Effect of Polymer Species and Reactive Processing Conditions on Upcycling of Thermoplastic Polyolefins into Reprocessable Polymer Networks

Permanent chemical cross-linking of polyolefins is practiced commercially at major scales. Cross-linked polyolefins made from thermoplastics exhibit upcycling of properties, e.g., improved impact strength. However, thermosets with permanent cross-links cannot be mechanically recycled because the cross-links prevent melt flow. In related work, we upcycled thermoplastic polyethylene into covalent adaptable networks (CANs) with dynamic covalent dialkylamino disulfide cross-links using a melt-state, free-radical grafting method. Here, we extend our method to a series of commodity polyolefins, with resulting CANs being recyclable multiple times without loss of cross-link density. Our method's utility is a function of repeat unit structure, chain architecture, and melt flow index, the latter related to molecular weight (MW). Most tested polyolefins were upcycled into reprocessable CANs, with polypropylene and very low MW polymer being exceptions. We also studied gel fraction and dynamic cross-linking characteristics as functions of initiator and dynamic cross-linker concentrations and processing temperature. Understanding these effects on dynamic cross-linking via reactive processing with a vinyl cross-linker can be an important step in improving the sustainability of plastics.