Effects of Hydrogen for Different Stereospecific Active Sites on Ultra Low TiCl3 Loading Supported Catalyst

The relationship between the stereospecificity of active sites and hydrogen effects on propylene polymerization was investigated for MgCl₂‐supported TiCl₃ catalysts at ultra‐low Ti contents. Hydrogen had no effect on the catalyst activities for isospecific sites and for aspecific sites. The efficiency of hydrogen as a chain‐transfer agent was found to depend on the stereospecificity of the active sites. Hydrogen was effective for isospecific sites but was not effective for aspecific sites. From the viewpoint of hydrogen dissociation, isospecific sites should have hydrogen dissociation sites in their surroundings. Therefore, isospecific sites may be in the island to be affected by hydrogen and isolated sites may be aspecific or have low isospecificity.

     

1H NMR titration study of stimuli-responsive supramolecular assemblies: inclusion complexes between PEG–b-PEI copolymer-grafted dextran and naphthalene-appended γ-cyclodextrin via double-strand inclusion

We have previously prepared a stimuli-responsive inclusion complex between PEG–b-PEI–g-dextran graft copolymer (PEG–PEI–dex) and γ-cyclodextrin (γ-CD) in order to investigate unique inclusion phenomena, double-axle inclusion. For further study, a γ-CD derivative, mono-6-O-(2-sulfonato-6-naphthyl)-γ-CD (SN-γ-CD) was additionally synthesized for ¹H NMR titration study, which is expected to induce the competition of pendant naphthyl group with external polymer guests. Consequently, ¹H NMR titration results of the inclusion complex of PEG–PEI–dex with SN-γ-CD showed stoichiometric changes, temperature-dependence, and reversibly pH-responsive properties of the inclusion complexes in terms of chemical shift variation.     

"O-acyl isopeptide method" for peptide synthesis: synthesis of forty kinds of "O-acyl isodipeptide unit" Boc-Ser/Thr(Fmoc-Xaa)-OH

The O-acyl isopeptide method has recently received attention as an efficient synthetic method for peptides. Herein, forty kinds of "O-acyl isodipeptide unit" Boc-Ser/Thr(Fmoc-Xaa)-OH (1-40) were effectively synthesized in two-steps without epimerization. The O-acyl isodipeptide units are important building blocks to enable the routine use of the O-acyl isopeptide method.     

Preparation of alpha-cyclodextrin-terminated polyrotaxane consisting of beta-cyclodextrins and pluronic as a building block of a biodegradable network

A beta-CD-based biodegradable polyrotaxane was prepared by capping both terminals of polypseudorotaxane consisting of hydrazide-terminated PEG-block-PPG-block-PEG (Pluronic P-105) and beta-CD-succinates with mono-aldehyde alpha-CDs. By decreasing pH, the fluorescent intensity of TNS was increased with time, indicating cleavage of the terminal hydrazone bonds followed by beta-CD-succinate release. The terminal alpha-CD moieties of the polyrotaxane are useful for self-assembled formation with some guest molecules. [Diagram: see text]     

Size effect on crystal structure and chemical bonding nature in BaTiO<Subscript>3</Subscript> nanopowder

Summary The size effect on the crystal structure including the chemical bonding nature has been investigated for several kinds of BaTiO₃ nanopowder with the particle sizes down to 50 nm in diameter, by means of powder diffraction using high-energy synchrotron radiation. The Rietveld refinement reveals that the BaTiO₃ nanopowder consists of tetragonal and cubic structure components at 300 K. The feature of coexistence can be illustrated by the core/shell model for the particle, in which the shell with a cubic structure covers the core with a tetragonal structure. The thickness of the cubic shell is almost constant irrespective of the particle sizes, and is estimated as approximately 8 nm. Hence, the critical particle-size, where the entire particle is covered with the cubic shell, is suggested as 16 nm. The charge density distributions of the BaTiO₃ nanopowder in the cubic phase at 410 K are revealed by the maximum entropy method. Changes in the bonding electron density and the ionic valence expected are not observed clearly even in the 50 nm crystal compared with the bulk crystal.     

Thermodynamic properties of Rb2ZnCl4 associated with three phase transitions

Transitions were observed by heat capacity measurements at 74.6 K, 195.2 K, and 303 K. They are a soft mode transition (ΔH_t = 30 J mol^?1, ΔS_t = 0.42 J K^?1 mol^?1), a first-order commensurate-incommensurate transition (ΔH_t = 6.2 J mol^?1ΔS_t = 0.032 J K^?1 mol^?1), and a second-order incommensurate-normal transition, respectively.     

