Chemistry Reference
In-Depth Information
temperature, however, are not always observed. In grafting polystyrene to poly(butyl acrylate) in
emulsion, for instance, there is no noticeable difference between 60 and 90
C by this technique [
289
].
The presence of sites with high transfer constants on the polymeric backbone enhances the
efficiency of grafting. Such sites can be introduced deliberately. These can, for instance, be mercaptan
groups [
290
] that can be formed by reacting H
2
S with a polymer containing epoxy groups:
O
2HS
OH
H
2
S
O
O
O
O
O
O
O
O
O
O
O
O
O
SH
HO
O
O
O
HS
HS
Free-radical polymerizations of acrylic and methacrylic esters in the presence of the above
backbones result in high yield of graft copolymers [
291
,
292
].
Another example is formation of mercaptan groups on cellulose in order to formgraft copolymers [
293
]:
OH
OH
HO
HO
S
O
+
O
O
O
HO
O
SH
Pendant nitro groups are also effective in chain transfer grafting reactions. Thus, graft copolymers
of polystyrene with cellulose acetate
p
p
-nitrobenzoate)
[
295
] form readily. Nitro groups appear to be more effective in formations of graft copolymers by
radical mechanism than are double bonds located as pendant groups [
294
].
-nitrobenzoate [
294
] and with poly(vinyl
9.6.2 Free-Radical Grafting Reactions to Polymers with Double Bonds
Carbon to carbon double bonds, either in the backbone or in the pendant groups, are potential sites for
free-radical attacks. In addition to the double bonds, the hydrogen atoms on the neighboring carbons
are allylic and potential sites for chain transferring. Because rubbers, natural and synthetic, possess
such unsaturations, they are used extensively as backbones for various grafting reactions. Whether the
reactivity of the initiating radical is important in determining grafting efficiency is not completely
established. Graft copolymers of poly(methyl methacrylate) on gutta-percha, however, form in good
yields when the initiator is benzoyl peroxide [
296
,
297
]. Yet, when azobisisobutyronitrile is used,
only a mixture of homopolymers forms. Work with
14
C-labeled initiators shows that the primary
radicals react both by addition to the double bonds and by transfer to the methylene group [
298
].
