Chemistry Reference
In-Depth Information
SbF
6
counterions the molecular weights of the products can be calculated directly from the ratios of
the initiators to the monomers. The molecular weight distributions of the polymers from such
polymerization reactions with PF
6
and, SbF
6
however, start out as narrow, but then broaden.
This is believed to be due to transfer reactions with ether oxygen. It is supported by evidence that with
SbF
6
initiation, both termination and transfer reactions take place [
65
]. In addition, polymerizations
of tetrahydrofuran, like those of the epoxides, can be accompanied by formations of some macrocy-
clic oligomers. This is often the case [
66
,
67
] when strong acids are used as initiators. The proposed
mechanism involves backbiting and chain coupling and results in linear polymers with hydroxyl
groups and oxonium ions at opposite chain ends as well as some macrocycles.
The absence of linear oligomers is due to rapid reactions of the hydroxyl and oxonium ion end
groups. This mechanism is quite general [
67
,
68
] for ring-opening polymerizations of cyclic ethers
initiated with string protonic acids. Substituted tetrahydrofurans generally resist polymerizations.
5.6 Polymerization of Oxepanes
Oxepanes are polymerized by various cationic initiators like (C
2
H
5
)
3
C
+
BF
4
,(C
2
H
5
)
3
C
+
SbCI
6
,
BF
3
-epichlorohydrin, and SbCl
6
-epichlorohydrin [
42
].The reactions take place in chlorinated
solvents, like methylene chloride. The rates of these reactions, however, are quite slow [
42
]. In
addition, these polymerizations are reversible. The rates of propagation of the three cyclic ether,
oxetane, tetrahydrofuran, and oxepane at 0
C fall is in the following order [
42
]:
oxetane
>
tetrahydrofuran
>
oxepane
At the same temperature oxetane is about 35 times as reactive as tetrahydrofuran, which in turn is
about 270 times as reactive as oxepane. This cannot be explained on the basis of ring strain, nor can it
be explained from considerations of basic strength. Saegusa suggested [
42
] that the differences in the
propagation rates are governed by nucleophilic reactivities of the monomers. They are also affected
by the reactivities of the cyclic oxonium ions of the propagating species and also by the steric
hindrances in the transition states of propagation. Higher activation energy of oxepane is explained by
increased stability of the seven-membered oxonium ion. The oxepane molecule has puckered
structure, and the strain that comes from the trivalent oxygen is relieved by small deformations of
the angles of the other bonds [
42
].
5.7 Ring-Opening Polymerizations of Cyclic Acetals
The cationic polymerizations of cyclic acetals are different from the polymerizations of the rest of the
cyclic ethers. The differences arise from great nucleophilicity of the cyclic ethers as compared to that
of the acetals. In addition, cyclic ether monomers, like epirane, tetrahydrofuran, and oxepane, are
stronger bases than their corresponding polymers. The opposite is tree of the acetals. As a result, in
acetal polymerizations, active species like those of 1,3-dioxolane, may exist in equilibrium with the
macroalkoxy carbon cations and tertiary oxonium ions [
69
]. By comparison, the active propagating
species in the polymerizations of cyclic ethers, like tetrahydrofuran, are only tertiary oxonium ions.
The properties of the equilibrium of the active species in acetal polymerizations depend very much
upon polymerization conditions and upon the structures of the individual monomers.
