UC Santa Barbara

Commodity thermosets are generally landfilled or incinerated due to the presence of covalent crosslinks that are effectively irreversible due to their high bond dissociation energy. This poses a pressing environmental concern. An emerging class of thermoset alternatives is covalent adaptable networks (CANs), which are crosslinked by dynamic covalent bonds that can be cleaved and regenerated upon application of an external stimulus (such as heat or light). By taking advantage of the on-demand reversibility of these CAN crosslinks, the material can undergo more environmentally sustainable end-of-life fates, such as reprocessing, upcycling, and chemical recycling. The activation and efficiency of the stimuli-responsive crosslink exchange of many CANs are contingent on externally added catalysts, which are not covalently bound to the polymer network. Catalyst loss via pathways such as leaching and degradation transforms reversible crosslinks into (effectively) irreversible linkages. This thesis tells three stories about developing polymer network platforms with catalyst-free reversible bonds. In the first section, a sulfur heteroatom capable of undergoing neighboring group participation (NGP) is embedded in the structure of a secondary alkyl halide bicyclo[3.3.1]nonane (BCN) crosslinker and used to generate ionic networks. This NGP—often called internal catalysis—affords thermally-induced solid-state bond exchange in crosslinkers that would otherwise be unreactive. Moreover, the use of externally added catalysts is avoided. This synthetically modular crosslinker platform is leveraged in the second story to incorporate other NGP heteroatoms (selenium and nitrogen) into the ionic CANs. The rate of the crosslink transalkylation exchange is controlled by the identity of the NGP atom [S, Se, or N-R] within the BCN crosslinker. The trivalent nitrogen atom in the aza-BCN crosslinkers enables further control of the anchimeric assistance efficiency by altering the steric parameters of the amine substituent. The temperature-dependent reversibility of these crosslinks allows the ionic CANs to be thermally reprocessed and chemically recycled. The final story incorporates dynamic boronic ester bonds into a vat photopolymerization 3D-printing resin to explore the application of catalyst-free reversible crosslinks in emerging manufacturing techniques. The photopolymerizable additive manufacturing resin combines dynamic boronic ester linkages (to provide reactive sites for post-printing modification) and a non-exchangeable crosslink diluent (to give the growing 3D-printed part dimensional stability). By capitalizing on the exchangeable crosslink, the 3D-printed constructs can undergo a series of post-manufacture modifications, such as tunable swelling and welding of separately printed parts to generate more structurally elaborate materials. As a result, this boronic ester-containing resin offers pathways to extend the lifecycle and utility of the parent 3D-printed polymer network.