Graft Polymerization of Vinyl Acetate onto Starch. Saponification to Starch-g-Poly(vinyl Alcohol)

Graft polymerizations of vinyl acetate onto granular corn starch were initiated by cobalt-60 irradiation of starch-monomer-water mixtures. and ungrafted poly(vinyl acetate) was separated from the graft copolymer by benzene extraction. Conversions of monomer to polymer were quantitative at a radiation dose of 1.0 IvIrad. However, over half of the polymer was present as ungrafted poly(vinyl acetate) (grafting efficiency less than 50%), and the graft copolymer contained only :34'1,) grafted synthetic polymer (:34% add-on). Lower irradiation doses produced lower conversions of monomer to polymer and gave graft copolymers with lower % add-on. Addition of minor amounts of acrylamide, methyl acrylate, and methacrylic acid as comonomers produced only small increases in % add-on and grafting efficiency. However, grafting efficiency was increased to 70% when a monomer mixture containing about 10% methyl methacrylate was used. Grafting efficiency could be increased to over 90% if the graft polymerization of vinyl acetate-methyl methacrylate was carried out near O°C, although conversion of monomers to polymer was low and grafted polymer contained 40-50',\0 poly(methyl methacrylate). Selected graft copolymers were treated with methanolic sodium hydroxide to convert starch-g-poly(vinyl acetate) to starch-g-poly(vinyl alcohol). The molecular weight of the poly(vinyl alcohol) moiety \vas about 30,000. The solubility of starch-g-poly(vinyl alcohol) in hot water was less than .50%; however, solubility could be increased by substituting either acid-modified or hypochlorite-oxidized starch for unmodified starch in the graft polymerization reaction. Vinyl acetate was also graft polymerized onto acid-modified starch which had been dispersed and partially solubilized by heating in water. A total irradiation dose of either 1.0 or 0.5 Mrad gave starch-g-poly(vinyl acetate) with about :35% add-on, and a grafting efficiency of about 40');, was obtained. A film cast from a starch-g-poly(vinyl alcohol) copolymer in which homopolymer was not removed exhibited a higher ultimate tensile strength than a comparable physical mixture of starch and poly(vinyl alcohol).