Catalytic deconstruction of waste polyethylene with ethylene to form propylene

Exploring biotechnology for plastic recycling, degradation and upcycling for a sustainable future

The persistent demand for plastic commodities, inadequate recycling infrastructure, and pervasive environmental contamination due to plastic waste present a formidable global challenge. Recycling, degradation and upcycling are the three most important ways to solve the problem of plastic pollution. Sequential enzymatic and microbial degradation of mechanically and chemically pre-treated plastic waste can be orchestrated, followed by microbial conversion into value-added chemicals and polymers through mixed culture systems. Furthermore, plastics-degrading enzymes can be optimized through protein engineering to enhance their specific binding capacities, stability, and catalytic efficiency across a broad spectrum of polymer substrates under challenging high salinity and temperature conditions. Also, the production and formulation of enzyme mixtures can be fine-tuned to suit specific waste compositions, facilitating their effective deployment both in vitro, in vivo and in combination with chemical technologies. Here, we emphasized the comprehensive strategy leveraging microbial processes to transform mixed plastics of fossil-derived polymers such as PP, PE, PU, PET, and PS, most notably polyesters, in conjunction with potential biodegradable alternatives such as PLA and PHA. Any residual material resistant to enzymatic degradation can be reintroduced into the process loop following appropriate physicochemical treatment.

A Broad-Spectrum Catalyst for Aliphatic Polymer Breakdown

Thermolysis of the well-defined aluminum fluoroalkoxide supported on silica (≡SiOAl(OC(CF3)3)2(O(Si≡)2), 1, 0.20 mmolAl g-1) at 200 °C forms a fluorinated amorphous silica-alumina (F-ASA) containing a distribution of Al(IV), Al(V), and Al(VI) sites that maintain relatively strong Lewis acidity. Small amounts of Brønsted sites are also present in F-ASA. Solid-state NMR studies show that a majority of the aluminum centers in F-ASA are not close to the Si-F groups that form during thermolysis. F-ASA is exceptionally reactive in cracking (or pyrolysis) reactions of neat polymer melts. Catalyst loadings as low as 2 wt % (0.017 mol % aluminum) efficiently break down isotactic polypropylene, high-density polyethylene, ethylene/1-octene copolymer, and postconsumer wastes. The major products of this reaction are hyperbranched liquid paraffins containing internal olefins and very small amounts of aromatics. Under continuous distillation of oils from the reaction mixtures, pyrolysis on 50 g reaction scales is feasible. F-ASA cokes and deactivates during this reaction but can be reactivated by calcination in air. These properties are complementary to other state-of-the-art catalysts for polymer breakdown, but unlike those catalysts F-ASA does not require an additional cofed reactant (e.g., H2, olefin, etc.) to drive the reaction.

Advances and Challenges in Low-Temperature Upcycling of Waste Polyolefins via Tandem Catalysis

Polyolefin waste is the largest polymer waste stream that could potentially serve as an advantageous hydrocarbon feedstock. Upcycling polyolefins poses significant challenges due to their inherent kinetic and thermodynamic stability. Traditional methods, such as thermal and catalytic cracking, are straightforward but require temperatures exceeding 400 °C for complete conversion because of thermodynamic constraints. We summarize and critically compare recent advances in upgrading spent polyolefins and model reactants via kinetic (and thermodynamic) coupling of the endothermic C─C bond cleavage of polyolefins with exothermic reactions including hydrogenation, hydrogenolysis, metathesis, cyclization, oxidation, and alkylation. These approaches enable complete conversion to desired products at low temperatures (<300 °C). The goal is to identify challenges and possible pathways for catalytic conversions that minimize energy and carbon footprints.

Efficient Upcycling of Polyolefin Waste to Light Aromatics via Coupling C─C Scission and Carbonylation

The upcycling of waste polyolefins into light aromatics has great potential to generate hundreds of millions of tons of valuable aromatic carbon feedstock. However, the conventional high-temperature radical cracking method for aromatizing polyolefins on zeolites faces challenges in aromaticity control and rapid deactivation due to coke. Here, we present a unique strategy that integrates the traditional cracking-aromatization process of PE with CO insertion, a key step of Fischer-Tropsch synthesis, achieving a rise of yield of light aromatics by four times, with an absolute value up to 67% by weight at only 280 °C. The insertion of CO into the Ru-alkyl intermediates formed during polyolefin cracking facilitates the generation of active oxygenate species, guarantees an ideal C─C chain length range, and smooths the way for subsequent efficient aromatization on Hol-ZSM-5@S1 with a short b axis. According to the technical economic analysis, this low-carbon-footprint and economic approach can reduce approximately 1/3 of carbon emissions compared to traditional naphtha cracking technologies and holds promise for reshaping the global aromatic hydrocarbon cycle of the petrochemical industry through polyolefin degradation.

On-demand, readily degradable Poly-2,3-dihydrofuran enabled by anion-binding catalytic copolymerization

Copolymerization with cleavable comonomers is a versatile approach to generate vinyl polymer with viable end-of-life options such as biodegradability. Nevertheless, such a strategy is ineffective in producing readily degradable 2, 3-dihydrofuran (DHF) copolymer with high-molecular-weight (>200 kDa). The latter is a strong and biorenewable thermoplastic that eluded efficient cationic copolymerization synthesis. Here, we show that an anion-binding catalyst seleno-cyclodiphosph(V)azanes enable the efficient cationic copolymerization with cyclic acetals by reversibly activating both different dormant species to achieve both high living chain-end retention and high-molecular-weight. This method leads to incorporating low density of individual in-chain acetal sequences in PDHF chains with high-molecular-weight (up to 314 kDa), imparting on-demand hydrolytic degradability while without sacrificing the thermomechanical, optical, and barrier properties of the native material. The proposed approach can be easily adapted to existing cationic polymerization to synthesize readily degradable polymers with tailored properties while addressing environmental sustainability requirements.

Population Balance Models for Catalytic Depolymerization: From Elementary Steps to Multiphase Reactors

ConspectusThe ongoing accumulation of plastic waste in landfills and in the environment is driving research on chemical processes and catalysts to recycle polymers. Traditional modeling strategies are not applicable to these processes because they involve too many reactants and intermediates, one for each molecular weight and each functionalization. To model the kinetics, we have developed population balance models (PBMs) that account for macromolecular reactants in the bulk and macromolecular catalytic intermediates. These PBMs couple to each other through polymer adsorption and desorption models and to traditional rate equations for small molecule products and co-reactants (like hydrogen or ethylene). The models, in combination with experimental data, are being used in many ways: (i) to test mechanistic hypotheses, (ii) to extract rate parameters, (iii) to quantitatively compare catalyst activities, (iv) to account for mass transfer and vapor-liquid partitioning in two-phase reactors, and (v) to design novel support architectures and catalysts that mimic the processive action of natural depolymerization enzymes. Some key theoretical advances allow PBMs to be constructed from elementary rates and mechanisms, as opposed to traditional formulations with pseudoelementary rate parameters invoked as fitting parameters. We discuss ways to build these models "bottom-up" from first-principles calculations and ways to extract model parameters from "top down" analyses of rate data. The combination provides a quantitative bridge between first-principles calculations and the kinetics of complex macromolecular transformations for polymer upcycling and beyond.

