ESR Study on Diffusion-Controlled Atom Transfer Radical Polymerization of Methyl Methacrylate and Ethylene Glycol Dimethacrylate

An electron spin resonance (ESR) spectrometer was applied on-line to the atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) initiated by methyl α-bromophenylacetate (MBPA) with copper bromide (CuBr) and 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA) as catalyst and ligand. The Cu(II) ESR signal was observable from the very beginning of the polymerization. The methacrylate radical spectrum appeared at a later stage due to the network formation of the system that imposed diffusion limitations on the fast radical deactivation and termination reactions. The radical and Cu(II) concentrations were measured and analyzed. The polymerization process appeared to have three definable stages. In the first stage, the Cu(II) concentration increased continuously but slowly. The methacrylate radical signal was not detectable because its concentration was below the sensitivity of the ESR machine. In the second stage, the Cu(II) concentration increased dramatically. The methacrylate radical signal started to appear and increased synchronously with the Cu(II) concentration. The autoacceleration was because the radical deactivation became diffusion-controlled. In the third stage, the Cu(II) and radical concentrations increased gradually and reached a steady state due to radical trapping in the network. The observation of the radical intermediate in ATRP further supported the radical mechanisms proposed for the transition-metal-mediated polymerization. The effects of comonomer composition, catalyst and initiator concentrations, and temperature on the radical concentration profiles were examined. The gel fraction, not the monomer conversion, determined the onset of the radical accumulation in this cross-linking ATRP system.