Oxidative polymerization of 2,6-dimethylphenol to poly(2,6-dimethyl-1,4-phenylene ether) (PPE) was catalyzed by copper-amine complexes which were prepared from cuprous oxide, hydrobromic acid, and amines. An unexpected synergistic phenomenon was observed for the mixed n-butylamine dibutylamine-copper complex. Under identical reaction conditions, the polymer produced with the mixed-amine copper complex had much higher molecular weight than those produced with the single-amine copper complexes. The best n-butylamine/dibutylamine composition for the mixed-amine copper complex was found to be at 1:1 molar ratio, which gave the highest intrinsic viscosity and the highest PPE yield with little change in 3,3’,5,5’-tetramethyldiphenoquinone yield and nitrogen content. The interaction of amines in the mixed-amine copper complex was observed using infrared spectroscopy.