Unsymmetrical Strategy on α-Diimine Nickel and Palladium Mediated Ethylene (Co)Polymerizations

Among various catalyst design strategies used in the α-diimine nickel(II) and palladium(II) catalyst systems, the unsymmetrical strategy is an effective and widely utilized method. In this contribution, unsymmetrical nickel and palladium α-diimine catalysts (Ipty/ⁱPr-Ni and Ipty/ⁱPr-Pd) derived from the dibenzobarrelene backbone were constructed via the combination of pentiptycenyl and diisopropylphenyl substituents, and investigated toward ethylene (co)polymerization. Both of these catalysts were capable of polymerizing ethylene in a broad temperature range of 0-120 °C, in which Ipty/ⁱPr-Ni could maintain activity in the level of 10⁶ g mol^-1 h^-1 even at 120 °C. The branching densities of polyethylenes generated by both nickel and palladium catalysts could be modulated by the reaction temperature. Compared with symmetrical Ipty-Ni and ⁱPr-Ni, Ipty/ⁱPr-Ni exhibited the highest activity, the highest polymer molecular weight, and the lowest branching density. In addition, Ipty/ⁱPr-Pd could produce copolymers of ethylene and methyl acrylate, with the polar monomer incorporating both on the main chain and the terminal of branches. Remarkably, the ratio of the in-chain and end-chain polar monomer incorporations could be modulated by varying the temperature.

Electronic Tuning of Sterically Encumbered 2-(Arylimino)Pyridine-Nickel Ethylene Polymerization Catalysts by Para-Group Modification

A collection of five related 2-(arylimino)pyridines, 2-{(2,6-(CH(C6H4-p-F)2)2-4- RC6H2)N=CMe}C5H4N, each ortho-substituted with 4,4′-difluorobenzhydryl groups but distinct in the electronic properties of the para-R substituent (R = Me L1, Et L2, i-Pr L3, F L4, OCF3 L5), were prepared and combined with (DME)NiBr2 to form their corresponding LNiBr2 complexes, Ni1–Ni5, in high yields. All the complexes were characterized by FT-IR, 19F NMR spectroscopy and elemental analysis, while Ni5 was additionally the subject of an X-ray determination, revealing a bromide-bridged dimer. The molecular structure of bis-ligated (L4)2NiBr2 (Ni4’) was also determined, the result of ligand reorganization having occurred during attempted crystallization of Ni4. On activation with either EtAlCl2 or MMAO, Ni1–Ni5 exhibited high catalytic activities (up to 4.28 × 106 g of PE (mol of Ni)−1 h−1 using EtAlCl2) and produced highly branched polyethylene exhibiting low molecular weight (Mw range: 2.50–6.18 kg·mol−1) and narrow dispersity (Mw/Mn range: 2.21–2.90). Notably, it was found that the type of para-R group impacted on catalytic performance with Ni5 > Ni4 > Ni3 > Ni1 > Ni2 for both co-catalysts, underlining the positive influence of electron withdrawing substituents. Analysis of the structural composition of the polyethylene by 1H and 13C NMR spectroscopy revealed the existence of vinyl-end groups (–CH=CH2) and high levels of internal unsaturation (–CH=CH–) (ratio of vinylene to vinyl, range: 3.1:1–10.3:1) along with various types of branch (Me, Et, Pr, Bu, 1,4-paired Me, 1,6-paired Me and LCBs). Furthermore, reaction temperature was shown to greatly affect the end group type, branching density, molecular weight and in turn the melting points of the resulting polyethylenes.

Ethylene polymerization of nickel catalysts with α-diimine ligands: factors controlling the structure of active species and polymer properties

α-Diimine and related complexes of late transition metals such as palladium and nickel have been attracting continuing interest as single-site catalysts of ethylene homopolymerization to branched polyolefins, having challenging mechanical properties. The state-of-the art catalysts demonstrate promising catalytic activities, and enhanced thermal stabilities, affording polyethylenes with a variable degree of branching and, in addition, are able to incorporate polar co-monomers into polyethylene structures. At the same time, fundamental understanding of the structure-reactivity relationships of such catalysts mostly remains at the phenomenological level, due to the lack of experimental data on the solution structures of intermediates that drive the polymerization process. In this perspective, we discuss recent advances of α-diimine nickel based catalysts of ethylene polymerization, focusing on the relationships between the catalyst structures on the one hand, and their thermal stabilities and properties of the resulting polyethylene, on the other hand. In addition, some intriguing novel mechanistic findings of these catalyst systems are presented.

