Polymerization of sterically hindered a-olefins with single-site group 4 metal catalyst precursors

A variety of group 4 metal catalytic systems (C2-symmetric {EBTHI}-, {SBI}-type zirconocene complexes (C2-1–4); C1-symmetric (C1-5–8) and Cs-symmetric (Cs-9) {Cp/Flu}-type zirconocene complexes; Cp*2ZrCl2 (Cp* 2-10)), half-metallocene complexes (CpTiCl3, HM-11), constrained-geometry (CGC-12) titanium catalysts) and post-metallocene catalysts (Dow’s ortho-metallated amido-pyridino hafnium complex (PM-13)) have been screened in the polymerization of the sterically demanding 3-methylbut-1-ene (3MB1) and vinylcyclohexane (VCH). All systems proved to be sluggishly active under regular conditions (toluene, 20°C; MAO as cocatalyst) towards 3MB1, with productivities in the range 0–15 kg.mol–1.h–1. Higher productivities (up to 75 kg.mol–1.h–1) were obtained in the polymerization of VCH with C1-symmetric metallocene catalysts under the same conditions, while Cs-symmetric systems were found to be completely inactive. For both 3MB1 and VCH, under all conditions tested, the most productive catalyst appeared to be Dow’s post-metallocene system PM-13/MAO. Optimization of the polymerization conditions led to a significant enhancement of the productivities of this catalyst system towards both 3MB1 and VCH up to 390 and 760 kg.mol–1.h–1, respectively (Tpolym = 70°C). 13C NMR spectroscopy studies revealed that all isolated P(3MB1) and P(VCH) polymers were isotactic, regardless the nature/symmetry of the (pre)catalyst used. The nature of the chain-end groups in P(3MB1) is consistent with two different chaintermination mechanisms, namely b-H elimination/transfer-to-monomer for C2-1/MAO and chain-transfer to Me3Al for PM-13/MAO systems, respectively. For polymerization of VCH with PM-13/MAO at 70°C, b-H elimination / transfer-to-monomer appeared to be the main chain termination reaction

CT-based follow-up following radiotherapy or radiochemotherapy for locally advanced head and neck cancer; outcome and development of a prognostic model for regional control

The purpose of this study is to make a prognostic model for regional relapse in head and neck cancer using clinical and Computed Tomography (CT) parameters.status: publishe

Mutual quenching of Er3+ photolurninescence under two laser excitation in GaN : Er

Erbium photoluminescence in GaN:Er was studied with above-band-gap excitation, provided by a He-Cd (lambda = 325 nm) laser and below-band-gap excitation by a tunable Ti-Sa laser. The spectra obtained with these two lasers exhibit different spectral shapes. When both lasers are used at the same time, we observe that the Er3+ photoluminescence induced by each of the lasers is partly quenched by the illumination of the other laser. In this experiment, one of the lasers is modulated and a lockin amplifier is used to filter the corresponding photoluminescence signal. The spectra recorded this way are found to be linear combinations of spectra obtained with each of the lasers used separately. This effect is explained by the presence of defects mediating the excitation towards the Er3+ ions. These defects act as electron traps, which can be populated by one specific laser excitation and are photo-ionized by the other laser leading to a large quenching of Er3+ emission. (C) 2004 Elsevier Ltd. All rights reserved

Experimental and Computational Investigations on Highly Syndioselective Styrene-Ethylene Copolymerization Catalyzed by Allyl ansa-Lanthanidocenes

