Remote Perfluoroalkyl Substituents are Key to Living Aqueous Ethylene Polymerization

Abstract

In various nickel(II) salicylaldiminato ethylene polymerization catalysts, which are a versatile mechanistic probe for substituent effects, longer perfluoroalkyl groups exert a strong effect on catalytic activities and polymer microstructures compared to the trifluoromethyl group. This effect is accounted for by a reduced electron density on the active sites, and is also supported by electrochemical studies. Thus, β‐hydride elimination, the key step of chain transfer and branching pathways, is disfavored while chain‐growth rates are enhanced. This enhancement occurs to an extent that enables living polymerizations in aqueous systems to afford ultra‐high‐molecular‐weight polyethylene for various chelating salicylaldimine motifs. These findings are mechanistically instructive as well as practically useful for illustrating the potential of perfluoroalkyl groups in catalyst design.

Efficient Suppression of Chain Transfer and Branching via Cs‐Type Shielding in a Neutral Nickel(II) Catalyst

“…Remarkably, the key 2,6‐disubstituted aniline is accessible in a single step from commercially available compounds in good yield (70 %, Scheme 3). This contrasts the multi‐step synthesis of the aniline component for state‐of‐the‐art N ‐terphenyl‐ and N ‐8‐arylnaphthyl‐ salicylaldiminato Ni II catalysts [21, 22, 27, 28] (cf. Scheme 1).…”

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.…”

Nickel(II) salicylaldiminates: Re-visiting a classic

“…Aryl amines are particularly diverse and their Ni(II) complexes have received considerable attention in the area of olefin oligomerisation and polymerisation. [2][3][4][5][6][7][8][9] The complexes have the general formula [Ni(sala-X)(R)(L)] and the tunability of both the phenyl and salicylaldimine rings permits substantial modification of catalytic behaviour.…”