Cationic copolymerization of tetrahydrofuran with ethylene oxide in the presence of diols: Composition,microstructure, and properties of copolymers

Cationic copolymerization of tetrahydrofuran (THF) with ethylene oxide (EO) in the presence of diols leads to dihydroxy terminated telechelic copolymers. In the present article the influence of copolymerization conditions on the copolymer structure was studied in view of conclusions derived from studies of copolymerization kinetics and mechanism. It was shown that according to established copolymerization mechanism, the number average molecular weights increase linearly with conversion up to M_n ≅ 2500, hydroxyl end groups are bound exclusively to EO units and copolymers are composed of [EO]–[THF]_y segments. Microstructure of copolymers may be to some extent regulated by changing reaction conditions. Some physical properties of copolymers also were studied. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3455–3463, 1999     

Polyethers containing stable ionic end-groups

Polyethers of low and medium molecular weight containing at both ends stable ionic groups (phosphonium ions) were obtained by living cationic polymerization of tetrahydrofuran initiated by difunctional initiator, trifluoromethanesulfonic anhydride, followed by termination with triphenylphosphine. It was shown, that products contain significant quantities of low molecular weight diphosphonium salt; a plausible explanation is presented. Alternative approach to the synthesis of diionically terminated polyethers was based on the conversion of hydroxyl end-groups of polyether diols, which can be obtained by cationic Activated Monomer (AM) polymerization of e.g. oxiranes, into phosphonium ion end-groups. Using this approach, poly(ethylene oxide)'s with M_n ranging from 300 to 3400, terminated at both ends with stable ionic groups, were prepared and characterized. Measurements of NMR relaxation times and viscosity measurements provide the evidence for the aggregation of ionic end-groups. The potential applications of inter- and intramolecular aggregation are discussed. It is shown, that intramolecular aggregation of ionic terminal groups in low molecular weight poly(ethylene oxide) leads to cyclic structures resembling crown ethers and showing comparable efficiency for complexing cations.     

Ion acceleration from aluminium plasma generated by a femtosecond laser in different conditions

Non-equilibrium plasma was obtained by irradiating Al foils in vacuum with a femtosecond (fs) laser at intensities of the order of 10¹⁸ W/cm². Protons and other light ions were accelerated in the forward direction by using the target-normal-sheath acceleration regime. Time-of-flight technique was employed to measure the ions' kinetic energy using SiC detectors placed at known distances and angles. The ion acceleration was monitored under different conditions of laser focal position, laser pulse energy, and laser contrast. The target was irradiated using different thicknesses and anti-reflecting graphene films. By optimizing the laser parameters, irradiation conditions, and target properties, it was possible to accelerate up to 2.3 MeV per charge state, as will be presented and discussed.     

Thermoelastic instability in friction clutches and brakes – Transient modal analysis revealing mechanisms of excitation of unstable modes

Sliding systems with frictional heating exhibit thermoelastic instability (TEI) when the sliding speed exceeds the critical value. TEI can lead to hot spots on contact surfaces and is generally of great practical importance in friction brakes and clutches. The phenomenon is well defined in terms of the theory of stability with a classic perturbation approach being commonly used. While the perturbation analysis determines the stability limit, recent interest extends further towards exploration of the unstable behavior. This is motivated by practical reasons, namely by the fact that many common friction brakes and clutches operate instantaneously at speeds exceeding the critical speed for TEI, i.e. in the unstable regime. In order to determine a transient solution, possible mechanisms of excitation of unstable modes of different nature need to be accurately defined and quantified. These mechanisms are normally not considered in stability analysis of the steady-state where an initial perturbation of the thermoelastic field is assumed. In many realistic situations, however, there is no indication of the existence of meaningful initial temperature variation. Lack of full understanding of these mechanisms has perhaps limited broader industrial applications of recent theoretical advances in TEI. In this paper a method of solving the transient thermoelastic process in frictional systems using finite element spatial discretization and modal superposition is presented. Then mechanisms that excite the unstable thermoelastic modes other than the initial perturbation of temperature are studied. The role of the background process (corresponding to nominal applied loads) in the excitation is shown in a clear form and illustrated by practical examples for automotive friction clutches. It is demonstrated, in particular, that while for some geometries and configurations of the sliding system the imperfections determine the excitation of unstable modes, with other configurations strong excitation occurs even in the absence of imperfections.     

Water-window microscopy using a compact, laser-plasma SXR source based on a double-stream gas-puff target

We report on a compact, desktop size, laboratory type microscopy setup, based on a short wavelength gas puff target soft X-ray source, which emits incoherent radiation in “water-window” spectral range. The microscope employs a Wolter type I reflective objective and allows capturing magnified images of objects with ~1-μm spatial resolution and exposure time as low as 5 s. A detailed characterization and optimization of both the source and the microscope setups are presented and discussed.     

Push–Pull Buta‐1,2,3‐trienes: Exceptionally Low Rotational Barriers of Cumulenic CC Bonds and Proacetylenic Reactivity

A variety of asymmetrically donor–acceptor‐substituted [3]cumulenes (buta‐1,2,3‐trienes) were synthesized by developed procedures. The activation barriers to rotation ΔG^≠ were measured by variable temperature NMR spectroscopy and found to be as low as 11.8 kcal mol^?1, in the range of the barriers for rotation around sterically hindered single bonds. The central C?C bond of the push–pull‐substituted [3]cumulene moiety is shortened down to 1.22 Å as measured by X‐ray crystallography, leading to a substantial bond length alternation (BLA) of up to 0.17 Å. All the experimental results are supported by DFT calculations. Zwitterionic transition states (TS) of bond rotation confirm the postulated proacetylenic character of donor–acceptor [3]cumulenes. Additional support for the proacetylenic character of these chromophores is provided by their reaction with tetracyanoethene (TCNE) in a cycloaddition‐retroelectrocyclization (CA–RE) cascade characteristic of donor‐polarized acetylenes.     

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.