ESR study and radical observation in transition metal-mediated polymerization: Unified view of atom transfer radical polymerization mechanism

Using an on-line electron spin resonance (ESR) technique, we studied the electron paramagnetic species of oxidized metal centers and carbon radicals in three representative catalyst/ligand systems of transition metal-mediated polymerization of methyl methacrylate. These systems were: (1) CuBr/Bpy, (2) RuCl2(PPh3)/Al(OiPr)(3), and (3) CuBr/N-pentyl-2-pyridylmethanimine. Ethylene glycol dimethacrylate was used as crosslinker to trigger the diffusion-controlled radical deactivation so that the radical population was accumulated to an ESR detectable level. Methacrylate radicals were observed in all the systems. Their signal hyperfine structures were typical of nine line and were identical to those observed in conventional free radical polymerization processes. There were also peculiar signals observed at low dimethacrylate levels in (2) and (3). These paramagnetic species were present without the monomer addition. The nature of the propagating center type was analyzed. This work provided a unified view for the radical mechanisms of atom transfer radical polymerization.