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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 into polypropylene homopolymer at the molecular level. By changing operating conditions, the reactor produces numerous grades of polymer in response to consumer requests. An appropriate multivariable control system is necessary to swiftly achieve grade transitions by changing pressure and concentrations of monomers in the reactor. Measurements of concentrations, available every 6 minutes, occur less frequently than pressure measurement does. The Model Predictive Control (MPC) scheme is extended to deal with the multirate output process. A 3Ã3 multirate multivariable model predictive control system is developed and installed in the reactor. The features of the control algorithm and a rigorous stability analysis are thoroughly described. Experimental results are also illustrated to show the performance of the developed control system.