Direct condensation of lactic acid in the presence or absence of supported zirconium sulfate

Abstract

Direct condensation of L-lactic acid was studied, aiming at highly crystalline and enantiopure high molecular weight (MW) products. Catalyst-free polylactic acid (PLA) with weight average MW of 80,000 g mol 21 , based on PLA standards, and a crystallinity of 75% is synthesized at 1408C in a fixed-bed reactor under vacuum or nitrogen purging. Supported zirconium sulfate is found to be an efficient and enantioselective catalyst for the melt and solid-state post-condensation of short-chain prepolymers. Small-molecule monohydroxy alcohols, aldehydes, di-and monocarboxylic acids (e.g., acrylic acid [1,700 ppm], propionic acid [1,800 ppm]) are detected in the lactic acid and prepolymers by high performance liquid chromatography, electron spray ionization, and electron impact mass spectroscopy. Thermal degradation of polymer chains, the crystallinity of the prepolymer, and the presence of monofunctional impurities are crucial parameters for the limitation of the MW of the resulting product.