Kinetics and thermodynamics of the blocking reaction of several aliphatic isocyanates

The oxime-blocking reaction of several aliphatic isocyanates, such as 1,6-Hexane diisocyanate (HDI), isophorone diisocyanate (IPDI), and dicyclohexylmethane-4,4′-diisocyanate (H₁₂MDI), is investigated. The reaction is carried on in various solvents that are divided into two categories: aromatic solvents and oxygen-contained solvents. In situ FT-IR is used to monitor the reaction and show the large difference of solvent and the structure of isocyanate. Kinetic studies indicate that the reaction rate appears faster in aromatic solvents although the polarity of aromatic solvents is lower. Then, thermodynamic parameters of the blocking reaction, such as activation energy (E_a), enthalpy (ΔH*) and entropy (ΔS*), are determined from the Arrhenius and Eyring equations. It is found that activation energy in aromatic solvents is higher, but the reaction rate is much faster, all of which are discussed corresponding to the reaction mechanism.