A new method for the control and estimation of the glass transition temperature (T-G) of free-radical copolymers is presented, and is intended to improve the on-line control of the quality of these products. As an example of this approach, a non-linear control law is used to manipulate the monomer feed flow rate of semi-batch copolymerisation in emulsion in order to pursue a predefined trajectory of T-G. The measured output required for an effective closed-loop system is provided through calorimetric measurements of the overall monomer conversion, and both the number of moles of monomer and the total number of radicals in the polymer particles are estimated using non-linear observation techniques. The global approach is based on a simplified kinetic model of the emulsion copolymerisation of water-insoluble monomers, which was validated through laboratory scale experiments. In addition to the overall and partial conversions, the model also predicts the time variations of the instantaneous copolymer composition and T-G and of the average particle size. The state and parameter observer was evaluated with both simulated and real polymerisation data, and is shown to perform very well. The tracking ability of the non-linear control strategy was found to be robust and accurate despite uncertainty and noise in both the simulated model and experimental measurements.