Simple upcycling of virgin and waste polyethylene into covalent adaptable networks: catalyst-free, radical-based reactive processing with dialkylamino disulfide bonds

Polyethylene (PE) is a ubiquitous commodity polymer that faces significant barriers to efficient recycling despite its thermoplastic nature. PE can be permanently cross-linked to enhance its properties and expand its applicability, but conventionally cross-linked PE (PEX) is not reprocessable in the melt-state and thus cannot be recycled for high-value use. Here, we have transformed thermoplastic PE into PE covalent adaptable networks (CANs) via reactive radical-based, melt-state processing with 1 wt% dicumyl peroxide and 5 wt% bis(2,2,6,6-tetramethyl-4-piperidyl methacrylate) disulfide (BiTEMPS methacrylate), a dynamic covalent cross-linker. The simple, catalyst-free, one-pot reactive process employing dialkylamino disulfide dynamic chemistry was used to upcycle both commodity and waste thermoplastic PE into thermally stable and reprocessable PE CANs, and the thermomechanical properties of resulting CANs are tunable without sacrificing their recyclability. Low-density PE CANs and high-density PE CANs fully recover cross-link densities and associated properties after multiple reprocessing steps, resist creep deformation at elevated temperature relative to their thermoplastic precursors, and, like PEX but unlike some PE vitrimers, exhibit no phase separation. This novel procedure opens the door to the development of CANs based on reactive processing of ethylene-based copolymers as well as cross-linked PE nanocomposites and foams. For the first time, virgin and waste polyethylene (PE) was upcycled using exclusively free-radical methods into reprocessable, thermally stable, and creep-resistant PE covalent adaptable networks capable of dialkylamino disulfide dynamic chemistry.