ORGANIC POLYMER CHEMISTRY


ORGANIC
POLYMER CHEMISTRY
AN INTRODUCTION TO THE ORGANIC CHEMISTRY
OF ADHESIVES, FIBRES, PAINTS, PLASTICS AND
RUBBERS
Second edition

K. J. SAUNDERS
Department of Applied Chemical and Biological Sciences
Ryerson Poly technical Institute, Toronto

LONDON

NEW YORK

CHAPMAN AND HALL

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First published in 1973 by
Chapman and Hall Ltd
11 New Fetter Lane, London EC4P 4EE
Second edition 1988
Published in the USA by
Chapman and Hall
29 West 35th Street, New York, NY 10001

© 1973, 1988 K. J. Saunders
Softcover reprint of the hardcover 2nd edition 1988
J. W. Arrowsmith Ltd. Bristol
ISBN -13:978-94-0 10-7031-7

All rights re~erved. No part of this book may be reprinted, or reproduced
or utilized in any form or by any electronic, mechanical or other means,
now known or hereafter invented, including photocopying and recording,
or in any information storage and retrieval system, without permission in
writing from the publisher.

British Library Cataloguing in Publication Data
Saunders, K. J. (Keith John)
Organic polymer chemistry: an introduction
to the organic chemistry of adhesives, fibres,
paints, plastics and rubbers.-2nd ed.
1. Polymers and polymerization
I. Title
547.7
QD381
e-ISBN-13:978-94-009-1195-6
ISBN-I3:978-94-010-7031-7
DOl: 10.1007/978-94-009-1195-6

Library of Congress Cataloging in Publication Data
Saunders, K. J. (Keith J.), 1931Organic polymer chemistry.
Includes index.
1. Polymers and polymerization.
I. Title.
TP1140.S32 1988

668.9
ISBN-13:978-94-010-7031-7

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2. Plastics.
87-31974


This book is dedicated with gratitude to
my parents, Leonard and Marjorie Saunders,
for their sacrifices in earlier years and to
my wife, Jeannette,for her steadfast encouragement
in recent times.

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CONTENTS

page IX

Preface

1. Basic concepts
2. Polyolefins

46

3. Polystyrene and styrene copolymers


76

4. Poly(vinyl chloride) and related polymers

90

5. Poly(vinyl acetate) and related polymers

113

6. Acrylic polymers

125

7. Fluoropolymers

149

8. Miscellaneous vinyl polymers

167

9. Aliphatic polyethers

171

10. Polyamides and polyimides

191


11. Polyesters

224

12. Other aromatic polymers containing p-phenylene groups

265

13. Cellulose and related polymers

286

14. Phenol-formaldehyde polymers

316

15. Aminopolymers

341

16. Polyurethanes

358

17. Silicones

388

18. Epoxies


412

19. Polysulphides

436

20. Polydienes

447

Index

492

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PREFACE

This book deals with the organic chemistry of polymers which find technological use as adhesives, fibres, paints, plastics and rubbers. For the most
part, only polymers which are of commercial significance are considered and
the primary aim of the book is to relate theoretical aspects to industrial
practice. The book is mainly intended for use by students in technical
institutions and universities who are specializing in polymer science and by
graduates who require an introduction to this field. There are available
several books dealing with the physical chemistry of polymers but the organic
chemistry of polymers has not received so much attention. In recognition of
this situation and because the two aspects of polymer chemistry are often
taught separately, this book deals specifically with organic chemistry and

topics of physical chemistry have been omitted. Also, in this way the book has
been kept to a reasonable size. This is not to say that integration of the two
areas of polymer science is undesirable; on the contrary, it is important that
the inter-relationship should be appreciated.
I was gratified by the favourable comments prompted by the first edition of
the book and I have therefore retained the same organization in this second
edition. Nevertheless, the book has been extensively revised to reflect the
developments which have taken place. The most noticeable features of the
period since the publication of the first edition have been the continued
dominance of the same bulk commodity polymers and the appearance of
several new speciality engineering thermoplastics. I have aimed to be comprehensive in scope and so both the well-established and the newer polymers are
dealt with in this edition.

KJ.S.
Department of Applied Chemical ana Biological Sciences,
Ryerson Poly technical Institute, Toronto, Ontario, Canada.

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1

BASIC CONCEPTS

1.1

DEFINITIONS

A polymer may be defined as a large molecule comprised of repeating
structural units joined by covalent bonds. (The word is derived from the

Greek: poly - many, meros - part.) In this context, a large molecule is commonly arbitrarily regarded either as one having a molecular weight of at least
1000 or as one containing 100 structural units or more. By a structural unit is
meant a relatively simple group of atoms joined by covalent bonds in a
specific spatial arrangement. Since covalent bonds also connect the structural
units to one another, polymers are distinguished from those solids and liquids
wherein repeating units (ions, atoms or molecules) are held together by ionic
bonds, metallic bonds, hydrogen bonds, dipole interactions or dispersion
forces.
The term macromolecule simply means a large molecule (Greek:
macros -large) and is often used synonymously with 'polymer'. Strictly
speaking, the terms are not equivalent since macromolecules, in principle,
need not be composed of repeating structural units though, in practice, they
generally are.
It may be noted that 'polymer' is often also used to refer to the massive
state. Then the term refers to a material whose molecules are polymers, i.e. a
polymeric material. Likewise, the term resin is sometimes used to refer to any
material whose molecules are polymers. Originally this term was restricted to
natural secretions, usually from coniferous trees, used mainly in surface
coatings; later, similar synthetic substances were included. Now the term is
generally used to indicate a precursor of a cross-linked polymeric material,
e.g. epoxy resin and novolak resin. (See later.)
1.2

