Chemistry Reference
In-Depth Information
period to 210
C, while the pressure is reduced to 20 mmHg. Reaction mixture is then heated to about
300
C for 5-6 h at 1 mmHg. Heating is stopped when desired viscosity is reached.
A synthetic route to polycarbonates was reported that uses crown ethers. Crown ethers generally
form stable complexes with metal cations, and by increasing the dissociation of ion pairs, provide
highly reactive, unsolvated anions. This led to direct preparations of new polycarbonates from
-
dibromo compounds, carbon dioxide, and potassium carbonate or salts of the diols [
36
] in the
presence of 18-crown-6-ether:
a
,
o
O
O
O
M
O
Br
n
O
O
+
2n CO
2
+
nK
2
CO
3
Br
18 crown-6-ether
Br
+
B
O
O
nKBr
n
O
When a potassium salt of a diol is present in the reaction mixture, mixed polyesters form [
36
]:
KO
Br
CO
2
+
Br
OK
O
M
O
O
O
+
2nBr
O
18 crown-6-ether
n
In addition to crown ethers, cryptates and polyglyme exhibit similar behavior [
37
].
Another direct route to polycarbonate was reported by Okuyama et al. [
36
]. The process consists of
an oxidative carbonylation procedure. It is catalyzed with a Pd complex system. {1,1
0
-Di-
-butyl-
3,3
0
-methylenediiimidazolin-2,2
0
,dylidene}palladium dibromide and produces a highest molecular
weight polymer, (
M
n
¼
9,600,
M
w
¼
24,000) in 80% yields.
Table
7.2
presents some
tert
T
g
and
T
m
values of some polycarbonates picked from the literature.
7.2.5 Polyesters from Lactones
produced commercially on a large scale. Judging from the patent literature, however, there is a
continuing interest in these materials, particularly in Japan. Because of fairly good hydrolytic
stability, polypivalyllactone is at the most advanced stages of commercial development:
O
O
