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

Abstract

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.