Page 25 - Introduction to Colloid and Surface Chemistry

16 The colloidal state

causes chain initiation to take place at many more points throughout
the reaction mixture.
Condensation polymerisation involves chemical reactions between
functional groups with the elimination of a small molecule, usually
water. For example,

-I- JtCOOH(CH 2 ) 4COOH -»
jcNH 2(CH 2 )gNH 2
hexamethylenediamine adipic acid
H[NH(CH 2 ) 6 NHCO(CH 2 ) 4 CO], OH+ (2x - 1)H 2O
nylon 66
If the monomers are bifunctional, as in the above example, then a
linear polymer is formed. Terminating monofunctional groups will
reduce the average degree of polymerisation. Polyfunctional mon-
omers, such as glycerol and phthalic acid, are able to form branching
points, which readily leads to irreversible network formation (see
Chapter 9). Bakelite, a condensation product of phenol and
formaldehyde, is an example of such a space-network polymer.
Linear polymers are usually soluble in suitable solvents and are
thermoplastic - i.e. they can be softened by heat without decomposi-
tion. In contrast, highly condensed network polymers are usually
hard, are almost completely insoluble and thermoset - i.e. they
cannot be softened by heat without decomposition.

Emulsion polymerisation and polymer latexes

A polymerisation method which is of particular interest to the colloid
scientist is that of emulsion polymerisation.
In bulk polymerisation, processing difficulties are usually encoun-
tered unless the degree of polymerisation is sharply limited. These
difficulties arise mainly from the exothermic nature of polymerisation
reactions and the necessity for efficient cooling to avoid the
undesirable effects associated with a high reaction temperature (see
page 15). Even at moderate degrees of polymerisation the resulting
high viscosity of the reaction mixture makes stirring and efficient heat
transfer very difficult.
The difficulties associated with heat transfer can be overcome, and
higher molecular weight polymers obtained, by the use of an
emulsion system. The heat of polymerisation is readily dissipated into
the aqueous phase and the viscosity of the system changes only
slightly during the reaction.