2.1 Reductive Hydrogenolysis to Hydrocarbon Fuels and Aromatics

Reductive catalytic strategies provide an effective approach for converting PS into value-added hydrocarbons via sequential hydrogenation and C–C bond cleavage. Within this context, Ni-based non-noble metal catalysts have emerged as promising systems. In 2024, Vlachos’ group reported the solvent-free hydrogenolysis of PS to produce lubricant base oils over Ni/SiO₂ catalysts [Reference Vance, Najmi and Vlachos40]. High-molecular-weight PS (~97 kDa) was converted into group IV base oils with yields up to 70%, exhibiting high viscosity indices (~130) and robust thermo-oxidative stability. Mechanistically, the hydrogenation of aromatic rings precedes C–C backbone cleavage, with structural sensitivity observed at the Ni sites (Figure 5a). Furthermore, catalyst recycling enhanced hydrogenation activity due to the enrichment of Ni(111) facets, although susceptibility to deactivation by additives (e.g., CaCO₃) in real wastes remains a challenge. Beyond direct hydrogenolysis, cascade strategies have been explored to transform PS into energy carriers. In 2025, An’s group developed a tandem route encompassing pyrolysis, hydrogenation, and dehydrogenation to produce liquid organic hydrogen carriers (LOHCs) [Reference Lee, Kim and Ryou46]. Specifically, styrene-rich pyrolytic mixtures were hydrogenated over Ru/C to yield cycloalkanes, which subsequently released hydrogen over Pt/Al₂O₃ catalysts at a rate of 215.5 mmol H₂ gmetal⁻¹ min⁻¹ at 275°C. Notably, LCA/TEA indicated that this PS-derived LOHC pathway could achieve a negative carbon footprint with competitive hydrogen costs (Figure 5b).

Figure 5.

Reductive/hydrogenolysis pathways for PS: (a) direct hydrogenolysis catalyzed by Ni/SiO₂. (b) two-step pyrolysis-hydrogenation-aromatization process. (c) One-pot synthesis of high-purity toluene catalyzed by RuSA/Co₃O₄.

Figure 5. Long description

The flowchart begins on the far left with the chemical structure of polystyrene, a benzene ring attached to a polymer chain.

Panel A shows a single-step Hydrogenolysis reaction using N i forward slash S i O sub 2 at 300 degrees Celsius and 70 bar H sub 2. This yields a variety of saturated cyclic and branched hydrocarbon structures labeled as Group I V lubricant base oils in red text.

Panel B shows a two-step process. Step 1 is Pyrolysis at 375 degrees Celsius under N sub 2, and step 2 uses R u forward slash C at 70 degrees Celsius under H sub 2. The intermediate products are cyclohexane derivatives which, when treated with P t forward slash N A dash A l sub 2 O sub 3, yield aromatic compounds labeled as Liquid organic hydrogen carriers in green text.

Panel C shows a one-pot synthesis. Step 1 occurs at 450 degrees Celsius under H sub 2, and step 2 uses R u S A forward slash C o sub 3 O sub 4 at 275 degrees Celsius. A dashed box contains Intermediates including styrene and ethylbenzene derivatives, which ultimately yield a single product molecule of high-purity toluene.

Recently, in 2026, Jiang’s group reported a tandem hydropyrolysis-hydrogenolysis strategy for the selective conversion of PS waste into toluene [Reference Wang, Zhang and Zhang41]. A dual-stage fixed-bed reactor was utilized to decouple initial pyrolytic deconstruction and catalytic upgrading: PS first undergoes hydropyrolysis at 475°C to form styrene-rich intermediates, which are subsequently converted via vapor-phase hydrogenolysis over a Ru single-atom catalyst supported on Co₃O₄ at 275°C. The catalyst achieved >99% selectivity toward toluene and delivered yields up to 83.5 wt% from post-consumer PS wastes with excellent stability over 100 h. Density Functional Theory（DFT） calculations revealed that isolated Ru sites effectively promote hydrogenation and C–C cleavage while suppressing over-hydrogenolysis. Collectively, these studies from distinct catalytic design perspectives highlight the remarkable versatility of reductive strategies in PS upcycling (Figure 5c). They enable the selective production of bulk commodity aromatics (e.g., toluene) as well as the synthesis of high-performance liquid fuels and hydrogen energy carriers, thereby providing a vital scientific foundation for the resource utilization of plastic waste.

2.2 Mechanochemical Activation and Solid-State Deconstruction

Mechanochemistry offers a highly efficient, solvent-free approach to overcoming the activation energy barriers of inert polymer degradation through mechanical force. In 2025, Wang’s group reported a mechanochemical strategy for the rapid degradation of PS to benzene using AlCl₃ as a catalyst under ambient conditions [Reference Wang, Li, Zelina, Pepper and Wang42]. Using ball milling as the energy input, high-molecular-weight PS pellets (165 kDa) were ground with an equal mass of AlCl₃ at 30 Hz for 15 min, achieving a degradation extent of 89%. Benzene and low-molecular-weight aliphatic fragments (<250 Da) were obtained as the major products. Mechanistic investigations, including isotope-labeling experiments, indicated that AlCl₃ acts as a Lewis acid to promote aromatic protonation, leading to polymer backbone cleavage via carbocation intermediates. Notably, a potential closed-loop recycling route was demonstrated by converting the generated benzene back into styrene via sequential bromination and iron-catalyzed coupling. Alternatively, direct upgrading of PS to benzophenone (40%–50% yield) was achieved through a one-pot cascade Friedel–Crafts acylation performed within the same milling jar (Figure 6). Ultimately, this solvent-free mechanochemical paradigm offers a highly sustainable and rapid pathway for the solid-state upcycling of inert polymers into versatile aromatic building blocks.

Figure 6.

Mechanism for acid-catalyzed PS degradation and upcycling. PS undergoes AlCl₃-catalyzed cleavage to benzene, which is then converted to benzophenone via Friedel–Crafts acylation.

2.3 Photothermal-Driven Depolymerization

Distinct from mechanical force, photothermal catalysis leverages localized heating induced by light irradiation to drive endothermic depolymerization processes under macroscopically mild conditions. In 2025, Stache’s group reported that carbon black additives, commonly present in commercial plastics, can act as efficient photothermal agents to convert light into localized heat under white-light irradiation [Reference Oh, Jiang, Kugelmass and Stache43]. In model systems, PS-carbon black composites afforded styrene yields of up to 60%, whereas neat PS showed no reaction under identical conditions. Mechanistically, the degradation proceeds through random chain scission followed by unzipping depolymerization. This method was successfully applied to various post-consumer black PS wastes (e.g., foam trays and coffee cup lids), producing styrene in yields up to 53% without the need for additional catalysts. Under focused solar irradiation, an 80% styrene yield was reached within 5 min from black PS foam. Moreover, the addition of only 10 wt% black PS enabled the rapid depolymerization of mixed-color PS wastes, affording styrene yields of 67%–72% under natural sunlight (Figure 7). By ingeniously repurposing intrinsic plastic additives as powerful photothermal susceptors, this solar-driven approach establishes a highly sustainable, self-catalyzed paradigm for closed-loop monomer recovery.

Figure 7.

Mechanism of sunlight-driven PS degradation under vacuum distillation. PS decomposes into styrene, oligomers, and aromatics (toluene, AMS) via chain scission, unzipping, and HAT pathways.

Figure 7. Long description

The diagram is divided into four horizontal panels.Top Panel: General degradation. A P S-C B sub x polymer chain undergoes sunlight and vacuum distillation to yield a mixture of styrene monomers, dimers, and trimers. The products include various phenyl-substituted alkanes and alkenes.Second Panel: Styrene formation. Labeled in red as Styrene formation. A polystyrene chain with three P h groups undergoes random scission, indicated by a right-pointing arrow. This creates two radical fragments in brackets. The fragments undergo chain-end unzipping to produce a shortened polymer radical and a styrene monomer.Third Panel: Dimer formation. Labeled in green as Dimer formation. A polymer chain radical undergoes a 1,7 H A T (Hydrogen Atom Transfer) reaction, followed by a 1,5 H A T reaction. Curved arrows show electron movement. The final step shows the release of a P h dimer and a shortened polymer radical.Fourth Panel: Trimer formation. Labeled in black as Trimer formation. A polymer chain radical undergoes a 1,5 H A T reaction to produce a P h trimer. Below this, a sub-panel enclosed in a dashed box shows further H A T pathways. A central radical fragment can react to form toluene (left), styrene (center), or A M S (alpha-methylstyrene) on the right.

Top panel: An overview reaction shows a P S-C B sub x polymer chain on the left. An arrow labeled Sunlight vacuum distillation points to a mixture of products including styrene monomers, dimers, trimers, and smaller aromatic molecules like toluene and A M S.

Second panel: Titled Styrene formation. A polystyrene chain undergoes random scission, indicated by a curved arrow over a carbon-carbon bond. This yields two radical fragments in brackets. A subsequent arrow labeled chain-end unzipping shows the radical fragment releasing a styrene monomer.

Third panel: Titled Dimer formation. A polymer radical chain with four phenyl groups (P h) undergoes a 1,7 H A T (Hydrogen Atom Transfer) reaction, shown by a long curved arrow from a hydrogen atom to the radical end. This is followed by a 1,5 H A T step. The final arrow shows the cleavage of the chain to produce a P h dimer and a shorter radical chain.

Fourth panel: Titled Trimer formation. Similar to the dimer panel, a 1,5 H A T process is shown on a radical chain, leading to the release of a P h trimer. Below this, a dashed box shows the further degradation of these fragments. Arrows labeled H A T point to the formation of toluene (a benzene ring with a M e group) on the left and A M S (alpha-methylstyrene) on the right, with styrene and radical intermediates in the center.

2.4 Acid- and Base-Promoted Deconstruction

Beyond reductive hydrogenolysis and mechanochemical approaches, acid- and base-promoted deconstruction offers complementary pathways for cleaving the inert PS backbone through carbocation-mediated (acid) or carbanion-mediated (base) mechanisms. Depending on the catalyst acidity/basicity and reaction conditions, these strategies enable selective conversion of PS into either styrene monomer or functionalized aromatic building blocks.

Early investigations into ionic-mediated degradation frequently explored single-function solid base catalysts like magnesium oxide (MgO) [Reference Oh, Ayala and Broadbelt47]. As demonstrated in foundational studies, while basic sites successfully enhance the initiation rate via proton abstraction from tertiary carbons, they simultaneously shorten the “zip length” during depropagation due to premature chain termination on the catalyst surface. Similarly, traditional solid acid catalysts effectively provoke carbocation-mediated bond cleavage, yet they traditionally lack high monomer selectivity. In 2023, Lee’s group investigated the mechanism of superacid-assisted degradation of PS using triflic acid under mild conditions [Reference Thigale, Trant and Ahn48]. By combining deuterated PS (d8-PS-50) with ¹H NMR analysis, proton–deuterium exchange was found to occur at the phenyl ring and β-hydrogens, whereas α-hydrogens remained unreactive, challenging the traditionally proposed β-scission pathway. The degradation at 20°C proceeds through multiple stages depending on acid concentration. At low acid loading, interchain coupling dominates, whereas higher acid concentrations promote intrachain cross-linking and the formation of poly(styrene-co-indan) nanoparticles (M_w ≈ 3–10 kDa). Upon prolonged reaction (>48 h), these intermediates further degrade into small aromatic molecules including benzene, alkylbenzenes, indan derivatives, and diphenylmethane, which can subsequently undergo Scholl-type reactions to generate polycyclic aromatic hydrocarbons such as pyrene and phenanthrene. Collectively, this work underscores the unique ability of strong Brønsted acids to direct PS deconstruction through carbocation-mediated pathways, revealing that product distributions are finely dictated by acid strength and concentration. To achieve higher selectivity toward hydrocarbon products, attention shifted toward solid base catalysts. In 2026, Zhang’s group developed a highly efficient defective zinc aluminate spinel catalyst [Reference Qin, Liu and Hu49], ZAS-16(Vo), for solvent-assisted polystyrene deconstruction. By introducing precise oxygen vacancies (Vo) through structural distortion and cation disorder, the catalyst successfully modulated surface electronic structures—weakening Zn–O bonds and concentrating well-matched Lewis acid–base site pairs. Coupled with toluene as a dynamic solvent that promotes polymer chain disentanglement and mitigates mass-transfer “cage effects,” this integrated Lewis acid–base synergy achieved an exceptional 98.82 wt% conversion and a 76.21 wt% styrene yield at a mild temperature of 350°C. Mechanistically, the adjacent Lewis acid sites initiate the process via hydride abstraction to form benzylic carbocations followed by β-scission, while the basic sites stabilize key carbanion intermediates through aromatic ring delocalization to steer selective cracking.

2.5 Summary

Overall, non-oxidative catalytic deconstruction strategies establish a robust framework for activating the inert C(sp³)–C(sp³) backbone of PS, enabling its precise deconstruction into value-added hydrocarbons and aromatic building blocks. Despite these conceptual and technological breakthroughs, formidable challenges remain in bridging the gap toward practical applications, particularly regarding the enhancement of catalytic selectivity, the optimization of energy efficiency, and the improvement of intrinsic tolerance toward the complex heterogeneities of real-world plastic wastes. From a sustainability perspective, non-oxidative strategies show considerable divergence. Reductive hydrogenolysis delivers high-value hydrocarbons but incurs energy penalties from H₂ atmosphere and elevated temperatures. Mechanochemical and photothermal approaches operate under milder conditions with minimal solvent use, yet the former relies on stoichiometric AlCl₃ and the latter is limited to dark-colored waste streams. Acid-promoted deconstruction avoids thermal costs but introduces corrosive reagent disposal burdens. These trade-offs suggest that strategy selection must be guided by feedstock characteristics and acceptable environmental burden.

3.1 Homogeneous Photocatalytic Oxidation Systems

Despite their diverse catalyst structures and activation modes, all homogeneous photocatalytic oxidation systems share a common mechanistic blueprint: benzylic C–H abstraction → O₂ trapping → peroxyl radical formation → β-scission → oxidative cascade to oxygenated aromatics. The core distinction among these systems lies primarily in the initiation step, which dictates the overall efficiency and selectivity. The following subsections focus on these distinctive initiation pathways, while the common mechanistic framework is summarized in Table 2. Homogeneous photocatalytic systems represent one of the most widely explored strategies for the oxidative upcycling of PS. Depending on the primary active species and the specific bond-activation mechanisms involved, these homogeneous systems can generally be categorized into ligand-to-metal charge transfer pathways, direct hydrogen atom transfer (HAT) pathways, reactive oxygen species (ROS)-mediated pathways, and metal-centered redox/single-electron transfer (SET) pathways.

Table 2.

