“Catch‐Store‐Release” Strategy for the Stabilization of 4,4′‐Methylene Diphenyl Diisocyanate (4,4′‐MDI)

Abstract

Abstract4,4′‐Methylene diphenyl diisocyanate (MDI) is an industrially crucial compound, being one of the most utilized linkers in the polyurethane industry. However, its long‐term stability is limited due to dimerization to form insoluble uretdione. Herein we demonstrate an organometallic “catch‐store‐release” concept aiming at improving the long‐term chemical stability of MDI. Treatment of MDI with two equivalents of selected N‐heterocyclic carbenes (NHC) forms stable MDI−NHC adducts. Treatment of the adducts with CuCl forms metastable di‐CuI complexes that undergo decomposition to re‐form MDI (up to 85 %), along with Cu−NHC complexes. The yield of re‐formed MDI can be improved (up to 95 %) by the release of the NHC ligands in the form of thiourea; this prevents subsequent MDI dimerization/polymerization by the carbenes. Furthermore, the need to separate MDI from the reaction mixture can be eliminated by the direct reaction of MDI−NHC complexes with alcohols (as models for diols), that form dicarbamate (as a model for polyurethane) quantitatively.