Methylaluminoxane’s Molecular Cousin: A Well-defined and “Complete” Al-Activator for Molecular Olefin Polymerization Catalysts

Sheet Models for Methylaluminoxane (MAO) Activators? A Theoretical Case Study involving rac-Me2Si(η5-C9H6)2Zr (SBIZr) Complexes

Activation of SBIZrMe2 or SBIZrMeCl and a sheet model for an active component of hydrolytic MAO, (MeAlO)16(Me3Al)6, (16,6) has been studied by DFT. Contact ion-pair formation occurs through the intermediacy of SBIZrMe(Cl) or SBIZrMe2 reacting with sheet 16,6 to furnish SBIZrMe-μ-X(MeAlO)16(Me3Al)6 (2, X=Me, Cl). Contact ion-pairs 2 would be in equilibrium with heterodinuclear catalyst precursors [SBIZrMe2AlMe2][(MeAlO)16(Me3Al)6X] (3 (X=Me, Cl) through reversible binding of Me3Al at higher Al : Zr ratios. Calculations show that formation of ion-pairs 3 from contact ion-pairs 2 is more favourable for the SBIZr compared with the parent Cp2Zr complexes. TD-DFT calculations were conducted on relevant SBIZr complexes to relate the results to earlier spectroscopic studies of catalyst activation using UV-Vis spectroscopy. Finally, propene insertion into ion-pairs 2, SBIZrMe-μ-MeB(C6F5)3 (6) and [SBIZrMe][B(C6F5)4] (7) was studied at M06-2X/TZVP level of theory. These studies suggest that contact ion-pairs 2 are significantly less reactive towards insertion than 6 or 7, in disagreement with experiment.

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.

The Dimerization and Oligomerization of Alkenes Catalyzed with Transition Metal Complexes: Catalytic Systems and Reaction Mechanisms

Dimers and oligomers of alkenes represent a category of compounds that are in great demand in diverse industrial sectors. Among the developing synthetic methods, the catalysis of alkene dimerization and oligomerization using transition metal salts and complexes is of undoubted interest for practical applications. This approach demonstrates substantial potential, offering not only elevated reaction rates but also precise control over the chemo-, regio-, and stereoselectivity of the reactions. In this review, we discuss the data on catalytic systems for alkene dimerization and oligomerization. Our focus lies in the analysis of how the activity and chemoselectivity of these catalytic systems are influenced by various factors, such as the nature of the transition metal, the ligand environment, the activator, and the substrate structure. Notably, this review particularly discusses reaction mechanisms, encompassing metal complex activation, structural and dynamic features, and the reactivity of hydride intermediates, which serve as potential catalytically active centers in alkene dimerization and oligomerization.

MAO- and Borate-Free Activating Supports for Group 4 Metallocene and Post-Metallocene Catalysts of α-Olefin Polymerization and Oligomerization

Modern industry of advanced polyolefins extensively uses Group 4 metallocene and post-metallocene catalysts. High-throughput polyolefin technologies demand the use of heterogeneous catalysts with a given particle size and morphology, high thermal stability, and controlled productivity. Conventional Group 4 metal single-site heterogeneous catalysts require the use of high-cost methylalumoxane (MAO) or perfluoroaryl borate activators. However, a number of inorganic phases, containing highly acidic Lewis and Brønsted sites, are able to activate Group 4 metal pre-catalysts using low-cost and affordable alkylaluminums. In the present review, we gathered comprehensive information on MAO- and borate-free activating supports of different types and discussed the surface nature and chemistry of these phases, examples of their use in the polymerization of ethylene and α-olefins, and prospects of the further development for applications in the polyolefin industry.

