On the Mechanism of the Steady-State Gamma Radiolysis-Induced Scissions of the Phenyl-Vinyl Polyester-Based Resins

The major societal problem of polymeric waste necessitates new approaches to break down especially challenging discarded waste streams. Gamma radiation was utilized in conjunction with varying solvent environments in an attempt to discern the efficacy of radiolysis as a tool for the deliberate degradation of model network polyesters. Our EPR results demonstrated that gamma radiolysis of neat resin and in the presence of four widely used solvents induces glycosidic scissions on the backbone of the polyester chains. EPR results clearly show the formation of alkoxy radicals and C-centered radicals as primary intermediate radiolytic products. Despite the protective role of the phenyl groups on the backbone of the radiation-induced polyester chains, the radiolytic-glycosidic scissions predominate. Among the following three solvents used in this study (water, isopropyl alcohol, and dichloromethane), the highest radiolytic yield of glycosidic scission was achieved using water. The •OH radicals produced in the radiolysis of phenyl unsaturated polyester aqueous suspensions very rapidly abstract H atoms from the methylene group, which is followed by a very rapid glycosidic scission. The lowest glycosidic yield was found in the dichloromethane solutions of these polyester resins due to scavenging by the fast electron capture reactions.

Solvent-controlled photocatalytic divergent cyclization of alkynyl aldehydes: access to cyclopentenones and dihydropyranols

Divergent synthesis is a powerful strategy for the fast assembly of different molecular scaffolds from the identical starting materials. We describe here a novel solvent-controlled photocatalytic divergent cyclization of alkynyl aldehydes with sulfonyl chlorides for the direct construction of highly functionalized cyclopentenones and dihydropyranols that widely exist in bioactive molecules and natural products. Density functional theory calculations suggest that an unprecedented N,N-dimethylacetamide-assisted 1,2-hydrogen transfer of alkoxy radicals is responsible for the cyclopentenone formation, whereas a C-C cleavage accounts for the selective production of dihydropyranols in acetonitrile. Given the simple and mild reaction conditions, excellent functional group compatibility, forming up to four chemical bonds, and tunable selectivity, it may find wide applications in synthetic chemistry.

The role of radical chemistry in the product formation from nitrate radical initiated gas-phase oxidation of isoprene

<p>Experiments at atmospherically relevant conditions were performed in the simulation chamber SAPHIR, investigating the reaction of isoprene with NO<sub>3</sub> and its subsequent oxidation. Due to the production of NO<sub>3</sub> from the reaction of NO<sub>2</sub> with O<sub>3</sub> as well as the formation of OH in subsequent reactions, the reactions of isoprene with O<sub>3</sub> and OH were estimated to contribute up to 15% of the total isoprene consumption each in these experiments. The ratio of RO<sub>2</sub> to HO<sub>2</sub> concentrations was varied by changing the reactant concentrations, which modifies the product distribution from bimolecular reactions of the nitrated RO<sub>2</sub>. The reaction with HO<sub>2</sub> or NO<sub>3</sub> was found to be the main bimolecular loss process for the RO<sub>2</sub> radicals under all conditions examined.</p><p>Yields of the first-generation isoprene oxygenated nitrates as well as the sum of methyl vinyl ketone (MVK) and methacrolein (MACR) were determined by high resolution proton mass spectrometry using the Vocus PTR-TOF. The experimental time series of these products are compared to model calculations based on the MCM v3.3.1,<sup>1</sup> the isoprene mechanism as published by Wennberg <em>et al.</em><sup>2</sup> and the newly developed FZJ-NO<sub>3</sub>-isoprene mechanism,<sup>3</sup> which incorporates theory-based rate coefficients for a wide range of reactions.</p><p>Among other changes, the FZJ-NO<sub>3</sub>-isoprene mechanism contains a novel fast oxidation route through the epoxidation of alkoxy radicals, originating from the formation of nitrated peroxy radicals. This inhibits the formation of MVK and MACR from the NO<sub>3</sub>-initiated oxidation of isoprene to practically zero, which agrees with the observations from chamber experiments. In addition, the FZJ-NO<sub>3</sub>-isoprene mechanism increases the level of agreement for the main first-generation oxygenated nitrates.</p><p> </p><p><sup>1</sup> M. E. Jenkin, J. C. Young and A. R. Rickard, The MCM v3.3.1 degradation scheme for isoprene, <em>Atmospheric Chem. Phys.</em>, 2015, <strong>15</strong>, 11433–11459.</p><p><sup>2</sup> P. O. Wennberg <em>at al.</em>, Gas-Phase Reactions of Isoprene and Its Major Oxidation Products, <em>Chem. Rev.</em>, 2018, <strong>118</strong>, 3337–3390.<span> </span></p><p><sup>3</sup> L. Vereecken <em>et al.</em>, Theoretical and experimental study of peroxy and alkoxy radicals in the NO3-initiated oxidation of isoprene, <em>Phys. Chem. Chem. Phys.</em>, submitted.</p>

Theoretical and experimental study of peroxy and alkoxy radicals in the NO3-initiated oxidation of isoprene

Under atmospheric conditions, nitrate-RO2 radicals are equilibrated and react predominantly with HO2, RO2 and NO. The nitrate-RO chemistry is affected strongly by ring closure to epoxy radicals, impeding formation of MVK/MACR.