Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.2 Solvent effects on free radical polymerization 793
The Bootstrap model may also be extended by assuming an alternative model (such as
the explicit penultimate model) as the baseline model, and also by allowing the Bootstrap
effect to vary as a function of monomer feed ratios. Closed expressions for composition and
sequence distribution under some of these extended Bootstrap models may be found in papers
by Klumperman and co-workers. 43,44
13.2.3.4.3 Experimental evidence
The Bootstrap model was introduced by Harwood, 116 who studied three solvent sensitive
copolymerizations (styrene/methacrylic acid, styrene/acrylic acid and styrene/acrylamide)
and found that the copolymers of the same composition had the same sequence distribution
irrespective of the solvent used. This meant that the conditional probabilities governing radical
propagation were independent of the solvent. On this basis, he argued that composition
and sequence distribution were deviating from their expected values because there was a
difference between the monomer feed ratios in the vicinity of the active chain end, and those
calculated on the basis of the bulk feed. In other words, the solvent was altering the rates of
the individual propagation steps by affecting the reactant concentrations and not, as in the
other solvent effects models, their reactivities. However, Fukuda et al. 31 have argued that
the NMR evidence provided by Harwood is not conclusive evidence for the Bootstrap
model, since Harwood’s observations could also be described by variation of the reactivity
ratios in such a way that their product (r1r2) remains constant. This has also been raised as an
issue by Klumperman and O’Driscoll. 43 They showed mathematically that a variation in the
local comonomer ratio is not reflected in the monomer sequence distribution versus copolymer
composition -this relationship being governed by the r1r2 product only. An alternative
explanation for Harwood’s experimental data may be the stabilization or destabilization of
the radicals by the solvent, an interpretation that would be analogous to the MCD model.
Simple energy stabilization considerations, as used by Fukuda et al. 119 to derive the penultimate
unit effect, also suggest the constancy of r1r2. Prior to Harwood’s work, the existence of a Bootstrap effect in copolymerization was
considered but rejected after the failure of efforts to correlate polymer-solvent interaction
parameters with observed solvent effects. Kamachi, 37 for instance, estimated the interaction
between polymer and solvent by calculating the difference between their solubility parameters.
He found that while there was some correlation between polymer-solvent interaction
parameters and observed solvent effects for methyl methacrylate, for vinyl acetate there
was none. However, it should be noted that evidence for radical-solvent complexes in vinyl
acetate systems is fairly strong (see Section 3), so a rejection of a generalized Bootstrap
model on the basis of evidence from vinyl acetate polymerization is perhaps unwise.
Kratochvil et al. 120
investigated the possible influence of preferential solvation in
copolymerizations and concluded that, for systems with weak non-specific interactions,
such as STY-MMA, the effect of preferential solvation on kinetics was probably comparable
to the experimental error in determining the rate of polymerization (±5%). Later,
Maxwell et al. 121 also concluded that the origin of the Bootstrap effect was not likely to be
bulk monomer-polymer thermodynamics since, for a variety of monomers, Flory-Huggins
theory predicts that the monomer ratios in the monomer-polymer phase would be equal to
that in the bulk phase. 122
Nevertheless, there are many copolymerization systems for which there is strong evidence
for preferential solvation, in particular, polymer solutions exhibiting cosolvency or
where one of the solvents is a non-solvent for the polymer. Preferential adsorption and