A series of polyether polyol (polyoxypropylene capped with polyoxyethylene), 4‐4′ diphenylmethane diisocyanate, and 1,4 butane diol urethanes were prepared by reaction injection molding with different levels of catalyst (dibutyl tin dilaurate). The degree of phase separation, morphology, and molecular weight were compared by dynamic mechanical spectroscopy, wide‐angle x‐ray scattering, optical microscopy, and gel permeation chromatography. Slowly reacted (uncatalyzed) samples displayed better phase separation, crystallinity, low molecular weight, and poor mechanical properties. Fast‐polymerized (highly catalyzed) samples showed a high degree of phase intermixing, low crystallinity, high molecular weight, and good mechanical performance. Differences between these two behaviors are discussed with the aid of a simplified phase diagram. Slowly reacted samples phase‐separate through nucleation and growth of large spherulites, while quickly reacted samples form a much finer, interconnected microstructure. Infrared studies of phase separation for slow systems are presented and used to discuss phase separation in fast polymerizations.