Handbook of Solvents - George Wypych - ChemTech - Ventech!

784 Michelle L. Coote and Thomas P. Davis
gen with the π-system of the acceptor monomer. Several workers have invoked frontier orbital
theory to rationalize such solvent effects in terms of radical-solvent complex
formation, and thus provide a theoretical base. 37,62
Many workers have suggested radical-solvent complexes as an explanation for the influence
of aromatic compounds on the homopolymerization of vinyl monomers. For instance,
Mayo 63 found that bromobenzene acts as a chain transfer agent in the polymerization
of STY but is not incorporated into the polymer. He concluded that a complex is formed between
the solvent molecule and either the propagating polystyryl radical or a proton derived
from it. The influence of halobenzenes on the rate of polymerization of MMA was detected
by Burnett et al. 8,64,65 They proposed that the efficiency of a number of different initiators increased
in various halogenated aromatic solvents and, since enhanced initiator or solvent incorporation
into the polymer was not observed, they concluded that
initiator-solvent-monomer complex participation affected the initiator efficiency.
Henrici-Olive and Olive 66-71 suggested that this mechanism was inadequate when the degree
of polymerization was taken into account and they proposed instead a charge transfer complex
between the polymer radical and aromatic solvent. The polymer radical can form a
complex with either the monomer or solvent molecule, but only the former can propagate.
Bamford and Brumby, 5 and later Burnett et al., 72,73 interpreted their solvent-effects data for
k p in terms of this donor-acceptor complex formation between aromatic solvents and propagating
radicals.
Radical-solvent complexes are expected to be favored in systems containing unstable
radical intermediates (such as vinyl acetate) where complexation may lead to stabilization.
In this regard Kamachi et al 18 have noted that solvent effects on vinyl acetate
homopolymerization result in a reduced k p. Kamachi et al. 74 also measured the absolute rate
constants of vinyl benzoate in various aromatic solvents and found that k p increased in the
order:
benzonitrile < ethyl benzoate < anisole < chlorobenzene < benzene < fluorobenzene < ethyl acetate
They argued that this trend could not be explained by copolymerization through the
solvent or transfer to the solvent because there was no correlation with the solvent dielectric
constant or polarity, or with the rate constants for transfer to solvent. However, there was a
correlation with the calculated delocalization stabilization energy for complexes between
the radical and the solvent, which suggested that the propagating radical was stabilized by
the solvent or monomer, but the solvent did not actually participate in the reaction.
As noted in the introduction to this section, radical-solvent complexes may enhance
the propagation rate if propagation through the complex offers an alternative, less-energetic
pathway for propagation. An example of this behavior is found in the homopolymerization
of acrylamide. The homopropagation rate coefficient for this monomer shows a negative
temperature dependence, which has been explained in terms of radical-complex formation.
Pascal et al. 11,75 suggested that propagation proceeds via a complex that enhances the propagation
rate, and this complex dissociates as temperature increases, thus explaining the normal
temperature dependence of the propagation rate at high temperatures. This
interpretation was supported by the observation that acrylamide behaves normally in the
presence of reagents such as propionamide, which would be expected to inhibit complex
formation.
Given the experimental evidence for the existence of radical-solvent complexes and
their influence on free-radical addition reactions such as homopropagation, it is likely that