The copolymerization of maleimide (MI) with alpha-ethylacrylic acid (EAA) and with ethyl alpha-ethylacrylate (EEA) in the presence of 2-phenylprop-2-yl dithiobenzoate (PPDB) was investigated. The copolymerization of MI and EAA was difficult to conduct with the reversible addition-fragmentation chain transfer (RAFT) mechanism because reinitiation of expelled radicals by fragmentation chain transfer was inhibited by the association of EAA in polar solvent and the strong interaction of the imino of MI with the carboxyl of EAA between the propagation chains. When the carboxylic group of EAA was esterified, then the copolymerization went well via RAFT, and alternating copolymers with controlled molecular weight were obtained. Combining by electron spin resonance showed a different result. It was found that before 30% of the comonomer conversion had occurred, the copolymer poly(EEA-co-MI) showed increasing molecular weight with the conversion and a rather narrow molecular weight distribution; then the molecular weight of the copolymer began to retard. This phenomenon of retardation was aggravated at high temperature. (C) 2004 Wiley Periodicals, Inc.