Mechanistic framework for polystyrene oxidative upcycling: contrasting initiation pathways, driving forces, and product fates.

Table 2. Long description

The table consists of four columns: Mechanistic dimension, Pathway/control type A, Pathway/control type B, and Key differences and impact on P S upcycling.

* Row 1: Initiation mechanism. Pathway A is the Direct H A T pathway where the catalyst abstracts a benzylic hydrogen atom. Pathway B is the R O S-mediated pathway where the catalyst activates O sub 2 into reactive oxygen species. Key difference: Direct H A T offers high site-selectivity based on B D E matching, while R O S pathways have faster kinetics but lower selectivity.

* Row 2: Reaction driving forces. Pathway A is Kinetic control determined by activation energy barriers. Pathway B is Thermodynamic control determined by overall free energy change Delta G. Key difference: Breaking C s p super 3-H and C-C bonds creates kinetic barriers, while forming stable oxygenates like benzoic acid provides a thermodynamic driving force.

* Row 3: Product fate. Pathway A is Selective oxidation yielding specific aromatic oxygenates. Pathway B is Over-oxidation or mineralization leading to C O sub 2 and H sub 2 O. Key difference: Successful upcycling requires a Kinetic Pause where products have a higher kinetic barrier to further oxidation than the parent polymer.

3.1.1 Ligand-to-Metal Charge Transfer (LMCT) Pathways

LMCT has emerged as a powerful photochemical mechanism for generating highly reactive halogen radicals to initiate polymer degradation. In a series of studies starting in 2021, Zeng’s group and Stache’s group independently developed inexpensive FeCl₃-catalyzed photocatalytic systems for PS oxidation [Reference Zhang, Zhang and Zeng44,Reference Zeng, Ke and Wang51]. Under visible-light irradiation (390 nm) in O₂, LMCT excitation of FeCl₃ generates ⦁Cl radicals that initiate the unified HAT/O₂/β-scission cascade (see Section 3.1), yielding benzoic acid (up to 67%) and benzaldehyde (Figure 9a). This strategy demonstrated excellent compatibility with commercial PS foams and led to dramatic molecular weight reduction (e.g., from >90 to <1 kDa). To further control product selectivity, the Stache’s group systematically compared FeCl₃ and FeBr₃ as radical precursors [Reference Oh and Stache52,Reference Oh and Stache53]. Mechanistic studies revealed that highly reactive ⦁Cl abstracts both secondary and tertiary C–H bonds indiscriminately, predominantly yielding benzoic acid. Conversely, the less reactive bromine radical (⦁Br) selectively abstracts tertiary C–H bonds. Furthermore, the intermediate primary alkyl radicals can undergo HAT with HBr (forming methyl chain ends), shifting the product distribution toward acetophenone. By adjusting the halogen precursor and introducing LiBr, an acetophenone-to-benzoic acid ratio of up to 2.3:1 was achieved, demonstrating that tuning LMCT-derived radical reactivity is a powerful tool for regulating product divergence (Figure 9b). This contrasting behavior―indiscriminate HAT by chlorine radicals versus selective tertiary C–H bond activation by bromine radicals―demonstrates that tuning the reactivity of LMCT-derived radicals enables controlled product divergence between benzoic acid and acetophenone.

Figure 9.

Mechanisms of Fe(III)-catalyzed PS photodegradation under LEDs. (a) FeCl₃/O₂ under white light forms oxidized oligomers and aromatic carbonyls. (b) FeBr₃/O₂ under 456 nm light yields acetophenone and benzoic acid (2.3:1 ratio). Both proceed via HAT, oxygen addition, and β-scission.

Figure 9. Long description

Panel A (Left): Top section shows polystyrene reacting with F e C l sub 3 and O sub 2 under white L E D s in acetone. Products include a modified polymer chain with less than 1 kg/mol and a benzoyl derivative where X equals H, O H, C H sub 3, or C l. The mechanism below shows a H A T process initiated by C l radical and H C l, leading to an alkyl radical on the polymer backbone. This reacts with O sub 2 to form a peroxy species, followed by F e super I I mediated formation of an alkoxy radical. Subsequent beta-scission steps lead to chain fragmentation, producing acetophenone and benzoic acid derivatives.Panel B (Right): Top section shows polystyrene reacting with F e B r sub 3 and O sub 2 under 456 nm L E D s in acetone. Products are acetophenone and benzoic acid with a 10 percent yield and an A P to B A ratio of 2.3 to 1. The mechanism mirrors Panel A but utilizes B r radical and H B r for the H A T step. It details a beta-chain scission pathway where the alkoxy radical breaks down into a terminal ketone (Ph-C=O-Me) and a secondary alkyl radical chain.

Panel A, top left, shows P S reacting with F e C l sub 3 and O sub 2 under white L E D s in acetone. This yields oxidized oligomers less than 1 kg per mol and aromatic carbonyls where X equals H, O H, C H sub 3, or C l. Below this, the mechanism starts with a P S chain undergoing H A T via a chlorine radical to form a polymer radical and H C l. Addition of O sub 2 forms a peroxy radical, which reacts with F e super I I to form an alkoxy radical and hydroxide. This leads to beta-scission, creating a ketone-terminated chain and a secondary radical. Further steps involving O sub 2, H A T, and light h nu lead to chain fragmentation into benzaldehyde and benzoic acid.

Panel B, top right, shows P S reacting with F e B r sub 3 and O sub 2 under 456 nm L E D s in acetone. This yields acetophenone and benzoic acid in a 2.3 to 1 ratio with a 10 percent yield. The mechanism below mirrors panel A but uses bromine radicals for H A T, forming H B r. The pathway proceeds through peroxy and alkoxy radicals via F e super I I. Beta-scission and beta-chain scission steps are highlighted, ultimately producing acetophenone and benzoic acid fragments.

3.1.2 Direct HAT Pathways

Organic HAT photocatalysts offer an attractive, metal-free alternative for PS oxidative deconstruction under mild conditions. Organic HAT photocatalysts abstract benzylic hydrogens upon photoexcitation, initiating the unified HAT/O₂/β-scission cascade.

In 2022, Reisner’s group reported the photocatalytic oxidative degradation of PS using fluorenone as a HAT photocatalyst. Under blue light irradiation (50°C) and ambient O₂, excited-state fluorenone abstracts benzylic hydrogen to form a ketyl radical [Reference Li, Vijeta and Casadevall63]. The concurrently generated PS benzylic radical undergoes rapid oxygenation and β-scission, efficiently converting high-molecular-weight PS (260 kDa) into benzoic acid (~40%) and other monocyclic aromatics. Gel permeation chromatography (GPC) analysis confirmed a rapid molecular weight reduction to 3.2 kDa within 4 h, leading to complete deconstruction after 16 h (Figure 10a). Building upon this metal-free paradigm, the Kokotos’s group developed an aerobic upcycling strategy in 2023 utilizing anthraquinone (AQ) as a photosensitizer [Reference Nikitas, Skolia, Gkizis, Triandafillidi and Kokotos64]. Under 390 nm irradiation, the highly electrophilic triplet excited state of AQ initiates the degradation via a similar semiquinone-mediated HAT process. Utilizing air as the sole oxidant, this protocol eliminated the need for strong acids or halogens and exhibited broad applicability across commercial PS wastes, affording benzoic acid yields ranging from 25% to 58%. More recently, in 2025, Xiao’s group achieved highly efficient plastic upcycling using a phenothiazine-based photocatalyst (PTH-3CN) [Reference Zhang, Wang, Su and Xiao65]. Operating via a consecutive photoinduced electron transfer (conPET) mechanism―where the catalyst absorbs two successive photons to access an exceptionally potent oxidative excited state―this system converted PS to benzoic acid (50%) and formic acid under 405 nm irradiation with an extremely low catalyst loading (500 ppm). Notably, this method demonstrated exceptional universality across diverse plastics (e.g., SAN, ABS, PE, PVC). While these organic HAT systems provide a versatile, metal-free platform for PS upcycling (Figure 10b).

Figure 10.

Mechanisms of photocatalytic PS degradation mediated by different ketone catalysts. (a) Fluorenone-catalyzed pathway: proceeds via HAT, oxygen addition, and β-scission to form carboxylic acids, which undergo further decarboxylation. (b) Anthraquinone-catalyzed pathway: involves direct HAT from sp³ C–H bonds, followed by oxidation and β-scission to yield aromatic carbonyl products.

Figure 10. Long description

Panel A shows a reaction involving a red-colored fluorenone catalyst. On the left, a catalytic cycle begins with fluorenone under h v light becoming an excited state with a red asterisk. This reacts via H A T (Hydrogen Atom Transfer) with a polystyrene chain, represented as a carbon backbone with a P h group and sub n subscript. The polystyrene becomes a radical. The catalyst is converted to a hydroxy-fluorenyl radical, which then reacts with O sub 2 via another H A T step to return to fluorenone, releasing H sub 2 O or H sub 2 O sub 2. The polystyrene radical follows a linear path to the right: reacting with O sub 2 to form a peroxy radical, then adding H plus and e minus to form a hydroperoxide. Loss of H sub 2 O and addition of H plus and e minus leads to an alkoxy radical. This undergoes beta-scission to break the chain into two fragments. One fragment reacts with H sub 2 O to form an alcohol, then O sub 2 to form a carboxylic acid. Decarboxylation releases C O and C O sub 2 to yield benzoic acid.Panel B shows a similar process using a green-colored anthraquinone catalyst. The catalytic cycle on the left follows the same h v and H A T steps. The polystyrene degradation pathway on the right proceeds through radical and alkoxy intermediates to beta-scission. The products shown include acetophenone (P h M e ketone), benzoic acid (P h O O H), and formic acid (H C O O H). A secondary path shows an aldehyde (P h C H O) reacting with O sub 2 to also form benzoic acid.

Panel B shows the anthraquinone-catalyzed pathway. A similar catalytic cycle on the left involves anthraquinone reacting with h v and undergoing H A T with the polymer. The polymer radical follows a linear sequence through O sub 2 addition and beta-scission. This process yields aromatic carbonyl products, including benzoic acid and acetophenone, through further oxidation cycles and subsequent beta-scission steps.

3.1.3 Reactive Oxygen Species (ROS)-Mediated Pathways

In ROS-mediated pathways, molecular oxygen is preferentially activated by the photocatalyst, rather than the polymer substrate being directly engaged. The photogenerated ROS―such as singlet oxygen or superoxide radical anions―serve as the primary active species to abstract benzylic hydrogen atoms, thereby initiating the radical cascade that leads to oxidative chain scission. In 2022, Das’s group reported a metal-free photocatalytic strategy for converting PS into aromatic building blocks [Reference Qin, Zhang, Ching, Raman and das66]. By utilizing N-bromosuccinimide (NBS) as a bromine source and sodium trifluoromethanesulfinate as an additive, PS was efficiently oxidized to benzoic acid (up to 73% yield) under 390 nm irradiation in an O₂ atmosphere (Figure 11). This mild protocol was successfully applied to various real-world PS wastes (e.g., foam plastics, yogurt containers), affording benzoic acid yields of 51%–68%. Mechanistically, it was suggested that photolysis generates bromine radicals and reactive sulfur species, which cooperatively facilitate the initial HAT process. Crucially, the involvement of superoxide radicals (⦁O₂⁻) was confirmed by electron paramagnetic resonance (EPR) spectroscopy, and the origin of the incorporated oxygen atoms from molecular O₂ was unambiguously verified via isotope-labeling studies. More recently, in 2025, Taton’s group re-examined the underlying photophysical mechanisms of acridinium-catalyzed PS upcycling [Reference De Abreu, Taton and Bassani17]. Using 9-mesityl-10-methylacridinium perchlorate under 456 nm irradiation, PS was degraded into benzoic acid (~40%) and formic acid. Through time-resolved emission and transient absorption spectroscopy, it was revealed that the excited-state MA is preferentially quenched by molecular oxygen rather than the PS substrate. Computational analyses further indicated that approximately 77% of absorbed photons are channeled into ROS generation (via energy or SET), whereas merely ~1% are consumed by direct polymer interaction. While these ROS-driven paradigms provide a highly sustainable, metal-free oxidation route, precise control over specific ROS generation efficiency remains a critical hurdle to mitigate over-oxidation and enhance product selectivity.

Figure 11.

Proposed mechanism for the photochemical degradation of PS to benzoic acid using NBS/CF₃SO₂Na under 390 nm light. The process involves dual HAT cycles (NBS and CF₃SO₂Na-derived radicals), oxygen addition, β-scission, and repeated oxidation steps to yield benzoic acid.

Figure 11. Long description

The diagram is divided into three horizontal sections. Top section: A general reaction shows polystyrene reacting with N B S (15 mol percent), C F sub 3 S O sub 2 N a (50 mol percent), Kessil 390 nm light, and Et O A c (0.1 M) at room temperature to yield benzoic acid.Middle section: - Top left: A catalytic cycle for C F sub 3 S O sub 2 N a activation. Under light (h v) and O sub 2, it forms C F sub 3-1, which undergoes light activation to a radical cation, reacting with O sub 2 to release superoxide and C F sub 3-2. This cycle involves a H A T (Hydrogen Atom Transfer) step.- Top right: A dashed box contains structures for C F sub 3-1 (trifluoromethyl sulfonate sodium salt), C F sub 3-2 (trifluoromethyl sulfonyl radical), and C F sub 3-3 (trifluoromethyl sulfonic acid).- Center: Polystyrene reacts via H A T with bromine radicals (Br dot) or N-succinimide radicals to form a polymer radical. - Bottom left: A bromine catalytic cycle where 1/2 Br sub 2 is activated by light to Br dot, which performs H A T and is regenerated from H Br via O sub 2 and superoxide.Bottom section: - The polymer radical reacts with O sub 2 and H plus to form a hydroperoxide, which under light releases a hydroxyl radical to form an alkoxy radical. - This alkoxy radical undergoes beta-scission to break the polymer chain into smaller fragments. - These fragments further oxidize through repeating O sub 2 additions and light-driven steps, releasing C O sub 2, C O, and hydroxyl radicals, eventually yielding benzoic acid and other small aromatic molecules.

Top section: A general reaction equation shows P S (polystyrene) reacting with N B S (15 mol percent), C F sub 3 S O sub 2 N a (50 mol percent), Kessil 390 nm light, and Et O Ac (0.1 M) at room temperature to yield benzoic acid.

Middle section: Two interconnected H A T (Hydrogen Atom Transfer) cycles.

- The top cycle involves C F sub 3 S O sub 2 N a reacting with O sub 2 and light (h v) to form C F sub 3 dash 1, which cycles through an excited state to C F sub 3 dash 2, releasing superoxide radicals.

