A novel strategy was used to synthesize poly(ethyl ene glycol) (PEG) with umbrella like structure containing a single reactive group at the handle of the umbrella 1 (Bis(2 hydroxyethyl)amino) 3 (1 ethoxyethoxy)propan 2 ol was used to initiate the ring opening polymerization (ROP) of ethyl ene oxide (EO) in the presence of diphenylmethylpotassium (DPMK) to obtain three arm PEG (PEG3) then terminated by benzyl bromide or ethyl bromide The resultant PEG3 was hydrolyzed to generate hydroxyl group at the conjunction point and the second step ROP of EO was carried out using PEG3 OH as macroinitiator in the presence of DPMK The obtained four arm PEG (PEG4) contained a functional hydroxyl group at the end of the fourth arm which could be easily modified to bioactive groups such as carboxyl active ester amino etc The well defined structure of umbrella like PEG was characterized by GPC H-1 NMR and MALDI TOF MS in detail Propionic acid succinimidyl ester of PEG4 (10 kDa) was utilized for protein conjugation with interferon alpha 2b (C) 2010 Wiley Periodicals Inc J Polym Sci Part A Polym Chem 48 5974-5981 2010