Α-diimine Nickel Catalysts Research Articles

Benzyltriboronates: Building Blocks for Diastereoselective Carbon-Carbon Bond Formation.

A highly diastereoselective carbon-carbon bond-forming reaction involving the tandem coupling of benzyltriboronates, enoates, and alkyl halides is described. This method was enabled by the discovery of α-diimine nickel catalysts that promote the chemoselective triborylation of benzylic C(sp3)-H bonds using B2Pin2 (Pin = pinacolate). The C-H functionalization method is effective with methylarenes and for the diborylation of secondary benzylic C-H bonds, providing direct access to polyboron building blocks from readily available hydrocarbons. Combination of the benzylic perborylation with a new deborylative conjugate addition-alkylation method enables a one-pot procedure in which multiple simple precursors are combined to generate diastereopure products containing quaternary stereocenters.

Synthesis and application of fluorinated α-diimine nickel catalyst for ethylene polymerization: deactivation mechanism

A fluorinated LTM catalyst of [bis(N,N′-2-fluorophenylimino)acenaphthene] nickel(ΙΙ) dibromide was prepared and applied for ethylene polymerization. We established a reasonable hypothesis, highlighting the role of fluorine in ligand structure. In addition, density functional theory (DFT) studies have been applied to strengthen our propositions. DFT calculations of the β-agostic cationic species at room temperature and 75 °C suggest that the β-agostic structure for the latter was destabilized by 19 eV which rendered it less prone to β-H transfer reactions than the species at room temperature. In addition, a mechanism for the catalyst deactivation for fluorinated α-diimine nickel based catalysts has been proposed.

α-Diamine Nickel Catalysts with Nonplanar Chelate Rings for Ethylene Polymerization.

A series of novel α-diamine nickel complexes, (ArNH-C(Me)-(Me)C-NHAr)NiBr2 , 1: Ar=2,6-diisopropylphenyl, 2: Ar=2,6-dimethylphenyl, 3: Ar=phenyl), have been synthesized and characterized. X-ray crystallographic analysis showed that the coordination geometry of the α-diamine nickel complexes is markedly different from conventional α-diimine nickel complexes, and that the chelate ring (N-C-C-N-Ni) of the α-diamine nickel complex is significantly distorted. The α-diamine nickel catalysts also display different steric effects on ethylene polymerization in comparison to the α-diimine nickel catalyst. Increasing the steric hindrance of the α-diamine ligand by substitution of the o-methyl groups with o-isopropyl groups leads to decreased polymerization activity and molecular weight; however, catalyst thermal stability is significantly enhanced. Living polymerizations of ethylene can be successfully achieved using 1/Et2 AlCl at 35 °C or 2/Et2 AlCl at 0 °C. The bulky α-diamine nickel catalyst 1 with isopropyl substituents can additionally be used to control the branching topology of the obtained polyethylene at the same level of branching density by tuning the reaction temperature and ethylene pressure.

Alkene Hydrosilylation Using Tertiary Silanes with α-Diimine Nickel Catalysts. Redox-Active Ligands Promote a Distinct Mechanistic Pathway from Platinum Catalysts

Combination of the readily available α-diimine ligand, ((ArN═C(Me))2 Ar = 2,6-iPr2–C6H3), (iPrDI) with air-stable nickel(II) bis(carboxylates) generated a highly active catalyst exhibiting anti-Markovnikov selectivity for the hydrosilylation of alkenes with a variety of industrially relevant tertiary alkoxy- and siloxy-substituted silanes. A combination of the method of continuous variations with stoichiometric studies identified the formally Ni(I) hydride dimer, [(iPrDI)NiH]2 as the nickel compound formed following reduction of the carboxylate ligands. For the hydrosilylation of 1-octene with (EtO)3SiH, a rate law of [Ni]1/2[1-octene][(EtO)3SiH] in combination with deuterium-labeling studies establish dissociation of the nickel hydride dimer followed by fast and reversible alkene insertion into (iPrDI)NiH, consistent with turnover-limiting C–Si bond formation. The hydrosilylation of 1-octene with triethoxysilane, a reaction performed commercially in the silicones industry on a scale of >5 000 000 kg/year...