- A dashed box on the right defines C F sub 3 dash 1 as sodium trifluoromethanesulfinate, C F sub 3 dash 2 as a sulfonyl radical, and C F sub 3 dash 3 as trifluoromethanesulfonic acid.

- The bottom cycle shows N B S reacting with light to form a nitrogen-centered radical and a bromine radical (Br dot). The bromine radical interacts with H Br and O sub 2 to regenerate 1/2 Br sub 2.

Bottom section: The degradation pathway of the polymer chain.

- A P S chain reacts via H A T to form a carbon-centered radical.

- Addition of O sub 2 and a proton (H plus) forms a hydroperoxide group.

- Light-induced cleavage of the O H radical leads to an alkoxy radical.

- Beta-scission breaks the polymer backbone into two fragments: a hydroxyl-terminated chain and a carbonyl-terminated chain.

- Repeated oxidation and light-driven steps involving the release of C O sub 2, C O, and H O radicals eventually convert the fragments into benzoic acid (P h C O O H).

3.1.4 Metal-Centered Redox and SET Pathways

Transition-metal complexes provide versatile platforms for mediating benzylic C-H activation by elegantly coupling photochemistry with metal-centered SET and redox cycling. Upon photoexcitation, highly oxidizing metal centers can directly abstract hydrogen atoms or initiate SET, driving subsequent oxidative C–C bond cleavage under macroscopically mild conditions.

In 2023, Soo’s group reported a visible-light-driven oxidative degradation system utilizing commercially available vanadyl acetylacetonate (VO(acac)₂) [Reference Li, Kong and Lyu67]. Under white LED irradiation and an O₂ atmosphere in dichloromethane, PS was smoothly converted into value-added platform chemicals (e.g., benzoic acid and formic acid). Mechanistic investigations―integrating kinetic analysis, EPR, XPS, and DFT calculations–revealed a tandem catalytic cycle. Specifically, the photo-induced oxidation of V(IV) generates a reactive V(V) species that facilitates rate-determining benzylic hydrogen abstraction, followed by rapid oxygen capture and C–C scission. Impressively, this protocol was successfully scaled up in a continuous-flow photoreactor and exhibited robust tolerance toward contaminated foams and multilayer packaging (e.g., PS-EVOH-PS).

In the same year, Jiang’s group developed an unconventional uranyl (UO₂²⁺) photocatalytic system for the selective degradation of styrenic plastics [Reference Rabot, Chen and Lin45]. Using uranyl nitrate under blue-light irradiation (460 nm), PS and its copolymers were efficiently degraded to benzoic acid. Mechanistic studies suggested that the highly oxidizing excited uranyl species (*UO₂²⁺) activates benzylic C–H bonds through a synergistic combination of direct HAT and SET pathways. The applicability of this method was further demonstrated across various styrenic blends (EPS, HIPS, SAN, ABS) and could even be extended to heterochain plastics (PET, PC). While these metal-centered redox pathways showcase exceptional mechanistic versatility, inherent environmental concerns regarding heavy metal toxicity underscore the pressing need for more benign and sustainable transition-metal alternatives (Figure 12).

Figure 12.

Mechanism of uranyl-catalyzed photodegradation of PS under 460 nm irradiation. The process involves HAT and SET steps, followed by oxygen addition and β-scission to yield benzoic acid.

Figure 12. Long description

The diagram consists of a central catalytic cycle and an outer degradation pathway.Central Catalytic Cycle:* At the center, U O sub 2 super 2 plus is excited by 460 nm light to form asterisk U O sub 2 super 2 plus.* This excited species undergoes H A T (Hydrogen Atom Transfer) with a polystyrene chain, converting a U super V species to a U super V species with a hydroxyl group.* The cycle involves reversible steps and a S E T (Single Electron Transfer) step that releases H plus to regenerate the catalyst.Outer Degradation Pathway:* Top-left: Polystyrene chain with phenyl (Ph) groups and a hydrogen (H) atom.* Bottom-left: After H A T, the polymer becomes a radical. It reacts with O sub 2 to form a peroxy radical.* Bottom-center: A S E T process involving U O sub 2 H super 2 plus and U O sub 2 super 2 plus converts the peroxy radical into an alkoxy radical, releasing H sub 2 O sub 2.* Bottom-right: The alkoxy radical undergoes beta-scission to produce a ketone-terminated fragment and a secondary carbon radical.* Top-right: The secondary radical reacts with O sub 2 via another S E T and beta-scission sequence, leading to benzaldehyde and eventually benzoic acid (Ph C O O H) at the top-center.

At the center is a catalytic cycle involving uranyl species. A U O sub 2 super 2 plus ion is excited by 460 nm light to an excited state asterisk U O sub 2 super 2 plus. This species undergoes a Hydrogen Atom Transfer H A T step with a polystyrene chain, converting the uranyl to a U super V species with an O H group and four L ligands. This U super V species can undergo a Single Electron Transfer S E T step, releasing an H plus ion and regenerating the U O sub 2 super 2 plus catalyst.

The first degradation pathway starts at the top-left with a polystyrene chain. Following the H A T step, it forms a polymer radical. Addition of O sub 2 leads to a peroxy radical. A subsequent S E T step involving U O sub 2 H super 2 plus yields a polymer alkoxy radical while releasing U O sub 2 super 2 plus and H sub 2 O sub 2. This radical undergoes beta-scission to produce a ketone fragment and a secondary radical.

The second pathway at the top-right shows a radical species undergoing a S E T step with O sub 2 to form an alkoxy radical. This is followed by beta-scission to yield an aldehyde, which is further oxidized to benzoic acid. The overall process is summarized by a dashed arrow from the initial polystyrene to the final benzoic acid product.

3.2 Heterogeneous Semiconductor Photocatalysis

Heterogeneous semiconductor photocatalysts offer a robust and sustainable platform for PS upcycling, characterized by their structural stability, recyclability, and ease of phase separation [Reference Yang, Li and Xiao68,Reference Zelekew, Wang, Gu and Wu69]. Unlike molecular catalysts, semiconductors utilize photo-induced charge carriers to activate molecular oxygen at the solid–liquid interface. Depending on the material composition and surface modification, these heterogeneous systems can be primarily categorized into carbon-based semiconductors and metal-oxide systems.

3.2.1 Metal-Free Carbon-Based Semiconductors

Graphitic carbon nitride (g-C₃N₄) has emerged as a cornerstone of metal-free PS upcycling due to its visible-light responsiveness and favorable band alignment for O₂ activation. In 2022, Ma’s group reported a thermal condensation-derived g-C₃N₄ system for the oxidative degradation of PS (Figure 13). Under visible light (150°C, 10 bar O₂), PS was successfully converted into aromatic oxygenates (benzoic acid, acetophenone, and benzaldehyde) [Reference Cao, Zhang and Hu70]. Detailed investigations revealed that photogenerated electrons on the g-C₃N₄ surface drive the formation of ⦁O₂⁻, which serve as the primary species for benzylic C–H abstraction. The system achieved a benzoic acid selectivity of >80%, demonstrating that pure organic semiconductors can rival metal-based systems in polymer deconstruction.

Figure 13.

Schematic of the stepwise photodegradation of PS mediated by g-C₃N₄ under oxygen. The process proceeds via initial oxidation to oxygenated PS (PS-O), followed by chain cleavage to aromatic intermediates, and final mineralization to benzoic acid, phenols, and CO₂.

Figure 13. Long description

The flowchart illustrates the degradation of P S (polystyrene) using C sub 3 N sub 4 and O sub 2. 1. Starting Material (Top-Left): A P S polymer chain with phenyl groups. Labels indicate C alpha and C beta on the backbone and C phenyl on the ring. 2. First Transformation: An arrow with C sub 3 N sub 4 and O sub 2 leads to P S dash O (oxygenated polystyrene). This structure shows the addition of hydroxyl (O H) and carbonyl (double-bonded O) groups to the backbone and rings.3. Second Transformation: A vertical arrow with h v (light), O sub 2, and C sub 3 N sub 4 leads downward to four smaller aromatic fragments. These include a tertiary alcohol, a ketone, a diol, and a diketone, all featuring a phenyl ring and R groups.4. Third Transformation: An arrow pointing left, also catalyzed by h v and O sub 2 over C sub 3 N sub 4, leads to a dashed box containing benzoic acid, benzaldehyde, and acetophenone. An internal arrow shows the progression toward benzoic acid.5. Final Products: A long arrow from the P S dash O intermediate and another from the dashed box both point to C O sub 2 at the bottom-right. A separate side product labeled Phenols is shown to the right of the main pathway.

1. Top-left: The starting material P S is a polymer chain with repeating phenyl groups. Labels indicate C alpha and C beta on the backbone and C phenyl on the ring.

2. Top-right: An arrow labeled C sub 3 N sub 4 and O sub 2 points to P S dash O, where the polymer chain now contains carbonyl and hydroxyl groups.

3. Bottom-right: A vertical arrow labeled with a blue h v circle, O sub 2, and C sub 3 N sub 4 points downward to four distinct aromatic intermediates. These include structures with hydroxyl, carbonyl, and alkyl substitutions on the phenyl ring.

4. Bottom-left: An arrow labeled with C sub 3 N sub 4, a blue h v circle, and O sub 2 points left to a dashed box containing benzoic acid, benzaldehyde, and acetophenone.

5. Final products: A long arrow from the dashed box and a vertical arrow from the top-right section both lead to C O sub 2 at the bottom-right. A separate side product labeled Phenols is shown on the far right with a phenyl ring substituted with R and H O groups.

3.3 Acid-Promoted Photosensitizer-Free Pathways

While the aforementioned photochemical strategies strictly rely on exogenous metal complexes or organic dyes to harvest light, an emerging frontier explores photosensitizer-free paradigms. In a conceptual breakthrough in 2022, Xiao’s group developed an acid-catalyzed photochemical system that drives PS oxidation without the need for any external photocatalysts [Reference Huang, Shanmugam and Liu61]. Utilizing strong Brønsted acids (e.g., trifluoromethanesulfonic acid, sulfuric acid, or p-toluenesulfonic acid) under 405 nm irradiation and 1 bar O₂, commercial PS was selectively converted into benzoic acid (36%–51% isolated yield) and formic acid (57%–72%). Mechanistic investigations revealed a unique autophotocatalytic pathway: the protonation of the PS backbone forms a distinct [PS···H⁺] ground-state complex. This transient complex directly absorbs light to generate ¹O₂ in situ, which subsequently abstracts tertiary benzylic hydrogen atoms to initiate the oxidative cascade. The viability of this methodology was further demonstrated through gram-scale deconstruction in a continuous-flow reactor using various real-world PS wastes. Although this pioneering work elegantly proves the feasibility of photosensitizer-free upcycling, the reliance on highly corrosive strong acids and the currently moderate product selectivity highlight the necessity for future optimizations in this nascent direction.

3.4 Thermally Driven Catalytic Oxidation Pathways

Beyond photochemical interventions, thermally driven catalytic oxidation represents a robust and scalable paradigm for PS upcycling. By leveraging elevated thermal energy to overcome the intrinsically high activation barriers of inert C–H bonds, these systems typically employ transition-metal-mediated autoxidation or in situ generated radical mediators to facilitate exhaustive oxygenative cleavage.

In 2023, Lim’s group reported the first metal-free thermal organocatalytic method for PS oxidation to benzoic acid (BA) and 4-nitrobenzoic acid (4-NBA) using N-hydroxyl catalysts (THICA or NHPI) with HNO₃ in acetic acid at 120°C under air [Reference Ong, Teo, Feng, Tan and Lim73]. The reaction proceeds via N-oxyl radical-mediated HAT from the PS backbone, followed by O₂ incorporation and β-scission, yielding BA (up to 32%) and 4-NBA (up to 10%). The exclusive para-nitration arises from steric hindrance during aromatic nitration prior to chain cleavage. This metal-free protocol tolerates colored additives and extends to PS derivatives (e.g., poly(4-methylstyrene) to terephthalic acid; PSS to 4-sulfobenzoic acid), establishing organocatalysis as a viable, additive-tolerant platform for PS upcycling. In 2024, Jiao’s group reported a Co/Mn/Br ternary catalytic system―conceptually reminiscent of industrial hydrocarbon oxidation―capable of universally converting mixed polyolefin wastes into valuable carboxylic acids [Reference Zhao, Tan and Yang74]. Utilizing CoCl₂·6H₂O and MnSO₄·H₂O with HBr as a promoter in n-butyl acetate, PS was oxidatively transformed into benzoic acid (up to 75% yield) at 120°C under ambient O₂ pressure. Remarkably, this robust protocol exhibited broad universality across diverse polyolefins, converting polyethylene (LDPE/HDPE) into C₄–C₇ dicarboxylic acids and polypropylene into acetic and formic acids. The practicality of this system was further underscored by its excellent catalyst recyclability and successful application to complex, mixed-plastic waste streams. More recently, in 2025, Mei’s group developed an exceptionally selective thermal oxidation strategy, utilizing nitrogen dioxide (NO₂) as a unique HAT catalyst alongside a CoCl₂ cocatalyst [Reference Yu, Zhu and Liu54]. In this system, NO₂―generated in situ from sodium nitrite and acetic acid―served as the primary HAT reagent. Under 180°C and 2 bar O₂, PS was completely degraded within 12 h, affording benzoic acid in an unprecedented yield of up to 90%, thereby outperforming the vast majority of existing photocatalytic and thermal systems. Mechanistic elucidations revealed a distinct dual-site activation pathway: NO₂ simultaneously activates both the C _α–H and C _β–H bonds, promoting the formation of a transient dioxetane intermediate. The subsequent O–O and C–C bond scissions selectively drive the reaction toward benzoic acid while effectively suppressing the formation of byproducts like acetophenone. In 2026, Chen’s group demonstrated that spent lithium manganese oxide (LiMn₂O₄) cathodes can be directly repurposed as heterogeneous catalysts for PS oxidation to benzoic acid [Reference Zhang, Chu and Zhang75]. The catalytic activity originates from asymmetric Mn³⁺–O–Mn⁴⁺ motifs that facilitate lattice oxygen activation and •O₂⁻ generation, driving selective C–C cleavage. The optimized system achieved 89% conversion with ~70% benzoic acid selectivity at 150°C within 1 h, and effectively converted real-world PS wastes. Techno-economic analysis revealed a minimum selling price of $0.89/kg—below market value—with substantially reduced environmental footprint. This work exemplifies a dual-waste co-recycling paradigm that simultaneously valorizes spent batteries and waste plastics.

While these thermally driven pathways deliver exceptional yields and demonstrate immense potential for processing realistic, mixed-plastic feedstocks, their inherent reliance on elevated temperatures and pressurized oxygen highlights the ongoing necessity for more energy-efficient catalytic designs (Figure 14).

Figure 14.

