Abstract
Six types of 2,8-bis(imino)-7,7-dimethyl-5,6-dihydroquinoline, 2-(ArN[double bond, length as m-dash]CMe)-8-(ArN)-7,7-Me2C9H6N (Ar = 2,6-Me2C6H3L1, 2,6-Et2C6H3L2, 2,6-iPr2C6H3L3, 2,4,6-Me3C6H2L4, 2,6-Et2-4-MeC6H2L5, 2,4,6-tBu3C6H3L6), distinguishable by their steric and electronic profile, are described that can readily undergo complexation with cobaltous chloride to form their corresponding LCoCl2 chelates, Co1-Co6. The molecular structures of Co2 and Co3 reveal square pyramidal geometries with ring puckering a feature of the gem-dimethyl section of their unsymmetrical N,N,N'-ligands. On activation with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all the cobalt complexes exhibited exceptionally high activities for ethylene polymerization with levels reaching up to 1.19 × 107 g PE per mol (Co) per h for mesityl-containing Co4. Significantly, these catalysts exhibited good thermal stability by displaying their optimal performance at temperatures up to 70 °C whilst also maintaining appreciable catalytic lifetimes. With the exception of that obtained using the most sterically hindered Co6 (2,4,6-t-butyl), the polyethylenes are of low molecular weight (Mw≤16.0 kg mol-1) and of narrow dispersity (Mw/Mn≤3.4). Moreover, end-group analysis of these highly linear polymer waxes reveals evidence for unsaturated as well as various levels of fully saturated materials highlighting the role of both β-H elimination and chain transfer to aluminum as termination pathways.
Highlights
Six types of 2,8-bis(imino)-7,7-dimethyl-5,6-dihydroquinoline, 2-(ArN=CMe)-8-(ArN)-7,7-Me2C9H6N (Ar = 2,6-Me2C6H3 L1, 2,6-Et2C6H3 L2, 2,6-iPr2C6H3 L3, 2,4,6-Me3C6H2 L4, 2,6-Et2-4-MeC6H2 L5, 2,4,6-tBu3C6H3 L6), distinguishable by their steric and electronic profile, are described that can readily undergo complexation with cobaltous chloride to form their corresponding LCoCl2 chelates, Co1 – Co6
With the exception of that obtained using the most sterically hindered Co6 (2,4,6-t-butyl), the polyethylenes are of low molecular weight (Mw ≤ 16.0 kg mol-1) and of narrow dispersity (Mw/Mn ≤ 3.4)
We target a new series of divalent cobalt chloride complexes bearing 2-(1-(arylimino)ethyl)-8-arylimino7,7-dimethyl-5,6-dihydroquinolines (F, Chart 1), in which a singly fused six-membered ring is appended with two methyl groups at the C-7 position
Summary
These observations highlight that following a rapid induction period, the rate of polymerization assumed a more stable profile with only a modest loss of catalytic activity by the one hour mark; similar short induction periods have been noted elsewhere for cobalt catalysts.[31,33,48] the molecular weights of the resultant polyethylenes steadily increased as the reaction time was extended (see Figures S14 - S17).
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