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  • Polymer and Materials Science  (100)
  • Inorganic Chemistry  (63)
  • Organic Chemistry  (50)
  • Industrial Chemistry
  • 1990-1994  (213)
  • 1
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Ethylene and propene were homopolymerized with 11 different metallocene-methylalumin-oxane (MAO) catalysts 1-11 using a constant set of parameters. The catalytic activity of the two unbridged and 9 ansa-metallocenes as well as the properties of the polymers, such as molecular weight, width of molecular weight distribution, melting point, and, with polypropene, chain microstructure, were investigated. It was found that structure variations of the catalysts cause rather different catalytic behaviour, which further diversifies the performances with the two different monomers. In some cases this comparison allows insight into the nature of the influencing factors.
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  • 2
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Handelsübliche Araldit™-Polymere wurden mit Zimtsäure- bzw. Furanacrylsäurechlorid modifiziert und über [2 + 2]-Cycloaddition photochemisch vernetzt. Die Glasübergangstemperaturen der entstandenen Polymeren betragen bis zu 130°C bei Dielektrizitätskonstanten von ∊r = 3.0. Daher können diese Polymeren in Mehrlagenmikroverdrahtungen verwendet werden. Zahlreiche wohldefinierte, multifunktionelle Modifikatoren wurden synthetisiert und deren Einfluß auf die Polymerisation und die Eigenschaften der Araldit™-Polymeren eingehend untersucht.
    Notes: Commercially available Araldite™-type polymers have been modified with cinnamic and furan acrylic acid chlorides. The polymers were photochemically crosslinked via a [2 + 2] cycloaddition. The resulting polymers show glass transition temperatures up to 130°C and dielectrical constants of ∊r = 3.0. Therefore, these polymers can be applied as permanent resists in multi-layered wiring structures. Numerous well-defined multifunctional modifiers have been synthesized. The effect of those modifiers on the polymerization and properties of the araldite-type polymers has been studied in some detail.
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  • 3
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 185 (1991), S. 75-86 
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Poly(2,6-dimethyl-1,4-phenylene oxide) with exactly one functional endgroup was synthesized by oxidative polymerization, halogen substitution polymerization, and by phase transfer catalysis. Reaction with 4-vinylbenzoyl chloride resulted in a quantitative conversion to a macromonomer as shown by GPC and endgroup analysis. The macromonomer is a glassy material with a Tg from 150 to 190°C depending on the molecular weight.
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  • 4
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 223 (1994), S. 101-120 
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Metallocenes have gained widespread interest as catalysts for olefin polymerization due to their high activity and versatility. A large range of new polymers with narrow molecular weight distribution and different microstructures can be produced with these “single-site” catalysts. These highly active catalysts which are able to produce 40 tonnes of polyethylene per gram zirconium per hour are even capable of incorporating remarkable amounts of sterically demanding olefins and dienes in copolymerization with ethylene. Isotactic, syndiotactic, stereoblock and isoblock polyolefins are accessible by variation of the metallocene ligands. Similarly, the isotacticity and molecular weight of the generated polyolefins are influenced by the ligand structure. By heterogenization of the metallocene on metal powder or silica polymers with substantially higher molecular weight, higher melting points and other new properties may be attained. Cyclic olefins like cyclopentene, norbornene or dimethanooctahydronaphthalene polymerize without ring-opening to yield highly melting and poorly soluble polymers. By separation of the racemic catalyst into enantiomers, optically active alkanes and alkenes are accessi-ble. Trimers are obtained with ee of 95%. These results provide new insights into the mechanism of Ziegler-Natta catalysis.
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  • 5
    Electronic Resource
    Electronic Resource
    New York : Wiley-Blackwell
    Biopolymers 33 (1993), S. 1271-1286 
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A Monte Carlo simulated annealing (MCSA) algorithm was used to generate the conformations of local regions in bovine pancreatic trypsin inhibitor (BPTI) starting from random initial conformations. In the approach explored, only the conformation of the segment is computed; the rest of the protein is fixed in the known native conformation. Rather than follow a single simulation exhaustively, computer time is better used by performing multiple independent MCSA simulations in which different starting temperatures are employed and the number of conformations sampled is varied. The best computed conformation is chosen on the basis of lowest total energy and refined further. The total energy used in the annealing is the sum of the intrasegment energy, the interaction energy of the segment with the local surrounding region, and a distance constraint to generate a smooth connection of the initially randomized segment with the rest of the protein.The rms deviations between the main-chain conformations of the computed segments in BPTI and those of the native x-ray structure are 0.94 Å for a 5-residue α-helical segment, 1.11 Å for a 5-residue β-strand segment, and 1.03, 1.61, and 1.87 Ã for 5-, 7-, and 9-residue loop segments. Side-chain deviations are comparable to the main-chain deviations for those side chains that interact strongly with the fixed part of the protein. A detailed view of the deviations at an atom-resolved level is obtained by comparing the predicted segments with their known conformations in the crystal structure of BPTI. These results emphasize the value of predetermined fixed structure against which the computed segment can nest. © 1993 John Wiley & Sons, Inc.
