According to the present invention, relatively high molecular weight aliphatic and/or aromatic polyamines having molecular weights of from 400 to 8,000 (preferably from 600 to 4,000) are mixed with substantially equivalent quantities of low molecular weight, solid, finely-divided diisocyanates having a melting point of >40.degree. C., (preferably >80.degree. C.), such as phenylene diisocyanate, naphth lene-1,5-diisocyanate, dimeric toluylene-2,4-diisocyanate or the urea diisocyanate of toluylene-2,4-diisocyanate. This mixture, having a pot life of at least several minutes, is subsequently heated to a relatively high temperature, such as from 80.degree. to 200.degree. C.The products obtained have excellent elastic properties over a broad temperature range. They have an exceptionally flat modulus of elasticity curve (G') in the torsional vibration test. The hard and soft segments are markedly segregated and are thus not substantially disturbed by reciprocal interaction causing "softening".