Semi-Crystalline Polar Polyethylene: Ester-Functionalized Linear Polyolefins Enabled by a Functional-Group-Tolerant, Cationic Nickel Catalyst.

A dibenzobarrelene-bridged, α-diimine Ni(II) catalyst (rac-3) was synthesized and shown to have exceptional behavior for the polymerization of ethylene. The catalyst afforded high molecular weight polyethylenes with narrow dispersities and degrees of branching much lower than those made by related α-diimine nickel catalysts. Catalyst rac-3 demonstrated living behavior at room temperature, produced linear polyethylene (Tm =135 °C) at -20 °C, and, most importantly, was able to copolymerize ethylene with the biorenewable polar monomer methyl 10-undecenoate to yield highly linear ester-functionalized polyethylene.

Immobilization of α-diimine nickel (II) catalysts on multi-walled carbon nanotube for slurry ethylene polymerization

Homogeneous α-diimine nickel (II) catalyst complexes, with and without amino para-aryl position functionality, were synthesized. These complexes were immobilized on carboxyl, hydroxyl, and acyl chloride functionalized multi-walled carbon nanotubes to form five novel heterogeneous α-diiminonickel catalysts. Immobilization was performed by covalent or electrostatic bonding via methylaluminoxane (MAO) linker or amide linkage. All the immobilized catalysts were fully characterized and their structures and supporting efficiencies were determined. Both the nature of α-diimine ligands and the kind of interaction between anchored catalyst complexes and multi-walled carbon nanotube surface influenced the catalytic performance, microstructure, and morphology of obtained polyethylenes. Catalysts immobilized by covalent bonds showed higher activities than physisorbed ones with the trend of activity rising versus temperature. Reaction between ligand amino para-aryl position and multi-walled carbon nanotube acyl chloride functionality resulted in immobilization through amide linkage. This procedure prepared catalyst with the lowest relative weight loss in thermogravimetry analysis and highest activities up to 5863 g PE mmol−1Ni h−1. The catalyst produced polyethylene with dense botryoidal morphology, highest molecular weight distributions and highest melting points up to 14.1 at 60 °C and 133 °C at 30 °C in polymerization trials, respectively.

Polymerization of α-Olefins Using a Camphyl α-Diimine Nickel Catalyst at Elevated Temperature

Chiral (1S)- and (1R)-camphyl α-diimine nickel complexes were synthesized respectively with (1S)-(+) camphorquinone and (1R)-(−) camphorquinone as raw reagents and used as catalyst precursors for olefin polymerizations. It is found that the ligand chirality has no influence on catalytic activity and regioselectivity for olefin polymerizations. Ethylene, propylene, 1-hexene, and 4-methyl-1-pentene polymerizations with the camphyl α-diimine nickel activated by AlEt2Cl can exhibit some living characteristics under the optimized conditions. The resultant polypropylenes and poly(1-hexene)s have significantly narrow molecular weight distributions (PDI < 1.2) in a wide temperature range, even at an elevated temperature of 70 °C. Sustainable period of the linear relationship of Mn vs polymerization time depends on temperature for propylene and 1-hexene polymerizations. Additionally, high 1,3-enchainment fraction of 45% is observed even at −60 °C for propylene polymerization using the camphyl α-diimine catalyst due to 2,1-insertion of propylene and chain walking.

Secondary Alkene Insertion and Precision Chain-Walking: A New Route to Semicrystalline “Polyethylene” from α-Olefins by Combining Two Rare Catalytic Events

While traditional polymerization of linear α-olefins (LAOs) typically provides amorphous, low T(g) polymers, chain-straightening polymerization represents a route to semicrystalline materials. A series of α-diimine nickel catalysts were tested for the polymerization of various LAOs. Although known systems yielded amorphous or low-melting polymers, the "sandwich" α-diimines 3-6 yielded semicrystalline "polyethylene" comprised primarily of unbranched repeat units via a combination of uncommon regioselective 2,1-insertion and precision chain-walking events.

