The instant invention is directed to the discovery that it is now possible to obtain high quality, elastomeric polyurethanes or polyureas, even when using highly reactive diamines as chain lengthening agents, without the formation of unwanted irregularities, if the mixture of reactants is exposed to a high velocity gradient by means of vigorously mixing kneading elements at an early phase of the reaction while the viscosity of the reaction mixture is still low. Generally, the viscosity is kept between 100 and 1000 poises and preferably between 200 and 700 poises during the action of the high velocity gradient.