Described is an oxidative polymerization of o-cresol catalyzed by (1,4,7-triisopropyl-1,4,7-triazacyclononane)copper complex, from which a mu-eta(2) : eta(2): eta(2)-peroxo dicopper(II) complex is formed under dioxygen. This complex reacts with o-cresol to give 'controlled' phenoxy radical-copper(I) complex exclusively without formation of 'free' phenoxy radicals. The resulting polymer was composed mainly of 2-methyl-1,4-phenylene oxide unit and showed high thermal stability. The polymerization catalyzed by (N,N,N',N'-tetramethylethylenediamine)copper complex,a typical-catalyst for 2,6-dimethylphenol polymerization, gave the polymer containing larger amounts of o-linkage structures and quinone moieties.