1,720,993 research outputs found
Solid-supported MADIX polymerization of vinyl acetate
No full textA simple, one-pot synthesis of a surface-tethered MADIX agent was established via the reaction of 1,1'-thiocarbonyl diimidazole (TCDI) with a thiol, followed by reaction with, the hydroxyl groups of a Wang resin. The supported RAFT agent was used to control the polymerization of vinyl acetate in toluene. The amount of surface-bound polymer was found to greatly increase with increasing content of solid MADIX agent, reaching values of almost 80%. The grafted PVAc chains were cleaved from the Wang resin support via radical reaction with tert-amyl peroxy acetate TAPA), and the molecular weight distributions exhibited the typical shape of polymers formed via a well-controlled living radical polymerization (LRP), with the free polymers showing a slightly higher molecular weight than the living fraction. The M-n values of both living polymer fraction (surface-bound polymer) and dead polymer (free polymer) increased steadily with monomer conversion-as expected for a well controlled MADIX polymerization-and were in good agreement with the theoretical molecular weights. PDI values of both polymer fractions scatter around 1.5, with a slight tendency of narrower molecular weight distributions of the living polymer, which shows a minimum value of 1.43. Addition of free MADIX agent to the solution phase led to a decrease in PDI values of both free and bound polymer, and. the experimental molar masses were in very good agreement with theoretical predictions
One-Pot RAFT/"Click" Chemistry via Isocyanates: Efficient Synthesis of alpha-End-Functionalized Polymers
No full textA new methodology has been developed for preparing alpha-functional polymers in a one-pot simultaneous polymerization/isocyanate "click" reaction. Our original synthetic strategy is based on the preparation of a carbonyl-azide chain transfer agent (CTA) precursor that undergoes the Curtius rearrangement in situ during reversible addition-fragmentation chain transfer (RAFT) polymerization yielding well-controlled alpha-isocyanate modified polymers. This strategy overcomes numerous difficulties associated with the synthesis of a polymerization mediator bearing an isocyanate at the R group and with the handling of such a reactive functionality. This new carbonyl-azide CTA can control the polymerization of a wide range of monomers, including (meth)acrylates, acrylamides, and styrenes (M-n = 2-30 kDa; D = 1.16-1.38). We also show that this carbonyl-azide CTA can be used as a universal platform for the synthesis of alpha-end-functionalized polymers in a one-pot RAFT polymerization/isocyanate "click" procedure
Polymers with well-defined end groups via RAFT - synthesis, applications and postmodification
No full textThe control over the chain-end functionality of a polymeric chain produced by\ud
controlled/‘living’ radical polymerization is inherent to the mechanism of the re- action. Indeed, \ud
the final product contains a majority of polymeric chains showing an ω-functional end group which is \ud
used to control the molecular weight growth. The growth of the molecular weight can be mediated \ud
either via the reversible homolytic cleavage of the covalent bond between the terminal carbon and \ud
the chain-end group (e.g. halogen for transition-metal-mediated living radical poly- \ud
merization/atom transfer radical polymerization (ATRP), nitroxide for nitroxide- mediated \ud
polymerization (NMP)) or via the degenerative transfer of chain-end groups between propagating \ud
radicals and dormant species (e.g. thiocarbonylthio groups for reversible addition–fragmentation \ud
chain transfer (RAFT)). Furthermore, α-functional end groups can also be introduced via the \ud
initiator/mediator of the polymerization, for example, halogen alkyls (ATRP), alkyl nitroxides \ud
(NMP) or\ud
dithioesters (RAFT)..
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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