Preparation and properties of carboxyl‐functional mixed esters of hydrolyzed starch

Abstract

AbstractMixed esters of hydrolyzed starch represent a new class of chemically modified natural polymers demonstrating a broad range of properties. Members of this class of polymers have both neutral aliphatic ester side chains and carboxyl‐functional half‐ester side chains. Use of hydrolyzed starch as the backbone polymer results in products that are considerably lower in molecular weight than whole starch derivatives, but which are still polymeric in character. Synthesis proceeds smoothly in pyridine using anhydrides as acylating agents and the pure solid mixed ester products are isolated by precipitation in water. Measurement of degree of substitution (D.S.) by NMR or hydrolysis characterizes the chemical composition of the polymers. The actual D.S. achieved in the synthesis depends upon the competition between starch and residual water for anhydrides, which can be quantitatively evaluated by monitoring the acid content of the reaction mixture. The Tg and Ts of starch mixed esters vary with both D. S. and length of the aliphatic ester side chain. As the composition changes from acetate—phthalate to butyrate—phthalate a Tg range of greater than 100°C is observed. Hydrolyzed starch mixed esters are hydrophobic and organic‐soluble, but may be readily solubilized in aqueous base through the half‐ester groups. Solutions show surface activity which varies according to the type and extent of substitution.