Zirconocene-catalyzed propene polymerization: a quenched-flow kinetic study

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.

Meso- and Rac-[bis(3-phenyl-6-tert-butylinden-1-yl)dimethylsilyl]zirconium Dichloride: Precatalysts for the Production of Differentiated Polyethylene Products with Enhanced Properties

Ansa-zirconocene complexes are widely employed as precatalysts for olefin polymerization. Their synthesis generally leads to mixtures of their rac and meso isomers, whose separation is often problematic. In this contribution, we report on the synthesis of a novel silyl-bridged bis(indenyl)-based metallocene, and on the separation of its rac and meso isomers by simple recrystallization from toluene. The two complexes, activated by methylaluminoxane (MAO), have been used as precatalysts in ethylene/1-hexene copolymerization. Regardless of the reaction conditions, the meso complex outperformed its rac congener. A similar trend was observed by performing the process in the presence of the silica-supported versions of the complexes. This is remarkable, since meso metallocenes generally display lower activities than their rac analogues. Furthermore, the meso isomer generates polymer products that are more in line with the targets for the preparation of a bimodal PE grade made of a lower-MW high-density (HDPE) fraction and a higher-MW linear low-density (LLDPE) fraction.

Polymerization kinetics of bicyclic olefins and mechanism with symmetrical ansa-metallocene catalysts associated with active center count: relationship between their activities and structure and activation path

Copolymerization of ethylene (E) with 5-vinyl-2-norbornene (VNB) catalyzed by ansa-metallocenes allows the precise control of essential polymeric properties such as comonomer incorporation, molecular weight (M w), and polydispersity (Đ). Significant efforts have been devoted to synthesizing and developing novel catalysts, cocatalysts, and activators, although the fundamental elements of catalytic processes remain unclear. For example, it is questionable how polymeric catalysts are divided across dormant and active sites and how this distribution affects the order of monomers for the propagation rate, which widely vary in the literature. Furthermore, although the empirical correlation between the monomers and average M w has been established in many systems, the fundamental processes of chain termination remain unknown. Furthermore, the involvement of ion-pairing in metallocene-catalyzed polymerization and the termination mechanisms are also contentious issues. In this study, we describe the use of a quenched-labeling technique based on acyl chloride to selectively quench the zirconium metal-polymeric bond, which can be used to study the kinetics, active site [Zr][C*] counting, copolymer microstructure, and molecular weight distribution (MWD) to determine the rate laws for chain initiation, chain propagation rate (R p), propagation rate constant (k p) and chain termination. In addition, we also predict previously unknown chemical characteristics of E/bicyclic copolymerization processes, where either a cis-endocyclic double bond with steric properties or a vinyl exocyclic double bond affects the activity, i.e., [Zr]/[*C], (R p) and (k p). All these properties require the implementation of a particular kinetic mechanism that assumes the low activity of the building copolymer chains incorporating a single ethylene/VNB unit, i.e., the Cp2Zr-C2H5 group, in the ethylene addition process in the Cp2Zr-C bond. Due to β-agostic stabilization, the Cp2Zr-C2H5 group exhibits a distinct feature. These effects were confirmed experimentally, such as the E/VNB co-polymer activity and VNB mol%, propagation rate decrease in the polymerization time (t p) of 120 s to 1800 s, crystalline properties, and significant increase in molecular weight. The active center [Zr]/[*C] fraction considerably increased in the initial (t p) 840 s, and subsequently tended to the steady stage of 33%, which is lower than previously reported E homo- and E/P copolymerization. The lower [C*]/[Zr] in both the early and stable stages, decrease in VNB mol%, and R p with t p can be associated with the more significant fraction of Cp2Zr-CH2CH3-type dormant site by the β-agostic hydrogen interaction with the Cp2Zr metal. The t p versus R pE, R pVNB, k pE, k pVNB, and [Zr]/[C*] count could be fitted to a model that invokes deactivation of the growing polymer chains. In the case of the thermal behavior of the copolymers (melting temperature (T m) and crystalline temperature (ΔH m)), T m varied from 101 °C to 121 °C, while ΔH m varied from 9 to 16 (J g-1).