Polyethylene Upcycling to Liquid Alkanes in Molten Salts under Neat and External Hydrogen Source-Free Conditions

Development of facile approaches to convert plastic waste into liquid fuels under neat conditions is highly desired but challenging, particularly without noble metal catalysts and an external hydrogen source. Herein, highly efficient and selective polyethylene-to-gasoline oil (branched C6-C12 alkanes) conversion was achieved under mild conditions (<170 °C) using commercially available AlCl3-containing molten salts as reaction media and to provide catalytic sites (no extra solvents, additives, or hydrogen feeding). The high catalytic efficiency and selectivity was ensured by the abundant active Al sites with strong Lewis acidity (comparable to the Al type in acidic zeolite) and highly ionic nature of the molten salts to stabilize the carbenium intermediates. Dynamic genesis of the Al sites was elucidated via time-resolved Al K-edge soft X-ray and 27Al NMR, confirming the tricoordinated Al3+ as active sites and its coordination with the as-generated alkene/aromatic intermediates. The carbenium formation and polyethylene chain variation was illustrated by inelastic neutron scattering (INS) and an isotope-labeling experiment. Theoretical simulations further demonstrated the successive hydride abstraction, β-scission, isomerization, and internal hydrogen transfer reaction pathway with AlCl3 as active sites. This facile catalytic system can further achieve the conversion of robust, densely assembled, and high molecular weight plastic model compounds to liquid alkane products in the diesel range.

Repurposing Post-Consumer Polyethylene to Access Cross-Linked Polyethylene with Reprocessability, Recyclability, and Tunable Properties

Polyethylene (PE) is the most widely produced plastic but accumulation and resistance to degradation has significantly contributed to the plastic waste crisis. Upcycling has presented promising solutions to transform PE waste into value-added products. In this study, mixed post-consumer PE was successfully repurposed into reprocessable and chemically recyclable cross-linked polyethylene (XLPE). This process involved converting PE into telechelic oligomers, followed by repolymerization using a hybrid cross-linking system consisting of a dynamic cross-linker 2,4,6-triethoxy-1,3,5-triazine (TETA) and non-dynamic cross-linker tris(6-isocyanatohexyl)isocyanurate (Tri-HDI). In the resulting XLPE, TETA facilitated iterative reprocessing with minimal property degradation across cycles, whereas Tri-HDI helped preserve functional performance throughout service life. Compared to PE, XLPE exhibited enhanced mechanical properties, reduced creep deformation under application-relevant temperatures, and high temperature structural stability. Notably, copolymerizing PE oligomers with commercial macrodiols was employed to create composite XLPEs, enabling tuning material properties. After use, XLPE can be efficiently and selectively depolymerized under mild conditions, even when mixed with commercial insulator cables. This depolymerization allows for the recovery of the constituent building blocks, enabling purification and subsequent repolymerization for reuse. This approach demonstrates the potential of repurposing plastic waste into sustainable materials and fostering the development of a circular economy.

Versatile C─H Alkylation and Alkylidenation via Catalytic Alkylidene Transfer of Enones

The alkylidene transfer reactions of alkenes are of particular significance but challenging. Here, we report that enones can serve as diverse alkylidene sources for catalyst-controlled selective C─H alkylation and/or alkylidenation of various nucleophiles. Treatment of a mixture of ketone (or lactam), enone, and diarylmethanol, with a catalytic amount of Y[N(TMS)2]3, gave the corresponding α-C─H bond alkylation products derived from the alkylidene transfer from enones to ketones/lactams, whereas the reaction of enones with various C-nucleophiles in the presence of KOH as a catalyst resulted in C─H alkylidenation. Moreover, the application of these strategies for the late-stage modification or structural simplification of some bioactive molecules is also presented. These alkylidene transfer reactions are characterized by operational simplicity, mild reaction conditions, and remarkable catalyst-controlled product outcomes. These results not only demonstrate a significant potential for easily accessible and recyclable enones to serve as versatile alkylidene sources in C─H alkylation and alkylidenation but also provide an attractive and concise method for hydrodealkylidenation of electron-deficient alkenes.

Enhancing Waste Plastic Hydrogenolysis on Ru/CeO2 Through Concurrent Incorporation of Fe Single Atoms and FeOx Nanoclusters

Ru-based catalysts have exhibited significant promise in converting waste plastics into valuable long-carbon chain products. However, their efficiency is hindered by the uncontrollable cascade hydrogenation, which stems from their exceptional reactivity for C─C cleavage. Herein, we reported a multi-scale regulation strategy by selectively anchoring Fe single atoms (SAs) and FeOx nanoclusters (NCs) by Ru NCs-decorated CeO2 substrates. This catalyst demonstrates an extraordinary performance, achieving nearly 100% low density polyethylene (LDPE) conversion under the conditions of 250 °C and 2 MPa hydrogen after 1 h, along with remarkably-improved liquid product selectivity of 86.4% compared to that of bare Ru/CeO2 (59.8%). Through a variety of spectroscopic studies, we revealed the unique interactions between FeOx NCs and Ru NCs, which leads to an increased Ru° content. More significantly, we also confirmed the crucial role of Fe SAs in adsorbing active hydrogen species, thereby increasing the hydrogen coverage. Such precise regulations towards both the intrinsic surface state of Ru and its adjacent chemical environment successfully inhibited the cascade hydrogenation, ultimately resulting in a significant enhancement in the selectivity of liquid products.

Gallium-catalyzed recycling of silicone waste with boron trichloride to yield key chlorosilanes

Chemical recycling to monomers is a key strategy for a sustainable circular polymer economy. However, most efforts have focused on polymers with carbon backbones. Recycling of silicone polymers and corresponding materials, featuring a robust inorganic backbone and tunable properties, remains in its infancy. We present a general method for depolymerization of a very wide range of silicone-based materials and postconsumer waste, including end-of-life cross-linked polydimethylsiloxane-based networks within formulated materials. The reaction proceeds at 40°C, harnessing an efficient gallium catalyst for a million-fold rate enhancement and boron trichloride as the chlorine source, to produce nearly quantitative yields of (methyl)chlorosilanes, key intermediates in the Müller-Rochow process that anchors the silicone industry.

