Chemistry Reference
In-Depth Information
78
C the following Lewis acids were rated in the order of their
In isobutylene polymerization at
efficiencies [
92
]:
BF
3
>
AlBr
3
>
TiBr
4
>
BBr
3
>
SnCl
4
The polymerization reactions can be complicated, sometimes, by two different types of propaga-
tion paths. Some chains may grow without a terminal counter-ion as free propagating species. Other
polymeric chains, however, may be paired off with counterions. It should be noted that when
references are made to free propagating ions, the ions are free from electrostatic influences of the
anions. They are, however, still associated with, and interact with polar or polarizable solvent
molecules or monomers.
4.3.3.1 Steric Control in Cationic Polymerization
Ionic polymerizations yield highly stereoregular polymer when control is exercised over monomer
placement. Much of the steric control in cationic polymerization occurs at low temperatures. The
earliest stereospecific vinyl polymerizations were observed in preparation of poly(isobutyl vinyl
ether) with a BF
3
-ether complex catalyst at
70
C. An isotactic polymer formed in this reaction [
93
].
The same catalyst was employed later to yield other stereospecific poly(vinyl ether)s [
93
-
96
]. The
amount of steric placement increases with a decrease in the reaction temperature and, conversely,
decreases with an increase in the temperature [
96
,
97
].
Various mechanisms were proposed to explain steric placement in cationic polymerization. Most
of them pertain to vinyl ethers. There is no general agreement. Some of the suggested mechanisms are
discussed in this section. Most were offered for homogeneous conditions with soluble initiators like
BF
3
-O(C
2
H
5
)
2
. There are, however, also some explanations of steric control with insoluble catalysts,
like Al(SO
4
)
2
H
2
SO
4
.
Control in Homogeneous Polymerizations
Not all explanations of steric control under homogeneous conditions give equal weight to the influence
of the counterion. A Bawn and Ledwith mechanism [
99
] for the polymerization of vinyl ethers is based
on data that suggests that only one mesomeric form of the ethers exists, presumably
trans
[
98
]
CH
2
OR
CH
2
R
O
O
R
An alkyl substituent composed of a three-carbon chain causes steric blocking of one side of the
bond:
H
H
O
A five-carbon substituent should exhibit the highest degree of steric hindrance, which, on the other
hand, should decrease with a decrease in the size of the group. There should be no blocking with an ethyl
group or with an isopropyl one [
98
]. This was, demonstrated experimentally [
99
]. It was suggested,
therefore, that in homogeneous polymerizations of vinyl ethers, the growing cations are stabilized by a
