Various effects on the coupling selectivity of the oxidative polymerization of 4-phenoxyphenol catalyzed by (1,4,7-triisopropyl-1,4,7-triazacyclononane)copper(II) halogeno complex [Cu(tacn)X-2] are described. With respect to the amount of the catalyst and the nature of the halide ion (X) of Cu(tacn)X-2, the coupling selectivity hardly changed. The Cu(tacn) catalyst possessed a turnover number greater than 1860. As the temperature of the reaction and the polarity of the reaction solvent were elevated, the C-O coupling at the o-position increased, but the C-C coupling was not involved. For the polymerization in toluene at 80 degreesC, poly(1,4-phenylene oxide), obtained as a methanol-insoluble part, showed the highest number-average molecular weight of 4000 with a melting point (T-m) of 195 degreesC. Only a slight change in the coupling selectivity was observed in the presence or absence of hindered amines as the base. Surprisingly, however, the C-O selectivity decreased from 100 to 24% with less hindered amines, indicating that the selectivity drastically changed from a preference for C-O coupling to a preference for G-C coupling.