Chemistry Reference
In-Depth Information
The styrene monomer-radical-ion is unstable and tends to dimerize:
CH
2
Na
Na
Na
2
Metal ketyls form in reactions of alkali metals with aromatic ketones in some polar solvents, like
dioxane or tetrahydrofuran. These ketyls exist in equilibrium mixtures of monomeric anion-radicals
and dimeric dianions [
136
]. Originally there was some controversy about the mechanism of initiation
of monomers like acrylonitrile or methyl methacrylate by sodium benzophenone. The following
mechanism was derived from spectral evidence. The initiation is by transfer of the electron and the
ion charge [
181
-
183
]:
O
O
O
O
+
+
O
O
4.4.1.3
Initiations by Alfin Catalysts
These are catalysts that are formed by combining an alkyl sodium with sodium alkoxide and with an
alkali metal halide [
184
-
189
]. A typical, effective catalyst for polymerization of dienes consists of
allyl sodium, sodium isopropoxide, and sodium chloride. The preparation of such a catalyst is carried
out by combining amyl chloride with sodium and then reacting the product with isopropyl alcohol.
After that, propylene is bubbled through the reaction mixture to convert amyl sodium to allyl sodium.
The reactions can be summarized as follows
1
:
5C
5
H
11
Cl
þ
3Na
!
1
:
5C
5
H
11
Na
þ
1
:
5NaCl
C
5
H
11
Na
þð
CH
3
Þ
2
CHOH
!ð
CH
3
Þ
2
CHONa
þ
C
5
H
12
C
5
H
11
Na
þ
CH
3
CH = CH
2
!
C
5
H
12
þ
CH
2
=CH
CH
2
Na
Diisopropyl ether may be used in place of isopropyl alcohol. In that case, the reaction does not
require addition of propylene because the olefin forms in situ [
189
]. These catalysts are particularly
effective in polymerizations of some conjugated dienes to very high molecular weight products.
Styrene can also be polymerized by these catalysts. Polymers of the dienes that form are high in
-
1,4 repeat units. Butadiene is polymerized by these catalysts much more rapidly than is isoprene. On
the other hand, 2,3-dimethylbutadiene fails to polymerize.
trans
