A novel profluorescent dinitroxide for imaging polypropylene degradation

Free-radical processes underpin the thermo-oxidative degradation of polyolefins. Thus, to extend the lifetime of these polymers, stabilizers are generally added during processing to scavenge the free radicals formed as the polymer degrades. Nitroxide radical precursors, such as hindered amine stabilizers (HAS),1,2 are common polypropylene additives as the nitroxide moiety is a potent scavenger of polymer alkyl radicals (R¥). Oxidation of HAS by radicals formed during polypropylene degradation yields nitroxide radicals (RRNO¥), which rapidly trap the polymer degradation species to produce alkoxyamines, thus retarding oxidative polymer degradation. This increase in polymer stability is demonstrated by a lengthening of the “induction period” of the polymer (the time prior to a sharp rise in the oxidation of the polymer). Instrumental techniques such as chemiluminescence or infrared spectroscopy are somewhat limited in detecting changes in the polymer during the initial stages of degradation. Therefore, other methods for observing polymer degradation have been sought as the useful life of a polymer does not extend far beyond its “induction period

Strategies for synthesis of yardsticks and abaci for nanometre distance measurements by pulsed EPR

Silvia Valera is grateful for support by EPSRC and Bela E. Bode acknowledges support by EastCHEM.Pulsed electron paramagnetic resonance (EPR) techniques have been found to be an efficient tool for elucidation of structure in complex biological systems as they give access to distances in the nanometre range. These measurements can provide additional structural information such as relative orientations, structural flexibility or aggregation states. A wide variety of model systems for calibration and optimisation of pulsed experiments has been synthesised. Their design is based on mimicking biological systems or materials in specific properties such as the distances themselves and the distance distributions. Here, we review selected approaches to the synthesis of chemical systems bearing two or more spin centres, such as nitroxide or trityl radicals, metal ions or combinations thereof and sketch their application in pulsed EPR distance measurements.Publisher PDFPeer reviewe

DFT Study of Nitroxide Radicals. 1. Effects of solvent on structural and electronic characteristics of 4-amino-2,2,5,5-tetramethyl-3-imidazoline-N-oxyl

Imidazoline-based nitroxide radicals are often used as spin probes for medium acidity and polarity in different systems. In this work, using the density functional theory (DFT) approach, we have studied how physico-chemical characteristics (geometry, atomic charges and electron spin density distribution) of pH-sensitive spin label 4-amino-2,2,5,5-tetramethyl-3-imidazoline-N-oxyl (ATI) depend on protonation and aqueous surroundings. Our calculations demonstrate that ATI protonation should occur at the nitrogen atom of the imidazoline ring rather than at the amino group. Protonation of ATI leads to a decrease in a spin density on the nitrogen atom of the nitroxide fragment >N-O. For simulation of ATI hydration effects, we have constructed a water shell around a spin label molecule by means of gradual (step-by-step) surrounding of ATI with water molecules (n = 2-41). Calculated spin density on the nitrogen atom of the nitroxide fragment increased with an extension of a water shell around ATI. Both protonation and hydration of ATI caused certain changes in calculated geometric parameters (bond lengths and valence angles). Investigating how structural and energy parameters of a system ATI-(H2O)n depend on a number of surrounding water molecules, we came to the conclusion that a hydrogen-bonded cluster of n ≥ 41 water molecules could be considered as an appropriate model for simulation of ATI hydration effects.Comment: 30 pages, 11 figures, 6 table

Synthesis and potential use of 1,8-naphthalimide type O-1(2) sensor molecules

New double (fluorescent and spin) sensor molecules containing 4-amino substituted 1,8-naphthalimide as a fluorophore and a sterically hindered amine (pre-nitroxide) or pyrroline nitroxide as a quencher and radical capturing moiety were synthesized. All sensors were substituted with a diethylaminoethyl side-chain to increase the water solubility. Steady state fluorescence properties of these compounds and their responses to ROS in vitro are reported with perspectives of plant physiology use in vivo

