The copper(II)-catalyzed polymerization of 2,4,6-trichlorophenol and 2,4,6-trichloro-3-methylphenol were studied. Their polymer structures were confirmed by elemental analysis, infrared (IR), and nuclear magnetic resonance (NMR). The 2,4,6-trichlorophenoxo-copper(II) complex and 2,4,6-trichloro-3-methylphenoxo-copper(II) complex intermediates were isolated. The elemental analysis and electron spin resonance (ESR) measurement of the phenoxo-copper(II) complexes were discussed in relation to their structures. Decem position of the phenoxo-copper(II) complex in refluxing benzene yielded poly(dichlorophenylene oxide). ESR measurements on the phenoxo-copper(II) complex in the solid state at 120 degrees C indicated that the phenoxy radical was generated during the period of decomposition and the intensity of the ESR spectra based on the copper(II) ion decreased with the measurement time. A single-electron transfer reaction mechanism was proposed for the phenoxo-copper(II) complex intermediate.