Iron(III)-Catalyzed C─H Hydroxylation of Low-Density Polyethylene Coupled with Short Chain Branching Growth

Low-density polyethylene (LDPE) is widely used in packaging applications, but after being discarded its environmental impact is a pressing concern due to a lack of effective chemical recycling strategies, especially owing to its chemical inertness and nonpolar nature. To address these challenges, we present a mild iron(III)-catalyzed oxidative upcycling of LDPE in which C─H hydroxylation of LDPE occurs coupled with the growth of methyl short chain branching (Me-SCB) and ethyl shortchain branching (Et-SCB). As a result, the resulting products achieve significant improvements in surface wettability, crystallinity, and mechanical properties despite a concomitant reduction in molecular weight. CH…F interactions and σ-π interactions are found between LDPE and the catalyst. Density functional theory (DFT) calculations elucidate the catalytic mechanism that fluorine on the ligand facilitates hydrogen peroxide activation and subsequent deprotonation, leading to the formation of high-valent ironoxo species. The growth of short-chain branching (SCB) involves the β-scission of CC bonds and a radical-mediated chain-walking mechanism. This work represents a transformative advancement in deep understanding of polyolefin upcycling and opens a new approach of polyolefin functionalization and architecture modulation.

Probing the Catalytic Degradation of Unsaturated Polyolefin Materials via Fe-Based Lewis Acids-Initiated Carbonyl-Olefin Metathesis

Degradation and recyclability of polymeric materials, including extensively used polyolefins, are becoming increasingly necessary. Chemically stable saturated polyolefin backbones make their degradation frustratingly challenging. The current effective strategy is to create cleavable defects, e.g., C═C double bonds along the backbone, and subsequently depolymerize them via cross-metathesis reaction with olefins. High-value chemicals or reusable polymeric segments are obtained. This two-step protocol provides operable means for alleviating plastics problems. There are several approaches to introduce unsaturation into a polymer backbone, like dehydrogenation or copolymerization of olefins and conjugated dienes. However, for the second step, to conduct a cross-metathesis reaction, only noble metal catalysts can be used most of the time. Regardless of their limited availability, the fact that these organometallics are unfavorably sensitive to impurities would raise barriers in industrial practices. Herein we employed earth-abundant and inexpensive iron-based Lewis acids to initiate carbonyl-olefin metathesis reactions between ketone/aldehyde reagents and unsaturated polyolefins. After explorations in poly(diene)s and industrial thermoplastic elastomers, we extended this protocol to degrade low-density polyethylene (LDPE). Low-molecular weight PE wax-like products were obtained as useful chemicals. This catalytic degradation system is expected to enable the development of more efficient metathesis strategies to promote degradation of polyolefins and pave sustainable ways for reuse of polymeric materials.

Electrochemical Commodity Polymer Up- and Re-Cycling: Toward Sustainable and Circular Plastic Treatment

The demand for commodity plastics reaches unprecedented dimensions. In contrast to the well-developed plethora of methods for polymer synthesis, sustainable strategies for the end-of-life management of plastics continue to be scarce. While mechanical re-cycling often results in downgraded materials, chemical re-cycling or up-cycling offers tremendous potential for an efficient and green approach, thereby addressing the precarious treatment of post-use plastics within a circular carbon economy. Recently, electrochemistry surfaced as a uniquely powerful tool for polymer up-cycling via polymer functionalization or degradation obtaining either novel polymers with valorized properties or high-value recycled small molecules, respectively. While discussing recent progress in that domain, future perspectives of electrochemical polymer modifications until January 2025 are outlined herein.

Obtaining the value of waste polyethylene mulch film through pretreatment and recycling technology in China

Polyethylene (PE) mulch film has been widely used in agricultural production. However, the residual PE mulch film fragments in the soil can cause severe pollution, affecting the quality of agricultural products and even the stability of the ecological environment. Moreover, PE mulch film has high calorific value and thermoplasticity, so recycling is necessary. This review provides an overview of the current pretreatment and recycling methods for waste PE mulch film, cites application examples from plastic recycling enterprises, and offers suggestions for future research directions. In the pretreatment technology, the research status of mechanical collection with high efficiency and mechanical collection with pretreatment function was summarized, and the advantages and disadvantages of different collection machines were pointed out. In the treatment technology, several technologies' advantages, disadvantages and research progress, including incineration, thermal pyrolysis, direct regranulation and modified regranulation, were summarized. It points out that improving the degree of resource recycling and optimizing the quality of recycled products is the key to reuse. In summary, this review points out that the research on collecting and recycling waste PE mulch film requires joint efforts in raising collecting awareness, promoting resource recycling technology, preventing secondary pollution, and making positive contributions to agricultural production and ecological environment protection.

Sustainable Diesel from Rapeseed Oil Esters by Sequential Semi-Hydrogenation, Double Bond Isomerization, and Metathesis

Rapeseed oil methyl esters (RME) have been converted to biofuel with a boiling point curve that fulfills the EN 590 specifications for modern diesel engines using a robust, three-step process. In the first step, the polyunsaturated esters of the RME were semi-hydrogenated in the presence of 20 ppm of a solvent-stabilized Pd0 colloid. The resulting mono-unsaturated fatty esters were further converted into a defined mixture of double-bond isomers by passing them over inexpensive, Brønsted-acidic Amberlyst 15 resin at high space-time yields (1.3 kg⋅L-1⋅h-1). The resulting mixture was then converted into a blend of terminally unsaturated olefins and monoesters, with <4.9 % diesters and <21 % saturated fatty esters by cross-metathesis with technical-grade ethylene. In this step, 50 ppm of a cyclic alkyl amino carbene (CAAC) Ru catalyst M1001 was used to achieve record-setting conversions (91 %) and selectivities (94 %). All three steps were conducted with neat feedstock at mild temperatures (60-100 °C). This demonstrates that sustainable diesel fuel for use in contemporary diesel engines is accessible from RME and ethylene via a short set of industrially viable reaction steps.

Ruthenium-Catalyzed Deconstruction of Polyolefins: A Strategy to Up-cycle Waste Polyethylene to Value-Added Alkene

Synthesis of value-added products from post-consumer waste polyolefins is fascinating as well as challenging. Here we report ruthenium-catalyzed up-cycling of the polyethylene to long-chain alkene derivatives. The developed methodology mainly involves two steps i.e., dehydrogenation of polyethylene through hydrogen atom transfer and its metathesis using the HG-II catalyst. The dehydrogenation of polyethylene using ruthenium catalysis derived up to 3.38 %, of double bonds; with 90 % of the recovered polyolefin material. The obtained unsaturated polyethylene was subjected to cross-metathesis with ethylene using HG-II catalytic system. This resulted in the synthesis of predominantly dodecene (C12) derivatives, with 58 % selectivity, along with other derivatives of varying chain lengths. The overall reaction produced terminal and internal olefins in the ratio 1:0.8 respectively. The dehydrogenation of polyethylene and its deconstruction was confirmed by NMR spectroscopy, Gel Permeation Chromatography (GPC) and Differential Scanning Calorimetry (DSC). The origin of C12 selectivity has been demonstrated by control experiments. The scope of the methodology was extended to post-consumer waste polyethylene which gave high conversion to value-added dodecene derivatives as a major product.

