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Avaliação do desempenho de catalisadores híbridos Ziegler-Natta/Metaloceno na homo- e copolimerização de etileno; The Performance Evaluation of Hybrid Ziegler-Natta/Metallocene Catalysts in Homo- and Copolymerization of Ethylene

Cunha, Fernanda O. Vieira da; Forte, Maria Madalena de Camargo; Zacca, Jorge Jardim
Fonte: Universidade Federal do Rio Grande do Sul Publicador: Universidade Federal do Rio Grande do Sul
Tipo: Artigo de Revista Científica Formato: application/pdf
POR
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Foram preparados catalisadores híbridos a partir de um catalisador do tipo Ziegler-Natta heterogêneo (MgCl2.TiCl4) e de um metalocênico (CpTiCl3), afim de se obter catalisadores com diferentes tipos de centros ativos. Foi proposta a formação de um novo centro ativo (CpTi(iBu)Cl2) na face 100 do suporte de MgCl2. Estes catalisadores foram avaliados na síntese de polietileno de alta densidade (PEAD) e de polietileno linear de baixa densidade (PELBD), utilizando-se somente trietilalumínio (TEA) como cocatalisador. Foi verificada a influência da razão molar Al/Ti, temperatura e tempo de polimerização na produtividade dos catalisadores e nas propriedades dos polímeros obtidos. Os catalisadores mistos apresentaram atividade catalítica da ordem de 5,0 kgPol/gcat.h na polimerização de etileno e da ordem de 7,0 kgPol/gcat.h na copolimerização etileno-1-buteno.; Hybrid catalysts formed by a heterogeneous Ziegler-Natta catalyst (MgCl2.TiCl4) and a metallocene (CpTiCl3) were prepared in this work. The formation of a new catalytic species CpTi(iBu)Cl2 on the (100) MgCl2 face has been proposed. These catalyst systems were applied to synthesize high density polyethylene (HDPE) and linear low density polyethylene (LLDPE), using only triethyl aluminium (TEA) as a cocatalyst. The influence of Al/Ti molar ratio...

Catalisadores metalogênicos suportados para a produção de poliolefinas : revisão das estratégias de imobilização; Supported metallocene catalysts for polyolefin production : review of the immobilization strategies

Fisch, Adriano Giraldi; Cardozo, Nilo Sérgio Medeiros; Secchi, Argimiro Resende; Santos, Joao Henrique Zimnoch dos
Fonte: Universidade Federal do Rio Grande do Sul Publicador: Universidade Federal do Rio Grande do Sul
Tipo: Artigo de Revista Científica Formato: application/pdf
ENG
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The inadequacy of strategies used for the heterogeneization of metallocene catalysts is pointed out as one of the main causes of the lack of industrial employability of such polymerization catalysts. The main problems are the necessity of large quantity of MAO (cocatalyst) and the inability to control molecular mass distribution of the polymers. Based on this background, the main strategies for the heterogeneization of metallocenes are here reviewed. The advantages and disadvantages of each strategy are presented and discussed on theoretical and practical perspective. Considering the results reported on the different researches, outcomes of heterogeneization strategies are pointed out.

Thermal and rheological behavior of reactive blends from metallocene olefin elastomers and polypropylene

