Ethylene Polymerization Catalysts Research Articles

Thermally resilient cobalt ethylene polymerization catalysts under the joint influence of co-catalyst, gem-dimethyl substitution and ortho-cycloalkyl ring size

The N,N,N′-chelated 2-(1-arylimino)ethyl-8-arylimino-7,7-dimethyl-5,6- dihydroquinoline-cobalt (II) chloride complexes, Co1 (aryl = 2-(C5H9)-6-MeC6H3), Co2 (aryl = 2-(C6H11)-6-MeC6H3), Co3 (aryl = 2-(C8H15)-6-MeC6H3), Co4 (aryl = 2-(C5H9)-4,6-Me2C6H2), Co5 (aryl = 2-(C6H11)-4,6-Me2C6H2) and Co6 (aryl = 2-(C8H15)-4,6-Me2C6H3), have been isolated in reasonable yield using a simple one-pot method. The molecular structures of Co1 and Co5 emphasize not only the gem-dimethyl-substitution present in the carbocyclic-fused tridentate ligand but also the substantial steric properties exerted by the ortho-cycloalkyl groups. Complexes Co1 – Co6, on interaction with MAO, were highly active catalysts for ethylene polymerization reaching a maximum level of 9.21 × 106 g PE mol−1 (Co) h−1 at 70 °C for cyclopentyl-containing Co1 and Co4. Strictly linear polyethylenes with low molecular weights (Mw range: 9.25–38.5 kg mol−1) and narrow dispersities were generated with selectivities for vinyl-terminated polymers approaching quantitative levels. By comparison, polymerizations conducted using Co1 – Co6 with MMAO as co-catalyst proved less thermally stable and less active, with vinyl chain-end selectivity proving temperature dependent. Common to both MAO and MMAO, the most sterically hindered precatalyst Co6 incorporating an ortho-cyclooctyl group gave the highest molecular weight polymer of the series.

Improved Single‐Site Chromium Catalysts with Electron Rich Indenyl Ligands for the Formation of Ultrahigh Molecular Weight Polyethylene

AbstractQuinolyl functionalized indenyl chromium complexes with different substitution patterns were synthesized and evaluated as single site catalysts in ethylene polymerization. Adding substituents like a fused cyclohexyl ring or a Si(CH3)3 group to the indenyl moiety improve complex stability, solubility and catalytic activity compared to the unsubstituted analogues. Furthermore, the ability to incorporate 1‐hexene into the polymer chain increased. Ultrahigh molecular weight polyethylene (UHMW‐PE) or linear low density polyethylene (LLD‐PE) with very high molecular weight was obtained. Although other catalysts are able to incorporate more 1‐hexene only a few systems are known, that combine substantial 1‐hexene incorporation with very high activity and such high molecular weights. Consequently, these catalytic systems are attractive candidates in industrial processes for the production of improved polyethylene materials.

Robust and Reactive Neutral Nickel Catalysts for Ethylene Polymerization and Copolymerization with a Challenging 1,1-Disubstituted Difunctional Polar Monomer

A series of methyl nickel complexes with phosphino-naphtholate chelate ligands have been strategically designed, synthesized, and fully characterized. These catalysts showed extremely high activity...

2,4,6‐Triphenylpyridinium: A Bulky, Highly Electron‐Withdrawing Substituent That Enhances Properties of Nickel(II) Ethylene Polymerization Catalysts

AbstractThe reactivity of NiII and PdII olefin polymerization catalysts can be enhanced by introduction of electron‐withdrawing substituents on the supporting ligands rendering the metal centers more electrophilic. Reported here is a comparison of ethylene polymerization activity of a classical salicyliminato nickel catalyst substituted with the powerful electron‐withdrawing 2,4,6‐triphenylpyridinium (trippy) group to the ‐CF3 analogue. The trippy substituent is substantially more electron‐withdrawing (σmeta=0.63) than the trifluoromethyl group (σmeta=0.43) which results in a ca. 8‐fold increase in catalytic turnover frequency. An additional advantage of trippy is the high steric bulk relative to the trifluoromethyl group. This feature results in a four‐fold increase in polymer molecular weight owing to enhanced retardation of chain transfer. A significant increase in catalyst lifetime is observed as well.

2,4,6-Triphenylpyridinium: A Bulky, Highly Electron-Withdrawing Substituent That Enhances Properties of Nickel(II) Ethylene Polymerization Catalysts.

