A polymerization process model for the continuous production of high impact polypropylene using Ziegler-Natta and metallocene catalysts has been developed and this is reported herein. The novel features of the model are the incorporation of the effects and interactions of residence-time distributions (RTD＇s) in each polymerization stage (homo and copolymerization stages), and the effects of thermal deactivation and poisoning of active sites on the distribution of the ratio of rubber (EPR)/polypropylene (i-PP) among the polymer particles leaving the copolymerization stage. It was shown that narrower RTD＇s narrow the distribution of EPR content in the polymer product. Poisoning can narrow the distribution as well. In addition, in our sample calculations, it was observed that I-CSTR in both polymerization stages with poison can have die same effect on the distribution of EPR in the polymer product as 3-CSTR＇s in both stages without poison.