Imide- and oxazolidinone-incorporated polyurethanes, based on hydroxy-terminated polybutadiene (HTPB), were synthesized and characterized. Reaction of the blocked isocyanate terminals of the HTPB prepolymer with diepoxy compounds, containing preformed imide groups, was the strategy followed. The diepoxy resins were derived through reaction of an aliphatic and an aromatic dicarboxylic acid with preformed imide groups with a diepoxy resin. The intermediates and the polyurethane-imide-oxazolidinone were characterized by chemical, spectral, and elemental analyses. Incorporation of these heterocyclic groups caused dramatic improvements in the thermal and mechanical properties and the thermomechanical profile of the system. The improvements in properties were proportional to the hard-segment content of the modified polyurethanes.