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.     

烷基铝对球形MgCl_2负载的茂金属催化剂催化乙烯聚合的影响

以甲基铝氧烷(MAO)为助催化剂的茂金属催化剂虽然具有催化活性高、分子量分布窄、聚合物化学组成均匀等优点,但其极高的Al/Zr比和聚合物颗粒形态差等缺点限制了其工业化应用,因此对茂金属催化剂的负载化成为近年来的研究热点.在众多的载体中,球形MgCl2是研究得很少的一类载体,文献中曾采用先负载主催化剂茂金属配合物,聚合时再加入助催化剂MAO的方法[1],由于加入的MAO与主催化剂的络合能力很强,会使部分载上的主催化剂溶解下来,成为均相聚合[2,3],导致聚合物颗粒形态差,且粘釜现象严重.我们则采用相反的思路,即先将助催化剂MAO负载在球形MgCl2上,制得MgCl2/MAO,在聚合前再将MgCl2/MAO与Et[Ind]2ZrCl2混合陈化,并立即在少量烷基铝活化下引发乙烯聚合[4],实验结果表明,该催化剂聚合活性高、聚合物的颗粒形态好、且不粘釜,是一种新型的载体催化剂.由于烷基铝的加入可使催化剂的活性大幅度提高,所以本文将烷基铝也称作助催化剂,来研究其对该载体催化剂催化乙烯聚合的影响.     

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.     

Ethyl‐iso‐butylaluminoxane activated metallocene catalyst for olefin polymerization

Ethylene and propylene were polymerized by different combinations of metallocenes and aluminoxanes. Ethyl‐iso‐butylaluminoxane (EBAO), which could be easily obtained by direct hydrolysis of Et₃Al/iso‐Bu₃Al in toluene with water, displayed as good polymerization activity as methylaluminoxane when it was paired with some metallocenes. The UV–visible spectroscopic study on different metallocene/aluminoxane pairs suggests that tightness of the ionic pairs generated from the reaction between metallocene and aluminoxane is different for a fixed metallocenium. The loose ionic pair derived from EBAO prefers the monomer propagation and hinders the interaction between active species, which results in high catalytic activity and molecular weight. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 998–1003, 2003     

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.     

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.     

Isospecific Polymerization of Propylene By ansa-Zirconocene Diamide Compound Cocatalyzed by Mao

Abstract

The kinetics of propylene polymerization initiated by racemic ethylene-1,2-bis(1-indenyl) zirconium bis(dimethylamide) [rac-(EBI) Zr(NMe₂)₂(rac-1)] cocatalyzed by methylaluminoxane (MAO) were studied. The polymerization behaviors of rac-1/MAO catalyst investigated by changing various experimental parameters are quite different from those of rac-(EBI) ZrCl₂ (rac-2)/MAO catalyst, due to the differences in the generation procedure of cationic actives species of each metallocene by the reaction with MAO. The activity of rac-1/MAO catalyst showed maximum when [Al]/[Zr] is around 2000, when [Zr] is 137.1 μM, and when polymerization temperature is 30°C. The negligible activity of rac-1/MAO catalyst at a very low MAO concentration seems to be caused by the instability of the cationic active species. The meso pentad values of polymers produced by rac-1/MAO catalyst at 30°C are in the range of 82.8% to 89.7%. The rac-1/MAO catalyst lost stereorigid character at the polymerization temperature above 60°C. The molecular weight of polymer decreased as [Al]/[Zr] ratio, polymerization temperature, and [Zr] increased. The molecular weight distributions of all polymers are in the range of 1.8–2.3, demonstrating uniform active species present in the polymerization system.     

Polymerization of styrene with supported half-sandwich complexes

Polymerizations of styrene were carried out with half-sandwich complexes supported on silica, CpTiX₃/MAO/SiO₂ (X = Cl, F). The optimum values for the polymerization time, the amount of cocatalyst and the Al_support/Ti ratio were found for the trichlorinated system. The highest activity obtained was 3,100 g sPS/(mol Ti × h × mol/L styrene). The trihalogenated complexes were compared to one another with respect to their polymerization rate. CpTiCl₃/MAO/SiO₂ and CpTiF₃/MAO/SiO₂ behave in a similar manner, suggesting that the active species of both half-sandwich complexes on the support are the same. Furthermore, aging experiments were carried out with CpTiCl₃/MAO/SiO₂ and, surprisingly, deactivation was observed, as opposed to supported zirconocenes which gain stability against deactivation reactions when anchored to a carrier. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2959–2968, 1999     

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.     

