The present invention is directed to poly-.alpha.-olefin/polyurethane block copolymers which are prepared by reactingA) isocyanate-reactive functionalized poly-(C.sub.2 -C.sub.10 -.alpha.-olefins) having a number average molecular weight M.sub.w of about 1000 to 350,000,B) polyisocyanates, isocyanate prepolymers or modified polyisocyanates, and, optionally,C) isocyanate-reactive synthesis units containing Zerewitinoff-active hydrogen atoms and having a molecular weight of 400 to about 5000 and, optionally,D) low molecular weight isocyanate-reactive chain extending agents having a molecular weight of 32 to 399.The present invention is also directed to this process for preparing the poly-.alpha.-olefin/polyurethane block copolymers and to their use as polymer dispersants for the production of polymer blends, as primers or modifiers or for the production of moldings or coatings.