From polyethylene waxes to HDPE using an α,α′-bis(arylimino)-2,3:5,6-bis(pentamethylene)pyridyl-chromium(iii) chloride pre-catalyst in ethylene polymerisation

Abstract

Five examples of α,α'-bis(arylimino)-2,3:5,6-bis(pentamethylene)pyridyl-chromium(iii) chlorides (aryl = 2,6-MePh Cr1, 2,6-EtPh Cr2, 2,6-i-PrPh Cr3, 2,4,6-MePh Cr4, 2,6-Et-4-MePh Cr5) have been synthesized by the one-pot template reaction of α,α'-dioxo-2,3:5,6-bis(pentamethylene)pyridine, CrCl·6HO and the corresponding aniline. The molecular structures of Cr1 and Cr4 reveal distorted octahedral geometries with the N,N,N-ligand adopting a mer-configuration. On activation with an aluminium alkyl co-catalyst, Cr1-Cr5 exhibited high catalytic activities in ethylene polymerization and showed outstanding thermal stability operating effectively at 80 °C with activities up to 1.49 × 10 g of PE (mol of Cr) h. Significantly, the nature of the co-catalyst employed had a dramatic effect on the molecular weight of the polymeric material obtained. For example, using diethylaluminium chloride (EtAlCl) in combination with Cr4 gave high density/high molecular weight polyethylene with broad molecular weight distributions (30.9-39.3). By contrast, using modified methylaluminoxane (MMAO), strictly linear polyethylene waxes of lower molecular weight and narrow molecular weight distribution (1.6-2.0) were obtained with vinyl end-groups.

Vinyl‐Polyethylene Waxes with Narrow Dispersity Obtained by Using a Thermally Robust [Bis(imino)trihydroquinolyl]chromium Catalyst

“…Despite using MMAO as the activator, there are, as with MAO, just two types of end‐group combinations (i.e., vinyl/methyl and methyl/methyl); no evidence for any isopropyl groups could be found in the high‐temperature 13 C NMR spectrum. This absence of isopropyl groups has previously been noted with other MMAO‐promoted chromium catalysts, and can be attributed to the active species displaying a preference to transfer the higher‐concentration methylaluminum.…”

“…On the other hand, the Cr2 /MMAO system shows higher activities at 20 or 30 °C, and the polymers exhibit lower melting temperatures as well as narrower molecular weight distributions. As with the MAO‐promoted system, short run times lead to increased activity with a maximum of 15.96 × 10 6 g (PE) · mol –1 (Cr) · h –1 reached after 5 min (Entry 13, Table ), and these findings are consistent with rapid formation of the active species followed by gradual deactivation on prolonging the reaction time , , …”

“…Thirdly, to explore the lifetime of the active species derived from Cr2 , run times of 5, 15, 30, 45, and 60 min (Entries 3 and 13–16, Table ) were conducted with the Al/Cr molar ratio kept at 1500 and the temperature at 80 °C. The best activity of 13.02 × 10 6 g (PE) · mol –1 (Cr) · h –1 was observed after 5 min (Entry 13, Table ), which is higher than that observed after 30 min [9.46 × 10 6 g (PE) · mol –1 (Cr) · h –1 ; Entry 10, Table ]; this would imply that the active species quickly formed following the addition of the cocatalyst and then was gradually deactivated over more extended reaction times , , . In addition, the molecular weights of the polyethylenes were found to slightly increase over longer reaction times (Figure ).…”

Exceptionally high molecular weight linear polyethylene by using N,N,N′‐Co catalysts appended with a N′‐2,6‐bis{di(4‐fluorophenyl)methyl}‐4‐nitrophenyl group

“…δ 1.41; 13 C ca . δ 30.0) with chemical shifts that are characteristic of linear polyethylene; Figures and show the corresponding spectra of the polyethylene obtained using Co1 /MAO (entry 1, Table ) while those obtained using Co1 /MMAO are given in Figures S5 and S6 (entry 1, Table ) . In addition, no evidence for signals corresponding to saturated or unsaturated end groups could be detected which further highlights the high molecular weight of the polymer.…”

Chromium complexes based on thiophene–imine ligands for ethylene oligomerization

“…Among the various catalytic systems available, chromium complexes bearing anionic and neutral polydentate Schiff base ligands have been utilized in oligomerization and polymerization of ethylene due to their advantages of easy preparation and tunable steric and electronic properties . For instance, a detailed study developed by Gibson and co‐workers based on Cr(III) complexes incorporating bidentate and tridentate Schiff base salicylaldimine ligands showed that the catalytic performances of these precatalysts (oligomerization versus polymerization behavior) are strongly influenced by the nature of the groups at the imine nitrogen donor .…”