The copolymerization of epoxy-terminated poly( ethylene glycol methyl ether) (CH(3)PEG-epoxide) with lactones such as epsilon-caprolactone (CL) was carried out to prepare the PEG graft poly(ester ether). The methanol-insoluble part of the reaction product was considered to be the graft copolymer. The graft copolymer was prepared using potassium tert-butoxide or sodium methoxide as a catalyst. The apparent number-average molecular weight ((M(n)) over bar) of the graft copolymers increased with an increase in CL concentration and a decrease in the dielectric constant of the solvents. beta-Propiolactone (PL) and delta-valerolactone (VL) were also copolymerized with CH(3)PEG-epoxide as well as was CL. The apparent composition of the graft copolymers (lactone units/CH(3)PEG) was greater than the feed ratio of the two monomers in all cases. On the other hand, the composition of copolymers prepared with the copolymerization of low molecular weight epoxides such as phenyl glycidyl ether with VL was almost consistent with the feed ratio of the two monomers.