Catalysts for Isocyanate Cyclotrimerization

The formation of isocyanurate via cyclotrimerization of isocya-nates is widely reported to provide a variety of polyurethane materials with improved chemical and physical properties such as weatherability, mechanical properties, thermal stability and flame retardancy. The demand for development of effective and selective catalysts for cyclotrimerization of isocyanates has been increasing. This review comprehensively summarizes catalysts for the cyclotrimerization of isocyanates that have been reported in peer-reviewed publications and provides a valuable guideline for choosing suitable catalysts to match specific requirements. The catalysts are categorized into two main classes: catalysts operating via a Lewis basic cyclotrimerization mechanism and metal-containing catalysts. Catalyst structures, reaction conditions, reaction time, catalytic effectivity as well as types of isocyanates whose trimerization is catalyzed are described in detail. In addition, featuring the findings and viewpoints from mechanistic studies, this review aims to stimulate the design and development of new, more efficient catalysts, and to guide further study of the trimerization mechanism with different classes of catalysts.