bluuebunny/crossref_metadata_2025_split huggingface dataset:
Precise arguments on the distribution of stereospecific active sites on MgCl2‐supported ziegler‐natta catalysts
Authors: Liu Boping , Nitta Takashi , Nakatani Hisayuki , Terano Minoru 
Published: June 2004

AbstractThe stereospecific nature of active sites on various MgCl2‐supported Ziegler‐Natta catalysts was investigated by stopped‐flow technique combined with temperature rising elution fractionation (TREF) method. A modified three‐sites model with precise description of the stereospecific nature of various types of active sites stemmed from surface titanium species, Al‐alkyl compounds, Mg‐compounds and electron donors has been proposed. It was demonstrated that the isospecificity of active sites strongly depends on the bulkiness of the ligands situated at the two most important ligand positions for construction of asymmetry and chirality of the active sites with steric hindrance. In general, there may exist both monometallic and bimetallic sites in heterogeneous Ziegler‐Natta catalyst system. The kinds of active titanium species with different chemical structures on this catalyst system should be limited, whereas, the non‐discrete distribution of isospecificity of active sites could be considered to generate from the numerous types of steric and electronic effects from the surroundings of active titanium species as well as large number of reversible and dynamic transformation reactions simultaneously occurred on the heterogeneous catalyst surface.
Surface physico‐chemical state of CO‐prereduced Phillips CrOx/SiO2 catalyst and unique polymerization behavior in the presence of Al‐alkyl cocatalyst
Authors: Liu Boping , Fang Yuwei , Nakatani Hisayuki , Terano Minoru 
Published: June 2004

AbstractIn this work, a CO‐prereduced Phillips CrOx/SiO2 catalyst (PC600/CO) was characterized by X‐ray photoelectron spectroscopy (XPS) and temperature programmed desorption with mass spectrometer (TPD‐MS) in comparison with a calcined catalyst (PC600). It was found that surface chromate Cr(VI) species had not been completely reduced into Cr(II) species, meanwhile, CO and CO2 still strongly adsorbed on the PC600/CO catalyst. Subsequently, the unique real‐time profiles of ethylene polymerization rate using PC600/CO in the presence of TEA indicated the existence of two types of active sites: the first type derived from the desorption of CO or CO2 from Cr(II) species by alkyl‐Al and featured with fast activation, higher activity and fast decay; the second type generated from the further reduction of residual chromate Cr(VI) species by alkyl‐Al and featured with slow activation, lower activity and slow decay.
New polymers by copolymerization of ethylene and norbornene with metallocene catalysts
Authors: Kaminsky Walter , Tran Phu‐Dennis , Werner Ralf 
Published: June 2004

AbstractTo increase the thermal stability of metallocene‐methylaluminoxane catalysts. pentalene bridged zirconocenes and a constrained geometry catalyst were used for the copolymerisation of ethene and norbornene. The constrained geometry catalyst is stable in solution up to 90°C. Surprisingly the molecular weight of the copolymers increases with higher incorporation rates of norbornene. Glass transition temperatures of 120°C are reached. The microstructure of the cyclic olefin copolymers material is more alternating.
Zirconocene‐catalysed propene polymerisation: kinetics, mechanism, and the role of the anion
Authors: Song Fuquan , Hannant Mark D. , Cannon Roderick D. , Bochmann Manfred 
Published: June 2004

AbstractThe olefin polymerisation activity of metallocene catalysts strongly depends on the counteranion provided by the activator system. The relative activities of a number of new diborate anions [Z(BAr3)2]− have been quantified (Z = CN, NH2, N(CN)2; Ar = C6F5 or o‐C6F4C6F5). The kinetic parameters for the initiation, propagation and termination steps of propene polymerisations catalysed by (SBI)ZrCl2 have been determined using quenched‐flow kinetic and batch techniques [SBI = rac‐Me2Si(1‐Ind)2]. Comparison of two activator systems, (i) CPh3[B(C6F5)4] / triisobutylaluminium (TIBA) and (ii) methylaluminoxane (MAO) shows, surprisingly, that the concentration of species actively involved in chain growth at any one time is comparable for both systems, although the MAO‐activated catalyst is about 20 times less active than the borate system. It is concluded that the counteranion remains sufficiently strongly bound to the metal centre throughout the chain growth sequence to modulate the energetics of monomer insertion. A model suggesting that the monomer binding follows an associative interchange (Ia) mechanism is proposed.
Highly active and stereoselective olefin polymerization catalysts generated by the transfer‐epimetallation of olefins or acetylenes with dialkyltitanium(IV) complexes: three‐membered metallocycles as active catalyst sites‡
Authors: Eisch John J. , Gitua John N. , Otieno Peter O. , Adeosun Adetenu A. 
Published: June 2004

