A mathematical model based on a slab configuration is employed to simulate fast step growth polymerizations. The effect of mixing pattern, molecular diffusivity, and catalyst concentration on the conversion profile is studied for reaction systems with different gel conversions. The results, which can be explained by the intensity and the scale of segregation, provide some conceptual guidelines in designing appropriate mixing methods for fast step growth polymerizations. The distribution of slab thickness and the benefit of using cascade impingement mixing in the reaction injection molding (RIM) process are explained by the phenomenon of flow transformation in the mixing chamber.