Abstract
A family of six unsymmetrical N,N'-diiminoacenaphthene-nickel(ii) bromide complexes, [1-{2,6-(Ph2CH)2-4-MeC6H2N}-2-(ArN)C2C10H6]NiBr2 (Ar = 2-(C6H11)-6-MeC6H2Ni1, 2-(C5H9)-6-MeC6H2Ni2, 2-(C8H15)-6-MeC6H2Ni3, 2-(C6H11)-4,6-Me2C6H2Ni4, 2-(C5H9)-4,6-Me2C6H2Ni5, 2-(C8H15)-4,6-Me2C6H2Ni6), each bearing one ring-size variable 4-R-2-methyl-6-cycloalkyl-substituted N-aryl group and one N'-4-methyl-2,6-dibenzhydrylphenyl group, have been prepared and fully characterized. The molecular structures of Ni1, Ni2, Ni3 and Ni5 reveal distorted tetrahedral geometries with different degrees of steric protection imparted by the two inequivalent N-aryl groups. On activation with either EASC or MMAO, all the precatalysts are highly active (up to 17.45 × 106 g PE mol-1 (Ni) h-1) for ethylene polymerization at 20-50 °C with their activities correlating with the type of cycloalkyl ortho-substituent: cyclooctyl (Ni6, Ni3) > the cyclopentyl (Ni5, Ni2) > cyclohexyl (Ni4, Ni1) for either R = H or Me. Moderately branched to hyperbranched polyethylenes (Tm's as low as 44.2 °C) can be obtained with molecular weights in the range 2.14-6.68 × 105 g mol-1 with the branching content enhanced by the temperature of the polymerization. Dynamic mechanical analysis (DMA) and monotonic tensile stress-strain tests have been employed on the polyethylene samples and reveal the more branched materials to show good elastic recovery properties (up to 75.5%).
Highlights
The incorporation of benzhydryl groups as the ortho-substituents (B, Chart 1),5 has led to reports of catalysts that display high productivity and operate with remarkable thermostability (90 oC or higher), a feature that has been attributed to the slowing down of chain transfer processes as well as inhibition of deactivation pathways.3a,4c,4f,6 More recently, nickel catalysts bearing unsymmetrical αdiimines have been disclosed which benefit from the steric properties of a N-4-R-2,6-dibenzhydrylphenyl group and the tunability of a less sterically bulky N-aryl group (C, Chart 1)
Methylphenylimino)acenaphthylenone, which can be further condensed with the corresponding aniline hydrochloride8g to form L1 – L6 in modest yields
Polyethylene microstructures Inspection of the two sets of polymerization data (Tables 3 and 4), reveals the Tm values of the polymers obtained in the runs performed at 30 °C or above to be under 80 °C suggesting a high branching content.12a,16c,18 To verify this, high temperature 13C NMR spectroscopy was performed on the polymers obtained using the more active catalysts Ni6/ethylaluminum sesquichoride (EASC) and Ni6/modified methylaluminoxane (MMAO) at 30 °C and compared with the data obtained at 50 °C
Summary
On increasing the molar ratio of Al/Ni from 400 to 900, with the temperature kept a 30 °C and the duration of the run at 30 minutes, the activity increased to a very high level of 17.45 × 106 g PE mol-1 (Ni) h-1 with an Al/Ni ratio of 700 (entries 1–6, Table 3), which is much higher than that observed with precatalysts of similar structure reported elsewhere.7a Notably as the number of molar equivalents of EASC is increased from 400 to 900 the molecular weight steadily drops.
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