Handbook of Solvents - George Wypych - ChemTech - Ventech!

788 Michelle L. Coote and Thomas P. Davis
the selection of each model should be on a case-by-case basis. There have been many more
systems for which the MCP model has been evaluated against the penultimate model on the
basis of kinetic behavior. These studies have been extensively reviewed by Hill et al. 90 and
Cowie 98 will therefore not be reviewed here. Instead a few additional sources of evidence
for the participation of the EDA complex will be highlighted.
UV and NMR evidence for the existence of EDA complexes
There is certainly a demonstrable existence of comonomer complexes in solutions of
electron donor acceptor monomer pairs. These complexes can be detected, and their equilibrium
constants measured, using UV or NMR spectroscopy. Techniques for this are described
in detail in reviews of comonomer complexes by Cowie 98 and Hill et al. 90 The latter
review 90 also includes a listing of the equilibrium constants for the numerous EDA complexes
that have been experimentally detected. The existence of comonomer complexes is
not sufficient evidence for their participation in the propagation step of copolymerization,
but the fact that they exist in solutions from which strongly alternating copolymers are produced
suggests that they play some role in the mechanism. Furthermore, the ability to measure
their strengths and quantify the effects of solvents on their observed equilibrium
constants without performing kinetic experiments, may provide the key to establishing their
role in the propagation mechanism. Since the alternative models for copolymerization include
(or in some cases omit) the equilibrium constant for these complexes in different
ways; if the equilibrium constant was to be measured separately and not treated as an adjustable
parameter in the kinetic analysis, more sensitive model discrimination would be possible.
To date, such an analysis does not appear to have been performed but it should be
included in subsequent kinetic analyses of these explicit systems.
Temperature effects
The study of the temperature dependence of copolymerization behavior may also provide
evidence for the role of comonomer complexes. As was seen previously in the study of
acrylamide, complexes dissociate at high temperatures and hence, if the complex is involved
in controlling an aspect of the polymerization behavior, a change in this behavior
should be observed at the temperature corresponding to the complete dissociation of the
complex. Such evidence has been obtained by Seymour and Garner 99,100 for the
copolymerization of MAH with a variety of vinyl monomers, including STY, VA, AN, and
α-MSTY. They observed that the copolymers undergo a change from strongly alternating to
random at high temperatures, and these temperatures are also the temperatures at which the
concentration of the EDA complex becomes vanishingly small. It is true that, since reactivity
ratios have an enthalpy component, they approach unity as temperature increases.
Hence, most models predict that the tendency of copolymers to form a random
microstructure increases as temperature increased. Indeed, more recent work by
Klumperman 101 has shown that for STY-MAH copolymerization, the reactivity ratios do
follow an Arrhenius type of temperature dependence. However, further work is required to
verify this for the other copolymerization listed above. Based upon the existing
copolymerization data, it appears that for many systems there are sudden transition temperatures
that correspond to the dissociation of the complex, which does suggests that the complex
is in some way responsible for the alternating behavior. 99,100
Stereochemical evidence for the participation of the complex
Stereochemical data may provide evidence for participation of the EDA complex. The
EDA complex will prefer a certain geometry -that conformation in which there is maximum