Transition Metal Complexes of Allylated α-Diimines as Self-Immobilizing Catalysts for the Polymerization of Ethylene

A series of allylated α-diimine complexes of Ti, Zr, V, Cr, Fe, Ni, and Pd was prepared and characterized. These complexes were activated with methylaluminoxane (MAO) and tested for catalytic ethylene polymerization. The allyl groups of the ligands can self-immobilize the catalysts and prevent reactor fouling. The catalyst activities and the properties of the obtained polyethylenes are influenced by the metal center and the structure of the corresponding catalyst. The allylated α-diimine nickel catalysts showed the highest activities in the series and produced polyethylenes with higher molecular weights compared to the analogous catalysts without allyl groups. The allylated α-diimine palladium catalysts showed no activities for ethylene polymerization.

Synthesis of Highly Branched Polyethylene Using “Sandwich” (8-p-Tolyl naphthyl α-diimine)nickel(II) Catalysts

Nickel(II) α-diimine dibromide complexes incorporating 8-p-tolylnaphthylimino groups have been prepared and used for ethylene polymerization by activation with modified methylalumoxane (MMAO). These catalysts possess increased axial bulk relative to standard diimine-nickel complexes, resulting in lower rates of chain transfer relative to chain propagation rates and thus higher polymer molecular weights and narrower PDIs. They yield the most highly branched PE produced by Ni catalysts seen to date. The easy synthesis of 8-aryl-1-naphthylamines provides ready access to a new class of α-diimine-based nickel catalysts in which the 8-substituent is ideally positioned to provide steric bulk at the axial sites.

Highly active ortho-phenyl substituted α-diimine Nickel(II) catalysts for “chain walking polymerization” of ethylene: Synthesis of the nanosized dendritic polyethylene

Three new α-diimine Ni(II) complexes {bis[N,N′-(4-fluoro-2,6-diphenylphenyl)imino]acenaphthene}dibromonickel 4a, {bis[N,N′-(4-chloro-2,6-diphenylphenyl)imino]acenaphthene}dibromonickel 4b, and {bis[N,N′-(4-methyl-2,6-diphenylphenyl)imino]acenaphthene}dibromonickel 4c, were synthesized and characterized. The crystal structure of the complex 4a was determined by X-ray crystallography. Complex 4a has pseudo-tetrahedral geometry about the nickel center, showing C2v molecular symmetry. These complexes, activated by diethylaluminum chloride (DEAC) were tested in the polymerization of ethylene under mild conditions. Complex 4a bearing 2,6-diphenyl and strong electron-withdrawing 4-fluorine groups, activated by diethylaluminum chloride (DEAC) shows highly catalytic activity for the polymerization of ethylene [4.95×106gPE/(molNihbar)] and produced dendritic polyethylene (153.3branches/1000C). The dendritic polyethylene particle size obtained by 4a/DEAC can be controlled in the 1–20nm under 0.2bar ethylene pressure, and could be expected to become a nano-targeted drug carrier after modified with water-soluble oligo(ethylene glycol) (OEG).

Copolymerization of ethylene with unsaturated alcohols and methylmethacrylate using a silylated α-diimine nickel catalyst: Molecular modeling and photodegradation studies

In this article, the experimental results obtained in the copolymerization of ethylene with polar monomers using a silylated α-diimine nickel catalyst are described and compared with those obtained with a non-silylate similar one. The results show that the introduction of a siliyl group in the para-position of the imine moiety turns the catalyst less sensitive toward polar functionalities. However, the reactivity observed in copolymerization reactions was found to be reduced both the global polymerization and incorporation rates. Molecular modeling calculation allowed us to understand both the preferred insertion mode and the intermediate product structures for E/MMA copolymerization. Photodegradation studies showed that the presence of polar groups in the copolymers increases their rate of oxidation and turn them more easily degradable. The stabilization performed by Tinuvin 770 showed that for long-exposure times, it is not very effective regardless it is only mixed or grafted to the copolymer. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Synthesis and characterization of ethylene-propylene copolymer and polyethylene using α-diimine nickel catalysts

