Chemistry Reference
In-Depth Information
When this reaction is applied to cyclopentene, a high molecular weight polymer forms [
164
]:
n
2
n
Tungsten hexachloride can apparently also act as a catalyst without the aluminum alkyl. In that
case it is believed to be activated by oxygen [
166
].The propagation reaction based on the tungsten
carbene mechanism can be shown as follows [
162
]:
Initiation
H
C
O
(CH
2
)
3
C
C
CH
O
2
WCl
6
W
O
W
O
W
O
W
Propagation
C
C
W
CH
CH
+
W
W
CH
It is significant that metal carbenes can act as catalysts for this reaction. Thus, a carbene (C
6
H
5
)
2
-
C
ΒΌ
W(CO)
5
will polymerize 1-methylcyclobutene to yield a polymer that is very similar in structure
to
cis
-polyisoprene [
162
]:
C
W(CO)
5
2
n
This carbene also yields high molecular weight linear polymers from bicyclo[4.2.0]octa-7-ene
monomer [
167
]:
H
C
C
H
CH
CH
n
The same product can also be obtained with WCl
6
/Sn(CH
3
)
4
catalyst. The molecular weight of the
product, however, is lower [
167
].
Some cycloolefins can undergo either a regular cationic polymerization or a metathesis one,
depending upon the catalyst. One of them is norbornene and its derivatives. For instance, 5-methylene-
2-norbornene polymerizes by a cationicmechanismwith a 1:1 combination of tungsten hexachloridewith
tetraalkyltin. A 1:4 combination of a tungsten halide with either C
2
H
5
AlCl
2
,orMoCl
5
, or TiCl
4,
or other
acidic catalysts [
166
] yields the same product. The polymer that forms has the repeat units:
