Dimeryl Diisocyanate (DDI) Curing Agent: Synthesis via Triphosgene reagent and Curtius Rearrangement of acyl hydrazide

Dimeryl diisocyanate (DDI), a bio-based cycloaliphatic isocyanate with high molecular weight and long carbon chains, is a suitable replacement for conventional isocyanates due to its advantages such as low toxicity, low vapor pressure, and moisture resistance. This compound has wide applications as a curing agent, plasticizer, and viscosity modifier in the formulations of propellants, polyurethanes, and elastomers. In this study, the synthesis of DDI was investigated through a three-step method based on the Curtius rearrangement using different reagents. In the intermediate steps, dimer amide and dimer hydrazide were synthesized with yields of 78% and 88%, respectively. In the final step, DDI synthesis was accomplished via two distinct routes: 1) the reaction of dimer amine with triphosgene, yielding the product with a 62% yield, and 2) the reaction of dimer hydrazide with sodium nitrite followed by the Curtius rearrangement, which achieved a yield exceeding 90%. The hydrazine-based method, due to its significant advantages such as high yield, the possibility of solvent-free reaction in some steps, cost-effectiveness, and the elimination of the need for triphosgene, is introduced as a promising non-phosgene strategy for the production of isocyanates. The structure of the products in each step was confirmed and characterized by FT-IR, ¹H NMR, and ¹³C NMR spectroscopy.