Chemistry Reference
In-Depth Information
The propagation can also involve an intramolecular halide migration. The polymerization of
3-chloro-3-methylbutene-1 proceeds at low temperatures by a chloride shift (about 50%) [
124
]:
n
Cl
Cl
Cl
Recently, Faust and coworkers [
125
] reported a study of the mechanism of isomerization and
polymerization of isobutylene in cationic polymerization. They concluded that
-proton elimination
from the tertiary polyisobutylene cation results in exo- and endo-olefins. Tetra-substituted olefins
form after initial cationic rearrangements leading to tertiary cations at the second and third positions,
followed by deprotonation. All three olefins possess regular carbon numbers. The formation of tri-
substituted olefins follows a different mechanism. First, a hydride shift between distant carbons
(backbiting) leads to a secondary cation, which undergoes a methyde shift to minimize the energy.
The resulting hindered tertiary cation undergoes chain scission, yielding the
b
-substituted
polyisobutylene olefin with irregular carbon number. This also results in an oligomeric byproduct,
which is known as “light polymer.”
n
4.3.4 Termination Reactions in Cationic Polymerizations
The termination reaction can occur by a transfer to a monomer at proper reaction conditions:
A
+
+
A
The termination reactions in cationic polymerizations can often lead to low molecular weight
products. This can be a result from various effects. Also, the counterions may be involved in the
terminations [
126
]. Thus, with some Lewis acids, the polymer cation may react with the counterion
by abstracting a halogen. An example of that is polymerization of isobutylene with BCl
3
. In this
reaction, termination by chain transferring is absent [
126
]. Instead, the following mechanism takes
place [
127
]:
BCl
3
OH
Cl
BCl
2
OH
+
Alkyl aluminum initiated reactions can terminate by a transfer of an alkyl group [
127
]:
(CH
3
)
3
AlX
+
(CH
3
)
2
AlX
where X is a halogen. When a hydrogen atom is located at
to the aluminum in an alkyl aluminum
moiety then termination by hydrogen transfer appears to occur preferentially [
125
]:
b
