Chemistry Reference
In-Depth Information
Some Lewis acids can form charge transfer complexes with monomers that yield cation radicals when
irradiated with ultraviolet light [
65
,
66
]. This was shown with such Lewis acids as VCl
4
,TiCl
4
,andTiBr
4
in polymerizations of isobutylene. The charge transfer complexes collapse after irradiation [
66
]:
VCl
4
þ½
M
monomer
н
VCl
4
M
!
VCl
4
M
½
VCl
4
M
h
v
½
VCl
4
M
monomer
2M
!
M--M
!
polymer
4.3.2.2 Radiation Initiated Polymerizations
Photochemical initiations
of cationic polymerizations [
67
-
71
] are used commercially. Irradiation by
ionizing radiation
of olefins forms several kinds of active, initiating species. Free-radical, cationic and
anionic polymerizations can potentially be initiated by these active species. Generally, the characteristics
of ionizing radiation induced polymerizations can be such that free-radical and ionic polymerizations
coexist [
72
]. Under dry conditions, polymerizations will proceed predominantly through cationic
intermediates with cyclopentadiene [
73
] and styrene [
74
,
75
]. Also, radiation-induced polymerizations
of some monomers in the liquid state are subject to retardation [
76
] by very low concentrations of ion
scavengers such as ammonia and water [
72
]. The efficiency of such retardation depends upon the proton
affinity of the scavengers [
72
]. In addition, radiation-induced cationic polymerizations of isobutylene
under anhydrous conditions occur through propagation by free ions [
77
].
4.3.2.3 Electro Initiation of Polymerization
These polymerizations, sometime also called
electrolytic polymerizations
, are carried out in an
electrical field. The field is applied to initiate chain growth. Passage of an electrical current through
solutions of monomers in suitable solvents can produce initiating species. Majority of these species,
however, are free-radical in nature [
78
-
80
].
Early studies on initiation of cationic polymerizations of styrene, isobutyl vinyl ether, and
-
vinylcarbazole were carried out by dissolving AgClO
4
in pure monomers or in nitrobenzene. Electric
current was then passed through them at room temperature [
81
,
82
]. Rubbery polymers formed as
well as some copolymers, suggesting a cationic path of the polymerization. A mechanism was
suggested based on anodic oxidation of the monomer:
N
e
ClO
4
!
ClO
4
þ
e
ClO
4
M
ClO
4
þ
M
!
þ
where, M is monomer. Later, however, it was concluded that the polymerizations are due to formation of
cation-radicals as the initiating species [
83
]. Further studies of the phenomenon led to a proposed
mechanism of initiation [
84
] of styrene polymerization in acetonitrile solution. Controlled potential
electrolysis at the anode, with the aid of a salt like tetrabutylammonium fluoroborate, is a result of direct
anodic oxidation. Electrons are transferred at the anode. This is accompanied by formations of radical-
cations:
