Control of Chain Walking by Weak Neighboring Group Interactions in Unsymmetrical Catalysts

• Published in: Journal of the American Chemical Society 2018-01, Vol.140 (4), p.1305-1312
• Main Authors: Falivene, Laura, Wiedemann, Thomas, Göttker-Schnetmann, Inigo, Caporaso, Lucia, Cavallo, Luigi, Mecking, Stefan
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.7b08975

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 [κ2-N,O-{2,6-(3′,5′-R2C6H3)2C6H3-NC­(H)-(3,5-X,Y-2-O-C6H2)}­NiCH3(pyridine)], namely hyperbranched oligomers for remote substituents R = CH3 versus high-molecular-weight polyethylene for R = CF3. 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 η2-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.