AbstractEfficient transfer‐epimetallations of simple olefins and acetylenes by R2TiL2 reagents (R = Bun, But; L = X) are readily achieved in THF at −78°C to generate titanacyclopropa(e)ne intermediates, readily capable of inserting various unsaturated addends (olefin, acetylene, nitrile). Analogous epimetallations conducted in hydrocarbons lead to the isotactic stereoselective polymerization of 1‐alkenes and the cyclotrimerization of acetylenes. In place of the widely accepted Arlman‐Cossee model for the active catalytic site, namely a Ti‐C bond on the TiCl3 crystal lattice, the 2‐substituted‐1‐halotitanacyclopropyl cation formed in hydrocarbon media is proposed as the active site for stereoselective olefin polymerization.
Polymerization of acrylate monomers by iron(II) complexes bearing bis(imido)pyridyl or phosphine ligands
Authors: Castro Pascal M. , Lankinen Mikko P. , Uusitalo Anne‐Marja , Leskelä Markku , Repo Timo 
Published: June 2004

AbstractIron(II) complexes bearing bis(imido)pyridyl or phosphine ligand were used for the polymerization of tert‐butylacrylate and other (meth)acrylates after activation with methylaluminoxane. Phosphine based catalysts turned to be more active than the bis(imido)pyridyl ones. tert‐Butylacrylate was the best polymerized monomer, while methylmethacrylate was slowly converted to poly(methylmethacrylate) and no activity was observed in the polymerization of styrene. Influence of the MAO to complex ratio was investigated and kinetics studies were conducted. Based on these results, a schematic representation of the polymerization mechanism was proposed.
Styrene polymerization by ziegler‐natta catalysts based on bis(salicylaldiminate)nickel(II) complexes and methyl aluminoxane
Authors: Carlini Carlo , Macinai Alessandro , Raspolli Galletti Anna Maria , Sbrana Glauco 
Published: June 2004

AbstractThe homopolymerization of styrene by using different catalytic systems based on bis(salicylaldiminate)nickel(II) and methylaluminoxane was investigated. In particular, the effect on catalyst activity and polymer characteristics by electron withdrawing groups located on the phenolic moiety was studied. The influence of the bulkiness of the substituents on the N‐aldimine ligand was also ascertained. Finally the catalytic performances were investigated as a function of the main reaction parameters, such as temperature, Al/Ni molar ratio and duration.
Polymerization of propene by post‐metallocene catalysts
Authors: Lamberti Marina , Mazzeo Mina , Pappalardo Daniela , Zambelli Adolfo , Pellecchia Claudio 
Published: June 2004

AbstractIn the last few years several non‐metallocene catalysts have been disclosed as efficient catalysts for the stereospecific polymerization of propene.In this paper we summarize some recent literature data and some new results concerning the stereochemical mechanism of propene polymerization promoted by late transition metal systems and group 4 metal bis(phenoxyimine) systems. NMR analysis of the fine structure of the polymers obtained, in some cases using isotopically enriched reagents, provides valuable information on the regiochemistry and stereochemistry of the polymerization.
Macromonomers and coordination polymerization
Authors: Lahitte Jean‐François , Peruch Frédéric , Isel François , Lutz Pierre Joseph 
Published: June 2004

AbstractThe present work discusses the synthesis of well‐defined comb‐shaped polymers or graft copolymer structures based on coordination (co)polymerization of macromonomers. Polystyrene macromonomers with various polymerizable entities were synthesized first by induced deactivation reactions. The homopolymerization of these macromonomers in the presence of selected early or late transition metal catalysts was examined. Comb‐shaped polymers could be obtained over a large range of DP values. The results were compared to those obtained by anionic homopolymerization. Some results on the copolymerization of these PS macromonomers with ethylene in the presence of VERSIPOLTM type catalysts were presented.
A combined density functional theory and molecular mechanics (QM/MM) study of single site ethylene polymerization catalyzed by [Cp{NC(tBu)2}TiR+] in the presence of the counterion, CH3B(C6F5)3−
Authors: Vanka Kumar , Xu Zhitao , Ziegler Tom 
Published: June 2004

