1,721,129 research outputs found
Modeling of Catalyzed Chain Growth (CCG) Polymerization of Styrene‐d8 using Cp 2ZrCl2 and Dibenzylmagnesium
No full textA kinetic scheme describing Catalytic Chain Growth (CCG) polymerization was developed and implemented into the computer program PREDICI, by which experimental concentration versus time traces of the participating individual species obtained from online NMR spectroscopy as well as full molecular weight distributions could successfully be modeled. The obtained kinetic coefficients are of high statistical significance. The method was demonstrated on the CCG of styrene-d8 in toluene-d8 using Cp 2ZrCl2 as the catalyst precursor and dibenzyl magnesium as the transfer agent. The quality of the modeling increased dramatically when assuming a chain-length dependent propagation. The extraordinarily good quality of the simulation for all participating species proves that the underlying kinetic model is very likely to be correct. The simulations also demonstrate that this CCG system is a controlled process and that increasing the concentration of the catalyst precursor may lead to a reduced overall polymerization rate
A Kinetic Investigation of the Initialization of Catalyzed Chain Growth of Styrene: The Reaction of Cp 2 ZrCl 2 with Dibenzylmagnesium
No full textThe system Cp 2ZrCl2 and dibenzyl magnesium in toluene, which is a model system for initialization in the catalyzed chain growth (CCG) of styrene using a bifunctional organometallic chain transfer agent, is studied via time-resolved NMR spectroscopy. Based on these data, a mechanism of initialization is proposed. The experimental concentration profiles of the individual species can successfully be modeled to the proposed kinetic scheme. Refined kinetic coefficients of high significance can be obtained by applying analytical methods using pseudo-first-order rate approximations. It is found that the adduct of the Zr catalyst and the Mg compound only slowly transforms via a ligand exchange reaction, yielding the alkylated Zr species. The temperature dependence of these reactions indicates a high reaction barrier of this type of rearrangement, which constitutes the core reaction of CCG in terms of molecular-weight control. image</graphi
Microplastic counts and polymer type in analysis-required aliquots of sediment samples from the Kuril Kamtchatka Trench
No full textThis data table provided information about the microplastic identification analysis carried out on the samples listed in the data set doi:10.1594/PANGAEA.928575. To guarantee a precise quantification of particles, the sample were analysed entirely by µFT-IR. This procedure requires an aliquoting of the samples in subsamples. The number and the volume of subsamples, in the table depict as % aliquot, were established after calculating the number of particles present in each sample, to avoid an overload of the filter necessary to carry out the spectroscopic measurement. By summing the outcomes of the aliquots of a sample, the total number of MP and polymer type per sample was established
Microplastic number and polymer types in sediment samples from the Kuril Kamtchatka Trench
No full textThe dataset below provides information about the microplastic contamination in the Kuril Kamchatka Trench in the NW Pacific Ocean. The samples were collected in summer 2016 during the Kurambio II expedition with RV Sonne. Each sample represents the first 5 cm of the sediment and the above sediment-water interface.
The data table depicts the results of the analysis carried out on 8 Sediment samples, collected with a Multicorer (OKTOPUS GmbH, Kiel, Germany). The samples represent the first five cm of the deep seafloor at four sampling stations of the Kuril Kamchatka trench
Diffusion of single molecular and macromolecular probes during the free radical bulk polymerization of MMA – towards a better understanding of the Trommsdorff effect on a molecular level
Get PDFFree radical bulk polymerizations exhibit complex kinetics due to the viscosity increase during the polymerization process. Especially the termination rate constant can be strongly influenced by the mobility of polymer chains in the polymerization mixture. As a consequence an autoacceleration period, the so-called Trommsdorff effect, can be observed often. In order to investigate this behaviour on a nanoscopic scale, we directly visualized the mobility of molecules and macromolecules in polymerizing MMA solutions using a combination of highly sensitive fluorescence correlation spectroscopy and widefield fluorescence microscopy. For this purpose, rather monodisperse PMMA chains were synthesized by RAFT polymerization and fluorescence-labelled with perylenediimide derivatives. The behaviour of the different fluorescent probes could be related to their size and flexibility. Our studies show that diffusional heterogeneities must be accounted for when modeling bulk polymerization.publishe
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
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