Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.2 Solvent effects on free radical polymerization 785
radical-solvent complexes will affect the copolymerization kinetics for certain
copolymerization systems, and indeed many workers have invoked the radical-complex
model in order to explain solvent effects in copolymerization. For instance, Heublein and
Heublein 76 have invoked a radical complex model in combination with a partitioning idea
(see Section 13.2.3.4) to explain solvent effects on the copolymerization of vinyl acetate
with acrylic acid. More recently, O’Driscoll and Monteiro 77 suggested that the effect of
benzyl alcohol on the copolymerization of STY-MMA was best described by an RC-type
model. This was supported by pulsed-laser studies 9 on the homopropagation reactions
where Ea values were found to be increased slightly by the presence of benzyl alcohol.
Czerwinski (see for example reference 78 and references cited therein) has also published a
variant of the RC model and has applied his model to a range of copolymerization experimental
data. In conclusion, there is a strong experimental evidence for the importance of
radical-solvent complexes in a number of specific copolymerization systems, especially
when there is a large disparity in the relative stabilities of the different propagating radicals.
13.2.3.3 Monomer-solvent complexes
13.2.3.3.1 Introduction
A solvent may also interfere in the propagation step via complexation with the monomer.
As was the case with radical-solvent complexes, complexed monomer might be expected to
propagate at a different rate to free monomer, since complexation might stabilize the monomer,
alter its steric properties and/or provide an alternative pathway for propagation. In examining
the effect of such complexation on copolymerization kinetics, there are a number
of different mechanisms to consider. In the case that the complex is formed between the
comonomers, there are three alternatives: (1) the monomer-monomer complex propagates
as a single unit, competing with the propagation of free monomer; (2) the monomer-monomer
complex propagates as a single unit, competing with the propagation of free monomer,
but the complex dissociates during the propagation step and only one of the monomers is incorporated
into the growing polymer radical; (3) the monomer-monomer complex does not
propagate, and complexation serves only to alter the free monomer concentrations. In the
case that the complex is formed between one of the monomers and an added solvent, there
are two further mechanisms to consider: (4) the complexed monomer propagates, but at a
different rate to the free monomer; (5) the complexed monomer does not propagate. Models
based on mechanisms (1) and (2) are known as the monomer-monomer complex participation
(MCP) and dissociation (MCD) models, respectively. Mechanisms (3) and (5) would
result in a solvent effect analogous to a Bootstrap effect, and will be discussed in Section
13.2.3.4. In this section, we review the MCP and MCD models, and conclude with a brief
discussion of specific monomer-solvent interactions.
13.2.3.3.2 Monomer-monomer complex participation model
The use of monomer-monomer charge transfer complexes to explain deviations from the
terminal model was first suggested by Bartlet and Nozaki, 79 later developed by Seiner and
Litt, 80 and refined by Cais et al. 81 It was proposed that two monomers can form a 1:1 donor
complex and add to the propagating chain as a single unit in either direction. The complex
would be more reactive because it would have a higher polarizability due to its larger π-electron
system that can interact more readily with the incoming radical. The complex would
also have a higher pre-exponential factor, as a successful attack may be achieved over a