Block copolymers are formulated with multifunctional chain extenders. The block copolymers include a soft segment and a hard segment made from a diisocyanate, an alkylene diamine chain extender, and a multifunctional chain extender which provides delayed crosslinking. The multifunctional chain extenders have a functionality ≥3 and typically have at least one OH group. The multifunctional chain extenders may be aliphatic or aromatic triols or polyols, or may have other configurations, as described. The resulting block copolymers have improved mechanical properties such as compression set. They may be used in medical applications, or in industrial applications such as seal and gasket applications, including O-rings, window seals, and automotive gaskets. The initially-formed polyurethane resin behaves as a thermoplastic processable material, while the configured end-use product is thermoset.