1,720,992 research outputs found
Borinic vitrimers : synthesis and applications
No full textLes vitrimères représentent une classe novatrice de polymères dévoilée par le Professeur Leibler et ses collègues en 2011. Ils définissent les vitrimères comme étant des « réseaux polymères capables de modifier leur topologie tout en conservant un nombre constant de liaisons chimiques ». Ces matériaux se distinguent par leur aptitude à fusionner les caractéristiques des thermoplastiques (réutilisabilité) et des thermodurcissables (propriétés thermomécaniques et résistance chimique supérieure) grâce à un réseau chimique incorporant des liaisons covalentes dynamiques. Des réactions d'échange telles que la transestérification boronique dans les vitrimères ont été documentées, mettant en lumière la remarquable propriété de la liaison B-O échangeable sans catalyseur. Bien que des vitrimères contenant des monomères d'acides boroniques aient été élaborés, aucune publication n'a jusqu'à présent exposé l'utilisation d'acides boriniques dans ces matériaux. La synthèse d'acides boriniques non symétriques est une tâche complexe, cependant, leur complexation avec des aminoalcools permet de les isoler sous forme d'aminoborinates, dans lesquels l'atome de bore est stabilisé par la paire d'électrons non liants de l'azote. Ces composés pourraient ouvrir la voie à des vitrimères présentant une liaison B–N plus labile que la liaison B–O des esters d'acides boroniques. L'objectif principal de ce projet est de concevoir de nouveaux dérivés d'acides boriniques fonctionnalisés pouvant être utilisés comme monomères ou agents réticulants dans les vitrimères. Ce document décrit la synthèse et l'utilisation de borinates non symétriques à travers une stratégie divergente. Ces composés ont ensuite été employés dans des réactions de post-fonctionnalisation telles que la cycloaddition 1,3-dipolaire et la condensation de Knoevenagel. Ces post-fonctionnalisations visent à générer des structures aptes à être incorporées dans des copolymères pour évaluer la formation de vitrimères.Vitrimers represent an innovative class of polymers introduced by Professor Leibler and colleagues in 2011. They define vitrimers as polymer networks capable of modifying their topology while maintaining a constant number of chemical bonds. These materials distinguish themselves by their ability to merge the characteristics of thermoplastics (reusability) and thermosets (superior thermomechanical properties and chemical resistance) through a chemical network incorporating dynamic covalent bonds. Exchange reactions such as boronic transesterification in vitrimers have been documented, highlighting the remarkable property of the B–O bond exchangeable without a catalyst. Although vitrimers containing boronic acid monomers have been developed, no publication has yet exposed the use of borinic acids in these materials. The synthesis of non-symmetric borinic acids is a complex task, however, their complexation with aminoalcohols allows their isolation in the form of aminoborinates, where the boron atom is stabilized by the lone pair of nitrogen electrons. These compounds could pave the way for vitrimers featuring an B–N bond more labile than the B–O bond in boronic acid esters. The main objective of this project is to design new functionalized borinic acid derivatives that can be used as monomers or crosslinking agents in vitrimers. This document describes the synthesis and utilization of non-symmetric borinates through a divergent strategy. These compounds were then employed in post-functionalization reactions such as 1,3-dipolar cycloaddition and Knoevenagel condensation. This aims to generate structures suitable for incorporation into copolymers to assess vitrimer formation
Photocleavable organoboron derivatives : preparation and use
No full textCe manuscrit présente la synthèse et l’utilisation de dérivés organoborés photoclivables. Des diols-1,2 et -1,3 porteurs de groupements photolabiles ont été préparés puis employés pour protéger un substrat modèle, l’acide mésitylboronique. La photodéprotection de ce composé a ensuite été étudiée à partir des esters boroniques synthétisés, conduisant à l’identification d’un groupement protecteur modèle, à partir duquel l’optimisation de la méthode a été réalisée. Les conditions finales ont permis de déprotéger de nombreux acides boroniques, sur des temps de réaction très courts (20 min), et avec des rendements modérés à excellents. Cette stratégie a été étendue à la déprotection d’un acide borinique et les premiers résultats sont prometteurs. Enfin, divers acides boroniques et boriniques ont été testés comme catalyseurs de polymérisation d’un prépolymère modifié, terminé silyl, le MS® 203H. Huit composés organoborés ont conduit à une modification importante de la viscosité du matériau. De premiers essais ont été menés dans le but de développer une version photoinduite de la polymérisation avec les composés organoborés photoclivables préparés.This manuscript presents the synthesis and use of photocleavable organoboron derivatives. 1,2- and 1,3- diols bearing photolabile groups were prepared and used to cage a model substrate, mesitylboronic acid. The photodecaging of this compound was then studied using the synthesized boronic esters, leading to the identification of a protecting group, from which, the optimization of the methodology was performed. The final conditions allowed the deprotection of many boronic acids on short reaction times (20 min) and with moderate to excellent yields. This strategy has been extended to a borinic acid and the preliminary results are promising. Finally, several boronic and borinic acids were tested as catalysts for the polymerization of a modified, silyl-terminated prepolymer, MS® 203H. Eight organoboron compounds led to a significant change in the viscosity of the material. Preliminary experiments were also conducted to develop a photoinduced version of the polymerization process with the synthesized photocleavable organoboron compounds
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
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
Les trifluoro(organo)borates de potassium : nouvelles perspectives pour la formation de liaisons carbone-carbone à l'aide de complexes du rhodium
No full textThis manuscript describes the use of potassium trifluoro(organo)borates in rhodiumcatalyzed carbon-carbon bond forming reactions. These easy to purify and very stable reagents turned out to constitute a valuable alternative to other boron reagents commonly used in transition metal catalyzed reactions. Asymmetric rhodium-catalyzed 1,4-additions of trifluoroborates were developed with the use of a chiral diphosphine which allowed a good control of the enantioselectivity. Under neutral conditions Michael adducts were obtained from various acceptors in nearly quantitative yield avoiding the use of a large excess of organometallic reagent. A new catalytic method for 1,2-addition allowed an easy and straightforward access to various alcohols. This general reaction proceeds under very mild conditions and was applied to the synthesis of substituted carbinols. Finally, a new cross-coupling reaction between organometallic reagents (boron and tin derivatives) and aldehydes has been discovered. This C-H bond activation has allowed an easy access to dissymmetric ketones using nonfunctionalized derivatives.Ce manuscrit présente l'utilisation de trifluoro(organo)borates de potassium dans des réactions de formation de liaisons carbone-carbone catalysées par des complexes du rhodium. Ces composés très stables, faciles à préparer et à purifier constituent une alternative intéressante aux dérivés du bore trivalent habituellement utilisés en catalyse. Un système catalytique général a été développé pour réaliser des additions de Michael asymétriques. L'emploi d'une diphosphine chirale assure un bon contrôle de l'énantiosélectivité et de bonnes conversions sont obtenues en milieu neutre sans ajout d'un large excès d'organométallique. Une nouvelle méthode catalytique d'addition-1,2 a permis d'obtenir des alcools dans des conditions très douces. Cette réaction très générale peut être appliquée à la synthèse de carbinols diversement substitués. Enfin, une réaction de couplage entre un aldéhyde et un organométallique a été découverte. L'activation formelle d'une liaison C-H permet ainsi un accès facile à de nombreuses cétones non symétriques à partir de dérivés peu fonctionnalisés
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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