A multivariable control strategy is developed to control the polymer molecular weight in emulsion polymerization processes while maximizing the reaction rate by manipulating the jacket temperature and the monomer flowrate. In order to take into account the process nonlinearity, input-output linearizing control is considered. A cascade of two nonlinear observers is developed to estimate the heat of the reaction and the number of moles of radicals in the polymer particles. By introducing these estimations and the available measurements into the process model, the polymer molecular weight could be estimated online. The online estimation and control strategy was validated experimentally in seeded emulsion polymerization of styrene for which the model of the molecular weight is well known. It was also validated for the methyl methacrylate monomer where a simplified model was identified. (C) 2011 Elsevier Ltd. All rights reserved.