Silica/MAO/(n-BuCp)2ZrCl2 catalyst: effect of support dehydroxylation temperature on the grafting of MAO and ethylene polymerization

This study reports the role of silica dehydroxylation temperature on the performance of Silica/MAO/(n-BuCp)₂ZrCl₂ catalysts in ethylene homopolymerization.

The design of a bipodal bis(pentafluorophenoxy)aluminate supported on silica as an activator for ethylene polymerization using surface organometallic chemistry

A well-defined solid activator for supported metallocene polymerization catalysts.

Toward Anisotropic Hybrid Materials: Directional Crystallization of Amphiphilic Polyoxazoline-Based Triblock Terpolymers

We present the design and synthesis of a linear ABC triblock terpolymer for the bottom-up synthesis of anisotropic organic/inorganic hybrid materials: polyethylene-block-poly(2-(4-(tert-butoxycarbonyl)amino)butyl-2-oxazoline)-block-poly(2-iso-propyl-2-oxazoline) (PE-b-PBocAmOx-b-PiPrOx). The synthesis was realized via the covalent linkage of azide-functionalized polyethylene and alkyne functionalized poly(2-alkyl-2-oxazoline) (POx)-based diblock copolymers exploiting copper-catalyzed azide-alkyne cycloaddition (CuAAC) chemistry. After purification of the resulting triblock terpolymer, the middle block was deprotected, resulting in a primary amine in the side chain. In the next step, solution self-assembly into core-shell-corona micelles in aqueous solution was investigated by dynamic light scattering (DLS) and transmission electron microscopy (TEM). Subsequent directional crystallization of the corona-forming block, poly(2-iso-propyl-2-oxazoline), led to the formation of anisotropic superstructures as demonstrated by electron microscopy (SEM and TEM). We present hypotheses concerning the aggregation mechanism as well as first promising results regarding the selective loading of individual domains within such anisotropic nanostructures with metal nanoparticles (Au, Fe3O4).

Constraints on an annihilation signal from a core of constant dark matter density around the milky way center with H.E.S.S

An annihilation signal of dark matter is searched for from the central region of the Milky Way. Data acquired in dedicated on-off observations of the Galactic center region with H.E.S.S. are analyzed for this purpose. No significant signal is found in a total of ∼9  h of on-off observations. Upper limits on the velocity averaged cross section, ⟨σv⟩, for the annihilation of dark matter particles with masses in the range of ∼300  GeV to ∼10  TeV are derived. In contrast to previous constraints derived from observations of the Galactic center region, the constraints that are derived here apply also under the assumption of a central core of constant dark matter density around the center of the Galaxy. Values of ⟨σv⟩ that are larger than 3×10^{-24}  cm^{3}/s are excluded for dark matter particles with masses between ∼1 and ∼4  TeV at 95% C.L. if the radius of the central dark matter density core does not exceed 500 pc. This is the strongest constraint that is derived on ⟨σv⟩ for annihilating TeV mass dark matter without the assumption of a centrally cusped dark matter density distribution in the search region.

Divinyl-end-functionalized polyethylenes: ready access to a range of telechelic polyethylenes through thiol-ene reactions

Telechelic α,ω-iodo-vinyl-polyethylenes (Vin-PE-I) were obtained by catalytic ethylene polymerization in the presence of [(C5 Me5 )2 NdCl2 Li(OEt2 )2 ] in combination with a functionalized chain-transfer agent, namely, di(10-undecenyl)magnesium, followed by treatment of the resulting di(vinylpolyethylenyl)magnesium compounds ((vinyl-PE)2 Mg) with I2 . The iodo-functionalized vinylpolyethylenes (Vin-PE-I) were transformed into unique divinyl-functionalized polyethylenes (Vin-PE-Vin) by simple treatment with tBuOK in toluene at 95 °C. Thiol-ene reactions were then successfully performed on Vin-PE-Vin with functionalized thiols in the presence of AIBN. A range of homobifunctional telechelic polyethylenes were obtained on which a hydroxy, diol, carboxylic acid, amine, ammonium chloride, trimethoxysilyl, chloro, or fluoroalkyl group was installed quantitatively at each chain end.

Site count: is a high-pressure quenched-flow reactor suitable for kinetic studies of molecular catalysts in ethylene polymerization?

Understanding the kinetics of olefin polymerization with Ziegler-Natta and related catalysts means identifying the main kinetic parameters such as k(p) and concentration of active sites. The high-pressure quenched-flow technique could be an absolute method for counting active sites and for activation studies of molecular catalysts. A kinetic study of metallocene catalysts and a phenoxy-imine titanium catalyst in the first instants of polymerization is presented in this paper.

Search for photon-linelike signatures from dark matter annihilations with H.E.S.S

Gamma-ray line signatures can be expected in the very-high-energy (E(γ)>100 GeV) domain due to self-annihilation or decay of dark matter (DM) particles in space. Such a signal would be readily distinguishable from astrophysical γ-ray sources that in most cases produce continuous spectra that span over several orders of magnitude in energy. Using data collected with the H.E.S.S. γ-ray instrument, upper limits on linelike emission are obtained in the energy range between ∼ 500 GeV and ∼ 25 TeV for the central part of the Milky Way halo and for extragalactic observations, complementing recent limits obtained with the Fermi-LAT instrument at lower energies. No statistically significant signal could be found. For monochromatic γ-ray line emission, flux limits of (2 × 10(-7) -2 × 10(-5)) m(-2) s(-1) sr(-1) and (1 × 10(-8) -2 × 10(-6)) m(-2) s(-1)sr(-1) are obtained for the central part of the Milky Way halo and extragalactic observations, respectively. For a DM particle mass of 1 TeV, limits on the velocity-averaged DM annihilation cross section ⟨σv⟩(χχ → γγ) reach ∼ 10(-27) cm(3)s(-1), based on the Einasto parametrization of the Galactic DM halo density profile.

Search for a dark matter annihilation signal from the galactic center halo with H.E.S.S

A search for a very-high-energy (VHE; ≥100  GeV) γ-ray signal from self-annihilating particle dark matter (DM) is performed towards a region of projected distance r∼45-150  pc from the Galactic center. The background-subtracted γ-ray spectrum measured with the High Energy Stereoscopic System (H.E.S.S.) γ-ray instrument in the energy range between 300 GeV and 30 TeV shows no hint of a residual γ-ray flux. Assuming conventional Navarro-Frenk-White and Einasto density profiles, limits are derived on the velocity-weighted annihilation cross section (σv) as a function of the DM particle mass. These are among the best reported so far for this energy range and in particular differ only little between the chosen density profile parametrizations. In particular, for the DM particle mass of ∼1  TeV, values for (σv) above 3×10(-25)  cm(3) s(-1) are excluded for the Einasto density profile.

A RAFT Analogue Olefin Polymerization Technique Using Coordination Chemistry

The present paper highlights analogies between one of the most efficient control radical polymerization techniques namely the reversible addition–fragmentation chain transfer process, and the catalyzed polyethylene chain growth, the only technique that can controlled olefins polymerization through coordination chemistry under catalytic conditions.