Chemistry Reference
In-Depth Information
cationic mechanism. They tend to resemble the schemes suggested for the polymerization of cyclic
ethers [
86
,
87
]. The initiation step involves an equilibrium that is followed by a ring-opening reaction:
O
O
O
R
R
+
(CH
2
)x
O
(CH
2
)x
O
RO
x
The propagation consists of many repetitions of the above step:
O
O
O
RO
RO
+
(CH
2
)
5
O
O
5
5
5
O
O
O
(CH
2
)
5
O
n
O
O
n+1
5
5
O
The polymerization of propiolactone in methylene chloride with an antimony pentachloride-
dietherate catalyst was investigated [
88
]. The results show that the concentration of the active centers
is dependent upon catalyst concentration and upon the initial concentration of the monomer. They also
support the concept that opening of the lactone rings includes initial formation of an oxonium ions [
88
]:
O
O
slow
O
+
O
O
O
O
O
2
2
O
O
fast
O
O
2
2
O
Because the carbonyl oxygen is the most basic of the oxygens in the lactone molecule, a reverse reaction is
O
O
C
+
O
C
O
C
O
C
O
Conductivitymeasurements during polymerizations of
b
-propiolactone with antimony pentafluoride-
etherate or
p
-toluenesulfonic acid show [
89
] that ion triplets form during the reaction. These are:
O
O
C
A
C
The triplets appear to be active centers throughout the course of the polymerizations. In addition, most
of the growing chain ends exist as ion pairs, depending upon the concentration of the monomer [
89
].
Bourissou et al. reported recently controlled cationic polymerization of lactones using a combina-
tion of triflic acid with a protonic reagent as the initiators [
90
]. The reaction was carried out in
CH
2
Cl
2
. Results indicated that the process is controlled is a linear relationship between the molecular
