Ethylene Polymerization Catalysts Research Articles

Highly Active Immobilized Catalyst for Ethylene Polymerization: Neutral Single Site Y(III) Complex Bearing Bulky Silylallyl Ligand.

Exploring the surface organometallic chemistry on silica of highly electrophilic yttrium complexes is a relatively uncommon endeavor, particularly when focusing on tris-alkyl complexes characterized by Y-C σ-alkyl bonds. A drawback with this class of complexes once grafted on silica, is the frequent occurrence of alkyl transfer by ring opening of siloxane groups, resulting in a mixture of species. Herein, we employed a more stable homoleptic yttrium allyl complex bearing bulky η3-1,3-bis(trimethylsilyl)allyl ligand to limit this transfer reaction. This strategy has been validated by comparing the reactivity between [Y{η3-1,3-C3H3(SiMe3)2}3] and [Y(o-CH2PhNMe2)3] with SiO2-700, where the undesired alkyl transfer reaction occurred for [Y(o-CH2PhNMe2)3] leading to a bipodal [(≡SiO)2Y(o-CH2PhNMe2)] as major surface species, 2, while [Y{η3-1,3-C3H3(SiMe3)2}3] resulted selectively in a monopodal species, [(≡SiO)Y{η3-1,3-C3H3(SiMe3)2}2], 1. The materials obtained were characterized by DRIFT, solid state NMR, mass balance analysis and EXAFS. Catalyst 1 showed high activity compared to 2 in ethylene polymerization. The catalytic performance of this neutral catalyst 1 was extended to pre-industrial scale in the presence of hydrogen and 1-hexene. An unprecedented activity, up to 7400 gPE·gcat-1·h-1 was obtained even with very low concentration of scavenger AliBu3 (TIBA/Y = 1.2). The obtained HDPE exhibited desired spherical particle morphology with broad molar mass distribution.

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.

Double stimulus-responsive palladium catalysts for ethylene polymerization and copolymerization

The external stimulus response strategy has been evolved rapidly in the field of olefin polymerization. In this work, we modularly synthesized three types of double stimulus responsive α-diimine palladium catalysts, combining redox regulation and other regulation together, such as light, Lewis acid and alkali cations. The catalytic activities and the molecular weight of polyethylene products can be regulated for 4 times in ethylene polymerization. These palladium complexes were also used for the copolymerization reaction of ethylene and polar monomers, such as methyl 10-undecylenate and methyl acrylate, effectively regulating the catalytic activities, the molecular weight and polar monomer incorporation of the prepared copolymers. The research on these dual-regulated palladium complexes makes full use of prepared catalysts and provides new inspirations for regulating olefin polymerization.

Research Progress of Iron Catalysts with N,N,N‐tridentate Ligands for Ethylene Polymerization

AbstractSome iron complexes are found to be effective catalysts of ethylene polymerization and have become a research hotspot due to their advantages of high activity, good thermal stability, and mild reaction conditions. Compared with other systems, iron catalysts bearing N,N,N‐tridentate ligand have good comprehensive performance in catalyzing ethylene polymerization. This review article focuses on changes in the framework of bis(imino)pyridyl ligand and the effect of different substituents on the catalytic performance of ethylene polymerization (activity, molecular weight, thermal stability, etc.) since 2019. Despite significant achievements, many opportunities and possibilities are yet to be fully addressed, and a brief outlook on the future of issues should be focused on is provided.

Synthesis of vanadium (V) complex with bulky phenoxy ligand and the study of catalysis behavior in ethylene (co)polymerization

A series of imido-vanadium complexes containing bulky aryloxo ligand have been isolated and employed as catalyst in ethylene (co)polymerization. The introduction of bulky ligand in the vanadium complex structure could promote the thermal stability of catalyst in ethylene polymerization. Especially the corresponding catalysts remain good activity in the ethylene copolymerization with polar monomers such as 10-undecen-1-ol (UOH) and methyl 10-undecenoate (UA). In the attempted copolymerization with acrylate monomer, the vanadium catalysts still show moderate activity but only afford polyethylene.

Lewis acid-induced homo- and heterogeneous nickel catalysts for ethylene polymerization and copolymerization with polar monomers

By introducing soluble and supported Lewis acids, homogeneous and heterogeneous nickel-catalyzed ethylene (co)polymerization was designed and studied.

An aza-cyclophane supported macrocyclic α-diimine nickel catalyst for ethylene polymerization

A novel aza-cyclophane-based macrocyclic α-diimine nickel(ii) catalyst prepared via a simplified synthesis route shows all-round improvements in ethylene polymerization compared to the acyclic analogue.

Predicting the catalytic activities of transition metal (Cr, Fe, Co, Ni) complexes towards ethylene polymerization by machine learning.

