Chain transfer reactions in metallocene catalyzed polymerization of allylbenzene

Summary Allylbenzene was polymerized in the presence of two types of homogeneous zirconocene catalysts (co-catalyst methylaluiminoxane). Selective chain termination through β-hydride elimination or chain transfer to aluminum was observed depending upon the catalysts employed. The rac-Et(Ind)₂ZrCl₂, and rac-Me₂Si(Ind)₂ZrCl₂, catalysts gave the polyallylbenzenes with saturated end groups due to chain transfer to aluminum, while the Cp₂ZrCl₂ catalyst gave the polyallylbenzenes with vinylidene end groups due to β-hydride elimination. Received: 7 December 1998/Revised version: 20 January 1999/Accepted: 29 January 1999     

Modification of a Ziegler-Natta catalyst with a metallocene catalyst and its olefin polymerization behavior

A Ziegler-Natta catalyst was modified with a metallocene catalyst and its polymerization behavior was examined. In the modification of the TiCl₄ catalyst supported on MgCl₂ (MgCl₂-Ti) with a rac-ethylenebis(indenyl)zirconium dichloride (rac-Et(Ind)₂ZrCl₂, EIZ) catalyst, the obtained catalyst showed relatively low activity but produced high isotactic polypropylene. These results suggest that the EIZ catalyst might block a non-isospecific site and modify a Ti-active site to form highly isospecific sites. To combine two catalysts in olefin polymerization by catalyst transitioning methods, the sequential addition of catalysts and a co-catalyst was tried. It was found that an alkylaluminum like triethylaluminum (TEA) can act as a deactivation agent for a metallocene catalyst. In ethylene polymerization, catalyst transitioning was accomplished with the sequential addition of bis(cyclopentadienyl)zirconium dichloride (Cp₂ZrCl₂)/methylaluminoxane (MAO), TEA, and a titanium tetrachloride/vanadium oxytrichloride (TiCl₄/VOCl₃, Ti-V) catalyst. Using this method, it was possible to control the molecular weight distribution (MWD) of polyethylene in a bimodal pattern. In the presence of hydrogen, polyethylene with a very broad MWD was obtained due to a different hydrogen effect on the Cp₂ZrCl₂ and Ti-V catalyst. The obtained polyethylene with a broader MWD exhibited more apparent shear thinning.     

Ethylene polymerization with catalysts on the base of Zr-cenes and methylaluminoxanes synthesized on zeolite support

Summary The fixed aluminoxanes have been prepared on zeolite support Na-form of ZSM-5 (Si/Al=42) in reaction of partial hydrolysis of trimethylaluminium (TMA) with inside zeolite water. It was shown that aluminoxanes synthesized on zeolite surface form the heterogenized complexes with Cp₂ ZrCl₂ and Et[Ind]₂ ZrCl₂ which are active in ethylene polymerization without addition of other aluminiumorganic cocatalyst for a long time. The activation energy of ethylene polymerization in the presence of ZSM-5(H₂O)/TMA - Et[Ind]₂ ZrCl₂ is equal to 32 kJ/mol. Molecular weight and melting point of polyethylene obtained with such zeolite supported Zr-cene catalysts are higher than those of polyethylene formed with appropriate homogeneous metallocene systems. Received: 15 February 2000/Revised version: 10 May 2000/Accepted: 10 May 2000     

Propylene polymerization using combined syndio‐ and isospecific metallocene catalysts supported on silica/MAO

Syndiotactic and isotactic polypropylene were produced using the metallocene compounds Ph₂C(Flu)(Cp)ZrCl₂ and SiMe₂(2‐Me,4‐Ph‐Ind)₂ZrCl₂ in homogeneous system and supported on silica/MAO. These catalysts were evaluated either isolated or as a binary system. In the latter case, the iso‐ and syndiospecific metallocene complexes were immobilized together during the preparation of the supported catalyst. In a further experimental set, the syndio‐ and isospecific isolated heterogeneous catalysts were mixed at the moment of propylene polymerization. The polypropylenes obtained were evaluated using differential scanning calorimetry. The catalytic activities were also investigated. At all the studied polymerization temperatures, the results showed that the binary catalyst produced polypropylenes with lower melting temperatures in comparison with those obtained when the mixture of isolated supported syndio‐ and isospecific catalysts was employed. Moreover, the activation energies for the polymerization of all catalysts systems were calculated, resulting in a lower value for the binary system when compared to that employing the catalyst mixture and to both the isolated supported metallocene catalysts. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 628–637, 2006     

