Copolymerization of Ethylene and Polar Monomers by Using Ni/IzQO Catalysts

The replacement of precious metals in catalysis by earth‐abundant metals is currently one of the urgent challenges for chemists. Whereas palladium‐catalyzed copolymerization of ethylene and polar monomers is a valuable method for the straightforward synthesis of functionalized polyolefins, the corresponding nickel‐based catalysts have suffered from poor thermal tolerance and low molecular weight of the polymers formed. Herein, we report a series of neutral nickel complexes bearing imidazo[1,5‐a]quinolin‐9‐olate‐1‐ylidene (IzQO) ligands. The Ni/IzQO system can catalyze ethylene polymerization at 50–100 °C with reasonable activity in the absence of any cocatalyst, whereas most known nickel‐based catalysts are deactivated at this temperature range. The Ni/IzQO catalyst was successfully applied to the copolymerization of ethylene with allyl monomers to obtain the corresponding copolymers with the highest molecular weight reported for a Ni‐catalyzed system.     

过渡金属乙烯齐聚与聚合催化剂研究进展

综述了近年来过渡金属配合物催化乙烯齐聚与聚合的最新进展;介绍了乙烯齐聚或聚合的反应时间、反应温度、乙烯压力、助催化剂用量等反应条件及配体上不同取代基对前过渡金属(铬,锆,钛,钒)和后过渡金属(铁,钴,镍,铜)配合物的催化活性和齐聚或聚合产物的影响;分别以钛和镍配合物催化剂为例,介绍了前过渡金属和后过渡金属催化乙烯齐聚或聚合的机理。     

Neutral Nickel Ethylene Oligo‐ and Polymerization Catalysts: Towards Computational Catalyst Prediction and Design

DFT calculations have been used to elucidate the chain termination mechanisms for neutral nickel ethylene oligo‐ and polymerization catalysts and to rationalize the kind of oligomers and polymers produced by each catalyst. The catalysts studied are the (κ²‐O,O)‐coordinated (1,1,1,5,5,5‐hexafluoro‐2,4‐acetylacetonato)nickel catalyst I , the (κ²‐P,O)‐coordinated SHOP‐type nickel catalyst II , the (κ²‐N,O)‐coordinated anilinotropone and salicylaldiminato nickel catalysts III and IV , respectively, and the (κ²‐P,N)‐coordinated phosphinosulfonamide nickel catalyst V . Numerous termination pathways involving β‐H elimination and β‐H transfer steps have been investigated, and the most probable routes identified. Despite the complexity and multitude of the possible termination pathways, the information most critical to chain termination is contained in only few transition states. In addition, by consideration of the propagation pathway, we have been able to estimate chain lengths and discriminate between oligo‐ and polymerization catalysts. In agreement with experiment, we found the Gibbs free energy difference between the overall barrier for the most facile propagation and termination pathways to be close to 0 kcal mol^?1 for the ethylene oligomerization catalysts I and V , whereas values of at least 7 kcal mol^?1 in favor of propagation were determined for the polymerization catalysts III and IV . Because of the shared intermediates between the termination and branching pathways, we have been able to identify the preferred cis/trans regiochemistry of β‐H elimination and show that a pronounced difference in σ donation of the two bridgehead atoms of the bidentate ligand can suppress hydride formation and thus branching. The degree of rationalization obtained here from a handful of key intermediates and transition states is promising for the use of computational methods in the screening and prediction of new catalysts of the title class.     

Regiocontrolled synthesis of poly(3‐alkylthiophene)s by Grignard metathesis

Poly(3‐octadecylthiophene)s (P3OTs) were prepared via Grignard metathesis polymerization of 2,5‐dibromo‐3‐octadecylthiophene in the presence of palladium and nickel catalysts. The effect of catalyst structure and reaction temperature on the regioregularity of P3OTs was investigated. Nickel catalysts provided P3OTs with higher regioregularity, while palladium catalysts gave lower regioregularity. Surprisingly, the regioregularity of P3OTs increased when the polymerization was conducted at higher temperature. The catalyst and temperature dependence of the regioregularity is consistent with two competing mechanisms. Polymerizations at higher temperature with nickel catalysts occur primarily via chain‐growth reactions, while polymerizations at lower temperature with palladium catalysts have competing step‐growth and chain growth reactions. P3OTs with higher regioregularity have longer wavelength visible absorptions, while P3OTs with lower regioregularity have shorter wavelength absorptions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5538–5547, 2004     

Effect of catalyst transition metal and ancillary ligand on syndiospecific polymerization of styrene

In the coordination polymerization of styrene, selected transition metal complexes of metals other than group 4 elements and non-metallocenes have been investigated in comparison to a known half-metallocene titanium complex with regard to the catalytic activity as well as to the thermal and molecular properties of the polymers synthesized. Whereas iron catalysts lead to syndiotactic polystyrenes, catalysts with nickel as the transition metal result only in atactic polymers with an enhanced isotactic content.In addition to the influence of the transition metal, the effect of a broad variation of the ancillary ligands of a specific half-sandwich titanocene, octahydrofluorenyl titanium trimethoxide, on polymerization activity and polymer properties has been investigated and discussed in detail.     

