Bidentate coordination effect on polycondensation of amino acid esters between metal triflates and methoxy groups

To study the bidentate coordination effect on the polycondensation of L ‐valinates between metal triflates as a Lewis acid and methoxy groups, we carried out the polycondensation of 2‐methoxy‐4‐nitrophenyl L ‐valinate ( 1a ) and 2‐methoxyphenyl L ‐valinate ( 1b ) in the presence of the various kinds of rare‐earth triflates in DMF solution at room temperature. The polymerizations of 1a did not proceed without any metal triflates. In the presence of 5 mol% triflates, especially Sc(OTf)₃, the polymerization proceeded effectively. After the reaction mixture was poured into water, the product was collected, which was recognized as poly(L ‐valine)s by FTIR spectrum and GPC measurement. The yield of the product from the polymerization of 1a with Sc(OTf)₃ was higher than that from the polymerization of 4‐nitrophenyl L ‐valinate ( 1c ) with Sc(OTf)₃. This result indicates that the polymerization of 1a was promoted to introduce the methoxy group on the o‐position of the phenyl ring at the ester group with the aim of the bidentate coordination effect between metal triflates and L ‐valinate. As a control experiment, we carried out the polycondensation of 1b in the presence of 5 mol% metal triflates; however, any polymerization did not proceeded. That reason is from the lower activity of activated L ‐valinate ( 1b ). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2864–2868, 2008     

Cationic ring‐opening copolymerization behavior of trioxane and seven‐membered cyclic carbonate

Cationic ring‐opening copolymerization behavior of trioxane (TOX) and a seven‐membered cyclic carbonate, 1,3‐dioxepan‐2‐one (7CC) is described. When TOX and 7CC were cationically copolymerized under various feed ratios using trifluoromethane sulfonic acid (TfOH) as an initiator in nitrobenzene at 30 °C, 7CC was consumed faster than TOX and the decarboxylation was accompanied to afford the corresponding polyacetal–polycarbonate type copolymers containing poly(oxytetramethylene) units. The copolymer composition could be controlled by the feed ratio of 7CC, whose increase resulted in the high copolymer composition of the 7CC unit. The solubility of the copolymers increased as the increase of the 7CC content. Thermogravimetric, size‐exclusion chromatographic, and X‐ray analyses of the copolymers suggest that the sequences of the copolymer chains consist of the segments containing the units originated from 7CC and those with TOX unit‐rich compositions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 733–739, 2008     

Differential scanning calorimetry study on thermal behaviors of freeze‐dried poly(L‐lactide) from dilute solutions

Glass transition, cold crystallization, and melting of freeze‐dried poly(L‐lactide) (PLLA) prepared from dilute 1,4‐dioxane solutions were investigated by differential scanning calorimetry (DSC). Conventional DSC measurements of heating scans revealed that freeze‐dried PLLA prepared from a 0.07 wt % solution undergoes a two‐step cold crystallization (or reorganization) with a lower exotherm appearing at about 78 °C and with a higher broad exotherm between 110–155 °C. The peak temperature of the former exotherm is about 50 K lower than that observed for a reference bulk sample. Step‐scan mode DSC, which provides information essentially equivalent to that obtained from the temperature‐modulated DSC, revealed that the glass‐transition temperature is about 6 K lower than that of the reference bulk. These findings suggest enhanced chain mobility for freeze‐dried PLLA. Freeze‐dried PLLA that crystallized at 80 °C for 40 min was revealed to contain a rather large amount of rigid amorphous material (42%). © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 115–124, 2005     

Chemical control of magnetism: oxidation-induced ferromagnetic spin coupling in the chromium dimer evidenced by photoelectron spectroscopy

The photoelectron spectrum of the dichromium oxide cluster anion, Cr2O-, and the analysis by the density-functional theory revealed that the spins of the two Cr atoms in Cr2O- are ferromagnetically coupled, and that its total spin magnetic moment is as large as 9 mu(B). This ferromagnetic spin coupling is induced by oxidation; the mixing of Cr 3d with O 2p orbitals plays an important role in a spin coupling between the localized electrons at the two Cr sites bridged by the O atom. The present finding is in marked contrast to the pure chromium dimer, which is known to be antiferromagnetic due to the strong sextuple Cr-Cr bond.     

Electric conductivities of 1:1 electrolytes in liquid methanol along the liquid-vapor coexistence curve up to the critical temperature. III. Tetraalkylammonium bromides

The molar conductivities of the dilute solutions of the tetraalkylammonium bromides have been measured in methanol along the liquid-vapor coexistence curve up to about 180 degrees C. The limiting molar conductivities and the molar association constants have been obtained from the analysis of the concentration dependence of the conductivity. On the basis of the present data together with the literature ones, the validity of the Hubbard-Onsager (HO) dielectric friction theory [J. Hubbard, J. Chem. Phys. 68, 1649 (1978)] derived from the continuum model has been examined for the translational friction coefficients of the tetraalkylammonium ions in methanol in the density range of 0.8232 g cm(-3) > or =rho > or =0.5984 g cm(-3) and the temperature range of -15 degrees C < or =t < or =180 degrees C. At high densities and low temperatures, the observed friction coefficients of Me(4)N(+) and Et(4)N(+) are remarkably smaller than the prediction of the HO theory (where Me stands for methyl group and Et for ethyl group); this kind of limitation of the HO theory has not been recognized for smaller ions, and can be attributed to the loosening of the solvent structure closely related to the weak charge effect for the large ions. The negative deviation from the HO theory gradually disappears with decreasing density and increasing temperature, and the friction coefficients of Me(4)N(+) and Et(4)N(+) are explained by the HO theory reasonably well at low densities and high temperatures. For Pr(4)N(+) and Bu(4)N(+) (where Pr stands for propyl group and Bu for butyl group), the experimental friction coefficients lay in the validity range of the HO theory in all the conditions studied here; the breakdown of the continuum theory at low densities and high temperatures has not been observed in this work. The density dependences of the molar association constants of the tetraalkylammonium bromides are qualitatively explained by the Fuoss theory based on the continuum model.     

