fundamentals of Polymer Chemistry
11.1 Historical introduction
THE CONCEPT OF A POLYMER
The differences between the properties of crystalline organic materials of low
molecular weight and the more indefinable class of materials referred to by
Graham in 1861 as ‘colloids’ has long engaged the attention of chemists. This
class includes natural substances such as gum acacia, which in solution are
unable to pass through a semi-permeable membrane. Rubber is also included
among this class of material.
The idea that the distinguishing feature of colloids was that they had a
much higher molecular weight than crystalline substances came fairly slowly.
Until the work of Raoult, who developed the cryoscopic method of estimating
molecular weight, and Van’t Hoff, who enunciated the solution laws, it was
difficult to estimate even approximately the polymeric state of materials. It also
seems that in the nineteenth century there was little idea that a colloid could
consist, not of a product of fixed molecular weight, but of molecules of a broad
band of molecular weights with essentially the same repeat units in each.
Vague ideas of partial valence unfortunately derived from inorganic chemistry
and a preoccupation with the idea of ring formation persisted until after
1920. In addition chemists did not realise that a process such as ozonisation
virtually destroyed a polymer as such, and the molecular weight of the ozonide,
for example of rubber, had no bearing on the original molecular weight.
The theory that polymers are built up of chain formulae was vigorously
advocated by Staudinger from 1920 onwards [1]. He extended this in 1929 to
the idea of a three-dimensional network copolymer to account for the insolubility
and infusibility of many synthetic polymers, for by that time technology
had by far outstripped theory. Continuing the historical outline, mention must
be made of Carothers, who from 1929 began a classical series of experiments
which indicated that polymers of definite structure could be obtained by the
use of classical organic chemical reactions, the properties of the polymer being
controlled by the starting compounds
[2]. Whilst this was based on research
in condensation compounds (see Section 1.2) the principles hold good for addition polymers
fundamental of polemer chemistry.
The last four decades have seen major advances in the characterisation of
polymers. Apart from increased sophistication in methods of measuring molecular
weight, such as the cryoscopic and vapour pressure methods, almost the
whole range of the spectrum has been called into service to elucidate polymer
structure. Ultraviolet and visible spectroscopy, infrared spectroscopy, Raman
and emission spectroscopy, photon correlation spectroscopy, nuclear magnetic
resonance and electron spin resonance all play a part in our understanding of
the structure of polymers; X-ray diffraction and small-angle X-ray scattering
have been used with solid polymers. Thermal behaviour in its various aspects,
including differential thermal analysis and high-temperature pyrolysis followed
by gas–liquid chromatography, has also been of considerable value. Other
separation methods include size exclusion and hydrodynamic chromatography.
Electron microscopy is of special interest with particles formed in emulsion
polymerisation. Thermal and gravimetric analysis give useful information in
many cases. There are a number of standard works that can be consulted [3–6].
1.2 Definitions
A polymer in its simplest form can be regarded as comprising molecules of
closely related composition of molecular weight at least 2000, although in
many cases typical properties do not become obvious until the mean molecular
weight is about 5000. There is virtually no upper end to the molecular
weight range of polymers since giant three-dimensional networks may produce
crosslinked polymers of a molecular weight of many millions.
Polymers (macromolecules) are built up from basic units, sometimes
referred to as ‘mers’. These units can be extremely simple, as in addition
polymerisation, where a simple molecule adds on to itself or other simple
molecules, by methods that will be indicated subsequently. Thus ethylene
CH2:CH2 can be converted into polyethylene, of which the repeating unit
is —CH2CH2—, often written as —CH2CH2n, where n is the number of
repeating units, the nature of the end groups being discussed later.
The major alternative type of polymer is formed by condensation polymerisation
in which a simple molecule is eliminated when two other molecules
condense. In most cases the simple molecule is water, but alternatives include
ammonia, an alcohol and a variety of simple substances. The formation of a
condensation polymer can best be illustrated by the condensation of hexamethylenediamine
with adipic acid to form the polyamide best known as nylon:
H2N(CH2)6NH
H
HOOC(CH2)4CO.OH HN(CH2)6NH2
H
= H2N(CH2)6NH.OC(CH2)4CONH(CH2)6NH2
+ +
+ H2O + H2O
The concept of a polymer 3
This formula has been written in order to show the elimination of water.
The product of condensation can continue to react through its end groups
of hexamethylenediamine and adipic acid and thus a high molecular weight
polymer is prepared.
Monomers such as adipic acid and hexamethylenediamine are described
as bifunctional because they have two reactive groups. As such they can
only form linear polymers. Similarly, the simple vinyl monomers such as
ethylene CH2:CH2 and vinyl acetate CH2:CHOOCCH3 are considered to
be bifunctional. If the functionality of a monomer is greater than two,
a branched structure may be formed. Thus the condensation of glycerol
HOCH2CH(OH)CH2OH with adipic acid HOOCCH24COOH will give a
branched structure. It is represented diagrammatically below:
HOOC(CH2)4COOCH2CHCH2OOC(CH2)4COOCHCH2O
O
CO
(CH2)4
CO
O
CH2
COOC(CH2)4COOCH2CHCH2O
CH2
O
CO(CH2)4COO
CH2O
H
O
The condensation is actually three dimensional, and ultimately a threedimensional
structure is formed as the various branches link up.
Although this formula has been idealised, there is a statistical probability of
the various hydroxyl and carboxyl groups combining. This results in a network
being built up, and whilst it has to be illustrated on the plane of the paper,
it will not necessarily be planar. As functionality increases, the probability of
such networks becoming interlinked increases, as does the probability with
increase in molecular weight. Thus a gigantic macromolecule will be formed
which is insoluble and infusible before decomposition. It is only comparatively
recently that structural details of these crosslinked or ‘reticulated’ polymers
have been elucidated with some certainty. Further details of crosslinking are
given in Chapter 5.
Addition polymers are normally formed from unsaturated carbon-to-carbon
linkages. This is not necessarily the case since other unsaturated linkages
including only one carbon bond may be polymerised
Fundamentals of polymer chemistry
Addition polymerisation of a different type takes place through the opening
of a ring, especially the epoxide ring in ethylene oxide CH2.CH2.
O
This opens as
—CH2CH2O—; ethylene oxide thus acts as a bifunctional monomer forming a
polymer as HCH2CH2On CH2CH2OH, in this case a terminal water molecule
being added. A feature of this type of addition is that it is much easier to
control the degree of addition, especially at relatively low levels, than in the
vinyl polymerisation described above.
Addition polymerisations from which polymer emulsions may be available
occur with the silicones and diisocyanates. These controlled addition polymerisations
are sometimes referred to as giving ‘stepwise’ addition polymers.
This term may also refer to condensation resins. Further details are given in
Chapter 7.
2 ADDITION POLYMERISATION
Addition polymerisation, the main type with which this volume is concerned,
is essentially a chain reaction, and may be defined as one in which only a small
initial amount of initial energy is required to start an extensive chain reaction
converting monomers, which may be of different formulae, into polymers.
A well-known example of a chain reaction is the initiation of the reaction
between hydrogen and chlorine molecules. A chain reaction consists of three
stages, initiation, propagation and termination, and may be represented simply
by the progression:
Activation +M +M +nM
M M* M2* M3* Mn+3 etc.
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