Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.2 Solvent effects on free radical polymerization 795
with acrylonitrile in various solvents. 44 This latter work confirmed the earlier observations
of Hill et al. 127 for the behavior of styrene with acrylonitrile in bulk, acetonitrile and toluene.
They had concluded that, based on sequence distribution data, penultimate unit effects were
operating but, in addition, a Bootstrap effect was evident in the coexistent curves obtained
when triad distribution was plotted against copolymer composition for each system. In the
copolymerization of styrene with acrylonitrile Klumperman et al. 44 a variable Bootstrap effect
was required to model the data. Given the strong polarity effects expected in this system
(see Section 13.2.2), part of this variation may in fact be caused by the variation of the solvent
polarity and its affect on the reactivity ratios. In any case, as this work indicates, it may
be necessary to simultaneously consider a number of different influences (such as, for instance,
penultimate unit effects, Bootstrap effects, and polarity effects) in order to model
some copolymerization systems.
13.2.4 CONCLUDING REMARKS
Solvents affect free-radical polymerization reactions in a number of different ways. Solvent
can influence any of the elementary steps in the chain reaction process either chemically or
physically. Some of these solvent effects are substantial, for instance, the influence of solvents
on the gel effect and on the polymerization of acidic or basic monomers. In the specific
case of copolymerization then solvents can influence transfer and propagation
reactions via a number of different mechanisms. For some systems, such as styrene-acrylonitrile
or styrene-maleic anhydride, the selection of an appropriate
copolymerization model is still a matter of contention and it is likely that complicated
copolymerization models, incorporating a number of different phenomena, are required to
explain all experimental data. In any case, it does not appear that a single solvent effects
model is capable of explaining the effect of solvents in all copolymerization systems, and
model discrimination should thus be performed on a case-by-case basis.
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