CAS: Dithioacetal as a Highly Tunable and Versatile Bond for the Design of Chemically Recyclable and Dynamic Covalent Polymers

在化学系大分子、超分子和纳米化学项目的支持下，内华达大学雷诺分校的杨颖正在使用开环和闭环聚合策略来制备可回收的多二硫代缩二醛。多硫缩醛是一种长链大分子，其中的主要聚合物主链除了在商品塑料中常见的碳之外，还含有化学元素硫。这类聚合物化学稳定，但具有高度可修饰的主链和侧基。因此，多硫代缩二醛有望成为性能可调的可回收聚合物的候选者。与该项目相关的研究将深入了解二硫缩醛聚合的聚合机理，研究聚合的热力学和动力学如何受到丰富的结构变化的影响，并探索使用这些化学方法来实现受刺激控制的热塑性和热固性多硫缩醛的动态结构转变。这些基础研究的结果有可能导致新型可回收聚合物的开发，这些聚合物可以在高温下进行再加工，并按需解聚回到环状大环中。从可持续发展的角度来看，与该项目相关的设计原则提供了一个非常有前景和可行的解决方案，以应对目前与广泛使用不可降解乙烯基塑料相关的挑战。因此，开发的方法可以用来制造易于降解和可持续的塑料，这些塑料符合循环经济的生命周期。研究小组将积极参与通过出版物传播成果，并在会议和当地活动中介绍工作。此外，将通过与拥有大量西班牙裔学生的Hug高中建立密切关系来开展社区外联活动。该项目还将通过将聚合物化学模块整合到大型普通化学课程中，促进内华达·里诺大学的聚合物科学研究和教学。这项研究将重点研究能够进行可逆熵驱动的开环聚合(ED-ROP)和开环解聚(RCD)的多硫缩醛，作为开发可回收聚合物的通用平台，以及动态共价体系。酸催化的二硫代缩醛交换反应将使动力学成为可能。该项目以可喜的初步成果为基础，旨在通过三个相互关联的目标详细了解ED-ROP对二硫代缩醛大环的化学和热力学：(1)了解不同引发剂/催化剂体系的聚合机理并获得受控的“活性”阳离子聚合；(2)改变聚二硫代缩醛主链和侧基/链的结构并研究它们对单体-聚合物平衡的影响；以及(3)开发可再加工和可解聚的可通过光和机械力控制降解的聚二硫代缩醛热固性材料。这些研究活动具有很强的可持续性，并有可能产生与使用ED-ROP开发固有的实用、可回收和功能聚合物相关的基础化学知识。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

With the support of the Macromolecular, Supramolecular and Nanochemistry program in the Division of Chemistry, Ying Yang of the University of Nevada at Reno is using ring-opening and -closing polymerization strategies to prepare recyclable polydithioacetals. Polydithioacetals are long chain macromolecules in which the main polymer backbone contains chemical element sulfur in addition to carbon commonly seen in commodity plastics. This class of polymers is chemically stable and yet has highly modifiable backbones and side groups. As a result, polydithioacetals are promising candidates for recyclable polymers with tunable properties. Research associated with this project will develop an in-depth understanding of the polymerization mechanisms of dithioacetal polymerization, investigate how thermodynamics and kinetics of polymerization are affected by the rich structural variations, and explore using these chemistries to enable dynamic architectural transformations of thermoplastic and thermoset polydithioacetals controlled by stimuli. The results from these fundamental studies have the potential to lead to the development of novel recyclable polymers which can be reprocessed at high temperatures and depolymerized on-demand back into cyclic macrocycles. From the sustainability point of view, the design principle associated with this project provides a very promising and viable solution to the current challenges associated with the widespread usage of non-degradable vinyl plastics. Hence, the developed methodology could be utilized to build readily degradable and sustainable plastics that are amenable to a circular economy life cycle. The research team will be actively involved in disseminating the results via publications and presenting the work at conferences and local events. Furthermore, community outreach will be conducted by establishing close relationships with Hug High School which has a large Hispanic student population. The project will also facilitate the research and education of polymer science at the University of Nevada Reno by integrating polymer chemistry modules into the large general chemistry classes.This research will focus on studying polydithioacetals that are capable of reversible entropy-driven ring-opening polymerization (ED-ROP) and ring-closing depolymerization (RCD) as a versatile platform for the development of recyclable polymers, as well as dynamic covalent systems. The dynamics will be enabled by acid-catalyzed dithioacetal exchange reactions. Building on promising preliminary results, the project aims to develop a detailed understanding of the chemistry and thermodynamics of ED-ROP for dithioacetal macrocycles through three interconnected objectives: (1) understand the polymerization mechanism with different initiator/catalyst systems and obtain controlled “living” cationic polymerization, (2) vary the structures of polydithioacetal backbones and side groups/chains and study their effects on the monomer-polymer equilibrium, and (3) develop reprocessable and depolymerizable polydithioacetal thermosets with controlled degradation by light and mechanical force. These research activities have a strong sustainability component and the potential to generate fundamental chemistry knowledge of relevance to using ED-ROP for the development of inherently practical, recyclable and functional polymers.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.