Mechanistic analysis by NMR spectroscopy: A users guide

A 'principles and practice' tutorial-style review of the application of solution-phase NMR in the analysis of the mechanisms of homogeneous organic and organometallic reactions and processes. This review of 345 references summarises why solution-phase NMR spectroscopy is uniquely effective in such studies, allowing non-destructive, quantitative analysis of a wide range of nuclei common to organic and organometallic reactions, providing exquisite structural detail, and using instrumentation that is routinely available in most chemistry research facilities. The review is in two parts. The first comprises an introduction to general techniques and equipment, and guidelines for their selection and application. Topics include practical aspects of the reaction itself, reaction monitoring techniques, NMR data acquisition and processing, analysis of temporal concentration data, NMR titrations, DOSY, and the use of isotopes. The second part comprises a series of 15 Case Studies, each selected to illustrate specific techniques and approaches discussed in the first part, including in situ NMR (^1/2H, ^10/11B, ¹³C, ¹⁵N, ¹⁹F, ²⁹Si, ³¹P), kinetic and equilibrium isotope effects, isotope entrainment, isotope shifts, isotopes at natural abundance, scalar coupling, kinetic analysis (VTNA, RPKA, simulation, steady-state), stopped-flow NMR, flow NMR, rapid injection NMR, pure shift NMR, dynamic nuclear polarisation, ¹H/¹⁹F DOSY NMR, and in situ illumination NMR.

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.

Comparative Analysis of Ethylene/Diene Copolymerization and Ethylene/Propylene/Diene Terpolymerization Using Ansa-Zirconocene Catalyst with Alkylaluminum/Borate Activator: The Effect of Conjugated and Nonconjugated Dienes on Catalytic Behavior and Polymer Microstructure

The copolymerization of ethylene‒diene conjugates (butadiene (BD), isoprene (IP) and nonconjugates (5-ethylidene-2-norbornene (ENB), vinyl norbornene VNB, 4-vinylcyclohexene (VCH) and 1, 4-hexadiene (HD)), and terpolymerization of ethylene-propylene-diene conjugates (BD, IP) and nonconjugates (ENB, VNB, VCH and HD) using two traditional catalysts of C₂-symmetric metallocene—silylene-bridged rac-Me₂Si(2-Me-4-Ph-Ind)₂ZrCl₂ (complex A) and ethylene-bridged rac-Et(Ind)₂ZrCl₂ (complex B)—with a [Ph₃C][B(C₆F₅)₄] borate/TIBA co-catalyst, were intensively studied. Compared to that in the copolymerization of ethylene diene, the catalytic activity was more significant in E/P/diene terpolymerization. We obtained a maximum yield of both metallocene catalysts with conjugated diene between 3.00 × 10⁶ g/mol_Mt·h and 5.00 × 10⁶ g/mol_Mt·h. ENB had the highest deactivation impact on complex A, and HD had the most substantial deactivation effect on complex B. A ¹H NMR study suggests that dienes were incorporated into the co/ter polymers’ backbone through regioselectivity. ENB and VNB, inserted by the edo double bond, left the ethylidene double bond intact, so VCH had an exo double bond. Complex A’s methyl and phenyl groups rendered it structurally stable and exhibited a dihedral angle greater than that of complex B, resulting in 1, 2 isoprene insertion higher than 1, 4 isoprene that is usually incapable of polymerization coordination. High efficiency in terms of co- and ter- monomer incorporation with higher molecular weight was found for complex 1. The rate of incorporation of ethylene and propylene in the terpolymer backbone structure may also be altered by the conjugated and nonconjugated dienes. ¹³C-NMR, ¹H-NMR, and GPC techniques were used to characterize the polymers obtained.

Chain Transfer to Solvent and Monomer in Early Transition Metal Catalyzed Olefin Polymerization: Mechanisms and Implications for Catalysis

Even after several decades of intense research, mechanistic studies of olefin polymerization by early transition metal catalysts continue to reveal unexpected elementary reaction steps. In this mini-review, the recent discovery of two unprecedented chain termination processes is summarized: chain transfer to solvent (CTS) and chain transfer to monomer (CTM), leading to benzyl/tolyl and allyl type chain ends, respectively. Although similar transfer reactions are well-known in radical polymerization, only very recently they have been observed also in olefin insertion polymerization catalysis. In the latter context, these processes were first identified in Ti-catalyzed propene and ethene polymerization; more recently, CTS was also reported in Sc-catalyzed styrene polymerization. In the Ti case, these processes represent a unique combination of insertion polymerization, organic radical chemistry and reactivity of a M(IV)/M(III) redox couple. In the Sc case, CTS occurs via a σ-bond metathesis reactivity, and it is associated with a significant boost of catalytic activity and/or with tuning of polystyrene molecular weight and tacticity. The mechanistic studies that led to the understanding of these chain transfer reactions are summarized, highlighting their relevance in olefin polymerization catalysis and beyond.

