UC Santa Barbara

ABSTRACTManipulating Chemistry at the Molecular Level for the Development of Stimuli Responsive Materials by Meghan Frances Nichol The ever-growing field of materials science has utilized its interdisciplinary nature to allow the advancements in research ranging from small molecule uses to polymer material discovery. Specifically, the development of stimuli responsive materials has become of interest while showcasing the importance of small molecule functionality and reactivity. Stimuli responsive materials, or materials that respond to their environment, was derived through studies to understand structure-activity relationships within materials science. Through the use of chemical and physical stimuli, this field has seen growth through the discovery of photoswitches, micelles, self-healing materials, and more. This thesis explores the utilization of acid and heat as tunable stimuli to enable new control and selectivity by which small molecule and materials can be manipulated. Acid catalysis is a well-studied field within organic chemistry where we see the utilization of the acid for altering reaction kinetics and product formation. Within this area of research, chiral acid catalyst not only take part in affecting kinetics and product formation, but also enable stereochemistry to be controlled. Stereochemistry is vital for organic syntheses due to the relationship between selectivity and reactivity where enantiomers can often have drastically different properties. Through the development of the asymmetric aza-Piancatelli reaction, I developed a new approach using chiral PCCP Brønsted acid catalyst leading to enantioselectivity above 85% ee. Building on my understanding of the aza-Piancatelli mechanism in small molecules we translated this work into a novel dual-responsive trigger for self-immolative polymers (SIPs). Utilizing the chemistry developed in the aza-Piancatelli rearrangement, we were able to manipulate the reaction’s use of acid catalysis and push towards the development of a new temporally controlled trigger unit for SIPs. This class of polymers is unique due to its characteristic ability to undergo end-to-end depolymerization in the presence of a given triggering stimuli. Previously, SIPs were known to undergo this depolymerization spontaneously when triggered; however, through the use of orthogonal chemistry—heat and acid—a trigger unit was developed that can now offer ON/OFF depolymerization control. The temporal control offered through thermally triggered materials was not only advantageous for work in SIPs, utilizing temperature as a stimulus encouraged the investigation into controlling the reactivity of isocyanates in polyurethane formation. Isocyanates are widely used in polyurethanes found in paints, sealants, adhesives, and polymer binders for energetic materials. Although well studied and used since the 1940’s, the processes used for curing of polymer binders is continuing to advance. With goals of pushing materials towards 3D printing, we are developing new formulations utilizing blocked isocyanate chemistry. This class of materials is produced via the addition of a blocking—or protecting—group to the isocyanate functionality to extend material pot life as well as add a handle of control over the isocyanate’s reactiveness. While still on going, this thesis will show the progress of these new materials. Access to new materials has become possible via detailed mechanistic studies of chemistry starting at the small molecule. This thesis presents the development of stimuli responsive materials through the utilization of chemical and physical stimuli for more control over reaction kinetics and selectivity.