Polyolefin products are often produced over solid catalysts in multistage reactor processes. As the polymerization proceeds, polymer encapsulates the original catalyst particle and expands with polymer yield. Ultimately a polymer particle 10-50 times as large as the original catalyst particle is produced. In each reactor a completely different polymer product may be produced adding to the previously formed polymer from prior reactors. During this time the morphology of the polymer particle may change dramatically and have a pronounced effect on the reaction/diffusion process in the catalyst pellet. In this paper we present a model of particle growth for multilayered "heterophasic" polymer products produced in a series of reactors. Simulation studies for impact polypropylene produced in a loop-fluidized bed process are shown. It is shown that reactor residence time distribution, catalyst deactivation, and particle porosity play a significant role in determining the product quality of impact copolymer. At high copolymer contents, severe diffusion diffusion limitations are predicted, producing a sticky copolymer on the surface of the granule and a propylene-rich copolymer in the interior.