Chemistry Reference
In-Depth Information
monomers. A prerequisite for this synthesis is that the active sites must form on the polymeric
backbones during the course of the reactions. Ideally, this occurs if the steps of initiations consist only
of attacks by the initiating radicals on the backbones:
Initiation:
I
I
2 I
+
I
A
A
A
I
H
+
A
A
A
Propagations then precede from the backbone sites:
(M)
n-1
M
Propagation:
nM
A
A
A
A
+
A
A
Terminations can take place in many ways. Of course, termination by combination will lead to
cross-linked insoluble polymers and that is undesirable. An ideal termination takes place by chain
transferring to another site on a polymer backbone to initiate another chain growth.
The above, however, is an ideal picture. In reality, the efficiency of grafting by this technique
depends upon the following:
1. Competitions between the different materials present in the reaction mixture, such as monomer,
solvent, and polymer backbone for the radical species. This includes competition between chains
growth and chain transferring to any other species present.
2. Competition between the terminating processes, such as disproportionation and chain transferring.
The conditions can vary considerably and it is possible to carry out the reactions in bulk, solution,
or in emulsion. When the reaction take place in emulsion, the success depends greatly on experimen-
tal techniques. The rate of diffusion is a factor and anything that affects this rate must be considered.
Because grafting efficiency depends upon chain transferring to the backbone, knowledge of the chain-
transferring constants can help predict the outcome of the reactions. Sometimes, the information on
the chain-transferring constants is not available from the literature. It may, however, be possible to
obtain the information from reactions of low molecular weight compounds with similar structures
[
284
-
286
]. One assumes equal reactivity toward attacking radicals. The validity of such an assump-
tion was demonstrated on oligomers [
281
-
283
].
The reactivity of the initiating radicals toward the backbones can vary and this can also vary the
efficiency of grafting. Benzoyl peroxide-initiated polymerizations of methyl methacrylate monomer,
for instance, in the presence of polystyrene [
284
] yield appreciable quantities of graft copolymers.
Very little graft copolymers, however, form when di-
t
-butyl peroxide initiates the same reactions.
Azobisisobutyronitrile also fails to yield appreciable quantities of graft copolymers. This is due to
very inefficient chain transferring to the polymer backbones by
t
-butoxy and isobutyronitrile radicals.
A better approach is work by Chung and coworkers [
288
] who grafted maleic anhydride to
polypropylene with the use of Borane/O
2
initiator. This initiator is claimed to form in situ a
monooxidized adduct (R-O* *O-BR
2
). These adducts then carry out hydrogen abstractions from
the polypropylene chains at ambient temperature. This results in formation of stable tertiary polymer
radicals that react with maleic anhydride to form graft copolymers:
Not all chain transferring to the backbones results in formations of graft copolymers. An example
is polymerization of vinyl acetate in the presence of poly(methyl methacrylate). No graft copolymers
form and this is independent of the reactivity of the initiators [
285
]. In fact, grafts of poly(vinyl
acetate) to poly(methyl methacrylate) and to polystyrene cannot be prepared by this technique [
286
].
Grafting efficiency may increase with temperature [
287
]. This could be due to higher activation
energy of the transfer reaction than that of the propagation reaction [
288
]. Meaningful effects of
