Heterogenized methylaluminoxane and isobutylaluminoxane as activators for metallocene catalysts

Heterogenized alkylaluminoxanes prepared in situ on the montmorillonite (MMT) surface by the partial hydrolysis of AIR₃ with water of the support are effective activators for metallocenes. The thermal destruction of isobutylaluminoxane molecules in the MMT-H₂O/Al(i-Bu)₃ system has been studied by the temperature-programmed desorption method coupled with mass spectrometry (TPD-MS). The process begins at a lower temperature and is more complicated than the destruction of methylaluminoxane (MAO) in MMT-H₂O/AlMe₃. Isobutyl-substituted aluminoxane and the ansa-metallocene Me₂Si(Ind)₂ZrCl₂ form metal-alkyl complexes that are more active in propylene polymerization than methyl-substituted aluminoxane. The TPD-MS study of the initial stages of gas-phase ethylene and propylene polymerization shows that the nature of the metallocene in the heterogenized metallocene catalysts is an essential factor in the distribution of active sites by the activation energy of the thermal destruction of active Zr-C bonds.     

Olefin polymerization on immobilized zirconocene catalysts containing alkylaluminoxanes synthesized on the support surface

Ethylene and propylene polymerization on immobilized catalysts of composition MMT-H₂O/AlR₃/Zr-cene (MMT = montmorillonite) and on the corresponding homogeneous catalysts of composition Zr-cene-MAO (Zr-cene = rac-Et(Ind)₂ZrCl₂, rac-Me₂Si(Ind)₂ZrCl₂, rac-Me₂Si(2-Me-4-Ph-Ind)₂ZrCl₂, rac-1-(9-η⁵-Flu)2-(5,6-Cp2-Me-1-η⁵-Ind)Et]ZrCl₂) is considered. Here, the activating support for the zirconocenes is montmorillonite containing methylaluminoxane or isobutylaluminoxane synthesized directly on the monmorillonite surface by the partial hydrolysis of an alkylaluminum (AlMe₃, Al(i-Bu)₃, Al(i-Bu)₂H) with the mobile water of the support (MMT-H₂O/AlR₃). The MMT-H₂O/AlR₃ supports are demonstrated to be effective activators for ansa-zirconocenes. The catalytic properties of the immobilized systems (process kinetics and efficiency, the molar mass of the resulting polymer, and the structure of the macromolecules) depend on the activating support and the zirconocene precatalyst. The complexes of the MMT-H₂O/Al(i-Bu)₃ support with all zirconocene precatalysts are more active in propylene polymerization than the same complexes of MMT-H₂O/AlMe₃. The zirconcenes immobilized on MMT-H₂O/AlR₃ afford polyethylene and polypropylene with a higher molar mass than the corresponding homogeneous systems. Furthermore, immobilization causes activesite heterogeneity. As compared to the homogeneous single-site catalysts of composition Zr-cene-MAO, the corresponding catalysts immobilized on MMT-H₂O/AlR₃/Zr-cene are more stereospecific in the case of rac-Me₂Si(Ind)₂ZrCl₂ (C ₂ symmetry) and are less stereospecific in the case of rac-[1-(9-η⁵-Flu)2-(5,6-Cp-2-Me-1-η⁵-Ind)Et]ZrCl₂ (C _s symmetry).     

Effect of the zeolite support on the polymerization of propylene with immobilized ansa-zirconocene catalysts

Anchored aluminoxanes are synthesized by the reaction of aluminum alkyls AlMe₃ and Al(i-Bu)₃ with water contained in the intracrystalline cavities of synthetic and natural zeolites (NaY (Si: Al = 5), HZSM-5 (Si: Al = 17 or 34), NH₄ZSM-5 (Si: Al = 32), NaZSM-5 (Si: Al = 42), and clinoptilolite-containing tuff) and are used for the synthesis of heterogenized complexes of ansa-zirconocenes (rac-C₂H₄(Ind)₂ZrCl₂, rac-Me₂Si(Ind)₂ZrCl₂, and rac-[1-(9-η⁵-Flu)-2-(5,6-cyclopenta-2-Me-1-η⁵-Ind)C₂H₄]ZrCl₂) active in the polymerization of propylene. The nature of the zeolite support determines the content of zeolite water and affects the formation of anchored alkylaluminoxanes and the activity of immobilized catalysts. Among the studied catalytic systems supported on zeolites, NaY and NaZSM-5 are the most efficient for the polymerization of propylene. PP synthesized with the supported zirconocene catalysts has a higher molecular mass and a wider molecular-mass distribution than those in the case of the corresponding homogeneous catalyst. The index of isotacticity and the content of pentads mmmm in PP prepared with immobilized metallocenes with the C ₂ symmetry, such as rac-C₂H₄(Ind)₂ZrCl₂ and rac-Me₂Si(Ind)₂ZrCl₂, are likewise higher. The stereoselectivity of supported catalysts depends on the zeolite nature.     

