A Guidebook to Mechanism in Organic Chemistry

Vinyl Polymerisation

it has been found to proceed very largely trans owing to the much less
free rotation about the C—C bond at this lower temperature. Thus
under these conditions cw-2-bromobut-2-ene was found to yield
92 per cent of the meyo-dibromide.
The addition of thiols, RSH, to olefines closely resembles that of
HBr in many ways. Heterolytic addition (of RSe) can take place but
radical additions may be initiated by the presence of peroxides and,
as with HBr, the two mechanisms generally lead to opposite orienta­
tions of addition.
(Hi) Vinyl pdfmeriaation. This reaction has probably received
more attention than any other involving radicals, not least because
of its commercial implications in the manufacture of polymers. It can
be said to involve three phases:

(a) Initiation: Formation of Ra* from peroxides, etc.

(b) Propagation:

CH,= CH,

Ra- + CH 2 =CHa -+ RaCH,—CH,- • Ra(CHjV etc.

(c) Termination:

(i) Ra(CHJ„-iCH,-+-Ra -> Ra(CH 2 )„Ra

(ii) Ra(CHJ„-iCH,-+-CH,(CHJ„-iRa -* Ra(CHa) 2 „Ra

The propagation stage is usually extremely rapid. #
As the olefine monomers readily absorb oxygen from the air, fortn-
ing peroxides which can themselves form radicals and so act as
initiators of polymerisation, it is usual to add some inhibitor, e.g.
quinone, to the monomer if it is to be stored. When, subse­
quently, the monomer comes to be polymerised sufficient radicals
must be produced to 'saturate' this added inhibitor before any
become available to initiate polymerisation; thus an induction period
is often observed before polymerisation begins to take place.
The radicals acting as initiators cannot properly be looked upon as
catalysts—though often referred to as such—for each one that ini­
tiates a polymerisation chain becomes irreversibly attached to the
chain and, if of suitable chemical structure, may be detected in the
final polymer. The efficiency of some radicals as initiators may be so
great that, after any induction period^very radical formed leads to a
polymer chain; the concentratiorTofinitiator radicals may thus be
kept very low. 0
Termination of a growing chain can result from reaction with
either an initiator radical or a second growing chain, but of these the
latter is normally the more important as the initiator radicals will have

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