The polycondensation reaction of molecularly uniform alpha,alpha’-bifunctional oligourethane macromonomers with alpha-(chlorocarbonyl)-omega-(chloroformyl)poly( oxytetramethylene) yielded graft copolymers with an exactly defined primary structure, i.e., with known graft distribution and length; corresponding oligomeric model compounds were also obtained from the macromonomers by reaction with a monfunctional chain extender. Above a certain length of the side chains the graft copolymers exhibited the typical properties of thermoplastic elastomers. A microphase-separated system was formed in which the grafts have segregated to form semicrystalline hard domains dispersed in a continuous polyether soft phase. The study of the thermal and mechanical properties of these thermoplastic graft copolymer elastomers and their model compounds in comparison with analogously built segmented poly(ether-urethanes) revealed a specific influence of the branching on the packing of the polyurethane grafts and comparable systematics in the elastomer properties with the number of repeat units in the polyurethane graft.