Chemistry Reference
In-Depth Information
d
cube
d
cube
CH
3
CH
3
O
Zn
O
Zn
O
CH
3
CH
2
CH
O
Zn
CH
3
Zn
O
CH
3
O
Zn
O
Zn
Zn
O
O
O
Zn
O
CH
3
CH
3
Zn
O
Zn
O
Zn
O
Zn
O
O
O
Zn
O
Zn
l cube
catalyst
complex
l cube
catalyst
complex
CH
3
d cube
CH
3
CH
3
Zn
O
Zn
O
d
cube
O
Zn
CH
3
O
Zn
CH
3
O
Zn
O
Zn
O
Zn
O
O
Zn
O
CH
3
CH
3
Zn
O
Zn
O
Zn
O
O
Zn
O
O
CH
3
O
Zn
O
Zn
l cube
transition state
l
cube
CH
3
product
Fig. 5.2
Tsuruta mechanism
5.4.1 The Initiation Reaction
Theoretically, any Lewis acid can catalyze oxetane polymerizations. However, these acids differ
considerably in their effectiveness. Boron trifluoride and its etherates are the most widely reported
catalysts. Moisture must be excluded, as it tends to be detrimental to the reaction [
35
].
Chlorinated hydrocarbon solvents, like methylene chloride, chloroform and carbon tetrachloride,
are common choices. The reactions are usually conducted at low temperatures and there are
indications that the lower the reaction temperature the higher the molecular weight of the product
It was reported that when oxetane polymerizations are carried out with boron trifluoride catalyst in
methylene chloride at temperatures between 0 and
27.8
C a cocatalyst is not required [
32
]. The
product, however, is a mixture of linear polymer and a small amount of a cyclic tetramer. This is in
agreement with an earlier observation that the polymerizations of oxetane are complicated by
formations of small amounts of cyclic tetramers [
33
]. Other catalysts, like protonic acids, capable
of generating oxonium ions, will also polymerize oxetane. Such acids are sulfuric, trifluoroacetic, and
fluorosulfuric. The initiation reaction can be illustrated as follows:
HA
+
H
O
O
A
