1,720,988 research outputs found
Coupling of CO 2 and epoxides catalysed by novel N -fused mesoionic carbene complexes of nickel( ii )
No full textWhile mesoionic carbenes have usually been shown to improve the catalytic potential of metal complexes, here, we report a switch of selectivity in the nickel-catalyzed coupling of CO 2 and epoxides, now favoring cyclic carbonates over polycarbonates.We report the syntheses of two rigid mesoionic carbene (MIC) ligands with a carbazole backbone via an intramolecular Finkelstein–cyclisation cascade and investigate their coordination behavior towards nickel( ii ) acetate. Despite the nickel( ii ) carbene complexes 4a,b showing only minor differences in their chemical composition, they display curious differences in their chemical properties, e.g. solubility. Furthermore, the potential of these novel MIC complexes in the coupling of carbon dioxide and epoxides as well as the differences in reactivity compared to classical NHC-derived complexes are evaluated.While mesoionic carbenes have usually been shown to improve the catalytic potential of metal complexes, here, we report a switch of selectivity in the nickel-catalyzed coupling of CO 2 and epoxides, now favoring cyclic carbonates over polycarbonates.We report the syntheses of two rigid mesoionic carbene (MIC) ligands with a carbazole backbone via an intramolecular Finkelstein–cyclisation cascade and investigate their coordination behavior towards nickel( ii ) acetate. Despite the nickel( ii ) carbene complexes 4a,b showing only minor differences in their chemical composition, they display curious differences in their chemical properties, e.g. solubility. Furthermore, the potential of these novel MIC complexes in the coupling of carbon dioxide and epoxides as well as the differences in reactivity compared to classical NHC-derived complexes are evaluated
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Intrinsic Switching Kinetics and Thermodynamics of Thermoresponsive Polymer Brushes
No full textSynthetic polymers represent one of the most important class of material in the 20th century and beyond. Particularly, stimuli-responsive polymers (so called smart polymers) build an outstanding and fascinating class of synthetic polymers which have become a versatile material in designing functional devices in material science, medicine and bioengineering. Controlled reaction schemes, such as ATRP (atomic transfer radical polymerization), allows one to have precise control over polymer architecture. One such example is the buildup surface-grafted polymer brush films with thicknesses down to the sub-100 nm range on almost any surface geometry. Due to their low film thickness, high sensitivities and fast response times can be achieved. However, in order to tailor efficient and powerful applications with high performance, a detailed fundamental knowledge of the switching thermodynamics and kinetics is essential. Unfortunately, kinetic and thermodynamic data of stimuli-responsive polymer brushes are still largely missing.
Tanaka and co-workers discovered in the 1980s that thermoresponsive polymer gels with dimensions in the micro and millimeter range exhibit response times on a second to hours timescale due to the diffusion-limited switching process. Nevertheless, much shorter response times in the milli/microsecond range are expected for ultrathin polymer films. Although, most previous studies revealed fairly long response times of several seconds or more for ultrathin polymer brushes.
Using a novel facile noncontact laser manipulation technique, this work reveals an unprecedented insight into the switching kinetics (rate constants, activation energies, frequency factors) and switching thermodynamics (change in Gibbs free enthalpy, free enthalpy, entropy and heat capacity) of thermoresponsive polymer brushes of PNIPAAm (poly(n-isopropylacrylamide)) and PDMAEMA poly(dimethylaminoethyl methacrylate). In detail, the impact of internal brush parameters such as grafting density, brush thickness and micro/nanopatterns are studied. Moreover, external parameters, i. e. the impact of salts, alcohols, gold nanoparticles and proteins (bovine serum albumin, BSA), are addressed. These results are expected to further support the progress on new innovative applications based on switchable polymer brushes and provide an essential reference on the way to unpuzzle the molecular driving forces of the switching mechanisms
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Entwicklung von funktionellen Polymerbürsten mit modularen Eigenschaften
