1,721,087 research outputs found
Lanthanide metal-organic frameworks as ziegler–natta catalysts for the selective polymerization of isoprene
No full textThe unprecedented ability of neodymium-based metal organic frameworks (MOFs) aspolymerisation pre-catalysts towards isoprene is reported. Combined with methylaluminoxane(MAO) or modified MAO (MMAO), they afford mainly cis-selective polyisoprene, up to90.7%. Both the activity and the selectivity are tentatively ascribed to the intrinsic microstructureof the starting materials. Compared to conventional carboxylates, MOFs associatedto an Al co-catalyst are less active but the selectivity is found to be higher, and itmay be modified by controlling the access to the pores, which would be favored at highertemperatures. Some residual crystallineMOF remains disseminated within thepolymer matrix, as shown by X-raydiffraction and X-ray absorption spectroscopystudies
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
Catalysts for the Controlled Polymerization of Conjugated Dienes
Full text linkSince the beginning of the 1960s, the coordinative polymerization of conjugated dienes has continuously improved. Today, chemists know how to polymerize conjugated dienes stereospecifically and in a controlled fashion, both petro-sourced (nowadays also bio-sourced) and those of natural origin. The industry has greatly improved the performances of the catalytic systems—covering a wide range of elements including metals from groups 4–6 and 8–10, and rare earths—with the aim of optimizing the preparation of synthetic polymers for a large range of industrial applications. Nowadays, there is a better understanding of the polymerization mechanism involving allyl-active species, thanks in particular to the support of more efficient calculation methods. In addition, statistical copolymerization of 1,3-dienes with olefin or styrene comonomers and innovative approaches to coordinative chain transfer polymerization allow the production of copolymers with controlled topology, while a last challenge is about to be solved with the preparation of stereoregular polydienes that are also end-functionalized. This issue brings together several important aspects of this chemistry that remain at the forefront of both academic and industrial research
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
Isoprene polymerisation by rare earth based catalysis in the presence of inorganic derivatives : access to new composite elastomers
No full textDeux systèmes catalytiques hétérogènes, l‘un à base de trisphénate de néodyme (Nd(O(C6H3tBu2)3 associé au MAO supporté sur kaolin, et l‘autre constitué d‘un « metal-organic framework » (MOF) à base de néodyme activé par des aluminoxanes (MAO et MMAO), ont été évalués pour la polymérisation par coordination contrôlée de l‘isoprène, de type « polymérisation in situ », dans le but de préparer des élastomères composites. Le trisphénate de néodyme, étudié de façon préliminaire comme pré-catalyseur dans des conditions conventionnelles, conduit à des catalyseurs « single-site » en association avec des co-catalyseurs aluminoxanes, alors que la formation de polymères hautement 1,4-cis stéréoréguliers est obtenue par activation avec AlEt2Cl. En conditions de polymérisation hétérogène, chacun des systèmes catalytiques préparés se révèlent actifs, et produisent des polyisoprènes hautement 1,4-cis. La stéréosélectivité est clairement reliée à la porosité du pré-catalyseur dans le cas des MOF. Les matériaux polymériques montrent la présence de résidus cristallins de type MOF ou kaolin, ce qui a permis de les classer comme matériaux microcomposites. La présence de ces résidus inorganiques, dispersés dans les matrices polymériques, induisent notamment une modification des températures de transition vitreuse. Ces microcomposites sont les premiers exemples de polyisoprènes composites obtenus par des MOFs à base de néodyme, et par polymérisation in situ par des argiles supportées en association avec des post-métallocènes de néodyme.Two types of heterogeneous catalytic systems, based on the neodymium trisphenoxide (Nd(O(C6H3tBu2)3, associated to a MAO supported kaolin, and on a neodymium metal-organic framework based (MOF), activated by aluminoxanes, were studied by controlled coordination polymerisation of isoprene, with the aim of preparing composite elastomers. The neodymium trisphenoxide, studied in a preliminary way as pre catalyst in conventional conditions, affords single site catalysts in association with aluminoxanes co catalysts. Polymers highly 1,4-cis regular are obtained with additional activation with AlEt2Cl. In heterogeneous polymerisation conditions, each of the catalytic systems were found active, and giving rise to highly 1,4-cis regular polyisoprenes. The stereoselectivity is clearly related to the pre catalyst porosity (in the MOF case). The polymeric materials showed the presence of crystalline residues of MOF type or kaolin, which allowed to classify them as microcomposites. The presence of inorganic residues, dispersed into the polymeric matrix, induces a modification of glass transition temperature. These microcomposites are the first example of polyisoprene based composites obtained with neodymium MOFs, and by in situ polymerisation by supported clays in association with neodymium post-metallocene
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
Lanthanide borohydrides : polymerisation and copolymerysation
No full textDe nouveaux complexes borohydrures de lanthanides ont été synthétisés avec pour objectif de valoriser en tant que précurseurs les trisborohydrures de lanthanides, composés stables et facilement accessibles. Pour cela, une nouvelle voie de synthèse qui s'apparente à une métathèse de liaison sigma et faisant intervenir l'alkylation in situ du trisborohydrure par un dialkylmagnésien, a été développée. Ces complexes ont été utilisés en combinaison avec un dialkylmagnésien pour la polymérisation 1,4- trans- stéréospécifique de l'isoprène et la copolyrnérisation statistique de l'isoprène avec le styrène. Les aspects mécanistiques de ce type de polyinérisation ont également été abordés.New lanthanide borohydride complexes were synthesized, aiming to increase the value as precursors of the lanthanide trisborohydrides, which are stable and easily accessible compounds. ln this frame, a new method of synthesis was developped, whichis apparented to a sigma bond metathesis and which introduces the in situ alkylation of the trisborohydride by a dialkylmagnesium. These complexes were used in combination with a dialkylmagnesium compound for the 1,4-trans-polymerisation of. isoprene and the statistical copolymerisation of isoprene with styrene. Mecanistic aspects of this type of polymerisation were also studie
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
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