SCOPE

A great variety of polymeric materials of many different types is to be found
throughout countless technological applications. For the purposes of this
book it is convenient to divide these materials according to whether they are

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2

BASIC CONCEPTS

inorganic or organic and whether they are naturally occurring or synthetic.
Using this classification, the diverse nature and widespread application of
polymeric materials is illustrated by Fig. 1.1. This book is concerned solely
with technologically useful organic polymeric materials. These materials are
commonly classified as adhesives, fibres, paints, plastics and rubbers according to their use. Although these are all polymeric materials, they clearly
possess a great diversity of properties about which few generalizations can be
made. It is significant, however, that no low molecular weight organic
compounds are useful in the above applications. The physical properties of
an individual polymeric material are largely determined by molecular weight,
strength of intermolecular forces, regularity of polymer structure and flexibility of the polymer molecule.

POLYMERIC MATERIALS

I

I

1

INORGANIC

I

I


NATURAL

I

I

.-----+-1------'1

I

Pottery Glass

I

I

Adhesives

Fig. 1.1

I

SYNTHETIC

I

1
1
I

I
I
Adhesives Fibres Pain~s Plas~ics Rubbers

Fibres

Polysaccharides

1.3

I

I

NATURAL

I

Sands

Cemen~

I

SYNTHETIC

Clays

Brick


ORGANIC

I

Fibres

Polyisoprene

Pro~eins

I

1

Adhesives

I

Fibres

I

Rubber

Some applications of polymeric materials.

RISE OF THE CONCEPT OF POLYMERS

Nowadays the concept of polymers (or, simply, big molecules) is easy to
accept but this has not always been the case. The rise of the concept is of

interest and a brief historical review is given below.
By the 1850s the existence of atoms and molecules was accepted but mainly
in respect of simple inorganic compounds. The application of these ideas to
more complex organic materials was not understood so clearly. At least, by
this time the notion that organic compounds all contained a mysterious 'vital
force' derived from living things had been finally abandoned as more and
more organic compounds were synthesized in the laboratory.

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RISE OF THE CONCEPT OF POLYMERS

3

In 1858 Kekule suggested that organic molecules were somewhat larger
than the simple inorganic molecules and consisted of atoms linked in chains
by bonds. This led to the realization that the order in which atoms are
arranged in the molecule is significant, i.e. the meaning of 'structure' was
appreciated. These ideas, aided by improving methods of elemental analysis,
resulted in the elucidation of the structure of many simple organic compounds such as acetic acid and alcohol. However, virtually nothing was
deduced about the structure of more complex organic materials such as
rubber, cellulose and silk. All that was clear was that these materials had
elemental analyses which were quite similar to those of the simple compounds whose structures were known.
The first significant information came in 1861 when Graham found that
solutions of such natural materials as albumin, gelatin and glue diffused
through a parchment membrane at a very slow rate. Materials of this kind
were called colloids (Greek: kolla-glue). In contrast, solutions of materials
like sugar and salts diffused readily; these substances were called crystalloids
since they were generally crystalline. The reason for this difference was not

clear, but it was generally supposed that the colloid solute particles were
rather large and therefore their passage through the membrane was hindered.
There were a few tentative suggestions that the colloids had high molecular
weights so that a solute particle was large simply because it comprised one
large molecule. However, this view was not at all acceptable to most scientists
of the day. At this time the current practices of organic chemistry demanded
the preparation of crystalline compounds of great purity with exact elemental
analyses and sharp melting points. It was generally felt that if the colloids
were 'cleaned up' they would crystallize and reveal themselves as 'normal' low
molecular weight compounds. This view was apparently reinforced by the
fact that many inorganic materials of low molecular weight can be prepared
so that they behave as colloids, e.g. colloidal arsenious sulphide, gold and
silver chloride. It was held, quite rightly, that in these cases the colloidal
particles are aggregates of smaller particles held together by secondary
valency forces of some kind. (Incidentally, the physically associated groups
were frequently called 'polymers' in the literature.) This view was very much
in keeping with the great emphasis which was being placed on van der Waals
forces in the 1890s and early 1900s. By analogy, the organic colloids were
assumed to be molecular aggregates or micelles, a concept still to be found in
the literature of the 1940s.
The first worker to take a clearly opposite view was Staudinger in
Germany in a paper of 1920. He maintained that the colloidal properties of
organic materials are due simply to the large size of the individual molecules
and that such macromolecules contain only primary valency bonds.
Staudinger's initial evidence was mainly negative. Firstly, he demonstrated
that the organic materials retain their colloidal properties in all solvents in

