extracted three times with a total of 80 ml of methylene chloride the organic phase was dried over sodium sulfate filtered to remove salt condensed on a rotary evaporator and added to 100 ml of ethyl ether the precipitate was collected by filtration and dried in vacuo yield 2 76 g 92 1 h nmr dmso d 6 u03b43 5 br m peg 2 54 d pegcooch ch 3 c h 2 cooh 5 1 h pegcooc h ch 3 ch 2 cooh 1 2 d peg cooch c h 3 ch 2 cooh 2 54 t pegoch 2 c h 2 coo pa 4 055 s pegoc h 2 coo cm mpeg ester butyric acid nhs ester cm u2014 hba u2014 nhs or pa u2014 hba u2014 nhs mpeg ester butyric acid 3000 1 g approx 0 33 mmole cm u2014 hba u2014 cooh or pa u2014 hba u2014 cooh and 42 mg n hydroxysuccinimide nhs 0 3 mmole was dissolved in 30 ml of dry methylene chloride to this solution was added dicyclohexylcarbodiimide dcc 80 mg 0 38 mmole in 5 ml of dry methylene chloride the solution was stirred under nitrogen overnight and the solvent removed by rotary evaporation the residual syrup was redissolved in 10 ml of dry toluene and the insoluble solid was removed by filtration the solution was then precipitated into 100 ml of dry ethyl ether the precipitate was collected by filtration and dried in vacuo yield 0 94 g 94 1 h nmr dmso d 6 u03b43 5 br.