A mathematical model was developed for a continuous reactor in which free radical polymerization of methyl methacrylate (MMA) occurred. Elementary reactions considered in this study were initiation, propagation, termination, and chain transfers to monomer and solvent. The reactor model took into account the density change of the reactor content and the gel effect. To measure the conversion an weight-average molecular weight on line, the on-line densitometer and viscometer were installed in such a way that the measured values of density were used to calculate the conversion and the viscosity measurement along with conversion data was used to determine the weight-average molecular weight. A control system was designed for a continuous reactor using an extended Kalman filter (EKF)-based nonlinear model predictive controller (NLMPC) to control both the conversion and the weight-average molecular weight of the polymer product. The control input variables were the jacket inlet temperature and the feed flow rate. For the purpose of validating the control strategy, an on-line digital control experiment was conducted with an on-line densitometer and viscometer installed. Despite the complex and nonlinear features of the polymerization reaction system, the EKF-based NLMPC performed quite satisfactorily for the property control of the continuous polymerization reactor.