Morphological structures formed by grafted polymers in poor solvents

We study a system of grafted polymers in a poor solvent by self-consistent-field methods as well as Monte-Carlo simulation methods. We observe a number of different morphological structures including an inverted solvent micelle or hole in the polymer layer, a lamella-like micelle structure, and fused, spherical micelle structures. These structures can be obtained by either varying the grafting density or chain length. We also develop a scaling theory for the existence of these structures and find reasonable agreement between this theory and our numerical calculations.     

Lewis base catalyzed aldol additions of chiral trichlorosilyl enolates and silyl enol ethers

[structures: see text] The consequences of double diastereodifferentiation in chiral Lewis base catalyzed aldol additions using chiral enoxysilanes derived from lactate, 3-hydroxyisobutyrate, and 3-hydroxybutyrate have been investigated. Trichlorosilyl enolates derived from the chiral methyl and ethyl ketones were subjected to aldolization in the presence of phosphoramides, and the intrinsic selectivity of these enolates and the external stereoinduction from chiral catalyst were studied. In the reactions with the lactate derived enolate, the strong internal stereoinduction dominated the stereochemical outcome of the aldol addition. For the 3-hydroxyisobutyrate- and 3-hydroxybutyrate derived enolates, the catalyst-controlled diastereoselectivities were observed, and the resident stereogenic centers exerted marginal influence. The corresponding trimethylsilyl enol ethers were employed in SiCl4/bisphosphoramide catalyzed aldol additions, and the effect of double diastereodifferentiation was also investigated. The overall diastereoselection of the process was again controlled by the strong external influence of the catalyst.     

Computer-assisted site-specific functionalization in the gibberellin series; norrish type II photocyclization via a 9-membered transition state

The site-specific Norrish II type reaction of the gibberellin oxo ester , prepared in two steps from tetraacetyl- GA₃, anhydride , to the photolactone , is reported. By potential energy calculations based on the X-ray analysis of , it is shown that the regio- and stereoselectivity of the photocyclization can be predicted involving a 9-membered cyclic transition state in the H-abstraction step. The structure of the photolactone was determined by spectroscopic data, especially NOE measurements.     

Précisions apportées au mécanisme de la polymérisation du chlorure de vinyle par le chlorure de tertiobutyle-magnésium en milieu tétrahydrofurannique. Etude par résonance magnétique nucléaire

NMR analysis of the low molecular weight poly(vinyl chloride) obtained from polymerizations initiated by t-BuMgCl in THF solution suggests that all the polymer chains have the structure: Enhanced signal/noise ratio by NMR spectrum accumulation confirms this structure in products of higher molecular weights. Examination of the NMR spectrum of vinyl chloride monomer in THF solution reveals the formation of a monomer–THF complex; it seems that the initiation step of the polymerization is governed by this complexation phenomenon.     

Proximal algorithm for minimization problems in l0-regularization for nonlinear inverse problems

Numerical Algorithms - In this paper, we study a proximal method for the minimization problem arising from l0-regularization for nonlinear inverse problems. First of all, we prove the existence of...     

The kinetics of the reaction between NH2 and propylene studied by laser resonance absorption

The reaction kinetics of NH₂ with propylene is studied by flash photolysis in the temperature range 300-500K. The NH₂ radicals are detected by resonance absorption, using a cw single mode dye laser. This method allows the detection of very small radical concentrations in a wide range of experimental conditions. The reaction of NH₂ with propylene is fairly slow and seems to correspond to the addition process. The Arrhenius expression obtained is (E in kcal/mole):k(NH₂ + C₃H₆) = 2.9 × 10⁸ exp[-4.3(± 0.2)[RT]M^?1s^?1.     

