Recent Advancements in the Synthesis of Ultra-High Molecular Weight Polyethylene via Late Transition Metal Catalysts.

Ultra-high molecular weight polyethylenes (UHMWPEs) are significant engineering plastics for their unique properties, such as high impact resistance, abrasion resistance, weatherability, lubricity, and chemical resistance. Consequently, developing a suitable catalyst is vital in facilitating the preparation of UHMWPE. The late transition metal catalysts have emerged as effective catalysts in producing UHMWPE due to their availability, enhanced tolerance to heteroatom groups, active polymerization characteristics, and good copolymerization ability with polar monomers. In this review, we mainly focus on the late transition metal catalysts, summarizing advancements in their application over the past decade. Four key metals (Ni, Pd, Fe, Co) for generating linear or branched UHMWPE will be primarily explored in this manuscript.

Merging σ-Bond Metathesis with Polymerization Catalysis: Insights into Rare-Earth Metal Complexes, End-Group Functionalization, and Application Prospects.

Polymers with well-defined structures, synthesized through metal-catalyzed processes, and having end groups exhibiting different polarity and reactivity than the backbone, are gaining considerable attention in both scientific and industrial communities. These polymers show potential applications as fundamental building blocks and additives in the creation of innovative functional materials. Investigations are directed toward identifying the most optimal and uncomplicated synthetic approach by employing a combination of living coordination polymerization mediated by rare-earth metal complexes and C-H bond activation reaction by σ-bond metathesis. This combination directly yields catalysts with diverse functional groups from a single precursor, enabling the production of terminal-functionalized polymers without the need for sequential reactions, such as termination reactions. The utilization of this innovative methodology allows for precise control over end-group functionalities, providing a versatile approach to tailor the properties and applications of the resulting polymers. This perspective discusses the principles, challenges, and potential advancements associated with this synthetic strategy, highlighting its significance in advancing the interface of metalorganic chemistry, polymer chemistry, and materials science.

Thermal-Robust Phenoxyimine Titanium Catalysts Bearing Bulky Sidearms for High Temperature Ethylene Homo-/Co- Polymerizations.

A family of titanium complexes (Ti1-Ti7) with the general formula LTiCl3, supported by tridentate phenoxyimine [O-NO] ligands (L1-L7) bearing bulky sidearms, were synthesized by treating the corresponding ligands with stoichiometric amount of TiCl4. All the ligands and complexes were well characterized by 1H and 13C NMR spectroscopies, in which ortho- methoxyl groups on N-aryl moieties shifted to downfield, corroborating the successful coordination reaction. Structural optimization by DFT calculations revealed that one of the phenyl groups on dibenzhydryl moiety could form π-π stacking interaction with the salicylaldimine plane, because of which the obtained titanium complexes revealed good thermal stabilities for high-temperature polymerization of ethylene. The thermal robustness of the complexes was closely related to the strength of π-π stacking interactions, which were mainly influenced by the substituents on the dibenzhydryl moieties; Ti1, Ti4 and Ti5 emerged as the three best-performing complexes at 110 °C. With the aid of such π-π stacking interactions, the complexes were also found to be active at >150 °C, although decreased activities were witnessed. Besides homopolymerizations, complexes Ti1-Ti7 were also found to be active for the high-temperature copolymerization of ethylene and 1-octene, but with medium incorporation percentage, demonstrating their medium copolymerization capabilities.

Nickel-Catalyzed Ethylene Copolymerization with Vinylalkoxysilanes: A Computational Study.

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.

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.

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.

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.

Ethylene polymerization using N-Heterocyclic carbene complexes of silver and aluminum.

Various transition metal catalysts have been utilized for ethylene polymerization. Silver catalysts have attracted less attention as the catalysts, but are potential for production of high molecular weight polyethylene. Herein, we report that silver complexes with various N-heterocyclic carbene (NHC) ligands in combination with modified methylaluminoxane (MMAO) afford polyethylene with high molecular weight (melting point over 140°C). SEM observation showed that the produced polyethylene has ultra-high molecular weight. NMR investigation of the reaction between the silver complexes with organoaluminums indicate that the NHC ligands transfer from the silver complex to aluminum to produce NHC aluminum complexes. Ph3C[B(C6F5)4] abstract methyl group from the NHC aluminum complex to afford cationic aluminum complex. The NHC aluminum complex promoted ethylene polymerization in the presence of Ph3C[B(C6F5)4] and organoaluminums. NHC ligand also promoted ethylene polymerization in combination with MMAO to produce polyethylene with high melting point (140.7°C). Thus, the aluminum complexes are considered to be the actual active species in silver-catalyzed ethylene polymerization.

Covalent Organic Framework-Supported Metallocene for Ethylene Polymerization.

The 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-Cp2 ZrCl2 for ethylene polymerization. The COF-supported catalyst demonstrates a higher catalytic activity of 31.1×106  g mol-1 h-1 at 140 °C, compared with 11.2×106  g mol-1 h-1 for the homogenous one. The resulting polyethylene (PE) products possess higher weight-average molecular weight (Mw ) and narrower molecular weight distribution (Ð) after COF supporting, that is, Mw increases from 160 to 308 kDa and Ð drops 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 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 highly efficient olefin polymerization and high-performance polyolefins.

