Handbook of Solvents - George Wypych - ChemTech - Ventech!

794 Michelle L. Coote and Thomas P. Davis
desorption are manifest where the polymer adjusts its environment towards maximum solvation.
With this knowledge it may be expected that Bootstrap effects based on preferential
solvation will be strongest where one of the monomers (or solvents) is a poor or non-solvent
for the polymer (such as copolymerization of acrylonitrile or N-vinyl carbazole). Indeed,
early experimental evidence for a partitioning mechanism in copolymerization was provided
by Ledwith et al. 123 for the copolymerization of N-vinyl carbazole with MMA in the
presence of a range of solvents.
Direct evidence for preferential solvation was obtained by Semchikov et al., 124 who
suggested that it could be detected by calculating, from measurements of the solution thermodynamics,
the total and excess thermodynamic functions of mixing. Six monomer pairs
were selected -Vac-NVP, AN-STY, STY-MA, Vac-STY, STY-BMA and MMA-STY. The
first four of these monomer pairs were known to deviate from the terminal model composition
equation, while the latter two were not. They found that these first four
copolymerizations had positive ΔG E values over the temperature range measured, and thus
also formed non-ideal polymer solutions (that is, they deviated from Raoult’s law). Furthermore,
the extent of deviation from the terminal model composition equation could be correlated
with the size of the ΔG E value, as calculated from the area between the two most
different composition curves obtained for the same monomer pair under differing reaction
conditions (for example, initiator concentration; or type and concentration of transfer
agent). For STY-MMA they obtained negative ΔG E values over the temperature range considered,
but for STY-BMA negative values were obtained only at 318 and 343K, and not
298K. They argued that the negative ΔG E values for STY-MMA confirmed the absence of
preferential solvation in this system, and hence its adherence to the terminal model composition
equation. For STY-BMA they suggested that non-classical behavior might be expected
at low temperatures. This they confirmed by polymerizing STY-BMA at 303K and
demonstrating a change in reactivity ratios of STY-BMA with the addition of a transfer
agent.
Based upon the above studies, it may be concluded that there is strong evidence to suggest
that Bootstrap effects arising from preferential solvation of the polymer chain operate
in many copolymerization systems, although the effect is by no means general and is not
likely to be significant in systems such as STY-MMA. However, this does not necessarily
discount a Bootstrap effect in such systems. As noted above, a Bootstrap effect may arise
from a number of different phenomena, of which preferential solvation is but one example.
Other causes of a Bootstrap effect include preferential solvation of the chain end, rather than
the entire polymer chain, 121,125 or the formation of non-reactive radical-solvent or monomer-solvent
complexes. In fact, the Bootstrap model has been successfully adopted in systems,
such as solution copolymerization of STY-MMA, for which bulk preferential
solvation of the polymer chain is unlikely. For instance, both Davis 125 and Klumperman and
O’Driscoll 43 adopted the terminal Bootstrap model in a reanalysis of the microstructure data
of San Roman et al. 126 for the effects of benzene, chlorobenzene and benzonitrile on the
copolymerization of MMA-STY.
Versions of the Bootstrap model have also been fitted to systems in which monomer-monomer
complexes are known to be present, demonstrating that the Bootstrap model
may provide an alternative to the MCP and MCD models in these systems. For instance,
Klumperman and co-workers have successfully fitted versions of the penultimate Bootstrap
model to the systems styrene with maleic anhydride in butanone and toluene, 43 and styrene