NMR and Macromolecules: Sequence, Dynamic, and Domain Structure (ACS Symposium Series)

NMR and Macromolecules ACS SYMPOSIUM SERIES 247 NMR and Macromolecules Sequence, Dynamic, and Domain Structure Jame...

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NMR and Macromolecules

ACS SYMPOSIUM SERIES

247

NMR and Macromolecules Sequence, Dynamic, and Domain Structure James C. Randall, E D I T O R Phillips Petroleum Company

Based on a symposium sponsored by the Division of Organic Coatings and Plastics Chemistry at the 185th Meeting of the American Chemical Society, Seattle, Washington, March 20-25, 1983

American Chemical Society, Washington, D.C. 1984

Library of Congress Cataloging in Publication Data N M R and macromolecules. ( A C S symposium series, I S S N 0097-6156; 247) "Based on a symposium sponsored by the Division of Organic Coatings and Plastics Chemistry at the 185th Meeting of the American Chemical Society, Seattle, Washington, March 20-25, 1983." Includes bibliographies and index. 1. Macromolecules—Analysis—Congresses. 2. Nuclear magnetic resonance spectroscopy— Congresses. I. Randall, James C . II. American Chemical Society. Division of Organic Coatings and Plastics Chemistry. III. Title: N.M.R. and macromolecules. IV. Series. QD380.N57 1984 I S B N 0-8412-0829-8

547.7'046

84-366

Copyright © 1984 American Chemical Society A l l Rights Reserved. The appearance of the code at the bottom of the first page of each chapter in this volume indicates the copyright owner's consent that reprographic copies of the chapter may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc., 21 Congress Street, Salem, M A 01970, for copying beyond that permitted by Sections 107 or 108 of the U . S . Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating a new collective work, for resale, or for information storage and retrieval systems. The copying fee for each chapter is indicated in the code at the bottom of the first page of the chapter. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by A C S of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. Registered names, trademarks, etc., used in this n without specific indication thereof, are not to be considered unprotected by law. PRINTED IN T H E UNITED STATES OF A M E R I C A

ACS Symposium Series M . Joan Comstock, Series Editor

Advisory Board Robert Baker

Geoffrey D. Parfitt

U.S. Geological Survey

Carnegie Mellon University

Martin L. Gorbaty

Theodore Provder

Exxon Research and Engineering Co.

Glidden Coatings and Resins

Herbert D. Kaesz

James C. Randall

University of California- Los Angeles

Phillips Petroleum Company

Rudolph J. Marcus

Charles N . Satterfield

Office of Naval Research

Massachusetts Institute of Technology

Marvin Margoshes

Dennis Schuetzle

Technicon Instruments Corporation

Ford Motor Company Research Laboratory

Donald E. Moreland U S D A , Agricultural Research Service

Davis L . Temple, Jr. Mead Johnson

W. H . Norton J. T. Baker Chemical Company

Charles S. Tuesday General Motors Research Laboratory

Robert Ory U S D A , Southern Regional Research Center

C. Grant Willson I B M Research Department

FOREWORD The A C S SYMPOSIUM SERIES was founded in 1974 to provide a medium for publishing symposia quickly in book form. The format of the Series parallels that of the continuing ADVANCES I N CHEMISTRY SERIES except that in order to save time the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. Papers are reviewed under the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are not accepted. Both reviews and

reports of research are acceptable since symposia may

embrace both types of presentation.

This book is dedicated to Frank A . Bovey, a pioneer researcher in NMR studies of polymers, on the occasion of his receipt of the American Chemical Society award in Applied Polymer Science, sponsored by the Phillips Petroleum Company, March 22, 1983.

FRANK A . BOVEY's research interests center a r o u n d the application o f N M R spectroscopy to the study o f structure, dynamics, and m o r p h o l o g y o f synthetic polymers and bio poly mers. H e has made major contributions to these fields, p r i m a r i l y through the development of techniques to determine microstructure in h o m o p o l y m e r and c o p o l y m e r chains and in the discovery and characterization of defect structures in v i n y l and related polymers. H e is the head of the P o l y m e r C h e m i s t r y Research Department at Bell L a b o r a t o ries, M u r r a y H i l l , N . J . D r . Bovey was b o r n in M i n n e a p o l i s in 1918. H e received a B . S . degree in chemistry from H a r v a r d in 1940, w o r k e d d u r i n g W o r l d W a r II for the N a t i o n a l Synthetic R u b b e r C o r p o r a t i o n , a 3 M subsidiary, and entered graduate school at the University of M i n n e s o t a as a R u b b e r Reserve F e l l o w in 1945. H i s thesis w o r k , carried out under the direction of I. M . Kolthoff, dealt with the mechanism o f free radical polymerization. D u r i n g this time he w o r k e d out the mechanism of oxygen i n h i b i t i o n and discovered oxygen sty re ne copolymers. After receiving his P h . D . in 1948, D r . Bovey returned to 3 M , now in the C e n t r a l Research Department, where he was made a research associate in 1955. It was at 3 M , d u r i n g the period from 1956 to 1962, that he conducted his pioneering investigations into the characterization o f polymers using N M R techniques. He reported some o f the first high-resolution p r o t o n N M R spectra of polymers in the late 1950s, at a time when it was still generally assumed that the spectra of macromolecules were too c o m p l e x to interpret. E x p l o i t i n g the rich detail in these spectra, he developed N M R techniques that are now used routinely in measuring the microstructure of polymer chains. These N M R methods made possible for the first time the determination and quantification o f the stereochemical configurations of noncrystallizable polymers. In 1962 D r . Bovey joined Bell Laboratories as a member of the technical staff, and was appointed to his present position in 1967. H e continued his detailed studies of polymer structure and c o n f o r m a t i o n at Bell Laboratories, and extended the scope of his w o r k to include investigations o f nuclei other than protons, branch analyses in polyethylene, and determination o f defect structures in v i n y l and related polymers. He continues to have a vigorous research p r o g r a m in the areas of polymer conformations in the solid state, polymer m o r p h o l o g y , and the mechanisms of polymer stabilization and degradation. D r . Bovey has published more than 125 papers o n his e x p l o r a t o r y research o n polymers and has contributed about 20 chapters to b o o k s in the field. He has written or has been coauthor of 10 books, including " M a c r o molecules, an Introduction to P o l y m e r Science" w i t h F. H . W i n s l o w in 1979 and " C h a i n Structure and C o n f o r m a t i o n of M a c r o m o l e c u l e s " in 1982. IX

He has served on an ad hoc panel of the National Academy of Sciences for the study of guayule rubber, and is currently serving on the nominating committee for a National Research Council panel on Polymer Science and Engineering. He has recently served on the Chemistry Panel of the National Research Council Fellowship Office; on the National Research Council Evaluation Panel for the Center for Materials Science of the National Bureau of Standards; on the Study Section for the National Institute of General Medicine (NIH) Shared Instrumentation Program; and both on the organizing committee and as a member of the U.S. delegation to the 1979 China U.S. Bilateral Polymer Symposium (Beijing, October 1979), spon sored by the National Academy of Sciences. He is a member of the award committee for the Baker Award of the National Academy of Sciences. In addition, he serves on the editorial boards of the Journal of Polymer Science—Polymer Chemistry and Polymer Physics Editions—and is an associate editor of Macromolecules. Many awards and honors attest to Dr. Bovey's contributions to poly mer science. He received the Union Carbide Award of the Minnesota Section of the A C S in 1962. In 1969 he received the Witco Award in Polymer Chemistry of the A C S and the Outstanding Achievement Award of the University of Minnesota. In 1974 he was awarded the Ford High Polymer Physics Prize of the American Physical Society. In 1978 he received the Nichols Medal Award of the New York Section of the A C S and delivered the Whitby Memorial Lectures at the University of Akron. His receipt in 1983 of the American Chemical Society Award in Applied Polymer Science, sponsored by Phillips Petroleum Company, was the occasion for the sympo sium in his honor on which this book is based. Dr. Bovey is a member of the American Chemical Society, the Ameri can Physical Society, the American Society of Biological Chemists, the New York Academy of Sciences, Sigma X i , and Phi Lambda Upsilon. He was elected to the National Academy of Sciences in 1975.

L. W. JELINSKY Bell Laboratories Murray Hill, N . J .

χ

PREFACE R . E C E N T E X P É R I M E N T A L I M P R O V E M E N T S in N M R spectroscopy have enabled the polymer chemist to determine macromolecular structure more definitively than was considered possible even a few short years ago. The sensitivity and range of N M R techniques are now such that investigations of polymer morphology and dynamic behavior are leading to information parallel to that from N M R solution studies of sequence distributions and configuration. For a number of years, progress in polymer synthesis and determination of physical properties far outpaced developments in establishing the microstructure of macromolecules. Advances in both liquid and solid N M R techniques have so changed this picture that it is now possible to obtain detailed information about the mobilities of specific chain units, domain structures, end groups, branches, run numbers, number average molecular weights, and minor structural aberrations in many synthetic and natural products at a level of 1 unit per 10,000 carbon atoms and below. This proliferation of macromolecular structural information is leading to new insights into the relationships between polymer molecular structure and the solid state structure and, ultimately, to an improved understanding of those molecular structural factors influencing polymer physical properties. The advent of a combination of quite different N M R techniques including cross polarization, magic angle spinning, improved probes, improved software in conjunction with more efficient computers, and sophisticated pulse sequencings not only has led to high-resolution N M R spectra of solids but also has stimulated the imaginations of those engaged in liquid polymer N M R analyses. A n appropriate sequence of pulses in solution studies of polymers leads to spectra where only specific carbon types are observed by selectively nulling those carbon nuclei that have different proton multiplicities. Subsequent spectral editings can lead to subspectra of single, specific carbon types. This technique of obtaining highly specific subspectra from the more complex overall N M R spectrum will have great utility in polymer characterization. This book presents to the polymer chemist illustrations of the most recent advances in N M R characterization of polymers while at the same time honoring Frank A . Bovey of Bell Laboratories. Dr. Bovey received the 1983 American Chemical Society Award in Applied Polymer Science, which was sponsored by the Phillips Petroleum Company and presented at the A C S National Meeting in Seattle in March 1983. Dr. Bovey is certainly the Xlll

pioneer of N M R studies of macromolecules influence

and has had a profound

on the progress and application of N M R in elucidating the

molecular structure of macromolecules. In honor of Dr. Bovey, a number of leading investigators in the fields of both liquid and solid state N M R have presented their work herein. Quite different experimental manifestations of the N M R phenomenon, which lead to either polymer sequence, dynamic, or domain structures, are reported. We hope that " N M R and Macromolecules" will serve as a reference book and guide to the polymer chemist who is interested

in polymer

characterization. We hope it will serve as well as a tribute to Frank A . Bovey by revealing the extensive and detailed molecular structural information available through a variety of Ν M R techniques in characterization studies of polymers. J A M E S C. R A N D A L L

Phillips Petroleum Company Bartlesville, Oklahoma

xiv

Table of Contents

FOREWORD ............................................................................ v Dedication ............................................................................... vii PREFACE ................................................................................ xiii OVERVIEW .............................................................................. 1 1 NMR and Macromolecules ............................................. 3 Stereochemical Configuration ..................................... 4 19F NMR Observations of Regioregularity ................. 7 Carbon—13 Study of Crystal Morphology................... 10 Chain Dynamics; Deuterium Spectoscopy .................. 12 Biopolymers ................................................................ 12 Literature Cited............................................................ 16 NMR OF SOLID POLYMERS .................................................. 19 2 An Introduction to NMR Spectroscopy of Solid Samples ............................................................................. 21 High Resolution NMR Spectroscopy of Solids ............ 22 Contributions to NMR Line Widths in the Solid State .. 22 Dipole-Dipole Interactions .................................... 22 Chemical Shift Anisotropy .................................... 22 Nuclear Quadrupole Effects ................................. 23 Line Narrowing Methods ....................................... 24 Dipolar Decoupling ............................................... 24 The Magic Angle ................................................... 24 Line Narrowing in Spin Space - WAHUHA ........... 28 Cross Polarization ................................................ 31 NMR Measurements of Motion in Polymers ......... 34 Line-Shape Analysis ............................................. 34 Relaxation and Motion in Solids Spin-Lattice Relaxation - T1 ..................................................... 35 Relaxation in the Rotating Frame-T1. ................... 37 Measurement of Relaxation .................................. 39 Conclusions................................................................. 39 Literature Cited............................................................ 41 3 Molecular Motion in Glassy Polystyrenes ....................... 43 Rotating-Frame Carbon Spin-Lattice Relaxation ........ 43 Dipolar Rotational Spin-Echo Experiment ................... 44 Typical Spectra in the Chemical Shift and Dipolar Dimensions ................................................................. 44 Comparison Between Experimental and Calculated Dipolar Sideband Patterns .......................................... 47 Small-Amplitude Low-Frequency Ring Motion ............ 50

Correlation Between and n2/n1 ............. 52 Conclusions for Ring Rotations in Polystyrenes ......... 52 Literature Cited............................................................ 54 4 Solid State 2H NMR Studies of Molecular Motion Poly(butylene terephthalate) and Poly(butylene terephthalate)-Containing Segmented Copolymers ......... 55 EXPERIMENTAL ........................................................ 57 RESULTS AND DISCUSSION.................................... 59 SUMMARY .................................................................. 63 ACKNOWLEDGEMENT.............................................. 65 LITERATURE CITED .................................................. 65 5 Spin Relaxation and Local Motion in a Dissolved Aromatic Polyformal .......................................................... 67 Experimental ............................................................... 68 Results ........................................................................ 68 Interpretation ............................................................... 70 Discussion ................................................................... 79 Acknowledgments ....................................................... 81 Literature Cited............................................................ 81 6 Characterization of Molecular Motion in Solid Polymers by Variable Temperature Magic Angle Spinning 13C NMR ............................................................ 83 Experimental ............................................................... 84 Results and Discussion ............................................... 84 Literature Cited............................................................ 94 NEW TECHNIQUES IN NMR OF LIQUID POLYMERS .......... 95 7 New NMR Experiments in Liquids .................................. 97 Spectral Editing Using J-Modulated Spin-Echos......... 98 Polarization Transfer Methods ....................................101 Two-Dimensional NMR ...............................................106 Literature Cited............................................................117 8 Application of the INEPT Method to 13C NMR Spectral Assignments in Low-Density Polyethylene and Ethylene-Propylene Copolymer .........................................119 Experimental ...............................................................120 Results and Discussion ...............................................120 Literature Cited............................................................126 NMR OF LIQUID POLYMERS.................................................129 9 13C NMR in Polymer Quantitative Analyses ..................131 Experimental Variables in Quantitative NMR Studies of Polymers .................................................................132 Conclusions.................................................................150 Literature Cited............................................................150 10 The Synthesis of Novel Regioregular Polyvinyl Fluorides and Their Characterization by High-Resolution

NMR ..................................................................................153 Experimental ...............................................................154 Results and Discussion ...............................................155 Literature Cited............................................................165 11 The Composition and Sequence Distribution of Dichlorocarbene-Modified Polybutadiene by 13C NMR ....167 Results ........................................................................168 Discussion ...................................................................171 Experimental ...............................................................176 Acknowledgments .......................................................179 Literature Cited............................................................179 12 NMR Spectra of Styrene Oligomers and Polymers ......181 Preparation and Separation of Styrene Oligomers .....182 NMR Spectra of the Dimer and Trimer........................183 Identification of the Tetramer and Pentamer ...............187 13C NMR Spectra of the Tetramer and Pentamer ......188 13C NMR Signal Assignment of Polystyrene ..............190 Literature Cited............................................................196 13 75-MHz 13C NMR Studies on Polystyrene and Epimerized Isotactic Polystyrenes .....................................197 Experimental ...............................................................200 Results and Discussion ...............................................202 Conclusions.................................................................218 Acknowledgments .......................................................221 Literature Cited............................................................221 14 Stereospecific Polymerization of α -Olefins: End Groups and Reaction Mechanism .....................................223 Mechanism of Addition to the Double Bond ................224 Regiospecificity ...........................................................224 Mechanism of the Steric Control .................................225 Stereoselective Polymerization of Ghiral α -Qlefins.....228 Conclusion ..................................................................230 Literature Cited............................................................230 15 Structural Characterization of Naturally Occurring cis-Polyisoprenes ..............................................................233 13C NMR Analysis of Model Compounds ...................234 Structural Characterization of Polyprenols...................236 Structure of Naturally Occurring Cis-Polyisoprenes ....238 Literature Cited............................................................244 16 A 13C NMR Study of Radiation-Induced Structural Changes in Polyethylene ...................................................245 Experimental ...............................................................247 Results ........................................................................253 Discussion ...................................................................264 Conclusions.................................................................266

Acknowledgments .......................................................267 Literature Cited............................................................267 INDEXES .................................................................................269 Author Index ......................................................................271 Subject Index .....................................................................271

OVERVIEW

1 NMR and Macromolecules F. A. BOVEY Bell Laboratories, Murray Hill, NJ 07974

1

13

19

High resolution nmr spectroscopy ( H, C, and F) of polymers in solution has been employed during the last twenty-five years for the elucidation of the microstructure of their chains. Examples of such studies carried out at Bell Laboratories are reviewed, including the measurement of stereochemical configuration and regioregularity (head-to-tail vs. head-to-head: tail-to-tail isomerism). The use of C nmr of epoxidized trans-1,4-polybutadiene crystals to establish the morphology of single crystals is described. Nmr is also a powerful means for the observation of chain dynamics; the use of deuterium quadrupolar echo spectroscopy for this purpose is illustrated. Finally, a large and exciting field of polymer nmr studies is that of the structure and dynamics of biomolecules, exemplified by proton nmr investigations of nucleic acid structure and function. 13

The first studies of polymers ( 0 were published only about a year after the first reports of the nmr phenomenon in bulk matter by Bloch and Purcell in 1946. The early work dealt with nuclear relaxation and chain dynamics in the solid state (2). In the mid-1950's, when the study of small molecules had reached a fairly advanced state, it was still generally assumed that very large molecules could not give useful spectra even in solution because of their supposedly slow motions, as evidenced by the very high viscosities of their solutions. There was also a feeling that their spectra would be too complex to interpret. In the late 1950's, scattered reports of high resolution spectra of synthetic and biological polymers began to appear (3-6). This trickle became a flood as investigators were able to show that N M R is uniquely powerful in the determination of polymer microstructure, including stereochemical configuration (7,8), geometrical isomerism (9,10), regioregularity (11,12), and monomer sequences in copolymers (6,13,14).

0097-6156/84/0247-0003S06.00/0 © 1984 American Chemical Society

4

NMR AND MACROMOLECULES

Stereochemical Configuration In Figure 1 is shown the 40 M H z *H spectrum of two samples of poly (methyl methacrylate), as reported by George Tiers and myself in 1960 (7). Spectrum a is that of a polymer prepared with a free radical initiator; spectrum b is that of a polymer prepared with an anionic initiator, w-butyllithium in toluene. As is now well known, the marked differences in the spectra arise from differences in stereochemical configuration. It is clear from fundamental considerations and from the spectra of small model molecules (e.g. the 2,4-disubstituted pentanes) that the methylene protons of racemic (r) monomer dyads, i.e. those characterizing a syndiotactic chain (Figure 2, upper left), are equivalent by reason of a two-fold symmetry axis and therefore have the same chemical shift. In the absence of vicinal coupling of main chain protons, as in poly (methyl methacrylate), they appear as a singlet. On the other hand, the methylene protons of the meso (m) monomer dyads composing an isotactic chain are non-equivalent and have differing chemical shifts; they are expected to appear as an A B quartet. It is evident in Figure 1 that the methylene proton spectrum of the anionically initiated polymer exhibits such a quartet ( J — —14.9 Hz) centered at 8.14 on the now outmoded τ scale (1.86 ppm from T M S ) and that therefore this polymer is predominantly isotactic. In the spectrum of the free radical polymer (a), the methylene proton spectrum is a broad singlet, and so this polymer must be predominantly syndiotactic. (Actually neither polymer is entirely stereoregular and at higher resolution both spectra show additional features from this cause.) Thus, proton nmr is an absolute method and no recourse to x-ray is necessary even if this were possible. g e m

The α-methyl resonances centered at ca. 9 τ can be interpreted to give quantitative estimates of isotactic (mm) and syndiotactic (rr) triad sequences of monomer units and also of the mixed unit mr, termed heterotactic, which must occur in chains which are not perfectly stereoregular. In Figure 1, we see three amethyl proton resonances having the same chemical shifts but very different intensities in each spectrum. They furnish a measure of the triad probabilities. In the years following these relatively primitive observations, a very large number of vinyl and related polymer systems have been studied by nmr in many laboratories. If the resolving power of the spectrometer is sufficient—that is, if the magnetic field strength is high enough—configurational sequences longer than triad may be observed. With respect to ^-methylene groups one may expect to resolve tetrad (Figure 2) and hexad sequences, appearing as a fine structure on the m and r dyad resonances. One may expect α-groups to be resolved into ten different pentad sequences or possibly as many as 36 heptad sequences. We have the following numbers N(n) of sequences of length n: η Ν (η)

n

2

2 2

m

3 3

1

4 6

5 10

6 20

7 36

8... 72

or

in general Ν (η) - 2 ~ + 2 ~~ where m = n/2 if η is even and m - (n-l)/2 if η is odd. Although these longer sequences can be resolved in some proton spectra at

1.

BOVEY

NMR and

5

Macromolecules

a. r

U

•J b.

ι

J

,

LmJ —J 640 T

Figure L

40

r

M H z proton spectra

• .,

X4 i1l Ô.I4/ I \ We m 9.05

of poly (methyl

10.00

methacrylate)

in

chloroform; (a) polymer prepared using free radical initiator; (b) polymer prepared using w-butyllithium initiator. F. Α.; Tiers, G . V. D. J. Polymer Sci., 1960, 44, 173).

(Bovey,

6

NMR AND MACROMOLECULES

CONFIGURATIONAL SEQUENCES DYADS DESIGNATION

PENTADS

PROJECTION

DESIGNATION

J ? j

mmmm (ISOTACTIC)

MESO m RACEMIC Γ

•m-

TRIADS ISOTACTIC, mm HETEROTACTIC, mr SYNDIOTACTIC, ΓΓ

44 +4 44 44 44 44

PROJECTION

mmmr rmmr mmrm mmrr rmrm (HETEROTACTIC) rmrr mrrm rrrm

rrrr (SYNDIOTACTIC) I

•m

44 44 Figure 2.

Configurational sequences in vinyl polymer chains shown in planar zigzag projection.

1.

BOVEY

NMR

and

1

Macromolecules

superconducting frequencies, the use of carbon-13 spectroscopy has proved much more effective, primarily because of the greater chemical shift range of C nuclei—over 250 ppm for structures of interest in polymers as compared to less than 10 ppm for protons. Thanks to the development of Fourier transform instruments with spectrum accumulation, carbon spectroscopy, despite the low natural abundance of C (1.1%), has become a method of fairly high sensitivity, able to establish the presence of structural features at a level of less than one carbon per 10000. In Figure 3 are shown the 90 M H z C spectra of the C H , 0 - C H and a - C H carbons of an atactic polypropylene (15). The * H — C J-coupling multiplicity has been removed by irradiation of the protons, as is customary in C nmr. One may clearly resolve 20 heptad configurational sequences out of a possible 36. Below each spectrum is a line spectrum in which is represented the predicted chemical shift based on the "7-effect" conformational model (16). In this theoretical model, it is predicated that two carbon atoms separated by three intervening bonds—that is, in a 7 position with respect to each other—will shift each others' resonances upfield by about 5 ppm when they are in a gauche conformation, as compared to their resonance positions when they are trans. Thus, stereochemical configuration influences C chemical shifts through its effect on the gauche content of the intervening bonds, which may be readily estimated by calculations based on the rotational isomeric state model of the polymer chain. 1 3

1 3

1 3

3

2

13

1 3

1 3

1 9

F N M R Observations of Regioregularity

1 9

F chemical shifts are also very sensitive to chain microstructure, sometimes even more so than those of C . In Figure 4 are shown 84.66 M H z F spectra of poly (vinyl fluoride) (17). Spectrum (a) is that of a commercial polymer; the four upfield groups of resonances at 190-200 ppm (from CC1 F), and some small resonances in the principal spectrum as well, correspond to inverted or head-tohead:tail-to-tail ("syndioregic" (18)) monomer units: 1 3

1 9

3

—CH —CHF—CH —CHF—CHF—CH —CH —CHF—CH —CHF— 2

2

2

2

2

Quantitative measurement shows about 11% of the monomer units to be inverted. The principal spectrum shows splitting into mm, mr, and rr triad resonances with some pentad fine structure. The polymer is nearly atactic. Assignment of inversion "defect" resonances is made easier by reference to spectrum (b), which is that of poly (vinyl fluoride) prepared by the following route (17):

free radical CH,=CFC1 ^

Bu SnH . ?

Initiator

-4-CH -CFCl% 2

, , (-CH.-CHF-^

8

NMR AND MACROMOLECULES

mmmm m m mmmmmr rmmmmr mmmmrr mmmm r m rmmmrr mrmmmr r r mm r r m r mm r r

Figure 3.

10, 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

m r mmrm mmmrrm mmmrrr rmmr r m r r rmmr mmmr m r mmmrmm r m rmmr mmrmmr r rm r r m r r rm r r m rm r r m r rrm r m

23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36.

r r m r mr mm r m r r m r m rmr mm r m r m m r r r rm r r r r rm r r r r r r r m r r rm r r r rmr m m r r rm r r r r mm r m r rmr r m r r mm mm r r mm

90.5 M H z C spectra of atactic polypropylene observed on a 20% (w/v) solution in heptane at 6 7 ° ; (a) C H ; (b) 0 - C H ; (c) α - C H . Line spectra appearing below each experimental spectrum correspond to theoretically calculated resonance positions for (a) heptad, (b) hexad, and (c) pentad configurational sequences. (Schilling, F. C, private communication). 3

2

1.

BOVEY

NMR and

Macromolecules

low-fieid

9

F NMR at 84.66 MHz

S T E R E O S E Q U E N C E SPLITTING

A

COMMERCIAL

REDUCED P V C F

180

Figure 4.

84.66

MHz

fluoride) ;

(b)

Φ 1 9

1

F spectrum poly (vinyl

9

PPM

0

of

(a)

fluoride)

2

0

0

commercial

poly (vinyl

prepared by reductive

dechlorination of poly (vinyl chlorofluoride) ; both observed at 130° in 8% (w/v) solution in Ν ,N—dimethy If or mamide (Cais, R. E.; Kometani, J., private communication).

10

NMR AND MACROMOLECULES

The steric requirements of the chlorine atom permit only a negligible proportion of syndioregic units in poly (vinyl chlorofluoride) (PVCF), and it is now observed that when the chlorine is reduced with tri-A2-butyltin hydride the resulting poly (vinyl fluoride) exhibits no upfield resonances, being now entirely regioregular. The random configuration evident in spectrum (b), however, does not reflect the stereochemistry of the precursor P V C F but is rather the result of racemization at the α-carbon during the reduction, which is a free radical reaction.

Carbon— 13 Study of Crystal Morphology A quite different use of high resolution nmr in polymer science is the application of C spectroscopy to the study of the morphology of chain-folded polymer single crystals grown from dilute solution (19). The problem posed here is the direct measurement of the lengths of the polymer folds at the crystal surface and of the crystal stems within the body of the crystal. The approach is a chemical one, the problem being to find a reagent which will react completely with the exposed folds but will not attack the crystal stems. No such reagent is known for polyethylene, the usual testbed for new morphological approaches, but for crystalline trans-\,4polybutadiene (m.p. 1 4 8 ° ) the formation of oxirane rings by reaction with mchloroperbenzoic acid appears to satisfy the requirements of selectivity and convenience: 1 3

The crystals, prepared from heptane solution, were suspended in toluene and reacted at 6° until the rate levelled off, which occurred at ca. 12-16% of completion. The reacted crystals were then dissolved in CDC1 and the C spectra observed at 50 MHz. The spectra were interpreted with the aid of the spectrum of trans-1,4-polybutadiene reacted in homogeneous solution. The statistics of the reaction proved strikingly different in the two cases. The chains of the reacted crystals were in effect block copolymers of quite regular structure with runs of oxirane units alternating with runs of unreacted monomer units, as shown in Figure 5, whereas the reaction in homogeneous solution was random. By comparison of the intensity of the "junction" methylene resonance, D in Figure 5, with that of the internal methylene Β (split because these runs are diastereoisomeric sequences of left- and right-handed oxirane rings), it is found that the fold length is only 2.5-3.0 butadiene units. This is the minimum number for a 180° turn of the chain, and appears to require adjacent re-entry. The deduced crystal structure is shown in Figure 6. The stem length can also be obtained by analagous nmr measurements and turns out to be 15 butadiene units for this particular preparation. (The inclination of the stems cannot be deduced from nmr but comes from x-ray). 1 3

3

1.

BOVEY

NMR and

Macromolecules

D

O /

B

11

B

O

\

D /

\

· · · - C H - C H = C H - C H - C H - C H - C H - C H - » « » » - C H - C H ~ C H - C H ~ C H ~CH = C H - C H - · · · 2

2

2

2

-CRYSTAL-

2

FOLD -

2

2

2

CRYSTAL

29.0

28.0

ppm vs TMS

Figure 5.

1 3

50.3 M H z C spectrum of methylene groups of trans-\,4polybutadiene epoxidized to 16.2% in the crystalline state; observed in CDC1 solution at 4 0 ° . Peak assignments are indicated in reference to the schematic block sequence above, with methylene D representing the junction between sequences. (Schilling, F. C ; Bovey, F. Α.; Tseng, S.; Woodward, A . E . Macromolecules, 1983, 16, 808). 3

'11 = 2.4 MONOMER UNITS

Figure 6.

Schematic diagram of a trans-1,4-polybutadiene crystal. The fold length is U and is approximately 3 butadiene units. The stem length L corresponds to 15 monomer units. The crystal s

thickness

L

c

is obtained

from this value and the x-ray

determined inclination angle of 114° (Schilling, F. C ; Bovey, F. Α.; Tseng, S.; Woodward, A . E . Macromolecules, 1983, 16, 808).

12

NMR AND MACROMOLECULES

Chain Dynamics; Deuterium Spectoscopy It is well known that nmr is a powerful means for the study of the dynamics of polymer chains both in solution and in the solid state. The relaxation of C nuclei has been extensively employed for this purpose in this and other laboratories. I illustrate here a different and particularly intriguing approach which as yet has seen only very limited application to synthetic polymers. This is deuterium quadrupolar echo spectroscopy, as employed in our laboratory by Dr. Lynn Jelinski and her collaborators (20). The presence of the nuclear electric quadrupole lifts the degeneracy of the two deuterium Zeeman transitions, and in the solid state produces a very broad (ca 200 kHz) powder pattern of transitions which can be interpreted to yield very specific motional information for those carbons labelled with deuterium. In Figure 7 are shown spectra of poly (butylène terephthalate) deuterated on the central carbons of the aliphatic chains: 1 3

X

By comparison of observed and theoretically calculated spectra it can be shown that these carbons are involved in gauche-trans conformational jumps of the C - D bond through a dihedral angle of 1 0 3 ° , and from the correlation times as a function of temperature an activation energy of 5.8 kcal/moi is found. Several seemingly plausible motional models are excluded by these results, but the data agree with models proposed by Helfand (21,22) for motion about three bonds.

Biopolymers Finally, a very large and exciting field of polymer nmr studies is that of the structure and dynamics of biomolecules. Here, proton spectroscopy (at superconducting frequencies) remains dominant. I illustrate this by an example of Dinshaw PateEs studies of nucleic acids and oligonucleotides, which shows the use of H—*H nuclear Overhauser enhancement (NOE) to explore the binding site of the antibiotic netropsin 1

1.

BOVEY

NMR

and

Macromolecules

13

to the dodecanucleotide d ( C G C G A A T T C G C G ) , which forms the double helical structures

C G C G A A T T C G C G 1 2 3 4 5 6 6 5 4 3 2 1 G C G C T T A A G C G C

(Here, A stands for adenosine, T for thymine, G for guanosine, and C for cytidine) Because of its sixth-power dependence on interproton distances, the N O E is a very sensitive means of exploring molecular contacts through interproton distances. For large molecules such as these, the enhancement is negative. In Figure 8 are shown difference spectra for the aromatic protons of the netropsin complex with this nucleotide,

14

NMR AND MACROMOLECULES

OBSERVED

CALCULATED

RATE CONSTANT 1

s"

J_J_LjLJ_X jJ.J_J_l_L_Ll 1 t i .)

-100

0

kHz Figure 7.

100

LXi^.^I.±„X^.^xJ_L.

-100

0

100

kHZ

Solid state deuterium N M R spectra of labelled poly (butylène terephthalate). Calculated spectra are for a two-site hopping model between two orientations of the C - D bond differing by 103° (Jelinski, L . W.; Dumais, J , J.; Engel, A . K . Macromolecules, 1983, J_6, 492).

1.

BOVEY

Figure 8.

NMR

and

15

Macromolecules

498 M H z proton N M R spectrum of the 1:1 netropsind ( C G C G A A T T C G C G ) complex in D 0 (pH 6.9, 3 0 ° ) . A negative N O E at the adenine H-2 proton of the dA-dT base pair 6 is observed on irradiation of either the 6.55 ppm or the 6.67 netropsin pyrrole H-3 proton. (Patel, D. J.; Pardi, Α.; Itakura, K . Science, 1982, 216, 581). 2

16

NMR AND MACROMOLECULES

exhibiting large negative peaks for the adenosine H - 2 protons of the central position 6 on each strand of the duplex upon saturation of the H-3 protons of each of the two pyrrole rings (they differ) in the bound netropsin (23). These and additional N O E measurements establish that the concave face of the bowshaped netropsin molecule binds in the minor groove of the A A T T tetranucleotide core of the complex.

Literature Cited 1.

Alpert, N. L. Phys. Rev., 1947, 72, 637.

2.

Slichter, W. P. Adv. Poly. Sci., 1958, 1, 35.

3.

Saunders, M.; Wishnia, Α.; Kirkwood, J. G. J. Am. Chem. Soc., 1957, 79, 3289.

4.

Saunders, M.; Wishnia, A. Ann, N.Y. Acad. Sci., 1958, 70 870.

5.

Odajima, A. J. Phys. Soc. Jap., 1959, 14, 777.

6.

Bovey, F. Α.; Tiers, G. V. D.; Filipovich, G. J. Polym. Sci., 1959, 38, 73.

7.

Bovey, F. Α.; Tiers, G. V. D. J. Polymer Sci., 1960, 44, 173.

8.

Nishioka, Α.; Watanabe, H.; Abe, K.; Sono, Y., 1960, 48, 241.

9.

Chen, Η. Y. Anal. Chem., 1962, 34, 1134, 1793.

10.

Golub, Μ. Α.; Fuqua, S. Α.; Bhacca, N. S. J. Am. Chem. Soc., 1962, 84, 4981.

11.

Wilson III, C. W. J. Polym. Sci., 1963, Part A1, 1305.

12.

Wilson III, C. W.; Santee, Jr., E. R. J. Polym. Sci., 1965, Part C8, 97.

13.

Bovey, F. A. J. Polymer Sci., 1962, 62, 197.

1. B O V E Y

17

NMR and Macromolecules

14.

Harwood, H . J.; Ritchey, W. M. J. Polym. Sci., 1965, Part B3, 419.

15.

Schilling, F. C., private communication.

16.

Tonelli, A . E.; Schilling, F. C. Acc. Chem. Res., 1981, 14, 233.

17.

Cais, R. E.; Kometani, J., Preprints of 28th I U P A C Macromolecular Symposium, Amherst, Mass. July, 12-16, 1982.

18.

Cais, R. E.; Sloane, N. J. A . Polymer, 1983, 24, 179.

19.

Schilling, F. C.; Bovey, F. Α.; Tseng, S.; Woodward, A . E . Macromolecules, 1982, in press.

20.

Jelinski, L . W.; Dumais, J. J.; Engel, A . K. Macromolecules, 1983, 16, 492.

21.

Helfand, E.; Wasserman, Z . R.; Weber, T. A . J. Chem. Phys., 1980, 13, 526.

22.

Helfand, E.; Wasserman, Z . J. Chem. Phys., 1981, 75, 4441.

23.

Patel, D. J.; Pardi, Α.; Itakura, K . Science, 1982, 216, 581.

R E C E I V E D November 10, 1983

R.;

Weber,

T.

Α.;

Runnels,

J.

H.

NMR OF SOLID POLYMERS

2 An Introduction to NMR Spectroscopy of Solid Samples DANIEL J. O'DONNELL Phillips Petroleum Company, Bartlesville, OK 74004

Complex solid sample NMR techniques used in other chapters of this text are discussed. These techniques are presented using conceptual arguments, rather than mathematical equations, so that those unfamiliar with NMR spectroscopy might get a quick grasp of the nature of the experiment. Figures and charts are given which depict the interactions present in the solid state and which show how these interactions are manipulated in the NMR experiment to yield the desired information. The p a p e r s p r e s e n t e d i n t h i s volume r e p r e s e n t a f r a c t i o n o f t h e a p p l i c a t i o n s d e v e l o p e d f r o m new t e c h n i q u e s i n NMR s p e c t r o s c o p y over t h e past decade. T h i s f l o o d o f new methods h a s g e n e r a t e d new terms w h i c h , t h o u g h m a t h e m a t i c a l l y w e l l d e f i n e d , a r e d i f f i cult tovisualize physically. As a r e s u l t , t h e advantages o f the newer methods ( a n d t h e i n f o r m a t i o n t o be g l e a n e d f r o m them) are o f t e n l o s t t o the s c i e n t i s t n o t i n t i m a t e l y f a m i l i a r w i t h NMR s p e c t r o s c o p y . The o b j e c t i v e o f t h i s c h a p t e r i s t o p r o v i d e a g u i d e f o r t h e NMR layman t o t h e methods u s e d i n t h e f o l l o w i n g c h a p t e r s . Mathem a t i c a l d e r i v a t i o n s h a v e been a v o i d e d i n f a v o r o f d e s c r i p t i o n s and d i a g r a m s t o p r o v i d e c o n c e p t u a l d e f i n i t i o n s . The d i s c u s s i o n will concentrate on t h e t e c h n i q u e s used i n the following c h a p t e r s , a n d t h e r e f o r e w i l l n o t a t t e m p t t o be c o m p r e h e n s i v e . R e a d e r s a r e r e f e r r e d t o s e v e r a l t e x t s and a r t i c l e s f o r d e t a i l e d treatments o f the subjects ( 1 - 5 ) .

0097 6156/84/0247 0021 $06.25/0 © 1984 American Chemical Society

22

NMR

High Resolution

AND MACROMOLECULES

NMR S p e c t r o s c o p y o f S o l i d s

Haeberlin (5) expressed the p r e v a i l i n g attitude o f the spectroscopist i n 1976 with the statement, "Narrow i s beautiful." Although t h i s a t t i t u d e s t i l l p r e v a i l s , i t has long been r e c o g n i z e d that a wealth of information i s contained i n NMR l i n e shapes b r o a d e n e d by s p e c i f i c i n t e r a c t i o n s i n s o l i d s . E x t r a c t i n g t h a t i n f o r m a t i o n has been d i f f i c u l t , s i n c e a v a r i e t y o f mechanisms c o n t r i b u t e t o t h e l i n e s h a p e , and e a c h must be selectively removed from the others to decipher the information. The methods u s e d t o d e c o n v o l u t e t h e c o m p l e x l i n e s h a p e s i n v o l v e m a n i p u l a t i o n s t o remove some b r o a d e n i n g w h i l e r e t a i n i n g other information. Several o f these techniques a r e d i s c u s s e d below. Contributions

t o NMR L i n e W i d t h s i n t h e S o l i d

Dipole-Dipole

Interactions

State

E a c h N M R - a c t i v e s p i n % n u c l e u s i n an e x t e r n a l m a g n e t i c f i e l d , H , a c t s as a m a g n e t i c d i p o l e w h i c h a l i g n s w i t h H i n specific states. F o r p r o t o n s , c a r b o n s and o t h e r s p i n h n u c l e i , two states e x i s t : p a r a l l e l or a n t i p a r a l l e l to H . Since each n u c l e u s , as a d i p o l e , h a s a l o c a l f i e l d a s s o c i a t e d w i t h i t , t h e a c t u a l f i e l d e a c h n u c l e u s e x p e r i e n c e s i s a sum o f t h e e x t e r n a l f i e l d , H , and c o n t r i b u t i o n s f r o m a l l t h e s u r r o u n d i n g d i p o l e s . T h i s d i p o l e - d i p o l e i n t e r a c t i o n i s h i g h l y d e p e n d e n t upon t h e a n g l e between t h e d i r e c t i o n o f H and t h e i n t e r n u c l e a r v e c t o r b e t w e e n a d i p o l e p a i r , and i s a l s o h i g h l y d e p e n d e n t upon t h e d i s t a n c e between t h e d i p o l e s . I n an abundant n u c l e a r species s u c h as p r o t o n s , e a c h p r o t o n h a s many n e i g h b o r s w h i c h i n t e r a c t w i t h i t a n d , b e c a u s e o f t h e dépendance o f a n g l e and d i s t a n c e , a wide v a r i e t y o f d i p o l e - d i p o l e i n t e r a c t i o n s are p o s s i b l e . In l i q u i d s , t h e s e s p e c i f i c i n t e r a c t i o n s a r e a v e r a g e d by m o t i o n s which c o n s t a n t l y change a n g l e s and d i s t a n c e s , resulting i n narrow l i n e s . I n s o l i d s , t h e a n g l e s and d i s t a n c e s a r e f i x e d , r e s u l t i n g i n an enormous number o f d i f f e r e n t l o c a l m a g n e t i c e n v i r o n m e n t s , e a c h o f w h i c h i s o b s e r v e d i n t h e NMR s p e c t r u m . The d i f f e r e n c e s i n l o c a l f i e l d s i n d u c e d by d i p o l e - d i p o l e i n t e r a c t i o n s r e s u l t i n a r a n g e o f s i g n a l s i n t h e NMR spectrum, c o v e r i n g 40 K H i n some c a s e s . As was m e n t i o n e d p r e v i o u s l y , i n f o r m a t i o n a b o u t i n t e r n u c l e a r d i s t a n c e s c a n be o b t a i n e d from the d i p o l e - d i p o l e i n t e r a c t i o n s between two i s o l a t e d d i p o l e s . The p r o b l e m i s i n d e c o u p l i n g a l l t h e o t h e r d i p o l e - d i p o l e i n t e r a c t i o n s , so t h a t one d i p o l e - d i p o l e c o u p l i n g c a n be o b s e r v e d . 0

Q

0

0

Q

Z

Chemical S h i f t

Anisotropy

The s e c o n d m a j o r c o n t r i b u t o r t o NMR l i n e w i d t h s i n s p e c t r a o f s o l i d m a t e r i a l s i s c h e m i c a l s h i f t a n i s o t r o p y , CSA. CSA r e s u l t s

2.

CTDONNELL

23

NMR Spectroscopy of Solid Samples

f r o m t h e i n t e r a c t i o n between m a g n e t i c f i e l d s f r o m e l e c t r o n s i n m o t i o n a r o u n d a n u c l e u s and t h e n u c l e a r s p i n . The d i s t r i b u t i o n of e l e c t r o n s around the nucleus w i l l depend upon c h e m i c a l b o n d i n g , and a s a r e s u l t w i l l n o t be u n i f o r m i n a l l d i r e c t i o n s . As an e x a m p l e , c o n s i d e r a c a r b o n y l bond. Electronically, this bond i s h i g h l y d i r e c t i o n a l , and how i t i n t e r a c t s w i t h t h e s t a t i c f i e l d , H , w i l l be s t r o n g l y d e p e n d e n t upon t h e a n g l e b e t w e e n t h e bond and t h e d i r e c t i o n o f H . R a p i d m o t i o n s i n t h e liquid state result i n t h e o b s e r v a t i o n o f a n e t average interaction (i.e., narrow l i n e s ) . I n the s o l i d s t a t e a l l possible orientations are "frozen" i n place, resulting i n a wide v a r i e t y o f l o c a l i n t e r a c t i o n s which a r e observed i n t h e spectrum ( i . e . , wide l i n e s ) . Although information concerning b o n d i n g and m o l e c u l a r symmetry a r e c o n t a i n e d i n t h e CSA l i n e s h a p e , t h e o v e r l a p o f CSA l i n e s h a p e s i n a c o m p l e x s y s t e m makes interpretation difficult. The l i n e w i d t h s i n d u c e d b y CSA i n t e r a c t i o n s a r e d i r e c t l y p r o p o r t i o n a l t o H , so t h a t h i g h e r f i e l d s do n o t improve r e s o l u t i o n . 0

Q

Q

N u c l e a r Quadrupole E f f e c t s The discussion so f a r h a s c e n t e r e d on t h e n o n - s y m m e t r i c distribution of local fields surrounding a given nucleus r e s u l t i n g f r o m o t h e r n u c l e i ( d i p o l e - d i p o l e i n t e r a c t i o n s ) and from e l e c t r o n s ( c h e m i c a l s h i f t a n i s o t r o p y , C S A ) . I n a d d i t i o n , t h e n u c l e u s i t s e l f may n o t be s y m m e t r i c , r e s u l t i n g i n a n o n symmetric nuclear charge distribution, i . e . , an electric q u a d r u p o l e moment. F o r n u c l e i w i t h a s p i n quantum number, I , o f h ( e . g . , * H a n d ^ C ) t h e q u a d r u p o l e moment i s z e r o , and no consideration need be g i v e n t o quadrupolar interactions. However, i n t h e c a s e o f d e u t e r i u m , I e q u a l s 1, and t h e e f f e c t s o f t h e q u a d r u p o l e must be t a k e n i n t o a c c o u n t . S i n c e t h e quadrupole i s e l e c t r i c i n nature, i tinteracts d i r e c t l y only with electric field g r a d i e n t s and n o t w i t h t h e magnetic field. However, t h e m a g n e t i c e n e r g y l e v e l s o f t h e n u c l e u s a r e c o u p l e d to t h e q u a d r u p o l a r e n e r g y l e v e l s , r e s u l t i n g i n s p l i t t i n g s i n t h e NMR s p e c t r a . These s p l i t t i n g s c a n be v e r y l a r g e , r e s u l t i n g i n s p e c t r a ~200 K H w i d e f o r D , and 5 M H f o r ^ N . A t t h i s p o i n t one m i g h t a s k why q u a d r u p o l a r i n t e r a c t i o n s s h o u l d be c o n s i d e r e d , s i n c e they a r e d i f f i c u l t t o o b s e r v e , and i n t h e case o f deuterium, r e q u i r e i s o t o p i c l a b e l l i n g . The answer l i e s in the fact that i f the e l e c t r i c f i e l d g r a d i e n t about t h e q u a d r u p o l e i s f l u c t u a t i n g due t o m o l e c u l a r m o t i o n , l i n e n a r r o w i n g o c c u r s w h i c h i s e x t r e m e l y s e n s i t i v e t o t h e f r e q u e n c y and amplitude o f that motion. The r e a d e r i s r e f e r r e d t o t h e chapter by J e l i n s k i and c o - w o r k e r s i n this v o l u m e , and r e f e r e n c e s t h e r e i n f o r examples and f u r t h e r d i s c u s s i o n s . 2

Z

Z

24

NMR

AND MACROMOLECULES

L i n e N a r r o w i n g Methods I n t h e above d i s c u s s i o n s o f t h e s o u r c e o f l i n e - b r o a d e n i n g i n s o l i d s t a t e NMR s p e c t r o s c o p y , i t was p o i n t e d o u t t h a t a l l o f t h e i n t e r a c t i o n s were h i g h l y o r i e n t a t i o n a l l y d e p e n d e n t . In the case o f d i p o l e - d i p o l e i n t e r a c t i o n s , the o r i e n t a t i o n of the i n t e r n u c l e a r v e c t o r between two d i p o l e s w i t h r e s p e c t t o H gave r i s e t o wide l i n e s . F o r CSA i n t e r a c t i o n s , t h e o r i e n t a t i o n o f t h e c h e m i c a l bond w i t h r e s p e c t t o H was i m p o r t a n t . Finally, the o r i e n t a t i o n o f t h e n u c l e a r e l e c t r i c q u a d r a p o l e w i t h r e s p e c t to t h e s u r r o u n d i n g e l e c t r i c f i e l d g r a d i e n t gave r i s e t o s p l i t t i n g s i n t h e NMR s p e c t r a . I n e v e r y c a s e i t was n o t e d that m o l e c u l a r m o t i o n , e s p e c i a l l y random r o t a t i o n a l and t r a n s l a t i o n m o t i o n as e x p e r i e n c e d i n t h e l i q u i d s t a t e , r e s u l t e d i n l i n e n a r r o w i n g by t i m e - a v e r a g i n g . To o b t a i n n a r r o w l i n e - w i d t h s i n t h e s o l i d s t a t e , i t i s t h e n up t o t h e s p e c t r o s c o p i s t t o m i m i c the e f f e c t o f motion i n a l i q u i d t o time-average t h e d i f f e r e n t interactions. Q

Q

Dipolar

Decoupling

The s i m p l e s t means o f r e m o v i n g d i p o l a r i n t e r a c t i o n s between two n u c l e i i s t o d e c o u p l e t h e i n t e r a c t i o n by a means entirely a n a l o g o u s t o s c a l a r d e c o u p l i n g u s e d i n ^ C NMR s p e c t r o s c o p y t o remove J - c o u p l i n g f r o m bonded p r o t o n s ( 6 ) . A strong radio frequency ( r f ) pulse a t the resonance frequency o f the protons i s t u r n e d on d u r i n g a c q u i s i t i o n o f t h e c a r b o n s i g n a l . The d e c o u p l i n g r f p u l s e promotes r a p i d s p i n t r a n s i t i o n s o r f l i p s between s p i n s t a t e s by t h e p r o t o n s p i n s , t h e r e b y a v e r a g i n g t h e static dipolar i n t e r a c t i o n s to zero. This decoupling c o n s t i t u t e s a r a p i d random m o t i o n i n " s p i n s p a c e " , as opposed to t h e random m o t i o n c h a r a c t e r i s t i c o f m o l e c u l e s tumbling i n r e a l space. U n f o r t u n a t e l y , i t c a n o n l y be u s e d t o remove heteronuclear dipole-dipole interactions. I n c a s e s where t h e i s o t o p i c c o n c e n t r a t i o n o f NMR a c t i v e s p e c i e s i s l o w , s u c h a s ( 1 . 1 % o f a l l c a r b o n s ) , homonuclear d i p o l e - d i p o l e i n t e r actions are not s i g n i f i c a n t . The l i n e - s h a p e s l e f t i n t h e NMR s p e c t r a o f s o l i d when d i p o l a r d e c o u p l i n g o f t h e p r o t o n s i s used therefore normally only reflect the chemical shift a n i s o t r o p y (CSA). The

Magic Angle

Although d i p o l a r d e c o u p l i n g removes d i p o l a r i n t e r a c t i o n s , i t does n o t remove CSA, n o r does i t p e r m i t o b s e r v a t i o n o f abundant s p i n s p e c i e s s u c h as p r o t o n s , s i n c e o b s e r v a t i o n and d e c o u p l i n g c a n n o t be done a t t h e same t i m e . D i f f e r e n t , more s e l e c t i v e means a r e needed t o remove t h e s e i n t e r a c t i o n s .

2.

O'DONNELL

NMR Spectroscopy of Solid Samples

25

I t c a n be shown ( 1 - 5 ) t h a t t h e m a g n i t u d e o f a n y o f t h e above anisotropic interactions have a very specific angular dépendance w i t h r e s p e c t t o : 1. the s t a t i c f i e l d ( C S A ) , 2. o t h e r n u c l e a r s p i n s ( d i p o l e - d i p o l e i n t e r a c t i o n s ) o r 3. w i t h surrounding e l e c t r i c f i e l d gradients (quadrupole i n t e r a c t i o n s ) . Among o t h e r terms d e s c r i b i n g t h e o r i e n t a t i o n a l dependence o f these a n i s o t r o p i c i n t e r a c t i o n s i n each o f t h e r e s p e c t i v e H a m i l t o n i a n o p e r a t o r s i s t h e t e r m (3cos ® - 1 ) . The a n g l e θ h a s a d i f f e r e n t m e a n i n g d e p e n d i n g upon t h e t y p e o f i n t e r a c t i o n b e i n g considered. For dipole-dipole interactions, t h e angle i s b e t w e e n a v e c t o r j o i n i n g two d i p o l e s and t h e d i r e c t i o n o f H (Fig. 1 A ) . I n t h e c a s e o f CSA i n t e r a c t i o n s , t h e a n g l e m i g h t be b e t w e e n t h e b o n d i n g a x i s and H ( F i g . l b ) . F i n a l l y , t h e a n g l e i n quadrupolar i n t e r a c t i o n s i s b e t w e e n t h e q u a d r u p o l e moment and t h e d i r e c t i o n o f t h e e l e c t r i c f i e l d g r a d i e n t ( F i g . 1 c ) . I n each case, i f t h e angle θ i s chosen such t h a t : 2

0

0

2

3 cos e

-1=0

a l l a n i s o t r o p i c c o n t r i b u t i o n s t o t h e NMR s p e c t r u m w i l l r e d u c e to z e r o . T h i s a n g l e , 54.7°, i s j u s t l y c a l l e d t h e m a g i c a n g l e . Since a l l values of θ are possible i n the n o n - c r y s t a l l i n e or powdered s o l i d , v e r y few i n t e r a c t i o n s n a t u r a l l y r e d u c e t o z e r o . However, i f , o v e r t i m e , t h e a v e r a g e v a l u e o f θ = 54.7°, t h e anisotropic static contributions w i l l again reduce t o zero. T h i s c a n be done i n r e a l s p a c e by h i g h speed sample r o t a t i o n a t an a n g l e 54.7° f r o m t h e d i r e c t i o n o f t h e f i e l d ( _ 3 ) ; or i n s p i n s p a c e , by m a n i p u l a t i o n o f t h e s p i n s u s i n g r f p u l s e s ( J 7 ) . In F i g . 2, a r e p r e s e n t a t i o n o f t h e e f f e c t o f s p i n n i n g on t h e t i m e a v e r a g e d v a l u e o f an i n t e r n u c l e a r v e c t o r i s shown. Rotation a b o u t t h e a x i s R c a u s e s t h e n u c l e i and t h e i n t e r n u c l e a r v e c t o r to c i r c l e t h e a x i s . O v e r a p e r i o d o f one r o t a t i o n , t h e a v e r a g e p o s i t i o n o f e a c h n u c l e i l i e s a l o n g R, and t h e t i m e a v e r a g e i n t e r n u c l e a r v e c t o r w i l l t h e r e f o r e a l s o be a l i g n e d w i t h R. The anisotropic static i n t e r a c t i o n s mentioned above will be c o h e r e n t l y m o d u l a t e d a t s p i n n i n g r a t e s l e s s t h a n ~ one h a l f o f the l i n e w i d t h ( i n H z ) , r e s u l t i n g i n s p i n n i n g s i d e bands. At s p i n n i n g r a t e s g r e a t e r t h a n t h e l i n e w i d t h , t h e s i d e bands disappear. T h e o r e t i c a l l y , i t i s p o s s i b l e t o remove a l l o f t h e above a n i s o t r o p i c i n t e r a c t i o n s b y sample s p i n n i n g a t t h e m a g i c angle. Realistically, s p i n n i n g r a t e s have b e e n l i m i t e d b y m a t e r i a l p r o b l e m s , so t h a t r a t e s o f 3 t o 5 K H a r e n o r m a l . These rates are sufficient to attenuate o r remove lineb r o a d e n i n g due t o CSA, b u t a r e f a r s h o r t o f t h e ~ 2 0 K H necessary t o remove dipolar interactions, with a few exceptions(_1 ) . Z

Z

26

NMR AND MACROMOLECULES

A .

X

Β .

F i g u r e 1 . O r i e n t a t i o n a l dependence o f a n i s o t r o p i c i n t e r a c t i o n s on t h e a n g l e Θ : a. D i p o l e - d i p o l e interac t i o n ; b. c h e m i c a l s h i f t a n i s o t r o p i c i n t e r a c t i o n ; c. e l e c t r i c q u a d rupolar interaction.

2.

O'DONNELL

27

NMR Spectroscopy of Solid Samples

Figure 2. The t i m e - a v e r a g e d r e s u l t randomly o r i e n t e d i n t e r n u c l e a r v e c t o r , t i o n a l a x i s , R.

of rotation ab, about a

of a rota-

NMR AND MACROMOLECULES

28 Line Narrowing i n Spin Space - WAHUHA

Sample spinning induces a time dependent e f f e c t on the r e l a tionship between a nuclear spin f i x e d along H and its "surroundings" by moving the "surroundings" about the magic angle. I f the "surroundings" are now f i x e d , the same type o f time dependent behavior can be induced by s p e c i f i c r o t a t i o n o f the nuclear spins about the magic angle. In F i g . 3, a repre s e n t a t i o n o f magic angle sample spinning i s given from the viewpoint o f the sample. Since the magic angle vector i s a locus o f points e q u i d i s t a n t from a l l three coordinates, a view down the magic angle r e v e a l s the three coordinate axis to be e q u a l l y spaced about the "magic" v e c t o r . As the sample s p i n s , the a p p l i e d f i e l d , H , appears to be " r o t a t i n g " through each o f the c o o r d i n a t e s . The nuclear spins remain a l i g n e d along H , so that the s p i n magnetization, M, a l s o appears to be r o t a t i n g through the three c o o r d i n a t e s . Q

0

0

In 1968, Waugh and co-workers(7) devised a means of " r o t a t i n g " M through each of three coordinate a x i s . Because nuclear spins precess about H a t the Larmor or resonance frequency, the co ordinate system used when s p i n manipulations are i n v e s t i g a t e d must also r o t a t e a t the Larmor frequency. This a l s o permits us to view r f pulses at the Larmor frequency as a p p l i e d f i e l d s along the axes o f t h i s r o t a t i n g frame (when the r o t a t i n g frame i s used, the axes w i l l be designated x , y and ζ ) . Q

f

f

1

C l a s i c a l l y , when an r f pulse i s a p p l i e d , e.g. along the x' a x i s , t h i s r f f i e l d , H]_, w i l l cause the net s p i n magnetization, M, a l i g n e d along H to precess as shown i n F i g . 4 a . A pulse of s p e c i f i c d u r a t i o n f o r a given H| w i l l cause M to precess e x a c t l y h r e v o l u t i o n , so that M l i e s on the y axis ( F i g . 4 b ) . Changing the phase of the o f pulse 180° w i l l cause M to precess i n the opposite d i r e c t i o n , r e t u r n i n g i t to the ζ ' axis ( F i g . 4c). The same type o f pulse sequence can be a p p l i e d along the y' a x i s , r o t a t i n g M into the x a x i s . By proper use of pulses and delays, M can be made to spend equal amounts of time on each of the r o t a t i n g frame axes. As can be seen, t h i s mimics i n s p i n space the r e s u l t s obtained by sample r o t a t i o n i n r e a l space. The necessary pulses are diagramed i n F i g . 4 d . I f the pulses are short enough and the delays, τ, are kept to a min imum, the " r o t a t i o n r a t e " o f the process can be made f a s t enough to e f f e c t i v e l y decouple homonuclear d i p o l e - d i p o l e i n t e r actions . This technique and other, more complicated pulse sequencesQ) have been used to narrow l i n e s i n proton s p e c t r a . The method i s also important i n that i t removes only homonuclear d i p o l e - d i p o l e broadening, but does not e f f e c t heteronuclear d i p o l e - d i p o l e i n t e r a c t i o n s . In the chapter by Schaefer and coworkers i n t h i s text, the WAHUHA pulse sequence 0

1

1

O'DONNELL

NMR Spectroscopy of Solid Samples

Figure 3. V i e w down m a g n e t i z a t i o n , M, and i s r o t a t i n g ccw w i t h The a n g l e o f R f r o m a l

t h e r o t a t i o n a l a x i s , R, o f t h e n e t t h e a p p l i e d f i e l d , Ho. The v i e w e r r e s p e c t t o the c o o r d i n a t e system. l three axis i s 5 4 . 7 ° .

NMR AND MACROMOLECULES

A .

fH

Ζ 0

M ' χ'-X' W/2 P U L S E

t

1

+ X ' W/2

PULSE

Β .

j

X

c.

W/2 X' 4

W/2 -X' W/2 !

4 Π

2\ψ

Y'

4

ψ

7Γ/2 -Y' 4

2Ψ

ACQUIRE DATA Figure 4. The WAHUHA e x p e r i m e n t : a . The a c t i o n o f a TT/2 r f p u l s e a p p l i e d t o a s p i n s y s t e m a l o n g t h e +x' a x i s i n t h e r o t a t i n g frame; b. t h e a c t i o n o f a ïï/2 p u l s e a p p l i e d a l o n g - x a x i s . c . The WAHUHA p u l s e s e q u e n c e . 1

2.

was

O'DONNELL

NMR

Spectroscopy of Solid Samples

u s e d t o remove 1 H - * H d i p o l e couplings.

couplings,

31

while

retaining ^-H-

F i n a l l y , i t s h o u l d be p o i n t e d o u t t h a t t h e above d i s c u s s i o n s are vast over-simplifications which are (hopefully) conceptually easy t o grasp. unfortunately, they are not theoretically satisfying. A d d i t i o n a l reading i s urged t o pro v i d e a more i n d e p t h understanding(1-5). Cross P o l a r i z a t i o n C l a s s i c a l l y , t h e n e t magnetization, M , o f a s p i n system r e s u l t s f r o m t h e sum o f t h e i n d i v i d u a l s p i n s w h i c h p r e c e s s a b o u t t h e applied field, H (see F i g . 5a). The f r e q u e n c y of the precession, called t h e Larmor f r e q u e n c y (OJL? F i g . 5 a ) , i s d e p e n d e n t upon t h e m a g n e t i c moment o f t h e i n d i v i d u a l s p i n s , and the a m p l i t u d e o f H . I n a d d i t i o n , the amplitude or p o l a r i z a t i o n o f M w i l l depend upon t h e v a l u e o f t h e n u c l e a r m a g n e t i c moment a s w e l l . I n t h e case o f a p r o t o n s p i n system, t h e n e a r l y 100% n a t u r a l abundance and l a r g e m a g n e t i c moment r e s u l t i n a p o l a r i z a t i o n that gives a large net magnetization, MJJ (Fig. 5a). The m a g n e t i c moment o f a l ^ C p i on t h e o t h e r h a n d , i s a b o u t one f o u r t h as l a r g e a s t h e p r o t o n moment, r e s u l t i n g i n a n e t m a g n e t i z a t i o n , M , t h a t i s one f o u r t h t h a t o f p r o t o n s i n t h e same H f o r an e q u a l number o f n u c l e i . To make m a t t e r s w o r s e , t h e n a t u r a l abundance o f ^ C n u c l e i i s o n l y 1.1%. F i n a l l y , t h e l e n g t h o f time f o r t h e carbon s p i n system t o r e c o v e r f r o m t h e p e r t u r b a t i o n n e c e s s a r y t o make a m e a s u r e ment c a n be 10 t o 100 t i m e s l o n g e r t h a n t h a t t i m e needed f o r a proton s p i n s y s t e m i n t h e same m o l e c u l e (see s p i n - l a t t i c e relaxation, below). This creates a time b o t t l e n e c k when repeated samplings a r e taken o f the ^ C magnetization. Q

Q

S

n

>

c

0

From t h e above d i s c u s s i o n , i t i s e v i d e n t t h a t i f t h e p r o t o n s p i n s y s t e m c o u l d be u s e d as a s o u r c e o f m a g n e t i z a t i o n and a s a means o f r e l a x a t i o n f o r t h e c a r b o n s p i n s y s t e m , a n enhancement o f t h e c a r b o n N M R s i g n a l and a s a v i n g s i n t i m e c o u l d be achieved. T h i s energy t r a n s f e r i n t h e s t a t i c f i e l d , H , i s n o t p o s s i b l e due t o t h e l a r g e m i s m a t c h i n t h e L a r m o r frequencies for t h e two d i f f e r e n t n u c l e i . I n order f o r a t r a n s f e r to o c c u r , t h e two s p i n s y s t e m s must have some p r e c e s s i o n a l compo nents that a r e equal i n frequency. As l o n g a s t h e s p i n s a r e precessing about t h e s t a t i c f i e l d , H , t h i s i s impossible. However, s p e c i f i c m a g n e t i c f i e l d s may be a p p l i e d by u s i n g r a d i o frequency ( r f ) pulses. I n F i g . 5a, the magnetization ofthe p r o t o n s was shown i n a c o o r d i n a t e s y s t e m s e t i n t h e l a b o r a t o r y ; the laboratory reference frame. An r f p u l s e a t a given f r e q u e n c y i n t h e l a b o r a t o r y frame w i l l a p p e a r a s a f i e l d o f magnitude H, r o t a t i n g a b o u t the Ζ axis a t the applied Q

Q

NMR A N D MACROMOLECULES

f

A .

"^3w'^ H

T

"

Ho

13

f

CROSS POLARIZATION

4/H = 4»C

F i g u r e 5. S p i n l o c k i n g o f t h e p r o t o n s p i n s y s t e m and subsequent c r o s s polarization between t h e p r o t o n and carbon s p i n s : a. The i n i t i a l p r o t o n m a g n e t i z a t i o n on t h e ζ' a x i s , a l i g n e d w i t h t h e f i e l d , Ho; b. Α π / 2 p u l s e i s a p p l i e d along the χ axis of the proton rotating f r a m e , f o l l o w e d by an r f f i e l d , H ç , a p p l i e d a l o n g t h e y a x i s o f t h e carbon r e f e r e n c e frame; c. Spin locking of t h e p r o t o n s p i n s b y H J J and c r o s s p o l a r i z a t i o n w i t h t h e carbon s p i n s . 1

2.

O'DONNELL

33

NMR Spectroscopy of Solid Samples

frequency. I f t h e c o o r d i n a t e s y s t e m were r o t a t i n g a t t h e a p p l i e d f r e q u e n c y ( a r o t a t i n g r e f e r e n c e f r a m e ) , t h e n Έ.\ w o u l d a p p e a r a s a f i e l d a l i g n e d a l o n g e i t h e r t h e x ' o r y' a x i s . In Fig. 5b, a n r f f i e l d a t t h e p r o t o n f r e q u e n c y , H J J , i s shown along the χ a x i s . T h i s f i e l d , a s shown i n f i g . 5b, a p p l i e s a torque t o MJJ, t h e n e t p r o t o n m a g n e t i z a t i o n , making i t r o t a t e into the y axis. A t t h a t p o i n t , t h e r f f i e l d c a n be s h i f t e d 90°, so t h a t i t a l s o l i e s a l o n g t h e y a x i s ( F i g . 5 c ) . Because HJJ i s a m a g n e t i c f i e l d t h a t i s c o - l i n e a r w i t h t h e n e t m a g n e t i z a t i o n , Mj|, t h e i n d i v i d u a l s p i n s r e p r e s e n t e d b y M|j w i l l p r e c e s s about Hj| a t a new p r e c e s s i o n a l f r e q u e n c y , that i s determined by t h e amplitude o f H . I f another r f f i e l d i s a p p l i e d a t t h e carbon f r e q u e n c y , such t h a t t h e p r e c e s i o n a l f r e q u e n c y , U ) , o f t h e c a r b o n s p i n s a b o u t H e q u a l s (jOg, t h e n an e f f i c i e n t t r a n s f e r o f magnetization can occur ( F i g . 5c). 1

1

1

H

C

c

The p r o c e s s o f h o l d i n g a n e t m a g n e t i z a t i o n a l o n g a n a x i s o f t h e r o t a t i n g frame u s i n g a n r f f i e l d i s c a l l e d s p i n - l o c k i n g . The match o f r f f i e l d i n t e n s i t i e s H and H s u c h t h a t (% = 0 ) i s c a l l e d a Hartman-Hahn m a t c h ( 8 ) . The p r o c e s s o f t r a n s f e r r i n g m a g n e t i z a t i o n from t h e p r o t o n s p i n s t o t h e carbon s p i n s i s called cross polarization(9). The r a t e of transfer i s characterized by a contact time, T , which S c h a e f e r and coworkers used t o s t u d y s p i n - s p i n c o n t r i b u t i o n t o T^p ( s e e b e l o w and c h a p t e r b y S c h a e f e r , e t a l , t h i s v o l u m e ) . C

H

c

c p

The c r o s s p o l a r i z a t i o n , o r CP, p r o c e s s may be u s e d w i t h a n y o r a l l o f t h e l i n e n a r r o w i n g t e c h n i q u e s t o o b t a i n NMR s p e c t r a o f s o l i d s w i t h r e s o l u t i o n approaching that of liquidsC3). A combination o f c r o s s p o l a r i z a t i o n ( C P ) , magic angle s p i n n i n g (MAS) and d i p o l a r d e c o u p l i n g were u s e d t o o b t a i n t h e s p e c t r u m o f a v e r y i n s o l u b l e p o l y p h e n y l e n e s u l f i d e ( R y t o n ) a s shown i n F i g . 6.

34

NMR

NMR M e a s u r e m e n t s o f M o t i o n

AND

MACROMOLECULES

i n Polymers

All o f the i n t e r a c t i o n s discussed above have a strong dependency on m o t i o n , ( o r l a c k o f m o t i o n ) i n s o l i d s . I t i s not s u r p r i s i n g then that the motions that a r e present i n a s o l i d c a n be d e t e c t e d by m o t i o n - s e n s i t i v e NMR m e t h o d s . The methods u s e d i n t h i s volume c a n be d i v i d e d i n t o two c a t a g o r i e s ; l i n e shape a n a l y s i s and r e l a x a t i o n s t u d i e s . Line-Shape A n a l y s i s B e c a u s e o f t h e s t r o n g o r i e n t a t i o n a l dependancy o f t h e l i n e b r o a d e n i n g mechanisms d i s c u s s e d a b o v e , t h e NMR l i n e shape w i l l r e f l e c t changes i n t h e o r i e n t a t i o n w i t h t i m e . I f the frequency o f a m o t i o n i n a s o l i d i s lower than the range o f f r e q u e n c i e s (i.e., line-width in H ) i n d u c e d b y an i n t e r a c t i o n (e.g., e l e c t r i c quadrupolar i n t e r a c t i o n s ) , a small, but measurable narrowing might occur. As t h e f r e q u e n c y and a m p l i t u d e o f t h e motion i s i n c r e a s e d by h e a t i n g t h e s a m p l e , a d d i t i o n a l line narrowing w i l l occur. D r a m a t i c l i n e - s h a p e changes o c c u r a s t h e motional frequency nears the l i n e - w i d t h frequency. Above t h e l i n e - w i d t h frequency, s u b s t a n t i a l l y narrower l i n e shapes a r e observed. C o n v e r s e l y , t h e m o l e c u l a r m o t i o n may be r a p i d comp a r e d t o t h e l i n e w i d t h a t room t e m p e r a t u r e ( n a r r o w l i n e s ) and the t e m p e r a t u r e dependency o f t h e l i n e shape may be b e s t i n v e s tigated by l o w e r i n g t h e t e m p e r a t u r e . In this t e x t , Dr. J e l i n s k i and c o w o r k e r s u s e d l i n e - b r o a d e n i n g from n u c l e a r e l e c t r i c q u a d r u p o l a r i n t e r a c t i o n s t o i n v e s t i g a t e motions about a bond b e t w e e n c a r b o n s isotopically enriched with deuterium. T h i s s t u d y was p a r t i c u l a r l y i n t e r e s t i n g , b e c a u s e two r e g i o n s o f t h e p o l y m e r e x i s t e d ; one t h a t gave a r e l a t i v e l y n a r r o w NMR l i n e - w i d t h ( " f a s t " m o t i o n ) and one t h a t y i e l d e d a much b r o a d e r NMR l i n e w i d t h ( " s l o w " m o t i o n ) . T h i s i n d i c a t e d two r e g i o n s i n the polymer w i t h d i s t i n c t l y d i f f e r e n t m o t i o n a l c h a r a c t e r i s t i c s . z

S c h a e f e r and c o w o r k e r s , i n a n o t h e r c h a p t e r i n t h i s t e x t , u s e d *H - l ^ C d i p o l e - d i p o l e " l i n e s h a p e s " o b t a i n e d i n a v e r y c l e v e r way t o i n v e s t i g a t e r o t a t i o n a l m o t i o n o f t h e a r o m a t i c r i n g s i n polystyrene. The method u s e d a WAHUHA p u l s e s e q u e n c e t o decouple proton-proton d i p o l a r i n t e r a c t i o n s , cross p o l a r i z a t i o n t o enhance s i g n a l a c q u i s i t i o n and an o v e r a l l s a m p l i n g t e c h n i q u e s y n c h r o n o u s w i t h t h e sample r o t a t i o n . The ^ C - *H d i p o l e d i p o l e i n t e r a c t i o n was mapped i n r o t a t i o n a l s i d e b a n d s p e c t r a o b t a i n e d f r o m 16 " n o r m a l " CP/MAS s p e c t r a . The method, t h o u g h somewhat i n v o l v e d , p r o v i d e d a measure o f d i p o l e - d i p o l e lineshapes w h i c h c a n be i n t e r p r e t e d i n terms o f s i d e - c h a i n r o t a t i o n i n the polymer.

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R e l a x a t i o n and M o t i o n i n S o l i d s S p i n - L a t t i c e R e l a x a t i o n - Ί·\ I n F i g . 5 a , a r e p r e s e n t a t i o n o f t h e n e t m a g n e t i z a t i o n , M, o f a n u c l e a r s p i n system i s g i v e n . The v a l u e o f M i s t h e sum o f t h e i n d i v i d u a l s p i n s p r e c e s s i n g a t t h e Larmor f r e q u e n c y , U)L, about the a p p l i e d field, H . The i n d i v i d u a l s p i n s r e p r e s e n t an e x c e s s o f s p i n s i n t h e l o w e n e r g y s t a t e o f a s y s t e m where t h e spins are distributed (Boltzman d i s t r i b u t i o n ) b e t w e e n two states (spin % nucleus). The s y s t e m , a s shown i n F i g . 5 a , i s at equilibrium. I f an r f p u l s e , Η χ , a t i s now a p p l i e d , t h e system w i l l a b s o r b some o f t h e e n e r g y f r o m t h e p u l s e ( t h e resonance c o n d i t i o n ) , t h a t i s , t h e system w i l l "heat up". I n F i g . 5b t h i s i s r e p r e s e n t e d by a t i p p i n g o f t h e m a g n e t i z a t i o n i n t h e r o t a t i n g f r a m e . The m a g n i t u d e o f M a l o n g H i s e q u a l t o z e r o i n F i g . 7b. The s y s t e m i s c l e a r l y n o t i n e q u i l i b r i u m w i t h H , and i f t h e r f f i e l d , H|, i s removed a t t h i s p o i n t , t h e s y s t e m w i l l be l e f t i n a d i s o r d e r e d s t a t e . To r e - e s t a b l i s h t h e equilibrium condition, some o f the i n d i v i d u a l spins must exchange e n e r g y , o r " c o o l down." The p r o b a b i l i t y o f a s p i n g i v i n g up e n e r g y i n t h e f o r m o f d i s c r e t e r f r a d i a t i o n ( a phonon) i s v e r y l o w . T h u s , s i n c e e n e r g y must be c o n s e r v e d , t h e e n e r g y must be d i s s i p a t e d i n some o t h e r f o r m . I t may be dissipated as thermal energy t o the atomic framework, o r l a t t i c e , i f a s u i t a b l e mechanism e x i s t s f o r t h e t r a n s f e r . The mechanism must be m a g n e t i c i n n a t u r e , and must f l u c t u a t e a t t h e L a r m o r f r e q u e n c y ( i . e . , i t must f l u c t u a t e a t a r a d i o f r e q u e n c y i n t h e megahertz r a n g e ) . The m a g n e t i c f i e l d s o u r c e s a v a i l a b l e on t h e a t o m i c s c a l e a r e n u c l e a r d i p o l e s and u n p a i r e d e l e c t r o n s . I f there i s r o t a t i o n , v i b r a t i o n or t r a n s l a t i o n i n the l a t t i c e a t t h e L a r m o r f r e q u e n c y , U3L, t h e n r e l a x a t i o n o f t h e s p i n s y s t e m to t h e e q u i l i b r i u m s t a t e can occur by passage o f t h e excess energy t o t h e l a t t i c e system; i . e . , s p i n - l a t t i c e relaxation occurs. The r e l a x a t i o n process usually appears t o be exponential i n n a t u r e , and i s u s u a l l y c h a r a c t e r i z e d by a s p i n l a t t i c e r e l a x a t i o n t i m e , T|. The v a l u e f o r Ύγ r e p r e s e n t s t h e t i m e n e c e s s a r y f o r t h e m a g n e t i z a t i o n t o r e t u r n t o w i t h i n ( 1( 1/e)) o r 6 3 % o f i t s m a g n i t u d e a t e q u i l i b r i u m . Thus, f o r f u l l r e s t o r a t i o n o f M a l o n g t h e f i e l d , H , one must w a i t several t i m e s t h e v a l u e o f Τ j ( u s u a l l y 5 · T]_ i s s u f f i c i e n t ) . Q

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Figure 6 . A CP/MAS NMR spectrum of Ryton (polyphenylene sulfide). The s i g n a l s occur at ~135 and ~133 ppm ( from an e x t e r n a l reference of TMS).

F i g u r e 7. A mapping of the carbon magnetization during a Τι measurement : a. The carbon magnetization, M Q , locked on the y axis by an r f f i e l d , H Q , a f t e r cross p o l a r i z a t i o n ; b. Mfj a l i g n e d along the a p p l i e d f i e l d , Ho, a f t e r a π/2 r f pulse along -x'; c. Mç a f t e r T j r e l a x a t i o n f o r a time p e r i o d , τ; d. Mç, a f t e r a π/2 r f pulse, a l i g n e d along y . This permits the amplitude of M Q to be measured. 1

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N M R Spectroscopy of Solid Samples

For the individual interested i n molecular motion, the important feature of spin-lattice relaxation (or other r e l a x a t i o n mechanisms) i s t h e dependency on m o l e c u l a r m o t i o n t o p r o v i d e an e f f i c i e n t e n e r g y pathway f o r r e l a x a t i o n . Thus, m o l e c u l a r motions a t t h e Larmor frequency f o r i n d i v i d u a l carbon atoms i n a m o l e c u l a r framework may be mapped b y T j m e a s u r e ments. Since t h e frequency o f m o l e c u l a r motion i s temperature d e p e n d e n t , a d d i t i o n a l thermodynamic and k i n e t i c i n f o r m a t i o n may be o b t a i n e d by m e a s u r i n g T\ v a l u e s f o r d i f f e r e n t c a r b o n s o v e r a range o f t e m p e r a t u r e s . I n t h e p a p e r b y L y e r l a and c o w o r k e r s i n t h i s v o l u m e , Ύγ measurements made f o r t h e f i r s t t i m e o v e r a range o f low temperatures y i e l d e d s p e c i f i c i n f o r m a t i o n about m o t i o n i n t h e backbone and s i d e c h a i n s o f a s e m i - c r y s t a l l i n e and a g l a s s y p o l y m e r . The d a t a was t a k e n a t a L a r m o r f r e q u e n c y o f 15.1 MHz f o r nuclei. I t was a l s o n o t e d i n t h i s p a p e r t h a t t h e T\ v a l u e s measured f o r t h e g l a s s y p o l y m e r showed n o n e x p o n e n t i a l b e h a v i o r . As s t a t e d i n t h e paper, t h i s r e p r e s e n t e d a d i s t r i b u t i o n o f T| v a l u e s due t o t h e many d i f f e r e n t e n v i r o n m e n t s , and t h e r e f o r e t h e many d i f f e r e n t T\ m e c h a n i s m s , p r e s e n t i n the polymer. R e l a x a t i o n i n t h e R o t a t i n g Frame-T]ρ As v a l u a b l e a s T| measurements c a n be i n a n a l y z i n g m o l e c u l a r motions, they suffer from a significant d r a w b a c k ; t o map motions a t other frequencies, different magnetic field s t r e n g t h s , H , must be u s e d . T h i s r e q u i r e s t h e use o f a d i f f e r e n t magnet ( i . e . , a d i f f e r e n t s p e c t r o m e t e r o p e r a t i n g a t a d i f f e r e n t Larmor f r e q u e n c y ) . I n addition, the l i m i t a t i o n of t h e Tji measurements t o m o l e c u l a r m o t i o n s i n t h e 10^ h e r t z f r e quency r a n g e ( f o r ^ C ) r e s t r i c t s t h e u s e o f T\ s t u d i e s t o o n l y a few t y p e s o f m o t i o n s . Motions i n the k i l o h e r t z r e g i o n a r e therefore inaccessible. Q

The f r e q u e n c y r a n g e o f m o t i o n a v a i l a b l e f o r s t u d y b y Ί\ mech anisms i s governed by t h e p r e c e s s i o n a l (Larmor) frequency o f the i n d i v i d u a l s p i n s about t h e a p p l i e d f i e l d , H . I f a means c o u l d be f o u n d t o make t h e s e s p i n s p r e c e s s a b o u t a much l o w e r field i n t h e presence o f H , then t h e f r e q u e n c y dependent n a t u r e o f t h e r e l a x a t i o n mechanism c o u l d be a l t e r e d t o a l l o w motions i n t h e KHz r a n g e t o be s t u d i e d . S u c h a means h a s a l r e a d y b e e n d i s c u s s e d — s p i n l o c k i n g ( s e e F i g . 5 ) . I n t h e CP e x p e r i m e n t , t h e p r o t o n s p i n s a r e f i r s t " l o c k e d " a l o n g an r f f i e l d , H||, r o t a t i n g a t t h e L a r m o r f r e q u e n c y . As shown i n F i g . 5b, H|| a p p e a r s a s a m a g n e t i c f i e l d a l i g n e d a l o n g an a x i s o f a Q

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NMR AND MACROMOLECULES

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coordinate system r o t a t i n g at the proton Larmor frequency about H (the *H r o t a t i n g frame). As shown i n 5c, the i n d i v i d u a l porton spins w i l l precess about H J J at a frequency, directly p r o p o r t i o n a l to the amplitude of H . In a d d i t i o n , the carbon spins at the end of the cross p o l a r i z a t i o n trans fer are a l s o " l o c k e d " along an r f f i e l d , Hç, such that the i n d i v i d u a l carbon spins are precessing about Hç at a frequency, U)fj, p r o p o r t i o n a l to H p There are two important features about the s p i n - l o c k c o n d i t i o n that make i t a t t r a c t i v e f o r r e l a x a t i o n s t u d i e s ; 0

W

1.

The l o c k i n g r f f i e l d s can have a range of amplitudes. Thus a range of motional frequencies can be i n v e s t i g a t e d by simply changing the amplitude of Hj| or H Q .

2.

