Recent advances in homogeneous chromium catalyst design for ethylene tri-, tetra-, oligo- and polymerization

This review focuses on recent progress made using well-defined molecular chromium complexes that, upon suitable activation, can catalyze the tri-, tetra, oligo- and/or polymerization of ethylene. In particular, emphasis will be placed on the tuning of the performance characteristics of these homogeneous catalysts through structural modifications made to the multidentate ligand manifold (e.g., donor atoms, charge, backbone and strain) and the effects these changes have on the resulting ethylene derivatives. While the ability of these catalysts to mediate the formation of high molecular weight linear polyethylene continues to see many developments, their capacity to form polyethylene waxes and oligomers has witnessed some major advances. Moreover, the impressive selectivity of some chromium systems to generate commercially important 1-hexene and more recently 1-octene has seen the implementation of this technology at the industrial level. The types of precatalysts to be discussed will be divided broadly on the basis of their ability to generate either polymers/oligomers or short chain α-olefins; the effects of co-catalyst and reaction conditions (e.g., temperature, pressure, solvent) on catalytic activity and selectivity, will be also developed. In addition, current proposals as to the mechanistic details displayed by these versatile chromium catalysts will be highlighted

Recent advancements in N-ligated group 4 molecular catalysts for the (co)polymerization of ethylene

Group 4 metal (Zr, Ti, Hf) catalysts for olefin polymerization and specifically those based on non-metallocene complexes have continued to be a subject of intense study in homogeneous catalysis. With a view to forming new or improved polyolefinic materials, complexes bearing N-donor anionic ligands such as β-diketiminate, amidinate, guanidinate, amido, imido as well as mixed N-donor ligands including N,C,C-azaallyl and N,O-phenoxy-imine, have been central to many key developments; high catalytic activities for homo- and copolymerization of ethylene have been a highlight of their catalysis. The fine tuning of these nitrogen-containing ligands significantly controls the catalytic performances of their metal catalysts as well as the structural properties of the resulting polymers with high molecular weight or even ultra-high molecular weight materials accessible. In this review the focus is on more recent publications in the field, in which we correlate the influence of ligand structure with the catalytic performance and microstructure of the polyethylenes. Furthermore, we examine the effects of co-catalyst on activity and thermostability of the precatalyst while efforts directed towards the copolymerization of ethylene with 1-hexene are also summarized. Overall, this work presents an overview of current knowledge pertaining to catalyst design and especially with regard to how the modulation of steric and electronic properties impact on the (co)polymerization process

Unimodal polyethylenes of high linearity and narrow dispersity by using ortho-4,4′-dichlorobenzhydryl-modified bis(imino)pyridyl-iron catalysts

Six different examples of 4,4′-dichlorobenzhydryl-substituted 2,6-bis(arylimino)pyridyl-iron(ii) chloride complex, [2-{{2,6-((p-ClPh)2CH)2-4-MeC6H2}N = CMe}-6-(ArN CMe)C5H3N]FeCl2 (Ar = 2,6-Me2C6H3Fe1, 2,6-Et2C6H3Fe2, 2,6-iPr2C6H3Fe3, 2,4,6-Me3C6H2Fe4, 2,6-Et2-4-MeC6H2Fe5, 2,6-((p-ClPh)2CH)2-4-MeC6H2Fe6), have been synthesized in good yield and characterized by various spectroscopic and analytical techniques. The molecular structures of Fe2 and Fe5 emphasize the uneven steric protection of the ferrous center imposed by the unsymmetrical N,N,N′-chelate. When treated with either MAO or MMAO (modified-MAO) as activators, Fe1-Fe5 exhibited very high productivities at elevated temperature with peak performance of 21.59 × 106 g PE mol−1(Fe) h−1 for Fe5/MMAO at 50 °C and 15.65 × 106 g PE mol−1(Fe) h−1 for Fe1/MAO at 60 °C. By contrast, the most sterically hindered Fe6 was either inactive (using MAO) or displayed very low activity (using MMAO). As a further feature, this class of iron catalyst was capable of displaying long lifetimes with catalytic activities up to 10.77 × 106 g PE mol−1(Fe) h−1 observed after 1 h. In all cases, strictly linear and unimodal polyethylene was formed with narrow dispersity, while the polymer molecular weight was strongly influenced by the aluminoxane co-catalyst (Mw using MAO > MMAO) and also by the steric properties of the second N-aryl group (up to 32.9 kg mol−1 for Fe3/MAO)

