Chemistry Reference
In-Depth Information
homopolymerizations after application of heat. A free-radical attack or irradiation with ultraviolet
light or with gamma rays is also effective [
198
].
It was proposed [
198
-
201
] that the greater reactivity is due to delocalization of the electrons of the
complexed molecules:
N
+
ZnCl
2
N
ZnCl
2
Molecular orbital calculations support this [
200
,
201
]. Polymerizations can also be carried out with
less than stoichiometric ratios of the Lewis acids to the monomers. As an illustration, coupling of an
acrylic or a methacrylic monomer with a Lewis acid can be shown as follows [
203
,
204
]
aM
þ
MeX
n
Ðð
M)
a
MeX
n
where, MeX
n
represents a Lewis acid.
Vinyl pyridine and vinyl imidazole also form complexes with zinc chloride. Here, conjugation
with the metal salt results in a super delocalizability and leads to spontaneous thermal polymerization
[
203
,
204
].
The increased reactivity toward polymerization by some monomers, even in the presence of less
than equimolar amounts of Lewis acids, was suggested to be due to one of two possibilities. It may be
due to greater reactivity of the portion of the monomer that complexes toward the growing chain end.
It may also be due to formation of new complexes between uncomplexed monomer and complexed
ones [
171
]:
N
N
+
N
N
ZnCl
2
ZnCl
2
The picture is more complicated when the Lewis acids are used in combinations with
donor-acceptor monomers. The donor-acceptor complexes are believed to form first and then react
with Lewis acids according to the following scheme [
203
,
204
]:
+
b
M
2
+
b
M
2
+
a
M
1
a
MeX
n
MeX
n
M
1
MeX
n
b
M
2
+ a
M
1
A structure of methyl methacrylate and styrene complexed with SnCl
4
was shown to be as follows
[
204
]:
O
O
Sn
O
O
