Alkoxy(acyloxy)silanes

Conclusions Alkoxychlorosilanes react with trimethylacyloxysilanes to give the difficultly accessible alkoxy(acyloxy)silanes of general formula (RO)_nSi(OCOR)_4-n (n=1–3) in 60–80% yields.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 671–674, March, 1989.     

Chimie des complexes de metaux de transition avec des hydrocarbures cyanes : III. Quelques observations sur la conformation et la reactivite de complexes dicyclopentadienyles du molybdene et du tungstene

A new class of cyclic telluronium salts has been prepared. All the salts are stable in solution in CHCl₃ or dimethylsulphoxide (DMSO). Conductivity measurements in DMSO and dimethylformamide (DMF) have shown that considerable ion pairing occurs in solution. Infrared, ¹H, ¹³C, ¹²⁵Te NMR, and mass spectra are reported and discussed.     

Origin of the lower critical flocculation temperature in polymer colloids stabilized by polydimethylsiloxane

Summary It has recently been reported (6, 7) that poly(methyl methacrylate) particles that are sterically stabilised by polydimethylsiloxane flocculate on cooling when dispersed in short chain n-alkanes. Since the LCFT was reported to be essentially independent of the nature of the dispersion medium, it was postulated (2) that incipient flocculation of these particles was caused by crystallisation of the polydimethysiloxane chains, which would have allowed the attractive van der Waals forces to become operative. This hypothesis has now been confirmed by low temperature X-ray studies.

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Zusammenfassung Von (6, 7) wurde kürzlich mitgeteilt, daß Poly(methylmethacrylat)-Teilchen, die mit Polydimethylsiloxan sterisch stabilisiert sind, in kurzkettigenn-Alkanen als Dispersionsmittel ausflocken. Da die LCFT im wesentlichen unabhängig von der Natur des Dispersionsmediums ist, wurde von (2) angenommen, daß die beginnende Ausflockung dieser Teilchen auf eine Kristallisation der Polydimethylsiloxan-Ketten zurückzuführen ist, bedingt durch die wirkenden van der Waals-Anziehungskräfte. Diese Hypothese wurde nun durch Röntgenuntersuchungen bei tiefer Temperatur bestätigt.

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With 1 figure     

Structural features of zinc hydroxyfluoride

Zinc hydroxyfluoride (ZnOHF), obtained by coprecipitation of a zinc salt in aqueous HF, exhibits a variable stoichiometry Zn(OH)_2−xF_x, where x is tuneable from 0.63 to 0.87 by controlling the pH of the solution. The structure was determined from Rietveld refinements using X-ray powder diffraction data. Crystallizing with the orthorhombic symmetry (SG : Pna2₁), the ZnOHF-type structure exhibits two different anionic sites. A bond valence analysis shows that fluorine exclusively occupies the anionic site that has shorter contacts to zinc. This site is split into two partially occupied sites, one corresponding to the position of a fluoride ion and the other to the position of a hydroxide ion. Bond valence calculations show that the split site model gives a more accurate picture of the local coordination of the anions on this site.     

Evolution of Statistical Ensembles of Microdomains on the Surface of Films of Rigid-Chain Polyimide during Thermal Imidization

Statistical ensembles of microdomains formed on the surface of polyimide films based on pyromellitic dianhydride and 2,7-diaminofluorene during thermal imidization were described in terms of the model of reversible aggregation using the electron-microscopic data. Parameters of the statistical distribution were determined for each ensemble depending on the film heating temperature.