Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.2 Solvent effects on free radical polymerization 781
homopropagation reactions, especially in copolymerization of highly electrophilic and
nucleophilic monomer pairs. Hence, when it is significant, charge transfer stabilization of
the transition state occurs to different extent in the cross- and homopropagation reactions,
and thus exerts some net effect on the monomer reactivity ratios. Now, it is known that polar
solvents can stabilize charged species, as seen in the favorable effect of polar solvents on
both the thermodynamics and kinetics of reactions in which charge is generated. 34 Therefore,
when charge transfer in the transition state is significant, the stability of the charge
transfer species and thus the transition state would be affected by the polarity of the solvent,
and thus a solvent effect on reactivity ratios would result.
13.2.3.1.2 Copolymerization model
There are two cases to consider when predicting the effect of solvent polarity on
copolymerization propagation kinetics: (1) the solvent polarity is dominated by an added
solvent and polarity is thus independent of the comonomer feed ratio, or (2) the solvent polarity
does depend on the comonomer feed ratio, as it would in a bulk copolymerization. In
the first case, the effect on copolymerization kinetics is simple. The monomer reactivity ratios
(and additional reactivity ratios, depending on which copolymerization model is appropriate
for that system) would vary from solvent to solvent, but, for a given copolymerization
system they would be constant as a function of the monomer feed ratios. Assuming of
course that there were no additional types of solvent effect present, these copolymerization
systems could be described by their appropriate base model (such as the terminal model or
the explicit or implicit penultimate models), depending on the chemical structure of the
monomers.
In the second case, the effect of the solvent on copolymerization kinetics is much more
complicated. Since the polarity of the reacting medium would vary as a function of the
comonomer feed ratios, the monomer reactivity ratios would no longer be constant for a
given copolymerization system. To model such behavior, it would be first necessary to select
an appropriate base model for the copolymerization, depending on the chemical structure
of the monomers. It would then be necessary to replace the constant reactivity ratios in
this model by functions of the composition of the comonomer mixture. These functions
would need to relate the reactivity ratios to the solvent polarity, and then the solvent polarity
to the comonomer feed composition. The overall copolymerization kinetics would therefore
be very complicated, and it is difficult to suggest a general kinetic model to describe these
systems. However, it is obvious that such solvent effects would cause deviations from the
behavior predicted by their appropriate base model and might therefore account for the deviation
of some copolymerization systems from the terminal model composition equation.
13.2.3.1.3 Evidence for polarity effects in propagation reactions
The idea of charge separation in the transition state of the propagation step of free radical
polymerization reactions, as suggested by Price, 35 was discounted by Mayo and Walling 36
and many subsequent workers. 37 Their rejection of this idea was based upon the absence of
any unambiguous correlation between the reactivity ratios of a system and the dielectric
constant of the solvent. For instance, in the copolymerization of STY with MMA, it was reported
that the reactivity ratios were independent of small quantities of water, ethyl benzene,
dodecylmercaptans or hydroquinone, or the presence or absence of air 30,38,39 and were
thus unaffected by the dielectric constant of the system. In contrast, other studies have found
a relationship between dielectric constant and the reactivity ratios in specific systems. 40,41