Synthesis of poly(L-lactic acid) with improved thermal stability by sulfonic acid-catalyzed melt/solid polycondensation

Melt/solid polycondensation (MP/SSP) is deemed as an alternative synthetic route besides ring-opening polymerization (ROP) in synthesizing poly(L-lactic acid) (PLEA). However, it is found that PLLA synthesized by MP/SSP has much poorer thermal stability than that by ROP due to more residual Sn(II) metallic catalyst in the former, but sulfonic acids does not show any detrimental effect on the thermal stability of PLEA. To synthesizing PLEA with good thermal stability by MP/SSP, a variety of commercially available sulfonic acids were screened as catalysts in MP/SSP of PLEA. Among these nonmetallic catalysts, it was found that 1,3-propanedisulfonic acid (PSA) and 1,5-naphthalene disulfonic acid (NSA) exhibited satisfactory catalytic reactivity and PLLAs with excellent thermal stability, high molecular weight, little coloration and good optical purity were successfully synthesized by MP/SSP. The decomposition temperature was increased by 80-100 degrees C in comparison to SnCl2-catalyzed PLEA, and the thermal stability is comparable to commercial PLEA produced by ROP. (C) 2013 Elsevier Ltd. All rights reserved.