Effects of chain end structures on pyrolysis of poly( l-lactic acid) containing tin atoms

Abstract

Thermal degradation of high molecular weight PLLA containing residual tin atoms was investigated as a means of controlling the reaction for feedstock recycling to l, l-lactide. To clarify the pyrolysis mechanism of the PLLA, three samples with different chain end structures were prepared, namely, as-polymerized PLLA-ap, precipitated-with-methanol PLLA-pr, and purified PLLA-H. From pyrolyzate and kinetic analyses, typical degradation mechanisms of Sn-containing PLLA were clarified. In other words, it was assumed that the pyrolysis of PLLA-ap proceeds through a zero-order weight loss process with the apparent E a =80–90 kJ mol −1 , and with the occurrence of backbiting and transesterification reactions caused by Sn-alkoxide chain ends. The pyrolysis of PLLA-pr was also assumed to proceed via a zero-order weight loss process with apparent E a =120–130 kJ mol −1 , with the proposed mechanism being Sn-catalyzed selective lactide elimination caused by Sn-carboxylate chain ends. Both pyrolysis of PLLA-ap and PLLA-pr produced l, l-lactide selectively. These degradation mechanisms and products are in contrast to those of PLLA-H, in which a large amount of diastereoisomers and cyclic oligomers were formed by random degradation. From this study, the complicated PLLA pyrolysis behavior as reported previously could be explained properly.