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4

BASIC CONCEPTS

which they dissolve. This is in contrast to the inorganic association colloids
which often lose their colloidal characteristics on change of solvent. Secondly,
he showed that, contrary to then current expectations, chemical modification
does not destroy the colloidal properties of the organic materials. At that time
it was commonly held that natural rubber was a cyclic material composed of
isoprene residues linked in rings of various sizes as, for example, in the
following structure:

Such molecules were then supposed to aggregate by virtue of secondary
valency forces arising from the presence of double bonds. (Incidentally, the
cyclic structure was held to account for the fact that no end-groups could be
detected.) However, Staudinger showed that the hydrogenation of natural
rubber produces a saturated material which still exhibits colloidal properties.
Thus he demonstrated that secondary forces originating from the unsaturation of natural rubber are unlikely to be responsible for colloidal behaviour. Staudinger proposed the long chain structures, which are accepted
today, for several polymers. Additional support for the existence of macromolecules came with the development of methods of molecular weight determination. Until this time, only cryoscopic methods were available and these
were inadequate for the very high molecular weights involved; also, it was
commonly held that the laws which hold for ordinary solutions were not
applicable to colloidal solutions.
Thus by about 1930 the concept of polymers was firmly established even if
not universally accepted. The macromolecular viewpoint was finally secured
largely by the work of Carothers in the USA. This work was begun in 1929
and had as its objectives, clearly stated at the outset, the preparation of
polymers of definite structure through the use of established reactions of
organic chemistry and the elucidation of the relationship between structure
and properties of polymers. These researches were brilliantly successful and
finally dispelled the mysticism surrounding this field of chemistry.

One outstanding result of Carothers' work was the commercial development of nylon. Nylon stockings came on the market in 1940, when polymers,
in terms of popular acceptance, might be said to have arrived. The theoretical, practical and economic foundations had been laid and since this date
progress has been phenomenal.

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GENERAL METHODS OF PREPARATION OF POLYMERS

1.4

5

GENERAL METHODS OF PREPARATION OF
POLYMERS

There are three general methods by which polymers may be prepared from
relatively simple starting materials (monomers). Each of these methods is
briefly described in this section; more detailed considerations are to be found
throughout later chapters.
1.4.1

Polymerization through functional groups

In this type of polymerization, reaction proceeds between pairs of functional
groups associated with two different molecules. Provided all the reacting
molecules have at least two reactive groups, a sequence of reactions occurs
and a polymer is formed. It will be apparent that the structural units of the
polymer will contain a group whose arrangement of atoms is not to be found
in the starting material. A polymerization of this kind occurs when

w-aminoundecanoic acid is heated:

or

-HN-(CH 2 )\O-CO-HN-(CH 2 )\O-CO-HN-(CH 2 )\O-COnH 2 N-(CH 2 ) 10-COOH

1
The resulting polymer (nylon 11) consists essentially of a long chain of
repeating -HN-(CH 2 )1O-CO- groups (the structural units); in a typical
commercial material n has a value of the order of too. The polymer contains
amide groups (-CO-NH-) (which are not present in the starting material) at
regular intervals along the chain and is, therefore, a polyamide. It may be
noted that in the representation of polymers it is common to leave unspecified
the nature of the end-groups (as above). This practice is justified since the
number of end-groups is very small compared to the number of repeating
units.
In the above example of polymerization through functional groups, the
single monomer (w-aminoundecanoic acid) contains two types of functional
group, namely amino and carboxyl. In practice it is more usual to use a

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6

BASIC CONCEPTS

mixture of two monomers, each having only one type of functional group;
such monomers are generally more readily obtainable. This technique is
illustrated by the formation of a polyamide (nylon 6,6) from hexamethylenediamine and adipic acid:


1
[-HN-(CH2)6-NH-OC-(CH2)4-C0--

1.

+

2nH 20

Further examples of polymerizations of this kind are as follows:

hexamethylenediamine

sebacic acid

!

[-HN-(CH2)6-NH-OC-(CH2)8-CO-].
polyamide (nylon 6.10)

nHO--CH 2-CH 2-OH

+

2nH 20

+ nHooc-o-COOH

ethylene glycol


1

terephthalic acid

[-O-CH2-CH2---O-C0-o-CO-].

+

2nH 20

polyester (poly(ethylene terephthalate))

CH 3

+

nHo---Q--{-Q-oH
CH 3
bisphenol A

nCOCI2
phosgene

1+

CH!
[- o - O - { - - O - O - - c o CH 3
polycarbonate


2nHCI

•

nCI-CH 2-CH 2-C1
ethylene dichloride

+ nNa 2S2
sodium disulphide

1 1. +

[ -CH 2-CH 2-S-Spolysulphide

2nNaCI

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GENERAL METHODS OF PREPARA nON OF POLYMERS

7

It will be noticed that in all the above examples the production of polymer is
accompanied by the formation of a secondary product; in each case there is
elimination of some small molecule as a by-product, e.g. water or hydrogen
chloride. However, polymerization through functional groups does not always result in such a by-product. For example, in the reaction between an
isocyanate, such as 1,6-hexamethylene diisocyanate, and a glycol, such as 1,4butanediol, polymer is the sole product:
nOCN-{CHz)6-NCO


+

nHO-{CH z)4-0H

------>

[-OC-HN-{CHz)6-NH-CO-O-{CHz)4-0-].