Coordination polymerization at very low amounts of methylaluminoxane as cocatalyst

The effect of very low amounts of methylaluminoxane as an activating cocatalyst in the coordination polymerization has been investigated in the syndiospecific polymerization of styrene with a half-sandwich metallocene catalyst in the presence of triisobutylaluminum at molar ratios of methylaluminoxane/transition metal from 0/1 to about 20/1 in comparison to the polymerization behavior at high molar methylaluminoxane (MAO)/metal ratios.As a result, there cannot be observed any polymerization reaction below a true molar ratio MAO/Ti of 6:1. At higher molar ratios until about 20, the polymerization conversion is increasing significantly with the MAO/Ti molar ratio.These observations and the results of the determination of the kinetic reaction order can be explained with Barron’s tert-butyl aluminoxane based model of MAO as a cage of six monomeric MAO units (AlOMe)₆ in contrast to Sinn’s MAO model of a cage of twelve monomeric units (AlOMe)₁₂ and are discussed with the results received at usually applied much higher MAO/transition metal ratios leading to a first-order dependence of the polymerization rate on the MAO concentration.From the thermal behavior of the syndiotactic polystyrenes synthesized it can be concluded, that the stereospecificity of the polymerization reaction is not affected by MAO at low MAO concentrations.     

Influence of Metallocene Type on the Order of Ethylene Polymerization and Catalyst Deactivation Rate in a Solution Reactor

The solution polymerization of ethylene using rac-Et(Ind)₂ZrCl₂/MAO and (Dimethylsilyl(tert-butylamido)(tetramethyl- cyclopentadienyl)titanium Dichloride)(CGC-Ti)/MAO was studied in a semi-batch reactor at 120 °C under different monomer pressures and catalyst concentrations. The kinetics of ethylene polymerization with rac-Et(Ind)₂ZrCl₂/MAO can be described with first order reactions for polymerization and catalyst deactivation. When (CGC-Ti)/MAO is used, however, second order kinetics are observed for catalyst decay and the order of polymerization changes from 2 to 1 with increasing ethylene pressure.     

Propene polymerization using homogeneous MAO-activated metallocene catalysts: Me2Si(Benz[e]Indenyl)2ZrCl2/MAO vs. Me2Si(2-Me-Benz[e]Indenyl)2ZrCl2/MAO

Propene was polymerized at 40°C and 2-bar propene in toluene using methylalumoxane (MAO) activated rac-Me₂Si(Benz[e]Indenyl)₂ZrCl₂ ( BI ) and rac-Me₂Si(2-Me-Benz[e]Indenyl)₂ZrCl₂ ( MBI ). Catalyst BI /MAO polymerizes propene with high activity to afford low molecular weight polypropylene, whereas MBI /MAO is less active and produces high molecular weight polypropylene. Variation of reaction conditions such as propene concentration, temperature, concentration of catalyst components, and addition of hydrogen reveals that the lower molecular weight polypropylene produced with BI /MAO results from chain transfer to propene monomer following a 2,1-insertion. A large fraction of both metallocene catalyst systems is deactivated upon 2,1-insertion. Such dormant sites can be reactivated by H₂-addition, which affords active metallocene hydrides. This effect of H₂-addition is reflected by a decreasing content of head-to-head enchainment and the formation of polypropylene with n-butyl end groups. Both catalysts show a strong dependence of activity on propene concentration that indicates a formal reaction order of 1.7 with respect to propene. MBI /MAO shows a much higher dependence of the activity on temperature than BI /MAO. At elevated temperatures, MBI /MAO polymerizes propene faster than BI /MAO. © 1995 John Wiley & Sons, Inc.     

Heterogenization of metalorganic catalysts of olefin polymerization and evaluation of active site non-uniformity

Heterogenized activators - “support-H₂O/AlR₃” (where R=Me, iBu, support=montmorillonite, zeolite), synthesized directly on the support, form with metallocenes metal alkyl complexes highly active in olefin polymerization without the use of commercial methylaluminoxane (MAO). It was shown by the method of temperature programmed desorption with the application of mass-spectrometry (TPD-MS) that the aluminumorganic compound in support-H₂O/AlR₃ is in general similar to the structure of commercial MAO. The heterogenization of Zr-cenes on support-H₂O/AlR₃ is accompanied by the appearance of the energy non-uniformity of active sites. The activation energy of thermal destruction of active Zr-C bonds in the active sites of prepared catalysts changes in the range from 25 to 32 kcal/mol.     

