Starch-cerium(IV) complexes in aqueous media: Formation, isolation, and stability

Abstract

Ever since Mino and Kaizerman (1) first reported that cerium(IV) salts initiate graft copolymerization of vinyl monomers on poly(vinyl alcohol), considerable attention has been given to the use of similar systems to graft copolymerize either cellulose or starch. Through kinetic studies various workers (2-5) have shown that cerium(IV) salts bond to alcohols and glycols to form complexes which disproportionate by single electron transfers and subsequently form free radical sites on the reducing agents. More recently, Pottenger and Johnson (6) showed by kinetic studies that cerium(IV) salts in IN acid react with D-glucose and methyII3-D-glucopyranoside, which served as model compounds of cellulose, at 1:5-1:40 cerium(IV) to substrate ratios to form chelate complexes. Graft reactions involving starch are usually conducted at 1:50-1 :200 cerium(IV) to anhydroglucose unit (AGU) ratios in reaction media whose acidities are on the order of O.OIN. Since rate of graft initiation on starch presumably depends on the rate of cerium(IV)-starch complex disproportionation, further knowledge of cerium(IV) complex properties is desirable. Previously, we (7) showed that by rheology we could detect the cationic charge of starch-cerium(IV) complexes made by adding Ce(NH4 h(N03k HN03 reagent to gelatinized dispersions of waxy corn starch (WCS) at 75: 1 AGU to cerium(IV) ratios. Rheology enabled us to follow the influence of HN03 concentration, age of the cerium(IV) reagent, and reaction time on the relative extent of complex formation. However, rheology did not give a quantitative measure of the rate and extent of complexing. Now we have developed a method for isolation and analysis of the complexes and have determined the effect of reaction conditions on extent of complexing. In our method the complexes are precipitated from aqueous solutions with absolute ethyl alcohol and the cerium(IV) content of the precipitate is determined.