Effect of the nature of an organoaluminum activator on catalytic properties of phenoxyimine zirconium complexes in homo- and copolymerization reactions of ethylene

Catalytic properties of the phenoxyimine zirconium complexes, viz., bis[N-(3,5-di-tert-butylsalicylidene)anilinato]zirconium(IV) dichloride (1) and its fluorinated analog, bis[N-(3,5-di-tert-butylsalicylidene)-2,3,5,6-tetrafluoroanilinato]zirconium(IV) dichloride (2), were studied. Ethylene homopolymerization and copolymerization of ethylene with α-olefins were chosen as catalytic reactions, and various organoaluminum compounds served as activators: commercial polymethylalumoxane (MAO) containing ∼35 mol.% of trimethylaluminum (TMA), MAO purified from TMA (“dry” MAO), and “classical” organoaluminum compounds, namely, TMA and triisobutylaluminum (TIBA). Complex 1 is not activated by “dry” MAO but is efficiently transformed into the catalytically active state by commercial MAO, “conventional” TMA, and TIBA. These processes give low-molecular-weight polyethylenes (PE) characterized by high values of polydispersity indices and by polymodal curves of gel permeation chromatography (GPC). The order of decreasing the efficiency of activation for the cocatalysts is MAO > TIBA > TMA. Fluorinated complex 2 exhibits a high activity after its treatment with MAO and “dry” MAO, the activity is much lower upon mixing with TIBA, and complex 2 is inactive when using TMA. In the copolymerization of ethylene with hex-1-ene and dec-1-ene, complex 1 treated with MAO is highly active but gives a low level of insertion of the comonomer (1–2 mol.% in the copolymer). Complex 2 activated with “dry” MAO is more efficient in the copolymerization of ethylene with propylene or hex-1-ene but, like complex 1, it does not produce copolymers with a high content of the comonomer. The both catalysts provide the insertion of α-olefin as isolated units separated by extended sections of the chain consisting of ethylene units.     

Novel Complex Catalyst of η3‐(Pentamethylcyclopentadienyl)dimethylaluminum/Titanium(IV) Butoxide (Cp*AlMe2/Ti(OBu)4) for Syndiotactic Polystyrene

A novel metallocene catalyst was prepared from the reaction of (η³‐pentamethylcyclopentadienyl)dimethylaluminum (Cp*AlMe₂) and titanium(IV) n‐butoxide Ti(OBu)₄. The resulting titanocene Cp*Ti(OBu)₃ was combined with methylaluminoxane (MAO)/tri‐iso‐butylaluminum (TIBA) to carry out the syndiotactic polymerization of styrene. The resulting syndiotactic polystyrene (sPS) possesses high syndiotacticity according to ¹³C NMR. Catalytic activity and the molecular weight of the resulting sPSs were discussed in terms of reaction temperature, concentration of MAO, amounts of scavenger TIBA added, and the hydrogen pressure applied during polymerization.     

1H NMR study of the catalytic system (rac-Me2Si(2-Me,4-PhInd)2ZrCl2— polymethylalumoxane)—triisobutylaluminum

The products of the reactions of polymethylalumoxane (MAO) with triisobutylaluminum (TIBA), rac-Me₂Si(2-Me,4-PhInd)₂ZrCl₂ (1) with MAO (1 + MAO), and (1 + MAO) + TIBA were studied by ¹H NMR at different molar ratios of the components. When the ratio Al_TIBA/Al_MAO is ∼6, the reaction between MAO and TIBA involves the replacement of the methyl group of MAO by isobutyl groups and the formation of isobutylmethylalumoxane or mixed isobutylmethylalumoxane structures. When the TIBA content in the system increases to 30 mol.%, these structures are rearranged to form products with a low degree of association. With the equimolar ratio of the reactants, the main reaction products are tetraisobutylalumoxane and polyisobutylalumoxane. The 1 + MAO system with the molar ratio Al_MAO/Zr = 50 affords a MAO-bonded monomethyl monochloride derivative [L₂ZrCl-μ-Me]^δ+[MAO]^δ−. An increase in this ratio to 150 produces intermediate binuclear complexes [L₂ZrCl-μ-Me-MeZrL₂]⁺[MAO]⁻ and [Me₂Al-(μ-Me)₂-ZrL₂]⁺[MAO]⁻. The addition of TIBA induces the replacement of the ZrMe groups by isobutyl groups at the first step of the interaction and formation of nonidentified reaction products at the subsequent steps. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 934–940, April, 2005.     