Upcycling Polynorbornene Derivatives into Chemically Recyclable Multiblock Linear and Thermoset Plastics

Synthetic polymers have found widespread use, but their ineffective end-of-life treatment is causing a significant environmental and human health crisis. Here, we demonstrate the upcycling of polynorbornene derivatives (pNBEs) through their deconstruction into distinct oligomeric buildings blocks that can be repolymerized into chemically recyclable pNBEs-like multiblock polymers via dehydrogenative polymerization. The resulting materials exhibit diverse mechanical properties, while integrating high melting temperatures (Tm as high as 133 °C). Notably, this method could also enable the selective deconstruction of permanently cross-linked polydicyclopentadiene (pDCPD) thermosets into telechelic-OH functionalized oligomers, overcoming the significant challenges posed by their robust network structure in recycling and degradation. The resulting pDCPD oligomers can subsequently be repolymerized with macrodiols to create multiblock thermosets with tunable mechanical properties, including Young's modulus and tensile elongation. After use, upcycled plastics could be effectively deconstructed back to the oligomers for recovery and repolymerization. Overall, this work establishes an approach that can be utilized to upcycle pNBEs into previously inaccessible multiblock thermosets and thermoplastics with full recyclability, and may be generalizable to a range of polymers to shift their end-of-life waste disposal toward sustainable recovery and reuse.

Highly adaptable oxidative upcycling of polyolefins to multifunctional chemicals containing oxygen and nitrogen

Highly adaptable upcycling of waste polyolefins was demonstrated to obtain high-value nitro-containing polycarboxylic acids in high carbon yields. This method is applicable to a wide range of polyolefins, mixed PP/PE in any ratio, as well as actual polyolefin products and their mixtures. Moreover, the obtained products are homogenized with similarity in molecular weight and functional groups, enabling direct reutilization as fine chemicals or feedstocks for preparation of recyclable high-performance/functional materials. This work provided a new universal and efficient upcycling strategy for waste polyolefins, which may reshape the model of waste plastics recycling, while providing alternative functional chemicals and materials to achieve sustainable development.

Optimization of low-temperature catalytic cracking of polyolefin waste in open-batch reactors using zeolite beta with controlled intrinsic properties

Environmental problems are worsening due to the complexity in managing plastic waste. Chemical recycling emerges as a pivotal technology that can suppress carbon introduction into the carbon cycle and provide petroleum alternatives for current petrochemical processes. The utilization of zeolites can reduce energy consumption by lowering the operation temperature for pyrolysis. Here, we demonstrate low-temperature catalytic cracking of polyethylene (PE) utilizing an open-batch reactor configuration and *BEA-type zeolite catalysts. With the optimized open-batch setup and zeolites, high PE conversion (~80%) and liquid selectivity (~70%) were achieved at 330 °C. We systematically explored the effects of aluminum (Al) site density and crystal size, revealing that zeolite crystal size is another critical factor determining the liquid production. This work not only demonstrates that an effective combination and optimization of reactor and catalysts can enhance the overall catalytic activity but also offers insights into designing catalysis systems for effective recycling of polyolefin wastes.

Upcycling polyolefins to methane-free liquid fuel by a Ru1-ZrO2 catalyst

Upcycling waste plastics into liquid fuels presents significant potential for advancing the circular economy but is hindered by poor selectivity and low-value methane byproduct formation. In this work, we report that atomic Ru-doped ZrO2 can selectively convert 100 grams of post-consumer polyethylene and polypropylene, yielding 85 mL of liquid in a solvent-free hydrocracking. The liquid (C5-C20) comprises ~70% jet-fuel-ranged branched hydrocarbons (C8-C16), while the gas product is liquefied-petroleum-gas (C3-C6) without methane and ethane. We found that the atomic Ru dopant in the Ru-O-Zr moiety functionalizes its neighboring O atom, originally inert, to create a Brønsted acid site. This Brønsted acid site, rather than the atomic Ru dopant itself, selectively governs the internal C-C bond cleavage in polyolefins through a carbonium ion mechanism, thereby enhancing the yield of jet-fuel-ranged hydrocarbons and suppressing methane formation. This oxide modulation strategy provides a paradigm shift in catalyst design for hydrocracking waste plastics and holds potential for a broad spectrum of applications.

Emerging Biochemical Conversion for Plastic Waste Management: A Review

In recent years, vast amounts of plastic waste have been released into the environment worldwide, posing a severe threat to human health and ecosystems. Despite the partial success of traditional plastic waste management technologies, their limitations underscore the need for innovative approaches. This review provides a comprehensive overview of recent advancements in chemical and biological technologies for converting and utilizing plastic waste. Key topics include the technical parameters, characteristics, processes, and reaction mechanisms underlying these emerging technologies. Additionally, the review highlights the importance of conducting economic analyses and life cycle assessments of these emerging technologies, offering valuable insights and establishing a robust foundation for future research. By leveraging the literature from the last five years, this review explores innovative chemical approaches, such as hydrolysis, hydrogenolysis, alcoholysis, ammonolysis, pyrolysis, and photolysis, which break down high-molecular-weight macromolecules into oligomers or small molecules by cracking or depolymerizing specific chemical groups within plastic molecules. It also examines innovative biological methods, including microbial enzymatic degradation, which employs microorganisms or enzymes to convert high-molecular-weight macromolecules into oligomers or small molecules through degradation and assimilation mechanisms. The review concludes by discussing future research directions focused on addressing the technological, economic, and scalability challenges of emerging plastic waste management technologies, with a strong commitment to promoting sustainable solutions and achieving lasting environmental impact.

Influence of ball milling parameters on the mechano-chemical conversion of polyolefins

Ball-milling of addition polymers such as polyolefins, polystyrene and polyacrylates can be used for depolymerization to obtain the respective monomers. However, absolute yields are typically low, especially from polyolefins which are notoriously difficult to depolymerize. To increase the viability of ball milling as a recycling technique, the effect of milling parameters on small hydrocarbon and monomer yields has to be understood. Herein, we systematically investigate the influence of sphere material, milling frequency, plastic filling degree, and milling temperature. Heavy spheres and high milling frequencies boost hydrocarbon yields by maximizing mechanical forces and frequency of collisions. While the dose of kinetic energy is commonly used to describe mechano-chemical processes, we found that it does not capture the mechano-chemical depolymerization of polyolefins. Instead, we rationalized the results based on the Zhurkov equation, a model developed for the thermo-mechanical scission of polymers under stress. In addition, low plastic filling degrees allow for high percentage yields, but cause significant wear on the grinding tools, prohibiting sustained milling. Milling below 40 °C is beneficial for brittle chain cleavage and depolymerization. This study provides a new approach to rationalize the influence of individual milling parameters and their interplay and serves as a starting point to derive design principles for larger-scale mechano-chemical depolymerization processes.