The magnetization locked along the a p p l i e d r f f i e l d s are at a magnitude generated by a much l a r g e r f i e l d , H . T h i s means that the magnetization, M , i s much too large i n prop o r t i o n to the r f f i e l d s , H J J and H Q , and M must d i m i n i s h v i a a r e l a x a t i o n mechanism to a l e v e l that matches the amplitude of H J J or H Q . 0

The process of r e l a x a t i o n of M to a value p r o p o r t i o n a l to the a p p l i e d r f f i e l d s i n the r o t a t i n g frame i s c a l l e d s p i n - l a t t i c e r e l a x a t i o n i n the r o t a t i n g frame. The mechanisms a v a i l a b l e f o r t h i s form of r e l a x a t i o n are e n t i r e l y analogous to those a v a i l able f o r simple s p i n - l a t t i c e r e l a x a t i o n as described above. S i m i l a r l y , r o t a t i n g frame r e l a x a t i o n i s c h a r a c t e r i z e d by a time constant analogous to T j , and i s c a l l e d T j p , or the spinl a t t i c e r e l a x a t i o n time i n the r o t a t i n g frame. Typically, Tjp values obtained from protons i n s o l i d samples are not of much use, since communication between the abundant protons tends to average the r e l a x a t i o n process, so that individual proton r e l a x a t i o n mechanisms cannot be observed. For ^ C , however, the n a t u r a l low abundance of l^C l i m i t s the degree of communic a t i o n , and separate T^p values can be obtained for each observed carbon s p e c i e s . As noted i n the papers by Schaefer and coworkers and by L y e r l a and coworkers, T j p data may be complicated by the f a c t that mechanisms other than s p i n - l a t t i c e i n t e r a c t i o n s (namely s p i n spin r e l a x a t i o n ) are p o s s i b l e which don't map motional charact e r i s t i c s . In the case of p o l y s t y r e n e , Schaefer and coworkers conclude that the spin-spin contributions are negligible, whereas L y e r l a and coworkers f i n d that the l e v e l s of s p i n - s p i n and s p i n - l a t t i c e c o n t r i b u t i o n s for i s o t a t i c polypropylene and a t a t i c polymethyl methacrylate were temperature dependent.

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Measurement o f R e l a x a t i o n The methods t o measure T j and T^p i n t h e s o l i d s t a t e a r e somewhat u n i q u e . The i n i t i a l step f o r both experiments c o n s i s t s o f enhancement o f t h e magnetization v i a - *H c r o s s p o l a r i z a t i o n ( F i g . 5 ) . A t t h e end o f t h e c r o s s p o l a r i z a t i o n p e r i o d , t h e carbon m a g n e t i z a t i o n i s e i t h e r allowed t o c o n t i n u e t o i n t e r a c t w i t h t h e r f f i e l d , Hç w h i l e t h e p r o t o n r f f i e l d i s removed ( F i g . 7 a ) , o r an r f p u l s e i s a p p l i e d t o r o t a t e it to align with H ( F i g . 7b). I n t h e f i r s t c a s e , t h e magnet i z a t i o n a l i g n e d a l o n g Hç d e c a y s v i a a T j p mechanism, and t h e r a t e o f d e c a y i s m o n i t o r e d by c h a n g i n g t h e l e n g t h o f t i m e H Q i s applied. The p u l s e s e q u e n c e u s e d i s o u t l i n e d i n t h e p a p e r by S c h a e f e r and c o - w o r k e r s and i s r e p e a t e d h e r e ( F i g . 8 c ) . The r e a d e r i s r e f e r r e d t o o t h e r r e f e r e n c e s f o r a more d e t a i l e d a c c o u n t (_3). Q

I f t h e m a g n e t i z a t i o n M Q i s moved t o t h e ζ a x i s t o a l i g n w i t h H , t h e i n d i v i d u a l s p i n s w i l l now p r e c e s s a t ωχ^ a r o u n d H ( M H frequencies). Remembering t h a t t h e m a g n e t i z a t i o n represented by M Q i s t h e r e s u l t o f a f o u r - f o l d enhancement f r o m t h e c r o s s p o l a r i z a t i o n w i t h t h e p r o t o n s p i n s , then i t i s e v i d e n t t h a t M Q does n o t r e p r e s e n t t h e n o r m a l e q u i l i b r i u m m a g n e t i z a t i o n f o r t h e carbon s p i n system. R e l a x a t i o n o c c u r s ( F i g . 7 c ) , and s a m p l i n g o f t h e p r o c e s s i s done by r o t a t i n g t h e r e m a i n i n g m a g n e t i z a t i o n v i a a ïï/2 r f p u l s e i n t o t h e x , y p l a n e ( F i g . 7 d ) . 1

0

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1

The p u l s e s e q u e n c e s f o r m e a s u r i n g T\ and T j p , enhancement p u l s e s , a r e g i v e n i n F i g . 8.

including

t h e CP

Conclusions T h i s s h o r t o v e r v i e w o f s o l i d sample NMR t e c h n i q u e s h a s been an attempt t o e x p l a i n t h e experiments used i n t h i s t e x t i n terms t h e layman c a n u n d e r s t a n d . A s c a n be s e e n b y t h e t i t l e s o f t h e p a p e r s on s o l i d sample NMR s p e c t r o s c o p y , t h e m a i n t h r u s t o f r e s e a r c h i n t h i s a r e a as a p p l i e d t o polymers i s t h e i n v e s t i g a t i o n o f motions i n polymers. The e x p e r i m e n t s o u t l i n e d i n t h i s and f o l l o w i n g c h a p t e r s h a v e been shown t o be u s e f u l f o r i n v e s t i g a t i n g m o t i o n s c o v e r i n g a f r e q u e n c y r a n g e o f 1 0 ^ t o 10^ H z . Most o f these experiments a r e pre-programmed into the commercial instruments a v a i l a b l e today. New a n d even more e x c i t i n g experiments are p o s s i b l e w i t h the f l e x i b i l i t y a f f o r d e d by t h e s t a t e o f t h e a r t c o m p u t e r c o n t r o l l e d s y s t e m s . Hopefully, as these instruments become more commonplace, these

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F i g u r e 8. P u l s e d i a g r a m s f o r s o l i d sample NMR e x p e r i ments: a. ^H-l^c cross polarization with dipolar d e c o u p l i n g ; b. WAHUHA % - *H d i p o l a r d e c o u p l i n g ; c . Τ r e l a x a t i o n sequence ( t i m e p e r i o d , τ, i s v a r i e d ) ; d. Τχ r e l a x a t i o n s e q u e n c e ( t i m e p e r i o d , t , i s v a r i e d ) .

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NMR Spectroscopy of Solid Samples

types o f experiments they deserve.

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use

Literature Cited 1.

Mehring, M. "High Resolution NMR Spectroscopy in Solids"; Diehl, P.; Fluck, Ε . ; Kosfeld, R . , Eds.; NMR, Vol. XI, Springer-Verlag, New York, 1976.

2.

Abragam, A. "The Principles of Nuclear Magnetism"; Clarendon Press: Oxford University, London, 1961.

3.

Schaefer, J ; Stejskal, Ε. O. in "topics in Carbon 13 NMR Spectroscopy"; Levy, G . , E d . ; Wiley-Interscience: New York, 1978; Chap. 4.

4.

Goldman, M. "Spin Temperature and Resonance in Solids"; Clarendon Press: London, 1970.

5. J.

Nuclear Magnetic Oxford University,

Haeberlin, U. in "Advances in Magnetic Resonance"; Waugh, S., Ed.; Acedemic: New York, 1976; V o l . I, pg. v.

6.

Block, F . Phys. Rev. 1958, III,

7.

Waugh, J. S.; Huber, L . M.; Haeberlen, Letters 1968, 20, 180.

8.

Hartmann, S. R.; Hahn, E . L .

9.

Pines, Α . ; Gibby, 1973, 59, 569.

RECEIVED November 18, 1983

841. U.

Phys. Rev. 1962,

M. G . ; Waugh,

J.

S.

J.

Phys.

Rev.

128, 2042. Chem. Phys.

3 Molecular Motion in Glassy Polystyrenes JACOB SCHAEFER, M. D. SEFCIK, E. O. STEJSKAL, and R. A. MCKAY—Monsanto Company, Physical Sciences Center, St. Louis, MO 63167 1

W. T. DIXON —Department of Chemistry, Washington University, St. Louis, MO 63130 R. E. CAIS—Bell Telephone Laboratories, Murray Hill, NJ 07974

The amplitudes of ring- and main-chain motions of a variety of polystyrenes have been established from the C NMR magic-angle spinning sideband patterns of dipolar and chemical shift tensors. The fre quencies of the same motions have been determined by T (C) and Τ ρ(C) experiments. The most prevalent motion i n these polymers is restricted phenyl rotation with an average t o t a l displacement of about 40°. Both the amplitude and frequency of this motion vary from one substituted polystyrene to another, and from site to site within the same polystyrene. A simple theory correlates the observed ring dipolar patterns with 's. 13

1

1

1

Rotating-Frame Carbon S p i n - L a t t i c e

Relaxation

B o t h s p i n - l a t t i c e ( m o t i o n a l ) and s p i n - s p i n p r o c e s s e s c o n t r i b u t e to Τ ρ(0). E x p e r i m e n t a l c r o s s - p o l a r i z a t i o n t r a n s f e r r a t e s from p r o t o n s i n the l o c a l d i p o l a r f i e l d t o carbons i n an a p p l i e d r f f i e l d c a n be u s e d t o d e t e r m i n e t h e r e l a t i v e c o n t r i b u t i o n s q u a n t i tatively. T h i s measurement a l s o r e q u i r e s a d e t e r m i n a t i o n o f t h e proton l o c a l f i e l d . Methods f o r m a k i n g b o t h measurements have b e e n d e v e l o p e d i n t h e l a s t few y e a r s [ 1 , 2 ] . For polystyrenes, the s p i n - l a t t i c e c o n t r i b u t i o n t o T p ( C ) ' s i s by f a r t h e l a r g e r . T h i s means t h a t t h e T j p C C ) ' s c a n be i n t e r p r e t e d i n terms o f r o t a t i o n a l motions i n the low-to-mid-kHz frequency range. χ

1

1

Current address: Mallinckrodt Institute of Radiology, Washington University Medical School, St. Louis, M O 63110.

0097 6156/84/0247 0043506.00/0 © 1984 American Chemical Society

44

NMR

AND MACROMOLECULES

D i p o l a r R o t a t i o n a l Spin-Echo Experiment The T j p C C ) ' s o f s u b s t i t u t e d p o l y s t y r e n e s show a l t e r a t i o n s i n m o t i o n due t o t h e s u b s t i t u e n t ( c f , b e l o w ) b u t do n o t show w h e t h e r t h e a l t e r a t i o n s i n v o l v e changes i n t h e a m p l i t u d e o r t h e f r e q u e n c y o f t h e m o t i o n . T h i s d i s t i n c t i o n c a n be made b y a s e p a r a t e e x p e r i ment w h i c h m e a s u r e s CH d i p o l a r c o u p l i n g . The s t r e n g t h o f a s t a t i c d i p o l a r i n t e r a c t i o n between an i s o l a t e d C and H s p i n p a i r i s known i f t h e i n t e r n u c l e a r d i s t a n c e i s known. T h i s i s t h e u s u a l s i t u a t i o n f o r a d i r e c t l y bonded CH f r a g m e n t i n a s o l i d p o l y m e r . The r e d u c t i o n i n t h e s t r e n g t h o f t h e CH d i p o l a r i n t e r a c t i o n by m o l e c u l a r motion ( o f frequency comparable t o o r g r e a t e r t h a n t h e d i p o l a r i n t e r a c t i o n i t s e l f ) t h e r e f o r e becomes a measure o f t h e a m p l i t u d e o f t h e m o t i o n . I n performing such a measurement o n a r e a l s y s t e m , t h e c o n d i t i o n t h a t t h e H - C s p i n p a i r be i s o l a t e d f r o m many-body p r o t o n d i p o l a r c o u p l i n g i s a c h i e v e d b y p h a s e - s h i f t e d m u l t i p l e - p u l s e (WAHUHA) H - H d e c o u p l i n g [3]. The t i m e e v o l u t i o n o f t h e c a r b o n m a g n e t i z a t i o n i s t h e n d e t e c t e d under t h e i n f l u e n c e o f H - C c o u p l i n g alone [4,5]. The r e s u l t i n g c a r b o n s i g n a l c a n be o b s e r v e d w i t h m a g i c - a n g l e spinning f o r high r e s o l u t i o n [6,7]. The p u l s e s e q u e n c e f o r t h i s e x p e r i m e n t i s shown i n F i g u r e 1 [ 8 ] . The e v o l u t i o n o f t h e c a r b o n m a g n e t i z a t i o n due t o c h e m i c a l s h i f t e f f e c t s i s r e f o c u s e d a f t e r two r o t o r p e r i o d s b y a c a r b o n 180° p u l s e a p p l i e d a f t e r t h e f i r s t r o t o r p e r i o d . Under h i g h - s p e e d s p i n n i n g c o n d i t i o n s , t h i s removes t h e e f f e c t o f t h e chemical s h i f t tensor. T h e H - C d i p o l a r e v o l u t i o n time i s v a r i e d w i t h t h e number o f WAHUHA p u l s e s e q u e n c e s . The s p i n n i n g s p e e d i s c h o s e n so t h a t a n i n t e g r a l number o f WAHUHA c y c l e s e x a c t l y f i t s i n t o one r o t o r p e r i o d . I n our experiments, t h i s number was s i x t e e n . ( E a c h WAHUHA c y c l e t o o k 33 p s e c , w i t h 3-psec 100° p u l s e s , so t h a t sample s p i n n i n g was a t 1894 Hz. M a t c h e d s p i n - l o c k t r a n s f e r s were p e r f o r m e d a t 60 k H z . ) 1 3

1

1

1

1

1

T y p i c a l Spectra

1 3

1

1 3

1 3

i n t h e C h e m i c a l S h i f t and D i p o l a r

Dimensions

Some c h e m i c a l - s h i f t s p e c t r a as a f u n c t i o n o f WAHUHA i r r a d i a t i o n f o r p o l y ( o - c h l o r o s t y r e n e ) a r e shown i n F i g u r e 2. P r o t o n a t e d c a r b o n m a g n e t i z a t i o n s r a p i d l y dephase u n d e r a s l i t t l e as two c y c l e s o f WAHUHA i r r a d i a t i o n , b u t a r e r e f o c u s e d a f t e r s i x t e e n c y c l e s (one r o t o r p e r i o d ) . Magic-angle spinning should r e f o c u s d i p o l a r c o u p l i n g j u s t a s i t does c h e m i c a l s h i f t a n i s o -

carbon

C ττ- pulse

one rotor period

chemical shifts

rcfocusis

,3

decoupling:

,3

1

spin-

,3

/

data acquisition begins

Η - C decoupling

one rotor period

'H- C dipolar modulation

Ή-Ή decoupling

F i g u r e 1. P u l s e sequence f o r a d i p o l a r echo e x p e r i m e n t .

preparation

CP contact

,5

'H- C

NMR A N D MACROMOLECULES

46

~fCH CH-)Cl 2

CI

—CH

0 WAHUHA cycles

200

2 cycles

100

4 cycles

16 cycles F i g u r e 2. D i p o l a r r o t a t i o n a l sρin-echo 15.1MHz - ^ C nmr s p e c t r a o f p o l y ( o - c h l o r o s t y r e n e ) as a f u n c t i o n o f t h e number o f WAHUHA c y c l e s used d u r i n g d i p o l a r e v o l u t i o n .

3.

SCHAEFER ET AL.

Molecular Motion in Glassy Polystyrenes

47

t r o p y , s o i n p r i n c i p l e , a n echo r e f o c u s e d f o l l o w i n g one r o t o r p e r i o d o f d i p o l a r m o d u l a t i o n s h o u l d have t h e same i n t e n s i t y as one w i t h o u t a n y d i p o l a r m o d u l a t i o n . I n f a c t , t h e f o r m e r echo i s o n l y a t h i r d t o a h a l f as l a r g e . The l o s s e s a r e due p r i m a r i l y t o i n c o m p l e t e H- H d e c o u p l i n g . U s i n g 16 s e p a r a t e c h e m i c a l - s h i f t s p e c t r a ( t h e number o f WAHUHA c y c l e s v a r y i n g f r o m z e r o t h r o u g h f i f t e e n ) , we p e r f o r m e d 1 6 - p o i n t F o u r i e r t r a n s f o r m s u s i n g t h e WAHUHA i r r a d i a t i o n p e r i o d as t h e t i m e v a r i a b l e and peak h e i g h t s as t h e i n t e n s i t y v a r i a b l e s . The r e s u l t i n g transforms f o r the p r o t o n a t e d aromatic carbons o f p o l y s t y r e n e , and o f c r y s t a l l i n e d i m e t h o x y b e n z e n e , a r e shown i n F i g u r e 3. To a r o u g h a p p r o x i m a t i o n , e a c h s p e c t r u m i s a Pake d o u b l e t [4,5] b r o k e n i n t o s p i n n i n g s i d e b a n d s [6,7,9] s e p a r a t e d by 1.894 kHz. Each sideband i s represented by a s i n g l e p o i n t i n t h e f r e q u e n c y domain. I n t e n s i t i e s o f t h e s i d e b a n d s c a n b e r e l i a b l y compared so l o n g a s s i d e b a n d shapes a r e t h e same. 1

1

Comparison Between E x p e r i m e n t a l and C a l c u l a t e d D i p o l a r Sideband Patterns The d i p o l a r p a t t e r n f o r p o l y s t y r e n e shown i n F i g u r e 3 ( l e f t ) i s n o t a Pake d o u b l e t r e f l e c t i n g o n l y t h e CH s t a t i c d i p o l a r c o u p l i n g ( a s s c a l e d b y WAHUHA i r r a d i a t i o n o f t h e p r o t o n s ) . R a t h e r , the p a t t e r n has been narrowed s l i g h t l y by m o l e c u l a r motion. T h u s , t h e p o l y s t y r e n e p a t t e r n h a s more i n t e n s i t y i n t h e z e r o t h and f i r s t s i d e b a n d s t h e n t h a t o f d i m e t h o x y b e n z e n e , and l e s s i n the second sideband. U s i n g t h e methods d e v e l o p e d b y H e r z f e l d and B e r g e r [ 1 0 ] , we c a l c u l a t e d how r i n g m o t i o n o f various types ( a l l r e s t r i c t e d r i n g r o t a t i o n s a t a rate f a s t compared t o t h e d i p o l a r c o u p l i n g ) a f f e c t e d t h e d i p o l a r s i d e b a n d patterns. R e s t r i c t e d r o t a t i o n o f a n a r o m a t i c CH v e c t o r a b o u t t h e r i n g C2 a x i s ( m o t i o n l y i n g o n a 60° c o n e , F i g u r e 4 ) p r o d u c e s a r e d u c t i o n i n i n t e n s i t y o f t h e s e c o n d , and i n c r e a s e s i n t h e z e r o t h and f i r s t d i p o l a r s i d e b a n d s , w i t h r e s p e c t t o t h e s t a t i c p a t t e r n ( T a b l e I , rows 3 and 4 ) . F o r p o l y ( g - i s o p r o p y l s t y r e n e ) , m o t i o n o f t h i s t y p e w i t h a n rms r o t a t i o n a n g l e o f a b o u t 20° p r o d u c e s a r e a s o n a b l e f i t w i t h e x p e r i m e n t ( T a b l e I , rows 2 and 3). I n p a r t i c u l a r , t h e c a l c u l a t e d r a t i o o f i n t e n s i t i e s o f the second t o f i r s t d i p o l a r s i d e b a n d s i s i n agreement w i t h e x p e r i m e n t . T h i s r a t i o i s an i m p o r t a n t parameter t o match s i n c e i t i n v o l v e s t h e r e l a t i v e i n t e n s i t i e s o f t h e l a r g e s t d i p o l a r s i d e b a n d s , a n d so i s f r e e from n o r m a l i z a t i o n u n c e r t a i n t i e s a r i s i n g from e r r o r s i n m e a s u r i n g t h e weak o u t e r s i d e b a n d s . The c h o i c e o f m o t i o n a l model i s n o t c r u c i a l f o r these sorts o f small-angle excursions. Motion o f t h e CH v e c t o r o n a c i r c l e r a t h e r t h a n a 60° cone g i v e s v i r t u a l l y t h e same r e s u l t .

48

NMR

A N D MACROMOLECULES

ά 3

0

-

3

SIDEBAND NUMBER

(crystalline)

3

0

-

3

SIDEBAND NUMBER

F i g u r e 3. E x p e r i m e n t a l d i p o l a r s i d e b a n d p a t t e r n s f o r t h e protonated aromatic carbon o f p o l y s t y r e n e ( l e f t ) and o f c r y s t a l l i n e dimethoxybenzene ( r i g h t ) under m a g i c - a n g l e s p i n n i n g a t 1.894 kHz.

Figure 4. Representation of the rotation of an aromatic CH vector about the r i n g C syirmetry axis through the aziimithal angle ce. 2

3. SCHAEFER ET A l

Table

I

49

Molecular Motion in Glassy Polystyrenes

C o m p a r i s o n s o f C a l c u l a t e d and E x p e r i m e n t a l D i p o l a r R o t a t i o n a l Sideband I n t e n s i t i e s for P o l y ( £ - i s o p r o p y 1 s t y r e n e ) w i t h Magic Angle S p i n n i n g a t 1894 Hz s i d e b a n d number

experiment or motional model observed observed, short-TiCC) component removed calculated, C2 r o l l s , e(rms)=21° calculated, static

0

1

2

3

4

5

.138

.142

.153

.079

.038

.014

.129

.135

.156

.083

.041

.015

.134

.153

.178

.063

.028

.009

.120

.124

.185

.075

.038

.013

Note than the e x p e r i m e n t a l sideband i n t e n s i t i e s f o r p o l y ( g - i s o p r o p y l s t y r e n e ) have b e e n c o r r e c t e d f o r t h e c o n t r i b u t i o n from a few s i t e s a t w h i c h t h e r i n g s a r e u n d e r g o i n g M H z - r a t e 180° flips. The c o n c e n t r a t i o n o f t h e s e s i t e s f o r v a r i o u s p o l y s t y r e n e s c a n be d e t e r m i n e d b y a Ί (C) e x p e r i m e n t , and v a r i e s from 0 t o 1 1 % f o r some t w e l v e p o l y s t y r e n e s e x a m i n e d t o d a t e [ 1 1 ] . C o n f i r m a t i o n t h a t t h e r i n g s a r e f l i p p i n g has b e e n made b y s t u d i e s o f o n - and o f f - C g a x i s c a r b o n c h e m i c a l s h i f t t e n s o r s . F o r our a n a l y s i s h e r e , i t i s p o s s i b l e t o use t h e Τ χ ( C ) r e l a x a t i o n b e h a v i o r as p a r t o f t h e c a r b o n p r e p a r a t i o n s t e p o f t h e p u l s e sequence o f F i g u r e 1, so t h a t d i p o l a r s i d e b a n d p a t t e r n s a r e o b t a i n e d w h i c h a r e f r e e from c o n t r i b u t i o n s from t h e l a r g e - a m p l i t u d e h i g h frequency s i t e s i n the polymer. Or, knowing the c o n c e n t r a t i o n o f f l i p p i n g r i n g s , the a p p r o p r i a t e l y weighted c a l c u l a t e d d i p o l a r p a t t e r n f o r a r i n g u n d e r g o i n g 180° f l i p s c a n be s u b s t r a c t e d from t h e o b s e r v e d p a t t e r n f o r t h e e n t i r e s a m p l e , and t h e d i f f e r e n c e 1

50

NMR

AND

MACROMOLECULES

d i p o l a r spectrum r e n o r m a l i z e d . Only such c o r r e c t e d p a t t e r n s w i l l be d i s c u s s e d i n t h e f o l l o w i n g . Table I I

C a l c u l a t e d R a t i o o f Second t o F i r s t D i p o l a r R o t a t i o n a l S i d e b a n d I n t e n s i t i e s f o r a CH P a i r U n d e r g o i n g M o l e c u l a r M o t i o n and M a g i c A n g l e S p i n n i n g (1894 Hz)

motional model

restricted ring rotation

<

t o t a l azimuthal angular d i s p l a c e ment (deg) 0 20 40 60 80 100 120 140

r i n g 71 f l i p s

n /n! 2

1.50 1.44 1.35 1.16 0.93 0.70 0.53 0.41 0.48

180

a r o m a t i c CH v e c t o r u n d e r g o i n g f a s t d i f f u s i o n a l on t h e s u r f a c e o f a 60° cone.

reorientation

S i n c e the second d i p o l a r s p i n n i n g sideband i s near the c u s p s o f t h e CH Pake d o u b l e t , t h e r a t i o o f i n t e n s i t i e s o f t h e second t o f i r s t sidebands i s a s e n s i t i v e m o n i t o r o f p a r t i a l c o l l a p s e o f t h e d i p o l a r t e n s o r by r e s t r i c t e d m o l e c u l a r m o t i o n . With a choice of s c a l i n g f a c t o r to give a s t a t i c r a t i o of n /n! s i d e b a n d s o f 1.50 [8,11] , t h e e f f e c t o f C a x i s r o t a t i o n o f i n c r e a s i n g a m p l i t u d e i s shown i n T a b l e I I . A t o t a l a n g u l a r e x c u r s i o n o f 120° r e d u c e s η / η by a f a c t o r o f 3, a b o u t t h e r e d u c t i o n a c h i e v e d by 180° f l i p s . The dependence o f t h e n / n i r a t i o on t h e t o t a l a z i m u t h a l a n g u l a r d i p l a c e m e n t i s r o u g h l y l i n e a r f o r d i s p l a c e m e n t s g r e a t e r t h a n a b o u t 20°. 2

2

2

χ

2

S m a l l - A m p l i t u d e Low-Frequency R i n g M o t i o n E v e n a f t e r t h e r e m o v a l o f t h o s e s i t e s engaged i n r i n g f l i p p i n g , m e a s u r a b l e r i n g m o t i o n p e r s i s t s i n p o l y s t y r e n e s . I n g e n e r a l , as shown i n T a b l e I I I , t h e s h o r t e r t h e < T p ( C ) > , t h e s m a l l e r t h e value of [ n / n ] . (The s t a r i n d i c a t e s t h e e x p e r i m e n t a l n / n r a t i o has b e e n c o r r e c t e d t o remove c o n t r i b u t i o n s f r o m r i n g s u n d e r g o i n g M H z - r a t e f l i p s as d e s c r i b e d i n t h e p r e v i o u s s e c t i o n . The f r e q u e n c y o f f l i p p i n g i s so h i g h t h e r e i s no s i g n i f i c a n t c o n t r i b u t i o n t o T p ( C ) . ) The f a c t t h a t b o t h T p ( C ) and [ n ^ n j o show a c o m p a r a b l e i n f l u e n c e o f l o w - f r e q u e n c y m o t i o n more t h a n h i n t s a t a s i m p l e c o n n e c t i o n b e t w e e n them. T h a t i s , we c a n be x

2

1

u

2

x

x

x

3

SCHAEFER ET AL.

TabLe I I I

51

Molecular Motion in Glassy Polystyrenes

,

P r o t o n a t e d A r o m a t i c < T p ( C ) > s o f some P o l y s t y r e n e s S c a l e d b y t h e A m p l i t u d e o f Root-MeanS q u a r e A n g u l a r F l u c t u a t i o n s Deduced f r o m Experimental D i p o l a r R o t a t i o n a l Sideband Intensities 1

37-kHz , msec

2

sin 0 χ <ΤτΡ(0>

a

l P

polymer poly(p-tbutylstyrene)

6.2

1.04

1.11

23

0.89

poly(p-isopropylstyrene)

8.8

1.08

1.15

21

1.06

1.20

1.25

18

polystyrene^ poly(|)-methylstyrene)

11.2

1.00

11.7

1.20

1.25

18

19.0

1.30

1.30

16

poly(styreneco-sulfone)

22.0

1.32

e

1.32

15

poly(o-chlorostyrene)

37.0

1.34

1.34

14

poly(a-methylstyrene)

1.04

1.35

1.38

2.02

s t r a i g h t - l i n e f i t t o o b s e r v e d d e c a y b e t w e e n 0.05 a n d 1.00 msec a f t e r t h e t u r n o f f o f H ^ H ) . r a t i o o f i n t e n s i t i e s o f second t o f i r s t d i p o l a r r o t a t i o n a l sidebands w i t h zero T i p ( C ) decay ( F i g u r e 1 ) . r a t i o o f i n t e n s i t i e s o f second t o f i r s t d i p o l a r r o t a t i o n a l sidebands w i t h c o n t r i b u t i o n s from r i n g s undergoing MHz-rate ^ f l i p s removed. a t a c t i c , quenched h i g h - m o l e c u l a r w e i g h t m a t e r i a l . c o n t r i b u t i o n t o n from t h e non-protonated a r o m a t i c carbon removed a s s u m i n g η / η = .13 f o r t h a t c a r b o n , t h e same r a t i o as i s o b s e r v e d f o r p o l y s t y r e n e . 1

Ί

0

52

N M R A N D MACROMOLECULES

r e a s o n a b l y s u r e t h a t t h e same c o o p e r a t i v e k H z - r e g i m e m o t i o n s r e s p o n s i b l e f o r T i P ( C ) , must a l s o be r e s p o n s i b l e f o r t h e p a r t i a l a v e r a g i n g o f t h e a r o m a t i c CH d i p o l a r t e n s o r . C o r r e l a t i o n Between and n / n i 2

For the p r o t o n a t e d a r o m a t i c carbons o f p o l y s t y r e n e s under r o t a t i o n a l r e o r i e n t a t i o n , we p r o p o s e T x p ( C ) = K s i n ^ ( Θ ) J ( u ) ) , where K i s a c o n s t a n t ( w h i c h i n c l u d e s powder a v e r a g i n g i n t h e s o l i d ) , s i n ( 0 ) i s the average d i p o l a r f l u c t u a t i o n o r t h o g o n a l to the a p p l i e d r f f i e l d ( F i g u r e 5 ) , and J(u)) d e s c r i b e s t h e s p e c t r a l d e n s i t y a s s o c i a t e d w i t h the r i n g motion at the carbon r o t a t i n g frame L a r m o r f r e q u e n c y [12] , i n t h i s i n s t a n c e , 37 k H z . I f we assume r i n g r o t a t i o n o n l y o c c u r s a b o u t t h e r i n g C a x i s , and i f we a l s o assume r i n g J(u)) s f o r a l l p o l y s t y r e n e s a r e t h e same, t h e n t h e r e l a t i v e T i p ( C ) ' s s h o u l d be a s i m p l e f u n c t i o n o f t h e amplitude of the r i n g motion. These a m p l i t u d e s c a n be e s t i m a t e d from t h e r e d u c t i o n i n t h e d i p o l a r CH p a t t e r n s as c h a r a c t e r i z e d b y t h e [ η / η χ ] ο r a t i o s , i f we assume t h a t t h e m o t i o n w h i c h r e d u c e s [n /n ] i s a l s o r e s p o n s i b l e f o r t h e T^p r e l a x a t i o n . The r e s u l t s o f such a comparison f o r seven s u b s t i t u t e d p o l y s t y r e n e s a r e shown i n T a b l e I I I . The p r o d u c t o f s i n ( 6 ) and < T p ( C ) > i s indeed roughly constant f o r a l l seven polymers. The p r o d u c t f o r the f i r s t s i x polymers i n T a b l e I I I i s constant t o w i t h i n about 50% e v e n t h o u g h t h e < T i p ( C ) > ' s t h e m s e l v e s v a r y b y a f a c t o r o f 4 . 2

2

2

2

1

2

2

1

0

2

x

Conclusions for Ring Rotations i n Polystyrenes The r i n g r o t a t i o n s g e n e r a t e t o t a l a n g u l a r d i s p l a c e m e n t s o f about 4 0 ° ( f o r s u b s t i t u e n t s i n the o r t h o p o s i t i o n ) t o 70° ( f o r b u l k y n o n - p o l a r s u b s t i t u e n t s i n the para p o s i t i o n ) . C o n s t r a i n t s on t h i s m o t i o n p r o b a b l y i n v o l v e b o t h i n t r a - c h a i n s t e r i c i n t e r a c t i o n s ( f o r an o - c h l o r o s u b s t i t u e n t ) and i n t e r chain packing (for polystyrene i t s e l f ) . We s u s p e c t t h e f r e q u e n c i e s o f many o f t h e s m a l l - a m p l i t u d e r i n g r o t a t i o n s a r e d e t e r m i n e d b y f l u c t u a t i o n s a r i s i n g from a l t e r a t i o n s i n l o c a l i n t e r c h a i n p a c k i n g . S i n c e t h e p a c k i n g changes when t h e m a i n c h a i n s move, t h e f r e q u e n c i e s o f s m a l l - a m p l i t u d e r i n g r o t a t i o n s and m a i n - c h a i n r e o r i e n t a t i o n s a r e c o m p a r a b l e . Thus, r i n g s u b s t i t u e n t s are important i n determining the amplitudes but not the frequencies o f these types o f c o o p e r a t i v e r i n g r o t a t i o n s . T h i s i s c o n s i s t e n t w i t h our assumption t h a t the J(u)) s f o r a l l p o l y s t y r e n e s a r e t h e same. We have c h o s e n m o t i o n a b o u t t h e C a x i s t o m o d e l r i n g motion i n polystyrenes. We a c k n o w l e d g e t h a t t h e s e m o t i o n s a r e l i k e l y t o be more c o m p l i c a t e d t h a n j u s t C rotations. However, the motions are s m a l l a m p l i t u d e . Small-amplitude w i g g l i n g c a n e q u a l l y w e l l be m o d e l e d b y a r o m a t i c CH m o t i o n on a c i r c l e or sphere, or by C r o t a t i o n s . T h u s , any o f t h e s e f

2

2

2

SCHAEFER ET AL.

Molecular Motion in Glassy

Polystyrenes

for relative comparisons of polystyrene aromatic (7^(C))s =

2

2

K sm ejia>)

θ measures f lucmaobn amplitude due to molecular motion Κ mehides powder averaging no rcLxxxliorLfor this orientation. j-l^

effective relaxation, for this orientation

M A S reduces o r k n u t i o n a i d i s p e r s i o n of rates

F i g u r e 5. G e o m e t r i c a l c o n s i d e r a t i o n s f o r a C H v e c t o r g i v i n g r i s e t o T^p(C) r e l a x a t i o n i n a s o l i d by r e s t r i c t e d r o t a t i o n a l r e o r i e n t a t i o n .

54

N M R A N D MACROMOLECULES

m o d e l s c a n be c o n s i d e r e d t o r e p r e s e n t a p p r o x i m a t e l y r o t a t i o n a l e x c u r s i o n s b y a r i n g as i t t r a c k s i t s a t t a c h e d m a i n c h a i n t h r o u g h complicated motions. We a l s o r e c o g n i z e t h a t p o l y s t y r e n e s a r e d y n a m i c a l l y h e t e r o geneous ( e v e n i g n o r i n g r i n g f l i p p e r s ) . T h u s , b o t h T p ( C ) s and a n g u l a r d i s p l a c e m e n t f l u c t u a t i o n p a r a m e t e r s must r e f l e c t a v e r a g e s over the e n t i r e sample. O f c o u r s e , t h e o b s e r v e d i s t h e weighted average o f a l l the T p ( C ) ' s p r e s e n t [ 1 ] . S i n c e the [ n 2 / n ] ο r a t i o has an a p p r o x i m a t e l y l i n e a r dependence on t o t a l a n g u l a r d i s p l a c e m e n t (and i s n o t c r i t i c a l l y m o d e l d e p e n d e n t ) , t h e o b s e r v e d rms f l u c t u a t i o n s p a r a m e t e r , Θ i s a l s o a s i m p l e weighted average. I n p a r t , the c o r r e l a t i o n of Table I I I ( b e t w e e n < T p ( C ) > and s i n 0 ) s u c c e e d s b e c a u s e we i g n o r e d e t a i l s of the d i s t r i b u t i o n s of motions. T h u s , w h e t h e r p o l y s t y r e n e has a f r a c t i o n of mobile r i n g s w i t h the remainder l e s s m o b i l e , or w h e t h e r a l l r i n g s have an i n t e r m e d i a t e m o b i l i t y , becomes immaterial. B o t h s i t u a t i o n s r e s u l t i n comparable average v a l u e s f o r < T p ( C ) > and Θ. The c o r r e l a t i o n o f T a b l e I I I f a i l s t o t h e extent there remain l a r g e - a m p l i t u d e high-frequency motions which r e d u c e [ ^ / n ^ o b u t do n o t c o n t r i b u t e t o T p ( C ) , o r 5 - 1 0 d e g r e e s m a l l - a m p l i t u d e l o w - f r e q u e n c y m o t i o n s w h i c h c a n s t i l l make s i g n i f i c a n t c o n t r i b u t i o n s t o b u t have o n l y a m i n o r e f f e c t on [ n / n ] . 1

x

x

1

1

2

x

x

x

x

2

1

0

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

J . Schaefer, E. O. Stejskal, T. R. Steger, M. D. Sefcik and R. A. McKay, Macromolecules, 13, 1121 (1980). J . Schaefer, M. D. Sefcik, E. O. Stejskal and R. A. McKay, Macromolecules, 14, 280 (1981). U. Haeberlen, "High Resolution NMR in Solids: Selective Averaging," (Adv. Magn. Reson., Suppl. 1), Academic Press, New York, 1976. R. K. Hester, J. L. Ackerman, B. L. Neff, and J. S. Waugh, Phys. Rev. Letters, 36, 1081 (1976). M. E. Stoll, A. J. Vega, and R. W. Vaughan, J. Chem. Phys. 65, 1093 (1976). M. G. Munowitz, R. G. G r i f f i n , G. Bodenhausen, and T. H. Haung, J. Am. Chem. Soc., 103, 2529 (1981). M. G. Munowitz and R. G. G r i f f i n , J . Chem. Phys., 76, 2848 (1982). J . Schaefer, R. A. McKay, E. O. Stejskal, and W. T. Dixon, J. Mag. Reson., 52, 123 (1983). M. Maricq and J. S. Waugh, J. Chem. Phys., 70, 3300 (1979). J. Herzfeld and A. E. Berger, J. Chem. Phys., 73, 6021 (1980). J . Schaefer, M. D. Sefcik, E. O. Stejskal, R. A. McKay, W. T. Dixon, and R. E. Cais, Macromolecules, March, 1984. A. Abragam, "The Principles of Nuclear Magnetism," Oxford University Press, London, 1961, p. 565.

RECEIVED November 18, 1983

4 2

Solid State H NMR Studies of Molecular Motion Poly(butylene terephthalate) and Poly(butylene terephthalate)Containing Segmented Copolymers L. W. JELINSKI and J. J. DUMAIS Bell Laboratories, Murray Hill, NJ 07974 A. K. ENGEL Ε. I. du Pont de Nemours and Company, Wilmington, DE 19898

Solid state deuterium NMR spectroscopy is used to provide information concerning the motional heterogeneity and homogeneity of segmented co-polyesters containing poly(butylene terephthalate) as the hard segment. The results presented here provide the first clear evidence that there are two distinct motional environments for the hard segments in the co-polyester. One of the environments is identical to that observed in the poly(butylene terephthalate) homopolymer, in which Helfand-type motions about three bonds (Helfand, E. J. Chem. Phys. 1971, 54, 4651) occur with a correlation time of 7 x 10 s at 20°C. The other motional environment is more-nearly isotropic. The residues in the mobile environment are attributed to short blocks of hard segments residing in the soft segment matrix, or to hard segments forming the loop regions in the poly(butylene terephthalate) lamellae. Approximately 10% of the hard segments reside in the mobile environment in the segmented copolymer with 0.87 mole fraction of poly(butylene terephthalate) hard segments. -6

Poly (butylène terephthalate) has been used as a model for observing motions about three bonds as an isolated motional mode in polymers. Early carbon N M R studies involving carbon N M R relaxation data (1,2) and carbon chemical shift anisotropy considerations (3) showed that the terephthalate residues can be considered static in comparison to the motions exhibited by the alkyl residues. Furthermore, the alkyl portion of poly (butylène terephthalate) contains the shortest sequence that is able to undergo motions about three bonds (4), with the terephthalate groups acting as "molecular anchors" to prevent the longer range motional modes. Poly (butylène terephthalate) is thus ideally set up to undergo three-bond types of motions. Solid state deuterium N M R spectroscopy was used to study these motions in detail (5), using the selectively labeled polymer: 0097 6156/ 84/ 0247 0055506.00/0 © 1984 American Chemical Society

56

N M R

A N D

MACROMOLECULES

C ~0-CH CD CD CH -Of2

2

2

2

X

The temperature-dependent spectra were interpreted in terms of a two-site hop model, in which the deuterons undergo jumps through a dihedral angle of 1 0 3 ° . This type of motion is consistent with gauche-trans conformational transitions. At -88 °C these motions appear static on the time scale of the deuterium N M R experiment, and at +85 ° C the motions are in the fast exchange limit. The rate constants for these motions were obtained from the calculated spectra. A n Arrhenius plot of these data show that the apparent activation energy is 5.8 kcal/mol. (Dynamic mechanical data (20 Hz) fall on the Arrhenius plot.) The transitions have an intermediate rate on the deuterium N M R time scale at 20 °C, with the correlation time for the motion being 7 x 1 0 s at this temperature. -6

The solid state deuterium N M R results for the selectively labeled poly (butylène terephthalate) homopolymer have been interpreted in terms of various models for polymer motion (5). The data are consistent with the models proposed by Helfand (6), in which counter rotation occurs about second neighbor parallel bonds. Termed gauche migration and pair gauche production, these motions are illustrated in Figure la and b, respectively. Gauche migration consists of a transition from g tt to ttg , and is part of the pathway for pair gauche production. In this latter motional mode, a ttt sequence goes to g tg . These motional models allow gauche-trans conformational transitions to occur without concomitant large-scale reorientation of the ends of the polymer chain. In addition, they require slightly more than one C - C bond rotational barrier height, and are thus consistent with the apparent activation energy of 5.8 kcal/mol. ±

±

±

T

Having established the rates and types of motion that occur in the poly (butylène terephthalate) homopolymer (5), it is of interest to extend these studies to the investigation of the types of motions that occur in segmented copolymers that contain poly (butylène terephthalate) as the hard segment. The structure of such a polymer is shown below, where m and η refer to the "hard" and "soft" segments, respectively. (This structure also illustrates the sites of the deuterium labels.)

4c

4.

JELINSKI ET AL.

2

Solid State H NMR Studies of Molecular Motion

57

The poly (butylène terephthalate) "hard" segments and the poly (tetramethyleneoxy) terephthalate "soft" segments are not completely miscible and thus lead to phase separation. However, in contrast to the discrete domain structures found in polystyrene — polybutadiene or polystyrene — polyisoprene, for example, the hard and soft segments of this copolyester are more intimately dispersed. This leads to a hard segment morphology that has been described as "continuous and interpenetrating lamellae." Such a morphology is shown in schematic form in Figure 2. This copolymer thus presents a unique opportunity to study a system in which there is a large interfacial area. The specific questions to be addressed by solid state deuterium N M R studies of this polymer are the following: (1) Are the motions of the C D groups in the segmented copolymer identical to the motions observed in the poly (butylène terephthalate) homopolymer? (2) What effect do variations in the hard:soft content ratio have on the motions of the C D groups? And (3), is there evidence for hard segments which reside in the soft segment matrix? If so, what per cent of them are in non-poly (butylène terephthalate) -like environments? 2

2

EXPERIMENTAL The selectively labeled poly (butylène terephthalate) -containing segmented copolymers were prepared according to literature methods (7), using 2,2,3,3-d butylene glycol (Merck) as the starting diol. The polymers used in this study correspond to m:n ratios (see the structure, above, for the meaning of m and n) of 24:1 and 7:1. The mole fractions of hard segments are 0.96 and 0.87, respectively, corresponding to 81 and 57 weight per cent of hard segments. The polymers were characterized by thermal measurements and by solution state deuterium and carbon N M R spectroscopy (8). No end groups were observed by carbon spectroscopy, and the deuterium N M R spectra in solution attested to the integrity of the labeling pattern. 4

The samples for N M R spectroscopy were melted into glass tubes and allowed to cool from the melt. The observed deuterium N M R spectra are reproducible with temperature cycling, thus providing evidence that the thermal history induced by acquiring temperature-dependent spectra of the samples does not greatly affect the properties that we are measuring. The solid state deuterium N M R spectra were recorded on a home-built spectrometer operating at 55.26 M H z for deuterium (360 M H z for protons). The spectrometer has been described previously (5). Routine spectra were obtained in quadrature using the quadrupolar echo pulse sequence (Figure 3) ( 9 0 - t - 9 0 - t ) (9-11), 4K data points, a digitization rate of 200 ns/point (5 M H z ) , and a 4.3 #s 90 degree pulse width. Unless otherwise noted, the length of t was generally set at 30 μ&. The value of t was set several microseconds shorter than the time needed to start digitization at the top of the quadrupolar echo maximum. After data accumulation, the FID was left-shifted by the correct number of points, so that for each spectrum, the part of the FID which was transformed began at the exact top of the echo. The quadrupolar echo pulse sequence is used to circumvent problems with receiver recovery times. The solid ± x

1

y

2

x

2

58

N M R A N D

MACROMOLECULES

Figure 1. Helfand-type motions about three bonds (6). The segments represented here correspond to the —OCH2CD2CD2CH2O— portion of the poly (butylène terephthalate) hard segments. These motions are consistent with the solid state deuterium N M R data for poly (butylène terephthalate) (5).

Figure 2. Schematic representation of continuous and interpenetrating hard segment lamellae. The hard segment blocks that are too short to crystallize are shown in the soft segment matrix.

4.

JELINSKI ET AL.

2

Solid State H NMR Studies of Molecular Motion

59

state deuterium N M R spectra are very broad (ca. 250 kHz), and thus the free induction decay signal dies away very rapidly. The quadrupolar echo pulse sequence causes the magnetization to refocus while the radio frequency circuits have time to recover from the transmitter pulses. Inversion-recovery deuterium N M R spectra were obtained by performing a 180°-r-90° pulse sequence, followed by the quadrupolar echo sequence (12). Spin lattice relaxation times were estimated from the null points in the inversionrecovery spectra. RESULTS AND DISCUSSION In Figure 4, the solid state deuterium N M R spectrum for poly(butylene terephthalate) at 20 °C is compared to the spectra for the segmented copolymers, also obtained at 20 °C. The spectrum of poly (butylène terephthalate) (Figure 4a) can be simulated by assuming the C - D bond jumps between two sites separated by a dihedral angle of 1 0 3 ° , with a rate constant of 1.4 x 10 s" . A quadrupolar splitting (A*>) of 124 kHz is used for this calculation (5). (The calculated deuterium spectrum is obtained by also taking into account pulse power fall-off as a function of frequency (13) and the distortions that arise when motions occur during the quadrupolar echo delay time (14,15).) Figure 4b shows the deuterium N M R spectrum of the segmented copolymer with 0.96 mole fraction hard segments, and below it in Figure 4c is the spectrum of the segmented copolymer with 0.87 mole fraction hard segments. The spectra of the segmented copolymers are clearly different from the spectrum of poly (butylène terephthalate), and suggest, particularly in the case of the softest segmented copolymer (Figure 4c), that there are at least two motional environments for the hard segments in the copolymers. 5

1

q

Inversion-recovery solid state deuterium N M R spectra can be used to show that the spectrum of the copolymer with 0.87 mole fraction of hard segments is composed of at least two components. Figure 5 shows such spectra. At an inversion-recovery delay time of 50 ms, the broad poly (butylène terephthalate)-like part of the line is almost nulled, yet the sharp component is positive (Figure 5b). At shorter inversion-recovery delay times, the sharp component goes through its null, and the broad component is negative. Inversion-recovery spectra such as these indicate that the solid state deuterium N M R spectra for the segmented copolymers shown in Figures 4b and 4c are composed of two components with different T values. The Ί of the sharp component in the copolymer with 0.87 mole fraction hard segments is estimated to be 10 ms, and that of the broad component is approximately 60 ms. The difference in these deuterium T values is a factor of six, and indicates that the deuterons that give rise to the broad component are in markedly different motional environments from those that give the sharp line. x

γ

{

It is noteworthy that the sharp component gives a signal with the quadrupolar echo pulse sequence. The observation of a quadrupolar echo can be interpreted to indicate that the constraints on the motion of the deuterons persist for a long time (16). Although the deuterium line for the T = 10 ms component is sharp, the observation of a quadrupolar echo proves that the line is not isotropically mobile on the deuterium N M R timescale. The sharp component also gives a spectrum with a r

60

N M R

A N D

MACROMOLECULES

Figure 3. The quadrupole echo pulse sequence. The time t is usually set at 30 /us, and t is approximately 25 MS. {

2

(a)

(b)

(c)

-100

0

100

kHz Figure 4. Solid state deuterium N M R spectra of (a) poly (butylène terephthalate) ; (b) segmented co-polyester containing 0.96 mole fraction of poly (butylène terephthalate) hard segments; and (c) segmented copolymer containing 0.87 mole fraction hard segments. (See text for specific deuterium labeling patterns.) A l l spectra were obtained with the quadrupole echo pulse sequence at 2 0 ° C and 55.26 M H z , using 30 MS at the t quadrupole echo delay time. {

4.

JELINSKI

ETAL.

2

Solid State H NMR Studies of Molecular

Motion

61

standard 90° — τ pulse sequence using a 30 MS receiver dead time (spectrum not shown). The width of this line at half-height is approximately 5 kHz. The spectra shown in Figure 5 clearly indicate that there are two distinct motional environments in the segmented copolymers. The broad component of the spectrum arises from the majority of the hard segments which show motional characteristics similar to that of poly (butylène terephthalate). The sharp component is attributed to hard segments which reside in the soft segment matrix, either by virtue of being very short blocks, or because they form loops on the surfaces of the hard segment lamellae. It is of interest to estimate the amount of hard segment which gives the sharp line, and further, to determine if the poly (butylène terephthalate) -like broad line represents deuterons which undergo motions which are identical to those observed in the poly (butylène terephthalate) homopolymer. These points are addressed for the copolymer containing 0.87 mole fraction hard segments, by the solid state deuterium N M R spectra shown in Figure 6. In Figure 6a is shown the quadrupole echo deuterium N M R spectrum of the segmented copolymer with 0.87 mole fraction of hard segments. Adjacent to it in Figure 6b, is the corresponding calculated spectrum. The dashed line represents the sum of the sharp and broad components. The sharp line is Lorentzian, and the broad line is Gaussian. The broad line is calculated assuming a quadrupolar splitting (Δί/ ) of 124 kHz, a two-site dihedral hop angle of 1 0 3 ° , and a rate constant for the hop of 1.4 x 10 s . Corrections have been made for pulse power fall-off (13) and for distortions that arise when motions occur during the quadrupolar echo pulse sequence (14,15). This calculation indicates that the sharp and broad components are present in an approximately 1:10 ratio. ς

5

_1

The spectrum shown in Figure 6c illustrates that the motions of the C D groups of poly (butylène terephthalate) in the segmented copolymer are identical to the motions in the homopolymer. This spectrum is a difference spectrum, obtained by subtracting the spectrum of the poly (butylène terephthalate) homopolymer (Figure 4a) from the spectrum of the segmented copolymer (Figure 4c). The difference spectrum (Figure 6c) shows an excellent null for the broad part of the deuterium N M R pattern. This result indicates that the motions of approximately 90% of the hard segment C D deuterons in the segmented copolymer are wellrepresented by the motions observed for the poly (butylène terephthalate) homopolymer. 2

2

It is emphasized that the semi-quantitative treatment above provides only an estimate of the amounts of hard segments in the two motional environments. A more-quantitative answer is probably unlikely, due to the quadrupolar echo pulse sequence necessary to obtain the data. The quadrupolar echo pulse sequence preserves the inhomogeneously broadened part of the solid state deuterium N M R pattern. Homogeneous T effects will cause some signal loss during the quadrupolar echo delay times. In comparing sharp and broad components such as those shown in Figure 6b, it is also necessary to know that both components have approximately the same homogeneous T . 2

2

N M R A N D

62

^

(a) (b)

A ,

1 1

1 1 1

I

-200

'~T~

^

'

1 1

-100

MACROMOLECULES

τ-, . I I . ι I !" 100

0

200

kHz Figure 5. Inversion-recovery solid state deuterium N M R spectra of the segmented copolymer containing 0.87 mole fraction hard segments. The spectra were obtained with a 1 8 0 ° — 1 - 9 0 ° — t j — 9 0 ° — 1 pulse sequence. The spectrum in (b) was obtained with a t of 50 ms. 3

±x

y

2

3

kHz Figure 6. Experimental (a), calculated (b), and difference (c) solid state deuterium N M R spectra for the segmented copolymer with 0.87 mole fraction of hard segments. The spectrum in (a) was obtained at 55.26 M H z and 20 °C, using the quadrupole echo pulse sequence. The dashed line in (b) represents the sum calculated for the broad and narrow components in a respective 10:1 ratio. (See text for details of the calculation.) The spectrum in (c) is the difference spectrum obtained by subtracting the spectrum of poly (butylène terephthalate) (Figure 4a) from the segmented copolymer spectrum (Figure 6a).

2

4. J E L I N S K I E T A L .

Solid State H NMR Studies of Molecular Motion

63

Homogeneous T effects can be studied by obtaining quadrupolar echo N M R spectra as a function of the quadrupolar echo delay times, t and t . Such experiments have been performed for all samples. The results indicate that the observed lineshapes are not markedly dependent upon the quadrupolar echo delay times, although signal intensity clearly is lost as the delay times are increased. Figure 7 shows a plot of the relative signal intensity versus quadrupolar echo delay time for the solid state deuterium N M R spectra of the segmented copolymer containing 0.87 mole fraction hard segments. This figure shows that the relative intensity is a linear function of the quadrupolar echo delay times. When extrapolated back to zero time, Figure 7 illustrates that approximately 2 5 % of the signal intensity is lost at the usual quadrupolar echo delay time of 30 MS. However, the lack of distortion in these spectra indicates that representative patterns are obtained at a quadrupolar echo delay time of 30 MS. 2

x

2

SUMMARY The results presented here show that there are two distinct motional environments for the "central" deuterons of poly (butylène terephthalate) in the segmented copolyesters containing poly (butylène terephthalate) as the hard segment. One of the motional environments is identical to that observed in the poly (butylène terephthalate) homopolymer, in which Helfand-type motions (6) about three bonds occur with a correlation time of 7 x 10~ s at 20 °C. The Ί for these deuterons is ca. 60 ms at 20 °C. Approximately 90% of the hard segments reside in these organized lamellar environments in the segmented copolymer with 0.87 mole fraction hard segments. 6

ι

The other 10% of the hard segments in this copolymer undergo motions that are more-nearly isotropic. The T for the "central" deuterons in the mobile regions is approximately 10 ms at 2 0 ° C . The mobile residues are attributed to short blocks of hard segments residing in the soft segment matrix, or to hard segments forming the loop regions of the poly (butylène terephthalate) lamellae. As the mole fraction of hard segments is increased from 0.87 to 0.96, a lower per cent of the hard segments are found to reside in mobile environments. t

Taken together, these solid state deuterium N M R experiments provide otherwise unobtainable answers to questions concerning motional homogeneity and heterogeneity in these segmented copolymer systems.

64

N M R A N D

20

60

MACROMOLECULES

100

QUADRUPOLAR ECHO DELAY TIME (ps) Figure 7. A plot of the relative intensity of the quadrupole echo deuterium N M R signal versus the echo delay time, tj, for the segmented copolymer containing 0.87 mole fraction hard segments. The line is extrapolated back to zero time.

4.

JELINSKI ET AL.

2

Solid State H NMR

Studies of Molecular Motion

65

ACKNOWLEDGEMENT We are grateful to Dr. F. A . Bovey for his interest and encouragement in this project.

LITERATURE CITED 1. Jelinski, L. W.; Dumais, J. J. Polym. Prepr., Am. Chem. Soc. Div. Polym. Chem. 1981, 22 (2), 273. 2.

Jelinski, L. W.; Dumais, J. J.; Watnick, P. I.; Engel, A. K.; Sefcik. M. D. Macromolecules 1983, 16, 409.

3.

Jelinski, L. W. Macromolecules 1981, 14, 1341.

4.

Schatzki, T. F. Polym. Prepr., Am. Chem. Soc. Div. Polym. Chem. 1965, 6, 646.

5.

Jelinski, L. W.; Dumais, J. J.; Engel, A. K. Macromolecules 1983, 16, 492.

6.

Helfand, E. J. Chem. Phys. 1971, 54, 4651.

7.

Wolfe, J. R., Jr. Polym. Prepr., Am. Chem. Soc. Div. Polym. Chem. 1978, 19 (1), 5.

8.

Jelinski, L. W.; Schilling F. C.; Bovey, F. A. Macromolecules 1981, 14, 581.

9.

Davis, J. H.; Jeffrey, K. R.; Bloom, M.; Valic, M. I.; Higgs, T. P. Chem. Phys. Lett. 1976, 42, 390.

10.

Blinc, R.; Rutar, V.; Seliger, J.; Slak, J.; Smolej, V. Chem. Phys. Lett. 1977, 48, 576.

11.

Hentschel, R.; Spiess, H. W. J. Magn. Resonance 1979, 35, 157.

12.

Batchelder, L. S.; Niu, C. H., Torchia, D. A. J. Am. Chem. Soc. 1983, 0000.

13.

Bloom, M.; Davis, J. H.; Valic, M. I. Can. J. Phys. 1980, 58, 1510.

14. 15.

Spiess, H. W.; Sillescu, H. J. Magn. Resonance 1981, 42, 381. Mehring, M. In "NMR-Basic Principles and Progress", Diehl, P.; Fluck, E.; Kosfeld, R.; Eds.; Springer-Verlag: New York, 11, 1976.

16.

Hentschel, D.; Sillescu, H.; Spiess, H. W. Macromolecules 1981, 14, 1605.

RECEIVED

December 10, 1983

5 Spin Relaxation and Local Motion in a Dissolved Aromatic Polyformal M. F. TARPEY, Y.-Y. LIN, and ALAN ANTHONY JONES Jeppson Laboratory, Department of Chemistry, Clark University, Worcester, MA 01610 P. T. INGLEFIELD Department of Chemistry, College of the Holy Cross, Worcester, MA 01610

Carbon-13 and proton spin-lattice relaxation times are reported for 10 wt% solutions of a dissolved aromatic polyformal. The relaxation times for both nuclei were determined at two Larmor frequencies and as a function of temperature from 0 to 120°C. These relaxation times are interpreted i n terms of segmental motion and anisotropic internal rotation. Segmental correlation functions by both Jones and Stockmayer, and Weber and Helfand were used to interpret the data. Internal rotation i s described by the usual Woessner approach, and restricted rotational diffusion, by the Gronski approach. Both segmental correlation functions lead to similar results; but, relative to the analogous polycarbonate, single bond conformational transitions are more frequent i n the polyformal. The phenyl groups i n the backbone undergo segmental rearrangements and internal anisotropic rotation at comparable rates. Motion in the formal linkage i s described by the same segmental correlation times plus restricted rotational diffusion about an axis between the oxygens of the formal group. The interpretation at the formal l i n k based on restricted rotational diffusion is d i s cussed i n terms of the conformations l i k e l y i n the l i n k which are commonly referred to as the anomeric effect. The choice of the axis of restricted rotation i n the formal unit i s only an approximation of the result of anisotropic single bond conformational t r a n s i tions occurring within that unit. 0097 6156/84/0247-0067S06.00/0 © 1984 American Chemical Society

68

NMR

A N D MACROMOLECULES

S p i n r e l a x a t i o n i n d i l u t e s o l u t i o n has been employed t o c h a r a c t e r i z e l o c a l c h a i n motion i n s e v e r a l polymers w i t h aromatic back bone u n i t s . The two g e n e r a l t y p e s examined so f a r a r e p o l y p h e n y l e n e o x i d e s (1-2) and a r o m a t i c p o l y c a r b o n a t e s ( 3 - 5 ) ; and t h e s e two t y p e s a r e t h e most common h i g h i m p a c t r e s i s t a n t e n g i n e e r i n g plastics. The p o l y m e r c o n s i d e r e d i n t h i s r e p o r t i s an a r o m a t i c p o l y f o r m a l ( s e e F i g u r e 1) where t h e a r o m a t i c u n i t i s i d e n t i c a l t o t h a t o f one o f t h e p o l y c a r b o n a t e s . T h i s p o l y m e r has a s i m i l a r d y namic m e c h a n i c a l s p e c t r u m t o t h e i m p a c t r e s i s t a n t p o l y c a r b o n a t e s ( 6 ) and i s t h e r e f o r e an i n t e r e s t i n g s y s t e m f o r c o m p a r i s o n o f c h a i n dynamics. I n a d d i t i o n , t h e f o r m a l u n i t i t s e l f o f f e r s a new o p p o r t u n i t y for monitoring chain motion r e l a t i v e to the polycarbonates s i n c e t h e c a r b o n a t e u n i t c o n t a i n s no p r o t o n s . The s p i n - l a t t i c e r e l a x a t i o n t i m e s , T j s o f a l l p r o t o n s and a l l c a r b o n s w i t h d i r e c t l y bonded p r o t o n s a r e r e p o r t e d f o r t h e p o l y f o r m a l . A l s o t h e c a r b o n and p r o t o n T j ' s a r e measured a t two d i f f e r e n t Larmor f r e q u e n c i e s t o expand t h e f r e q u e n c y r a n g e c o v e r e d by t h e s t u d y . In a d d i t i o n t o d e t e r m i n i n g the time s c a l e s f o r s e v e r a l l o c a l m o t i o n s i n p o l y f o r m a l , two d i f f e r e n t i n t e r p r e t a t i o n a l models f o r s e g m e n t a l m o t i o n w i l l be e m p l o y e d . An o l d e r model by J o n e s and S t o c k m a y e r (7 ) , based on t h e a c t i o n o f a t h r e e bond jump on a t e t r a h e d r a l l a t t i c e i s compared w i t h a new model by Weber and H e l f a n d ( 8 ) , based on computer s i m u l a t i o n s o f p o l y e t h y l e n e t y p e chains. These two models f o r s e g m e n t a l m o t i o n have been compared b e f o r e ( 5 ) f o r two p o l y c a r b o n a t e s b u t somewhat d i f f e r e n t r e s u l t s are seen i n the p o l y f o r m a l i n t e r p r e t a t i o n . 1

Experimental H i g h m o l e c u l a r w e i g h t samples o f t h e p o l y f o r m a l were k i n d l y s u p p l i e d by G e n e r a l E l e c t r i c . The s t r u c t u r e o f t h e r e p e a t u n i t i s shown i n F i g u r e 1 a s w e l l as t h e s t r u c t u r e o f a p a r t i a l l y d e u t e r a t e d f o r m w h i c h was s y n t h e s i z e d (9) t o r e d u c e p r o t o n c r o s s relaxation. A 10 w e i g h t p e r c e n t s o l u t i o n o f t h e p o l y m e r i n d e u t e r a t e d t e t r a c h l o r o e t h a n e was p r e p a r e d i n an NMR t u b e , s u b j e c t e d t o f i v e f r e e z e , pump, thaw c y c l e s and s e a l e d . S p i n l a t t i c e r e l a x a t i o n measurements were c o n d u c t e d on two s p e c t r o m e t e r s w i t h a s t a n d a r d π-τ-π/2 p u l s e s e q u e n c e . The 30 and 90 MHz p r o t o n measurements as w e l l as t h e 22.6 MHz c a r b o n - 1 3 meas u r e m e n t s were made on a B r u k e r SXP 20-100. The 250 MHz p r o t o n and 62.9 c a r b o n - 1 3 measurements were made on a B r u k e r WM-250. Results S p i n l a t t i c e r e l a x a t i o n times a r e c a l c u l a t e d from the r e t u r n o f t h e m a g n e t i z a t i o n t o e q u i l i b r i u m u s i n g a l i n e a r and n o n - l i n e a r l e a s t s q u a r e s a n a l y s i s o f t h e d a t a . The two a n a l y s e s y i e l d T| v a l u e s w i t h i n 10% o f e a c h o t h e r and a v e r a g e v a l u e s a r e r e p o r t e d . No e v i d e n c e o f c r o s s - r e l a x a t i o n o r c r o s s - c o r r e l a t i o n were o b s e r v e d

5.

T A R P E Y ETA L ,

Spin Relaxation

Cl

and Local

Motion

69

Cl

\/ c —c—

Cl

ν

H

H

\ / 0— c — 0 —

Cl

S

D D

O-i-<0>-<>-<-°F i g u r e 1. S t r u c t u r e o f t h e p o l y f o r m a l r e p e a t u n i t and t h e p a r t i a l l y deuterated analogue.

70

NMR

A N D MACROMOLECULES

i n t h e f o r m o f n o n l i n e a r b e h a v i o r when t h e r e t u r n o f t h e magne t i z a t i o n was p l o t t e d i n t h e s t a n d a r d f r o m 1η(Αοο-Α ) v e r s u s τ. The p r e s e n c e o f c r o s s - r e l a x a t i o n i n t h e p r o t o n d a t a was f u r t h e r c h e c k ed by c o m p a r i n g t h e p h e n y l p r o t o n T j i n t h e p a r t i a l l y d e u t e r a t e d a n a l o g u e w i t h t h e p h e n y l T| i n t h e f u l l y p r o t o n a t e d p o l y m e r . The p h e n y l p r o t o n T j i s a b o u t 10% l o n g e r i n t h e d e u t e r a t e d p o l y m e r i n d i c a t i n g a s m a l l amount o f c r o s s - r e l a x a t i o n t h o u g h t h e 10% change i s e s s e n t i a l l y t h e same a s t h e e x p e r i m e n t a l u n c e r t a i n t y . A 10% u n c e r t a i n t y i s p l a c e d on a l l T j v a l u e s , r e f l e c t i n g e r r o r c o n t r i b u t i o n s f r o m c o n c e n t r a t i o n , t e m p e r a t u r e and p u l s e w i d t h s a s w e l l a s random f l u c t u a t i o n s i n i n t e n s i t y . T a b l e I c o n t a i n s p r o t o n and c a r b o n - 1 3 T j s as a f u n c t i o n o f t e m p e r a t u r e and L a r m o r f r e q u e n c y . τ

1

Interpretation 1

f

The s t a n d a r d r e l a t i o n s h i p s between T j s and s p e c t r a l d e n s i t i e s J s are employed. F o r carbon-13, the e x p r e s s i o n s a r e ( 4 ) l/Ίι

W

= W

=

0

+ 2W

0

EY 2Y 2h2j ( C

H

+ W

1C

1

a ) ( )

2

)/20r 6 i

j W

(la) 2

2

2

= E3Y Y h J (u) )/40rj6

lc

C

H

1

C

j W

2

2

=

2

2

23Y Y h J (w )/10rj6 c

H

2

2

j U)Q

and

f o r protons 1/Ti

= u)|j -

(ùq

ω

2

=

+

U)ç

i t is

= 2(9/8)Y hr -6[( /i5)j ( 4

j

2

1

a ) H

) + (8/15)J (2a> )] 2

H

(lb)

j The i n t e r n u c l e a r d i s t a n c e s employed a r e 1.095 Â f o r t h e p h e n y l C-H d i s t a n c e , 1.125 Â f o r t h e f o r m a l C-H d i s t a n c e , 2.4 Â f o r t h e 2-3 p h e n y l p r o t o n d i s t a n c e , and 1.75 Â f o r t h e f o r m a l p r o t o n - p r o t o n distance. The 2-3 p h e n y l p r o t o n d i s t a n c e used h e r e i s c o m p a r a b l e t o t h e d i s t a n c e o f 2.41 Â u s e d i n t h e p o l y c a r b o n a t e i n t e r p r e t a tions. The c h o i c e o f 2.4 Â i s based on t h e p h e n y l p r o t o n T| m i n i mum and t h e s l i g h t l y s m a l l e r v a l u e i s c o n f i r m e d by a l a r g e r Pake doublet s p l i t t i n g observed i n the s o l i d s t a t e spectrum o f the phenyl protons i n the p a r t i a l l y deuterated analogue ( 1 0 ) . E x p r e s s i o n s f o r t h e s p e c t r a l d e n s i t y c a n be d e v e l o p e d f r o m m o d e l s f o r l o c a l m o t i o n i n r a n d o m l y c o i l e d c h a i n s . Two g e n e r a l t y p e s o f l o c a l m o t i o n w i l l be c o n s i d e r e d , and t h e y a r e s e g m e n t a l m o t i o n and a n i s o t r o p i c r o t a t i o n . S e g m e n t a l m o t i o n i t s e l f w i l l be

0 20 40 60 80 100 120

T a b l e I:

548 499 522 628 794 1168 1411

90 MHz

Phenyl

30

153 189 274 432 553 763 939

MHz

Protons

(ms)

137 174 243 377 543 798 1113

62.9 MHz 72 106 156 349 448 679 936

MHz

Carbons

22.6

Protonated Phenyl

S p i n - L a t t i c e R e l a x a t i o n times

MHz

335 282 268 295 350 467 628

250

Formal 90 129 114 133 198 258 320 433

MHz

Protons

81 91 115 158 229 339 403

62.9

MHz

Formal

36 52 82 123 193 275 377

22.6

Carbons MHz

72

NMR

A N D

MACROMOLECULES

d e s c r i b e d i n two ways. The f i r s t d e s c r i p t i o n i s d e r i v e d f r o m t h e a c t i o n o f a t h r e e bond jump on a t e t r a h e d r a l l a t t i c e (7_) and t h e s e c o n d i s d e v e l o p e d f r o m c o n s i d e r a t i o n o f computer s i m u l a t i o n s o f backbone t r a n s i t i o n s i n p o l y e t h y l e n e c h a i n s 0 8 ) . A n i s o t r o p i c r o t a t i o n c a n a l s o be c h a r a c t e r i z e d i n s e v e r a l ways. I t can be d e s c r i b e d as jumps between two minima ( 1 1 ) , jumps between t h r e e m i n ima ( 1 2 ) o r s t o c h a s t i c d i f f u s i o n ( 1 2 ) . I n t h e t h r e e bond jump model f o r s e g m e n t a l m o t i o n t h e r e a r e two p a r a m e t e r s . The t i m e s c a l e i s s e t by t h e h a r m o n i c a v e r a g e c o r r e l a t i o n time, and t h e e f f e c t i v e d i s t r i b u t i o n o f c o r r e l a t i o n t i m e s i s s e t by t h e number o f c o u p l e d bonds m. The s h a r p c u t o f f o f c o u p l i n g s o l u t i o n o f t h e t h r e e bond jump model i s employed here. The c o m p o s i t e s p e c t r a l d e n s i t y f o r i n t e r n a l r o t a t i o n by jumps o r s t o c h a s t i c d i f f u s i o n p l u s s e g m e n t a l m o t i o n by t h r e e bond jump i s J.(ω.) = 2ZG

Α τ k

k-1

1

+

^0

T

A

= WÀ

k

1

2

^ k O

x

B T

+

k0

+

_ 1

w

bkO i

2 T

= tk"

C T

+

bko

2

1

+

W

i

ckQ 2 T

ck0

2

1

s = (m + l ) / 2

k

2

k

= 4 s i n [ ( 2 k - 1)π/2(πι + 1 ) ] x -l

=

h

2W

s-1 G

k

= 1/s + ( 2 / s ) E e x p ( - y q ) cos [ ( 2 k - 1)πς/2β] q-i γ = In 9 2

A = (3 c o s

Δ -l ) / 4 2

Β = 3(sin

2

2Δ)/4

C = 3(sin

4

Δ)/4

for stochastic diffusion T

T

for a threefold

jump

bk(f

τ

1

= \Γ

1

τ

1

c k O = \Γ

+

^ir""

1

+ (Tir/4)-

1

(2)

5.

Spin Relaxation and Local

TARPEY ET AL.

73

Motion

The a n g l e Δ i s between t h e i n t e r n u c l e a r v e c t o r and t h e a x i s o f internal rotation. The t h r e e bond jump s e g m e n t a l m o t i o n d e s c r i p t i o n c a n a l s o be combined w i t h a d e s c r i p t i o n o f r e s t r i c t e d a n i s o t r o p i c r o t a t i o n a l diffusion (13-14). I n t h i s case, the composite s p e c t r a l d e n s i t y equation i s s J.(u).) = 2ZG, ι ι k τ~ k=l 2

{ [ ( 1 - c o s I)

+ sin

A T k

Q

5

ι 2

r

1}

l

2

n=l

(1 - Ξ Ι ) s i n ( £ + rm)} ,}2

T

Ί

(1+21)

{ I [(1 - cos 2 J 0

2

2

1

+ sin

2

+

ω .

1 = JL + λ nkO

n

- (Bl) I

2

% r

ÏÏDL) I

T

and

(3)

^

k

k

o

2

τ, ku

Q

+ 2

2 +

I

(2 +

1 = _J_ kO k

+

+

[ ( [ 1 - c o s ( 2 i - η π ) ] + [1 - c o s ( 2 £ + η π ) ] > (2 - Ξ 1 ) (2 + Β ! )

(2 - TL> I

2

2

1 + ω. ι

2

λ

z

J 2

T

21]

2

(sin(2Jt - η π ) + s i n ( 2 £ + η π ) } ]

r

kO

nkO

ι

T

T

(1 + Ξ 1 )

2

+

(1 -f)

JL 2l

2

[ f [ l ~ cos(J6 - η π ) ] + [1 - c o s U + η π ) ] }

| s i n ( £ - ηπ)

where

k0

1 + ω.

Σ

00 Σ n=l

+

kO

l

1

τ

T

nkO

1 + ω. ι

| 2

τ

, nkO Λ

2

+

74

N M R

A N D

MACROMOLECULES

The new parameters f o r r e s t r i c t e d a n i s o t r o p i c r o t a t i o n a l d i f f u s i o n are the angular amplitude over which r o t a t i o n d i f f u s i o n occurs, i, and the r o t a t i o n a l d i f f u s i o n constant f o r r e s t r i c t e d a n i s o t r o p i c r o t a t i o n a l d i f f u s i o n , D-^. A second d e s c r i p t i o n of segmental motion can be combined with the various types of i n t e r n a l a n i s o t r o p i c i n t e r n a l r o t a t i o n , Weber and Helfand (8^) c h a r a c t e r i z e segmental motion i n terms of a c o r r e l a t i o n time f o r s i n g l e conformational t r a n s i t i o n s , T Q , and a c o r r e l a t i o n time f o r cooperative conformational t r a n s i t i o n s , τ^. This model has been a p p l i e d to nuclear spin r e l a x a t i o n before ( 5 ) and the form of the s p e c t r a l density f o r a composite segmental motion and a n i s o t r o p i c i n t e r n a l r o t a t i o n i s w r i t t e n r

J

w

i( i)

= A^ia^O»

τ

ω

B J

1» ί> +

T

τ

ib( bO>

Α = (3 cos

2

ω

CJ

T

τ

1> ί ) + ie( eO»

ω

1> ί>

2

Δ - 1) /4

Β = 3 (sin

2

2Δ)/4

(4)

4

C = 3 ( s i n Δ)/4 for

stochastic diffusion T

b0"

TcO" for

1

1

1+

= το""

= το"

1 +

Tir""

1

4

(τ^/ )"

1

a three bond jump T

b0~

1

= TcO"

1

= TO"

1

+ Tir""

1

The form of J± , J-j^ and J-^ i s the same as J ^ j given below with T Q replaced by T Q , T ^ Q and T Q r e s p e c t i v e l y . a

c

C

+ 21^)

J i j ( ^ i ) = 2{[(τ -1)(τ -1 0

0

χ cos [1/2 a r c t a n ( 2 ( x " 0

1

-

2 ω

ι

2 2 - I / ] +[2(τ ""1 + τ Γ ^ ) ] } 0

+ τj" )ω /το"" (TQ" 1

1

1

1

2

+ 2 τ ~ ) - ω± ] 1

1

This d e s c r i p t i o n of segmental motion can also be combined restricted anisotropic rotational diffusion

with

4

5.

Spin Relaxation and Local Motion

T A R P E Y E TA L .

Ji(o)i) JL 2

= AJ

2

{ [ ( 1 - c o s I)

0 1

(

i

+ sin

W l

) +

1} J .

2

0

1

(ω.) +

t

Α

Σ

- cos(£ - ηπ)] + [1 - cos(Z + η π ) ] >

2

n=l

(1 - EL)

+

(1 + EL)

ι

I

λ n ( . ) |+ sin(£ + η π ) } 2, ] j / 2

{ s i n U - ηπ)

τ

φ

τ

ω

( 1 + IHL)ι

(1 - IE.)

{ I [(1 - cos 2JO

JL 2i

2

2

1

+ sin

1

01 21} J , (ω.) +

2

2

Σ

[ { [ 1 - c o s ( 2 £ - ηπ)] + [1 - c o s ( 2 £ + η π ) ] > (2 - EL)

n=l

+

(2 + EL)

I f s i n ( 2 £ - ηπ)

ζ

I s i n ( 2 £ + ηπ)> ] j ^ n ( . ) } ] 2

+

ω

(2 - EL> ι

1

(2 + EL) I

1

where Ji

(2

0

1

(on) = { [ τ - 1 ( τ 0

2 ~ UK)}

1

/

0 1

-1 +

-

W i

2]2 +

_

2x 9J_

4

χ cosfi. arctan V ^ o f

1

+

i

τ

••Γ

1

}

2

χ cos[I arctan 2

(τ,, 0 τοΓ

1

0 1

+

+ λ Κ τ " η 01 /

= 0 τ

λ

AH

1

2(τ - 1

1

χ

τ

+ 1

λ ) ω η

1

±

+ λ ) - ω. η' ι

1

= ( £ 1 ) D. 2

o f t h e terms have been d e f i n e d i n e q s . 2-4.

} 2

ω

ι

2

-1/4

76

NMR A N D

MACROMOLECULES

To a p p l y t h e models t o t h e i n t e r p r e t a t i o n o f t h e d a t a , t h e a p p r o a c h d e v e l o p e d f o r t h e p o l y c a r b o n a t e s w i l l be f o l l o w e d . The p h e n y l p r o t o n T j s a r e i n t e r p r e t e d f i r s t i n terms o f s e g m e n t a l motion. For these protons, the d i p o l e - d i p o l e i n t e r a c t i o n i s p a r a l l e l t o t h e c h a i n backbone and t h e r e f o r e r e l a x e d o n l y by s e g mental motion. I n t h e t h r e e bond jump model t h e p a r a m e t e r s τ and m a r e a d j u s t e d t o a c c o u n t f o r p h e n y l p r o t o n d a t a , and i n t h e W e b e r - H e l f a n d model t h e p a r a m e t e r s T Q and T J a r e a d j u s t e d . T a b l e I I c o n t a i n s t h e t h r e e bond jump p a r a m e t e r s , and T a b l e I I I , t h e W e b e r - H e l f a n d model p a r a m e t e r s . Both models can s i m u l a t e t h e d a t a w i t h i n 10% w h i c h i s e q u i v a l e n t t o t h e e x p e r i m e n t a l e r r o r . P h e n y l g r o u p r o t a t i o n c a n be c h a r a c t e r i z e d f r o m t h e p h e n y l c a r b o n T j s by a s s u m i n g t h e s e g m e n t a l d e s c r i p t i o n d e v e l o p e d f r o m the p r o t o n d a t a ( 5 ) . E i t h e r s e g m e n t a l model c a n be u s e d , and t h e c o r r e s p o n d i n g c o r r e l a t i o n t i m e s f o r i n t e r n a l r o t a t i o n o f t h e phe n y l g r o u p by s t o c h a s t i c d i f f u s i o n , T - j ^ p ' s , a r e d i s p l a y e d i n T a b l e I I and T a b l e I I I . A g a i n b o t h a p p r o a c h e s match t h e o b s e r v e d c a r b o n - 1 3 d a t a w i t h i n t h e 10% u n c e r t a i n t y . T

η

1

Table I I :

P h e n y l Group M o t i o n S i m u l a t i o n P a r a m e t e r s U s i n g t h e T h r e e Bond Jump M o d e l

°C

m

0 20 40 60 80 100 120

1 1 1 1 3 5 7

E (

kj/mole) x 10 (s) Correlation Coefficient a

Too

1 4

τ

η

(ns)

2.69 1.30 0.79 0.49 0.180 0.080 0.049 30 0.59 0.99

T

i

r

p

(ns)

1.85 1.19 0.73 0.299 0.247 0.192 0.145 20 30 0.99

5.

TARPEY ET AL.

