A prepolymer comprising acid functionality is made by a process comprising the step of contacting: (i) a di-isocyanate, (ii) a polyol containing an acid group, (iii) a polyol without an acid group, and (iv) a metal salt catalyst, the contacting conducted under reaction conditions and in a solvent consisting essentially of: (A) a dialkyl amide, and (B) optionally, an aprotic glycol ether. The prepolymer is useful in the preparation of water-based polyurethane dispersions, and certain of the solvent blends consisting essentially of a dialkyl amide and an optional aprotic glycol ether are azeotropic or pseudo-azeotropic.