Department of Chemistry, Center of Excellence in Polymer Chemistry (CEPC), University of Houston, 3589 Cullen Boulevard, Houston, Texas 77204, United States.
Williams A. Brookshire Department of Chemical and Biomolecular Engineering, University of Houston, 4226 Martin Luther King Blvd, Houston, Texas 77204, United States.
J Am Chem Soc. 2023 May 3;145(17):9755-9770. doi: 10.1021/jacs.3c01513. Epub 2023 Apr 18.
This work elucidates a long-standing unexplained paradox commonly observed within the polymerization of α-olefin using palladium (Pd)(II)-diimine catalysts, in which isomerization and living polymerization of α-olefins are both observed. With a classical mechanistic understanding of these complexes, this behavior is often dismissed and interpreted as experimental error. Herein, we present a comprehensive mechanistic investigation into this phenomenon that supports the existence of a novel mechanistic pathway for Pd(II)-diimine complexes. Part one of the mechanistic study lays the foundation of the proposed mechanism, in which neutral Pd(II)-diimine complexes were found to exhibit a moderate to good catalytic activity for olefin isomerization of α-olefins despite the established notion that catalyst activation is required. Extensive experimental and computational studies reveal the possibility of a partial dissociation of the diimine ligand, which frees up one coordination site and enables coordination-insertion. This finding is significant as the coexistence of two reactive coordination sites at the palladium center becomes a valid proposal for the activated cationic Pd(II)-diimine complexes. In part two, we examined and validated the simultaneously observed α-olefin isomerization and living polymerization using the cationic Pd(II)-diimine catalyst, which supports the presence of two independent reaction pathways of isomerization and polymerization, respectively. Moreover, the addition of a strong Lewis acid, such as AlCl, accelerates the ligand dissociation and the consequential isomerization as it weakens the palladium-nitrogen bond through competitive binding. In part three, Lewis acid-triggered olefin isomerization-polymerization is employed to prepare living olefinic block copolymers and further synthesize novel polyolefin-polar block copolymers with unique architectures, distinct levels of branching, crystallinity, and polar functionality in a one-pot manner.
本工作阐明了使用钯（Pd）（II）-二亚胺催化剂聚合α-烯烃时普遍存在的一个长期未解之谜，其中观察到α-烯烃的异构化和活性聚合。基于对这些配合物的经典机理理解，这种行为通常被忽略并解释为实验误差。在此，我们对这一现象进行了全面的机理研究，支持了 Pd（II）-二亚胺配合物存在一种新的机理途径。机理研究的第一部分为所提出的机理奠定了基础，其中发现中性 Pd（II）-二亚胺配合物对α-烯烃的烯烃异构化具有中等至良好的催化活性，尽管存在催化剂活化的概念。广泛的实验和计算研究揭示了二亚胺配体部分解离的可能性，这释放了一个配位位并能够进行配位插入。这一发现意义重大，因为钯中心的两个反应性配位位的共存成为了活性阳离子 Pd（II）-二亚胺配合物的有效提议。在第二部分中，我们使用阳离子 Pd（II）-二亚胺催化剂检查和验证了同时观察到的α-烯烃异构化和活性聚合，这分别支持了异构化和聚合的两个独立反应途径的存在。此外，添加强路易斯酸（如 AlCl）会通过竞争性结合削弱钯-氮键，从而加速配体解离和随后的异构化。在第三部分中，路易斯酸引发的烯烃异构化-聚合被用于制备活性烯烃嵌段共聚物，并进一步以一锅法合成具有独特结构、不同支化程度、结晶度和极性官能度的新型聚烯烃-极性嵌段共聚物。
