Chemistry Reference
In-Depth Information
units in the final polymers are structurally different from the original monomers. Such rearrangements
are not limited to cationic polymerizations. In this section, however, are only discussed the
isomerizations in cationic polymerizations.
The isomerization polymerizations were classified by Kennedy according to the type of rearrange-
ment that accompanies the propagation and by the particular processes [
116
]:
1. Propagation reactions accompanied by bond or electron rearrangement:
A. Intra-intermolecular polymerization
B. Transanular polymerization
C. Polymerization by strain relief
2. Propagation reactions accompanied by migration of one or more atoms.
Examples of
intra intermolecular polymerizations
are cyclic polymerizations of nonconjugated
dienes. This can resemble cyclopolymerization by free-radical mechanism:
R
R
R
+
In this group of monomers, aliphatic dienes of various types can also be found . The propagation
proceeds through an internal attack by the electrophile on another part of the molecule yielding a new
carbon cation. The following is an example of such a reaction [
58
,
59
]:
R
R
+
R
Polynortricyclene [
116
-
118
] forms by cationic polymerization of norbornadiene by a
transanular
polymerization
.
The reaction is best carried out below
100
C to prevent crosslinking. Two propagation paths are
possible
R
+
R
n
or
R
R
+
n
NMR and IR spectra demonstrated that both propagations shown above take place during the
polymerization and the resultant polymer is in effect a copolymer of both structures [
119
]. Similarly,
cationic polymerization of 2-methylene, 5-norbornene involves a transanular addition of the initially
formed carbon cation to the strained endocyclic double bond. The product
is a polymeric
nortricyclane [
119
].
Polymerization by strain relief
occurs by opening of strained rings or by rearrangements of
internal double bonds during propagation. An example is the polymerization of
-pinene. The
propagation is accompanied by rearrangement of the initially formed carbon cation to a tertiary
cation that relieves the ring strain [
119
]:
a
+
H