Plastomeric-like polyethylenes achievable using thermally robust N,N'-nickel catalysts appended with electron withdrawing difluorobenzhydryl and nitro groups

A new set of five unsymmetrical N,N'-diiminoacenaphthenes, 1-[2,6-{(4-FC₆H₄)₂CH}₂-4-NO₂C₆H₄N]-2-(ArN)C₂C₁₀H₆ (Ar = 2,6-Me₂C₆H₃L1, 2,6-Et₂C₆H₃L2, 2,6-ⁱPr₂C₆H₃L3, 2,4,6-Me₃C₆H₂L4, 2,6-Et₂-4-MeC₆H₂L5), have been synthesized and used to prepare their corresponding nickel(ii) halide complexes, LNiBr₂ (Ni1-Ni5) and LNiCl₂ (Ni6-Ni10). The molecular structures of Ni3(OH₂) and Ni4 reveal distorted square pyramidal and tetrahedral geometries, respectively, while the ¹H NMR spectra of all the nickel(ii) (S = 1) complexes show broad paramagnetically shifted peaks. Upon activation with either methylaluminoxane (MAO) or ethylaluminum sesquichloride (Et₃Al₂Cl₂, EASC), Ni1-Ni10 displayed very high activities for ethylene polymerization with the optimal performance being observed using 2,6-dimethyl-containing Ni1 in combination with EASC (1.66 × 10⁷ g PE mol^-1 (Ni) h^-1 at 50 °C) which produced high molecular weight plastomeric polyethylene (M_w = 3.93 × 10⁵ g mol^-1, T_m = 70.6 °C) with narrow dispersity (M_w/M_n = 2.97). Moreover, Ni1/EASC showed good thermal stability by operating effectively at an industrially relevant 80 °C with a level of activity (6.01 × 10⁶ g of PE mol^-1 (Ni) h^-1) that exceeds previously disclosed N,N'-nickel catalysts under comparable reaction conditions. This improved thermal stability and activity has been ascribed to the combined effects imparted by the para-nitro and fluoride-substituted benzhydryl ortho-substituents.

A continuing legend: the Brookhart-type α-diimine nickel and palladium catalysts

Here we will summarize some of the recent advances in α-diimine ligand developments, as well as some new and challenging monomers that this class of catalysts can address through these ligand improvements.

N,N-chelated nickel catalysts for highly branched polyolefin elastomers: a survey

The physical properties and end applications of polyolefin materials are defined by their chain architectures and topologies. These properties can, in part, be controlled by a judicious choice of the steric and electronic properties of the catalyst and, in particular, the ligand framework. One major achievement in this field is the discovery of thermoplastic polyolefin elastomers that combine the processing and recyclable characteristics of thermoplastics with the flexibility and ductility of elastomers. These polymers are highly sought after as alternative materials to thermoset elastomers. In this perspective, works in the literature related to the development of nickel catalysts as well as their implementations for the synthesis of polyolefin elastomers are summarized in detail. Throughout the perspective, attention has been focused on developing the relationship between catalyst structure and performance, on strategies for the synthesis of polyolefin elastomer using nickel catalysts, on properties of the resultant polyolefin, such as degree of branching and crystallinity, as well as on their effects on mechanical properties. The future perspective regarding the most recent developments in single-step production of polyethylene elastomers will also be presented.

Advancing polyethylene properties by incorporating NO2 moiety in 1,2-bis(arylimino)acenaphthylnickel precatalysts: synthesis, characterization and ethylene polymerization

