Unraveling the mechanism of substrate-induced reactivity change in the trimerization of isocyanates: A comprehensive DFT study

Abstract

Cyclotrimerization of isocyanates is a widely used method for producing isocyanurates, which can enhance the thermal and mechanical properties of various polyurethanes (PU). The nature of the substituent moiety (aliphatic groups versus aromatic groups) on the isocyanate backbone can significantly impact the catalytic performance in terms of the trimerization reaction. Under catalyst-free conditions, both one-step and two-step proposals were firstly considered and the DFT calculations revealed that the two-step mechanism operates in the system. With the acetate catalyst, the possible reaction pathways to produce isocyanurate trimers were further investigated to comprehend the different reactivities of aliphatic and aromatic isocyanates. The greater reactivity of the aromatic isocyanate is attributed to the stronger electrophilicity of the carbon in the isocyanate group resulting from the conjugative effect between the phenyl group and the isocyanate group. The reaction path for the formation of the aromatic by-product is also proposed.