Upcycling Virgin and Waste Polyolefins to Reprocessable Dynamic Covalent Networks

Plastics enable modern life through their advantageous properties and broad applicability. Regardless of their type or use, plastics are challenging to recycle efficiently. Current methods for recycling spent thermoplastics such as re-extrusion with additives result in property degradation over time and the relegation of these downcycled polymers to low-value applications. Plastics may be permanently cross-linked into thermosets, yet permanent cross-links prevent these plastics from being processed and molded into new shapes at high temperature. An emerging avenue to mitigate these sustainability problems involves enriching waste plastics with dynamic covalent bonds as chemical cross-links. By introducing dynamic covalent cross-links, previously thermoplastic materials exhibit robust mechanical properties characteristic of conventional thermosets yet maintain their reprocessability at high temperatures. Not only can the incorporation of dynamic bonds be achieved during polymer synthesis to produce reprocessable step-growth networks like polyurethanes and addition-type networks like polymethacrylates, but also it can be achieved via post-polymerization modification to upcycle spent polyolefins for higher value applications. Using reactive batch processing, we upcycled various virgin and waste polyolefins (e.g., low-density and high-density polyethylene, random and multiblock ethylene/1-octene copolymers, etc.) into covalent adaptable networks (CANs) via melt-state, free-radical grafting of a cross-linker capable of dynamic dialkylamino disulfide (BiTEMPS) chemistry onto polymer chains. Unlike ethylene-based thermoset polymers, our ethylene-based CANs are reprocessable and recover their thermomechanical properties after reprocessing via successive compression molding cycles. We have further shown that, in the absence of crystallinity, high-temperature creep and stress relaxation behaviors of the CANs are dominated by the exclusively dissociative reversible dynamic chemistry of the cross-linker. This observation also demonstrates the utility of this dissociative dynamic chemistry of high activation energy at suppressing creep in networks exhibiting different viscoelastic behavior.