The role of peroxyl radicals in polyester degradation – a mass spectrometric product and kinetic study using the distonic radical ion approach

Abstract

Mass spectrometric techniques were used to obtain detailed insight into the reactions of peroxyl radicals with model systems of (damaged) polyesters. Using a distonic radical ion approach, it was shown that N-methylpyridinium peroxyl radical cations, Pyr(+)OO˙, do not react with non-activated C-H bonds typically present in polyesters that resist degradation. Structural damage in the polymer, for example small amounts of alkene moieties formed during the manufacturing process, is required to enable reaction with Pyr(+)OO˙, which proceeds with high preference through addition to the π system rather than via allylic hydrogen atom abstraction (kadd/kHAT > 20 for internal alkenes). This is due to the very fast and strongly exothermic subsequent fragmentation of the peroxyl-alkene radical adduct to epoxides and highly reactive Pyr(+)O˙, which both could promote further degradation of the polymer through non-radical and radical pathways. This work provides essential experimental support that the basic autoxidation mechanism is a too simplistic model to rationalize radical mediated degradation of polymers under ambient conditions.

Reaction of Distonic Aryl and Alkyl Radical Cations with Amines: The Role of Charge and Spin Revealed by Mass Spectrometry, Kinetic Studies, and DFT Calculations

“…and 4‐Pyr +. ( m / z 93) were obtained from the respective N ‐methyl iodo pyridines 7 a and 7 b through collision‐induced dissociation (CID), as described previously (Scheme a,b) …”

Environmental Polymer Degradation: Using the Distonic Radical Ion Approach to Study the Gas-Phase Reactions of Model Polyester Radicals

“…19,20 We have recently employed this methodology to study the reaction of distonic N-methylpyridinium peroxyl radical cations (Pyr + OO • ), 12,13 which were used as a simplified model for ROO • , with various substrates that exhibit typical structural motifs in intact or defective polyesters. 21 Scheme 1 outlines the reaction of Pyr + OO • with cyclohexene, which shows that allylic hydrogen atom transfer (HAT; pathway (a)) is only a minor pathway.…”

“…The major reaction channel involves addition of Pyr + OO • to the alkene π system, followed by rapid homolytic O−O bond fragmentation to yield an epoxide and Pyr + O • (pathway (b)), which is a reactive oxygen-centered radical species (RO • ) that reacts rapidly through HAT and radical addition. 21 Formation of highly reactive RO • through reaction of ROO • with π systems in alkenes has not been considered as pathway in radical polymer degradation so far. This not only highlights the deficiencies of the current mechanistic rationale but also demonstrates the power of the distonic radical ion approach, which allows the discovery of new, previously unidentified degradation pathways.…”

“…The chemistry of any charged species in the reaction system can be directly monitored in a time-resolved fashion to obtain kinetic data for individual reaction steps. This approach has been used to investigate radical reactions of relevance to numerous fields, for example, organic synthesis, , atmospheric chemistry, − and biological chemistry. , We have recently employed this methodology to study the reaction of distonic N -methylpyridinium peroxyl radical cations (Pyr + OO • ), , which were used as a simplified model for ROO • , with various substrates that exhibit typical structural motifs in intact or defective polyesters Scheme outlines the reaction of Pyr + OO • with cyclohexene, which shows that allylic hydrogen atom transfer (HAT; pathway (a)) is only a minor pathway.…”

“…The major reaction channel involves addition of Pyr + OO • to the alkene π system, followed by rapid homolytic O–O bond fragmentation to yield an epoxide and Pyr + O • (pathway (b)), which is a reactive oxygen-centered radical species (RO • ) that reacts rapidly through HAT and radical addition . Formation of highly reactive RO • through reaction of ROO • with π systems in alkenes has not been considered as pathway in radical polymer degradation so far.…”