Interpreting "acidity" as a global property controlling comonomer reactivity in olefin polymerization

A possible rationale for the different catalytic behaviors of systems based on rac-(ethylenebis(1-indenyl))zirconium dichloride (rac-EBIZrCl2), rac-(ethylenebis(1-indenyl))hafnium dichloride (rac-EBIHfCl2), and rac-(isopropylidenebis(1-indenyl))zirconium dichloride (rac-iPrBIZrCl2) toward ethene\u2013styrene copolymerization has been sought by studying related active systems. For this purpose, the metallocene ion pairs (IPs) rac-EBIZrMe\u2014MeB(C6F5)3, rac-EBIHfMe\u2014MeB(C6F5)3, and rac-iPrBIZrMe\u2014MeB(C6F5)3 have been synthesized and their structures in solution explored with ROESY and pulsed gradient NMR spectroscopy. The energetics of dynamical processes relevant for catalysis that can be used as indicators of the cation acidity have been studied with variable-temperature NMR experiments and density functional theory (DFT). NMR experiments successfully provided IP structural details in solution and also indicated the presence of an intricate dynamic behavior for all the IPs. DFT results, instead, indicated quantitatively how changing the metal and/or the ancillary ligand bridge influences the energetics of the active species and modifies the reaction energy profile. The theoretical results also drew attention to the fact that finding a rationale for the ligand influence on the catalytic behavior of rac-EBIZrCl2/MAO and rac-iPrBIZrCl2/MAO in ethene\u2013styrene copolymerization requires not only considering the steric effects but also determining how subtle changes in the ligand sphere affect the capability of the metal center to accept electrons from the counteranion or the olefins

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

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

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

Beta catenin and cytokine pathway dysregulation in patients with manifestations of the "PTEN hamartoma tumor syndrome"

Background. The "PTEN hamartoma tumor syndrome" (PHTS) includes a group of syndromes caused by germline mutations within the tumor suppressor gene "phosphatase and tensin homolog deleted on chromosome ten" (PTEN), characterized by multiple polyps in the gastrointestinal tract and by a highly increased risk of developing malignant tumours in many tissues. The current work clarifies the molecular basis of PHTS in three unrelated Italian patients, and sheds light on molecular pathway disregulation constitutively associated to PTEN alteration. Methods. We performed a combination of RT-PCR, PCR, sequencing of the amplified fragments, Real Time PCR and western blot techniques. Results. Our data provide the first evidence of β-catenin accumulation in blood cells of patients with hereditary cancer syndrome caused by germ-line PTEN alteration. In addition, for the first time we show, in all PHTS patients analysed, alterations in the expression of TNFα, its receptors and IL-10. Importantly, the isoform of TNFRI that lacks the DEATH domain (TNFRSF1β) was found to be overexpressed. Conclusion. In light of our findings, we suggest that the PTEN pathway disregulation could determine, in non-neoplastic cells of PHTS patients, cell survival and pro-inflammatory stimulation, mediated by the expression of molecules such as β-catenin, TNFα and TNFα receptors, which could predispose these patients to the development of multiple cancers

Lunar Gravitational-Wave Antenna

Monitoring of vibrational eigenmodes of an elastic body excited by gravitational waves was one of the first concepts proposed for the detection of gravitational waves. At laboratory scale, these experiments became known as resonant-bar detectors first developed by Joseph Weber in the 1960s. Due to the dimensions of these bars, the targeted signal frequencies were in the kHz range. Weber also pointed out that monitoring of vibrations of Earth or Moon could reveal gravitational waves in the mHz band. His Lunar Surface Gravimeter experiment deployed on the Moon by the Apollo 17 crew had a technical failure rendering the data useless. In this article, we revisit the idea and propose a Lunar Gravitational-Wave Antenna (LGWA). We find that LGWA could become an important partner observatory for joint observations with the space-borne, laser-interferometric detector LISA, and at the same time contribute an independent science case due to LGWA's unique features. Technical challenges need to be overcome for the deployment of the experiment, and development of inertial vibration sensor technology lays out a future path for this exciting detector concept.Comment: 29 pages, 17 figure

Effect of molecular architecture on physical properties of tree-shaped and star-shaped poly(methyl methacrylate)-Based copolymers

The synthesis of star-like A(B)n copolymers based on the hydrophilic poly(ethylene glycol) monomethyl ether (m-PEG, block A) and the hydrophobic poly(methyl methacrylate) (PMMA, blocks B) is reported. We obtained copolymers made of one m-PEG chain and 2 or 4 PMMA blocks using a combined "arm first"-"core first" approach. Such structures were called tree-shaped copolymers where the m-PEG was considered as the trunk and PMMA arms as the branches. Star-like copolymers (B)nA-A(B) n built by two tree-shaped fragments with a poly(propylene oxide) (PPO) as the central junction, were also synthesized according to a previously reported procedure. The latter were called star-shaped structures and the synthesis was performed to obtain architectures different from the tree-shaped one but characterized by a similar length of the PMMA arms. Microstructural analysis was carried out through 1H-NMR and GPC, and the thermal and transport properties (sorption and diffusion) to liquid water were investigated and correlated to the molecular architecture of the two classes of copolymers. \ua9 2014 Taylor and Francis Group, LLC

Dendrimer biocompatibility and toxicity

The field of biomedical dendrimers is still in its infancy, but the explosion of interest in dendrimers and dendronised polymers as inherently active therapeutic agents, as vectors for targeted delivery of drugs, peptides and oligonucleotides, and as permeability enhancers able to promote oral and transdermal drug delivery makes it timely to review current knowledge regarding the toxicology of these dendrimer chemistries (currently under development for biomedical applications). Clinical experience with polymeric excipients, plasma expanders, and most recently the development of more "classical polymer"-derived therapeutics can be used to guide development of "safe" dendritic polymers. Moreover, in future it will only ever be possible to designate a dendrimer as "safe" when related to a specific application. The so far limited clinical experience using dendrimers make it impossible to designate any particular chemistry intrinsically "safe" or "toxic". Although there is widespread concern as to the safety of nano-sized particles, preclinical and clinical experience gained during the development of polymeric excipients, biomedical polymers and polymer therapeutics shows that judicious development of dendrimer chemistry for each specific application will ensure development of safe and important materials for biomedical and pharmaceutical use