Polylactones, 43. Polymerization of L-lactide catalyzed by zinc amino acid salts

The usefulness of the zinc salts of five different amino acids or amino acid derivatives as polymerization catalyst of L-lactide was studied and compared. The five amino acid (derivatives) were: N-acetyl-glycine, N-tosylglycine, L-proline, L-pyroglutamic acid and N-acetyl-4-aminobenzoic acid. All polymerizations were conducted in bulk. Due to the low reactivity of all Zn catalysts a reaction temperature of 150°C was needed to obtain yields around 90%. While the basic Zn prolinate caused partial racemization, the neutral Zn salts yielded poly(L-lactides) with 97–100% optical purity. Regardless of catalyst and reaction conditions only moderate molecular weights were obtained.