Depolymerization by transition metal complexes: strategic approaches to convert polymeric waste into feedstocks

At present, plastic pollution is a global environmental catastrophe and a major threat to mankind. Moreover, the increasing manufacture of various plastic products is causing rapid depletion of precious resources. Thus, transforming plastic waste into feedstock, which can maintain a circular economy, has emerged as a significant technique for waste management and carbon resource conservation. Furthermore, the urgent development of effective depolymerization methods is vital to save our planet from man-made plastic pollution. Among various chemical depolymerization techniques developed thus far, cleavage of the polymeric skeleton by transition metal complexes is a highly emerging, effective and exciting strategy. In this context, herein, we have summarized mechanistic approaches for cleaving various polymeric bonds using organometallic catalysts. The recently developed strategies, catalyst design and mechanisms for depolymerization of synthetic and natural polymers with polar (C-N, C-O, C-Cl, and Si-O) and non-polar (C-C) skeletal bonds are systematically discussed in detail.

Room Temperature Ethene to Propene (ETP) Tandem Catalysis using Single Crystalline Solid-State Molecular Pre-Catalysts

A tandem catalytic ensemble of solid-state molecular organometallic (SMOM) crystalline pre-catalysts are deployed under batch or flow conditions for the ethene to propene process (ETP). These catalysts operate at ambient temperature and low pressure, via sequential ethene dimerization, butenes isomerization and cross-metathesis. Under flow conditions the on-stream ethene conversion (55 %), initial propene selectivity (92 %), stability (71 % selectivity after 7 h) and low temperature/pressures are competitive with the best-in-class heterogeneous systems, marking a new, in crystallo, approach to ETP.

Highly selective upcycling of plastic mixture waste by microwave-assisted catalysis over Zn/b-ZnO

7 billion of 9.2 billion tons of plastic produced becomes waste while conventional catalytic plastic recycling methods are vulnerable with degraded performance and intensive energy input. Here, a hybrid Zn/b-ZnO catalyst, together with the specially-designed microwave reaction system, has achieved fast plastic waste upgrading under atmospheric pressure without using H2. Bifunctional ZnO acts as a microwave absorber and substrate catalyst, and in-situ formed Zn clusters promote C-C bond cleavage and nearly 100% upcycle landfilled plastic mixtures into lubricant base oil precursors and monomers. Unprecedented turnover number (250 gplastic g-1catalyst) of plastic depolymerisation and long-time stability over 50 successive cycles have been demonstrated, together with 8-time higher energy efficiency compared with conventional catalysis, indicating this strategy is an economical approach to efficient upcycling of plastics towards valuable products. Moreover, the catalyst can tolerate high contaminates, even the landfilled plastics can still be converted to lubricant base oil precursors, which has never been reported before.

Production of Branched Alkanes by Upcycling of Waste Polyethylene over Controlled Acid Sites of SO4/ZrO2-Al2O3 Catalyst

Branched alkanes, which enhance the octane number of gasoline, can be produced from waste polyethylene. However, achieving highly selective production of branched alkanes presents a significant challenge in the upcycling of waste polyethylene. Here, we report a one-pot process to convert polyethylene into gasoline-range hydrocarbons (C4-C13) with yield of 73.3 % over SO4/ZrO2-Al2O3 catalyst at 280 °C. The proportion of branched alkanes reaches 90.1 % within the C4-C13 fraction. Incorporation of sulfate group endows the catalyst with strong Lewis acid sites and weak and moderate Brønsted acid sites. In situ X-ray absorption, in situ infrared spectroscopy, in situ small angle neutron scattering, and DFT calculations reveal that polyethylene activation occurs through the synergy between sulfate groups and strong Lewis acid sites (Zr sites). The weak and moderate Brønsted acid sites preferentially catalyze the isomerization and type A β-scission processes, which favors the formation of branched alkanes, while suppressing competing reactions that produce straight-chain alkanes.

Integrated Ziegler-Natta/Brookhart-Ni Catalysts for the Synthesis of Sutured Polar High-Impact Polypropylenes

The direct synthesis of polar high-impact polypropylenes using industrially-preferred heterogeneous catalysts is challenging due to the poisoning of polar functional groups towards metal center and the high stereo-selectivity requirement. In this work, dual-site catalysts combining Ziegler-Natta and Brookhart-Ni catalysts were used to produce polar polyolefin ionomers, followed by polar high-impact polypropylenes containing isotactic polypropylene and branched polyethylene as toughening agents. Three combination modes between these catalysts were investigated, including mixed, core-shell, and integrated types. The integrated dual-site catalyst achieved the optimal material properties because the polyolefin ionomer acted as a suture molecule that stitched different components into a whole network. This produced sutured polar high-impact polypropylenes with excellent mechanical properties and compatibility with polar substances. The restraining effect of the suture molecules greatly reduced the release of microplastic particles after aging. Moreover, the obtained polar high-impact polypropylene can serve as an efficient compatibilizer to recycle polyethylene/polypropylene mixed-waste plastics. This work provides an appealing and potentially practical strategy to upgrade the widely used polypropylenes and to alleviate the ever-growing plastic pollution issue.

Microwave-Powered Liquid Metal Degradation of Polyolefins

Upcycling waste plastics is highly promising to tackle global white pollution while achieving sustainable development. However, prevailing approaches often encounter challenges in scalable engineering practices due to either insufficient plastic upcycling capability or arduousness in the separation, recovery, and purification of catalysts, which inevitably augments the cost of plastic upcycling. Here, the microwave-powered liquid metal synergetic depolymerization is presented to facilitate low-cost plastic upcycling. By leveraging the fluidity of liquid metals and their exceptional chemical-bond activation ability under microwave field, this method efficiently converts various polyolefins into narrowband hydrocarbon oil (Oil yield: 81 wt.% for polypropylene (PP), 85.9 wt.% for polyethylene (PE)) and high-value olefin monomers (C2-4  selectivity: 50% for PE, 65.3% for PP) over 30 successive cycles, resulting in a high turnover frequency of 2.83 kgPlastic mLLiquid metal -1. These captivating advantages offered by electromagnetically-powered liquid metals are also supported by their self-separation features, thereby paving the way for large-scale engineering solutions in waste plastic upcycling.

