Chemistry Reference
In-Depth Information
Termination through a rearrangement of the propagating ion-pair is also possible. Such
terminations are sometimes called
spontaneous terminations
or as chain transfers to counterions.
The ion couple is rearranged by an expulsion of hydrogen leaving the terminal unsaturated [
120
]:
BF
3
OH
+
H
BF
3
OH
Many substances which act as catalysts (in combination with Lewis acids), like water, are also
good chain-transferring agents. A solvent or an impurity can act in the same way.
4.3.5 Living Cationic Polymerizations
In some cationic polymerizations, when conditions are carefully controlled, quasi termination less or
termination less systems can be achieved [
130
-
136
]. The “living” polymers, or “quasiliving,” form as
chain transfer to monomer and all other forms of termination are greatly decreased or made reversible
throughout reaction. The important aspect of living cationic polymerization is that the propagating
centers are sufficiently low in reactivity. Transfer and termination reactions are suppressed. The
propagation, however, is maintained. The molecular weights must increase in proportion to the cumula-
tive amount of the monomer added. The lifetimes of the propagating species can be extended in such
polymerizations by carrying out continuous slow additions of the monomer. Because chain-transferring
reactions are not completely eliminated in all these systems, the term “quasi” is sometimes used.
Living cationic polymerizations have been carried out with a number of monomers, such as
isobutylene, styrene,
-vinylcarbazole, and others [
137
,
138
].
To achieve living conditions, it is necessary to match the propagating carbon cation with the
counterion, the solvent polarity, and the reaction temperature. Some examples are offered in Table
4.2
.
By stabilizing the inherently unstable carbocationic growing species and preventing chain transfer and
termination, living cationic polymerizations are achieved. This canbedonebytwomethods:(1)useof
suitable nucleophilic counterions, or (2) external additions of weak Lewis bases [
139
]. The ion pairs are very
tight in these reactions and may border on being covalent. The propagation can be illustrated as follows:
p
-methyl styrene,
p
-methoxystyrene,
N
R
R
δ
R
δ
I
δ
I
δ
R
R
ZnI
2
+
R
I
δ
R
R
R
I
ZnI
2
δ
δ
I
ZnI
2
δ
R
