PRODUCTION OF POLY(LACTIC ACID)

PURPOSE: To obtain a high-mol. poly(lactic acid) in a short time from inexpensive lactic acid by heating an acetylated lactic acid oligomer in a screw extruder under a reduced pressure to conduct deacetylation and polycondensation. CONSTITUTION: An acetylated lactic acid oligomer (preferably one obtained by reacting lactic acid with an acetylating agent) is deacetylated and condensation-polymerized with heating under a reduced pressure in a screw extruder to produce poly(lactic acid) having a weight-average mol.wt. of 10,000-50,000. Thus, oligomerization and the subsequent polycondensation can be carried out smoothly in a short time, because the hydroxyl groups of the lactic acid are converted to acetyl groups, which are more reactive, before the lactic acid is reacted. Further due to the high volatile-removing function of the extruder, the acetic acid, water, etc., generated during the polycondensation are easily and almost completely removed and, hence, a lower temp. and a shorter period than conventional ones are sufficient for the treatment. As a result, high-quality poly(lactic acid) can be obtained which does not suffer the discoloration, mol.wt. decrease, etc., caused by oxidation or pyrolysis.