Using polypropylene-ether urethanes, we studied for the various factors which had the effect on the elastic properties of polyurethane elastomers. The factors contained were the preparation conditions and the structures of the polymers. Prepolymers were prepared from polypropylene ether glycol (PPG), Actol (Allied Chem. Co.) and 2, 4-2, 6-isomers ratio 80/20-tolylene diisocyanate (TDI), Nacconate-80 (Allied Chem. Co.) at various ratios of [NCO]/[OH], K₁, and mixed with diamine, 4, 4′-methylene-bis-2-chloroaniline (MOCA) at various ratios of [NH₂]/[free NCO of prepolymer] K₂, to form the elastomers.<br>TDI-PPG reactions at 50 to 140°C, 1.0 to 2.0 of K₁ values, resulted in apparently second order and 12 kcal/mol of apparent energy of activation, Ea, but at higher than 150°C temperature ranges the second order reaction was not formed, wherein elastic properties were better in relatively lower temperature reactions. When the mixing of the prepolymer and MOCA was run at 60 to 150°C temperature ranges, 80 to 110°C was found to be better for the elastic properties. At higher than 200°C, gases were evolved by the thermal decomposition of the polymer.<br>On the other, the effects of aromatic rings, urethane and urea groups concentrations on elastic properties were investigated at a constant primary crosslink density, biuret groups concentration, with the various values of K₁ and K₂ which respectively were chosen so properly that the mean molecular weight per cross-linked unit, Mc, might fall in 6000. Consequently, an increasing of those groups concentrations, especially urea groups, resulted in an increasing of tensile strength, tear resistance, modulus, hardness and Gehman freezing point and a decreasing of elongation, due to their hydrogen bondings and cohesive energies. Gehman torsional testing showed that a tendency to crystallization was promoted by strong intermolecular forces of those polar groups.<br>The retractile forces attributable to crosslinkage of primary bond were calculated from effective chaines in network according to the statistical theory of rubberlike elasticity and those attributable to secondary bonding were recognized as the residue by which the former differed from the actually surveyed retractile forces. The crosslinkage based on primary bond was caused by biuret groups and the secondary bond caused by the intermolecular forces such as hydrogen bonding of polar groups and dispersion effects. Effective chains in network were confirmed by appling to Flory and Rehner′s formulas for the values of dv₂/dT gained by measuring volume fraction of the polymer in swelling equilibrium in cyclohexanone at various temperatures. The examination of 100% Modulus and equilibrium swelling proved that most of retractile forces were dependent upon the secondary bonding in the polypropylene ether urethane elastomers.