Ionen-Einfang bei der kationischen Polymerisation

Ohne Zusammenfassung     

Structural and electronic evolution of Cr2O3 on compression to 55 GPa

Synchrotron single-crystal x-ray diffraction experiments have been performed on corundum-type Cr₂O₃ up to a pressure of 55 GPa in Ne and He pressure transmitting media. Diffraction experiments were complemented by measurements of optical absorption spectra with single crystal samples up to 60 GPa. Results of the diffraction data analysis rule out the earlier reported monoclinic distortion at 15–30 GPa, but indicate evidence of two discontinuous transitions of electronic or magnetic nature, most likely associated with a change in magnetic ordering and charge transfer. The compression mechanism established from single crystal refinements indicates much smaller distortion of the Cr³⁺ coordination environment than was previously assumed.     

Antibacterial Activity of Peptides Related to Alafosfalin. The Effect of Varying the Aminophosphonate Fragment

Abstract

A series of 32 dipeptides containing N-terminal alanine or leu-cine and a variety of racemic 1-aminoalkanephosphonic acids vere prepared by standard procedures and tested for growth inhibition of six bacterial species (Escherichia coli, Klebsiella aerogenes, Serratia mercescens, Staphylococcus aureus, Streptococcus faeca-lis and Bacillus subtilis). The aminophosphonate residues were racemic and included Va1P, LeuP, ProP, PheP, α-methyl-AlaP, Glu-α-P, O-methyl-DOPAP, cyclohexane-1-amino-1-phosphonic acid, t-LeuP, O-acetyl-SerP, and GlyP derivatives RCH(NH₂)PO₃H₂ where R=cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl and adaman-tyl. N-Ala and N-Leu peptides of racemic AlaP were used as positive control. MIC and IC₅₀ values indicate that the peptides containing 4-amino-4-phosphonobutyric acid (Glu-α-P) and α-methyl-AlaP are potent antibiotics, comparable in activity with LeuAlaP and AlaAlaP (Alafosfalin). Weak activity was observed for peptides of ProP, LeuP, ValP, PheP, cyclohexane-1-amino-1-phosphonic acid and 1-aminocyclopentylmethanephosphonic acid. While the activity of the α-methyl-AlaP peptides may be explained by inhibition of alanine racemase, the mechanism of action of the Glu-α-P peptides remains unknown.     

Azines and Azoloazines,Part 1: Reactions of Triazolo[3,4‐a]phthalazine and Its Derivatives with Carbanions

Triazolo[3,4‐a]phthalazine as well as their chloro and nitro derivatives were subjected to the reactions with the carbanions typical for the vicarious nucleophilic substitution (VNS) of hydrogen. The reactions were strongly dependent on the substituents present on the triazolo[3,4‐a]phthalazine ring and resulted not only in the substitution of hydrogen but also in exchange of chlorine atom and pyridazine ring scission; the latter process dominated for the unsubstituted triazolophthalazine. Two of the products showed promising stimulating activity towards the central nervous system with no significant toxic effects.     

Hyperbranched polyethers by ring‐opening polymerization: Contribution of activated monomer mechanism

Propagation in the cationic ring‐opening polymerization of cyclic ethers involves nucleophilic attack of oxygen atoms from the monomer molecules on the cationic growing species (oxonium ions). Such a mechanism is known as the active chain‐end mechanism. If hydroxyl groups containing compounds are present in the system, oxygen atoms of HO? groups may compete with cyclic ether oxygen atoms of monomer molecules in reaction with oxonium ions. At the proper conditions, this reaction may dominate, and propagation may proceed by the activated monomer mechanism, that is, by subsequent addition of protonated monomer molecules to HO? terminated macromolecules. Both mechanisms may contribute to the propagation in the cationic polymerization of monomers containing both functions (i.e., cyclic ether group and hydroxyl groups) within the same molecule. In this article, the mechanism of polymerization of three‐ and four‐membered cyclic ethers containing hydroxymethyl substituents is discussed in terms of competition between two possible mechanisms of propagation that governs the structure of the products—branched polyethers containing multiple terminal hydroxymethyl groups. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 457–468, 2003     

Polyepichlorohydrin diols free of cyclics: Synthesis and characterization

Polymerization of epichlorohydrin (ECH) in the presence of diols, catalyzed by Lewis or protic acids, proceeds by activated monomer mechanism (AMM), i.e., by subsequent additions of protonated monomer molecules to the terminal hydroxyl groups of the growing chain. As opposed to the typical active chain end mechanism, side reactions, including cyclization, are strongly suppressed in the polymerization by AMM and well-defined linear product are obtained. It follows from kinetic considerations, that in order to achieve the high contribution of AMM, the reaction should be carried out at low instantaneous concentration of monomer, and this can be accomplished by slowly adding ECH to the reaction mixture. Using this approach, polyepichlorophydrin diols have been prepared in the M?_n ～ 2500 products with DP _n = [M]₀/[I]₀ can be obtained practically free of cyclic by-products with the yields approaching quantitative. Polyepichlorohydrin diols obtained by AM polymerization are strictly bifunctional, regular head-to-tail polymers containing mainly (≥ 95%) secondary hydroxyl and groups.