Radiochemical studies of free-radical vinyl polymerizations. VI. Polymerization of methyl methacrylate in the presence of organotin PVC stabilizers

¹⁴C-Azoisobutyronitrile was used to initiate polymerizations of methyl methacrylate in the presence of the organotin compounds: tetrabutyltin, dibutyltin diacetate, dibutyltin dilaurate, dibutyltin di(ethyl mercaptide), dibutyltin di(dodecyl mercaptide), and dibutyltin dichloride. Only dibutyltin dichloride affected rates of polymerization significantly, and this was ascribed to an increase in the velocity constant k_p for the propogation reaction. No evidence was obtained for radical displacement reactions of the polymer radicals with bonds between tin and carbon, oxygen, sulfur, or chlorine. Transfer activity exhibited by the mercaptides was ascribed to traces of thiol impurity, possibly formed during storage. The relevance of these results to the mechanism of stabilization of poly(vinyl chloride) is briefly discussed.     

Die Kristallstruktur von U2Cr3Si,U4Mn5Si3 und U2Co3Si

Zusammenfassung In den Dreistoffen: Uran–T (Ti,V,Cr,Mn,Co,Nb,W)–{Si,Al} werden Legierungen auf dem Schnitt U(T,Si)₂ bzw. U(T,Al)₂ aus den Komponenten hergestellt und röntgenographisch identifiziert^*. Es bestehen die ternären Verbindungen U₂Cr₃Si, U₄Mn₅Si₃ und U₂Co₃Si, die sich als mit MgZn₂ isotyp erweisen.     

Syntheses of seven-membered rings: ruthenium-catalyzed intramolecular [5+2] cycloadditions

The Ru-catalyzed intramolecular [5+2] cycloaddition of cyclopropylenynes is investigated with respect to the regio- and diastereoselectivity as well as the functional group compatibility of the reaction. Evidence for the mechanism as occurring through a ruthenacyclopentene intermediate is elucidated from 1) the study of the diastereoselectivity of the cycloaddition; 2) the effect of variation of substituents on the regioselectivity of cyclopropyl bond cleavage in 1,2-trans- and 1,2-cis-disubstituted cyclopropanes and 3) examples that clearly do not involve ruthenacyclohexene as intermediates as products still incorporate the cyclopropyl moiety. The scope and limitations of the Ru-catalyzed cycloaddition are discussed and compared with the Rh-catalyzed reaction. The potential power of this methodology towards natural product total synthesis is demonstrated by the formation of several polycyclic systems with the chosen reaction conditions and readily available cyclopropylenyne substrates.     

Interfacial and intrafacial linkage isomerizations of rhenium complexes with aromatic molecules

The mechanisms for the interconversion of facial diastereomers of a variety of TpRe(CO)(L)(eta(2)-L(Ar)) complexes [L = (t)BuNC, pyridine (py), PMe(3), or 1-methylimidazole (MeIm); L(Ar) = benzene, anisole, naphthalene, 1-methylpyrrole, furan, or thiophene; Tp = hydridotris(pyrazolyl)borate] have been investigated by (1)H NMR spin saturation experiments. In addition, the rates and free energies of activation for these processes were calculated from spin saturation experiments and T(1) measurements. The operative mechanisms for interconversion of the pi diastereomers were found to be nondissociative, undergoing either an interfacial or intrafacial linkage isomerization. A comparison of the kinetic parameters for isomerization of related eta(2)-olefin complexes of the [TpRe(CO)(PMe(3))] and [CpRe(NO)(PPh(3))](+) fragments is also presented.     

Chlorine derivatives of octaorganylsilasesquioxanes

The chlorination of saturated and unsaturated octaorganylsilasesquioxanes proceeds through two pathways, leading to the formation of volatile and nonvolatile products, whose ratio depends on the reaction conditions. The volatile products of the chlorination of R-T₈ (R=C₂H₅ and CH=CH₂) are compounds with the general formula Cl_nR_8–n[SiO_1.5]₈, (n=1–7), obtained independently of the nature of the organic substituent and chlorinating agent. The nonvolatile chlorination products are silasesquioxanes, containing chlorine and chloroalkyl substituents on the atoms of the silicon-oxgyen core. Analysis of the vibrational spectral data indicated that the most significant changes in the molecular vibrational system are made by the halogenation of the silicon-oxygen core Si₈O₁₂.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 446–450, February, 1990.     

Neutron activation for precise nuclear data

Techniques for extracting neutron flux parameters pertaining to the irradiation positions and nuclear data pertaining to the isotopes concemed from measured reaction rates have been developed. An adjustment procedure based on the generalized least squares method with incorporates the, evaluated literature nuclear data, estimates of flux parameters and the experimentally measured reaction rates using high resolution -ray spectrometry is applied. More precise values of the nuclear data, i.e., thermal neutron cross sections, resonance integrals and -ray emission probabilities are thus generated. The irradiation of a multi-element standard (MES) containing 24 elements is carried out in six diffieret positions in two diverse types of reactor in the UK and Russia. It is shown that the improvement in nuclear data is revelaed in moire than 90% of the cases.     

Fission track mapping of uranium in black spruce (Picea Mariana) twigs

Fission track analysis (FTA) has been used to investigate the biological uptake of uranium. Examination of fission track maps of black spruce twigs (from areas of uranium mineralization and mill tailings) shows uranium to be concentrated in the inner bark while decreasing in the outer bark of the twigs. Track clusters in the bark, too large to be uraniferous airborne particulate matter, are identified as being associated with resin canals in the twigs. The results of this work help clarify the mechanisms by which uranium is incorporated in black spruce twigs. The observation of decreasing uranium content with the increasing age of black spruce twigs by an earlier worker, can now be explained by the relative reduction in volume of the uranium enriched inner bark with age.     

The dependence of the fatty-acid composition of cottonseed oil on the degree of unsaturation

Summary 1. Results of thiocyanometric and spectrophotometric analysis of cottonseed oil confirm the relationship found previously in catalytic hydrogenation using potentiometric monitoring of the state of the catalyst.2. Together with Eqs. (3) and (4), these results counterbalance the results of the chromatographic analysis of cottonseed oil.3. This contradiction leads to the posing of the question of the probable inaccuracy of individual methods of chromatographic analysis for substances with similar absorption properties, i.e., for linoleic and oleic acids.Alma-Ata Medical Institute. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 697–700, November–December, 1975.     

The interference of heteronuclear chromium(III) tartrate complexes in the EDTA titration of cobalt(II), copper(II), zinc(II) and cadmium(II)

When present together in solution, chromium(III) and tartrate can interfere seriously in the titration of Co(II), Cu(II), Zn(II) and Cd(II) with EDTA. Ternary (heteronuclear) tartrate complexes containing Cr(III) and bivalent metal ion in the ratio 1:1 are formed. The conditions for the formation of these complexes have been investigated. Cadmium(II) can be determined without interference by employing potentiometric end-point detection.     

Synthesis and oxidation of 2,6-dimethyl-3, 5-di(phenoxycarbonyl)-1,4-dihydropyridines

A number of new aryl esters of acetoacetic acid were obtained in the reaction of phenols with diketene in the presence of triethylamine. Reaction of the esters thus obtained with hexamethylenetetramine gave, for the first time, aryl esters of 1,4-dihydropyridine-3,5-dicarboxylic acid, the oxidation of which with chloranil gave the corresponding pyridines.