Handbook of Solvents - George Wypych - ChemTech - Ventech!

782 Michelle L. Coote and Thomas P. Davis
The apparent lack of a general relationship between the dielectric constant of the system and
the monomer reactivity ratios does not necessarily discount a polarity effect on reactivity ratios.
A polarity effect is only expected to occur if the charge transfer configurations of the
transition state are sufficiently low in energy to contribute to the ground state wave function.
Since this is not likely to occur generally, a comprehensive correlation between reactivity
ratios and the solvent dielectric constant is unlikely. Furthermore, even in systems for
which a polarity effect is operating, a correlation between solvent dielectric constant and
monomer reactivity ratios may be obscured by any of the following causes.
• The operation of additional types of solvent effect, such as a Bootstrap effect, that
would complicate the relationship between solvent polarity and reactivity ratios.
• Errors in the experimental data base from which the correlation was sought.
• The recognized inadequacy of simple reactivity - dielectric constant correlations,
that take no account of specific interactions between the solvent and solute
molecules. 34
In fact, recent theoretical 33 and experimental studies 42 of small radical addition reactions
indicate that charge separation does occur in the transition state when highly
electrophilic and nucleophilic species are involved. It is also known that copolymerization
of electron donor-acceptor monomer pairs are solvent sensitive, although this solvent effect
has in the past been attributed to other causes, such as a Bootstrap effect (see Section
13.2.3.4). Examples of this type include the copolymerization of styrene with maleic
anhydride 43 and with acrylonitrile. 44 Hence, in these systems, the variation in reactivity ratios
with the solvent may (at least in part) be caused by the variation of the polarity of the
solvent. In any case, this type of solvent effect cannot be discounted, and should thus be
considered when analyzing the copolymerization data of systems involving strongly
electrophilic and nucleophilic monomer pairs.
13.2.3.2 Radical-solvent complexes
13.2.3.2.1 Basic mechanism
Solvents can also interfere in the propagation step via the formation of radical-solvent complexes.
When complexation occurs, the complexed radicals are more stable than their corresponding
uncomplexed-radicals, as it is this stabilization that drives the complexation
reaction. Thus, in general, one might expect complexed radicals to propagate more slowly
than their corresponding free-radicals, if indeed they propagate at all. However, in the special
case that one of the comonomers is the complexing agent, the propagation rate of the
complexed radical may instead be enhanced, if propagation through the complex offers an
alternative less-energetic reaction pathway. In any case, the complexed radicals would be
expected to propagate at a rate different to their corresponding free-radicals, and thus the
formation of radical-solvent complexes would affect the copolymerization propagation kinetics.
13.2.3.2.2 Copolymerization model
A terminal radical-complex model for copolymerization was formulated by Kamachi. 37 He
proposed that a complex is formed between the propagating radical chain and the solvent
(which may be the monomer) and that this complexed radical has a different propagation
rate constant to the equivalent uncomplexed radical. Under these conditions there are eight
different propagation reactions in a binary copolymerization, assuming that the terminal
unit is the only unit of the chain affecting the radical reactivity. These are as follows.