This paper addresses reactor modeling and recipe optimization of semibatch ring-opening polymerization processes for making block copolymers. Two rigorous reactor models are developed on the basis of the population balance and method of moments, respectively. The complete polymerization process model also includes vapor-liquid equilibrium equations from applying Flory-Huggins theory. The accuracies of both reactor models are validated against historical plant data by adjusting model parameters such as kinetic rate constants. The recipe optimization problem is formulated to design the optimal reactor operating policy to minimize polymerization time and incorporate additional process constraints in accordance with final product properties and process safety requirements. The resulting dynamic optimization problem is translated to a nonlinear program by using the simultaneous collocation method, and further solved by the interior point method. In the case study example, both reactor models show satisfactory matches between their predictions and the historical plant data. The recipe optimization with both models demonstrates significant process improvement and reductions in batch operating time. Moreover, the moment model shows superiority over the population balance model in terms of computational efficiency.