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Rumi L.,Albert Ludwigs University of Freiburg | Scheuermann G.M.,Albert Ludwigs University of Freiburg | Mulhaupt R.,Freiburg Institute for Advanced Studies FRIAS | Bannwarth W.,Albert Ludwigs University of Freiburg
Helvetica Chimica Acta

Pd 2+-Exchanged graphite oxide (GO) serves as a precatalyst for the formation of Pd-nanoparticles which are then deposited on the highly functionalized carbonaceous support. This versatile, air-stable, and ligand-free system was applied successfully to Suzuki-Miyaura couplings of some aryl chlorides and to the Mizoroki-Heck as well as the Sonogashira reaction showing relatively high activities and good selectivities. Like with other ligand-free supported systems, the reaction proceeded dominantly by a homogeneous mechanism, but attack of an aryl iodide to Pd-nanoparticles can be excluded as substantial contribution to the entire catalytic process. Beside its straightforward preparation and its stability in air, the system combines the advantages of both homogeneous and heterogeneous catalysis. © 2011 Verlag Helvetica Chimica Acta AG, Zürich, Switzerland. Source

Kurek A.,Albert Ludwigs University of Freiburg | Xalter R.,Albert Ludwigs University of Freiburg | Sturzel M.,Albert Ludwigs University of Freiburg | Mulhaupt R.,Albert Ludwigs University of Freiburg | Mulhaupt R.,Freiburg Institute for Advanced Studies FRIAS

Highly active single- and dual-site catalysts supported on silica nanofoams (NF) enable the control of both polyethylene (PE) morphology and tailoring of bimodal PE molar mass distribution in catalytic ethylene polymerization. In a templating process, aqueous polystyrene (PS) nano suspensions are mineralized and calcinated at 600 C, thus producing NF with specific surface area of 1200 m2/g and average pore diameter varying between 20 and 80 nm. Mineralization of the aqueous PS nano suspensions in a water-in-oil emulsion affords spherical NF with average diameter of 40 μm and control of pore size. The methylalumoxane- (MAO-) tethered NF immobilizes single-site catalysts such as metallocene (nBuZr), bisiminopyridine iron(II) (Fe) and constrained geometry chinolyl cyclopentadienyl chromium(III) complexes (Cr-1). Immobilization of binary blends of Fe/Cr-1 and nBuZr/Cr-1 affords NF-supported dual-site catalysts. The catalyst activity, PE particle size and molecular weight distribution varies as a function of the NF pore size. Typically, macroporous NF (pore size of 75 nm) are effective supports for Cr-1, producing ultrahigh molecular weight polyethylene (UHMWPE, Mw > 106 g/mol). Dual -site catalysts such as nBuZr/Cr-1 on mesoporous NF (pore size of 20 nm) enable tailoring of bimodal PE molar mass distribution with UHMWPE content increasing with increasing Cr-1/nBuZr molar ratio. © 2013 American Chemical Society. Source

Kurek A.,Albert Ludwigs University of Freiburg | Mark S.,University of Heidelberg | Enders M.,University of Heidelberg | Kristen M.O.,Basell Polyolefine GmbH | And 2 more authors.
Macromolecular Rapid Communications

A ternary blend of the bisiminopyridine chromium (III) (Cr-1) with the bisiminopyridine iron (II) (Fe-2) postmetallocenes with the quinolylsilylcyclopentadienyl chromium (III) halfsandwich complex (Cr-3) was supported on mesoporous silica to produce novel multiple single-site catalysts and polyethylene reactor blends with tailor-made molecular weight distributions (MWDs). The preferred cosupporting sequence of this ternary blend on MAO-treated silica was Fe-2 followed by Cr-1 and Cr-3. Cosupporting does not impair the single-site nature of the blend components producing polyethylene fractions with M̄w = 104 g · mol-1 on Cr-1, M̄w = 3 × 105 g · mol-1 on Fe-2, and M̄w = 3 × 106 g mol-1 on Cr-3. As a function of the Fe-2/Cr-1/Cr-2 mixing ratio it is possible to control the weight ratio of these three polyethylenes without affecting the individual average molecular weights and narrow polydispersities of the three polyethylene fractions. Tailor-made polyethylene reactor blends with ultra-broad MWD and polydispersities varying between 10 and 420 were obtained. When the molar ratio of Fe-2/Cr-1 was constant, the ultra-high molecular polyethylene (UHMWPE, M̄w 106 g-mol-1) content was varied between 8 and 16 wt.-% as a function of the Cr-3 content without impairing the blend ratio of the other two polyethylene fractions and without sacrificing melt processability. When the molar ratio Fe-2/Cr-3 was constant, it was possible to selectively increase the content of the low molecular weight fraction by additional cosupporting of Cr-1. Due to the intimate mixing of low and ultra-high molecular weight polyethylenes (UHMPEs) produced on cosupported single-site catalysts a wide range of melt processable polyethylene reactor blends was obtained. (Figure Presented) © 2010 WlLEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Gall B.T.,Albert Ludwigs University of Freiburg | Thomann R.,Albert Ludwigs University of Freiburg | Mulhaupt R.,Albert Ludwigs University of Freiburg | Mulhaupt R.,Freiburg Institute for Advanced Studies FRIAS
Journal of Polymer Science, Part A: Polymer Chemistry

iPS-b-PDMS-b-iPS triblock copolymers were prepared by hydrosilylation of vinyl-terminated isotactic polystyrenes (iPS) with α,Iω- bis(dimethylsilane)-terminated poly(dimethylsiloxane)s (PDMS). As a function of the molecular weights of the two components, the triblock copolymer composition was varied between 9.0 and 98 wt % iPS. The resulting triblock copolymers remained soluble during block copolymer synthesis due to slow iPS crystallization in solution. At iPS content exceeding 31 wt %, the iPS crystallization was achieved by postpolymerization annealing and melt processing. The triblock copolymers melted above 200 °C with melting temperatures very similar to those of the corresponding iPS homopolymers. Nanostructure and microstructure formation of both amorphous and semicrystalline triblock copolymers were examined by means of light microscopy, atomic force microscopy, and TEM measurements. © 2011 Wiley Periodicals, Inc. Source

Raue A.,Albert Ludwigs University of Freiburg | Kreutz C.,Albert Ludwigs University of Freiburg | Theis F.J.,Helmholtz Center Munich | Timmer J.,Albert Ludwigs University of Freiburg | And 2 more authors.
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences

Increasingly complex applications involve large datasets in combination with nonlinear and highdimensional mathematical models. In this context, statistical inference is a challenging issue that calls for pragmatic approaches that take advantage of both Bayesian and frequentist methods. The elegance of Bayesian methodology is founded in the propagation of information content provided by experimental data and prior assumptions to the posterior probability distribution of model predictions. However, for complex applications, experimental data and prior assumptions potentially constrain the posterior probability distribution insufficiently. In these situations, Bayesian Markov chain Monte Carlo sampling can be infeasible. From a frequentist point of view, insufficient experimental data and prior assumptions can be interpreted as non-identifiability. The profile-likelihood approach offers to detect and to resolve non-identifiability by experimental design iteratively. Therefore, it allows one to better constrain the posterior probability distribution until Markov chain Monte Carlo sampling can be used securely. Using an application from cell biology, we compare both methods and show that a successive application of the two methods facilitates a realistic assessment of uncertainty in model predictions. © 2012 The Author(s) Published by the Royal Society. All rights reserved. Source

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