A re-examination of the polyurethane reaction

Abstract

The general base-catalyzed formation of polyurethanes is shown to involve the attack of the nucleophilic oxygen of the alcohol on the electrophilic carbon of the isocyanate. Although the base probably functions by increasing the nucleophilicity of the oxygen through hydrogen-bonding to the adjacent proton, this is, at present, unclear, because of the large number of side reactions, the existence of complexes, and the poorly understood role of the solvent. The metal complex-catalyzed formation of polyurethanes appears to occur in one of two ways: one for Sn⁴⁺ complexes and the other, for all other octahedral, as well as planar, complexes. The mechanism for the tin complexes involves an initial reaction with alcohol to form tin alkoxides, while the other mechanism involves the reaction of a metal-isocyanate complex with uncomplexed alcohol. The general base catalysis/metal complex catalysis synergism is discussed and is shown to fit the proposed mechanisms.