Polyurethane formation in a fully intermeshing, counterrotating twin-screw extruder (Leistritz) has been studied. This polymerization is based on a long polyether polyol (poly(tetramethylene oxide)), a short diol (1, 4-butanediol), and a liquid form of an aliphatic diisocyanate (dicyclohexylmethane 4, 4’-diisocyanate or hydrogenated MDI). A model previously developed by Stuber was modified to introduce the urethane kinetics and rheology. This model predicts pressure, flow rate, and polymer properties as a function of position along the extruder. The chambers are approximated in a twin-screw extruder as a series of continuous stirred tank reactors connected by leakage flows through the mechanical clearances. When compared with experimental data, the model predicts the proper trends in polymer molecular weight, conversion of isocyanate groups, and extruder pressure generation.