Spin-frustrated (VO)36+-triangle-sandwiching octadecatungstates as a new class of molecular magnets

Spin-frustrated polyoxometalates, K(11)H[(VO)(3)(SbW(9)O(33))(2)].27H(2)O (1) and K(12)[(VO)(3)(BiW(9)O(33))(2)].29H(2)O (2), containing approximately equilateral and isosceles (VO)(3)(6+)-triangles (V(IV)...V(IV) separation of 5.4-5.5 A) sandwiched by two diamagnetic alpha-B nonatungstate ligands ([SbW(9)O(33)](9)(-) and [BiW(9)O(33)](9)(-)) with approximate D(3)(h) symmetry, are found to show magnetization jumps with distinct hysteresis for the S = (1)/(2) <--> S = (3)/(2) level crossing under fast sweeping pulsed magnetic fields (approximately 10(3) T/s) at T < or = 0.5 K. This unusual phenomenon is attributed to the theoretical prediction of half step magnetization, which is expected for an antiferromagnetic spin triangle with antisymmetrical Dzyaloshinky-Moriya interaction. The degeneracy of the S = (1)/(2) states for 1 is removed by slightly lower symmetry effects of triangular structure for 2. The calorimetry of 1 and 2 shows the heat capacity anomaly at 2 < or = T < or = 20 K which is associated with a thermal excitation from the S = (1)/(2) ground states to the S = (3)/(2) state at zero field. Zero-field splitting energies (5-7 K) between S = (1)/(2) and S = (3)/(2) states for 1 and 2, readily estimated by the level-crossing field for the magnetization, allow us to measure the hyperfine-structural 22 lines due to three equivalent I = (7)/(2) (51)V nuclei, the fine-structural triplet line of the S = (3)/(2) excited state, and the g anisotropy on the high-frequency ESR spectra. The spin-frustrated (VO)(3)(6+)-triangle for 1 and 2 is a good model of the magnetization between pure quantum states S = (1)/(2) and (3)/(2) and provides a new class of single-molecule magnets.     

Thermogravimetry of metal chelates of 1,1,1-trifluoropentane-2,4-dione in a flow of helium and helium containing ligand vapor

Thermogravimetry of 22 metal chelates of 1,1,1-trifluoropentane-2,4-dione, H(tfa) in a flow of helium and helium containing H(tfa) vapor is described. Samples were placed in an inserted tube in the inlet port of a gas chromatograph and heated stepwise. The presence of H(tfa) vapor in the helium flow was very effective in suppressing thermal dissociation of hydrated tfa chelates, which decomposed in a helium flow but could be completely vaporized, after dehydration, in a flow of helium containing H(tfa) vapor. The relationship between the volatility and the structure of the tfa chelates is discussed. Results obtained by the proposed method and with a conventional thermobalance are compared.     

Spectroscopic studies of electron spin resonance and Raman scattering on novel hybrid poly-p-phenylenebenzobisoxazole (PBO) fibers with copper phthalocyanine

As was shown in the previous study using X-ray analyses, poly-p-phenylene benzobisoxazole fibers can accommodate copper phthalocyanine molecules with a molecularly-dispersed state in the fiber structure. It is necessary for us to investigate the presence/absence of chemical interactions between the two molecules mentioned above for the purpose to make clear the mechanism why such characteristic structures with the well dispersion of copper phthalocyanine molecules in the hybrid fiber were realized. Spectroscopic analyses based on electron spin resonance and Raman scattering were adopted. Because the copper phthalocyanine molecule take a plane form having D_4h symmetry in an ideal state, the spectra from the molecule would express the consequences based on its symmetry as the shape of the spectrum; the presence of characteristic bands in the spectra would be a proof of the existing state of copper phthalocyanine and poly-p-phenylene benzobisoxazole molecules in the hybrid fiber. It is found that both the spectroscopic methods suggested that there were no chemical bond observed between the two molecules of copper-phthalocyanine and poly-p-phenylene benzobisoxazole in the hybrid fiber.     

Effect of atmospheric pressure on the phase transitions of α,α-trehalose dihydrate DTA study of the dehydration behavior in open systems

The phase transitions of α,α-trehalose dihydrate (T _h) were investigated by either differential thermal analysis (DTA) with an in-house apparatus of variable-pressure type equipped with an open sample holder or commercially available TG (thermal gravimetry)-DTA apparatus for comparison under the same experimental conditions as to the heating rate (2°C min⁻¹), the type of pan (open), and the particle size of T _h (63 μm). The former DTA measurements were carried out under five different total pressures, 101, 75, 61, 48 and 35 kPa, which provided quite helpful information necessary for confirmative assignments of the endothermic peaks due to either melting or dehydration of T _h. The usage of largely different amount of T _h, 126 and 14 mg for the DTA and TG-DTA measurements respectively, led to their different DTA traces, showing that there were largely different extents of the influence by the measured sample surface exposed to the surrounding atmosphere on its dehydration behavior. In addition the high thermal sensitivity achieved with such mass of T _h gave rise to an interesting discovery of an unidentified thermal event at 92°C prior to either melting or dehydration of T _h.