UC Santa Barbara

Natural network polymers are ubiquitous, leveraged for a diverse range of functions, and controlled by modular chemical motifs at the molecular level. Synthetically, there is a desire to borrow many of nature’s methods to build network materials with modular approaches for screening and engineering materials properties. Yet in spite of the range of chemical transformations available, most network development follows the classic approach of network formulation, formation, and examination. This conventional methodology often entails challenges in precise control of network structures, as well as limitations in functional group compatibility with network polymerization approaches. Consequentially, there is a need for modular, scalable synthetic strategies which can be performed as post polymerization modifications of network pendant motifs for facile derivatization and screening of materials properties. To address this, we can turn to active ester chemistry as a versatile, scalable platform for control and manipulation of network functionalities using mild conditions and simple wash protocols. Through integration of active ester containing monomer units into widely used free radical network polymerizations, this thesis demonstrates the utility of this straightforward approach for materials screening and design. Key discussions highlight the ability to install biomimetic metal-ligand crosslinking motifs; modulate an array of properties including mechanical, magnetic, and fluorescent behavior; and lastly leverage simple derivatization strategies to interrogate the impact of chemical substituents on nanoscale water transport behavior. To begin we can take direct structural inspiration from well characterized iron-catechol crosslinks of marine mussels and adapt these natural motifs to synthetic networks for tuning and patterning mechanical properties. Through utilization of a pentafluorophenyl acrylate as an active ester monomer unit, we can directly install catechol containing dopamine into the network, crucially without the need for protection or deprotection of phenolic moieties and avoiding their oxidation during substitution. Through this synthetic approach, large scale films could be prepared with an array of catechol and covalent crosslinking contents, followed by complexation and crosslinking of catechol groups with Fe3+ salts to achieve increases in elastic moduli of nearly two orders of magnitude. Moreover, by conceptually mirroring the mussels’ core-shell iron localization in the byssal thread, we can design diffusion controlled gradients in ion incorporation in catechol functionalized networks to prepare materials with patterned ion installation and heterogeneous, composite-like mechanical properties. Results demonstrated sub-millimeter scale resolution control and appreciable changes in mechanical behavior. Building from these approaches, we can turn to other biological inspirations for preparation of oxidation resistant materials which provide even greater degrees of tunability through increased metal chelation scope. While literature examples abound of potential ligands of interest, hydroxypyridinone (HOPO) motifs, adapted from the L-mimosine amino acid used in plants, present an enticing moiety to work with. Structurally, these ligands are similar to that of catechol moieties, but they have added benefits of increased oxidative stability and wider metal ion chelation scope through their altered ring structure. Present work builds on recent literature methods to prepare an active ester compatible, amine terminated HOPO ligand in a one pot approach, without the need for multiple protection and deprotection steps. Leveraging the active ester network platform, ligands could be installed and complexed with an array of metal salts, allowing for comparison mechanical reinforcement differences between catechol and HOPO functionalized materials after complexation. By building on prior patterning methods and utilizing the improved properties of the HOPO motif, materials with heterogeneous mechanical, magnetic, and fluorescent responses could be prepared, opening new doors for material preparation for soft robotic and encryption applications. Lastly, this topology consistent approach for network substituent modification was leveraged in hydrogel systems for measuring the impact of chemical identity on nanoscale water transport behavior. To conduct such measurements, Overhauser Dynamic Nuclear Polarization (ODNP) was leveraged as a combined electron paramagnetic and nuclear magnetic resonance (EPR and NMR) technique to measure interactions between passing water molecules and a nitroxide radical spin probe tethered to the network. Critically, ODNP measurements place significant restriction on sample geometry and must be conducted in narrow bore (0.6mm ID) capillaries. To address this challenge, prior work on active ester network polymerizations was extended to a biphasic flow synthesis, enabling the formation of microparticle networks with varied crosslinker and active ester content to modulate final water content and network substituents. Moreover, via this approach, large batches of parent materials could be prepared, split, and derivatized with both nitroxide spin labels and hydrophobic or hydrophilic sidechains, allowing for measurement of final hydrogel materials with consistent network topology and repeat unit sequence. Present results provide fundamental insight into nanoscale water behavior at the polymer network surface and highlight the impact of chemical identity on nanoscale water diffusivity, providing guiding design rules for next generation membrane materials.