We develop an observer-based control structure for a standard nonlinear model of polymerization reactors. We use a classical input/output linearization technique for the controller synthesis, and prove theoretically its global asymptotic stability. Because our stabilizing feedback laws require knowledge of the state of the model, we solve the estimation problem by designing an exponentially converging observer, the equations of which are close to those of the extended Kalman filter. Finally, by proving a nonlinear separation principle, we conclude that our observer-based control structure is capable of stabilizing polymerization reactors. As a consequence, polymerization reactors can be operated even on open-loop-unstable steady states.