Scattering from supramacromolecular structures

We study theoretically the scattering imprint of a number of branched supramacromolecular architectures, namely, polydisperse stars and dendrimeric, hyperbranched structures. We show that polydispersity and nature of branching highly influence the intermediate wavevector region of the scattering structure factor, thus providing insight into the morphology of different aggregates formed in polymer solutions.Comment: 20 pages, 8 figures To appear in PR

Homer 2 tunes G protein–coupled receptors stimulus intensity by regulating RGS proteins and PLCβ GAP activities

Homers are scaffolding proteins that bind G protein–coupled receptors (GPCRs), inositol 1,4,5-triphosphate (IP3) receptors (IP3Rs), ryanodine receptors, and TRP channels. However, their role in Ca2+ signaling in vivo is not known. Characterization of Ca2+ signaling in pancreatic acinar cells from Homer2−/− and Homer3−/− mice showed that Homer 3 has no discernible role in Ca2+ signaling in these cells. In contrast, we found that Homer 2 tunes intensity of Ca2+ signaling by GPCRs to regulate the frequency of [Ca2+]i oscillations. Thus, deletion of Homer 2 increased stimulus intensity by increasing the potency for agonists acting on various GPCRs to activate PLCβ and evoke Ca2+ release and oscillations. This was not due to aberrant localization of IP3Rs in cellular microdomains or IP3R channel activity. Rather, deletion of Homer 2 reduced the effectiveness of exogenous regulators of G proteins signaling proteins (RGS) to inhibit Ca2+ signaling in vivo. Moreover, Homer 2 preferentially bound to PLCβ in pancreatic acini and brain extracts and stimulated GAP activity of RGS4 and of PLCβ in an in vitro reconstitution system, with minimal effect on PLCβ-mediated PIP2 hydrolysis. These findings describe a novel, unexpected function of Homer proteins, demonstrate that RGS proteins and PLCβ GAP activities are regulated functions, and provide a molecular mechanism for tuning signal intensity generated by GPCRs and, thus, the characteristics of [Ca2+]i oscillations

Influence of silica calcination temperature on the performance of supported catalyst SiO2–nBuSnCl3/MAO/(nBuCp)2ZrCl2 polymerizing ethylene without separately feeding the MAO cocatalyst

Abstract The effects of support calcination temperature, an important catalyst synthesis parameter, on the overall performance of the supported catalyst [silica ES70–nBuSnCl3/MAO/(nBuCp)2ZrCl2], polymerizing ethylene without separately feeding the MAO cocatalyst, were studied. The silica was calcined at 250, 450, 600, and 800 8C for 4 h. nBuSnCl3 was used to functionalize the silica. Ethylene was polymerized using the synthesized catalysts at 8.5 bar(g) in hexane for 1 h. No reactor fouling was observed. Free-flowing polymer particles with bulk density (0.23–0.27 g/ml) and a fairly spherical morphology similar to that of the catalyst particles were obtained. Also, the particle size distribution of the polymer resembled that of the catalyst. Therefore, the replication phenomenon from catalyst to polymer took place. The narrow PSD span (1.41) indicates that the resulting polyethylenes are suitable for various mixing-intensive polymer applications. The MAO cocatalyst-free ethylene polymerization instantaneously formed a polymer film around the catalyst particle, which coated/immobilized the catalyst constituents; this is how leaching was in situ prevented which favored heterogeneous catalysis to occur. The catalysts showed fairly stable polymerization kinetics. The catalyst activity, as a function of the silica calcination temperature, varied as follows: 250 8C > 600 8C > 800 8C > 450 8C. This finding has been explained considering the relevant surface chemistry phenomena. The calcination temperature did not significantly affect the bulk density and the PDI (3.4 PDI 3.8) of the resulting polyethylenes. The low PDI substantiates the retention of single-site catalytic behavior of the experimental supported catalysts. # 2007 Elsevier B.V. All rights reserved. Keywords: Supported zirconocene catalysts; Silica functionalization; Calcination temperature; Particle size distribution; Bulk densit

Influence of silica calcination temperature on the performance of supported catalyst SiO2–nBuSnCl3/MAO/(nBuCp)2ZrCl2 polymerizing ethylene without separately feeding the MAO cocatalyst

Abstract The effects of support calcination temperature, an important catalyst synthesis parameter, on the overall performance of the supported catalyst [silica ES70–nBuSnCl3/MAO/(nBuCp)2ZrCl2], polymerizing ethylene without separately feeding the MAO cocatalyst, were studied. The silica was calcined at 250, 450, 600, and 800 8C for 4 h. nBuSnCl3 was used to functionalize the silica. Ethylene was polymerized using the synthesized catalysts at 8.5 bar(g) in hexane for 1 h. No reactor fouling was observed. Free-flowing polymer particles with bulk density (0.23–0.27 g/ml) and a fairly spherical morphology similar to that of the catalyst particles were obtained. Also, the particle size distribution of the polymer resembled that of the catalyst. Therefore, the replication phenomenon from catalyst to polymer took place. The narrow PSD span (1.41) indicates that the resulting polyethylenes are suitable for various mixing-intensive polymer applications. The MAO cocatalyst-free ethylene polymerization instantaneously formed a polymer film around the catalyst particle, which coated/immobilized the catalyst constituents; this is how leaching was in situ prevented which favored heterogeneous catalysis to occur. The catalysts showed fairly stable polymerization kinetics. The catalyst activity, as a function of the silica calcination temperature, varied as follows: 250 8C > 600 8C > 800 8C > 450 8C. This finding has been explained considering the relevant surface chemistry phenomena. The calcination temperature did not significantly affect the bulk density and the PDI (3.4 PDI 3.8) of the resulting polyethylenes. The low PDI substantiates the retention of single-site catalytic behavior of the experimental supported catalysts. # 2007 Elsevier B.V. All rights reserved. Keywords: Supported zirconocene catalysts; Silica functionalization; Calcination temperature; Particle size distribution; Bulk densit