Structural investigations of polypropylene glycol (PPG) and isophorone diisocyanate (IPDI) based polyurethane prepolymer by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF)-mass spectrometry

The wide application of polyurethane (PU) prepolymers in the reactive hot melt adhesives, sealant and coating sector has made it necessary to understand the changes occurring during polymerization. NCO terminated PU of polypropylene glycol (PPG-2000) and isophorone diisocyanate (IPDI) with NCO/OH ratio 2:1 were prepared. The reaction was continued up to 111 h and the prepolymer obtained at different reaction times were partly removed from the reaction kettle and end-capped with methanol. The end-capped samples were subjected to MALDI-TOF-mass spectral investigation. The peaks at different reaction times in the MALDI-TOF-mass spectra were identified as different PPG homologues. These results suggest that initially the reaction produces monourethane, which subsequently convert to diurethane during the course of reaction.