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

The N, N, N′-ferrous chloride complexes, [2-{CMeN(2,4-(CHPh)2-6-FC6H2)}-6-(CMeNAr)C5H3N]FeCl2 (Ar = 2,6-Me2C6H3 Fe1, 2,6-Et2C6H3 Fe2, 2,6- iPr2C6H3 Fe3, 2,4,6-Me3C6H2 Fe4 and 2,6-Et2-4-MeC6H2 Fe5), each possessing one N-2,4-dibenzhydryl-6-fluorophenyl group, were readily synthesized from their respective unsymmetrical bis(imino)pyridines, L1–L5. Structural identification of Fe2 highlighted the variation in the steric properties provided by the dissimilar N-aryl groups. Following pre-treatment with either MAO or MMAO, complexes Fe1–Fe5 all displayed, at an operating temperature of 80 °C, high activities for ethylene polymerization with levels falling in the order: Fe4 > Fe1 > Fe5 > Fe2 > Fe3. Notably, Fe4/MAO displayed the highest activity of 1.94×107 gPE·molFe−1·h−1 of the study with only a modest loss in performance at 90 °C. Generally, the resulting polyethylenes were highly linear (T m range: 122–132 °C), narrowly disperse and of low molecular weight (M w range: 6.73–46.04 kg·mol−1), with the most sterically hindered Fe3 forming the highest molecular weight polymer of the series. End-group analysis by 1H- and 13C-NMR spectroscopy revealed saturated alkyl (n-propyl and i-propyl) and unsaturated vinyl chain ends indicative of the role of both β-H elimination and chain transfer to aluminum as termination pathways. By comparison with previously reported iron precatalysts with similar tridentate ligand skeletons, it is evident that the introduction of a large benzhydryl group in combination with a fluorine as the ortho-substituents of one N-aryl group has the effect of enhancing thermal stability of the iron polymerization catalyst whilst maintaining appreciable polymer molecular weight.</p

A comparative kinetic study of ethylene polymerization mediated by iron, cobalt and chromium catalysts bearing the same N,N,N-bis(imino)trihydroquinoline

The iron(II), cobalt(II) and chromium(III) chlorides, [2-{(2,4,6-Me3C6H2)NCMe}-8-{N(2,4,6-Me3C6H2)}C9H8N]MCln (n = 2, M = Fe LFeCl2, Co LCoCl2; n = 3, M = Cr LCrCl3), each bearing the same N,N,N-bis(imino)trihydroquinoline chelating ligand, have been employed as precatalysts for ethylene polymerization with modified methylaluminoxane (MMAO) as the co-catalyst. The kinetic profiles for these homogeneous polymerizations are reported in addition to the properties of the resultant polymers under comparable reaction conditions. All the experimental data indicate that the active metal center plays a key role on the catalytic performances of the complexes, especially the polymerization activity, thermal stability and lifetime of the active species. Under optimized conditions the iron catalyst displays the highest rate of polymerization but displays this for only a short period, while the chromium catalyst shows a lower maximum polymerization rate but sustains its performance over a longer period and at a higher temperature. In terms of the polymer properties, all three metal catalysts afford highly linear polymers with the metal center influencing the molecular weight and type of end group. Specifically, the cobalt and chromium catalysts produce narrowly dispersed low molecular weight polymers incorporating vinyl end groups, while the iron catalyst affords polymers of higher molecular weight displaying broad molecular weight distributions, with both fully saturated and unsaturated chain ends

Co-catalyst effects on the thermal stability/activity of N,N,N-Co ethylene polymerization Catalysts Bearing Fluoro-Substituted N-2,6-dibenzhydrylphenyl groups