Domingues Junior, Nei Sebastiao; Forte, Maria Madalena de Camargo; Riegel, Izabel Cristina
Fonte: Universidade Federal do Rio Grande do Sul Publicador: Universidade Federal do Rio Grande do Sul
Tipo: Artigo de Revista Científica Formato: application/pdf
POR
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Reactive blends of metallocene polyolefin elastomers (POE)/polypropylene (PP) with 60/40 composition were prepared with an organic peroxide, 2,5-dimethyl-2,5-di-(t-butylperoxy)hexane, and a bis-azide derivative, diphenyloxid-4,4’-bis(sulfonylazide) (BSA). Ethylene-1-butene (EB) and ethylene-1-octene (EO) copolymers and elastomeric polypropylene (ePP) were used as the elastomeric phase. The effect of elastomeric phase on the thermal, rheological, morphological and mechanical properties of the thermoplastic vulcanizates (TPVs) or dynamic vulcanizates were studied. All TPVs depicted pseudoplastic behavior and blends cured with azide curative showed higher viscosities. The TPVs showed both dispersed and continuous phase morphology that depends on the elastomeric phase type revealing a limited degree of compatibility between PP and the elastomers EO or EB. On the other hand, the TPV PP/ePP showed a uniform morphology suggesting an improved compatibility. Substantial changes observed in physical properties were explained on the basis of blends’ morphology and dynamic vulcanization. The results confirm that the mechanical properties are more influenced by the elastomeric phase than by the curative agent. This study revealed a broad new range of opportunities for POE-based TPVs.

Metallocene Supported on Inorganic Solid Supports: an Unfinished History

Ouros,Aline C. dos; Souza,Michèle O. de; Pastore,Heloise O.
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/12/2014 EN
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The production of polyolefin nanocomposites has been the field of many studies for improving polymer properties for many applications. The most important challenge on the area is the preparation of the fillers randomly dispersed on polymer matrix. It is general agreement, however, that the dispersion obtained through in situ polymerization is the most efficient and, additionally, confers to the system the advantages of heterogeneous catalysis. This contribution offer a review of characteristics of the most employed fillers, their use as metallocene catalysts supports and the application on in situ production of nanocomposites.

The role of defect microstructure in the crystallization behavior of metallocene and MgCl2-supported Ziegler-Natta isotactic poly(propylenes)

Alamo,Rufina G.
Fonte: Associação Brasileira de Polímeros Publicador: Associação Brasileira de Polímeros
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/12/2003 EN
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The intermolecular defect distribution of poly(propylenes) of the Ziegler-Natta (ZN) and metallocene (M) types is assessed by classical fractionation and analysis of the fractions by GPC and 13C NMR. In addition, the linear growth rates, a property sensitive to differences in defects distribution in the poly(propylene) chain, are used to infer the stereoblock-like intramolecular distribution of defects in industrial type ZN iPPs and their fractions. The behavior of fractions from a matched metallocene iPP provide direct evidence of the single-site nature of the catalyst. Metallocene poly(propylenes) enable a quantitative assessment of the molecular, thermodynamic and kinetic factors that govern the formation and concentrations of the a and g polymorphs. Short continuous crystallizable sequences and high crystallization temperatures favor the formation of this polymorph in homo poly(propylenes) and random propylene copolymers. Differences in the partitioning of the comonomer between the crystalline and non-crystalline regions leads to contents of the gamma phase that differ among the copolymers at any given crystallization temperature. Qualitatively these differences can be used to assess the degree to which a counit participates in the crystallite.

Ansa-metallocene polymerization catalysts derived from [2+2]cycloaddition reactions of bis(1-methylethenyl-cyclopentadienyl)zirconium systems

Paradies, Jan; Kehr, Gerald; Fröhlich, Roland; Erker, Gerhard
Fonte: National Academy of Sciences Publicador: National Academy of Sciences
Tipo: Artigo de Revista Científica
EN
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Bis(1-methylethenyl-cyclopentadienyl)zirconium dichloride (7a) was prepared by a fulvene route. Photolysis at 0°C with Pyrex-filtered UV light resulted in a rapid and complete intramolecular [2+2]cycloaddition reaction to yield the corresponding cyclobutylene-bridged ansa-zirconocene dichloride isomer (8a). This is one of the rare examples of an organic functional group chemistry that leads to carbon–carbon coupling at the framework of an intact sensitive group 4 bent metallocene complex. More sterically hindered open metallocenes that bear bulky isopropyl or tert-butyl substituents at their Cp rings in addition to the active 1-methylethenyl functional group undergo the photochemical ansa-metallocene ring closure reaction equally facile. The metallocene systems used and obtained in this study have served as transition metal components for the generation of active metallocene propene polymerization catalysts.