Effects of cocatalyst on structure distribution of propylene polymers catalyzed by rac-Me2Si(Ind)2ZrCl2/aluminoxane

Propylene polymerization and propylene/1-octene copolymerization were studied using rac-Me₂SiInd₂ZrCl₂(1)/MAO or rac-Me₂SiInd₂ZrCl₂/(MAO + TIBA) as catalyst (methyl aluminoxane, MAO; AliBu₃, TIBA). The structure distribution of the polymers was characterized by temperature gradient extraction fractionation or precipitation fractionation, as well as by DSC analysis of the thermal segregated samples. By comparing the structure distribution of polypropylene and propylene-1-octene copolymer synthesized by 1/MAO and 1/(MAO + TIBA), it is found that adding TIBA in the catalyst system increase the blockiness of the polymer chain, especially in the copolymerization system. It is assumed that, when iso-butyl is incorporated in the aluminoxane, ion pair of the active center and the aluminoxane counter ion may exist in different states that show different catalytic behaviors, resulting in the formation of polymers with block structure.     

Syndiospecific polymerization of styrene in the presence of new titanium complexes with dialkanolamines: Titanocanes and bistitanocanes

The syndiospecific polymerization of styrene is studied in the presence of titanium complexes with dialkanolamines—bistitanocanes and titanocane—activated by individual MAO or the combined cocatalyst MAO/TIBA. It is shown that these catalysts are more active and stereospecific after their activation with the combined cocatalyst ([TIBA]: [MAO] ≤ 0.13) than that in the case of the activation with MAO: The activities of the catalysts are ≤18 and 9 kg PS/(mol Ti h), and the syndiotacticities of PS are ≤76 and 60%, respectively. Polymers synthesized in the presence of bistitanocanes are characterized by M _n ≤ 4.5 × 10⁴ and T _m ≤ 268°C and a narrow molecular-mass distribution (≤2.5).     

Effects of ethylene polymerization conditions on the activity of SiO2-supported zirconocene and on polymer properties

The effects of polymerization conditions were evaluated on the production of polyethylene by silica-supported (n-BuCp)₂ZrCl₂ grafted under optimized conditions and cocatalyzed by methylaluminoxane (MAO). The Al : Zr molar ratio, reaction temperature, monomer pressure, and the age and concentration of the catalyst were systematically varied. Most reactions were performed in toluene. Hexane, with the addition of triisobutilaluminum (TIBA) to MAO, was also tested as a polymerization solvent for both homogeneous and heterogeneous catalyst systems. Polymerization reactions in hexane showed their highest activities with MAO : TIBA ratios of 3 : 1 and 1 : 1 for the homogeneous and supported systems, respectively. Catalyst activity increased continuously as Al : Zr molar ratios increased from 0 to 2000, and remained constant up to 5000. The highest activity was observed at 333 K. High monomer pressures (≈ 4 atm) appeared to stabilize active species during polymerization, producing polyethylenes with high molecular weight (≈ 3 × 10⁵ g mol⁻¹). Catalyst concentration had no significant effect on polymerization activity or polymer properties. Catalyst aging under inert atmosphere was evaluated over 6 months; a pronounced reduction in catalyst activity [from 20 to 13 × 10⁵ g PE (mol Zr h)⁻¹] was observed only after the first two days following preparation. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1987–1996, 1999     

Synthesis and characterization of ethylene‐propylene‐1‐pentene terpolymers

Ethylene (E), propylene (P), and 1‐pentene (A) terpolymers differing in monomer composition ratio were produced, using the metallocenes rac‐ethylene bis(indenyl) zirconium dichloride/methylaluminoxane (rac‐Et(Ind)₂ZrCl₂/MAO), isopropyl bis(cyclopentadienyl)fluorenyl zirconium dichloride/methylaluminoxane (Me₂C(Cp)(Flu)ZrCl₂/MAO, and bis(cyclopentadienyl)zirconium dichloride, supported on silica impregnated with MAO (Cp₂ZrCl₂/MAO/SiO₂/MAO) as catalytic systems. The catalytic activities at 25 °C and normal pressure were compared. The best result was obtained with the first catalyst. A detailed study of ¹³C NMR chemical shifts, triad sequences distributions, monomer‐average sequence lengths, and reactivity ratios for the terpolymers is presented. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 947–957, 2008     

