Effect of [bpy]/[Cu(I)] Ratio, Solvent, Counterion, and Alkyl Bromides on the Activation Rate Constants in Atom Transfer Radical Polymerization

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] ∼ 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 + CuBr 2 -, correspondingly. The highest values of k act for Cu(I)PF 6 were observed at [bpy]/[Cu(I)] ∼ 2/1 in both more polar and less polar solvents, which was explained by the formation of the same active Cu(bpy) 2 + PF 6 - complex. Compared to Cu(I)Br, the use of Cu(I)PF 6 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. An 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) (∼44) ≫ methyl 2-bromopropionate (MBrP) (∼3) ≅ ethyl 2-bromopropionate (EBrP) (∼3) > tert -butyl 2-bromopropionate ( t -BBrP) (∼1).