Full text linkIm Rahmen dieser Arbeit wurden neuartige, temperatursensitive Polymerbürsten mit terminaler Click-Funktionalität hergestellt, die nicht nur im Detail untersucht bzw. charakterisiert wurden, sondern deren Eigenschaften zusätzlich durch die polymeranaloge Umsetzung mit einem entsprechend modifizierten Fluoreszenzfarbstoff, Polymer oder mit superparamagnetischen Nanopartikeln erweitert werden konnten. Mittels kontrolliert radikalischer Polymerisation wurde mono- und bi-funktionalisiertes Poly(N-isopropylacrylamid) mit unterschiedlichen Endgruppen und Molekulargewichten synthetisiert und über das "grafting to"-Verfahren an eine Modeloberfläche angebunden. Die kovalente Anbindung der entsprechenden Komponenten erfolgte entweder durch die Kupfer-katalysierte oder thermisch induzierte Alkin-Azid-Cycloaddition. Neben den physiko-chemischen Eigenschaften wurde im Besonderen das temperaturabhängige Schaltverhalten dieser funktionellen Polymerbürsten in situ untersucht, wobei neben der spektroskopischen Ellipsometrie als Hauptmethode, zusätzlich Rasterkraftmikroskopie, UV/VIS- und Fluoreszenzspektroskopie, sowie Elektronenmikroskopie verwendet wurden. Im Fall der Farbstoff-modifizierten Polymerbürsten zeigte sich, dass nicht nur die Eigenschaften des Farbstoffs durch das Polymer beeinflussbar sind, sondern auch das Quellverhalten der Polymerbürsten durch den Farbstoff. Die mit Nanopartikeln modifizierten Polymerbürsten zeigten eine homogene Verteilung der Partikel auf der Oberfläche, sowie eine veränderte Schaltcharakteristik, die durch das Auftreten eine dynamischen Wechselwirkungszone zwischen den Polymerbürsten und den Nanopartikeln erklärt werden kann. Mit Hilfe der thermisch induzierten Azid-Alkin-Cycloaddition konnten erstmalig definierte Kettenverlängerungen an Polymerbürsten durchgeführt und gleichzeitig der Nachteil der limitierten Pfropfungsdichte der „grafting to“-Methode überwunden werden. Darüber hinaus konnte der temperaturinduzierte Schalteffekt nicht nur erhalten bleiben, sondern in Bezug auf die Schaltamplitude auch signifikant vergrößert werden und ergab ein sensitiveres, verbessertes Polymerbürstensystem
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Polymer networks as carrier for organo catalysts within continuously driven microfluidic reactor systems : final report under the DFG Individual Grants Programme: Research Grants Programme
No full textDie kontinuierliche Flussreaktionen mit gelgebundener Organokatalysatoren in einem mikro-fluidischen Reaktor ist ein attraktiver Ansatz für die organische Synthese. Hier haben wir ver-schiedene katalytische Monomere aus verschiedenen funktionellen Organokatalysatoren syn-thetisiert und mit Hilfe der Photolithographie polymerisiert, um polymere Gelpunkte mit unter-schiedlichen Zusammensetzungen von Katalysator, Gel-bildendes Monomer und Vernetzer zu bilden. Die Morphologie der Gelpunkte wurde durch optische und Laser-3D-Mikroskopie be-wertet. Die Quelleigenschaften der Gelpunkte wurden im verschiedenen Reaktionslösungs-mittelmischungen und verschiedenen Temperaturen bestimmt. Die synthetisierten gel-gebun-denen Organokatalysatoren wurden in einen mikrofluidischen Reaktor (MFR) eingesetzt und für die Durchführung verschiedener organischer Reaktionen wie Knoevenagel-Reaktion, Al-dol-Reaktion und Baylis-Hillman-Reaktion unter milden Bedingungen verwendet. Der Umsatz der Reaktion wurde hauptsächlich durch kernmagnetische Resonanz (NMR) als Offline-Me-thode bestimmt, nachdem das Eluat aus dem MFR für entsprechende Zeiten (abzüglich der Verweilzeit) gesammelt wurde. Die Reaktionsparameter wie Lösungsmittel, Durchflussrate, Konzentration der Reaktanten und Temperatur wurden optimiert. Insgesamt konnten die Re-aktionen mit weniger Katalysator und mit ähnlicher Selektivität wie die entsprechenden Batch-Reaktionen durchgeführt werden.Continuous flow reactions using gel-bound organo-catalysts in a microfluidic reactor is an at-tractive approach in organic synthesis. Here, we have synthesized various catalytic monomers from various functional organo-catalyst and polymerized using photolithography to form poly-meric gel dots with different compositions of catalyst, gelling agent and crosslinker. The mor-phology of the gel dots was evaluated by optical and Laser 3D microscopy. Swelling properties of the gel dots were determined in different reaction solvent mixtures and at different temper-atures. The synthesized gel-bounded organocatalysts were inserted into a microfluidic reactor (MFR) and used for conducting different organic reactions like Knoevenagel reaction, Aldol reaction, and Baylis-Hillman reaction under mild conditions. The conversion of the reaction was determined mainly by nuclear magnetic resonance (NMR) as an offline method after col-lecting elute from the MFR for respective times (less the residence time). Reaction parameters like solvent, flowrate, concentration of reactants, and temperature were optimized. In total re-actions could be performed with less catalyst and with similar selectivity as respective batch reactions.Prof. Dr. Dirk KucklingDFG reference number: KU 1280/14-1; Project number: 463498885DFG KU 1280/14-1 463498885Abschlussbericht 01.08.2021 - 31.01.202
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