Table I I I :

Spin Relaxation

τι ( n s )

0 20 40 60 80 100 120

3.80 1.89 1.09 0.49 0.259 0.142 0.070

kj/mole) x 10 (s) Correlation Coefficient a

Too

1 4

11

Motion

P h e n y l Group M o t i o n S i m u l a t i o n P a r a m e t e r s U s i n g t h e W e b e r - H e l f a n d Model

°c

E (

and Local

30 1.04 0.99

TQ

(ns)

T

2.15 1.15 0.72 0.280 0.240 0.170 0.150

6.01 3.7 2.34 2.00 1.99 1.86 1.70 9 97 χ 1 0 0.94

i r p (ns)

2

21 20 0.99

Now t h e i n t e r p r e t a t i o n d i v e r g e s f r o m t h e p o l y c a r b o n a t e p a t t e r n as t h e f o r m a l g r o u p i s considered· As m e n t i o n e d , t h e s t r u c t u r a l analogue t o t h e f o r m a l group i n t h e p o l y c a r b o n a t e i s the c a r b o n a t e g r o u p , and t h e l a t t e r c a n n o t be d i r e c t l y s t u d i e d by s o l u t i o n s p i n r e l a x a t i o n s t u d i e s s i n c e i t has no d i r e c t l y bonded protons. I f t h e f o r m a l i s f i r s t viewed i n d e p e n d e n t l y from the p h e n y l g r o u p d a t a , one m i g h t a t t e m p t t o employ s e g m e n t a l m o t i o n d e s c r i p t i o n s a l o n e s i n c e t h e f o r m a l g r o u p l i e s i n t h e backbone. P u r s u i n g t h i s a p p r o a c h , b o t h t h e t h r e e bond jump and t h e WeberH e l f and models were a p p l i e d t o s i m u l a t e t h e p r o t o n and c a r b o n - 1 3 d a t a i n T a b l e I . N e i t h e r model i s a b l e t o a c c o u n t f o r t h e d a t a , w i t h s y s t e m a t i c d i s c r e p a n c i e s up t o 70% i n b o t h attempts· The l a r g e s t d i s c r e p a n c i e s o c c u r a t l o w t e m p e r a t u r e s w i t h o n l y somewhat b e t t e r simulations p o s s i b l e at higher temperatures. I n one s e n s e i t i s r e a s s u r i n g t o d e t e r m i n e t h a t models f o r segmental motion cannot account f o r a l l d a t a s e t s . On t h e o t h e r h a n d , i t i s s t i l l d e s i r a b l e t o d e v e l o p some d e s c r i p t i o n o f m o t i o n w h i c h w i l l a c c o u n t f o r t h e d a t a a t hand, s i n c e t h e f a i l u r e t o s i m u l a t e i m p l i e s some p o t e n t i a l l y i n t e r e s t i n g i n f o r m a t i o n a l content· The s u c c e s s f u l p h e n y l g r o u p i n t e r p r e t a t i o n c a n a s s i s t t h e e f f o r t to a c c o u n t f o r t h e f o r m a l d a t a . The s e g m e n t a l m o t i o n d e s c r i p t i o n s a p p l i e d t o t h e p h e n y l p r o t o n d a t a a r e based on i s o t r o p i c a v e r a g i n g of t h e d i p o l e - d i p o l e i n t e r a c t i o n s by t h e s e g m e n t a l m o t i o n . One c o u l d assume t h a t t h e same s e g m e n t a l m o t i o n d e s c r i p t i o n o c c u r r i n g at t h e p h e n y l groups a l s o o c c u r s a t t h e f o r m a l group s i n c e b o t h g r o u p s a r e a d j a c e n t i n t h e backbone. I f t h i s a s s u m p t i o n i s made, some a d d i t i o n a l m o t i o n must be c o n s i d e r e d t o match t h e o b s e r v e d f o r m a l r e l a x a t i o n t i m e s . I n t h e c o n t e x t o f t h e models b e i n g a p p l i e d , t h e added m o t i o n c o u l d be a a n i s o t r o p i c r o t a t i o n o r r e stricted rotation. F o r t h e f o r m a l group, t h e f i r s t guess i s r o t a -

78

NMR

A N D MACROMOLECULES

t i o n or r e s t r i c t e d r o t a t i o n about the C-0 a x i s . This would be a s i n g l e backbone conformational t r a n s i t i o n o c c u r r i n g as an anisot r o p i c motion on top of the segmental motion of say the WeberHelf and model determined from the phenyl proton data. Complete a n i s o t r o p i c r o t a t i o n about the C-0 bond adequately accounts f o r the higher temperature data, but f a i l s to simulate the lower temperature data by about 40%. A r e s t r i c t e d r o t a t i o n model at lower temperatures i s also not able to simulate the observed T| s though i t comes c l o s e r . Adding a r o t a t i o n or r e s t r i c t e d r o t a t i o n about the C-0 axis to the three bond jump model i s e q u a l l y unsuccessful as might be expected since so f a r the three bond jump and WeberHelf and model have p a r a l l e d each other. The next motion considered i s r o t a t i o n or r e s t r i c t e d r o t a t i o n of the 0CH 0 u n i t about the 0-0 axis of the u n i t . The i n i t i a l l o g i c here was that the l a r g e r aromatic groups were slower moving anchors and the formal group was a n i s o t r o p i c a l l y r o t a t i n g r e l a t i v e to the two oxygens which were the connections to the more s l u g g i s h phenyl groups. At higher temperatures, complete a n i s o t r o p i c r o t a t i o n about the 0-0 axis i n a d d i t i o n to a segmental motion d e s c r i p t i o n using the Weber-Helfand model developed from the phenyl proton data accounted f o r the formal data but d i s c r e p a n c i e s of 30% s t i l l remained at lower temperatures. The lower temperature data could be accounted f o r by allowing f o r incomplete a n i s o t r o p i c r o t a t i o n a l d i f f u s i o n about the 0-0 axis i n a d d i t i o n to segmental mot i o n . With complete r o t a t i o n at higher temperatures and r e s t r i c t ed r o t a t i o n at lower temperatures, a l l formal proton and carbon-13 data can be simulated w i t h i n the experimental u n c e r t a i n t y of the T | s . The a n i s o t r o p i c r o t a t i o n simulation parameters are reported i n Table IV f o r the case where segmental motion i s c h a r a c t e r i z e d with the Weber-Helfand model on the basis of the phenyl proton data. A s u b s t i t u t i o n of the three bond jump model f o r the WeberHelf and model leads to nearly the same results· f

2

f

Table IV:

°c 0 20 40 60 80 100 120 (a)

Formal Group Simulation Parameters Using the WeberHelfand M o d e l a )

JL 86 119 164 360 360 360 360

D

i r

x

10~

1 0

1

(s""" )

0.100 0.110 0.130 0.100 0.160 0.210 0.230

The values of T j and T Q reported i n Table I I I are used here as w e l l as the parameters l i s t e d .

5.

T A R P E Y ET A L .

Spin Relaxation and Local

Motion

79

Discussion As the f i r s t p o i n t , the dynamics of the phenyl group i n the poly formal can be considered. Motional d e s c r i p t i o n s from the two seg mental models can be compared as they have been before f o r the polycarbonates ( 5 ) . In the three bond jump model the primary parameter i s the harmonic mean c o r r e l a t i o n time, τ^; and i n the Weber-Helfand model the primary parameter i s the c o r r e l a t i o n time f o r cooperative backbone t r a n s i t i o n s , τ ι. At the lower tempera tures s t u d i e d , T Q plays an i n c r e a s i n g r o l e i n the Weber-Helfand model but T J i s s t i l l the major f a c t o r . This i s an i n t e r e s t i n g point i n i t s e l f since cooperative t r a n s i t i o n s were also found to predominate when the Weber-Helfand model was a p p l i e d to the p o l y carbonates . Here i n the polyformal, s i n g l e bond conformational t r a n s i t i o n s do play a l a r g e r r o l e ; and t h i s can be seen i n the three bond jump model as w e l l by the drop of m to 1 at lower tem peratures. Since T J and τ are both measures of the time s c a l e f o r cooperative motions, i t i s i n t e r e s t i n g to note that the Arrhenius summaries of the two c o r r e l a t i o n times i n Tables II and I I I are very s i m i l a r . This s i m i l a r i t y , taken together with the domination of cooperative t r a n s i t i o n s i n the i n t e r p r e t a t i o n s , sup ports the u t i l i t y of both models though the Weber-Helfand model i s developed from a more d e t a i l e d a n a l y s i s of chain motion. One i n t e r e s t i n g d i f f e r e n c e between the Weber-Helfand i n t e r p r e t a t i o n of the polyformal and the polycarbonates i s the r e l a t i v e apparent a c t i v a t i o n energies f o r τ ι and T Q . For the polycarbo nates , the a c t i v a t i o n energies f o r T Q and τ ι were about the same ( 5 ) as would be expected i f the cooperative t r a n s i t i o n s occurred s e q u e n t i a l l y as opposed to simultaneously (15-17)· For the polyformal, the a c t i v a t i o n energy f o r the cooperative process i s much higher than f o r the s i n g l e t r a n s i t i o n s which i s more i n d i c a t i v e of simultaneous cooperative t r a n s i t i o n s such as a crank shaft · Since the s i n g l e t r a n s i t i o n s are minor processes i n both the polycarbonates and to a l e s s e r extent i n the polyformal, dwelling on the a c t i v a t i o n energy d i f f e r e n c e s may be r i s k y . It i s worth noting that the d e s c r i p t i o n of phenyl group r o t a t i o n i s not s i g n i f i c a n t l y i n f l u e n c e d by changing d e s c r i p t i o n s of segmental motion. This too supports the u t i l i t y of both models and the v a l i d i t y of the general a n a l y s i s of l o c a l motion f o r phe n y l groups as being d i v i d e d between segmental motion and i n t e r n a l rotation. Segmental motion and phenyl group r o t a t i o n i n the polyformal can be compared to that of the polycarbonates. R e l a t i v e to the analogous C h l o r a l polycarbonate Ç5), the cooperative segmental mot i o n i n the polyformal i s s i m i l a r i n general time s c a l e but has a s i g n i f i c a n t l y higher a c t i v a t i o n energy. Phenyl group r o t a t i o n i n the polyformal and the polycarbonate are n e a r l y i d e n t i c a l . This suggests phenyl group r o t a t i o n i s a very l o c a l i z e d process not g r e a t l y i n f l u e n c e d by r e p l a c i n g the carbonate l i n k with a formal link. On the other hand, i t i s hard to imagine phenyl group r o t a η

80

NMR

AND

MACROMOLECULES

t i o n as a s i m p l e p r o c e s s w i t h i n the b i s p h e n o l u n i t s i n c e MNDO c a l c u l a t i o n s (18) i n d i c a t e a h i g h b a r r i e r w i t h i n t h i s u n i t . A n o t h e r i n t e r e s t i n g p o i n t about p h e n y l g r o u p r o t a t i o n i n t h e p o l y f o r m a l and p o l y c a r b o n a t e s i s t h a t i t i s b e s t modeled i n s o l u t i o n as s t o c h a s t i c d i f f u s i o n r a t h e r t h a n two f o l d jump (π f l i p s ) . I n s o l i d BPA p o l y c a r b o n a t e , b o t h d e u t e r i u m ( 1 9 ) and c a r b o n - 1 3 ( 2 0 ) l i n e s h a p e a n a l y s i s p o i n t t o two f o l d jumps o r π f l i p s as t h e p r i mary p r o c e s s . C a l c u l a t i o n s by T o n e l l i ( 2 1 - 2 2 ) a l s o p o i n t t o l o w b a r r i e r s t o p h e n y l g r o u p r o t a t i o n f o r i s o l a t e d BPA c h a i n s . I f t h e i n t r a m o l e c u l a r b a r r i e r f o r p h e n y l g r o u p r o t a t i o n i s i n d e e d l o w as i n d i c a t e d by t h e s o l u t i o n s t u d i e s and t h e c a l c u l a t i o n s , t h e change t o a h i g h e r b a r r i e r ( 6 , 1 8 ) and π f l i p s i n t h e s o l i d must r e f l e c t intermolecular interactions. T h i s i s indeed p l a u s i b l e s i n c e the new c o n f o r m a t i o n f o l l o w i n g a π f l i p i n t h e s o l i d r e q u i r e s no change i n t h e s u r r o u n d i n g s (no change i n f r e e v o l u m e ) y e t t h e s u r r o u n d i n g s c o u l d p r o v i d e an a p p r e c i a b l e b a r r i e r t o t h e t r a n s i t i o n . As m e n t i o n e d , t h e f o r m a l l i n k p r o v i d e s new d y n a m i c i n f o r m a t i o n r e l a t i v e t o the p o l y c a r b o n a t e s where no d e t a i l e d a n a l y s i s o f the carbonate u n i t i s p o s s i b l e . In the i n t e r p r e t a t i o n , a r a t h e r complex d e s c r i p t i o n i s r e q u i r e d to account f o r the f o r m a l r e l a x a t i o n d a t a . A c c o r d i n g t o t h e i n t e r p r e t a t i o n , t h e f o r m a l g r o u p un d e r g o e s s e g m e n t a l m o t i o n as d e t e r m i n e d a t t h e p h e n y l g r o u p p l u s a n i s o t r o p i c r o t a t i o n about t h e o x y g e n - o x y g e n a x i s o f t h e f o r m a l group. At low temperatures t h i s a n i s o t r o p i c r o t a t i o n i s d e s c r i b e d as r e s t r i c t e d r o t a t i o n a l d i f f u s i o n . The m a i n q u e s t i o n i s w h e t h e r t h e r e i s any p h y s i c a l s e n s e t o s u c h a p i c t u r e . S i n c e t h e segment a l m o t i o n i s somewhat c o o p e r a t i v e and t h e p h e n y l g r o u p i s a d j a c e n t , i t seems r e a s o n a b l e t o assume t h a t t h i s m o t i o n e x t e n d s o v e r b o t h t h e p h e n y l and f o r m a l g r o u p s . The r e a l q u e s t i o n i s t h e a n isotropic restricted rotation. To p u r s u e t h i s a s p e c t , c o n f o r m a t i o n a l e n e r g y maps o f d i m e t h o x y m e t h a n e were r e v i e w e d ( 2 3 - 2 4 ) . The lowest conformations are gg and g g and t h i s u n u s u a l s i t u a t i o n r e l a t i v e t o p o l y e t h y l e n e c h a i n s i s commonly c a l l e d t h e a n o m e r i c effect. E a c h o f t h e s e c o n f o r m a t i o n s has two c o n f o r m a t i o n s w h i c h a r e o n l y 4 k J h i g h e r i n e n e r g y . The t g and g t c o n f o r m a t i o n s a r e e n e r g e t i c a l l y near the g g c o n f o r m a t i o n and t h e t g and g ' t c o n f o r m a t i o n s a r e e n e r g e t i c a l l y n e a r t h e g'g c o n f o r m a t i o n . The g ' g , gg and t t s t a t e s a r e c o n s i d e r a b l y h i g h e r i n e n e r g y . The most f a c i l e c o n f o r m a t i o n a l changes f r o m t h e l o w e s t s t a t e s c o u l d be r e p r e s e n t e d by T

f

f

f

1

t g

t

β

g

t

=

g

g

t

s

g t

g =

t g

(6) g

t

t

A t l o w e r t e m p e r a t u r e s where a g i v e n f o r m a l u n i t i s l i k e l y t o be e i t h e r gg or g'g, t h e t r a n s i t i o n s r e p r e s e n t e d by eq. 6 w o u l d r e s u l t i n r e s t r i c t e d r o t a t i o n a l averaging. T h i s would g e n e r a l l y agree w i t h the r e s u l t s o b t a i n e d from the s i m u l a t i o n of the f o r m a l r e l a x a t i o n d a t a f r o m 0 t o 40 d e g r e e s where t h e a n g u l a r a m p l i t u d e f

5.

TARPEY ET AL.

Spin Relaxation

and Local

81

Motion

o f r e s t r i c t e d r o t a t i o n , I, r a n g e s f r o m 86 t o 164 d e g r e e s . A t h i g h e r t e m p e r a t u r e s p o p u l a t i o n s i n s t a t e s o t h e r t h a n g g o r g'g w o u l d become l a r g e r a l l o w i n g f o r t h e more common o c c u r r e n c e o f c o n f o r m a t i o n a l changes o t h e r t h a n t h o s e l i s t e d i n e q . 6. T h i s w o u l d r e s u l t i n e f f e c t i v e l y c o m p l e t e r o t a t i o n i n agreement w i t h t h e s i m u l a t i o n f r o m 60 t o 120 d e g r e e s . These arguments w o u l d a c c o u n t f o r t h e s h i f t f r o m r e s t r i c t e d r o t a t i o n t o c o m p l e t e a n i s o t r o p i c r o t a t i o n , b u t why i s t h e c h o i c e o f t h e o x y g e n - o x y g e n a x i s made? I n f a c t , i t c a n o n l y be a r o u g h a p p r o x i m a t i o n , s i n c e t h e ends o f t h e f o r m a l g r o u p must move d u r i n g these c o n f o r m a t i o n a l changes. The t i m e s c a l e f o r t h e f o r m a l g r o u p c o n f o r m a t i o n a l changes a r e o n l y somewhat more r a p i d r e l a t i v e t o t h e t i m e s c a l e o f s e g m e n t a l m o t i o n and p h e n y l g r o u p r o t a t i o n , s o p h e n y l g r o u p s a r e o n l y somewhat s l u g g i s h w i t h r e s p e c t t o t h e f o r m a l g r o u p . A more d e t a i l e d and a c c u r a t e model f o r t h e f o r m a l g r o u p m o t i o n c o u l d be u n d e r t a k e n b u t t h e d a t a i n hand do n o t warrant i t . The p r e s e n t p i c t u r e p o i n t s t o s i n g l e c o n f o r m a t i o n a l t r a n s i t i o n s a t t h e f o r m a l group which r e s u l t i n o n l y p a r t i a l s p a t i a l averaging of d i p o l a r i n t e r a c t i o n s a t lower temperatures. f

Acknowledgments The r e s e a r c h was c a r r i e d o u t w i t h f i n a n c i a l s u p p o r t o f N a t i o n a l S c i e n c e F o u n d a t i o n G r a n t DMR-790677, o f N a t i o n a l S c i e n c e F o u n d a t i o n Equipment G r a n t No. CHE 77-09059, o f N a t i o n a l S c i e n c e F o u n d a t i o n G r a n t No. DMR-8108679, and o f t h e U.S. Army R e s e a r c h O f f i c e G r a n t DAAG 29-82-G-0001.

Literature Cited (1)

A.A. Jones and R.P. Lubianez, Macromolecules (1978) 11, 126.

(2)

R.P. Lubianez, A.A. Jones and M. B i s c e g l i a , Macromolecules (1980) 12, 1141.

(3)

A.A. Jones and M. B i s c e g l i a , Macromolecules (1979) 12, 1136.

(4)

J . F . O'Gara, S.G. Desjardin and A.A. Jones, Macromolecules (1980) 14, 64.

(5)

J . J . Connolly, E. Gordon and A.A. Jones, submitted to Marcomolecules.

(6)

A . F . Yee and S.A. Smith, Macromolecules (1981) 14, 54.

(7)

A.A. Jones and W.H. Stockmayer, J . Polym. Sci., Polym. Phys. Ed. (1977) 15, 847.

(8)

T.A. Weber and E. Helfand, submitted to J . Chem. Phys.

82

(9)

NMR A N D MACROMOLECULES

A.S. Hay, F.J. Williams, G.M. Loucks, H.M. Relies, B.M. Boulette, P.Ε. Donahue and D.S. Johnson, Polym. Prepr. Am. Chem. Soc. Div. Polym. Chem. ( 1 9 8 2 ) 2 3 ( 2 ) , 117.

(10)

A.A. Jones and M.F. Tarpey, unpublished

(11)

A.A. Jones, J . Polym. Sci., Polym. Phys. Ed.

(12)

D.E. Woessner, J . Chem. Phys.

(13)

W. Gronski and N. Murayama, Makrmol. Chem.

(14)

W. Gronski, Makromol. Chem.

(15)

E. Helfand, J . Chem. Phys.

(16)

E. Helfand, Z.R. Wasserman, and T.A. Weber, Macromolecules (1980)

13,

(1962)

(1979) (1971)

36,

180, 54,

results. (1977)

15,

863.

1. (1978)

179,

1521.

1119.

4651.

526.

(17)

J . Skolnik and E. Helfand, J . Chem. Phys.

(18)

A.A. Jones, J . F . O'Gara, P.T. Inglefield, J.T. Bendler, A . F . Yee, and K . L . Ngai, Macromolecules ( 1 9 8 3 ) 16, 6 5 8 .

(19)

H.W. Spiess, C o l l o i d . Polym. S c i . (1983) 2 6 1 , 1 9 3 .

(20)

P.T. Inglefield, R.M. Amici, J . F . O'Gara, C.-C. Hung and A.A. Jones, submitted to Macromolecules.

(21)

A . E . T o n e l l i , Macromolecules (1972) 5, 5 5 8 .

(22)

A . E . T o n e l l i , Macromolecules (1973) 6, 5 0 3 .

(23)

I . Tvaroska and T. Bleha, J . Mol. Struct. (1975) 2 4 , 2 4 9 .

(24)

G.A. Jeffrey and R. Taylor, J. Comp. Chem.

RECEIVED September 22, 1983

(1980)

72,

1 (1980)

5489.

99.

6 Characterization of Molecular Motion in Solid Polymers by Variable Temperature Magic Angle Spinning CNMR 13

W. W. FLEMING, J. R. LYERLA, and C. S. YANNONI IBM Research Laboratory, San Jose, CA 95193 The inclusion of a variable temperature magic-angle spinning capability for solid state C NMR spectroscopy makes feasible the investigation by C relaxation parameters of structural and motional features of polymers above and below Tg and in temperature regions of secondary relaxations. Herein, we report variable temperature (50K to 323K) spectral data on semicrystalline poly(propylene) and glassy PMMA. Illustrative of the data are the T and T results for isotactic poly(propylene) over the temperature range 50K to 300K. All carbons in the repeat unit show minima in T and T which reflect methyl group reorientation motion at the appropriate measuring frequencies (15 MHz and 57 kHz). The T data for CH and CH carbons indicate the importance of spin-spin as well as spin-lattice pathways in their rotating frame relaxation over much of the temperature interval studied. An interesting spectral observation is the strong motional broadening of the methyl group in the temperature region of the T minimum. These and other facets of the poly(propylene) data as well as similar data for PMMA are discussed with respect to their implications for insight into polymer chain dynamics in the solid state. 13

13

1

1ρ

1

1ρ

1ρ

2

1ρ

One of the principal advantages of C P M A S experiments is that resolution in the solid state allows individual-carbon relaxation experiments to be performed. If a sufficient number of unique resonances exist, the results can be interpreted in terms of rigid-body and local motions (e.g., methyl rotation, segmental modes in polymers, etc.) (1,2). This presents a distinct advantage over the more common proton relaxation measurements, in which efficient spin diffusion usually results in averaging of relaxation behavior over the ensemble of protons to yield a single relaxation time for all protons. This makes interpretation of the data in terms of unique motions difficult.

0097-6156/ 84/ 0247^0083506.00/ 0 © 1984 American Chemical Society

84

N M R A N D

MACROMOLECULES

Relaxation parameters of interest for the study of polymers include 1) C and * H spin-lattice relaxation times ( T and T ) , 2) the spin-spin relaxation time T , 3) the nuclear Overhauser enhancement ( N O E ) , 4) the proton and carbon rotating-frame relaxation times ( T ^ and T ^ ) , 5) the C - H cross-relaxation time T , and 6) the proton relaxation time in the dipolar state, T (2). Not all of these parameters provide information in a direct manner; nonetheless, the inferred information is important in characterizing motional frequencies and amplitudes in solids. The measurement of data over a range of temperatures is fundamental to this characterization. The initial studies of carbon relaxation in polymers have emphasized T j and T measurements, which provide information on molecular motions in the M H z and kHz frequency ranges, respectively. Schaefer and Stejskal have carried out the pioneering work in their investigations of glassy polymers (1). In particular, they stress the utility of T measurements for probing the dynamic heterogeneity of the glassy state and as a potential source of insight into the mechanical and other physical properties of polymers at the molecular level. Garroway and co-workers (3) reported the first variable-temperature ( V T - M A S ) T j results in their study of epoxy resins, and together with VanderHart, (4) have detailed the complications in extracting information on molecular motion from T experiments. In this paper, we report the first extensive sub-ambient V T - M A S C Tj and T data on macromolecules. The emphasis of the study was placed on isotactic poly (propylene) (PP) and atactic poly (methylmethacrylate) ( P M M A ) as they represent semi-crystalline and glassy polymers, respectively. Specifics of the investigation were directed to the issue of elucidating sidechain and backbone motions from the high frequency relaxation experiments. 1

3

1 C

1 H

2

p

C

1

l

p

H

D

p

l

l

p

p

1

l

3

p

Experimental 1

3

The C data at 15.1 M H z were acquired on a modified Nicolet TT-14 N M R system. The features of this spectrometer and of the spinning assembly have been reported previously (5). Samples were machined into the shape of Andrew-type rotors and used directly for the various studies. Temperature variation was achieved by cooling or heating the helium gas used for driving the rotor. The temperature was controlled to ± 2 ° C with a home built temperature sensing and heater/feedback network. Spin-lattice relaxation times, Τ j were collected using a pulse sequence developed by Torchia (6) which allows cross-polarization enhancement of the signals. The T data were determined at 57 kHz using T methodology of Schaefer et al. (1). The PP examined was a 90% isotactic, 70% crystalline sample. The P M M A was an atactic commercial polymer. l

l

p

p

Results and Discussion 1

3

Figure 1 shows the C P M A S C spectra of PP as a function of temperature. The interesting feature is the progressive broadening of the methyl resonance

6.

FLEMING ETAL.

Molecular Motion in Solid

Figure 1. C P M A S temperature.

1

3

Polymers

C spectra of poly (propylene) as a function o f

85

86

N M R A N D

MACROMOLECULES

as the temperature is lowered. At « 1 1 0 K , the resonance is broadened to the point of disappearing from the spectrum. However, at temperatures below 77K, the methyl resonance narrows and reappears in the spectrum. This broadening phenomenon arises as the reorientation rate of the methyl group about the C3 axis becomes insufficient to average (stochastically) dipolar interaction with the methyl protons. A t the onset of broadening, the methyl motion has a correlation time comparable to the inverse of the strength (in frequency units) of the proton decoupling field. This reduces the efficiency of the rf decoupling and leads to a maximum linewidth of the carbon when the motions occur at the frequency corresponding to the amplitude of the proton decoupling field. Rothwell and Waugh (7) have developed the theory for T (the inverse of the C linewidth) for an interplay between stochastic and coherent motions. For such a system, the profile of linewidth vs. temperature shows a maximum when the correlation time for molecular motion, T , is equal to the modulation period of the decoupling, (Ι/ω^). In the "short correlation time" limit ( ω τ < < 1 ) (high temperature), the linewidth is reduced by the rapid motional averaging, while in the "long correlation time" limit ( ω τ , > > 1 ) (low temperature), the linewidth is reduced by efficient decoupling of C - H dipolar interactions. The spectra of PP in Figure 1 are consistent with the progression of the methyl resonance through the linewidth regions as the temperature is lowered. The reappearance (narrowing) of the methyl resonance at 77K indicates that the "long correlation time" regime has been reached (7). Further proof of the progressive changes in correlation time for methyl rotation as the temperature is lowered is provided by the decoupling-field dependence of the linewidth. A t about 160K, the methyl linewidth is independent of decoupling field, while at 77K the linewidth varies as the inverse square of the decoupling field. This is the expected dependence for the transition between the extreme narrowing and long correlation time regimes (7). Finally, from the expression for the C linewidth derived by Rothwell and Waugh [Eq. (1)] and the correlation time and temperature of the T minimum observed by McBrierty et al. (8), 2

1

3

c

1

0

1

1

(

3

l

2

_ l . yhl*

(

T

c

\

p

( 1 )

we calculate, for ω^=5Ί kHz (the value of the decoupling field used to obtain the spectra in Figure 1), that the maximum broadening for the methyl resonance would occur at 109K, in excellent agreement with our observations. This broadening of the methyl resonance observed in PP is also found in polycarbonate, P M M A , and epoxy polymers. It should be a general phenomenon for rapidly reorienting side groups or main-chain carbons in polymers. For semicrystalline systems, where the local molecular structure is relatively homogeneous, severe broadening should result in the "disappearance" of resonance lines from the spectra. For glassy systems, where there is more heterogeneity in the local molecular environment, the

6.

F L E M I N G ET A L .

87

Molecular Motion in Solid Polymers

effect may result in significant changes in resonance lineshape as a function of temperature as the carbons in differing environments undergo severe broadening. Of course, the phenomenon may be used to determine T for the group undergoing the motion (7); however, the severe broadening does limit the ability to measure high-frequency relaxation times in such temperature intervals. The C spin-lattice relaxation times for isotactic PP are shown in Figure 2. Primarily, the data represent that of the crystalline component. The semilog plots of intensity vs. time were nearly exponential for each of the carbons at all temperatures. Over the temperature range, each carbon in the repeat unit displays an individual relaxation time. The methyl relaxation appears to be dominated by methyl C reorientation. If it is assumed that a C - H heteronuclear relaxation mechanism is operative, a calculation of the methyl carbon relaxation time based on a Bloembergen-Purcell-Pound (BPP) formalism and the correlation time at the proton T j minimum (8) at - 1 1 0 ° C gives a value of 10 ms at - 1 1 0 ° C , in good agreement with the observed value of 17 ms. In addition, the methyl motion also seems to dominate the backbone relaxation. This is evidenced by the shorter T j observed for the methine carbon relative to methylene (despite there being two direct C - H interactions for the methylene carbon). Apparently, backbone motions are characterized by such small amplitudes and low frequencies that contributions from the direct C - H interactions to spectral density in the M H z region of the frequency spectrum are minor relative to those from side groups. The 1 / r distance dependence of dipolar relaxation thus accounts for both the long T j values of C H and C H carbons (one to two orders of magnitude) relative to the methyl carbon and the shorter T j values for methine carbons relative to methylene carbon. The fact that the observed T j minimum for C H and C H carbons is close to that reported for a proton T j minimum (at 30 M H z ) (8) in PP that was assigned to methyl reorientation provides unequivocal support for the dominance of the T j relaxation by methyl protons. c

1

3

3

6

2

2

The T data (Figure 3) for the C H and C H carbons also give an indication of methyl group rotational frequencies. As the temperature is lowered below 163K, the T j for these carbons increases and the T j decreases by roughly an order of magnitude between 163K and 95K, suggesting that the contribution of methyl proton motion to M H z spectral density is decreasing, while increasing in the kHz regime. The C H and C H T do not change greatly over the temperature interval from 163K to ambient, and, in contrast to the T j behavior, the C H carbon has the shorter T . The interpretation of the carbon T data is complicated by the fact that spin-spin (cross-relaxation) processes, as well as rotating-frame spin-lattice processes, contribute to the relaxation (4). Only the latter provide direct information on molecular motion. Although both processes show a dependence on the number of nearest-neighbor protons, the relative insensitivity of T to temperature and the approximate 2:1 ratio of C H / C H T values also suggest that spin-spin processes dominate the relaxation above 163K. (If spin-spin effects dominate the rotating-frame relaxation and the carbon cross-relaxation to the proton l

p

2

2

2

l

l p

p

l

2

C

H

p

l

p

N M R A N D

1

3

MACROMOLECULES

Figure 2. The C spin-lattice relaxation times at 15 M H z for isotactic poly(propylene) methylene ( · ) , methine (O), and methyl (A) carbons.

Molecular Motion in Solid

FLEMING ETAL.

Polymers

11 I I I 1—J I 1 1—ι—ι—ι—ϊ 2 4 6 8 10 12 14 1000/Τ(°Κ) Figure 3. The T data at 57 kHz for C H (O), C H ( · ) , and C H carbons in poly (propylene). l

p

2

3

(•)

90

N M R A N D

MACROMOLECULES

dipolar reservoir is less efficient than the corresponding proton spin-lattice relaxation T , the observed T for the C H carbon will be about 1 . 7 - 2 . 0 χ that of the C H carbon, based on the approximate twofold difference in the second moments (due to protons) for the two types of carbons. This is roughly the result observed in the data displayed in Figure 3.) Below 163K, the T of both carbons shorten and tend toward equality, indicating that spin-lattice processes derived from methyl reorientation are becoming competitive with the spin-spin process in relaxing the backbone carbon magnetization. McBrierty et al (8) report a proton T minimum at 97K, which reflects methyl reorientation at kHz frequencies. No clear minimum is observed in the C data, perhaps due to the interplay of the spin-spin and spin-lattice processes. Nonetheless, it is apparent that the methyl protons are responsible for the spin-lattice contributions to the C H and C H T values. 1

D

l

p

2

l

p

l

1

p

3

2

l

p

Further evidence for the effect of spin-spin processes on T in PP is given in Figure 4, which shows T plotted against the reciprocal of the temperature as a function of the rotating frame field. As indicated in Table 1, in the case of motion dominating T , there is a square dependence of T on field. For spin-spin domination, there is an exponential dependence. The results at room temperature clearly display a dependence greater than the 4 χ suggested for motion and the field variation. Only at temperatures less than 150K with large rotating-frame fields are strong motional effects observed. As previously discussed, these arise from methyl rotation. The domination of both spin-lattice relaxation times for C H and C H carbons in PP by methyl reorientation is clearly disappointing, since the potential for information on backbone motion due to the high resolution of the C P M A S experiments is not realized. The implication is that it may not be possible to observe backbone motion in crystalline materials having rapidly reorienting side groups without resorting to deuterium substitution of these side groups. The T j data for various carbons in P M M A are given in Figure 5. Cleai deviations from nonexponential behavior of the magnetization were often observed. Behavior different from that observed for PP presumably arises because the high degree of stereoregularity and high crystallinity of the PP provide a more homogeneous local environment than in glassy P M M A , where distributions of relaxation times are commonly observed, owing to site heterogeneity. For P M M A , the reported relaxation times represent the long-time portion of the magnetization decay curves. The results for P M M A tend to cluster over the temperature range studied, except for the α-methyl carbon. The rapid relaxation for this carbon in the temperature range from 2 0 ° C to - 7 0 ° C is consistent with the proton T j minimum at about -23 ° C assigned to α-methyl rotation at M H z frequencies (9). The T data for P M M A are summarized in Table 2. As in the case of PP, the a - C H undergoes motional broadening and disappears from the spectrum near the minimum in T . In P M M A , severe broadening occurs in the temperature range between 140K and 200K. At lower temperatures, the l

l

l p

p

p

l

p

2

l

p

3

l p

6.

FLEMING ETAL.

Molecular Motion in Solid Polymers

Ο CH Δ CH ©A 33 KHz 2

1001

•

ΟΔ45 •A 63

KHz

KHz

50 r •

A 6

J 8

L 10

(1000/T)

Figure 4. T h e T j for methine (circles) and methylene (triangles) carbons poly(propylene) as a function of the rotating-frame field and temperature; (a) 33 k H z , (b) 45 k H z , and (c) 63 k H z .

92

N M R A N D MACROMOLECULES

T A B L E 1.

Motion:

1

3

CT

l

p

formalism.

BPP dipolar

_ l . = ΞίφΙ f(r ) = { c

2

«

ω

ι

)

J

(

<

0

+

" "^

}

+

2

+

f(Tc)

3J(

where Ηω ) {

= 2 T / ( 1 + ω?τ*) . c

For long T , - J — oc - L c

Spin effects:

T

1P

J

D

( « )

=

i r T

1 — —

D

e -

,

e

i c

2πν τ ίη

oc

e

1 C

D

ΊΛ

|

T

D

W

H

)

+

3Ι(ω

Η

+ω,)}

Molecular Motion in Solid

FLEMING E T A L .

PMMA, T ^ 60

-73

c p

Polymers

vs T e m p e r a t u r e

-130

-162

-182

-196

T., (sec)

• = c=o 0 = CH

2

* = OCH ο = _co = aCHo

3

10"

2

i \ l pi 4

i_J 6

I 8

I

1 10

1

1 12

L

14

1000/T(°K)

Figure 5. The T j data for various carbons in poly (methyl methacrylate).

94

N M R A N D

T A B L E 2.

3

C T

l

p

data for poly(methyl methacrylate) ( P M M A ) .

Temperature (K)

Carbon Τ (ms) 1

-CH

293 253 213 193 173 133 113 78

MACROMOLECULES

3

26 14 5 —

~

11 31 23

-C=0

- C -

271 343 134 139 102 294 366 314

-OCH3

135 227 164 156 124 157 132 82

187 174 118 98 104 158 186 178

α-methyl carbon line narrows as the decoupling becomes effective. The T data for the quaternary carbonyl and methoxy carbons also reflect the methyl motion. Again, this demonstrates the effectiveness of methyl motion in relaxing other carbons in solids via long-range C - H dipolar interactions. Thus, despite the resolution of the C P M A S experiment, the extraction of information on local motional rates from carbon T j and T data on P M M A is not straightforward. These initial results on the temperature dependence of C spin-lattice relaxation times indicate the importance of V T - M A S experiments in the interpretation of relaxation pathways and suggests such experiments will help sort out the various motional processes. l

l

p

p

1

3

Literature Cited 1. Schaefer, J.; Stejskal, E. O.; Buchdahl, R. Macromolecules 1975, 8, 291-296 and 1977, 10, 384-405. 2. Schaefer, J.; Sefcik, M. D.; Stejskal, E. O.; McKay, R. A. Macromolecules 1981, 14, 188-192 and references therein. 3. Garroway, A. N.; Moniz, W. B.; Resing, H. A. ACS Symposium Series 1979, 103, 67-87. 4. Garroway, A. N.; VanderHart, D. L. J. Chem. Phys. 1979, 71, 2772-2787. 5. Fyfe, C. Α.; Mossbrugger, H.; Yannoni, C. A. J. Magn. Reson. 1979, 36, 61-68 and Lyerla, J. R. Contemp. Top. Polym. Sci. 1979, 3, 143-213. 6. Torchia, D. A. J. Magn. Reson. 1978, 30, 613-616. 7. Rothwell, W. P.; Waugh, J. S. J. Chem. Phys. 1981, 74, 2721-2732. 8. McBrierty, V. J.; Douglass, D. C.; Falcone, D. R. J. Chem. Soc., Faraday Trans. II 1972, 68, 1051-1059. 9. Powles, J. G.; Mansfield, P. Polymer 1962, 3, 336-339. RECEIVED December 10, 1983

NEW TECHNIQUES IN NMR OF LIQUID POLYMERS

7 New NMR Experiments in Liquids GEORGE A. GRAY Varian Associates, Palo Alto, CA 94303

The recent past has been the most e x c i t i n g and productive i n the short h i s t o r y of NMR as an a n a l y t i c a l technique. The previous developments of homogeneous and higher f i e l d magnets, multinuclear c a p a b i l i t e s and the introduction of pulsed and fourier methods were spaced over a much longer time and new experiments took years to gain widespread attention and use. New experiments were t i e d to new hardware and generation of new approaches r e l i e d on performing instrument development. Because of the "home-built" and specialized nature of technique development, rapid deployment of new experiments was impossible. P a r t i c u l a r l y within the l a s t five years, new instruments have been introduced which have accelerated the development of new experiments, and at the same time made t h e i r widespread use p o s s i b l e . The r e s u l t has been an explosion i n the number of strategies and applicable approaches. This chapter is intended to give an overview of these new developments and to provide general c l a s s i f i c a t i o n s by which advantages and drawbacks may be understood. On modern instruments the instrument time which any one experiment a c t u a l l y consumes i s a c t u a l l y shrinking, while the time necessary to i n t e l l i g e n t l y design and carry out the spectroscopic approach i s growing. Fortunately, the information gleaned from the e f f o r t i s growing at a much faster r a t e . Instrumental Requirements. The topic "New Experiments" i s s p e c i f i c a l l y intended to cover new ways of generating spectroscopic information. That i s , the techniques are not s p e c i f i c a l l y oriented toward one 0097-6156/84/0247 0097S06.50/0 © 1984 American Chemical Society

98

N M R A N D MACROMOLECULES

nucleus or type o f sample, but r e l y on fundamentally new m e t h o d s o f p e r t u r b i n g t h e n u c l e a r s p i n s y s t e m . The r e s u l t i n g s i g n a l s a r e d e t e c t e d i n t h e t r a d i t i o n a l manner. H e n c e , t h e f o c u s i n "New E x p e r i ments" i s on the events p r i o r to s i g n a l a c q u i s i t i o n . There i s nothing unique about pulses o f r f i n t h i s classification. CW m e t h o d s c a n a n d h a v e b e e n i m p l e mented. However, the i n s t r u m e n t a l c o n v e n i e n c e and the a b i l i t y to "instantaneously" affect nuclear s p i n s g i v e s h i g h - p o w e r , p u l s e d NMR a d e c i d e d a d v a n tage. T h u s , t h e t e r m " p u l s e s e q u e n c e " h a s come t o symbolize the r e c i p e for "preparing" a nuclear spin system. V a r i a t i o n s o f p l a c e m e n t , t i m i n g and phases o f t h e s e p u l s e s , on one o r more n u c l e i , p r o v i d e the r i c h n e s s and d i v e r s i t y upon w h i c h the r a p i d d e v e l o p ments o f the l a s t f i v e years rest. Spectral

Editing

Using

J-Modulated Spin-Echos

The f a v o r a b l e p r o p e r t i e s o f 1 8 0 ° r e f o c u s s i n g p u l s e s have been e x p l o i t e d i n two m a i n e f f o r t s ; o b t a i n i n g more s p e c t r a l i n f o r m a t i o n b y c a u s i n g s p e c t r a to depend on J and the number o f c o u p l e d n u c l e i , and secondly, discussed later, i n polarization transfer methods. Of c o u r s e , t h e s i m p l e s t way t o make s p e c t r a depend on J , e i t h e r homonuclear o r h e t e r o n u c l e a r , is t o r u n a c o u p l e d s p e c t r u m . The l o w s e n s i t i v i t y o f l ^ C , along w i t h the severe o v e r l a p i n a coupled s p e c t r u m , make t h i s a p p r o a c h n o t v e r y u n i v e r s a l . F r e q u e n t l y , an i n t e r m e d i a t e l e v e l o f i n f o r m a t i o n i s required, e.g., j u s t a k n o w l e d g e o f the number o f attached protons for the v a r i o u s carbons i n a NMR s p e c t r u m . F o r m a n y y e a r s t h e m e t h o d o f c h o i c e was t h e s i n g l e - f r e q u e n c y o f f - r e s o n a n c e d e c o u p l i n g ( S F O R D ) t e c h n i q u e (.1) w h i c h y i e l d s p a r t i a l l y c o l lapsed m u l t i p l e t s i n the X spectrum for X H spin systems. The m u l t i p l e t s a r e b r o a d due t o u n c o l l a p s e d l o n g - r a n g e c o u p l i n g s and are d i f f i c u l t to i n t e r p r e t i n complex m o l e c u l e s . The d e s i r a b l e features o f broadband-decoupled spectra are compell i n g , p a r t i c u l a r l y f o r small samples o r congested spectra. n

1

3

Fortunately, C s p i n echo p u l s e sequences have been developed (2-17) which a l l o w broadbandd e c o u p l i n g and c a r b o n m u l t i p l i c i t y s e l e c t i o n (the number o f a t t a c h e d p r o t o n s ) . Patterned after an experiment introduced for studying proton-proton

7. G R A Y

New NMR Experiments in Liquids

c o u p l i n g s ( 1 J 3 ) , the c e n t r a l theme i s the amplitude and phase m o d u l a t i o n o f , f o r example signals, due t o h e t e r o n u c l e a r s p i n c o u p l i n g . This modulation describes the a c t i o n o f magnetization generated by a pulse under the c o n d i t i o n o f c o u p l i n g to another spin — therefore, the d e c o u p l e r must be t u r n e d o f f f o r some p e r i o d o f t i m e . The e f f e c t may be generated i n e i t h e r o f two ways, gated (inter rupted) decoupling or by applying simultaneous pro t o n and C 1 8 0 ° p u l s e s . The former p u l s e sequence i s noted i n Figure 1 along w i t h the effects o f v a r i a t i o n o f Τ w i t h i n the pulse sequence. Three useful c h a r a c t e r i s t i c s o f t h i s approach are obvious: (1) q u a t e r n a r y c a r b o n s a r e u n m o d u l a t e d , (2) a t = 1/2J a l l p r o t o n a t e d c a r b o n s a r e n u l l e d , a n d ( 3 ) a t t? = l / j q u a t e r n a r i e s and methylene carbons are at oppo s i t e phase from m e t h y l and m e t h i n e carbons. 1

3

The i n v e s t i g a t o r t h u s h a s t h e o p t i o n o f s e a r c h i n g f o r q u a t e r n a r i e s w h i c h might be masked by p r o tonated carbons by s e t t i n g = 1/2J; o r , i t might be more d e s i r a b l e t o s o r t b y CH/CH3 and CH2/C. It is a l s o p o s s i b l e t o combine n o r m a l s p i n - e c h o and modu lated spin-echo spectra to a r r i v e at edited subspec t r a c o n t a i n i n g o n l y carbons o f one type (6,14-17). P a t t and S h o o l e r y (8) have a d d r e s s e d the p r o b lem o f o p t i m i z i n g s e n s i t i v i t y i n t h i s experiment i n t h e i r APT ( A t t a c h e d P r o t o n T e s t ) e x p e r i m e n t . The normal use o f a 9 0 ° p u l s e t o b e g i n the sequence has t h e e f f e c t o f s a t u r a t i n g l o n g e r Τχ n u c l e i a n d t h e r e fore requires longer equilibrium delays. In normal e x p e r i m e n t s i t i s customary t o use an intermediate p u l s e angle set r e l a t i v e t o the expected Τ χ ' s and r e p e t i t i o n rate o f the experiment. By i n c o r p o r a t i n g a second 1 8 0 ° pulse j u s t p r i o r to a c q u i s i t i o n they show t h a t z - m a g n e t i z a t i o n l e f t a f t e r an initial s u b - 9 0 ° pulse i s restored to the z - a x i s by the last 180° pulse, thereby allowing faster repetition of the experiment. An example o f t h e use o f t h e APT i s g i v e n i n F i g u r e 1 f o r an e t h y l e n e - 1 - h e x e n e copolymer. I n v e r t e d s i g n a l s a r i s e f r o m C H a n d CH3 carbons. C o n t r a s t t h i s d i r e c t d e t e r m i n a t i o n t o the more p o o r l y - r e s o l v e d and l o w e r s e n s i t i v i t y o f f - r e s o n a n c e data also given. The d e l a y e d technique can be

detection aspects used to e l i m i n a t e

o f the spin-echo protonated

100

NMR A N D MACROMOLECULES

Ethylene-1 -Hexene Copolymer (97%)

(3%)

15% in trichlorobenzene/C6D6 10mm/130C/XL-400

OFF-RESONANCE SPECTRUM

13C Chemical Shift (ppm)

F i g u r e 1. 1 0 0 MHz C spectra of ethylene-1hexene. The APT spectrum was o b t a i n e d w i t h a t a u o f 7 m i l l i s e c o n d s , i n v e r t i n g t h e methylene carbons. F i g u r e courtesy of V a r i a n A s s o c i a t e s . 1 3

7.

GRAY

New NMR

Experiments in Liquids

carbons from a C spectrum. However, no J m o d u l a t i o n p e r i o d need be u s e d . Rather, the proton d e c o u p l e r i s p l a c e d o f f - r e s o n a n c e a n d i s e i t h e r CW or broadband-modulated. The d i s t a n c e o f f - r e s o n a n c e must be enough t o s e v e r e l y b r o a d e n t h e r e s o n a n c e s o f protonated carbons, without s i g n i f i c a n t l y affecting t h e l i n e w i d t h ( o r T2) of nonprotonated carbons. By d e l a y i n g a c q u i s i t i o n f o r a s h o r t p e r i o d f o l l o w i n g an i n i t i a l 9 0 ° p u l s e the signals for protonated car bons d e c a y r a p i d l y and a r e n o t d e t e c t e d . The q u a t e r n a r y c a r b o n s a r e d e t e c t e d e a s i l y and p h a s i n g t h e s p e c t r u m i s made s i m p l e r b y i n s e r t i n g a r e f o c u s s i n g 1 8 0 ° p u l s e midway b e t w e e n t h e 9 0 ° p u l s e and a c q u i s i t i o n . C o o k s o n and S m i t h (14) h a v e applied t h i s technique to petroleum mixtures, c l e a r l y d e t e c t i n g the quaternary aromatic c a r b o n s . E x t e n s i o n o f t h i s method t o n o n - p r o t o n a t e d c a r b o n s i n p o l y m e r s i s d i r e c t and s t r a i g h t f o r w a r d , allowing d i r e c t d e t e c t i o n o f branching s i t e s , f o r example, which are hidden underneath a large protonated c a r bon e n v e l o p e . 1

J

Beloeil, et. a l . ( 1 2 ) h a v e i n t r o d u c e d a new type o f spin-echo sequence which produces e i t h e r C/CH2 o r CH/CH3 C subspectra. It i n v o l v e s an i n i t i a l 4 5 ° p u l s e f o l l o w e d b y f - 1 8 0 ° - T p e r i o d and a f i n a l m o n i t o r i n g 4 5 ° p u l s e e i t h e r i n phase o r 1 8 0 ° out o f phase w i t h r e s p e c t to the o r i g i n a l 4 5 ° p u l s e . The p r o t o n d e c o u p l e r i s g a t e d o f f d u r i n g t h e s e c o n d *£(= l / j ) p e r i o d . 1

Polarization

3

Transfer

Methods

Those f a m i l i a r w i t h the r o u t i n e a c q u i s i t i o n o f C NMR s p e c t r a a r e a w a r e o f t h e c o n s e q u e n c e s o f t h e nuclear Overhauser e f f e c t (NOE). Saturation of pro tons has the e f f e c t o f i n c r e a s i n g the net C mag n e t i z a t i o n o f those carbons r e l a x e d by the protons o f up t o a f a c t o r o f t h r e e t i m e s t h e e q u i l i b r i u m magnetization. Most a n a l y t i c a l o r s u r v e y C spec t r a are obtained with continuous broadband proton d e c o u p l i n g and any r e s u l t a n t NOE. Characteristics o f t h i s mode o f o p e r a t i o n a r e , (1) t h e p o s s i b i l i t y o f v a r i a b l e NOE, (2) r e p e t i t i o n r a t e g o v e r n e d b y ^- C Τχ a n d (3) b o t h p r o t o n a t e d a n d n o n - p r o t o n a t e d c a r bons are d e t e c t e d . T h e f i r s t a s p e c t makes q u a n t i t a tion difficult. The s e c o n d a f f e c t s n e t sensitivity, and the t h i r d h a s t h e p r o s p e c t o f h a v i n g u n d e s i r a b l e signals in certain situations. 1

3

1 3

1 3

3

102

NMR

A N D MACROMOLECULES

S i n c e q u a n t i t a t i o n i s c r u c i a l i n many polymer a n a l y s e s , i t i s i m p o r t a n t t o o b t a i n d a t a w i t h Τχ and NOE i n mind. H i g h l y f l e x i b l e p o l y m e r s u n d e r g o i n g r a p i d segmental m o t i o n t y p i c a l l y g i v e narrow C l i n e s . O f t e n t h e s e c a r b o n s c a n have Τχ's o f s e v e r a l seconds and f u l l o r n e a r l y f u l l NOE. O t h e r , more r i g i d p o l y m e r s may e x h i b i t b r o a d l i n e s and l i t t l e NOE. I n t h e case o f e t h y l e n e - l - h e x e n e copolymer t h e r e i s c o n s i d e r a b l e NOE f o r t h e 1-hexene p o r t i o n ( F i g u r e 2 ) . R e l a t i v e peak a r e a s c a n produce good c o n c e n t r a t i o n s o n l y i f Τχ and NOE a r e p r o p e r l y con sidered . 1 3

A fundamentally d i f f e r e n t approach t o s i g n a l e x c i t a t i o n i s present i n p o l a r i z a t i o n t r a n s f e r methods. These r e l y on t h e e x i s t e n c e o f a r e s o l v a b l e J c o u p l i n g between two n u c l e i , one o f w h i c h (normally t h e proton) serves as a p o l a r i z a t i o n s o u r c e f o r t h e o t h e r . The e a r l i e s t o f t h e s e type o f e x p e r i m e n t s were t h e SPI ( S e l e c t i v e P o p u l a t i o n I n v e r s i o n ) type ( 1 9 ) i n w h i c h low-power s e l e c t i v e pulses are applied to a s p e c i f i c X - s a t e l l i t e i n the p r o t o n spectrum f o r an X-H system. The r e s u l t a n t p o p u l a t i o n i n v e r s i o n p r o d u c e s an enhanced m u l t i p l e t i n t h e X spectrum i f d e t e c t i o n f o l l o w s t h e i n v e r s i o n . A b a s i c improvement w h i c h removes t h e need for s e l e c t i v e p o s i t i o n i n g o f t h e proton frequency was t h e i n t r o d u c t i o n o f t h e INEPT ( I n s e n s i t i v e Nucleus E x c i t a t i o n by P o l a r i z a t i o n Transfer) t e c h n i q u e b y M o r r i s and Freeman (20 ) . T h i s t e c h n i q u e uses s t r o n g n o n - s e l e c t i v e p u l s e s and g i v e s g e n e r a l s e n s i t i v i t y enhancement. The p u l s e sequence h a s a b a s i c p o l a r i z a t i o n t r a n s f e r p o r t i o n ( F i g u r e 3) w h i c h p r o d u c e s a n e t i n v e r s i o n o f one o f t h e p r o t o n s p i n s t a t e s . F o l l o w ing an X n u c l e u s 90° p u l s e t h e r e e x i s t s enhanced m a g n e t i z a t i o n i n t h e X m u l t i p l e t . The s i g n a l enhancement i s p r o p o r t i o n a l t o t h e r a t i o o f t h e magn e t o g y r i c r a t i o s o f t h e two n u c l e i i n v o l v e d , a f a c tor o f 4 f o r C and 10 f o r N f o r X- H e x p e r i ments. The r e p e t i t i o n r a t e o f t h e e x p e r i m e n t i s d i c t a t e d by t h e Τχ o f t h e p o l a r i z a t i o n s o u r c e nucleus. S i n c e t h i s i s t y p i c a l l y t h e p r o t o n , more s i g n a l p e r u n i t t i m e o f t e n c a n be o b t a i n e d t h a n b y NOE. S i n c e t h e r e l a x a t i o n mechanism f o r t h e X n u c l e u s i s n o t i n v o l v e d , v a r i a b l e o r n e g a t i v e NOE i s n o t a p r o b l e m , as c a n be t h e case f o r ^ N and 2 9 g i One drawback i s t h e e f f e c t o f s h o r t T 2 ' s on t h e sen s i t i v i t y improvement. S i n c e t h e sequence r e q u i r e s 1

3

1 5

1

1

β

7.

GRAY

New NMR Experiments in Liquids

Ethylene-1-Hexene Copolymer

15%in TCB/C6D6

ct£

10mm/130°C/XL-400 90

sec

delay

w/o NOE

1

3

F i g u r e 2. Comparison o f C s p e c t r a w i t h and w i t h o u t n u c l e a r o v e r h a u s e r enhancement. F i g u r e courtesy of Varian Associates.

103

104

NMR AND MACROMOLECULES

F i g u r e 3.

Polarization transfer

excitation

INEPT. F i g u r e c o u r t e s y o f V a r i a n A s s o c i a t e s .

using

7. G R A Y

New NMR Experiments in Liquids

delays on the order o f l / j before d e t e c t i o n , (X-H c o u p l e d systems w i t h J o f 10-100 Hz r e q u i r e d e l a y s o f f r o m 1 0 0 - 1 0 ms) s h o r t p r o t o n Τ 2 s o r X n u c l e u s T2's i n decoupled p o l a r i z a t i o n transfer experiments can r a d i c a l l y lower expected g a i n s . For example, i f t h e p r o t o n s h a v e a Τ 2 o f 3 0 ms ( l i n e w i d t h i n t h e p r o t o n s p e c t r a o f ^10 H z ) , 2/3 o f t h e p r o t o n magnet i z a t i o n w i l l d e c a y away i n 30 ms. T h i s would be the same t i m e n e c e s s a r y t o a t t a i n p o l a r i z a t i o n t r a n s f e r f o r a s y s t e m h a v i n g an X - H J c o u p l i n g o f 16 H z . For t y p i c a l one-bond c o u p l i n g s o f 40-250 Hz f o r a n d C , t h i s l o s s i s l e s s important because o f the l / J delay dependence. A l t h o u g h NOE f a l l s o f f a s field s t r e n g t h and m o l e c u l a r s i z e i n c r e a s e s , i t s t i l l may have a net advantage for macromolecules because o f 1

1

1 3

W h i l e t h e f i r s t r e p o r t e d INEPT e x p e r i m e n t d w e l t on a g e n e r a t i o n o f enhanced coupled s p e c t r a , later reports (21-23) extended the b a s i c sequence to allow broadband decoupling during d e t e c t i o n , realiz ing the g o a l o f g e n e r a l s e n s i t i v i t y enhancement i n a more w i d e l y u s e f u l form. As a s i d e b e n e f i t , through proper selection of a refocussing period, spectral editing is possible. No q u a t e r n a r y c a r b o n s a r e u s u a l l y observed since delays are t y p i c a l l y set for J's appropriate for protonated X nuclei. XH o n l y s p e c t r a c a n be o b t a i n e d e a s i l y , as w e l l as s p e c t r a i n w h i c h XH2 i s i n v e r t e d w i t h r e s p e c t t o X H a n d XH3. The c o m b i n a t i o n o f t h e s e two s p e c t r a t h e n a l l o w s d i r e c t i d e n t i f i c a t i o n o f X H , X H 2 a n d XH3 s i g n a l s i n even the most complex m o l e c u l e s i n a s i m i l a r manner as i n the J modulated s p i n echo e x p e r i m e n t s described above. An a n a l o g o u s c a s e i s p r e s e n t i n t h e l^C-^H INEPT e x p e r i m e n t (24 ) . Here, only deuterated car bons are o b s e r v e d . While the magnetogyric r a t i o f a c t o r s h o u l d g i v e o n l y a t h e o r e t i c a l 3/5 enhance ment, the s h o r t Τχ a l l o w s v e r y r a p i d a c c u m u l a tion. The t e c h n i q u e s h o u l d be u s e f u l f o r d e t e r m i n a tion of deuterated X nuclei spectra without i n t e r f e r e n c e s from p r o t o n a t e d X s i g n a l s . The s e n s i t i v i t y g a i n s h o u l d be v e r y d r a m a t i c f o r f u l l y deu t e r a t e d X n u c l e i where normal X nucleus Τ χ ' s can be extremely long because o f i n e f f i c i e n t d i p o l a r r e l a x a t i o n from H or other mechanisms. This experiment offers the c a p a b i l i t y o f d e t e c t i o n o f deuterated s i t e s at small concentration i n , for example, 2

106

NMR

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s t u d i e s o f t h e mechanism o f d e u t e r a t i o n ( o r r e a c t i o n u s i n g d e u t e r a t e d r e a c t a n t s , e.g., o x i d a n t s ) o f macromolecules. W h i l e p o s s e s s i n g s e v e r a l a d v a n t a g e s , INEPT has p r o p e r t i e s w h i c h a r e u n d e s i r a b l e : (1) c o u p l e d spec t r a have, a t t i m e s , m i s s i n g l i n e s and a n t i p h a s e com p o n e n t s ; (2) t h e r e i s a f a i r l y s t r o n g J dependence i n c a s e s where s i g n a l s s h o u l d n u l l , l e a d i n g t o r e s i d u a l unwanted s i g n a l s ; and, (3) r e f o c u s s e d INEPT d e l a y p e r i o d s a r e v a r i a b l e , depending on m u l t i p l i c i t y desired, leading to p o s s i b l e v a r i a t i o n i n i n t e n s i t y due t o v a r i a t i o n s i n T2 among the r e s o nances. D o d d r e l l , Pegg and B e n d a l l ( 2 5 , 2 6 ) have d e v e l o p e d a D i s t o r t i o n l e s s Enhancement by P o l a r i z a t i o n T r a n s f e r (DEPT) p u l s e sequence ( F i g u r e 4) w h i c h a d d r e s s e s t h e s e problems t o a l a r g e d e g r e e . Coupled X n u c l e i DEPT s p e c t r a have t h e normal b i n o m i a l d i s t r i b u t i o n o f i n t e n s i t i e s , t h e J dependence f o r XH s e l e c t i v i t y i s b e t t e r than f o r INEPT, and the m u l t i p l i c i t y s e l e c t i o n r e l i e s on a v a r i a b l e p r o t o n f l i p angle r a t h e r than v a r i a b l e d e l a y s , thus f a c t o r i n g out T2 dependence. By c h o o s i n g θ = 45°, 90°, and 135° i n s e p a r a t e s p e c t r a , s p e c t r a l e d i t i n g i s p o s s i b l e through proper combinations o f these s p e c t r a to g i v e XH, X H and XH3 s u b s p e c t r a . DEPT i s more sen s i t i v e o v e r a l l t o s p i n r e l a x a t i o n d u r i n g the p u l s e t r a i n s i n c e the r e l e v a n t d e l a y i s o f magnitude 3/2J w h i l e c o r r e s p o n d i n g p e r i o d s i n INEPT r a n g e s from l / J t o 5/4J t y p i c a l l y . 2

Two-Dimensional

NMR

P r o b a b l y t h e g r e a t e s t r e c e n t change i n the p r a c t i c e o f NMR has been the e x p l o s i v e growth i n t e c h n i q u e s and a p p l i c a t i o n s o f 2D NMR. 2D NMR i s an e x t e n s i o n o f o r d i n a r y NMR. The b a s i c p r i n c i p l e was i n v e n t e d by J e n n e r (27) and cov e r s e s s e n t i a l l y a l l 2D e x p e r i m e n t s i n NMR, as w e l l as o t h e r a r e a s . The g e n e r a l 2D NMR e x p e r i m e n t i s c h a r a c t e r i z e d by up t o f o u r t i m e p e r i o d s : Preparation...Evolution(t^)... Mixing(t )...Detection(t2) m

P r e p a r a t i o n t i m e i s n e c e s s a r y t o b r i n g t h e system t o a known s t a t e , e.g., e q u i l i b r i u m m a g n e t i z a t i o n , and i s u s u a l l y a f i x e d d e l a y t i m e . At t h e b e g i n n i n g o f t h e E v o l u t i o n p e r i o d , t h e s p i n s a r e p e r t u r b e d and

GRAY

New NMR Experiments in Liquids

DEPT Pulse Sequence ο

·

90 equil.

·

180

1/2J

Ο

1/2J ο

90

1/2J

180

J U U

Ί

10

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F i g u r e 4. The e d i t e d s p e c t r a a r e t h e r e s u l t s p e c i f i c combinations o f d i f f e r e n t spectra o b t a i n e d f o r θ = 4 5 ° , 90° a n d 135© u s i n g a solution of cholesteryl acetate i n CDCI3. Figure courtesy of Varian Associates.

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F i g u r e 5. The d a t a f r o m t h e 2D e x p e r i m e n t i s d i s p l a y e d i n contours o f i n t e n s i t y , each a x i s represents a proton s h i f t axis. The o f f - d i a g o n a l i n t e n s i t i e s show h o m o n u c l e a r c o u p l i n g s . For i n s t a n c e , t h e p r o t o n a t 950 Hz ( a s i n d i c a t e d b y an a r r o w ) h a s o f f - d i a g o n a l i n t e n s i t y s h o w i n g c o u p l i n g s to the four other i n d i c a t e d protons. Figure

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a l l o w e d t o e v o l v e w i t h i n a s p e c i f i c environment t h a t may be d i f f e r e n t from any environment i n t h e o t h e r time p e r i o d s . This E v o l u t i o n time i s r e g u l a r l y v a r i e d t h r o u g h a range o f v a l u e s from z e r o t o some maximum, each r e s u l t i n g i n a s e p a r a t e FID r e c o r d e d f o r a f i x e d time d u r i n g the d e t e c t i o n p e r i o d (Cer t a i n 2D e x p e r i m e n t s use a m i x i n g , o r s p i n exchange period). The r e s u l t i s a c o l l e c t i o n o f FID's, t h e number o f w h i c h e q u a l s t h e number o f d i f f e r e n t v a l u e s o f t h e E v o l u t i o n t i m e p e r i o d . A l l FID's a r e t h e n t r a n s f o r m e d i n t h e u s u a l way, r e s u l t i n g i n a c o l l e c t i o n o f spectra (the f a m i l i a r inversionr e c o v e r y Τχ e x p e r i m e n t i s an example o f t h i s s t a g e o f t h e p r o c e s s ) . A new c o l l e c t i o n o f FID's i s assembled b y t a k i n g d a t a p o i n t s from each spectrum at given frequency v a l u e s . For example, v a l u e s o f s p e c t r a l i n t e n s i t y i n each spectrum found a t a f r e q u e n c y f a r e t a k e n and arranged i n a data t a b l e . This i s repeated f o r e v e r y f v a l u e i n t h e spectrum, r e s u l t i n g i n Ν FID's when Ν i s t h e number o f p o i n t s i n t h e o r i g i n a l spec trum. These Ν FID's a r e t h e n f o u r i e r t r a n s f o r m e d , r e s u l t i n g i n Ν s p e c t r a . These Ν s p e c t r a , p r e s e n t e d on a t w o - d i m e n s i o n a l p l o t , a r e now c h a r a c t e r i z e d b y two f r e q u e n c y a x e s . One f r e q u e n c y (F2) i s always t h a t o f t h e normal o b s e r v e spectrum s i n c e i t r e s u l t s from FT o f normal FID's. The s i g n i f i c a n c e o f t h e remaining ( F l ) frequency i s determined by t h e p u l s e sequence used. There have been m a j o r advances i n t h e l a s t few y e a r s b o t h i n t h e number and v a r i e t y o f t h e p u l s e sequences used t o p e r t u r b t h e s p i n s , and i n t h e com p u t i n g power and d a t a p r o c e s s i n g t e c h n i q u e s employed. A l l two d i m e n s i o n a l e x p e r i m e n t s use e s s e n t i a l l y t h e same d a t a r e d u c t i o n p r o c e s s . S e v e r a l y e a r s ago t h e d a t a a c q u i s i t i o n phase o f 2D e x p e r i m e n t s was u s u a l l y the s h o r t e s t , w i t h double t r a n s f o r m a t i o n r e q u i r i n g up t o s e v e r a l h o u r s f o l l o w e d by, a g a i n , comparable p l o t t i n g t i m e . O f f - l i n e d a t a p r o c e s s i n g was o f t e n used f o r f a s t e r p r o c e s s i n g , b u t t h i s s e v e r e l y l i m i t e d any w i d e s p r e a d use o f 2D methods. 2D p r o c e s s i n g c a p a b i l i t i e s became g e n e r a l l y a v a i l a b l e on commercial s p e c t r o m e t e r s a f t e r 1979 and w i t h t h e i n t r o d u c t i o n o f f l e x i b l e p u l s e programmers t h e

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u s e o f 2D m e t h o d s h a s g r o w n q u i c k l y . Very recently, new c o m p u t e r h a r d w a r e , i n c l u d i n g A r r a y P r o c e s s o r s , h a s r e v o l u t i o n i z e d 2D d a t a p r o c e s s i n g , a l l o w i n g c o m p l e t e d a t a r e d u c t i o n , i n c l u d i n g f u l l 2D d i s p l a y i n l e s s t h a n 30 s e c o n d s ( f o r a t y p i c a l 512 χ 512 matrix). This speed, coupled w i t h responsive c o l o r graphics, has permitted d i r e c t , o n - l i n e data pro c e s s i n g and removed the f o r m i d a b l e t i m e - b a r r i e r to f u l l u s e o f 2D NMR s p e c t r o s c o p y . J - R e s o l v e d 2D N M R . T h e k e y t o t h e n a t u r e o f t h e F χ domain l i e s i n the environment i n which the spins p r e c e s s d u r i n g t h e e v o l u t i o n p e r i o d t]_. This e n v i r o n m e n t may i n c l u d e t h e i n t e r a c t i o n s o f s p i n coupling, the chemical s h i f t — which governs the r a t e o f p r e c e s s i o n ( r e l a t i v e to the t r a n s m i t t e r fre quency as zero) — and magnet i n h o m o g e n e i t y w h i c h causes i d e n t i c a l n u c l e i to have d i f f e r e n t precession rates. R e f o c u s s i n g p u l s e s c a n remove some, o r a l l , of these interactions. In p a r t i c u l a r , after a 9 0 ° p u l s e use o f a s t r o n g n o n - s e l e c t i v e 1 8 0 ° r e f o c u s s i n g pulse on the observe nucleus midway through the evo l u t i o n period eliminates the effect o f chemical s h i f t d i f f e r e n c e s f o r a l l observe s p i n s as detected i n t-2 f o r a n y t ^ . H e n c e , t h e r e s u l t a n t 2D s p e c t r u m might be thought t o be r a t h e r s i m p l e , as i t is indeed for non-coupled n u c l e i - - singlets are cen tered at coordinates ( F j = 0 , F2 = v g ) . However, t h e m u t u a l J c o u p l i n g o f s p i n s d u r i n g t^ does affect t h e p h a s e o f t h e s i g n a l w h e n d e t e c t e d i n t2# and hence J i n f o r m a t i o n i s encoded i n the detected sig nals . T h e h e t e r o n u c l e a r J - r e s o l v e d 2D e x p e r i m e n t p r o duces c o m p l e t e l y separate i n f o r m a t i o n i n both domains, X n u c l e u s c h e m i c a l s h i f t i n F2 (since b r o a d b a n d d e c o u p l i n g i n t2 removes t h e e f f e c t o f J c o u p l i n g ) and AX c o u p l i n g i n F j . This is particu l a r l y v a l u a b l e , for example, i n C - H cases since complete c o u p l i n g p a t t e r n s can be e x t r a c t e d for each C without overlap of adjacent patterns. Informa t i o n can be e x t r a c t e d e a s i l y , i n c o n t r a s t to d i r e c t o b s e r v a t i o n o f h i g h l y o v e r l a p p e d c o u p l e d ID s p e c t r a . 1

3

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T h e h o m o n u c l e a r J - r e s o l v e d 2D s p e c t r u m i s dif f e r e n t i n t h a t AX c o u p l i n g i s a c t i v e i n b o t h t^ and 2, and thus i s r e f l e c t e d i n the e x i s t e n c e o f s p i n multiplets i n both dimensions. t

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E a c h s p i n p a t t e r n i s p r e s e n t a l t h o u g h no one s l i c e p e r p e n d i c u l a r to F2 c o n t a i n s a whole p a t t e r n . A c o m p u t e r p r o c e s s known a s " t i l t i n g " , "rotating", o r " s h e a r i n g " can be used to r e a l i g n the s p i n p a t terns ( 4 5 ° t i l t ) perpendicular to F 2 · This permits d i s p l a y a n d p l o t t i n g o f " s l i c e s " o f t h e 2D d a t a — a n F± s p e c t r u m a t some s p e c i f i e d F 2 v a l u e . These s l i c e s are the f u l l y coupled s p i n p a t t e r n s . The spin-echo nature o f the experiment produces narrow l i n e s , and t h u s v e r y h i g h l y r e s o l v e d m u l t i p l e t s . In highly congested, but f i r s t - o r d e r , proton spectra t h i s t e c h n i q u e c a n b e o f immense v a l u e . P o l a r i z a t i o n t r a n s f e r may b e u s e d i n h e t e r o n u c l e a r J - r e s o l v e d 2D NMR ( 2 8 ) f o r s i g n a l e n h a n c e m e n t , s h o r t e r e q u i l i b r a t i o n p e r i o d s and e l i m i n a t i o n o f n o n - p r o t o n a t e d r e s o n a n c e s - - a l l t h e same f e a t u r e s a s i n INEPT o r D E P T . In t h i s e x p e r i m e n t t h e refocussing period following polarization transfer becomes the e v o l u t i o n t i m e , r a t h e r t h a n the 1/4J, 1/2J, a n d 3/4J v a l u e s n o r m a l l y u s e d f o r r e f o c u s s e d INEPT o r v a r i a b l e f l i p a n g l e # f o r D E P T . The p o l a r i z a t i o n t r a n s f e r p a r t o f the sequence i n c l u d e s a 1/2J p e r i o d , a s u s u a l , a n d t h u s t h e 2D s e q u e n c e c a n be t a i l o r e d to s p e c i f i c types (or J ' s ) o f XH c o u p l e d p a i r s , e . g . , c e r t a i n long-range couplings for q u a t e r n a r y c a r b o n s whose n o r m a l C Τ χ ' s may b e t o o l o n g t o p e r m i t o r d i n a r y h e t e r o n u c l e a r 2D J e x p e r i ments . 1

3

C h e m i c a l S h i f t C o r r e l a t i o n 2D NMR. Probably the m o s t w i d e l y u s e d 2D NMR e x p e r i m e n t s a r e t h o s e w h i c h r e l a t e chemical s h i f t s of d i f f e r e n t n u c l e i . That i s , F]_ a n d F 2 r e p r e s e n t c h e m i c a l s h i f t a x e s a n d t h e c l a s s i n c l u d e s b o t h h o m o n u c l e a r and h e t e r o n u c l e a r categories. A g a i n , the environment d u r i n g the e v o l u t i o n t i m e d i c t a t e s t h e i n t e r p r e t a t i o n o f t h e new information. The s i m p l e s t t o c o n s i d e r i s t h e homonuclear chemical s h i f t c o r r e l a t i o n (27,29-31), a l s o known i n o n e f o r m a s COSY ( c o r r e l a t i o n s p e c troscopy) . The most w i d e l y u s e d p u l s e s e q u e n c e i s r a t h e r s i m p l e , c o n s i s t i n g of 9 0 ° - t x - 9 0 ° followed by acquisition. T h e r e s u l t i n g p e a k s i n t h e 2D p l o t f a l l i n s e v e r a l c a t e g o r i e s : a x i a l , d i a g o n a l and off-diagonal. T h e a x i a l p e a k s l i e a l o n g F^ =0 a n d r e s u l t from l o n g i t u d i n a l m a g n e t i z a t i o n sampled by the l a s t p u l s e . The d i a g o n a l p e a k s a r e due t o m a g n e t i z a t i o n s w h i c h r e m a i n a s s o c i a t e d w i t h t h e same s p i n s b e f o r e and a f t e r t h e s e c o n d 9 0 ° p u l s e , t h a t i s t h e m a g n e t i z a t i o n h a s t h e same f r e q u e n c y d u r i n g t^

NMR A N D MACROMOLECULES

F i g u r e 6. E a c h p e a k i n t h e 2D s p e c t r u m a r i s e s from a C - H bond. Extrapolation to the relevant axes g i v e the c o r r e s p o n d i n g chemical s h i f t s . The d a t a was o b t a i n e d o n 90 mg i n 1 0 . 5 h o u r s u s i n g a 1 0 mm p r o b e a t 75 M H z ( X L - 3 0 0 ) . F i g u r e c o u r t e s y Varian A s s o c i a t e s .

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and t 2 · The o f f - d i a g o n a l p e a k s c o n f i r m t h e f a c t t h a t m a g n e t i z a t i o n p r e s e n t a t o n e f r e q u e n c y i n t^ i s t r a n s f e r r e d to another frequency a f t e r the second 9 0 ° "mixing" pulse. This i s o n l y p o s s i b l e i f the two n u c l e i a t t h e two f r e q u e n c i e s s h a r e t h e same spin system, t h a t i s , they are J - c o u p l e d . This last f e a t u r e forms the b a s i s o f the w i d e s p r e a d use o f h o m o n u c l e a r s h i f t c o r r e l a t e d 2D NMR s i n c e i t gives equivalent information to spin-decoupling. The s p r e a d o f i n f o r m a t i o n i n two d i m e n s i o n s a c t u a l l y makes t h e i n t e r p r e t a t i o n e a s i e r . P r o p e r p h a s e c y c l i n g (31) c a n remove t h e a x i a l p e a k s , c o n s i d e r a b l y s i m p l i f y i n g the experiment. V a r i a t i o n s on the b a s i c experiment i n c l u d e the 9 0 ° - t x - 4 5 ° - a c q u i r e e x p e r i m e n t (29 ) . T h i s v e r s i o n n a r r o w s t h e p a t t e r n s on the d i a g o n a l p e r m i t t i n g a n a l y s i s o f c o r r e l a t i o n s o f n u c l e i c l o s e l y spaced i n chemical s h i f t . T h e s e c o n d m a j o r f o r m o f s h i f t c o r r e l a t i o n 2D NMR i s h e t e r o n u c l e a r 2D s h i f t c o r r e l a t i o n . The e x p e r i m e n t as p r o p o s e d b y M a u d s l e y and E r n s t (32) can be v i s u a l i z e d as g e n e r a t i n g p r o t o n m a g n e t i z a t i o n , f o r e x a m p l e , b y a 9 0 ° p u l s e and l e t t i n g t h e m a g n e t i z a t i o n p r e c e s s f o r a time t^. The e x t e n t o f p r e c e s s i o n ( i n the r o t a t i n g frame) i s p r o p o r t i o n a l t o the d i s t a n c e o f f - r e s o n a n c e from the p r o t o n t r a n s m i t t e r f r e q u e n c y and t h e r e f o r e i t s phase i s proton s h i f t dependent. Simultaneous 9 0 ° p u l s e s on t h e p r o t o n and X o b s e r v e n u c l e u s t r a n s f e r m a g n e t i z a t i o n t o X — j u s t a s i n t h e INEPT e x p e r i m e n t . The phase o f the X n u c l e u s m a g n e t i z a t i o n i s hence coded with the chemical s h i f t o f the attached p r o t o n . The e x p e r i m e n t i s c a r r i e d o u t f o r t h e f u l l r a n g e o f t^ v a l u e s t o p r o d u c e t h e 2D s p e c t r u m . Since the X n u c l e u s s h i f t i s p r e s e n t i n F2# t h e XH p a i r h a s b o t h chemical s h i f t s i d e n t i f i e d by e x t r a p o l a t i o n to the i n d i v i d u a l a x e s f r o m t h e XH p e a k i n t h e 2D d a t a . Broader u t i l i z a t i o n o f heteronuclear chemical s h i f t 2D h a s r e a l l y j u s t b e g u n . Amman e t . a l . , (33) h a v e i l l u s t r a t e d how 2D m e t h o d s c a n b e u s e d t o a s s i g n p r o t o n s a n d c a r b o n s i n l u p a n e , a C30 triterpene c o n t a i n i n g o n l y c a r b o n and h y d r o g e n . Heteronuc l e a r J - r e s o l v e d 2D was u s e d t o a s s i g n t h e number o f p r o t o n s t o e a c h c a r b o n , and h e t e r o n u c l e a r c h e m i c a l s h i f t c o r r e l a t i o n allowed assignment o f the a s s o c i a t e d p r o t o n s h i f t s a n d many o f t h e h o m o n u c l e a r H-H coupling constants. I k u r a and H i k i c h i (34) u s e d t h e same t e c h n i q u e s on d - b i o t i n . A mixture of a l l y l n i c k e 1 c o m p l e x e s was a l s o s t u d i e d by Benn (3JO , and

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M o r r i s and H a l l (35) have examined a s e r i e s o f c a r bohydrates. Larger molecules are, of course, feasi ble. Chan and M a r k l e y (36) have a s s i g n e d h i s t i d i n e , t y r o s i n e and p h e n y l a l a n i n e p r o t o n a t e d c a r b o n r e s o n a n c e s i n u n i f o r m l y e n r i c h e d (20%) o x i d i z e d ferrod i x i n b y h e t e r o n u c l e a r H / C 2D s h i f t c o r r e l a t i o n N M R . Polypeptide resonances have a l s o been assigned using C / H 2D NMR ( 3 7 , 3 8 ) . I n t h e s e s y s t e m s Η / Η h o m o n u c l e a r , H / C h e t e r o n u c l e a r a n d Ν / Η h e t e r o n u c l e a r 2D techniques have allowed d i r e c t c o n f i r m a t i o n o f a s s i g n m e n t s a n d c o n n e c t i v i t i e s , i n p a r t i c u l a r t h e NH n i t r o g e n s and p r o t o n s . Heteronuclear chemical shift correlation tech niques can be used to i n f e r s p i n - l a t t i c e r e l a x a t i o n times o f the protons attached to the observe nucleus (39.) · T h i s i s a c c o m p l i s h e d b y s a t u r a t i o n o f t h e p r o tons and observe n u c l e u s f o l l o w e d by a v a r i a b l e time t (saturation recovery) during which the observe nucleus i s c o n t i n u e d at s a t u r a t i o n (by repeated p u l s i n g ) and the a t t a c h e d p r o t o n r e m a g n e t i z e s . This p r o c e s s i s f o l l o w e d b y t h e n o r m a l H / X 2D s h i f t correlation experiment. T h e t - d e p e n d e n c e o f t h e 2D peak i n t e n s i t i e s i s then used to e x t r a c t ^H Τχ s by exponential analysis. This approach can be used to extract proton T i ' s i n polymers v i a observation of t h e i r attached ^ C ' s , obtaining motional data nor m a l l y i m p o s s i b l e t o o b t a i n b y p r o t o n NMR. 1

A l l o f the above h e t e r o n u c l e a r s h i f t c o r r e l a t i o n techniques produce spectra which have H-H spin m u l t i p l e t s i n the dimension. These can be i n v a l u a b l e i n c e r t a i n s i t u a t i o n s where the pattern i s o b s c u r e d i n t h e ID s p e c t r u m . At other times sen s i t i v i t y c o n s i d e r a t i o n s would argue for c o l l a p s i n g these m u l t i p l e t s , thereby gaining at least a factor o f two i n i n t e n s i t y . In other cases, h i g h l y con g e s t e d 2D s p e c t r a c o u l d h a v e o v e r l a p p e d H / C c o r r e l a tions. Bax (40) has d e v e l o p e d a method f o r c o l l a p s ing these m u l t i p l e t s which e s s e n t i a l l y replaces the s i n g l e X nucleus 180° pulse at the m i d - p o i n t o f the e v o l u t i o n time w i t h the element 90° (H)-l/2J180° (H)180° (X)-1/2J-90° (H). T h e r e s u l t a n t 2D s h i f t c o r r e l a t i o n spectra are characterized by s i n gle peaks for a X - H bond. S l i c e s i n F^ show p r o t o n s i n g l e t s at the appropriate chemical s h i f t . Of c o u r s e , a p r o j e c t i o n o n t o t h e F]_ a x i s w o u l d g i v e a proton "stick" spectrum. x

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M a g n e t i z a t i o n T r a n s f e r 2D N M R . O n e o f t h e m o r e e x c i t i n g a s p e c t s o f 2D NMR i s t h e c l a s s o f e x p e r i ments which can probe i n t r a - m o l e c u l a r interactions, p r o x i m i t y o f n u c l e i and c h e m i c a l exchange. As o p p o s e d t o s o m e o f t h e a b o v e 2D e x p e r i m e n t s where transverse (X,Y) m a g n e t i z a t i o n t r a n s f e r occurs, t h e r e are e x p e r i m e n t s w h e r e i n l o n g i t u d i n a l (Z) magnetization is transferred, i . e . , energy l e v e l popul a t i o n s change. The n u c l e a r O v e r h a u s e r e f f e c t is one example o f p o p u l a t i o n r e d i s t r i b u t i o n w i t h i n a s p i n system v i a an i n c o h e r e n t p r o c e s s . Other types i n c l u d e s a t u r a t i o n t r a n s f e r experiments where the mechanism i s c h e m i c a l exchange. O t h e r means o f s t u d y i n g c h e m i c a l exchange o r NOE h a v e i n c l u d e d l i n e s h a p e a n a l y s i s a t o r n e a r c o a l e s c e n c e and s e l e c t i v e s a t u r a t i o n o r i n v e r s i o n with subsequent following of propagation o f magnetiz a t i o n throughout the spin system. The f o r m e r technique can be v e r y d i f f i c u l t f o r broad coalescence peaks and r e q u i r e c e r t a i n t e m p e r a t u r e s t o be e s t a blished. The l a t t e r i s v e r y u s e f u l f o r a l i m i t e d number o f l i n e s t o be a f f e c t e d b u t g e t s t i m e c o n s u m i n g f o r more t h a n a few l i n e s and v e r y d i f f i cult for closely-spaced lines. T h e 2D m e t h o d r e q u i r e s no p a r t i c u l a r s p e c i a l c o n d i t i o n s and t h e r e fore has a g r e a t deal o f a t t r a c t i v e n e s s for studies o f c h e m i c a l and b i o c h e m i c a l d y n a m i c s . This class of e x p e r i m e n t s was p r o p o s e d b y J e n n e r e t . a l . (41,42) and e l a b o r a t e d on b y M a c u r a and E r n s t ( 4 3 ) . The b a s i c pulse sequence i s 9 0 ° - t - 9 0 ° - t ( M I ) C T - 9 0 . The second 9 0 ° p u l s e can be thought o f as r e s t o r i n g to the Z - a x i s transverse magnetizations created by the first 90° pulse. The component and d i r e c t i o n o f t h e resultant Z - a x i s magnetization i s a function o f the p r e c e s s i o n o f the spins during t^. Some c o m p o n e n t s w i l l b e p o s i t i v e , some n e g a t i v e a n d some z e r o . These components w i l l o s c i l l a t e as a f u n c t i o n o f t j . The r e s u l t a n t Z - m a g n e t i z a t i o n s w i l l be i n a n o n e q u i l i b r i u m s t a t e and s p i n - l a t t i c e r e l a x a t i o n processes w i l l work toward e q u i l i b r i u m . For the case of protons r e l a x a t i o n i s p r i m a r i l y d i p o l a r v i a other protons, this non-equilibrium magnetization w i l l then be r e d i s t r i b u t e d by mutual s p i n f l i p to other protons. The f i n a l 9 0 ° p u l s e m o n i t o r s the extent of magnetization transfer. T h e 2D e x p e r i m e n t samples a l l d e g r e e s o f m a g n e t i z a t i o n t r a n s f e r as i t increments t^. o

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T h e 2D s h i f t c o r r e l a t i o n s p e c t r u m t h u s produced is c h a r a c t e r i z e d by the usual diagonal peaks coming f r o m m a g n e t i z a t i o n r e m a i n i n g a t t h e same frequency i n ti and t 2 · P h a s e - c y c l i n g can remove the axial p e a k s , j u s t as i n homonuclear s h i f t c o r r e l a t i o n 2D NMR. The o f f - d i a g o n a l p e a k s come f r o m magnetization which has been t r a n s f e r r e d i n a s p i n - l a t t i c e relaxat i o n sense from one t y p e o f s p i n t o a n o t h e r . This c o u l d be because o f t r u e c h e m i c a l exchange o c c u r r i n g d u r i n g the mix t i m e , o r i t c o u l d be s i m p l y from s p i n s c l o s e enough i n space to p r o v i d e mutual dipolar relaxation. T h i s sequence has been used to e s t a b l i s h NOE's i n p o l y p e p t i d e s and p r o t e i n s (45,46). It i s p a r t i c u l a r l y e f f e c t i v e i n macrom o l e c u l e s where t h e r e i s much s l o w e r m o l e c u l a r tumb l i n g and more f a v o r a b l e NOE s. V a r i a t i o n of t(MIX) allows probing of the rates o f magnetization t r a n s f e r and t h u s p r o x i m i t y o f p r o t o n s f o r NOE, o r chemical k i n e t i c s for true exchange (47). 1

Carbon-13 chemical exchange networks have been e x p l o r e d by Huang, Macura and E r n s t (48) u s i n g the above t e c h n i q u e s . Normal 9 0 - t - 9 0 ° - t T M I X ) - 9 0 sequences were used, i n the presence o f proton d e c o u p l i n g ( f o r NOE, s e n s i t i v i t y and simplicity). R e f o c u s s e d INEPT was a l s o used t o p r e p a r e the init i a l magnetization, i n place o f the f i r s t 9 0 ° pulse, to gain additional s e n s i t i v i t y . This extension sugg e s t s f u t u r e f l e x i b i l i t y and s e l e c t i v i t y s i n c e the INEPT p o r t i o n o f the sequence c a n be t a i l o r e d for specific J's ( l o n g - r a n g e and d i r e c t ) , t h u s a l l o w i n g v e r y p r e c i s e c o n t r o l over the s i t e from w h i c h magn e t i z a t i o n can evolve. Huang, e t . a l (48) applied these s t r a t e g i e s to the c l a s s i c exchange problems o f r i n g - p u c k e r i n g i n d e c a l i n , bond s h i f t i n b u l l v a l e n e and s o l v a t i o n s h e l l exchange i n aluminum complexes. The l o n g e r r e l a x a t i o n t i m e s o f c a r b o n - 1 3 c a n actua l l y be put to advantage i n these s t u d i e s s i n c e t h e y permit longer mix times for slower exchange processes. T h e o t h e r m a j o r a d v a n t a g e o f t h e 2D t e c h n i q u e i s t h a t i t can be performed on a v e r y s l o w l y e x c h a n g i n g system whose l i n e s a r e s t i l l narrow. o

o

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GRAY

New NMR

Experiments in Liquids

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2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

E r n s t , R . R . , J. Chem. Phys. 1966, 45, 3845; Reich, H.J.; J a n t e l a t , M . ; Meese, M . T . ; Weigert, F.J.; J . D . Roberts, J . Am. Chem. Soc. 1969, 91, 7445. Anet, F.A.L.; J a f f e r , N.; Strouse, J., 21st Experimental NMR Conference, 1980, Tallahassee, FL. Rabenstein, D . L . ; Nakashima, T . K . , A n a l . Chem. 1979, 51, 1465A. L e v i t t , M . H . ; Freeman, R . , J. Magn. Reson. 1980, 39, 533. LeCocq, D . ; Lallemand, J.-Υ., J. Chem. Soc. Chem. Commun. 1981, 150. Cookson, D.J.; Smith, B.E., Org. Magn. Reson. 1981, 16, 11. Brown, D.W.; Nakashima, T . K . ; Rabenstein, D . L , J. Magn. Reson. 1981, 45, 302 P a t t , S . L . ; Shoolery, J.N., J. Magn. Reson. 1982, 46, 535. Cookson, D.J.; Smith, B.E., Fuel 1983, 62, 34. P e i , Feng-Kui, Freeman, R . , J. Magn. Reson. 1982, 48, 318. Jakobsen, H.J.; Sorensen, O.W.; Brey, W.S.; Kanyha, P . , J. Magn. Reson. 1982, 48, 328. B e l o e i l , J.-C.; LeCocq, C . ; Lallemand, J.-Υ., Org. Magn. Reson. 1982, 19, 112. Nakashima, T . K . ; Rabenstein, D . L . , J. Magn. Reson. 1982, 47, 339. Cookson, D.J.; Smith, B.E., Fuel 1983, 62, 39. B e n d a l l , M . R . ; D o d d r e l l , D . M . ; Pegg, D . T . , J . Amer. Chem. Soc. 1981, 103, 4603. B e n d a l l , M . R . ; Pegg, D . T . ; D o d d r e l l , D . M . ; Johns, S . R . ; Willing, R . , J . C . S . Chem. Commun., 1982, 1138. B e n d a l l , M . R . ; Pegg, D . T . ; D o d d r e l l , D . M . ; W i l l i a m s , D . H . , J. Org. Chem. 1982, 47, 3021. Campbell, I . D . ; Dobson, C . M . ; W i l l i a m s , R . J . P ; Wright, P.E., FEBS L e t t . 1975, 57, 96. Pachler, Κ.G.R.; Wessels, P.L., J. Magn. Reson. 1977, 28, 53; Jakobsen, H.J.; W.S. Brey, J . Amer. Chem. Soc. 1979, 101, 760. M o r r i s , G . A . , J. Amer. Chem. Soc. 1980, 102, 428. Burum, D . P . ; E r n s t , R . R . , J. Magn. Reson. 1980 39, 163. D o d d r e l l , D . M . ; Pegg, D . T . , J. Amer. Chem. Soc. 1980, 102, 6388. B o l t o n , P . H . , J. Magn. Reson. 1980, 41, 287.

