Chemistry Reference
In-Depth Information
There is no general agreement about the mechanism of these polymerizations. Both anionic and
free-radical mechanisms were proposed [
184
-
189
] as the most probable reaction paths. The role of
sodium chloride is not clear in this mixture, though it was shown that it is essential [
184
-
189
]. It may
act, perhaps, as a support for the catalyst and may be a part of some sort of lattice involving both
sodium alkoxide and allyl sodium. The anionic mechanism is pictured as follows [
184
-
189
]:
R
R
monomer
CH
2
R
Na
Na
Na
Na Cl Na
Cl
Na
Cl Na Cl
Na Cl Na Cl
catalytic bed
initiation
-1,4 placement in
polymerization of butadiene [
189
]. According to that mechanism a complex of sodium isopropoxide
and allyl sodium forms first:
The free-radical mechanism was suggested due to high predominance of
trans
O
O
Na
CH
2
Na
Na
Na
CH
2
The monomer adsorbs on the surface and is visualized as displacing the allyl anion from the
complex to form an ion pair first and then, through an electron rearrangement, a radical [
189
]:
CH
2
CH
2
+
CH
2
Na
CH
2
Na
CH
2
The polymerization is assumed to go on from this point via free-radical mechanism until a
combination with an allyl radical takes place. Because the allyl radical is bound to the catalyst
surface, combination does not take place readily and high molecular weights are attained.
4.4.1.4 Electroinitiation of Anionic Polymerizations
Electrolytic polymerizations were described in the section dealing with cationic polymerizations.
Anionic polymerizations can also be initiated in an electric field. When LiAlH
4
or NaAl(C
2
H
5
)
4
are
used as electrolytes in tetrahydrofuran solvent “living” polymers can be formed from
-methylstyrene
[
190
]. The deep red color of carbanions develops first at the cathode compartments of divided cells.
a