Heat Stabilizers as Activators in Coordination Polymerization

Summary: The solvent‐free syndiospecific styrene polymerization as an example of a coordination polymerization has been investigated with a catalyst system consisting of η⁵‐octahydrofluorenyl titanium trimethoxide as a transition metal catalyst, MAO as a cocatalyst, and TIBA, in the presence of reaction products of sterically hindered phenolic compounds, usually applied as heat stabilizers of polymers. Unexpectedly, such reaction products led to a significant increase in polymerization activity of the catalyst system. Second, after deactivation of the catalyst system, such activators result in a significantly enhanced thermal stability of the syndiotactic polymers received.

Effect of the P₈‐activator on polymerization activity in dependence on polymerization time (molar ratio–styrene:MAO:TIBA:P:Ti = 700 000:50:25:25:1; molar ratio–phenolic compound:TIBA = 1:3.2; polymerization temperature: 50 °C).     

Poly(10-undecene-1-ol) characterized by MALDI-TOF MS and NMR spectroscopy

Poly(10-undecene-1-ol)s were synthesized by metallocene-catalyzed polymerization. MALDI-TOF MS allows obtaining detailed information on the monomer units in the polymer chains and the nature of the head and end groups of these polymers in dependence on the Al alkyl triisobutyl aluminum (TIBA) as well as methylalumoxan (MAO), both used as protecting agent for the hydroxyl groups of the monomer. The peak-to-peak distances of the main peaks could be distinctly assigned to the monomer unit 10-undecene-1-ol. Evaluating the MALDI-TOF peak distributions, polymers with -H, -C₄H₉, -CH₃ head groups in combination with vinylidene and saturated (–CH₃) end groups could be detected. By ¹H NMR spectroscopy, it was verified that with the used catalysts polymers with vinylidene end groups were obtained predominantly. The presence of saturated end groups could be proved qualitatively by combination of ¹H and ¹³C NMR spectroscopy for polymers produced by TIBA protection, which strikingly confirm the results from MALDI-TOF MS. For the polymers prepared with only MAO protection, saturated groups are also proved but discrimination between head and end groups was not possible. A polymerization mechanism corresponding to the detected different head and end groups is proposed.     

Triisobutylaluminum as cocatalyst for zirconocenes. I. Sterically opened zirconocene/triisobutylaluminum/perfluorophenylborate as highly effective ternary catalytic system for synthesis of low molecular weight polyethylenes

Ethylene polymerizations with catalytic systems Me₂SiCp*N^tBuZrX₂ ( 1 ) [Cp* = C₅(CH₃)₄; X = Cl ( 1Cl ), Me ( 1Me )], triisobutylaluminum (TIBA), perfluorophenylborate CatB(C₆F₅)₄ [Cat = CPh₃ ( 3 ), Me₂NHPh ( 4 )], or Me₂SiCp₂ZrX₂ [X = Cl ( 2Cl ), Me ( 2Me )]/TIBA/ 3 ( 4 ) were performed within a wide range of ethylene pressures of different Al/Zr ratios, and Zr/B = 1. Catalytic systems 1Cl ( 2Cl )/TIBA/ 3 led to the formation of very high linear molecular weight polyethylene (PE) of M_η ∼2,000,000 with low activity. The replacement of both chlorine ligands in the precatalyst for the methyl ones led to the formation of active species producing low molecular weight PE with high activity. Chain transfer to ethylene was shown to be the main reaction controlling PE chain propagation: k_p/k_tr ∼20–30 for 1Me /TIBA/ 3 and k_p/k_tr ∼350–500 for 2Me /TIBA/ 3 . It was suggested that TIBA was present in the active center first in the form of a neutral heterobimetallic Zr–Al bridged complex followed by the formation of a partially polarized Zr–Al(Cl)R₂ (R = ⁱBu) or an unreactive Zr–AlR₃ cationic complex by abstraction of the alkyl ligand under the action of borate. It was concluded that AlR₃ from the latter cationic complex may be easily reversibly replaced under the specific coordination of ethylene or accumulated α-olefin, giving rise to highly labile and sterically accessible cationic species. Experiments on ethylene polymerization with the catalytic systems 1Cl ( 1Me )/TIBA/ 3 /Ph₂NH, 1Cl ( 1Me )/TIBA/ 4, 2Cl ( 2Me )/TIBA/ 3 /Ph₂NH, and 2Cl ( 2Me )/TIBA/ 4 were performed to confirm the suggestion. Catalytic systems derived from dichloride complexes in the presence of a σ-donor substrate also produced low molecular weight PEs with molecular weight characteristics similar to those of products obtained with the dimethylated precatalysts. The specific feature of active species derived from 2Me complexes to isomerize coordinated α-olefin into trans-vinylene polymer chains was also revealed. The catalytic behavior of the ternary catalytic system based on 2Me relative to 2Me or 2Cl precatalysts activated with polymethylaluminoxane at different Al/Zr ratios was compared. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1901–1914, 2001     