Task-Specific Deep Eutectic Solvent for Efficient Dissolution and Further Accelerating Alkaline Hydrolysis of Polyesters Into Their Monomers

Polyester plastics have brought great convenience to modern society. However, the continuous accumulation of their production increasingly threatens human health. Polyethylene terephthalate (PET) is one of the largest type of polyester plastics and its recycling is a major challenge. In this work, deep eutectic solvent (DES) composed of thenyl alcohol and choline chloride (ChCl) was designed for efficient dissolution of PET at 165 °C for 20 min, and further accelerating complete alkaline hydrolysis of PET into its monomer terephthalic acid (TPA) and ethylene glycol (EG) with a high TPA monomer yield (98.2 %) in 25 min at 100 °C. Moreover, the designed DES is also efficient for dissolution and alkaline hydrolysis of other polyester plastics, including poly(trimethylene terephthalate) (PTT) and poly(ethylene furanoate) (PEF) into their monomers. This work provides a feasible and sustainable solution for the recycling of polyester wastes.

Synthesis of Polyamides Bearing Directing Groups and Their Catalytic Depolymerization

We report a directing group (DG)-enabled strategy for polyamide depolymerization. Pyridine-based DGs selectively interact with In(III) catalysts, activating amide bonds for catalytic cleavage via alcoholysis. The process achieves efficient depolymerization of DG-introduced polyamides into recyclable monomers, providing a sustainable chemical recycling approach for robust polyamides.

Upcycling Polyethylene to High-Purity Hydrogen under Ambient Conditions via Mechanocatalysis

Polyethylene (PE) is the most abundant plastic waste, and its conversion to hydrogen (H2) offers a promising route for clean energy generation. However, PE decomposition typically requires high temperatures due to its strong chemical bonds, leading to significant carbon emissions and low H2 selectivity (theoretically less than 75 vol % after accounting for further steam-reforming reactions). Here, we report a mechanocatalytic strategy that upcycles PE into high-purity H2 (99.4 vol %) with an exceptional H2 recovery ratio of 98.5 % (versus 15.7 % via thermocatalysis), using manganese as a catalyst at a low temperature of 45 °C. This method achieves a reaction rate 3 orders of magnitude higher than thermocatalysis. The marked improvement in H2 recovery ratio is mainly due to metal carbides formation induced by the mechanocatalytic process, which does not catalyze hydrocarbons formation. This work is expected to advance studies of the conversion of polyolefins to high-purity H2 with net-zero carbon emissions.

Hydroxyl Spillover in Fe-Se Dual-Site Catalysts for Mixed Plastics Assay

The complex composition of real plastic wastes poses a significant challenge for their large-scale disposal. A responsive on-site compositional analysis of plastics is informative in choosing downstream processing methods. Nanocatalyst-based assay kit is highly qualified for this scene; however, there remain no efficient nanocatalysts for plastics due to their highly inert chemistry. Herein, we first unveiled the hydroxyl spillover effect in an Fe-Se dual-site catalyst (FeSe/NC) and devised a prototype colorimetric assay kit for mixed plastics. Experimental and theoretical results unveiled that Fe sites acted as the main active sites for H2O2 activation to produce adsorbed hydroxyl (*OH) intermediates, which subsequently desorb as hydroxyl radicals (•OH) and transfer to Se sites, supports, and even plastics for further catalysis. Specifically, •OH transferred to different plastics shows varying activities, where signal outputs were hereby used as the fingerprint for plastic identification. Moreover, the remaining *OH could respond to redox interferences in the samples for enhanced accuracy. In contrast to traditional techniques involving precise apparatus and complex pretreatments, our approach enables a rapid assay (∼10 min) of raw powdery mixed plastic wastes with an ultralow cost (0.0012 $). This discovery fills a crucial gap in the plastic assay, offering new technical guidance for diverse upcycling and recycling strategies to tackle the global plastic waste crisis.

Polyethylene Recycling via Water Activation by Ball Milling

Polyethylene (PE) is the most prevalent type of plastic waste and also the most challenging to depolymerize because of its inert carbon-carbon (C-C) bonds.[1] High temperature and noble metals are usually required for depolymerization.[2] To avoid using noble metals, costly reagents and harsh reaction conditions, it is worthwhile but challenging to explore new reaction pathways.[3] We report an unprecedented mechanochemical reaction of PE and water to result in shorter-chain alkanes, alkenes, alcohols, and ketones (Cn, where n≲50), with above 80 % of starting carbon converted into these products, which could be a valuable feedstock for re-entering chemical value chains. This reaction is driven solely by ball milling, without heating and pressurizing. No costly catalysts are used. Instead, only earth-abundant Al2O3 was milled with reactants.

Chemical transformation of polyurethane into valuable polymers

Polyurethanes are an important class of synthetic polymers, widely used in a variety of applications ranging from everyday items to advanced tools in societal infrastructure. Their inherent cross-linked structure imparts exceptional durability and flexibility, yet this also complicates their degradation and recycling. Here we report a heterogeneous catalytic process that combines methanolysis and hydrogenation with a CO2/H2 reaction medium, effectively breaking down PU waste consisting of urethane and ester bonds into valuable intermediates like aromatic diamines and lactones. These intermediates are then converted into functional polymers: polyimide (PI), noted for its exceptional thermal and electrical insulation, and polylactone (P(BL-co-CL)), a biodegradable alternative to traditional plastics. Both polymers exhibit enhanced performance compared to existing commercial products. This approach not only contributes to the valorization of plastic waste but also opens new avenues for the creation of high-performance materials.

Polyethylene hydrogenolysis by dilute RuPt alloy to achieve H2-pressure-independent low methane selectivity

Chemical recycling of plastic waste could reduce its environmental impact and create a more sustainable society. Hydrogenolysis is a viable method for polyolefin valorization but typically requires high hydrogen pressures to minimize methane production. Here, we circumvent this stringent requirement using dilute RuPt alloy to suppress the undesired terminal C-C scission under hydrogen-lean conditions. Spectroscopic studies reveal that PE adsorption takes place on both Ru and Pt sites, yet the C-C bond cleavage proceeds faster on Ru site, which helps avoid successive terminal scission of the in situ-generated reactive intermediates due to the lack of a neighboring Ru site. Different from previous research, this method of suppressing methane generation is independent of H2 pressure, and PE can be converted to fuels and waxes/lubricant base oils with only <3.2% methane even under ambient H2 pressure. This advantage would allow the integration of distributed, low-pressure hydrogen sources into the upstream of PE hydrogenolysis and provide a feasible solution to decentralized plastic upcycling.

Review of key issues and potential strategies in bio-degradation of polyolefins

Polyolefins are the most widely used plastic product and a major contributor to white pollution. Currently, studies on polyolefin degradation systems are mainly focused on microorganisms and some redox enzymes, and there is a serious black-box phenomenon. The use of polyolefin-degrading enzymes is limited because of the small number of enzymes; in addition, the catalytic efficiency of these enzymes is poor and their catalytic mechanism is unclear, which leads to the incomplete degradation of polyolefins to produce microplastics. In this review, three questions are addressed: the generation and degradation of action targets that promote the degradation of polyolefins, the different modes by which enzymes bind substrates and their application scenarios, and possible multienzyme systems in a unified system. This review will be valuable for mining or modifying polyolefin degradation enzymes and constructing polyolefins degradation systems and may provide novel ideas and opportunities for polyolefin degradation.

