摘要:

Asymmetric amphiphilic diblock copolymers self-assemble in selective solvents. Since 1995, when we first reported the systematic preparation of a sequence of various "crew-cut" aggregate morphologies from this class of copolymer in solution (1), we have identified a vast array of structures and have begun a detailed investigation of the thermodynamic and kinetic parameters that induce morphogenesis. Not only spheres, rods, bilayer and bicontinuous architectures, as well as inverted structures are observed, but also a selection of mixed, combined and much more complex aggregates is documented. All of these aggregates have a phase-separated insoluble core and a crew-cut soluble corona. Thus, all parameters that permit selective modification of the component of either phase or of the interface provide a window for morphological control. By carefully adjusting the polymer chain environment, it has been possible to develop a systematic understanding of morphogenic parameters, which include, among others, polymer composition, common solvent, initial concentration, temperature, type and concentration of added ions, method of preparation, and added homopolymer. To date, more than 30 publications have appeared in the literature from our group alone on this subject. One of the problems inherent with such a complicated system is the taxonomy or classification: which morphologies correspond to equilibrium positions and which are intermediate or trapped? An attempt at a logical presentation of the observed aggregates is given, preceded by a qualitative discussion of the thermodynamic framework for this system. Where possible, the transitions between morphologies are explained in the context of the thermodynamic parameters. Finally, parallels are drawn between the copolymer aggregates and biological architectures.Key words: crew-cut, morphology, block copolymer, self-assembly, amphiphile.

ISSN:0008-4042    

DOI:10.1139/v99-141

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

The following complexes, in which CpIr(CO)2acts as a 2e donor ligand, have been prepared: Cp(OC)2IrW(CO)5(1), Cp(OC)2IrRu(CO)3(SiCl3)2, Cp(OC)2IrOs(CO)3(GeCl3)(Cl) (2), Cp(OC)2IrOs(CO)3(X)2(X = Cl, Br (3), I). The characterization of the complexes included the crystal structures of1,2, and3,which reveal that all contain an unbridged metal-metal bond. The carbonyl ligands of1in solution undergo exchange on the NMR time scale above -40°C. With the exception of2,all the complexes dissociate in solution at room temperature, some rapidly so. Only in the case of3is an equilibrium with the dissociation products established. The results indicate that CpIr(CO)2is a weak ligand.Key words: tungsten, osmium, iridium, binuclear

ISSN:0008-4042    

DOI:10.1139/v99-131

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

A procedure was developed for the annelation of a butenolide to an aromatic ketone that highlighted a manganese(III) oxidation of an aromatic ketone. The merit of this procedure was illustrated in a synthesis of Chromolaenin (Laevigatin) (1) from 4,7-dimethyl-1-tetralone (2c). Oxidation of tetralone and indanone with manganese(III) acetate in the presence of chloro- or bromopropionic acid or their Mn(II) salts furnished a &agr;-haloesters3a-d. An Arbuzov reaction of haloesters with triethylphosphite and an intramolecular Horner-Watsworth-Emmons cyclization of the resulting phosphonate gave butenolide5a,bin 77-79% yield. This methodology was also applied to 4,7-dimethyl-1-tetralone (2c), and the corresponding butenolide6is synthesized in 85% yield, which was then converted to Chromolaenin (1) using DIBAL-H.Key words: chromolaenin (Laevigatin), manganese(III) acetate oxidation, aromatic ketones, butenolide annelation, lactone, Horner-Watsworth-Emmons cyclization, Arbuzov reaction.

ISSN:0008-4042    

DOI:10.1139/v99-136

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

This present work deals with the formation of polyparaphenylene (PPP) electrodes in which the polymer is associated with conductive carbons. Regular and reproducible coatings of PPP on different types of carbon electrodes were obtained by catalytic electrosynthesis (Fauvarque method) and were examined by electron microscopy. Electrochemical properties of PPP, coated on conductive carbons, were studied in sulfolane-NBu4BF4electrolyte. Coatings on carbon lead to a good cyclability and a high apparent current density (as high as 2 mA/cm2).Key words: conductive polymers, polyparaphenylene, electrolytic coating.

ISSN:0008-4042    

DOI:10.1139/v99-132

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

The photodecomposition of &agr;-guaiacoxyacetoveratrone (GAV) in homogeneous solvents and in aqueous cyclodextrin solutions was studied by following the fluorescence emission of the photoproducts formed. The same qualitative behavior as previously observed for the photodegradation quantum yield measurements was reproduced in the fluorescence studies in dry or wet methanol and acetonitrile. The yield for GAV photodegradation in water is smaller than in the organic solvents. Complexation of GAV to &bgr;- and &ggr;-cyclodextrins leads to an increase in the photodecomposition yield, especially during the early part of the photodecomposition kinetics. The transients formed in the photolysis of GAV in water are similar to those observed in acetonitrile, and the lifetime of triplet GAV in water is 130 ns. In addition, solvated electrons were formed when GAV was photolyzed in water. Complexation of GAV to cyclodextrins leads to an increase in the triplet yield compared to the radical yield and a moderate shortening of the triplet lifetime.Key words: &agr;-guaiacoxyacetoveratrone, cyclodextrin, excited singlet states, excited triplet states, radicals.

