Dendrimers as scaffolds for multifunctional reversible addition–fragmentation chain transfer agents: Syntheses and polymerization

The synthesis and characterization of novel first‐ and second‐generation true dendritic reversible addition–fragmentation chain transfer (RAFT) agents carrying 6 or 12 pendant 3‐benzylsulfanylthiocarbonylsulfanylpropionic acid RAFT end groups with Z‐group architecture based on 1,1,1‐hydroxyphenyl ethane and trimethylolpropane cores are described in detail. The multifunctional dendritic RAFT agents have been used to prepare star polymers of poly(butyl acrylate) (PBA) and polystyrene (PS) of narrow polydispersities (1.4 < polydispersity index < 1.1 for PBA and 1.5 < polydispersity index < 1.3 for PS) via bulk free‐radical polymerization at 60 °C. The novel dendrimer‐based multifunctional RAFT agents effect an efficient living polymerization process, as evidenced by the linear evolution of the number‐average molecular weight (M_n) with the monomer–polymer conversion, yielding star polymers with molecular weights of up to M_n = 160,000 g mol^?1 for PBA (based on a linear PBA calibration) and up to M_n = 70,000 g mol^?1 for PS (based on a linear PS calibration). A structural change in the chemical nature of the dendritic core (i.e., 1,1,1‐hydroxyphenyl ethane vs trimethylolpropane) has no influence on the observed molecular weight distributions. The star‐shaped structure of the generated polymers has been confirmed through the cleavage of the pendant arms off the core of the star‐shaped polymeric materials. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5877–5890, 2004     

Fluoride effect on the palladium–phenanthroline catalyzed carbonylation of nitroarenes to carbamates

Fluorides promote the palladium–phenanthroline catalyzed carbonylation of nitroarenes to carbamates. The effect is more evident on the rate of the reaction at short reaction times, but a positive effect on selectivity is also observed under certain conditions. The effect is observed even under conditions under which chloride inhibits the reaction. Tetraethylammonium is a better countercation than sodium. Copyright © 2007 John Wiley & Sons, Ltd.     

Dimensionally and thermally stable polymer,containing disordered graphitic structure and adamantane

In an attempt to develop a low‐k interlayer dielectric, adamantane‐diphenyldiethynyl moiety containing oligomer is prepared. Oligomerization of 1,3,5,7‐tetrakis[3/4‐ethynylphenyl]adamantane ( 4 ) is accomplished by a Glaser–Hay oxidative coupling with 1,3,5‐triethynylbenzene and phenylacetylene end‐capping agent. The CHCl₃ soluble oligomer is then thermally treated by step‐curing at 200, 300, 380, and 450 °C for 30 min at each temperature under nitrogen flow to render a shiny void‐free black polymer. TGA analysis indicates that the polymer is stable under nitrogen up to 500 °C with a marginal decomposition up to 800 °C. Solid‐state ¹³C NMR, Raman scattering, and FTIR are used to characterize the structure of the polymer. The polymer consists of amorphous carbon networks with the adamantane moieties and nanosized graphitic regions (clusters), which are generated from the thermal crosslinking of the diphenyldiethynyl units. It shows a remarkably low linear coefficient of thermal expansion (～25 ppm/°C), presumably due to the presence of the disordered graphitic structure. Its high density (～1.21 g/cm³), refractive index (～1.80 at 632 nm), and Young's modulus (～17.0 GPa) are also consistent with the interpretation. This study reveals important details about the effect of microscopic structure on the macroscopic properties of the highly crosslinked polymer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6909–6925, 2006     

Polymers Containing Lactone Groups

Polymers containing intact lactone groups are a new class of macromolecules with reactive groups, which are relatively easy to obtain by polymerization, polycondensation and polyaddition, as well as by reactions on existing macromolecules. Polymers with β-lactone Groups in particular can enter into numerous addition reactions, which can be used, for example, to obtain macromolecules containing hydroxy acid or amino acid groupings. The reactions proceed under mild conditions, and can even be carried out in aqueous media, frequently giving water-soluble polymers. The polymers can be cross-linked at low temperatures, even from the aqueous phase, by the addition of bifunctional or oligofunctional reagents. Polymers containing β-lactone groups can also be used as a basis for graft co-polymers; polyester or polyether branches can be grafted on, depending on whether monomeric lactones or monomeric epoxides are used.     

