King Abdullah University of Science and Technology, Chemical and Life Sciences and Engineering, Kaust Catalysis Center , Thuwal 23955-6900, Saudi Arabia.
Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz , 78464 Konstanz, Germany.
J Am Chem Soc. 2018 Jan 31;140(4):1305-1312. doi: 10.1021/jacs.7b08975. Epub 2018 Jan 23.
A combined theoretical and experimental study shows how weak attractive interactions of a neighboring group can strongly promote chain walking and chain transfer. This accounts for the previously observed very different microstructures obtained in ethylene polymerization by [κ-N,O-{2,6-(3',5'-RCH)CH-N═C(H)-(3,5-X,Y-2-O-CH)}NiCH(pyridine)], namely hyperbranched oligomers for remote substituents R = CH versus high-molecular-weight polyethylene for R = CF. From a full mechanistic consideration, the alkyl olefin complex with the growing chain cis to the salicylaldiminato oxygen donor is identified as the key species. Alternative to ethylene chain growth by insertion in this species, decoordination of the monomer to form a cis β-agostic complex provides an entry into branching and chain-transfer pathways. This release of monomer is promoted and made competitive by a weak η-coordination of the distal aryl rings to the metal center, operative only for the case of sufficiently electron-rich aryls. This concept for controlling chain walking is underlined by catalysts with other weakly coordinating furan and thiophene motifs, which afford highly branched oligomers with >120 branches per 1000 carbon atoms.
一项理论与实验相结合的研究表明，相邻基团的弱吸引相互作用如何能强烈促进链行走和链转移。这解释了先前在乙烯聚合中观察到的非常不同的微观结构，[κ-N,O-{2,6-(3',5'-RCH)CH-N═C(H)-(3,5-X,Y-2-O-CH)}NiCH(pyridine)]得到的是超支化低聚物，取代基 R = CH，而 R = CF 则得到高分子量聚乙烯。从全面的机理考虑，确定具有生长链顺式到水杨醛亚胺氧供体的烷基烯烃配合物为关键物种。在该物种中，通过插入进行乙烯链增长的替代方案是单体去配位形成顺式β-桥接络合物，从而提供支化和链转移途径。这种单体的释放通过远端芳环与金属中心的弱η配位来促进和竞争，这种配位只有在芳基具有足够的富电子时才起作用。通过具有其他弱配位呋喃和噻吩结构的催化剂来控制链行走的这一概念，得到了具有 >120 个支链/1000 个碳原子的高度支化低聚物。