H-, C-NMR and ethylene polymerization studies of zirconocene/MAO catalysts: effect of the ligand structure on the formation of active intermediates and polymerization kinetics

Using ¹H- and ¹³C-NMR spectroscopies, cationic intermediates formed by activation of L₂ZrCl₂ with methylaluminoxane (MAO) in toluene were monitored at Al/Zr ratios from 50 to 1000 (L₂ are various cyclopentadienyl (Cp), indenyl (Ind) and fluorenyl (Flu) ligands). The following catalysts were studied: (Cp-R)₂ZrCl₂ (R=Me, 1,2-Me₂, 1,2,3-Me₃, 1,2,4-Me₃, Me₄, Me₅, n-Bu, t-Bu), rac-ethanediyl(Ind)₂ZrCl₂, rac-Me₂Si(Ind)₂ZrCl₂, rac-Me₂Si(1-Ind-2-Me)₂ZrCl₂, rac-ethanediyl(1-Ind-4,5,6,7-H₄)₂ZrCl₂, (Ind-2-Me)₂ZrCl₂, Me₂C(Cp)(Flu)ZrCl₂, Me₂C(Cp-3-Me)(Flu)ZrCl₂ and Me₂Si(Flu)₂ZrCl₂. Correlations between spectroscopic and ethene polymerization data for catalysts (Cp-R)₂ZrCl₂/MAO (R=H, Me, 1,2-Me₂, 1,2,3-Me₃, 1,2,4-Me₃, Me₄, Me₅, n-Bu, t-Bu) and rac-Me₂Si(Ind)₂ZrCl₂ were established. The catalysts (Cp-R)₂ZrCl₂/AlMe₃/CPh₃⁺B(C₆F₅)₄⁻ (R=Me, 1,2-Me₂, 1,2,3-Me₃, 1,2,4-Me₃, Me₄, n-Bu, t-Bu) were also studied for comparison of spectroscopic and polymerization data with MAO-based systems. Complexes of type (Cp-R)₂ZrMe⁺←Me⁻-Al≡MAO (IV) with different [Me-MAO]⁻ counteranions have been identified in the (Cp-R)₂ZrCl₂/MAO (R=n-Bu, t-Bu) systems at low Al/Zr ratios (50-200). At Al/Zr ratios of 500-1000, the complex [L₂Zr(μ-Me)₂AlMe₂]⁺[Me-MAO]⁻ (III) dominates in all MAO-based reaction systems studied. Ethene polymerization activity strongly depends on the Al/Zr ratio (Al/Zr=200-1000) for the systems (Cp-R)₂ZrCl₂/MAO (R=H, Me, n-Bu, t-Bu), while it is virtually constant in the same range of Al/Zr ratios for the catalytic systems (Cp-R)₂ZrCl₂/MAO (R=1,2-Me₂, 1,2,3-Me₃, 1,2,4-Me₃, Me₄) and rac-Me₂Si(Ind)₂ZrCl₂/MAO. The data obtained are interpreted on assumption that complex III is the main precursor of the active centers of polymerization in MAO-based systems.     

Effect of trimethylaluminum on the formation of active sites of the catalytic system bis[N-(3,5-di-tert-butylsalicylidene)-2,3,5,6-tetrafluoroanilinato]titanium(IV) dichloride—MAO and catalytic isomerization of hex-1-ene