Heterogeneous reactions : VI. Reactions of trimethylchlorosilane,dimethyldichlorosilane and chloromethyldimethylchlorosilane with potassium t-butoxide and sodium methoxide

The heterogeneous gas-solid reaction of trimethylchlorosilane and an equal molar mixture of potassium t-butoxide and sodium methoxide gave exclusively trimethyl-t-butoxysilane. A similar reaction utilizing dimethyldichlorosilane as the vapor phase substrate gave predominant incorporation of butoxide. The heterogeneous reaction of chloromethyldimethylchlorosilane with these two bases was extensively studied. The parameters investigated for the latter system include the dependence of the product distribution on temperature, nitrogen flow rate and the relative hydroxide content of the solid alkoxide base. These studies allow the assessment of the relative reactivities of these bases under the conditions of heterogeneous gas/solid reaction. Under these conditions and with partially hydrolyzed bases a substantial amount of displacement of the chloromethyl group from chloromethyldimethylchlorosilane is observed. The role of hydroxide ion in this reaction is discussed. Convenient syntheses of t-butyl methyl ether and dimethylchloromethoxysilane are reported.     

Differential capacity curves at adsorption of organic ions: Model approach I

Theoretical analysis of the double-layer model has been carried out in the presence of the specific adsorption of organic cations accompanied by considerable increase of the inner-layer dimensions. The formulae for calculation of the differential capacity curves of an electrode have been derived. A flat minimum at high negative charges, caused by the diffuse structure of the double layer, has been predicted on the capacity curves. The presence of such minima has been verified experimentally on the mercury and bismuth electrodes. By computer calculation it has been shown that, although. relatively good agreement of the theoretically calculated capacity curves with the experimental curves could he obtained, a physically unrealistic interaction parameter of the specifically adsorbed ions had to be used. As demonstrated, this result is a result of the double-layer model assuming linear dependence of the inner-layer integral capacity of the surface coverage and its independence from the electrode charge.     

Röntgenkleinwinkeluntersuchungen an Lösungen von Schweineserum-Albumin bei verschiedenen pH-Werten

Zusammenfassung Schweineserum-Albumin wurde nach sorgfältiger Reinigung bei den pH-Werten 7,0, 5,1 und 3,7 mit Hilfe der Röntgenkleinwinkelstreuung untersucht. Es wurde bei allen Messungen praktisch dasselbe Molekulargewicht gefunden: der Mittelwert liegt bei 70 300±2 500. Die Ermittlung der Streumassenradien ergab bei pH 7,0 und pH 5,1 ähnliche Werte (33,0 Å bzw. 31,1 Å), bei pH 3,7 dagegen einen Wert von 37,8 Å. Die Formbestimmung ergab bei pH 7,0 und pH 5,1 identische Achsenverhältnisse (0,75 : 1 : 2), während bei pH 3,7 ein solches von 0,2 : 1 : 1 gefunden wurde.

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X-ray small-angle scattering investigations on solutions of porcine serum albumin at differentpH-values

Higly purified samples of porcine serum albumin were measured by the X-ray small-angle scattering method at pH 7.0, 5.1, and 3.7. The molecular weight is found to be the same in all three cases: the mean value is 70 300±2 500. The radius of gyration is 31.1 Å and 33.0 Å at pH 5.1 and 7.0 resp., whereas at pH 3.7 a value of 37.8 Å is found. The determination of the molecular conformation yields a similar axial ratio at pH 5.1 and pH 7.0 of 0.75 : 1 : 2; at pH 3,7 we find an axial ratio of 0.2 : 1 : 1 to be consistent with the scattering curves.

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Herrn Prof. Dr.H. Nowotny gewidmet.     