AbstractA density functional study was conducted on the approach and insertion of ethylene monomer into the Ti‐Cα bond of the catalyst system, CpNC(tBu)2RTi‐μ‐Me‐B(C6F5)3 (R= methyl, propyl). A validated QM/MM model was used to represent the counterion. Solvent effects were incorporated with single point solvent calculations done with cyclohexane (ϵ = 2.023) as the solvent. For R=Me (the initiation step), approach and insertion of the ethylene was found to be endothermic, with the barrier for insertion being 12.7 kcal/mol for the most favourable case. For R=Pr (the propagation step), the insertion barrier was found to decrease slightly (11.5 kcal/mol for the most favorable case), corroborating experimental evidence of decrease in insertion barrier with increase in chain length. Termination by chain transfer to monomer was also considered, and found to be unfavourable, in comparison to insertion, by 8.6 kcal/mol for the propagation step. Solvent effects were found to be significant for the propagation step, changing the rate determining step from insertion to uptake for the most favorable case of insertion.
On‐line monitoring and fingerprint technology: new tools for the development of new catalysts and polyolefin materials
Authors: Tuchbreiter Arno , Marquardt Jürgen , Kappler Bernd , Honerkamp Josef , Mülhaupt Rolf 
Published: June 2004

AbstractThe High‐Output Polymer Screening (HOPS) combines process‐relevant automated reactor systems and rapid polymer characterization with on‐line polymerization monitoring and automated data acquisition (“electronic notebook”) in order to make effective use of advanced data mining tools. This has led to the development of fingerprint technology based upon correlations between spectroscopic data and polymerization process conditions, catalyst compositions, as well as polymer end‐use properties. Infrared spectroscopic fingerprints proved to be very useful for accelerating polymer analyses including characterization of polymer molecular architectures as well as non‐destructive testing of the mechanical, thermal and other end‐use polymer properties. Such spectroscopic fingerprints represent important components of effective on‐line quality control systems. With ATR‐FT‐IR probes on‐line monitoring of catalytic olefin copolymerization was performed in solution to measure in real time copolymerization kinetics, catalyst productivities, catalyst deactivation as well as copolymerization parameters and copolymer sequence distributions. Monomer consumption and comonomer incorporation were monitored simultaneously. Advanced fingerprint technology can reduce significantly the need for time‐ and money consuming polymer testing and can also stimulate the search for new catalyst systems and polymeric materials.
Synthesis of polyolefin based materials with improved thermo‐oxidative stability
Authors: Dolatkhani Marc , Cramail Henri , Deffieux Alain , Ribeiro Maria Rosário , Santos Jorge M. , Bordado João M. 
Published: June 2004

AbstractPolyolefin based materials with chemically bonded phenol compounds have been synthesized. Two unsaturated polyolefins: an ethylene/propylene/5,7‐dimethyl‐1,6‐octadiene terpolymer and an ethylene/5,7‐dimethyl‐1,6‐octadiene copolymer, were used as starting materials. The functionalisation method involves a two‐step procedure that consists in the hydrochlorination of the substituted pending double bonds, followed by grafting of the phenol compound. This procedure was found to be more effective for the terpolymer than for the corresponding copolymer. The reasons for this are discussed. A thermogravimetric study, in air atmosphere shows that, poly(ethylene‐co‐5,7‐dimethyl‐1,6‐octadiene) grafted with 2‐tert‐butyl‐4‐methylphenol present improved thermo‐oxidative stability when compared to a polyethylene reference.
Equilibrium Swelling of an Epoxy‐Resin in Contact with Water – A Molecular Dynamics Simulation Study
Authors: Dömötör Gyula , Hentschke Reinhard 
Published: June 2004

AbstractFull Paper: We have studied the equilibrium swelling of an idealized model network based on a realistic epoxy‐resin in contact with water using the molecular dynamics simulation method. We use a novel method to measure the water chemical potential inside the network as function of water concentration. The equality of the water chemical potential computed in this fashion with the bulk water chemical potential under the same thermodynamic conditions determines the equilibrium water content and the swelling ratio. We study the dependence of the water content as function of temperature and as function of the size of the atomic partial charges.Schematic representation of the simulation volume. Spheres represent epoxy units with n = 0. The dashed lines represent curing agent units with m = 5.imageSchematic representation of the simulation volume. Spheres represent epoxy units with n = 0. The dashed lines represent curing agent units with m = 5.
Herstellung verbundverstärkter Aluminiumprofile für ultraleichte Tragwerke durch Strangpressen
Authors: Kleiner M. , Schomäcker M. , Schikorra M. , Klaus A. 
Published: June 2004

