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The Use of Spin Traps for the Kinetic Investigation of Elementary Events of Pseudoliving Radical Reversible Addition-Fragmentation Chain-Transfer Polymerization

Abstract (Expand)

The spin-trapping technique is used for the first time to study the kinetics and mechanism of addition and fragmentation elementary events in reversible addition-fragmentation chain-transfer pseudoliving … radical polymerization. As shown by the example of the spin-trap-reversible addition-fragmentation chaintransfer agent model system, the constants of addition (substitution) of the model tert-butyl radical to polymeric reversible addition-fragmentation chain-transfer agents (poly(styrene dithiobenzoate), poly(n-butyl acrylate) dithiobenzoate, etc.) are one to two orders of magnitude higher than the constants of addition reactions involving low-molecular-mass reversible addition-fragmentation chain-transfer agents (tert-butyl dithiobenzoate, benzyl dithiobenzoate, di-tert-butyl trithiocarbonate, and dibenzyl trithiocarbonate). This circumstance makes it possible to significantly widen the synthetic possibilities of reversible addition-fragmentation chain-transfer polymerization. Rate constants of the fragmentation reaction for a number of intermediates are estimated, and the relationship between their structure and stability is ascertained. For the model reaction of the interaction (addition and fragmentation) of the tert-butyl radical with low-molecular-mass reversible addition-fragmentation chain-transfer agents, equilibrium constants are calculated via the methods of computational chemistry.

Authors: V. B. Golubev, A. N. Filippov, E. V. Chernikova, M. L. Coote, C. Y. Lin, G. Gryn’ova

Date Published: 1st Sep 2011

Publication Type: Journal

Evaluation of the Dependence of Aqueous Solubility of Nitro Compounds on Temperature and Salinity: A COSMO-RS Simulation

Abstract (Expand)

The solubility in pure and saline water at various temperatures was calculated for selected nitro compounds (nitrobenzene, 1,3,5-trinitrobenzene, 2-nitrotoluene, 3-nitrotoluene, 4-nitrotoluene, … 2,4-dinitrotoluene, 2,6-dinitrotoluene, 2,3-dinitrotoluene, 3,4-dinitrotoluene, 2,4,6-trinitrotoluene) using the Conductor-like Screening model for Real Solvents (COSMO-RS). The results obtained were compared with experimental values. The COSMO-RS predictions have shown high accuracy in reproducing the trends of aqueous solubilities for both temperature and salinity. The proposed methodology was then applied to predict the aqueous solubilities of 19 nitro compounds in the temperature range of 5–50 °C in saline solutions. The salting-out parameters of the Setschenow equation were also calculated. The predicted salting-out parameters were overestimated when compared to the measured values, but these parameters can still be used for qualitative estimation of the trends.

Authors: Yana A. Kholod, Ganna Gryn’ova, Leonid Gorb, Frances C. Hill, Jerzy Leszczynski

Date Published: 1st Apr 2011

Publication Type: Journal

Revising the Mechanism of Polymer Autooxidation

Abstract (Expand)

The basic scheme for autooxidation of polymers, originally developed by Bolland, Gee and co-workers for rubbers and lipids, is now widely applied to all types of polymeric materials. According to their … scheme, the reaction that makes this process autocatalytic, referred to as the propagation step, is a hydrogen abstraction from the next substrate by the peroxyl radical (ROO˙ + RH → ROOH + R˙). In this study, using advanced quantum-chemical methods, we have shown that this step is actually characterised by largely positive Gibbs free energy (10–65 kJ mol−1) for most regular polymers with saturated chains (polypropylene, polyethylene, polyvinyl chloride, polyvinyl acetate, polyurethane, poly(methyl methacrylate)etc.) and even some polymers with unsaturated fragments (polystyrene, polyethylene terephthalate). Neither elevated temperature, nor solvation makes this process thermodynamically favourable. Only when the formed radical centre is conjugated with adjacent double bonds (as in polybutadiene) or captodatively stabilised by two suitable functional groups (such as a carbonyl and a lone pair donor such as oxygen or nitrogen), is the propagation step exoergic. Instead, we show that it is the presence of structural defects, such as terminal or internal double bonds, formed either during polymerisation or in the degradation process itself, that is responsible for the autooxidation of most polyesters and most polyalkenes. Recognition of the real mechanism of autooxidation in polymers is a key to developing strategies for the prevention of their degradation.

Authors: Ganna Gryn'ova, Jennifer L. Hodgson, Michelle L. Coote

Date Published: 2011

Publication Type: Journal

Oxidation of 4-Substituted TEMPO Derivatives Reveals Modifications at the 1- and 4-Positions

Abstract (Expand)

Potenital pathways for the deactivation of hindered amine light stabilisers (HALS) have been investigated by observing reactions of model compounds—based on 4-substituted derivatives of … 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)—with hydroxyl radicals. In these reactions, dilute aqueous suspensions of photocatalytic nanoparticulate titanium dioxide were irradiated with UV light in the presence of water-soluble TEMPO derivatives. Electron spin resonance (ESR) and electrospray ionisation mass-spectrometry (ESI-MS) data were acquired to provide complementary structural elucidation of the odd- and even-electron products of these reactions and both techniques show evidence for the formation of 4-oxo-TEMPO (TEMPONE). TEMPONE formation from the 4-substituted TEMPO compounds is proposed to be initiated by hydrogen abstraction at the 4-position by hydroxyl radical. High-level ab initio calculations reveal a thermodynamic preference for abstraction of this hydrogen but computed activation barriers indicate that, although viable, it is less favoured than hydrogen abstraction from elsewhere on the TEMPO scaffold. If a radical is formed at the 4-position however, calculations elucidate two reaction pathways leading to TEMPONE following combination with either a second hydroxyl radical or dioxygen. An alternate mechanism for conversion of TEMPOL to TEMPONEvia an alkoxyl radical intermediate is also considered and found to be competitive with the other pathways. ESI-MS analysis also shows an increased abundance of analogous 4-substituted piperidines during the course of irradiation, suggesting competitive modification at the 1-position to produce a secondary amine. This modification is confirmed by characteristic fragmentation patterns of the ionised piperidines obtained by tandem mass spectrometry. The conclusions describe how reaction at the 4-position could be responsible for the gradual depletion of HALS in pigmented surface coatings and secondly, that modification at nitrogen to form the corresponding secondary amine species may play a greater role in the stabilisation mechanisms of HALS than previously considered.

Authors: David L. Marshall, Meganne L. Christian, Ganna Gryn'ova, Michelle L. Coote, Philip J. Barker, Stephen J. Blanksby

Date Published: 2011

Publication Type: Journal

Unusual Complexes of Trapped Methanol with Azacryptands

Abstract (Expand)

Structural analysis of sulfate complexes of two different azacryptands crystallized under identical conditions in the presence of methanol and water reveals that one methanol is selectively trapped in … each cavity, assisted by the specific arrangements of three external sulfates close to one tren unit.

Authors: Md. Alamgir Hossain, Musabbir A. Saeed, Ganna Gryn'ova, Douglas R. Powell, Jerzy Leszczynski

Date Published: 2010

Publication Type: Journal

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