Chemistry Reference
In-Depth Information
widely used in formations of graft copolymers of poly(vinyl alcohol) and particularly of cellulose and
starch. The grafting reaction fails, however, when attempted on polysaccharides that lack free
hydroxyl groups on the second and third carbons. This led to speculation [
351
] that the bond between
these carbons cleaves. In the process, free radicals, presumably, form on the second carbons and
aldehyde structures on the third carbons of the glucose units. This point of view, however, is not
generally accepted. Instead, it was proposed that more likely positions for attacks by the ceric ions are
at the C
1
carbons of the glucoses at the end of the polysaccharide chains [
352
]. This is supported by
observation that oxidation of cellulose is an important prerequisite for the formation of graft
copolymers [
353
].
Graft copolymerizations by redox mechanism can also take place with the aid of other ions. This
includes grafting on cellulose backbones with ferrous ions and hydrogen peroxide [
354
]. Redox
grafting reactions can also take place on nylon and on polyester. For instance, graft copolymers of
methyl methacrylate on nylon 6 can be prepared with manganic, cobaltic, and ferric ions [
355
].
Another example is grafting poly(glycidyl methacrylate) on poly(ethylene terephthalate) fibers with
the aid ferrous ion-hydrogen peroxide. The reaction depends on the concentration of the monomer,
hydrogen peroxide, time, and temperature [
356
].
9.6.5 Photochemical Syntheses of Graft Copolymers
Photo labile groups on polymers can serve as sites for photoinitiated graft copolymerizations. For
instance, when polymers and copolymers of vinyl ketone decompose in ultraviolet light in the
presence of acrylonitrile, methyl methacrylate or vinyl acetate graft copolymers form [
357
]:
O
O
O
CH
3
+
O
O
The free radicals that are unattached to the backbone polymers, like the methyl radical shown
above, also initiate polymerizations and considerable amounts of homopolymers form as well.
In some instances, graft copolymers form as a result of chain transferring that takes place after
photodecomposition of the photo labile materials. An example is formation of graft copolymers of
polyacrylamide on natural rubber, poly(vinyl pyrrolidone), or dextrin with the aid of benzophenone
and ultraviolet light [
358
]. The free radicals from photodecomposition of benzophenone react with
the polymers by chain transferring. The growth of acrylamide is subsequently initiated from the
polymer backbones. Photo tendering dyes can be used in this manner with cellulose [
359
]. Thus,
anthraquinone dyes can be adsorbed to cellulose. Upon irradiation, proton abstractions take place,
creating initiating radicals on the backbone polysaccharide:
