KINETICS AND CATALYSIS OF CONSECUTIVE ISOCYANATE REACTIONS - FORMATION OF CARBAMATES, ALLOPHANATES AND ISOCYANURATES

Organic isocyanates (I) react with alcohols and phenols (HX) in a sequ ence of reactions to give carbamates, allophanates and isocyanurates. Rate and equilibrium constants of the individual steps have been deter mined. The partitioning of the reaction products predominantly depends on the I:HX ratio and the nature of the catalyst applied. At equimola r I:HX ratios, the carbamate is the dominating product in the absence and in the presence of catalysts like tin carboxylates and common tert iary amines including 1,4-diazabicydo[2.2.2] octane (DABCO). In these cases, the rate constant of carbamate formation (k(1)) is larger than those of allophanate and isocyanurate formation (k(2) and k(3)). With catalysts such as aminals, aminoalcohols, amidines and carboxylate, ph enolate and alkoxide anions, however, the isocyanurate is mainly forme d. With these catalysts the ratio of rate constants is k(1) < k(2) app roximate to k(3). For base catalysts, the k(1):k(2):k(3) ratios depend on the mechanism of catalysis. Tertiary amines react in the alcohol-i socyanate 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 allop hanates, all base catalysts react by the stepwise anionic mechanism, b ut the anionic ones are more efficient because their conjugate acids f orm 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 o f cyclotrimerisation is mainly governed by the k(1):k(2):k(3) ratios a nd the magnitude of the equilibrium constants K-1, K-2 and K-3. The ki netic order with respect to isocyanate may vary from 1 to 3.