Chromium ethylene polymerization catalysts bearing sterically enhanced α,α′-bis(imino)-2,3:5,6-bis(pentamethylene)pyridines: Tuning activity and molecular weight

The ortho-benzhydryl-substituted α,α′-bis(arylimino)-2,3:5,6-bis(pentamethylene)pyridine-chromium(III) chlorides, [2,3:5,6-{C4H8C(N(2-R1-4-R2-6-(CHPh2)C6H2)}2C5HN]CrCl3 [R1 = R2 = Me Cr1, R1 = Me, R2 = CHPh2Cr2, R1 = Et, R2 = CHPh2Cr3, R1 = i-Pr, R2 = CHPh2Cr4, R1 = Cl, R2 = CHPh2Cr5, R1 = F, R2 = CHPh2Cr6], differing in the electronic and/or steric properties of their aryl-R1 and -R2 groups, have been prepared by a one-pot template approach involving α,α′-dioxo-2,3:5,6-bis(pentamethylene)pyridine, the corresponding aniline and CrCl3(THF)3 in acetic acid. The molecular structure of six-coordinate Cr1 reveals the carbocyclic-fused N,N,N-ligand to adopt a mer configuration with the puckered sections of the two fused rings arranged mutually cis. On activation with MAO or MMAO, Cr1 - Cr6 displayed high activities (up to 1.83 × 106 g (PE) mol−1 (Cr) h−1) for the polymerization of ethylene with the MAO-promoted polymerizations in most cases more productive than with MMAO. In general, the chromium complexes appended with ortho-halide substituents (Cr6 (F)) and (Cr5 (Cl)), proved the most active with the overall order being: Cr6 > Cr5 > Cr1 > Cr2 > Cr3 > Cr4. All catalysts formed linear polyethylene displaying a wide range of molecular weights (from 2.17 to 300.4 kg mol−1) that were highly dependent on the nature of the ortho-R1 substituent with fluoride Cr6 forming the lowest molecular weight and the most sterically demanding Cr4 (i-Pr) the highest. The ortho-benzhydryl-substituted α,α′-bis(arylimino)-2,3:5,6-bis(pentamethylene) pyridine-chromium(III) chlorides, on activation with MAO or MMAO, displayed high activities for ethylene polymerization producing linear polyethylenes; variation in the ortho-R1 substituent (halide vs. alkyl) had a striking effect on the Mw of the polymer. [Display omitted] •The novel benzhydryl-containing α,α′-bis(imino)-2,3:5,6-bis(pentamethylene)pyridine-chromium chlorides.•Highly active catalysts for ethylene polymerization.•Remarkable control of polymer molecular weight through judicious choice of ortho-substituent.•Linear polyethylenes: from waxes to high molecular weight polymers.•ortho-Halide enhancement effects on catalytic activity.