Unambiguous 13C-NMR assignments for isocyanate carbons of isophorone diisocyanate and reactivity of isocyanate groups in Z - and E-stereoisomers

Unambiguous ¹³C-NMR assignments for the primary (prim-) and secondary (sec-) isocyanate carbons of isophorone diisocyanate (IPDI) have been made by using two-dimensional NMR measurements. On the basis of the assignments, relative reactivity of the prim- and sec-isocyanate groups with n-butanol was studied by quantitative ¹³C-NMR analysis. The individual stereoisomers of IPDI (Z-IPDI and E-IPDI) and their equimolar mixture were reacted with n-butanol (IPDI/n-butanol = 2/1 molar ratio) at 50°C for 3 days. It was found that the sec-NCO is about 1.6 times more reactive than the prim-NCO in both Z- and E-isomers. Reactivity of the E-isomer was found to be slightly higher than that of the Z-isomer. When di-n-butyltin dilaurate (DBTDL) was used as a catalyst, the reactivity of the sec-NCO became about 12 times higher than that of the prim-NCO with both isomers. In the case of 1,4-diazabicyclo 2.2.2] octane (DABCO) catalyst, the prim-NCO was 1.2 times more reactive than the sec-NCO with both isomers.