(385t) Making Non-Recyclable Polymers Recyclable: Novel Advances in Obtaining Value, Circularity, and Sustainability from Spent Polymer Networks and Thermoplastics

Here, we will discuss recent advances from our research group regarding circularity, including chemical recycling and small-molecule recovery, from covalent adaptable networks (CANs) that can serve as recyclable replacements for non-recyclable polymer thermosets with permanent covalent cross-links. Examples will be drawn from two classes of materials.

The first class is networks from addition-type polymers, e.g., polyethylene (PE), ethylene-based copolymers (EBCs), and polymethacrylates. We will discuss the use of a dynamic covalent cross-linker which can be to develop CANs either by free-radical copolymerization at low levels with other monomers or by radical-based, melt-state reactive processing, e.g., extrusion, with thermoplastics such as PE, EBCs, or polymethacrylates with hexyl (e.g., poly(n-hexyl methacrylate) or longer side groups. Such networks can be recycled (reprocessed) multiple times with full recovery of cross-link density and associated properties. Additionally, the radical-based reactive processing approach allows for spent or waste thermoplastic PE, EBCs, etc. to be upcycled into higher value, reprocessable networks with enhanced properties.

The second class is condensation-type networks from biobased non-isocyanate polythiourethanes (NIPTUs), which can serve as benign replacements for conventional polyurethane networks made from toxic isocyanates. This is important because polyurethane is the sixth most produced polymer, with the vast majority being in network form. NIPTU CANs exhibit full recovery of cross-link density and associated properties after multiple reprocessing steps, providing an excellent route to recycle spent materials into valuable products. Additionally, NITPU CANs exhibit rapid synthesis and reprocessability due to the “click”-like nature of the chemistry. We have also demonstrated that robust properties can be obtained with NIPTU networks made from biowaste-based starting materials, e.g., cashew nutshell liquid and glycerol, the latter being a waste byproduct of biodiesel production. Additionally, we will show that end-of-life NIPTU CANs may allow for facile recovery of valuable small molecules by chemical recycling, with a specific example providing for 97% yield of recovered small molecule.