Chemistry Reference
In-Depth Information
the polymerization is slowed but not entirely suppressed. This occurs if (1) the
rate of either alternative process is close to that of the monomer addition reaction
(8-91a) and the new radicals from steps (8-91b) or (8-91c) do not reinitiate, or (2)
if the alternative processes are fast compared to ordinary monomer addition but
the new radicals formed reinitiate slowly.
Retardation and inhibition differ only as to whether the production of high
polymer is slowed or completely eliminated. When the retarder or inhibitor is a
chain transfer agent it is distinguished from an effective transfer agent in that it
transfers an atom to the propagating radicals very readily to produce a new radical
that reinitiates slowly or not at all.
Figure 8.2
compares the effects of inhibitors and retarders on the rate of free-
radical polymerization. Polymerization at normal rates is resumed when and if the
inhibitor or retarder has been consumed.
Inhibition can be achieved by three processes:
1.
Atom transfer to form a stable radical which does not reinitiate
polymerization, as in the reaction of poly(vinyl acetate) radical and
diphenylamine to yield a diphenyl nitrogen radical which will not add vinyl
acetate but may terminate a macroradical.
2.
Stable radicals such as diphenyl picryl hydrazyl, which are reactive enough to
terminate propagating radicals but not to initiate polymerization.
3.
Addition of primary or macroradicals to nonradical substances to yield free
radicals that do not add monomer.
Inhibition
Time
FIGURE 8.2
Conversion time plots for normal, retarded, and inhibited free radical polymerizations.
