Multinuclear late transition metal catalysts for olefin polymerization

Binuclear Enamino-Oxazolinate Rare-Earth Metal Complexes: Synthesis and Their Catalytic Performance in Isoprene Polymerization

We have synthesized a series of binuclear rare-earth metal complexes bearing the newly designed enamino-oxazolinate ligands that feature bridging para-phenyl, meta-phenyl, 1,5-naphthalenyl, and 1,5-anthracenyl moieties. NMR and X-ray diffraction analyses confirmed the binuclear structures of the obtained complexes with two enamino-oxazolinate-metal units located at a trans position against the bridged aryl plane. After activation by [Ph3C][B(C6F5)4], all the rare-earth metal complexes served as efficient catalysts for isoprene polymerization, producing polymers with high cis-1,4 regularity (up to 96.1%) and high molecular weight. The steric and electronic effects exerted on the active metal centers, as well as the radius of metal centers, were the major contributing factors for determining both the catalytic activity and cis-1,4-selectivity of the binuclear catalytic systems. Compared to its mononuclear analogue, the binuclear yttrium catalytic system with a para-phenyl bridge exhibited a higher thermostability and catalytic efficiency during polymerization, revealing a special binuclear effect in this binuclear catalytic system.

Ambient Catalytic Spinning of Polyethylene Nanofibers

A novel single-atom Ni(II) catalyst (Ni-OH) is covalently immobilized onto the nano-channels of mesoporous Santa Barbara Amorphous (SBA)-15 particles and isotropic Anodized Aluminum Oxide (AAO) membrane for confined-space ethylene extrusion polymerization. The presence of surface-tethered Ni complexes (Ni@SBA-15 and Ni@AAO) is confirmed by the inductively coupled plasma-optical emission spectrometry (ICP-OES) and X-ray photoelectron spectroscopy (XPS). In the catalytic spinning process, the produced PE materials exhibit very homogeneous fibrous morphology at nanoscale (diameter: ~50 nm). The synthesized PE nanofibers extrude in a highly oriented manner from the nano-reactors at ambient temperature. Remarkably high Mw (1.62×106  g mol-1 ), melting point (124 °C), and crystallinity (41.8 %) are observed among PE samples thanks to the confined-space polymerization. The chain-walking behavior of surface tethered Ni catalysts is greatly limited by the confinement inside the nano-channels, leading to the formation of very low-branched PE materials (13.6/1000 C). Due to fixed supported catalytic topology and room temperature, the filaments are expected to be free of entanglement. This work signifies an important step towards the realization of a continuous mild catalytic-spinning (CATSPIN) process, where the polymer is directly synthesized into fiber shape at negligible chain branching and elegantly avoiding common limitations like thermal degradation or molecular entanglement.

Enhancing isoprene polymerization with high activity and adjustable monomer enchainment using cyclooctyl-fused iminopyridine iron precatalysts

In this study, a series of structurally rigid cyclooctyl-fused iminopyridine iron complexes, [L2FeCl][FeCl4] and [2L3Fe][Cl][3FeCl4], was synthesized via a one-pot method and investigated as precatalysts in conjunction with methylaluminoxane for isoprene (Ip) polymerization. Combined characterization through FTIR analysis, elemental analysis and single crystal XRD analysis fully verified the structure of these complexes. The most active iron complex, FeH, exhibited a trisligated nature, with its cation adopting an octahedral geometry around the metal center. In contrast, all the other iron complexes (Fe2Me, Fe2Et, Fe2iPr, Fe3Me, Fe2Et,Me) displayed bisligated configurations, with distorted trigonal bipyramidal geometry of cations. During isoprene polymerization, the extent of steric hindrance of the ligand framework exerted a significant impact on catalytic performance. The FeH precatalyst with less steric hindrance demonstrated excellent performance, producing high molecular weight polyisoprenes with conversions exceeding 99% for 4000 equiv. of monomer. Even at very low catalyst loadings, as low as 0.0025 mol% (Fe/Ip), the polymerization of isoprene could proceed smoothly with an exceptionally high activity of 4.0 × 106 gPI (molFe, h)-1. Moreover, this precatalyst exhibited good thermal stability, maintaining high activity levels (typically 105 gPI (molFe, h)-1) across a broad temperature range from -20 °C to 100 °C. Additionally, by adjusting steric substituents and the reaction temperature, the 1,4/3,4 regioselectivity could be modulated from 9/91 to 69/31 while maintaining a high stereoselectivity of cis-1,4 structures (cis/trans: >99/1).

