1,720,972 research outputs found
The First Molecularly Characterized (Isotactic Polypropylene)-block-Polyethylene Obtained via “Quasi-Living” Insertion Polymerization
No full textZiegler-Natta Catalysts: Regioselectivity and "hydrogen Response"
No full textAdvanced High Throughput Experimentation (HTE) methods were applied to quantify the regioselectivity and the so-called "hydrogen response" in propene polymerization of four Ziegler-Natta catalysts representative of the three latest industrial generations (i.e., modified with a dialkyl phthalate, 1,3-dimethoxypropane or dialkyl succinate Internal Donor), used in combination with AlEt3 and a library of eight alkoxysilane External Donors with large structural diversity. At odds with stereoselectivity, regioselectivity turned out to be only modestly impacted by the chemical structure of the electron donors used as surface modifiers. A clear and simple relationship between regioselectivity and "hydrogen response", and, therefore, polymer molecular weight modulation was highlighted
The First Molecularly Characterized (Isotactic Polypropylene)-block-Polyethylene Obtained via “Quasi-Living” Insertion Polymerization
No full text
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
THE FIRST MOLECULARLY CHARACTERIZED (ISOTACTIC POLYPROPYLENE)-BLOCK-POLYETHYLENE OBTAINED VIA QUASI-LIVING INSERTION POLYMERIZATION
No full textThe synthesis of PE-block-iPP with a Zr-based postmetallocene catalyst was carried out. 13C NMR provided direct and complete evidence of a true block nature for a polyolefin-based material containing a polypropylene block of any tacticity
The Effect of Residence Time Distribution on the Slurry-Phase Catalytic Ethylene Polymerization: An Experimental and Computational Study
No full textThis work is focused on the development and validation of a model accounting for the impact of the reactor residence time distribution in well-stirred slurry-phase catalytic polymerization of ethylene. Particle growth and morphology are described through the Multigrain model, adopting a two-site model for the catalyst and a conventional kinetic scheme. Particle size distribution and polymer properties (average molecular weights and polydispersity) are computed as a function of particle size through a segregated model, assuming that neither breakage nor aggregation occur. Reactors are modeled by means of fundamental mass conservation equations. The model is applied to a system constituted by a series of two ideal continuous stirred tank reactors, where the synthesis of polyethylene with bimodal molecular weight distribution is performed, employing the initial catalyst size distribution as the only adjustable parameter. The model provides insights at the single particle scale for each specific size, thus highlighting the inhomogeneity which arises from the synergic effects of chemical kinetics and residence time distributions in both reactors. The satisfactory agreement between model results and experimental data, in terms of particle size distribution and average molecular weights, confirmed the suitability of the model and underlying assumptions
Improving the Performance of Methylalumoxane: a Facile and Efficient Method to Trap “Free” Trimethylaluminum
No full textVariations 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
BLOCK COPOLYMERS OF HIGHLY ISOTACTIC POLYPROPYLENE VIA CONTROLLED ZIEGLER-NATTA POLYMERIZATION
No full textThe block copolymerization polypropylene under highly isotactic stereocontrol, using a new catalyst obtained by means of rational ligand design, was discussed. Moderately isotactic polypropylenes of very low average molecular mass in reasonable yields was obtained. NMR analysis of samples prepared at different propene concentrations revealed a 1,2 insertion regiochemistry and pointed to trans-alkylation by the Al-alkyl and β-H transfer to the monomer as the dominating chain transfer pathways. The results are in agreement with quantum mechanics/molecular mechanics (QM/MM) calculations
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