All-cis poly(p-phenylene vinylene)s with high molar masses and fast photoisomerization rates obtained through stereoretentive ring-opening metathesis polymerization of [2,2]paracyclophane dienes with various aryl substituents

Poly(p-phenylene vinylene)s (PPVs) are some of the most widely studied conjugated polymers in academia and industry, but most production methods lack precise control over molar masses and alkene stereochemistry. These critical parameters have a large influence on processability, polymer morphology, and optical properties. Herein, we report the stereoretentive ring-opening metathesis polymerization (ROMP) of [2.2]paracyclophane dienes bearing either a linear alkyl substituent or a branched alkoxy substituent to form all-cis PPVs with great solubility in organic solvents. The introduction of 2-ethylhexyloxy side-chains permits the preparation of PPV with uncharacteristically high molar masses (up to 108.8 kg/mol) for a polymerization with living characteristics. Exposure to UV light (365 nm) rapidly induces isomerization of all-cis alkenes leading to the formation of all-trans PPV. This study proposes an attractive strategy to synthesize soluble all-cis and all-trans PPVs with tunable, high molar masses through careful design of the ROMP monomer.     

An Orthogonal Dynamic Covalent Chemistry Tool for Ring-Opening Polymerization of Cyclic Oligochalcogenides on Detachable Helical Peptide Templates

A model system is introduced as a general tool to elaborate on orthogonal templation of dynamic covalent ring-opening polymerization (ODC-TROP). The tool consists of 3₁₀ helical peptides as unprecedented templates and semicarbazones as orthogonal dynamic covalent linkers. With difficult-to-control 1,2-dithiolanes, ODC-TROP on the level of short model oligomers occurs with high templation efficiency, increasing and diminishing upon helix stabilization and denaturation, respectively. Further, an anti-templated conjugate with mispositioned monomers gave reduced templation upon helix twisting. Even with the “unpolymerizable” 1,2-diselenolanes, initial studies already afford mild templation efficiency. These proof-of-principle results promise that the here introduced tool, recyclable and enabling late-stage side chain modification, will be useful to realize ODC-TROP of intractable or unknown cyclic dynamic covalent monomers for dynamer materials as well as cellular uptake and signaling applications.     

Recent advances in integrating platinum group metal-free catalysts in proton exchange membrane fuel cells

Platinum group metal (PGM)-free catalysts are promising low-cost materials for the oxygen reduction reaction in proton exchange membrane fuel cells (PEMFCs). A variety of chemical precursors and synthesis methods have been proposed to increase their catalytic activity. In comparison, significantly less attention has been dedicated to the integration of these PGM-free catalysts into operating electrodes by investigating the role of the membrane electrode assembly (MEA) fabrication on the PEMFC performance. We discuss here some remarkable performance improvements recently achieved by tuning catalyst loading, ionomer content, and ink solvent composition, and call for further explorations of the ink processing and MEA fabrication to improve performance.     

Left-handed properties of erbium-doped crystals

We show that lanthanide-doped crystals can be made left handed for a finite spectral range. The electronic transitions in the dopant ions are both dipolar electric and dipolar magnetic allowed. This enables tuning the permittivity and the permeability at the same frequency. The analysis focuses on erbium-doped crystals where left-handed properties are predicted inside an experimentally reachable domain of parameters.     

Fully Data-Driven Time-Delay Interferometry with Time-Varying Delays

Raw space-based gravitational-wave data like laser interferometer space antenna's (LISA) phase measurements are dominated by laser frequency noise. The standard technique to make this data usable for gravitational-wave detection is time-delay interferometry (TDI), which cancels laser noise terms by forming suitable combinations of delayed measurements. To do so, TDI relies on inter-spacecraft distances and on how laser noise enters the interferometric data. The basic concepts of an alternative approach which does not rely on independent knowledge of temporal correlations in the dominant noise recently introduced. Instead, this automated principal component interferometry (aPCI) approach only assumes that one can produce some linear combinations of the temporally nearby regularly spaced phase measurements, which cancel the laser noise. Then the data is let to reveal those combinations, thus providing a set of laser-noise-free data channels. The authors' previous approach relies on the simplifying additional assumption that the filters which lead to the laser-noise-free data streams are time-independent. In LISA, however, these filters will vary as the constellation armlengths evolve. Here, a generalization of the basic aPCI concept compatible with data dominated by a still unmodeled but slowly varying dominant noise covariance is discussed. Despite its independence on any model, aPCI successfully mitigates laser frequency noise below the other noises' level, and its sensitivity to gravitational waves is the same as the state-of-the-art second-generation TDI, up to a 2% error.     

