Page 24 - Introduction to Colloid and Surface Chemistry

The colloidal state 15

methacrylate)), terylene (poly (ethylene terephthalate)) and the
nylons, e.g. (poly (hexamethylene adipamide)). Only brief mention
of some of the more general aspects of polymerisation will be made.
The reader is referred to the various specialised texts for details of
preparation, properties and utilisation of these products.
High polymers contain giant molecules which are built up from a
large number of similar (but not necessarily identical) units (or
monomers) linked by primary valence bonds. Polymerisation reactions
can be performed either in the bulk of the monomer material or in
solution. A further technique, emulsion polymerisation, which
permits far greater control over the reaction, is discussed on page 16.
There are two distinct types of polymerisation: addition polymer-
isation and condensation polymerisation.
Addition polymerisation does not involve a change of chemical
composition. In general, it proceeds by a chain mechanism, a typical
series of reactions being:

t. Formation of free radicals from a catalyst (initiator), such as a
peroxide.
2. Initiation: for example,

CH 2 = CHX + R -» RCH 2 - CHX
vinyl free
monomer radical
3. Propagation:

RCH 2 - CHX -I- CH 2 = CHX -» RCH 2 - CHX - CH 2 - CHX, etc.

to R(CH 2 - CHX)., CH 2 - CHX
vinyl polymer

4. Termination. This can take place in several ways, such as reaction
of the activated chain with an impurity, an additive or other
activated chains, or by disproportionation between two activated
chains.

A rise in temperature increases the rates of initiation and
termination, so that the rate of polymerisation is increased but the
average chain length of the polymer is reduced. The chain length is
also reduced by increasing the catalyst concentration, since this