The effects of SnCl4 on the radical polymerization of N-allyl-N-phenylmethacrylamide (APM) and N-allyl-N-phenylacrylamide (APA) were investigated. The polymerizations of APM and APA with dimethyl 2,2-azobisisobutyrate (MAIB) were carried out at 50 degrees C in benzene at various concentrations (0-1.0 mol/L) of SnCl4. The polymerization rates showed a maximum on varying the SnCl4 concentration, while the molecular weights of the resulting poly(APM) and poly(APA) were decreased with increasing SnCl4 concentration. In both systems, the degree of cyclization of polymers were decreased with the SnCl4 concentration. From the IR results, the cyclic structure of the resulting poly(APM)s was confirmed to be five-membered, whereas poly(APA)s contained not only five-membered but also six-membered rings. The H-1-NMR examination on the interactions of APM and APA with SnCl4 revealed that these monomers form 1:1 and 2:1 complexes with SnCl4 with fairly large stability constants. Copolymerizations of APM (M(1)) with methyl methacrylate (MMA) and styrene (St) (M(2)) were investigated at 60 degrees C in benzene in the absence of SnCl4. APM units were found to be incorporated exclusively as five-membered rings in the resulting copolymer. Monomer reactivity ratios were estimated to be r(1) = 0.29, r(2) = 4.88 for APM/MMA and r(1) = 0.66, r(2) = 5.39 for APM/St. The presence of equimolar (to APM) SnCl4 was found to enhance the reactivity of APM toward poly(MMA) radical; r(1) = 0.24, r(2) = 2.56.