Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.2 Solvent effects on free radical polymerization 789
overlap between the highest occupied molecular orbital (HOMO) of the donor and the lowest
unoccupied molecular orbital (LUMO) of the acceptor. 102 If the complex adds to the
propagating polymer chain as a unit, then this stereochemistry would be preserved in the
polymer chain. If, however, only free monomer addition occurs, then the stereochemistry of
the chain should be completely random (assuming of course that there are no penultimate
unit effects operating). Hence, it is possible to test for the participation of the monomer
complexes in the addition reaction by examination of the stereochemistry of the resulting
polymer.
Such stereochemical evidence has been collected by a number of workers. For instance,
Iwatsuki and Yamashia 103 observed an unusually high percentage of cis units in
MAH/butadiene copolymers. Olson and Butler 104
studied the EDA system
N-phenylmaleimide (NPM)/2-chloroethyl vinyl ether (CEVE) and found that the
stereochemistry at succinimide units in NPM-CEVE copolymers is predominantly cis, and
random elsewhere. Furthermore, they noted that the proportion of cis units was correlated
with those variables with which the concentration of the EDA complex was also correlated.
In these examples, the cis geometry is that which is most stable for the complex. However,
Rätzsch and Steinert 105 have argued that this preference for cis geometry may also be explained
by propagation occurring via a complex between the reacting free monomer and the
chain end, as in an RC model. Thus this evidence should be used in conjunction with other
evidence for model discrimination.
Further stereochemical evidence for the MCP model has been obtained by Butler et
al. 106 They predicted that the usual preference for head-tail addition in free-radical polymerization
would be overcome if propagation occurred via the EDA complex, and its favored
geometry was a head-head conformation. They noted that for most EDA pairs head-tail geometry
was favored and hence the predominance of head-tail linkages in these copolymers
could not discriminate between free monomer addition and complex participation. To solve
this problem, they designed and synthesized two monomer pairs for which a head-head conformation
would be expected in their EDA complexes. These were the systems dimethyl
cyanoethylene dicarboxylate (DMCE) with CEVE, and dimethyl cyanoethylene
dicarboxylate (DMCE) with CEVE. They then showed that there were significant
head-head linkages in the resulting copolymer and the proportion of these linkages was correlated
with same types of variables that had previously affected the cis content of
NPM/CEVE copolymers -that is, those variables which affected the concentration of the
EDA complex. Thus they concluded that there was strong stereochemical evidence for the
participation of the EDA complex in the propagation step.
ESR evidence for the participation of the complex
ESR studies have also been suggested as a means for providing information about the
participation of the EDA complex. Since the addition of the complex is likely to occur more
readily in one direction, if propagation occurs as the repeated addition of the complex then
the propagating radical should be predominantly of one type. However, if free monomer
addition predominantly occurs, both types of radical are likely to be present at any time.
Thus ESR can be used to distinguish between the two mechanisms. This approach was used
by Smirnov et al. 107 to show that, in the system phenyl vinyl ether/MAH, alternating addition
of the free monomer predominates, but participation of EDA complexes is important
for the system butyl vinyl ether/MAH. They argued that the difference in the behavior of the
two EDA systems was a result of the different strengths of their EDA complexes. In another