2,4,6-Triphenylpyridinium: A Bulky, Highly Electron-Withdrawing Substituent That Enhances Properties Of Nickel(Ii) Ethylene Polymerization Catalysts

The reactivity of Ni-II and Pd-II olefin polymerization catalysts can be enhanced by introduction of electron-withdrawing substituents on the supporting ligands rendering the metal centers more electrophilic. Reported here is a comparison of ethylene polymerization activity of a classical salicyliminato nickel catalyst substituted with the powerful electron-withdrawing 2,4,6-triphenylpyridinium (trippy) group to the -CF3 analogue. The trippy substituent is substantially more electron-withdrawing (sigma(meta)=0.63) than the trifluoromethyl group (sigma(meta)=0.43) which results in a ca. 8-fold increase in catalytic turnover frequency. An additional advantage of trippy is the high steric bulk relative to the trifluoromethyl group. This feature results in a four-fold increase in polymer molecular weight owing to enhanced retardation of chain transfer. A significant increase in catalyst lifetime is observed as well.