This paper describes a modeling and simulation study for the polyurethane synthesis by bulk copolymerization of dicyclohexyl methane 4,4'diisocyanate with a mixture of a polyether diol and 1,4-butane diol, in a co-rotating twin-screw extruder. The process model is based on linear chain growth theory, a kinetic expression previously published in literature and the axial dispersion model, in both the hypothesis of non-segregated and totally segregated fluid structures. The numerical simulations evidenced that the effect of segregation phenomenon appears to be significant, particularly in the evaluation of the product polydispersity. Also, the polymerization process features an important sensitivity of the polymer molecular weight in respect to the inlet monomer molar ratio. (c) 2005 Elsevier B.V. All rights reserved.