The polymer contains urethane groups along the chain and is, therefore, a
polyurethane. The significance of the absence of by-products in reactions of
this kind is referred to later in this chapter (section 1.4.5).
The chemistry involved in polymerizations through functional groups is
essentially the chemistry of simple organic reactions wherein the corresponding functional groups are present in small molecules. These polymerizations
nearly always proceed in a stepwise manner and the polymer chain is
therefore built up relatively slowly by a sequence of discreet interactions
between pairs of functional groups. Each interaction is chemically identical
since each involves the same kind of functional group. It may also be noted
that the reactivity of a functional group at the end of a polymer chain is
similar to that of the corresponding functional group in a monomer molecule.
Thus a functional group at the end of a polymer chain can react with a
functional group which is either in a monomer molecule or at the end of
another polymer chain. The growth of a polymer chain is therefore somewhat
fitful. Since there is no inherent termination reaction, the molecular weight of
the polymer continues to increase with time until, ideally, no more functional
groups remain available for reaction. By far the greatest majority of polymerizations through functional groups proceed in a stepwise manner but a
few have been found to involve a chain reaction (see section 1.4.5).
1.4.2

Polymerization through multiple bonds


This method of polymer preparation may be simply regarded as the joining
together of unsaturated molecules through the multiple bonds. There is
essentially no difference in the relative positions of the atoms in the unsaturated molecules and in the structural units of the polymer and there is no
change in composition. Polymerizations of this kind may be divided into the
various categories which follow.

(a)

Vinyl polymerization

The most common unsaturated compounds which undergo polymerization
through their multiple bonds contain carbon-carbon double bonds and are

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BASIC CONCEPTS

8

ethylene derivatives. The simplest monomer is ethylene itself, the polymerization of which may be written:

The resulting polymer, polyethylene, consists essentially of a long chain of
repeating -CH 2-CH 2- groups (the structural units); in a typical commercial
material n has a value of the order of 1000. Further examples of polymerizations of ethylene derivatives are as follows:
CH 3

- l
- l
- [-CH2-~IH-t

CH 3 ]
-CH 2--tH- "
polypropylene

I

nH 2C==CH
propylene
C6H S

I

nH 2C=CH
styrene
CI

I

nH 2C==CH
vinyl chloride

H
C6 S]
-CH 2-tH- "
polystyrene

poly(vinyl chloride)

nH 2C=CCI 2
vinylidene chloride

CH 3

I
I

nH 2C=C
COOCH 3
methyl methacrylate

nF 2 C CF 2
tetrafluoroethylene

[-CH 2-CCI-]
2"
poly(vinylidene chloride)

-

I
CH,

1

-CH -C2 I
COOC.H3 n
poly(methyl methacrylate)
[-CF-CF
- ]"
2
2

polytetrafluoroethylene

Polymerizations of the above type are often referred to as vinyl polymerizations although, strictly speaking, vinyl compounds are only those containing the H 2 C=CH- group. It is convenient, however, to consider all these
polymerizations under one heading.
Vinyl polymerization, as illustrated by the above reactions, involves a
three-part process, namely initiation, in which is formed an active species
capable of starting polymerization of the otherwise unreactive vinyl compound; propagation, in which high molecular weight polymer is formed; and
termination, in which deactivation occurs to produce the final stable polymer.
The active species in vinyl polymerizations may be of three different types,

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GENERAL METHODS OF PREPARA nON OF POLYMERS

9

namely free radicals, anions and cations and these possibilities give rise to
three distinct methods of accomplishing polymerization.

Free radical polymerization
In free radical polymerization, initiation may be brought about by light or
heat; most commonly, however, it is achieved by the addition of a material
which, on heating, decomposes into free radicals (which may be defined as
molecules containing an unpaired electron). Examples of frequently used
initiators are benzoyl peroxide and azobisisobutyronitrile which give rise to
free radicals as follows:

0


o

0
II~

11

~ tf -C-O-O-C~

--

o

F\~

2LJ-O.CH 3

CH 3

CH 3

I

I

1 -CH3 -

H3C-9-N =N

CN


2H3C-t.

