UC Santa Barbara

Radical ring-opening polymerization (rROP) of α-lipoic acid (LA) has facilitated the synthetic accessibility of functional comonomers to impart degradability into polymers. LA is a commercially available and renewable monomer with a 1,2-dithiolane ring that can undergo rROP in the presence of external stimuli (heat, light, radicals). This thesis aims to highlight the novel approaches to designing degradable and recyclable materials through the rROP with LA.Pressure-sensitive adhesives (PSAs) based on poly(acrylate) chemistry are common in a wide variety of applications, but the absence of backbone degradability causes issues with recycling and sustainability. Here, we report a strategy to create degradable poly(acrylate) PSAs using simple, scalable, and functional 1,2-dithiolanes as drop-in replacements for traditional acrylate comonomers. Our key building block is α-lipoic acid, a natural, biocompatible, and commercially available antioxidant found in various consumer supplements. α-Lipoic acid and its derivative ethyl lipoate efficiently copolymerize with n-butyl acrylate under conventional free-radical conditions leading to high–molecular-weight copolymers (Mn > 100 kg mol–1) containing a tunable concentration of degradable disulfide bonds along the backbone. The thermal and viscoelastic properties of these materials are practically indistinguishable from non-degradable poly(acrylate) analogues, but a significant reduction in molecular weight is realized upon exposure to reducing agents such as tris(2-carboxyethyl)phosphine (e.g. Mn = 198 kg mol–1 → 2.6 kg mol–1). By virtue of the thiol chain ends produced after disulfide cleavage, degraded oligomers can be further cycled between high and low molecular weights through oxidative repolymerization and reductive degradation. Transforming otherwise persistent poly(acrylates) into recyclable materials using simple and versatile chemistry could play a pivotal role in improving the sustainability of contemporary adhesives. Here, we present the synthesis and characterization of statistical and block copolymers containing α-lipoic acid (LA) using reversible addition–fragmentation transfer (RAFT) polymerization. LA, a readily available nutritional supplement, undergoes efficient radical ring-opening copolymerization with vinyl monomers in a controlled manner with predictable molecular weights and low molar-mass dispersities. Because lipoic-acid diads present in the resulting copolymers include disulfide bonds, these copolymers efficiently and rapidly degrade when exposed to mild reducing agents such as tris(2-carboxyethyl)phosphine (Mn = 56 kg mol–1 → 3.6 kg mol–1). This scalable and versatile polymerization method affords a facile way to synthesize degradable polymers with controlled architectures, molecular weights, and molar-mass dispersities from α-lipoic acid, a commercially available and renewable monomer. Here, we introduce a versatile synthetic strategy to form selectively degradable bottlebrush networks from well-defined poly(4-methylcaprolactone) (P4MCL) macromonomers that allow us to synthesize bottlebrush polymers with a range of molecular weights (Mn = 45 kg mol–1 – 2200 kg mol–1) to achieve a variety of physical properties. α-lipoic acid (LA) can be installed on the P4MCL hydroxy chain end to act as a crosslinker due to the facile ring-opening of the dithiolane ring under UV light. The resulting redox-active disulfide crosslinks enables selective electrochemical or chemical degradation of the crosslinks while the P4MCL sidechains and disulfides can be simultaneously degraded using more exhaustive reducing conditions. This platform presents a versatile strategy to synthesize highly tunable and discrete bottlebrush elastomers with the potential for selective degradation of the crosslinks or sidechains. The promise of ABC triblock terpolymers for improving the mechanical properties of thermoplastic elastomers is demonstrated by comparison with symmetric ABA/CBC analogs having similar molecular weights and volume fraction of B and A/C domains. It is shown that the ABC architecture enhances elasticity (up to 98% recovery over 10 cycles) in part through essentially full chain bridging between discrete hard domains leading to minimization of mechanically unproductive loops. In addition, the unique phase space of ABC triblocks also enables the fraction of hard-block domains to be higher (fhard ≈ 0.4) while maintaining elasticity, which is traditionally only possible with non-linear architectures or highly asymmetric ABA triblock copolymers. These advantages of ABC triblock terpolymers provide a tunable platform to create materials with practical applications while improving our fundamental understanding of chain conformation and structure–property relationships in block copolymers.