1,721,050 research outputs found
Comparing Pd-catalyzed CO/vinyl arene and ethylene/acrylate copolymerization
No full textPd(II) complexes bearing -diimines as ancillary ligands have been widely applied as catalysts to CO/alkene1 and ethylene/methyl acrylate (MA)2 copolymerization reactions. However, up to now there is a lack of a homogeneous comparison between these two reactions, hampering to highlight their analogies and differences.
For this purpose, we have studied two -diimines with an acenaphthene (BIAN) and two with a diazabutadiene (DAB) skeleton, bearing 1-naphthyl (1-napht) or 2-naphthyl (2-napht) groups on the N-donors. The corresponding Pd(II) complexes [Pd(N-N)(Me)(L)]0/+ (N-N = 1-naphtBIAN, 2-naphtBIAN, 1-naphtDAB, 2-naphtDAB; L = Cl-, MeCN) were characterized both in solid state and in solution. For [Pd(Me)Cl(1-naphtBIAN)], syn and anti isomers (Figure) were found in the crystal structure, in agreement with the characterization in solution, that shows the equilibrium between the two conformers. On the other hand, for the complexes bearing 2-naphthyl-substituted ligands a single species was observed in solution.
The cationic derivatives were tested as precatalysts both in CO/styrene and ethylene/MA copolymerizations under mild conditions, highlighting that: i. in CO/styrene copolymerization BIAN ligands generate more productive and more stable catalysts than DAB derivatives (2.73 kg CP/g Pd vs 0.79 kg CP/g Pd); ii. in ethylene/MA copolymerization opposite trend is found: DAB-containing catalysts are better performing than BIAN-analogues (201.3 g P/g Pd vs 27.0 g P/g Pd); iii. for both reactions, regardless to the ligand skeleton, catalysts with 2-naphthyl-substituted ligands show a catalytic behaviour analogous to that of catalysts with meta-substituted aryl-BIANs, while catalysts with 1-naphthyl-substituted ligands show a catalytic behaviour analogous to that of catalysts with ortho-substituted aryl-BIANs. Catalysts with 1-naphtBIAN or 1-naphtDAB, detected in solution as syn/anti isomers, lead to CO/styrene polyketones with isotactic/atactic stereoblocks, while atactic CO/styrene copolymers are produced with catalysts having the 2-napht-ligands. Ethylene/MA cooligomers, obtained with 1-naphDAB-containing catalyst, are highly branched with 29 % of inserted polar monomer.
1 a) A. Scarel, M.R. Axet, F. Amoroso, F. Ragaini, J.C. Elsevier, A. Holuigue, C. Carfagna, L. Mosca, B. Milani Organometallics 2008, 27, 1486; b) B. Binotti, C. Carfagna, C. Zuccaccia, A. Macchioni Chem. Commun. 2005, 92; c) J.H. Groen, J.G.P. Delis, P.W.N.M. van Leeuwen, K. Vrieze, Organometallics 1997, 16, 68.
2 L. K. Johnson, S. Mecking, M. Brookhart J. Am. Chem. Soc. 1996, 118, 267
'Crystallization and melting behaviour of a main-chain thermotropic copolyester
No full textThe crystallization and melting behavior of a main-chain thermotropic copolyester has been investigated by differential scanning calormetry (DSC). The effect of annealing time and temperature on the transition temperatures and enthalpies has been evidenced. Two melting peaks are observed and the first one clearly develops on annealing. Hypotheses are suggested about the crystallization mechanism
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
'Phase transitions of a main-chain thermotropic copolyester: crystallization, melting behaviour and glass-transition studied by DSC
No full textThe thermal behaviour of a main-chain thermotropic co-polyester has been investigated by Differential Scanning Calorimetry. Crystallization and melting have been studied through the effect of the annealing time and temperature on the transition temperatures and enthalpies. Two melting peaks are observed, the first one clearly developping on annealing. Hypotheses are suggested about the crystallization mechanism. The glass-transition (rate-dependent) appears broadened and does not show hysteresis on heating and cooling through the transition temperature
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
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