki M., Armes S., Pople JA., Gast AP (2002). Macromolecules, 35, 8540. [23] Lehninger., AL Reynafarje B., Alexandre A., Villalobo A (1980). Ann NY Acad Sci, 341, 585. [24] Liu S., Weaver JVM., Tang Y., Billingham NC., Armes SP (2002). Macromolecules, 35, 6121. [25] Liu S., Zhang G., Jiang M (1999). Polymer, 40, 5449. [26] Liu YC., Wu LM., Chen P (1985). Tetrahedron Lett, 26, 4201. [27] Lowe AB., Billingham NC., Armes SP (1997). Chem Commun, 1035. [28] Marquardt DW (1963). J Soc Indust Appl Math, 11, 431. [29] Martin TJ., Prochazka K., Munk P., Webber SE (1996). Macromolecules, 29, 6071. [30] McClain JB., Canelas DA., Samulski ET., DeSimone JM., Londono JD., Cochran HD., Wignall GD., Chillura-Martino GD., Triolo R (1996). Science, 274, 2049. [31] Miyazawa T., Endo T., Shiihashi S., Ogawara M (1985). J Org Chem, 50,1332. [32] Neradovic, D., Nostrum, C. F., Hennink, W. E. (2001). Macromolecules, 34, 7589. [33] Paleos CM., Dais P (1977). Chem Commun, 10, 345. [34] Paper I., Bobbitt JM., Cecile M., Flores L (1988). Heterocycles, 27, 509. [35] Rozantsev, E. G. & Golubev, V. A. (1966). Izv Akad Nauk SSSR Ser Khim, 891. [36] Rozenberg VI., Piotrovskii VK., Golubev VA., Gvon KI., Nikanorov VA., Bundel YG., Reutov OA (1975). Bull Acad Sci USSR Chem Ser, 24, 2508. [37] Slater EC (1983). Trends Biochem Sci, 8, 239. [38] Wang RL., Tam KY., Compton RG (1997). J Electroanal Chem, 434, 105. [39] Weaver JVM., Armes SP., Butun V (2002). Chem Commun, 2122. [40] Yoshida E., Kunugi S (2002). Macromolecules, 35, 6665. [41] Yoshida, E. & Ogawa, H. (2007). J Oleo Sci, 56, 297. [42] Yoshida E., Tanaka M., Takata., T (2005). Collid Polym Sci, 284, 51. [43] Yoshida E., Tanaka T (2006). Colloid Polym Sci, 285, 135. [44] Yoshida, E. & Tanaka, T. (2008). Colloid Polym Sci, 286, 827. [45] Yoshida E., Takata T., Endo T (1992). Macromolecules, 25, 7282.
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[46] Yoshida E., Takata T., Endo T., Ishizone T., Hirao A., Nakahama S (1994). Chem Lett, 1827. [47] Yoshida E., Terada Y (2005). Collid Polym Sci, 283, 1190. [48] Yoshida E., Sugita A (1996). Macromolecules, 29, 6422. [49] Zhao H., Douglas EP (2002). Mater Res Soc Symp Proc, 43. [50] Zhdanov RI., Golubev VA., Gida VM., Rozantsev EG (1971). Dokl Akad Nauk SSSR, 196, 856. [51] Zhou S., Chu B (1998). Macromolecules, 31, 5300.
2. INDUCED SELF-ASSEMBLY BY PHOTOLYSIS Light is a handy, easily available, and environmentally clean stimulant to cause selfassembly. In vivo, the photoreceptor proteins in animal eye cells change their highdimensional structure by receiving photons [1], while artificial polymers responsive to light contain photochromic compounds such as azobenzene [2-4], spiropyran [5,6], stilbene [7-9], cinnamate [10], and triphenylmethane leuco residues [11]. The polymers reversibly change their structure through the cis-trans isomerization, dimerization, and conformational changes of the photochromic compounds. This reversible behavior is manipulated by UV wavelength of the compounds or sometimes temperature. Compared to these reversible reactions required as a function of the on-off switches, irreversible reactions are convenient to fix the spatial structure changed by photo irradiation. The structure change effects by the photo irreversible reaction have been investigated on the photolysis of diazosulfonates [12-14], 1iminopyridinium ylides [15], [4(4'-alkoxybenzoyl)phenylmethyl]phosphonic acids [16], and didecyl-2-methoxy-5-nitrophenyl phosphate [17]. The former three kinds of surfactants loose their surface-active ability by photolysis, resulting in the destruction of the micelles and vesicles. On the other hand, didecyl-2-methoxy-5-nitrophenyl phosphate formed vesicles by the photolysis. The self-assembly induced by the photolysis was determined for a poly(4-tert-butoxystyrene)block-polystyrene diblock copolymer (PBSt-b-PSt) [18] (Figure 2-1). In this photolysis-induced self-assembly, a diblock copolymer produced by the photolysis formed micelles. This new way of molecular self-assembly induced by photoirradiation has the potential to produce new applications for optical memory materials and optical devices using the photoirreversible reaction.
Figure 2-1. The PBSt-b-PSt diblock copolymer.
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The PBSt-b-PSt diblock copolymer shows no self-assembly in dichloromethane since the PBSt and PSt blocks are solvophilic to it. Light scattering studies have demonstrated that the copolymer is self-assembled into micelles in dichloromethane by irradiation in the presence of a photoacid generator. Figure 2-2 shows the variation in the hydrodynamic diameter (DH) and the relative scattering intensity (I/I0) of the copolymer with the molecular weight of Mn(PBSt-b-PSt) = 15,000-b-97,000 during the irradiation using bis(alkylphenyl)iodonium hexafluorophosphate (BAI) as a photoacid generator. The molar ratio of BAI to the BSt unit was 0.38. The hydrodynamic diameter and scattering intensity showed a good correlation. They increased at 4.5 h and became constant over 5 h, indicating that the micellization was completed over 5 h. The hydrodynamic diameter of the micelles averaged 63.0 nm, while that of the isolated copolymer, which is a unimer, was 16.6 nm based on the cumulant analysis. The observation of the jump and the constant state within the short time period suggests the rapid micellization due to the strong aggregation force.
Figure 2-2. The variation in the hydrodynamic diameter (DH), relative scattering intensity (I/I0), and conversion of the copolymer during the irradiation using BAI. Mn(PBSt-b-PSt) = 15,000-b-97,000, [copolymer]0 = 3.30 g/L.
The variation in the scattering intensity distribution of the hydrodynamic diameter also supported the formation of the micelles by the rapid association. Figure 2-3 shows the
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scattering intensity distribution obtained by the Marquadt analysis. The distribution was shifted to the higher side of the hydrodynamic diameter over time by the irradiation. The slight shift in the distribution at 3.5 h implies that aggregates with a lower aggregation number were formed during the first stage and those associated into micelles, rather than that the unimers inserted step by step into the micelles.
Figure 2-3. Scattering intensity distributions of the hydrodynamic diameter of the copolymer. [copolymer]0 = 3.30 g/L
Figure 2-4. A TEM image of the micelles. Mn(PBSt-b-PSt) = 15,000-b-97,000,
TEM observation confirmed the formation of spherical micelles through the irradiation. The TEM image of the micelles is shown in Figure 2-4. The diameter of the micelles was
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estimated to average 40.6 nm based on the TEM. Compared to the micellar size determined by the cumulant analysis, the TEM exhibited a smaller diameter of the micelles than the dynamic light scattering. The estimation of the micelles as the smaller size can be accounted for by the fact that the micelles in the solution contracted when isolated in air. The irradiation of the copolymer in the absence of BAI and the dark reaction in its presence produced no changes in the hydrodynamic diameter and scattering intensity. These two control experiments suggest that the structure of PBSt-b-PSt was changed by the irradiation on BAI. The 1H NMR confirmed that the micellization was caused by the elimination of the tert-butyl groups in the copolymer. Figure 2-5 shows the 1H NMR spectra of the copolymer before and after the irradiation. The 1H NMR measurements were performed in 1,4-dioxane-d8. Signals at 1.29 ppm based on the tert-butyl groups were hardly observed after the irradiation. The disappearance of the signals implies that the tert-butyl groups were eliminated from the copolymer. PBSt-b-PSt should have been converted into poly(4-vinyl phenol)-block-PSt (PVPh-b-PSt) by the hydrolysis of the tert-butoxy groups with the photoacid generator as a catalyst (Figure 2-6), based on the mechanism of the hydrolysis of poly(4-tert-butoxystyrene) [19]. A signal based on the hydroxyl groups of the PVPh blocks could not be discerned due to the fact that it overlapped with the signals of the aromatic protons and had too low an intensity. In addition, it is clear that the disappearance of the butyl proton signals and no observation of the hydroxyl signal were not based on the self-assembly of the copolymer into micelles. This is because PVPh-b-PSt showed no selfassembly in 1,4-dioxane-d8 and existed as unimers. The conversion of the BSt units into the VPh units was estimated based on the signal intensity of the tert-butyl protons to that of the aromatic protons at 6.3-7.7 ppm. The time-conversion plots are shown in Figure 2-2. The conversion started increasing at an earlier stage than the scattering intensity. The scattering intensity jumped when the conversion reached 50%, indicating that the micellization was dependent on the degree of the VPh unit formation.
Figure 2-5. 1H NMR spectra of the copolymer before (bottom) and after the irradiation (upper, the irradiation time = 5.5 h). Solvent: 1,4-dioxane-d8.
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Figure 2-6. The micellization induced by photolysis of PBSt-b-PSt.
Figure 2-7. The variation in the hydrodynamic diameter, scattering intensity, and conversion of the copolymer during the irradiation in the presence of BAI (o), DPI (z), and TPS (▲). [copolymer]0 = 3.30 g/L. Photoacid generator/BSt unit = 0.38.
Research and Review and Studies Induced Self-Assembly of Diblock Copolymers
25
The study of the micellization using different kinds of photoacid generators demonstrated that the micellization, coupled with the conversion, were dependent on the ability of the photoacid generator. The micellization by the irradiation was evaluated using diphenyliodonium hexaflurophosphate (DPI) and triphenylsulfonium triflate (TPS). Figure 27 shows the variation in the hydrodynamic diameter, scattering intensity, and conversion during the irradiation by BAI, DPI, and TPS. The conversion for DPI started increasing slightly earlier than that for BAI, although there is a negligible difference in the transition of the scattering intensity and hydrodynamic diameter. On the other hand, TPS needed a longer irradiation time to promote the micellization as compared to BAI and DPI. This difference in promoting the micellization was clarified on the basis of the UV analysis of the photoacid generators. Figure 2-8 shows the UV spectra of the photoacid generators and the PBSt-b-PSt copolymer, coupled with the illumination intensity of the irradiation versus the wavelength for the high-pressure mercury lamp. It is considered that the irradiation reaction of the photoacid generators occurred around 290 nm, because at this wavelength, the absorption of the photoacid generator overlapped at a highest proportion with the illumination intensity of the lamp without any obstruction by the copolymer. The absorbance of the photoacid generators decreased in the order of BAI > DPI > TPS. In particular, TPS had a slight absorption at 290 nm. It can be deduced that the difference in the absorption intensity among the photoacid generators was reflected in the irradiation time needed to initiate the micellization.
Figure 2-8. UV spectra of BAI (a), DPI (b), TPS (c), and PBSt-b-PSt (d) with the illumination intensity of the irradiation of the high-pressure mercury lamp (e). Solvent: dichloromethane. Mn(PBSt-b-PSt) = 15,000-b-97,000.
The efficiency of the micellization was also dependent on the concentration of the photoacid generator. Figure 2-9 shows the variation in the hydrodynamic diameter, scattering intensity, and conversion during the irradiation in the presence of DPI at the DPI/BSt molar ratios of 0.38 and 1.00. More sharply and earlier jumping was observed at 1.00, indicating that the micellization was promoted more effectively at 1.00. Consequently, the irradiation time needed for the micellization was manipulated by the concentration of the photoacid generator. The block length of the copolymer had an effect not only on the micellar size and scattering intensity, but also on the conversion. For the identical PBSt block length (Mn = 15,000), the effect of the PSt block length on the micellization was explored. Figure 2-10 shows the variation in the hydrodynamic diameter, scattering intensity, and conversion during the irradiation using copolymers with the different PSt block lengths: Mn = 63,000 and
26
Eri Yoshida
170,000. Regarding the hydrodynamic diameter and the scattering intensity, it was observed that the copolymer with the shorter PSt block showed a transition at an earlier stage than that with the longer PSt. The copolymer with the shorter PSt more easily aggregated into micelles. The shorter PSt sample produced smaller micelles with a higher aggregation number due to the shorter length of the PSt blocks. The conversion of the shorter PSt sample also started increasing slightly earlier than that for the longer sample. The reason why the shorter PSt sample initiated the micellization more earlier is also based on the faster conversion.
Figure 2-9. The variation in the hydrodynamic diameter, scattering intensity, and conversion during the irradiation at 0.38 (O) and 1.00 (z) of DPI/BSt. [copolymer]0 = 3.30 g/L, Mn(PBSt-b-PSt) = 15,000-b97,000.
Research and Review and Studies Induced Self-Assembly of Diblock Copolymers
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Figure 2-10. The variation in the hydrodynamic diameter, scattering intensity, and conversion during the irradiation using copolymers with Mn(PSt block) = 63,000 (z) and 170,000 (O). Mn(PBSt block) = 15,000, [copolymer]0 = 3.30 g/L, DPI/BSt = 0.38.
REFERENCES [1]
[2] [3] [4] [5] [6]
Palczewski, K., Kumasaka, T., Hori, T., Behnke, C., Motoshima, B., Fox, A., Le Trong, I., Teller, T., Okada, T., Stenkamp, R. E., Yamamoto, M. & Miyano, M. (2000). Science, 289, 739 Lovrien, R. (1967). Proc Natl Acad Sci USA, 57, 236 Matejka, L., Dusek, K. (1998). Makromol Chem, 182, 3223 Pieroni, O. & Fissi, A. (1992). J Photochem Photobiol B Biol, 12, 125 Menju, A., Hayashi, K. & Irie M. (1981). Macromolecules, 14. 755 Taguchi, M., Li, G., Gu, Z., Sato, O. & Einaga, Y. (2003). Chem Mater, 15, 4756
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Eri Yoshida Meier, H. (1992). Angew Chem Int Ed Engl, 31, 1399 Szczubialka, K. & Nowakoaska, M. (2003). Polymer, 44, 5269 Eastoe, J., Sanchez, M., Dominguez, S., Wyatt, P., Beeby, A. & Heenan, R. (2002). Langmuir, 18, 7837. Szczubialka, K. & Nowakoaska, M. (2003). Polymer, 44, 5269 Irie, M. & Hosoda, M. (1985). Makromol Chem Rapid Commun, 6, 533 Dunkin, I. R., Gittinger, A., Sherrington, D. C. & Whittaker, P. (1994). J Chem Soc Chem Commun, 2245. Mezger, T., Nuyken, O., Meindl, K. & Wokaun, A. (1996). Prog Org Coatings, 29, 147. Nuyken, O. & Voit, B. (1997). Macromol Chem Phys, 198, 2337. Haubs, M. & Ringsdorf, H. (1987). New J Chem, 11, 151. Okamoto, Y., Yoshida, H. & Takamuku, S. (1988) Chem Lett, 569. Veronese, A., Berclaz, N. & Luisi, P. L. (1998). J Phys Chem B, 102, 7078. Yoshida, E., Kuwayama, S. (2007). Colloid Polym Sci, 285, 1287. Conlon, D. A., Crivello, J. V., Lee, JL. & O'Brien, M. J. (1989). Macromolecules, 22, 509.
In: Encyclopedia of Polymer Research Ed: Caroline E. Jones
ISBN: 978-1-61761-926-7 © 2011 Nova Science Publishers, Inc.
Chapter 2
A NOVEL THERMOSENSITIVE COMPOSITE HYDROGEL BASED ON POLY(ETHYLENE GLYCOL)POLY(Ε-CAPROLACTONE)-POLY(ETHYLENE GLYCOL) (PECE) COPOLYMER AND PLURONIC F127 ++
Chang Yang Gong, Shuai Shi, Peng Wei Dong, Ma Ling Gou, Xing Yi Li, Yu Quan Wei and Zhi Yong Qian* State Key Laboratory of Biotherapy, West China Hospital, West China Medical School, and School of Life Sciences, Sichuan University, Chengdu, 610041, China.
ABSTRACT A novel kind of biodegradable thermosensitive composite hydrogel was successfully prepared in this work, which was a flowing sol at ambient temperature and became a nonflowing gel at body temperature. The composite hydrogel was composed of poly(ethylene glycol)-poly(ε-caprolactone)-poly(ethylene glycol) (PEG-PCL-PEG, PECE) and Pluronic F127 copolymer. By varying the composition of above two copolymers, in vivo degradation rate and in vitro drug release behavior could be controlled. Histopathological study of tissue at injection site showed no significant inflammatory reaction and toxicity, which means that the composite hydrogel might serve as a safe candidate as in situ gelforming controlled drug delivery system.
++
This work was financially supported by National Natural Science Foundation (NSFC20704027), National 863 Project (2007AA021900), Sichuan Prominent Young Talents Program (07ZQ026-033), and Sichuan Key Project of Science and Technology (2007SGY019), and Chinese Key Basic Research Program (2004CB518807). * Corresponding author: Tel: 86-28-85164063, Fax: 86-28-85164060, E-mail:
[email protected].
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1. INTRODUCTION Cancer is one of the most severe diseases and causes millions of death each year in the world. More than 1 million new cancer cases and 500,000 deaths from cancer are projected to occur in 2008 in USA [1]. Chemotherapy, as conventional cancer treatment, was widely used in clinical. However, conventional chemotherapy had severe side effects, which are very painful for patients to tolerate. Novel controlled drug delivery systems (DDS) are intended to deliver drugs at predetermined rates for predefined periods of time, which might overcome the shortcomings of conventional drug formulations, therefore could diminish the side effects and improve the life quality of the patients [2-3]. Thus, a suitable controlled drug delivery system is extremely important for chemotherapy. An optimal controlled drug delivery system should sustained release drugs in an extended period, and should be biodegradable, biocompatible, and non-toxic. Hydrogels are a special series of materials that could absorb a considerable amount of water while maintaining their three-dimension integrity in water. Over the past decades, the stimuli-sensitive copolymer hydrogel has attracted increasing attention owing to their smart responsibility to the environmental stimulus including chemical substances and changes in temperature, pH, or electric field [4-10]. Especially, the biodegradable thermosensitive physical crosslinked hydrogels consisted of hydrophobic and hydrophilic blocks have been extensively studied owing to their smart responsibility to the environmental stimulus, biodegradability, and biocompatibility. Therefore, biodegradable thermosensitive hydrogels, which might have potential biomedical applications, have been investigated as in situ gelforming system, such as controlled drug delivery, tissue repair, and cell encapsulation [1123]. In previous contributions, many biodegradable thermosensitive hydrogels have been studied, such as poly(ethylene glycol)-poly(D,L-lactic acid-co-glycolic acid) (PEG-PLGA) copolymers [24-27], poly(ethylene glycol)-poly(L-lactic acid) (PEG-PLLA) copolymers [10, 28], chitosan derivatives [29-32], methylcellulose, polyphosphazene, and etc. Poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol) triblock copolymer (PEG-PPG-PEG), known as Pluronic or Poloxamer, has been extensively studied as a potential drug delivery vehicle due to their excellent biocompatibility and thermosensitivity [33-34]. These copolymers have been widely used as emulsifiers, wetting agents, and solubilizers [35]. However, due to weak hydrophobicity of PPG block and high PPG content, the Pluronic copolymer forms a fast-eroding gel and can not persist longer than a few hours in vivo, which greatly restricted its application as in situ gel-forming systems. In our previous study, we prepared a new kind of biodegradable and injectable thermosensitive poly(ethylene glycol)–poly(ε-caprolactone)–poly(ethylene glycol) (PEGPCL-PEG, PECE) hydrogel controlled drug delivery system that undergoes sol-gel-sol transition. In vitro drug release behavior and in vivo gel formation and degradation test upon subcutaneous injection of PECE hydrogel in the mice model were also conducted [36]. PEG and PCL are materials that are biocompatible and have benn used in several FDA-approved products [37-42]. And PCL is lack of toxicity and has great permeability [43]. Due to combination of great advantages of PEG and PCL, the PECE copolymer might have great potential application in biomedical field. In this study, we prepared a new kind of biodegradable and injectable composite hydrogel based on PECE and Pluronic F127 copolymer. The composite hydrogels are free-flowing sol
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at room temperature and becomes gel at body temperature, and the influence of composition of the composite hydrogel on the sol-gel-sol transition was investigated. We wonder whether composite hydrogel with different composition would result in different drug release rate and in vivo gel degradation rate. Therefore, in vitro drug release behavior and in vivo gel formation and degradation test were conducted in this paper. The result showed that by altering the composition of composite hydrogel, in vitro drug release rate and in vivo gel degradation rate could be controlled, which was very important for their further application as injectable in situ gel-forming controlled drug delivery system.
2. MATERIALS AND METHODS 2.1. Materials Poly(ethylene glycol) methyl ether (MPEG, Mn=550, Aldrich, USA), ε-caprolactone (εCL, Alfa Aesar, USA), Pluronic F127 (Sigma, USA), hexamethylene diisocyanate (HMDI, Aldrich, USA), stannous octoate (Sn(Oct)2, Sigma, USA), bovine serum albumin (BSA, BR, BoAo Co. Ltd, China), and VB12 (Sigma, USA) were used without any further purification. All the materials used in this article were analytic reagent (AR) grade and used as received, expect BSA. BALB/c mice, at weight of 20±2g, were used for in vivo gel formation and degradation test. The animals were purchased from the Laboratory Animal Center of Sichuan University. The animals were housed at controlled temperature of 20-22oC, relative humidity of 50-60% and 12h light-dark cycles. Free access to food and water was allowed. All the animals would be in quarantine for a week before treatment. All animal care and experimental procedures were conducted according to Institutional Animal Care and Use guidelines.
2.2. Synthesis and Characterization of PECE Copolymer PECE copolymer was synthesized as described in our previous work according to Figure 1. Briefly, PEG-PCL diblock copolymer was prepared by ring opening polymerization of ε-CL initiated by MPEG using stannous octoate as catalyst; PEG-PCL-PEG triblock copolymer was synthesized by coupling PEG-PCL diblock copolymer using HMDI as the coupling agent [18, 36]. The just-obtained PECE copolymer was first dissolved in AR grade dichloromethane, and reprecipitated from the filtrate using AR grade excess cold petroleum ether. Then the mixture was filtered and vacuum dried to constant weight at room temperature. The purified copolymer was kept in air-tight bags before further use. The obtained PECE copolymer was characterized by FTIR (NICOLET 200SXV, Nicolet, USA), 1H-NMR (Varian 400 spectrometer, Varian, USA), and GPC (Agilent 110 HPLC, USA).
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2.3. Preparation of Composite Hydrogel Preparation of composite hydrogels was described in Figure 2. Different amount of Pluronic F127 copolymer were dissolved in icy cold deionized water to a transparent solution, and PECE solution at different concentration were prepared by dissolution of PECE copolymer in deionized water at a designated temperature then cooled to 4oC. Subsequently, the two prepared solutions were mixed together under mild agitation to obtain a homogeneous liquid solution. The final solution contained a given concentration and composition of the two copolymers to form different composite hydrogel samples. The composite hydrogel prepared in this work were listed in Table 1. Table 1. The composite hydrogel prepared in this work Code S1 S2 S3 S4 S5
PECE: Pluronic F127 (w/w) 100:0 70:30 50:50 30:70 0:100
Phase Transition behavior Sol-gel-sol Sol-gel-sol Sol-gel-sol Sol-gel-sol Sol-gel-sol
The concentration region with phase transition behavior (Wt%) 15% to 35% 25% to 35% 20% to 35% 20% to 35% 15% to 35% O
O
H3 C
x
H O
O
Y
(MPEG)
(epsilon-CL)
130oC
Sn(Oct)2
O O
O
H
O
H 3C
OCN(CH2 )6NCO
X
Y
(HMDI)
(MPEG-PCL)
130oC
Sn(Oct)2 O
O O H3C
OY
O
O
O OCHN(CH2)6 NHCO
X
PEG Figure 1. Synthesis scheme of PECE copolymer
PCL
XO
Y
PEG
CH3
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33
Figure 2. Preparation of composite hydrogel.
2.4. Sol-Gel-Sol Phase Transition Behavior Study Sol-gel-sol phase transition diagrams of composite hydrogel were recorded using test tube-inverting method [18, 36]. The sol-gel-sol transition was visually observed by inverting the vials, and conditions of sol and gel were defined as “flow liquid sol” and “no flow solid gel” in one minute respectively.
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In this study, the volume of the composite hydrogel solution was kept 0.5 mL in total regardless of the concentration. After incubated in water bath at 0oC for 20 minutes, the hydrous samples were slowly heated at a rate of 0.5oC/min, from 0oC to the temperature when precipitation occurred.
2.5. Scanning Electron Microscopy (SEM) of Composite Hydrogel SEM was employed to investigate morphology of composite hydrogel. The composite hydrogels were frozen in liquid nitrogen and lyophilized for 72 h. The hydrogels were sputtered with gold before observation. In this study, morphology of prepared composite hydrogels was examined on JEOL SEM (JSM-5900LV, JEOL, Japan).
2.6. In vitro Drug Release Behavior Freshly prepared VB12 loaded composite hydrogel were used to assay in vitro release behavior of hydrophilic drugs. In detail, 200µl of prepared VB12 loaded composite hydrogel containing 1mg VB12 were placed into 4 mL EP tubes and allowed to gel in an incubator at 37oC for 12 h. Then, the gels were immersed in 1 mL of PBS (pH=7.4) and were shaken at 100rpm at 37oC. At specific time intervals, all of the release media were removed and replaced by fresh release media. After centrifugation at 13000rpm for 10min, the supernatant of the removed release media were collected and stored at -20oC until analysis. The collected supernatants were detected on UV spectrophotometer at 362 nm to determine the concentration of VB12. BSA was used as model protein drug to determine the release behavior of protein or peptide from protein loaded composite hydrogel in vitro. The detail procedure was similar to the study of VB12 release, but the initial drug loading amount were 4mg. The amount of BSA present in the supernatant was determined by bicinchoninic acid (BCA) assay and BCATM Protein Assay Kit (PIERCE, USA) was used. The SDS-polyacrylamide gel electrophoretic (PAGE) analysis was used to assay the stability of BSA during the release period.
2.7. In vivo Gel Formation and Degradation Study In vivo gel formation and degradation tests were performed in BALB/c mice (20±2g). The composite hydrogel (S1, S2, S3, S4 and S5) at concentration of 30wt% were prepared. Then, each animal was subcutaneously injected with 0.5 mL of composite hydrogel solutions by a syringe with a 25 gauge needle. At predetermined time (2h, 6h, 1d, 3d, 7d, 10d, and 14d), the animals were sacrificed by cervical dislocation. And then the injection site was carefully cut open and the in situ formed gel was taken photo.
2.8. Histopathologic Study Tissue samples of injection site were obtained after taken photo at each time point (2h, 6h, 1d, 3d, 7d, 10d, and 14d), which described in section 2.7. Control group was only given
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similar volume of normal saline at the same time and same way. All the tissue samples were preserved in 10% buffered formaldehyde and were subsequently embedded in paraffin. Then, paraffin sections were stained with haematoxylin-eosin (HE) for histopathologic examination. In order to investigate the inflammatory reaction and toxicity of composite hydrogel at injection site, the histopathological changes of tissue at injection site were observed on light microscope. At day 14, composite hydrogels would be physiologically metabolized and removed from injection site. Therefore, we save the picture at day 14 as an illustration.
3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of PECE Copolymer The synthesis of PECE triblock copolymer has been described in our previous work [18, 36]. Briefly, ring-opening copolymerization of ε-CL onto MPEG was performed to synthesize PEG-PCL diblock copolymer using stannous octoate as catalyst. PEG-PCL diblock copolymer was then coupled using HMDI as coupling agent to produce the biodegradable PEG-PCL-PEG triblock copolymer. FTIR, 1H-NMR and GPC were used to characterize the chemical structure of PECE copolymer (data not shown). The results of FTIR, 1H-NMR and GPC indicated that the PECE triblock copolymer designed by controlling the feed composition was synthesized successfully. The number-average molecular weight (Mn) and the PEG/PCL block ratios of PECE triblock copolymer synthesized in this work were calculated from 1H-NMR spectra. The Mn of PECE triblock copolymer was 3408 and the PEG/PCL block ratios was 960/2448, which was consistent with theoretical value calculated from feed ratio (Mn=3300, PEG/PCL= 1100/2200). Mn and polydispersity (PDI, Mw/Mn) of PECE copolymer determined by GPC were 4391 and 1.30 respectively.
3.2. Temperature-Dependent Sol-Gel-Sol Transition Behavior The composite hydrogel prepared in this work were composed of PECE and Pluronic F127 hydrogel, which individually presented sol-gel-sol transition. Due to combination of PECE hydrogel and Pluronic F127 hydrogel, the composite hydrogel also showed sol-gel-sol transition. As presented in Table 1, composite hydrogel based on PECE and Pluronic F127 copolymers from S1 to S5 showed temperature-dependent reversible sol-gel-sol phase transition. The composite hydrogels flowed freely at lower temperature, but became gelation at body temperature about 37oC (Figure 3). Figure 4 presented the sol-gel-sol phase transition diagram of prepared composite hydrogel. When the copolymer concentrations are above the critical gelation concentration (CGC), composite hydrogel changed from “sol” phase to “gel” phase with increase in temperature to the lower critical gelation temperature (LCGT). With further increase of temperature to upper critical gelation temperature (UCGT), the sol phase occurs.
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o
Temperature ( C)
Figure 3. Photograph of composite hydrogel (30wt%). S1 at 10oC (A) and 37oC (B); S3 at 10oC (C) and 37oC (D); S5 at 10oC (E) and 37oC (F);
120 110 100 90 80 70 60 50 40 30 20 10 0
S1 (ECE:100%, F127:0%) S2 (ECE:70%, F127:30%) S3 (ECE:50%, F127:50%) S4 (ECE:30%, F127:70%) S5 (ECE:0%, F127:100%)
Sol Gel Sol 0
5
10
15 20 25 30 35 Concentration (wt%)
40
Figure 4. Sol-gel-sol transition phase diagram of composite hydrogel .
According to Figure 4, pure PECE hydrogel (S1) and pure Pluronic F127 hydrogel (S5) both have a CGC of approximately 15wt%, but S5 have a much wider gelation window than that of S1. The UCGT of S5 at the concentration of 30wt% and 35wt% was not detected in the temperature range of 0oC to 100oC. The CGC of S2, S3, and S4 were 25wt%, 20wt%, and 20wt% respectively, which were much higher than that of two pure hydrogels. By mixing the two hydrogel together, the CGC of the composite hydrogel increased accordingly. CGC of S2 increased approximately 10wt% than that of S1, whereas CGC of S4 increased approximately 5wt% compared with that of S5. This phenomenon indicated that with regard to CGC the influence of Pluronic F127 hydrogel on PECE hydrogel was more dramatic than that of PECE hydrogel on Pluronic F127 hydrogel.
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As shown in Figure 4, with increase in ECE hydrogel content in composite hydrogel, the UCGT decreased significantly, whereas the LCGT increased slightly. The UCGT of S5 hydrogel at concentration of 30wt% and 35wt% could not detected in the range of 0oC to 100oC, but the UCGT at concentration of 30wt% and 35wt% were detected in S4 and S3 hydrogel respectively due to the increase content of PECE hydrogel. Therefore, it was obvious that sol-gel-sol transition behavior of composite hydrogel was depended on the composition of the PECE and Pluronic F127 hydrogel. In fact, by altering the composition of composite hydrogel, the temperature range of sol-gel-sol phase transition could be broadened to a certain extent, which might be very useful for their further application as injectable in situ gel-forming drug delivery system in different situation.
3.3. Morphology of Composite Hydrogel Interior morphology of composite hydrogel was investigated by SEM. The composite hydrogels were frozen in liquid nitrogen and lyophilized for 72 h before the test. Figure 5-A, B and C showed the morphology of S1, S3 and S5 hydrogel respectively. All the hydrogel samples showed porous three-dimension structure, but the shape and mesh size of pores in the hydrogel were different. S1 showed approximately spherical pore with small mesh size, whereas S5 showed cylindrical-like shape with relatively larger mesh size compared to S1. S3 hydrogel, composed of 50% PECE hydrogel and 50% Pluronic F127 hydrogel, also showed spherical pore, but have a larger mesh size compared to pure PECE hydrogel (S1). The morphology of S3 hydrogel suggested that two compositions of S3 hydrogel both have great influence on S3 hydrogel interior structure.
Figure 5. SEM photograph of composite hydrogel (×500). A: S1 hydrogel (30wt%); B: S3 hydrogel (30wt%); B: S5 hydrogel (30wt%)
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3.4. In vitro Drug Release Profile of Composite Hydrogel In vitro release profiles of VB12 from VB12 loaded composite hydrogel in PBS solution were investigated, and the results were shown in Figure 6. According to Figure 6, VB12 could release from composite hydrogel in an extended period and the composition of composite hydrogel had great effect on VB12 release profile. S5 hydrogel disappeared completely in PBS solution in 12h, whereas S1 and S3 hydrogel could maintain their integrity in the whole release period. VB12 released faster and reached higher cumulative release rate (94.2%) from S5 hydrogel compared to S1 and S3 hydrogel. In S1 hydrogel, an initial burst release of 20.2% of loaded VB12 occurred in the first one hour, followed by release of 82.9% in 7 days, whereas, in S3 hydrogel, the cumulative release rate of one hour and 7 days were 22.6% and 87.2% respectively. In this study, BSA was used as a model drug to investigate in vitro release behavior of protein or peptide drugs from composite hydrogel, and the data were summarized in Figure 7. As presented in Figure 7-A, higher PECE hydrogel content in composite hydrogel resulted in slower release rate of BSA from composite hydrogel. The cumulative release rate of BSA from S1, S3, and S5 hydrogel were 32.7%, 46.1%, and 90.7% respectively. SDS-PAGE was performed to evaluate the stability of BSA in the in vitro release period. According to Figure 7-B, the major band for BSA appeared at about 67KD (lane2 to lane10) according to the protein marker, which means that BSA was stable in all the composite hydrogel and the whole release period. Thus, composition of composite hydrogel substantially affected the drug release behavior of composite hydrogel, where higher Pluronic F127 content resulted in higher cumulative release rate and higher initial burst release rate. By altering the composition of PECE and Pluronic F127 copolymers in composite hydrogel, drug release rate could be varied and controlled.
3.5. In vivo Gel Formation and Degradation Behavior The application of composite hydrogel as in situ gel-forming drug delivery system was tested by subcutaneous injection with 0.5 mL of hydrogel solution (from S1 hydrogel to S5 hydrogel, 30wt%) into BALB/c mice. In Figure 8, from A to E showed the photograph took at 1st day, 3rd day, 7th day, 10th day and 14th day after composite hydrogel injection respectively ( From left to right: S4, S3, S2 and S1 hydrogel). F and G presented the photograph took at 2nd hour and 6th hour after S5 hydrogel injection. The injectable sol state of composite hydrogel became gel in mice just in a few seconds and maintained its integrity in vivo in an extended period. The formed gel of S1 and S2 hydrogel were spherical shaped rather than spread to form sheet shaped. In the process of time after composite hydrogel injection, the in vivo formed gel decreased in size and finally disappeared due to the degradation. Hydrogels of S5, S4, S3, and S2 were disappeared in vivo at 6th hour, 7th day, 10th day, and 14th day after injection respectively. At 14th day after injection (Figure8-E), only a small mount of S1 hydrogel was found in vivo, which indicated that S1 hydrogel could sustain longer than 14days.
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Cumulative release (%)
100 80 60 40
S1 (ECE:F127=100:0) S3 (ECE:F127=50:50) S5 (ECE:F127=0:100)
20 0
0
24
48
72 96 120 144 168 Time (h)
Figure 6. In vitro release behavior of VB12 from composite hydrogel.
Cumulative release (%)
100 S1 (ECE:F127=100:0) S3 (ECE:F127=50:50) S5 (ECE:F127=0:100)
80 60 40 20 0
A 0
48
96 144 192 240 288 336 384 Time (h)
Figure 7. In vitro release behavior of BSA from composite hydrogel (A) and SDS-PAGE results of BSA in vitro release profile (B) (Lane 1: marker; Lane 2: BSA standard; Lane 3: S1 at 24th hour; Lane 4: S1 at 168th hour; Lane 5: S1 at 360th hour; Lane 6: S5 at 1st hour; Lane 7: S5 at 12th hour; Lane 8: S3 at 24th hour; Lane 9: S3 at 168th hour; Lane 10: S3 at 360th hour;).
According to Figure 8, we could find that the sustained time of the composite hydrogel in vivo could be controlled by altering the composition of above two hydrogel, which might be very useful for its potential application in injectable in situ gel-forming drug delivery system.
3.6. Histopathological Study All samples were histopathological observed by light microscope, and no significant histopathological changes were observed due to composite hydrogels (Figure 9). As shown in Figure 9, the morphology of the composite hydrogel-treated tissue (Figure 9-B, C) did not show significant difference when compared with control group (Figure 9-A). Histopathological study of tissue at injection site showed no significant inflammatory reaction and histopathological changes, which mean the composite hydrogel were nontoxicity and might serve as a safe candidate as in situ gel-forming drug delivery system.
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Figure 8. In situ gel formation and degradation behavior of composite hydrogels. From A to E showed the photograph took at 1d, 3d, 7d, 10d and 14d after injection respectively ( From left to right: S4, S3, S2 and S1). F and G presented the photograph took at 2h and 6h after Pluronic F127 hydrogel injection.
Figure 9. Photograph of tissue samples from injection site after composite hydrogel injection for 14d ( ×400). A: Control group; B: S3 hydrogel injection group; C: S1 hydrogel injection group.
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CONCLUSION A series of novel biodegradable and injectable thermosensitive composite hydrogel were successfully prepared. The obtained composite hydrogel underwent sol-gel-sol transition as temperature increased, which was a flowing sol at ambient temperature and became a nonflowing gel at physiological temperature, and the sol-gel-sol phase transition behavior of the copolymers aqueous solutions was determined using the test tube inverting method. By varying the composition of PECE and Pluronic F127 copolymers, the in vivo degradation rate and in vitro drug release profile of composite hydrogel could be controlled, which was very useful for its potential applications as an in situ gel-forming controlled drug delivery system. Histopathological study suggested the composite hydrogel is a safe candidate for its applicantion in biomedicine fields.
REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[13]
Jemal, A; Siegel, E; Ward, E; Hao, Y; Xu, J; Murray, T; Thun, MJ. Cancer statistics, CA Cancer J Clin, 2008, 58, 71-96. Qiu, Y; Park, K. Environment-sensitive hydorgels for drug delivery. Adv Drug Deliver Rev, 2001, 51, 321-339. Hatefi, A; Amsden, B. Biodegradable indectable in situ forming drug delivery systems. J Control Rel, 2002, 80, 9-28. Suzuki, A; Tanaka, T. Phase transition in polymer gels induced by visible-light. Nature, 1990, 346, 345-347. Galaev, IY; Mattiasson, B. ‘Smart’ polymers and what they could do in biotechnology and medicine. Trends in Biotechnol, 1999, 17, 335-340. Nanjawade, BK; Manvi, FV; Manjappa, AS. In situ-forming hydrogels for sustained ophthalmic drug delivery. J Control Rel, 2007, 122, 119-134. Miyta, T; Asami, N; Uragami, T. A reversibly antigen-responsive hydrogel. Nature, 1999, 399, 766-769. Lee, KY; Mooney, Dj. Hydrogels for tissue engineering. Chem Rev, 2001, 101, 18691879. Kamath, KR; Park, K. Biodegradable hydrogels in drug delivery. Adv Drug Deliv Rev, 1993, 11, 59-84. Jeong, B; Bae, YH; Lee, DS; Kim, SW. Biodegradable block copolymers as injectable drug-delivery systems. Nature 1997, 388, 860-862. Jeong B; Kim SW; Bae YH. Thermosensitive sol-gel reversible hydrogels. Adv Drug Deliver Rev, 2002, 54, 37-51. Kissel, T; Li, Y; Unger, F. ABA-triblock copolymers from biodegradable polyerster Ablocks and bydrophilic poly(ethylene oxide) B-blocks as a candidate for in situ forming hydrogel delivery systems for proteins. Adv Drug Deliver Rev, 2002, 54, 99-134. Gariépy, ER; Leroux, JC. In situ-forming hydrogels—review of temperature-sensitive systems. Eur J Pharm Biopharm, 2004, 58, 409-426.
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[14] Dimitrov, I; Trzebicka, B; Muller, AH; Dworak, A; Tsvetamov, CB. Thermosensitive water-soluble copolymers with doubly responsive reversibly interacting entities. Progress Polym Sci, 2007, 32, 1275-1343. [15] Song, MJ; Lee, DS; Ahn, JH; Kim, DJ; Kim, SW. Thermosensitive sol-gel transition behaviors of poly(ethylene oxide)/aliphatic polyester/poly(ethylene oxide) aqueous solutions. J Polym Sci, Part A, Polym Chem, 2004, 42, 772-784. [16] Liu, CB; Gong, CY; Huang, MJ; Wang, JW; Pan, YF; Zhang, YD; et al. Thermoreversible Gel-Sol Behavior of Biodegradable PCL-PEG-PCL Triblock Copolymer in Aqueous Solutions. J Biomed Mater Res B, 2008, 84, 165-175. [17] Choi, SW; Choi, SY; Jeong, B; Kim, SW; Lee, DS. Thermoreversible gelation of poly(ethylene oxide) biodegradable polyester block copolymers.II. J Polym Sci, Part A, Polym Chem, 1999, 37, 2207-2218. [18] Gong, CY; Qian, ZY; Liu, CB; Huang, MJ; Gu, YC; Wen, YJ; et al. A Thermosensitive Hydrogel Based on Biodegradable Amphiphilic Poly(ethylene glycol)polycaprolactone-poly(ethylene glycol) block Copolymers. Smart Mater Struct, 2007, 16, 927-933. [19] Liu, CB; Gong, CY; Pan, YF; Zhang, YD; Wang, JW; Huang, MJ; et al. Synthesis and Characterization of a Thermosensitive Hydrogel Based on Biodegradable Amphiphilic PCL-Pluronic(L35)-PCL block Copolymers. Colloids Surfaces A, 2007, 302, 430-438. [20] Bromberg, LE; Ron, ES. Temperature-responsive gels and thermogelling polymer matrices for protein and peptide delivery. Adv Drug Deliver Rev, 1998, 31, 197–221. [21] Li, J; Li, X; Ni, X; Leong, KW. Synthesis and Characterization of New Biodegradable Amphiphilic Poly(ethylene oxide)-b-Poly[(R)-3-hydroxybutyrate]-b-Poly(ethylene oxide) Triblock Copolymers. Macromolecules, 2003, 36, 2661–2667. [22] Lee, JW; Hua, FJ; Lee, DS. Thermoreversible gelation of biodegradable poly(εcaprolactone) and poly(ethylene glycol) multiblock copolymers in aqueous solutions. J Control Rel, 2001, 73, 315-327. [23] Li, J; Ni, X; Leong, KW. Injectable Drug-Delivery Systems Based on Supramolecular Hydrogels Formed by Poly(ethylene oxide)s and Cyclodextrin. J Biomed Mater Res, Part A, 2003, 65, 196-202. [24] Jeong, B; Bae, YH; Kim, SW. Biodegradable thermosensitive micelles of PEG-PLGAPEG triblock copolymers. Colloid Surface B, 1999, 16, 185-193. [25] Lee, DS; Shim MS; Kim SW; Lee H; Park I; Chang T. Novel thermoreversible gelation of biodegradable PLGA-block-PEO-block-PLGA triblock copolymers in aqueous solution. Macromol Rapid Commun, 2001, 22, 587-592. [26] Chung, YM; Simmons, KL; Gutowska, A; Jeong, B. Sol-gel transition temperature of PLGA-g-PEG aqueous solutions. Biomacromolecules, 2002, 3, 511-516. [27] Jeong, B; Bae, YH; Kim, SW. Thermoreversible Gelation of PEG-PLGA-PEG Triblock Copolymer Aqueous Solutions. Macromolecules, 1999, 32, 7064-7069. [28] Choi, SW; Choi, SY; Jeong, B; Kim, SW; Lee DS. Thermoreversible gelation of poly(ethylene oxide) biodegradable polyester block copolymers.II. J Polym Sci, Part A, Polym Chem, 1999, 37, 2207-2218. [29] Schuetz, YB; Gurny, R; Jordan, O. A Novel Thermoresponsive Hydrogel Based on Chitosan. Eur J Pharm Biopharm, 2008, 68, 19-25.
A Novel Thermosensitive Composite Hydrogel Based on…
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[30] Bhattarai, N; Ramay, HR; Gunn, J; Matsen, FA; Zhang, M. PEG-grafted chitosan as an injectable thermosensitive hydrogel for sustained protein release. J Control Release, 2005, 10, 609-624. [31] Berger, J; Reist, M; Mayer, JM; Felt, O; Peppas, NA; Gurny, R. Structure and interactions in covalently and ionically crosslinked chitosan hydrogels for biomedical applications. Eur J Pharm Biopharm, 2004, 57, 19–34. [32] Chenite, A; Chaput, C; Wang, D; Combes, C; Buschmann, MD; Hoemann, CD; et al. Novel injectable neutral solutions of chitosan form biodegradable gels in situ. Biomaterials, 2000, 21, 2155-2161. [33] Gilbert, JC; Richardson, JL; Davies, MC; Palin, KJ; Hadgraft. The Effect of Solutes and Polymers on The Gelation Properties of Pluronic F-127 Solutions for Controlled Drug Delivery. J Controlled Release, 1987, 5, 113-118. [34] Xiong, XY; Tam, KC; Gan, LH. Synthesis and aggregation behavior of Pluronic F127/Poly(lactic acid) block copolymers in aqueous solutions. Macromolecules, 2003, 36, 9979-9985. [35] Rangelov, S; Dimitrov, P; Tsvetanov, CB. Mixed block copolymer aggregates with tunable temperature behavior. J Phys Chem B, 2005, 109, 1162-1167. [36] Gong, CY; Shi, S; Dong, PW; Kan, B; Gou, ML; Wang, XH; et al. Synthesis and Characterization of PEG-PCL-PEG Thermosensitive Hydrogel. Int J Pharm, 2009, 365, 89-99. [37] Chung, YM; Simmons, KL; Gutowska, A; etc. Sol-gel transition temperature of PLGAg-PEG aqueous solutions. Biomacromolecules, 2002, 3, 511-516. [38] Chen, X; Qian, ZY; Gou, ML; Chao, GT; Zhang, YD; Gu, YC; Huang, MJ; Wang, JW; Pan, YF; Wei, YQ; Chen, JP, Tu, MJ. Acute Oral Toxicity Evaluation of Biodegradable and pH-sensitive Hydrogel based on Polycaprolactionem, Poly(ethylene glycol) and Methylacrylic acid (MAA). J Biomed Mater Res, Part A, 2008, 84, 589-597. [39] Bea, SJ; Suh, JM; Sohn, YS; Bae, YH; Kim, SW; Jeong, B. Thermogelling Poly(caprolactone-b-ethylene glycol-bcaprolactone) Aqueous Solutions. Macromolecules, 2005, 38, 5260-5265. [40] Li, J; Li, X; Ni, X; Wang, X; Li, H; Leong, KW. Self-assembled supramolecular hydrogels formed by biodegradable PEO-PHB-PEO triblock copolymers and εcyclodextrin for controlled drug delivery. Biomaterials, 2006, 27, 4132-4140. [41] Li, Z; Ning, WWJ; etc. Controlled gene delivery system based on thermosensitive biodegradable hydrogel. Pharmacol Res, 2003, 20, 884-888. [42] Iza, M; Stoianovici, G; Viora, L ; etc. Hydrogels of poly(ethylene glycol): mechanical characterization and release of a model drug. J Control Rel, 1998, 52, 41-51. [43] Zhou, SB; Deng, XM; Yang, H. Biodegradable poly(ε-caprolactone)-poly(ethylene glycol) block copolymers: characterization and their use as drug carriers for a controlled delivery system. Biomaterials, 2003, 24, 3563-3570.
In: Encyclopedia of Polymer Research Ed: Caroline E. Jones
ISBN: 978-1-61761-926-7 © 2011 Nova Science Publishers, Inc.
Chapter 3
NITROGEN-CONTAINING LIGANDS ANCHORED ONTO POLYMERS AS CATALYST STABILIZER FOR CATALYTIC ENANTIOSELECTIVE REACTIONS Christine Saluzzo *and Stéphane Guillarme Université du Maine UCO2M UMR-CNRS 6011, Av. O. Messiaen, 72085 Le Mans Cedex 9, France
ABSTRACT This paper reviews the recent progress made in insoluble polymer supported amino alcohols, amino thiols, oxazolines, salens, sulphonamides, oxazaborolidines and diamines ligands. This paper deals also with various approaches of stabilization of the catalytic system by immobilization of the chiral catalyst onto the polymer by the way of immobilization of the chiral ligand. Different types of ligand immobilization are presented: pendant ligands anchored on a polymer prepared by a polymer reaction, ligands on the backbone prepared by copolymerization and molecular imprinting technique. Examples of their use, performance and recyclability in a variety of enantioselective reactions such as alkylation and reductions of C=O bonds (hydrogenation, hydrogen transfer reduction) reduction of C=N bonds, C-O bond formations (epoxidation, dihydroxylation), C-C bond formations (Diels Alder, cyclopropanation, aldolisation, allylic substitution) and oxidation … are presented.
1. INTRODUCTION The need to develop effective methods for enantioselective synthesis is becoming ever more important as only a single enantiomer of a racemic bioactive compound is generally required for pharmaceuticals [1], agrochemicals [1,2], flavour [1,3] or fragrance [1,3]. Homogeneous asymmetric catalysis has been used to perform a variety of transformations *
Corresponding author: e-mail :
[email protected]
46
Christine Saluzzo and Stéphane Guillarme
under mild conditions with high enantioselectivity. As these fine and specialty chemicals are manufactured to meet high and well defined standards of purity compatible with the desired performance, chemical industry had to adopt efficient and clean technology [4]. From an industrial point of view, homogeneous methods remain unpratical, particularly due to the high cost of the chiral metal catalysts (range of the chiral catalyst loadings from 1 to 10% and in some cases up to 30%) and the difficulty of their recovery and reuse. Thus, from an economic, environmental and technical point of view, homogeneous supported catalysis is preferable to homogeneous catalysis because of the handling, separation and recycling abilities [5]. Many approaches have been employed to immobilize the homogeneous catalyst. [6]. In the classical immobilization with organic polymer, the chiral ligand units are anchored onto polymers (type 1, Scheme 1). The incorporation of chiral ligands on the main chain of the polymers is another approach (type 2, Scheme 1) and is done by copolymerization of the chiral ligand with a linker. The last but less known approach consists in the use of an imprinted polymer (type 3, Scheme 1), an extensively used methodology to form artificial antibodies. The imprinted polymer is based on a copolymerization and a crosslinking of a polymer around a molecule used as a printed molecule (PM). This latter included into the polymer by means of an interaction with functional groups is then removed, thus leaving its imprint in the polymeric matrix. If a chiral PM is used, the formed chiral cavity is able to act as a center of molecular recognition [7]. L GP
type 1
L M
PM
L
Ligand Ligand L
M M type 2 Ligand
L M Ligand
M M PG: polymerizable group M: metal PM: printed molecule
PM
L L L M L
type 3
Scheme 1.
The aim of this review is to discuss the contribution of soluble polymer-supported ligands and insoluble polymer-supported ligands to asymmetric catalysis in the field of reduction of C=O bonds, cyclopropanation, Diels-Alder, alkylation, allylation, dihydroxylations, epoxydation reactions, kinetic resolution of terminal epoxide ….. by means of nitrogen containing ligands complexed with metal as asymmetric catalyst. Each type of asymmetric homogeneous supported catalyst will be examined successively from the perspective of using phosphine-free nitrogen-containing ligands complexed with a metal. Results were analyzed in comparison to the results in the literature for similar systems.
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
47
2. REDUCTION OF C=O BOND The three major catalytic reduction procedures which have emerged are enantioselective hydride reduction, hydrogenation and hydrogenation transfer reduction (HTR) close to the hydrosilylation which represent only few examples in asymmetric polymer-supported catalyst.
2.1. Hydrogenation In homogeneous catalysis, various phosphines have been developped for asymmetric hydrogenation [8]. However, for hydrogenation of unfunctionalized ketone, a remarkably high reactivity emerges when the Ru compounds are further complexed with a 1,2 diamine ligand. Thus, the precatalyst RuCl2(phosphine)2(1,2-diamine) combined with an inorganic base in isopropanol is considered as one of the most powerful systems for the hydrogenation of ketones (Scheme 2, i-PrOH as solvent) [9, 10].
O Ar
1,2-diamine-(S)-BINAP-RuCl2
R1
+ H2 (1 MPa) t-BuOK, i-PrOH or i-PrOH-DMF
OH H R Ar 1
Scheme 2.
Close to polymer bound-BINAP/diamine ruthenium precatalyst [11], complexes of BINAP or their derivatives in the presence of polymer bound diamine have been employed. These catalytic systems have been mainly studied by Itsuno with immobilized diphenylethylenediamine (DPEN). As the use of N-substituted 1,2-diamine ligands decreased the catalytic activity of hydrogenation of simple ketones contrary to free diamine ligands, they have been less employed. However, N-substituted 1,2-diamine ligands are rather effective in the hydrogenation of α-substituted ketone, such as α-amido ketones. Thus, two types of immobilized DPEN have been performed. Functionalized DPEN for subsequent grafting onto polymer or copolymerization are presented (Scheme 3). If free primary amines were maintained, grafting was realized by the modification of phenyl groups into phenolic ones [12, 13] and copolymerization by using a phenolic ether group containing a vinylphenyl functionality as monomer. ((S,S)-2, (R,R)-3 and (S,S)-4, -5, 6; (R,R)-5, -6, -7, -9 respectively (Scheme 3) [13, 14, 15]. The first type of polymers was formed by reaction of the phenoxide derived from DPEN with 1% crosslinked chloromethylated polystyrene (Schemes 4 and 5). It has been observed that in the cases of higher content of chloromethyl group or a higher degree of crosslinking, some unreacted chloromethyl groups remain.
48
Christine Saluzzo and Stéphane Guillarme Homogeneous ligands Functionalized DPEN for subsequent grafting or copolymerization OH
HO
H2 N
O
OH
NH2 H2N NHBoc (R,R)-3
H2N NH2 (S,S)-2
(S,S)-DPEN (S,S)-1
CH2
CH2 NH2 H2 N (R,R)-DPEN (R,R)-1
O
OMe
H2C
O
MeO
MeO
O
CH2
n
O
n
OMe H2C
CH2
BocHN NHBoc n = 1, (R,R)-5; (S,S)-5 n = 4, (R,R)-6; (S,S)-6 O OR
O
O
n
BocHN NHBoc (S,S)-4 H2N NHBoc (R,R)-7a, n = 1, R = Me (R,R)-7b, n = 1, R = Bn (R,R)-7c, n = 4, R = Me (R,R)-7d, n = 4, R = Bn H2N
NH Bn
H2N
NH
(R,R)- N-benzyl DPEN (R,R)-8
O
CH2
(R,R)-9
Scheme 3.
x
y
CH2Cl +
z
CH2 O
NaH, DMF
H2N H2N
O CH2
(
(S,S)-2
z
)
x
y
(S,S)-10a: x = 0.01, y = 0.2, z = 0.79 (S,S)-10b: x = 0.05, y = 0.2, z = 0.75
Scheme 4.
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
x
y
x
z
cross linkage 'CH2)nX
y
49
z
cross linkage
1) NaH, DMF 2) HCl, THF 3) Et3N, THF
'CH2)n O
+ (R,R)-3
O H2N crosslinkage: (R,R)-11a: x = 0.10, y = 0.01 (R,R)-11b: x = 0.10, y = 0.05 crosslinkage: (R,R)-12a: x = 0.10, y = 0.05 (R,R)-12b: x = 0.10, y = 0.10
NH2
CH2(OCH2CH2)2OCH2
Scheme 5.
To perform the catalytic homogeneous-supported hydrogenation of acetophenone with precatalyst ((S,S)-10a, (S,S)-10b, (R,R)-11a, (R,R)-11b, (R,R)-12a or (R,R)-12b)-(R)-BINAPRuCl2, it has been shown that the suitable solvent is a mixture of i-PrOH/DMF/ (1/1, v/v). If i-PrOH is employed alone, hydrogenation does not take place due to little swelling of the polymer catalyst (Scheme 2, Ar = Ph). Moreover, a lower degree of crosslinking gave better reactivity; higher ees and conversion are obtained (Table 1). Table 1. Enantioselective hydrogenation of acetophenone with diamine polymersupported-BINAP-RuCl2 Ligand-BINAP-RuCl2 10aa 10ba 11ab 11bb 12ab 12bb
Yield (%) 100% 39% 91% 46% >99% 70%
Ee (%) 73% 68% 70% 67% 23% 65%
Ref. [12] [12] [13] [13] [13] [13]
a
Diamine/BINAP/RuCl2/ketone/t-BuOK : 1/1/1/200/20 Diamine/BINAP/RuCl2/ketone/t-BuOK : 2/1/1/1000/20
b
The best results are observed if xylBINAP is used instead of BINAP (ie 93% ee, 100% yield in the presence of 10a). The immobilized (S,S)-10a-(R)-BINAP-RuCl2 can be reused at least four times without loss of activity and selectivity (from the first to the fourth reuse ee is about 73-74% with 100% yield) [12]. Enantioselectivities obtained with cocatalyst DPEN modified by a vinylphenyl group copolymerized under radical conditions (polymers 13 [14] and 14, 15 (Schemes 6 and 7) [15] and 16 (Scheme 8) [13] also exhibited almost the same level as those obtained from the low-
50
Christine Saluzzo and Stéphane Guillarme
molecular-weight counterpart in solution system (73-80% and 80% ee respectively with conversions always >99% for polymer containing less than 5% DVB).
x
y
CH2 O
x
OMe
MeO
y
H2C O
MeO
OMe CH2
+ (S,S)-2
H2C
O
O
H2N
NH2
13a: x = 0.05, y = 0.95 13b: x = 0.10, y = 0.90 Scheme 6.
)
R2
( R1 CH2 O
n
H2C O
n
R2
CH2 O
+ R1
n
O H2C
n
O
O
a: MMA: R1 = OMe R2 = Me BocHN NHBoc b: HEMA: R1 = O(CH2)2OH H N NH2 2 R2 = Me 5: n = 1 14: n = 1 c: BMA: R1 = OBu 6: n = 4 15: n = 4 R2 = Me Scheme 7.
Catalytic systems derived from 13 and 14, 15 and 16a have successfully been recycled from four (for 13 [14]), 14 [15] and 15 [15]) to sixteen times (for 16a [13]) without any change of activity and selectivity. They were also tested with other aromatic ketones and high yields and enantioselectivities were observed (some examples are presented in Table 2).
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
x
+
1) AIBN 2 mol% THF 70°C 2) HCl, THF 3) Et3N, THF
y
51
z
) (
(CH2)n O
(R,R)-7 OR H2N NH2 16a: R = Me, n = 1, x = 0.10, z = 0.05 16b: R = Bu, n = 1, x = 0.10, z = 0.10
Scheme 8.
Table 2. Hydrogenation of several ketones with polymeric diamine/BINAP/RuCl2 catalyst Ketone
Diamine
Time (h)
acetophenone
13aa or 13ba 14ab 15cb
propiophenone
13aa 14ab 15cb 16a 13aa 14ab 15cb 16ac
1’-acetonaphtone
Ee (%) (config.) 77 (R) 78 (R) 79 (R)
Ref.
18 5 5
Conv. (%) 100 >99 >99
4 12 12 24 18 12 12 24
100 >99 >99 99 100 >99 >99 >99
82 (R) 84 (R) 83 (R) 82 96 (R) 96 (R) 96 (R) 97
[14] [15] [15] [13] [14] [15] [15] [13]
[14] [15] [15]
a
Diamine/BINAP/RuCl2/ketone/t-BuOK : 2/1/1/1000/20 Diamine/BINAP/RuCl2/ketone/t-BuOK : 2/1/1/200/20 c Diamine/BINAP/RuCl2/ketone/t-BuOK : 1/1/1/200/20 b
For the hydrogenation of the racemic α-(N-benzoyl-N-methylamino)propiophenone, (Scheme 9) polymers containing N-substituted DPEN were prepared by radical copolymerization of (R,R)-9 (Scheme 3) with various crosslinking agents (Scheme 10) [16]. O
OH
Ph
+ H2 (1 MPa) Ph
N O
Scheme 9.
1,2-diamine-(R)-BINAP-RuCl2 t-BuOK, i-PrOH-DMF
Ph N
Ph O
52
Christine Saluzzo and Stéphane Guillarme R1 0.10
R2
R3 0.88
R4
0.02
R3 O HN H2N 17a: R1 = R3 = H, R2 = Ph, R4 = C6H4 17b: R1= H, R2 = Ph, R3 = Me, R4 = CO2CH2CH2OCO 17c: R1= Me, R2 = CO2CH2CH2OH, R3 = Me, R4 = CO2CH2CH2OCO 17d: R1= Me, R2 = CO2CH2CH2OH, R3 = H, R4 = C6H4 17e: R1 = R3 = Me, R2 = CO2Me, R4 = CO2CH2CH2OCO 17f: R1 = H, R2 = CONHi-PrMe, R3 = Me, R4 = CO2CH2CH2OCO 17g: R1 = H, R2 = C6H4CH2OH, R3 = Me, R4 = CO2CH2CH2OCO
Scheme 10.
For the catalytic reduction, the solvent of choice was the mixture 1/1 (v/v) of i-PrOH/DMF (conditions for good swelling of the polymeric ligands 17). The polarity of the crosslinking agent has a direct influence on the results of the catalysis. The precatalyst obtained with 17a produced a low conversion (7%) and an excellent diastereoselection (>99%) for an enantiomeric excess of 61%. A slight improvement was observed for 17b formed with EGDMA a polar crosslinker (conversion: 10%, ee: 88%). Compared to them 17e present a better conversion (89%) with >99% of de and ee. The precatalysts prepared from 17c, 17d, 17f, 17g from better hydrophilic polymer support have exhibited excellent catalytic performance for this dynamic kinetic resolution of racemic α-amide ketone via hydrogenation to yield the corresponding syn β-amide alcohol (100% conversion, de >99% and ee ranging from 90 to >99%). In addition, the polymer-supported catalyst could be reused several times without loss of catalytic activity.
2.2. Hydrogenation Transfer Reduction Hydrogenation transfer reduction (HTR) is defined as a reduction of mainly C=O and C=N bonds employing an hydrogen donor (DH2) such as cyclohexene, cyclohexadiene, alcohols, formic acid or hydrazine, in the presence of a catalyst, avoiding all the risks inherent to molecular hydrogen (Scheme 11). For this reason and for a procedural simplicity, HTR can be regarded as an alternative for the asymmetric hydrogenation [17]. For HTR, the first heterogeneous catalytic systems were formed with homogeneous-supported phosphines, which were also oxygen sensitive contrary to amine ligands. Moreover, nitrogen-containing ligands could be easily polymerized. O R1 Scheme 11.
OH
cat*
R2
+ DH2
OH +
R1
R2
R1
R2
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
53
2.2.1. 1,2-diamine and derivatives As for hydrogenation, most of the chiral ligands involved in heterogeneous HTR have been prepared starting from functionalized DPEN (Scheme 12). In 1993, Lemaire found that N,N-dimethyl-1,2-diphenylethylenediamine (18) complexed with Rh could be employed as ligand for asymmetric HTR, leading to 67% ee and 100% for the reduction of acetophenone and methyl phenyl glyoxylate respectively [18]. Due to the presence of the amino groups, this C2 diamine could be easily transformed into diurea and dithiourea derivatives.
HN
HN
(S,S)-18
H2N
NH2
NH SO2
NH2
(S,S)-19
(R,R)-20
BocHN
NH SO2
COOH (R,R)-21
BocHN
NH SO2
O ( ) COOH n (R,R)-22
Scheme 12.
As good results for the asymmetric HTR of acetophenone were obtained (conversion 100% and 91% ee) with diurea [19], not only copolymerization of diamine 18 has been performed but 1,2-cyclohexyldiamine 19 was also used. Thus pseudo-C2 polyamide 23, polyureas 24, 24 and 26 or polythioureas 27 were prepared by polycondensation with diacid chloride, diisocyanate [20] or dithioisocyanate respectively (Scheme 12) [21]. With rhodium complexes the conversions varied from 22% to 100% and ee from 0% to 60% for HTR with acetophenone at 70°C, in the presence of [Rh(cod)Cl]2 with diamine polymer unit (23-26)/Rh ratio of 10, 2-propanol and KOH. Polyamide 23 proved to be useless (only 22% conversion and 28% ee) contrary to polyurea 25 which presents similar ee to those observed with diamine 18 when (Rh(cod)Cl)2 was employed as the catalytic precursor for HTR (ie 100% conversion for both of them and 55% and 59% ee respectively). Polyureas 24a, 24b, 25 and crosslinked 26 led to better conversions, 80, 50, 97 and 100% with respectively 0, 13, 39 and 60% ee. Moreover, the chiral crosslinked polyurea 26 presented a slight increase in enantioselectivity over the monomer analog 18 (55% ee and 94% conversion under similar conditions) and the reaction rate appeared to be even higher than in the homogeneous phase. Catalytic system from 25 showed a capacity to recycle (Scheme 13) [20]. For thioureas, it has already been shown that under homogeneous conditions, they are better ligands than ureas, when using ruthenium complexes as precursors [22]. Thus, polythioureas were good candidates for the HTR reaction [21]. For polythioureas 27, studies of the DP were performed and it had been showed that DP depends on the ratio of the reagents. During the polymerization, a guanidine moiety could be formed, stopping the chain growing. To avoid its formation, dithioisocyanate must be used in excess.
54
Christine Saluzzo and Stéphane Guillarme R1
R1
R2 NH HN R2 ClOC Ph
XCN (R)
COCl
Ph
R1
N N C O (R) O C R2 R2 O O n
NCX
R1
N N C R2 R2 X
NH (R) NH C X n
23: R1 = Ph, R2 = Me
24: R1 = -(CH2)4-, R2 = H, X = O Me Me
R=
a: R = 25: R1 = Ph, R2 = Me, X = O R=
b: R =
26: crosslinked 25 formed with 70/30 mixture of di and triisocyanatodiphenylmethane
27: R1 = Ph, R2 = H, X = S a: R = -(CH2)4b: R = Me Me c: R = d: R = Me Me
Scheme 13.
O Ph
cat* 5% i-PrOH/KOH 70°C
cat*: 24a (Rh(cod)Cl)2 1st use conv. 100% 60% ee 2nd use conv. 100% 59% ee 3rd use conv. 100% 60% ee
OH Ph cat*: 27c (Ru(benzene)Cl2)2 1st use conv. 92%, 70% ee 2nd use conv. 98%, 67% ee 3rd use conv. 99%, 66% ee 4th use conv. 99%, 63% ee 5th use conv. 98%, 61% ee
Scheme 14.
The HTR of various ketones was then performed in isopropanol with a substrate/ligand/tBuOK/Ru ratio of 20/1.5/4/1. The chiral induction depended on the flexibility of the linker. The more flexible the linker, the less enantioselective the reaction: 31%, 70%, 70% and 65% ee for respectively 27a, 27b, 27c, 27d ruthenium complexes with conversion up to 90% for 27a, 27b, 27c and only 47% for 27d. For this latter ligand which was fully crosslinked, the decrease in conversion was due to the low accessibility of the catalytic site although a ligand/metal ratio of 8 had to be used. The recovery and reusing of the 27c polymeric catalyst has been performed with practically no loss of either catalytic activity or enantioselectivity (Scheme 14). These results could be
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
55
explained by the rigidity of the active site which is crucial for the selectivity and the stability of the catalytic system. Compared to the polyureas most of the polythioureas tested were more efficient. Modification of 1,2-diphenylethylenediamine into N-p-toluenesulfonyl-1,2-diphenylethylenediamine (TsDPEN) led to one of the most selective ligand for the homogeneous HTR of ketones (97% ee for acetophenone) [23]. The heterogeneization of this ligand was first carried out with N-(vinylbenzene-psulfonyl)-1,2-diphenylethylenediamine (20), styrene and divinylbenzene as crosslinker (Scheme 15).
+ NH2
AIBN CH2Cl2
+
NH
O S NH O
NH2
SO2
20
1 1
/ /
10 10
/ /
0 0.5
AIBN CH2Cl2
28 77% yield 29 71% yield
Scheme 15.
When tested as ligands for the IrI and RuII catalyzed HTR [24] of acetophenone, in presence of 2.5% of metal precursor and 2-propanol/base, at 70°C, Ir complexes led to higher ee (28: 92% and for the crosslinked 29: 94%) compared to the homogeneous analog for which only 75% of conversion and 89% ee were obtained. Nevertheless, the Ir catalyst was more efficient than ruthenium (Ir: 92% ee, Ru: 64% ee crosslinked Ir: 94% ee, crosslinked Ru: 84% ee), but for Ru the conversions were of about 20% in both cases. On the other hand the homogeneous Ir complex analog led to only 75% conversion at 89% ee. This difference could be explained by the formation of chiral microenvironments upon polymerization and stabilization of the reactive Ir complex. Attempts to reuse the catalysts showed that the one containing Ru which is less selective is more stable upon reuse than the Ir derivative. Itsuno [25] has also shown that polymer-supported DPEN monosulfonamides containing sulfonated pendent group (Scheme 16) are able to catalyze the HTR reduction of ketones in water with sodium formiate as hydrogen donor (S/C = 100). However, TsDPEN immobilized on polystyrene crosslinked or not, polymer 30 and 31 respectively, shrank in water. Sodium p-styrene sulfonate was copolymerized with chiral N-(vinylbenzene-p-sulfonyl)-DPEN (20) under radical polymerization conditions with or without DVB leading respectively to ligand 32 and 33. Control of the balance hydrophilicity/hydrophobicity of the polymer support is carried out by changing the salt from Na+ to quaternary ammonium. All of these polymers swelled in water, and their respective ruthenium, rhodium or iridium complexes were prepared. Compared to sodium salt polymer-supported catalyst from 32a and 33a, ammonium
56
Christine Saluzzo and Stéphane Guillarme
supported catalyst from 32b and 33b have produced the best results in terms of activity and selectivity for the reaction in aqueous media. Moreover, almost the same activity was observed for the reduction of acetophenone with the ammonium non crosslinked catalyst 32b prepared from [RhCl2Cp]2 and [RuCl2(p-cymene)]2 (100% and 99% conversion; 98% and 98% ee respectively) while the catalyst from [IrCl2Cp]2 decreased both reactivity and enantioselectivity (77% conversion, 89% ee).
0.1
Ph
NH SO2
Ph
NH2
(R,R)-30
0.1
Ph Ph
0.1
0.9
NH SO2 NH2
Ph
NH SO2
Ph
NH2
0.1
(R,R)-31
0.1
0.9
Ph
SO3- X+
Ph
(R,R)-32 +
0.8
0.1
0.8
SO3-X+
NH SO2 (R,R)-33
NH2
+
a: X = Na ; b: + N
Scheme 16.
Enantioselectivities of 97-98% were obtained in five recycle runs by using (R,R)-33b derived catalyst. Earlier, Bayston [26] has synthesized the polymer 34 (Scheme 17) starting from the aminostyrene Merrifield-type polymer. By means of ruthenium precursor, 88% conversion with 91% ee was obtained for reduction of acetophenone. Compared with the preceding ligand 29 complexed with ruthenium, better conversion and enantioselectivities are shown; these latter results were comparable with the Ir catalyst. O
O2S NH NH2 34
Scheme 17.
N H
PS
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
57
A similar heterogeneization of TsDPEN was made by Wang [27] with N-(carboxy, carbomethoxy or carboethoxy) benzene -p-sulfonyl)-1,2-diphenylethylenediamine (21 and 22 scheme 12). The acid function permitted the immobilization of the ligand to an aminomethylated polystyrene via an amide function (Scheme 18). O O2S NH
1) aminomethylated polystyrene, DCC, DMAP pentafluorophenol 2) TFA, CH2Cl2
(R,R)-20
CONH n
31 n = 1 32 n = 2 NH2
aminomethylpolystyrene: 1.07 mmol/g; DVB 1% CONH O2 S NH 33 NH2 O Ph
R
L* (RuCl2(p-cymene)2), 30°C HCOOH/Et3N, CH2Cl2 (1/1) Ketone/L*/Ru: 100/1.2/1
OH Ph *
R
R = CO2Et, CONHCH3, CN Scheme 18.
Ruthenium complexes of 31, 32 and 33 using the azeotrope triethylamine/formic acid as hydrogen donnor permitted the reduction of β-keto ester, amide and nitrile with high conversion (> 95%) and ee (> 90%) (Scheme 18). The electronic character and the spacer length between the polystyrene part and the benzene ring had very little effect on the reduction outcome, for a same substrate conversion and ee are similar. Here also, it has been shown that the catalyst formed with ligand 32 could be reused at least three times without loss of activity and enantioselectivity. Xiao proposed a poly(ethylene glycol)-supported ligand (34) in order to perform the HTR of aromatic ketones in water, using as hydrogen donor, the azeotrope triethylamine/formic acid [28] or sodium formate/water (Scheme 19) [29]. These ligands were formed by immobilization of TsDPEN by functionalization of the phenyl group by a hydroxy group in the meta position followed by a nucleophilic reaction with a monomethylether mesylate PEG (Mn 2000) (Scheme 19). The Ru-catalyst formed was soluble under the conditions of the reaction. It could be isolated from the reaction mixture by precipitation, though addition of a low polarity solvent,
58
Christine Saluzzo and Stéphane Guillarme
allowing recycling. Up to 80% ee and up to 85% ee were achieved with all of the ketones tested using the azeotrope triethylamine/formic acid or sodium formate/water systems respectively. Nevertheless, convertion depended on the structure of the ketone and mainly on the position of the withdrawing group of the aromatic group (Cl, OMe, CF3).
OH
HO
H2N
HN Ts
1) MeO PEG OMs CsCO3, DM F, 50°C
O
O
2) CF 3 COOH, CH 2 Cl2 , H 2 O, r.t.
H2N
L* (RuCl2 (p-cym ene) 2), 50°C HCOOH/Et 3 N/H 2 O (1/1/1 v/v/v), or NaCOOH/H 2 O S/C: 100/1
O Ar
HN Ts 34 OH
Ph *
Ar: p-Cl, o-Cl, p-Me, o-Me, m-OM e, m-Br, p-CF 3 Phenyl; 1-naphtyl, 2-naphtyl
Scheme 19.
For the sodium formate/water reduction system [29], several parameters were studied. A 20°C decrease of the temperature led to a slightly higher ee at a longer time. The lower the formate concentration, the slower the reduction is and the lower ee. Moreover, the reaction was not affected by the presence of a surfactant. The ligand itself probably acted as a phase transfer catalyst. The presence of an organic solvent, inducing a biphasic system, led to a slower reaction and a decrease in ee. Concerning the catalytic system stability, it appeared that in sodium formate/water medium, a possible decomposition occurred because catalytic recycle led quickly to loss of catalytic activity and ee, which was not the case when formate/water system was used: more than ten recycles with no loss of activity and enantioselectivity. The same approach was involved in 2008 by Chan [30]. As an alternative method for attaching a PEG chain onto TsDPEN ligand, a PEG-750 (ie Mn = 750) was anchored at the para-position of an aryl sulphate group leading to polymer 35 in 51% overall yields starting from chiral DPEN (Scheme 20). Polymer 35 was examined in the Ru-catalyzed asymmetric HTR of acetophenone in water using HCOONa as the hydrogen donor at room temperature. Best ee (96%) was obtained with 5 equivalents of HCOONa and the high ratio of substrate to catalyst (S/C = 1000) led to slower reaction with a slight decrease in ee (90% ee). Other aryl ketones have been reduced. Compared with the precedent catalytic system formed with polymer 34, similar results were obtained in term of enantioselectivity. Once again, the catalyst could be easily recovered and reused at least eight times. Ph
Ph
Ph
HN HN Boc
SO2
1) MeO PEG OTs CsCO3, acetone
HN
SO2
2) CF3COOH, CH2Cl2
OH
Scheme 20.
H2N
Ph
35
O
O
O
16
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
59
2.2.2. Imprinting technique for hydrogen transfer reduction with 1,2-diamines In 1995, Lemaire [32] reported the immobilized transfer hydrogenation catalyst using a tetramine-rhodium complex 35 in order to study the effect of molecular imprinting. The upper rhodium complex was formed by copolymerization with a diisocyanide in the presence of sodium (S)-phenylethanolate (PM) leading to the imprinted polymers 36a and 36b (Scheme 21). The PM is then removed by adding 2-propanol. The resulting polymer as well as the nontemplated polymer were tested in the transfer hydrogenation of acetophenone and phenylethylketone in the presence of 5 mol % of Rh-polymer with KOH/[Rh] ratio of 4 at 60°C. Results are presented in Table 3.
Ph HN Me
H NaO Ph + NH Me
Me OCN
R
Me
NCO
CH2Cl2, r.t. Ar
N
HO
Rh N
Rh(cod)Cl 2 Me 36a R =
OH
36
HO H Me
particle size : 80-120 μm
Ph
Me N
36b R = HO
Me
HO
Rh N
R
O
R Ph OH
Ph O
R N
HO
Rh N
Scheme 21.
Table 3. Molecular imprinting effect in reduction by using Rh catalyst with templated and non-templated polymers 35a and 35b. Substrate acetophenone acetophenone acetophenone acetophenone phenylethylketone phenylethylketone
Chiral polymer-Rh catalyst 36a polymerized 36a (S) templated 36b polymerized 36b (R) templated 36b polymerized 36b (R) templated
Inductor configuration (R,R) (R,R)
Conv. (%) 44 42
Time (days) 1 1
Ee (%) (config.) 33 (S) 43 (S)
(S,S) (S,S)
98 98
1 1
25 (R) 43 (R)
(S,S) (S,S)
96 91
6 9
47 (R) 67 (R)
60
Christine Saluzzo and Stéphane Guillarme
In homogeneous catalysis using chiral diamine 18 complexed with Rh, the acetophenone was reduced quantitatively with 55% ee, in 7 days. In the case of polymerized complex 36a, acetophenone reduction leads to 33% ee and with its templated analog 43% ee. With 36b, an increase of about 20% ee is observed between polymerized and templated ligand. These increases in ee were ascribed to a favourable molecular imprinting effect of the PM, creating chiral pockets within the polymer network. Nevertheless, in the case of sterically demanding substrates little or no reduction occured, indicating a substrate stereospecificity different from that observed in homogeneous catalysis or when using catalyst precursor belonging to the polymer backbone [32]. For the reduction of acetophenone, crosslinked templated polymers were studied. Optimization of the crosslinking ratio led to the best compromise between activity and selectivity (70% ee for a crosslinking ratio of 50/50 of triisocyanate/diisocyanate) [33, 34]. This selectivity could be explained by a certain rigidity of the cavity, permitting good accessibility to the reaction sites. This molecular imprinting technique was also used by Severin [35] [36] who proposed the synthesis of phosphonato complexes 37 (Scheme 22) in order to mimic the six membered cyclic transition structure suggested for HTR. This immobilized catalyst was prepared in a three step reaction (Scheme 22). In order to compare the effect of such polymeric organometallic transition state analog (TSA 38), the polymer 40 was prepared (Scheme 22) by copolymerization of the non phosphonato complexed catalyst with ethylene glycol dimethacrylate (EGDMA) (Ru/EGDMA: 1/99). The ability of such polymers to catalyse the reduction of benzophenone was tested, using the azeotrope formic acid/Et3N as hydrogen donor.
R R N Ru O NH2 R1 P R O 37
1
N Ru O NH2 R1 P R1 O EGDMA, CH2Cl2 V-70, 35°-65°C
AgO2PPh2 CH2Cl2, r.t.
TSA 38 [BnNEt3]Cl, MeOH
R 1) EGDMA, CH2Cl2 V-70, 35°-65°C 2) [BnNEt3]Cl, MeOH
N Ru Cl NH2 R1 R1 R = SO2C6H4CH=CH2
R N Ru Cl NH2 R1 R1
non-imprinted polymer 40
a: R1 = H b: R1 = -(CH2)4Scheme 22.
39
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
61
The molecular imprinted polymer 39a was significantly more active than polymer 40a (TOF (40a) = 51.4 h-1, TOF (39a) = 16.5 h-1). With this enhancement of the rate of the reaction using the imprinting polymer, substancially higher specificity for benzophenone was observed when completion of HTR of an equimolar amont of benzophenone and a ketonic cosubstrate was performed. This behaviour was slightly larger for aliphatic ketones as compared to aromatic ketones. These results were consistent with the activity and selectivity using organometallic TSA as template with an imprinting technique. A similar study was made with the polymer including rhodium complexes coordinated with phosphinato ligands (polymer 38b) and the imprinted polymer 39b. After 6 hours, the reduction of acetophenone led to 81% yields and 95% ee, and ruthenium complexes gave up to 70% ee for the reduction of the ethyl phenyl ketone [37]. Due to the structurally defined transition state analogs, included in a polymer, a shapeselective cavity close to the catalytic active center was formed. This cavity is highly selective and an enhancement of the activity was observed Furthermore, it has also been shown that activity and selectivity depended on how the metal complex was attached to the polymer backbone: a rigid connection by two styrene side chains is superior [36].
2.2.3. Aminoalcohol and aminothiol as ligand Chiral β-amino alcohols can also be used for homogeneous HTR of ketones and are known to be effective for reduction of aromatic ketones [38] but, only few examples of organic polymer-supported ligands have been reported. Recently, poly((S)-glycidylmethacrylate-co-ethyleneglycol dimethacrylate) (41) (poly((S)-GMA-co-EGDMA)), [39][40] (poly((S)-PhGMA-co-EGDMA)) (42), [41] poly((S)glycidylmethacrylate-co-divinylbenzene (poly((S)-GMA-co-DVB)) (43) [41] [42] and ((S)thiiranylmethylmethacrylate-co-ethyleneglycol dimethacrylate) (44) [42] different precursor polymers were synthesized. They were formed by radical suspension polymerisation of enantiopure (S)-GMA (30 wt % for 41a or 70 wt % for 41b) with EGDMA or DVB (70 wt % for 43), or (S)-PhGMA (40 wt % for 42) with EGDMA or (S)-TMA (30 wt % for 45) with EGDMA in presence of AIBN, with a mixture of cyclohexanol/dodecanol (91/9 wt/wt) and 2% of aqueous polyvinylpyrrolidone (PVP) as stabilizer (Scheme 23). Chiral beads of 41 and 43 presented respectively a specific surface area of 82 and 275 m2/g, and in both the cases, 2.11 mmol/g of epoxyde function. A specific area of 50 m2/g with 4.92 mmol/g of epoxyde function, a specific area of 35 m2/g with 1.83 mmol/g of epoxyde function and a specific area of 92 m2/g with 1.72 mmol/g of epoxyde function were respectively observed with 42, 44 and 45. The subsequent epoxide or episufide ring opening with diverse amines led to the corresponding polyaminoalcohols and polyaminothiols. Their ruthenium complexes were applied to the HTR of benzophenone (Scheme 23). It is noteworthy that the nature of the amine of the aminoalcohol played a crucial role. The best results in terms of conversion and enantioselection are obtained with polyaminoalcohols and polyaminothiols derived from methylamine and benzylamine (Table 4) [39]. It appeared that the efficiency of the catalyst depends on the nature and the proportion of the crosslinking agent (catalyst presenting 70% of EGDMA was more efficient than this with 30% of EGDMA and for DVB a decrease in conversion and ee was observed). It depended also on the specific surface, the nature of the ligand (aminoalcohol or aminothiol) and on the
62
Christine Saluzzo and Stéphane Guillarme
steric hindrance (aminoalcohol and ligands from GMA compared with PhGMA were more efficient) (Table 4) [41]. Moreover, attempts to recycle ruthenium complexe of 46a led to a marked decrease in activity from 94 to 27% and lowered selectivity from 70 to 54%. O
R1 +
O X X = O: GMA X = S: TMA
R2
O
R1 * H O X R4 41 - 45 R3HN R5 (3 eq)
AIBN, PVP dodecanol/cyclohexanol
R R2
DMF 100°C, 22 h
X = O 41, 42, 46 and 47: X = S 45 and 48:
O
O
O
O
R1 * N O HX R3 R5 46 - 60 H R4 O
R1 = H, R2 = Me;
R=
and R1 = H, R2 = H
X = O 43 and 60: R = O
O
O
O
C-X configuration: S if X = O; R if X = S C-R1 configuration: S if R1 = Ph
R1 = Ph, R2 = Me;
X = O 44 and 49: R = O Ph
Me
i-PrOH, t-BuOK, 80°C [RuCl2(p-cymene)]/46-60
OH H Me Ph
acetophenone/Ru/aminoalcohol polymer unit/t-BuOK: 20/1/4/5
Scheme 23.
Table 4. HTR of acetophenone using several aminoalcohols and aminothiols (L*/Ru/Ketone/t-BuOH: 4/1/20/5, 80°C) R1 * O HX HN CH2R5 O
Phenylethanol
X (C-X config.) O (S)
R1 (C-R1 config.) H
R5
Ligand
Functionality (mmol/g)
Ph
46a
O(S)
H
H
O (S)
H
S (R) S (R) O (S) O (S) O (S)
Time (h)
Conv. (%)
Ee (%) (config.)
1,46
3
94
71 (R)
46b
0,85
1
95
65 (R)
Ph
47a
2.90
72
51
57 (R)
H
Ph
48a
1.11
22
55
50 (S)
H Ph (S) Ph (S) H
H Ph H Ph
48b 49a 49b 60a
0,90 0.65 0.51 1.28
22 22 72 22
50 61 <5 58
39 (S) 21 (R) 29 (R) 44 (R)
Wills [43] has tested a copolymer formed by a 7/3 methylmethacrylate and hydroxyethylmethacrylate ratio soluble in a range of organic solvent as backbone of a
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
63
norephedrine pendent polymer ligand, an aminoalcohol ligand selected for its good results in HTR of ketones when complexed with RuII (Scheme 24). O H
n
O
X
for control solubility
X/Y
m
O
Y
for attachment of functional group
O H
7 MMA + 3 HEMA
AIBN, sodium bis(2-ethylhexyl) sulfosuccinate 0.06 mol % CoBF, 4h, rt
7
O
OR
3
OMe O
O
R = Me or CH2CH2OH
Pol1
OH
Pol1
O Ph NH
51
OH
OH
O H 7
H O
O ( +
O)3Et
AIBN, sodium bis (2-ethylhexyl) sulfosuccinate 0.06 mol % CoBF, 4h, rt
3 HEMA Ph
Pol2 N H
7
O
O (
OR
3
O
O
O)3Et
OH
Pol2
OH 52
OH
Scheme 24.
With ephedrine supported ligand 51, the best results were obtained for the HTR of acetophenone when 0.44 mol% of catalyst in dichloromethane is used (20% yield, 77% ee). Evaluation of ligand 52 for this reaction with 2 mol% of catalyst gave up to 85% yield with 81% ee. Three additions of acetophenone showed a deterioration of the catalytic system. Few attemps have been made by Masuda to utilize the main chain chirality of polymers [44]. They reported the synthesis of an L-threonine-based helical poly(N-propargylamide)-Ru complex (53-Ru-complex) and evaluation of its catalytic activity in the asymmetric HTR of ketones (Scheme 25). The corresponding alcohols were obtained in 23-48% yields and the ee values were 12-36%; the highest ee was achieved in the reduction of 2-acetonaphtone. This reaction was achieved in homogeneous conditions with 54-Ru-complex and acetophenone led
64
Christine Saluzzo and Stéphane Guillarme
to phenylethanol with no ee. This suggested that the helical structure was effective to enhance the catalytic ability for chiral induction. HC C
1) (nbd)Rh+ [η6-C6H5B-(C6H5)3 2 mol%, THF
NH NHBoc
O
n
NH
2) HCl M, AcOH 3) MeOH, Et3N
NH2
O
CH3(CH2)17 OH
53 OH
NH NH2
O
54 OH O Ar
R
OH
L* (RuCl2(p-cymene)2 i-PrOH/MeOH (95/5), t-BuOK Ketone/L*-Ru/t-BuOK: 100/1/10
Ar * R
Scheme 25.
2.2.4. Dialdimine ligand as ligand One example of the use of heterogeneized dialdimine ligand 55 (Scheme 26) was reported for the asymmetric HTR of acetophenone [45]. Enantioselectivities of up to 70% were obtained for a degree of crosslinking of 71%. The rise of the degree of crosslinking to 100% gave no significant difference contrary to a crosslinking less than 71% which gave a decrease in enantioselectivity.
N
N
N +
O
+
O
N
AIBN MeOH
O
O
55
Scheme 26.
2.3. Enantioselective Hydride Reduction The enantioselective hydride reduction of prochiral ketone is a well documented reaction which consists in the preparation of optically active alcohols. Thus, a large number of reducing agents and chiral catalysts have been developed.
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
65
In 1981, Itsuno [46] then Corey in 1987 [47] described the use of oxazaborolidine in the presence of a hydride for ketone reductions which is one of the most efficient system for this type of reaction. [48] The chiral ligands are mostly aminoalcohols such as chiral prolinol or ephedrine-type compounds. Thus, from the beginning of the eighties, polymers containing chiral aminoalcohols and their derivatives were used as chiral auxilliaries for LAH [49], BH3 [50] [51] or NaBH4 [52]. One of the first experiment involved a continuous flow system using a crosslinked polystyrene gel supporting a chiral aminoalcohol 56 [53] (Scheme 27), prepared from Ltyrosine and 2% crosslinked chloromethylated polystyrene gel [51]. The reducing agent is formed by a polymeric complex of aminoalcohol 56 and BH3. Valerophenone and BH3 are continuously added and passed through the polymeric complex. With this method the destruction of the polymeric gel was not observed (no stirring) and large quantities of the corresponding optically active alcohol could be formed. With valerophenone, 83% to 91% ee are obtained. With the soluble polymer-supported catalyst 57 (Scheme 27), the reduction of acetophenone was performed in a continuous operated membrane reactor equipped with a nanofiltration membrane [54]. An enhancement of total TON from 10 to 560 (equivalent to 0.18 mol % catalyst for an average ee of 91% and an almost quantitative conversion) was observed. Excellent space-time yields of up to 1.4 kg per day were reached
O
Ph Ph H2N
OH 56
m
n
Ph
O
Ph Ph H2N
OH
5 eq styrene, AIBN, toluene, 60°C, 50h (53%) then BH3.Me2S, THF
O
Ph Ph N 57
H3 B
O B H
functionalization: 1.18 mmol/g, Mn 13800 g/mol
Scheme 27.
More recently, several polymer versions of the efficient Corey-Bakshi-Shibata (CBS) catalyst have been prepared [47]. Different approaches have been used to bind the catalytic moieties to the polymer support: the diphenyl prolinol reacted with beads bearing benzene sulfonyl chloride residues (catalytic moieties bound to the nitrogen atom as part of a sulphonamide link), the diphenyl prolinol is bound to a polymer support or the aminoalcohol
66
Christine Saluzzo and Stéphane Guillarme
reacted with a polymer-supported boronic acid [55]. As in the last case the aminoalcohol is not supported, only the first two possibilities are reported. In order to prepare polymer-supported CBS catalyst, monomeric precursors 59, 60 or 61 or bromides 62 and 63 have been synthesized (Scheme 28). The oxazaborolidine could be formed in situ or not before performing the hydride reduction of ketones. C B S lig a n d s
58a: R = H or 58b: R = M e N B
O
R C B S s up p o rte d p re c u rs ors
N HO
N H HO
N H HO R 5 9 a : R = p - i-P r 59b: R = H Br
60
61
Br
N H HO
N H HO 62
63
Br
Scheme 28.
Shore [56] synthesized a pendent (64) and a crosslinked (65) CBS catalyst in bead form from monomeric precursors 59a and 60 respectively (Scheme 29) but also in monolith polymer form with a crosslinking level of 2% [57]. Monolith polymers were then cut to form CBS disks (64D and 65D) roughly 2 mm in height and 5 mm in diameter. Ar N Ar Boc OH
1) styrene, 2% DVB, (PhCO2)2 PhCl, H2O, Δ (29%) 2) MeOH, DMSO, KOH 3) trimethylboroxine
64 0.40 meq arylprolinol/g incorporation of ~ 4.5% of distyrylprolinol crosslinker
Boc 60
Ar
1) styrene, 55% DVB, (PhCO2)2 PhCl, H2O, Δ (73%) 2) MeOH, DMSO, KOH 3) trimethylboroxine
N R Boc OH Boc 59a Ar = p-vinylphenyl R = p-i-PrC6H4 Scheme 29.
N B O
N B O R 65 0.32 meq arylprolinol/g
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
67
In 2004, Degni anchored chiral styrenic prolinol 60 and 61 on chemically inert polyethylene (PE) fibers by electron beam (EB) preirradiation, inducing graft copolymerization leading respectively to polymer 66 and 67 (Scheme 30) [58]. The corresponding oxazaborolidines were prepared in situ with borane from different sources: NaBH4 in the presence of Me3SiCl or BH3.Et2O and BH3.SMe2.
PE fibers, EB
N H HO
N H HO
(graft co-polymerization)
66
60
N
N PE fibers, EB
HO
HO
(graft co-polymerization)
61
67
Scheme 30.
Involving a Suzuki reaction, Hodge prepared “chemical robust” polymer-supported diphenyl prolinol based catalysts 68 and 69 from 62 and 63 and polystyrene boronic acid prepared via a direct lithiation of 1% crosslinking gel-type polystyrene beads (2.21 mmg/mol of boronic acid residue) (Scheme 31) [59]. Br
Suzuki reaction
+
N H HO
B(OH)2 Br
63
+ N H
HO 68a 50%
Br
N H HO
+
Suzuki reaction
N H HO
B(OH)2 69 62 Suzuki reaction conditions: Na2CO3, Pd(PPh3)4, (CH2OMe)2
Scheme 31.
N H
HO 68b 50%
Br
68
Christine Saluzzo and Stéphane Guillarme
Comparative results for the reduction of oxazaborolidines are presented in Table 5.
acetophenone
with
these diverse
Table 5. Reduction of acetophenone with several oxazaborolidines isolated or formed in situ from aminoalcohols and borane Aminoalcohol or oxazaborolidine 58a 64 65 64D 65D 58b 66 67 68 69
Borane source BH3.THF BH3.THF BH3.THF BH3.THF BH3.THF BH3.SMe2 BH3.SMe2 BH3.SMe2 BH3.SMe2 BH3.SMe2
Mol % 5 5 5 50 50 10 10 10 30 30
T (°C) r.t. r.t. r.t. r.t. r.t. 45 45 45 22 22
Yield (%) 100 100 100 97 100 99 90 99 > 95 > 95
Ee (%) 85 84 62 99 63 98 95 44 94 96
Recycling (ee %) 3 (58) 1 (78) 16 8
Ref.
[56] [56] [57] [57] [58] [58] [59] [59]
In general, the crosslinked CBS (64, 64D or 66) afforded enantioselectivities almost identical to those of the CBS in homogeneous conditions (58a and 58b) and superior to the pendent-linker equivalent ligand (ie respectively 65, 65D and 67). These results could be ascribed to the superior swelling characteristics of the crosslinked catalyst in the solvent of the reaction which permits a better diffusion of the reagents into the catalyst [56-58]. For polymer 67, the weaker coordination with borane, due to the steric hindrance on the nitrogen atom, may be responsible of the slower catalysis. With 68 and 69 the results of the reaction were almost similar. After the 20th reuse, analysis of the polymeric mixture 68 showed that the overall weigh fell to 57% of its original value with an average loss per reaction of <3%. Moreover, nitrogen analysis was almost the same, suggesting that the catalyst groups were firmly bound and that the losses are probably due to physical attrition [59]. Concerning the reuse of the crosslinking polymer 64D, the recycle failed. This suggested a disassembly of the oxazaborolidine and a structural breakdown of the polymer itself [57]. Ketones [60], β-functionalized ketones [61] [62] and meso cylic imide [63] have been enantioselectively reduced using polymer-supported chiral sulfonamides in presence of boranes (NaBH4/Me3SiCl or BH3.SMe2), leading in situ to the corresponding oxazaborolidine polymer-supported chiral sulphonamide 70 (Scheme 32).
Ph N H HO
C lS O 3 H C H C l3 co p olym e r styre n e /D V B
Ph
E t 3 N , C H 2C l2
SO 2C l
SO2
N
HO Ph
Scheme 32.
Ph 70
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
69
The reduction of unfunctionalized ketone was investigated at various conditions reactions: THF or toluene as solvent, 1% or 2% of crosslinking polystyrene resin and several degrees of functionalization. The best results in terms of enantioselection (92.5%) and conversion (95%) were obtained in reflux toluene, with a catalyst amount of 15% polymer crosslinking of 2% and an aminoalcohol loading of 2.29 mmol/g. With lower degree of functionalization and 1% crosslinking the ee was of about 10 to 20% lower. The catalyst has been reused at least five times with minimal loss of catalytic performance. It is noteworthy that under these conditions, homogeneous supported catalyst showed better results than homogeneous catalyst. The best conditions were also involved successfully for the reduction of other ketones. With the same polymer-supported chiral sulphonamide 70 (200-400 mesh, 2% DVB, 2.29 mmol/g functionalization) the reduction of several β- ketonitriles [61] and β-ketosulfones [62] were tested. It is noteworthy that aromatic β-ketonitriles and β-ketosulfones were reduced in good enantioselectivity (85-96% ee). Electron density (substituant in the para position) had little effect on enantioselectivity. For aliphatic β-ketonitriles or sulfones, both chemical yield and ee were lower compared to those obtained with aromatic analogs. Meso-cyclic imide precursor of d-biotin has successfully been reduced with BH3.SMe2 in the presence of polymer-supported sulphonamide catalyst 70 in THF under reflux for 6 hours. (Scheme 33) [63]. The enantioselective hydroxylactam was obtained in 91% yield with an ee > 98.5%. The recycling of the chiral supported-sulfonamide in the reduction of the meso precursor of d-biotin could be involved at least 5 times with non change in activity and enantioselectivity. Ph O
O S
Ph OH
N (0.15 eq)
O
O
70
NH H
NH H
BH3.SMe2 THF reflux 6h
O
N Bn
O
NH NH H H H O HO N Bn
Scheme 33.
Recently, Wang [64] prepared by radical copolymerization a cinchona alkaloid copolymer: the methyl acrylate-co-quinine (PMA-QN (71)) (Scheme 34). Complexed with palladium(II), its catalytic activity in the heterogeneous catalytic reduction of aromatic ketones by sodium borohydride was studied. High yields in their corresponding alcohols are obtained but it is found that the efficiency of the catalyst depended on the nature of the solvent and the ketone which related to the accessibility of the catalytic active site. The optical yields in methanol and ethanol 95% were lower than in ethanol. This ability was attributed to a bad coordination between PMA-QN-PdCl2 and sodium borohydride and a reaction rate which was very rapid. The stability of the chiral copolymer catalyst was studied
70
Christine Saluzzo and Stéphane Guillarme
via the study of the recycling efficiency. Only 2 wt% loss of metal was observed after repeated use (at least 5 times) with only a little decrease in enantioselectivity.
n
m
COOMe
N OH
H
N
MeO
OH
H N
MeO
QN
PMA-QN
N 71 Scheme 34.
2.4. Hydrosilylation with Ligands as Pendent Group Asymmetric hydrosilylation of ketone had been studied in homogeneous supported catalyst with nitrogen containing ligands and ony few examples have been reported. In 1998, Enders reported immobilized triazolium salts as precursors to chiral carbene-rhodiumcatalyzed asymmetric hydrosilylation (Scheme 35). This Rh complex gave 24% ee compared to 17% ee for the homogeneous reaction (Scheme 34). The recycling of the solid supported catalyst 71 was successful with only slowly decreasing yields which were comparable to the yields of the homogeneous reactions with catalyst 72 [65].
N N
O
O
+
ClO4 H
N
ClO4 H
N N N
p-TSA CH2Cl2
HO ( ) t-Bu n
O
O
O ( ) t-Bu n
[CODRhCl]2 THF, Et3N
N N
Cl N N
Rh
N O
O
Ph Scheme 35.
Rh
N
O ( ) t-Bu n 72 O
Cl
1) Ph2SiH2 (1eq) THF 72 or 73 2) MeOH, p-TSA
BnO
t-Bu 73
OH Ph *
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
71
3. REDUCTION OF C=N BOND Although highly effective asymmetric reduction of carbonyl compounds has been extensively investigated, enantioselective reduction of imine derivatives to amines has been less studied, mainly in homogeneous supported catalysis [66]. With polymer containing nitrogen (2-amino-3-(p-hydroxyphenyl)-1,1-diphenylpropan-1ol [67], 2-piperazinemethanol), Itsuno [68] has shown that in stoechiometric conditions, oxime ethers could be reduced with borane with excellent ee (up to 99%). More recently, in organocatalysis, Kočovsky [69] has reported asymmetric reduction of imine with trichlorosilane catalyzed by an (N-methylvaline)-derived formamide anchored to a polymer. Under the best conditions, with 15 mol% of the catalyst enantiomeric excesses were about 85% and the catalyst could be reuse. Then, in 2009, Itsuno [70] utilized for HTR of N-benzylimine the polymer-immobilized chiral ligands 31, 33a and 33b (Scheme 16) as their RuII complexes. These complexes were already used to catalyze the HTR reduction of ketones in water [25]. The HTR was carried out in organic solvent (dichloromethane) and in water using as hydrogen donor respectively the azeotrope HCOOH/Et3N and HCOONa. Catalytic system with ligand 31 was effective for the HTR of N-benzyl imines in organic solvent. The corresponding amines were formed in good yields and ee (92-96% yields and 84-88% ee). In contrast, RuII complexes obtained from amphiphilic polymer 33a and 33b were found to be effective for the HTR of cyclic imines in water (50-95% yields 86-94% ee). The catalytic activity, in water, seemed to be controlled by the hydrophilic-hydrophobic balance in a polymer-supported catalyst.
4. CYCLOPROPANATION Highly functionalized cyclopropanes are important building blocks for obtaining numerous natural compounds (terpenes, pheromones ….), and compounds which are of high value in biological (ie: agrochemical active species) and medicinal chemistry. A wide number of methods have emerged in order to produce optically active cyclopropanes [71]. From all of them, highly enantioselective cyclopropanes are formed by asymmetric catalytic insertion of carbene to prochiral olefins. This method, first employed by Noyori in 1966 [72] consists in the metal-catalyzed decomposition of substituted diazo compounds in the presence of various prochiral alkenes. Under homogeneous conditions [73], various chiral Schiff base-Cu(II or I) or Ru(II) complexes have been used as chiral catalyst precursors and from all of them the most efficient systems are formed with oxazolines and bis-oxazolines. Thus after their first use by Masamune [74], oxazoline-derived ligand have been extensively employed in homogeneous enantioselective catalytic reaction particularly in cyclopropanation [75]. Thus nitrogenanchoring ligands on organic polymers are mainly formed by immobilized oxazolines.
72
Christine Saluzzo and Stéphane Guillarme
4.1. Oxazolines Ligands Bis-oxazolines (Box), azabis-oxazolines (azaBox) and pyridine bis-oxazoline (PyBox) with a C2 symmetry form the great majority of various oxazoline-derived ligands which are used in immobilized catalytic cyclopropanation reactions (Scheme 36). For these reactions, immobilized ruthenium and copper complexes which are able to form carbene intermediates have been tested. Most of the time, the reaction involved styrene and ethyl diazoacetate (EDA). However other alkenes (diphenyl ethylene …) and other diazo compounds (i.e. tertiobutyl and ethyl 2-phenyl diazoacetate) have also been investigated (Scheme 36).
O
O N
N
N R
R
H N
O
R1
+
N2 C
R PyBox 76
a: R = Ph b: R = t-Bu c: R = Bn
a: R = i-Pr
R2 R2
R4 COOR3
CuL*
N
R R AzaBox 75
a: R = Ph b: R = t-Bu c: R = Bn d: R = Et e: R = i-Pr
O
N N
N
R
Box 74
O
O
R1 R2 R1
R4 A
COOR3
R2 +
COOR3 C R4
COOR3
R1 B
R4
R1 R2
R4
D
COOR3
Scheme 36.
First, heterogeneous catalytic systems involved N-anchoring ligands on inorganic supports such as zeolithes [76], clays [77] …. Immobilization of the catalyst on organic support leading to insoluble or soluble polymers was made by grafting or copolymerization of the chiral ligand. In all cases, the immobilization is performed via alkylation of the methylene bridge or of the nitrogen of the bridge (AzaBox, PyBox).
4.1.1. Insoluble supported-polymer Box as ligands In 2000, Mayoral [78] described the first immobilization of Box on insoluble polymer. For a same ligand, grafting and several methods of polymerization (Scheme 37) were reported in order to compare the catalytic efficiency of these supported ligands after their complexation with Cu. The study was based on immobilization of bis-(oxazoline) 74a-c (Scheme 36) and their corresponding Cu-complexes were tested in the cyclopropanation of styrene with EDA, a well known reaction in homogeneous catalysis [79].
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
73
The Box methylene bridge of 74a, [78, 80] 74b [78, 80, 81, 82] or 74c [78, 80] was first functionalized with two p-vinylbenzyl groups in order to perform homo-polymerization [78, 80] (polymer 79), or co-polymerization with styrene [78, 80, 81] (polymer 78), DVB [78, 80, 81, 82] (polymer 80α) divinylbenzyl polyethylene glycol [80, 81, 82] (polymer 80β) and divinylbenzyl resorcinol [80, 81] (polymer 80γ) or dendrimers 80δ (Scheme 2). The best results were observed with the homopolymer 79b (R = t-Bu) [78, 80]. This homopolymer 79 prepared by thermal polymerization [78] was able to coordinate a large amount of Cu and presented a better activity than the same homopolymer prepared in presence of AIBN (respectively 51% and 34% yield, 35/65 and 39/61 trans/cis ratio, 75% ee and 77% ee for the trans cyclopropane, 70% ee and 73% ee for the cis cyclopropane) [78]. For all the copolymers, the low efficiency of the copper complexes (cyclopropanation of styrene: yield up to 40%) may be explained by the non accessible core of polymer by Cu probably due to the high degree of crosslinking [78, 80]. But moderate to high ee were observed (up to 78% ee) and contrary to homopolymers, the cis cyclopropane is mainly formed. Attempts to use copolymers from monofunctionalized Box (Scheme 38) as ligand did not improve the efficiency (yield up to 16%); the diastereoselectivity is in favour of the trans stereoisomer. As for copolymer 80α, yields obtained with 80β−δ, do not exceed 35% and the diastereoselection gave mainly the trans cyclopropane. With 80β the formation of trans isomer is up to 67% [81, 83]. The presence of oxygen atom in the crosslinking agents could induced more coordination with the metal. This effect is detrimental for the catalytic properties for the resulting Cu-loaded resin [81]. The homopolymer 79b could be reused up to five times without loss of ee but with a slight decrease in activity [80] In order to avoid the non accessibility to the core of polymer by the metal due to excessive crosslinking, a monofunctionalization of the Box leading to Box pendant ligand 81 was performed [80] (Scheme 38). Unfortunately, the incorporation of copper into polymers 81 was not found to be improved (Cu loading: 0.07-0.08 mmol/g). Furthermore, a higher loading was observed with the less crosslinked polymer 81a. It is noteworthy that if the ligand 81c is transformed into a polymer with bisoxazoline moieties by treatment with MeLi and MeI, the results obtained for the cyclopopanation reaction were slightly worse that those obtained with 81c itself (yields 16% vs 16%; trans/cis ratio 60/40 vs 57/43, ee trans 23% vs 29%; ee cis 22% vs 34%). Salvadori [84] showed that a copolymer structure presenting a single spacer, but flexible one linking the ligand moiety to the inert polystyrene backbone was effective with solutionlike behaviour (Scheme 39). For the cyclopropanation of styrene with EDA, Cu-polymerized 82 complexes afforded similar results than the non supported Box. In the first case 61% and 60% yield, 67/33 and 71/29 dr in favour of the trans, 93% and 94% ee were respectively observed. The catalytic system could be reused without loss of its catalytic properties. Moreover, it is noteworthy that the immobilization method did not influence the catalytic behaviour: catalytic systems formed with copolymerized (route A) [84] or graft methodology (route B) [85] were substantially equivalent (variation of about 1 to 2% of the yield, ee or dr). In 2004, Yifei reported Merrifield (3.8 mmol/g) polymer-supported Box 74c-e [85] (Schemes 36 and 40) for the cyclopropanation of 1,1-diphenylethene with EDA.
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Christine Saluzzo and Stéphane Guillarme
The highest enantioselectivity of 85% ee with 82% yield was achieved by using 83d showing as in homogeneous conditions that the steric hindrance of the R group of the oxazoline had a great influence on the ee and the yield. Under recycling, the catalytic system led to an important decrease in enantioselectivity. In order to restrict the conformational mobility of the groups on the bridge of the Box, a 1,3-dioxane group was introduced, then the Box was grafted onto a Wang resin (0.7 mmol Br/g) (Scheme 41) [86]. In that condition, only 0.26 mmol/g has been grafted onto the active site and the copper content was 0.018 mmol/g. The ligand 84 was evaluated in the cyclopropanation of styrene with EDA. The trans cyclopropane was obtained in 65% ee. The high ee was due to less steric hindrance between the two oxazoline groups, as the consequence of the chair conformation of the dioxane ring. AzaBox as well as PyBox have the advantage towards Box to present a donor nitrogen atom in the bridge allowing to a higher coordinating ability. Thus a better stabilization and a better recoverability, with regard to their analogous Box ligand are observed. Additionally, AzaBox and PyBox ligands have only one link point in the bridge which makes grafting easier.
4.1.2. Insoluble supported-polymer AzaBOX as ligands Recently, AzaBox ligands were attached to a TentaGel (polymer with a polystyrene backbone and PEG periphery) (87) or to polystyrene (88 and 89) by direct grafting of the ligand onto the polymer or by copolymerization of the ligand functionalized with a styryl group (Scheme 42) [87]. The corresponding catalyst presented a copper content of 0.2-0.3 mmol/g. Cyclopropanation of styrene and 1,1-diphenylstyrene with EDA was studied with 11.5 mol% of the catalyst. In these experiments, catalyst formed with copolymerized ligand 89 did not show any advantage over grafting ones. Moreover, Merrifield resin was a better support than TentaGel. With 87a the yield was up to 51% but its diastereoselection (64/34) and and its ee (up to 62%) are lower than those obtained with 88 or 89 (yield up to 32%, dr 70/30, ee up to 88%) for the cyclopropanation of styrene. 4.1.3. Insoluble supported-polymer PyBox as ligands The first immobilization of PyBox was made by Mayoral [88] by means of radical copolymerization of 4-vinylPyBox with styrene and DVB in presence of AIBN and a porogen (toluene or a mixture toluene/dodecanol) (ligand 90 Scheme 43). PyBox was incorporated in the polymer with high yield from 75 to 95%. The corresponding Ru-complex was formed at about 50-60% showing that all the ligands were not accessible. The nature of the porogen solvent was important. A polymeric ligand prepared in presence of dodecanol gave a lower Ru functionalization and was detrimental to the trans/cis selectivity (77/23). The best catalytic system, high trans/cis selectivity (85/15) and ee (trans: 85%), was formed by the polymer prepared in toluene. However, it has been observed that with a lower crosslinking degree, dodecanol has less importance. The best catalysts were reused twice, but a marked decrease in both selectivities and activities were observed for the second recycle.
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
O
O N
1) MeLi
N 2)
R
R
Cl
74a-c
1) MeLi
O
O N
77 [11]
N R
R 2) n
m
78 [9, 11]
Cl
AIBN
O
O N
N
N R
R
O
O
a: m = 20, n = 80 b: m = 50, n = 50 c: m = 80, n = 20 d: m = 90, n = 10
N
n AIBN or heat
R
R AIBN
O
O N
crosslinker
R 79 [78, 80]
R x
z
y
crosslinker : α:
x=20, y=70, z=10 [78, 80, 81]
β:
80
O O 7.7O x=20, y=70, z=10 [78, 80, 81]
O
O N
N γ:
R
R
O
O R1O
O x=20, y=70, z=10 [78, 80, 81]
OR1 δ:
O
O
O
x=10, y=85, z=5
OR1
[80, 81]
R1 = R1O
N
O
O OR1
R1 =
O
O O
OR1
Scheme 37.
[81, 82]
75
76
Christine Saluzzo and Stéphane Guillarme
y
x
AIBN
H N
Styrene
H
DVB
O
O
81 a: x = 0, y = 90 b: x = 40, y = 50 c: x= 70, y = 20
10
O
O
N
N t-Bu
t-Bu
N t-Bu
t-Bu
Scheme 38. (
)(
) ( y
x
)
z
Ph ) (
O (
O +
+
route A ref. [84]
O
O N
82
AIBN
)n
O N
N
N
t-Bu t-Bu route A n = 1, x/y/z ~4/43/53 route B n = 2, x/y/z ~6/92/2
t-Bu
t-Bu
O
route B ref. [85] 1) 9-BBN, THF 2) H2O2, NaOH 3) KH, THF 4) Merrifield resin cat. KI and C[18-6]
O(
)2 O
O N t-Bu
N t-Bu
Scheme 39.
R
O O
O
Cl
N R
N
R 74 c: R = Bn d: R = Et e: R = i-Pr
Scheme 40.
Merrifield's resin (3.8 mmol Cl/g)
N C H2
83
N O
R
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
77
O
OH O
O
1) NaH
O
O
2) Br-Wang
O
O N i-Pr
O
O
O O
O
N
O
O i-Pr
N
N
N
N i-Pr
i-Pr i-Pr
i-Pr 84
85
Scheme 41.
