Base-assisted elemental sulfur-based multicomponent polymerizations to synthesize asymmetric polythioureas

Polythioureas with great potential in precious metal recovery materials, dielectric materials, light refractive materials, self-healing materials, and adhesives have become a group of popular polymer materials. While various synthetic approaches have been reported for different polythiourea structures, which generally involve expensive and toxic monomers, several polythiourea structures, such as asymmetric aromatic polythioureas, still have limited access. Herein, a base-catalyzed multicomponent polymerization (MCP) of elemental sulfur, diisocyanides, and diamines was developed, which was generally applicable for both aromatic amines with low reactivity and aliphatic amines with high nucleophilicity, affording polythioureas with great structural diversity, high M _ws of up to 107,700 g/mol and high yields of up to 99%. Moreover, aromatic polythioureas with different aromatic spacers installed on each side of the thiourea moiety could be facilely synthesized. Amine exchange reactions were studied for thiourea compounds or polythioureas, indicating that the C-N bonds of the thiourea moiety on the aromatic substitute side were more labile compared with those on the aliphatic substitute side, rendering the potential controllable degradation of different polythiourea structures. The polythioureas generally possess high thermal stability, with the glass transition temperatures ranging from 121 to 169 °C. These polythioureas could efficiently absorb Hg²⁺ from polluted water, and aromatic polythioureas generally showed better performance compared with semi-aromatic and aliphatic polythioureas. This work has hence provided a general synthetic approach for various aliphatic, aromatic, and semi-aromatic polythioureas with symmetric or asymmetric thiourea structures. The dynamic covalent bond nature of thioureas endowed these polymer materials with controllable degradation, making them sustainable functional polymer materials.