Novel titanium complexes bearing two chelating phenoxy‐imine ligands and their catalytic performance for ethylene polymerization

Three substituted salicylaldimine ligands ( 1a, 2a, 3a ) and their titanium complexes bis[N‐(5‐nitrosalicylidene)‐2,6‐diisopropylanilinato]titanium(IV)dichloride ( 1 ), bis[N‐(5‐chlorosalicylidene)‐2,6‐diisopropylanilinato]titanium(IV)dichloride ( 2 ) and bis[N‐(5‐bromosalicylidene)‐2,6‐diisopropylanilinato]titanium(IV)dichloride ( 3 ) were synthesized and characterized by mass spectra, ¹H NMR and elemental analyses, as well as complex 1 by X‐ray structure analysis. In the presence of methylaluminoxane (MAO), 1, 2 and 3 are efficient catalysts for ethylene polymerization in toluene. Under the conditions of T = 60 °C, p = 0.2 MPa, and n(MAO)/n(cat) = 1500, the activities of 1–3 reached 4.55–8.80 × 10⁶ g of PE (mol of Ti h bar)^?1, which is much higher than that of the unsubstituted complex bis[N‐(salicylidene)‐2,6‐diisopropylanilinato]titanium(IV)dichloride ( 4 ). The viscosity‐average molecular weight of polyethylene ranged from 24.8 × 10⁴ to 44.9 × 10⁴ g/mol for 1–3 and the molecular weight distribution M_w/M_n from 1.85 to 2.34. The effects of reaction conditions on the polymerization were examined in detail. The increase in ethylene pressure and rise in polymerization temperature are favorable for 1–3 /MAO to rise the catalytic activity and the molecular weight of polyethylene. Copyright © 2007 John Wiley & Sons, Ltd.     

Improvement of Structure and Properties of Polypropylene/Poly(ethylene-co-propylene) In-reactor Alloy by Modifying the Cocatalyst

Summary : A series of polypropylene/poly(ethylene-co-propylene) in-reactor alloy were synthesized by a TiCl₄/MgCl₂/SiO₂/diester type Ziegler-Natta catalyst, using triethylaluminium (TEA), triisobutylaluminium (TIBA) or TEA/TIBA mixtures of different molar ratio as cocatalyst. Mechanical properties of the alloy are strongly influenced by the cocatalyst. Toughness-stiffness balance of the alloy synthesized using a 50/50 TEA/TIBA mixture as cocatalyst is much better than that of the alloy based on pure TEA cocatalyst. Changes in copolymer chain structure and composition distribution are thought to be the main reason for this improvement of properties.     

Homopolymerization of Methyl Methacrylate by Novel Ziegler‐Natta‐Type Catalysts Based on Bis(chelate)‐nickel(II) Complexes and Methylaluminoxane

Novel catalytic systems based on bis‐(chelate)nickel(II) precursors, such as bis(α‐nitroacetophenonate)nickel(II) [Ni(naph)₂] and bis(2,6‐diisopropylbenzenesalicylaldiminate)nickel(II) [Ni(dipbs)₂], and methylaluminoxane (MAO) as the cocatalyst were employed for the polymerization of methyl methacrylate (MMA). Reaction parameters were examined. Under proper conditions, the Ni(dipbs)₂/MAO system allowed to obtain poly(MMA) with a very high productivity (TOF up to 70 000 h^–1) and a remarkable syndiospecificity degree (rr > 80%) at room temperature without addition of an ancillary Lewis base.     

