Ethylene-propylene copolymerization initiated with solubilized ziegler-natta macromolecular complexes. II. Characterization of [styrene]-b-[ethylene-co-propylene] copolymers

Copolymerization of ethylene (E) with propylene (P) initiated with soluble (polystyrene)₁₀₀-butadiene₃-Li/TiCl₄ complexes leads to [St]-b-[E-co-P] diblock copolymers with a random [E-P] block. GPC and ¹³C-NMR analyses of copolymers with various compositions and block lengths disclose well-defined linear structures. From DSC measurements of such materials, three different endothermal transition regions are observed: two glass transition regions corresponding, respectively, to the T_g of styrenic and olefinic blocks, and one [E-P] block melting region in which propylene units play a primary role. Furthermore, an additional endotherm is observed, depending on the thermal history of the sample.     

Semiclassical treatment of ro-vibrational relaxation in the large j limit. Application to CO + 4He collisions

A semiclassical model for the vibrational relaxation of a diatomic molecule in collision with an atom is presented in the coupled-state framework in the large j limit. Our model realizes an exact quantum resolution of the rotational dynamics, the vibrational dynamics being integrated within a first-order perturbational perturbed-stationary-state approach. The application to CO + ⁴He confirms an appreciable role of the rotational energy transfers which produce an increase of the rate constant by a factor of ≈ 2.5.     

Theories egalitaires dans les Langages sur types de graphes

Sans résumé Ce travail doit son origine et son développement, aux préoccupations de recherche qui animent le groupe de “Logique et théorie des catégories,” dirigé par le Professeur A. Preller à l'U.E.R. de Luminy (Université d'Aix-Marseille II). Presented by J. D. Monk.     

Stereoelectronic Properties of Tetrahedral Species derived from Carbonyl Groups. Ab initio study of the hydroxymethanes

Analysis of the electronic structure of the hydroxymethanes provides a consistent picture of stereoelectronic effects in such molecules: The average C? O bond length decreases in the series methanol, methanediol, methanetriol. An oxygen (O′) lone pair, which is trans-anti-periplanar (app) to another oxygen (O″), shortens and strengthens the C? O′ bond and simultaneously lengthens and weakens the C? O′ bond. This is consistent with solid state structural evidence and with the reactivity patterns of tetrahedral species resulting from nucleophilic addition to a carbonyl group.     

Theoretical study of the Diels-Alder reaction

The cycloaddition of ethylene to butadiene has been studied by theab initio LCAO-SCF-MO method of Roothaan using STO-3G and 7s-3p basis sets. The potential energy hypersurface of the supersystem formed by the reactants has been calculated in order to determine the reaction path. It was found that, during the approach of the partners, the planes of the molecules form an angle around 70 °. The activated complex has a geometry which prefigures the half-chair conformation of cyclohexene and exhibits no biradical character. Our theoretical results are in reasonable agreement with the corresponding experimental ones.     

EXAFS investigation of nickel and cobalt precursors of homogeneous ziegler-type hydrogenation catalysts

A preliminary EXAFS study is reported of precursors of homogeneous hydrogenation catalysts obtained by addition of AlEt₃ to nickel or cobalt octoate solutions in benzene. For a given nickel octoate solution studied before reduction, monodentate coordination of the nickel cations to at least four carboxylic anions was established and accurate Ni...O₁, Ni...C₁ distances (i.e. 2.06±0.01 Å, 3.01±0.02 Å) were determined. The spectra of the reduced solutions are however clearly dominated by intense Ni^*...Ni or Co^*...Co signals at distances comparable to the relevant distances of the bulk metal, and thus suggest the presence of rather disordered, amorphous-like metal clusters. Strong evidence is also produced in favour of additional metal...carbon bonds which were often assumed to play an important role in the catalytic activity of these or related solutions.     

MO-Calculation of the Optical Activity of Oligopeptides. I. Computational Procedure and Application to Small Helices

All-valence electron calculations on the chiroptic properties of oligopeptides rapidly become intractable, as the size of the molecule increases. A Frozen Core procedure is here proposed, which is inspired by the PPP method. It takes explicitly into account only the (pseudo) π electrons and oxygen n electrons of the amide moieties, as well as the π electrons of eventual long-wavelength absorbing sidegroups. Local n-π interaction is taken into account by an adjustable parameter. Other parameters are calibrated on the isolated amide and carbonyl chromophores. The method then follows the usual SCF-CI procedure. Rotatory strengths and f-values for the transitions are computed without further approximations, as previously described. Comparisons with results from exciton calculations and experimental data on polypeptides show the computed quantities to have a consistent and satisfactory order of magnitude.     

Synthesis of (±)-negamycin and of (±)-epinegamycin

The antibiotic negamycin and its diastereomer have both been synthesized in racemic form. The latter has been found to possess antibacterial activity, but at a lower level than negamycin itself.     

The Poincare-Dulac theorem for nonlinear representations of nilpotent lie algebras

The aim of this Letter is to show that the Poincare-Dulac theorem for holomorphic finite-dimensional representation, is valid for any nilpotent Lie algebrag. We reduce the classification problem of representations with a semisimple linear part satisfying the Poincaré condition to an algebraic problem. We develop a complete computation in a particular case.