Poly(ethylene glycol) (PEG) having a polymerizable methacryloyl group at one end and an aldehyde group at the other end was quantitatively synthesized. Potassium 3,3-diethoxy-1-propanoxide initiated the anionic polymerization of ethylene oxide to form an acetal-ended PEG having potassium alkolate at the omega chain end. By adding methacrylic anhydride to the reaction mixture, the omega end was quantitatively converted to the methacryloxy group. When the PEG with an acetal group at one end and a methacryloyl group at the other end was treated with 90% acetic acid solution, the hydrolysis of the acetal end group proceeded to form a PEG macromonomer possessing an aldehyde end group. The end groups were characterized by H-1 NMR and MALDI-TOF-MS spectroscopies. Copolymerization of the aldehyde-PEG macromonomer with methacrylate monomers quantitatively gave the PEG graft polymer with an aldehyde group at the free end of the graft chain.