Thermodynamics of titanium and vanadium reduction in non-aqueous environment calculated at various levels of theory

Reduction of titanium and vanadium compounds is a process accompanying the activation of coordinative olefin polymerization catalysts. Four density functional theory (DFT) functionals, coupled cluster with single, double, and perturbative triple excitations method CCSD(T) as well as complete active-space second-order perturbation theory method CASPT2 with a complete active-space self-consistent field CASSCF reference wave function were applied to investigate the thermodynamics of titanium and vanadium reduction. The performance of these theoretical methods was assessed and compared with experimental values. The calculations indicate that vanadium(IV) chloride is more easily reduced by trimethylaluminum than the corresponding titanium compound; the energies of reaction calculated at the CCSD(T) level are equal -57.21 and -33.10 kcal/mol, respectively. The calculations deal with the redox reactions of metal chlorides in the gas phase, rather than solvated ions in the aqueous solution. This approach may be more appropriate for olefin polymerization, usually carried out in nonpolar solvents.     

Stabilized Vanadium Catalyst for Olefin Polymerization by Site Isolation in a Metal–Organic Framework

Vanadium catalysts offer unique selectivity in olefin polymerization, yet are underutilized industrially owing to their poor stability and productivity. Reported here is the immobilization of vanadium by cation exchange in MFU‐4l, thus providing a metal–organic framework (MOF) with vanadium in a molecule‐like coordination environment. This material forms a single‐site heterogeneous catalyst with methylaluminoxane and provides polyethylene with low polydispersity (PDI≈3) and the highest activity (up to 148 000 h^?1) reported for a MOF‐based polymerization catalyst. Furthermore, polyethylene is obtained as a free‐flowing powder as desired industrially. Finally, the catalyst shows good structural integrity and retains polymerization activity for over 24 hours, both promising attributes for the commercialization of vanadium‐based polyolefins.     

Facile synthesis of sterically demanding SHOP‐type nickel catalysts for ethylene polymerization

The P,O‐chelated shell higher olefin process (SHOP) type nickel complexes are practical homogeneous catalysts for the industrial preparation of linear low‐carbon α‐olefins from ethylene. We describes that a facile synthetic route enables the modulation of steric hindrance and electronic nature of SHOP‐type nickel complexes. A series of sterically bulky SHOP‐type nickel complexes with variable electronic nature, {[4‐R‐C₆H₄C(O) = C‐PArPh]NiPh (PPh₃); Ar = 2‐[2′,6′‐(OMe)₂C₆H₃]C₆H₄; R = H ( Ni1 ); R = OMe ( Ni2 ); R = CF₃ ( Ni3 )}, were prepared and used as single component catalysts toward ethylene polymerization without using any phosphine scavenger. These nickel catalysts exhibit high thermal stability during ethylene polymerization and result in highly crystalline linear α‐olefinic solid polymer. The catalytic performance of the SHOP‐type nickel complexes was significantly improved by introducing a bulky ortho‐biphenyl group on the phosphorous atom or an electron‐withdrawing trifluoromethyl on the backbone of the ligand, indicating steric and electronic effects play critical roles in SHOP‐type nickel complexes catalyzed ethylene polymerization.     

Design of Thermally Stable Amine–Imine Nickel Catalyst Precursors for Living Polymerization of Ethylene: Effect of Ligand Substituents on Catalytic Behavior and Polymer Properties

Nickel complexes bearing amine–imine ligands with various backbone substituents were synthesized and employed as ethylene polymerization catalysts on activation with Et₂AlCl. The substituent on the backbone carbon atom of the amine moiety is decisive for the living nature of ethylene polymerization. A bulky amine–imine nickel precursor with a tert‐butyl group on the carbon atom of the amine group can polymerize ethylene in a living fashion at an elevated temperature of 65 °C, which is the highest temperature of living polymerization of ethylene with late transition‐metal catalysts. The wide applicable temperature range for living polymerization and sensitivity of the branch structure of the polyethylene to temperature enable precise synthesis of di‐ and triblock polyethylenes featuring different branched segments by sequential tuning of the polymerization temperature.     