Synthesis of Mono- and Dinuclear Aluminum Complexes Bearing Aromatic Amino-Phenolato Ligands: A Comparative Study in the Ring-Opening Polymerization of Cyclohexene Oxide

Dinuclear aluminum methyl complexes bearing aromatic diamine-bridged tetra(phenolato) ligands and the mononuclear aluminum methyl complex with the phenylamine-bridged bis(phenolato) ligand have been synthesized and characterized. Structure determination revealed that the Al-Al distances in these dinuclear aluminum complexes are tunable by the choice of the suitable aromatic backbone of the diamine-bridged tetra(phenolato) ligands. The catalytic behaviors of these mono- and dinuclear aluminum complexes for cyclohexene oxide (CHO) polymerization were investigated. The activities of these dinuclear Al complexes were observed to increase with the decrease of Al-Al distances, and the dinuclear Al complexes appeared to have better catalytic activity than the mononuclear Al complex, even if the Al-Al distance is as long as 9.401 Å. Dinuclear aluminum complex 2, with the shortest Al-Al distance (7.236 Å), showed the highest activity toward CHO polymerization with TOFs up to 6460 h-1 in neat CHO at 30 °C. Furthermore, comparative kinetic studies revealed that the polymerization is first-order for CHO concentration, and the reaction orders for initiator concentration are different for the mono- and dinuclear Al complexes. The polymerization mechanism study revealed that both the methyl and phenolate groups were involved in the initiation process.

Ethylene polymerization using heterogeneous multinuclear nickel catalysts supported by a crosslinked alpha diimine ligand network

A crosslinked alpha diimine ligand supporting a nickel metal center polymerizes ethylene to produce polyethylene with controlled microstructures, high activities, and can be removed from the product.

Structural evolution of iminopyridine support for nickel/palladium catalysts in ethylene (oligo)polymerization

The interest in the late transition metal catalyst based design of new architectures of polyethylene (PE) has continuously been increasing over the last few years. The structure of these catalysts is predominantly important in controlling the morphological and architectural properties of the resulting polyethylene. Particularly, iminopyridine is a versatile bidentate support for Ni and Pd catalysts in ethylene (oligo)polymerization providing a wide variety of products ranging from volatile oligomers to ultra-high molecular weight polyethylene. Extensive structural modifications have been induced in the iminopyridine ligand through steric and electronic substitution, tuning the catalyst behavior in terms of activity and properties of the resulting polymer. Carbocyclic-fused iminopyridine and N-oxide iminopyridine are the new state of the art iminopyridine ligand designs. In this review, we aim to summarize all the developments in mononuclear iminopyridine-nickel and -palladium catalysts for ethylene (oligo)polymerization since the first report published in 1999 to present, focusing on the correlation among the pre-catalyst, co-catalyst type, thermal stability and polymer/oligomer structure. For comparison, the structural variations in the binuclear iminopyridine-nickel catalysts are also described. The detailed comparison of the structural variations in these catalysts with respect to their polymerization performance will give deep understanding in the development of new efficient catalyst designs.

4,4'-Dimethoxybenzhydryl substituent augments performance of bis(imino)pyridine cobalt-based catalysts in ethylene polymerization

A series of cobalt complexes with bis(imino)pyridine derivatives featuring unsymmetrical substitution with bulky groups has been synthesized and characterized. The molecular structures of two representatives have been determined by the single-crystal X-ray diffraction study, revealing distorted tetrahedral geometry with different degrees of steric hindrance imparted by the two inequivalent aryl groups attached to the imine nitrogen atoms. On activation with either MAO or MMAO, these complexes display high activity toward ethylene polymerization, reaching 8.71 × 10⁶ g of PE (mol of Co)⁻¹ h⁻¹ at 60 °C and produce polyethylene of high molecular weight (M_w = 5.27 × 10⁵ g mol⁻¹) and low dispersity. The presence of the methoxy-substituent noticeably enhances the activity of the cobalt catalyst and increases the molecular weight of the resultant polyethylene.

Employing ligands with 4,4′-dimethoxybenzhydryl groups, the cobalt precatalysts display high activities toward ethylene polymerization and produce highly linear polyethylenes, the high density polyethylene (HDPE).

Cation-π interactions enabling hard/soft Ti/Ag heterobimetallic cooperativity in lactide ring-opening polymerisation

The combination of a Ti-salen complex with AgBArF reveals unique hard/soft heterobimetallic cooperativity in lactide ring-opening polymerisation (ROP), enabling significant activity at room temperature. Reactivity, mechanistic and computational studies highlight the role of cation-π interactions in the formation of heterobimetallic species and provide key insights into the role of both metals in ROP.

Synthesis and Magnetic Properties of a Dimerized Trinuclear Ni String Complex, [Ni6Cl2(dpa)8](I5)2·0.25I2 (dpa- = 2,2'-Dipyridylamide Anion)

Metal string complexes, linearly aligned transition metal arrays coordinated with the multidentate organic ligands, have gained much attention both in unique electronic/structural properties and in potential applications as conductive molecular nanowires. Here we report on a dimerized Ni^II trinuclear complex, [Ni₆Cl₂(dpa)₈](I₅)₂·0.25I₂ (dpa^- = 2,2'-dipyridylamide anion). X-ray structural analysis revealed that two trinuclear moieties are bridged by a Cl anion to form a dimerized string structure. This is the first example of two Ni string complexes that are connected. In the electronic absorption and Raman spectra, characteristic absorption bands and a vibration mode based on the dimer string structure were observed. In the solid state, dimer complexes align in one dimension in an MMMXMMMX (M = metal, X = halogen) manner, leading to the intra- and interdimer antiferromagnetic interactions.