Selective Upcycling of Polyolefins into High-Value Nitrogenated Chemicals

The selective upcycling of polyolefins to create products of increased value has emerged as an innovative approach to carbon resource stewardship, drawing significant scientific and industrial interest. Although recent advancements in recycling technology have facilitated the direct conversion of polyolefins to hydrocarbons or oxygenated compounds, the synthesis of nitrogenated compounds from such waste polyolefins has not yet been disclosed. Herein, we demonstrate a novel approach for the upcycling of waste polyolefins by efficiently transforming a range of postconsumer plastic products into nitriles and amides. This process leverages the catalytic properties of manganese dioxide in combination with an inexpensive nitrogen source, urea, to make it both practical and economically viable. Our approach not only opens new avenues for the creation of nitrogenated chemicals from polyolefin waste but also underscores the critical importance of recycling and valorizing carbon resources originally derived from fossil fuels. This study provides a new upcycling strategy for the sustainable conversion of waste polyolefins.

Electrochemical Postpolymerization Modification and Deconstruction of Macromolecules

Electrolysis is an emerging approach to polymer postpolymerization modification, deconstruction, and depolymerization. Electrochemical reactions are particularly appealing for macromolecular transformations because of their high selectivity, ability to be externally monitored, and intrinsic scalability. Despite these desirable features and the recent resurgent use of small-molecule electrochemical reactions, the development of macromolecular electrolysis has been limited. Herein, we highlight recent examples of polymer transformations driven by heterogeneous redox chemistry. Throughout our exploration of macromolecular electrolysis, we provide our perspective on opportunities for continued investigation in this nascent field. Specifically, we highlight how targeted reaction development through deeper mechanistic insight will expand the scope of materials that can be (de)constructed with electrochemical methods. As this insight is developed, we expect macromolecular electrolysis to emerge as a high-functioning and complementary tool for macromolecular functionalization and deconstruction.

Installing a Trigger to Upcycle High-Density Polyethylene

Creating C═C bonds as "weak" sites in the stable C-C chains of polyethylene (PE) is an appealing strategy to promote sustainable development of the polyolefin industry. Compared to methods, such as dehydrogenation and postpolymerization modification, the copolymerization of ethylene (E) and butadiene (BD) should be a convenient and direct approach to introduce C═C bonds in PE, whereas it encounters problems in controlling the composition and regularity of the copolymer due to the mismatched activities and mechanisms between the two monomers. Herein, we report by employing the amidinate gadolinium complex, controllable E/BD copolymerization was achieved, where BD was incorporated in the uniformly discrete 1,4 mode. The obtained copolymer possesses the same physical, mechanical, processing, and antioxygen (aging at 100 °C for 28 days) properties as commercial high-density-PE, which, strikingly, were degraded by C═C bonds into α,ω-telechelic oligomers with narrow distribution. These degraded functional products were transferred to compatibilizers via atom-transfer radical polymerization or immortal ring-opening polymerization, achieving upcycling.

Controlled degradation of chemically stable poly(aryl ethers) via directing group-assisted catalysis

To establish a sustainable society, the development of polymer materials capable of reverting into monomers on demand is crucial. Traditional methods rely on breaking labile bonds such as esters in the main chain, which limits applicability to polymers that consist of robust covalent bonds. We found that the integration of directing groups allowed the engineering of resilient polymers with built-in recyclability. Our study showcases phenylene ether-based polymers fortified with directing groups, which can be selectively disassembled under nickel catalysts via selective cleavage of carbon-oxygen bonds. Notably, these polymers exhibit exceptional chemical stability towards acids, bases, and oxidizing agents, while being degradable to well-defined, repolymerizable molecules in the presence of a catalyst. Our findings allow for the development of next-generation polymer materials that are chemically recyclable by design.

Closed-loop recyclable polymers: from monomer and polymer design to the polymerization-depolymerization cycle

The extensive utilization of plastic, as a symbol of modern technological society, has consumed enormous amounts of finite and non-renewable fossil resources and produced huge amounts of plastic wastes in the land or ocean, and thus recycling and reuse of the plastic wastes have great ecological and economic benefits. Closed-loop recyclable polymers with inherent recyclability can be readily depolymerized into monomers with high selectivity and purity and repolymerized into polymers with the same performance. They are deemed to be the next generation of recyclable polymers and have captured great and increasing attention from academia and industry. Herein, we provide an overview of readily closed-loop recyclable polymers based on monomer and polymer design and no-other-reactant-involved reversible ring-opening and addition polymerization reactions. The state-of-the-art of circular polymers is separately summarized and discussed based on different monomers, including lactones, thiolactones, cyclic carbonates, hindered olefins, cycloolefins, thermally labile olefin comonomers, cyclic disulfides, cyclic (dithio) acetals, lactams, Diels-Alder addition monomers, Michael addition monomers, anhydride-secondary amide monomers, and cyclic anhydride-aldehyde monomers, and polymers with activatable end groups. The polymerization and depolymerization mechanisms are clearly disclosed, and the evolution of the monomer structure, the polymerization and depolymerization conditions, the corresponding polymerization yield, molecular weight, performance of the polymers, monomer recovery, and depolymerization equipment are also systematically summarized and discussed. Furthermore, the challenges and future prospects are also highlighted.

Surface-Activated Mechano-Catalysis for Ambient Conversion of Plastic Waste

Improved recycling technologies can offer sustainable end-of-life options for plastic waste. While polyolefins can be converted into small hydrocarbons over acid catalysts at high temperatures, we demonstrate an alternative mechano-catalytic strategy at ambient conditions. The mechanism is fundamentally different from classical acidity-driven high-temperature approaches, exploiting mechanochemically generated radical intermediates. Surface activation of zirconia grinding spheres creates redox active surface sites directly at the point of mechanical energy input. This allows control over mechano-radical reactivity, while powder catalysts are not active. Optimized milling parameters enable the formation of 45% C1-10 hydrocarbons from polypropylene within 1 h at ambient temperature. While mechanochemical bond scission is undesired in plastic production, we show that it can also be exploited for chemical recycling.

Polyolefin waste to light olefins with ethylene and base-metal heterogeneous catalysts

The selective conversion of polyethylene (PE), polypropylene (PP), and mixtures of these two polymers to form products with high volume demand is urgently needed because current methods suffer from low selectivity, produce large quantities of greenhouse gases, or rely on expensive, single-use catalysts. The isomerizing ethenolysis of unsaturated polyolefins could be an energetically and environmentally viable route to propylene and isobutylene; however, noble-metal homogeneous catalysts and an unsaturated polyolefin are currently required and the process has been limited to PE. We show that the simple combination of tungsten oxide on silica and sodium on gamma-alumina transforms PE, PP, or a mixture of the two, including postconsumer forms of these materials, to propylene or a mixture of propylene and isobutylene in greater than 90% yield at 320°C without the need for dehydrogenation of the starting polyolefins.