Study on chain end structures of polypropylenes prepared with different symmetrical metallocene catalysts

Propylene homopolymerizations were conducted by using three kinds of metallocenes: Cp₂ZrCl₂, En(Ind)₂ZrCl₂ and iPr(Cp)(Flu)ZrCl₂, all of which were activated with methylaluminoxane. Detailed NMR analyses of the chain ends in the resulting polymers were carried out to discuss the chain end structures of the polypropylenes and the mechanism of polymerization. The characteristic of each metallocene for the mechanism of polymerization was also described.     

Polymer-supported metallocene catalysts for gas-phase ethylene polymerization

The use of hydroxylated chloromethylated-styrene/divinylbenzene copolymer as a support for three different catalysts, Cp₂ZrCl₂, [Ind]₂ZrCl₂ and (CH₃)₂Si[Ind]₂ZrCl₂ has been examined for the polymerization of ethylene in gas phase. The gas phase polymerization experiments were performed in a horizontal reactor by using Box-Behnken experimental design [Box and Wilson, 1951] to study the effects of temperature, ethylene partial pressure, and MAO cocatalyst level on polymerization. The measured average catalyst activities were empirically correlated with these three factors. Temperature appears to be the most important factor, which shows a first and second order effect on activity and also interacts with pressure and MAO. The kinetic study shows that these supported catalysts might contain two types of active sites, and the deactivation of sites follows a first order kinetic. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Chain transfer reaction in metallocene catalyzed ethylene copolymerization with allyltrimethylsilane

Summary In order to investigate the polymerization behavior of allytrimethylsilane as a comonomer, ethylene was copolymerized with allyltrimethylsilane at 80°C in toluene using methylaluminoxane (MAO) activated metallocene catalysts. The catalytic activity of the polymerization strongly depended on both the type of the catalysts and the concentration of allyltrimethylsilane. End group analysis of the copolymers by means of ¹H and ¹³C NMR spectroscopy revealed that allyltrimethylsilane rather act as a chain transfer agent in the copolymerization, even though considerable amount of allyltrimethylsilane was incorporated in the polymer chain with rac-Et(Ind)₂ZrCl₂ catalysts. The chain transfer reaction influence strongly the molecular weight and comonomer content of the copolymers. Received: 22 June 1999/Revised version: 9 September 1999/Accepted: 30 September 1999     

Functionalization of polyethylene using borane reagents and metallocene catalysts

A new method to prepare functionalized polyethylene involving borane intermediates and transition metal catalysts is described. Two processes, direct and post polymerizations, were employed to prepare borane-containing polyethylene (PE-B), which can be transformed to functionalized polyethylene (LLDPE-f) with various functional groups, such as ? BR₂, ? OH, ? NH₂, ? OSi(CH₃)₃. In the direct process, the PE-B copolymers were prepared in one step by copolymerization of ethylene with a borane monomer (ω-borane-α-olefin). The post polymerization process requires two steps: copolymerization of ethylene and 1,4-hexadiene, and subsequential hydroboration reaction of unsaturated PE. Three transition metal catalysts, including two homogeneous metallocene (Cp₂ZrCl₂ [bis(cyclopentadienyl) zirconium dichloride] and Et(Ind)₂ZrCl₂ [1,1′-ethylenedi-η⁵-indenyl-zirconium dichloride] with MAO (methylaluminoxane)) and one heterogeneous (TiCl₃·AA/Et₂AlCl) ones, were studied in the copolymerization reactions. The single site Et(Ind)₂ZrCl₂/MAO homogeneous catalyst, with a strained ligand geometry and opened active site, is by far the most effective system in the incorporation of high olefins into polyethylene structures.     

