Chemistry Reference
In-Depth Information
The above shown cations form crystalline salts with anions. These anions are ClO
4
, SbCl
6
,
BF
4
,PF
6
, SbF
6
, FeCl
4
, and AsF
6
[
40
].
The initiation reactions by stable cations, designated as (X
L
), may proceed by several possible
mechanisms [
41
]:
(A) By direct additions to unsaturated systems:
R
R
+
X
X
(B) By hydride abstractions:
R
R
+
+
X
XH
(C) By formations of cation-radicals through electron transfer or through some other mechanisms:
R
R
+
X
+
X
In (A) the reaction will predominate on the left-hand side. Exceptions appear to be olefins with
strong electron releasing substituents that confer thermodynamic stability to the newly formed cation,
~CH
2
-CHR
L
[
40
]. This either results from a suitable charge delocalization over the
-electron system
or from a presence of a heteroatom. Accordingly, only those olefins that possess relatively strong
nucleophilic characteristics can be polymerized by stable carbon cations. Such olefins are alkyl vinyl
ethers,
p
N
p
a
-vinyl carbazole,
-methoxystyrene, indene, and vinylnaphthalenes. Styrene and
-methyl
styrene, however, will not polymerize, because they are less reactive.
The initiation mechanism, as suggested by Ledwith [
40
], follows the path of reaction (A) and is a
result of a direct addition of the cation to the olefin. This is based on observations of the reaction of the
xanthylium cation with 1,1-diphenylethylene. It is also based on the reaction of the tropilium cation
with
-vinyl carbazole. The last may, perhaps, be influenced by steric factors. The high initiating
efficiency of the tropilium ion, which has a stable six
N
p
-electrons system, may be a result of formation
of a charge transfer complex [
39
]:
R
R
+
C
7
H
7
C
7
H
7
C
7
H
7
R
Based on the above, polymerization of
N
-vinylcarbazole can be shown as follows [
40
]:
charge
transfer
complex
+
A
A
N
N
N
N
A
N