I

CN

+ N2

CN

The initiation of polymerization is therefore a two-step sequence. The first
step is the dissociation of the initiator, as illustrated above. This may be
represented as:
I-I

----->

21'

The second step is the addition of the initiator fragment radical (I·) to a vinyl
monomer molecule (H 2 C=CHR) to give an initiated monomer radical:
I'

+

H 2C=CHR

----->


I-CH2-CHR

This new radical then adds further monomer molecules in rapid succession to
form a polymer chain. In this propagation the active centre remains, being
continuously relocated at the end of the chain:
H,C==CHR

•

I-CH 2-CHR-CH 2-CHR

1

H,C=CHR

I-CH 2-CHR-CH 2-CHR-CH 2-CHR

1

nH,C=CHR

I

f-CH2-CHR~CH2-CHR

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10


BASIC CONCEPTS

Propagation continues until the growing long chain radical becomes deactivated. Such termination is commonly by reaction with another long chain
radical in one of two ways:
(i) Combination:
+ RHC-CH 2-

-CH 2-CHR

----4

-CH 2-CHR-CHR-CH 2-

(ii) Disproportionation:
-CH 2-CHR

+

RHC-CH 2-

----+

-CH 2-CH 2R

+

CHR=CH-

where -CH 2-CHR represents the last unit in a growing polymer chain.
During a polymerization reaction both of these mechanisms may operate

together or one only may occur to the exclusion of the other. The actual mode
of termination depends on the experimental conditions and the monomer
involved. Termination of a growing polymer chain may also occur by
transfer. In this case, however, deactivation of the chain radical results in the
formation of a new free radical. Transfer reactions have the general form:
-CH 2-CHR

+

AB

----+

-CH 2-CHRA

+

B'

where AB may be monomer, polymer, solvent or added modifier. Depending
on its reactivity, the new free radical (Bo) mayor may not initiate the growth
of another polymer chain. The transfer of hydrogen from monomer to
growing polymer may be given as an example of termination by transfer:
-CH 2-CHR

+ H 2C=CHR

----4

-CH 2-CH 2R


+ H 2C=CR

Anionic polymerization
In anionic polymerization of vinyl monomers the active centre is a car bani on.
Substances which initiate this type of polymerization are of two kinds:
(i) Ionic compounds of the type X+Y- or ionogenic compounds of the type
XHya- where the actual or potential anion Y- is able to add to the
carbon--carbon double bond to form a carbanion which can then propagate.
Initiators of this kind include alkali metal alkyls, aryls, alkoxides and amides.
The initiation step which occurs with potassium amide will serve to illustrate
the mode of reaction of such initiators:
K +H2~-

+ H 2C=CHR

----4

H2N-CH2-~HR K +

(ii) Free metals which are able to transfer an electron to the monomer with
the consequent formation of an anion-radical. Alkali metals (M 0) are the most
common initiators of this type and may initiate polymerization by two
different kinds of electron-transfer reactions:
(a) Direct transfer of an electron to the monomer to form the initiating
species:

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GENERAL METHODS OF PRE PARA nON OF POL YMERS

11

In most solvents the resultant anion-radical rapidly dimerizes to give a
dicarbanion which functions as the actual initiating species:
2Hi-~HR M+

------>

M+RH~-CH2-CH2-~HR M+

(b) Transfer of an electron to an intermediate compound (A) to form an ion
radical which subsequently transfers the electron to the monomer to form
the initiating species:
M'

+

M +A'-

+

A

------>

M+ A'-

H 2C=CHR


------>

Hi:-~HR M +

+

A

The resultant anion-radical generally dimerizes to give a dicarbanion which
functions as the actual initiating species (see above). An example of this type
of initiator system is the solution obtained by adding sodium to naphthalene
in an inert solvent such as tetrahydrofuran. The reactions involved are:

Na·

+

- [CO

(0'<: .:
~

I

I.. I .-.
""'- ""'- . etc.1Na +
-·co'<::.::
- lH c=CHR


~

#

2

It might be supposed that whatever the nature of the initiation of anionic
polymerization, the propagation step could be regarded as analogous to the
free radical propagation step, being represented as:
-CH2-~HR

+

H 2C=CHR

------>

-CH2-CHR-CH2-~HR

However, in general the presence of a counter (or gegen) ion in close
proximity to the active centre has a profound effect. Thus, whilst in free
radical polymerization the growth of the propagating chain is independent of
the initiator used, the same cannot be said of anionic polymerization. In
particular, the separation between the carbanion end-group and the counter
ion is the primary factor determining the stereochemistry of the propagation
reaction. Also in contrast to free radical polymerization, true termination
reactions are absent from anionic polymerizations. Under vigorous reaction
conditions the active centre may be destroyed by hydride elimination:
-CH2-~HR M+


------>

-CH=C!-lR

+

MH

A similar reaction may also result in transfer of activity to monomer:
-CH2-~HR M+

+ H 2 C=CHR

------>

-CH=CHR

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+ H3C--~HR M+


BASIC CONCEPTS

12

The presence of a solvent may provide another mode of transfer. The
mechanism of such a reaction cleafly depends on the nature of the solvent;
one possibility is proton transfer trom the solvent (S-H):
-CH2-~HR M+


+

S-H

--->

-CH 2-CH 2 R

+

S-M+

Impurities containing active hydrogen also participate in transfers of the
above type. Carbon dioxide inhibits polymerization by forming a carboxylate
anion which is not sufficiently reactive to initiate further polymerization:

?!

