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.     

Relative reactivity enhancement of (Me5Cp)2ZrCl2 in mixture with (1,2,4-Me3Cp)2ZrCl2 during ethene/1-hexene copolymerization

Dual-site ethene/1-hexene copolymerizations with MAO-activated (1,2,4-Me₃Cp)₂ZrCl₂ and (Me₅Cp)₂ZrCl₂ catalysts were performed. Copolymers with narrow molecular weight distributions and bimodal short chain branching distributions could be produced. The combined catalyst system demonstrates a number of discrepancies from an expected average behavior of the individual sites. Dual-site (1,2,4-Me₃Cp)₂ZrCl₂/(Me₅Cp)₂ZrCl₂ systems produce copolymers with lower incorporation than expected. Clear evidences for relative activity enhancement of the (Me₅Cp)₂ZrCl₂ catalyst in the mixture were observed in melting endotherms and Crystaf profiles. Molecular weights obtained by the mixture were higher than for any of the individual catalysts. A similar effect is observed for a dual-site system of the (1,2,4-Me₃Cp)₂ZrCl₂ catalyst together with the Me₄Si₂(Me₄Cp)₂ZrCl₂ catalyst as an alternative to (Me₅Cp)₂ZrCl₂.     

Effect of substituents on the catalytic properties of bis(cyclopentadienyl) zirconocene dichlorides in polymerization of ethene

Comparative analysis of catalytic activity of substituted bis(cyclopentadienyl)zirconium dichlorides with the general formula (R _n Cp)₂ZrCl₂ (Cp₂ZrCl₂, (MeCp)₂ZrCl₂, (PrⁱCp)₂ZrCl₂, (Prⁱ ₂Cp)₂ZrCl₂, (BuⁿCp)₂ZrCl₂, (BuⁱCp)₂ZrCl₂, (Bu^tCp)₂ZrCl₂, Cp^* ₂ZrCl₂ (Cp^*=Me₅C₅), (Me₃SiCp)₂ZrCl₂, (cyclo-C₆H₁₁Cp)₂ZrCl₂, and [(cyclo-C₆H₁₁)₂Cp]₂ZrCl₂) in ethene polymerization using polymethylalumoxane as the cocatalyst was performed. The molecular mass characteristics of the polyethylene samples obtained were determined. A linear correlation of the specific activity of the catalysts and the turnover number with the electronic and steric characteristics of substituents at the Cp ring of the complexes was established for the first time. Analysis of the polymerization kinetics and the obtained correlation between the specific activity of the complexes and molecular mass characteristics of the polyethylene samples suggest that alkyl substituents participate in reactions responsible for the restriction of the polymer chain growth and regeneration of the active center. These interactions most likely involve associates of AlMe₃ with polymethylalumoxane molecules.     

Polymerization of ethylene over metallocenes confined inside the supercage of a NaY zeolite

Metallocene catalysts entrapped inside the supercages of NaY zeolite were prepared by reacting NaY with methylaluminoxane (MAO) or trimethylaluminium (TMA) and then with Cp₂ZrCl₂ (Cp: cyclopentadienyl) or Cp₂TiCl₂. NaY/MAO/Cp₂ZrCl₂ and NaY/MAO/Cp₂TiCl₂ catalysts could polymerize ethylene. The amount of additional MAO for the polymerization was lowered to a mole ratio of Al/Zr of 186. Molecular weights and melting points of polyethylene polymerized with NaY-supported catalysts were higher than those of polyethylene obtained with homogeneous metallocene catalysts. It could be confirmed by extraction experiments that the metallocene catalyst was confined securely inside the supercage of the NaY zeolite.     

Binary metallocene supported catalyst for propylene polymerization

The binary silica supported catalyst system comprising the Cp₂ZrCl₂ and SiMe₂(Ind)₂ZrCl₂ metallocene compounds was prepared with different immobilization methods and evaluated at different propylene polymerization conditions. The performance results of the homogeneous isolated catalysts and also the homogeneous catalyst mixture were also included for comparison. High activities were obtained with the supported systems and the molecular weight of the produced polypropylene was invariably higher than that obtained using the homogeneous precursor.     

