(420e) Recyclable Cross-Linked Polymer Networks Via One-Step Controlled Radical Polymerization

Conventional polymer networks cannot be recycled into high-value products because of permanent, covalent cross-links. Rubber tires illustrate well the issues ranging from economic loss to environmental problems that arise with spent, cross-linked polymers. Here, we developed a one-step strategy using nitroxide-mediated polymerization (NMP) to synthesize recyclable network polymers with dynamic alkoxyamine cross-links that allows for reprocessing in the melt state through repeated bond formation and dissociation. With this method, we use stable nitroxide radicals containing a polymerizing moiety (carbon-carbon double bond) as both radical regulators during reaction and branch units on the polymers or cross-linker chains. We also use bifunctional initiators to generate chains with radical sites on both sides to connect the nitroxide branch units, resulting in cross-linked polymer. This NMP-based approach can be applied to any monomers or polymers that contain at least one carbon-carbon double bond that is amenable to radical polymerization. For example, through this strategy, we designed a network polymer using polybutadiene and styrene monomer as a model for recyclable tires, showing full property recovery after multiple melt-reprocessing recycles; we also produced polymer networks from monomers with good reprocessability and relative uniformity. With nitroxide groups serving as thermally reversible cross-links and simple functionalization of monomers, this one-step strategy provides for both robust, sustainable recyclability of cross-linked polymers and design of networks for advanced technologies.