The polymerization of E-caprolactone in fully-filled conveying elements of co-rotating twin-screw extruders was analyzed with three-dimensional numerical simulation method. The effects of screw rotational speed, geometry of screw element, and initial conversion at the channel inlet on polymerization progression were studied. The simulation results show that polymerization is accelerated with increasing screw pitch, due to the increase in mixing intensity. With increasing screw rotational speed, the reaction could either slow down or speed up, depending on the viscosity of the reaction system. It is found that the advancement of polymerization depends on the competition among heat from reaction, viscous dissipation and heat loss through the wall surfaces. (c) 2005 Elsevier Ltd. All rights reserved.