Kinetics and catalysis of consecutive isocyanate reactions. Formation of carbamates, allophanates and isocyanurates

Abstract

Organic isocyanates (I) react with alcohols and phenols (HX) in a sequence of reactions to give Carbamates, allophanates and Isocyanurates. Rate and equilibrium constants of the individual steps have been determined. The partitioning of the reaction products predominantly depends on the I : HX ratio and the nature of the catalyst applied. At equimolar I : HX ratios, the carbamate is the dominating product in the absence and in the presence of catalysts like tin carboxylates and common tertiary amines including 1,4-diazabicyclo[2.2.2]octane (DABCO). In these cases, the rate constant of carbamate formation (k₁) is larger than those of allophanate and isocyanurate formation (k₂ and k₃). With catalysts such as aminals, aminoalcohols, amidines and carboxylate, phenolate and alkoxide anions, however, the isocyanurate is mainly formed. With these catalysts the ratio of rate constants is k₁ < k₂≈k₃. For base catalysts, the k₁ : k₂ : k₃ ratios depend on the mechanism of catalysis. Tertiary amines react in the alcohol-isocyanate reaction by a concerted termolecular mechanism, whereas the anionic catalysts react by a stepwise mechanism via alcoholate anions. In the reactions of the Isocyanate with phenols, Carbamates and allophanates, all base catalysts react by the stepwise anionic mechanism, but the anionic ones are more efficient because their conjugate acids form hydrogen-bonded homoassociate complexes in higher extend.

At high I : HX ratios, the isocyanurate is the only final product formed via the carbamate and allophanate as detectable intermediates. The kinetics of cyclotrimerisation is mainly governed by the k₁ : k₂ : k₃ ratios and the magnitude of the equilibrium constants K₁, K₂ and K₃. The kinetic order with respect to Isocyanate may vary from 1 to 3.