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Metallocene
Terms modified by Metallocene Selected AbstractsThe discovery of metallocene catalysts and their present state of the artJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 16 2004Walter Kaminsky Abstract Metallocene and other transition metal complexes activated by methylaluminoxane are highly active catalysts for the polymerization of olefins, diolefins, and styrene, which was discovered at the University of Hamburg about 25 years ago. These catalysts allow the synthesis of polymers with a highly defined microstructure, tacticity, and stereoregularity, as well as new copolymers with superior properties such as film clarity, tensile strength, and lower extractables. A better understanding of the mechanism of olefin polymerization leads to findings of other new single site catalysts. The development of the metallocene/MAO-catalysts from their discovery to their present state of the art is presented. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3911,3921, 2004 [source] Copolymerization of Ethylene with 2,7-Octadienyl Methyl Ether in the Presence of Metallocene and Nickel Diimine CatalystsMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 7 2009Mércia Fernandes Abstract In this work, copolymers of ethylene with 2,7-octadienyl methyl ether have been synthesized in the presence of three single-site catalysts. The obtained copolymers not only have a polar ether function but also a double bond in the side chains that is useful for secondary reactions. The polymers were characterized by GPC, EA, DSC, NMR, and FT-IR. The catalytic activity depends on the kind of catalysts, the concentration of the polar monomer, and the concentration of the protecting agent. Up to 7.3 wt.-% of MODE could be incorporated by the metallocene catalyst. These are in average 120 functional side groups in a polymer molecule with a molecular mass of 230,000. [source] Metallocene based polyolefin: a potential candidate for the replacement of flexible poly (vinyl chloride) in the medical fieldPOLYMERS FOR ADVANCED TECHNOLOGIES, Issue 9 2010M. C. Sunny Abstract A comparative assessment of the performance properties of metallocene polyolefin (m-PO) with those of plasticized poly (vinyl chloride) (pPVC) and ethylene vinyl acetate (EVA) copolymer having 18% vinyl acetate content (EVA-18), the two common polymers used for flexible medical products, is carried out. The preliminary evaluation of the processability, mechanical properties, and thermal stability of the new material, m-PO is described. The processability parameters like mixing torque and melt viscosity of m-PO are found to be comparable with those of pPVC and EVA-18. Mechanical properties such as tensile strength, elongation at break, and tear strength (TS) of m-PO are much higher than that of pPVC and EVA-18. Thermo gravimetric analysis (TGA) indicates that the thermal degradation of m-PO takes place only at temperatures above 340°C and can be processed at 170°C without much damage. Oxygen and carbon dioxide permeabilities of m-PO at three different temperatures (10, 25, and 40°C) are evaluated and compared with those of pPVC and EVA-18. It could be seen that the permeabilities of both the gases for m-PO at three temperatures were lower than those of pPVC and EVA. Biological evaluation of m-PO is carried out by assessing its cytotoxicity, hemolytic property, and blood clotting initiation. The cytotoxicity studies indicate that m-PO is non-toxic to the monolayer of L929 mammalian fibroblast cell lines on direct contact or the exposure of its extract. Non-hemolytic property of m-PO by direct contact as well as test on extract is revealed both in static and in dynamic conditions. Blood clotting time experiments indicate that the initiation of blood clotting due to m-PO is faster than that of pPVC and EVA-18. Copyright © 2009 John Wiley & Sons, Ltd. [source] The Effect of Orientation on Extrusion Cast Metallocene Polyethylenes and the Role of Processing Conditions in the Die Swell of Metallocene and Conventional PolyethylenesASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 1-2 2004B.G. Millar Cast films were prepared from a range of metallocene polyethylenes (mPEs) of varied co-monomer types (hexene, octene) using a Killion single-screw extruder, using different haul off speeds (8,4 m/min) and die gaps (700,250 m,m). Samples with greater orientation in one direction had increased tensile strength and shrinkage in that direction. DSC analysis showed crystallinity to decrease with decreasing haul of speed. Extrusion of mPEs and conventional linear low density polyethylenes (LLDPEs) by single capillary rheology showed that die swell increased with increasing extrusion rate and decreasing melt temperature. Increased die swell was found for the broader molecular weight distribution (M.W.D.) LLDPEs and in the higher molecular weight resins. Furthermore, long chain branching was found to increase die swell. [source] The Extraordinary Cocatalytic Action of Polymethylaluminoxane (MAO) in the Polymerization of Terminal Olefins by Metallocenes: Chemical Change in the Group 4 Metallocene Dimethyl Derivatives Induced by MAO,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 20 2005John J. Eisch Abstract In the polymerization of olefins with Group 4 metallocene dichlorides or dimethyl derivatives as procatalysts the use of polymethylaluminoxane (MAO) as the cocatalyst, especially in extreme excess (102,103 