Mechanism of Co(II)-catalyzed oxidative degradation of polystyrene with NaNO₂/AcOH under oxygen. Three parallel pathways proceed via HAT, oxygen addition, and β-scission/ring-opening, ultimately yielding benzoic acid, nitrobenzoic acid, and other aromatic carbonyl products.

Figure 14. Long description

A multi-panel chemical diagram divided into three main sections.Top section: A general reaction shows polystyrene reacting with C o C l sub 2, N a N O sub 2, A c O H, and O sub 2 at 180 degrees Celsius for 12 hours. The products are benzoic acid, 4-nitrobenzoic acid, and a phenyl ketone where R equals H, Me, or C O P h.Middle section: Titled Generation path of intermediate state. It features three parallel horizontal pathways starting from a central polystyrene chain. 1. Top blue pathway: Hydrogen Atom Transfer (H A T) leads to an oxygen-centered radical, followed by beta-scission to produce a hydroxyl-terminated chain and a shorter polymer fragment.2. Middle red pathway: H A T involving N O sub 2 leads to a nitro-substituted chain, followed by ring-opening to produce aldehyde and ketone-terminated fragments.3. Bottom black pathway: Similar to the blue path, H A T leads to an oxygen radical and beta-scission, resulting in ketone and hydroxyl-terminated fragments.Bottom section: Titled Transformation of the intermediate state. The intermediate fragments from the middle section are grouped in a large box. Arrows point downward to show repeating degradation steps. - On the left, a nitro-substituted benzaldehyde transforms into 4-nitrobenzoic acid.- In the center, benzoyl radicals transform into benzoic acid and benzaldehyde.- On the right, benzophenone is formed, and C O sub 2 is released as a byproduct.

Top section: A general reaction equation shows polystyrene reacting with C o C l sub 2, N a N O sub 2, A c O H, and O sub 2 at 180 degrees Celsius for 12 hours. This yields benzoic acid, 4-nitrobenzoic acid, and aromatic ketones where R equals H, Me, or C O P h.

Middle section: Titled Generation path of intermediate state. It branches into three color-coded pathways starting from a common polystyrene backbone.

- The top blue pathway involves O sub 2 addition and H plus protonation, leading to beta-scission and producing hydroxylated polymer fragments.

- The middle red pathway involves H A T (Hydrogen Atom Transfer) with N O sub 2, leading to ring-opening and producing nitro-substituted and oxygenated polymer chains.

- The bottom black pathway follows a similar beta-scission route as the blue path but without the initial N O sub 2 interaction.

Bottom section: Titled Transformation of the intermediate state. The intermediate fragments from the three pathways are grouped in a large box. Arrows point downward to show the repeating degradation steps.

- On the left, nitro-substituted intermediates yield 4-nitrobenzaldehyde and then 4-nitrobenzoic acid.

- In the center, benzoyl radicals and intermediates yield acetophenone, benzoic acid, and benzaldehyde.

- On the right, further oxidation yields benzil and releases C O sub 2.

3.5 Photoelectrochemical and Synergistic Energy Pathways

To transcend the thermodynamic and kinetic limitations of monomodal energy inputs (e.g., purely photochemical or thermal systems), the integration of photochemistry and electrochemistry has recently emerged as a formidable frontier for polymer upcycling. This multi-energy coupling enables highly complementary activation pathways, thereby maximizing catalytic efficiency and enabling the co-generation of valuable energy carriers. In a seminal 2026 study, Ackermann’s group pioneered a photo–electro–iron cooperative catalytic system for the exhaustive degradation of PS coupled with green hydrogen production [Reference Hourtoule, Skumial and Trienes55]. Utilizing iron(III) tetraphenylporphyrin chloride as the molecular catalyst, PS was oxidatively depolymerized under synergistic 390 nm irradiation and constant-current electrolysis. Under optimized conditions, this triple-activation protocol afforded benzoyl products in an impressive total yield of up to 73.7%. Detailed mechanistic elucidations revealed a sophisticated interplay between the applied energy fields: electrochemistry primarily initiates the anodic cleavage of the polymer backbone, whereas light irradiation plays a decisive role in accelerating the subsequent deep oxidation of the oligomeric intermediates. Crucially, it was discovered that cathodic oxygen reduction generates hydrogen peroxide in situ, which subsequently reacts with the iron porphyrin to form highly reactive high-valent iron–oxo species that vigorously promote benzylic HAT. Concurrently, anodic oxidation generates benzylic radicals, perfectly paired with the cathodic hydrogen evolution reaction (HER). The robustness of this protocol was further validated by its exceptional tolerance toward real post-consumer PS wastes―readily processing commercial items contaminated with dyes, additives, and food residues (affording 44%–73% product yields). By elegantly coupling anodic waste polymer valorization with cathodic hydrogen generation, this photoelectrocatalytic paradigm charts a highly sustainable and economically compelling trajectory for next-generation plastic upcycling (Figure 15).

Figure 15.

Mechanism of electrophotocatalytic PS degradation mediated by Fe(TPP)Cl. The process combines anodic oxidation, cathodic reduction, and Fe-catalyzed HAT, followed by oxygen addition and β-scission, to yield benzoic acid and other aromatic products.

Figure 15. Long description

The diagram is divided into two sections by a dashed horizontal line. Top section: A general reaction shows polystyrene reacting with Fe T P P Cl 0.02 mmol, Ac O H 10 mmol, and Li C l O sub 4 1 mmol in an undivided cell with G F and Pt electrodes. Conditions include 4 m A, room temperature, O sub 2, 390 nm light, and 24 hours in a 1,2-D C B or M e C N solvent mixture, yielding benzoic acid.Bottom section: A detailed mechanism. 1. Anodic oxidation: Polystyrene undergoes hydrogen atom transfer H A T mediated by an iron porphyrin catalyst cycle. The catalyst cycles between Fe super I V and Fe super V states. 2. Radical formation: The polymer radical reacts with O sub 2 to form a peroxy radical, which undergoes cathodic reduction involving h v and H plus to form an alkoxy radical. 3. Chain scission: The alkoxy radical undergoes beta-scission, breaking the polymer chain into smaller fragments. 4. Further oxidation: Subsequent steps labeled [O] and beta-scission lead to the formation of acetophenone and eventually benzoic acid. 5. Side pathways: A secondary path shows the formation of phenylglyoxylic acid, which under h v light releases C O or C O sub 2 to form benzaldehyde and then benzoic acid. An X A T pathway involving a benzoyl radical and chlorine is also indicated.

Top section: A general reaction scheme shows polystyrene on the left reacting under conditions of F e (T P P) Cl 0.02 m mol, Ac O H 10 m mol, Li C l O sub 4 1 m mol, G F and P t electrodes, 4 m A, room temperature, O sub 2, 390 n m light, for 24 hours in a 1,2-D C B:Me C N 2:1 solvent mixture. The reaction yields benzoic acid on the right.

Bottom section: A detailed catalytic cycle labeled F e-catalysis.

- At the top left, anodic oxidation of a polystyrene chain involves the loss of an electron and a proton.

- This connects to a H A T (Hydrogen Atom Transfer) step mediated by an iron-porphyrin complex, F e super I V (T P P) Cl.

- The resulting radical reacts with O sub 2 to form a peroxy radical, which undergoes cathodic reduction (H plus, e minus, and light h v) to form an alkoxy radical and release a hydroxyl ion.

- This leads to beta-scission, breaking the polymer chain into smaller fragments.

- On the right side, subsequent oxidation [O] and further beta-scission steps transform these fragments into acetophenone and eventually benzoic acid.

- A secondary pathway involves X A T (Halogen Atom Transfer) where a benzoyl radical reacts to form benzoyl chloride.

- The iron catalyst cycles through different oxidation states, shown as four interconnected porphyrin structures in the lower left quadrant, facilitating the continuous H A T process.

3.6 Summary

The preceding subsections have surveyed diverse catalytic systems—spanning homogeneous photocatalysis, heterogeneous semiconductors, thermal oxidation, and photoelectrochemical approaches—all of which operate through a common mechanistic blueprint: benzylic C–H activation followed by oxidative C–C cleavage. Yet, as summarized in Table 2, a deeper analysis reveals fundamental distinctions across three key mechanistic dimensions. The choice between direct HAT and ROS-mediated initiation dictates the inherent selectivity–kinetics trade-off: direct HAT offers higher site selectivity through precise BDE matching, whereas ROS pathways accelerate reaction rates at the cost of indiscriminate oxidative attacks. The reaction trajectory is governed by the competition between kinetic barriers (C–H activation and β-scission) and thermodynamic driving forces (the deep exergonic well of stable oxygenate formation), which together create a thermodynamic sink that channels product distributions predominantly toward benzoic acid. Preventing over-oxidation to CO₂ requires a “kinetic pause”—ensuring that the desired products possess a higher kinetic barrier to further oxidation than the parent polymer. From a sustainability standpoint, oxidative upcycling presents a clear dichotomy. Homogeneous photocatalysis offers mechanistic tunability under mild conditions, but relies on organic solvents and faces catalyst recovery challenges. Heterogeneous systems enable facile separation yet often require elevated temperatures or pressurized O₂. Thermal oxidation delivers the highest yields and tolerates real-world impurities, but operates at 120°C–180°C under pressurized O₂ with corrosive promoters. Photoelectrochemical approaches offer ambient operation with H₂ cogeneration but remain early in development. Despite these advances, most systems remain trapped in the benzoic acid sink. Breaking this bottleneck requires a conceptual departure from direct oxidative scission—a direction explored in the following section through skeletal rearrangement and pre-functionalization strategies that bypass conventional thermodynamic preferences to access diverse high-value aromatics. Translating these fundamental protocols to real-world mixed waste streams under scalable conditions will ultimately define the success of sustainable PS upcycling.

4.1 Hock-Type Rearrangement Pathways to Phenolic Compounds

The Hock rearrangement, a classical industrial route for phenol production via the acid-promoted cleavage of cumene hydroperoxide, has recently been elegantly extended to the oxidative upcycling of PS. In 2025, Stahl’s group reported the first thermally driven Hock-type transformation of PS [Reference Baek, Al Abdulghani and Walsh76]. Employing an NHPI-derived HAT mediator and AIBN under 10 atm O₂, PS dimers and trimers underwent autoxidation, followed by perchloric-acid-promoted rearrangement of the resulting hydroperoxide intermediates. While the dimer afforded phenol in 30% yield, the trimer exhibited a lower yield (17%) due to steric constraints hindering oxidation at internal benzylic sites. When applied to the PS polymer, approximately 25% of benzylic positions were oxygenated, delivering phenol in a 12.7% yield alongside polymer chain scission. Almost concurrently, Shen’s group developed a photocatalytic variant that circumvents the harsh thermal requirements, enabling the one-step conversion of PS into phenol derivatives under ambient conditions [Reference He, Xu and Wan77]. Utilizing sodium anthraquinone sulfonate as the photocatalyst and triflic acid to suppress competing benzoic acid pathways, PS was transformed into phenyl acetate (22% yield) under blue-light irradiation in air. This protocol involves initial benzylic hydroperoxidation, subsequent acid-catalyzed Hock rearrangement, and in situ acetylation. Notably, coupling this photochemical step with electrochemical oxidation enabled the synthesis of 1,4-diacetoxybenzene with an overall yield of 14.1% under solar-driven conditions, showcasing a highly sustainable route to diverse phenolic building blocks (Figure 16). Almost concurrently, Stache’s group developed a mild photooxidative HAT strategy using Eosin Y as a metal-free photocatalyst for PS-to-phenol conversion under blue light (456 nm) at room temperature and 1 atm O₂ [Reference Jiang, Oh and Stache78]. The reaction proceeds via benzylic C–H abstraction by photoexcited Eosin Y, followed by O₂ trapping to form hydroperoxyl PS (PS-OOH), which undergoes acid-catalyzed Hock rearrangement to afford phenol (up to 21% yield from pristine PS). Notably, this protocol was successfully applied to eight post-consumer PS plastics (including cups, lids, foams, and LEGO blocks), achieving phenol yields up to 23% after purification—comparable to the industrial cumene process. While additives such as TiO₂ and carbon black diminished yields due to light scattering/absorption, the system proved compatible with plasticizers (DEHP), flame retardants (HBCD), and processing aids (CaSt₂). This work establishes a mild, metal-free photocatalytic platform for converting waste PS into phenol, complementing thermal Hock strategies. Ultimately, adapting the classical Hock rearrangement to macromolecular systems marks a leap in the advanced upcycling of PS. By circumventing unselective fragmentation via targeted benzylic oxygenation and skeletal rearrangement, these methodologies unlock a direct pathway to high-value phenols, elegantly bridging plastic waste valorization with mainstream industrial synthesis.

Figure 16.

Photoelectrocatalytic degradation to phenol and aryl acetates via anthraquinone-mediated HAT and chain scission.

Figure 16. Long description

The diagram is divided into two main sections. Top section: Two reaction schemes show polystyrene being converted into acetoxy-functionalized monomers. Both use Blue L E D s (40 W, 455-460 nm), P C-5 (20 mol%), air, and a solvent mixture of Ac O H, Ac sub 2 O, C H sub 2 Cl sub 2, and T f O H in a 2 to 1 to 2 to 0.05 ratio for 60 hours. The right-hand scheme adds an electrochemical step using Ccloth (+) and Ni foam (-) electrodes with T B A C l O sub 4 for 10 hours at a cell voltage of 2.6 V. A dashed box identifies P C-5 as sodium 9,10-anthraquinone-2-sulfonate.Bottom section: A mechanistic cycle situated between a blue ‘cathode’ bar at the top and a red ‘anode’ bar at the bottom. - At the cathode, 2 H plus and 2 e minus combine to form H sub 2.- The photocatalytic cycle on the left shows anthraquinone being excited to a triplet state (indicated by brackets and an asterisk), which then abstracts a hydrogen atom from the polystyrene chain. The resulting radical species is regenerated via anodic oxidation, O sub 2, or H A T.- The polystyrene radical reacts with O sub 2 to form a peroxy radical, leading to successive chain scission via 1,5- or 1,7- or 1,n H A T. - This leads to a Hock rearrangement and hydrolysis, breaking the polymer into oxygenated fragments and phenol.- At the anode, the fragments undergo further oxidation. A sequence shows an acetoxy-benzene radical cation losing an electron and reacting with Ac O H to form various intermediate radical and cationic species, eventually yielding the final acetoxylated products after losing a proton (minus H plus).

Top Section: Two reaction arrows originate from a polystyrene polymer structure. The left arrow shows conversion to phenyl acetate under Blue L E D s, P C-5 catalyst, air, and an acid mixture. The right arrow shows conversion to a di-acetate product using the same conditions followed by electrolysis with a carbon cloth cathode and nickel foam anode at 2.6 V. A dashed box on the far right identifies P C-5 as a sodium anthraquinone sulfonate molecule.

