The purpose of this work is to model the bulk polymerization of epsilon -caprolactone in a corotating twin screw, extruder: Starting from kinetic data and rheological laws previously defined, we nse a twin screw flow modelling software to calculate the progress of the polymerization along the extruder Experiments are performed to assess the influence of processing conditions (feed rate, screw speed, barrel temperature, initiator concentration) and to validate the model. They put in evidence the control of the polymerization process by the temperature and the residence time in the extruder. It is shown that a good correlation between theoretical and experimental results can only be obtained if accurate kinetic and rheological data are used in the model.