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-{C 4 H 8 C(N(2-R 1 -4-R 2 -6-(CHPh 2 )C 6 H 2 )} 2 C 5 HN]CrCl 3 [R 1 = R 2 = Me Cr1, R 1 = Me, R 2 = CHPh 2 Cr2, R 1 = Et, R 2 = CHPh 2 Cr3, R 1 = i-Pr, R 2 = CHPh 2 Cr4, R 1 = Cl, R 2 = CHPh 2 Cr5, R 1 = F, R 2 = CHPh 2 Cr6], differing in the electronic and/or steric properties of their aryl-R 1 and -R 2 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 CrCl 3 (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 × 10 6 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-R 1 substituent with fluoride Cr6 forming the lowest molecular weight and the most sterically demanding Cr4 (i-Pr) the highest.