Middle Section: A catalytic cycle and mechanism situated between a blue cathode bar at the top and a red anode bar at the bottom.

1. Cathode Reaction: At the top left, 2 H plus and 2 e minus combine to form H sub 2.

2. Anthraquinone Cycle: On the left, anthraquinone is excited to a triplet state, which abstracts a hydrogen atom from the polystyrene chain to form a polymer radical and a semiquinone radical. The semiquinone is regenerated to anthraquinone via anodic oxidation, O sub 2, or H A T.

3. Polymer Degradation: The polymer radical reacts with O sub 2 to form a peroxy radical. This undergoes 1,5- or 1,7- or 1,n H A T and successive chain scission to form a hydroperoxide. Protonation leads to a Hock rearrangement and hydrolysis, yielding phenol and a ketone-terminated polymer fragment.

4. Anode Reaction: At the bottom, phenol reacts with an acetate group and undergoes electron loss at the anode to form a radical cation. Subsequent steps involving Ac O H and further electron loss lead to the final aryl acetate product, releasing a proton.

4.2 Tandem Electrophilic Functionalization and Oxidative Cleavage

Beyond phenol production, another compelling alternative strategy involves functionalizing the aromatic backbone prior to oxidative deconstruction, enabling access to high-value aromatic polycarboxylic acids. In 2024, De Vos’s group reported a tandem strategy integrating Friedel-Crafts functionalization with oxidative degradation [Reference Giakoumakis, Marquez, de Oliveira-Silva, Sakellariou and de Vos79]. In this sequence, PS phenyl rings were initially modified via either acylation or alkylation, and subsequently oxidatively cleaved to yield terephthalic acid (TA), isophthalic acid, and trimesic acid. In the acylation route, AlCl₃ and acetic anhydride achieved nearly quantitative para-acylation of the PS rings, delivering TA in an isolated yield of 55 mol% after oxidation. Alternatively, an alkylation route catalyzed by InCl₃ (6 mol%) with isopropyl acetate achieved ~60% polymer functionalization, yielding a mixture of TA, isophthalic acid, and benzoic acid. Impressively, the obtained TA could be directly esterified to dimethyl terephthalate (97% purity) and repolymerized into virgin PET, demonstrating a remarkable closed-loop upcycling trajectory from waste PS to high-performance PET (Figure 17a). Ultimately, this ingenious pre-functionalization sequence elegantly illustrates how macromolecular editing prior to cleavage can fundamentally reroute degradation pathways, establishing a formidable “cross-polymer” upcycling bridge between distinct commodity plastic value chains.

Figure 17.

(a) Lewis acid-catalyzed functionalization and oxidation to aromatic carboxylic acids. (b) AlCl₃-catalyzed rearrangement to polyindane, followed by oxidation to phthalic acid derivatives.

4.3 Skeletal Rearrangement to Ortho-Functionalized Aromatics

The intrinsic steric hindrance of the PS backbone dictates that direct electrophilic substitutions predominantly occur at the para-position. In a remarkable departure from this paradigm, Lim’s group reported in 2025 the first conversion of PS into 1,2-disubstituted aromatics (e.g., phthalic anhydride) via a macro-to-micro skeletal rearrangement strategy [Reference Ong, Zhang, Teo, Lee and Lim56]. Using AlCl₃ at room temperature, PS underwent skeletal reorganization to form polyindane (73% conversion). This polyindane intermediate was subsequently oxidized in an AcOH/HNO₃ system at 100°C, utilizing N-hydroxy catalysts (THICA or NHPI) to afford phthalic acid and phthalic anhydride. Mechanistically, this ingenious sequence proceeds via protonation of the phenyl ring and subsequent phenyl abstraction to form carbocations, which undergo intramolecular cyclization to circumvent the native spatial constraints. Demonstrated on a 10 g scale with mixed PS wastes, this protocol yields phthalic anhydride that can be readily purified via sublimation and further derivatized into premium fine chemicals like fluorescein and phenolphthalein (Figure 17b). By rewriting the polymer’s topology prior to cleavage, this macro-to-micro indanization elegantly bypasses polystyrene’s spatial limitations, unlocking new chemical space for fine chemical synthesis.

4.4 Summary

Overall, recent breakthroughs underscore that the chemical space accessible from PS waste can be significantly expanded beyond rudimentary aromatic monomers through versatile upgrading strategies. By harnessing Hock-type rearrangements, tandem Friedel-Crafts functionalizations, and pre-oxidative skeletal reorganizations, researchers have successfully bypassed thermodynamic sinks to synthesize structurally diverse, high-value aromatics (e.g., phenols, terephthalic acid, and phthalic anhydride). Nevertheless, these advanced synthetic cascades frequently suffer from multi-step complexities and rely heavily on stoichiometric or highly corrosive reagents (e.g., AlCl₃, HNO₃, perchloric acid). Consequently, future innovations must focus on developing robust, multifunctional, and benign catalytic systems capable of orchestrating these multi-pathway transformations in a more streamlined, one-pot fashion. Sustainability-wise, these advanced routes unlock products with significantly higher market value than benzoic acid, but at the cost of multi-step complexity and stoichiometric corrosive reagents (AlCl₃, HNO₃). AlCl₃ generates substantial aluminum waste, while HNO₃ produces NO_x emissions. Carbon efficiency is higher than exhaustive oxidation, yet cumulative losses at each step remain a concern. Future work should prioritize one-pot tandem systems to minimize intermediate purification.

5.1 Nitridative Scission and Oxidative Amination Pathways

The direct insertion of nitrogen atoms into the robust non-polar backbone of polyolefins poses distinct thermodynamic challenges. In 2024, Jiao’s group reported a pioneering nitridative scission strategy, enabling the conversion of diverse polyolefin wastes (including PS) into nitriles and amides [Reference Yang, Li and Xiao68]. Utilizing MnO₂ as the catalyst and urea as a ubiquitous nitrogen source under an oxygen atmosphere (200°C, 15 bar O₂), PS was selectively converted into benzonitrile and benzamide. Detailed kinetic profiling revealed that the cascade is initiated by the rapid chain scission of PS to form amide-terminated oligomers, which subsequently undergo deep oxidation. Impressively, this protocol was successfully applied to discarded LEGO bricks (yielding 2.84 g of N-containing products) and various styrenic copolymers (SBS, ABS, HIPS). Subsequently, in 2025, the Huang’s group developed an alternative nitridative degradation paradigm leveraging a CuBr catalyst with acetonitrile acting dually as the nitrogen source and solvent [Reference Zhao, Hu and Sun57]. Under 200°C and 2 MPa O₂, commercial PS was converted to benzonitrile and benzamide in a combined yield of 70%. Mechanistic elucidations highlighted that a trace amount of water (40 μL) acts as a crucial promoter, triggering the formation of hydroxyl radicals that facilitate the thermal decomposition of acetonitrile to release in situ ammonia. Notably, the homogeneous CuBr catalyst undergoes a phase transition into a recoverable heterogeneous solid during the reaction, enabling excellent recyclability. While these oxidative amination strategies achieve unprecedented skeletal functionalization, their ubiquitous reliance on extreme thermobaric conditions (e.g., 200°C, 15–20 bar O₂) and aggressive oxidative environments highlights the pressing need to develop lower-energy catalytic alternatives (Figure 18a). Nevertheless, by successfully breaching the formidable thermodynamic barriers of direct nitrogen incorporation, these pioneering methodologies fundamentally expand the chemical space of polymer deconstruction, providing a powerful blueprint for the direct transformation of inert plastics into high-value, heteroatom-functionalized chemical portfolios.

Figure 18.

(a) Mechanism of CuBr-catalyzed conversion of PS to benzonitrile and benzamide. (b) Sulfur-mediated radical cascades and tandem sulfenylation in sunlight-driven PS degradation.

Figure 18. Long description

Panel A shows the catalytic upcycling of polystyrene. The top section displays a general reaction where polystyrene reacts with C u B r, C H sub 3 C N, H sub 2 O, and O sub 2 at 200 degrees Celsius for 15 hours to yield benzonitrile, benzamide, and benzoic acid. Below this, a mechanistic breakdown titled preoxygenation and nitrogen installation is shown. It begins with polystyrene reacting with a hydroxyl radical to form a polymer radical. This undergoes beta-scission via two paths. Path a leads to an acetophenone-type radical which reacts with N H sub 3 and copper/oxygen to form benzonitrile. Path b leads to benzaldehyde, which can either react with N H sub 3 to form benzonitrile or be oxidized to benzoic acid. Benzoic acid then reacts with C H sub 3 C N or N H sub 3 to form benzamide. Panel B shows the degradation of polystyrene using sulfur and sunlight. The top section shows polystyrene reacting with S sub 8 under sunlight for 2 minutes to produce a thiophene derivative, a phenyl-substituted benzene, and styrene. A central box shows S sub 8 breaking down under heat into a sulfur radical chain. The bottom mechanism shows a dimer of polystyrene reacting with sulfur radicals. The pathway involves hydrogen abstraction, formation of double bonds, and cyclization. One branch leads to the formation of a 2,4,5-triphenylthiophene. Another branch leads to a dihydronaphthalene radical which further reacts to form a substituted naphthalene. A vertical arrow on the right indicates a favored pathway involving an H S sub 4 radical leading to a specific sulfur-containing intermediate.

Panel B: Top section shows P S reacting with S sub 8 in sunlight to yield a phenyl-substituted thiophene, a triphenylbenzene, and styrene. A central box shows S sub 8 yielding a sulfur radical chain under heat. The lower mechanism shows a dimer of P S reacting with sulfur radicals (dot S sub 8 dot) to form unsaturated intermediates. These intermediates undergo radical cascades involving H S sub 4 dot and H S sub 3 dot species, leading to the formation of the phenyl-thiophene product and a naphthalene derivative.

5.2 Sulfur-Mediated Radical Cascades and Tandem Sulfenylation

Paralleling the advancements in nitrogen incorporation, the targeted installation of sulfur atoms into the PS backbone has unlocked new avenues for producing valuable organosulfur building blocks. In 2023, Liu’s group [Reference Munyaneza, Posada and Xu82] designed a sophisticated tandem “degradation-upgrading” cascade. Initially, PS was exhaustively unzipped into benzene (97.6% yield) via an AlCl₃-catalyzed random chain-scission mechanism. This in situ generated benzene subsequently served as a platform intermediate, undergoing consecutive Friedel–Crafts reactions with various acyl chlorides or thionyl chloride (SOCl₂). Remarkably, when SOCl₂ was employed, polycyclic aromatic hydrocarbons (e.g., naphthalene) concurrently formed during degradation served as in situ hydrogen donors via a Scholl reaction, driving the highly selective reduction of sulfoxide intermediates to diphenyl sulfide (88% selectivity).

More recently, in 2025, Chen’s group demonstrated a brilliant solar-driven photothermal co-upcycling strategy, utilizing elemental sulfur (S₈) dually as a photothermal susceptor and a reactant [Reference Liu, Liu and Wang83]. By concentrating sunlight via a concave mirror under solvent-free conditions, the PS/S₈ mixture rapidly achieved localized temperatures exceeding 450°C, driving the reaction to completion within mere minutes to afford 2,4-diphenylthiophene and 1,3,5-triphenylbenzene. DFT calculations and trapping experiments confirmed that photothermally generated sulfur radicals abstract hydrogen atoms from PS to yield alkene intermediates, which then undergo thermodynamically favored radical addition and cyclization. Although these sulfur-incorporation cascades ingeniously bypass traditional oxidative sinks, the necessity for highly corrosive Lewis acids (AlCl₃, SOCl₂) in tandem pathways or extreme localized heating in photothermal designs presents significant engineering hurdles for continuous, large-scale processing (Figure 18b). Nevertheless, by unlocking direct access to complex organosulfur architectures, these pathways significantly broaden the versatile upcycling portfolio of polystyrene.

5.3 Summary

In summary, directly constructing C–N and C–S bonds onto inert PS backbones marks a conceptual breakthrough, elevating plastic upcycling from rudimentary oxidative deconstruction to the targeted synthesis of high-value heteroaromatics (e.g., nitriles, amides, thiophenes). By orchestrating radical cascades, nitridative scissions, and photothermal cyclizations, researchers have successfully breached the severe thermodynamic barriers of C–H/C–C functionalization. However, this nascent field remains confined by harsh operational windows, relying heavily on hyperbaric oxygen, extreme temperatures, and corrosive reagents. Designing robust catalytic microenvironments capable of operating under mild conditions to circumvent these limitations is crucial for broad industrial implementation. The sustainability tension here is acute: nitrogen/sulfur-containing aromatics command premium prices, but their synthesis requires exceptionally harsh conditions (200°C, 15–20 bar O₂) or stoichiometric corrosive reagents. Energy inputs rank among the highest of all PS upcycling strategies. While homogeneous-to-heterogeneous catalyst recovery in the CuBr/acetonitrile system is promising, it remains laboratory-scale. Future efforts must focus on earth-abundant heterogeneous catalysts operating under significantly milder conditions.

6.1 Photo-Mediated C(Sp ² )–H Functionalization and Graft Initiation

Directly editing the inert aromatic rings of PS without inducing backbone degradation requires exquisite catalytic control. In a pioneering example, Leibfarth’s group reported a photoredox C(sp²)–H functionalization strategy leveraging electrophilic fluoroalkyl radicals for the precise post-polymerization modification of PS [Reference Lewis, Wilhelmy and Leibfarth58]. Utilizing Ru(bpy)₃Cl₂ as the photocatalyst, paired with pyridine N-oxide and trifluoroacetic anhydride, this system successfully transfigured native aromatic C–H bonds into trifluoromethylated units under ambient blue-light irradiation. The functionalization density could be precisely tuned (19–71 mol%) by modulating reagent stoichiometry. Crucially, GPC confirmed that the polymer’s molecular weight distribution remained remarkably intact, validating the complete suppression of deleterious chain-scission or cross-coupling pathways. While ¹⁹F NMR indicated a statistical distribution of modifications primarily at the para and meta positions, the protocol exhibited exceptional robustness across diverse aromatic polymers (e.g., PET, polycarbonate) and post-consumer EPS foams. Most notably for macromolecular engineering, substituting the reagent with bromodifluoroacetic anhydride introduced bromodifluoromethyl handles (15 mol%) directly onto the PS backbone. These dormant halide sites function as highly efficient macroinitiators for subsequent atom transfer radical polymerization (ATRP), seamlessly enabling the “grafting-from” synthesis of complex PS-graft-polyacrylate architectures. While highly precise, this protocol’s reliance on expensive precious metal catalysts (Ru) and specialized fluorinated reagents limits its scalability. Future efforts must prioritize earth-abundant, cost-effective catalytic systems for practical upcycling (Figure 19a). Ultimately, by demonstrating the feasibility of non-destructive structural editing, this approach redefines waste polystyrene as a highly versatile macromolecular scaffold for advanced materials engineering.