The study aims to execute machine learning (ML) method for building an intelligent prediction system for catalytic activities of a relatively big dataset of 1056 transition metal complex precatalysts in ethylene polymerization. Among 14 different algorithms, the CatBoost ensemble model provides the best prediction with the correlation coefficient (R2 ) values of 0.999 for training set and 0.834 for external test set. The interpretation of the obtained model indicates that the catalytic activity is highly correlated with number of atom, conjugated degree in the ligand framework, and charge distributions. Correspondingly, 10 novel complexes are designed and predicted with higher catalytic activities. This work shows the potential application of the ML method as a high-precision tool for designing advanced catalysts for ethylene polymerization.

Active Sites in Cr(III)-Based Ethylene Polymerization Catalysts from Machine-Learning-Supported XAS and EPR Spectroscopy.

The ethylene polymerization Phillips catalyst has been employed for decades and is central to the polymer industry. While Cr(III) alkyl species are proposed to be the propagating sites, there is so far no direct experimental evidence for such proposal. In this work, by coupling Surface organometallic chemistry, EPR spectroscopy, and machine learning-supported XAS studies, we have studied the electronic structure of well-defined silica-supported Cr(III) alkyls and identified the presence of several surface species in high and low-spin states, associated with different coordination environments. Notably, low-spin Cr(III) sites are shown to participate in ethylene polymerization, indicating that similar Cr(III) alkyl species could be involved in the related Phillips catalyst.

Active Sites in Cr(III)‐Based Ethylene Polymerization Catalysts from Machine‐Learning‐Supported XAS and EPR Spectroscopy**

AbstractThe ethylene polymerization Phillips catalyst has been employed for decades and is central to the polymer industry. While Cr(III) alkyl species are proposed to be the propagating sites, there is so far no direct experimental evidence for such proposal. In this work, by coupling Surface organometallic chemistry, EPR spectroscopy, and machine learning‐supported XAS studies, we have studied the electronic structure of well‐defined silica‐supported Cr(III) alkyls and identified the presence of several surface species in high and low‐spin states, associated with different coordination environments. Notably, low‐spin Cr(III) sites are shown to participate in ethylene polymerization, indicating that similar Cr(III) alkyl species could be involved in the related Phillips catalyst.

Synthesis of polyolefin elastomers from amine-imine nickel-catalyzed ethylene polymerization

The catalytic synthesis of polyolefin elastomers by using ethylene as a sole feedstock is fascinating, while currently reported chain walking catalysts are only limited to α-diimine type. In this contribution, we developed alternative amine-imine nickel catalysts for ethylene polymerization to prepare polyolefin elastomers. The cooperative effect of steric hindrance between the less bulky amine moiety and the bulky imine moiety significantly enhanced the performance of amine-imine nickel-catalyzed ethylene polymerization. The amine-imine nickel catalysts also showed excellent controlled behavior for ethylene polymerization, and living ethylene polymerization was also realized to generate narrowly distributed polyethylenes. The microstructure of the resultant polyethylenes could be tuned by catalyst structure, reaction temperature, and ethylene pressure. The polyethylenes produced by Ni2 with the cyclic amine have higher long-chain branching contents, therefore showing better elastic properties. The strain-at-break value of 888.0 % and the strain recovery value of > 79.0 % are super to most values of polyolefin elastomers generated from α-diimine nickel catalysts.

XAS and DFT investigation of atomically dispersed Cu/Co alloyed Pt local structures under selective hydrogenation of acetylene to ethylene

In this study, we focused on the semihydrogenation of acetylene (SHA) in an ethylene-rich stream to produce clean ethylene (devoid of acetylene that poisons the ethylene polymerization catalysts) as an incentive to increase polymer production. Two alloyed Pt catalysts namely PtCu and PtCo were synthesized via functionalization with oxalate ligands and supported on ZSM-5 (PtCuOx/Z and PtCoOx/Z), characterized, and tested for the SHA under a typical industrial condition. While there are instances of single-atom Pt alloyed with Cu in the PtCuOx/Z catalyst, nanoclusters of alloyed Pt and Co prevailed in PtCoOx/Z according to the EXAFS and HAADF-STEM results. Threshold energies (E0) of 11566.8 and 11563.8 eV obtained from the first derivative of the XANES spectra were observed for PtCuOx/Z and PtCoOx/Z, respectively under experimental conditions. Accordingly, the high E0 implied a higher unoccupied density of Pt d-orbital which is a key indicator for enhanced SHA activity leading to a higher ethylene yield (YC2H4) of 87.6 % over PtCuOx/Z compared to 62.9 % over PtCoOx/Z at 200 °C. These results were confirmed by a DFT computation which showed that Pt in the catalysts contains more electrons in PtCuOx/Z than in PtCoOx/Z, thus shifting the d-band center downward of the Fermi level in the former than the latter which allows only a weakly π-bonded C2H4 that desorbs before the secondary reactions leading to high YC2H4, while PtCoOx/Z favor higher YC2H6. This result showed that alloying Pt with Cu is more promising in synthesizing industrial relevant SHA catalyst.