Metallocene Antimalarials: The Continuing Quest

Blackie, Margaret A. L.; Chibale, Kelly
Fonte: Hindawi Publishing Corporation Publicador: Hindawi Publishing Corporation
Tipo: Artigo de Revista Científica
EN
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Over the last decade, a significant body of research has been developed around the inclusion of a metallocene moiety into known antimalarial compounds. Ferroquine is the most successful of these compounds. Herein, we describe our contribution to metallocene antimalarials. Our approach has sought to introduce diversity sites in the side chain of ferroquine in order to develop a series of ferroquine derivatives. The replacement of the ferrocenyl moiety with ruthenocene has given rise to ruthenoquine and a modest series of analogues. The reaction of ferroquine and selected analogues with Au(PPh3)NO3, Au(C6F5)(tht), and [Rh(COD)Cl2] has resulted in a series of heterobimetallic derivatives. In all cases, compounds have been evaluated for in vitro antiplasmodial activity in both chloroquine-sensitive and chloroquine-resistant strains of Plasmodium falciparum. Preliminary structure-activity relationships have been delineated.

Electronic Structure of the d1 Bent-metallocene Cp2VCl2: A Photoelectron and Density Functional Study

Cranswick, Matthew A.; Gruhn, Nadine E.; Enemark, John H.; Lichtenberger, Dennis L.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 15/04/2008 EN
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The Cp2VCl2 molecule is a prototype for bent metallocene complexes with a single electron in the metal d shell, but experimental measure of the binding energy of the d electron by photoelectron spectroscopy eluded early attempts due to apparent decomposition in the spectrometer to Cp2VCl. With improved instrumentation, the amount of decomposition is reduced and subtraction of ionization intensity due to Cp2VCl from the Cp2VCl2/Cp2VCl mixed spectrum yields the Cp2VCl2 spectrum exclusively. The measured ionization energies provide well-defined benchmarks for electronic structure calculations. Density functional calculations support the spectral interpretations and agree well with the ionization energy of the d1 electron and the energies of the higher positive ion states of Cp2VCl2. The calculations also account well for the trends to the other Group V bent metallocene dichlorides Cp2NbCl2 and Cp2TaCl2. The first ionization energy of Cp2VCl2 is considerably greater than the first ionization energies of the second- and third-row transition metal analogues.

Hafnium metallocene compounds used as cathode interfacial layers for enhanced electron transfer in organic solar cells

Park, Keunhee; Oh, Seungsik; Jung, Donggeun; Chae, Heeyeop; Kim, Hyoungsub; Boo, Jin-Hyo
Fonte: Springer Publicador: Springer
Tipo: Artigo de Revista Científica
Publicado em 09/01/2012 EN
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We have used hafnium metallocene compounds as cathode interfacial layers for organic solar cells [OSCs]. A metallocene compound consists of a transition metal and two cyclopentadienyl ligands coordinated in a sandwich structure. For the fabrication of the OSCs, poly[3,4-ethylenedioxythiophene]:poly(styrene sulfonate), poly(3-hexylthiophene-2,5-diyl) + [6,6]-phenyl C61 butyric acid methyl ester, bis-(ethylcyclopentadienyl)hafnium(IV) dichloride, and aluminum were deposited as a hole transport layer, an active layer, a cathode interfacial layer, and a cathode, respectively. The hafnium metallocene compound cathode interfacial layer improved the performance of OSCs compared to that of OSCs without the interfacial layer. The current density-voltage characteristics of OSCs with an interfacial layer thickness of 0.7 nm and of those without an interfacial layer showed power conversion efficiency [PCE] values of 2.96% and 2.34%, respectively, under an illumination condition of 100 mW/cm2 (AM 1.5). It is thought that a cathode interfacial layer of an appropriate thickness enhances the electron transfer between the active layer and the cathode, and thus increases the PCE of the OSCs.