Synthesis of ultra-high molecular weight poly(ethylene)-co-(1-hexene) copolymers through high-throughput catalyst screening

A family of permethylindenyl titanium constrained geometry complexes, Me₂SB(^R′N,^3-RI*)TiX₂ ((3-R-η⁵-C₉Me₅)Me₂Si(^R′TiX₂)), supported on solid polymethylaluminoxane (sMAO) are investigated as slurry-phase catalysts for ethylene/H₂ homopolymerisation and ethylene/1-hexene copolymerisation by high-throughput catalyst screening. Me₂SB(^tBuN,I*)TiCl₂ supported on sMAO [sMAO-Me₂SB(^tBuN,I*)TiCl₂] is responsive to small quantities of H₂ (<1.6%), maintaining high polymerisation activities (up to 4900 kg_PE mol_Ti⁻¹ h⁻¹ bar⁻¹) and yielding polyethylenes with significantly decreased molecular weight (M_w) (from 2700 to 41 kDa with 1.6% H₂). In slurry-phase ethylene/1-hexene copolymerisation studies, a decrease in polymerisation activity and polymer molecular weights compared to ethylene homopolymerisation is observed. Compared to many solid supported system, these complexes all display high 1-hexene incorporation levels up to a maximum incorporation of 14.2 mol% for sMAO-Me₂SB(^iPrN,I*)TiCl₂). We observe a proportionate increase in 1-hexene incorporation with concentration, highlighting the ability of these catalysts to controllably tune the amount of 1-hexene incorporated into the polymer chain to produce linear low-density polyethylene (LLDPE) materials.

Permethylindenyl titanium CGCs supported on solid polymethylaluminoxane (sMAO) are investigated as slurry-phase catalysts by high-throughput catalyst screening (copolymerisation of ethylene/1-hexene with incorporation levels up to 14.2 mol%).

Polyolefin catalysis of propene, 1-butene and isobutene monitored using hyperpolarized NMR

Polymerization reactions of the dissolved gases propene, 1-butene, and isobutene catalyzed by [Zr(Cp)₂Me][B(C₆F₅)₄] were characterized using in situ NMR. Hyperpolarization of ¹³C spins by the dissolution dynamic nuclear polarization (DNP) technique provided a signal enhancement of up to 5000-fold for these monomers. For DNP hyperpolarization, liquid aliquots containing monomers were prepared at a temperature between the freezing point of the solvent toluene and the boiling point of the monomer, mixed with the polarizing agent α,γ-bis-diphenylene-β-phenylallyl free radical, and subsequently frozen. The hyperpolarized signals after dissolution enabled the observation of reaction kinetics, as well as polymer products and side products within a time of 30 s from the start of the reaction. The observed kinetic rate constants for polymerization followed a decreasing trend for propene, 1-butene, and isobutene, with the lowest rate constant for the latter explained by steric bulk. For all reactions, partial deactivation was further observed during the measurement time. The line shape and the chemical shift of the monomer signals with respect to a toluene signal were both dependent on catalyst concentration and reaction time, with the strongest dependence observed for isobutene. These changes are consistent with the characteristics of a rapid binding and unbinding process of the monomer to the catalyst occurring during the reaction.

Hyperpolarization by dissolution dynamic nuclear polarization (DNP) enhances ¹³C NMR signals of normally gaseous olefins. The polymerization reactions of these dissolved gases catalyzed by a metallocene catalyst are characterized in real time.

Real-time analysis of methylalumoxane formation

Methylalumoxane (MAO), a perennially useful activator for olefin polymerization precatalysts, is famously intractable to structural elucidation, consisting as it does of a complex mixture of oligomers generated from hydrolysis of pyrophoric trimethylaluminum (TMA). Electrospray ionization mass spectrometry (ESI-MS) is capable of studying those oligomers that become charged during the activation process. We have exploited that ability to probe the synthesis of MAO in real time, starting less than a minute after the mixing of H₂O and TMA and tracking the first half hour of reactivity. We find that the process does not involve an incremental build-up of oligomers; instead, oligomerization to species containing 12–15 aluminum atoms happens within a minute, with slower aggregation to higher molecular weight ions. The principal activated product of the benchtop synthesis is the same as that observed in industrial samples, namely [(MeAlO)₁₆(Me₃Al)₆Me]⁻, and we have computationally located a new sheet structure for this ion 94 kJ mol⁻¹ lower in Gibbs free energy than any previously calculated.