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R i n a l d i , P.L.; Baldwin, N.J., J. Amer. Chem. Soc. 1982, 104, 5791. B e n d a l l , M . R . ; Pegg, D . T . ; D o d d r e l l , D . M . ; Field, J., J. Amer. Chem. Soc. 1981, 103, 934. D o d r e l l , D . M . ; Pegg, D . T . ; B e n d a l l , M . R . , J . Magn. Reson. 1982, 48, 323. Jeener, J.; Ampere I n t e r n a t i o n a l Summer School, Basko P o l j e , Yugoslavia 1971. Thomas, D . M . ; B e n d a l l , M . R . ; Pegg, D . T . ; D o d d r e l l , D . M . ; Field, J., J . Magn. Reson. 1981, 42, 298, Rutar, V.; Wong, T.C., J. Magn. Reson. 1983, 53, 495. Aue, W.P.; B a r t h o l d i , Ε . ; E r n s t , R . R . , J. Chem. Phys. 1976, 64, 2229. Nagayama, N.; Kumar, A.; Wuthrich, K . ; E r n s t , R . R . , J . Magn. Reson. 1980, 40, 321. Bax, Α . ; Freeman, R . ; M o r r i s , G . , J. Magn. Reson. 1981, 42, 164. Maudsley, A.A.; E r n s t , R . R . , Chem. Phys. L e t t . 1971, 50, 368. Ammann, W.; R i c h a r z , R.; W i r t h l i n , T . ; Wendisch, D . , Org. Magn. Reson. 1982, 20, 260. Ikura, M . ; Hikichi, Κ., Org. Magn. Reson. 1982, 20, 266. Benn, R . , Z. Naturforsch 1982, 37b, 1054. M o r r i s , G . A . ; Hall, L.D., J. Amer. Chem. Soc. 1981, 103, 4703. Chan, T.-M.; Markley, J.L., J. Amer. Chem. Soc. 1982, 104, 4010. K e s s l e r , H . ; H e h l e i n , W.; Schuck, R . , J. Amer. Chem. Soc. 1982, 104, 4534. Gray, G . A . , Org. Magn. Reson. 1983, 21, 111. Avent, A.G.; Freeman, F., J. Magn. Reson. 1980, 39, 169. Bax, Α . , J. Magn. Reson. 1983, 53, 517. Jeener, J.; Meier, B . H . ; Bachmann, P . ; Ernst, R.R. J . Chem. Phys. 1979, 71, 4546. Meier, B.H.; Ernst, R . R . ; J. Amer. Chem. Soc. 1979, 101, 6441. Macura, S . ; E r n s t , R . R . , M o l . Phys. 1980, 41, 95. Kumar, Α . ; E r n s t , R . R . ; Wutrich, Κ., Biochem. Biophys. Res. Commun. 1980, 95, 1. Bosch, C . ; Kumar, A.; Baumann, R.; E r n s t , R . R . ; Wutrich, Κ., J . Magn. Reson. 1981, 42, 159. Kumar, A.; Wagner, G . ; E r n s t , R . R . ; Wutrich, Κ., J. Amer. Chem. Soc. 1981, 103, 3654. Huang, Y.; Macura, S . ; E r n s t , R . R . , J . Amer. Chem. Soc. 1981, 103, 5327.

RECEIVED

October 31, 1983

8 13

Application of the INEPT Method to C NMR Spectral Assignments in Low-Density Polyethylene and Ethylene-Propylene Copolymer KUNIO HIKICHI, TOSHIFUMI HIRAOKI, and SHINJI ΤΑΚEMURA—Department of Polymer Science, Hokkaido University, Sapporo 060, Japan MUNEKI OHUCHI—NMR Application Laboratory, Scientific Instrument Division, JEOL Ltd., Akishima, Tokyo 196, Japan ATSUO NISHIOKA—Faculty of Engineering, Fukui Institute of Technology, Gakuen, Fukui 910, Japan Carbon 13 NMR spectra of low density-polyethylene and ethylene propylene copolymer were observed at a freqeuncy of 125.77 MHz. The INEPT method was applied to identify methyl, methylene, and methine carbons. I t was demonstrated that the INEPT method provides a very useful technique for characterization of polymers. 13 I t has been w e l l e s t a b l i s h e d t h a t C NMR s p e c t r o s c o p y provides an i m p o r t a n t t e c h n i q u e f o r c h a r a c t e r i z a t i o n o f v a r i o u s p o l y m e r s ( 13). S h o r t - c h a i n b r a n c h e s i n l o w - d e n s i t y p o l y e t h y l e n e s a n d monomer sequence d i s t r i b u t i o n s i n e t h y l e n e p r o p y l e n e c o p o l y m e r s h a v e b e e n the s u b j e c t o f a number o f s t u d i e s ( 4 - 2 7 ) , b e c a u s e t h e t y p e , distribution, and c o n c e n t r a t i o n o f branches i n a low-density p o l y e t h y l e n e a n d t h e sequence d i s t r i b u t i o n i n a copolymer should g r e a t l y a f f e c t t h e thermodynamic, m o r p h o l o g i c a l , and p h y s i c a l p r o p e r t i e s o f the polymers. A l t h o u g h many C NMR s t u d i e s h a v e been r e p o r t e d o n l o w - d e n s i t y p o l y e t h y l e n e s a n d e t h y l e n e propylene c o p o l y m e r s , t h e r e s u l t s a r e s t i l l ambiguous. The r e a s o n f o r t h i s is, the difficulty i n o b t a i n i n g complete assignments f o r each resonance i n terms o f c o n t r i b u t i n g branches o r sequences. So f a r t h e p e a k a s s i g n m e n t h a s b e e n made t h r o u g h v a r i o u s means: a comparison o f t h e observed chemical s h i f t values w i t h those calculated by empirical a d d i t i v i t y rules(28-30), a consideration of internal consistency o f intensities, a comparison o f alkane model compounds, a n d s p e c i f i c l a b e l l i n g . F o u r y e g s ago, M o r r i s a n d Freeman (31) r e p o r t e d a m e t h o d f o r enhancing C NMR s i g n a l s b y u s i n g p o l a r i z a t i o n t r a n s f e r f r o m p r o t o n s ( INEPT). Subsequently, D o d d r e l l a n d Pegg (32) showed t h a t this INEPT method c o u l d b e u s e d t o i d e n t i f y m e t h i n e , m e t h y l e n e , and m e t h y l g r o u p s o f a c o m p l e x m o l e c u l e when c o m b i n e d w i t h a p p r o p r i a t e d e l a y t i m e (Δ) p r i o r t o d a t a a c q u i s i t i o n a n d b r o a d band decoupling. I f Δ=1/2J ( J i s t h e c o u p l i n g c o n s t a n t b e t w e e n C a n d Ή ) , o n l y methine carbon resonances appear i n t h e 0097-6156/ 84/0247-0119S06.00/ 0

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spectrum, and i f Δ = 3 / 4 J , methyl and methine carbon resonances appear upward and methylene carbon resonances appear downward. Since then, the IlgpT method has proven t o be q u i t e u s e f u l when a s s i g n i n g complex C NMR s p e c t r a . The purpose o f t h i s paper i s t o demonstrate the usefulness o f the INEPT method f o r the characterization of low-density polyethylene and ethylene propylene copolymers. Experimental The low-density polyethylene (LDPE) was obained from the Rubber and Plastic Research A s s o c i a t i o n (RAPRA). The polyethylene was d i s s o l v e d i n a mixture o f 1,2,4-trichlorobenzene and benzened (4:l) at a concentration o f 25%(w/v). The LDPE sample was the same as used i n the previous work (16 ). The measurements were made at a temperature o f 120 ° C . The ethylene-propylene copolymer (EP) was provided by Japan Synthetic Rubber Co. ( JSR). The propylene monomer content o f this EP copolymer was reported t o be 36.3 mole% by the manufacturer. The copolymer was d i s s o l v e d i n a mixture o f o-dichlorobenzene and benzene-dg ( 3:1 ) at a concentration o f 15%(w/v). The copolymer measurements were made at a temperature o f 100°C. Carbon 13 NMR s p e c t r a were obtained using a JEOL GX-500 spectrometer with a quadruture d e t e c t i o n operating at a frequency of 125.77 MHz i n a pulse-Fourier transform mode. FID s were acquired with a 16 b i t A/D converter and s t o r e d on 32 o r 64 Κ memory l o c a t i o n s with 32 b i t word l e n g t h . The chemical s h i f t was measured from an i n t e r n a l standard, hexamethyldisiloxane, which was taken as 2.03 ppm from t e t r a m e t h y l s i l a n e ( l M S ) . Internal lock was p r o v i d e d by an a d d i t i o n o f benzene-d^. 6

1

Results and D i s c u s s i o n Low-Density .^Polyethylene Figure 1 contains an "Sî noisedecoupled C spectrum o f LDPE. A p a r t i a l spectrum from 28.5 t o 32.0 ppm i s shown with reduced i n t e n s i t y j u s t below the whole spectrum. T h i s spectrum was obtained w i t h a s p e c t r a l range o f 10 KHz and with 6837 FID accumulations a t a temperature o f 120 C . A p u l s e width o f approximately 10 ^is corresponding t o a f o r t y f i v e degree p u l s e angle and delay times between pulses o f 3.6 s were used during data accumulation. The FID s were s t o r e d on 32 Κ memory l o c a t i o n s . As p r e v i o u s l y reported(4-16), there appear t o be a number o f peaks on both s i d e s o f the main methylene peak centered at 30.00 ppm. I t i s c l e a r l y demonstrated t h a t t h i s high magnetic f i e l d strength (corresponding t o the proton resonance frequency o f 500 MHz) remarkably enhances resolution and sensitivity. The major methylene resonances appearing i n the range from 29.4 t o 30.6 ppm c o n s i s t o f more than 10 peaks, and a t l e a s t 80 peaks can be d i s c e r n e d i n the whole spectrum. Figure 2 shows (a) the methine and methylene r e g i o n o f the o

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F i g u r e 2. C INEPT s p e c t r u m w i t h t h e d e l a y t i m e o f A=3/4J(a) a n d t h e common s p e c t r u m ( b ) f o r t h e l o w - d e n s i t y polyethylene.

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INEPT s p e c t r u m f o r LDPE, w h i c h was o b t a i n e d w i t h t h e d e l a y t i m e o f A = 3 / 4 J ( 6 ms) a n d 6000 F I D a c c u m u l a t i o n s a n d (b) t h e s p e c t r u m o b t a i n e d b y t h e common method, w h i c h i s shown f o r c o m p a r i s o n . I n t h i s f i g u r e one c a n e a s i l y d i s t i n g u i s h m e t h i n e resonances(upward peaks) from methylene resonances(downward p e a k s ) . Combined w i t h t h e INEPT s p e c t r u m w i t h t h e d e l a y t i m e A = 1 / 2 J , a l l m e t h y l , m e t h y l e n e , a n d m e t h i n e r e s o n a n c e s c a n te i d e n t i f i e d . Of p a r t i c u l a r i n t e r e s t a r e r e s o n a n c e s f o r t h e m e t h i n e a n d methyl carbons, which a r e l o c a t e d a t branch p o i n t s and are branch end groups, r e s p e c t i v e l y . The o b s e r v e d 15 d i f f e r e n t r e s o n a n c e s for m e t h i n e c a r b o n s s u g g e s t t h a t a t l e a s t 15 d i f f e r e n t branch types are present. Among t h e s e , t h e most i n t e n s e resonance a p p e a r s a t 38.06 ppm a n d a s e c o n d one o c c u r s a t 38.11 ppm w h i c h are p r o b a b l y a s s o c i a t e d w i t h branch p o i n t methine carbons from b u t y l a n d l o n g e r c h a i n b r a n c h e s . The h i g h e s t field methine r e s o n a n c e a p p e a r s a t 31.48 ppm. C a l c u l a t i o n s u s i n g t h e LindemanAdams p a r a m e t e r s (29) s u g g e s t t h a t t h i s p e a k may b e a s s o c i a t e d w i t h m e t h i n e c a r b o n s a t t a c h e d t o t h e m e t h y l g r o u p s o f a n y o f 1,3dimethyl branches, 1,3-paired m e t h y l , e t h y l o r longer branches. I n t h e v i c i n i t y o f 33 ppm t h e r e a p p e a r t h r e e m e t h i n e r e s o n a n c e s , o n e o f t h e s e i s p r o b a b l y due t o i s o l a t e d m e t h y l b r a n c h e s . The l o w e s t f i e l d m e t h i n e r e s o n a n c e a p p e a r s a t 40.74 ppm, b e l o w w h i c h we c o u l d n o t observe any methine resonances. C h e m i c a l s h i f t and peak h e i g h t i n t e n s i t y a r e l i s t e d T a b l e 1. Peak h e i g h t i n t e n s i t y i s e x p r e s s e d i n terms o f p e r c e n t o f t h e t o t a l i n t e n s i t y . and nuclear O v e r h a u s e r enhancement (NOE) measurements were made a t t h i s h i g h f i e l d a n d a t 120 C f o r t h e m a j o r m e t h y l e n e r e s o n a n c e , a n d we o b t a i n e d T^=2.1 s a n d NOE=2.6. Because n o NOE measurements a n d n o T^ measurements f o r e a c h r e s o n a n c e w e r e made, i n t e n s i t y v a l u e s l i s t e d i n the table are only qualitative. I n t h e range below 38.5 ppm, a t l e a s t t h r e e m e t h y l e n e r e s o n a n c e s c a n b e f o u n d a t 38.93, 39.48, a n d 40.08 ppm. T h e s e may o r i g i n a t e f r o m a C-P0 m e t h y l e n e c a r b o n i n a 1 , 3 - d i e t h y l p a i r , a C-6 m e t h y l e n e c a r b o n i n a 5-ethylhexyl branch, a C-3 methylene carbon i n a t e t r a f u n c t i o n a l m e t h y l , p r o p y l branch o r any o f t h o s e branch t y p e s i l l u s t r a t e d by Axelson e t al.(13). In t h e high f i e l d methyl region, 14 m e t h y l r e s o n a n c e s c a n b e i d e n t i f i e d as l i s t e d i n Table I . The h i g h e s t f i e l d methyl r e s o n a n c e s a p p e a r a t 7.85 ppm a n d 8.12 ppm a n d h a v e b e e n a s s i g n e d t o methyl carbons o f t e t r a f u n c t i o n a l e t h y l , b u t y l branches and t e t r a f u n c t i o n a l e t h y l branches, r e s p e c t i v e l y ( 1 6 ) . An i n t e n s e methyl resonance a p p e a r i n g a t 14.1 ppm h a s b e e n a s s i g n e d t o m e t h y l t e r m i n a l s o f b u t y l a n d l o n g e r c h a i n b r a n c h e s when o b s e r v e d a t lower magnetic f i e l d s . I n t h e present study performed a t a higher magnetic f i e l d , t h i s resonance s p l i t s i n t o two peaks a t 14.10 a n d 14.15 ppm. S i m i l a r b e h a v i o r was f o u n d f o r m e t h i n e r e s o n a n c e s a t 38.06 a n d 38.11 ppm. T h i s i s a consequence o f t h e e n h a n c e d r e s o l u t i o n due t o t h e h i g h m a g n e t i c f i e l d s t r e n g t h , a n d i n d i c a t e s t h e p o s s i b i l i t y o f t h e s e p a r a t i o n o f methine and methyl resonances o f b u t y l branches from those o f l o n g c h a i n branches.

8.

H I K I C H I ET A L ,

13

The INEPT Method in C NMR Spectral Assignments

123

T a b l e I . Chemical S h i f t V a l u e s a n d Peak H e i g h t I n t e n s i t i e s o f the Low-Density P o l y e t h y l e n e Methine Chemical shift 31.48 33.12 33.27 33.58 35.76 37.17 37.20 37.87 38.06 38.11 38.46 38.53 39.89 40.02 40.74

Intensity

0.10 0.02 0.02 0 06 0.13 0.17 0.17 0.05 0.91 0.58 0.07 0.08 0.03 0.05 0.02

Methyl Chemical shift 7.85 8.12 10.74 10.89 10.90 11.00 11.16 11.22 12.88 14.10 14.15 14.39 14.66 20.03

Intensity

0.06 0.06 0.04 0.06 0.06 0.19 0.07 0.08 0.02 0.57 1.12 0.05 0.03 0.03

The r e s o n a n c e a p p e a r i n g n e a r 20.0 ppm c o n s i s t s o f t w o p e a k s , t h e one a t 20.03 ppm i s t h e m e t h y l g r o u p r e s o n a n c e f r o m m e t h y l b r a n c h e s a n d t h e o t h e r a t 20.14 ppm i s a s s o c i a t e d w i t h C-2 methylene o f p r o p y l b r a n c h e s . An i n t e g r a t i o n o f a l l m e t h y l peaks i n d i c a t e s t h e p r e s e n c e o f 24 m e t h y l c a r b o n s p e r 1000 c a r b o n s , c o m p a r a b l e t o a v a l u e p r e v i o u s l y o b t a i n e d o f 19.8 ( 1 6 ) . Our r e s u l t s i n d i c a t e t h a t t h e p r e v i o u s a s s i g n m e n t s f o r m e t h y l peaks are probably correct. I n t h i s study, i t i s p o s s i b l e t o c l a s s i f y a l l resonances i n t o methyl, methylene, a n d methine c a r b o n s . However, d e t a i l e d a s s i g n m e n t s t o s p e c i f i c structural u n i t s a r e s t i l l ambiguous, a n d more w o r k i s n e e d e d . L 13 E t h y l e n e P r o p y l e n e Copolymer The H noise-decoupled C s p e c t r u m o f a n EP c o p o l y m e r i s shown i n F i g u r e 3. The s p e c t r u m was o b t a i n e d o v e r a s p e c t r a l r a n g e o f 8 KHz w i t h 8352 F I D a c c u m u l a t i o n s a t a t e m p e r a t u r e o f 100°C. F o r t y f i v e d e g r e e p u l s e s a n d 6 s d e l a y t i m e s between p u l s e s were u s e d . The F I D ' s were s t o r e d o n 64 Κ memory l o c a t i o n s . A s e x p e c t e d ( 1 7 - 2 7 ) , more t h a n 80 p e a k s a p p e a r i n a r a n g e f r o m 19 t o 47 ppm. F i g u r e 4 shows t h e m e t h i n e r e g i o n o f t h e INEPT s p e c t r u m o b s e r v e d w i t h t h e d e l a y t i m e o f Δ =3/4J ( a ) . T h r e e thousand a c c u m u l a t i o n s w e r e s t o r e d o n 32 Κ memory l o c a t i o n s . The common spectrum(b) i s a l s o d e p i c t e d i n t h i s figure. We c a n e a s i l y separate methine r e s o n a n c e s (upward) from methylene

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F i g u r e 4. C INEPT s p e c t r u m w i t h t h e d e l a y t i m e o f A=3/4J(a) a n d t h e common s p e c t r u m ( b ) f o r t h e e t h y l e n e propylene copolymer.

8.

HIKICHI

T a b l e II.

Methine Chemical shift 28.33 28.37 28.44 28.54 28.61 28.67 28.76 30.81 31.07 31.09 31.16 32.87 32.98 33.05 33.10 33.20 33.32 33.48 33.55 33.77 33.81

ET A L .

The INEPT

13

Method in C NMR Spectral Assignments

125

Chemical S h i f t V a l u e s a n d Peak Height I n t e n s i t i e s o f the E t h y l e n e - P r o p y l e n e Copolymer

Intensity

0.7 0.7 0.9 0.3 0.5 0.4 0.3 6.5 0.8 0.9 0.7 0.1 0.3 0.3 0.3 15.8 0.9 2.7 2.5 0.5 0.7

Methyl Chemical shift 19.97 20.00 20.07 20.18 20.43 20.59 20.66 20.70 20.74 20.81 20.85 20.90 21.36 21.54 21.57 21.78 21.82

Intensity

13.2 8.6 4.0 0.9 0.2 0.7 3.2 1.7 1.0 0.9 0.7 0.5 0.3 0.3 0.3 0.2 0.3

r e s o n a n c e s (downward). T a k i n g i n t o a c c o u n t t h e INEPT s p e c t r u m w i t h t h e d e l a y t i m e o f Δ=1/23, we i d e n t i f i e d a l l m e t h i n e , methylene, and methyl resonances. The m e t h i n e r e s o n a n c e s o f t h i s c o p o l y m e r c a n b e o b s e r v e d i n 3 different regions, 28.3-28.7 ppm, 30.8-31.2 ppm, a n d 32.834.0 ppm. R a y e t a l . (22) c o r r e c t l y a s s i g n e d t h e s e methine r e s o n a n c e s . O f most i n t e r e s t i s t h e d i f f e r e n t i a t i o n o f t w o p e a k s a p p e a r i n g a t 30.81 ppm a n d 30.75 ppm. The INEPT s p e c t r a c l e a r l y i n d i c a t e t h a t t h e o n e a t 30.75 ppm c a n b e a s s i g n e d t o m e t h y l e n e c a r b o n s a n d t h e o t h e r a t 30.81 ppm t o m e t h i n e c a r b o n s . A c c o r d i n g t o t h e a s s i g n m e n t s made b y R a y e t a l . (22) t h e p e a k a t l o w e r field is a s s o c i a t e d w i t h t h e m e t h i n e c a r b o n s o f t h e P P E sequence a n d the o n e a t h i g h e r f i e l d t o ÏY m e t h y l e n e c a r b o n s o f t h e PEEP sequence. The m e t h i n e r e s o n a n c e s w h i c h a p p e a r a t 28.3-28*4 ppm h a v e b e e n a s s i g n e d t o t h e m e t h i n e c a r b o n s i n t h e PPP s e q u e n c e . A spectral i n t e g r a t i o n o f the methyl region indicates that t h e c o m p o s i t i o n o f p r o p y l e n e u n i t s i s 37 m o l e p e r c e n t comparable to the provided value. T a b l e I I shows c h e m i c a l s h i f t v a l u e s a n d

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A N D MACROMOLECULES

peak h e i g h t i n t e n s i t i e s f o r methine a n d m e t h y l resonances. Values of a n d NOE f o r 8 m a j o r r e s o n a n c e s w e r e f o u n d t o b e 1.2-2.6 s a n d 2.3-2.6, r e s e p c t i v e l y . The present study mostly confirms the r e s u l t s o f previous assignments made b y R a y e t a l . (22) w i t h r e s p e c t t o t h e i d e n t i f i c a t i o n o f methyl, methylene, and methine peaks. In c o n c l u s i o n , t h e INEPT m e t h o d p r o v i d e s a p o w e r f u l t e c h n i q u e f o r c h a r a c t e r i z a t i o n o f polymers. Acknowledgments The a u t h o r s w i s h t o e x p r e s s t h e i r t o Mr. K . A r a i o f J S R f o r p r o v i d i n g t h e EP s a m p l e s .

appreciation

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

Bovey, F. A. "High Resolution NMR of Macromolecules"; Academic: New York, 1972. Randall, J.C. "Polymer Sequence Determination:Carbon 13 NMR Method"; Academic: New York, 1978; Chap. 3, 6. Randall, J . C. "Polymer Characterization by ESR and NMR"; Woodward, A. E.; Bovey, F. Α., Eds.; American Chemical Society: Washington, D.C., 1980; Symp. Ser. No.142, Chap. 6. Dorman, D. E.; Otocka, E. P.; Bovey, F. A. Macromolecules 1972, 5, 574-7. Randall, J . C. J. Polym. Sci. Polym. Phys. Ed. 1973, 11, 27587. Randall, J . C. J. Polym. Sci. Polym. Phys. Ed. 1975, 13, 9018. Hama, T.; Suzuki, T.; Kosaka, K. Kobunshi Ronbunshu 1975, 32, 91-6. Bovey, F. A . ; Shilling, F. C.; MaCrackin, F. L . ; Wagner, H. L. Macromolecules 1976, 9, 76-80. Cudby, M. E. A.; Bunn, A. Polymer 1976, 17, 345-7. Nishioka, A . ; Ando, I . ; Matsumoto, J . Bunseki Kagaku 1977, 26, 308. Cutler, D. J.; Hendra, P. J.; Cudby, Μ. A. E . ; Willis, H. A. Polymer 1977, 18, 1005-8. Randall, J . C. J. Appl. Polym. Sci. 1978, 22, 585-8. Axelson, D. E.; Levy, G. C.; Mandelkern, L. Macromolecules 1979, 12, 41-52. Dechter, D. E . ; G. C.; Mandelkern, L. J. Polym. Sci. Polym. Phys. Ed. 1980, 18, 1955-61. Nishioka, A . ; Mukai, Y . ; Oouchi, M.; Imanari, M. Bunseki Kagaku 1980, 29, 774-80. Ohuchi, M.; Imanari, M.; Mukai, Y , ; Nishioka, A. Bunseki Kagaku 1981, 30, 332-8. Carman, C. J.; Wilkes, C. E. Rubber Chem. Technol. 1971, 44, 781. Wilkes, C. E . ; Carman, C. J.; Harrington, R. A. J. Polym. Sci. Polym. Symp. 1973, 43, 237-50. Tanaka, Y , ; Hatada, K. J. Polym. Sci. Polym. Chem. Ed. 1973, 11, 2057-68.

8. HIKICHI ET AL. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.

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The INEPT Method in C NMR Spectral Assignments 127

Carman, C. J.; Baranwal, K. C. Rubber Chem. Technol. 1975, 48, 705. Carman, C. J.; Harrington, R. A . ; Wilkes, C. E. Macromolecules 1977, 10, 536-44. Ray, G. J.; Johnson, P. E . ; Knox, J. R. Macromolecules 1977, 10, 773-8. Randall, J. C. Macromolecules 1978, 11, 33-6. Smith, W. F. J. Polym. S c i . Polym. Phys. Ed. 1980, 18, 157385. Smith, W. V. J. Polym. S c i . Polym. Phys. Ed. 1980, 18, 1587. Prabhu, P . ; Schindler, Α.; T h e i l , M. H . ; Gilbert, R. D. J. Polym. S c i . Polym. Lett. Ed. 1980, 18, 389-94. Randall, J. C.; Hsieh, Ε. T. Macromolecules 1982, 15, 1584-6. Grant, D. M . ; Paul, E. G. J. Am. Chem. Soc. 1964, 86, 298490. Lindeman, L . P . ; Adams, J. Q. Anal. Chem. 1971, 43, 1245-52. Carman, C. J.; Tarpely, A. R.; Goldstein, J . H. Macromolecules 1973, 6, 719-24. Morris, G. A.; Freeman, R. J. Am. Chem. Soc. 1979, 101, 7602. Doddrell, D. M . ; Pegg, D. T. J. Am. Chem. Soc. 1980, 102, 6388-90.

RECEIVED October 31, 1983

NMR OF LIQUID POLYMERS

9 13

C NMR in Polymer Quantitative Analyses

J. C. RANDALL and E. T. HSIEH

Phillips Petroleum Company, Research and Development, Bartlesville, OK 74004 Carbon 13 nuclear magnetic resonance can be used quantitatively in analyses of polymers to measure conveniently comonomer concentrations, average sequence lengths, run numbers and comonomer triad distributions. The identification of both short chain and long chain branches in polyethylene at concentrations of 1 per 10,000 carbon atoms has become feasible with the availability of improved probes and improved computer hardware/ software capabilities. Reviewed in this chapter are the methods and computations as well as the basic requirements for sound quantitative analyses: namely, correct choice of solvent, a consideration of concentration effect on line widths and satisfying nuclear Overhauser effects and spin lattice relaxation time requirements. Finally, the NMR generated structural information is put to use in correlations with polyethylene physical properties and measurements of number average molecular weight. T h e i m p o r t a n c e of 1 3 c N M R i n d e t e r m i n i n g p o l y m e r s t r u c t u r e has b e c o m e c l e a r l y e v i d e n t t h r o u g h the abundance of l i t e r a t u r e on the subject d u r i n g the past s e v e r a l y e a r s (1)(2). Sequence d i s t r i b u t i o n s i n v o l v i n g as many as s e v e n c o n t i g u o u s m o n o m e r u n i t s have been r e p o r t e d i n studies o f p o l y m e r c o n f i g u r a t i o n (3)(4) and t r i a d i n f o r m a t i o n has been c o n v e n i e n t l y e x t r a c t e d f r o m 1 3 c N M R s p e c t r a o f many c o p o l y m e r s (5-9). M u c h o f the e a r l y emphasis i n 1 3 c N M R studies o f p o l y m e r s was on c h e m i c a l shift assignments (2)(10)(11). A f t e r a l l , t h i s was an e s s e n t i a l p r e r e q u i s i t e t o any p o l y m e r 1 3 c N M R s t r u c t u r a l a n a l y s i s . R e l a x a t i o n t i m e and n u c l e a r O v e r h a u s e r e f f e c t m e a s u r e m e n t s w e r e also o f c o n s i d e r a b l e i n t e r e s t because o f the i n f o r m a t i o n p r o v i d e d about p o l y m e r d y n a m i c s and r e q u i s i t e e x p e r i m e n t a l c o n s i d e r a t i o n s . T h e last a r e a i n N M R studies t o be f u l l y e x p l o i t e d was the q u a n t i t a t i v e m e t h o d . It is n a t u r a l f o r the p o l y m e r c h e m i s t t o e x p e c t q u a n t i t a t i v e i n f o r m a t i o n once the p o w e r o f the m e t h o d i n d e c i p h e r i n g p o l y m e r s t r u c t u r e has b e c o m e e v i d e n t .

0097-6156/ 84/ 0247-0131 $06.25/ 0 © 1984 American Chemical Society

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Improvements in instrumentation have led to increased s p e c t r a l s e n s i t i v i t y and t o m o r e r e l i a b l e methods f o r d a t a i n t e g r a t i o n . These i m p r o v e m e n t s c o u p l e d w i t h l a r g e r , more e f f i c i e n t and f a s t e r c o m p u t e r s have g i v e n q u a n t i t a t i v e 1 3 c N M R a p p l i c a t i o n s the boost r e q u i r e d f o r b r o a d useage. A s e n s i t i v i t y o f at least one s t r u c t u r a l unit p e r 10,000 c a r b o n a t o m s is r e q u i r e d f o r useful m o l e c u l a r w e i g h t and b r a n c h i n g m e a s u r e m e n t s and t o a l l o w s p e c i e s p r o d u c e d by o x i d a t i o n and i r r a d i a t i o n (12)(13) t o be d e t e c t e d e a r l y i n the p r o c e s s . T h e r e are a n u m b e r o f c o n s i d e r a t i o n s t h a t must be addressed w h e n f o r m u l a t i n g q u a n t i t a t i v e 1 3 c N M R p r o c e d u r e s - these i n c l u d e s o l v e n t e f f e c t s , s p e c t r a l o v e r l a p , l i n e w i d t h s , d y n a m i c and n u c l e a r O v e r h a u s e r e f f e c t s and d e t a i l e d a s s i g n m e n t s . T h e steps r e q u i r e d t o d e v e l o p sound q u a n t i t a t i v e methods w i l l be the subject o f t h i s c h a p t e r . It is i m p e r a t i v e t h a t e x c e l l e n t q u a n t i t a t i v e methods be e s t a b l i s h e d so t h a t NMR c a n be u t i l i z e d i n studies o f p o l y m e r structure-property r e l a t i o n s h i p s . P o l y m e r m o l e c u l a r s t r u c t u r e needs t o be r e l a t e d t o the i n c i p i e n t s o l i d s t a t e s t r u c t u r e and u l t i m a t e l y t o o b s e r v e d s o l i d s t a t e p h y s i c a l p r o p e r t i e s such as d e n s i t y , f l e x u r a l m o d u l i , e n v i r o n m e n t a l stress c r a c k i n g b e h a v i o r , t o n a m e a f e w . T h e usefulness o f 1 3 c N M R i n q u a n t i t a t i v e analyses o f p o l y m e r s is q u i t e b r o a d and c o v e r s a w i d e number of b o t h a d d i t i o n and c o n d e n s a t i o n p o l y m e r s . F o r the sake o f b r e v i t y , the p r e s e n t d i s c u s s i o n c o n c e r n i n g t h e d e v e l o p m e n t o f q u a n t i t a t i v e p r o c e d u r e s w i l l be l i m i t e d t o e t h y l e n e - 1 o l e f i n c o p o l y m e r s and a f e w high d e n s i t y p o l y e t h y l e n e s . These p o l y e t h y l e n e s a l l o w the t y p i c a l N M R p r o b l e m s e n c o u n t e r e d t o be e x p l o r e d w h i l e , at the s a m e t i m e , they s e v e r e l y t e s t the i n s t r u m e n t a l d y n a m i c range c a p a b i l i t i e s . A l t h o u g h p o l y e t h y l e n e s have a r e l a t i v e l y s i m p l e r e p e a t unit s t r u c t u r e , t h e y may c o n t a i n a v a r i e t y o f short c h a i n branches, l o n g c h a i n branches and d i f f e r e n t t y p e s o f end groups. In l i n e a r l o w d e n s i t y p o l y e t h y l e n e s , the c o m o n o m e r s e q u e n c i n g may o f f e r o v e r l a p and a s s i g n m e n t p r o b l e m s . The p r i n c i p l e s e s t a b l i s h e d i n t h e s e q u a n t i t a t i v e studies c a n be u s e f u l l y a p p l i e d t o o t h e r p o l y m e r s y s t e m s o f i n t e r e s t because t h e s a m e b a s i c c o n s i d e r a t i o n s must be addressed. In the f o l l o w i n g s e c t i o n s , the c h o i c e of s o l v e n t , c o n c e n t r a t i o n e f f e c t s , d y n a m i c e f f e c t s , w a y s t o a v o i d assignment p r o b l e m s , sequence analyses, l o n g c h a i n b r a n c h i n g and m o l e c u l a r w e i g h t m e a s u r e m e n t s w i l l be discussed in d e t a i l . E x p e r i m e n t a l V a r i a b l e s i n Q u a n t i t a t i v e N M R Studies o f P o l y m e r s C h o i c e o f S o l v e n t . T h e most a p p r o p r i a t e s o l v e n t f o r N M R studies o f p o l y m e r s w o u l d a l l o w a range of p o l y m e r c o n c e n t r a t i o n s t o be i n v e s t i g a t e d , be f r e e o f o v e r l a p p r o b l e m s and h o p e f u l l y p r o v i d e a s i g n a l for i n t e r n a l l o c k . N o t a l l o f these c o n d i t i o n s c a n u s u a l l y be met as m a n y h i g h m o l e c u l a r w e i g h t p o l y m e r s pose s o l u b i l i t y p r o b l e m s and c a n be e x a m i n e d in o n l y a l i m i t e d n u m b e r of s o l v e n t s . D e u t e r i u m resonance is the t y p i c a l c h o i c e f o r an i n t e r n a l l o c k s i g n a l on most m o d e r n N M R spectrometers. U n f o r t u n a t e l y , the m a j o r i t y o f a v a i l a b l e d e u t e r a t e d s o l v e n t s are poor s o l v e n t s f o r many a d d i t i o n p o l y m e r s such as the p o l y o l e f i n s w h i l e i t is g e n e r a l l y possible t o f i n d a n u m b e r o f a p p r o p r i a t e d e u t e r a t e d s o l v e n t s f o r many o f the c o n d e n s a t i o n p o l y m e r s . The

9.

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13

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Analyses

133

c h l o r i n a t e d b e n z e n e s a r e t h e best s o l v e n t s f o r N M R s t u d i e s o f p o l y e t h y l e n e s a n d v i n y l p o l y m e r s . In p a r t i c u l a r , 1 , 2 , 4 - t r i c h l o r o b e n z e n e has a h i g h b o i l i n g point (213.5 C ) , poses no o v e r l a p p r o b l e m s i n t h e 0-100 ppm (TMS) range, allows a significant range o f p o l y e t h y l e n e c o n c e n t r a t i o n s t o be i n v e s t i g a t e d and is s t a b l e o v e r t h e l o n g d a t a acquisition periods required for 1 3 c N M R analyses. Usually a small a m o u n t o f p e r d e u t e r o b e n z e n e c a n be added t o 1 , 2 , 4 - t r i c h l o r o b e n z e n e t o p r o v i d e a s i g n a l f o r l o c k purposes. If o n l y a f e w hours o f d a t a a c c u m u l a t i o n a r e r e q u i r e d , m a n y s u p e r c o n d u c t i n g m a g n e t s y s t e m s have s u f f i c i e n t s t a b i l i t y t o g i v e e x c e l l e n t s p e c t r a w i t h o u t u s i n g any l o c k s o l v e n t . F o r o v e r n i g h t and l o n g e r runs, a l o c k s o l v e n t is p r e f e r r e d . Effects of P o l y m e r Concentration. A normal approach when attempting t o d e t e c t l o w q u a n t i t i e s o f s t r u c t u r a l units i n p o l y o l e f i n s is t o p r e p a r e a s o l u t i o n as c o n c e n t r a t e d as possible f o r t h e N M R s t u d y . T h e p o l y m e r signal strength p e r free induction decay will i m p r o v e as t h e concentration increases. A f a c t o r not o f t e n c o n s i d e r e d t o be o f i m p o r t a n c e i n q u a n t i t a t i v e N M R studies o f p o l y m e r s is t h e e f f e c t o f c o n c e n t r a t i o n upon r e s o n a n c e l i n e w i d t h . F o r most p o l y o l e f i n s , t h e resonance l i n e w i d t h i n c r e a s e s as the c o n c e n t r a t i o n o f t h e p o l y m e r is increased. C a r b o n 13 N M R s p e c t r a o f a M a r l e x 6003 h i g h d e n s i t y p o l y e t h y l e n e ( M = 140,000, M = 20,000) a r e shown i n F i g u r e l a at a c o n c e n t r a t i o n o f 5 0 % by w e i g h t i n 1 , 2 , 4 - t r i c h l o r o b e n z e n e and i n F i g u r e l b at a c o n c e n t r a t i o n o f 15% by w e i g h t w i t h t h e r e m a i n d e r o f t h e experimental conditions being the same. (The n o m e n c l a t u r e is g i v e n under S e c t i o n 3.) T h e r e s o n a n c e l i n e w i d t h at o n e - h a l f h e i g h t f o r t h e major recurring methylene resonance, δ+δ+ , changed from a p p r o x i m a t e l y 1.0 H z at 15 w e i g h t % t o a p p r o x i m a t e l y 10 H z at 50 w e i g h t % . A l t h o u g h t h e end groups r e s o n a n c e s have l a r g e r a r e a s at a c o n c e n t r a t i o n o f 5 0 % r e l a t i v e t o 15%, t h e s i g n a l t o noise is a c t u a l l y b e t t e r f o r t h e s p e c t r u m w i t h t h e more n a r r o w l i n e w i d t h s . T h e s p e c t r u m in F i g u r e l a was o b t a i n e d a f t e r 4,992 F I D s w h i l e t h a t i n F i g u r e l b was o b t a i n e d a f t e r 5,518 F I D ' s . If the F I D a c c u m u l a t i o n f o r t h e 15% by w e i g h t s a m p l e is a l l o w e d t o c o n t i n u e t o 20,683, t h e s p e c t r u m shown i n F i g u r e 2 is o b t a i n e d . T h e s i g n a l t o noise is now s u c h t h a t one s t r u c t u r a l unit i n 10,000 c a r b o n a t o m s c a n be d e t e c t e d . Evidence that indicates the p r e s e n c e o f l o n g c h a i n b r a n c h i n g has now been o b s e r v e d as i n d i c a t e d by t h e weak r e s o n a n c e s f o r t h e m e t h i n e , and β δ + c a r b o n s f o r a l o n g chain branch. w

n

f

βδ + « δ +

α

δ + βδ+

-CH2-CH2-CH-CH2-CH2-

CH2

CH

2

Methine

38.19 p p m ( f r o m T M S ) 34.55

βδ +

27.20

134

NMR A N D MACROMOLECULES

Figure 1

50.3 M H z 13c N M R S p e c t r a o f 6003 P E at 1 2 5 ° C at (a) 50% by Weight and (b) 15% by Weight i n 1,2,4-Triehlorobenzene

3s

40 Figure 2

!

30

20

PPM, TMS

A 50.3 M H z 1 3 c N M R S p e c t r u m o f 6003 P E at 1 2 5 ° C at 15% Weight i n 1 , 2 , 4 - T r i e h l o r o b e n z e n e a f t e r an A c c u m u l a t i o n o f 20,683 FID's

9.

13

R A N D A L L A N D HSIEH

C NMR in Polymer Quantitative Analyses

135

F r o m a c o m p a r i s o n o f peak heights, t h e l o n g c h a i n b r a n c h i n g i s a p p r o x i m a t e l y 1 per 10,000 carbons. Another desirable result from spectra obtained with narrow line w i d t h s is t h a t o v e r l a p is r e d u c e d not o n l y f o r c l o s e l y s p a c e d r e s o n a n c e s but also f r o m t h e L o r e n t z i a n " t a i l s " w h i c h i n f l u e n c e s m a l l r e s o n a n c e s w i t h i n 10 p p m o f a major resonance s u c h as the δ+δ + r e s o n a n c e i n p o l y e t h y l e n e . I n t e g r a t i o n o f the end group r e s o n a n c e s i n F i g u r e 2 leads to a n u m b e r a v e r a g e m o l e c u l a r w e i g h t o f 18,900. A t polymer c o n c e n t r a t i o n s above 1 5 % by weight, t h e m e a s u r e d number a v e r a g e m o l e c u l a r w e i g h t b e c o m e s a f u n c t i o n o f c o n c e n t r a t i o n as t h e o v e r l a p f r o m the s t r o n g t a i l o f the δ+δ+ r e s o n a n c e b e c o m e s m o r e severe w i t h increasing concentration. F o r p o l y e t h y l e n e s , 1 5 % by weight is a n o p t i m u m c o n c e n t r a t i o n i n t e r m s o f line w i d t h versus s i g n a l s t r e n g t h . F o r o t h e r p o l y m e r s , t h e c o n c e n t r a t i o n e f f e c t s w o u l d have t o be e s t a b l i s h e d independently d e p e n d i n g upon the p r o x i m i t y o f the v a r i o u s resonances a n d the r e l a t i v e s i g n a l s t r e n g t h s . A c c o r d i n g l y , i t may w e l l be possible t o w o r k s u c c e s s f u l l y w i t h h i g h e r c o n c e n t r a t i o n s i n p o l y m e r systems other than polyethylene. R e l a x a t i o n T i m e a n d N u c l e a r O v e r h a u s e r E f f e c t s . It is w e l l k n o w n t h a t f o r 90° pulse angles, a pulse s p a c i n g of 5 χ Τχ w i l l ensure t h a t 99% o f r f e x c i t e d n u c l e i w i l l be f u l l y r e l a x e d b e t w e e n pulses (14). S u c h pulse spacings w i l l ensure r e l i a b l e q u a n t i t a t i v e r e s u l t s a n d are r e c o m m e n d e d a l t h o u g h i t is possible to o b t a i n q u a n t i t a t i v e l y r e l i a b l e r e s u l t s w i t h l o w e r pulse angles. T h e f o l l o w i n g r e l a x a t i o n t i m e (Τχ) d a t a was o b t a i n e d f o r a 97/3 ethylene-1-hexene c o p o l y m e r : γδ+ βδ+ « δ + 1.7 1.1 1.3 1.3 1.8 2.9 8 8 7 ~CH2-CH2-CH2-CH-CH2-CH2-CH2-(CH2)n-CH2-CH2-CH2-CH3 4B4

CH

2

1.1

2

2.0

δ + δ+

4s

3s

2s

Is

I

3B4

2B4 IB4

CH

I C H 2 4.4 I CH3 7

It is evident f r o m these d a t a that use o f only t h e p o l y m e r b a c k b o n e c a r b o n resonances i n a q u a n t i t a t i v e t r e a t m e n t c o u l d l e a d t o an e f f i c i e n t NMR experiment. The spin-lattice relaxation times increase p r o g r e s s i v e l y f o r carbons t o w a r d s the ends of the b u t y l b r a n c h e s and the c h a i n ends. S i m i l a r s p i n - l a t t i c e r e l a x a t i o n t i m e d a t a was o b t a i n e d f r o m the i n v e r s i o n r e c o v e r y (15) and p r o g r e s s i v e s a t u r a t i o n (16) methods. T h e p e r f e c t 9 0 ° pulse angle e x p e r i m e n t w o u l d have a pulse s p a c i n g o f 40 seconds. T h i s p a r t i c u l a r ethylene-1-hexene c o p o l y m e r h a d a n u m b e r a v e r a g e m o l e c u l a r weight o f 5,300 and a w e i g h t a v e r a g e m o l e c u l a r weight o f 31,100 w h i c h a f f o r d e d an o p p o r t u n i t y t o o b t a i n the end group s p i n - l a t t i c e r e l a x a t i o n times. N u c l e a r O v e r h a u s e r e f f e c t s (14) c a n a r i s e f r o m energy t r a n s f e r f r o m the p r o t o n n u c l e a r spin r e s e r v o i r t o the 1 3 c n u c l e a r spin r e s e r v o i r

136

NMR AND MACROMOLECULES

d u r i n g p r o t o n h e t e r o n u c l e a r spin d e c o u p l i n g . It is possible f o r the N O E t o v a r y a m o n g v a r i o u s types o f p o l y m e r carbons, p a r t i c u l a r l y f o r p r o t o n a t e d versus n o n p r o t o n a t e d c a r b o n s i n s y s t e m s h a v i n g h i g h l y restricted molecular mobilities. Q u i t e o f t e n f u l l N O E ' s (-3.0) are o b s e r v e d f o r p o l y m e r s because the d i p o l e - d i p o l e r e l a x a t i o n m e c h a n i s m (14) w i l l p r e d o m i n a t e because t h e r e is a r e s t r i c t e d o v e r a l l m o l e c u l a r m o b i l i t y y e t t h e r e are s u f f i c i e n t i n t e r n a l s e g m e n t a l m o t i o n s t o a l l o w f o r a f u l l N O E . When N O E ' s do v a r y a m o n g p o l y m e r carbons of d i f f e r e n t t y p e s , t h e r e are s e v e r a l courses o f a c t i o n . T h e N O E c a n be m e a s u r e d d i r e c t l y t h r o u g h g a t e d d e c o u p l i n g e x p e r i m e n t s (17) and e a c h o b s e r v e d resonance i n t e n s i t y c a n t h e n be c o r r e c t e d f o r N O E d i f f e r e n c e s . If the s i g n a l t o noise r a t i o is such t h a t a c c u r a t e N O E ' s cannot be o b t a i n e d , it w o u l d be w i s e to u t i l i z e g a t e d d e c o u p l i n g t o p r e c l u d e the p o s s i b i l i t y o f differences in N O E contributions. N u c l e a r O v e r h a u s e r e f f e c t m e a s u r e m e n t s w e r e made f o r the 97/3 ethylene-l-hexene copolymer through gated decoupling experiments (17). T h e N O E was f u l l (-3.0) f o r a l l c a r b o n s i n c l u d i n g the ends o f branches and end groups. T h i s r e s u l t i n d i c a t e s t h a t the d i p o l e - d i p o l e r e l a x a t i o n m e c h a n i s m p r e d o m i n a t e s s u g g e s t i n g , once a g a i n , a l i m i t e d o v e r a l l m o l e c u l a r m o b i l i t y but indicating appreciable segmental m o t i o n s . T h i s is a most f o r t u n a t e o c c u r r e n c e s i n c e f u l l N O E ' s g r e a t l y s i m p l i f y the n u m b e r o f steps r e q u i r e d i n p e r f o r m i n g q u a n t i t a t i v e analyses on 1 3 c N M R d a t a f r o m p o l y m e r s . W i t h N O E and Τ χ i n f o r m a t i o n about the ethylene-l-hexene c o p o l y m e r , we are i n a p o s i t i o n t o d e s i g n an e f f i c i e n t e x p e r i m e n t f o r free i n d u c t i o n d e c a y (FID) a c c u m u l a t i o n s . Two experiments were p e r f o r m e d : (a) a 9 0 ° pulse a n g l e , (pulse w i d t h = 10.5 ps) a 15 s e c o n d pulse d e l a y and a 1.0 s e c o n d a c q u i s i t i o n t i m e , and (b) - 3 0 ° pulse a n g l e , (pulse w i d t h = 3.5 ps) no pulse d e l a y and a 1.0 s e c o n d a c q u i s i t i o n t i m e . C a r b o n 13 N M R s p e c t r a are p r e s e n t e d i n F i g u r e 3a f o r the 9 0 ° pulse angle e x p e r i m e n t and i n F i g u r e 3b f o r the - 3 0 ° pulse angle e x p e r i m e n t . The o b s e r v e d peak heights and r e l a t i v e areas are p r e s e n t e d in T a b l e I. The n u m b e r of F I D ' s a c c u m u l a t e d f o r the - 3 0 ° pulse angle e x p e r i m e n t was 14,400 w h i l e o n l y 1,441 w e r e a c c u m u l a t e d for the 9 0 ° pulse angle e x p e r i m e n t . T h e - 3 0 ° pulse angle e x p e r i m e n t r e q u i r e d four hours o f i n s t r u m e n t t i m e whereas the 9 0 ° pulse angle e x p e r i m e n t r a n f o r 6.4 hours. S i g n i f i c a n t t i m e savings c a n be r e a l i z e d w i t h l o w e r pulse angles when 1 3 c n u c l e i are present w i t h s u b s t a n t i a l l y l o n g r e l a x a t i o n t i m e s . A s c a n r e a d i l y be seen i n F i g u r e 3a, the s i g n a l t o noise r a t i o is m u c h b e t t e r f o r the 9 0 ° pulse angle s p e c t r u m . (The pulse angle at 3.5 ps is l i k e l y less t h a n 3 0 ° . A p p r o x i m a t e l y 1 ps is r e q u i r e d t o t u r n the pulse on.) T h e m o l e p e r c e n t 1-hexene, u s i n g a m e t h o d w h i c h w i l l be p r e s e n t e d s h o r t l y , was 3.1 f o r the 9 0 ° pulse angle e x p e r i m e n t and 3.5 f o r the - 3 0 ° pulse angle e x p e r i m e n t . T h e m e t h y l resonance w i t h the longest Τ χ was not used i n t h i s d e t e r m i n a t i o n o f m o l e p e r c e n t 1-hexene a l t h o u g h 2 B 4 was u t i l i z e d . If one only uses resonances w h i c h have a Τ χ less t h a n 2.0 seconds, s i m i l a r r e s u l t s are o b t a i n e d w i t h a v a l u e of 3.3 m o l e p e r c e n t for the 9 0 ° e x p e r i m e n t and 3.8 m o l e p e r c e n t f o r the - 3 0 ° e x p e r i m e n t . These r e s u l t s i n d i c a t e t h a t a pulse s p a c i n g o f less t h a n 5 χ Τχ w i l l not l e a d t o serious e r r o r s i f the pulse s p a c i n g is s t i l l g r e a t e r t h a n 3 χ Τ χ f o r the s l o w e s t r e l a x i n g nucleus and most o f the Ï 3 c n u c l e i are t o t a l l y

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Table I R e l a t i v e P e a k H e i g h t s and I n t e g r a t e d A r e a s f r o m 50.3 M H z 1 3 c N M R Spectra of a 97/3 E t h y l e n e - l - H e x e n e Copolymer U t i l i z i n g Different P u l s e A n g l e s and P u l s e S p a c i n g s .

Line

PPM.TMS

Areas

Areas

0.014

0.015

0.048

0.054

0.008

0.011

0.860

0.844

1.

38.15

2.

34.55

3.

34.15

4.

32.17

5.

30.47

6.

29.98

7.

29.55

8.

29.52

9.

27.28

0.029

0.031

10.

23.36

0.014

0.016

11.

22.86

0.008

0.008

12.

14.08 0.020

0.022

13.

14.03

138

NMR AND MACROMOLECULES

satisfied dynamically by the experimental pulse spacing. Quantitatively, the results are similar for the two experiments utilizing different pulse angles. The determination of mole percent 1-hexene in the previous example was based on relative areas measured by spectral integration. The use of peak heights gives mole percents 1-hexene of 2.8 from the 9 0 ° data and 2.3 from the - 3 0 ° data. Relative areas are normally recommended because any difference in resonance line widths will be taken into consideration through a use of relative areas. Once a reliable 13c N M R spectrum of a polymer is obtained, one must still face problems associated with resonance overlap and detailed assignments. If the polymer solution is too concentrated and strong signals are present, overlap from Lorentzian "tails" can lead to large errors in a quantitative determination. A satisfactory way to avoid some of these problems is given in the following section. Method of Collective Assignments. One way to avoid errors introduced into a quantitative analysis of 13c N M R data of polymers by spectral overlap and complex fine structure is to divide the observed spectrum into spectral regions sufficiently broad to contain overlapping resonances and fine structure. Closely spaced resonances will often result from a high chemical shift sensitivity to either configuration or comonomer sequences. Configurational sensitivities often lead to observable, but incompletely resolved pentads and heptads where the various pentads and heptads are grouped according to common central triads which are sufficiently resolved as separate multiplets. The methyl region of polypropylene is a good example of such behavior. As shown in Figure 4, sequence sensitivity is to both pentads and heptads yet the three basic triad centers are well resolved as separate complex multiplets. Under such circumstances a triad assignment is all that is required because of the necessary triad-pentad and pentad-heptad relationships. Two examples are given below: mm = mmmm + m mmr + rmmr

(1)

mmmm - mmmmmm + mmmmmr + rmmmmr

(2)

The same principle applies to comonomer sequence chemical shifts. One needs only to identify the basic dyad in tetrads, hexads, etc., or the basic triad in pentads and heptads, etc. The final equations can be expressed in any adjacent complete distribution as desired through use of the appropriate necessary relationships (2) and as dictated by the number of independent spectral observations. The spectral region to be defined should be broad enough to include any unusual chemical shift behavior. A n example best illustrates such an approach; the 97/3 ethylene-l-hexene copolymer shown in Figure 3 and an 83/17 ethylene-l-hexene copolymer shown in Figure 5 serve as useful prototypes. The spectral regions are defined below in terms of spectral range, sequence assignments and contributing carbons. The nomenclature is that used throughout where Greek symbols are used to denote the nearest branched carbons in both directions from a

9.

13

R A N D A L L A N D H Si E H

C NMR in Polymer Quantitative Analyses

139

(A)

βδ3B

2B,

S

4s

(B)

40

20

30

PPM, TMS

10

1 3

F i g u r e 3 50.3 MHz C NMR S p e c t r a o f a 97/3 E t h y l e n e - 1 - H e x e n e Copolymer a t 125° a n d 1 5 % b y W e i g h t i n 1 , 2 , 4 - T r i e h l o r o b e n z e n e a f t e r ( a ) a 90 P u l s e A n g l e and 16 s e c P u l s e I n t e r v a l a n d ( b ) a 30 P u l s e A n g l e a n d 1 s e c P u l s e I n t e r v a l

mmrm

22

21 1 3

+ rmrr

20

PPM, TMS

F i g u r e 4 A 50.3 MHz C NMR S p e c t r u m o f t h e M e t h y l R e g i o n o f an A t a c t i c P o l y p r o p y l e n e a t 125°C i n 1 , 2 , 4 - T r i e h l o r o b e n z e n e (Sample c o u r t e s y o f W a l t e r K a m i n s k y , U n i v . o f Hamburg, W.Germany)

140

NMR AND MACROMOLECULES

D I

1

Ε

Η

A

h

1

40 Figure 5

30

20

PPM, TMS

10

A 50.3 MHz 13c N M R Spectrum of an 83/17 Ethylene-l-Hexene Copolymer at 1 2 5 ° C and 15% by Weight in 1,2,4-Triehlorobenzene

9.

R A N D A L L A N D HSIEH

I3

C NMR in Polymer Quantitative Analyses

141

methylene carbon of interest. The backbone methylene carbons in poly(l-hexene), for example, are all « α . (For consistency, the final equations are expressed in terms of triads only.) Region " A "

39.5 to 42 ppm

<χα C H 2 from H H H H , H H H E , E H H E

The oca methylene carbons normally display a tetrad sensitivity. In this case, the H H H H carbons are broadened by the presence of closely spaced resonances from sequences of different configurations (see Figure 5). Collectively, the entire region is simply proportional to the H H dyad concentration, that is, Total Area

= T A = k [(HHHH) + (HHHE) + (EHHE)] = k(HH)

(3)

and, in terms of triads through the "necessary relationships" (2) becomes TA

Region "B"

38.1 ppm

= k [ (HHH) + (1/2) (EHH) ]

(4)

Methine from E H E sequences Τβ

= k (EHE)

(5)

Region " C " is somewhat more complicated because it consists of both methine and methylene carbon resonances. Region " C "

33 to 36 ppm

Methine carbons from E H H and H H H 4B4 carbons from E H E , E H H and H H H <χδ+ from E H E E and H H E E <χγ from E H E H and H H E H

and, finally, Tc

= k [2 (HHH) + 3 (EHH) + 3 (EHE)]

(6)

Region "D" contains the strong δ+δ+ resonance from the recurring methylene units and interferes with nearby resonances which originate from the 3B4 branch carbon, and the γγ, γδ+ backbone carbons close to a butyl branch. Region "D"

28.5 to 31 ppm

γγ from H E E H γδ+ from H E E E δ + δ+ from (EEE)n 3B4 from E H E , E H H and H H H

and T D = k [2 (EEE) + (1/2)(HEE) + (EHE) + (EHH) + (HHH)]

(7)

where the following necesary relationships (7) were needed to obtain equation 7.

142

NMR A N D MACROMOLECULES

( E E E ) = ( 1 / 2 ) δ+δ+ +

( 1 / 4 ) γδ+

(8)

( 1 / 2 ) (HEE) = ( H E E H ) + ( 1 / 2 ) ( H E E E )

(9)

R e g i o n s " E " and " F " i n v o l v e m u l t i p l e r e s o n a n c e s but f r o m o n l y one t y p e of c a r b o n i n e a c h c a s e . Region " E "

2 6 . 5 to 2 7 . 5 ppm

βδ+ f r o m E H E E and H H E E

T E = k(HEE) Region " F "

2 4 to 2 5 ppm

(10)

ββ f r o m E H E H E ,

E H E H H

and

H H E H H

T p = k(HEH)

(11) f f

T h e H E H - c e n t e r e d p e n t a d s i n r e g i o n F " a r e not c o m p l e t e l y r e s o l v e d , thus t h e use o f c o l l e c t i v e a s s i g n m e n t s is most v a l u a b l e i n t h i s c a s e w h e r e one a c t u a l l y needs o n l y t h e H E H t r i a d t o c o m p l e t e t h e t r i a d s e t . R e g i o n s " H " and " G " c o n t a i n s i n g l e r e s o n a n c e s f r o m t h e f i r s t a n d s e c o n d carbons o f t h e b u t y l b r a n c h , r e s p e c t i v e l y . A l t h o u g h t h e y r e l a t e d i r e c t l y t o t h e t o t a l 1 - h e x e n e c o n t e n t i n t h e c o p o l y m e r , t h e y c a n be expressed as t r i a d s f o r s o l u t i o n o f t h e c o m p l e t e set o f s i m u l t a n e o u s equations. Region " G "

2B4 f r o m " H "

2 3 . 4 ppm

T Q = k[(EHE) + ( E H H )+ ( H H H ) ]

Region " H "

(12)

IB4 (methyl) from " H "

1 4 . 1 ppm

Τ Η = k[(EHE) + ( E H H )+ ( H H H ) ]

(13)

O t h e r p r o b l e m s c a n d e v e l o p i n t h e use o f E q u a t i o n s 4 t h r o u g h 1 3 b e c a u s e o v e r l a p c a n o c c u r f r o m r e s o n a n c e s f r o m sources o t h e r t h a n r e p e a t u n i t resonances. F o r e x a m p l e , t h e a l l y l i c c a r b o n r e s o n a n c e at 3 3 . 9 1 p p m w h i c h arises f r o m a t e r m i n a l v i n y l group w i l l p r o b a b l y o v e r l a p w i t h t h e 4B4 b r a n c h m e t h y l e n e r e s o n a n c e f o r E H E at 3 4 . 1 3 p p m . In F i g u r e 5 , t h e s a t u r a t e d end group r e s o n a n c e s a r e so weak t h a t o v e r l a p w i t h c o r r e s p o n d i n g r e s o n a n c e s f r o m t h e b u t y l b r a n c h e s is not a p r o b l e m . F o r l o w e r m o l e c u l a r w e i g h t c o p o l y m e r s s u c h as i n F i g u r e 3 , c o r r e c t i o n s must be made w h e r e n e c e s s a r y i n t h e equations d e s c r i b i n g t h e s p e c t r a l r e g i o n w h e r e t h e end group r e s o n a n c e s r e s i d e . T h e use o f equations f o r s p e c t r a l regions A , B , D , F and G do not o f f e r possible o v e r l a p p r o b l e m s w i t h end group r e s o n a n c e s and g i v e t h e f o l l o w i n g r e l a t i o n s h i p s f o r t h e triad concentrations: k (EHE) =Τβ

(14)

k (EHH) = 2 ( T - T G

B

-

T A )

k (HHH) = 2 T A + Τβ - T G

(15) (16)

9.

R A N D A L L A N D LIS! F H

13

k (HEH) =

C NMR in Polymer Quantitative

T F

(17)

k (HEE) = 2(TG- Τ Α k

143

Analyses

( E E E ) = (1/2)

( T A + T

T F )

D

+ T

(18)

F

- 2TQ)

(19)

The integrated areas from the 1 3 c N M R spectrum in Figures 3 and 5 lead t o the f o l l o w i n g results f o r the triad distribution: 83/17

Copolymer

9 7 / 3 Copolymer

( E H E )

0.098

0.031

( E H H )

0.053

0.000

( H H H )

0.022

0.000

( H E H )

0.043

0.000

( H E E )

0.164

0.061

( E E E )

0.620

0.908

T h e " E " s p e c t r a l r e g i o n was n o t used i n t h e above analyses b e c a u s e phasing d i f f i c u l t i e s l e d t o c o n s i s t e n t l y h i g h r e s u l t s . T h i s w i l l be a t y p i c a l p r o b l e m i n 1 3 c N M R s p e c t r a w h e r e t h e r e is a s t r o n g d o m i n a n t resonance close t o substantially weaker resonances of interest. A t r i a d d i s t r i b u t i o n i s u s e f u l because i t gives t h e r e l a t i v e c o n c e n t r a t i o n s o f e a c h o f t h e possible c o n n e c t i n g sequences. W i t h t h i s i n f o r m a t i o n , t h e c o m o n o m e r c o n c e n t r a t i o n s , n u m b e r a v e r a g e sequence lengths and t h e "run number" c a n be c a l c u l a t e d as f o l l o w s : (H) = ( H H H ) + ( E H H ) + ( E H E )

(20)

(E) = ( E E E ) + ( H E E ) + ( H E H )

(21)

Run Number

=

(1/2)(HE)

= ( E H E ) + (1/2)(EHH) = ( H E H ) + ( 1 / 2 ) ( H E E )

(22) (23)

A v e r a g e "E" Sequence L e n g t h ~ (E)/Run N u m b e r

(24)

A v e r a g e " H " Sequence L e n g t h = (H)/Run N u m b e r

(25)

T h e c o n c e p t o f r u n n u m b e r was f i r s t i n t r o d u c e d by H a r w o o d ( 1 8 ) a n d w a s d e f i n e d as t h e n u m b e r o f l i k e m o n o m e r runs p e r 1 0 0 p o l y m e r c h a i n u n i t s . In t h e above equations, t h e r u n n u m b e r i s d e f i n e d i n t e r m s o f r e l a t i v e c o n c e n t r a t i o n s a n d c a n be c o n v e r t e d t o t h e H a r w o o d d e f i n i t i o n by simply m u l t i p l y i n g by 1 0 0 . T h e d e v e l o p m e n t o f t h e c o n c e p t s o f r u n number, a v e r a g e sequence lengths and t r i a d d i s t r i b u t i o n s would be o f l i t t l e more t h a n a c a d e m i c i n t e r e s t i f t h e y c o u l d not be u s e f u l l y a p p l i e d . T h e c o n c e p t o f r u n number is most v a l u a b l e i n a c o n s i d e r a t i o n o f t h e e f f e c t o f c o m o n o m e r c o n t e n t versus b r a n c h l e n g t h i n a f f e c t i n g p o l y e t h y l e n e d e n s i t y . T h e following section u t i l i z e s the run number i n a c o r r e l a t i o n w i t h a number of p o l y e t h y l e n e p h y s i c a l p r o p e r t i e s .

144

NMR AND MACROMOLECULES

Structure-Property Relationships. The choice of comonomer and its density-concentration relationship has been of considerable interest among polymer chemists engaged in the preparation of ethylene-l-olefin copolymers. A graph of mole percent 1-olefin versus density for the 1butene, 1-hexene and 1-octene comonomers in ethylene-l-olefin copolymers is presented in Figure 6. The run numbers were determined from 13c N M R data using the appropriate method of collective assignments and integrated resonance areas. The densities were determined from a density gradient column. A clear trend can be seen in spite of the scatter in the data. As the mole percent 1-olefin increases, the density of the polymer decreases. A n effect from branch length is also indicated. The 1-octene comonomer appears to be the most effective in reducing density while the 1-butene comonomer appears to be the least effective. The points for the ethylene-l-hexene copolymers are in between those for the 1-butene and 1-octene copolymers. These same copolymers are presented in Figure 7, but this time the density is plotted versus the run number instead of mole percent 1olefin. The run number gives the average number of comonomer disruptions per 100 chain units irrespective of the run length of the disrupting units. Thus, the tendency of a comonomer to "cluster" in contiguous sequences is taken into account. A dimer or trimer of contiguous comonomer sequences would count as a disrupting unit in the same way as an isolated comonomer unit. It is quite apparent in Figure 7 that the scatter of points associated with different branch lengths is much reduced. The density clearly decreases as the run number increases but the effect of branch length now is much less pronounced than in Figure 6. It would appear that the ethylene-l-olefin copolymer density is more dependent upon chain disruptions as indicated by the run number than upon the difference in branch lengths. The fact that 1butene is less effective in reducing the density in an ethylene copolymer than either 1-hexene or 1-octene may be more related to its tendency to form "clusters" than to its shorter branch length. The 1-octene comonomer shows very little tendency to cluster because the relationship observed in the graph of mole percent versus polymer density is quite similar to that observed for run number versus density. The largest differences between the two graphs were shown by those ethylene-l-butene copolymers where the EBB and BBB sequences gave measureable concentrations. The extent of clustering for any ethylene-l-olefin copolymer can be defined by dividing the run number by the mole percent 1-olefin. This result is called the "monomer dispersity" as shown below: Monomer Dispersity = 100x[(l/2)(EH)/(H)]

(26)

A value of 100 will be obtained for the monomer dispersity whenever the comonomer exists solely as an isolated unit. In Figures 6 and 7, the monomer dispersities of the 1-butene copolymers varied from 60 to 99, the 1-hexene copolymers varied from 85 to 99 and the 1-octene copolymers had monomer dispersities which varied from 96 to 100. This

9.

13

R A N D A L L A N D HSIEH

C NMR in Polymer Quantitative Analyses

• 0.94

1-Butene

Δ 1-Hexene

-I

o 1-Octene

ο Δ

Δ • ο ο ο

0.93

Ο

Δ ΔΔ

c

Ο

DENSITY Δ 0.92

Ο

Ο

0.91

2.0

1.0

4.0

3.0

5.0

MOLE P E R C E N T 1 - O L E F I N Mole Percent 1-Olefin Versus Density for a Series of Ethylene-1-Butene, Ethylene-l-Hexene and Ethylene-1-Octene Copolymers

Figure 6

0.94

•

ο

0.93

ο ο

Δ

•

1-Butene

Δ

1-Hexene

o

1-Octene

Δ

•

ο

ΔΟ ο ο Δ Δ Ο Δ Δ Ο

DENSITY

•

Δ

Δ

Ο

0.92

Δ Ο •

Δ

0.91 1.0

2.0

3.0 RUN

Figure 7

4.0

5.0

NUMBER

Run Number Versus Density for a Series of Ethylene-1-Butene, Ethylene-l-Hexene and Ethylene-1-Octene Copolymers

145

146

NMR AND

MACROMOLECULES

q u i t e l a r g e d i f f e r e n c e i n m o n o m e r d i s p e r s i t i e s suggests t h a t an e f f e c t o f b r a n c h l e n g t h on r e d u c i n g d e n s i t y c o u l d o n l y be i n v e s t i g a t e d t h r o u g h an e x a m i n a t i o n o f c o p o l y m e r s w i t h s i m i l a r (near 100) m o n o m e r d i s p e r s i t i e s . T h e p o i n t s i n F i g u r e s 6 and 7 w h i c h had m o n o m e r d i s p e r s i t i e s o f 94 t o 100 are r e p l o t t e d i n F i g u r e 8. A l t h o u g h the 1-octene p o i n t s f a l l at the l o w e r d e n s i t i e s , it is v e r y d i f f i c u l t t o a s c e r t a i n any s i g n i f i c a n t difference between the ethylene-l-butene and ethylene-l-hexene copolymers. T h e s c a t t e r i n the p o i n t s , p a r t i c u l a r l y at the h i g h e r d e n s i t i e s , are r e l a t e d t o d i f f e r e n c e s i n m o l e c u l a r w e i g h t and m o l e c u l a r weight d i s t r i b u t i o n . T h i s c o r r e l a t i o n w i l l be r e p e a t e d i n a new s t u d y using samples f r o m f r a c t i o n a t e d p o l y m e r s . F o r those c o p o l y m e r s where o t h e r p h y s i c a l p r o p e r t y d a t a was a v a i l a b l e , an a t t e m p t was made t o c o r r e l a t e the r u n n u m b e r w i t h the f l e x u r a l modulus and the t e n s i l e s t r e n g t h at y i e l d . R e s u l t s are g i v e n i n F i g u r e s 9 and 10 f o r a l i m i t e d n u m b e r o f p o i n t s . T h i s d a t a should be o b t a i n e d f r o m c a r e f u l l y a n n e a l e d samples or ones w i t h s i m i l a r t h e r m a l histories; h o w e v e r , a c l e a r t r e n d is seen i n b o t h f i g u r e s . The run number does r e l a t e t o c r y s t a l l i n i t y and to a t t e n d a n t p h y s i c a l p r o p e r t i e s such as f l e x u r a l modulus and t e n s i l e s t r e n g t h . W i t h the d e t a i l e d s t r u c t u r a l i n f o r m a t i o n p r o v i d e d t h r o u g h 1 3 c N M R analyses of p o l y m e r s , the p o l y m e r s c i e n t i s t is i n a p o s i t i o n t o determine physical property - structure relationships. Many excellent studies w i l l be f o r t h c o m i n g d u r i n g t h i s d e c a d e , l a r g e l y because o f the development of N M R spectrometers with high sensitivity and sophisticated pulsing techniques which greatly simplify quantitative studies. One f i n a l a r e a f o r d i s c u s s i o n is the use o f N M R f o r the d e t e r m i n a t i o n o f p o l y m e r number a v e r a g e m o l e c u l a r w e i g h t s . E n d group resonances were c l e a r l y v i s i b l e i n the 1 3 c N M R s p e c t r u m o f the e t h y l e n e - l - h e x e n e c o p o l y m e r i n F i g u r e 3. A n o p p o r t u n i t y to d e t e r m i n e p o l y m e r degrees of p o l y m e r i z a t i o n or number a v e r a g e m o l e c u l a r w e i g h t s should not be o v e r l o o k e d . M e a s u r e m e n t s o f N u m b e r A v e r a g e M o l e c u l a r W e i g h t . C a r b o n 13 N M R lends i t s e l f t o a m e a s u r e m e n t o f the degree o f p o l y m e r i z a t i o n or n u m b e r average m o l e c u l a r w e i g h t because a r a t i o o f the number o f r e p e a t u n i t s t o end units is a l l t h a t is r e q u i r e d for these d e t e r m i n a t i o n s . The a v a i l a b i l i t y of f l o a t i n g point a r i t h m e t i c d u r i n g F o u r i e r t r a n s f o r m a t i o n s and the use o f double p r e c i s i o n d u r i n g d a t a a c q u i s i t i o n and m a n i p u l a t i o n s have v i r t u a l l y e l i m i n a t e d p r o b l e m s a s s o c i a t e d w i t h d y n a m i c r a n g e . When evaluating the N M R technique for molecular weight measurements, it is prudent t o use k n o w n m o l e c u l a r w e i g h t s t a n d a r d s and N B S 1475 ( M = 53,100; M = 18,300) is a good c h o i c e f o r s u c h an i n v e s t i g a t i o n . N B S 1475 is r e p o r t e d t o be a l i n e a r , h i g h d e n s i t y (0.9784) p o l y e t h y l e n e . A 1 3 c N M R s p e c t r u m is shown i n F i g u r e 11. A r e s o n a n c e at 33.91 ppm r e v e a l s the presence o f v i n y l t e r m i n a l groups (10) a l t h o u g h it is c l e a r f r o m the r e l a t i v e i n t e n s i t i e s t h a t the s a t u r a t e d end groups are more abundant. T h e s i g n a l t o noise r a t i o i n F i g u r e 11 is such t h a t the end group and m a i n c h a i n resonances c a n be measured by s p e c t r a l i n t e g r a t i o n . T h i s affords an o p p o r t u n i t y t o c o m p a r e r e l a t i v e a r e a s t o r e l a t i v e peak heights as g i v e n b e l o w : w

n

9.

13

R A N D A L L A N D HSIEH

C NMR in Polymer Quantitative Analyses

• Δ ο

0.94

1-Butene 1-Hexene 1-Octene

•

•

0.93

c Ο

Δ Δ Δ Ο Δ Ο

D E N S I T Y

Δ

0.92

Ο

Ο

0.91 1.0

2.0

3.0 RUN

Figure 8

5.0

4.0 NUMBER

Run Number Versus Density for a Series of Ethylene 1-Butene, Ethylene-l-Hexene and Ethylene-l-Octene Copolymers Possessing Monomer Dispersities of 94-100 • Δ

600 H

1-Butene 1-Hexene

500 H F L E X U R A L

MODULUS MPa 400

300 1.0

2.0

3.0 RUN

Figure 9

4.0

5.0

NUMBER

Run Number Versus Felxural Modulus for a Few Ethylene-l-Hexene and Ethylene-l-Butene Copolymers

147

148

NMR AND MACROMOLECULES

at

3S

I9

*HP \

PPM,

TMS

F i g u r e 11

40

30

2 S

1 S

20

10

A 50.3 M H z 1 3 c N M R S p e c t r u m o f N B S 1475 P o l y e t h y l e n e at 1 2 5 ° C and at 15 W e i g h t P e r c e n t i n 1,2,4-Triehlorobenzene

R A N D A L L A N D HSIEH

9.

