Cationic polymerization of 1,3-pentadiene I. infrared, 1H-NMR and 13C-NMR investigations on the microstructure of poly(1,3-pentadiene) synthesized with cationic catalysts

This study deals with cationic polymerization of the cis- and trans-isomers of 1,3-pentadiene. The microstructure of the polymer chains is studied by ¹H-NMR, ¹³C-NMR and IR spectroscopies. It is shown that the trans-diene gives strictly trans-1,4 and trans-1,2 residual linear insaturations, whereas the cis-isomer yields also cis-1,4, cis-1,2 and 3,4-units whose overall content can reach 10 mol-%. According to the cyclization degree of the macromolecules, ranging from 30 to 70 mol-%, the number of trans-(1,2+1,4) units varies between 33 to 65 mol-% and that of trans-1,2 units between 4 and 20 mol-%. An analytical method is proposed to evaluate the average number of rings present in the polycyclic sequences. It is found that the cyclic fragments of the polymer chains consist of bi- or tri-cyclohexane fused rings containing α tetrasubstituted double bond.     

2,3- and 4,5-Secodopa,the Biosynthetic Intermediates Generated from L-Dopa by an Enzyme System Extracted from the Fly Agaric,Amanita muscaria L., and Their Spontaneous Conversion to Muscaflavin and Betalamic Acid,Respectively, and Betalains

An enzyme is extracted from the red peel of Amanita muscaria which cleaves the C(2)–C(3) and the C(4)–C(5) bond of the aromatic ring of L -dopa ( 1 ) to form a mixture of 4,5-secodopa (= salt of 6-amino-2-hydroxy-4-(2′-oxoethylidene)hept-2-enedioic acid; 2 ) and 2,3-secodopa ( = salt of 7-amino-5-formyl-2-hydroxyocta-2,4-dienedioic acid; 3 ), two hitherto hypothetical biosynthetic intermediates (see Scheme). Though isolation of these products has not been possible, structural evidence is inferred from reaction products, kinetics, and spectroscopical characteristics in comparison with known compounds. Secodopas 2 and 3 are characterized in dilute solution by HPLC and UV/VIS spectroscopy (anions; λ_max 424 and 414 nm, resp., ?₄₂₀ = 25500; on acidification, shift to 380 and 372 nm, resp.). They cyclize without enzyme catalysis, optimally at pH 4.5–5; 3 produces muscaflavin ( 5 ) and 2 betalamic acid ( 4 ). The products arc identified by direct comparison with authentic samples in HPLC, by ¹H-NMR of 5 , and by condensation of 4 with L -proline to form the well known betalain indicaxanthin ( 7 ). The enzymatic conversion of L -dopa ( 1 ) via 2 to betalamic acid ( 4 ; (S)) and its condensation with L -proline leads to pure natural indicaxanthin ( 7 ; (2S,115)); correspondingly, the enzymatic conversion of D -dopa to (R)-betalamic acid and its condensation with L -proline produces isoindicaxanthin ((2S,11R)) which is unknown in nature. Particularly relevant is the fact that the same enzyme cleaves pyrocatechol to produce a solution of the enolate form of the known 2-hydroxy-6-oxohexa-2,4-dienoate (secopyrocatechol; 9 ; see Fig. 5). Dissociation constants of the corresponding enolic functions in the cleavage products are determined by spectrometric titration and compared to those of known systems.     

Isomerisation and phase transitions in small sodium clusters

Using a distance-dependent tight-binding hamiltonian, we have studied the influence of the temperature on the geometries of small alkali clusters (Na₄, Na₈, and Na₂₀). We have applied a Monte-Carlo thermodynamical method which consists in performing canonical samplings for various temperatures, these samplings being reexpressed in the microcanonical ensemble. This method provides thermodynamical values such as the entropy and the specific heat. Their behaviour shows one phase transition in the case of Na₄ and Na₈, and two phase transitions for Na₂₀. As concerns Na₄ and Na₈, the transition occurs at 200 K, between a solid-like phase and a phase for which the geometry of these clusters oscillates between numerous shapes. In the case of Na₂₀, the two observed phase transitions can be described as a melting of the surface atoms (at 200 K) preliminarily to the fluctuation of an inner icosahedron seed (at 300 K).     

