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 [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.

Remote Perfluoroalkyl Substituents are Key to Living Aqueous Ethylene Polymerization

“… Catalyst precursor, with C 6 F 13 ‐substituents at remote positions, capable of truly living ethylene polymerization in aqueous media (left) . Interaction promoting chain transfer and branch formation in the case of electron‐donating substituents, exemplified by methyl substituents (right) …”

“…The strong influence of substituents at these remote positions of the chelating ligand, distant from the active center, can be related to a weak interaction between the distal aryl rings and the metal atom (Figure , right), and it promotes decoordination of ethylene and favors β‐hydride elimination (BHE) . BHE is the key step in chain transfer and branch formation.…”

Tailored Strength Neighboring Group Interactions Switch Polymerization to Dimerization Catalysis

“…The electron density on the Ni-atom is directly correlated to the catalytic properties. 15 To this end, forward peak potentials obtained by cyclic voltammetry (CV) measurements of the Ni(II)/Ni(III) pair are a probe for the electronic properties of the complexes ( Table 1). The forward peak potentials are comparable to structurally related Ni(II) salicylaldiminato complexes bearing a N-terphenyl group.…”

“…Alternatively, from this species ethylene can be decoordinated to form a cis ß-agostic complex providing an entry to further branching and chain transfer pathways. 2,15,[26][27] 12 13 By comparison, for the reference system 8-CH 3 -I2 no such assisted two-step pathway is available, neither an interaction of the methylene bridge nor the distal aryl ring with Ni is available, and 1--C forms directly from 2-Coor-C with a corresponding energy barrier. The key transition state for the olefin displacement with 4-CH 3 -I2 system is a five coordinated complex (Figure 7a) with the leaving ethylene occupying an apical position and the oxygen coordinating in the ligand plane (O … Ni distance 2.17 Å).…”

“…This has been traced to a weak interaction of the distal aryl rings with the Ni(II) center, which promotes decoordination of ethylene and hereby favors BHE and consequently chain transfer and branch formation. 15 Only with electron-rich substituted aryls this weak 2 interaction is operative. Notably, also for other late transition metal catalyst structures the role of substituents' attractive 'secondary' interactions with substrates and the active site is being revealed.…”