The reactivity of NiII and PdII olefin polymerization catalysts can be enhanced by introduction of electron-withdrawing substituents on the supporting ligands rendering the metal centers more electrophilic. Reported here is a comparison of ethylene polymerization activity of a classical salicyliminato nickel catalyst substituted with the powerful electron-withdrawing 2,4,6-triphenylpyridinium (trippy) group to the -CF3 analogue. The trippy substituent is substantially more electron-withdrawing (σmeta =0.63) than the trifluoromethyl group (σmeta =0.43) which results in a ca. 8-fold increase in catalytic turnover frequency. An additional advantage of trippy is the high steric bulk relative to the trifluoromethyl group. This feature results in a four-fold increase in polymer molecular weight owing to enhanced retardation of chain transfer. A significant increase in catalyst lifetime is observed as well.

Nickel complexes based on BIAN ligands: transformation and catalysis on ethylene polymerization.

Treatment of bis(arylimino)acenaphthene (ArBIAN) with Ni(COD)2 in toluene afforded dmpBIANNi(COD) (2a, dmp = 2,6-Me2C6H3) and dippBIANNi(COD) (2b, dipp = 2,6-iPrC6H3), respectively, in moderate yields. Complexes 2a and 2b can be oxidized by a small amount of oxygen at low temperature leading to oxygen-bridged dinuclear Ni(ii) complexes (dmpBIANNi)2(μ-O)2 (4a) and (dippBIANNi)2(μ-O)2 (4b), respectively, as a purple powder. The reaction of ArBIAN with 0.5 equiv of Ni(COD)2 or Ni(Ph3P)4 gave bisligated complexes (dmpBIAN)2Ni (3a) and (dippBIAN)2Ni (3b), which can be considered as Ni(0) complexes supported by two neutral BIAN ligands. Oxidation of the bisligated nickel complexes 3a and 3b with [Cp2Fe][B(C6F5)4] afforded cationic Ni(i) complexes [(dmpBIAN)2Ni][B(C6F5)4] (5a) and [(dippBIAN)2Ni][B(C6F5)4] (5b), respectively, in which the Ni(i) centre is chelated by two neutral Ar-BIAN ligands. These complexes were characterized by NMR and IR spectroscopy and DFT calculation, and the molecular structures of 3b, 4b, and 5b were well established by X-ray diffraction analysis. These complexes were evaluated as catalysts for ethylene polymerization in which 2b showed high activity in the presence of AlMe3. 13C NMR analysis of polymers showed that the 2b/AlMe3 catalytic system gave less-branched polymers when compared to that obtained with dippBIANNiBr2 under the same conditions.

МЕТАЛЛООРГАНИЧЕСКИЕ КАТАЛИЗАТОРЫ ПОЛИМЕРИЗАЦИИ ЭТИЛЕНА

The use of organometallic catalysts in ethylene polymerization at low pressure is considered. A comparative characteristic of different generations of catalysts and their influence on the physical and mechanical properties of polymers is given.

Ruthenium–nickel heterobimetallic complex as a bifunctional catalyst for ROMP of norbornene and ethylene polymerization

A new multifunctional Ru–Ni heterobimetallic catalyst for ROMP and ethylene polymerization.

Doubly fused N,N,N-iron ethylene polymerization catalysts appended with fluoride substituents; probing catalytic performance via a combined experimental and MLR study

In agreement with the MLR analysis, the fluorinated iron precatalyst, R1 = F, R2 = R3 = CH(p-FPh)2, proved the most active at 70 °C generating strictly linear polyethylene waxes.

Cr[CH(SiMe3)2]3/SiO2 catalysts for ethene polymerization: The correlation at a molecular level between the chromium loading and the microstructure of the produced polymer

Several spectroscopic techniques have been applied in a concerted way to investigate the local structure and the function of the chromium sites in a series of Cr[CH(SiMe3)2]3/SiO2 catalysts for ethene polymerization that differ in chromium loading and to correlate them with the microstructure of the produced polyethylene (PE). According to previous studies, the density of the produced PE is proportional to the Cr loading: the higher the Cr loading, the lower the PE density, because of an increase in the content of short-chain branching. Here, the nature of the branches and their enchainment in the polymer have been further investigated to determine the different properties of the produced polymers, and the differences detected have been traced back by spectroscopic analysis to the presence of two different types of Cr sites working in tandem: monografted SiOCr[CH(SiMe3)2]2 species that are responsible for ethene oligomerization, and bis-grafted (SiO)2CrCH(SiMe3)2 species that account for ethene homo- and co-polymerization. The relative abundance of the two species depends on the chromium loading. Moreover, we have also found that carbon monoxide selectively poisons the sites responsible for ethene oligomerization, indicating a possible strategy for the fine regulation of the properties of the produced PE.