Controlled polymerization of activated glycine esters by copper(II) chelate

We describe a novel method of polymerization, via the insertion of activated glycine esters into N‐salicylideneglycinato‐aquo‐copper(II) chelate ( 1 ), that uses the reactivity of the metal chelate. In the absence of 1 , a high molecular weight polyglycine was formed as a white precipitate after triethylamine was added to an N,N‐dimethylformamide solution of 4‐nitrophenyl glycinate ( 3a ). In the presence of 5 mol % 1 , however, the polymerization proceeded homogeneously. After the reaction mixture was poured into tetrahydrofuran, a condensation product of glycine was obtained. According to gel permeation chromatography analysis, the product consisted of high and low molecular weight fractions. The former and latter were obtained by self‐polycondensation and polycondensation via the insertion of 3a into 1 , respectively. So that the self‐polycondensation of activated glycinates would be depressed, 2‐chlorophenyl ( 3b ), 3‐chlorophenyl ( 3c ), 4‐chlorophenyl, and phenyl glycinates were used as less activated glycine esters. For the polymerization of 3b and 3c , the polymerization via the insertion of activated glycinates into 1 was promoted. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1504–1510, 2003     

Swept excitation model of superradiance from multi-level system

A simple model of superradiance from the multi-level system, in which the propagation effects is fully taken into account, is presented. With applying this model to the three level system, some typical effects peculiar to the multi-level system are discussed.     

Carbon-13 nuclear magnetic resonance spectroscopy IX—monosubstituted ethylenes

Carbon-13 chemical shifts of sixteen monosubstituted ethylenes were obtained. In order to explain the chemical shifts, σ and π electron densities of these compounds are calculated by the σ-included ω-HMO method.

1 See Ref. 8.

A linear relationship exists between carbon-13 chemical shifts and the calculated electron densities, and also between substituent constants and electron densities. A slope of unity is obtained between the chemical shifts of α carbons of monosubstituted ethylenes and those of carbons adjacent to the substituents in monosubstituted benzenes. On the other hand, a plot of chemical shifts of C_ortho of benzene derivatives against that of the β carbon in ethylene derivatives gives a slope of 3. These slopes can be explained by the calculated electron densities. A slope of 4/3 is obtained between the direct coupling constant ¹J(C? H) of the α carbon in monosubstituted ethylenes and that in the corresponding substituted methanes.     

Conversion from arenes having a benzene ring to those having a picolinic acid by simple growing cell reactions using Escherichia coli that expressed the six bacterial genes involved in biphenyl catabolism

The comprehensive bioconversion of aromatic compounds with a benzene ring to a picolinic acid was achieved with a recombinant Escherichia coli strain that expressed the six genes involved in biphenyl catabolism, these being the bphA1(2072)A2A3A4 genes encoding the evolved biphenyl dioxygenase, the bphB gene encoding dihydrodiol dehydrogenase, and the bphC gene encoding catechol 2,3-dioxygenase.     

Electric conductivities of 1:1 electrolytes in liquid methanol along the liquid-vapor coexistence curve up to the critical temperature. II. KBr and KI solutions

The molar conductivities Lambda of KBr and KI in dilute methanol solutions were measured along the liquid-vapor coexistence curve up to the critical temperature (240 degrees C). The concentration dependence of Lambda in each condition was analyzed by the Fuoss-Chen-Justice equation to obtain the limiting molar conductivities and the molar association constants. Using the present data together with the literature ones, the validity of the Hubbard-Onsager (HO) dielectric friction theory based on the sphere-in-continuum model was examined for the translational friction coefficients zeta of the halide ions (the Cl(-), Br(-), and I(-) ions) in methanol in the density range of 2.989rho(c)> or =rho> or =1.506rho(c), where rho(c)=0.2756 g cm(-3) is the critical density of methanol. For all the halide ions studied, the friction coefficient decreased with decreasing density at rho>2.0rho(c), while the nonviscous contribution Deltazeta/zeta increased; Deltazeta was defined as the difference between zeta and the friction coefficient estimated by the Stokes law. The density dependence of zeta and Deltazeta/zeta were well reproduced by the HO theory at rho>2.0rho(c). The HO theory also explained the ion-size dependence of Deltazeta/zeta which decreased with ion-size at rho>2.0rho(c). At rho<2.0rho(c), on the other hand, the HO theory could not explain the density and the ion-size dependences of zeta and Deltazeta/zeta. These results indicated that the application limit of the HO theory lied about rho=2.0rho(c) which is the same as the application limit observed for the alkali metal ions. The present results were also compared with the results in subcritical aqueous solutions.