Chemistry Reference
In-Depth Information
In formations of ternary complexes, the acceptor vinyl compound must have a double bond
conjugated to a cyano or to a carbonyl group. Such acceptors are acrylonitrile, methacrylonitrile,
acrylic and methacrylic esters and acids, methyl vinyl ketone, acrylamide, etc. Donor monomers are
styrene,
-methyl styrene, butadiene, 2-3-dimethyl butadiene, isoprene, chloroprene, etc.
One proposed mechanism [
171
,
204
] is that such charge-transfer polymerizations are in effect
homopolymerizations of the charge-transfer complexes [D
L
A_
a
MeX
n
]. In other words, the metal
halide is complexed with the electron acceptor monomer and acts as an acceptor component.
The above opinion, however, is not universal. Others hold that the increased susceptibility to
ultraviolet radiation or to initiating radicals [
205
] is due to increased reactivity of the propagating
radicals of complexed monomers toward incoming uncomplexed ones.
Arguments against the ternary complex mechanism are as follows: (1) the physical evidence that
proves the existence of the ternary molecular complexes is weak; (2) the ternary molecular complexes
can have no bearing on the copolymerizations because the equilibrium concentration of the
complexed monomers is low, compared to the uncomplexed ones [
206
].
A third opinion is that a complex of an acceptor monomer with a Lewis acid copolymerizes
alternately with the donor monomer and with an uncomplexed acceptor monomer [
207
,
208
]. This
presumably takes place according to the conventional chain-growth polymerization scheme of radical
copolymerization. The alternate placement of monomers is due to highly enhanced values of cross-
propagation constants. It results from complexing acceptor monomers with Lewis acids. Such a
mechanism fails to explain satisfactorily the completely alternating incorporation of monomers and
the inefficiency of chain-transfer reagents. It also fails to explain the spontaneous initiation of
alternating copolymerization.
Kabanov suggested [
209
] that during the primary free-radical formation of the Lewis
acid-monomer complex, both the uncomplexed and the complexed monomers may participate in
chain propagation. This would result in appearance of complexed propagating radicals besides the
usual ones. In the complexed ones, the last unit carrying the valence is a ligand of coordination
complex:
...
M
......
MeX
n
It excludes, however, all electron transfer reactions that may take place due to ultraviolet light
irradiation.
3.13 Steric Control in Free-Radical Polymerization
In free-radical polymerization reactions, the propagating radical chain has a great amount of freedom.
Atactic polymers, therefore, are usually formed. Some control that the reaction conditions exercise
over the propagating species increases at lower temperatures due to lower mobilities. This leads to
increased syndiotactic placement, as was discussed in the section on propagation. Special techniques,
however, such as the use of canal complexes can be employed to form stereoregular polymers by free-
radical mechanism. Urea and thiourea were used originally for such purposes [
210
,
211
]. Monomers
such as butadiene or others form complexes within the voids, or canals of the crystal lattices of these
compounds. Brief exposure to high energy radiation initiates chain growth. In the canals, the
monomer molecules are held in fixed positions, so chain growth is restricted in one direction only.
Steric control is exercised because in these fixed positions the monomer molecules tend to align
uniformly. It was suggested that in the canal complexes the monomers are not just lined up end to end,
but packed in an overlapping arrangement. For molecules such as isobutylene or vinylidene chloride,
