Search Results (17)

The development of earth-abundant nickel-based catalysts is currently one of the greatest challenges for the straightforward synthesis of functionalized polyolefins. With environmental protection concerns, controllable copolymerizations of ethylene with biosourced comonomers derived from castor oil, such as methyl 10-undecenoate (U-COOMe), 10-undecen-1-ol (U-OH), or 10-undecenyl bromide (U-Br), were realized using α-diimine nickel catalyst (Ni-DBB) with dibenzobarrelene backbone. Catalyst Ni-DBB was highly tolerant toward polar comonomers, and functional polyethylenes were successfully prepared. The influences of the polar group, temperature, and comonomer concentration were studied in detail. Catalyst Ni-DBB was able to catalyze the copolymerization of ethylene with U-OH to afford high-molecular-weight (~180 kg/mol) functional polyethylene in a controlled fashion. NMR analysis showed that the produced functional polyethylenes were highly branched and had broad melting peaks ranging from 0 to 30 °C. Water contact angle (WCA) measurements showed that the surface of the obtained hydroxyl-functionalized polyethylene changed from hydrophobic to hydrophilic with the introduction of the comonomer U-OH. Full article

Late transition metal olefin polymerization catalysts have received more attention in the field of catalytic olefin polymerization. Barrelene-based α-diimine nickel and palladium olefin polymerization catalysts are rising stars because of their backbone structure and catalytic properties. In this review, we present a comprehensive review of barrelene-derived α-diimine nickel and palladium olefin polymerization catalysts. α-Diimine nickel and palladium catalysts are introduced from two aspects: barrelene-derived backbone and aniline derivatives with different substituents. The relationship between catalyst structure and catalytic properties is also emphasized. This review attempts to provide an inspiration for the design of high-performance barrelene-based catalysts. Full article

The homopolymerization of norbornene and the copolymerization of norbornene and ethylene were carried out using the neutral α-sulfonate-β-diimine nickel catalyst SD-Ni. The neutral α-sulfonate-β-diimine catalyst is highly active in the homopolymerization of norbornene, producing vinyl-addition polynorbornene (PNB) with a high molecular weight. The copolymerization of norbornene (NB) and ethylene (E) using the catalyst SD-Ni was also investigated. The α-sulfonate-β-diimine catalyst SD-Ni shows distinctive catalytic copolymerization properties to produce high-molecular-weight E-NB copolymers with low norbornene incorporation. Importantly, microstructure analyses confirm that the resultant E-NB copolymers are branched cyclo-olefin copolymers (COCs) with branched polyethylene units. Full article

The α-diimine late transition metal catalyst represents a new strategy for the synthesis of atactic polypropylene elastomer. Taking into account the properties of the material, enhancing the molecular weight of polypropylene at an elevated temperature through modifying the catalyst structure, and further increasing the activity of α-diimine catalyst for propylene polymerization, are urgent problems to be solved. In this work, two α-diimine nickel(II) catalysts with multiple hydroxymethyl phenyl substituents were synthesized and used for propylene homopolymerization. The maximum catalytic activity was 5.40 × 10⁵ gPP/molNi·h, and the activity was still maintained above 10⁵ gPP/molNi·h at 50 °C. The large steric hindrance of catalysts inhibited the chain-walking and chain-transfer reactions, resulting in polypropylene with high molecular weights (407~1101 kg/mol) and low 1,3-enchainment content (3.57~16.96%) in toluene. The low tensile strength (0.3~1.0 MPa), high elongation at break (218~403%) and strain recovery properties (S.R. ~50%, 10 tension cycles) of the resulting polypropylenes, as well as the visible light transmittance of approximately 90%, indicate the characteristics of the transparent elastomer. Full article