AbstractAuf Grund ähnlicher spezifischer Steifigkeits‐ und Festigkeitseigenschaften von Aluminium und Stahl lassen sich in der Anwendung bei leichten Rahmenstrukturen durch gegenseitige Werkstoffsubstitution nur geringe Leichtbaugewinne erzielen. Lediglich die Verwendung von z. B. Kohlefaserwerkstoffen oder Höchstleistungsstählen lässt eine Gewichtsreduktion für den Einsatz von Rohren oder Profilen in ultraleichten Strukturen erwarten. Am Lehrstuhl für Umformtechnik (LFU) der Universität Dortmund ist ein Verfahren entwickelt worden, das durch ein modifiziertes Strangpressen zur Herstellung von Verbundprofilen geeignet ist. Ausgehend von konventionellen Aluminium‐Pressblöcken werden hierbei verschiedenartige endlose Verstärkungselemente innerhalb der Wandstärke der Profilmatrix eingebettet. Für das Verfahren, das im Rahmen des Sonderforschungsbereiches SFB/TR10 erforscht wird, ist die Entwicklung neuartiger Strangpresswerkzeuge erforderlich. Durch experimentelle Untersuchungen auf einer 2,5 MN Laborstrangpresse wurden erfolgreich erste Verbundprofile hergestellt. Zur Vertiefung des Prozessverständnisses und der Prognose der Wirksamkeit neuer Werkzeugkonzepte wurden parallel FEM‐Simulationen durchgeführt. Die mit Hilfe des Verfahrens hergestellten geraden Verbundprofile lassen sich auf Grund möglicher Gefügeschädigungen nicht biegen. In Kombination mit dem ebenfalls am LFU entwickelten Verfahren Runden beim Strangpressen werden diese Verbundprofile jedoch auch mit gekrümmter Kontur herstellbar sein.
Hydromechanisches Tiefziehen und Hochdruckblechumformung als Verfahren zur Herstellung komplexer Bauteile aus Magnesiumfeinblechen des Typs AZ31B‐0
Authors: Viehweger B. , Richter G. , Düring M. , Karabet A. , Sviridov A. , Hartmann H. , Richter U. 
Published: June 2004

AbstractEine gute Umformbarkeit von Blechhalbzeugen aus Magnesiumknetlegierungen stellt sich bekanntlich bei Anwendung von Umformtemperaturen im Bereich von 175 °C – 240 °C ein. Anhand der wirkmedienbasierten Umformverfahren hydromechanisches Tiefziehen und Hochdruckblechumformung ist an handelsüblichen AZ31B‐0 Feinblechen die Einstellung unterschiedlicher Umformparameter erprobt worden. Unter Verwendung von Umformtemperaturen von 175 °C, 200 °C, 225 °C und 240 °C und entsprechend angepassten Niederhalterdrücken ist eine praxisnahe Versuchsgeometrie „Minihood”︁ ausgeformt worden. Im Hinblick auf angewendete Umformverfahren und Versuchsparameter wurde an den Versuchsbauteilen die Blechdickenabnahme und die Radienausprägung bewertet und verglichen. Für ausgewählte Versuchsbauteile, umgeformt mittels der Hochdruckblechumformung bei Temperaturen von 175 °C und 225 °C, ist eine ergänzende Untersuchung hinsichtlich der durch den Umformvorgang verursachten Veränderungen von Oberflächenkennwerten durchgeführt worden. Anhand dieser Ergebnisse werden Hinweise auf die Grenzen des Einsatzes der o.g. Umformverfahren für Magnesiumblech des Legierungstyps AZ31B‐0 gegeben.
Untersuchungen zur Herstellung flächig gekrümmter Bauteile durch den Einsatz der Robotertechnik beim Freiformschmieden
Authors: Ziegelmayer O. , Schneider V. , Kopp R. , Dürr O. 
Published: June 2004

AbstractDer Prozess des Freiformschmiedens zeichnet sich durch seine hohe Flexibilität der einsetzbaren Werkstoffe und der mittels einfacher Werkzeuge erzielbaren Halbzeuggeometrien aus. Um eine höhere Komplexität bei gleichzeitig höherer Reproduzierbarkeit gegenüber den in heutigen industriellen Anlagen produzierbaren Bauteilformen schaffen zu können, wurde vor einigen Jahren am Institut für Bildsame Formgebung (IBF) der RWTH Aachen ein Sechs‐Achsen Schwerlast‐Industrieroboter als Schmiedemanipulator (Tragfähigkeit ∼300 kg) als Ergänzung zu einer hydraulischen Freiformschmiedepresse (Maximalkraft: 6,3 MN) installiert.Diese Anlage bietet die Voraussetzung, Bauteile mit einer flächigen Krümmung im industrienahen Maßstab mittels Warmumformung herzustellen, wobei die Teile sich mittels gezielter Massenvorverteilung für nachfolgende Gesenkschmiede‐Operationen nutzen lassen. Eine industrielle Anwendung hierfür stellt bspw. ein Rumpfspant für die Luftfahrtindustrie dar, der aus Aluminium hergestellt wird und u. a. auch strengen werkstofflichen Anforderungen unterliegt.
Temperatureinfluss auf den Adhäsionsverschleiß beim Umformen von Aluminiumblechen
Authors: Groche P. , Nitzsche G. 
Published: June 2004