The Nature and Role of the Nickel Species in the Ni(II) α-Diimine-based Ethylene Polymerization Catalyst Systems (A Review)

The Nature and Role of the Nickel Species in the Ni(II) α-Diimine-based Ethylene Polymerization Catalyst Systems (A Review)

Exploring Long Range para-Phenyl Effects in Unsymmetrically Fused bis(imino)pyridine-Cobalt Ethylene Polymerization Catalysts

Unsymmetrical 11-phenyl-1,2,3,7,8,9,10-heptahydrocyclohepta[b]quinoline-4,6-dione, incorporating a para-phenyl substituted pyridine unit fused by both 6- and 7-membered carbocyclic rings, has been prepared on the gram-scale via a multi-step procedure involving cyclization, hydrogenation and oxidation. Templating this diketone, in the presence of cobalt(II) chloride hexahydrate, with the corresponding aniline afforded in good yield five examples of doubly fused bis(arylimino)pyridine-cobalt(II) chlorides, Co1 (aryl = 2,6-dimethylphenyl), Co2 (2,6-diethylphenyl), Co3 (2,6-diisopropylphenyl), Co4 (2,4,6-trimethylphenyl) and Co5 (2,6-diethyl-4-methylphenyl). Structural characterization of Co1, Co2 and Co3 highlights the flexible nature of the inequivalent fused rings on the NNN’-ligand and the skewed disposition of the para-phenyl group. On activation with MAO, Co1–Co5 exhibited high activity for ethylene polymerization at 30 °C (up to 5.66 × 106 g (PE) mol−1 (Co) h−1) with the relative order being as follows: Co4 > Co1 > Co5 > Co3 > Co2. All polyethylenes were strictly linear, while their molecular weights and dispersities showed some notable variations. For Co1, Co2, Co4 and Co5, all polymerizations were well controlled as evidenced by the narrow dispersities of their polymers (Mw/Mn range: 1.8–2.7), while their molecular weights (Mw range: 2.9–10.9 kg mol−1) steadily increased in line with the greater steric properties of the N-aryl ortho-substituents. By contrast, the most hindered 2,6-diisopropyl counterpart Co3 displayed a broad distribution with bimodal characteristics (Mw/Mn = 10.3) and gave noticeably higher molecular weight polymer (Mw = 75.5 kg mol−1). By comparison, the MMAO-activated catalysts were generally less active, but showed similar trends in molecular weight and polymer dispersity. End group analysis of selected polymers via 13C and 1H NMR spectroscopy revealed the presence of both saturated and unsaturated polyethylenes in accordance with competing chain transfer pathways. Notably, when comparing Co3/MAO with its non-phenyl substituted analogue (E2,6-iPr2Ph)CoCl2/MAO, the former, though less controlled, displayed higher activity and molecular weight, a finding that points towards a role played by the remote para-phenyl group.

High Temperature Iron Ethylene Polymerization Catalysts Bearing N, N, N′-2-(1-(2,4-Dibenzhydryl-6-fluorophenylimino)ethyl)-6-(1-(arylphenylimino)ethyl)pyridines

High Temperature Iron Ethylene Polymerization Catalysts Bearing N, N, N′-2-(1-(2,4-Dibenzhydryl-6-fluorophenylimino)ethyl)-6-(1-(arylphenylimino)ethyl)pyridines

Preparation of polyethylene thermoplastic elastomers using CS-symmetric nickel catalysts in ethylene polymerization

Recently, the structural symmetry of α-diimine nickel catalysts was demonstrated to have a pronounced effect on the microstructure of the resulting polyethylene. In this study, two plane symmetric α-diimine Ni(II) complexes were designed and used in ethylene polymerization. The plane-symmetric Ni(II) complexes exhibited very high activities (well above 106 g mol−1 h−1) and yielded high molecular weight (up to 1088 kg/mol) polyethylenes with tunable branching density (49–87/1000C). Compared to the corresponding centrosymmetric Ni(II) complex, the plane-symmetric Ni(II) complex yielded significantly higher branched polyethylene (77 vs 29/1000C) under the same conditions. The polyethylene products obtained by plane-symmetric Ni(II) complexes exhibited low to moderate stress (3.7–21.6 MPa) and high strain (617–4058%) at break values as well as tunable recovery values (SR = 35–79%). Compared to the centrosymmetric Ni(II) complex, the plane-symmetric Ni(II) complexes yielded polyethylene materials with higher strain and lower stress at break values, and most importantly, significantly higher SR values (68% vs. 28%) under the identical conditions. Anyway, elastomer-like polyethylene materials were obtained from plane-symmetric Ni(II) complexes while centrosymmetric Ni(II) complex yielded linear low-density polyethylene-like materials.