Figure 19.

Site-selective C–H functionalization of PS: (a) Photochemical bromination with fluorinated reagents. (b) Modular cross-coupling reactions (Suzuki, Heck, Sonogashira, borylation, and C–N coupling) using the brominated polymer. (c) Direct Mn-catalyzed C–H phosphonylation with various phosphite reagents.

Figure 19. Long description

Panel A shows a reaction where a polymer with a brominated side chain reacts with 1.1 equiv of a perfluoroalkanoic anhydride (R sub F O R sub F) using 1.0 equiv pyridine N-oxide and 1 mol% Ru(bpy) sub 3 Cl sub 2 in D C M under blue light. This yields a polymer with an R sub F group. A dashed box lists R sub F variants: C F sub 3, C sub 2 F sub 5, C sub 3 F sub 7, C C l F sub 2, and C B r F sub 2, with corresponding mol% yields of 31, 45, 37, 26, and 15.Panel B features a central reaction where a polystyrene-like polymer reacts with N-bromosuccinimide (N B S) and 1 mol% AlCl sub 3 to produce a brominated intermediate. From this central intermediate, five divergent reaction paths lead to different functionalized polymers:- Top left: Reaction with vinyl trifluoroborate and Pd(PPh sub 3) sub 2 Cl sub 2 yields P 1 (92% yield, 95% functionalization).- Top right: Reaction with aryl boronic acid and Pd(PPh sub 3) sub 4 yields P 2 (93% yield, 89% functionalization).- Middle right: Reaction with LiHMDS and Pd(dba) sub 2 yields P 7 (79% yield, 92% functionalization).- Bottom right: Two paths using styrene or methyl acrylate with Pd catalysts yield P 3 (94% yield) and P 4 (87% yield).- Bottom left: Reaction with 1-ethynyl-3-methylbenzene and Pd(PPh sub 3) sub 4 yields P 6 (94% yield).- Middle left: Reaction with B sub 2 (pin) sub 2 and Pd(dppf)Cl sub 2 yields P 5 (94% yield).Panel C shows the reaction of a polymer with a phosphite (H P O R sub 2) using 20 mol% Mn(OAc) sub 2 to attach a phosphonate group. A dashed box displays five phosphite structures: D E P, B F P, D M P, D B P, and D P P.

Panel A at the top shows the photochemical bromination of a polystyrene chain using fluorinated reagents. A central reaction arrow points from a polymer with a phenyl group to a polymer with a brominated naphthyl-like structure. A dashed box on the right lists five R sub F groups including C F sub 3 and C sub 2 F sub 5 with corresponding mol% values ranging from 15 to 45.

Panel B in the center shows modular cross-coupling. A central box contains the reaction of polystyrene with N-bromosuccinimide and A l C l sub 3 to form a brominated intermediate. Six arrows radiate outward to different products:

- Top left: Suzuki coupling to form P 1 with a methyl ester group.

- Top right: Heck coupling to form P 2 with a vinyl group.

- Middle left: Borylation to form P 5 with a B pin group.

- Middle right: C-N coupling to form P 7 with an N H sub 2 group.

- Bottom left: Sonogashira coupling to form P 6 with an alkyne group.

- Bottom right: Heck coupling to form P 3 or P 4 with styrene or methyl acrylate.

Each product includes yield, functionalization percentage, M sub n, and dispersity values.

Panel C at the bottom shows direct M n-catalyzed C-H phosphonylation. Polystyrene reacts with a phosphite reagent and M n (O A c) sub 2 to yield a phosphonylated polymer. A dashed box on the right displays five phosphite reagents: D E P, B F P, D M P, D B P, and D P P.

6.2 Transition-Metal-Catalyzed C(sp ² )–H Activation and Late-Stage Diversification

Transition-metal catalysis offers a powerful toolkit for the site-selective functionalization of aromatic C–H bonds, allowing for the installation of versatile functional handles without compromising the polymer backbone. In 2023, Wang’s group reported a seminal AuCl₃-catalyzed, para-selective bromination of PS C(sp²)–H bonds using NBS under mild conditions (40°C) [Reference Dou, Xu and Wang84]. By modulating the NBS stoichiometry, the bromination density could be precisely tuned (24–93 mol%) while rigorously maintaining the pristine molecular weight distribution―confirming the absence of deleterious chain scission or cross-linking. Crucially, the resulting brominated PS functioned as a versatile macromolecular scaffold for late-stage diversification via cross-coupling cascades (e.g., Suzuki, Heck, and Sonogashira). This enabled the modular introduction of amino, boronate, aryl, and alkenyl moieties, thereby affording precise control over the polymer’s thermal properties (elevating the glass transition temperature, T_g, from 99°C to 135°C) and surface wettability (Figure 19b).

Extending this precision engineering to heteroatom functionalization, in 2024, Shen’s group developed an earth-abundant manganese-catalyzed strategy for the direct phosphonation of PS to fabricate advanced flame-retardant materials [Reference Liu, Lin and Zou85]. Utilizing inexpensive Mn(OAc)₂ and phosphites in DMF (80°C), phosphonate groups were covalently grafted onto the aromatic rings via robust C–P bonds. GPC verified the exceptional preservation of the polymer architecture during this modification (6.8–13.6 mol% functionalization). Macroscopically, this targeted grafting profoundly enhanced the material’s thermal stability: the modified polymer exhibited a nearly six-fold increase in char yield (from 2.48% to 14.44%), alongside a ~40% reduction in both peak heat release rate (pHRR) and heat release capacity (HRC). Notably, this protocol demonstrated excellent applicability to real-world plastic wastes, including foamed PS and SAN copolymers (Figure 19c). Extending the Au-catalyzed bromination platform, in 2026, Skrydstrup’s group developed a two-step protocol for converting PS into amine-functionalized CO₂ adsorbents [Reference Ebenbauer, Møller and Bjørn86]. PS-Br with tunable bromination degrees (11%–93%) was subjected to Cu-catalyzed Ullmann-type amination with aliphatic diamines (EDA, TAEA), yielding macroporous solid sorbents with CO₂ capacities up to 2.5 mmol g^–1 at 25°C (0.77 mmol g^–1 under DAC conditions). The protocol was validated on five real-world PS wastes and, notably, the amine source could be derived from depolymerized polyurethane waste—enabling a closed-loop upcycling paradigm. Solid-state NMR revealed carbamate/carbamic acid formation at primary amine sites as the primary adsorption mechanism. This work demonstrates the translation of precision C–H functionalization into functional materials for environmental applications, bridging plastic upcycling with carbon capture.

Transition-metal-catalyzed C–H activation provides unmatched regioselectivity and enables sophisticated late-stage macromolecular diversification. However, the rigorous removal of trace transition metal residues―which can detrimentally alter the long-term optical, electrical, and degradation properties of the functionalized polymers-remains a critical purification hurdle that must be addressed before large-scale industrial translation.

6.3 Electrochemical Modification: From Dearomatization to C–H Functionalization

Harnessing the electron as a traceless reagent, electrochemical C–H functionalization offers a highly sustainable, reagent-economical paradigm for precision macromolecular engineering. In 2021, Sarlah’s group reported the first systematic investigation into the electroreductive dearomatization of PS [Reference Siddiqi and Sarlah87]. Using a constant current electrolysis setup with LiBr as electrolyte and HMPA as additive in THF, the aromatic rings of PS were selectively reduced to 1,4-cyclohexadiene moieties, delivering poly(vinylcyclohexadiene) (PVCHD). Remarkably, the degree of reduction could be precisely tuned (0%–100%) by adjusting the charge passed (1.0–6.0 F mol^–1), and the transformation proceeded without significant chain scission or cross-linking across a wide molecular weight range (10³–10⁶ Da). The dearomatized products serve as versatile platforms for downstream olefin chemistry: chemoselective reduction with diimide yielded poly(vinylcyclohexene), while epoxidation with mCPBA afforded epoxidized materials with tunable glass transition temperatures (up to 162°C)—functionalized polymers that would be challenging to prepare by any other means. The practicality of this approach was demonstrated on a 20 g scale and on commercial foam cups. This work established electrochemistry as a powerful tool for accessing novel polymer architectures through dearomatization. In 2025, Naumann’s group reported an innovative electro-oxidative platform for the late-stage thianthrenation of PS [Reference Naumann, Clerk, Hernandez and Romero88]. Through the anodic oxidation of TA at a graphite electrode, highly reactive TA radical cations are generated, which subsequently undergo targeted cross-coupling with the benzylic aromatic rings to yield rigid thianthrenium salt polyelectrolytes. By modulating the TA-to-PS feed ratio, the degree of functionalization (DF) was precisely tuned from 18% to 87% without inducing backbone cleavage, maintaining the polymer’s pristine molecular weight distribution. This platform proved highly robust, accommodating high-molecular-weight EPS waste and various TA derivatives bearing carboxyl, imide, and ester moieties (DF 59%–79%). The resulting cationic polyelectrolytes displayed unique phase separation and crystallization behaviors, and successfully functioned as macromolecular photoacid generators for epoxy curing. Most remarkably, this functionalization is dynamically reversible: under photoredox conditions, the thianthrenium handles undergo facile reductive elimination, enabling the traceless recovery of the original PS architecture with an 83% isolated yield and >99% defunctionalization (Figure 20).

Figure 20.

Electrochemical and photoredox-mediated C–H sulfenylation of PS with phenothiazine derivatives.

Figure 20. Long description

The diagram is divided into two horizontal sections by a dashed line. Top row: C H sulfenylation of P S. 1. Reactants: Polystyrene (P S) with a repeating unit ‘n’ and a thianthrene molecule (red). 2. First Reaction Arrow: Conditions include C (+) / Pt (-) divided cell, D C M / Me C N (3:2), T B A B F sub 4, T F A A, T F A, and constant potential. 3. Intermediate: A P S chain where the phenyl ring is bonded to a thianthrene cation, balanced by a B F sub 4 minus anion. 4. Second Reaction Arrow: Conditions include H E H (1.5 equiv), D C M / Me C N, Ir (p p y) sub 3 (3 mol percent), and light at 365 nm or 440 nm. 5. Products: Regenerated P S and thianthrene. Bottom row: 1. Reactants: P S and a functionalized thianthrene (green) containing an imide group and a green sphere representing a solid support or specific functional group. 2. Reaction Arrow: Conditions include 1.2 equiv of the thianthrene, C (+) / Pt (-), D C M / Me C N (3:2), T B A B F sub 4, T F A A, T F A, constant potential (E sub cell = + 5.0 V), and 2.4 F / mol. 3. Product: A P S chain bonded to the functionalized thianthrene cation with a B F sub 4 minus counterion. 4. Right-hand box: Two variations of the final product are shown with different alkyl chains (Me and branched chains) attached to the imide nitrogen.

Top section: C H sulfenylation of P S.

1. Left: Polystyrene (P S) with a repeating unit ‘n’ reacts with a thianthrene molecule (red).

2. Center: An arrow indicates electrochemical conditions using a C plus / P t minus divided cell, D C M / Me C N (3 to 2 ratio), T B A B F sub 4, T F A A, T F A, and constant potential. This yields a P S-thianthrene intermediate with a B F sub 4 minus counterion.

3. Right: A second arrow indicates photoredox conditions using H E H (1.5 equivalents), D C M / Me C N, I r (p p y) sub 3 (3 mol percent), and light at 365 nm or 440 nm. This yields the final sulfenylated P S and regenerated thianthrene.

Bottom section: Sulfenylation with functionalized derivatives.

1. Left: P S reacts with a green-colored phenothiazine derivative containing an imide group and a green circular polymer support.

2. Center: An arrow specifies electrochemical conditions: C plus / P t minus, D C M / Me C N (3 to 2), T B A B F sub 4, T F A A, T F A, constant potential (E sub cell equals plus 5.0 V), and 2.4 F / mol. This yields a green-colored intermediate complex.

3. Right: A dashed box contains two variations of the product where the green circular support is replaced by linear and branched alkyl chains (Me groups).

While electrochemical thianthrenation offers an elegant, closed-loop approach to reversible macromolecular engineering, the inherent insulating nature of PS and the sluggish mass transport of massive polymer coils to the electrode surface present severe kinetic bottlenecks, necessitating advanced flow-cell reactor designs for future industrial scale-up.

6.4 Mechanochemical Post-Polymerization Modification

Mechanochemistry has emerged as a compelling, solvent-minimized paradigm for macromolecular engineering, utilizing mechanical force rather than thermal energy to drive transformations. In 2018, Kim’s group established the viability of this approach by demonstrating a solvent-free, ball-milling-enabled Schiff-base condensation on aldehyde-functionalized PS [Reference Ohn and Kim89]. Operating at 30 Hz for 30–60 min, the protocol achieved quantitative imine formation while strictly preserving the molecular weight distribution, proving highly selective even in block copolymers.

Advancing from pre-installed functional handles to the direct activation of inert C(sp²)–H bonds, in 2024, Golder’s group reported an elegant liquid-assisted grinding (LAG) strategy for the catalytic trifluoromethylation of commercial PS [Reference Skala, Zeitler and Golder90]. To circumvent the ubiquitous issue of mechanically induced chain scission, trace amounts of solvent (0.2–1.2 μL mg⁻¹) were introduced. This LAG approach effectively plasticized the polymer matrix, dissipating destructive mechanical energy and thereby preserving the macromolecular architecture of high-molecular-weight PS (M_n approx 26 kDa) while achieving 0.6–1.2 mol% functionalization. Remarkably, intrinsic additives in post-consumer waste (e.g., EPS foams and colored cup lids) acted as endogenous plasticizers, further suppressing chain degradation (Figure 21a).

Figure 21.

Materialization transformation: Functionalized and crosslinked PS. (a) Mechanochemical trifluoromethylation of brominated PS. (b) Mechanochemical crosslinking of PS using metal additives.

Figure 21. Long description

Panel A shows the modification of polystyrene sub n. The reaction arrow is topped with three spheres representing milling balls and the conditions N sub 2, 4 h, milling, 30 H z. Below the arrow are reagents 10 mol% T M S C F sub 3, 40 mol% Ag O T f, 40 mol% K F, and D C E 0.2 u L / m g. The product is a copolymer with subunits x and y. Subunit x is the original styrene, and subunit y is a trifluoromethylated styrene where a C F sub 3 group in red is attached to the aromatic ring.Panel B shows a more general modification of polystyrene sub n. The reaction arrow is topped with three milling balls and the text metal, additives. Below the arrow are conditions 30 H z, r t dot, time, air, 10 m L jar, and 10 m m ball. The product is a complex copolymer with four subunits labeled a, b, c, and d. Subunit a is the original styrene. Subunit b shows a modified aromatic ring. Subunit c shows a partially saturated ring. Subunit d shows a modified aromatic ring with a double bond. The modified rings in subunits b, c, and d are highlighted in dark green.

