A prepolymer comprising acid functionality is made by a process comprising the step of contacting: (i) a di-isocyanate, (ii) a diol containing an acid group, and (iii) a polyol without an acid group, the contacting conducted under reaction conditions and in a solvent consisting essentially of dipropylene glycol dimethyl ether. The prepolymer is useful in the preparation of water-based polyurethane dispersions.