The unsymmetrical bis (arylimino)pyridines, 2-[CMeN{2,6-{(4-FC6H4)2CH}2–4-t-BuC6H2}]-6-(CMeNAr)C5H3N (Ar = 2,6-Me2C6H3 L1, 2,6-Et2C6H3 L2, 2,6-i-Pr2C6H3 L3, 2,4,6-Me3C6H2 L4, 2,6-Et2–4-MeC6H2 L5), each containing one N-aryl group bedecked with ortho-substituted fluorobenzhydryl groups, have been employed in the preparation of the corresponding five-coordinate cobalt (II) chelates, LCoCl2 (Co1 – Co5); the symmetrical comparator [2,6-{CMeN(2,6-(4-FC6H4)2CH)2–4-t-BuC6H2}2C5H3N]CoCl2 (Co6) is also reported. All cobaltous complexes are paramagnetic and have been characterized by 1H/19F NMR spectroscopy, FT-IR spectroscopy and elemental analysis. The molecular structures of Co3 and Co6 highlight the different degrees of steric protection given to the metal center by the particular N-aryl group combination. Depending on the aluminoxane co-catalyst employed to activate the cobalt precatalyst, distinct variations in thermal stability and activity of the catalyst towards ethylene polymerization were exhibited. In particular with MAO, the resultant catalysts reached their optimal performance at 70 °C delivering high activities of up to 10.1 × 106 g PE (mol of Co)−1 h−1 with Co1 > Co4 > Co2 > Co5 > Co3 >> Co6. On the other hand, using MMAO, the catalysts operate most effectively at 30 °C but are by comparison less productive. In general, the polyethylenes were highly linear, narrowly disperse and displayed a wide range of molecular weights [Mw range: 18.5–58.7 kg mol−1 (MAO); 206.1–352.5 kg mol−1 (MMAO)]

Branched polyethylenes attainable using thermally enhanced bis(imino)acenaphthene-nickel catalysts: Exploring the effects of temperature and pressure

The 4,4′-difluorobenzhydryl-containing nickel(II) bromide and chloride chelates, [1-[2,6-{(4-F-C6H4)2CH}2-4-{C(CH3)3}-C6H2N]-2-(ArN)C2C10H6]NiX2 (X = Br: Ar = 2,6-Me2C6H3 Ni1, 2,6-Et2C6H3 Ni2, 2,6-i-Pr2C6H3 Ni3, 2,4,6-Me3C6H2 Ni4, 2,6-Et2-4-MeC6H2 Ni5 and X = Cl: Ar = 2,6-Me2C6H3 Ni6, 2,6-Et2C6H3 Ni7, 2,6-i-Pr2C6H3 Ni8, 2,4,6-Me3C6H2 Ni9, 2,6-Et2-4-MeC6H2 Ni10), have been prepared and fully characterized. The solid-state structures of representative Ni3 and Ni7 display distorted tetrahedral geometries which are maintained in solution with broad paramagnetically shifted resonances a feature of all the 1H and 19F NMR spectra; the effect the halide (Br/Cl) ligand has on the proton and fluorine chemical shifts presents a further point of interest. All ten nickel complexes displayed, on activation with either MAO (methylaluminoxane) or EASC (ethyl aluminum sesquichloride), very high activities (up to 1.36 × 107 g PE mol−1 (Ni) h−1) for ethylene polymerization at either 1 or 10 atm C2H4 with the structural features of the N,N’-ligand influential. Significantly, with EASC as co-catalyst, Ni5 was capable of operating effectively at 90 °C without comprising too much catalytic activity [ca. 4.34 × 106 g PE mol−1 (Ni) h−1]. All the polyethylenes are highly branched with the branching content and type of branch strongly affected by a combination of temperature, pressure and the class of co-catalyst employed. Moreover, good tensile strength (εb up to 2839.5%) and elastic recovery (up to 74%) have been displayed, properties that are characteristic of thermoplastic elastomers (TPEs)

Iron ethylene polymerization catalysts incorporating trifluoromethoxy functionality: Effects on PE molecular weight and productivity

The capacity to broaden the range of molecular weights displayed by polyolefinic materials is an important factor to be considered in the design of polymerization catalysts. Herein, the 2,6-dibenzhydryl-4-trifluoromethoxy modified bis(imino)pyridyl-ferrous chlorides, [2-[CMeN{2,6-{(C6H5)2CH}2-4-(F3CO)C6H2}]-6-(CMeNAr)C5H3N]FeCl2 [Ar = 2,6-Me2C6H3 Fe1, 2,6-Et2C6H3 Fe2, 2,6-i-Pr2C6H3 Fe3, 2,4,6-Me3C6H2 Fe4, 2,6-Et2-4-MeC6H2 Fe5], are used as precatalysts in the solution polymerization of ethylene. On the activation with either MAO or MMAO, all complexes displayed high productivity [up to 18.4 × 106 g (PE) mol-1 (Fe) h-1 for Fe5/MAO], generating highly linear polyethylenes with a wide range of molecular weights (Mw range: 0.85 × 103 to 8.80 × 105 g mol-1). Notably, higher activity was achieved in hexane than in toluene under MAO activation, while the opposite trend was seen with MMAO, highlighting the key role played by solvent in the polymerization process. By comparison with structurally related iron catalysts, the presence of the electron withdrawing para-trifluoromethoxy group has the effect of increasing the molecular weight of the polyethylene. In addition to the polymerization studies, full synthetic and characterization details are presented for Fe1-Fe5 including the X-ray structures of Fe1 and Fe2

