A Monte Carlo method was used to take thorough account of the influences of different reactivity ratios and initial feed compositions on copolymer microstructure. The model proves the lack of azeotropic behavior in systems in which rA 1 and rB 1 or vice versa; it is also able to calculate the drift in the copolymer properties: copolymer composition, and randomness parameter. Moreover, for each reactivity ratio pair given, there is a unique reaction conversion, at which macromolecules produced inherit their maximum allowed alterations. This critical conversion declines as initial feed composition increases. However, for systems with rA 1 and rB 1, as well as those with rA 1 and rB 1, the azeotropic behavior of the reactions is clearly observed. Besides, copolymer composition reaches azeotrope point at the end of the reaction when rA 1 and rB 1. Finally, for systems in which rA 1 and rB 1, randomness parameter becomes maximum at azeotrope point when rA equals rB.