International audienceThe syndioselective copolymerization of styrene with ethylene (in bulk or in aliphatic hydrocarbon solutions, (nBu)2Mg as scavenger, Tpolym = 60-140 °C) was achieved in the presence of a series of ansa-lanthanidocenes of the type {R2C(C5H4)(R′R′Flu)}Ln(1,3-C3H3(SiMe3)2)(THF)x (1-Nd-K-allyl, 2-7-Nd, and 2-Sc,La,Sm,Pr). While precursors based on small ionic radius metals (2-Sc) or bearing bulky substituents in 3,6-positions of the fluorenyl moieties (3-Nd and 5-Nd) were poorly or not active under standard polymerization conditions (60 °C), 2-La,Pr,Nd,Sm which bear 2,7-tBu2 substituents on the Flu ligand produced efficiently sPSE materials (productivity 300-400+ kg mol(Ln)-1 h-1, [r]5 = 71%; controlled amount of ethylene inserted in the range 1-15 mol %). Under harsher conditions (Tpolym = 100-140 °C, [St]0/[Nd]0 = 40 000-147 000 equiv), 2-Nd produced similar sPSE materials with a productivity increased by 1 order of magnitude (up to 5430 kg mol(Nd)-1 h-1). Theoretical DFT investigations including the solvent model, performed on the first three insertion steps for the benchmark catalysts {(Me2C(C5H4)(Flu)}Nd(C3H5)(THF) (I), the putative 1-Nd, and the most effective 2-Nd, allowed to corroborate the nature of the obtained copolymers, with ethylene units randomly distributed within long sPS sequences. These studies established also that the presence of bulky substituents on the fluorenyl ligands is crucial in the activity, but the nature of the substituents on the allyl group has no effect on the chemistry and the nature of the resulting styrene-ethylene copolymer but only influences the initiation step in which the first ethylene insertion will be more or less favored with respect to the styrene insertion

Substitution Effects in Highly Syndioselective Styrene Polymerization Catalysts Based on Single-Component Allyl ansa-Lanthanidocenes: An Experimental and Theoretical Study

International audienceA series of allyl ansa-lanthanidocenes, [{Me2C(C5H4)-(Flu)}Nd(1,3-C3H3(SiMe3)(2))(2)]K (Flu = 9-fluorenyl; 1-Nd-K(allyl)) and {R2C(C5H4)(R'R'Flu)}Ln(1,3-C3H3(SiMe3)(2))(THF)(x) (R = Me, R' = 2,7-tBu(2), Ln = Y (2-Y), Sc (2-Sc), x = 0; Ln = La (2-La), Pr (2-Pr), Nd (2-Nd), Sm (2-Sm), x = 1; R = Me, R' = oct = octamethyloctahydrodibenzo, Ln = Nd, x = 1 (3-Nd); R = Ph, R' = H, Ln = Nd, x = 1 (4-Nd); R = Me, R' = 3,6-tBu(2), Ln = Nd, x = 1 (5-Nd)), were prepared in good yields and characterized by NMR spectroscopy (for diamagnetic complexes 2-Sc, 2-Y, and 2-La) and by single-crystal X-ray diffraction (1-Nd-K(allyl), 2-La, 2-Pr, 2-Nd, 2-Sm, and 4-Nd). Those complexes, especially 1-Nd-K(allyl), 2-Nd, 4-Nd, 2-La, and 2-Sm, act as single-component catalyst precursors for polymerization of styrene (in bulk or in aliphatic hydrocarbon solutions, (nBu)(2)Mg as scavenger, T-polym = 60-140 C), affording highly syndiotactic polystyrene (sPS) ([r](5) = 63-88%; T-m up to 260 C). High productivities (up to 4560 kg(sPS) mol(Ln)(-1) h(-1)) were achieved at 120-140 C, at low catalyst loadings ([St]/[Nd] = 20000-76000 equiv), with 2-Nd and 2-Pr. On the other hand, precursors having bulky substituents on the fluorenyl moieties in 3,6-positions (3-Nd, 5-Nd) or based on small ionic radius metals (2-Y, 2-Sc) were poorly or not active under standard polymerization conditions. These results have been rationalized by DFT computations, which included the solvent, carried out on the putative 1-Nd, and the isolated 2-Nd and 5-Nd complexes. Three consecutive styrene insertions were studied, and it was revealed that (i) the formation of sPS is thermodynamically controlled by two effects-minimization of repulsions between fluorenyl/styrene phenyl ring and (in the initiation phase) fluorenyl/SiMe3 substituents of the allyl ligand-and (ii) the presence of bulky substituents on the fluorenyl moiety does not influence the activation barrier of monomer insertion, but it may destabilize thermodynamically the insertion product

Long-Chain Branched Polyethylene via Coordinative Tandem Insertion and Chain-Transfer Polymerization Using rac-{EBTHI}ZrCl2/MAO/Al-alkenyl Combinations: An Experimental and Theoretical Study

