Role of Acetate Anions in the Catalytic Formation of Isocyanurates from Aromatic Isocyanates

Abstract

The formation of isocyanurates via cyclotrimerization of aromatic isocyanates is widely used to enhance the physical properties of a variety of polyurethanes. The most commonly used catalysts in industries are carboxylates for which the exact catalytically active species have remained controversial. We investigated how acetate and other carboxylates react with aromatic isocyanates in a stepwise manner and identified that the carboxylates are only precatalysts in the reaction. The reaction of carboxylates with an excess of aromatic isocyanates leads to irreversible formation of corresponding deprotonated amide species that are strongly nucleophilic and basic. As a result, they are active catalysts during the nucleophilic anionic trimerization, but can also deprotonate urethane and urea species present, which in turn catalyze the isocyanurate formation. The current study also shows how quantum chemical calculations can be used to direct spectroscopic identification of reactive intermediates formed during the active catalytic cycle with predictive accuracy.

Towards high-performance polyurethanes: a mechanism of amine catalyzed aromatic imide formation from the reaction of isocyanates with anhydrides

“…However, the more nucleophilic the co-catalyst was, the more likely it was to catalyze the cyclotrimerization of isocyanates after the isocyanate–dianhydride reaction (entries 5-8 to 5-10). 45…”

“…However, the more nucleophilic the co-catalyst was, the more likely it was to catalyze the cyclotrimerization of isocyanates after the isocyanate-dianhydride reaction (entries 5-8 to 5-10). 45 Based on results shown in Table 2, N,N-dimethylcyclohexylamine (DMCHA) was chosen as the optimal co-catalyst and various secondary amines were evaluated as a pre-catalyst (Table 3). N-Methylaniline turned out to be a better secondary amine pre-catalyst than dibutylamine.…”

Catalysts for Isocyanate Cyclotrimerization

“…[45] Tetrabutylammonium acetate (TBAA) (4) is an efficient cyclotrimerization catalyst for aryl isocyanates and is also effective for cyclotrimerization of alkyl isocyanates. [46,47] Trimethylammonium acetate ( 5) is an effective cyclotrimerization catalyst for phenyl isocyanates. [29] Furthermore, several papers report that n-hydroxy-alkyl quaternary ammonium acetates (6 and 7) are cyclotrimerization catalysts with high catalytic [a] Proton donors such as alcohol, phenols or carbamates are required.…”

Synthesis of polyisocyanurate prepolymer and the resulting flexible elastomers with tunable mechanical properties

“…2,4,6-Tris(dimethylaminomethyl)phenol (TDMAMP) 6,31 was found to be the most suitable trimerization catalyst for PIR prepolymer preparation in terms of mildness of reaction conditions, and product processability and solubility. While some catalysts, for example, tetrabutylammonium acetate (TBAA), 32,33 potassium 2-ethylhexanoate [34][35][36][37] and 2-{[2-(dimethylamino)ethyl]methylamino}ethanol (DMAEMAE) 38 were too active to be controlled, others, such as sodium p-toluenesulfinate ( p-TolSO 2 Na) 39 and potassium acetate 40,41 had poor solubility in either solvents or isocyanate. N,N′,N″-tris(3-dimethylaminopropyl)hexahydro-1,3,5-triazine (TDMAPHT) 7,42,43 provided more allophanate and less isocyanurate than TDMAMP (Fig.…”