Lattice Boltzmann simulations of flow past a cylindrical obstacle

We present lattice Boltzmann simulations of flow past a cylindrical obstacle. Our study is based on the Lévy walk model of turbulence in a lattice Boltzmann model. We discuss pressure around the cylinder with laminar and turbulent incident flows, as well as the dependence of the von Karman street on the analog of integral scale in our model.     

Critical velocity of continuous vortex formation in rotating<Superscript>3</Superscript>He-A

Vortex lines with continuous (singularity-free) core structure are usually formed when superfluid³He-A is accelerated to rotation. The critical counterflow velocityv _c (T), at which vortex formation starts, has been measured with cw NMR. The onset of vortex formation is found to depend on the order parameter texture and differs from that of singular core vortices in⁴He-II or³He-B due to much larger length scale.     

Nucleation of vortices in superfluid<Superscript>3</Superscript>He-B

In a smooth-walled rotating container vortices nucleate spontaneously in³He-B at a critical velocityv _c (T, P) which is related to the bulk superflow instability velocityv _cb (T, P). This process takes place at the wall of the container. Vortices are also observed to nucleate at lower velocities in the bulk liquid in ionizing radiation. For this thermal neutrons are most practical since for them the absorption cross section is large in³He. An absorption event creates local overheating, a hot bubble in the cold superflow. An increasing number of vortex rings is observed to emerge from the bubble as a function of the superflow velocity.     

Effects of ions and detergents in drug partition chromatography on liposomes

We have determined drug partitioning into phospholipid bilayers by immobilized-liposome chromatography (ILC). Electrostatic effects on the drug partitioning were observed on neutral bilayers at low ionic strength. The size of the counterions affected the partitioning. When liposomes were supplemented with ionic detergents the partitioning of charged drugs was strongly affected, allowing complete separation of drugs of different charges which showed similar retention on neutral bilayers. Partial separation was obtained on bilayers containing fatty acid. Detergent ions or fatty acid inserted into phospholipid bilayers affected the partitioning of drugs much more than did free ions or phospholipid head group charges.     

Lifetime simulation and thermal characterization of PVC cable insulation materials

Thermal behavior of commercial PVC cable insulation both before and after extraction of plasticizers, fillers and other agents were tested by TG/DTG and DSC during heating in the range 20-800°C in air. The ultrasound enhanced hexane extraction and dissolution in THF with subsequent precipitation of PVC were used to prepare 'extracted' and 'precipitated' samples. The total mass loss measured for the 'non-treated', 'extracted' and 'precipitated' PVC samples was 71.6, 66.6 and 97%, respectively. In the temperature range 200-340°C the release of dioctylphthalate, HCl and CO₂was observed by simultaneous TG/FTIR. From TG results measured at different heating rates (1.5, 5, 10, 15 K min^-1) in the range 200-340°C the non-isothermal kinetics of the PVC samples degradation was determined. Activation energy values of the thermal degradation processes calculated by ASTM E 698 method, for 'non-treated', 'extracted' and 'precipitated' PVC samples were 174.6±17 kJ min^-1, 192.8±19 kJ min^-1, 217.1±20 kJ min^-1, respectively. These kinetic parameters were used for the lifetime simulation of the materials.     

ESR study of mechanically destructed methyl methacrylate-maleic anhydride copolymer

Methyl methacrylate-maleic anhydride copolymers were destructed mechanically; their ESR spectra were studied over the range ?50° to +50° using three copolymers of different compositions. From theoretical spectra, radicals present in the system were identified at various temperatures; their decay was investigated. Radicals in the mechanically destructed copolymer were compared with those trapped in gamma-irradiated copolymer. At ?50° in the copolymer destructed, some radicals are stabilized which do not occur in the gamma-irradiated copolymer; at room temperature, the differences between the spectra are not substantial.