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Polymerization of ethene and propene at bis(phenoxyamine)Zr-based catalysts: so similar, so different!
No full textMetallocene-Catalyzed Propene Polymerization: From Microstructure to Kinetics. 1. C2-Symmetric ansa-Metallocenes and the "Trigger" Hypothesis
No full textA microstructural method for the determination of the reaction order with respect to the monomer in coordinated 1-alkene polymerizations is proposed. The method is applicable whenever enantioselective monomer polyinsertion competes with an intramolecular process introducing stereodefects of known and recognizable nature in the growing polymer chains at a rate independent of monomer concentration, [M] (e.g., chain or site epimerization for C-2-symmetric and C-s-symmetric ansa-metallocene catalysts, respectively). Indeed, in such cases, the stereoregularity of the polymer obtained at a given temperature results from the balance between the rates of the two competing processes, and is obviously a function of [M] because so is the rate of polyinsertion, . Therefore, from the way polymer stereoregularity changes with [M], the corresponding change of can be inferred. Compared with the conventional macroscopic approach based on measurements of monomer consumption, the microstructural route is more reproducible, and allows one to discriminate between "authentic" and apparent higher-order polyinsertion rate laws. In this paper, in particular, we show that the "trigger" hypothesis invoked to explain the observed pseudo-second-order kinetics of propene polymerization promoted by C-2-symmetric ansa-metallocene catalysts is inconsistent with the experimental results and that an alternative explanation assuming first-order kinetics at catalytic species which interconvert between a propagating and a "resting" state is more plausible
High-Throughput Screening in Olefin-Polymerization Catalysis: From Serendipitous Discovery Towards Rational Understanding.
No full textHigh-throughput-screening (HTS) tools and methods are used more and more, especially in industry, in the search for new, selective organometallic catalysts. In most cases, the approach is, in essence, empirical, and the strategy is to increase the number of experiments that can be run at a given place in a given time. Highly miniaturized, parallel reaction setups have been implemented for the rapid assessment of whether novel catalysts resulting from the structural amplification of a basic framework are "good" or "bad" with respect to the properties of interest, and, depending on the response, worthy of a subsequent, more-careful evaluation. In this article, we demonstrate that it is possible to utilize these state-of-the-art HTS platforms with a different strategy: the rapid generation of reliable kinetic data for mechanistic studies in view of a thorough understanding and rational catalyst design. Ziegler-Natta-type catalytic olefin polymerization will be used throughout as an example
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
METALLOCENE-CATALYZED PROPENE POLYMERIZATION: FROM MICROSTRUCTURE TO KINETICS, 2 - CS-SYMMETRIC ANSA-ZIRCONOCENES
No full textHighly syndiotactic propene polymn. in the presence of Cs-sym. metallocene cations is much less obvious than it may appear based on symmetry considerations alone. Effectively preventing the generation of m stereo-defects requires the assistance of a counterion able to slow site epimerization more than monomer insertion, but also seconding chain-migratory insertion in such a way that the pathway are not competitive. A good success in this delicate balance was achieved at the first attempt with MAO, in turn serendipitously discovered
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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