Chemistry Reference
In-Depth Information
placement in the resultant chains. The adducts, just as in the initiation reactions, are resonance
hybrids of two structures, A and B:
Me
Me
O
O
O
O
A
B
where Me represents a metal. Furukawa and coworkers concluded that due to the extremely low
tendency of the adducts to dissociate [
157
], the carbonyl absorption can only be ascribed to
undissociated ion pairs. The magnitude of the carbonyl absorption and the shifts to higher frequencies
show the degree of contribution of structure B, shown above. The absorption and the shifts were also
explained by the configurations of the electrons in the anti-bonding orbitals of the carbonyl groups.
The higher the stretching frequencies, the nearer are the positions of the counterions to the carbonyl
groups of the terminal units [
207
]. This is accompanied by higher tacticity [
157
]. The carbanions on
the terminal units in the transition states are located near the
-carbons of the incoming monomers.
At the same time, rotations around the axis through these two carbons may be quite restricted when
the cations are in the vicinity of the carbonyl groups of the terminal unit and near the incoming
monomers. In this manner, isotactic placement is enhanced [
157
]:
b
Li
RO
OR
O
O
The same mechanism was proposed for the polymerization of methacrylonitrile [
157
].
Cram and Kopecky [
219
] offered a different mechanism of steric control. According to their
mechanism, during a methyl methacrylate polymerization the growing ether enolate possesses a
complete alkoxide character:
Li
O
Li
O
O
O
O
O
O
O
Attacks by the alkoxide ion on the carbonyl groups of the penultimate units lead to formations of
six-membered rings:
Li
O
O
O
O
O
O
O
O
Li
