The invention relates to a process for the preparation of cellular, microcellular and noncellular polyurethane polyurea molded parts through the reaction of organic polyisocyanates, polyols having at least 2 reactive hydrogen atoms, and a chain extender containing at least 70 percent by weight of a mixture of aromatic diamines comprising from 80 to 50 weight percent of at least one primary aromatic diamine whose amino groups are sterically hindered relative to reaction with polyisocyanates, and 20 to 50 weight percent of at least one unsubstituted or substituted reactive primary aromatic diamine whose amino groups do not exhibit reduced reactivity to polyisocyanates.These reactive mixtures produce polyurethane polyurea polymers with excellent demoldability without the use of external mold release agents.