We have observed that earlier models of the reactive extrusion Of E-caprolactone (CL), which do not include a consideration of mechanical degradation, significantly overpredict the molecular weights of the product. We have performed an engineering analysis of the increase in the molecular weight and the shear-induced reduction of the molecular weight after the polymerization of CL during reactive extrusion. The variation of the molecular weight has been correlated to the polymerization rate and the mechanical power input into the Poly(is an element of-caprolactone) (PCL) melt. We first compute the increase in the molecular weight of PCL during reactive extrusion as a function of the mean residence time until the conversion is completed. Subsequently, we compute the shear-induced reduction of the molecular weight after polymerization through the reactive extrusion of CL. The influence of the mechanical load on the PCL melt with increasing shear stress is important for predicting the molecular weight of the final product, particularly after polymerization. Our approach to the prediction of the reduction of the molecular weight is based on the correlation between the specific mechanical energy (i.e., the energy consumed per unit of mass of the material extruded) and the molecular weight. (C) 2004 Wiley Periodicals, Inc.