Metallocene/mao polymerization catalysts supported on cyclodextrin

By treating cyclodextrin(CD) with methylaluminoxane (MAO such as PMAO or MMAO) or trimethylaluminium (TMA) followed by Cp₂ZrCl₂, CD/PMAO/Cp₂ZrCl₂, CD/MMAO/Cp₂ZrCl₂ and CD/TMA/Cp₂ZrCl₂ catalysts were prepared. The catalysts were analyzed by ¹³C-CP/MAS NMR spectrometer and ICP to examine the structure of catalyst and content of Zr and Al. Ethylene polymerization was conducted with MAO or TMA as cocatalyst. Styrene polymerization was also carried out with α-CD/MMAO/Cp*TiCl₃ and α-CD/TMA/Cp*TiCl₃ catalysts. While the ordinary trialkylaluminium such as TMA as well as MAO can be used as cocatalyst for ethylene polymerization, only MAO could initiate the styrene polymerization with α-CD supported catalysts.     

A novel zirconocene/ultradispersed diamond black powder supported catalytic system for ethylene polymerization

A novel carrier of ultradispersed diamond black powder (UDDBP) was used to support metallocene catalyst. Al₂O₃ was also used as carrier in order to compare with UDDBP. Supported catalysts for ethylene polymerization were synthesized by two different reaction methods. One way was direct immobilization of the metallocene on the support, the other was adsorption of MAO onto the support followed by addition of the metallocene. Four supported catalysts Cp₂ZrCl₂/UDDBP, Cp₂ZrCl₂/Al₂O₃, Cp₂ZrCl₂/MAO/UDDBP and Cp₂ZrCl₂/Al₂O₃/MAO were obtained. The content of the zirconium in the supported catalyst was determined by UV spectroscopy. The activity of the ethylene polymerization catalyzed by supported catalyst was investigated. The influence of Al/Zr molar ratio and polymerization temperature on the activity was discussed. The polymerization rate was also observed.     

ESR studies on oxidation state of titanocene and zirconocene catalysts

An on‐line electron spin resonance (ESR) technique was applied to investigate the oxidation states of the metallocene catalysts CpTiCl₃, CpZrCl₃, Cp₂TiCl₂, and Cp₂ZrCl₂. These metallocene catalysts were activated by modified methylaluminoxane (MMAO). It was found that the titanocene catalysts (CpTiCl₃ and Cp₂TiCl₂) were readily reduced to the trivalent state while the zirconocene catalysts (CpZrCl₃ and Cp₂ZrCl₂) were quite stable with respect to reduction. The concentrations of the trivalent species Ti(III) and Zr(III) showed linear relationships with the concentrations of metallocene catalyst precursors. However, their slopes were always smaller than unity indicating the existence of bimetallic interactions of the active sites. The ESR detectable Ti(III) and Zr(III) concentrations initially increased with the MAO/catalyst ratio and then leveled off after an 800–1000 Al/catalyst molar ratio. The deactivation processes were followed as a function of aging time over a range of temperature (25–100°C). The decay curves strongly depended on aging temperature with higher temperature giving faster decay rates. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1465–1472, 1999     

Copolymerization of ethylene with bicyclo[2.2.1]hept-5-ene-2-methanol: A facile route to hydroxyl functional poly(ethylene)s

A direct and efficient route to the synthesis of hydroxyl functional polyethylene was identified by copolymerization of ethylene with bicyclo[2.2.1]hept-5-ene-2-methanol prereacted with trimethylaluminium using the catalyst system metallocene/MAO. Copolymerization studies were conducted using three metallocenes, namely, Cp₂ZrCl₂, Et(ind)₂ZrCl₂ and Me₂SiCp₂ZrCl₂ the last one of which gave a copolymer containing as much as 6.2 mol-% of alcohol. The effect of the Al/Zr ratio as well as of temperature was studied. The copolymers were characterized by means of ¹H NMR, differential scanning calorimetry, and intrinsic viscosity measurements.     

