The control of PVC reactors is usually accomplished by using a PID control algorithm, despite attempts at using different advanced control algorithms and strategies. This paper presents simulation results from the application of a PID on-line auto-tuning controller based on robust adaptive control theory. PVC reactor control performance achieved using the combined PID auto-tuning controller is compared with that obtained using a conventional fixed parameter PID algorithm. Stable control behavior is achieved, and the response settles quickly with the PID controller parameters adapted to achieve generalized minimum variance control. Also, a method for estimating the turning point in a polymerization process is proposed.