Method for producing an aliphatic polyester having increased molecular weight

A method for producing an aliphatic polyester having an increased molecular weight comprising the steps of:

• (i) introducing into a mixing tank equipped with an inlet for injecting a diisocyanate:
• (1) an aliphatic polyester prepolymer having a number average molecular weight of 5000 or higher and having terminal hydroxyl groups, wherein the aliphatic polyester prepolymer is melted at a temperature not lower than a melting point of the prepolymer and is in a liquid state, and
• (2) at least one acid component comprising a succinic acid compound, wherein the aliphatic polyester prepolymer is introduced above the position of the inlet for injecting the diisocyanate;
• (ii) injecting the diisocyanate into the aliphatic polyester prepolymer, wherein the diisocyanate comprises isocyanate groups in an amount equivalent to between one tenth and two times the amount of the hydroxyl groups in the aliphatic polyester prepolymer and wherein the diisocyanate is injected into the aliphatic polyester prepolymer below a vapor phase portion of the mixing tank;
• and
• (iii) reacting the aliphatic polyester prepolymer with the diisocyanate,
• to obtain the aliphatic polyester having the increased molecular weight.