Unraveling the Mechanism of Trimerization Reaction of Hexamethylene Diisocyanate: DFT Calculation and Experiments

The hexamethylenediisocyanate (HDI) trimer synthesis process is challenging to control. It is of great significance to clarify the mechanism of the HDI synthesis process for inhibiting the production of byproducts and improving the yield of target products. The synthesized HDI-trimer was characterized by FTIR, GPC, and NMR, and the structured of the HDI-trimer and its byproducts in the product were determined. The DFT method was used to study the transition states, reaction paths, and thermodynamic properties of HDI-trimers and their byproducts. The polymerization mechanism was analyzed from an energy point of view, and a two-step reaction mechanism of HDI cyclotrimerization was proposed. First, two HDI monomers reacted with the isocyanate (−NCO) group to form a quaternary ring 11-dimer, and the reaction energy barrier was 126.3 kJ/mol in the gas phase. Further, the 11-dimer reacts with the HDI monomer to form a six-membered-ring transition state with 149.9 kJ/mol energy and finally forms a six-membered HDI-trimer. Similarly, the formation process of byproducts in the HDI cyclotrimerization reaction is analyzed, and it is proposed that the byproducts (including HDI asymmetric trimers, pentamers, heptamers, and other polymers) are all two-step reaction mechanisms. With an increase in the number of polymerization units (n) in the polymer, the reaction energy barrier increases, and the reaction becomes more challenging.