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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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