times the metallocene equivalent), has been shown to have an extraordinary accelerating effect on the rate of olefin polymerization, when compared with the cocatalytic action of alkylaluminum halides. In attempts at explaining the greatly superior catalytic activity of MAO in olefin polymerization (the MAO conundrum), hypotheses have generally paralleled the steps involved in the cocatalytic action of RnAlCl3,n, namely the alkylation of Cp2MtCl2, ionization of Cp2Mt(R)Cl into the metallocenium cation, [Cp2Mt,R]+, and anion, [Rn,1AlCl4,n], and subsequent ion-pair separation. In order to understand any differences in catalytic action between such cocatalysts, we have studied the individual action of MAO (100 equiv.) and of MeAlCl2 (1,2 equiv.) on each of the Group 4 metallocene derivatives, Cp2TiCl2, Cp2ZrCl2, Cp2Ti(CH3)2 and Cp2Zr(CH3)2. With MeAlCl2 each of the metallocene derivatives appeared to form the cation, [Cp2Mt,CH3]+, with greater (Ti) or lesser (Zr) ease, because an alkyne such as diphenylacetylene was then found to insert into the Mt,CH3 bond stereoselectively. In striking contrast, treatment of each metallocene with MAO gave two reactions very different from MeAlCl2, namely a steady evolution of methane gas upon mixing and a finding upon hydrolytic workup that the diphenylacetylene present had undergone no insertion into the Mt,CH3 bond but instead had been reductively dimerized completely to (E,E)-1,2,3,4-tetraphenyl-1,3-butadiene. To account for this astonishing difference in chemical behavior between MAO and MeAlCl2 in their cocatalytic activation of Group 4 metallocenes to olefin polymerization, it is necessary to postulate a novel, unique sequence of reaction steps occurring between MAO and the metallocene. If one starts with the metallocene dichloride, then the free TMA present in the MAO would generate the Cp2Mt(CH3)2. This metallocene dimethyl derivative, complexed with an oligomeric MAO unit, would undergo a transfer-epimetallation with added olefin or acetylene to form a metallacyclopropane or metallacyclopropene, respectively. With added diphenylacetylene the resulting 2,3-diphenylmetallacyclopropene would be expected rapidly to insert a second alkyne to form the 2,3,4,5-tetraphenyl-1-metallacyclopentadiene. Simple hydrolysis of the latter intermediate would generate (E,E)-1,2,3,4-tetraphenyl-1,3-butadiene while alternative workup with D2O would give the 1,4-dideuterio derivative of this butadiene. Both such expectations were confirmed by experiment. In the case of added olefin, similar metallacyclopropane and metallacyclopentane intermediates should be produced until ring opening of the latter five-membered ring leads to an open-chain zwitterion, a process having ample precedent in the research of Gerhard Erker. The solution to the MAO conundrum then, namely the extraordinary cocatalytic activity of MAO in olefin polymerization by metallocenes, lies in the unique catalytic activation of the Group 4 metallocene dimethyl derivative, which occurs by transfer-epimetallation of the olefin monomer by the Cp2Mt(CH3)2,MAO complex. The most advantageous Lewis acidic sites in the MAO,oligomeric mixture for such metallocene,MAO complexation are suggested to be terminal Me2Al,O,AlMe, segments of an open-chain oligomer. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Etorphine-Related Ferrocenyl-Substituted Morphinan AlkaloidsHELVETICA CHIMICA ACTA, Issue 10 2003Gerhard Laus The two diastereoisomeric ferrocenyl-substituted orvinols 2 and 3 were prepared. The modified alkaloids are still able to interact with opioid receptors (see Table). The ferrocene moiety allows highly selective and sensitive electrochemical detection. The X-ray crystal structure of the major isomer 2 was determined. The combination of a metallocene and a morphinan alkaloid holds promise for useful antitumor activity. [source] Catalytic activity during the preparation of PE/clay nanocomposites by in situ polymerization with metallocene catalystsJOURNAL OF APPLIED POLYMER SCIENCE, Issue 4 2009Paula Zapata Abstract Catalytic activity during the formation of polyethylene (PE)/clay nanocomposites by in situ polymerization with metallocenes was studied. Ethylene polymerization was carried out with the homogeneous metallocene in the presence of the clay particles and using the clay-supported metallocene catalyst. It was found that the catalytic activity of the homogeneous metallocene does not decrease in the presence of the clay particles and only a slight decrease of activity occurs using the clay-supported catalyst. The modification of the clay with MAO cocatalyst as well as its intercalation with ODA surfactant were found to play an important role during the in situ formation of the PE/clay nanocomposite. ODA-intercalated clay apparently facilitates the activation and monomer insertion processes on zirconocene centers located in internal sites of the clay structure. Although metallocene supported on MAO-treated clay exhibited somewhat lower catalytic activity than that supported directly on the ODA-intercalated clay, both systems favored the production of PE nanocomposites containing highly exfoliated clay particles. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009 [source] Highly active copolymerization of ethylene with 10-undecen-1-ol using phenoxy-based zirconium/methylaluminoxane catalystsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 23 2005Xiaofan Zhang Abstract Activated with methylaluminoxane (MAO), phenoxy-based zirconium complexes bis[(3- tBu-C6H3 -2-O)-CHNC6H5]ZrCl2, bis[(3,5-di- tBu-C6H2 -2-O)-PhCNC6H5] ZrCl2, and bis[(3,5-di- tBu-C6H2 -2-O)-PhCN(2-F-C6H4)]ZrCl2 for the first time have been used for the copolymerization of ethylene with 10-undecen-1-ol. In comparison with the conventional metallocene, the phenoxy-based zirconium complexes exhibit much higher catalytic activities [>107 g of polymer (mol of catalyst),1 h,1]. The incorporation of 10-undecen-1-ol into the copolymers and the properties of the copolymers are strongly affected by the catalyst structure. Among the three catalysts, complex c is the most favorable for preparing higher molecular weight functionalized polyethylene containing a higher content of hydroxyl groups. Studies on the polymerization conditions indicate that the incorporated commoner content in the copolymers mainly depends on the comonomer concentration in the feed. The catalytic activity is slightly affected by the Al(MAO)/Zr molar ratio but decreases greatly with an increase in the polymerization temperature. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5944,5952, 2005 [source] Stereospecific polymerization of methacrylates by metallocene and related catalystsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 14 2004Eugene Y.-X. Abstract Stereospecific,isospecific, syndiospecific, and diastereospecific,polymerizations of methacrylates using group 4 metallocene and related catalysts produce polymethacrylates with controlled stereo-microstructures. The versatility and stereospecificity of these cat- alysts for methyl methacrylate polymerization were demonstrated not only in solution-phase polymerization, but also in polymerizations on silica surfaces and inside silicate nanogalleries. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3395,3403, 2004 [source] Copolymerization of propylene with various higher ,-olefins using silica-supported rac -Me2Si(Ind)2ZrCl2JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 19 2001Jun-Ting Xu Abstract The copolymerization of propylene with 1-hexene, 1-octene, 1-decene, and 1-dodecene was carried out with silica-supported rac -Me2Si(Ind)2ZrCl2 as a catalyst. The copolymerization activities of the homogeneous and supported catalysts and the microstructures of the resulting copolymers were compared. The activity of the supported catalyst was only one-half to one-eighth of that of the homogeneous catalyst, depending on the comonomer type. The supported catalyst copolymerized more comonomer into the polymer chain than the homogeneous catalyst at the same monomer feed ratio. Data of reactivity ratios showed that the depression in the activity of propylene instead of an enhancement in the activity of olefinic comonomer was responsible for this phenomenon. We also found that copolymerization with ,-olefins and supporting the metallocene on a carrier improved the stereoregularity and regioregularity of the copolymers. The melting temperature of all the copolymers decreased linearly with growing comonomer content, regardless of the comonomer type and catalyst system. Low mobility of the propagation chain in the supported catalyst was suggested as the reason for the different polymerization behaviors of the supported catalyst with the homogeneous system. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3294,3303, 2001 [source] The tenth anniversary of Suzuki polycondensation (SPC)JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 10 2001A. D. Schlüter Abstract This article describes the successful transfer of the Suzuki cross-coupling (SCC) reaction to polymer synthesis, one of the major developments within the last decade of polymer synthesis. The polymers prepared by Suzuki polycondensation (SPC) and its Ni-catalyzed reductive counterpart are soluble and processable poly(arylene)s that, because of their rigid and conjugated backbones, are of interest for the materials sciences. Achievable molar masses easily compete with those of traditional polyesters and polyamides. This article also provides insight into some synthetic problems associated with the transfer of SCC from low molar mass organic chemistry to high molar mass polymer chemistry by addressing issues such as monomer purity, stoichiometric balance, achievable molar masses, and defects in the polymer structure. Although the emphasis of this article is synthetic and structural issues, some potential applications of the polyarylenes obtained are briefly mentioned. Together with the enormous developments in the areas of metallocene, ring-opening metathesis, and acyclic diene metathesis polymerization, the success of SPC impressingly underlines the increasing importance of transition-metal-catalyzed CC-bond-forming reactions in polymer synthesis. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1533,1556, 2001 [source] Video Microscopy for the Investigation of Gas Phase CopolymerizationMACROMOLECULAR MATERIALS & ENGINEERING, Issue 11 2005Daniela Ferrari Abstract Summary: Video microscopy as a tool for investigating olefin gas phase copolymerization is presented for the first time in this paper. The central theme of this work is the study of the comonomer effect shown by an unbridged metallocene catalyst supported on silica. By using video microscopy, it is possible to observe the increase in catalytic activity in terms of particle growth as well as monomer consumption. The observation that a more pronounced induction period in the particle growth profile is shown with increasing propylene concentration led us to investigate the copolymers obtained at different polymerization times using 13C NMR analysis and single particle energy dispersive X-ray (EDX mapping). This allowed us to adapt the "polymer growth and particle expansion model" to the copolymerization. Besides physical causes for the comonomer effect, we wanted to determine whether the catalyst structure plays an important role in the comonomer effect. To this end we investigated two metallocenes bearing the same long bridging unit but differing in the ligand bound to the zirconium center. One metallocene bears a cyclopentadienyl ring, while the other bears an indenyl group. From a close analysis of the 13C NMR, it is clear that both catalysts insert ethylene more easily then propylene, probably due to the long bridging unit that results in a narrower aperture angle of the ligand. In addition to this, the indenyl ligand does not allow the formation of propylene blocks even at high propylene concentration. Snapshot of the polymer particles taken after 165 min of ethylene-1-butene copolymerization with catalyst 1. [source] Defined Poly[styrene- block -(ferrocenylmethyl methacrylate)] Diblock Copolymers via Living Anionic PolymerizationMACROMOLECULAR RAPID COMMUNICATIONS, Issue 17 2009Markus Gallei Abstract Ferrocenylmethyl methacrylate (FMMA) is one of the very few metallocene-based monomers that are promising candidates for truly living anionic polymerization. Nevertheless, FMMA homopolymers with a narrow polydispersity, or block copolymerization studies that result in satisfying blocking efficiencies, are unknown so far. Here we describe a procedure that leads to highly regular FMMA-based polymers for the first time, characterized by polydispersity indices (PDI) of less that 1.05 and very high blocking efficiencies (>95%) in sequential copolymerization with styrene. Some of the obtained poly[styrene- block -(ferrocenylmethyl methacrylate)]s show unusual microphase morphologies, presumably the consequence of high Tgs causing ,frustrated' non-equilibrium states. [source] Novel Polyolefins Containing Crystallizable Isotactic Polystyrene Side ChainsMACROMOLECULAR RAPID COMMUNICATIONS, Issue 18 2008Barbara T. Gall Abstract Vinyl-terminated isotactic polystyrene macromonomers were copolymerized with ethylene and 1-octene to new polyolefin graft copolymers with crystallizable polystyrene side chains. The iPS content was varied from 4 to 78 wt.-% using different metallocene and halfsandwich catalysts. No solubility problems of the stereoregular macromonomer occured, because iPS does not crystallize during polymerization. In addition the polymers show nanophase separation of the polyolefin backbone and the iPS side chains. The crystallization of iPS, having a melting point of about 200,°C, can be achieved by annealing. This synthesis strategy allows the fabrication of a new class of thermoplastic elastomers with improved heat distortion temperatures. [source] Supported (nBuCp)2ZrCl2 Catalysts: Effects of Selected Lewis Acid Organotin Silica Surface Modifiers on Ethylene PolymerizationMACROMOLECULAR REACTION ENGINEERING, Issue 4 2008Muhammad N. Akhtar Abstract This study investigated the effects of several organotin silica surface modifiers on the ethylene polymerization performance of (nBuCp)2ZrCl2 -based supported catalysts in which MAO and metallocene were sequentially loaded. Each organotin compound acted as a spacer, increasing the catalyst activity. However, the catalyst activity and of the resulting polyethylenes varied as follows: Activity and fractional Sn+ charge: nBuSn(OH)2Cl,>,MeSnCl3,>,nBuSnCl3,>,Reference catalyst; and, : Reference catalyst,>,nBuSnCl3,>,MeSnCl3,>,nBuSn(OH)2Cl. The above catalyst activity rating was explained considering the influence of the Lewis acidity, that is, the fractional Sn+ charge of the organotin modifiers on the generation, concentration, and electron density at the active [(nBuCp)2ZrMe]+ cation. All the catalysts showed fairly stable kinetic profiles and produced narrow molecular weight distribution resins; 2.8,,,PDI,,,3. [source] Characterization of Ethylene-1-Hexene Copolymers Made with Supported Metallocene Catalysts: Influence of Support TypeMACROMOLECULAR SYMPOSIA, Issue 1 2007Beatriz Paredes Abstract Summary: It is known that the nature of the support, as well as the technique used to anchor the metallocene onto it, play important roles on catalytic activity and on the properties of the polymers produced with supported metallocenes. In the present work, the effect of different support types on the microstructure of ethylene/1-hexene copolymers made with supported metallocene catalysts has been investigated through the analysis of molecular weight and chemical composition distributions using high temperature gel permeation chromatography (GPC) and crystallization analysis fractionation (Crystaf). The copolymer samples obtained using commercial carriers (silica and silica-alumina) had unimodal chemical composition distributions and were used to create a linear calibration curve relating the peak crystallization temperature from Crystaf and the comonomer content as determined by 13C NMR. This calibration curve is useful to determine the 1-hexene fractions for each peak in the resins showing bimodal chemical composition distributions, such as those obtained with catalysts supported on MCM-41 and SBA-15 materials. The structure and chemistry