Macromolecular Chemistry Volume 2

A Review of the Literature Published during 1979 and 1980

By A. D. Jenkins, J. F. Kennedy

The Royal Society of Chemistry

Copyright © 1982 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-866-0

Contents

Chapter 1 Introduction By A. D. Jenkins and J. F. Kennedy, 1,
Chapter 2 Chain Reaction Polymerization,
Chapter 3 Step Growth Polymerization,
Chapter 4 Plasma Polymerization By D. Shuttleworth, 77,
Chapter 5 Polysaccharides and Glycoproteins By R. J. Sturgeon, 84,
Chapter 6 Natural Polymers: Proteinsand Enzymes By D. R. Thatcher, 111,
Chapter 7 Natural Polymers: Nucleic Acids By J. T. Knowler and J. P. Goddard, 138,
Chapter 8 Inorganic Polymers By K. M. Hoch, 162,
Chapter 9 Configurations By S. B. Boss-Murphy, 174,
Chapter 10 Nuclear Magnetic Resonance Spectroscopy By F. Heatley, 190,
Chapter 11 Neutron Scattering Studies By D. G. H. Ballard and E. Janke, 203,
Chapter 12 Polymer Crystallization By J. N. Hay, 214,
Chapter 13 Characterization of Synthetic Polymers By J. M. G. Cowie, 234,
Chapter 14 Engineering and Technology By B. J. Briscoe, S. M. Richardson, and D. J. Walsh, 251,
Chapter 15 Reactions on Polymers: Polymer Modification By G. G. Cameron, 271,
Chapter 16 Polymer Degradation,
Chapter 17 Reactions in Macromolecular Systems By D. A. Crombie and M. I. Page, 322,
Chapter 18 Biomedical Applications of Polymers By B. J. Tighe, 347,
Chapter 19 Computer Applications By A. H. Fawcett, 361,
Author Index, 387,

CHAPTER 1

Introduction

BY A. D. JENKINS AND J. F. KENNEDY

It is in the nature of macromolecules that it is hard to define any of their properties in precise terms, indeed it is no easy matter to define ‘macromolecule’ in a way that would be universally accepted even by those intimately concerned with such materials. Consequently, in setting out to delineate the areas to be covered in a report of recent research in the field of macromolecules, many demarkation issues have to be faced and resolved.

In aiming at a list of contents for this volume, the Senior Reporters have had to pay attention not only to the subject matter itself but also to the existence of other series of Specialist Periodical Reports dealing with contiguous areas of chemistry. Without the least desire to poach on other people’s preserves, some small degree of overlap seems to be the only reasonable solution. This occurs in dealing with fields such as colloids, carbohydrates, proteins, and nucleic acids. It has seemed reasonable in the interests of providing a comprehensive treatment of the range of macromolecules to feel justified in including modest discussion in the context of ‘macromolecules’ rather than simply referring the reader to the individual volumes in which the information he requires may be buried within a large bulk of (to him) irrelevant material.

Another problem concerns the frequency with which individual topics should be examined. The more global subjects, like polymerization chemistry, will no doubt be treated in each issue but smaller topics, for example specific techniques for characterization, may adequately be dealt with if they are reviewed at intervals. Of course, much depends on whether a particular topic is advancing rapidly, in which case we recognize that there is an obligation to bring the reader as up-to-date as possible.

It will be apparent from the foregoing paragraph that the list of chapter headings cannot be a constant factor although, as a general principle, we maintain a watching brief over the following broad areas: Polymerization Chemistry; Particular Classes of Polymers; Natural Polymers; Degradation; Polymers as Catalysts and Reagents; Properties of Solid Polymers, Crystalline and Amorphous; Properties of Polymer Solutions; Characterization Techniques; Theoretical Treatment of Polymers; Applications of Polymers; Polymer Engineering.

Inclusion of the last two areas states our intention to embrace technology as well as pure science. Macromolecules, in the shape of synthetic plastics, fibres, films, paints, adhesives and the like, make an enormous contribution to everyday life; they occupy a large slice of the chemical industry in preparation and processing operations and the borderline between polymer science and plastics technology is very diffuse. It is fully in accordance with the attitude of many people in the field, and certainly of the Science and Engineering Research Council, that one should as far as possible integrate the more academic and the more practical aspects of research on polymers, and that is the stand adopted here.

In keeping with the original plan of biennial frequency of publication, the literature survey represented in this volume is principally concerned with the years 1979 and 1980. Work earlier than 1977, the initial year of coverage of the Series, was only cited in Volume 1 where it provided an important basis for current papers and an introduction of certain phenomena into the Series.