Subsequent studies involving not only copolymerized ligands but also polymers grafted onto a Merrifield resin (Scheme 43) were performed [89]. For the latter methodology, functionalization of the PyBox was required, thus 4-aminophenoxyPyBox and 4mercaptoPyBox were synthesized, leading to the corresponding ligands supported polymers 91-93. For the Ru catalysts, the introduction of a spacer is important. The longer the spacer is, the higher Ru functionalization. This is consistent with the better accessibility of the grafted ligands contrary to copolymerized ones. Concerning their evaluation for the cyclopropanation of styrene with EDA it has been found that to obtain high catalytic performance and good recyclability, the heterogeneous catalyst must have a gel type organic support with a low cross linking (2% DVB) and a support with no electron-donating effect on the pyridine ring (C-C bond between the support and the ligand). Thus taking into account these parameters, a highly active (40-65% yield) and enantioselective (87-91% ee) catalyst was obtained. Reused at least 3 times of the catalytic system with the same performance was possible. Mayoral [90] also demonstrated that the PyBox-Ru supported complexes on macroporous monolithic polymers were efficient catalysts for the cyclopropanation reaction. PyBox monolithic minireactors were prepared in a stainless column by radical copolymerization of 4-vinyl or 4-styryl PyBox in presence of styrene and DVB using 10% toluene/50% dodecanol as the precipitating porogeneous mixture (polymers 94 and 95 Scheme 44). To form the Ru complex, a solution of [RuCl2(p-cymene)]2 was passed through the column at low flow then washed with dichloromethane to remove the non complexed Ru. For the cyclopropanation of styrene with EDA, several parameters have been studied: flow rate, morphology of the catalyst and solvent. An increase of the flow rate induced a decrease of the cyclopropanation and a dimerization of EDA which is one of the poisons of the catalyst. Besides both region and enantioselectivities gave good values: 80/20 trans/cis, cis isomer 48 ± 5% ee, trans isomer 75 ± 5% ee. The catalyst could be used for several runs (4-6 times) without change in catalytic performance. Concerning the morphology of the catalyst, a slight decrease in catalytic
78
Christine Saluzzo and Stéphane Guillarme
activity was found with ligand 95 containing an aryl spacer. If the crosslinking degree was higher (51% DVB instead of 20%), the activity and chemoselectivity were improved. The reaction performed without solvent provided an increase in both efficiency and chemoselectivity for a mixture styrene/EDA: 1/7 and minimize the dimerization of EDA. With scCO2 as solvent, yields were about 25% but chemoselectivities and ee of both diastereoisomers rise. Moreover, the total number of turnover (TON) is greatly increased. O n-BuLi, -78°C-0°C THF
O Tenta- O Gel
TentaO Gel N
O Br O
H N
N
O
O N
poly styrene
O
87 R a: R = i-Pr b: R = t-Bu loading 0.1-0.12 mmol/g poly styrene
N R
R X
N
R
N N R 75a R 75b
H N
O
N
O
75a 75b
N
O N R
R
X =Cl X = Br
88 a: R = i-Pr b: R = t-Bu loading 0.5-0.56 mmol/g poly styrene N
O N i-Pr
O
styrene (6eq), DVB (7.3 eq) AIBN, 80°C, toluene (toluene/monomers: 40/60 w/w)
N
O N
N i-Pr
O N
i-Pr 89 loading 0.52 mmol/g
i-Pr
Scheme 42.
4.1.4. Immobilization of PyBox on natural polymers Some natural polymers have also been used as support for Schiff base ligands. A starch support has also been used as support for a PyBox. A telomerized starch 96 [91] presenting a final double bound reacted with a thiol-PyBox in the presence of AIBN (Scheme 45) to form the corresponding immobilized PyBox. [92] Although the Ru-97 complex exhibited lower activity (up to 44% after the third reuse) and selectivity (trans/cis: 76/24 with 50% ee for the
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
79
trans and 16% ee for the cis) towards the homogeneous complex formed with 98 (yield 67%, trans/cis: 89/11 with 77% ee for the trans and 63% ee for the cis) this supported catalyst showed better performance than that immobilized on silica [93]. These results pointed out a great influence of the support.
7
x
93-x
Ph O
O +
N N
AIBN toluene
+
O
N
O +
N N
i-Pr
i-Pr
N
i-Pr
i-Pr
90
O N O
O O NH2
N
CHO
O
Br
O
N N
92 0.45-0.5 mmol ligand/g
N O
i-Pr
i-Pr
O
N N
N i-Pr
i-Pr SH
S NaH, DMF, 100°C
O i-Pr
Scheme 43.
O
N N
i-Pr
NH
O
O
N 91
i-Pr
EtOH, reflux
O
N
O
N
Br i-Pr
N i-Pr
O
N N
i-Pr 93 0.45-0.5 mmol ligand/g
80
Christine Saluzzo and Stéphane Guillarme
O
O
N N
O
N
N
i-Pr
i-Pr
N
O
O
N
N i-Pr
95
i-Pr
i-Pr
O
N N
N
i-Pr
i-Pr
94
1) AIBN toluene/1-dodecanol 2) washing with THF
+
+
O
N
i-Pr
or
O
N
Scheme 44. OH O
(
OH
HO
(
(
O OH O
96
(
O O HO
1) Ac 2 O, pyridine 2) AIBN, CHCl3 , reflux
O N HS
N
SH N
O O
OH O
N
OH
HO
(
(
O N
S
( )5
O O HO
OH O
N 97 N
Scheme 45.
O
non supported ligand
O
(
(
O
N N
98
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
81
The homogeneous catalyst formed by reaction of non supported oxazoline 98 with RuCl2 was very efficient, the ratio cis/trans was about 89/11 and enantioselectivities were 63% and 71% respectively. These values are lower than those achieved using 4H PyBox-Ru catalyst (88 and 70%) respectively, owing to the presence of a slightly electron-withdrawing group on the pyridine moiety of the ligand. The Ru-complex of the polymer supported ligand 97 exhibited lower activity and selectivity toward the cyclopropanation reaction of styrene. Three recycles were performed showing a trans/cis ratio and the enantioselectivity constants.
4.1.5. Soluble supported-polymer Box as ligands Box 74a and 74b have also been immobilized onto the soluble polymer MeOPEG (a monomethylether of polyethylene glycol, Mn > 2000 Da) and employed as ligand in the Cucatalyzed cyclopropanation of styrene and 1,1-diphenylethylene with EDA with up to 93% ee. Taking into account the best parameters found with insoluble polymer, which were necessary to enhance the stereoselectivity of Box promoted reaction [94], i.e. disubstitution at the Box bridging atom and the necessity of introducing a spacer in order to separate the polymer moiety (PEG) to the ligand itself, Cozzi synthesized the supported polymers 99a and 99b (Scheme 46) [95]. With styrene, ligand 99b gave a trans/cis stereoselectivity in favour of the trans product (70%), the trans isomer presenting 87% ee. An increase of the yield (about 20%) was observed if the time of addition of EDA and of the reaction increased. In the same conditions, the results were inferior to those obtained with the gem dimethyl Box 74b but the two MeOPEG-supported catalysts presented similar behaviour. Recycling of the catalytic system was possible with marginal erosion of the catalytic activity and very limited loss in ee. OH
1) NaH or Cs2CO3 or Bu4N+OH-
2)
O
O
O N
N
O
O spacer
R
R
O
OMs
N
N R
R 99
: MeOPEG
a: R = Ph b: R = t-Bu
Scheme 46.
4.1.6. Soluble supported-polymer AzaBox as ligands In 2000, Reiser [96] reported the first immobilization of aza-bis(oxazolines). Azabis(oxazoline) 74b has been easily grafted to a soluble polymeric support ((methoxy(polyethyleneglycol) MeOPEG 5000) in order to form soluble catalysts which could be recovered by precipitation. This polymer (100 Scheme 47) was obtained successfully (55% yield) if a benzylidene spacer linked to the PEG was employed. Complexed with Cu(II), this ligand was able to promote asymmetric cyclopropanation of styrene (Scheme 36: R1 = Ph, R2 = H, R3 = Me, R4 = H) and 1,1-diphenylethene (scheme 36:
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Christine Saluzzo and Stéphane Guillarme
R1 = R2 = Ph, R3 = Me, R4 = H) with methyl diazoacetate in dichloromethane. With styrene, a diastereoselectivity in favour of the trans cyclopropane with a predominance of C over D was observed (C/B: 71/29, C: 91% ee B: 87% ee) with a yield of 69% (Scheme 36). Compared to the non-supported catalytic system (ligand 75b), the results in terms of diastereoselection and ee were 10% lower. The possibility of the reuse of this catalytic system was examined. 13 cycles were conducted without loss of ee, but to keep a good yield after the 10th use, it was necessary to reactivate the catalytic system with phenyl hydrazine. O
O N t-Bu
O
H N
O
n
Br
N
n
N
O N
n-BuLi, -78°C-0°C
t-Bu 75b
O
O
t-Bu
O N
100
t-Bu
Scheme 47.
4.2. Porphyrins More recently, chiral Ru-porphyrin polymers were used for catalytic asymmetric carbene transfer. A C2-symmetric group containing two norbornane moieties fused to the central vinyl substituted benzene ring of a porphyrin was chosen in order to induce the chirality. Then the chiral ruthenium vinylporphyrin 101 was involved into radical copolymerization with styrene and DVB or EGDMA to lead respectively to monolithic resins chiral 102 and 103 which were crushed (Scheme 48). In order to form cyclopropane esters and cyanocyclopropanes, EDA and diazoacetonitrile were respectively used in the cyclopropanation of various substituted styrenes. Although less enantioselective than their homogeneous counterpart, these catalysts in presence of EDA led to high stereoselectivities. For example with 102a, up to 92/8 for styrene and pmethoxystyrene and 97/3 for p-bromostyrene, in favour of the trans isomer were obtained. Yields were respectively 77%, 88% and 75%. The best ee (90%) was obtained with the pbromo derivative. Moreover, diastereoselectivity and enantioselectivity were influenced by the nature of the porogenic solvent employed during the formation of the catalyst. Compared to 102b, better ee and diastereoselectivities were obtained with catalyst 102a (trans/cis ratio: 82/18 and 92/8; 82% ee and 71% ee respectively), these two catalysts having the same DVB/styrene ratio. This behaviour was attributed to the reduced accessibility of the catalytic sites, also evidenced with polymers prepared from higher DVB/styrene ratio. With 102 slightly lower results than those of 102a were observed. Concerning the cyclopropanation with diazonitrile, moderate activities from 35 to 53% yield and correct enantioselectivities around 70% were obtained, the best results being found with catalyst 102a. These catalysts were stable upon recovery and recyclability (no leach of metalloporphyrins) but a sligh decrease in activity was observed [97].
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
83
R
N CON Ru N N
N CON Ru N N
styrene DVB or EGDMA R AIBN, porogen
R
R
101 catalyst 102 crosslinker: DVB R = porogene: 102a: toluene 102b: CH2Cl2/dodecane
( catalyst 103, crosslinker: EGDMA R = porogene: toluene
)
O
O
O O
Scheme 48.
Supported metalloporphyrins have also been synthesized by electropolymerization leading to films of polymers. They were formed on the platinium working electrode during the oxidative electrosynthesis (Scheme 49) [98]. Contrary to the preceding catalysts 102 and 103, these electropolymerized catalysts led to low enantioselectivities for the bench-mark reaction between styrene and EDA. Up to 53% ee at - 40°C was reached in the presence of 105a. Moreover, at room temperature, the reactions proceeded efficiently (yields 80-90%). Seven recycling of polymer 105a were carried out without a significant decrease in enantioselectivity and activity.
4.3. Miscellaneous Instead of Box ligand, carboxamidate coordinate with rhodium have been tested successfully for the cyclopropanation of some functionalized alkenes [99] (Scheme 50).The carboxamidate anchored to the NovaSyn Tentagel hydroxyl resin (TG) or Merrifield resin (Scheme 50) was treated with chiral dirhodium tetrakis[methyl 2-oxypyrrolidine-5(S)carboxylate] (Rh2((S,S)MEPY)4) leading to the corresponding Rh-106 and Rh-107 precatalyst. With these catalysts, intramolecular cyclopropanation of allyldiazoacetate was investigated (Scheme 51). Compared with homogeneous catalyst, yield of bicyclolactone was similar to those obtained with the two catalysts i.e. 75% and 95% ee.
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Christine Saluzzo and Stéphane Guillarme
Catalyst with the Merrifield support (Rh-107) presented a yield of about 75% and a slight decrease in ee after 10 runs contrary to the TG one (Rh-106) which showed 10% of the yield and of about 20% ee decrease. Ligand loading had an influence only on the yield but not on the selectivity of the reaction. A similar study was performed with the cyclopropanation of styrene and EDA. Once again, the best catalytic system in terms of reproducibility of the results on the reuse corresponds to the Merrified one. For the first use they were quite equivalents.
R1
N CON Ru N N
R2
Anodic oxidation spirobifluorene
N CON Ru N N
(R1)n
104
105 x
R=
R3
R2 =
(
)
R3 = x
(
104a: R1 = R, R2, 105a: n = 1, R1 = R, R2 104b: R1 = R2, 105b: (R1)n = R3
Scheme 49.
In 2003, Xia [100] has reported the cyclopropanation of styrene with alkyl diazoacetates by copper complexes of Shiff bases, derived from chitosan and substituted salicylaldehydes as the catalyst (Scheme 52). This natural polymer is an environmental friendly natural material: biocompatible, biodegradable, non toxic… The influence of several parameters: nature of the solvent, temperature, the molar ratio styrene/EDA and the copper content [100] has been studied. Better yields were obtained with high temperature but better ee were found at 60°C, temperature for which the nature of the solvent had a great influence. From all the solvent tested, 1,2-dichloroethane, ethylacetate, acetonitrile and toluene, the best yield (91.5%) was obtained with 1,2-dichloroethane; toluene led only to 41.5% yield. Better ee were also found in 1,2-dichloroethane. When the molar ratio styrene/EDA was about 17/1 the results of this reaction were fairly good; as this ratio diminished, the yield and ee decreased and for a ratio of 2/1, the yield reached only 45.3%. Taking into account of all these parameters, it has been shown that to obtain the best compromise between the yield (81.5%), the best cis/trans ratio (35.3/64.7) and the best ee (cis: 20.7% ee, trans: 10%ee) for the cyclopropanation of styrene, a copper content
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
85
(Cu(OAc)2) of 4.4 mol% was necessary with ligand 108a. More or less amount of the copper content led to a decrease of all these reaction results. It is noteworthy that neither the electronwithdrawing nor the bulky substituents on the salicylaldehyde of the catalysts had much influence on the yield, the ee of the cis and trans isomers but the nature of the diazoacetate had a great influence. Using t-Bu diazoacetate instead of the Et one resulted in a important increase of the ee for both the cis and trans isomers but with a dramatic drop of the yield. This behaviour could be a result of the steric hindrance of the t-Bu groups which provoked a difficulty to form metal carbene species by the reaction of t-Bu diazoacetate with the Schiff base copper complexes. These catalytic systems have also been use to transform other alkenes into cyclopropyl derivatives. In some cases, compared to the styrene reaction the cyclopropanation of 1-hepten or 1-octene gave better ee.
O(
) OH + O n
N H
TG 0.27 mmol OH/g
Cl
+ O
N H
1 mmol Cl/g
O
DCC HOBt DMF
COOH
O(
)O n
106 HN O
DMF
COO- Cs+
O
O 107 HN
O
O O(
)O
N
n
Rh2
O O(
)O
MeOOC
n
or O
O
N H
O
N H
O 3
+ Rh2((S,S)MEPY)4 PhCl
or
reflux
O
N
Rh-106 O
O O
N
O
Rh2 MeOOC
N
O 3
Rh-107
Scheme 50.
N catalyst
O O Scheme 51.
CH2Cl2 reflux
O O
86
Christine Saluzzo and Stéphane Guillarme CH2OH O
CH2OH O O
OH
O
OH derivatives of salicylaldehyde
N
HO NH2
n
R1
n
108 a: R1=R2=H b: R1=H, R2=Cl c: R1=H, R2=Br d: R1=H, R2=NO2 e: R1=R2=Cl f: R1=R2=t-Bu
R2 Scheme 52.
To conclude, for the cyclopropanation reaction, the difficulties consist in the achievement of high enantioselectivity close to a good stereoselectivity. To be useful in organic synthesis, the last requirement is preponderant and depends on the nature and the number of the substituents of the cyclopropane ring. It is the reason why, most of the catalytic systems used in heterogeneous supported catalysis cyclopropanation are formed by bis oxazolines complexes which have shown a excellent efficiency in non supported conditions. In most of the cases, this reaction seems to be very sensitive to the polymer morphology controlled by crosslinking and the nature of the porogen agent which determine the performance of the immobilized catalyst.
5. CYCLOADDITIONS AND HETEROCYCLOADDITIONS Discovered in 1928, the Diels-Alder reaction is one of the powerful synthetic methods for the construction of substituted cyclohexene that fits the modern concept of atom economy [4]. For this reason, in the last two decades, the asymmetric Diels-Alder reaction, leading to carbon-carbon bond formation, has received considerable attention and enantioselectivities greater than 90% have been reported [101]. In homogeneous conditions, a wide range of metal, ligands and dienophiles have been studied. Because of their high stabilities, most of the studies are focused on the use of N-containing complexes. Concerning the Diels-Alder reaction, polymeric supports were mostly involved in solid phase synthesis where the diene and the dienophile are grafted onto the polymer. Compared to these procedures, only few examples involving supported catalysts were reported in the literature. Most of these supported catalysts contain nitrogen as ligands such as Box, aminoalcohol, N-sulfonylamino acid and salen.
5.1. Supported Box Ligands Moberg [102] developed a polymer bound bis(oxazoline) 109 (Scheme 53) as a zinc complex catalyst for the Diels-Alder reaction of cyclopentene with 3-(2-propenoyl)-2-oxazolidinone
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
87
(Scheme 54). This supported catalyst was formed by grafting a phenolic bis(oxazoline) derivative onto an ArgoGel Wang-Cl resin and presented 0.071 mmol of ligand/g polymer. OH
O
1) K2CO3
O
2) Argogel Wang-Cl
O
O N Ph
O
O
N
N Ph
Ph
109
O
O
N
N Ph
N Ph
Ph
Scheme 53.
Compared to the monomer (conversion 100%, exo/endo ratio: 94/6, ee: 85% for the exo adduit), the polymer supported catalyst gave no selectivity (racemic compounds are formed) and lower reactivity (13% at -40°C). This behaviour is probably due to the low swelling and the rigidity of the polymer at low temperature. At the same time, Cozzi described the immobilization of Box ligands on PEG polymer (polymer 99, Scheme 49), and the corresponding copper catalyst used for the cyclopropanation reaction was also involved in Diels–Alder cycloaddition between cyclopentadiene and N-acryloyloxazolidinone (Scheme 54) [95]. This reaction was performed with 15 mol% each of Box ligand and Cu(OTf)2 in DMF in order to obtain homogeneous conditions. The reaction occurred in good yield (up to 83%) and excellent diastereoselectivity in favour of the endo stereoisomer (ratio endo/exo >98/2). However, the maximum of enantioselectivity was only 45% contrary to the non-supported Box equivalent catalyst which led to complete end at 95% ee and 88% yield. H
O
O N
H
O
O N
O (S)
O
+ O
O
N
O
(R)
Scheme 54.
In 2002, Lemaire chosed to immobilize indaBox, not only for its rigidity but also for its good enantioselectivities for the Diels-Alder cycloadditions [103]. The heterogeneization was carried out by polymerization as part of the main chain of a polymer backbone (Scheme 55). Mixed with Cu(OTf)2, polymer 110 (8 mol% relative to the oxazolidinone) was allowed to react with methacrolein and cyclopentadiene at -78°C (Scheme 56). The reaction was quantitative and led to the major endo diastereoisomer with 51-56% ee with 87-90% yields for the first three reaction cycles. Itsuno synthesized supported aminoalcohols 111-113 (Scheme 57) [104] and Nsulfonylamino acid 114 [105-107] (Scheme 58) by radical suspension copolymerization of the corresponding monomer chiral styryl aminoalcohols and styryl sulfonylamino acid derivatives with styrene and eventually with a crosslinker.
88
Christine Saluzzo and Stéphane Guillarme OH O
HO O N
+
N
OCN
NCO
DMF, dibutyltin dilaurate
O
HO O O
O N
O H
N H
N H
O
N
66%
110
n
Scheme 55. H + O
Polymeric catalyst
CHO Me
Scheme 56.
5.2. Supported Aminoalcohol and N-Sulfonylamino Acid-Derived Oxazaborolidine or Oxazaborolidinone The styryl chiral sulfonylamino acid derived from L-valine 114a, L-isoleucine 114b, Lthreonine 114c or D-2-phenylglycin 114d (Scheme 58). The loading of aminoacid residue was about 0.78-0.88 mmol/g for a polymer formed with 10% of the styryl chiral sulfonylamino acid, 80% styrene and 10% of DVB with a yield range of 85-96%. In the case of valine, for a polymer formed with 50% of the styryl chiral sulfonylamino acid, 40% styrene and 10% of DVB, it could be up to 2.55 mmol/g and 73% yield [105]. The Diels-Alder reaction was performed with cyclopentadiene and methacroleine (Scheme 56) in presence of 15 mol% of the oxazaborolidine and oxazaborolidinone catalysts derived respectively from supported aminoalcohols and from N-sulfonylamino acid polymers. The oxazaborolidine and oxazaborolidinone catalysts were formed in situ by action of BH3, BH2Br, BHBr2 or BBr3. The diastereoselectivity was excellent in favour of the exo adduct and yields from 65 to 99%. It is noteworthy that higher loading of chiral catalyst site in the polymer, lower exo selectivity and enantioselectivity. The diastereoselection depended not only on the nature of the supported ligand, the crosslinker but also on the borane and the solvent. Results are summarized in Table 6. From all the oxazaborolidine catalyst, the L-valine derived catalyst 115a gave the highest enantioselectivity and an excellent ratio endo/exo. Moreover oxazaborolidinone polymeric catalysts having oxyethylene crosslinkages (116) present better performance.
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
0.1
89
0.2
0.7
styrene DVB
O H
O
Ph Ph
Ph Ph
H
OH
OH
H2N 111
H2N
0.2
0.8 styrene
NH
OH NH
OH 112
0.8
0.1
N
styrene DVB
HO Ph Ph
0.1
N HO Ph Ph
113
Scheme 57.
5.3. Supported Salen as Ligand Seebach [108] prepared non-dendritic 117 and dendritic 118 and 119 styryl-substituted salen, monomers for crosslinking suspension copolymerization with styrene (Schemes 59 and 60). Complexed with Cr, these resulting copolymers were involved in Diels-Alder reaction of Danishefsky’s diene with several aldehydes (Scheme 61). In all cases, these new Cr-salen catalysts led to slightly lower enantioselectivities than those observed with their homogeneous analogs under the same conditions. Surprisingly, upon reuse, enantioselectivities generally increased around 5% from the first to the fifth catalytic cycle. With caproaldehyde and with benzaldehyde enantioselectivities are respectively higher than 85% and 75%. Recently, Schulz was carried out electropolymerization, an original methodology to prepare polymer supported salen 113 [109][110]. It consisted in the introduction of a thiophene moiety, an electropolymerizable functionality, on the salen backbone (Scheme 62) followed by electropolymerization under cyclic voltammetry conditions on a platinum grid. These polymeric catalyst 120a and 120b were then evaluated in hetero Diels-Alder reaction (HTR) between several aldehydes and 1-methoxy-3-[(methylsilyl)oxy]-1,3-butadiene
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Christine Saluzzo and Stéphane Guillarme
(Scheme 61) using 4 mol% 120a or 6 mol% 120b. It has been shown that the counter ion has quite no influence on the selectivity but the activity is lower with 120b containing BF4.
1) styrene DVB 2) Borane
O2S HN R1
OH 114
R2 O
O2S
N
R1 R2
X
B
115
O O
crosslinkage
1) styrene crosslinker 2) BH3.Me2S
O2 S
O2S HN i-Pr
OH
N B H O 116
i-Pr
O
X = Br, X = H a: R1 = H, R2 = i-Pr b: R1 = H, R2 = t-Bu c: R1 = Ph, R2 = H d: R1 = H, R2 = CH(OH)Me
O
crosslinker: a
(CH2)8
b: n = 0; c: n = 3; d: n = 7.7 O
O
n
O
Scheme 58
Table 6. Asymmetric cycloaddition reaction of cyclopentadiene and methacroleine in presence of 15 mol% polymeric boron catalyst, at -78°C Polymer 111 112 113 113 113 113 115a 115b 115c 115d 116a 116b 116c 116d
Borane BH2Br BH2Br BH2Br BH3 BH3 BBr3 BH3 BH3 BH3 BH3 BH3 BH3 BH3 BH3
Solvent CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2/THF CH2Cl2/THF CH2Cl2/THF CH2Cl2/THF CH2Cl2/THF CH2Cl2/THF CH2Cl2/THF CH2Cl2/THF CH2Cl2/THF CH2Cl2/THF
Yield (%) 96 98 99 95 89 96 87 99 99 65 86 85 93 88
Endo/exo 8/92 4/96 < 1/99 < 1/99 < 1/99 5/95 < 1/99 3/97 4/96 8/92 8/92 5/95 1/99 4/96
Ee (%) 16 25 44 57 65 54 65 49 49 10 84 77 92 95
Ref. [104] [104] [105] [104] [104] [105] [106] [107] [106] [106] [106] [107] [107] [107] [107] [107]
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
N
91
N
OH HO t-Bu + Styrene
t-Bu
suspension copolymerization benzene, THF/H2O, PVA, AIBN 90°C, 20h
copolymer 117 Scheme 59.
Compared to the homogeneous catalytic system analog, heterogeneous one is less selective. The difference in enantioselectivity between homogeneous catalyst and these electropolymerized ones were possibly due to the steric hindrance around the active site containing the metal and the electronic effects providing by the delocalization due to the conjugaison of the polymerized catalyst. Seven or fifteen recycles of the catalytic system have been performed with heptanal and cyclohexane carbaldehyde respectively without showing any change in enantioselectivity and activity.
6. ADDITION OF ORGANOMETALLIC REAGENTS TO ALDEHYDES, KETONES AND IMINES The carbon-carbon bond forming reaction is one of the most useful chemical processes for the construction of complex natural or synthetic organic molecules. Addition of organometallic reagents to carbonyl or imine compounds is among the most fundamental reaction [111]. Its enantioselective version producing simultaneously a carbon-carbon bond and a chiral center is particularly important. The reaction of Grignard reagents with carbonyl compounds proved to be one of the best methods for forming carbon-carbon bond. Unfortunately, Grignard reagents are unsuitable for asymmetric alkylation because of their high reactivity in opposite to organozinc reagents such as diethylzinc, diphenylzinc or alkynylzinc which are excellent nucleophiles in presence of chiral ligands (Scheme 63). Numerous studies about the asymmetric addition of these reagents to carbonyl or imine derivatives have been performed and various chiral ligands have been synthesized [112]. Among the ligands used, chiral ligands bearing nitrogen chelating donor atom have been extensively studied due certainly to their easy preparation and availability. Furthermore, a lot of chiral ligands have proved to be highly efficient for the asymmetric alkylation of carbonyl or imine compounds in homogeneous phase. But, for the reasons explained in introduction, the use of homogeneous supported or heterogeneous ligand is preferable. Among the polymer-supported ligands, derived-aminoalcohol ones have been largely used in this reaction but few are derived either from N-sulfonamideaminoalcohols, or oxazoline or salen.
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Christine Saluzzo and Stéphane Guillarme
Ph
Ph
N R
N suspension copolymerization benzene, THF/H2O,PVA, AIBN 90°C, 20h
OH HO
R
t-Bu + Styrene
Ph
copolymer 118
t-Bu
Ph
N
N
OH HO
R
R
t-Bu + Styrene
t-Bu
suspension copolymerization benzene, THF/H2O, PVA, AIBN 90°C, 20h
copolymer 119 R= O
O
O Scheme 60.
O TMSO +
O R
polymeric catalyst
H
O OMe
Scheme 61.
*
R
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
N
93
N
OH HO S
S t-Bu
t-Bu
1) CrCl2, THF 2) AgBF4, TBME 3) 50 mA, Pt/Cgraphite NBu4NBF4, CH3CN
N
N Cr O X O S
S t-Bu
t-Bu
n
120 a:X = Cl, 77% b: X = BF4, 64%
Scheme 62.
X + R
R''2M or R''M
Ligand Solvent, T°C
XH * R R"
X=O, N-R' Scheme 63.
6.1. Supported Aminoalcohols as Ligands The first work about polymer-supported β-aminoalcohol used in the asymmetric nucleophilic addition to aldehydes was reported by Fréchet [113]. The best result (95% ee) was obtained by using a polymeric catalyst derived from N,N-dialkylated (-)-3-exoaminoisoborneol 124b for the asymmetric addition of diethylzinc to o-methoxybenzaldehyde in toluene at 0°C (Scheme 64). This catalyst was prepared through reaction of aminoalcohols with 1-2% crosslinked chloromethylated polystyrene. The chiral polymeric catalyst of the reaction could be used several times in further asymmetric reactions.
Me H
Ph H
N
OH
121
Scheme 64.
O
Ph
R2N OH 122a, R=H 122b, R=Me
OH N
N R OH 123
124a, R=H 124b, R=Me
94
Christine Saluzzo and Stéphane Guillarme
Soai has described the enantioselective addition of dialkylzinc reagent to aromatic and aliphatic aldehydes by using the supported-ligand 121 [114]. This latter was also prepared from (1R,2S)-(-)-ephedrine and chloromethylated polystyrene (1% divinylbenzene, 0.8 mmol Cl/g, 100-200 Mesh). In the case of benzaldehyde, the enantioselectivity was better than those reported with 124a (89% vs 80%) and with the N-alkylephedrine-based homogeneous catalyst. The authors mentioned that the catalyst could be easily recovered and reused. They also studied the reaction with norephedrine homolog and reported enhanced enantioselectivity in the case of aliphatic aldehydes [115]. Polymer-supported ephedrine 121 was also employed in the first heterogeneous enantioselective alkylation of N-diphenylphosphinoylimines and the best results (80% yield and 80 % ee) were obtained when the reaction was carried out in toluene at room temperature for 2 days [116]. Itsuno has reported interesting work about the effect of the difference of the structure of the polymeric part on the enantioselectivity of the reaction and the stability of the support [117]. They first prepared the chiral aminoalcohols 125 which could react with a crosslinked chloromethylated polystyrene or 126 which could be copolymerised with styrene and some styrene-derived crosslinking compound (Scheme 65).
OH O Ph Ph H 2N
125
Ph Ph
OH H2N
OH 126
O
O
O 3
127
Scheme 65.
Five different chiral polymers containing the same aminoalcohol moiety were synthesized (Scheme 66). Reaction of aminoalcohol 125 with a chloromethylated polystyrene crosslinked with 2% of divinylbenzene led to chiral polymer 128. When the highly crosslinked polystyrene containing 20% of DVB 129 was used, no reaction between 125 and this resin because of the lack of the swellability of the resin in organic solvents occurred. To avoid this problem, the crosslinking agent containing an oxyethylene chain 127 and the corresponding crosslinked chloromethylated polystyrene 131 were prepared. This latter swelled very well in organic solvents such as toluene and THF, even in the case of a highly crosslinked resin. Phenoxide anion of 125 could react with the chloromethyl group of poly mer 131 to give the chiral supported ligand 132. Three other chiral polymers have been synthesised by copolymerization of the chiral monomer 126 with styrene in the presence or not of crosslinking agent (Scheme 66). Chiral polymer 133 was prepared by suspension copolymerization of aminoalcohol 125 with styrene and DVB in a 1/7/2 molar ratio. Polymer 133 swelled well in organic solvents such as toluene, benzene and THF and that unusual swellability could be due to the bulkyness and polarity of the chiral monomer 125. The replacement of DVB by 127 as the crosslinking agent afforded chiral polymer 134 after copolymerization. This one also swelled well in
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
95
organic solvents and presented interesting mechanical stability. The solution polymerization of 127 and styrene led to the soluble linear chiral polymer 135. The main problem of polymers containing a high degree of crosslinking is their stability with stirring over a long period of time. Indeed, their spherical beads are not enough stable and their breakdown during stirring lead to formation of a fine powder of the insoluble polymer. So the filtration at the end of the reaction is difficult and the reuse of the chiral polymer was not possible. The authors studied the effect of the magnetic stirring on the three highly cross-linked polymer beads and found that chiral 20% 127-crosslinked polymers 132 and 134 were more stable than polymer 133 prepared by using 20% DVB. For example, when stirring at 350 rpm for 4 days in toluene, 50% of polymer 133 was transformed into a fine powder contrary to 132 and 134 which kept their original bead form. The five chiral polymers 128 and 132-135 were then tested in asymmetric addition of diethylzinc to benzaldehyde. Good enantioselectivities were obtained with chiral polymers 133-135 in opposite to the two others polymers 128 and 132. In the case of pchlorobenzaldehyde, an excellent ee of 99% was obtained with polymer 134 in oposite to oalkoxybenzaldehydes (21-54%) and longer reaction times were necessary to have satisfactory conversions with aliphatic aldehydes. The recyclability of the chiral polymers was studied in a batch system using a simple loading of polymeric ligand 134 and both yield and ee were highly reproducible. Large scale of a chiral product could be obtained by using a continuous flow system. This technique have been tested with chiral polymer 133 and showed that a column containing 5 mmol of the polymer could produce about 90 mmol of (S)-1-(p-chlorophenyl)propanol with 94% ee. The main advantage of this technique is that it avoids the stirring which destroy the polymer during repeated reactions. Soai and Watanabe have studied the effect of the introduction of a spacer between the ligand and the polymeric support [118]. In fact, when the polymer is directly connected to the nitrogen atom of the aminoalcohol, the polymer part could have an influence on the enantioselectivity since it is known that nitrogen atom is strongly involved in the formation of intermediate chiral zinc complex. The two chiral polymers 137 (content of aminoalcohol moiety is up to 0.085 mmol/g) and 138 (content of aminoalcohol moiety up to 0.25 mmol/g) were prepared from polymer 136 (1% DVB; Cl: 0.8 mmol/g; 100-200 mesh) and the corresponding chiral aminoalcohols (Scheme 67). The enantioselective addition of diethyzinc to aldehyde at 0°C with chiral polymers 137140 was examined. With aromatic and aliphatic aldehydes, higher enantioselectivities were obtained with chiral polymers 137 and 138 containing a six-methylene spacer compared to the two chiral polymers 139 and 140. The same level of enantioselectivity was found with recycled chiral polymers. The authors assumed that the spacer has here two effects. The steric repulsion between the aminoalcohol part and the polystyrene is reduced and the spacer acts as a subtituent of the nitrogen atom which assists the stereochemical control of the reaction. The four-methylene chain analog of chiral polymer 137 and chiral polymer 139 were also tested in the asymmetric alkylation of N-diphenylphosphonylbenzaldimines [119]. Contrary to aldehydes, both yield and enantioselectivity were lower with chiral polymer containing a spacer between the polymeric moiety and the catalytic part. The authors then examined the reaction conditions with 139. The result of their screening was that the best yields and ee’s
96
Christine Saluzzo and Stéphane Guillarme
were obtained when the reaction was carried out in toluene at room temperature in presence of 1 equivalent of the polymeric ligand.
a
Cl
R*O
128
a
129
Cl
R*O
130
a
Cl
O O
O
O
3
O
3
131
+
OR*
O
132
b
+
R*O
OR*
133
b
+
+ O
O
O
OR*
R*O
O O
3
O
3
134 b
+
Ph Ph
R* = OR*
135
OR*
H 2N
OH
Scheme 66. (a) R*OH, NaH, DMF. (b) AIBN, THF Ephedrine or norephedrine-derived copolymers 142a-d have also been used in the same reaction [120]. These chiral polymers were prepared by copolymerization of monomer 141a-d with styrene and DVB in a molar ratio 141a-d/styrene/DVB of 1/7/2 (Scheme 68).
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer… Ph H
Me H HN
CH 2 O
OH Bu
N
OH
5
Bu
137
CH 2O(CH 2 )6I + 136
Ph H
Me H
K 2 CO 3
K 2CO 3
HN
CH 2O 5
HO
97
N
138
PhPh
HO Ph H
Me H N
OH
PhPh
N
Bu 139
HO PhPh 140
Scheme 67.
The asymmetric addition of diethylzinc to N-diphenylphosphinylbenzaldimine was carried out in toluene at room temperature in presence of the chiral copolymerized ligands 142a-d. Although the yields were low to moderate, the enantioselectivity was moderate to high (64%-88% ee) particularly with the chiral ligands 142b and 142c. The enantioselectivities with these two polymers were comparable with those observed with the corresponding monomer 141b and 141c. The recyclability of polymer 142b was also tested and this one could be reused without loss of enantioselectivity.
CH 2
H C
m
H2 C
H C
n
DVB / AIBN
+
N R
Me *
141a-d
* Ph
OH
Benzene / THF H 2O
N R
Me *
* Ph
OH
(m/n = 1/7)
(1S,2R)-142a: R=H (1S,2R)-142c: R=Et (1R,2S)-142b: R=Me(1S,2R)-142b: R=Bu
Scheme 68.
Hosoya has reported the preparation of polymer-supported ligand which has only moderate enantioselectivity because the aim of the work was to show the effect of the type of immobilization [121]. The polymeric ligands were prepared by two different techniques, the Mix Method and the Add Method. The Mix Method was a classical polymerization: the ligand-containing monomer was dissolved in the mixture of monomers and porogenic solvent was used for the second-step swelling. After completion of this step, the polymerization step was carried out at 70°C with slow stirring for 24h to lead to the resulting polymers 144 and
98
Christine Saluzzo and Stéphane Guillarme
145. The Add Method consisted in adding the same amount of monomers 143 in the aqueous polymerization medium 4h after the initiation of the polymerization to give the polymers 146 and 147 (Scheme 69). The two monomers were quantitatively incorporated in the beads produced by the Mix Method and only 44% of monomers for the Add Method. R N
OH
144: 145: 146: 147:
AIBN
PS-PG-M PS-PA-M PS-PG-A PS-PA-A
PS=polystyrene PG=phenylglycinol PA=phenyalanilol M=Mix Method A=Add Method
143a: R=Ph 143b R=PhCH 2
Scheme 69.
The catalytic activity of the different polymeric ligands was evaluated in the asymmetric ethylation of aldehyde in hexane. The polymers derived from the Mix Method gave better results in terms of activity and enantioselectivity than those from the Add Method. However, both yields and ee were slightly lower than those obtained with the monomeric ligands 137. When a mixture of toluene/hexane (1/1) was used as solvent, chemical yield dropped significantly. The effect of the level of functionalization and crosslinkage of different Merrifield resins was also tested [122]. Chiral polymers 150 were prepared in two steps from enantiopure epoxyalcohol 148 (Scheme 70). X X O Ph
NaH, DMF
OH 148
Cl Merrifield resin
R
O O
Ph 149
N H LiClO 4 CH 3CN
R
R Ph
N
R O
OH 150a: X=CH 2 , R=H 150b: X=NMe, R=H 150c: X=CH 2 , R=Me
Scheme 70.
These chiral polymers 150 with different levels of crosslinking and ligand funtionalization were evaluated in the asymmetric addition reaction of diethylzinc to benzaldehyde and the results are summarized in Table 7. Although the conversions were higher than 95%, the enantioselectivities were low to moderate and significantly lower than those observed with the corresponding homogeneous ligand [123]. In all cases, the best results were obtained with both lower level of cross-linking (1%) and level of chlorine per gram of resin (1.2 mmol) of the chloromethylated polystyrene. Higher the level of aminoalcohol residue is, more important the formation of dimeric zinc alkoxide is. These
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
99
dimeric species are suspected to be inactive in catalysis, requiring a preliminary dissociation before the catalysis step. Table 7. Enantioselectivity for the ethylation of benzaldehyde with chiral polymers 150 Ligand (%DVB ; fa) Amount (%)b ee (%) 150a (1; 0.8) 5 36 150a (2; 1.6) 4 22 150a (2; 1.8) 3 20 150b (1; 0.9) 5 39 150b (2; 1.6) 4 20 150b (2; 1.7) 3 19 150c (1; 1.0) 3 69 150c (2; 1.6) 4 57 150c (2; 1.7) 3 57 a f=mmol of ligand/g resin (calculated by elemental analysis of nitrogen). b amount of catalyst (% molar with respect tobenzaldehyde)
. It was assumed that the low enantioselectivity was probably due to a lack of bulky susbstituents on the primary hydroxy group of the chiral aminodiol. The cis 2,6dimethylpiperidine-derived aminoalcohol 151 was anchored to 2-chlorotrityl chloride resin (Barlos resin) leading to polymeric ligand 152 (Scheme 71, Table 8). The use of an amount of 5 mol% of funtionalized Barlos resin 152 in the asymmetric addition of Et2Zn to benzaldehyde in toluene was highly efficient and (S)-1-phenylpropanol could be isolated with an excellent ee (entry 2, Table 8. When the reaction was carried out at 0°C, the enantioselectivity was slightly improved (entry 3, Table 8). The asymmetric addition of Et2Zn to different aromatic and aliphatic aldehydes using the functionnalized Barlos resin 146 was also studied and both conversions (> 82%) and ee’s (>86 %) were high. X
X R
N
R
Ph
+ OH
Ph (o-Cl)-Ph
R
Cl Barlos resin
N
R
Ph
O
Ph (o-Cl)-Ph
OH 152
OH 151
Scheme 71.
Table 8. Asymmetric addition of diethylzinc to benzaldehyde using 152 Entry 1 2 3 4
f (mmol ligand/ g of resin) 0.9 1.1 1.1 1.2
% molar ligand 4 5 5 5
T (°C) rt rt 0 0
Ee (%) 79 92 93 94
100
Christine Saluzzo and Stéphane Guillarme
Hodge has also studied the reaction by using chiral polymers from ephedrine-derived monomer 141b [124]. Homopolymerization of monomer 141b in presense of AIBN in toluene and copolymerisation with styrene or 4-methylstyrene in various proportions led respectively to chiral polymers 153, 154 and 155 (Scheme 72). styrene or methylstyrene
Me H
Ph H
N
OH
1-x
AIBN Toluene
x
Me H
Ph H
N
OH
R
(1S,2R)-141b
153: x=1 154: R=H, x=0.01-0.66 155: R=CH3, x=0.08-0.52 156: R=H, x=0.06-0.51; prepared from crosslinked polymers beads
Scheme 72.
Chiral polymers 153-155 were used for the asymmetric addition reaction of diethylzinc to benzaldehyde. The reactions were carried out in toluene at room temperature for 24 hours. The results are summarized in Table 9. Although the major product was the 1phenylpropanol, benzyl alcohol was detected as a minor product with each polymer 153 (9%), 154 (0-6%) and 155 (6-7%). As we can noticed in Table 9, the higher the loading of catalyst is, the lower the yield and the ee are. It can be explained for highly loading polymers by the presence of dimeric zinc alkoxide species which drop the catalytic activity of the catalyst. Higher yield and ee were obtained with Zn-complex derived from 154f and 154g which were soluble in toluene (entries 7-8, Table 9). In the case of polymers 155 (entries 9-13, Table 9), the reaction was heterogeneous and yield and ees increased as the loading decreased. The enantioselectivities obtained with polymers 154d-154g (loading ≤ 1.32 mmol.g-1) and 154d155e (loading ≤ 1.32 mmol.g-1) were similar or better than those obtained with the model ligand, (1R,2S)-N-benzylephedrine. Table 9. Effect of the ligand loading of polymers 153-155 on the catalytic activity Entry
Polymer
1 2 3 4 5 6 7 8 9 10 11 12 13
153 154a 154b 154c 154d 154e 154f 154g 155a 155b 155c 155d 155e
Fraction of monomer 135b (x) 1 0.66 0.52 0.28 0.18 0.05 0.03 0.01 0.52 0.46 0.36 0.17 0.08
Loading (mmol.g-1) 3.52 2.98 2.65 1.81 1.32 0.44 0.27 0.09 2.56 2.38 2.03 1.16 0.61
Yield (%)
ee (%)
47 56 66 80 95 98 100 100 65 67 74 92 92
44 56 67 79 81 82 83 83 69 74 76 81 86
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
101
The authors then turned their attention to the preparation of crosslinked polymer beads which are more insoluble and are more convenient to use than the linear polymers 153-155. Different chiral crosslinked polystyrene beads 156 were prepared by reaction of various crosslinked polystyrene beads with (1R,2S)-N-benzylephedrine in presence of potassium carbonate. The catalytic properties of these various crosslinked polymer-supported ephedrines have been tested in the asymmetric addition reaction of diethylzinc to benzaldehyde and the results are summarized in Table 10. The reactions were carried out either in toluene or hexane at 0°C or 20-23°C. As it can be noticed in Table 10, the enantioselectivities were higher in toluene. Compared to linear polymers 153-155, lightly crosslinked chiral polymers led to the best results in terms of yield and ee. The use of Amberlite XAD-4 and Polyhipe™ as support proved to be not efficient. It was assumed that probably for both supports the catalytic sites are more in the inner surfaces of the pores and that result in a high local concentration of aminoalcohol residue which lead to a significant site-site interaction. Chiral polymer 156a was also used in the asymmetric ethylation of 2-methoxybenzaldehyde and 4chlorobenzaldehyde under similar conditions and ees reached 90-93% when the reaction was performed at 0°C. Three other polymers 158 were prepared by homopolymerisation and copolymerisation of aminoalcohols 157 and by grafting of the corresponding aminoalcohol onto 1% crosslinked polystyrene beads containing 0.70 mmol.g-1 of chloromethyl group leading to a polymer containing 0.64 mmol.g-1 of ligand residue (Scheme 73) [124]. Table 10. Asymmetric diethylzinc addition to benzaldehyde using crosslinked polymer beads 156. ee (%) Catalyst Degree of Yield (%) (ee)a loading substitution (yield)a b 1 156a 1% crosslinked gel 0.93 0.12 81 (81) 81 (64) 2c 156a 1% crosslinked gel 0.93 0.12 85 (-) 78 (-) b 3 156b 1% crosslinked gel 1.13 0.15 77 (83) 77 (62) 4c 156b 1% crosslinked gel 1.13 0.15 85 (81) 74 (65) b 5 156c 1% crosslinked gel 2.30 0.40 80 (82) 74 (62) 6c 156d 1% crosslinked gel 2.30 0.40 86 (97) 73 (65) b 7 156d 2% crosslinked gel 1.06 0.14 70 (-) 72 (-) 8b 156e 2% crosslinked gel 2.62 0.51 71 (76) 69 (62) 9b 156f Amberlite XAD-4 0.90 0.11 59 (62) 28 (39) c 10 156g Amberlite XAD-4 0.50 0.06 75 (70 28 (39) 11b 156h Polyhipe™ 1.51 0.21 59 (57) 42 (23) Under brackets, reaction was carried out in hexane. b Reaction was carried out at 0°C for 70h. Reaction was carried out at 20-23°C for 20h. Entry
a
Polymer
Starting polymer
c
Contrary to ephedrine-derived polymers 153-156, the use of a high loading of the catalytic residue (≥ 2.48 mmol.g-1) for chiral polymers 158a and 158b gave excellent enantioselectivies for the ethylation of benzaldehyde and similar to those obtained with the corresponding non-polymeric chiral aminoalcohol. This surprising excellent result with a high loading of aminoalcohol unit polymer is probably due to their high solubility in toluene. The
102
Christine Saluzzo and Stéphane Guillarme
enantioselectivity obtained with the crosslinked polymer beads 158c with a ligand loading of 0.64 mmol.g-1 was also high (97% ee).
x
1-x
N N OH 157
OH 158a: x=1 158b: x=0.66 158c: x=0.11; prepared from crosslinked polymers beads
Scheme 73.
The authors were then interested to use the two enantiomers of ephedrine- and camphorderived polymeric ligands 153-156 and 158 in a flow system [125]. When polymeric ligand 156a and its enantiomer analog (1% crosslinking, 1.11 mmol/g of aminoalcohol residue) were employed with a flow rate of 2 mmol.h-1 of benzaldehyde and a diethylzinc/benzaldehyde ratio of 2.5/1, the ee were respectively 81% and 72%. Lightly crosslinked beads 156 (0.2% crosslinking, 1.78 mmol/g loading) produced the (R)-alcohol in a yield of 98% and in 98% ee. With a flow rate of 2.5 mmol.h-1 of benzaldehyde, the ee dropped slightly (91% vs 98%). Supported-ligand 158c (with a ligand loading of 0.64 mmol/g) was also tested in a flow system because this polymer-supported catalyst gave better results than ephedrine one 156. A loss of catalyst performance was noticed here; indeed, the chemical yields and enantioselectivity dropped significantly after aproximatively twelve runs (run 1: 95% yield and 97% ee; run 12: 59% yield and 87 % ee). It was suggested that the deterioration in performance could be due to the oxidation of the hydroxyl group of the aminoalcohol unit. (S)-aziridinyldiphenylmethanol 159 was immobilized to polymer-bound triphenylchloromethane (1.1 mequiv/g) to lead to the new chiral polymer 160 (Scheme 74) [126]. This latter was employed in the enantioselective addition of diethylzinc to aromatic and aliphatic aldehydes. For example, (S)-1-phenylpropanol was isolated in a yield of 92% and an ee of 96% when the reaction was performed in a 1/1 mixture of toluene and dichloromethane. The presence of dichloromethane proved to be essential for swelling of the polymer and making the catalytic sites accessibles to substrates. With other aromatic aldehydes and cyclohexane carboxaldehyde, the same range of chemical yield and ee was obtained. But with acyclic aliphatic aldehydes the chemical yield and ee dropped significantly. The recyclability of the chiral polymer 160 has been investigated and it was noticed a slightly decrease of the enantioselectivity of 1% per run. Styrenic L-prolinol-derived ligand N-Boc-60 was immobilized on chemical inert, mechanically stable polyethylene fibers by electron beam induced preirradiation grafting using styrene as a co-monomer to lead to polymer fiber 161 with a loading of 0.2 mmol/g [127]. Carbamate function was then removed under classical conditions to produce polymer 162 (Scheme 75). Chiral polymer 162 was used as ligand in the diethylzinc addition to
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
103
benzaldehyde and yield and ee were very low (36% and 24% respectively). A higher yield of 55% could be obtained in a titanium-mediated addition of diethylzinc to benzaldehyde by using the protected chiral polymer 161 with the same enantioselectivity.
-Tr-Cl
HN HO
Ph Ph
N
Et3N, CH 2Cl2
159
160 HO
Ph Ph
Scheme 74.
In order to improve the catalytic properties of this supported-α,α-diphenylprolinol polymer fibers, Degni has tested the immobilized ligand 67 with a loading of 0.4 mmol/g (Scheme 30) [128] in the diethylzinc addition to benzaldehyde. The chemical yields were largely superior to those obtained with polymer 162 (95-98% vs 30-36%) and the enantioselectivities were also higher and reached 40% which was comparable to the corresponding non-polymeric ligand.
OH N
O
O N-Boc 60
OH N
O
O 161
OH NH 162
Scheme 75.
Chiral polymer 67 was also tested in another enantioselective reaction, the Carreira reaction. The phenylacetylene addition to benzaldehyde in presence of zinc triflate using 67 as polymeric ligand was carried out. The corresponding propargyl alcohol was isolated in a moderate yield but with an excellent enantioselectivity of 91%. This supported ligand was also recycled for this reaction whithout loss of catalytic activity.
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Christine Saluzzo and Stéphane Guillarme
Br
OH
OH
OH N
N
Ph
N
Ph
Ph
OR
OR
OR
OR
OR
OR
OR
OR
Br 163 +
Pd(PPh 3)4 K 2CO 3 THF
OR (HO)2B
165 B(OH)2 OR
164: R= (CH 2)5CH 3
Ph
Ph
N OH
N OH
B(OH)2
RO 163 + OR
B(OH)2
OH N
OH N
166
Ph
Pd(PPh 3 )4 K 2 CO 3 THF
Ph
OR
OR
OR
OR
OR
OR
167 R=(CH 2 )5 CH3 Ph
N OH
Ph
N OH
Scheme 76.
A new strategy for optically active chiral polymers was envisaged and based on a Suzuki coupling polymerisation of (1R,2S)-ephedrine unit-bearing monomer with rigid linkers [129]. The Suzuki coupling of ephedrine-derived dibromide 163 (which was prepared in four steps from (1R,2S)-ephedrine) with the diboronic acid 164 in presence of Pd(PPh3)4 afforded chiral
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
105
polymers 165 (Scheme 76). Synthesis of chiral polymer 167 containing a longer linker between two chiral units has been performed by using diboronic acid 166. Both polymers were soluble in common solvents such as CH2Cl2, THF, CHCl3… The chiral polymeric ligands 165 and 167 were then employed in the diethylzinc addition to benzaldehyde and anisaldehyde. The enantioselectivities obtained with both polymers were between 74 and 76% for the two aldehydes. The polymers could be recovered by precipitation from methanol and the recycled polymeric ligand showed the same catalytic properties. Chiral bicyclic aminoalcohol-containing polymers 170 and 173 were produced from the industrial waste material 168a generated in the industrial process of synthesizing the ACE inhibitor Ramipril at Aventis. Supported ligand 170 was obtained by grafting of compound 168b (obtained in two steps from 168a) onto a Merrifield resin (1.1mmol Cl/g) and after reaction of the resulting polymer with phenyl Grignard (Scheme 77) [130]. Excellent yields and ee (83 and 89 % respectively) were observed for the diethylzinc addition to benzaldehyde for the heterogeneous ligand 170 and were similar to those obtained for the homogeneous reaction. In a second time, the authors turned their attention on the preparation of flow systems. They considered that in some cases the chiral supported ligands isolated by polymerisation led to higher enantioselectivity. The monolithic column was selected as polymerisation technique because the resulting material is more suitable for the preparation of flow systems because of its better porosity properties. Monoliths 173 with the desired morphology and properties were obtained by mixing 40 wt% of monomers (10 mol% of 172/90 mol% of DVB; no styrene) and 60 wt% of toluene/1-dodecanol as the porogenic mixture (10 wt% of toluene). A flow system was performed and after 24 h, (R)-1-phenylethanol was isolated with an excellent ee of 99%. This enantioselectivity is better than those obtained with the corresponding homogeneous ligand and with the grafted ligand 170 and is one of the best ee observed with a polymeric ligand. The authors assumed that this efficiency could be due to the formation of more appropiated chiral cavities because of the polymerization process or to the high level of crosslinking agent used which isolates the aminoalcohol residue. Chiral polymer 173 could be reused for four successive runs without loss of enantioselectivity. Reduction of the ester function of polymer 169 or treament with a dimagnesium bromide reagent led to the two immobilized aminoalcohols 174 and 175 (Scheme 77). Chiral polymer 174 proved to be not efficient for the diethylzinc addition to benzaldehyde (11% yield and no ee) and with chiral polymeric ligand 175, for the same reaction, the yield was excellent but the enantioselectivity was low (40%) [131]. The same group has developped a small library of supported chiral aminoalcohols from different amino acid [132]. The hydrochloride salt of different amino acid methyl esters 176 reacted with a chloromethylated polystyrene-divinylbenzene (1 mmol Cl/g, 1% DVB) under basic conditions to afford the polymeric amino acid ester 177. Reduction of the ester function or addition of Grignard reagents led to supported-amino alcohols 178-181 (Scheme 78). The nitrogen atom of these polymers was then methylated to afford the N-methylaled polymeric aminoalcohols 182-185. Chiral polymers 178-181 were first tested for the diethylzinc addition to benzaldehyde using 10 mol% of the supported catalyst. In all cases, the chemical yield and the selectivity (based on the 1-phenylethanol/benzyl alcohol ratio) were moderate and the ee were around 10 % except for the (S)-proline-derived polymer 180e (45% ee). The low catalytic activity could be due to the presence of the NH group in polymers 178a-d – 181a-d.
106
Christine Saluzzo and Stéphane Guillarme H
H
H
H CO 2Me
N
b
H
N
a
170: Grafted 174: Polymerised e H
H
H
N H
169
H CO 2 R
168a: R=Ph 168b: R=Me
H
c
H
N
H CO 2Me
d
N
H
LiAlH 4, THF or BrMg(CH 2 )4 MgBr THF
H
H Ph Ph OH
172
171
169
H Ph Ph OH
H
N
H R 174: R=R'=H R' 175: R=R'=-(CH ) 2 4 OH
Scheme 77. Reagents and conditions: (a) Merrifield resin (1.1mequiv/g, 1%cross-linking), NaHCO3, THF. (b) PhMgX, THF. (c) 4-vinylbenzyl chloride, NaHCO3. (d) PhMgCl, THF. (e) A49/DVB/Toluene/1-dodecanol, AIBN.
In order to improve the catalytic activity of these suppported ligands, the nitrogen atom of these polymers was then methylated to afford the N-methylaled polymeric aminoalcohols 182-185. The catalytic activity of these heterogeneous ligands was evaluated and the results are summarized in Table 11. The analysis of the results showed that an increase in selectivity is accompanied by an increase in the enantioselectivity. Polymers 182 were poor catalysts and that is probably due to the non-presence of substituent in α position. The most surprising results are those obtained with polymers 183 bearing two phenyl groups in α position since this type of aminoalcohols give generally high enantioselectivity. The best results were obtained with immobilized aminoalcohols 184a and 185b which are derived from respectively valine and leucine. This last one gave a higher selectivity (91 vs 86 % for 184a) but a lower enantioselectivity (74 vs 80 % for 184a) (entries 9 and 14, Table 11). Supported chiral bispidine-derived aminoalcohols were screened in the addition of diethylzinc to benzaldehyde [133]. These immobilized ligands were obtained in three steps from compound 186 isolated from N-Boc-piperidin-4-one (Scheme 79). The hydroxyl function of 186 was first linked onto chloromethylated polystyrene to afford polymer 187. The carbamate was then removed under classical conditions and reaction of the resulting polymer with chiral epoxides led to chiral polymeric bispidine-derived aminoalcohols 188.
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer… R
O
a, R=(CH3)2CH, R'=H b, R= (CH3)2CHCH2, R'=H H2N Cl R' c, R=Ph, R'=H d, R=PhCH2, R'=H e, R=R'=-(CH2)4-
OCH3 170
+ Cl NaHCO3 DMF
R
LiAlH4 THF
PhMgX THF
R
R OH
N
OCH3
N 171
Ph
OH
N
R'
Ph
R'
O
R' XMg(CH2)4MgX THF
R
R OH
N
BuMgX THF
R'
173a-d, R'=H 174a-d, R'=H 172a-d, R'=H 174e, R=R'=-(CH2)4172e, R=R'=-(CH2)4- 173e, R=R'=-(CH2)4172a-d, R'=H 176a-d, R'=Me CH3I 173a-d, R'=H 177a-d, R'=Me 174a-d, R'=H K2CO3 178a-d, R'=Me 175a-d, R'=H THF 179a-d, R'=Me
OH
N R'
175a-d, R'=H 175e, R=R'=-(CH2)4-
Scheme 78.
Table 11. Enantioselective diethylzinc addition to benzaldehyde using 182-185. Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Polymer 182a 182b 182c 182d 183a 183b 183c 183d 184a 184b 184c 184d 185a 185b 185c 185d
Yield (%) 89 94 93 82 89 90 83 97 99 81 90 92 92 98 94 95
Selectivity (%) 55 65 56 48 63 70 58 70 86 53 68 70 73 91 86 85
ee (%) 10 15 7 2 15 24 11 24 80 22 23 20 34 74 17 45
107
108
Christine Saluzzo and Stéphane Guillarme
Catalytic activity of both chiral polymers 188 was then evaluated in addition of diethylzinc to benzaldehyde. Although yield reached 95%, the enantioselectivity was poor (25% ee for 188a and 5% ee for 188b) contrary to the corresponding non-polymeric ligand homolog of 188a (96% ee).
1) TFA, CH 2Cl2
NaH,DMF
N Merrifield Boc resin
N
HO
4
N
O
4
N 2) n-BuLi, THF Boc
O
chiral epoxide
N R
187
186
N
4
R1 R2 188a: R=Ph, R 1 =OH, R 2 =H 188b: R=Et, R 1 =H, R 2 =OH
Scheme 79.
Pericas has reported the immobilization of (2R)-2-piperidino-1,1,2-triphenylethanol analog 190 onto several resins with different levels of crosslinking and functionalization [134]. These different chiral supported aminoalcohols 191 and 192 were prepared by attaching the aminoalcohol 190 via the benzylic hydroxyl group to different Merrifield resin (0.63-1.49 mmol Cl/g; 1 or 2 % DVB) and to two Barlos resin (1.24-1.6 mmol Cl /g) (Scheme 80). These functionalized resins were then tested in the enantioselective diethylzinc addition to benzaldehyde and the results are summarized in Table 12. The five polymer-supported aminoalcohols exhibited high enantioselectivities (>92 %) and except 192b, they presented a similar profile of catalytic activity. After six consecutive runs, the recovered 191c showed no loss of performance. This supported-ligand 191c was also used with other aldehydes including aromatic and aliphatic aldehydes and both conversion and enantioselectivities were excellent (>95%). O R
OH
O Ph
H
3 steps
Ph
Ph
189
CN
DIEA,C H 2Cl2
HO Ph
H N
190
Merrifield or Barlos resin
HO Ph Ph
H N
Merrifield resin 191a-c: R=
Barlos resin 192a-b: R=
Scheme 80.
Ph Cl
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
109
Table 12. Effect of functionnalized resins 191 and 192 on the enantioselectivity Entry
Functionalized resin (fb)
Starting resin
1 Merrifield (1% DVB; f0a = 0.63) 191a (0.375) 2 Merrifield (1% DVB; f0 = 1.49) 191b (0.850) 3 Merrifield (1% DVB; f0 = 0.63) 191c (0.389) 4 Barlos (f0 = 1.6) 192a (0.925) 5 Barlos (f0 = 1.24) 192b (0.689) a f0=mmol Cl/g. f=mmol/g (calculated by elemental analysis of nitrogen with the following f=0.714% N).
Ee 95 93 95 93 92 formula:
The asymmetric phenylation of different aldehydes was performed with chiral supported ligand 191 and the corresponding diarylmethanols were isolated in good to excellent yields (74-99%) and high enantioselectivities (85-91%) were observed but were slighly lower to those recorded with the homogeneous ligand [135]. Although the enantioselectivity obtained previously were among the best for supported ligands, the preparation of the aminoalcohol unit just before anchoring required up to five steps from commercial precursors. The supported-ligand 195 was prepared in only two steps from (S)-triphenylethylene oxide 193 (Scheme 81) [136]. The aminoalcohol 194 was anchored to a Merrifield resin (2% DVB, fo = 0.84) with an anchoring yield of 97 %. O Ph Ph
193
H Ph
HN
NH
LiClO 4
HO Ph Ph
H Ph N
N 194 H
Cs 2CO 3
HO Ph
Merrifield resin
Ph
H Ph N
195
N
Scheme 81.
The catalytic activity of the polymer-supported aminoalcohol 195 was then evaluated for the diethylzinc addition of aldehydes. After optimization of the reaction (temperature, amount of catalyst), two types of reaction conditions (2 mol% of 195, 0°C, 8h and 4 mol% of 195, 0°C, 6h) were tested with 15 different aldehydes. With the first reaction condition type, the conversion and the enantioselectivity were already high (≥ 95%) for most aromatic adehydes, the second one permit a complete conversion for all aromatic and aliphatic aldehydes and a better one for the α-substituted aldehydes. In order to evaluate the effect of the polymer part on the catalytic activity, two parallel kinetic measurements on the diethylzinc addition to benzaldehyde using either 195 or the Nbenzyl non-polymeric ligand were performed. The profiles for both ligands are pratically similar and at short reaction times, the polymer-supported ligand 195 showed a slightly better activity than its homogeneous analog (83% conversion vs 80% after 30 min). This supported ligand proved to be highly recyclable in five consecutive runs without any loss of catalytic activity (> 99% conversion) and enantioselectivity (average of 95.1 %).
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Christine Saluzzo and Stéphane Guillarme
The enantioselective phenyl transfer to different aldehydes was also carried out using the chiral polymer 195 with 5 mol % of catalyst amount and the enantioselectivities were also high (≥ 82 %) for aromatic aldehydes and moderate for aliphatic ones (68-79 %) [135]. Polymeric regioisomer of 191 and 195 was also synthesized and evaluated in the ethylation of aldehydes since the homogeneous analog was a highly active ligand [135]. Aminoalcohol 197 which was synthesised in six steps from epoxide 196 was anchored to Barlos resin with fo = 1.6 to lead to polymeric ligand 198 (f = 0.90) (Scheme 82). O Ph
H
Ph Ph
Six steps
Ph
DIEA,C H 2Cl2
OH H
Barlos resin
N
CN
196
Ph Ph N
OH
197
OH H
O
198
Ph Cl
Scheme 82.
The ethylation of aromatic and aliphatic aldehydes in toluene at 0°C in presence of an amount of 8 mol% of the supported-ligand 198 was performed. Except for α-susbituted aldehydes (around 50% conversion), both conversion and ee were high. The phenylation of aromatic aldehydes was also tested with polymeric ligand 198. Although conversions were excellent, the enantioselectivities were moderate (38-48% ee) and lower than those obtained with functionalized resin 191 and 195. In order to avoid the limitation of the use of the anchoring step as a source of diversity to prepare libraries of supported ligands containing the same type of aminoalcohol as 195, click chemistry was employed. This more convenient and efficient technique is a copper(I)catalyzed cycloaddition between an alkyne and an azide [137]. Aminoalcohol-derived alkynes 199 were prepared by reaction of 194 with different bromine reagent. The reaction between these alkynes and a variety of azido resins with different degree of functionalization (f = 0.74 – 2.25 mmol N3/g) was performed to afford the supported-ligands 200 (Scheme 83). Merrifield resin
Br 188
n
Cs2CO3
HO Ph Ph
HO Ph
H Ph N3
N
CH 3CN
N
CuI, DIPEA DMF/THF (1/1) 35°C
N 193a, n=1 193b, n=4 193c, n=8
Ph
H Ph
n
N 194a, n=1 194b, n=4 194c, n=8
n
N N N
Scheme 83.
The new functionalized resin 200a was first tested in the phenylation of p-tolualdehyde and p-anisaldehyde in toluene at 10°C using 5 mol% of the catalyst and the dependance of enantioselectivity on the level of functionalisation was studied (entries 1-6, Table 13). The
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
111
first analysis of the results was that the enantioselectivity depended strongly on the level of functionalisation of the resin. For the phenylation of p-tolualdehyde (entries 1-4), the higher the degree of functionalization, the lower the enantioselectivity. When a high ligand-loaded polymer was employed, the diarylcarbinols were isolated in their racemic form (entry 4). In this case, the triazole ring could interfere with the catalytic site involving a drop of the enantioselectivity. In order to avoid that, catalytic activity of resins 200b and 200c bearing a spacer were evaluated in the phenylation of p-tolualdehyde and p-anisaldehyde (entries 7-14, Table 13). It can be noticed that the two polymeric ligands 200a and 200b gave the best results in terms of conversion and ee. Resin 200b with a level of functionalization of 0.72 mmol/g exhibited the best catalytic activity (entries 9 and 13). This last one was chosen for phenylation of other aromatic and aliphatic aldehydes and the same range of conversion and enantioselectivity was obtained. The recyclability of this supported-ligand was studied on panisaldehyde and no decrease of the conversion was observed after three consecutive runs while a slightly drop in the enantioselectivity was recorded. The enantioselectivities were around 20% lower than those obtained with similar supported-ligand 195 with no triazole ring and for that reason the idea that the triazole ring could interact with zinc reagent is reinforced. Until now, supported diphenylprolinol were build either by copolymerisation with a vinylbenzyl group on nitrogen atom or a vinyl group on phenyl or by anchoring the ligand onto a Merrified resin via its nitrogen atom. Chen has reported a strategy to support this type of ligands using 4-hydroxydiphenylprolinol 201 [138]. Basic treatment of 201 with polymers 202 prepared in two steps from Merrifield resin (1% DVB, 100-200 mesh, 1 mmol Cl /g) led to supported-aminoalcohols 203a and 203b with a loading of aminoalcohol residue of 0.66 and 0.42 mmol/g respectively (Scheme 84). Table 13. Effect of the level of functionalisation of polymer 200 on activity and selectivity. Entry Polymeric ligand F (mmol L/g) Conversion (%) 1 200a 0.59 95 2 200a 0.74 >97 3a 200a 0.71 89 4 200a 1.16 >99 5 200a 0.59 64 6b 200a 0.71 92 7 200b 0.66 100 8c 200b 0.66 68 9 200b 0.72 95 10b 200b 0.72 99 11 200c 0.67 79 12 200b 0.66 68 13 200b 0.72 97 14 200c 0.67 54 a 3.6 % of catalyst. b10% of catalyst. cHigher crosslinked polymer (2% DVB).
ee (%) 74 68 63 0 77 71 66 66 76 78 75 77 82 76
These two immobilized ligands were tested in the diethylzinc addition to benzaldehyde and the best results were obtained when the reaction was performed in hexane at rt using an
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Christine Saluzzo and Stéphane Guillarme
amount of 10 mol% of catalyst. Polymeric ligand 203a showed a slightly better activity than 203b (91 % yield and 63% ee vs 85% yield and 57% ee). The optimized reaction conditions in the presence of 203a as ligand were used with different aldehydes. Except for salicylic aldehyde, p-nitrobenzaldehyde and cyclohexanecarboxaldehyde, the same range of yield and ee were observed. Five consecutive diethylzinc addition to benzaldehyde reactions using the same batch of polymer 203a were carried out without loss of the catalytic activity for the three first ones while a slight decrease of the catalytic was detected for the fourth and fifth run. The replacement of the ether bond between the 4-hydroxyprolinol and the spacer by an ester link has been performed [139]. Wang has prepared the polystyrene-supported diphenyl prolinol 205 and the soluble polymer-supported analog 206 in two steps from aminoalcohol 201 (Scheme 84). The loading of aminoalcohol was evaluated to be at 0.76 mmol/g for supported ligand 205 and 0.42 mmol/g for the soluble one. With polystyrene-supported ligand 205 for the enantioselective diethylzinc addition to aldehyde, the best result (92 % yield, 68% ee) was obtained when the reaction was carried out in toluene at 25°C using 15 mol% of the catalyst. Both yield and ee were similar to those obtained with the ether-linked polymeric ligand 203 but the reaction time was shorter. The optimized conditions were also used with other aromatic aldehydes and the enantioselectivities were moderate to good (34-71%). Soluble polymer-supported ligand 206 was also tested and for all the aldehydes used both yield and enantioselectivity were higher than those using the polystyrene-supported 205. This latter could be reused at least 3 times without significant loss of performance contrary to its soluble analog. The first immobilization of a pyridine-based tridentate ligand has also been investigated since the tridentate can stabilize the zinc complex and reduce the effect of the polymer blackbone [140]. The aminoalcohol 207 was prepared in six steps from 2,5-dibromopyridine and was anchored onto a 2% DVB crosslinked Merrifield resin to afford the supportedtridentate ligand 208 (Scheme 85). This immobilized ligand was then employed in the diethylzinc addition to benzaldehyde and the excellent catalytic activity was recorded when the reaction was carried out in toluene at rt using 5 mol% of catalyst and a low benzaldehyde/Et2Zn ratio of 1/1.1 (>99% conversion and 93% ee). The best reaction conditions were used with other aromatic aldehydes and yields and ees were similar to those observed with benzaldehyde except for pyridine carboxaldehyde. In this case, nitrogen atom of pyridine ring was suspected to be too chelating. Excellent enantioselectivity (93%) was also obtained with an enolizable aldehyde, n-octanal. Furthermore the catalyst could be recovered by simple filtration and could be reused five times without loss of catalytic activity. The results obtained are interesting since the supported-ligands containing a NH group gave generally poor to moderate enantioselectivities. El-Shehawy studied the effect of the loading of the ephedrine moiety and the degree of crosslinking of polymer-supported ephedrine for the diethylzinc addition to benzaldimine [141]. Different chiral polymers were prepared by copolymerisation of monomers 141 with styrene and DVB. The compositions of these polymers are summarized in Table 14. These chiral polymers 209-213 were then tested in the asymmetric addition of diethyzinc to benzaldimines. First, the catalytic activity of the copolymers 209, 210a, 211 and 213 having a molar ratio of copolymerization of 1/7/2 were evaluated and the best result was obtained with polymer 210a (92% yield and 82% ee) when the reaction was carried out in
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
113
toluene at – 10 °C. The enantioselectivity of the reaction with polymeric ligand 210a was highly temperature-dependant and surprisingly a significant decrease of the chemical yield when the reaction was performed at 0°C or rt was observed.
O
O n
N OH
203a: n=4 203b: n=6 CH2O(CH2)nCH2Cl
NaH DMF
202a: n=4 202b: n=6 HO N OH
201 Succinic anhydride DMAP
O
HO2C O
N OH
204 NH2 MeOPEG, DMAP, DCC
DIC HOBt
O N H
O O O
O
O 2
n
O
O
N
N OH
205
Scheme 84.
OH
206
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Christine Saluzzo and Stéphane Guillarme O
HO KI, DMF
N
207
HN
R
HO
Ph Ph
N
Merrifield resin
HN
R
HO
Ph Ph
208
Scheme 85.
Table 14. Compostions of polymers 209-213. Chiral Polymer 209 210a 210b 210c 210d 210e 210f 210g 210h 211 212 213 a
Chiral Monomer
Molar Ratioa
(1S,2R)-141a (1S,2R)-141b (1S,2R)-141b (1S,2R)-141b (1S,2R)-141b (1S,2R)-141b (1S,2R)- 141b (1S,2R)-141b (1S,2R)-141b (1S,2R)-141c (1S,2R)-141d (1R,2S)-141b
1:7:2 1:7:2 1:8:1 1:6:3 1:5:4 2:6:2 3:5:2 4:4:2 5:3:2 1:7:2 1:7:2 1:7:2
Loading of ligand (mmol/g) (DF)b 0.764 (0.10) 0.771 (0.10) 0.750 (0.10) 0.821 (0.10) 0.735 (0.10) 1.292 (0.20) 1.863 (0.30) 2.127 (0.40) 2.449 (0.50) 0.792 (0.10) 0.721 (0.10) 0.807 (0.10)
Molar ratio of copolymerization: chiral monomer:styrene:DVB. b Degree of functionalization
The increase of the crosslinking reagent (DVB) involved a significant decrease of both yield and ee. The same effect was observed when the loading degree of the aminoalcohol unit increased. The optimized procedure with chiral polymer 210a was employed with other aromatic aldimines and the same range of enantioselectivity was recorded. El-Shehawy has reported the synthesis of original polystyrene-supported dendritic chiral ephedrine [142]. Chain end-functionalized polystyrenes bearing a number of 2, 4, 8 or 16 BnBr residues at their chain-ends, PS(BnBr)2-PS(BnBr)16 were first isolated (Scheme 86). Reaction of polymers of PS(BnBr)2-PS(BnBr)16 having two-sixteen BnBr moieties with ephedrine led to the corresponding polymers PS(Ephed)2-PS(Ephed)16 bearing the same numbers of ephedrine at their chain-ends (Scheme 87). The catalytic activity of these four chiral polymer-supported dendritic aminoalcohols was evaluated in the enantioselective diethylzinc addition to N-diphenylphosphinoylbenzaldimine. The reaction was first carried out in toluene at rt for 48 h using an equimolar amount of supported-ligand (based on the total numbers of ligand unit). The best chiral polymer, PS(Ephed)8a, was also tested with other N-diphenylphosphinoylarylimine under the same reaction conditions and similar level of yield and enantioselectivity were observed.
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
115
Br Br Br n
n
Br PS(BnBr)2
Br Br
PS(BnBr)4
Scheme 86.
The same trend as with imine was observed for the enantioselective diethylzinc addition to benzaldehyde and the best results (94% yield and 90% ee) were also obtained with PS(Ephed)8 [143].
Br
Me
Ph
OH
N Me
* * HN OH Me
PS
Me
*
*
Ph
PS
K 2 CO 3/DMF
Br
N Me *
Me
*
Ph
OH
* * Me
N
OH
Ph
Me
PS
(1R,2S)-PS(Ephed)2
PS
PS
PS
(1R,2S)-PS(Ephed)4 (1R,2S)-PS(Ephed)8a (1S,2R)-PS(Ephed)8b
Scheme 87.
(1R,2S)-PS(Ephed)16
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Christine Saluzzo and Stéphane Guillarme
Table 15. Enantioselective diethylzinc addition to N-diphenylphosphinoylbenzaldimine using polymer-supported dendritic aminoalcohols. Entry
Supported-ligand
Reaction Yield (%) ee (%) time (h) 1 PS(Ephed)2 48 53 81 (R) 2 PS(Ephed)2 90 85 83 (R) 3 PS(Ephed)4 48 71 84 (R) 4 PS(Ephed)4 72 90 85 (R) a,b 5 PS(Ephed)8a 48 92 90a, b (R) 6 PS(Ephed)16 48 78 79 (R) c 7 PS(Ephed)16 48 93 80 (R) 8 PS(Ephed)8b 48 91 90 (S) a 91% yield and 92% ee when using 1.0 molar of (1R,2S)-N-benzylephedrine as chiral ligand. b74% yield and 89% ee when using 1.0 molar of (1R,2S)-N-vinylbenzylephedrine copolymerized with styrene and DVB as chiral ligand. c1.5 molar equivalent of chiral polymer.
Although high catalytic activity was observed with polymer-supported aziridine-derived ligand 160, the attachment on this type of chiral ligand through the phenyl group was studied [144]. The protected monomers 214 were prepared and copolymerized with styrene and DVB to afford the chiral aminoalcohol-containing copolymers 215 with a loading of 0.1 mmol/g (Scheme 88). The three polymeric ligands were then examined in the diethylzinc addition to benzaldehyde. Protected polymer-supported ligand 215a and 215b were slighty less active than the Zwannenburg’s polymeric ligand 160. The trityl-protected supported-ligand 215a was also recycled and showed a complete loss of catalytic activity (3%) and enantioselectivity (0%). It is important to pointed out the high enantioselectivity (84% ee) obtained with the unprotected polymer-supported ligand 215c while the conversion was moderate (52 %) after 24h of reaction. Polymer-supported chiral Schiff-base amino alcohols have also been used as chiral ligands. The four polymeric Schiff-base ligands 216a, 216b, 217a and 217b have been anchored onto Merrifield resin (1 mmol Cl/g, 1% DVB, 200 mesh) using the corresponding chiral ligands (Scheme 89) [145]. These chiral polymers have been first tested as chiral ligand in the phenylacetylene addition to acetophenone and chiral polymer 217a induced the best enantioselectivity (63%) when the reaction was performed in toluene at rt using 10 mol% of the catalyst. When the reaction temperature decreased to – 18 °C, a significant increase of the enantioselectivity (75%) was observed. The optimized reaction conditions with chiral supported-ligand 217a were then applied to different functionalized acetophenone and enantioselectivities were comprised between 62 and 89%. The polymeric ligand 217a could be reused in five successive reactions with an average loss of enatioselectivity of 1% per run.
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
117
Br
Me
Ph
OH
N Me
* * HN OH Me
PS
Me
*
*
Ph
PS
K 2CO 3/DMF
Br
N Me *
Me
*
Ph
OH * *
Me
N
OH
Ph
Me
PS
(1R,2S)-PS(Ephed)2
PS
PS
PS
(1R,2S)-PS(Ephed)4 (1R,2S)-PS(Ephed)8a (1S,2R)-PS(Ephed)8b
(1R,2S)-PS(Ephed)16
Scheme 87.
R N HO
214a: R=CPh 3 214b: R=CHPh 2 Scheme 88.
styrene, DVB Benzoyl peroxide Polyvinylalcohol H 2O, PhCl
R N HO
215a: R=CPh3 215b: R=CHPh 2 215c: R=H
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Christine Saluzzo and Stéphane Guillarme Ph H N
Ph Ph
Ph H
Ph Ph
N
OH
OH
216a NaH, DMF
HO
O Ph H N
Ph Ph
Cl THF
OH
Ph H
Ph
N
OH
Ph
216b HO
O
HO O
Ph Ph
H N
Ph Ph
H
OH
N
OH 217a
NaH, DMF
Cl THF
HO H N
Ph Ph OH
O H
Ph Ph
N OH 217b
Scheme 89.
6.2. Supported Sulfonamide and N-Sulfonylated Amino Alcohols Itsuno has described the first enantioselective allylation of aldehydes using crosslinked polymer-supported N-sulfonylated aminoalcohols as chiral ligands [146]. Polymer-supported ligands 220 and 221 were prepared by copolymerisation of the norephedrine-derived monomer 218 or D-camphor-derived 219 with styrene and divinylbenzene (DVB) in a [chiral monomer]/[styrene]/[DVB] molar ratio of 1/8/1 (Scheme 90).
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer… Ph
Me * * O HO HN S O 218a: (1R,2S) 218b: (1S,2R) O * NH S O * OH 219a: (1R,2S,3R,4S) 219b: (1S,2R,3S,4S)
119
Ph
Styrene/DVB
Me * * O HO HN S O 220a: (1R,2S) 220b: (1S,2R) O * NH S O * OH 221a: (1R,2S,3R,4S) 221b: (1S,2R,3S,4S)
Scheme 90.
Reaction of these chiral polymer-supported ligands with triallyborane led to the corresponding allyboron reagents which were allowed to react with aldehydes. When the norephedrine-derived ligands 220 were employed in the asymmetric allylation of benzaldehyde, homoallylic alcohol (S) was isolated in a 93% yield and in 75% ee. The enantioselectivity was slightly higher with the camphor-derived analog 221a (85% ee). The polymer-supported ligands induced better enantioselectivities than the non-polymeric analogs. High enantioselectivities were also observed in the allylation of aliphatic aldehydes (92% for acetaldehyde and 84% for pivalaldehyde). Liskamp has reported the synthesis and the screening in asymmetric catalysis of a library of polymer-supported peptidosulfonamide [147]. (R,R) or (S,S)-pyrrolydines 222 prepared in six steps from D or L-tartaric acid were anchored onto Argonaut resin (0.41 mmol/g) to afford the corresponding supported pyrrolidines 223 (Scheme 91). The polymeric peptidosulfonamides 224 and 225 were then synthesized in four steps from the immobilized pyrrolidines 223. The different chiral polymers were tested in the titanium-mediated diethylzinc addition to benzaldehyde, p-chlorobenzaldehyde, cyclohexanecarboxaldehyde and phenylacetaldehyde. Both yield and ee were very low with all these supported-ligands for the enantioselective reaction with the two aliphatic aldehydes. With aromatic aldehydes, the best results were observed with both leucine-derived supported peptidosulfonamides 224d and 225d. Isoborneolsulfonamide-containing trans 1,2-diaminocyclohexane have been immobilized on solid support [148]. The four polymer-supported ligands 227 were synthesized by suspension copolymerization in polyvinyl alcohol (average Mw=85,000-146,000) of chiral monomer 226 with styrene and DVB (Scheme 92). These four chiral polymers 227 were then employed as chiral ligands in the enantioselective titanium-mediated diethylzinc addition to acetophenone. Although enantioselectivities were excellent (99%) with these four polymersupported ligands, the chemical yields were low (16 (227a), 23 (227b) and 36% (227d)) to moderate (56% (227c) even after a very long reaction time (17 days). A more reactive zinc reagent, the ethyl phenyl zinc reagent prepared by transmetallation of diethylzinc with triphenylboron was the employed. This reagent was used with pbromoacetophenone in the presence of 5 mol% of polymeric ligand 227 and an excess of titanium tetraisopropoxide and the corresponding tertiary alcohol was isolated with good enantioselectivities (71-86%). The amount of the crosslinking agent had a strong influence on
120
Christine Saluzzo and Stéphane Guillarme
the catalytic activity and the enantioselectivity and the best results were obtained with the soluble polymer-supported ligand 227d. O HO
O 2
NHBoc
O
NH2
N
NHBoc (R,R) or (S,S)-216
O
NHBoc
N N 2 H NHBoc (R,R) or (S,S)-217
BOP DIPEA
1) TFA, Et3N
R 2) ClO2S
a, R=H b, R=Me i c, R= Pr d, R=iBu
NMM, CH2Cl2 3) Piperidine, DMF 4) Boc2O, NMM, CH2Cl2
e, R=Bn O N H
R O O S NHBoc NH
O 2
NHFmoc
N NHBoc
HN
S O O
R
218a-e (R,R) 219a-e (S,S) Scheme 91.
This ligand was employed in the enantioselective phenyl addition to different ketones, and the same level of chemical yield and enantioselectivity was observed for aromatic ketones while the enantioselectivity dropped significantly with hexan-2-one (38% ee). The reuse of the polymer-supported ligand 227d was studied and the catalytic activity decreased rapidly after only two runs. Gau has prepared three different types of polymer-supported N-sulfonylated aminoalcohols. The first one 230 was produced by anchoring the aminoalcohol 229 through the nitrogen atom to a chlorosulfonated resin (1 mmol Cl/g; 2% DVB) with a ligand loading of 2.30 mmol/g (Scheme 93) [149]. In order to decrease the ligand unit loading, the authors have also synthesized chiral monomer 231 which was allowed to copolymerize with DVB and styrene to lead to polymeric ligands 232a and 232b with a ligand loading of 1.28 mmol 1.76 mmol, respectively. It was not surprising to observe moderate yield (62%) and enantioselectivity (44%) for the titanium-mediated diethylzinc addition to benzaldehyde with the polymeric ligand 230 since a high ligand-loaded polymer is generally less active because of the proximity of the catalytic sites. The best result (100% yield and 92% ee) was obtained when 10 mol% of the low ligand-loaded 232a in the presence of 10 equivalents of titanium tetraisopropoxide
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
121
(relative to the ligand). This polymer-supported ligand was also employed with different aldehydes and both yield an ee were high in all cases. This latter could be also reused five times and the enantioselectivity decrease from 92 to 86%.
Styrene + DVB + O 2 S NH HN S O2 226 AIBN Polyvinylalcohol H 2O, THF, PhH
n
99
1
Ph
n
O2S 227a: n=32 227b: n=8 227c: n=1 227d: n=0
N H
HN
S O2
Scheme 92.
In the aim to discover other polymer-supported catalysts having a better activity and reusability, chiral polymers 234a and 234b were prepared by copolymerization with styrene and DVB with respectively a ligand loading of 0.93 mmol/g and 0.95 mmol/g [150]. The heterogeneous 234a/Ti(OiPr)4 system proved to be better than the 232/Ti(OiPr)4 systems for the enantioselective diethylzinc to aldehydes and a highly recoverable catalyst. Indeed, this polymeric ligand/Ti(OiPr)4 catalyst could be reused 9 times with enantioselectivities ≥ 87% ee. The polymer-supported tridentate ligand 234b was employed in the enantioselective triethylaluminium addition to aldehydes. To obtain the same level of enantioselectivity as precedent system, 28 equivalents of Ti(OiPr)4 relative to the chiral ligand was necessary. With most aldehydes used, the enantioselectivities were slightly lower than those obtained with the heterogeneous 234a/Ti(OiPr)4 system.
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Christine Saluzzo and Stéphane Guillarme Ph
Ph 0.98
H2N
0.02
229
OH
0.98
0.02
Et3N, CH2Cl2
SO2Cl
O2S HO
228
NH 230
Ph
Ph x
SO2 Ph HN
+ styrene + DVB
0.02
AIBN
H2O/Benzene
O2 S HO
HO
y
Ph
Ph
231
NH Ph 232a: x=0.20, y=0.78 232b: x=0.40, y=0.58
O 0.2
0.78
0.02
AIBN
+ styrene + DVB R O2S
H2O/Benzene
H N
O HO
Ph
233a: R=
R O2S
OH
H N
234a: R= OH
Cl
233b: R=
HO Cl
Ph
Cl
234b: R= Cl
Scheme 93.
6.3. Supported Oxazoline or Oxazolidine as Ligands Two polymer-supported chiral aminooxazoline 236 have been prepared because of better chelating properties of this type of ligand due to the higher nucleophilicity of the amino group [151]. Polymers 236 were produced after reaction of polymer-supported isatoic anhydride 235 (isolated from chloromethylated styrene-DVB polymer (4.8 eq. Cl/g)) and the corresponding amino alcohol under acidic conditions (Scheme 94).
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
N
O HO O
+
NH 2 R
O
123
NH Acidic clay
N
PhCl
R
O 236a, R=Bn 236b, R=i-Bu
Scheme 94.
The catalytic activity of these polymeric oxazolines was examined in the enantioselective diethylzinc addition to benzaldehyde. Chiral supported-oxazoline 236b exhibited slightly better enantioselectivity (89%) and yield (90%) than its analog 236a when the reaction was carried out in a 20% CH2Cl2/toluene mixture at 0°C. This chiral supported-ligand has been recycled three times without a significant loss of performance. O
Ph Cl Ph
Fe
Bu N CPh2 OH O 238
O
Fe
Bu N CPh 2OH O
6
O HO
Ph
O
OH
237
Ph
O 6
Fe
O 2
Bu N CPh2 OH O O
O-MPEG
6
O
O 2
O-MPEG
239
Scheme 95.
Chiral ferrocenyl oxazoline-based ligand which is often used in asymmetric catalysis have been immobilized onto two different supports, an insoluble trityl chloride polystyrene and a soluble polymer, a polyethylene glycol monomethyl ether [152]. The linker-containing compound 237 has been anchored to both polymers to produce respectively chiral insoluble polymer 238 and chiral soluble polymer 239 (Scheme 95). These polymer-supported ligands were then tested in the enantioselective phenyl and ethyl addition to p-chlorobenzaldehyde and benzaldehyde respectively. Polystyrene-supported oxazoline 238 was totally inefficient for the asymmetric phenyl transfer to p-chlorobenzaldehyde while a high enantioselectivity (87%) was observed for the ethyl addition to benzaldehyde. Contrary to the insoluble one, the soluble polymer 239 exhibited high enantioselectivity for both reactions (97 and 86% respectively). In general, the enantioselectivities were slightly lower than those obtained with the non-polymeric ligand. Futhermore, no significant loss of enantioselectivity was detected after five consecutive phenyl additions to p-chlorobenzaldehyde with the recovered polymer.
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Christine Saluzzo and Stéphane Guillarme
Portnoy focused his attention on the immobilization of PyBox ligands [153]. The polymer-supported PyBox 240 were prepared in five steps from the Wang trichloroacetamidate resin (Scheme 96). The supported-ligands 240 were allowed to react with copper (I) triflate for 24h to afford the corresponding catalysts 241. These catalysts were then used in the first heterogeneous catalyzed addition of phenylacetylene to imine. The best enantioselectivity (83%) was obtained when the reaction was performed in dichloromethane at 40°C for 24h using 10 mol% of the catalyst 241f which possesses a steric group on the oxazoline ring. When a recovered catalyst was employed for a second run, a drop of the catalytic was observed which is probably due to the oxidation of the metal. The use of other solvents such as THF in order to improve the recyclability was attempted but a decrease of the catalytic activity (80 to 56%) was detected during three consecutive reactions while the enantioselectivity increased (54 to 60%). To reduce the oxidation, ascorbic acid was added to the reaction mixture and proved to preserve the catalytic activity but the enantioselectivity was totally lost. Another strategy to immobilize a PyBox ligand has been described by Moberg and Levacher [154]. Supported PyBox 243 has been prepared using click-chemistry reaction between alkyne 241 and polystyrene-supported azide (1.2 mmol N3/g) (Scheme 97). The PyBox loading was estimated at 0.8 mmol/g. This polymer-supported PyBox/Cu(I) catalyst was then tested in the phenylacetylene to N-benzylideneaniline. The enantioselectivity was lower than that observed with the non-polymeric but slightly better than those with Portnoy’s immobilized PyBox 240. Compared to catalyst 241 similar trend was observed concerning the recyclability. When the immobilized PyBox-Cu(I) catalyst was only filtered, a drop of both catalytic activity and enantioselectivity were detected after five consecutive runs. When the supported-ligand was washed several times and reloaded with copper triflate, catalytic activity and enantioselectivity were recovered. The same level of conversion and ee were observed with other aromatic imines. The use of polymer-supported oxazolidine derived from (1R,2S)-cis-1-amino-2-indanol for the asymmetric alkynylation of aldehyde has been reported [155]. Three chiral supportedoxazolidines 244 were prepared in two steps from Merrifield resin (2.5 mmol Cl/g) (Scheme 98). The optimization of the enantioselective addition of phenylacetylene to benzaldehyde has been done with the polymeric ligand 244a. The best result (82% yield and 79% ee) was obtained when the reaction was performed in THF at room temperature for 24 h in the presence of Et2Zn, Ti(OiPr)4 and 10 mol% of the ligand. The two other polymeric ligands 244b and 244c induced higher enantioselectivity (83% and 90% respectively) under the optimized reaction conditions. The same level of catalytic activity and enantioselectivity was observed with a variety of aromatic and heteroaromatic aldehydes when the polymer-supported oxazolidine 244c was employed. This supported ligand could be reused three times without significant loss of performance.
6.4. Supported Salen as Chiral Ligands The catalytic activity of polymer-supported chiral salen has been also studied in the enantioselective diethylzinc addition to aldehydes since the importance of this type of ligand in asymmetric catalysis. Soluble polymer-supported salens have been synthesized by Venkataraman
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
125
[156]. Salen 245 was prepared and was then anchored on MeO-PEG changing only the spacer between the PEG matrix and the salen to lead to the polymer-supported salens 246-248 (Scheme 99). R
O N O
CCl3
Five steps
O
NH Wang resin
O
N
240a: R=Me 240b: R=Et O 240c: R=i-Bu 240d: R=Bn 240e: R=i-Pr 240f: R=t-Bu 240g: R=Ph
N R
CuOTf
R
O N O
N Cu OTf
O
N O
241
R
Scheme 96.
N N N N3 O N Ph
O
O
N
N
N 242
Ph
Ph
O
N
CuI/DIEA/THF
N 243
Ph
Scheme 97.
The enantioselective diethylzinc addition to benzaldehyde was then carried out using 10 mol% of polymeric salens 246-248 and the best result (90% yield and 82% ee) was obtained when the chiral polymer was used. With other functionalized aromatic aldehydes, both yield and ee were lower. Futhermore the soluble polymers could be recovered and reused whithout significant loss in selectivity or reactivity. Chiral salen-type linear polymers have been prepared and tested in the diethylzinc addition to aldehydes [157]. The symetrical dialdehyde-containing compound 249 was first synthesized in five steps from 2-tertbutylphenol and allowed to react with 1,2-diaminocyclohexane and DPEN to afford the linear polymers 250 (Scheme 100). The catalytic activity of these chiral
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Christine Saluzzo and Stéphane Guillarme
linear polymers was evaluated in the diethylzinc addition to m-nitrobenzaldehyde using 1 mol% of 250 (with respect to the monomeric unit). Although yields were satisfactory the ee were moderate and chiral linear polymer 250b proved to be more efficient for the reaction. The recovered polymer has been reused without loss of performance.
O 244a
H N O
H N 2 steps
O
244b O
CH2Cl
O H N
244c O
Scheme 98.
7. ASYMMETRIC π-ALLYLIC SUBSTITUTION Another largely studied asymmetric reaction to form new carbon-carbon bond is the palladium-catalyzed allylic substitution using particularly homogenous ligand (Scheme 101) [158]. Among the ligands employed, numerous phosphine-containing ones have been reported and we report here only the phosphorus-free nitrogen-containing ligands. The use of heterogeneous ligands has not been studied extensively and the use of some immobilized nitrogen-containing ligands has been published. To the best of our knowledge, the first polymeric ligand for this reaction was those of Lemaire’s group [20]. Their chiral poly(amide) 23 and poly(urea) 24 (Scheme 13) was employed for the catalytic allylic alkylation. Using 23 the conversion was low (38%) and the ee reached 80% while with 24, the conversion was better (72%) and enantioselectivity was lower (38%). In both cases, conversion and ee were lower than those obtained with the corresponding monomeric diamine. Moberg focused her attention on the immobilzation of oxazoline-derived ligands for the enantioselective allylic alkylation [159]. Pyridine-containing oxazolines 251 were first prepared and were supported on TentaGel resin to afford the corresponding polymeric oxazolines 252 (Scheme 102). These polymer-supported ligands were then tested in the
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
127
palladium-catalyzed allylic substitution and the sterically bulky ones 252b and 252c proved not to be active (< 5% yield).
N O
1) glutaric anhydride DMAP, CH2Cl2 2) MeO-PEG, DCC, DMAP, CH2Cl2
N
O O
O
R
R
R
O
N
OH HO
R
Cs2CO3
R
DMF
245 R R=t-Bu O
R
246
N
N
OH HO R
OH HO
3
PEG-OMs
HO
N
247
R
Cs2CO3 DMF
OMs
N
O
N
3
O
OH HO
R
3
R
248
R
Scheme 99.
When the chiral polymer 252a was used as chiral ligand, the product was isolated in a yield comprised between 60 and 100% and in 80% ee; this result was closed to that obtained with the monomeric ligand. Polymer-supported bis(oxazoline) 109 having a degree of functionalization of 0.071 mmol/g have been tested in the same reaction [102]. This polymeric ligand exhibited better enantioselectivity (94-95%) than the previous chiral polymers 252. The asymmetric π-allylic alkylation using a polymer-supported pyridyldiamide as chiral ligand was applied to the synthesis of the (R)-baclofen which is an agonist of the GABAB receptor [160]. The unsymmetrical cyclohexanediamine 253 was anchored onto a TentaGel HL-COOH resin using a classical coupling reaction to give the polymer-supported ligand 254 (Scheme 103). This one was then tested in the molybdenum-catalyzed allylation reaction of dimethylmalonate with 3-phenylprop-2-enyl methyl carbonate. A high branched-to-linear ratio (35/1) and enantioselectivity were obtained with the polymeric ligand and this result was quite similar to those observed with the non-supported ligands. The ligand could be recovered by filtration and could be reused for seven consecutive cycles without loss of catalytic activity and enantioselectivity. Bandini has described the grafting of diamino-oligothiophene compounds onto the monomethyl ether of PEG5000 to give the soluble polymer 256 (Scheme 104) [161]. High yield (98%) and ee (99%) were observed with soluble polymer 256a when the allylic alkylation reaction between 1,3-diphenylallyl and dimethyl malonate was performed in THF
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Christine Saluzzo and Stéphane Guillarme
at room temperature using cesium carbonate as the base. This result is comparable to that obtained with the non-polymeric ligand. Polymer-supported ligand 256b proved to be less efficient than 256a. The recyclability of the supported-ligand/Pd catalyst was also evaluated and loss of performance was detected only after three cycles.
OC 8H 17
N
N
OC 8H 17
N
OH HO
N
OH HO n
C 8H 17O
t-Bu
t-Bu
C 8H 17O
t-Bu
t-Bu
250a
CHCl3
H2N OHC
NH2
OC 8H 17
CHO
HO
OH
t-Bu
C 8H 17 O
t-Bu 249
CHCl3
Ph
Ph
N
N
OC8H 17
Ph
Ph
H2N
NH 2 Ph
Ph
N
N
OC 8H 17
OH HO
OH HO n
C 8H 17 O
t-Bu
t-Bu
C 8H 17 O
t-Bu
t-Bu
250b
Scheme 100.
LG R
R''
R' acyclic or cyclic LG: Leaving group Scheme 101.
Pd/L * NuH, Base Solvent
Nu R R'
*
R''
R and/or R'
R'' * Nu
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
129
O O
R' R
N
+ HO2C
O
TG
OH
N
Ph
251a: R=R'=H 251b: R=t-Bu, R'=H 251c: R=H, R'=t-Bu Scheme 102.
CO2H
O NH HN
N
N
O HN
NH HN
O DCC, DMAP
NH
TG
252a: R=R'=H O 252b: R=t-Bu, R'=H 252c: R=H, R'= t-Bu
O
NH2 N
O
O
N
Ph
O
R' R
N
N
NH
254
253 Scheme 103. R R
R S
S
R MsO
S
3
NH
O
NH
Cs2CO3 / DMF
S
S
PEG-OMe
NH NH S
OH 255a: R=H 255b: R-=O-CH2-CH2-O-
O 256a: R=H 256b: R-=O-CH2-CH2-O-
3
PEG O
Scheme 104.
8. DIHYDROXYLATION AND AMINOHYDROXYLATION The osmium-catalyzed asymmetric dihydroxylation (AD) is an excellent method to prepare chiral vicinal diols [162]. Most of the ligands employed are cinchona alkaloids and are derived from quinine or quinidine like ligand 257 for example. In 1992, Sharpless has described the improvement of the osmium-catalyzed asymmetric dihydroxylation by using bis cinchona alkaloids such as 1,4-bis(9-0-dihydroquinyl)phthalazine (258) as chiral ligand (Scheme 105) [163]. This type of ligand has then received much attention since high enantioselectivities were observed. Although the amount of OsO4 has been reduced with the possibility to use cooxidants, the high cost of osmium and chiral ligands, the toxicity, the possible contamination of the chiral products by osmium avoid the use of this process in industry. The possibility to support and to recycle the osmium/ligand catalyst and particularly
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Christine Saluzzo and Stéphane Guillarme
the use of soluble or insoluble supported ligands has been explored. In order to recycle the catalyst, number of research groups has investigated the immobilization on soluble and insoluble polymers of this type of alkaloids.
O H
N MeO
N O H
N N O
H
N O
MeO N 257
OMe N
N 258
Scheme 105.
8.1. Insoluble Supported Alkaloid as Ligand The first work about AD using heterogeneous ligand has been reported by Sharpless [164]. The polymeric alkaloids 259-262 (Scheme 106) were prepared by copolymerisation of the corresponding monomer with acrylonitrile in presence of the radical initiator AIBN. The performance of these polymer-supported cinchona were evaluated in the asymmetric dihydroxylation of trans stilbene using OsO4 and NMO or K3Fe(CN)6 as secondary oxidant. The reaction rate was slow and no ee was detected using the polymeric ligand 259 probably because of the proximity of the active site to the polymer matrix. The best results (81-87% yield and 85-93% ee) were observed when the supported alkaloid 260 was used as chiral ligand in presence of only 1 mol% of OsO4 and NMO in acetone/water at 10°C. It is interesting to note that when the OsO4/supported ligand complex was reused a second time without further addition of OsO4, the yield and enantioselectivity dropped slightly. Both yield and ee were improved when K3Fe(CN)6 was used as cooxidant with polymers 261 and 262. When the same reaction was tested with different olefins using the acetate analog of polymer 259, the polymer-supported alkaloid 263, low enantioselectivities were observed (Scheme 107) [165]. High chemical yields could be obtained when low alkaloid-loaded copolymers were used (< 15 mol%). Moreover high degree of alkaloid incorporation seemed to inhibit the reaction. Use of chiral polymers 264 and 265 did not improve the enantioselectivity of the dihydroxylation of trans stilbenes and ee were very low with polymer 264 compared to those obained with polymers 259-260. Dihydroquinidine 266 containing a p-vinylbenzoate was prepared and was copolymerized either with styrene or with 4-phenylstyrene (Scheme 108) [166]. The effect of the loading of the alkaloid monomer on the activity of the corresponding polymer for the AD of trans stilbenes was examined and the results are summarized in Table 16. As already observed with different supported ligands, higher the loading of the alkaloid monomer, more important drop of chemical yield, reaction rate and enantioselectivity were. Polymer 267a proved to be more efficient than its homolog 267b and the complex 267a/OsO4 could also be reused without
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
131
significant loss of performance. This system was employed for the asymmetric dihydroxylation of other olefins including non-aromatic and α,β-unsturated alkenes and moderate enantioselectivities were obtained while chemical yields were high. In order to improve the enantioselectivity obtained with their supported alkaloid for the AD, Salvadori has prepared polymer 269 which contained a spacer between the alkaloid and the polymer backbone [167]. This one was isolated after copolymerization of quinine-based monomer 268 with styrene and DVB in a 1/7/2 molar ratio (Scheme 109). This polymersupported quinine was then tested in the AD of trans stilbene, styrene and (E)-βmethylstyrene. The enantioselectivities were higher than those obtained with polymers 263264 with a shorter reaction time at rt (4-6h) or 0°C (7h). At 0°C, the enantioselectivity reached 87% for the dihydroxylation of trans stilbene. This polymer was also reused after recovering without significant loss of the activity. CN
CN O
S
O N
H
O
O
Cl O H
O
MeO N N CN
259
O
H
Cl O H
MeO
O S O
N
260
O O
O N
H
N
NMe2 O H
H
O H
MeO
MeO 261
N N
Scheme 106.
CN N
H
OCOR H
MeO N 263: R=CH3 264: R=p-Cl-C6H4 265: R=m-(CH3O)-C6H3
Scheme 107.
262
CN 6
132
Christine Saluzzo and Stéphane Guillarme Table 16. AD of trans stilbene using polymer 267. Entry
Polymer 267a (267b) Reaction Yield (%) ee (%) n m Time 1 9.0 1.0 24 h 86 (68)a 82 (71)a b 2 9.0 1.0 24 h 79 78b 3 8.0 2.0 24 h 72 (65) 78 (68) 4 5.0 5.0 2d 50 (50) 45 (40) 5 0.0 1.0 2-3 d 20 a Under brackets, yields and ee obtained with 267b. b Polymer containing dihydroquinine was used.
m
n
O
N O H
Styrene or
R
O
4-phenylstyrene
MeO
N O H
MeO N 266
N 267a, R=H 267b, R=Ph
Scheme 108.
Lohray has reported the first example of immobilized bisdihydroquinylpyridazine [168]. The monomer 270 has been prepared and was allowed to react with ethylene glycol methacrylate in a 1/9 ratio to afford the chiral polymer 271 (Scheme 110). When trans stilbene and styrenic olefins were dihydroxylated using supported ligand 271, OsO4 as oxidant and K3Fe(CN)6-K2CO3 as cooxidant, both yields and ee were high, ees being slightly lower than those observed with the homogeneous ligand. In opposite, the enantioselectivities dropped significantly with aliphatic alkenes. The excellent results could be due to larger pore size and hydrophily of this type of polymer which facilitate the approach of the reactants to the catalytic site. Since excellent results were obtained in the asymmetric aminohydroxylation in homogeneous phase by Sharpless [169], heterogeneous systems appeared to be of great interest. Nandanan has reported the first heterogeneous osmium tetroxide-catalyzed asymmetric aminohydroxylation of various olefins using polymer-supported bisdehydroquinine ligand 273 (Scheme 111) [170]. When chloramine T was used as nitogen source, yields and ee were moderate with all olefins. Monomer 268 was also copolymerized with hydroxyethyl methacrylate and ethylene glycol dimethacrylate as crosslinking agent in a 1/7/2 ratio to lead to the new polymersupported alkaloid 274 (Scheme 112) [171]. This one was insoluble in all solvents and swelled very well in protic polar solvents. The catalytic activity of chiral polymer 274 was tested in the AD of various aliphatic or aromatic olefins using two different reaction conditions (NMO or K3Fe(CN)6 as cooxidant in respectively acetone/H2O or t-BuOH/H2O).
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
133
With all alkenes used, the enantioselectivities were higher when K3Fe(CN)6 was employed as secondary oxidant and comparable to those obtained with the non-polymeric alkaloid. Concerning the aliphatic olefins, the enantioselectities remained low to moderate.
7
1
2
Styrene DVB
O
O
2S O O
AIBN
2R
O
O
268 R=
H
N
MeO
2S O O
2R
269
OCOC6H4p-Cl H
N Scheme 109.
N
H
N
N
O
O
H
N
MeO
OMe N
270
N O 1
N
H
N
O 9
O
N
O
O
O
H
MeO
N OMe
N
271
N
Scheme 110.
In order to improve the enantioselectivity in the AD of aliphatic alkenes, the two dihydroquinidine-derived polymers 275 and 276 were prepared by radical copolymerization of the corresponding monomers with hydroxyethyl methacrylate and ethylene glycol dimethacrylate as crosslinking agent (Scheme 112) [172]. These two polymers 275 and 276 exhibited enhanced enantioselectivities (88 and 86% vs 75% with polymer 274) for the dihydroxylation of two aliphatic alkenes, 1-decene and 5-decene compared to those observed with 274. Furthermore, when polymer 276 was used as chiral ligand in the dihydroxylation of
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Christine Saluzzo and Stéphane Guillarme
aromatic alkenes, ee were higher than 91% in all cases and close to those observed with the non-polymeric analog [173]. O S H
N
N N O
O H
O
N
MeO
OMe N
272
N EGDMA 9
O
O O
O
1
O
S
O
N
H
N N O
O H
MeO
N OMe
N
273
N
Scheme 111.
Salvadori has employed the polymer-supported bis(quinidyl)phthalazine 277 in the asymmetric aminohydroxylation of isopropyl cinnamate 278 in presence of Nchloromethanesulfonamide sodium salt and K2OsO4 in n-propanol/H2O (Scheme 113) [174]. The chemoselectivity, regioselectivity and ee are summarized in Table 17. Polymer 277 proved to be as efficient as homogeneous ligand in terms of activity, chemoselectivity and regioselectivity. Furthermore, the enantioselectivity was higher (87%) than those obtained with 273. Table 17. Asymmetric aminohydroxylation with polymer 277. Conversiona Chemoselectivityb (%) Regioselectivityc Ee of 279 (%) (%) (%) 1 98 94 91 87 2d 73 91 89 83 3d 58 84 89 81 e 4 59 79 88 78 a 279+280+281. b(279+280)/( 279+280+281). c279/(279+280). dRecycling run without addition of K2OsO4. eRecycling run with addition of 10 mol% of K2OsO4. Entry
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
7
1
O
O
2
O
O
O
OH
O
O
O 2S
135
O
O
274: R'=OC(O)-C6H4-p-Cl, R''=H 275: R'=H, R'' = O
N
H
R' R''
MeO N
S O2 N N
276: R'=H, R'' = O
H O
H
2
O
O
N OMe
N
N N 277: R''=H, R'= O
H
N
O
OMe N
Scheme 112.
When the osmium/ligand complex was recycled, both activity and chemoselectivy dropped significantly while chemoselectivity was remained. The enantioselectivity decreased slightly even when an initial amount of K2OsO4 was added to the mixture.
COOiPr
317
316 (10%) 3 MeSO 2 N(Cl)Na K 2 OsO 2 (4%) n-PrOH/H 2 O (1/1)
NHSO 2 Me COOiPr
OH COOiPr
OH 318
NHSO 2 Me 319
OH
COOiPr OH 320 Scheme 113.
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Christine Saluzzo and Stéphane Guillarme
The polymer backbone polarity could influence the outcome of the reaction since the catalytic activity is strongly dependent on the solvent system. For this reason, the catalytic activity of homopolymers 284 and 285 and the copolymers 286 and 287 having a polar group on polymer backbone were compared [175]. Homopolymers 284, 285a and 285b were prepared by polymerization of dihydroquinidine acrylate 282a, dihydroquinidine 4vinylbenzoate 283a and dihydroquinine 4-vinylbenzoate 283b respectively using AIBN as radical initiator (Scheme 114). These supported cinchona were tested in AD of trans metyl cinnamate and trans stilbenes using either NMO in acetone/H2O or K3Fe(CN)6 in tBuOH/H2O. Analysis of the results showed that better enantioselectivities were observed with alkaloid 4-vinylbenzoate-derived homopolymers 285. When the K3Fe(CN)6/t-BuOH/H2O system was used with homopolymers, enantioselectivities were excellent with both alkenes. Song assumed that in the acetone/H2O solvent system, the homopolymers 285 formed a viscous lump which did not allow a high concentration of the substrate near the active site while in the t-BuOH/H2O system these polymers swelled well. The accessibility of the active site is highly dependent on the compatibility between the polymer support and the liquid reaction medium. For this reason, copolymers 286 and 287 having a polar polymer structure were prepared by radical copolymerization of monomer 283 with methylmethacrylate or 2-hydroxyethyl methacrylate in presence of AIBN (Scheme 114). Indeed, these polymers seemed to be more compatible with the liquid reaction medium because both yield and enantioselectivity were higher with the two NMO/acetone/H2O and K3Fe(CN)6/t-BuOH/H2O systems in the AD of trans methyl cinnamate. The polymer 286 and 287/OsO4 complexes could be easily filtered from the reaction mixture and could be reused without significant loss of performance. Since 1,4-bis(9-O-dihydroquininyl)phtalazine ((DHQ)2-PHAL) 258 gave better results than the benzoate analog 257 (Scheme 105) in homogeneous catalysis, the heterogeneous ones were designed in order to check if the same trend was observed [176]. The copolymers 289a and 289b were prepared by copolymerisation of the monomer 288 with methylmethacrylate or 2-hydroxyethyl methacrylate respectively (Scheme 115). These two polymer-supported alkaloids were employed in the AD of trans metyl cinnamate and trans stilbene using OsO4 as oxidant and K3Fe(CN)6-K2CO3 in t-BuOH/H2O (1/1) at 10°C. Excellent enantioselectivities (>98%) were observed in all cases. The OsO4/289a complex was easily filtered off from the reaction mixture and reused a second time in the dihydroxylation of trans stilbene. Although the enantiosectivity remained excellent (99%), the reaction rate was slower (65% yield after 32h vs 93% yield after 15h). In opposite, the polymer 289b was difficult to filter because of its high swellability. An analog of monomer 288 has been prepared replacing the phthalazine by a 2,5diphenylpyrimidine and the corresponding monomer 290 was copolymerized with methyl methacrylate or ethylene glycol dimethacrylate to lead to the copolymers 291a and 291b respectively (Scheme 116) [177]. Unfortunately, the enantioselectivities were around 15-45% lower than those observed with the phthalazine-derived supported dihydroquinidine 289. Another type of insoluble polymer-bound cinchona alkaloid was tested in the AD of olefins. Salvadori has prepared copolymers 294 by solution or suspension copolymerization of chiral monomers 292 with the two styrenic compound 293 and 294 with different monomer ratio (Scheme 117) [178]. Both yield and ee were very high and copolymer 295a proved to be more efficient than 295b and 295c. The better swellability due to the lower crosslinking degree has certainly more influence on the catalytic properties and the
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
137
enantioselectivity than the solvent or the technique polymerisation. The supported alkaloid could be recovered and reused several times without significant loss of the enantioselectivity.
O
O 9 H C
O
8
N
MeO
O 9 H C
N
MeO N
N 283a: DHQD: (8R,9S) 283b: DHQN: (8S,9R) (CH2-CH)n (CH2-C)m
282a: DHQD: (8R,9S) 282b: DHQN: (8S,9R) (CH2-CH)n
(CH2-C)m
O C O 9 H C
COOR
COOCH3 8
O C
N
MeO
8
O 9 H C
N
8
N
MeO
284: n=1, m=0, (8R,9S)
N 285a: n=1, m=0, (8R,9S) 285b: n=1, m=0, (8S,9R) 286a: R=CH3, n=0.2, m=0.8, (8R,9S) 286b: R=CH3, n=0.2, m=0.8, (8S,9R) 287a: R=CH2CH2OH, n=0.06, m=0.94, (8R,9S)
Scheme 114.
N
N N O
H
N O
MeO
OMe 288
N
N
MMA or HEMA AIBN/Benzene
CO2R
RO2C
N H
N N O
N O
MeO
OMe N 289a: R=CH N 3 289b: R=CH2CH2OH
Scheme 115.
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Christine Saluzzo and Stéphane Guillarme
MeO2C
N
N
CO2Me
N
N
O
H
O
MeO
OMe N
N
291a
MMA, AIBN
N
N
N
N
O
H
O
MeO
OMe N
N
290
EGDMA, AIBN
O O
O
O
O N
N
O
N
O
H
O O
N O
MeO
OMe N
291b
N
Scheme 116.
8.2. Soluble Supported Alkaloid as Ligand The immobilization of a ligand on a MeO-PEG leads to the corresponding soluble polymer and it is known that the solubility could have an influence on the outcome of the reaction. For this reason, soluble polymer 296 was prepared and employed in the AD of olefins (Scheme 118) [179]. Similar yield and ee were observed for the reaction with trans stilbene than those obtained with the insoluble polymer-supported alkaloid 262 but with this soluble polymer the reaction time was very short (5h vs 48h). With other olefins used, the yield and ee were less satisfactory. The addition of diethylether allowed the precipitation of the supported alkaloid which could be reused without decrease of the catalytic activity and enantioselectivity. The soluble polymer containing the highly active ((DHQD)2-PHAL) 297 was also prepared and employed in the dihydroxylation of olefins (Scheme 118) [180]. The same level of
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
139
enantioselectivitity (> 97% for all alkenes) than that obtained with the insoluble analog 289a was observed. S
S
RO
O
6
N N
N
N
O
O
OMe
MeO N
292a: R=H 292b: R=
N
2
O 293
O
O
O
O
O O HO 6
O S O
S O O
N N
N
O
N
O
OMe
MeO N
N
295a: R=H, 292a/293/294 :5/ 93/2 (solvent: chlorobenzene) 295b: R= 292b/293/294 : 5/85/10 (solvent: toluene) 295c: R=
Scheme 117.
