(291h) 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. Research began over a decade ago to produce network polymers with dynamic or reversible bonds that can be self-healing or recyclable. Such networks can not only maintain good mechanical performance but also change their chemical structures or physical shapes in response to external stimuli (e.g., temperature) through reversible rearrangement reactions. Much of this research has focused on reversible reactions between functional groups, including thermoreversible Diels-Alder reactions,¹ transesterification reactions,² disulfide exchange reactions,³ among others.^4,5 Such approaches often require complex synthesis of specific chemical units as precursors or suffer exchange inefficiency in the solid state.No previous study has achieved direct formation of recyclable networks from commercial monomer/polymer without pre-synthesis and pre-functionalization or demonstrated full property recovery (within error) for elastomers reflecting full recovery of cross-links after multiple melt-reprocessing steps.

Here, we developed a one-step strategy using nitroxide-mediated polymerization (NMP) to synthesize recyclable network polymers with dynamic alkoxyamine cross-links^6,7 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.

References

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