CAREER: Stimuli-Responsive Dynamic Macromolecular Assemblies

技术概述：本研究利用二醇和硼酸之间可逆键形成的强大特性，制备和研究自组装和响应的大分子材料。两种特殊的系统将被研究：(1)刺激响应嵌段共聚物组件和(2)动态共价自组装材料。在第一种体系中，将通过控制自由基聚合和其他有效的聚合后转化相结合，制备具有永久亲水嵌段（如聚乙二醇或聚丙烯酰胺）和含有丙烯酰胺或苯乙烯嵌段的硼酸的二嵌段共聚物。在没有二醇和硼酸部分的pHpKa处，有机硼段将脱水且不溶，导致两亲性嵌段共聚物自组装成胶束和囊泡。引入二醇会导致硼酸的pKa降低，由此产生的阴离子硼酸酯形成会引起溶解度转变和聚集体解体。研究将阐明诱导聚集体解离所需的临界二醇浓度、pH值和温度，并将深入了解控制组装和聚合的动力学。第二类材料依靠硼酸-二醇酯化反应，通过末端或悬垂二醇和硼酸官能团的（co）聚合物自组装来构建共价大分子结构。分子刷，星形和其他支链拓扑结构将通过在体态和有机或水溶液中形成硼或硼酸酯来构建。硼酸酯的水解将导致拓扑复杂的大分子可逆解离成单独的线性聚合物组分。在可逆共价体系固有的平衡条件下，引入第二含二醇的聚合物，形成更稳定的硼酸酯配合物，将导致大分子构建块的交换。有效交换所需的选择性将通过设计具有显著不同络合电位的聚合物来实现。因此，在大分子解离后，在存在竞争平衡的情况下重建将导致聚合物构建块的交换，从而产生新的材料。通过组装-拆卸来重新洗牌组件的能力也将用于诱导溶液中戏剧性的建筑重排（例如，从刷到星的过渡）。本研究旨在重新定义刺激响应聚合物的传统概念，包括改变其化学功能和整体链拓扑结构以响应环境压力的大分子结构。非技术总结：通过制备纳米级物体，使其在暴露于局部环境变化时发生破裂和重建，可以获得许多控制治疗药物递送的机制以及新的自愈和自适应材料的行为的基本见解。由于这些研究需要材料科学、化学和工程方面的各种技能，因此参与这项研究的学生和初级科学家可以获得真正的跨学科技能，从而增强必要的劳动力，以加速新的先进和特殊材料市场的发展。该研究的外展部分旨在通过与当地社区学院和独立的K-12学区建立合作关系，直接解决美国竞争力倡议的许多任务，以促进将代表性不足的少数民族学生纳入南卫理公会大学化学系的实习职位。

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 pHpKa 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.