The alternating copolymerization of N-phenyl maleimide (NPMI) with ethyl alpha-ethylacrylate (EEA) by the reversible addition fragmentation chain-transfer process was investigated. The monomer reactivity ratios were measured and r(1) = 0.19 +/- 0.03 for NPMI and r(2) = 0.20 +/- 0.04 for EEA. It was found that before about 45% of the comonomer conversion, the molecular weight of the copolymer increased with the conversion, the molecular weight distribution was rather narrow, and the molecular weight of the copolymer approached a constant value, irrespective of the length of the polymerization time. Electronic spin resonance determined that the radical signal disappeared quickly after the conversion of comonomer exceeded 45%, which may be attributed to the coupling termination of the propagating polymer chains with the EEA end with the intermediate radicals when the concentration of comonomers decreased. (C) 2004 Wiley Periodicals, Inc.