Chemistry Reference
In-Depth Information
Once added, however, the reaction proceeds at a normal rate [
158
]. Polymerizations of methyl
methacrylate in polar solvents, on the other hand, proceed in what might be described as an ideal
manner with formations of only one kind of ion pairs [
159
-
162
].
Other monomers can also exhibit abnormal behavior in some anionic polymerizations. Thus, for
instance, organomagnesium initiation of methacrylonitrile polymerization results in formation of two
types of active centers [
163
,
164
]:
N
MgX
R
N
RMgX
+
R
MgX
N
Methyl methacrylate polymerizations, initiated by organomagnesium compounds, also yield
abnormal products. Here, the active centers are unusually persistent and stable. In addition, the
a
-carbon atoms of the monomers were found to assume tetrahedral configurations [
165
-
167
]. This
suggests that the active centers contain covalent magnesium carbon bonds. Also, gel permeation
chromatography curves of the products show that more than one active center operates independently
[
166
,
167
]. A “
pseudo anionic
” mechanism was, therefore, postulated for polymerizations of acrylic
and methacrylic esters [
111
,
112
] by Grignard reagents.
Three different simultaneous reactions appear to be taking place in butyllithium-initiated
polymerizations of vinyl ketones in benzene [
168
]:
(C
4
H
9
Li)
6C
4
H
9
Li
C
4
H
9
O
Li
1,4
2
3
+
C
4
H
9
Li
1
4
O
1,2
Li
C
4
H
9
3,4
O
Li
O
C
4
H
9
The initiations of the polymerizations of the conjugated dienes in inert hydrocarbons are also
believed to be preceded by coordination of the organometallic compounds with the
p
-electron clouds
of the monomers:
H
H
+
R
Li
H
H
Li
H
R