Ionization of Cp2 ZrMe2 and Lewis Bases by Methylaluminoxane: Computational Insights

The interactions of the Lewis bases CO, octamethyltrisiloxane (OMTS) and 2,2'-bipyridine (bipy) with a sheet model for the principal activator (MeAlO)₁₆ (Me₃ Al)₆ (16,6) in hydrolytic methylaluminoxane (MAO) were investigated by DFT. These studies reveal that OMTS and bipy form adducts with Me₃ Al prior to methide abstraction by 16,6 to form the ion-pairs [Me₂ Al(κ² -L)][16,6] (5: L=OMTS, 6: L=bipy, [16,6]^- =[(MeAlO)₁₆ (Me₃ Al)₆ Me]^- ) while CO simply binds to a reactive edge site without ionization. The binding and activation of Cp₂ ZrMe₂ with 16,6 to form both neutral adducts 1 Cp₂ ZrMe₂ ⋅16,6 and contact ion-pairs 4 and 7, both with formula [Cp₂ ZrMe][μ-Me(MeAlO)₁₆ (Me₃ Al)₆ ], featuring terminal and chelated MAO-anions, respectively was studied by DFT. The displacement of the anion with either excess Cp₂ ZrMe₂ or Me₃ Al was also studied, forming outer-sphere ion-pairs [(Cp₂ ZrMe)₂ μ-Me][16,6] (2) and [Cp₂ Zr(μ-Me)₂ AlMe₂ ][16,6] (3). The theoretical NMR spectra of these species were compared to experimental spectra of MAO and Cp₂ ZrMe₂ and found to be in good agreement with the reported data and assignments. These studies confirm that 16,6 is a very suitable model for the activators present in MAO but highlight the difficulty in accurately calculating thermodynamic quantities for molecules in this size regime.

Transition Metal-(μ-Cl)-Aluminum Bonding in α-Olefin and Diene Chemistry

Olefin and diene transformations, catalyzed by organoaluminum-activated metal complexes, are widely used in synthetic organic chemistry and form the basis of major petrochemical processes. However, the role of M−(μ-Cl)−Al bonding, being proven for certain >C=C< functionalization reactions, remains unclear and debated for essentially more important industrial processes such as oligomerization and polymerization of α-olefins and conjugated dienes. Numerous publications indirectly point at the significance of M−(μ-Cl)−Al bonding in Ziegler−Natta and related transformations, but only a few studies contain experimental or at least theoretical evidence of the involvement of M−(μ-Cl)−Al species into catalytic cycles. In the present review, we have compiled data on the formation of M−(μ-Cl)−Al complexes (M = Ti, Zr, V, Cr, Ni), their molecular structure, and reactivity towards olefins and dienes. The possible role of similar complexes in the functionalization, oligomerization and polymerization of α-olefins and dienes is discussed in the present review through the prism of the further development of Ziegler−Natta processes and beyond.

Effect of Borate Cocatalysts toward Activity and Comonomer Incorporation in Ethylene Copolymerization by Half-Titanocene Catalysts in Methylcyclohexane

Ethylene copolymerizations with 2-methyl-1-pentene, 1-dodecene (DD), vinylcyclohexane (VCH), [Me₂Si(C₅Me₄)(N ^t Bu)]TiCl₂ (1), Cp*TiMe₂(O-2,6- ⁱ Pr₂-4-RC₆H₂) [R = H (2), SiEt₃ (3)]-borate, and [A(H)]⁺[BAr₄]^- [Ar = C₆F₅; A(H)⁺ = N⁺(H)Me(n-C₁₈H₃₇)₂, N⁺(H)(CH₂CF₃)(n-C₁₈H₃₇)₂, HO⁺(n-C₁₄H₂₉)₂·O(n-C₁₄H₂₉)₂, HO⁺(n-C₁₆H₃₃)₂·O(n-C₁₆H₃₃)₂; Ar = C₁₀F₇, A(H)⁺ = HO⁺(n-C₁₄H₂₉)₂·O(n-C₁₄H₂₉)₂ (B5), N⁺(H)(CH₂CF₃)(n-C₁₈H₃₇)₂] catalyst systems conducted in methylcyclohexane (MCH) exhibited better comonomer incorporation than those conducted in toluene (in the presence of methylaluminoxane (MAO) or borate cocatalysts). The activity was affected by the borate cocatalyst and 1,3-B5 catalyst systems in MCH and showed the highest activity in the ethylene copolymerizations with VCH and DD.

