The base-free multicomponent polymerization of elemental sulfur, difluoromethylene phosphobetaine and amines toward electron-deficient aromatic polythioureas

Abstract

Polythioureas are emerging materials with fascinating properties, such as self-healing and adhesion, high refractive indices, high dielectric constants, and heavy metal ion adsorption.With the increasing requirement for polymer structures and properties suiting a wide range of potential applications, versatile synthetic approaches are demanded to access a great diversity of polythiourea structures efficiently and economically. In this work, a commercially available difluorocarbene precursor, difluoromethylene phosphobetaine (PDFA), was selected to react with sulfur and electron-deficient aromatic amines to enable the efficient syntheses of electron-deficient polythioureas from amine monomers with low reactivity based on the high reactivity of a difluorothiocarbonyl intermediate. A one-pot catalyst-free multicomponent reaction of sulfur, PDFA and an amine was designed, which could take place efficiently in DMAc at 60 °C under nitrogen and was applicable for aromatic amines with both electron-donating and electron-withdrawing groups, producing thioureas in high yields (up to 93%). Most importantly, catalyst-free multicomponent polymerizations of sulfur, PDFA, and diamines were also developed in DMAc at 60 °C with commercially available monomers, showing high efficiency (M_ws up to 65 900 g mol⁻¹ and yields up to 99%) and wide monomer applicability, providing an efficient synthetic approach for syntheses of polythioureas. Moreover, these polythiourea thin films showed high refractive indices (n_{633 nm} up to 1.8133), suggesting their potential application in optical devices.