1,720,976 research outputs found
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
Tavola Rotonda "Il mercato dell'ICT"
No full textL’annuale rapporto sull’ICT stilato da Assintel e Nextvalue fotografa un mercato statico e ancora ‘in ritardo’: occorre far ripartire la ‘macchina’ e ‘digitalizzare’ il Paese. Ma vediamo il parere di alcuni esperti del settor
Catalysts for olefins polymerization
No full textPolyolefins are still protagonist of an exciting innovation, due to a continuous development of new catalysts, processes and products. The positive solutions given by polyolefins to the environmental and energetic issues are among the factors responsible for their success. The most relevant breakthrough occurred in the last years is the discovery of metallocenes, and more in general, of single centre catalysts. They are, in most cases, highly active catalysts and are already employed on the industrial scale for the preparation of both "drop-in" products with improved properties, and of totally new materials
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
13C-NMR Analysis of alfa-olefins copolymers with 1,3-butadiene obtained with zirconocenes / methylaluminoxane catalysts
No full textVariations 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
POLYMERIZATION OF PROPENE IN THE PRESENCE OF MGCL2-SUPPORTED ZIEGLER-NATTA CATALYSTS .1. THE ROLE OF ETHYL BENZOATE AS INTERNAL AND EXTERNAL BASE
No full textThe effect of BzOEt [93-89-0] as a component of the solid catalyst (internal base) and/or of the cocatalyst (external base) on polymn. of propene in the presence of MgCl2-supported Ziegler-Natta catalysts is discussed. Internal BzOEt mainly prevents TiCl4 from coordination on MgCl2 crystal faces where mostly non-stereospecific active sites would be formed, whereas external BzOEt inhibits non-stereospecific centers selectively and decreases the reaction of the Al alkyl with the internal base. A model of active sites for isotactic 1-alkenes in the presence of MgCl2-supported catalysts is presented, which agrees well with exptl. data
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