Each chapter opens with an introduction which is specialized with respect to the contents of the chapter and others outline the context of the chapter particularly for those not completely familiar with the subject treated. Reference to the corresponding chapter in Volume 1 will also be helpful in this respect.

Our Reporters have been asked to collate rather than to criticize but they have not been debarred from offering a personal opinion on points of particular interest. It is our hope that the reader will find this book a useful guide to the most important recent literature on the chemistry of macromolecules, and that future volumes will keep the major topics under perpetual review. The Senior Reporters will be glad to examine comments, suggestions, and offers of contributions to help the series to live up to their hopes.

CHAPTER 2

Chain Reaction Polymerization

BY D. J. DUNN, J. M. ROONEY, R. N. YOUNG, J. C. BEVINGTON, D. C. BLACKLEY, AND B. M. TIDSWELL

PART I Cationic Polymerization

by D. J. Dunn and J. M. Rooney

1 Introduction

This survey deals with homogeneous addition polymerizations involving positively-charged propagating species, typically carbenium, oxonium, sulphonium, or immonium ions.

General reviews were published on the polymerization of vinyl monomers, and cyclic monomers. Gandini and Cheradame provided an extensive and critical review of the initiation step in cationic polymerization, while Ledwith reviewed the formation and reactivity of free propagating ions.

The 5th International Symposium on Cationic and Other Ionic Polymerizations was held in Kyoto, Japan, in April 1980.

2 Initiation Systems

Metal Alkyl and Metal Halide Compounds. — Interactions between metal alkyl or metal halide compounds and cationically polymerizable monomers have been studied recently in detail. Experimental difficulties arise primarily owing to the extreme sensitivity of these initiation systems to trace amounts of impurities. Water is the most ubiquitous impurity and exerts a marked co-catalytic effect on the 1,1-diphenylethylene (DPE)–AlCl3 system, resulting in a two-stage initiation process. Direct initiation of isobutylene polymerization by aluminium halide cations generated through self-ionization in highly purified solvents has been demonstrated, and the existence of stable tertiary carbenium tetrahaloaluminates (analogous to spectroscopically observed derivatives of gallium halides) verified by conductivity measurements. Theoretical stabilities of the counterions in such systems have been analysed by quantum mechanical methods.

The problem of determining the exact nature of the initiating species in cationic polymerizations of alkenes conducted in the presence of metal perchlorates led to the use of a sterically hindered proton scavenger, 4-methyl-2,6-di-t-butyl pyridine. Since initiation occurred in solutions containing this compound it was concluded that protons derived from impurities were not responsible for the reaction and direct initiation by the metal perchlorates was postulated. However, results from experiments with the BCl3/H2O system suggest that hindered pyridines act as proton transfer agents. Differences in mechanism between protonic acid – and metal halide – initiated oligomerizations of styrene and DPE have been defined.

Neither chromyl chloride nor the mixed initiator diethyl zinc/phosphoryl chloride were found to induce stereoregularity in the polymerization of alkyl vinyl ethers.

Organic Cation Salts. — In general, organic cation salts, which can be preformed or generated in situ, operate through relatively simple initiation mechanisms. Consequently the efficacy of these initiators can often be traced directly to structural features in the cation, the counterion, or the monomer.

Significant differences in reactivity were observed between p-methylbenzyl bromide and benzyl bromide in the formation of carbenium ions by reaction with silver hexafluorophosphate. The former compound in conjunction with AgPF6 was found to be a highly efficient initiator for the cationic polymerization of tetrahydrofuran (THF) at — 10 °C. In situ formation of the benzyl ion was much slower, rendering syntheses of monodisperse poly(THF) impossible.

Theoretical calculations of the stabilities of various hexahaloantimonates have shown that the rupture of an Sb–Cl bond requires more energy than that of an Sb-Br bond, and the relative positions of chlorine and bromine atoms within a mixed halogen counterion can affect the ease of bond breakage. Model compound studies of the reaction between p-methyl benzyl chloride, triethyl aluminium, and 2,4,4-trimethylpent-l-ene (dimeric isobutylene) indicated that the principal chain termination reactions involve hydridation and ethylation resulting from counterion decomposition.

Pulse radiolytic techniques facilitated estimates of the rate constants for reactions between benzyl and diphenylmethyl carbenium ions and a series of alkenes and dienes. Differences in reactivity were influenced by monomer structure, primarily the position and strength of electron-donating substituents.

Photochemical and Radiation Initiation. — The topics of photochemical initiation of cationic polymerizations, radiation-induced initiation, and their interrelationships have been reviewed recently.