Cationic α-Diimine Nickel and Palladium Complexes Incorporating Phenanthrene Substituents: Highly Active Ethylene Polymerization Catalysts and Mechanistic Studies of syn/anti Isomerization

α-Diimine palladium complexes incorporating phenanthryl- and 6,7-dimethylphenanthrylimino groups have been synthesized and characterized. The (diimine)PdMeCl complexes prepared from 2,3-butanedione...

Post-functionalization of narrowly dispersed PE waxes generated using tuned N,N,N′-cobalt ethylene polymerization catalysts substituted with ortho-cycloalkyl groups

Six structurally related bis(arylimino)trihydroquinolyl-cobalt (II) chloride complexes [2-(ArNCCH3)-8-(ArN)-5,6,7-C9H8N]CoCl2 (Ar = 2-(C5H9)-6-MeC6H3Co1, 2-(C6H11)-6-MeC6H3Co2, 2-(C8H15)-6-MeC6H3Co3, 2-(C5H9)-4,6-Me2C6H2Co4, 2-(C6H11)-4,6-Me2C6H2Co5, 2-(C8H15)-4,6-Me2C6H2Co6), distinguishable by the ring size of the ortho-cycloalkyl substituent and type of para-R group, have been synthesized and characterized. A distorted square pyramidal geometry is a feature of the molecular structure of Co4 with the two ortho-cyclopentyl groups located on neighboring N-aryl groups trans-configured. Compounds Co1 – Co6, on activation with methylaluminoxane (MAO) or modified MAO (MMAO), proved highly productive catalysts for ethylene polymerization at 60 °C [up to 17.1 × 106 g (PE) mol−1(Co) h−1 for cyclopentyl-containing Co4/MAO]; even at 90 °C significant activity was attainable (up to 6.75 × 106 g (PE) mol−1(Co) h−1). Strictly linear polyethylene waxes of low molecular weight (ca. 1.50 kg mol−1), narrow dispersity (Mw/Mn range: 1.1–2.4) and incorporating high levels of vinyl end-groups were generated. Post-functionalization of these PE waxes by epoxidation, thiol-ene addition and cross-olefin metathesis to form e-PE, PE-S-CH2CH2NH2·HCl and PE-MMA, respectively, has been demonstrated. For comparative purposes, [2-(ArNCCH3)-8-ArN-5,6,7-C9H8N]CoCl2 (Ar = 2,4,6-Me3C6H2Coo-Me, 2-(Ph2CH)-4,6-Me2C6H2Coo-CHPh2) have also been prepared and evaluated as polymerization catalysts.

Versatile PNPO ligands for palladium and nickel catalyzed ethylene polymerization and copolymerization with polar monomers

Asymmetric PO-type ligands are versatile platforms for palladium and nickel catalyzed ethylene polymerization as well as copolymerization of ethylene with polar monomers. In this work, a series of phosphinoamine phosphoramidate/phosphoramide ligands and the corresponding palladium/nickel catalysts were prepared, characterized and investigated in ethylene polymerization as well as copolymerization of ethylene with various polar monomers. The palladium and nickel complexes exhibited high activities in ethylene (co) polymerization, generating high-molecular-weight polyethylenes and copolymers of ethylene and polar monomers. The introduction of electron-donating substituents in the palladium catalytic system can significantly increase polymer molecular weights while maintaining high ethylene (co)polymerization activities. The less electronic donating substituents in the nickel catalysts led to increased catalytic activities in both ethylene polymerization and copolymerization with polar monomers. These results demonstrate the importance of ligand electronic effect in tuning the properties of nickel and palladium catalysts in ethylene polymerization and copolymerization reactions.

