1,720,959 research outputs found
ON THE MECHANISM OF SYNDIOTACTIC PROPENE POLYMERIZATION IN THE PRESENCE OF TI-BASED HETEROGENEOUS ZIEGLER-NATTA CATALYSTS
No full textResults of a 13C NMR study of polypropene and ethene (I)-propene copolymers (I content <2%) prepd. in the presence of MgCl-supported TiCl4-AlEt3 catalysts confirm that significant amts. of syndiotactic polymer are obtained, under proper conditions. Formation of mainly syndiotactic sequences occurs via 1-2 (primary) monomer addn., possibly with a chain-end stereocontrol. At appropriate catalytic sites, sequences can be formed having high syndiotactic stereoregularit
PROPENE POLYMERIZATION IN THE PRESENCE OF MGCL2-SUPPORTED ZIEGLER-NATTA CATALYSTS .4. EFFECTS OF LEWIS-BASES ON POLYMER STEREOCHEMISTRY
No full textThe stereochemistry of polypropenes obtained in the presence of several MgCl2-supported Ziegler-Natta catalyst systems comprising different Lewis bases has been investigated by C-13 NMR spectroscopy, with special attention paid to the ''less tactic'' (heptane-soluble) fractions. These have been found to consist of ''tendentially isotactic'' and ''tendentially syndiotactic'' stereosequences, in largely variable amounts (depending on the catalyst system). The microstructure of the former is in accordance with the enantiomorphic site model, whereas that of the latter suggests a stereocontrol arising from the asymmetry of the growing chain end. In both cases, propene insertion is primary (1-2). Polymerization conditions leading to the formation of ''syndiotacticrich'' polymers showing syndiotactic crystallinity have been identified
PROPENE POLYMERIZATION IN THE PRESENCE OF MGCL2-SUPPORTED ZIEGLER-NATTA CATALYSTS .4. EFFECTS OF LEWIS-BASES ON POLYMER STEREOCHEMISTRY
No full textThe stereochemistry of polypropenes obtained in the presence of several MgCl2-supported Ziegler-Natta catalyst systems comprising different Lewis bases has been investigated by C-13 NMR spectroscopy, with special attention paid to the ''less tactic'' (heptane-soluble) fractions. These have been found to consist of ''tendentially isotactic'' and ''tendentially syndiotactic'' stereosequences, in largely variable amounts (depending on the catalyst system). The microstructure of the former is in accordance with the enantiomorphic site model, whereas that of the latter suggests a stereocontrol arising from the asymmetry of the growing chain end. In both cases, propene insertion is primary (1-2). Polymerization conditions leading to the formation of ''syndiotacticrich'' polymers showing syndiotactic crystallinity have been identified
POLYMERIZATION OF PROPENE IN THE PRESENCE OF MGCL2-SUPPORTED ZIEGLER-NATTA CATALYSTS .2. EFFECTS OF THE COCATALYST COMPOSITION
No full textAs part of a study on the polymn. of propene [115-07-1] in the presence of MgCl2-supported catalysts, results were obtained for polymg. at variable Et benzoate (I) [93-89-0]-alkylaluminum and alkylaluminum-Ti mole ratios. The pretreatment of MgCl2 with the Lewis base is essential to achieve high stereoregularities without an unacceptable decrease of polymn. rate, apparently because it prevents the formation of nonstereospecific active sites of low Lewis acidity, which require high I-alkylaluminum mole ratios in the cocatalyst to be poisoned. The data can be explained in the framework of a model of catalytic sites, presented in Part 1 of this series
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
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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