Syntheses of Silylene-Bridged Thiophene-Fused Cyclopentadienyl ansa-Metallocene Complexes for Preparing High-Performance Supported Catalyst

We synthesized a series of Me2Si-bridged ansa-zirconocene complexes coordinated by thiophene-fused cyclopentadienyl and fluorenyl ligands (Me2Si(2-R1-3-R2-4,5-Me2C7S)(2,7-R32C13H6))ZrMe2 (R1 = Me or H, R2 = H or Me, R3 = H, tBu, or Cl) for the subsequent preparation of supported catalysts. We determined that the fluorenyl ligand adopts an η3-binding mode in 9 (R1 = Me, R2 = H, R3 = H) by X-ray crystallography. Further, we synthesized a derivative 15 by substituting the fluorenyl ligand in 9 with a 2-methyl-4-(4-tert-butylphenyl)indenyl ligand, derivatives 20 and 23 by substituting the Me2Si bridge in 12 (R1 = Me, R2 = H, R3 = tBu) and 15 with a tBuO(CH2)6(Me)Si bridge, and the dinuclear congener 26 by connecting two complexes with a –(Me)Si(CH2)6Si(Me)– spacer. The silica-supported catalysts prepared using 12, 20, and 26 demonstrated up to two times higher productivity in ethylene/1-hexene copolymerization than that prepared with conventional (THI)ZrCl2 (21–26 vs. 12 kg-PE/g-(supported catalyst)), producing polymers with comparable molecular weight (Mw, 330–370 vs. 300 kDa), at a higher 1-hexene content (1.3 vs. 1.0 mol%) but a lower bulk density of polymer particles (0.35 vs. 0.42 g/mL).

Formation and Structure of Hydrolytic Methylaluminoxane Activators

Methylaluminoxane (MAO) activators have sheet structures which form ion-pairs on reaction of neutral donors such as octamethyltrisiloxane (OMTS). The ion-pairs can be detected by electrospray ionization mass spectrometry (ESI-MS) in polar media. The growth of these reactive precursors during hydrolysis of Me₃ Al can be monitored using ESI-MS. Density functional theory, combined with numerical simulation of growth, indicates that this process involves rapid formation of low MW oligomers, followed by assembly of these species into low MW sheets. These can grow through further addition of low MW oligomers or by fusion into larger sheets. The mechanism of these growth processes leads to the prediction that even-numbered sheets should be favored, and this surprising result is confirmed by ESI-MS monitoring experiments of both activator growth and MAO aging.

Active Sites in a Heterogeneous Organometallic Catalyst for the Polymerization of Ethylene

Heterogeneous derivatives of catalysts discovered by Ziegler and Natta are important for the industrial production of polyolefin plastics. However, the interaction between precatalysts, alkylaluminum activators, and oxide supports to form catalytically active materials is poorly understood. This is in contrast to homogeneous or model heterogeneous catalysts that contain resolved molecular structures that relate to activity and selectivity in polymerization reactions. This study describes the reactivity of triisobutylaluminum with high surface area aluminum oxide and a zirconocene precatalyst. Triisobutylaluminum reacts with the zirconocene precatalyst to form hydrides and passivates -OH sites on the alumina surface. The combination of passivated alumina and zirconium hydrides formed in this mixture generates ion pairs that polymerize ethylene.

Are Methylaluminoxane Activators Sheets?

