Olefin Polymerization Catalysts Research Articles

Positional heterogenization effect in salicylaldimine nickel catalyzed ethylene polymerization

Positional heterogenization effect in salicylaldimine nickel catalyzed ethylene polymerization

Orbital-scale understanding of the surface effect of the MgCl2 in the Ziegler–Natta catalyst for ethylene polymerization: A computational study

Ziegler-Natta catalyst is a widely used olefin polymerization catalyst in petrochemical industries. However, the role of each catalyst component is not clear due to the complexity of composition and structure. MgCl2 is a suitable support for Ziegler-Natta catalyst because its crystal structure is similar to that of TiCl3, and the surface effect of MgCl2 in Ziegler-Natta catalyst system was explored in ethylene polymerization at the orbital scale. A series of the stable surface-supported catalysts with unsaturated coordination Mg2+ were used to simulate the mechanism of ethylene polymerization by the periodic density functional theory. This study found that MgCl2 with different surfaces regulates the activity of the catalyst by adjusting the d band centre of the catalyst active centre. This concept can be used to explain the surface effect of support in general catalysts.

Homo- and hetero-polymerization of ethylene catalyzed by α-diimine nickel complexes with hydroxyl groups

Heterogeneous olefin polymerization catalysts have received wide interests from academia and industry because their distinct advantages in thermostability, avoidance of reactor fouling and continuous production operations. In this work, hydroxy functional groups are introduced to the α-diimine nickel framework, which can serve as reactive functionality to react with MgCl2/AlRn(OEt)3-n and form ionic anchoring interaction that bind the catalyst strongly to the surface of the supports. These heterogeneous catalysts are highly active in ethylene polymerization to produce polyethylene with higher molecular weight (up to 280.5 × 104 g·mol−1), lower branching density and higher tensile strength (up to 26.7 MPa) than those generated by the homogeneous counterparts. More importantly, the nickel complexes with multiple hydroxy groups lead to high loading capacity on MgCl2/AlRn(OEt)3-n supports, affording polyethylene products with minimum inorganic contamination.

Recent advancements in multinuclear early transition metal catalysts for olefin polymerization through cooperative effects

Abstract The pursuit of high-performance polyolefin materials has been an objective for chemists. Recently, the development of multinuclear catalysts has significantly enabled the preparation of high-performance polyolefin materials. In this review, we mainly focus on the cooperative effects of multinuclear early transition metal catalysts and provide a comprehensive summary of the research progress in binuclear and multinuclear early transition metal catalysts over the past decade. Compared with the catalytic performance and polymer structure of these catalysts with mononuclear catalysts, we have found that various factors, such as steric hindrance effect, electronic effect, heteroatom effect, hydrogen bonding interaction, the distance between metal centers in multinuclear metal catalysts, and the use of additives, have distinctive and significant impacts on catalyst performance. These effects give rise to notable cooperative effects. This review offers crucial insights and serves as a valuable reference for shaping the future development directions of binuclear and multinuclear early transition metal catalysts.

Mono(phenoxy-imine) Group 4 Trialkyl Complexes: A Family of Olefin Polymerization Catalysts

Mono(phenoxy-imine) Group 4 Trialkyl Complexes: A Family of Olefin Polymerization Catalysts

Mono- and bi-nuclear phosphine-phenolate neutral nickel catalysts via simple Schiff-Base condensation for ethylene polymerization