Titanium Salan/Salalen Complexes: The Twofaced Janus of Asymmetric Oxidation Catalysis

Optically pure chiral epoxides and sulfoxides are ubiquitous building blocks in fine organic synthesis, employed in the pharmaceutical, agrochemical, and cosmetic industries. On the road to chiral epoxides and sulfoxides, efficient and stereoselective transition metal‐based catalysts are the most promising guides. Among transition metals, we favor titanium for its cheapness and availability, nontoxicity, and well‐known ability to catalyze a variety of stereoselective transformations, including oxidations with environmentally benign H₂O₂. In this personal account, we summarize the state‐of‐the‐art of rational design of chiral titanium(IV) salan and salalen catalysts, and investigations of their catalytic reactivities and stereoselectivities in the epoxidations of olefins and oxidations of thioethers, unraveling the peculiarities and mechanisms of their catalytic action.     

Nickel‐Catalyzed Allylic Alkylation with Diarylmethane Pronucleophiles: Reaction Development and Mechanistic Insights

Palladium‐catalyzed allylic substitution reactions are among the most efficient methods to construct C?C bonds between sp³‐hybridized carbon atoms. In contrast, much less work has been done with nickel catalysts, perhaps because of the different mechanisms of the allylic substitution reactions. Palladium catalysts generally undergo substitution by a “soft”‐nucleophile pathway, wherein the nucleophile attacks the allyl group externally. Nickel catalysts are usually paired with “hard” nucleophiles, which attack the metal before C?C bond formation. Introduced herein is a rare nickel‐based catalyst which promotes substitution with diarylmethane pronucleophiles by the soft‐nucleophile pathway. Preliminary studies on the asymmetric allylic alkylation are promising.     

Multi‐frequency (S,X, Q and W‐band) EPR and ENDOR Study of Vanadium(IV) Incorporation in the Aluminium Metal–Organic Framework MIL‐53

Doping the well‐known metal–organic framework MIL‐53(Al) with vanadium(IV) ions leads to significant changes in the breathing behaviour and might have repercussions on the catalytic behaviour as well. To understand the properties of such a doped framework, it is necessary to determine where dopant ions are actually incorporated. Electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) are applied to reveal the nearest environment of the paramagnetic vanadium(IV) dopant ions. EPR spectra of as‐synthesised vanadium‐doped MIL‐53 are recorded at S‐, X‐, Q‐ and W‐band microwave frequencies. The EPR spectra suggest that at low dopant concentrations (1.0–2.6 mol %) the vanadium(IV) ions are well dispersed in the matrix. Varying the vanadium dopant concentration within this range or the dopant salt leads to the same dominant EPR component. In the ENDOR spectra, hyperfine (HF) interactions with ¹H, ²⁷Al and ⁵¹V nuclei are observed. The HF parameters extracted from simulations strongly suggest that the vanadium(IV) ions substitute Al in the framework.     

Nickel N‐heterocyclic carbene complexes in the vinyl polymerization of norbornene

Vinyl polymerized norbornene has some useful properties such as good mechanical strength, optical transparency and heat resistance. Several transition metal complexes have been described in the literature as active catalysts for the vinyl polymerization of norbornene. We now report the use of three types of nickel(II) complexes with N‐heterocyclic carbene (NHC) ligands in the catalytic vinyl polymerization of norbornene under a range of conditions. Specifically, two nickel complexes bearing a chelating bis(NHC) ligand, two nickel complexes bearing two chelating anionic N‐donor functionalized NHC ligands as well as one diiodidonickel(II) complex with two monodentate NHC ligands were tested. The solid‐state structure of bis(1,3‐dimethylimidazol‐2‐ylidene)diiodidonickel(II), as determined by X‐ray crystallography, is presented. The highest polymerization activity of 2.6 × 10⁷ g (mol cat)^?1 h^?1 was observed using the latter nickel complex as catalyst, activated by methylaluminoxane. The norbornene polymers thus obtained are of high molecular weight but with rather low polydispersity. Copyright © 2010 John Wiley & Sons, Ltd.     

Polymerization of cyclobutene rings. VI. Influence of the organometallic compound of the Ziegler-Natta catalysts on the mechanism of polymerization of cyclobutene and 3-methylcyclobutene

The polymerization of cyclobutene and 3-methylcyclobutene in the presence of Ziegler-Natta catalysts was studied especially in view of the influence of the metal alkyls on the course of polymerization. Several metal alkyls of the main groups I–IV of the periodic table of elements were investigated, vanadium halides being used as transition metal compounds. The influence of the metal alkyl was found not to be limited to the activity of the catalyst system, which is maximum for aluminum alkyl, but also to extend to the polymerization mechanism. Depending on the metal alkyl, the two cyclobutene monomers can in fact be polymerized by two different mechanisms, i.e., by opening of the double bond or by ring opening with retention of the double bond. No migration of the double bond is involved in these polymerizations. An interpretation of the different experimental observations is tentatively proposed, and the relationship with the stereospecific polymerization of the α-olefins is stressed.     

