Patent ID: 7622546

Claim:
A process for producing an aliphatic polyester by subjecting a cyclic ester to bulk ring-opening polymerization, which comprises i) providing a cyclic ester purified to the extent that a water content is at most 50 ppm, an α-hydroxycarboxylic acid content is at most 100 ppm, and linear α-hydroxycarboxylic acid oligomers content is at most 1,000 ppm, ii) controlling an overall proton concentration in a ring-opening polymerization system by adding a molecular weight control agent consisting of water to the cyclic ester to control the overall proton concentration in the cyclic ester to be within a range of higher than 0.09 mol % but lower than 2.0 mol %, wherein the overall proton concentration in the cyclic ester is calculated out on the basis of the total amount of hydroxycarboxylic compounds consisting of the α-hydroxycarboxylic acid and the linear α-hydroxycarboxylic acid oligomers as impurities in the cyclic ester, water contained as impurities in the cyclic ester, and water added to the cyclic ester, and iii) subjecting the cyclic ester to ring opening polymerization in the closed state and in the presence of a ring-opening polymerization catalyst, thereby controlling melt viscosity of the resulting aliphatic polyester, wherein the resulting aliphatic polyester has a melt viscosity of 50 to 6000 Pa·s as measured at a temperature of 240° C. and a shear rate of 121 sec −1 .