Recovering Valuable Chemicals from Polypropylene Waste via a Mild Catalyst-Free Hydrothermal Process

Waste polypropylene (PP) presents a significant environmental challenge, owing to its refractory nature and inert C-C backbone. In this study, we introduce a practical chemical recovery strategy from PP waste using a mild catalyst-free hydrothermal treatment (HT). The treatment converts 64.1% of the processed PP into dissolved organic products within 2 h in an air atmosphere at 160 °C. Higher temperatures increase the PP conversion efficiency. Distinct electron absorption and emission characteristics of the products are identified by spectral analysis. Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) reveals the oxidative cracking of PP into shorter-chain homologues (10-50 carbon atoms) containing carboxylic and carbonyl groups. Density functional theory (DFT) calculations support a reaction pathway involving thermal C-H oxidation at the tertiary carbon sites in the polymer chain. The addition of 1% H2O2 further enhances the oxidation reaction to produce valuable short-chain acetic acids, enabling gram-scale recycling of both pure PP and disposable surgical masks from the real world. Techno-economic analysis (TEA) and environmental life cycle costing (E-LCC) analysis suggest that this hydrothermal oxidation recovery technology is financially viable, which shows significant potential in tackling the ongoing plastic pollution crisis and advancing plastic treatment methodologies toward a circular economy paradigm.

Advances in Nonreactive Polymer Compatibilizers for Commodity Polyolefin Blends

Recycling mixed polyolefin plastics is a significant challenge due to the limitations in sorting and degraded mechanical properties of blends. Nonreactive compatibilization by adding a small amount of polymeric additive is a widespread approach to restoring the performance and value of recycled plastics. Over the past several decades, synthetic advances have enabled access to low-cost copolymers and precision architectures for deepening the understanding of compatibilization mechanisms in semicrystalline polyolefins. This review covers the design parameters of a polymeric compatibilizer, the testing of blends, the synthetic methods of producing economically viable additives, and surveys the literature of blends of compatibilized HDPE, LLDPE, LDPE, and iPP. From this, readers should gain a comprehension of the polymer mechanics, synthesis, and macromolecular engineering of processable polyolefin blends, along with the field's future directions.

Catalytic Upcycling of Polyolefins

The large production volumes of commodity polyolefins (specifically, polyethylene, polypropylene, polystyrene, and poly(vinyl chloride)), in conjunction with their low unit values and multitude of short-term uses, have resulted in a significant and pressing waste management challenge. Only a small fraction of these polyolefins is currently mechanically recycled, with the rest being incinerated, accumulating in landfills, or leaking into the natural environment. Since polyolefins are energy-rich materials, there is considerable interest in recouping some of their chemical value while simultaneously motivating more responsible end-of-life management. An emerging strategy is catalytic depolymerization, in which a portion of the C-C bonds in the polyolefin backbone is broken with the assistance of a catalyst and, in some cases, additional small molecule reagents. When the products are small molecules or materials with higher value in their own right, or as chemical feedstocks, the process is called upcycling. This review summarizes recent progress for four major catalytic upcycling strategies: hydrogenolysis, (hydro)cracking, tandem processes involving metathesis, and selective oxidation. Key considerations include macromolecular reaction mechanisms relative to small molecule mechanisms, catalyst design for macromolecular transformations, and the effect of process conditions on product selectivity. Metrics for describing polyolefin upcycling are critically evaluated, and an outlook for future advances is described.

A Merger of Relay Catalysis with Dynamic Kinetic Resolution Enables Enantioselective β-C(sp3)-H Arylation of Alcohols

The conceptual merger of relay catalysis with dynamic kinetic resolution strategy is reported to enable regio- and enantioselective C(sp3)-H bond arylation of aliphatic alcohols, forming enantioenriched β-aryl alcohols typically with >90 : 10 enantiomeric ratios (up to 98 : 2 er) and 36-74 % yields. The starting materials bearing neighbouring stereogenic centres can be converted to either diastereomer of the β-aryl alcohol products, with >85 : 15 diastereomeric ratios determined by the catalysts. The reactions occur under mild conditions, ensuring broad compatibility, and involve readily available aryl bromides, an inorganic base, and commercial Ru- and Pd-complexes. Mechanistic experiments support the envisioned mechanism of the transformation occurring through a network of regio- and stereoselective processes operated by a coherent Ru/Pd-dual catalytic system.

Closed-Loop Recycling of Mixed Plastics of Polyester and CO2-Based Polycarbonate to a Single Monomer

Physical blending is an effective strategy for tailoring polymeric materials to specific application requirements. However, physically blended mixed plastics waste adds additional barriers in mechanical or chemical recycling. This difficulty arises from the intricate requirement for meticulous sorting and separation of the various polymers in the inherent incompatibility of mixed polymers during recycling. To overcome this impediment, this work furthers the emerging single-monomer - multiple-materials approach through the design of a bifunctional monomer that can not only orthogonally polymerize into two different types of polymers - specifically lactone-based polyester and CO2-based polycarbonate - but the resultant polymers and their mixture can also be depolymerized back to the single, original monomer when facilitated by catalysis. Specifically, the lactone/epoxide hybrid bifunctional monomer (BiLO) undergoes ring-opening polymerization through the lactone manifold to produce polyester, PE(BiLO), and is also applied to ring-opening copolymerization with CO2, via the epoxide manifold, to yield polycarbonate, PC(BiLO). Remarkably, a one-pot recycling process of a BiLO-derived PE/PC blend back to the constituent monomer BiLO in >99 % selectivity was achieved with a superbase catalyst at 150 °C, thereby effectively obviating the requirement for sorting and separation typically required for recycling of mixed polymers.

Industrial and Laboratory Technologies for the Chemical Recycling of Plastic Waste

Synthetic polymers play an indispensable role in modern society, finding applications across various sectors ranging from packaging, textiles, and consumer products to construction, electronics, and industrial machinery. Commodity plastics are cheap to produce, widely available, and versatile to meet diverse application needs. As a result, millions of metric tons of plastics are manufactured annually. However, current approaches for the chemical recycling of postconsumer plastic waste, primarily based on pyrolysis, lag in efficiency compared to their production methods. In recent years, significant research has focused on developing milder, economically viable methods for the chemical recycling of commodity plastics, which involves converting plastic waste back into monomers or transforming it into other valuable chemicals. This Perspective examines both industrial and cutting-edge laboratory-scale methods contributing to recent advancements in the field of chemical recycling.