Structure-property relationship of blocked diisocyanates: synthesis of polyimides using imidazole-blocked isocyanates

Imidazole, 2-methylimidazole and benzimidazole-blocked aromatic and aliphat ic diisocyanates have been prepared and polymerized with pyromellitic dianh ydride in the presence of a basic catalyst. The polymers are characterized with FTIR, H-1 NMR and C-13 NMR spectroscopy and GPC, DSC and TGA. The stru cture-property relationship of blocked diisocyanates are discussed in terms of molecular weight of the polyimides obtained. Considering the blocking a gent, GPC results show that the benzimidazole blocked adduct yields higher molecular weight polymer than the 2-methylimidazole-blocked adduct which, i n turn, yields higher molecular weight polymer than the imidazole-blocked a dduct. Considering the structure of the isocyanate, the molecular weight of polymer increases from isophorone diisocyanate to hexamethylene diisocyana te and to toluene diisocyanate (TDI). DSC traces of the polymers derived fr om TDI show glass transitions (T-g) in the temperature range 152-180 degree s C and the values increase from the polymer based on imidazole-blocked TDI to 2-methylimidazole-blocked TDI and to benzimidazole-blocked TDI. (C) 200 0 Society of Chemical industry.