Stereospecific ethylene-styrene block copolymerization with ansa-zirconocene-based catalyst

rac-[Methylene(3-tert-butyl-1-indenyl)(2)]ZrCl2 activated by MAC, employed as catalyst in the ethylene-styrene copolymerization, shows a peculiar behavior. Actually, it produces block copolymers, whereas the homogeneous catalytic systems known to copolymerize these monomers preferentially give rise to alternating sequences of ethylene and styrene. The polystyrene blocks are isotactic and show crystallinity, also when their average length is as short as 10 units. Some evidence suggests the primary styrene insertion into the metal-carbon bond of the growing chain: this unusual regiochemistry could account for the observed tendency to give homosequences

Ethylene-propene copolymerization. Monomer reactivity and reaction mechanism

The relative reactivity of ethylene and propene in the first insertion steps of Ziegler-Natta copolymerization has been evaluated by analyzing via 13CNMR the %enriched end groups of copolymer fractionsofdifferentstereoregularity. Somepossibleinferencesconcerningreactionmechanismarereported

Enantioselectivity of Cs- and C2-symmetric ansa-metallocene catalysts in the styrene insertion

The correlation between relative reactivity of the two styrene enantiofaces and geometry of ansa-zirconocene systems has been determined by C-13 NMR microstructural analysis of the poly(propylene-co-styrene-ethylene) (P/S-E) obtained in the presence of C-2- and C-s-symmetric zirconocene catalysts. The chemical shifts assignment accomplished by using configurational additivity rules suggests that secondary styrene insertion with C-2-symmetric metallocene occurs with the opposite enantioface with respect to primary propylene insertion. On the contrary the analysis of C-13 enriched copolymer obtained in the presence of C-s-symmetric catalyst seems to indicate that the secondary styrene insertion occurs with same enantioface with respect to propylene. This stereochemistry is what one expects for a simple olefin inserting in secondary fashion despite the peculiar behavior of the aromatic monomer toward zirconocene-based catalysts

Structural characterization of syndiotactic propylene-styrene-ethylene terpolymers

The structural features of propylene-styrene-ethylene terpolymer (sP/(S-E)) prepd. using syndiospecific Cs-sym. metallocene catalyst, isopropylidene(cyclopentadienyl)-(9-fluorenyl)zirconium dichloride with MAO co-catalyst were studied. The polymorphism studies of as-prepd., melt-crystd., and fiber sP/(S-E) confirmed that ethylene units are mainly bound to styrene units arising from catalyst reactivation after the secondary insertion of styrene. Styrene-ethylene units are mainly segregated in the amorphous phase. The bulky styrene units, bound to ethylene, prevent the inclusion of ethylene units in the crystals of syndiotactic polypropylene (sPP), as generally occurs for copolymers of sPP with ethylene. This explains the fact that the effect of styrene-ethylene units on polymorphism of sPP is smaller than that obsd. in copolymers of sPP contg. only ethylene or butene comonomer units

Zirconocene-based catalysts for the ethylene-styrene copolymerization: reactivity ratios and reaction mechanism

A kinetic study of the ethylene-styrene copolymerization in the presence of rac-(ethylenebis- (1-indenyl)zirconium dichloride activated by MAO shows the peculiar behavior of this catalytic system that is known to produce crystalline nearly alternating copolymers. Some inferences on the monomer- metal interactions are made on the basis of this study. The parallel investigation on the catalyst based on isopropylidene(1-cyclopentadienyl)(9-fluorenyl)zirconium dichloride allows us to produce a stereoregular nearlyalternatingethylene-styrenecopolymeralsowiththiscatalyticsystem. Thesamemaintacticity seems to characterize the copolymers obtained with the C2 and Cs symmetric catalysts in accordance with the generally accepted mechanism of stereocontrol in the 1-alkene polymerizations

Crystal structure of the stereoregular ethylene-alt-styrene copolymer synthesized with a zirconocene-based catalyst

A stereoregular alternating ethylene-styrene copolymer, also stretchable in fiber form, has been obtained with rac-isopropylidenebis( l-indenyl)zirconium dimethyldiamide activated by the methylalumoxane catalytic system. From X-ray diffraction fiber spectra, an isotactic structure has been assigned to this copolymer as well as the previously described stereoregular ethylene-styrene copolymers. The polymer has zigzag planar chain conformation with phenyl groups oriented perpendicularly to the chain axis. The crystallographic symmetry in B2/m. Lattice constants (monoclinic, unique axis c): a = 10.23, b = 15.53, c = 5.12 Angstrom; gamma = 98.6 degrees. The disagreement index for measured reflections is R-1 = 0.10

Investigation collision of the particles, moving in the stream of gas with the firm wall

