Non-Isocyanate Synthesis of Covalent Adaptable Networks Based on Dynamic Hindered Urea Bonds: Sequential Polymerization and Chemical Recycling

The development of environmentally sustainable processes for polymer recycling is of paramount importance in the polymer industry. In particular, the implementation of chemical recycling for thermoset polymers via covalent adaptable networks (CANs), particularly those based on the dynamic hindered urea bond (HUB), has garnered intensive attention from both the academic and industrial sectors. This interest stems from its straightforward chemical structure and reaction mechanism, which are well-suited for commercial polyurethane and polyurea applications. However, a substantial drawback of these CANs is the requisite use of toxic isocyanate curing agents for their synthesis. Herein, we propose a new HUB synthesis pathway involving thiazolidin-2-one and a hindered amine. This ring-opening reaction facilitates the isocyanate-free formation of a HUB and enables sequential reactions with acrylate and epoxide monomers via thiol-Michael and thiol-epoxy click chemistry. The CANs synthesized using this methodology exhibit superior reprocessability, chemical recyclability, and reutilizability, facilitated by specific catalytic and solvent conditions, through the reversible HUB, thiol-Michael addition, and transesterification processes. A non-isocyanate synthetic pathway for dynamic hindered urea bonds was demonstrated using a ring-opening reaction between thiazolidine-2-one and 1,3-di(piperidin-4-yl)propane. The resulting thiol groups formed covalent adaptable networks (CANs) via click reactions. These CANs can be selectively controlled by activating each dynamic bond, providing multiple pathways for reprocessing, chemical recycling, and reutilization. image