Chemistry Reference
In-Depth Information
+
+
+
+
+
+
+
In the above initiation the monomer itself is the initiator and the anode can be regarded as the
“co-initiator.”
If a supporting electrolyte is first oxidized to a radical, indirect cationic polymerization can result.
The radical in a subsequent step oxidizes the monomer to a first initiating entity, the cation-radical
[
85
]. Such an indirect initiation was also suggested for electropolymerization of isobutyl vinyl ether
in the presence of BF
4
supporting electrolyte [
85
]:
e
BF
4
BF
4
anode
OR
OR
BF
4
+
BF
4
+
4.3.3 Propagation in Cationic Polymerization
In the propagation step the polarity of the medium affects strongly the reaction because the intimacy
of the ion pair depends upon solvent polarity. The bond between the two ions can vary from a high
degree of covalence to that of a pair of free, solvated ions [
86
]:
C
A
C
A
C / / A
+
A
C
covalent
intimate ion pair
solvent separated ion pair
free, solvated ion pair
The chemical structures of the monomers also determine their reactivity toward cationic
polymerizations. Electron donating groups enhance the electron densities of the double bonds.
Because, the monomers must act as nucleophiles or as electron donors in the course of propagation,
increased electron densities at the double bonds increase the reaction rates. It follows, therefore, that
electron withdrawing substituents on olefins will hinder cationic polymerizations. They will, instead,
enhance the ability for anionic polymerization. The polarity of the substituents, however, is not the
only determining factor in monomer reactivity. Steric effects can also exert considerable controls
over the rates of propagation and the modes of addition to the active centers. Investigation of
polymerizations of alkyl vinyl ethers, for instance, with BF
3
O(C
2
H
5
)
2
in toluene or in methylene
chloride at
79
C, showed that the rate of monomer consumption falls in the following order [
87
]:
Alkyl group
¼ t
- butyl
>i
- propyl
>
ethyl
>n
- butyl
>
methyl
