Specific features of liquid-crystalline comb-like polymers with mesogenic groups

The results of structural investigation of the new type of the liquid-crystalline thermotropic methacrylic polymers are discussed. These polymers contain mesogenic groups as models for cholesteric, nematic and smectic types of low-molecular liquid crystals. The groups are attached to the backbone through methylene bridges of various lengths. The polymers of the first group have amorphous structure; the polymers of the second group are characterized by liquid crystalline structure and those of the third group can exist in both crystalline and liquid-crystalline states, manifesting properties of enantiotropic liquid crystals. The temperatures and heats of phase transitions have been determined. It is shown that the ability to realize the liquid-crystalline state of comb-like polymers with mesogenic groups depends on and is determined by the proceess of ordering of these groups.     

Structural Phase Transformations in Al/Pt Bilayer Thin Films during the Solid-State Reaction

A sequence of phases forming during the solid-phase reaction in Al/Pt bilayer thin films has been investigated by in situ electron diffraction. It is shown that the amorphous PtAl₂ phase forms first during the solid-phase reaction initiated by heating. Upon further heating, PtAl₂, Pt₂Al₃, PtAl, and Pt₃Al crystalline phases sequentially form, which is qualitatively consistent with an effective formation heat model. The content of phases forming during the reaction has been quantitatively analyzed and the structural phase transformations have been examined.

     

Force constants and vibration frequencies and forms of the coordination center of cysteine-containing chromium (III) trischelates

For chromium(III) tris-hydrocysteinate [Cr(LH)₃], the structural problem is solved by normal coordinate analysis. The structural model of the complex involving three N,O-cysteinate rings is confirmed by comparing the calculated and experimental frequencies of normal vibrations. Translated fromZhumal Strukturnoi Khimii, Vol. 39, No. 4, pp. 626–632, July–August, 1998.     

Free and Sulfated Sterols of Two Far-East Leptasterias Starfish

NMR spectroscopy, capillary GLC, and GLC-MS are used to study the composition of free and sulfated sterols from the far-east starfishLeptasterias alaskensis asiatica(Fischer) andL. Fisheri(Djakonov). The total free sterols of both species are shown to have similar qualitative and quantitative compositions and contain mainly⁷ -sterols. Sterol sulfate fractions contain cholesterol sulfate as the main component but differ in the ratios of⁵ :⁰ :⁷ -sterol derivatives. Possible reasons for these differences are discussed. A new steroid, 3-hydroxycholest-5-en-7-one sulfate, was isolated.     

Chemical composition optimization and characterization of the NiO/B<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system as a catalyst for ethylene oligomerization

The samples of the NiO/B₂O₃-Al₂O₃ system with NiO contents from 0.48 to 38.30 wt % were synthesized by the impregnation of borate-containing alumina (20 wt % B₂O₃). It was found that nickel oxide occurred in an X-ray amorphous state in the samples containing to 23.20 wt % NiO. At a NiO content of 4.86 wt % or higher, the support was blocked by the modifier to cause a decrease in the specific surface area from 234 to 176 m²/g and in the amount of acid sites from 409–424 to 333 μmol/g. An extremal character of the dependence of catalyst activity in ethylene oligomerization on NiO content was found with a maximum in the range of 4.86–9.31 wt %. Based on spectroscopic data, it was found that ethylene activation on the NiO/B₂O₃-Al₂O₃ catalyst can be associated with the presence of Ni²⁺ cations, which chemically interact with the support. The catalyst containing 4.86 wt % NiO at 200°C, a pressure of 4 MPa, and an ethylene supply rate of 1.1 h⁻¹ provided almost complete ethylene conversion at the yield of liquid oligomerization products to 90.0 wt %; the total concentration of C⁸⁺ alkenes in these products was 89.0 wt %.