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 (Mw) 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 (Mv) ranging from 2 x 10(4) to 6.8 x 10(5). The monomer conversion and Mv showed a maximum at a catalyst concentration around 0.05 wt%. The monomer conversion and Mv increased almost linearly with polymerization time up to a monomer conversion of 80%, but both the conversion and Mv 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 Mv 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.