O 294
AIBN 2) cat. OsO4, NMO
RO
O
OH
1)
292b/293/294 : 5/85/10 (solvent:chlorobenzene/H2O)
2
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Christine Saluzzo and Stéphane Guillarme
O H
N
O
O H
O
O
n
OMe
MeO N 296 N
O
N N O
H
S O O
N
NH-PEG-OMe
O
MeO
OMe N
297
N
Scheme 118.
Bolm has reported the first anthraquinone-derived supported dihydroquinidine for the dihydroxylation reaction [181]. Two different ways about attaching the anthraquinone ligand were studied: either via the anthraquinone part or via the double bond of the natural quinidine (Scheme 119). The two families of soluble supported bisdihydroquinidine 298 and 299 were prepared and tested in the AD of allyl iodide and indene using 0.4 mol% of K2[OsO2(OH)4] as oxidant and K3Fe(CN)6-K2CO3 as cooxidant in t-BuOH/H2O (1/1). In the case of allyl iodide, the soluble supported ligands gave the same enantioselectivities than the nonpolymeric ligand. Although the yields were better for the AD of indene, the ee were slightly lower for all the supported ligands employed. The anchoring or the binding site had absolutely no influence on the enantioselectivity. RO
O
O-DHQD
N DHQD: MeO
O
O-DHQD O O
298a: R= MeO
N
O
m
O
O
298b: R= MeO RO
m
OR
S
S N
N O
H
O
MeO
OMe N
N 0
299a: R= MeO 299b: R= MeO
Scheme 119.
O
H
O m
O O
O m
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
141
After addition of MTBE, the supported alkaloid 298a and 299a could be recovered and reused in another dihydroxylation reaction and no loss of performance was detected.This type of supported anthraquinone-derived alkaloid has been also applied in a continuous flow Sharpless dihydroxylation using the continuously run chemzyme membrane reactor (CMR) in which the catalyst attached to the soluble polymeric ligand 300 is retained by a membrane (Scheme 120) [182]. The two cooxidant systems have been tested and although the NMO one gave lower ee, this system was more compatible with the CMR. The different solutions were delivered in the reactor and after a residence time of 85 minutes, fractions from the reaction mixture were collected. At the beginning, both conversion and ee were around 80% and after only six residence times the conversion dropped to 18% suggesting that a leaching of osmate occurred. Excellent results were obtained for the asymmetric aminohydroxylation using the soluble polymer-bound bisdehydroquinine 301, prepared in two steps from dehydroquinine (Scheme 121) [183]. When analogs of trans cinnamate 278 were used using K2[OsO2(OH)4] as oxidant, AcNHBr as nitrogen source in presence of LiOH in t-BuOH/H2O (1/1), regioselectivity (>20/1) and ee (> 95%) were excellent particularly with 301b and 301c. Results were less satisfactory when the acetone/water was used as solvent. After precipitation and filtration, the PEG-bound ligand 301c could be reused in five consecutive runs with an average decrease of 0.8% per run for the enantioselectivity. DHQ O
O
DHQ O
O
O
O O ca. 450 300
O
O-DHQ
O
O-DHQ
N DHQ:
O
H MeO N
Scheme 120. N N O
H
O PEG O
MeO
H N
N
Scheme 121.
OMe
O
301a: HO-PEG-OH, MW ca. 4000 301b: HO-PEG-OH, MW ca. 6000 301c: HO-PEG-OH, MW ca. 8000
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Christine Saluzzo and Stéphane Guillarme
Anthraquinone-derived quinine has also been anchored onto PEG-OMe to produce soluble polymeric alkaloid 302 (Scheme 122) [184]. Good to excellent yields (80-94%) and ee (80-99%) were observed with different olefins using K2[OsO2(OH)4] as oxidant, K3Fe(CN)6-K2CO3 as cooxidant in t-BuOH/H2O (1/1).
N
O
MeO
O-PEG-OMe
O
O
N 302 Scheme 122.
9. EPOXIDATION Epoxides constitute a class of versatile intermediates, as they can be easily transformed into a wide variety of functional groups involving regioselective ring opening reactions [185]. Thus, asymmetric epoxidation (AE) of olefins is a key reaction for the synthesis of enantiomerically enriched compounds. Complexed with Mn, Cr or Co, N,N-ethylenebis(salicylidene aminato) derivatives known as salen compounds, first described in 1990 by Jacobsen, are widely used as catalysts. From them, manganese complexes have been reported as highly active and enantioselective catalysts in the epoxidation of unfunctionalized alkenes in homogeneous phase using a wide range of oxidants [186]. Number of structure presenting the same backbone have been synthesized and involved in these oxidation reactions (Scheme 123). Easy to handle and to obtain, rather cheap, uses of Jacobsen type catalysts represent one of the most relevant methods of building chiral epoxides. Immobilization of such a compound was extensively studied because it restrains the formation of μ-oxoMn(IV) dimers by isolation of the catalytic sites. R
R
N
N
M: Mn
M O X O
R1 R2
R3
R1 R3
R2
Scheme 123.
The immobilization of salen onto an organic polymer was achieved according to two strategies: a copolymerization of a salen monomer or by grafting onto a polymer followed by
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
143
subsequent solid phase synthesis of the salen. In the latter case, only unsymmetrical salen are formed.
9.1. Supported Salen as Ligand The first example of a polymer-supported Jacobsen catalyst was reported by Dhal [187]. It involved the radical copolymerization of the corresponding distyryl monomer with EGDMA in a ratio of 10/90 to give the macroporous polymer 303 (Scheme 124). The epoxidation of unfunctionalized olefins led to moderate yields (55-72%) and low ee (up to 30%). Moreover, this catalytic system could be used at least five times without any significant loss of its catalytic activity. Ph
Ph
N
O
N Mn O Cl O t-Bu
O
t-Bu EGDMA, toluene, AIBN
Ph N O
Ph N
Mn O Cl O
O 303
t-Bu
t-Bu
Scheme 124.
A similar approach permitted to prepare a macroporous polystyrene-based polymer 306 via a radical copolymerization of a divinylsalen 304 or of a “distyryl spaced” salen 305 with styrene and DVB (ratio 10/75/15) (Scheme 125) [188]. The oxidation was performed with m-CPBA/NMO instead of PhIO as oxidant because this latter was transformed into PhIO2 its disproportionation product which was insoluble in the medium. The catalytic reaction was performed with 10 mol% of the catalyst at 0°C. After 30 minutes, modest to excellent conversions (67-99%) with low to moderate ee (10-62%) were observed. Moreover, the catalytic system could be recycled 5 times, the activity and the selectivity remained identical. Compared to homogeneous systems, enantioselectivity was still low. The epoxide of styrene, cis-β-methylstyrene and indene were formed in 16%, 62% and 60% ee respectively with conversions up to 99% in each case with polymer 305a as ligand. First it was considered that as the salen ligand was localized at the crosslink it may induced a steric hindrance and a conformational rigidity responsible of the low ee. Therefore, Laibinis [189-190] and Sherrington [191] synthesized non symmetrical immobilized salen attached specifically to the polymer in a pendent fashion. The polymer was a styrene base resin.
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N
N Mn O Cl O t-Bu R N
304
N
Mn O Cl O
S
t-Bu O CO
Mn Cl +
t-Bu R
S
t-Bu
OO C
305 a: R = R'= -(CH2)4b: R = R' = Ph
+
AIBN toluene Δ
Mn Cl 75 10
15
306
Scheme 125.
Laibinis used a three steps synthesis for the polystyrene-supported salen 307 (Scheme 126) [189-190]. Asymmetric epoxidation was carried out in biphasic conditions with NaOCl as the oxidant in presence of 307Mn as catalyst. The enantiomeric excesses and the yields of AE with styrene (9% ee, 7% yield), cis-β-methylstyrene (79% ee, 2% yield) or dihydronaphtalene (42% ee, 46% yield) were low to modest. Sherrington [191] synthesized gel type and macroporous resins from 4-(4vinylbenzyloxy)salicylaldehyde (Scheme 126). The gel type catalyst was rather inactive due to the formation of the oxo-bridged dimer of the catalytic center contrary to the macroporous one with an effective site isolation of the catalytic center. With the macroporous catalyst 310 (Scheme 127) although yields were good, ee were always low (up to 20% ee). With an EGDMA polymer matrix (Scheme 128), polymer 311 afforded the epoxidation of 1-phenylcyclohex-1-ene with a high ee (91%) at 49% conversion. With the soluble Jacobsen catalyst, 92% ee and 72% conversion were observed [192-193]. The key factors for this result could be the low loading of Mn sites (0.08 mmol/g) and the high surface area. From all these results a highly active and selective immobilized salen ligands should result in a molecular structure close to those of Jacobsen’s catalyst. Sherrington [193] thought also that to provide good catalytic performance of the polymer-supported catalyst the following design criteria could be met. The complex could be attached to a single flexible linkage to the polymer support in order to minimize the steric hindrance. Moreover, a low
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
145
loading of the catalyst linked to the polymer was required to maximize site isolation of catalytic centres and hence to minimize the formation of the inactive oxo bridge dimer. The polarity of the resin should be such as to provide the optimum microenvironment for the catalyst and finally, the morphology of the resin must permit a high level of mass transfer to the active sites. OH CHO
HO
CH2Cl
R
H2N
NH2
t-Bu CHO
Merrifield resin
t-Bu OH
HO 307Mn M = Mn 307Cr M = Cr O
N
N M O Cl O
t-Bu
t-Bu
Scheme 126.
In 2000, taking into account the criteria edicted by Sherrington, Janda [194] described a convergent strategy to attach a salen ligand containing an appropriate linker to a low loading preformed polymer. A glutarate spacer (5 carbon atoms) was then used and the polymer 312 was prepared from styrene and polyTHF-derived crosslinker to form beads (Scheme 129). The AE in presence of m-CPBA as oxidant was effective. Goods yields were obtain in the AE of styrene and cis-β-methylstyrene with 51% ee and 88% ee respectively. These results were similar to those achieved with the homogeneous Jacobsen catalyst. The supported catalyst could be reused 3 times without loss of activity and selectivity. Another approach, keeping the C2 symmetry, consisted in the immobilization of the catalyst on a TentaGel amine resin via the pyrrolidine part of a pyrrolidine-salen instead of the aromatic rings (Scheme 130) [195]. In presence of NaOCl or m-CPBA, with 4 mol% of catalyst 313 all underwent AE of 2,2-dimethylchromene, 6-cyano-2,2-dimethylchromene and 1-phenylcyclohex-1-ene in high yields, more than 70% and with enantioselectivities of 82%, 86% and 68% respectively. In these conditions, decomposition of this catalyst was observed. To conserve the C2 symmetry, Zhen [196-197] formed poly-salen complexes by condensation of a slight excess of chiral diamino cyclohexane with subsequent insertion of Mn ion (Scheme 131). These catalysts were soluble in CH2Cl2 but almost insoluble in ether. The oxidation reaction was performed in dichloromethane. To avoid any degradation of the polymer NaOCl was used in the presence of 4-phenylpyridine-N-oxide (4-PPNO) or m-CPBA in the presence of NMO as cooxidants. With m-CPBA/NMO, enantioselectivities and activities are higher than those obtained with NaOCl/4-PPNO. With salen 314a or 315 (Scheme 131), the catalytic system was similar to Jacobsen’s one; at 0°C, up to 45% ee were observed for the AE of styrene. The lower temperature, the higher catalytic efficiency. For
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styrene epoxide formation, at room temperature and at -78°C the yields were 61% and 91% respectively with no significant improvement of the enantioselectivity of about 40%. Recovery and reuse of the catalytic system by precipitation was possible with only slight loss in enantioselectivity. The behaviour of catalyst 314b (Scheme 131) was a little different [197]. At -78°C, it presented much lower activity but at -22°C, 83% yield and 51% ee were observed which was represented the best ee for the epoxidation of styrene. This catalytic system unstable in the reaction conditions was not reused.
+
O
suspension polymerization
+
CHO
O CHO 308
OH
OH
H2N
N
N Mn O Cl O O
t-Bu
1) salicylaldehyde derivative dioxane 18C6 2) Mn(II) 3) air 4) NaCl
O
310 t-Bu
NH2
N OH 309
Scheme 127.
N
N Mn O Cl O
O
t-Bu
t-Bu
t-Bu 311
O
O O
O
OAc O O
EGDMA 68%
Scheme 128.
19%
13%
NH2
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
O O
N
N Mn O Cl O
O O
t-Bu
t-Bu
t-Bu 312 Scheme 129.
O O
O NH
N N
t-Bu
N Mn O Cl O
t-Bu
t-Bu
t-Bu 313 Scheme 130.
314 a: R = H b: R = Me
N
N Mn O Cl O
R C R
t-Bu
t-Bu
n
N
H2C O H2C
N Mn O Cl O t-Bu
t-Bu 315 Scheme 131.
CH2 O CH2
n
147
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Christine Saluzzo and Stéphane Guillarme
Kureshy [198] had also carried AE with a poly-salen. The poly-salen 316 derived from chiral diphenylethylenediamine (2 mol%) (Scheme 132) and NaOCl in the presence of cooxidant (NMO, 4-PPNO, PyNO, DMSO) forming the oxidative reagent were used for the epoxidation of styrene, indene and 6-cyano-2,2-dimethylchromene. In all cases, the best results in term of activity (yields up to 99%) and enantioselectivity were obtained with PyNo as cooxidant. The epoxide of styrene, indene and 6-cyano-2,2-dimethylchromene were formed in 32%, 78% and 100% ee respectively. Comparatively, Poly-salen 314a (n = 12, Scheme 131) led to 55% ee for epoxystyrene but for epoxyindene and epoxy 6-cyano-2,2-dimethylchromene 69% and 100% ee were observed. The turn over frequency (TOF) was higher with poly-salen 314a (Scheme 131). The activity of the recycled catalyst gradually decreased upon successive use possibly due to minor degradation under epoxidation conditions and/or weight loss during recovery process of the chiral catalyst, but the ee remained constant. Ph
Ph
N
N Mn O Cl O
CH2
t-Bu
t-Bu
n
316 n = 10 Scheme 132.
N
N Mn +
O O Ph Ph
X
-
317: R = CH2CH2CH2OH 318: R = H 319: R = CH2CH2CH2OCOC6H4 a: X = PF6 b: X- = AcO-, c: X- = CF3SO3Scheme 133.
R
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
149
Smith [199] involved immobilized Katsuki-type (salen)Mn complex 318 (R = H, Scheme 133) for the AE of 1,2-dihydronaphtalene with NaOCl/4-PPNO. The Katsuki-type (salen) 317 presenting an hydroxyalkyl group (R = CH2CH2CH2OH Scheme 133) on the 6-position was grafted onto a polystyrene resin via an ester link by means of polystyrene carboxyl chloride (1% crosslinked, 200-400 mesh, 1.17 mg/mol of chloride) (salen 319, Scheme 133). The counter ion seemed to have no influence on the catalytic efficiency and selectivity. For all of them, yields were about 40% with excellent ee (upper than 90%). Each catalytic system was reused after filtration, ee and yields remained quite unchanged. To increase the yield upon the recycle (up to 70%), concentration of the reaction mixture was required. The molecular imprinting polymer (MIP) was tested by Gohdes [200]. The manganese salen precatalyst 320 (Scheme 134) having two polymerizable acryloyl groups could be covalently embedded in the crosslinked polymer network. In order to mimic the bound styrene substrate, phenylacetate ligand was fixed on the Mn atom which after its removal will leave behind the substrate-binding cavity. The MIP synthesis was performed with the precatalyst 320 in presence of EGDMA as the crosslinking agent, styrene as a non linking polymer and dichloroethane as a porogen. Exploration of porosity and crosslinking effects on activity on epoxidation of the styrene in presence of PhIO or m-CPBA was carried out. In the optimum conditions 40-50% porogen and 70% crosslinking and m-CPBA as oxidant, the ee was up to 14%. This imprinted polymer could be reuse at least 3 times without loss of ee. Rigid macroporous polymer substrate analog
shape selective binding pocket
O O L M L
L M L
Ligands covalently attached to the polymer matrix N
N Mn + O O O O O O
O 320 molecular precatalyst
O
Scheme 134.
Another way to immobilize the salen catalyst was to use glucose as a linker and to graft it on CHO modified Wang resin (Scheme 135) [201]. The activity of the catalyst 322 in the AE of styrenes has been investigated with four oxidants: H2O2, NaOCl, (n-Bu4N)HSO5 and mCPBA. The best results in terms of activity (99%) have been achieved with m-CPBA/NMO;
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the respective ee were 30% and 80% for styrene and cis β-methylstyrene. As the reaction was accompanied by extensive lack of metal from the solid, no recycle has been considered. HO
O O
O
HO
O
O N
+
O N
N
O t-Bu
+
N
Mn
Mn O
O PF6-
t-Bu
t-Bu
homogene catalyst 321
O PF6t-Bu
supported catalyst 322
Scheme 135.
OH O t-Bu
R
R
NH2 H2N Mn(OAc)2 t-Bu O HO
OH t-Bu
O
OAc t-Bu Mn O O
t-Bu N R
N R
323 a R = Ph b R = -(CH2)4-
Scheme 136.
A new approach consisting in the formation of rigid and amorphe salen-polymer (323, Scheme 136) was reported by Gothelf [202]. It involved a condensation of a rigid trisalicylaldehyde with substituted ethylene diamines (Scheme 136). The AE of cis βmethylstyrene at 0°C, with m-CPBA in presence of NMO and polymer 323a led to 78% conversion with a cis/trans epoxide ratio of 17.0, with a catalyst loading of 15 mol%. Up to
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
151
67% ee for the cis stereoisomer and 23% ee for the trans diastereoisomer are obtained. After isolation from the reaction mixture, the catalyst was reused at least 6 times without significant drop in reactivity and selectivity. Weck [203] developed a monomer salen complex linked to a norbornene via a stable phenylene-acetylene linker and its subsequent polymerization by means of the controlled ROMP method using 3rd generation Grubb’s catalyst (Scheme 137). This polymerization methodology led to fully functionalized immobilized metal-salen catalyst. By this way, the supported catalyst showed catalytic activities and stereoselectivities similar to the nonsupported Jacobsen catalyst. Moreover, activities and selectivities seemed to depend on the density of the catalytic moieties: homopolymer 324 were less selective than their copolymer analogs 325. For example, AE of 1,2-dihydronaphtalene led in both cases to total conversion and 76% ee for the homopolymer 324 vs 81% ee for copolymer 325a. Recycle was possible and after 3 recyles a drastic decrease in ee was observed. AE of dihydronaphtalene led to 81% ee for the first cycle vs 6% ee for the third one.
N
O O
O t-Bu
n
N
M
t-Bu
O
[Ru]
t-Bu
Mes N N Mes Cl Cl Ru N N Ph
324 M = Mn-Cl, n = 50 [Ru] CDCl3 50°C
N
O O
O t-Bu
M
Br
Br
O
N
O n-octyl
t-Bu
O t-Bu
[Ru] CDCl3 rt, 2h
O O n-octyl x
N
O O
y
O t-Bu
M
N O t-Bu
t-Bu 325Mn a M = Mn-Cl, x/y = 1.1, x+y = 50 b M = Mn-Cl, x/y = 3.1, x+y = 100 c M = Mn-Cl, x/y = 9.1, x+y = 100 325Co a M = Co, x/y = 1.1, x+y = 50 b M = Co, x/y = 3.1, x+y = 100 c M = Co, x/y = 9.1, x+y = 100
Scheme 137.
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Christine Saluzzo and Stéphane Guillarme
9.2. Porphyrins Polymer RuCO-porphyrins 102 and 103 (Scheme 48) were used for the cyclopropanation reaction of styrenes but they were also tested in the epoxidation reaction. Contrary to cyclopropanation for which moderate enantioselectivities and low activities were observed, these Ru porphyrin complexes gave good ee (up to 76%) and activity (up to 89%) for AE of unfunctionalized olefines [204]. Recently, Simmoneaux showed that iron catalyst 326 derived from electropolymerized tetraspirofluorenyl porphyrin (Scheme 138) led to moderate yields without chiral induction [205]. CH2OAc
N Cl N Fe N N
AcOH2C
CH2OAc
CH2OAc )
n
)
anodic oxidation n
H2COAc Fe(Cl)
)n )
n(
AcOH2C
CH2OAc
n(
n
)n )
n
CH2OAc
326
CH2OAc CH2OAc
Scheme 138.
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
153
9.3. Soluble Polymers In 2006, Yu [206] prepared a polymeric chiral salen Mn(III) complex containing chiral diamine spacer, acting as a solvent regulated phase transfer catalyst (scheme 139). This catalyst presented a Mn ion content of 1.49 mmol/g and a molecular weight of 9100. Evaluation of its catalytic behaviour in AE of styrene was performed in the presence of the oxidative system m-CPBA/PyNO, m-CPBA/4-PPNO, m-CPBA/4-PPPyNO or mCPBA/NMO. Enantiomeric excesses up to 35% were found similar to those observed in the same reaction conditions with Jacobsen catalyst. This was probably due to the solubility of the polymeric catalytic system in the reaction mixture acting thus as an homogeneous catalyst by minimizing the diffusion of the reactants to the active sites of the catalysts. The nature and the amount of the N-oxide cocatalyst showed some impact on both yield and ee. Without this additive, the yield was rather low and no ee was detected; and its presence led to an increase of both yield and ee. The optimum yield and ee were obtained when 2 equivalents of each additive were used. But NMO was the best one leading to 98% yield and 43% ee. Recycle was possible but after the third one, poor yields and ee were observed.
O ClH2C
1) K2CO3 H2N NH2
OH t-Bu
2) Mn(OAc).4H2O 3) LiCl
N
N Mn O Cl O NH t-Bu
NH
t-Bu
327 n ~14
n
Scheme 139.
Recently, Liese [207-208] used a hyperbranched polyglycerol support as high loading Mn-salen complex (chemzyme) as a catalyst fo AE 6-cyano-2,2-dimethylchromene for a continuous application in a membrane reactor. This polymer supported catalyst was performed using a polyglycerol which molecular weight is 8000 g/mol and OH group loading 13.5 mmol/g. The polymer purification was carried out by ultrafiltration leading to 65% of the desired polymer 328 (Scheme 140). The following metal insertion led to polymersupported catalyst 329. The oxidative reagent was formed by m-CPBA and NMO. Upon recycling it was observed an enhancement of the stability of the catalyst, the total turnover number increasing from 23.5 for a single batch to 80 in four repetitive batches. Moreover, some metal leaching occurred because the maximum conversion decreased from 98 to 75%, but enantioselectivities were less affected; they decreased from 95 to 88% ee from the first to the fourth batch. Nevertheless, in a continuously operated chemical membrane reactor, the TTN reached up to 240 after 20 residence times with conversions up to 70% and ee up to 92%.
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Christine Saluzzo and Stéphane Guillarme
-
O
O
+
OH
+
ClH3N
N t-Bu
HO
t-Bu Et3N
N
t-Bu
N
OH
HO
O t-Bu
t-Bu
t-Bu loading 1.6 mmol/g Yield 65%
328 metal insertion
N
+
N
M O
O X- O
t-Bu
t-Bu
t-Bu 329Co M = Co, X = OAc, conversion = 54%, Co loading: 0.76 mmol/g 329Mn M = Mn, X = Cl, conversion = 75%, Mn loading 1 mmol/g Scheme 140.
10. KINETIC RESOLUTION OF TERMINAL EPOXIDES Close to Mn-salen complexed which were effective in the epoxidation of unfunctionalized alkenes Co or Cr-salen complexes have shown their ability in asymmetric ring opening such as hydrolytic kinetic resolution (HKR) of racemic epoxides. This reaction, performed in homogeneous conditions was able to furnish chiral epoxides and diols with high enantioselectivities. Numbers of inorganic supports have been used however some organic supports have also been studied. Other kinetic resolution could be performed such as ring opening epoxide with other nucleophiles and dynamic resolution. Contrary to the catalytic systems employed for the epoxidation, mechanistic studies of asymmetric ring opening have shown that cooperative interactions between catalyst units are needed [209]. Thus, high local concentration of catalyst was necessary and high-loading support should increase the catalytic reactivity. In 1999, Jacobsen [210] prepared a polystyrene-supported Co-salen complex 332 by grafting a monophenol derivative 330 of an highly efficient chiral salen onto hydroxymethyl polystyrene beads (90 μm) derivatized as their p-nitrophenyl carbonate 331 (Scheme 140).
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
155
The same polystyrene complex 332 could be synthesized by resin capture of the salen 330. The preparation of salen 330 was carried out using an excess of di-tert-butyl salicylaldehyde relative to 2,5-dihydroxy-3-tert-butylbenzyldehyde and enantiopure cyclohexane diamine (ratio: 3/1/2) yielding a 9/6/1 ratio of ditertiobutyl, monophenolic (330) and diphenolic ligands. Then, addition of the carbonate derived polystyrene resin to the mixture allowed the selective capture of the two phenolic ligands. After washing, the polymeric mixture is formed by the incorporation of the diphenolic ligand in the resin bound ligand close to ligand 332. Complexed with Co, this small amount of the incorporated diphenolic ligand in the resin catalyst did not affect the rate and the enantioselectivity; compared to material prepared with pure ligand 332, similar results were observed.
H N
H +
OH HO
HO
t-Bu
330
H
O 331
H N
N
O
O
1) DMF, DIPEA, 1.5h 2) Co(OAc)2, MeOH/PhMe 3) PhMe/HOAc (9/1)/ air
t-Bu
t-Bu
NO2
O
N
Co O
O
O
O OAc t-Bu t-Bu 332
OH = 90 μm hydroxymethylpolystyrene 160 μmol of Co-salen per gram of resin
Scheme 140.
The HKR of epichlorhydrine (X = Cl, Scheme 141) and 4-hydroxy-1-butene oxide (X = OH, Scheme 141), the dynamic kinetic resolution of epibromhydrin and the enantioselective ring opening of epoxides by phenol were examined. In the first experiment, combination of the crude organic soluble products of the five recycle reactions and concentration led to the (S)-epichlorhydrine in 41% overall yield and > 99% ee and the (R)-chlorodiol in 93% ee. In the second one, the sum of five experiments provided (S)-triol in 36% overall yield and 94.4% ee while the enantioselectivity of the epoxide was only 59% ee. The dynamic resolution of epibromohydrine provided only the bromodiol in 94% overall yield and 96% ee (sum of the five experiments). Excellent results were also obtained for the ring opening epoxides with phenol, ee could reach more than 99% and yield 98%. The kinetic resolution of other epoxides with phenol was also studied in enantioselective parallel synthesis with the same catalyst, providing efficient access to important precursors of pharmacologically active compounds [211]. For all the studied substrates, the catalytic system was proved to be very efficient. β−aryloxyalcohols were formed in high yield (90-100%), purity ((93-100) and ee (81->99%).
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Christine Saluzzo and Stéphane Guillarme
H H N N Co O O OAc t-Bu t-Bu
O O O
O
X
OH
OH
OH
X +
O
X X = Cl, OH HKR conditions
X = Br Dynamic resolution conditions
Co-salen (0.25 mol%) H2O (0.7 eq.), CH2Cl2, rt, 3h
Co-salen (0.5 mol%) H2O (1.5 eq.), rt, 24h
HO
O
R
R
PhO
R = Br or n-Pr Co-salen (1 mol%) (CF3)3OH (0.2 eq.), rt, 2h
Scheme 141.
t-Bu O OHC
t-Bu
CHO
O O
t-Bu O O
CHO t-Bu
t-Bu t-Bu CHO t-Bu t-Bu + O
333 O
O
O
O
O
O
CHO t-Bu
t-Bu
O
O O
334
1) (R,R)-diaminocyclohexane, THF 2) Co(OAc)2.4H2O, toluene/MeOH 2,6-lutidinium p-toluenesulfonate
polymeric salenCo(III)complexes O
O
salen
salen
en
335
O
O 336
n=6
salen:
sal
n le sa Scheme 142.
O
O O
O
O
O
O
O
N O
O
N M
O
t-Bu t-Bu M = Co(III)OTs
O O
n
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
157
Zheng [212] prepared crosslinked polymeric Co(III)-salen complexes in order to induce cooperative effect. Trialdehyde 333 and dialdehyde 334 are formed via condensation of 2,5dihydroxy-3-tert-butylbenzyldehyde with diacid and triacid derived from hydroquinone and chloroglycinol. Different proportions of tri and di aldehydes (tri/di) were subsequently condensed with enantiopure cyclohexane diamine to afford polymeric ligands 335 (Scheme 142). An average molecular weight between 4000 and 10000 were observed for these polymers. For the HKR of epichlorhydrine, styrene oxide, and phenylglycidyl ester, in a wide range of tri/di proportion, similar behaviour of the catalytic system was observed in the same reaction conditions. For example, with epichlorhydrine conversion based on racemic mixture varied from 50-52%, epoxide ee from 98-99% and diol ee from 91-97% using 0.02 mol% of the catalyst, at 10°C. Most of the crosslinked polymeric catalyst presented better activities than those of the oligomer 336, showing a positive effect on the cooperation on catalytic centers with the crosslinked polymer. But with the complete crosslinked polymeric catalyst 335 activities and enantioselectivities are slightly lower, behaviour probably due to the poor solubility of the catalyst. Moreover, attempts to recycle the catalytic system were unsuccessful. This fact was ascribed to the sensitive ester linkage to the reaction medium.
N
N
OH HO HO
OH
O
O
O
t-Bu t-Bu
n
337 N
N
O N
OH HO
Cl
t-Bu
Cl
t-Bu
OH HO O
O
OH
t-Bu t-Bu
338 HO
N O
n
OH
OH
N
O
OH HO O
HO OH
N
t-Bu t-Bu
O 339
O
n
Scheme 143.
Kim [213] studied the effect of the counter ion on the HKR of epichlorhydrine, 1,2epoxybutane, 1,2-epoxyhexane and epoxystyrene. The salen polymer catalyst were synthesized by copolymerization of salen bearing chloromethyl groups with sodium phenoxide derivatives of hydroquinone, 1,3,5-trihydroxybenzene or 1,1,1-tris(phydroxyphenyl)ethane in presence of N-methylpyrrolidine and NaH in THF (Schemes 143 and 144). Co(II) type polymeric chiral salen ligands were formed by reaction of the corresponding salen ligand with hydrous Co(II) acetate. To obtained Co(III) polymeric chiral
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Christine Saluzzo and Stéphane Guillarme
salen catalysts, the Co(II) polymer salen was treated with ferrocenium hexafluorophosphate or ferrocenium tetrafluoroborate noted respectively Co(III).(PF6), Co(III).(BF4). The HKR of epichlorhydrine, 1,2-epoxybutane, 1,2-epoxyhexane and epoxystyrene was performed. With all these substrates, excellent enantioselectivities (up to 99.8%) were observed with 337-, 338-, 339-, 340-Co(III).(PF6), -Co(III).(BF4) catalysts. Both type of catalysts exhibited almost the same ee as monomeric salen catalyst. These catalytic systems were stables; they could be recovered and reused at least seven times without further treatment without loss of activity and selectivity and without regeneration of the catalyst. However, it is noteworthy that 340Co(III)OAc catalyst, the regeneration with AcOH/air is necessary to avoid an ee decreased to 20% in the second hydrolysis. Moreover with 340-Co(III)OAc catalyst a racemization during the reaction and the product isolation by distillation could occurred The ee decreased slowly via racemization which is not the case with 340-Co(III).(PF6).
O HO t-Bu
O CH2
OH t-Bu
N H2N
NH2
N
OH HO t-Bu
t-Bu
CH2 n
340
Scheme 144.
Other 340-Co-salen complexes containing Lewis acid were studied [214](Scheme144). Monomeric salen Co(II) or Co(III) unit are supposed to be attached to the polymer salen catalyst by means of the Lewis acid. Thus, salen units were linked together as a dimeric form (polysalen 346 and 348, Scheme 145). It was observed a great increase of the catalytic activity for the HKR reaction of various epoxides. For example, in presence of 0.4 mol% polysalen catalysts 346 and 348 the HKR reaction of epichlorhydrine led to about 98% ee, after 6h whereas with complex 345 ee was up to 55%. As dissociation of monomeric salens from the polymer backbone partially occurred after their first use, the polymeric salen 346 and 348 lost their catalytic activity. It is noteworthy that the activity of the salen catalyst could be recovered upon treatment with monomeric salen. Contrary to Mn salen 256Mn (Scheme 126), which is used for the AE, Laibinis used the heterogeneous Cr salen 256Cr catalyzed ring epoxide opening in the presence of TMSN3 [190]. In the conditions of ring opening epoxide, ee reach 36% and yield 47% for the propylene oxide. But the catalytic system is stable and could be reused 3 times without loss of activity and enantioselectivity. Weck [203] has performed the HKR of several epoxides by means of polymer-supported cobalt salen catalysts containing different counterions (325/Co-OAc, 325/Co-I, 325/Co-OTs, 325/Co Scheme 137). 325/Co-I and 325/Co-OTs catalysts have shown higher activities than 325/Co-OAc and ee could be up to > 99% with a conversion of 55% after 1h with 325/Co-I for the HKR of epichlorhydrine. These catalysts can only be recycled once as a result of their decreased solubility after the reoxidation step. Weck [215] has also described polystyrene-supported chiral salen ligands 349 and 350 synthesized respectively by radical homopolymerization or copolymerization of an
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
159
unsymmetrical styryl-substituted salen monomer (Scheme 146). They were easily converted into their Co(III) acetate complexes and then tested in the HKR of epichlorhydrine at room temperature. The reaction was carried out with 0.5 mol% of the catalyst and after 1h, conversion is about 55% and ee >98% even after four recycling (catalyst 350b). OH N N Co O O t-Bu t-Bu 341
t-Bu N F
BF3.2H2O t-Bu
O
t-Bu F B F
Co
t-Bu
O
BF3.Et2O
t-Bu t-Bu N
t-Bu
Co
t-Bu
N
O Cl t-Bu t-Bu O
t-Bu
N N Co O O t-Bu
N
F t-Bu F B F 342
t-Bu
343
N Co O
N
N O
t-Bu BF3 t-Bu
344
Co O
N O
O
t-Bu
t-Bu
N
Co
BF3.Et2O MC/N2
345
n
N
N
n
N
N
O
O
O
O
t-Bu BF3 t-Bu n
345
t-Bu t-Bu F B F F t-Bu t-Bu H t-Bu O O Co N t-Bu N
N
N
O
O
BF3 t-Bu
t-Bu
N Co
Co
Co
t-Bu
346
O
t-Bu t-Bu F B F F t-Bu H t-Bu O O N Co N t-Bu n
N
N
N
Co OCl O
O
t-Bu BF3 t-Bu 347
N
N
n
Cl
O
O
t-Bu t-Bu F B F F t-Bu t-Bu H t-Bu O O N Co N t-Bu
N
N
t-Bu
O
O Cl BF3 t-Bu t-Bu
348
N Co
Co
Co
Cl
O
t-Bu t-Bu F B F F t-Bu H t-Bu O O Co N t-Bu N n
Scheme 145.
Polyethylene glycol–salen-Co-OAc (Scheme 140) were applied in the HKR of several terminal epoxides with a catalyst loading of 0.5 mol% using 0.55 equiv. of water in THF. A positive dendritic effect was observed; the dendritic backbone was supposed to increase the intramolecular cooperation between Co active site by bringing the reactive site of the reactant
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Christine Saluzzo and Stéphane Guillarme
adjacent to each other (cooperative bimetallic catalysis) [208]. This reason was evoked to explain that better results were obtained with the polymeric catalyst contrary to the non polymeric one. With epichlorhydrine, the conversion reached 52.5% and the enantioselectivity 90.5%.
N
N (
t-Bu
OH
HO )
t-Bu
349 a: n = 12, Mn = 7200 b: n = 24, Mn = 14600
AIBN
N t-Bu
OH t-Bu
n
t-Bu
N HO t-Bu styrene AIBN
N OH
HO t-Bu
)(
t-Bu
N (
t-Bu
350 a: n = 15, m = 14, Mn = 10200 b: n = 6, m = 48, Mn = 8600
n
)
m
Scheme 146.
11. MISCELLANEOUS Few other asymmetric reactions have been performed using insoluble or soluble polymersupported ligands. The first example is a Mukaiyama-aldol condensation between silyl ketene acetal and different aldehydes using polymeric Box analog of 99 as chiral ligands and Cu(OTf)2 as metal source in water (Scheme 147) [216]. When using benzaldehyde as substrate, yields were very low (12-34%) and ee were moderate (40-62%) whatever the polymer-supported Box. The same level of enantioselectivity was observed with other aldehydes while the yield was better with all the ligand/Cu complexes used. Insoluble polymer-supported Box 82 (n=1) (Scheme 40) was also used in an enantioselective glyoxylate-ene reaction (Scheme 148) [217]. In most cases, although the yields and the ee were satisfactory, they were slightly slower than those observed with the corresponding non-polymeric Box.
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
O
O
O
161
O
N
N
R
R
351a: R=i-Pr 351b: R=Bn
= MeO-PEG5000
352: R=Bn
=
(OCH 2 CH 2)40 -
t-Bu
OTMS + RCHO
OMe
OH
Cu(OTf)2 L* (20 mol%)
CO 2Me
R
H 2O
Scheme 147.
R2 R1
R3 +
H
R2
cat. Box/Cu(OTf)2
O CO2Et
CH2Cl2
R1
OH CO2Et
R3 Scheme 148.
The possibility to recycle the catalyst was examined in the ene reaction of αmethylstyrene and ethyl glyoxylate. The conversion dropped from 90 to 60% after the fourth recycle (conversion was evaluated after six hours for each run) while the enantioselectivity remained constant. Furthermore, a continuous flow-system has been developed using the polymer 82 and overall 23 mmol of α-methylstyrene have been transformed into the corresponding product after 5 consecutive batches. The enantioselectivity did not change between the successive runs (close to 90% ee). The TTN was estimated at 44 or 51 versus the supported Box and copper salt respectively. The complexes of polymer 243 (Scheme 97) with YbCl3 and LuCl3 have been used for the ring opening of cyclohexene oxide by trimethylsilylcyanide [154]. Although the conversions were excellent, the ee were low to moderate. Furthermore a significant drop of the enantioselectivity was observed when the recovered supported Yb-complex was reused. The same complexes were then evaluated for the enantioselective TMSCN addition to benzaldehyde. For this reaction, both yield and ee were better with the Lu-complex. The asymmetric TMSCN addition to benzaldehyde was also performed in a microreactor using the polymeric catalyst 353 (Scheme 149) [218]. A normal batch procedure was first carried out and the protected cyanohydrin was obtained with 91% conversion and 73% ee.
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Christine Saluzzo and Stéphane Guillarme
When a microreactor using this catalyst 353 was employed, same result was observed with a flow rate of 0.8µL. Enantioselective sulfoxidation has been also studied using polymeric aminoalcoholderived Schiff bases. Polymers 354-357 have been prepared by copolymeristation of the corresponding monomer with either MMA and EGDMA or styrene and DVB (Scheme 150) [219]. These four polymers were then stirred with VO(acac)2 for 4h at rt to lead to the vanadium complexes. For the enantioselective oxidation of thioanisole, although the yields were similar to those obtained with the homogeneous analogs, the enantioselectivity were lower. The best selectivity was obtained with the catalyst derived from 355. N
N Ti O O O
O
N O O O O
O N
N
Ti O O O O O O Ti N
353 N N
N
N OH HO
OH
O
OH 3
HO
O
O
Scheme 149. NO2 HO
HO OH
HO HO
OH
N
OH
N
N
H
H
OH
OH
N
H
H O
O
O O
O
O
MMA EGDMA AIBN Toluene
styrene DVB AIBN Toluene
MMA EGDMA AIBN Toluene
styrene DVB AIBN Toluene
HO
HO OH
NO2
HO HO
OH
N
OH
N
N
H
H
OH
OH
H O PS
PS
O PA2
PA1 355
356
PA=polyacrylate, PS=polystyrene
Scheme 150.
N H
O
O 354
O
357
Nitrogen-Containing Ligands Anchored onto Polymers as Catalyst Stabilizer…
163
12. CONCLUSION The performance of the soluble catalysts supported on a polymer or of the heterogeneous catalysts is highly dependant on the polymer morphology which is, in the last case, mainly controlled by the degree of crosslinking, the nature of the crosslinker and nature of the porogen. For soluble polymer catalysts, the catalytic performances are generally close to that observed with homogeneous non-supported catalysts. This behaviour is mainly inherent to high mobility of the bound catalyst and a good mass transport. However, the stability of the catalyst depends on the reaction conditions, some catalytic systems being not stable in some solvents. Their recovery needs filtrations or precipitation, sometimes under inert atmosphere. But, their use could be interesting for an industrial point of view, particularly in continuous flow systems involving membranes where important conversions of the substrate with good selectivity could be involved. As for insoluble catalysts they could be separated into two series.The ligand could be anchored onto swellable or unswellable polymers. In the first case, the support is formed by a crosslinked polymer, mostly a polystyrene crosslinked with 0.5 to 3% DVB. In the second case, pratically unswellable polymers formed the support. This latter is a highly crosslinked polymer. A large variety of solvent could be used without changing the texture of the catalyst. In order to obtain a good mass transport, it is generally necessary to obtain immobilized catalysts with a pore size enough larger to include the size of the metal complex catalyst and the substrate. This parameter depends on the nature of the porogen. These immobilized catalysts must present a large surface area For the specific molecular imprinting polymer, the catalyst is efficient if a good mass transport is possible and particularly if the chiral cavity fits perfectly with the size and the structure of the substrate. It is the reason why in most of the cases the shape of the polymer is specific to a single substrate. Reaction performed with monolithic polymer-supported catalysts, lead generally to good to excellent behaviour in terms of catalytic activity and enantioselectivity and as the reaction is performed without stirring, the catalytic system is generally very stable. The catalyst ability for recycling and reuse is in correlation to the stability of the catalytic system itself. A simple change of the nature of the polymer support could increase the stabilization and thus the recyclability of the catalytic system. Concerning the nature of the nitrogen-containing ligand itself, immobilized derivatives of DPEN revealed to be the most efficient in the reduction of prochiral ketones by catalytic hydrogenation in the presence of BINAP or by HTR and CBS-supported ligands for the enantioselective hydride reduction. As for C-C bond formation via an addition of organometallic reagent to aldehyde, ketone or imine or allylation, supported-complexes of prolinol, ephedrine and oxazoline derivated constitute the best catalytic systems. Most of the polymeric-catalytic systems used for the dihydroxylation or aminohydroxylation of C=C bond is issued from alkaloid, mainly cinchona which has been proved to be the best ligand for this type of reaction.
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For the epoxidation or kinetic resolution of terminal epoxide, polymer-supported salens have shown to be either as efficient as or more efficient than their non-supported homologs. It is noteworthy that concerning the loading, low loading in ligand and metal are required to achieve asymmetric hydrogenation, C-C bond formation or dihydroxylation or aminohydroxylation. But, high loading is necessary for epoxidation or resolution of terminal epoxides if salen derivatives ligands are employed. To conclude, the nitrogen-containing ligands are easy to handle, to synthesize and to support them on polymers, thus this constitutes a crucial advantage for using them in various separation systems (precipitation, ultrafiltration, membrane reactor, biphasic solid/liquid or liquid/liquid).
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In: Encyclopedia of Polymer Research Ed: Caroline E. Jones
ISBN: 978-1-61761-926-7 © 2011 Nova Science Publishers, Inc.
Chapter 4
SMALL MOLECULE STABILIZATION: A NOVEL CONCEPT FOR THE STABILIZATION OF SMALL INORGANIC NANOPARTICLES Georg Garnweitner* TU Braunschweig, Institute for Particle Technology, Braunschweig, Germany
ABSTRACT In the last 20 years, the synthesis of nanoparticles with defined size and shape has been studied with strongly growing interest, leading to a multitude of synthetic approaches and strategies. Whereas the synthesis of the nanocrystals has been studied in great detail, far less effort has been directed towards the stabilization of the obtained materials against agglomeration. This is surprising as the stabilization determines their dispersibility in various solvents, which is a crucial parameter for most applications. For conventional colloids, the classical theories of electrostatic, steric and electrosteric stabilization are well established, but application of these theories to the stabilization of small nanomaterials leads to some peculiarities and at the same time has some limitations, which is known from experimental experience but has not been studied in a systematic fashion yet. One important conclusion from the theories is that short organic molecules sufficiently serve to provide steric stabilization of nanoparticles less than about 50 nm in size, without a need for long-chain polymeric stabilizers. This concept has been successfully applied using commercial metal oxide nanoparticles in the 50 nm size range, and it is even possible to tailor nanoparticle dispersions with respect to their rheological properties by adjustment of the stabilizer size. Through proper choice of the stabilizer, nanoparticle slurries with high solids content but at the same time low viscosity can be realized, which is highly advantageous for applications especially in the field of ceramic processing. For ultrasmall nanoparticles in the sub-10-nm regime, the picture is somewhat different. On the one hand, the dispersions of such particles in a stabilized state show *
Corresponding author, Prof. Dr. G. Garnweitner, TU Braunschweig, Institute for Particle Technology, Volkmaroder Str. 5, 38104 Braunschweig, Germany
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Georg Garnweitner very special properties on the verge to molecular solutions, rendering them highly relevant for applications and thus their preparation highly important. On the other hand, due to the lack of suitable model materials, the fundamentals of interaction and stabilization of such small nanoparticles remains largely in the dark. Only a small number of reports were specifically directed to adress these problems and systematically investigate the effects of stabilizer chemistry and structure as well as solvent influence. A brief overview of these studies is provided to show that first concepts have been presented, but the general applicability of these concepts still remains to be seen, and to demonstrate the substantial need for further research in this field in order to develop concepts for the rational stabilization and preparation of dispersions with tailored nanoparticle interactions and thus tailored properties.
INTRODUCTION The strong tendency of nanoparticles and nanomaterials to agglomerate and aggregate constitutes probably the most important challenge for their synthesis and application [1, 2]. Already since the work of Graham and Ostwald [3] it is known that dispersions of particles in aqueous (but also gaseous) media are thermodynamically unstable. The agglomeration of particles, which is termed coagulation in the field of colloids chemistry, results in a reduction of the surface free energy, and therefore occurs instantly [4]. In the case of nanoparticles, which by common definition are colloidal particles less than 100 nm in size, the formation of agglomerates usually is observed already during the particle synthesis [5, 6]. The agglomerates, also termed secondary particles, in many cases exceed the size of the individual nanocrystals, or primary particles, by orders of magnitude [1]. For many applications, such agglomerates are undesired as they corrupt the materials’ performance and complicate processing and storage, especially if the nanoparticles are to be processed or applied as dispersions. In these cases, agglomerates usually lead to an increase in viscosity of the system, are prone to sedimentation (as the sedimentation velocity increases with the square of the particle diameter), and strongly increase the turbidity. Another important application of nanoparticles where the agglomeration state has a crucial importance is the field of nanocomposites. Commonly, nanocomposites are fabricated by incorporation of inorganic nanoparticles into a polymer matrix, resulting in a material that can be processed and shaped like a polymer but possesses strongly enhanced mechanical or optical properties. The mechanical properties of nanocomposites on the one hand were reported to be strongly dependent on the secondary particle size of the nanomaterial filler [2], and on the other hand of course also the optical properties, especially transparency, are greatly influenced by aggregation of the nanoparticles [7]. Since the beginning of colloids science, however it is also known that the agglomeration of colloids and dispersed particles can be prevented or controlled by stabilization [8]. The attractive interactions between the colloidal particles, caused by van-der-Waals forces, need to be compensated by repulsive interactions. The latter can be based either on electrostatic repulsion due to same-sign surface charges (electrostatic stabilization), or on repulsion via a polymer shell formed through adsorption of polymers to the particle surface (steric stabilization, in presence of polyelectrolytes termed electrosteric stabilization due to additional charged-induced repulsion) [9, 10]. The stabilization by control of the interaction forces between colloidal particles has been in the focus of extensive research efforts. Already
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in the early days of colloid science, the forces between particles were studied theoretically [11], and for electrostatic stabilization the so-called DLVO theory was proposed [12, 13] and its validity for a broad range of systems and conditions is accepted until today [14]. A special focus of research has been the steric stabilization of colloidal particles by adsorption of organic polymers to the particle surface [15-17]. Typically used polymer stabilizers possess a molecular weight ranging between 5,000 Da and 15,000 Da. These polymers are targeted to adsorb to the particle surface and form an organic layer around the particles (often termed adlayer), of about 3-10 nm thickness. The general unterstanding of steric stabilization is that when two particles approach each other, the polymer layers interpenetrate, creating an osmotic pressure that counteracts the attractive interparticle force, thereby efficiently preventing close contact of the particles that would result in agglomeration [17]. The stabilization of colloidal particles can be achieved by addition of the stabilizer in a distinct stabilization step following the synthesis (post-synthesis method) [18], or by addition to the reaction system already prior to the particle formation (in-situ method) [19-22]. The latter strategy has the potential benefit of a perfect stabilization as the forming particles are instantly capped with the stabilizer, but in many cases possesses the disadvantage of a modification of the particle formation process, as strongly binding stabilizers may lead to altered particle sizes, different morphologies or even completely prevent the particle formation [23-25]. In the post-synthesis method, particle fabrication and stabilization are carried out independently, thus rendering this strategy suitable also for solventless hightemperature particle synthesis methods that would prevent the use of a stabilizer during the formation process. During their synthesis in the absence of stabilizers, however, the nanoparticles may strongly agglomerate or aggregate, which in many cases necessitates the use of mechanical dispersion methods such as ultrasonic treatment additionally to a chemical process to achieve a successful stabilization. For many applications, steric stabilization is the strategy of choice due to the higher stability, its suitability also for hydrophobic media and matrices, and the lower sensitivity towards pH and additives. Besides the stabilization effect, the adsorbed organic layer around inorganic particles can additionally be used for functionalization, which is a more encompassing concept that is based on achieving chemical functionality of the particles, resulting in functions such as selective binding to biological molecules, affinity to certain media or surfaces, environmentally responsive behavior, etc. [26-28].
STERIC STABILIZATION OF NANOPARTICLES Whilst a great deal of research has been carried out to study the interactions of particles featuring adsorbed polymer chains, especially with respect to the chemistry and conformation of the polymer chains, the dependence of these interactions on the size of the involved particles is an aspect that has not so much been in the focus of research activities. Whereas the particle interaction is relatively independent of the particle size for larger colloids, for particles smaller than 100 nm in size, this is no longer true [29]. The thickness of the adlayer formed by common polymer stabilizers for such particles starts to become considerable
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compared to the particle size, and for small nanoparticles can even exceed the size of the core particle, which has severe, and mainly undesired, consequences [20]. On the other hand, it is generally known that the stabilization of particles with sizes below the critical range of 100 nm also follows some special principles. Already from Hamaker’s theory of the attraction between particles, it can be inferred that for very small particles, the range of attractive potential decreases, and already for 50 nm sized particles only amounts to a few nm. This has severe consequence on electrostatic interaction, as is described by the DLVO theory, as well as for steric stabilization [13, 30]. It needs to be stated, however, that the attractive potential strongly depends on the nature of the material, as is illustrated in Figure 1, presenting calculations by Sigmund et al. of the attractive potential between two 100 nm sized particles of SiO2 and Al2O3 [31]. It is clearly visible that the attraction beween Al2O3 particles is much more long-ranged than beween silica spheres, which would imply that alumina particles need a stabilizing layer with substantially larger thickness to prevent them from reaching an interparticle distance small enough to result in a high attractive potential. The horizontal line in Figure 1 indicates the stabilization range, as the thermal energy is sufficient to easily overcome an attractive interaction of 2 k·T. Hence, the critical separation distance for the SiO2 spheres amounts to about 3.5 nm, whereas for the Al2O3 spheres it is increased to 12 nm [31].
Figure 1. Comparison of van der Waals interaction energies for SiO2 (– – –) and Al2O3 (- - -) spheres of 100 nm radius in water, and of SiO2 spheres in n-dodecane (––––). Reproduced from Ref. [31] with kind permission of Wiley-VCH.
These results imply that for the steric stabilization of SiO2 particles, polymers with substantially lower molecular weight, corresponding to a thinner stabilizing layer, are required than for Al2O3. The molecular weight MW of an adsorbed homopolymer can be approximately related to the adlayer thickness by [17] L [nm] = 0.06·(MW)0.5
(Eq. 1),
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indicating that the polymer used for the formation of an adlayer of sufficient thickness would require a minimum molecular weight of 40,000 Da for the alumina particles, but only of about 3,000 Da for the silica. For smaller particles, the minimum required stabilizing layer thickness decreases substantially. Lu performed theoretical calculations of the particle-particle interaction energies of aqueous dispersions of Al2O3 nanoparticles with a particle size of 20-45 nm stabilized by a protective adlayer of an adsorbed polymer [32]. The distance-dependent steric interaction energy, which was calculated based on the work of Vincent and Edwards [33] assuming a uniform polymer segment density at small interparticle distances, as well as the total interaction energy are presented for various protective layer thicknesses in Figure 2. For this system, a protective layer of 5 nm in thickness was found to be more than sufficient to achieve stabilization of the particles, with repulsion steadily increasing as the particles approach each other, whilst for a layer thickness of 2.5 nm and less, the interparticle forces become dominantly attractive at low separation distance. The system with a stabilizing layer thickness of 2.5 nm however still shows an energy barrier of several k·T in total interaction energy at intermediate separation (Fig. 2, b). Consequently, a layer thickness of 2.5 nm should be sufficient to prevent coagulation of the particles, and these findings nicely correspond to experimental results of successful stabilization when using a polymer with a molecular weight of 1,800 g mol-1 [32].
Figure 2. Theoretical calculation of the dependence of the steric interaction energy Ester (a) and the total interaction energy ET (b) on interparticle distance for different values of polymer layer thickness. Reproduced from Ref. [32] with kind permission of Elsevier.
As the particle size is further reduced, successful stabilization is achieved with even smaller stabilizers. Here, an adlayer of sufficient thickness may even be formed by adsorption of olilgomers or even of common organic molecules [34]. Experimentally, the stabilization of such small nanoparticles by adsorption of simple organic molecules such as surfactants or complexating agents is indeed broadly applied. Many well-known and successful strategies to nanoparticle dispersions involve the use of surfactants as stabilizers, such as fatty acids or alkylammonium salts, for the preparation of dispersions of small nanoparticles, especially for metal nanoparticles [35-37]. In many cases, e. g. for the famous citrate stabilization technique for gold nanoparticles [38], it has been shown that in fact the binding of the utilized surface modifier results in the immobilization of charges to the particle surface, leading to stabilization via an essentially electrostatic rather than steric mechanism [39-41]. The preparation of magnetic fluids and other nanoparticle dispersions in hydrophobic media on the other hand prove the feasibility of steric stabilization via molecular species, as in these cases an electrostatic mechanism can be excluded due to the absence of charges. In order to
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distinguish the mechanisms of steric stabilization by adsorption of low-molecular weight species from polymer-based stabilization concepts, the term “small molecule stabilization” has been coined and shall also be used in this Chapter.
UTILIZATION OF SMALL MOLECULE STABILIZATION CONCEPTS FOR TAILORING THE PROPERTIES OF NANODISPERSIONS Even though the concepts of small molecule stabilization are hence in broad use for many applications, knowledge about the underlying fundamentals and mechanisms is by no means complete. Often the stabilization is applied in a trial-and-error fashion, without any general rational concept for a directed stabilization that would be optimized for a specific system. Additionally, the usual goal of a perfect stabilization in fact does not always represent the optimum case with respect to handling, processing and application issues. For example, in ceramic processing, but also in many other fields, weakly flocculated dispersions are highly advantageous [42, 43]. Such dispersions are easy to purify, due to the presence of larger structures that can be centrifuged or filtered, and afterwards can be restabilized via simple processes [44]. Therefore, concepts need to be developed that reach beyond the usual blackand-white image of stabilization or agglomeration, rather targeting improved control and even tunability of the particle interactions [45]. This offers the possibility of tailoring dispersions with respect to their rheological properties [46, 47], and it has been shown that also processes such as the fractionation and size selection of particles are greatly facilitated or even only made possible through customization of the particle interactions [48]. One of the most straightforward strategies to tailor the particle interactions for a given material and particle size is an adjustment of the thickness of the stabilizing steric layer. It is generally known that a thinner stabilizing layer, equivalent to smaller chain length or lower molecular weight of the stabilizer, results in increased attractive interaction between the particles, whereas a thicker layer vice versa leads to better shielding of the core particles and thus to lower attractive particle interaction, although it needs to be stated that this rule-of-thumb implies analogous chemistry of the stabilizers which does not apply in all cases. However, it becomes clear that tailoring of the stabilizer and its size is an essential means to control and optimize the properties of nanoparticle dispersions [31, 44]. One important case illustrating the practical relevance of controlled stabilization is found in dispersions with high solid content, as used in ceramic processing. Here, solid bodies are formed from particle slurries via various shaping methods, followed by drying and calcination to result in a dense and dry rigid body. A high solid content of the slurry is generally desired to facilitate the drying process, leading to a lower risk of cracks and lower shrinkage. In the recent years, exact control of the size of the utilized particles and of their surface interactions has become more and more important for the fabrication of advanced ceramics with enhanced properties [49, 50]. One particular focus has been the emerging field of nanoceramic processing, where nanostructured ceramic bodies are obtained through special processing strategies [51, 52]. For the fabrication of such ceramics, two important requirements are a high solid content to allow fast and facile drying, and on the other hand a relativley low viscosity in order to enable processing of the slurry. Both of these requirements strongly depend on the stabilizing layer thickness. The maximum solid content that can be achieved in
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stable slurry is realized when the particles are packed in an optimized fashion. As illustrated in Figure 3, the packing of small particles is however strongly influenced by the adlayer thickness Δ. For a large Δ in the range of the particle radius, within a given volume less particles can be packed (solid lines) as compared to small Δ (dashed lines). This can also be expressed as the effective volume fraction Φeff which is based on the entire particle including the adlayer and is related to the true volume fraction Φ of the core particles by
Φ eff
Δ⎞ ⎛ = Φ ⎜1 + ⎟ r⎠ ⎝
3
(Eq. 2),
where r refers to the particle radius [53]. Hence, the maximum achievable solids volume fraction based on the core particles increases as Δ is decreased. Vice versa, the maximum allowable layer thickness for reaching a random close packing state (Φrcp=0.64) at a given volume fraction Φ can be calculated from [54]
⎡ Φ rcp ⎤ Δ max = r ⎢3 − 1⎥ ⎢⎣ Φ ⎥⎦
(Eq. 3).
Figure 3. Illustration of the packing of spherical particles dependent on the thickness Δ of a stabilizing layer [54]. Image reprinted with kind permission of Elsevier.
On the other hand, the adlayer thickness has also an influence on the viscosity of the dispersion. It is generally known that the viscosity η of a particle dispersion depends on its solids volume fraction Φ. The particles start to form a network as Φ is increased, which leads to an increase in relative viscosity ηr as described in the modified Krieger-Dougherty equation
η ⎡ Φ ⎤ η r = = ⎢1 − η l ⎣ Φ max ⎥⎦
−n
(Eq. 4),
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where ηl is the viscosity of the liquid medium, Φmax is the maximum filling level at which any shearing is blocked, and n is a fitting parameter [55, 56]. The combined effect of a decrease in dispersion viscosity together with higher maximum loading levels creates a strong impetus for investigations on the influence of stabilizer size on the dispersion properties [43]. Due to the high practical relevance of the rheological properties of suspensions and slurries, it is thus no surprise that the tailoring of interparticle properties by variation of adlayer thickness has been increasingly studied. Most studies were directed towards ceramic processing and utilized metal oxide nanoparticles in the 50 nm size range that by now are also commercially easily available. As an example, the validity of the modified KriegerDougherty equation (Eq. 4) for highly concentrated suspensions of nanoscale ZrO2 particles about 40-50 nm in size was investigated by Renger et al. [54]. The authors used different stabilizers, including triammonium citrate (TAC) and 2-(2-(2-methoxyethoxy)ethoxy)acetic acid (TODA). The relative viscosity ηr of suspensions stabilized with each stabilizer was thereby determined for various high solid contents, as shown in Figure 4. It is clearly visible that TODA is capable of stabilizing dispersions with much higher volume fraction of solids than TAC. The authors applied fittings to the obtained data points utilizing the modified Krieger-Dogherty equation, and it is clearly visible that the TODA-stabilized system follows this equation (Φmax = 0.43, n = 8.24). For the TAC stabilizer, the fit is rather bad, indicating that the particles in this case probably were not well-dispersed, which agrees with the observed higher viscosity of dispersions of TAC-stabilized zirconia nanoparticles as compared to TODA-stabilized dispersions at the same solids content. Moreover, due to the smaller thickness of the repulsive barrier layer, the maximum achievable filling level for TODA is much higher as compared to TAC, both experimentally and in theory [54]. This example illustrates that the suitability of a stabilizer cannot only be judged on the size of the repulsive layer that is formed upon its adsorption, but also other factors such as the adsorption strength and affinity play a key role.
Figure 4. Relative viscosity of suspensions of ZrO2 nanoparticles stabilized with TODA (♦) and TAC (▲) vs. solid content. The experimental data were fitted using Eq. 4 (---) [54]. Image reprinted with kind permission of Elsevier.
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Studart and Gauckler performed extensive studies on the stabilization of 65 nm-sized Al2O3 nanoparticles, and first investigated the minimum adlayer thickness sufficient for stabilization to then be able to prepare dispersions with highest solids concentrations [57, 58]. They stabilized the alumina particles with alkyl chains as well as polyether chains that were linked to the particle surface in a very stable and controlled fashion via a gallol linker. The minimum thickness of the stabilizing shell was determined to 3.6 nm, corresponding to a molar weight of about 500 g mol-1; smaller stabilizers resulted in agglomeration of the nanoparticles and a significantly higher viscosiy of the medium [57]. Optimization of the stabilizing shell allowed the authors to prepare suspensions with volume concentrations above 40 % [58]. However, it was also demonstrated that the nature of binding to the nanoparticles is of crucial importance to stability: besides the gallol linker, also poly(methacrylic acid) was used as linker, and additionally purely electrostatic stabilization was employed (Figure 5). Whereas under certain conditions, the poly(methacrylic acid)-coupled polymer provided good stabilization, and also the electrostatic stabilization mechanism was efficient at low salt concentrations, only the strongly binding gallol-coupled polymer stabilizers provided high stability at all tested conditions. These results by Gauckler et al. show that only a strong, in the best case covalent linkage of the stabilizer can ensure stability in a broad range of salt concentrations, pH levels, temperatures, etc.
Figure 5. Stabilization of alumina nanoparticles with PEG chains via gallol linkers (a), with block copolymer chains (b), or electrostatic stabilization (c); left: schematic, right: obtained particle size distributions at various salt concentrations. Reprinted from Ref. [58] copyright 2007 American Chemical Society.
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Aside from aqueous and hydrophilic systems, also the rheology of suspensions of alumina nanoparticles in hydrophobic organic solvents has been of high interest [53, 59, 60]. Again, slurries with high solid contents but low viscosity are desired, which also sparked investigations as to the optimization of the utilized stabilizers. Bell et al. performed extensive tests of the rheology of suspensions of alumina nanoparticles in decalin (cis/transdecahydronaphthalene). Various fatty acids were employed as stabilizers, and the authors clearly observed a dependence of the stability on the chain length of the used stabilizer (Figure 6). Propionic up to heptanoic acid were found not to create sufficiently thick layers, leading to strongly flocculated systems with elastic properties, whereas the use of oleic acid resulted in only a very week overall attraction between the particles, thus creating stable fluid-like dispersions [53].
Figure 6. Interparticle total energy potential for alumina nanoparticles stabilized with various fatty acids in decaline (a…particle radius). Reprinted from ref. [53] with kind permission of Elsevier.
An interesting schematic by Studart et al. illustrating the general correlation beween adlayer thickness and the maximum achievable volume fraction is depicted in Figure 7. At small stabilizing layer thickness, the particles can be regarded as attractive spheres, with strong attraction beween the particles. As the adlayer thickness increases, the attraction is diminished, leading to higher volume fractions that can be realized for a given viscosity. As the adlayer thickness reaches and then exceeds a critical value, full stabilization is realized, and the particles can be considered as hard spheres. However, the increasing exclusion volume results in a decrease in maximum volume fraction. This nicely illustrates that indeed, an optimum adlayer thickness needs to be found if one wants to realize concentrated ceramic suspensions with low viscosity.
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Figure 7. Volume fraction of Al2O3 nanoparticles resulting in a relative viscosity ηr of 1000 in dependence of adlayer thickness. Image reproduced from Ref. [58] copyright 2007 American Chemical Society.
SMALL MOLECULE STABILIZATION OF ULTRASMALL NANOPARTICLES: PROBING THE LIMITS Within the world of nanoscale objects, smallest nanoparticles in the size regime below 10 nm play a special role [61]. In this size range, a significant amount of atoms is located at the particle surface, resulting in extreme surface energies, and additionally quantum effects start to emerge [62, 63]. Also dispersions of such ultrasmall nanoparticles are unique, in that they have the appearance of molecular solutions even at high solid concentrations due to the small dimensions of the particles, bringing about very low light scattering and low viscosity. This renders the dispersions of ultrasmall nanoparticles difficult to characterize with regard to the nanoparticle properties, but on the other hand leads to exceptional and fascinating properties that enable numerous new applications. Therefore, the preparation of such dispersions by stabilization of ultrasmall nanoparticles is of special importance. Again, the elaboration of rational concepts for the stabilization, ideally being able to specifically tailor the particle interactions to realize desired properties, is an ambitious but highly important goal. For many applications, especially in the optical field, agglomeration of the nanoparticles is practically not tolerable, whereas for others, weakly flocculated systems are advantageous also in this case. The stabilization of nanoparticles in the size range below 10 nm on the other hand is still also of fundamental interest. As stated before, all calculations based on the colloidal models of particle interaction predict that the thickness of any stabilizing adlayers would now lie in the Ångström regime and could in principle be reached already by the adsorption of organic solvent molecules. The colloidal
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models are, however, based on assumptions that reach their limitations when the particle size approaches the lower nanometer regime, such as treatment of the solvent as continuum [64] or approximating the surfaces of polymer-coated particles as planes [34]. Recent theoretical work by Roan and Kawakatsu on the interaction of nanoparticles with end-grafted homopolymers challenges the results of earlier models of steric stabilization such as the Edwards model [16], suggesting attractive forces between particles at large interparticle distance, repulsive forces at intermediate distance, and again attractive forces at small separation [65-69]. Even though the stabilization of ultrasmall metal nanoparticles with small organic species has already been applied since the early days of colloids chemistry, the stabilization of metal oxide nanoparticles at the frontier to the molecular scale is still only partially explored. Although numerous reports deal with the preparation of dispersions from nanoparticles, the specific study of the interactions between stabilizer and the nanoparticle surface, as well as of their precise effect on the particle agglomeration and disagglomeration and the general influence of stabilizer size and geometry, has not been a strong focus of research, much in contrast to the particle synthesis itself. Moreover, due to the general lack of suitable materials with required homogeneity, the systematic and reliable study of size effects on the particle stabilization and the elucidation of fundamentals have proven difficult. To the knowledge of the author, no studies exist so far as to the effects of a systematic variation of stabilizer size and chemistry on the properties of small nanoparticle dispersions. Only a small number of studies have been explicitly devoted toward the investigation of specific aspects of the stabilization of ultrasmall nanoparticles, and some examples are presented below to highlight recent findings in this area. Most of these investigations were performed on metal oxide nanostructures due to their high application relevance. Of these systems, titania nanoparticles certainly represent one of the most explored systems, due to their high application potential especially in photocatalysis. Numerous synthesis approaches have been reported, ranging from gas-phase pyrolysis to solvent-based approaches, especially aqueous and nonaqueous sol-gel chemistry, and mainly lead to ultrasmall nanoparticles only a few nm in diameter. The further processing of these nanoparticles requires the preparation of particle dispersions, and therefore it is no surprise that also the stabilization of these particles against agglomeration has been a focus of research. TiO2 nanoparticles used for the preparation of aqueous dispersions are usually charge-stabilized, whilst diverse strategies are pursued for their stabilization in organic media. In the latter case, most often organic polymers or surfactants are added to adsorb to the nanoparticles, for example fatty acids [70, 71]. In basically all cases, this results in substantial deagglomeration of the nanoparticles, however in the most cases no full stabilization, equivalent to the exclusive presence of individual nanoparticles, is observed. The reason for this might in fact be an only partial suitability of the stabilizer, which forms an insufficiently thick shell to be able to prevent attraction between the particles, or even induces these interactions, or shows insufficient binding, thereby not forming a compact and uniform shell. Not many studies are dedicated to a detailed investigation on these mechanisms, whilst generally a still very empirical approach is pursued here, often solely being based on trialand-error principles [72].
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Figure 8. TEM images of TiO2 nanoparticles without stabilizer (a) and stabilized with both propionic acid and n-hexylamine (c); the inset images (b) and (d) present the corresponding high-resolution images of one particle [73]. Reproduced with kind permission of Elsevier.
Further insight into the stabilization of small nanoparticles can be gained by looking at a very interesting study by Nakayama and Hayashi, who investigated the stabilization of TiO2 nanoparticles prepared via a chemical method. Their aim was to realize good stability of the nanoparticles using organic surfactants of the smallest possible quantity in weight by reducing the chain length of alkyl surfactants [73]. Thereby, the binding of the surfactants to the nanoparticle surface turned out to be a crucial aspect: the authors identified the surface of titania to show different acid sites, both Lewis acid sites (Ti+) and Brönsted acid sites (Ti– OH). Therefore, a combination of two stabilizers, both a carboxylic acid and an alkyl amine, was used in order to have binding and saturation of both sites. Indeed, when applying a twostep approach with both classes of stabilizers, much better stabilization was achieved as compared to the individual agents. Figure 8 shows images of transmission electron microscopy (TEM) indicating that the individual nanoparticles, about 5 nm in size and highly crystalline, are strongly agglomerated after the synthesis, whereas after the treatment they are much more evenly distributed on the TEM grid after drying, indicating their lower agglomeration state and lower attractive interaction. This is also evident in photographs of dispersions of the stabilized nanoparticles in various solvents (Figure 9). Due to the formation of hydrochloric acid as side product of the synthesis, a good dispersion of the nanoparticles in water is achieved even in the absence of stabilizers (a). In this case, the mechanism of stabilization is clearly of electrostatic nature. Such “bare” TiO2 nanoparticles however cannot be stabilized at all in organic solvents, necessitating the use of a stabilizer. Whereas the sole use of hexanoic acid (b, c) and n-hexylamine (d) however resulted in significant turbidity, the two-step strategy employing a mixed stabilization led to a completely transparent dispersion (e), proving that the utilized combination of stabilizers strongly reduces or even fully prevents the agglomeration. An important conclusion from this work therefore must be a careful study of the various surface sites of the particles, as the use of a stabilizer only binding to one particular type of site may result in an insufficiently dense stabilizing layer.
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Figure 9. Photograph (top) and UV-vis spectra (bottom) of bare TiO2 nanoparticles in water (a) and of TiO2 nanoparticles stabilized with alkyl carboxylic acid in methanol and chloroform (b, c) and with an n-alkylamine (d) as well as the mixed stabilizers in chloroform (e). Reproduced from Ref. [73] with kind permission of Elsevier.
As mentioned in the Introduction section, the adsorption of stabilizers to achieve stabilization of particle dispersions can not only be carried out in a separate step, but also already during the particle synthesis (in-situ modification or in-situ functionalization), which
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can be especially beneficial for small nanoparticles with their extremely high specific surface area. For titania nanoparticles, thorough studies on the in-situ modification with organic adsorbands were pursued, not only with the goal of optimizing the particle stabilization, but also as this is a means to alter and enhance the photocatalytic properties [74-76]. It has been shown that special ligands can form charge-transfer complexes with titania surface species, which results in a shift of the absorption edge towards the visible range and thus, a change in color of the material [77]. In these studies, ortho-catechols were identified as an especially suitable class of ligands, because they bind to the titania surface in a very stable fashion due to the confined geometry of the two hydroxyl groups [77]. Usually, they cannot be replaced by other ligands and thus can be considered as covalently bound [78]. This brings about a great advantage, as the stabilization of titania nanoparticles modified with such ligands can be studied independently of the effect of ligand affinity or exchange. Niederberger and the author utilized this concept to perform studies of the dependence of stabilization on the chemical structure of the stabilizer [78]. It was shown that via modification of the side chain of catechol ligands, the stabilization of the nanoparticles could be optimized for a specific solvent. By using an aminoethyl sidechain, the nanoparticles were tailored to form highly stable aqueous dispersions, being redispersible even after full drying; when employing more hydrophobic side groups, transparent dispersions were formed in various organic solvents [78]. Figure 10 shows a photograph depicting dispersions of nanoparticles capped with two kinds of catechol species in the respectively suitable solvent. It however also needs to be stated that due to the employed synthesis conditions, also a high quantity of charges (protons) was located on the nanoparticle surface, allowing the preparation of aqueous dispersions also in the absence of ligands under certain pH conditions [79, 80].
Figure 10. Titania nanoparticles modified with various catechol ligands as stabilizers in the dry state (a) and dispersed in water (b) and tetrahydrofuran (c). Reproduced with alterations from Ref. [78] copyright 2004 American Chemical Society.
Stabilization studies have also been carried out with other metal oxide nanoparticles. Due to the demanding requirements as to transparency and homogeneity of nanocomposites for optical applications, which are greatly determined by the agglomeration state of the
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nanoparticles, the stabilization of nanoparticles of high-refractive index materials such as zirconia is of high practical relevance. The author has recently performed an extensive study of the stabilization of ultra-small zirconia nanoparticles about 3 nm in size that were utilized for the fabrication of holographically structured optical devices [81]. The particles were prefabricated via the benzyl alcohol-based nonaqueous synthesis strategy, which represents one convenient route to highly crystalline, well defined and homogeneous metal oxide nanoparticles and nanostructures [82, 83]. The synthesis was performed in the absence of stabilizers; however the used solvent was detected to bind to the particle surface to a considerable extent. As the solvent has insufficient stabilizing properties, the particles were initially obtained as weak agglomerates, but could easily be stabilized via the addition of a suitable stabilizer. Stabilization experiments in polar organic solvents such as THF revealed that silanes are highly suitable for particle stabilization due to their stable linkage to the particle surface, and the results suggested that the stability of the formed dispersion can be correlated to the amount of stabilizer adsorbed to the particle surface [84]. Investigations in hydrophobic organic solvents on the other hand mainly were focused on long-chain fatty acids, the addition of which resulted in instant stabilization and even disintegration of previously formed agglomerates without any need for additional mechanical treatment, as is illustrated in the photograph and the TEM micrographs presented in Figure 11. The nanoparticles could be redispersed in very small volumes of solvent, which resulted in concentrated dispersions reaching up to 200 mg mL-1 solids content whilst being completely transparent. The TEM image of a dried dispersion (Figure 11, b) proves that the particles are completely free-standing in the dispersions, and the repulsive forces even prevent their reagglomeration on the sample grid upon drying [81].
Figure 11. Photograph (left) and TEM images (right) of ZrO2 nanoparticles before (a) and after (b) stabilization with oleic acid. Images taken from Ref. [81] with kind permission of Wiley-VCH.
In this system, the adsorption of the stabilizer was characterized throroughly employing various spectroscopic techniques. Especially, 1H and 13C liquid state NMR spectroscopy proved as a useful probe for the surface chemistry of nanoparticles in concentrated dispersions, as species adsorbed to the surface can be identified, however the functional groups directly adjacent to the surface are motionally hindered, which results in spectral broadening [85]. It is hence possible to assess the amount of surface-bound species, determine the functional groups binding to the particle surface, and qualitatively investigate the chemistry of both particle surface and bulk solution. In the zirconia case, it was detected that indeed only partially the initially bound benzyl alcohol solvent is replaced by the stabilizer
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arachidic acid (Figure 12, a). The stabilizer is bound via a deprotonated carboxylic acid functional group, and no free arachidic acid was detected. A quantitative evaluation of the adsorbed species was possible via complementary TGA measurements (Figure 12, b), and revealed further interesting details of the stabilization process. For example, it turned out that the addition of higher amounts of stabilizer did not increase the total amount of organics on the particle surface but merely led to an increased exchange of the arachidic acid stabilizer with previously adsorbed benzyl alcohol, indicating that after the initial synthesis, the particle surface is saturated with benzyl alcohol [81].
a)
b)
Figure 12. Characterization of the binding mode of fatty acid stabilizers to the surface of ZrO2 nanoparticles can be carried out via 13C NMR spectroscopy (a; BA=benzyl alcohol) and thermogravimetric analysis (b; AA=arachidic acid, HD=hexadecanoic acid, OD=octadecanoic acid, OA=oleic acid). Image (a) reproduced from Ref. [81] with kind permission of Wiley-VCH; image (b) is courtesy of the author.
CONCLUSION In this Chapter, the concept of small molecule stabilization of inorganic nanoparticles has been introduced. It has been shown that whilst the steric stabilization of colloids by adsorption of polymers has been intensively studied both experimentally and in theory, the stabilization of nanoparticles is a much less understood process. The established theories of colloidal stabilization, based on electrostatic or steric stabilization mechanisms, can also be applied for nanoparticles, revealing that already small adsorption layers created by small molecules rather than polymers can suffice to achieve stabilization, which has also been confirmed by experimental results. Indeed, practical experience even in the early days of colloids chemistry has already shown that the small molecule stabilization of nanoparticles is a feasible and highly advantageous strategy. Despite this long-time utilization, only selective aspects of the precise influence of the stabilizer on the microscopic (particle size and agglomeration) and macroscopic (rheology) properties of nanoparticle dispersions have however been studied in more detail. For larger nanoparticles in the 50 nm size range, investigations have been performed by several groups in the last years, showing the applicability of concepts that utilize a specific tailoring of stabilizer length and chemistry to achieve particle dispersions with tunable rheological properties.
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For small nanoparticles in the 10 nm size range that are of key interest for novel applications, only some aspects of small molecule stabilization have been explored so far, such as the advantage of using mixed stabilizers that can provide a true, dense particle adlayer via the specific saturation of several different surface sites, or the applicability of a “twoface” concept where double-functional stabilizers are used, featuring a head group that selectively and in a stable manner can bind to the particle surface, and a tail that is specifically adapted to be compatible with the solvent. Many other fundamental aspects still remain unclear, as it is known that the colloidal theories reach their limitations precisely in the low nanometer size regime, with novel studies predicting severe deviations and a higher interaction complexity when applying more elaborate and realistic models. Moreover, smallest nanoparticles are on the limit to the molecular scale, and the general question arises as to the validity of the conventional concepts of stabilization for such structures. It remains to be seen whether it will be possible to bridge the gap between theoretical explanation and practical experience in this field, whilst in view of the high relevance of particle stabilization for almost all areas of applications, the development of rational concepts for tailoring the particle interactions would be highly desirable and will certainly create a strong research impetus in the future as small nanoparticles in high quality and homogeneity will become increasingly available.
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In: Encyclopedia of Polymer Research Ed: Caroline E. Jones
ISBN: 978-1-61761-926-7 © 2011 Nova Science Publishers, Inc.
Chapter 5
MOLECULAR IMPLICATIONS IN THE SOLUBILIZATION OF THE ANTIBACTERIAL AGENT TRICLOCARBAN BY MEANS OF BRANCHED POLY (ETHYLENE OXIDE)-POLY (PROPYLENE OXIDE) POLYMERIC MICELLES Diego A. Chiappetta1,2, José Degrossi3, Ruth A. Lizarazo4, Deisy L. Salinas4, Fleming Martínez4 and Alejandro Sosnik1,2* 1
The Group of Biomaterials and Nanotechnology for Improved Medicines (BIONIMED), Department of Pharmaceutical Technology, Faculty of Pharmacy and Biochemistry, University of Buenos Aires, Buenos Aires, Argentina 2 National Science Research Council (CONICET), Buenos Aires, Argentina 3 Department of Toxicology, Faculty of Pharmacy and Biochemistry, University of Buenos Aires, Buenos Aires, Argentina 4 Department of Pharmacy, Faculty of Sciences, National University of Colombia, Bogotá, Colombia
ABSTRACT Aiming to gain further insight into the complexity of drug/polymeric micelle interaction phenomena, the present chapter investigated the incorporation of the poorly water-soluble topical antibacterial agent triclocarban (TCC) into polymeric micelles of the branched pH/temperature-responsive poly(ethylene oxide)-poly(propylene oxide) block copolymers Tetronic® 1107 (MW = 15 kDa, 70 wt% PEO) and 1307 (MW = 15 kDa, 70 wt% PEO). Solubility extents showed a sharp increase of up to 4 orders of magnitude. Due to the pH*
Corresponding author: Department of Pharmaceutical Technology, Faculty of Pharmacy and Biochemistry, University of Buenos Aires, 956 Junín St, 6th Floor, Buenos Aires CP1113, Argentina, Phone #: 54-11-49648273, Fax #: 54-11-4964-8273, Email:
[email protected].
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dependent character of both the carrier and the drug, studies were performed under different pH conditions. Due to a more efficient poloxamine aggregation at higher pH-values, a clear increase in the solubilization capacity was apparent under these conditions. However, ionization of TCC at pH 12.7 constrained the formation of hydrogen bonds between the urea moieties and the polyether chain, leading to a decrease in solubility above this pH. The size and size distribution of drug-loaded micelles was evaluated by Dynamic Light Scattering (DLS). Findings indicated the increase in the size of the aggregates with the incorporation of the drug. The morphology of the nanostructures was visualized by transmission electron microscopy (TEM). The stability of the systems over time was also evaluated. Finally, the antibacterial activity of different TCC/poloxamine complexes was assayed on different bacteria collections. For example, while a poloxamine-free TCC aqueous solution (pH 7.4) was not effective on Staphylococcus aureus, a 10% drug-containing T1307 system inhibited the bacterial growth to some extent. These results supported the release of the drug from the polymeric reservoir. However, as opposed to previous reports, overall findings indicated the limited intrinsic activity of TCC against the investigated pathogens.
1. INTRODUCTION Adhesion of bacteria to the surface of biomedical devices, tissues and working areas and the later progression of the colonization process involves a first stage (phase one) of physical and reversible surface/pathogen interactions [1]. Then, a second stage (phase two), entails a permanent time-dependent adhesion and the secretion of a slimy extracellular matrix known as biofilm [2,3]. Microorganisms embedded in a biofilm frequently display higher minimal inhibitory concentrations (MIC) and resist common antibiotic chemotherapy [4,5]. This phenomenon would stem from a modified metabolism. Biomaterial-centered infections (BCIs) originate in the colonization of biomaterial-made devices by bacteria and the generation of a biofilm [3,6]. Among the microorganisms, Staphylococcus epidermidis and other staphylococci were indicated as highly prevalent actors in the development of BCIs [4,7,8]. Generation of biofilm by antibiotic-resistant clinical strains (e.g. methicillin-resistant staphylococci or MRS and vancomycin-resistant enterococci or VRE) plays also a central role in the spreading of hospital-acquired infections [9,10]. The low aqueous solubility displayed by more than 50% of the FDA-approved drugs constitutes a main obstacle towards the development of formulations for oral, parenteral and topical administration [11,12]. Triclocarban (TCC, 3,4,4´-trichlorocarbanilide, see structure in Table 1) is a synthetic antibacterial agent used as antiseptic in cosmetic and health-consumer products [13,14]. It presents low acute and chronic toxicity. Due to its molecular nature, TCC remains unionized in a broad range of pH-values (pKa = 12.7). Contrary to triclosan (TS), another well-known topical and broadly used biocide, reports indicated that TCC displays a more limited activity (essentially against Gram positive microorganisms) [15,16]. The extremely low solubility of TCC in water (~50 ng/mL) constrains the development of water-based antibacterial TCCcontaining formulations [17]. In order to enhance the soubility of TCC, different approaches have been investigated [18]. For example, complexation of the drug with different cyclodextrins (CDs) led to good solubility improvements [19,20]. In this context, whether the
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low antibacterial activity stems from the intrinsic resistance to the drug or, by the contrary, from the extremely low concentrations attainable in aqueous media remains unclear. Polymeric micelles are nanostructures generated by the self-assembly of polymer amphiphiles in water [21]. Micelles are conformed by two zones: (A) an inner and hydrophobic core and (B) an outer and hydrophilic shell. The hydrophobicity of the core enables the inclusion and solubilization of poorly water-soluble molecules [22]. Solubilization is governed by two main parameters, namely the micellization ability of the polymer and the drug/micelle interaction. Poly(ethylene oxide)-poly(propylene oxide) block copolymers (PEO-PPO) are one of the most extensively investigated groups of micelleforming biomaterials [23,24]. A number of advantageous features support their application: (A) a broad range of commercially available molecular weights and EO/PO ratios, (B) a proven cell and tissue-compatibility and (C) the approval of several derivatives by different regulatory agencies (e.g., FDA, EMEA) as additives in pharmaceutical and cosmetic products [25-27]. The most thorough work was performed on micelles of the linear PEO-PPO-PEO poloxamers (Pluronic®) [24]. On the other hand, the 4-arm poloxamines (Tetronic®) present a central ethylenediamine segment that renders the molecule two tertiary amine moieties. Thus, molecules are both temperature and pH sensitive [28]. Also, amine groups enable further chemical modification to produce more cell-adhesive substrates [29]. Our research group dedicates efforts to investigate the application of polymeric micelles as nanocarries for poorly-water soluble biocides in the context of hospital biofilm prevention and treatment. In a recently published work, we investigated the inclusion of TS into polymeric micelles of the poloxamine Tetronic® 1107 at different pH-values [30]. Solubility levels increased up to 4 orders of magnitude. Microbiological assays demonstrated that TSloaded micelles were active against a broad spectrum of pathogens, including MRS and VRE. Moreover, TS/poloxamine systems were effective against Staphylococcus epidermidis biofilm. Aiming to gain further insight on the molecular aspects controlling the solubilization process of TCC the present chapter reports on the inclusion of the drug into polymeric micelles of two branched PEO-PPO block copolymers, Tetronic® 1107 and 1307. The size and size distribution of drug-loaded micelles were studied by Dynamic Light Scattering (DLS) and the morphology of the aggregates visualized by Transmission Electron Microscopy (TEM). Finally, the antibacterial activity of TCC-containing systems was investigated on different collections and clinical pathogens.
2. MATERIALS AND METHODS Materials. Tetronic® T1107 (MW = 15 000 Da, 70 wt% PEO, HLB 18-23) and 1307 (MW = 18 000 Da, 70 wt% PEO, HLB > 24) were a kind gift of BASF (NJ, USA). Triclocarban (TCC, Sigma, St. Louis, MO), KH2PO4 (Carlo Erba, Italy), KCl (Merck, Germany), NaOH (Merck, Germany) and concentrated HCl (Anedra, Argentina) were used as received. Buffers of pH 2.0, 5.8 and 7.4 were prepared in distilled water according to the USP 24 edition. Buffer pH 12.7 was prepared using Na2HPO4 (Merck, Germany) and the pH adjusted with NaOH 0.2M.
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Thermal analysis. In order to establish the enthalpy of fusion of TCC (ΔHf) a sample (~ 5 mg) of the drug was sealed in a 40 μL Al-cruicible pan and analyzed by Differential Scanning Calorimetry (DSC, Mettler TA-400 differential scanning calorimeter) in a single heating ramp (25 to 300oC, 10oC/min.). The value of enthalpy was normalized to the weight of the sample and expressed in kJ/mol. Preparation of poloxamine micelles. Poloxamine micellar systems (1 to 10%) were prepared by dissolving the required amount of polymer in the different media at 4oC. Systems were equilibrated at 23oC, 24 hours prior to use in order to allow the formation of the micelles. Concentrations are expressed in % w/v. Preparation of TCC-loaded micelles. TCC (in excess, ~ 20 mg/mL poloxamine solution) was added to T1107 or T1307 micellar systems (3 mL, 1 to 10% w/v) in dark glass vials (10 mL) and sealed appropriately with Parafilm®. Specimens were vigorously shaken (48 h) in a temperature-controlled horizontal shaker at 23oC (Minitherm-Shaker; Adolf Kuhner AG, Switzerland). Suspensions were filtered through clarifying filters (0.45 μm, cellulose nitrate membrane, Microclar, Argentina) and dried in a vacuum oven at room temperature. Dry samples were re-dissolved in ethanol and the concentration adjusted as required. Drug concentrations were determined by measuring the absorbance in a UV spectrophotometer (265 nm, CARY [1E] UV–Visible Spectrophotometer Varian, USA) at 23oC using a calibration curve of TCC solutions in ethanol covering the range between 2.7 to 13.7 μg/mL (correlation factor was 0.9985). Mean concentrations are expressed in μg/mL ± S.D. (n = 3). Ethanol was used as blank. Molar solubilization ratios (MSR) were calculated by ratioing the molar concentration of the drug by the molar concentration of the polymer, at every condition. Measurement of the micellar size by Dynamic Light Scattering (DLS). The average hydrodynamic diameter of drug-loaded poloxamine micelles was measured in a Zetasizer Nano Series (Zetasizer Nano-Zs, Malvern Instruments, UK) provided with a He-Ne (633 nm) laser and a digital correlator ZEN3600. Measurements were performed in PMMA disposable cuvettes at 23°C. Samples were filtered by clarifying filters (0.45 μm) prior to the assay. Results are expressed as the average of five measurements. Visualization of the drug-containing micelles. In order to investigate the morphology of drug-containing micelles, samples were visualized by Transmission Electron Microscopy (TEM, EM 109T Zeiss Transmission Electron Microscope, Karl Zeiss, Berlin, Germany). Briefly, TCC-containing poloxamine systems were placed on grids covered with Fomvar film and stained with 2% w/v phosphotungstic acid solution in water. Then, samples were dried in a closed container with silicagel and observed under the microscope. Stability of the TCC-loaded micelles upon storage. In order to study the stability of the drug-containing micelles along the time, specimens previously prepared were stored at room temperature for 1 month and the concentration of TCC in the different samples determined by UV (see above). Samples were worked up as previously described. Results of % of remaining TCC (% TCC) are expressed as Mean ± S.D. (n = 3).
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Antibacterial activity of TCC/poloxamine systems. Antibacterial evaluation of different TCC-containing T1107 and T1307 micelles (pH 7.4) was evaluated in plate experiments. Bacteria were cultured on Tryptone Soy Agar (TSA, Britannia, Argentina) plates and were incubated at 35oC for 24 h. Then, microorganisms were removed from the isolation medium, suspended in 0.9% NaCl to a final concentration equivalent to an optical density of 0.6 (at 600 nm) and diluted 1/10 (0.9% NaCl). Suspensions (0.1 mL) were diluted in cold molten TSA (10 mL) containing 2,3,5-triphenyltetrazolium hydrochloride (TTC, 0.007% final concentration, Sigma, USA), plated in sterile Petri dishes and allowed to solidify. Paper discs (6 mm diameter) were embedded in the corresponding samples, namely, poloxamine-free saturated TCC solution in buffer (pH 7.4, control), drug-free poloxamine T1107 and T1307 micelles (blank) and TCC/poloxamine systems, incubated (35oC, 24 h) and the inhibition zone in disc diffusion tests was measured. Results are expressed in diameter. Inhibition zones of <6 mm indicate no antibacterial activity. Table 1. Physicochemical properties of triclocarban (TCC).
Molecular structure
X 2id /
M/ g⋅mol–1
ΔHfus / kJ⋅mol–1
Tfus / K
x 10-3
Swater / μg/mL
315.6
41.94
528
4.25
0.049
X2/ x 10-9 2.86
0 0 ΔGsoln -ideal / ΔG Exc /
kJ mol-1
kJ mol-1
13.54
35.22
Table 2. Estimation of solubility parameters of triclocarban by means of Fedors and van Krevelen methods (a) TCC Group Disubstituted phenylene group Trisubstituted phenylene group –Cl >=O –NH–
(a)
Quant
Fedors V/ cm3 mol–1
Van Krevelen Fd / J1/2 cm3/2 mol–1
Fp2 / J cm3 mol–2
Uh / J mol–1
1
1 × 52.4
1 × 1270
1 × (110)2
-
1
1 × 33.4
1 × 1110
1 × (110)2
-
3 1 2
3 × 24.0 1 × 10.8 2 × 4.5 177.6
3 × 450 3 × (550)2 1 × 290 1 × (770)2 2 × (210)2 2 × 160 4340 1612800 δd = δp = ((1612800)1/2/177.6) (4340/177.6) = 7.15 MPa1/2 = 24.44 MPa1/2 δ = (24.442 + 7.152 + 7.282)1/2 = 26.48 MPa1/2
Calculated according procedures described by Barton [31].
3 × 400 1 × 2000 2 × 3100 9400 δh = (9400/177.6)1/2 = 7.28 MPa1/2
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TCC solubility (μg/mL)
700 600 500
2
400
5.8
300
7.4
200
12.7
100 0 0
2
4
6
8
10
12
T1107 concentration (% w/v) A
TCC solubility (μg/mL)
600 500 400 2 5.8 7.4 12.7
300 200 100 0 0
2
4
6
8
10
12
T1307 concentration (% w/v) B Figure 1. Sa of TCC versus poloxamine concentration at four different pHs. A) T1107 and B) T1307.
3. RESULTS AND DISCUSSION 3.1. Water-Solubilization of Triclocarban The goal of the present work was to investigate the molecular implications governing the solubilization process of TCC into the core of poloxamine polymeric micelles and the further understanding of the phenomena associated with the limited antibacterial performance of the drug.
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Table 3. Triclocarban/poloxamine molar ratios (MSR) in poloxamine systems between 1 and 10% at four different pH-values.
Poloxamine
T1107
T1307
Polymer concentration (%) 1 3 5 7 10 1 3 5 7 10
2.0
5.8
pH 7.4
12.7
13.88 5.55 3.55 2.74 3.09 21.16 9.32 6.89 8.62 20.90
7.76 5.69 4.63 4.84 6.02 10.04 6.03 7.63 24.44 67.39
30.18 11.49 20.42 42.60 196.08 20.22 99.54 166.46 264.07 289.39
11.74 8.75 12.27 24.03 60.66 17.17 36.90 119.50 103.24 241.57
The ideal solubility of a solute is controlled by the melting point and the enthalpy of fusion [32]. The molecular form of TCC could interact with solvents and among itself by hydrogen bonding, namely, as hydrogen-donor through the N-H groups present in the urea moiety, whereas it could act as hydrogen-acceptor through the carbonyl-oxygen in the same functional group. In addition it could interact through weak interactions, such as London dispersion forces due to the presence of highly hydrophobic aromatic rings. The ideal solubility of a crystalline solute in a liquid solvent can be calculated by Eq. 1:
ln X 2id = −
ΔH fus (Tfus − T ) ⎛ ΔC p + ⎜⎜ RTfusT ⎝ R
⎛ T ⎞⎤ ⎞ ⎡ (Tfus − T ) ⎟⎟⎥ ⎟⎟ ⎢ + ln⎜⎜ T T ⎠⎣ ⎝ fus ⎠⎦
(1)
Where, X 2 is the ideal solubility of the solute as mole fraction, ΔHfus is the molar enthalpy id
of fusion of the pure solute (at the melting point), Tfus is the absolute melting point, T is the absolute solution temperature, R is the universal gas constant (8.314 J⋅mol–1⋅K–1), and ΔCp is the difference between the molar heat capacity of the crystalline form and the molar heat capacity of the hypothetical super-cooled liquid form, both at the solution temperature. Since ΔCp cannot be easily determined, in this investigation it is assumed that ΔCp may be approximated to the entropy of fusion (ΔSfus = 79.4 J⋅mol-1⋅K-1). Values of ideal and aqueous solubility are presented in Table 1. Since the thermal behavior is a function of the solutesolute interactions in the crystalline lattice, the stronger these forces are, the more insoluble the solute is. As opposed to triclosan that melts around 56oC (and shows an enthalpy of fusion of 17.75 kJ/mol) [33], TCC displays an extremely high melting point around 255oC, supporting the presence of strong solute-solute forces. In order to characterize the ideal solubility of crystalline TCC, the enthalpy of fusion was determined by DSC. The high value observed (41.94 kJ/mol) supported the very poor solubility of the drug observed in water (~50 ng/mL) (Table 1). The low solubility of TCC was also supported by calculations according to the group contribution methods of Fedors and van Krevelen (Table 2). Based on the analogy
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that ‘like dissolves like’, the gap between the value of the parameter of solubility (δ ) of TCC (δ = 26.5 MPa1/2) and the one of water (δ = 48.0 MPa1/2) supported the limited drug solubility in aqueous medium. Even though some minimal solubilization improvement could be theoretically attained below the critical micellar concentration (by means of the interaction between isolated amphiphile molecules known as unimers and drug molecules), maximization of the inclusion phenomenon relies on the presence of the micelles. Due to their structure, poloxamines are both temperature and pH-responsive. In general, pKa values are between 4 and 6 for the first amine group and around 6-8 for the second [34]. Thus, depending on the pH of the medium, from diprotonated to unprotonated forms (going through a monoprotonated one) are available. When positively-charged (pH < pKa), central ethylenediamine groups repel each other and hinder micellization, leading to an increase in the CMC; repulsion is stronger for diprotonated than for monoprotonated forms. Once the molecule displays an uncharged form (at pH > 8), the CMC usually decreases sharply. Consequently, the efficiency of the solubilization usually increases at higher pH-values. On the other hand, the affinity of TCC/poloxamine affinity also plays a fundamental role. Thus, the present study focused on polymer concentrations above the CMC for both T1107 and T1307, in the whole range of pHvalues [30,34]. Two main factors influencing the aggregation phenomenon and the solubilization were evaluated: (A) the concentration of the polymer and (B) the pH. Due the high enthalpy of fusion shown by TCC and the tremendously low solubility in water, a more limited solubilization of the drug in the hydrophobic polymeric core in comparison to triclosan (a previously investigated biocide) was predicted. The apparent solubility (Sa) of TCC in T1107 and T1307 micellar systems (1 to 10%) saturated with the drug was determined at pH-values between 2.0 and 12.7 (Figure 1). Findings indicated a sharp increase as the concentration of both amphiphiles rose. For example, for T1107 at pH 7.4, Sa increased from ~ 50 ng/mL in poloxamine-free medium to 6.4, 21.48 and 412.6 μg/mL for 1, 5 and 10% systems, respectively. It is worth stressing that these results meant an increase in solubility of up to more than 3 orders of magnitude. A decrease in the pH (see pH-values between 2.0 and 7.4) hindered the micellization due to the repulsion between the positivelycharged ethylenediamine blocks and rendered lower Sa levels. These findings were in agreement with previous reports that indicated the concomitant decrease in solubility as the pH decreased [35]. Due to the unprotonated character of poloxamine molecules at pH 12.7, an even higher solubility was a priori envisioned. However, TCC contains a ureide moiety in the structure that displays the ability to form hydrogen bonds with the polyether chain. Ionization of this functional group at pH-values above the pKa (~ 12) is supported by the increase in the intrinsic solubility of the drug above this pH from 50 to 300 ng/mL. Thus, even if micellization was favored under these conditions, a more limited interaction between the drug and the polymer resulted in lower solubilization extents between 2.5 and 127.6 μg/mL for 1 and 10% systems, respectively. T1307 displays a higher molecular weight and a consequent enlargement of the micellar core (from about 4500 to 5400 Da) that could favor a more efficient solubilization. On the other hand, HLB data indicate that T1307 presents, overall, a more hydrophilic structure. Results with T1307 followed a similar trend for similar polymer concentrations and pH conditions; solubility gradually increased from pH 2.0 to 7.4 and then decreased at pH 12.7. It is worth remarking that in this case values increased utmost 4 orders of magnitude. The sharp increase in the apparent solubility of TCC as the pH rose can be
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further evaluated in Table 3, where calculations of MSR are presented. In general, values steadily increased in the pH range between 2.0 and 7.4 due to the higher aggregation tendency of the amphiphiles. Then, under more basic conditions (pH 12.7), where the drug was ionized, a sharp decrease was apparent. A similar trend was observed as a function of the polymer concentration in the range 3-10% and pH-values between 5.8 and 12.7. The analysis of (A) 1% systems and (B) specimens at pH 2.0 (at every concentration) was less straightforward. In both conditions, the apparent solubility (expressed in μg/mL) was relatively low. However, MSR values were, often, higher than those calculated for 3 and 5%, indicating a higher number of solubilized TCC molecules per poloxamine molecule. An interesting phenomenon is also the one observed for T1107 at pH 2.0 where MSR values gradually decreased between 1 and 7% and then showed a slight recovery for a 10% sample. T1307 (pH 2.0) showed a similar tendency though the recovery was apparent at a lower concentration: 7%. These results were particularly observed under conditions where micellization was strongly hindered due to (A) very low polymer concentrations slightly above the CMC (~0.5-1%) and (B) repulsion of poloxamine molecules derived from diprotonation. Previous investigations showed that the lower the pH the smaller the size of poloxamine aggregates is [35,36]. In the proximity of the CMC (1% systems) at pH 2.0, the presence of very small aggregates (including a high incidence of unimers in the composition) probably led to higher MSR values. This, even with very low Sa levels. Then, as a consequence of the increase in the polymer concentration, more micelles were generated and a sudden decrease in the molar ratio was found (even if higher TCC amounts were solubilized). Finally, additional increase in the poloxamine concentration led to a substantial recovery in MSR. These findings also suggested the presence of a phenomenon where larger micelles are formed, enabling the inclusion of a higher number of TCC molecules per polymer molecule. On the other hand, the solubilization ability of poloxamines for TCC was lower than the one found for TS [30]. These findings expectedly emerged from the stronger solute-solute forces showed by the former and a more limited tendency to interact with the liquid-like hydrophobic micellar core.
3.2. Effect of TCC on the Aggregation of Poloxamine Inclusion of TCC into the polymeric micelles was expected to affect the size and size distribution of the aggregates [37]. TCC-loaded micelles of 3, 5 and 7% poloxamine systems (at pH 7.4) were evaluated by means of DLS (Figure 2). An increase in the micellar size due to the enlargement of the core associated with the incorporation of the solute molecules could be anticipated [37,38]. Findings of T1107 and T1307 indicated the presence of two main populations (Table 2). Small size aggregates (~6-8 nm) were mainly consistent with the presence of regular drug-loaded micelles. However, this population decreased in intensity as the polymer concentration rose (and consequently the drug loading); the area of the small-size peaks was 82.9 to 56.5% for 3 and 7% samples, respectively, for T1107 micelles. By the contrary, for the same amphiphile, the presence of larger aggregates (~65-110 nm) suggested the enlargement of the micelles due to the incorporation of TCC into the core. In addition, the higher the polymer concentration was, the higher the incidence of this large-size fraction. Results for T1307 followed a similar trend as the recurrence of the large-size micelles increased for high polymer concentrations from 46.3 to 63.0% for 3 and 7% T1307 systems, respectively. On the other hand, these aggregates displayed a smaller size than those
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generated by T1107 and a slight decrease in the size of the aggregates was found as the polymer concentration rose. This behavior was probably associated with both the higher hydrophilicity and the larger shell of T1307 micelles that rendered more stable aggregates. It is worth mentioning that TCC/poloxamine systems were completely transparent and supported the fact that micellar fusion was not involved in the aggregates enlargement process [30]. Visualization of TCC-loaded systems by means of TEM is exemplified for 10% T1107 micelles in Figure 3. Findings showed the presence of spherical aggregates of different sizes and supported the DLS data. Also here, no evidence of secondary aggregation was found [30]. It is noteworthy that size determination by TEM was unreliable due to the nonrepresentative character of the sample.
12
Intensity (%)
10 8 3% 5% 7%
6 4 2 0 1
10
100
1000
Size (nm)
(A) 10
Intensity (%)
8 6
3% 5% 7%
4 2 0 1
10
100
1000
Size (nm)
(B) Figure 2. Micellar size and size distribution (% intensity) of TCC-loaded 3, 5 and 7% T1107 (A) and T1307 (B) systems.
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Figure 3. TEM micrographs of TCC-saturated 10% T1107 micelles negatively stained with 2% phosphotungstic acid at pH 7.4. Different sizes are apparent. Scale bar = 100 nm (microscope mag. = 100,000x).
Table 4. Micellar size and size distribution (% intensity) of TCC-loaded 3, 5 and 7% T1107 and T1307 systems Poloxamine
T1107
T1307
Poloxamine concentration (%) 3 5 7 3 5 7
Peak 1 Size (nm) %
Peak 2 Size (nm) %
8.03 6.89 6.51 8.05 6.97 6.05
89.81 64.11 112.7 76.4 54.3 50.3
82.9 68.8 56.5 52.2 53.3 37.0
17.1 31.2 43.5 47.8 46.7 63.0
3.3. Stability of the TCC-Loaded Micelles upon Storage Solubilization encompasses the inclusion of TCC molecules into the hydrophobic core of the aggregates. Usually, under regular storing conditions, temperature can fluctuate, affecting the micellization of the thermo-responsive poloxamines. In this context, a decrease in temperature would lead to the disassembly of the micelles and a decrease in the micellar concentration, often to concentration levels close or below the CMC. Moreover, according to
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the thermodynamic calculations (strong solute-solute interactions), TCC displays a high tendency to aggregate. Consequently, in the case of TCC/poloxamine complexes, a precipitation phenomenon and a titer loss over time was envisioned. Different drugcontaining specimens (1-10% of pH 7.4) were maintained at room temperature and the concentration of the drug determined after 1 month. A T1107 1% system displayed a sharp drug-load loss of about 64% and the appearance of insoluble drug crystals. Then, higher polymer concentrations improved the stability upon storage. For example 10% systems of T1107 and T1307 lost only 16.6 and 15.6% of the initial TCC loading, respectively. Also, a slight increase in stability was observed for T1307, consistent with its more stable structure in water due to a higher hydrophilicity [24]. It is remarkable that TCC/poloxamine complexes presented a more limited stability than the highly stable TS/poloxamine ones that remained almost unchanged for at least 3 months [30]. Solubilization relies on the generation of strong solute-solvent interactions in order to overcome solute-solute and solvent-solvent ones. The ΔHf displayed by TCC was substantially higher than the reported for TS and indicated much stronger solute-solute forces. Thus, as expected, weaker drug/core and stronger drug/drug interactions resulted in a higher sensitivity of the complex to small changes in temperature and a higher tendency of the drug to precipitate.
3.4. Antibacterial Activity of TCC/Poloxamine Systems According to previous reports, TCC displays a relatively limited antibacterial spectrum and effectiveness. Studies of the antibacterial activity of the complex aimed to explore if the low activity of the drug was an intrinsic limitation or, otherwise, it stemmed from the poor solubility attainable under regular conditions (e.g., in water). In contrast to a previously published study on TS where 5% polymer were used, herein, due to the more limited solubility of TCC and its lower antibacterial activity, more concentrated poloxamine solutions (10%) were tested. Plate assays indicated a slightly broadening of the inhibition area when Gram positive Staphylococcus aureus and methicilin-resistant Staphylococcus aureus (clinical strain) were exposed to the substantially higher biocide concentrations attained in micellar systems. On the other hand, findings stressed the substantially lower activity of this agent compared to the previously investigated TS and suggested that the poor antibacterial activity relied on the intrinsic resistance of the different pathogens to the antibacterial agent. These results contrasted with previous reports indicating minimal inhibitory concentrations of TCC in the 0.5-5 μg/mL range. In addition, inhibition for drug-loaded T1107 specimens was more limited probably due to the lower concentration attained.
4. CONCLUSION Inclusion of the poorly water-soluble drug triclocarban into poloxamine polymeric micelles enhanced the aqueous solubility in up to 4 orders of magnitude. In agreement with previous reports, higher solubilization extents were found at higher pH values in the range between 2.0 and 7.4. In contrast, a pronounced decrease in solubility was found at pH 12.7, regardless the more efficient micellization of the polymers under these conditions. This
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phenomenon could be explained by the pH-sensitive ionization of the -NH-CO-NH- moiety of TCC under this pH. At a lower pH (e.g., 7.4) where aggregation was less favored, unionized ureide groups formed H bonds with the poly(ether) chain and solubilization was enhanced. By the contrary, once unprotonated, TCC molecules displayed a weaker interaction with the polymer and a lower solubility was found. Finally, the activity of the complexes was assayed on a number of bacteria. Even though complexation slightly increased the effectiveness of the antibacterial agent under study, activity was dramatically lower than that demonstrated by TS. Overall findings supported the relatively limited activity of TCC even at concentrations significantly higher than those attainable in poloxamine-free media.
ACKNOWLEDGMENTS This work was partially supported by the University of Buenos Aires and the CONICET (Grant UBACyT-B424).
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[26] Kabanov, A. V. & Alkhov, V. Yu. (2002). Pluronic® block copolymers in drug delivery: From micellar nanocontainers to biological response modifiers. Critical Rev Therap Drug Carrier Syst, 19, 1-72. [27] Kibbe, A. H. (2000). Handbook of Pharmaceutical Excipients, American Pharmaceutical Association: Washington D. C.; 386-388. [28] Dong, J., Chowdhry, B. Z. & Leharne, S. A. (2003). Surface activity of poloxamines at the interfaces between air–water and hexane–water. Colloid Surface A: Physicochem Eng Aspects, 212, 9-17. [29] Sosnik, A. & Sefton, M. V. (2006). Methylation of poloxamine for enhanced cell adhesion. Biomacromolecules, 7, 331-338. [30] Chiappetta, D. A., Degrossi, J., Teves, S., D’Aquino, M., Bregni, C. & Sosnik, A., (2008). Triclosan-loaded poloxamine micelles for enhanced antibacterial activity against biofilm. Eur J Pharm Biopharm, 69, 535-545. [31] Barton, A. (1991). Handbook of Solubility Parameters and Other Cohesion Parameters. 2nd Ed. New York: CRC Press; 157-193. [32] Martínez, F., Ávila, C. M. & Gómez, A. (2003). Thermodynamic study of the solubility of some sulfonamides in cyclohexane. J Braz Chem Soc, 14, 803-808. [33] Veiga, M. D., Merino, M., Cirri, M., Maestrelli, F. & Mura, P. (2005). Comparative study on triclosan interactions in solution and in the solid state with natural and chemically modified cyclodextrins. J Incl Phenom Macrocycl Chem, 53, 77-83. [34] Gonzalez-Lopez, J., Alvarez-Lorenzo, C., Taboada, P., Sosnik, A., Sandez-Macho, I. & Concheiro A. (2008). Self-associative behavior and drug solubilizing ability of poloxamine (Tetronic®) block copolymers. Langmuir, 24, 10688-10697. [35] Alvarez-Lorenzo, C., Gonzalez-Lopez, J., Fernandez-Tarrio, M., Sandez-Macho, I. & Concheiro, A. (2007). Tetronic micellization, gelation and drug solubilization: Influence of pH and ionic strength. Eur J Pharm Biopharm, 66, 244-252. [36] Kabanov, A. V., Nazarova, I. R., Astafieva, I. V., Batrakova, E. V., Alakhov, V. Y., Yaroslavov, A. A. & Kabanov, V. A. (1995). Micelle formation and solubilization of fluorescent probes in poly(oxyethylene-β-oxypropylene-β-oxyethylene) solutions. Macromolecules, 28, 2303-2314. [37] Riess, G. (2003). Micellization of block copolymers. Prog Polym Sci, 28, 1107-1170. [38] Allen, C., Maysinger, D. & Eisenberg, A. (1999). Nano-engineering block copolymer aggregates for drug delivery. Colloid Surface B: Biointerfaces, 16, 3-27.
In: Encyclopedia of Polymer Research Ed: Caroline E. Jones
ISBN: 978-1-61761-926-7 © 2011 Nova Science Publishers, Inc.
Chapter 6
SILOXANE-CONTAINING COMPOUNDS AS POLYMER STABILIZERS Carmen Raclesa* , Thierry Hamaideb and Etienne Fleuryb a
“Petru Poni” Institute of Macromolecular Chemistry, Iasi, Alea Gr. Ghica Voda 41A, RO700487, Romania. b Université de Lyon F-69003 Lyon, France. Ingénierie des Matériaux Polymères. CNRS, UMR 5223. 69622 Villeurbanne, France.
ABSTRACT Generally, surfactants are used as stabilizers of interfaces or particles and their applications are very wide, from foams or adhesion modifiers to the orientation of chemical reactions. Siloxane surfactants are known for their ability to decrease the surface tension of liquids in such extent that is comparable only with some fluorinated compounds, which are thought to exhibit potential toxicological problems. On the other hand, polysiloxanes are unique by their set of properties, like for example low glass transition temperature, hydrophobic behavior, transparency to visible and UV light, high permeability to various gases (especially oxygen), physiological inertness, excellent blood compatibility (low interaction with plasma proteins). In addition, their chemistry is very versatile, and as a result, a very broad range of siloxane-organic compounds can be synthesized, including amphiphilic macromers or polymers. The most commonly known siloxane surfactants are the so called „silicone polyethers”, but other nonionic, as well as ionic surface active agents have been prepared and used over the years in cosmetics, textile conditioning, foam stabilization, coatings or agriculture. Recent developments in this research field and especially our experimental results on the synthesis, properties and applications of siloxane-containing surfactants will be reviewed. Our main interest is to propose new surfactants or alternative synthetic procedures, and new stabilization systems for polymeric nanoparticles. Carbohydrate modified (poly)siloxanes with different architectures have particularily been studied and tested, due to their biocompatibility and bioavailability. *
Corresponding author: e-mail:
[email protected]
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INTRODUCTION Surfactants are crucial for certain applications in the modern technique, from laundry and heavy washing to personal care or medicine. The most recent applications of surfactants are related to nanotechnologies; they are used as templates for nano-patterning, as well as for the stabilization of nanoparticles -the „new wave” in science- [1]. Various surface active agents have been synthesized and investigated, as well as their interaction with polymers [2]. New surfactants continue to appear, and even slight modifications of the molecular structure are very useful for understanding of their properties [3]. In the domain of polymer stabilization (polymerization in dispersed media as well as using physico chemical procedures), it becomes of first importance to anchor the surfactant onto the surface of the particles not only to avoid flocculation, but also to limit water pollution. In the case of emulsion polymerization, a good way to overcome these drawbacks is to use polymerizable surfactants (also called "surfmers") [4-6]. Polymeric surfactants, and particularly block and grafted copolymers, offer another quite versatile way to stabilize nanoparticles [7], with enhanced properties with respect to the molecular emulsifiers. The consequence is excellent surface properties at low concentrations, which allows using lower amounts of emulsifier for the stabilization of emulsions [8]. This is an obvious economical benefit, and the fact that the residual concentration of emulsifier in the aqueous phase is lower, brings another advantage limiting secondary effects such as air entrapment and foaming, or adsorption at various interfaces. Among polymeric surfactants, polyethylene oxide - polypropylene oxide (PEO-PPO) diblock and triblock copolymers (Pluronics® or Poloxamers®) are well-known and their properties and uses are broadly documented [9, 10]. The hydrophobic PPO block can advantageously be replaced by a biodegradable block polyester such as polycaprolactone or polylactide [11-16]. In the field of surface active compounds, one cannot disregard the unique properties of siloxanes as hydrophobic components in connection with such applications. Siloxane surfactants are used since the 1960s in textile conditioning, polyurethane foams, as adjuvants in agriculture, paint additives, cosmetic formulations and so on. Their uniqueness is their solubility both in water and in organic solvents, reducing the surface tension of all these media and exhibiting hydrophobic as well as oleophobic properties. The reasons for this unique behavior are the high flexibility and low cohesive energy of the siloxane bond. The siloxane backbone can adapt to the interface geometry very easily, without creating steric hindrance due to this flexibility [17, 18]. On the other hand, there are some additional properties that make siloxane compounds very attractive. For example, polydimethylsiloxanes (PDMS) have a very low glass transition temperature, transparency to visible and UV light, high permeability to various gases (especially oxygen), physiological inertness, excellent blood compatibility (low interaction with plasma proteins) [19]. The very low surface tension of the polysiloxanes, along with their versatile chemistry and biocompatibility increased the interest for siloxane-containing surfactants. Thus, amphiphilic compounds of all kinds of molecular architectures and variable chemical structures have been synthesized and studied as surface active materials. A wide range of organic moieties, namely nonionic, anionic, cationic and zwitterionic organic groups, have been linked to polysiloxane backbones in order to get soluble products [20]. Siloxane surfactants are usually required in small amounts and often provide better performances then their purely organic counterparts. For example, it was calculated that one
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215
Si(CH3)2 unit can be compared to 4 CH2 units, while a C(CH3)2 unit is equivalent to 2 CH2 groups in terms of surface activity [21]. Many of these surfactants are largely used and commercially available. The first and thus the most studied were those having polyethyleneoxide as hydrophilic part, either as block copolymers or as trisiloxanes (Schemes 1-3). Only some of the most recent developments in this class of surfactants will be reviewed here, since the subject is quite vast and other comprehensive references are available. In various domains, the choice of surfactants is not dictated only by their surface properties, but is limited by many other constraints. This is particularly true for applications that concern living organisms, environment and humans. That is why a more and more important topic is that of biocompatibility and / or biodegradability. Carbohydrate-containing siloxane surfactants have attracted much attention, since they can be obtained from a natural resource and thus be more eco-friendly. The reports on this particular class of siloxane surfactants will be summarized, including our own contribution.
