The effects of [bpy]/[Cu(I)] ratio, polarity of the medium, counterion, and nature of alkyl bromides on the activation rate constants (k(act)) in atom transfer radical polymerization were investigated. The highest values of k(act) for Cu(I)Br were obtained at [bpy]/[Cu(I)Br] similar to 2/1 and 1/1 in more polar and less polar solvents, respectively. This was ascribed to different structures of the complex, Cu(bpy)(2)Br-+(-) and Cu(bpy)(2)+CuBr2-, correspondingly. The highest values of k(act) for Cu(I)PF6 were observed at [bpy]/[Cu(I)] similar to 2/1 in both more polar and less polar solvents, which was explained by the formation of the same active Cu(bpy)(2)+PF6-complex. Compared to Cu(I)Br, the use of Cu(I)PF6 results in an increased rate of activation by a factor of 1.8. The k(act) was larger in more polar solvents compared to less polar solvents. Am increase in k(act) was observed in the presence of water, and a decrease in k(act) was noticed in the presence of solvents containing oxyethylene groups. The relative rates of k(act) of alkyl bromides in all solvents follow the order: ethyl 2-bromoisobutyrate (EBriB) (similar to44) much greater than methyl 2-bromopropionate (MBrP) (similar to3) congruent to ethyl 2-bromopropionate (EBrP) (similar to3) > tert-butyl 2-bromopropionate (t-BBrP) (similar to1).