In catalytic olefin polymerization, a significant amount of effort has been invested to modify either phosphine substituent or ligand backbone in recently advanced phosphine-phenolate [P,O] neutral nickel catalysts. Unlike other commonly used ligands like phenoxy-imines [N,O] and α-diimines [N,N], [P,O] ligands often require a laborious synthesis technique, which has hindered the rational design of ligand structures and the development of new catalysts. Here we present a novel and effective strategy for creating catalysts using [N,O,P] hybrid ligands. The substituent adjacent to the phenoxy group can be efficiently modified through straightforward Schiff-base condensation reactions with various amines that have established synthetic routes and diverse structures. This process results in the preparation of a series of highly active [P,O] nickel catalysts (up to 1.7 × 107 g mol-1h−1) that are capable of producing ultrahigh molecular weight polyethylene (UHMWPE) (Mw = 243.7 × 104 g/mol) in ethylene polymerization. The additional binding site greatly facilitates modulation of catalytic properties by additives, and parameters such as catalytic activity, polymer molecular weight, and polymer branching can be tuned in this way. Furthermore, binuclear [P,O] neutral nickel catalysts for olefin polymerization are synthesized for the first time using a sequence of diamines. The strategy presented in this work greatly improves the efficiency for the design and synthesis of [P,O] nickel and related catalysts. These findings will be advantageous for future catalyst design and screening.

Donor Ability of Bisphosphinemonoxide Ligands Relevant to Late-Metal Olefin Polymerization Catalysis.

Late-transition-metal catalysts for polymerization of olefins have drawn a significant amount of attention owing to their ability to tolerate and incorporate polar comonomers. However, a systematic way to experimentally quantify the electronic properties of the ligands used in these systems has not been developed. Quantified ligand parameters will allow for the rational design of tailored polymerization catalysts, which would target specific polymer properties. We report a series of platinum complexes bearing bisphosphinemonoxide ligands, which resemble those used in the polymerization catalysts of Nozaki and Chen. Their electronic properties are investigated experimentally, and trends are rationalized by using computed spectral properties. Benchmarking computational data with known experimental parameters further enhances the utility of both methods for determining optimal ligands for catalytic application.

Dinuclear group IV metal complexes based on a bis(indenyl)-(E/Z)-stilbene platform: a potential prototype of "photoswitchable" catalysts for olefin polymerization.

The preparation of dizirconium complexes based on a novel bis(indenyl)-(E/Z)-stilbene platform was explored. Negishi coupling between the in situ-generated diorganozincates obtained from the respective o/m/p-(E/Z)-dibromostilbenes and the bromo-functionalized zirconocene (η5-Cp*)(η5-2-methyl-4-bromoindenyl)ZrCl2, or, alternatively, the preparation of bis(indene)stilbene pro-ligands {o/m/p-(E/Z)-BisIndSB}H2 through Negishi coupling of the corresponding dibromostilbenes with 4-bromoindene and subsequent metallation/transmetallation with Cp*ZrCl3 or Zr(NMe2)4, allowed the preparation of a series of dinuclear complexes. These were analyzed by NMR spectroscopy and some of them by iASAP-mass spectrometry and by X-ray diffraction studies. Experimental results were compared with DFT modelling of the targeted dinuclear complexes evidencing that the (E)-complexes are more stable by 7-11 kcal mol-1 than their (Z)-analogues. Thermal, uncontrolled isomerization of (Z)- to (E)-stilbene platform was observed experimentally for some systems, in the course of their synthesis, either from the (Z)-dibromostilbene reagent or on the dinuclear complexes resulting from the Negishi coupling. Photoisomerization of the (E)- and (Z)-{BisIndSB}H2 proligands and of complexes {o-(E)-BisIndSB}(Zr(NMe2)3)2 and {m-(E)-BisIndSB}(ZrCl2Cp*)2 was investigated under a variety of conditions. It proved effective for the proligands but induced decomposition of the dizirconium complexes. Time-dependent DFT (TD-DFT) computations were performed to identify unambiguously the nature of the observed absorption bands and account for decomposition of the complexes. Preliminary ethylene/1-hexene homo- and copolymerization investigations did not evidence putative cooperativity phenomena within these dinuclear systems nor significantly differentiated behavior between the (Z)- and (E)-isomers of a given type of complex under the reaction conditions investigated.

Cocatalyst effects in Hf-catalysed olefin polymerization: taking well-defined Al-alkyl borate salts into account.