Density functional theory calculations on neutral sheet models for methylaluminoxane (MAO) indicate that these structures, containing 5‐coordinate and 4‐coordinate Al, are likely precursors to ion‐pairs seen during the hydrolysis of trimethylaluminum (Me₃Al) in the presence of donors such as octamethyltrisiloxane (OMTS). Ionization by both methide ([Me]⁻) and [Me₂Al]⁺ abstraction, involving this donor, were studied by polarizable continuum model calculations in fluorobenzene (PhF) and o‐difluorobenzene (DFB) media. These studies suggest that low MW, 5‐coordinate sheets ionize by [Me₂Al]⁺ abstraction, while [Me]⁻ abstraction from Me₃Al‐OMTS is the likely process for higher MW 4‐coordinate sheets. Further, comparison of anion stabilities per mole of aluminoxane repeat unit (MeAlO)_n, suggest that anions such as [(MeAlO)₇(Me₃Al)₄Me]⁻=[7,4]⁻ are especially stable compared to higher homologues, even though their neutral precursors are unstable.

Polymethylaluminoxane organic frameworks (sMAOF) – highly active supports for slurry phase ethylene polymerisation

A series of modified solid polymethylaluminoxane (sMAO) catalyst supports have been developed for slurry phase ethylene polymerisation, using aryl di-ol modifier groups.

Rapid kinetic evaluation of homogeneous single-site metallocene catalysts and cyclic diene: how do the catalytic activity, molecular weight, and diene incorporation rate of olefins affect each other?

The kinetics and mechanism of ethylene and cyclic diene 5-ethylidene-2-norbornene (ENB) copolymerization catalyzed by rac-Et(Ind)₂ZrCl₂/[Ph₃C][B(C₆F₅)₄]/triisobutylaluminium (TIBA) were investigated using a quench-labeling procedure using 2-thiophenecarbonyl chloride (TPCC). The E/ENB copolymers were characterized by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and ¹H nuclear magnetic resonance (NMR) spectroscopy and sulfur analysis. To reduce the errors of the ethylene–diene copolymerization for the kinetics study, we selected E/ENB with steric and electronic features that permit us to elucidate the metallocene catalyst behavior against dienes. A quantitative approach of catalyst speciation, stereodynamics, and micro-kinetics assisted the resolution of mechanistic problems, such as the elastomeric synthesis of ethylene propylene diene monomer rubber (EPDM), the catalyst resting state nature, and how much ion-pairing occurs during polymerization. We report here the precise observation of metal–polymer species, explanation of the dynamics of their initiation, propagation, and termination, and ethylene ENB copolymer development. An approach based on acyl chloride was used to selectively quenched transition metal–polymer bonds to evaluate the polymeric catalyst in terms of its reaction rate, R_p, propagation rate content, k_p, and mole fraction of active centers. It is noted that the decline in catalytic activity in the range of 1800 s, and the active center [Zr]/[*C] fraction significantly increased during the initial 1000 s and then tended towards a steady figure of 86%. It is suggested that nearly complete initiation of all olefins catalysts can be obtained after a sufficiently extended reaction. The quick increase in active sites in the first stage can be described by the immediate initiation of active sites positioned on the surfaces of catalyst particles. The initial polymerization rate, R_p, is high and the crystalline properties of the E/ENB copolymer are low due to the greater incorporation of ENB in the polymer backbone, and later the polymerization reaction rates remained stable with a lower mol% of ENB. The melting temperature (T_m) ranges from 108 to 127 °C, whereas the crystalline temperature ranges from 63 to 108 (J g⁻¹). In the E–ENB copolymers, the value of k_pE is much greater than that of k_pENB; at 120 s, the k_pE and k_pENB values are 9115 and 431 L mol⁻¹ s⁻¹, respectively, implying smaller diffusion barriers in the early stages, which are close to the actual propagation rate constant.

The kinetics and mechanism of ethylene and 5-ethylidene-2-norbornene copolymerization catalyzed by rac-Et(Ind)₂ZrCl₂ were investigated using 2-thiophenecarbonyl chloride.