AbstractDie Adhäsionsneigung von Aluminiumblechen führt beim Tiefziehen zu verstärktem Werkzeugverschleiß in Form von Aufschweißungen. Um wirksame Maßnahmen zur Reduzierung des Adhäsionsverschleißes treffen zu können, müssen die Mechanismen bekannt sein, die zur Adhäsionsbildung führen. Die Temperatur in der Umformzone wird dabei in den existierenden Theorien nicht berücksichtigt. Zur Untersuchung dieses Parameters werden die in der Umformzone vorliegenden thermischen und mechanischen Beanspruchungen grundlegend numerisch analysiert sowie experimentell im Modellversuch überprüft. Mit Hilfe temperierter Werkzeuge wird anschließend eine definierte Temperatur in der Umformzone eingestellt, die den Werkzeugtemperaturen im realen Presswerk entspricht. Während bei einem konventionellen System ohne Erwärmung kein Adhäsionsverschleiß festzustellen ist, kommt es mit erwärmten Werkzeugen zur Bildung von Aufschweißungen. Um den Adhäsionsverschleiß zu reduzieren, müssen daher Maßnahmen getroffen werden, die den vorhandenen Temperatureinfluss berücksichtigen.
Delayed gadolinium‐enhanced MRI of cartilage (dGEMRIC) and T2 characteristics of human knee articular cartilage: Topographical variation and relationships to mechanical properties
Authors: Kurkijärvi J.E. , Nissi M.J. , Kiviranta I. , Jurvelin J.S. , Nieminen M.T. 
Published: June 2004

AbstractThe macromolecular structure and mechanical properties of articular cartilage are interrelated and known to vary topographically in the human knee joint. To investigate the potential of delayed gadolinium‐enhanced MRI of cartilage (dGEMRIC), T1, and T2 mapping to elucidate these differences, full‐thickness cartilage disks were prepared from six anatomical locations in nonarthritic human knee joints (N = 13). Young's modulus and the dynamic modulus at 1 Hz were determined with the use of unconfined compression tests, followed by quantitative MRI measurements at 9.4 Tesla. Mechanical tests revealed reproducible, statistically significant differences in moduli between the patella and the medial/lateral femoral condyles. Typically, femoral cartilage showed higher Young's (>1.0 MPa) and dynamic (>8 MPa) moduli than tibial or patellar cartilage (Young's modulus <0.9 MPa, dynamic modulus <8 MPa). dGEMRIC moderately reproduced the topographical variation in moduli. Additionally, T1, T2, and dGEMRIC revealed topographical differences that were not registered mechanically. The different MRI and mechanical parameters showed poor to excellent linear correlations, up to r = 0.87, at individual test sites. After all specimens were pooled, dGEMRIC was the best predictor of compressive stiffness (r = 0.57, N = 77). The results suggest that quantitative MRI can indirectly provide information on the mechanical properties of human knee articular cartilage, as well as the site‐dependent variations of these properties. Investigators should consider the topographical variation in MRI parameters when conducting quantitative MRI of cartilage in vivo. Magn Reson Med 52:41–46, 2004. © 2004 Wiley‐Liss, Inc.
Deconvolution of dynamic contrast‐enhanced MRI data by linear inversion: Choice of the regularization parameter
Authors: Sourbron Steven , Luypaert Rob , Van Schuerbeek Peter , Dujardin Martine , Stadnik Tadeusz , Osteaux Michel 
Published: June 2004

AbstractTruncated singular value decomposition (TSVD) is an effective method for the deconvolution of dynamic contrast‐enhanced MRI. Two robust methods for the selection of the truncation threshold on a pixel‐by‐pixel basis—generalized cross validation (GCV) and the L‐curve criterion (LCC)—were optimized and compared to paradigms in the literature. The methods lead to improvements in the estimate of the residue function and of its maximum and converge properly with SNR. The oscillations typically observed in the solution vanish entirely and perfusion is more accurately estimated at small mean transit times. This results in improved image contrast and increased sensitivity to perfusion abnormalities, at the cost of 1–2 min in calculation time and isolated instabilities in the image. It is argued that the latter problem may be resolved by optimization. Simulated results for GCV and LCC are equivalent in terms of performance, but GCV is faster. Magn Reson Med 52:209–213, 2004. © 2004 Wiley‐Liss, Inc.