A copolymerized poly(4-vinyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl) (PTVE) was synthesized by cationic polymerization of a vinyl monomer bearing a 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical and an ethylene oxide or tri(ethylene oxide). The charge/discharge properties of copolymerized PTVE improved with the length of the ethylene oxide chain, and high rate and high power properties of the PTVE synthesized by copolymerization with tri(ethylene oxide) were much better than those of the PTVE homopolymer. Electrochemical measurements showed that the ionic conductivity of the polymer synthesized by copolymerization with tri(ethylene oxide) was higher than that of the PTVE homopolymer.