[C:1]([OH:6])(=[O:5])[CH:2]([CH3:4])[OH:3].[C:7]([OH:11])(=[O:10])[CH2:8][OH:9].C(O)C.ClC(OCC)=O>O.ClCCl.C(N(CC)CC)C>[CH3:7][CH2:8][O:5][C:1]([CH:2]([OH:3])[CH3:4])=[O:6].[C:7]([OH:11])(=[O:10])[CH2:8][OH:9]9 g of a lactic acid/glycolic acid copolymer having a weight-average molecular weight of about 7,500 (lactic acid/glycolic acid=75/25 (mol %)) (Wako Pure Chemical Industries, Ltd.) was dissolved in a mixed solvent consisting of 20 ml of dichloromethane and 20 ml of ethanol. While thus obtained solution was cooled and stirred, 0.45 ml of triethyl amine, 0.29 ml of ethyl chloroformate and 0.36 g of N,N-dimethylaminopyridine were added. After the resulting mixture was stirred for another 2 hours, 50 ml of dichloromethane and 50 ml of water were added and then dichloromethane phase was separated. The dichloromethane phase was washed 2 times with 50 ml of 10% ethanol-water, and then dried with magnesium sulfate. After magnesium sulfate was separated by filtration, the mixture was concentrated to dryness under reduced pressure to yield 7.7 g of the ethyl ester of lactic acid/glycolic acid copolymer.
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