13

C NMR in Polymer Quantitative Analyses

149

NBS 1475 Polyethylene Standard Carbon

Relative Areas*

Relative Peak Heights

Is 2s 3s at δ+δ+

30.4 29.1 43.7 16.0 30,000.0

40.5 50.7 56.9 19.4 30,000.0

*The relative areas were determined by digital integration and normalized to δ+δ+ = 30,000 for comparison purposes. t"a is the allylic carbon resonance at 33.91 ppm for terminal vinyl groups. ft

It is apparent that the two N M R data sets (areas versus peak heights) will not lead to the same number average molecular weight. The following equation can be used to determine the number average molecular weight: M

n

-

Total Carbon Intensity Single Carbon Intensity

χ

1

(27)

4

In the case of NBS 1475, the total carbon intensity and the single carbon intensity are given by Total Carbon Intensity = (Is + 2s + 3s + a + δ δ )

(28)

Single Carbon Intensity = (1/2) (s + a)

(29)

+

+

where s is the average intensity for Is, 2s, and 3s. The number average molecular weight using Equations 27-29 and peak height data is 12,300, while a result of 16,700 is obtained using integrated areas. If one assumes that 3s is affected by the strong Lorentzian tail from the δ δ resonance and is omitted from the calculation, a number average molecular weight of 18,400 is obtained from the area data. This latter result is in excellent agreement with the number average molecular weight reported by NBS. The difference in values obtained for the number average molecular weight according to the choice of peak intensity measurement arises from a difference in linewidths for the more mobile carbons. The following results were obtained for linewidths at one-half height for the observed NBS 1475 resonances at a solution concentration of 15% by weight in 1,2,4-trichlorobenzene and a temperature of 1 2 5 ° C . +

+

150

NMR AND

Carbon Is 2s 3s a +

δ δ

+

MACROMOLECULES

L i n e w i d t h at 1/2 H e i g h t 1.1 H z 1.0 0.9 (too l o w f o r a r e l i a b l e measurement) 1.2

T h e end group resonances are s u f f i c i e n t l y more n a r r o w (10-20%) t o c r e a t e a s u b s t a n t i a l e r r o r when d e t e r m i n i n g r e p e a t unit versus end group r a t i o s by peak h e i g h t s . S p e c t r a l i n t e g r a t i o n is r e q u i r e d and m o d e r n s p e c t r o m e t e r s equipped w i t h d i g i t a l i n t e g r a t i o n c a n do the j o b . A n o t h e r reason t h a t s u c h good a g r e e m e n t was o b t a i n e d b e t w e e n the N M R m e t h o d and the g e l p e r m e a t i o n c h r o m a t o g r a p h y m e t h o d used by N B S is t h a t N B S 1475 has a f a i r l y n a r r o w m o l e c u l a r w e i g h t d i s t r i b u t i o n ( M / M = 2.9). It is possible t h a t p o l y e t h y l e n e s h a v i n g v e r y broad m o l e c u l a r weight d i s t r i b u t i o n s w i l l g i v e d i f f e r e n t n u m b e r a v e r a g e m o l e c u l a r w e i g h t s by the t w o methods. T h e N M R m e t h o d is n o n d i s c r i m i n a t i n g as far as o l i g o m e r s are c o n c e r n e d and w i l l g i v e the same end group resonances f o r a 36 c a r b o n o l i g o m e r and a h i g h m o l e c u l a r weight p o l y m e r . T h e G P C m e t h o d is v e r y i n s e n s i t i v e t o the presence o f o l i g o m e r s w h i c h c o u l d be present as a c o n s t i t u e n t i n polyethylenes having broad m o l e c u l a r weight d i s t r i b u t i o n s . A c o m p a r i s o n o f number a v e r a g e m o l e c u l a r w e i g h t s o b t a i n e d f r o m t h e s e t w o t e c h n i q u e s t h e r e f o r e c o u l d be u s e f u l . w

n

Conclusions U n t i l the last f e w y e a r s , our a b i l i t y to s y n t h e s i z e and d e v e l o p new p o l y m e r s c o m m e r c i a l l y f a r e x c e e d e d our a b i l i t y t o o b t a i n d e t a i l e d s t r u c t u r a l i n f o r m a t i o n , w h i c h was l i m i t e d l a r g e l y t o c o m o n o m e r c o n c e n t r a t i o n s , c o n f i g u r a t i o n , c r y s t a l l i n i t y and a d e t e r m i n a t i o n o f i n f r a r e d s e n s i t i v e f u n c t i o n a l groups. W i t h the c a p a b i l i t y o f F o u r i e r t r a n s f o r m 1 3 c N M R t o d e t e c t c o n n e c t i n g carbons b e t w e e n m o n o m e r s o f d i f f e r e n t t y p e s , we c a n now measure average sequence lengths, r u n numbers, d y a d , t r i a d and h i g h e r c o m o n o m e r d i s t r i b u t i o n s , and b r a n c h l e n g t h d i s t r i b u t i o n s . T h i s i n f o r m a t i o n w i l l g i v e t r e m e n d o u s insight i n t o s t r u c t u r e - p r o p e r t y r e l a t i o n s h i p s and s o l i d s t a t e s t r u c t u r e s . With a s e n s i t i v i t y t o s t r u c t u r a l e n t i t i e s at a l e v e l o f one per t e n thousand carbons or l o w e r , 1 3 c N M R w i l l be v a l u a b l e i n d e t e c t i n g t h e onset of d e g r a d a t i o n and l i n k i n g r e a c t i o n s . A c h a p t e r on r a d i a t i o n i n d u c e d s t r u c t u r a l changes i n p o l y e t h y l e n e appears l a t e r i n t h i s m o n o g r a p h . T h e r e c a n be l i t t l e doubt t h a t we are on the t h r e s h o l d of e s t a b l i s h i n g s t r u c t u r e - p r o p e r t y r e l a t i o n s h i p s w h i c h were not c o n s i d e r e d possible a few short y e a r s ago.

Literature Cited 1. Bovey, F. A. "Chain Structure and Conformation Macromolecules"; Academic Press: New York, 1982.

of

9. 2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12. 13.

14. 15. 16. 17. 18. 19.

RANDALL. A N D HSIEH

13

C NMR in Polymer Quantitative Analyses

Randall, J. C. "Polymer Sequence Determination: Carbon-13 NMR Method"; Academic Press: New York, 1977. Schilling, F. C.; Tonelli, A. E. Macromolecules 1980, 13, 270. Randall, J . C. "Polymer Sequence Determination: Carbon-13 NMR Method"; Academic Press: New York, 1977; p 116. Sato, H.; Tanaka, Y . Chapter 12, this Monograph. Chen, T. K.; Harwood, H. J. Chapter 13, this Monograph. Hsieh, E. T.; Randall, J. C. Macromolecules 1983, 15, 353. Ray, G. J.; Spanswick, J.; Knox, J. R.; Serres, C. Macromolecules 1981, 14, 1323. Ray, G. J.; Johnson, P. E.; Knox, J. R. Macromolecules 1977, 10, 773. Hsieh, E. T.; Randall, J . C. Macromolecules 1982, 15, 1402. Randall, J. C.; Hsieh, Ε. T. Macromolecules 1982, 15, 1584. Carman, C. J.; Tarpley, A. R . , Jr.; Goldstein, J. H. Macromolecules 1973, 6, 719. Clague, A. D. H.; Van Broekhoven, J . A. M.; Blaauw, L. P. Macromolecules 1974, 7, 348. Bovey, F. Α.; Schilling, F. C.; Cheng, Η. N. "Advances in Chemistry Series No. 169"; Allara, D. L.; Hawkins, W. L., Eds.; American Chemical Society: Washington, D.C., 1978; pp. 133-141. Randall, J . C.; Zoepfl, F. J.; Silverman, J. Makromol. Chem., Rapid Commun. 1983, 4, 149. Randall, J. C.; Zoepfl, F. J.; Silverman, J. Chapter 16, this Monograph. Farrar, T. C.; Becker, E. D. "Pulse and Fourier Transform NMR"; Academic Press: New York, 1969. Gupta, R.; Ferretti, J.; Becker, E.; Weiss, G. J. Magn. Reson. 1980, 38, 447. Freeman, R.; Hill, H. D. W. J. Chem. Phys. 1971, 54, 3367. Freeman, R.; Hill, H. D. W.; Kaptein, R. J . Magn. Reson. 1972, 7, 327. Harwood, H. J.; Ritchey, W. M. Polymer Letters 1964, 2, 601. Randall, J. C. "Polymer Characterization by ESR and NMR", Woodward, A. E.; Bovey, F. Α., Eds.; ACS SYMPOSIUM SERIES No. 142, American Chemical Society: Washington, D.C., 1980; pp. 93-118.

RECEIVED December 10, 1983

151

10 The Synthesis of Novel Regioregular Polyvinyl Fluorides and Their Characterization by High-Resolution NMR RUDOLF E. CAIS and JANET M. KOMETANI Bell Laboratories, Murray Hill, NJ 07974

We report thefirstsynthesis of poly vinylfluoride(PVF) having a pure head-to-tail (isoregic) sequence of monomer units. The procedure involves dechlorination of appropriate isoregic precursor polymers which can be chosen to either allow or prevent racemization of the product PVF. We have also prepared head-to-head, tail-to-tail (syndioregic) PVF by the copolymerization of ethylene with 1,2-difluoroethylene, and predominantly syndiotactic PVF by polymerization in an urea clathrate complex. The chemical microstructures of these polymers, as well as that of commercial regioirregular (aregic) PVF, have been analyzed by 188 MHz fluorine-19 NMR. Quantitative statistical descriptions of both the regiosequence and stereosequence distributions have been deduced from the NMR measurements. We show that free-radical homopolymerization of VF typically involves 11% of reverse monomer addition which results in aregic defect structures. The regiosequence distribution follows first-order Markov statistics, whereas the stereosequence distribution is Bernoullian.

The majority of vinyl polymers are regioregular with a head-to-tail sequence of monomer units 0 ) , i.e. they are isoregic (2). Some of the most notable exceptions are the polymers obtained by free-radical addition reactions of the fluoroethylenes vinyl fluoride (VF), vinylidene fluoride ( V F ) , and trifluoroethylene (F E), which incorporate significant amounts of head-to-head and tail-to-tail structural defects caused by reverse monomer addition (3). We have devised a general procedure for the synthesis of isoregic polyfluoroethylenes, namely by the reductive dechlorination (or debromination) of appropriate precursor polymers in which head-to-head addition is sterically blocked by chlorine (or bromine) substituents. In this manner we have been able to prepare isoregic P V F , P V F , and P F E for the first time (4). The present report will deal with P V F , and details of the P V F and P F E syntheses will be given in subsequent publications. 2

2

3

3

2

0097-6156/84/ 0247-0153S06.00/ 0 © 1984 American Chemical Society

3

154

NMR AND MACROMOLECULES

P V F is a commercial polymer which exhibits unique weathering, mechanical, electrical, and chemical properties (5). It finds many applications in film form, including protective coverings on building and transportation materials, release coatings for molds, and glazing for solar panels. As we show here these materials typically contain about 11% of inverted monomer units, which create structural defects capable of influencing physical properties insofar as they may reduce crystallinity. One incentive for preparing pure isoregic polyfluoroethylenes is that they may exhibit improved properties compared to their aregic counterparts. PVF has an additional source of structural irregularity owing to pseudoasymmetric —CFH— methine carbons in the polymer backbone. In the commercial polymer these occur in nearly equal numbers of meso and racemic dyad pairs arranged in stereoirregular (atactic) sequences (6), as we shall see from the N M R analyses. A n equilibrium stereosequence distribution is obtained after epimerization (7). When isoregic P V F is prepared by the reductive dechlorination of poly ( 1 -chloro-1 -fluoroethene), racemization takes place so that the stereosequence distribution may approach the equilibrium state. However racemization of the —CFH— centers does not take place during the reductive dechlorination of poly (1 -chloro-2-fluoroethene), and we contrast the tacticities resulting from these two alternative routes to isoregic PVF, and the synthesis of PVF in urea at low temperature which produces a more stereoregular polymer. Heretofore the tacticity of P V F could not be measured accurately by fluorine19 N M R owing to the overlap of resonances from aregic sequences with those from stereoconfigurational triads and pentads in isoregic sequences (8). The initial assignments proposed by Weigert (9) were made on the basis of rather questionable chemical shift analogies with the carbon-13 N M R spectrum of polypropylene. We are now in a position to deduce more definitive assignments from the spectra of the novel polymers described herein combined with accurate measurements of sequence probabilities. Experimental Commercial P V F was obtained from Aldrich Chemical Company. PGR Research Chemicals Inc. supplied 1 -chloro-1 -fluoroethene (vinylidene chlorofluoride, V C F ) , which was degassed and distilled under high vacuum and sealed in glass ampules with 1 mol% of azobis(isobutyronitrile) initiator. Polymerization took place at 45 °C for 24 h with 95% conversion to the polymer P V C F . Complete details on V C F polymerizations over a range of temperatures will be published separately. The monomer 1 -chloro-2-fluoroethene (CFE) was likewise obtained from PGR and vacuum distilled. It was polymerized for 5 days at 6 0 ° C in a sealed ampule with 1 mol% of acetyl peroxide to yield 89% of P C F E . The Matheson Company Inc. provided V F monomer, which was passed through a column packed with silica gel and molecular sieves (for removal of inhibitor and water), and distilled under vacuum. Urea (Fischer, certified A C S ) was recrystallized three times from anhydrous ethanol, and 250 mmol was placed in a bulb attached to a vacuum manifold. Then 160 mmol of purified V F was added to the bulb, followed by a trace of methanol (0.2 ml) to initiate complex

10.

CAIS A N D

Synthesis of Regioregular Polyvinyl Fluorides

KOMFTANI

155

formation (10). The bulb was held at -65 °C for 11 days, during which time the urea crystals became opaque as the VF-urea clathrate complex formed. The clathrate complex was exposed to cobalt-60 7 radiation for 24 h at -80 °C for an absorbed dose of 6.5 Mrad to yield 16% of P V F after the urea was washed away with water. P C F E was prepared in urea in a similar manner. Ethylene (Matheson, C P grade) and 1,2-difluoroethene (mixed cis and trans from PCR) were sealed as an equimolar mixture in a heavy-wall ampule and exposed to cobalt-60 7 radiation (effective dose rate = 0.27 Mrad/h) for 20 h at -80 ° C . A poor yield (ca. 1%) of a waxy solid was obtained. Essentially complete reductive dechlorination of the precursor poly (chlorofluoroethylene) s to P V F was achieved after 24 h at 60 ° C in tetrahydrofuran with a molar excess of tri(n-butyl)tin hydride and 1 mol% of sizobis(isobutyronitrile) (11). The reaction mixture was homogeneous until dechlorination was almost complete, after which the mixture turned cloudy owing to the insolubility of PVF. Usually less than 0.1% of chlorine remained after the above treatment according to analysis by X-ray fluorescence spectroscopy. Fluorine was not removed from the polymer owing to its inertness compared to chlorine (12). A n elemental analysis performed by Galbraith Laboratories Inc. confirmed these findings (anal, calcd for C H F : C , 52.2; H , 6.5; F, 41.3; found: C , 52.2, H , 6.2; F, 41.6). More vigorous reducing conditions (24 h at 9 0 ° C in dimethylformamide solution) caused yellowing of the polymer, probably due to dehydrohalogenation. Fluorine-19 N M R data were acquired at a frequency of 188.22 M H z with a Varian XL-200 spectrometer. Typically, 100 transients were accumulated from a 5% polymer solution by volume in dimethylformamide-d placed in a 5 mm sample tube at 120°C with internal hexafluorobenzene as a reference (Φ = 163 ppm). A sweep width of 8000 Hz was used with 8 Κ computer locations (acquisition time 0.5s) and a 5.0 s delay between 90° pulses (9.0 MS duration). Proton heteronuclear coupling was removed by broad-band irradiation centered at 200 M H z . A modified Bruker WH-90 spectrometer allowed carbon-13 N M R spectra to be obtained with simultaneous proton and fluorine-19 broadband decoupling (13). 2

3

7

Results and Discussion Microstructures of Poly (chlorofluoroethylene) s The carbon-13 N M R spectrum of P V C F consists of a — C H — resonance at 54.1 ppm and a —CFC1— resonance at 108.8 ppm. There is no splitting of these lines due to tacticity, nor are there any other resonances to indicate the presence of regioirregular monomer sequences. However the polymer is stereoirregular, as shown by the fluorine-19 N M R spectrum in Figure 1. There are three principal resonances spread by 3 ppm owing to triad stereosequences, with some pentad fine structure which is barely resolved. The triad components have been assigned to syndiotactic (rr), heterotactic (mr), and isotactic (mm) sequences in order of increasing field. Peak areas are consistent with a Bernoullian sequence distribution having a p(m) value of 0.49 2

(6).

156

NMR AND MACROMOLECULES

The fluorine-19 N M R spectrum of P C F E appears far more complicated. Figure 2 shows the spectra from two P C F E samples, one prepared at 60 °C (a) and one prepared at - 8 0 ° C in urea (b). Each backbone carbon is a pseudoasymmetric center in P C F E , compared to every second carbon in P V C F , so that the dispersion of fluorine-19 chemical shift from stereoirregularity is much larger. This dispersion is almost 15 ppm for P C F E , and is similar to the spread observed in the fluorine-19 N M R spectrum of poly ( 1,2-difluoroethene) (14). Polymerization of C F E in urea at -80 °C has a mild stereoregulating influence compared to bulk polymerization at 60 °C, although the spectra in Figure 2 show that both polymers are substantially atactic. Three major triad resonances can be discerned more readily in spectrum (b), and these are assigned to mm, mr, and rr stereosequences as shown simply by analogy with the poly ( 1,2-difluoroethene) assignments ( 14,1_5). At present we have no basis to make definitive assignments, but those indicated are at least consistent with the known syndiotactic bias exerted by urea on the polymerization of complexed vinyl monomers (10). General Features of P V F Spectra The proton- and fluorine-decoupled 22.62 M H z carbon-13 N M R spectra of P V F prepared from P V C F (a) and commercial P V F (b) are shown in Figure 3. There are five additional peaks present in spectrum (b) from the commercial polymer which are absent in spectrum (a). These are due to aregic monomer sequences, which have been assigned according to Tonelli et al. (16). Monomer sequence triads are resolved and are denoted by the binary regiosequence pentad notation in Table 1 (1 = C F H , Ο — C H ) . The absence of these aregic defect resonances in spectrum (a) confirms that P V F obtained by the reductive dechlorination of P V C F is indeed isoregic. The 10101 and 01010 peaks have fine structure due to stereoirregularity, although this is not well resolved by carbon-13 N M R (13). Fluorine-19 N M R is more sensitive to the microstructure of P V F , as indicated in Figure 4, which compares the spectra from the above P V F samples at 84.66 M H z . Most of the resonances from aregic sequences occur at high field (spectrum b, 190 to 200 ppm), with the major peak intensity concentrated in the isoregic resonances which are split into a stereosequence triad from 179 to 183 ppm. Again it is evident from spectrum (a) that P V F derived from P V C F is regioregular, although stereoirregular. We now examine the tacticity of P V F more closely. 2

Stereosequence Microstructure of P V F Figure 5 shows in detail the 188 M H z fluorine-19 N M R signals from isoregic sequences in commercial P V F (a) and P V F prepared at -80 °C in urea (b). The resolution at the higher spectrometer frequency is better (cf Figure 4), and it is further improved at 282 M H z (17). The resonance lines are much broader from the urea polymer because this material does not dissolve completely in dimethylformamide. We believe that crosslinking takes place through urea molecules during gamma irradiation, since P V F prepared under similar conditions but without urea is soluble. The major splitting of the isoregic resonance into the three peaks is due to triad stereosequences, as noted above. Assuming that polymerization at low temperature in urea favors syndiotactic propagation, we assign these peaks to isotactic, heterotactic, and syndiotactic sequences in order of increasing field as

10.

CAIS A N D KOMETANI

Synthesis of Regioregular

Polyvinyl Fluorides

157

mr

—f Φ

Q

^ — ^ o o ppm

188 M H z Fluorine-19 N M R of P V C F , ( C H - C C l F ) , observed at 2 0 ° C as a 10% solution in acetone-d . 2

n

6

NMR AND MACROMOLECULES

158 —

CFH

— —

a

100

90

50

80

CH

2

—

5

40

30

20

ppm

Figure 3.

22.62 M H z Carbon-13 N M R spectra obtained with broad-band proton and fluorine decoupling of P V F obtained by reductive dechlorination of (a) P V C F and (b) commercial P V F . Spectra observed at 1 0 0 ° C with dimethylsulfoxide-d solvent (S solvent peak). The peak notation is explained in Table I. β

6

180

190

φ

Figure 4.

200

ppm

84.66 M H z Fluorine-19 N M R spectra of P V F obtained by reductive dechlorination of (a) P V C F and (b) commercial P V F . The peaks marked with an asterisk are due to aregic sequences not present in the pure isoregic sample (a).

10.

CAIS A N D K O M E T A N I

Figure 5.

Synthesis of Regioregular Polyvinyl Fluorides

159

Detailed expansion of the 188 M H z fluorine-19 resonances from the A

5

(10101)

and B

5

(00101)

sequences in (a) commercial P V F and

(b) P V F prepared at -80 ° C in urea.

160

N M R A N D

MACROMOLECULES

shown. These assignments are supported by calculations published by Tonelli et al. (15). Peak areas show that the probability of meso dyad formation, p(m), is 0.46 for the commercial P V F and 0.35 for the urea P V F . The triad stereosequence distribution is Bernoullian for both polymers. Fine structure from pentad stereosequences is clearly resolved in spectrum (a), although there is some overlap from peaks due to the 00101 (B ) regiosequence. The stereosequence fine structure is seen more clearly in spectra from the pure isoregic P V F samples, which are shown in Figure 6. The spectra of P V F derived from P C F E (a) and P V C F (b) are virtually identical, and show that the stereosequence distributions are Bernoullian with p(m) values of 0.49 and 0.48, respectively. The pentad assignments have been made according to these statistics by examining relative peak intensities and by comparison with Figure 5(b), where the pronounced syndiotactic bias distinguishes ambiguous cases (e.g. rmrr and mmrm) which arise when p(m) is very close to 0.5. The weak peaks denoted by arrows in Figure 6(a) are probably due to end groups, since the precursor polymer P C F E had a relatively low degree of polymerization. The bar graph in Figure 7 compares the triad stereosequence probabilities for the P V F samples examined here with values calculated for an equilibrium stereochemical distribution which would result from epimerization at 50 °C (18). All P V F samples, except for the urea polymer, rather fortuitously have nearly an equilibrium stereosequence distribution. Furthermore, both the precursor P V C F and final P V F derived therefrom have nearly identical p(m) values and stereosequence distributions, so that the effect of racemization during reductive dechlorination (19) is not apparent. 5

Regiosequence Microstructure of P V F In this final section we will be concerned with the regiosequence distributions in the aregic P V F samples. The A and B regiosequence pentads (Table 1) were identified in Figure 5(a), and we now examine the high-field resonances corresponding to C and D sequences. These are shown in Figure 8 for commercial P V F (a), urea P V F (b), and the copolymer of ethylene with 1,2-difluoroethene which serves as a model for syndioregic P V F (c). 5

5

5

5

There are six principal resonances from the two regiosequences C and D owing to further splitting from stereochemical effects. This splitting is larger when the —CHF— carbons are adjacent (11) than when they are two bonds apart (101). We saw that m dyads are less likely than r dyads in the 101 sequence of urea P V F , so that the peaks denoted by the arrows in Figure 8(b) can be associated with m dyads. The two major resonances from the syndioregic polymer (c) establish the 00110 sequence assignments, with due regard for the complication of these signals by end group effects in the copolymer which has a very low degree of polymerization. Finally, we have proven that the B and C regiosequences must have equal probability, irrespective of the sequence statistics, whereas peak D will have a different probability except in the special case of Bernoullian statistics (2). 5

5

5

5

5

Given the above considerations we arrive at the consistent set of assignments shown in Figure 8(a), which are supported by the calculations of Tonelli et al.

10.

CAIS A N D KOMETANI

Figure 6.

Synthesis of Regioregular Polyvinyl Fluorides

161

Detailed expansion of the 188 M H z fluorine-19 resonances from isoregic P V F samples prepared by reductive dechlorination of (a) P C F E and (b) P V C F .

The minor peaks marked with arrows in

spectrum (a) are most likely due to end groups.

Figure 7.

The isotactic (mm), heterotactic (mr), and syndiotactic (rr) triad stereosequence probabilities for (1)

commercial P V F , (2) P V F

derived from P V C F , (3) P V F derived from P C F E , (4) urea P V F and

(5) P V F epimerized at 50 °C (18).

heterotactic probability is mr + rm.

Note that the observed

162

NMR AND MACROMOLECULES

Table I.

Set of Nonequivalent 1- and O-Centered Regiosequence Pentads (O - C H , 1 « C H F ) 2

A

5

B

5

c

5

D

5

10101

«5

01010

00101

β

11010

10110

75

01001

00110

Figure 8. Detailed expansion of the 188 M H z fluorine-19 resonances from the C (10110) and D (00110) sequences in (a) commercial PVF, (b) urea PVF, and (c) the copolymer model for syndioregic PVF. 5

5

11001

5

φ

ppm

195

200

10.

CAIS A N D KOMETANI

Synthesis of Regioregular Polyvinyl Fluorides

163

(15). These differ in detail from the assignments reported by Weigert (9) and Gorlitz et al. (8). The present assignments are summarized in Table II. The probabilities of the regiosequence pentads for commercial P V F and urea P V F are shown in Table III. For the former sample it is apparent simply by inspection that the regiosequence distribution is not Bernoullian, since P (C ) and P ( D ) are different (2). The distributions conform to first-order M a r k o v statistics, characterized by two reactivity ratios r and r where r = k ^ / k ^ and T\ = k / k (ky is the rate constant for monomer addition to terminal radical i which generates the new terminal radical j ) . The present pentad data is insufficient to check the validity of this model, but it is unlikely that there is any deviation, as the same model has been tested and found adequate to describe the regiosequence distribution in P V F (2). o b s

o b s

0

u

5

5

l 5

0

1 0

2

Table I V shows the reactivity ratios r and v derived from the probabilities in Table III in accord with a first-order M a r k o v model (2), where it is assumed that the more likely propagating terminal radical structure is 1 ( — C H F ) and not 0 ( — C H ) . This assumption is consistent with gas phase reactions of V F with mono-, d i - , and trifluoromethyl radicals, which add more frequently to the C H carbon than to the C H F carbon (20). The reactivity ratio product is unity i f Bernoullian statistics apply, and we see this is not the case for either P V F sample, although the urea P V F is more nearly Bernoullian in its regiosequence distribution. Polymerization of V F in urea at low temperature also reduces the frequency of head-to-head and tail-to-tail addition, which can be derived from the reactivity ratios according to % defect « 100(1 + r ) / ( 2 + r + η ) . O u r analysis of the fluorine-19 N M R spectrum shows that commercial P V F has 10.7% of these defects, which compares very well with the value of 10.6% obtained from carbon13 N M R (13). Therefore the values of 26 to 32% reported by Wilson and Santee (21) are in error. 0

x

2

2

0

0

Conclusions The free-radical polymerization of V F produces a regioirregular and stereoirregular polymer. The degree of irregularity can be reduced by lowtemperature polymerization in urea. Isoregic P V F can be prepared by reductive dechlorination of either P V C F or P C F E . Preliminary studies by D S C show that the isoregic P V F melts at 2 1 7 ° C compared to 1 9 5 ° C for aregic P V F . One might expect that a stereoregular isoregic P V F would be even more crystalline. Whereas the stereosequence distribution in isoregic and aregic P V F is nearly ideal random (Bernoullian with p(m) — 0.5), the latter has a first-order Markov regiosequence distribution. Accordingly the monomer sequence isomerism in P V F cannot be described by a single parameter such as the % defect, and requires two reactivity ratios for complete specification.

NMR AND MACROMOLECULES

Table II.

Assignment of the 188 M H z Fluorine-19 N M R Spectrum of Commercial PVF to Regio-and Stereo-Sequence Pentads (observed in dimethylformamide at 120 ° C, internal hexafluorobenzene — 163 ppm, 11 stereochemical dyads — M and R, 101 stereochemical dyads — m and r).

Regiosequence A

5

(10101)

Φ ppm 179.1 (mmmm), 179.1 (mmmr), 179.1 (rmmr), 181.0 (rmrr), 181.1 (rmrm), 181.3 (mmrr), 181.4 (mmrm), 182.5 (rrrr), 182.7 (mrrr), 182.9 (mrrm).

B (00101)

179.8 (m), 181.7 (r)

5

C

5

(10110)

189.8 (mM), 191.8 (rM), 196.0 (mR), 198.8 (rR).

D

5

(00110)

192.3 ( M ) , 196.8 (R).

Table HI. Observed Probabilities of Regiosequence Pentads ( P ( S ) as defined in reference 2) for Aregic P V F Samples obs

Sequence

5

P

o5s

(S ) 5

Commercial P V F

Urea P V F

A

5

0.349

0.398

B

5

0.047

0.034

c

5

0.047

0.034

D

5

0.057

0.034

Table IV.

Reactivity Ratios and Percent Defect (head-to-head and tail-to-tail) for Aregic P V F Samples

Parameter

Commercial P V F

Urea P V F

0.033

0.068

7.62 r T, 0

% defect

0.25 10.7

12.7 0.86 7.2

10.

CAIS A N D KOMETANI

Synthesis of Regioregular Polyvinyl Fluorides

165

Literature Cited 1. Bovey, F. A. "Chain Structure and Conformation of Macromolecules"; Academic: New York, 1982; chap. 6. 2. Cais, R.E.; Sloane, N . J. A. Polymer, 1983, 24, 179-87. 3. Koenig, J. L. "Chemical Microstructure of Polymer Chains"; WileyInterscience: New York, 1980, chap. 9. 4. Cais, R. E.; Kometani, J. M . Proc. Org. Coat. Appl. Polym. Sci. Div. Amer. Chem. Soc., 1983, 48, 216-20. 5. Brasure, D. E. "Poly(Vinyl Fluoride)" in Kirk-Othmer Encyclopedia of Chemical Technology; Wiley-Interscience: New York, 1980, 3rd. Edition, Grayson, M., Exec. Ed., 11, 57-64. 6. Bovey, F. A. "High Resolution N M R of Macromolecules"; Academic: New York, 1972; chap. 3. 7. Suter, U. W. Macromolecules, 1981, 14, 523-8. 8. Görlitz, M . ; Minke, R.; Trautvetter, Angew. Makromol. Chem. 1973, 29/30, 137-62.

W.;

Weisgerber, G.

9. Weigert, F. J. Org. Magn. Reson, 1971, 3, 373-7. 10. White, D. M . J. Amer. Chem. Soc., 1960, 82, 5678-85. 11. Hjertberg, T.; Wendel, A. Polymer, 1982, 23, 1641-5, and references therein. 12. Carlsson, D. J.; Ingold, K. U. J. Amer. Chem. Soc., 1968, 90 7047-55. 13. Schilling, F. C. J. Magn. Reson., 1982, 47, 61-67. 14. Cais, R. E. Macromolecules, 1980, 13, 806-8. 15. Tonelli, A. E.; Schilling, F. C.; Cais, R. E. Macromolecules, 1982, 15, 84953. 16. Tonelli, A. E.; Schilling, F. C., Cais, R. E. Macromolecules, 1981, 14, 5604. 17. Lin, Fu-Mei Chen, Ph.D Thesis, The University of Akron, Ohio, 1981. 18. Tonelli, A. E., personal communication. 19. Davies, D. I.; Parrott, M . J. "Free Radicals in Organic Synthesis"; Springer-Verlag: New York, 1978; p. 18. 20. Sloan, J. P.; Tedder, J. M.; Walton, J. C. J. Chem. Soc. Perkin II, 1975, 1846-50. 21. Wilson, C. W.; Santee, E. R. J. Polym. Sci. C, 1965, 8, 97-112. RECEIVED September 22, 1983

11 The Composition and Sequence Distribution of Dichlorocarbene-Modified Polybutadiene by C NMR 13

1

CHARLES J. CARMAN, RICHARD A. KOMOROSKI, and SAMUEL E. HORNE, JR. The B.F. Goodrich Research and Development Center, Brecksville, OH 44141

Carbon-13 NMR i s being used to characterize the microstructure of a variety of chlorine-containing polymers. Among these are homopolymers, copolymers, and chemically modified polymers. In the last category i s the series of polymers obtained by addition of dichlorocarbene to the double bonds of polybutadiene. Here we use C NMR to examine a number of dichlorocarbene adducts of c i s - and trans-polybutadiene prepared i n a two phase system with phase transfer catalysis. Monomer compositions, comonomer sequence lengths, and stereochemical information are obtained for the resulting polymers. The polymers examined here were stereochemically pure and were treated as simple copolymers. Samples prepared using aqueous NaOH, CHCl and phase transfer catalyst can be described as essentially random copolymers over the entire range of monomer composition. Samples prepared using s o l i d instead of aqueous a l k a l i metal hydroxides contain a higher fraction of blocked units than a polymer of comparable composition prepared using aqueous NaOH. This blockiness can coincide with the presence of two glass transition temperatures and a two-phase morphology. Fractionation of a substantiall y blocked sample yielded a chlorine-poor fraction which was a random copolymer and a chlorine-rich fraction which was more blocked than the o r i g i n a l unfractionated material. 13

3

Portions of this work are reprinted from: R. A. Komoroski, S. E. Home, Jr., and C. J. Carmen, J. Polym. Sci, Polym. Chem. Ed, 21, 89 (1983), copyright 1983, John Wiley and Sons, Inc. Reprinted by permission of John Wiley and Sons, Inc. 1

Current address: Polysar, Inc., Stow, O H 44224. 0097 6156/84/0247 0167S06.00/0 © 1984 American Chemical Society

168

NMR AND

MACROMOLECULES

1 3

C NMR has long been the technique of choice f o r the c h a r a c t e r i z a t i o n of the molecular s t r u c t u r e of homopolymers and copolymers (1) . Recently, i t has been used with success to study c h l o r i n e containing polymers and copolymers (1-6). Among these are p o l y ( v i n y l c h l o r i d e ) (PVC) and various copolymers of v i n y l c h l o r ide and other monomers both with and without c h l o r i n e . I t has also proven to be powerful f o r c h a r a c t e r i z i n g modified polymers, i n p a r t i c u l a r the products obtained from c h l o r i n a t i o n of polyvinyl c h l o r i d e ) (7,8) and polyethylene (9). Although both c h l o r i n a t e d PVC and c h l o r i n a t e d polyethylene are simple i n t h e i r basic composition, c o n s i s t i n g predominantly of CC1 , C H C 1 , and Ch*2 groups, the many p o s s i b l e arrangements of these groups produce very complex macromolecules. Both sequence d i s t r i b u t i o n and CHC1 c o n f i g u r a t i o n must be considered. A more simple modified polymer i s c h a r a c t e r i z e d i n t h i s report. Dichlorocarbene, :CC1 , generated i n s i t u , can add to polydienes to produce novel polymer containing d i c h l o r o c y c l o p r o pane rings at the points of a d d i t i o n (10-13). The r e a c t i o n i s i l l u s t r a t e d below f o r cis-polybutadiene. 2

2

CI

CI

Polymers with a r e l a t i v e l y broad range of p h y s i c a l p r o p e r t i e s can be prepared by varying the extent of r e a c t i o n and hence c h l o r i n e content. These modified polymers can be viewed as a s e r i e s of copolymers of varying comonomer composition and treated as such with regard to sequence d i s t r i b u t i o n . We have examined the m i c r o s t r u c t u r e of a number of d i c h l o r o carbene adducts of both c i s - and trans-polybutadiene using C NMR spectroscopy. Samples were prepared i n a two phase system where dichlorocarbene was generated by the r e a c t i o n of e i t h e r concentrated aqueous or s o l i d a l k a l i metal hydroxide with chloroform i n the presence of a phase t r a n s f e r c a t a l y s t (14). Monomer compositions and sequence lengths were obtained as f o r true copolymers and were c o r r e l a t e d with g l a s s t r a n s i t i o n temperature and phase morphology. 1 3

Results In Table I are some p e r t i n e n t data f o r a number of dichlorocarbene adducts of c i s - and trans-polybutadiene. Figure 1 shows the C NMR spectra of three cis-polybutadiene-based adducts having widely d i f f e r e n t c h l o r i n e contents. At low percent CI, i . e . low extent of r e a c t i o n , the spectrum i s e s s e n t i a l l y that of cis-polybutadiene (15) with some a d d i t i o n a l smaller resonances due to the presence of modified butadiene units (Figure 1A). With i n c r e a s i n g c h l o r i n e content, these a d d i t i o n a l resonances grow r e l a t i v e to those due to 1 3

11.

Dichlorocarbene-Modified

CARMAN ETAL.

169

Polybutadiene

Table I . Glass T r a n s i t i o n Temperatures, Compositions, and R e a c t i o n C o n d i t i o n s o f Some D i c h l o r o c a r b e n e A d d u c t s o f C i s - a n d Trans- Polybutadiene

Sample 1 2 3 4 5 6 7 8 8 A

d 8B 9 10 11 12 13

d

e

e

a) b) c) d) e) f)

8

Tg ( « C )

1

-85 -93 -41 -31 -5 31 57 -78, 50 45 -76 -61 -89 -68 -56 - 3 1 , -19

Weight % CI Chemical NMR Analysis 15.,0 14..8 33 37.,8 42..1 47 51. .2 33. .9 41..9 22..0 17,.4 16 .0 31 .3

15,,9 16.,3 32.,5 39..4 43. Λ 48..0 51..2 33..6 43. .3 22,.7 25,.1 19 .0 30 .4 17 .4 31 .3

% RDB 14. 9 15.,4 40. 0 55,,8 67.,2 83. .5 97.,1 42..2 66..8 23. .6 27..1 18,.6 35,.9 16 .7 37 .7

Reaction ^ Conditions 5 0 % aqueous NaOH s o l i d NaOH 5 0 % aqueous NaOH If tf ft

s o l i d NaOH s o l i d KOH tt tt

5 0 % aqueous KOH s o l i d C OH s o l i d RS0H 5 0 % aqueous NaOH tf

S c h o n i g e r o x y g e n f l a s k method. s e e r e f e r e n c e 14. a c e t o n e - s o l u b l e f r a c t i o n o f sample 8. a c e t o n e - i n s o l u b l e f r a c t i o n o f sample 8. Adducts o f trans-polybutadiene. M e a s u r e d u s i n g a P e r k i n E l m e r DSC-2 d i f f e r e n t i a l s c a n n i n g calorimeter. g) The s t a r t i n g p o l y m e r s were >98% c i s - o r t r a n s - p o l y b u t a d i e n e .

NMR AND MACROMOLECULES

JL

u

J

L

JAUJL L

140

130

120

_L

no

100

90

80 ppm

60

70

50

40

30

20

F i g u r e 1. C a r b o n - 1 3 NMR s p e c t r a o f t h r e e d i c h l o r o c a r b e n e m o d i f i e d c i s - p o l y b u t a d i e n e s i n C D C 1 ; (A) Sample 2, 15.4% o f d o u b l e bonds r e a c t e d ; (B) Sample 5, 6 7 . 2 % o f d o u b l e bonds r e a c t e d ; (C) Sample 7, 9 7 . 1 % o f d o u b l e bonds r e a c t e d . ( R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f . 16.) 3

11.

C A R M A N ET A L .

Dichlorocarbene-Modified

171

Polybutadiene

the cis-polybutadiene (Figure I B ) . When a l l double bonds have reacted, the spectrum i n Figure 1C i s obtained. Peak assignments were made f o r the spectra i n Figure 1 on the basis of model compounds (Table I I ) , substituent e f f e c t s , s i n g l e frequency off-resonance decoupling, and peak i n t e n s i t i e s . The r e l a t i v e l y large a- and β-substitutent e f f e c t s f o r chlorine produce C NMR spectra of c h l o r i n a t e d polymers that can span the range from 25 to 100 ppm. Substituent e f f e c t s on the C s h i f t of the c e n t r a l carbon are given below f o r c h l o r i n e s u b s t i t u t i o n at various locations. Substituent e f f e c t s due to v i c i n a l l y or germinally s u b s t i t u t e d 1 3

1 3

±0.5 CI

+9.0 CI

-3.2 CI

C - C - C - C - C - C - C t CI +34.0 chlorines are not a d d i t i v e , a feature which can sometimes compli cate assignments f o r h i g h l y c h l o r i n a t e d polymers (8). I t i s i n t e r e s t i n g to note that the CC1 carbon i n the small molecular weight models (Table II) and the polybutadiene adducts occurs about 20 ppm u p f i e l d of i t s usual p o s i t i o n (5). This i s due to i t s occurrence i n a cyclopropyl r i n g . Our assignments f o r the cis-adducts are i n Figure 2. A table of chemical s h i f t s and assignments f o r both the c i s and trans-adducts has been given elsewhere (16). The m u l t i p l e resonances observed f o r carbons of a given f u n c t i o n a l i t y i n d i c a t e that the C NMR chemical s h i f t s are s e n s i t i v e to the sequence d i s t r i b u t i o n of monomer u n i t s . From the changes i n the i n t e n s i t i e s of the various peaks with changing extent of r e a c t i o n , we have assigned peaks to copolymer t r i a d structures such as those i n Figure 3 . The CC1 carbon i s s e n s i t i v e to t r i a d s t r u c t u r e s , while the vinylene CH of the diene u n i t and the CH carbon of the dichlorocyclopropane u n i t i s s e n s i t i v e to dyads. The CH carbon of the dichlorocyclopropane u n i t d i s p l a y s no s e n s i t i v i t y to sequence d i s t r i b u t i o n under the present condi tions . 2

1 3

2

2

Discussion Isomerization during the course of the dichlorocarbene modifica t i o n of polybutadienes i s a p o t e n t i a l concern. Isomerization i s p o s s i b l e i n both the unreacted butadiene p o r t i o n and the c h l o r i n e containing p o r t i o n . Although the a d d i t i o n of dichlorocarbene to double bonds i s know to be s t e r e o s p e c i f i c and syn, (17) i t i s p o s s i b l e that small amounts of t r a n s - s u b s t i t u t e d cyclopropyl groups might be present i n the modified c i s polymer. We saw no a d d i t i o n a l resonances suggestive of mixed stereochemistry of the cyclopropyl group i n e i t h e r the c i s - or trans-adducts. The C 1 3

172

NMR A N D MACROMOLECULES

Table I I .

1 3

C NMR Chemical S h i f t s of Some Model Compounds

Compound

θ (ppm)

Carbon

67.4 25.9 19.0 20.4

3 CI CI

CI

CI 1 2

νΟΦν CI

CI

CH - C H

72.3 31.9,32.2 (mixture o f isomers)

CI 89.7

CClc

- C - C H - CH I CI (vinyl chloride-vinylidene c h l o r i d e copolymer) (5) 2

2

r

r

CH (AAA),CH (4)(AAB) 2

2

CH(ABA,BBA,BBB)-y

= CH(AAA,BAA)

CCI

2

ppm 1 3

F i g u r e 2. C NMR p e a k a s s i g n m e n t s f o r t h e d i c h l o r o c a r b e n e adducts o f c i s - polybutadiene.

11.

C A R M A N ET AL.

Dichlorocarbene-Modified

Polybutadiene

173

spectrum f o r the m o d i f i e d t r a n s - p o l y b u t a d i e n e i s s u f f i c i e n t l y different from t h a t of m o d i f i e d c i s - p o l y b u t a d i e n e t h a t each s t r u c t u r e can be d e t e c t e d s e p a r a t e l y i n a c o p o l y m e r ( 1 6 ) . A l though the C s p e c t r u m o f t r a n s - p o l y b u t a d i e n e homopolymer o v e r l a p s w i t h t h a t o f t h e m o d i f i e d c i s - p o l y b u t a d i e n e , r e l a t i v e peak a r e a s i n d i c a t e t h a t l i t t l e , i f any, t r a n s - p o l y b u t a d i e n e i s p r e s e n t (16). R e a c t i o n o f an e m u l s i o n p o l y b u t a d i e n e containing c i s , t r a n s , and v i n y l groups y i e l d s a C NMR s p e c t r u m t h a t i s much more c o m p l i c a t e d t h a n t h a t o f e i t h e r t h e t o t a l l y c i s o r t r a n s a d d u c t s , even when t h e p r e s e n c e o f v i n y l groups i s t a k e n i n t o account. We can o b t a i n monomer c o m p o s i t i o n and w e i g h t p e r c e n t CI f r o m t h e C s p e c t r a u s i n g t h e a s s i g n m e n t s i n F i g u r e 2. The amount o f r e a c t e d d o u b l e b o n d s , % RDB, i s g i v e n by t h e f o r m u l a 1 3

1 3

1 3

% RDB

=

h

A (CH, 32.4

A (CH, 32.4 pgn) ppm) + A ( a l l v i n y l e n e

1 0 0

(1)

CH)

The A terms a r e t h e a r e a s o f t h e NMR p e a k s . T a b l e I shows t h e v a l u e s f o r % RDB, as c a l c u l a t e d u s i n g C NMR d a t a . Values f o r w e i g h t % c h l o r i n e c a l c u l a t e d from NMR a r e a l s o g i v e n i n T a b l e I and a r e compared t o t h e c o r r e s p o n d i n g v a l u e s o b t a i n e d by t h e Schdniger method. Agreement between % c h l o r i n e by chemical a n a l y s i s and by NMR r a n g e s from f a i r t o e x c e l l e n t , d e p e n d i n g on sample. A number o f f a c t o r s a f f e c t s t h e a c c u r a c y and p r e c i s i o n o f q u a n t i t a t i v e FT NMR measurements o f c o m p o s i t i o n . Among t h e s e a r e signal-to-noise ratio, digitization of the frequency domain spectrum, p u l s e r e p e t i t i o n r a t e , the n u c l e a r Overhauser e f f e c t (NOE), and s p e c t r a l r e s o l u t i o n . The s i g n a l - t o - n o i s e r a t i o , t h e NOE, s p e c t r a l r e s o l u t i o n , and s p e c t r u m d i g i t i z a t i o n may be f a c t o r s t h a t reduce the accuracy of the c o m p o s i t i o n r e s u l t s . The low s i g n a l - t o - n o i s e r a t i o o f some o f t h e s p e c t r a i s p r o b a b l y a m a j o r f a c t o r , p a r t i c u l a r l y a t low e x t e n t o f r e a c t i o n . Differential NOE's and s h o r t p u l s e r e p e t i t i o n r a t e s can be m a j o r f a c t o r s when c a l c u l a t i o n s r e l y on a r e a s o r i n t e n s i t i e s o f b o t h p r o t o n a t e d and nonprotonated carbons (18). For example, the area of the CC1 p e a k o f sample 7 ( F i g u r e 1C) i s o n l y 5 7 % o f t h a t e x p e c t e d , b a s e d on t h e a r e a s o f t h e CH and CH c a r b o n p e a k s . The p u l s e r e p e t i t i o n r a t e used here w i l l not s i g n i f i c a n t l y a f f e c t the accuracy of the r e s u l t s i n T a b l e I s i n c e none o f o u r q u a n t i t a t i v e r e s u l t s r e l y on c o m p a r i s o n s o f p r o t o n a t e d and n o n p r o t o n a t e d c a r b o n a r e a s . Number-average sequence l e n g t h s f o r b o t h t h e d i c h l o r o c y c l o p r o p a n e and t h e c i s - b u t a d i e n e u n i t s can be c a l c u l a t e d from t h e C d a t a u s i n g t h e a s s i g n m e n t s i n F i g u r e 2 and s t a n d a r d e q u a t i o n s . F o r t h e c h l o r i n e - c o n t a i n i n g u n i t s (Β), n^ can be o b t a i n e d f r o m dyad c o n c e n t r a t i o n s u s i n g (I) 1 3

2

2

1 3

B

n

N =

BB

N

* ** B A \ N M

=

A(24.1 ppm

+ 24.2 ppm) + A ( 2 5 . 0 ppm) A(25.0 ppm)

(2)

174

NMR

H e r e N_._ where

and

Ν

are

A

N

Œ

BB

the

A

(

2

concentrations

4

*

1

PP

ra

+

V a l u e s f o r n ^ c a n a l s o be o b t a i n e d the CC1 resonance ( 1 ) .

2

4

·

2

AND

of the

M A C R O M O L E C U L E S

designated

Ppm)

from t r i a d c o n c e n t r a t i o n s

dyads,

(3) using

2

B

n

- K 6 5 . 6 npm) + 1 ( 6 5 . 3 ppm) + 1(65.0 ppm) 1(65.6 ppm) + \ I (65.3 ppm)

(4)

Peak i n t e n s i t i e s ( I ) were u s e d f o r t r i a d c o n c e n t r a t i o n s s i n c e t h e y were more e a s i l y and a c c u r a t e l y measured t h a n t h e c o r r e s p o n d i n g areas i n t h i s p a r t i c u l a r case. An e q u a t i o n ( E q u a t i o n 5) a n a l o g o u s t o 2 was u s e d f o r b u t a d i e n e sequence l e n g t h s . A

=

n

A ( 2 7 . 4 ppm) + A ( 2 6 . 3 ppm) A ( 2 6 . 3 ppm)

(5)

The s e q u e n c e l e n g t h s o f b o t h t h e d i c h l o r o c y c l o p r o p y l and t h e butadiene u n i t s f o r the polymers s t u d i e d here are given i n Table III. The agreement b e t w e e n n^ ( d y a d ) and n^ ( t r i a d ) , although good a t low s e q u e n c e l e n g t h s , i s p o o r a t h i g h e r s e q u e n c e l e n g t h s . The c a u s e o f t h i s d i s c r e p a n c y i s n o t known. The s p e c t r a a r e t o o s i m p l e t o a l l o w f o r t h e p r e s e n c e o f a s u b s t a n t i a l amount o f some a d d i t i o n a l s t r u c t u r a l f e a t u r e t h a t c o u l d be r e s p o n s i b l e f o r t h e discrepancy (16). The use o f peak h e i g h t s i n one c a s e and a r e a s i n a n o t h e r c o u l d a c c o u n t f o r t h e l a c k o f good a g r e e m e n t . Another p o s s i b i l i t y i s t h a t a v a r i a t i o n of n u c l e a r Overhauser enhancement i s occurring with composition f o r the peak at 24.1 ppm. T h i s r e s o n a n c e i s due t o C H c a r b o n s o f b o t h end and i n t e r i o r Β u n i t s and t h e o b s e r v e d NOE may change as t h e f r a c t i o n o f l o n g b l o c k s c h a n g e s . A l o w e r NOE m i g h t be e x p e c t e d f o r c a r b o n s i n a l o n g (>3) b l o c k t h a n i n a s h o r t b l o c k on t h e b a s i s o f e x p e c ted chain m o b i l i t y (18). S u c h an o c c u r r e n c e c a n s e v e r e l y c o m p l i c a t e q u a n t i t a t i v e NMR a n a l y s e s . As e x p e c t e d , n^ i n c r e a s e s and n ^ d e c r e a s e s w i t h i n c r e a s i n g e x t e n t o f r e a c t i o n . F o r t h e s a m p l e s p r e p a r e d u s i n g aqueous NaOH, the observed s e q u e n c e l e n g t h s a r e i n r e a s o n a b l e agreement w i t h those expected f o r a random c o p o l y m e r d i s p l a y i n g B e r n o u l l i a n s t a t i s t i c s ( T a b l e I I I ) (_1 ). The random c o p o l y m e r sequence l e n g t h s were c a l c u l a t e d u s i n g E q u a t i o n 6, where Ρ i s t h e mole f r a c t i o n o f dichlorocyclopropane units. 2

β

A

fi

=

1 / P

B

δ

Β

1 / ( 1 =

"V

(6)

F o r t h e o t h e r p o l y m e r s t h e n^ v a l u e s a r e h i g h e r t h a n c a l c u l a t e d u s i n g B e r n o u l l i a n s t a t i s t i c s , i n d i c a t i n g the presence of l a r g e r f r a c t i o n s of blocked d i c h l o r o c y c l o p r o p y l - c o n t a i n i n g u n i t s i n p o l y m e r s n o t p r e p a r e d w i t h aqueous NaOH t h a n i n c o m p a r a b l e p o l y m e r s p r e p a r e d w i t h aqueous NaOH. C o m p a r i s o n o f s a m p l e s 3 and

11.

Cl

I

175

Dichlorocarbene-Modified Polybutadiene

C A R M A N ET AL.

Cl

4

BBA / V_V \2V \=J Y Y ci ci ci ci

\

F i g u r e 3 . Two p o s s i b l e comonomer triads f o r dichlorocarbene-modified cis-polybutadiene. (Reproduced w i t h p e r m i s s i o n f r o m Réf. 16. )

T a b l e I I I . Number-Average Sequence L e n g t h s o f Some Dichlorocarbene Adducts o f C i s - P o l y b u t a d i e n e s

l

e

1 2 3 4 5 6 7 8 8A 8B 9 10 11 12 13 a) b)

A

A

n (dyad) Â

5.8 6.8 2.2 1.8 1.4 1.3

-

2.9 2.0 3.9 3.2 6.0 2.8 6.0 2.4

fi (theor.)b A

6. 7 6 , ,5 2 . .5 1, ,8 1. . 5 1. . 2 1, .0 2. .4 1, . 5 4, .2 3, .7 5. . 4 2 , .8 6 . .0 2, J

n^Idyâd)

ng(triad)

1. . 3 1. .5 1. . 6 2 . .1 2 . .5 5. .3

1. , 2 1. ,8 1. ,8 2 . ,4 3 . ,0 6. . 6

2. .5 3 . .4

3, .3 4 . .9 1, , 3 1, . 5 1, . 8 1, . 8 1 .3 1 .7

1, . 3 1, . 3 1 .4 1 .6 1 .2 1 .6

Adducts of t r a n s - p o l y b u t a d i e n e . Calculated using Bernoullian s t a t i s t i c s

(1).

n (theor.)b B

1. , 2 1. . 2 1. J 2 . .3 3 . .0 6 . ,1 34 1, . 7 3 , .0 1, . 3 1, . 4 1, . 2 1. . 6 1, . 2 1 .6

176

NMR

AND MACROMOLECULES

8 well illustrates this point. B o t h have t h e same elemental c o m p o s i t i o n , y e t sample 8, p r e p a r e d w i t h s o l i d KOH, h a s a much l a r g e r f r a c t i o n o f b l o c k e d u n i t s t h a n sample 3. The d i f f e r e n c e i s c l e a r l y r e f l e c t e d i n the s p e c t r a (Figure 4 ) . A s i m i l a r comparison can be made b e t w e e n sample 12, a n a d d u c t o f t r a n s - p o l y b u t a d i e n e prepared u s i n g aqueous NaOH, and sample 2, a c i s - p o l y b u t a d i e n e a d d u c t p r e p a r e d u s i n g s o l i d NaOH. F o r t h e s a m p l e s p r e p a r e d u s i n g aqueous NaOH, Tg i n c r e a s e s i n a smooth f a s h i o n w i t h i n c r e a s i n g e x t e n t o f r e a c t i o n and n ^ , as e x p e c t e d f o r random c o p o l y m e r s . The s i t u a t i o n f o r s a m p l e s p r e p a r e d w i t h s o l i d b a s e i s more c o m p l e x . I n some c a s e s two Tg's a r e observed. T h i s has been c o r r e l a t e d w i t h t h e o b s e r v a t i o n o f a l a r g e d o m a i n , two-phase m o r p h o l o g y u s i n g t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y (TEM) ( 1 9 ) . I n t h e c a s e s where two Tg's a r e p r e s e n t , the f r a c t i o n o f blocked d i c h l o r o c y c l o p r o p y l u n i t s i s h i g h g i v e n the r e l a t i v e l y low o v e r a l l extent o f r e a c t i o n . One example i s sample 8 i n T a b l e s I and III. A q u e s t i o n a r i s e s concerning t h e homogeneity i n s o l u t i o n o f t h e p o l y m e r s w i t h two T g ' s . Can b l o c k e d segments be i s o l a t e d f r o m more random c o p o l y m e r c h a i n s ? And c a n t h e l o w and h i g h Tg's be a s s i g n e d t o b l o c k e d b u t a d i e n e and b l o c k e d d i c h l o r o p r o p y l u n i t s , respectively? Sample 8, p r e p a r e d w i t h s o l i d KOH and w i t h Tg's o f -78°C and 50°C, was f r a c t i o n a t e d u s i n g a c e t o n e . The r e s u l t s a r e i n T a b l e s I and I I I and F i g u r e 5. The s o l u b l e p o r t i o n ( 8 A ) h a d a h i g h c h l o r i n e c o n t e n t , a Tg o f 45°C, and a h i g h l y b l o c k e d s t r u c t u r e [ I g ( t r i a d ) = 4 . 9 ] . The i n s o l u b l e p o r t i o n ( 8 B ) h a d a l o w c h l o r i n e c o n t e n t , a Tg o f -76°C, and was a random c o p o l y m e r [ n ^ ( t r i a d ) = 1.3]. This experiment l i n k s the m i c r o s t r u c t u r a l f e a t u r e s , as d e t e r m i n e d b y C NMR, t o t h e t h e r m a l a n a l y s i s r e s u l t s i n a d i r e c t way. The C NMR r e s u l t s f o r t h e f r a c t i o n a t e d s a m p l e s a g r e e w i t h TEM r e s u l t s f o r o t h e r s a m p l e s ( 1 4 ) . TEM showed p h a s e s e g r e g a t i o n o f c h l o r i n e - r i c h and c h l o r i n e - p o o r r e g i o n s ( 1 9 ) . I t i s p o s s i b l e , h o w e v e r , t h a t a h i g h l y h e t e r o g e n e o u s sample l i k e 8 c o u l d be s e p a r a t e d i n t o f r a c t i o n s w i t h a continuous range o f c o m p o s i t i o n and b l o c k i n e s s and s t i l l d i s p l a y a two-phase morphology. F o r such heterogeneous polymers, s t a t i s t i c a l d e s c r i p t i o n s o f t h e s e q u e n c e d i s t r i b u t i o n s c a n be u s e d t o compare p o l y m e r s t r u c t u r e s , b u t c a n n o t be u s e d t o draw d e t a i l e d c o n c l u s i o n s a b o u t the chemistry o f p r e p a r a t i o n . A l t h o u g h o n l y a l i m i t e d number o f s a m p l e s were p r e p a r e d u s i n g a l k a l i m e t a l h y d r o x i d e s o t h e r t h a n NaOH, t h e r e a p p e a r s t o be no s t r o n g e f f e c t due t o t h e c a t i o n i n s o l u t i o n o r i n t h e s o l i d . 1 3

1 3

Experimental D e t a i l s o f t h e s a m p l e p r e p a r a t i o n a r e g i v e n i n R e f e r e n c e 16. The C NMR s p e c t r a were o b t a i n e d on a B r u k e r HX-90E F o u r i e r t r a n s f o r m s p e c t r o m e t e r a t 22.6 MHz a n d a p p r o x i m a t e l y 30°C. Samples u s u a l l y c o n s i s t e d o f 0.5 g o f p o l y m e r i n 2 m l o f C D C 1 i n 10 mm o.d. NMR tubes. C h e m i c a l s h i f t s were r e f e r e n c e d t o i n t e r n a l ( C H ) S i . The s p e c t r a l c o n d i t i o n s were : 90° r a d i o - f r e q u e n c y p u l s e s , 15 p s e c ; 1 3

3

3

4

11.

C A R M A N ET A L .

Dichlorocarbene-Modified

Polybutadiene

A)

Β)

1 3

F i g u r e 4. C NMR s p e c t r a o f two a d d u c t s w i t h t h e same % CI. ( A ) Sample 3, η ( t r i a d ) = 1.8. ( B ) Sample 8, η ( t r i a d ) = 3.3. B

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NMR AND MACROMOLECULES

F i g u r e 5. C a r b o n - 1 3 NMR s p e c t r a o f Sample 8 (A) and i t s a c e t o n e s o l u b l e ( 8 a ) (B) and i n s o l u b l e ( 8 b ) (C) f r a c t i o n s i n C D C 1 . ( R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f . 16.) 3

11.

C A R M A N

Dichlorocarbene-Modified Polybutadiene

E T A L .

179

s p e c t r a l w i d t h , 6 k H z ; number o f p o i n t s ( F I D ) , 16k; p u l s e r e p e t i t i o n r a t e , 5 s ; l i n e b r o a d e n i n g due t o e x p o n e n t i a l filtering, I . 5 H z ; number o f s c a n s , 1000. Acknowledgments We t h a n k Dr.C J . Singleton f o r providing t h e Tg r e s u l t s , R. W. S m i t h f o r p r o v i d i n g t h e TEM r e s u l t s , a n d R. E. S c o u r f i e l d f o r a c q u i r i n g t h e C NMR s p e c t r a . 1 3

Literature Cited 1.

Randall, J. C. "Polymer Sequence Determination", Academic: New York, 1977. 2. K e l l e r , F.; Zepnik, S.; Hösselbarth, B. Faserforsch. T e r t i l tech. 1978, 29, 490. 3. K e l l e r , F.; Mügge, C. Faserforsch. Textiltech. 1976, 27, 347. 4. Carman, C. J. ACS Symp. Ser. 1980, 142, 81. 5. Komoroski, R. Α.; Shockcor, J. P. Macromolecules 1983, 16, xx. 6. Kaplan, S. J . Polym. Sci., Polym. Chem. Ed. 1980, 18, 3307. 7. K e l l e r , F.; Hösselbarth, B. Faserforsch. Textiltech. 1978, 29, 152. 8. Komoroski, R. Α.; Parker, R. G . ; Lehr, M. H. Macromolecules 1982 15, 844. 9. K e l l e r , F.; Mügge, C. Plaste u. Kautschuk 1977, 24, 88. 10. Pinazzi, C.; Levesque, G. C. R. Acad. S c i . 1965, 260, 3393. 11. Lishanskii, I . S.; Tsitskhtsev, V. Α.; Vinogradova, N. D. Vysokom. Soedin. 1966, 8, 186. 12. L a i , J.; Saltman, W. J. Poly. S c i . A-1 1966, 4, 1637. 13. DeWitt, W.; Hurwitz, M. J.; Albright, F . J . Poly. S c i . A-1 1969, 7, 2453. 14. Horne, S. Ε . , Jr., to be published. 15. Clague, A. D. H.; van Broekhoven, J . A. M . ; Blaauw, L . P. Macromolecules 1974, 7, 348. 16. Komoroski, R. Α.; Horne, S. Ε . , Jr.; Carman, C. J. J. Polym. Sci., Polym. Chem. Ed. 1983, 21, 89. 17. Morrison, R. T . ; Boyd, R. N . "Organic Chemistry", Allyn and Bacon: Boston, 1973; p. 311. 18. Komoroski, R. A. J. Polym. Sci., Polym. Phys. Ed. 1979, 17, 45. 19. Horne, S. Ε . , Jr.; Singleton, C. J.; Smith, R. W., unpub lished results. RECEIVED September 22, 1983

12 NMR

Spectra of Styrene Oligomers and Polymers

HISAYA SATO and YASUYUKI TANAKA Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan Styrene oligomers having propyl end groups were prepared by the oligomerization of styrene i n i t i a t e d with ethyllithium and terminated with 1-bromopropane. The oligomers were separated according to molecular weight by GPC and the 2-5 mers were fractionated into diastereomers by l i q u i d chromatography. The diastereomers of the 2- and 3-mers were identified by 1H NMR and those of the 4- and 5-mers by using the results of the 2- and 3-mers. The 13C NMR signals of the oligomers were assigned by selective decoupling measurements and also by comparing the chemical shifts with each other. The methylene and phenyl C(1) signals of polystyrene were assigned on the basis of the oligomer. Polystyrenes prepared with benzoyl peroxide, n-butyllithium, and t r i f l u o roboron etherate had a random d i s t r i b u t i o n of r and m dyads with P values of 0.54, 0.56, and 0.45, respectively. r

The 13C NMR s p e c t r u m o f p o l y s t y r e n e was f i r s t r e p o r t e d by Bovey e t a l . i n 1970 ( 1 ) . The m e t h y l e n e and p h e n y l C ( l ) c a r b o n r e s o n a n c e s d i s p l a y s p l i t t i n g s w h i c h r e f l e c t t h e t a c t i c i t y d i s t r i b u t i o n and have been a s s i g n e d t o c o n f i g u r a t i o n a l s e q u e n c e s by many a u t h o r s ( 2 - 5 ) . However, t h e a s s i g n m e n t s d i f f e r e d f r o m e a c h o t h e r and d i f f e r e n t v a l u e s , r a n g i n g f r o m 0.45 t o 0.72, were r e p o r t e d f o r t h e probability o f a racemic dyad (P ) i n radically prepared polystyrene. These d i f f e r e n c e s may a r i s e f r o m t h e f a c t t h a t t h e o n l y p o l y s t y r e n e w i t h a known s t r u c t u r e i s i s o t a c t i c p o l y s t y r e n e prepared w i t h a Z i e g l e r - N a t t a c a t a l y s t . U s i n g t h i s p o l y m e r one c a n o n l y a s s i g n r e s o n a n c e s due t o t h e mmm t e t r a d and t h e mmmm p e n t a d , w h i l e o t h e r s i g n a l s a r e a r b i t r a r i l y a s s i g n e d by a s s u m i n g a Bernoullian o r other s t a t i s t i c a l d i s t r i b u t i o n . I n o r d e r t o a s s i g n r e s o n a n c e s i n t h e 1H and 13C NMR s p e c t r a o f p o l y s t y r e n e , s t y r e n e o l i g o m e r s h a v i n g m e t h y l end g r o u p s have been p r e p a r e d and s e p a r a t e d i n t o diastereomers up t o t e t r a m e r r

0097 6156/ 84/0247 0181 $06.00/0 © 1984 American Chemical Society

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NMR AND MACROMOLECULES

( 6 - 9 ) . However, t h e s e o l i g o m e r s f a i l e d t o p r o v i d e u s e f u l i n f o r m a t i o n c o n c e r n i n g t h e s p e c t r a l a n a l y s i s o f p o l y s t y r e n e , because t h e m e t h y l end g r o u p s r e s u l t e d i n s p e c i a l c h e m i c a l s h i f t s f o r t h e 1H and 13C atoms l o c a t e d n e a r b y . A p p a r e n t l y , t h e s e o l i g o m e r s had d i f f e r e n t c o n f o r m a t i o n a l d i s t r i b u t i o n s than the polymer. I n t h i s c h a p t e r 1H and 13C NMR s p e c t r a o f p o l y s t y r e n e a r e a n a l y s e d u t i l i z i n g s t y r e n e o l i g o m e r s h a v i n g p r o p y l end g r o u p s . P r e p a r a t i o n and S e p a r a t i o n o f S t y r e n e

Oligomers

Styrene oligomers h a v i n g p r o p y l end g r o u p s were p r e p a r e d by i n i t i a t i n g o l i g o m e r i z a t i o n w i t h e t h y l l i t h i u m and t e r m i n a t i n g w i t h 1-bromopropane i n e t h y l e t h e r : CH CH L i + CH =CH * Ph 3

1

CH CH CH Br 3

2

2

(1)

The a v e r a g e d e g r e e s o f o l i g o m e r i z a t i o n were c o n t r o l l e d by v a r y i n g t h e m o l a r r a t i o o f s t y r e n e and t h e i n i t i a t o r . This anionic o l i g o m e r i z a t i o n method i s much e a s i e r t h a n t h e G r i g n a r d and o t h e r c o u p l i n g methods u s e d f o r t h e p r e p a r a t i o n o f o l i g o m e r s w i t h m e t h y l end g r o u p s , a l t h o u g h t h i s p r o d u c t i s a m i x t u r e o f o l i g o m e r s h a v i n g d i f f e r e n t degrees o f o l i g o m e r i z a t i o n . I n o r d e r t o s e p a r a t e t h e o l i g o m e r m i x t u r e , we examined t h e e l u t i o n b e h a v i o r o f t h e o l i g o m e r u s i n g a s t y r e n e - d i v i n y l benzene c o p o l y m e r g e l as a s t a t i o n a r y p h a s e and s e v e r a l e l u e n t s h a v i n g d i f f e r e n t p o l a r i t y a s shown i n F i g u r e 1. E l u e n t s h a v i n g t h e same p o l a r i t y as t h e g e l ( s o l u b i l i t y p a r a m e t e r ( S P ) = c a . 9 . 1 ) , s u c h a s c h l o r o f o r m and t e t r a h y d r o f u r a n c a u s e d t h e o l i g o m e r t o s e p a r a t e e a s i l y from h i g h e r m o l e c u l a r weight polymer. The o l i g o m e r s e l u t e d much e a r l i e r when t h e s e s o l v e n t s were u s e d t h a n when o t h e r t y p e s o f e l u e n t s were u s e d . This i n d i c a t e s that the oligomers are s e p a r a t e d by GPC by a s i z e e x c l u s i o n mechanism. N o n p o l a r e l u e n t s ( 2 , 2 , 4 - t r i m e t h y l p e n t a n e and i s o p r o p y l e t h e r ) and p o l a r e l u e n t s ( m e t h a n o l and a c e t o n i t r i l e ) e l u t e d t h e o l i g o m e r s more s l o w l y t h a n c h l o r o f o r m by a f a c t o r o f two. I t i s c l e a r t h a t t h e s e e l u e n t s c a u s e s e p a r a t i o n t o be c o n t r o l l e d by an a d s o r p t i o n mechanism w i t h a n e g l i g i b l e s i z e e x c l u s i o n e f f e c t . Using these e l u e n t s , we o b s e r v e d peak s e p a r a t i o n o r b r o a d e n i n g due t o t h e d i a s t e r e o m e r s f o r t h e d i m e r and h i g h e r o l i g o m e r s . U s i n g c y c l o h e x a n e and a c e t o n e , w h i c h have s i m i l a r SP v a l u e s t o t h a t o f t h e g e l , we s e p a r a t e d t h e o l i g o m e r s i n t o two g r o u p s ; one a t an e l u t i o n volume a r o u n d 20 ml due t o h i g h e r m o l e c u l a r w e i g h t o l i g o m e r s and t h e o t h e r a r o u n d 30 - 40 ml due t o l o w e r molecular weight ones. This discontinuous molecular weighte l u t i o n volume r e l a t i o n c a n be e x p l a i n e d by a h y b r i d mechanism o f s i z e e x c l u s i o n and a d s o r p t i o n e f f e c t s . Larger oligomers cannot p e r m e a t e i n t o t h e g e l and c o n s e q u e n t l y e l u t e e a r l i e r , e x p e r i e n c i n g

12.

SATO A N D TAN A K A

NMR Spectra of Styrene Oligomers and Polymers

183

o n l y a s l i g h t a d o r p t i o n e f f e c t , w h i l e s m a l l e r o l i g o m e r s permeate into t h e g e l and s u f f e r b o t h s i z e e x c l u s i o n and a d s o r p t i o n effects. They e l u t e a t a l m o s t t h e same e l u t i o n volume r e g a r d l e s s of t h e m o l e c u l a r weight. The o l i g o m e r m i x t u r e p r e p a r e d a c c o r d i n g t o E q u a t i o n ( 1 ) was f i r s t s e p a r a t e d i n t o p u r e η-mers by G P C u s i n g c h l o r o f o r m as an eluent. The p u r e 2 - 5 mers were s e p a r a t e d b y a d s o r p t i o n chroma tography u s i n g a r e c y c l i n g technique. D i i s o p r o p y l e t h e r was u s e d as an e l u e n t , b e c a u s e t h i s e l u e n t p r o v i d e d a column w i t h a h i g h e r number o f t h e o r e t i c a l p l a t e s t h a n d i d t r i m e t h y l p e n t a n e , a c e t o n i t r i l e , and m e t h a n o l . The d i m e r s and t r i m e r s were s e p a r a t e d i n t o 2 and 3 f r a c t i o n s , r e s p e c t i v e l y , a f t e r 12 r e c y c l e s ( F i g . 2 a and b) . The t e t r a m e r , w h i c h h a s 6 d i a s t e r e o m e r s , was s e p a r a t e d a c c o r d i n g l y i n t o 6 f r a c t i o n s a f t e r 24 r e c y c l e s , a l t h o u g h t h e s e p a r a t i o n o f t h e second and t h e t h i r d f r a c t i o n s was n o t s o good ( F i g . 2 c ) . The s e p a r a t i o n o f t h e p e n t a m e r was c a r r i e d o u t i n two stages o f f r a c t i o n a t i o n . I n t h e f i r s t s t a g e , t h e f i r s t and t h e n i n t h f r a c t i o n s were c o l l e c t e d and t h e r e m a i n i n g p o r t i o n was s e p a r a t e d i n t o 3 p a r t s l a b e l l e d A , B , and C. The t h r e e p a r t s were s u b s e q u e n t l y s e p a r a t e d i n t o f r a c t i o n s 2 and 3, f r a c t i o n s 4, 5, and 6, and f r a c t i o n s 7 and 8, r e s p e c t i v e l y , d u r i n g t h e second s t a g e o f fractionation (Fig. 2 d ) . NMR S p e c t r a o f t h e D i m e r and T r i m e r The d i a s t e r e o m e r s o f t h e d i m e r were i d e n t i f i e d t h r o u g h t h e s p l i t t i n g p a t t e r n o b s e r v e d f o r t h e m e t h y l e n e p r o t o n s f l a n k e d b y two methine protons; t h e methylene protons o f t h e r dyad a r e e q u i v a l e n t , w h i l e t h o s e o f t h e m dyad a r e n o n e q u i v a l e n t . Ph H a H CH CH CH -Ç - Ç - Ç-CH.CH CH H H a Phi ^ r isomer 0

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F i g u r e 3 shows m e t h i n e and m e t h y l e n e H - l NMR s p e c t r a o f t h e 2 d i m e r f r a c t i o n s w h i c h g a v e m e t h y l e n e p r o t o n r e s o n a n c e s a r o u n d 1.8 ppm. F r a c t i o n 2-1 showed a n AA'X m e t h y l e n e r e s o n a n c e p a t t e r n , w h i l e f r a c t i o n 2-2 showed a n ABX s y s t e m . The m e t h y l e n e p r o t o n s i g n a l s d e c o u p l e d f r o m t h e m e t h i n e p r o t o n d i s p l a y more c l e a r l y t h e d i f f e r e n c e between t h e s p l i t t i n g p a t t e r n s ; F r . 2-1 e x h i b i t e d AA* s p l i t t i n g s , w h i l e F r . 2-2 e x h i b i t e d a n AB q u a r t e t . Therefore, f r a c t i o n s 2-1 and 2-2 were a s s i g n e d t o t h e r and m i s o m e r s , respectively. S i m i l a r l y , t h e t r i m e r f r a c t i o n s , F r . 3-1, 3-2, and 3-3, were i d e n t i f i e d a s rr, rm, and mm i s o m e r s b y t h e s p l i t t i n g p a t t e r n o f t h e i r m e t h y l e n e p r o t o n s i g n a l s a s shown i n F i g u r e 4. The m e t h i n e p r o t o n s o f t h e d i m e r and t h e t r i m e r r e s o n a t e d i n two c h e m i c a l s h i f t r a n g e s . One o c c u r r e d between 2.5-2.2 ppm, w h i c h i s a s s i g n e d t o t h e m end and t h e mm c e n t e r and t h e o t h e r b e t w e e n 2.2-2.0 ppm, w h i c h i s a s s i g n e d t o t h e r end and t h e ( r r + rm) c e n t e r . I t i s n o t e w o r t h y t h a t t h e c e n t r a l m e t h y l e n e p r o t o n o f

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t h e rm i s o m e r r e s o n a t e d a t t h e same f r e q u e n c y a s t h a t o f t h e rr i s o m e r and h i g h e r t h a n t h a t o f t h e mm i s o m e r . This finding i s c o n s i s t e n t w i t h t h e r e s u l t o f S h e p h e r d e t a l . ( 1 0 ) , who r e p o r t e d t h a t t h e m e t h i n e p r o t o n r e s o n a n c e o f p o l y s t y r e n e i s s p l i t i n t o two p e a k s , t h e one a t t h e l o w e r m a g n e t i c f i e l d i s a s s i g n e d t o t h e mm t r i a d , w h i l e t h e o t h e r i s a s s i g n e d t o t h e rm and rr t r i a d s . However, i n t h e c a s e o f t h e s t y r e n e t r i m e r w i t h m e t h y l end g r o u p s , t h e c e n t r a l m e t h i n e p r o t o n s o f mm, rm, and rr i s o m e r s were o b s e r v e d a t c h e m i c a l s h i f t s o f 2.5, 2.3, and 2.1 ppm, r e s p e c t i v e l y (8). T h i s d i s c r e p a n c y may be e x p l a i n e d by t h e s m a l l end g r o u p s , w h i c h may p r o v i d e t h e o l i g o m e r a s p e c i a l c o n f o r m a t i o n o r r e n d e r a s p e c i f i c chemical s h i f t t o the proton. The s t y r e n e d i m e r w i t h p r o p y l end g r o u p s h a s t h e end m e t h y l ( A ) , end m e t h y l e n e (Β) , e x t e r n a l m e t h y l e n e (C) , end m e t h i n e (D) and i n n e r m e t h y l e n e c a r b o n s ( E ) i n t h e main c h a i n and t h e end p h e n y l group ( i ) as t h e s i d e c h a i n . The t r i m e r h a s one new t y p e o f c a r b o n atom, i . e . , i n n e r m e t h i n e (D') , and t h e i n n e r p h e n y l g r o u p ( i i ) a l o n g w i t h t h e c a r b o n s and t h e p h e n y l g r o u p c o n t a i n e d i n the dimer. Furthermore, the tetramer possesses a c e n t r a l m e t h y l e n e c a r b o n ( F ) , and t h e ρentamer p o s s e s s e s a c e n t r a l m e t h i n e ( D ) and a c e n t r a l p h e n y l g r o u p ( i i i ) . M

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C a r b o n s A, B, C, D + D , and Ε were a s s i g n e d by s e l e c t i v e d e c o u p l i n g measurements. F o r example, t h e rm i s o m e r o f t h e t r i m e r showed 7 s i g n a l s i n t h e r e g i o n o f 46 - 38 ppm ( F i g . 5 a) . By d e c o u p l i n g t h e p r o t o n s on c a r b o n C a t 1.45 ppm s i g n a l s 6 and 7 a p p e a r e d a s s h a r p p e a k s ( F i g . 5 b) . T h e r e f o r e , t h e two s i g n a l s were a s s i g n e d t o t h e c a r b o n C. A f t e r comparison w i t h t h e o t h e r i s o m e r s , s i g n a l 6 was a s s i g n e d t o c a r b o n C on t h e r s i d e and signal 7 to the m side. D e c o u p l i n g t h e p r o t o n s on c a r b o n s Ε i n t h e ? and m s i d e s a t 1.75 and 1.92 ppm, r e s p e c t i v e l y , l e d t o s h a r p l i n e s f o r r e s o n a n c e s 1 and 4 ( F i g . 5 c and d) , s h o w i n g t h a t t h e s e s i g n a l s a r e due t o c a r b o n s Ε o f t h e r and m s i d e s . Upon d e c o u p l i n g t h e m e t h i n e p r o t o n s , we were a b l e t o a s s i g n r e s o n a n c e s

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NMR Spectra of Styrene Oligomers and Polymers

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2, 3, and 5 t o t h e m e t h i n e c a r b o n s D and D . R e s o n a n c e s 2 and 3 were a s s i g n e d t o c a r b o n D and r e s o n a c e 5 t o D by c o m p a r i s o n w i t h t h e o t h e r i s o m e r s and t h e d i m e r . I n t h e d i m e r and t r i m e r , c a r b o n s A and Β r e s o n a t e d a t 14 a n d 20 ppm, r e s p e c t i v e l y , r e g a r d l e s s o f t h e m o l e c u l a r w e i g h t and configuration. C a r b o n C r e s o n a t e d around 40 ppm f o r t h e r s i d e and 38 ppm f o r t h e m s i d e . C a r b o n s D and D r e s o n a t e d a t 4 3 ppm and 41 ppm r e g a r d l e s s o f c o n f i g u r a t i o n . Carbon Ε r e s o n a t e d i n a wide chemical shift range o f 46 t o 40 ppm, r e f l e c t i n g a s e n s i t i v i t y t o c o n f i g u r a t i o n a l sequences. E a c h i s o m e r o f t h e t r i m e r h a s two o r t h r e e t y p e s o f p h e n y l C ( l ) carbons. The p h e n y l C ( l ) r e s o n a c e s o f t h e rr and mm i s o m e r s were a s s i g n e d by t h e s i g n a l i n t e n s i t y r a t i o . Those o f t h e rm Isomer were a s s i g n e d t h r o u g h c o m p a r i s o n o f t h e s e c h e m c i a l s h i f t s with other isomers. The p h e n y l C ( l ) c a r b o n o f t h e end u n i t r e s o n a t e d f r o m l o w e r m a g n e t i c f i e l d ; mm < mr < rr < rm. The i n n e r u n i t gave an o r d e r ; ™ < rvrç < rr. This order also held f o r the p h e n y l C ( l ) c a r b o n o f t h e t e t r a m e r and t h e p e n t a m e r . 1

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The d i a s t e r e o m e r s o f t h e t e t r a m e r and t h e pentamer a r e d i f f i c u l t to i d e n t i f y u s i n g o n l y t h e s p l i t t i n g p a t t e r n o f t h e methylene protons. S e v e r a l m e t h y l e n e p r o t o n r e s o n a n c e s o v e r l a p around 1.8 ppm, and t h e m e t h y l e n e p r o t o n s a d j a c e n t t o t h e m dyad ( e g . t h e rm p r o t o n i n t h e rm i s o m e r o f t h e t r i m e r ) d i s p l a y an AB t y p e s p l i t t i n g p a t t e r n i n t h e d i m e r and t h e t r i m e r . Therefore, the i s o m e r s o f t h e t e t r a m e r and t h e p e n t a m e r were i d e n t i f i e d by u s i n g t h e f o l l o w i n g i n f o r m a t i o n c o n c e r n i n g t h e d i m e r and t r i m e r : 1) t h e e l u t i o n o r d e r o f l i q u i d c h r o m a t o g r a p h y : t h e more r d y a d s an i s o m e r h a s , t h e e a r l i e r i t e l u t e s ( e l u t i n g o r d e r was r <m f o r t h e d i m e r and rr
NMR A N D MACROMOLECULES

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13C NMR S p e c t r a o f t h e T e t r a m e r and P e n t a m e r The a l i p h a t i c c a r b o n s i g n a l s o f t h e t e t r a m e r and t h e pentamer were a s s i g n e d b y s e l e c i v e d e c o u p l i n g measurements t o c a r b o n s A, B, C, D, Ε + F, D + D". I n t h e t e t r a m e r a n d p e n t a m e r r e s o n a n c e s f o r c a r b o n s A» B, C, D and D had t h e same c h e m i c a l s h i f t s a s o b s e r v e d f o r t h e d i m e r and t r i m e r . The r e s o n a n c e s f o r c a r b o n D" o c c u r r e d a t 41 ppm, w h i c h i s t h e same a s c a r b o n D , r e g a r d l e s s o f c o n f i g u ration. D e p e n d i n g on t h e c o n f i g u r a t i o n a l s e q u e n c e s , c a r b o n F r e s o n a t e d o v e r a w i d e c h e m i c a l s h i f t r a n g e o f 4 7 t o 41 ppm, w h i c h i s almost equal t o t h e c h e m i c a l s h i f t range observed f o r carbon E. The r e s o n a n c e s f r o m c a r b o n s Ε and F were a s s i g n e d b y t h e s i g n a l i n t e n s i t y r a t i o i n t h e case o f symmetric d i a s t e r e o m e r s o f t h e t e t r a m e r , i . e . , rrr, rmr, mrm, and mmm i s o m e r s . I n t h e case o f the o t h e r i s o m e r s , a s s i g n m e n t s were made b y c o m p a r i s o n o f t h e observed chemical s h i f t s t o those o f t h e corresponding carbons i n the d i m e r and t r i m e r a s shown i n F i g u r e 6. Good c o r r e l a t i o n s a r e o b s e r v e d among t h e c h e m i c a l s h i f t s o f t h e c a r b o n s c o r r e s p o n d i n g t o Ε and F, w h i c h i n d i c a t e s t h e v a l i d i t y o f t h e i d e n t i f i c a t i o n o f t h e diastereomers. The p h e n y l C ( l ) s i g n a l s o f t h e e n d , t h e i n n e r and t h e c e n t r a l u n i t s were a s s i g n e d b y c o m p a r i s o n o f t h e c h e m i c a l s h i f t s o f t h e c o r r e s p o n d i n g c a r b o n s o f t h e i s o m e r s a s shown i n F i g u r e 7. The p h e n y l C ( l ) c a r b o n s i g n a l o f t h e end u n i t a p p e a r e d f r o m l o w e r magnetic f i e l d ; mm< mr < jrr < rm. The r e s o n a t i n g o r d e r o f t h e p h e n y l C ( l ) s i g n a l o f t h e i n n e r u n i t was mm
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190

NMR AND MACROMOLECULES

o f t h e mm t r i a d h a s been a s s i g n e d t o t h e l o w e s t m a g n e t i c f i e l d b y comparison w i t h t h e spectrum o f i s o t a c t i c p o l y s t y r e n e , i t i s p e c u l i a r t h a t t h e mmm i s o m e r o f t h e m e t h y l end o l i g o m e r gave an i n n e r phenyl C ( l ) resonance i n t h e middle o f those f o r the o t h e r isomers. T h e r e f o r e , t h e m e t h y l end o l i g o m e r c a n n o t be a good model compound f o r a n a l y z i n g t h e phenyl C ( l ) resonances o f polystyrene. 13C NMR S i g n a l A s s i g n m e n t o f P o l y s t y r e n e As shown i n F i g u r e 6 t h e c e n t r a l m e t h y l e n e c a r b o n o f t h e t e t r a m e r and t h e p e n t a m e r r e s o n a t e d f r o m l o w e r m a g n e t i c f i e l d ; rmr < rrr < rmm < rrm + mmm < mrm. I n the pentamer the c e n t r a l methylene carbon i n t h e mmr and mmm t e t r a d s r e s o n a t e d o v e r a s m a l l c h e m i c a l s h i f t r a n g e , w h i l e t h a t i n t h e rmr, rrr, rrm, and mrm t e t r a d s r e s o n a t e d o v e r a w i d e c h e m i c a l s h i f t r a n g e . T h e r e f o r e , i t i s presumed t h a t the m e t h y l e n e c a r b o n o f p o l y s t y r e n e r e s o n a t e d f r o m l o w e r m a g n e t i c f i e l d ; rmr < rrr < rmm < rmm + mmm < mrm and t h a t t h e s i g n a l s due t o the rmr, rrr, rrm, and mrm t e t r a d s show s p l i t t i n g s due t o t h e hexad c o n f i g u r a t i on. F i g u r e 8 shows 13C NMR s p e c t r a o f p o l y s t y r e n e p r e p a r e d w i t h b e n z o y l p e r o x i d e c a t a l y s t and measured i n c h l o r o f o r m - d a t room t e m p e r a t u r e ( a ) and i n 1 , 2 - d i c h l o r o b e n z e n e a t 150° C (b) . The s p e c t r u m measured a t room t e m p e r a t u r e was a s s i g n e d a s shown i n t h e f i g u r e on t h e b a s i s o f t h e r e s u l t s f r o m t h e o l i g o m e r . The s p e c t r u m measured a t 150°C showed w e l l r e s o l v e d s i g n a l s , w h i c h were a l s o a s s i g n e d on t h e b a s i s o f r e s u l t s f r o m t h e o l i g o mer. S i g n a l s 1, 2, and 3 were a t t r i b u t e d t o t h e rmr t e t r a d s , w h i c h were a s s i g n e d t o t h e mrmrm, mrmrr, and rrmrr f r o m t h e l o w e r m a g n e t i c f i e l d , b e c a u s e i n t h e c a s e o f t h e s t y r e n e pentamer t h e m e t h y l e n e c a r b o n o f t h e rmrm i s o m e r r e s o n a t e d a t l o w e r m a g n e t i c f i e l d t h a n t h a t o f t h e rmrr i s o m e r . S i m i l a r l y , s i g n a l s 4, 5, and 6 were a s s i g n e d t o t h e hexad c o n f i g u r a t i o n s h a v i n g t h e rrr t e t r a d , and s i g n a l s 1 0 , 1 1 , 12 t o t h e hexads h a v i n g t h e mrm t e t r a d . Sign a l 8 and p a r t o f s i g n a l 9 were a l l o t t e d t o t h e rrm t e t r a d and were a s s i g n e d t o t h e hexad s e q u e n c e s c o n s i d e r i n g t h e s i g n a l i n t e n sities. S i g n a l s due t o t h e mmr and mmm t e t r a d s ( s i g n a l s 7 and 9 ) showed no d i s c r e t e s p l i t t i n g due t o hexad c o n f i g u r a t i o n s . This s i g n a l a s s i g n m e n t i s a l m o s t t h e same a s one r e p o r t e d b y Chen e t a l . (11). The i n t e n s i t y o f e a c h s i g n a l o f t h e s p e c t r u m measured a t 150°C was d e t e r m i n e d f o r p o l y s t y r e n e s p r e p a r e d w i t h b e n z o y l p e r o x i d e , n - b u t y l l i t h i u m , and t r i f l u o r o b o r o n e t h e r a t e c a t a l y s t s . The o b s e r v e d r e l a t i v e i n t e n s i t i e s o f t h e s i g n a l s were i n good agreement w i t h t h e c a l c u l a t e d v a l u e s a s s u m i n g B e r n o u l l i a n s t a t i s t i c s w i t h Pr o f 0.54, 0.56, and 0.45 f o r t h e r a d i c a l , a n i o n i c , and c a t i o n i c p o l y s t y r e n e s , r e s p e c t i v e l y ( T a b l e I I ) . The Pr v a l u e o f the r a d i c a l p o l y s t y r e n e was n e a r l y t h e same a s t h a t r e p o r t e d b y Shepherd e t a l . ( 1 0 ) . The p h e n y l C ( l ) c a r b o n r e s o n a n c e o f p o l y s t y r e n e was r e p o r t e d

NMR Spectra of Styrene Oligomers and Polymers

SATO A N D T A N A K A

191

(a) 2 mer /

3 mer -

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rm

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4 mer -

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5 mer -

mrm

S 146.0

(b)

rmr^rmm

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Chemical Shift (ppm)

3 mer -

rmr mrr

4 mer -

rrr rrm

5 mer -

Figure 7. C h e m i c a l s h i f t o f t h e p h e n y l C ( l ) c a r b o n i n t h e end ( a ) and i n n e r u n i t s ( b ) .

