Polyester (PEPA) and polyether (PTMO) prepolymers with NCO end groups were synthesized. The chain extension reaction of these prepolymers with MOCA diamine was studied by FTIR spectrometry and found to be a second-order reaction. Data on chain extension reaction rate constants and activation energy showed that the chain extension reaction rate of the PEPA prepolymer with MOCA. was faster than that of PTMO prepolymer with MOCA. Such a difference of chain extension reaction rate for PEPA and PTMO prepolymers with MOCA can be explained by hydrogen bonding interaction and miscibility between hard and soft segments in the cast polyurethane elastomer. The hydrogen bonding interactions of MOCA with PEPA prepolymer and PTMO prepolymer without chain extension reaction convincingly demonstrated the difference of chain extension reaction in the polyurethane-urea systems.