CHAIN BRANCHING IN POLY-BETA-ALANINE

Proton magnetic resonance and infrared absorption studies have been carried out on poly-β-alanines prepared by t-butoxide-initiated polymerization of acrylamide in pyridine, nitrobenzene, dimethyl sulfoxide, and dioxane solutions in the presence of phenyl-β-naphthylamine free-radical inhibitor. The water-soluble fractions were found to be branched copolymers of β-alanine and β,βʹ-iminodipropionic acid with primary amide end groups. Carboxyl end groups were detected as well, possibly resulting from the hydrolysis of primary amide groups. These carboxyl groups were the probable cause of a polyelectrolyte expansion effect in viscosity studies of a salt-free aqueous solution of a highly branched sample (PBA-S1) at pH 6. Flow birefringence studies showed that PBA-S1 consisted of readily orientable, nonspherical molecules in aqueous solution, suggesting that relatively short branches occur along a rather extended “backbone.” The water solubility of the polymers increased with increasing branching and decreasing molecular weight. It is suggested that branch growth is initiated during polymerization by the formation of secondary amide anions on the polymer chain. © 1969, American Chemical Society. All rights reserved.