+ c=o-

?i

-CH 2-CHR-C-g-M+

Provided inert solvents and pure reactants are used, most monomers under
approPriate conditions will give rise to systems in which active carbanion
end-groups are always present. The indefinite activity of the growing chains
has led to the rather inappropriate term, 'living polymers' for these materials.
Anionic polymerizations of a rather different kind to those discussed above

may be effected by the use of co-ordinqtion catalysts. This general type of
catalysis was discovered by Ziegler who used it to polymerize ethylene (1953);
Natta extended its use to the polymerization of other unsaturated monomers
(1954-60). (See later.) Hence these catalysts are often referred to as 'ZieglerNatta catalysts'. The catalysts are typically obtained by mixing an alkyl or
aryl of a metal from Groups I - III of the Periodic Table with a compound
(commonly a halide) of a transition metal of Groups IV-VIII. Several
thousand permutations are possible and a great number of combinations are
cited in the literature. Examples of organometallic compounds which have
been used are phenyllithium, diethylberyllium, diethylzinc, and triethylaluminium. Examples of transition metal compounds which have been used are
titanium tetrachloride, vanadium oxychloride (VOCI), molybdenum pentachloride and tungsten hexachloride. Probably the best-known example of
this class of catalyst is that obtained from triethylaluminium and titanium
tetrachloride. The structure of these catalysts and their mode of operation
have been the subject of extensive investigation and much information has
resulted. However, the matter is by no means settled and this information has
not yet been integrated into a generally accepted theoretical framework. By
way of illustration of proposals which have gained wide acceptance the
system based on triethylaluminium and titanium tetrachloride will be discussed specifically. When triethylaluminium and titanium tetrachloride are
mixed together in a hydrocarbon solvent (e.g. n-heptane) at about room
temperature, hydrocarbon gases are evolved and there is precipitated a blackbrown solid which is the active polymerization catalyst. The gases comprise
mainly ethane with small amounts of ethylene and n-butane; some polyethylene is also formed. The initial reaction between the two components is

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GENERAL METHODS OF PREP ARA nON OF POLYMERS

13

generally regarded as involving a series of alkylations and reductive dealkylations of the following kind:
(C 2H shAl

(C2HshAl

+ TiCl 4

---->

(C 2H shAlCl

C 2H STiCl 3

---->

°C 2H s

+ TiCl 3

---->

(C2Hs)2AlCl

+ C 2H sTiCl 3

+ TiCl 3

+ C 2H sTiCl 2
+ TiCl 2
(C2Hs)2AlCl + (C2HSh TiCl2
°C 2H s + C 2Hs TiCl 2

C2HS TiCl 2 ----> °C 2H s

(C 2H shAl

+ C2HS TiCl 3

---->

(C 2H sh TiCl 2 ---->

Several other alkylation reactions may be written. The formation of ethyl free
radicals by decomposition of unstable alkyltitanium compounds accounts for
the evolution of the hydrocarbon gases. Thus the final product is a complex
mixture of organo-aluminium and -titanium compounds, lower titanium
chlorides and some organic fragments. The actual composition of the product
is dependent on the relative proportions of the starting materials and the time
and temperature of reaction.
It is generally supposed that the active component ofthe catalyst mixture is
a complex formed between titanium trichloride and triethylaluminium. This
complex is envisaged as forming at the surfaces of titanium trichloride
crystals; its formation may be regarded as the strong adsorption of the
aluminium compound on to the crystal surfaces. By analogy with the bridged
structures known to be present in dimeric aluminium alkyls, the complex is
assumed to have the following structure:
b-

CL,h/Cl.··· ... b+,...... C2HS
,......TI)I1 b- .....:A l'-.,
CI

"'SH


2

C2

H
S

CH 3

In such a complex the titanium has unfilled 3d-orbitals to which may be
co-ordinated n-electrons from the double bond of the vinyl monomer. Thus a
possible representation of initiation and propagation is as follows:

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BASIC CONCEPTS

14

The essential feature of this mechanism is that monomers are inserted, one
after the other, into a polarized titanium--carbon bond. The polymer therefore grows out of the active centre, rather as a hair grows from the root. It will
be noticed that the propagating end of the polymer chain is negatively
charged and therefore the reaction may be regarded as an anionic polymerization. Chain growth may be terminated by several types of transfer, e.g.

Internal hydride transfer:
Cat-CH z-CHR+CH z-CHR+,CH z-CH 3 ----+

+


Cat-H

H zC=CRTCH 2-CHR+.CH 2-CH 3

Transfer to monomer:
+

Cat-CH 2-CHR+CH 2-CHR+.CH 2-CH 3

+

Cat-CH 2-CH 2R

H 2C=CHR----+

H 2C=CR+CH 2-CHR+.CH 2-CH 3

True (kinetic) termination may be brought about by the addition of an active
hydrogen compound such as an alcohol:

+

Cat-CH 2-CHR+CH 2-CHR+.CH 2-CH 3

+

Cat-OR'

R'-OH----+


H 3C-CHR+CH 2-CHR+.CH 2-CH 3

Cationic polymerization
In cationic (or more specifically, carbo cationic) polymerization of vinyl
monomers the active centre is a carbenium ion. Substances which initiate this
type of polymerization are of three kinds:
(i) Pro tic acids, e.g. sulphuric, perchloric and tritluoroacetic acids, The initiation step consists of the transfer of a proton to the monomer:
+

+ H 2C=CHR ----+ H3C-CHR A-

HA

Propagation then proceeds via the carbenium ion produced:
+

H3C-CHR A -

+

+

H 2C=CHR ----+ H3C-CHR-CH2-CHR A - etc.

