Polyurethane prepolymers are widely used in reactive hot melt adhesives and moisture cured coatings. NCO-terminated polyurethane of polypropylene glycol (PPG)-1000 and isophoron diisocyanate (IPDI) with an NCO/OH ratio of 1.2:1 were prepared without any catalyst or solvent. 1D and 2D NMR spectroscopy was used for the structural investigation of the synthesized prepolymer. H-1 NMR spectra and dibutylamine back-titration were used to monitor the reaction of isocyanate groups with the hydroxyl function of PPG and to allow a good description of the evaluation of the urethane groups. The data on percentage conversion were crosschecked with C-13 NMR spectroscopy in the urethane zone for the methanol endcapped prepolymer. The observations show a higher reactivity of the secondary NCO group than of the primary group. (C) 2005 Wiley Periodicals, Inc.