Chemistry Reference
In-Depth Information
Oxetane compounds also polymerize with the aid of aluminum trialkyl-water acetylacetone
catalysts [
45
,
46
]. The reactions can take place at 65
C in heptane and yield very high molecular
weight polymers. These polymerizations, however, are ten times slower that similar ones carrier out
with propylene oxide, using the same catalyst. The reaction conditions and the high molecular weights
of the products led to assumption that coordinated mechanisms of polymerizations take place [
46
].
Also, it was reported [
218
] that quaternary onium salts coupled wit bulky organoaluminum
diphenolates initiate controlled (living) coordinate anion polymerizations of oxetane to give narrow
molecular weight distribution polyethers The catalyst system consists of onium salts, such as
quaternal
ammonium phosphonium halides
that
are
combined with sterically hindered
methylaluminum diphenolates [
47
]
R
2
R
2
R
1
Al
R
3
R
4
R
4
R
3
Crivello reported frontal photopolymerization of oxetane [
48
]. He proposed a mechanism for the
polymerization that is shown in the following scheme:
MtX
n
+
Ar
ArI
hν
Ar
2
I
MtX
n
ArI
+
Ar
HMtX
n
MtX
n
+
O
HMtX
n
H
O
MtX
n
HO
MtX
n
O
H
O
MtX
n
+
O
H
O
MtX
n
+
O
MtX
n
O
HO
O
Crivello reported that only diaryliodonium salts were used as cationic photoinitiators in this study.
Photo activation was carried out by UV irradiation prior to thermal initiation. Initiation was carried
out using an electrically heated wire immersed in the monomer. The velocity of the resulting
propagating front is quite high with the temperature of the front reaching 110
C.
5.5 Polymerization of Tetrahydrofurans
Lewis acids, carbon cations, salts of oxonium ions, and strong protonic acids initiate polymerizations
of tetrahydrofuran. The reactions can be conducted in solution or without a solvent It was originally
polymerized [
49
,
50
] with a trialkyloxomum salt, R
3
OBF
4
.
