Vibration spectra of rare earth metal tantalates

Infrared and Raman spectra of TRTa₇O₁₉ (TR = La, Nd, Eu) have been considered. Their structures have not been determined yet. Vibration spectra enable to assume the presence of TaO₄-tetrahedrons in tantalates. Vibration spectra of TRTaO₄ have been considered as well. Vibration spectra of TR₃TaO₇ have been recorded. Crystals of Le₃TaO₇, Nd₃TaO₇ are isostructural to veberite, while those of Eu₃TaO₇ to pyrochlorine. Factor-group analysis of the vibrations of the above samples of rare earth metal tantalates has been performed.     

Elastic backward scattering of negative pions on protons at 25 and 38 GeV/c

The backward elastic scattering reaction π^?p → pπ^? at momenta 25 and 38 GeV/c have been measured using a magnetic spectrometer with hybrid chambers. The experimental data on the dependence of the cross section dσ/du on the momentum transfer u as well as the energy dependence dσ/du at u = 0 are given.     

Effect of catalysts on the yield of products formed in biomass gasification

The effect of catalysts on the yield of products formed in thermal treatment of wood in the filtration combustion mode was experimentally studied. Natural zeolite of TsPS brand and K₂CO₃ were used as catalysts. The products were analyzed and the results were compared with those for noncatalytic systems. With the catalysts, the combustion temperature decreased by 100–200°C. The yield of liquid products formed in wood pyrolysis decreased with increasing amount of a catalyst in the mixture, and the appearance of unburned carbon on catalyst particles was also observed.     

Tritium measurement in the Western Caucasus

Measurement of tritium in water of rains, springs, wells, mud volcanoes and rivers, lakes of the Western Caucasus (Krasnodar region) has been carried out since 1997 for hydrogeology, engineering geology, ecology and seismology. Electrolytic cells with spiral electrodes and the big multiwire proportional chamber were used for low tritium concentration measurements on expeditions. With the new design of the cell the enrichment factor of 64.0 ± 1.5 % was obtained during the electrolytic process. Correlation of tritium concentration is observed in mud volcanoes and spring water with regional seismicity. The long-term tritium data are shown in natural waters in South Russia.     

Homo-and copolymerization of vinylcyclohexane with α-olefins in the presence of heterogeneous and homogeneous catalytic systems

The polymerization and copolymerization of vinylcyclohexane with α-olefins in the presence of several heterogeneous and homogeneous catalytic systems were studied. It was shown that, with respect to activity in the polymerization of vinylcyclohexane, the tested catalysts can be arranged in the following order: α-TiCl₃ < titanium-magnesium catalyst < metallocene catalyst. Poly(vinylcyclohexane) prepared with heterogeneous catalytic systems is a solid semicrystalline polymer. The properties of polymers synthesized with homogeneous systems differ substantially depending on the type of the metallocene used. In the presence of metallocenes with a C ₂ symmetry, crystalline powderlike products arise, while in the case of metallocenes with C ₁ and C _s symmetries, polymerization yields amorphous viscous products. Molecular-mass distributions of poly(vinylcyclohexane) samples prepared using both heterogeneous titanium-magnesium catalysts and homogeneous metallocene complexes show a bimodal pattern, indicating the heterogeneity of active centers of these catalysts. Upon introduction of a comonomer (ethylene, propylene, and 1-hexene) into the reaction mixture, the activity of all studied catalytic systems increases. When Me₂C(3-Me-Cp)(Flu)ZrCl₂ and rac-Me₂SiInd₂ZrCl₂ are used as catalysts, the degree of crystallinity of the copolymers grows owing to the presence of ethylene or propylene units in poly(vinylcyclohexane) chains.     

Triisobutylaluminum as cocatalyst for zirconocenes. II. Triisobutylaluminum as a component of a cocatalyst system and as an effective cocatalyst for olefin polymerization derived from dimethylated zirconocenes

An investigation of the catalytic behavior of the dimethylated zirconocenes Me₂SiCp*N^tBuZrMe₂ [Cp* = C₅(CH₃)₄; 1Me ], Me₂SiCp₂ZrMe₂ ( 2Me ), Cp₂ZrMe₂ ( 3Me ), Ind₂ZrMe₂ ( 4Me ), Me₂SiInd₂ZrMe₂ ( 5Me ), Et(2-MeInd)₂ZrMe₂ ( 6Me ), and Me₂Si(2-MeInd)₂ZrMe₂ ( 7Me ) with the combined activator triisobutylaluminum (TIBA)/CPh₃B(C₆F₅)₄ (Al/Zr = 250; B/Zr = 1) in ethylene polymerizations at increased monomer pressures (5–11 bar, 30 °C) was carried out. Sterically opened zirconocenes in ternary catalysts gave rise to active species effective in the formation of low molecular weight polyethylenes (PEs). These active species tended to increase the PE molecular weight [ 1Me (2100) < 2Me (20,000) < 5Me (89,000) < 3Me (94,500)] under similar conditions. PE obtained with 4Me showed a bimodal gel permeation chromatography curve with a 64% peak area [weight-average molecular weight (M_w) = 43,000] and a 36% peak area (M_w = 255,000). The increase in sterical demands from the zirconocenes was also demonstrated by the reduction of the chain transfer to monomer, the reinsertion of vinyl-ended PE chains, and their ability for isomerization. These reactions were most pronounced for the zirconocenes 1Me and 2Me . The active species responsible for the formation of low molecular weight PEs deactivated quickly. The zirconocenes 6Me , 7Me , and (2-PhInd)₂ZrMe₂ ( 8Me ) bearing substituent at the 2-position of the indenyl ring was activated with TIBA alone, yielding active species effective in ethylene and propylene polymerizations. PEs formed with 6Me – 8Me complexes activated with TIBA had high molecular weights. An increase in the Al/Zr ratio in the catalytic system 8Me /TIBA from 50 to 300 led to an enhancement of the molecular weight of polypropylene (PP) samples from oligomeric products to an viscosity-average molecular weight of 220,000. The increase in the molecular weights of PPs with an increase in the propylene concentration was also observed. An analysis of the catalytic performance of the 8Me /TIBA system showed first-order dependency of the initial polymerization rates on the TIBA concentration and close to second-order dependency on propylene. The second-order dependency on the monomer concentration is explained in terms of the monomer participation in the initiation step of the polymerization reaction. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1915–1930, 2001