Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.2 Solvent effects on free radical polymerization 779
the radical addition reaction. The largest solvent effects observed on kp for
homopropagations have been for vinyl acetate 18 and for α-(hydroxymethyl) ethyl acrylate. 12
In the former case the radical is highly unstable and some form of π-complexation between
the vinyl acetate radical and aromatic solvents seems plausible. However, the large solvent
effect cannot be explained by a simple radical stabilization argument (because of the early
transition state for free radical propagation reactions) 19 and again the evidence points towards
a change in the geometry of the transition state. The solvent effects on
α-(hydroxymethyl) ethyl acrylate are in the order of 300% on kp and there are large changes
in both the Arrhenius parameters as the solvent medium is changed. 12 In monomers exhibiting
a strong solvent effect on propagation it is plausible that some control of the
stereochemistry of the chains is possible by manipulating the solvent and possibly utilizing
Lewis acids as additives. This approach is already being successfully applied to the control
of radical reactions in conventional organic chemistry. 20
13.2.2.3 Transfer
Solvent effects on transfer reactions have not received too much attention. It would be expected
(owing to the similarities between the transition states for radical addition and abstraction
reactions) that these solvent effects should emulate those found in propagation
reactions. However, there is potential for significant polar interactions in transfer reactions.
Odian 21 has suggested that polar interactions play a significant role in the transfer reactions
between styrene and carbon tetrachloride. More recent work supports this idea. 22 Significant
solvent effects have been observed in catalytic chain transfer reactions using
cobaloximes where the transfer reaction appears (in some cases at least) to be diffusion controlled
and therefore the speed of the reaction is governed, in part, by the viscosity of the
polymerizing medium. 23 In transfer reactions involving organometallic reagents then solvent
effects may become important where ligand displacement may occur. This is thought
to happen in catalytic chain transfer when pyridine is utilized as a solvent. 24
13.2.2.4 Termination
The solvent effects on the termination reaction have been extensively studied. In early
work, it was established that the radical-radical termination reaction is diffusion controlled
and the efficacy of termination was found to have a strong relationship with the solvent viscosity.
25 Subsequently, more complex models have been developed accounting for the quality
of the solvent (hence the size of the polymer coil). 26 The current debate centers on the
relative roles played by segmental and translational diffusion at different stages of conversion
for a variety monomers. Clearly in both cases the nature of the solvent becomes important.
Solvent effects are known to play a significant role in determining the strength and
onset conversion of the gel effect. This work originated in the classical paper by Norrish and
Smith 27 who reported that poor solvents cause an earlier gel effect in methyl methacrylate
polymerization. Recent careful studies of the gel effect by Torkelson and co-workers 28 have
reinforced observations made by Cameron and Cameron 29 over two decades ago concluding
that termination is hindered in poor solvents due to formation of more tightly coiled polymer
radicals.
13.2.3 COPOLYMERIZATION
When solvent effects on the propagation step occur in free-radical copolymerization reactions,
they result not only in deviations from the expected overall propagation rate, but also
in deviations from the expected copolymer composition and microstructure. This may be