Since the discovery of α-diimine catalysts in 1995, an extensive series of Brookhart-type complexes have shown their excellence in catalyzing ethylene polymerizations with remarkable activity and a high molecular weight. However, although this class of palladium complexes has proven proficiency in catalyzing ethylene copolymerization with various polar monomers, the α-diimine nickel catalysts have generally exhibited a much worse performance in these copolymerizations compared to their palladium counterparts. Recently, Brookhart et al. reported a notable exception, demonstrating that α-diimine nickel catalysts could catalyze the ethylene copolymerization with some vinylalkoxysilanes effectively, producing functionalized polyethylene incorporating trialkoxysilane (-Si(OR)₃) groups. This breakthrough is significant since Pd-catalyzed copolymerizations are commercially less usable due to the high cost of palladium. Thus, the utilization of Ni, given its abundance in raw materials and cost-effectiveness, is a landmark in ethylene/polar vinyl monomer copolymerization. Inspired by these findings, we used density functional theory (DFT) calculations to investigate the mechanistic study of ethylene copolymerization with vinyltrimethoxysilane (VTMoS) catalyzed by Brookhart-type nickel catalysts, aiming to elucidate the molecular-level understanding of this unique reaction. Initially, the nickel complexes and cationic active species were optimized through DFT calculations. Subsequently, we explored the mechanisms including the chain initiation, chain propagation, and chain termination of ethylene homopolymerization and copolymerization catalyzed by Brookhart-type complexes. Finally, we conducted an energetic analysis of both the in-chain and chain-end of silane enchainment. It was found that chain initiation is the dominant step in the ethylene homopolymerization catalyzed by the α-diimine Ni complex. The 1,2- and 2,1-insertion of vinylalkoxysilane exhibit similar barriers, explaining the fact that both five-membered and four-membered chelates were identified experimentally. After the VTMoS insertion, the barriers of ethylene reinsertion become higher, indicating that this step is the rate-determining step, which could be attributed to the steric hindrance between the incoming ethylene and the bulky silane substrate. We have also reported the energetic analysis of the distribution of polar substrates. The dominant pathway of chain-end -Si(OR)₃ incorporation is suggested as chain-walking → ring-opening → ethylene insertion, and the preference of chain-end -Si(OR)₃ incorporation is primarily attributed to the steric repulsion between the pre-inserted silane group and the incoming ethylene molecule, reducing the likelihood of in-chain incorporation. Full article

For covalent attachment-supported α-diimine catalysts, on the basis of ensuring the thermal stability and activity of the catalysts, the important problem is that the active group on the catalyst can quickly react with the support, anchoring it firmly on the support, shortening the loading time, reducing the negative impact of the support on the active centers, and further improving the polymer morphology, which makes them suitable for use in industrial polymerization temperatures. Herein, we synthesized a α-diimine nickel(II) catalyst bearing four hydroxyl substituents. The hydroxyl substituents enable the catalyst to be immobilized firmly on silica support by covalent linkage in 5–10 min. Compared with the toluene solvent system, the homogeneous catalysts show high activity and thermal stability in hexane solvent at the same conditions. Compared with homogeneous catalysts, heterogeneous catalysis leads to improvements in catalyst lifetime, polymer morphology control, catalytic activity, and the molecular weight of polyethylene (up to 679 kg/mol). The silica-supported catalysts resulted in higher melting temperatures as well as lower branching densities in polyethylenes. Even at 70 °C, the polyethylene prepared by S-CatA-2 still exhibits dispersed particle morphology, and there is no phenomenon of reactor fouling, which is suitable for industrial polymerization processes. Full article

Recently, polyolefin thermoplastic elastomers can be obtained directly using ethylene as a single feedstock via α-diimine nickel-catalyzed ethylene chain walking polymerization. Here, a new range of bulky acenaphthene-based α-diimine nickel complexes with hybrid o-phenyl and -diarylmethyl anilines were constructed and applied to ethylene polymerization. All the nickel complexes under the activation of excess Et₂AlCl exhibited good activity (level of 10⁶ g mol⁻¹ h⁻¹) and produced polyethylene with high molecular weight (75.6–352.4 kg/mol) as well as proper branching densities (55–77/1000C). All the branched polyethylenes obtained exhibited high strain (704–1097%) and moderate to high stress (7–25 MPa) at break values. Most interestingly, the polyethylene produced by the methoxy-substituted nickel complex exhibited significantly lower molecular weights and branching densities, as well as significantly poorer strain recovery values (48% vs. 78–80%) than those by the other two complexes under the same conditions. Full article