Effect of cocatalyst on the chemical composition distribution and microstructure of ethylene-hexene copolymer produced by a metallocene/Ziegler-Natta hybrid catalyst

A silica-magnesium bisupport (SMB) was prepared by a sol-gel method for use as a support for metallocene/Ziegler-Natta hybrid catalyst. The SMB was treated with methylaluminoxane (MAO) prior to the immobilization of TiCl₄ and rac-Et(Ind)₂ZrCl₂. The prepared rac-Et(Ind)₂ZrCl₂/TiCl₄/MAO/SMB catalyst was applied to the ethylenehexene copolymerization with a variation of cocatalyst species (polymerization run 1: triisobutylaluminum (TIBAL) and methylaluminoxane (MAO), polymerization run 2: triethylaluminum (TEA) and methylaluminoxane (MAO)). The effect of cocatalysts on the chemical composition distributions (CCDs) and microstructures of ethylene-hexene copolymers was examined. It was found that the catalytic activity in polymerization run 1 was a little higher than that in polymerization run 2, because of the enhanced catalytic activity at the initial stage in polymerization run 1. The chemical composition distributions (CCDs) in the two copolymers showed six peaks and exhibited a similar trend. However, the lamellas in the ethylene-hexene copolymer produced in polymerization run 1 were distributed over smaller sizes than those in the copolymer produced in polymerization run 2. It was also revealed that the rac-Et(Ind)₂ZrCl₂/TiCl₄/MAO/SMB catalyst preferably produced the ethylene-hexene copolymer with non-blocky sequence when TEA and MAO were used as cocatalysts.     

Polymerization of ethylene using zirconocenes supported on swellable cation-exchanged fluorotetrasilicic mica

Supported catalysts consisting of Cp₂ZrCl₂ and cation-exchanged fluorotetrasilicic mica (Mⁿ⁺-mica, Mⁿ⁺ = Na⁺, Mg²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺) were prepared and employed in the ethylene polymerization or ethylene/1-hexene copolymerization in the presence of R₃Al. The catalysts consisting of swellable Mg²⁺-mica and Zn²⁺-mica (both calcined at 200 °C) and Cp₂ZrCl₂ displayed high activity for the polymerization reaction. By contrast, when Mg²⁺-mica and Zn²⁺-mica were calcined above 250 °C, the swellability was lost, and the activities of Cp₂ZrCl₂ supported on these non-swellable micas were significantly reduced. The relationship between the activity and swellability of mica was clearly observed both in ethylene polymerizations employing (n-BuCp)₂ZrCl₂ in place of Cp₂ZrCl₂ and in ethylene/1-hexene copolymerizations using Cp₂ZrCl₂. The role of Mⁿ⁺-mica for the activation of the metallocene complex was investigated by surface observation using a scanning electron microscope and by XRD measurements of the catalysts after polymerization of ethylene for a short time. The results of the surface observations indicated that polyethylene was produced on the edges of Mⁿ⁺-mica lamellas at the initial stage of the polymerization. The XRD measurements show that the regularity of the stacked lamellas was immediately lost at this stage. The catalyst prepared by removing free Cp₂ZrCl₂ (i.e., unsupported Cp₂ZrCl₂, Cp₂ZrCl₂ dissolved into the catalyst slurry) showed extremely low activity, suggesting that the most of the active sites were formed through the reactions of Mⁿ⁺-mica and free Cp₂ZrCl₂. These results indicate that the lamellas of Mⁿ⁺-mica are peeled off at the initial stage of the polymerization and that exposed metal cations react with free-Cp₂ZrCl₂ to form additional active species. The swellability of Mⁿ⁺-mica strongly affects the formation of additional active sites, and therefore the supported catalysts based on non-swellable Mⁿ⁺-mica displayed only low activities.     

Synthesis and characterization of ethylene-1-hexene copolymers using homogeneous Ziegler-Natta catalysts

Summary This study investigated the copolymerization of ethylene with 1-hexene using the homogeneous Et[Ind]₂ZrCl₂ and [Ind]₂ZrCl₂ catalysts. The Et[Ind]₂ZrCl₂ catalyst gave a higher catalytic activity than the [Ind]₂ZrCl₂ and also showed a better incorporation of 1-hexene for the same comonomer concentration in the feed. Thermal analysis (DSC) and viscosity measurements showed that an increase of the 1-hexene incorporated in the copolymer results in a decrease of the melting point, crystallinity and molecular weight of the polymer formed. The reactivity ratios for ethylene and 1-hexene confirmed the more successful incorporation of the comonomer for the polymerization catalyzed by Et[Ind]₂ZrCl₂.     

