An objective function to determine the optimal operating conditions to produce polymers with a specified molecular weight distribution (MWD) in a continuous stirred tank reactor (CSTR) was formulated. Two polymerization cases were studied: free radical styrene polymerization and cationic isobutylvinylether polymerization. The optimization results of objective function J(1) in view of finite weight fraction stopped near a curve that passes through a predefined optimal point instead of the target point for both cases. Objective function J(2) based on two MWD parameters, was formulated to investigate the optimization result of J(1) In the case of styrene polymerization, the optimization searches of J(2) converged on the optimal point. On the other hand. in the case of isobutylvinylether polymerization, the optimal point cannot be obtained by minimizing J(2). In this case, an additional constraint such as polymer concentration is needed to determine an optimal solution.