Capillary isoelectric focusing of proteins utilizing poly(vinylpyrrolidone)- and plexiglas-coated columns

Fused-silica capillaries chemically derivatized with silane/poly(vinylpyrrolidone) (PVP) or dynamically modified with plexiglas [poly(methyl methacrylate)] were prepared and evaluated with regard to column stability and separation performance for capillary isoelectric focusing of standard proteins. The PVP coating showed the better stability and was good for at least 100 runs while the plexiglas coating started to deteriorate after about 30 runs. The time spent for the plexiglas coating is about 40 minutes while the PVP coating requires two days. The migration time reproducibility was better with the PVP capillary (RSD 0.7-1.6%, n = 5) compared to the plexiglas-coated column (RSD 1.2-2.9%, n = 5) while peak area and height varied over a similar interval (RSD 2-28.1% area; 0.9-22.7% height, n = 5). The two most consistent proteins in this evaluation, viz. myoglobin A and carbonic anhydrase II, showed linear dynamic ranges between 5-150 and 5-50 microg/mL, and limits of detections at 2 and 1 microg/mL, respectively, employing UV detection at 280 nm.     

Amino acid analysis by micellar electrokinetic chromatography with laser-induced fluorescence detection: application to nanolitre-volume biological samples from Arabidopsis thaliana and Myzus persicae

Amino acids were derivatised with 4-fluoro-7-nitrobenzo-2,1,3-oxadiazol (NBD-F), separated by micellar electrokinetic chromatography (MEKC), and detected by argon-ion (488 nm) laser-induced fluorescence. The optimised MEKC background electrolyte conditions were: 40 mM sodium cholate, 5 mM beta-cyclodextrin in 20 mM aqueous borate buffer, pH 9.1, with 7% v/v acetonitrile. Using these conditions, 19 amino acids were separated within 17 min. The limits of detection were in the range of 7.6-42.2 pmol/mL and limits of quantitation from 0.05-0.14 nmol/mL. The method was systematically validated for injection volume error, migration time variation, calibration linearity, accuracy, precision, and recovery. Nanolitre volume samples of phloem sap of individual sieve element cells from the plant Arabidopsis thaliana and honeydew from the aphid Myzus persicae were directly analysed with this method. Quantitative amino acid concentrations in these two biological matrices were profiled for the first time. This method is particularly important because it allows the complete profile of the amino acids obtained from individual phloem elements, allowing cell to cell and plant to plant variation to be quantified, which to date has not been possible with Arabidopsis thaliana.     

Confirmation of the "long-lived" tetra-nitrogen (N4) molecule using neutralization-reionization mass spectrometry and ab initio calculations

Tetra-nitrogen (N(4)), which has been the subject of recent controversy [Cacace, d. Petris, and Troiani, Science 295, 480 (2002); Cacace, Chem. Eur. J. 8, 3839 (2002); Nguyen et al., J. Phys. Chem. A 107, 5452 (2003); Nguyen, Coord. Chem. Rev. 244, 93 (2003)] as well as of great theoretical interest, has been prepared from the N(4) (+) cation and then detected as a reionized gaseous metastable molecule with a lifetime exceeding 0.8 micros in experiments based on neutralization-reionization mass spectrometry. Moreover, we have used the nature of the charge-transfer reaction which occurs between a beam of fast N(4) (+) ions (8 keV translational energy) and various stationary gas targets to identify the vertical neutralization energy of the N(4) (+) ion. The measured value, 10.3+/-0.5, most closely matches that of the lowest energy azidonitrene (4)N(4) (+)C(s)((4)A(')) ion, resulting in the formation of the neutral bound azidonitrene (3)N(4)C(s)((3)A(")). Neutralization of the global minimum (2)N(4) (+)D( infinity h)((2)Sigma(u) (+)) ion leads to a structure 166 kJ mol(-1) above the dissociation products [N(2)((1)Sigma(g) (+))+N(2)((1)Sigma(g) (+))]; moreover, it was not possible to find a minimum on the (1)N(4) neutral potential energy surface for a covalently bonded structure. Ab initio calculations at the G3, QCISD/6-31G(d), and MP2/AUG-cc-pVTZ levels of theory have been used to determine geometries and both vertical neutralization energies of ions (doublet and quartet) and ionization energies of neutrals (singlet and triplet). In addition, we have also described in detail the EI ion source for the Ottawa VG ZAB mass spectrometer [Holmes and Mayer, J. Phys. Chem. A 99, 1366 (1995)] which was modified for high-pressure use, i.e., for the production of dimer and higher number cluster ions.