Radicals and Their Reactions
been largely used up in setting the chains going in the first place. It
should be emphasised that such mutual interaction of radicals can
result not only in reaction as above but also lead to disproportionation
(p. 236). The chain length, i.e. the molecular weight, of the polymer
may be controlled by addition of terminators or of chain transfer
agents. These are usually compounds, XH, which react with a growing
chain by loss of a hydrogen atom, so terminating the chain:
A new radical, X>, is formed and in the case of terminators X is chosen
so that this radical is of low reactivity and hence not capable of
initiating addition polymerisation in more monomer. In the case of
chain transfer agents, however, X is chosen so that X- is reactive
enough to initiate a new reaction chain so that the length (molecular
weight) of individual chains is then controlled without at the same
time slowing down the overall rate at which monomer undergoes
polymerisation. Thiols, RSH are often used as chain transfer agents
yielding RS- radicals as the initiators of the new chains.
Vinyl polymerisation, proceeding via ionic mechanisms, may also
be initiated by acids and bases and by Lewis acids, e.g. BF 8 , etc. These
reagents have recently become of increasing importance in the
manufacture of oriented polymers, e.g. polypropylene in which the
methyl substituents are arranged in a regular pattern on the same side
of the 'backbone' chain of carbon atoms (isotactic polymers). Such
oriented polymers have notable advantages in crystalline structure,
melting point, mechanical strength etc. over comparable species in
which the alkyl substituents are arranged at random (atactic polymers).
(c) Displacement reactions: (i) Halogenation. The displacement
reactions on carbon that proceed via a radical mechanism are not in
fact direct displacements or substitutions but involve two separate
stages. This may be seen in the photochemically catalysed chlorin-
ation of a hydrocarbon: '
Ra(CH,)„CM,-+HX -• Ra(CH 1 )„CHa+ Xa—a
R—H+'Cl -* R- + HC1 %•a + R—ci