Cationic polymerization with boron halides. V. Synthesis of poly(isobutylene-b-styrene) and poly(styrene-b-isobutylene)

Block copolymers of isobutylene and styrene, PIB-b-PSt and PSt-b-PIB, have been prepared by a two-step synthesis involving (1) preparation of terminally chlorinated (telechelic) polyisobutylene or polystyrene “prepolymers” by using the H₂O/BCl₃ initiator system and (2) blocking from these telechelic prepolymers a second polymer segment by using an alkyaluminum, e.g., Et₂AlCl coinitiator. The telechelic polyisobutylene and polystyrene contain tertiary and benzylic chlorine termini, respectively. Block copolymer characterization included detailed selective solvent extraction procedures, coupled with GPC determinations, PMR, solubility, and intrinsic viscosity studies. The synthesis of PIB-b-PSt and PSt-b-PIB provides direct chemical evidence for the presence and position of active chlorine termini in BCl₃-coinitiated olefin polymerizations.     

Techniques in chemometrical modeling for micro-pollutants assessment

Summary Chemometrics have been described as effective tools for exploring chemical data, and many software packages are now available on micro-computers. This work evaluate their suitability for environmental analytical chemistry. Guidelines for multivariate method selection are proposed. They are based on the type of variables and the goal of the study. Two examples are proposed to illustrate these methods and their efficiency. Firstly it is shown that a global assessment of Rhine basin mercuric pollution in the Alsace region is possible with Multiple Correspondence Factor Analysis. Several goals are simultaneously reached: a mapping of pollution, a detection of pollution changes during the study period and an evaluation of bio-accumulation as a function of fish species. Secondly the modeling of industrial soil pollution by heavy metals is studied by Multiple Linear Regression. It demonstrates that chemometrics provide us with necessary tools for environmental analytical chemistry but also for toxicological studies or ecosystem modeling.     

Crystallographic evidence for different modes of interaction of BF3/BF4 species with water molecules and 18-crown-6

We report the crystal and molecular structures of the complex of 18-C-6 with H₃O⁺BF₄⁻ (I) and the complex of 18-C-6 with BF₃OH₂·H₂O (II). The different modes of appearance of the “BF₃” species as BF₃, BF₃OH₂, BF₃OH₂·H₂O and BF₄⁻, as well as their structurally significant intermolecular and intramolecular interactions, are discussed. In complex I the oxonium ion is bound at the centre of the 18-C-6 macrocycle. The oxonium oxygen is located practically equidistant (2.68–2.73 Å) from the six macrocyclic ethereal oxygens. The BF₄⁻ counter-ion is positioned 7.3 Å away from the oxonium ion in the same general plane of the crown ether. This anion is not involved in any direct intermolecular contacts, a fact that may explain why it is spherically disordered. In complex II there is no guest molecule (or ion) present in the “cavity” of the macrocycle, but there are two hydrogen-bonded systems of BF₃OH₂·H₂O that are interacting with the crown ether on either side of the general macrocyclic plane. Complex II features three types of hydrogen bonds—the O(water)-HO(crown) bonds (2.83 and 2.85 Å), the O(water)H-O(BF₃) bond (2.49 Å) and the O(BF₃)-HO(crown) bond (2.65 Å). The strong intermolecular O(crown)O(water)O(BF₃) and O(crown)O(BF₃) interactions stabilize the normally unstable BF₃OH₂·H₂O species.     

The synthesis of biphenylisopropyloxycarbonyl-amino acid salts

A number of biphenylisopropyloxycarbonyl-blocked amino acids have been synthesized as their cyclohexylamine or dicyclohexylamine salts, and their properties described. A facile method for regenerating the Bpoc-amino acids from their salts is detailed, along with the conditions used to determine that the syntheses are free of racemization.     

“Free” constituent quarks and dilepton production in heavy-ion collisions

An approach is suggested that invokes vitally the notion of constituent massive quarks (valons) that can survive and propagate rather than hadrons (except of pions) within hot and dense matter formed below the chiral-transition temperature in the course of heavy-ion collisions at high energies. This approach is shown to be quite good for describing the experimentally observed excess in the dilepton yield at masses 250≤M_ee≤700 MeV over the prompt-resonance-decay mechanism (CERES cocktail) predictions. In certain aspects, it appears to be even more successful than conventional approaches: it seems to match the data somewhat better at dilepton masses below the two-pion threshold and below the ρ-meson peak, as well as at higher dilepton masses (beyond the φ-meson one). The approach implies no specific assumptions on the special features of phase transitions in expanding nuclear matter, and the ideal gas approximation is motivated to be still workable for describing the pion-valonic system under consideration.     

Photochemical reaction of hydrogen with germanosilicate glass initiated by 3.4 5.4-eV ultraviolet light

By measuring the photosensitivity of Ge-doped silica fibers over the 3.4-5.4-eV spectral range and from T=95 K to T=375 K , we demonstrate that different physical mechanisms take place depending on whether H(2) is present. Without H(2) the principal photosensitivity pathway involves excitation of a germanium oxygen-deficient center to its triplet state. In H(2) -loaded fibers the UV light excites the regular Ge-O bond, which then reacts with H(2) to produce Si-OH or Ge-OH and a GeE(') center.