Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.2 Solvent effects on free radical polymerization 787
As an example, the system STY with maleic anhydride (MAH) has been perhaps the
most widely studied EDA system and yet there is still uncertainty concerning the role of the
EDA complex in its propagation mechanism. Early studies 82,83 concluded that its behavior
was best modelled by a penultimate model, despite the spectroscopic evidence for EDA
complexes in this system. Later Tsuchida et al. 84,85 fitted an MCP model, based on the evidence
that the rate went through a maximum at 1:1 feed ratio in benzene or CCl4 but in
strong donor solvents no such maximum occurred and instead the rate increased with the
content of MAH in the feed. They argued that maximum in rate at 1:1 feed ratios was due to
the fact that propagation occurred via the complex, which had a maximum concentration at
this point. In strong donor solvents, the maximum rate moved to higher concentrations of
MAH due to competition between the donor and STY for complexation with the MAH.
However, a few years later, Dodgson and Ebdon 86,87 conducted an extensive study of
STY-MAH in various solvents and discounted the MCP model on the basis of an absence of
a dilution effect with the inert solvent MEK. In an MCP model a dilution effect would be expected
due to the decrease in the relative concentration of the comonomer complex and the
enhanced participation of the free monomer. 88 Later, Farmer et al. 89 reanalyzed this data and
concluded that the composition data was consistent with both models and suggested sequence
distribution may provide the answer. They also pointed out that there was a small dilution
effect in MEK -greater than that predicted by the penultimate model and less than that
predicted by the MCP model. Hill et al. 90 has suggested that interpretation of the effect of
solvents is complicated by the fact that no solvent is truly inert, hence such results such be
treated with caution. More recently Sanayei et al. 91 have performed a pulsed-laser polymerization
study on STY-MAH copolymerization in butanone and acetonitrile. They concluded
that whilst the complex participation model described the copolymer composition it failed
to predict the average kp data. Consequently the best description of this copolymerization
was given by the penultimate unit model.
There have been many other systems for which the MCP model has been proposed as
an alternative to the penultimate unit model. For instance, Litt and Seiner used the MCP
model to describe the composition of a number of systems, including MAH with
1-diphenylethylene, β-cyanacrolein with styrene, 92
and vinyl acetate with
dichlorotetrafluoroacetone and with hexafluoroacetone. 80 An MCP model has also been
suggested for the system STY-SO 2. 39,83,93-95 In this system, the composition changes with dilution
or with solvent changes, strongly alternating behavior is observed across a range of
feed ratios, and one of the comonomers (SO 2) does not undergo homopolymerization.
However, while the MCP model appears to be appropriate for some systems, in other
strongly alternating copolymerizations it is clearly not appropriate. For instance, there are
many strongly alternating copolymerizations for which there is no evidence of complex formation.
36,39,88,96 Even when complex formation is known to occur, results cannot always be
explained by the MCP model. For instance, measurements of sequence distribution data revealed
that, while both the MCP and penultimate model could provide an adequate description
of the composition of STY with acrylonitrile (AN), only the penultimate model could
account for the sequence distribution data for this system. 97 As will be seen shortly, there is
evidence that in some systems the heat of propagation would be sufficient to dissociate the
EDA complex and hence it could not add to the monomer as unit. In this case an MCD
model would be more appropriate. Thus, it might be concluded that the MCP, MCD and the
penultimate models are needed to describe the behavior of strongly alternating systems, and