Macrocyclic Binuclear α-Diimine Nickel Catalysts for Ethylene Polymerization

Macrocyclic Binuclear α-Diimine Nickel Catalysts for Ethylene Polymerization

Synthesis of α-Diimine Complex Enabling Rapidly Covalent Attachment to Silica Supports and Application of Homo-/Heterogeneous Catalysts in Ethylene Polymerization.

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.

DFT insight into metals and ligands substitution effects on reactivity of phenoxy-imine catalysts for ethylene polymerization

The reaction mechanism of ethylene (ET) polymerization catalyzed by the phenoxy-imine (FI) ligands using DFT calculations was studied. Among five possible isomers, isomer A which has an octahedral geometry and a (cis-N/trans-O/cis-Cl) arrangement is the most stable pre-reaction Ti-FI dichloride complex. The isomer A can be activated by MAO to form the active catalyst and the active form was used for the study of the mechanism for Ti-FI. The second ethylene insertion was found to be the rate-determining step of the catalyzed ethylene polymerization. To examine the effect of group IVB transition metals (M = Ti, Zr, Hf) substitutions, calculated activation energies at the rate-determining step (EaRDS) were compared, where values of EaRDS of Zr < Hf < Ti agree with experiments. Moreover, we examined the effect of substitution on (O, X) ligands of the Ti-phenoxy-imine (Ti-1) based catalyst. The results revealed that EaRDS of (O, N) > (O, O) > (O, P) > O, S). Hence, the (O, S) ligand has the highest potential to improve the catalytic activity of the Ti-FI catalyst. We also found the activation energy to be related to the Ti-X distance. In addition, a novel Ni-based FI catalyst was investigated. The results indicated that the nickel (II) complex based on the phenoxy-imine (O, N) ligand in the square-planar geometry is more active than in the octahedral geometry. This work provides fundamental insights into the reaction mechanism of M − FI catalysts which can be used for the design and development of M − FI catalysts for ET polymerization.

N,N-Bis(2,4-Dibenzhydryl-6-cycloalkylphenyl)butane-2,3-diimine-Nickel Complexes as Tunable and Effective Catalysts for High-Molecular-Weight PE Elastomers.

Four examples of N,N-bis(aryl)butane-2,3-diimine-nickel(II) bromide complexes, [ArN=C(Me)-C(Me)=NAr]NiBr2 (where Ar = 2-(C5H9)-4,6-(CHPh2)2C6H2 (Ni1), Ar = 2-(C6H11)-4,6-(CHPh2)2C6H2 (Ni2), 2-(C8H15)-4,6-(CHPh2)2C6H2 (Ni3) and 2-(C12H23)-4,6-(CHPh2)2C6H2 (Ni4)), disparate in the ring size of the ortho-cycloalkyl substituents, were prepared using a straightforward one-pot synthetic method. The molecular structures of Ni2 and Ni4 highlight the variation in the steric hindrance of the ortho-cyclohexyl and -cyclododecyl rings exerted on the nickel center, respectively. By employing EtAlCl2, Et2AlCl or MAO as activators, Ni1-Ni4 displayed moderate to high activity as catalysts for ethylene polymerization, with levels falling in the order Ni2 (cyclohexyl) > Ni1 (cyclopentyl) > Ni4 (cyclododecyl) > Ni3 (cyclooctyl). Notably, cyclohexyl-containing Ni2/MAO reached a peak level of 13.2 × 106 g(PE) of (mol of Ni)-1 h-1 at 40 °C, yielding high-molecular-weight (ca. 1 million g mol-1) and highly branched polyethylene elastomers with generally narrow dispersity. The analysis of polyethylenes with 13C NMR spectroscopy revealed branching density between 73 and 104 per 1000 carbon atoms, with the run temperature and the nature of the aluminum activator being influential; selectivity for short-chain methyl branches (81.8% (EtAlCl2); 81.1% (Et2AlCl); 82.9% (MAO)) was a notable feature. The mechanical properties of these polyethylene samples measured at either 30 °C or 60 °C were also evaluated and confirmed that crystallinity (Xc) and molecular weight (Mw) were the main factors affecting tensile strength and strain at break (εb = 353-861%). In addition, the stress-strain recovery tests indicated that these polyethylenes possessed good elastic recovery (47.4-71.2%), properties that align with thermoplastic elastomers (TPEs).