Ethylene polymerization by chromium complexes with phosphorous‐bridged bisphenoxy ligand

Chromium catalysts combined with phosphorous‐bridged bisphenoxy ligands were found to be highly active for ethylene polymerization. The most efficient catalyst precursor among them, generated by combining bis[3‐tert‐butyl‐5‐methyl‐2‐hydroxyphenyl](phenyl)phosphine hydrochloride ( 1a ) and CrCl₃(THF)₃, was characterized. X‐ray analysis of (3‐tert‐butyl‐5‐methyl‐2‐phenoxy)(3‐tert‐butyl‐5‐methyl‐ 2‐hydroxyphenyl)(phenyl)phosphine bis(tetrahydrofuran)chromium dichloride ( 6 ), obtained by the reaction of 1a and CrCl₃(THF)₃ in the presence of NaH, revealed a unique structure in which one phenol moiety of the bisphenol did not coordinate to the chromium center. Complex 6 showed higher activities than those observed in the in situ catalyst system. Polyethylene of various molecular weights was obtained with differing activators. The highest activity (113.5 kg mmol (cat)^?1 h^?1) was observed when TIBA/TB was used as a cocatalyst. A medium molecular weight polymer with narrow molecular weight distribution (M_w = 128,700, M_w/M_n = 1.8) was obtained using a 6 ‐TIBA/B(C₆F₅)₃ system. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3668–3676, 2007     

Ethylene/hexene copolymerization with the heterogeneous catalyst system SiO2/MAO/rac‐Me2Si[2‐Me‐4‐Ph‐Ind]2ZrCl2: The filter effect

The main focus of this study is the ethylene/hexene copolymerization with the silica supported metallocene SiO₂/MAO/rac‐Me₂Si[2‐Me‐4‐Ph‐Ind]₂ZrCl₂. Polymerizations were carried out in toluene at a reaction temperature of 40°C–60°C and the cocatalyst used was triisobutylaluminium (TIBA). The kinetics of the copolymerization reactions (reactivity ratios r_E/H, monomer consumption during reaction) were investigated and molecular weights M_w, molecular weight distributions MWD and melting points T_m were determined. A schematic model for the blend formation observed was developed that based on a filtration effect of monomers by the copolymer shell around the catalyst pellet.     

Synthesis and microstructural study of stereoblock elastomeric polypropylenes from metallocene catalyst (2‐PhInd)2ZrCl2 activated with cocatalyst mixtures

Various elastomeric polypropylenes (PPs) are synthesized through homogeneous propylene polymerization with metallocene catalyst (2‐PhInd)₂ZrCl₂ in the presence of different cocatalyst mixtures: triethylaluminum (TEA)/methylaluminoxane (MAO) or triisobutylaluminum (TIBA)/MAO in the range of Al_AlR3/Al_MAO = 0.0–0.9. The cocatalyst formulation impacts the resultant polymer microstructure and the thermal and dynamic mechanical properties of the produced PPs. ¹³C NMR analysis of the polymers reveals essentially atactic PP, with mmmm = 7.9%, when Al_AlR3/Al_MAO = 0.0. The mmmm pentad content is maximized when Al_AlR3/Al_MAO = 0.8; for TIBA, mmmm = 23.5%; and for TEA, mmmm = 17.6%. Differential scanning calorimetry analysis and dynamic mechanical thermal analysis corroborate these findings. Specifically, T_m, ΔH_m, and T_g are essentially maximized under these conditions, and the minimum damping is observed for Al_AlR3/Al_MAO = 0.6–0.8. ¹H NMR analysis of the mixtures of catalyst and cocatalysts (without monomer) shows very minor differences for [Zr]:Al_AlR3 in the range of 1:1–1:5. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Alkylaluminum activator effects on polyethylene branching using a N,N′‐nickel precatalyst appended with bulky 4,4′‐dimethoxybenzhydryl groups

