The copolymerization of N-butyl maleimide (BMI) and ethyl alpha-phenyl acrylate (EPA) was successfully carried out without an initiator. A high alternating tendency was observed. The Q, e values were derived by Alfrey-Price equations: Q = 0.09, e = 0.81 for BMI and Q = 0.21, e = -0.5 for EPA, and the monomer reactivity ratios were r(BMI) = 0.15 +/-0.01 and r(EPA) = 0.18 +/- 0.08, respectively. In this system BMI was donor and EPA was acceptor. The maximum copolymerization rate and molecular weight appeared at 70 mol % (BMI) in the feed ratio. The spontaneous alternating copolymerization was considered to be completed by a contact-type charge-transfer complex formed by the monomer pairs. (C) 2004 Wiley Periodicals, Inc.