The effects of copper(I) and copper(II) metal centers on the atom transfer radical polymerization (ATRP) of styrene and methyl acrylate were investigated. The free-radical polymerizations were initiated by AIBN in the presence of copper(I) and copper(II) complexes. For methyl acrylate, the rate of the polymerization was reduced in the presence of (CuBr)-Br-I/dNbpy and (CuOTf)-O-I/dTbpy but was unaffected by the presence of Cu-II(OTf)(2)/dTbpy. For styrene, under conditions which yield relatively low molecular weight polymer (16000), no effect was observed in the presence of Cu-II(OTf)(2)/dNbpy; however, under conditions which yield high molecular weight polystyrene (50000-100000), the polymerization was limited in the molecular weight attainable and stopped at partial conversion. No effect was observed for the free-radical polymerization of styrene in the presence of copper(I) complexes. These results indicate that control in ATRP does not originate in interactions of growing radicals with copper complexes but in the reversible halogen atom transfer.