Effect of N-Aryl Para-Benzhydryl Substituent on the Thermal Stability of α-Diimine Nickel Catalyst.

The thermal stability of α-diimine nickel catalysts has always been the focus of research. The introduction of large groups in the backbone or N-aryl ortho-position is a relatively mature solution. However, the question of whether the N-aryl bond rotation is a factor affecting the thermal stability of nickel catalysts is still open. In this work, the effects of N-aryl para-benzhydryl substitutes on catalyst thermal stability are investigated, and the results of ethylene polymerization and the factors affecting thermal stability (steric effect, electronic effect, five-membered coordination ring stability, N-aryl bond rotation, etc.) are systematically analyzed. It is believed that the introduction of large steric hindrance groups at the N-aryl para-position hinders the rotation of the N-aryl bond. This obstacle effect is beneficial to improving catalyst thermal stability, and the obstacle capacity is weakened with the increase of ortho-substituent size.

Direct Synthesis of Polyethylene Thermoplastic Elastomers Using Hybrid Bulky Acenaphthene-Based α-Diimine Ni(II) Catalysts.

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.

The Highly Controlled and Efficient Polymerization of Ethylene.

The highly controlled and efficient polymerization of ethylene is a very attractive but challenging target. Herein we report on a Coordinative Chain Transfer Polymerization catalyst, which combines a high degree of control and very high activity in ethylene oligo- or polymerization with extremely high chain transfer agent (triethylaluminum) to catalyst ratios (catalyst economy). Our Zr catalyst is long living and temperature stable. The chain length of the polyethylene products increases over time under constant ethylene feed or until a certain volume of ethylene is completely consumed to reach the expected molecular weight. Very high activities are observed if the catalyst elongates 60 000 or more alkyl chains and the polydispersity of the strictly linear polyethylene materials obtained are very low. The key for the combination of high control and efficiency seems to be a catalyst stabilized by only one strongly bound monoanionic N-ligand.

Unsymmetrical Strategy on α-Diimine Nickel and Palladium Mediated Ethylene (Co)Polymerizations.

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.

Unusual Effect of α-olefins as Chain Transfer Agents in Ethylene Polymerization over the Catalyst with Nonsymmetrical Bis(imino)pyridine Complex of Fe(II) and Modified Methylalumoxane (MMAO) Cocatalyst.

Ethylene polymerization with bis(imino)pyridlyiron precatalysts generally produces linear polyethylene (PE) even with the presence of α-olefins because α-olefins are not incorporated into polymeric products. Interestingly, α-olefins, such as hexene-1 or butene-1, have been found to act as effective chain transfer agents in the ethylene polymerization promoted by nonsymmetrical bis(imino)pyridyliron complexes with modified methylalumoxane (MMAO), resulting in higher catalytic activities with higher amounts of polymers with lower molecular weights, and, more importantly, narrower molecular weight distributions of the resultant polyethylenes (PE). This phenomenon confirms the assistance of α-olefins in the chain-termination reaction of iron-initiated polymerization and regeneration of the active species for further polymerization. Besides higher activities of the catalytic system, the formation of linear PE with trans-vinylene terminal groups and lower molecular weights are explained. The observation will provide a new pathway for enhancing catalytic activity and improving the quality of polyethylenes obtained by regulation of molecular weights and molecular weight distribution.

Catalytic Performance of Cobalt(II) Polyethylene Catalysts with Sterically Hindered Dibenzopyranyl Substituents Studied by Experimental and MLR Methods.

Given the great importance of cobalt catalysts supported by benchmark bis(imino)pyridine in the (oligo)polymerization, a series of dibenzopyran-incorporated symmetrical 2,6-bis(imino) pyridyl cobalt complexes (Co1-Co5) are designed and prepared using a one-pot template approach. The structures of the resulting complexes are well characterized by a number of techniques. After activation with either methylaluminoxane (MAO) or modified MAO (MMAO), the complexes Co1-Co4 are highly active for ethylene polymerization with a maximum activity of up to 7.36 × 106 g (PE) mol-1 (Co) h-1 and produced highly linear polyethylene with narrow molecular weight distributions, while Co5 is completely inactive under the standard conditions. Particularly, complex Co3 affords polyethylene with high molecular weights of 85.02 and 79.85 kg mol-1 in the presence of MAO and MMAO, respectively. The 1H and 13C NMR spectroscopy revealed the existence of vinyl end groups in the resulting polyethylene, highlighting the predominant involvement of the ß-H elimination reaction in the chain-termination process. To investigate the mechanism underlying the variation of catalytic activities as a function of substituents, multiple linear regression (MLR) analysis was performed, showing the key role of open cone angle (θ) and effective net charge (Q) on catalytic activity.

Hydrophilic Catalysts with High Activity and Stability in the Aqueous Polymerization of Ethylene to High-Molecular-Weight-Polyethylene.

