Copolymerization of 2-butyl-1,3-dioxepane (2-Bu-DOP) with 1,3-dioxolane (DOL) was carried out with boron trifluoride etherate (BF₃.O(C₂H₅)₂) as an initiator and the mechanism was investigated theoretically using semiempirical modified intermediate neglect of differential overlap (MINDO/3), modified neglect of diatomic overlap (MNDO), and Austin Model 1 (AM₁) methods. The nucleophilicity and the reactivity of cyclic acetals were explained by the negative charge on the oxygen atom and the LUMO energy of propagating species of cyclic acetals. The GC determination of unreacted monomers during the copolymerization indicated that the preferential polymerization of 2-Bu-DOP had occurred. The values of reactivity ratios, calculated by the Fineman-Ross method, were r_A=2.13 ±0.05 and r_B=0.12 ±0.05 at -10 ℃. Relative equilibrium concentration the of cyclic oxonium ions (1) and the open carbenium ion (2) is considered very important to determine the copolymerization mechanism. The reactivity of 2-Bu-DOP containing butyl substituent at 2-position can affect the relative computational stability of oxonium ion by 5-7 kcal/mol favoring the carbenium ion. Considering the rapid equilibrium of these two cations and the reaction coordinate based on calculations, the chain growth via SN₁ mechanism might be as fast as that via SN₂ mechanism.