水杨醛亚胺钒（III）化合物催化乙烯聚合反应机理

钒系烯烃聚合催化剂在工业上有着不可替代的位置,它可用于制备高活性窄分布的聚合物、乙烯与α-烯烃共聚物和间规聚丙烯等。但由于实验手段难以确定钒催化剂活性物种的结构,进一步对催化机理的确认及催化剂结构的改进十分困难。本文运用密度泛函方法对水杨醛亚胺钒配合物催化乙烯聚合的活性物种结构进行了理论研究。对多种活性物种模型的比较研究结果表明,对此催化反应最有利的活性物种为中性双金属物种a1, a1结构中包含两个连接铝原子与钒中心的氯桥结构。研究同时表明,助催化剂AlEt2Cl的存在不仅加速了钒配合物前体的烷基化反应,同时其对活性物种a1结构中氯桥的形成至关重要。最后还研究了该催化体系的链终止反应机理。     

Polymerization of Substituted Acetylenes Carrying Non‐Polar and Polar Groups with Transition Metal Acetylide Catalysts

A new series of single‐component air‐stable transition metal acetylide catalysts for the polymerization of substituted acetylenes carrying non‐polar and polar groups was developed. The catalytic properties of transition metal acetylides are related to transition metals, phosphines and acetylenic ligands. Nickel acetylides show higher catalytic activity towards the polymerization of substituted acetylenes containing non‐polar groups, while palladium acetylides show higher catalytic activity towards the polymerization of substituted acetylenes carrying polar groups. Palladium acetylides with PPh₃ ligand are efficient catalysts for the polymerization of substituted acetylenes bearing both non‐polar and polar groups.     

Remote Perfluoroalkyl Substituents are Key to Living Aqueous Ethylene Polymerization

In various nickel(II) salicylaldiminato ethylene polymerization catalysts, which are a versatile mechanistic probe for substituent effects, longer perfluoroalkyl groups exert a strong effect on catalytic activities and polymer microstructures compared to the trifluoromethyl group. This effect is accounted for by a reduced electron density on the active sites, and is also supported by electrochemical studies. Thus, β‐hydride elimination, the key step of chain transfer and branching pathways, is disfavored while chain‐growth rates are enhanced. This enhancement occurs to an extent that enables living polymerizations in aqueous systems to afford ultra‐high‐molecular‐weight polyethylene for various chelating salicylaldimine motifs. These findings are mechanistically instructive as well as practically useful for illustrating the potential of perfluoroalkyl groups in catalyst design.     

A Second‐Coordination‐Sphere Strategy to Modulate Nickel‐ and Palladium‐Catalyzed Olefin Polymerization and Copolymerization

Transition‐metal‐catalyzed copolymerization reactions of olefins with polar‐functionalized comonomers are highly important and also highly challenging. A second‐coordination‐sphere strategy was developed to address some of the difficulties encountered in these copolymerization reactions. A series of α‐diimine ligands bearing nitrogen‐containing second coordination spheres were prepared and characterized. The properties of the corresponding nickel and palladium catalysts in ethylene polymerizations and copolymerizations were investigated. In the nickel system, significant reduction in polymer branching density was observed, while lower polymer branching densities, as well as a wider range of polar monomer substrates, were achieved in the palladium system. Control experiments and computational results reveal the critical role of the metal−nitrogen interaction in these polymerization and copolymerization reactions.     

Ni‐Catalyzed Amination Reactions: An Overview

Nitrogen‐containing organic compounds are valuable in many fields of science and industry. The most reliable method for the construction of C(sp²)–N bonds is undoubtedly palladium‐catalyzed amination. In spite of the great achievements made in this area, the use of expensive Pd‐based catalysts constitutes an important limitation for large‐scale applications. Since nickel is the least expensive and most abundant among the group 10 metals, the interest in Ni‐based catalysts for processes typically catalyzed by palladium has grown considerably over the last few years. Herein, we revise the development of Ni‐catalyzed amination reactions, emphasizing the most relevant and recent advances in the field.     

Tuning of nickel 2-phosphinophenolates - catalysts for oligomerization and polymerization of ethylene

This short personal account summarizes recent work concerning the syntheses and structural aspects of neutral and cationic (organo)nickel and (organo)palladium 2-phosphinophenol(ate) complexes and catalysts for the oligo- or polymerization of ethylene.