Chain growth continues until either chain transfer or termination occurs. In
chain transfer, there is no loss of active centres and the kinetic chain remains
operational. Various types of chain transfer are possible, e.g.

Transfer to counter ion:
+


~CH2-CHR

A - ----+

~CH=CHR

+

HA

Transfer to monomer:
+

~CH2-CHR

A-

+

H 2C=CHR ----+

~CH=CHR

+

+

H3C-CHR A-


In true (kinetic) termination there is irreversible loss of propagating ability.
This may be brought about ill several ways, the most important of which is

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GENERAL METHODS OF PREPARA nON OF POLYMERS

15

neutralization. In this process, the propagating carbenium ion and counterion interact to give an electrically neutral species. With trifluoracetic acid, for
example, termination occurs by ester formation:
+

-CH 2-CHR CF 3COO-

---+

-CH 2-CHR-OCO-CF 3

(ii) Carbenium ion salts, e.g. trityl hexachloroantimonate (Ph 3 C+SbCli).
Here, initiation involves direct cationation of the monomer, e.g.

Propagation and chain transfer then occur as discussed above. The nature of
termination reactions is obscure. The use of carbenium ion salts as polymerization initiators appears to be restricted to aromatic and vinyl ether monomers. The salts do not initiate polymerization of aliphatic olefins.
(iii) Metal halides of the type which catalyses the Friedel-Crafts reaction, e.g.
aluminium trichloride, boron trifluoride and ferric chloride. The pure, anhydrous metal halides do not initiate polymerization; they are active only in
the presence of co-initiators. Co-initiators are commonly compounds
containing active hydrogen, e.g. alcohols, protic acids and water: The coinitiator (QH) co-ordinates with the metal halide (MX.) to form a complex
protic acid which transfers a proton to the monomer:

MX.
[QMX.rW

+

+

QH

---+

H 2 C=CHR

[QMX.rW

---+

+

H3C-CHR [QMX.r

Propagation then proceeds as follows:

Chain transfer takes place as discussed above. The nature of termination
reactions has not been fully established. The advantage of the metal halide
initiators over protic acid initiators is their ability to extend the lifetime of the
kinetic chain and thus give polymers of higher molecular weight. For
commercial purposes, the metal halides are by far the most important type of
initiators used for cationic polymerization.
By way of conclusion to this short discussion of vinyl polymerization, it

may be noted that not all vinyl monomers can be polymerized to high
molecular weight polymers by all three of the general methods described,
namely free radical, anionic and cationic polymerization. Table 1.1 indicates
the general applicability of the three methods in homogeneous systems to
some vinyl monomers. Often the effectiveness, or otherwise, of the methods
can be related to the polarity of the monomer double bond. Electronreleasing substituents favour the formation of carbenium ions and render the
monomer susceptible to cationic polymerization. Thus isobutene (with two
electron-releasing methyl groups) and vinyl ethers (in which the resonance

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16

BASIC CONCEPTS

Table 1.1 General applicability of polymerization methods in homogeneous systems
to some vinyl monomers
Monomer

Polymerization method

Structure

Cationic

Isobutene
Vinyl ethers
Ethylene
Vinyl esters

Vinyl halides
Acrylic esters
Acrylonitrile
Vinylidene halides
1-Nitro-l-alkenes
Vinylidene cyanide
Styrene

H 2 C=C(CH 3 h
H 2 C=CHOR
H 2 C=CH 2
H 2 C=CHOCOR
H 2 C=CHX
H 2 C=CHCOOR
H 2 C=CHCN
H 2 C=CX 2
H 2 C=CRN0 2
H 2 C=C(CNh
H 2 C=CHC6 H s

Yes
Yes
Yes
No
No
No
No
No
No
No

Yes

Free
radical

Anionic

No
No
Yes
Yes
Yes
Yes
Yes
Yes
No
No
Yes

No
No
No
No
No
Yes
Yes
Yes
Yes
Yes
Yes


Monomers are arranged in approximate order of increasing susceptibility to anionic polymerization.

effect, due to delocalization of an un shared electron pair on the oxygen atom,
outweighs the inductive effect exerted by the ether group to give an overall
electron-releasing effect) undergo cationic polymerization exclusively. On the
other hand, electron-withdrawing substituents favour the formation of carbanions and render the monomer susceptible to anionic polymerization.
Thus vinylidene cyanide (with two electron-withdrawing cyano-groups) and
I-nitro-l-alkenes (in which the resonance and inductive effects exerted by the
nitro-group reinforce one another and outweigh the inductive effect exerted
by the alkyl group to give an overall electron-withdrawing effect) undergo
anionic polymerization exclusively. Between these two extremes lie those
monomers wherein the electron-releasing and -withdrawing effects are less
pronounced; these monomers undergo free-radical polymerization and show
a tendency to undergo either cationic or anionic polymerization, depending
on the nature of the substituent. A few monomers such as styrene (in which
the phenyl group can function as either an electron source or electron sink)
can be polymerized by all three methods. It may be noted here that the fact
that a monomer has a high tendency to polymerize does not necessarily mean
it forms high molecular weight polymer. The molecular weight is also
governed by factors such as transfer and termination reactions.