1. SILICONE POLYETHERS (SPE) The most commonly known siloxane surfactants contain polyoxyalkylenes (polyethers) as hydrophilic parts. They have been investigated since the 1960s and silicone polyethers have practically become the definition itself of siloxane surfactants. Their effectiveness in organic systems as well as in water and their use in cosmetics, textile conditioning, foam stabilization, coatings and agriculture has been reported [22-28]. The polyether groups are polyethylene oxide (PEO) and/or polypropylene oxide (PPO), and are attached as side chains to the siloxane backbone through a hydrosilylation or condensation process. They can have a linear or grafted (comb) architecture and a great diversity of structures are available (Schemes 1-4), since molecular weight and composition of the polyether chain (PEO/PPO) as well as the functionality of the end group and the ratio of siloxane to polyether can also be varied. Siloxanes with a very high molecular weight are more suitable for forming water/oil emulsions [29]. Depending on their solubility parameters, silicone polyethers can stabilize foams or act as antifoaming agents [30].
Me3SiO
Me3SiO
Me
O
Si
(CH2)3 O CH2 CH2 O
SiMe3 y
m
R
Scheme 1. Polyether trisiloxane surfactants
Me Si
O
(CH2)3 O CH2 CH2 O
SiMe3 y
m
CH2CH O H CH3 n Bluestar Silicone H3303.
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Carmen Racles, Thierry Hamaide and Etienne Fleury CH3
CH3
(CH3)3-Si-O
Si O
Si O
x
y
CH3
CH2
Si-(CH3)3
3
(OCH2CH2) OR n
CH3
CH3
RO(CH2CH2O) (CH2)3 Si O n CH3
CH3
Si O x
CH3
Si (CH2)3 (OCH2CH2)n OR CH3
Scheme 2. General structures of the main types of SPE copolymers
Me3SiO
Me Si Me
O
Me Si
O
x (CH ) 2 2
Me Si
Me
O
Si
y (CH ) 2 3
O Si(CH3)2 CH2 O n CH 2 SiMe3 OH
O
(CH)p
Me3SiO
SiMe3 z
Si
O
Me
Me
CH3
Me
O
Si x (CH ) 2 2 Si(CH3)2 O SiMe3
m
O
Si y
(CH2)3
SiMe3 z
O CH2 n CHOH CH2
3
OH
Shin-Etsu - KF 6038 Me3SiO
Me
Shin-Etsu - KF 6104 Me Si
Me
O
Me
O
Si x
(CH2)3 O CH2 CH2 O
SiMe3 y
m
CH2CH O R CH3 n
Wacker-Belsil® DMC 6031 PEG/PPG - 25/25 Dimethicone Goldschmidt. ABIL B 8851. n = 20, m =5, x/y = 75/25 Goldschmidt. ABIL B 8873. n = 20, m =5, x/y = 35/65 Bluestar silicone SP3300. n = m = 22. x/y = 90/10 Scheme 3. Examples of commercial surfactants
Extensively studied nonionic surfactants are PDMS / polyether block copolymers [31] and poly(ethylene oxide) substituted trisiloxanes [32-35]. They are able to achieve a maximum surface tension depression to about 21 mN/m and a low critical micelle concentration (CMC) [36]. These siloxane surfactants can self-assemble into a variety of
Siloxane-Containing Compounds as Polymer Stabilizers
217
ordered morphologies in the melt as well as in solution, depending on the volume fractions of the hydrophilic units [37].
1. A. RECENT DEVELOPMENTS IN THE SYNTHESIS AND INVESTIGATION OF SPES The preparation, uses, and physical chemistry of silicone surfactants have been described in a well-known reference [38], focusing on silicone polyoxyalkylene copolymers. This book explores many aspects of SPEs as surfactants, like interfacial processes, surface viscoelasticity, and aggregation, explains the unusual wetting behavior of the trisiloxane surfactants and the ternary phase behavior of mixtures of silicone surfactants with water and silicone oils. That is why we only try to emphasize some newer insights into the subject, reported mainly after year 2000. From the chemical point of view, some improvements in the structure and synthesis of SPEs have been made. In order to avoid Si-O-C linkages, which are susceptible to hydrolytic instability, hydrosilylation reactions are used to prepare these compounds, starting from unsaturated precursors [21, 39-41]. Triblock PEO-PDMS-PEO copolymers have been obtained by anionic polymerization of ethylene oxide using a PDMS diol as transfer agent. These copolymers have been fully characterized by NMR and Maldi-Tof mass spectrometry. As expected, the solubility depends on the PEO/PDMS ratio [42]. It was found that, when the hydrophilic functional groups are linear or when the endcapped units of the hydrophilic groups are of hydroxy type, the hydrolytic stability decreases and chemical attack of silicone surfactants backbone occurs easily in the aqueous solution. A way to avoid this is to synthesize grafted copolymers or to use PEO blocks having methoxy end groups [43]. Although such siloxane surfactants were extensively exploited, their surface active properties are still investigated [21, 27, 44]. All types of aggregates known for hydrocarbon surfactants comprising micelles, lyotropic liquid crystalline phases from lamellar to the inverse hexagonal phase, vesicles, and "sponge" (L3) phases have been found in aqueous solution [37, 45]. It was found that the minimum ratio which insures water solubility in siloxane surfactants is 9 PEG units to 29 siloxane groups. The critical aggregation concentration decreases systematically with the decrease in the hydrophilic PEG substitutions. The surface and selfassembling properties have been investigated with surface tension, viscosity, fluorescence, and contact angle measurements [21]. Recent investigations still approach different aspects of this complex subject, like selfassembling behavior, adsorption on different substrates, rheology. Many studies have been undertaken for siloxane surfactants in dilute aqueous solutions, but their potential applications in organic solvents or as nanoreactors are not yet fully investigated. The CMC, micelle hydrodynamic diameter, microviscosity in the micelle core in water and in mixtures of water with polar solvents have been investigated for a polyoxyalkylene siloxane surfactant, by means of fluorescence [46].
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The self-aggregation in long PEO-PDMS surfactants showed their ability to form inverse micelles and liquid crystals in nonpolar oils, including octamethylcyclotetrasiloxane (D4). These structures, which are not found in conventional hydrocarbon surfactant systems, were explained by the segregation tendency of the PEO chains and the flexibility of the siloxane chains [27]. A polyethyleneoxide-b-polydimethylsiloxane-polyethyleneoxide surfactant, (EO)15(DMS)15-(EO)15, was studied with freeze-fracture transmission electron microscopy and pulsed-field gradient nuclear magnetic resonance spectroscopy, in order to establish the effect of glycerol on the permeability of vesicle membranes. Small vesicles with diameters of less than 25 nm and multilamellar vesicles with diameters larger than 250 nm were observed in pure water, which were modified when water was gradually replaced with glycerol [47]. Polydisperse (PPO-b-PEO) allyl ether siloxane surfactants were synthesized by the hydrosilylation reaction of 1,1,1,3,5,7,7,7 octamethyltetrasiloxane (MD2'M) with oligo(PO-bEO) allyl ethers. In this series, the surface tension increased with increasing the EO chain length and it decreased with increasing the PO ratio, while the sedimentation time of the aqueous solution showed opposite trend. The cloud point temperatures tended to increase with the increase in the EO chain length and decrease of the PO ratio [48]. Recently, a series of well defined amphiphilic poly[methyl(3,3,3-trifluoropropyl)siloxane]-b-poly(ethylene oxide) (PMTFPS-b-PEO) diblock copolymers with different block lengths have been prepared by hydrosilylation [49]. Trisiloxane surfactants containing polyethyleneoxide chains of different lengths, known as superwetters, have been studied by soft-contact AFM imaging and direct force measurements at the solid-liquid interface, using different substrates [50]. The surface aggregate structures for these siloxane surfactants correlate with those of their hydrocarbonbased equivalents and resemble bulk structures. Surface force measurements of ABA type SPEs (Scheme 4), compared to hydrocarbonbased Pluronic surfactants, at the solid/liquid interface in water/ethanol mixtures showed that SPEs are capable of providing steric stabilization at a hydrophobic surface up to 80% and even up to 95% ethanol level (depending on structure), while in the case of hydrocarbonbased surfactants this barrier was 40% [51]. CH3
CH3 Cap
(CH2CH2O) n
(CH2-CH-O) m (CH2)3 CH3
Si O CH3
x
Si CH3
(CH2)3 (OCH2CH2) n
(OCH-CH2)
m
Cap
CH3
Cap: OH or OAc Scheme 4. General formula of triblock SPE surfactants [51]
Although the nature of superwetting remains enigmatic, the surface aggregate structures are studied in order to better understand colloidal phenomena and to approach nanotechnologies by self-assembly of hierarchical structures [50].
Siloxane-Containing Compounds as Polymer Stabilizers
219
1.B. SPEs as Polymer Stabilizers: Applications of SPEs Oil-in-water emulsions obtained with certain SPEs are stable in the presence of alcohol, which is an important advantage for many emulsion and dispersion formulations, where alcohols are common co-solvents or additives. At the same content of ethanol, hydrocarbonbased Pluronic surfactants do not emulsify oil-in-water [51]. Silicones give a unique dry-lubricity feel to surfaces such as textiles, hair, and skin [38]. That is why graft polysiloxane-poly(oxyalkylene) copolymers are used for making water-insilicone oil (w/o) emulsions for personal care products [52]. The question of how to solubilize polymeric silicone oils, preferentially in microemulsions, is of high technological importance [53] and has received special attention. Silicone surfactants alone or in combination with short-chain alkanols are efficient emulsifiers for short chain siloxanes. It was found that organic surfactants are not as suitable as silicone surfactants for stabilizing silicone/water emulsions [54]. The chemical affinity of the hydrophobic part of the surfactant to the silicone oil is a benefic factor, helping the solubilization process [53]. Silicone oil in water (o/w) emulsions are more difficult to stabilize. To prepare such dispersions, nonionic PDMS-polyoxyalkylene copolymers with high degree of polyoxyalkylene substituents are generally used, in order to render the surfactant more hydrophilic. Thus, the resulting emulsions are sterically stabilized by the polyether chains [54]. Viscoelastic and viscosimetic measurements have been performed on model systems and on commercial products [29]. Water-in-silicone emulsions have been prepared, in which the oil phase consisted of decamethylcyclopentasiloxane (cyclomethicone, D5) and the surfactant used was a branched type silicone copolymer. The interfacial rheology study of such surfactants revealed that the ability of these copolymers to effectively stabilize water-in-D5 emulsions is a result of a process involving the nucleation, growth, and accumulation of surfactant-rich particulates at the D5/water interface [55]. The w/o/w or o/w/o emulsions are important, in particular when the protection of sensitive ingredients or controlled release of active substances is required [54]. Silicone-based emulsifiers are suited for these applications. They are strongly adsorbed at the oil interface, and do not migrate from one interface to the other, thus preventing the destabilization. Two emulsifiers have to be used to stabilize these multiphase emulsions. The hydrophilic– lipophilic balance (HLB) values of the emulsifiers should be above 10 for the hydrophilic emulsifier and below 6 for the hydrophobic emulsifier [54]. The ability of SPEs to form vesicles, which are structurally comparable to liposomes, has opened a wide range of possibilities for the incorporation of active ingredients. Silicone vesicles can have a diameter from about 0.05 to 1 micron and an internal volume of 10-6 μm3. The membrane thickness of these vesicles is about 3 to 4 nm [56]. The main application of siloxane-based surfactants vesicles is in cosmetics. For example, using silicone vesicles, hydrophilic and hydrophobic active substances can be separated and protected from each other, thus reducing for example skin irritancy. Active delivery systems include non-aqueous emulsions of polyols in silicone fluids, multiple-phase emulsions, and polar solvent-in-oil emulsions. Silicone-based surfactants offer many benefits in waterborne coatings. The coating surface tension can be modified to improve substrate wetting, in particular when trisiloxane polyethers are used, since these structures provide excellent wetting to low-energy substrates,
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like plastics. Copolymers with linear or grafted structures give moderate wetting, but can provide mar resistance, slip and de-aeration [30]. For waterborne coatings and ink formulations, polymeric surfactants are used in conjunction with polar organic solvents. The numerous patents covering this area of research are discussed in a review [57]. In view of such applications, the adsorption of a grafted (rake-type) polymeric siloxane surfactant containing 48% PDMS, 39% PEO, and 13% PPO on carbon black particles dispersed in mixtures of water with polar organic solvents has been investigated [58]. The adsorption was found to obey the Langmuir isotherm below the critical micelle concentration and a sharp increase in the adsorbed amount was observed at higher surfactant concentrations. DLS and SANS data indicate that the structure of the adsorbed layer is similar to that of micelles. An aqueous laundry stain pretreater composition which would provide outstanding cleaning performance on both oil and water based stains on a variety of fabrics has been developed using polyalkylene oxide modified trisiloxanes [59]. Trisiloxane surfactants have been used as adjuvants in agriculture. They increase the foliar uptake of pesticides, due to reduction of surface tension and superspreading. A method for improving the performance of agricultural compositions under conditions of low humidity has been developed, by using silicone surfactants that reduce the surface tension to less than 30 mN/m in 0.10% (w/w) aqueous solution, without concomitant spreading of the spray solution [60]. SPEs can act as stabilizers in chemical reactions. For example, silver nanoparticles with an average diameter of about 3 nm have been obtained inside the aggregates of a PDMS-gPEO copolymer, without adding a reducing agent [61]. Polymerization of halogen-containing monomers in aqueous medium using siloxane surfactants instead of potentially toxic fluoroalkyl surfactants has been described [62].
2. SILOXANE SURFACTANTS WITH CARBOHYDRATE MOITIES Siloxane-polyethers usually have a poor emulsification activity at the siloxane-water interface, which has been explained by the weak intermolecular interactions between the surfactant molecules [63]. This is one reason for seaking surfactants with stronger intermolecular interactions. Carbohydrates are bioavailable, biocompatible, biodegradable, biologically active and suitable for molecular recognition. They are hydrophilic and capable of self-association. So, there are a lot of reasons for using them as partners for siloxane compounds, in order to produce more environmentally friendly surfactants with superior properties. Glycopolymers are often defined as synthetic polymers bearing sugar moieties [64-68]. This definition can also be enlarged to any macromolecular structure resulting from chemical modification of natural polymers (cellulose, dextran ...). They are used in a lot of biomedical application fields including drugs and drug delivery devices because of the ability of sugar moieties to interact with protein receptors (the so-called cluster glycoside effect). In most cases, glycopolymers are amphiphilic polymers because of the presence of hydrophilic and hydrophobic units and they can act as polymer surfactants. These glycopolymers can therefore offer a unique combination of macromolecular recognition and colloidal
Siloxane-Containing Compounds as Polymer Stabilizers
221
stabilisation of nanoparticles in dispersed media to get opportunities for the production of novel targeted drug delivery systems. For instance, maleic copolymers with different contents of galactose moieties and dodecyl chains were synthesized and used as both a stabilizer and a surface coating for the preparation of poly(ε-caprolactone) nanoparticles by the emulsification-diffusion technique. The surface modification of nanoparticles was confirmed by ζ-potential measurements. Nanoparticles were also shown to be recognized by a galactose-specific lectin, demonstrating the presence of galactose units on the particle surface [69]. More recently, 8-amino-3,6-dioxaoctyl α-D-mannopyranoside and 8-amino-3,6dioxaoctyl β-D-galacto-pyranoside were prepared for coupling with a copolymer made from ε-caprolactone macromonomers and maleic anhydride. The glycopolymers thus obtained were used in conjunction with Pluronic® F-68 for the stabilization of PCL nanoparticles prepared by the emulsification-diffusion technique [70]. The combination of carbohydrates and silicones has received a great attention in both scientific and economic media [71]. Hybrid materials, from small molecules, to polymers, oligomers and nanostructured composites have been prepared and their applicability especially in biomedical fields has been proved.
2.A. Synthetic Methods for Carbohydrate – Modified Siloxane Surfactants A broad variety of carbohydrate-modified siloxane surfactants have been synthesized and tested as surfactants [72-75] or in cosmetic formulations [76-79]. Such compounds are currently produced on an industrial scale (see for example Scheme 5) and their use has gradually increased due to their excellent biodegradability, good dermatological compatibility, and the absence of toxic effects. The chemistry of carbohydrate modified polysiloxanes is not easy, starting from the totally different solubility of the reagents and continuing with the instability of the siloxane and glycosidic bonds in certain conditions. Nevertheless, various structures have been obtained, sometimes applying very ingenious solutions to overcome such problems. Different carbohydrate moieties have been used, as well as various HO-protecting procedures (mainly using acetyl or trimethylsilyl derivatives) and subsequent deprotection methods. The most common method for linking carbohydrate derivatives with siloxanes is the hydrosilylation. An alternative is the ring-opening reaction between an amino-functional siloxane and a carbohydrate lactone [79, 81-83], but other methods have also been described and are still developed. Allyl glycosides, allyl ethers and allyl amides of glucose, gluconic acid and glucuronic acid-γ-lactone with protected hydroxyl groups have been used to obtain silane precursors for modified polysiloxanes [84]. A great variety of carbohydrate-modified siloxane surfactants have been prepared by Wagner and coll. [72-75, 78, 79]. They described a synthetic path yielding to straight-chained glycosides-functionalized siloxanes with amide linkages, via allyl-glycidyl ether –modified intermediates [78]. Similar compounds have been prepared from silanes, carbosilanes, polysilanes and non-permethylated siloxanes [73] and the role of the key intermediates for the surface properties of the surfactants has been thoroughly investigated.
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Carmen Racles, Thierry Hamaide and Etienne Fleury Me
Me3SiO
Si
Me
O
Me
Si
Me
O
x (CH ) 2 3
Si y
O
SiMe3 z
(CH2)w
O
CH3 CH2 OH H2C H O O O OH HO ~ 1,8
Scheme 5. A commercially available carbohydrate-modified polysiloxane (Wacker Belsil SPG128VP): Caprylyl Dimethicone Ethoxy Glucoside, 20% in cyclopentasiloxane (D5)) [80]
Different approaches have been used for the hydrosilylation of allylglycosides with various H-functional polysiloxanes [71, 78, 85-87]. Glucopyranosyl- and cellobiosylterminated oligodimethylsiloxanes with thioether or ether linkages have been prepared and tested as transdermal penetration enhancers [87]. The monosaccharide-modified compounds exhibited a pronounced permeation acceleration for antipyrine, while the disaccharide ether siloxane had no enhancing activity. High molecular weight polysiloxanes modified with mono-, di- or oligosaccharides showed different aggregation behavior in solution, depending on the detailed chemical structure of the carbohydrate moiety and its stereochemistry [88]. Gemini surfactants have been obtained from a trisiloxane amine, D-gluconic acid δ−lactone and oligoethylene glycol diglycidyl ethers [89] (Scheme 6). These surfactants reduced the surface tension of water to approximately 21 mN/m at a concentration of around 10-5 mol/L and showed two critical aggregation concentration values. H3C H3C
H3C
CH3
CH3 Si
Si O
CH3
CH3
CH3
H3C H3C
Si CH3 O
O
N
O(CH2CH2O) OH
HO
OH H
H
OH
H
OH CH2OH
Scheme 6. Gemini surfactants [89]
O
N
n OH
NH O
O H
Si
Si
Si
HN
CH3
H3C
H HO
OH H
H
OH
H
OH CH2OH
CH3
Siloxane-Containing Compounds as Polymer Stabilizers
223
Amphiphilic polysiloxanes have been recently obtained by reacting a water-soluble poly(3-aminopropyl)siloxane with fatty acid chlorides to attach hydrophobic moieties and subsequently with gluconolactone which provided the hydrophilic groups. The regularity of the nanoaggregates in water formed from the amphiphilic polysiloxanes was relatively controlled by the chain lengths and functionalities of the hydrophobic parts [90]. Glucose functionalized polysiloxanes have been obtained by transacetalation [63]. Different acidic catalysts have been tested and activated clay was found to be the best, due to the great advantage of avoiding the contamination of the final product. We have described the synthesis of mannose, glucose, galactose or cellobiose modified siloxanes: telechelic difunctional oligosiloxanes, siloxane copolymers with pendant monosaccharide groups and cyclic oligosiloxanes (Scheme 7) by a method involving hydrosilylation and cation exchangers as deprotecting catalysts [91-93]. OH
OH O
O
HO HO
OH
O R
OH
OH O
HO HO
Si O Si n
O
OH
OH
Si
N
Me O
CH 3
OH OH
I
HO
Me
HO O
R O
Me O
Si
N
Me
n
H 3C
Ce llo bio se
II
O
Cellobiose
Si O
Si O R
OH O Si
O
O
III
x
O
Si y
O
OH OH
HO
Si
O
OH
HO OH OH
IV
Scheme 7. Carbohydrate modified siloxanes [91-93]
In this approach, the protection-deprotection method was based on the fragile solubility equilibrium. In a first version [91, 92], the trimethylsiloxy groups were removed in the presence of a cation exchange resin which was used in wet state (gel form), in order to prevent the cleavage of the Si-O bonds. Indeed, cation exchangers with similar exchange capacity are largely used for the polymerization of cyclosiloxanes, in laboratory and in industry [94-97]. The only difference is that these polymerization catalysts are thoroughly dried before use, by azeotropic distillation of toluene, and only traces of water remain in their structure. Thus, the highly hydrophobic siloxanes have access to the active sites. For the deprotection of the saccharide OH groups, the catalyst that we used had a high content of water (50%) and in this situation the active sites are not available for the hydrophobic part of
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the molecules: in other words, the water has a screening effect between the siloxane backbone and the catalytic sites and therefore prevents the breakage. On the other hand, the protected compounds were dissolved in a mixture of THF and methanol. The deprotection occurs gradually and the solubility of the deprotected compounds shifts towards methanol. The higher the content in hydrophilic groups the better the methanol and water solubility. In this case, cleavage of the siloxane bonds may occur, but the phenomenon is limited, especially if the reaction time is strictly controlled. The polymer conformation and self-assembling in solution have probably a great importance in preserving the integrity of the siloxane chains. On the contrary, when the same cation exchanger was used, but after drying to constant weight, the siloxane chain break became more important [93]. The reaction time was significantly reduced, but the chain length control was not possible anymore. However, the glycosidic bonds in cellobiose moieties were not affected by the deprotection catalyst. This deprotection method proved to be as „tricky” as effective, but the overall advantages of ion exchangers have to be considered, and especially the easy removal from the reaction mixture, without significant contamination of the final product. OH O
HO HO
Si
OH HO O
O N Ac
O
OH
N3
O
Si O
OH
OH
Si
OH O
HO HO
OH HO O
N Ac
O
OH
OH
H
Si O
N
N
O
N
Si O
OH
Si OH OH
HO
O O
HO HO
HO m
OH O HO HO
HO HO
O OH OH HO
O
OH HO O O
OH O OH
O
O HO
O O
OH
OH HO O
O OH
N Ac
H
OH
Si O
N
N N
O n
O
O
O OH
Si O Si
OH
Scheme 8. Cellobiose and xyloglucan modified heptamethyltrisiloxane (MD’M) obtained by click chemistry [98]
Although the procedure based on Fischer glycosidation and deprotection of HO- groups using a cation exchanger may be adjusted to produce as less damage to the siloxane chain as possible, it remains dangerous for the integrity of oligosaccharide segments, since those are also cleaved and reformed according to an equilibrium process. Oligosaccharide modified
Siloxane-Containing Compounds as Polymer Stabilizers
225
polysiloxanes can be obtained, but the precise control of their structure remains uncertain with this procedure. An alternative may be the „click chemistry” applied to silicon-based materials. This reaction is quite versatile and appropriate to be used with compounds with different solubilities. Thus, the application of click chemistry to generate surfactants appears to be promising and particularly valuable. Well-defined glyco-polyorganosiloxanes with cellobiose and xylogluco oligosaccharide have been synthesized by „click” chemistry (Scheme 8) [98]. The method is based on Huisgen 1,3-dipolar cycloaddition from azido-containing PDMS with plant polysaccharide xyloglucan and cellobiose bearing a terminal alkyne functionality. It involves non-protected sugar derivatives, easy purification steps and it allows regioselective introduction of the alkyne function onto the carbohydrate moiety.
2.B. Applications of Carbohydrate Modified Siloxanes in Polymer Stabilization Attempts have been made to prepare microemulsions of hexamethyldisiloxane or longer PDMS, using carbohydrate modified siloxane surfactants [78, 79], but the approach was not succesful due to the siloxane insolubility of the amphiphile [53]. The addition of ethanol was helpful, but only in certain cases. The surface active properties of a xyloglucane modified trisiloxane (Scheme 8) at the dodecane/water interface, as well as at the silicone/water interface, were studied and compared with those obtained with other surfactants usually involved in cosmetic formulations. The tests indicated that this glyco-organosiloxane was a poor emulsifier, but its foaming behavior is promising for detergent and cosmetic applications [98]. Glycopolysiloxanes with cellobiose moieties have been used for the steric stabilization of vinyl acetate miniemulsion polymerization [93]. The first attempts to get stable PVAc nanoparticles by using only the glycosiloxane derivatives as polymer surfactants failed, but stable nanoparticles were obtained using a mixture of glycosiloxane and a non-ionic triblock PEO-b-PPO-b-PEO copolymer. The observation that glycosiloxanes employed alone are unable to stabilize miniemulsion polymerizations is in good agreement with the fact that mannose derivatives grafted onto vinylic polymer backbones are ineffective in assuring the colloidal stability of polycaprolactone nanoparticles [70]. This may tentatively be interpreted in terms of the layer thickness of the polymer surfactant adsorbed onto the particles. The siloxane chain is highly hydrophobic and forms a flattened alignment with the interface. The sugar moieties grafted either onto the backbone or at the chain ends are solvated by the aqueous phase but their small molecular size does not allow a sufficient extension into the solution to give a thick enough polymer layer to ensure a good steric stabilisation, contrary to silicone polyethers. Adding a triblock copolymer such as Pluronics is therefore needed to provide a good colloidal stability since the PEO segments extend further into water. Polymer nanoparticles are more and more investigated and one of their most spectacular applications is in the medical field, where they are largely used for encapsulation of drugs [10]. For such a purpose, the polymers have to be biodegradable, biocompatible or bioresorbable [10, 99, 100]. Siloxane surfactants containing monosaccharides, as well as
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potassium salts of siloxane-aliphatic carboxylic acids, have been used for the stabilization of polymer nanoparticles by nanoprecipitation [91, 92]. In the emulsification process, it was assumed that the chemical affinity of the hydrophobic part of the surfactant to the silicone oil promotes solubilization [53]. Nevertheless, this compatibility seems to be insufficient and other factors also intervene. For the same surfactant and experimental parameters, we observed differences in the size and size distribution from one polymer to another, and especially the stability after drying was greatly influenced by the thermo-mechanical properties of the polymer core. Well-defined spherical nanoparticles were obtained with polysulfone (PSF), a rigid polymer with high Tg. Polycaprolactone, PCL (negative Tg, but positive Tm) formed stable monodisperse particles in water, which exhibit a tendency of aggregation after drying. The nanoparticles obtained by nanoprecipitation of high molecular weight PDMS (both negative Tg and Tm collapsed) [91, 92]. In our attempt to obtain stable silicone nanoparticles, we have tested the possibility of crosslinking polysiloxane chains in the presence of siloxane surfactants [101]. The crosslinking reactions occur in the formed particles and improve their colloidal and dry-state stability. However, the average diameter and size distribution of the particles measured by DLS were not very good, which may be due to the insufficient hydrophilic content, or insufficient sterical repulsion. Other chemical reactions can be performed in the presence of carbohydrate modified siloxane surfactants. For example, a glucose substituted cyclosiloxane has been successfully used to stabilize silver nanoparticles (Figure 2) [102].
3. OTHER SYSTEMS Alkali metal salts of siloxane-aliphatic acids can play the role of anionic surfactants. Such compounds have been described in a patern long time ago and their possible use as anticorrosion agents and emulsifiers has been claimed [103]. We studied potassium salts of mono- and di-substituted sebacomethyl disiloxane (Scheme 9) as surface active compounds and we used them as stabilizers for polymeric nanoparticles [92, 101]. H3 C
H3C
O Si
CH3 Si
O
O K
(CH2)8
+
CH3 H3C
O
O
Scheme 9. Example of anionic siloxane surfactant
For example, polymer nanoparticles have been obtained combining nanoprecipitation with polycondensation. We have prepared poly(siloxane-azomethine) nanoparticles in a one step procedure, using a siloxane surfactant [101]. These particles were small and monodisperse (Figure 1d), as in the case of polysulfone. The core polymer has a positive Tg, as well, and this seems to be an important criterion for obtaining small and stable polymeric nanoparticles.
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a
bb
c d
5 μm
Figure 1. SEM and AFM images of nanoparticles stabilized with nonionic or anionic siloxane surfactants: (a) polycaprolactone, (b) polysulfone, (c) crosslinked PDMS, and (d) poly(siloxaneazomethine)
Figure 2. UV-Vis spectrum and AFM (phase) image of Ag nanoparticles obtained with a glucosemodified cyclosiloxane
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Well-defined particles have been obtained from PDMS containing different additives, using the surfactant depicted in Scheme 9. For example, metal nanoparticles, or crystalline compounds may be encapsulated in PDMS and the resulting particles are better defined and stable after drying (Figure 3), probably due to a reinforcing effect. The same result was observed in the case of PCL nanoparticles with encapsulated indomethacine. In that case, the colloidal stability was visibly increased compared to the neat polymer [92]. Another class of siloxane surface active agents is that of cationic surfactants, which usually contain amine groups (Scheme 10) and are widely used in cosmetic products, especially as hair softeners and conditioners [54]. A quaternary ammonium chloride has been used to modify montmorillonite, in order to obtain nanocomposites with polymathylsilsesquioxane [104].
Figure 3. TEM images of crosslinked polysiloxane nanoparticles reinforced with metal nanoparticles (left) and with an organic crystalline compound (20%) (right)
Si HO
O
Si
Si m
O
nO
Si O H Si
O
HO
Si O
Si a
O
NH
NH
NH2
NH2
CH3
CH3
(CH3)3-Si-O
Si
m
CH3
O
Si n
CH2
O Si-(CH3)3
3 CH2CH3
O CH2
+
CH CH2 N OH
Scheme 10. Cationic siloxane surfactants.
b
CH3 Cl
CH2CH3
Si O
c
Si OH
O O d
H
Siloxane-Containing Compounds as Polymer Stabilizers
229
The ability of siloxane surfactants to stabilize different chemical reactions allows the preparation of different nano-materials. For example, gemini surfactants containing a siloxane moiety have been used as templates for the preparation of mesoporous metal oxides such as zirconium, titanium, and vanadium oxides. The siloxane segment seems to play an important nano-propping role during the surfactant removal by direct calcination [105]. Amphiphilic ABA-type triblock copolymers containing PDMS, two poly(2-methyloxazoline) –PMOXA- side blocks and methacrylate terminal groups have a complex selfassembling behavior, which allows for the synthesis of covalently cross-linked hydrogels, with applications as contact lenses [106]. Cross-linked vesicles of such polymers are nanoand microcapsules which act as stable nanoreactors with controlled permeability [107-109]. Siloxane copolymers with polar groups may form aggregates in nonaqueous media, too. Their behavior is similar to that of surfactants in water. The relationship between nanostructure and rheological properties of such systems, and their potential use as nanoreactors are still under investigation [61, 110, 111]. For example, the micellization of a siloxane- ketimine in organic media may lead to metal complex nanoparticles [112].
CONCLUSION Although siloxane surfactants and especially silicone polyethers are known and produced for some decades, they continue to incite the interest of the scientific world. New structures are emerging, new methods are used for their investigation and new applications are discovered. In particular, biocompatible surfactants are of great importance nowadays, as well as those obtained from renewable sources. Nano-materials may be prepared with siloxanecontaining surfactants, which are active both in water and in organic media.
ACKNOWLEDGMENT The financial support from the Romanian Ministry of Education and Research under grants: Idei_233 no. 5/2007 and Bilateral Franco-Romanian Program Brancusi (14760TM) is gratefully acknowledged. We acknowledge the support provided by Région Rhone-Alpes for a postdoctoral fellowship (C.R.), which allowed the begining of our collaboration a few years ago. The authors are fully indebted to Prof. Sylvie Boileau for fruitfull discussions.
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Reviewed by Prof. Sylvie Boileau, Institut de Chimie et des Materiaux, Paris-Est (ICMPE), LRP-CNRS, 2-8 Rue Henri Dunant, 94320 Thiais, France
In: Encyclopedia of Polymer Research Ed: Caroline E. Jones
ISBN: 978-1-61761-926-7 © 2011 Nova Science Publishers, Inc.
Chapter 7
AMPHIPHILIC BLOCK COPOLYMERS: POTENT EFFLUX PUMP INHIBITORS FOR DRUG DELIVERY AND CANCER THERAPY Martin Werle and Hirofumi Takeuchi Gifu Pharmaceutical University, Laboratory of Pharmaceutical Engineering, 5-6-1 Mitahora, Gifu, Japan.
ABSTRACT The ability of amphiphilic block copolymers to modulate multi drug resistance related processes has been demonstrated the first time more than 10 years ago. Nowadays, the efflux pump inhibitory activity of amphiphilic block copolymers is used in two main areas. First, to improve the transport of efflux pump substrates across the blood brain barrier (BBB) and second, in cancer therapy. It has been shown that in the presence of amphiphilic block copolymers higher concentrations of certain anticancer drugs, which are known as efflux pump substrates, can be found in the brain. Within the current chapter, recent developments in the field of amphiphilic block copolymer mediated efflux pump inhibition are discussed. Besides presenting data from in vitro and vivo studies, also the mechanisms involved in efflux pump inhibition are addressed. In addition, the influence of hydrophilicity/lipophilicity of various amphiphilic block copolymers as well as factors such as micelle formation on the efflux pump inhibitory activity are explained.
1. INTRODUCTION Efflux pumps are transmembrane located transporter proteins which are expressed in various tissues including liver, placenta, the proximal tubule in the kidney, capillary endothelial cells of brain and testis, and epithelial cells of the intestine [1,2]. In addition, they are over-expressed in cancer cells and are involved in the multi drug resistance (MDR) mechanisms of tumors. Besides other mechanisms such as CYP3A, efflux pumps constitute an integral part of the body´s natural detoxification system. Due to their localisation in various tissue they affect absorption, distribution, metabolism and elimination [3]. They are
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known to actively transport a wide range of structurally diverse compounds out of the cells; including anticancer agents [4], immunosuppressants [5], steroid hormones [6], calcium channel blockers [7], beta-adrenoreceptor blockers [8] and cardiac glycosides [9]. The most prominent representative of the group of efflux transporters is P-glycoprotein (P-gp)[10], but also other efflux pumps such as the multidrug resistant proteins (MRP) 1 and 2 as well as the breast cancer resistant protein (BCRP) [11] gain increasing attention. Efflux pumps are ATPdependent proteins which are encoded by the ATP Binding Cassette (ABC) gene family. It has been demonstrated that a broad variety of substances are capable of inhibiting efflux pumps. As MDR can be regarded as a main problem in cancer therapy, an inhibition of efflux pumps can contribute to the development of improved cancer therapeutics by increasing the transport of efflux-pump substrates into MDR cells. Another promising approach of efflux pump inhibition is to improve the transport of efflux pump substrates through physiological barriers, such as the blood-brain barrier (BBB) or the small intestine. Various therapeutic and already marketed drugs were identified to exhibit efflux pump inhibitory properties, in addition to their main pharmacological activity. Representatives of these so called “first generation” inhibitors are for example Verapamil or Tamoxifen [7,12]. In the following years, effort has been put in the development of inhibitors that lack therapeutic effects but still retain their efflux pump inhibitory properties. Therefore, “second generation inhibitors” where synthesised by chemically modifying first generation efflux pump inhibitors. Examples of second generation efflux pump inhibitors, which are based on the lead structure of first generation inhibitors are KR30031, a verapamil analogue, and PSC833 (Valspodar®), which is based on cyclosporine D [13,14]. Finally, “third generation inhibitors” which often lack CYP3A4 interactions and which are capable of inhibiting P-gp in a non-competitive manner were developed. Representatives which entered clinical trials are Elacridar (GF120918; GlaxoSmithKline), XR9576 (Tariquidar; Xenova), LY335979 (Zosuquidar; Eli Lilly), R101933 (Laniquidar) and ONT-093 (Ontogen). Recently, it has been discovered that some widely used polymeric auxiliary agents can interact with efflux pumps. Many of these polymeric efflux pump inhibitors have been used for decades because of their proposed lack of pharmacological activity. The use of polymeric efflux pump inhibitors in pharmaceutical applications has been comprehensively reviewed previously [15,16]. Representatives of polymeric or polymer based efflux pump inhibitors are anionic gums and sodium alginate [17], polyethylene glycol (PEG)[18,19], PEG based detergents [2023], thiolated polymers [24-26] and poloxamers such as pluronics [27]. Among the amphiphilic copolymers which are capable of inhibiting efflux pumps, pluronics have been evaluated in most detail so far. By modifying the length of the ethylene oxide (EO) and propylene oxide (PO) segments which are arranged in the basic A – B - A structure, polymers with various hydrophilicity/lipophilicity ratios can be synthesized. These “functional excipients” are known to affect immune response [28] and wound healing [29]. In addition, it was discovered already 10 years ago, that pluronics can modulate MDR related processes.
2. EFFLUX PUMP INHIBITION MECHANISMS OF PLURONICS Various mechanisms which are responsible for the efflux-pump inhibitory properties of pluronics have been identified and discussed in detail previously [30]. Among them are inhibition mediated by ATP depletion, membrane fluidization and ATPase inhibition. As
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already mentioned, efflux pumps are ATP dependent, which means that ATP is required in order to sustain the function of efflux pumps. It was demonstrated that the intracellular ATP level of cancer cells was depleted following a 2 hour incubation period with pluronic P85 [31]. Additional studies by Batrakova et al. using various MDR and non-MDR cell lines showed that the pluronic mediated ATP depletion is reversible and that MDR cells are much more responsive to pluronic than non-MDR cells [32,33]. Another mechanism which appears to be responsible for the efflux pump inhibitory activity of pluronics is mediated by the ability of pluronics to be absorbed on cell membranes. This effect leads to changes in the microviscosity of cell membranes, due to alterations in the structure of the lipid bilayers [34]. In particular, the presence of pluronic P85 leads to fluidization of membranes of healthy and cancer cells [35]. It has been suggested that non-specific changes in lipid and protein conformation and mobility can lead to ATPase inhibition, which would consequently lead to decreased efflux pump activity [36]. Studies showed that exposure of cells to pluronic P85 led to dramatically decreased ATPase activity in comparison to the control which contained no polymer [35].
3. FACTORS INFLUENCING THE EFFLUX-PUMP INHIBITORY ACTIVITY OF PLURONICS The hydrophilicity/lipophilicity ratio was identified to have a direct impact on the effluxpump inhibitory activity of pluronics [37]. In a study focussing on this topic, the efficacy of 20 pluronics with different hydrophilic (EO) / lipophilic (PO) ratios on the accumulation of rhodamine 123 into MDR cell lines (KBv) was investigated. Based on their efficacy, pluronics were divided into three groups. Group 1 comprised of hydrophilic pluronics with a hydrophilic/lipophilic balance (HLB) between 20 and 29. Representatives of this group are F68, F88, F108 or F127. Group 1 pluronics showed no or only minor effects on P-gp. The most effective P-gp inhibitors belonged to group 2 and were identified to be hydrophobic pluronics with a HLB below 19 and with 30 – 60 repeating PO units in the PO block. In this group, P85, L81 and L61 can be found. Group 3 pluronics, which are less effective than group 2 pluronics, are also hydrophobic copolymers with a HLB below 19, but the number of PO units is either lower than 30 or higher than 60. Representatives of group 3 pluronics are L35, L44 or L121 [37]. It was shown that only the group 2 pluronics where transported to the inside of the cells, and that group 2 pluronics showed the strongest effect on ATP depletion as well as on ATPase inhibition. The properties of the 3 groups are summarized in Table 1. Another important factor which is not directly related to the hydrophilicity/lipophilicity ratio of various pluronics but which strongly influences the efflux pump inhibitory activity of pluronics is micelle formation. Above a certain concentration, the so called “critical micelle concentration” (CMC), amphiphilic block copolymers self assemble into micelles. It has been demonstrated, that the efflux pump inhibitory activity of pluronics increases with increasing pluronic concentrations, but only until the CMC is reached. Above the CMC, substrate accumulation in cancer cells could not be further increased or was found to even decrease [37]. Therefore, the occurrence of pluronic unimers can be regarded as the crucial prerequisite for the efflux pump inhibitory activity of pluronics [37]. As one mechanism for efflux pump inhibition has been identified to be ATP depletion, it seems necessary that the pluronic unimers are transported into the cells in order to exert this action.
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Martin Werle and Hirofumi Takeuchi Table 1. Properties of the three groups of pluronics Group 1 F68, F88, F108, F127 hydrophilic 20 - 29
Group 2 P85, L81, L61 hydrophobic < 19
Group 3 L35, L44, L121 hydrophobic < 19
29 - 65
30 - 60
30 < or > 60
Cell accumulation behaviour
No cell accumulation
Accumulation in the cytoplasma
Inhibitory activity
-
+++
Representatives Character HLB Number of PO units per PO Block
Accumulation in the intracellular endosomal compartments, not released in the cytoplasma +
4. APPLICATION IN CANCER THERAPY AND DRUG DELIVERY Based on their ability to inhibit efflux pump transporters, pluronics have been utilized in cancer therapy as well as in order to overcome the blood brain barrier (BBB). The BBB is known to prevent the transport of certain drugs to the brain. Brain microvessel endothelial cells which form tight extracellular junctions and display low pinocytic activity as well as efflux pumps that remove drugs from the epithelial cells have been identified to be responsible for the limited transport [38]. As pluronics are capable of inhibiting efflux pumps, an improved transport of efflux pump substrates through the blood brain barrier in presence of pluronics was anticipated [39]. The potential of pluronics for BBB delivery has been reviewed in detail previously [30]. Miller et al. [27] were maybe the first who demonstrated a concentration dependent inhibitory activity of pluronic P85 by monitoring the accumulation of rhodamine 123 in brain microvessel endothelial cell (BMVEC) monolayers. Extensive data of in vitro permeation studies using bovine brain microvessel endothelial cells (BBMEC) and a broad variety of efflux pump substrates including e.g.: etoposide, doxorubicin and paclitaxel that show an improved apical to basolateral drug transport are available [40]. In addition, in vivo studies that demonstrate the efficacy of pluronics to improve the transport of efflux pump substrates through the BBB have been performed. As way of example, the brain accumulation of digoxin in wild type mice, mdr1a knockout mice and wild type mice treated with pluronic P85 was investigated and it was reported that in the presence of pluronic a prolonged residence time and an increased concentration of digoxin in the brain was observed [39]. Another physiological barrier besides the BBB which displays efflux pump expression and which can limit drug transport is the intestinal mucosa. Drug transporters located in the intestine have been identified to be responsible for the low oral bioavailability of a number of efflux pump substrates. Moreover, it has already been demonstrated in various studies, that the oral bioavailability of efflux pump substrates can be improved by co-administrating efflux pump inhibitors [20,26]. A review article focussing on the applications of pluronics in cancer therapy has been published previously [41]. Apart from their efflux pump inhibitory activity, pluronics are promising agents in cancer therapy because they affect drug sequestration and the GSH/GST detoxification system, which is also discussed in the mentioned review article [41]. The in vivo efficacy of pluronic formulated doxorubicin for the treatment of various different kinds of cancer has been demonstrated [42]. It was concluded that the enhanced cytotoxicty of
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anticancer agents in the presence of pluronics can at least partly be assigned to P-gp inhibition. An enhanced doxorubicin accumulation in P-gp expressing cells was observed in the presence of pluronics, whereas such an effect was not observed when using non P-gp expressing cell lines [37,43]. These findings were supported by studies in which it was shown that the accumulation and permeation of various P-gp substrates was enhanced in the presence of pluronics when using MDR1 transfected cell lines. In contrast, no or only minor effects were observed in non-transfected cells [39,44]. In addition, no enhanced accumulation of non-P-gp substrates in neither resistant nor sensitive cell lines was observed [27,39,45]. Besides the capability of pluronics to inhibit P-gp, there is evidence that pluronics can also inhibit other efflux pumps including MRP1 and MRP2. However, a comparison of the effect of pluronics on P-gp ATPase activity and MRP1/MRP2 ATPase activity revealed that the effect on P-gp ATPase activity was more pronounced than that on MRP1/MRP2 ATPases [46].
CONCLUSION The potential of pluronics to inhibit efflux pumps and their efficacy in cancer therapy as well as drug delivery has been demonstrated in numerous studies. Especially an intended use of pluronics for cancer therapy is very promising and a plethora of data is already available. In conclusion, the ability of pluronics to inhibit efflux pumps is believed to play an important role in the development of novel drug formulations in the near future.
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[22] Friche, E., Jensen, P. B., Sehested, M., Demant, E. J. & Nissen, N. N. (1990). The solvents cremophor EL and Tween 80 modulate daunorubicin resistance in the multidrug resistant Ehrlich ascites tumor. Cancer Commun, 2, 297-303. [23] Liu, C., Wu, J., Shi, B., Zhang, Y., Gao, T. & Pei, Y. (2006). Enhancing the bioavailability of cyclosporine a using solid dispersion containing polyoxyethylene (40) stearate. Drug Dev Ind Pharm, 32, 115-123. [24] Werle, M. & Hoffer, M. (2006). Glutathione and thiolated chitosan inhibit multidrug resistance P-glycoprotein activity in excised small intestine. J Control Rel, 111, 41-46. [25] Föger, F., Schmitz, T. & Bernkop-Schnürch, A. (2006). In vivo evaluation of an oral delivery system for P-gp substrates based on thiolated chitosan. Biomaterials, 27, 42504255. [26] Föger, F., Hoyer, H., Kafedjiiski, K., Thaurer M. & Bernkop-Schnürch, A. (2006). In vivo comparison of various polymeric and low molecular mass inhibitors of intestinal P-glycoprotein. Biomaterials, 27, 5855-5860. [27] Miller, D. W., Batrakova, E. V., Waltner, D. O., Alakhov, V. & Kabanov, A. V. (1997). Interactions of pluronic block copolymers with brain microvessel endothelial cells: evidence for two potential pathways for drug absorption. Bioconjug Chem, 8, 649 - 657. [28] Hunter, R. L., McNicholl, J., Lal, A. A. (1994). Mechanisms of action of nonionic block copolymer adjuvants. AIDS Res Hum Retroviruses, 10, 95-98. [29] Rodeheaver, G., Turnbull, V., Edgerton, M. T., Kurtz, L. & Edlich, R. F. (1976). Pharmacokinetics of a new skin wound cleanser. Am J Surg, 132, 67-74. [30] Kabanov, A. V., Batrakova, E. V. & Miller, D. W. (2003). Pluronic block copolymers as modulators of drug efflux transporter activity in the blood-brain barrier. Adv Drug Deliv Rev, 55, [31] Slepnev, V. I., Kuznetsova, L. E., Gubin, A. N., Batrakova, E. V., Alakhov, V. & Kabanov, A. V. (1992). Micelles of poly(oxyethylene)-poly(oxypropylene) block copolymer (pluronic) as a tool for low-molecular compound delivery into a cell: phosphorylation of intracellular proteins with micelle incorporated [gamma-32P]ATP. Biochem Int, 26, 587-595. [32] Batrakova, E., Li, S., Alakhov, V. & Kabanov, A. V. (2000). Selective energy depletion and sensitization of multiple drug resistant cancer cells by Pluronic block copolymers. Polym Prep, 41, 1639-1640. [33] Batrakova, E., Li, S., Elmquist, W. F., Miller, D. W., Alakhov, V. & Kabanov, A. V. (2001). Mechanism of sensitization of MDR cancer cells by Pluronic block copolymers: selective energy depletion. Br J Cancer, 85, 1987-1997. [34] Melik-Nubarov, N. S., Pomaz, O. O., Dorodnych, T. Y., Badun, G. A., Ksenofontov, A. L., Schemchukova, O. B., Arzhakov, S. A. (1999). Interaction of tumor and normal blood cells with ethylene oxide and propylene oxide block copolymers. FEBS Lett, 446, 194-198. [35] Batrakova, E. V., Li, S., Vinogradov, S. V., Alakhov, V., Miller, D. W. & Kabanov, A. V. (2001). Mechanism of pluronic effect on p-glycoprotein efflux system in blood brain barrier: contributions of energy depletion and membrane fluidization. Pharmacol Exp Ther, 299, 483 - 493. [36] Regev, R., Assaraf Y. G. & Eytan G. D. (1999). Membrane fluidization by ether, other anesthetics, and certain agents abolishes P-glycoprotein ATPase activity and modulates efflux from multidrug-resistant cells. Eur J Biochem, 259, 18-24.
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[37] Batrakova, E., Lee, S., Li, S., Venne, A., Alakhov, V. & Kabanov, A. V. (1999). Fundamental relationships between the composition of pluronic block copolymers and their hypersensitization effect in MDR cancer cells. Pharm Res, 16, 1373-1379. [38] Pardridge W. M., editors Introdcution to the Blood-Brain Barrier. Methodology, Biology and Pathology ed., Cambridge: University Press., 1998; 486. [39] Batrakova, E. V., Miller, D. W., Li, S., Alakhov, V., Kabanov, A. V. & Elmquist, W. F. (2001). Pluronic P85 enhances the delivery of digoxin to the brain: in vitro and in vivo studies. J Pharmacol Exp Ther, 296, 551-557. [40] Batrakova, E. V., Li, S., Miller, D. W. & Kabanov, A. V. (1999). Pluronic P85 increases permeability of a broad spectrum of drugs in polarized BBMEC and Caco-2 cell monolayers. Pharm Res, 16, 1366-1372. [41] Kabanov, A. V., Batrakova, E. V. & Alakhov, V. Y. (2002). Pluronic block copolymers for overcoming drug resistance in cancer. Adv Drug Deliv Rev, 54, 759-779. [42] Alakhov, V., Klinksi, E., Li, S., Pietrzynski, G., Venne, A., Batrakova, E., Bronitch, T. & Kabanov, A. V. (1999). Block co-polymer based formulation of doxorubicin. From cell screen to clinical trials. Colloids Surf B: Biointerfaces, 16, 113-134. [43] Venne, A., Li, S., Mandeville, R., Kabanov, A. V. & Alakhov, V. (1996). Hypersensitizing effect of pluronic L61 on cytotoxic activity, transport, and subcellular distribution of doxorubicin in multiple drug-resistant cells. Cancer Res, 56, 3626-3629. [44] Evers, R., Kool, M., Smith, A. J., van Deemter, L., de Haas, M. & Borst, P. (2000). Inhibitory effect of the reversal agents V-104, GF120918 and Pluronic L61 on MDR1 Pgp-, MRP1- and MRP2-mediated transport. Br J Cancer, 83, 366-374. [45] Batrakova, E. V., Han, H. Y., Alakhov, V., Miller, D. W. & Kabanov, A. V. (1998). Effects of pluronic block copolymers on drug absorption in Caco-2 cell monolayers. Pharm Res, 15, 850-855. [46] Batrakova, E. V., Li, S., Li, Y., Alakhov, V. Y. & Kabanov, A. V. (2004). Effect of pluronic P85 on ATPase activity of drug efflux transporters. Pharm Res, 21, 22262233.
In: Encyclopedia of Polymer Research Ed: Caroline E. Jones
ISBN: 978-1-61761-926-7 © 2011 Nova Science Publishers, Inc.
Chapter 8
THE ABSENCE OF PHYSICAL AGING EFFECTS IN THE SURFACE REGION OF GLASSY POLYMERS Z. Yang Department of Physics, The Hong Kong University of Science and Technology, Clearwater Bay, Kowloon, Hong Kong
ABSTRACT The effects of physical aging near the surface region of glassy polymers are studied via the relaxations of (1) surface topographic features created by rubbing, and (2) the rubbing induced birefringence (RIB). Extensive experimental results are presented to show that physical aging processes that would have drastic effects on the relaxations of bulk polymers have little effects on the relaxations of rubbed surfaces. We also found that surface topographic features, such as ditches and ridges created by rubbing, relax at temperatures at about 20 °C below the bulk glass transition temperature of the polystyrene for the molecular weight of 442 kg/mol, even though the Laplace Pressure driving the relaxation is 1/500 of the yield limit. The relaxation of RIB in polystyrene (PS), its derivatives with modified side group, and polycarbonate (PC), involves only the length scale of the order of an individual segment. A phenomenological model based on individual birefringence elements is proposed for the RIB relaxation. The relaxation times (RT’s) of the elements are found to be independent of the thermal or stress history of the samples, either before or after the formation of the birefringence. The RT’s are also independent of the molecular weight, rubbing conditions, and film thickness, while the RT’s distribution function does depend on the molecular weight and rubbing conditions. The model provides quantitative interpretations that agree very well with all the reported experimental results, and sheds important light to the novel behaviors of the RIB relaxation. The absence of physical aging effects is probably due to the combined effects of small length scale of the RIB relaxation, and the accelerated aging speed in the near surface region. This is consistent with the mobility enhancement in the surface layer previously reported in the literature.
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PART I. INTRODUCTION Upon cooling a polymer melt or rubber, its volume (enthalpy) begins to deviate from the equilibrium liquid value at glass transition temperature (Tg) that depends upon the cooling rate. When the cooling is stopped the glass spontaneously evolves towards its equilibrium thermodynamic state (volume or enthalpy) [1]. Associated with the changes in thermodynamic structure are the changes in the mechanical, dielectric, etc., responses of the material that have come to be referred to as physical aging [2]. In general, the behavior of the physical aging response for the linear viscoelastic properties of polymers is widely accepted to follow the broad outlines put forward by Struik [2] and often referred to as following a time-aging time superposition principle analogous to the time-temperature superposition found in viscoelasticity of polymer melts [3]. For creep compliance, typical value of the characteristic time τ0 of polystyrene (PS) at 60 ºC in thermodynamic equilibrium state is well beyond the realistic experimental time scale [3], but its value of a sample freshly quenched from above Tg (~ 100 ºC) is many orders of magnitude smaller than the one in equilibrium state [1]. PS pre-strained below Tg also exhibits much shorter τ0 at 40 ºC [4]. The compliance curve then shifts to longer time along the logarithmic time axis as the aging time increases, following the time-aging time superposition principle. Compliance curves at different aging times can be shifted to form a master curve. If the evolution of the creep compliance is viewed as the relaxations of microscopic segmental units, then all the time constants of the relaxation elements are shifted by the same factor (thermorheologically simple) [1]. Thermal quench aging and stress aging behave qualitatively the same [1] [2] [4]. Quantitatively there are differences. The time-aging time exponential factor is smaller for increasing strain [4]. It is modeled in the strain dependent aging clock theory [5]. Recent constitutive theory based on Holmholtz free energy can fit a wide range of physical aging phenomenon quantitatively [6]. Aging in glassy polymer films were also investigated in the frame of solvent induced glass transition [7] [8]. Results are utterly different from those obtained with equivalent temperature cycles. Hysteresis at small activity as well as complex behavior of the melting time depending on the quench amplitude had been observed. This behavior was attributed to the possible coupled evolution of the free volume and the solvent fraction. Physical aging also exists in thin films. The gas permeability of 400 nm thick polysulfone decreases when the films continued to densify [9]. Thin films age faster in gas permeability [10]. Kawana and Jones reported [11] thin supported films of PS aged below Tg exhibit clear overshooting in expansivity temperature curves when reheated for thicknesses of 18 nm or more, but not for a thickness of 10 nm, suggesting the existence of a surface layer of the order of 10 nm in which aging is complete within the time scale of 103 s, while the rest of the film essentially has thermal properties identical to bulk samples. In this article we review the work on the effects of physical aging in the region near the surface of glassy polymers. In Part-II the relaxation of topographic features of PS surface is examined. In Part-III the relaxation of RIB in PS is examined. In Part-IV the relaxation of RIB in PS derivatives and polycarbonate (PC) is examined. In Part-V we conclude.
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 245
PART II. TOPOLOGICAL RELAXATION OF POLYSTYRENE SURFACE 1. Introduction The dynamics of polymer surfaces is an intriguing subject in polymer physics, in addition of being of significant technological importance. There has been much experimental evidence that demonstrate the existence of a surface mobile layer [12 – 24]. Positron annihilation [12] and muon spin probes [13] indicated the presence of a surface mobile layer about 2 nm in thickness near the polystyrene (PS) surface. The Tg of PS thin films were found to decrease drastically with the reduction of thickness [14] [15], and later studies revealed the broadening of the glass transition in polymer thin films with depth [16] [17] [18]. These phenomena were attributed to the existence of a liquid-like surface layer that extended deeper with the rise of temperature and diverged at bulk Tg. Surface friction force microscopy also showed a significant decrease in Tg of the surface layers [19] [20] [21]. The higher mobility of the surface layer was attributed to the chain ends enrichment at the surface, which was confirmed by experiments [20]. Gold spheres of 10 nm and 20 nm in diameter placed on PS surfaces were found to sink into PS at temperatures well below bulk Tg [22] [23], and the phenomenon was interpreted as due to the presence of a liquid-like surface layer. Rough PS surfaces were found to turn mobile at temperatures well below Tg under the driving force of Laplace Pressure [24]. Despite of the rich evidence listed above, there are a few equally strong experimental evidence that seem to contradict the presence of such mobile layer [25] [26]. Using a vigorous viscoelastic mechanic contact analysis, Hutcheson and McKenna showed that the sinking of gold spheres on PS surface could actually take place without the decrease in surface Tg [25]. The creep compliance of 30 nm thick PS films was found to have a lower Tg but a much stiffened rubbery plateau than the bulk value [26]. A direct measurement of the local segmental mobility as a function of depth from surface is strongly desired. But such an experiment without disturbing the segmental motions near the surface would be a rather difficult challenge. However, if physical aging is fast in the near surface region and can come to completion within reasonably short time (~ 103 s), the complication of physical aging effects on the dynamic properties of surface will no longer be present and the analysis of the properties of surface relaxation could be simplified. Gentle rubbing with a velvet cloth on PS surfaces can create molecular segment alignment and distortion [27] [28], in addition to surface ridges and ditches with depths and heights of the order of nanometers. The relaxations of the rubbing induced optical birefringence [29] [30] [31] and the segmental alignments [27] [28] [32] were investigated. These relaxations, however, occurred at even lower temperature than the surface topography relaxations [31]. Preliminary experimental evidence showed that the birefringence relaxation is also independent of the aging effects [31]. We study the evolution of the topological features of rubbed surfaces of samples undergoing various aging processes. The approach is similar to creep experiments that are widely employed in the study of segmental relaxations in bulk polymers, but with two distinctions. One is that here the stress due to the Laplace Pressure (LP) from the surface topographic features, such as ditches and ridges created by rubbing, is present at all time until
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the surface is mostly flattened. The other more important one is that the process to generate the initial roughness (by rubbing with a velvet cloth) introduces large mechanical deformation on the surface, and deformation is known to greatly accelerate the relaxations in bulk samples [4]. Extensive experimental results will be presented to reveal the relaxations of PS surfaces deformed by rubbing with velvet cloth. We found that the dominant driving force of the relaxations is the LP due to the surface topographic features. The surface layer turns to rubbery at about 20 °C below the bulk glass transition temperature for the molecular weight of 442 kg/mol, even though the LP is only about 1/500 of the yield limit. However, this is not due to the accelerated relaxations caused by large mechanical deformation often observed in bulk samples, because the relaxation is independent of the thermal aging before the rubbing process, and post rubbing aging below 55 °C. In other words, the effects of these physical aging processes that would have great influence on the relaxations of bulk polymers are completely absent in the relaxations of surface strain.
2. Experiments Mono-dispersed PS with molecular weights 442 kg/mol (Mw/Mn < 1.1) in toluene solution was spin coated on silicon wafers with 20 nm of thermal SiO2 to form 62 nm films. The samples were then annealed in vacuum at 150°C for several days to completely drive off the solvent before slowly cooled down (< 0.1 ºC/min) to room temperature. These will be referred to as fresh samples from now on. Rubbing was done at normal pressure of 90 kg/m2 and at a constant speed of 1.0 × 10-2 m/s at 23 ºC. Each rubbing run consisted of 90 passes, with each pass covering 2.0 cm distance. All were done in ambient atmosphere so no solvent induced effects occurred [7]. The samples were thick enough that possible influence of the PS/substrate interface was avoided. This was confirmed by PS films on two types of substrates, namely the SiO2 which is known to have weak interaction, and hydrogen terminated Si which is known to have strong interaction with the PS films [33]. No difference in surface strain relaxations was observed for the films on the two types of substrates. For the sake of completeness, the samples investigated were divided into two series, namely the A-series and the B-series. Starting from fresh samples, the A-series samples underwent thermal aging processes before rubbing, while the B-series samples underwent thermal aging processes after rubbing. There are three possibilities of aging effects. The first is that the aging prior to rubbing affects the morphology of the rubbed surfaces and their subsequent relaxations. The second is that due to large mechanical deformation the previous thermal history is ‘erased’ by rubbing so the relaxation depends on post rubbing aging only. The third is that the aging has no effect on the surface relaxation. The study of the A-series samples will substantiate or rule out possibility-1. If possibility-1 is ruled out, the study of the B-series samples will then decide whether possibility-2 or -3 is true. The surface topography was measured on a scanning probe microscope (SPM) in tapping mode. The lateral resolution is about 10 nm and the height resolution is about 0.1 nm. An onstage heater provided the controlled thermal processes (aging and annealing). The measurement sequences were the same for both the A-series and the B-series samples. The samples were measured using either type-I or type-II measurement sequences. In the type-I measurement sequence, the sample was first raised quickly from 23 ºC to the annealing
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 247 temperature (say 40 ºC), kept there for 0.5 hours, then quickly cool down to 23 ºC. SPM measurements were then carried out on a selected 5 × 5 μm2 area and completed within 1 hour at 23 ºC. The process was then repeated but with a higher annealing temperature, until the surface had flattened. In the type-II measurements, the sample temperature was quickly raised to a fixed value (say 85 ºC) for a certain duration (say 0.5 hours), then quickly cooled down to 23 ºC for SPM measurements. The process was then repeated except that the duration (annealing time) the sample temperature was maintained at the given value was progressively longer. In all, the annealing times were 0.5 hours, 1.0 hours, 3.0 hours, and 10 hours. This resulted in the accumulative annealing time of the samples as 0.5 hours, 1.5 hours, 4.5 hours, and 14.5 hours. In both measurement sequences the total time taken to measure the viscoelastic response of the samples (< 20 hours) was much shorter than the aging times (up to 7 days). Several images of nearly the same area were analyzed in both sequences. Using recognizable topological features, a smaller but identical area was selected from the set of micrographs for analysis. This enabled us to track the evolution of particular topological features such as ditches and ridges, as well as statistical averages such as the root mean square roughness < h 2 > − < h > 2 , over the whole area. The main reason SPM was carried out only at 23 ºC was that the PS surfaces remained frozen at such low temperature, where no detectable change of the topographic features was found in any samples up to 7 days. The surfaces undergoing relaxation at elevated temperatures were frozen once they were quickly cooled to 23 ºC, allowing ‘snap shots’ to be taken without the surfaces going through continuous changes.
3. Results and Discussions 3.1. Typical surface relaxation behavior A reference sample (Sample-A1), which was a fresh sample rubbed and then measured within one hour, was studied first. Figure 1 shows a typical topographic image of the sample surface just after rubbing (Figure 1(a)), and after consecutive 30-minute annealing at 40 °C, 50°C, 60°C, 70°C, 80°C, 90°C, and 100°C (Figure 1(b)), following the type-I measurement sequence. The fresh surface contains ditches and ridges along the rubbing direction. A ‘roughness spectrum’ obtained from the Fourier transform of a line scan across the rubbing direction is shown in Figure 1(c). The dominant features are about a fraction of micrometer in width. Features less than 20 nm are nearly absent. However, it is possible that there are smaller features that cannot be resolved by the limited lateral resolution of the AFM tip. Our analysis is therefore limited to the features that can be resolved in our instrument, which is comparable to the study reported in Ref. [24]. The surface remained unchanged up to 80°C. The deep ditches that had survived the annealing at 100°C can be matched well with the original ones before annealing. The images taken after annealing above 80°C show that the general evolution pattern of the surface was a nearly uniform reduction of the height of the ridges and the depth of the ditches, so the shallow ditches and low ridges disappeared first. No lateral (horizontal and perpendicular to ditch/ridge) movement of the ditch/ridge walls was observed. The center positions of the ridges and ditches remained unchanged. At 100°C the reduction in height or depth across the entire area ranged between 2 – 3 nm, and the shallow ditches with original depth < 2 nm were smeared out. There was no irregular local
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stress driven relaxation [34], indicating that the LP was the main driving force [24]. This is consistent with the statistical aspect ratio data in Ref. [24]. The average height differences < Δh >line between several ditch/ridge pairs, a typical one being marked by a pair of small arrows at the bottom of Figure 1(a), are plotted as a function of annealing temperature in Figure 1(d). It is seen that the reduction of < Δh >line with annealing temperature follows the same trend, regardless of the initial height differences of the pairs. In particular, the reduction from 23 °C to 100 °C is 3.4 ± 0.4 nm for all the pairs with initial height difference ranging from 5 nm to 11 nm. This is true for all other samples reported in this study. Figure 2 depicts the average roughness over the whole area < Δh > area as a function of annealing temperature together with that of the other four samples underwent various aging processes to be described later. The behavior of the average roughness resembles that of the height differences of the ridge/ditch pairs shown in Figure 1(d). Noticeable changes of morphology features (≥ 0.1 nm) occurred only at ≥ 80 ºC, and the decrease in roughness was rather rapid beyond this point. Because the average roughness < Δh > area and the individual ridge/ditch < Δh >line relaxations behaved the same, the average roughness < Δh > area will be used as the indicator of the surface relaxations in the rest part of the article. The main driving force to flatten a rough polymer surface is the Laplace Pressure P = γ 0 / ρ [24], where γ0 = 0.035 N/m is the surface energy of PS [25] [35], and ρ is the surface curvature along the direction normal to the surface. In the case of an indent of 2 radius w and depth h, the effective radius is approximately given by ρ = w , assuming that 4h the arc passes through the middle point of the bottom and the edge of the indent. The same applies to a long ditch of width w and depth h. The LP becomes P = 4γ 0 h / w2
(1)
Typical depth (height) and width of ditches (ridges) in Figure 1 (h ≈ 10 nm, w ≈ 100 nm) gives rise to LP ≈ 1.4 × 105 N/m2, and varies by a factor of < 4 across the surface. This is about 1/500 of the yield stress of PS [4], so the relaxation was under a driving force well below the yielding limit [36]. The reduction of roughness on the surface is given by, in analogy to conventional creep theory, C = PJ (T , t )
(2)
where C is the ‘relative deformation’ with the freshly rubbed surface as the reference , P is the Laplace Pressure, t is the time and J(T, t) is the surface effective compliance (SEC), which is expected to be a sensitive function of temperature T. Its counterpart, the bulk creep compliance, also depends sensitively on the thermal history of the sample, i. e., the effects of physical aging [1]. As the width w remains unchanged the average LP of the area under investigation is proportional to the roughness, and the ‘relative deformation’ is proportional to the change of roughness.
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 249 23 ºC
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(b)
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Figure 1. (a) SPM image of a rubbed PS surface; (b) SPM image of the same area as (a) after annealing at consecutive temperatures up to 100 ºC described in the text; (c) A roughness spectrum obtained from the Fourier transform of a typical line scan across the ridges and ditches in Figure 1(a); (d) The height difference of selected ridge/ditch pairs as a function of annealing temperature. All the images were taken from a reference Sample-A1, i. e., it was in thermal equilibrium before rubbing and the measurements were taken right after rubbing.
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<Δh>area (nm)
4 3 2 1
A-1 (Ref) A-2 A-3
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<Δh>area (nm)
4 3 2 1
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0 20
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Temperature (°C) Figure 2. (a) The surface average roughness versus temperature curves of the reference sample and the A-series samples underwent various aging processes before rubbing. The points for A-1 have been shifted upwards by 1.0 nm, while that of the A-3 have been shifted down by 0.5 nm. (b) The surface average roughness versus temperature curves of the reference sample and the B-series samples underwent various aging processes after rubbing. The points for A-1 have been shifted upwards by 1.0 nm, while that of the B-3 have been shifted down by 0.5 nm.
Consider the red curve in Figure 1(d) as an example. At 90 °C the reduction of height difference < Δh >line as compared to the fresh surface is about 1.5 nm from the initial 6.0 nm value. For the sake of discussions we take two extreme scenarios. If the entire 62 nm film is involved in the relaxation one obtains a relative change along the film depth of about 2.5 %. Since the LP is only 1.4 × 105 Pa, it results in an average compliance over the entire film depth of about 1.8 × 10-7 Pa-1, much higher than the bulk compliance in glassy state (JG = 8.5 × 10-10 Pa-1). If only the initially deformed layer (~ 6.0 nm) is taken as reference the relative change is then 25 %, and the compliance is close to that of the bulk rubbery plateau (JR = 6.1 × 10-6 Pa-1). The actual situation is probably somewhere in between, but even in the extreme case the effective compliance is still much higher than JG. In the meantime, as no lateral movement of ditches and ridges was observed, and considering the fact that the spacing between ditches is hundreds of nanometers as compared to the film thickness of 62 nm, the randomly distributed local stress [34], if ever present, must be at least 200 times smaller than the LP.
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 251 Vs
A
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A B2 B3 B 23 ºC
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Tg
T
*
Figure 3. (a) The schematics of the free volume evolution of A-series samples. (b) The schematics of the free volume evolution of B-series samples.
The change of surface topographic features in glassy state under the present LP is expected to be about ~ 1.2 × 10-4 in relative change and ~ 8 × 10-3 nm in actual displacement, which is well below the SPM detection limit. Therefore, only when the surface is near the rubbery state can the displacement be detected. For a ditch 10 nm deep, a height reduction of 3 nm due to the increase of temperature from 23 °C to 100 °C reduced the original LP by only 30 %, while the SEC changed by several orders of magnitude. It is therefore a good approximation to take LP as nearly constant when analyzing the drastic change of SEC with temperature, and the roughness (height difference) versus annealing temperature curves in Figure 1(d) and Figure 2 therefore represent a measure of the viscoelastic SEC as a function of temperature within a fixed time scale of 30 minutes. In the following, we examine the physical aging effects on the roughness versus temperature curves of samples undergone different thermal processes. In particular, we concentrate on the onset temperature of the glassy-to-rubbery transition, and examine the curves up to 110 ºC when there was still considerable roughness left and the driving force, while somewhat weakened, was still present.
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3.2. Aging before rubbing In Figure 3(a) the bulk specific volume of the surface layers of the PS samples undergoing various physical aging processes is shown schematically. Following Struik [1] [2] we use the term ‘specific volume’ to describe the degree of deviation from thermodynamic equilibrium of the system and the enhancement of segmental mobility in general term, rather than the actual excessive volume, since mechanical deformation may not actually decrease the actual mass density but does increase the segmental mobility, or ‘free volume’ [4]. In the following discussions the speculation about the thermodynamic states of the samples is based upon the assumptions that (i) the aging behavior of the surface region of the rubbed PS follows the general trend of bulk PS undergoing the same processes; and (ii) the rubbing process places the samples away from thermodynamic equilibrium but not enough to ‘erase’ the effect of previous thermal history. As shown in Figure 3(a), Sample-A1 was in thermodynamic equilibrium before rubbing, while after rubbing it was away from the thermodynamic equilibrium state as indicated by the dashed line below Tg. During the subsequent measurement sequence, its specific volume evolved close to the thermodynamic equilibrium line. It served as a reference sample and its surface relaxation has already been presented in sub-section 3.1. Sample-A2 was quenched from 130°C to 23°C and rubbed within 10 minutes. Its specific volume is expected to be far away from the equilibrium line. During the subsequent measurement sequence, it might age slightly towards equilibrium, but it would evolve far away from the equilibrium line, and its segmental mobility is expected to be well above that of Sample-A1. As a result, its surface is expected to turn rubbery at lower temperature than Sample-A1. Sample-A3 was quenched from 130 ºC to 70 ºC, aged there for 7 days, then quenched to 23 ºC, and rubbed. Due to the 7 days aging its specific volume is expected to be closer to the equilibrium line than Sample-A2, but may not be as close as Sample-A1, in the subsequent SPM measurements. Although the three samples had been under different thermal treatments before rubbing, their surface morphology characteristics after rubbing were the same. The typical ditches and ridges shown in Figure 1 were observed on all three surfaces, and the average roughness over some typical areas was about the same. This implies that before rubbing the surface of Sample-A2 was not much ‘softer’ than Sample-A1 in thermal equilibrium, which is in stark contrast with the bulk cases where a freshly quenched sample would have much higher creep compliance [1]. The subsequent relaxation behaviors of the three surfaces were the same. All the surfaces remained frozen below 80 ºC, and started to turn rubbery above 80 ºC. Figure 2(a) shows the < Δh > area versus temperature curves of the three A-series samples. For clarity, the red points for the Sample-A1 are shifted up by 1 nm, and the green points for Sample-A3 are down shifted by 0.5 nm. As the LP was comparable in all three samples, their surface roughness evolution is a direct measure of the SEC as a function of temperature. Adopting the approach in determining Tg from thermal expansion curves, we use the cross point of two lines, one horizontal when the surface remains frozen and the other when the surface is relaxing, as the onset temperature of the glassy-to-rubbery transition TR. The values of TR are (86 ± 2) ºC, (88 ± 2) ºC, and (86 ± 2) ºC for samples A-1, A-2, and A-3, respectively. The results imply that the SEC remained unchanged by the thermal history before rubbing. Such results rule out
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 253 possibility-1 speculated in Figure 3, leaving possibility-2 and -3 for further investigations. One is that the rubbing process had effectively erased the effects of all the previous thermal history, and placed the surface layers of all three samples in nearly the same state far from thermodynamic equilibrium, such as in the neighborhood of point-A in Figure 3(a), which is well above all the volume curves due to various thermal quenches. The other possibility is that the surface layers of all samples were at point-B (in thermodynamic equilibrium) before the measurement sequence was started, due to the quick aging of the surface layer despite of the large mechanical deformation caused by rubbing. The original viscoelastic properties of the surface were therefore restored before the start of the SPM measurements.
3.3. Aging after rubbing As discussed above, rubbing introduces large mechanical deformation to the surface, which may effectively ‘erase’ all the previous thermal history and place the surface in nearly the same non-equilibrium state. Here we study the effects of aging after rubbing. Since we found that surface features would change significantly at above 80 ºC, all post rubbing aging processes reported below were done at < 60 ºC. Similar to the discussions in sub-section 3.2, in Figure 3(b) the bulk specific volume of PS samples undergoing various physical aging processes is shown schematically. We take the first scenarios as suggested at the end of sub-section 3.2, and assume that the rubbing processes placed the samples far away from thermal equilibrium, i. e, at point-A in Figure 3(b). Therefore, the reference Sample-A1 was at point-A after rubbing. Sample-B2 was aged at 23 ºC for 4 days after rubbing, before being tested in the same way as the A-series samples. Assuming that the surface layer aged at a rate comparable to bulk, Sample-B2 should be closer to the thermodynamic equilibrium line than Sample-A1. Sample-B3 was aged at 50 ºC for 2 days after rubbing, then quenched to 23 ºC and tested. Right before testing the sample was probably closer to the equilibrium line than Sample-B2. Despite of the large differences in their thermal history after rubbing, the observed surface morphology evolution of the two samples is the same as the A-series samples. Their roughness at different annealing temperature as plotted in Figure 2(b). The transition temperature for the three samples are (86 ± 2) ºC, (84 ± 2) ºC, and (88 ± 2) ºC for samples A-1, B-2, and B-3, respectively. These experimental results rule out the possibility that the surface layer of PS follows the aging behavior of bulk polymers. Physical aging affects the temperature dependence of a number of dynamic responses of bulk polymers. It is well known that in differential scanning calorimetry there is a large enthalpy peak right below Tg for quenched samples, and the peak reduces as the samples age [37]. Physical aging of PS was investigated by dynamic viscoelasticity measurements [38]. The effect of physical aging was observed for samples aged at 60 and 80 ºC after quenched from 190 ºC. The temperature curves of the dynamic viscoelastic functions of the bulk samples right after quench and that of well aged were significantly different. The curves of the samples partially aged at 60 ºC coincided with that of the quenched sample in the low temperature range. It then started to deviate from the curve at above 65 ºC and gradually merged to the curve of the well aged sample at high temperature. The ‘merge temperature’ increased with the aging time. On the other hand, the curves of the samples aged at 80ºC differed from the curves of the well aged ones and the quenched samples in the entire temperature range. This is significantly different from the aging behavior of the surface viscoelastic response of our samples.
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We now study the post rubbing aging using the type-II measurements, which according to the discussion in subsection 3.1, are effectively the measurements of the viscoelastic SEC versus time at a fixed temperature. The annealing temperature is 85 ºC. Sample-C1 is a reference sample similar to Sample-A1 and tested within one hour after rubbing. Before the measurements Sample-C2 was aged at 50 ºC for 2 days, and then quenched to 23 ºC. SampleC3 was aged at 23 ºC for 7 days after rubbing. The speculation on their thermodynamic states follows the general arguments on the samples underwent similar processes. Figure 4 shows the isothermal temporal evolution of the average roughness at 85 ºC of the three samples. Individual surface topological features behaved the same as the average roughness. Following the same argument above, it is clear that at 85 ºC the surface is near rubbery state. Furthermore, the temporal evolutions of the three samples are identical within experimental uncertainties. In particular, since the aging times of Sample-C1 and Sample-C3 differed by about 170 times, even at a shift rate of 0.6 [2], the bulk creep compliance curve of Sample-C3 should have shifted by a factor of 22 (= 1700.6) relative to Sample-C1 along the time axis. Had the SEC followed the bulk creep compliance, it would have taken 22 hours for the roughness of Sample-C3 to decrease by the same amount as Sample-C1 in 1.0 hour. Similarly, the bulk shift factor at 50 ºC is about unity, and the aging time of Sample-C2 was 48 times of that of Sample-C1, so the time shift factor between Sample-C2 and Sample-C1 is about 48, i. e., it would have taken more than 48 hours for the roughness of Sample-C2 to decrease by the same amount as Sample-C1 in 1.0 hour. No detectable drop in its roughness should have been observed within the experimental time span of 14.5 hours. The fact that the SEC of all three samples behaved the same is consistent with a surface layer in which aging effects are absent.
<Δh>area (nm)
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0
4
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Time (Hrs.) Figure 4. The isothermal temporal evolution at 85°C of the surface average roughness of three samples aged according to the description in the text.
3.4. Discussions The experimental results in this paper show that the aging effects are absent in the relaxation of surface of PS. This implies that in the surface region either aging was complete within an hour at room temperature, or aging was extremely slow that even after 7 days no noticeable effects could be observed, a very unlikely scenario that is in stark contradiction to
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 255 the previous reports [9] [12 – 24]. As the surface ages quickly, the deformed surface recovers its thermodynamic properties within the 1 hour time scale. We therefore suggest that their viscoelastic properties determined from the evolution of surface topographic features at elevated temperatures are the intrinsic properties of PS surfaces. Indeed, relaxations of surfaces deformed in two different ways, namely by rubbing below Tg (like cold rolling), and by deformation above Tg (like hot drawing) [24], behaved much the same. It is consistent with the findings in Ref. [11] that physical aging effects were absent in PS films 10 nm thick, and is consistent with the general expectation of the enhanced mobility in the surface layer. The observed surface relaxation behaviors, apart from their independence on physical aging, are themselves intriguing. For example, in bulk creep experiments, the deformation of the specimen creates internal stress that counter balances the external force until reaching the balance point before the rubbery plateau is reached over a long period. In the surface relaxation studied here, the only ‘counter balance’ is the reduction of LP due to the reduction of roughness. As seen in Figure 4, the roughness decrease in the first 4 hours is about 0.2 nm, which is twice the decrease (0.1 nm) in the following 10 hours. In other words, the relaxation rate became slower with increasing time. Such behavior cannot be accounted for by a film with uniform compliance that remains constant or increases with time as in the bulk, because it cannot explain why under nearly the same LP (reduced by only 10 %), the reduction of roughness becomes much slower at longer time. Instead, it hints a depth dependent compliance which is much enhanced at the surface. As the absence of aging effects has been established in this work, the stage is set for the detailed analysis of these surface relaxation behaviors, which will be reported in our subsequent publications.
4. Conclusion Extensive experimental results have been presented to reveal the relaxations of polystyrene surface deformed by rubbing with velvet cloth. We found that the relaxations were mostly driven by the Laplace Pressure due to the surface topographic features, such as ditches and ridges created by rubbing. The relaxation at elevated temperatures was dominated by a nearly uniform reduction of the depth of the ditches and the height of the ridges across the surface, while the centers of these topological features remained fixed. There was no lateral movement (perpendicular to the ditches/ridges) of ditch/ridge walls. The randomly distributed stress, if present, was at least 200 times smaller than the Laplace Pressure. For PS with molecular weight of 442 kg/mol the surface layer turned to rubbery at about 20 °C below the bulk glass transition temperature, even though the Laplace Pressure was only about 1/500 of the yield limit. This, however, was not due to the accelerated relaxations due to mechanical deformation, a phenomenon well known in bulk polymer samples, because the relaxation was independent of the thermal history before the rubbing process, and post rubbing history below 50 °C. Physical aging processes at 23 °C for up to 7 days and at 50 °C for 2 days, which would have drastic effects on the relaxations of bulk polymers, have little effects on the relaxations of strained surfaces. This is consistent with the observed mobility enhancement in the surface layer. The observed enhanced mobility of the surface layer is most likely the intrinsic property of a polymer surface, which is consistent with many previous experimental results.
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PART III. RELAXATION OF RUBBING INDUCED BIREFRINGENCE IN POLYSTYRENE 1. Introduction The relaxations and related dynamic properties of glass forming polymers have been extensively studied in the past decades [39 - 48]. The birefringence of polystyrene (PS) uniaxially drawn at above the glass transition temperature (Tg) remains almost unchanged at temperatures below Tg. Above Tg there are three well separated relaxation steps with distinct time scales τ1, τ2, and τ3 [40]. The relaxation of the segmental alignment dichroism of cold rolled PS at 60 °C can be fitted by two single exponential functions of time with relaxation times of 76 s and 3600 s [39]. Many other relaxation processes below Tg follow the Kohlrausch-Williams-Watts (KWW) stretched exponential function [41] [42] [43]:
φ (t ) = φ (0)e − (t / τ
0
)β
(1)
These include creep compliance [41], density fluctuations [42], volume relaxation of quenched PS [43], etc.. Unlike the cold rolled or hot drawn PS where the polymer specimen is usually elongated by several times of its original length, rubbing with a velvet cloth on a PS surface at a modest pressure of (~ 9 g/cm2) and speed (~ 1 cm/s) creates segmental alignment and segmental distortion, mostly within the top 20 nm layer near the surface [27] [31]. The surface layer is severely deformed, with ditches up to 5 nm deep, but the average thickness of the PS film, as measured by ellipsometry, remains unchanged, implying that little materials are removed by the rubbing cloth. The majority (> 89 %) of the rubbing induced birefringence (RIB) in PS comes from the segmental distortion. Only less than 11 % comes from the segmental alignment [27]. For a 20 nm thick PS, the RIB can be as large as 20 % of the intrinsic value of undistorted PS segments. The RIB relaxations of a number of polymers were first studied by Kovacs and Hobbs [44], and more recently by a number of groups [27] [28] [30] [31] [45], but there is no consistent theoretical interpretation [30] [45] [46]. Upon cooling a polymer melt, its volume (enthalpy) begins to deviate from the equilibrium liquid value at Tg that depends upon the cooling rate. When the cooling is stopped the glass spontaneously evolves towards its equilibrium thermodynamic state (volume or enthalpy) [1]. Associated with the changes in thermodynamic structure are changes in the mechanical, dielectric, etc., responses of the material that have come to be referred to as physical aging [2]. In general, the behavior of the physical aging response for the linear viscoelastic properties of polymers is widely accepted to follow the broad outlines put forward by Struik [2] and often referred to as following the timeaging time superposition principle analogous to the time-temperature superposition in the viscoelasticity of polymer melts [3].
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 257
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Δn (10 )
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40
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Figure 5. Rubbing induced birefringence versus rubbing pass of two standard samples.
The characteristic time τ0 of the creep compliance of PS at 60 ºC in thermodynamic equilibrium state is well beyond the realistic experimental time scale, but its value of a sample freshly quenched from above Tg (~ 100 ºC) is many orders of magnitude smaller [1]. PS pre-strained below Tg also exhibits much shorter τ0 at 40 ºC [4]. The compliance curve then shifts to longer time along the logarithmic time axis as the aging time increases, following the time-aging time superposition principle. Compliance curves at different aging times can be shifted to form a master curve. If the evolution of the creep compliance is viewed as the relaxations of microscopic segmental units, then all the time constants of the relaxation elements are shifted by the same factor (thermorheologically simple) [1]. Thermal quench aging and stress aging behave qualitatively the same [5]. Quantitatively there are differences. The time-aging time exponential factor is smaller for increasing strain. It is modeled in the strain dependent aging clock theory [5]. Recent constitutive theory based on Holmholtz free energy can fit a wide range of physical aging phenomenon quantitatively [6]. This is the characteristics of the cooperative relaxation that it depends sensitively on global thermodynamic states. As the RIB is due to segmental alignment and distortion, it is most relevant to examine the RIB relaxation in the context of segmental relaxations below Tg. And in such regime physical aging is expected to play an essential role. Our preliminary results indicated [31], however, that the RIB relaxes in a way that is characteristically different from any other known forms of relaxation in glass forming polymers. In this part, we explore further along this line. In particular, conventional segmental relaxation times (RT’s), such as the parameter τ0 in Eq. (1), strongly depend on the thermal and strain history, in addition to temperature [1] [4]. Extensive results will be presented to show that the RIB RT’s are independent of the thermal and strain history, while having about the same temperature dependence as the conventional segmental relaxations. This part is divided in the following way. After Introduction (Section-1) and Experiments (Section-2), experimental evidence to prove the absence of physical aging effects on the relaxation of RIB is presented in section-3. In section-4 a phenomenological model based on a generic energy barriers distribution is presented. In section-5 extensive experimental results are analyzed based on the model. The model provides quantitative interpretations that agree very well with all the reported experimental results, which in turn further confirms the absence of physical aging effects. And in section-VI we summarize the main conclusions.
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2. Experiment The details of the experiments can be found in Ref. [46]. Briefly stating, PS in toluene solutions was spin-coated on thermally grown SiO2 on silicon substrates. The samples were then annealed at 130 °C in vacuum for at least 24 hours, and slowly cooled down (< 0.1 °C/min) and stored at room temperature before testing. The thickness of the resulting PS films ranged from 7 nm to 50 nm depending on the solution concentration and spin speed. These samples will be referred to as fresh samples. As many samples were used in the study, a standard film thickness of 30 nm is taken unless otherwise specified. Fresh samples rubbed to saturation but without going through any other rubbing and thermal processes will be referred to as reference samples. Rubbing was done on a home-made apparatus. Each rubbing pass covered 2 cm distance at a speed of 1 cm/s in one direction, with a normal pressure of 9 g/cm2. The optical RIB of the PS films was measured using the reflectance difference spectroscopy at 633 nm in wavelength. Various thermal and rubbing processes were imposed on the test samples, and their effects on the RIB relaxation behaviors of the samples were studied by using the following ‘standard’ tests. The first was to measure the normalize birefringence (NB) as a function of temperature when the sample temperature was raised continuously at 2 K/min. The results are referred to as the Continuous Curves Ω(T). The second was the isothermal relaxation of NB, i. e., the NB of a sample was measured as a function of time at a fixed temperature. Note that the isothermal relaxation process also served as physical aging as both are expected to take place. The third was the combination of the two, i. e., a continuous temperature increase at 2 K/min after an isothermal process at a given temperature for a given period of time.
3. Physical Aging In this section, we examine the effects of physical aging on the relaxation of RIB.
3.1. Rubbing induced birefringence Figure 5 depicts a typical plot of RIB vs rubbing pass at 23 ºC of two fresh PS samples with molecular weights (Mw) of 99 Kg/mol. For the first sample, the RIB was recorded after each rubbing pass up to 10 passes. For the second sample the birefringence was recorded at every 5 passes. The birefringence increased quickly with the first few rubbing passes, and then gradually approached the saturated value. More than 70 % of the total birefringence was generated by the first 20 passes, and the subsequent 65 passes created the remaining birefringence. Further rubbing after saturation resulted in destroying roughly the same amount of birefringence as creating it, and the net birefringence fluctuated around the saturation value with amplitude of about 1 % of the saturation value. There was also a birefringence transient, which is not shown in the figure, of Δn ≈ 1.0 × 10-4 that relaxed within 30 s after each pass. The net birefringence value of each sample was measured within 100 s after the completion of the relaxation of the transient birefringence due to the last rubbing pass. It was then used as the normalization factor in determining the NB in the subsequent relaxation studies. After the completion of the transient, the birefringence relaxed slowly, decreasing by about Δn = 1.0 × 10-4 in the first hour after rubbing. The maximum
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 259 saturation birefringence was about 20 % of the intrinsic segmental alignment value [40]. However, since only about 10 % of the birefringence was from the segmental alignment and the rest from the segmental distortion [27], the degree of segmental alignment was around 2 % [40]. This could be highly non-uniform, however, with higher concentration close to the surface and decreasing with depth. The intrinsic maximum birefringence of segmental distortion is not known to the best of our knowledge, leaving the degree of segmental distortion still undetermined.
3.2. Typical relaxation behaviors of RIB and surface morphology Besides the RIB, rubbing also creates grooves on the surface which flatten and finally disappear at elevated temperatures, as reported in Part-II. One issue of significance is whether the relaxations of RIB and surface morphology are related. A typical Continuous Curve of RIB of a reference sample with Mw = 99 Kg/mol is shown in Figure 6(a), along with the evolution of surface morphology roughness (points) measured by atomic force microscope (AFM) in tapping mode. The RIB had completely relaxed near 114 ºC. If a freshly rubbed sample is heated to 130 ºC within seconds, all its RIB will disappear within 30 s. A portion of the AFM images are shown in Figures 6(b) and 6(c). The micrographs of nearly the same 5 × 5 μm2 area were recorded at 23 ºC after each annealing at the given temperature for 30 minutes. Using recognizable topological features, such as the crossing of two ditches as markers, a smaller but identical area was selected from the set of micrographs for analysis. The surface Laplace Pressure is estimated as 1.4 × 105 N/m2, and varies by a factor of < 4 across the surface. This is about 1/500 of the yield stress of PS [4]. Careful tracking of the individual topographic features showed that noticeable relaxation of morphology features (≥ 0.1 nm) occurred only at ≥ 85 ºC, and the decrease in roughness was rather rapid beyond this point. On the other hand, at 80 ºC about 30 % of the RIB had already relaxed, while no noticeable change ≥ 0.1 nm in any morphology features occurred. Also, no change in morphology could be detected if the sample was left at 23 ºC for up to 7 days, but during the same period there was significant RIB relaxation (See subsection 3.4 below). It is seen that significant RIB relaxation occurred at much lower temperatures than surface morphology relaxations, a first sign that the two are not directly related. The relaxations of RIB can take place without any collective segmental movement beyond 0.1 nm in length, which is an indication of their very small length scale. The possibility that the morphology birefringence (MB) is or contributes significantly to the observed RIB has been ruled out in our previous works [31] [46]. For the sake of completeness in the discussion, we revisit the main findings. First, the amplitude of MB can be estimated from the surface morphology, and the result showed that the amplitude is too small to account for the observed birefringence [46]. Second, we performed the following experiments [31]. Poly(vinyl alcohol) (PVA) dissolved in water was spin coated on top of some PS samples of different molecular weights after rubbing to saturation. The samples were then dried at 20 ˚C in dry air for two weeks, together with the reference samples rubbed under the same condition but without PVA cover. This resulted in a flat surface PVA cover of about 15 nm with a refractive index (= 1.5) almost the same as PS. The MB will be completely masked even though the PS/PVA interface still preserves the original PS surface morphology. However, the observed birefringence of the PS samples after PVA coating remained the same as before, varying by no more than 1 % of the original Δn, indicating that the observed birefringence was not due to surface morphology, and little birefringence was disturbed by the PVA cover. Third, if the RIB of PS was concentrated only within the very top layer, the PVA coating would certainly alter its relaxation behavior at elevated
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temperature. However, no noticeable difference in relaxation behavior at elevated temperatures was observed for samples covered with PVA as compared to the reference samples. For example, the Continuous Curves for both the PVA covered and the reference samples were essentially the same. This has two implications. First, the relaxations of PS surface grooves are expected to be hindered by the PVA, due to the reduction of Laplace Pressure and the presence of PVA that impedes the movements. But this has no effects on the RIB relaxations. Second, the RIB relaxation is mainly from the segments below the very top PS surface layer, away from the influence of the PVA cover. Combining all the evidence, we conclude that the RIB is not due to the MB of surface, and the relaxations of surface morphology and RIB are not directly related. 4 1.0
NB
0.6
2 Roughness Birefringence
0.4
1
0.2
(a)
0.0 20
40
60
80
100
Temperature (°C)
0 140
120
100 ºC
23 ºC
500 nm
500 nm 430nm
(b)
(c)
1.0
140
0.8
120 100
0.6
80 0.4 60 0.2
40
(d)
0.0 0
2
4
6
8
Tempearture (°C)
NB
Roughness (nm)
3
0.8
10
Time (KS) Figure 6. (a) The surface average roughness (points, right axis) versus annealing temperature, together with a typical Continuous Curve (solid curve, left axis) of a Mw = 99 Kg/mol reference sample. The dashed curve is for guide of eyes only. (b) AFM image of a fresh rubbed surface of PS; (c) AFM image of the same surface area of (b) after annealing at 100 °C for 30 minutes. (d) Normalized birefringence as a function of time (solid curve), with the temperature shown as the dashed curve.
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 261
0.6 90 °C
0.5
NB
0.4 0.3 0.2 0.1
Sample-A Sample-B (a)
0.0 1 10
2
10
3
4
10
10
Time (S)
1.0
NB
0.8 0.6 0.4 0.2
Sample-A' Sample-B' (b)
0.0
40
60
80
Tempearture (°C)
100
Figure 7. (a) The isothermal relaxation curves at 90 °C of a reference sample and a sample which underwent repeated rubbing process as described in the text. (b) The Continuous Curves of two PS samples. The solid curve is for a reference sample, while the dashed curve is for a sample which underwent repeated rubbing process.
Figure 6(d) shows the NB of a reference sample under a step-like temperature rise sequence. The solid curve is the NB as a function of time as its temperature underwent the sequence shown as the dashed curve in the figure. When the sample was subjected to a fast rise in temperature from 45 °C to 60 °C, a sizable portion of its NB disappeared along with it, followed by a slower and smaller decay as the sample temperature was maintained at 60 °C. After 30 minutes at 60 °C, subsequent rise-then-hold in temperature from 60 °C to 80 °C and from 80 °C to 100 °C brought about similar responses of the NB, until all RIB disappeared. An isothermal relaxation curve of a reference Sample-A is shown in Figure 7(a). The temperature was raised at t = 0 from 23 °C to 90 °C in less than 3 seconds, and then was maintained there for 10 hours. Within the first 10 s about 52 % of the original birefringence had relaxed. The NB then decayed slowly, approaching logarithm in time. Simple isothermal relaxations of reference samples at other elevated temperatures behaved in similar way in that there was always an initial quick drop in NB followed by a slow decay [31].
3.3. Thermal quench before rubbing Several fresh samples were first heated up to 130 ºC and quenched to 23 ºC in less than 300 s, and then rubbed to saturation. Their saturation RIB and Continuous Curves show no
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noticeable difference from the typical ones presented in Figure 5 and Figure 6. This is inconsistent with the established physical aging theory, according to which the samples quenched to 23 ºC should have preserved the majority of the segmental RT’s at 130 ºC. If a reference sample is quickly heated to 130 ºC, all its RIB will disappear within 30 s, implying that at 130 ºC the RIB RT’s are all shorter than 30 s. Therefore, had the RIB RT’s followed the segmental RT’s, all of them in the quenched sample would have been short (≤ 30 s). All RIB in the sample would have relaxed quickly even at 23 ºC, and no net RIB should have even been obtained. This is in clear contradiction to the experimental results.
3.4. Repeated rubbing Conventional segmental relaxations are greatly sped up by high strain/stress [4]. In rubbed PS the RIB is due to the segmental alignments and distortion, so it is taken as a measure of the degree of distortion in the discussion here. More experimental results are presented in Figure 7. Here sample-A was one of the reference samples. Sample-B was from group-B where the samples were first rubbed to saturation (> 80 passes), heated up at 2K/min till 90 ºC, then quenched to 23 ºC and rubbed again until the RIB was back to its original saturation value at the end of the first rubbing run. The entire quenching and rubbing process was completed within 30 minutes. The molecular weight of the samples in both groups was 99 Kg/mol. The isothermal relaxation curves, the dashed curve for sample-B from group-B and the solid curve for a reference sample-A, are depicted in Figure 7(a). The Continuous Curves of a reference sample-A’ and a sample-B’ from group-B are depicted in Figure 7(b). The two curves coincide well in both cases. No significant difference is seen. All these again show that once the RIB is created, its RT’s are not affected by further deformation, in clear contradiction with the behavior of the conventional segmental relaxations. 3.5. Disrupted continuous curve The experimental results presented so far demonstrate that the RIB relaxation does not depend on the rubbing and thermal history before the final rubbing process and the start of the subsequent relaxation measurements. There could be a small possibility that somehow the final rubbing process ‘resets the clock’ and puts the PS all to a state that relax in a very similar way, i. e., similar to mechanical ‘rejuvenation’ in segmental relaxations even though the term itself is not exact [5] [47]. To clarify, we studied the effects of thermal processes conducted after rubbing. The NB curves as a function of temperature of two originally identical samples are shown in Figure 8. For a reference sample-A, the temperature was raised at 2 K/min from 23 °C to 130 °C without interruption. The result is a typical Continuous Curve. The second sample, sample-C, underwent more complicated temperature sequence. First the temperature was raised continuously from 23 °C to 90 °C at 2 K/min. Its NB decreased in almost the same way as sample-A up to 90 °C. Upon reaching 90 °C the temperature was lowered at 2 K/min till 44 °C (when ambient cooling could no longer maintain the 2 K/min rate). The NB continued to decrease by a small amount until the temperature reached about 80 °C, then maintained its value from 80 °C to 44 °C. Upon reaching 44 °C the temperature was raised again to 90 °C at 2 K/min. The NB remained unchanged during the second temperature rise to 80 °C. Upon reaching 90 °C again, the sample was dropped into liquid nitrogen, and then let to warm up in a dry air ambient environment (~ 23 °C) for 16 hours. During the period no RIB change was observed. Finally the temperature was raised continuously from 23 °C to 130 °C at 2 K/min. The NB again
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 263 traced the curve where the temperature was raised from 44 °C to 90 °C, and joined the NB of sample-A after 94 °C. Several points are noted in the above temperature sequence. First, the relaxation of NB of sample-C was identical to that of sample-A for T > 94 °C, i. e., the relaxation above 94 °C was not affected by the thermal history below 90 °C, within the time scale of ~20 hours. Second, no relaxation of NB was observed below 80 °C after the first temperature rise, indicating that by the first time the temperature reached 90 °C the relaxation of NB below 80 °C was already complete. That is also the reason why the NB curves of the second (before liquid nitrogen quench) and the last temperature rise (after quench and annealing at 20 °C for 16 hours) are identical. Third, above 80 °C the relaxation was still active, and the relaxation of NB was again activated when 80 °C was reached from below in both the second and the last temperature rises. Fourth, unlike in the case of physical aging where further nonequilibrium can be introduced by additional thermal process, quenching or other thermal process below 90 °C on rubbed PS brought no increase to the remaining RIB. According to the conventional physical aging theory, the quench in the above case will freeze the thermodynamic state and the segmental RT’s at 90 °C. If the linear compliance of the sample is tested at 20 °C, its characteristic time will be close to that at 90 °C, and many orders of magnitude shorter than the one in thermodynamic equilibrium [1]. If the RIB RT’s had followed the trend of the conventional segmental RT’s in the quench process, then the measured NB would have decreased during the 16 hours at 20 °C and when the temperature was raised again from 20 °C, because the RT’s should have been close to those at 90 °C. The portion of curve-C above 94 °C should have shifted horizontally to the left (lower temperature), and the amount of shift would have depended on the elapse time at 20 °C, as is indicated by the imaginative curve labeled ‘Following PA’ (PA stands for physical aging) in the figure. When the temperature reached 90 °C for the third time the thermodynamic state of sample-C was certainly different from that of sample-A at the same temperature. The fact that the RIB in sample-C relaxed in the same way as sample-A at above 94 °C indicates that although the thermodynamic state of sample-C was different from sample-A, the portion of RIB that would relax at above 94 °C was still the same as before, unaffected by the quenching and reheating.
1.0
NB
0.8 Sample-C
0.6
Sample-A
0.4 PA
0.2 0.0 20
40
60
80
Temperature (°C)
100
120
Figure 8. Normalized birefringence versus temperature of two PS samples. The solid curve is for a reference sample-A, while the dashed curve is for sample-C which underwent several temperature rise and fall sequences as described in the text.
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Z. Yang 1.0
TLag
NB
0.8 0.6
56 K, 70 rubs
6 K, 44 rubs 6 K, 5 rubs
0.4
99 K, 10 rubs
0.2 0.0 20
99 K, 80 rubs
56 K, 10 rubs
40
60
80
Temperature (°C)
100
120
Figure 9. Normalized birefringence of three pairs of samples versus temperature at a heating rate of 2 K/min after annealing at 23 ºC for 7 days. The sample parameters are given in the text.
3.6. The temperature lag phenomenon The Continuous Curve of any rubbed sample following a period of isothermal annealing differs from the ones without the annealing. This is the temperature lag phenomenon first reported in Ref. [31]. Three pairs of samples were studied here. The molecular weights of the pairs were 6.4 Kg/mol, 56 Kg/mol, and 99 Kg/mol, respectively. For each pair of samples, one was rubbed for only a few passes (5 for the Mw = 6.4 Kg/mol sample and 10 for the other two samples) and the other one till saturation (44 passes for the 6.4 Kg/mol sample, 70 passes for the 56 Kg/mol sample, and 80 passes for the 99 Kg/mol sample). The net RIB of the lightly rubbed samples was about 40 % of the saturation values (Δn = 0.012 for the 6.4 Kg/mol sample, Δn = 0.017 for the 56 Kg/mol sample, and Δn = 0.021 for the 99 Kg/mol sample). The rubbed samples were then left at 23 ºC for 7 days, during which they aged and the RIB relaxed but no surface morphology change could be detected by AFM. Figure 9 shows the Continuous Curves of the samples after the aging. Several features are noted. The temperature at which the NB vanished was around 90 ºC for the Mw = 6.4 Kg/mol samples, 104 ºC for the Mw = 56 Kg/mol samples, and 110 ºC for the Mw = 99 Kg/mol samples. These temperatures are close to but do not strictly follow the Tg’s of PS at these molecular weights. Also, they are almost rubbing condition independent. This effectively rules out the possibility of significant chain scissions by rubbing, as shorter chains are equivalent to smaller molecular weights which would lower the Tg and the temperature at which RBI disappears. The same were observed for the reference samples without aging. Within the 7 days, for samples of the same molecular weight, the decrease in NB for lightly rubbed sample was larger than the saturated one. For example, for the 99 Kg/mol samples, the RIB of the lightly rubbed one reduced to 81 % of its original value right after the rubbing, a drop of 19 %, while that of the saturated one reduced to 91 % of its original value, a drop of 9 %. This is in contradiction with the strain accelerated relaxation mechanism [4]. The NB’s did not immediately decrease as the temperature was raised above the aging temperature TA until it reached a higher temperature TD. Above TD the Continuous Curves resembled those reference samples rubbed in the same ways but without the 7-day aging at 23 °C. Despite the large difference in molecular weights and in the number of rubbing passes, the temperature lag, defined as TLag ≡ TD – TA, of the six samples were the same within the experimental uncertainty of ± 1ºC.
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 265 More results measured under similar conditions on other samples are summarized in Table 1. The aging/annealing times tA ranged from 1 hour to 7 days. The molecular weights ranged from 6.4 Kg/mol to 550 Kg/mol. The film thicknesses ranged from 7 nm to 30 nm. The annealing temperatures were 23 ºC and 60 ºC. At a given annealing temperature, TLag increased with the logarithm of annealing time, in a similar way as reported earlier [31]. At a given annealing temperature and for a given annealing duration tA, the TLag’s of all the samples were the same within the experimental uncertainty of ± 1ºC, regardless of the molecular weight, the film thickness, and the rubbing passes. In a similar experiment as in subsection 3.4, several group-B samples, after repeated rubbing and heating, were tested for their TLag at 60 ºC, and the outcome showed no discrepancy to the normal values shown in Table-1 and in Figure 10. Therefore, the TLag is also independent of the thermal and rubbing history. Table 1. The temperature lags of the PS samples investigated. Film Thickness (nm)
TA (°C)
Duration tA (s)
3600
22
23 36,000
6.05 × 105
30 7 10
60
36,000
Molecular Weight (kg/mol) 6.4 6.4 12 12 56 56 99 99 6.4 6.4 34 34 6.4 6.4 56 56 99 99 6.4 22 99 550 99 99
TLag (°C)
15.3 15.7 15.4 15.7 15.8 16.1 15.8 15.6 24.2 25.1 25.9 24.6 29.1 28.8 28.4 29.6 27.1 29.1 20.0 20.8 18.9 19.0 20.1 20.1
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14
Ln(Time (s))
13 12 11 10
60ºC, 7 nm 60ºC, 10 nm 60ºC, 30 nm 90ºC, 30 nm 90ºC, 18nm* 22ºC, 30 nm
9 8 7 5
10
15
20
TLag (°C)
25
30
35
Figure 10. Logarithm of annealing time tA versus temperature lag TLag at several annealing temperatures. The 18* nm sample was quenched from 130 °C to 23 °C several minutes before it was rubbed at 23 °C.
The logarithm of annealing time tA as a function of TLag is plotted in Figure 10 at several annealing temperatures. A nearly linear relation between the two quantities can be confirmed. More quantitative analysis will be present in later sections. Here we concentrate the discussions on its implication on the effects of physical aging. The temperature lag phenomenon is one of the most important characteristics of RIB relaxation, and plays a critical role in the quantitative analysis of its relaxation behaviors. The RIB remains unchanged when the temperature is still in the range of TLag, while above the range the RIB decreases with the increasing temperature in exactly the same way as a sample without going through the annealing and aging. For example, for a sample annealed (and aged) at 23 ºC for 7 days, there was no change in RIB in the temperature range of the subsequent Continuous Curve from 23 ºC to 50 ºC, while the portion of the Continuous Curve above 60 ºC was the same as the one in Figure 5. In other words, the annealing at 23 ºC for 7 days eliminated all the low temperature RIB relaxation up to about 50 ºC, but left the one well above 50 ºC unchanged. This is in stark contradiction with the effects of physical aging, where the segmental relaxations at all temperatures sufficiently below Tg, regardless whether they are above or below the aging temperature, are affected by the aging process [1].
4. Model We have conducted many similar experiments on many other samples. The outcome, together with the experimental results presented in Section-3, can be summarized in the following. (i) The relaxation of RIB does not depend on thermal history before rubbing. Representative experimental evidence is presented in subsection 3.3. (ii) The relaxation of RIB does not depend on the thermal and rubbing history prior to the last rubbing run. Representative experimental evidence is presented in subsection 3.4.
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 267 (iii) The dependence of the RIB relaxation on the thermal history after the last rubbing run is governed by the temperature lag effect. If a rubbed sample is aged at temperature TA for a duration of tA, then a ‘lag temperature’ TLag can be found experimentally from its subsequent Continuous Curve such that below TA + TLag the Continuous Curve is flat, and the portion of Continuous Curve above TA + TLag resembles the one obtained from reference samples without aging. Furthermore, TLag is proportional to the logarithm of tA. Representative experimental evidence is presented in subsections 3.5 and 3.6. Optical probes have been used to study the relaxations of PS and other polymers, and substantial physical aging effects were observed. Thurau and Ediger reported photobleaching studies on the segmental relaxations of PS [48]. The rotational probe relaxation followed the KWW function, while the translational probe decayed in a simple exponential form. After quenching from above Tg, the RT’s at 33 ºC were much shorter than the equilibrium values. The aging effect was much less drastic than the typical ones seen in creep compliance experiments [1]. Aging from 103 s to 105 s, a change of time scale of 100 folds, brought RT increase by only three times. The fluorescence intensity of chromophore-doped/labeled PS thin films studied by Ellison et al. [49] was insensitive to aging, changing by only 6 % in total intensity over 5 hours of aging. These results indicate that the relaxations involving smaller length scales are not as sensitive to aging as the ones involving larger scales in creep experiments. Besides the small length scale, the absence of physical aging effects in the RIB relaxation could also be due to the accelerated aging speed in the near surface region. In fact, the aging speed could be depth dependent, similar to the speculated depth dependent Tg [50]. Kawana and Jones reported [11] thin supported PS films aged below Tg that exhibited clear overshooting in expansivity temperature curves when reheated for thicknesses of 18 nm or more, but not for a thickness of 10 nm, suggesting the existence of a surface layer of the order of 10 nm in which aging was complete within ~103 s, while the rest of the film essentially had thermal properties identical to bulk samples. The absence of the physical aging effects in the RIB relaxations could therefore be due to the combined effects of accelerated aging of the near surface region and the small length scales involved in the relaxation process. As was pointed out earlier [31], the RIB relaxation does not follow the KWW function. One essential element for the KWW function to be applicable is that the start and end values of the relaxation observable must be well defined, as in the case of photobleaching where the values are 1 and 0, or in the glass to rubbery transition of creep compliance where the end values are the well established JG and JR values. In isothermal RIB relaxation, the only well defined values are 0 and 1 for NB and there is no ‘level off’ in between. Taking the short time (~ a few hours) end value as ‘level off’ is mistaking [31]. Since KWW function does not apply here, we must start with the generic distribution function and proceed to determine it using experimental results. One of the characteristics of the relaxation processes in glass forming polymers is the heterogeneity. A monotonic relaxation process can in general be expressed as ∞
α (t ) = ∫ g (τ )e −t / τ dτ 0
(2)
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where α is the relaxation function of an observable, such as the segmental alignment dichroism, the optical birefringence, etc., τ is the relaxation time (RT), and g(τ) is the RT density distribution. In thermodynamically simple cases g(τ) = G(κτ) where the shift factor κ is temperature and thermal history dependent but the functional form of G(x) remains fixed, following the well known time-temperature superposition law and the time-aging time superposition law. There is no general and systematic way to extract G(κτ) from experimental results, but fortunately in many cases [41] [42] [43] G(κτ) takes the simple form of KWW function in the time scale within which the change of κ can be ignored. The G(κτ) after various length of aging times can then be determined and the shift factor κ extracted. The dominant effect of physical aging in the conventional cases is on the time shift factor κ, which leads to the well known time-aging time superposition principle [1]. Extensive experimental evidence in the last section has shown clearly that the RIB relaxation depends strongly on temperature but is insensitive to physical aging. The RIB relaxation can therefore be described by an RT distribution function g(τ) that depends only on temperature but not on thermal or strain history. This is the most significant difference between the conventional segmental relaxations and the RIB relaxation. As the RIB g(τ) depends on temperature only, the existing models that predict the physical aging effects [5] [6] are no longer applicable here. Instead, a new and systematic approach [31] can be devised to determine the distribution function from carefully designed experiments. The phenomenological model we propose is based on the generic model for the relaxations in glass forming polymers that consists of individual relaxing units overcoming an energy barrier under thermal excitations. In the model, the total RIB of a PS sample is taken as the combined contribution of individual RIB elements. To remove its contribution to the total RIB each element must overcome an energy barrier E. The RT is expressed in the conventional Arrhenius form E
τ = Ae ( 273+T ) R
(3)
where R = 8.315 J/ ( K ⋅ mol ) is the gas constant, A is a pre-exponential factor, and T is the temperature in the unit of °C. Once an RIB element is created by rubbing, its RT will not be altered by further thermal process or strain, until it has relaxed and disappeared. By expressing the energy barrier in the form
E = (273 + ξ )η 0
(4)
where η0 is the same for all RIB elements, and ξ (in the unit of °C) is element specific, the RT becomes
τ (T , ξ ) = τ 0 e γ (ξ −T )
(5A)
τ 0 ≡ Aeη / R
(5B
with 0
and
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 269
γ ≡ η 0 / R(273 + T )
(5C).
The parameters γ and τ0 in Eq. (5A) are the same for all elements and depend weakly on temperature, while ξ is generally different for different RIB elements and is independent of temperature. At a given temperature T the parameter ξ alone determines the RT of the element. The prefactor A for all the RIB elements is the same at any given temperature, which means that all elements have the same ‘vibration frequency’ in their potential wells. This results in the RT’s in the ‘threshold temperature’ form as expressed in Eq. (5A), with the ‘threshold temperature’ proportional to the well depth, as is hinted by the step-like drop in NB following a step-like rise of temperature in Figure 6(d). At a given temperature T, the elements with ξ ≤ T will have short RT’s (~ seconds) and relax quickly, while those with ξ > T will have RT’s that increase exponentially with ξ. The rubbing process generates an initial density distribution N (ξ , 0) of elements, where ∞
∫ N (ξ , 0)d ξ = 1 . After spending tA at temperature T the distribution becomes
0
N (ξ , t A ) = N (ξ , 0)e
−
tA
τ (T ,ξ )
(6A)
The isothermal relaxation of the normalized RIB is then given by ∞
NB(T , t ) = ∫ N (ξ , 0)e
−
t
τ (T ,ξ )
dξ
(6B)
0
The expression is equivalent to Eq. (2).