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  • 6
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two trans-syn cyclobutane photodimers of thymidylyl (3′-5′) deoxyuridine were formed by deamination of the corresponding trans-syn cyclobutane photodimers of thymidylyl(3′-5′) deoxycytidine and were examined by 1H-, 13C-, and 31P-nmr spectroscopy. Correlation spectroscopy, nuclear Overhauser enhancement spectroscopy, and one-dimensional heterodecoupling experiments allowed a more complete assignment of the 1H spectra, compared with previous reports by Koning et al. [ (1991) European Journal of Biochemistry, Vol. 195, pp. 29-40] and Liu and Yang [(1978) Biochemistry, Vol. 17, pp. 4865-4876]. Deoxyribose ring conformations were calculated from 1H coupling constants by pseudorotational analysis, and rotamer distributions of exocyclic bonds were calculated from the observed homonuclear and heteronuclear coupling constants. The cyclobutane ring configuration (CB) of each isomer was identified, using arguments based upon observed scalar and dipolar couplings. Glycosidic bond conformation was ascertained from nuclear Overhauser enhancements observed between base and deoxyribose protons. Isomer I (S-type class; CB-; SYN-ANTI) and isomer II (N-type class; CB+; ANTI-SYN) exhibit markedly different conformational features. 31P chemical shifts and exocyclic bond rotamer distributions indicate diminished backbone flexibility for both photoproducts relative to parent thymidylyl (3′-5′) deoxyuridine. Isomer I (SYN-ANTI) is particularly rigid, while isomer II (ANTI-S YN) maintains some flexibility. Also, 13C spectra were acquired and assigned unequivocally with the aid of short- and long-range two-dimensional heteronuclear shift correlation experiments. © 1993 John Wiley & Sons, Inc.
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  • 7
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: On treatment with H2SO4/MeOH, epimerization of hexamethyl cis-5,6-dihydroxycobyrinate c,8-lactam (3) takes place quantitatively at C(6), yielding the corresponding trans-diol 4. The corresponding lactone 7, whose structure has been established by X-ray analysis, is obtained from xanthocorrinoids 5 and 6 under similar conditions.
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  • 8
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The titanates derived from α,α,α′,α′-tetraaryl-1,3-dioxolane-4,5-dimethanols (TADDOLs, prepared from tartrate) act as catalysts for enantioselective additions of dialkylzinc compounds to aldehydes. For the standard reaction chosen for this investigation of the mechanism, the addition of diethylzinc to benzaldehyde, there is very little change of selectivity with different aryl substituents on the TADDOLate ligands (Tables 2-4, examples). With 0.02 to 0.2 equiv. of the chiral titanates, selectivities above 90% are observed only in the presence of excess tetraisopropyl titanate! According to NMR measurements (Fig. 2), the chiral bicyclic titanate and the achiral titanate do not react to give new species under these conditions. From experiments with different stoichiometries of the components, and with different achiral or chiral OR groups on the Ti-atom of the seven-membered ring titanate, it is concluded (i) that a single chiral titanate is involved in the product-forming step, (ii) that the bulky TADDOLate ligand renders the Ti-center catalytically more active than that of (i-PrO)4Ti, due to fast dynamics of ligand exchange on the sterically hindered Ti-center (Table 5, Fig. 3), and (iii) that the role of excess (i-PrO)4Ti is to remove - by ligand exchange - the product alkoxides (R*O) from the catalytically active Ti-center (Scheme 4, Table 6). Three new crystal structures of TADDOL derivatives (two clathrates with secondary amines, and a dimethyl ether) have been determined by X-ray diffraction (Figs. 5-7), and are compared with those previously reported. The distances between the C(aryl)2O oxygen atoms in the C2- and C1-symmetrical structures vary from 2.58 to 2.94 Å, depending upon the conformation of their dioxolane rings and the presence or absence of an intramolecular H-bond (Fig. 8). A single-crystal X-ray structure of a spiro-titanate, with two TADDOLate ligands on the Ti-atom, is described (Fig. 9); it contains six different seven-membered titanate-ring conformations in the asymmetric unit (Fig. 10), which suggests a highly flexible solution structure. The structures of Ti TADDOLate complexes are compared with those of C2-symmetrical Ru, Rh, and Pd disphosphine chelates (Table 7). A common topological model is presented for all nucleophilic additions to aldehydes involving Ti TADDOLates (Si attack with (R,R)-derivatives, relative topicity unlike; Fig. 11). Possible structures of complexes containing bidentate substrates for Ti TADDOLate-mediated ene reactions and cycloadditions are proposed (Fig. 12). A simple six-membered ring chair-type arrangement of the atoms involved can be used to describe the result of TADDOLate-mediated nucleophilic additions to aldehydes and ketones, with Ti, Zr, Mg, or Al bearing the chiral ligand (Scheme 6). A proposal is also made for the geometry of the intermediate responsible for enantioselective hydrogenation of N-(acetylamino)cinnamate catalyzed by Rh complexes containing C2-symmetrical diphosphines (Fig. 13).
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  • 9
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reactions of 5H,2λ5-Azaphospholes with Arylazocarbonitriles and Dialkyl AcetylenedicarboxylatesAzaphospholes 1a-c react with activated arylazocarbonitriles to 1,5,2λ5-diazaphosphorines 2a-c and 3a-c. The reaction of 1a-c with diethyl or dimethyl acetylenedicarboxyiates yields 7H-1,4λ5-azaphosphepines 4a-c. The structures of 2b, 3a, and 4a are established by an X-ray diffraction analysis.
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  • 10
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Treatment of ten monocyclic dienols 8-11 with an excess of fluorosulfonic acid in 2-nitropropane at -90° afforded diastereoisomeric mixtures of racemic tricyclic ethers 12-14 in 81-91% yield (see tables 1 and 2). These transformations represent further examples of biomimetic acid-mediated cyclisations in which an OH group serves as the internal nucleophilic terminator. A non-synchronous process is postulated, and the examples described strongly re-inforce our working mechanistic hypothesis, whereby the stereochemical course of cyclisation is directed by the orientation of the side chain vicinal to the intermediate cyclohexyl cation (see Schemes 4 and 5). It is also demonstrated that the efficiency of this process is independent of the nature of the OH group, which may be primary, secondary, or tertiary. In addition, the organoleptic properties of 12-14, Me homologs of known odorants such as Ambrox® ((-)-3a) and its diastereoisomer, are briefly discussed.
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