A mathematical model has been developed for studying the multistep synthesis of polyamic acid from pyromellitic dianhydride and oxydianiline. This synthesis process consists of two consecutive semibatch polymerization steps : prepolymerizing and finishing. Using a diffusive mixing theory, the model combines the mixing effect with the chemical kinetics to give a quantitative description of (1) the wide molecular weight distribution of the resulting polymer and (2) the drop of weight-average molecular weight during the holdup. The modeling results have been compared with actual process observations, and excellent agreement has been obtained.