Chemistry Reference
In-Depth Information
Alkyl substituents on the ethylene oxide ring enhance the process of cationic polymerization. For
instance, ethylene oxide yields only low molecular weight oils with strong Lewis acids. Tetramethy-
lethylene oxide, on the other hand, is converted readily by BF
3
into high molecular weight polymers
that are insoluble in common solvents [
10
].
When proton donors initiate the polymerizations of epoxides, only low molecular weight
products result. The reaction is quite straightforward. Oxonium ions form during the initiation
step as follow:
O
O
H
A
HA
+
Propagation is the result of a ring-opening attack by a monomer:
A
O
HO
H
O
A
+
O
Chain-growth can terminate by a reaction with water:
A
H
O
H
O
n
+
H
2
O
HA
O
OH
+
O
n
In cationic polymerizations of propylene oxide the ring-opening step involves a direct attack on
the oxonium ion at the carbon that bears a more labile bond to the oxygen:
A
O
H
O
A
O
+
H
O
Cationic ring-opening polymerization of oxiranes can also be carried out photochemically (photo-
chemical reactions are discussed in Chap.
10
). Yagci and coworkers reported polymerizations of
cyclohexene oxide with the aid of highly conjugated thiophene derivatives [
12
]. The reaction is
illustrated as follows:
O
O
n
visible light
Ph
2
I
n
PF
6
S
N
N
O
O
O
O
