Dynamic Covalent Polyurethane Networks with Excellent Property and Cross-Link Density Recovery after Recycling and Potential for Monomer Recovery

Dynamic urethane chemistry, which involves both associative and dissociative mechanisms, can be used to achieve reprocessability in cross-linked polyurethane (PU) networks. We discovered that property recovery and thermal stability of dynamic covalent PU networks can be enhanced by incorporating excess free hydroxyl groups, thus performing polymerization reactions at slight stoichiometric imbalance of hydroxyl and isocyanate groups, and by increasing the cross-linker functionality from three to four. While free hydroxyl groups suppress the reversion of urethane links and minimize side reactions associated with liberated isocyanate groups under reprocessing conditions, tetrafunctional cross-linkers help to maintain network integrity in the presence of small levels of side reactions. Using these strategies, we developed a PU network that, within experimental error, exhibits 100% recovery of cross-link density and tensile properties after multiple reprocessing steps, which has not been reported before. We also demonstrated the potential of recovering monomer from PU networks by alcoholysis under relatively mild conditions. Our study not only offers a solution to the long-standing issue of PU network recycling but also provides simple strategies to improve property recovery of reprocessable networks.