A one-dimensional physically motivated dynamic model of a twin-screw extruder for reactive extrusion has been developed. This model predicts the transient and stationary behavior of the extruder for pressure, filling ratio, temperature, and molar conversion profiles as well as residence time distribution under various operating conditions. The model consists of a cascade of perfectly stirred reactors that can be either fully filled with backflow or partially filled according to the operating conditions. Each reactor is described by the reactant concentrations and the melt temperature. A piece of barrel and screw, described by their temperature, is associated with each reactor. Living polymerization of c-caprolactone with tetrapropoxytitanium as the initiator is chosen as an example of application. The flow representation aspect of the model is validated by using experimental residence time distributions. Validation of the model is derived from simulation results as well as comparison with experimental data.