Synthesis of polylactides with different molecular weights

The synthesis of poly(lactic acid) through polycondensation of the lactic acid monomer gave weight average molecular weights (M-w) lower than 1.6 x 10(4), whereas ring-opening polymerization of lactides in bulk at 130 degrees C for 72 h using stannous octoate as catalyst in the concentration range from 0.003 to 0.8 wt% produced polylactides with viscosity average molecular weight (M-v) ranging from 2 x 10(4) to 6.8 x 10(5). The monomer conversion and M-v showed a maximum at a catalyst concentration around 0.05 wt%. The monomer conversion and M-v increased almost linearly with polymerization ti me up to a monomer conversion of 80%, but both the conversion and M-v decreased after passing through a maximum, when the polymerization reaction was allowed to proceed for longer periods of time. This time dependence was pronounced at higher polymerization temperatures. The decrease in M-v at prolonged polymerization and higher polymerization temperatures was attributed to thermal depolymerization of resultant polylactides, but no significant optical rotation of poly(L-lactide) was noticed. (C) 1997 Elsevier Science Limited. All rights reserved.