1,721,002 research outputs found
Catalyst activation and the dimerization energy of alkylaluminium compounds
No full textAlkylation of olefin polymerization catalyst precursors with Me3Al (or MAO) is a crucial first step in the activation of many polymerization catalysts. However, structures with alkyls and/or halides bridging between aluminium atoms, are known problem cases for DFT methods. We have explored the scope of the problem and possible solutions. The performance of various DFT methods for a range of bridged dimers II-12Mi2(1L-X)2 and [Me2Mi2(1-X)2 (M = Al TI, X = H, Me, NMe2, OMe, F, Cl) and related compounds was investigated in a systematic fashion. In the absence of reliable experimental data, highly accurate calculated dimerization and complexation energies (extrapolated to the CCSD(T,Full)/aug-ccpVQZ level of theory: fully correlated Coupled-Cluster Theory with Single and Double and Perturbative Triple excitations) for 26 model systems relevant to Ziegler Natta catalysis and related chemistry serve as reference. Most functionals (strongly) underestimate the dimerization energy for both electrondeficient (hydride, alkyl) and regular (halide, alkoxide, amide) bridges, and for both main-group and transition metals. To reach "chemical accuracy", a basis set of VTZ quality (with BSSE correction) or VQZ (without BSSE correction) is recommended, in combination with the M06-2X functional, or with the TPSSTPSS functional plus Grimme's D2 correction. Subsequently, we explored alkylation of olefin polymerization catalyst precursors LTiX2 (L = bis(cyclopentadienyl), Salan or Salalen) with Me3Al to LTiMe2. Using our recommended procedure, we find that the process is exergonic for most electronegative groups X (F, Cl, OR, NR2) provided the dimerization energies of Me3Al and Me2AIX are taken into account. Moreover, alkylation is more exergonic for metallocenes, which we ascribe to the more saturated nature of the Cp2M fragment. Alkylation is predicted to be more favourable for amides and alkoxides than for the widely used chloride precursors, and failure to exploit those precursors in catalysis is likely due to kinetic issues caused by steric factors. In particular, employing metal alkoxides or amides with small alkyl groups (0Me, OEt) rather than the commonly used bulky 01Pr and OtBu groups might yield catalyst precursors that can be activated more easily than chlorides
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
Accurate Prediction of Copolymerization Statistics in Molecular Olefin Polymerization Catalysis: The Role of Entropic, Electronic, and Steric Effects in Catalyst Comonomer Affinity
No full textAccurate in silica prediction of copolymerization performance of olefin polymerization catalysts is demonstrated. It is shown by the example of 19 metallocene and post-metallocene group IV metal (Ti, Zr, Hf) systems that DFT (M06-2X(PCM)/TZ//TPSSTPSS/DZ) can accurately describe the copolymerization factor r(e): i.e., the competition of ethene and propene for insertion in metal n-alkyl bonds. Experimental r(e) values were computationally reproduced with a mean average deviation (MAD) and maximum deviation of only 0.2 and 0.5 kcal/mol, respectively. Both dispersion and solvent corrections play a crucial role in achieving this accuracy. Ethene insertion is found to be entropically favored for all catalysts due to a combination of symmetry factors and less congested insertion geometries. The enthalpic preference for either ethene or propene is catalyst dependent. The predictions are based on straightforward calculation of relevant insertion transition state energies; there are no indications for a shift in rate-limiting step from insertion to e.g. olefin capture or chain rotation
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
Polyolefin chain shuttling at ansa-metallocene catalysts: legend and reality
No full textSeveral papers have claimed the amenability for binary metallocene systems to undergo coordinative chain transfer polymerization (CCTP) to produce stereoblock isotactic/syndiotactic polypropylene materials. In this work, we explored through high throughput experimentation (HTE) tools and methods the propensity of the isotactic-selective rac-Me2Si(2-Me-4-Ph-1-Ind)2ZrCl2 (2) catalyst to mediate propene CCTP, individually or in tandem with the syndiotactic-selective ansa-zirconocene Ph2C(Cp)(2,7-di-tBu-9-Flu)ZrCl2 (3), using ZnEt2 and methylalumoxane (MAO) as chain shuttling agents. The results demonstrated that 2 is not prone to propene CCTP, and therefore does not yield stereoblock PP materials when used in racemic form or in tandem with other metallocenes. An unambiguous method to demonstrate the occurrence of CCTP in isotactic propene polymerization, based on the microstructural characterization of the sample by 13CNMR spectroscopy, is also proposed
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