Characterization of the Zr-polymeryl Species Present during Ethylene and Propylene Polymerizations by Homogeneous Zirconocene Catalysts

Vatamanu, MIHAELA
Fonte: Quens University Publicador: Quens University
Tipo: Tese de Doutorado
EN; EN
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A detailed investigation of Zr-polymeryl intermediates, present during ethylene and propylene polymerization by metallocene and non-metallocene catalysts, is presented. In this regard, the potential use of bromine as a quenching agent to give polymers with brominated end groups is investigated. Using NMR spectroscopy, these groups can be identified as primary or secondary and quantified, thus providing information on the nature of the Zr-polymeryl linkage(s) and their relative concentration(s). Bromine quenching was examined for ethylene polymerization by Cp2ZrMe2/B(C6F5)3, and for propylene polymerization by Cp2ZrMe2/B(C6F5)3, (indenyl)2ZrMe2/B(C6F5)3, and rac-{C2H4(1-indenyl)2}ZrMe2/B(C6F5)3. Polymers with primary alkyl bromide end groups were obtained. In the case of propylene polymerization by [ONNO]Zr(CH2C6H5)2/MAO/2,6-di-tbutylphenol, both primary and secondary alkyl bromide end groups were detected. Primary alkyl bromide end groups result from the cleavage by bromine of primary Zr-CH2~polymeryl linkages while secondary alkyl bromides are the result of the cleavage by bromine of secondary Zr-CH(Me)CH2~polymeryl linkages. The concentration of Zr-CH2~polymeryl intermediates was found to be low, suggesting that the catalyst deactivates during the polymerization. A second objective of this research is to investigate the formation of Cp2Zr+-allyl complexes as these species are proposed to arise during the polymerization and deactivate the catalyst. Compounds of type [Cp2ZrMe]+ were found to react with vinylidene compounds of type CH2=C(Me)R...

The Effects of Silica Support on Kinetic Behavior and Polymer Properties of Heterogonous Metallocene Catalyst

ASHRI, ABDULRAHMAN
Fonte: Quens University Publicador: Quens University
Tipo: Tese de Doutorado
EN; EN
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The heterogeneous metallocene catalyst is becoming a very competitive industrially due to its ability to produce tailor-made polymers. The main advantage of the metallocene polymer product is the narrow molecular weight distribution (MWD) and the systematic comonomer distribution along the polymer chains. Therefore, the metallocene polymer product has well-defined mechanical and optical properties. The aim of this thesis is to investigate the effects of the silica support on the reaction kinetics and micro properties of the heterogeneous metallocene catalyst system. These investigations include studying the influence of the pore volume, surface area, particle size distribution, and the surface chemical characteristics of silica support on the catalyst performance. The experiments showed that the silica type has an influence on the kinetic behavior. For instance, silica with a lower pore volume shows an induction period when compared with higher pore volume silicas. Moreover, the silica type has a clear influence on catalyst activity and polymer morphology. The smallest silica particles produced the highest activity among the other sizes regardless of silica type. The supported catalysts were characterized and linked to the silica type and size in terms of catalyst activity and polymer morphology. Each catalyst in terms of silica type behaved similarly regardless of type of alkylaluminum used in the formulation. The micro properties of the produced polymers...

Metallocene Polypropylene Crystallization Kinetic During Cooling in Rotational Molding Thermal Condition

SARRABI, Salah; BOYER, S;A.E; LACRAMPE, Marie-France; KRAWCZAK, P; TCHARKHTCHI, Abbas
Fonte: Wiley Publicador: Wiley
EN
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This article is part of an ambitious project. The aim is to simulate mechanical properties of rotomolded part from micro-structure consideration. Main objective here is to consider metallocene polypropylene crystallization kinetic (PP) during cooling stage in rotational molding. Crystallization kinetic of metallocene PP is so rapid that microscopy cannot help to observe nucleation and growth. Crystallization rate can be estimated by a global kinetic. Given that cooling in rotational molding is dynamic with a constant rate, Ozawa law appears more appropriate. Ozawa parameters have been estimated by differential scanning calorimetry. In rotational molding thermal condition, Avrami index identifies a complex nucleation intermediate between spontaneous and sporadic. Ozawa rate constant is 68 times higher than this obtained for Ziegler–Natta PP. By coupling transformation rate from Ozawa model and a thermal model developed earlier, the difference between theory and experimental is less than 1%. To optimize rotational molding, study has been completed by sensitivity to adjustable parameters.