Chromium ethylene polymerization catalysts bearing sterically enhanced α,α′-bis(imino)-2,3:5,6-bis(pentamethylene)pyridines: Tuning activity and molecular weight

The ortho-benzhydryl-substituted α,α′-bis(arylimino)-2,3:5,6-bis(pentamethylene)pyridine-chromium(III) chlorides, [2,3:5,6-{C 4 H 8 C(N(2-R 1 -4-R 2 -6-(CHPh 2 )C 6 H 2 )} 2 C 5 HN]CrCl 3 [R 1 = R 2 = Me Cr1, R 1 = Me, R 2 = CHPh 2 Cr2, R 1 = Et, R 2 = CHPh 2 Cr3, R 1 = i-Pr, R 2 = CHPh 2 Cr4, R 1 = Cl, R 2 = CHPh 2 Cr5, R 1 = F, R 2 = CHPh 2 Cr6], differing in the electronic and/or steric properties of their aryl-R 1 and -R 2 groups, have been prepared by a one-pot template approach involving α,α′-dioxo-2,3:5,6-bis(pentamethylene)pyridine, the corresponding aniline and CrCl 3 (THF) 3 in acetic acid. The molecular structure of six-coordinate Cr1 reveals the carbocyclic-fused N,N,N-ligand to adopt a mer configuration with the puckered sections of the two fused rings arranged mutually cis. On activation with MAO or MMAO, Cr1 - Cr6 displayed high activities (up to 1.83 × 10 6 g (PE) mol −1 (Cr) h −1 ) for the polymerization of ethylene with the MAO-promoted polymerizations in most cases more productive than with MMAO. In general, the chromium complexes appended with ortho-halide substituents (Cr6 (F)) and (Cr5 (Cl)), proved the most active with the overall order being: Cr6 > Cr5 > Cr1 > Cr2 > Cr3 > Cr4. All catalysts formed linear polyethylene displaying a wide range of molecular weights (from 2.17 to 300.4 kg mol −1 ) that were highly dependent on the nature of the ortho-R 1 substituent with fluoride Cr6 forming the lowest molecular weight and the most sterically demanding Cr4 (i-Pr) the highest

Naphthalenyl-Substituted α,α′-Bisimino-2,3 : 5,6-Bis(pentamethylene)pyridines as Thermally Robust Supports for Iron Ethylene Polymerization Catalysts

Six examples of N,N,N-chelated iron(II) chloride complex, [2,3 : 5,6-{C4H8C(NAr)}2C5H3N]FeCl2 (Ar=1-C10H7 Fe1, 2-Me-1-C10H6 Fe2, 2-(CHMePh)-1-C10H6 Fe3, 2-Cl-1-C10H6 Fe4, 4-Br-1-C10H6 Fe5, 4-NO2-1-C10H6 Fe6), differing in the steric/electronic properties of the substituents appended to the N-naphthalen-1-yl groups, have been synthesized in reasonable yield via a facile one-pot route. The molecular structure of five-coordinate Fe3 emphasizes the steric hindrance imposed on the iron center by the CHMePh-substituted bicyclic aromatic groups. With either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO) as activator, the complexes displayed a wide range in catalytic activities for ethylene polymerization with the order for MMAO being: Fe3>Fe2≫Fe1>Fe4≫Fe5 while Fe6 was almost inactive. In particular, Fe3/MMAO exhibited exceptionally high activity at 70 °C (up to 15.7×106 g (PE) mol−1 (Fe) h−1) affording low molecular weight (2.97 kg mol−1) polyethylene of high linearity and narrow dispersity (Mw/Mn=1.38). Moreover, the same catalyst displayed excellent thermostability by maintaining high activity at 90 °C (up to 9.26×106 g (PE) mol−1 (Fe) h−1). Similar activity trends were observed with MAO though the catalysts were in general less active but formed distinctly higher molecular weight polyethylene