ABSTRACTIn the first part of this sequel, an experimental investigation on the synthesis of linear segmented polyurethanes was presented, that included estimation of the kinetic constants at 60°C for the prepolymerization and finishing stages. This work presents two comprehensive mathematical models that simulate the mentioned prepolymerizations, in reactions between two poly(tetramethylene oxide) PTMO macrodiols and an excess of methylene diphenyl diisocyanate. The models require to input the molar mass distribution (MMD) of the initial macrodiol. The single‐phase (or homogeneous) model calculates a final MMD of approximately 40,000 different molecular species, and somewhat underestimates the observed molar mass dispersities. The double‐phases (or heterogeneous) model produces a better fit of the observed MMDs by simulating two independent polymerizations carried out in parallel. The double‐phases model contains three adjustable parameters: the reaction imbalances into both phases, and an “effective” rate constant. In part III of this sequel, the presented models will be extended to simulate the finishing stages. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 46946.