Comparison of the Reactivity and Structures for the Neutral and Cationic Bis(imino)pyridyl Iron and Cobalt Species by DFT Calculations

Density Functional Theory (DFT) method was adopted to investigate and compare the reaction mechanisms of ethylene polymerization catalyzed by neutral, cationic bis(imino)pyridyl (PDI) iron and cobalt derivatives. The electronic structure and the oxidation states of the metal center and the PDI ligand were analyzed by taking spin states, natural bond orbital (NBO) charge distribution, etc. into consideration, revealing that the reactivity is closely related to the valence electron numbers instead of the charge numbers. The neutral Co(0) had the lowest reactivity as it possessed the most electrons. During the formation of the cationic Co(+)/Fe(+), one electron was mainly lost from PDI ligand rather than the metal center while the metal center maintained +II valence state through the process. Moreover, a special unsymmetrically bidentate N^N coordination manner was found to provide the deficient metal surroundings with 14e, which may initiate the reactivity of some unsymmetrical species with rich electrons. Finally, an anion [AlMe4]− participating process was proposed to explain the presence of the experimentally observed LCo(+)B(C2H4). A special intermediate, Co(+)B(C2H4) [AlMe4]− with Co in +I and absence of Co–C σ bond, was obtained. These calculation results may provide fundamental information for further understanding and designing the ethylene polymerization catalysts.

Kinetics and Mechanism Comparison between Cr/Ti‐Based Bimetallic and Ti‐Based Monometallic Catalysts for Ethylene Polymerization

AbstractTwo (SiO2/MgR2/MgCl2)·TiClx model catalysts are made by refluxing TiCl4 with 0.35 wt% Cr modified silica gel/alkyl Mg adducts or silica gel/alkyl Mg adducts, which are named as Cr/Ti‐based bimetallic Cat‐1 and Ti‐based monometallic Cat‐2, respectively. The kinetics, active center counting, morphology, and polymer characterizations are studied to disclose the effect of low loading Cr active sites on the Cr/Ti‐based bimetallic Cat‐1 polymerization under mild conditions. The activity of Cat‐1 is 120.4% higher than that of Cat‐2, with a 114.1% higher [C*]/[M] value. Morphology results show the Cat‐1 fragmentation in the first 3 min is highly accelerated, which helps to release buried clustered Ti sites. Differential scanning calorimetry results show that low‐temperature heat absorbing shoulder of polyethylene (PE) from Cat‐2 demonstrates the signal of low crystallinity polymer made by Cat‐2 during the first 60 s, verifying the fluffy polymer in morphology results. GPC results show PE from Cat‐1 has a higher Mw in the first 3 min while a lower Mw in the end. The Cat‐1, which release active sites faster, has a high Mw in the early time. Lower Mw in the 900 s attributes to the effect of relative lower Mw polymer made by Cr sites, compared with Cat‐2.

Selective Hydrogenation of Acetylene to Ethylene Over Nanosized Gold and Palladium Supported Catalysts.

Ethylene, the main raw material for polyethylene production, is a by-product produced by thermally decomposing naphtha and it contains a small amount of acetylene. The acetylene reacts as a permanent catalyst poison for the ethylene polymerization catalyst. In this study, we wanted to improve the acetylene conversion and the ethylene selectivity by selective hydrogenation of acetylene for removing acetylene contained in ethylene. Catalyst was prepared by loading nanosized gold (Au) and palladium (Pd) particles on support (Al₂O₃, TiO₂). Deposition order Au and Pd particles was changed. The activity of the catalyst was investigated using a flow-typed fixed bed reactor under atmospheric pressure. Au and Pd particles deposited on TiO₂ were oxidized to Au₂O₃ and PdO due to strong metal support interaction (SMSI). It was considered that the Au/Pd/Al₂O₃ catalyst was more active than the Pd/Au/Al₂O₃ catalyst due to the formation of the interface between Au particles and Pd particles (or support). But Pd/Au/Al₂O₃ catalyst is considered to have poor activity because Pd particles cover part of the interface between Au and the support. Au/Pd/Al₂O₃ catalyst showed the best catalytic activity, and acetylene conversion and ethylene selectivity were 100% and about 80% at 40 °C, respectively.

Study of Silica‐Supported Chromocene Catalysts for Ethylene Polymerization

AbstractUltrahigh molecular weight polyethylene (UHMWPE) has drawn great interest from researchers because it possesses many excellent properties such as superb strength and impact resistance, which other polyolefin cannot achieve. A silica‐supported chromocene catalyst, for the production of UHMWPE, is successfully developed. The polyethylene (PE) produced by the chromocene catalyst can achieve a molecular weight (MW) of over 3 × 106 g mol−1 with narrow molecular weight distribution (MWD) a value of approximately three. The activity calculated by Cr per unit reaches the maximum when the loading of Cr is 1.7 wt%. With the increasing loading of chromocene, the MW shows a small increase and the MWD becomes narrower. The chromocene catalyst for ethylene polymerization also shows a significant hydrogen response. With 0.01 MPa hydrogen added into the ethylene polymerization, the MW of the PE decreases immensely to only 0.7 × 106 g mol−1 and the MWD is broadened from <3 to ≈12. If the amount of hydrogen increases, the MW continues to decrease and the MWD becomes wider. The MW of the PE produced by the chromocene catalyst can be regulated easily by adjusting the amount of hydrogen.

