Kinetics of the cationic polymerization of 1-substituted aziridines, such as 1-methoxycarbonylmethyl aziridine, 1-methoxycarbonylethyl aziridine, and 1-benzyl aziridine, initiated with 3-hydroxy-1-propane sulfonic acid sultone have been investigated, and the results are compared with the results of the polymerization of 1-β-cyanoethyl aziridine. The course of polymerization of 1-methoxycarbonylmethyl aziridine involved a termination reaction due to the reaction between the growing endgroup and the imino group in the polymer chain. On the other hand, the polymerizations of 1-methoxycarbonylethyl aziridine and 1-benzyl aziridine were terminated by a backbiting reaction with the formation of a piper-adinium ring on the polymer end. The propagation and termination constants were obtained at different temperatures, and the enthalpies of activation (ΔH*) and the entropies of activation (ΔS*) for this polymerization were calculated.