5. Analysis We now proceed to the analysis of the experimental results presented in Section-III.
5.1. Range of ξ where N (ξ , 0) is non-zero According to Eq. (5A), any elements with ξ < 23 °C created by rubbing will have relaxed quickly before any subsequent measurements are carried out. The relaxation of these elements is the origin of the transient birefringence right after each rubbing pass reported in subsection 3.1. On the other hand, the RIB relaxation is complete when the temperature is near Tg (see Figure 6). Therefore, taking into account of the experimental Continuous Curve in Figure 6, we expect N (ξ , 0) to be a smooth and continuous function of ξ in the range of 23 °C ≤ ξ ≤ ~Tg. According to the experimental results in Figure 9, in the Mw = 6.4 Kg/mol samples there were no RIB elements with ξ > 90 °C, while in the Mw = 99 Kg/mol samples there were RIB elements with ξ > 90 °C but not the ones with ξ > 114 °C. When the temperature of a freshly rubbed sample is raised quickly from 23 °C to 90 °C there must be a quick drop of birefringence because the elements with barrier height 23 °C ≤ ξ ≤ 90 °C will relax within ~ 10 s. This quick drop of NB has indeed been observed if one keeps track of the original
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birefringence before temperature rise, as is shown in Figure 7(a). Likewise, a step-like rise of temperature will bring a step-like drop in NB, which was indeed observed (See Figure 6(d)). More analysis on the distribution function will be presented in subsection 5.7. Note that although ξ is in the unit of °C, it describes the potential well depth and itself is independent of temperature.
5.2. Interpretation of temperature lag phenomenon The RT of an RIB element increases exponentially with ξ at a given temperature T. Equation (6) then predicts the ‘temperature lag’ phenomenon reported in subsection 3.6. For a freshly rubbed sample, its initial density distribution is N (ξ , 0) . As its temperature T(t) is raised at a constant rate, the elements with ξ = T(t) will relax quickly as their RT’s become short enough. As a result, the NB decreases with temperature, leading to a typical Continuous Curve as shown in Figure 6(a). However, after a freshly rubbed sample has gone through an isothermal annealing process for certain duration tA at temperature TA, subsequent temperature rise at a constant rate will not bring immediate decrease in NB until a higher temperature TD is reached. This is because the elements with ξ0 ≥ ξ, where τ(TA, ξ0) = tA, have already relaxed. Subsequent rise in temperature will not bring any significant decrease in NB until it reaches TD ≥ ξ0. By measuring the temperature lag TLag (≡ TD – TA) at different annealing time tA, the relation between τ(TA, ξ) and ξ can be obtained, namely tA = τ(TA, ξ = TA + TLag). According to Eq. (5A), TLag is linearly dependent on the logarithm of tA. Using linear fit we obtained the parameters [31] for τ(TA, ξ) at TA = 60 °C, namely γ = 0.42 ± 0.05 K-1 and τ0 = 14 ± 8 s, which lead to η 0 / R = 140, and E ranging from 340.4 kJ/mol to 445.7 kJ/mol. Similarly, in the subsequent part of the paper the parameters at other temperatures from 23 °C to 90 °C and their possible dependence on PS properties will be extracted from the experimental results presented in Section-3. We found that Eq. (5) is followed, and the parameters γ and τ0 can be extracted in all the cases studied.
5.3. Molecular weights, film thickness, and rubbing distance dependence As presented in subsection 3.6, at a given temperature the TLag of samples with different thermal histories, thicknesses, molecular weights, and prepared by different rubbing distances were all the same. This implies that the dependence of τ(T, ξ) on ξ, the energy barriers the RIB elements must overcome, is the same regardless of the molecular weight, the film thickness, thermal history, and the rubbing conditions. This indicates that only the local environment immediately surrounding an individual element determines its energy barrier, rather than the global parameters such as the chain length, the net RIB, or the film thickness. 5.4. Temperature dependence of γ and τ0 For a fixed annealing time tA, TLag is smaller at higher temperatures. The NB curves of four samples are plotted against (T – TA) in Figure 11, where T was the temperature at a constant heating rate of 2 K/min and TA was the annealing temperature prior to the temperature rise. The annealing time was tA = 10 hours for all the samples. The (T – TA) at which NB started to drop was the largest for TA = 45 ºC and smallest for TA = 90 ºC. The TLag data extracted from the curves are plotted against TA in the insert of Figure 11. A linear relation is seen between TLag and TA.
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 271 0.90 60 ºC 45 ºC
0.85
NB
90 ºC 22
Tlag (°C)
20
0.80
18
75 ºC
16 14 12 10 40
0.75 0
50
60
70
80
TA (°C)
5
90
100
10
15
20
25
T - TA (K) Figure 11. The NB curves of four PS samples vs (T – TA), where T is the temperature at a constant heating rate of 2 K/min and TA is the annealing temperature prior to the temperature rise. The insert shows the TLag obtained from the curves as a function of TA. The molecular weight of the samples is 550 Kg/mol.
Table 2. The parameters of the relaxation times obtained from the linear fitting in Figure 6. The 18* nm sample was quenched from 130 °C to 23 °C several minutes before it was rubbed at 23 °C. Film Thickness (nm) 7 10 30 30 18* 30
TA (°C) 60 60 60 90 90 23
τ0 (s) 15 ~ 44 15 ~ 33 11 ~ 24 4.5 ~ 5.5 3.3 ~ 33 5.5 ~ 45
γ (K-1)
η0 /R
0.35 ± 0.05 0.37 ± 0.04 0.41 ± 0.03 0.64 ± 0.02 0.64 ± 0.09 0.37 ± 0.04
116 ± 17 123 ± 13 136 ± 10 232 ± 7 232 ± 32 110 ± 12
According to Eq. (5), the above results indicate that either or both the parameters γ and τ0 increases with temperature. To quantitatively determine the two parameters, TLag’s at fixed annealing temperatures were measured at different annealing time tA, and the results are shown in Figure 10. Straight lines of ln(tA) vs. TLag are found. The linear fitting results of the data in Figure 10 are summarized in Table 2. The value of τ0 in all the cases are of the order of 5 ~ 50 s, although at 90 ºC the parameter τ0 does seem to be smaller than at lower temperatures. The data at 60 ºC hint some weak thickness dependence of γ, but the difference is still within the experimental error. The value of γ increases by about 50 % from 60 ºC to 90 ºC. Also to be noted is the 18* nm sample, which was quenched from 130 °C to 23 °C several minutes before it was rubbed at 23 °C. The entire quenching and rubbing process was completed in 30 minutes. Compared to conventional physical aging, such drastic thermal quench had surprisingly little, if any, effects on the RIB RT’s. Summarizing the results presented so far, we conclude that the RT’s of the RIB elements follow Eq. (5A) in the temperature range investigated. The parameter τ0 is at most weakly dependent on temperature, changing by no more than a factor of 10 over the temperature range from 23 ºC to 90 ºC. As a comparison, the actual RT’s change by about 1011 times in
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the same temperature range. The RT’s are independent of the molecular weights, the number of rubbing passes, the film thickness down to 7 nm, and the thermal history. The parameter γ increases linearly with temperature at a rate of 7 × 10-3 K-2, and its value at 90 ºC is 0.64 K-1. According to Eq. (5C) an increase of γ leads to η0 to increase with temperature, indicating that the well depth of the RIB elements not yet relaxed increases with temperature. But since the pre-factor A, according to Eq. (5B), decreases exponentially with η0, an RIB element with ξ = 100 ºC, for example, will still relax quickly only when the temperature reaches 100 ºC.
5.5. Disrupted Continuous Curve If the temperature is lowered shortly after T1 is reached, the RIB elements with ξ ≤ T1 will have relaxed and those above T1 will simply go ‘hibernation’ until the temperature is raised back to the neighborhood of T1. This explains the relaxation behavior of sample-C in subsection-3.5. By the first time the temperature reached 90 °C, all elements with ξ ≤ 90 °C in sample-C had already relaxed while most of those with ξ ≥ 90 °C were still intact. During the first and the second temperature drop and rise, the NB remained constant because there were no ξ ≤ 90 °C RIB elements left, and the RT’s of the remaining elements with ξ ≥ 90 °C were very long. Therefore there were few elements that would relax in that temperature range within the time scale of the experiment. The small decrease in NB above 80 °C was due to the slow relaxations of the elements with ξ slightly above 90 °C. Once the temperature was finally raised above 90 °C, the elements with ξ ≥ 90 °C then relaxed in earnest in the same manner as the reference sample, because their RT’s remained unaltered by the thermal processes. 5.6. Density distribution N(ξ, 0) The analysis so far shows that the dependence of the RT of an RIB element τ(T, ξ) follows Eq. (5A), and the parameters γ and τ0 are the same for all the PS samples investigated so far. As shown in Eq. (6B), τ (T , ξ ) and N (ξ , 0) together determines the relaxation of RIB in a sample. The difference in the relaxations of samples with different molecular weights or rubbing conditions lies in the difference between their density distributions. For example, the reduction of NB at a fixed temperature is faster if there are more elements with lower barrier heights and therefore shorter RT’s. As N (ξ , 0) approaches zero when ξ is near Tg, the upper bound of ξ, above which N (ξ , 0) is zero, of a lower molecular weight sample is lower than that of a higher molecular weight sample. In this part, we concentrate on the extraction of the density distribution N (ξ , 0) from the experimental data, using the relaxation times obtained in subsection 5.5. In our early work [31], a qualitative argument was given to show how to extract the density distribution from the experimental Continuous Curve Ω(T). Here we present a quantitative derivation. It is noted that Ω(T) is a special form of NB(T, t) with
T (t ) = T0 + αt
(7)
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 273 At 2 K/min, α = 1 K/s, and T0 is the starting temperature at t = 0. For Ω(T) after 30 annealing at temperature TA for a certain duration, T0 = TA. To obtain the initial density distribution N (ξ , 0) , the measurement of Ω(T) must start from the rubbing temperature within 100 s after rubbing. The general expression for the relaxation of NB with given temperature T(t) is ∞
NB(T (t ), t ) = ∫ N (ξ , 0)e − f (ξ , t ) d ξ
(8A)
0
with
dt ' 0 τ (T (t '), ξ ) t
f (ξ , t ) = ∫
(8B)
The relaxation time τ (T , ξ ) is given by Eq. 5(A). We take τ0 = 15 s, and γ = 0.64 K-1 at 90 °C and increases linearly with temperature at a rate of 7 × 10-3 K-2, and proceed to show how to extract the density distribution from the measurements of Ω(T). We define the relaxation function as
K (t , ξ ) ≡ e − f (ξ ,t )
(9)
where T(t) is given by Eq. (7). From the numerical results with α = 1 K/s shown in Figure 30 12(a), it is seen that K(t, ξ) is nearly a step function of ξ, the rise being at the point where ξ = T(t). The dashed curve is the negative derivative − ∂K = − ∂K . It is a narrow peak function α∂t ∂T with half width at half height proportional to 1/γ and a total area of unity. It then follows that ∞
dΩ = dT
d ( ∫ N (ξ , 0) K (t , ξ )d ξ ) 0
α dt
∞
= ∫ N (ξ , 0) 0
∂K d ξ ≈ − N (ξ , 0) |ξ =T ∂T
(10)
i. e., the density distribution N (ξ , 0) can be extracted from the experimentally measured Continuous Curve by taking its derivative with respect to temperature. The same conclusion was reached in the qualitative argument [31]. A typical density distribution N (ξ , 0) of a reference sample is shown in Figure 12(b), extracted from the experimental Continuous Curve in the same figure. The density distribution was then put back to Eq. (8), and by using Eq. (5A) for the relaxation times, to generate a calculated Continuous Curve at the heating rate of 2 K/min. It is seen that the calculated one coincides with the experimental one very well. It should be noted that the relaxation times used in the calculation were extracted from the TLag measurements from other samples. The fact that the two curves agree well with one another provides a strong evidence to support the theoretical model.
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1.0
0.20
0.8
0.15
0.6 0.10 0.4 0.05
0.2 0.0 40
20 1.0
(a)
50
40
60
70
ξ (°C) ξ (°C) 60
80
90
80
100
2K/min
0.00 100
120 4 3
0.2K/min
0.6
2 0.4
Density Function
1
0.2 0.0 20
N(ξ)
NB
0.8
Derivative of K(t, ξ)
K(t, ξ)
274
(b)
40
60
80
100
0 120
Tempearture (°C) Figure 12. (a) The calculated decay function at a constant heating rate of 2 K/min (solid curve) along with its derivative (dashed curve) as a function of the energy barrier parameter ξ at the time when the corresponding temperature is 60 ºC. (b) The experimental Continuous Curve and the density distribution N(ξ) together with the numerical simulation results. The solid curve is the experimental curve (left and bottom axes). The circular points are the density distribution N(ξ) obtained from the experimental Continuous Curve by taking derivative versus temperature (right and top axes). The two dashed curves are numerical simulations of the Continuous Curve at two heating rates. The molecular weight of the sample is 550 Kg/mol.
Using the same data, we also simulated the Continuous Curve for heating rate at 0.2 K/min, 10 times slower than the one used in the measurements. It is seen that because the elements now have more time to relax, the amount of remaining elements at any temperature is smaller than at a faster heating rate. Extracting the density distribution from such Continuous Curve therefore requires more complicated numerical work than simple derivative operations.
5.7. Rubbing dependence of N (ξ , 0) In the sections above we have shown that the RT’s are independent of the rubbing conditions. In this section, we explore the effects of rubbing on the density
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 275 distribution N (ξ , 0) . Shown in Figure 13 are the density distributions of the six samples obtained from the Continuous Curves in Figure 9. It is seen that below ξ ≈ 45 ºC the density is zero for all the samples, due to the relaxation for 7 days at 23 ºC. The ‘temperature lag’ phenomenon is clearly manifested. For the two Mw = 6.4 Kg/mol samples, the density distributions are about the same, and are nearly uniform between ξ ≈ 60 ºC and ξ ≈ 80 ºC. Both drop to zero at ξ ≈ 90 ºC. For the Mw = 56 Kg/mol and Mw = 99 Kg/mol samples, the density distribution of the lightly rubbed ones are nearly constant between ξ ≈ 50 ºC and ξ ≈ 100 ºC, but the distribution of the ones rubbed to saturation have a hump above ξ ≈ 90 ºC. The results indicate that in relatively high molecular weight PS, the rubbing process first creates birefringence elements with a nearly uniform distribution of energy barriers, and further rubbing creates elements with barriers more concentrated between ξ ≈ 90 and 100 ºC. For light molecular weight PS, the rubbing process creates a uniform density distribution all the way to the saturation birefringence. This could be due to the entanglements of the PS molecules, as 6.4 Kg/mol is below the entanglement limit of Mw ≈ 20 Kg/mol [51] while 56 Kg/mol is above it. Further study is being carried out to understand the microscopic mechanism of the rubbing process.
5.8. Repeated rubbing As has already been shown above, the RT’s of the RIB elements, once created, are independent of the subsequent thermal or rubbing processes until the elements have relaxed. Repeated rubbing processes were attempted in the hope to generate higher concentration of deeper barriers in the samples. As presented in subsection 3.4, a rubbed sample was first raised to 90 ºC and quenched to 23 ºC to eliminate all the RIB elements except for the ones with the barriers ξ ≥ 90 °C. It was rubbed again, in the hope that if the chance of generating barriers of all depths had been the same, then there would have been more barriers with ξ ≥ 90 °C after the second rubbing run than the first run. The experimental results presented in Figure 7, however, imply that the distribution density function N (ξ , 0) at the end of the second rubbing run was nearly the same as the first run. This is somewhat surprising. A likely scenario is that the second rubbing run destroyed and created about the same amount of elements with ξ ≥ 90 °C. As a result, the relaxations of the repeatedly rubbed samples were the same as the reference ones. 5.9. Discussions The major findings presented so far seem to contradict the cooperative motion characteristics of segmental relaxations. For example, the RT’s of creep compliance can change by many orders of magnitude during aging. Yet such change has little effects on the local environment of the remaining RIB elements, such as their energy barrier heights and/or well width, either before or after they are formed. Such finding, however, is consistent with the very small length scale involved in the RIB relaxations. The change in thermodynamic state changes the large scale cooperative motions, but has diminishing effects on the smaller length scale RIB relaxations. The small length scale of RIB relaxation could imply that it is of the nature of βrelaxation due to side group motions. However, the β-relaxation relaxation times are much shorter than 1 s at 20 ºC and above, while the RIB relaxation times can exceed 1010 s, comparable to that of the cooperative segmental relaxations (α-relaxation). This effectively
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rules out the possibility that the RIB relaxation is of the nature of β-relaxation. The microscopic picture of RIB is still largely unknown other than that most of it is due to the segmental distortions, a subject that is still poorly understood.
1000 x N(ξ, 0)
20 Mw = 6.4 K 5 rubs 44 rubs
15 10 5 (a)
0 20
40
60
80
100
120
80
100
120
100
120
ξ (°C)
1000 x N(ξ, 0)
20 15
Mw = 56 K 10 rubs 70 rubs
10 5 0 20
(b)
40
60
ξ (°C)
1000 x N(ξ, 0)
20 15
Mw = 99 K 10 rubs 80 rubs
10 5 (c)
0 20
40
60
80
ξ (°C)
Figure 13. The density distributions of the three pairs of samples extracted from the Continuous Curves in Figure 8.
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 277
6. Summary A systematic study of the relaxation of rubbing induced birefringence in PS has been conducted. Extensive and clear experimental evidence have been found that show the absence of the physical aging effects in the relaxation of RIB, and the relaxation of RIB involves very small length scales. The RIB relaxation is then modeled by a relaxation times distribution function that depends only on temperature but not on thermal or strain history. An individual birefringence elements model has been proposed and a systematic way has been devised to extract the parameters in the model from specifically designed experiments, namely the Temperature Lag measurements and the Continuous Curve measurements. The results predicted by the model agree well with experiments. The absence of physical aging effects in RIB relaxation is in stark contrast to the conventional segmental relaxations, where the relaxation times can be changed by many orders of magnitude by thermal processes or stress, a phenomenon that has been extensively studies in the last four decades. This is probably due to the much accelerated aging process in the near surface region, and the very small length scale of the RIB relaxation. A model for the RIB relaxation and for the absence of the physical aging effects based on microscopic first principles is still lacking. The results presented here provide a concrete test ground for the development of future theoretical models.
PART IV. RIB IN OTHER GLASSY POLYMERS 1. Introduction The viscoelastic properties and segmental relaxations of glass forming polymers above the glass transition temperature (Tg) are generally well understood. The viscoelasticity follows the time-temperature superposition principle [52]. In the segmental alignment relaxation of polystyrene (PS) drawn at above Tg there are three well separated relaxation steps with distinct time scales τ1, τ2, and τ3 [40]. The relaxation time constants in these cases all follow the so-called Vogel–Fulcher temperature law [3] [52] [53]. Below Tg the relaxation phenomena are much more diversified and without a unified picture as in the cases above Tg. The segmental relaxation of polycarbonate (PC) still follows the time-temperature superposition, and the relaxation time follows the Arrhenius form with activation energy of 958 KJ/(mol·K). The relaxation becomes much slower at temperatures below Tg – 20 °C [52]. Additional α-relaxation besides the normal α-relaxation peak was observed in dielectric relaxations [54] [55]. The relaxation times follow the Arrhenius form with activation energies in the range of 17 – 55 KJ/(mol·K). Dielectric probe molecules were mixed with PS and their size dependence of dielectric relaxations was investigated [56] [57]. Below a particular size-1, no coupling between the probe and host PS was found. Above size-1 but below size-2, both α-relaxation and β-relaxation coupled to the probes occurred. Above size-2 no β-relaxation coupling was observed [56]. In Ref. [57], the shortest probes revealed β*-relaxation in additional to the normal α-relaxation and β-relaxation. Reorientation of detrapped molecules dissolved in PS revealed by electron parametric resonance is much faster even at low temperatures (190 GHz at 200 K) and encounters much lower barriers (~600 J/(mol·K)) [58].
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Non-linear optics probe molecules dispersed in PS [59] revealed that below Tg the relaxation time τ follows the Arrhenius form with activation energies of 45 – 50 kcal/(mol·K), or 188 – 209 kJ/(mol·K). Similar relaxation has been found in labeled PS [60]. Other non-linear optical probe molecules have much longer decay time (~ days) [61]. Relaxation of poled order in dye doped PS was probed by isothermal and non-isothermal current measurements [62]. Neutron scattering revealed that the fast process of phenyl rings starts at 200 K, and main chain starts at 250 K [63]. The relaxation of the segmental alignment dichroism of cold rolled PS at 60 °C can be fitted by two single exponential functions of time with RT’s of 76 s and 3600 s [39]. The rubbing induced birefringence (RIB) relaxations of a number of polymers were first studied by Kovacs and Hobbs [44]. Significant relaxations were observed below Tg. In PartIII, we found that the physical aging effects are absent in the relaxation of RIB in PS, and the relaxation involves very small length scale. A phenomenological model based on individual birefringence elements was proposed for the RIB relaxations. The relaxation times (RT’s) of the elements were found to be independent of the thermal or stress history of the samples, either before or after the formation of the birefringence. The RT’s were also independent of the molecular weight, rubbing conditions, and film thickness, while the initial RT’s distribution function did depend on the molecular weight and rubbing conditions. The model provided quantitative interpretations that agreed very well with all the reported experimental results, and shed important light to the novel behaviors of the RIB relaxation in polymers below Tg. In this part, we report on the RIB relaxations of other glass forming polymers, including PC and PS derivatives with various modifications to the phenyl ring side group. It is found that the RIB relaxations follow qualitatively the same way as PS, but there is quantitative difference in minor details among them. On the other hand, they are qualitatively different from any of the other reported relaxations mentioned above [39, 57 – 68]. No physical aging effects in the relaxations of RIB are found. The relaxations are then analyzed under the same theoretical framework as in Part-III. The energy barrier height decreases with decreasing temperature in all the cases, leading to a sub-Arrhenius form of RT dependence on temperature at temperatures below Tg.
2. Experiments Polymers in toluene solutions were spin-coated on thermally grown SiO2 on silicon substrates. The samples were then annealed at 20 °C above Tg in vacuum for at least 24 hours, and slowly cooled down (< 0.1 °C/min) and stored at room temperature before testing. The thickness of the resulting polymer films ranged from 38 nm to 70 nm. Rubbing was done on a home-made apparatus. Each rubbing pass covered 2 cm distance at a speed of 1 cm/s in one direction, with a normal pressure of 9 g/cm2, except for the B5 samples which required 25 g/cm2. The optical RIB of the PS films was measured using the reflectance difference spectroscopy at 633 nm in wavelength. The RIB vs rubbing pass at 23 ºC of the polymers behaved in a similar way as PS. The birefringence increased quickly with the first few rubbing passes, and then gradually approached the saturated value. For PC about 10 % of the total birefringence was created in the first 5 rubbing passes, and the subsequent 75 passes created the remaining birefringence.
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 279 Further rubbing after saturation resulted in eliminating roughly the same amount of birefringence as creating it, and the net birefringence fluctuated around the saturation value with amplitude of about 1 % of the saturation value. Other polymers behaved almost the same. The RIB approached saturation at about 80 rubbing passes, except for B2 samples, the saturation pass of which being 60 due to its low molecular weight. The net birefringence value of each sample was measured within 100 s after the last rubbing pass. It was then used as the normalization factor in determining the normalized birefringence (NB) in the subsequent relaxation experiments. Details about the samples parameters and the maximum birefringence are listed in Table 3. The maximum birefringence ranges from 8.0 × 10-3 for B2 samples (poly(4-methylstyrene), Mw = 2.7 kg/mol) to nearly 6.0 × 10-2 for B3 samples (poly(4-methoxystyrene), Mw = 10.5 kg/mol), as compared to the maximum birefringence of 2.5 × 10-2 for PS. This indicates that the side groups contribute significantly to the RIB. The RIB relaxation behaviors of the samples were studied by using the following ‘standard’ methods. The first was to measure the NB as a function of temperature when the sample temperature was raised continuously at 2 K/min. The results are referred to as the Continuous Curves Ω(T). The second was the isothermal relaxation of NB, i. e., the NB of a sample was measured as a function of time at a fixed temperature. The third was the combination of the two, i. e., continuous temperature rise at 2 K/min immediately following an isothermal process at a given temperature for a given period of time.
3. Results and Discussions 3.1. Disrupted continuous curve Rubbing deforms the surface region of the sample and puts the sample in nonequilibrium. To verify the absence of physical aging effects, we studied the effects of thermal processes conducted after rubbing, namely the disrupted Continuous Curve experiment, in the same approach as in Part-III. The NB curves as a function of temperature of two originally identical PC samples are shown in Figure 14. For a reference sample-A, the temperature was raised at 2 K/min from 23 °C to 180 °C without interruption. The result is a typical Continuous Curve. The second sample, sample-B, underwent more complicated temperature sequence. First the temperature was raised continuously from 23 °C to 100 °C at 2 K/min. Its NB decreased in almost the same way as sample-A up to 100 °C. Upon reaching 100 °C the temperature was lowered at 2 K/min till 30 °C. The NB increased slightly and linearly with the lowering of temperature due to thermal expansion. Upon reaching 30 °C the temperature was raised again to 100 °C at 2 K/min. The NB traces back the nearly flat line formed during the lowering of temperature. Upon reaching 100 °C again, the sample was dropped into liquid nitrogen, and then let to warm up in a dry air ambient environment (~ 23 °C) for 16 hours. During the period no RIB change was observed. Finally the temperature was raised continuously from 30 °C to 180 °C at 2 K/min. The NB again traced the straight line where the temperature was raised from 30 °C to 100 °C for the second time, and joint the NB of sample-A after 104 °C.
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1.0 PC Sample-A Sample-B
NB
0.8 0.6 0.4 PA
0.2 0.0 20 40
60 80 100 120 140 160 180
Temperature (ºC)
Figure 14. Normalized birefringence versus temperature (Continuous Curve) of two PC samples. The solid curve is for a reference sample-A, while the dashed curve is for sample-B which underwent several temperature rise and fall sequences as described in the text.
Table 3. Physical parameters of the samples used in the study. Mw (kg/mol) 2.7 K
Thickne ss (nm) 58
RIB (10-3)
10.5 K
40
B4
poly(4-methoxy styrene) poly(4-methyl styrene)
90 K
49
B5
poly(α-methyl styrene)
112 K
38
B6 B7
poly(4-methyl styrene) poly(4-tertbutylstyrene) Poly(4-hydroxyl styrene)
50 K 3.2 M
45 40
47.4 ~ 59.3 23.6 ~ 27.4 16.4 ~ 19.6 11.6 42.0
69.5 K
50
Sample Name B2 B3
T3
PC
Polymer poly(4-methyl styrene)
Polycarbonate
Polydisperse d
70
8.0
13.0 ~ 16.8
11.4 ~ 15.2
TA (ºC) 45
τ0 (s)
η(C-1)
30 ~ 90
85
16 ~ 25
95
22 ~ 33
140
37 ~ 180
N/A 130
N/A 1.7 ~ 3.7
60 60
0.33 ~ 2.5 74 ~ 134
80
90 ~ 245
100
15 ~ 40
120
5.5 ~ 15
0.36 ± 0.04 0.54 ± 0.02 0.45 ± 0.02 0.32 ± 0.04 N/A 0.59 ± 0.02 0.38 ± 0.04 0.24 ± 0.01 0.25 ± 0.02 0.41 ± 0.03 0.49 ± 0.03
Several points are noted in the above temperature sequence. First, the relaxation of NB of sample-B was identical to that of sample-A for T > 104 °C, i. e., the relaxation above 104 °C was not affected by the thermal history below 100 °C, within the time scale of ~ 20 hours. Second, no relaxation of NB was observed below 95 °C after the first temperature rise, indicating that by the first time the temperature reached 100 °C the relaxation of NB below 95 °C had already completed. That is also the reason why the NB curves of the second (before
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 281 liquid nitrogen quench) and the last temperature rise (after quench and annealing at 23 °C for 16 hours) are identical. Third, above 95 °C the relaxation was still active, and the relaxation of NB was again activated when 95 °C was reached from below in both the second and the last temperature rises. Fourth, unlike in the case of physical aging where further nonequilibrium can be introduced by additional thermal process, quenching or other thermal process below 100 °C brought no increase to the remaining RIB. According to the conventional physical aging theory, the quench in the above case will freeze the thermodynamic state and the segmental RT’s at 100 °C. If the linear compliance of the sample is tested at 30 °C, its characteristic time will be close to that at 100 °C, and many orders of magnitude shorter than the one in thermodynamic equilibrium [1]. If the RIB RT’s had followed the trend of the conventional segmental RT’s in the quench process, then the measured NB would have decreased during the 16 hours at 23 °C and when the temperature was raised again from 30 °C, because the RT’s should have been close to those at 100 °C. The portion of curve-B above 104 °C should have shifted horizontally to the left (lower temperature), and the amount of shift would have depended on the elapse time at 23 °C, as is indicated by the imaginative curve labeled ‘PA’ (PA stands for physical aging) in the figure. When the temperature reached 100 °C for the third time the thermodynamic state of sample-B was certainly different from that of sample-A at the same temperature. The fact that sample-B relaxed in the same way as sample-A at above 104 °C indicates that although the thermodynamic state of sample-B was different from sample-A, the portion of RIB that would relax at above 104 °C was still the same as before, unaffected by the quenching and reheating.
3.2. Continuous Curves The two solid curves in Figure 15 are the Continuous Curves of two PC samples, one rubbed for 5 passes and the other rubbed till saturation. The curves are smooth and monotonic, without step-like features which could be caused by possible discrete RIB domain relaxations. The temperature T0 at which RIB reaches zero is 156 ºC, which is close to the glass transition temperature Tg of PC. The curve for the lightly rubbed sample is lower than that of the one rubbed to saturation. The density distribution functions N (ξ , 0) of the two samples (dashed curves in Figure 15) obtained by using Eq. (10) in Part-III are depicted as well. Unlike low molecular weight PS or lightly rubbed high molecular weight PS, where the density function is nearly flat between ξ ~ 23 ºC and ξ ~ Tg , the density function of even the lightly rubbed PC sample has a peak right below T0, and the rest of the density is nearly flat. For the rubbed to saturation sample, the peak is even higher. This indicates that unlike in PS, initial rubbing on PC already creates more favorably the high energy barrier birefringence elements. Like high molecular weight PS, further rubbing creates even more high-barrier elements and relatively less low-barrier elements. The width of the peak of PC, however, is narrower than that of PS. More Continuous Curves for other polymers rubbed till saturation are shown in Figure 16(a). All curves behave qualitatively the same. The RIB approaches zero near the Tg of the corresponding polymers, except for poly(4-tert-butylstyrene) (B7) which has a tail extending well above its Tg. Again, all curves are without step-like features, which rules out domain-like relaxation behavior. The density functions of the polymers obtained from the curves in Figure 16(a) are depicted in Figure 16(b). It is seen that except for poly(α-methyl styrene) (B5), all the others show a peak in their density functions. The shape of the density function of the
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poly(4-methyl styrene) (B4) sample is close to that of high molecular weight PS, with a peak near ξ = 90 ºC. For the same type of polymer (B2) with lower molecular weight (Mw = 2.7 kg/mol) the peak of the density is near the barrier value of ξ = 50 ºC. The density functions of poly(4-methoxy styrene) (B3) and poly(4-tert-butylstyrene) (B7) have high peaks below their Tg’s but with somewhat different shapes. The detailed differences between these polymers indicate different microscopic details of the birefringence elements in different polymers, which remain to be further explored.
ξ (°C) 20 1.0
40
60
60 Passes 5 Passes
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30 25 20
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15 0.4 0.2 0.0 20
3
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N(ξ) × 10
NB
80 100 120 140 160
5 40
60
80 100 120 140 160
0
Temperature (°C)
Figure 15. Continuous Curve (left and lower axes) and distribution of barriers (right and upper axes) of two PC samples, one lightly rubbed and the other rubbed till saturation.
1.0 B2 B3 B4 B5 B7
NB
0.8 0.6 0.4 0.2 (a)
0.0 20 40 60 80 100 120 140 160 180
Temperature (°C) Figure 16 (Continued)
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 283
N(ξ) × 10
3
40 B2 B3 B4 B5 B7
30 20 10 (b)
0 20 40 60 80 100 120 140 160 180
ξ (°C)
Figure 16. (a) Continuous Curves and (b) Density distribution of barriers of several polymers.
The surface layer of PS-like polymers is expected to have a gradient of mobility enhancement, with the top layer at the surface being much more mobile than the one deep in the bulk. The density function of the polymers presented here might be related to such depth dependent mobility, with the birefringence elements with higher barriers being in the deeper region from the surface, and the elements with lower barriers closer to the surface. Further investigations are required to verify the hypothesis.
3.3. Isothermal relaxation curves Figure 17 shows the RIB isothermal relaxations of several samples as a function of time, together with a PS sample for comparison. The NB(t) curve for B2 has been shifted upward by 0.3 for clarity. All curves behave qualitatively the same. For example, when the temperature is raised quickly from 23 °C to the isothermal relaxation temperature TA (60 °C for PC, PS and T3, 45 °C for B2), there is a quick drop of NB within ~ 10 s, because the elements with barrier height 23 °C ≤ ξ ≤ TA will readily relax. The NB then decayed slowly, approaching logarithm in time. The RT’s of the polymers span a wide range. Some are shorter than 10 s, while others are longer than 104 s. For example, for PC, 17 % of the elements have RT’s shorter than 10 s, while over 70 % of the elements have RT’s exceeding 104 s. At such rate, it would probably take a time in the geological time scale for the remaining RIB elements of PC to relax completely. Similar conclusions can be drawn for the other polymers. 3.4. Temperature lag The temperature lag phenomenon has been observed in the RIB relaxations of all the polymers we have investigated in this study. Figure 18 shows a typical set of experimental temperature lag Continuous Curves. Notice that after the samples had stayed at a certain temperature TA for certain duration tA, subsequent temperature rise above TA did not bring the RIB to decrease immediately. Instead, the RIB remained unchanged until a higher temperature TA + TLag was reached. Beyond TA + TLag the RIB started to decrease again, in much the same way as the sample without having spent time tA at TA. The temperature lag TLag was larger for longer annealing time tA. According to the discussion in Part-III, the dependence of RT of the birefringence elements on the barrier height parameter ξ, in the form
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of Eq. (5A), can readily be obtained from the measured TLag and tA, namely t A = τ 0 e
γ TLag
. The
experimental tA vs TLag data are presented in Figure 18 together with the least square fittings. The obtained parameters τ 0 and γ are listed in Table 3. It is seen that the value of τ 0 differs by up to a factor of 10 for all the polymers except for T3, its smaller than the rest of the polymers. The exponential parameter
τ 0 being about 100 times
γ varies by a factor less than
3, ranging from 0.23 for PC at 60 °C to 0.59 for B7 at 130 °C. The variation is within that of the temperature dependent γ of PS. There is no correlation between γ and T0 of the corresponding polymer, even though their T0 varies from about 80 °C (B2) to nearly 180 °C (B7), indicating that the relaxation of RIB is influenced only by a very small scale local environment. Taking into account the fact that in PS nearly 90 % of the RIB is due to segmental distortion and the fact that the RIB relaxations of all the polymers studied here behave in much the same way, we speculate that the majority of RIB in these polymers is due to the segmental distortion in the polymers.
1.0
PC, 60°C T3, 60°C PS, 60°C B2, 45°C
0.9
NB
0.8 0.7 0.6 0.5 0.4 1 10
10
2
3
10
10
4
5
10
Time (S) Figure 17. Isothermal relaxation curves of RIB of several polymers.
The samples listed in Table 3 are all thick enough that the finite thickness effects are avoided. This can be verified by the fact that none of their T0 is substantially lower than their Tg. In Part-III, we have shown that the amount of temperature lag of PS is independent of the film thickness. We expect the same for the polymers presented here. We want to emphasize again that the temperature lag phenomenon is in contradiction with the effects of physical aging, proving once again that aging effects are absent in the relaxation of RIB. In the temperature lag phenomenon, the RIB remains unchanged when the temperature is still in the range of TA + TLag, while above TA + TLag the RIB decreases with the increasing temperature in the same way as a sample without going through the annealing (aging) at TA. The annealing at TA eliminated all the RIB elements up to about ξ ≤ TA + TLag, but left the ones with ξ’s well above TA + TLag unchanged. This is in stark contrast with the effects of physical aging, where the segmental relaxations at all temperatures sufficiently
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 285 below Tg, regardless whether they are above or below the aging temperature, are affected by the aging process [1].
3.5. Temperature dependence of energy barriers At fixed annealing times the temperature lag TLag is usually temperature dependent. A typical one is shown in Figure 19(a). The general trend is that TLag decreases with increase temperature. Figure 19(b) summarizes TLag at fixed annealing time of 10 hours at different temperatures for several polymers. It is consistent with the increase of parameter γ with temperature shown in Tab. 3. For PC, TLag is nearly temperature independent above 100 ºC, and increase with decreasing temperature below 100 ºC at about the same rate as PS. For B6, the TLag increases with decreasing temperature in the entire temperature range of investigation and at a rate faster than PS. For T3, TLag is nearly constant below 80 ºC and then decreases with increasing temperature at a rate nearly the same as B6. These detailed differences among the polymers suggest the influence of detailed microscopic environment on the RT’s of the RIB elements.
0.9 PC
NB
0.8
30 Min. 3 hrs. 10 hrs. 3 days
0.7 TLag
0.6 0.5
(a)
0.4 60
70
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90
100
Temperature (ºC)
110
120
13
Ln[tA (s)]
12 11
PC
10
60°C 80°C 100°C 120°C
9 8 7 5
Figure 18 (Continued)
(b)
10
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25 TLag(°C)
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40
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Ln[tA (s)]
12 11 10
T3 B2 B3 B4 B5 B7
9 8
(c)
7 5
10
15
20
25
30
35
40
TLag (°C) Figure 18. (a) The Tlag curves of PC at fixed temperature and different annealing times; least square fittings of logarithmic annealing time tA vs Tlag for (b) PC at different temperatures, and (c) other polymers.
1.0
TLag
T3 45 °C 60 °C 75 °C 90 °C 110 °C 130 °C 150 °C
NB
0.9 0.8 0.7
(a)
0.6 0
20
40
60
80
100
120
Tlag (°C)
TLag (°C)
40 B6 T3 PS PC
30 20 10
(b)
40
60
80
100
120
140
160
Termperature (°C) Figure 19. (a) Typical Continuous Curves of PC showing Tlag at fixed annealing time of 10 hours and at different temperatures; (b) Tlag vs temperature at fixed annealing time of 10 hours of several polymers.
The Absence of Physical Aging Effects in the Surface Region of Glassy Polymers 287 The highest energy barrier for PC at T = 120 ºC for the RIB elements with ξ = 150 ºC is E = 677 kJ/mol, which is lower than the value for segmental relaxations of 958 KJ/(mol·K) [52]. The energy barriers of the same elements at 60 ºC are E = 273 kJ/mol, which is less than half of the value at 120 ºC, mainly due to the temperature dependence of γ . Similar results are obtained for other polymers. Such results have an interesting implication. It is well known that above Tg the segmental RT’s follow the Vogel – Fulcher form [3] [52] [53], which increases faster than the Arrhenius form with decreasing temperature. It is equivalent to an Arrhenius form with an energy barrier that increases with decreasing temperature. Below Tg the relaxation times of other forms of relaxation reported so far are in Arrhenius form with a smaller but temperature independent barrier ranging from 50 kJ/mol to 209 kJ/mol [54] [55] [59] [60]. What we have observed here is an energy barrier that decreases with decreasing temperature. The RT still increases with decreasing temperature but at a rate that is slower than the Arrhenius form, because the energy barrier is also decreasing. We call it subArrhenius form of relaxation times. Figure 20 shows the simulation RT in an Arrhenius form and that of PS RIB elements with ξ = 100 ºC, based on the temperature dependent RT’s obtained in Part-III. The Arrhenius energy barrier E and the pre-factor are so chosen such that the two curves have the same slope and value at T = 100 ºC. It is clear that the increase of RT of RIB elements with ξ = 100 ºC is indeed slower than the Arrhenius form at temperatures much lower than 100 ºC. Above T = 100 ºC, however, the RT of RIB increases faster than Arrhenius with temperature.
40
Ln(τ (s))
30
RIB Arrhenius
20 10 0 -10 -20 40
60
80
100
120
Temperature (°C) Figure 20. Simulation of RIB relaxation times of PS (solid curve) and an imaginative one following Arrhenius form as a function of temperature.
4. Summary In summary, we have shown that RIB can be generated in many glass forming polymers. Significant relaxations of RIB can take place well below the glass transition temperature Tg, similar to what Kovacs and Hobbs have reported [44]. The relaxation times span a wide range from ~ 10 s to probably geological time scale. Physical aging effects are absent in the RIB relaxations. The model proposed for the interpretation of RIB in PS works well for all the
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polymers investigated here. The energy barriers are of the order of a few hundred kJ/mol and decrease with decreasing temperature. It is in opposite of the trend of Vogel – Fulcher form for polymer segmental relaxations above Tg, and is different from the barriers found in other relaxation processes below Tg, which are temperature independent [39, 57, 60 – 68]. The relaxation behaviors of different polymers are qualitatively similar but somewhat different in quantitative details, such as in the values of the saturated birefringence, the shape of the initial barrier density distribution functions, the rates of barrier decrease with decreasing temperature, and the dependence of relaxation times on temperature and parameter ξ, etc. They are different from any of the relaxations below Tg that have been reported in the literature [39, 57, 60 – 68]. In view of the substantial amount of experimental results, a microscopic model for the relaxations of RIB is much desired.
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[48] Courtney, T. & Thurau, M. D. (2002). Ediger, J. of Polymer Science: Part B: Polymer Physics, 40, 2463. [49] Ellison, C. J. Kim, S. D. Hall, D. B. & Torkelson, J. M. (2002). Eur. Phys. J. E 8, 155. [50] Dalnoki-Veress, K. Forrest, J. A. Murray, C. Gigault, C. & Dutcher, J. R. Phys. Rev. E., 63, 31801 (2001). [51] Nicole Raymonde Demarquette, Jose´ Carlos Moreira, Renato Norio Shimizu, Mazen Samara, Musa R. Kamal, Journal of Applied Polymer Science, 83, 2201 (2002). [52] Paul, A. O’Connell and Gregory, B. McKenna, Journal of Chemical Physics, 110, 11054 (1999). [53] Vogel, H. & Phys. Z. (1921). 22, 645 G. S. Fulcher, J. Am. Ceram. Soc. 8, 339 (1925 [54] Veronica Lupascu, Stephen J. Picken, Michael Wubbenhorst, Journal of NonCrystalline Solids, 352 5594 (2006). [55] Rodney, D. Priestley, Linda J. Broadbelt, John M. Torkelson, and Koji Fukao, (2007). Physical Review E, 75, 61806. [56] Urakawa, O., Ohta, E., Hori, H. & Adachi, K. (2006). Journal of Polymer Science: Part B: Polymer Physics, 44, 967. [57] Otto van den Berg, Michael Wubbenhorst, Stephen J. Picken, Wolter F. Jager, (2005). Journal of Non-Crystalline Solids, 351, 2694. [58] Bercu, V. Martinelli, M. Massa, C. A. Pardi, L. A. & Leporini, D. (2005). Europhys. Lett., 72 (4), 590. [59] Ali Dhinojwal, George, K., Wong, John, M. & Torkelson, (1994). J. Chem. Phys., 100, 6046 [60] Rodney, D., Priestley, Linda, J., Broadbelt, John, M., Torkelson, & Koji Fukao, (2007). Physical Review E, 75, 061806. [61] Akiyoshi Suzuki and Yoshihiko Matsuoka, J. Appl. Phys. 77, 965 (1995). [62] Sergei Fedosov, Jose, A., Giacometti, G. F., Leal Ferreira, & Mauro, M. (1997). Costa, J. Appl. Phys. 82, 4355. [63] Kanaya, T., Kawaguchi, T. & Kaji, K. (1996). J. Chem. Phys. 104, 3841.
In: Encyclopedia of Polymer Research Ed: Caroline E. Jones
ISBN: 978-1-61761-926-7 © 2011 Nova Science Publishers, Inc.
Chapter 9
CURRENT DEVELOPMENTS IN DOUBLE HYDROPHILIC BLOCK COPOLYMERS G. Mountrichas and S. Pispas Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Ave., 11635, Athens, Greece
ABSTRACT Double hydrophilic block copolymers (DHBCs) constitute a novel class of watersoluble macromolecules with potential utilization in a wide range of applications. The exceptional combination of features, coming from their block copolymer structure and their ability to be stimuli responsive, establishes this class of copolymers as a core of intense research interest, aiming at elucidating aspects regarding their targeted synthesis, solution behavior and application possibilities. In this chapter, the current developments in the field of double hydrophilic block copolymers are discussed. In particular, synthetic strategies leading to the preparation of DHBCs are described. Moreover, their aqueous solution behavior is examined in respect to their ability to self assemble, due to changes in the solution temperature, and/or pH, as well as due to complexation. Additionally, the potential applications of DHBCs in mineralization processes, nanomedicine, nanotechnology and so on are mentioned. Finally, future perspectives in the field of DHBCs regarding general polymer science and nanotechnology issues, as well as open scientific questions, on synthesis and solution behavior of this class of materials, are also discussed.
1. INTRODUCTION Double hydrophilic block copolymers (DHBCs) are a class of polymers that combine the self assembly ability of block copolymers with the water solubility of hydrophilic macromolecular chains. Numerous sophisticated works have been already described in the literature, indicating the potential of this class of copolymers in emerging technologies. The synthesis of novel DHBCs, using either new monomers or post polymerization functionalization schemes, is the subject of intense investigation during the current years.
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Moreover their self-assembly in aqueous media has been also studied in considerable detail. Emphasis has been given to the stimuli responsive character of the DHBCs upon environmental changes, like solution pH, ionic strength or temperature. Moreover, the potential utilization of DHBCs in a wide range of applications has been demonstrated in a number of publications. The synthesis of DHBCs has been realized by several synthetic strategies. Typically, controlled polymerization schemes are used in order to ensure the formation of block copolymers. A plethora of monomers have been polymerized following a number of polymerization mechanisms, involving, but not limited to, anionic polymerization, atom transfer radical polymerization (ATRP) or reversible addition-fragmentation chain transfer polymerization (RAFT). The formation of supramolecular structures, like core-shell micelles or vesicles, has been indentified for a numerous block copolymers upon changes in pH, salinity or temperature. The combination of blocks, able to respond at different stimuli, within the chemical structure of the copolymer chain has immerged as an appealing way for controlling and enriching the self assembly behavior of DHBCs. Finally, DHBCs have been established as a key component in a wide gamut of applications, related to drug and gene delivery, surface functionalization or creation of smart surfaces and mineralization of inorganic matter. In this chapter recent developments in the field of DHBCs are described. Excellent review articles have already outlined the major features of DHBCs [1,2]. A number of excellent reviews summarize specific aspects of DHBCs in detail [3,4,5]. This chapter is focused on recent advances in the particular research area. The synthetic strategies followed for the preparation of DHBCs, important investigations on their aqueous solution behavior and a number of potential applications are presented. The field is growing rapidly. Therefore, the creation of a complete list of works, concerning DHBCs, is practically impossible and beyond the goal of this chapter. The works presented here have been selected in order to representatively describe the current developments in DHBC research.
2. SYNTHESIS OF LINEAR DIBLOCK DHBCS The synthesis of a variety of linear diblock DHBCs structures has been realized by the vast majority of the so called controlled polymerization techniques. Herein, we describe recent achievements in the synthesis of DHBCs categorized according to the followed polymerization mechanism.
2.1. Synthesis via Anionic Polymerization Methodologies Anionic polymerization is known to give model block copolymers with controlled molecular weights, narrow molecular weight distributions and versatile architecture. Anionic polymerization has been used for the synthesis of DHBCs in several cases although this type of polymerization technique is relatively intolerant to the presence of polar functionalities on the monomers utilized. A recent example has been described by Hadjichristidis and coworkers [6]. They have presented the synthesis of a series of poly (2-vinylpyridine-b-
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ethylene oxide) (P2VP-PEO) block copolymers with different molecular weights and composition. The synthesis was performed by sequential polymerization of the vinyl pyridine monomer followed by the polymerization of the ethylene oxide, in a polar solvent, Figure 1. The high enough molecular weights, as well as the low polydispersity index of the obtained copolymers, indicate the successful utilization of anionic polymerization techniques for the synthesis of P2VP-PEO DHBCs. The synthesis of P2VP-PEO block copolymers has been reported several years ago [7,8] with rather poor results concerning the purity of the final block copolymers. Combination of anionic polymerization and post polymerization reactions has been used for the synthesis of poly(acrylic acid-b-Ν,Ν-diethylacrylamide) (PAA-PDEA) copolymers [9]. Initially the synthesis of a precursor poly(tert-butylacrylate-b- Ν,Ν-diethylacrylamide) (PtBMA-PDEAAm) block copolymer was realized via sequential anionic polymerization of the tert-butyl acrylate and diethylacrylamide monomers. However, an amount of PtBMA homopolymer was detected in the crude reaction product. In order to remove the vast majority of the homopolymer, the authors proposed the precipitation of the crude product in hexane, where the homopolymer is highly soluble, in contrast to the block copolymer. The purified block copolymer was subjected to deprotection of the tert-butyl group in acidic media, leading to the desirable DHBC. The final block copolymer showed pH and thermosensitive solution aggregation. DHBCs of the type poly(p-hydroxystyrene-b-methacrylic acid) (PHOS-PMAA) were also synthesized via anionic polymerization followed by acidic hydrolysis [10]. Both blocks of the precursor poly(p-tert-butoxystyrene-b-tert-butylmethacrylate) (PtBOS-PtBMA) copolymers, formed by sequential addition of the protected monomers, could be deprotected in a single step giving the desired pH-responsive block copolymers, Figure 2. Hydrolysis was found to be nearly quantitative and resulted in a series of copolymers with well defined molecular characteristics and of variable composition.
Figure 1. Synthesis of P2VP-PEO DHBCs by anionic polymerization. Reproduced from [6] by permission of Elsevier.
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Figure 2. Synthesis of PHOS-PMAA by combination of anionic polymerization and post polymerization deprotection reaction. Reproduced from [10] by permission of the American Chemical Society.
A new family of neutral-ionic block copolymers were synthesized by post polymerization functionalization of anionically prepared poly(isoprene-b-ethylene oxide) amphiphilic diblock copolymers. Reaction of the double bonds of the polyisoprene block with excess chlorosulfonylisocyanate, followed by alkaline hydrolysis of the intermediate lactam functionalized copolymers, led to the synthesis of poly[sodium (sulfamate/carboxylate) isoprene-b-ethylene oxide] (PSCI-PEO) DHBCs [11]. Degrees of functionalization of the polyisoprene block were found in the range 0.70-0.78 molar. The PSCI block of the copolymers carries surfamate and carboxylate groups that can change the state of their ionization depending on the solution pH, giving to the copolymer variable degrees and type of charges. Different series of DHBCs were prepared from anionically synthesized poly(p-tertbutoxystyrene-b-ethylene oxide) (PtBOS-PEO) precursors [12]. Post polymerization acidic hydrolysis of the PtBOS block resulted in poly(p-hydroxystyrene-b-ethylene oxide) (PHOSPEO) copolymers. Further functionalization of the PHOS block via a Mannich type aminomethylation reaction gave the poly[3,5-bis(dimethylaminomethylene)hydroxystyreneb-ethylene oxide] (PNHOS-PEO) copolymers, as testified by FT-IR and NMR experiments. In these copolymers the PNHOS block carries two dimethylamino groups per monomeric unit that can be protonated in acidic media and weakly acidic phenolic groups that have their own pH sensitivity. The PNHOS-PEO block copolymers were further quaternized with
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methyliodide to give the corresponding cationic-neutral block copolymers that retain their cationic nature at all pH values.
2.2. Synthesis via Atom Transfer Radical Polymerization Methodologies The synthesis of an interesting DHBC, namely poly (4-vinylbenzoic acid-block-2(diethylamino)ethyl methacrylate) (PVBA-PDEAEMA), has been presented by Armes and Liu [13]. The synthesis was performed by ATRP using protecting group chemistry in three steps. Initially the polymerization of a tert-butyl protected PVBA macro-initiator was performed followed by the polymerization of the second monomer, DEAEMA. Finally the hydrolysis of the tert-butyl protected block was realized giving rather monodisperse block copolymers. A similar procedure has been presented by the same group for the synthesis of another block copolymer, namely poly[4-vinylbenzoic acid-block-2-N-(morpholino)ethyl methacrylate] (PVBA-PMEMA). The protection of the VBA monomer with a tert-butyl group is essential for the synthesis of the desired copolymer, otherwise a mixture of PVBA-PMEMA diblock and PVBA homopolymer was obtained. The particular copolymer is expected to have interesting solution properties since the PVBA block is a week polyacid, while PMEMA is a conjugated acid, soluble over a wide pH range at room temperature and in the absence of salt [14]. The synthesis of an ampholytic block copolymer, namely PMAA-PDEAEMA, carrying carboxylic and tertiary amino side groups, has been also realized by ATRP, as has been reported by Tam and coworkers [15]. Initially, the synthesis of the tert-butyl protected PMAA block was performed using p-toluenesulfonyl chloride as an initiator and CuCl complexed with N,N,N’,N’,N”,N”-hexamethyltriethylenetetraamine as a catalyst in 50 vol % anisole at 90 °C. The obtained polymer was used as the macroinitiator for the subsequent polymerization of the second monomer, DEAEMA, under similar reaction conditions, Figure 3. The resulted copolymer was subjected to selective hydrolysis, under acidic conditions, for removal of the tert-butyl protecting group. In the works mentioned, protecting group chemistry has been utilized for the synthesis of ampholytic/zwitterionic block copolymers. However, Armes and coworkers have also presented the synthesis of zwitterionic block copolymers without using protecting groups. In particular, various tertiary amine methacrylate monomers were copolymerized with 2hydroxyethyl methacrylate to produce diblock copolymers with a hydroxyl functionalized block. The synthesis of the amine containing block was realized in bulk, followed by the polymerization of the hydroxyl group containing monomer in methanol [16]. The obtained diblock precursor was then derivatized using succinic anhydride in order to introduce the desired carboxylic groups on the hydroxyl functional block. The procedure followed led to high monomer conversion and to copolymers having fairly low polydispersities. However, as has been revealed by the solution behavior of the synthesized polymers, the existence of some amine containing segments in the carboxylated block could not be excluded. In order to improve the synthetic procedure, the polymerization of the first block was performed in protic media under mild conditions [17]. Moreover, the order of monomer addition has been also inversed, because of the low blocking efficiency of the amine block, when it was polymerized in methanol. Finally, pure block copolymers, with polydispersities ca. 1.2, were obtained following the above synthetic procedure.
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Figure 3. Synthesis of the ampholytic PMAA-PDEAEMA by ATRP. Reproduced from [15] by permission of the American Chemical Society.
The ATRP mediated synthesis of non-zwitterionic copolymers, without the use of protecting groups, has been also presented in the literature [18]. An interesting example is the case of the poly(N-isopropylacrylamide)-b-poly(4-vinylpyridine) (PNIPAM-P4VP) copolymers. The synthesis of the particular type of copolymer was achieved by using conventional ATRP techniques. The molecular characteristics and the data obtained by size exclusion chromatography (SEC) analysis indicate the successful synthesis of copolymers with polydispersity index values ca.1.2.
2.3. Synthesis via RAFT Polymerization Laschewsky and coworkers have presented the synthesis of a series of DHBCs using RAFT polymerization methodologies, which is a “controlled” free radical polymerization. The system, that they have reported, is a temperature responsive block copolymer, namely poly(N-isopropylacrylamide-b-3-[N-(3-methacrylamidopropyl)-N,Ndimethyl]ammoniopropane sulfonate) (PNIPAM-PSPP) [19]. The synthesis was performed by sequential polymerization of NIPA and SPP monomers. The molecular weight was determined to be close enough to the theoretically calculated one. Interestingly, the followed polymerization procedure led to macromolecular chains with high enough molecular weight, up to about 60,000 g/mol. The same polymerization technique has been also utilized for the synthesis of other polyacrylamide containing DHBCs, like PNIPAM-PDEAEMA [20] or
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poly(N,N-dimethylacrylamide-b-N-isopropylacrylamide) (PDMA-PNIPAM) [21]. In the last case, Liu and coworkers have presented the synthesis of a PDMA-PNIPAM, where some azide-containing monomers (AzPAM) have been copolymerized within the PNIPAM block [21]. The above azide-containing monomer seems to be polymerized quite easily in dioxane at elevated temperatures, while the azide groups are relatively inert and compatible with RAFT processes. The authors assumed that the structural similarity of AzPAM and PIPAM monomers has as a result the statistical copolymerization of the two monomers. The RAFT process has been also used in the synthesis of novel ionic liquid containing DHBCs [22]. The polymerization of ionic liquid monomers was performed in methanolic solutions at elevated temperatures. The synthesis of copolymers with predictable molecular characteristics was described, indicating the living nature of the polymerization procedure. The synthesis of block copolymers of the type poly(ionic liquid-b-acrylamide) (PIL-PAm) and PMAA-PIL was successfully realized by using the as synthesized first block as a macrochain transfer agent for the polymerization of the second block. The final copolymer could be molecularly dissolved in water. However, chemical modification, by either esterification of the carboxylic groups of the PMAA block or ion exchange in the PIL block, could be used for the introduction of hydrophobic character in one of the blocks, leading to the formation of supramolecular self-assembled structures in aqueous media. The synthesis of a well defined block copolymer, containing a poly(N-vinylimidazole) block (PVim), has been demonstrated by Ge et al. [23]. The polymerization was realized via a RAFT procedure employing xanthates as chain transfer agents. Initially, the polymerization of NIPAM monomer, affording PNIPAM macro-CTA, was realized followed by the polymerization of Vim, Figure 4. The obtained block copolymers had controllable molecular characteristics with a Mw/Mn value ca. 1.2. The living nature of the Vim polymerization has been confirmed by kinetic experiments, for the first 150 min of the polymerization reaction. It has to be noted that the above procedure was the first example of the synthesis of PVim containing block copolymers.
Figure 4. Synthesis of a PVim containing DHBC by RAFT. Reproduced from [23] by permission of the American Chemical Society.
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2.4. Synthesis via Other Types of Polymerization Methodologies Group transfer polymerization (GTP) has been recognized as a powerful alternative methodology, toward the synthesis of methacrylate containing copolymers, with well defined structure and narrow molecular weight distribution. Armes and coworkers have presented the synthesis of a diblock copolymer, namely poly[2-(dimethylamino)ethylmethacrylate-b-2(diethylamino)ethylmethacrylate] (PDMAEMA–PDEAEMA), utilizing the GTP technique [24]. The particular synthesis was the first effort to synthesize a block copolymer in which both component blocks are polybases. The molecular weight of the synthesized polymers was up to about 20,000 g/mol with narrow molecular weight distributions (typically less than 1.15). The chemical modification of the copolymer by quaternization of the PDMAEMA block has been also presented by the same group [25]. Moreover, the synthesis of block copolymers consisting of DMAEMA and other tertiary methacrylate blocks has been also described [26,27]. Finally, Tsitsilianis and coworkers have presented the synthesis of an A-b(B-co-C) DHBC where A is PDEAEMA and B-co-C is poly[(oligo(ethylene glycol) methacrylate)-co-methyl methacrylate] (P(EGMAco-MMA)) [28]. A series of copolymers with different content of MAA were synthesized in order to obtain macromolecules with different hydrophilicity. An interesting class of DHBCs is the case of the polypeptide diblock copolymers. Lecommandoux and coworkers have presented the synthesis of such a type of copolymer, i.e. poly(γ-benzyl-L-glutamate-b-N-trifluoroacetyl-L-lysine) (PBLG-PTFALys). The synthesis was performed by sequential ring-opening polymerization of the corresponding R-amino acid N-carboxyanhydrides. The PTFALys block was synthesized first, followed by the polymerization of the second monomer. The polymerization took place in organic media and at room temperature. The final DHBC copolymer was obtained upon removing the benzyl (Bz) and trifluoroacetyl (TFA) protective groups of L-glutamic acid and L-lysine using KOH in THF. The copolymer is a zwitterionic diblock copolymer with only about 15 repeating units in each block [29]. The synthesis of a hybrid DHBC, containing both a polypeptide block and a polyacrylamide block, has been also presented [30]. The synthesis was realized by using an amine terminated PNIPAM as the macroinitiator in the ring-opening polymerization of γbenzyl-L-glutamate, followed by deprotection of the benzyl groups of the polypeptide block, Figure 5. Interestingly, samples with high enough molecular weights were obtained following the aforementioned method; however, the polydispersity of the copolymers was also relatively high. The synthesis of a novel, primary amine containing, DHBC has been described by Liu and coworkers [31]. The synthesis of the amine-containing monomer was performed by a two step click reaction. The preparation of the block copolymer, namely poly(Nisopropylacrylamide)-b-1-(3-aminopropyl)-4-acrylamido-1,2,3-triazole hydrochloride) (PNIPAM-PAPAT), was realized by RAFT. In the first step, the polymerization of PNIPAM was achieved and the as synthesized polymer was used as the macroinitiator for the polymerization of APAT. However, in order to polymerize primary amine containing monomers, acidic media are needed in order to eliminate some side reactions (the primary amine moieties have to be protonated in order to suppress the side reactions).
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Figure 5. Synthesis of a polypeptide containing DHBC by ring opening polymerization. Reproduced from [30] by permission of the American Chemical Society.
The synthesis of a well defined poly(vinyl alcohol)-b-poly(acrylic acid) (PVA-PAA) DHBC has been recently reported in the literature [32] by a two step synthetic scheme. First the synthesis of a poly(acrylonitrile) (PAN) block was realized via cobalt-mediated radical polymerization, using a poly(vinyl acetate) (PVAc) macroinitiator, followed by hydrolysis of both blocks. The polymerization was performed in DMF, a very good solvent for PAN, and at low temperature, where block copolymers with low polydispersity were obtained. The polymerization procedure led to well defined macromolecules with relatively high molecular weights. The obtained copolymers were transformed to the desired DHBCs by hydrolysis, using large excess of potassium hydroxide in a water/ethanol mixture. The successful completion of the hydrolysis reaction was monitored by NMR and IR spectroscopy. An additional macroscopical indication of the DHBC formation was the aqueous solubility of the reaction product. An interesting approach for the synthesis of DHBCs has been presented by Shi and coworkers [33]. In particular, end functionalized homopolymers, namely P4VP and PNIPAM, were synthesized by conventional RAFT polymerization, using the appropriate initiators. Mixing equimolar amounts of the above polymers led to the formation of well defined noncovalent bonded DHBCs through inclusion complexation between the β-cyclodextrin and the adamantyl end groups, Figure 6. The copolymer seems to behave as a typical diblock copolymer, i.e. forms supramolecular self-assembled structures under appropriate solution conditions. Furthermore, the synthesis of a “covalent analogous” polymer via ATRP methodologies has been also described in another work [34].
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AD-PNIPAM70
-CD-P4VP30
Complex
Figure 6. Schematic illustration of DHBCs formation through inclusion complexation. Reproduced from [33] by permission of The Royal Society of Chemistry.
A similar approach has been also used for the synthesis of “covalently linked” DHBCs. Polysaccharide based DHBCs were prepared by end-to-end coupling of two readily available biocompatible water-soluble homopolymers [35]. The synthesis was a two step reaction where a) a terminal aldehyde group of a dextran homopolymer was oxidized and b) a monoamine end functionalized PEG reacted with the oxidized dextran, via a lactone aminolysis reaction. Interestingly, the obtained polymer could be chemically modified, in a controlled way, in order to produce neutral-cationic or neutral-anionic DHBCs. Finally, the controlled aqueous polymerization of various monomers, via nitroxide mediated polymerization has been recently demonstrated by Phan and coworkers [36]. The polymerization was realized by using a chemically modified nitroxide, which bears a carboxylic acid function. The introduction of carboxylic groups offers water solubility, when the groups are in the ionized form, allowing the controlled polymerization of water soluble monomers. The controlled (co)polymerization of nonionic, anionic and cationic monomers has been demonstrated under mild conditions. The polymerization scheme presents the characteristics of controlled polymerizations, such as first-order kinetics up to high conversions, linear increase of the molecular weight with monomer conversion and good agreement between experimentally obtained and theoretically expected molecular weights. However, the relatively high polydispersity index values of the products discourage the use of the above polymerization scheme for the preparation of well defined block copolymers for structure-properties relationship investigations. Additionally, the synthesis of block copolymers was accompanied by the presence of unreacted homopolymer, due to some termination reactions.
3. SYNTHESIS OF DHBCS WITH OTHER ARCHITECTURES 3.1. Linear Multiblock DHBCs The synthesis of an ABA triblock copolymer, where A is a P2VP block and B is a PEO block, was described in the literature [6]. The synthesis of the copolymer was performed via anionic polymerization high vacuum techniques. Initially a diblock copolymer was synthesized by sequential polymerization of vinyl pyridine and ethylene oxide monomers. Subsequently, the as synthesized diblock copolymer was reacted with p-dibromoxylene. A portion of the diblock copolymer reacts with the difunctional reagent leading to the formation of a triblock, namely P2VP-PEO-P2VP, having double the molecular weight of the precursor
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diblock. Fractionation of the crude reaction product took place at elevated temperature in order to remove the unreacted excess diblock. Anionic polymerization has been also used for the synthesis of another ABA DHBC, where A is PAA and B is P2VP [37,38]. The synthesis of a precursor polymer was realized by using a bifunctional initiator and by sequential addition of 2VP and tert-butyl acrylate monomers (tBA). The obtained SEC data indicate the formation of well defined triblock chains, without the presence of neither P2VP homopolymer, nor P2VP-PtBA diblock copolymer. The formation of the desired DHBC triblock was finally achieved by acidic hydrolysis of the PtBA blocks in order to remove the tert-butyl protecting group. A similar procedure has been used by the same group for the preparation of another ABA block copolymer, namely PEO-P2VP-PEO. However, in this case the deprotection step is not necessary, since the PEO block is already hydrophilic [39]. The synthesis of an ABA triblock double hydrophilic copolymer, where A= PEO and B= PSCI has been also reported via the utilization of chlorosulfonylisocyanate functionalization reaction on an anionically synthesized poly(ethylene oxide-b-isoprene-b-ethylene oxide) triblock precursor [11]. Finally, the synthesis of a BAB triblock copolymer, where A is a PDMA block and B is a PNIPAM block, has been realized via RAFT polymerization using a symmetrical bistrithiocarbonate as the bifunctional chain transfer agent for the polymerization of the middle block [40]. The polydispersity indices for the synthesized triblocks, determined by SEC, were in the range 1.19-1.31. The high Mw/Mn values could be also correlated with the possible interaction of the copolymers with the column material, as well as to the association of block copolymer chains in the carrier solvent. A series of copolymers with an identical middle block and outer blocks with different molecular weight have been synthesized. However, the presence of diblocks or unreacted homopolymers in the final product could not be excluded. An ABC triblock terpolymer was successfully synthesized by a sequential three-step RAFT polymerization process of N-n-propylacrylamide, N-isopropylacrylamide and N,Nethylmethylacrylamide monomers [41]. The different cloud points of the respective blocks, present in the terpolymer, are responsible for the rich temperature depended solution selfassembly of the sample in aqueous media. In the concept of water soluble ABC block copolymers one has also to mention a huge amount of work that has been presented on the synthesis and solution behavior of copolymers with two hydrophilic blocks and one hydrophobic. However, the presence of a permanently hydrophobic block in this type of block polymers makes difficult their categorization as DHBCs. The synthesis of ABC triblock terpolymers with at least two hydrophilic blocks has been realized via a number of polymerization methodologies, like GTP and cationic polymerization, and has been studied in detail [42,43,44,45,46,47,48,49]. In most of the cases, the terpolymers were based on suitably functionalized methacrylate monomers and have been produced by the sequential monomer addition method. Finally, the synthesis of multiblock DHBCs has been also reported. The desired multiblocks have been obtained via RAFT polymerization, using polytrithiocarbonate as the chain transfer agent [50]. The synthesis of two multiblocks with different molecular characteristics was achieved. Both block copolymers were consisted of PDMA and PNIPAM sequences. The molecular characteristics of the synthesized macromolecules were studied
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both by NMR and SEC. Polymers with quite high molecular weight were obtained. However, it has to be noted that the polydispersity of the samples was about 1.5.
3.2. Non Linear Double Hydrophilic Block Copolymers One of the first syntheses of non-linear DHBCs has been described by Armes and coworkers [51]. They have prepared a series of A2B miktoarm star copolymers via ATRP of several hydrophilic methacrylate monomers by utilizing Jeffamine macroinitiators. The macroinitiators (which were the B blocks of the copolymers) were synthesized by reaction of the terminal amino group of the polyalkyleneoxide chain with two equivalents of 2-hydroxyethylmethacrylate, followed by esterification with 2-bromoisobutyryl bromide. The A blocks carried pH responsive substituted amine groups (water soluble and positively charged at low pH) or zwitterionic phosphatidylamine groups. The expected chemical structures for the macroinitiators and the resulting star copolymer where confirmed by NMR, MALDI-TOF-MS and SEC techniques. Liu and coworkers [52] have described the synthesis of two different types of branched block copolymers, namely A2BA2 and A4BA4. In both cases, the B block was poly(propylene oxide) (PPO) while the A block was PDEAEMA. The synthesis was realized via ATRP using a commercially available diamine terminated PPO as the backbone of the copolymer. The initial PPO central block was functionalized in order to introduce multi-ATRP initiator functionalities at both free ends of the polymer. Subsequently the polymerization of DEAEMA monomer was performed following typical ATRP procedures. The followed synthesis scheme is shown in Figure 7.
Figure 7. Synthesis of an A2BA2 DHBC by ATRP. Reproduced from [52] by permission of the American Chemical Society.
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ATRP has been also utilized for the synthesis of an AB4 miktoarm star copolymer, where A is a PNIPAM block and B is a PDEAEMA block [53]. At first, a mono-amino terminated PNIPAM homopolymer was synthesized. Subsequently, an addition reaction on the amino group using glycidol was realized, followed by esterification with excess 2-bromoisobutyryl bromide. The end functionalized PNIPAM formed in this way could be used as a tetrafunctional macroinitiator for the polymerization of DEAEMA monomers via ATRP. The formation of pure AB4 miktoarm star copolymer was assumed by the authors based on results from size exclusion chromatography. However, the polydispersity index was quite high (Mw/Mn= 1.23), therefore, SEC alone could not unambiguously confirm the level of purity of the miktoarm stars. The synthesis of a miktoarm star copolymer of the type AnBn has been also demonstrated. The synthesis was performed via ATRP using divinylbenzene, as the core cross-liking agent. PEO macroinitiator chains were utilized for the polymerization of divinylbenzene forming a star polymer, with a random number of branches. The above star polymer was used as a multi-functional initiator for the polymerization of methacrylate monomers. Therefore, the synthesis of an amphiphilic miktoarm star copolymer was realized [54]. Finally, the hydrolysis of the protected methacrylate block led to the preparation of the desired DHBCs, namely the PEOn-PMAAn stars. SEC analysis of the precursor PEOn-PMMAn copolymer revealed a relatively broad molecular weight distribution. Nevertheless, this is a good example for the synthesis of AnBn double hydrophilic star copolymers. The synthesis of a series of star copolymers using GTP has been described by Patrickios and coworkers [55]. The general route for the synthesis of the copolymers was the combination of the conventional polymerization of the desired monomer by GTP and the use of a bi-functional coupling agent, as has been described before. The synthesis of four different types of star copolymers, composed of DMAEMA and methoxy hexa(ethylene glycol) methacrylate (HEGMA) was presented. In particular, isomeric star copolymer, heteroarm, star block and statistical star copolymers were realized by varying the sequences of the monomers and coupling agent additions. In all cases a constant ratio of the two monomers (DMAEMA / HEGMA = 9/1) was utilized. The presence of linear (co)polymer precursors was evident by SEC analysis of the samples, indicating the incomplete formation of star copolymers. Finally, the synthesis of a double hydrophilic linear-hyperbranched block copolymer has been realized by a four step reaction [56]. Initially a polyethylene oxide macroiniator was synthesized either by anionic polymerization of ethylene oxide using Cs as the counterion or by deprotonation of a commercially available monohydroxyl terminated PEO via interaction with cesium hydroxide. Subsequently, the anionic polymerization of ethoxyethyl glycidyl ether monomer (EEGE) was performed under argon and at elevated temperature. The obtained diblock copolymer was subjected to acidic hydrolysis in order to recover the hydroxyl groups, by removing the acetal protecting groups on the PEEGE block. Finally, the desired linear-hyperbranched copolymer was synthesized by initiating the polymerization of glycidol from the hydroxyl groups of the diblock, using cesium hydroxide as the activating agent. The resulting copolymers exhibited low polydispersities, while the molecular weight was up to almost 1.6x104 g/mL. It has to be noted that the presented methodology allows for variation of the molecular weight in both the linear PEO block and of the hyperbranced block.
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4. SOLUTION SELF-ASSEMBLY OF DHBCS Several intriguing and interesting self-assembled nanostructures can be produced in aqueous solutions of DHBC, due to the stimuli responsive character of the blocks comprising the copolymer that in turn determines the amphiphilicity of the system. The solution properties of DHBCs can be tuned via the chemical structure of the copolymer and external parameters including solution pH, ionic strength, temperature, as well as the addition of oppositively charged chemical entities. These features open possibilities for understanding the self-assembly motifs in natural occurring systems and facilitates the utilization of DHBCs in a number of advanced nano(bio)technological applications. In the following sections we focus on selective studies concerning the aqueous solution behavior of this type of copolymers.
4.1. Temperature Responsive Self-Assembly One of the most interesting examples of DHBCs which form self-assembled structures upon changes of solution temperature has been presented by Laschewsky and coworkers [19]. They have developed a system where one of the blocks presents a LCST and the other a UCST. Therefore the copolymer, namely PNIPAM-PSPP, could either be molecularly soluble or associate into aggregates at low and high temperature, Figure 8. The nature of the hydrophobic core of the formed aggregates depends on the solution temperature. In particular the PSPP block is soluble for temperatures higher than 20oC (depending on the molecular weight), while the PNIPAM block is soluble for temperatures lower than 32-34oC (almost independent of the chain molecular weight). In this way, by controlling the molecular characteristics of the copolymer, the desired aggregation/disaggregation profile could be achieved. The studied copolymer is soluble in the whole temperature range under investigation. However, at the low temperature aggregation limit, the insoluble microdomains are relatively polar and probably they have low solubilization efficiency for hydrophobic molecules, a property that would have been useful in the field of nanosized drug carriers.
Figure 8. Micellization behavior of PNIPAM-PSPP upon changes in the solution temperature. Reproduced from [19] by permission of the American Chemical Society.
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An interesting work on the temperature induced self-assembly of DHBCs has been presented by Pasparakis and Alexander. They have proposed the formation of vesicles by copolymers consisting of a highly hydrophilic poly(2-glucosyloxyethyl methacrylate) block and a water-soluble poly(diethyleneglycol methacrylate) block [57]. The above copolymer is molecularly dissolved at 15oC but forms aggregates at higher temperatures. The size of the aggregates depends on the solution temperature, due to the increased hydrophobicity of poly(2-glucosyloxyethyl methacrylate) upon increasing the temperature of the solution. Interestingly, the aforementioned aggregates were found to be vesicles, as has been demonstrated by transmission electron microscopy. Schubert and coworkers observed a temperature induced cylinder-to-vesicles transition in aqueous solutions of P2VP-PEO copolymers [58]. By cooling the polymer solution down to 4oC the initial vesicles are transformed into wormlike micelles, through intermediate formation of basket-like aggregates. A subsequent increase in solution temperature allows for the appearance of vesicles through intermediate discoid and octopus-like structures. The preparation of unilamellar and nearly monodisperse vesicles with controlled sizes, in the range 60 to 500 nm, is possible in these systems by appropriate variation of heating rates, polymer concentration, ionic strength, as well as the incubation time at the limits of the transition temperatures. The authors also demonstrated that several model drugs could be encapsulated in the nanostructures formed in each case. The formation of core-crosslinked, temperature responsive DHBCs micelles have been presented in the literature. The micelles have been prepared by the self assembly of PDMA-bP(NIPAM-co-AzPAM) chains followed by crosslinking, utilizing a difunctional propargyl ether. The formation of the crosslinked micelles can be performed by following two different pathways. In the first one, the micelles were formed upon increasing the solution temperature, since the PNIPAM block becomes hydrophobic at temperatures higher than 32oC, and the rich PNIPAM cores were subsequently crosslinked via click chemistry. In the second approach, the copolymer was molecularly dissolved in DMF, a good solvent for both blocks, and the crosslinking reagent was added leading to the formation of supramolecular structures. After dialysis, of the DMF solution against water, stable polymeric micelles were observed. Both of the above methods had as a result the formation of polymeric micelles, which were not destroyed even at room temperature where both blocks are soluble, Figure 9. In contrast, the micellar core tents to swell at low temperature, leading to relatively larger micelles, indicating the possible use of core-crosslinked DHBCs micelles in drug delivery applications [21]. An insightful work on the temperature induced micellization kinetics has been presented by Zhang et al. [20]. The study was performed by utilizing a PNIPAM-b-PDEAEMA copolymer upon jumps of the solution temperature from 20 to 45 oC, where micelles with a PNIPAM core were formed. The solution pH was set at 4, a pH value where the PDEAEMA block is soluble in the whole experimental temperature range. The experimental results indicate the existence of two relaxation processes. The first, fast process was attributed to association of unimers into a large amount of small micelles and the formation of quasi-equilibrium micelles. The second, slow process was almost independent of the polymer concentration, indicating that unimer insertion/expulsion is the main mechanism for the slow process. The authors attribute the above unimer insertion/expulsion mechanism to electrostatic interactions, due to the charged PDEAEMA block. However, the influence of copolymer molecular weight has to be taken into account, as has been demonstrated by the same group later on [59].
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Figure 9. Formation of thermoresponsive crosslinked micelles following two different pathways. Reproduced from [21] by permission of Wiley.
The temperature induced micellization has been also studied for a linear triblock BAB, where A= PDMA and B= PNIPAM [40]. It is well known that the PNIPAM block is temperature responsive, i.e. is soluble for temperatures lower than the LCST. Therefore, the formation of supramolecular structures was observed at elevated temperatures. Interestingly, the aggregation process was not confined to a small temperature interval, but stretches over a range of 10 to 20oC, or even more. Moreover, the micellization temperature was strongly dependent on the molecular characteristics of the copolymer chains, like the composition and the molecular weight of the B block. Strong kinetic effects have been recorded during the micellization process, since larger aggregates and broader size distributions were observed for slower heating rates. Finally, the solution behavior of thermoresponsive multiblock copolymers has been reported [50]. Multiblocks of PDMA and PNIPAM with sequences of different molecular weights were studied upon increasing the solution temperature. At elevated temperatures the PDMA block is water soluble, while the PNIPAM block becomes insoluble. Depending on the molecular weight of the sequences, the multiblock copolymer can adopt unimolecular flower-like micelle conformation or can aggregate through intermolecular interactions.
4.2. pH Responsive Self-Assembly One of the first and more characteristic examples of pH responsive DHBCs is the solution behavior of P2VP-PEO [8]. The above copolymer is molecularly dissolved at low pH values due to the protonation of the P2VP block, considering that the PEO block is a neutral, water soluble polymer in the whole pH range. However, upon increasing the solution pH, deprotonation of the P2VP block leads to the formation of micelles at pH higher than 4. This behavior, i.e. the formation of micelles upon changes in the solution pH, is characteristic for a number of polymers, as it will be demonstrated below. The micellization of a similar system, namely PHEGMA-PDEAEMA, upon changes in the protonation degree of the amine containing block has been studied in detail by light scattering and NMR [60]. The above polymer was molecularly dissolved when the degree of
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protonation was higher than 30%, although a small fraction of aggregates was also observed, even at protonation degrees higher than 30%. However, the tendency for aggregation of the copolymer chains was increased by decreasing the percentage of the ionized amine groups to 20%. A further decrease of the protonation degree led to the formation of precursor micelles, while the equilibrium micelles were formed only upon complete deprotonation of the PDEAEMA block. The above, step-by-step investigation of the micelle formation reveals a detailed picture of the copolymer solution properties. Changes of solution pH can induce micellization not only because of a decrease in the ionization degree of one block, but also because of the development of secondary interactions, like hydrogen bonds. An example of the above situation is the solution behavior of the PVA-PAA block copolymer [32]. The particular copolymer was almost molecularly dissolved in aqueous media at neutral or alkaline environments. However, the formation of aggregates was observed at low pH values, not only because of deprotonation of the PAA block but also due to the development of intermolecular hydrogen bonds between the PAA and PVA blocks. In a similar context the aggregation behavior of PHOS-PMAA block copolymers was found to be pH dependent [10]. In this particular system both blocks can be considered as acidic, but the pKa values of PHOS and PMAA differ substantially. Loose polydisperse aggregates were observed at pH>9 where phenolate and carboxylate groups are dissociated. At 4< pH<9 micellar like aggregates were formed having a more compact structure with PHOS cores and PMAA coronas. The copolymers precipitated at pH<4 due to the protonation of the PMAA and possibly hydrogen bond formation. The micellization pathway is shown schematically in Figure 10. A pH dependent self-assembly behavior was also observed for PNHOS-PEO diblock copolymers [12]. The presence of basic and acidic groups on the same segment of the PNHOS block allows for variation of the solubility and the internal structure of the aggregates formed at different pH values, as it was confirmed by dynamic light scattering and fluorescence spectroscopy.
Figure 10. Micellization behavior of the PHOS-PMAA pH-responsive DHBC. Reproduced from [10] by permission of the American Chemical Society.
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A diblock consisting of two polybase blocks has been also studied. The copolymer, PDMAEMA–PDEAEMA, could be directly dissolved in water at acidic environment or at neutral pH if the PDMAEMA content is greater than 50%. The polymer was molecularly dissolved at pH 2, however micelles with a PDEAEMA core, and a mean hydrodynamic diameter of ca. 20 nm, were formed at pH 9.5 [24]. The block copolymer exhibited a completely reversible pH-induced micellization. Additionally, the micellar structure of the PDMAEMA-PDEAEMA block copolymer has been studied in detail, using a plethora of complimentary techniques, upon changes in solution pH and in ionic strength, since the increase of salt concentration allows micellization at higher degrees of protonation because of charge screening [61]. The first example of micelle formation by a so called “schizophrenic” block copolymer has been given by Armes and coworkers [13]. The term “schizophrenic polymer” was proposed by the authors in order to describe macromolecules that can form micelles or inverse micelles upon changes of the environmental conditions. In particular, the copolymer PVBA-PDEAEMA was observed to form well defined micelles both in acidic and basic environments, while it precipitated at pH values between 6.6 and 8.3. In acidic environment, the micellar core was formed by the PVBA blocks, while the inverse micelle was observed for alkaline solutions, Figure 11. Interestingly, well defined micelles were observed only in the case of very low and very high pH values, i.e pH<2 and pH >10. At all the other pH ranges, where the polymer was still water soluble, non-micellar aggregates were formed, with hydrodynamic diameters up to 150nm. Schizophrenic solution behavior has been also observed in a PMAA-PDEAEMA DHBC. The particular copolymer formed core-shell micelles at extreme pH ranges, i.e. pH < 5.5 or pH > 9.2, while it was phase separated at intermediate pH values. Micelles having PMAA cores were formed in acidic solutions, while micelles with PDEAEMA cores were formed in alkaline solutions [15].
Figure 11. Schizophrenic micellization behavior of a PVBA-b-PDEAEMA DHBC. Reproduced from [13] by permission of Wiley.
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The micellization behavior of a series of zwitterionic DHBCs with an amine group containing block and a carboxylic group containing block has been also studied. The copolymers formed supramolecular aggregates in acidic and alkaline solutions, while an isoelectric point (IP) was observed at intermediate pH values. The well defined structure of these aggregates strongly depends on the purity of the samples used [17]. Therefore, the formation of large aggregates at low pH values, which could not be explained by the formation of simple core-shell micelles, indicates the existence of amine containing monomers in the carboxylic block, due to inefficiencies in the synthetic procedure utilized [16]. The micellization behavior of a triblock DHBC, namely PAA-P2VP-PAA, has been studied upon changes in solution pH [37]. At low pH values the polymer can be either molecularly dissolved or can form transient networks, depending on the polymer concentration. The formation of a network was possible because of the development of physical cross-links, which arise from electrostatic interactions among the oppositively charged PAA and P2VP blocks. By increasing the solution pH the isoelectric point (IP) was reached. Close to the IP, i.e. 4 < pH < 7, precipitation of the copolymer chains was observed. However, further increase of the pH had as a result the redissolution of the macromolecule and the formation of compact micelles with a P2VP core. The aforementioned phase transformation upon pH changes was found to be reversible. A comprehensive study of the micellization kinetics has been performed by Liu and coworkers on an ABC double hydrophilic triblock terpolymer upon alteration of the solution pH [62]. The studies have been performed by applying the stopped-flow light scattering technique on a poly(glycerol monomethacrylate-b-2-(dimethylamino)ethyl methacrylate-b2(diethylamino)ethyl methacrylate) (PGMA-PDMAEMA-PDEAEMA) terpolymer, which has a block soluble at all pH values (PGMA) and two others blocks (PDMAEMA and PDEAEMA) with pKa’s of 7.0 and 7.3 respectively. The particular terpolymer formed micelles upon changing the solution pH from 4 to values higher than 7. When the pH of the final copolymer solution was between 7 and 7.5, two relaxation processes were observed, one with positive and one with negative amplitude. This suggests that the micelle size and/or the micelle number density were initially increased, whereas the opposite occurred during the second relaxation mode, which is associated with micellar formation-breakup. In contrast, for a final pH value higher than 8, two processes with positive amplitude were recorded. The fast process was attributed to the aggregation of unimers, forming some transient micelles, and to intermicelle fusion, forming quasi-equilibrium micelles. The second, slower, process was related to micelle formation-break up because of unimer insertion/expulsion and seems to be concentration independent. The situation was different in the presence of salt. In that case, the two processes were much slower or leveled off, depending on the salt concentration. The authors concluded that the micellar fusion/fission mechanism is favored instead of the unimer insertion/expulsion mechanism [63]. Recently, the same group has presented also the micellization kinetics of a diblock copolymer, namely PEO-PDEAEMA [59]. Several samples of this type of copolymers have been synthesized having identical PEO blocks and PDEAEMA block with varying degrees of polymerization. The block copolymers tend to create micelles with PDEAEMA cores in basic conditions. Therefore, the micellization kinetics studies were performed using a stop flow technique in order to introduce micelles upon a pH-jump from 3 to 12. The observation of two processes, i.e. a fast one attributed to the formation of quasi-equilibrium micelles and a slow one attributed to the relaxation into final equilibrium micelles, were observed, as
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expected. Interestingly, the authors concluded that the reorganization mechanism followed, during the slow process, was connected to the degree of polymerization. In particular, the unimer insertion/expulsion mechanism mainly dominated the relaxation from quasiequilibrium micelles into stable micelles, in the case of copolymers with low degree of polymerization. In contrast, the same process was dominated by micelle fusion/fission in the case of copolymers with high enough molecular weight. Finally, interesting results have been also obtained by the study of a diblock polypeptide [29]. The copolymer, poly(L-glutamic acid)-b-poly(L-lysine) (PGA-PLys), self-assembled spontaneously in aqueous basic (pH 12) and acidic (pH 3) solutions. At pH 3, the PGA block was neutralized adopting a compact R-helical structure, instead of the charged coil conformation at pH >4. In contrast, under basic conditions (pH >9), the PGA block was charged, while the PLys block was transformed into a neutral and insoluble block. Therefore aggregates with either the one or the other block as the core forming block could be obtained depending on the solution pH. The formed aggregates were determined to be vesicles. This was the first time that this schizophrenic behavior was observed in vesicle forming block copolymers, Figure 12. The vesicle formation was tentatively explained, according to the authors, by the systematic presence of a polypeptide in a rodlike conformation, in the hydrophobic part of the membrane, which induces a low interfacial curvature and as a result a hollow supramolecular structure.
4.3. Self-Assembly via Complexation with Other Building Blocks The complexation of two DHBCs has been recently described in the literature. The complex formation between a miktoarm star copolymer PEOn-PMAAn and a quaternized PEO-PDMA diblock was studied upon changes in solution pH [54]. At elevated pH values, where PMAA blocks were ionized, the formation of electrostatic complexes was recorded between the oppositely charged PMAA and quaternized PDMAEMA blocks. However, the aggregates were soluble due to the presence of the PEO uncomplexed blocks originating from both of the copolymers. The situation was dramatically different at low pH values, where the PMAA block was not ionized. At the aforementioned pH values the development of hydrogen bonds between the PMAA and PEO blocks took place. In the later case, the aggregates were electrostatically stabilized by the quaternized PDMAEMA polyelectrolyte block. The micellization/complexation behavior is schematically illustrated in Figure 13. Complexation induced micellization can occur not only by using an oppositely charged polyelectrolyte but also by using a salt with divalent ions. Tam and coworkers have described in detail the mechanism of salt induced micellization [64] of a PEO-PAA diblock upon addition of CaCl2. The formation of supramolecular aggregates was strongly dependent on the Ca2+/COO- ratio. When the ratio was 0.5, the formation of coil-like globules was observed due to charge neutralization. An increase of the above ratio to values higher than 2.5, led to the creation of aggregates with a mean diameter of ca. 50 nm. Interestingly, the data reveal that the aggregate formation was not caused by electrostatic interaction between the ions but it was the disruption of the water structure, due to the solvation of CaCl2 excess. It was concluded that the copolymer chains formed clusters in order to compensate the entropic changes, caused by changes in the structure of water.
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Figure 12. Schizophrenic behavior in the formation of vesicles by a polypeptide containing DHBC. Reproduced from [29] by permission of the American Chemical Society.
Figure 13. Complexation induced micellization of a star DHBC. Reproduced from [54] by permission of Wiley.
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An example of electrostatically assembled complexes formed between a PEO-PMAA DHBC and poly(amidoamine) dendrimers of different generations was presented very recently [65]. An increase in the size of the complexes was observed by increasing the amount of G4 dendrimer present in the solutions. The structure of the core of the aggregates was dependent on the dendrimer concentration, while the overall structure of the assemblies could not be described as a micellar-like structure. At low dendrimer concentration a close aggregation of the PMAA chains and G4 dendrimer was evident, while at higher G4 concentration the PMAA chains efficiently fill the aggregate core. The behavior was the opposite for the G0 dendrimer. The size distribution of the aggregates was narrow and the structure of the assemblies was dependent on the solution pH as a result of the different ionization degree of the components. A large amount of work, concerning the interaction and nanostructure formation in DHBC/surfactant and DHBC/polyelectrolytes mixed solutions has been published [66,67,68,69,70,71,72,73,74,75,76,77,78,79, 80,81]. The structural characteristics of the aggregates in such systems can be tuned and controlled by the chemical structure of the block copolymer, the surfactant and the polyelectrolyte utilized. A very important parameter that determines the structure of the nanosystems is the charge ratio between the two components. Electrostatic interactions are the main reason for structure formation, but secondary interactions, like hydrophobic interactions and hydrogen bonding, play also a decisive role. Various morphologies including micellar core-shell like aggregates, cylindrical superstructures and vesicles have been observed. The mixed structures can be also influenced or be responsive to changes in the physicochemical parameters of the system (pH, salinity, temperature). The great versatility of DHBC/surfactant and DHBC/polyelectrolytes systems, concerning structural diversity and properties, makes them a very active field of current research with great potential towards nanoapplications, including drug and gene delivery and surface modification. A special and very interesting class of complexes containing suitably designed and synthesized DHBCs, especially in terms of potential biotechnological applications, are the complexes formed in the presence of oppositely charged proteins and nucleotides [82,83,84,85,86,87,88,89,90,91]. The use of such complexes for delivery of protein drugs, separation and purification of proteins and for gene delivery has been proposed. Several mixed systems have been reported and the fundamental properties of the superstructures in aqueous solutions have been elucidated. Structural characteristics are depending on the structure of the DHBC and a number of physicochemical parameters. Resposiveness of the created nanostructures to stimuli that are primarily encountered under physiological conditions in living organisms, is of primary interest due to the desired functionality of the ensembles. Hydrophobic interactions in conjunction to electrostatic ones are determining the stability and structural and functional tunability of the nanoaggregates.
4.4. Self-Assembly via Combination of Stimuli An interesting example of a DHBC system that can self assemble under the influence of two different stimuli is the PVBA-PMEMA copolymer, presented by Liu and Armes [14]. The particular system formed small well defined micelles with a PVBA core at pH values lower than 5.5 at room temperature and in the absence of salt. No precipitation was observed at the isoelectric
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point. However, in the presence of a sufficient amount of Na2SO4 (0.80 M) or at elevated temperatures (more than 80 oC), well defined micelles with PMEMA cores were formed in alkaline media, because of salting out of the PMEMA block. By choosing the appropriate values of pH, salinity and temperature the observation of micelles with PVBA cores, PMEMA cores micelles or precipitation (at the isoelectric point) was possible. Moreover, the micellization temperature of the system was altered upon changes in the salinity. The micellization kinetics, of the above rather complicated system, has been also studied [92]. A system analogous to the PVBA-PMEMA copolymer, where a PDEAEMA block has been used instead of the PVBA block, has been also presented by the same group [26]. The particular copolymer could form micelles with either PDEAEMA or PMEMA cores depending on the solution pH and salinity. At low ionic strength the PMEMA block was soluble in the whole pH range, in contrast to the PDEAEMA block which is not soluble at pH higher than 7-8. Therefore, at zero salt concentration and pH=8, PDEAEMA core micelles were observed. However, when the solution pH was carefully adjusted to 6.7 and the salinity was increased by addition of NaSO4, then the PMEMA block was no longer soluble in water and PMEMA-core micelles were observed, Figure 14. The temperature of the solution plays a crucial role on the stability and polydispersity of the formed micelles. Another example of a DHBC, which responds to two different stimuli, is the PNIPAPSPP copolymer [93]. This copolymer presents both UCST (PSPP block) and LCST (PNIPAM block) in aqueous solutions. However, the addition of salt increases the hydrophilicity of the PSPP block leading to a disappearance of the UCST. In contrast, the hydrophilicity of the PNIPAM block, is decreased upon addition of salt. Therefore, the size of the supramolecular aggregates formed is altered not only upon changes in solution temperature but also upon changes in the salinity. Copolymer concentration was also found to play a crucial role on the dimensions of the self-assembled structures. The micellization of a polypeptide containing DHBC has been also studied under variable solution pH and temperature [30]. A schizophrenic solution behavior was observed for a PNIPAM-PGA block copolymer. At room temperature and low pH values micelles having polypeptide cores were formed, while at elevated temperatures and increased pH values micelles with PNIPAM cores were observed. It has to be noted that the formation of PGA core micelles was accompanied by a coil-to-helix transition of the polypeptide sequence. The above transition had as a result a different micellization kinetic profile for the system in comparison with other conventional pH responsive systems.
Figure 14. Micellization of MEMA-DEAEMA DHBC upon changes of more than one stimulus. Reproduced from [26] by permission of the American Chemical Society.
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Figure 15. Schematic illustration of the combination of stimuli for producing different types of DHBC aggregates. Reproduced from [9] by permission of Wiley.
Another example of schizophrenic behavior has been observed in the case of A-b-(B-coC) DHBCs. The copolymers P(EGMA-co-MMA)-PDEA (with various contents of MAA) associated in water either at elevated temperature and low pH values or at low temperature and increased pH values [28]. Inverse micelles were observed in each case. Moreover, both the solution salinity and the content of MAA had a great influence on the critical micellization temperature, as was revealed by the experimental observations. The PAA-PDEA block copolymer has a pH responsive block (PAA) and a temperature responsive block (PDEA) [9]. The particular copolymer could be molecularly dissolved in alkaline aqueous solutions at room temperatures. However, upon increasing the solution temperature the formation of crew-cut micelles with a PDEA core were formed, mainly due to the high asymmetry in the lengths of the blocks. This rather effective strategy allows the formation of crew-cut micelles in water without using a cosolvent. Moreover, the formation of inverse micelles can be also achieved at room temperature and at low pH values. In the latter case, star-like core shell micelles are observed, Figure 15. Temperature and pH responsiveness has been also recorded for a non covalently bonded DHBC inclusion complex, namely P4VP-PNIPAM [33]. The non-covalently connected copolymer tends to create micelles with PNIPAM cores at low pH values and at elevated temperatures. However, at room temperature and high pH values, the polymer formed vesicles instead of core-shell micelles. The formation of vesicles was confirmed by a number
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of complementary techniques, including TEM and light scattering. The pH induced vesicle formation by a non-covalently connected DHBC is an attractive approach for the facile synthesis of drug and gene delivery systems. Moreover, the solution behavior of the analogous covalently connected PNIPAM-P4VP copolymer has been also reported [34]. This copolymer was molecularly dissolved at low temperature and low pH. However, increasing of either the temperature or the pH led to the formation of core-shell micelles with PNIPAM or P4VP cores respectively. The formation of core-shell micelles instead of vesicles is probably caused by the differences in the molecular characteristics of the two polymers. Additionally, the nature of the non-covalent connection between the constituting blocks may allow for a more efficient reorganization of the polymer chains under appropriate conditions. The solution behavior of an ABA block copolymer, namely PEO-P2VP-PEO, has been also studied under variable pH, temperature and salinity conditions [39]. In the present case, the polymer tended to form star like micelles with P2VP core at high pH values and room temperature. However, the situation was different at low pH values, increased salinity and elevated temperature. Under those conditions the formation of flower like micelles was observed, since the outer blocks became hydrophobic. Block copolymers with non-linear structure have been also studied under the concept of formation of multi-stimuli responsive DHBCs supramolecular structures [52]. The solution behavior of A4BA4 and A2BA2 copolymers, where A is the pH responsive PDEAEMA block and B the temperature responsive PPO block, has been studied upon changes in solution pH and temperature. According to the authors, the experimental results indicate that the copolymers tend to form large, flower-like aggregates at low temperature and in a basic environment, where the water soluble PPO block forms loops surrounding the PDEAEMA cores of the aggregates. In contrast, unimolecular micelles are formed at higher temperatures and at pH 6.4, where the PPO is insoluble. Finally, the solution behavior of another schizophrenic non linear DHBC has been also investigated. An AB4 miktoarm star copolymer, where A is a temperature responsive PNIPAM block and B is a pH responsive PDEAEMA block, has been studied under varying solution pH and temperature [53]. The copolymer forms PNIPAM core micelles at high temperatures and at low pH values, and the reverse PDEAEMA core micelles at room temperature and high pH values. The micelles with a PDEAEMA core have a much smaller size in comparison with the micelles formed by a linear diblock copolymer with similar composition and molecular weight. Finally, different micellization kinetics has been recorded for the miktoarm star and the linear block copolymer upon pH jumps from 4 to 10, using a stop flow light scattering technique.
5. CURRENT AND POTENTIAL APPLICATIONS The versatile synthetic procedures for the preparation of a wide variety of DHBCs, as well as their intriguing solution properties have promoted and suggested the utilization of such polymers in several demanding and “smart” technological applications. Some of them are discussed in the following sections.
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5.1. Mineralization of Inorganic Compounds The use of double hydrophilic block copolymers in biomimetic mineralization processes has been investigated in recent years. In contrast to rigid templates (like carbon nanotubes and porous aluminum templates which predefine the final structure) water soluble polymers could be used as soluble species at various hierarchy levels. Usually, in the case of DHBCs, one of the block acts as scaffold for the development of the crystal, while the other acts as a solublestabilizing matrix. Therefore, both of the blocks play a crucial role on the development of the crystals. There is a plethora of reports on the emerging bio-inspired mineralization field. Various crystal structures have been presented during the last years, following versatile synthetic routes. A very detailed and illustrious review has been recently given by Colfen [3]. The above review describes in detail all aspects of the specific field. Herein, we present just a few selected examples. Antonietti and coworkers have demonstrated the designed synthesis of barium sulfate crystallites using a large gamut of DHBCs with different functional groups [94]. In particular, the growth of BaSO4 into novel morphologies has been achieved by using block copolymers with varying binding and solvating blocks. Morphologies like rods, peanuts, peaches, nanofibers and flowers have been observed using different polymeric stabilizers and morphology promoters/modifiers. The development of CaCO3 tablet-like arrays was achieved at the air/water interface through the cooperative mineralization regulated by a polypeptide and a DHBC, namely PEO-PMAA [95]. The experimental data indicate that the role of the block copolymer is focused on the regulation of the arrangement and orientation of the CaCO3 tablets. The cooperative action of a polypeptide is essential for the formation of CaCO3 tablet at the air/water interface. In the absence of the polypeptide, the formation of calcite CaCO3 particles in the water phase was observed.
Figure 16. Schematic demonstration of the formation of single crystals, mesocrystals, and polycrystalline aggregates in the presence of DHBCs. Reproduced from [96] by permission of the American Chemical Society.
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Experimental evidence for a unifying model of copolymer-directed crystallization has been recently presented by Colfen and coworkers [96]. According to the proposed model a continuous transition in particle structures and crystallization mechanisms occurs between polycrystalline aggregates, mesocrystals, and single crystals, Figure 16. Particularly, a number of crystals with different shapes could be obtained by aggregation, and the observed aggregation (with low or high degree of orientation) depends on the solution supersaturation. Therefore, a single crystal, a mesocrystal or a polycrystal could be obtained by changing the solution conditions.
5.2. Nanomedicine The utilization of DHBCs as key-playing component in potential nanomedicinal applications has been demonstrated by Kataoka and coworkers [4, 82-90]. An interesting example of using DHBCs for encapsulation and targeted release of oligonucleotides has been demonstrated some years ago [97]. Partially thiolated PEO-PLys block copolymer has been used for the formation of polyion complex (PIC) micelles with a specific oligonucleotide. The formed micelles are stabilized in aqueous media because of the presence of the polyethylene oxide block, while the nucleotide is located at the micellar core in the form of a complex with PLys. After the formation of the PIC micelles, crosslinking of the core took place, by creation of disulfide bonds due to the oxidation of thiol groups preattached on the PLys chain. The resulting core-crosslinked PIC micelles are an effective carrier for the transportation and delivery of the oligonucleotide, since the stability against nuclease was appreciably increased, compared to that of free oligonucleotide and that in the micelles without crosslinking. Selective release of the oligonucleotide in the intracellular environment could be achieved. The increased concentrations of glutathione, an agent that could cleave the disulfide bonds, inside the cells leads to the located release of the oligonucleotide. The present example is featuring the potential of thiolated block copolymers in nucleotide delivery systems. The formation of a model system for targeted drug delivery and smart release has been proposed by Liu and coworkers [98]. The model system is a shell crosslinked micelle formed by a triblock copolymer, namely Ald-PEGMA-PDMAEMA-PDEAEMA where Ald is an aldehyde end-group. The above polymer forms micelles in acidic solutions with a PDEAEMA core, a PDMAEMA inner shell and a Ald-PEGMA outer corona. The PDMAEMA block can be crosslinked, leading to stable structures. In parallel, the core is pH responsive, thus a tunable swelling/shrinkage, for controlled release of encapsulated guest molecules, can be achieved. The formed micelles are biocompatible due to the PEGMA outer corona. Finally, the micelles are surface functionalized with aldehyde groups, which can be used for the controlled conjugation with a number of biomolecules. The authors have already demonstrated the conjugation of the above micelles with the model protein lysozyme, Figure 17. An outstanding approach for the development of drug delivery systems has been recently presented by Zhang and Ma [99]. In this work, the use of a β-cyclodextrin (β-CD) containing DHBC has been proposed for the inclusion of many hydrophobic substances. In their work, the synthesis of a block copolymer with a PEO block and a polyaspartamide block carrying some β-CD units has been described. The β-CD units are used as host sites for a plethora of hydrophobic small molecules, like pyrene and coumarine, as well as for hydrophobic polymers. However, the formation of inclusion complexes leads to the creation of located hydrophobic areas within the copolymer domain, which, in turn, has as a result the formation
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of multi-chain, core-shell supramolecular structures. Moreover, the inclusion of charged molecules has been also achieved leading to the formation of micelles after complexation with oppositely charged polymers. The novelty in the use of CD containing block copolymers is the versatility in choosing various guest molecules, since the solubilization effect of CD toward a wide range of hydrophobic compounds has been well documented. Another approach for the stabilization, transport and delivery of the hydrophobic nonsteroidal anti-inflammatory drug indomethacin (IND) has been presented by Giacomelli et al [100]. They have utilized the covalent attachment of the drug on DHBCs by an esterification reaction. The above reaction leads to an amphiphilic block copolymer/drug conjugate. The block copolymer/drug conjugate forms supramolecular structures in water with variable size and morphology (micelles or vesicles) depending on the molecular characteristics of the DHBCs and the content of IND in the conjugate. Moreover, unbounded IND moieties can be also encapsulated in the polymer aggregates, leading to a further increase of the drug load in the conjugate (up to 58% w/w), Figure 18. However, it has been found that high drug loading has as a result the formation of vesicles with an accompanying undesired increase of the aggregate size. It has been also observed that the drug release is a pH-dependent process. At neutral pH the release of charged unbounded IND is favored, while at acidic conditions, where the ester bond between drug and polymer is no longer stable, the slow release of both free and chemically attached IND was possible. The interaction of aggregates of DHBCs with bacteria has been also recently demonstrated [57]. The interaction of a glucose containing copolymer, which forms vesicles at temperatures higher than 15oC, with E. coli has been studied in detail. The obtained data demonstrate that specific interactions between synthetic vesicles and cells can be developed. However, the most interesting finding was the controllable nature of the interactions according to the size of the polymeric vesicle. The authors assumed that by changing the vesicles’ size it is possible to change the interactions, from bulk aggregation to individual associations. The incorporation of ether phospholipid drugs, used for the treatment of leishmania, into supramolecular structures, formed by chemically modified DHBCs has been also studied [101]. A PEO-PDMAEMA block copolymer, where a portion of the tertiary amine groups of the PDMAEMA block was transformed into phosphorozwitterions, was used for the incorporation of the drug. The above copolymer can form both micelles and/or vesicles in aqueous solutions, depending on solution pH, temperature and the molecular characteristics of the copolymer chains. The phospholipid drug could be encapsulated in both micelles and vesicles. However, the introduction of an increased amount of drug favors the formation of vesicles. Moreover, the vesicles formed have a reduced size, probably due to the better organization of the aggregates in the presence of the drug. A series of star DHBCs were evaluated for their ability to transfect human cervical HeLa cancer cells with the modified plasmid pRLSV40, bearing the enhanced green fluorescent protein as the reporter gene [55]. The copolymers utilized were composed of PDMAEMA and PHEGMA blocks (where PDMAEMA is an ionizable block, while PHEGMA is a non-ionic water soluble block). The experimental data indicate a decreased toxicity for the star copolymer, compared to a reference PDMAEMA star homopolymer, for the same amounts of star polymer tested. Moreover, it has been found that the architecture of the star copolymer, i.e. star block, miktoarm star etc, plays a decisive role on the transfection efficiency. The best performance, for all star copolymers tested, was observed for a star block copolymer with
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PDMAEMA as the outer block. Interestingly, the transfection efficiency of the particular copolymer is comparable with that of the commercially available transfection reagent SuperFect.
Figure 17. Schematic demonstration of the micelle formation by an Ald-PEGMA-PDMAEMAPDEAEMA DHBC and interaction of the micelles with lysozyme. Reproduced from [98] by permission of the American Chemical Society.
Figure 18. Inclusion and release of a drug in a micelle formed by a PEO-(PG2MA-IND) DHBC. Reproduced from [100] by permission of the American Chemical Society.
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5.3. Other Nanotechnological Applications Liu and coworkers have presented the synthesis of gold nanoparticles in crosslinked micelles. They have succeeded in selectively crosslinking either the core or the shell of the micelles, formed by a primary amine containing block copolymer, namely PNIPAM-PAPAT [31]. The gold nanoparticles have been synthesized by addition of HAuCl4, which is selectively located at the crosslinked, primary amine containing block. Subsequently, reduction of the gold ions leads directly to the formation of gold nanoparticles at either the core or the shell of the polymeric aggregate. The synthesis of Pt nanoparticles has been also reported in the literature [102]. Three different types of DHBCs, namely PEO-PV2P, PHEGMA-b-PDEAEMA, and PEO-bPDEAEMA, have been used as templates for the development of Pt nanoparticles. All the copolymers tent to self-assemble in solution at high pH values due to the deprotonation of the P2VP or PDEAEMA blocks. Therefore, in both cases, micelles with cores containing amine moieties were formed. The amine groups could be used as coordination sites for Pt ions leading to the preferential location of the ions in the micellar core. In another approach, Pt ions could be used in order to induce micellization in solutions containing molecularly dissolved block copolymers. Pt ions could electrostatically interact with the positively charged block copolymer at acidic solutions. The electrostatic attractions lead to the formation of supramolecular structures. In both cases, addition of Pt ions in polymeric micelles or molecularly dissolved copolymers, leads to core-shell micelles with Pt ions preferentially located in the cores. In these micellar nanoreactors, metal nanoparticles nucleate and grow, upon reduction, with sizes in the range of a few nanometers.
5.4. Miscellaneous Properties and Applications of DHBC Systems The adsorption of DHBCs on solid surfaces has attracted considerable scientific interest due to the possibilities that offers for the construction of smart/responsive surfaces with tunable properties. Biggs and coworkers have studied the behavior of PDMAEMA-PDEAEMA micelles, adsorbed onto mica surfaces, upon changes in solution pH using an in situ AFM instrumentation set-up [103]. In aqueous solutions, the copolymers tend to be molecularly dissolved in acidic environment and to form micelles at elevated pH values [103]. The absorbed polymer chains were found also to respond to changes of the solution pH. However, unlike micelles in bulk solution, the adsorbed copolymer micelles do not dissociate at low pH, but remain attached to the solid/solution interface. Furthermore, the micellar core can open or close upon changes in the solution pH – due to this behavior the term “nanoanemones” was coined in order to describe the specific nanosystem. The above behavior leads to smart surfaces with tunable lubricity, which can be used in a number of emerging technologies. However, the situation is dramatically different in the case where the PDMAEMA block has been quaternized. Increasing the number of permanent charges, in the copolymer, leads to the formation of random surface structures which are irreversibly damaged at low pH values [25]. Gohy and coworkers have studied the adsorption of PMAA-PDMAEMA copolymers on silicon surfaces under varying solution pH [104]. A series of copolymers with different
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molecular weights and composition have been utilized in order to elucidate the influence of polymer molecular characteristics on the adsorbed amount. The experimental results indicate that all the parameters (copolymer molecular weight, composition and pH) play their role on the final adsorbed amount and other characteristics of the adsorbed polymer layer. The investigation of another system, namely P2VP-PDMAEMA, by the same group has led to similar results [105]. The latter copolymer formed micelles at pH higher than 5, while it precipitated at pH higher than 8. The adsorption profile upon changes in the solution pH indicates two adsorption maxima: at the begging of the micelle formation and at the precipitation regime. The great influence of pH on the adsorption was correlated with the ionization degree of the polyelectrolyte chains. Preliminary results have been also presented by Armes and coworkers on the adsorption of zwitterionic block copolymers, both on silica and mica surfaces [17]. The copolymer forms negatively and positively charged micelles depending on the solution pH. Positively charged micelles can easily be adsorbed on oppositely charged silica particles and also on planar mica surfaces. However, the adsorption of even negatively charged micelles on mica has been also observed. The later observation was assigned to the relatively weak hydrophobic interactions between the micelle cores and the mica surface, rather than to strong electrostatic interactions. Nevertheless, the ability of pH responsive zwitterionic micelles to be adsorbed on surfaces has been demonstrated. In a detailed study of the adsorbed DHBCs chain conformation, Hamley et al. have presented experimental results on adsorbed Pluronic triblock copolymers onto both silica and mica substrates [106]. The experimental data were obtained for a wide range of bulk polymer concentrations, even below the critical micelle concentration. The results indicate that, depending on the hydrophilicity of the substrate, the copolymer chains are adsorbed in a completely different way. Moreover, it was concluded that the polymer concentration plays a significant role on the conformation of the adsorbed polymeric layer. The catalytic activity of PVim containing DHBCs has been also demonstrated [23]. A PNIPAM-PVim copolymer was found to form micelles with PNIPAM cores at elevated temperatures. The above micelles showed catalytic activity toward the hydrolysis of pnitrophenyl acetate. Interestingly, molecular dissolved copolymer chains do not show the same catalytic activity. The Arrhenius plot for the PVim-based DHBCs exhibited a pronounced upward curvature above the critical micellization temperature. The ability of DHBCs to encapsulate metalloporphyrins under different pH environments has been recently studied [107]. The polymer utilized was a PAA-P4VP block copolymer which forms micelles both in acidic (PAA core) and alkaline (P4VP core) solutions. The encapsulation of porphyrin molecules in both types of micelles has been observed. However, interactions between the P4VP block and metalloporphyrins leads to higher encapsulation ability for the P4VP core micelles. Depending on the initial micellar type, release of the metalloporphyrins can occur either by increasing or decreasing the solution pH. A similar system, more specifically designed for bioapplications, has been reported by Kataoka and coworkers some years earlier. The formation of core-shell micelles of dendrimer metalloporphyrin with DHBCs has been reported by these authors [108]. The micelle formation took place due to electrostatic interaction of charges at the periphery of the dendrimeric metalloporphyrins with opposite charged block copolymers. Depending on the porphyrin/DHBC system, the formed micelles were stable against NaCl solutions (due to the formation of hydrogen bonds) or dissociated at increased salt concentration. Such kind of
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systems could be used in the battle against cancer, as photosensitizers in photodynamic therapies. Kataoka and coworkers have illustrated the potential of mixed porphyrin/DHBCs systems on this kind of therapies [109]. Particularly, the formation of PIC between dendrimer metalloporphyrins and DHBCs can effectively deliver the porphyrin moieties into cancer cells. Importantly, no toxicity effects were recorded, at least for the concentration range utilized.
6. SOME REMARKS ON FUTURE PERSPECTIVES ON DHBC RESEARCH The existing capabilities on sophisticated synthetic procedures for the designed preparation of DHBCs are enormous and they are expanding rapidly. The knowledge on DHBC self-assembly behavior under varying conditions is continuously enriched with new behavioral motifs, tuned by the chemistry of the systems. However, novel and demanding technological applications require the further development of new DHBC systems. The synthesis of new DHBCs has to be realized by utilizing novel monomers, efficient polymerization schemes or/and post polymerization functionalization methodologies. A future goal could be the synthesis of multicomponent DHBCs with linear and non-linear macromolecular architectures. Surely the development of novel polymers will help to establish more precise structure–properties relationships that will aid the application potential of such systems. A number of aspects have to be elucidated in more detail, in order to improve the design and properties of the block copolymers. Theoretical calculations may help substantially and to some extent orient the experimental work. The application spectrum of DHBCs is extended into a wide range of nanomedicine and nanotechnology oriented fields. Considering, the increasing interest for water compatible systems, further increase of DHBCs utilization in emerging technological applications is expected. The outstanding aqueous solution properties of DHBCs have already started to be utilized in a number of applications and these trends will be enhanced in the future.
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In: Encyclopedia of Polymer Research Ed: Caroline E. Jones
ISBN: 978-1-61761-926-7 © 2011 Nova Science Publishers, Inc.
