Handbook of Solvents - George Wypych - ChemTech - Ventech!

780 Michelle L. Coote and Thomas P. Davis
true even in bulk copolymerization, if either of the monomers exerts a direct effect or if
strong cosolvency behavior causes preferential solvation. A number of models have been
proposed to describe the effect of solvents on the composition, microstructure and propagation
rate of copolymerization. In deriving each of these models, an appropriate base model
for copolymerization kinetics is selected (such as the terminal model or the implicit or explicit
penultimate models), and a mechanism by which the solvent influences the propagation
step is assumed. The main mechanisms by which the solvent (which may be one or both
of the comonomers) can affect the propagation kinetics of free-radical copolymerization reactions
are as follows:
(1) Polarity effect
(2) Radical-solvent complexes
(3) Monomer-solvent complexes
(4) Bootstrap effect
In this chapter we explain the origin of these effects, show how copolymerization
models for these different effects may be derived, and review the main experimental evidence
for and against these models. Throughout this review the baseline model for
copolymerization is taken as the terminal or Mayo-Lewis model. 30 This model can be used
to derive well-known expressions for copolymer composition and copolymerization propagation
kinetics. Deviations from this model have often been interpreted in terms of either
solvent effects or penultimate unit effects, although the two are by no means mutually exclusive.
Deviations which affect both the copolymer composition and propagation kinetics
have been termed explicit effects by Fukuda 31 in deriving penultimate unit models, whereas
deviations from the kinetics without influencing the copolymer composition have been
termed implicit effects. In this review we use the same terminology with respect to solvent
effects: that is, a solvent effect on kp only is termed an implicit solvent effect, while a solvent
effect on composition, microstructure and kp is termed explicit. The relatively recent discovery
by Fukuda and co-workers 32 of the seemingly general failure of the terminal model
to predict kp, even for bulk copolymerizations that follow the terminal model composition
equation, led them to propose an implicit penultimate unit effect as a general phenomenon
in free-radical copolymerization kinetics. We conclude this review with a brief examination
of the possibility that a implicit solvent effect, and not an implicit penultimate unit effect,
may instead be responsible for this failure of the terminal model kp equation.
13.2.3.1 Polarity effect
13.2.3.1.1 Basic mechanism
One type of solvent effect on free-radical addition reactions such as the propagation step of
free-radical polymerization is the so-called ‘polarity effect’. This type of solvent effect is
distinguished from other solvent effects, such as complexation, in that the solvent affects
the reactivity of the different types of propagation steps without directly participating in the
reaction. The mechanism by which this could occur may be explained as follows. The transition
states of the different types of propagation steps in a free-radical copolymerization
may be stabilized by charge transfer between the reacting species. The amount of charge
transfer, and hence the amount of stabilization, is inversely proportional to the energy difference
between the charge transfer configuration, and the product and reactant configurations
that combine to make up the wave function at the transition state. 33 Clearly, the
stability of the charge transfer configuration would differ between the cross- and