Design and Continuous (Re)Processing of Thermally Resilient Poly(Styrene<i>-co-</i>Maleic Maleate)-Based Covalent Adaptable Networks

Cyclic anhydrides, classical monomers for bulk synthetic materials, have recently gained attention in covalent adaptable networks (CANs). They undergo reversible ring-opening with polyols, forming dynamic monoester linkages. This inherent property makes cyclic anhydrides ideal for industrially relevant polymer matrices with CAN-like recycling and reprocessing capabilities. Our study presents a rational design for poly(styrene-co-maleic maleate) monoester (PS-MME)-based polymer networks, achieved by cross-linking commercially available poly(styrene-co-maleic anhydride) (PS-co-MA) with a series of diols. The network synthesis is not only feasible on a large scale and under solvent-free conditions but also allows for the tailoring of the final cross-linking degree and properties by independently tuning the PS-co-MA maleic anhydride content, its chain length, and/or the loading of cross-linker diols used. Previous limitations in the thermal stability of MME-based CANs, stemming from remaining free carboxyl acids in monoester linkages, are circumvented through the dilution of reactive sites in an apolar matrix and switching to a cyclic anhydride ring embedded into the polymer backbone. This robust macromolecular architecture enhances the kinetics and thermodynamics of reversible monoester formation, resulting in PS-MMEs with exceptional thermal resilience. Even at temperatures exceeding 210 °C, PS-MMEs exhibit fast cross-link exchange without compromising their rheological, mechanical, or thermal properties. This feature not only enables continuous (re)processing via extrusion and injection molding but also allows the solvent-assisted recovery of the pure starting polyol monomers and PS-co-MA linear polymers, facilitating their chemical recycling and use in new polymer materials. In other words, this research paves the way for industrially applicable CANs by leveraging widely used bulk monomers and polymers.