AbstractIn studying the mechanism of the oxidation coupling polymerization of 2,6‐dimethylphenol, the effect of the copper counter‐ion was also investigated. This polymerization is catalyzed by copper(II)‐amine complexes. In these experiments, tmed*** is used as amine ligand. The counter‐ions were divided into strongly coordinating (viz. Cl−and Br−), weakly coordinating (viz. NO3−) and non‐coordinating counter‐ions (viz. ClO4−, CF3SO3−and BF4−).Catalyst solutions, prepared from copper salts with coordinating counter‐ions, yield very high maximum activities at OH/Cu ≈︁ 1, whereas the non‐coordinating counter‐ions give much lower maximum reaction rates at OH/Cu ≈︁ 2 or higher. Copper‐amine complexes of weakly and non‐coordinating counter‐ions were found to yield hydroxo‐bridged complexes with a very low catalytic activity under all conditions. Due to this stable dihydroxo bridge and the absence of coordinating counter‐ions (Cl−, Br−) to replace them, only phenolate anion can readily attack this complex and occupy a coordination site at the expense of a coordinated OH−ion, thus explaining the optimum activity at an OH/Cu ratio of 2 or higher. However, even under these conditions, catalytic activity of these complexes is still low compared to catalysts prepared from CuCl2or CuBr2, further suggesting that preformation of a chloro‐bridged or bromo‐bridged copper complex is beneficial. Thus, these experiments indicate that the active catalyst, or its precursor, is the chloro‐bridged, or bromo‐bridged, dinuclear copper complex with hydroxide acting as a co‐catalyst required for the formation of strongly coordinating phenolate anions.Although large differences in reactivity were observed, the product composition,i.e.the fraction polymer (by C‐O coupling) and diphenoquinone (DPQ, undesired side‐product formed by C‐C coupling), was not affected by the counter‐ion used,viz. 3.5% DPQ. These results are a further indication