Polymer, Vol.171, 87-95, 2019
Chromium ethylene polymerization catalysts bearing sterically enhanced alpha, alpha '-bis(imino)-2,3:5,6-bis(pentamethylene)pyridines: Tuning activity and molecular weight
The ortho-benzhydryl-substituted alpha, alpha'-bis(arylimino)-2,3:5,6-bis(pentamethylene) pyridine-chromium(III) chlorides, [2,3:5,6-{C4H8C(N(2-R-1-4-R-2-6-(CHPh2)C6H2)}(2)C5HN] CrCl3 [R-1 = R-2 = Me Cr1, R-1 = Me, R-2 = CHPh2 Cr2, R-1 = Et, R-2 = CHPh2 Cr3, R-1 = i-Pr, R-2 = CHPh2 Cr4, R-1 = Cl, R-2 = CHPh2 Cr-5, R-1 = F, R-2 = CHPh2 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 a, a'-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 x 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 (C1)), 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.
Keywords:Ethylene polymerization;Chromium precatalysts;alpha-alpha '-bis(arylimino)-2,3:5,6-bis(pentamethylene)pyridine;Electronic and steric effects