Hafnium catalysts for olefin polymerization are often very sensitive to the nature of cocatalysts, especially if they contain "free" aluminium trialkyls. Herein, cocatalyst effects in Hf-catalysed propene polymerization are examined for four Hf catalysts belonging to the family of CS-symmetric (Hf-CS-Met) and C2-symmetric (Hf-C2-Met) metallocenes, as well as of octahedral (Hf-OOOO) and pentacoordinated (Hf-PyAm) "post-metallocenes". The performance of the recently developed {[iBu2(PhNMe2)Al]2(μ-H)}+[B(C6F5)4]- (AlHAl) cocatalyst is compared with that of established systems like methylalumoxane, phenol-modified methylalumoxane and trityl borate/tri-iso-butylaluminium. The worst catalytic performance is observed with MAO. Conversely, the best cocatalyst varies depending on the Hf catalyst used and the performance indicator of interest, highlighting the complexity and importance of selecting the right precatalyst/cocatalyst combination. AlHAl proved to be a suitable system for all catalysts tested and, in some cases, it provides the best performance in terms of productivity (e.g. with hafnocenes). Furthermore, it generally leads to high molecular weight polymers, also with catalysts enabling easy chain transfer to Al like Hf-PyAm. This suggests that AlHAl has a low tendency to form heterodinuclear adducts with the cationic active species, therefore preventing the formation of dormant sites and/or termination events by chain transfer to Al.

Quantitative structure–property relationship study of constrained geometry catalysts for olefin polymerization

The quantitative structure–property relationship (QSPR) of constrained geometry catalysts (CGCs) has been analyzed by combining density functional theory (DFT) and multivariate linear regression (MLR).

Triptycene as a scaffold in metallocene catalyzed olefin polymerization.

A set of metallocene olefin polymerization catalysts bearing triptycene moieties in either position 4-5 (complexes Ty1-Ty5) or in position 5-6 (complexes Ty6-Ty8) of the basic dimethylsilyl-bridged bis(indenyl) system has been tested in propene polymerization and in ethene/1-hexene copolymerization. Comparison of the results with QSPR (quantitative structure-property relationship) predictions not parametrized for these exotic ligand variations demonstrates that trends can still be identified by extrapolation. Interestingly, Ty7, upon suitable activation, provides a highly isotactic polypropylene with an exceptional amount of 2,1 regio-errors (8%). The previously developed QSPR type models successfully predicted the low regioselectivity of this catalyst, despite the fact that the catalyst structure differs significantly from the benchmark set.

Molecular field analysis in half-titanocene complexes: computational study towards data-driven in silico optimization of single-site olefin polymerization catalysts

The control of multiple reaction outcomes in olefin polymerization has been demonstrated through data-driven catalyst optimization based on molecular field analysis, using data collected from DFT-based transition-state calculations.

Operando spectroscopies and heterogeneous olefin polymerization catalysis: A rare marriage with a lot of potential beyond usual understanding

This short review summarizes the main results achieved in the field of operando spectroscopies applied to heterogeneous catalysts for olefin polymerization. Following a chronological order, starting from the first spectroscopic measurements applied to the Phillips catalyst, the review retraces the main stages of this exciting story, reporting also some details on the experimental set-ups adopted from time to time, the difficulties encountered, the failures, and the solutions taken to address the problems. What emerges is a dynamic picture with still many unexplored potentialities, even though unfortunately limited to a few research groups systematically engaged in this field.

Visualizing the Structure, Composition and Activity of Single Catalyst Particles for Olefin Polymerization and Polyolefin Decomposition.

