Chemistry Reference
In-Depth Information
As mentioned above, the polystyryl carbanions are particularly stable and persist for weeks in non-
polar solvents. Yet, even in the absence of terminating agents, the concentration of the carbanion
active centers decreases with time [
159
]. The mechanism of decay is not fully understood. Based on
spectral evidence, it is believed to consist of a hydride-ion elimination [
219
]:
Na
+
NaH
The above reaction can be followed by an abstraction of an allylic hydrogen from the product of
elimination by another active center:
+
+
Polystyryl carbanions are much less stable in polar solvents. They decay within a few days at room
temperature. At lower temperatures, however, the stability is considerably better. The termination in
polar solvent occurs by a mechanism of abstracting
a
-hydrogens and/or by a nucleophilic attack on
the carbon-oxygen bonds.
Polar monomers, like methyl methacrylate, acrylonitrile, or methyl vinyl ketone contain
substituents that react with nucleophiles. This can lead to terminations and side reactions that
compete with both initiation and propagation [
219
]. An example is a nucleophilic substitution by
an intramolecular backbiting attack of a propagating carbanion:
O
O
O
COOCH
3
COOCH
3
+
CH
3
O
COOCH
3
COOCH
3
Side reactions like the one shown above can be minimized by using less nucleophilic initiators and
low temperatures. This can yield “living” polymerizations of acrylic and methacrylic monomers. In
addition, it is possible to add common ions like LiCl to alkyl lithium to tighten the ion pairs of the
propagating anion-counterion species. That also increases the tendency to form “living” polymers
[
221
]. This approach, however, offers only limited success.
In addition. it was found that Lewis acid assisted polymerizations of methyl methacrylate with
aluminum porphyrin initiators yield “living” polymers [
222
]. The polymerizations of methacrylate
esters with alkylaluminum porphyrin initiators occur through formations of enolate aluminum
porphyrin intermediate as the growing species [
222
]. For the sake of illustration, the methylaluminum
porphyrin molecules (see Fig.
4.3
) can be designated as:
Al
CH
3