Oligomerisation of 1-pentene with metallocene catalysts

The homo-oligomerisation of 1-pentene in the presence of various bridged and non-bridged metallocenes and methylaluminoxane (MAO) at room temperature and at 60°C, respectively, has been studied. The use of the bridged catalysts rac-[C₂H₄(Ind)₂]ZrCl₂ ( 1 ) and [(CH₃)₂Si(Ind)₂]ZrCl₂ ( 2 ) leads to the formation of isotactic poly(1-pentene). The use of Cp₂ZrCl₂ ( 3 ), Cp₂HfCl₂ ( 4 ) and [(CH₃)₅C₅]₂ZrCl₂ ( 5 ) leads to the formation of atactic poly(1-pentene). The stereoregularity of the isotactic poly(1-pentene) obtained with 1 was higher than that of the poly(1-pentene) synthesised with 2 . The degree of polymerisation was highly dependent on the metallocene catalyst. Oligomers ranging from the dimer of 1-pentene to poly(1-pentene) with a number-average molar mass M_n = 5100 g mol^–1 were formed. The ¹H NMR spectra of the samples were analysed with regard to functional groups and these were attributed to different chain termination processes. A MALDI-TOF spectrum of low-molar-mass poly(1-pentene) could be recorded using dithranol as matrix and adding silver trifluoroacetate to promote ion formation.     

Synthesis and functionalization of poly(ethylene‐co‐dicyclopentadiene)

Copolymerizations of ethylene with endo‐dicyclopentadiene (DCP) were performed by using Cp₂ZrCl₂ (Cp = Cyclopentadienyl), Et(Ind)₂ZrCl₂ (Ind = Indenyl), and Ph₂C(Cp)(Flu)ZrCl₂ (Flu = Fluorenyl) combined with MAO as cocatalyst. Among these three metallocenes, Et(Ind)₂ZrCl₂ showed the highest catalyst performance for the copolymerization. From ¹H‐NMR analysis, it was found that DCP was copolymerized through enchainment of norbornene rings. The copolymer was then epoxidated by reacting with m‐chloroperbenzoic acid. ¹³C‐NMR spectrum of the resulting copolymer indicated the quantitative conversion of olefinic to epoxy groups. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 103–108, 1999     

A comparative study of tetraisobutyldialuminoxane and methylaluminoxane as cocatalysts for Cp2ZrCl2 catalyzed ethylene polymerization

Summary The effect of [A1]/[Zr] mol ratio and temperature on the cocatalytic effects of tetraisobutyldialuminoxane (TIBDAO) and methylaluminoxane (MAO) for ethylene polymerization using Cp₂ZrCl₂ catalyst were studied. The decay type kinetic profile was observed for both TIBDAO and MAO cocatalyzed ethylene polymerizations. Catalytic activity and rate of polymerization were found to be low for TIBDAO cocatalyzed ethylene polymerization when compared to MAO cocatalyzed ethylene polymerization. The differences in catalytic activity and rate of polymerization for ethylene polymerization catalyzed by Cp₂ZrCl₂-TIBDAO and Cp₂ZrCl₂-MAO were discussed with respect to the structures of MAO and TIBDAO. An active species for Cp₂ZrCl₂-MAO and Cp₂ZrCl₂-TIBDAO catalyzed ethylene polymerizations was also discussed. The polyethylene was characterized by intrinsic viscosity measurements.     

Preparation of nanopolyethylene wire with carbon nanotubes‐supported Cp2ZrCl2 catalyst

Nanowires were prepared by ethylene polymerization in situ with carbon nanotubes (CNTs)‐supported Cp₂ZrCl₂ catalyst. It was found that the metallocene catalyst was first adsorbed on the surface of the CNTs and then the polymerization of ethylene was initiated. The resulting PE could encapsulate on the surface of the CNTs to form nanowire. The diameter of nanowire could be controlled easily by the polymerization conditions. The possible mechanism of formation of nanopolyethylene wire is proposed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1291–1294, 2006     

Preparation of branched polyethene using a soluble zirconocene catalyst

[Me₂C(Cp) (Ind)]ZrCl₂ metallocene catalyst has been prepared and employed in a study of ethene polymerization in the presence of the cocatalyst methylaluminoxane. C₁ and C₂ signals are detected in the ¹³C NMR spectra of the resultant polymers; this reveals that the resultant polymer is a branched polyethene (polyethylene). The influence of polymerization temperature, catalyst concentration and [Al]/[Zr] ratio on catalytic activities and polymerization kinetics is investigated. A plausible mechanism for forming branched polyethene is suggested. © 2000 Society of Chemical Industry     