The transformations of bis[N-(3,5-di-tert-butylsalicylidene)-2,3,5,6-tetrafluoroanilinato]-titanium(iv) dichloride (L₂TiCl₂) occurring in toluene under the action of methylalumoxane (MAO) were studied by ¹H NMR spectroscopy. The commercially available MAO containing trimethylaluminum (AlMe₃) and MAO free of AlMe₃ (the so called “dry” MAO) were used. The catalytic transformations of hex-1-ene involving the systems L₂TiCl₂-MAO were studied. We proposed the structures of the cationic titanium complexes formed in the absence and in the presence of hex-1-ene under the action of MAO. In the absence of olefin, neutral and cationic titanium complexes are decomposed under the action of AlMe₃ according to the exchange reaction of the complex ligand with the methyl groups of AlMe₃ to form LAlMe₂. The neutral complexes react considerably faster than the cationic ones. In the presence of olefin, decomposition of complexes under the action of AlMe₃ is suppressed. The titanium complex activated by “dry” MAO isomerizes hex-1-ene to hex-2-ene. In the presence of large amounts of TMA (commercial MAO), this reaction does not take place.     

Investigation of the microstructure of poly(propylene) in dependence of the polymerization temperature for the systems iPr[3‐RCpFlu]ZrCl2/MAO,with R = H,Me, Et,iPr, tBu,and iPr[IndFlu]ZrCl2/MAO

With six different metallocenes, namely ⁱPr[CpFlu]ZrCl₂ I , ⁱPr[3‐MeCpFlu]ZrCl₂ II , ⁱPr[3‐EtCpFlu]ZrCl₂ III , ⁱPr[3‐ⁱPrCpFlu]ZrCl₂ IV , ⁱPr[IndFlu]ZrCl₂ V and ⁱPr[3‐^tBuCpFlu]ZrCl₂ VI propene polymerizations were carried out at different polymerization temperatures. MAO was used as a cocatalyst for all polymerizations. In case of metallocenes II, III and IV an increase in isotacticity with increasing polymerization temperature was observed. This is due to the increased rotation and, as a consequence, to the increased steric demand of the substituent at the cyclopentadienyl residue. With metallocene V a catalyst of in principle the same type was synthesized, but rotation of the substituent is not possible. Here, in the contrary, the assumed effect was observed, that the stereospecificity of the metallocene decreases, while raising the polymerization temperature. In metallocene I there is no rotatory substituent at the cyclopentadienyl residue and therefore a more stereoirregular polymer is formed at higher polymerization temperatures. Metallocene VI produces poly(propylene) with slightly increased isotacticity at higher polymerization temperature.     

Solution Structure and Reactivity with Metallocenes of AlMe2F: Mimicking Cation–Anion Interactions in Metallocenium–Methylalumoxane Inner‐Sphere Ion Pairs

The solution structure of AlMe₂F and its reactivity with a prototypical ansa‐metallocene have been investigated by advanced NMR techniques, in an attempt to indirectly shed some light on the structure and working principles of methylalumoxane (MAO) mixtures in olefin polymerization. In solution, AlMe₂F gives rise to a complex equilibrium of oligomeric species, including a heterocubane [(Me₂Al)₄F₄] tetramer, resembling the behavior of MAO. This complex mixture reacts with (ETH)ZrMe₂ (ETH=rac ‐[ethylenebis(4,5,6,7‐tetrahydro‐1‐indenyl)]) to afford [(ETH)ZrMe^δ+(μ‐F)(AlMe₂F)_nAlMe₃^δ−] inner‐sphere ion pairs through successive insertions/deinsertions of AlMe₂F units into the Zr⋅⋅⋅(μ‐F) bond.     

Mechanistic aspects of the olefin polymerization with metallocene catalysts

With C₁-, C₂- or C_s-symmetric metallocenes, different intermediates and types of copolymers can be obtained from randomly distributed to alternating structures. Substitution of the Cp-ring in [Me₂C-(tert-Bu Cp)(Flu)]ZrCl₂ yields ethene/norbornene copolymers with an alternating structure, because the rigid norbornene can only be inserted from the open side of the metallocene. By variation of the polymerization parameters, copolymers with glass transition temperatures above 180°C and molecular weights > 100 000 are synthesized. By supporting different metallocenes on a silica/methylaluminoxane (MAO) carrier the deactivation reaction under electron and hydrogen transfer can be suppressed. This is proved for different Al/Zr ratios when trimethylaluminum (TMA) is used as cocatalyst by the lack of methane evolution by metallocenes and by near independence of the polymerization activity on the prereaction time, after reaching maximum activity. Aluminumalkyls and MAO leach Cp₂ZrCl₂ from the carrier, the leached metallocene is only active in polymerization by adding MAO.     

