Synthesis of polylactides with different molecular weights

Abstract

The synthesis of poly(lactic acid) through polycondensation of the lactic acid monomer gave weight average molecular weights ( M w ) lower than 1.6 × 10 4, whereas ring-opening polymerization of lactides in bulk at 130 °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 × 10 4 to 6.8 × 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 time 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 W 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.