This paper describes the results of a joint university-industry study to control a semi-commercial impact copolymerization reactor that produces ethylene-propylene-rubber, and mixes it with polypropylene homo-polymer at the molecular level. By changing operating conditions, the reactor can produce numerous grades of polymer according to consumer requests. An appropriate multivarible control system is necessary to achieve swift grade changeovers by changing pressure and concentrations of monomers in the reactor. Because measurements of concentrations, available every 6 minutes, occur less frequently than pressure measurements, we extend the Model Predictive Control (MPC) scheme to deal with this multirate output-sampled process. A 3×3 multirate MPC is developed and installed in the reactor. A thorough description of the features of this control algorithm is also included as well as experimental results to illustrate its performance.