Carbodiimide Ring-opening Metathesis Polymerization: Precision Synthesis of Nitrogen-Rich Polymer Backbones

在化学系大分子、超分子和纳米化学项目的支持下，位于查佩尔山的北卡罗来纳州大学的Aleksandr Zhukhovitskiy正在开发用于合成富氮聚合物骨架的开环聚合反应。 聚合物-塑料和橡胶材料的大分子成分-在我们的日常生活中是必不可少的，无处不在：从食品包装到服装再到汽车零件。 目前生产的绝大多数塑料主要沿聚合物主链沿着含有碳原子。 与此同时，进一步开发与含有富氮聚合物主链的系统相关的化学是迫切需要的。这改善了对它们的组装、折叠、热力学和介电性质以及催化活性的控制。 这些概念在蛋白质中得到了很好的说明，蛋白质是天然存在的聚合物。 然而，将氮掺入聚合物主链中难以合成实现，并且大多数当前方法是非选择性的并且通常缺乏结构精确性。 在本研究中，基于铱金属的高活性催化剂将被用于各种聚酰亚胺的开环聚合，以制备具有丰富且精确定位的氮原子沿着聚合物主链的聚合物。 重点将放在详细的动力学和催化循环，单体设计和控制聚合物链的长度机械的理解。 补充计算方法也将被用来指导实验设计和了解这些反应如何工作，以及如何改善它们。 这项研究提供了独特的机会，教育学生在机械分析，物理有机化学，有机金属化学和高分子化学。 该研究还将改变学术和工业相关的富氮聚合物主链的设计和合成，为更深入地了解这些聚合物的结构-性能关系打开大门。 此外，该团队将继续扩大其以聚合物化学为重点的教育计划，该计划针对K-12学生和科学教师，重点关注STEM领域资源不足和代表性不足的人才库。 该计划包括在查佩尔山的K-12学校进行虚拟和面对面的科学演示，并将扩大到包括北卡罗来纳州的达勒姆。 这些努力将有助于培养未来几代学生对高分子科学研究的欣赏和认同。本研究将重点关注胍铱配合物催化的碳二亚胺开环易位聚合（CDI ROMP），用于精确合成富氮聚合物主链。 第一个目标将侧重于通过实验和理论工具相结合的反应机制的调查。以下几个方面将进行探讨：验证链增长，类型的复分解过程中，能量学的crossimide-iridium协调，营业额限制步骤的能量障碍的性质，以及相应的过渡态的几何形状。在第二个目标中，将开发受控的和活性的CDI ROMP，并用于制备嵌段共聚物。 计算和实验研究将阐明铱催化剂中辅助配体结构对背咬/链转移过程的影响。 最后一个目标将集中在精确的主链聚酰亚胺的衍生化，以形成其他类别的富氮聚合物主链，包括聚脲，聚硫脲，聚胍，聚脒和聚脒基化合物的库。 本研究中的插入/消除机制与传统的通过环加成/消除进行的C-C键复分解有根本不同。 因此，所获得的机理见解将对其他类型的复分解转化的未来发展具有指导意义，例如二氮烯复分解和闭环碳二亚胺复分解。该奖项反映了NSF的法定使命，并被认为值得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来支持。

With the support of the Macromolecular, Supramolecular and Nanochemistry program in the Division of Chemistry, Aleksandr Zhukhovitskiy of the University of North Carolina at Chapel Hill is developing ring-opening polymerization reactions for the synthesis of nitrogen-rich polymer backbones. Polymers—macromolecular constituents of plastics and rubber materials— are essential and ubiquitous in our daily lives: from food packaging to clothing to vehicle parts. The vast majority of plastics that are currently being produced contain predominantly carbon atoms along the main polymer backbone. Meanwhile, further developing the chemistry associated with systems containing nitrogen-rich polymer backbones is critically needed. This improved control over their assembly, folding, thermodynamic and dielectric properties, as well as catalytic activity. These concepts are illustrated well in proteins, which are naturally occurring polymers. However, incorporation of nitrogen into the polymer backbone is difficult to achieve synthetically, and the majority of current methodologies are non-selective and generally lack structural precision. In this research, highly active catalysts based on iridium metal will be used in ring-opening polymerization of a variety of carbodiimides to prepare polymers with abundant and precisely positioned nitrogen atoms along the polymer backbone. Strong emphasis will be placed on detailed kinetic and mechanistic understanding of the catalytic cycle, monomer design, and control over the length of polymer chains. Complementary computational methods will also be utilized to guide experimental design and understand how these reactions work and how to improve them. This research provides unique opportunities to educate students in mechanistic analysis, physical organic chemistry, organometallic chemistry and polymer chemistry. The research will also transform the design and synthesis of academically and industrially relevant nitrogen-rich polymer backbones, opening the door to a deeper understanding of the structure-property relationships in these polymers. In addition, the team will continue to expand its educational polymer-chemistry focused program that targets K-12 students and science teachers with a focus on talent pools that remain under-resourced and underrepresented in the STEM fields. This program includes virtual and in-person science demonstrations at K-12 schools in Chapel Hill and will expand to include Durham, NC. These efforts will be instrumental to nurturing within future generations of students an appreciation for and identification with polymer science research.This research will focus on carbodiimide ring-opening metathesis polymerization (CDI ROMP) catalyzed by iridium guanidinate complexes for the precise synthesis of nitrogen-rich polymer backbones. The first objective will focus on the investigation of the reaction mechanism through a combination of experimental and theoretical tools. The following aspects will be explored: verification of chain growth, the type of metathesis process, energetics of the carbodiimide-iridium coordination, the nature of energy barrier of turnover limiting step, and the geometry of the corresponding transition state. Controlled and living CDI ROMP will be developed in the second objective and utilized to prepare block copolymers. Computational and experimental studies will shed light on the effect of ancillary cyclopentadienyl ligand structure in the iridium catalyst on the backbiting/chain transfer process. The last objective will focus on the derivatization of precise main-chain polycarbodiimides to form libraries of other classes of nitrogen-rich polymer backbones, including polyureas, polythioureas, polyguanidines, polyamidines and polyamidinates. The mechanism of insertion/elimination in this research is fundamentally different from traditional metathesis of C-C bonds that proceeds via cyclo-addition/elimination. As such, the gained mechanistic insights will be instructive in the future development of other types of metathesis transformations, such as diazene metathesis and ring-closing carbodiimide metathesis.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.