Synthesis and structure investigation of hexamethylene diisocyanate (HDI)-based polyisocyanates

Solvent-free hexamethylene diisocyanate (HDI)-based polyisocyanates were prepared by cyclotrimerization of HDI monomer using tri-n-butylphosphine (TBP) as a catalyst, and characterized by gel permeation chromatography (GPC), nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), and mass spectrometry (MS). The GPC results revealed the effects of reaction time, temperature and concentration of TBP on the molecular weight distribution of the polyisocyanates. The structures of the polyisocyanates were identified and confirmed by the combined analysis of MS, FTIR and NMR. There are dimer, pentamer, heptamer and higher-molecular-weight oligomers presented in the product, beyond the main product of trimer oligomers.