Nature of DC conductivity in polyamides

The dc conductivity of poly(sebacyl piperazine), a polyamide prepared from the secondary diamine piperazine, in which no N? H groups are present and no hydrogen bonding can occur, has been examined and compared with that of a normal 610 polyamide. The results obtained point clearly to the conduction in the 610 polyamide being electronic below about 100°C but probably involving protons as well as electrons above this temperature. This is largely consistent with the findings of earlier work and clarifies the nature of conduction below about 80°C as being almost certainly electronic where previously it was in doubt. A definite and sometimes marked hysteresis in the conductivity was observed with regard to raising and lowering the temperature of the polyamides. This is explained in terms of the space-charge polarization developed in the materials at higher temperatures and which becomes clearly evident in their dielectric behavior. This shows the importance of discharging specimens at a sufficiently high temperature before making conductivity measurements. The polarization is a bulk and not an electrode effect, and it will probably depend to a marked extent on the morphology of the polyamides.     

The synthesis of a single enantiomer of a major alpha-mycolic acid of Mycobacterium tuberculosis

We report a synthesis of a single enantiomer of a dicyclopropane containing mycolic acid from Mycobacterium tuberculosis; this method can be simply varied to modify the chain lengths or the absolute stereochemistry of either cyclopropane.     

Copolymerization of 1‐hexene and ethylene catalyzed by the Ziegler catalyst Cp*TiMe3/B(C6F5)3

The Ziegler–Natta system Cp*TiMe₃/B(C₆F₅)₃ catalyzed the copolymerization of ethylene and 1‐hexene in toluene into materials that were characterized by ¹H and ¹³C{¹H} NMR spectroscopy, differential scanning calorimetry, and gel permeation chromatography. The effects of temperature and ethylene/1‐hexene and olefin/catalyst ratios on catalyst activities and copolymer molecular weights and molecular weight distributions were studied; the ethylene proportions varied from less than 5% to 85% or more. In addition, significant amounts of 1‐hexene were incorporated into the growing polymer chain in a 2,1‐fashion; consequently, conventional ¹³C NMR analytical methodologies for deducing monomer proportions and dispersions and polymer microstructures, based on a low 1,2‐incorporation of α‐olefin, did not work very well. A soluble (in toluene at ambient temperature) but very high molecular weight (weight‐average molecular weight ∼ 8 × 10⁵, weight‐average molecular weight/number‐average molecular weight = 1.8) rubbery copolymer that formed at −78 °C exhibited a predominantly alternating microstructure. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3966–3976, 2000     

Effective and Selective Mild Catalytic Hydrodehalogenation of Halocyclopropanes: Preparative Capabilities and Mechanistic Aspects

This review surveys both data obtained by the authors and published data on the partial or full hydrodehalogenation of di- and polyhalocyclopropanes (chlorides and bromides) with Grignard reagents catalyzed by titanium or zirconium compounds. The factors affecting the efficiency and selectivity of the hydrodebromination of bromocyclopropanes are considered: the nature of Grignard reagents (including isotopically labeled reagents), their transformations and effects in catalyzed and uncatalyzed reactions, the participation of solvents, catalytic and stoichiometric amounts of the catalyst, etc. A scheme is proposed in which the key steps of the mechanism of hydrodebromination of bromocyclopropanes includes three blocks of reactions: (a) the generation of a catalytically active Ti(II) species; (b) the hydrodehalogenation of bromocyclopropanes involving electron transfer from a low-valent catalyst species, formation of the cyclopropyl radical, and stabilization of this radical as a result of hydrogen atom transfer from the solvent molecule; and (c) transformations of previously formed radical species, such as dimerization and disproportionation (for example, of radical species generated from Grignard reagents or ether molecules) or the linking of alkyl radicals to radical species produced from solvent molecules.     

Q-curvature flow on 4-manifolds

We formulate an appropriate gradient flow in order to study the evolution of the Q-curvature to a prescribed function on a 4-manifold. For a class of prescribed functions, we show convergence and describe the asymptotic behaviour at infinity.     

Current Challenges to Appropriate Clinical Use of New Genetic Knowledge in Different Countries

Objective: To describe the challenges facing countries all over the world regarding the appropriate clinical use of genetics in their health care systems. Methods and Results: Aspects of the economic and social contexts in different countries which are of particular relevance to shaping the existing challenges are outlined. Issues which are relevant (but of different prominence) in all countries in providing genetic services are discussed. Conclusions: The challenges facing the provision of appropriate genetic services differ markedly in four major groups of countries. These challenges range from controlling inappropriate commercialization and the overuse of genetic approaches to putting in place even minimal basic community genetics services in countries where the infant mortality rate has fallen to a range where genetic and congenital disorders contribute substantially to ongoing handicap and early mortality. Copyright 2001 S. Karger AG, Basel     

Synthetic epoxy-mycolic acids

We report the synthesis of single enantiomers of epoxy-mycolic acids containing an α-methyl-trans-alkene or a cis-cyclopropane with structures that match those of major isomers of such molecules present in complex mixtures in Mycobacteria such as Mycobacterium fortuitum or Mycobacterium smegmatis     

Sulfimide-mediated Asymmetric Synthesis

Enantiomerically enriched sulfimides are used to prepare enantiomerically enriched epoxides, aziridines and substituted tetrahydro-furans and -pyrans. A new ‘asymmetric sulfimidation’ procedure is described.