A new family of nickel halides (bromides Ni1-Ni5 and chlorides Ni6-Ni10) ligated by 1-(2,6-dibenzhydryl-4-nitrophenylimino)-2-(arylimino)acenaphthylene (Aryl = 2,6-Me₂C₆H₃L1, 2,6-Et₂C₆H₃L2, 2,6-ⁱPr₂C₆H₃L3, 2,4,6-Me₃C₆H₂L4, and 2,6-Et₂-4-MeC₆H₂L5) have been prepared and well characterized, and the scope of their catalytic properties toward the polymerization of ethylene has been investigated. Upon activation with either Et₂AlCl or EASC, the nickel bromide complexes displayed better activities than their nickel chloride counterparts and produced higher-molecular-weight polyethylene in the range of 10⁶ g mol^-1 with a very narrow range of molecular weight distributions. In comparison with reference precatalysts with non-nitro substituents (CH₃, F or Cl), the title complexes experienced a modest negative effect on catalytic activity upon replacement with a NO₂ moiety (activity up to 4.61 × 10⁶ g PE (mol Ni)^-1 h^-1 at 20 °C). Conversely, the NO₂ moiety exerted a positive effect to increase the molecular weight of the resulting polyethylene, and Ni4/Et₂AlCl gave polyethylene with a maximum molecular weight of as high as 32.8 × 10⁵ g mol^-1, which is not only the highest among the title complexes but also higher than any literature values reported with 1,2-diiminoacenaphthylnickel halides.

Synthesis, characterization and olefin polymerization behaviors of phenylene-bridged bis-β-carbonylenamine binuclear titanium complexes

Binuclear and multinuclear complexes have attracted much attention due to their unique catalytic performances for olefin polymerization compared with their mononuclear counterparts. In this work, a series of phenyl-bridged bis-β-carbonylenamine [O⁻NS^R] (R = alkyl or phenyl) tridentate ligands and their binuclear titanium complexes (Ti²L₁–Ti²L₅) were synthesized and characterized by ¹H NMR, ¹³C NMR, FTIR and elemental analysis. The molecular structure of ligand L₂ (R = ⁿPr) and its corresponding Ti complex Ti²L₂ were further investigated by single-crystal X-ray diffraction, which showed that each titanium coordinated with six atoms to form a distorted octahedral configuration along with the conversion of the ligand from β-carbonylenamine to β-imino enol form. Under the activation of MMAO, these complexes catalyzed ethylene polymerization and ethylene/α-olefin copolymerization with extremely high activity (over 10⁶ g mol (Ti)⁻¹ h⁻¹ atm⁻¹) to produce high molecular weight polyethylene. At the same time, wider polydispersity as compared with the mononuclear counterpart TiL₆ was observed, indicating that two active catalytic centers may be present, consistent with the asymmetrical crystal structure of the binuclear titanium complex. Furthermore, these complexes possessed better thermal stability than their mononuclear analogues. Compared with the complexes bearing alkylthio sidearms, the complex Ti²L₅ bearing a phenylthio sidearm exhibited higher catalytic activity towards ethylene polymerization and produced polyethylene with much higher molecular weight, but with an appreciably lower 1-hexene incorporation ratio. Nevertheless, these bis-β-carbonylenamine-derived binuclear titanium complexes showed much higher ethylene/1-hexene copolymerization activity and 1-hexene incorporation ratios as compared with the methylene-bridged bis-salicylaldiminato binuclear titanium complexes, and the molecular weight and 1-hexene incorporation ratio could be flexibly tuned by the initial feed of α-olefin commoners and catalyst structures.

Phenyl-bridged bis-β-carbonylenamine binuclear titanium complexes were synthesized, characterized and used to catalyze ethylene (co)polymerization with extremely high activity.

Ultra-high molecular weight elastomeric polyethylene using an electronically and sterically enhanced nickel catalyst

Highly active para-t-Bu-containing 1,2-bis(imino)acenaphthene-Ni(ii) catalysts are disclosed which afford hyper-branched PEs with M_w's up to 3.1 × 10⁶ g mol⁻¹; high tensile strength, excellent shape fixity as well as high elongation at break are a feature.

Ligand steric effects on α-diimine nickel catalyzed ethylene and 1-hexene polymerization

α-Diimine nickel complexes with systematically varied ligand sterics were used as a precatalyst for ethylene and 1-hexene polymerizations. The catalytic activities, molecular weights and branching densities could be tuned over a very wide range.

Enantioselective construction of imidazolines having vicinal tetra-substituted stereocenters by direct Mannich reaction of α-substituted α-isocyanoacetates with ketimines

Enantioselective direct Mannich-type reaction of ketimines with α-substituted α-isocyanoacetates gave imidazolines having vicinal tetra-substituted stereocenters with high stereoselectivity.