Chapter 10
THERMO-OXIDATION STABILITY OF POLY (BUTYLENE TEREPHTHALATE) AND CATALYST COMPOSITION Antonio Massaa*, Valeria Bugattib, Arrigo Scettria and Socrate Contessac a
Dipartimento di Chimica, Università di Salerno, Via Ponte Don Melillo 84084 Fisciano, Salerno, Italy. b Dipartimento di Ingegneria Chimica ed Alimentare, Università di Salerno, Via Ponte Don Melillo, Fisciano Salerno. c NGP S.p.A.,R&D, Contrada Pagliarone, Acerra, Napoli.
1. SYNTHESIS AND THE STABILIZATION OF POLYESTERS Polyesters are heterochain macromolecular substances characterized by the presence of carboxylate ester groups in the repeating units of their chains. Predominant in terms of volume and products value are those based on poly(ethylene terephthalate) (PET), long established as basis of fibers, films, molding plastics and containers for liquids, and poly(butylene terephthalate) (PBT) largely used to produce fibers as well as for special applications in motor and electric industry.[1-3] PET and PBT can be conveniently synthesized, both at laboratory and industrial scale, in the presence of metal catalysts in a two-step-polyesterification of the diols, ethylene glycol or 1,4-butanediol respectively, and dimethyl terephthatate (DMT) or terephthalic acid (TPA). The first step is basically a transesterification or esterification process depending on the use of DMT or TPA respectively, while the second step is the polycondensation of the resulting oligomers. The polycondensation step occurs at higher temperature and at reduced pressure to facilitate the distillation of the diols and consequently the polymerisation of the oligomers.
*
Corresponding author: e-mail:
[email protected]; tel.: +39 089 969565.
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There are important differences in the synthesis of these two polymers, depending also on the choice of dimethyl terephthatate (DMT) or terephthalic acid (TPA) as starting materials. A single catalyst, usually a titanium(IV) alcolate, is used in the synthesis of PBT for both ester-interchange and polycondensation steps. Two different catalytic systems are necessary in PET synthesis if DMT is used as starting material: Mn(II), Co(II), Mg(II) as acetate salts are the common catalysts for the transesterification step and Sb(III), usually Sb2O3, is the catalyst for the polycondensation step. Titanium (IV) alcolates are also good polycondensation catalysts for PET synthesis but they are not often used because of the yellowing of the obtained polymer. One of the most accredited hypothesis for the explanation of this phenomenon is the formation of coloured titanium-based complexes during the high temperature polycondensation stage. For these reasons only small amounts of a titanium(IV) alcolates are usually used as co-catalyst in addition to Sb(III) catalyst. However the exploitation of Ti(IV) based compounds for PET synthesis is a hot subject and several questions are still open in order to solve these problems.[4] Direct esterification of TPA is catalysed by COOH terminal groups and ester-interchange metal-catalysts are not necessary. The synthesis from TPA is particularly convenient for the production of PET because the use of the metals is limited only to polycondensation catalytic system. For PBT synthesis, TPA is less convenient because the presence of acidic groups leads to the decomposition of 1,4-butanediol to the formation of tetrahydrofurane. A typical example of the two-step synthesis of PBT is reported in Scheme 1. O
O
H3COC
2 HOCH2CH2CH2CH2OH
COCH3 +
Ti(IV) catalyst T = 150 - 210°C
DMT
Transesterification
O
O
HO(CH2)4OC
Ti(IV) catalyst
CO(CH2)4OH
+ 2 CH3OH
T = 240 - 250° C P = 0.1-1.0 mbar Polycondensation
BHBT
O
O
C
CO(CH2)4O
+
HOCH2CH2CH2CH2OH
n PBT
Scheme 1.
Since the early application of polyesters, great importance was attributed to the stability and shelf-life of these materials.[5-19] The stabilization for poly(ethylene terephthalate) (PET) is mostly related to the stabilization of ester-interchange catalysts. When these metals are not stabilized, a strong degradation and yellowing of the PET occurs at higher temperature in the following polycondensation stage. In this case the addition of phosphates or phosphonates at the end of transesterification stage suppresses the undesired processes and
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increases the stability of the PET. These additives probably form inert complexes or salts with the ester-interchange catalysts.[12-19] Even if several studies demonstrate that PBT is less stable than PET, less attention has been paid to the improvement of the stability of poly(butylene terephthalate).[7,8] The usual strategy to increase PBT stability is the addition, during the synthesis or the manufacturing, of anti-oxidizers, like Ultranox and Irganox the typical phenol-type radical scavengers.[20] However, during the last years, a good number of new catalytic systems are claimed to catalyse efficiently PBT synthesis, but in these studies very few information are available about possible relationships between catalyst composition and polymer stability.[21-23] Only recently Colonna et al. reported that in PBT synthesis, the use of mixtures of titanium(IV) tetrabutoxide and Hafnium(IV) acetylacetonate as catalysts in the presence of metal phosphates as co-catalysts improved the thermal stability of the polymers.[24] Unfortunately these interesting results have not received significant attention and no hypothesis has been formulated about the relationships between the improved stability and the catalyst composition. It is worthy to note that at the end of the typical synthesis of polyesters, the catalytic system is a residual dispersed in the polymer matrix. But what are the effects of these metal impurities? Is this metal residual still active? Can these metal impurities catalyse the degradation processes and reduce the life of the material? For example, solid state polycondensation (SSP), a way to increase the molecular weight directly on pellets of solid PET, is the evidence that Sb(III) residual is still active in solid polymer.[25-28] On the contrary less information are available about the effect of Ti(IV) residual on SSP for PBT. It is a matter of fact that in all the studies about thermal and thermooxidation degradation, the effects of the catalytic system are usually ignored because the catalyst is always present as residual in every PBT sample. It is simply a background effect.
2. RELATIONSHIPS BETWEEN CATALYST COMPOSITION AND PBT STABILITY: THE CASE OF Ti (acac)2(O-iPr)2 Following the idea that the decomposition processes in PBT can be related to catalyst residual and that degradation can be inhibited by “metal deactivators”, we started to test a series of new potential catalytic systems and additives with specific characteristics for polyester synthesis. A “metal deactivator” can be defined like a kind of chelating agent which forms inactive complexes with residuals of metal catalysts or impurities.[20,29] In our investigations the chelating agents, the additives are introduced in the synthesis as complexes with the metal catalysts. For this reason the catalysts must be effective in the synthesis of the polymer and at the same time should reduce the degradation reactions. As a result of these considerations and after a wide screening of several chelating agents and Ti(IV) complexes, we found that commercially available Ti(acac)2(O-iPr)2 (titanium bisacetylacetonate diisopropylate) can be conveniently used in the synthesis of poly(butylene terephthalate) (PBT 1) both at pilot plant and industrial scale.[30] This new catalyst showed higher activity than standard Ti(O-nBu)4 and very interestingly the obtained polymers showed higher thermo-oxidation stability than PBT 2 synthesized in the presence of Ti(O-nBu)4. Furthermore, PBT 3 stabilized with Ultranox 626 and synthesised with Ti(O-nBu)4 was less stable than new PBT 1, but more stable than that synthesized only in the presence of Ti(O-nBu)4 (PBT 2).[30]
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Even the mixture of acetylacetone and Ti(O-nBu)4 was effective in PBT synthesis and the obtained polymer 4 showed again improved thermo-oxidation stability, similar to PBT 1.[31] OiPr O
O
Ti O
O OiPr
Ti(acac)2(O-iPr)2 Figure 1.
Table 1. characteristics of different PBTs PBT polymer
Catalyst
EI stage Time(min.) 72 86 85 67
PC Stage Time(min.) 78 87 85 83
I.V. (dl/g)a
Ti(acac)2(O-iPr)2b 0.889 Ti(O-nBu)4b 0.851 Ti(O-nBu)4 + U626 0.829 Ti(O-nBu)4 + 0.851 acetylacetoned a Determined on a solution of PBT in o-chlorophenol with an auto viscosimeter Shotte-Gerate b The complex was supplied from Aldrich. c The stabilizer U626 was added to the reaction mixture at the end of the transesterification step at a concentration of 0.1% w/w. d The catalytic system was prepared according to the procedure reported in the experimental section. 1 2 3c 4
Regarding the experimental details, these four different types of PBTs were synthesized in a 20 L pilot plant under standardized conditions and they were submitted to standard oven testing for the evaluation of thermo-oxidation stability. In particular, experimental data about reaction time for ester-interchange and polycondensation steps and the values of the intrinsic viscosity (I.V.) of the PBTs are summarized in table 1. It is worthy to note that both Ti(acac)2(O-iPr)2 and the mixture acetylacetone/Ti(OnBu)4 were more effective for both EI and PC stages and the resulting polymers were obtained with a higher I.V. and consequently with a higher molecular weight. This is very important from economical point of view because in the presence of a more active catalyst there is a significant reduction of the manufacturing costs of the materials. Moreover a shorter reaction time involves a reduced permanence of the polymer at high temperature with the consequent reduction of the thermal decomposition processes. The effects of thermo-oxidation were evaluated by means of the standard method of ageing a large quantity of pellets of the four PBTs in forced circulating air oven at 160°C.[32] This is considered the most reliable method for ageing acceleration tests.[33] Then at different time, we measured colour-L (Figure 2), colour-b (Figure 3), intrinsic viscosity (I.V.; Figure 4) and carboxyl end groups (C.E.G.; Figure 5) for the treated samples. All these parameters are reasonably related to the “level” of degradation of macromolecules. Degradation causes the cleavage of the polymer chains and consequently the decrease of molecular weight, the decrease of the related I.V. and the increase of C.E.G. The colour, the
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appearance is also important and in the practical experience of everyone the yellowing is commonly related to the ageing. The colour of polymers can be numerically measured in terms of L-value and b-value directly on the pellets, according to Cielab scale. L-Value is related to the degree of whiteness with a greater numerical value showing a higher (desirable) whiteness. The whiteness and consequently colour L is usually related to the level of crystallinity. The b-value is related to the degree of yellowness, and a higher numerical value shows a higher (undesirable) degree of yellowness. A particular importance is attributed to colour b for the relationship with the level of degradation and oxidation of macromolecules.[13] 92
90
"PBT 1: Ti(acac)2(O-iPr)2 88
"PBT 2: Ti(O-nBu)4 86
"PBT 3: Ti(O-nBu)4 + U626 84
"PBT 4: Ti(O-nBu)4 + acetylacetone
82
80
78
0
10
20
30
40
50
Ageing time (h)
Figure 2. Colour L of PBTs aged at 160°C 25
20
Colour b
Ti(acac)2(O-iPr)2 Ti(O-nBu)4
15
Ti(O-nBu)4 + U626 10
Ti(O-nBu)4 + acetylacetone 5
0 0
10
20
30
Time (h)
Figure 3. Colour b of PBTs aged at 160°C
40
50
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Antonio Massa, Valeria Bugatti, Arrigo Scettri et al.
The comparison of the obtained data shows that, after prolonged ageing time, PBTs 1 and 4 keep their properties practically intact. PBTs 1 and 4 suffer a very slight discolouring with a constant degree of whiteness (Colour-L, Figure 2), and a very reduced increase of yellowness (colour-b, Figure 3); a constant value of C.E.G. (carboxyl end groups, Figure 4) and a slight increase of I.V. (Figure 5). However PBT 3, stabilised with U626, is more stable than 2 and less stable than 1 and 4. In this case both the discolouring, the decrease of the intrinsic viscosity (I.V.) and the increase of C.E.G. are not as strong as in 2. Therefore the use of the stabilizer has a positive effect on the stability of the polymer obtained in the presence of Ti(O-nBu)4, but it is useless when Ti(acac)2(O-iPr)2 is employed as catalyst. 40
35
C.E.G. (eq/ton)
30 Ti(acac)2(O-iPr)2 Ti(O-nBu)4
25
Ti(O-nBu)4 + U626 Ti(O-nBu)4 + acetylacetone
20
15
10 0
10
20
30
40
50
Ageing time (h)
Figure 4. C.E.G. of PBTs aged at 160°C.
0,92 0,9
Ti(acac)2(O-iPr)2
0,88
Ti(O-nBu)4
0,86 0,84
Ti(O-nBu)4 + U626 0,82
Ti(O-nBu)4 + acetylacetone
0,8 0,78 0,76 0
10
20
30 T ime ( h)
Figure 5. I.V. of PBTs aged at 160°C.
40
50
Thermo-Oxidation Stability of Poly (Butylene Terephthalate) and Catalyst…
333
Moreover the analysis of the data shows some peculiar features for PBTs 1 and 4 that have to be noted. The significant improvement of colour L (and of the related level of whiteness), after the first 7 h of heating, can be related to the annealing that crystallizes the otherwise crystallinity-retarded polymer (Figure 1). This behaviour as well as the lower value of colour L at t=0 of freshly synthesised PBT 1 and 4, can be explained through the formation and incorporation of a small amount of an additional co-monomer as di(butanediol). The trend of I.V. in PBT 1 and 4, since it cannot be related to the accuracy of the method (0.002 dL/g), is very interesting. If on one hand the slight decrease from 7 to 47 h can be attributed to a slight degradation, the increase after the first 7 h can be related to a small increase of molecular weight. This effect is very intriguing and can be related both to the possibility of a solid state polycondensation on PBT (SSP) and reticulation processes. In addition to acetylacetone other additives and well-known complexing molecules, like those depicted in Figure 6, were tested in the synthesis of PBT. The new potential catalytic systems were obtained as a reaction mixture of the additives with Ti(O-nBu)4 in 1,4-butanediol. This mixtures were used in PBT synthesis without any modification or purification.[31,32] In particular, among the tested additives, it is worthy to note that commercially available titanium complexes are reported for ethylacetoacetate;[34] EDTA is one of the most universally employed chelating agent. A different approach was considered in the choice of TEPF and TEPA. TEPA as a phosphonate based compound, can be used as stabilizer for transesterification catalysts in PET synthesis.[35-37] In this case the aim was to verify if this type of compounds have the same effect on the stability of PBT as widely described for PET. O O
O
P
EtO
P
EtO
O
OEt EtO
EtO
OEt
triethyphosphonformiate (TEPF)
triethyphosphonacetate (TEPA)
NaOOC
O
O
HOOC
N N
COOH
OCH2CH3 COONa
ethylacetoacetate (EAA)
ethylenediamminotetraacetic disodium salt (Na2EDTA)
Figure 6.
Comparing the experimental data about reaction time and the intrinsic viscosity (I.V.) of these new PBTs (Table 2), the Ti(O-nBu)4/acetylacetone catalyst showed again a significant higher activity than the other systems both for ester-interchange and polycondensation steps (Table 2). Only in the case of Ti(O-nBu)4/TEPF the polymer was obtained with a higher I.V., even if in longer reaction time. Then thermo-oxidation stability of the PBTs was tested in the usual way by oven ageing, and the results are summarized in the following figures 7-9.
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Entry
Table 2. characteristics of different PBT EI stage PC Stage Time(min.) Time(min.) Ti(O-nBu)4 86 87 Ti(O-nBu)4 + acetylacetone 67 83 Ti(O-nBu)4 + EDTA 75 90 Ti(O-nBu)4 + EAA 84 91 Ti(O-nBu)4 + TEPF 85 91 Ti(O-nBu)4 TEPA 81 89
Catalyst
I.V. (dl/g)a
2 0.855 4 0.851 5 0.843 6 0.828 7 0.875 8 0.849 a I.V.s were measured on a solution of PBT in o-chlorophenol employing an auto viscosimeter ShotteGerate. b According to Cielab scale, colours were determined employing a colorimeter BYK Gardner. 92 90
Colour L
88
Ti(O-nBu)4 Ti(O-nBu)4 + acetylacetone
86
Ti(O-nBu)4 + EDTA Ti(O-nBu)4 + EAA
84
Ti(O-nBu)4 + TEPF Ti(O-nBu)4 + TEPA
82 80 78 0
10
20
30
40
50
Tme (h)
Figure 7. Colour L of PBTs aged at 160°C. 25
20
Colour b
Ti(O-nBu)4 Ti(O-nBu)4 + acetylacetone
15
Ti(O-nBu)4 + EDTA Ti(O-nBu)4 + EAA
10
Ti(O-nBu)4 + TEPF Ti(O-nBu)4 + TEPA
5
0 0
10
20
30 Time (h)
Figure 8. colour b of PBTs aged at 160°C.
40
50
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The comparison of the reported data for colour L, colour b and I.V. shows again that PBT 4, synthesized in the presence of Ti(O-nBu)4 / acetylacetone, is the most stable. All the other catalytic systems influenced negatively the stability of PBT even if Ti(O-nBu)4 is the worse. However, there are significant differences among the several catalysts on PBT properties. These effects are probably related to the strength of the complexes formed between Ti(IV) and the complexing agents. In particular it should be noted that Ti(O-nBu)4 / ethylacetoacetate is less deleterious: in this case we noted a constant level of colour L, and the decrease of I.V. and color b was less pronounced than in the other cases. 0,9
0,88
0,86 Ti(O-nBu)4
I.V. (dl/g)
0,84
Ti(O-nBu)4 + acetylacetone Ti(O-nBu)4 + EDTA
0,82
Ti(O-nBu)4 + EAA Ti(O-nBu)4 + TEPF
0,8
Ti(O-nBu)4 + TEPA
0,78
0,76
0,74 0
10
20
30
40
50
Time (h)
Figure 9. I.V. of PBTs aged at 160°C. However, it is very difficult to propose a mechanistic explanation of the obtained results, but a look to the general features of the accepted thermo-oxidation mechanism of organic materials and polymers can be helpful to have an idea about the processes involved during the degradation. This subject will be explored in the next sections. In conclusion it is important to say that the use of Ti(acac)2(O-iPr)2 at industrial scale led to a higher quality and cheaper PBT. This polymer has been commercialised for several years until the bankruptcy of our old company. The main characteristic of this product was of course the high stability, achieved without the use of any stabilizer.
3. THERMO-OXIDATION OF ORGANIC MATERIALS AND POLYMERS The study of degradation processes of polymers and organic materials is a fundamental topic both from academic and industrial point of view and it can be expected that thermal and thermo-oxidative decomposition mechanisms are reasonably well established.[20,38] These complex reactions proceed via a typical free-radical chain as outlined in scheme 2.
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However the details of the initiating process (step 1), the very early formation of radicals, . are not yet fully understood. Usually it is assumed that the oxidation initiating radical R is formed by heat, light, by mechanical stress or by reactions with radicals originating from foreign sources. Then, in the presence of oxygen the radicals lead to the formation of hydroperoxides (step 3). These are the key intermediates of the thermo-oxidation process because the thermally induced homolytic decomposition of -OOH groups into different radicals renews oxidation chains (stage 4).[39] Since homolytic decomposition of hydroperoxides into free radicals (stage 4) requires relatively high activation energies, this process is rather slow and becomes effective only at temperature of about 120°C or higher.[20] However, in the presence of catalytic amounts of certain metal ions, hydroperoxides decompose at a lower temperature and/or at higher rate. This degradation mechanism involves a series of redox processes in which the most active catalysts are those derived by metals easy to oxidize or to reduce by one-electron, such as a series of transition metals like Fe, Co, Mn, Cu, Ce, V. Then several competitive termination couplings between different radicals (step 5) or the decompositions of peroxy radicals (step 6) lead to radical destruction. INITIATION
1)
R.
RH
PROPAGATION
2)
R.
O2
+
ROO .
HYDROPEROXIDE FORMATION
3)
ROO . + RH
ROOH
+
R.
CHAIN BRANCHING
4)
RO
ROOH
.
+
. OH
TERMINATION
5)
R.
6)
ROO .
+
ROO .
ROOR Inactive products
Scheme 2.
The best way to contrast these degradation reactions is the use of an antioxidant. The effectiveness of antioxidants is based on the fact that they are able to stop the reactions described in scheme 2. The majority of antioxidants belong to the class of sterically hindered phenols like those known with the trade names Ultranox or Irganox. These compounds are peroxy-radicals decomposers, according to the reaction reported in scheme 3. The
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stabilization is achieved by the fact that the reaction of scheme 3 competes with reaction 3 of scheme 2, transforming the reactive peroxy radical into the much less reactive phenoxy radical. In this way the chain branching in scheme 2 is inhibited and the radical chain process is terminated. The phenoxy radical is also capable to react with another peroxy radical to give a peroxycyclohexanedione as widely established in literature (Scheme 4).[33, 40, 41]. OH t-Bu
. ROO
O t-Bu
.
t-Bu
t-Bu
+
ROOH
+
phenoxy radical
Scheme 3. O t-Bu
.
O
O t-Bu
t-Bu
t-Bu
t-Bu
.
+
ROO
t-Bu
. OOR
phenoxy radical
peroxycyclohexanedienone
Scheme 4.
O O
O
P
P O
O
O
U626: bis-(2,4-ditertbutylphenyl)-pentaerythryl-diphosphite
Figure 10.
P(OR)3
+
R'OOH
OP(OR)3
+
R'OH
Scheme 5.
The thermal stability of peroxycyclohexanedienones is limited. Their decomposition leads to new reaction chains at temperatures below 150°C. Also phenoxy radicals can initiate new radical chains at high temperature. Of course these features do not affect the performances of stabilizers during the normal life of stabilized materials, but these effects should be taken into account especially in high temperature application and testing of antioxidants-added materials. Otherwise this means that the effectiveness of sterically
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hindered phenols decreases with increasing the temperature of application and manufacturing of the stabilized materials. Even within the limits of the testing conditions, the positive effect of a typical antioxidant, Ultranox U626, for the stabilization of PBT synthesised in the presence of Ti(O-nBu)4 was previously highlighted in section 2. The structure of U626 is reported in Figure 10. U626 is a rather complex molecule. Very interestingly the phosphite moieties in U626 can have also an important role in the stabilization. Phosphites are well-known stabilizers and they are called secondary antioxidants, while hindered phenol-based stabilizers are known as primary antioxidants. Phosphites have the ability to react with hydroperoxides to yield phosphates according to scheme 5.[20,38] U626 combines primary and secondary stabilizers in the same molecule.
4. THERMO-OXIDATIVE DEGRADATION OF PBT: THE ROLE OF TITANIUM (IV) RESIDUAL An experimental investigation directly on polymers in order to find out the degradation products is difficult to realize. However, both the study of degradation of model compounds like butylene dibenzoate [5-7], and the general mechanism of thermo-oxidation of organic molecules (scheme 2), can be very helpful to propose a specific mechanism for the thermooxidative degradation of PBT (scheme 6). In order to simplify the discussion, the contribution of thermal degradation has not been considered. The first step of thermo-oxidative degradation is believed to be the homolytic cleavage of α−CH bonds, a process that leads to the formation of α-radicals (step 1).[5-7] Then in the presence of oxygen the α−radicals react to form hydroperoxides that are the key intermediates of the degradation mechanism (step 2). In effect the decomposition of the hydroperoxides (step 4) is the beginning of complicated radical chains that lead to the formation of a great number of products and functionalities that are difficult to isolate and characterize (step 5).[511,42,43] However the results of the these degradation reactions are mainly C-C and C-O cleavages of the bonds adjacent to radical moieties. One of the consequences of these cleavages is the decrease of the molecular weight and the increase of both aliphatic and aromatic carboxyl end groups. As emphasized in section 3, the decomposition of hydroperoxides, the key step of the degradation mechanism, is rather slow and becomes effective only at temperature of about 120°C or higher. However in the presence of catalytic amounts of certain metal ions, such as a series of transition metals like Fe, Co, Mn, Cu, Ce, V, hydroperoxides decompose at a lower temperature and/or at higher rate. According to this concept, the catalyst residual in PBT can have an important role to catalyze the degradation of hydroperoxides (step 4 of scheme 6). In fact it is well-known the capability of titanium(IV) to catalyze important oxidation reactions, such as epoxidation of allylic alcohols and sulfoxidation, in which hydroperoxides are involved as oxidants.[44,45] The role of titanium center in these reactions is the coordination and activation of the hydroperoxides to favor oxygen atom transfer. Ti(IV) compounds are also able to catalyze hydroperoxides decomposition into ketonealcohol mixtures, a well-known industrial process for acetone and phenol production.[46-48] In particular, a kinetic study reported by Sukin et al. showed that, in titanium(IV) catalysed
Thermo-Oxidation Stability of Poly (Butylene Terephthalate) and Catalyst…
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radical decomposition of peroxides, the decomposition increases in the order diacyl peroxides ≤ peroxy esters < peroxy ketones = hydroperoxides. In these reactions, sterically hindered and chelated titanium(IV) compounds catalyse hydroperoxide decomposition only very slowly.[46] Moreover, even Ti(III) based compounds play an important role in the radical decomposition of t-butyl hydroperoxide.[48] Therefore the possibility of the different redox species, Ti(IV)/Ti(III), can also affect radicals formation and polymer decomposition. On the basis of these considerations we can confidently assume a relationship between titanium residual and PBT degradation. The different effect of Ti(O-nBu)4 and the chelated Ti(acac)2(O-iPr)2 on PBT stability could be explained with a different capability of the two metal residuals to coordinate the OOH groups and/or a different ability to catalyse hydroperoxides decomposition. Therefore Ti(O-nBu)4 and the chelated Ti(acac)2(O-iPr)2 should affect in a different way the key step 4 of the degradation mechanism (scheme 6) and consequently the rates of the entire degradation process are very different in the two cases. Then, the decrease of I.V., the increase of CEG and the yellowing of standard PBT 1 (see section 2) are related of the further decomposition the alkoxyl radicals (step 5). The observed yellowing could be correlated both to the formation of products with conjugated functional groups and to new coloured titanium species formed during high temperature oven testing. Of course it is difficult to determine experimentally the catalytic species and to establish the role of different Ti(IV) compounds on the degradation process in solid PBT. Probably the thermooxidation of the model compounds in the presence of different titanium based complexes can be useful. In conclusion, even if a specific mechanistic model is not available yet for the obtained results, the improved stability of PBT in the presence of Ti(acac)2(O-iPr)2 can be attributed to the inhibition of the degradation of hydroperoxides by means of metal deactivation using a chelating agent.
CONCLUSION AND OUTLOOK Excellent progresses have been made over past decades in the development of new efficient catalysts for polyesters synthesis, and in the understanding of the properties of these materials. Even if an impressive quantity of information are available, several challenges are still open. One of the most important purpose in PET research is, without no doubt, the substitution of the toxic Sb(III) based catalyst with more friendly compounds. One of these possibilities, as described in section 1, is related to titanium (IV) based compounds. In effect Ti(O-nBu)4 is an active catalyst in PET synthesis but it is not often used because of the yellowing of the obtained polymers. However, many studies are evaluating the use of different additives and/or several Ti(IV) based complexes in PET synthesis in order to solve these problems and to understand the reasons of the discoloration. The different effect of titanium catalysts on PET and PBT colour could be related to the thermal degradation. Polycondensation temperature for PET is significantly higher than that of PBT, and titanium(IV) can affect their thermal decomposition in different ways. However, the role of titanium catalysts on PET properties is not clear yet, but, all these problems, the stability, the discoloration, are correlated directly or indirectly to the catalyst composition.
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Antonio Massa, Valeria Bugatti, Arrigo Scettri et al. O
O
O
.
O
O
C
COCHCH2CH2CH2CO
*
C
COCH2CH2CH2CH2CO
*
*
1
2
*
O
O OOH
C
COCHCH2CH 2CH2CO
O
.
+H
*
*
O
.
-H
O2
.
O
O OO
C
COCHCH2CH2CH2CO
O *
3
4
*
.
-OH
.
O
O O
C
COCHCH2CH2CH2CO
O *
5
Further oxidation products; formation of different radicals; C-C C-O cleavage products, etc. etc.
Scheme 6.
REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Mark, HF; Ed., Encyclopedia of Polymer Science and Engineering, Vol. 12, 2nd ed.; Wiley-Interscience: New York, 1988. Margolis, JM; Ed.; Engineering Thermoplastics: Properties and Applications; Marcel Dekker: New York, 1985. Brozenick, NJ. Modern Plastics Encyclopedia; McGraw-Hill: New York, 1986-1987. Klausner, E; Santostefano, A; Monti, R; Barchielli, G. European Patent, n°, 1998, 1, 035 916. Buxbaum, LH. Angew Chem Int Ed Engl, 1968, 7, 182. Allen, NS; Edge, M; Mohammadian, M; Jones, K. Polym Degrad Stab, 1993, 41,191. Botelho, G; Queiros, A; Liberal, S; Gijsman, P. Polymer Degradation and Stability, 2001, 74, 39-48. Carroccio, S; Rizzarelli, P; Scaltro, G; Pugliesi C. Polymer, 2008, 49, 3371–3381. Nealy, DL; Adams, LJ. J Polym Sci, 1971, 9, 2063. Goodings E.P. Soc Chem Ind Monograph No., 1961, 13, London, 211. Lum R.M; J Polym Sci Polym Chem Ed, 1979, 17, 203. Dzieciol, M; Trzeszczynski, J. J Appl Polym Sci, 1998, 69, 2377. Edge, M; Allen, NS; Wiles, R; McDonald, W; Mortlock, SV. Polymer, 1995, 36, 227. Zimmerman, H; Kim, NT;. Polym Eng Sci, 1980, 20, 680. Zimmerman, H. Faserforsch u Textiltechn, 1962, 13, 481. The Resinous Products and Chem Co. Brit. Pat., 1947, 5,888,834. Zimmerman, H. Faserforsch u Textiltechn 1968, 19, 372. E. I. Du Pont de Nemours Co. U.S. Pat. 3,406,153 (1968).
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[19] Celanese Corporation, U.S. Pat., 3, 784,507, 1974. [20] Gachter, R; Muller, H. Plastics Additives Handbook; Hanser/ Gardner Publishers: Munchen–New York, 1988. [21] Walter, M; Boerner, F; Rafler, G; Thiele, UK; Stibal, W; Hagen, R. (Inventa-Fischer G.m.b.H. & Co. KG., Germany). Ger. Offen. 2005, 11, Application: DE, 200310339742 20030828. [22] Banach, TE; Fiorini, M; Patel, BR; Pilati, F; Berti, C; Marianucci, E; Messori, M; Colonna, M; Toselli, M. (General Electric Company, USA). PCT Int. Appl. (2000), 24 pp. Application: WO 99-US25685. [23] Putzig, DE. (USA). U.S. Pat. Appl. Publ. (2005), 8 pp. Application: US 2004-810403. [24] Colonna, M; Banach, TE; Berti, C; Fiorini, M; Marianucci, E; Messori, M; Pilati, F; Toselli, M. Polymer, 2003, 44, 4773. [25] Pilati, F; Gostoli, C; Sarti, GC. Polym Process Eng, 1986, 3, 303. [26] Fortunato, B; Pilati, F; Manaresi, P. Polymer, 1981, 22, 655. [27] Guo, B; Chan, CM. J Appl Polym Sci, 1999, 71, 1827. [28] Duh, B; Tallmadge, O. J Appl Polym Sci, 2003, 89, 3188. [29] Typically, metal deactivators are used in the stabilization of polyolefins in contact with copper (see ref. 19). [30] Massa, A; Scettri, A; Contessa, S; Bugatti, V; Concilio, S; Iannelli, P. Journal of Applied Polymer Science, 2007, 104, 3071. [31] Massa, A; Bugatti, V; Scettri, A; Contessa, S. Macromolecules: An Indian Journal, 2008, 4 (1), 45-50. [32] All the samples for ageing tests refers to PBTs synthesized in 20 L pilot plant. All the catalytic systems were tested twice. For experimental details about polymers and catalysts synthesis, and for ageing tests see ref 29. [33] Pospisil, J; Horak, Z; Pilar, J; Billingham, NC; Zweifel, H; Nespurek, S. Polym Degrad Stab, 2003, 82, 145. [34] Puchberger, M; Rupp, W; Bauer, U; Schubert U. New J . Chem., 2004, 28,1289. [35] Hiroshi, K; Shinichi, T. (Teijin Ltd., Japan). Jpn. Kokai Tokkyo Koho (2004) Application: JP, 2003-5564 20030114. [36] Takase, T; Tsukamoto, R. (Teijin Fibers Ltd., Japan). Application: JP, 2003-284767 20030801-2005. [37] Katamani, H; Konagaya, S; Nakamura, Y. Polymer, 1980, 12, 125. [38] Zhang, X. M; Mantzaris, J. Recent Res Dev Org Chem, 1998, 2, 453. [39] Bolland, JL; Gee, G. Trans Faraday Soc, 1946, 42, 236. [40] Pospisil, J, Adv. in Pol. Sci., 1980, 36, 69. [41] Koch, J. Angew. Makromol. Chem., 1971, 20, 7. [42] Lum, R.M. J Polym Sci Polym Chem Ed, 1979, 17, 203. [43] Passalacqua, V; Pilati, F; Zamboni, V; Fortunato, B; Manaresi, P. Polymer, 1976,17,1044. [44] Johnson, RA; Sharpless, KB. In Catalytic Asymmetric Synthesis; Ojima, I., Ed; VCH: New York, 1993, 227. [45] Kagan, HB; Luukaas, T. In Transition Metals for Organic Synthesis; Bolm, C., Beller, Eds; Wiley/VCH: Weinheim, 1999, Vol. 2, p 361.
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[46] Sukin, AV; Bulatov, MA; Spasskii, SS; Suvorov, AL; Khrustaleva, EA; Kochneva, MA. Zhurnal Fizicheskoi Khimii, 1977, 51, 2093. [47] Stepovik, LP; Gulenova, MV; Martynova IM. Russian Journal of General Chemistry, 2005, 75(4), 507 Translated from Zhurnal Obshchei Khimii, 2005, 75 (4), 545. [48] Citterio, A; Arnoldi, A; Griffini, A. Tetrahedron, 1982, 38(3), 393.
In: Encyclopedia of Polymer Research Ed: Caroline E. Jones
ISBN: 978-1-61761-926-7 © 2011 Nova Science Publishers, Inc.
Chapter 11
HINDERED AMINE STABILIZERS AS SOURCES OF MARKERS OF THE HETEROGENEOUS PHOTOOXIDATION / PHOTOSTABILIZATION OF CARBON CHAIN POLYMERS J. Pilař and J. Pospíšil Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, v. v. i.; 162 06 Prague 6, Czech Republic.
ABSTRACT Application of Hindered Amine Stabilizers (HAS) is the state-of-the-art approach to protection of carbon-chain polymers such as polyolefins and polystyrene or blends containing these against weathering. During outdoor exposure, the polymers loose their material properties due to solar radiation-triggered photooxidation. The complex mechanism of the stabilization involving cyclic oxidation-triggered transformation of HAS is outlined. Monitoring of the formation of the HAS-developed key transformation products, HAS-related nitroxides, responsible within the regenerative mechanism for the effective stabilization was used to confirm the heterogeneous character of photooxidation of two carbon-chain polymers, polypropylene and a specific polyethylene copolymer. Depth profiles of nitroxides were monitored in a long-term photooxidation regime using Electron Spin Resonance Imaging (ESRI) technique. The shape of concentration profiles of the nitroxides accumulated in the equilibrium state upon filtered Xenon lamp-equipped Weather-Ometer exposure was interpreted in terms of the oxygen diffusion limited oxidation and radiation penetration in oxidation-stressed polymer surfaces. The data indicate differences in the character of the heterogeneous process in dependence on the polymer matrix and on the used stabilizer system based on secondary HAS and Oalkylhydroxylamine HAS and/or HAS combination with UV absorbers. Imaging of nitroxides is a precise tool for marking heterogeneous oxidation of polyolefins.
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INTRODUCTION AND BASIC CONSIDERATION Carbon-chain commodity and engineering polymers (PH) suffer during all phases of their lifetime in the earth atmosphere from oxidative degradation, either thermal or phototriggered. Mechanism of oxidation was studied in detail [1, 2]. The oversimplified free radical oxidation scheme involves initiation (1), propagation (2 and 3) and termination steps (4) followed by reactions of primary intermediates and products.
⎯→ P• PH ⎯⎯ ⎯
(1)
P• + O2 → POO•
(2)
POO• + PH → POOH + P•
(3)
P•, POO• → Products
(4)
initiation
The last two steps are rather well understood. There exist some uncertainties encountering initiation steps due to the character and distribution of initiation centers in the polymer mass such as sensitizing impurities, charge transfer complexes (CTC) and/or participation of active environmental pollutants [3-5]. Generally this does not affect however the structure of the oxidation products having oxidation properties formed in the consecutive steps to initiation [alkylhydroperoxides POOH, peroxyacids PC(O)OOH] and their oxygen-centered free radical precursors [POO•, PC(O)OO•] that play an important role in the fate of polymer additives, stabilizers in particular. Phototriggered reactions such as photolysis and photooxidation play a decisive role in outdoor exposure (weathering) of polymers [1, 4-6]. Degradation of real polymer systems was shown to be a heterogeneous process strongly affected by the sample morphology and geometry. Analysis of the physical chemistry of polymer oxidation and stabilization confirmed [7] differences between oxidation of amorphous and crystalline parts of the material. This results in degradation heterogeneity due to the oxygen impermeability of the crystalline areas. The heterogeneity on molecular and supramolecular level is primarily affected by the random localization of initiation and chromophoric centers in the polymer mass followed by infectious oxidative spreading from the centers [8]. Concentration of activated centers depends on the intensity of UV radiation penetrating through the polymer that depends on polymer transparency. Moreover the oxidation rate and localization of oxidation products are dependent on the availability of oxygen within the system. Consequently rate of oxygen diffusion into the material, rate of oxygen consumption and the possibility to supplement the consumed oxygen by diffusion are decisive factors affecting the depth of oxidation of thick walled materials. Diffusion limited oxidation (DLO) results in differences in oxidative changes between the heavily damaged surface layers and central parts of the materials [5]. The differences are dependent on the thickness of the material and are observed as concentration gradients from the surface to the bulk of the material using step-bystep analyses [5, 9-11]. DLO is a process characteristic of both thermal and photooxidation of thick-walled materials and results mostly in U-shape formed spatial distribution of changes [5, 9, 11].
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Oxygen insufficiency, such as in the central parts of the thick samples of the material accounts for accumulation and subsequently for autoreactions of polymeric alkyl radicals generated according to (1) from the less oriented taut tie molecules located between crystallites. Autoreactions of alkyls, disproportionation and recombination result in chain scission and crosslinking of polyolefins. The ratio between scission and crosslinking depends on the basic structure of polyolefins and has a mostly U-shaped gradient character similarly as concentration gradients of oxygenated products or polymer unsaturation [10, 12, 13]. Macroalkyls remaining in the polymer bulk are not only examples of the heterogeneity of polymer oxidation. Their reactivity with suitable additives accounting for alkyl annihilation may contribute to the polymer stabilization process differing from that based on deactivation of oxygenated structures. For thermal stabilization of carbon-chain polymer, chain-breaking donor antioxidants such as sterically hindered phenols are generally used. They scavenge alkylperoxy radicals generated in the propagation step (2), but are unable to deactivate polymeric alkyls. Chain breaking antioxidants are conventionally used with hydroperoxide deactivating antioxidants, e.g., organic phosphites or sulfides [14]. Both mentioned classes of antioxidants are rather inactive against photooxidation. Compounds acting by other mechanisms are used as photostabilizers. They consist either of additives with preventive function, such as UV absorbers competing with the polymer in preferential absorption of the actinic radiation, preventing thus polymer degradation, or are photoantioxidants or quenchers of excited states having the function of curative stabilizers deactivating products of the photochemistry/photophysics of the polymer matrix and blocking thus their participation in the propagation steps of the chain oxidation [4, 14]. Hindered amine stabilizers (HAS) are the most common class of the curative additives and their application is the state-of-the-art in photoprotection of carbon-chain polymers, polyolefins in particular. HAS shape future polymer development, promote their consumption in new areas and expand material performance by increasing its lifetime. Application of HAS is based on a long-term effective development and is connected with commercial benefits for polymers. An optimized technical application of HAS required explanation of their chemistry and activity mechanisms in different phases of the oxidative degradation of polyolefins [1417]. There is not a full consensus what everything is involved in the HAS mechanism because it is difficult to confirm some of the proposed mechanistic steps in the polymer matrix. Formation of HAS-derived nitroxides (>NO•) (5) is the assumed primary step of the stabilizing activity connected with the sacrificial transformation as shown for a secondary HAS (having active function >NH) [15, 16].
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Identification of nitroxides in the oxidized hydrocarbon substrate doped with HAS is considered as a proof of the photoantioxidant activity of HAS. The latter involves unique mechanistic features in polymer stabilization differing thus from the activity of conventional chain-braking antioxidants. The developed nitroxides are able to scavenge polymeric alkyls formed not only in the surface layers but also in areas of oxygen insufficiency in deeper polymer layers where they may be formed due to presence of residual oxygen. Participation of the polymeric alkyls in oxidative transformation into alkylperoxyls POO• (2) is thus disabled. Resulting O-alkylhydroxylamines >NOP are formed not only from >NH but also from various N-substituted HAS (>NX, X=alkyl, aryl, alkoxyl) and are considered as active reservoirs of HAS efficiency (6). (6) Nitroxide oxidative regeneration (Klemchuk-Gande mechanism) from >NOP based on its interaction with oxygen-centered radical oxidation products of polyolefins (polymeric alkylperoxyls and acylperoxyls) was proposed (7) where symbol CHR1R2 represents “P” in >NOP [18]. According to this scheme no peroxidic by-products with pro-oxidative character are formed within the resulting products.
(7) Formation and location of nitroxides, the key HAS-developed transformation products, was monitored using electron spin resonance imaging (ESRI) technique that confirms the heterogeneous character of photooxidation of polyolefins.
EXPERIMENTAL Materials Polymer plaques containing stabilizers were prepared using additive-free polypropylene (PP) PH6 (Polymer Institute, Brno), Mw = 308 000, Mn = 62 500, of ca. 50 % crystallinity or recently developed amorphous polyolefin poly(ethylene-co-norbornene) Topas® 8007 (TP, TOPAS Advanced Polymers GmbH, Frankfurt am Main, Germany), volume flow index at 260 oC/2.16 kg: 30 ml/10 min, density 1.02 g/cm3, norbornene content ca 17 weight %. HASs Tinuvin®770 and Tinuvin®123, UVAbs Tinuvin®327 as preventive light stabilizer acting by an excited state intramolecular proton transfer (ESIPT) mechanism and benzoate photoantioxidant Tinuvin®120 were used as additives. All photostabilizers were supplied by Ciba Specialty Chemicals, Basel, Switzerland.
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Polymer-stabilizer mixtures were homogenized in the W 50 EH chamber of a Brabender Plasti-Corder laboratory mixer at 190 °C and 60 rpm for 10 min. After removing the material from the chamber, 6 mm thick plaques 8 x 8 cm were molded in a laboratory hot press at 200 °C for 2 min under 50 kN for deairation plus another 2 min under 100 kN followed by water cooling to 70 °C for ca. 15 min under 150 kN.
Accelerated Photooxidation and Oven Test The plaques were exposed to accelerated photooxidation in an Atlas Ci 3000+ Weather-Ometer under the following test conditions: filtered Xenon light wavelength bandpass 295-800 nm (inner and outer filtercombination Type S Boro/Type S Boro), irradiance 0.5 Wm-2nm-1 at 340 nm, black panel temperature (bpt) 60 °C (used PP and TP were non-pigmented, accordingly the surface sample temperature was lower), dry bulb temperature (dbt) 30 °C, RH 20%. Oven tests were performed in a heating box at temperature 60 °C. Cylindrical samples (diameter 3 mm, length 6 mm) were bored out from the plaques after appropriate time periods of exposure. Between periods of exposure in Weather-Ometer or in oven the plaques were stored in the dark at room temperature. Tests proved a negligible change of concentration profiles in the samples bored out from the plaques during the storage.
Electron Spin Resonance Imaging ESRI experiments were performed with a commercial Bruker ELEXSYS E540 X-band spectrometer equipped with a pair of eight-shaped Lewis gradient coils delivered by George Associates (Berkeley, USA) that are able to produce vertical gradient perpendicular to the external magnetic field Gmax~320 G/cm. Concentration profiles of nitroxides in the samples along the axes of the cylinders coinciding with the direction of irradiation incident on the plaque in the Weather-Ometer were calculated using the 1D ESRI and deconvolution procedure described previously [19]. Matlab software tool was used for the data manipulation and programs targeting the deconvolution procedure in 1D experiment and filtered back projection procedure in 2D experiment based on the Matlab scripting language were developed. Spatial dependence of ESR spectral line shapes was excluded by checking spatial-
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spectral images calculated using back projection algorithm from 2D ESRI data. Experimental scheme is given in Fig. 1. In order to obtain intensity of images in absolute units, usual spectrometer calibration for a given sample setup with the standard of known concentration was performed. In the 1D experiment we calibrated each sample spectrum (taken with gradient coils off) used for calculation of the particular one-dimensional profile using the spectrum of the standard taken under similar conditions. As the second integrals of the EPR spectra are directly proportional to the number of spins present in the measured sample, we were able to assess the total concentration of spins in the given sample and ascribe this number to the first integral of the resulting 1D profile. Anticipating axial symmetry of nitroxide distribution in the cylindrical sample we obtained the true concentration profile in number of spins per volume unit in particular depth of the sample after normalization of the calibrated 1D profile to the sample cross-section.
Figure 1. Experimental arrangement.
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Nitroxide Formation and Location in Oxidized Polypropylene In spite of some uncertainties in the individual steps of the HAS mechanism in polymer stabilization due to the specific effects of the polymer matrix and the environmental stress, the HAS-based nitroxides are considered the key intermediate in the HAS reactivity mechanism. Detection and quantification of the formed nitroxides using ESRI spectroscopic technique has been exploited for confirmation of the primary transformation step in HAS mechanism [15, 16, 20], as a consequence of interactions of HAS with oxygenated radical and molecular products of polyolefins (5). Monitoring of the nitroxide development enables tracing of the oxidation process within the polymer matrix. Consequently it is also a tool for marking the heterogeneity of the oxidative transformation of semicrystalline carbon chain polymers [polypropylene (PP), polyethylenes (PE)] or amorphous polymers [copolymers of ethylene with norbornene, polystyrene (PS), high impart polystyrene (HIPS), acrylonitrilebutadiene-styrene polymer (ABS)]. Most of the experimental studies dealing with tracing of nitroxide formation were performed using thin sectioned polymers, films of PP in particular [20, 21]. The determined nitroxide level is generally low [15]. Time-dependent ESR analysis of photooxidized and HAS-doped PP films revealed an increase in the amount of formed nitroxides from the beginning of the exposure [20, 21]. After reaching a maximum, a gradual decrease approaching slowly the steady state concentration characterizes the in-polymer behavior of nitroxides in consecutive steps, such as scavenging of polymeric alkyl radicals P•, formation of >NOP (5) and nitroxide regeneration (7). The process is affected by the structure of the used HAS and by the external stress [4, 20]. Differences between accelerated and natural aging in PE doped with oligomeric HAS [22] or differences between thermal and photooxidation in ABS [23] were observed. Increase of the concentration of the HAS originally added to the polymer results in an increase of the concentration maximum of the nitroxide in the initial phase of the process [22] and prolongs the period of the effective material protection. The trend in the change of the nitroxide concentration indicating an approach to the equilibrium concentration is analogous at various in practice used concentrations between 0.5 and 3.0 % and confirms generally the overall cyclic HAS stabilization mechanism. Development trends in the nitroxide level analogous to that in plastics were found in HAS doped automotive clearcoats [24]. Determination of the heterogeneity of oxidative damages increases the insight into the material properties of aged polymers because the damaged regions influence seriously the overall properties of the material. For example the heavily degraded polymer surface layer deteriorates seriously tensile fracture properties of the whole material or infects components of polymer blends containing recyclates. Monitoring of nitroxides in thick materials (plates several mm thick) after oxidative stress is a sensitive method to localize the degradation within the material depth. Using HAS of various structures in different polymer materials provides moreover important information about specific mechanistic features of the process. In this paper we concentrate on secondary HAS Tinuvin® 770, O-alkylhydroxylamine-type HAS Tinuvin® 123, a combination of Tinuvin® 770 with UV absorber Tinuvin® 327 and photoantioxidant Tinuvin® 120 in semicrystalline additive-free PP. For matter of comparison, experiments with an amorphous polyethylene, poly(ethylene-co-norbornene) copolymer and a HAS-developed nitroxide additive were performed.
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Several studies of the spatial resolution of oxidation processes in HAS stabilized polymer appeared since the pioneering paper [25] and deal with events in PP [26 - 28] or commercial poly(acrylonitrile-co-butadiene-co-styrene) (ABS) copolymer plaques with thickness from 2 to 6 mm [27, 29]. We preferred for our experiments plaques of 6 mm thickness providing more detailed information on the spatial distribution of nitroxides in both thermal and photooxidative stress situations [28]. Results of experiments performed with oxidized PP doped with secondary HAS Tinuvin® 770 show high concentration of nitroxides in the vicinity of both surfaces due to the DLO. This was observed after continuous exposure to radiation in the Weather-Ometer on the irradiated (front) and non-irradiated (back) surfaces as well as on the both surfaces of the samples exposed thermally in hot air oven. Very low concentration of nitroxides was present inside of the samples. The concentration profiles are of characteristic U-shape and indicate preferential surface oxidation of PP, with a specific response to thermal and photochemical stress (Fig. 2). The assumed complex HAS mechanism is thus more explicitly confirmed in thick samples than by monitoring nitroxide concentration in PP films. It is consistent with surface consumption of oxygen in thick plaques and lower availability of oxygenated products in the depth of the PP matrix necessary for a direct development of nitroxides from HAS as well as for nitroxide regeneration from O-alkylhydroxylamine >NOP within the regenerative cycle (7). The comparison of the content of nitroxides generated in PP plaques containing 1 % of Tinuvin® 770 as a sole additive measured after 73 days exposure indicates faster degradation (monitored by nitroxide formation) during the phototriggered process (Fig. 2a) in comparison with oven aging (Fig. 2b). Generation of nitroxides in the oven aged samples confirms at the same time a heat stabilizing contribution of HAS in PP at used conditions (60 °C).
Figure 2. Dependence of nitroxide concentration profiles measured in PP plaques stabilized by 1 % of Tinuvin®770 as a sole additive and exposed to accelerated photodegradation (a) or oven test (b) on the net exposure time. The arrow indicates direction of the incident light.
The nitroxide level in samples containing from 0.5 to 2.0 % of Tinuvin® 770 increased with increasing HAS concentration. Approximately symmetrical U-shape of the nitroxide concentration profile shown in Fig. 2a was confirmed by comparison of shapes of the nitroxide concentration profiles from the front and back surfaces of PP plaques oxidized for overall net exposure 87 days in the presence of different amounts of HAS (Fig. 3). A
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significant decrease in the nitroxide concentration with exposure time in samples doped with the lowest concentration (0.5 %) of HAS was observed indicating consumption of considerable portion of the stabilizer during the degradation process.
Figure 3. Dependence of concentration of nitroxides generated in the irradiated (full symbols and solid lines) and unexposed (empty symbols and dashed lines) surface layers of the PP plaques stabilized by 0.5 % (!, ∀), 1 % (Β, Χ), and 2 % (,, −) of Tinuvin®770 as a sole additive and exposed to accelerated photodegradation on the net exposure time.
A simplified model explains the observed shapes of nitroxide profiles in the thick polymer plates exposed to the environmental oxidative stress characteristic of DLO and radiation penetration. Oxygen is able to diffuse from both sides of the sample into the polymer mass and is consumed into radical and molecular oxygenated species. The latter are concentrated predominantly in the near surface areas as confirmed experimentally by formation of surface-centered concentration profiles of carbonyl species, accounting for transformations of the primary peroxidic species [2, 5, 12]. Within these areas the secondary HAS Tinuvin® 770 (>NH) is transformed into nitroxide (>NO•). Formation of Oalkylhydroxylamine (>NOP) after trapping polymeric alkyl radicals and regeneration within the cyclic mechanism (7) [18] follow in the next steps and result in equilibrium nitroxide concentration. The maximum equilibrium nitroxide concentration found in irradiated surface layer of the PP plaque stabilized by 1 % of Tinuvin® 770 equals approximately to 1 % of concentration of the stabilizer added. Intensity of the UV radiation penetrating through the
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polymer and participating in formation of the polymeric alkyl radical depends on polymer transparency. Due to the high transparency of PP there is sufficient intensity of light to trigger oxidation also in the (rich in oxygen) back surface layer of the sample where the concentration of nitroxides similar as in the irradiated surface layer is generated. Due to the preferential surface oxygen consumption, the central part of the sample suffers from oxygen deficiency. The HAS developed >NOP, if present, has a limited possibility for nitroxide regeneration. The U-shape nitroxide concentration profile arises consequently in the PP plaque (Fig. 2a). We found in an earlier study [28] and confirmed in new experiments that the shapes and time-dependent change of the nitroxide concentration profiles inside polymer plaques subjected to accelerated aging as measured by ESRI depend on the polymer matrix (comparison between PP and polystyrene) and on the responses of the HAS structure to oxidizing counterparts of the matrix. Nitroxide concentration profiles measured in TP plaque stabilized with 1.0 % of Tinuvin® 770 oxidized for overall net exposure 147 days shows slower increase of nitroxide concentration and lower concentration of nitroxides in the back surface layer when compared with PP plaque. Both observations are similar to the results obtained in polystyrene [28], but even more pronounced (Fig. 4). It indicates the lower transparency of TP in comparison with PP and even with PS which is confirmed by optical measurements.
Figure 4. Dependence of nitroxide concentration profiles measured in TP plaques stabilized by 1 % of Tinuvin®770 as a sole additive and exposed to accelerated photodegradation on the net exposure time. The arrow indicates direction of the incident light.
Generation of nitroxides in the PP matrix is dependent on the intensity of the incident UV radiation. By using effective UV absorbers (light stabilizers with preventive mechanism) in combination with HAS [4] the efficiency of the oxidative phototriggered process decreases with depth in the sample due to the light filtration effect. We studied two light stabilizers with
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different activity mechanisms (Fig. 5), the benzotriazole-based UV absorber Tinuvin® 327 and the phenolic photoantioxidant Tinuvin® 120 for overall net exposure 73 days. Decrease of efficiency of the oxidative phototriggered process in the back sides of the exposed plaques is markedly shown by the changed shape of the nitroxide concentration profile. The gradually reduced penetration of the radiation through the sample results in reduced oxidation of the non-irradiated side of the sample shown by formation of lower concentration of nitroxides in the back side surface layer. The photostabilizing effect depends on the structure of the light stabilizer, i.e., on its activity mechanism and efficiency. The benzotriazole-based UV absorber Tinuvin® 327 is comparatively more effective (Fig. 5a) than the phenolic photoantioxidant Tinuvin® 120 (Fig. 5b) in decreasing nitroxide concentration in the irradiated surface layer.
Figure 5. Dependence of nitroxide concentration profiles measured in the PP plaques stabilized by (a) 1 % of Tinuvin®770 + 0.5 % of Tinuvin®327 and (b) 1 % of Tinuvin®770 + 0.5 % of Tinuvin®120 and exposed to accelerated photodegradation on the net exposure time. The arrow indicates direction of the incident light.
Participation of the protective mechanism affected by the modified structure of HAS was evidenced by comparison of nitroxides developed in PP plaques doped with secondary HAS Tinuvin® 770 (Fig. 2) and NOR-HAS Tinuvin® 123 (Fig. 6). The latter was developed for polymer photostabilization in acid environment [15, 16]. Very high concentration of nitroxides was found in thermally pressed Tinuvin® 123 containing plaques prepared for oxidation experiments. Nitroxides were distributed almost homogeneously within the mass of the plaque. The high nitroxide concentration remained in thermally oxidized PP (Fig. 6b). A fully different progress in the development of the nitroxide distribution within the sample was observed in the phototriggered process (Fig. 6a, 7). The initial concentration decreased with increasing exposure time. At the same time the shape of the nitroxide profile changed from the nearly homogeneous distribution to the asymmetric U-shaped profile characterized by a lower amount of nitroxides in the irradiated front surface in comparison with the back surface and a low amount of nitroxides in the central part of the sample.
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Figure 6. Dependence of nitroxide concentration profiles measured in the PP plaques stabilized by 1 % of Tinuvin®123 as a sole additive and exposed to accelerated photodegradation (a) or oven test (b) on the net exposure time. The arrow indicates direction of the incident light.
Figure 7. Dependence of the amount of nitroxides generated in normalized volume of the cylinder (1 mm2 cross-section) bored out from the PP plaques stabilized by: 1 % of Tinuvin®770 (!,∀), 1 % of Tinuvin®770 + 0.5 % of Tinuvin®327 (,,−), 1 % of Tinuvin®123 as a sole additive (Β,Χ), and exposed to accelerated photodegradation (full symbols and solid lines) or to the oven test (empty symbols and dashed lines) on the net exposure time.
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Development of the high nitroxide level generated from Tinuvin® 123 in the sample from the beginning of the experiment is explained by cooperation of thermochemical and photochemical degradation of NOR-HAS. The thermolysis participates even in the phase of the sample preparation and accounts for formation of nitroxides from the NOR-HAS according to (8) [15, 30]. Δ
>NOR ⎯ ⎯→ >NO• + olefin
(8)
Differences in the shape of nitroxide concentration profiles during photo and thermal degradation of the sample containing Tinuvin® 123 are most probably due to presence of polymer alkyl radicals generated during photodegradation proces in the whole volume of the PP sample. Nitroxide concentration in the central part of the sample subjected to photodegradation is reduced by the reaction of the nitroxides with polymer alkyl radicals (6) and cannot be renewed by reaction (5) due to the oxygen deficiency. Negligible concentration of polymer alkyl radicals are generated during thermodegradation process when practically no change of the nitroxide concentration profiles is observed.
CONCLUSION In PP plaques, 6 mm thick, stabilized with secondary HAS Tinuvin® 770 as the sole additive, with the combination of Tinuvin® 770 with UV absorber Tinuvin® 327 or with photoantioxidant Tinuvin® 120, U-shaped concentration profiles of nitroxides were determined during exposure to accelerated photooxidation in a Weather-Ometer already after the first day of irradiation. This type of the nitroxide distribution results from the DLO and penetration of the incident radiation that initiates degradation of PP through the plaques and reaches the unexposed side of the plaque with sufficient intensity. Due to the DLO, radical and molecular oxidation products are formed from the alkyls predominantly near to the front and back surfaces of the PP plaques indicating the depth of oxygen diffusion. These oxidation products participate in transformation of the added HAS marking thus clearly the heterogeneous character of oxidation. Oxygen insufficiency in the central part of the plaques (accounting not only for limited oxygen diffusion but also for the effective and preferential oxygen consumption in the surface layers) allows polymeric alkyls to survive. Consequently, oxidation of the originally added HAS or oxidative regeneration of its transformation product >NOP into nitroxide are practically hindered. Moreover, small amounts of nitroxides are effectively scavenged by the alkyls. Low concentration of nitroxides confirms unequivocally limited degradation in the central part of the plaques. Application of a combination of HAS with UV absorber indicates efficient cooperation between the two types of light stabilizers acting by different stabilization mechanisms as well as the heterogeneity of the degradation process. The latter is affected in the presence of UV absorbers by the lower radiation intensity of the UV light penetrating into the back (not directly irradiated) side of the plaque in comparison with the intensity of the incident light. A lower level of nitroxides than in photooxidized PP plaques characterized by the concentration profile confirming slightly heterogeneous oxidation was detected in plaques
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doped with Tinuvin® 770 and thermally treated in oven. This is a proof of the long term heating stabilization influence of HAS at experimental conditions. Monitoring concentration of nitroxides in PP stabilized with the NOR-HAS Tinuvin® 123 reveals their high concentration already from the beginning of the oxidative stress. This is due to the nitroxides generated during thermal pressing of the plaques. Concentration of nitroxides decreased during the photooxidative stress. Scavenging of nitroxides by polymeric alkyls generated by the phototriggered degradation is assumed. Performed experiments proved the heterogeneous character of oxidation of the PP plaques containing HAS by profiling nitroxide formation using ESRI method.
ACKNOWLEDGMENT The financial support from the Grant Agency of the Academy of Sciences of the Czech Republic (project IAA400500804) and gifts from Ciba Specialty Chemicals (stabilizers) and from TOPAS (Topas®) are acknowledged. We thank Dr. Jiří Pfleger for characterization transparency of the plaques by diffusion transmission measurements and Dana Michálková, MS (both IMC Prague), for preparation of the plaques and Weather-Ometer operation.
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Rabek, J. F.. Polymer Photodegradation: Mechanisms and Experimental Methods. Chapman & Hall, London, England, 1995. Gillen, K. T.; Wise J.; Clough R. L. Polym Degrad Stab. 1995, 47 (1), 149-161. Gijsman P.; Meijers G.; Vitarelli G. Polym Degrad Stab. 1999, 65 (1), 433-441. Pospíšil J.; Nešpůrek S. Prog Polym Sci. 2000, 25 (9), 1261-1335. Pospíšil J.; Nešpůrek S.; Pilař J. Effect of environmental stress and polymer microenvironment on efficiency trials and fate of stabilizers. In Service Life Prediction of Polymeric Materials; Martin, J. W.; Ryntz R. A.; Chin J.; Dickie R. A.; Eds.; Springer, New York, USA, 2009, pp 493-520. Pospíšil J.; Nešpůrek S.; Kruliš Z.; Pilař J. Effect of atmospheric impurities on polymer weathering mechanism and longevity. In Natural and Artificial Ageing of Polymers; Reichert, T.; Ed.; Gesellschaft fűr Umweltsimulation: Pfinztal, Grmany, 2007; pp 125139. Billingham, N. C. The physical chemistry of polymer oxidation and stabilization. In Atmospheric Oxidation and Antioxidants; Vol. IV; Scott, G.; Ed.; Elsevier: Amsterdam, The Netherlands, 1993; pp 219-277. George, G. A.; Celina, M. Homogeneous and heterogeneous oxidation of polypropylene. In Handbook of Polymer Degradation, 2nd Edition; Halim Hamid, S.; Ed.; Dekker, M.: New York, USA, 2000; pp 277-314. Gillen K. T.; Clough, R. L. Techniques for monitoring heterogeneous oxidation of polymers. In Handbook of Polymer Science and Technology, Vol. 2, Performance Properties of Plastics and Elastomers; Cheremisinoff, N. P.; Ed.; Dekker M.: New York, USA, 1989; pp 167-202.
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[10] White, J. R. Plast Rubber Compos Process Appl. 1998, 27 (3), 124-131. [11] Pospíšil, J.; Pilař, J.; Billingham, N. C.; Horák, Z.; Nešpůrek, S. Polym Degrad Stab. 2006, 91 (3), 417-422. [12] White, J. R.; Shyichuk, A. V. Polym Degrad Stab. 2007, 92 (11), 2095-2101. [13] Shyichuk A. V.; White, J. R. Scission and crosslinking rates during photooxidation of thick-sectioned polyolefins: depth profiles for different exposures. In Natural and Artificial Ageing of Polymers; Reichert, T.; Ed.; Gesellschaft fűr Umweltsimulation: Pfinztal, Germany, 2007; pp 175-184. [14] Pospíšil, J.; Nešpůrek, S. Highlights in the inherent chemical activity of polymer stabilizers. In Handbook of Polymer Degradation, 2nd Edition; Halim Hamid, S.; Ed.; Dekker, M.: New York, USA, 2000; pp 191-276. [15] Pospíšil, J. Adv Polym Sci. 1995, 124, 87- 189. [16] Pospíšil; J.; Pilař, J.; Nešpůrek, S. J Vinyl Addit Technol. 2007, 13 (3), 119-132. [17] Pfaendner, R. C R Chimie. 2006, 9 (11-12), 1338-1344. [18] Klemchuk P.; Gande, M. E.; Cordola, E. Polym Degrad Stab. 1990, 27 (1), 65-74. [19] Marek, A.; Labský, J.; Koňák, Č.; Pilař, J.; Schlick, S. Macromolecules 2002, 35 (14), 5517-5528. [20] Wiles, D. M.; Jensen, J. P.; Carlsson, D. J. Pure Appl Chem. 1983. 55 (10), 1651-1659. [21] Gugumus, F. 3rd International Conference on Polymer Photochemistry, Sestri Levante, Italy, September 5-10,1993. [22] Scoponi, N.; Simmino C.; Kaci, M. Polymer 2000, 41 (22), 7969-7980. [23] Motyakin, M. V.; Schlick, S. Macromolecules 2001, 34 (9), 2854-2864. [24] Gerlock, J. L.; Kucherov, A. V.; Smith, C. A. Polym Degrad Stab. 2001, 74 (2), 201210. [25] Lucarini, M.; Pedulli, G. F.; Borzatta, V.; Lelli N. Polym Degrad Stab. 1996, 53 (1), 917. [26] Franchi, P.; Lucarini, M.; Pedulli, G. F.; Borzatta, M.; Vitali M. Macromol Chem Phys. 2001, 202 (7), 1246-1256. [27] Lucarini, M.; Pedulli, G. F.; Motyakin, M. V.; Schlick S. Prog Polym Sci. 2003, 28 (2), 331-340. [28] Marek, A.; Kaprálková, L.; Schmidt, P.; Pfleger, J.; Humlíček, J.; Pospíšil, J.; Pilař, J. Polym Degrad Stab. 2006, 91 (3). 444-458. [29] Motyakin, M. V.; Gerlock, J. L.; Schlick, S. Macromolecules. 1999, 33 (16), 54635467. [30] Berger, H.; Bolsman, T. A. B. M.; Brouwer, D. M. Catalytic inhibition of hydrocarbon autoxidation by secondary amines and nitroxides. In Development in Polymer Stabilization – 6; Scott, G.;Ed.; Applied Science Publishers: London, England, 1983; pp 1-78.
Reviewed by Dr. Jacques Sampers, DSM, Geleen, The Nederlands.
In: Encyclopedia of Polymer Research Ed: Caroline E. Jones
ISBN: 978-1-61761-926-7 © 2011 Nova Science Publishers, Inc.
Chapter 12
A REVIEW OF THE HAZARDOUS EFFECTS OF POLYCYCLIC AROMATIC HYDROCARBONS ON LIVING ORGANISMS Maria Aparecida Marin-Morales1,2∗; Daniela Morais Leme1,2,3; Dânia Elisa Christofoletti Mazzeo1,2 1
Department of Biology, Institute of Biosciences, São Paulo State University, Rio Claro, SP, Brazil 2 Researcher of the Programa de Formação de Recursos Humanos em Geologia e Ciências Ambientais do Setor de Petróleo, Gás and Biocombustíveis – PRH05 (UNESP/MCT/FINEP/ANP) 3 Researcher of the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
ABSTRACT Polycyclic aromatic hydrocarbons (PAHs) are one of the most important, abundant and dangerous class of persistent organic contaminants found in air, water, food and waste sites. The great concern with this class of pollutants is due to the fact that these chemicals cause various deleterious effects, which could jeopardize the genome of exposed organisms or even lead them to death. Many studies have been conducted in several laboratories distributed around the world and with different organisms to quantify the impact of those pollutants and to diagnose their action mechanisms. Thus, there is a real necessity to compile the data already described in the literature, obtained from tests in situ, in vivo and in vitro, derived from simulation experiments, occupational exposure or environmental disasters, in order to produce a report with the main effects already described for that class of pollutant. This review article intends to address the following issues: toxicity, cytotoxicity, genotoxicity, mutagenicity, teratogenicity and carcinogenicity of PAHs, besides information about estrogenicity and metabolism of these substances. In this context, it will be also presented the ∗ Tel: +55 19 3526 4143; Fax: +55 19 3534 0009. E-mail address:
[email protected] (M.A. Marin-Morales).
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test systems that betteresponded to the assessments of the PAHs effects, in an attempt to show the most sensitive and efficient organisms to detect the contamination of these agents.
1. INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are characterized by a diversified group of organic compounds constituted of two or more condensed rings. These substances present a wide distribution in the environment, having their levels already determined in soils, drinking water, rivers, lakes, marine sediments and also in air analyses (Harvey, 1997). PAHs are originated from several sources, being possible find them, mainly, in cigarette smoke, incomplete combustion of organic matter during industrial activities and other human activities such oil refining, fossil fuel burning resulted from vehicle traffic and coal production (Stanley, 1992; Thirman et al., 1994; Mitchell et al., 1999; Annas et al., 2000; Bihari et al., 2006). Composition and complexity of PAHs mixtures depends on the emission sources, which makes available a great variety of these substances, in different concentration levels (Pereira Netto et al., 2000). PAHs present low solubility in water, due to its lipophilic nature and, in consequence of this characteristic, concentrations of these chemicals in sediments can be much higher than from those found in water. However, the low bioavailability of the PAHs present in sediments, reduces, significantly, the toxic potential in this environment. In the air, PAHs tend to react with the particulate matter, which can be the contamination vehicle of other sources such as soil and food, by deposition of these particles in crops, fruits and vegetables (Albers, 2002; WHO, 2000). Due to its wide distribution in the environment, great part of the living beings is exposed to these pollutants, having some of the adverse effects already detected. Detection of PAHs hazardousness to the exposed organisms have characterized these chemicals as one of the most dangerous classes of environmental pollutants (Aina et al., 2006), being also considered as one of the most toxic among all the hydrocarbons (Catoggio, 1991).
2. EFFECTS ON THE LIVING BEINGS Toxic environmental pollutants can cause several alterations in the normal physiologic process of the living beings. To a pollutant exercise its deleterious effects in one exposed organism it must first enter in the individual to after reach its target site (Landis and Yu, 1998). Thus, assessment of the damages caused in the organisms must consider, besides the substance physico-chemical properties, the magnitude, duration and frequency of the exposition, introduction via and the susceptibility of the biological systems, intra and interspecies (Stanley, 1992, 1994; Landis and Yu, 1998; Pedrozo et al., 2002). PAHs are characterized by their high lipophilic affinity, which allows their fast absorption by the biological membranes and, consequently, their accumulation in the tissues (Tuvikene, 1995). Biological membranes are characterized as barriers between the cell and the environment, regulating the passage of toxic agents into the cellular interior and exterior (Stanley, 1992, 1994). Independently from the introduction via used, the chemical compound
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must cross several cellular membranes, until reaching their target site and produce its harmful effect (Pedrozo et al., 2002). Comprehension of the distribution and the storage places of these chemicals inside the organism is another important issue. According to Landis and Yu (1998), organs such as liver, lungs, kidney, bone and adipose tissues are considered as storage deposits. Nevertheless, the relation between the target site of the substance is not always true since the agent can be stored in one determined tissue/organ, but act in another place. In case of the PAHs, due to its lipophilic nature these chemicals are easily stored in adipose tissues (Ballantyne and Sullivan, 1992). Living beings present certain mechanisms that help in the elimination of the toxic substances introduced in their organic system such as the biotransformation process (Guecheva and Henriques, 2003). However, for some pollutants classes such as PAHs, biotransformation can result in a more drastic effect since the reactions conjunct involved in this process lead to a formation of metabolites highly reactive, which become responsible for the toxic effect of the original substances.
2.1. Biotransformation: PAHs Bioactivation Most living beings have the capacity to metabolize xenobiotics by the process denominated biotransformation (Tuvikene, 1995). Biotransformation is characterized as a conjunct of enzymatic reactions, responsible for the conversion of the liposoluble substances in hydrosoluble facilitating, thus, their excretion process. However, although the purpose of the xenobiotics biotransformation is detoxification not always the originated metabolite is less toxic than the own chemical. Thus, xenobiotics biotransformation can increase the toxicity of the chemical products by the formation of electrophilic metabolites, extremely reactive, which can present potential to bind, covalently, with macromolecules inside the cells with DNA, RNA and proteins, which can result in several alterations such as disturbance in the immune system, mutations and even the organism death (Stanley, 1992, 1994; Landis and Yu, 1998; Guecheva and Henriques, 2003). PAHs constitute a group of chemical substances that characterize, perfectly, the situation exposed above. These agents per si not only confer toxic effects to the living beings, but, after the oxidative metabolization process, they become bioactivated, exerting then their deleterious effects (Tuvikene, 1995; Luczyński et al., 2005). The main PAHs metabolization via involves the cytochrome P-450 (CYP) system, which is constituted of mixed function oxidases (MFO). These enzymes are bound to the membrane and present a wide distribution in the organism, with elevated concentration in the endoplasmic reticulum of the liver, kidney, lungs, nasal vias and intestine. Additionally, CYP enzymes are characterized by an ancient proteic family, possibly the bigger ever found in nature, with a wide distribution among bacteria, fungi, plants and animals. It is known, nowadays, that there are about 215 different families of the CYP enzymes being 72 in bacteria, 29 in lower eukaryote, 50 in plants and 67 in higher animals (Guecheva and Henriques, 2003). According to Stegeman and Hahn (1994) and Luczyński et al. (2005), isoenzymes CYP1A1 and CYP1A2 of the subfamily CYP1A are the main responsible for the biotransformation process of the PAHs.
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Biotransformation process, in general occurs in two phases: phase I and phase II reactions. In the phase I, one reactive polar group such as OH, NH2, COOH or SH, is introduced in the molecule of the chemical agent by oxidation, reduction or hydrolysis reactions, generating metabolites highly reactive (bioactivation). In phase II, occurs the detoxification process, where the metabolites generated in phase I are conjugated with endogenous substrates like glycine, cisteine, glutathione, glicuronic acid and sulphates by transferases enzymes, making them hydrophilic and passible of being excreted (Stanley, 1992, 1994; Tuvikene, 1995; Landis and Yu, 1998; Pereira Netto et al., 2000; Pedrozo et al., 2002) (Figure 1).
Figure 1. Representative diagram of the xenobiotic biotransformation.
Monooxigenases of the CYP1A family are the responsible for the PAHs enzymatic oxidation. Firstly, the PAHs are activated in arene oxides, in specific regions by the monooxigenases CYP action. Arene oxides can be both spontaneously isomerized in phenols or converted, enzymatically, to dihydrodiols by the epoxide hydrolases action. Dihydrodiols can suffer new oxidation forming metabolites highly reactive. Thus, this last stage plays an important role in the formation of the carcinogens, which can react together with other metabolites, with DNA, and initiate the malign transformation process of the cell (Jacob, 1996). Among the PAHs known until now, benzo[a]pyrene is, undoubtedly, the most studied, having its metabolic via already reported. In vitro experiments with B[a]P have indicated that dihydrodiols is capable of binding to DNA characterizing more than75% of the metabolites formed by the CYP via (Stegman and Klopper-Sams, 1987; Jimenez and Stegeman, 1990). These experiments have still shown that an increase in the monooxygenases activity, due to exposition to environmental pollutants, can rise the quantity of reactive potentially chemicals that, consequently, can react with DNA, originating the DNA adducts. Identification of the formed metabolites from the PAHs exposition in different groups of living beings is an important issue, since it allows a correlation between the test-system and extrapolation of the obtained results to other organisms. According to Ahokas (1979) and Ronis et al. (1992) microsomes of the trout (Salmo trutta lacustris), starry flounder (Platychthys stellatus) and coho salmon (Oncorhynchus kisutch) liver, metabolize B[a]P in metabolites similar to those produced by rat liver microsome. The main metabolites produced
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by fish microsome are 7,8 and 9,10-dihydrodiols, which are precursor of the main B[a]P carcinogenic metabolites. Many are the metabolites originated by the PAHs biotransformation. As already exposed, some of these metabolites can be excreted by the organism or even not be reactive to the organisms not conferring, therefore, hazard to the exposed organism. However, many of them, when transformed can cause several alterations in the biological system and put in danger individuals, populations or even entire ecosystems exposed to their action.
2.2. Toxicity Toxicity tests are reliable techniques to predict the impacts of contaminant agents over several communities of various environments. With these tests it is possible to determine in which fractions each substance can be available to exercise toxic effects on the organisms. Chemical compounds of anthropogenic origin suffer several interactions and can act in many ways on the exposed communities. Toxicity tests can be developed in laboratories (in vivo or in vitro) or in field (in situ) and use several methodologies since the more basic such as the dose answer curve, observation of behavior, mental, physiologic and lethal changes, even more sophisticated methods such as biochemical, cytological, genetic and teratogenic alterations. The observed effects also vary according to the characteristic of the used methodology, the biology of the specie and the type of the used compound. The PAHs most common action mechanism that result in toxicity for the organisms is related with the interference in the cellular membranes function by the bond in the membrane hydrophobic sites, causing deformity and molecular perturbance, besides alterations in the enzymatic system (Tuvikene, 1995). As a consequence of these bonds, the membrane permeability is altered and the substances transport in and out the cells is damaged. Moreover, as many enzymatic systems are associated with structure of the membrane, other essential process for the cells can be damaged and the normal energy balance becomes debilitated (Knutzen, 1995). Although the PAHs can result in a direct toxic effect, the major concern resides in the formation of reactive metabolites such as epoxides and dihydrodiols, which can bind to the genetic material and to the cellular proteins (Albers, 2002). PAHs with high molecular weight do not result in high toxicity for the organisms, but the effect can be potentialized by exposition to the ultraviolet solar radiation (Arfsten et al., 1996), which leads to the formation of new photoproducts with toxicological properties different from the original molecule (Huang et al., 1993; Krylov et al., 1997). The absorption of light energy lead the PAHs molecules to an excited state, whose energy can be transferred to biological molecules, generating with this, the reactive oxygen species (ROS), intermediary reactive, free radicals or photo-metabolite that can damage these cellular constituents, resulting in toxicity (Betowski et al., 2002; Yu, 2002). Chemicals potentially dangerous such as the PAHs and their metabolites, endanger not only human health, but also aquatic and terrestrial ecosystems by affecting, directly, the soil biota or, after lixiviation and percolation, the aquatic biota (Bispo et al., 1999). PAHs affect the organisms in different ways, since some species are more sensitive to these contaminants than others. Thus, for a better comprehension of the PAHs toxic potentiality it must be considered their effects over several species, in different trophic levels,
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and in all life stages. Besides this, an important factor to be considered ís the investigation of the movement of these molecules in the environment and their action mode on the organisms. The PAHs toxic effect and action mode on plants are not very known and seem to vary according to the biologic specie and the PAH own chemical constitution (Wittig et al., 2003; Baek et al., 2004). Smith et al. (2006) studied the toxic effects of soils contaminated with PAHs on the germination and growth of four graminaceous species and on three leguminous species. No significative effect was observed for the germination index of the studied species during the ten days of exposition, however, the subsequent growth was significantly reduced being this effect more pronounced in the leguminous. The toxicity of soils contaminated with PAHs was evaluated by laboratory assays (in vivo test), using four different plant species (Zea mays L., Hordeum vulgare L., Sinapis alba L. and Helianthus annus L.). The authors considered that the root length was the parameter that best answered to the toxicity tests, being barley (Hordeum vulgare L.) the most sensitive specie among all the species tested, which indicates that this specie can be characterized as an efficient test-organism for PAHs toxicity assay (Smreczak and Maliszewska-Kordybach, 2003). Studies carried out with Brassica napus (canola), to assess the toxic and phototoxic effects of 3 PAHs, have shown that these compounds present direct toxic action on the roots growth, not being transported to the other organs of the plant (Ren et al., 1996). The authors have also concluded that, between the tested PAHs, anthracene was the most toxic, followed by B[a]P and fluoranthene. This same toxicity order was maintained when these hydrocarbons were exposed to light. In order to detect the toxicity of sediment samples of the Rovinj marine coast, in Croatia, contaminated with PAHs, Bihari et al. (2006) used the Microtox® test with the marine bacteria Vibrio fisheri. All the tested sediments were potentially toxic, nevertheless the PAHs were not considered the most toxic chemicals present. The toxicity of lixiviate extracts of soils contaminated with PAHs extracted by water and methanol was evaluated by assays with the Vibrio fischeri bacteria (Microtox® test), with the alga Pseudokirchneriella subcapitata and with micro crustaceans Daphnia magna and Thamnocephalus platyurus. The toxicity of the lixiviated soils with water was much lower than the extracts lixiviated with methanol, mainly, for the tests carried out with bacteria and alga, which were considered by the authors as the organisms more suitable for the detection of the toxicity of soils contaminated by PAHs (Bispo et al., 1999). Erstfeld and Snow-Aihbrook (1999) assessed the chronic effect of soil contaminated with low PAHs levels on the soil microfauna (represented by the nematode), on the mesofauna, (by the microarthropod of the Collembola and Acarina orders) and on macrofauna (by the earthworm Eisenia fetida andrei). The observed PAHs contamination levels (5.28 to 80.46 mg/Kg) seem to have a stimulatory effect on the growth of the invertebrates population present in the soil. According to the authors, this answer can be due to the increase in the microbial population that is capable of metabolizing the PAHs, breaking into smaller molecules that can be easily assimilated by the soil biota. Saint-Denis et al. (1999) studied the effect of different concentrations and different exposition times to PAHs, on the earthworm Eisenia fetida andrei. According to the authors, this chemical was metabolized, both by the CYP via, and by the free radicals oxidation via,
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promoting the generation of free radicals that can stimulate the lipids peroxidation, resulting in toxicity for this organism. PAHs tend to accumulate in the bottom sediments, which can become a constant source of toxic contaminants putting in danger, continuously, the biota health and viability, mainly, of the benthic organisms that suffer constant expositions to PAHs by the direct contact with the soil (Bihari et al., 2006; Boscolo et al., 2007). Due to the fast accumulation and to the small metabolization capacity the bivalve mollusks have been used, frequently, as sentinel organisms for the PAHs retention (Baussant et al., 2001). According to Boscolo et al. (2007), the benthic mollusk specie Tapes philippinarum is a good indicator of the PAH contamination, because it concentrates and tolerates high levels of these chemicals in its tissues. PAHs sub-lethal toxic effects were registered for the mollusk Tapes semidecussatus, that were exposed to sediments containing high levels of these chemical substances (Byrne and O’Halloran, 2004). Toxicity tests are very useful tools to identify the possible impacts of the PAHs on aquatic organisms. Lethality tests are considered good indicators of the toxicity of different chemicals and for different species, while the sub-lethality tests are interesting since they consider the impacts on the population by evaluating the effects on the organisms reproduction and growth (Wake, 2005). Individual toxicity of the PAHs present in aquatic environments, for plants and animals, increase when the molecular weight of these substances are next to 202 (e.g. fluoranthene, pyrene). Values superior to this weight configure a less soluble molecule, which promotes a reduction in the PAHs concentration to lower levels than of the considered lethal (Albers, 2002). However, exposition to PAHs with high molecular weight even in small concentrations, can lead to sub-lethal effects to the organisms (Neff, 1985). Kim et al. (2007) analyzed the PAHs toxic action on the genic expresion of the genes recA (related with DNA repair), KatG (that responds to oxidative stress), grpE (that is expressed in response to damages in the proteins) and fabA (that responds to damages in the membrane) of Escherichia coli, using in real time PCR (RT-PCR). The analyses showed that naphthalene and B[a]P caused damages in the E. coli DNA, while phenanthrene did not present any significative effect on the expression of these genes. Reece and Burks (1985) observed that the 11 PAHs present in petroleum refinery effluents caused lethal acute effects in Daphnia magna and that these substances were acting in an additive or synergistic way. Holst and Giesy (1989) studied the chronic effect of the exposition to anthracene and its photoproducts on the cladoceran Daphnia magna reproduction. In the experiments carried out both in the absence and in the presence of the UVR light, there was a reduction in the neonate number and an increase in the mortality number but these effects were more conspicuous when there was exposition to the light. An important result was observed by Allred and Giesy (1985), in studies about the anthracene phototoxicity on Daphnia pulex. The exposition of these organisms to anthraquinone, which is the first and more stable photoproduct of the anthracene, did not result in toxicity but in the experiments where the animals were previously treated with anthracene in the presence of light and after were transferred to water, the toxicity was significative. By the results, the authors suggest that the toxicity was due to the activation, by the light energy, of the own anthracene molecule that binds to the molecules that are
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constituents of the D. pulex cells, and not of the metabolites of this substance present in the medium. The contaminants bond to specific receptors alters the metabolic and functional capacities of the tissues, by the induction or suppression of important proteins, causing harmful alterations to the organism. Thus, the induction or suppression of proteins can be used as biomarkers of effect and/or exposition (Roos et al., 2004). Bellas et al. (2008) studied the toxic and phototoxic potentials of 6 PAHs, individually and in mixtures, on embryos and larvae of mussels, sea urchin and ascidian. All the PAHs were toxic in the tested concentrations for sea urchin embryos and all, with the exception of fluoranthene, were toxic for mussels embryos. In relation to the ascidian embryos, only naphthalene presented significative toxic effect. However, exposition to fluorescent light increased the toxicity of phenanthrene, fluoranthene, pyrene and hydroxypyrene but not of the naphthalene and fluorene. The PAHs mixture did not present additive effect of the toxicity. B[a]P and anthracene effects, on the activity of several enzymes of the fish Pomatoschistus microps, was evaluated by Vieira et al. (2008). After 96h of exposition to several concentrations, it was observed that these substances inhibited acetylcholinesterase, leading to an interruption of the cholinergic transmission and induced the activity of lactate dehydrogenase, catalase, superoxide dismutase, gluthatione peroxidase and gluthationereductase. It was observed an induction of the glutathione S-transferase activity in fishes exposed to B[a]P and an inhibition of this enzyme after exposition to anthracene, suggesting an involvement of glutathione S-transferase in the B[a]P detoxification. Studies carried out by McCloskey and Oris (1993) showed an increase of the hematocrit, evidences of hemolysis and a decrease in the presence of hemoglobin in the blood, besides a significative enzymatic inhibition of Na, K-ATPase and Mg-ATPase in the gills of the bluegill fish, after 96 hours of exposition to anthracene and ultraviolet solar radiation. The authors concluded that this toxicity is associated with a perturbation in the cellular membrane functions, promoted by anthracene. It was evaluated the PAHs effects on the neotropical fish Oreochromis mossambicus, by enzymatic bioindicators. Expositions carried out with low concentrations of PAH (<0.44µg/g) led to sub-lethal toxic effects, observed in the liver of this organism, identified by the increase of the activity of the serum sorbitol dehydrogenase enzyme, which is characterized to be an indicator of damages in the liver. In this way, the authors concluded that the quantification of the EROD (ethoxyresorufin-O-deethylase) and serum sorbitol dehydrogenase enzymes characterize to be an excellent tool to monitor sub-lethal effects in fishes, resulted from pollution by PAHs (Shailaja and D’Silva, 2003). Ke et al. (2007) studied the toxic effects of the water of a lake contaminated with PAHs, using an in vivo test with crucian carp (Carassius auratus) and in vitro test with rat hepatoma cells H4IIE. PAHs induced activity of EROD presenting a good correlation between the in vivo and in vitro assays, indicating that there was an activation of the enzyme CYP1A1 by the aryl hydrocarbon receptor (AhR). Previous studies carried out with sediments contaminated with PAHs of the same lake (Meiliang Bay), also showed a significative induction of the EROD activities in rat cells H4IIE maintained in culture (Qiao et al., 2006). Ingestion of contaminated soils with PAHs by rats resulted in an increase of the CYP1A1 levels in the duodenum, liver, kidney, lung and spleen, indicating that these contaminants can cross the intestinal barrier and be distributed to other regions of the body (Roos et al., 2004). B[a]P ingestion by mice resulted in acute effect in the hematopoietic system leading to a
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drastic alteration in the bone marrow with the almost complete destruction of the pluripotent cells (Anselstetter and Heimpel, 1986). The target organs of the PAHs toxic action in mammals are the skin, intestine, kidney and mammary gland, beyond the hematopoietic and lymphoid tissues, immunological system and gametic tissues (Albers, 2002). PAHs phototoxic effects difficultly are observed in mammals since the skin confers a barrier against the penetration of the UV rays, blocking the absorption of the energy light by the PAHs molecules (Arfsten et al., 1996). However, according to Kornhauser et al. (1991), phototoxicity generally results in direct effects in the skin such as allergic reactions, dermatitis and erythemas. Oral exposition to high doses of PAHs provokes several hematological effects in mammals. Aplastic anaemia, pancytopenia, drastic reduction in the leukocyte of the peripheral blood and severe diminishing in the production of red globules are effects resulted from the mammals exposition to PAHs (WHO, 2000). Several studies have demonstrated that PAHs, principally those found in cigarettes, are strongly related with the incidence of cardiovascular diseases in humans (Korashy and ElKadi, 2006). In humans, acute intoxication by PAHs is related with expositions to concentrations around 100 to 200 ppm. These concentrations promote nonspecific clinic signs and symptoms such as dizziness, nausea, headaches and difficulties in the motor coordination, by the action of these substances in the nervous system. Chronis effects can lead to toxic manifestations in several organs and systems, mainly, hematopoietic, renal, hepatic and nervous (Pedrozo et al., 2002). The effects of the B[a]P inhalation by humans result in patch opacities, prominent bronchiovascular markings and pleural effusions, besides vomits with blood, problems in the respiration, pain and irritation on the chest, irritation in the throat and cough (Gupta et al. 1993). Studies of several environmental matrices such as combustion process in general, cigarette smoke, petroleum and its derivatives, among others, have demonstrated that PAHs, present in these mixtures, are the principal responsible for the toxicity potential. As aforementioned, several studies have related adverse effects produced by the PAHs, specifically their toxic effects, but still arise the necessity to establish new techniques to this methodologic set already existent and to intensify the studies of these contaminants actions, in the attempt to solve problems resulted from their impacts on the environment and on the human health.
2.3. Cytotoxicity Cytotoxicity is characterized by the induction of damages in the cellular structure and/or function that can lead to disturbances in the cell cycle and in its physiology resulting in, many times, in the cellular death. The PAHs cytotoxic activity has been described in many studies carried out with several organisms including man. Many assays, principally in vitro tests, have been developed to evaluate the cytotoxic effects and action of the PAHs on cells. The knowledge of the chemical properties of the molecules of these hydrocarbons is very important to the comprehension of their action mechanisms on the cells.
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Studies carried out with microorganisms exposed to PAHs showed that the majority of these compounds, when present in small concentrations (5 to 100 ppb) in the environment, can stimulate or inhibit the cell division of bacteria and algae. In higher concentrations (0.2 to 10 ppm) these chemicals interfere, negatively, on the cell division of these organisms being capable of even leading them to death (Albers, 2002). Alkio et al. (2005) also proved that PAHs can promote death in plants. According to the authors, phenanthrene induced cell death in Arabidopsis thaliana, observed by the cytotoxicity evaluation with the trypan blue dye. The number of cells in cellular death process was dependent on the concentration and exposition time to phenanthrene. The authors suggested an inhibition in the mitotic division process, by the observation of the reduction in growth of the stem and root. According to Leme et al. (2008), mixtures of PAHs, present in water of rivers impacted by petroleum, can induce cell death in the meristematic cells of root tips of the plant Allium cepa. Babich and Borenfreund (1987), when studying the in vitro cytotoxicity of several PAHs in the fish bluegill BF-2 cells, observed a low cytotxicity of these substances for the tested cells. However, when the S-9 microsomal fraction was incorporated to the mixture, they identified a significant increase of the cytotoxicity that, according to the authors, was due to the formation of active metabolites. Another factor described as important to the increase of the toxicity of the PAHs was the temperature that, according to them, potentiated the cytotoxic effects of the PAHs for this culture. The direct cytotoxic effect of sixteen PAHs was evaluated by Schirmer et al. (1998a) in a cell line of gills of the rainbow trout (RT Gill-W1) by assays with fluorescent dyes. Only the PAHs composed by 2 or 3 benzene rings (naphthalene, acenaphthylene, acenaphthene, fluorene and phenanthrene) presented cytotoxic effects on the cells without metabolic activation. In this way, the authors suggest that factors such as solubility in water and the lipophilic property are characteristics directly related with the cytotoxicity of the PAHs, which make them capable of altering the integrity of the cellular membrane when they are accumulated in it. The capacity of a chemical to induce damages or cause cell death as a result of photochemical reactions, known as photocytotoxicity (Schirmer et al., 1997) was evaluated for PAHs in assays with RT Gill-W1 cells. Of the 16 PAHS tested by Schirmer et al. (1998b), thirteen were photocytotoxic for the cited cells. Nevertheless, tests carried out with fish by the same authors showed that only the fluoranthene and pyrene offered risks to these organisms in the environment, since the phototoxic potential for fishes were observed in concentrations below of their solubility in water. According to Schirmer et al. (1998a), naphthalene is a PAH potent in inducing direct cytotoxicity in fishes since it was also capable of causing cellular death even when it was found in concentrations below its solubility limit, being considered dangerous in environmental contaminations. The cytotoxicity of anthracene (Oris and Giesy, 1985) and fluoranthene (Weinstein et al., 1997), for fishes, was evaluated by assays developed with dorsal epidermis and gill epithelium since these systems were considered by these authors as good indicators of the photocytotoxicity induced by PAHs. The PAHs induced cytotoxic alterations in Chinese hamster V79 cells, having their potentiality enhanced when these agents were exposed to the light (Utesch et al., 1996). Studies report that B[a]P promoted cytotoxic alterations in the cell cycle of the Chinese hamster V79 cells maintained in culture. According to Ochi et al. (1985), these cells are
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highly susceptible to the B[a]P cytotoxic action, when they are found in the G1/S phases or beginning of the S phase but these action decrease when the cell cycle progress. According to Pandey et al. (2006), the B[a]P cytotoxicity is dependent on the activation of the CYP. When this activation in the cell culture does not happen by the absence of monooxygenases activity no cytotoxic effect is observed (Babich et al., 1988; Lee et al., 1993). PAHs extracted from a mixture of heavy oil were tested as regards to their potential in causing interruption of the cellular mitotic division process in lung cells of V79 Chinese hamster (Hadnagy et al., 1997). These agents showed a strong dose-dependent relation in relation to the inhibition of the mitotic index, being considered potentially cytotoxic. Besides this, these authors observed the presence of alterations in the index of the division phases, quantifying an increase of cells in the metaphase phase and a decrease in cells in the anaphase and telophase phases. Hader et al. (1996) also reported a cytotoxic effect of the PAHs in V79 cells synchronized in a same cell division phase. After the treatment, 50% of the analyzed cells were in the G2/M phase. This high percentage in this division phase can be associated with an inhibition of the progress of the mitotic activity of these cells. Detmar et al. (2008), when analyzing the effects of the PAHs present in the cigarette smoke on placenta cells of murine observed alterations in the cell death rates (decrease of the rates) and changes in the expression and in activity of the key proteins of the physiologic control of cell death (increase in the expression of the antiapoptotic levels and decrease of the proapoptotic levels). According to the authors, several PAHs are ligands of the aryl hydrocarbon receptors (AhR), a cellular sensor of xenbiotics responsible for the activation of the metabolic machinery that is involved in innumerous cellular process such as death, survival and cellular stress answers. Studies showed that the induction of cytotoxicity in the bone marrow of mice caused by anthracene was dependent on the metabolic activation of the CYP1B1 (Heidel et al., 2000). Hiura et al. (1999) suggested that organic compounds, including the PAHs, present in diesel exhaust particles can induce apoptosis in macrophages of rats and man, resulting from the formation of the reactive oxygen species. Chou et al. (2003) observed that the PAHs present in jet fuels presented a high cytotoxicity in keratinocytes of the human epidermis leading to the death of almost all cells present in the culture after 24 hours of exposition, even when in concentrations smaller than 0.5%. The authors concluded that the increase in the cytotoxicity is related with the increase in number and size of the sequence of the aromatic rings chains. Among the sixteen PAHs considered priority pollutants by the United States Environmental Protection Agency (USEPA), nine were considered by Wang et al. (2007) as highly photocytotoxic for keratinocytes of human skin (HaCaT). These researchers concluded that the phototoxicity of the PAHs is dependent on the molecule size and on the rings arrangement, molecules with four or more benzene rings (except chrysene) present a higher photocytotoxic potential. Frying oil, when reused several times, can also be a source of PAHs and cause damages to the cell. Pandey et al. (2006) identified nine different types of PAHs in the reused frying oil. This study suggests that the reused oil present a significative cytotoxic effect for the human hepatoma cell line (HepG2) by the induction of the catalytic activity of the enzime CYP1A1 leading to a metabolic activation of the present PAHs.
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By the neutral red assay in vitro in HepG2, it was possible to evaluate the effects of the PAHs over the cell viability (Babich, 1988). The cytotoxic potential was induced by B[a]P, benzo[k]fluoranthene, benzo[b]fluoranthene, chrysene, benzo[a]anthracene, pyrene, phenanthrene, fluoranthene but, however, the fluorene, anthracene, acenaphthene and acenaphthylene were not cytotoxic for the same cells.
2.4. Genotoxicity and Mutagenicity Genotoxic agents are characterized as agents capable of inducing primary lesions on DNA, such as formation of DNA adducts, oxidation and dimerization of nitrogenous bases (Majer et al., 2005). These effects are considered transitory since they are susceptible to be repaired, in other words, they do not characterize permanent alterations in the content or structure of the genetic material of an organism (Dearfield et al., 2002). Nevertheless, if these effects are not repaired or wrongly repaired, they can progress to permanent lesions in the DNA molecule, named mutations, which characterize the mutagenic events. Several chemical agents are considered genotoxic and mutagenic. Amongst them there are the alkylanting agents, the intercalating and the reactive oxygen species (ROS). The alkylating agents are defined, briefly, as electrophilic agents that react with DNA substituting one hydrogen atom for an alkyl group resulting in the modification of this base (Stanley, 1994). In general, in the repair process of this lesion, DNA fragmentation, induction of mutations and formation of nucleotidic bridges can occur. Adducts are defined as molecules that bind covalently to the DNA or other molecules, becoming strongly attached to them. The DNA alkylation with a posterior covalent bond can form DNA adducts, which can influence, crucially, the biological process contributing for the toxic and mutagenic effects (Friedberg et al., 1995; Sturla, 2007). The ROS, resulted from the oxidative stress, can have their production induced by several chemical agents present in the environment (Picada et al., 2003). According to Collins et al. (1997), ROS are involved in the carcinogenesis process and act on the DNA molecules. The PAHs are substances capable of promoting several deleterious effects on organisms. The genotoxicity and mutagenicity of the PAHs can be established by different ways such as the adducts formation originated by the binding of their metabolites, resulted from the biotransformation via CYP with the DNA; by the ROS production, from the quinone reaction, product of the PAHs metabolization via CYP, with the O2; among others. For the detection of environmental genotoxicity and mutagenicity various short-term tests, using a great variety of organisms, have been developed. The use of this great diversity of test organisms is possible due to the high conservation of the DNA structure (Majer et al., 2005). Such characteristic is also relevant for correlation studies between the test-systems since it makes possible the extrapolation, in a cautiously way, of the results obtained in one test-system to other living beings. Great part of the environmental samples is defined as complex mixtures since they are constituted by a diversity of chemical agents potentially dangerous. The evaluation of the pollutants present in complex mixtures is complicated due to the difficulty in recognizing the chemical group harmful to the organisms or the synergistic effect exercised by the present substances. Nonetheless, the evaluation of the complex sample is extremely important since the obtained results are more faithful to the reality of the environmental expositions.
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For White (2002), the non-identification of the toxic compounds present in complex environmental samples, the paucity of knowledge of the chemical behavior of genotoxic substances present in environmental samples and the lack of scientific data, characterize a great difficulty in the risk evaluation that these substances can cause to the organisms. This author assessed the effects of PAHs present in complex mixtures estimating the genotoxicity of the PAHs individually and in 13 combinations of more than 14 PAHs by the SOS chromotest, which is based in the evaluation of the SOS functions by monitoring the levels of the enzyme ß-galactosidase in Escherichia coli lines with the fusion of the operon sulA::lacZ. The obtained results showed the highest potentiality of the mutagenic effects for PAHs mixtures, probably, equal or inferior to the sum of the hazards observed for each compound of the mixture analyzed individually. Down to the broad distribution of the PAHs in the environment, many researchers have demonstrated a special preoccupation with the contamination by these pollutants in aquatic ecosystems since, being water an essential asset to life, any alteration in the hydric resources can expose countless organisms even those that do not have their life restricted to this environment. Chromosome aberrations (CA) are characterized by changes in the normal structure of a chromosome or in the total number of chromosomes that can happen spontaneously or as result of exposition to physical or chemical agents (Russel, 2002). In the last 30 years, the CA test has been widely used in environmental and occupational monitoring, as biomarker of the genotoxic effects induced by carcinogens (Hagmar et al., 2004). For the assessment of the genotoxic influence of a petroleum field in Nigeria, Odeigah et al. (1997) applied the CA test on the plant test system A. cepa. The authors showed that the analyzed effluent was capable of inducing CA in the meristematic cells of the A. cepa roots, even after carrying out several dilutions of the sample. The micronucleus test (MN) is considered by many authors as one of the most promising techniques of evaluating the mutagenic effects induced by chemical agents (Landolt and Kocan, 1983; Heddle et al., 1983; Ribeiro, 2003; Matsumoto et al., 2006). Such event is due to the fact that MN are resulted from damages, repaired or wrongly repaired, in the parental cells (Ribeiro, 2003) being easily visualized in the daughter cells as a structure similar to the main nucleus, however, with a reduced size. Leme and Marin-Morales (2008) evaluated the impact of petroleum hydrocarbons in a river of the North coast of São Paulo State-Brazil, after the occurrence of an oil pipeline leakage. In this work it was used the CA and MN test in two different cell types of A. cepa (meristematic cells and F1) with the aim to evaluate if the genotoxic effects observed by the presence of CA in the meristematic cells progressed to mutagenic effects (presence of MN in F1 cells). The authors carried out, together with the bioassays, chemical analyses for total petroleum hydrocarbons (TPHs) and PAHs, where they could establish a positive correlation between CA and MN with the exposition to the samples contaminated with PAHs indicating a genotoxic and mutagenic action of the PAHs. These data are also corroborated by other work carried out by Leme et al. (2008), where the authors showed that petroleum, being a complex mixture of hydrocarbons, can present both a clastogenic and an aneugenic action or even induce cellular death on meristematic cells of A. cepa. For Leme et al. (2008), these actions are, principally, due to the presence of PAHs detected by chemical analyses in the tested sample.
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Çavaş and Ergene-Gözükara (2005) applied the MN test in the Oreochromis niloticus species (Pisces: Perciformes), for the assessment of a petroleum refinery effluent and a chromium processing plant. In this work, it was evaluated the presence of MN in epithelial cells of gills and erythrocytes of peripheral blood, after exposition of the fishes to different concentrations of the effluent (5, 10 and 20% v/v) for a period of 3, 6 and 9 days. The authors reported mutagenicity for both tested effluents, although the mutagenic effects observed were much higher for the petroleum refinery effluent than for the chromium processing plant, this fact can be related to the presence of PAHs in the refinery effluent samples. The authors also show a higher sensitivity of the MN test in gills, probably, due to a higher direct exposition to the environmental contaminants. Another important question reported by the authors is the values, detected by the chemical analyses, of the pollutants present in the studied effluents. Albeit the visualized levels showed to be inferior of the established by the Turkish Environmental Standards, both tested effluents were considered harmful in relation to the mutagenicity. In this way, the authors believe that the mutagenicity tests must be included in the standards for the effluents emission in water bodies for a better evaluation of the hazard caused to the living beings. The comet assay consists, particularly, in a valuable and cheap technique since it allows the detection of different cellular answers resulting from DNA damages and repair, which can be observed by the migration, in electrophoresis, of any cellular type obtained from a cell suspension (Sasaki et al., 1997, Mitchelmore and Chipman, 1998). Due to a great occurrence of petroleum and its derivatives spills, that impact the Brazilian rivers, and the paucity of information about the answers of neotropical fishes to these situations, Vanzella et al. (2007) developed laboratorial tests with Prochilodus lineatus exposing these fishes to diesel water soluble fraction (DWSF) compound that has the PAHs as one of its main constituents. P. lineatus specimens were exposed to DWSF for 6, 24 and 96 hours, in order to characterize the acute effect and for 15 days, for a sub-chronic answer. By the obtained results, the authors concluded that both expositions to DWSF, acute and subchronic, cause genotoxic and mutagenic damages in P. lineatus blood cells. The authors also affirm that the combination of the comet assay and MN test techniques was sensitive and adequate for the evaluation of diesel genotoxicity, due to the complementary answer generated by these two methodologies. Hoshina et al. (2008) carried out the MN test in O. niloticus (Nile tilapia) erythrocytes, aiming to assess the efficiency of the treatments performed in a petroleum refinery effluent and the quality of the river that receives the effluent. The authors collected effluents samples in different phases of the industrial process, such as after the physico-chemical treatment, entrance of the stabilization pond (after the biological treatment), exit of the pond (final effluent), upstream and downstream of the effluent discharge. With the collected water samples, the authors exposed O. niloticus specimens for a period of 72 hours. It was observed by the realized assays that the treatments used by the refinery promoted a diminishing in the effluent mutagenic potential, however, these treatments were not totally efficient, since the final effluent still presented a potential to induct damages in the genetic material of the test organism. This can be explained, by Tatem et al. (1978) citation, which says that refinery effluents tend to present a smaller quantity of light hydrocarbons than the crude oil, but a higher quantity of PAHs.
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The marine environment has been a constant target of PAHs pollution, mainly by the accidental petroleum spills and its derivatives and by the routinely contamination by fuel oils such as diesel caused by the marine traffic. The evaluation of complex mixtures is a problematic issue, because it is difficult to identify which chemical is the responsible for the deleterious effect or recognize a synergistic effect, among the several pollutants present in the mixture. Thus, it is not easy to establish a relation between the harmful effects and the contamination by any agent, including the PAHs. Akcha et al. (2003), to assess the genotoxic effects of the marine contamination, used the comet assay and the DNA adducts detection (nuclease P1-enhanced post-labelling technique) in hepatic tissue of female dab specimens (Limanda limanda), collected in different sites of the English Channel (France) east region, area known by the PAHs contamination and by polychlorinated biphenyl (PCBs). By the obtained results, the authors concluded that the comet assay is a sensitive test for environmental biomonitoring since it was observed a concordance between the percentage of the comet tail size and the degree of chemical contamination in the collection sites. In other conclusion, the authors affirm that the observed DNA adducts were not related with the exposition to PAHs, due to the knowledge of the diol epoxides migration pattern, metabolites highly electrophilics induced in dab after exposition to B[a]P, substance that was not present in any site. Taban et al. (2004) observed, by the comet assay carried out with mussels (Mytilus edulis L.) and sea urchin (Strongylocentrotus droebachiensis), an increase in the percentage of the comet tail size, in relation to the negative control, after 4-5 days of exposition of the organisms to the crude oil. These data indicated that the comet assay is a good tool to biomonitor damages in the DNA of marine invertebrates exposed to oil contamination. Bolognesi et al. (2006), aiming to evaluate the genotoxic risk, in a long term, caused by oil spills, used as a case study the Haven oil spill that affected all the Ligurian coast, in Italy. For this, the authors assessed the damages in the DNA and the MN frequency in several testorganisms such as mussels, oysters and benthic fishes. By the obtained results, it was observed that even after several years of the accident occurrence the genotoxic effects persisted in different levels and in different living beings indicating the existence of residual sources of mutagenic and carcinogenic agents in the local. To investigate the effects of crude oil contamination on native marine fishes, Baršienė et al. (2006), applied the MN test in peripheral blood erythrocytes and in cephalic kidney cells of the turbot (Scophthalmus maximus) and of the Atlantic cod (Gadus morua) exposed, in in vivo simulations, to this pollutant. Besides the exposition to crude oil, the authors also carried out assays with the addition of alkylphenol and PAHs into the oil with the objective to estimate the potential risk caused by the produced polluted water from the petroleum platforms since the presence of these chemicals is described in these areas. The results obtained in this work indicated a mutagenicity caused by the exposition to crude oil in both cellular types and species of fishes tested. The authors reported that the MN frequency of the cephalic kidney cells was two times higher than of the peripheral blood erythrocytes, which can be due to the fact that the cephalic kidney is the main hematopoietic tissue in teleost fishes. The authors also showed that the addition of alkylphenol and PAHs elevated, significantly, the MN frequency indicating an increase in the induced mutagenicity. The food chain is characterized as one of the contamination via by PAHs and it has been a preoccupation focus, mainly, due to the human exposition potentiality. In this way, with the purpose the evaluate the PAHs contamination by the food via, Lemiere et al. (2005), carried
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out a study in order to estimate the damages in mammals DNA resulted from the trophic via by the ingestion of mussels contaminated with oil (Erika ship spill, France). Contaminated mussels were offered in rats daily food supply for a period of two to four weeks. DNA damage was evaluated by the comet assay carried out with hepatic, blood and bone marrow cells of rats treated with different PAHs contamination levels (312 µg, 569 µg and 870 µg of 16 PAHs/kg dry weight). The results showed a dose-effect-time relation between the quantity of DNA damage in the rats hepatic and hematopoietic cells and the PAHs contamination levels of the mussels. The genotoxic effects of the highest tested concentrations were growing between the 15th and 30th day of the treatment. According to the authors, it seems that there is efficiency in the DNA repair capacity for lower concentrations of PAHs since there was no induction of significative damages in the hepatic and hematopoietic cells of the exposed rats to lower concentrations of PAHs for a 30 days period, opposite of the observed for the 15 days. It was concluded by this work that foods contaminated with oil can cause damages to the DNA of organisms that are in superior trophic levels by the transference via food chain. The evaluation of sediments and soils contaminated with PAHs is very important since these compartments have been characterized as pollutants deposits, which can characterize themselves in potential sources of dangerous chemicals. Many works carried out with several test-organisms have been developed in order to estimate the PAHs effects present in soils and sediments, on exposed organisms (Bispo et al., 1999; Békaert et al., 1999; Cotelle et al., 1999; Monarca et al., 2002; Plaza et al., 2005; Bihari et al., 2006). Bispo et al. (1999) used the Mutatox® test to evaluate the genotoxicity of industrial soils impacted by PAHs. Such test uses the mutant bacteria Vibrio fischeri M169 as test-organism and is based on the light emission given by the bacteria reversion to the original form thus indicating the occurrence of damages in the DNA. In this work, the bioassays were carried out with the lixiviate extract and solvent extract (specific to assess pollutants of low solubility in water) of three soil samples contaminated with PAHs. According to the authors, the different soil samples presented genotoxicity even in the absence of the exogenous metabolization system (S9 mixture), resulted from the presence of molecular structures such as the quinones and diol epoxides formed during the PAHs biodegradation process in the soil. Békaert et al. (1999) carried out a study to evaluate the genotoxicity of aqueous lixiviate of a soil contaminated with PAHs and heavy metals, by the Ames and Mutatox® in vitro tests, and the MN test in vivo in amphibians of the Xenopus laevis species. The authors observed by the three tests that this contaminated soil can cause damages to the aquatic environment. The authors affirmed that biological tests are important complementary tools for the physicochemical analyses since it allows the evaluation of the chemical substances biodisponibility and bioaccumulation, besides showing the possible interaction between the pollutants. Békaert et al. (1999) also alert for the tests that require previous filtration process of the samples before its execution. The authors affirm that this procedure can masquerade the real results by the loss of some pollutants that are important during the preparation process of the sample. The authors affirm that 0.45 µm membranes can already eliminate many micropollutants potentially genotoxic. Monarca et al. (2002) observed by the Ames test and MN test in Tradescantia that soil samples of an area highly industrialized in Italy presented mutagenic activity that, according to the authors, can be related with high concentrations of PAHs and heavy metals detected in the chemical analyses of these samples.
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Evaluation of contaminated soils by petroleum hydrocarbons, followed by bioremediation by the biopile aerobic process, developed by bacteria assays of short-term, SOS chromotest and umu-test by Plaza et al. (2005) showed that this bioremediation process can reduce in almost 81% the quantity of petroleum hydrocarbons, including the PAHs present in the samples that led to an absence of genotoxic effects for the analyzed soils, both for the tests with the presence of S9 and with absence of this metabolization system (Plaza et al., 2005). Bihari et al. (2006) evaluated the quality of the marine sediment of the North region of the Adriatic Sea, in Croatia. For this study, superficial marine sediments were collected in eight distinct sites of the region in order to quantify and qualify chemically the PAHs and assess their toxic and genotoxic potentials. According to the authors, the chemical analyses revealed a clear difference from the natural areas to the urban-industrialized areas and harbor zones, being the latter the most contaminated with PAHs. It was not reported any genotoxic actions for the organic extracts of the sampled sediments, submitted to the umu-test. In this test, the genotoxicity is measured by the ß-galactosidase activity of the Salmonella typhimurium TA1535/psk1002 line and is expressed by this enzymatic activity induction rate, between the sediment and the negative control treatment. The DNA damage, evaluated in the mussel (M. galloprovincialis) haemolymph was related with the total PAHs present in four sampled sites, although, the highest verified DNA damage rate does not correspond to the sample of the site that presented the highest PAHs contamination. It is presumed that the atmospheric pollution displays a fundamental role in the incidence of respiratory diseases and cancers. Aerotransportated particles are capable of penetrating deeply in the lungs. Amongst these particles, it is found a wide variety of chemical substances, including several mutagens and carcinogens such as the PAHs. Du Four et al. (2004) evaluated the genotoxic and mutagenic activities of air samples collected in Belgium. The collections were carried out in two seasons of the year (summer and winter) in the vicinity of a busy street; in industrial sites and in a rural community by two sampling systems, one using quartzfilters (QF) and other with polyurethane foam (PUF). It was observed a higher mutagenicity for the air collected during winter, probably, as a consequence of the PAHs high levels registered in this season. The increase in PAHs was attributed to a greater necessity of domestic heating and the specific meteorological conditions such as solar radiation intensities and ozone concentrations. The authors related the variation in the mutagenicity rates between the summer and winter seasons with the presence of direct and indirect mutagens in the collected samples because, according to them, in winter the indirect mutagenic activities are higher than the direct activities indicating a considerable contribution of the PAHs in the observed effects. In this work it was observed that all the analyzed environments presented contamination by PAHs showing that, independently from the sampled area (rural, urban and industrial), the exposition to PAHs was always significant. In one another evaluation the genotoxic and mutagenic activities of the Belgium air, carried out by the Salmonella mutagenicity standard plate-incorporation and Vitotox® assays, it was possible to observe that the highest mutagenicity values were registered out of the big cities and in the rural area, contaminated by the pollution of a chemical industry located 4 Km of distance. However, the authors reported that, based on the concentrations of 10 mutagenic PAHs and on the supposition of their specific mutagenicities additive effect, only a small percentage of the indirect mutagenic activity could be explained. The results alert for the fact that the majority of the observed mutagenic effects could be related with the action of other
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substances, that were not identified or measured by the chemical analyses, which shows the importance of the bioassays application since, with these tests, it is possible to monitor the pollutant and estimate the air quality (Du Four et al., 2005). Samples of the particulate material, collected in two different region of the Mexico city – industrial and residential, were processed in two final extracts, one aqueous and one organicsoluble extract, in order to evaluate the health risks promoted by the particulate material resulted from the atmospheric pollution (Roubicek et al., 2007). By the samples chemical analyses, the authors have found that the highest PAHs concentrations were registered for the residential area, although both extracts, industrial and residential, induced a significative increase in the MN frequency, indicating the mutagenic potential for all the analyzed samples. Additionally, the authors have shown that, in the association between MN presence and chemical compounds detected, cadmium and PAHs are the most related agents, significantly, with MN induction. Studies of the mutagenicity assessment (Ames test with the TA98 and YG1041 lines, with and without S9 addition) and the detection of DNA adducts (32P-postlabelling analyses) of fractions of the particulate atmospheric material from São Paulo city- Brazil were carried out by Umbuzeiro et al. (2008). Air samples were collected in the vicinity of a highway and in a park located in the urban area of São Paulo city. The particulate material was fractioned in order to obtain the PAH, nitro-PAH and oxy-PAH fractions. The PAHs fractions, of both sites, presented insignificant mutagenicity opposite of the observed for the nitro-PAHs fraction. Both sites presented the same mutagenic activity levels, probably, due to the related effects of the nitro and oxy-PAHs action. The use of the 32P-postlabelling analyses technique for the detection of DNA adducts in HepG2 cells showed a genotoxic potential of the air samples particulate material, collected in a central area of the Prague city, Czech Republic. It was observed for several tested periods a positive correlation between B[a]P and carcinogenic PAHs present in the particulate material with the total levels of DNA adducts of the exposed cells (Sevastyanova et al., 2008). These results pointed out for the hypothesis that B[a]P and other carcinogenic PAHs are more important factors to determine the genotoxic potential. Thus, the genotoxicity estimative of the air environment and the evaluation of health risk must be based, principally, on carcinogenic PAHs concentration and in the reactivity of these substances with the biological material, while the particles mass and the particulate material type do not seem to be important factors to estimate the atmospheric genotoxicity. To investigate the DNA adduct levels in workers exposed to traffic and in the general population of Florence city (Italy), Palli et al. (2008) carried out a study with 152 healthful adult inhabitants and 62 non-smokers workers exposed to traffic, for 1-2 weeks. In this work, it was carried out the DNA adducts evaluation, by the 32P-postlabelling technique, in blood samples collected from all the sampled individuals. By the obtained results, the authors concluded that the DNA adduct levels tend to reflect the exposition levels to the particulate material being the workers exposed to the traffic the greater bearers of these alterations. Cigarette smoke can be considered as one environmental exposition and constitute a risk factor for the development of several illnesses such as lung cancer and vascular diseases (Mitchell et al., 1999). Among the PAHs, B[a]P is a substance related with smoke that is associated with endothelial damages in blood vessels (Thirman et al., 1994). Studies to assess the B[a]P potential to induce DNA damages in human endothelial cells were carried out by Annas et al. (2000). In this work, it was evaluated the action of B[a]P different concentrations
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(0.1, 1.0 and 10µM) in human umbilical cord cells cultivated in medium without addition of an exogenous metabolization system, in order to verify the capacity of inducing damages in the genetic material and the biotransformation via involved in this process. The authors concluded that B[a]P induce a significative quantity of DNA damage, even in the absence of an exogenous metabolization system suggesting that the bioactivation of this agent is not totally dependent on the activity induced by CYP1A. As explained above, several works have been developed in order to measure the genotoxic and mutagenic damages induced by the exposition to PAHs. The detection of these effects can be carried out with several techniques, using a great diversity of test-organisms, being reported, for most of the cases, positive results. In this way, we can note the real importance in the genetic risk assessment of the PAHs and the use of biological systems in the environmental monitoring of these pollutants.
