Abstract: The fast (Reaction injection molding, RIM) polymerization was carried out by reacting ethylene oxide capped poly (propyl oxide) polyether diol (PPO) with 4,4-diphenyl-methylene diisocynate (MDI) or liquefied MDI (L-MDI). Catalyst activity was compared for dibutyltin dilaurate (DBTDL), starmous octoate (SnOct), alkyl tin mercaptide (UL-29) and corresponding organotin/triethylene diamine (DABCO) systems in the polymerization process. Kinetic parameters were calculated with a non -linear optimization technique for a simple nth order model with an Arrhenius temperature dependence. The results suggest that activation energy is 130-170 kJ/mol and reaction heat is 61-72 kJ/eq. NCO. The catalyst activity is ordered as SnOct>DBTDL>UL - 29 and DBTDL/DABCO>UL - 29/DAB-CO>SnOct/DABCO. Overall reaction order is about 2 that is not related to catalyst activity.