Panel A: On the left is a polystyrene repeat unit with subscript n. A reaction arrow points right, topped with three spheres and text: N sub 2, 4 h, milling, 30 H z. Below the arrow are reagents: 10 mol% T M S C F sub 3, 40 mol% Ag O T f, 40 mol% K F, and D C E (0.2 u L / m g). The product on the right shows a modified polymer chain with two repeat units, x and y. Unit y has a C F sub 3 group attached to the phenyl ring in red text.

Panel B: On the left is the same polystyrene repeat unit with subscript n. A reaction arrow points right, topped with three spheres and the text: metal, additives. Below the arrow are conditions: 30 H z, r t, time, air, 10 m L jar, 10 m m ball. The product on the right is a crosslinked polymer chain with four repeat units labeled a, b, c, and d. The rings of units a, b, and d are connected to form a fused polycyclic system highlighted with green outlines.

In a parallel breakthrough involving extreme structural remodeling, in 2024, Peterson demonstrated an ultrafast mechanochemical Birch reduction of PS using a Li/ethylenediamine (EDA) system [Reference Kim, Park and Kim91]. Within just one minute of milling in air, the inert aromatic rings were dearomatized into reactive skipped-diene motifs (up to 96% conversion) without significant backbone cleavage or cross-linking. Mechanistic insights revealed that THF acts as a crucial ligand to facilitate electron transfer rather than a bulk solvent. The resulting skipped-diene polymers serve as highly reactive platforms, readily undergoing thiol–ene click chemistry to yield flexible, swellable polymer networks (Figure 21b).

While mechanochemical functionalization provides remarkably rapid, green, and waste-compatible routes for polymer upgrading, the inherent competition between targeted bond formation and deleterious mechanical chain-tearing remains a fundamental challenge. Furthermore, transitioning these processes from batch ball-milling to continuous twin-screw extrusion will be pivotal for the industrial realization of scalable mechanochemical upcycling.

6.5 Summary

Collectively, precision macromolecular grafting offers a thermodynamically elegant alternative to total depolymerization. By harnessing orthogonal activation modalities-spanning photoredox, transition-metal, electro-oxidative, and mechanochemical pathways―researchers can execute surgical C–H bond editing while strictly preserving the polymer’s structural integrity. This enables the controlled installation of diverse chemical handles, seamlessly transforming inert PS waste into versatile platforms for late-stage diversification and advanced material fabrication. Despite these conceptual leaps, the field faces formidable translational bottlenecks. Current protocols are constrained by expensive reagents, trace transition-metal residues, sluggish mass transport, and mechanically induced chain-tearing. To overcome these hurdles, future trajectories must prioritize earth-abundant catalysts and transition from batch processing to scalable continuous-flow or twin-screw extrusion architectures, ultimately merging precision functionalization with rational materials engineering. From a sustainability perspective, grafting offers unique advantages: complete carbon retention and low energy input (room temperature to 80°C). However, product markets for functionalized polymers are smaller than those for bulk monomers, limiting waste volume absorption. Reliance on precious metals (Ru, Au) raises cost concerns, though Mn-catalyzed phosphonation offers a more abundant alternative. Heteroatom introduction may complicate end-of-life disposal. Nonetheless, grafting remains a strategically important complement to deconstruction, particularly for PS streams too contaminated for monomer recovery.

7.1 Porous Carbon Materials for Supercapacitor Electrodes

In 2020, Xiao’s group reported a template-directed carbonization and activation strategy to upcycle waste PS into hierarchical porous carbon nanosheets for high-performance supercapacitors [Reference Ma, Liu and Min59]. Utilizing porous MgO as a structural template, PS was first pyrolyzed at 700°C (34.1% yield), followed by KOH activation (700°C–900°C) to sculpt a hierarchical micro/mesoporous architecture (ACNS-X). The optimized material (ACNS-800) exhibited an extraordinary specific surface area of 2650 m² g^–1, a vast pore volume of 2.43 cm³ g^–1, and an intrinsic oxygen doping level of 14.97 at%. This synergistic combination of an immense surface area and heteroatom doping significantly enhanced the electrochemical kinetics. In a 6 M KOH aqueous electrolyte, ACNS-800 delivered a high specific capacitance of 323 F g^–1 at 0.5 A g^–1, excellent rate capability (222 F g^–1 at 20 A g^–1), and 92.6% capacitance retention over 10,000 cycles. Furthermore, a symmetric supercapacitor assembled with an EMIMBF₄/PC organic electrolyte achieved a remarkable energy density of 44.1 Wh kg^–1 at a power density of 757.1 W kg^–1, retaining 81.5% of its initial capacitance after 10,000 cycles. This work establishes a highly viable trajectory for converting carbon-rich PS waste into premium energy storage scaffolds (Figure 22a).

Figure 22.

Porous carbon and functional materials: (a) Chemical modification to PS-PBI for blue-emitting LED and WLED device packaging. (b) Reversible RAFT-based repair of PS. (c) FeCl₃-mediated knitting of PS into crosslinked networks.

Figure 22. Long description

Panel A shows a process flow starting with a photo of white Waste E P S foam and its chemical structure. An arrow labeled Chemical modification leads to the chemical structure of P S dash P B I. Below this, a syringe labeled P S dash P B I solution performs drop-casting onto a glass substrate. This results in a glowing blue P S dash P B I film under U V light. The final step, Device packaging, shows two L E D devices. The top is a Blue-emitting L E D device with a P S dash P B I film over a 365 nm U V chip. The bottom is a W L E D device using tricolor films over a 365 nm U V chip.Panel B depicts a circular reaction for Repeated Repair. It starts with a polystyrene chain reacting with a sulfur-based crosslinker. A circular arrow at the top shows the addition of a styrene monomer under h v light. A return arrow at the bottom shows the regeneration of the polymer chain with the sulfur crosslinker.Panel C illustrates two knitting reaction pathways from a polystyrene backbone. The top path, Knitting Reaction with External Crosslinker using D C E, F e C l sub 3, and F D A, leads to a dense red-colored polymer network. The bottom path, Solvent Knitting Reaction using D C E and F e C l sub 3, leads to a green-colored polymer network with a different crosslinking density.

7.2 Optoelectronic Functional Materials: Blue-Emitting LED Films

Expanding the horizon of polymer upcycling into solid-state lighting, in 2023, Liu’s group showcased an elegant strategy to transform waste expanded polystyrene (EPS) into blue-emitting luminescent matrices [Reference Su, Lin and Li92]. Following standard purification, the inert PS backbone was chemically activated via para-selective chloromethylation. These newly installed electrophilic handles served as reactive sites for the covalent grafting of 2-(2-pyridyl)benzimidazole chromophores. This targeted side-chain engineering endowed the resulting polymer (PS-PBI) with tailored photophysical properties. The PS-PBI film exhibited an excitation maximum at 356 nm―ideally overlapping with commercial 365 nm near-UV chips―and delivered a strong blue emission centered at 468 nm. Demonstrating its macroscopic utility in advanced device fabrication, the PS-PBI film was seamlessly integrated with a Eu³⁺-complex red-emitting film and a Tb³⁺-complex green-emitting film onto a single 365 nm UV chip. This modular assembly successfully yielded a high-performance white LED device with an exceptional color rendering index (CRI) of 93.6, underscoring a lucrative and highly innovative pathway from everyday plastic packaging to advanced optoelectronic devices.

7.3 Porous Adsorbent Materials for Environmental Remediation

Leveraging the high density of aromatic rings inherent to PS, in 2025, Xiao’s group developed a Friedel-Crafts hypercrosslinking strategy to upcycle waste EPS into high-performance porous adsorbents [Reference Varyambath, Hu and Chen93]. Utilizing FeCl₃ as the catalyst and formaldehyde dimethyl acetal (FDA) as an external crosslinker in dichloroethane (DCE), the linear PS chains were spatially knitted into a methylene-bridged hierarchical micro/mesoporous network (PREPS-FDA/DCE). This profound structural reconfiguration yielded a remarkable BET specific surface area of 1021 m²/g, vastly outperforming networks crosslinked solely by the DCE solvent (373 m²/g), as structurally corroborated by solid-state ¹³C NMR and FTIR analyses. Functionally, the abundant aromatic nodes within this rigid network engendered a highly hydrophobic, pi-electron-rich microenvironment. Consequently, PREPS-FDA/DCE exhibited exceptional uptake capacities for aromatic volatile organic compounds (VOCs)―reaching 537 mg g^–1 for xylene and 470 mg g^–1 for toluene―driven primarily by robust pi–pi interactions, while demonstrating negligible affinity for aliphatic VOCs. These metrics significantly eclipsed those of commercial activated carbon. Crucially, the intrinsic hydrophobicity of the hypercrosslinked resin effectively shielded the adsorption sites from moisture interference. Under harsh 80% relative humidity, the xylene adsorption capacity retained an impressive 94.8% of its original value, in stark contrast to activated carbon, which plummeted to 48.4%. Furthermore, the material demonstrated excellent recyclability across multiple thermal regeneration cycles (120°C), thereby establishing a highly robust, waste-derived platform for practical environmental remediation (Figure 22b).

7.4 Mechanoradical Capture and Macroscopic Polymer Repair

In 2025, Hill’s group reported a novel mechanochemical strategy for the efficient closed-loop recycling and macroscopic repair of PS and poly(methyl methacrylate) (PMMA) via mechanoradical capture [Reference Cunningham, Wilusz, Lee, Estock and Hill94]. Rather than viewing ball-milling-induced chain scission as a degradative dead-end, the team harnessed the nascent mechanoradicals by trapping them in situ with bis(butyl trithiocarbonate) under solvent-free conditions. This rapid interception quantitatively capped the severely cleaved, low-molecular-weight fragments (<20 kDa) with trithiocarbonate (TTC) end groups. Functioning as dormant macroinitiators, these TTC-terminated prepolymers underwent photoinduced iniferter polymerization, successfully driving precise chain extension to restore the molecular weight to over 150 kDa. Macroscopically, rigorous rheological assessments confirmed the complete reconstruction of topological chain entanglements and the recovery of the rubbery plateau, verifying the authentic “healing” of the bulk material properties. Remarkably, this mechanochemical repair cycle could be robustly iterated three times without performance fatigue. Furthermore, this platform offered an alternative pathway for targeted deconstruction. For PMMA, ball-milling without the TTC trapping agent directed the mechanoradicals toward disproportionation, generating terminally unsaturated chains. This defect-rich chain-end profile radically suppressed the thermal depolymerization threshold from 400°C to 220°C, facilitating a 44% monomer recovery yield under anaerobic conditions. Mechanistic investigations elucidated that radical trapping efficiency was finely dictated by the liquid-to-solid ratio, while the subsequent photochemical chain extension strictly adhered to pseudo-first-order kinetics. Ultimately, this dual-pathway paradigm elegantly bridges mechanochemistry with reversible deactivation radical polymerization, establishing a powerful blueprint for the macroscopic repair of vinyl polymers (Figure 22c).

7.5 Summary

This chapter highlights a paradigm shift in PS upcycling: from molecular-level deconstruction to macroscopic materialization. Exploiting its carbon-rich aromatic backbone, PS waste is reconfigured into high-value functional architectures, including supercapacitor electrodes, LED matrices, VOC adsorbents, and mechanochemically repaired polymers. However, industrial translation faces critical hurdles. Real-world waste impurities, coupled with the substantial logistical and preprocessing costs of post-consumer feedstocks, compromise device consistency and overall economic viability. Moreover, converting plastics into heavily crosslinked or multi-component devices poses secondary recycling challenges, risking “delayed disposal” rather than true circularity. Future innovations must inextricably integrate comprehensive LCA/TEA to ensure these material-centric approaches provide a genuinely sustainable, closed-loop destination for waste PS.

8.1 Catalytic Oxidation Combined with Fungal Synthesis for Natural Products

In 2023, Williams’s group developed a tandem chemo-biocatalytic pipeline for the upcycling of waste PS [Reference Rabot, Chen and Lin45]. Using a Co/Mn autoxidation system with O₂, diverse post-consumer PS substrates were catalytically degraded into benzoic acid, achieving yields up to 71%. Bypassing extensive downstream purification, the crude oxidation stream was directly utilized as the sole carbon source for engineered Aspergillus nidulans. Integration of exogenous biosynthetic gene clusters into the AFO regulatory locus under an inducible promoter enabled the heterologous expression of three fungal natural products (ergothioneine, pleuromutilin, and mutilin), yielding fermentation titers exceeding 170 mg L^–1. Metabolomic analysis verified specific carbon assimilation dynamics, wherein the fungal strain exclusively metabolized benzoic acid while exhibiting physiological tolerance to unreactive PS oligomers. Expanding this platform to agricultural biocontrol, cultivation of the non-aflatoxigenic Aspergillus flavus Af36 strain on PS-derived benzoic acid induced a 5.2-fold increase in sporulation relative to glucose control media. This work validates a systemic pathway for transforming recalcitrant polyolefins into structurally complex bio-therapeutics and agricultural formulations (Figure 23a).

Figure 23.

Materialization transformation: biologically derived and industrial platform chemicals: (a) Catalytic oxidation to benzoic acid, followed by fungal metabolism to bioactive natural products (ergothioneine, pleuromutilin, mutilin). (b) Conversion to cis,cis-muconic acid via engineered Pseudomonas putida, or to adipic acid through benzene hydrogenation/oxidation pathways.

Figure 23. Long description

Panel A shows a chemical reaction starting with polystyrene (a benzene ring attached to a polymer chain). A horizontal arrow points to benzoic acid. Reagents listed above the arrow are Co(N O sub 3) sub 2 plus Mn(N O sub 3) sub 2 (5 weight percent each). Reagents below the arrow are N H P I (5 weight percent), O sub 2 (4 bars, 3 aliquots), Ac O H (15 m L g-1 P S), and 150 degrees Celsius. A vertical arrow labeled Fungal metabolism points from benzoic acid down to three complex molecular structures labeled Ergothioneine, Pleuromutilin, and Mutilin. Panel B shows a similar starting point with polystyrene. A horizontal arrow points to benzoic acid with reagents O sub 2, Mn forward slash Br, and 165 degrees Celsius. A vertical arrow labeled Engineered P. putida points down to cis,cis-muconic acid. A final horizontal arrow pointing left, labeled H sub 2, P t forward slash C 40 degrees Celsius, shows the conversion of cis,cis-muconic acid into adipic acid.