Factors influencing the reactivity of triarylsulphonium salts include counterion type and the nature of ring substituents. For a series of triphenyl sulphonium salts, reactivity was found to vary with the counterion in the order SbF-6 > AsF-6 > PF-6 > BF-4. A reaction scheme proposed to account for the activity of these salts as initiators of both free-radical and cationic polymerizations is outlined in reactions (1) — (3),

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

in which S–H represents a solvent molecule. By eliminating the possibility of acid formation through selection of a suitable counterion (e.g., replacing BF-4 with BPh-4) cationic polymerization is suppressed and free-radical polymerization proceeds alone. Salts with extended conjugation due to ring substitution showed increased activity, providing an explanation for the enhanced performance of impure samples of triphenyl sulphonium salts. Dialkyl-4-hydroxyphenyl sulphonium salts were believed to form protonic acids by a reversible dissociation to an ylid through a mechanism analogous to that outlined for dialkylphenacyl sulphonium salts in reaction (4).

[FORMULA NOT REPRODUCIBLE IN ASCII] (4)

The use of free-radical sources that undergo a one-electron transfer oxidation in conjunction with cationic photo-initiators has been examined. Regeneration of potential initiators leads to high yields of active centres from each photon absorbed by the system.

Cationic polymerization of isobutylene induced by β radiation from tritium resulted in high-molecular-weight polymer at -78 °C. The process involved diffusion of cations from the gas phase into the liquid phase and the rate of tritium decay was found to be independent of temperature.

Polymerizations induced by γ radiation proceed by both cationic and free-radical mechanisms. Predominance of the cationic process can be assured by exhaustive purification of monomers and solvents or by employing high radiation dose rates (from electron beam sources). The rate of cationic polymerization has been shown to be strongly influenced by the dielectric constant of the medium, giving rise to discrepancies between radiation-induced bulk polymerizations and chemically initiated solution polymerizations. These discrepancies have been attributed to specific solvation of propagating ions by monomer and polymer.

3 Propagating Species and their Reactivities

Useful reviews include preparation, reactivity, and spectroscopic identification of carbocations and the use of pico- and nano-second pulse radiolysis for observing both monomer and polymer cations. Theoretical discussions on the use of Mayo plots in cationic polymerizations have appeared. Morawetz derived an expression for [??]w/[??]n for a system involving instant and complete initiation with termination but no transfer and showed why this is normally less than the most probable distribution of [??]w/[??]n = 2. Heublein et al. continued their mathematical analysis of cationic homo and copolymerizations using non-linear parameter functions.

Styrene and Styrene Derivatives. — The conceptually very simple polymerizations initiated by Bronsted acids continue to be plagued by conflicting data and interpretations. Thus, the system styrene/CF3SO3H/CH2Cl2 was described as being kinetically rather simple, but could give polymers with uni-, bi-, or tri-modal molecular-weight distributions depending on the initial concentration of initiator. The kinetic data was challenged and the challenge refuted. In contrast, other authors who studied the same polymerization in ethylene dichloride using stop-flow techniques with rapid-scanning u.v. spectroscopy, reported that the kinetics could be explained by a mechanism involving propagation from free ions and ion-pairs in dynamic equilibrium. By making three basic assumptions: (a) the total concentration of free-ions and ion-pairs can be measured by their absorption at 340 nm; (b) only 1 to 4% of the initiator is consumed; (c) ion-dissociation is totally suppressed by the addition of a common-anion salt, it was possible to derive propagation rate constants for free-ions (kp+) and ion-pairs (kp+) of 3 × 106 and 1 × 105 dm3 mol-1 s-1. These values are two orders of magnitude higher than any previous values reported in chemically initiated systems and future workers should question the validity of the assumptions. Using the same techniques Sawamoto and Higashimura measured overall kp values for p-methoxystyrene in (CH2Cl)2 or (CH2Cl)2/CCl4 mixtures initiated by CF3SO3H or CH3COClO4, although they report an invisible (i.e., non u.v. absorbing) propagating species in these systems. Oligomer formation from styrene and CF3SO3H in CCl4 and benzene was studied by Hamaya and Yamada. Higashimura reviewed the subject of dissociated and non-dissociated propagating species. The effect of the counterion on the styrene/HClO4/(CH2Cl) system was studied by adding silver salts of BF-4, ClO-4, PF-6, AsF-6, and SbF-6. The fact that the initial rate and conversion increased as the nucleophilicity of the anion decreased was ascribed to the presence of two propagating species. Hatada et al. determined the transfer constants to monomer and solvent for the polymerization of perdeuteriated styrene by TiCl4 or EtAlCl2 in toluene at –78 °C. New polymers were reported from the cationic copolymerization of styrene with α-olefins and p-benzoquinone.

(Continues…)Excerpted from Macromolecular Chemistry Volume 2 by A. D. Jenkins, J. F. Kennedy. Copyright © 1982 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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