Catalytic dehydrogenation of cyclooctane with titanium, zirconium and hafnium metallocene complexes

Metallocene complexes in combination with cocatalysts like methylalumoxane (MAO) are not only excellent catalysts for olefin polymerization but also appropriate catalysts for the activation of alkanes in homogeneous (autoclave) and heterogeneous (fixed bed reactor) reactions. The activities of the catalysts depend on the temperature, the cocatalysts, additives, the central metal and the ligand structure. Generally, complexes with low steric demands and MAO as cocatalyst gave the highest activities. The comparison of different π-ligands resulted in the following activity order: cyclopentadienyl > indenyl > fluorenyl. The influence of σ-ligands and n-donor ligands gave the following activity order: -Cl > -PMe₃ > -CH₂Ph > -(CH₂)₄CH₃ > -NPh₃. The activities depended on the nature of the cocatalyst and decreased in the following order: MAO ? AlMe₃ > AlEt₃. The addition of aluminum powder and the Lewis base NPh₃ increased the activity of the Cp₂ZrCl₂/MAO catalyst. The Cp₂ZrCl₂/MAO/NPh₃ catalyst showed the highest activity in homogeneous reactions with 458 turnovers in 16 h at 300 °C. The Cp₂ZrCl₂/MAO/NPh₃/SI1102 catalyst gave the highest activity in heterogeneous catalysis with 206 turnovers in 5 h at 350 °C. None of the catalysts required a hydrogen acceptor like an external olefin.     

Ethylene Polymerization in the Presence of Iron(II) 2,6-Bis(imine)pyridine Complex: Structures of Key Intermediates

The structures of intermediates generated by the activation of 2,6-bis[1-(2,6-dimethylphenylimino)ethyl]pyridineiron(II) chloride (1) with various cocatalysts, methylalumoxane (MAO), trimethylaluminum (TMA), and TMA in combination with B(C₆F₅)₃and Ph₃CB(C₆F₅)₄, is studied by ¹H and ²HNMR spectroscopy. The 1/AlMe₃system exhibits a higher catalytic activity in ethylene polymerization than the 1/MAO system. The activity of the latter decreases sharply with a decrease in the amount of AlMe₃in MAO. Neutral Fe(II) complexes rather than cationic intermediates are suggested to be active components in both catalytic systems.     

Hydrogen transfer reactions of supported metallocene catalysts

The evolution of methane from methylaluminoxane (MAO) solutions is enhanced in the presence of homogeneous metallocenes. This reaction serves as a model for the deactivation of metallocene catalysts. By supporting different metallocenes on a silica/MAO carrier the deactivation reaction by alpha-hydrogen transfer among metallocene active sites and aluminum alkyls can be suppressed. The suppression of alpha-hydrogen transfer is proven for different Al/Zr ratios and by near independence of the polymerization activity on the catalyst aging time, after reaching maximum activity. Aluminum alkyls and MAO leach Cp₂ZrCl₂ from the carrier, the leached metallocene is only active in polymerization if MAO is present.     

Triboluminescence of zirconium η<Superscript>5</Superscript>-complexes

This work describes a comparative study of tribo- and photoluminescence of metallocenes (Cp₂HfCl₂, Cp₂TiCl₂, Cp₂ZrCl₂, (CpMe)₂ZrCl₂, rac-Me₂C(3-Bu^t-Cp)₂ZrCl₂, Ind*₂ZrCl₂). Occurring under mechanodestruction of polycrystals, triboluminescence of zirconium bis-cyclopentadienyl complexes, Cp₂ZrCl₂, (CpMe)₂ZrCl₂, and rac-Me₂C(3-Bu^t-Cp)₂ZrCl₂ has been revealed for the first time. The triboluminescence spectrum in air is similar to the photoluminescence spectrum of zirconocene polycrystals. The triboluminescence spectrum does not show *N₂ luminescence. Ne and Ar lines are observed in the triboluminescence spectrum in the atmosphere of these gases. An increase in the number of substituents in zirconocene ligands leads to a bathochromic shift of the emission band maximum in triboluminescence spectra of the complexes. Possible mechanisms of zirconocene triboluminescence excitation are discussed.     

