1,721,011 research outputs found
"Chain-end-controlled isotactic" and "stereoblock-isotactic" polypropylene: Where is the difference?
No full textThis communication describes the microstructure of "stereoblock isotactic" polypropylene obtained with sterically hindered "oscillating" metallocene catalysts, points out in which respects it differs from that of "chain-end-control led isotactic" polypropylene, and explains why the two definitions cannot be used as synonymous (as is commonly found in the literature)
“Chain-End-Controlled Isotactic” and “Stereoblock-Isotactic” Polypropylene: Where Is the Difference?
No full textEffect of Cation-Complexing Agents on the Ba(II)-Assisted Basic Ethanolysis of Phenyl Acetate: From Cation Deactivation to Cation Activation
No full text"Supramolecular Catalysis of 1,4-Thiol Addition by Salophen-Uranyl Complexes"
Get PDFour catalysts mimic the behavior of a metalloenzyme,
in that they bind 2-cyclopenten-1-one and promote
reaction of the bound substrate. Release of the addition product,
followed by preferential binding to another enone molecule,
ensures the onset of a catalytic cycle with high turnover efficiency
OSCILLATING" METALLOCENE CATALYSTS: WHAT STOPS THE OSCILLATION?
No full textThe 150 MHz C-13 NMR microstructural analysis of polypropylene samples produced with two representative "oscillating" metallocene catalysts of largely different steric hindrance, namely [(2-(3,5-di-tert-butyl-4-methoxyphenyl)indenyl)(2)ZrP](+) and [(2-phenylindenyl)(2)ZrP](+) (P = polymeryl), and the implications on the origin of the stereocontrol are presented and discussed in detail. The original mechanistic proposal of an "oscillation" between a rac-like (isotactic-selective) and a meso-like (nonstereoselective) conformation cannot explain the observed polymer configuration. The isotactic-stereoblock nature of the polymers obtained with the former catalyst proves unambiguously that the active cation "oscillates" between the two enantiomorphous rac-like conformations at an average frequency that, even at high propene, concentration, is only slightly lower than that of monomer insertion. The less-hindered [(2-phenylindenyl)(2)ZrP](+) gives instead a largely stereoirregular polypropylene, which is the logical consequence of a faster ligand rotation; however, depending on the use conditions (in particular, on the nature of the cocatalyst and the polarity of the solvent), the polymerization products may also contain appreciable amounts of a fairly isotactic fraction. The peculiar microstructure of this fraction, with isotactic blocks of the same relative configuration spanned by very short atactic ones, rules out the possibility that the latter are due to an active species in meso-like conformation and points rather to a conformationally "locked" rac-like species with restricted ring mobility. The hypothesis of a stereorigidity induced by the proximity to a counteranion, which would play the role of the interannular bridge in the rac-bis(indenyl) ansa-metallocenes, was tested by computer modeling on a [rac-(2-phenylindenyl)(2)ZrMe(C3H6)][B(C6F5)(4)] ion couple and found viable
THE STRANGE CASE OF THE "OSCILLATING" CATALYSTS
No full textThe field of stereoselective propene polymerization has been dramatically innovated by the discovery of homogeneous metallocene-based catalysts with well-defined and tunable molecular structure. Of all, "oscillating" metallocenes are probably the most ingenious and challenging example of catalyst design. Their catalytic species were built to "flip-flop" between a chiral and an achiral conformation, at a rate intermediate between those of monomer insertion and chain transfer. The result of this molecular switching would be a polypropylene with an isotactic/atactic stereoblock structure, performing as a thermoplastic elastomer. This essay discusses how the real polymerization mechanism differs from what the catalyst inventors had in mind, but also how - through fortunate circumstances their optimism has been rewarded
"New Insight into the Mechanism of Coniugate Addition of Benzenethiol to Cyclic and Acyclic Enones and of the Corresponding Uranyl-Salophen-Catalysed Version"
No full textChiral supramolecular capsule by ligand promoted self-assembly of resorcinarene-zn porphyrin conjugate
No full textCavitand-Zn porphyrin conjugates self-assemble to give supramolecular (1 + 1) structures upon coordination of bifunctional ligands such as 4,4'-bipyridine and the like. The formation of the capsule depends on key structural factors, such as the size of the cavity, and the possibility of the onset of hydrogen bonds, π-π and π-cation interactions. The extension of this protocol to chiral bifunctional ligands, such as (+)-cinchonine and (-)-cinchonidine, and cinchona alkaloid derivatives, results in the achievement of supramolecular structures with chiral cavities, whose configuration is dependent on the asymmetry of the bound ditopic ligand. MM calculations, gave further insights into the plausible geometry of the structures. Copyright © 2008 Society of Porphyrins & Phthalocyanines
137-Cs Determination and Mutageniticy Tests in Wild Mus musculus domesticus Before and After the Chernobyl Accident
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