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

Abstract

The effects of [bpy]/[Cu(I)] ratio, polarity of the medium, counterion, and nature of alkyl bromides on the activation rate constants (kact) in atom transfer radical polymerization were investigated. The highest values of kact 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+CuBr2-, correspondingly. The highest values of kact for Cu(I)PF6 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+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 kact was larger in more polar solvents compared to less polar solvents. An increase in kact was observed in the presence of water, and a decrease in kact was noticed in the presence of solvents containing oxyethylene groups. The relative rates of kact 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).