The first synthesised examples of di-siloxy-substituted cyclopentadienyl zirconocenes,their synthesis,structure and activity in ethylene polymerisation

Bis-(1,2-(RMe₂SiO)₂)ZrCl₂ complexes with R = Me-(5a), iPr-(5b) and tBu-(5c) have been synthesised in good yields and characterised. Compounds 5a–c are the first synthesised examples of multi-siloxy-substituted cyclopentadienyl metallocenes. The siloxy-substituents have a for siloxy substituted metallocenes unique steric arrangement, with one almost in the same plane, and the other nearly perpendicular to the plane of the cyclopentadienyl ligand of the zirconocene. The ethylene/1-hexene co-polymerisation activity using methylalumoxane as co-activator gives low activities, with compound 5c (262.92 kg(PE)/g(Zr)/h) being the most active.     

Methylalumoxane (MAO)-Derived MeMAO− Anions in Zirconocene-Based Polymerization Catalyst Systems – A UV-Vis Spectroscopic Study

Reaction of Me₂Si(Ind)₂ZrCl₂ with excess methylalumoxane (MAO) gives rise to ion pairs containing zirconocenium cations of the type [Me₂Si(Ind)₂ZrMe]⁺ in contact with two types of anions, MeMAO _A ⁻ and MeMAO _B ⁻, which differ in their coordinative strengths: More strongly coherent contact-ion pairs [Me₂Si(Ind)₂ ZrMe^+..MeMAO _B ⁻] are converted by a sufficiently high excess of MAO to more easily separable contact-ion pairs [Me₂Si(Ind)₂ZrMe^+..MeMAO _A ⁻], which react with AlMe₃ to form the outer-sphere ion pairs [Me₂Si(Ind)₂Zr(µ-Me)₂AlMe₂]⁺ MeMAO _A ⁻, and are likely to be required also for the formation of the olefin-containing reaction complexes responsible for catalytic activity.     

A DFT quantum‐chemical study on the structures and active sites of polymethylaluminoxane

The electronic structure and geometry of polymethylaluminoxane (MAO) [—Al(CH₃)O—]_n with different size (n = 4–12) have been studied using quantum‐chemical DFT (density functional theory) calculations. It has been found: 1) Starting from n = 6, the three‐dimensional oxo‐bridged (cage) structure of MAO is more stable than the cyclic structure. 2) Both for cage structure and for cyclic structure the Lewis acidity of Al atoms characterized by their net positive charge amplifies with increasing size of MAO (n). 3) Trimethylaluminium (AlMe₃) reacts with the cage structure of MAO with cleavage of an Al‐O dative bond and formation of acidic tri‐coordinated Al^v and basic di‐coordinated O^v atoms in the MAO molecule. Two molecules AlMe₃ are associated with acidic Al^v and basic O^v centers. As the MAO increases in size, the acidity of Al^v centers amplifies and the distance Al^v‐(AlMe₃) shortens; on the contrary, interaction of AlMe₃ with O^v centers weakens and the distance O^v‐(AlMe₃) increases with increasing n value. The total heat of Al₂Me₆ interaction with MAO (sum interaction of Al^v‐(AlMe₃) and O^v‐(AlMe₃)) noticeably decreases as the size of MAO increases (from 50.9 kcal/mol for n = 4 to 20.2 kcal/mol for n = 12). It is proposed that acidic Al^v and basic O^v centers formed in the cage structure of MAO interact with zirconocene yielding ‘cation‐like’ zirconium active centers.     

Mao based catalysts for syndiotactic polystyrene (sPS)

Several non-metallocene (Ti, Zr) and substituted mono-Cp titanium metallocenes have been tested in the presence of methylalumoxane (MAO) as catalyst for syndiospecific polymerization of styrene. Effect of substitutions on the titanium and Cp ligand, molar ratio of Al/Ti, TMA and temperature on activity, Mwt. and % sPS were studied. CpTi(OiPr)₃ gives a less active catalyst than Cp*Ti(OiPr)₃ and the resulting sPS is less stereoregular and of lower molecular weight.     