What controls the pore spacing in porous anodic oxides?

In this paper, we use energy-based perturbation criteria to examine whether strain or electrostatic energy acts as a driving force for porosity initiation in anodic oxides. By doing so, we also succeeded to rationalise the dependence of pore spacing on anodising conditions. Our experimental approach consists of measuring in-situ the internal stress in anodic oxide films grown galvanostatically on aluminium in phosphoric acid, and to correlate these data with the measured pore spacing of the obtained porous films. Our results indicate that the possibility of a strain energy-induced surface instability is unlikely, as for this case the constitutive dependence of pore spacing on internal stress was not verified. Instead, the measured pore spacing, electric field and barrier oxide thickness obtained on our anodic alumina films indicate that electrostatic energy is the main driving force for pore initiation, as well as the factor controlling the pore spacing. Corroborative quantitative evidence for this novel electrostatic-based scaling law is provided by data compiled from the literature for a range of other anodic oxide systems, including nanoporous alumina and nanotubular titania films.     

Magnetic defect structure of iron-rich Fe x O

Mössbauer spectra were recorded at 80 and 4.2 K for Fe _x O (x> 0.95) samples that had been synthesized at 1000 C and pressures up to 10 GPa. The spectra, which consist predominantly of six broadened lines, were fit to three magnetic components: bulk Fe²⁺, defect cluster Fe²⁺ and defect cluster Fe³⁺. Mean hyperfine parameter values for bulk Fe²⁺ were calculated using second-order perturbation theory, and correlations between fluctuations were determined from the mean-square linewidths. Implications for the magnetic defect structure of Fe _x O are discussed.     

Preparation of trimethylsilyl ethers of 3-azetidinols. Scope and Limitations

Preparation of the trimethylsilyl ethers of 1-alkyl-3-azetidinols from non-hindered primary amines and epichlorohydrin by conversion of the intermediate 1-(alkylamino)-3-chloro-2-propanols to their trimethylsilyl ethers by either N-(trimethylsilyl)acetamide or by 1-(trimethylsilyl)imidazole followed by ring closure in acetonitrile is described. This sequence of reactions fails for aromatic amines, but appears to be general for all primary aliphatic amines, although the condensation of hindered amines with epichlorohydrin occurs slowly. Several novel azetidinols, in which the N-alkyl substituent itself contains a second heterocyclic system, are reported. In addition, the pK_A's of several m. and p-substituted 1-benzylazetidinols correlates well with the Hammett equation.     

Bestimmung von Thiosulfat in schwefelhaltigen Mineralw?ssern

Zusammenfassung Zur quantitativen Ermittlung kleinster Thiosulfatmengen in schwefelhaltigen Mineralwässern wurden zwei Methoden ausgearbeitet, die auf der Methylenblau-Ausbleichung und auf der Verzögerung der Methylenblau-Bildung von Sulfid mit Dimethyl-p-phenylendiamin durch Thiosulfat beruhen. Mit diesen Verfahren können noch 0,1 bzw. 0,01 mg/l S₂O₃ ^2– bestimmt werden.

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Summary Two methods have been worked out for the quantitative determination of very small concentrations of thiosulphate ion in mineral waters containing sulphur. The first method is based on the bleaching of methylene blue solution and the second one uses the retarding effect of thiosulphate on the methylen blue reaction between sulphide and dimethyl-p-phenylenediamine. 0.1 and 0.01 ppm of S₂O₃ ^2– resp., can be determined by these methods.