Mechanistic Insights Enable Divergent Product Selectivity in Catalyst-Controlled Photooxidative Degradation of Polystyrene

Abstract

Polystyrene upcycling to valuable commodity feedstocks is essential for reducing plastic waste. Photooxidative degradation has emerged as a method for converting polystyrene to oxidized aromatic compounds. Investigating the mechanism of photooxidative degradation is essential for understanding the pathways to generating different small molecules. Here, we leveraged the reactivity differences between chlorine and bromine radicals to study the degradation mechanism of polystyrene. While degradation with chlorine radical yields primarily benzoic acid (50 mol %), bromine radical shows an increased preference for acetophenone (4 mol % for both products). After conducting several mechanistic studies, we discovered that activation of 3°C− H bonds is necessary to form acetophenone. However, for acetophenone production, a second hydrogen atom transfer from H−Br enables the reformation of a methyl group on the polymer chain end. Lastly, with the mechanistic insights in hand, we added exogenous bromine sources to increase the concentration of HBr and improved the acetophenone yield, becoming the major degradation product. We envision that our insights into the polystyrene degradation mechanism will further enable divergent product selectivity.

Selective Electrocatalytic Degradation of Ether‐Containing Polymers

“…This result demonstrates that a constant electrochemical stimulus is needed for polymer degradation. Based on this result, our control experiments which show the necessity of each component, and literature precedent, [32,45,55] we propose that NHPI is deprotonated by pyridine and oxidized at the anode to form a phthalimide‐ N ‐oxyl radical, which can subsequently abstract a hydrogen atom from an electron‐rich C−H bond on the PEVE backbone (Figure 3b). The resulting carbon‐centered radical is then quenched by molecular oxygen, which leads to the β ‐scission of an adjacent C−C bond (Figures S69, S71, and S73) and a decrease in M n .…”

“…Photodegradation has emerged as a promising strategy for polymer upcycling [19–25] . For example, the Stache group and several others have recently reported the photooxidative degradation of polystyrene to benzoic acid through a hydrogen atom transfer (HAT) mechanism [26–33] . Knowles and co‐workers have also developed light‐driven depolymerizations of hydroxylated polymers through proton‐coupled electron transfer (PCET) [34,35] .…”

Enhancing the photocatalytic upcycling of polystyrene to benzoic acid: a combined computational-experimental approach for acridinium catalyst design

“…While Br − is easier to oxidize than Cl − to generate their respective radicals ( = 1.60 V vs. SCE, = 2.03 V vs. SCE), 27 HAT from PS by Cl˙ is favoured due to the stronger H–Cl bond formed (bond dissociation energy for H–Cl = 103 kcal mol −1 ; H–Br = 87 kcal mol −1 ). 28,29 The best BA yields were obtained using DCE/MeCN 6 : 1 (v/v), with MeCN, CHCl 3 and different DCE-containing solvent mixtures affording much less BA (Table S1†). Finally, the reaction is tolerant to different excitation wavelengths, with 440 nm irradiation affording similar BA yields (entry 9) as 390 nm.…”

Selective Electrocatalytic Degradation of Ether‐Containing Polymers

“…The authors have cited additional references within the Supporting Information. [32,62,64,[75][76][77][78][79][80][81][82][83][84][85][86][87]…”

“…[19][20][21][22][23][24][25] For example, the Stache group and several others have recently reported the photooxidative degradation of polystyrene to benzoic acid through a hydrogen atom transfer (HAT) mechanism. [26][27][28][29][30][31][32][33] Knowles and coworkers have also developed light-driven depolymerizations of hydroxylated polymers through proton-coupled electron transfer (PCET). [34,35] However, while photochemical methods have gained much attention for plastic valorization, the use of electrochemistry to drive polymer degradation is far less explored.…”