Abstract: Segmented polyurethane/ureas useful in the production of spandex with improved heat set are produced by chain extending an isocyanate-terminated prepolymer with a diamine chain extender that includes: (1) greater than 25 to 75 equivalent percent of an asymmetric aliphatic and/or cycloaliphatic diamine and (2) a linear diamine such as ethylene diamine in the presence of a solvent. The isocyanate-terminated prepolymer having an isocyanate group content of from about 2.25 to about 4 is produced by reacting a stoichiometric excess of an isocyanate with an isocyanate-reactive component that includes: (1) from about 25 to about 75 equivalent percent of a polyoxypropylene diol having a molecular weight in excess of 1500 Da and an average unsaturation level of less than about 0.03 meq/g and (2) from about 25 to about 75 equivalent percent of a polytetramethylene ether glycol having a molecular weight of at least 600 Da.