The cationic polymerizations of substituted oxiranes which have pendant energetic groups such as azido, and nitrato, are investigated theoretically using the semiempirical MNDO, and $AM_1$ methods. The nucleophilicity and basicity of substituted oxiranes can be explained by the negative charge on oxygen atom of oxiranes. The reactivity of propagation in the polymerization of oxiranes can be represented by the positive charge on carbon atom and the low LUMO energy of active species of oxiranes. Ring opening of the complexed cyclic oxonium ion to the open chain carbenium ion is expected computational stability of the oxonium and carbenium ion by 30∼40 kcal/mol favoring the carbenium ion. The relative equilibrium concentration of cyclic oxonium and open carbenium ions will be a major determinant of mechanism. The chain growth $SN_1$, mechanism will be at least as fast as that for $SN_2$ mechanism.