1,720,958 research outputs found
Indenyl-amido titanium and zirconium dimethyl complexes: improved synthesis and use in propylene polimerization
No full textThe synthesis of a series of indenyl amido titanium dimethyl complexes, by means of the direct synthesis from the ligand, a 2-fold excess of MeLi, and TiCl4 is reported. The 1H NMR spectra of the complexes show a quartet structure for the metal-bound methyl groups, due to through-metal proton-proton coupling. Coupling of Ti-methyl protons with protons on the Cp ring is also revealed by COSY 2D-NMR. The performance of the Ti complexes in propylene polymerization, including [Me2Si(Me4C5)(t-BuN)]TiMe2 (1-TiMe2), [Me2Si(Ind)(t-BuN)]TiMe2 (2-TiMe2) and six other methyl titanium complexes bearing substituted indenyl ligands, has been investigated with different cocatalysts and at different polymerization temperatures and propylene concentrations. All complexes produce amorphous polypropylene (am-PP). The catalytic activity and molecular weight strongly depend on the
substitution of the Cp ring: 2-TiMe2 gives polymers of lower molecular weight, while the presence of a methyl group in position 2 (as in 3-TiMe2) determines up to 4-fold increase in molecular weight. The type of cocatalyst influences mainly the catalytic activity, the borates being better activators than MAO, but also molecular weight, with again the borates giving higher molecular weights than MAO. 5-TiMe2-Ph3CB(C6F5)4 shows an overall activation energy of polymerization of 7.35 kcal mol-1. The rate of chain release is first order in monomer. The following activation energies for overall chain release have been calculated: DDE** 2-TiMe2=3.4 kcalmol-1, DDE** 5-TiMe2=3.8 kcal mol-1, DDE** 3-TiMe2=6.3 kcal mol-1. Even if all the polymers produced are amorphous, 2-TiMe2 and 5-TiMe2 show a microstructure unbalanced towards isotacticity, while 3-TiMe2, 6-TiMe2 and 8-TiMe2 are syndiotacticenriched. Chiral induction comes mainly from a weak enantiomorphic site control
Fun&Co: identification of key functional differences in transcriptomes
No full textMotivation: Microarray and other genome-wide technologies allow a global view of gene expression that can be used in several ways and whose potential has not been yet fully discovered. Functional insight into expression profiles is routinely obtained by using gene ontology terms associated to the cellular genes. In this article, we deal with functional data mining from expression profiles, proposing a novel approach that studies the correlations between genes and their relations to Gene Ontology (GO). We implemented this approach in a public web-based application named Fun&Co. By using Fun&Co, the user dissects in a pair-wise manner gene expression patterns and links correlated pairs to gene ontology terms. The proof of principle for our study was accomplished by dissecting molecular pathways in muscles. In particular, we identified specific cellular pathways by comparing the three different types of muscle in a pairwise fashion. In fact, we were interested in the specific molecular mechanisms regulating the cardiovascular system (cardiomyocytes and smooth muscle cells).
Results: We applied here Fun&Co to the molecular study of cardiovascular system and the identification of the specific molecular pathways in heart, skeletal and smooth muscles (using 317 microarrays) and to reveal functional differences between the three different kinds of muscle cells
Ethene/propene copolymerisations with rac-EBTHIZrR2/alumoxane: σ-ligands effect
No full textWe report here about the role played by the sigma-ligands of a metallocene in ethene/propene (E/P) copolymerisations promoted by rac-[ethylenebis(4,5,6,7-tetrahydro-1-indenyl)]zirconium dichloride (1), and its corresponding dimethyl derivative rac-EBTHIZrMe2 (2) and binaphtholate rac-EBTHIZr(BNP) (3) derivative, in the presence of several alumoxanes.
Alumoxanes alternative to the traditional polymethylalumoxane (MAO) were synthesised and characterised. They are based on Al(2,4,4-trimethylpentyl)3 (TIOA) (4) and were prepared by reaction of (4) with water to give either tetraisooctylalumoxane (TIOAO) (5) or polyisooctylalumoxane (TAO) (6) depending on the stoichiometry of the reaction. H-1 NMR spectra of these products are characterised by the presence of broad bands and of resolved multiplets. Their relative amount in the region between 1.9 and 2.5 ppm was identified as the "fingerprint" of the polymerisation activity. A comparison of the results obtained in polymerisation with metallocenes having different sigma-ligands allowed us to shed more light on the role played by those species.
In conclusion, the nature of the sigma-ligands of a metallocene affects its catalytic activity in polymerisation, as a function of the alumoxane employed and of the relative ratio with the metallocene, and this effect is enhanced in "critical conditions," i.e. with a weakly activating cocatalyst or at a low Al/Zr ratio
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
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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