46 44 Chemical Shift

42 ( ppm)

40

F i g u r e 8. M e t h y l e n e c a r b o n s p e c t r a of r a d i c a l p o l y s t y r e n e measured i n c h l o r o f o r m - d a t room t e m p e r a t u r e ( a ) and i n 1 , 2 - d i c h l o r o b e n z e n e a t 150 °C ( b ) , Reproduced w i t h p e r m i s s i o n f r o m R e f . 13. C o p y r i g h t 1983, J o h n W i l e y & Sons, I n c .

46.73 46.44 46.22 45.61 45.22 44.95 44.79 44.05

43.59

42.70 42.66 42.55

1 2 3 4 5 6 7 8

9

10 11 12 mmrmr mmrmm

rmrmr

mmm

rrrmm

rrrmr

mrrmm

mrrmr

mmr

mrrrm rrrrm rrrrr

rrmrr

mrmrm mrmrr

Assignment

2.7 6.7 2.9

23.9

2.6 6.4 4.0 3.6 7.6 4.3 21.5 12.8

3.3 5.7 2.4

24.2

2.8 6.1 3.9 3.4 7.8 4.6 22.9 12.4

BPO o b s d . (Pr=0.54)

2.4 5.3 2.3

24.6

2.4 6.1 4.5 4.0 10.1 5.4 21.5 11.3

3.4 5.3 2.1

24.0

2.7 6.8 4.3 3.4 8.7 5.5 21.7 12.1

2.6 6.5 5.1

24.4

29.4 12.5

3.8 5.5 2.6 2.9 4.7

2.8 5.7 4.1

26.7

29.0 12.2

3.4 5.5 2.3 2.8 4.5

I n t e n s i t y (%) BuLi Et 0 o b s d . (Pr=0.56) o b s d ? (Pr=0.45)

C

Assignment o f Methylene Carbon S i g n a l (Measured i n 1 , 2 - D i c h l o r o b e n z e n e a t 1 5 0 C )

S o u r c e : R e f . 1 3 . U s e d w i t h p e r m i s s i o n o f J o h n W i l e y & S o n s , I n c . C o p y r i g h t 1983.

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Signal

Table I I .

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m

Π

m

ο

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12.

SATO A N D ΤΑ Ν A Κ A

NMR Spectra of Styrene Oligomers and Polymers

193

t o show s p l i t t i n g s due t o p e n t a d c o n f i g u r a t i o n s ( 1 , 2, 3, 5) . T h e r e f o r e , t h e p e n t a m e r i s r e q u i r e d a s t h e s m a l l e s t model compound to analyze these resonances. F i g u r e 9 shows t h e c h e m i c a l s h i f t behavior o f t h e phenyl C ( l ) carbon i n t h e c e n t r a l u n i t o f t h e p e n t a m e r as measured a t room t e m p e r a t u r e i n c h l o r o f o r m - d . The c e n t r a l phenyl C ( l ) carbon s i g n a l appeared i n o r d e r o f i n c r e a s i n g m a g n e t i c f i e l d a s mm
•;

mmmm

mmmr

rmmr

rrrr

mrrm

Ô

Ο

Qrrrm

φ mmrr mmr

m

φ

φ 146

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rrmr

φ φ

mrrnn

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(ppm)

F i g u r e 9. C h e m i c a l s h i f t o f t h e c e n t r a l p h e n y l C ( l ) c a r b o n o f t h e pentamer.

194

F i g u r e 10, P h e n y l C ( 1 ) c a r b o n s p e c t r a of r a d i c a l p o l y s t y r e n e measured i n c h l o r o f o r m - d a t room t e m p e r a t u r e (a) and i n 1 , 2 - d i c h l o r o b e n z e n e a t 150 °C (b). Reproduced w i t h p e r m i s s i o n from R e f . 13. C o p y r i g h t 1983, J o h n W i l e y & Sons, I n c .

NMR AND MACROMOLECULES

I

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NMR

Spectra of Styrene Oligomers and

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196

NMR AND MACROMOLECULES

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

L . F. Johnson, F. Heatley, and F. A. Bovey, Macromolecules, 3, 175 (1970). Y. Inoue, A. Nishioka, R. Chûjo, Makromol. Chem., 156, 207 (1972). K. Matsuzaki, T. Uryu, Κ. Osada, T. Kawamura, Macromole cules, 5, 816 (1972). J . C. Randall, J . Polym. Sci., Polym. Phys. E d . , 13, 889 (1975). S. Sparno, J . Lacoste, S. Raynal, J . F. Regnier, F. Schué, R. Sempere, J . Sledz, Polymer J., 12, 861 (1980). F. A. Bovey, F. P. Hood I I I , E . W. Anderson, L . C. Snyder, J. Chem. Phys., 42, 3900 (1965). D. Doskocilová, B. Schneider, J. Polym. Sci., Part B 3, 213 (1965). D. Lím, M. Kolínský, J . Petránek, D. Doskocilová, B. Schneider, J. Polym. Sci., Part B 4, 645 (1966). B. Jasse, F. Laupretre, L . Monnerie, Makromol. Chem., 178, 1987 (1977). L . Shepherd. T. K. Chen, H. J. Harwood, Polym. Bull., 1, 445 (1979). T. K. Chen, T. A. Gerken, H. J. Harwood, Polym. Bull., 2, 37 (1980). H. Sato, K. Saito, K. Miyashita, Y . Tanaka, Makromol. Chem., 182, 2259 (1981). H. Sato, Y. Tanaka, K. Hatada, J. Polym. Sci., Polym. Phys. Ed., 21 (1983).

RECEIVED November 3, 1983

13 13

75-MHz C NMR Studies on Polystyrene and Epimerized Isotactic Polystyrenes 1

H. JAMES HARWOOD, TENG-KO CHEN, and FU-TYAN LIN

Institute of Polymer Science, The University of Akron, Akron, OH 44325 Isotactic polystyrene was epimerized to various extents by reaction with KOtBu i n hexamethylphosphoramide solution at 100°. The C-NMR spectra of the epimerized samples and of polystyrene i n 9:1 trichlorobenzene:nitrobenzene-d solution at 150° were recorded and analyzed using stereosequence distributions that were calculated for the samples by Monte Carlo simulation of the epimerization process. Triad, pentad and heptad assignments were developed for the aromatic C-1 carbon resonance patterns and t r i a d assignments previously proposed for the methine carbon resonances of polystyrene were v e r i fied. Based on the assignments developed, the C-NMR spectrum of polystyrene indicates it to be almost a t a c t i c , P(m)=0.45. 13

6

13

P o l y s t y r e n e was one o f t h e f i r s t p o l y m e r s t o be s t u d i e d b y h i g h r e s o l u t i o n NMR s p e c t r o s c o p y C O , b u t i t s s p e c t r a a r e i n c o m p l e t e l y u n d e r s t o o d e v e n t o d a y . The H-NMR s p e c t r a o f p o l y s t y r e n e a r e p o o r l y d e f i n e d ; t h e methine and ortho aromatic p r o t o n s p e c t r a a r e i n f l u e n c e d s i g n i f i c a n t l y by p e n t a d o r l a r g e r s t e r e o s e q u e n c e e f f e c t s but they c o n t a i n o n l y three o r f o u r b r o a d , b a d l y overlapped r e s o n ances. I n c o n t r a s t , t h e C-NMR s p e c t r a o f p o l y s t y r e n e a r e r i c h i n d e t a i l , b e i n g s e n s i t i v e t o hexad and h e p t a d s t e r e o s e q u e n c e effects. S i n c e t h e r e a r e 20 d i s t i n g u i s h a b l e h e x a d c o m b i n a t i o n s and 36 d i s t i n g u i s h a b l e h e p t a d c o m b i n a t i o n s , i t h a s been e x t r e m e l y d i f f i c u l t t o a s s i g n t h e v a r i o u s r e s o n a n c e s o b s e r v e d i n t h e C-NMR spectrum o f p o l y s t y r e n e . The d i f f i c u l t y h a s b e e n compounded b y the f a c t t h a t f r e e r a d i c a l i n i t i a t e d p o l y m e r i z a t i o n s o f s t y r e n e y i e l d p o l y m e r h a v i n g t h e same m i c r o s t r u c t u r e , r e g a r d l e s s o f p o l y m e r i z a t i o n t e m p e r a t u r e . Many i o n i c p o l y m e r i z a t i o n s o f s t y r e n e a l s o y i e l d p o l y m e r s t h a t have t h e same m i c r o s t r u c t u r e a s t h o s e p r e p a r e d 1

13

13

7

Current address: Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15261. 0097 6156/84/0247 0197$07.50/0 © 1984 American Chemical Society

198

NMR

AND

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by f r e e r a d i c a l p o l y m e r i z a t i o n . Thus, i t has not been p o s s i b l e t o p r e p a r e , by d i r e c t p o l y m e r i z a t i o n , a s e r i e s o f p o l y s t y r e n e s a m p l e s h a v i n g m i c r o s t r u c t u r e s t h a t v a r i e d i n some r a t i o n a l way. Studies on t h e s p e c t r a o f s u c h s a m p l e s , i n w h i c h t h e i n t e n s i t i e s o f t h e v a r i o u s s i g n a l s w o u l d v a r y as t h e p o l y m e r s t r u c t u r e c h a n g e d , w o u l d have been v e r y u s e f u l f o r making resonance a s s i g n m e n t s . Cationic and some a n i o n i c p o l y m e r i z a t i o n s o f s t y r e n e h a v e y i e l d e d p o l y m e r s t h a t have s p e c t r a t h a t d i f f e r from the spectrum of f r e e r a d i c a l p o l y m e r i z e d p o l y s t y r e n e ( 2 - _ 3 ) . However, t h e u s e f u l n e s s o f s u c h s a m p l e s was dampened by t h e p o s s i b i l i t y t h a t s i d e r e a c t i o n s accomp a n i e d t h e c a t i o n i c p o l y m e r i z a t i o n s and by t h e p o s s i b i l i t y t h a t m u l t i - s t a t e p r o p a g a t i o n mechanisms w e r e i n v o l v e d i n some o f t h e anionic polymerization reactions. Our r e c e n t u n d e r s t a n d i n g o f t h e NMR s p e c t r a and s t r u c t u r e o f p o l y s t y r e n e i s b a s e d on t h e r e s u l t s o f model compound s t u d i e s ( 4 - 1 1 ) , t h e o r e t i c a l c a l c u l a t i o n s ( 1 2 - 1 4 ) , s t u d i e s on p o l y s t y r e n e d e r i v a t i v e s (15) and a n a l o g u e s ( 1 6 ^ ) , s t u d i e s on t h e NMR s p e c t r a o f e p i m e r i z e d i s o t a c t i c p o l y s t y r e n e s a m p l e s ( 1 7 - 1 9 ) and on r e s u l t s o b t a i n e d u s i n g other approaches(20-22). The m o d e l compound a p p r o a c h , e x e m p l i f i e d by t h e e x c e l l e n t p a p e r c o n t a i n e d i n t h i s v o l u m e ( 1 1 ) , i n v o l v e d i s o l a t i n g p o l y s t y r e n e o l i g o m e r s o f known c o n f i g u r a t i o n and r e c o r d i n g t h e i r NMR s p e c t r a . C h e m i c a l s h i f t a s s i g n m e n t s made f o r t h e o l i g o mers a r e t h e n u s e d i n m a k i n g a s s i g n m e n t s f o r p o l y s t y r e n e . This a p p r o a c h r e q u i r e s t h a t a l a r g e number o f d i f f e r e n t o l i g o m e r s be s e p a r a t e d and s t u d i e d i f a s s i g n m e n t s a r e t o be made a t p e n t a d o r h i g h e r l e v e l s and c a r e must be t a k e n t o d e s i g n t h e o l i g o m e r s so t h a t t h e y a d o p t c o n f o r m a t i o n s s i m i l a r t o t h o s e a d o p t e d by t h e p o l y m e r . T h i s t e d i o u s a p p r o a c h has b e e n v e r y f r u i t f u l and rewarding(11). E p i m e r i z a t i o n of s t e r e o r e g u l a r polymers(17-19,23-33) p r o v i d e s a way t o o b t a i n p o l y m e r s h a v i n g c o n t r o l l e d s e q u e n c e d i s t r i b u t i o n s . The p r o d u c t s o f p o l y m e r e p i m e r i z a t i o n r e a c t i o n s can be u s e f u l f o r e s t a b l i s h i n g NMR a s s i g n m e n t s b e c a u s e t h e i r s t r u c t u r e s can be c a l c u l a t e d by s i m u l a t i n g t h e e p i m e r i z a t i o n p r o c e s s . They c a n t h u s serve as models f o r s t r u c t u r e assignment purposes. A l t h o u g h f r e e r a d i c a l c h e m i s t r y h a s b e e n u s e d f o r some p o l y m e r e p i m e r i z a t i o n experiments(30), s t r o n g bases are g e n e r a l l y used to epimerize p o l ymers. S h e p h e r d ( 3 4 ) , made a m a j o r c o n t r i b u t i o n t o t h e s t u d y o f p o l y s t y r e n e s t r u c t u r e by s h o w i n g t h a t i s o t a c t i c p o l y s t y r e n e c o u l d be e p i m e r i z e d by p o t a s s i u m t - b u t o x i d e i n h e x a m e t h y l p h o s p h o r a m i d e s o l u t i o n at m i l d temperatures. T h i s p r o c e s s c a n be e n v i s i o n e d a s shown by t h e e q u a t i o n s g i v e n on t h e f o l l o w i n g page. These e q u a t i o n s show how a mmmm s e q u e n c e i n p o l y s t y r e n e c a n be c o n v e r t e d i n t o a mrrm s e q u e n c e by a s i m p l e e p i m e r i z a t i o n event. Should the c o n f i g u r a t i o n of the f o u r t h carbon from the l e f t i n t h e l a s t s t r u c t u r e a l s o be a l t e r e d , a rmrm p e n t a d w o u l d result. T h u s , by a s e r i e s o f e p i m e r i z a t i o n s t e p s i t i s p o s s i b l e t o change i s o t a c t i c p o l y s t y r e n e g r a d u a l l y i n t o a p o l y m e r t h a t e x h i b i t s t h e same NMR s p e c t r a a s t h e p o l y s t y r e n e t h a t was p r e p a r e d by f r e e r a d i c a l p o l y m e r i z a t i o n ( 1 7 - 1 9 ) . A Monte C a r l o p r o g r a m h a s

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b e e n w r i t t e n t o s i m u l a t e t h i s e p i m e r i z a t i o n p r o c e s s and t o c a l c u l a t e the r e l a t i v e concentrations o f stereosequences present i n the p o l y m e r s a f t e r v a r i o u s e x t e n t s o f epimerization(17)· By c o r r e l a t i n g t h e resonance p a t t e r n s o f e p i m e r i z e d p o l y s t y r e n e s t h a t were prepared by Shepherd s procedure w i t h stereosequence d i s t r i b u t i o n s c a l c u l a t e d f o r t h e p o l y m e r s u s i n g t h e Monte C a r l o m e t h o d , a s s i g n ments have b e e n d e v e l o p e d f o r t h e m e t h i n e p r o t o n ( 1 _ 7 ) , m e t h i n e c a r bon (18) , m e t h y l e n e c a r b o n ( 1 8 ) a n d a r o m a t i c C - l c a r b o n r e s o n a n c e s of p o l y s t y r e n e ( 1 9 ) . These a s s i g n m e n t s agree v e r y w e l l w i t h a s s i g n ments d e v e l o p e d i n t h e m o d e l compound s t u d i e s , a n d t h e y l e a d t o the c o n c l u s i o n t h a t p o l y s t y r e n e , a s prepared by f r e e r a d i c a l polym e r i z a t i o n methods, h a s a B e r n o u l l i a n s t e r e o s e q u e n c e d i s t r i b u t i o n w i t h a meso c o n t e n t , P(m) o f a b o u t 0.46. I n o t h e r w o r d s , t h e polymer i s almost a t a c t i c . Our p r e v i o u s r e p o r t s c o n c e r n e d 20 MHz C-NMR s p e c t r a o f p a r t i a l l y epimerized polystyrenes. The a r o m a t i c C - l c a r b o n r e s o n a n c e s w e r e r e c o r d e d a t room t e m p e r a t u r e a n d w e r e p o o r l y d e f i n e d when t h e e x t e n t o f e p i m e r i z a t i o n was h i g h . A need f o r r e m e a s u r i n g these resonances a t higher temperature, using higher f i e l d spectro m e t e r s was c l e a r l y e v i d e n t . This paper i s t h e r e f o r e concerned w i t h t h e 75 MHz C-NMR s p e c r r a o f p a r t i a l l y e p i m e r i z e d i s o t a c t i c polystyrenes. 1

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Experimental Isotactic Polystyrene. I s o t a c t i c p o l y s t y r e n e was p r e p a r e d by h e a t i n g a 1 0 % s o l u t i o n o f s t y r e n e i n b e n z e n e a t 60° f o r 24 h o u r s i n t h e p r e s e n c e o f a c a t a l y s t p r e p a r e d i n t h e p r e s e n c e o f monomer f r o m e q u i m o l a r amounts o f ( h e p t a n e s o l u t i o n ) a n d T1CI3-AA (heptane suspension). The r e a c t i o n m i x t u r e was d i l u t e d w i t h b e n zene a n d p o u r e d i n t o a n e x c e s s o f i s o p r o p a n o l . The r e s u l t i n g p r e c i p i t a t e was d i s s o l v e d i n warm m e t h y l e n e c h l o r i d e . The s o l u t i o n was f i l t e r e d a n d added t o h o t m e t h y l e t h y l k e t o n e . The s o l u t i o n was c o n c e n t r a t e d a n d c o o l e d t o o b t a i n t h e i s o t a c t i c p o l y m e r i n t h e f o r m o f a w h i t e powder t h a t was washed w i t h m e t h y l e t h y l k e t o n e and d r i e d u n d e r vacuum. F i n a l p u r i f i c a t i o n was a c h i e v e d b y r e p r e c i p i t a t i o n o f the polymer from benzene s o l u t i o n i n t o methanol, f o l l o w e d b y d r y i n g a t 60° u n d e r vacuum.

(C2H5)3AI

E p i m e r i z a t i o n Procedure. P o t a s s i u m t - b u t o x i d e t h a t had been heated u n d e r vacuum a t 130°C f o r t h r e e h o u r s a n d h e x a m e t h y I p h o s p h o r a m i d e t h a t h a d been d r i e d o v e r C a H a t 60° f o r 24 h o u r s a n d t h e n d i s t i l l e d from L i A l H i * under reduced p r e s s u r e were used. S o l u t i o n s cont a i n i n g 10g. i s o t a c t i c p o l y s t y r e n e a n d 56g. (0.5 m o l e ) KOtBu p e r l i t e r o f hexamethyIphosphoramide ( C a u t i o n - suspected carcinogen) w e r e p r e p a r e d u n d e r n i t r o g e n a n d were h e a t e d a t 100°C f o r p e r i o d s t h a t r a n g e d f r o m 1 t o 20 h o u r s . The r e a c t i o n m i x t u r e s w e r e i n i t i a l l y p i n k and u l t i m a t e l y developed a v i o l e t c o l o r t h a t i n t e n s i f i e d w i t h r e a c t i o n time. The r e a c t i o n m i x t u r e s w e r e p o u r e d i n t o m e t h a n o l t o i s o l a t e t h e p o l y m e r s . These w e r e r e p r e c i p i t a t e d f r o m b e n z e n e s o l u t i o n i n t o m e t h a n o l and were d r i e d u n d e r vacuum a t 40°C. The u s e o f t e m p e r a t u r e s h i g h e r t h a n 100°C c a u s e s e x t e n s i v e m o l e c u l a r w e i g h t r e d u c t i o n . The s a m p l e s employed i n t h i s s t u d y a r e t h e same o n e s u s e d i n o u r p r e v i o u s C-NMR s t u d i e s . NMR M e a s u r e m e n t s . In a l l cases, ten percent s o l u t i o n s of the p o l y s t y r e n e s a m p l e s d i s s o l v e d i n a 9:1 1 , 2 , 4 - t r i c h l o r o b e n z e n e m i t r o b e n z e n e - d m i x t u r e were u s e d f o r t h e NMR s t u d i e s . 75 MHz C-NMR """H-decoupled s p e c t r a o f t h e e p i m e r i z e d p o l y s t y r e n e s a m p l e s w e r e r e c o r d e d a t 150-160° u s i n g a B r u k e r WH-300 NMR S p e c t r o m e t e r . A 70° p u l s e w i d t h , a n a c q u i s i t i o n t i m e o f 0.82 s e c o n d s w i t h a 16K d a t a s i z e , a n d a p u l s e d e l a y o f 0.1 s e c o n d w e r e e m p l o y e d . The number o f t r a n s i e n t s c o l l e c t e d v a r i e d f r o m 3000 t o 10,000 a n d t h e d a t a w e r e p r o c e s s e d w i t h a l i n e b r o a d e n i n g o f 0.8-1.0 Hz. A T s t u d y done on t h e a r o m a t i c C - l a n d a l i p h a t i c c a r b o n r e s o n a n c e s o f p o l y s t y r e n e a t 200°C, u s i n g a V a r i a n XL-400 NMR S p e c t r o m e t e r , r e v e a l e d t h a t w i t h i n e x p e r i m e n t a l e r r o r t h e i n d i v i d u a l components o f t h e s e r e s o n a n c e p a t t e r n s h a d t h e same r e l a x a t i o n t i m e ( 3 5 ) . This i n d i c a t e s t h a t t h e c o n d i t i o n s d e s c r i b e d above a r e a p p r o p r i a t e f o r o b t a i n i n g resonance p a t t e r n s t h a t c o u l d be a n a l y z e d q u a n t i t a t i v e l y . R e s o n a n c e a r e a measurements were made b y c u t t i n g a n d w e i g h i n g s p e c t r a t r a c i n g s . S p e c t r a s i m u l a t i o n s w e r e done u s i n g a Calcomp p l o t t e r w i t h t h e a i d o f a p r o g r a m w r i t t e n by D r . B.L. B r u n e r o f the U n i v e r s i t y o f Kentucky, u s i n g stereosequence d i s t r i b u t i o n s c a l c u l a t e d f o r t h e p o l y m e r s b y Monte C a r l o s i m u l a t i o n s o f t h e e p i m e r i 2

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Polystyrene and Epimerized Isotactic Polystyrenes

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z a t i o n p r o c e s s and u s i n g c h e m i c a l s h i f t s o b t a i n e d by methods t o be d e s c r i b e d l a t e r i n t h i s p a p e r . A l i n e shape t h a t was a c o m b i n a t i o n o f L o r e n t i z i a n and G a u s s i a n f u n c t i o n s was assumed f o r e a c h line. T h i s combined f u n c t i o n h a s worked w e l l i n o u r p r e v i o u s s i m u l a t i o n s t u d i e s . A c o n s t a n t l i n e - w i d t h was u s e d f o r a l l l i n e s w i t h i n a g i v e n spectrum. Monte C a r l o S i m u l a t i o n s . The e p i m e r i z a t i o n r e a c t i o n was s i m u l a t e d u s i n g t h e Monte C a r l o p r o g r a m we d e s c r i b e d e a r l i e r ( 1 7 ) . A 5000 e l e m e n t a r r a y was a l l o c a t e d t o s t o r e i n f o r m a t i o n a b o u t t h e c o n f i g u r a t i o n s o f monomer u n i t s a t v a r i o u s p o s i t i o n s i n a 5000 u n i t p o l y mer c h a i n . The p o s i t i o n s w e r e i n d e x e d i n s u c h a way t h a t t h e p o l y m e r c o u l d be c o n s i d e r e d c y c l i c . T h i s was done t o a v o i d e n d group e f f e c t s . The c o n f i g u r a t i o n s ( R o r S) a t i n d i v i d u a l s i t e s w e r e i n d i c a t e d by 0 o r 1 v a l u e s . The p o l y m e r c h a i n was made i s o t a c t i c b y g i v i n g a l l e l e m e n t s o f t h e a r r a y i n i t i a l v a l u e s o f 0. The c h a i n was t h e n " e p i m e r i z e d " b y c o n d u c t i n g a s e r i e s o f " e p i m e r i z a t i o n e v e n t s , " each o f which i n v o l v e d t h e f o l l o w i n g s t e p s ; 1.

A s i t e o n t h e c h a i n was s e l e c t e d t o e x p e r i e n c e a n e p i m e r i z a t i o n e v e n t b y c a l l i n g a random number t h a t r a n g e d f r o m 1 t o 5000. 2. The s t r u c t u r e o f t h e s i t e s e l e c t e d , i n c l u d i n g s t r u c t u r e s o f n e a r e s t n e i g h b o r s was t h e n d e t e r m i n e d . The p o s s i b l e s t r u c t u r e s w e r e 0 0 0 , 0 0 1 , 1 0 1 , 1 1 0 , 011 a n d 1 1 1 . 3. D e p e n d i n g on t h e s t r u c t u r e f o u n d , a p r o b a b i l i t y ( P ) t h a t t h e s i t e s e l e c t e d would have c o n f i g u r a t i o n 1 a t t h e c o m p l e t i o n o f t h e e p i m e r i z a t i o n e v e n t was e s t a b l i s h e d b y u s e o f T a b l e I . The V v a l u e i n t h i s t a b l e (R i n p r e v i o u s p a p e r s ) h a s a range o f 0 - 1 and i s t h e o n l y parameter r e q u i r e d f o r t h e simulation. 4. A s e c o n d random number h a v i n g a r a n g e o f 0 - 1 was t h e n c a l l e d a n d compared t o P. When t h i s number was l e s s t h a n o r e q u a l t o Ρ, t h e a r r a y e l e m e n t r e p r e s e n t i n g t h e s i t e was g i v e n a v a l u e o f 1. O t h e r w i s e i t was g i v e n a v a l u e o f 0. 5. An e p i m e r i z a t i o n e v e n t c o u n t e r was t h e n i n c r e m e n t e d and compared t o a n o u t p u t s c h e d u l e . I f o u t p u t was d e s i r e d a f t e r t h i s e v e n t , t h e c h a i n ( a r r a y ) was analyzed f o r a l l p o s s i b l e dyad, t r i a d , t e t r a d , p e n t a d , hexad and heptad s t e r e o s e q u e n c e s and t h e r e s u l t s were s t o r e d . E p i m e r i z a t i o n e v e n t s were r e p e a t e d u n t i l t h e c h a i n r e a c h e d equilibrium. The c a l c u l a t i o n was r e p e a t e d t e n t i m e s a n d t h e s t e r e o s e q u e n c e c o n c e n t r a t i o n s e v a l u a t e d a f t e r s p e c i f i e d numbers o f e p i m e r i z a t i o n e v e n t s w e r e t h e n a v e r a g e d , p r i n t e d a n d punched f o r use i n s p e c t r a s i m u l a t i o n s . I n r e l a t i n g Monte C a r l o and e x p e r i m e n t a l r e s u l t s , t h e v a l u e o f t h e e p i m e r i z a t i o n event counter can be used a s a q u a n t i t y t h a t i s

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110

P r o b a b i l i t y (P) V 0.5 1-V 1-V 0.5 V

p r o p o r t i o n a l t o time i n a c t u a l e p i m e r i z a t i o n experiments. In the p r e s e n t s t u d y , h o w e v e r , Monte C a r l o r e s u l t s w e r e c o r r e l a t e d w i t h e x p e r i m e n t a l r e s u l t s by m a t c h i n g mm-contents c a l c u l a t e d b y t h e Monte C a r l o method t o mm-contents e v a l u a t e d f r o m t h e 300 MHz meth ine proton resonances of the polymers. Two d i f f e r e n t V v a l u e s w e r e u s e d f o r t h e Monte C a r l o c a l c u l a tions. A v a l u e o f 0.5 was u s e d t o p r e d i c t t h e e f f e c t o f a com p l e t e l y random e p i m e r i z a t i o n p r o c e s s t h a t y i e l d s a p e r f e c t l y a t a c t i c p o l y m e r when e q u i l i b r a t i o n i s c o m p l e t e . Our p r e v i o u s work employed a V v a l u e o f 0.65 a n d t h i s was a l s o u s e d h e r e . The 0.65 v a l u e , w h i c h y i e l d s a s i i g h t l y s y n d i o t a c t i c p o l y m e r (P(m)=0.43), was s e l e c t e d i n i t i a l l y b e c a u s e i t p r e d i c t s a r r - c o n t e n t f o r t h e completely e q u i l i b r a t e d polymer that agrees w i t h t h e o r e t i c a l pre d i c t i o n s o f F l o r y and W i l l j a m s ( 3 6 ) . R e s u l t s and D i s c u s s i o n A r o m a t i c C - l R e s o n a n c e . F i g u r e s 1-6 show a r o m a t i c C - l c a r b o n r e sonance p a t t e r n s o b s e r v e d f o r v a r i o u s e p i m e r i z e d i s o t a c t i c p o l y s t y r e n e s a m p l e s . The p a t t e r n s a r e e a s i l y d i v i d e d i n t o s i x r e s o n a n c e a r e a s , t h a t a r e d e s i g n a t e d A-F i n o r d e r o f i n c r e a s i n g f i e l d . A r e a s A, C a n d Ε a r e c l e a r i n t h e s e F i g u r e s w h i l e a r e a s B, D a n d F a r e cross-hatched. Except f o r the resonance p a t t e r n observed f o r atac t i c p o l y s t y r e n e ( F i g u r e 6 ) , t h e resonances appear t o be a s s i g n a b l e as f o l l o w s : A - mmmm Β - (mmmr + rmmm) C - rmmr D - (mmrr 4- rrmm) + (mmrm + mrmm) Ε - (rmrm + mrmr) + ( r m r r + r r m r ) F - rr T h u s , t r i a d s e q u e n c e d i s t r i b u t i o n s c a n b e measured b y c o m b i n i n g t h e s e f r a c t i o n a l r e s o n a n c e a r e a s : mm=A+B+C; (mr+rm)=D+E; r r = F . I n t h e c a s e o f t h e a t a c t i c p o l y s t y r e n e p a t t e r n ( F i g u r e 6) t h e demar c a t i o n between t h e D and Ε a r e a s i s n o t c l e a r and i t i s e a s i e r t o d i s t i n g u i s h D a n d Ε a r e a s t h a t may b e a s s i g n e d a s f o l l o w s : f

1

13.

H A R W O O D ET A L .

147

Polystyrène

and Epimerized Isotactic Polystyrenes 203

145

146 ôc(ppm)

F i g u r e 1.

O b s e r v e d and S i m u l a t e d 75 MHz A r o m a t i c C - l Carbon Resonance P a t t e r n o f an E p i m e r i z e d Isot a c t i c P o l y s t y r e n e Sample H a v i n g a mm-Content of 0.70.

204

NMR AND MACROMOLECULES

F i g u r e 2.

O b s e r v e d and S i m u l a t e d 75 MHz A r o m a t i c C - l C a r bon R e s o n a n c e P a t t e r n o f a n E p i m e r i z e d I s o t a c t i c P o l y s t y r e n e Sample H a v i n g a mm-Content o f 0.59.

13.

H A R W O O D ET A L .

F i g u r e 3.

Polystyrene and Epimerized Isotactic Polystyrenes

O b s e r v e d and S i m u l a t e d 75 MHz A r o m a t i c C - l C a r bon R e s o n a n c e P a t t e r n o f a n E p i m e r i z e d I s o t a c t i c P o l y s t y r e n e Sample H a v i n g a mm-Content o f 0 . 5 5 .

205

F i g u r e 4.

O b s e r v e d and S i m u l a t e d 75 MHz A r o m a t i c C - l C a r bon R e s o n a n c e P a t t e r n o f an E p i m e r i z e d I s o t a c t i c P o l y s t y r e n e Sample H a v i n g a mm-Content o f 0.41.

13.

H A R W O O D ET A L .

A

147

Polystyrene and Epimerized Isotactic Polystyrenes

B

O

D

E

146

F

145 ôc(ppm)

F i g u r e 5.

O b s e r v e d and S i m u l a t e d 75 MHz A r o m a t i c C - l C a r bon R e s o n a n c e P a t t e r n o f an E p i m e r i z e d I s o t a c t i c P o l y s t y r e n e Sample H a v i n g a mm-Content of 0.34.

207

208

NMR AND MACROMOLECULES

F i g u r e 6.

O b s e r v e d and S i m u l a t e d 75 MHz A r o m a t i c C - l C a r b o n R e s o n a n c e P a t t e r n o f P o l y s t y r e n e (mm M3.20).

13.

H A R W O O D ET A L .

Polystyrene and Epimerized isotactic Polystyrenes

D Ε

1

1

209

- (mmrr + rrmm) - (mmrm + mrmm) + (rmrm + mrmr) + (rmrr + rrmr).

In e v a l u a t i n g t r i a d stereosequence d i s t r i b u t i o n s f o r a t a c t i c p o l y styrene, then, the f o l l o w i n g combinations of f r a c t i o n a l resonance a r e a s c a n b e u s e d : mm=A+B+C, (mr+rm)=D'+Ε'; r r - F . T a b l e I I com pares t r i a d stereosequence d i s t r i b u t i o n s evaluated a s described above w i t h v a l u e s c a l c u l a t e d f o r t h e p o l y m e r s b y Monte C a r l o s i m u l a t i o n o f t h e e p i m e r i z a t i o n p r o c e s s . C a l c u l a t e d r e s u l t s b a s e d on V v a l u e s o f 0.50 a n d 0.65 a r e p r o v i d e d f o r c o m p a r i s o n . T h e r e i s l i t t l e d i f f e r e n c e b e t w e e n c a l c u l a t e d r e s u l t s b a s e d o n V=0.50 o r 0.65 when mm>0.35, b u t when mm<0.35 b e t t e r c o r r e l a t i o n s b e t w e e n observed and c a l c u l a t e d resonance a r e a s ( o r resonance p a t t e r n s , v i d e i n f r a ) a r e o b t a i n e d u s i n g V=0.65. T a b l e I I I c o m p a r e s t h e r e l a t i v e r e s o n a n c e a r e a s o f s i g n a l s A-F w i t h r e l a t i v e a r e a s e x p e c t e d based on t h e pentad resonance a s s i g n m e n t s d i s c u s s e d above and on s t e r e o s e q u e n c e d i s t r i b u t i o n s c a l c u l a t e d f o r t h e p o l y m e r s u s i n g V=0. 6 5 . The good a g r e e m e n t o b t a i n e d b e t w e e n o b s e r v e d a n d c a l c u l a ted resonance areas i n d i c a t e s that the g e n e r a l i n t e r p r e t a t i o n pre s e n t e d above i s p r o b a b l y c o r r e c t . I t s h o u l d be noted t h a t t h e c o n c e n t r a t i o n s o f mmrm and rmrm s t e r e o s e q u e n c e s a r e a p p r o x i m a t e l y t h e same i n a l l t h e p o l y m e r s , so t h a t t h e s e s t e r e o s e q u e n c e s c o u l d be i n t e r c h a n g e d i n t h e a s s i g n m e n t s p r o p o s e d above w i t h o u t a f f e c t i n g the agreement between c a l c u l a t e d and o b s e r v e d resonance a r e a s . T a b l e I I . T r i a d S t e r e o s e q u e n c e D i s t r i b u t i o n s Measured f o r E p i m e r i z e d I s o t a c t i c P o l y s t y r e n e Samples f r o m A r o m a t i c C - l C a r b o n R e s o n a n c e s a n d C a l c u l a t e d b y Monte C a r l o Simulation Triad

Method

Distributions

0.70 0. 70 0.70 0.70

0.59 0.59 0.59 0.59

0.55 0.52 0.55 0.55

0.41 0.39 0.41 0.41

0.35 0.34 0.35 0.35

0.23 0.20 0.25 0.19

C Calcf* Calc

0.24 0.20 0.20

0.29 0.27 0.27

0.34 0.30 0.29

0.44 0.39 0.37

0.45 0.43 0.41

0.50 0.50 0.49

Calcf* Calc

0.07 0.10 0.10

0.11 0.14 0.14

0.14 0.15 0.16

0.17 0.19 0.22

0.21 0.22 0.24

0.30 0.25 0.33

13

C

Calcf

Calc mr+rm

T r i a d Stereosequence

b

1 3

b

a) Monte C a r l o M e t h o d , V=0.50 b) Monte C a r l o M e t h o d , V=0.65

I t i s i n t e r e s t i n g that the aromatic C - l carbon resonance p a t t e r n s o f t h e e p i m e r i z e d p o l y s t y r e n e samples a r e t y p i c a l o f t h e

210

NMR AND MACROMOLECULES

Table I I I .

C o r r e l a t i o n o f A-F A r o m a t i c C - l C a r b o n R e s o n a n c e A r e a s Measured f o r P o l y s t y r e n e and E p i m e r i z e d I s o t a c t i c P o l y s t y r e n e Samples w i t h S t e r e o s e q u e n c e D i s t r i b u t i o n s C a l c u l a t e d by Monte C a r l o S i m u l a t i o n (V=0.65)

mm-Content

1

( H-NMR)

F i g u r e No.

0 .70 1

0. 59 2

Area A mmmm

0 .49 0 .55

0. 39 0. 41

Area Β (mmmr+rmmm)

0 .17 0 .14

Area C rmmr

0,.55 3

0 .41 4

0.,34 5

0. 20(PS) 6

-

0,.31 0,.36

0 .16 0 .22

0.,08 0,,16

0. 03

0. 17 0. 16

0,.17 0 .17

0 .17 0 .16

0,,16 0.,15

0. 11 0. 09

0 .04 0 .01

0. 03 0. 02

0,.05 0 .02

0 .06 0 .04

0.,10 0.,04

0. 09 0. 06

Area D mmrr+rrmnij mmrm+mrmm

0 .17

0. 20

0 .22

0 .25

0.,23

0. 15

0 .16

0. 20

0 .21

0 .23

0,,24

0. 12

Area Ε mmrm+mrmm^ rmrm+mrmr -j rmrr+rrmr

0 .07

0.,09

0 .11

0 .19

0,.23

0. 35

0 .04

0.,07

0 .08

0 .14

0,,17

0. 37 _

0 .07 0 .10

0..11 0.,14

0 .14 0 .16

0 .17 0 .21

0,,21 0,.24

0. 30 0. 33

Area F rr

p a t t e r n s t h a t a r e e n c o u n t e r e d i n s t u d i e s o n t h e NMR s p e c t r a o f many p o l y m e r s and a l t e r n a t i n g c o p o l y m e r s . I t i s not unusual t o note that the r r - c e n t e r e d pentad resonances occur c l o s e t o g e t h e r , t h a t the mr-centered pentad resonances are spread f u r t h e r a p a r t , o f t e n o c c u r r i n g i n two g r o u p s a n d t h a t t h e mm-centered p e n t a d r e sonances a r e w e l l s e p a r a t e d from each o t h e r . T h i s i s r e a s o n a b l e i f s h i e l d i n g b y n e x t n e a r e s t n e i g h b o r monomer u n i t s i s more v a r i a b l e when t h e n e a r e s t n e i g h b o r h a s t h e same c o n f i g u r a t i o n a s t h e monomer u n i t u n d e r s t u d y t h a n when t h e n e a r e s t n e i g h b o r h a s t h e opposite configuration. T h i s c a n b e e x p l a i n e d more c l e a r l y b y m a k i n g r e f e r e n c e t o F i g u r e 7. Suppose t h a t s h i e l d i n g by n e a r e s t n e i g h b o r i n t e r a c t i o n s c a u s e s t h e mm, (mr+rm) a n d r r r e s o n a n c e p a t t e r n s t o be w e l l s e p a r a t e d a n d l e t t h e mmmm, (mmrm+mrmm) a n d mrrm r e s o n a n c e s have c h e m i c a l s h i f t s i n d i c a t e d b y t h e l i n e s d e p i c t e d a t the t o p o f F i g u r e 7. Now l e t t h e s h i e l d i n g due t o n e x t n e a r e s t n e i g h b o r i n t e r a c t i o n s b e s u c h t h a t r e p l a c e m e n t o f a t e r m i n a l mm(or -mm) sequence b y a rm- ( o r -mr) sequence c a u s e s a l a r g e u p f i e l d c h e m i c a l s h i f t change a n d l e t r e p l a c e m e n t o f a t e r m i n a l mr(or -rm) sequence b y a r r - ( o r - r r ) sequence c a u s e a s m a l l downf i e l d c h e m i c a l s h i f t change. T h i s would cause the complete pentad r e s o n a n c e p a t t e r n t o b e t h a t shown a t t h e b o t t o m o f F i g u r e 7, w h i c h i s a n a l o g o u s t o t h a t seen i n F i g u r e 5. A s i m i l a r p a t t e r n would r e s u l t w i t h o t h e r s h i e l d i n g t e n d e n c i e s ; t h e important c o n s i -

13.

H A R W O O D ET A L .

Polystyrene and Epimerized Isotactic Polystyrenes

211

d e r a t i o n i s t h a t s h i e l d i n g a s s o c i a t e d w i t h -mr ( o r rm-) t e r m i n a l c o n f i g u r a t i o n s must b e d i f f e r e n t i n m a g n i t u d e ( n o t n e c e s s a r i l y d i r e c t i o n ) from t h a t a s s o c i a t e d w i t h a - r r (or r r - ) t e r m i n a l con figuration. Although the aromatic C - l carbon resonance p a t t e r n s observed for p a r t i a l l y epimerized polystyrenes are r e a d i l y i n t e r p r e t e d u s i n g t h e above c o n s i d e r a t i o n s , t h i s i s n o t t h e c a s e f o r t h e p a t t e r n observed f o r p o l y s t y r e n e (or f o r the completely e q u i l i b r a t e d polymer). The s i x - a r e a p a t t e r n t h a t i s s o c l e a r l y e v i d e n t i n t h e s p e c t r a of the p a r t i a l l y e p i m e r i z e d polymers i s not e v i d e n t i n the spectrum of p o l y s t y r e n e . I t seems t h a t r e s o n a n c e s o f h e p t a d s o r nonads w i t h h i g h r - c o n t e n t s have c h e m i c a l s h i f t s t h a t correspond to v a l l e y s o b s e r v e d i n t h e s p e c t r a o f t h e p a r t i a l l y e p i m e r i z e d polymers. T h i s causes the d e m a r c a t i o n between pentad resonance p a t t e r n s t o become o b s c u r e when t h e r - c o n t e n t i s a b o u t 0.5. This c o m p l i c a t i o n s h o u l d d i s a p p e a r a s t h e r - c o n t e n t i n c r e a s e s f r o m 0.5 and r e s o n a n c e s due t o h e p t a d s o r n o n a d s w i t h h i g h m - c o n t e n t s d i minish i n concentration. U n f o r t u n a t e l y the u n a v a i l a b i l i t y o f p o l y s t y r e n e s w i t h h i g h r - c o n t e n t s at the p r e s e n t time p r e v e n t s t h i s p o s s i b i l i t y from being pursued. H e p t a d C - l R e s o n a n c e s . The a r o m a t i c C - l c a r b o n r e s o n a n c e p a t t e r n s c o n t a i n components due t o h e p t a d s t e r e o s e q u e n c e s . Those e v i d e n t i n the s p e c t r a of p o l y s t y r e n e s w i t h low r - c o n t e n t s are e a s i l y as signed from t h e i r r e l a t i v e i n t e n s i t i e s . An a t t e m p t t o a s s i g n the o t h e r h e p t a d r e s o n a n c e s can b e made b y g e n e r a t i n g e m p i r i c a l s h i f t r u l e s , u s i n g p r o c e d u r e s t h a t have been d e s c r i b e d p r e v i o u s l y ( 1 8 , 1 9 ) . T h i s a p p r o a c h assumes t h a t s h i e l d i n g o f a q u a t e r n a r y a r o m a t i c c a r bon b y monomer u n i t s i n one d i r e c t i o n o f the p o l y m e r c h a i n i s i n d e p e n d e n t o f t h e c o n f i g u r a t i o n s of t h e monomer u n i t s g o i n g i n t h e opposite d i r e c t i o n . Although t h i s i s probably not s t r i c t l y t r u e , i t a l l o w s one t o d e v e l o p a n I n i t i a l s e t o f a s s i g n m e n t s w i t h a minimum number o f a d j u s t a b l e p a r a m e t e r s . The a s s i g n m e n t s can t h e n be r e f i n e d t o o b t a i n i m p r o v e d c o r r e s p o n d e n c e b e t w e e n o b s e r v e d and c a l c u l a t e d resonance p a t t e r n s . Seven p a r a m e t e r s a r e needed t o c a l c u l a t e t h e c h e m i c a l s h i f t s o f 36 h e p t a d s b y t h i s a p p r o a c h and f o u r of these can be e v a l u a t e d d i r e c t l y from the resonance p a t t e r n s of polymers e p i m e r i z e d to low e x t e n t s . The s e v e n p a r a m e t e r s w i l l be r e p r e s e n t e d b y t h e s y m b o l A x y z , where x , y and ζ c a n b e r o r m. U s i n g oabcdef to r e p r e s e n t the c h e m i c a l s h i f t of a g i v e n h e p t a d , where a,b,c,d,e and f c a n a l s o b e e i t h e r r o r m, t h e f o l l o w i n g definition prevails: A x y z = ommmxyz - ommmmmm = o a b c x y z - ôabcmmm, e t c . = 6zyxmmm - omminmmm = o z y x a b c - ômmmabc, e t c . ommmmmm can b e measured f r o m t h e c h e m i c a l s h i f t o f i s o t a c t i c p o l y s t y r e n e or from t h a t of the l a r g e s t s i g n a l i n p o l y m e r s e p i m e r i z e d t o l o w e x t e n t s and i s 146.40 ppm f o r t h e c o n d i t i o n s employed i n the p r e s e n t study. A c c o r d i n g l y , t h e c h e m i c a l s h i f t o f any h e p t a d can b e c a l c u l a t e d b y t h e f o l l o w i n g g e n e r a l f o r m u l a s and s p e c i f i c examples.

212

NMR AND

ôabcdef ommmrrm ôrrmmmr δmmrrmm

146.40 146.40 146.40 146.40

+ + + +

MACROMOLECULES

Acba + Adef Arrm Amrr + Ammr 2Armm

When t h e c h e m i c a l s h i f t s o f o b v i o u s l y a s s i g n a b l e r e s o n a n c e s a r e known, t h e y c a n b e u s e d , w i t h e q u a t i o n s l i k e t h o s e w r i t t e n above, t o e v a l u a t e Axyz parameters. The n a t u r e o f t h e e p i m e r i z a t i o n p r o c e s s makes i t p o s s i b l e t o e v a l u a t e Ammr, A m r r , A r r m a n d Armm d i r e c t l y f r o m t h e s p e c t r a o f p o l y m e r s e p i m e r i z e d t o l o w e x tents. F i g u r e 8 shows a s e c t i o n o f a n i s o t a c t i c p o l y m e r c h a i n t h a t c o n t a i n s one i n v e r t e d monomer u n i t . I t c a n be seen t h a t (mmmmmr + rmmmmm), (mmmmrr + rrmmmm), (mmmrrm + mrrmmm) a n d mmrrmm h e p t a d s i n c l u d e t h e i n v e r t e d monomer u n i t and t h a t n o o t h e r h e p t a d stereosequences a r e present. T h u s , r e s o n a n c e s due t o t h e s e h e p t a d s should be prominent i n t h e s p e c t r a o f i s o t a c t i c p o l y s t y r e n e s t h a t have been e p i m e r i z e d t o o n l y l o w e x t e n t s ( e . g . , F i g u r e 1 and 2 ) . The s i g n a l due t o mmrrmm h e p t a d s s h o u l d b e e a s i l y r e c o g n i z e d b e c a u s e i t must h a v e a n i n t e n s i t y t h a t i s o n e - h a l f t h a t o f t h e o t h e r heptads. P r o v i d e d t h a t t h e o t h e r r e s o n a n c e s o c c u r between t h e mmmmmm ( l a r g e s t ) a n d mmrrmm s i g n a l s , i t i s r e a s o n a b l e t o e x p e c t t h e o t h e r m a j o r r e s o n a n c e s t o o c c u r i n t h e f o l l o w i n g o r d e r : mmmmmm, (mmmmmr + rmmmmm), (mmmmrr + rrmmmm), (mmmrrm + mrrmmm), mmrrmm. Based on t h i s e x p e c t a t i o n , r e s o n a n c e s o f the h e p t a d s mentioned c a n be i d e n t i f i e d and t h e i r c h e m i c a l s h i f t s c a n be u s e d t o e v a l u a t e Ammr, A m r r , A r r m and Armm. E v a l u a t i o n o f t h e r e m a i n i n g t h r e e p a r a m e t e r s , Amrm, Δrmr and A r r r , i s n o t a s e a s i l y done a s i s t h e e v a l u a t i o n o f t h e f o u r p a r a meters d i s c u s s e d above, b u t a s y s t e m a t i c approach i s p o s s i b l e . I t i s b a s e d on t h e o b s e r v a t i o n t h a t t e n h e p t a d s , h a v i n g c o n c e n t r a t i o n s l e s s than those d i s c u s s e d above, b u t h a v i n g v e r y s i m i l a r concen t r a t i o n s , make m i n o r c o n t r i b u t i o n s t o t h e s p e c t r a o f p o l y m e r s e p i m e r i z e d t o l o w e x t e n t s . These h e p t a d s a r e l i s t e d i n T a b l e IV. The c h e m i c a l s h i f t s o f f o u r o f t h e s e h e p t a d s ( l i n e s 2,3,6 a n d 8 ) c a n b e c a l c u l a t e d u s i n g v a r i o u s c o m b i n a t i o n s o f Ammr, A m r r , Δrmm and Armm. The s i x r e m a i n i n g h e p t a d s f a l l i n t o two g r o u p s . The c h e m i c a l s h i f t s o f t h e members o f g r o u p A ( l i n e s 1,4 and 7) w i l l be s e p a r a t e d f r o m t h a t o f t h e mmmmmm s i g n a l b y Amrm, Armr a n d A r r r and t h o s e o f Group Β ( l i n e s 5,9 a n d 1 0 ) w i l l b e s e p a r a t e d f r o m t h a t o f t h e (mmmrmm + mmrmmm) s i g n a l ( 6 must b e c a l c u l a t e d ) b y Amrm, Armr and A r r r a l s o . T h e s e p a r a m e t e r s c a n be e s t i m a t e d b y attempting s i m u l a t i o n s of the experimental resonance p a t t e r n u s i n g c a l c u l a t e d h e p t a d i n t e n s i t i e s , measured c h e m i c a l s h i f t s f o r m a j o r heptad resonances, chemical s h i f t s c a l c u l a t e d f o r f o u r of the minor h e p t a d r e s o n a n c e s , ( l i n e s 2,3,6 a n d 8) a n d v a r i a b l e c h e m i c a l s h i f t s f o r t h e other s i x resonances w i t h the c o n s t r a i n t t h a t w i t h i n Groups A a n d Β t h e r e l a t i v e s p a c i n g o f t h e r e s o n a n c e s , must b e t h e same. S i n c e t h e i n t e n s i t i e s o f t h e s i x l i n e s i n Group A a n d Β w i l l b e a p p r o x i m a t e l y t h e same i n p o l y m e r s e p i m e r i z e d t o l o w e x t e n t s , a n C

H A R W O O D ET A L .

Polystyrene and Epimerized Isotactic Polystyrenes

mmrm mrmm

mmmm

mmmr rmmm

mmrr rrmm

rmrm mrmr

mmrm mrmm

F i g u r e 7.

rrrr

mrrm

mrrr rrrm

rmrr rrmr

I n f l u e n c e o f L o n g Range S h i e l d i n g b y S t e r e o s e q u e n c e s on R e s o n a n c e P a t t e r n s .

Inversion

Point

I ^

m

m

m

m

m

{

m

m

m

r

m

m

r

m

r

r

m

m

m

r

r

m

^ m

m

r

m

m

r

r

r

r

r r

m

F i g u r e 8.

m

m

m

m

m m

m

m

m

m

m

m

^

m

m

m }

m

m

m

m

m

m

m

m

ι

m

r

H e p t a d s P r e s e n t i n t h e V i c i n i t y o f an I n v e r t e d Monomer U n i t i n an I s o t a c t i c P o l y m e r .

213

214

Table

NMR AND MACROMOLECULES

IV.

Heptad Stereosequences Responsible f o r Minor S i g n a l s i n P a r t i a l l y E p i m e r i z e d I s o t a c t i c P o l y s t y r e n e Samples h a v i n g mm-Contents B e l o w 0.50.

Line

Heptad

1 2 3 4 5 6 7 8 9 10

mrmmmm + mmmmrm rrmmmr + rmmmrr rrmmrr mmmrmr + rmrmmm mrrormm + mmrmrm rmmrrm + mrrmmr mmmrrr + rrrmmm rrmrrm + mrrmrr rmr rmm + mmr rmr rrrrmm + mmrrrr

Parameters Required t o C a l c u l a t e 6q Amrm Δ mmr + Amrr 2 Amrr Armr Amrm + Armm Ammr + A r r m Arrr A r r m + Amrr Armr + Armm A r r r + Armm

Comment Group A Calculated Calculated Group A Group Β Calculated Group A Calculated Group Β Group Β

a s s u m p t i o n w i l l h a v e t o be made a b o u t t h e r e l a t i v e m a g n i t u d e s o f Amrm, Armr and A r r r . I n t h e p r e s e n t work i t was assumed t h a t Armr > A r r r > Amrm. F i g u r e 9 may h e l p t o i l l u s t r a t e t h i s a p p r o a c h . Once a l l s e v e n A x y z p a r a m e t e r s h a v e b e e n e v a l u a t e d , c h e m i c a l s h i f t s f o r t h e r e m a i n i n g 21 h e p t a d s c a n b e c a l c u l a t e d and u s e d f o r s i m u l a t i n g the s p e c t r a o f polymers epimerized t o h i g h e r e x t e n t s , as w e l l a s t h e s p e c t r u m o f p o l y s t y r e n e . M i n o r a d j u s t m e n t s c a n t h e n b e made i n c a l c u l a t e d c h e m i c a l s h i f t s t o f i l l i n o r deepen v a l l e y s o r t o e l i m i n a t e d i s t o r t i o n s caused by c o i n c i d e n c e s o f c a l culated chemical s h i f t s . T h i s approach worked r e a s o n a b l y success f u l l y when a p p l i e d t o q u a t e r n a r y a r o m a t i c c a r b o n r e s o n a n c e s o b s e r ved f o r e p i m e r i z e d i s o t a c t i c p o l y s t y r e n e s a s measured a t 20 MHz and room t e m p e r a t u r e ( 1 9 ) . I t proved n e c e s s a r y , however, t o s h i f t heptads c o n t a i n i n g a c e n t r a l rmrr (or rrmr) pentad u p f i e l d by 0.256 ppm t o o b t a i n good a g r e e m e n t b e t w e e n o b s e r v e d and s i m u l a t e d s p e c t r a . T h i s s u g g e s t s t h a t t h e s h i e l d i n g e x p e r i e n c e d by a n u c l e u s f r o m one d i r e c t i o n o f t h e p o l y m e r c h a i n may n o t b e e n t i r e l y i n d e pendent o f t h e s t r u c t u r e o f t h e c h a i n t h a t proceeds i n the opposite d i r e c t i o n . T h i s same g e n e r a l a p p r o a c h was u s e d i n t h e p r e s e n t s t u d y t o d e v e l o p A x y z v a l u e s f o r p o l y s t y r e n e s p e c t r a r e c o r d e d a t 150°. The values o b t a i n e d , together w i t h those developed p r e v i o u s l y f o r s p e c t r a r e c o r d e d a t room temper a ture(]L9) a r e g i v e n i n T a b l e V. The most s i g n i f i c a n t d i f f e r e n c e b e t w e e n t h e two s e t s o f v a l u e s i s the dramatic i n c r e a s e i n A r r r w i t h an i n c r e a s e i n temperature. S i n c e Armm, A r r m , Armr and A r r r a l l h a v e s i m i l a r and l a r g e n e g a t i v e v a l u e s compared t o t h e o t h e r A x y z v a l u e s a t 150°, i t i s u n d e r s t a n d able t h a t p o l y s t y r e n e s p e c t r a recorded a t h i g h temperature can p r o v i d e r e l i a b l e measures o f r r - t r i a d c o n c e n t r a t i o n s . M i n o r a d j u s t m e n t s were made i n c h e m i c a l sh i f t s c a l c u l a t e d u s i n g Axyz v a l u e s t o improve the q u a l i t y o f f i t between s i m u l a t e d and o b s e r v e d s p e c t r a . The h e p t a d c h e m i c a l s h i f t s u s e d t o s i m u l a t e

H A R W O O D ET A L .

Polystyrene and Epimerized Isotactic Polystyrenes

-p| mmmmmm - mmmmmr

=-Ammr

I mmmmmm - mmmmrr = - A m r r

mmmmmm - mmmrrm = - A r r m

mmmmmm - mmrrmm = - 2 A r m m

•

I—*\ I

>j

h Group F i g u r e 9.

I—*| (

A

xxxmmm - xxxmrm =-Δmrm *|

^ I

^calculated line

^|

xxxmmm - xxxrrr = - Δ π τ xxxmmm - xxxrmr =-Δ rmr

B

E s t i m a t i o n o f Aabc P a r a m e t e r s f r o m t h e S p e c t r a o f E p i m e r i z e d P o l y m e r s H a v i n g H i g h mm-Contents.

216

NMR A N D MACROMOLECULES

T a b l e V.

E m p i r i c a l Chemical S h i f t Parameters f o r P o l y s t y r e n e a t 20° and 150°

Parameter Δ mmr Δπιπιι Armm Amrr Arrm Armr Arrr rmrr

correction

(Axyz) E v a l u a t e d

V a l u e (ppm) 20°

150°

-0.080 -0.288 -0.752 -0.192 -0.695 -0.621 -0.496

-0.077 -0.266 -0.728 -0.196 -0.637 -0.665 -0.616

-0.256

-0.112

the s p e c t r a a r e g i v e n i n Table V I , along w i t h those c a l c u l a t e d u s i n g t h e p a r a m e t e r s l i s t e d i n T a b l e V. T a b l e V I a l s o compares the r e l a t i v e order o f these heptad resonance assignments w i t h t h e r e l a t i v e order o f methyl carbon heptad resonances c a l c u l a t e d f o r p o l y p r o p y l e n e by S c h i l l i n g a n d T o n e l l i ( 3 7 ) , u s i n g t h e r o t a t i o n a l i s o m e r i c s t a t e m o d e l . The c o r r e s p o n d e n c e b e t w e e n t h e two s e t s o f a s s i g n m e n t s i s s u r p r i s i n g l y good. A l t h o u g h some a r o m a t i c C - l c a r bon p e n t a d r e s o n a n c e p a t t e r n s o v e r l a p i n t h e p o l y s t y r e n e s p e c t r a , the r e l a t i v e o r d e r i n g o f the pentad resonance p a t t e r n s e x a c t l y matches t h a t o f t h e methyl carbon pentad resonances o f p o l y p r o p y l e n e , a s c a l c u l a t e d b y S c h i l l i n g a n d T o n e l l i ( 3 7 ) [mmmm, (mmmr + rmmm), rmmr, (mmrr + rrmm), (mmrm + mrmm), ( r m r r + r r m r ) , (rmrm + mrmr), r r r r , ( m r r r + r r r m ) , mrrm, i n o r d e r o f i n c r e a s i n g f i e l d ] . T h i s o r d e r i n g i s a l s o i n agreement w i t h t h e a s s i g n m e n t s d e v e l o p e d f o r t h e a r o m a t i c C - l r e s o n a n c e s o f p o l y s t y r e n e by S a t o a n d Tanaka (11). I t i s i n o n l y f a i r agreement w i t h o r d e r i n g b a s e d on T o n e l l i s recent c a l c u l a t i o n s f o r p o l y s t y r e n e ( 3 8 ) , which, i n con t r a s t t o t h e p o l y p r o p y l e n e c a l c u l a t i o n s ( 3 7 ) , t e n d t o group t h e l i n e s i n t o f o u r g e n e r a l a r e a s . R e s o n a n c e s o f mm-centered p e n t a d s a r e c a l c u l a t e d t o o c c u r i n t h r e e o f t h e s e a r e a s a n d t h e r e i s more e x t e n s i v e o v e r l a p p i n g o f mm- and ( m r + r m ) - c e n t e r e d p e n t a d r e s o n a n c e r e g i o n s t h a n seems r e a s o n a b l e b a s e d on o u r s i m u l a t i o n s t u d i e s . F i g u r e s 1 - 6 compare o b s e r v e d a r o m a t i c C - l c a r b o n r e s o n a n c e s p e c t r a w i t h s i m u l a t e d s p e c t r a based on t h e heptad c h e m i c a l s h i f t s g i v e n i n T a b l e V I and on heptad stereosequence c o n c e n t r a t i o n s c a l c u l a t e d by Monte C a r l o s i m u l a t i o n o f t h e e p i m e r i z a t i o n p r o c e s s , u s i n g V=0.65. The s i m u l a t i o n s p e c t r a r e p r o d u c e t h e g e n e r a l f e a t u r e s o f t h e o b s e r v e d s p e c t r a v e r y w e l l a n d c a n be c o n s i d e r e d t o be i n a t l e a s t s e m i - q u a n t i t a t i v e agreement w i t h t h e o b s e r v e d s p e c tra. The agreement b e t w e e n o b s e r v e d and s i m u l a t e d s p e c t r a m i g h t be i m p r o v e d i f s p e c t r a w i t h h i g h e r S/N r a t i o s were employed a n d i f a d d i t i o n a l p a r a m e t e r a d j u s t m e n t s w e r e made. I t seems, h o w e v e r , t h a t t h e h e p t a d a s s i g n m e n t s d e v e l o p e d i n t h i s work a r e r e a s o n a b l y correct. f

13.

H A R W O O D ET A L .

Polystyrene and Epimerized Isotactic Polystyrenes

111

Table V I . Heptad Assignments f o r A r o m a t i c C - l Carbon Resonances of P o l y s t y r e n e Heptad

6 (Calc) c

<S (Simulation) R e l a t i v e Order Polystyrene > Polypropylene C

3

b

mmmmmm rmmmmm + mmmmmr rmmmmr

146.40 146.32 146.25

146.40 146.32 146.25

l 2 3

l 2 3

1 2 3

+ + + +

mmmmrm rmmmrm mmmmrr rmmmrr

146.13 146.06 146.20 146.13

146.13 146.06 146.20 146.18

6 7 4 5

9 11-13 7,8 10

5 7 4 6

mrmmrm rrmmrm + mrmmrr rrmmrr

145.87 145.94 146.01

145.91 145.92 146.01

10 9 8

18 17 16

10,11 9 8

mmmrrm rmmrrm mmmrrr rmmrrr

+ + + +

mrrmmm mrrmmr rrrmmm rrrmmr

145.76 145.69 145.78 145.71

145.77 145.70 145.85 145.76

12 14 11 13

11-13 14,15 11-13 14,15

mmmrmm rmmrmm mmmrmr rmmrmr

+ + + +

mmrmmm mmrmmr rmrmmm rmr mmr

145.67 145.59 145.74 145.66

145.67 145.60 145.74 145.63

16 18 15 17

5 7,8 4 6

16 18 15 17

mrmrmm rrmrmm mrmrmr rrmrmr

+ + + +

mmrmrm mmrmrr rmr mrm rmrmrr

145.41 145.48 145.47 145.54

145.36 145.41 145.50 145.53

25 24 20 19

22 20,21 20,21 19

26 24 25 23

mrmrrm rrmrrm mrmrrr rrmrrr

+ + + +

mrrmrm mrrmrr rrrmrm rrrmrr

145.39 145.46 145.41 145.48

145.35 145.45 145.43 145.48

26 22 23 21

26 23,24 25 23,24

21 19 22 20

mrrrrm rrrrrm + mrrrrr rrrrrr

145.13 145.15 145.17

145.27 145.15 145.20

27 29 28

27 28 29

27 28 29

+ + + +

mmrrrm mmrrrr rmrrrm rmr r r r

145.04 145.06 145.10 145.12

145.07 145.06 145.11 145.12

32 34 31 30

32 33 31 30

32 33 30 31

mmrrmm rmrrmm + mmr rmr rmr rmr

144.94 145.01 145.07

144.94 145.00 145.08

36 35 33

36 35 34

36 35 34

mrmmmm mrmmmr rrmmmm rrmmmr

mrrrmm rrrrmm mrrrmr rrrrmr

a

b

10,11 13 12 14

(a) B a s e d o n l i n e s u s e d f o r s i m u l a t i o n s shown i n F i g u r e s 1-6. (b) B a s e d o n c a l c u l a t i o n s o f T o n e l l i ( 3 8 ) . (c) Based on c a l c u l a t i o n s o f T o n e l l i and S c h i l l i n g ( 3 7 ) .

1

218

NMR A N D MACROMOLECULES

S p e c t r a s i m u l a t i o n s b a s e d on s t e r e o s e q u e n c e distributions c a l c u l a t e d f o r t h e p o l y m e r s u s i n g V=0.50 a l s o matched t h e e x p e r i mental s p e c t r a very w e l l , except f o r p o l y s t y r e n e ( o r the completely epimerized polymer). I t was n o t p o s s i b l e t o d e v e l o p h e p t a d r e s o n ance a s s i g n m e n t s t h a t a f f o r d e d u n i f o r m agreement b e t w e e n o b s e r v e d and s i m u l a t e d s p e c t r a f o r a l l t h e s a m p l e s s t u d i e d when s t e r e o s e quence d i s t r i b u t i o n s b a s e d on V=0.50 w e r e u s e d . Since resonance a s s i g n m e n t s c a n be made u s i n g e p i m e r i z e d p o l y m e r s w i t h h i g h mm c o n t e n t s u s i n g s t e r e o s e q u e n c e c o n c e n t r a t i o n s b a s e d on e i t h e r V=0.50 o r V=0.65, t h e s e a s s i g n m e n t s c o u l d be u s e d t o d e t e r m i n e what V v a l u e was a p p r o p r i a t e f o r p o l y s t y r e n e ( o r t h e c o m p l e t e l y e p i m e r i z e d polymer). V v a l u e s r a n g i n g f r o m 0.62 t o 0.65 a f f o r d e d good a g r e e ment b e t w e e n s i m u l a t e d and o b s e r v e d s p e c t r a o f p o l y s t y r e n e . Assuming B e r n o u l l i a n s t a t i s t i c s , ν=(1-σ) /σ , where σ i s t h e p r o b a b i l i t y o f a meso p l a c e m e n t i n t h e c o m p l e t e l y e q u i l i b r a t e d p o l y m e r ( o r i n p o l y s t y r e n e ) . V v a l u e s o f 0.62-0.65 t h u s i m p l y t h a t p o l y s t y r e n e c a n be c h a r a c t e r i z e d b y a σ v a l u e o f 0.44. One d i s t u r b i n g a s p e c t o f t h i s p o r t i o n o f t h e s t u d y i s t h e f a c t t h a t t h e mmrrmm s i g n a l ( 6 = v L 4 5 . 3 ppm) i s n o t a s i n t e n s e i n some o b s e r v e d s p e c t r a ( F i g u r e s 1-4) a s i t s h o u l d be b a s e d on s i m u l a t i o n and on a r g u m e n t s p r e s e n t e d e a r l i e r i n t h i s p a p e r . T h i s may i n d i c a t e an i n f l u e n c e o f n o n a d s , o r a d i m i n i s h e d s e n s i t i v i t y o f c a r b o n s i n t h i s e n v i r o n m e n t due t o r e l a x a t i o n t i m e o r NOE d i f f e r ences. A d d i t i o n a l study of t h i s p o i n t i s merited. M e t h y l e n e and M e t h i n e C a r b o n R e s o n a n c e s . F i g u r e 10 compares t h e m e t h y l e n e and m e t h i n e c a r b o n r e s o n a n c e p a t t e r n s o b s e r v e d f o r p o l y s t y r e n e and t h e e p i m e r i z e d p o l y m e r s . The m e t h y l e n e c a r b o n s p e c t r a are too noisy t o j u s t i f y q u a n t i t a t i v e study. An a n a l y s i s o f s p e c t r a r e c o r d e d u s i n g a l a r g e r number o f FID a c c u m u l a t i o n s w i l l be reported subsequently. However, t h e p a t t e r n s o b s e r v e d a r e q u a l i t a t i v e l y s i m i l a r t o those d i s c u s s e d e a r l i e r t h a t were r e c o r d e d w i t h a 20 MHz s p e c t r o m e t e r ( 1 8 ) . The m e t h i n e c a r b o n r e s o n a n c e p a t t e r n s o c c u r r e d o v e r a s m a l l c h e m i c a l s h i f t r a n g e a n d were t h e r e f o r e adequately d e f i n e d f o r q u a n t i t a t i v e study. We r e p o r t e d p r e v i o u s l y that the methine carbon resonance of these polymers occurs i n two g e n e r a l a r e a s a n d a s s i g n e d t h e l o w e r f i e l d a r e a t o m m - t r i a d s . F i g u r e 11 compares t h e p r o p o r t i o n o f m e t h i n e c a r b o n r e s o n a n c e o b s e r v e d i n t h i s l o w e r a r e a w i t h mm-contents m e a s u r e d f o r t h e p o l y mers f r o m t h e i r m e t h i n e p r o t o n r e s o n a n c e p a t t e r n s . I t c a n be s e e n t h a t t h e r e i s a 1:1 c o r r e s p o n d e n c e b e t w e e n t h e s e two q u a n t i t i e s , t h u s p r o v i n g t h a t t h e l o w e r f i e l d m e t h i n e c a r b o n r e s o n a n c e i s due to mm-triads. 2

2

c

Conclusions Although the aromatic C - l carbon resonance of p o l y s t y r e n e i s very complex, r e l a t i v e l y simple aromatic C - l carbon resonance s p e c t r a are observed f o r p a r t i a l l y epimerized i s o t a c t i c p o l y s t y r e n e s . S t u d i e s on s u c h "model p o l y s t y r e n e s " p r o v i d e t h e i n f o r m a t i o n needed to i n t e r p r e t t h e spectrum o f p o l y s t y r e n e i t s e l f . B a s e d on a s s i g n -

H A R W O O D ET A L .

F i g u r e 10.

Polystyrene and Epimerized Isotactic Polystyrenes

75 MHz M e t h y l e n e and M e t h i n e C a r b o n R e s o n a n c e o f P o l y s t y r e n e and o f E p i m e r i z e d I s o t a c t i c P o l y s t y r e n e Samples.

219

220

NMR AND MACROMOLECULES

13.

H A R W O O D ET A L .

Polystyrene and Epimerized Isotactic Polystyrenes

221

1

merits made i n t h i s p a p e r a n d i n o t h e r s i n t h i s s e r i e s , t h e H and C-NMR s p e c t r a o f p o l y s t y r e n e i n d i c a t e t h a t i t c a n be c h a r a c t e r i z e d by a σ v a l u e o f ^ 0 . 4 5 . 13

A c k n o w l e dgment s T h i s s t u d y was s u p p o r t e d i n p a r t b y a g r a n t f r o m t h e N a t i o n a l S c i e n c e F o u n d a t i o n (DMA-80-10709).

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16 17. 18. 19. 20. 21.

Bovey, F . A . ; Tiers, G.V.D.; F i l i p o v i c h , G . , J. Polymer S c i . 1959 38, 73. Kawamura, T.; Uryu, T.; Matsuzaki, Κ., Makromol. Chem., Rapid Comm. 1982, 3, 651 and references cited therein. Suparno, S.; Lacoste, J.; Raynal, S.; Sledz, J.; Schue, F . , Polymer J. 1981, 13, 313. Jasse, B . ; Laupretre, F . ; Monnerie, L., Makromol. Chem. 1977, 178, 1987. Nguyen-Tran, T . M . ; Laupretre, F . ; Jasse, Β . , Makromol. Chem. 1980, 181, 125. Elgert, K.F.; Henschel, R . ; Schorn, H . ; Kosfeld, R., Polymer B u l l e t i n 1981, 4, 105. Tanaka, Y.; Sato, H . ; Saito, K . ; Miyashita, Κ., Makromol. Chem., Rapid Comm. 1980, 1, 551. Sato, H . ; Tanaka, Y.; Hatada, Κ., Makromol. Chem., Rapid Comm. 1982, 3, 175. Sato, H . ; Tanaka, Y.; Hatada, Κ., Makromol. Chem., Rapid Comm. 1982, 3, 181. Tanaka, Y.; Sato, H . ; Saito, K . ; Miyashita, Μ., Rubber Chem. and Technol. 1981, 54, 686. Sato, H . ; Tanaka, Y., paper published in the present volume. T o n e l l i , A.E., Macromolecules 1979, 12, 252. Yoon, D . Y . ; Flory, P.J., Macromolecules, 1977, 13, 562. Fujiwara, Y . ; Flory, P.J., Macromolecules 1970, 3, 43. Trumbo, D . L . ; Chen, T . K . ; Harwood, H.J., Macromolecules 1981, 14, 1138. Trumbo, D . L . ; Suzuki, T.; Harwood, H.J., Polymer B u l l e t i n 1981, 4, 677. Shepherd, L.; Chen, T . K . ; Harwood, H.J., Polymer B u l l e t i n 1979, 1, 445. Chen, T . K . ; Gerkin, T . A . ; Harwood, H.J., Polymer B u l l e t i n 1980, 2, 37. Chen, T . K . ; Harwood, H.J., Makromol. Chem., Rapid Comm. 1983, 4, 463. Randall, J . C . "Polymer Sequence Determination - Carbon-13 NMR Method," Academic Press: New York, 1977, pp 87-92, 116-119, and references cited therein. Suparno, S.; Lacoste, J.; Raynal, S.; Regnier, J.F.; Schue, F.; Sempere, R.; Sledz, J., Polymer J. 1980, 12, 861.

NMR AND MACROMOLECULES

222 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.

Inone, Y.; Nishioka, Α.; Chujo, R . , Makromol. Chem. 1972, 156, 207. Ueno, Α.; Schuerch, C . , J . Polym. Sci., Part Β 1965, 3, 53. Clark, E.G., J . Polym. Sci., Part C 1968, 16, 3455. Flory, P.J.; Williams, A . D . , J. Am. Chem. Soc. 1968, 91, 3118. Mercier, J.; Smets, G . , J. Polym. Sci., Part A 1963, 1, 1491. Hogen-Esch, T . E . ; Tien, C . F . , J. Polym. Sci., Part Β 1979, 17, 431. Hogen-Esch, T . E . ; Tien, C . F . , Macromolecules 1980, 13, 207. Suter, U.W.; Pucci, S.; Pino, P . , J . Am. Chem. Soc. 1975, 97, 1018. Stehling, F . ; Knox, J . R . , Macromolecules 1975, 8, 595. Suter, U.W.; Neuenschwander, P . , Macromolecules 1981, 14, 528. Dworak, Α.; Harwood, H.J., to be published. Williams, A . D . ; Brauman, J.I.; Nelson, N.J.; Flory, P.J., J . Am. Chem. Soc. 1967, 89, 4807. Shepherd, L., Ph.D. Thesis, University of Akron, Akron, Ohio, 1979. Gray, G . , private communication of spectra. Williams, A . D . ; Flory, P.J., J . Am. Chem. Soc. 1969, 91, 3111. S c h i l l i n g , F . C . ; T o n e l l i , A.E., Macromolecules 1980, 13 270. T o n e l l i , A.E., Macromolecules 1983, 16 604.

RECEIVED

November 3, 1983

14 Stereospecific Polymerization of α-Olefins: End Groups and Reaction Mechanism 1

A. ZAMBELLI and P. AMMENDOLA Universita di Napoli, Naples, Italy 2

1

2

M. C. SACCHI and P. LOCATELLI Istituto di Chimica delle Macromolecole, Consiglio Nazionale delle Ricerche, Rome, Italy

The achievements concerning reaction mech anisms of α - o l e f i n polymerizations are summarized. The contributions of CNMR in this f i e l d , p a r t i c u l a r l y concerning the stereochemical structure of C enriched end groups, are discussed. 13

13

S i n c e t h e l a t e I 9 6 0 * s h i g h r e s o l u t i o n NMR became a n i n c r e a s i n g l y i m p o r t a n t method f o r i n v e s t i g a t i n g t h e structure o f macromolecules(1). The d e t a i l e d know ledge o f t h e molecular s t r u c t u r e so achieved had a l a r g e impact i n t h e f i e l d o f c o r r e l a t i o n s between s t r u c t u r e and p r o p e r t i e s o f s y n t h e t i c polymers and g r e a t l y h e l p e d t h e u n d e r s t a n d i n g o f t h e mechanism of polymerization reactions. P r o t o n and^3c NMR a n a l y s e s have been e x t e n s i v e l y used f o r s t u d y i n g t h e mechanism o f s t e r e o s p e c i f i c p o l y m e r i z a t i o n o f (X-ole f i n s (1). I s o t o p i c s u b s t i t u t i o n has a l s o been v e r y h e l p f u l i n order to s i m p l i f y the spectra, t o increase t h e s e n s i t i v i t y a n d , even more i m p o r t a n t , i n remov i n g s t r u c t u r e degeneracy. I n t h i s c h a p t e r , we w i l l b r i e f l y s u m m a r i z e some of the r e s u l t s reported i n the l i t e r a t u r e concerning the mechanisms o f s t e r e o s p e c i f i c p o l y m e r i z a t i o n s o f o t - o l e f i n s a n d d i s c u s s some o f t h e l a t e s t r e s u l t s o b t a i n e d b y o u r r e s e a r c h g r o u p s v i a 13c NMR a n a l y s e s . 1

2

Current address: V i a Mezzocannone 4-80134, Naples, Italy. Current address: V i a Bassini 15A-20133, M i l a n , Italy.

0097-6156/ 84/ 0247-0223506.00/ 0 © 1984 American Chemical Society

NMR AND

224

Mechanism o f A d d i t i o n

MACROMOLECULES

t o t h e Double Bond

The s t r u c t u r e s o f t h e monomer u n i t s which c o u l d be ob t a i n e d by e i t h e r c i s o r t r a n s a d d i t i o n of t h e r e a c t i v e m e t a l - c a r b o n bond o f t h e a c t i v e s i t e o r t h e double bonds o f t h e C C - o l e f i n s a r e d e g e n e r a t e . Degeneracy can be removed by a p r o p e r i s o t o p i c s u b s t i t u t i o n on the monomer which i s t o be p o l y m e r i z e d . For instance, cis-l-d^-propene c o u l d g i v e polymer c h a i n u n i t s h a v i n g d i f f e r e n t s t r u c t u r e s ( 2 ) depending on t h e s t e r e o c h e m i c a l mechanism o f a d d i t i o n , as shown i n t h e f o l l o w i n g scheme ( F i s h e r p r o j e c t i o n s ) !

p

CH~

Q 2.