Chlorotitanium (IV) tetradentate Schiff‐base complex immobilized on inorganic supports: Support type and other factors having effect on ethylene polymerization activity

A titanium complex with [O,N,N,O]‐type tetradentate Schiff base (LTiCl₂), never used before in polymerization of olefins, was immobilized on silica‐ and magnesium‐type carriers, and it was used in ethylene polymerization. The conducted research revealed that the catalytic properties of the complex LTiCl₂ supported on those carriers were different for both the catalytic systems studied, and simultaneously they turned out different from those of the unsupported system. The supported catalysts require the use of Me₃Al, Et₃Al, or MAO as the activator to be able to offer high catalytic activities, whereas Et₂AlCl is needed for the nonsupported catalyst. This finding, together with considerable changes in polymerization yields and in properties of polymers versus composition of the catalytic system, suggest that there are different types of active sites in the studied catalysts. The catalyst anchored on the carrier produced in the reaction of MgCl₂·3.4EtOH with Et₂AlCl is definitely the most active one within the support systems tested. Its activity remarkably increases with the increasing reaction temperature. Moreover, that catalyst does not undergo deactivation over the studied period of time, irrespective of the type of the activator used and of the process temperature. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4811–4821, 2009     

Recent advances in olefin polymerization

Various metallocenes were tested for syndiotactic polymerization of propylene. Reduction in the amount of costly methylalumoxane (MAO) or replacement with the other components was accomplished. Small modifications in the metallocene ligands brought about remarkable differences in the resulting catalysts concerning their storage stability, H₂ response as well as stereo-specificity. Syndiotactic polypropylene (SPP) in the melt state behaves very differently compared to isotactic polypropylene (IPP). The notion that these polymers have different distributions of molecular entanglements are pursued, and verified for the first time. Excellent dispersibility of other olefin polymers in SPP matrix contributed to improve the usefulness of SPP.     

Alumina as support for metallocene catalyst in ethylene polymerization

Aluminas thermally and/or chemically treated were used as support for Cp₂ZrCl₂ and evaluated in ethylene polymerization at constant reaction conditions. Two different calcination temperatures were employed, and the metallocene was fixed either directly or after support pretreatment with MAO, TMA, or NaOH solutions. The obtained alumina‐supported catalysts showed activities comparable to the homogeneous precursor. It was noticed that the textural properties of the supports strongly influenced the catalyst performance. The direct fixation of the metallocene on alumina produced catalysts presenting lower activities in comparison to the ones obtained from the chemically treated supports. The chemical pretreatment of hydrated alumina with TMA originated catalysts whose activities were superior to those obtained by pretreatment with MAO. The pretreatment with NaOH produced the more active catalyst and generated branched polymer. The molecular weight of the PE produced by the supported catalysts was higher than the ones obtained with the homogeneous system. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 9–21, 2004     

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     

Polymerization of 3-methyl-1-butene promoted by metallocene catalysts

3-Methyl-1-butene was polymerized in the presence of a number of homogeneous metallocene catalysts (co-catalyst methylalumoxane). Contrary to literature reports, it was found that even the simplest C₂-symmetric metallocenes promote the isotactic polymerization of this monomer with reasonable productivities. Quite surprisingly, a prevailingly isotactic polymer was also obtained in the presence of C_s-symmetric metallocenes, which are instead syndiotactic-specific in propene polymerization.     

AlR2Cl/MgR2 combinations as universal cocatalysts for Ziegler–Natta,metallocene, and post‐metallocene catalysts

Combinations of dialkylaluminum chlorides and dialkylmagnesium compounds, when used at molar [AlR₂Cl]:[MgR₂] ratios ≥ 2, act as universal cocatalysts for all three presently known types of alkene polymerization catalysts—Ziegler–Natta, metallocene, and post‐metallocene. When these cocatalysts are used with supported Ti‐based Ziegler–Natta catalysts, they produce catalyst systems which are 1.5–2 times more active than the systems utilizing AlR₃ compounds as cocatalysts. Combinations of AlR₂Cl/MgR₂ cocatalysts and various metallocene complexes produce single‐center catalyst systems similar to those formed in the presence of MAO. The same cocatalysts activate numerous post‐metallocene Ti complexes containing bidentate ligands of a different nature and produce multicenter systems of very high activity. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3271–3285, 2009     

Polymerization of methyl methacrylate using a metallocene catalyst system

Methyl methacrylate was polymerized with Cp₂YCl(THF) or IVB group metallocene compounds (i.e., Cp₂ZrCl₂ and Cp₂HfCl₂, etc.), in the presence of a Lewis acid like Zn(C₂H₅)₂. The Lewis acid was complexed with methyl methacrylate, which avoided the metallocene compounds being poisoned with a functional group. A living polymerization was promoted through the use of metallocene/MAO/Zn(C₂H₅)₂, which gave tactic poly(methyl methacrylate) with a high molecular weight. The polymer yield increases with polymerization time, which indicates that the propagation rate is zero in order in the concentration of the monomer. The polymer yield increases also with the concentration of Cp₂YCl(THF), which indicates the yttrocene to be the real catalyst. When the polymerization temperature exceeds room temperature, the poly(methyl methacrylate) cannot be synthesized by the Cp₂YCl(THF) catalyst. When the reaction temperature reachs −60 °C, the poly(methyl methacrylate) is high syndiotatic and molecular weight by the Cp₂YCl(THF)/MAO catalyst system. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1184–1194, 2000