Graft copolymerization onto starch. I. Complexes of Mn3+ as initiators



Advantages in using the pyrophosphate complex of trivalent managanese over the sulfate complex as initiator for graft copolymerization onto starch are discussed. The first successful attempts to graft copolymerize acrylonitrile, methyl methacrylate, and acrylamide to starch and starch derivatives are described using managanic pyrophosphate as initiator. Selective solvent extraction of the reaction products and very low conversions of monomer to homopolymer in absence of starch substrates provide evidence for high grafting efficiencies obtained with acrylonitrile and methyl methacrylate. With acrylamide as monomer, however, low grafting efficiencies and considerable amounts of homopolymer are obtained under the experimental conditions investigated. Reaction mechanisms responsible for initiation of graft copolymerization are discussed. These are (a) glycol cleavage in the anhydroglucose units by Mn3+ ions leading to formation of a radical, and (b) enolization and further oxidation of oxidized starch by Mn3+ ions also leading to radical species. Mechanisms are also proposed for homopolymerization through Mn3+ oxidation of enols which probably are formed by “vinylogous” addition of water molecules to acrylamide and methyl methacrylate.