International audienceIn situ synthesis of topologically modified linear polyethylenes (PE) using single-site polymerization catalysis is a challenging task, but it can enable the production of valuable advanced polymer materials with tailored properties. Described herein is an investigation aimed at the efficient generation of long-chain branches (LCB) in linear PEs using Al-alkenyl species, namely, iBuAl(oct-7-en-1-yl)2 (Al-1), in combination with homogeneous rac-{EBTHI}ZrCl2 (Zr-1)/MAO or heterogeneous MAO on silica-supported-rac-{EBTHI}ZrCl2 (supp-Zr-1)/TIBAL catalytic systems. As corroborated by extensive rheological studies and 13C NMR spectroscopy, the Al-alkenyl reagent was found to be quite efficient in the formation of LCB, via a mechanistic pathway involving both insertion and transmetallation reactions. Formation of LCB has been rationalized by density functional theory (DFT) computations carried out on the putative [rac-{EBTHI}Zr-R]+ (R = Me, nPr, and pentyl) cationic species and including a solvent model. Of the three possible isomers of Al/Zr heterobimetallic complexes derived from the cationic species [rac-{EBTHI}Zr-R]+ and Al-1, only one was identified, on kinetic and thermodynamic grounds, as the key intermediate. The DFT study also unveiled that (i) insertion of ethylene into the Zr-alkyl bond of the growing PE chain is accompanied by a reversible decoordination of the Al-vinyl transfer agent (AVTA), (ii) the vinyl 1,2-coordination/insertion of the alkenyl moieties of Al-1 into the Zr-alkyl bond, resulting in the formation of branching, is in direct kinetic competition with the insertion of ethylene, and (iii) the recoordination of the AVTA after either insertion step is thermodynamically favored and mostly responsible for the transmetallation phenomenon

Performance assessment of a rapid molecular respiratory syncytial virus point-of-care test : a prospective community study in older adults

BACKGROUND: Respiratory syncytial virus (RSV) causes a substantial burden in older adults. Viral load in RSV-infected adults is generally lower compared to young children, which could result in suboptimal sensitivity of RSV diagnostics. Although the Xpert® Xpress Flu/RSV assay has been used in routine clinical care, its sensitivity to diagnose RSV infection in older adults is largely unknown. We aimed to compare the performance of the Xpert® Xpress Flu/RSV assay with real-time reverse-transcription polymerase chain reaction (RT-PCR) in home-dwelling older adults (≥60 years of age). METHODS: Nasopharyngeal swabs were tested with Xpert® Xpress Flu/RSV and compared to RSV RT-PCR in older adults with acute respiratory tract infections with different levels of disease severity. RESULTS: We studied 758 respiratory samples from 561 older adults from 2 consecutive RSV seasons. Thirty-five (4.6%) samples tested positive for RSV by at least 1 of the assays, of which 2 samples were negative by Xpert® Xpress Flu/RSV and 3 samples by real-time RT-PCR. The positive percentage agreement (PPA) was 90.9% (95% confidence interval [CI], 76.4%–96.8%) and negative percentage agreement was 99.7% (95% CI, 99.0%–99.9%). Viral loads were low (≤103 copies/mL or cycle threshold value ≥34) in all cases with discordant results for the 2 assays. CONCLUSIONS: The PPA of Xpert® Xpress Flu/RSV compared to routine RT-PCR is high for RSV detection in home-dwelling older adults. The assay is fast and easy to use at the point of care. CLINICAL TRIALS REGISTRATION: NCT03621930

Thermal stability and relaxation mechanisms in compressively strained Ge0.94Sn0.06 thin films grown by molecular beam epitaxy

© 2016 Author(s). Strained Ge1-xSnx thin films have recently attracted a lot of attention as promising high mobility or light emitting materials for future micro- and optoelectronic devices. While they can be grown nowadays with high crystal quality, the mechanism by which strain energy is relieved upon thermal treatments remains speculative. To this end, we investigated the evolution (and the interplay) of composition, strain, and morphology of strained Ge0.94Sn0.06 films with temperature. We observed a diffusion-driven formation of Sn-enriched islands (and their self-organization) as well as surface depressions (pits), resulting in phase separation and (local) reduction in strain energy, respectively. Remarkably, these compositional and morphological instabilities were found to be the dominating mechanisms to relieve energy, implying that the relaxation via misfit generation and propagation is not intrinsic to compressively strained Ge0.94Sn0.06 films grown by molecular beam epitaxy.status: publishe