Chemistry Reference
In-Depth Information
cationization process. This reaction, termed “priming” by Kennedy and Marechal [
9
] is a process of
ion formation in a non-nucleophilic media through: (1) dissociation of protonic acids to form protons
and counterions, (2) through reactions of Lewis acids with Bronsted acids, (3) through dissociation of
dimeric Lewis acids, (4) through complexation of Lewis acids with water or with alkyl halides or with
ethers, and so on. These reactions may take place through a series of complicated steps. The second
reaction, the cationization of the olefins may also include several intermediate steps that will
eventually lead to propagating species.
4.3.1.1
Initiation by Protonic Acids
The ability of a proton from a protonic acid to initiate a polymerization depends upon the nucleophi-
licity of the conjugate base, A
[
10
,
11
]. If it is low, protonation or cationization of the olefin is a step
of initiation [
12
]:
+
HA
H
A
R
R
HA
+
A
R'
R'
In protonic acids, where the complex anion assists ion generation, the acid may be considered as
providing its own counterion, A
. When such counterions are highly nucleophilic, then complexing
agents such as metal salts, or metal oxides have to be used to immobilize the anions. Such complexing
agents are in effect coinitiators.
Thus, the differences in activities of protonic acids are due to the quality of the corresponding
anion or to its tendency to form chemical bonds with the carbon cation. If the anion is unable to form
such bonds without extensive regrouping or decomposition, the addition of the proton is followed by
polymerization. Should the reactivity of the anion be suppressed by solvation, the tendency to
polymerize is enhanced. Consequently, the efficiencies of protonic acids depend very much upon
the polarities of the media and upon the reaction conditions [
12
,
13
]. Also, the stronger the protonic
acid the higher the reaction rate and the resultant degree of polymerization [
14
]. Generally, hydrogen
halide acids do not initiate polymerizations of alkyl-substituted olefins. They may, however, initiate
polymerizations of aryl-substituted olefins and vinyl ethers in polar solvents. The same is true of
sulfuric acid [
15
].
4.3.1.2 Lewis Acids in Cationic Initiations
Complexation of Lewis acids with water is another case of formation of electrophiles that can initiate
chain growth. MeX
n
represents a Lewis acid
MeX
n
+
H
2
O
H
MeX
n
OH
R
R
+
H
MeX
n
OH
MeX
n
OH
R'
R'
R
R
+
n
MeX
n
OH
Polymer
R'
R'