Ethylene polymerization using diatomite as support for zirconocene dichloride/modified methylaluminoxane

In this paper, we report on a zirconocene dichloride/modified methylaluminoxane catalyst system supported on a natural diatomite in order to provide ethylene polymerization catalysts for gas phase or slurry processes. Such natural diatomite support exhibited similar properties of silica-based molecular sieve because of their similar chemical composition and porous structure. The supported catalysts were shown to be highly active and to form flocculent and fibrous polyethylene as proven by scanning electron microscopy. A natural resource diatomite was successfully applied as a support for zirconocene by us. Such catalyst system performed as a silica-based molecular sieve supported zirconocene in the ethylene polymerization. Polyethylene with flocculent and fibrous structure was obtained.

Why could the CrOx/SiO2 and VOx/SiO2 catalysts show so different behaviors in ethylene polymerization? A theoretical approach

Phillips catalyst (CrOx/SiO2) is one of the most famous industrial catalysts for ethylene polymerization, while VOx/SiO2 catalyst is a promising catalyst capable of making UHMWPE, which was recently developed in our group. These two catalysts were known to show completely deferent behaviors in ethylene polymerization in terms of activity, H2 response and copolymerization ability. However, a molecular insight into the mechanism is not well understood. Herein, a theoretical comparison between them via the propagation through insertion and termination through chain transfer reaction pathways is performed. The results show the transition states for olefins (ethylene, 1-butene and 1-hexene) insertion are energetically more stable in VOx/SiO2. However, the binding energies of the olefins by VOx/SiO2 is obviously lower, which results in the insertion barriers for VOx/SiO2 about 2–3 kcal mol−1 higher than those for CrOx/SiO2. As to the termination, CrOx/SiO2 is more probably to terminate via the BHE (β-H elimination to Cr center) pathway, which generally has lower barriers than those for BHT (β-H elimination to monomer) in the VOx/SiO2 system. Meanwhile, VOx/SiO2 is superior to CrOx/SiO2 in hydrogenolysis, because it is more accessible by the H2, and the reaction barrier is also much lower. The energy gap between the barriers for propagation and termination is generally smaller for CrOx/SiO2, implying its tendency to produce lower MW PE when compared with VOx/SiO2. Furthermore, for CrOx/SiO2, the insertion barriers between C2H4 and α-olefin are bigger and the promotion effect of the incorporated α-olefins on termination is more obvious, which might account for the lower incorporation efficiency for comonomer by CrOx/SiO2, despite of its lower insertion barriers.

Tuning the Redox Chemistry of a Cr/SiO2 Phillips Catalyst for Controlling Activity, Induction Period and Polymer Properties.

The Cr/SiO2 Phillips catalyst has taken a central role in ethylene polymerization ever since its discovery in 1953. This catalyst is unique compared to other ethylene polymerization catalysts, since it is active without the addition of a metal‐alkyl co‐catalyst. However, metal‐alkyls can be added for scavenging poisons, enhancing the catalyst activity, reducing the induction period and altering polymer characteristics. Despite extensive research into the working state of the catalyst, still no consensus has been reached. Here, we show that by varying the type of metal‐alkyl co‐catalyst and its amount, the Cr redox chemistry can be tailored, resulting in distinct catalyst activities, induction periods, and polymer characteristics. We have used in‐situ UV‐Vis‐NIR diffuse reflectance spectroscopy (DRS) for studying the Cr oxidation state during the reduction by tri‐ethyl borane (TEB) or tri‐ethyl aluminum (TEAl) and during subsequent ethylene polymerization. The results show that TEB primarily acts as a reductant and reduces Cr6+ with subsequent ethylene polymerization resulting in rapid polyethylene formation. TEAl generated two types of Cr2+ sites, inaccessible Cr3+ sites and active Cr4+ sites. Subsequent addition of ethylene also revealed an increased reducibility of residual Cr6+ sites and resulted in rapid polyethylene formation. Our results demonstrate the possibility of controlling the reduction chemistry by adding the proper amount and type of metal‐alkyl for obtaining desired catalyst activities and tailored polyethylene characteristics.