Thiol-Thioester Dynamic Covalent Chemistry in Polymer Networks

日常使用的合成聚合物有两大类。热塑性塑料（通常简称为“塑料”）用途广泛，可以熔化、溶解、重塑和回收。相比之下，热固性材料通常是较硬的材料，具有溶剂性和耐热性，但在初始形成后通常不能再加工或回收。科罗拉多大学的Christopher Bowman教授正在探索使用动态共价化学方法在物质的这两种状态之间转换。在这里实现的动态共价化学中，在施加期望的刺激（如暴露在光或热下）后，化学键结构被激活-以一种方式将一种类型的聚合物转化为另一种类型的聚合物结构内的化学键反复断裂和重组。本项目研究聚合物结构与动态共价化学内在方面之间的基本关系，包括活性化学官能团、用于促进反应的催化剂、反应动力学和动态共价键的浓度。该项目研究合成和聚合物化学的基本现象，以及应用科学的明显进步，同时为研究生和本科生提供深刻的教育经验。此外，该项目以化学合成、材料科学和聚合物化学的独特组合培养了包括大量本科生和研究生在内的多元化人才。在美国国家科学基金会化学部大分子、超分子和纳米化学项目的支持下，Bowman的研究团队致力于在交联聚合物网络中实现硫-硫酯交换反应，作为光活化和光失活的动态共价化学反应。该项目的重点是了解随着硫酯、催化剂和聚合物分子结构的变化而产生的化学结构-性质关系及其对共价键交换速率的影响。对巯基-硫酯交换反应及其在非极性、相对不移动环境中的催化动力学的基本了解，使研究人员能够微调所需的键交换过程和相应的动态聚合物性质。各种外源传递的刺激（即，不同波长的光，添加或去除热量）的组合被用来控制网络内的化合物的活化，催化悬垂巯基和硫酯交联之间的动态共价化学反应，并阻止与正交刺激的交换反应。具体来说，开发了用于启动和停止动态共价化学的光和热刺激的双手柄。利用巯基/硫酯交换反应，Bowman研究小组的目标是创造共价交联聚合物，这种聚合物能够从用户导向的、可逆的从刚性热固性转变为可塑性热塑性，应用于光学材料、增材制造和粘合剂。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

There are two broad categories of synthetic polymers familiar to everyday use. Thermoplastics (typically abbreviated simply as "plastics") are versatile and can be melted, dissolved, remolded and recycled. In contrast, thermosets are typically harder materials that are solvent and heat resistant and but cannot generally be reprocessed or recycled after their initial formation. Prof. Christopher Bowman at the University of Colorado is exploring the use of dynamic covalent chemistry approaches to transition between these two states of matter. In dynamic covalent chemistry as implemented here, upon application of a desired stimulus such as exposure to light or heat, chemical bond structures become activated - repeatedly breaking and reforming the bonds within the polymer structure in a manner that converts one type of polymer to the other. This project investigates the fundamental relationships between the polymer structure and the inherent aspects of the dynamic covalent chemistry including the reactive chemical functional group, the catalysts used to facilitate the reaction, the kinetics of the reaction, and the concentration of the dynamic covalent bond. This project investigates fundamental phenomena of synthetic and polymer chemistry as well as demonstrable advancement of applied science all while providing profound educational experiences for graduate and undergraduate students alike. Further, the project trains diverse personnel, comprising a large number of undergraduate and graduate students, in a unique combination of chemical synthesis, materials science, and polymer chemistry.Supported by the Macromolecular, Supramolecular and Nanochemistry Program of the NSF Chemistry Division, the Bowman research team aims to implement the thiol-thioester exchange reaction as a photoactivatable and photodeactivatable dynamic covalent chemistry in crosslinked polymer networks. The project focuses on understanding chemical structure-property relationships that arise with changes to the thioester, catalyst and polymer molecular structures and their impact on the covalent bond exchange rate. Fundamental understanding of the thiol-thioester exchange reaction and the kinetics of its catalysis in a non-polar, relatively immobile environment allows researchers to fine-tune the desired bond exchange process and the corresponding dynamic polymer properties. Combinations of various exogenously delivered stimuli (i.e., different wavelengths of light, addition or removal of heat) are used to control the activation of compounds within the network that catalyze the dynamic covalent chemistry between pendant thiol groups and thioester crosslinks and to arrest the exchange reaction with orthogonal stimuli. Specifically, dual handles of light and heat-based stimuli for starting and stopping the dynamic covalent chemistry are developed. Taking advantage of the thiol/thioester exchange reaction the Bowman research team aims to create covalently crosslinked polymers that are capable of user-directed, reversible transitions from rigid thermoset to malleable thermoplastic that has applications in optical materials, additive manufacturing, and adhesives.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.