UCLA

With a rapidly growing global population, increased urbanization, and the desertification of arable land, solutions towards creating more sustainable and productive agricultural practices are necessary for food security. Polymers are tunable technologies that can enhance the utilization efficiency of biological or chemical active agents used in agriculture. Through encapsulation, adsorption, or conjugation, polymer carriers can modify the solubility, stability, and specificity of active ingredients. Ultimately, these benefits prevent loss of vulnerable cargo and mitigate deleterious effects on the environment and off-target species. This dissertation reports efforts towards creating effective polymer scaffolds for the stabilization of animal feed enzymes, controlled delivery of water to crops, and the reduced soil mobility of a small molecule herbicide. The first project focuses on the thermostabilization of animal feed enzymes using a trehalose hydrogel (Chapter 2). The hydrogel was prepared through a two-step synthesis where trehalose, a natural saccharide, was mono- and di-substituted with styrenyl moieties that could then undergo a redox-initiated radical polymerization. After investigating various purification strategies and solvents, we were able to produce the gel on a multi-gram scale at high yields and with green or amber solvents. The top three animal feed enzymes used in the livestock industry phytase, -glucanase, and xylanase, were then encapsulated within the gel and heated to 90 �C to simulate industrial processing. Compared to controls, the activity of all three enzymes was vastly improved and almost quantitative. Finally, we demonstrated release of phytase within four hours, demonstrating the hydrogel’s potential to effectively enhance the use of enzymes in the livestock industry. Next, the same trehalose hydrogel was applied for another important sector of agriculture by evaluating its soil conditioning ability for tomatoes subjected to drought (Chapter 3). Tomato plants were transplanted into individual pots with soil that contained trehalose hydrogel, a polyacrylate-based hydrogel, or no hydrogel and then subjected to various watering schedules, with regular watering, droughts or recoveries. The health of tomato plants was monitored using Soil Plant Analysis Development (SPAD), leaf water potential (Ψleaf�), stomatal conductance (gs), and relative growth rate (RGR). While the polyacrylate-based hydrogel did improve tomato plant health in drought conditions, as indicated by improved stomatal conductance and relative growth rates, the trehalose hydrogel did not affect the plants’ health. In the next section, small molecule conjugation strategies of a selective, but unstable herbicide, mesotrione, were explored (Chapter 4). Five different conjugates were created and three demonstrated hydrolytic reversibility in environmentally-relevant conditions. These three candidates were linked through a phenyl ester or thioether linkages and proved to be promising proherbicide candidates. All three conjugates showed reduced mobility of mesotrione in acidic soil. Finally, the phenyl ester conjugate exhibited good herbicidal activity against a common weed, Chenopodium album. Lastly, the conjugation strategies developed for mesotrione were applied to create polymeric propesticides (Chapter 5). Using enamine, thioether, and ester linkages, a variety of polymeric carriers were created. While the enamine linkages were easily formed, only a small percentage of mesotrione was released from the conjugates. However, preliminary weed efficacy studies demonstrated that the conjugate may still be herbicidally-active. Past efforts to create a thioether conjugate polymer were unsuccessful, but a small molecule thioether mesotrione derivative with a clickable-functional group was synthesized and could potentially serve as a handle to post-modify polymers with alkyne pendants. Finally, a hydrogel was formed through ester conjugates and with mesotrione acting as a cross-linker. This hydrogel was degradable and sustainably released mesotrione in acidic conditions. Additionally, the gel reduced mesotrione’s soil mobility, demonstrating promise as a polymer proherbicide.