The activator methylaluminoxane is made by hydrolysis of trimethylaluminum. Analysis using ESI-MS reveals rapid formation of small oligomers is followed by slower aggregation to the larger precursors most capable of releasing [Me₂Al]⁺.

Mechanistic Studies of Hafnium-Pyridyl Amido-Catalyzed 1-Octene Polymerization and Chain Transfer Using Quench-Labeling Methods

Chromophore quench-labeling applied to 1-octene polymerization as catalyzed by hafnium-pyridyl amido precursors enables quantification of the amount of active catalyst and observation of the molecular weight distribution (MWD) of Hf-bound polymers via UV-GPC analysis. Comparison of the UV-detected MWD with the MWD of the "bulk" (all polymers, from RI-GPC analysis) provides important mechanistic information. The time evolution of the dual-detection GPC data, concentration of active catalyst, and monomer consumption suggests optimal activation conditions for the Hf pre-catalyst in the presence of the activator [Ph₃C][B(C₆F₅)₄]. The chromophore quench-labeling agents do not react with the chain-transfer agent ZnEt₂ under the reaction conditions. Thus, Hf-bound polymeryls are selectively labeled in the presence of zinc-polymeryls. Quench-labeling studies in the presence of ZnEt₂ reveal that ZnEt₂ does not influence the rate of propagation at the Hf center, and chain transfer of Hf-bound polymers to ZnEt₂ is fast and quasi-irreversible. The quench-label techniques represent a means to study commercial polymerization catalysts that operate with high efficiency at low catalyst concentrations without the need for specialized equipment.

Preparation of "Constrained Geometry" Titanium Complexes of [1,2]Azasilinane Framework for Ethylene/1-Octene Copolymerization

The Me₂Si-bridged ansa-Cp/amido half-metallocene, [Me₂Si(η⁵-Me₄C₅)(N^tBu)]TiCl₂, termed a "constrained-geometry catalyst (CGC)", is a representative homogeneous Ziegler catalyst. CGC derivatives with the [1,2]azasilinane framework, in which the amide alkyl substituent is joined by the Si-bridge, were prepared, and the catalytic performances of these species was studied. Me₄C₅HSi(Me)(CH₂CH=CH₂)-NH(C(R)(R')CH=CH₂) (R, R' = H or methyl; Me₄C₅H = tetramethylcyclopentadienyl) was susceptible to ring closure metathesis (RCM) when treated with Schrock's Mo-catalyst to afford -Si(Me₄C₅H)(Me)CH₂CH=CHC(R)(R')NH- containing a six-membered ring framework. Using the precursors and the products of RCM, various CGC derivatives, i.e., [-Si(η⁵-Me₄C₅)(Me)CH₂CH=CHC(R)(H)N-]TiMe₂ (13, R = H; 15, R = Me), [-Si(η⁵-Me₄C₅)(Me)CH₂CH₂CH₂CH₂N]TiMe₂ (14), [(η⁵-Me₄C₅)Si(Me)(CH₂CH=CH₂)NCH₂CH=CH₂]TiMe₂ (16), [(η⁵-Me₄C₅)Si (Me)(CH=CH₂)NCH₂CH=CH₂]TiMe₂ (17), and [(η⁵-Me₄C₅)Si(Me)(CH₂CH₃)NCH₂CH₂CH₃]TiMe₂ (18), were prepared. The catalytic activity of the newly prepared complexes was lower than that of CGC when activated with [Ph₃C][B(C₆F₅)₄]/iBu₃Al. However, the catalytic activity of these species was improved by using tetrabutylaluminoxane ([iBu₂Al]₂O) instead of iBu₃Al and the activity of 14/[Ph₃C][B(C₆F₅)₄]/[iBu₂Al]₂O was comparable to that of CGC/[Ph₃C][B(C₆F₅)₄]/iBu₃Al (4.7 and 5.0 × 10⁶ g/mol-Ti, respectively). Advantageously, the newly prepared complexes produced higher molecular weight poly(ethylene-co-1-octene)s than CGC.

Exploiting directional long range secondary forces for regulating electrostatics-dominated noncovalent interactions

It has been well established that long range secondary electrostatic interactions (SEIs) have a significant effect on the stability of supramolecular complexes. However, general rules for exploiting SEIs in the rational design of diverse supramolecular complexes have been difficult to obtain. In this work, we outline a quantum chemical approach for understanding the strength of electrostatic interactions. This approach is seen to provide excellent correlation between the electrostatic force and the binding energy between two partners in hydrogen-bonded complexes, as well as that between two ions in ion-pair complexes. Furthermore, we illustrate how the understanding of the binding allows for the rational design of new complexes where the association constant between the two partners can be increased or decreased, as desired, by several orders of magnitude. Hence, the current work showcases a general, simple and powerful method of understanding and exploiting long range secondary electrostatic interactions.