Metallocene catalysis in the complex-radical polymerization of methyl methacrylate

The initiation stage of methyl methacrylate polymerization in the presence of benzoyl peroxide-metallocene (MC) systems is considered, with MC = Cp₂Fe, (C₅Me₅)₂Fe, (AcC₅H₄)(C₅H₅)Fe, Cp₂TiCl₂, Cp₂ZrCl₂, and (C₅Me₅)₂ZrCl₂. The decomposition of benzoyl peroxide in the presence of a metallocene proceeds via the formation of its complex with the metallocene. The catalytic effect of the metallocenes on the initiation of methyl methacrylate polymerization is due to the formation of a metallocene-benzoyl peroxide complex and its decomposition yielding primary radicals. The chain propagation stage is metallocene-dependent, which is explained by the formation of complex sites. Their formation pathway and structures are analyzed using quantum-chemical calculations.     

Copolymerization of ethylene with 1‐hexene over metallocene catalyst supported on complex of magnesium chloride with tetrahydrofuran

The study of ethylene/1‐hexene copolymerization with the zirconocene catalyst, bis(cyclopentadienyl)zirconium dichloride (Cp₂ZrCl₂)/methylaluminoxane (MAO), anchored on a MgCl₂(THF)₂ support was carried out. The influence of 1‐hexene concentration in the feed on catalyst productivity and comonomer reactivity as well as other properties was investigated. Additionally, the effect of support modification by the organoaluminum compounds [(MAO, trimethlaluminum (AlMe₃), or diethylaluminum chloride (Et₂AlCl)] on the behavior of the MgCl₂(THF)₂/Cp₂ZrCl₂/MAO catalyst in the copolymerization process and on the properties of the copolymers was explored. Immobilization of the Cp₂ZrCl₂ compound on the complex magnesium support MgCl₂(THF)₂ resulted in an effective system for the copolymerization of ethylene with 1‐hexene. The modification of the support as well as the kind of organoaluminum compound used as a modifier influenced the activity of the examined catalyst system. Additionally, the profitable influence of immobilization of the homogeneous catalyst as well as modification of the support applied on the molecular weight and molecular weight distribution of the copolymers was established. Finally, with the successive self‐nucleation/annealing procedure, the copolymers obtained over both homogeneous and heterogeneous metallocene catalysts were heterogeneous with respect to their chemical composition. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2512–2519, 2004     

Initiation of complex-radical polymerization of methyl methacrylate in the presence of metallocenes

The initiation of radical polymerization of methyl methacrylate in the presence of benzoyl peroxide-metallocene (Cp₂Fe, Cp₂ZrCl₂, and Cp₂TiCl₂; (C₅Me₅)₂Fe, (C₅Me₅)₂ZrC_l2, and (AcC₅H₄)(C₅H₅)Fe) systems is studied. It is shown that a metallocene affects the rate of initiation and the initial rate of polymerization. On the basis of quantum-chemical calculations, a new mechanism of the initiation reaction may be advanced: Namely, the decomposition of benzoyl peroxide proceeds via the stage of complexation with a metallocene, while the nature of a metallocene determines the probability of complexation and decomposition.     

Copolymerization of ethene with cyclic and other sterically hindered olefins

Sterically hindered olefins like norbornene, dimethanooctahydronaphthalene (DMON), 4‐methylpentene, and 3‐methylbutene can be copolymerised with ethene by metallocene/MAO catalysts. Different C₂‐, C_s‐ and C₁‐symmetric and meso‐zirconocenes were used. Only isolated and alternating norbornene sequences but no norbornene blocks are formed by substituted [Me₂C(Cp‐R)(Flu)]ZrCl₂ catalysts. The alternating microstructure leads to melting points up to 270°C for ethene‐norbornene copolymers and up to 380°C for the semi‐crystalline alternating copolymer of ethene and DMON. Other sterically hindered olefins such as 3‐methylpentene build more blocky structures with high glass transition temperatures. The mechanism for the insertion reaction of the different catalysts is discussed.     

PP/PMMA Block Copolymers by Combination of ATRP and Metallocene Catalysis

PP-b-PMMA has been synthesized by a combination of metallocene catalysis and the controlled radical polymerization technique ATRP. Cp₂ZrCl₂/MAO and (Me₄Cp)SiMe₂(N-tert-Bu)TiCl₂/MAO were used for the synthesis of atactic polypropylene. By a series of chemical modifications pp macroinitiators for the ATRP polymerization of MMA were synthesized. The PP-b-PMMA with polydispersities from 1.8–2.8 and an M_n ranging from 8 to 26 kg/mol was characterized by ¹H-NMR,SEC and DSC.     

