Bhusal Shusil, Oh Changjun, Kang Youngjong, Varshney Vikas, Ren Yixin, Nepal Dhriti, Roy Ajit, Kedziora Gary
Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright Patterson Air Force Base, Dayton, Ohio 45433, United States.
Universal Technology Corporation, 1270 N Fairfield Rd., Beavercreek, Ohio 45432, United States.
J Phys Chem B. 2021 Mar 11;125(9):2411-2424. doi: 10.1021/acs.jpcb.0c10403. Epub 2021 Feb 26.
Recently, thermoset vitrimer polymers have shown significant promise for structural applications because of their ability to be reshaped and remolded due to their covalent adaptive network (CAN). In these vitrimers, the transesterification reaction is responsible for the CAN, where the efficiency of the reaction is controlled either by organic or by organometallic catalysts. Understanding the mechanism of the transesterification reaction in the bulk phase using direct experimental techniques is extremely difficult due to the highly cross-linked complex structure of thermosetting vitrimers. Therefore, we use solution-phase experiments to investigate the catalytic efficiency and to guide density functional theory (DFT) simulations of the transesterification reaction mechanism with catalysts triazabicyclodecene (TBD), zinc acetate (Zn(OAc)), 1-methylimidazole (1-MI), and dibutyltin oxide (DBTO). The estimated catalytic efficiency from the detailed DFT reaction path calculations follows the order TBD ≳ DBTO ≳ Zn(OAc) > 1-MI, which agrees with the experimental results. In addition to reaction path modeling, the mechanism and the relative rates of the transesterification reaction are analyzed with the assistance of Fukui indices as a measure of electrophilicity and nucleophilicity of atomic sites and with partial charges. It was found that the sum of the nucleophilicity index of the base and the electrophilicity index of the acid of the bifunctional catalysts correlates with the S2 transition state and tetrahedral intermediate energies, which are related to the barrier of the rate-limiting step. This correlation provides a hypothesis for computational prescreening of potentially better catalysts that have an index in a range of values. These results provide a basis for understanding an important part of the mechanism of transesterification in vitrimer systems and may assist with designing new catalysts.
最近，热固性玻璃态弹性体聚合物因其共价自适应网络（CAN）赋予的可重塑和再成型能力，在结构应用方面展现出巨大潜力。在这些玻璃态弹性体中，酯交换反应构成了CAN，该反应的效率由有机催化剂或有机金属催化剂控制。由于热固性玻璃态弹性体高度交联的复杂结构，利用直接实验技术理解本体相中酯交换反应的机理极具挑战性。因此，我们采用溶液相实验来研究催化效率，并指导对催化剂三氮杂双环癸烯（TBD）、醋酸锌（Zn(OAc)）、1-甲基咪唑（1-MI）和二丁基氧化锡（DBTO）的酯交换反应机理进行密度泛函理论（DFT）模拟。从详细的DFT反应路径计算得出的催化效率估算结果顺序为TBD ≳ DBTO ≳ Zn(OAc) > 1-MI，这与实验结果一致。除了反应路径建模，还借助福井指数作为原子位点亲电性和亲核性的度量以及部分电荷，分析了酯交换反应的机理和相对速率。研究发现双功能催化剂的碱亲核性指数与酸亲电性指数之和与S2过渡态和四面体中间体能量相关，而这些能量与限速步骤的势垒有关。这种相关性为计算预筛选具有特定指数范围内值的潜在更好催化剂提供了一种假设。这些结果为理解玻璃态弹性体体系中酯交换反应机理的重要部分提供了基础，并可能有助于设计新型催化剂。