The structural and morphological characterization of individual catalyst particles for olefin polymerization, as well as for the reverse process of polyolefin decomposition, can provide an improved understanding for how these catalyst materials operate under relevant reaction conditions. In this review, we discuss an emerging analytical toolbox of 2D and 3D chemical imaging techniques that is suitable for investigating the chemistry and reactivity of related catalyst systems. While synchrotron-based X-ray microscopy still provides unparalleled spatial resolutions in 2D and 3D, a number of laboratory-based techniques, most notably focused ion beam-scanning electron microscopy, confocal fluorescence microscopy, infrared photoinduced force microscopy and laboratory-based X-ray nano-computed tomography, have helped to significantly expand the arsenal of analytical tools available to scientists in heterogeneous catalysis and polymer science. In terms of future research, the review outlines the role and impact of in situ and operando (spectro-)microscopy experiments, involving sophisticated reactors as well as online reactant and product analysis, to obtain real-time information on the formation, decomposition, and mobility of polymer phases within single catalyst particles. Furthermore, the potential of fluorescence microscopy, X-ray microscopy and optical microscopy is highlighted for the high-throughput characterization of olefin polymerization and polyolefin decomposition catalysts. By combining these chemical imaging techniques with, for example, chemical staining methodologies, selective probe molecules as well as particle sorting approaches, representative structure-activity relationships can be derived at the level of single catalyst particles.

Octahedral Zirconium Salan Catalysts for Olefin Polymerization: Substituent and Solvent Effects on Structure and Dynamics.

Group 4 metal-Salan olefin polymerization catalysts typically have relatively low activity, being slowed down by a pre-equilibrium favoring a non-polymerization active resting state identified as a mer-mer isomer (MM); formation of the polymerization active fac-fac species (FF) requires isomerization. We now show that the chemistry is more subtle than previously realized. Salan variations bearing large, flat substituents can achieve very high activity, and we ascribe this to the stabilization of the FF isomer, which becomes lower in energy than MM. Detailed in situ NMR studies of a fast (o-anthracenyl) and a slow (o-tBu) Salan precursors, suitably activated, indicate that preferred isomers in solution are different: the fast catalyst prefers FF while the slow catalyst prefers a highly distorted MM geometry. Crystal structures of the activated o-anthracenyl substituted complex with a moderately (chlorobenzene) and, more importantly, a weakly coordinating solvent (toluene) in the first coordination sphere emphasize that the active FF isomer is preferred, at least for the benzyl species. Site epimerization (SE) barriers for the fast catalyst (ΔS > 0, dissociative) and the slow catalyst (ΔS < 0, associative) in toluene corroborate the solvent role. Diagnostic NMe 13C chemical shift differences allow unambiguous detection of FF or MM geometries for seven activated catalysts in different solvents, highlighting the role of solvent coordination strength and bulkiness of the ortho-substituent on the isomer equilibrium. For the first time, active polymeryl species of Zr-Salan catalysts were speciated. The slow catalyst is effectively trapped in the inactive MM state, as previously suggested. Direct observation of fast catalysts is hampered by their high reactivity, but the product of the first 1-hexene insertion maintains its FF geometry.

Structural Characterization and Spatial Mapping of Active Sites on Supported Metallocene Catalysts for Olefin Polymerization: A Luminescence-based Approach

Structural Characterization and Spatial Mapping of Active Sites on Supported Metallocene Catalysts for Olefin Polymerization: A Luminescence-based Approach

Covalent Organic Framework-Supported Metallocene for Ethylene Polymerization.

Loading of homogeneous catalysts with support can dramatically improve their performance in olefin polymerization. However, the challenge lies in the development of supported catalysts with well-defined pore structures and good compatibility to achieve high catalytic activity and product performance. Herein, we report the use of an emergent class of porous material -- covalent organic framework material (COF) as a carrier to support metallocene catalyst -- Cp2ZrCl2 for ethylene polymerization. The COF-supported catalyst demonstrates a higher catalytic activity of 31.1 × 106 g/(mol·h) at 140 °C, compared with 11.2 × 106 g/(mol·h) of the homogenous one. The resulting polyethylene (PE) products possess higher weight-average molecular weight (Mw) and narrower molecular weight distribution (Ɖ) after COF supporting, i.e., Mw increases from 160 kDa to 308 kDa and Ɖ reduces from 3.3 to 2.2. The melting point (Tm) is also increased by up to 5.2 °C. Moreover, the PE product possesses a characteristic filamentous microstructure and demonstrates an increased tensile strength from 19.0 MPa to 30.7 MPa and elongation at break from 350% to 1400% after catalyst loading. We believe that the use of COF carriers will facilitate the future development of supported catalysts for high-efficient olefin polymerization and high-performance polyolefins.