Nickel-based catalysts of dibromo[N,N′-bis(2,6-diisopropylphenyl)-2,3-butanediimine]nickel(II) (A) and dibromo[N,N′-(phenanthrene-9,10-diylidene)bis(2,6-diisopropylaniline)]nickel(II) (B) were synthesized under controlled conditions. The catalysts A, B and mixed of 1:1 weight ratio of them were used for ethylene-propylene (EP) copolymerization and ethylene polymerization. Methylaluminoxane (MAO) was used as a cocatalyst. The polymers were characterized by means of FT-IR, 1H NMR and DSC (Differential scanning calorimetry). In ethylene polymerization using mixed of 1:1 w/w of the catalysts A and B degree of branching polymer chain decreased with increasing monomer pressure while increased with enhancing polymerization temperature. In ethylene-propylene copolymerization using mixed of 1:1 w/w of A and B, the catalytic system showed the maximum activity at about [Al]/[Ni] = 2000:1 ratio and further addition of MAO did not affect the activity of the catalyst. Increasing the copolymerization temperature from 20 to 80 °C decreased the productivity of the catalytic system while increased the ethylene content of the copolymer. Decreasing relative pressure of P Pr /P Et from 9:1 to 1:1 increased the ethylene contents of the copolymers from 51 % to 72 % while decreased the viscosity average molecular weight (M v ) and glass transition temperature (T g ) of the products. A copolymer with more rubber-like behavior was obtained in relative pressure of P Pr /P Et (5:1) and total pressure of 5 bars.

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In the field of olefin coordination polymerization there is a demand to design new catalysts with high activity, high stability, controlled polymer structure and properties. Late transition metal catalysts have attracted much attention due to their ease of modification, the capability of controlling branching-structure of olefin polymer and tolerance towards polar monomers. In this paper, we summarized the recent work of our group on designing nickel catalysts with [ N,N ]-bidentate ligands for olefin polymerization, including monoamidinate, α-diimine, 2-amine pyridine, amine-imine, β-diimine and anilido-imine nickel catalysts. Living polymerization of olefins could be successfully realized via subtle modification of catalyst structure.

Control of morphology in olefin polymerization catalyzed by nickel diimine complexes encapsulated into polybutadiene based support

A commercially available hydroxyl terminated low molecular cis-1,4-polybutadiene was modified by carboxylic functions and used as an effective support in single-site α-olefin polymerization. Dynamic light scattering analysis of support solution in heptane confirmed increased ability to self-assembly of modified polybutadienes in the presence of methylaluminoxane cocatalyst (MAO). α-Diimine nickel and bis(indenyl)zirconium dichloride catalysts activated by trimethylaluminium or MAO cocatalysts were encapsulated into polymer micellar aggregates and used for ethylene and octadec-1-ene polymerization. Furthermore, the effects of cocatalyst type and concentration on the micelle-formation process, activity and morphology of polyethylene (polyoctadec-1-ene) particles were investigated. In the presence of polybutadiene supports, formation of spherical micrometric PE beads was observed during MAO cocatalyzed ethylene polymerization, whereas the activity of polymerization was not decreased.

Polymerization of 4-methyl-1-pentene catalyzed by α-diimine nickel catalysts: Living/controlled behavior, branch structure, and mechanism

Branched α-olefin, 4-methyl-1-pentene (4 MP), was polymerized with classical α-diimine nickel complexes in the presence of MAO. Influences of structure of α-diimine nickel catalysts and polymerization parameters including temperature and [Al]/[Ni] mole ratio were evaluated. At 0 °C, 4-methyl-1-pentene can be polymerized in a living/controlled manner. The obtained poly(4-methyl-1-pentene)s are amorphous elastomers with low glass transition temperature (Tg). Nuclear magnetic resonance (NMR) and distortionless enhancement by polarization transfer (DEPT) analyses show that various types of branches and microstructural units are present in the polymers. On the basis of assignment of microstructures, mechanistic models that involves the 1,2- and 2,1-insertion, and chain walking were constructed. The influences of temperature and [Al]/[Ni] mole ratio on branching degree, branch type, and insertion pathways were also discussed in detail.

