Kinetics of Uncatalyzed Reactions of 2,4′‐ and 4,4′‐Diphenylmethane‐Diisocyanate with Primary and Secondary Alcohols

The kinetics of the uncatalyzed reaction of an industrially important 50/50 blend of isomers of 4,4′‐diphenylmethane‐diisocyanate (4,4‐MDI) and 2,4′‐diphenylmethane‐diisocyanate (2,4′‐MDI) with primary and secondary alcohols was studied using high‐performance liquid chromatography coupled with photodiode array detector. The alcohols such as 1‐propanol, 2‐propanol, 1‐hexanol, 2‐hexanol, 3‐hexanol, 1‐methoxy‐2‐propanol, and 3‐methoxy‐1‐propanol were used in high molar excess to diisocyanate in toluene at 80°C, and pseudo–first‐order dependences on the concentrations of 4,4′‐MDI and 2,4′‐MDI were found. Appropriate treatments of the kinetic data allowed us to determine the corresponding pseudo–first‐order rate constants. According to the kinetic results, the reactivity of the isocyanate group in the para‐position is about four to six times higher than that of the ortho‐positioned isocyanate group, depending on the reacting alcohol. Furthermore, the substitution effect, i.e., change in the reactivity of the free isocyanate group after the other has been reacted, was found for both 4,4′‐MDI and 2,4′‐MDI isomers. The differences in the reactivities of the isocyanate groups of 2,4′‐MDI and 4,4′‐MDI isomers before and after one of two isocyanate groups has been reacted are explained in terms of partial positive charges on the corresponding carbonyl carbon atom calculated by high‐level quantum chemical calculations. In addition, the UV‐spectral properties of the products obtained by quenching the reaction mixture with methanol are also discussed in light of practical implications.