A copolymerization of macromonomer poly(ethylene oxide) (PEO) with a styryl end group (PEOS) and styrene was successfully carried out in the presence of poly(epsilon-caprolactone) (PCL) with 2,2,6,6-tetramethylpiperidinyl-1-oxy end group (PCL,). The resulting copolymer showed a narrower molecular weight distribution and controlled molecular weight. The effect of the molecular weight and concentration of PCLT and PEOS on the copolymerization are discussed. The purity of PEOS exerted a significant effect on the copolymerization; when the diol contents of PEO macromonomer were greater than 1%, the crosslinking product was found. (C) 2004 Wiley Periodicals, Inc.