Donor- and acceptor-modified metallocene-based homogeneous Ziegler-Natta catalysts

The polymerization kinetics of propene polymerization using metallocene/methylaluminoxane (MAO) homogeneous catalysts have been investigated to explore the role of donor/acceptor interactions and to enhance the catalyst productivities. In the case of the non-stereospecific Cp₂ZrCl₂/MAO model system it has been demonstrated that, in addition to the well known irreversible deactivation, reversible deactivations, which are second order relative to the zirconium active site concentration, account for the decay of the polymerization rate. While MAO injection during polymerization enhances the polymerization rate, zirconocene addition deactivates the catalyst which can be reactivated by injecting additional MAO. A sequence of dynamic equilibria involving the formation of active cationic metallocene intermediates as well as inactive zirconocene species, e.g., zirconocene dimers, is proposed. Lewis base and Lewis acid additives have been added as probes to examine the role of such equilibria in the case of metallocene-based catalyst systems such as MAO-activated Cp₂ZrCl₂, racemic ethylenebisindenyl zirconium dichloride (EBIZrCl₂), and racemic ethylenebis (4,5,6,7-tetrahydroindenyl) zirconium dichloride (EBTHIZrCl₂). While the conventional donors such as 2,6-ditert.butyl-4-methylphenol (BHT) and 2,2,6,6-tetramethylpiperidine (TMP) reduce catalyst productivities, even at very low donor/Al molar ratios, increasing propene concentration and addition of trimethylboroxine (TMB) substantially enhance catalyst productivities and affect molecular weights of the polypropylene produced with metallocene/MAO catalysts.     

Activation of Metallacyclopropenes,five-membered Metallacyclocumulenes and Metallacyclopentynes of Zirconium with iBu2AlH

The recently described reaction products of zirconacyclopropenes Cp^′₂Zr(η²-Me₃SiC₂SiMe₃) and five-membered zirconacyclocumulenes (zirconacyclopenta-2,3,4-trienes) Cp*₂Zr(η⁴-1,2,3,4-RC₄R), Cp* = η⁵-pentamethylcyclopentadienyl, R = Me, Me₃Si and Ph, with i-Bu₂AlH are active catalysts in the polymerization of ethylene and in the ring opening polymerization of ε-caprolactone. Here we describe the different activity of these complexes after thermal activation or if additional i-Bu₂AlH together with water are added. These results are compared to those which were obtained with the complexes Cp₂Zr(η⁴-1,2,3,4-H₂C₄H₂), rac-(EBTHI)ZrF₂, rac-(EBTHI)ZrCl₂, [rac-(EBTHI)Zr(H)(µ-H)]₂ and rac (EBTHI)Zr(F)CH₂-CH₂(2-Py) after activation with i-Bu₂AlH together with water.     

Polypropylene fractions produced by binary metallocene catalysts

Because of the great economic interest in propylene‐based polymers and the possibility of designing materials with desired properties with metallocene catalyst mixtures, we investigated the characteristics of polypropylenes produced by mixtures of SiMe₂Ind₂ZrCl₂: dimethylsilane‐bis(indenyl) zirconocene ( 1 ) and Et(Flu)(Cp)ZrCl₂: ethylidene (fluorenyl cyclopentadienyl) zirconocene ( 2 ) in different proportions. The polymers were fractionated with solvents, and the fractions were characterized. We observed that the polymers produced by the different mixed systems showed lower weight‐average molecular weights and only slightly broader molecular weight distributions than polypropylenes synthesized by the individual catalysts. We concluded that catalyst 1 acted independently of catalyst 2 , producing polymers with the same isotacticity. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1478–1485, 2003     

Controlling molecular weight distributions of polyethylene by combining soluble metallocene/MAO catalysts