2.5. Teratogenicity and Embryotoxic Effect Teratogenic alterations are manifestations of the action of chemical agents over the fetus or over the organisms’ reproduction. These actions lead to structural and/or functional malfunctions that are due to the interference in the organism development, still in the embryonary phases. Congenital anomalies in human embryos are adverse consequences of a pregnancy that can be associated with exposition to environmental pollution (Dolk and Vrijheid, 2003). Analyses of blood samples, obtained from human umbilical cord indicated the presence of 287 chemicals, which confirmed that there is an intense exposition to agents with teratogenic potentiality during the gestational period (Houlihan et al., 2005). Beyond their toxic, genotoxic, mutagenic and carcinogenic actions the PAHs are recognized teratogenic compounds (Wassenberg and Di Giulio, 2004). The PAHs teratogenic action has been registered for many different organisms and in the different ecosystem levels. Studies with invertebrates, fishes, amphibians, reptiles and birds, in their majority, restrict themselves to the exposition of the eggs to PAHs mixtures or to a specific hydrocarbon. In rodents, most of the studies use intraperitoneal applications or oral ingestion in pregnant females. In humans, the studies are made, principally, by the environmental or occupational exposition to the PAHs. Studies made with aquatic invertebrates contaminated with PAHs report a variety of alteration such as reduction in the reproductive rates, inhibition of the larvae and embryos development, besides delay in the larval emergence (Neff, 1985; Eisler, 2000). Teratogenic alterations regarding anomalies in the shell and the mantle formation were found by Wessel et al. (2007) in oysters embryos (Crassostrea gigas) exposed to B[a]P. According to Geffard et al. (2003), bioassays using oysters embryos and larvae contribute to a deep knowledge of the PAHs teratogenicity. Pelletier et al. (2000), studying the maternal transference of the photoreactive PAHs in benthic bivalves, observed that this transference would result in phototoxicity for the pelagic larvae, when they were exposed to ultraviolet light, which would represent a significative adverse effect over the populations of the organisms that live in sediments contaminated by this type of pollutant. Many studies were made in order to study the PAHs influence in the embryonary development and in the reproduction of many species of fishes. Among the proved
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alterations, we can cite the reduction in the maturation of the gonads and eggs (Collier et al., 1993; Kirby, 1994; Ghosh and Thomas, 1995; Thomas and Budiantra, 1995); decrease of the reproductive success, with reduction of the percentage of incubated eggs and survival of the recent eclosed (Black et al., 1988; Casillas et al. 1991; Tilghman Hall and Oris, 1991; Kocan et al., 1996). Exposition to PAH, according to Collier et al. (1993), can damage the initial phases of the reproduction process of females of the English sole. Nicolas (1999) in a review article recognizes the PAHs as substances with deleterious effect in all the stages of fish vitellogenesis, which could endanger the succession of their descendants. Van Meter et al. (2006) suggest that the hydrocarbons accumulation in the embryos vitelline sac become a continuous source of teratogenicity for the larva along its development. It was observed by Pollino and Holdway (2002), deformities in the larval stage of the fish Melanotaenia fluviatillis after exposition to PAHs such as edema in the pericardium, anomalies in the jaw and alteration in the flexion and curvature of the vertebrae. Many authors consider the use of embryos of fish as an excellent test organism for the elucidation of vascular defects in vertebrates induced by chemicals potentially teratogenic, such as the PAHs (Hose et al., 1984, Antkiewicz et al., 2006; Carney et al., 2006; Goldstone and Stegeman, 2006). According to Billiard et al. (2008), one of the advantages of the use of these embryos would be the presence of a transparent chorion, which allows a direct and not invasive observation during all the embryogenesis. Bioassays developed with embryos of Danio rerio exposed to several solutions of organic films containing PAHs, collected in the urban area of Toronto (Canada), showed that these compounds were capable of inducing cardiovascular, hematopoiesis, neural and pigmentary abnormalities, besides behavioral alterations in these fishes (Diamond et al., 2000). Incardona et al. (2005) affirm that embryos of fishes exposed to wheathered ANS oil and to mixtures of PAHs developed a pericardium syndrome, yolk sac edema, reduction in the craniumfacial structure and body axis defects. Many of the defects were induced by individual exposition to the tricyclic aromatic hydrocarbons that is one the most abundant class between the wheathered ANS oil, causing secondary impacts in the cardiac function of the embryo, coincident with the development of the cardiac conduction system. Studies carried out by Incardona et al. (2006), about induction of alterations in zebrafish embryos by PAHs constituted by four aromatic rings, pointed out that chrysene did not show any embryotoxic and teratogenic effect in these embryos. The exposition to benzo[b]anthracene resulted in mortality in all the concentrations between 8 and 10 µM and defects in the development as a failure of head straightening, lack of finfold development and poor cardiac looping with pericardial edema hemorrhage, for concentrations between 2 and 6 µM. The continuous expositions to doses superior to 5 µM of pyrene resulted in systemic toxicity that occurred in the initial phases of the larval development, including dorsal curvature of the body axis, reduction in the peripheral circulation, anemia and pericardial edema. Tuvikene (1995) reports that eggs of fishes contaminated with anthracene, after exposition to ultraviolet radiation, presented alterations in the development such as edema, malfunction of the yolk sac and deformities in the eyes. According to Tilghman Hall and Oris (1991), PAHs, specifically anthracene, can be transferred by the mother to her offspring. Tests carried out with alevins of the rainbow trout (Salmo gairdneri Richardson) exposed to B[a]P showed an incidence significantly high of cranium and vertebral column deformities and microphtalmia (Hose et al., 1984). Deformities in the embrionary development such as
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malformation of tail and head, beyond pericardium edema, were reported by Wassenberg et al. (2002) in the marine fish Fundulus heteroclitus, after exposition to several concentrations of B[a]P. Exposition of eggs of turtles (Chelydra serpentina) to PAHs caused significant alterations in the embryos development, varying from mild deformities (tail deformities, absence of pigmentation, malfomation of the shell), moderate (absence of the members or eyes and asynchronous development) until severe such as absence of two or more members, cerebral abnormalities and not closing of the cranium (Van Meter et al., 2006). The effects caused by the PAHs in embryos of birds include death, abnormalities in the development and a diversity of cellular and biochemical answers (Albers, 2006). Lethal and sub-lethal effects were observed in embryos of birds in experiments carried out with eggs recovered with PAHs fractions (Hoffman, 1990). According to Matsumoto and Kashimoto (1986), these chemicals produce embryotoxic effects in birds which vary according to the quantity and chemical structure of the PAHs. Mayura et al. (1999), when carrying out experiments with ingestion of fractionated mixtures of PAHs in egg yolks of chicken, found out severe teratogenic alterations in their embryos and according to Hoffman and Gay (1981), high rates of teratogenic deformities were observed in embryos of ducks, whose eggs were exposed to PAHs. Feuston et al. (1997) observed deleterious effects in the development of embryos of pregnant rats, after their exposition to crude oil. According to the authors, the observed effects would be directly related with the presence of PAHs in the samples. According to Sanyal and Li (2007), introperitoneal applications of PAHs in pregnant rats lead to an efficient absorption of the mother and a more direct exposition of the fetus, resulting in toxic effects for the mother and deformities resulted from alteration in the fetus development. However, Hansen et al. (2000) reported that fetus born from females of mice submitted by gavage to carbon black oil that contains PAHs, in five different concentrations (100, 200, 300, 350, 400 mg/kg/day) did not present any structural deformation in the viscera and in the skeleton being observed only a decrease in the weight of the embryo. Sanyal and Li (2007) observed that the prenatal exposition of rats to PAHs induced adverse effects on the blood vascular system, principally during the angiogenesis, which can lead to grave malformations and dysfunctions in vital organs. Shum et al. (1979) reported the presence of congenital defects and growth retard in fetus of mice, whose mothers were treated, orally, with doses of B[a]P between 50 and 300 mg per Kg of weight of the animal, between the seventh and tenth day of the gestation, confirming the descriptions of Nebert (1989) that classify the B[a]P as a teratogenic chemical for mice. The presence of abnormalities in the fetus, resulted from the exposition of the mothers to PAHs, would be related to the fact that these substances are toxic to the placenta, which facilitates the transference of toxic agents for the fetus, besides causing lose of the regulatory function, with a decrease in the nutrients supplement and other placental factors essential for the correct growth and development of the fetus (Sanyal and Li, 2007). Studies carried out by Mackenzie and Angevine (1981), exposing pregnant females of mice CD-1 to gavage of 10 mg/Kg/day of B[a]P, during 10 days, showed that the descendants presented alterations in the gonad morphology and in the development of the germ cells. Lineages of mices Ah-responsive and Ah-non responsive, presented an increase in the abortion incidence, reabsorption of the fetus and malformations after oral administrations of B[a]P in pregnant females, being these alterations more expressive in Ah-non responsive than
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in embryos Ah-responsive (Legraverend et al., 1984). These authors also concluded that the teratogenicity is directly related with the maternal and/or embryo genotype controlled by the gene Ah that codifies the cytosolic Ah receptor that regulates the induction of the CYP enzymes. Intraperitoneal injections of B[a]P in pregnant females of mice caused alterations in the follicular growth, ovulation and formation of the luteous body (Swartz and Mattison, 1985). Prenatal exposition to B[a]P caused severe effects in the offspring of the mice such as testicular atrophy and tumors in the interstitial cells (Payne, 1958). According to Sanyal and Li (2007), the dimethylbenzo[a]anthracene is more potent in producing deleterious effects in pregnant females of rats than the B[a]P. Exposition of pregnant rats to PAHs induced toxicity to the placenta, that resulted from abnormal development of the fetus and hemorrhages in different parts of the body, mainly intradermic and cranial, which demonstrated that the fetal vascularity is the main target of the toxic action of the PAHs (Sanyal and Li, 2007). According to Genuis (2006), exposition to PAHs present in the environment during the human embrionary development is quite expressive. These agents penetrate in the body of the mother by different ways and can pass through the placenta and become source of deleterious alterations in the offspring (Sanyal and Li, 2007). The formation of DNA adducts, as well as the occurrence of oxidative damages in these macromolecules, are key events for the triggering of the teratogenicity molecular mechanisms (Wessel, 2007). Perera et al. (2005), when studying pregnant women exposed to different concentrations of PAHs in 4 regions of the world, observed that the fetuses can be 10 times more susceptible to the PAHs than the mothers. It is known that some PAHs are transmitted by the mother to the fetus via placenta, causing alterations in the development, liver, lung, lymphatic and nervous system (Rice and Ward, 1982; Perera et al., 2003). Studies indicated that PAHs can induce a number of adverse effects being capable of jeopardizing even future generations (WHO, 2000). Other studies report a decrease in the head circumference of babies, whose mothers were frequently exposed to PAHs, which could affect the cognitive development and the scholar performance of these children (Hack et al., 1991; Dam et al., 1998; Perera et al., 2006). According to Choi et al. (2006), based in studies carried out in New York (USA) and in Krakow (Poland), human exposition to low concentrations of PAHs induced adverse reproductive effects, principally in relation to the fetal growth.
2.6. Carcinogenicity According to Pedrozo et al. (2002), carcinogenicity is the capacity of certain agents induce the formation of tumors, by the direct action in DNA, which progress to biochemical, histological, functional and clinic alterations of the target cells and tissues. Replication of the cells and tissues, that present carcinogenic adducts in their DNA, can result in gene mutations that regulate the cellular growth and differentiation occasionating a progressive genetic instability, which leads to an uncontrolled growth, lose of cellular differentiation, invasion of dedifferentiated cells in the tissues and metastasis (Stewart and Kleihues, 2003). Induction of damages in DNA, responsible for the adverse effects such as mutations and chromosome
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rearranges, are considered primary mechanisms of cancer induction (Jacobson-Kram et al., 1993). PAHs carcinogenic activity is dependent on the structural characteristics of these substances, such as the number of benzene rings, fusion sites, extent of condensation, degree and local of the methylation (Ross and Nesnow, 1999). According to Jacob (1996), PAHs that contain more than 3 aromatic rings are, potentially, the most carcinogenics. According to Baird et al. (2005), PAHs are known as complete carcinogens since they can, by their action, induce tumors in the skin. However, according to Luczyński et al. (2005) and Pashin and Bakhitova (1979), PAHs do not have direct actions like carcinogens since their carcinogenic activity is promoted by metabolizations that these chemicals suffer. There are two main PAHs metabolization process in the cells, which lead to damages in the DNA. One activation via is characterized by the dihydrodiol epoxides formation, that bind, covalently, to amino heterocyclics groups of purine bases, to form stable adducts. Another via involves the formation of cation radicals, which bind to the N7 or C8 of the purine bases to form instable adducts and generate apurine sites in the DNA, by spontaneous depurination (Melendez-Colon et al., 1999). Pereira Netto et al. (2000) cite another two mechanisms to explain PAHs activation. The first would be related with the formation of benzilic esters, electrophilic, by a series of substitution reactions. The second would be the de-hydrogenation of dihydrodiols metabolites, which lead to the production of quinones, substances that are capable of reacting, directly, with DNA or with O2, generating reactive oxygen species such as hidroxil radicals or superoxide anions, which attack DNA. PAHs are metabolized by a variety of xenobiotic-metabolizing enzymes (Baird et al., 2005). CYP and epoxide hydrolase convert the PAH into PAH-diols and these products are transformed in metabolites, potentially, carcinogenic, by the CYP action, forming PAH diolepoxides or by the Aldo-keto-reductase action, generating PAH o-quinones. PAHs can be also activated by CYP and by peroxidases, forming the reactive cations radicals, which bind, covalently, to DNA. Intermediary products formed are still metabolized by enzymes of phase II, resulting in metabolites more polars and soluble in water becoming ready to be excreted by the body (Shimada, 2006). Adducts ability to bind to DNA and persist to form mutations is dependent on the adduct conformation inside the DNA sequence and of the identification of the lesion by the repair system (Wu et al., 2002). Formation of the PAH-DNA adducts can lead to a induction of mutations, which activate the protoongenes (ras) or inactivate the tumor suppressor genes (p53) that can trigger the tumorization initiation process (Prahalad et al., 1997; Straif et al., 2005). PHAs induce the activation of the CYP1A1 and CYP1B1, via the aryl hydrocarbons receptor (AhR). This cytosolic receptor, after the aryl hydrocarbon nuclear translocator (ARNT) dimerization, enters in the nucleus and interact with genes that contain xenobiotic response element (XRE), resulting in the induction or hyper regulation of the proteins involved in the xenobiotics detoxification (Whitlock, 1999). PHA-DNA adducts can be formed in several genes, however, these adducts have been found, frequently, in the ras genes, which when mutated are capable of transforming the cells and inducing the appearance of malign tumors (Maronpot et al., 1995). Three different types of the ras gene were identified and sequenced: Ha-ras (chromosome 11); Ki-ras (chromosome 12) and N-ras (chromosome 1). About 20% of the majority of the malign
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tumors in humans seems to be related with the proliferation of aberrant cells, with alteration of the cell checkpoint control and cellular differentiation (Ross and Nesnow, 1999). There are some structural arrangements in the benzene rings that interfere in the carcinogenic properties of the PAHs and derivates. Bay region consists of 3 aromatic rings, connected in an angular way (Figure 2), creating a recoil that looks like a bay. Pseudo bay region is composed by two aromatic rings and one terminal aliphatic ring (Figure 2). A fjord region is resultant from an angular disposition of four aromatic rings (Figure 3) and, in the case of a pseudo fjord region, the terminal ring is saturated (Figure 3) (Luczyński et al., 2005).
Figure 2. Scheme showing PAH with bay region, K region (e.g. Benz[a]anthracene) and showing the pseudo bay region (e.g. 16,17-Dihydro-15H-cyclopenta[a]phenanthrene).
Figure 3. Scheme showing PAH with fjord region, K region (e.g. 2,3:5,6-Dibenzophenanthrene) and showing the pseudo fjord region (e.g. 2,3-Dihydro-1H-cyclopenta[c]phenanthrene).
According to Baird et al. (2005), PAHs that present a structure with bay regions are potent carcinogens but those who have fjord regions, like dibenzo[a,l]pyrene, are even more potent. Metabolites derived from PAHs with bay region present a pronunciated tendency to form carbonium ions, becoming much more reactive (Jacob, 1996). When the bay region changes to fjord region the molecule plane is also deformed. This change facilitates the access
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to an epoxide ring of the diolepoxide molecule, allowing the covalent bond in the DNA molecule (Luczyński et al., 2005). Another chemical characteristic of the PAHs that contribute to their carcinogenicity is the presence of a region highly electrondense designated K region (Figure 2 and 3). This region allows the formation of the K region epoxides, which makes it a site highly reactive, for the occurrence of double definite and localized bonds. (Samanta et al., 2002; Luczyński et al., 2005), besides being the main local of metabolic reactions (Bücker et al., 1979). Observation of the carcinogenic potentiality, individual, of several PAHs was registered by the presence of malign and benign tumors in animals, by different application vias of these compounds (oral, intraperitoneal, subcutaneous, epicutaneous, among others). The in vitro assays, carried out with culture animal cells and tissues, contributed, in a significative way, for the elucidation of the carcinogenic potential of these chemicals. In humans, the PAHs carcinogenic results were registered, mainly, by occupational exposition (Jacob, 1996). The assessment and control of the human exposition to PAHs and derivates are made, generally, by the monitoring of the environmental levels of these substances, knowledge of the penetration vias in the organism, their metabolism, as well the early assessment of their biological effects (Pereira Netto et al., 2000). Since the DNA adducts formation is a critic stage in the chemical carcinogenesis caused by the PAHs, the identification of these alterations can serve as a precocious bioindicator of cancer (Beach and Gupta, 1992). According to Pereira Netto et al. (2000), the most used assays are the 32P postlabeling, imunoassays, fluorescent spectroscopy and detection of 1hydroxypyrene. Okona-Mensah et al. (2005), suggest the use of nasal cells for the investigation of cancer induction in lungs by atmospheric pollution, as well as the use of mass spectroscopy to quantify specific adducts. Metabolic activation vias and the types of damages induced by PAHs dibenzo[a,l]pyrene (DB[a,l]P), 7,12-dimethylbenzo[a]anthracene (DMBA) and the B[a]P in mice epidermis were evaluated by the analysis of the formation of stable adducts and AP sites in the DNA of this tissue (Melendez-Cólon, 1999). The study revealed that the topic exposition of mice to these hydrocarbons resulted in a formation dose-dependent of high levels of stable dihydrodiol epoxide-DNA adducts but lower levels of AP sites in the DNA of the epidermis cells. The registered levels for stable adducts were higher for B[a,l]P, followed by DMBA and B[a]P, for all the doses and times tested. In this way, the authors affirmed that the dihydrodiol epoxide-DNA adducts are the main responsible for the cancer initiation in skin of mice, induced by PAHs. According to Ross and Nesnow (1999), the identification of specific mutations in the ras gene makes possible to diagnose which adduct induced the alterations. The authors reported that transversion mutations of the A→T type, in the codon 61, are related to the deoxyadenosime adducts action, while transversions G→T, in the codon 12, are frequently induced by the diol-epoxide, which binds to deoxyguanosine and to cyclopenta-ring oxides, inducing transversions G→C, in the codon 12. In studies carried out about tumorigenesis of the PAHs coal tars and of the B[a]P in females of mice B6C3F1, fed with these substances, it was verified that PAHs present in the coal tars induced the formation of tumors in the small intestine, liver, lung and forestomach, besides skin sarcomas, histosarcomas and hemangiosarcomas in several organs. Evaluations made with B[a]P, showed that this substance was tumorigenic in tongue, larynx, esophagus and forestomach. The DNA adduct levels, measured in lung, liver and forestomach, were
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dose dependent indicating that both B[a]P and PAHs coal tars were transported to other organs, although B[a]P have not induced tumor in lung and liver of mice exposed to this PAH. B[a]P and PAHs coal tar induced mutation in the positions 1 and 2 of the guanine in the codons 12 and 13 of the K-ras oncogenes in forestomach tumors. Mutations in guanine were also found in lung tumors, but not in small intestine tumors. Thus, mutations in the K-ras gene must be involved with lung and forestomach tumors, but not in small intestine tumors (Warshawsky, 1999). There are epidemiological evidences that suggest that occupational groups exposed frequently to complex mixtures of PAHs present a more elevated risk in the development of lung cancer (Okona-Mensah et al., 2005). Studies carried out by Straif et al. (2005) showed that, in mice, B[a]P caused lung tumors, by the formation of diol-epoxide and skin tumors, triggered by diol-epoxide mechanisns and cations radicals. According to these same authors, few PAHs are considered carcinogens more potent than B[a]P. However, dibenzo(a,l)pyrene seems to be a carcinogenic 10 times more potent for rats than the own B[a]P (Okona-Mensah et al., 2005). Another PAH highly potent, according to Platt et al. (1990) is the dibenzo(a,h)anthracene. Pavanello et al. (1999) used the HPLC/fluorescence method to detect the anti -BPDE– DNA adduct in mononuclear leukocytes exposed to PAHs. The authors concluded that this methodology can be used for the evaluation of the chronic exposure to PAHs and that the inhalatory via contributed, substantially, for the adducts formation in the lymphocyte plus monocyte fraction in coal factories workers and chimney cleaners. Jacobson-Kram et al. (1993) studied the cancer risk in peripheral lymphocytes of a group of firefighters induced by occupational exposition to products potentially carcinogenic resulted from the combustion and organic matter burning, by the presence of adducts in the DNA of these lymphocytes. In relation to the PAH-DNA adducts levels and the presence of benzo[a]pyrene diol epoxide (BPDE)-DNA in the peripheral lymphocytes of these professionals, the results found were not significative, however, when other factors, such as smoke and alcohol consumption, were associated, the PAH-DNA levels were increased. As presented above, PAHs are carcinogens in potential, both for humans and for the rest of the animals. Nevertheless, the chemical complexity of the PAHs, the diversity of the metabolic routes of the biological systems, the several enzymes involved in the metabolization process of these substances, besides the several exposition vias, to which the animals can be exposed, constitute difficulties for the assessment of the action and effects of these chemicals in the biological environment. For a better understanding of the PAHs activities it is necessary that new methodologies be applied in studies with these chemicals, besides re-evaluations that can confirm the obtained effects on some organisms.
2.7. Estrogenicity Existence of chemical substances that are capable of inducing alterations in the hormonal homeostasis is already known for many years (Rivas et al., 1997), however, the concern with this pollutant class has been growing, due to the adverse effects related to their exposition. According to the U.S. Environmental Protection Agency (USEPA), these chemicals, named endocrine disrupting chemicals (EDCs), are defined as exogenous agents that interfere in the synthesis, secretion, transport, bond, action or elimination of natural body hormones,
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which are responsible for the maintenance of the homeostasis, reproduction, development and/or behavior of the organism (Belgiorno et al. 2007). It is known that chemicals suspect of causing alterations in the endocrine system such as EDCs are potentially associated with several diseases such as testicle, breast and prostate cancer, reduction in the spermatozoid rate, deformities in the reproductive organs, thyroid dysfunctions and alterations related to the neurological system (Rivas et al., 1997; Jimènez, 1997; Ghiselli and Jardim, 2007). Many of the EDCs are synthetic organic chemicals introduced in the environment by anthropogenic discharges (Belgiorno et al., 2007). Among the EDCs acknowledged nowadays, we can cite the phthalates, pesticides, polychlorinated biphenyls (PCBs), dioxins, alkyl phenols, bisphenols, and PAHs (Jimènez, 1997; Ghiselli and Jardim, 2007; Belgiorno et al., 2007). PAHs action like EDCs has been described in some studies, which have reported both the estrogenic and anti-estrogenic actions. According to Ghiselli and Jardim (2007), PAHs, by presenting similar structures to sexual steroids, can interfere in the natural function of the endocrine system, binding to hormonal receptors and resulting in an antagonistic effect, in other words, the occurrence of the blockage of the natural hormone action, by obstructing the interaction with its respective receptor. Additionally, PAHs action as anti-estrogens can be given by the bond to the aryl hydrocarbon receptor (AhR), triggering the induction of genes associated with the activation of this receptor (Ah-responsive genes), which results in a wide spectrum of anti-estrogenic responses, including the increase in the 17-β estradiol (E2) metabolism and decrease of the nuclear estrogen receptor (ER) (Santodonato, 1997; Arcaro et al., 1999). Arcaro et al. (1999) used the MCF-7 focus assay, which measures the relation estrogendependency in the cellular growth; recombinant human estrogen-receptor (ER) binding assay, which evaluates the binding capacity of the environmental chemicals to ER; whole-cell ER binding and 17-ß estradiol (E2) metabolism assay, to investigate the estrogenicity and antiestrogenitity of sediments contaminated with PAHs, originating from two localities in New York, one industrial and other non-industrial. The authors have reported the detection, by chemical analyses of 14 PAHs, being the levels observed in the industrial area much more elevated than in the non-industrial. In this work, the bioassays were carried out with fractions of the PAHs collection obtained from extracts of the sediment samples; with two reconstituted mixtures and with the 14 PAHs detected, tested separately. The authors reported anti-estrogenicity for the PAHs fractions of both tested sediments, for the reconstituted mixtures and for 8 of the 14 PAHs tested individually. The authors also investigated the processes that are involved in the anti-estrogenicity induced by the PAHs, showing a relation with, at least, two mechanisms: the first involving competition of the ER for the PAHs metabolites, showing that PAHs can exert this activity by their metabolic inactivation, and the second involving diminishing of E2 by induction of the metabolism. Vitellogenesis is a process hormonally controlled and that can be affected by the exposition to xenoestrogens. In most fish species, oocyte maturation process occurs seasonally, in which one spawn occurs in each annual reproductive cycle. In this way, factors that affect vitellogenesis can cause a drastic reduction in the reproductive success of the individual and, consequently, affect an entire species population. According to a review carried out by Nicolas (1999), deleterious effects in fishes vitellogenesis were reported for PAHs both for wild populations and laboratorial experiments. Amongst the effects cited, it is
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included reduction in the circulation of hormones and plasmatic vitelogenin (species-specific protein synthesized by hepatocytes involved in the oocyte maturation); estrogenic and antiestrogenic effects; delays in the oocyte maturation and reduction of the reproductive success. Based in this review, Nicolas (1999) showed that PAHs action mechanisms in the vitellogenesis process in fishes are not totally clear and that the issue still requires several investigations. Fertuck et al. (2001), to study the PAHs habilities and their monohydroxy derivates to interact with the α and β isoforms of the ER, used the in vitro competitive binding and the reporter gene expression techniques in breast cancer cells MCF-7. Interactions between these two different ER isoforms have presented a particular interest, due to the functions that they fulfill. ERα has showed to be critical in the fertility maintenance and in the maternal behavior. ERβ, probably, is more involved with the maintenance of the bone mineral density of the organism. From the obtained results, the authors have shown that the nonhydroxylated compounds are incapable of competing with the ER ligands, on the contrary of the monohydroxylated metabolites. For example, B[a]P is incapable of competing with the ER ligands, while its monohydroxylated derivates are capable of interacting and inducing genic expression in a much more elevated degree, and preferably with ERβ. According to the authors, taking in consideration the structural similarity of the studied chemicals with B[a]P, probably, the response obtained for the assay with MCF-7 cells is due to the metabolites produced in the PAHs bioactivation by the CYP isoenzymes, which produce monohydroxylated and other metabolites. Finally, the authors have concluded that several PAHs can interact, positively or negatively, with the in vivo ER signaling, probably, in a cellspecific way. Wang et al. (2003) used the human estrogen receptor (ER) recombinant yeast bioassay to assess the estrogenic activities from different fossil fuel emissions (soot from petroleum fired car, soot from coal fire stove, soot from diesel fired agrimotor and soot from coal fired electric power station). In the present work, the authors showed a weak, but clear, estrogenic activity of the fossil fuels, including coal, petroleum and diesel. Although the estrogenicity impacts of these chemicals in the ecosystem and in human health are not fully known, the authors affirm that the estrogenic activity phenomenon, originated from pollutant sources, still requires plenty of investigation in order to evaluate the real effects of these chemical substances on the endocrine system of the exposed organisms. Plíšková et al. (2005) assessed the estrogenic activities of some PAHs (B[a]P, benzo[a]anthracene - B[a]A, fluoranthene - Fla and dibenzo[a,l]pyrene - DB[a]P) on the cellular cycle progression and on the cellular proliferation, in the human breast carcinoma. For these analyses, it was used the E-screen assay (cellular proliferation test in MCF-7 cells); neutral red cytotoxicity assay (test to measure the cell viability using a vital dye); cell cycle and BrdU incorporation analyses; cell death detection; western blotting (that detects the p53 and β-actin proteins). According to the authors, B[a]P and B[a]A, even in low concentrations (100nM), stimulated the MCF-7 cells proliferation, in a ER-dependent way. These results were contradictory from other authors that have found an inhibition of the MCF-7 cells development focus in culture. Plíšková et al. (2005) infer that this discrepancies are related with the cell growth conditions used, which are, significantly, different from those used in post-confluent cells model and, also, different from the experimental configurations. The authors conclude that, both B[a]P and B[a]A can contribute to a tumor progression and, in final analysis, to carcinogenity of the estrogen-sensitive tissues.
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Lipzig et al. (2005) investigated the metabolites formation of the B[a]P and chrysene (CHN) by the CYP microsome via in rat livers. After the metabolites akcnowledge, the authors have tested 8 mono- and tri- dihydroxylate metabolites and a mixture of 7 monohydroxylate metabolites, with regard to their ER affinities, using radioligand binding assay. In this regard, it was also evaluated the metabolites estrogenic activities, by the cell proliferation (modified E-screen) and gene reporter (ER-CALUX) assays, using trasnfected ER-positive T47D stable cells (human breast adenocarcinoma cell line), which expresses the luciferase actitivity, by ER activation. From the obtained results, it was observed that B[a]P and CHN metabolites, generated by the CYP isoenzymes, presented a higher affinity by the ER than from the B[a]P and CHN. To verify the functional answer of these metabolites ligands to ER, the authors carried out a E-screen modified assay and showed that none of the B[a]P metabolites, just like this pure chemical, have the capacity of inducing cell proliferation similar to that observed to E2 (pattern). However, by the obtained results with the ERCALUX assay, it was observed estrogenic activity for B[a]P, CHN and some of their metabolites. For the metabolites mixture, that presented, individually, weak estrogenic activity it was observed a significative increase in the estrogenic effect. These data conffirmed the risks to animals and human health of the exposition to several environmental xenoestrogens, even if they present weak estrogenic activity when assessed individually. Patel et al. (2006) evaluated the B[a]P effects in mRNA expression of the CYP aromatase (CYP19) and in the activity of this enzyme, by exposing embryos and adults of the Fundulus heteroclitus species (Pisces: Cyprinodontidae) to waters polluted by B[a]P, for a period of 10 and 15 days, respectively. CYP19 is a key steroidogenic enzyme, responsible for the conversion of androgens to estrogens, its expression in lower vertebrate is, typically, restrict to the gonads, neural tissues (brain and retina) and hypophysis. For mammals, in general, there is only one CPY19 gene with multiple promoters. On the other hand, teleosts present two distinct CYP19 genes, CYP19A1 and CYP19A2, which are regulated, differently, and codify proteins, also, with different enzymatic characteristic. CYP19A1 is expressed in the ovary and is involved with sexual differentiation and oocyte maturation, while the CYP19A2 expression occurs in elevated levels in the brain, retina and hypophysis, and is related with the neuronal differentiation process, survival, morphology, synaptogenesis and sexual behavior. From the obtained results, it was observed that AhR ligands such as B[a]P, present pontential to cause alteration in the endocrine system by modulation of the aromatase activity and this expression differs, with regards to tissue and sex. The authors concluded that B[a]P can interfere in the esteroids homeostasis and cause harmful alterations in the fish reproductive process. Martin-Skilton et al. (2008), when studying the effects caused by the Prestige oil fuel spill, with regards to the alteration conferred to hepatic enzymatic biotransformation and to endocrine system, exposed turbot juveniles (Scophthalmus maximus) to a food diet with different concentration of the Prestige fuel oil, for a period of 6 weeks. To evaluate the alterations in the endocrine system, the authors measured the testosterone and E2 levels in the blood plasma and assessed aromatase gonadal activity and glucuronodation of testosterone by liver microsomal fractions. Acute exposition to Prestige fuel oil reduced, significantly, the testosterone quantity in the plasma, which led to deleterious effects in the reproductive capacity of the exposed individuals. According to the authors, exposition to Prestige fuel oil altered the hepatic enzymatic biotransformation and the levels of the endogenous hormones in the tested organisms. The authors still affirmed the necessity of a further investigation to
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estimate the real effect of these chemicals over the sexual development and the reproductive process in fishes. Kummer et al. (2008) used the in vivo immature rat uterotrophic assay to investigate the estrogenic and anti-estrogenic acute effects of some environmental PAHs (B[a]P, B[a]A, Fla and benzo[k]fluoranthene (B[k]F)), that were previously reported as modulators of ERdependent genic expression and promoters of the cell proliferation in in vitro tests. By the analyses, B[a]P, B[a]A and Fla acted as weak estrogens in immature rat uterotrophic assay, being this estrogenicity demonstrated by the increase of the uterine weight and by a hypertrophy significative of the luminal epithelium , whose effects are, probably, mediated by ERα, which are the main ER subtype present in uterus. Taking in consideration that PAHs can cause several deleterious effects in the functioning of the living beings endocrine system, it is important to emphasize the necessity of studies in this area, in order to proportionate a better understanding of the real consequences that these effects can provoke in organisms of the natural environments and also to the man.
3. FINAL CONSIDERATIONS PAHs and their derivates are organic pollutants of great environmental persistence (POPpersistent organic pollutants), being that many of them can induce, in the organisms exposed, toxic, cytotoxic, genotoxic, mutagenic, teratogenic and/or carcinogenic effects, which vary according to the PAHs molecular structure (Table 1). Table 1. Summary of the PAHs effects on several organisms. PAHs acenaphthene acenaphthene acenaphthylene acenaphthylene anthracene anthracene
Organisms in vitro test with RT Gill-W1 (gill cells of rainbow trout) in vitro test with HepG2 cells in vitro test with RT Gill-W1 (gill cells of rainbow trout) in vitro test with HepG2 cells in vitro test with HepG2 cells rapeseed (Brassica napus, canola)
Effects
non-cytotoxic non-cytotoxic
References Schirmer et (1998a) Babich (1988) Schirmer et (1998a) Babich (1988) Babich (1988)
photo/toxic
Ren et al. (1996)
cytotoxic non-cytotoxic cytotoxic
anthracene
bluegill fish
photo/toxic
anthracene
fish
photocytotoxic
anthracene
microcrustacean (Daphnia pulex) fishes microcrustacean (Daphnia magna) fish (Pomatoschistus microps)
benzo[a]pyrene
epidermis of mice
carcinogenic
benzo[a]pyrene benzo[a]pyrene benzo[a]pyrene benzo[a]pyrene benzo[a]pyrene
mice mice in vitro test with HepG2 cells in vitro tests with MCF-7 cells in vitro tests with MCF-7 cells
carcinogenic carcinogenic cytotoxic estrogenic estrogenic
anthracene anthracene anthracene
phototoxic teratogenic toxic toxic
al.
al.
McCloskey and Oris (1993) Oris and Giesy (1985) Allred and Giesy (1985) Tuvikene (1995) Holst and Giesy (1989) Vieira et al. (2008) Melendez-Cólon (1999) Warshawsky (1999) Straif et al. (2005) Babich (1988) Plíšková et al. (2005) Lipzig et al. (2005)
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Table 1. Summary of the PAHs effects on several organisms (Continued) benzo[a]pyrene benzo[a]pyrene benzo[a]pyrene benzo[a]pyrene benzo[a]pyrene PAHs benzo[a]pyrene
in vivo test - immature rat uterotrophic assay human umbilical cord cells in vitro test with Chinese hamster V79 cells in vitro test with Chinese hamster V79 cells rapeseed (Brassica napus, canola) Organisms alevins of the rainbow trout (Salmo gairdneri Richardson)
estrogenic genotoxic
Annas et al. (2000)
photo/cytotoxic
Utesch (1996)
photo/cytotoxic
Ochi et al. (1985)
photo/toxic
Ren et al. (1996)
Effects
References
teratogenic
Hose et al. (1984)
benzo[a]pyrene
fish (Fundulus heteroclitus)
teratogenic
benzo[a]pyrene
mice
teratogenic
benzo[a]pyrene
mice
teratogenic
benzo[a]pyrene
mice
teratogenic
benzo[a]pyrene benzo[a]pyrene benzo[a]pyrene
mice bacteria (Escherichia coli) fish (Pomatoschistus microps)
teratogenic toxic toxic
benzo[a]pyrene
mice
toxic
benzo[a]pyrene benzo[a]anthracene benzo[a]anthracene
humans in vitro test with HepG2 cells in vitro tests with MCF-7 cells in vivo test - immature rat uterotrophic assay
toxic cytotoxic estrogenic
benzo[a]anthracene
Kummer et al. (2008)
estrogenic
Wassenberg et al. (2002) Shum et al. (1979) Mackenzie and Angevine (1981) Legraverend et al. (1984) Payne (1958) Kim et al. (2007) Vieira et al. (2008) Anselstetter and Heimpel (1986) Gupta et al. (1993) Babich (1988) Plíšková et al. (2005) Kummer et al. (2008)
benzo[b]anthracene
embryos of zebrafish
teratogenic
benzo[b]fluoranthene benzo[k]fluoranthene
cytotoxic cytotoxic non-estrogenic
Kummer et al. (2008)
chrysene chrysene
in vitro test with HepG2 cells in vitro test with HepG2 cells in vivo test - immature rat uterotrophic assay in vitro test with HepG2 cells in vitro tests with MCF-7 cells
Incardona et (2006) Babich (1988) Babich (1988)
chrysene
embryos of zebrafish
dibenz[a,h]anthracene dibenzo[a,l]pyrene
rat in vitro tests with MCF-7 cells
cytotoxic estrogenic non-embryotoxic/nonteratogenic carcinogenic non- estrogenic
dibenzo[a,l]pyrene
epidermis of mice
carcinogenic
dibenzo[a,l]pyrene
rat
carcinogenic
dimethylbenzo[a]anthracene 7,12dimethylbenzo[a]anthracene fluoranthene
mice
teratogenic
epidermis of mice
carcinogenic
in vitro test with HepG2 cells in vivo test - immature rat uterotrophic assay in vitro tests with MCF-7 cells embryos of mussels rapeseed (Brassica napus, canola) embryos of sea urchin in vitro test with RT Gill-W1 (gill cells of rainbow trout)
cytotoxic
Babich (1988) Lipzig et al. (2005) Incardona et al. (2006) Platt et al. (1990) Plíšková et al. (2005) Melendez-Cólon (1999) Okona-Mensah et al. (2005) Sanyal and Li (2007) Melendez-Cólon (1999) Babich (1988)
estrogenic
Kummer et al. (2008)
non-estrogenic non-toxic
Plíšková et al. (2005) Bellas et al. (2008)
benzo[k]fluoranthene
fluoranthene fluoranthene fluoranthene fluoranthene fluoranthene fluoranthene fluoranthene
fish
al.
photo/toxic
Ren et al. (1996)
photo/toxic
Bellas et al. (2008) Schirmer et al. (1998b) Weinstein et al. (1997)
photocytotoxic photocytotoxic
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Maria Aparecida Marin-Morales, Daniela Morais Leme et al.
fluorene fluorene fluorene hydroxypyrene naphthalene PAHs naphthalene naphthalene phenanthrene phenanthrene phenanthrene phenanthrene phenanthrene pyrene pyrene pyrene pyrene mixture mixture mixture
embryos of sea urchin and mussels in vitro test with RT Gill-W1 (gill cells of rainbow trout) in vitro test with HepG2 cells embryos of sea urchin and mussels in vitro test with RT Gill-W1 (gill cells of rainbow trout) Organisms bacteria (Escherichia coli) embryos of sea urchin, mussels and ascidian higher plant (Arabidopsis thaliana) in vitro test with RT Gill-W1 (gill cells of rainbow trout) in vitro test with HepG2 cells bacteria (Escherichia coli) embryos of sea urchin and mussels embryos of sea urchin and mussels in vitro test with HepG2 cells in vitro test with RT Gill-W1 (gill cells of rainbow trout) embryos of zebrafish in vitro test with fish bluegill BF-2 cells in vitro test with Chinese hamster V79 cells embryos of ducks
toxic
non-cytotoxic
Schirmer et (1998a) Babich (1988)
photo/toxic
Bellas et al. (2008)
cytotoxic
Effects toxic toxic
Bellas et al. (2008)
cytotoxic
cytotoxic non-toxic photo/toxic
Bellas et al. (2008)
photo/toxic
Bellas et al. (2008)
cytotoxic
Babich (1988) Schirmer et al. (1998b) Incardona et al. (2006) Babich and Borenfreund (1987)
cytotoxic
photocytotoxic teratogenic cytotoxic cytotoxic
Hader et al. (1996)
embryotoxic /teratogenic
Hoffman (1981)
mutagenic
White (2002)
teratogenic
mixture
humans
teratogenic
mixture mixture
teratogenic teratogenic
environmental sample environmental sample - air
humans humans earthworm (Eisenia fetida andrei) embryos of sea urchin, mussels and ascidian neotropical fish (Oreochromis mossambicus) zebrafish (Danio rerio) yeast bioassay
environmental sample - air
HepG2 cells
genotoxic
mixture
mixture mixture mixture
environmental sample - air environmental sample - air environmental sample - air
bacteria assays – Salmonella test and Vitotox® in vitro tests with A549human type II alveolar epithelial cells Ames test and DNA adducts (32P-postlabelling analyses)
Alkio et al. (2006) Schirmer et al. (1998a) Babich (1988) Kim et al. (2007)
mixture
mixture
al.
Schirmer et al. (1998a) References Kim et al. (2007)
cytotoxic
bacteria assay SOS chromotest embryos of the fish specie Melanotaenia fluviatillis turtles (Chelydra serpentina) embryos of rats
mixture
Bellas et al. (2008)
teratogenic teratogenic
toxic toxic toxic teratogenic estrogenic
and
Gay
Pollino and Holdway (2002) Van Meter et al. (2006) Sanyal and Li (2007) Rice and Ward (1982) Perera et al. (2003) WHO (2000) Saint-Denis et al. (1999) Bellas et al. (2008) Shailaja and Classy D’Silva (2003) Diamond et al. (2000) Wang et al. (2003) Sevastyanova et al. (2008)
mutagenic
Du Four et al. (2004)
mutagenic
Roubicek (2007)
mutagenic
Umbuzeiro (2008)
et et
al. al.
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391
Table 1. Summary of the PAHs effects on several organisms (Continued) environmental sample sediments environmental sample sediments environmental sample sediments PAHs environmental sample sediments environmental sample - soil
-
in vitro test – MCF-7 focus assay
anti-estrogenic
Arcaro et al. (1999)
mussel (M. galloprovincialis)
genotoxic
Bihari et al. (2006)
bacteria assay umu-test
non-genotoxic
Bihari et al. (2006)
Effects
genotoxic
References Byrne and O’Halloran (2004) Bispo et al. (1999)
mutagenic
Békaert et al. (1999)
mutagenic
Monarca et al. (2002)
environmental sample - soil
Organisms mollusk (Tapes semidecussatus) bacteria Vibrio fischeri M169 frog (Xenopus laevis) and in vitro tests - Ames and Mutatox® higher plant (Tradescantia) and Ames test earthworm (Eisenia andrei), microarthropod and nematode leguminous
environmental sample - soil
barley (Hordeum vulgare L.)
-
-
environmental sample - soil environmental sample - soil environmental sample - soil
algae (Pseudokirchneriella subcapitata) and bacteria (Vibrio fischeri) rat higher plant (Allium cepa) higher plant (Allium cepa) mussels, oysters and benthic fishes
environmental sample - soil environmental sample - soil environmental sample - water environmental sample - water environmental sample - water
toxic
non-toxic toxic toxic
toxic
Bispo et al. (1999)
toxic cytotoxic genotoxic
Roos et al. (2004) Leme et al. (2008) Odeigah et al. (1997) Bolognesi et al. (2006) Leme and MarinMorales (2008) Çavaş and ErgeneGözükara (2005) Hoshina et al. (2008) Reece and Burks (1985)
genotoxic
environmental sample - water
higher plant (Allium cepa)
genotoxic/mutagenic
environmental sample - water
fish (Oreochromis niloticus)
mutagenic
environmental sample - water
fish (Oreochromis niloticus) microcrustacean (Daphnia magna) crucian carp (Carassius auratus) and in vitro test with rat hepatoma cells H4IIE in vitro test with rat hepatoma cells H4IIE
mutagenic
environmental sample - water environmental sample - water environmental sample - water environmental water/sediments
sample
–
dab (Limanda limanda)
Erstfeld and SnowAihbrook (1999) Smith et al. (2006) Smreczak and MaliszewskaKordybach (2003)
toxic toxic
Ke et al. (2007)
toxic
Qiao et al. (2006)
non-genotoxic
Akcha et al. (2003)
genotoxic
Taban et al. (2004)
genotoxic/mutagenic
Vanzella et al. (2007)
mutagenic
Baršienė et al. (2006)
mutagenic
occupational exposure
mussel (Mytilus edulis L.) and sea urchin (Strongylocentrotus droebachiensis) neotropical fish (Prochilodus lineatus) fishes turbot (Scophthalmus maximus) and Atlantic cod (Gadus morua) humans
occupacional exposure
humans
non-carcinogenic
occupational exposure coal tars cigarette smoke
humans mice placenta cells of murine macrophages of rats humans HepG2 cells
toxic carcinogenic cytotoxic
Palli et al. (2008) Jacobson-Kram et al. (1993) Pedrozo et al. (2002) Warshawsky (1999) Detmar et al. (2008)
cytotoxic
Hiura et al. (1999)
cytotoxic
Pandey et al. (2006)
simulation experiment - water (crude oil) simulation experiment – water (diesel) simulation experiment - water (crude oil)
diesel exhaust particles frying oil
and
392
Maria Aparecida Marin-Morales, Daniela Morais Leme et al. in vitro test with Chinese hamster V79 cells keratinocytes of the human epidermis rat mice Organisms embryos of rats
heavy oil jet fuels food chain carbon black oil PAHs crude oil wheathered ANS mixtures
oil
and
embryos of fishes
cytotoxic
Hadnagy et al. (1997)
cytotoxic
Chou et al. (2003)
genotoxic non-teratogenic Effects teratogenic
Lemiere et al. (2005) Hansen et al. (2000) References Feuston et al. (1997) Incardona et al. (2005)
teratogenic
Carcinogenic and mutagenic potentialities of the different PAHs are significative for those who have more than 4 aromatic rings and even higher, for those who have 5 or 6 rings. Substitution of hydrogen for chemical groups, can also affect, drastically, the PAHs activity depending on the position where the substitution occurs and of the substituent group (Combariza et al., 1995). Simultaneous presence of several of these compounds in the environment makes the assessment of their genotoxicities from environmental samples, very difficult, besides also difficulting the correlation studies between the exposition via and the observed carcinogenic effect (Tuvikene, 1995). These characteristics vary from one species to another but, in the human species, the respiratory via is considered the most important, particularly for individuals occupationally exposed, although, in several cases, the dermic via is so or more important (Harvey and Dune, 1978). Another effect observed, resulted from the exposition to PAHs, is that many of them have a negative effect on the immunological system, characteristic that seems to be associated with carcinogenicity (Luster and Rosenthal, 1993; Fiala et al., 2000, 2004; Borská et al. 2006). One of the most studied PAHs and, consequently most known in regard to their effects, is the B[a]P, which was, firstly, identified in domestic soot and, subsequently, in particulate matter. This chemical was characterized as a cancerigenous agent of large distribution (Finlayson-Pitts, 1997), however, in the 70 decade, it was acknowledged the carcinogenicity of atmospheric particles extracts, so far attributed to B[a]P, not only for this substance but, also, due to the presence of other PAHs still unknown, in regards to their action (Pitts, 1987; Finlayson-Pitts, 1997). From this epoch, much attention has been given to the assessment of PAHs in environmental and biological matrices. The environmental importance of the PAHs and their derivates is recognized and accepted by the scientific community. Studies about this class of compounds englobe a great variety of aspects such as: reactivity of these compounds, development of analytical methodologies and field studies about concentration, synthesis, biological effects and removal process of these contaminants of the environment. Among the PAHs, 16 (Figure 4) are considered dangerous by the USEPA, being recommended to be evaluated in environmental and biological monitoring. Contamination of rivers, lakes, seas, forests and atmosphere can cause irreparable damages to nature and human health. PAHs malefic action on living beings can be exerted directly and, mainly, by their metabolites, many of them still unknown (Tuvikene, 1995). The search of reliable bioindicators for exposition studies of human and all the other living beings groups to PAHs and their metabolites still constitutes an issue for many researches. As exposed in this work, some specific indicators have been proposed and used
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with success in assessment studies of the deleterious effects induced by PAHs in the exposed organisms.
Figure 4. Chemical structure of the PAHs indicated by USEPA for environmental monitoring.
ACKNOWLEDGMENTS The authors would like to thank the Programa de Formação de Recursos Humanos em Geologia e Ciências Ambientais do Setor de Petróleo, Gás and Biocombustíveis – PRH05 (UNESP/MCT/FINEP/ANP) for the financial support of the researches developed in the thematic of the present review.
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In: Encyclopedia of Polymer Research Ed: Caroline E. Jones
ISBN: 978-1-61761-926-7 © 2011 Nova Science Publishers, Inc.
Chapter 13
POLYCYCLIC AROMATIC HYDROCARBONS TOXICITY IN ANIMALS AND HUMANS Pierina Visciano and Monia Perugini Department of Food Science, University of Teramo, Teramo, Italy
ABSTRACT Polycyclic aromatic hydrocarbons (PAHs) are a group of about 100 different chemicals that are formed during incomplete combustion or heat-induced decomposition of organic matter. Humans can be exposed to PAHs through different routes. For nonsmokers, the major route of exposure is from food with a minor contribution from inhaled air. Food can be contaminated from environmental sources, industrial food processing and from home food preparation. The European regulator has found that there is a low risk to consumers from PAHs in foods based on average dietary exposure but risk management action may be required for high dietary intake. Several PAHs have been shown to be genotoxic carcinogens. In 2002, the Scientific Committee on Food (SCF) reviewed PAH toxicity. For 15 compounds it concluded that there was clear evidence for their toxicity. In 2005, the Joint FAO/WHO Expert Committee on Food Additives (JECFA) performed a risk assessment on PAHs, and basically agreed with the SCF selection. Also the US Environmental Protection Agency has classified some PAHs as priority-pollutants based on toxicity, potential for human exposure, frequency of occurrence and the extent of information available. Seven of these PAHs (benzo(a)anthracene, chrysene, benzo(a)pyrene, benzo(b)fluoranthene, benzo(k)fluoranthene, dibenzo(a,h)anthracene and indeno(1,2,3-cd)pyrene) are considered as probable human carcinogens, but benzo(a)pyrene is the most studied and used as a marker for PAHs in ambient air and food. PAHs are themselves chemically inert but undergo metabolic activation in mammalian cells to diol-epoxides that bind covalently to cellular macromolecules, including DNA, causing errors in DNA replication and mutations that initiate the carcinogenic process. Other reactive intermediates can be generated by one-electron oxidation process that can result in chemically unstable alkylation of DNA, which leads to depurination, a potentially mutagenic event. In this paper the most important effects of PAHs on animals and humans health are reported and discussed to define new possible preventive and regulatory programs for these pollutants.
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INTRODUCTION Polycyclic aromatic hydrocarbons are a class of structurally different organic molecules, composed of two or more fused aromatic rings, found in a wide variety of sources. They can be present as complex mixtures in several worldwide ecosystems (air, water, sediments and soil), as a consequence of natural sources (forest fires and volcanic eruptions) and human activities (municipal waste, industrial effluents, petroleum spills, creosote oil, automobile exhaust, coal tar production). In the atmosphere PAHs tend to have low vapour pressure, so they are usually adsorbed on particulate matter. The vapour pressure of PAHs is inversely proportional to the number of rings contained and thus almost all five-ring PAH compounds are particulate bound, while three-ring PAHs are also present as vapour in the atmosphere. The solubility of PAHs in water is inversely proportional to the number of rings the PAH molecule contains. Thus, in aqueous environments, PAHs are generally found adsorbed on particulates and on humic matter, or dissolved in any oily contaminant that may be present in water, sediment and soil. PAHs are hydrophobic compounds and their persistence in the environment is mainly due to their chemical properties. Higher molecular weight PAHs exhibit greater environmental persistence than lower molecular weight PAHs due to an increase in hydrophobicity and an increase in stability with increasing mass. This relationship of environmental persistence and increasing number of benzene rings is consistent with the results of studies correlating biodegradation rates and PAH molecule size (Cerniglia, 1992). PAHs are an important group of chemical compounds some of which are mutagenic and carcinogenic (JECFA, 2006). Furthermore many of them are also potential endocrine disruptors causing adverse effects on hormone homeostasis (Gozgit et al., 2004; Fertuck et al., 2001a). The metabolism of PAHs leads to the formation of toxic as well non-toxic metabolites. Therefore, data on the biological fate of these chemicals and on the nature of the metabolites formed by different species are needed to predict the potential hazard posed to organisms. Most of the data pertains to mammalian and fish species (Varanasi et al., 1986; Yuan et al., 1999; Viau et al., 1999; Ramesh et al., 2000; Billiard et al., 2002) and only little information is available on metabolism by other species (Honey et al., 2000). The health effects of polycyclic aromatic hydrocarbons may be apparent under circumstances of high exposure. However, the great majority of people during their lifetime are exposed to complex mixtures of organic compounds at low concentrations. Long-term exposure may be more important in terms of overall public health. Many single PAHs are carcinogenic in rodent bioassays, and they are suspected carcinogens in humans. PAHs exhibit their biological effects through metabolic activation by cytochrome P450-related enzymes (CYP) to electrophilic species, capable of reacting with the nucleophilic sites of DNA to form adducts. The formation and persistence of carcinogen–DNA adducts are believed to be critical events for the initiation of neoplasia in target cells. The risk of mutations arising from DNA adducts does not depend only on the adduct level but also on their mutagenic potential as well as on the DNA sequence of neighbouring adducts (Binková and Šrám, 2004). In 2008 the European Food Safety Authority (EFSA) Panel on Contaminants in the Food Chain (CONTAM Panel) reviewed the available data on occurrence and toxicity of PAHs and concluded that the risk should be based upon the PAHs (benzo(a)pyrene, benzo(a)anthracene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(ghi)perylene, chrysene, dibenzo(a,h)-
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anthracene and indeno(1,2,3-cd)pyrene) for which oral carcinogenicity data were available. The CONTAM Panel concluded that these PAHs, either individually or in a combination, are currently the only possible indicators of the carcinogenic potency of PAHs in food (EFSA, 2008).
Absorption, Metabolism and Excretion of Pahs Humans are exposed to PAHs by various pathways. Occupational PAH exposure doses (coal tar, aluminium production, carbon electrode manufacture, coal gasification and its conversion to coke and coal tar and the processing and use of coal-tar derived products) exceed by far those encountered in the general environment. For non-smoking people with no occupational exposure to these contaminants, the main source of PAH exposure comes from ingested food and from the breathing of both indoor and outdoor air. PAHs presence in food arises from both environmental pollution (atmospheric deposition on crops, releasing in aqueous environment and adsorption on sediments) and pyrolysis of food components during cooking or food processing (grilling, roasting, frying, smoking). The two highest contributors to the dietary exposure were found to be cereals and cereal products, and seafood and seafood products (EFSA, 2008). Absorption of PAHs from the diet is determined by the size and the lipophilicity of the molecule, the presence of bile in the digestive tract, the dose ingested and the lipid content of the diet. Animals and humans have evolved inducible enzymes and transporters to facilitate the biotransformation and elimination of toxic compounds encountered in the environment. The enzymatic components of this inducible biotransformation system are well-known and include monooxygenases in the cytochrome P450 superfamily as well as conjugating enzymes such as the glutathione transferases and glucuronosyl transferases. The transporter components of this system include the ATP binding cassette (ABC) proteins, which act as efflux pumps to remove metabolites of endogenous and xenobiotic chemicals from cells (Dean et al., 2001). The component of this system consists of soluble receptors that regulate the expression of the biotransformation and transporter genes in response to environmental chemicals. These receptors include several members of the steroid/nuclear receptor superfamily (Honkakoski and Negishi, 2000) as well as the aryl hydrocarbon receptor (AhR), a member of the basic-helix-loop-helix (bHLH)–PerARNT-Sim (PAS) gene superfamily (Hahn, 2002). The AhR is considered an ‘induction receptor’ that controls the expression of aryl hydrocarbon hydroxylase activity. The mechanism by which the AhR regulates the induction of adaptive enzymes, especially CYP1A1, has been studied extensively (Withlock, 1999). Although originally thought to have a relatively narrow structural specificity, the AhR is now known to recognize an impressive range of chemical structures, including non-aromatic and non-halogenated compounds (Hahn, 2002). It has been estimated that there are about 58 different cytochrome P450 genes within the human genome, among these enzyme families CYP 1-3 are involved in xenobiotic metabolism (Guengerich, 2001). While some of these genes are constitutively expressed, the expression of others can be induced by their substrates. Several mechanisms of induction have been characterized, and CYP1A1 has been shown to be under the control of the AhR (Safe, 1995).
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For PAHs metabolic activation is required for their toxic, mutagenic and carcinogenic action (Harvey, 1991). In most cases, cytochrome P450 catalyzed epoxide formation is an initial step in the activation process, followed by formation of other, highly reactive electrophilic metabolites capable of binding to cellular macromolecules, in particular proteins and nucleic acids. In total, three principal metabolic pathways have been described for PAHs (Xue and Warschawsky, 2005). Historically, the bay region dihydrodiol epoxide pathway was the first discovered as being crucial for the mutagenicity of many PAHs (Hall and Grover, 1990; Harvey, 1991). Later, the one-electron oxidation pathway leading to reactive PAH radicals was discovered (Cavalieri and Rogan, 1992; Cavalieri and Rogan, 1995). More recently, the ortho-quinone pathway, also resulting in reactive eletrophilic PAH metabolites, has been described (Penning et al., 1996). The mechanism of PAHs metabolism has been mainly investigated using benzo(a)pyrene (BaP) as a model compound. BaP is known to be converted to 3-hydroxybenzo(a)pyrene (3OH BaP), 9-hydroxybenzo(a)pyrene (9-OH BaP), benzo(a)pyrene-4,5-dihydrodiol (BaP 4,5diol), benzo(a)pyrene-7,8-dihydrodiol (BaP 7,8-diol), benzo(a)pyrene-9,10-dihydrodiol (BaP9,10-diol) and benzo(a)pyrene quinones (BaP quinones) by mammalian and fish liver microsomes (Miranda et al., 2006). In many animal species, including humans (Shou et al., 1994), 3-OH BaP is a major metabolite of BaP, and is the major metabolite measured in the aryl hydrocarbon hydroxylase assay. In humans, CYP1A1, CYP1B1, CYP1A2, CYP2C9 and CYP2C19 are the catalysts involved in the formation of BaP 7,8-diol from BaP (Gautier et al., 1996; Kim et al., 1998; Shimada et al., 1999). BaP 7,8-diol is further oxidized to the ultimate carcinogenic metabolite, 7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene (BaPDE), mainly catalyzed by CYP1A1 (Gautier et al., 1996; Kim et al., 1998), but also by human CYPs 1B1, 1A2, 2C8, 2C9, 2C19, 2E1 and 3A4 forms (Shou et al., 1994; Gautier et al., 1996; Kim et al., 1998). Many of the enzymes involved in the metabolism of PAHs have been shown to be polymorphic. A genetic polymorphism is a genetic difference occurring in at least 1% of the population. A genetic polymorphism in xenobiotic metabolising enzymes may change the ratio of activation/deactivation of PAHs. An increased risk of certain cancers has been linked to certain genetic polymorphisms (IARC, 1999). The exact role of genetic polymorphism on risk for PAH carcinogenesis has not been completely elucidated as compensatory mechanisms or alternative enzymatic pathways may also be involved. Currently, no clear picture of the effects of genetic polymorphisms of certain CYPs on the carcinogenic risk of humans exposed to PAHs can be derived. It seems likely that the influence of polymorphisms depends on the level of exposure being more important at low doses of exposure (SCF, 2002). After initial metabolism, most of the detoxified portion of PAH compounds are excreted into the bile as metabolites, then subsequently eliminated in the faeces. A smaller proportion is excreted in urine. The two major metabolites of benzo(a)pyrene found in urine are 3hydroxy- and 9-hydroxybenzo(a)pyrene, part of them being conjugated to sulphate or glucuronic acid (Bouchard and Viau, 1997; Ramesh et al., 2001). The process of excretion of PAH compounds into the intestine via the bile, reabsorption, and return to the liver by the portal circulation (enterohepatic recirculation) has been demonstrated to occur for 7,12dimethylbenzo(a)anthracene, benzo(a)pyrene, anthracene and pyrene (Ramesh et al., 2004). Enterohepatic circulation extends the residence time of PAHs in the body and may lead to long half-lives of reactive PAH metabolites. Though enterohepatic recycling of PAHs has
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been studied in animal models, it has not been confirmed in human studies (de Kok and van Maanen, 2000).
Cancerogenicity and Genotoxicity PAHs have been described as the largest group of known environmental carcinogens and are probably the most widely distributed environmental contaminants. In the past decade (WHO/IPCS, 1998; SCF, 2002; FAO/WHO, 2005) they were evaluated by the International Programme on Chemical Safety (IPCS), the Scientific Committee on Food (SCF) and by the Joint FAO/WHO Expert Committee on Food Additives (JECFA). SCF concluded that 15 PAHs, namely benzo(a)anthracene, benzo(b)fluoranthene, benzo(j)fluoranthene, benzo(k)fluoranthene, benzo(ghi)perylene, benzo(a)pyrene, chrysene, cyclopenta(cd)pyrene, dibenzo(a,h)anthracene, dibenzo(a,e)pyrene, dibenzo(a,h)pyrene, dibenzo(a,i)pyrene, dibenzo(a,l)pyrene, indeno(1,2,3-cd)pyrene and 5-methylchrysene show clear evidence of mutagenicity/ genotoxicity in somatic cells in experimental animals in vivo and with the exception of benzo(ghi)perylene have also shown clear carcinogenic effects in various types of bioassays in experimental animals (SCF, 2002). Thus, SCF reasoned that these compounds may be regarded as potentially genotoxic and carcinogenic to humans and therefore represent a priority group in the assessment of the risk of long-term adverse health effects following dietary intake of PAHs. SCF suggested to use benzo(a)pyrene as a marker of occurrence and effect of the carcinogenic PAHs in food, based on examinations of PAH profiles in food and on evaluation of a carcinogenicity study of two coal tar mixtures in mice (Culp et al., 1998). Using the assessments of IPCS and SCF as starting points and taking into account newer studies, the JECFA re-evaluated PAHs in 2005. Overall, the JECFA concluded that 13 PAHs are clearly genotoxic and carcinogenic. Except benzo(ghi)perylene and cyclopenta(cd)pyrene, the compounds were the same as those stated by SCF. The JECFA also concluded that benzo(a)pyrene could be used as a marker of exposure to, and effect of, the 13 genotoxic and carcinogenic PAHs (JECFA, 2006). There is a considerable amount of data on levels of PAHs in food, covering a large number of different members of the group as there are differing views on the priority to be given to the individual PAHs. The PAHs analyzed tend to be those with 4- to 5- rings as some members with these structures have been found to be carcinogens, teratogens and mutagens. However, benzo(a)pyrene is usually highlighted, although it comprises less than 5 percent of the total amount of PAHs present in the atmosphere, since it is thought to be the most toxic PAH. The International Agency for Research on Cancer (IARC) considers BaP a known animal carcinogen and a probable human carcinogen (Group 2A). The carcinogenicity of BaP has been attributed to the reaction of BaP metabolites, primarily the diol epoxides, with DNA. There are four enantiomers of BaP diol epoxide that can form adducts with DNA, namely: (+)-anti-BaPDE; (−)-anti-BaPDE; (+)-syn-BaPDE and (−)-syn-BaPDE (Lau and Baird, 1994). Each enantiomer can react via cis or trans addition at the C10 position of BaP with the exocyclic amino groups of guanine and adenine residues (Banasiewicz et al., 2004). Carcinogenesis is a multi-step process, at least two stages of action of chemical compounds being distinguished in it: initiation and promotion. Certain compounds have been found to be initiators, while some others possess only promoting functions. Initiation is mediated by the interaction of an electrophilic metabolite with DNA leading to changes in
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DNA structure followed by mutation. The principal promoter function consists of ‘‘switching-off’’ of the initiated cells from the regulatory control of adjacent normal cells, thus providing conditions for preferential growth of the initiated cells. The ‘‘promotion’’ stage is also referred to as epigenetic, since it is not associated with modification of the genetic apparatus and is induced by non-genotoxic compounds (Kobliakov, 1998; Rakitsky et al., 2002). In the past more attention was given to the genotoxic nature of carcinogens but little has been done to study the epigenetic effects. Recently, however, the epigenomic component of environmental carcinogens has been suggested to be very important in human carcinogenesis due to altering gene expression that leads to an imbalance of cell proliferation, differentiation and apoptosis (Thilly, 2003; Trosko and Upham, 2005). ‘‘Complete’’ carcinogens, i.e., compounds that induce tumors without the need for additional exposure, are able to stimulate both the initiation and promotion stages. Some PAHs are found to be complete carcinogens. The initiation process for PAHs is relatively well studied. The most important step is the binding of the active metabolites of PAHs to DNA, predominantly to amino groups of guanine and adenine. The major stable adduct is formed at the N2 position of desoxyguanosine. DNA adduct formation is generally regarded as one of the first steps in carcinogenicity of the mutagenic PAHs (EFSA, 2008). PAH-DNA adducts have been detected in a variety of human tissues, including target organs of PAH and tobacco-associated cancers. A large number of studies clearly show that lowering exposure to carcinogenic PAH results in decreasing PAH-DNA adduct levels (Kriek et al., 1998). Recent developments hold at least two important potential new applications for cancer molecular epidemiology. First, the capability to sequence the genes that are part of the molecular pathological pathway leading to the initiation of tumors in individuals, provides the basis for a whole new categorization of disease outcomes. This is exemplified by the results of Denissenko and others (Denissenko et al., 1996), who studied the distribution of BaPDE adducts along exons of the p53 gene. Thus, adduct formation at specific sites appears to determine partly the p53 mutational spectrum found in lung cancer cases. The detection of mutational hotspots and genetic fingerprints caused by carcinogen-induced DNA modifications may be useful for molecular epidemiologists to disclose previous exposures and retrospectively identify causative agents. Secondly, new technology being developed will enable us to measure the cellular frequency of known types of mutations along known cancer pathways in otherwise normal tissue obtained from biopsies or samples of exfoliated normal cells from particular tissues. This approach may allow the identification of high risk subjects and early interventions through preventive measures.
Pahs as Endocrine Disruptor Compounds The International Program for Chemical Safety defines endocrine disrupters as “exogenous substances that alter function(s) of the endocrine system and consequently cause adverse health effects in an intact organism, its progeny, or (sub)populations”. Endocrine disrupters interfere with the functioning of the endocrine system in at least three possible manners: by mimicking the action of a naturally produced hormone, binding to their hormone receptors; by blocking the receptors in target cells for these hormones and therefore preventing the action of natural hormones; by altering the synthesis and function of hormone
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receptors and modifying the synthesis, transport, metabolism and excretion of hormones (Ropero et al., 2006). A growing body of literature identifies PAHs as potential environmental endocrine disruptors (Santodonato, 1997). While the mutagenicity and carcinogenicity of this class of compounds has been studied extensively, more recent reports have also suggested that PAHs are suspected to exhibit both estrogenic and antiestrogenic activity. Some 4- and 5-ring PAHs can interact with the estrogen receptor (ER) and signalling pathways in a variety of test systems (Clemons et al., 1998; Arcaro et al., 1999) potentially interfering with estrogen signaling in important developmental and reproductive processes. The antiestrogenicity of PAHs generally is based on three possible mechanisms: by binding to the Ah receptor (Clemons et al. 1998; Arcaro et al. 1999) leading to induction of Ah responsive genes that result in a broad spectrum of antiestrogenic responses (Chaloupka et al., 1992; Santodonato, 1997), by blocking activation of the estrogen receptor (ER), as reported in transfected yeast cells (Tran et al., 1996) or by binding of PAHs or metabolites to the ER resulting in an increase of estrogen catabolism. This last activity has been shown to occur in some cases (Ebright et al., 1986; Arcaro et al., 1999) but not in others (Toft and Spelsberg, 1972; Keightley and Okey, 1973). However, the most common mechanism is that PAHs bind to the AhR, and upregulate CYP1A1, which enhances the metabolism of PAHs resulting in the formation of hydroxylated metabolites that bind the ER. The antiestrogenic effects are widely reported for BaP and benzo(a)anthracene (Charles et al., 2000; Fertuck et al., 2001a, 2001b; Vondracek, 2002), but some authors reported also this activity for benzo(b)fluoranthene, benzo(k)fluoranthene, chrysene, and indeno(1,2,3cd)pyrene in vitro test systems (Tran et al., 1996; Arcaro et al., 1999). It should be highlighted that in vitro systems are highly artificial and the extrapolation to specific in vivo responses is challenging. On the other hand, it is possible to underline an estrogenic activity of PAHs and their metabolites. One of the best described modes of action is the crosstalk between activated AhR and ER (Safe and Wormke, 2003). PAHs have been shown to be active in various ERmediated systems that measure either the displacement of 3H-Estradiol-17 beta from the ER in a whole-cell binding assay (Arcaro et al., 1999) or the induction of ER-mediated reporter activity (Charles et al., 2000; Fertuck et al., 2001a, 2001b; Tran et al., 1996; Vondracek, 2002), thus indicating the potential role for estrogenicity. Although certain PAHs or their metabolites may bind the ER and induce transcription in reporter gene assays, it is questionable whether they induce a full estrogen response, for example, cell proliferation and tissue restructuring, by turning on estrogen-dependent genes. For example, hydroxylated BaP metabolites that were ER agonists in vitro were not found to affect mouse uterine weight and uterine lactoferrin mRNA expression, two estrogen-inducible endpoints (Fertuck et al., 2001b). Nevertheless PAHs impact on ER-dependent cell proliferation in target tissues in vivo remains controversial. The in vitro estrogenic activity of BaP and benzo(a)anthracene was reported in the literature (van Lipzig et al., 2005; Kummer et al., 2008), but they both also behaved as estrogen-like compounds in immature rats, as reported by Kummer and others (Kummer et al., 2008). Nevertheless, the results of these studies should be interpreted with caution as the in vivo assay may not reflect possible effects of chronic exposure to low doses of PAHs or impact of co-exposure to other organic pollutants being present in complex environmental mixtures (Ssempewba et al., 2004; Carpenter et al., 2002). Other authors (Kortenkamp and
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Altenburger, 1999) already found that for a number of estrogenic compounds, their combined effect could be explained by ‘concentration addition’ based on similar action of the mixture components. Their results showed that mixtures of weakly estrogenic PAH metabolites induced significant estrogenic effects. Hence, while the risk from exposure to individual weak estrogenic compounds seems negligible, combined they may affect hormone regulation in humans and wildlife. Due to the widespread and continuous emission of the endocrine disruptors into natural, domestic and occupational environments, potential health effects as increased prevalence of cryptorchidism and hypospadias, testicular, breast and prostate cancer, endometriosis, polycysticovarian syndrome and declines in semen quality in the human population seem to be linked to environmental contaminants with hormone-like activity exposure (Foster, 2008).
Other PAHs Toxic Effects Polycyclic aromatic hydrocarbons are potent immunosuppressive environmental contaminants. Immunotoxic effects of PAHs, which may contribute to their carcinogenic potential, have been established in various animal models and in human immune cells (van Grevenynghe et al., 2004). Inhibition of the production of immunocompetent cells (lymphocytes and monocytes) is likely one of the mechanisms contributing to the immunosuppression due to PAHs. Exposure to PAHs inhibits the differentiation of monocytes into dendritic cells and macrophages; it also induces apoptosis of both pre-T cells in the thymus and pre-B cells in the bone marrow, which may account for thymic atrophy and decreased lymphoid cell recovery from the spleen, lymph nodes, and bone marrow in PAHexposed mice (Lutz et al., 1998; Page et al., 2002). These toxic effects towards precursors of immune cells have been linked, at least in part, to the PAHs metabolism into toxic reactive intermediates triggering apoptosis of pre-B cells (Mann et al., 1999). Mature differentiated immune cells also constitute targets for PAHs. PAH exposure therefore likely reduces the total population of macrophagic cells by triggering their apoptotic death. Moreover, PAHs inhibit generation of macrophages from monocytes (van Grevenynghe et al., 2003). Besides benzo(a)pyrene, other PAHs such as 7,12dimethylbenzo(a)anthracene, benzo(a)anthracene, dibenzo(a,h)anthracene, benzo(k)fluoranthene, benzo(b)fluoranthene and indeno(1,2,3-c,d)pyrene were found to exert toxicity towards macrophages. These PAHs are widely detected in air and food and have been reported or suspected to be carcinogenic for humans according to the International Agency for Research on Cancer; by contrast, other PAHs as anthracene, fluorene, fluoranthene and pyrene, which were found to not alter macrophagic features are considered as noncarcinogenic (IARC, 1983). Taken together, these data suggest a direct link between carcinogenicity and immunotoxicity of PAHs. In addition, these environmental contaminants reduce proliferation of lymphocytes in response to mitogenic signals (Davila et al., 1996) and diminish T cell-related cytokine production and B cell-mediated Ab secretion. They also impair some functional properties of differentiated macrophages such as cellular adhesion (van Grevenynghe et al., 2003) and cytokine production (Lyte and Bick, 1986). Interestingly, proinflammatory cytokines, such as TNF-α or interleukin-1β have been reported to act as promoting factors in cell transformation and carcinogenesis induced by PAHs both in vitro and in vivo (Krelin et al., 2007).
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In addition to the carcinogenic and immunotoxic effects, PAHs exposure has also been suggested to be involved in human cardiovascular dysfunction (Gustavsson et al., 1996; Thirman et al., 1994). It has been reported that benzo(a)pyrene is involved in atherogenesis (Thirman et al., 1994) and induce vascular smooth cells proliferation (Zhang and Ramos, 1997). However, the direct evidence related with the association between exposure of PAHs and cardiovascular dysfunction is still weak. Some low molecular weight PAHs such as naphthalene, fluoranthene and fluorene induced vasorelaxation in isolated rat aorta in an endothelium dependent manner (Kang and Cheng, 1997), suggesting that the PAHs with smaller ring numbers might have direct effect on vascular endothelium. Moreover, Li and others (Li et al., 2004) showed that the above mentioned PAHs could enhance nitric oxide formation in human umbilical vein endothelial cells, as they activated and up-regulated the endothelial nitric oxide synthase expression. Another effect on vascular system is the atherosclerosis. Some reports showed that PAHinduced atherosclerosis was linked to the metabolic activation of PAHs by CYPs (Majesky et al., 1983, Paigen et al., 1986). PAHs have been shown to cross the placenta exposing the foetus to the contaminant body burden of the mother. Consequently, a gestational exposure may result in increased adverse health outcomes, possibly affecting cognitive performance (Wormley et al., 2004). Saunders and others (Saunders et al., 2002; Saunders et al., 2003) showed that exposure to BaP and fluoranthene produced a variety of behavioral deficits specific to the nervous system, including decreased motor activity, neuromuscular, physiological and autonomic abnormalities, and decreased responsiveness to sensory stimuli. Children born in years of maximal air pollution in the Czech republic had learning disorders that were attributed to elevated levels of polyaromatic hydrocarbons in the atmosphere from the mining and combustion of coal (Otto et al., 1997). Similarly, in Poland children exposed in utero to PAHs released into the environment as a result of industrial emissions and combustion of coal had impaired neurocognitive development (Jedrychowski et al., 2003). Maternal exposure to airborne PAHs during pregnancy in the inner city African-American population of New York was associated with a reduction in head circumference in children that may correlate with poorer cognitive functioning (Perera et al., 2003). Paternal occupational exposure to polycyclic aromatic hydrocarbons was associated with an increased risk of neuroectodermal tumors in children from Italy, France, and Spain (Cordier et al., 1997). Similarly, an association between paternal exposure to creosote and diagnosed cases of neuroblastoma in children was seen (Kerr et al., 2000). Toxic effects of PAHs in tissues such as placenta have been demonstrated to be due to their metabolites which interact with DNA. Neurotoxicity of PAHs was also studied in adults. Plant workers in Poland employed to process coke developed neurotoxic symptoms with vegetative dysregulation and loss of shortterm memory; their prevalence depended on the level of exposure to BaP (Majchrzak et al., 1990). Neurological symptoms were also reported from a community chronically exposed to BaP, benzo(a)anthracene, chrysene, naphthalene, fluorene and pyrene dumped at a site in Texas from the 1960s until the 1970s (Dayal et al., 1995). Similarly, residents near a combustion site in Louisiana displayed neurophysiological and neuropsychological impairments (Kilburn and Warshaw, 1995). Zhang and others showed that benzo(a)pyrene could affect the autonomic nervous function of coke oven workers mainly by downregulating the parasympathetic nervous function (Zhang et al., 2008).
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CONCLUSION The number of chemicals to which humans are exposed has dramatically increased in the past 100 years. Although the health effects of single contaminants may be apparent under circumstances of high exposure, most people are exposed to chemical mixtures of organics and inorganics at lower concentrations. PAHs are generally found as mixtures in the environment (e.g., products of partial combustion, coal tars, environmental cigarette smoke), and the interactions among PAHs may be quite complex. The toxicity and carcinogenicity of PAH mixtures may depend on the net activities of CYP inducers (AhR agonists or antagonists), the prevalence of enzyme inhibitors versus substrates for bioactivation, and the activation or inhibition of metabolic pathways leading to nontoxic/noncarcinogenic metabolites (Carpenter et al., 2002). The interpretation of the studies on toxicological risk from PAH exposure is complicated by several factors. The toxicity of PAH mixtures can not be accurately predicted based on chemical analysis, due to chemical interactions or to the presence of unidentified chemicals. In addition, it is practically impossible for epidemiology to disentangle the risk posed by each individual PAH. Indeed PAHs occur invariably as mixtures, whose composition depends mainly on the raw material and the combustion circumstances, and they are often adsorbed to particles in the environment. As a consequence epidemiologic data are inadequate to evaluate the possible interaction between PAHs and particles in the induction of cancer in humans. Finally, exposure to PAHs is ubiquitous and the risk associated with complex mixtures remains a challenge to predict. In recent years, evidence has emerged for a contribution of environmental agents to a number of diseases that have not previously been considered to have environmental causes. This is especially true for some general categories of diseases linked to endocrine disruption, as well as a number of chronic diseases. In many of these situations we have some evidence for a variety of chemicals having a relationship to the disease, but currently little or no evidence of the nature and effects of interactions among chemicals related to the disease state has been demonstrated.
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van Grevenynghe, J., Rion, S., Le Ferrec, E., Le Vee, M., Amiot, L., Fauchet, R. & Fardel, O. (2003). Polycyclic aromatic hydrocarbons inhibit differentiation of human monocytes into macrophages, The Journal of Immunology, 170, 2374–2381. van Grevenynghe, J., Sparfel, L., Le Vee, M., Gilot, D., Drenou, B., Fauchet, R. & Fardel, O. (2004). Cytochrome P450-dependent toxicity of environmental polycyclic aromatic hydrocarbons towards human macrophages. Biochemical and Biophysical Research Communications, 317, 708-716. van Lipzig, M. M. H., Vermeulen, N. P. E., Gusinu, R., Legler, J., Frank, H., Seidel, A. & Meerman, J. H. N. (2005). Formation of estrogenic metabolites of benzo(a)pyrene and chrysene by cytochrome P450 activity and their combined and supra-maximal estrogenic activity. Environmental Toxicology and Pharmacology, 19, 41–55. Varanasi, H., Nishimoto, M., Reichert, W. L. & Eberhalt, B. (1986). Comparative metabolism of benzo(a)pyrene and covalent binding to hepatic DNA in English sole, Starry Flounder, and rat. Cancer Research, 46, 3817-3824. Viau, C., Bouchard, M., Carrier, G., Brunet, R. & Krishnan, K. (1999). The toxicokinetics of pyrene and its metabolites in rats. Toxicology Letters, 108, 201-207. Vondracek, J. (2002). Modulation of estrogen receptor-dependent reporter construct activation and G0/G1-S-phase transition by polycyclic aromatic hydrocarbons in human breast carcinoma MCF-7 cells. Toxicological Sciences, 70, 193– 201. Whitlock, J. P. (1999). Induction of cytochrome P4501A1, Annual Review of Pharmacology and Toxicology, 39, 103-125. WHO/IPCS (1998). World Health Organization/International Programme on Chemical Safety. Selected Non-heterocyclic Polycyclic Aromatic Hydrocarbons. Environmental Health Criteria 202. International Programme on Chemical Safety, World Health Organization, Geneva. Wormley, D. D., Ramesh, A & Hood, D. B. (2004). Environmental contaminant–mixture effects on CNS development, plasticity, and behaviour. Toxicology and Applied Pharmacology, 197, 49-65. Xue, W. & Warschawsky, D. (2005). Metabolic activation of polycyclic and heterocyclic aromatic hydrocarbons and DNA damage: A review. Toxicology and Applied Pharmacology, 206, 73-93. Yuan, Z.-X., Honey, S. A., Kumar, S. & Sikka, H. C. (1999). Comparative metabolism of dibenzo(a,l)pyrene by liver microsomes from rainbow trout and rat. Aquatic Toxicology, 45, 1-8. Zhang, H.-M., Nie, J.-S., Wang, F., Shi, I.-T., Zhang, L., Antonucci, A., Liu, H.-J., Wang, J., Zhao, J., Zhang, Q.-L., Wang, L.-P., Song, J., Xue, C.-E, Di Gioacchino, M. & Niu, Q. (2008). Effects of benzo(a)pyrene on autonomic nervous system of coke oven workers. Journal of Occupational Health, 50, 308-316. Zhang, Y. & Ramos, K. S. (1997). The induction of proliferation vascular smooth muscle cell phenotypes by benzo(a)pyrene does not involved mutational activation of ras genes. Mutation Research, 373, 285–292.
In: Encyclopedia of Polymer Research Ed: Caroline E. Jones
ISBN: 978-1-61761-926-7 © 2011 Nova Science Publishers, Inc.
Chapter 14
POLYCYCLIC AROMATIC HYDROCARBONS AND CARCINOGENIC RISK ASSESSMENT IN DEVELOPING COUNTRIES Mahmoud A. Hassanien Air Pollution Department, National Research Center, Dokki, Cairo, Egypt
INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are a group of organic compounds composed of fused aromatic rings. Some high molecular weight PAHs are considered to be carcinogenic, mutagenic, teratogenic even at low concentrations and have received more environmental concern. PAHs are resistant to biodegradation and can bioaccumulate in the environment due to their hydrophobic nature and low solubility. PAHs are ubiquitous environmental contaminants found in air, soil and water. The main sources of PAHs in the environment and for general population exposure are natural and intentional combustion of organic matter and petroleum manipulation. Atmospheric emissions will arise from the processing of coal and petroleum products and incomplete combustion during industrial processes. In the developing countries, present information concerning the sources and inputs of PAHs in these countries is rather limited. PAHs concentrations have been reported in a number of urban areas as well as in remote places and are consistently higher in winter than in summer, and in urban than in rural areas. The higher values are found in particular places like stack gases, tunnels of motorways, open burning sites, etc. There are no ambient air quality standards and no specific guideline value can be recommended for PAHs in air. PAHs in air are attached to particles, which may also play a role in their carcinogenicity. Although food is thought to be the major source of human exposure to PAHs, part of this contamination may arise from air pollution with PAHs. Exposure to PAHs mainly occurs from atmospheric pollution through inhalation, ingestion and dermal contact. PAHs half-lives in humans are in fact in the range of days/hours, and metabolism is responsible of the formation of carcinogenic metabolites. Risk assessments and potency assessments of various individual and complex mixtures of PAHs
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have been attempted. BaP is the only PAH for which a database is available, allowing a quantitative risk assessment. PAH potencies are used to determine quantitative health risks posed by PAH exposure. The risks posed by a mixture of PAHs are based on an assumption of additively of the individual risks posed by the PAHs. The objectives of this chapter are to: 1) report PAH concentrations in the environmental media in developing countries; 2) collate published and unpublished PAH information from environmental agencies and write a report on the state of knowledge of PAHs; 3) characterize the priority PAHs, carcinogenic risk assessment and potential health impacts.
DEFINITION AND SOURCES Polycyclic Aromatic Hydrocarbons (PAHs) are a ubiquitous environmental contaminants found in air, soil, and water. Some of which are harmful to human and animals and considered environmental contaminants. PAHs have been shown to be persistent in various media in nature at significant levels or at levels to which exposure to human and animal life over a continued period is toxic. A lot of PAHs are recalcitrant and thus their continued presence in the nature is a constant source of risk to human, animal and plant health, where they could be carcinogenic, mutagenic or induce gentotoxic effects in cells of plants and animals. PAHs are a group of organic compounds composed of fused aromatic rings. Some high molecular weight (greater than four fused rings, HMW) PAHs are considered to be carcinogenic, mutagenic and teratogenic even at low concentrations (IARC, 1983, Ciganek et al., 2002). Because of their hydrophobic nature and low solubility, PAHs are resistant to biodegradation and can bioaccumulate in the environment through the food chain. Therefore, PAHs in the environment represent a long-term threat to human health and have received more and more environmental concern. PAHs are complex mixtures of hundreds of chemicals, including derivatives of PAHs, such as nitro-PAHs and oxygenated products, and also heterocyclic PAHs. The biological properties of the majority of these compounds are as yet unknown There are natural and anthropogenic sources of PAHs. Natural PAHs are formed as sideproducts of natural and anthropogenic carbon combustion. During combustion there is the thermal alteration of organic compounds in the organic material being combusted, this alteration produces PAHs and the type of PAH produced depends on the pyrolysis temperature. Natural sources of PAHs include volcanoes, the earths’ geologic permutations, and crude oil (Zander, 1980, Baek et al., 1991, Juhasz et al., 2000). Other natural sources of PAHs include PAH formation during thermal and geologic production of materials, emissions from trees, burning of vegetation in forest or bush fires and biogenic precursions during early digenesis (Cerniglia 1980). Anthropogenic sources of PAHs include incomplete combustion of fuels such as wood, petroleum, oil and gas. Approximately 90% of PAHs emissions are estimated to be anthropogenic sources typically including the combustion of fossil fuel, industrial processes and domestic heating systems. Vehicular traffic is considered to be the major contributor to the urban atmosphere (Fang et al., 2004; Yang and Chen, 2004). Anthropogenic sources of PAHs account for significant amount of PAHs present in nature today, “concentration of PAH found in soils around urban areas are sometimes up to two orders of magnitude higher that those in less-developed areas (derived from forest fires and
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air pollution)” (Dabestani and Ivanov, 1999). Vehicle emissions alone are believed to be responsible for 35% of the total PAH emission in highly populated and industrialized urban areas of the United States (Bjoerseth et al. 1985). Anthropogenic sources are rapidly increasing the amount of PAHs in the environment as PAH accumulation rate far exceeds the degradation rate of PAHs. Industrialization of developing countries and continued industrial expansion of developed countries is projected to increase the PAH production as projections on PAH production are forecasted to increase in the coming decades (Han and Naeher, 2006). Other sources include grilling meat, incineration of wastes, and use of industrial power generators. Industrial processes can produce PAHs as can the use of chemical solvents and the various incineration, or high thermal processes. Chemical processes such as the production of coal tar, petroleum, fractional distillation of crude oil, creosote production (Harrad et al., 2006; Lin et al., 2008) and various industrial processes are significant point sources for PAHs. Most of PAHs are released into the environment from anthropogenic sources such as combustion of fossil fuels in power plants, domestic heating, waste incineration, industrial processes and, most importantly, motor vehicle exhaust (Lim et al., 1999; Marr et al., 1999; Lehndorff and Schwark, 2004; Chang et al., 2006). Additional contributions to ambient air levels arise from tobacco smoking, while the use of invented PAHs can be found in various compartments of the environment: air, surface water, sediment, soil, food and in lipid tissues of both aquatic and terrestrial organisms' sources can increase. Because of such widespread sources, PAHs are present almost everywhere (Zhang et al., 2008; Lin et al., 2008). In 1992, the National Academy of Sciences identified PAHs as among the highest priorities for exposure research, in part because these compounds are frequently constituents of fine aerosol and some of them are mutagens and probable human carcinogens. Determining exposure to PAH is still a new area of research. It is still largely unknown in what ways they are distributed among the vapor and particulate phases in air or the aqueous and nonaqueous phases in water. Likewise, their distributions and levels in other media, such as food or soil, are largely unknown (EPA, 1997). PAH carcinogenicity and toxicity is induced during the metabolism of PAHs by living cells (Cerniglia 1984), for this reason knowledge of PAH metabolism or degradation is very important to help us assess correctly the risk involved in exposure to PAHs. Exposure to PAHs is nature is always to multiple PAHs and rarely to an individual PAH. A better assessment of the risk involved in PAH metabolism would involve understanding the degradation of PAHs in mixtures as it occurs in nature, the complex substrate interactions and its effects on the magnitude of risk assessed.
DESCRIPTION OF PAHS PAHs are organic chemicals containing two or more fused aromatic (benzene) rings (Kanaly et al. 2000; Juhasz et al. 2000). They range from the simple two (2) fused benzenering, naphthalene, to complex hydrocarbons such as the ten (7) fused ring, coronene. Parent compound PAHs are fused benzene rings without any chemical moiety or group added to them. They are the basic skeletal structure of PAHs and serve as the template for modified PAHs. PAHs usually exist in the modified form different from the parent compound through the addition of different chemical groups to the parent compound such as the alkyl, nitro and
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sulfur groups. These additions can significantly change the character of the parent compound they can confer previously unknown properties to the parent compound such as increased or decreased solubility, generally affecting properties already present in the parent compound such as hydrophobicity. Addition of these groups considerably modify action and effects due changes in the physical, stearic and chemical properties of the PAH, in effect changing the metabolism and actions of the parent compound. Table 1. Classification of polycyclic aromatic hydrocarbons. Low Molecular Weight; LMW (PAHs). Two or three rings Naphthlene , Acenaphtylene, Acenaphthene, Fluorene, Phenanthrene, Anthracene.
Middle Molecular Weight MMW (PAHs). Four or five rings Fluoranthene , Pyrene, Benzo(a)anthracene a, Chrysene a,
High Molecular Weight, HMW (PAHs). Six or seven rings Benzo(b)fluoranthenea, Benzo(k)fluoranthene , Benzo(e)pyrene, Benzo(a)pyrenea, Perylene, Indeno(1,2,3-cd)pyrenea, Dibenz(a,h)anthrathenea, Benzo(b)chrycene, Benzo(ghi)perylenea, Coronene.
a
Carcinogenic polycyclic aromatic hydrocarbon compounds ( IARC, 1983 and Kipopoulou et al., 1999, Hassanien, 2006)
CHEMICAL AND PHYSICAL CHARACTERISTICS The fused benzene rings of PAHs could be in linear, angular, or clustered arrangements (Kanaly et al. 2000). PAHs with less than three benzene rings are termed low molecular weight PAHs (LMW PAHs) while those with more than three benzene rings are called high molecular weight PAHs (HMW PAHs) as shown in Table 1. The chemical properties, and hence environmental fate of an individual PAH, is dependent in part upon its molecular size i.e. number of aromatic rings, this determines its solubility, octanol water partitioning coefficient (Kow), configuration and electrochemical stability (Dabestani and Ivanov, 1999, Kanaly et al. 2000, Juhasz et al. 2000). This general grouping of PAHs as HMW and LMW PAHs becomes useful because each group member share similar characteristics, though with slight variations. The physicochemical and environmental properties interactions of parent compounds of PAHs in both groups are characteristic of the groups, in the low molecular PAH group behavior within similar range includes, low boiling and melting points, high water solubility which translates into them being easily degradable, similar octanol-water partition co-efficient, similar fugacity values, vapor pressure and other properties. In the high molecular weight PAH group physico-chemical and environmental properties apart from their molecular weights are also shared such as high boiling and melting points, low solubility in water, low octanol-water partition co-efficient, not being easily degradable, higher toxicity to plants and animals, similar fugacity, vapor pressure and other physical properties. It is noticed that this grouping
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might not hold when parent compounds are substituted as these functional groups greatly alter the properties of the parent compound. Generally, PAHs are hydrophobic compounds measured directly by their aqueous solubility in water (Bjoerseth 1985). PAH hydrophobicity increases with an increase in the number of fused benzene rings. PAH hydrophobicity is a major factor contributing to the persistence of PAHs in the different polar media in the environment and in lipids (Cerniglia 1984; Dabestani and Ivanov 1999; Kanaly et al. 2000; Juhasz et al 2000). The increase in size of PAHs with increasing benzene rings along with increasing angularity confers greater electrochemical stability to PAHs making them be more stable in nature (Kanaly et al. 2000). The Octanol-water partition constant (Kow) is the ratio of the concentration of a substance in octanol to that in water at equilibrium at a specified temperature (Asante-Duah 2002), (Kow) is used to predict the fate of PAHs either in the environment or in biological tissues. (Kow) increases with increasing benzene rings, from LMW PAHs to HMW PAHs. The (Kow) is used to determine the phase which the PAH shall partition to, this ability is very useful in human exposure analysis to PAHs (Asante-Duah 2002). Measurements of (Kow) of individual PAHs correlate perfectly with measurements of solubility and hydrophobicity, with increasing hydrophobicity is low solubility and an increasing Octanol-Water partition coefficient.
SOURCES OF PAHS IN DEVELOPING COUNTRIES The main sources of PAHs in the environment are natural and intentional combustion of organic matter (e.g. forest fires and fuel combustion) and petroleum manipulation (extraction, transport and refining). The contributions, therefore, range from diffused chronic inputs (e.g. atmospheric deposition and terrestrial run-off) to large point releases (i.e. tanker accidents or accidents at coastal installations). Present information concerning the sources and inputs of PAHs in these countries is rather limited, but the Mediterranean Sea and its region has been considered to be relatively more polluted by oil than any other sea from which data are available (UNEP, 2002). Sea transportation appears to be one of the main sources of PAHs pollution into the Mediterranean. The land-based discharges are both industrial and urban (Viana et al., 2005). The number of major refineries located in the Mediterranean region is more than 40 with a combined capacity, in 2000, of about 438 million tones. An additional source of PAHs is the result of the sewage sludge management strategies. Moreover, accediental sources in the Mediterranean region showed that over the last decades, about 55000 tonnes of oil have been spilled at the Mediterranean Sea with 3 major accidents amounting to 75% of the quantity spilled. 82 accidents involving oil spills were recorded during the period Jan. 1990 to Dec. 1999 while the quantity of spilt oil was 22150 tonnes (UNEP, 2002).
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Table 2. Estimated PAH inputs from diverse sources, assuming two Different PAH contents in the case of oil discharges (UNEP, 2002) Type Shipping discharges
Accidents Refinery effluents
Annual oil input rate (tones) 25000 (low estimation) 500000 (high estimation) 3667 900
PAH content (g/tonne) 1.28 2000 1.28 2000 1.28 2000 1.28 2000
Urban and river discharges Sludge sea dumping Sludge applications in soils Sludge landfilling Atmospheric Sludge incineration
PAH input (tonne) 0.32 500 0.64 1000 0.005 7.3 0.001 1.8 30-40 0.2 1.2 1.2 48 0.2
The estimates of the relative importance of the various sources of PAHs in the region are summarised in Table 2. Atmospheric emissions appear to have a relatively high importance while the actual PAHs input from shipping activities remains uncertain. Another source, difficult to estimate, is the accidental fires which are rather current in the region.
DISTRIBUTION OF PAHS Effective transport facilitates PAHs through different media. For example winds can aid in the transference of airborne PAHs from the foci point to regions thousand of miles away from the focal point depending on the wind current and strength, this is a common occurrence as seen in areas with high smoke stacks (Smith and Frankeberg 1975). Also flowing polluted streams might be vectors of pollution as they can transport the pollutants from its source to other water bodies (within medium) or could cause deposition in soil and sediments far off (intermedia) (Chiou et al. 1998), also atmospheric deposition of PAHs is a major source of soil pollution (Chiou et al. 1998; Grimmer 1983), as volatilization of LMW PAHs from the soil could be a source of atmospheric PAH. There are a lot of computational and mathematical models that effectively model transport of PAHs in a medium and through media.
PHASES OF PAHS AND THEIR FATE IN THE ENVIRONMENT Historically, small quantities of PAHs have been found in samples of atmospheric smoke collected in both urban and rural parts of Wales, Cheshire and Lancashire (UK). The results showed that the concentration of hydrocarbons were much lower in rural than in urban areas (Commins, 1958). In the atmosphere, PAHs display a wide range of gas-particle partitioning characteristics. Vapor-particle partitioning in the atmosphere can be approximated from a
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compound’s subcooled liquid vapor pressure (PL) and/or its octanole air partition coefficient (KOA). Compounds with PL greater than ca. 0.01 Pa (log KOA < 10) are vapor phase constituents; compounds with PL less than ca. 0.000001 Pa (log KOA > 12) are almost entirely particle associated, and compounds with PL (log KOA) between these values can be considered as mixed phase atmospheric constituents (Lohmann et al., 2001). PAHs with two or three rings, which have relatively low log- KOA values, are mainly associated with the vapor phase, 4-ring PAHs occur in both the gas and particle phase, whereas PAHs with five rings or more, which have high log KOA values, are mostly bound to particles. However, a higher temperature probably increases the gas partitioning of the PAHs with four rings or more (Soderstrom et al., 2005). Some high molecular weight (more than four fused rings) PAHs are considered to be carcinogenic, mutagenic and teratogenic even at low concentrations (IARC, 1983; Bostrom et al., 2002; Yu et al., 2006). However, lighter PAHs are most abundant in the urban atmosphere and react with other pollutants to form more toxic derivatives (Chang et al., 2006). Because of their hydrophobic nature and low solubility, PAHs are more resistant to biodegradation and can bioaccumulate in the environment through the food web (Ohura et al., 2004a; Conde et al., 2005; Motelay-Massei et al., 2005; Bartkow et al., 2006; Cupr et al., 2006; Huckins et al., 2006; de Souza Pereira et al., 2007; Zhu et al., 2008). Therefore, PAHs in the environment represent a long-term threat to human health and have received more and more environmental concern. Regulation of PAH emission and reliable monitoring of PAH concentration in ambient air is thus of paramount importance for public health (Lehndorff and Schwark, 2004).
LEVELS OF PAHS IN THE ENVIRONMENTAL MEDIA ATMOSPHERIC AIR No specific guideline value can be recommended for PAHs as such in air. These compounds are typically constituents of complex mixtures. Some PAHs are also potent carcinogens, which may interact with a number of other compounds. In addition, PAHs in air are attached to particles, which may also play a role in their carcinogenicity. Although food is thought to be the major source of human exposure to PAHs, part of this contamination may arise from air pollution with PAHs. The levels of PAHs in air should therefore be kept as low as possible. In view of the difficulties in dealing with guidelines for PAH mixtures, the advantages and disadvantages of using a single indicator carcinogen to represent the carcinogenic potential of a fraction of PAH in air were considered. Evaluation of, for example, BaP alone will probably underestimate the carcinogenic potential of airborne PAH mixtures, since co-occurring substances are also carcinogenic. Nevertheless, the well-studied common constituent of PAH mixtures, BaP, was chosen as an indicator, although the limitations and uncertainties in such an approach were recognized The determination of atmospheric PAHs is hindered by many problems, the most common derived from sampling. Therefore, it is difficult to integrate the existing information into a coherent data set. PAHs concentrations have been reported in dry deposition in a number of urban areas as well as in remote places, including the open Mediterranean Sea and developing countries Table 3. Despite the variety of components analysed, some seasonal and spatial trends are apparent. Concentrations are consistently higher in winter than in summer,
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and in urban than in rural areas. The higher values are found in particular places like stack gases, tunnels of motorways, open burning sites, etc. To compare between PAHs concentration in developed countries with other developing ones Table 4 showing data of estimated indoor and outdoor air concentrations for PAHs (ng/m3) at each concerned area using sampling rates of 2.5 m3/day (indoor) and 3.5 m3/day (outdoor) (Bohlin et al., 2008). Using sampling rates suggested in the literature, the sums of 13 PAHs in the different sites were estimated to be 6.1–180 ng/m3, with phenanthrene as the predominant compound. Indoor PAH levels tended to be higher in Gothenburg and outdoor levels higher in Mexico City. Table 3. Concentration of PAHs in aerosol samples from different cities*. Location Algeria Algiers
Sampling period
PAHs
Conc. (ng/m3)
Wint./sum. 1999
Oued Smar landfill
Wint./sum. 1999
∑18 PAHs BaP ∑18 PAHs BaP
59.9/13.7 0.4-2.3 136.5/24.9 0.3-3.9
Egypt Cairo(urban area)
Wint/sum
Alexandria
Summer 97-98
∑18 PAHs BaP ∑18 PAHs BaP
14.8/7.5 0.1-0.2 32 0.95
Greece Thessaloniki
1987-1988
∑9 PAHs BaP ∑13 PAHs ∑10 PAHs BaP
22-55 2-6 5-60/0.4-8.1 11.3(V), 0.8(P) 0.003(V),0.025(P)
∑7 PAHs BaP BaP PAHs BaP ∑4 PAHs BaP
5.96-7.33 1.2 0-174/(av.=22.7) 2-130 0.31-3.8 0.8-19.1 1.1-2.0
North Greece town Crete inokalia Italy Rome
wint/sum 96-97 2000-2001
Bolgan Naples
1991-1993 1996-1997
Genoa (traffic site)
1997-1999 1994-1999
Syria Damascus
1999-2000
∑18 PAHs BaP
14.8-45.7 0.9-2.7
FR Yugoslavia Belgrade
2000
BaP
0.2-0.6
1993-1998
*(modified from UNEP, 2002)
Phenanthrene was the predominant PAH compound and the 3–4 ring PAHs which are mainly in the gas phase contributed to >96% of ∑13PAH. The indoor levels in Mexico City ranged from 6.1 to 92 ng/m3, with median levels being 28 ng/m3 at the urban area, and 25 ng/m3 at the semi-rural area. The indoor levels of PAHs tended to be higher in Gothenburg (median 37 ng/m3) than in Mexico City and Lancaster (median 28 ng/m3). The estimated levels of indoor air in Kuwait, both in terms of the composition profile and ∑PAHs (1.3–16 ng/m3). The levels herein are also in agreement with estimated indoor air
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levels of PAHs from an area in Sweden where domestic wood burning is widespread (20–250 ng/m3) (Strandberg et al., 2006). The outdoor levels in Mexico City ranged from 32 to 92 ng/m3. Highest median levels (82 ng/m3) were found in the semi-rural area where both point sources of PAHs (i.e. main roads, backyard burning and forest fires), and wind drift from the urban plume occur. As expected, outdoor levels tended to be lower in Gothenburg and Lancaster compared to Mexico City (Coleman et al., 1997; Motelay-Massei et al., 2005). Table 4. Indoor and outdoor concentrations of PAHs in different areas
Mexico City urban In Out Mexico City semi-rural In Out Gottenburg In Out Lancaster In Out
Range
Mean
∑13 PAHs Median
6.1-92 32-64
34 47
28 44
12-37 72-92
25 82
25 82
14-180 7.7-68
69 37
37 33
8.5-60 6.8
30 6.8
28 6.8
The levels of PAHs released into the environment from various human activities especially from industrial sources in the Niger Delta area of Nigeria shows that the most polluted medium is the air followed by the soil and water media. Comparing the study locations there is an indication that Eleme (the most industrialized) as compared to Ahoada East (one of the least industrialized) recorded PAH levels 7- million fold higher in the air. Levels of PAHs in Eleme were 3-fold higher in surface water and 7 -fold higher in soil than the levels found in Ahoada East as clearly shown in Fig. 1.
Figure 1. Environmental levels of PAH in the Niger Delta Area.
PAHs occurrence and situation in atmosphere of Egypt is well shown in the following section which represent a case study of developing country. Since the 1960s, Egypt has witnessed rapid development and industrialization, a by-product of which has been the generation of large amounts of air pollutants. As a result, air quality has decreased drastically.
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In Egypt as an example of developing countries, most air toxics originate from human-made sources, including; mobile sources (e.g., cars, trucks, buses) and stationary sources (e.g., factories, refineries, power plants); as well as indoor sources (e.g., building materials and activities such as cleaning). Toxic air pollutants are emitted from many industrial and nonindustrial sources; fuel combustion, open burning of organic material, solvent uses, storage and transport of petroleum products, volatilization from the use of consumer products, and various manufacturing and commercial processes. These sources are categorized as point, area or non-point, non-road mobile, and on-road mobile sources. Several new industrial areas were established such as 6th of October, 10th of Ramadan and Sadat City. These new industrial cities were founded to divert industrial development from the Nile Valley and Delta towards the adjacent desert areas. PAHs are at the present time one of the most documented class of organic pollutants in the environment due to their toxicity carcinogenicity, and mutagenicity, PAHs continue to attract the curiosity of researchers. PAH concentrations found in the atmosphere are highly dependent of the size of particles and the highest concentrations are found in breathable particle. Certain PAHs are suspected of being carcinogens and some are associated with acute and chronic health problems. The main sources of PAHs in Egypt are diesel exhausts, the petroleum and printing industries, and road transport. However, there are few available studies on PAHs in ambient air in Cairo city. PAH concentrations in Sadat city as an example of new industrial area were measured, beside old industrial areas in Egypt. The seasonal and annual mean concentrations of ∑9PAHs in Sadat City during the year 2003 were 403.79 and 143.82 ng/m³, respectively at two measuring sites. The ∑9PAH reached the maximum during winter season (786.69 ng/m³), while the minimum value was recorded during autumn season (33.09 ng/m³). The annual average concentration of benzo(a)pyrene was 18.464 ng/m3 at the industrial area in Sadat city during the period under study. The mean concentrations of benzo(a)pyrene in the present study were 6 times higher than the annual limit set by Italy (2.5 ng/m3), and the current guideline limit of the Federal Environmental Agency of Germany (1 ng/m3) (Ezzo, 2006). In the same way, the concentration of PAHs were measured in the industrial area in Greater Cairo, the obtained values were much higher than recorded for the industrial areas for many countries such as New Jersey, Canada and Belgium. This may be due to the higher local emission of PAHs from the coke factory, iron and steel factory and other industries in and around this area. PAHs are normally associated with small particles that generally have long residence times in atmosphere and, for that reason have the potential to be transported to quite long distances (Hassan, 2006). The annual mean concentrations of atmospheric PAH over the 4 investigated sites in Cairo from autumn 2001 to autumn 2003 were illustrated in Table 5 (Hassan, 2006). It can be noted that the highest levels were recorded in Shoubra Elkhiem (industrial area) followed by El-Tebeen (industrial area), then City center, and finally Faysal (residential area). Worldwide, regulations and standards regarding permissible levels of PAHs in ambient air vary somewhat. The maximum permissible concentration in ambient air is 1 ng/m3 in Soviet Union former and Hungary (Hassanien et al, 1996). In the US, no national ambient air quality standard exists. However, several states have established ambient standards. For B(a)P, these range from 0 to 100 ng/m3 (8-hour average). In the Netherlands, the Environmental Programme 1988-1991 recommended a draft tolerable B(a)P concentration of 5 ng/m3 (annual average) (RIVM, 1989) see Table 6.
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Table 5. Seasonal and annual mean concentrations of atmospheric PAH in different sites in Egypt Location
PAHs
Sampling year
Cairo
∑12 PAHs
Sadat City New Indus. city Greater Cairo Soubra ElKheima (old indus. Area)
∑9 PAHs
1997 Wint/Sum 2003 Win./sum
Recorded value (ng/m3) 22.32 (annual) 14.79/7.53 273.8 (annual) 786.7/145.2
2001-2002 Wint/Sum
4460 (annual) 7040/1771
2001-2002 Wint/Sum
3552 (annual) 5416/1378
Elattaba
2001-2002 Wint/Sum
3477 (annual) 248/134
Faysal (resid..area)
2002-2003 Wint/Sum 2005
1965 2274/1789 37.3 (outdoor) 29.7 (indoor) 20.6 (outdoor 8.8 (indoor)
City Center ∑16 PAHs
∑12 PAHs Cairo (traffic
∑carcino.PAHs
References Hassanien et al, 2001 Ezzo, 2006
Hassan, 2006
Hassanien, 2006
An average of 32 ng/m3 of total PAHs in the ambient air found in a study in heavy traffic areas in Alexandria City (Barakat, 2002). A summary of the data obtained from analysis of polycyclic aromatic hydrocarbons in suspended dust collected during 1997 in Cairo Egypt (Hassanien et al., 2001 and recently Hassanien, 2006) is given in Table 5. It can be seen from the analysis of the 12 components that the sum of PAH concentrations at the sampling site was higher during the winter season 1997 (14.79 ng/m3) than in summer during 1997 (7.53 ng/m3). It was found that the carcinogenic materials constitute about 20%; 33% and 25% of the total PAH for the winter, summer and annual periods, respectively. A highly significant difference between the estimated concentrations of PAHs of winter and summer was observed. It was found that PAHs concentrations in outdoor air generally exceeded that in the corresponding indoor air. The sum of PAHs concentrations in outdoor air was 37.3 ng/m3; the corresponding concentrations in indoor air was 29.7 ng/m3. Carcinogenic compounds measured in b-oth outdoor and indoor represent 20.6 and 8.8 ng/m3, respectively with a contribution 55.2% and 29.6% from the total of PAHs. In addition, from the eighteen PAHs, fluoranthene, pyrene, chrysen, benzo(b)fluroanthene, dibenzo(a,h)anthracene, benzo(ghi)perylene, and benzo(a)pyrene were the most abundant PAHs found in indoor and outdoor air. They contributed more than 55-65%, respectively to the sum of PAHs (Hassanien, 2006).
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Table 6. Ambient air quality standard for the PAHs*.
Netherlands PAH WHO-AQG PAH
Limit 5 ng/m3
Guide value 5 ng/m3 1.0 ng/m3
Measuring period Years as ng/BaP/m3 Years as ng/BaP/m3
*(Hassanien, 2006)
INDOOR VERSUS OUTDOOR PAHS LEVELS IN SOME CITIES In recent decades, there has been a tendency toward atmospheric pollution in the developing countries due to fast urbanization and industrialization. In past years, many studies have stressed the characterization of inorganic components of aerosols in developing cities in Asia and Africa because the desert land profile for these countries which considered the largest source of particulate matter in the world (Baek et al., 1991; MaliszewskaKordybach, 1999; Mastral and Callen, 2000; Ohura et al., 2004b). Indoor emission sources of PAHs include smoking, cooking, heating, and furniture (Liu et al., 2001; Ohura et al., 2002). Ratios between indoor and outdoor air concentrations (I/O ratios) may be used as a tool to indicate whether there are indoor sources (I/O> 1) or outdoor sources (I/O<1). Spatially and temporally consistent indoor and outdoor samples were taken at some sites in different cities. The I/O ratios were considerably >1 in Gothenburg, and generally close to 1 in Mexico City. The high I/O ratios for ∑PAH in Gothenburg together with somewhat elevated indoor levels may indicate that PAH emissions in these homes are mainly from indoor sources while the opposite pattern was observed in Mexico City (Motelay-Massei et al., 2005, Strandberg et al., 2006). An interesting observation was that low molecular weight PAHs in Gothenburg in general show an I/O ratio well above 1 while the 5–7 ring PAHs are found in more samples outdoors than indoors, giving an I/O < 1. Lower I/O ratios for the 5–7 ring PAHs were also seen in Mexico City. This may indicate the presence of emission sources of mainly low molecular weight PAHs (vapour phase PAHs) indoors. PAH levels in the PM2.5 fraction of Beijing urban outdoor air were lowest in summer and highest in winter (He et al., 2006). The size distributions of ambient particles showed no significant difference between summer and winter, either outdoors or indoors. This suggested that the distributions were strongly affected also by the sources. On the other hand, ∑PAH associated with particles was predominantly (>80%) found in PM2.5 fractions. The proportion of ∑PAH in PM2.5 fractions was about one order of magnitude higher than in coarse particles (PM>10 and PM2.5–10) (Ohura et al., 2004a). The concentrations of indoor ∑PAH associated with PM2.5 (∑PAH– PM2.5) measured in his study ranged from 1.6 to 23.7 ng/m3). In outdoors, the ∑PAH– PM2.5 concentrations ranged from 1.1 to 29.5 ng/m3. The concentrations of indoor ∑PAH associated with PM>10 (∑PAH–PM>10) and PM2.5–10 (∑PAH– PM2.5–10) ranged from 0.13 to 1.8 and 0.18 to 2.05 ng/m3, respectively. Outdoors, ∑PAH–PM>10 and ∑PAH– PM2.5– 3 10concentrations ranged in 0.10–1.64 and 0.17–2.48 ng/m , respectively. The variability of outdoor ∑PAH associated with particles was considerably larger than that of the corresponding particles. This may be due to the difference of emission sources, lifetime, and physical properties, between PAH and particles. For light molecular PAHs, concentrations were strongly affected by seasonal factors because of their high vapor pressures.
