From my point of view, the best way would be to use a PEG-NHS ester, which can be obtained commercially from different companies.

http://www.rapp-polymere.com/#index.php?id=1202¤cy=asd

Hi Kyungsun,

You can prepare peg-carboxylate using TEMPO, NaBr and Na-hypochlorite (hypo, chlorox).

Dissolve peg in hypo and and add NaBr and a couple of percent of tempo in a fridge.

doi:10.1021/la9609231; 10.1021/ma050290+

 The last one was reproduced successfully by myself in the preparation of CD-rotaxanes.

Good luck,

Laszlo