[C:1]([OH:6])(=[O:5])[CH:2]([CH3:4])[OH:3].[C:7]([OH:11])(=[O:10])[CH2:8][OH:9].C(I)C.C(=O)([O-])[O-].[K+].[K+]>CC(C)=O>[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]15 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.) and a 7.8 g of ethyl iodide were dissolved in 150 ml of acetone. To thus obtained solution was added 1.38 g of potassium carbonate and then the resulting mixture was refluxed for 6 hours. After the resultant solution was cooled, inorganic substances were removed by filtration and then the filtrate was concentrated under reduced pressure. The concentrate was dissolved in 100 ml of dichloromethane, washed 3 times with 100 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 12.5 g of the ethyl ester of lactic acid/glycolic acid copolymer.
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