Commercially important impact polypropylene is usually:produced in a cascade of reactors in series using a solid catalyst. The product is made in situ by the addition of an ethylene-propylene copolymer phase to isotactic polypropylene. Since all industrial processes are continuous in nature, reactor residence-time distributions imply a natural distribution of polymer properties from polymer particle to particle as they exit the process. In this paper we apply the methods of population balances to impact polypropylene produced in several industrially significant multistage processes. The presence of monomer diffusion, due to the changing morphology of the growing copolymer particles is also included. It is shown that catalyst residence-time distribution has a significant effect on the copolymer content, ethylene content and total yield distribution of the product. The performance of each commercial process is compared, and issues such as the role of diffusion and particle sticking in the reactor are discussed.