Homogeneous hydrolysis of 4,4’-methylenediphenyl diisocyanate (MDI) in water

The environmental effects of 4,4’-methylenediphenyl diisocyanate (MDI) are linked with its potential hydrolytic conversion to 4,4’-methylenedianiline (MDA). Likewise, the reactions of MDI with biological macromolecules govern the absorption, distribution, metabolism, and excretion processes associated with sensitization and toxicological effects, and the hydrolytic formation of MDA in the body would be a source of additional concern. To obtain better insight into the influence of the reactive behavior of MDI on its potential health and environmental hazards, its hydrolysis in water was studied at very high dilution (120 nmol/L) under homogeneous conditions that minimized potential interference by solvents, buffers, and strong N- or S-centered nucleophiles. The reaction proceeds in two distinct steps, converting one isocyanate group at a time. Pseudo-first-order rate constants for the conversion of MDI were determined at 293 K and pH values between 4 and 9, and at temperatures between 283 and 303 K at pH 7. The half-life of MDI in water under neutral conditions and at 298 K was found to be 11 s. This newly-generated kinetic and mechanistic information provides a basis for refining the conceptual and quantitative models used to predict the environmental and toxicological effects of MDI and MDI-related substances.