Chuanbing Huang

Highly branched unsaturated polyethylenes achievable using strained imino-cyclopenta[b]pyridyl-nickel precatalysts

A new family of strained imino-cyclopenta[b]pyridines, 7-(ArN)-6-Me2C8H5N (Ar = 2,6-Me2Ph (L1), 2,6-Et2Ph (L2), 2,4,6-Me3Ph (L3), 2,6-Et2-4-MePh (L4), 2,6-i-Pr2Ph (L5)), have been synthesized in reasonable yield by a sequence of reactions from 2-chloro-cyclopenta[b]pyridin-7-one. Treatment of L1 and L3 with NiCl2·6H2O generates mononuclear bis-ligated [7-(ArN)-6-Me2C8H5N]2NiCl2 (Ar = 2,6-Me2Ph (Ni1), 2,4,6-Me3Ph (Ni3)), while with L2 and L4, the chloride-bridged binuclear complexes [7-(ArN)-6-Me2C8H5N]2Ni2(μ-Cl)2Cl2 (Ar = 2,6-Et2Ph (Ni2), 2,6-Et2-4-MePh (Ni4)), have been isolated; no apparent reaction occurred with L5. On activation with either MAO or MMAO, Ni1–Ni4 exhibited high activities towards ethylene polymerization with Ni3 the most active (5.02 × 106 g PE per mol Ni per h at 20 °C); rapid regeneration of the active species (3096–5478 h−1 at 20 °C) is a feature of their catalytic performance. A detailed microstructural analysis of the polyethylenes reveals the presence of vinyl and higher levels of internal vinylene groups indicative of high rates of chain isomerization, e.g., the ratio of (–CHCH–) to (H2CCH–) groups is 2.2u2006:u20061 using Ni3/MAO at 60 °C. Agostic interactions involving γ-, δ- and higher-hydrogens are inferred in addition to β-hydrogen elimination to account for the vinylene groups and the longer chain alkyl branches. The molecular structures of Ni1 and Ni2·2MeOH are also reported.

Rigid geometry 8-arylimino-7,7-dimethyl-5,6-dihydroquinolyl nickel bromides: single-site active species towards ethylene polymerization

The fused ring heterocyclic ketone, 5,6-dihydro-7,7-dimethylquinolin-8-one, was prepared and employed for the synthesis of a series of 8-arylimino-7,7-dimethyl-5,6-dihydroquinoline derivatives (aryl = 2,6-Me2Ph (L1), 2,6-Et2Ph (L2), 2,4,6-Me3Ph (L3), 2,6-Et2-4-MePh (L4), 2,6-i-Pr2Ph (L5)). The reaction of L1–L4 with (DME)NiBr2 (DME = 1,2-dimethoxyethane) gave the corresponding cationic bis-chelates, [(Lx)2NiBr][Br] (Lx = L1 (Ni1), L2 (Ni2), L3 (Ni3), L4 (Ni4)), as bromide salts; no such complex could be isolated with the most sterically bulky L5. All new compounds were characterized using IR spectroscopy, elemental analysis and in the case of L1–L5 using 1H and 13C NMR spectroscopy. Furthermore, the molecular structures of Ni1 and Ni3 have been determined and they reveal cation–anion pairs based on a trigonal bipyramidal nickel-containing cation charge balanced by a bromide counterion. In addition, X-ray photoelectron spectroscopy (XPS) was used to probe the solid state structures of L1, L3 and Ni1–Ni4; this technique provided valuable information regarding the net charge on nickel within the complexes. Upon activation with either methylaluminoxane (MAO) or ethylaluminium sesquichloride (EASC), all nickel complexes exhibited high activities towards ethylene polymerization and produced polyethylene waxes with low molecular weights. The catalytic activities, Ni1 [2,6-di(Me)] > Ni3 [2,4,6-tri(Me)] > Ni4 [2,6-di(Et)-4-Me] > Ni2 [2,6-di(Et)], correlated well with the trend in net charges observed in XPS analysis. The polydispersities (1.7–2.0) obtained for polyethylenes are narrow and indicate genuinely single-site active species for these catalysts. These performance characteristics have been attributed to the influence of the rigid geometry imparted by L1–L5 that, due to the presence of 7,7-dimethyl-substituents, prevents imine–enamine tautomerization.

Accessing highly linear polyethylenes by 2-(1-aryliminoethyl)-7-arylimino-6,6-dimethylcyclopenta[b]pyridylchromium(III) chlorides

A series of 2-(1-aryliminoethyl)-7-arylimino-6,6-dimethylcyclopenta[b]pyridylchromium(III) chlorides (aryl = 2,6-Me2Ph (Cr1), 2,6-Et2Ph (Cr2), 2,6-i-Pr2Ph (Cr3), 2,4,6-Me3Ph (Cr4), 2,6-Et2-4-MePh (Cr5)) was synthesized and characterized using FT-IR and elemental analysis, and the molecular structure of complex Cr1 was confirmed by using single-crystal X-ray diffraction to present a distorted octahedral geometry around the chromium center. Upon activation with modified methylaluminoxane (MMAO) or ethylaluminium sesquichloride (EASC), all chromium pre-catalysts exhibit good activities towards ethylene polymerization for the highly linear polyethylene.