The reaction of isophorone diisocyanate (IPDI) with poly(hexane neopentyl adipate glycol) (HNA) and dimethylol propionic acid (DMPA) was researched through an analysis of the contents of the NCO groups of the prepolymers through which water-borne polyurethanes (PUs) could be formed. The reaction activity of DMPA with IPDI was studied and compared to that of HNA on the basis of an investigation into the effects of the temperature and catalyst on the prepolymerization and the viscosity of the prepolymers. The results showed that the reaction of IPDI with DMPA was much slower than the reaction of HNA, and so DMPA was concluded to be a relatively inert nucleophilic reagent for attacking IPDI. A temperature of 80 C was suitable, with dibutyl tin dilaurate as a catalyst, for producing prepolymers from IPDI and mixtures of HNA and DMPA, whereas 60 C without catalysis was enough to complete the reaction of IPDI with HNA. This study may be helpful in understanding the prepolymerization of waterborne PUs and in manufacturing them. (C) 2004 Wiley Periodicals, Inc.