Effect of Restricted Geometry on the Structure and Phase Transitions in Potassium Nitrate Nanoparticles

The effect of restricted geometry and thermal prehistory of sample preparation on phase transitions in nanocomposites on base of porous glasses with average pore diameters 7 and 46 nm filled by potassium nitrate have been studied by X-rays and neutron diffraction. The nanoparticle sizes have been determined and phase diagrams (on cooling) for these nanoparticles have been described. It is shown that there is a critical nanoparticle size in the interval from 30 till 20 nm, at which in nanocomposite the ferroelectric phase is realized only regardless of preparation method.     

Comparative Study of Aerobic Alkene Epoxidations Catalyzed by Optically Active Manganese(iii) and Cobalt(ii) Salen Complexes

A comparative study of aerobic alkene epoxidations in the presence of pivalaldehyde catalyzed by the optically active Mn(III)Salen and Co(II)Salen complexes has been performed. The nature of the asymmetric induction is discussed.     

Dual mechanisms of an organocatalytic homodimerization reaction

The l-proline organocatalytic homodimerization of 2-cyclohexenone has been studied and it is concluded that the mechanism of the reaction follows competing pathways involving either a two-step imine/enamine addition or a concerted Diels-Alder cycloaddition where the ultimate product distribution is dependent upon the ratio of the organocatalyst to 2-cyclohexenone and a base present.     

Study of the compositional heterogeneity of ethylene-1-hexene copolymers via thermal fractionation with the use of differential scanning calorimetry

The compositional heterogeneity of ethylene-1-hexene copolymers synthesized with various types of supported catalysts, namely, the titanium-magnesium catalyst TiCl₄/MgCl₂ and the zirconocene catalyst SiO₂(MAO)/Me₂Si(Ind)₂ZrCl₂, is studied via the method of successive self-nucleation-annealing (SSA) with the use of differential scanning calorimetry. On the basis of the data on the temperatures of individual peaks on SSA curves, the thickness of lamellas composed of macromolecules with a certain degree of short-chain branching is estimated. The copolymer synthesized with the zirconocene catalyst has a narrower range of fusion and does not contain large lamellas composed of molecules with a low degree of short-chain branching. With the use of the broadness index, it is shown that the copolymer synthesized with the zirconocene catalyst has a more uniform distribution of the comonomer than does the copolymer synthesized with the titanium-magnesium catalyst. For the copolymers synthesized with the titanium-magnesium catalyst, the compositional heterogeneity increases with an increase in the content of 1-hexene.     

Conservative methods for the Toda lattice equations

We are concerned with the numerical integration of the Todalattice equations by using different conservative methods. Numericalexperiments suggest that the global error for isospectral schemesdecreases exponentially with time but it is almost constantfor either symplectic or more general integrators. We providea theoretical explanation for these experimental findings.     

Study of Multi-Site Nature of Supported Ziegler-Natta Catalysts in Ethylene-Hexene-1 Copolymerization

Summary: Heterogeneity of active centers (AC) of titanium-magnesium catalysts (TMC) and vanadium-magnesium catalyst (VMC) in ethylene-hexene-1 copolymerization has been studied on the base of data of polymer molecular weight distribution (MWD) deconvolution technique and copolymer fractionation onto narrow fractions. It was found that 3 and 4 Flory components (groups of active centers) are required to describe experimental MWD curves of copolymers produced over TMC with different Ti content. In the case of VMC MWD of homopolymer and copolymer are characterized by set of 5 Flory components (5 groups of AC). Different character of inter-relationship between MW and short chain branching (SCB) was found for ethylene-hexene-1 copolymers produced over different catalysts: heterogeneous type in the case of TMC and more uniform for copolymer prepared over VMC. The content of Ti affects on the slope of that profile in copolymers produced over TMC. The results indicated that TMC and VMC are different greatly on the heterogeneity of active centers to the formation of polymers with different molecular weights and to formation of copolymers with different inter-relationship between MW and short chain branching. TMC produces polymers with more narrow MWD but it contains highly heterogeneous centers regarding comonomer reactivity ratios. VMC produces polymers with broad and bimodal MWD but it contains more homogeneous centers regarding comonomer reactivity ratios.