Thermal Degradation of Hydroxyl-Terminated Poly(L-Lactic Acid) Oligomer into L-Lactide
This study investigates the thermal degradation of poly(L-lactic acid) (PLLA) oligomers into L-lactide（LA）by modification of the oligomer terminal groups. Pure PLLA oligomer (OLLA) was synthesized by condensation polymerization of L-lactic acid. Poly-hydroxyl-terminated OLLA (OLLAPOH) and two-carboxylic acid-terminated OLLA (OLLACOOH) were prepared by adding a small amount of polyfunctional substance, such as 1,4-butanediol, pentaerythritol, or adipic acid, to L-lactic acid. The thermal degradation behaviors of these oligomers were compared at 210-220 °C under 0.3-0.5 kPa. The results show that OLLAPOH has a 10-20% higher yield than OLLA and OLLACOOH. The order of decreasing yield is as follows: OLLAPOH>OLLA >OLLACOOH. The hydroxyl ends of the chains induce the reaction. OLLAPOH simultaneously starts depolymerization from each hydroxyl end to speed up the reaction, to shorten the time the substance remains at high temperature, and to reduce the byproducts. This results in an increased L-lactide yield. A practical approach for L-lactide production by thermal degradation of OLLAPOH is adopted by using a degradation mechanism, which is an unzipping reaction beginning from the hydroxyl ends of the chains.
Zhengyi Jiang, Jingtao Han and Xianghua Liu
J. Shen et al., "Thermal Degradation of Hydroxyl-Terminated Poly(L-Lactic Acid) Oligomer into L-Lactide", Advanced Materials Research, Vols. 152-153, pp. 222-228, 2011