ISSN:0008-4042    

DOI:10.1139/v99-137

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

Successive photocyanation was found to be a general reaction when chlorinated aromatic compounds were photolyzed with sodium cyanide. The products were polycyanated hydroxychloro compounds with various degrees of chlorine replacement. Although the products from some substrates could be isolated, identified, and characterized, most reactions proceeded with low regioselectivity, which limits their synthetic potential. Quantum yields of substrate disappearance increased with the number of chlorine substituents on a substrate, and followed the expected relationship φ-1is proportional to [CN-]-1. In some cases, φ depended also on the concentration of the chloro compound, indicating the involvement of excimers, although the major reaction channel appears to be SN2Ar*. Sensitization and quenching experiments established the triplet excited state to be reactive for all substrates tested.Key words: photocyanation, chlorinated aromatic compounds.

ISSN:0008-4042    

DOI:10.1139/v99-127

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

Irradiation of borinates derived from 9-borabicyclo[3,3,1]nonane and 1,3-diketones at their &pgr;-&pgr;* transition band caused a rearrangement from their triplet excited state to give boronate intermediates that could be hydrolyzed to the corresponding aldols. The primary photolysis involves the scission of one of the B—C bonds followed by a 1,3- or 1,5-migration and the formation of a more stable B—O bond by radical pathways. The triplet state reaction was established by quenching and heavy atom effect experiments. The overall reaction pattern is an irreversible 1,2- or 1,4-addition of an alkyl-boron group to an &agr;,&bgr;-conjugated ketone system by radical processes. Evidence showed that the non-isolated boronates also underwent secondary photoreactions followed by hydrolysis to afford enones corresponding to dehydration products of aldols; mechanisms are suggested for these secondary steps. The photoreaction was more facile in nonpolar solvents than in polar solvents; different solvents also gave different product compositions. In the presence of dilute acetic acid, the photolysis gave a high yield of aldols, suggesting a rapid acetolysis of the B—C bond in boronates to acetylborates. Under oxygen, the photolysis was complicated by radical oxidation to give totally different products.Key words: borinate photochemistry, charge transfer absorption, alkylborane addition, boronate intermediates, B—C bond homolytic scis

ISSN:0008-4042    

DOI:10.1139/v99-139

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

High-temperature X-ray diffraction (293T(K)1773) is used to investigate the reaction between strontium monoxide and uranium dioxide under controlled atmosphere (105(Pa)10-24), with lattice parameter measurements and composition estimates of the different uranates obtained. Thus, with a Sr/U = 1 sample, we successively observe the phases: (a) orthorhombic &agr;-"SrUO4", whose reduction (3.67O/U3.62) is shown by a constant volume of the cell (V= 0.367 nm3) between 1173 et 1373 K; (b) rhombohedral &bgr;-"SrUO4", which shows a large composition variation between the metastable oxidized form (&bgr;-SrUO3.60) below 1108 K and the stable conjugate reduced form (&bgr;'-SrUO3.11) at whatever temperature; (c) "SrUO3" of constant composition (O/U3) between 293 and 1533 K, then variable (O/U < 3) above 1533 K, with a probable second-order transformation (&agr;-Pnma, &bgr;-Imma) for this distorted perovskite near 1073 K; (d) fluorite type U1-&dgr;Sr&dgr;O2-&dgr;solid solution for which a maximal account in SrO (&dgr;0.25) induces a 0.5 % thermal expansion parameter in comparison with UO2.00. A pseudo-binary "SrUO3"-"SrUO4" phase diagram is propounded. With a sample compound of Sr/U = 3, the monoclinic "Sr3UO6" perovskite is stable under105Pa up to 1373 K. On the other hand, in reducing atmosphere (10-19Pa), it becomes orthorhombic "Sr3UO5" with much greater lattice constants at every temperature.Key words: high-temperature X-ray diffraction, reactivity in metallic oxides, U-Sr-O system, nuclear fuels.

ISSN:0008-4042    

DOI:10.1139/v99-134

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

We report the synthesis and conformational analysis of a series of cyclic and bicyclic decapeptide templates for combinatorial chemistry. The peptides were synthesized via solid phase synthesis and followed by solution cyclization. The conformation of the peptides was studied by proton NMR spectroscopy in DMSO and in TFE-water. The structure of the peptide template was calculated with the program DIANA and followed by SA from the NMR experimental constraints. The peptide adopts a fold comprising two &bgr;-strands and two type II &bgr;-turns. The design of such a restained cyclic decapeptide template will be discussed along with Template Assembled Synthetic Proteins (TASP).Key words: solid phase peptide synthesis, cyclic decapeptide, NMR, conformational analysis, &bgr;-sheet.

ISSN:0008-4042    

DOI:10.1139/v99-128

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

Activation of nucleoside donors5by sulfuryl chloride followed by the addition of 5'-thionucleoside acceptors3yields 5'-thioformacetal dinucleotide analogs6with in situ trapping of liberated methanesulfenyl chloride with cyclohexene. Purine as well as pyrimidine derivatives can be part of a coupling reaction as nucleoside donors or acceptors. The dimethoxytrityl protecting group is compatible with the present coupling methodology allowing differential 3',5'-end protection. The synthesis of longer oligonucleotides is also possible as shown by the preparation of trinucleotide analog23.Key words: oligonucleotides, 5'-thioformacetal.

ISSN:0008-4042    

DOI:10.1139/v99-135

出版商:NRC Research Press    

年代:1999

数据来源: NRC