of the support used had a large influence on comonomer incorporation and the shape of the chemical composition distribution of the polymer, which suggests that the supporting process creates different types of active sites. [source] Fluorenyl based syndiotactic specific metallocene catalysts structural features, origin of syndiospecificityMACROMOLECULAR SYMPOSIA, Issue 1 2004Abbas Razavi Abstract The stereochemistry of propylene insertion/propagation reactions with a variety of Cs symmetric fluorenyl- containing single site catalysts is discussed. Our recent results indicate that independent of the chemical composition of the ancillary ligand fragments, or nature of the transition metal, active sites with local Cs symmetry and enantiotopic coordination positions behave syndioselectively in the general context of chain migratory insertion mechanism. Perfect bilateral symmetry neither exists nor is required in these processes. In this context the mechanism of syndiospecific polymerization is revisited by taking into account the structural characteristics and catalytic behavior of the original metallocene based (,5 -C5H4 -CMe2 -,5 -C13H8) MCl2/ MAO; M = Zr (1), Hf (2) catalyst systems and new syndiotactic specific systems including (,5 -C5H4 -CPh2-,5-3,6-di-tBut-C13H6)ZrCl2 (3), ,1,,5 -(,Me2Si)(3,6-di-tBut-Flu)(t-ButN)MCl2/ MAO; M =Ti (4), Zr (5) and ,1,,5 -(,Me2Si)(2,7-di-tBut-Flu)(t-ButN)MCl2/ MAO; M = Ti (6), Zr (7). [source] Processing characteristics and mechanical properties of metallocene catalyzed linear low-density polyethylene foams for rotational moldingPOLYMER ENGINEERING & SCIENCE, Issue 4 2004E. Archer The object of this work is to assess the suitability of metallocene catalyzed linear low-density polyethylenes for the rotational molding of foams and to link the material and processing conditions to cell morphology and part mechanical properties (flexural and compressive strength). Through adjustments to molding conditions, the significant processing and physical material parameters that optimize metallocene catalyzed linear low-density polyethylene foam structure have been identified. The results obtained from an equivalent conventional grade of Ziegler-Natta catalyzed linear low-density polyethylene are used as a basis for comparison. The key findings of this study are that metallocene catalyzed LLDPE can be used in rotational foam molding to produce a foam that will perform as well as a Ziegler-Natta catalyzed foam and that foam density is by far the most influential factor over mechanical properties of foam. Polym. Eng. Sci. 44:638,647, 2004. © 2004 Society of Plastics Engineers. [source] Gross melt fracture of polyethylene.POLYMER ENGINEERING & SCIENCE, Issue 3 2002II: Effects of molecular structure The effect of molecular structure (MW, MWD and LCB) on the critical tensile stress (,c) for the onset of gross melt fracture (OGMF), proposed in Part I (1) as a material-dependent criterion for fracture, was determined for a group of polyethylenes varying in structure. These included linear low and high-density polyethylenes and several materials produced using metallocene and constrained geometry catalysts. It was found that the critical stress is independent of MW, for constant polydispersity but increases with increasing long chain branching and polydispersity. The addition of boron nitride particles had no effect on the ,c up to a level of 0.5% by weight. [source] Propylene polymerization in a semibatch reactor.POLYMER ENGINEERING & SCIENCE, Issue 12 2001Analysis of soluble metallocene catalyst behavior through reactor modeling We study the process involved in metallocene activation and further propylene polymerization. In this paper, we begin by analyzing the behavior of soluble metallocene in propylene polymerization before advancing to the study of the heterogeneous polymerization. Experimental data obtained in a semibatch laboratory polymerization reactor using ethylenbisindenylzirconium dichloride (EtInd2ZrCl2)/ methylaluminoxane (MAO) are combined with a mathermatical model providing useful information such as number of active sites and their activation patterns. We present a mathematical model for the reactor that predicts not only reactor productivity but also the molecular properties of the product. We apply the model to soluble systems in order to find the optimal parameters for the catalyst itself and in the presence of different types of additives such as aluminum chloride (AlCl3) and ethyl benzoate (E.B.). [source] Styrene/substituted styrene copolymerization by Ph2Zn,metallocene,MAO systems: homo- and copolymerization of p -methoxystyrene with styrene,POLYMER INTERNATIONAL, Issue 5 2008Franco M Rabagliati Abstract BACKGROUND: The present work is part of a general study regarding the homo- and copolymerization of styrene using diphenylzinc,additive initiator systems, with the aim of improving the properties of commercial atactic polystyrene. The study is focused on syndiotactic polystyrene and/or copolymers of styrene (S) with substituted styrene, styrene derivatives or various ,-olefins. This research has been ongoing over the last 15 years. RESULTS: The reported experiments show that binary metallocene,methylaluminoxane (MAO) and ternary Ph2Zn,metallocene,MAO, depending on the metallocene employed, are capable of inducing both homo- and copolymerization of styrene and p -methoxystyrene (p -MeOS). The results indicate that for a styrene/p -MeOS mole ratio with p -MeOS > 25% the product obtained has only a minor incorporation of styrene units. The efficiency of the metallocenes studied follows the order bis(n -butylcyclopentadienyl)titanium dichloride ((n -BuCp)2TiCl2) > indenyltitanium trichloride (IndTiCl3) > Cp2TiCl2. CONCLUSION: Metallocenes (n -BuCp)2TiCl2, Cp2TiCl2 and IndTiCl3 in binary systems combined with MAO, as well as in ternary systems combined with Ph2Zn and MAO, induce the homopolymerization of p -MeOS and its copolymerization with styrene. The styrene/p -MeOS copolymer obtained was enriched in p -MeOS with respect to the initial feed, in agreement with the I+ inductive effect of the methoxy group in the para position of styrene. As already reported, the role of Ph2Zn was nullified by its complexation with the p -MeOS comonomer. Copyright © 2008 Society of Chemical Industry [source] Styrene,substituted-styrene copolymerization using diphenylzinc,metallocene,methylaluminoxane systemsPOLYMER INTERNATIONAL, Issue 8 2006Franco M Rabagliati Abstract Homopolymerization of disubstituted styrenes (2,4- and 2,5-dimethylstyrene) and trisubstituted styrene (2,4,6-trimethylstyrene) and their copolymerization with styrene were carried out using diphenylzinc,metallocene,methylaluminoxane initiator systems for metallocene (n -BuCp)2TiCl2 and for half-metallocene CpTiCl3. The studied comonomers were found to be less reactive than p -tertbutylstyrene, p -methylstyrene and styrene. The results indicate that, even though the methyl group has I+ inductive effect, di- and tri-methylstyrenes are reluctant to undergo either homopolymerization or copolymerization. This behavior suggests that the reactivity is regulated not only by the inductive effect of the alkyl group but also by the steric impediment caused by the crowding of the substituents on the benzene ring. Copyright © 2006 Society of Chemical Industry [source] Short communication: A thermochromism study in the ferrocenyl-benzopyran seriesAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 2 2004Stéphane Anguille Abstract The synthesis and the photochromic behaviour of a series of benzopyrans (otherwise called 2H -chromenes), substituted in the 2-position by a metallocene, have been previously described. Some of these compounds also present thermochromic properties. As the thermochromism has been scarcely studied in the benzopyran series, we describe here a study of the thermochromic properties of ferrocenyl benzopyrans. The correlations between the structure of the compounds investigated and their thermochromic properties are discussed. Copyright © 2004 John Wiley & Sons, Ltd. [source] The Reductive Elimination of Methane from ansa -Hydrido(methyl)metallocenes of Molybdenum and Tungsten: Application of Hammond's Postulate to Two-State ReactionsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 15 2005José-Luis Carreón-Macedo Abstract The energetic profile of the methane reductive elimination from a selected number of hydrido(methyl)molybdenocene and -tungstenocene derivatives has been calculated by DFT methods. The calculations were carried out for the CH2(C5H4)2M (a -M), SiH2(C5H4)2M (a -H2Si,M), and SiMe2(C5Me4)2M (a -Me2Si,M*) ansa -metallocene systems for M = Mo, W. They include the full optimization of minima [the hydrido(methyl) starting complexes, M(H)(CH3), the intermediate methane complexes, M(CH4), and the metallocene products in the singlet and triplet configurations, (3M and 1M)], transition states (for the methyl hydride reductive elimination, M,TSins, and for the hydrogen exchange, M,TSexch), and the minimum energy crossing point (M,MECP) leading from the singlet methane complexes to the corresponding triplet metallocenes. The results are compared with those previously obtained for the simpler (C5H5)2M (Cp2M) systems (J. C. Green, J. N. Harvey, and R. Poli, J. Chem. Soc., Dalton Trans.2002, 1861). The calculated energy profiles, notably the relative energies of M,TSins and M,MECP, are in agreement with available experimental observations for the a -Me2Si,M* systems. The comparison of the energies and geometries of the rate-determining M,TSins and M,MECP structures with those of the thermodynamically relevant minima for the various systems show the applicability of Hammond's postulate to two-state reactions. However, one notable exception serves to show that the principle is only quantitatively reliable when all the potential energy surfaces for the set of analogous reactions have similar shapes. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Orbital Splitting and Pairing Energy in Open-Shell Organometallics: A Study of Two Families of 16-Electron Complexes [Cp2M] (M = Cr, Mo, W) and [CpM(PH3)] (M = Co, Rh, Ir)EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 12 2005Rinaldo Poli Abstract The singlet,triplet gap for two families of 16-electron organometallic complexes has been examined in detail by DFT calculations at the B3LYP level with polarized basis sets on both metal and ligands. For the first family, the group 6 metallocenes (Cp2M with Cp = ,5 -C5H5 and M = Cr, Mo, W), the singlet,triplet gap (ES , ET) is always positive and decreases continuously on going from Cr to Mo to W. For the family of group 9 CpM(PH3), on the other hand, there is a decrease on going from Co to Rh, followed by a slight increase on going further to Ir. These trends have been analyzed in qualitative monoelectronic terms as a function of the competition between the pairing energy and the orbital gap. While the pairing energy decreases as expected in the order 3d >> 4d > 5d, the orbital gap varies in a different way for the two families and, though quantitatively less important, is responsible for the different trends. It is argued that changes in orbital gap are system-dependent for open-shell organometallic systems, thus it is not possible to establish a universal trend of singlet,triplet gaps for a homologous series of complexes with a group of transition metals. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] The Extraordinary Cocatalytic Action of Polymethylaluminoxane (MAO) in the Polymerization of Terminal Olefins by Metallocenes: Chemical Change in the Group 4 Metallocene Dimethyl Derivatives Induced by MAO,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 20 2005John J. Eisch Abstract In the polymerization of olefins with Group 4 metallocene dichlorides or dimethyl derivatives as procatalysts the use of polymethylaluminoxane (MAO) as the cocatalyst, especially in extreme excess (102,103 times the metallocene equivalent), has been shown to have an extraordinary accelerating effect on the rate of olefin polymerization, when compared with the cocatalytic action of alkylaluminum halides. In attempts at explaining the greatly superior catalytic activity of MAO in olefin polymerization (the MAO conundrum), hypotheses have generally paralleled the steps involved in the cocatalytic action of RnAlCl3,n, namely the alkylation of Cp2MtCl2, ionization of Cp2Mt(R)Cl into the metallocenium cation, [Cp2Mt,R]+, and anion, [Rn,1AlCl4,n], and subsequent ion-pair separation. In order to understand any differences in catalytic action between such cocatalysts, we have studied the individual action of MAO (100 equiv.) and of MeAlCl2 (1,2 equiv.) on each of the Group 4 metallocene derivatives, Cp2TiCl2, Cp2ZrCl2, Cp2Ti(CH3)2 and Cp2Zr(CH3)2. With MeAlCl2 each of the metallocene derivatives appeared to form the cation, [Cp2Mt,CH3]+, with greater (Ti) or lesser (Zr) ease, because an alkyne such as diphenylacetylene was then found to insert into the Mt,CH3 bond stereoselectively. In striking contrast, treatment of each metallocene with MAO gave two reactions very different from MeAlCl2, namely a steady evolution of methane gas upon mixing and a finding upon hydrolytic workup that the diphenylacetylene present had undergone no insertion into the Mt,CH3 bond but instead had been reductively dimerized completely to (E,E)-1,2,3,4-tetraphenyl-1,3-butadiene. To account for this astonishing difference in chemical behavior between MAO and MeAlCl2 in their cocatalytic activation of Group 4 metallocenes to olefin polymerization, it is necessary to postulate a novel, unique sequence of reaction steps occurring between MAO and the metallocene. If one starts with the metallocene dichloride, then the free TMA present in the MAO would generate the Cp2Mt(CH3)2. This metallocene dimethyl derivative, complexed with an oligomeric MAO unit, would undergo a transfer-epimetallation with added olefin or acetylene to form a metallacyclopropane or metallacyclopropene, respectively. With added diphenylacetylene the resulting 2,3-diphenylmetallacyclopropene would be expected rapidly to insert a second alkyne to form the 2,3,4,5-tetraphenyl-1-metallacyclopentadiene. Simple hydrolysis of the latter intermediate would generate (E,E)-1,2,3,4-tetraphenyl-1,3-butadiene while alternative workup with D2O would give the 1,4-dideuterio derivative of this butadiene. Both such expectations were confirmed by experiment. In the case of added olefin, similar metallacyclopropane and metallacyclopentane intermediates should be produced until ring opening of the latter five-membered ring leads to an open-chain zwitterion, a process having ample precedent in the research of Gerhard Erker. The solution to the MAO conundrum then, namely the extraordinary cocatalytic activity of MAO in olefin polymerization by metallocenes, lies in the unique catalytic activation of the Group 4 metallocene dimethyl derivative, which occurs by transfer-epimetallation of the olefin monomer by the Cp2Mt(CH3)2,MAO complex. The most advantageous Lewis acidic sites in the MAO,oligomeric mixture for such metallocene,MAO complexation are suggested to be terminal Me2Al,O,AlMe, segments of an open-chain oligomer. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Catalytic activity during the preparation of PE/clay nanocomposites by in situ polymerization with metallocene catalystsJOURNAL OF APPLIED POLYMER SCIENCE, Issue 4 2009Paula Zapata Abstract Catalytic activity during the formation of polyethylene (PE)/clay nanocomposites by in situ polymerization with metallocenes was studied. Ethylene polymerization was carried out with the homogeneous metallocene in the presence of the clay particles and using the clay-supported metallocene catalyst. It was found that the catalytic activity of the homogeneous metallocene does not decrease in the presence of the clay particles and only a slight decrease of activity occurs using the clay-supported catalyst. The modification of the clay with MAO cocatalyst as well as its intercalation with ODA surfactant were found to play an important role during the in situ formation of the PE/clay nanocomposite. ODA-intercalated clay apparently facilitates the activation and monomer insertion processes on zirconocene centers located in internal sites of the clay structure. Although metallocene supported on MAO-treated clay exhibited somewhat lower catalytic activity than that supported directly on the ODA-intercalated clay, both systems favored the production of PE nanocomposites containing highly exfoliated clay particles. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009 [source] Styrene/1,3-butadiene copolymerization by C2 -symmetric group 4 metallocenes based catalystsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 4 2008Mariagrazia Napoli Abstract C2 -symmetric group 4 metallocenes based catalysts (rac -[CH2(3- tert -butyl-1-indenyl)2]ZrCl2(1), rac -[CH2(1-indenyl)2]ZrCl2(2) and rac -[CH2(3- tert -butyl-1-indenyl)2]TiCl2(3)) are able to copolymerize styrene and 1,3-butadiene, to give products with high molecular weight. In agreement with symmetry properties of metallocene precatalysts, styrene homosequences are in isotactic arrangements. Full determination of microstructure of copolymers was obtained by 13C NMR and FTIR analysis and it reveals that insertion of butadiene on styrene chain-end happens prevailingly with 1,4- trans configuration. In the butadiene homosequences, using zirconocene-based catalysts, the 1,4- trans arrangement is favored over 1,4- cis, but the latter is prevailing in the presence of titanocene (3). Diad composition analysis of the copolymers makes possible to estimate the reactivity ratios of copolymerization: zirconocenes (1) and (2) produced copolymers having r1 × r2 = 0.5 and 3.0, respectively (where 1 refers to styrene and 2 to butadiene); while titanocene (3) gave tendencially blocky styrene,butadiene copolymers (r1 × r2 = 8.5). The copolymers do not exhibit crystallinity, even when they contain a high molar fraction of styrene. Probably, comonomer homosequences are too short to crystallize (ns = 16, in the copolymer at highest styrene molar fraction). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1476,1487, 2008 [source] Stereoregular P(MMA)-clay nanocomposites by metallocene catalysts: In situ synthesis and stereocomplex formationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 13 2007Wesley R. Mariott Abstract This contribution reports the synthesis and characterization of stereochemically controlled, as well as crystalline stereocomplex, P(MMA)-clay nanocomposites using metallocene complexes and alane-intercalated clay activators. The ligand elimination and exchange reactions involving Lewis acids E(C6F5)3 (E = Al, B) and an organically modified montmorillonite clay were employed to synthesize the alane-intercalated clay activators. When combined with dimethyl metallocenes of various symmetries, these clay activators brought about efficient MMA polymerizations leading to in situ polymerized, stereochemically controlled P(MMA)-intercalated clay nanocomposites. The most noticeable thermal property enhancement observed for the clay nanocomposite P(MMA), when compared with the pristine P(MMA) having similar molecular weight and stereomicrostructure, has a considerable increase in Tg (,10 °C). Mixing of dilute THF solutions of two diastereomeric nanocomposites in a 1:2 isotactic to syndiotactic ratio, followed by reprecipitation or crystallization procedures, yielded unique double-stranded helical stereocomplex P(MMA)-clay nanocomposites with a predominantly exfoliated clay morphology. Remarkably, the resulting crystalline stereocomplex P(MMA) matrix is resistant to the boiling-THF extraction and its clay nanocomposites exhibit high Tm of 201 to 210 °C. Furthermore, the stereocomplex P(MMA)-clay nanocomposite shows a one-step, narrow decomposition temperature window and a single, high maximum rate decomposition temperature of 377 °C. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2581,2592, 2007 [source] Video Microscopy for the Investigation of Gas Phase CopolymerizationMACROMOLECULAR MATERIALS & ENGINEERING, Issue 11 2005Daniela Ferrari Abstract Summary: Video microscopy as a tool for investigating olefin gas phase copolymerization is presented for the first time in this paper. The central theme of this work is the study of the comonomer effect shown by an unbridged metallocene catalyst supported on silica. By using video microscopy, it is possible to observe the increase in catalytic activity in terms of particle growth as well as monomer consumption. The observation that a more pronounced induction period in the particle growth profile is shown with increasing propylene concentration led us to investigate the copolymers obtained at different polymerization times using 13C NMR analysis and single particle energy dispersive X-ray (EDX mapping). This allowed us to adapt the "polymer growth and particle expansion model" to the copolymerization. Besides physical causes for the comonomer effect, we wanted to determine whether the catalyst structure plays an important role in the comonomer effect. To this end we investigated two metallocenes bearing the same long bridging unit but differing in the ligand bound to the zirconium center. One metallocene bears a cyclopentadienyl ring, while the other bears an indenyl group. From a close analysis of the 13C NMR, it is clear that both catalysts insert ethylene more easily then propylene, probably due to the long bridging unit that results in a narrower aperture angle of the ligand. In addition to this, the indenyl ligand does not allow the formation of propylene blocks even at high propylene concentration. Snapshot of the polymer particles taken after 165 min of ethylene-1-butene copolymerization with catalyst 1. [source] | |||||||||||||