(b) Diene polymerization
Conjugated dienes comprise the second group of unsaturated compounds
which undergo polymerization through their multiple bonds. The most

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GENERAL METHODS OF PREPARATION OF FOLYMERS


17

common dienes used for the preparation of commercially important polymers are butadiene, chloroprene and isoprene. These are normally represented by the following structures:
CH)
H 2C=CH-CH=CH 2
butadiene

H 2C=t-CH=CH2
isoprene

Cl

I

H 2C=C-CH=CH 2
chloroprene

Such monomers can give rise to polymers which contain various isomeric
structural units. Each of the above structures contains a 1,2- and a 3,4-double
bond and there is thus the possibility that either double bond may participate
independently in polymerization, giving rise to I,2-units and 3,4-units respectively:

x
I
I

-CH -C-

-CH-CH-


CH

c-x

CH 2

CH 2

2

2

II

II

X = H, CH) or Cl

1,2-unit

I

3,4-unit

With symmetrical dienes such as butadiene, these two units become identical.
A further possibility is that both bonds are involved in polymerization
through conjugate reactions, giving rise to l,4-units. A l,4-unit may occur as
either the cis- or the trans-isomer:
-CH 2 ,


C=C

x/

/CH 2-

'H

cis-I,4- uni t

-CH 2,

/H

_./C=C,

x

CH 2 -

trans-l,4- unit

Generally speaking, the polymer obtained from a conjugated diene contains more than one of the above structural units. The relative frequency of
each type of unit is governed by the nature of the initiator and the experimental conditions as well as the structure of the diene. Each of the three
general methods of accomplishing vinyl polymerization, described above,
may be used for the polymerization of conjugated dienes.
In free radical polymerization, the various structural units may be envisaged as arising from the addition of one or other end of a monomer molecule
to one of the resonant forms of the growing polymeric radical, e.g.


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BASIC CONCEPTS

18

1

H,CJ-CH==CH,

X
I

X
I

II

II

H,CJ-CH=CH,

X

X

I

I


-CH 2-C=CH-CH 2-CH 2---)'

- CH c-<[-CH2---)'
CH
CH
CH 2

1

CH
D
CH 2

CH 2

1,4-unit

1,2-unit

and

l,4-unit

3,4-unit

In practice, a preponderance of 1,4-units is found; this is attributable to the
greater accessibility of the primary radical compared to that of the secondary
radical. The trans-form of the growing radical is slightly more favoured
energetically than the cis-form and hence l,4-trans-units predominate in the

polymer, particularly when polymerization is carried out at low temperatures.
With regard to anionic polymerization of conjugated dienes, alkali metal
alkyls and free alkali metals are the most commonly used initiators. The
resultant polymers generally have much higher contents of 3,4-units compared to the polymers prepared by free,radical polymerization. When an
alkali metal alkyl (M+R -) is the initiator, the initiation reaction may be
represented as:
M+R-

+

f

H2C=CH-C=CH 2 -

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GENERAL METHODS OF PRE PARA TION OF POLYMERS

19

Addition of a monomer molecule to one of the resonant forms of the anion
then results in the formation of the corresponding structural unit. When an
alkali metal (Mo) is the initiator, the initiation reaction may be represented as:

M'

+

f

H2C=C-CH=CH 2

X

-

M

+H2~~CH----CH2

The anion-radical produced may either dimerize or add on another alkali
metal atom; in either event a dicarbanion is formed and this functions as the
actual initiating species:

Polymerization then proceeds in the manner of the alkyl-initiated reaction
described above. It may be noted here that lithium (both as the free metal and
as alkyls) stands in marked contrast to the other alkali metals in giving rise to
polymers with high proportions of l,4-units. This is attributable to the
covalent character of the lithium--carbon bond, as is discussed later
(Chapter 20).
The use of Ziegler-Natta catalysts in the polymerization of conjugated
dienes has been widely investigated. It is characteristic of these catalysts that
the resulting polymers often contain a very high proportion of one type of
structural unit. By appropriate choice of catalyst, polydienes comprised
almost exclusively of cis-I,4-, trans-I,4-, 1,2-, and 3,4-units have been obtained. The mechanisms of such reactions are, at present, somewhat obscure
but presumably the diene molecule co-ordinates with the metal--carbon bond
of the catalyst in a manner similar to that involving the lithium--carbon bond
mentioned above.
In contrast to free radical and anionic polymerization, the cationic polymerization of conjugated dienes has received little study. Generally, rather
low molecular weight polymers are produced and these have not attained

commercial significance. A feature ofthe products obtained by the polymerization of conjugated dienes using metal halide catalysts is that they contain an
appreciable proportion of cyclized structures. The linear portions of the
polymers consist mainly of trans-I,4-units.
The profound effect of the initiator on the microstructure of the products
obtained by polymerization of conjugated dienes is illustrated by Table 1.2.

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