The uncatalyzed and catalyzed polymerization of polypropylene glycols (MW 400-2000) with toluene diisocyanate has been studied in toluene solution at three different temperatures. Discontinuity was observed in the second-order plots of uncatalyzed reactions in contrast to the catalyzed reactions which showed straight-line plots. The rate constants (k1, k2) and the activation parameters (Ea1Ea2, δS#1, δS#2) for the isocyanate groups in the 4- and 2-positions were calculated. In the uncatalyzed and diethylcyclohexylamine-catalyzed reactions, the rate constants were found to decrease with increasing molecular weight of the polyols, whereas in the dibutyltin-dilaurate-catalyzed reactions the rate constants and the activation parameters were found to be independent of the molecular weight of the polyols.