CAREER: Stimuli-Responsive Dynamic Macromolecular Assemblies

Southern Methodist University, Dallas TX

Investigators

Abstract

TECHNICAL SUMMARY: This research capitalizes on the robust nature of reversible bond formation between diols and boronic acids for the preparation and investigation of self-assembled and responsive macromolecular material. Two particular systems will be investigated: (1)stimuli-responsive block copolymer assemblies and (2)dynamic covalent self-assembled materials. In the first system, diblock copolymers with a permanently hydrophilic block (e.g.,poly(ethylene glycol) or polyacrylamide) and a responsibe boronic acid containing acrylamido or styrenic block will be prepared by combination of controlled radical polymerization and other efficient postpolymerization transformations. In the absence of diol and at pH<pKa of the boronic acid moieties, the organoboron segments will be dehydrated and insoluble, leading to self-assembly of the amphiphilic block copolymers into micelles and vesicles. Introduction of diols will lead to a reduced pKa of the boronic acids, and the resulting anionic boronate ester formation will cause a solubility transition and aggregate disassembly. Investigations will elucidate the critical diol concentration, pH, and temperature necessary to induce aggregate dissociation, and insight will be gained into the kinetics governing assembly and sissociation. The second class of materials relies on boronic acid-diol esterification to construct covalent macromolecular architectures via self-asembly of (co)polymers with either terminal or pendant diol and boronic acid functionality. Molecular brushes, stars, and other branched chain topologies will be constructed via boronic or boronate ester formation in the bulk state and in organic or aqueous solutions. Hydrolysis of the boronate esters will lead to reversible dissociation of the topologically complex macromolecules into individual linear polymer components. Under the equilibrium conditions inherent to reversible covalent systems, introduction of a second diol-containing polymer that forms a more stable boronate ester complex will lead to an exchange of macromolecular building blocks. The selectivity requred for efficient exchange will be implemented by designing polymers with dramatically different complexation potentioals. Thus, after macromolecular dissociation, reconstruction in the presence of a competing equilibrium will result in exchange of polymer building blocks to yield a new materials. The ability to reshuffle constituents through assembly-disassembly will also be employed to induce dramatic architectural rearrangements in solution (e.g., brush to star transitions). This research seeks to redefine the traditional concept of stimuli-responsive polymers to include macromolecular constructs that change both their chemical functionality and overall chain topology in response to environmental stress. NON-TECHNICAL SUMMARY: By preparing nanoscale objects that undergo rupture and reconstruction when exposed to changes in their local environment, fundamental insight can be gained into many of the mechanisms governing the controlled delivery of therapeutics and the behavior of new self-healing and adaptive materials. Because these studies require a diverse set of skills from materials science, chemistry, and engineering, students and junior scientists involved in this research are provided with a truly interdisciplinary set of skills that can enhance the workforce necessary to accelerate development of new advanced and sepciality materials market. An outreach component of the research is desinged to directly address many of the mandates of the American Competitiveness Initiative by establishing collaborations with local community colleges and independent K-12 school districts to facilitate the inclusion of underrepresented minority students for internship positions within the Department of Chemistry at Southern Methodist University.