A novel room-temperature stable diamagnetic nickel complex 2 was detected upon activation of Brookhart-type ethylene polymerization pre-catalyst LNiBr₂ (1, L = 1,4-bis-2,4,6-trimethylphenyl-2,3-dimethyl-1,4-diazabuta-1,3-diene) with AlMe₃. Using in situ ¹H, ²H, and ¹³C NMR spectroscopy, as well as DFT calculations, this species has been identified as an antiferromagnetically coupled homodinuclear complex [LNi^II(μ-Me)(μ-CH₂)Ni^IIL]⁺Br⁻. Its behavior in the reaction solution is characteristic of the resting state of nickel catalyzed ethylene polymerization. Full article

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. Full article

The less-expensive and earth-abundant nickel catalyst is highly promising in the copolymerization of ethylene with polar monomers and has thus attracted increasing attention in both industry and academia. Herein, we have summarized the recent advancements made in the state-of-the-art nickel catalysts with different types of ligands for ethylene copolymerization and how these modifications influence the catalyst performance, as well as new polymerization modulation strategies. With regard to α-diimine, salicylaldimine/ketoiminato, phosphino-phenolate, phosphine-sulfonate, bisphospnine monoxide, N-heterocyclic carbene and other unclassified chelates, the properties of each catalyst and fine modulation of key copolymerization parameters (activity, molecular weight, comonomer incorporation rate, etc.) are revealed in detail. Despite significant achievements, many opportunities and possibilities are yet to be fully addressed, and a brief outlook on the future development and long-standing challenges is provided. Full article

Four supported α-diimine nickel(II) catalysts covalently linked to silica via hydroxyl functionality on α-diimine acenaphthequinone-backbone were prepared and used in slurry polymerizations of ethylene to produce branched polyethylenes. The catalytic activities of these still reached 10⁶ g/molNi·h at 70 °C. The life of the supported catalyst is prolonged, as can be seen from the kinetic profile. The molecular weight of the polyethylene obtained by the 955 silica gel supported catalyst was higher than that obtained by the 2408D silica gel supported catalyst. The melting points of polyethylene obtained by the supported catalysts S-C1-a/b are all above 110 °C. Compared with the homogeneous catalyst, the branching numbers of the polyethylenes obtained by the supported catalysts S-C1-a/b is significantly lower. The polyethylenes obtained by supported catalyst S-C1-a/b at 30–50 °C are free-flowing particles, which is obviously better than the rubber-like cluster polymer obtained from homogeneous catalyst. Full article

Nickel(II) complexes with bidentate N,N-α-diimine ligands constitute a broad class of promising catalysts for the synthesis of branched polyethylenes via ethylene homopolymerization. Despite extensive studies devoted to the rational design of new Ni(II) α-diimines with desired catalytic properties, the polymerization mechanism has not been fully rationalized. In contrast to the well-characterized cationic Ni(II) active sites of ethylene polymerization and their precursors, the structure and role of Ni(I) species in the polymerization process continues to be a “black box”. This perspective discusses recent advances in the understanding of the nature and role of monovalent nickel complexes formed in Ni(II) α-diimine-based ethylene polymerization catalyst systems. Full article

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. Full article

Homo- and copolymers of propylene and 1-decene were synthesized by controlled chain-walking (co)polymerization using phenyl substituted α-diimine nickel complexes activated with modified methylaluminoxane (MMAO). This catalytic system was found to polymerize propylene in a living fashion to furnish high molecular weight ethylene-propylene (EP) copolymers. The copolymerizations proceeded to give high molecular weight P/1-decene copolymers with narrow molecular weight distribution (M_w/M_n ≈ 1.2), which indicated a living nature of copolymerization at room temperature. The random copolymerization results indicated the possibility of precise branched structure control, depending on the polymerization temperature and time. Full article

Catalysts that employ late transition-metals, namely Ni and Pd, have been extensively studied for olefin polymerizations, co-polymerizations, and for the synthesis of advanced polymeric structures, such as block co-polymers. Unfortunately, many of these catalysts often exhibit poor thermal stability and/or non-living polymerization behavior that limits their ability to access tailored polymer structures. Due to this, the development of catalysts that display controlled/living behavior at elevated temperatures is vital. In this manuscript, we describe a Ni α-diimine complex that is capable of polymerizing ethylene in a living manner at temperatures as high as 75 °C, which is one of the highest temperatures reported for the living polymerization of ethylene by a late transition metal-based catalyst. Furthermore, we will demonstrate that this catalyst’s living behavior is not dependent on the presence of monomer, and that it can be exploited to access polyethylene-based block co-polymers. Full article