Chemistry Reference
In-Depth Information
The above mechanism [
29
] is based on an observation that strong Lewis acids can form stable
molecularly bound complexes with hindered alkenes, as for instance with adamantyladamantane:
S
b
F
6
SbF
6
The carbon cation, which forms, from the above shown complex, actually cannot initiate a cationic
polymerization of excess alkene molecules that are present in the reaction mixture, due to steric
hindrance [
29
,
31
]. The less hindered alkenes, however, like isobutylene polymerize rapidly under the
same conditions with antimony pentafluoride. It was, therefore, suggested [
29
,
31
] that initiation
involves an opening of a previously formed
p
complex of SbF
5
with the double bond, through halide
participation. The
-haloalkylantimony tetrafluoride ionizes with excess Lewis acid to form a
related carbon cation and is capable of initiating cationic chain growth:
a
,
b
n
SbF
5
SbF
5
+
Polymer
SbF
4
F
SbF
4
SbF
6
F
SbF
4
Alternately, the intermediate could be shown as splitting of HF that can then react with another molecule
of Lewis acid, because the metalated alkene is a tertiary halide and can eliminate hydrogen halide:
SbF
5
SbF
4
+
SbF
4
SbF
4
F
HF
SbF
6
The initiationmechanismof olefins byAlB
3
was explained similarly [
29
]. The intermediate compound
of aluminum bromide and the olefin is expected to lose HBr. Although initially the reaction mixture is free
from protonic acid, it could form under conditions where initiation takes place by a conjugate acid
catalyzed system. In addition, all cationic polymerizations of olefins should be considered as typical
examples of general carbocationic reactivity in electrophilic reactions [
29
].Theseparatemechanismsare
to be looked upon as various examples that differ only in the nature of the initial electrophile. They always
lead to the related trivalent alkyl cation when polymeric chain growth is initiated [
29
]:
R
n
R
R
R'
+
E
E
Polymer
R'
R'
(Electrophile)
Much of the evidence gathered to date, however, supports the concept of initiation due to auto
ionization of Lewis acids. This was originally proposed in 1948 [
30
]. Lewis acids can aggregate,
generally into dimers, and then auto ionize. The electrophilic portion adds to the olefin:
2 AlBr
3
AlBr
2
AlBr
4
R
R
AlBr
4
+
AlBr
2
AlBr
4
AlBr
2
R'
R'
