The effects of monophenol chain terminators on the structure–property relationships of controlled epoxy networks

Five families of new controlled epoxy thermosets (CENs) using three monophenol chain terminators were prepared to study systematic changes in the structure and amount of the monophenol and the initial molecular weight between crosslinks (M_c,i) on the properties of epoxy thermosets. Glass transition temperature (T_g) decreases with monophenol mole fraction (χ) in proportion to both the concentration and flexibility of the chain terminator. Distinct serial relations for T_g depression were observed for the three M_c,i families. Dynamic mechanical analysis (DMA) shows significant perturbations of the relaxation behavior with added terminator as evidenced by decrease in peak tan δ and in post T_g damping. The rubbery coefficients of thermal expansion (CTE) increases with monophenol concentration only at χ > 0.05 and shows distinct curvature versus temperature, but is largely invariant with monophenol flexibility. The thermal stability of terminated CENs decreases only slightly with χ and little difference was found with monophenol structure. Most surprisingly, fracture toughness decreases markedly and discontinuously with χ depending on M_c,i. The values of the critical monophenol concentration at which fracture toughness markedly decreases (χ_c) are inversely proportional to M_c,i but are independent of monophenol flexibility. No correlation of χ_c with any of the calculated network structure parameters was apparent. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1632–1640, 2008     

1H magnetic resonance spectroscopy of some substituted acetophenones

Measurements are reported on the nuclear magnetic resonance spectra of the acetyl protons of a series of substituted acetophenones. Although the extreme values of the chemical shifts, δ for the meta- and para- substituted compounds differ only by 0·2 units, the values themselves are linearly related to the Hammett substituent constants. No such relationship exists for the ortho-substituted compounds. The magnitude of the chemical shift is independent of substrate concentration over a five-fold variation.     

A new method for the fluorometric determination of micro quantities of citrate

Summary An extremely sensitive fluorometric procedure has been developed for the micro determination of citrate in various types of samples, based on the quenching effect of citrate on the fluorescence at 450 nm of the tungstate-flavanol complex. The calibration curve is linear in the 0 to 200 ppb range of citrate. With a slight modification of the method, citrate may be analyzed in the 0 to 5 ppm concentration range.

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Zusammenfassung Eine außerordentlich empfindliche, fluorimetrische Methode zur Mikrobestimmung von Citrat in verschiedenartigen Proben wurde ausgearbeitet. Sie beruht auf dem Fluoreszenzlöscheffekt des Citrats gegenüber dem Wolfram-Flavanol-Komplex bei 450 nm. Bis 200 ppb verläuft die Eichkurve linear. Mit einem geringfügig modifizierten Verfahren kann man auch bis zu 5 ppm Citrat bestimmen.

     

Reactions of diborane with aromatic heterocycles—21: Reactions with nitrogen-containing heterocycles related to pyridine

The reactions of solutions of B₂H₆ in ethers with a variety of aromatic heterocycles containing one or more six-membered rings with only one nitrogen per ring have been examined. Quinoline and isoquinoline form monoborane adducts which hydroborate in the presence of an excess of B₂H₆. Protonolysis gives the 1,2,3,4-tetrahydro-compounds in both cases. N-Methylquinolinium and N-methylisoquinolinium iodides both hydroborate. Hydroboration and halogen loss occur with 2-, 3- or 4-haloquinolines (chlorine or bromine). Prior to hydroboration 8-hydroxyquinoline forms a borane adduct which rapidly eliminates one equivalent of hydrogen in a probable intramolecular ring closure. Sodium 8-hydroxyquinolinate combines with B₂H₆ in an unsymmetrical cleavage reaction to form B₂H₇^? or BH₄^?, and the same ring closed product formed by 8-hydroxyquinoline. Phenanthridine and N-methylphenanthridinium iodide both readily undergo hydroboration. The products formed by the latter with B₂H₆ are 5-methyl-5,6-dihydrophenanthridine-borane and an iodoborane etherate. 1,8-Naphthyridine forms a transient borane complex which rapidly undergoes hydroboration; protonolysis gives 1,2,3,4-tetrahydro-l,8-naphthyridine. 2,2′-Bipyridyl undergoes a complex reaction with one equivalent of BH₃ giving a mixture of bpy·2BH₃ and hydroborated products, but 4,4′-bipyridyl does not appear to hydroborate. 1,10- and 1,7-Phenanthroline both hydroborate giving a mixture of reduced products on protonolysis. Pyridine-borane and 2-phenylpyridine-borane both decompose in refluxing diglyme, but protonolysis gives no reduced products in either case.