Kinetic study of cytochrome P450 3A4 activity on warfarin by capillary electrophoresis with fluorescence detection

The use of capillary electrophoresis (CE) for the determination of cytochrome P450 3A4 (CYP3A4) activity with R-warfarin as a substrate was investigated. CYP3A4 activity was determined by the quantitation of the product, 10-hydroxywarfarin, based on separation by CE. The separation conditions were as follows: capillary, 80.5 cm (75 microm i.d., 60 cm effective length); 50 mM sodium phosphate buffer (pH 6.5); 23 kV (90 microA) applied voltage; fluorescence detection, excitation wavelength, 310 nm, emission wavelength, 418 nm; capillary temperature, 37 degrees C. With the developed CYP3A4 activity assay and the Lineweaver-Burk equation, the Michaelis-Menten parameters Km and Vmax for formation of 10-hydroxywarfarin from R-warfarin in the presence of CYP3A4 were calculated to be 166 +/- 12 microM and 713 +/- 14 pmol/min/nmol (or 91.4 pmol/min/mg) CYP3A4, respectively.     

The influence of intermolecular interactions on polymerization—2. Polymers of 1-naphthyl methacrylate and 2-naphthyl methacrylate obtained by radical polymerization

1-Naphthyl methacrylate and 2-naphthyl methacrylate were polymerized with azoisobutyronitrile in n-hexane, cyclohexane and benzene in the absence and in the presence of tetracyanoethylene. Products were transformed into poly(methyl methacrylate) by hydrolysis and methylation; tacticities were then determined by proton NMR.     

Surface plasmon resonance within ion implanted silver clusters

Surface plasmon resonance (SPR) belongs to the most sensitive indicators for changes in analyte concentrations or other sample properties, which depend on the refractive index in the medium. Surface plasmons represent collective electron oscillations in metal cluster or metal layers of diameter or thickness in the nanometer range. Such layers or clusters are used in many optical sensors in order to enhance the interaction between electromagnetic radiation and analyte. Clusters are preferred to enhance Raman scattering and IR absorption, whereas layers are used for SPR in the visible range. We tested the applicability of ion implanted clusters in order to enhance the stability of the metal coatings of the SPR sensor elements. A model based on the effective media theory was developed in order to enhance the sensor capabilities. The potential of the SPR with ion implanted metal clusters consists in durable resonance layers for biochemical sensors. Received: 21 December 1997 / Revised: 6 March 1998 / Accepted: 12 March 1998     

A study of HPLC separation and spectrophotometric, polarographic and voltammetric detection of 4-substituted derivatives of N-nitroso-N-methylaniline

Optimised conditions have been found for the separation of N-nitroso-N-methylaniline derivatives substituted in the position 4 (4-R-NMA, where R =- H,- CH₃, - OCH₃, - Cl, - CN, - OH and - NO₂), using reversed phase HPLC with a C18 chemically bonded stationary phase. Suitable mobile phases are mixtures of aqueous 0.01 mol l^–1 KH₂PO₄ with methanol (4+6) or acetonitrile (7+3), or a mixture of water, methanol, acetonitrile and 80% H₃PO₄ (200+200+200+3). Four detection techniques have been investigated: Direct UV photometry, polarography on a hanging mercury drop electrode, anodic voltammetry on a glassy carbon fibre array electrode and indirect anodic voltammetric detection after photolytic denitrosation of the analytes. The UV photometry is the most universal and yields limits of detection around 10^–6 mol l^–1. Polarography exhibits the poorest sensitivity (with a limit of detection of ca. 10^–5 mol l^–1) but it can be used for selective detection of 4-NO₂-NMA. Direct voltammetric detection is selective for the oxidizable derivatives and the limits of detection attained are lower than those obtained by UV photometry (for 4-OH-NMA) or comparable with (for 4-OCH₃-NMA). When the analytes are photolytically denitrosated to yield oxidizable derivatives, the voltammetric detection of 4-H-NMA, 4-Cl-NMA, 4-CH₃-NMA and 4-NO₂-NMA has limits of detection one order of magnitude lower than those obtained UV photometrically.