Thermal and rheological behavior of reactive blends from metallocene olefin elastomers and polypropylene

Domingues Junior,Nei S.; Forte,Maria M. de C.; Riegel,Izabel C.
Fonte: Associação Brasileira de Polímeros Publicador: Associação Brasileira de Polímeros
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2012 EN
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Reactive blends of metallocene polyolefin elastomers (POE)/polypropylene (PP) with 60/40 composition were prepared with an organic peroxide, 2,5-dimethyl-2,5-di-(t-butylperoxy)hexane, and a bis-azide derivative, diphenyloxid-4,4'-bis(sulfonylazide) (BSA). Ethylene-1-butene (EB) and ethylene-1-octene (EO) copolymers and elastomeric polypropylene (ePP) were used as the elastomeric phase. The effect of elastomeric phase on the thermal, rheological, morphological and mechanical properties of the thermoplastic vulcanizates (TPVs) or dynamic vulcanizates were studied. All TPVs depicted pseudoplastic behavior and blends cured with azide curative showed higher viscosities. The TPVs showed both dispersed and continuous phase morphology that depends on the elastomeric phase type revealing a limited degree of compatibility between PP and the elastomers EO or EB. On the other hand, the TPV PP/ePP showed a uniform morphology suggesting an improved compatibility. Substantial changes observed in physical properties were explained on the basis of blends' morphology and dynamic vulcanization. The results confirm that the mechanical properties are more influenced by the elastomeric phase than by the curative agent. This study revealed a broad new range of opportunities for POE-based TPVs.

Viscoelastic relaxations in poly(ethylene-co-1-octadecene) synthesized by a metallocene catalyst

Pérez, E.; Yazdani-Pedram Zobeiri, Mehrdad; Quijada Abarca, Raúl; Benavente, Rosario
Fonte: ELSEVIER SCI LTD Publicador: ELSEVIER SCI LTD
Tipo: Artículo de revista
EN
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The viscoelastic relaxations of four copolymers of ethylene and I-octadecene, with different comonomer contents, synthesized with a metallocene catalyst, have been analysed and compared to those of the corresponding polyethylene homopolymer. Similar to other ethylene copolymers, the temperature for alpha relaxation decreases with the increase in the comonomer content, attributed to the reduction of the crystal size. In fact, a linear relation has been found between the temperature location of alpha relaxation and the crystal thickness estimated from the SAXS long spacing. However, the position of beta relaxation experiences very little dependence with the comonomer content and it overlaps to alpha relaxation for the higher comonomer contents. As to gamma relaxation, there is also a clear effect of the comonomer content on the breadth and on the location of this relaxation.

Study of the polymerization of 1-octadecene with different metallocene catalysts

Guevara, Juan Luis; Yazdani-Pedram Zobeiri, Mehrdad; Galland, Griselda B.; Ribeiro, D.; Quijada Abarca, Raúl
Fonte: SPRINGER-VERLAG Publicador: SPRINGER-VERLAG
Tipo: Artículo de revista
EN
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1-Octadecene (C18) was polymerized by using different metallocene catalysts. The rac-Et(Ind)(2)ZrCl2/MAO (I) and rac-Me2Si(Ind)(2)ZrCl2/MAO (III) presented the highest C, activity as compared with rac-Et(2-Me-Ind)(2)ZrCl2/MAO (II) and Ph2C(Flu)(Cp)ZrCl2/MAO (IV) catalysts. Catalyst IV produced polymers with highest molecular weights. The microstructure of the polymers was determined by C-13-NMR spectro scopy. Catalyst systems I, II and III produced isotactic polymers while catalyst IV produced polymers with mainly syndiotactic structures but with large amount of stereoregular error.