Silica/MAO/(n-BuCp)2ZrCl2 catalyst: effect of support dehydroxylation temperature on the grafting of MAO and ethylene polymerization

This study reports the role of silica dehydroxylation temperature on the performance of Silica/MAO/(n-BuCp)₂ZrCl₂ catalysts in ethylene homopolymerization.

Olefin polymerisation catalysts: when perfection is not enough

Despite decades of thorough mechanistic investigations, it is still hard to predict the activity of a novel olefin polymerisation catalyst, even when the precursor is a well-defined molecular entity. In the present study, we highlight the crucial importance of activation entropy on the polymerisation rate and how weak interactions of the catalytic species with electron donating species in the reaction pool can ultimately lower the activation free energy.

The behaviour of β-triketimine cobalt complexes in the polymerization of isoprene

Cobalt complexes of β-triketimines activated by Et₂AlCl polymerize isoprene to give up to 80%cis-1,4 with 20% 3,4 polymer, at 35 °C.

Mass spectrometric characterization of methylaluminoxane-activated metallocene complexes

Electrospray-ionization mass spectrometric studies of poly(methylaluminoxane) (MAO) in the presence of [Cp2 ZrMe2 ], [Cp2 ZrMe(Cl)], and [Cp2 ZrCl2 ] in fluorobenzene (PhF) solution are reported. The results demonstrate that alkylation and ionization are separate events that occur at competitive rates in a polar solvent. Furthermore, there are significant differences in ion-pair speciation that result from the use of metallocene dichloride complexes in comparison to alkylated precursors at otherwise identical Al/Zr ratios. Finally, the counter anions that form are dependent on the choice of precursor and Al/Zr ratio; halogenated aluminoxane anions [(MeAlO)x (Me3 Al)y-z (Me2 AlCl)z Me](-) (z=1, 2, 3…︁) are observed using metal chloride complexes and under some conditions may predominate over their non-halogenated precursors [(MeAlO)x (Me3 Al)y Me](-) . Specifically, this halogenation process appears selective for the anions that form in comparison to the neutral components of MAO. Only at very high Al/Zr ratios is the same "native" anion distribution observed when using [Cp2 ZrCl2 ] when compared with [Cp2 ZrMe2 ]. Together, the results suggest that the need for a large excess of MAO when using metallocene dichloride complexes is a reflection of competitive alkylation vs. ionization, the persistence of unreactive, homodinuclear ion pairs in the case of [Cp2 ZrCl2 ], as well as a change in ion pairing resulting from modification of the anions formed at lower Al/Zr ratios. Models for neutral precursors and anions are examined computationally.

Formation of octameric methylaluminoxanes by hydrolysis of trimethylaluminum and the mechanisms of catalyst activation in single-site α-olefin polymerization catalysis

Hydrolysis of trimethylaluminum (TMA) leads to the formation of methylaluminoxanes (MAO) of general formula (MeAlO)n (AlMe3)m. The thermodynamically favored pathway of MAO formation is followed up to n=8, showing the major impact of associated TMA on the structural characteristics of the MAOs. The MAOs bind up to five TMA molecules, thereby inducing transition from cages into rings and sheets. Zirconocene catalyst activation studies using model MAO co-catalysts show the decisive role of the associated TMA in forming the catalytically active sites. Catalyst activation can take place either by Lewis-acidic abstraction of an alkyl or halide ligand from the precatalyst or by reaction of the precatalyst with an MAO-derived AlMe2(+) cation. Thermodynamics suggest that activation through AlMe2(+) transfer is the dominant mechanism because sites that are able to release AlMe2(+) are more abundant than Lewis-acidic sites. The model catalyst system is demonstrated to polymerize ethene.

Site count: is a high-pressure quenched-flow reactor suitable for kinetic studies of molecular catalysts in ethylene polymerization?

Understanding the kinetics of olefin polymerization with Ziegler-Natta and related catalysts means identifying the main kinetic parameters such as k(p) and concentration of active sites. The high-pressure quenched-flow technique could be an absolute method for counting active sites and for activation studies of molecular catalysts. A kinetic study of metallocene catalysts and a phenoxy-imine titanium catalyst in the first instants of polymerization is presented in this paper.