The processes of extracting particulate matter include air, generally, the deposition of particles on dry or wetted surfaces and removing deposits from deposition surfaces. In the dust collector and the separation devices following methods of separating suspended solids from the weighing medium used: sedimentation in the gravitational field, sedimentation under the influence of inertial forces, as well as the deposition in a centrifugal field. Shock interaction of particles with the wall is a subject of research for a long time. Much of the research has been devoted to "dry" particles collision with the wall, that is a collision in a vacuum or an environment with little resistance. An important parameter characterizing the process of collision is elastic recovery ratio, defined as the ratio of the speed after impact to the velocity before impact. The calculation results show that the transfer of particles substantially in the horizontal direction depends on the particle diameter. Indeed, with an increase in particle size increases their inertia and the flow can not carry them over long distances. Furthermore, increase in size leads to an increase of deposition rate on the surface. Thus, smaller particles are transported over long distances than larger. This trend is observed for the particle size of less than 100mikron (seen in the movement of air). Next, mode of interaction with the wall varies greatly. Beat ceases to be completely inelastic. Particles incident on the wall is obtained momentum directed away from the wall and carried into the stream. Then, under the influence of gravitational forces they rise speed is slowed down, and the particles begin to move back towards the wall. The number of collisions increases with particle size. Thus, in the hammer mode of interaction the length to which the particles are transported, with their diameter increases

Insights into the G-rich VEGF-binding aptamer V7t1: when two G-quadruplexes are better than one!

The G-quadruplex-forming VEGF-binding aptamer V7t1 was previously found to be highly polymorphic in a K+-containing solution and, to restrict its conformational preferences to a unique, well-defined form, modified nucleotides (LNA and/or UNA) were inserted in its sequence. We here report an in-depth biophysical characterization of V7t1 in a Na+-rich medium, mimicking the extracellular environment in which VEGF targeting should occur, carried out combining several techniques to analyse the conformational behaviour of the aptamer and its binding to the protein. Our results demonstrate that, in the presence of high Na+ concentrations, V7t1 behaves in a very different way if subjected or not to annealing procedures, as evidenced by native gel electrophoresis, size exclusion chromatography and dynamic light scattering analysis. Indeed, not-annealed V7t1 forms both monomeric and dimeric G-quadruplexes, while the annealed oligonucleotide is a monomeric species. Remarkably, only the dimeric aptamer efficiently binds VEGF, showing higher affinity for the protein compared to the monomeric species. These findings provide new precious information for the development of improved V7t1 analogues, allowing more efficient binding to the cancer-related protein and the design of effective biosensors or theranostic devices based on VEGF targeting

miRNA signature predittivo di metastasi linfonodali da carcinoma della laringe

The discovery that miRNAs are frequently deregulated in tumours offers the opportunity to identify them as prognostic and diagnostic markers. The aim of this multicentric study is to identify a miRNA expression profile specific for laryngeal cancer. The secondary endpoint was to identify specific deregulated miRNAs with potential as prognostic biomarkers for tumour spread and nodal involvement, and specifically to search for a miRNA pattern pathognomonic for N+ laryngeal cancer and for N-tissues. We identified 20 miRNAs specific for laryngeal cancer and a tissue-specific miRNA signature that is predictive of lymph node metastases in laryngeal carcinoma characterized by 11 miRNAs, seven of which are overexpressed (upregulated) and four downregulated. These results allow the identification of a group of potential specific tumour biomarkers for laryngeal carcinoma that can be used to improve its diagnosis, particularly in early stages, as well as its prognosis

GIANO-TNG spectroscopy of red supergiants in the young star cluster RSGC2

The inner disk of the Galaxy has a number of young star clusters dominated by red supergiants that are heavily obscured by dust extinction and observable only at infrared wavelengths. These clusters are important tracers of the recent star formation and chemical enrichment history in the inner Galaxy. During the technical commissioning and as a first science verification of the GIANO spectrograph at the Telescopio Nazionale Galileo, we secured high-resolution (R~50,000) near-infrared spectra of three red supergiants in the young Scutum cluster RSGC2. Taking advantage of the full YJHK spectral coverage of GIANO in a single exposure, we were able to identify several tens of atomic and molecular lines suitable for chemical abundance determinations. By means of spectral synthesis and line equivalent width measurements, we obtained abundances of Fe and other iron-peak elements such as V, Cr, Ni, of alpha (O, Mg, Si, Ca and Ti) and other light elements (C, N, Na, Al, K, Sc), and of some s-process elements (Y, Sr). We found iron abundances between half and one third solar and solar-scaled [X/Fe] abundance patterns of iron-peak, alpha and most of the light elements, consistent with a thin-disk chemistry. We found a depletion of [C/Fe] and enhancement of [N/Fe], consistent with CN burning, and low 12C/13C abundance ratios (between 9 and 11), requiring extra-mixing processes in the stellar interiors during the post-main sequence evolution. Finally, we found a slight [Sr/Fe] enhancement and a slight [Y/Fe] depletion (by a factor of <=2), with respect to solar.Comment: Paper accepted on A&