Steric and electronic effects in cationic pyridine carboxamidate nickel mediated ethylene polymerization and copolymerization with methyl 10-undecenoate

Improvement of catalysts performance by structural modification is a well recognised and more reliable way in catalysis. Amide derived catalysts were already used in olefin polymerization but possessed many limitation regarding to their poor thermal stability and mostly generating low molecular weight oligomers. Steric and electronic factors have played a characteristic role to control the catalyst stability, activity, molecular weight and branch densities of the polymers. So in continuity of our interest to improve the performance of olefin polymerization catalysts, we contributed here by synthesized cationic nickel catalysts (Ni1–Ni3) by simple amidation of picolinoyl chloride with sterically bulky and electronically varied anilines. Effect of steric and electronic factors also has been explored. Compared with previously reported sterically open pyridine carboxamidate nickel catalysts, these bulky nickel catalysts have two orders of magnitude higher activity of up to 3.73× 106 g (mol Ni)−1 h−1 towards ethylene polymerization and thermal stability up to 80 °C, producing higher molecular weight semi-crystalline polyethylene with tunable branching densities (17–67 branches/1000C) and melting points (65–121 °C). Moreover, these cationic nickel catalysts also promote the copolymerization of ethylene and methyl 10-undecenoate (MU) to afford polar functionalized polyethylenes with maximum incorporation of MU up to 1.53 mol%.

Role of Titanium in Ti/SiO2‐Supported Metallocene‐based Olefin Polymerization Catalysts. Part 1: Genesis of Active Sites and Catalytic Performance

AbstractPerformance enhancement of metallocene‐based olefin polymerization catalysts can be obtained, among others, by the chemical modification of their support. We investigated a titanated SiO2 (Ti/SiO2) as an improved support material for zirconocene catalysts, which lead to ∼35 % higher ethylene polymerization yields when compared to the unmodified SiO2‐supported metallocene. The improved catalytic performance of the modified catalyst was found to be related to the presence of a Ti‐based active site, second to the Zirconocene centers, as well as to a higher Lewis acidity in the titanated supported‐MAO, which improved the activation of the Zr‐based centers. The Ti active species were generated by reduction of the Ti/SiO2 support with the methylaluminoxane (MAO) co‐catalysts, which introduced oxygen vacancies (VO) and Ti3+ species, as confirmed by probe molecule Fourier Transform Infrared (FT‐IR) spectroscopy and UV‐Vis Diffuse Reflectance Spectroscopy (DRS). The higher Lewis acidity was linked to 5‐coordinated Ti4+, as detected by probe molecules FT‐IR spectroscopy.

Role of Titanium in Ti/SiO2‐Supported Metallocene‐based Olefin Polymerization Catalysts. Part 2: Particle Fragmentation

AbstractA commercial SiO2 modified by the addition of a TiO2 layer on its surface and pore walls offers a support for metallocene‐based ethylene polymerization catalysts with a 35 % improved catalytic activity. The effect of such support modification on the catalyst particle fragmentation during polymerization reactions were assessed. This was performed by exposing the internal cross‐section of several SiO2 and Ti/SiO2‐based metallocene catalyst particles, polymerized at a wide range of reaction conditions, with a Focused‐Ion Beam Scanning Electron Microscope (FIB‐SEM), followed by a semi‐quantitative analysis of polymer, catalyst and macropores of each 2D image obtained. The titanation of the SiO2 lead to a support framework that fractures earlier and more efficiently. This was evidenced by the presence of several empty (polymer‐free) cracks along the support dense‐shell, found more extensively in the Ti‐modified polymerized catalyst particle, which also resulted in a more sustained polymerized particles macroporosity during the course of the reaction.