Panel A: Polystyrene yields benzoic acid via catalytic oxidation using Co(N O sub 3) sub 2, Mn(N O sub 3) sub 2, N H P I, O sub 2, and Ac O H at 150 degrees Celsius. A downward arrow labeled Fungal metabolism leads from benzoic acid to three bioactive products: Ergothioneine, Pleuromutilin, and Mutilin.

Panel B: Polystyrene yields benzoic acid via O sub 2 and Mn forward slash Br at 165 degrees Celsius. A downward arrow labeled Engineered P. putida leads to cis,cis-muconic acid. A leftward arrow labeled H sub 2, P t forward slash C at 40 degrees Celsius leads from cis,cis-muconic acid to adipic acid.

8.2 Thermocatalytic Oxidation and Bacterial Metabolic Conversion for Adipic Acid

In 2025, Beckham’s group established a tandem chemo-biological process for the conversion of waste PS into adipic acid. The protocol initiates with the thermocatalytic autoxidation of PS utilizing a Mn/Br catalyst [Reference Moon, Des Veaux and Van Roijen60]. By employing a benzoic acid/water binary solvent system, the reaction capitalizes on the high solvation capacity of benzoic acid for PS. This configuration substantially enhances substrate loading and reaction kinetics compared to conventional acetic acid systems, affording a maximum benzoic acid yield of 94%. The resulting benzoic acid stream was subsequently introduced as the carbon substrate for an engineered Pseudomonas putida KT2440-CJ074 strain. Harnessing the native ortho-cleavage metabolic pathway via a catechol intermediate, the microbial cell factory quantitatively funnelled benzoic acid into cis,cis-muconic acid. A subsequent downstream catalytic hydrogenation step reduced the muconic acid intermediate into adipic acid, thereby bridging plastic deconstruction with the synthesis of high-value nylon monomers. Furthermore, integrated TEA and LCA indicated that this hybrid process could reduce greenhouse gas emissions by 61% relative to traditional fossil-derived adipic acid synthesis, substantiating the environmental and industrial viability of this upcycling trajectory.

8.3 Photocatalytic Degradation and Microbial Conversion for Nylon-6,6 Precursors

Pushing tandem upcycling boundaries, in 2025, Wittmann’s group converged dual waste streams (PS and textiles) into virgin-grade nylon-6,6. Initially, PS underwent photocatalytic oxidation into benzoic acid [Reference Kohlstedt, Weiland and Pearson62]. Bypassing purification, this crude extract directly fed a catBC-deleted P. putida KT2440 chassis. Using ¹³C metabolic flux analysis, the team identified substrate toxicity as a critical metabolic bottleneck. By implementing a dissolved oxygen (DO)-stat pulsed feeding strategy to mitigate this, they achieved a record cis,cis-muconic acid titer exceeding 65 g L^–1. Downstream, the fermentation intermediate was catalytically converted into a complete suite of nylon monomers. Utilizing textile-derived glucose as a fermentation co-substrate, subsequent polycondensation yielded nylon-6,6 commensurate with petrochemical standards, establishing a highly integrated paradigm for valorizing heterogeneous bulk waste (Figure 23b).

8.4 Summary

This chapter elucidates a transformative PS upcycling paradigm via a “chemical deconstruction―metabolic funnel” cascade. Advanced oxidation deconstructs the recalcitrant PS backbone into bio-assimilable intermediates, predominantly benzoic acid, which are subsequently utilized by engineered microbial cell factories. This tandem approach seamlessly bridges the synthesis of complex natural products and biocontrol agents with the scalable production of bulk monomers like adipic acid. Synergizing the deconstructive power of chemical catalysis with the structural precision of synthetic biology, this pipeline circumvents the limitations of pure chemical degradation. However, mitigating the microbial toxicity of crude streams, resolving the kinetic mismatch between chemo- and biocatalysis, and integrating LCA remain critical imperatives to transition these hybrid platforms from bench-scale concepts to industrially viable realities. The sustainability merits are compelling: the PS-to-adipic acid route achieves 61% GHG reduction versus fossil-derived production. Yet challenges remain. The chemocatalytic oxidation step still requires elevated temperatures and pressurized O₂. Fermentation demands sterile conditions, nutrients, and extended residence times. A kinetic mismatch exists between relatively slow chemical deconstruction and rapid microbial metabolism, necessitating integrated process design (e.g., continuous-flow photoreactors coupled with membrane bioreactors). These bottlenecks must be resolved for industrial translation.

9.1 Substrate Heterogeneity and Catalyst Poisoning Mechanisms in Authentic PCR Streams

Transitioning from pristine model PS to authentic post-consumer recycled (PCR) streams introduces severe chemical challenges that remain largely unaddressed in academic literature. Real-world PS waste is rarely pure; high-impact polystyrene (HIPS) and expanded polystyrene (EPS) are heavily formulated with architectural additives (Table 4), most notably decabromodiphenyl ethane as a brominated flame retardant specific to PS packaging, alongside residual pentane blowing agents. During multi-energy-driven deconstruction, thermal or electro-catalytic fields inevitably induce the leaching of corrosive bromide ions from DBDPE. These halogen species possess a high affinity for active metal centers, poisoning transition-metal catalysts (e.g., Ru, Pt, or homogeneous iron porphyrins) via irreversible coordination or surface fouling. Furthermore, the volatility of trapped pentane gas alters localized pressures and interfaces in batch systems, frequently causing mass transport bottlenecks or catalyst delamination. Future research must move away from “ideal waste” imagination and prioritize the design of poisoning-resistant catalytic microenvironments capable of selectively sequestering or tolerating halogenated and aliphatic impurities intrinsic to commercial styrenic waste.

Table 4.

Impurity types, sources, deactivation mechanisms, and pretreatment strategies.

Table 4. Long description

The table consists of five columns and six data rows.

* Row 1: Brominated flame retardants (D B D P E) sourced from H I P S or E P S. Deactivation occurs via irreversible poisoning where Br super minus binds to metal sites. It affects Ru, Pt, Fe, and Ni catalysts. Pretreatment involves thermal debromination or solvent extraction.

* Row 2: Inorganic fillers such as Ca C O sub 3 and Ti O sub 2 sourced from colorants and processing aids. Deactivation occurs through physical fouling and pore blockage. It affects Zeolites and supported metals. Pretreatment involves filtration or dissolution-precipitation.

* Row 3: Carbonblack sourced from black P S products. Deactivation occurs via surface coverage and light scattering. It affects Photocatalysts. Pretreatment involves mechanical separation.

* Row 4: Nitrogen-containing compounds like caprolactam and aniline sourced from co-mingled polyamides and nitrile rubbers. Deactivation involves reversible aniline or irreversible caprolactam binding to acid sites. It affects Zeolites like H-Z S M-5. Pretreatment involves improved sorting and distillation.

* Row 5: Polycyclic aromatic hydrocarbons sourced from P S pyrolysis intermediates. Deactivation occurs through coke formation, pore blockage, and site encapsulation. It affects Pt all over Al sub 2 O sub 3 and supported metals. Pretreatment involves distillation.

* Row 6: Polymerization retarders (benzyl-containing molecules) sourced from catalytic depolymerization products. Deactivation occurs via chain transfer inhibition. It affects repolymerization catalysts. Pretreatment involves distillation yielding approximately 92 percent styrene.

9.2 Green Energy Input Mechanisms: Expansion of Electrocatalytic and Photoelectrocatalytic Synergies

Among existing energy input modes, thermocatalysis is typically limited by harsh conditions of high temperature and pressure, while homogeneous photocatalysis faces engineering bottlenecks in scaling up. For PS in particular, the high energy demand of conventional pyrolysis (>400°C) and the thermodynamic preference for benzoic acid formation in photochemical oxidation represent two intertwined challenges that are uniquely pronounced for this aromatic polymer.

As a method where SET processes can be precisely controlled by adjusting the potential, electrocatalysis provides a highly promising green alternative for the degradation of inert polymers at room temperature and ambient pressure. In the future, clean electrocatalytic and photoelectrocatalytic systems should not only be widely applied to the controllable oxidative deconstruction of PS, but these reaction mechanisms are also expected to be extended to the directional conversion of engineering plastics with stronger aromatic backbones and stable ether bonds (such as polyphenylene oxide). However, for PS-specific applications, two additional considerations merit attention: (i) the insulating nature of PS necessitates the development of redox mediators or conductive supports to facilitate electron transfer from the bulk polymer to the electrode surface; and (ii) the tendency of aromatic intermediates to form coke on electrode surfaces calls for pulsed electrolysis or flow-cell designs that periodically regenerate the active interface.

9.3 Integration of In-Depth Precise Functionalization and Biochemical Hybrid Upcycling

In the high-value transformation of PS, current modification methods still face issues such as limited conversion rates and poor site selectivity; meanwhile, a ceiling often exists for the economic added value of simple chemical deconstruction products (such as benzoic acid). An effective pathway to break this limitation lies in the integration of synthetic biology. The introduction of small-molecule mixtures generated by chemical deconstruction into the “metabolic funnels” of microorganisms has been proven to be an efficient route for synthesizing high-value natural products and fine chemicals [Reference Meng, Zhou, Li and Jiang39].

For PS in particular, the aromatic-rich nature of its degradation products presents both an opportunity and a challenge. On one hand, benzoic acid and related aromatics are readily metabolized by engineered Pseudomonas putida strains via the ortho-cleavage pathway [Reference Ji, Meng, Li, Wang and Jiang37,Reference Li, Meng and Jiang38]. On the other hand, the toxicity of aromatic intermediates to microbial hosts at elevated concentrations necessitates sophisticated fed-batch or DO-stat feeding strategies to maintain cell viability [Reference Ji, Meng, Li, Wang and Jiang37]. Future efforts should be dedicated to cultivating engineered strains with higher tolerance to aromatic toxicity, and more precise C–H bond activation and heteroatom introduction systems must be developed to adapt to and convert the complex carbon sources generated from the degradation of different batches of waste plastics. Furthermore, the kinetic mismatch between the relatively slow rates of chemical deconstruction and the rapid metabolism of engineered microbes must be resolved through integrated process design—for instance, by coupling continuous-flow photoreactors with membrane bioreactors to achieve steady-state operation.

9.4 Feedstock Logistics and the True Cost of Post-Consumer PS: Bridging Laboratory Metrics and Industrial Reality

Evaluating the genuine viability of PS upcycling demands an integrated TEA and LCA to bridge the gap between laboratory metrics and industrial reality. A pervasive blind spot in current research is the idealized assumption regarding feedstock cost. As highlighted by Liu’s group, post-consumer PS is fundamentally not a “free” resource. For EPS, the ultra-low bulk density (0.01–0.02 t m^–3) incurs disproportionately high freight costs [Reference Xu, Sun and Xu97]. Moreover, municipal solid waste streams necessitate extensive, cost-intensive sorting and washing protocols. Consequently, the actual market price of PS waste in regions such as Europe can exceed €800/tonne, establishing a severe economic threshold even before any catalytic conversion begins.

Methodological challenges in cross-study LCA/TEA comparisons. Comparing LCA and TEA results across different PS upcycling studies is complicated by several methodological factors that are often overlooked. First, system boundaries vary considerably: some analyses adopt a “cradle-to-gate” perspective encompassing feedstock acquisition, pretreatment, and catalytic conversion, while others employ a narrower “gate-to-gate” scope that excludes upstream and downstream stages. The energy required for distillation of styrene monomer from pyrolysis oil, for example, accounts for approximately 39% of the total energy consumption in the recycling process, yet this is frequently underrepresented in simplified assessments. Second, the choice of functional unit profoundly influences outcomes. Products of different upcycling strategies range from bulk commodities (styrene, ~$1.5–2.0/kg) to specialty chemicals (benzoic acid, ~$3–5/kg) to advanced functional materials (>$10/kg), making mass-based comparisons inherently problematic. Third, data quality and scaling uncertainty arise from extrapolating laboratory-scale results to industrial scale; sensitivity analyses reveal that global warming potential (GWP) estimates for PS-based processes can vary by over 40% depending on assumed energy consumption and feedstock purity. Fourth, co-product allocation methods—whether by mass, economic value, or system expansion—can produce GWP differences of 30%–50% for the same process. Finally, temporal and geographical context adds further complexity, as energy mixes, labor costs, and regulatory frameworks differ substantially across regions and evolve over time. Collectively, these limitations do not invalidate LCA and TEA as tools for guiding research and development, but they necessitate that both analysts and readers approach cross-study comparisons with caution, paying close attention to underlying assumptions and system boundaries.

At the processing stage, LCA models reveal substantial variances in energy consumption across distinct upcycling trajectories. Complete depolymerization―exemplified by high-temperature pyrolysis―is highly endothermic and demands energy-intensive downstream distillation, resulting in a considerable GWP. Conversely, “skeletal preservation” strategies, such as ambient-temperature electrocatalysis or photocatalysis, theoretically mitigate these energy penalties by circumventing the high activation barrier of total C–C backbone cleavage. In practice, however, this environmental advantage is frequently offset by severe operational bottlenecks. Most homogeneous catalytic protocols rely on large volumes of hazardous or high-boiling-point solvents, and LCA consistently shows that the energy required for solvent recovery, product isolation, and catalyst separation often negates the thermodynamic savings achieved under mild conditions.

To break this thermodynamic and economic deadlock, inspiration can be drawn from recent macro-scale demonstrations in other polyolefin upcycling. For instance, an oxygen-mediated tandem strategy was recently developed to drive the endothermic aromatization of polyethylene by selectively oxidizing in-situ generated hydrogen—a kinetic coupling principle that could be adapted to PS systems where similar thermodynamic trade-offs govern product selectivity. This kinetic coupling successfully shifted the thermodynamic equilibrium, achieving an unprecedented 78 mol% aromatic selectivity under mild conditions without relying on massive solvent volumes. Importantly, pilot-scale demonstrations (500 g) and subsequent TEA revealed a highly competitive minimum selling price of $1.05/kg. This underscores that intelligently coupling endothermic and exothermic cascades is a viable engineering route to offset scaling costs [Reference Li, Tu and Zhang98].

To translate molecular-level upcycling into tangible environmental and economic benefits, future paradigms must simultaneously address these front-end (feedstock logistics) and back-end (downstream separation) bottlenecks. From a technological standpoint, priority should be given to solvent-free or high-solid paradigms (e.g., mechanochemistry, liquid-assisted grinding), the development of robust heterogeneous catalysts to preclude metal leaching, and tandem reaction cascades that minimize intermediate purification steps. Economically, alternative upcycling strategies should strategically bypass conventional multi-step synthesis to deliver high-value pharmaceutical scaffolds and fine chemical building blocks. Only by generating a substantial economic delta through premium products can the process offset both the unavoidable logistical burdens of waste procurement and the capital costs of industrial scale-up.