Iminopyridine Ni(II) Catalysts Affording Oily Hyperbranched Ethylene Oligomers and/or Crystalline Polyethylenes Depending on the Reaction Conditions: Possible Role of In Situ Catalyst Structure Modifications

Abstract

Nickel-based ethylene polymerization catalysts have unique features, being able to produce macromolecules with a variable content of branches, resulting in polymers ranging from semicrystalline plastics to elastomers to hyperbranched amorphous waxes and oils. In addition to Brookhart’s α-diimine catalysts, iminopyridine Ni(II) complexes are among the most investigated systems. We report that Ni(II) complexes bearing aryliminopyridine ligands with bulky substituents both at the imino moiety and in the 6-position of pyridine afford either hyperbranched low molecular weight polyethylene oils or prevailingly linear crystalline polyethylenes or both, depending on the ligand structure and the reaction conditions. The formation of multiple active species in situ is suggested by analysis of the post-polymerization catalyst residues, showing the partial reduction of the imino function. Some related arylaminopyridine Ni(II) complexes were also synthesized and tested, showing a peculiar behavior, i.e., the number of branches of the produced polyethylenes increases while ethylene pressure increases.

From tetramerization to oligomerization/polymerization of ethylene by dinuclear pyridyl‐imine Co‐ and Ni‐based catalysts

“…This may imply that the active center species are highly sensitive against temperature as it may cause activation or deactivation of these species. 24,53 A similar scenario can be suggested for high concentration of cocatalyst that augments the chain transfer reactions affording lower crystallinity and slightly broader M w /M n .…”

“…These results are consistent on the similar iminopyridyl structures that have broadening of the molecular weight distributions were along with production of mixtures of different products. 41,[44][45][46]53 One of the suggestions in this regard is the correlation of different active species formation by activation with cocatalyst. 24,53 On the other side, it mostly has been observed for multinuclear or dinuclear catalysts that the broadening of M w /M n is inevitable due to the presence of different active centers.…”

“…41,[44][45][46]53 One of the suggestions in this regard is the correlation of different active species formation by activation with cocatalyst. 24,53 On the other side, it mostly has been observed for multinuclear or dinuclear catalysts that the broadening of M w /M n is inevitable due to the presence of different active centers. 5,22 This dependency not only led to broad distribution (2.93-5.41) but also oily product (Entry 11) was obtained at high polymerization temperature (50 C).…”

Metal Catalyzed Polymerization: From Stereoregular Poly(α‐olefins) to Tailor‐Made Biodegradable/Biorenewable Polymers and Copolymers

“…This finding prompted us to synthesize and test some corresponding aminopyridine Ni(II) complexes. The results suggested the formation of multiple active species owing to ligand structure modification in the activation process under certain conditions [46b] …”

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“…Vegetable oils, complexed with cyclodextrins, can be regarded as a novel therapeutic option in treatments involving painful processes and in the development of new antinociceptive agents, due to an improved bioavailability associated with solubility, dissolution, and permeability, as well as the reduction in doses and the side effects [23]. On the other hand, hyperbranched, low-molecular-weight polyethylene oils were synthesized using Ni(II) complexes bearing aryliminopyridine ligands, and it was found that the degree of branching and the molecular weight of the produced samples were hardly sensitive to the reaction conditions [24].…”