Highly Active Bipyridine-Based Ligands for Atom Transfer Radical Polymerization

A series of 2,2'-bipyridines with 4,4'-substituents (R-bpy) were investigated for atom transfer radical polymerization (ATRP) of methyl acrylate (MA) and methyl methacrylate (MMA). Ligand substituents with a large range of Hammett parameters (R = Cl, H, Me, dinonyl (dN), MeO, and (Me)₂N) were studied with cyclic voltammetry (CV), revealing that increasing the strength of electron donating groups (EDGs) resulted in more stable Cu^II complexes and larger ATRP equilibrium constants. Normal ATRP experiments confirmed the obtained CV data by showing the fastest rates of polymerization with R-bpy ligands containing EDGs ((Me)₂N and MeO) and the slowest with electron withdrawing Cl. A 400-fold increase in the polymerization rate was observed with bpy ligands containing p-(Me)₂N compared to H substituents. Linear increases in molecular weight with monomer conversion, and narrow molecular weight distributions were obtained with (Me)₂N-bpy and MeO-bpy ligands. Low catalyst concentrations of 50 to 100 parts-per-million (ppm) were successfully employed with highly active R-bpy ligands (R = MeO and (Me)₂N) and found to be effective in polymerizing MA and MMA, respectively, with narrow molecular weight distributions <1.3.