Design of Chemically Recyclable Thermosets: Strategies with Commodity and Synthesized Polymers

Thermosets are a class of polymeric materials widely used in applications requiring high strength and thermal resistance. These superior properties arise from their irreversible covalent cross-linking, which also makes thermosets non-meltable and insoluble, substantially complicating their recycling. Consequently, developing new strategies for creating chemically recyclable thermosets has become a focus of a significant research effort. This thesis explores two approaches to address this challenge: modifying commodity polymers and designing specialized, synthesized polymers. Since the recyclability challenge for thermosets stems from the irreversible covalent bonds, introducing cleavable cross-linking offers a promising solution. This strategy allows even commodity polymers, which were not originally designed for chemical recycling, to gain the ability to recover their polymeric starting materials by incorporating cleavable cross-linkers. We introduce a novel cleavable linkage, beta-(1,3-dioxane) ester (DXE), specifically designed for polymers containing 1,3-diol moieties. For example, we demonstrate that poly(vinyl alcohol) (PVA) can be cross-linked to form high-performance, yet chemically recyclable, thermosets. The second approach focuses on using monomers that are inherently chemically recyclable. While this method still involves irreversible covalent linkers, it enables the selective recovery of the non-cross-linked monomers. Since cross-linked components typically constitute only a small portion of the material, even if discarded, the reduced recycling steps, along with savings in energy and chemical consumption, present a trade-off worth considering. We first describe the synthesis of a series of polyesters and polycarbonates, followed by evaluating their thermal properties and recyclability through ring-closing depolymerization (RCDP). Additionally, we demonstrate the feasibility of copolymerizing allyl-functionalized cyclic carbonates with alkyl cyclic carbonates and cross-linking them via a thiol-ene reaction. This approach results in chemically recyclable thermosets with the potential for direct monomer recovery, highlighting a promising direction for advancing a circular polymer economy.