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Smoking is a major emission source of indoor pollutants (Liu et al., 2001; Ohura et al., 2002). Humans spend about 90% of their daily lives indoors; thus, estimating indoor pollution is critical to ensuring a healthy environment. BaP is widely used as an indicator of human health risks and pollution levels because of its strong carcinogenicity (Finlayson-Pitts and Pitts, 2000). In Guanzhou (China), the seasonal concentration for ∑PAH ranged from 8.11 to 106.26 ng/m3. The average ∑PAH measured were highest in winter and lowest in summer. 5-6 ring PAHs were the abundant compounds, which accounted for 65-90% of ∑PAH and benzo[b+]fluoranthene dominated in the four seasons. Moreover, the total PAHs in Guangzhou were significantly higher than those in Hong Kong, Naples, Rom and Seoul; however, the distribution patterns of PAHs of PAHs in Guangzhou showed some similarities. In these cities high molecular weight (HMW) compounds dominated with Bbkf as maxima (except Naples) and Ant had the lowest concentration in all PAHs (Tan et al., 2005). Fourteen polycyclic aromatic hydrocarbons were identified and quantified in PM10 rang in Guangzhou during the period from March 2002 and June 2003. The ∑PAH ranged from 8.11 to 106 ng/m3. A strong seasonal trend of PAHs abundance was observed. The highest level was in winter and the lowest was in summer. The average ∑PAH in winter was 3.8 times higher than that in summer; the Asian monsoon had a strong influence on the seasonal variation. The distribution pattern of PAHs varied with the season. PM10-bound PAHs were dominated by high molecular weight compounds. LMW PAHs tend top have a higher contribution in the winter while HMW PAHs tend to have a higher contribution in the summer. Based on the diagnostic ratios, it is concluded that vehicular were the predominant source of airborne PAHs in Guangzhou (Tan et al., 2005).
OCCURRENCE OF PAHS IN DUST Settled house dust can be a source of human exposure to toxic PAHs through non-dietary ingestion and dermal contact. Information regarding the concentrations of various contaminants in house dust would be useful in estimating the risk associated with exposure to these compounds. PAHs in settled house dust were collected from homes in three locations: Sumgayit, Azerbaijan; Shanxi Province, China; and southern Texas, United States. The highest PAH floor surface loadings were observed in China, followed by Azerbaijan and Texas. Median concentrations of high molecular weight (four ring and larger) PAHs ranged from a low of 0.11 μg/m2 in Texas, to 2.9 μg/m2 in Azerbaijan and 162 μg/m2 in China as listed in Table 7. These trends in total surface loading and relative carcinogenicity indicate that the risk of health effects from exposure to PAHs in house dust is highest in the Chinese population and lowest in the Texas population (Naspinski et al., 2008). PAHs in house dust may originate from both outdoor and indoor sources. Typical indoor sources include combustion of fossil fuels and biomass for heating and cooking; burning of tobacco products, candles and incense; and cooking fumes. Maertens et al. (2004) estimated the excess lifetime cancer risk from non-dietary ingestion of PAH in settled house dust to exceed the acceptable range for pre-school children in homes with dust PAH loading at the 95th percentile. Chuang et al. (1999) reported that ingestion of house dust and soil contributed 24% of the estimated daily dose of B2 PAH in a study of children in low-income
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households. The floor dust collection procedure used in this study is quick, inexpensive, easily portable, and requires no specialized equipment. Although surface loading cannot be used directly in an exposure calculation, it is valuable for the hazard identification step of risk assessment. Carcinogenic PAHs in the house dust collected from different places was found in general, that the heavier PAHs are more toxic. All of the carcinogenic PAHs (cPAHs) contain 4 or more rings and are therefore in the HMW fraction. The percentage of HMW PAHs was highest in China and lowest in Texas (Naspinski et al., 2008). Table 7. Concentrations of total and high molecular weight PAHs of dust samples collected from houses.
Total PAHs Median Min Max High molecular weight PAHs Median Min Max EPA B2 carcinogen PAHs Median Min Max
Azerbaijan
China
µg/m2 Texas
6.1 0.7 49.6
232 42 6720
1.34 0.24 35.3
2.9 0.5 20.0
162 30 4440
0.11 0.03 2.90
1.0 0.2 5.4
53 12 1620
BD BD 0.27
In Egypt, road dust PAHs was received relatively little attention to date and studies focused on heavy metals almost. Consequently, background level of PAHs in dust of Egyptian roads is not available. Recently, a study carried out by Hassanien and Abdel-Latif (2008) on the road dust collected from 17 sites over greater Cairo (Fig. 2). The concentration of PAHs was determined by using HPLC technique. Twelve common environmental PAHs were found to be distributed. The data illustrated that the total average of PAHs over the investigated sites was ranged from 0.045 mg/kg to 2.6 mg/kg. On individual scale, the highest concentrations were 1.031 and 1.028 mg/kg for pyrene and phenanthrene, meanwhile the lowest was benzo(a)pyrene with value 0.0001 mg/kg. The obtained results showed that the carcinogenic content of PAHs (benzo(a)anthracene, chrysene, benzo(a)pyrene, and benzo(k)fluoranthene) ranged from 0.2 % to 23.9% of total PAHs. The highest average concentrations for PAH species in Cairo's road dust were for PHE (phenanthrene) of 0.404 mg/kg followed by PYR (pyrene) and FLA (flouoranthene) with average levels of 0.166 and 0.068 mg/kg, respectively, in consistent with those results found in Beijing (Li et la., 2006). Moreover, Bostrom et al. (2002) found PHE (phenanthrene) to be the most abundant of 14 different PAHs compounds monitored at the street-level site in Stockholm. Meanwhile the lowest measurable species in dust were BaP (benzo(a)pyrene), ACE (acenaphthene), BaA (benzo(a)anthracene), and BbF (benzo(b)flouranthene) with average concentrations of 0.0024, 0.0027, 0.0028, and 0.0035 mg/kg, respectively.
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Figure 2. Map of the Greater Cairo with location of road dust sampling sites (solid circles).
The spatial variability in road dust PAHs concentration in Cairo was presented in Figure 3. It involves 17 representative locations, with site-specific information gathered at each location Distribution patterns of PAH in Cairo's road dust is obtained by processing data of species concentrations rang between 0.045 and 2.6 mg/kg, as shown in Fig. 3. Spatial distribution of PAHs highlighted sites of high concentration, and as it has revealed from results, these hot-spot-sites are distributed through out whole Cairo, i.e. sites 4 and 5 in the north, site 6 in the down town, site 11 in the west and site 13 in the south. These sites are heavily traffic areas, in addition to the industrial activities at sites 4 and 13. A moderate pollution was recorded also in sites 5, 15 and 16. This is the first comprehensive information on PAH spatial distribution pattern in different district areas of Greater Cairo, Egypt. This work revealed the road dust contamination with PAHs residues in different district areas of Cairo. It extends our understanding of the current PAHs contamination status in this mega city. The average concentration of 12 PAHs was 0.704 mg/kg with carcinogenic content of 17.1% of total PAHs. The present study confirms the contribution of vehicular traffic, industrial emissions and the incomplete combustion of open waste burning to road dust PAHs. Plotting PAH results on a map of city can be used to advantage in investigating road dust pollution and its potential deleterious effects. This study suggested that the pollution of PAHs in Cairo should emphasize on controlling the industrial emissions and traffic exhausts.
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Concentration (mg/kg) 3 2.5 2 1.5 1 0.5
Total PAH (mg/kg)
Carcinogenic % of total PAH
Carcinogenic % 50 45 40 35 30 25 20 15 10 5 0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Invesrigated Sites
Figure 3. Total concentrations of PAH and carcinogenic percent at investigated sites.
PAHS IN FOOD MATERIALS Although many PAHs are included in the “priority pollutant” listings, there are many uncertainties over the significance of PAHs in relation to their transfer from the natural environment into the human food chain. There are very few available studies concerned with PAHs in food materials in developing countries. Therefore, it should be important to see the experiences from developing countries. Based on the premise that the plant leaf contents can provide a ‘time integral’ of the airborne concentration (Carberg et al., 1983, Eriksson et al., 1989, Bacci et al., 1991, Franzaring et al., 1992) the paddy leaves were selected as passive samplers to observe contemporary levels of PAHs in the atmosphere. PAHs reach the surface of plant leaves primarily from the atmosphere (Nash and Beall, 1970, Bacci and Gaggi, 1986, Trapp et al., 1990) PAHs in the atmosphere exist in either the gas or the particulate phase or both. Thus, the mechanism of transfer to plant leaves may be through gaseous dry deposition, particulate dry deposition, or wet deposition (including rain, snow and fog). Gaseous dry deposition has been shown to be the main process for many organic contaminants (Umlauf et al., 1994, McLachlan et al., 1995) PAHs with a vapor phase component in air are regarded as being subject to an air-plant exchange process, moving toward equilibrium over time. In addition, the paddy root was used for observing the adsorption of PAHs from soil and water because of root uptake, bioconcentration and translocation. In Japan, a study was carried out to estimate the PAHs content in rice as component of food chain. The total PAHs content in rice ranged from 46 to 77 µg/kg dry weight with an arithmetic mean value of 58 µg/kg dry weight. Phenanthrene was the most abundant in most cases. For analyzed PAHs, the arithmetic mean value of acenaphthylene, fluorene, anthracene, pyrene, weight (152–202) to heavier molecular weight (228– 252) PAHs in paddy leaf (73 : 27) was similar to that in rice (70 : 30) (Liu et al., 2001). The mobility of light molecular weight PAHs through the atmosphere was greater than that of the heavy molecular
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weight PAHs because of the greater gas phase component. Conversely, heavy molecular weight PAHs were considered to be only particle-bound and will be subject to deposition processes that effectively remove them from the atmosphere as air masses move away from source regions (Eriksson et al., 1989, Bacci et al., 1991, Franzaring et al., 1992).
POLYCYCLIC AROMATIC HYDROCARBONS IN MARINE BIOTA Concentrations of PAHs in marine biota indicate a high degree of contamination of certain areas of the region. Data given for the Ligurian Sea for Mytilus sp. range from 14 to 571 ng/g dw. In other studies, the 14 priority PAHs in mussel tissues (Mytilus sp.) and fish livers (Serranus and Mullus sp.) of specimens collected in the French and Spanish Mediterranean coasts and found values of 25-82, 27-87 and 15-49 ng/g dw, respectively, n values up to 390 ng/g dw were found in mussels collected inside harbours. Similar determinations carried out in deep sea fish from the NW Mediterranean basin (Mora sp.) showed liver concentrations of 7-16 ng/g ww (UNEP, 2002). A large survey of mussels carried out during 1995-99 in the French Mediterranean and Atlantic coasts gave mean values of 590 and 411 ng/g dw (Σ16 PAHs), respecively. A survey carried out recently in bivalves (Perna perna) from 18 stations along the Atlantic coast of Morocco gave results from 3.4- 34.6 ng/g (Σ14 PAHs) with values up to 87-109 ng/g ww in the vicinities of the cities of Agadir and El Jadida. Values of 170-750 ng/g dw were found in mussels from the Thermaikos Gulf (Greece). The concentrations of 16 PAHs in mussels collected in 1999 from Izmit Bay (Turkey) were found in the range 1.25-34.0 ng/g. Blubber samples were collected in 1993 and 1996 from live specimens of fin whales (Balenoptera physalus) and stripped dolphins (Stenella coeruleoalba) from the Ligurian and Ionian Seas. In whales, the median value of total PAHs was 350 ng/g ww while median carcinogenic PAH values were 114 ng/g ww, in dolphins. The median values of total and carcinogenic PAHs were 938 and 670 ng/g ww, respectively (UNEP, 2002).
HEALTH AND ENVIRONMENTAL IMPLICATIONS The hazards caused by toxic chemicals are devastatingly dangerous in the developing countries because:
People and governments are unaware of the inherent dangers of pesticides and other chemicals; The necessary financial resources are either scarce or do not exist; Facilities either for containing the wastes or for its destruction are not available Appropriate legal measures do not exist or, if they do, are either impossible or difficult to implement; Environmental activists are either absent or too weak to stage public demonstration, and Expertise or skilled manpower is not available. However, some of the adverse health and environmental impacts, include
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Direct exposure of workers and farmers, local residents, to unsecured pesticides stocks Indirect exposure through contaminated food and water, thus endangering public health Poisoning of farm lands and other workers Pollution of ground and surface water sources, which impacts on fauna and flora Impacts on land use resulting in limiting the potential of development of the area surrounding any chemical site Destruction of beneficial insects Risks and costs of destruction and incineration.
The effects associated with exposure to PAHs are enormous. They could be classified into environmental and health effects. While the environmental effects are associated with the pollution of the air, water and soil ecosystems, the later being the most dominant is associated with disturbances and disorders ranging from the disorders of the nervous or immune system, to increases in the risk of certain cancers. They persist in the environment, and may bioaccumulate and magnify in the food web due tof the tendency of these chemicals to accumulate in fatty tissue. There may be special windows of vulnerability in the development of fetuses when these chemicals can have longterm, irreversible effects on the reproductive and neurological systems.
CARCINOGENIC ASSESSMENT OF PAHS TOXIC EFFECT OF PAHS Short-term studies showed adverse hematological effects. Systemic effects caused by long-term treatment with PAHs have been described only rarely, because the end-point of most studies has been carcinogenicity. Significant toxic effects are manifested at doses at which carcinogenic responses are also triggered. In studies of adverse effects on the skin after dermal application, non- or weakly carcinogenic PAHs were inactive, whereas carcinogenic compounds caused hyperkeratosis. Benz[a]anthracene, benzo[a]pyrene, dibenz[ah]anthracene, and naphthalene are embryotoxic to mice and rats. Benzo[a]pyrene also have teratogenic and reproductive effects (Ravindra et al., 2008). PAHs have also been studied extensively in assays for genotoxicity and cell transformation; most of the PAHs are genotoxic or probably genotoxic. The only compounds for which negative results were found in all assays were anthracene, fluorene, and naphthalene. Owing to inconsistent results, phenanthrene and pyrene could not be reliably classified for genotoxicity. PAHs have generally been reported to have immunosuppressive effects (Ravindra et al., 2001). The toxicity of PAHs represents a risk to human health, and there are varied risk assessment approaches to quantifying the risk posed by PAHs based on exposure routes and scenarios. PAHs are not carcinogenic until they are metabolically activated as the body attempts to break them down and forms reactive metabolites that bind to the DNA causing subsequent replication in the cells. Fundamental to assessing risk posed by PAHs is understanding metabolism of PAHs. Since exposure to PAHs is never to single PAHs,
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understanding what differences may occur in mixtures of PAHs gives accurate assessment of the dangers of PAHs. Understanding the dynamics of complex metabolism vis-à-vis single metabolism of PAHs and possible effects on the toxicity expression of PAHs is a necessary advancement to accurately impact and guide remediation strategies. The bioaccumulation of PAHs and their bioamgnification along successive trophic levels is another means of exposure to human and animal life. PAHs can cause mutations, malignant and benign tumors, among a host of genotoxic effects. The link between PAHs and harmful effects in humans was made since 1775 by Sir. Percival Pot, surgeon to St. Bartholomew’s Hospital in London. He observed the increased incidence of scrotal cancer in chimney sweepers originated from their exposure to soot; (Cerniglia 1980). This observation provided an important link into studying occupational related diseases and their causes. Increasingly, links were made between tumors and occupation that had PAH exposure, in workers such as miners, chemical plant workers, tar-producing industry workers.
HUMAN EXPOSURE Air pollution is often suggested as being partly accountable for an increased incidence of lung cancer in cities. Over the past decade, an increasing body of scientific evidence has accrued associating outdoor air pollution with certain types of cancer. In many researching scientists have found out that exposure to air pollution, even at low levels, can lead to a wide range of adverse health effects including lung cancer. An individual’s total exposure to air pollution depends on indoor as well as outdoor exposures. Indoor air quality has large potential health implications because people may spend considerable amounts of time indoors. Indoor air pollution may stem from incoming outdoor air or may originate indoors from tobacco smoking, building materials, soil gases, household products, and combustion from heating and cooking. The exposure of human populations to carcinogens in outdoor air may be the result of proximity to more localized sources such as industrial facilities, small businesses, municipal facilities, or areas with high vehicular traffic. PAHs are lipophilic compounds and can be absorbed by the lung and gastrointestinal tract tissues and by the skin. Several studies have been carried out to determine the levels of exposure of humans to PAHs (Ciganek et al., 2002). The dietary sources of PAHs are cereals and vegetables, rather than meat, excepting populations with a high consumption of meat cooked over an open flame. The recently developed biomonitoring procedures allow the assessment of human exposure to PAHs, as well as identification of diet as the major exposure source. The U.S. Environment Protection Agency published “The Exposure Factors Handbook” in which available statistical data on various factors used in assessing human exposure are summarized (http://www.epa.gov/ncea/exposfac.htm). The major steps in exposure assessment include: (1) determination of exposure pathways, (2) identification of environmental media conveying the contaminant, (3) determination of the contaminant concentration, (4) determination of exposure time, frequency, and duration, and (5) identification of the exposed population.
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EXPOSURE AND ROUTES OF EXPOSURE Exposure to chemical hazards may occur via all types of exposure: inhalation, oral ingestion, absorption through the skin, absorption through the eyes, and placental transfer from pregnant woman to the fetus, inoculation and direct penetration to target organs and from mother to child through breastfeeding. In the non occupational environment, ingestion of substances containing chemicals is the most common route of exposure. In the workplace, because of the nature of exposure, duration of the workday, and character of the compounds; inhalation is the most significant route of entry, followed by skin absorption and ingestion. Health effects, of course, derive from exposure and thus it can be expected that global illhealth from air pollution is dominated by indoor exposures in developing countries as well. The precise extent of impact, unfortunately, is not known yet, but enough information is available to make reasoned estimates for India, where more data are available than in most other developing countries (Smith, 2000). The main sources of general population exposure are: vehicular traffic, residential heating, smoke from open fireplaces and cooking, tobacco smoke, contaminated food and drinking-water, and the use of PAHs contaminated products. The calculated total daily intake of PAHs from food has been estimated to be 3 µg/day (1.4 µg/day carcinogenic PAHs) per person in Italy (UNEP, 2002). In addition to a lack of studies on the risks of exposures, there is a dearth of studies on the distribution and magnitude of exposure to PAHs in developing countries itself. Such information is needed both to estimate the total burden of disease and to identify the regions and populations where interventions need to be focused. Compared with the thousands of studies on outdoor and indoor air pollution in developed countries, only few studies have been carried out in the developing world (African countries). Although striking because of the high exposures to major pollutants that have been demonstrated, the available studies have not been conducted in a fashion that allows the results to be confidentially extrapolated to large populations.
PAH POTENCIES PAH potencies are used to determine quantitative health risks posed by PAH exposure. The risks posed by a mixture of PAHs are based on an assumption of additivity of the individual risks posed by the PAHs. The IPCS monogram on PAHs (IPCS, 1998) describes three approaches used to calculate PAH potencies: 1) toxicity equivalence factors approach is based on expressing of individual potencies relative to benzo(a)pyrene, 2) comparative potency approach, which does not identify or quantify the individual compounds but determines the potency of the mixture of compounds and 3) benzo(a)pyrene surrogate approach assumes that benzo(a)pyrene is an indicator of all the PAHs. As the toxicities (carcinogenicity and mutagencity) of individual PAHs differ considerably, toxicity assessments of PAHs are complex. A number of approaches have been developed for evaluating the potencies of the various PAHs with regard to the possible inhalation cancer risk to humans (IARC, 1983). One approach is to calculate the inhalation unit risk for excess lung cancer over the risk posed by BaP for each of its copollutant carcinogenic PAH in the polluted ambient air divided the particular PAH’s risk by the risk
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for BaP to obtain their individual potency equivalence factors (PEFs); thus, the PEF for BaP would be, by definition, 1.00. These PEFs can be used to estimate the relative carcinogenicity of ambient air with a known distribution of PAHs (Ohura et al., 2004a,b). The risk associated with the inhalation of particulate PAHs indoors showed that the contribution of BaP to the total carcinogenic potential was dominant in the range of 51% to 64%. Human activity patterns indicate that people tend to spend 90% or more their time indoors. In addition, indoor BaP concentrations are strongly affected by the corresponding outdoor ones. Thus, the risk assessment for inhalation of PAH could be roughly estimated by only the outdoor BaP concentrations. PAHs half-lives in humans are in fact in the range of days/hours, and metabolism is responsible of the formation of toxicologically active (carcinogenic) metabolites. Because of the lack of persistence and to the complexity of PAH mixtures, human exposure is usually characterised in terms of “internal dose” rather than of “body-burden”, by the use of biomarkers of exposure, mainly 1-OH pyrene in urine and DNA adducts in peripheral blood lymphocytes.
RISK ASSESSMENT OF PAHS RISK RESEARCH NEEDS Hundreds of urban developed-country epidemiological studies have been carried out in recent decades, allowing in some cases large-scale meta-analyses and reviews for important pollutants like small particulates (e.g. WHO, 1999; Kreweski et al., 2000; Samet et al., 2000). Extrapolating these relatively well-characterized results to the populations using solid fuels indoors is difficult, however, because of different pollutant mixtures, health conditions, agedistributions, exposure patterns and so on, as well as the scarcity of actual exposure measurements. There is clearly need to find ways to make these extrapolations with more confidence (McMichael and Smith, 1999). In addition, however, there are needs to conduct additional research directly within the communities of interest. Given what is known about the relative importance of the major air-pollution-related diseases in developing countries, the risks from existing studies, and the relative difficulties of measuring effects in studies of reasonable duration, the following kinds of studies would seem to have the highest priority. Although many people associate public exposures to air pollution primarily with urban outdoor settings, readers of this chapter are well aware that indoor environments can also be contaminated, both from pollution penetrating from outside and from indoor sources. General population exposure to PAHs mainly occurs from atmospheric pollution and diet, this latter accounting for about 90% of the total exposure (Hassanien et al., 2001 ). The largest exposures to health-damaging air pollution probably occur in the developing world. As a result, much of the health impacts from air pollution worldwide seem to occur among the poorest and most vulnerable populations, largely women and young children who are most exposed to the indoor pollution sources of importance in poor countries. For many PAHs, toxicological effects are still unknown or not fully clarified, and mechanistic information is lacking. Moreover, even for PAHs whose human health hazard has been characterised, no risk assessment is actually feasible due to the lack of exposure data in most countries. Food being the main exposure route, data on PAHs daily intakes are of major relevance and represent the necessary basis to develop an adequate risk assessment.
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These high individual and population exposures are the result of a set of factors:
About half the world’s households use unprocessed solid fuels for cooking, ranging roughly from near zero in developed countries to more than 80% in China, India, and Sub-Saharan Africa (Holdren et al., 2000). The individual peak and mean exposures experienced in such settings are large by comparison with WHO guidelines and national standards.
BaP is one of the polycyclic aromatic hydrocarbons formed when gasoline, garbage or any animal or plant materials burn incompletely. It was first isolated from coal tar. People may be exposed to BaP from air, water, soil, cigarette and other plant product smoke, food and some work environments through inhalation, ingestion and skin contact. Exposure to PAHs may cause harmful health effects. When humans are exposed to BaP at high concentrations for relatively short periods of time, BaP may cause red blood cell damage leading to anemia and suppressed immune system. Mixtures of PAHs including BaP such as coal tar were shown to be dermal carcinogens in animals as early as 1918. BaP has caused tumors in laboratory animals when administered in the diet, when applied to their skin or when inhaled for a long period of time. Humans exposed to mixtures of PAHs and other compounds at high concentrations over long periods of time can also develop cancer. To set priorities with respect to control, an excess lifetime cancer risk, expressed in terms of the BaP concentration and based on observations in coke oven workers exposed to mixtures of PAHs, is presented here. It must be emphasized that the composition of PAHs to which coke-oven workers are exposed may not be similar to that in ambient air, although it was noted that similar risks have been derived from studies of individuals exposed to other mixtures containing PAHs. Having also taken into consideration some recent animal data from which a unit risk of the same order of magnitude can be derived, it was concluded that the occupational epidemiology data should serve as the basis for the risk estimate. Based upon epidemiological data from studies in coke-oven workers, a unit risk for BaP as an indicator in air constituent is estimated to be 8.7 × 10-5 (ng/m3)-1 which is the same as that established by WHO (1987). The corresponding concentrations of BaP producing excess lifetime cancer risks of 1/10 000, 1/100 000 and 1/1 000 000 are 1.2, 0.12 and 0.012 ng/m3, respectively. To assess health-risks associated with PAH exposures, Lin et al. (2008) reported that it is important to know the total carcinogenic potency arising from the exposures of various PAH compounds. In principle, the carcinogenic potency of a given PAH compound can be assessed according to its benzo[a]pyrene equivalent concentration (BaPeq). Calculating the BaPeq concentration for a given PAH compound requires the use of its toxic equivalent factor (TEF; using benzo[a]pyrene as a reference compound) to adjust its original concentration (Chu and Shen, 1984; Thorslund and Farrer, 1991; and Nisbet and LaGoy, 1992). Among the available TEFs lists, the one established by Nisbet and LaGoy has been demonstrated to best reflect the actual state of knowledge of the toxic potency of each individual PAH species (Nisbet and LaGoy, 1992). By using the TEF list the carcinogenic potency of total PAHs (total BaPeq) can be determined as the sum of BaPeq concentrations of the 21 selected PAH compounds. For estimating the lung cancer risk associated with inhalatory PAH exposures, the World Health Organization (WHO) has suggested a unit risk of 8.7×10−2 (µg/m3)−1 for the lifetime (70 years) PAH exposure, assuming one was exposed to BaP concentration of 1 µg/m3
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(WHO, 1987). It is worth noting that the above unit risk was proposed for lifetime exposure, therefore, it has been adopted for assessing the exposure of general adults to the ambient atmospheric PAHs (Zmirou et al., 2000). For occupational exposure, Pott established a relationship between BaP exposure and lung cancer risk (Pott, 1985), based on a data bank provided by an epidemiological study conducted by Redmond et al. (1976). He suggested the unit risk of 7.0×10−2 (µg/m3)−1 for a 25-year occupational PAHs exposure with the averaged BaP concentration of 1 µg/m3. By using the same data bank, the US Environmental Protection Administration (EPA, 1984), however, suggested a different unit risk of 6.4×10−4 (µg/m3)−1 for PAHs exposure based on its total PAH content (expressed as the benzene soluble fractions). Since another study has indicated BaP could be a better indicator than total PAH content on characterizing the carcinogenic potency of PAHs (Petry et al., 1996).
INTERVENTIONS AND RISK RESEARCH NEEDS In spite of the clear need to conduct more research on risks and exposures, the current, if imperfect, knowledge of the health burden imposed by air pollution in developing countries argues that action is warranted now. Unfortunately, however, there also is a severe lack of good information on the interventions that might be best applied to effectively reduce the risks. Better ventilation, better stoves, better fuels, and behavioral changes would seem to encompass the range of potential interventions, but remarkably little systematic work has been performed on any of these, considering again the potential scale of the problem and consequent potential benefit. There is research needed not only on the technologies and their acceptance at the household level, but also on the policy tools needed to promote exposure-reducing changes in household fuels and stoves.
How to develop confounding for exposure-reduction efforts among developing sectors (health, energy, housing, and environment) in the worst affected regions. Conduct research on ways that different sectors can implement programs with exposure reduction potential, for example, fuel pricing in the energy sector. Conduct systematic program of research to evaluate the effectiveness of various policy measures
CONCLUSION AND FUTURE RESEARCH The environmental pollution mixtures in major population centers should be characterized both in terms of physical and chemical constituents and in terms of sources of major constituents. If possible, retrospective characterization of levels of certain constituents could be accomplished. Although a program of risk, exposure, and intervention studies as outlined above would not be inexpensive by some measures, it is likely to be highly costeffective as a means to address the large exposures now being experienced widely in developing countries.
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Better estimates of the amount of effect will need additional epidemiologic studies. Large numbers of cases will be essential to measure the effects of environmental pollution by PAHs and to measure combined effects of pollutants and factors such as occupation, smoking and such studies will probably require pooling data from multiple locales. New studies will need to develop and apply improved methods and to compare the effect of exposure to the pollutant mixtures on lung cancer in different cities while effectively controlling confounding factors including different exposure sources. Also we need methods for the retrospective estimation of lifetime exposure to environmental pollutants.
ACKNOWLEDGMENT The author would like to report that this chapter is the result of the work of many dedicated professionals who helped him in design, concept, and research. The author wishes heartily to thank his colleagues in Air Pollution Department, National Research Center for their sincere support and encouragement. He would like also to thank Prof. Alia A. Shakoiur, Dr. Mamdouh El melegy, Dr. Nasser M. Abdel-Latif and Aliaa A. Ezoo for supplying him with important data and information. Finally, the author thanks his sons for editorial assistance and their patience.
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In: Encyclopedia of Polymer Research Ed: Caroline E. Jones
ISBN: 978-1-61761-926-7 © 2011 Nova Science Publishers, Inc.
Chapter 15
ADIPOSE TISSUE AS A KEY CONTRIBUTOR TO POLYCYCLIC AROMATIC HYDROCARBON-INDUCED CARCINOGENESIS P. Irigaray1∗ and D. Belpomme2 1
Cancer Research Center, Association for Research and Treatments Against Cancer (ARTAC), Paris, France 2 Department of Medical Oncology, European Hospital Georges Pompidou (HEGP), University of Paris, Paris, France
ABSTRACT We have previously shown that benzo[a]pyrene used at extremely low dose can cause weight gain in mice and that the increase in adipose tissue mass is due to inhibition of β-adrenergic stimulation of lipolysis. Moreover we have suggested that adipose tissue is a reservoir for many carcinogens including Polycyclic Aromatic Hydrocarbons (PAHs).In this paper we demonstrate that the two C4 PAHs pyrene and phenanthrene can accumulate into adipocytes, but at the difference of benzo[a]pyrene have no impact on lipolysis.Since benzo[a]pyrene has been proved to be mutagenic while C4 PAHs are tumor promoters, we consider that a mixture of the three PAHs may act as a complete carcinogen. Furthermore, because adipose tissue can be induced by benzo[a]pyrene, is a reservoir for many PAHs and other carcinogens and can stimulate proliferation of normal or cancer cells, through the secretion of adipokines by adipocytes, we therefore propose that adipose tissue is a key contributor to chemical carcinogenesis by acting not only upon initiation and promotion but also upon progression.
Low molecular weight polycyclic aromatic hydrocarbons (l-PAHs) (3 to 4 rings) such as the C4 PAHs, pyrene and phenanthrene have been shown to be non genotoxic tumor ∗
P. Irigaray, PhD,ARTAC ,57-59 rue,de la convention,75015 Paris, Phone: +33 (0)1 45 78 53 53,Fax: +33 (0)1 45 78 53 50,E-mail:
[email protected].
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promoters, while high molecular weight PAHs (h-PAHs) (5 to 7 rings), such as the C5 PAH benzo[a]pyrene (B[a]P) are mutagenic though DNA adduction and may act as tumor initiators and/or tumor promoters [1]. We have previously shown that in addition to its genotoxic properties, B[a]P can inhibit adipocyte lipolysis and contribute to the hypertrophy/hyperplasia of adipose tissue [2], and have consequently hypothesized that in humans, B[a]P can generate both obesity and cancer [3]. Furthermore we have proposed the concept that the adipose tissue in addition to its endocrine function acts as a reservoir for many lipophilic liposoluble exogenous carcinogens and that these carcinogens can be permanently released in the blood circulation and target peripheral tissues for carcinogenesis. [3, 4]. In this paper, we attempt to further elaborate the concept that adipose tissue act as a reservoir for chemical carcinogens by analysing the ex vivo incorporation of PAH into adipocytes and by testing whether the C4 PAHs pyrene and phenanthrene can inhibit lipolysis as the C5 PAH B[a]P does. In this study, we confirm that adipocytes can incorporate h-PAH and l-PAH in a similar manner and that B[a]P in addition to its mutagenic carcinogenic properties can significantly inhibit lipolysis. So because adipocytes can incorporate both mutagenic and promoting PAHs, we conclude that PAHs cocktails may be equivalent to complete carcinogens and that the adipose tissue may play a key role in PAH-induced carcinogenesis.
MATERIALS AND METHODS Isolated Adipocyte Preparation Adipocytes were isolated from mice adipose tissue using Rodbell’s method as previously described [2].
Lipogenesis Assays Lipogenesis assays were performed as previously described [5]. Primary adipocytes freshly isolated from murine white adipose tissue were incubated with insulin (90 nM) and HAPs. This insulin concentration permits to obtain good lipogenesis [5]. More precisely, cell suspensions were aliquoted in Eppendorf tubes containing Krebs Ringer. Bicarbonate Buffer (KRBB) supplemented with 4% (w ⁄ v) bovine serum albumin (BSA), 1mM glucose and 0,278 µCi of 14C-phenanthrene (0.0066 µmol/l) or 0.296 µCi of 14 C-pyrene (0.007 µmol/l) or 0,272 µCi of 14C-benzo[a]pyrene (0,0065 µmol/l). After different incubation time at 37°C under gentle agitation (40 r.p.m) with or without insulin (90 nM), aliquots of the media were removed for radioactivity determination by liquid scintillation (Ultima Gold) counting in a Packard Tricarb in order to determine the percent of extra cellular radioactivity.
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Lipolysis Assays Lipolysis assays were performed as previously described [2]. Primary adipocytes freshly isolated from murine white adipose tissue were incubated with increasing concentrations of HAPs and with a dose of 10µM of epinephrine. This process was followed by measurement of the Free Fatty Acids (FFA) released in the media. Preliminary experiments using different concentrations of epinephrine (Figure 1A) and incubation times with epinephrine (Figure 1B) allowed us to determine the epinephrine concentration of 10 µM and incubation time of 45 minutes that permit obtaining good lipolysis.
Figure 1. Effect of different concentrations and incubation times of epinephrine on the release of FFA from mice adipocytes. Freshly isolated mice adipocytes were incubated (A) for 60 min at different concentrations of epinephrine and (B) for different times with a previously determined concentration of 1.8 mg/l (10 µM) of epinephrine. Each point represents the mean ± SEM of 4 experiments during which FFA levels were measured in duplicate.
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Figure 2. Kinetics of incorporation into adipocytes of B[a]P (■), pyrene (○) and phenanthrene (▲) in the presence of 90 nM of insulin. Each point represents the mean ± SEM of 4 experiments during which FFA levels were measured in duplicate. Data are expressed as the percent of molecules incorporated into mice adipocytes.
Cell suspensions were aliquoted in Eppendorf tubes containing KRBB supplemented with 4% (w⁄v) BSA, 5 mM glucose and one of the three PAHs, i.e. B[a]P, pyrene and phenanthrene at different concentrations during 45 min. After this preincubation period of 45 min, an epinephrine solution was added to the medium at a final concentration of 10 µM for 45 minutes, before measurements of the FFA released in the media were made.
RESULTS 1. Incoporation study As reported in figure 2, the kinetics of incorporation was identical for the three PAHs. PAHs incorporate into adipocytes uniquely during the first ten minutes (p = 0.001). This rapid incorporation suggests that PAHs incorporation is not insulino-dependant [5]. In addition, as indicated in tables 1 and 2 incorporation appears to depend upon the lipophilicity of molecules. Table 1. Incorporation rates of PAHs after 60 minutes of incubation at a concentration of 90 nM of insulin. Values indicated are the mean ± SEM of 4 experiments during which FFA levels were measured in duplicate. Compounds Benzo[a]pyrene Pyrene Phenanthrene
Percent of PAHs introduced in or adsorbed on adipocyte 35.04±2.28 20.62±2.55 23.08±2.89
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Table 2. Physico-chemical properties of the three PAHs tested. Compounds
Benzo[a]pyrene Pyrene Phenanthrene
Number benzenoïd rings 5 4 3
of
Molecular weight (g/mol) 252.3 202.3 178.2
Water solubility (mg/l at 25°C)
Lipophilicity Kow)
3.80.10-3 1.30.10-1 1.21
6.31 4.88 4.6
(log
2. Lipolysis Inhibition A significant inhibitory effect on epinephrine-induced FFA release (P < 0.01) was achieved with 1 mg/l B[a]P concentration (Figure 3A). The estimated Kd value for B[a]P inhibition of lipolysis is 0.9 mg/l (3.5 µM). This value is in the same range as that measured for the known β blocker atenolol (Kd = 0.4 mg/l or 1.5 µM for β1-adrenoreceptor and 2.3 mg/l or 8.6 µM for β2-adrenoreceptor) [6]. The inhibitory effect of 1.8 mg/l of B[a]P on epinephrine-induced lipolysis was detectable within 5 min of preincubation (Figure 3B). This suggests that B[a]P directly affects the cellular signal transduction pathway rather than affecting gene expression. However, for the two C4 PAHs (pyrene and phenanthrene), no effect was observed on the epinephrine-induced FFA release (Figures 4A and 4B).
Figure 3. Effect of B[a]P on the release of FFA from mice adipocytes. Freshly isolated mice adipocytes were incubated (A) for 45 min with different concentrations of B[a]P and (B) during different preincubation periods with a predetermined concentration of 2.5 mg/l (10 µM) of B[a]P. Epinephrine (1.8 mg/l i.e. 10 µM) was then added and the incubation continued for 45 min. Each point represents the mean ± SEM of the 4 experiments during which FFA levels were measured in duplicate.
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Figure 4. Effect of the two C4 PAHs, pyrene and phenanthrene on the release of FFA from mice adipocytes. Freshly isolated mice adipocytes were incubated for 45 min with different concentrations of Pyrene (A) or Phenanthrene (B). Epinephrine (1.8 mg/l i.e. 10 µM) was then added and the incubation continued for 45 min. Each point represents the mean ± SEM of 4 experiment during which FFA levels were measured in duplicate.
DISCUSSION B[a]P is a well-known ubiquitous carcinogen and consumption of B[a]P-rich foods have been suggested to contribute to the overall cancer burden affecting human populations [7]. The carcinogenicity of B[a]P is well-documented. This pollutant is metabolized via the cytochrome P450 system into reactive dihydrodiol epoxide derivatives (e.g. B[a]P-7,8dihydrodiol-9,10-epoxide, BPDE). These metabolites bind covalently to DNA and form BPDE adducts, that may lead to mutations and consequently to uncontrolled cell growth and tumour formation in various tissues (lung adenocarcinoma, mammary adenocarcinoma, lymphoma and hepatocellular carcinoma) [8]. B[a]P has also been shown to display immunotoxic properties that affect macrophage function [9] and increase local inflammatory response which may contribute to increased atherosclerotic lesion size [10]. In this paper, we show that in ex vivo experiments, B[a]P can inhibit epinephrine-induced FFA release whereas the two C4 PAHs pyrene and phenanthrene cannot. This confirms our previous observation that the administration of a single dose of B[a]P (0.5 mg/kg in IP) in male C57BL6/J mice induces a significant reduction of the epinephrine-induced lipolysis. This phenomenon occurs in less than 30 minutes with maximum inhibition 24 hours later before returning to the basal lipolytic capacity levels 72 hours after the injection. In addition, we have shown that repeated intraperitoneal injections of B[a]P (0.5 mg/kg/48h) for 14 days induce a significant body weight gain (+43%, p>0.1) in mice without detectable changes in food consumption [2]. We previously proposed that the inhibition of lipolysis by B[a]P most likely results from physical perturbations of the plasma membrane phospholipid bilayer. Our interpretation is that B[a]P strongly and rapidly inhibits the signalling capacity of the β1-, β2-, β3-adrenergic receptors and of the ACTH receptors. These receptors share common features: all contain
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seven transmembrane spanning domains and are coupled to G-proteins which are themselves anchored to the inner leaflet of the plasma membrane. In contrast atrial natriuretic peptideinduced lipolysis via the stimulation of natriuretic peptide receptor A, that contains a single transmembrane-spanning domain, was not affected by B[a]P [2] Yet, physicochemical studies using differential scanning calorimetry, infrared spectroscopy and small-angle X-ray diffraction have shown that B[a]P incorporates into phospholipid bilayers and localizes in the most apolar region of the phospholipid matrix. This phenomenon may account for the observation of an expanded and swollen membrane [11]. We have therefore, proposed that distortion of the physiochemical properties of the adipocyte plasma membrane by B[a]P decreases the signalling capacity of G-coupled receptors intimately linked to the phospholipid bilayer, via their seven transmembrane domains. Our data tends to show that the capacity of PAHs to incorporate into adipocytes depends upon their Coeficient Octanol Water (Kow). We therefore suggest that because the capacity of lipophilic molecules to be incorporated into the phospholipids bilayer depends on their Kow, the lack of lipolysis-inhibiting effect of the two C4 PAH pyrene and phenanthrene might be related to their low Kow. A striking observation in our study is that the concentrations needed to achieve the B[a]Pinduced lipolysis inhibition effect are 2000-fold lower than those causing carcinogenesis in rodents and 20-fold lower than those causing alterations of the epidermal growth factor receptors in cultured human uterine cells, RL95-2 [12]. Another observation is that B[a]P can directly inhibit epinephrine-induced lipolysis, whereas in order to adduct DNA and produce mutagenic and carcinogenic effects, B[a]P needs to be activated through the CYP1A1 cytochrome P450 process. It has been shown that in the ob⁄ob obese mouse model, CYP1A1 levels are decreased [13]. We thus assume that because B[a]P contribute to the hypertrophy/hyperplasia of the adipose tissue, it may simultaneously increase its bioaccumulation in this tissue by reducing its metabolic bioactivation. Since pyrene and phenanthrene are also activated through CYP1A1, a similar process of storage in the adipose tissue may also concern these two C4 PAHs, so B[a]P might in fact generate the storage in the adipose tissue of both types of mutagenic and promoting PAHs. In our study, all three PAHs incorporated adipocytes in a similar manner. Our results are similar to those found by Victor, who obtained a complete incorporation of the 2,3,7,8-TCDD in hepatocytes suspension within 2 minutes [14]. These data reinforce the concept that adipose tissue may act as a reservoir for many different lipophilic exogenous chemicals including PAHs. This concept is opposed to the hypothesis according to which adipose tissue might be a ‘‘protective reservoir’’ [15]. This assumption was based on the opinion that because adipose tissue is able to bioaccumulate chemical pollutants, it might protect the organism against their deleterious effects [15-18] Indeed, we believe that this concept needs to be revised. Following environmental exposure to low-dose chemical pollutants during many years, accumulation of carcinogens in the adipose tissue might in fact correspond to a stronger exposure, as it is observed in hot spot polluted area. On the basis of many experimental toxicological data, it is assumed that prolonged chronic exposure to low dose xenochemicals may in fact lead to carcinogenesis, as it is the case for occupational cancers during hotspot exposure [19, 20]. As a result of this new concept, it clearly appears that the carcinogenic effects of environmental xenochemicals, be they mutagens and/or promoters may be the consequence of a chronic exposure to repetitive lower doses than those at which no effect level is observed in classical
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rodent tests. This concept is based on the fact that many exogenous chemical carcinogens are persistent organic pollutants accumulating in fat and that they can be released in the organism from adipose tissue at doses which do not correspond to those observed in the environment. Therefore, during prolonged environmental exposure, cancer may be induced by repetitive lower doses than it is usually expected [20, 21]. Furthermore, it has been shown that mixtures of pollutants may have synergic effects and that some types of ‘‘cocktail effects’’ that occur in adipose tissue may contribute to carcinogenesis [22] and/or cancer cell proliferation [23]. Since our study shows that C4 as well as C5 PAHs can incorporate into adipocytes, we hypothesize that PAH cocktails that bioaccumulate in the adipose tissue may be equivalent to complete carcinogens (Figure 5). As summarized in figure 5, we believe that the concept of chronic toxicity must also be totally revised in the light of the role of reservoir of the adipose tissue for lipophilic carcinogens. Even in lean people, white adipose tissue (WAT) represents about 15 to 25 % of body weight, and this percentage can increase by more than 50 % in the case of obese patients. Environmental lipophilic pollutants can enter the organism via food, water and air, bioaccumulate and often concentrate in the adipose tissue, to be permanently released in the blood circulation and elicit their effect in peripheral tissues at convenient doses according to their potential biologic effects. The amount of pollutants that bioaccumulate in WAT may depend on dietary exposure and on individual susceptibility [24]. The postnatal stock of innate preadipocytes, the volume of WAT and the capacity of the organism to assure energy homeostasis might be risk factors, inter alia. Also weight loss may result in an increased concentration of pollutants in reduced WAT [25] and in their decreased elimination rate [26]. Finally we conclude that in addition to its endocrine properties, the adipose tissue can act as a reservoir for many exogenous chemical molecules including PAHs and that B[a]P may play a pivotal role in carcinogenesis not only by inducing cancer through its mutagenic carcinogenic effects but also by increasing the bioaccumulation capacity of the adipose tissue mass.
Figure 5. Adipose tissue as a reservoir for PAHs. Hypothetic mechanism according to which repeated low doses of PAHs can induce carcinogenesis.
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[2]
[3] [4] [5] [6] [7]
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Trosko, JE; Upham, BL. (2005).The emperor wears no clothes in the field of carcinogen risk assessment: ignored concepts in cancer risk assessment. Mutagenesis, 20, 81–92. Irigaray, P; Ogier, V; Jacquenet, S; Notet, V; Sibille, P; Mejean, L; et al. (2006). benzo[a]pyrene impairs beta-adrenergic stimulation of adipose tissue lipolysis and causes weight gain in mice: a novel molecular mechanism of toxicity for a common food pollutant. FEBS J, 273, 1362-1372. Irigaray, P; Newby, JA; Lacomme, S; Belpomme, D. (2007). Overweight/obesity and cancer genesis: more than a biological link. Biomed Pharmacother, 61, 665-678. Irigaray, P; Belpomme, D. (2009). Mechanism of Environmental Carcinogenesis. In Encyclopedia of Environmental Health, Elsevier, accepted for publication in 2009. Irigaray, P; Mejean, L; Laurent, F. (2005). Behaviour of dioxin in pig adipocytes. Food Chem Toxicol, 43, 457-460. Smith, C; Teitler, M. (1999). Beta-blocker selectivity at cloned human beta 1- and beta 2-adrenergic receptors. Cardiovasc Drugs Ther, 13, 123–126. Kazerouni, N; Sinha, R; Hsu, CH; Greenberg, A; Rothman, N. (2001). Analysis of 200 food items for benzo[a]pyrene and estimation of its intake in an epidemiologic study. Food Chem Toxicol, 39, 423–436. IARC. (1983) IARC Monographs on the Evaluation of Carcinogenic Risk of Chemicals to Humans – Polynuclear Aromatic Compounds, Vol. 32 (33-91), pp. 211–224. International Agency for Research into Cancer, Lyon. van Grevenynghe, J; Rion, S; Le Ferrec, E; Le Vee, M; Amiot, L; Fauchet, R; Fardel, O. (2003). Polycyclic aromatic hydrocarbons inhibit differentiation of human monocytes into macrophages. J Immunol; 170, 2374–2378. Curfs, DM; Lutgens, E; Gijbels, MJ; Kockx, MM; Daemen, MJ; van Schooten, FJ. (2004). Chronic exposure to the carcinogenic compound benzo[a]pyrene induces larger and phenotypically different atherosclerotic plaques in ApoE-knockout mice. Am J Pathol, 164, 101–108. Jimenez, M; Aranda, FJ; Teruel, JA; Ortiz, A. (2002). The chemical toxic benzo[a]pyrene perturbs the physical organization of phosphatidylcholine membranes. Environ Toxicol Chem, 21, 787–793. McGarry, MA; Charles, GD; Medrano, T; Bubb, MR; Grant, MB; CampbellThompson, M; Shiverick, KT. (2002). Benzo(a)pyrene, but not 2,3,7,8tetrachlorodibenzo-p-dioxin, alters cell adhesion proteins in human uterine RL95-2 cells. Biochem Biophys Res Commun, 294, 101–107. Roe, AL; Howard, G; Blouin, R; Snawder, JE. (1999). Characterization of cytochrome P450 and glutathione S-transferase activity and expression in male and female ob ⁄ ob mice. Int J Obes Relat Metab Disord, 23, 48–53. Victor, J; Wroblewski, VJ; Olson, JR. (1985). Hepatic metabolism of 2,3,7,8Tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) in the rat and guinea pig. Toxicol Appl Pharmacol, 81, 231-240. Lassiter, RR; Hallam, TG. (1990). Survival of the fattest: implication for acute effects of lipophilic chemicals on aquatic populations. Environ Toxicol Chem, 9, 585-595.
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[16] Geyer, HJ; Schramm, KW; Scheunert, I; Schughart, K; Buters, J; Wurst, W; et al. (1997). Considerations on genetic and environmental factors that contribute to resistance or sensitivity of mammals including humans to toxicity of 2,3,7,8tetrachlorodibenzo-p-dioxin (TCDD) and related compounds. Part 1: Genetic factors affecting the toxicity of TCDD. Ecotoxicol Environ Saf , 36, 213-230. [17] Geyer, HJ; Scheuntert, I; Rapp, K; Kettrup, A; Korte, F; Greim, H; et al. (1990). Correlation between acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and total body fat content in mammals. Toxicology, 65, 97-107. [18] Geyer, HJ; Scheunert, I; Rapp, K; Gebefugi, I; Steinberg, C; Kettrup, A. (1993). The relevance of fat content in toxicity of lipophilic chemicals to terrestrial animals with special reference to dieldrin and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Ecotoxicol Environ Saf, 26, 45-60. [19] Belpomme, D; Irigaray, P; Hardell, L; Clapp, R; Montagnier, L; Epstein, S; Sasco, AJ. (2007). The multitude and diversity of environmental carcinogens. Environ Res, 105, 414-429. [20] Howard, CV; Newby, JA. (2004). Could the increase in cancer incidence be related to recent environmental changes?. In: Nicolopoulou-Stamati P, Hens L, Howard CV, Van Larenbeke L, editors. Cancer as an environmental disease. Dordrecht, The Netherlands: Kluwer Academic Publishers; 2004. p. 39-56. [21] Hutcheon, DE; Kantrowitz, J; Van Gelder, RN; Flynn, E. (1983). Factors affecting plasma benzo[a]pyrene levels in environmental studies. Environ Res, 32, 104-110. [22] Knerr, S; Schrenk, D. (2006). Carcinogenicity of ‘‘non-dioxinlike’’ polychlorinated biphenyls. Crit Rev Toxicol, 36, 663-694. [23] Payne, J; Scholze, M; Kortenkamp, A. (2001). Mixtures of four organochlorines enhance human breast cancer cell proliferation. Environ Health Perspect, 109, 391-397. [24] Smeds, A; Saukko, P. (2001). Identification and quantification of polychlorinated biphenyls (PCBs) and some endocrine disrupting pesticides in human adipose tissue from Finland. Chemosphere, 44, 1463-1471. [25] Jandacek, RJ; Anderson, N; Liu, M; Zheng, S; Yang, Q; Tso, P. (2005). Effects of yoyo diet, caloric restriction, and olestra on tissue distribution of hexachlorobenzene. Am J Physiol, 288, G292-G299. [26] MichalekK, JE; Tripathi, RC. (1999). Pharmacokinetics of TCDD in veterans of operation ranch hand: 15-year follow-up. J Toxicol Environ Health, 57, 369-378.
In: Encyclopedia of Polymer Research Ed: Caroline E. Jones
ISBN: 978-1-61761-926-7 © 2011 Nova Science Publishers, Inc.
Chapter 16
CHLORINATE POLYCYCLIC AROMATIC HYDROCARBONS: A NEW CLASS OF ENVIRONMENTAL TOXIN? Takeshi Ohura∗ and Hiroyuki Sakakibara Institute for Environmental Sciences, University of Shizuoka, Japan
ABSTRACT A review has provided the environmental sources and behavior of chlorinated 3- to 5-ring polycyclic aromatic hydrocarbons (ClPAHs). ClPAHs have been known to be ubiquitous contaminants, which can be found in urban air, vehicle exhaust gas, snow, tap water and sediments. The concentrations in the each environment are generally higher than those of chlorinated dioxins, although which are significantly lower than the concentrations of parent compounds, PAHs. Based on the environmental data and emission sources analysis for the ClPAHs, the dominant production process is by incineration of organic materials including chlorine like as the case of dioxins. On the other hand, treatment of PAHs with chlorine in water also produced ClPAHs. Indeed, such secondary reaction for ClPAHs generation could occur in the gaseous phase. For the biological effects, the toxicities such as mutagenicity and aryl hydrocarbon receptor activity of certain ClPAHs are emphasized than those of parent PAH, in which the toxicities of exposure to ClPAHs based on AhR activity were estimated to be approximately 30-50 times higher than that of dioxins. To investigate the sources and environmental behavior of ClPAHs is of great importance to assess human health risks.
∗ Address correspondence to T. Ohura,Institute for Environmental Sciences, University of Shizuoka, 52-1 Yada, Shizuoka 422-8526, Japan.Telephone: +81 54 264 5789. Fax: +81 54 264 5798.E-mail:
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INTRODUCTION To date, the studies on the environmental behaviors and biological effects of PAHs have been carried out in the world, and the numbers of papers are more than 10,000. On the other hand, those studies on chlorinated congeners, ClPAHs, are very limited [1]. One of the reasons will be due to the lack of purified, individual ClPAH analytical standard. Recently, Ohura et al. succeeded to synthesize 12 individual ClPAHs with 3 to 5 ring as analytical standards (Figure 1) [2]. It has been advanced the studies on environmental behaviors and biological effects of ClPAHs. This section outlines the latest advance of ClPAHs study. Cln Cln
Cln Cln
chloroanthracen e (n=1−2)
chlorophenanthrene (n=1−3)
chlorofluoranthene (n=1−2)
chlorochrysene (n=1−2)
Cl
Cln 1-chloropyrene
Cl
chlorobenz[a]anthracene 6-chlorobenzo[a]pyrene (n=1−2)
Figure 1. Structures of ClPAH derivatives.
Occurrences of Clpahs in the Environment The first report of environmental ClPAHs will be on the survey of tap water performed by Shiraishi et al., which were performed as long ago as 1980s [3]. In this paper, relatively low molecular weight ClPAHs (< 4-ring) such as chlorophenanthrene and chloropyrene derivatives have been detected, which have been suggested to produce during chlorine treatment of water as the possible production mechanism. Indeed, Oyler et al. have confirmed to be caused production of low molecular weight ClPAHs by process of chlorine disinfection in water [4, 5]. Some time later, Nilsson et al. have reported that ClPAHs with 3 to 5-ring were present in both gas and particle phase in urban street and road tunnel air [6]. Based on the profile analysis, the authors suggested that the ClPAHs were formed in secondary reactions with PAH and leaded gasoline scavenger containing dichloroethane. In the mid2000s, Ohura and co-workers have been carrying on the studies on atmospheric behaviors and sources of ClPAHs [2, 7-9]. In a Japanese urban air, 26 species of ClPAHs have been detected in either the gaseous or particulate phase. The ambient (gaseous + particulate) concentration of total ClPAH ranged from 18 to 330 pg/m3 [8]. Those ClPAHs levels tended to be approximately ~1000 times lower than PAHs. A seasonal variability of both gaseous and particulate ClPAHs concentrations was observed, with higher concentrations in winter
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than in summer, which was different profile from PAH. Only under the study, the natural logarithm of total concentrations of particulate ClPAHs were significantly correlated (p < 0.05) with the reciprocal mean temperatures, but not for gaseous ClPAHs. It implies that the gaseous ClPAHs, rather than particulate ClPAHs, would be affected by various factors such as long-range transport, photodecay reactions, and sources. According to factor analysis using particulate ClPAH in addition to particulate PAH concentrations, the predominant source of particulate ClPAHs was suggested to be incineration. Indeed, Ohura and Kannan group have been vigorously carried out the analysis of possible emission sources for ClPAHs, and detected much of ClPAHs in fly ash and bottom ash from waste incinerators [10, 11]. With regard to the occurrences of ClPAHs in other environments such as soil and sediment, there is not report, yet.
Biological effects of ClPAHs AhR activity A majority of the adverse effects of xenobiotics such as PAHs and dioxins is thought to be mediated through AhR activation. Therefore, to investigate AhR-mediated activity of ClPAHs will be important to assess the exposure risks. Ohura et al. have investigated the AhR activities of 18 ClPAHs with 3 to 5 rings using the yeast assay system and compared the potencies to the corresponding PAHs [12]. All ClPAHs tested showed appreciable dosedependent increases in AhR-mediated reporter gene expression. Among the ClPAHs, 3,8dichlorofluoranthere was the most potent AhR ligand, followed by 6-chlorochrysene and 7chlorobenz[a]anthracene, 3,9,10-trichlorophenanthrene, having activities that were about 6, 2, and 0.8 times higher than that of benzo[a]pyrene.
AhR-mediated cross-talk In the addition of AhR activation described above, some xenobiotics such as dioxins and 3-methylcholanthrene have been recently reported to stimulate estrogen response elements (EREs)-mediated transcription without binding directly to estrogen receptors (ERs) or affecting expression levels of ERs, and consequently exert estrogen-related effects [13]. These activations were verified to be exerted via directly association of ligand-binding AhR/Arnt heterodimer with ERs, so-called AhR-ER cross-talk. One of our recent topics is whether ClPAHs affect AhR-ER cross-talk pathway or not. As reported by many researchers including our group [12, 14], phenanthrene itself have vastly weak effect on AhR activation. On the other hand, mono-, di-, and tri-chlorination increased the AhR activation and consequently activate the transcription of genes related to the drug-metabolizing enzymes such as cytochrome P-450s with the number of chlorine atom on the phenanthrene skelton [12]. However, these chlorinated phenanthrene derivatives did not affect the gene expression of cathepsin D, which is one of the target genes expressed via ER pathway, in spite of there ligand activity to AhR. These results suggest that estrogenic action mediated ER signaling through AhR activation could not necessarily occur in every AhR activated ligands. We are now under investigation on AhR-ER cross-talk activity of other chlorinated PAHs, such as chlorinated benzo[a]pyrene, and also other halogenated PAHs.
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Risk of Exposure to Clpahs To evaluate the risk of human exposure to ClPAHs, it is important to demonstrate the relationship between residue levels in the environment and toxicity. Most of the adverse effects of halogenated aromatic compounds are thought to be mediated through AhR activation. Therefore, the toxic potencies of these compounds can be expressed in terms of relationship between congener-specific concentrations and a relative potency (REP), which is specific to an experiment, or a TCDD equivalency factor (TEF), which is consensus-derived from many relative potency values. Ohura et al. established the REP values of each ClPAH relative to BaP (REPBaP) in a yeast assay system [12]. In a similar system, the intensity of the AhR-associated toxicity of TCDD was 60-fold higher than that of BaP [15]. Therefore, the overall toxicity or toxic equivalents (TEQs) of a mixture of ClPAHs relative to TCDD can be defined as follows: TEQ =∑[Ci] × REPBaPi / 60
(1)
where Ci represents the concentration of an individual ClPAH. For air samples collected from a Japanese urban city, the calculated TEQ concentration of total ClPAHs based on equation 1 was 1.18 pg-TEQ/m3, whereas the TEQ concentrations of dioxins ranged from 0.024 to 0.043 pg-TEQ/m3 in 2005 [12]. Thus, it shows that the toxicity of exposure to ClPAHs would be approximately 30 to 50 times higher than that of dioxins. On the other hand, the exposure risk of ClPAHs and PAHs in the air was also calculated using the individual REPBaP and corresponding mean concentrations obtained from ambient particles in Japanese urban air [16]. The calculated TEQ concentrations of total ClPAHs and PAHs were 2.95 and 650 pg-TEQ/m3, respectively, indicated that the contribution of ClPAHs to PAHs was less than 1 %.
Genotoxicity of ClPAHs The study on genotoxicity of ClPAHs had long been performed in comparison to other biological effects as well as the environmental surveys. The first report will be to investigate the mutagenicity of ClPAHs using Salmonella typhimurium TA98 and TA100 in the presence or absence of S9 activation enzyme system [17]. In the study, some ClPAHs such as 1chloropyrene showed the direct mutagenicity, which was apparently different pattern from parent PAH, pyrene. The reason of such toxic activation occurs remain unclear. Fu et al. have investigated the tumogenicity of ClPAH, especially focused on 7-chlorobenz[a]anthracene (7ClBaA), indicated that 7-ClBaA are potent carcinogen and that bay-region diol epoxides are the ultimate metabolites that lead to DNA adduct formation and tumor initiation [18, 19]. Conversely, 6-chlorobenzo[a]pyrene (6-ClBaP) reduced the level of mammary epithelial tumors of female Sprague-Dawley rats, compared to parent PAH, BaP [20]. It was suggested that the weak activity of 6-ClBaP could be due to poor substrate for metabolism by cytochrome P-450.
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CONCLUSION The risk assessments based on relative potencies of each AhR activity showed that ambient ClPAHs was extremely low contributors, as well as order of the ambient concentrations. In general, the significant contributions of PAHs to AhR activities in the environmental samples have been often observed in the assessment of persistent organics from sediments, which were 5~50 times order higher than that of polychlorinated dibenzo-pdioxins and dibenzofurans (PCDD/Fs). Similarly, the contribution of PCBs to the total TEQ in sediment was less than 1%, PAHs occupied the considerable portion, whereas PCBs accounted for a greater proportion (> 95%) of total TEQs in coastal and tidal flatfishes. PCDD/Fs and PCBs are extremely persistent and bioaccumulative compared to PAHs. Although metabolic characteristics of halogenated PAHs remain unknown, they are expected to be more lipophilicity and be difficult to metabolize rather than origin PAHs. Thus, such ClPAHs may be accumulated at high level in biological tissues, and exert the toxic action. In order to reveal the toxicities of ClPAHs, further studies concerning the metabolic dispositions will be needed.
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Ohura, T. (2007) Environmental behavior, sources, and effects of chlorinated polycyclic aromatic hydrocarbons. ScientificWorldJournal. 7, 372-380. Ohura, T.; Kitazawa, A.; Amagai, T. & Makino, M. (2005) Occurrence, profiles, and photostabilities of chlorinated polycyclic aromatic hydrocarbons associated with particulates in urban air. Environ. Sci. Technol. 39, 85-91. Shiraishi, H.; Pilkington, NH.; Otsuki, A. & Fuwa, K. (1985) Occurrence of chlorinated polynuclear aromatic hydrocarbons in tap water. Environ. Sci. Technol. 19, 585-590. Oyler, AR.; Liukkonen, RJ.; Lukasewycz, MK.; Cox, DA.;Peake, DA. & Carlson, RM. (1982) Implications of treating water containing polynuclear aromatic hydrocarbons with chlorine: A gas chromatographic-mass spectrometric study. Environ. Health Perspect. 46, 73-86. Oyler, AR.; Liukkonen, RJ.; Lukasewycz, MT.; Helkklla, KE.; Cox, DA. & Carison, RM. (1983) Chlorine "disinfection" chemistry of aromatic compounds. Polynuclear aromatic hydrocarbons: Rates, Products, and Mechanisms. Environ. Sci. Technol. 17, 334-342. Nilsson, UL. & Ostman, CE. (1993) Chlorinated polycyclic aromatic hydrocarbons: Method of analysis and their occurrence in urban air. Environ. Sci. Technol. 27, 18261831. Kitazawa, A.; Amagai, T. & Ohura, T. (2006) Temporal trends and relationships of particulate chlorinated polycyclic aromatic hydrocarbons and their parent compounds in urban air. Environ. Sci. Technol. 40, 4592-4598. Ohura, T.; Fujima, S.; Amagai, T. & Shinomiya, M. (2008) Chlorinated Polycyclic Aromatic Hydrocarbons in the Atmosphere: Seasonal Levels, Gas-Particle Partitioning, and Origin. Environ. Sci. Technol. 42, 3296-3302.
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Takeshi Ohura and Hiroyuki Sakakibara Ohura, T.; Kitazawa, A. & Amagai, T. (2004) Seasonal variability of 1-chloropyrene on atmospheric particles and photostability in toluene. Chemosphere. 57, 831-837. Horii, Y.; Ok, G.; Ohura, T. & Kannan, K. (2008) Occurrence and Profiles of Chlorinated and Brominated Polycyclic Aromatic Hydrocarbons in Waste Incinerators. Environ. Sci. Technol. 42, 1904-1909. Ohura, T.; Kitazawa, A.; Amagai, T. & Shinimiya, M. (2007) Relationships between chlorinated polycyclic aromatic hydrocarbons and dioxins in urban air and incinerators. Organohalogen Compd. 69, 2902-2905. Ohura, T.; Morita, M.; Makino, M.; Amagai, T. & Shimoi, K. (2007) Aryl hydrocarbon receptor-mediated effects of chlorinated polycyclic aromatic hydrocarbons. Chem. Res. Toxicol. 20, 1237-1241. Ohtake, F.; Takeyama, K.; Matsumoto, T.; Kitagawa, H.; Yamamoto, Y.; Nohara, K.; Tohyama, C.; Krust, A.; Mimura, J.; Chambon, P.; Yanagisawa, J.; Fujii-Kuriyama, Y. & Kato, S. (2003) Modulation of oestrogen receptor signalling by association with the activated dioxin receptor. Nature. 423, 545-550. Lampen, A.; Ebert, B.; Stumkat, L.; Jacob, J. & Seidel, A. (2004) Induction of gene expression of xenobiotic metabolism enzymes and ABC-transport proteins by PAH and a reconstituted PAH mixture in human Caco-2 cells. Biochim. Biophys. Acta. 1681, 3846. Kawanishi, M.; Sakamoto, M.; Ito, A.; Kishi, K. & Yagi, T. (2003) Construction of reporter yeasts for mouse aryl hydrocarbon receptor ligand activity. Mutat. Res. 540, 99-105. Ohura, T.; Sawada, K.; Amagai, T. & Shinomiya, M. (2009) Atmospheric behaviors and exposure risks of novel halogenated polycyclic aromatic hydrocarbons associated with particulates. submitted. Colmsjo, A.; Rannug, A. & Rannug, U. (1984) Some chloro derivatives of polynuclear aromatic hydrocarbons are potent mutagens in Salmonella typhimurium. Mutat. Res. 135, 21-29. Fu, PP.; Von Tungeln, LS.; Zhan, D-J. & Bucci, T. (1996) Potent tumorigenicity of 7chlorobenz[a]anthracene and 7-bromobenz[a]anthracene in the neonatal B6C3F1 male mouse. Cancer Lett. 101, 37-42. Xia, Q.; Yi, P.; Zhan, D-J.; Von Tungeln, LS.; Hart, RW.; Heflich, RH. & Fu, PP. (1998) Liver tumors induced in B6C3F1 mice by 7-chlorobenz[a]anthracene and 7bromobenz[a]anthracene contain K-ras protooncogene mutations. Cancer Lett. 123, 2125. Cavalieri, E.; Rogan, E.; Cremonesi, P.; Higginbotham, S. & Salmasi, S. (1988) Tumorigenicity of 6-haloganated derivatives of benzo[a]pyrene in mouse skin and rat mammary gland. J. Cancer Res. Clin. Oncol. 114, 10-15.
In: Encyclopedia of Polymer Research Ed: Caroline E. Jones
ISBN: 978-1-61761-926-7 © 2011 Nova Science Publishers, Inc.
Chapter 17
RECENT TRENDS IN ANALYTICAL METHODS TO CHEMICALLY MONITOR PAH POLLUTION Verónica Pino Department of Analytical Chemistry, Nutrition and Food Science, University of La Laguna, Spain
ABSTRACT Polycyclic aromatic hydrocarbons (PAHs) are a large group of organic compounds with two or more fused aromatic rings. They have received increased attention in recent years in pollution studies because some of these compounds are highly carcinogenic or mutagenic. Some, while not carcinogenic, may act as synergists. These compounds are widespread in the environment. Their occurrence is largely a result of anthropogenic and natural emissions such as fossil fuel burning, motor vehicles, waste incinerators, oil refining, coke and asphalt production, aluminum production, etc. PAHs have relatively low solubility in water but are highly lipophilic. Most of the PAHs with low vapor pressure in the air are adsorbed on particles. When dissolved in water or adsorbed on particulate matter, PAHs can undergo photodecomposition when exposed to UV light. In the atmosphere, PAHs can react with many other pollutants such as ozone, nitrogen oxides and sulfur dioxide, yielding diones, nitro- and dinitro-PAHs, and sulfonic acids, respectively. PAHs may also be degraded by some microorganisms in soils. PAHs in soils/sediments can be taken up by biota, and some bio-magnification of these toxic compounds may occur as they pass through the food chain. The US Environmental Protection Agency (EPA) has identified 16 un-substituted PAHs as priority pollutants. The European Food Security Authority (EFSA) has classified 15 PAHs by their food toxicity. Thus, potential exposure to PAHs must be assessed by testing contaminated soils, sediments, air, water and foods, and estimating the degree of intake of each of these media into the human body, as well as the overall toxicity.There are several main problems associated with the determination of PAHs in environmental and food samples: their low concentrations, the complexity of the samples, and the necessity for fast and reliable analytical methods. To achieve successful results, many extraction-separation techniques have been developed depending on the nature of the sample. An in-depth description of the most commonly-used techniques, together with recent trends in analytical methods to monitor PAHs levels in different kind of samples, will be the main
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Verónica Pino focus of this chapter. Among those that will be cited are Soxhlet extraction, liquid-liquid extraction (LLE), microwave-assisted extraction (MAE), ultrasound-assisted extraction (UAE), pressurized fluid extraction (PFE), supercritical fluid extraction (SFE), solidphase extraction (SPE), and solid-phase microextraction (SPME), mainly in combination with chromatographic techniques (HPLC and GC). Moreover, the utilization of active or passive samplers in pollution monitoring, as well as the increased utilization of semipermeable membrane passive samplers (SPMDs), will be described.
1. INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) refers to a group of a few hundred organic compounds consisting of two or more condensed benzene rings that can incorporate rings with an odd number of carbon atoms, sharing at least two atoms in each pair of contiguous rings. Table 1 shows several of the most significant PAHs. In general, PAHs are originated during the incomplete combustion of organic matter in natural processes (i.e., forest fires, volcanoes, and hydrothermal processes) [1,2] and in anthropogenic activities (fossil fuel burning, motor vehicles, waste incinerators, oil refining, coke and asphalt production, aluminum production, heat, food processing, etc.) [3-5]. The formation mechanism of PAHs involves several steps, not well established, but probably related with pyrolysis and pyrosynthesis reactions. It has been established that the rupture of organic matter at high temperatures in combustion processes forms radicals of low molecular weight, which later will recombine by pyrosynthesis to form PAHs [6]. In 1973, Crittenden et al. [7] proposed a PAH formation mechanism in which the acetylene molecules react to form unsaturated radicals C4, C6, C8 and C2n. These species can interact to form polyacetylenes, or can suffer a ring closure reaction to form radicals constituted by an aromatic ring attached to a 2-carbon chain (phenyl-acetylene or styrene). PAHs are then formed by reactions involving the radicals phenyl-acetylene and styrene. In 1983, Bockhorn et al. [8] established a similar mechanism, but proposed that the phenyl-acetylene is formed by the reaction between acetylene and 1,3-butadiene, as can be observed from Figure 1. The authors described in depth the different formation reactions of PAHs starting with phenyl-acetylene. Other authors have suggested the formation of PAHs starting with cyclopentadienyl radicals [9,10]. In spite of the difficulty in establishing the formation mechanism of PAHs [3], it can be ensured that, in general, the first PAH to be formed is naphthalene, mainly due to its high volatility and stability. Heavier PAHs, especially the ones with higher thermodynamic stability, are formed by addition of acetylene or by intra-molecular ring closure of lighter PAHs when the combustion temperature is increased [9-12].
Table 1. Names and structures of several significant PAHs. PAH (Abbreviation)
Structure
PAH (Abbreviation)
Structure
PAH (Abbreviation)
Structure
Naphthalene (N)
5-Methylchrysene (5-MeChy)
Anthracene (A)
Me
Fluoranthene (Ft)
Acenaphthene (Ace)
Acenaphthylene (Acy)
Benzo(b)fluoranthene (B(b)Ft)
Benzo(k)fluoranthene (B(k)Ft)
Pyrene (Py)
Fluorene (Fl)
Benzo(j)fluoranthene (B(j)Ft)
Benzo(a)anthracene (B(a)A)
Phenanthrene (Phe)
Chrysene (Chy)
Perylene (Per)
Table 1 (Continued) Names and structures of several significant PAHs. PAH (Abbreviation)
Structure
PAH (Abbreviation)
Benzo(e)pyrene (B(e)Py)
Structure
PAH (Abbreviation)
Structure
Dibenzo(a,c)anthracene (diB(a,c)A)
Dibenzo(a,l)pyrene (diB(a,l)Py)
Benzo(a)pyrene (B(a)Py)
Dibenzo(a,e)pyrene (diB(a,e)Py)
Dibenzo(a,h)anthracene (diB(a,h)A)
Dibenzo(a,h)pyrene (diB(a,h)Py)
Indeno(1,2,3-cd)pyrene (I(1,2,3-cd)Py)
Dibenzo(a,i)pyrene (diB(a,i)Py)
1-Hydroxypyrene (1-OHPy)
Cyclopenta(c,d)pyrene (cyPent(c,d)Py)
Benzo(ghi)perylene (B(ghi)Per)
OH
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· C 2H
CH
H
CH
·C
C
C
C
C
CH
CH
+ C 2 H2
+
CH
H C
C
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CH
C
C
C
HC ·
C
C
·
CH
· CH CH C
CH
CH + C 2 H2
·
·
C
·
+ C 2 H2
CH
C H -· C 2 H
- H·
PAHs of higher molecular weight
Figure 1. Formation mechanism of PAHs proposed by Bockhorn et al. [8].
1.1. Physicochemical Properties PAHs are compounds of high chemical stability, due to the conjugated systems of π orbital. Table 2 shows some physical properties of several significant PAHs. PAHs are in general solids at room temperature. Many of them can be classified as semivolatile organic compounds. They are soluble in many different organic solvents, but practically nonsoluble, or slightly soluble, in water. Nevertheless, the aqueous solubility can be increased by the presence of detergents, salts, fatty acids, or organic solvents [13]. In the same manner, the aqueous solubility can be three times higher when increasing the temperature. Their physicochemical properties, especially nonpolarity and hydrophobicity, are responsible for their persistence in the environment. Their lipophilicity justifies the bioaccumulation of these compounds in the lipid tissues of living organisms. Table 2. Physicochemical properties of several PAHs PAHs Naphthalene Acenaphthylene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthene
Vapor pressure (Pa) 10.4 0.90 0.30 0.09 0.02 1.0·10-3 1.2·10-3
Boiling point (ºC) 218.0 270.0 277.5 295.0 339.0 340.0 375.0
Solubility (mg·L-1) 31.0 16.1 3.80 1.90 4.57 0.05 0.26
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Table 2 (Continued) Physicochemical properties of several PAHs PAHs Pyrene Benz(a)anthracene Chrysene Benzo(b)fluoranthene Benzo(k)fluoranthene Benzo(a)pyrene Indeno(1,2,3-cd)pyrene Dibenzo(a,h)anthracene Benzo(g,h,i)perylene Benzo(j)fluoranthene Cyclopenta(c,d)pyrene Dibenzo(a,e)pyrene Dibenzo(a,h)pyrene Dibenzo(a,i)pyrene Dibenzo(a,l)pyrene 5-Methylchrysene
Vapor pressure (Pa) 6.0·10-4 2.8·10-5 5.7·10-7 6.7·10-8 5.2·10-8 7.0·10-7 1.4 10-8 3.7·10-10 1.3·10-11 2.4·10-6 2.4·10-5 1.5·10-9 1.5·10-9 1.5·10-9 1.5·10-9 1.0·10-5
Boiling point (ºC) 360.0 435.0 407.0 481.0 481.0 495.0 536.0 524.0 525.0 480.0 438.3 552.3 552.1 552.1 552.1 449.4
Solubility (mg·L-1) 0.13 0.01 9.6·10-3 1.5·10-3 8.0·10-4 3.8·10-3 1.9·10-3 6.0·10-4 2.6·10-4 5.0·10-3 0.014 2.7·10-4 2.7·10-4 2.7·10-4 2.7·10-4 9.7·10-3
1.2. Carcinogenic Effects The first evidence of the carcinogenic effect of PAHs was reported in 1775, when the British medical doctor Percival Pott (1714–1788) pointed out the relationship between the scrotum cancer in chimney sweeps and their long-term exposure to tar and soot. In 1930 it was demonstrated that the PAH benzo(a)pyrene, contained in the soot, was carcinogenic [14]. Up to the present, several studies have evaluated the risk for cancer development in humans exposed to PAHs via the lungs or dermis [15-17]. Recent epidemiological studies suggest that PAH-DNA adduct formation in blood and breast tissue may play an important role in breast cancer development [18]. Given their carcinogenic characteristics, several agencies have included PAHs in their list of priority pollutants. The US Environmental Protection Agency (EPA) has selected 16 PAHs as contaminants of high priority. In the same fashion, the Food Scientific Committee of the European Union (FSC), now named the European Food Security Authority (EFSA), has selected 15 PAHs by their potential toxicity in relation to foods. In 2002, the former European Commission’s Scientific Committee on Food (SCF) considered the study of the 33 PAHs included in an Environmental Health Criteria document on PAHs [19]. Not all PAHs have been demonstrated to be carcinogenic. In many instances, the carcinogenic effect is in fact due to the presence of more than one member of the PAH family, as well as several of their derivatives. Hence, some PAHs, while not carcinogenic, may act as synergists. Thus, the implication of human exposure to mixtures of PAHs, rather than to individual substances, is important. The toxicity equivalent factor (TEF) is a useful tool to quantitatively estimate the overall carcinogenic effect of an environmental sample containing several PAHs. Based on toxicological studies, a toxicity value is given to each PAH by comparison with a well-known carcinogenic PAH, benzo(a)pyrene, which has a
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reference TEF value of 1. The overall carcinogenic effect of a sample is then determined considering both the amount of each PAH in the sample and their respective TEF values. Table 3 shows several TEF values for a group of PAHs, proposed by different authors and/or agencies. It can be observed that hydrocarbons with a high number of condensed rings in the structures (between 5 and 6) are the ones with higher TEF values. The potential toxicity of a sample, expressed as equivalents of benzo(a)pyrene (B(a)Py), depends on the TEF values selected for each hydrocarbon. The carcinogenic and mutagenic activities of PAHs are closely related with their chemical structures. In general, planar PAHs are more stable and less toxic than non-planar PAHs. In addition to this, the introduction of an alkyl group into a PAH moiety generates a structural distortion of the hydrocarbon molecule, which can provoke activation or deactivation effects [20]. PAHs are apparently not carcinogenic by themselves, but they are converted into active carcinogens when metabolized by a living organism. The induction process of cancer in mammals involves a group of enzymes able to convert PAHs in water soluble compounds [21]. These oxydase enzymes of multiple functions belong to the group of cytochrome P450. As shown in Figure 2, PAHs are transformed into diol-epoxide adducts, followed by a rapid transformation in carbocations, able to covalently bind with DNA macromolecules, and therefore modifying the genetic message during the formation of new cells [22,23]. Similar reactions have also been observed with other macromolecules such as albumin and hemoglobin [20]. The presence of PAHs-DNA adducts in human tissues has been conclusively established [24]. A recent study has shown that chromosomal aberrations in cord blood are associated with prenatal exposure to carcinogenic polycyclic aromatic hydrocarbons [25]. There are various industrial workplaces which present a significant risk of certain cancer diseases. This may be attributed to an unusually high exposure to PAH [17]. Furthermore, as highly exposed occupational groups, chimney sweeps, roadmen (pavement-tarring) and roofers (roof-tarring) are also under increased risk [23]. OH O
OH
OH
OH
OH
OH
OH
OH
OH O
Macromolecule
Figure 2. Schematic representation of the metabolic activation of benzo(a)pyrene.
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Table 3. Toxicity equivalent factors proposed by different authors for the 16 PAHs listed by the EPA (numbered from 1 to 16) and for the 15 PAHs listed by the EFSA (numbered from 9 to 23). PAHs 1. Naphthalene 2. Acenaphthylene 3. Acenaphthene 4. Fluorene 5. Phenanthrene 6. Anthracene 7. Fluoranthene 8. Pyrene 9. Benz(a)anthracene 10. Chrysene 11. Benzo(b)fluoranthene 12. Benzo(k)fluoranthene 13. Benzo(a)pyrene 14. Indeno(1,2,3-cd)pyrene 15. Dibenzo(a,h)anthracene 16. Benzo(g,h,i)perylene 17. Benzo(j)fluoranthene 18. Cyclopenta(c,d)pyrene 19. Dibenzo(a,e)pyrene 20. Dibenzo(a,h)pyrene 21. Dibenzo(a,i)pyrene 22. Dibenzo(a,l)pyrene 23. 5-Methylchrysene
Nisbet [26] 0.001 0.001 0.001 0.001 0.001 0.01 0.001 0.001 0.1 0.01 0.1 0.1 1 0.1 1 0.01 — 0.1 — — — — —
EPA [27] — — 0 — — — — — 0.1 0.001 0.1 0.01 1 0.1 1 — — — — — — — —
Kalberlah [28] — 0.01 0.001 0 0 0.01 0.01 0.001 0.1 0.01 0.1 0.1 1 0.1 1 0.01 0.1 — — — — — —
Cecinato [29] — — — — — — — — 0.06 — 0.07 0.07 1 0.08 0.6 — — — — — — — —
Larsen [30] — — — — 0.0005 0.0005 0.05 0.001 0.005 0.03 0.05 0.1 1 0.1 1.1 0.02 0.05 0.02 0.2 1 0.1 1 —
1.3. Environmental Distribution of PAHs Once PAHs are formed in the atmosphere, many of them can suffer photo-oxidation reactions with nitrogen oxides, ozone, and hydroxyl radicals, with reaction rates varying between hours and days [31]. PAHs, as semivolatile organic compounds, exist in both the gaseous and the particulate phase in air. The distribution of PAHs between the gas phase and the particulate matter (PM) is a complex process which depends on the temperature, the relative humidity, the composition of the particulate matter, and the nature of the PAH, mainly, the molecular weight and vapor pressure. In general, it has been demonstrated that PAHs of lower molecular weight tend to stay in the gas phase whereas PAHs of higher molecular weight tend to exist associated to the particulate matter, especially with particulate matter having diameters lower than 10 μm (PM10) [32,33]. In general terms, heavy PAHs are associated with the particulate matters by adsorption and absorption mechanism [34]. Independently on the sorption process that is taken place, the distribution constant is inversely dependent on the vapor pressure of the hydrocarbon.
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The presence of PAHs in the aquatic ecosystems is mainly as a result of anthropogenic inputs [35]. PAHs can be transported by direct effluents, lixiviation of soils, dry atmospheric deposition, and, mainly, humid atmospheric deposition (rain). For some watersheds, atmospheric deposition represents a significant input to the total surface water PAH burden, and may be independent of adjacent land use [36]. Once PAHs are in the aqueous medium, they can associate or bind to colloids, and so being transported in water. This justify the presence of PAHs in living organisms that are located in areas far away of an intensive industrial activity, and justify the fact that sediments are much richer in PAHs than the surrounding aqueous media [37-39]. Atmospherically derived PAHs in marine sediments have been linked to industrial and athropogenic sources [40,41]. The degradation of PAHs in sediments is in general a very slow process [42,43]. PAHs can accumulate in aquatic living organisms [44], entering into their biologic processes and affecting physiological lethal and sub-lethal levels [44,45]. Evidently, this effect is not isolated, because everything belongs to the ecological equilibrium. PAHs are metabolized by fishes [46] and into a lesser extent by mollusks or crustaceans, generating polar metabolites that can be excreted. In addition to this, the presence of PAHs may change the mechanisms of metal uptake by several plants, thus influencing kinetics and extent of metal phytoextraction [47]. It results clear that the biomagnification of PAHs into higher trophic levels across the food chains results into dispersal of pollutants far from their point of origin and from areas of significant human activity. Figure 3 illustrates a general scheme of the environmental distribution of PAHs. Atmosphere Particulate matter - Gas phase
Volatilization / Aerosolization
Dry/Wet deposition
Water
Soils
Vegetal
Colloids
Sediments
Biota
Human life
Figure 3. Environmental distribution of PAHs.
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1.4. Monitoring PAHs Given all these precedents, the wide environmental distribution and the carcinogenic risk associated to PAHs, it results necessary to monitor these contaminants in all kind of environmental samples, as well as to establish their impact in living organisms [48]. They must therefore be measured in the atmosphere for air quality assessment, in biological tissues for health-effects monitoring, in waters, sediments and mollusks/fishes for environmental monitoring, and in foodstuffs for safety reasons. To understand the extent of human exposure to PAHs, reliable sampling and analytical methods are necessary. The monitoring of PAHs in environmental samples presents several problems. One of the main limitations is related with the low concentration levels of PAHs to be monitored in the samples, with the consequent difficulty for being detected and quantified. Another limitation is related with the inadvertent errors during the sampling. In addition to this, the complexity of the samples, the undoubtedly presence of interfering substances, and the necessity of accurate and fast analysis, do the monitoring of PAHs an analytical challenge. In the analysis of PAHs, chromatographic techniques, mainly gas chromatography (GC) and liquid chromatography (LC), are often the preferred approach for separation, identification and quantification. Spectrofluorimetric techniques have also been used with screening purposes. In general, capillary GC is preferred over LC for the determination of PAHs mainly because of favorable combination of greater selectivity and resolution [49-51]. On the other hand, HPLC with fluorescence detection (FD) presents the best detection limits for PAHs [15]. Nevertheless, to reach the adequate sensitivity limits in gas chromatography or liquid chromatography, independently on the detection technique used (mass-spectrometry or fluorescence, respectively), it results necessary to carry out preconcentration procedures as well as clean-up steps [52]. The utilization of clean-up and preconcentration procedures introduces more sources of error in the analytical method, as well as increases the analysis time. The most common extraction-preconcentration procedures are Soxhlet extraction, liquid-liquid extraction (LLE), ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), pressurized fluid extraction (PFE), dialysis, solid-phase extraction (SPE), and solid-phase microextraction (SPME). The sampling is a critical step that must be carried out carefully, with the appropriate sampling design and sampling device. The increased utilization of semi-permeable membrane passive samplers (SPMDs) during the sampling stage of PAHs should be highlighted. The main aim of this chapter is to describe the most common procedures to monitor PAHs in environmental samples, with emphasis to the analytical methods used in extraction and determination procedures. This chapter is divided in analytical monitoring depending on the nature of the sample, though some methods turn out to be common for different samples.
2. AIR MONITORING The highest concentrations of atmospheric PAHs can be found in the urban environment, due to the traffic and the small dispersion of the atmospheric pollutants [15,53]. Hence, the cancer risk development in humans is much higher in the cities, considering the population
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density [54]. Sources of PAHs in urban atmosphere also include re-suspended soils, refineries and power plants [55-57]. Lighter PAHs, which have weaker carcinogenic effects by themselves, are the most abundant in the urban atmosphere; however, their presence is quite risky because they can react with other air pollutants to form more toxic derivatives [58], i.e., they can react with ozone, nitrogen oxides and sulfur dioxide, yielding diones, nitro- and dinitro-PAHs, and sulfonic acids, respectively [31]. Despite the efforts conducted to reduce urban particulate pollution in cities, the traffic still contributes to increase urban particulate pollution. Besides, particles produced by cars are much smaller than coal particles and can be found in the breathable size fraction [59]. Heavier PAHs, with higher carcinogenic effects, are usually associated to particulate matter in the atmosphere. Particles of concern to human health are those known as inhalable particles (PM2.5, PM with aerodynamic diameter less than 2.5 μm). Most particulate-phase PAH are adsorbed onto fine particles, which are deposited slowly and, depending on atmospheric conditions and chemical reactivity, may be transported over long distances, being placed even in remote areas. The European Union requires continuous air-quality monitoring with longterm assessment of benzo(a)pyrene (B(a)Py) and other relevant PAHs [60]. The Council Directive 2004/107/EC establishes a target value for B(a)Py of 1 ng·m−3, and a long-term objective of 0.1 ng·m−3, applying to the B(a)Py content in the particulate matter fraction with an aerodynamic diameter lower than 10 μm (PM10) [60]. Other sources of PAHs are the incomplete biomass combustion which can take place in both natural processes (as fires or volcanic eruptions) and anthropogenic processes (as elimination of residues, cooking or smoking of foods). Smokes coming from biomass combustion not only increases the amount of PAHs in the atmosphere but also constitute an important source of particulate matter in the atmosphere [61], contributing to increase the risk of sorption of PAHs. The composition of PAHs in association with airborne particles depends significantly on the emission sources. PAH concentration ratios can be used to identify possible emission sources: combustion of vegetal biomass or combustion of fossil fuels. [62]. These ratios are usually restricted to PAHs with equal molecular weights, which are normally present in combustion processes due to their chemical stability [63-65]. Table 4 shows several isomeric ratios of PAHs used to distinguish among combustion sources. Table 4. Isomeric ratios of PAHs used to distinguish among different combustion sources. Isomeric ratio Ft / (Ft + Py) = Ft/202 A / (A + Phe) = A/178 1,7-diMePhe / (1,7-diMePhe + 2,6-diMePhe) = 1,7-diMePhe/206 I(1,2,3-cd)Py / (I(1,2,3-cd)Py + B(ghi)Per) = I(1,2,3-cd)Py/276
Combustion source Petroleum Biomass 0.40-0.50
> 0.50
< 0.10
> 0.10
< 0.45
> 0.70
0.20–0.50
> 0.50
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Indoor air quality is also of great interest in the world, especially if considering that most people spend approximately 90% of their time indoors (in their residences, at work, or elsewhere), where scarce ventilation usually helps in increasing PAHs contents. Indoor air in houses is polluted by PAHs that come from not only outdoor air but also indoor emission sources such as cooking [66] and gas home appliances [67,68]. Additional contributions to ambient air levels arise from tobacco smoking [69-71]. To monitor air quality, it results therefore important not only to determine the PAHs contents in the gas phase but also the content associated to the particulate matter. In addition to this, the monitoring methods for airborne PAHs maybe classified in those concerned with either direct occupational exposures or with ambient exposures (outdoor) [72]. Occupational environment is usually monitored by active sampling, whereas ambient exposure is usually monitored by passive sampling, but not exclusively. Active sampling methods require pumping the air stream through a medium (e.g., filter, sorbent, impinger, cryogenic trap) which will retain the PAHs. The analysis of PAHs from air samples in active sampling is followed by an extraction step, clean-up procedures and/or exchange of solvents, and then chromatographic separation and quantification. Sample preparation is considered to be the bottleneck of most analytical methodologies, as it constitutes the principal source of error in the analytical process and remains one of the most time-consuming steps [73]. Figure 4 shows a general scheme of the usual procedure for the monitoring of PAHs in air. The active sampling methods commonly used to determine and quantify PAHs in air are combination of fiber filters, to retain the particle-bound PAH fraction, whereas impregnated filters, impingers, solid sorbents like polyurethane foam plugs (PUF) and XAD resins, and cryogenic traps are used to trap the semivolatile and volatile PAH fractions, respectively [74-76]. After sampling, the most common extraction and preconcentration methods are Soxhlet [77] or sonication [78] of the systems used to trap the PAHs. These routine methods are time and solvent consuming. Apart of these common extraction procedures, many times other clean-up techniques are necessary given the complexity of the samples, such as a cleaning using a chromatographic column (filled with silica gel, Florisil or alumina, as sorbent phases), or a solid-phase extraction (SPE) step. Passive sampling techniques rely on the controlled transport of the analyte along a concentration gradient towards a surface which acts as a sink for the PAHs. Passive air samplers are preferred to conventional active air samplers since they do not require electricity or maintenance and are less expensive. The main disadvantage is the long sampling time required due to the low PAH levels and limited analytical sensitivity [79-81]. Followed passive sampling, extraction-preconcentration, clean-up, and chromatographic procedures, identical to the ones used in active sampling approaches, are conducted.
2.1. Sampling Systems The most critical step to ensure an accurate monitoring of PAHs in air is the choice of the sampling strategy. The main problems associated to the sampling systems of PAHs in air are related to insufficient sorption on the sampling devices, the re-evaporation of already retained PAHs, and even the chemical destruction of the PAHs already retained. Then, PAHs losses
Passive sampling
General scheme
Common procedures
Other procedures
PAHs in air Pump. Active sampling
·Fibre filters ·Impinger solutions ·Adsorbents (PUF or XAD resins)
Sampling
Extraction
·Soxhlet ·Ultrasound-assisted extraction
·Dialysis (passive sampling with SPMDs) ·Microwave-assisted extraction ·Pressurized-fluid extraction ·Supercritical-fluid extraction
Clean-up
·Chromatographic column (i.e. silica gel) ·Solid-phase extraction ·Solvent exchange
·Solid-phase microextraction
GC or HPLC Figure 4. Common steps of the analytical method to monitor PAHs in air.
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are practically unavoidable due to their chemical reactivity and volatility. The importance of the sampling is such that, indeed, the posterior utilization of any extraction and/or chromatographic method does not turn out to be so critical in the analysis of PAHs for air monitoring.
2.1.1. Active Sampling While diffusive passive sampling offers advantages in terms of cost and simplicity, active-pumped sampling remains the most versatile option for simultaneous monitoring of multiple target PAHs. The major drawbacks of active sampling are the high cost of the equipment and requirement of power and training; these restrict its application to widespread locations especially to remote regions. Given that large air volumes (>100 L) may be drawn through the tubes in many cases, pumped active sampling also offers better detection limits than other air monitoring techniques. There are multiple pump models on the market, from a wide variety of brands, designed to fulfill different needs depending on the specific sampling. High/low volume pumps designed for active sampling typically operate at flow rates ranging from 0.5-1400 L min-1. The sampling time is normally a few hours up to one day. Sampling of airborne PAHs requires special equipment since PAHs with a lower molecular weight are found mainly as vapor, while those with a higher molecular weight are to be found in the condensed phase. Consequently, an adequate sampling method usually combines a filter for the collection of particulate, and one or more sorbents such as XAD-2 and/or polyurethane foam (PUF) for the collection of the vapor phase. Table 5. Sampling systems recommended by standard methods. Standard method ASTM D 6209-98 (2004) ISO 11338-2 (2003)
ISO 12884 (2000)
ISO 16362 (2005)
Application Standard test method for determination of gaseous and particulate PAHs in ambient air Stationary source emissions— determination of gas and particlephase PAHs—Part 2: sample preparation, clean-up, and determination Ambient air—Determination of total (gas and particle phase) PAHs—Collection on sorbentbacked filters with gas chromatographic–mass spectrometric analyses Ambient air—Determination of particle-PAHs by HPLC
Sampling system Fine particulate filter followed by a vapor trap containing PUF or resin XAD-2. Filters to collect the particulate phase whereas gas phase is trapped on an adsorbent (XAD-2 or PUF). Fine-particle filter followed by a vapor trap containing PUF or XAD-2.
Fine-particle filters.
Historically, two types of filters, glass fiber (GFF) and silver membrane have been utilized most often in the sampling of airborne PAHs in both active and passive sampling. Glass filters have mainly been used in active sampling whereas silver membranes have mainly been utilized in passive sampling [72].
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New materials have recently been developed for filters [82], like silica quartz (QFF), teflon-coated glass (TGFF), reagents-coated glass [83], PTFE membrane bonded to polyethylene net, and PTFE member supported by PTFE fibers, among others. Improvements in sampling methods designed to minimize the losses of PAHs volatized during sampling have also relied on addition of solid sorbent back-up cartridges and cryogenic traps. For the sampling of PAHs, the sorbents most commonly used include Tenax, XAD-2, polyurethane foam (PUF), Florisil, Chromosorb 102, Chromosorb 106 and SepPakC18 on Porasil [84]. The procedures UNE-77250 and EPA TO-13A described a sampling system to determine PAHs in ambient air based on the utilization of a particulate fiber filter followed by a PUF cartridge or a XAD-2 resin [85,86]. Table 5 shows several sampling devices utilized in standard methods. The most common sampling devices utilized in recent years for the trapping of PAHs in air monitoring are XAD-2 resins [87], glass-quartz filters [88-90], combinations of filters and PUF [15,91], combination of glass-quartz filters, PUF, and XAD-2 [92], combination of PTFE filters and XAD-2 [93], or combination of denuders and filter pack (XAD-4 impregnated quartz) [94]. With respect to the PAHs determination in biomass smoke, Table 6 includes the most common sampling systems utilized in recent years. It can also be observed that glass-quartz filters, PUF and XAD-2 are the preferred trapping systems. Table 6. Sampling procedures to determine PAHs in biomass smoke. Combustion source Wood, barley, corn, rice and wheat Woods Woods Woods residues Pines wood and needle Agricultural and sylvicultural debris Cork residues Cattle residues Urban residues Medical residues Pine and coal Coal Coal * G-Q: glass or quartz
Sampling procedure G-Q* filter, XAD-2 and impingers G-Q* filter and XAD-2 G-Q* filter, XAD-2 and impingers Impingers (glass wool and CH2Cl2) G-Q* filter and PUF/XAD-2/PUF G-Q* filter and PUF/XAD-2/PUF G-Q* filter and XAD-2 G-Q* filter and cartridge XAD-2/PUF G-Q* filter and PUF G-Q* filter and cartridge XAD-16/PUF G-Q* filter, XAD-2 and active coal XAD-2 impregnated with CH2Cl2 G-Q* filter and PUF
Ref. [95] [5] [96] [97] [98] [99] [100] [101] [102] [4] [74] [103] [104]
A recent method has been reported for the ambient air sampling of the 16 EPA PAHs plus B(e)Py. The method is based on active sampling on sorption tubes consisting of polydimethylsiloxane (PDMS) foam, PDMS particles and a TENAX TA bed [105]. It was demonstrated that both the high molecular weight PAHs and the more volatile PAHs, such as naphthalene, are efficiently trapped and quantitatively desorbed, without breakthrough and chemical reaction artifacts. An important result is that the total toxicity equivalence value (TEQ) for PAHs was ca. two times higher compared to the conventional sampling method, illustrating that the concentrations of PAHs in ambient air have been underestimated until
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now [105]. This important conclusion points out the need to develop accurate and quantitative methods of sampling in order to report authentic values of PAHs contents in the air. In any case, the best exposure and risk assessments for PAHs for the general population and the working population can be done using personal monitors. Ideally, this should consider a sampling time relevant to exposure scenarios, and hence be representative for possible health effects. These personal samplers are usually equipped with a battery to facilitate the pumping, and are available for a variety of brands. The personal sampler is characteristics for being portable and should be carried by a person as close to the breathing zone as possible [16,17,93,106]. In addition to this, people do not need to be excessively trained on the utilization of these systems. These personal samplers are equipped with filters and/or sorbents, as any sampling device above described.
2.1.2. Passive Air Samplers (PAS) Diffusive samplers, also called diffusive monitors, passive samplers or passive monitors, are utilized for sampling without the need for an air mover, that is, without a pump. Manmade diffusive sampling operates by allowing gas or vapor molecules to diffuse through a defined volume of still air or through a polymer membrane, until they reach a sorbent bed. The principles of uptake are to consider that the passive sampling medium is uniform and “porous” and that it traps PAHs from the atmosphere by gaseous diffusion, and sorption. The mass collected is a function of the external concentration and the diffusion coefficient of the molecules. The diffusion coefficient varies in a known manner with temperature and pressure, and so the result can be corrected for these parameters [107]. Passive samplers provide information about long-term contamination. They can be used in a screening method for semi-quantitative comparison of different sites contamination with the advantage of low sensitivity to accidental short-time changes in concentration of pollutants. Other advantages of PAS are the simplicity and easiness of handle. In addition to this, they are cheaper because they do not need a pump. The main disadvantage is the long sampling time required due to the low PAH levels and limited analytical sensitivity. Most of the passive air sampling measurements have been performed using semipermeable membrane devices (SPMDs) [80,108,109], polyurethane foam disks (PUFs) [79,110-112] and XAD resins [113], which can be exposed over the period of several weeks or months. SPMDs were initially developed by Huckins et al. [114]. The importance of SPMDs for air monitoring is expected to increase in the near future. SPMDs were initially designed to mimic the behavior of aquatic animal tissues in bioconcentrating contaminants, that is, the purpose was that only the bioavailable fraction of contaminants is absorbed and not compounds bonded to macromolecules [80]. As pointed out by Esteve-Turrillas et al. [80], semi-permeable membrane devices comprise a low-density polyethylene (LDPE) tube, 70-90 μm wall thickness, filled with triolein (1,2,3-tris-cis-9-octadecenoyl glycerol). Standard, these devices are 106 cm long, 2.54 cm wide, and contain 1 mL of triolein. Such devices are usually located in chambers to avoid external contamination. Due to both their high membrane surface area and their relatively large volume of receiving medium, SPMDs proved to be most effective in their capacity to accumulate lipophilic contaminants such as PAHs. The main disadvantage of the SPMD technique is the complex sample preparation procedure required to recover the accumulated pollutants from the collecting phase (triolein). This is achieved by dialysis using considerable amounts of organic solvents, followed by
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concentration of the extracts and an expensive cleanup before the chromatographic analysis. SPMDs composed of some other materials for the membrane and for the internal solvent have also been described in recent years [80]. Photo-degradation of PAHs has been described in several SPMDs [115], so recent studies have focused in developing the best sorption chamber to avoid such photo-degradation [116]. SPMDs have been used not only to monitor PAHs in air [117], but also to monitor PAHs and nitro-PAHs in a wide survey which includes five European countries [118]. SPMDs must be calibrated in the laboratory, and the equations to relate the extracted concentrations with environmental concentrations must be established. Some other devices for low-flow sampling are the denuders, which consist of sections of capillary gas chromatographic columns in which the vapor-phase of the semi-volatile organic compounds are sorbed by diffusion into the stationary phase, while the particulates pass through and are collected on a filter. Advantages of the method are the more accurate measurement of the vapor-particle partitioning and the more quantitative sampling of the most volatile compounds. Disadvantages are the low sampling flows resulting in very long sampling times (several days) [94,119]. Many studies utilize biomaterials as pollutant sorbents, such as plant samples, e.g. bark [120-123], lichen or pine needles [117,123-125], to monitor ambient air concentrations assuming that their spatial pollution distribution in the plants and the atmosphere will be closely related. However, the concentration capacity and sampling rate of plant tissues vary with the plant species and age, location and season. Therefore, these kinds of plant data are still subject to several uncertainties, which complicate their interpretation and limit the potential of plants as monitoring tools. For this reason, man-made passive air samplers are preferred up to date, despite their additional costs and that they have to be deployed, since the variability of samplers at different locations is low, and the sampling period can be controlled.
2.2. Extraction Procedures After sampling, convenient transport and storage of PAHs must be conducted, to avoid losses by volatilization, sorption on recipients, or photo-degradation. In general, the sampling devices are protected from light until they reach the laboratory. It is also advisable to transport the filters and adsorbents that contain the PAHs in a refrigerated bag or icebox [94,99]. Once in the laboratory, samples are conserved at ≤ 4ºC, in absence of light, during at period not longer than 7 days prior to extraction. The most common extraction methods when monitoring PAHs in air are Soxhlet and ultrasound-assisted extraction (Figure 4).
2.2.1. Soxhlet Extraction Soxhlet has been a standard technique during more than one century and it is still the main reference procedure to which the performance of other analytical methods is compared. The Soxhlet extraction is based on the apparatus invented in 1879 by Franz von Soxhlet, and is mainly used when extracting solid and complex samples. Figure 5 shows a typical scheme of a Soxhlet apparatus. The typical Soxhlet procedure starts by placing the sample in a cellulose thimble, which is loaded into the main chamber of the extractor. The Soxhlet extractor is then placed onto a distillation flask containing the organic extraction solvent, being acetone, toluene, and methylene chloride the most common organic solvents used.
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Once the organic solvent is heated to reflux, the solvent vapor travels up to the distillation arm and floods into the chamber housing the thimble. When the Soxhlet chamber is almost full, the chamber is automatically emptied by a siphon side arm, with the solvent running back down to the distillation flask. This cycle is repeated several times, over hours or days. It is obvious that during each cycle a portion of the PAHs dissolves in the solvent. After many cycles the PAHs are then concentrated in the extracting solvent. After extraction the solvent is removed, typically by means of a rotary evaporator, to decrease the amount of organic solvent and so to preconcentrate the analytes. The non-soluble portion of the extracted sample remains in the thimble, and is usually discarded. Soxhlet extraction is still today the preferred extraction technique to ensure quantitative extraction of complex samples. Table 7 shows significant examples of works which utilize Soxhlet extraction in the determination of PAHs for air monitoring. Soxhlet is applied in the extraction of the solid systems used to trap PAHs in air monitoring. Table 7 also includes the most common organic solvents used, and the extraction time required to complete the extraction step. It should be noted the high extraction times (around 24 h following official procedures) and the high volumes of toxic organic solvents required in this traditional extraction approach, which are not only expensive to dispose off but which cause additional environmental issues. These are undoubtedly the main drawbacks associated with the Soxhlet extraction. Official procedures also recommend the utilization of Soxhlet extraction in PAHs air monitoring, in the methods ISO 11338-2 (2003) “Stationary source emissions — determination of gas and particle-phase PAHs—Part 2: sample preparation, clean-up, and determination”; and ISO 12884 (2000) “Ambient air—Determination of total (gas and particle phase) PAHs—Collection on sorbent-backed filters with gas chromatographic–mass spectrometric analyses”. The ASTM D 6209-98 (2004) “Standard test method for determination of gaseous and particulate PAHs in ambient air” also recommends Soxhlet extraction. It is also recommended by EPA TO-13A (toxic compounds in ambient air). Protection from light is always recommended during Soxhlet extraction performance, to avoid photo-degradation of PAHs. Given the above mentioned disadvantages of Soxhlet, several authors have tried to improve the conventional Soxhlet device, either by modifying its design or the operational procedure [126]. The main updates of this “old-fashioned” technique have been related to the miniaturization (and therefore diminution of the organic solvent consumption), automation, and particularly, the combination with microwaves [127]. Main applications have been found when combining focused microwave-assisted extraction with Soxhlet, which is based on the same principles as conventional Soxhlet extraction but using microwaves as auxiliary energy to accelerate the process. However, up to date there are no reported applications of the focused microwave-assisted-Soxhlet in the analysis of PAHs in air monitoring. There are few works which utilize devices based on automatic Soxhlet in air-monitoring of PAHs [79,112]. It is therefore necessary that the recent developments in analytical procedures are followed by an implementation in monitoring techniques.
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1.- Thimble 2.- Distillation path 3.- Siphon
2
3 1
Figure 5. Typical scheme of a Soxhlet extractor.
Table 7. Recent works which utilize Soxhlet for the monitoring of PAHs in air. Extraction time (hours) 20 18 13 8 (each) 8 24 16 72 18
Organic solvent hexane:acetone (1:1) or toluene diethyl ether:hexane (1:9) hexane:methylene chloride (1:1) methylene chloride methylene chloride methylene chloride methylene chloride methylene chloride diethylether:hexane (1:9)
Organic solvent volume (mL) 150 200 90 1st 250 and 2nd 150 — 200 — — 300
Reference [77] [90] [121] [94] [15] [122] [16] [104] [99]
2.2.2. Ultrasound-Assisted Extraction (UAE) Any sonication waves with a frequency higher than the ones detected by the human hear are called ultrasounds (US), that is, ultrasounds have wave frequencies higher than 16 kHz. The production of ultrasounds is possible due to the discovery of Marie and Pierre Curie in 1880 of the piezoelectric effect and its inverse. Nevertheless, the first commercial equipment of ultrasounds was not developed until 1917. Such equipment was curiously proposed in a contest of ideas in 1912, with the purpose of detecting icebergs right after the Titanic shipwreck. The utilization of ultrasounds in analytical chemistry laboratories did not take place until 1970, which appears to be illogical considering that it is obvious that the
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propagation of the sonication waves in any medium is going to increase the mobility of the molecules present in such medium. The most common ultrasounds devices in an analytical laboratory are the sonication bath and the sonication probe. The ultrasonic bath is so common in the chemical laboratories that it does not need presentation. It is widely used due to their economical feasibility, ensuring at the same time a relatively good energy distribution in the reactor placed in the bath. The sonication probe has a higher power, which can be modulated. The main applications of the ultrasounds in analytical chemistry are the ultrasound-assisted extraction and the ultrasound-assisted digestion, among others [128-130]. The most common procedure to carry out an ultrasound-assisted extraction is to place the sample in a recipient containing the extractant organic solvent, and to activate the ultrasounds during a fixed period of time. Protection from light of recipients-containing PAHs during the sonication extraction is always recommended. EPA Method 3550 C describes the utilization of sonication for solid samples, which can be extended to several samples coming from air monitoring. Recent applications of sonication in the extraction of PAHs from the sampling devices used in air monitoring are summarized in Table 8. The extraction times are lower compared to traditional Soxhlet, and also the amounts of organic solvents used, but higher enough to be an environmental issue. Sonication probes or the most-recent US-devices have not been used up to date in the analysis of PAHs in air monitoring. A paradoxical situation has emerged as most of the analytical methods employed to investigate environmental problems also generate chemical wastes, like high amounts of organic solvents, resulting in an environmental impact. In some circumstances, the chemicals employed are even more toxic than the species being monitored. In this sense, the work of some analytical chemists has focused on the development of methods less harmful to humans and to environment. Great concern over the disposal of the toxic organic solvents employed in the described method and their effect to the environment has led to move towards cleaner extraction methods such as microwave-assisted extraction and pressurized fluid extraction, among others. Improvements in clean-up methods also include solid-phase extraction (SPE) and solid-phase microextraction (SPME). Table 8. Recent works reporting the utilization of ultrasound-assisted extraction to remove PAHs from sampling devices in air monitoring Sampling device/sample Extractant Filters 5 mL cyclohexane XAD-2 sorbents 1 mL acetonitrile Pine needles/bark 30 mL hexane:CH2Cl2 1:1 Glass filter cyclohexane 20 mL hexane:acetone 1:1 SRMa 1648 or 1649a SRMa 1648 or 1649a 20 mL toluene Aerosol 25 mL CH2Cl2 Filters 150 mL CH2Cl2 a Standard reference material (certified)
Extraction time (min) 30 min 30 min 10 min (2 times) 30 min 30 min (2 times) 30 min (2 times) 10 min (4 times) 30 min (3 times)
Ref. [93] [93] [123] [88] [77] [77] [89] [104]
2.2.3. Microwave-Assisted Extraction (MAE) Microwaves are non-ionizing radiation with a frequency between 300 and 300000 MHz. Molecules under microwaves are moved by ion migration and dipole rotation. Commercial
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microwaves operate at 2450 MHz, which generate the alignment of molecules under the radiation followed by return to disorder 4.9·109 times per second. This quick movement-relax is consequently followed by a fast heating. The only condition for a molecule to suffer such heating is to be a dielectric compound. Fastest heating are obtained with analytes of high dielectric constants. The first analytical application of microwaves was carried out by Abu-Samra in 1975 [131]. The utilization of microwaves to extract organic compounds from a number of samples (soil, seeds, foods and feeds) was carried out at the end of the 80s utilizing a domestic microwave [132]. The analytical applications in extraction procedures are based on the heating of the extractant by microwaves. The extractant is usually an organic solvent characteristic for being a dielectric compound. The use of solvent mixtures with and without dipoles opens up a variety of potential solvent mixtures. The extractant is placed in a reactor altogether with the sample, followed by heating under microwaves during a certain period of time. It should be highlighted that microwave heat the sample and the extractant without the need of heating the reactor, therefore reaching high temperatures with lower extraction times. After the application of the microwaves, the analytes originally bound to the sample are dissolved in the extractant solvent. The efficiency of the microwave extraction process depends on the nature of the sample and the extractant. While domestic microwaves are still used in several analytical laboratories, commercial microwaves specifically-designed for laboratory use are preferred, especially attending to safety reasons. Most analytical applications have been carried out using cavity-diffuse or, to a lesser extent, focused microwave systems. Usually they are referred to as closed- or openvessel systems, respectively, but this terminology is not correct as the so-called open vessels in focused systems are not completely open, and as it is also possible to operate closed vessels in a focused microwave [133]. The utilization of diffuse microwaves needs of a cavity (an oven) where samples are placed in a rotor to be moved for ensuring reproducibility in microwave irradiation, because the electric field is not uniform inside the cavity. Several samples can be placed simultaneously in a cavity-microwave. Focused microwaves are more efficient in terms of homogeneous heating. Figure 6 shows a scheme with the differences between diffuse and focused microwaves devices. The main variables to be optimized in an analytical extraction by microwaves are: the microwave power, the extraction temperature and/or extraction pressure, the extraction time, the nature and volume of the extractant, and the characteristics of the matrix such as amount of water or presence of interferences. The optimization of these variables is time consuming unless an experimental design is used [134]. Microwave-assisted extraction has successfully been used in the extraction of sampling devices utilized for monitoring of PAHs in air, as it can be observed in Table 9. It can be seen that shorter extraction times and lower amounts of organic solvents are needed, especially when compared to Soxhlet extraction. In spite of the advantages of the microwave-assisted extraction method, including high extraction efficiencies, its utilization in samples coming from air monitoring is not widespread. EPA Method 3546 utilizes microwaves to extract solid samples, which can be extended to several samples coming from air monitoring.
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Table 9. Microwave-assisted extraction procedures used in the monitoring of PAHs in air. Sample SRM1648 SRM1649a SRM1649a
Extractant 15 mL hexane:acetone (1:1)
Power-Time Microwave 400 W – 20 min commercial
Ref. [77]
400 W – 20 min commercial 15 mL toluene (10% water) or 15 mL CH2Cl2 XAD-2 resins 10 mL ethyleneglycol:NaCl 1M 120 W – 40 min domestica (7:3) 513 W – 30 min domestica bark / needles 90 mL hexane:CH2Cl2 (1:1) a modified domestic oven
[77] [87] [123]
Microwave oven
Diffuse microwaves
Extraction vessels Extraction vessel
Magnetron Focused microwaves
Wave guide
Focused microwave Rotor
Cavity microwave
Figure 6. Differences between cavity and focused microwave devices.
2.2.4. Pressurized Liquid Extraction (PLE) The technique pressurized liquid extraction (PLE), also named pressurized fluid extraction (PFE), was originally launched by Dionex Inc. in 1996 under the name accelerated solvent extraction (ASETM) [135,136]. PLE is a solid-liquid extraction process performed with organic solvents in stainless steel closed-vessels into which solvent is pumped and brought to relatively elevated temperatures, between 80 and 200ºC, and elevated pressures, between 10 and 20MPa [137]. The sample must be placed in the extraction vessel typically dispersed in a drying or inert sorbent such as sodium sulfate, Hydromatrix (Isco, Lincoln, NE, USA), or diatomaceous earth. During the heating cycle, solvent is pumped in and out of the cell to maintain the pressure and to perform the number of static cycles indicated by the user. In PLE, the organic solvents used as extractant are still below their critical conditions but with enhanced solvation power and with lower viscosities and surface tensions. The combination of high temperature and pressure results in better extraction efficiency, so minimizing solvent use and expediting the extraction process. PLE is like Soxhlet extraction, the only difference being the use of solvents that are raised to the near-supercritical region, where they show better extraction properties.
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Water can also be employed as the extraction solvent in PLE, and so the technique is called pressurized hot water extraction, subcritical water extraction (SWE), super-heated water extraction, or high temperature water extraction [138]. The pressure in PLE is not a significant variable to optimize, because its role is simply to maintain the solvent in its liquid state. The main variables to optimize are extraction t