A critical look at the possibility of controlling the molecular weight distribution (MWD) of polyolefins by combining metallocene/methylalumoxane (MAO) catalysts is offered. Catalysts investigated were bis(cyclopentadienyl)zirconium dichloride (Cp₂ZrCl₂), its titanium and hafnium analogues (Cp₂TiCl₂ and Cp₂HfCl₂), as well as rac-ethylenebis(indenyl)zirconium dichloride (Et(Ind)₂ZrCl₂). As observed by other researchers, the MWD of polyethylene can be manipulated by combining soluble catalysts, which on their own produce polymer with narrow MWD but with different average molecular weights. Combined in slurry polymerization reactors, the catalysts in consideration produce ethylene homopolymer just as they would independently. Unimodal or bimodal MWDs can be obtained. This effect can be mimicked by blending polymers produced by the individual catalysts. We demonstrate how a variability in catalyst activity translates into a variability in MWD when mixing soluble catalysts in polymerization. Such a variability in MWD must be considered when setting goals for MWD control. We introduce a more quantitative approach to controlling the MWD using this method. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 831–840, 1998     

Borane chain transfer reaction in olefin polymerization using trialkylboranes as chain transfer agents

This article discusses a new borane chain transfer reaction in olefin polymerization that uses trialkylboranes as a chain transfer agent and thus can be realized in conventional single site polymerization processes under mild conditions. Commercially available triethylborane (TEB) and synthesized methyl‐B‐9‐borabicyclononane (Me‐B‐9‐BBN) were engaged in metallocene/MAO [depleted of trimethylaluminum (TMA)]‐catalyzed ethylene (Cp₂ZrCl₂ and rac‐Me₂Si(2‐Me‐4‐Ph)₂ZrCl₂ as a catalyst) and styrene (Cp*Ti(OMe)₃ as catalyst) polymerizations. The two trialkylboranes were found—in most cases—able to initiate an effective chain transfer reaction, which resulted in hydroxyl (OH)‐terminated PE and s‐PS polymers after an oxidative workup process, suggesting the formation of the B‐polymer bond at the polymer chain end. However, chain transfer efficiencies were influenced substantially by the steric hindrances of both the substituent on the trialkylborane and that on the catalyst ligand. TEB was more effective than TMA in ethylene polymerization with Cp₂ZrCl₂/MAO, whereas it became less effective when the catalyst changed to rac‐Me₂Si(2‐Me‐4‐Ph)₂ZrCl₂. Both TEB and Me‐B‐9‐BBN caused an efficient chain transfer in the Cp₂ZrCl₂/MAO‐catalyzed ethylene polymerization; nevertheless, Me‐B‐9‐BBN failed in vain with rac‐Me₂Si(2‐Me‐4‐Ph)₂ZrCl₂/MAO. In the case of styrene polymerization with Cp*Ti(OMe)₃/MAO, thanks to the large steric openness of the catalyst, TEB exhibited a high efficiency of chain transfer. Overall, trialkylboranes as chain transfer agents perform as well as B? H‐bearing borane derivatives, and are additionally advantaged by a much milder reaction condition, which further boosts their applicability in the preparation of borane‐terminated polyolefins. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3534–3541, 2010     

Homo-and copolymerization of vinylcyclohexane with α-olefins in the presence of heterogeneous and homogeneous catalytic systems

The polymerization and copolymerization of vinylcyclohexane with α-olefins in the presence of several heterogeneous and homogeneous catalytic systems were studied. It was shown that, with respect to activity in the polymerization of vinylcyclohexane, the tested catalysts can be arranged in the following order: α-TiCl₃ < titanium-magnesium catalyst < metallocene catalyst. Poly(vinylcyclohexane) prepared with heterogeneous catalytic systems is a solid semicrystalline polymer. The properties of polymers synthesized with homogeneous systems differ substantially depending on the type of the metallocene used. In the presence of metallocenes with a C ₂ symmetry, crystalline powderlike products arise, while in the case of metallocenes with C ₁ and C _s symmetries, polymerization yields amorphous viscous products. Molecular-mass distributions of poly(vinylcyclohexane) samples prepared using both heterogeneous titanium-magnesium catalysts and homogeneous metallocene complexes show a bimodal pattern, indicating the heterogeneity of active centers of these catalysts. Upon introduction of a comonomer (ethylene, propylene, and 1-hexene) into the reaction mixture, the activity of all studied catalytic systems increases. When Me₂C(3-Me-Cp)(Flu)ZrCl₂ and rac-Me₂SiInd₂ZrCl₂ are used as catalysts, the degree of crystallinity of the copolymers grows owing to the presence of ethylene or propylene units in poly(vinylcyclohexane) chains.