Ten unsymmetrical N,N'‐bis (imino) acenaphthene‐nickel (II) halide complexes, [1‐[2,6‐{(4‐MeOC₆H₄)₂CH}₂–4‐MeC₆H₂N]‐2‐(ArN)C₂C₁₀H₆]NiX₂, each appended with one N‐2,6‐bis(4,4'‐dimethoxybenzhydryl)‐4‐methylphenyl group, have been synthesized and characterized. The molecular structures of Ni1 , Ni3 , Ni5 and Ni6 highlight the variation in steric protection afforded by the inequivalent N‐aryl groups; a distorted tetrahedral geometry is conferred about each nickel center. On activation with diethylaluminum chloride (Et₂AlCl) or methylaluminoxane (MAO), all complexes showed high activity at 30°C for the polymerization of ethylene with the least bulky bromide precatalysts ( Ni1 and Ni4 ), generally the most productive, forming polyethylenes with narrow dispersities [M_w/M_n: < 3.4 (Et₂AlCl), < 4.1 (MAO)] and various levels of branching. Significantly, this level of branching can be influenced by the type of co‐catalyst employed, with Et₂AlCl having a predilection towards polymers displaying significantly higher branching contents than with MAO [T_m: 33.0–82.5°C (Et₂AlCl) vs. 117.9–119.4°C (MAO)]. On the other hand, the molecular weights of the materials obtained with each co‐catalyst were high and, in some cases, entering the ultra‐high molecular weight range [M_w range: 6.8–12.2 × 10⁵ g mol^?1 (Et₂AlCl), 7.2–10.9 × 10⁵ g mol^?1 (MAO)]. Furthermore, good tensile strength (ε_b up to 553.5%) and elastic recovery (up to 84%) have been displayed by selected more branched polymers highlighting their elastomeric properties.     

Experimental and theoretical investigations of the structure of methylaluminoxane (MAO) cocatalysts for olefin polymerization

The structure of methylaluminoxane (MAO), used as a cocatalyst for olefin polymerization, has been investigated by Raman and in situ IR spectroscopy, polymerization experiments, and density functional calculations. From experimental results, a number of quantum chemical calculations, and bonding properties of related compounds, we have suggested a few Me₁₈Al₁₂O₉ cage structures, including a highly regular one with C_3h symmetry, which may serve as models for methylaluminoxane solutions. The cages themselves are rigid but may contain up to three bridging methyl groups on the cage surfaces that are labile and reactive. Bridging methyls were substituted with Cl atoms to form a compound otherwise similar to MAO. Chlorinated MAO is unable to activate a metallocene catalyst, even in the presence of trimethylaluminum (TMA), but allows subsequent activation by regular MAO. With bis(pentamethylcyclopentadienyl)zirconium dichloride, MAO and TMA seem to influence chain termination independently. Several findings previously poorly explained are rationalized with the new model, including the observed lack of reaction products with excess TMA. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3106–3127, 2000     

(η5-Pentamethylcyclopentadienyl)trimethyltitanium as a precursor for the syndiospecific polymerization of styrene

An equimolar mixture of Cp*Ti(CH₃)₃ (2) and Ph₃C⁺[B(C₆F₅)₄]^? (1) forms a highly active and syndioselective catalyst for the polymerization of styrene, producing 96% syndiotactic polystyrene (PS) at an activity of 0.91 × 10⁷ g PS (mol Ti)^?1 (mol styrene)^?1 h^?1. Both activity and syndioselectivity can be increased using tri–isobutylaluminum (TIBA) to scavenge the system. ESR measurements indicate that the polymerization proceeds via titanium(IV) intermediates. Catalysts derived from 2/methylaluminoxane (MAO) as well as Cp*TiCl₃/MAO also function as syndioselective styrene polymerization catalysts, but are less active than the ‘cationic’; system derived from 1 and 2.     

Activation of hafnocene catalyzed polymerization of 1-hexene with MAO and borate

A study of 1-hexene polymerization with ethylene-bis(9-fluorenyl) hafnium dichloride has been carried out using two different cocatalyst systems, methyl-aluminoxane/trimethylaluminum (MAO/TMA) and tris-isobutyl-aluminum/N,N-dimethylanilinium tetrakis(pentafluorophenyl) borate (TIBA/borate). When MAO/TMA was used, 1-hexene polymerized into a low molar mass poly(1-hexene) with low catalytic activity. Activation with TIBA/borate increased polymerization activity drastically as well as the molar mass of the polymers. In order to analyze differences in the activity profiles, UV-Vis spectroscopy was employed to investigate ligand to metal charge transitions (LMCT) of the hafnocene dichloride during the activation process. The low catalytic activity and the fast chain transfer to the cocatalyst with MAO/TMA may originate from strong bonding between the metallocene cation and the MAO/TMA species thus obstructing monomer coordination and insertion.