2,3-Dihydropyran (DHP) and ethyl vinyl ether (EVE) were co-polymerized with maleic anhydride (MA) with benzoyl peroxide at 60°C, and 1:1 alternating copolymers were obtained. The rates were maximum at 1:1 monomer composition. Spontaneous copolymerization and solvent effect on the rate were observed in the copolymerization of DHP with MA, in which initial rates were slower in more polar solvents. Participation of charge transfer complex was considered. EVE copolymerized rapidly with MA, reaching the theoretical limiting conversion of 1:1 alternating copolymerization. Although DHP-MA comonomer pair and EVE-MA comonomer pair formed similar 1:1 charge transfer complexes, DHP copolymerized slowly with MA to produce a low molecular weight copolymer, and the limiting conversion was much lower than the theoretical one. To explain these, degradative chain transfer to DHP monomer is proposed as the initial rate of DHP-MA copolymerization is proportional to the initiator concentration to the power 1.1. Q and e values of DHP were calculated to be 0.013 and -0.93, respectively, from the monomer reactivity ratios of copolymerization of DHP with acrylonitrile [r1(DHP)=0.003 ± 0.006 and r2(AN)=3.6 ± 0.3].