Study of the effect of branching in degradation of polyethylenes obtained via metallocene catalyst

Quijada Abarca, Raúl; Campos Vallette, Marcelo; Escudero Acevedo, Mario
Fonte: SOC CHILENA QUIMICA Publicador: SOC CHILENA QUIMICA
Tipo: Artículo de revista
EN
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Polyethylenes, homo and copolymers obtained via metallocene catalyst, were synthesized and used to study the effect of comonomer size and content in the main linear backbone polymer on its degradation. Homoethylene metallocene polymer, ethylene-1-hexene and ethylene-1-octadecene metallocene copolymer films were characterized and analyzed during their aging in a forced draft even at 60 degrees C for one year. The polymers were characterized before and after ageing using GPC, DSC, FTIR, and tensile tests in order to detect changes in chemical structure, size, and molecular weight distribution as well as to quantify the effect of time over their useful life. As a qualitative reference parameter, the carbonyl index, the ratio of the infrared absorbance of the CO stretching band at 1715 cm(-1) and the absorbance of a reference band at 718 cm(-1), was determined. Results showed that kinetic thermooxidation is related to comonomer size and content in the main backbone polymer. As comonomer size decreases or comonomer content increases, degradation rate increases. This can be observed through the scission factor (S) and carbonyl index (CI) graphs of each material which show a slope increasing related to the autocatalytic rate of oxidation.

Use of functionalized metallocene copolymers from ethylene and polar olefins as compatibilizers for low-density-polyethylene/starch and low-density-polyethylene/dextran blends

Quijada Abarca, Raúl; Yazdani-Pedram Zobeiri, Mehrdad; Domínguez Barceló, Ana María
Fonte: WILEY-V C H VERLAG GMBH Publicador: WILEY-V C H VERLAG GMBH
Tipo: Artículo de revista
EN
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Functionalized metallocene copolymers synthesized from ethylene with 5-hexen-1-ol and ethylene with 10-undecen-1-ol were used as compatibilizers in LDPE/starch and LDPE/dextran blends in order to improve the interfacial adhesion between hydrophobic LDPE and hydrophilic natural polymers. An increase in tensile modulus and a slight decrease in tensile strength was observed when poly [ethylene-co-(10-undecen-1-ol)] was added to a 70:30 wt.-% LDPE/dextran blend, whereas the addition of poly[ethylene-co-(5-hexen-1-ol)] as compatibilizer resulted in obtaining a more rigid material with a slightly higher modulus. Scanning electron microscopy of modified dextran blends containing 3 wt.-% of both compatibilizers showed some degree of phase cocontinuity. Enhanced interfacial adhesion and decrease in particle size of starch was observed when 5 wt.-% of poly [ethylene-co(5-hexen-1-ol)] copolymer was used as the compatibilizer in starch blends. The crystallization temperature of LDPE, determined by DSC, was shifted to a slightly higher temperature as a consequence of the addition of the compatibilizers. The existence of phase segregation was also revealed by thermal analysis when 5 wt.-% of the copolymers were used as blend modifiers.

Ethylene polymerization using dealuminated ZSM-2 zeolite nanocrystals as an active metallocene catalyst support