Water-soluble synthetic transition metal catalysts have been studied extensively for many reactions, but for olefin polymerization such catalysts have been lacking. We report herein a straightforward synthesis of phosphinephenolato NiII catalysts endowed permanently with a hydrophilic sulfonate moiety bound to the chelating ligand. These catalysts' hydrophilic active sites promote aqueous ethylene polymerization with high activity (TOF up to 6.3×104  molEthylene molNi -1 h-1 ) to high molecular weight polyethylene (HDPE), with half-lives on the order of hours also at elevated temperatures. The obtained polyethylene dispersions feature narrow particle size distributions without any aggregates.

Neutral Unsymmetrical Coordinated Cyclophane Polymerization Catalysts.

Cyclophane structures can control steric pressure in the otherwise open spaces of square-planar d8 -metal catalysts. This elegant concept was so far limited to symmetrical coordinated metals. We report how a cyclophane motif can be generated in ligands that chelate via two different donors. An ancillary second imine in the versatile κ2 -N,O-salicylaldiminato catalyst type enables ring closure via olefin metathesis and selective double bond hydrogenation to yield a 30-membered ring efficiently. Experimental and theoretical analyses show the ancillary imine is directed away from the active site and inert for catalysis. In ethylene polymerization the cyclophane catalyst is more active and temperature stable vs. an open structure reference, notably also in polar solvents. Increased molecular weights and decreased degrees of branching can be traced to an increased energy of sterically demanding transition states by the encircling cyclophane while chain propagation remains highly efficient.

Polymerization via Insertion of Ethylene into Al-C bond under Mild Conditions: Mechanistic Studies on the Promotion Exerted by a Catalytic Amount of Cationic Gadolinium Metallocene.

The cationic gadolinium metallocene [(C5 Me5 )2 Gd][B(C6 F5 )4 ], when combined with an excess amount of Al(i Bu)3 , efficiently produces polyethylene at 80 °C under 0.8 MPa pressure of ethylene. After quenching, the resulting polyethylene has ethyl group at one end and isobutyl group at the other terminal. Because no Gd-alkyl species appears to be involved, a mechanism with conventional coordinative chain transfer polymerization (CCTP) is not feasible. Density functional theory (DFT) analyses indicate a novel mechanism in which the cationic Gd plays a crucial role by coordinating ethylene and assists the insertion of the coordinated ethylene into Al-C bond.

Synthesis and ethylene polymerization reaction of dendritic titanium catalysts.

The 1.0 G dendrimer (C22H48N10O4),3,5-di-tert-butylsalicylaldehyde and TiCl4 · 2THF were used as the synthetic materials, and the dendritic salicylaldehyde imide ligand with substituent hindrance and its titanium catalyst were synthesized by the condensation reaction of schiff base. The structure of the synthesized products was characterized by infrared spectroscopy, nuclear magnetic resonance hydrogen spectroscopy, ultraviolet spectroscopy, electrospray mass spectrometry, and inductively coupled plasma mass spectrometry, The actual structure is consistent with the theoretical design structure. Activated methylaluminoxane (MAO) was used as a catalyst precursor for ethylene polymerization in the process of ethylene catalytic. The effects of ethylene polymerization were studied in terms of the Al/Ti molar ratio, reaction time, reaction temperature, polymerization pressure, and ligand structure of the catalyst. The results show at the reaction temperature of 25°C, the reaction time was 30 min, and the ethylene pressure was 1.0 MPa and Al/Ti was 1,000, the catalytic activity can reach 78.56 kg PE/(mol Ti.h). Furthermore, high-temperature GPC-IR, DSC, and torque rheometer were used to characterized the microstructure, thermal properties, and viscoelastic state of polyethylene samples obtained. The results showed that the product was ultra-high-molecular-weight polyethylene.

Influence of Pore Structure and Metal-Node Geometry on the Polymerization of Ethylene over Cr-Based Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) have received increasing interest as solid single-site catalysts, owing to their tunable pore architecture and metal node geometry. The ability to exploit these modulators makes them prominent candidates for producing polyethylene (PE) materials with narrow dispersity index (Ð) values. Here a study is presented in which the ethylene polymerization properties, with Et2 AlCl as activator, of three renowned Cr-based MOFs, MIL-101(Cr)-NDC (NDC=2,6-dicarboxynapthalene), MIL-53(Cr) and HKUST-1(Cr), are systematically investigated. Ethylene polymerization reactions revealed varying catalytic activities, with MIL-101(Cr)-NDC and MIL-53(Cr) being significantly more active than HKUST-1(Cr). Analysis of the PE products revealed large Ð values, demonstrating that polymerization occurs over a multitude of active Cr centers rather than a singular type of Cr site. Spectroscopic experiments, in the form of powder X-ray diffraction (pXRD), UV/Vis-NIR diffuse reflectance spectroscopy (DRS) and CO probe molecule Fourier transform infrared (FTIR) spectroscopy corroborated these findings, indicating that indeed for each MOF unique active sites are generated, however without alteration of the original oxidation state. Furthermore, the pXRD experiments indicated that one major prerequisite for catalytic activity was the degree of MOF activation by the Et2 AlCl co-catalyst, with the more active materials portraying a larger degree of activation.