Q

ι '

ι

H

H

cis

addition

ι 1 r\

I

— L/ — — I

1 H

trans

D

3

-S

- CI

H

addition

The s t r u c t u r e s o f t h e monomer u n i t s a c t u a l l y r e s u l t i n g from b o t h i s o t a c t i c and s y n d i o t a c t i c Z i e g l e r - N a t t a p o l y m e r i z a t i o n s o f c i s - l - d i -propene have been d e t e r mined by IR and 3-H NMR. The a d d i t i o n s have been found

to be c i s ( 3 , M . Regiospecificity The i n s e r t i o n o f OC - o l e f i n s on t h e m e t a l - c a r b o n bond (Mt-R) o f t h e a c t i v e s i t e s c o u l d be e i t h e r p r i m a r y (or m e t a l t o C I ) o r s e c o n d a r y ( o r m e t a l t o C 2 ) .

Mt-R

+

cyi

primary i n s e r t i o n

Mt-CH CH(CH-)-R

secondary i n s e r t i o n

Mt-CK(CH~)CH -R

?

6

p

As r e p o r t e d i n t h e l i t e r a t u r e , t h e problem o f d e t e r m i n i n g t h e a c t u a l type o f i n s e r t i o n f o r s y n d i o t a c t i c p o l y p r o p y l e n e has been f a c e d by o b s e r v i n g t h e amount o f i r r e g u l a r l y a r r a n g e d monomer u n i t s i n p o l y p r o p y l ene and i n e t h y l e n e - p r o p y l e n e c o p o l y m e r s ( 5 - ? ) · The problem o f t h e r e g i o s p e c i f i c i t y o f t h e i n s e r t i o n has been r e c e n t l y i n v e s t i g a t e d by a n a l y z i n g t h e s t r u c t u r e o f t h e end groups o f b o t h s y n d i o t a c t i c and i s o t a c t i c polypropylene. S e l e c t i v e l y 13c e n r i c h e d polymers

14.

Z A M B E L L I ET AI

Stereospecific Polymerization of a-Olefins

225

have been p r e p a r e d i n t h e presence o f d i f f e r e n t c a t a l y t i c s y s t e m s a n d a n a l y z e d b y 3 c NMR(8)· The 1 3 c NMR s p e c t r u m o f a s a m p l e o f s y n d i o t a c t i c poly-3-13Cp r o p y l e n e ( s a m p l e 1) p r e p a r e d i n t h e p r e s e n c e o f t h e c a t a l y t i c s y s t e m , VC14-A1(CH3)2^1 i s r e p o r t e d i n F i g ure 1 . 1 . T h e r e s o n a n c e s a t 20.69, 2 0 . 8 7 , 21.54» a n d 2 1 . 7 4 PPm f r o m HMDS(9) h a v e b e e n a t t r i b u t e d t o t h e presence o f e n r i c h e d m e t h y l s o f i s o b u t y l end groups w h i l e t h e r e s o n a n c e a t 1 2 . 3 7 ppm h a s b e e n a t t r i b u t e d t o 3Q e n r i c h e d m e t h y l s o f η-propyl e n d g r o u p s . The l a s t resonance i s also observed i n t h e spectrum o f s y n d i o t a c t i c p o l y - 3 ~ 3c-prοpy1ene p r e p a r e d i n t h e p r e s e n c e o f t h e c a t a l y s t , Y C I 4 - A I (C2H <) 2 C I ( s a m p l e 2, F i g u r e 1.2) w h i l e t h o s e b e t w e e n 2 0 . 6 9 a n d 2 1 . 7 4 P P a r e n o t . These i n t e r p r e t a t i o n s have been based on t h e c o n s i d e r a t i o n t h a t most polymer c h a i n s a r e bonded t o t h e m e t a l atom o f t h e c a t a l y t i c c o m p l e x e s o r t o aluminum atoms (after chain transfer processes). Hydrolysis o f this b o n d a f f o r d s CH3 e n r i c h e d η-propyl g r o u p s . T h e r e f o r e , the i n s e r t i o n o f t h e l a s t p o l y m e r i z e d u n i t o f each macromolecule i s secondary. On t h e o t h e r h a n d , t h e i n s e r t i o n o f t h e f i r s t monomer u n i t ( i n i t i a t i o n ) o n a m e t a l - m e t h y l bond g i v e s e n r i c h e d i s o b u t y l g r o u p s . As a consequence t h e i n i t i a t i o n i s p r i m a r y . The i n i t i a t i o n o n a m e t a l - e t h y l bond (sample 2) s h o u l d a f f o r d a 2,-13c-2-methylbutyl group. I n t h e l a s t case t h e r e s onances o f t h e e n r i c h e d methyls o v e r l a p w i t h t h e r e s o n a n c e s o f t h e i n n e r monomer u n i t s ( F i g u r e 1 . 2 ) . T h e s e r e s u l t s agree w i t h t h e h y p o t h e s i s t h a t t h e i n s e r t i o n of p r o p y l e n e i n s y n d i o t a c t i c p o l y m e r i z a t i o n s i s a f i r s t o r d e r Markovian p r o c e s s . The p r o b a b i l i t y f o r primary i n s e r t i o n i s higher than f o rsecondary i n s e r t i o n when i t o c c u r s o n a m e t a l - p r i m a r y c a r b o n b o n d . I t s h o u l d a l s o be noted t h a t t h e p r o b a b i l i t y o f sec o n d a r y i n s e r t i o n b e c o m e s h i g h e r when o c c u r r i n g o n a m e t a l - s e c o n d a r y c a r b o n bond(7 10)· T h e i n t e r e s t e d reader i s r e f e r r e d t o t h e quoted papers f o r a d e t a i l e d discussion. I n t h e s p e c t r u m o f i s o t a c t i c p o l y - 3 - 3cp r o p y l e n e p r e p a r e d w i t h t h e c a t a l y s t , STiCl3-Al(CH3)3 ( s a m p l e 3, F i g u r e 1.4) o n l y e n r i c h e d i s o b u t y l e n d groups a r e d e t e c t e d . T h e r e f o r e i n t h i s case t h ei n s e r t i o n o f t h e monomer i s a l w a y s p r i m a r y . 1

1

m

t

Mechanism o f t h e S t e r i c

Control

Stereoregular polymerization requires that the faces o f t h e p r o c h i r a l monomer m u s t h a v e a d i f f e r e n t r e a c t i v i t y toward one g i v e n c h i r a l r e a c t i v e s i t e . By u s i n g ^3ο NMR t o e x a m i n e t h e s t e r e o c h e m i c a l s e q u e n c e s o f t h e c o n f i g u r a t i o n s o f t h e monomer u n i t s o f p o l y p r o p y l -

NMR

226

AND MACROMOLECULES

ene and e t h y l e n e - p r o p y l e n e c o p o l y m e r s p r e p a r e d i n t h e presence o f d i f f e r e n t c a t a l y t i c systems, i t has been possible to recognize that f o r isotactic polymerization the s t e r i c c o n t r o l a r i s e s from t h e c h i r a l s t r u c t u r e o f the a c t i v e s i t e s . Instead, f o rs y n d i o t a c t i c polymeriz a t i o n , t h e s t e r i c c o n t r o l a r i s e s from t h e c h i r a l c a r bon o f t h e l a s t u n i t o f t h e g r o w i n g c h a i n e n d ( 7 , 1 1 ) . F u r t h e r i n s i g h t i n t o t h e mechanism o f t h e s t e r i c cont r o l has been achieved by l o o k i n g a t t h e s t e r e o c h e m i c a l p l a c e m e n t o f t h e e n r i c h e d c a r b o n on end g r o u p s formed by i n i t i a t i n g i s o t a c t i c p o l y m e r i z a t i o n i n t h e presence of enriched organometallic cocatalysts (12). The m e t h y l c a r b o n s o f t h e i s o b u t y l e n d g r o u p s o f p o l y p r o p y l e n e a r e d i a s t e r e o t o p i c ( 1 3 ) and t h e c h e m i c a l s h i f t depends on t h e s t e r e o c h e m i c a l p l a c e m e n t w i t h r e s p e c t t o t h e m e t h y l g r o u p s o f t h e n e i g h b o r i n g monomer u n i t s ( 4 , 1 3 ) . A t 2 2 . 6 3 MHz, f o u r d i f f e r e n t *C r e s o nances a r e detected f o r these methyl groups depending on t h e s p a t i a l r e l a t i o n s h i p s w i t h t h e m e t h y l g r o u p s o f t h e f o l l o w i n g two monomer u n i t s . These f o u r r e s o nances a r e observed i n t h e spectrum o f t h e a t a c t i c poly-3- p r o p y l e n e ( s a m p l e 4 ) shown i n F i g u r e 1 . 3 · The s p e c t r a g i v e n i n F i g u r e 2 a r e f r o m two s a m p l e s o f i s o t a c t i c polypropylene prepared, r e s p e c t i v e l y , i n the p r e s e n c e o f é T i C l 3 - A l ( C H 3 ) 3 ( s a m p l e 5) a n d i T i C l j Al( ^ΟΗ2θΗ3)3 (sample 6 ) . I n t h e spectrum o f sample 5 ( F i g u r e 2.5) o n l y two r e s o n a n c e s a r e d e t e c t e d f o r t h e e n r i c h e d methyl groups a r i s i n g from t h e primary i n s e r t i o n o f p r o p y l e n e o n t h e m e t a l - ^01-13 b o n d o f t h e a c t i v e sites. By c o m p a r i s o n w i t h m o d e l c o m p o u n d s ( 8 ) , t h e resonance a t 2 1 . 7 6 P P has been a t t r i b u t e d t o t h e 3 Q H 3 S having i s o t a c t i c placements with respect t o t h e m e t h y l s u b s t i t u e n t s o f t h e s u c c e e d i n g monomer u n i t s and t h e r e s o n a n c e a t 20.60 ppm i s a s s i g n e d t o t h e 1 3 c H 3 * s h a v i n g s y n d i o t a c t i c placements(14). 1

1 J

m

1

,

1 3

G G C I I I -C-C-C-C-C-C-C

G

C I I -C-C-C-C-C-C-C "h

S i n c e t h e s e r e s o n a n c e s i n F i g u r e 2.5 h a v e t h e same r e l a t i v e i n t e n s i t i e s , the i n s e r t i o n o f the f i r s t pro p y l e n e u n i t i s s t e r e o i r r e g u l a r . The s u b s e q u e n t i n s e r t i o n s (on m e t a l - i s o b u t y l and on m e t a l - 2 , 4 - d i m e t h y l p e n t y l groups) are h i g h l y s t e r e o s p e c i f i c . The r e s o nances o f t h e enriched methylene carbons o f 2-methylb u t y l groups, r e s u l t i n g a f t e r i n i t i a t i o n o f propylene polymerization i n the presence o f 6TiCl3-Al( 3cH2CH3)3, are o b s e r v a b l e i n t h e spectrum o f sample 6 ( F i g u r e 2 . 6 ) 1

14.

Z A M B E L L I ET A L .

22 20 18 1*6 Î4 12

Stereospecific Polymerization of a-Olefins

22 20 18 16 î*4 Î2

227

F i g u r e 1. C NMR s p e c t r a ( 2 2 , 6 3 MHz) o f s a m p l e s 1-4 (9) ( m e t h y l r e g i o n ) . Reproduced w i t h p e r m i s s i o n o f t h e a u t h o r s f r o m R e f . 8.

Lii)

zo

io

ppm

Figure 2. C NMR s p e c t r a ( 2 2 , 6 3 MHz) o f s a m p l e s 5-7 ( 9 ) . R e p r o duced w i t h p e r m i s s i o n o f the a u t h o r s from R e f . 1 2 .

228

NMR A N D MACROMOLECULES

s o n a n c e a t 27.72 Ls t w i c e a s i n $2 ppm ( ^Cîi2 i n i s o t a c t i c p l a c e m e n t s ) (12). R e p r o d u c e d i n F i g u r e 2.7 i s t h e s p e c t r u m o f i s o t a c t i c p o l y - l - b u t e n e ( s a m p l e 7) p r e p a r e d i n t h e presence o fS T i G ^ - A l ( 0 E ^ ) j . H e r e t h e 2-methylb u t y l e n d g r o u p s a r e e n r i c h e d o n t h e 2* m e t h y l g r o u p . Two r e s o n a n c e s a t 18.I3 a n d 17.84 ppm a r e d e t e c t e d f o r the ^GH^'s i n d i f f e r e n t s t e r e o c h e m i c a l placements(8). T h e y h a v e t h e same i n t e n s i t y . As d i s c u s s e d i n d e t a i l i n r e f e r e n c e s 12 a n d 1 4 , t h e s e d a t a c a n b e e x p l a i n e d b y a s s u m i n g t h a t i s o t a c t i c c o n t r o l o f α-olefin p o l y m e r i z a t i o n i s due t o t h e p r e s e n c e o f c h i r a l a c t i v e s i t e s . However t h e e x t e n t o f s t e r i c c o n t r o l i n c r e a s e s w i t h an i n c r e a s i n g s i z e o f t h e a l k y l group (methyl < e t h y l < i s o - b u t y l ) o n w h i c h t h e i n s e r t i o n o f t h e monomer u n i t occurs. Work i s i n p r o g r e s s c o n c e r n i n g t h e i n s e r t i o n o f propylene between m e t a l - p h e n y l bonds. We c a n a n t i c i p a t e t h a t t h i s i n s e r t i o n o f p r o p y l e n e w i l l be h i g h l y i s o t a c t i c specific(15). x

l j

Stereoselective

Polymerization o fGhiral

QC-Qlefins

As d i s c o v e r e d b y N a t t a , P i n o a n d coworkers(16,17), i s o t a c t i c polymerization o f c h i r a l oc-olefins i sstereo s e l e c t i v e (e.g., i s o t a c t i c poly-(R,S)-3-methyl-l-pentene c o n s i s t s o f enantiomeric macromolecules which c a n be p a r t i a l l y r e s o l v e d . ) I f one c o n s i d e r s t h a t t h e i s o t a c t i c s t e r i c c o n t r o l a r i s e s from t h e enantiosel e c t i v i t y o f t h ec h i r a l c a t a l y t i c s i t e s toward t h e e n a n t i o t o p i c c a r b o n o f t h e monomer(7»11)» t h e n s t e r e o s e l e c t i v i t y s i m p l y means t h a t t h e i n s e r t i o n i s d i a stereos elective. I n o t h e r words, t h e d i a s t e r e o t o p i c f a c e s o f t h e monomer m u s t h a v e a d i f f e r e n t r e a c t i v i t y i n t h e i n s e r t i o n ( F i g u r e 3). F i g u r e 4 s h o w s t h e NMR s p e c t r u m o f i s o t a c t i c p o l y - ( R , S ) - 3 - m e t h y l - l - p e n t e n e o b t a i n e d i n t h e p r e s e n c e o f ST1CI3-AI( ^CKj)j (sample 8 ) . The r e s o n a n c e s a t $·°9 , 1J ppm a r e d u e t o t h e Qllj s o f 2 - C-2,3-dim e t h y l - p e n t y l end groups formed i n t h e i n i t i a t i o n

13.24, 13*57»

15.27

1J

1

a

n

d

9

step(l8). Mt-

1 3

GH

3

+ CH =GH-CH(GH )-C H 2

3

2

>.

5

Mt-GH -GH( 2

1 3

GH )-CH(CH )-C H ^ 3

3

2

13 A c c o r d i n g t o r e f e r e n c e 18, t h es t e r i c placement o f C methyl group w i t h r e s p e c t t o t h e3* methyl group (see F i g u r e 3) c a n b e e v a l u a t e d f r o m t h e NMR c h e m i c a l

Ζ A M BELLI E T A L .

Stereospecific Polymerization of a-Olefins

^ r-*C—C •--jp—C--c U

S ' R fac H\ H\

W

X

/JC—c—c X

H

H

x

ρ—a—c Η-' Ή

C

Figure 3. D i a s t e r e a m e r i c end groups r e s u l t from t h e a t t a c k o f the d i a stereotopic faces o f 3 -methyl-l~pentene. For the conf i g u r a t i o n a l n o t a t i o n see R e f . 1 8 . Reproduced w i t h p e r m i s s i o n o f t h e a u t h o r s f r o m R e f . 18

PPM

13 Figure 4. C NMR s p e c t r u m ( 2 2 , 6 3 MHz) o f s a m p l e 8 (9). Reproduced w i t h p e r m i s s i o n o f the authors from R e f . 1 8 .

230

NMR AND MACROMOLECULES

shift. lj

The r e s o n a n c e s a t

I5.O9

and

15.27

arise

from

,

^ C K 3 s i n an e r y t h r o r e l a t i o n s h i p (S'R u n i t on bottom o f F i g u r e 3) w i t h r e s p e c t to t h e 3* m e t h y l groups and t h o s e a t 13.24 a n d 1 3 « 5 7 P p m c o r r e s p o n d t o t h e a n a l o g o u s t h r e o r e l a t i o n s h i p ( R Ή u n i t a t t h e top o f F i g u r e 3)· In t h e spectrum o f F i g u r e 4 , one can o b s e r v e t h a t t h e i n t e n s i t y o f the r e s o n a n c e s o f t h e e r y t h r o - i ^ C H V is ι roughly, twice that o f those o f t h e threo CH3~*s. The placement o f the ^CKj s on t h e end groups i s r e l a t e d to t h e f a c e o f the monomer which r e a c t e d as shown i n F i g u r e 3· 'The c o n c l u s i o n r e a c h e d i s t h a t the more r e a c t i v e f a c e s o f the monomer a r e t h o s e which, a f t e r c o o r d i n a t i o n t o a t r a n s i t i o n m e t a l atom, g i v e a new asym m e t r i c c a r b o n h a v i n g t h e same a b s o l u t e c o n f i g u r a t i o n as t h a t o f t h e s u b s t i t u e n t (the f r o n t f a c e o f F i g u r e 3). Conclusion s

lj5

l

9

The s t e r e o s p e c i f i c p o l y m e r i z a t i o n o f Q£-olefins i s one o f the b e s t examples which i l l u s t r a t e t h e p o s s i b l e ap p l i c a t i o n s o f h i g h r e s o l u t i o n NMR t o t h e d e t e r m i n a t i o n o f r e a c t i o n mechanisms. As a m a t t e r o f f a c t , t h e s t e r e o r e g u l a r s t r u c t u r e o f t h e r e a c t i o n p r o d u c t s and the s e n s i t i v i t y o f Q c h e m i c a l s h i f t s t o s t e r e o c h e m i c a l environments make i t p o s s i b l e t o o b t a i n c o n s i d e r a b l e and important i n f o r m a t i o n about v e r y s u b t l e de t a i l s o f the r e a c t i o n mechanism. T h i s i s e s p e c i a l l y t r u e when NMR i s used i n c o n j u n c t i o n w i t h i s o t o p i c substitution. 1

3

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

Bovey, F . A. "High Resolution NMR of Macromole cules"; Academic Press: New York, 1 9 7 2 . Natta, G . ; F a r i n a , M.; Peraldo, M. Chim. Ind. (Milan) 1960, 42, 2 5 5 . Mijazawa, T.; Ideguchi, Τ. J. Polymer S c i . 1 9 6 3 , B1, 3 8 9 . Zambelli, A.; Giongo, M. G.; Natta, G. Makromol. Chem. 1968, 1 1 2 , 1 8 3 . Zambelli, A.; T o s i , C.; Sacchi, M. C. Macromolecules 1 9 7 2 , 5, 6 4 9 . Asakura, Τ . ; Ando, I.; Nishioka, A.; D o i , Y.; Keii, Τ. Macromol. Chem. 1977, 178, 7 9 1 . Zambelli, A.; Bajo, G . ; Rigamonti, E.; Makromol. Chem. 1978, 179, 1 2 4 9 . Zambelli, A.; L o c a t e l l i , P . ; Rigamonti, E . Macro molecules 1979, 1 2 , 1 5 6 . A l l o f the chemical s h i f t s reported i n t h i s paper are with respect to the HMDS s c a l e . Each o f the

14. ZAMBELLI ET AL.

10. 11. 12. 13. 14. 15. 16. 17. 18.

231 Stereospecific Polymerization of α-Olefins

polymers have been examined in 1 , 2 , 4 - t r i c h l o r o benzene s o l u t i o n at high temperature. The reader is r e f e r r e d to the references reported throughout this paper for s p e c i f i c experimental d e t a i l s . Zambelli, A.; A l l e g r a , G. Macromolecules 1980, 13, 42. Z a m b e l l i , A. in "NMR Basic P r i n c i p l e s and Progress" Springer: Heidelberg, 1971; V o l . 4, p. 101. Z a m b e l l i , A.; S a c c h i , M. C.; Locatelli, P . ; Zannoni, G. Macromolecules 1982, 15, 211. Carman, G. J.; Tarpley, A. R. Jr.; G o l d s t e i n , J . H . Macromolecules 1973, 6, 719. Z a m b e l l i , Α.; Locatelli, P . ; Bajo, G. Macromole cules 1979, 12, 154. Unpublished data from our l a b o r a t o r i e s . Natta, G.; Pino, P . ; Mazzanti, G.; C o r r a d i n i , P . ; G i a n n i n i , U. Rend. Acc. Naz. L i n c e i (VIII), 1955 19, 397. Pino, P.; Ciardelli, F.; Montagnoli, G. J. P o l y mer Sci. Part C, 1969, 16, 3256. Z a m b e l l i , A.; Ammendola, P . ; S a c c h i , M. C.; Locatelli, P.; Zannoni, G. Macromolecules 1983, 16, 341.

RECEIVED

October 24, 1983

15 Structural Characterization of Naturally Occurring cis-Polyisoprenes YASUYUKI TANAKA Department of Material Systems Engineering, Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan The chemical structure of naturally occurring cis polyisoprenes was determined by 13C NMR spectroscopy using acyclic terpenes and polyprenols as model compounds. The arrangement of the isoprene units along the polymer chain was estimated to be i n the order: dimethylallyl terminal unit, three trans units, a long block of cis units, and cis isoprenyl terminal unit. This result demonstrates that the biosynthesis of cis-polyisoprenes i n higher plants starts from trans,trans,trans-geranylgeranyl pyro phosphate. Polyisoprenes are synthesized by thousands of plant species c o v e r i n g most g e n e r i c f a m i l i e s . Usually, the polyisoprenes are rubbery and have p r e d o m i n a n t l y a cis-1,4 structure. Only a r e l a t i v e l y f e w p l a n t s p e c i e s p r o d u c e trans - 1 , 4 p o l y i s o p r e n e s . In e a r l y studies the presence o f the 3,4 structure i n natural rubber was e s t i m a t e d a t 1 - 2 % b y i n f r a r e d a n a l y s i s . T h i s c o n c l u s i o n was challenged after studies utilizing 1 H NMR s p e c t r o s c o p y , which r e p o r t e d t h a t n a t u r a l r u b b e r ( f r o m Hevea brasiliensis) and g u t t a percha a r e a t l e a s t 9 9 . 0 - 9 9 . 5 % cis-1,4 a n d trans-1,4 polyiso prenes . The 3 , 4 i s o m e r i c s t r u c t u r e was p r e s e n t i n l e s s t h a n t h e minimum d e t e c t a b l e amount ( 1 , 2 ) . Guayule rubber from Parthenium argentatum w a s f o u n d t o h a v e a s t r u c t u r e n e a r l y 1 0 0 % cis-1,4 and i d e n t i c a l t o t h a t o f n a t u r a l r u b b e r a s d e t e r m i n e d u s i n g 3 0 0 MHz 1 H NMR spectroscopy (3). The d i f f e r e n c e s i n physical properties between n a t u r a l r u b b e r a n d s y n t h e t i c cis-1,4 p o l y i s o p r e n e s have been a s c r i b e d i n p a r t to the s t r u c t u r a l p u r i t y o f the repeating units. S y n t h e t i c p o l y i s o p r e n e s prepared w i t h A l - T i c a t a l y s t s were f o u n d t o b e 99% cis-1,4, t h e r e m a i n d e r b e i n g 0 - 0 . 7 % trans-1,Λ and 0.3-1.0% 3,4. The p r e s e n c e o f b r a n c h e s and c r o s s - l i n k i n g h a s been considered to be another characteristic of natural rubber. H o w e v e r , l i t t l e s t r u c t u r a l i n f o r m a t i o n i s known a b o u t t h e t e r m i n a l u n i t s , b r a n c h i n g , and s o - c a l l e d abnormal groups i n n a t u r a l r u b b e r and how t h e y r e l a t e t o s p e c i f i c p h y s i c a l p r o p e r t i e s .

0097 6156/84/0247 0233S06.00/0 © 1984 A m e r i c a n C h e m i c a l S n e i e t v

NMR

234

AND MACROMOLECULES

The b i o s y n t h e s i s o f n a t u r a l r u b b e r h a s been s t u d i e d f r o m t h e viewpoint o f an e l u c i d a t i o n o f i n i t i a t i o n and propagation mechanisms m a i n l y by t r a c e r t e c h n i q u e s . The s t e p s i n the formation of isopentenyl pyrophosphate from acetyl-coA v i a m e v a l o n a t e a r e now w e l l e s t a b l i s h e d i n t h e in v i t r o s y n t h e s i s o f rubber. I t has a l s o been c o n f i r m e d t h a t c h a i n e x t e n s i o n o c c u r s on t h e s u r f a c e o f e x i s t i n g r u b b e r p a r t i c l e s by s u c c e s s i v e a d d i t i o n s o f i s o p e n t e n y l p y r o p h o s p h a t e t o b u i l d up c h a i n s o f 5000-7000 isoprene u n i t s (4,5). The i n i t i a t i o n s t e p o f r u b b e r f o r m a t i o n , however, r e m a i n s unknown due t o t h e l a c k o f d e t a i l e d i n f o r m a t i o n concerning the d i r e c t precursor of the chain extension. This paper d e s c r i b e s t h e s t r u c t u r a l a n a l y s i s o f n a t u r a l l y o c c u r r i n g c i s -1,4 polyisoprenes using 13C NMR spectroscopy. F i r s t , t h e s t r u c t u r a l c h a r a c t e r i z a t i o n o f p o l y p r e n o l s , which a r e l i n e a r i s o p r e n o i d compounds c o n t a i n i n g 30 t o 100 c a r b o n s , was c a r r i e d o u t on t h e b a s i s o f i n f o r m a t i o n o b t a i n e d f r o m a c y c l i c terpenes h a v i n g v a r i o u s c i s and trans i s o p r e n e u n i t s as model compounds. This method was a l s o a p p l i e d t o t h e s t r u c t u r a l analysis of polyisoprenes. The e l u c i d a t i o n o f t h e s t r u c t u r e o f t h e end g r o u p s and t h e a r r a n g e m e n t o f i s o p r e n e u n i t s p r o v i d e s i n f o r m a t i o n on t h e mechanism o f t h e b i o s y n t h e s i s o f p o l y p r e n y l compounds i n n a t u r e . 13C

NMR A n a l y s i s o f Model Compounds

By a n a l o g y w i t h t h e mechanism o f t h e b i o s y n t h e s i s o f f a r n e s y l p y r o p h o s p h a t e and all-trans t e r p e n o i d s , i t was deduced t h a t r u b b e r formation p r o c e e d s by t h e s u c c e s s i v e a d d i t i o n o f i s o p e n t e n y l pyrophosphate t o d i m e t h y l a l l y l pyrophosphate ( 4 ) . f ^C=CHCH 0PP + η CH =CCH CH 0ΡΡ Un ^ d d d d CH

3

3 N

»

o

CH CH

' 3

C=CHCH -(CH C=CHCH )-0ΡΡ d d d η

( I ) (0ΡΡ: p y r o p h o s p h a t e )

According to this mechanism, n a t u r a l r u b b e r c h a i n s a r e e x p e c t e d t o have one d i m e t h y l a l l y l t e r m i n a l u n i t and one i s o p r e n y l pyrophosphate t e r m i n a l u n i t ; t h e l a t t e r may g i v e r i s e t o a h y d r o x y l group by h y d r o l y s i s . From t h i s p o i n t o f v i e w , a c y c l i c terpenes i n t h e g e n e r a l i z e d s t r u c t u r e ( I I ) may be a p p r o p r i a t e models f o r t h e s t r u c t u r a l c h a r a c t e r i z a t i o n o f n a t u r a l p o l y i s o p r e n e s by 13C NMR s p e c t r o s c o p y . ψ3 f3 ^C=CHCH -(CH C=CHCH ) CH C=CHCH 0H Un d d d d d 3 n-2 ω-terminal internal a-terminal CH

n

n

n

o

(II)

15.

TANAKA

Naturally Occurring

Table I . Model compounds f o r 13C NMR assignment compounds ( 8 ) . n=2 ω-Τ-ΟΗ (geraniol) ω-C-OH (nerol)

235

cis-Polyisoprenes

of p o l y i s o p r e n o i d

n=3

n=4

ω-Τ-Τ-ΟΗ (farnesol) ω-1-C-OH ω-C-T-OH ω-C-C-OH

ω-Τ-Τ-Τ-ΟΗ (geranylgeraniol) ω-C-T-T-OH ω-Τ-e-T-OH ω-C-C-T-OH ω-Τ-Τ-C-OH ω-C-T-C-OH ω-Τ-C-C-OH ω-C-C-C-OH

The a b b r e v i a t i o n ω, τ, and C correspond to ω-terminal» trans, cis u n i t s , r e s p e c t i v e l y .

and

Geraniol and n e r o l (n=2), farnesol isomers (n=3), and g e r a n y l g e r a n i o l isomers (n=4) having various combinations of trans and cis isoprene u n i t s were used as model compounds, as l i s t e d i n Table I . Here, the geometric isomers of f a r n e s o l were i s o l a t e d from s y n t h e t i c f a r n e s o l , which i s a mixture of four isomers, by l i q u i d chromatography (6) . In a s i m i l a r way, the g e r a n y l g e r a n i o l isomers were separated from a mixture prepared by the isomeriza tion of n a t u r a l l y o c c u r r i n g trans,trans,trans-geranylgeraniol under UV i r r a d i a t i o n ( 7). The a l i p h a t i c carbon s i g n a l s , observed at 50.1 MHz i n these compounds, were assigned through a c o n s i d e r a t i o n of chemical s h i f t c o r r e l a t i o n s among these compounds as well as by the usual 13C NMR techniques (8) . In these compounds, the methyl carbon atoms i n the i n t e r n a l trans and cis u n i t s resonated at 16.0 and 23.4 ppm, r e s p e c t i v e l y , while those i n the ω-terminal u n i t resonated at 17.6 and 25.6 ppm. The C - l methylene carbon atoms i n the cis and trans units showed s i g n a l s p l i t t i n g s r e f l e c t i n g the geometric isomerism of the u n i t l i n k e d to the C - l methylene carbon atom, that i s , r e f l e c t i n g the dyad sequences of cis and trans u n i t s . Here, the carbon atoms are designated as f o l l o w s : 5 C -C-C=C-C12 3 4 The C - l methylene carbon atoms i n trans u n i t s resonated at 39 .6 ppm (trans-trans( α ) ) , 39.7-39.8 ppm ( ω - t r a n s and trans trans), 39.9 ppm (cis-trans(a)) and 40.0 ppm (cis-trans), while those i n cis u n i t s resonated at 32.0-32.1 ppm (trans-cis , ω-cis, and trans-cis( a ) ) and 32.3-32.4 ppm (cis-cis and cis-cis(a)). In f

NMR AND MACROMOLECULES

236

these c o r r e l a t i o n s i t was observed that the ω-terminal u n i t has the same s h i e l d i n g e f f e c t on the subsequent C-l methylene carbon atom as the i n t e r n a l trans unit. The C-4 methylene carbons i n the trans and cis α-terminal u n i t s gave s i g n a l s a t 59.4 and 59.1 ppm, r e s p e c t i v e l y . The geometric isomerism of the u n i t l i n k e d to the ω-terminal u n i t could be determined by the observed chemical s h i f t o f the ω C-2 carbon atom; the ω - t r a n s linkage showed about 0.2-0.4 ppm u p f i e l d s h i f t compared with that i n the ω-cis linkage. The assignment o f the s i g n a l s c h a r a c t e r i s t i c o f the terminal u n i t s and the alignment of isoprene u n i t s i n the geranylgeraniol isomers are l i s t e d i n Table I I . S t ru c t u r a l C har ac t e r i ζ a t i on o f Ρolypggnols Polyisoprenoid alcohols c o n s i s t i n g o f 9 to 20 isoprene u n i t s have a widespread occurrence as i n d i c a t e d by t h e i r presence i n the leaves of higher p l a n t s , mammalian t i s s u e s , and microorganisms. Most of the polyprenols i s o l a t e d from higher plants c o n s i s t o f trans and cis isoprene u n i t s with the exception o f s o l a n e s o l , which i s composed o f all-trans isoprene u n i t s . The arrangement o f trans and cis u n i t s i n these polyprenols has been determined from a consideration o f the mechanism o f the formation o f b e t u l a prenols, C(30)-C(45), isolated from wood tissue of Betula verrucosa (9) . However, up to the present there has been no d i r e c t evidence to prove the l o c a t i o n o f the i n t e r n a l trans and cis units. The arrangement o f the trans and cis units i n typical polyprenols was determined according to information obtained from the 13C NMR study o f a c y c l i c terpenes mentioned above. Polyprenol-11 C(55), i s o l a t e d from the leaves of Ficus elastica, was found to contain the ω-terminal u n i t , three i n t e r n a l trans units, six i n t e r n a l cis u n i t s , and a cis α-terminal u n i t from 1H NMR observations. Polyprenol-12 C(60), i s o l a t e d from Ficus elastica and silkworm feces, showed a s i m i l a r composition except that there were seven i n t e r n a l cis u n i t s . As shown i n F i g . 1, the s i g n a l due to the C-l methylene carbon atom i n cis u n i t s i s s p l i t i n t o two peaks a t 32.37 and 32.12 ppm, which were assigned to the cis u n i t s i n the cis-cis (cis-cis{a))

and trans-cis

(ω-cis)

linkages, respectively.

On the

other hand, the C-l methylene carbon atom i n trans u n i t s showed a s i n g l e peak a t 39.86 ppm, corresponding to the trans u n i t i n the trans-trans

( ω -trans

)

linkage.

The

presence

o f the

^-trans

linkage was confirmed by the c h a r a c t e r i s t i c C-2 o l e f i n i c carbon s i g n a l o f the ω-terminal u n i t a t 131.20 ppm. The CH OH s i g n a l at 59.00 ppm i n d i c a t e d that the α-terminal u n i t has the cis configuration. The observed i n t e n s i t y r a t i o of the trans-trans (ω-trans)

0.9

j

trans-cis,

and cis -cis

(cis-eis(ct))

signals

was 2.8 :

: 6.3 f o r polyprenol-11 and 2.7 : 1.2 : 7.1 f o r polyprenol-12.

131.47

131.51

131.49

131.55

131 .22

131 .30

131,.24

131,.29

ωΤ

59.47 59.43 59.39 59.43

T

59.08 59.06 59.06 59.08

C

2

α C-4 (-CH 0H)

40,.01

40,,01

CT a

39,.72 39,.76

39,.78

TT ωΤ

TT a

39,.58 39 .76 39,.60

2

C - l (-CH ~)

39,.89 39 .88 39,.78

CT

trans

2

CC CC a

TC TC

a

C - l (~CH ~)

units,

32,.34 32.03 32.01 32.03

32..30 32.04 32..29 31.97 32.03 32.07

cis

isomers ( 8 ) .

*The a b b r e v i a t i o n s ω, Τ, and C correspond to ω-terminal, trans, and cis respectively. The s u b s c r i p t α means the u n i t with hydroxyl group.

CCT-OH TCT-OH CTT-OH TTT-OH

CCC-OH TCC-OH CTC-OH TTC-OH

coC

ω C-2 (=CH-)

T a b l e I I . C h e m i c a l s h i f t s o f 13C NMR s i g n a l s i n g e r a n y l g e r a n i o l

NMR A N DMACROMOLECULES

238 These s i g n a l s values. On t h e polyprenols-11 i n t e r n a l trans units aligned

showed T^ v a l u e o f l e s s t h a n 1 s and n e a r l y f u l l NOE b a s i s o f these findings, i t i s concluded that and -12 a r e composed o f t h e ω-terminal, t h r e e , s i x o r s e v e n i n t e r n a l c i s , and c i s α-terminal i n t h a t o r d e r a s shown b e l o w :

H H C H C H H C H ι 3ι 3j ι 3ι ι /C=CCH -(CH C=ÇCH_)-(CH C=CCH )-CH C=CCH OH (III) CH 2 2 ^ 2 2 ,7 2 ω-terminal trans cis c i s α-terminal r H

CH

n u

0

0

0

2

3

3

2

6

T h i s a l i g n m e n t o f t h e trans and c i s u n i t s c l e a r l y i n d i c a t e s t h a t t h e s e p o l y p r e n o l s a r e s y n t h e s i z e d in vivo by t h e c i s a d d i t i o n o f isopentenyl pyrophosphate to trans,trans,tpans-geranylgeranyl pyrophosphate ( 1 0 ) . S i m i l a r l y , p o l y p r e n o l s - 1 5 t o -20, i s o l a t e d from t h e l e a v e s o f Ginkgo v i l o b a , showed t h e same s p l i t t i n g o f C - l m e t h y l e n e c a r b o n s i g n a l s , as shown i n F i g . 2. From t h e i n t e n s i t y r a t i o s o f t h e s e s i g n a l s i t was f o u n d t h a t t h e s e p o l y p r e n o l s c o n t a i n e d two i n t e r n a l trans and 11 t o 16 i n t e r n a l c i s u n i t s a l i g n e d as g i v e n i n ( I V ) (11). H H C H C H CH I Sj 3,| C=CCH_-(CH C=CCH_)-(CH_C=CCH )CH 2 2 2 2 ^ 2 _ n u

y

3

ω-terminal

Q

H C H 31

Q

à

2

n

trans

cis

1

-CH_C=CCH 0H ^ 2 2 Q

1

6

(IV)

cis a-terminal

The p o l y p r e n o l s composed o f trans and c i s i s o p r e n e u n i t s isolated so f a r have been classified into two t y p e s : t h e f i c a p r e n o l t y p e and t h e b e t u l a p r e n o l - t y p e , c o n t a i n i n g t h r e e and two i n t e r n a l trans u n i t s , r e s p e c t i v e l y ( 1 2 ) . The f i n d i n g s d e s c r i b e d above s u g g e s t t h a t trans trans trans-geranylgerany1 pyrophosphate o r t r a n s , t r a n s - f a r n e s y l p y r o p h o s p h a t e a c t s as an i n i t i a t o r o f t h e biosynthesis of polyprenols. f

t

Structure of Naturally Occurring

Cig-Polyisoprenes

The c h e m i c a l shifts o f the c h a r a c t e r i s t i c carbon s i g n a l s i n acyclic t e r p e n e s , p o l y p r e n o l s , and c i s - t r a n s i s o m e r i z e d poly i s o p r e n e s a r e p l o t t e d i n F i g . 3. H e r e , t h e c h e m i c a l s h i f t s a r e c o r r e l a t e d u s i n g t h e ω C-5 m e t h y l c a r b o n s i g n a l a t 17.66 ppm as an i n t e r n a l standard (except f o r isomerized polyisoprenes) i n order t o compensate f o r t h e e f f e c t o f s o l u t i o n c o n c e n t r a t i o n . It is c l e a r that these chemical s h i f t s a r e independent o f the c h a i n l e n g t h o f t h e compounds and c a n be u s e d f o r t h e d e t e r m i n a t i o n o f t h e a r r a n g e m e n t o f i s o p r e n e u n i t s as w e l l as t h e t e r m i n a l u n i t s i n v a r i o u s i s o p r e n o i d compounds ( 8 ) . Cis-polyisoprene isolated from the leaves o f goldenrod (Solidago altissima) was s e p a r a t e d i n t o two f r a c t i o n s by GPC, S - l (Mn=76,000) and S-2 (Mn=120,000). The 13C NMR s p e c t r a o f b o t h

TANAKA

Naturally Occurring cis- Polyisoprenes

cis-cis ω -trans trans-trans

trans-cis

40.0

32

39.5

ppm(TMS)

F i g u r e 1. C - l m e t h y l e n e c a r b o n s i g n a l s i n p o l y p r e n o l - 1 1 f r o m Ficus e l a s t i c a . Reproduced w i t h p e r m i s s i o n from R e f . 1 0 . C o p y r i g h t 1979, The B i o c h e m i c a l S o c i e t y .

cis-cis

trans-trans ω -trans

40

39

trans-cls

33

32

ppm(TMS)

F i g u r e 2. C - l metylene carbon s i g n a l s i n p o l y p r e n o l - 1 8 f r o m Ginkgo viloba. Reproduced w i t h p e r m i s s i o n from R e f . 1 1 . C o p y r i g h t 1 9 8 3 , The B i o c h e m i c a l S o c i e t y .

240

NMR AND MACROMOLECULES

f r a c t i o n s showed f i v e s i g n a l s corresponding to the carbon atoms i n the cis isoprene u n i t . In the expanded spectrum of S - l , obtained by 34,730 accumulations, small s i g n a l s were observed, as shown i n F i g . 4. The s i g n a l at 59.00 ppm was assigned to the CH^OH carbon atom i n the cis α-terminal u n i t from the r e l a t i o n s h i p shown i n F i g . 3. S i m i l a r l y , the s i g n a l s at 17.67 and 16.00 ppm were ascribed to the C-5 methyl carbon atoms i n the ω -terminal and i n t e r n a l trans u n i t s , r e s p e c t i v e l y . The C - l methyl carbon s i g n a l due to the i n t e r n a l trans u n i t s was observed at 39.74 ppm, which i s c h a r a c t e r i s t i c of the trans u n i t s i n the trans-trans and ω-trans linkage. The C-2 o l e f i n i c carbon atom i n the ω-terminal u n i t resonated at 131.14 ppm, showing the presence of the ω-trans linkage. The absence of the s i g n a l c h a r a c t e r i s t i c of the cis-trans linkage, which i s presumed to resonate around 39.9 ppm, i n d i c a t e s that a l l the trans u n i t s are s i t u a t e d f o l l o w i n g the ω-terminal u n i t . Although the C - l methylene carbon s i g n a l due to the trans-cis linkage was not observed because of overlap with the strong s i g n a l from the cis-cis linkage, the f i n d i n g s shown above s t r o n g l y support the idea that a block of cis u n i t s i s l i n k e d to the ω-(trans) - sequence. S i m i l a r l y , small s i g n a l s due to the ω-terminal, czs α-terminal, and i n t e r n a l trans u n i t s were observed i n the spectrum of the sample S-2. The r e l a t i v e i n t e n s i t i e s of these s i g n a l s , together with the NOE and T^ values, are l i s t e d i n Table I I I . The i n t e n s i t y r a t i o from the s i g n a l s of the ω-terminal u n i t ( ω C-5) and α -terminal u n i t (α C-4) was 0.8-0.9 f o r both samples. The i n t e n s i t y r a t i o of the corresponding s i g n a l s i n polyprenol-11 was 0.84, i n which case the d i f f e r e n c e i s ascribed to a T^ of 7.5 versus 1.9 s f o r the ω C-5 and α C-4 carbon atom, r e s p e c t i v e l y , as well as to the NOE values. Therefore, i t seems resonable to assume that polymers S - l and S-2 contain the same amount of ω - and α-terminal u n i t s . The number of i n t e r n a l trans u n i t s determined from the i n t e n s i t y r a t i o of the trans C - l and α C-4 methylene carbon s i g n a l s was 3.4 and 3.6 f o r S - l and S-2, r e s p e c t i v e l y . On the other hand, 2.5 trans u n i t s were estimated f o r both samples from the i n t e n s i t y r a t i o of the trans C-5 and α C-4 carbon s i g n a l s . The former i s considered to be a more r e l i a b l e value i n view of the f a c t that the C-5 carbon atom shows smaller NOE and longer Τ values than the trans C-l methylene carbon atom i n polyprenols-11 and -12. This i n d i c a t e s that the polymers contain approximately three to four i n t e r n a l trans units. The number of the i n t e r n a l cis u n i t s determined from the i n t e n s i t y r a t i o of e i t h e r the cis C - l or C-4 methylene s i g n a l to the α C-4 methylene carbon s i g n a l , was approximately 1000 and 2200 f o r the polymers S - l and S-2, respectively. S i m i l a r NOE and shorter T^ values f o r these carbon atoms i n d i c a t e that an almost quantitative evaluation i s p o s s i b l e f o r the i n t e n s i t y r a t i o s between these methylene carbon s i g n a l s . The degree of polymeri z a t i o n determined from the α-terminal u n i t agrees with that from GPC f o r these samples, i . e . , 1100 f o r S - l and 1800 f o r S-2. This

Naturally Occurring cis- Polyisoprenes

TANAKA

Isoprene

ω C-2

Units

α C-4

C-5

C-1

u>C u?T

2 3

4

1

9

1

11 12 17 18 Polymer 132

1 3 1 60

59 40

39

32

26

25 24

2 3 18

ppm(TMS)

Figure 3. Chemical s h i f t s of the s i g n a l s c h a r a c t e r i s t i c of the arrangement of the isoprene u n i t s (8, 13).

cis

CIS -CH

-CH

2

cis 2

-CH

3

-CH

3

cis α -CH

-CH OH 2

3

Χ: X 8 , Y : X 2 5 6

LJJLX 60

50

40

30

X 2 , Y: X 1 2 8

20

ppm(TMS)

Figure 4. A l i p h a t i c carbon s i g n a l s i n c i s - p o l y i s o p r e n e from Solidago altissima observed with a pulse r e p e t i t i o n time of 7 s f o r 45° pulse (^denotes s i g n a l s due to i m p u r i t i e s ) . Reproduced with permission from Ref. 13. Copyright 1983, The American Chemical S o c i e t y .

242

NMR AND MACROMOLECULES

Table I I I . R e l a t i v e i n t e n s i t i e s o f the a l i p h a t i c carbon s i g n a l s (13) . ( * V a l u e s d e t e r m i n e d f o r p o l y p r e n o l - 1 1 as a model compound) Chemical

shift

A s s i |inment

S-2

S-l α trans cis cis cis ω trans

59.00 39.74 32.23 26.43 23.41 17.67 16.00

C-4 C-l C-l C-4 C-5 C-5 C-5

NOE

Relative intensity

(ppm)

2.62* 2.62* 2.71 2.78 2.49 2.22* 2.22*

1 3.6 2200 2200 2100 0.9 2.5

1 3.4 1000 1000 980 0.8 2.5

T

i (s) 1.9* 0.9* 0.7 0.8 1.7 7.5* 3.9*

i s important evidence t h a t these polymers a r e l i n e a r p o l y i s o p r e n e s h a v i n g ω- and α-terminal u n i t s . These facts clearly indicate that the cis-polyisoprene i s o l a t e d f r o m g o l d e n r o d h a s t h e a l i g n m e n t o f i s o p r e n e u n i t s as shown i n (V) ( 1 3 ) . H H C H C H H C H j 3ι 3ι ι 31 ι /C=CCH -(CH C=ÇCH )-(CH C=CCH )-CH C=CCH 0H 3 H m η ω-terminal trans cis cis a - t e r m i n a l CH C

H

2

2

2

2

2

2

2

(V)

(m=3-4 and n=1000-2200) A similar alignment o f i s o p r e n e u n i t s was o b s e r v e d f o r c i s - p o l y i s o p r e n e i s o l a t e d f r o m t h e l a t e x o f Ficus elastica. This is significant evidence clarifying t h e d e t a i l e d mechanism o f rubber formation i n higher p l a n t s . The p r e s e n c e o f t h e s e q u e n c e c o n s i s t i n g o f t h r e e t o f o u r trans u n i t s l i n k e d t o t h e ω-terminal u n i t demonstrates t h a t the primer o f c i s - p o l y i s o p r e n e i s a p r e n y l p y r o p h o s p h a t e p o s s e s s i n g an a l l - t r a n s c o n f i g u r a t i o n . Geranyl g e r a n i o l i s one o f t h e most common p r e n y l compounds i n n a t u r e , whereas g e r a n y l f a r n e s o l ( C ( 2 5 ) trans,trans,trans,trans) occurs only r a r e l y . Therefore, i t c a n be presumed t h a t t h e c h a i n e x t e n s i o n t o t h e cis p o l y m e r o c c u r s by t h e a d d i t i o n o f i s o p e n t e n y l p y r o - p h o s p h a t e t o all-trans g e r a n y l g e r a n y l p h r o p h o s p h a t e , a s shown i n F i g . 5. I t i s w o r t h n o t i n g t h a t t h i s s t r u c t u r e i s i d e n t i c a l t o t h a t o f f i c a p r e n o l - t y p e p o l y p r e n o l s i s o l a t e d mainly from t h e l e a f t i s s u e s o f a n g i o s p e r m s , e x c e p t f o r t h e number o f cis u n i t s . On the other hand, the low molecular weight fraction (Mn=10,000) o f n a t u r a l r u b b e r , w h i c h was o b t a i n e d by f r a c t i o n a t i o n o f Hevea r u b b e r purified by d e p r o t e i n i z a t i o n o f a c o m m e r c i a l l a t e x , showed s m a l l s i g n a l s c h a r a c t e r i s t i c o f t h e C - l m e t h y l e n e and C-5 m e t h y l c a r b o n atoms i n i n t e r n a l trans u n i t s a t 39.80 ppm and 16.02 ppm, r e s p e c t i v e l y , as shown i n F i g . 6. The s i g n a l s due t o t h e t e r m i n a l u n i t s were n o t d e t e c t e d a t 59.0 ppm and 17.7 ppm,

TAN A Κ A

243

Naturally Occurring cis- Polyisoprenes

IPP O-PP

c i s a d d . xn

trans add. x 3

DMAPP ι

ψ

/n^/^o-PP

*'

V

\v V

OPP

40-PP

3

ri-1|

trans

ω-end

cis

cis-a-end

n=1000 or 2200 F i g u r e 5. (13, 1 4 ) .

B i o s y n t h e s i s mechanism o f c i s - p o l y i s o p r e n e s

cis

cis

-CH

CH ι "k

C

2

cis

-CH -CH 2

3

3 3

W

C

H

bΟa

60

"

*

trans

trans-tran»

CH2

I " i 50

40

-CH

y 30

20

3

10

ppm(TMS)

F i g u r e 6. A l i p h a t i c c a r b o n s i g n a l s i n c i s - p o l y i s o p r e n e f r o m Hevea brasiliensis ( n a t u r a l r u b b e r ) (^denotes s i g n a l s due t o i m p u r i t i e s ) ( 1 4 ) .

N M R A N D MACROMOLECULES

244

whereas the s i g n a l s from epoxide group were observed a t 64.51 and 60.76 ppm. The trans C - l methylene carbon s i g n a l showed the chemical s h i f t c h a r a c t e r i s t i c of the trans -trans and ω-trans linkages and had a r e l a t i v e i n t e n s i t y of approximately 1/500 compared to the c i s C - l methylene carbon s i g n a l . Taking into account the degree o f p o l y m e r i z a t i o n of the sample, one expects about three trans u n i t s per molecule to occur as an i s o l a t e d sequence (14). These f a c t s suggest that natural rubber i s synthesized in vivo i n a s i m i l a r manner to goldenrod rubber and that the terminal units subsequently undergo some p a r t i c u l a r r e a c t i o n to form f u n c t i o n a l groups. One may speculate that the r e a c t i v e f u n c t i o n a l groups are r e s p o n s i b l e f o r the formation of branches and c r o s s - l i n k s , which are believed to occur i n s i g n i f i c a n t amounts i n n a t u r a l rubber. Very r e c e n t l y , Archer et al. found that a l l - t r a n s geranylgeranyl pyrophosphate can a c t as an i n i t i a t o r to synthesize new rubber molecules (15). This r e s u l t s t r o n g l y supports the suggested mechanism f o r the formation of n a t u r a l rubber as described above.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Golub, Μ. Α.; Fuqua, F. Α.; Bhacca, Ν. S. J . Am. Chem. Soc. 1962, 84, 498 (1962). Chen, H. Y. J . Polym. Sci. 1966, B4, 891. Campos-Lopez, E . ; Palacios, J . J . Polym. Sci. Polym. Lett. Ed. 1976, 14, 1561. Lynen, F . ; Henning, U. Angew Chem. 1960, 72, 820. Archer, B. L . ; Ayrey, G.; Cockbain, Ε. G.; McSweeney, G. P. Nature (London) 1961, 189, 663. Sato, H; Kageyu, A.; Miyashita, Κ.; Tanaka, Y. J. Chromatogr. 1982, 237, 178. Tanaka, Y. ; Sato, H.; Kageyu, A. Polym. Prep. Japan 1981, 30, 1834. Tanaka, Y.; Sato, H.; Kageyu, A. Polymer 1982, 23, 1087. Wellburn, A. R.; Hemming, F. W. Nature (London) 1966, 212, 1634. Tanaka, Y.; and Takagi, M. Biochem. J. 1979, 183, 163. Ibata, Κ.; Mizuno, M.; Takigawa, T.; Tanaka, Y. Biochem. J. 1983, 213, 305. Hemming, F. W. "Biochemistry of Lipids (Biochemistry Series 1)"; Goodwin, T. W., Ed; Butterworth: London, 1974; Vol. 4, p. 39-98. Tanaka, Y.; Sato, H.; Kageyu, A. Rubber Chem. Technol. 1983, 56, 299. Tanaka, Y. Polym. Prep. Japan 1983, 32, 75. Archer, Β. L., personal communication.

RECEIVED

November 3, 1983

16 13

A C NMR Study of Radiation-Induced Structural Changes in Polyethylene J. C. RANDALL—Phillips Petroleum Company, Bartlesville, OK 74004 F. J. ZOEPFL—Pickard, Lowe and Garrick, Inc., Washington, DC 20036 JOSEPH SILVERMAN—University of Maryland, College Park, MD 20742 Polyethylenes, irradiated to doses just short of the gel dose, have been examined for structural changes using C NMR in a study of the effects of ionizing radiation on polyethylene. Radiation gelled polyethylenes do not produce high resolution NMR resonances possibly because NMR dipolar interactions in the gel phase are not effectively averaged by the available molecular motions. One of the major structural units formed during irradiation of polyethylene is the Y type of long chain branch. Other structural entities monitored and followed versus radiation conditions were trans and cis double bonds, terminal vinyl end groups, saturated end groups, hydroperoxide groups and carbonyl groups. In addition to NMR measurements, molecular weight changes were monitored through intrinsic viscosity, gel permeation chromatography and low angle laser light scattering measurements. It was possible through the appropriate experimental conditions both before and during irradiation, to convert a linear, high density polyethylene to a medium density, exclusively long chain branched polymer, which was free of gel. Attempts to detect the Η-link in polyethylenes irradiated to doses short of the gel dose met with little success. 13

T h e d i r e c t d e t e c t i o n o f r a d i a t i o n i n d u c e d c r o s s l i n k s i n p o l y e t h y l e n e has been a major g o a l o f r a d i a t i o n c h e m i s t s for many y e a r s . It was r e c o g n i z e d as e a r l y as 1967 t h a t s o l u t i o n 1 3 c n u c l e a r m a g n e t i c resonance ( N M R ) s p e c t r o s c o p y c o u l d be used t o d e t e c t s t r u c t u r e s p r o d u c e d i n p o l y m e r s f r o m i o n i z i n g r a d i a t i o n . F i s c h e r and L a n g b e i n ( l ) r e p o r t e d the first direct detection of radiation induced crosslinks (H-links) in p o l y o x y m e t h y l e n e using 1 3 c N M R . B e n n e t t et αί.(2) used 1 3 c N M R t o detect radiation induced crosslinks in n-alkanes irradiated in vacuum in the m o l t e n s t a t e . B o v e y et ai.(3) used this t e c h n i q u e to i d e n t i f y b o t h r a d i a t i o n i n d u c e d H - l i n k s and l o n g c h a i n branches ( Y - l i n k s ) i n n - a l k a n e s

0097 6156/84/0247 0245S06.75/0 © 1984 American Chemical Society

246

NMR AND MACROMOLECULES

irradiated in the molten state. Bovey et ah also determined that few, if any, H-links and long chain Y branches form in n-alkanes irradiated in the solid state. Previous investigators have had little success in obtaining resonances arising from the gel structure formed during irradiation of polyethylene. Partially gelled polyethylene can be swollen with an appropriate solvent to produce a "solution" which appears suitable for 1 3 c NMR liquid measurements. However, the results of this study demonstrate that only the mobile, soluble component produces observable resonances under these conditions. It is possible that NMR dipolar interactions in the gel phase are not effectively averaged through the available polymer molecular motions. Under such circumstances, the resonances would become so broad as not to be observed in normal high resolution liquid 1 3 c NMR spectra. The problems involved in observing resonances from gelled polyethylene were also encountered in this study. We decided, therefore, to examine polyethylene samples irradiated with absorbed doses less than the gel dose. By so doing, some of the radiation induced structural changes produced prior to gelation could be detected. It may be instructive at this point to review some of the advantages provided by the 1 3 c N M R technique for examining soluble irradiated polyethylenes: (a) Measurements at 50 MHz and 398 Κ at solution concentrations between 10 and 20 percent by weight can yield high resolution spectra displaying a sensitivity of approximately 0.5 structural units per 10,000 total carbon atoms. (b) Each separate polyethylene structural entity gives rise to a unique array of resonances; the number and relative intensities depend upon the number and types of carbon atoms associated with each particular structural unit. Thus a number of different resonances will be associated with a structural unit. Model compounds and model polymers can lead to unequivocal assignments for the various types of polyethylene carbons. (c) Each resonance intensity is directly proportional to the number of contributing carbon atoms. Thus there is only one proportionality constant for all resonances and there is no need to determine various extinction coefficients as required for infrared and ultraviolet spectral measurements. (d) Saturated end groups and both short and long chain branches can be monitored independently using the 1 3 c N M R technique. (e) Internal cis and trans double bonds and terminal vinyl groups can be monitored directly and independently with the 1 3 c N M R method. Advances in high resolution 13c NMR equipment, such as higher magnetic field strengths, improved probes, improved dynamic ranges and sophisticated dedicated computer systems have greatly increased the sensitivity of 1 3 c N M R measurements. New instruments, operating at frequencies of either 300 or 500 MHz, have extended the capability to detect long chain branching in polyethylene to as few as 1-5 branches per 100,000 carbon atoms.

16.

R A N D A L L ET A L .

Radiation-Induced

Changes in

Polyethylene

247

The c o n c e n t r a t i o n s of s t r u c t u r e s p r o d u c e d i n i r r a d i a t e d p o l y e t h y l e n e a r e on t h e o r d e r o f 1 p e r 10,000 c a r b o n a t o m s f o r absorbed doses o f approximately 2.0 M r a d . Although the approach of examining p o l y e t h y l e n e s i r r a d i a t e d w i t h absorbed doses less t h a n t h e g e l dose p l a c e d a p r e m i u m on s e n s i t i v i t y , we w e r e able t o d e t e c t the f i r s t d i r e c t r a d i a t i o n induced l o n g c h a i n b r a n c h e s i n h i g h d e n s i t y p o l y e t h y l e n e (4). Experimental Irradiation. A l l irradiations were performed at the U n i v e r s i t y of M a r y l a n d w i t h a 25,000 c u r i e c o b a l t 60 g a m m a source. T h e a b s o r b e d dose r a t e o f 1.2 M r a d per h r was d e t e r m i n e d by f e r r o u s s u l f a t e d o s i m e t r y . A l l s a m p l e s w e r e i r r a d i a t e d under s e c o n d a r y e l e c t r o n e q u i l i b r i u m c o n d i t i o n s . F o l l o w i n g i r r a d i a t i o n , a l l s a m p l e s i r r a d i a t e d i n v a c u u m w e r e annealed i n v a c u u m a t 380 Κ f o r 24 hours; this treatment reduces t h e long-lived radical concentration to undetectable levels. M a t e r i a l s . T h e n - h e x a t r i a c o n t a n e ( H T C , n-C36, M.P. 75°C) s a m p l e s w e r e obtained from A l d r i c h C h e m i c a l Company. Irradiations were performed in v a c u u m i n both t h e s o l i d a n d m o l t e n s t a t e s t o r e p r o d u c e t h e r e s u l t s r e p o r t e d by b o t h B e n n e t t et αί.(2) and B o v e y et αϊ.(3) w h e r e t h e H - l i n k was r e p o r t e d t o f o r m . T h e Η-link c h e m i c a l s h i f t d a t a f r o m t h e m o d e l a l k a n e i r r a d i a t i o n s (at 100 Mrad) c o u l d be used t o i d e n t i f y t h e Η-link a f t e r c o r r e s p o n d i n g N M R m e a s u r e m e n t s on i r r a d i a t e d p o l y e t h y l e n e s . P h i l l i p s M a r l e x 6003, a high d e n s i t y p o l y e t h y l e n e ( M = 53,000; M = 18,000) possessing p r e d o m i n a n t l y s a t u r a t e d end groups was i r r a d i a t e d t o 4.0 M r a d in v a c u u m in p e l l e t f o r m as supplied. N B S 1475 is a c o m m e r c i a l p o l y e t h y l e n e c o n t a i n i n g 111 ppm o f Irganox 1010. O t h e r e x p e r i m e n t s i n v o l v e d h e a t i n g i n v a c u u m t o 500 Κ f o r 24 h p r i o r t o i r r a d i a t i o n t o 3.0 M r a d at 500 K. T h e N B S 1475 s a m p l e r e q u i r e d a g r e a t e r a m o u n t o f i r r a d i a t i o n to r e a c h the g e l dose than d i d M a r l e x 6003. w

n

G e l F r a c t i o n Determinations. The irradiated polyethylene samples were measured f o r g e l c o n t e n t b y e x t r a c t i o n w i t h b o i l i n g x y l e n e f o r 72 h. O n l y one sample, N B S 1475 i r r a d i a t e d t o 8.0 M r a d in v a c u u m a t 300 K, showed a measurable g e l c o n t e n t . 13ç N M R M e a s u r e m e n t s . A V a r i a n X L - 2 0 0 N M R s p e c t r o m e t e r , l o c a t e d at t h e P h i l l i p s R e s e a r c h C e n t e r , was used t o make a l l o f the 1 3 c N M R m e a s u r e m e n t s a t 50.3 M H z . T h e s a m p l e s were d i s s o l v e d i n 1,2,4t r i c h l o r o b e n z e n e a t 15 w e i g h t p e r c e n t a n d m a i n t a i n e d under a n i t r o g e n a t m o s p h e r e a t 398 Κ i n the probe c a v i t y d u r i n g d a t a a c q u i s i t i o n . O t h e r c o n d i t i o n s w e r e as f o l l o w s : P u l s e angle: P u l s e delay: Acquisition time: Spectral width D a t a points/spectrum: Free Induction Decays (FID's) a c c u m u l a t e d :

90° 15.0 seconds 1.0 seconds 8000 H z 32,000 5,000 - 20,000

248

NMR AND MACROMOLECULES

D o u b l e p r e c i s i o n a r i t h m e t i c was e m p l o y e d d u r i n g d a t a a c q u i s i t i o n and the s o f t w a r e i n c l u d e d a f l o a t i n g point F o u r i e r t r a n s f o r m c a p a b i l i t y . A f a c t o r c o n t r o l l i n g the u l t i m a t e s e n s i t i v i t y a v a i l a b l e is the l i n e w i d t h s at o n e - h a l f height of the p o l y e t h y l e n e r e s o n a n c e s . It is d e s i r a b l e t o c r e a t e c o n d i t i o n s w h i c h l e a d to the most n a r r o w l i n e w i d t h s possible when e x a m i n i n g the i r r a d i a t e d p o l y e t h y l e n e s by NMR. At c o n c e n t r a t i o n s o f 15 w e i g h t p e r c e n t and at t e m p e r a t u r e s o f 398 K, l i n e w i d t h s o f 1.0 - 1.5 H z w e r e o b t a i n e d at o n e - h a l f height f o r the major p o l y e t h y l e n e resonances f o r the r e c u r r i n g e q u i v a l e n t m e t h y l e n e u n i t s , δ+δ+, at 29.98 ppm f r o m T M S as shown in T a b l e I. S u b s t a n t i a l l y l a r g e r l i n e w i d t h s at o n e - h a l f height w e r e o b t a i n e d f o r s o l u t i o n c o n c e n t r a t i o n s higher t h a n 15 w e i g h t p e r c e n t . N a r r o w resonances l e a d t o g r e a t e r peak heights, w h i c h i n t u r n l e a d to g r e a t e r s e n s i t i v i t y . A n o t h e r p r o b l e m e n c o u n t e r e d d u r i n g q u a n t i t a t i v e m e a s u r e m e n t s was w h e t h e r peak h e i g h t s c o u l d be r e l i a b l y used f o r i n t e n s i t i e s of weak resonances. T h e end group resonances w e r e o f s u f f i c i e n t s t r e n g t h t o a l l o w a c o m p a r i s o n o f peak heights t o i n t e g r a t e d areas. It was c l e a r f r o m the n a r r o w o b s e r v e d l i n e widths (<1.0 H z @ è height) o f the end group resonances t h a t peak heights w o u l d l e a d to l o w results when used to measure m o l e c u l a r w e i g h t . A r e a m e a s u r e m e n t s by s p e c t r a l i n t e g r a t i o n led to n u m b e r a v e r a g e m o l e c u l a r w e i g h t s of 18,900 and 17,800 f o r M a r l e x 6003 and N B S 1475, r e s p e c t i v e l y , i n good a g r e e m e n t w i t h the v a l u e s o b t a i n e d by G P C . N u m b e r a v e r a g e m o l e c u l a r w e i g h t s of a p p r o x i m a t e l y 12,000 w e r e o b t a i n e d f o r both s a m p l e s when r e l a t i v e peak heights w e r e used f r o m the s a m e s p e c t r a l d a t a . T h e most n a r r o w l i n e w i d t h s c o n s i s t e n t l y o c c u r r e d f o r c a r b o n resonances f r o m c h a i n ends and ends o f b r a n c h e s . C o n s e q u e n t l y , r e l a t i v e peak heights are more r e l i a b l e f o r weak resonances a s s o c i a t e d w i t h s t r u c t u r a l m o i e t i e s f r o m i n t e r i o r sequences o f the p o l y m e r c h a i n . M e a s u r e m e n t s o f the Y b r a n c h , c i s and t r a n s double bonds, c a r b o n y l and h y d r o p e r o x i d e group c o n c e n t r a t i o n s i n a range o f 1-5 per 10,000 carbons w e r e made by a c o m p a r i s o n of peak heights b e c a u s e these resonances were g e n e r a l l y too w e a k f o r r e l i a b l e integral measurements. W e i g h t i n g f u n c t i o n s c a n be e m p l o y e d t o s m o o t h the a c c u m u l a t e d free i n d u c t i o n d e c a y s i g n a l to i m p r o v e the s i g n a l to noise r a t i o upon F o u r i e r t r a n s f o r m a t i o n o f the d a t a . S u c h s m o o t h i n g c a n l e a d to i m p r o v e d peak height r a t i o s because it tends to r e d u c e d i f f e r e n c e s a m o n g resonance l i n e w i d t h s as r e s o l u t i o n is lost w i t h the r e d u c t i o n i n noise l e v e l . E s s e n t i a l l y no d i f f e r e n c e s were o b t a i n e d in the q u a n t i t a t i v e r e s u l t s for l o n g c h a i n b r a n c h i n g , c i s and t r a n s double bonds, c a r b o n y l and h y d r o p e r o x i d e groups based on peak heights for the w e i g h t e d versus u n w e i g h t e d d a t a . R e s u l t s f o r the end groups based on peak h e i g h t s w e r e i m p r o v e d , h o w e v e r , when a s m o o t h i n g f u n c t i o n was used t o i n c r e a s e s i g n a l to noise. T h i s r e s u l t w o u l d t e n d to v a l i d a t e the use of peak h e i g h t s f o r those weak resonances from interior chain structural moieties. U n f o r t u n a t e l y , the a v a i l a b l e s e n s i t i v i t y was pushed to the l i m i t i n t h i s study and no r e c o u r s e o t h e r t h a n the use o f peak heights a p p e a r e d f e a s i b l e for i n t e n s i t y m e a s u r e m e n t s of weak resonances. T h e d a t a i n T a b l e s II t h r o u g h VI were d e r i v e d f r o m u n w e i g h t e d d a t a .

16.

249

Radiation-Induced Changes in Polyethylene

R A N D A L L ET A L .

Table I Linewidths at One-Half Height for the δ+δ+ Resonance at 29.98 ppm as a Function of Radiation Dose.

Sample NBS 1475 NBS 1475 NBS 1475 NBS 1475 NBS 1475 Marlex Marlex Marlex Marlex

(Fig. 9) (Fig.10) (Fig.12) (Fig. 11)

6003 6003 6003 6003

(Fig. (Fig. (Fig. (Fig.

4) 5) 8) 6)

Marlex 6003 (Fig.7)

LineWidth at έ Height Radiation Dose 1.2 H z None 1.2 2.0 Mrad at 398 Κ 1.3 4.0 Mrad at 398 Κ 1.1 8.0 Mrad at 398 K * 1.4 3.0 Mrad (Melt, 500 K) (after 24 h. at 500 K) 1.3 H z None 1.6 2.0 Mrad at 398 Κ 1.0 4.0 Mrad at 398 Κ 1.3 None (after 24 h at 550 K) 1.2 1.0 Mrad at 550 Κ (after 24 h at 550 K)

FID's Completed 9,699 4,482 14,500 5,104 7,271 20,683 9,617 5,250 4,314 9,302

*Sample partially gelled; soluble

Table II 1 3 c NMR Chemical Shifts Associated with the H-link

Carbon Methine α CH2 β CH2 γ CH2

Chemical Shifts wrt TMS, p p m This Study Bennett et al.(2) 41.01 30.47 28.62 30.60

Bovey et

39.49 30.70 28.22 30.19

γ β α α β γ ~CH2-CH2-CH2-CH2-CH-CH2-CH2-CH2-CH2~ I ~CH2-CH2-CH2-CH2-CH-CH2-CH2-CH2-CH2~ γ β α α β γ

40.5 31.9 28.6

al.(Z)

250

NMR A N D MACROMOLECULES

T a b l e III Concentrations F o l l o w i n g Irradiation of n-Hexatriacontane ( H T C )

N u m b e r of U n i t s per 10,000 C a r b o n A t o m s I r r a d i a t e d 100 M r a d I r r a d i a t e d 200 M r a d in V a c u u m at 298 Κ i n V a c u u m at 355 Κ

Structural Unit

Saturated End Groups^ T r a n s D o u b l e Bonds C i s D o u b l e Bonds L o n g C h a i n B r a n c h e s (Y) H-Links

a

558 4.9 1.5 5.4

562 8.6 Not D e t e c t e d Not Detected Trace

3.3

B y integrated area measurements; a l l other measurements from relative peak h e i g h t s . C a l c u l a t e d H T C s a t u r a t e d end group c o n c e n t r a t i o n p r i o r t o i r r a d i a t i o n is 556 per 10,000 c a r b o n a t o m s .

T a b l e IV Structural Concentrations Following Irradiation of M a r l e x

S o l i d S t a t e 298 Κ N u m b e r of U n i t s per 10,000 C a r b o n A t o m s Irradiated Irradiated 4.0 M r a d 2.0 M r a d Before in A i r in V a c u u m Irradiation

Structural Unit

Saturated End Groups^ Vinyl End G r o u p s L o n g C h a i n B r a n c h e s (Y) T r a n s D o u b l e Bonds C i s D o u b l e Bonds Hydroperoxide Groups Carbonyl Groups a

M

w

M

n

χ

x 10-3

M /M w

10-3

n

7.3 7.8 1.2 1.7 1.7 1.8 0.7 140b

8.6

10.1

3.9 2.2 3.1 2.5 2.8

4.1 N.D.

N.D. 164C

3.1 3.1 4.6 1.7

58.3C

20 7.0

-7

>4

a C o n c e n t r a t i o n s d e t e r m i n e d by i n t e g r a t e d a r e a m e a s u r e m e n t s . R e m a i n i n g c o n c e n t r a t i o n s d e t e r m i n e d by peak height m e a s u r e m e n t s . b

M e a s u r e d by G P C

c

M e a s u r e d by L A L L S

16.

R A N D A L L ET AL.

Radiation-Induced Changes in Polyethylene

251

Table V Structural Concentrations Following Thermal Degradation and Subsequent Irradiation of Marlex 6003 Polyethylene

Structural Unit

Saturated End Groups^ Vinyl End Groups^ Long-Chain Branches (Y) Butyl Branches Trans Double Bonds Cis Double Bonds Hydroperoxide Groups Carbonyl Groups M

w

M

n

χ 10-3 x 10-3

M /M w

n

Melt State 550K Number of Units per 10,000 Carbon Atoms Heated 550 Κ Heated 550 Κ 24 h in Vacuum 24 h in Vacuum 1.0 Mrad a 550 Κ 18.6 14.8 1.9 2.3 1.6 2.0 1.9 N.D. 166b

18.6 9.4 3.6 3.1 1.9 1.8 1.9 N.D. 192b

13.1

14.8

12.4

12.9

a Concentrations determined by integrated area measurements. Remaining concentrations determined by peak height measurements, b Measured by G P C

252

NMR AND MACROMOLECULES

T a b l e VI S t r u c t u r a l C o n c e n t r a t i o n s F o l l o w i n g I r r a d i a t i o n o f N B S 1475

Structural Unit

N u m b e r o f U n i t s per 10,000 C a r b o n A t o m s 3.Mradb 8. M r a d Before 2. M r a d 4. M r a d 500 Κ Irradiation 298 Κ 298 Κ 298 Κ In V a c . In V a c . In V a c . In V a c .

Saturated End Groups c Vinyl End Groups^ L o n g C h a i n Branches (Y) C i s D o u b l e Bonds T r a n s D o u b l e Bonds E t h y l Branches Butyl Branches Hydroperoxide Groups Carbonyl Groups M χ 10-3 M χ 10-3 M /M w

n

w

n

10.4 5.3 0.7 1.4 1.4 2.5 N.D. 1.8 N.D. 52.8d 18.1 2.9

13.0 2.0 0.9 3.6 2.7 3.2 N.D. 2.7 2.2 116d 21.8 5.3

12.9 2.8 1.0 1.6 1.5 4.0 N.D. 1.2 1.3 128d 22.3 5.7

15.1 —

1.3 2.9 5.1 4.3 N.D. 4.6 N.D. — — —

94.3 16.1 43.5C 1.9 2.6 4.1 4.4 2.5 1.3 35.8d 5.5 6.5

a S a m p l e p a r t i a l l y g e l l e d ; o b s e r v e d soluble c o m p o n e n t o n l y , b S a m p l e h e a t e d i n v a c u u m {§. 500 Κ f o r a p p r o x i m a t e l y 24 hours p r i o r to i r r a d i a t i o n , c C o n c e n t r a t i o n s d e t e r m i n e d by i n t e g r a t e d a r e a m e a s u r e m e n t s ; o t h e r c o n c e n t r a t i o n s d e t e r m i n e d by peak height m e a s u r e m e n t s , d M e a s u r e d by G P C .

16.

R A N D A L L ET A L .

Radiai ion-Induced

Changes in

Polyethylene

253

T h e n o m e n c l a t u r e e m p l o y e d t o d e s i g n a t e the v a r i o u s c a r b o n a t o m s in the s t r u c t u r a l m o i e t i e s m o n i t o r e d i n t h i s study is shown i n F i g u r e 1. T h e q u a n t i t a t i v e r e s u l t s g i v e n i n T a b l e s III t h r o u g h VI w e r e o b t a i n e d by d i v i d i n g the i n t e n s i t y o f one c a r b o n f r o m a s t r u c t u r a l m o i e t y by the t o t a l c a r b o n i n t e n s i t y f o r the e n t i r e s p e c t r u m and m u l t i p l y i n g by 10,000. T h i s g i v e s the r e s u l t i n t e r m s o f s t r u c t u r a l u n i t s per 10,000 c a r b o n a t o m s . F o r e x a m p l e , the t o t a l c a r b o n i n t e n s i t y (TCI) f o r a 1 3 c N M R s p e c t r u m o f h i g h d e n s i t y p o l y e t h y l e n e g i v e n by T C I = δ+δ+ + 3(s+a)

(1)

w h e r e " s " is the a v e r a g e i n t e n s i t y o b s e r v e d f o r the Is, 2s and 3s c a r b o n s and " a " is the a l l y l i c c a r b o n i n t e n s i t y f o r a t e r m i n a l v i n y l group. T h e δ+δ+ t e r m d o m i n a t e s because the r e p e a t i n g m e t h y l e n e units are by f a r the l a r g e s t c o n t r i b u t o r s to the N M R s p e c t r u m . It was set at 30,000 m m . P r o p o r t i o n a l i n t e n s i t i e s f o r the r e m a i n i n g s t r u c t u r a l e n t i t i e s r a n g e d f r o m 5 t o 15 m m . F o r t u n a t e l y , f o r m e a s u r e m e n t s of l o n g c h a i n b r a n c h i n g , t h e r e are t h r e e α and t h r e e β carbons per Y b r a n c h ; Thus the α and β c a r b o n resonance i n t e n s i t i e s need o n l y t o be 9 m m i n o r d e r t o h a v e a s e n s i t i v i t y of one b r a n c h per 10,000 c a r b o n a t o m s . Molecular Weight Measurements A Waters M 150C gel permeation c h r o m a t o g r a p h ( G P C ) equipped w i t h four porous s i l i c a c o l u m n s : t w o S E 4000 s, one S E 500 and one P S M 60s ( a v a i l a b l e f r o m D u P o n t ) was used f o r molecular weight measurements. A Wilkes variable wavelength high t e m p e r a t u r e i n f r a r e d d e t e c t o r , also f r o m D u P o n t , was used i n s t e a d o f a r e f r a c t i v e index d e t e c t o r . L o w angle l a s e r l i g h t s c a t t e r i n g ( L A L L S ) m e a s u r e m e n t s w e r e made w i t h a C h r o m a t i x K M X - 6 unit c o u p l e d t o a D u P o n t M o d e l 830 s i z e e x c l u s i o n c h r o m a t o g r a p h . T h e c o l u m n set was the s a m e as t h a t e m p l o y e d i n the W a t e r s M 1 5 0 C G P C u n i t . T h e m o b i l e phase used i n these m e a s u r e m e n t s was 1,2,4-trichlorobenzene and the t e m p e r a t u r e was m a i n t a i n e d at 403 K . B o t h the c h r o m a t o g r a p h y and l i g h t s c a t t e r i n g m e a s u r e m e n t s w e r e made at the P h i l l i p s R e s e a r c h C e n t e r . f

Results T h e s t r u c t u r a l e n t i t i e s m o n i t o r e d as a f u n c t i o n o f absorbed dose and i r r a d i a t i o n c o n d i t i o n s are p r e s e n t e d i n F i g u r e 1. T h e n o m e n c l a t u r e f o r d e s c r i b i n g the v a r i o u s types o f c a r b o n a t o m s , as w e l l as the o b s e r v e d c h e m i c a l shifts ( f r o m an i n t e r n a l T M S standard), are also i n c l u d e d . T h e c h e m i c a l shifts o b s e r v e d f o r the Η - l i n k s t r u c t u r e are shown i n T a b l e II.

I r r a d i a t i o n of n - H e x a t r i a c o n t a n e ( E T C ) . Results from 1 3 c N M R m e a s u r e m e n t s on H T C i r r a d i a t e d t o 100 M r a d i n v a c u u m and at r o o m t e m p e r a t u r e (298 K ) in the s o l i d s t a t e and at 353 Κ i n the m o l t e n s t a t e are shown i n T a b l e III. T h e s p e c t r a f r o m w h i c h these d a t a w e r e o b t a i n e d

are shown i n F i g u r e s 2 and 3. T h e i r r a d i a t e d H T C was d i s s o l v e d i n p e r d e u t e r o b e n z e n e to f o r m a s a t u r a t e d s o l u t i o n ; it was s t i l l p o s s i b l e to

254

NMR A N D MACROMOLECULES

Vinyl End Groups

Saturated End Groups

— C H , — C H = CH

— CH.—ÇH —CH, ?

k

4%

a 33.89

3s 2s 1s 32.17 22.85 14.05

Trans Double Bonds

Cis Double Bonds

— CH.

CH = C H —CH,

;

*

/

\ CH -

a 27.45

\

a.

2

CH = CH

a

\

CH-

a. 32.52 Carbonyl Groups

Hydroperoxide Groups

β -CH —CH —CH—CH —CH 2

2

2

2

I

4

0

33.14 26.83

*

8

ψ

«

β

α

2

2

/ * 30.47 27.30

2

2

2

?