Polymerization of ethylene by (α-Diimine) nickel catalyst and statistical analysis of the effects of reaction conditions

This article regards the ethylene polymerization catalyzed by a nickel catalyst and activated by ethylaluminum sesquichloride (EASC). The effects of the reaction conditions [polymerization temperature, cocatalyst (EASC) concentration, and ethylene concentration] on the average molecular weights of the final polymers and reaction yields were evaluated with the help of empirical statistical models. It is shown that reaction temperature and cocatalyst (EASC) concentration exert the most important effects on average molecular weights and catalyst activity. The polydispersities of the obtained polyethylenes are larger than the polydispersities of polyethylenes obtained with typical Brookhart catalysts. The analysis of polymer branching frequencies shows new types of short branching and significant amounts of long branches, which may explain the relatively large polydispersities of the obtained polymer samples. POLYM. ENG. SCI., 50:1797–1808, 2010. © 2010 Society of Plastics Engineers

Gas Phase Polymerization of Ethylene with Supported α-Diimine Nickel(II) Catalysts

An efficient synthetic strategy for new 2,5- and 2,6-substituted unbridged and 1,4-dithiane bridged ligands is presented. The reaction of the latter compounds with Ni(acac)2 and trityl tetrakis(pentafluorophenyl)borate gave the corresponding Ni(II) complexes in high yields. The structure of one of these complexes was determined by X-ray analysis. These complexes were supported on silica without a chemical tether and were used as catalysts for ethylene polymerization reactions in the gas phase. Furthermore, ethylene was polymerized with the unsupported 2,5-complexes in homogeneous solution for comparison. The influence of the ligand structure, hydrogen and temperature on the polymerization performance was investigated. The supported catalysts showed moderate to high activities and produced polyethylenes ranging from HDPE to LLDPE, without further addition of an 1-olefin comonomer. In contrast to 2,6-complexes, which generate high molecular weight polyethylene, the 2,5-compounds afford materials of lower molecular weight comprising terminal and internal double bonds. In addition, video microscopy experiments allowed to investigate the growth of single polyethylene particles. Electron microscopy was applied to show that their morphology is a replicate of the starting catalyst grains.

Thermostable α-Diimine Nickel(II) Catalyst for Ethylene Polymerization: Effects of the Substituted Backbone Structure on Catalytic Properties and Branching Structure of Polyethylene

On the basis of the strategy of promoting thermostability of α-diimine nickel catalyst by ligand backbone framework, a series of α-diimine nickel(II) complexes with bulky camphyl or diaryl-substituted backbones, [2,6-(R2)2C6H3−N═C(R1)−C(R1)═N−2,6-(R2)2C6H3]NiBr2 (1a, R1 = Ph, R2 = CH3; 2a, R1 = 4-methylphenyl, R2 = CH3; 3a, R1 = 4-fluorophenyl, R2 = CH3; 4a, R1 = camphyl, R2 = CH3; 4b, R1 = camphyl, R2 = i-Pr), were synthesized and used as catalyst precursors for ethylene polymerization. Crystallographic analysis revealed that the bulky camphyl backbone has a valid steric-effect on the nickel center by blocking the axial site for the metal center and suppressing the potential rotation of the CAr−N bond. Ethylene polymerizations catalyzed by these nickel α-diimine complexes activated by MAO were systematically investigated and the influences of the substituted backbones as well as reaction temperature on the catalytic activity, molecular weight and branching structure of the polymers were evaluated. It was...

Ethylene Polymerizations with Unsymmetrical (α-Diimine)nickel(II) Catalysts

Four unsymmetrical (α-diimine)nickel(II) catalysts were synthesized by sequential stepwise condensation of acenaphthenequinone with the corresponding aniline compounds and subjected to ethylene polymerizations with MAO. Interestingly, when the catalysts have the same composition of the ortho substituents, the catalyst with two ortho isopropyl substitients on one side and two ortho methyl substituents on the other side produced polyethylenes with higher branching densities and lower molecular weights than the catalyst with one ortho isopropyl and one ortho methyl substituent on each side. However, branch distribution was not changed by the position of the ortho substituents.