The effects of three iodide additives on the performances of two 2,6-dimethylphenol polymerization catalysts based on copper bromide were studied. The iodide additives were sodium iodide, potassium iodide and methyl iodide. The polymerization catalysts were CuBr2/n-butylamine and Cu2O/HBr/n-butylamine. Iodide addition resulted in the higher polymer molecular weight which decreased in the following order: NaI > KI > CH3I. Polymerization induction period increased significantly with NaI addition, but was insensitive to the concentrations of KI and CH3I. Addition of quaternary ammonium salt to the NaI-added catalyst dramatically shortened the induction period and greatly decreased the amount of copper catalyst needed. UV-VIS absorption spectroscopy showed that the Br-->I exchange occurred with the addition of NaI into the copper bromide-based catalyst. Besides, I-->Cl exchange was observed with the addition of methyltrioctylammonium chloride to the NaI-added catalyst. The I-->Cl exchange was caused by the binding of the iodide anion to the quaternary ammonium cation. Copper chloride/n-butylamine complex was generated after the I-->Cl exchange.