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

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 that the pyrolysis of PLLA-ap proceeds through a zero-order weight loss process with the apparent E-a = 80-90 U 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 U 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. (C) 2004 Elsevier Ltd. All rights reserved.