Hydroboration 95 — Relative Effectiveness of the 2-Organylapoisopinocampheylboranes for the Asymmetric Hydroboration of Representative Prochiral Alkenes

Enantiomerically pure and sterically-varied 2-organylapoisopinocampheylboranes (RapBH₂; R=Me, IpcBH₂; R=Et, EapBH₂; Pr, PraBH₂; i-Bu, i-BapBH₂; R=Ph, PapBH₂; and R=i-Pr, i-PraBH₂) were prepared from their corresponding 2-organylapopinenes (2-R-apopinenes; R=Me, Et, Pr, i-Bu, Ph, and i-Pr) and the relative efficiency of these reagents for the asymmetric hydroboration of representative prochiral alkenes compared. With the exception of Ph, the results reveal simple relationships between the steric requirements of the groups R (Me, Et, Pr, i-Bu, Ph, and i-Pr) in these reagents and the moderate to excellent enantioselectivities achieved in the asymmetric hydroboration of six representative prochiral alkenes, such as 2-methyl-1-butene, cis-2-butene, trans-2-butene, 2-methyl-2- butene, 1-methyl-1-cyclopentene, and 1-methyl-1-cyclohexene.     

1,2- and 3,4-rich polyisoprene synthesized by Mo(VI)-based catalyst with phosphorus ligand

Polyisoprene with relatively high content of 1,2/3,4 structure was synthesized using a novel catalyst system composed of MoO₂Cl₂ supported by phosphorus ligand and Al(OPhCH₃)(i-Bu)₂ as co-catalyst. The effects of phosphites, phosphates and phosphoric acid as ligands were investigated in the coordination polymerization of isoprene in the chosen catalyst system. The studied ligands significantly affected the catalytic activity of the Mo–Al catalytic active center without significant effect on the stereoselectivity. Mo(VI)-based catalyst system was proved to be highly effective in the polymerization of isoprene even at low [Al]/[Mo] ratio (10), affording polyisoprene with 1,2- and 3,4-% structural units in the range of 44.6–52.5%, high molecular weights M_n ~ 10⁵, and relatively broad molecular weight distributions (M_w/M_n = 3.0–4.4). The effect of molar ratio of phosphorous ligand to Mo-catalyst on catalyst activity of isoprene polymerization was discussed, and the structures of Mo–phosphite complexes were preliminarily studied by IR.     

Ethylene polymerization and ethylene-1-hexene copolymerization over immobilized metallocene catalysts

Ethylene polymerization and ethylene-1-hexene copolymerization in the presence of metallocene catalysts based on Cp₂ZrCl₂, rac-Et(Ind)₂ZrCl₂, rac-Me₂Si(2-Me-4-Ph-Ind)₂ZrCl₂, and rac-Me₂Si(2-Me-4-Ph-Ind)₂HfCl₂ been investigated. The catalysts have been immobilized on montmorillonite (MMT) containing methylaluminoxanes (MMT-H₂O)/AlMe₃ or isobutylaluminoxanes (MMT-H₂O)/ Ali-Bu₃ synthesized directly on the support surface. The immobilized catalysts, with the general formula (MMT-H₂O)/AlR₃/Zr(Hf)-cene, show a high activity comparable with the activity of the respective homogeneous systems, which depends on the nature of the metal and on the metallocene composition and structure. The catalytic properties of the metallocene systems depend strongly on the nature of the activator as a component of the catalytic complex. (MMT-H₂O)/Ali-Bu₃ is a more effective activator of the hafnocene precatalyst in the polymerization processes than oligomeric methylaluminoxane or methylaluminoxane synthesized on the support. The immobilization of the metallocenes on (MMT-H₂O)/AlR₃ leads to an increase in the molar mass of polyethylene and ethylene-1-hexene copolymers relative to the molar mass of the polymers synthesized using the respective homogeneous systems. The immobilized metallocene catalysts display high selectivity toward the insertion of a higher α-olefin (1-hexene) into the polymer chain, retaining this important property of their homogeneous counterparts.     

Ethylene polymerization catalyzed by metallocene/methylaluminoxane systems: quantum-mechanical study on the role of olefin separated ion pairs (OSIP) in the polymerization mechanism

DFT (density-functional theory) calculations were performed to investigate the thermodynamics of formation of Olefin Separated Ion Pairs (OSIP) Cp₂MtCH₃⁺/C₂H₄/Cl₂Al[O(AlMe₃)AlHMe] (Cp = η₅-C₅H₅, Mt = Ti, Zr, Me = CH₃) from ethylene and Cp₂MtMe · Cl₂Al[O(AlMe₃)AlHMe]₂, a model of adduct produced by metallocence/methylaluminoxane (MAO) systems for olefin polymerization. The results account for the high cocatalytic activity of MAO and show that titanium complexes are potentially more active than zirconium homologues, as confirmed by low temperature polymerization tests.