Rojas, René; Quijada Abarca, Raúl; Covarrubias, Cristián
Fonte: ELSEVIER Publicador: ELSEVIER
Tipo: Artículo de revista
EN
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The preparation of dealuminated (DEAL) ZSM-2 zeolite nanocrystals for use as an active metallocene polymerization catalyst support is presented. The DEAL-ZSM-2 zeolite form was prepared by using steam treatment; metallocene catalyst was directly supported on the zeolite: and evaluated in the polymerization of ethylene using either methylaluminoxane (MAO) or an alkylaluminun as cocatalysts. in order to elucidate the activator effect of the acidic zeolite support. a detailed material characterization was performed. Metallocene catalyst supported on DEAL-ZSM-2 zeolite exhibited high activity values. The metallocene activation is attributed to the action of soluble cocatalyst (MAO or alkylaluminun) into the reactor and to the activator effect of extraframework aluminum species with strong Lewis acidity existing in the DEAL-ZSM-2 zeolite structure. High external surface area of the nanosized zeolite also contributes to reduce the diffusion effects commonly observed in microsized zeolite supports. The results of this work demonstrate that DEAL-ZSM-2 zeolite support does not necessitate to be treated with MAO previous fixation of the metallocene catalyst, and that polymerization activity can be also achieved using a trialkyaluminum as cocatalyst. Thus...

Role of solvent reorganization dynamics in electron-transfer processes. Theory-experiment comparisons for electrochemical and homogeneous electron exchange involving metallocene redox couples

Gennett, Thomas; Milner, David; Weaver, Michael
Fonte: The American Chemical Society: Journal of Physical Chemistry Publicador: The American Chemical Society: Journal of Physical Chemistry
Tipo: Abstract Formato: 31371 bytes; application/pdf
EN_US
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Electrochemical rate constants and activation parameters are reported for the electron exchange of five metallocene couples and bis(benzene)chromium(I/0) in eight solvents at mercury electrodes. The solvents (acetonitrile, acetone, methylene chloride, formamide, N-methylformamide, N,N-dimethylformamide, dimethyl sulfoxide, and benzonitrile) were chosen so to provide substantial variations in their dynamical as well as dielectric properties. The metallocene couples are of the form M(Cp)2+/0, where M = Fe, Co, or Mn, and Cp = cyclopentadiene or pentamethylcyclopentadiene. The inner-shell (Le., bond distortional) barriers are calculated for the metallocene and arene couples from bond-distance and vibrational data to be small (~<0.25 kcal mol-1) yet metal dependent. Detailed comparisons of the observed solvent-dependent kinetics are made with the rate parameters calculated from contemporary theoretical treatments of outer-sphere electron transfer. Considerably better agreement between the experimental and theoretical kinetic parameters was obtained when the latter take into account the influence of solvent friction upon the barrier-crossing frequency. A comparison between the corresponding experimental and theoretical rate parameters for ferrocenium-ferrocene self-exchange in eight solvents yielded a similar finding. These results indicate that the conventional transition-state theory may not apply to electron-transfer reactions where the free-energy barrier is due chiefly to solvent reorganization...

Double-layer effects on electrochemical kinetics in nonaqueous media. Dependence reactant charge for some metallocene redox couples.

Gennett, Thomas; Weaver, Michael
Fonte: Elsevier: Journal of Electroanalytical Chemistry Publicador: Elsevier: Journal of Electroanalytical Chemistry
Tipo: Abstract Formato: 37365 bytes; application/pdf
EN_US
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The effect of altering the double-layer composition on standard rate constants, kob, for metallocene and related redox couples has been examined at mercury electrodes in dimethylformamide and acetonitrile containing tetraalkylammonium supporting electrolytes. For cationic metallocene couples, M(Cp)+/02, where M = Fe, Mn, or Co, and Cp = cyclopentadienyl or pentamethylcyclopentadienyl, kob is virtually unaffected by changes in the double-layer structure brought about via alterations in M, as well as by variations in the ionic strength and the nature of the supporting electrolyte cation. In contrast, rate constants for the structurally related anionic couple Co(Cp)0/−2, along with those for several other anionic metallocene and organic couples, display the usual sensitivity to such changes in double-layer structure even though equal and opposite double-layer effects for M(Cp)+/02 and M(Cp)0/−2 couples are predicted from conventional treatments. The behavioral disparities between cationic and anionic metallocene couples are rationalized in terms of the charge distribution between the Cp rings and the metal core. These findings illustrate the limitations of employing the usual Frumkin model to estimate double-layer corrections for such redox couples in nonaqueous media.