A detailed model for particle size distribution in a vinyl acetate/butyl acrylate emulsion co-polymerization system is developed, employing the pseudo-homopolymerization approach. Significant changes have been made in modeling the average number of radicals per polymer particle (n) over bar. The partial differential equation describing the particle population (population balance equation) is discretized using orthogonal collocation on finite elements, and the complete system of differential algebraic equations is solved using DDASSL. Simulation results under semi-batch operations are described and model predictions are compared with experimental results generated from an experimental reactor. A parametric sensitivity study is described with connection to the issue of parameter identification.