II

k k

0

42.83 24.31

Isolated Butyl Branches 38.15 „

7

r

β

ρ

v

—CH —CH —CH —CH—CH —CH —CH 2

— CH —CH —

2

Isolated Ethyl Branches

~~~r

β

«

-CH —CH-—C

?

- C H - C H - C H - C H - C H ; , -

/

+ i 34.06 C H 26.74

30.47

2

I «

«

α

/

27.30

4

34.55

β

r

-CH;, — C H , -

ι

CH., 34.17

C H 29.51 i C H 23.36

CH3H.I8

;

!

*

C H , 14.09

Recurring Methylenes

Isolated LongChain Branches

r -CH.-

β

«

β

γ^α

-CH —CH —CH —CH —CH.?

:

r -CH 2

?

-(CH,) 29.98 n

« C H . 34.55 β C H , 27.30 7 C H 30.47 2

F i g u r e 1. Carbon-13 N M R C h e m i c a l Structural Entities Found in Polyethylene.

Shifts

and

Nomenclature

of

16.

R A N D A L L ET A L .

Radiât ion-Induced Changes in Polyethylene

n-HTC 100 Mrad @ 300 K

C6D6

255

I V||

4S

3S

1S

2S

TRANS -CH =

128

PPM, TMS

10

30

40

Figure 2. Carbon-13 N M R S p e c t r u m Irradiated in the Solid State.

of

n-Hexatriacontaine (HTC)

n-HTC 100 Mrad @ 355 K (MELT)

3S

C6D6

4S

2S

1S

-CHr

TRANS^

, S

128 4 i

c°

1

28.62 V ac

E-

57

4

1

JL

OUI. 20

PPM, TMS

Figure 3. Carbon-13 N M R Spectrum Irradiated in the Molten State.

of

10

n-Hexatriacontane

(HTC)

256

NMR A N D MACROMOLECULES

o b t a i n t h e N M R d a t a a t 398 K, t h e t e m p e r a t u r e o f t h e p o l y e t h y l e n e measurements. B e n n e t t e t al.(2) i d e n t i f i e d t h e Η-link i n n-alkanes i r r a d i a t e d i n v a c u u m i n the m o l t e n s t a t e i n an i n i t i a l 1 3 c N M R s t u d y and r e p o r t e d t h e f i r s t N M R assignments. A c o r r e s p o n d i n g Η-link s p e c t r a l p a t t e r n was i d e n t i f i e d i n t h e p r e s e n t study a f t e r i r r a d i a t i o n o f H T C i n v a c u u m and i n t h e m o l t e n s t a t e . A m e t h i n e resonance, c o n f i r m e d by t h e p r e s e n c e o f a d o u b l e t upon o f f - r e s o n a n c e d e c o u p l i n g , was o b s e r v e d a t 41.01 p p m a t 398 K. A m e t h y l e n e r e s o n a n c e f r o m c a r b o n s β t o t h e m e t h i n e c a r b o n s o f t h e Η-link, was o b s e r v e d at 28.62 ppm. T h e r e l a t e d α and Y m e t h y l e n e c a r b o n resonances w e r e n o t e d a t 30.47 and 30.60 ppm, r e s p e c t i v e l y , as i n d i c a t e d by B e n n e t t et al f o r n-alkanes. A s o m e w h a t d i f f e r e n t s i t u a t i o n is e n c o u n t e r e d i n p o l y e t h y l e n e because t h e s t r o n g b a c k b o n e m e t h y l e n e r e s o n a n c e at 29.98 ppm and lines a s s o c i a t e d w i t h Y b r a n c h e s and end groups t e n d t o obscure a p o s i t i v e i d e n t i f i c a t i o n o f t h e α and γ m e t h y l e n e r e s o n a n c e s f r o m t h e Η-link. T h e o v e r a l l o b s e r v e d r e s o n a n c e p a t t e r n f o r t h e H T C Η-link is s i m i l a r t o t h a t r e p o r t e d by B e n n e t t f o r l o w e r m o l e c u l a r w e i g h t n-alkanes a l t h o u g h t h e m e t h i n e c h e m i c a l s h i f t was a p p r o x i m a t e l y 2 ppm t o l o w e r f i e l d . B y l o w e r i n g the t e m p e r a t u r e t o 323 Κ and p r e p a r i n g t h e s a m p l e i n a m i x e d s o l v e n t o f p e r d e u t e r o c h l o r o f o r m and 1,2,4t r i c h l o r o b e n z e n e , we w e r e able t o observe t h e Η-link m e t h i n e r e s o n a n c e s h i f t f r o m 41.01 t o 39.95 ppm, w h i c h is c l o s e r t o t h e 39.5 p p m v a l u e r e p o r t e d by B e n n e t t et al f o r r o o m t e m p e r a t u r e m e a s u r e m e n t s i n p e r d e u t e r o c h l o r o f o r m . Thus under t h e c o n d i t i o n s s e l e c t e d f o r a s t u d y o f p o l y e t h y l e n e s , t h e Η-link m e t h i n e resonance should be e x p e c t e d t o r e s i d e near 41 ppm. T h e β m e t h y l e n e resonance should be v i s i b l e near 28.6 ppm a l t h o u g h t h e α and Y r e s o n a n c e s would l i k e l y be t o t a l l y o b s c u r e d by t h e v e r y s t r o n g δ δ r e s o n a n c e a t 29.98 ppm. N o t e also t h a t t h e m i n i m u m d e t e c t a b l e c o n c e n t r a t i o n f o r H - l i n k s should be h i g h e r t h a n t h a t f o r Y branches because t h e f o r m e r has f o u r α, β, γ carbons p e r s t r u c t u r a l u n i t , w h e r e a s t h e l a t t e r has t h r e e α, β, γ carbons per s t r u c t u r a l u n i t . B o v e y et αΖ.(3) also o b s e r v e d b o t h H - l i n k s and Y branches i n a n N M R study o f a n o r m a l a l k a n e i r r a d i a t e d i n v a c u u m i n t h e m o l t e n s t a t e . T h e l o n g c h a i n Y b r a n c h was easy t o r e c o g n i z e i n t h e present s t u d y b e c a u s e o f the p r e s e n c e o f a m e t h i n e r e s o n a n c e at 38.19 ppm, an α m e t h y l e n e r e s o n a n c e a t 34.55 ppm and a β m e t h y l e n e resonance a t 27.30 ppm. T h e s e a s s i g n m e n t s are i n e x c e l l e n t a g r e e m e n t w i t h those o f B o v e y et al. (3) and c o r r e s p o n d c l o s e l y t o the m e t h i n e and α, β m e t h y l e n e r e s o n a n c e s o b s e r v e d f o r an e t h y l e n e - l - o c t e n e c o p o l y m e r as shown below: +

Carbon Methine α Methylene β Methylene

+

C h e m i c a l Shifts Y B r a n c h (3) 38.19 ppm (TMS) 34.55 27.30

Backbone C h e m i c a l Shifts Ethylene-l-Octene Copolymer 38.16 ppm ( T M S ) 34.53 27.27

16.

R A N D A L L ET A L .

Radiation-Induced Changes in Polyethylene

257

The results shown in Table III demonstrate that when H T C is irradiated in the molten state both H-links and Y branches as well as cis and trans double bonds are formed. In contrast, irradiation at room temperature produces principally trans double bonds. The linked structures formed during melt irradiation are present in reasonably comparable amounts. The number of saturated end groups, as determined by spectral integration, was not significantly affected by irradiations to 100 Mrad in either the molten or solid state. Finally, it should be pointed out that the radiation yields for structures produced by a 100 Mrad irradiation of H T C are quite low. The G values for the most abundant radiation products are less than one. Irradiation of Marlex 6003 Polyethylene. Results of 13c N M R measurements on Phillips Marlex 6003 polyethylene both prior to and just following a 2.0 Mrad irradiation in vacuum and 298 Κ are presented in Table IV. The spectra from which these data were obtained are shown in Figures 4 and 5. These results indicate that irradiation of high density polyethylene in vacuum in the solid state reduces the concentration of terminal vinyl unsaturations and increases the concentrations of long chain Y branches, saturated end groups and trans double bonds. The H link could not be detected following an irradiation of Marlex 6003 in the solid state. The 13c N M R spectrum of thermally degraded Marlex 6003 is shown in Figure 6. The sample was heated in vacuum at 550 Κ for 24 hours. It is clear that this treatment produced additional terminal vinyl unsaturations, saturated end groups and both short and long chain branches. A subsequent irradiation to only 1.0 Mrad in vacuum at 550 Κ resulted in a reduction in terminal vinyl unsaturation and an increase in the number of Y branches. No resonances from an Η-link could be detected. Judging from the yield of H-links in the model H T C irradiations, we should not expect to detect the presence of H-links because the level should be below 0.5 per 10,000 carbons. The copious yield of Y branches formed after irradiation may be related to the substantial quantity of terminal vinyl unsaturation produced by thermal degradation. Complete quantitative data is given in Table V. It is interesting that irradiations more extensive than 1.0 Mrad at 550K produced partially gelled polyethylene samples. A spectrum of non-gelled polyethylene produced by the combination of thermal degradation and 1.0 Mrad irradiation is shown in Figure 7. Results of measurements on Marlex 6003 irradiated to 4.0 Mrad in air are also included in Table IV. The spectrum from which these data were obtained is shown in Figure 8. These results indicate that the radiolytic oxidation of solid high density polyethylene produces an increase in the concentration of hydroperoxide and carbonyl groups with an accompanying drastic reduction in both the number average and weight average molecular weights of the polymer. Irradiation of NBS 1475 Polyethylene. Results of 13c NMR measurements on NBS 1475 both prior to and following 2.0 and 4.0 Mrad irradiations in vacuum at room temperature are shown in Table VI. The spectra which were used to determine the concentrations listed in Table VI are shown in

258

NMR AND MACROMOLECULES

6003 ΡΕ

UNIRRADIATED

a

'

,-δΥ

HP3S

"

PPMJMS

40

»...

2 5

30

1 5

20

10

F i g u r e 4. C a r b o n - 1 3 N M R S p e c t r u m o f P h i l l i p s M a r l e x 6003 P r i o r Irradiation.

to

6003 P E 2 Mrad @ 300 Κ

3S

at I

PPM, TMS

40

'

30

2S

20

10

F i g u r e 5. C a r b o n - 1 3 N M R S p e c t r u m o f P h i l l i p s M a r l e x 6003 I r r a d i a t e d i n the S o l i d S t a t e .

16.

R A N D A L L ET A L .

Radiât ion-Induced Changes in Polyethylene

6003 P E

+

24h @ 550 K N O IRRADIATION

Λ

ΗΡ

l

/

d

259

+ à

3S at

2S

1S

CH

PPM, TMS

10

40

F i g u r e 6. C a r b o n - 1 3 N M R S p e c t r u m of P h i l l i p s M a r l e x 6003 H e l d at K i n V a c u u m for 24 H o u r s .

6003 PE 24h @ 550 Κ 1 Mrad @ 550 Κ

j I

550

A

t

at

3S

2S

1S

CH

PPM, TMS 40

10

F i g u r e 7. C a r b o n - 1 3 N M R S p e c t r u m of P h i l l i p s M a r l e x 6003 H e l d at Κ i n V a c u u m for 24 H o u r s and I r r a d i a t e d at 550 K .

550

260

NMR AND MACROMOLECULES

F i g u r e s 9 and 10. T h e r e s u l t s f o r N B S 1475 are s i m i l a r t o those o b t a i n e d f r o m the study o f M a r l e x 6003 a f t e r i r r a d i a t i o n i n the s o l i d s t a t e . O n c e a g a i n , the c o n c e n t r a t i o n s o f l o n g c h a i n Y branches, s a t u r a t e d end groups and t r a n s double bonds a p p a r e n t l y i n c r e a s e w i t h i r r a d i a t i o n w h i l e the c o n c e n t r a t i o n s o f t e r m i n a l v i n y l groups d e c r e a s e w i t h i r r a d i a t i o n . T h e r e is some s c a t t e r i n the d a t a as might be e x p e c t e d f o r c o n c e n t r a t i o n s measured i n the range o f 1-3 per 10,000 c a r b o n a t o m s . A s was t h e ease for M a r l e x 6003, the p r e s e n c e o f any a p p r e c i a b l e q u a n t i t y o f H - l i n k s was not d e t e c t e d . Thus the Η - l i n k c o n c e n t r a t i o n must be w e l l b e l o w one per 10,000 c a r b o n a t o m s . A p r e f e r e n c e f o r Y b r a n c h f o r m a t i o n has been c o n s i s t e n t l y o b s e r v e d i n the studies of i r r a d i a t e d M a r l e x 6003 and N B S 1475 p o l y e t h y l e n e s . N B S 1475 p o l y e t h y l e n e was also s u b j e c t e d to t h e r m a l d e g r a d a t i o n for 24 hours at 500 Κ and t h e n i r r a d i a t e d i n v a c u u m also at 500 Κ t o 3.0 M r a d . T h e q u a n t i t a t i v e r e s u l t s are g i v e n i n T a b l e IV and the 1 3 c N M R s p e c t r u m w h i c h y i e l d e d these d a t a is shown i n F i g u r e 11. A s r e m a r k a b l y i n d i c a t e d i n F i g u r e 11, the f o r m e r l y l i n e a r N B S 1475 is now e x t e n s i v e l y long chain branched. F r o m a c o m p a r i s o n of r e l a t i v e peak a r e a s , the c o n c e n t r a t i o n o f l o n g c h a i n branches is now a p p r o x i m a t e l y 44 per 10,000 carbon atoms. T h e r e are t w o more p r o d u c t s f o r m e d under t h e s e conditions: the c o n c e n t r a t i o n of s a t u r a t e d end groups is now 94 per 10,000 c a r b o n a t o m s and the c o n c e n t r a t i o n o f t e r m i n a l v i n y l groups is 16 per 10,000 c a r b o n a t o m s . T h e c o m b i n e d t h e r m a l and i r r a d i a t i o n t r e a t m e n t s c o n v e r t e d a high d e n s i t y p o l y e t h y l e n e (0.978) t o a m e d i u m d e n s i t y (0.947) p o l y e t h y l e n e w i t h o u t i n t r o d u c i n g short c h a i n b r a n c h e s or d r a m a t i c a l l y i n c r e a s i n g the m o l e c u l a r w e i g h t . T h e m o l e c u l a r weight m e a s u r e m e n t s made on t h i s s a m p l e show t h a t both the number a v e r a g e and w e i g h t average m o l e c u l a r w e i g h t s d e c r e a s e d f o l l o w i n g the c o m b i n e d t h e r m a l t r e a t m e n t and i r r a d i a t i o n . E x t r e m e l y weak resonances w e r e p e r c e p t i b l e near 41.1 ppm and 28.6 p p m , w h i c h suggests t h a t H - l i n k s w e r e also f o r m e d d u r i n g the h i g h t e m p e r a t u r e m e l t i r r a d i a t i o n . T h e Η - l i n k c o n c e n t r a t i o n is e s t i m a t e d at a p p r o x i m a t e l y 1 per 10,000 c a r b o n a t o m s . R e s u l t s o f m e a s u r e m e n t s on a s a m p l e o f N B S 1475 i r r a d i a t e d t o 8.0 M r a d i n v a c u u m at 300 Κ are also shown in T a b l e V I . T h e s p e c t r u m f r o m w h i c h these d a t a w e r e o b t a i n e d is shown i n F i g u r e 12. T h i s e x p e r i m e n t was the o n l y one p e r f o r m e d i n this study w h e r e the g e l dose was e x c e e d e d . A n a t t e m p t was made t o d i s s o l v e the e n t i r e s a m p l e i n 1,2,4t r i c h l o r o b e n z e n e but o n l y a c l e a r s w o l l e n g e l was the r e s u l t . The subsequent 1 3 c N M R s p e c t r u m i n d i c a t e d t h a t o n l y the m o b i l e , s o l u b l e component p r o d u c e d o b s e r v a b l e resonances. The only significant s t r u c t u r a l unit i d e n t i f i e d in the s o l u b l e c o m p o n e n t was t r a n s double bonds; a t r a c e amount o f l o n g c h a i n Y branches was also o b s e r v e d , but H - l i n k s c o u l d not be d e t e c t e d i n the soluble f r a c t i o n .

16.

R A N D A L L ET A L .

Radiation-Induced Changes in Polyethylene

6003 P E 4 M r a d IN A I R 300 K

261

M /

at,3S

PPM, TMS 40

^

a

3

C

= 2S 0

1S

20

0

10

F i g u r e 8. C a r b o n - 1 3 N M R S p e c t r u m of P h i l l i p s M a r l e x 6003 I r r a d i a t e d i n A i r i n the S o l i d S t a t e .

N B S 1475 UNIRRADIATED

++

δ

β

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40

ac ο

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F i g u r e 9. C a r b o n - 1 3 N M R S p e c t r u m o f N B S 1475 P r i o r to I r r a d i a t i o n .

262

NMR AND MACROMOLECULES

N B S 1475 4 Mrad @ 300 Κ

f$

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PPM, TMS

Figure 10. State.

-ÇH

a

c

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c=ο ,

rt'\ a

40

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10

Carbon-13 NMR Spectrum of NBS 1475 Irradiated in the Solid

NBS1475 24h @500K 3 Mrad @ 500 Κ

3S

1S

-CH

37.51 ,at

PPMJMS

20

Figure 11. Carbon-13 NMR Spectrum of NBS 1475 Held at 500K for 24 Hours and Irradiated at 500K.

16.

R A N D A L L ET A L .

Radiation-Induced Changes in Polyethylene

N B S 1475 8 Mrad @ 300 K

PPM, T M S

40

>

263

μψ

ι

30

20

10

F i g u r e 12. C a r b o n - 1 3 N M R S p e c t r u m of N B S 1475 I r r a d i a t e d t o 8 M r a d in the S o l i d S t a t e (Soluble P o r t i o n O n l y ) .

264

NMR A N D MACROMOLF.CULES

Discussion Radiation induced structural changes i n p o l y e t h y l e n e have been i n v e s t i g a t e d f o r o v e r 30 y e a r s . These studies have shown t h a t the p r i n c i p a l c h e m i c a l changes w h i c h o c c u r d u r i n g i r r a d i a t i o n are: (1) p r o d u c t i o n o f m o l e c u l a r h y d r o g e n , (2) p r o d u c t i o n o f t r a n s double bonds, (3) d i s a p p e a r a n c e o f t e r m i n a l v i n y l u n s a t u r a t i o n s and (4) p r o d u c t i o n o f i n t e r m o l e c u l a r l i n k s . A l t h o u g h the p r o d u c t i o n of m o l e c u l a r h y d r o g e n was not m o n i t o r e d i n t h i s study, 13c N M R was an e x c e l l e n t t e c h n i q u e to f o l l o w the p r o d u c t i o n o f i n t e r n a l double bonds, i n t e r m o l e c u l a r l i n k s and the d i s a p p e a r a n c e o f t e r m i n a l v i n y l groups. A d d i t i o n a l l y , it was o b s e r v e d t h a t s a t u r a t e d end groups are a p p a r e n t l y p r o d u c e d d u r i n g i r r a d i a t i o n o f p o l y e t h y l e n e . T h e p r o d u c t i o n o f t r a n s double bonds d u r i n g i r r a d i a t i o n o f p o l y e t h y l e n e has been s t u d i e d by many i n v e s t i g a t o r s p r i n c i p a l l y t h r o u g h infrared spectroscopy. A l t h o u g h c i s double bonds w e r e probably c o n s i d e r e d as a p r o d u c t o f i r r a d i a t i o n , c o n v e n t i o n a l i n f r a r e d methods c o u l d not be used to m o n i t o r the f o r m a t i o n of c i s double bonds. T h e 1 3 c N M R r e s u l t s r e p o r t e d i n t h i s study show t h a t b o t h c i s and t r a n s double bonds are p r o d u c e d d u r i n g i r r a d i a t i o n of m o l t e n p o l y e t h y l e n e w h i l e trans double bond formation predominates during irradiation of solid p o l y e t h y l e n e . In the i r r a d i a t e d c r y s t a l l i n e H T C and i n the soluble p o r t i o n of a p a r t i a l l y g e l l e d N B S 1475 p o l y e t h y l e n e , w h i c h had been i r r a d i a t e d to 8.0 M r a d , t r a n s double bonds were the major p r o d u c t f o r m e d . These r e s u l t s suggest t h a t t r a n s double bonds are a major p r o d u c t p r o d u c e d upon i r r a d i a t i o n i n the c r y s t a l l i n e regions of p o l y e t h y l e n e w h i l e b o t h c i s and t r a n s double bonds are f o r m e d i n amorphous r e g i o n s . T h e d i s a p p e a r a n c e o f t e r m i n a l v i n y l groups d u r i n g i r r a d i a t i o n o f p o l y e t h y l e n e has also been s t u d i e d by many i n v e s t i g a t o r s who u t i l i z e d i n f r a r e d methods. T h e 1 3 c N M R r e s u l t s r e p o r t e d i n this study are s i m i l a r to the i n f r a r e d r e s u l t s r e p o r t e d by O k a d a and M a n d e l k e r n (6). L y o n s (7,8) and M a n d e l k e r n (6) have proposed t h a t v i n y l d i s a p p e a r a n c e is r e l a t e d t o the f o r m a t i o n o f r a d i a t i o n i n d u c e d l i n k s i n p o l y e t h y l e n e . M a n d e l k e r n has also proposed t h a t v i n y l d i s a p p e a r a n c e is r e l a t e d to a d e c r e a s e i n the p r o d u c t i o n o f m o l e c u l a r h y d r o g e n d u r i n g i r r a d i a t i o n (6). O n e o f the major new s t r u c t u r a l units i d e n t i f i e d i n the p r e s e n t study of i r r a d i a t e d p o l y e t h y l e n e s is the l o n g c h a i n Y b r a n c h , w h i c h f o r m s d u r i n g i r r a d i a t i o n o f p o l y e t h y l e n e s i n v a c u u m b o t h in the s o l i d s t a t e and i n the high t e m p e r a t u r e m o l t e n s t a t e . L o n g c h a i n Y b r a n c h e s are also f o r m e d i n H T C i r r a d i a t e d i n v a c u u m i n the m o l t e n s t a t e , as p r e v i o u s l y n o t e d by B o v e y et ah (3) a f t e r i r r a d i a t i o n of n-alkanes i n the m o l t e n s t a t e . T h e N M R r e s u l t s shown i n T a b l e s IV, V and VI d e m o n s t r a t e t h a t the t e r m i n a l v i n y l group c o n c e n t r a t i o n d e c r e a s e s as the l o n g c h a i n Y b r a n c h c o n c e n t r a t i o n i n c r e a s e s . U p o n c o m p a r i n g the r e s u l t s f r o m the i r r a d i a t e d N B S 1475 to those o b t a i n e d f r o m i r r a d i a t e d M a r l e x 6003, it was noted t h a t the y i e l d o f l o n g c h a i n Y branches was g r e a t e r for M a r l e x 6003, w h i c h had a g r e a t e r i n i t i a l t e r m i n a l v i n y l c o n c e n t r a t i o n . A l t h o u g h some c a u t i o n should be e x e r c i s e d because N B S 1475 c o n t a i n e d an a n t i o x i d a n t w h e r e a s M a r l e x 6003 d i d not, these o b s e r v a t i o n s do suggest t h a t some o f

16.

R A N D A L L ET A L .

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265

the t e r m i n a l v i n y l groups m a y be r e a c t i n g w i t h s e c o n d a r y a l k y l r a d i c a l s t o f o r m l o n g c h a i n Y b r a n c h r a d i c a l s . S u c h a hypothesis was f i r s t a d v a n c e d by B . J . Lyons(7) as shown b e l o w : ~CH2 ^CH- + ~CH2

CH2=CH-CH2~-

~ CH2. > ^CH-CH2-CH-CH2^

(2)

~CK2

T h e subsequent r e a c t i o n s o f the Y b r a n c h r a d i c a l are u n k n o w n at t h i s t i m e ; a l t h o u g h c h a i n t r a n s f e r t o f o r m a l o n g c h a i n Y b r a n c h appears t o be a likely possibility. Thus Y b r a n c h f o r m a t i o n w o u l d r e s u l t f r o m a p r o p a g a t i n g s t e p i n the f r e e r a d i c a l c h e m i s t r y , as opposed t o a t e r m i n a t i n g s t e p w h i c h is r e q u i r e d t o p r o d u c e H - l i n k s . T h e r e is also no e x p l a n a t i o n f o r the o b s e r v a t i o n t h a t the d i s a p p e a r a n c e o f t e r m i n a l v i n y l groups e x c e e d s t h e f o r m a t i o n o f l o n g c h a i n Y b r a n c h e s . O t h e r means o f v i n y l d i s a p p e a r a n c e and o t h e r l i n k i n g r e a c t i o n s are p o s s i b l e . The establishment of a satisfactory mechanism accounting for a l l the observed p r o d u c t s must a w a i t f u r t h e r e x p e r i m e n t a l e v i d e n c e . S t r o n g support f o r L y o n s ' h y p o t h e s i s is p r o v i d e d by the e x p e r i m e n t a l r e s u l t s o f M a n d e l k e r n et αϊ.(6), w h i c h show t h a t the g e l dose f o r h y d r o g e n a t e d p o l y e t h y l e n e is a p p r o x i m a t e l y t h r e e t i m e s t h a t of the s a m e p o l y e t h y l e n e c o n t a i n i n g one t e r m i n a l v i n y l group per m o l e c u l e . T h e r e s u l t s o f the present study, w h i c h p r o d u c e d the f i r s t d i r e c t d e t e c t i o n o f r a d i a t i o n i n d u c e d l o n g c h a i n b r a n c h i n g i n p o l y e t h y l e n e , also supports L y o n s ' h y p o t h e s i s . T h e most i m p o r t a n t f i n d i n g i n t h i s study is t h a t the y i e l d o f l o n g c h a i n Y branches appears t o be m u c h g r e a t e r t h a n the y i e l d o f H - l i n k s w h e n p o l y e t h y l e n e is i r r a d i a t e d i n v a c u u m i n the s o l i d s t a t e w i t h absorbed doses less t h a n the g e l dose. T h e Η - l i n k c o u l d p o s s i b l y be d e t e c t e d i n o n l y one p o l y e t h y l e n e s a m p l e and t h a t was a f t e r N B S 1475 was h e a t e d t o 500 Κ for 24 hours i n v a c u u m and subsequently i r r a d i a t e d i n v a c u u m t o 3.0 M r a d at 500 K . T h e r e are t w o possible e x p l a n a t i o n s as to w h y the Η - l i n k went u n o b s e r v e d i n the 13c N M R s p e c t r a of the o t h e r i r r a d i a t e d p o l y e t h y l e n e s : (1) the Η - l i n k c o u l d have r e m a i n e d b e l o w the d e t e c t i o n l i m i t o f a p p r o x i m a t e l y 0.5 units p e r 10,000 c a r b o n a t o m s or (2) the H - l i n k s possessed so l i t t l e m o l e c u l a r m o b i l i t y as to go u n o b s e r v e d b e c a u s e o f e i t h e r d i p o l a r b r o a d e n i n g or r e l a x a t i o n e f f e c t s . T h e l a t t e r e x p l a n a t i o n is not e n t i r e l y p l a u s i b l e because the p o l y e t h y l e n e m a t r i x s u r r o u n d i n g t h e fl u n k is s u f f i c i e n t l y m o b i l e t o p e r m i t an o b s e r v a t i o n o f o t h e r s t r u c t u r a l s p e c i e s . A l s o the e x i s t e n c e o f p e r s i s t e n t d i p o l a r i n t e r a c t i o n s w i t h i n a n H l i n k w o u l d r e q u i r e it t o be s u b s t a n t i a l l y less m o b i l e t h a n a l o n g c h a i n Y b r a n c h . It is reasonable to c o n c l u d e t h a t the f o r m a t i o n o f l o n g c h a i n Y branches i n p o l y e t h y l e n e i r r a d i a t e d w i t h absorbed doses less t h a n the g e l dose is s i g n i f i c a n t l y more i m p o r t a n t t h a n t h e f o r m a t i o n o f H - l i n k s . P e r h a p s the most s u r p r i s i n g r e s u l t o b t a i n e d in t h i s study was the c o n s i s t e n t o b s e r v a t i o n o f an apparent s m a l l i n c r e a s e i n s a t u r a t e d end group c o n c e n t r a t i o n f o l l o w i n g i r r a d i a t i o n o f s o l i d p o l y e t h y l e n e s i n v a c u u m , w i t h no a c c o m p a n y i n g d e c r e a s e i n the number a v e r a g e m o l e c u l a r w e i g h t o f the p o l y e t h y l e n e s (Tables IV and VI). T h i s a p p a r e n t i n c r e a s e i n s a t u r a t e d end group c o n c e n t r a t i o n is not w e l l u n d e r s t o o d at t h i s t i m e but c o u l d a r i s e f r o m s a t u r a t i o n o f t e r m i n a l v i n y l groups d u r i n g i r r a d i a t i o n or f r o m m i g r a t i o n o f the t e r m i n a l v i n y l group to an i n n e r p o s i t i o n i n the

NMR AND MACROMOLECULES

266

chain. Radiation induced scission through β cleavage could also lead to saturated end groups but appears unlikely because of the observed reduced content of terminal vinyl groups and the absence of sufficient linking products to account for the difference:

— CH2CH2CHCH2CH2

^ ~CH2CH2CH=CH2

+

-CH2~

(3)

This reaction is known to occur during thermal degradation of polyethylene but proof of its contribution to the products formed during irradiation of polyethylenes in the solid state will have to await further evidence. As can be seen in Tables IV and V, high temperature thermal degradation of polyethylene in vacuum leads to the formation of terminal vinyl unsaturation, saturated end groups, butyl branches and long chain Y branches. Following thermal degradation the resulting changes in the concentrations of terminal vinyl groups and Y branches are collectively close to the change in the observed concentration of saturated end groups. This result strongly supports the occurrence of β cleavage during thermal degradation in vacuum at temperatures of 500-550 K . A subsequent irradiation in vacuum at 500-550 Κ leads to an increase in molecular weight, a substantial increase in long chain Y branches and a loss of terminal vinyl groups. These results demonstrate that long chain Y branches can be a product of thermal degradation and that this reaction can be enhanced by irradiation although there is no longer a material balance between the formation of Y branches and the loss of terminal vinyl groups. There must be a relationship between the initial terminal vinyl content and radiation induced formation of Y branches because the yield of radiation induced Y brances is directly proportional to the initial concentration of terminal vinyl groups in the polyethylene. Finally, irradiation of polyethylene in the solid state in the presence of air precludes many of the products observed following irradiation in vacuum. There is a drastic reduction in molecular weight and a substantially reduced yield of long chain Y branches. There is still a loss of terminal vinyl groups and an apparent increase in the number of saturated end groups as shown in Table IV. The hydroperoxide and carbonyl content increase as expected. Overall, these observations demonstrate that radiolytic oxidation reactions are effective competing reactions to the linking reactions observed following vacuum irradiations. v

Conclusions 1.

2. 3.

4.

Long chain Y branches are one of the principal products formed during irradiation of high density polyethylenes in vacuum both in the solid and molten states at absorbed doses below the gel dose. The concentration of H-links remained below 0.5 per 10,000 carbon atoms for absorbed doses less than the gel dose. Cis double bonds are formed during both ambient and high temperature irradiations of polyethylenes but appear restricted to amorphous regions of solid polyethylenes. Saturated end groups are apparently produced during irradiation of polyethylene.

16. 5. 6.

7.

8.

R A N D A L L

E T A L .

Radiation-Induced Changes in Polyethylene

267

T r a n s double bonds appear to be a major p r o d u c t f o r m e d i n the c r y s t a l l i n e regions o f i r r a d i a t e d p o l y e t h y l e n e s . I r r a d i a t i o n o f s o l i d p o l y e t h y l e n e i n the p r e s e n c e o f a i r leads t o a r e d u c t i o n i n the c o n c e n t r a t i o n s o f branches, an i n c r e a s e i n the c o n c e n t r a t i o n s o f h y d r o p e r o x i d e and c a r b o n y l groups and a d e c r e a s e in m o l e c u l a r w e i g h t . T h e r m a l d e g r a d a t i o n o f p o l y e t h y l e n e i n v a c u u m leads t o c h a i n s c i s s i o n w i t h the p r o d u c t i o n o f a d d i t i o n a l s a t u r a t e d and v i n y l end groups and b o t h short and l o n g c h a i n b r a n c h e s . T h e f o r m a t i o n o f l o n g c h a i n Y b r a n c h e s appears t o be r e l a t e d t o the d i s a p p e a r a n c e of t e r m i n a l v i n y l groups i n i r r a d i a t e d p o l y e t h y l e n e .

T h i s study p r o v i d e s a m e t h o d o f c h a r a c t e r i z a t i o n t h a t c a n be u s e f u l l y a p p l i e d by o t h e r s i n studies o f i r r a d i a t e d p o l y e t h y l e n e s and o t h e r polymers. U s e o f the p o w e r f u l N M R t e c h n i q u e w i l l u n d o u b t e d l y y i e l d f u r t h e r s i g n i f i c a n t i n f o r m a t i o n about the r a d i a t i o n c h e m i s t r y o f p o l y m e r s .

A c k n o w l e dgments T h e a u t h o r s thank M r . J . R . D o n a l d s o n of the P h i l l i p s P e t r o l e u m C o m p a n y for p e r f o r m i n g the l i q u i d 13c N M R m e a s u r e m e n t s . T h e authors also w i s h to thank M r . C . H . L e i g h of the P h i l l i p s P e t r o l e u m C o m p a n y f o r p e r f o r m i n g b o t h the G P C and L A L L S analyses. S e v e r a l s t i m u l a t i n g discussions w i t h D r . D . L . V a n der H a r t of the U . S . N a t i o n a l B u r e a u of Standards and w i t h D r . J . D . H o f f m a n o f the U n i v e r s i t y o f M a r y l a n d also c o n t r i b u t e d to t h i s w o r k .

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8.

Fischer, H.; Langbein, W. Kolloid Ζ 1967, 216-217, 329. Bennett, R. L.; Keller, Α.; Stejny, J.; Murray, M. J. Polym. Sci., Polym. Chem. Ed. 1976, 14, 3027. Bovey, F. Α.; Schilling, F. C.; Cheng, Η. N. in "Advances in Chemistry Series No. 169"; Allara, D. L.; Hawkins, W. L., Eds.; American Chemical Society: Washington, D.C., 1978; pp. 133-141. Randall, J. C.; Zoepfl, F. J.; Silverman, J. Makromol, Chemie, Rapid Commun. 1983, 4, 149. Hoeve, C. A . J.; Wagner, H. L.; Verdier, P. H. J. Res. Nat. Bur. Stand., Part A 1972, 76, 137. Mandelkern, L. in "The Radiation Chemistry of Macromolecules," Vol. 1, Dole, M. Ed.; Academic Press: N.Y., 1972; Chapter 13. Lyons, B. J . Polym. Prepr., Am. Chem. Sec., Div. Polym. Chem. 1967, 8, 672. Lyons, B. J . in "International Conference on Radiation Processing for Plastics and Rubber"; The Plastics Institute: London (UK), 1981; Chapter 5.

RECEIVED October 24, 1983

INDEXES

Author Index L i n , Y.-Y., 67 L o c a t e l l i , P., 223 L y e r l a , J . R., 83 McKay, R. Α., 43 N i s h i o k a , A t s u o , 119 O ' D o n n e l l , D. J . , 21 Ohuchi, Muneki, 119 R a n d a l l , J . C., 131,245 S a c c h i , M. C., 223 S a t o , H i s a y o , 181 S c h a e f e r , J a c o b , 43 S e f c i k , M. D., 43 S i l v e r m a n , J o s e p h , 245 S t e j s k a l , Ε. 0., 43 Tanaka, Y a s u y u k i , 181,233 Takemura, S h i n j i , 119 Tarpey, M. F., 67 Y a n n o n i , C. S., 83 Z a m b e l l i , A., 223 Z o e p f l , F. J . , 245

Ammendola, P., 223 Bovey, F. A., 3 C a i s , R u d o l f Ε., 43,153 Carman, C. J . , 167 Chen, Teng-Ko, 197 D i x o n , W. T., 43 Dumais, J . J . , 55 E n g e l , Α. Κ., 55 F l e m i n g , W. W., 83 Gray, George A., 97 Harwood, H. James, 197 H i k i c h i , K u n i o , 119 H i r a o k i , T o s h i f u m i , 119 Home, S. E. , J r . , 167 H s i e h , Ε. Τ., 131 I n g l e f i e l d , P.T., 67 J e l i n s k i , L. W., 55 J o n e s , A l a n Anthony, 67 Kometani, J a n e t Μ., 153 K o m o r o s k i , R. A., 167 L i n , Fu-Tyan, 197

Subject Index A r e g i c PVF, r e g i o s e q u e n c e d i s t r i b u t i o n s , 160-63 A r o m a t i c C-1 r e s o n a n c e , e p i m e r i z e d i s o t a c t i c PS, 202-11 A t a c t i c p o l y p r o p y l e n e , ^C s p e c t r a , 7 A t t a c h e d p r o t o n t e s t (APT), o p t i m i z i n g s e n s i t i v i t y , 99

A A c q u i s i t i o n t i m e , i r r a d i a t e d ΡΕ, 247 A c y c l i c terpenes C NMR o f n a t u r a l p o l y i s o p r e n e s , 234 c h e m i c a l s h i f t s , 238 Adsorption e f f e c t s , separation o f s t y r e n e o l i g o m e r s , 182 A i r , e f f e c t , PE i r r a d i a t i o n , 266 A l i p h a t i c carbon s i g n a l s i n c i s p o l y i s o p r e n e , 241f,243f A l k y l r a d i c a l s , secondary, i r r a d i a t e d PE, 265 Aluminum c o m p l e x e s , 2D NMR, 116 Amplitude modulation, h e t e r o n u c l e a r s p i n c o u p l i n g , 99 Angular displacements, r i n g r o t a t i o n s i n PS, 52 A n i o n i c i n i t i a t o r , PMMA Η s p e c t r a , 4 A n i o n i c o l i g o m e r i z a t i o n , s t y r e n e , 182 Anisotropic interactions, orientat i o n a l dependence, 2 6 f Anisotropic rotation, polyformal spin r e l a x a t i o n , l o c a l m o t i o n , 70,78t A n i s o t r o p y , c h e m i c a l s h i f t (CSA), s o l i d sample NMR, 22 A n t i p h a s e components, u n d e s i r a b l e INEPT p r o p e r t i e s , 106 A r e a s , i n t e g r a t e d , q u a n t i t a t i v e NMR s t u d i e s , 137t 1 3

Β Backbone methylene c a r b o n s , p o l y ( 1 - h e x e n e ) , 141 Benzoyl peroxide c a t a l y s t , C NMR o f PS, 190 B i n d i n g s i t e s , n e o t r o p s i n , 12-16 Biopolymers, Η s p e c t r o s c o p y , 12 Biosynthesis, cis-polyisop r e n e s , 233-44 B r a n c h e s , n a t u r a l r u b b e r , 244 Broadband d e c o u p l i n g s p i n echo p u l s e sequences, 98 INEPT, 105 B u l l v a l e n e , 2D NMR, 116 D

1« J

B u t y l branches, C chemical s h i f t s , PE, 2 5 4 f _n-Butyllithium C NMR s p e c t r a o f PS, 190 t o l u e n e i n i t i a t o r , PMMA H spectra, 4 1 3

1

271

272

NMR AND

C C-1 m e t h y l c a r b o n r e s o n a n c e , c i s p o l y i s o p r e n e s t r u c t u r e , 240 C-1 methylene c a r b o n resonance model compounds f o r p o l y i s o p r e n e s , 235 p o l y p r e n o l - 1 1 , 239f p o l y p r e n o l - 1 8 , 239f C-1 resonance aromatic, epimerized i s o t a c t i c PS, 202-11 heptad, epimerized i s o t a c t i c PS, 211-18 C-2 a x i s r o t a t i o n , e f f e c t on d i p o l a r s p i n n i n g s i d e b a n d , 50 C-2 o i e f i n i e c a r b o n r e s o n a n c e , c i s p o l y i s o p r e n e s t r u c t u r e , 240 C-5 m e t h y l c a r b o n r e s o n a n c e , c _ i s p o l y i s o p r e n e s t r u c t u r e , 240 Carbon backbone m e t h y l e n e , p o l y ( 1 - h e x e n e ) , 141 methylene, s t y r e n e o l i g o m e r , 189f phenyl C ( 1 ) , c h e m i c a l s h i f t , 194f q u a t e r n a r y a r o m a t i c , s p i n echo sequence, 101 Carbon-13 NMR 180° s i m u l t a n e o u s w i t h p r o t o n p u l s e , 99 assignments INEPT method, 119-26 p o l y i s o p r e n o i d model compounds, 234 PS, 190-93 c h e m i c a l s h i f t s , 237t e f f e c t s o f i o n i z i n g r a d i a t i o n on PE, 245-67 l i n e w i d t h , e x p r e s s i o n , 86 spectra d ichlorocarbene-mod i f i e d PBD, 1 7 0 f , 1 7 7 f , 1 7 8 f epoxidized trans-1,4p o l y b u t a d i e n e , 11f ethylene-1-hexene copolymer, 1 0 0 f , l 4 0 f NOE c o m p a r i s o n , 1 0 3 f v a r i o u s p u l s e a n g l e s , 139Γ ethylene-propylene copolymer, 124f irradiated polymers, 255f,258f,261f i s o t a c t i c p o l y - ( R , S ) - 3 - m e t h y 1-1pentene, 228 l o w - d e n s i t y PE, 121f PE, I 4 8 f PP, α-olefin p o l y m e r i z a t i o n , 227f PP, a t a c t i c , 8 f PVF, 158f 2D, c h e m i c a l exchange n e t w o r k s , 116 VTMAS, s o l i d p o l y m e r s , 83-93

MACROMOLECULES

Carbon-13 NMR s p e c t r o s c o p y c i s - p o l y i s o p r e n e s t r u c t u r e , 233-44 c r y s t a l morphology, 10 overview, 7 PS, e p i m e r i z e d i s o t a c t i c , 197-220 Carbon-13 s p i n echo p u l s e sequences, broadband-decoupled s p e c t r a , 98 Carbon-13 s p i n s y s t e m , s o l i d sample NMR, 31 Carbon-13 s p i n - l a t t i c e r e l a x a t i o n time d i s s o l v e d a r o m a t i c p o l y f o r m a l , 67-81 i s o t a c t i c PP, 87 PMMA, 89 r o t a t i n g - f r a m e , g l a s s y PS, 43 Carbon heptad r e s o n a n c e s , m e t h y l , 216 Carbon l i n e w i d t h s , m o b i l e , q u a n t i t a t i v e NMR s t u d i e s , 149 Carbon m a g n e t i z a t i o n g l a s s y PS, 44 s o l i d sample NMR, 3 6 f Carbon r e s o n a n c e s , v a r i o u s , q u a n t i t a t i v e NMR s t u d i e s , 141-43 Carbon s i g n a l s , PS, v a r i o u s , 192t,195t C a r b o n y l groups C c h e m i c a l s h i f t s , PE, 254f i r r a d i a t e d PE, 250t C a t a l y s t s , v a r i o u s , ^C NMR o f PS, 190 Chain dynamics, ^H s p e c t r o s c o p y , 12 Chain-folded s i n g l e c r y s t a l , morphology, 10 Chain t r a n s f e r , Y branch r a d i c a l , i r r a d i a t e d PE, 265 C h e m i c a l changes, PE i r r a d i a t i o n , 264 Chemical s h i f t backbone, 256 C NMR g e r a n y l g e r a n i o l i s o m e r s , 237t m e t h i n e , i r r a d i a t e d PE, 249t PE, 254f c o r r e l a t i o n , 2D NMR, 111 e t h y l e n e - p r o p y l e n e c o p o l y m e r , 125t h e p t a d , 211-18 isoprene u n i t s , 241f l o w - d e n s i t y p o l y e t h y l e n e , 123t methylene c a r b o n , s t y r e n e o l i g o m e r , 189 f p a r a m e t e r s , PS, 2 l 6 t p h e n y l C(1) c a r b o n PS, 194f s t y r e n e o l i g o m e r , 191 f s p e c t r a , WAHUHA i r r a d i a t i o n , g l a s s y PS, 44 C h e m i c a l - s h i f t a n i s o t r o p y (CSA), s o l i d sample NMR, 22,26f C h i r a l α-olefins, s t e r e o s e l e c t i v e p o l y m e r i z a t i o n , 228-30 C h i r a l s i t e s , s t e r i c c o n t r o l , α-olefin p o l y m e r i z a t i o n , 226 C h l o r i n e weight p e r c e n t , d i c h l o r o c a r b e n e - m o d i f i e d PBD, 173 1 3

1

1 3

273

INDEX

C h l o r o f l u o r o e t h y l e n e (CFE), p o l y m e r i z a t i o n i n u r e a , 156 1 - C h l o r o - 2 - f l u o r o e t h e n e , 156 C h o l e s t e r y l a c e t a t e , DEPT s p e c t r a , 107f C i s d o u b l e bonds, i r r a d i a t e d PE, 250t Cis-trans isomerized polyisoprenes, c h e m i c a l s h i f t s , 238 C i s u n i t s , arrangement i n p o l y p r e n o l s , 236-38 β-Cleavage, i r r a d i a t e d PE, 266 C l u s t e r i n g , q u a n t i t a t i v e NMR s t u d i e s , 144 C o l l e c t i v e assignments, q u a n t i t a t i v e NMR s t u d i e s , 138-45 Composition, dichlorocarbene-modified PBD, I 6 9 t Conformational t r a n s i t i o n s gauche-trans, H s p e c t r o s c o p y , 12 s i n g l e - b o n d , p o l y f o r m a l m o t i o n , 79 Copolymers, c o n t a i n i n g poly(butylène t e r e p h t h a l a t e ) , s o l i d s t a t e H NMR s t u d i e s , 55-64 C o r r e l a t i o n s p e c t r o s c o p y (COSY), 2D NMR c h e m i c a l s h i f t , 111 CPMAS NMR s p e c t r a 13c, a s f u n c t i o n o f t e m p e r a t u r e , PP, 8 5 f Ryton, 3 6 f C r o s s - l i n k i n g , n a t u r a l r u b b e r , 244 C r o s s - p o l a r i z a t i o n , s o l i d sample NMR, 31-33 C r o s s - p o l a r i z a t i o n magic-angles p i n n i n g — S e e CPMAS Cross-relaxation i s o t a c t i c PP, 87 p o l y f o r m a l m o t i o n , 70 C r y s t a l morphology, 3 Q s p e c t r o s c o p y , 10 C r y s t a l l i n i t y , and r u n number, 146 1

D e u t e r i u m q u a d r u p o l a r echo s p e c t r o s c o p y , c h a i n d y n a m i c s , 12 D i a s t e r e o m e r , s t y r e n e o l i g o m e r s , NMR s p e c t r a , 183 D i a s t e r e o m e r i c end g r o u p s , PP,α-olefin p o l y m e r i z a t i o n , 229f Dichlorocarbene-mod i f i e d p o l y b u t a d i e n e , '^c NMR, 167-79 Dimer, s t y r e n e o l i g o m e r s , NMR s p e c t r a , 183 Dimethoxybenzene, d i p o l a r sideband patterns, 48f D i p o l a r d e c o u p l i n g , s o l i d sample NMR, 24 D i p o l a r i n t e r a c t i o n s , 265 D i p o l a r r o t a t i o n a l spin-echo e x p e r i m e n t , g l a s s y PS, 44 D i p o l a r sideband p a t t e r n s , g l a s s y PS, 47-50 Dipole-dipole interactions, solid sample NMR, 2 2 , 2 6 f D i p o l e - d i p o l e r e l a x a t i o n mechanism, q u a n t i t a t i v e NMR s t u d i e s , 136 D i s t o r t i o n l e s s enhancement by p o l a r i z a t i o n t r a n s f e r (DEPT), 106 Dodecanucleotide, double h e l i c a l s t r u c t u r e , 13 Dodecanucleotide-neotropsin complex, 'H NMR s p e c t r u m , 1 5 f Double bond c i s , i r r a d i a t e d PE, 250t α-olefin, mechanism o f a d d i t i o n , 224 PE 3c c h e m i c a l s h i f t s , 2 5 4 f PE i r r a d i a t i o n , 264 reacted, dichlorocarbene-modified PBD, 173 t r a n s , i r r a d i a t e d PE, 250t,264 Dyad c o n c e n t r a t i o n s , d i c h l o r o c a r b e n e m o d i f i e d PBD, 173 1

Ε

D Data r e d u c t i o n p r o c e s s , 2D NMR, 109 D e c a l i n , 2D NMR, 116 D e c h l o r i n a t i o n , r e d u c t i v e , PVCF, 156 Decoupling r f , t e m p e r a t u r e dependence o f m o l e c u l a r m o t i o n , 86 s t y r e n e o l i g o m e r s , 186,188 D e f e c t , p e r c e n t , a r e g i c PVF samples, I64t Degeneracy, α-olefins, 224 Delay p e r i o d s , v a r i a b l e , INEPT, 106 D e n s i t y v s . mole p e r c e n t , 1 - o l e f i n , v a r i o u s copolymers, I40f D e u t e r a t e d s i t e s , C NMR, INEPT, 105 Deuterium NMR s p e c t r o s c o p y (^H NMR), s o l i d s t a t e molecular m o t i o n , 55-64 1 3

Echo p u l s e sequence, q u a d r u p o l e , 5 8 f E l e c t r i c quadrupolar i n t e r a c t i o n , s o l i d sample NMR, 2 6 f Eluent, e f f e c t , separation o f styrene o l i g o m e r s , 182 E l u t i o n volume and m o l e c u l a r w e i g h t , s t y r e n e o l i g o m e r s , 184Γ End group α-olefin p o l y m e r i z a t i o n , 225 C c h e m i c a l s h i f t s , PE, 2 5 4 f d i a s t e r e o m e r i c , PP, α-olefin p o l y m e r i z a t i o n , 229f4 r e s o n a n c e s , i r r a d i a t e d PE, 248 s a t u r a t e d , i r r a d i a t e d PE, 264,265 v a r i o u s , i r r a d i a t e d PE, 250t Epimerization PS, e q u a t i o n s , 198 1 3

274

NMR AND

Epimerization—Continued PVF, t r i a d s t e r e o s e q u e n c e s , 160 Epimerized i s o t a c t i c p o l y s t y r e n e , C NMR s t u d i e s , 197-220 E t h y l branches, C chemical s h i f t s , PE, 254f Ethylene-1-butene copolymer, run number v s . f l e x u r a l modulus and t e n s i l e s t r e n g t h , 148f Ethylene-1-hexene copolymer APT, 99 C NMR s p e c t r a , 1 0 0 f , l 4 0 f v a r i o u s p u l s e a n g l e s , 139f mole p e r c e n t 1 - o l e f i n v s . density, l45f NOE, 102,136 q u a n t i t a t i v e NMR, 138 r e l a x a t i o n t i m e , 135 E t h y l e n e - 1 - o c t e n e copolymer backbone c h e m i c a l s h i f t s , 256 mole p e r c e n t 1 - o l e f i n v s . d e n s i t y , I45f E t h y l e n e - p r o p y l e n e c o p o l y m e r , INEPT C NMR, 119-26 E v o l u t i o n p e r i o d , 2D NMR, 106 Excitation, polarization transfer, INEPT, 104f 1 3

1 3

13

13

F F a r n e s o l i s o m e r s , model compounds f o r p o l y i s o p r e n e s , 235 Ficus e l a s t i c a p o l y p r e n o l - 1 1 , C-1 methylene carbon, 239f r u b b e r f o r m a t i o n , 242 F i e l d s t r e n g t h , e f f e c t , NOE, 105 F l e x u r a l modulus v s . run number, v a r i o u s c o p o l y m e r s , 147f F l u o r i n e - 1 9 NMR s p e c t r a PCFE, 157f PVCF, 157f PVF, 9f,158f F l u o r i n e - 1 9 NMR . s p e c t r o s c o p y , overview, 7 Fluorine-19 resonances, expansion,

PVF, 159f J 6 l f , l 6 2 f Formal c a r b o n s , s p i n - l a t t i c e r e l a x a t i o n t i m e s , 71t Formal group s i m u l a t i o n p a r a m e t e r s , Weber-Helfand model, 78t Formal p r o t o n s , s p i n - l a t t i c e r e l a x a t i o n times, 71t Free i n d u c t i o n decay (FID) a c c u m u l a t i o n s , q u a n t i t a t i v e NMR, 136 i r r a d i a t e d PE, 247 Free r a d i c a l i n i t i a t o r , PMMA H spectra, 4 1

MACROMOLECULES G

Gated d e c o u p l i n g , q u a n t i t a t i v e NMR s t u d i e s , 136 Gauche m i g r a t i o n , s o l i d s t a t e NMR s t u d i e s , 56 Gauche-trans c o n f o r m a t i o n a l jumps s p e c t r o s c o p y , 12 poly(butylène t e r e p h t h a l a t e ) , 56 G e l dose, hydrogenated PE, 265 Gel f r a c t i o n determinations, i r r a d i a t e d PE samples, 247 G e l p e r m e a t i o n chromatography, s e p a r a t i o n o f s t y r e n e o l i g o m e r s , 182 G e l phase, C NMR, i r r a d i a t e d PE, 246 G e r a n i o l , model compounds f o r p o l y i s o p r e n e s , 235 G e r a n y l g e r a n i o l isomers C NMR c h e m i c a l s h i f t s , 237t model compounds f o r p o l y i s o p r e n e s , 235 t r a n s - G e r a n y l g e r a n y l pyrophosphate, r u b b e r m o l e c u l e s y n t h e s i s , 244 Ginkgo v i l o b a , p o l y p r e n o l - 1 8 , C-1 methylene c a r b o n , 2 3 9 f Glass t r a n s i t i o n temperatures, d i c h l o r o c a r b e n e - m o d i f i e d PBD, I 6 9 t Glassy p o l y s t y r e n e , molecular m o t i o n , 43-54 Goldenrod r u b b e r , b i o s y n t h e s i s , 244 13

1 3

H H-links i r r a d i a t e d HTC, 256 i r r a d i a t e d NBS 1475, 260 i r r a d i a t e d P h i l i p s M a r l e x 6003, 257 H e l f a n d - t y p e m o t i o n s , poly(butylène t e r e p h t h a l a t e ) , 58f Heptad a s s i g n m e n t s , PS a r o m a t i c C-1 c a r b o n r e s o n a n c e s , 217t Heptad C-1 r e s o n a n c e s , e p i m e r i z e d i s o t a c t i c PS, 211-18 Heptad c o n f i g u r a t i o n a l sequences, PP C spectra, 7 Heteronuclear s p i n coupling, amplitude and phase m o d u l a t i o n , 99 H e t e r o n u c l e a r 2D NMR J - r e s o l v e d , 110 s h i f t c o r r e l a t i o n , 113 H e t e r o t a c t i c stereosequences PVCF, 155 PVF, 156 t r i a d p r o b a b i l i t i e s , PVF, l 6 l f Hevea b r a s i l i e n s i s , c i s - p o l y i s o p r e n e , a l i p h a t i c c a r b o n s i g n a l s , 243f Hexamethylphosphoramide, i s o t a c t i c PS e p i m e r i z a t i o n , 198,200 n - H e x a t r i a c o n t a n e (HTC) i r r a d i a t e d , C s p e c t r a , 255f 1 3

1 3

INDEX

275

n-Hexatriacontane (HTC)—Continued irradiation, 250t,253 m a t e r i a l s , i r r a d i a t e d ΡΕ, 247 Hydrogen, m o l e c u l a r , i r r a d i a t e d PE, 264

J-modulated s p i n echo p u l s e , s p e c t r a l e d i t i n g , 98-101 J - r e s o l v e d two d i m e n s i o n a l NMR, 110 Jump model, t h r e e - b o n d , p h e n y l group motion s i m u l a t i o n , 7 6 t

L

H y d r o p e r o x i d e , i r r a d i a t e d PE, 250t

I I n s e n s i t i v e n u c l e u s e x c i t a t i o n by p o l a r i z a t i o n t r a n s f e r (INEPT) C NMR s p e c t r a e t h y l e n e - p r o p y l e n e c o p o l y m e r , 124f l o w - d e n s i t y PE, 121f method, C NMR, 119-26 p o l a r i z a t i o n t r a n s f e r , 102 u n d e s i r a b l e p r o p e r t i e s , 106 Interactions anisotropic, orientational dependence, 2 6 f d i p o l e - d i p o l e , s o l i d sample NMR, 22 Intermolecular l i n k s , irradiated PE, 264 I n t e r n u c l e a r v e c t o r , time-averaged rotation, 27f I n t r a m o l e c u l a r i n t e r a c t i o n s , 2D NMR, 115 Inversion-recovery s o l i d s t a t e H NMR, 59 s p i n - l a t t i c e r e l a x a t i o n time d a t a , 135 I r r a d i a t i o n , PE s t r u c t u r a l changes, 247 Isomerization, dichlorocarbenem o d i f i e d PBD, 171 Isopentenyl pyrophosphate, p o l y p r e n o l s y n t h e s i s , 238 Isoprene u n i t s , chemical s h i f t s , 241f I s o r e g i c PVF, 156 I s o t a c t i c poly-(R,S)-3-methy1-1 p e n t e n e , ' C NMR s p e c t r u m , 228 Isotactic polystyrenes, e p i m e r i z e d , ' C NMR s t u d i e s , 197-220 I s o t a c t i c stereosequences PVCF, 155 PVF, 156 I s o t a c t i c s t e r i c c o n t r o l , α-olefin p o l y m e r i z a t i o n , 228 I s o t a c t i c t r i a d sequences, PMMA, 4 I s o t a c t i c t r i a d stereosequence p r o b a b i l i t i e s , PVF, 161f 1 3

1 3

2

3

3

J J-coupling m u l t i p l i c i t y , PP C NMR, 7 s o l i d sample NMR, 24 J-dependence, INEPT, 106 1 3

L a m e l l a e , poly(butylène t e r e p h t h a l a t e ) c o p o l y m e r , 57,58f Larmor f r e q u e n c y p o l y f o r m a l m o t i o n , 70 s o l i d sample NMR, 28,35,37 Line narrowing i n s p i n s p a c e , s o l i d sample NMR, 28 s o l i d sample NMR, 24 L i n e - s h a p e a n a l y s i s , s o l i d sample NMR, 34 Linewidth q u a n t i t a t i v e NMR s t u d i e s , 149 vs. r a d i a t i o n dose, i r r a d i a t e d PE, 249t s o l i d s t a t e NMR, 22-33 t e m p e r a t u r e v s . m o l e c u l a r m o t i o n , 86 L i q u i d p o l y m e r s , v a r i o u s NMR e x p e r i m e n t s , 97-116 L o c a l motion, d i s s o l v e d aromatic p o l y f o r m a l , 67-81 Long c h a i n b r a n c h e s , PE C c h e m i c a l s h i f t s , 254f Long c o r r e l a t i o n t i m e , t e m p e r a t u r e dependence o f m o l e c u l a r m o t i o n , 86 1 3

M M a g i c - a n g l e , s o l i d sample NMR, 24-27 M a g i c - a n g l e s p i n n i n g (MAS) g l a s s y PS, 44 v a r i a b l e temperature, s o l i d p o l y m e r s , 83-93 M a g n e t i c moment, s o l i d sample NMR, 31 M a g n e t i z a t i o n , s o l i d sample NMR, 31 M a g n e t i z a t i o n t r a n s f e r 2D NMR, 115-16 M a r l e x 6003, P h i l l i p s C NMR s p e c t r a , 2 5 8 f irradiation, 250t,257 m a t e r i a l s , i r r a d i a t e d PE, 247 t h e r m a l d e g r a d a t i o n and i r r a d i a t i o n , 251t Meso dyad f o r m a t i o n , PVF, 160 M e t a l - p h e n y l bonds, PP i n s e r t i o n , 228 Methine c a r b o n C chemical s h i f t s , irradiated PE, 249t INEPT method, 119-26 resonance i r r a d i a t e d HTC, 256 PS, 218 s t y r e n e o l i g o m e r s , NMR s p e c t r a , 183 t e m p e r a t u r e v s . m o l e c u l a r m o t i o n , 87 1 3

1 3

276

NMR A N D MACROMOLECULES

Methyl carbon resonance c i s - p o l y i s o p r e n e s t r u c t u r e , 240 heptad , 216 model compounds f o r p o l y i s o p r e n e s , 235 temperature v s . m o l e c u l a r m o t i o n , 87 M e t h y l end g r o u p s , s t y r e n e o l i g o m e r s , 186 M e t h y l g r o u p s , INEPT method, 119-26 M e t h y l r e l a x a t i o n , t e m p e r a t u r e depen dence o f m o l e c u l a r m o t i o n , 87 M e t h y l resonance α -olefin p o l y m e r i z a t i o n , 225,226 a t a c t i c PP, 138,139f b r o a d e n i n g , v a r i o u s p o l y m e r s , 86 PMMA, 4 , 5 f t e m p e r a t u r e e f f e c t , CPMAS C o f PP, 84 Methylene carbon C c h e m i c a l s h i f t s , PE, 2 5 4 f INEPT method, 119-26 p o l y ( 1 - h e x e n e ) , 141 p o l y p r e n o l , 239f PS, a s s i g n m e n t , 192t 1 3

1 3

resonance C-1, model compounds f o r p o l y i s o p r e n e s , 235 PS, 218 s t y r e n e o l i g o m e r , 183,189Γ temperature v s . m o l e c u l a r m o t i o n , 87 Methylene resonance i r r a d i a t e d HTC, 256 q u a n t i t a t i v e NMR s t u d i e s , 1 33 Microstructure r e g i o s e q u e n c e , PVF, 160-63 s t e r e o s e q u e n c e , PVF, 156-60 Mole p e r c e n t v s . d e n s i t y , 1 - o l e f i n , v a r i o u s copolymers, I40f M o l e c u l a r motion g l a s s y PS, 43-54 s o l i d p o l y m e r s , VTMAS C NMR, 83-93 s o l i d s t a t e H NMR s t u d i e s , 55-64 M o l e c u l a r s i z e , e f f e c t , NOE, 105 M o l e c u l a r weight and e l u t i o n volume, s t y r e n e o l i g o m e r s , 184f M o l e c u l a r w e i g h t measurements, i r r a d i a t e d PE, 253 Monomer, i n s e r t i o n e x - o l e f i n p o l y m e r i z a t i o n , 225 Monomer c o m p o s i t i o n , d i c h l o r o c a r b e n e m o d i f i e d PBD, 173 d i c h l o r o c a r b e n e - m o d i f i e d PBD, 174 c i s - p o l y i s o p r e n e s t r u c t u r e , 240 Nuclear quadrupole e f f e c t s , s o l i d sample NMR, 23 Nuclear s p i n system, p e r t u r b a t i o n , 97-116 Number-average m o l e c u l a r w e i g h t , q u a n t i t a t i v e NMR s t u d i e s , 146-50 1 J

2

Monte C a r l o s i m u l a t i o n s , PS e p i m e r i z a t i o n , 198,201 M o t i o n i n s o l i d s , 35-40 M o t i o n a l e n v i r o n m e n t s , segmented c o p o l y m e r s , 61

Ν N a t u r a l rubber a l i p h a t i c carbon s i g n a l s i n c i s p o l y i s o p r e n e , 243f b i o s y n t h e s i s , 234,244 NBS 1475 C NMR s p e c t r a , 261 f i r r a d i a t i o n , 252t,257 m a t e r i a l s , i r r a d i a t e d PE, 247 N e o t r o p s i n , b i n d i n g s i t e s , 12-16 Neotropsin-dodecanucleotide complex, Η NMR s p e c t r u m , 1 5 f N e r o l , p o l y i s o p r e n e model compounds, 235 Nomenclature, PE, 2 5 4 f N u c l e a r Overhauser e f f e c t (NOE) consequences, 101 d i c h l o r o c a r b e n e - m o d i f i e d PBD, 173 q u a n t i t a t i v e NMR s t u d i e s , 135-38 2D NMR, 115 N u c l e a r Overhauser enhancement (NOE) a n t i b i o t i c n e o t r o p s i n , 12 comparison o f C s p e c t r a , 103Γ Number-average sequence l e n g t h , dichlorocarbene-modified PBD, 173,175t 1 3

1

3

0 Qf-Olefin, s t e r e o s p e c i f i c p o l y m e r i z a t i o n , 223-30 1 - O l e f i n , mole p e r c e n t v s . d e n s i t y , various copolymers, I40f O l e f i n i c c a r b o n atom r e s o n a n c e , C-2, 240 Oligomerization, styrene, preparation and s e p a r a t i o n , 182 O l i g o m e r s , s t y r e n e , NMR s p e c t r a , 181-93 O r g a n o m e t a l l i c c o c a t a l y s t s , α-olefin p o l y m e r i z a t i o n , 226 O r i e n t a t i o n a l dependence, a n i s o t r o p i c interactions,26f Oxirane u n i t s , trans-1,4p o l y b u t a d i e n e , 10 Oxygen-oxygen a x i s , p o l y f o r m a l s p i n r e l a x a t i o n and l o c a l m o t i o n , 80 Ρ 2

P a i r gauche m i g r a t i o n , s o l i d s t a t e H NMR s t u d i e s , 56 Peak a s s i g n m e n t s , C NMR, d i c h l o r o c a r b e n e - m o d i f i e d PBD, 172f 1 3

INDEX

277

Peak h e i g h t i n t e n s i t y ethylene-propylene copolymer, 125t l o w - d e n s i t y p o l y e t h y l e n e , 123t Pentad regiosequence n o n e q u i v a l e n t , 162t observed p r o b a b i l i t i e s , 164t resonance e p i m e r i z e d i s o t a c t i c PS, 210 PS, 216 sequence, PVF, F NMR, l 6 4 t Pentamer, s t y r e n e o l i g o m e r s , NMR s p e c t r a , 187-90 Perturbation, nuclear spin system, 97-116 P e t r o l e u m m i x t u r e s , s p i n echo sequence, 101 Phase m o d u l a t i o n , h e t e r o n u c l e a r s p i n c o u p l i n g , 99 Phenyl C(1) carbon c e n t r a l , chemical s h i f t , 194f PS s i g n a l a s s i g n m e n t , 195t styrene oligomer, 191f P h e n y l group motion s i m u l a t i o n three-bond jump model, 7 6 t Weber-Helfand model, 7 7 t Phenyl proton p o l y f o r m a l m o t i o n , 70 s p i n - l a t t i c e r e l a x a t i o n times, 71t P h i l l i p s M a r l e x 6 0 0 3 — See M a r l e x 6003, Phillips P l a n a r z i g z a g p r o j e c t i o n , sequences i n v i n y l polymer c h a i n s , 6 f Polarization transfer h e t e r o n u c l e a r J - r e s o l v e d 2D NMR, 1 1 1 s p e c t r a l e d i t i n g , 101-106 t r a n s - 1 , 4 - P o l y b u t a d i e n e (PBD) ^ N M R , 10 c r y s t a l , 11f P o l y b u t a d i e n e (PBD), d i c h l o r o c a r b e n e m o d i f i e d , C NMR, 167-79 Poly(butylène t e r e p h t h a l a t e ) , s o l i d s t a t e H NMR s t u d i e s , 55-64 Poly(p-t-butylstyrene), rotating-frame s p i n - l a t t i c e r e l a x a t i o n , 52t P o l y c a r b o n a t e , s o l i d BPA, t w o f o l d jumps, 80 P o l y ( c h l o r o f l u o r o e t h y l e n e ) (PCFE) 1 9 NMR s p e c t r a , 157f m i c r o s t r u c t u r e s , 155 Poly(o-chlorostyrene) d i p o l a r r o t a t i o n a l spin-echo C NMR spectra, 46f rotating-frame s p i n - l a t t i c e r e l a x a t i o n , 52t P o l y e t h y l e n e (PE) C NMR s p e c t r a , 148f h i g h - d e n s i t y , 134f l o w - d e n s i t y , 121f INEPT, 119-26 1 9

1 3

2

F

1 3

1 3

Polyethylene (PE)--Continued radiation-induced structural changes, 245-67 t h e r m a l d e g r a d a t i o n , 266 P o l y f o r m a l , d i s s o l v e d a r o m a t i c , sp i n r e l a x a t i o n and l o c a l m o t i o n , 67-81 P o l y ( 1 - h e x e n e ) , methylene c a r b o n s , 141 Polyisoprene, c i s - t r a n s isomerized, c h e m i c a l s h i f t s , 238 P o l y i s o p r e n e , l i n e a r , ω - and Q f - t e r m i n a l u n i t s , 242 cis-Polyisoprene a l i p h a t i c carbon s i g n a l s , 241f,243f b i o s y n t h e s i s mechanism, 2 4 3 f s t r u c t u r a l c h a r a c t e r i z a t i o n , 233-44 Poly(g-isopropylstyrene) d i p o l a r r o t a t i o n a l sideband i n t e n s i t i e s , 49t rotating-frame s p i n - l a t t i c e relaxation, 52t Polymer c h a r a c t e r i z a t i o n , C NMR INEPT method, 119-26 concentrations, quantitative NMR, 133-35 l i q u i d , NMR, 97-116 m o t i o n , NMR, 34 preparation e p i m e r i z e d i s o t a c t i c PS, 200 n o v e l r e g i o r e g u l a r PVF, 154 s o l i d s t a t e H NMR s t u d i e s , 57 quantitative analyses, C NMR, 131-50 s o l i d m o l e c u l a r m o t i o n , VTMAS C 1 3

2

1 3

1 3

NMR, 83-93 Polymerization d e g r e e s , q u a n t i t a t i v e NMR, 146 stereospecific, α-olefins, 223-30 Ziegler-Natta, c i s - l - d ^ p r o p e n e , 224 P o l y ( m e t h y l m e t h a c r y l a t e ) (PMMA) atactic, spin-lattice i n t e r a c t i o n s , 38 H s p e c t r a , 4,5f t e m p e r a t u r e v s . m o l e c u l a r m o t i o n , 89 Poly-(R,S)-3-methy1-1-pentene, i s o t a c t i c , C NMR, 228 P o l y ( m e t h y l s t y r e n e ) (PMS), r o t a t i n g frame s p i n - l a t t i c e r e l a x a t i o n , 5 2 t P o l y p h e n y l e n e s u l f i d e ( P P S ) , CPMAS NMR s p e c t r u m , 36f Polyprenol b i o s y n t h e s i s , 238 c h e m i c a l s h i f t s , 238 s t r u c t u r a l c h a r a c t e r i z a t i o n , 236-38 P o l y p r o p y l e n e (PP) atactic, C spectra, 7 C NMR s p e c t r a , 227f CPMAS C s p e c t r a v s . temperature, 8 5 f 1

1 3

1 3

1 3

1 3

278

NMR

Polypropylene (PP)—Continued heptad assignments, 217t isotactic, spin-lattice i n t e r a c t i o n s , 38,87 α -olefin p o l y m e r i z a t i o n , 225,226 s y n d i o t a c t i c , α-olefin p o l y m e r i z a t i o n , 224 t e m p e r a t u r e v s . m o l e c u l a r m o t i o n , 84 P o l y s t y r e n e (PS) a t a c t i c , rotating-frame s p i n - l a t t i c e ^ r e l a x a t i o n , 52t C NMR s i g n a l a s s i g n m e n t , 190-93 c NMR s t u d i e s , 197-220 d i p o l a r sideband p a t t e r n s , 4 8 f g l a s s y , m o l e c u l a r m o t i o n , 43-54 NMR s p e c t r a , 181-93 r i n g m o t i o n , 50 s p i n - l a t t i c e i n t e r a c t i o n s , 38 Poly-co-styrenesuIfone), rotatingframe s p i n - l a t t i c e r e l a x a t i o n , 5 2 t P o l y v i n y l c h l o r o f l u o r i d e ) (PVCF), s y n d i o r e g i c u n i t s , 10 P o l y ( v i n y l f l u o r i d e ) (PVF) *C NMR s p e c t r a , 158f F NMR s p e c t r a , 7,158f e x p a n s i o n o f ^°F r e s o n a n c e s , 159f,161f,162f i s o r e g i c , ^F r e s o n a n c e s , 1 6 1 f n o v e l r e g i o r e g u l a r , 153-64 Poly(vinylidene chlorofluoride) (PVCF), r e d u c t i v e d e c h l o r i n a t i o n , 156 P o t a s s i u m t - b u t o x i d e , i s o t a c t i c PS e p i m e r i z a t i o n , 198,200 Potassium hydroxide, s o l i d , d i c h l o r o c a r b e n e - m o d i f i e d PBD, 176 Progressive saturation, spin l a t t i c e r e l a x a t i o n time d a t a , 135 Propene, c i s - l - d ^ - , Z i e g l e r - N a t t a p o l y m e r i z a t i o n , 224 Property-structure relationships, q u a n t i t a t i v e NMR s t u d i e s , 144-46 P r o p y l end g r o u p s s t y r e n e o l i g o m e r s , 182,186 Propylene a t a c t i c , methyl r e g i o n r e s o n a n c e s , 138,139Γ i n s e r t i o n , α-olefin p o l y m e r i z a t i o n , 225,226,228 Proton decoupling f i e l d , temperature dependence o f m o l e c u l a r m o t i o n , 86 P r o t o n f l i p a n g l e , v a r i a b l e , DEPT, 106 P r o t o n moment, s o l i d sample NMR, 31 P r o t o n NMR ( 'H NMR) s p e c t r o s c o p y , o v e r v i e w , 12 P r o t o n NMR ( H NMR) s p e c t r u m , neotropsin-dodecanucleotide complex, 15f Proton p u l s e , simultaneous with ^ C 180° p u l s e , 99 13

l

1 9

3

AND MACROMOLECULES

Proton s p i n - l a t t i c e r e l a x a t i o n times, dissolved aromatic p o l y f o r m a l , 67-81 Proton s p i n s t a t e s , i n v e r s i o n , p o l a r i z a t i o n t r a n s f e r , 102 Protonated phenyl carbons, s p i n l a t t i c e r e l a x a t i o n times, 71t Pulse angle and d e l a y , i r r a d i a t e d PE, 247 various q u a n t i t a t i v e NMR, 136 r e l a t i v e peak h e i g h t s and i n t e g r a t e d a r e a s , 1371 P u l s e d i a g r a m s , s o l i d sample NMR, 4 0 f Pulse r e p e t i t i o n r a t e , d i c h l o r o c a r b e n e - m o d i f i e d PBD, 173 P u l s e sequence COSY, 111 d i p o l a r s p i n - e c h o , g l a s s y PS, 4 5 f P u l s e s p a c i n g , v a r i o u s , r e l a t i v e peak h e i g h t s and i n t e g r a t e d a r e a s , 1371 Q Q u a d r u p o l a r echo d e l a y , s o l i d s t a t e Η NMR s t u d i e s , 63 Quadrupole echo H NMR s p e c t r a , c a l c u l a t e d v s . e x p e r i m e n t a l , 61 Quadrupole echo p u l s e sequence, s o l i d s t a t e H NMR s t u d i e s , 5 8 f 2

2

1λ J

Q u a n t i t a t i v e a n a l y s e s , polymer, C NMR, 131-50 Quaternary aromatic carbons, spin-echo sequence, 101 R Racemization, reductive d e c h l o r i n a t i o n , PVCF, 160 R a d i a t i o n dose v s . l i n e w i d t h s , 2 4 9 t Radiation-induced s c i s s i o n , i r r a d i a t e d PE, 266 R a d i a t i o n - i n d u c e d s t r u c t u r a l changes, PE, 245-67 R a d i a t i o n y i e l d s , i r r a d i a t e d HTC, 257 Radiolytic oxidation i r r a d i a t e d PE, 266 s o l i d h i g h - d e n s i t y , 257 Reacted d o u b l e bonds, d i c h l o r o c a r b e n e m o d i f i e d PBD, 173 Reaction c o n d i t i o n , dichlorocarbenem o d i f i e d PBD, I 6 9 t R e a c t i o n mechanism, α-olefin p o l y m e r i z a t i o n , 223-30 R e a c t i v i t y r a t i o s , a r e g i c PVF s a m p l e s , 164t R e d u c t i v e d e c h l o r i n a t i o n , PVCF, 156 R e f o c u s i n g p e r i o d , INEPT s p e c t r a l e d i t i n g , 105 R e f o c u s i n g p u l s e , 180°, 98-106

INDEX

279

Regioregular p o l y ( v i n y l f l u o r i d e s ) ( P V F ) , s y n t h e s i s and c h a r a c t e r i z a t i o n , 153-64 Regioregularity, F NMR, 7 Regiosequence m i c r o s t r u c t u r e , PVF, 160-63 Regiosequence p e n t a d s nonequivalent, l62t observed p r o b a b i l i t i e s , 164 f PVF, F NMR, I 6 4 t R e g i o s p e c i f i c i t y , α-olefin p o l y m e r i z a t i o n , 224 R e l a t i v e peak h e i g h t s , q u a n t i t a t i v e NMR s t u d i e s , 137t Relaxation rotating-frame carbon s p i n l a t t i c e , g l a s s y PS, 43 s o l i d s t a t e , 35-40 R e l a x a t i o n time q u a n t i t a t i v e NMR s t u d i e s , 135-38 s p i n - l a t t i c e , heteronuclear chemical s h i f t c o r r e l a t i o n , 114 Resonance c a r b o n , q u a n t i t a t i v e NMR s t u d i e s , 141-43 gauche-trans s h i f t , 7 v a r i o u s , model compounds f o r p o l y i s o p r e n e s , 235 Resonance l i n e w i d t h , c o n c e n t r a t i o n e f f e c t , 133 Restricted rotation, polyformal spin r e l a x a t i o n and l o c a l m o t i o n , 77 Rf d e c o u p l i n g , t e m p e r a t u r e dependence o f m o l e c u l a r m o t i o n , 86 Ring motion g l a s s y PS, 47 s m a l l - a m p l i t u d e l o w - f r e q u e n c y , 50 Rotating-frame r e l a x a t i o n c a r b o n s p i n - l a t t i c e , g l a s s y PS, 43 e f f e c t s , i s o t a c t i c PP, 87 s o l i d sample NMR, 37-38 R o t a t i o n a l i s o m e r i c s t a t e model, me thy 1 c a r b o n heptad r e s o n a n c e s , 216 Rubber F i c u s e l a s t i c u s , 242 g o l d e n r o d , b i o s y n t h e s i s , 244 n a t u r a l , b i o s y n t h e s i s , 234,244 Run number vs. density, various c o p o l y m e r s , 145 f v s . f l e x u r a l modulus and t e n s i l e strength a t y i e l d , various copolymers, I48f q u a n t i t a t i v e NMR s t u d i e s , 144 R y t o n , CPMAS NMR s p e c t r u m , 3 6 f 1 9

1 9

S

Scission, radiation-induced, i r r a d i a t e d PE, 266 Segmental m o t i o n , p o l y f o r m a l s p i n r e l a x a t i o n and l o c a l m o t i o n , 70 v a r i o u s models, 72-81 Selective population inversion (SPI), p o l a r i z a t i o n t r a n s f e r method, 102 Sequence, h e p t a d c o n f i g u r a t i o n a l , PP C spectra, 7 Sequence l e n g t h s , number-average, dichlorocarbene-modified PBD, 173,175t S h i e l d i n g , l o n g - r a n g e , PS, 210 Short c o r r e l a t i o n time, temperature dependence o f m o l e c u l a r m o t i o n , 86 S i d e - c h a i n r o t a t i o n . p o l y m e r , 34 S i g n a l a s s i g n m e n t , C NMR, PS, 190-93 Signal excitation, polarization trans f e r method, 102 Signal-to-noise r a t i o , d i c h l o r o c a r b e n e - m o d i f i e d PBD, 173 Single-bond conformational t r a n s i t i o n s , p o l y f o r m a l m o t i o n , 79 Single c r y s t a l , chain-folded, morphology, 10 S i n g l e - f r e q u e n c y o f f - r e s o n a n c e decoup l i n g (SFORD), 98 S i z e e x c l u s i o n mechanism, s e p a r a t i o n o f s t y r e n e o l i g o m e r s , 182 Sodium h y d r o x i d e , d i c h l o r o c a r b e n e m o d i f i e d PBD, 176 S o l i d polymer NMR t e c h n i q u e s , 21-40 VTMAS C NMR, 83-93 S o l i d s t a t e H NMR m o l e c u l a r m o t i o n , 55-64 poly(butylène t e r e p h t h a l a t e ) , I 4 f Solidago a l t i s s i m a , cis-polyisoprene, a l i p h a t i c carbon s i g n a l s , 241f Solvent q u a n t i t a t i v e NMR s t u d i e s , 132 s e p a r a t i o n o f s t y r e n e o l i g o m e r s , 182 Spectral density c o m p o s i t e , p o l y f o r m a l , 72 p o l y f o r m a l m o t i o n , 70 Spectral editing J-modulated s p i n echo p u l s e , 98-101 p o l a r i z a t i o n t r a n s f e r , 101-106 Spectral resolution, dichlorocarbenem o d i f i e d PBD, 173 S p e c t r a l w i d t h , i r r a d i a t e d PE, 247 Spin-echo pulse ' C , broadband-decoupled s p e c t r a , 98 d e l a y e d d e t e c t i o n , 99 d i p o l a r r o t a t i o n a l , g l a s s y PS, 44 J-modulated, s p e c t r a l e d i t i n g , 98-101 Spin-lattice relaxation C , i s o t a c t i c PP, 87 heteronuclear chemical s h i f t c o r r e l a t i o n , 114 1 3

1 3

2

3

1 3

S a t u r a t e d end g r o u p s , i n c r e a s e , i r r a d i a t e d PE, 265

280

NMR

Spin-lattice relaxation—Continued q u a n t i t a t i v e NMR, 135 r o t a t i n g - f r a m e c a r b o n , g l a s s y PS, 43 s o l i d sample NMR, 35-40 S p i n - l o c k i n g , s o l i d sample NMR, 3 2 f , 3 3 Spin pattern realignment, h e t e r o n u c l e a r J - r e s o l v e d 2D NMR, 111 Spin r e l a x a t i o n DEPT, 106 d i s s o l v e d a r o m a t i c p o l y f o r m a l , 67-81 Spin system, n u c l e a r , p e r t u r b a t i o n , 97-116 S p i n n i n g r a t e s , s o l i d sample NMR, 25 Stereoregulation, polymerization of CFE i n u r e a , 156 S t e r e o s e q u e n c e d i s t r i b u t i o n , PS, f r e e - r a d i c a l p o l y m e r i z a t i o n , 199 Stereosequence m i c r o s t r u c t u r e , PVF, 156-60 S t e r e o s e q u e n c e p e n t a d s , PVF, F NMR, I 6 4 t S t e r i c c o n t r o l , α-olefin p o l y m e r i z a t i o n , 225-28 Stochastic d i f f u s i o n , polyformal spin r e l a x a t i o n and l o c a l m o t i o n , 72 Structure-property r e l a t i o n s h i p s , q u a n t i t a t i v e NMR, 144-46 S t y r e n e o l i g o m e r s , NMR s p e c t r a , 181-93 Syndioregic units PVCF, 10 PVF monomer, 7 S y n d i o t a c t i c p o l y p r o p y l e n e , α-olefin p o l y m e r i z a t i o n , 224 S y n d i o t a c t i c stereosequences PMMA, t r i a d , 4 PVCF, 155 PVF, 156 1 9

triad probabilities,

I6lf

Τ Temperature, e f f e c t , PP CPMAS 13C, 84 Tensile strength a t y i e l d vs. run number, v a r i o u s c o p o l y m e r s , I 4 8 f Terpenes, a c y c l i c 13C NMR o f n a t u r a l p o l y i s o p r e n e s , 234 a c y c l i c , c h e m i c a l s h i f t s , 238 T e t r a m e r , s t y r e n e o l i g o m e r s , NMR s p e c t r a , 187-90 Tetranucleotide core, neotropsin b i n d i n g , 16 Thermal d e g r a d a t i o n , v a r i o u s p o l y e t h y l e n e s , 251t,257,260,266

AND MACROMOLECULES

Three-bond jump model, m o t i o n , 72,76t T r a n s d o u b l e bonds, i r r a d i a t e d PE, 250t,264 Trans-gauche c o n f o r m a t i o n a l t r a n s i t i o n s , H NMR, 12 Trans-geranylgeranyl pyrophosphate, r u b b e r m o l e c u l e s y n t h e s i s , 244 T r a n s u n i t s , p o l y p r e n o l s , 236-38 T r i a d , comonomer, d i c h l o r o c a r b e n e m o d i f i e d PBD, 175f Triad concentrations, dichlorocarbenem o d i f i e d PBD, 174 T r i a d sequences e p i m e r i z e d i s o t a c t i c PS, 202 PMMA, 4 Triad stereosequences d i s t r i b u t i o n s , a t a c t i c PS, 209 p r o b a b i l i t i e s , PVF, 161f PVCF, 155 T r i f l u o r o b o r o n e t h e r a t e , PS 13c NMR s p e c t r a , 190 T w o - d i m e n s i o n a l NMR (2D NMR) d i s c u s s i o n , 106-16 2

U U r e a , p o l y m e r i z a t i o n o f CFE, 156 V Vacuum, e f f e c t , PE i r r a d i a t i o n , 266 V a r i a b l e temperature magic-angle s p i n n i n g c a r b o n - 1 3 NMR (VTMAS 13c NMR), s o l i d p o l y m e r s , 83-93 Vector, i n t e r n u c l e a r , time-averaged rotation, 27f V i n y l u n s a t u r a t i o n s , d i s a p p e a r a n c e , PE i r r a d i a t i o n , 264 W WAHUHA i r r a d i a t i o n g l a s s y PS, 44 s o l i d sample NMR, 28,30f Weber-Helfand model, m o t i o n , 74-79 Y Y b r a n c h , i r r a d i a t e d PE, 256-260 Ζ

Z - m a g n e t i z a t i o n , r e s t o r a t i o n , APT, 99 Ziegler-Natta polymerization, cis-1d -propene , 224