A triplet biradical with double bidentate sites based on tert-butyl pyridyl nitroxide as a candidate for strong ferromagnetic couplers

We synthesised a new biradical, 34bpybNO, having two chelating radical sites and demonstrated its crystal structure and ground triplet nature. Frozen-solution magnetic measurement and electron spin resonance results revealed a strong intramolecular ferromagnetic coupling reaching 2J/kB = ∼+170 K, which is supported by density functional theory calculations

Synthesis of N-arylpyridinium salts bearing a nitrone spin trap as potential mitochondria-targeted antioxidants

The generation of excess reactive oxygen species (ROS) in mitochondria is responsible for much of the oxidative stress associated with ageing (aging), and mitochondrial dysfunction is part of the pathology of neurodegeneration and type 2 diabetes. Lipophilic pyridinium ions are known to accumulate in mitochondria and this paper describes a general route for the preparation of nitrone-containing N-arylpyridinium salts having a range of lipophilicities, as potential therapeutic antioxidants. The compatibility of nitrones with the Zincke reaction is the key to their synthesis. Their trapping of carbon-centred radicals and the EPR spectra of the resulting nitroxides are reported

High molar mass segmented macromolecular architectures by nitroxide mediated polymerisation

A straightforward synthetic pathway based on nitroxide mediated polymerisation (NMP) for the synthesis of a variety of high molar mass segmented copolymers comprising both polystyrene (PS) and polyether segments is reported. First, various precursors such as linear or star-shaped polyether macromonomers, containing either a-methylstyrene or styrene functions at one polymer terminus, as well as PS and polyether macroalkoxyamines bearing either 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) or N-tert-butyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide (SG1) end-groups, were prepared. In a second step, these prepolymers were used to design different copolymer architectures such as block, graft, star-grafted, toothbrush and palm tree structures, in which PS constituted the backbone and polyether the side chains. Block copolymers were obtained by NMP of styrene initiated with polyether macroalkoxyamines. Copolymerisation of styrene with linear and star-shaped polyether macromonomers by NMP resulted in graft and star-grafted copolymers, respectively. A toothbrush copolymer was produced in a similar way with the exception of the initiator, which was a PS macroalkoxyamine. Likewise, palm tree architectures were obtained by homopolymerising polyether macromonomers initiated by PS macroinitiators. Advanced characterisation of the different polymer structures was performed, including 2D chromatography

EPR Study of Spin Labeled Brush Polymers in Organic Solvents

Spin-labeled polylactide brush polymers were synthesized via ring-opening metathesis polymerization (ROMP), and nitroxide radicals were incorporated at three different locations of brush polymers: the end and the middle of the backbone, and the end of the side chains (periphery). Electron paramagnetic resonance (EPR) was used to quantitatively probe the macromolecular structure of brush polymers in dilute solutions. The peripheral spin-labels showed significantly higher mobility than the backbone labels, and in dimethylsulfoxide (DMSO), the backbone end labels were shown to be more mobile than the middle labels. Reduction of the nitroxide labels by a polymeric reductant revealed location-dependent reactivity of the nitroxide labels: peripheral nitroxides were much more reactive than the backbone nitroxides. In contrast, almost no difference was observed when a small molecule reductant was used. These results reveal that the dense side chains of brush polymers significantly reduce the interaction of the backbone region with external macromolecules, but allow free diffusion of small molecules

Synthesis of Nitroxide-Annulated Carbocycles and Heterocycles

New, pyrroline nitroxide annulated lactones, polycycles, and maleimide were synthesized by classical and microwave-assisted methodology. We report the application of the metathesis reaction in the presence of nitroxide yielding a pyrroline nitroxide condensed 1,4-benzoquinone as a paramagnetic dienophile. The formation of the isoselenazolone-fused pyrroline ring system was examined and different reactivity was observed with pyrrolin-1-oxyl and pyrroline derivatives