Factors Affecting Dehydrogenation and Catalytic Activity: Methyl Substituent

Metallocene compound is supported on methylaluminoxane (MAO) and the carrier to form the metallocene catalyst, where (n-BuCp)₂ZrCl₂ and (1,3-Me,n-BuCp)₂ZrCl₂ are two typical representatives of the metallocene compounds. The difference in the performances of the two compounds (n-BuCp)₂ZrCl₂ and (1,3-Me,n-BuCp)₂ZrCl₂ was calculated by DFT. The introduction of methyl groups led to an increase in the number of Mulliken charge and a decrease in the dipole distance. The results fully confirmed that the methyl group showed an electron-donating effect. The catalyst formed by the metallocene compound (n-BuCp)₂ZrCl₂ in the olefin polymerization process followed the a-agostic mechanism of ground state and transition state, and dehydrogenation occurred during the polymerization process. In contrast, the catalyst formed by the metallocene compound (1,3-Me,n-BuCp)₂ZrCl₂ in the olefin polymerization process was based on the Green–Rooney mechanism (hydrogen anion migration), and metal–hydrogen bond and small molecule alkane were formed during the polymerization reaction. However, there were some differences in the mechanism of the homopolymerization and copolymerization when using metallocene catalysts for ethylene polymerization. Ethylene/1-alkene reacted to produce copolymers containing unsaturated bonds, and the results were confirmed by FT-IR analysis. The significant effect of the ligand structure on the metallocene catalyst is beyond imagination, which will lead us to find a suitable ligand structure to meet the high catalytic activity and low hydrogen evolution.

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Polymerization of ethylene using metallocene and aluminoxane systems

This paper describes ethylene polymerization using a number of metallocene and aluminoxane catalyst systems, Cp₂MR₂ and methylaluminoxane [M=Zr, W, Nb; R=Cl, CH₃]. Two types of methylaluminoxane, MAO (1) and MAO (2), were used as cocatalysts. The polymerization activities of the complexes Cp₂WCl₂ and Cp₂NbCl₂ were compared with that of Cp₂ZrCl₂. The Nb and W complexes were found to be less active than the Zr complex. Polyethylene characterization was also carried out by the following methods: gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR). ©1997 SCI     

Retrofitting of industrial olefin polymerization plants: producing broad MWDs through multiobjective periodic operation

The drop‐in of metallocene catalysts (MCs) in existing industrial polymerization plants is the current goal of most polymer producers. However, the narrow molecular weight distribution (MWD) of the polymers produced by MCs prevent them of moving into commodities market dominated by conventional Ziegler–Natta catalysts, where ease of processing is an essential property. Broader MWDs may be obtained through mixing of different MCs or blending of different resins, but resin‐compatibility problems and complex undesirable catalyst interactions pose technological problems that have yet to be solved. For these reasons, modern olefin polymerization plants have to work with both catalysts to respond to market demands, resulting in costly operations of grade/catalyst change. In this article, we describe how periodic control of short residence‐time reactors operating with an MC (Me₂Si(2‐Me‐Benz[e]Ind)₂ZrCl₂/MAO) can lead to polymers with broad MWD and, consequently, to high processability. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 437–452, 2000     

Synthesis of ethylene and norbornene copolymer with metallocene catalysts and characteristic analysis

To synthesis ethylene (E) and norbornene (NB) copolymer with high glass transition temperature and transparency, three metallocene catalysts with different symmetric structure were evaluated, respectively. The catalyst activity, NB fraction in copolymer and the transparency of copolymers produced under various conditions were investigated. It has been found that C₂ symmetric catalyst such as rac‐[En(Ind)₂]ZrCl₂ was the best choice to produce copolymer with high NB fraction while keeping high catalyst activity. Furthermore, the effects of reaction conditions on activity of rac‐[En(Ind)₂]ZrCl₂ and the resultant copolymer structure have also been thoroughly studied. The results indicate that increasing the NB/E ratio is the effective way to increase NB content of copolymer when NB/E ratio is less than 20. However, when NB/E ratio is over 20, further increase in NB/E ratio will lead to significant lower catalyst activity and very limited increase in NB content of copolymer. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008