Fluorinated Nickel(II) Phenoxyiminato Catalysts: Exploring the Role of Fluorine Atoms in Controlling Polyethylene Productivities and Microstructures

Abstract

A series of neutrally charged Ni(II) phenoxyiminato catalysts with fluorine atoms at different positions on the N-terphenyl motif are synthesized, and their abilities to polymerize ethylene are compared. At 25°C, the orthofluorinated Ni-5F, Ni-3F′, and Ni-2F achieve significantly higher polymerization activities than Ni-3F and Ni-0F. In addition, branch density and molecular weight of the obtained polyethylenes vary gradually in the order of Ni-5F, Ni-3F, Ni-3F′, Ni-2F, and Ni-0F. Based on the X-ray crystal structure and 19 F NMR spectra, the ortho fluorine atoms are found to make terphenyl groups more rigid and bulky. Theoretical calculations suggest that the increased steric bulk of terphenyl motif leads to an increase in the ground state energy of the resting state species relative to the migratory insertion transition state, and consequently, lowered migratory insertion barriers are expected in Ni-5F, Ni-3F′, and Ni-2F. On the other hand, the weak hydrogen bonding between the ortho fluorine atoms and coordinated ethylene in insertion transition state is also proposed in favor of insertion. Similar to previous reports, polyethylene microstructure was mainly related to electronic effects of fluorine atoms. ■ INTRODUCTIONSince the pioneering discovery of cationic Ni(II)-and Pd(II)-based α-diimine catalysts by Brookhart 1 and neutral Ni(II) phenoxyiminato catalysts by Grubbs, 2 catalytic polymerization of olefins by late transition metal complexes 3 has attracted much attention due to their lower oxophilicity in comparison with early transition metal catalysts. Under ethylene homopolymerization conditions, these catalysts yield polyethylene that range from linear to highly branched, through a process termed as "chain walking". 4 Of various cationic and neutral catalyst systems, the nickel phenoxyiminato system stands out in their tolerance toward polar additives 2b and even can be carried out in aqueous emulsion. 5 Catalytic properties are commonly modified by adjusting two specific properties: steric and electronic effects of the ligands. One major breakthrough in designing Ni(II) and Pd(II) catalysts was the discovery of the role of bulky groups in ligands. 1 These bulky groups are crucial for blockage of the axial coordination sites and reduction of chain transfer. 3b Ligand electronic structures of both Grubbs-type and Brookhart-type catalysts were also found to have a significant effect on microstructure of the polyethylene: depending on the substituent electronics, high molecular weight linear polyethylene or low molecular weight branched polyethylene are formed. 2a,b,6 Mecking reported a series of single-component terphenyl phenoxyiminato Ni(II) ethylene polymerization catalysts bearing remote electron-withdrawing or electrondonating groups. 7 It was suggested that polymer branching and molecular weight are mainly controlled by electronics of these substituents, despite their remoteness from the nickel center.On the other hand, weak attractive interactions were also employed in modulating catalytic prop...

Remote dibenzocycloheptyl-substitution of an iminotrihydroquinoline-nickel catalyst as a route to narrowly dispersed branched polyethylene waxes with alkene functionality

“…This finding suggested that the ortho-F substitution suppressed chain-walking and favored β-H elimination. The explanation for this observation is uncertain but may be due to weak hydrogen bonding between the ortho-fluorine atoms and the coordinated ethylene during the insertion transition state; similar results have been observed elsewhere [39][40][41]. This lower branching content of the polyethylene formed by Ni4 was also consistent with the higher Tm of the polymer.…”

Elaborate Tuning in Ligand Makes a Big Difference in Catalytic Performance: Bulky Nickel Catalysts for (Co)polymerization of Ethylene with Promising Vinyl Polar Monomers

“…Note that bond length of Ni(1)−O(2) and Ni(1)−O(3) in 3 b is 3.817 and 4.436 Å, respectively, either is longer than the sum of the van der Waals radii of nickel and oxygen (3.15 Å) . In addition, bond length of Ni(1)−F(1) and Ni(1)−F(2) in 3 h is 3.791 and 4.487 Å, respectively, while the sum of van der Waals radii between Ni and F atom is 3.31 Å . This result reveals that the interaction between −F or −OMe group and the nickel center for both 3 b and 3 h is negligible.…”

Fluorinated α‐Diimine Nickel Mediated Ethylene (Co)Polymerization

“…Fluorine substituents in these catalysts is also effective in tuning elementary reaction pathways, likewise. Phenoxy‐imine nickel catalysts bearing fluorine (F) substituents or fluorocarbon (C n F 2n+1 ) substituents have been thoroughly studied in ethylene polymerization (Scheme 1B) [67–74] . The effect of fluorine substituents on thermal stability and activity of catalyst, and molecular weight and branching density of polymer is observed.…”