Precise Structural Design for Chemically Recyclable Polymers

Abstract

The global production and consumption of plastics generate vast amounts of waste, creating a pressing challenge for sustainable management. Varieties of solutions have been developed to convert discarded plastics into valuable resources, which can be primarily categorized into mechanical and chemical approaches. This review focuses on chemically recyclable polymers, with a particular emphasis on recent progress in precise chemical structural design, aiming at enabling or enhancing the recyclability of polymers. We first examine strategies for endowing linear polymers with recyclability, including the incorporation of renewable linkages, the redesign of monomers with tailored recyclability, and the chemical transformation of otherwise unrecyclable polymers. Precision chemistry provides a versatile toolkit to balance the often-conflicting requirements of material stability and recyclability. At last, we discuss recycling strategies for traditionally nonrecyclable polymer networks, with particular attention to covalent adaptable networks (CANs) that exploit dynamic covalent bonds.

1. Introduction

Plastic pollution has been a persistent issue for nearly six decades and is expected to endure as long as the materials themselves. A striking example is the recent discovery of massive plastic accumulation in the North Atlantic, which has drawn both public and scientific concern , and reflects the broader crisis of marine plastic pollution. At the same time, global plastic waste generation reached 353 million tonnes (Mt) in 2019, yet only 9% was recycled. This tremendous imbalance highlights the urgency to increase the efficiency of polymer waste recycling to construct a more sustainable and circular society. Conventional recycling methods for plastic wastes, mostly mechanical recycling, fall short of meeting the scale and complexity of today’s plastic waste challenge. Mechanical recycling relies on physical reprocessing, such as melting and remolding, and inevitably causes polymer chain degradation, contamination, and downcycling into materials of lower quality. Unlike the inefficient and often uneconomical mechanical recycling route, chemical recycling offers a transformative alternative: polymers can be depolymerized into their constituent monomers or converted to other valuable chemicals, enabling true closed-loop recycling or even upcycling into high-value chemicals.

Precision chemistry is an emerging paradigm that emphasizes exact control over chemical structures and processes. And, as articulated by Bernard L. Feringa, a Nobel laureate of 2016, precision chemistry represents “the art of building small”constructing complex functional systems from molecular-level building blocks with foresight and precision. This philosophy, when applied to polymer science, translates to the deliberate design of monomer structures and polymer architectures. By enabling the rational design of polymer structures at the molecular level, precision chemistry allows for the development of materials that can be easily depolymerized or reprocessed without compromising their functional properties. It is poised to redefine the landscape of polymer recycling by programming their lifecycle, including their degradation and reassembly. Specifically, introduction of deliberately selected linkages to the proper position, which only broke under certain conditions, can impart the recyclability to polymer materials without affecting their functionality and durability. In addition, precision chemical design facilitates fine-tuning of monomer structures, where even subtle modifications, such as the addition of a single methyl group, can dramatically influence depolymerization behavior and unlock the recyclability. Overall, precise chemical design opens new possibilities for creating sustainable materials that can maintain performance and functionality.

This review highlights precise structural design strategies for developing next-generation recyclable polymers, examining both the underlying principles and the recent progress. Our discussion centers on two primary classes: recyclable linear polymers and cross-linked ones. For linear polymers, recyclability can be achieved by the incorporation of chemically cleavable linkages, the redesign of monomers to permit appropriate depolymerization, or the structural transformation of polymers into recyclable ones. In the case of cross-linked polymers, while some strategies parallel those used for linear systems, some of them rely on the integration of dynamic covalent bonds. The dynamic bonds allow polymer networks to undergo a distinctive transition of polymer networks from viscoelastic solids to viscoelastic liquids under appropriate conditions, thereby conferring reprocessability to permanently cross-linked materials.

This review involves several terms related to polymer recycling, including chemical recyclability, degradation, depolymerization, closed-loop recycling, and reprocessability. These terms appear throughout the manuscript, and although closely related, they describe distinct concepts. To avoid confusion, we clarify them here at the outset.

Chemical recyclability describes the capability of polymers to be converted through chemical processes into monomers or other valuable chemicals that can be reused.

Degradation broadly refers to the breakdown of polymer chains into smaller fragments and does not necessarily imply the recovery of useful products.

Depolymerization is a specific type of degradation in which polymers are converted back to their original monomers, typically via reversal of the polymerization pathway.

Closed-loop recycling represents the ideal case of chemical recyclability, in which depolymerization regenerates the original monomers or macromonomers with high fidelity, allowing repeated polymerization–depolymerization cycles without significant loss of performance.

In contrast to the chemical recyclability, reprocessability refers to the ability of polymer materials to be reshaped or remolded without destroying their structure, often through dynamic covalent chemistry.

2. Precise Structural Design for Chemically Recyclable Linear Polymers

Linear polymers account for the vast majority of plastics encountered in daily life, including polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), polyethylene terephthalate (PET), and polyamides such as Nylon. Their widespread use is attributed to an optimal combination of exceptional mechanical strength, thermal stability, and processability. Yet, these very stabilities create a challenge for sustainability: most linear polymers are highly resistant to degradation and poorly compatible with existing recycling technologies. Addressing this paradoxpreserving material performance while enabling circularityremains a central goal in polymer science. In response, researchers have developed a broad spectrum of molecular strategies to render these materials recyclable or to design structurally related analogues with intrinsic recyclability. These efforts can be conceptually grouped into three complementary pathways, each representing a distinct approach toward truly sustainable polymeric materials (Figure ).

1.

Fundamental strategies for designing recyclable linear polymers. Path A: Incorporating renewable linkages into polymer backbones. Path B: Redesigning monomers with proper depolymerizability. Path C: Transforming polymers into recyclable ones.

2.1. Incorporating Renewable Linkages into Polymer Backbones

Polymers such as PE and PS present considerable challenges for chemical recycling due to their highly stable all-carbon backbones. Recently, promising strategies employing metal catalysts have been developed to break down these materials into value-added products via upcycling processes. − For example, the upcycling of PS has achieved a carbon recovery of approximately 60%, producing industrially relevant chemicals, such as benzoic acid and formic acid. By contrast, the upcycling efficiency for PEthe most widely used polymerremains low, with a carbon recovery rate of only about 10%. These disparities underscore the urgent need for more efficient and scalable methods to chemically recycle these persistent polyolefins.

A promising solution lies in the introduction of renewable chemical bonds along the polymer backbones. Such bonds, including esters, carbonates, and other cleavable linkages, are designed to be both synthetically accessible and selectively degradable under mild conditions, enabling controlled depolymerization and upcycling. Among renewable chemical linkages, the ester bond is perhaps the most widely recognized and utilized. For example, the ester bond can be readily formed through the condensation of carboxylic acids and alcohols, and easily cleaved via acid- or base-catalyzed hydrolysis. Due to their synthetic accessibility and cleavability, ester bonds have been extensively incorporated into the design of chemically recyclable polymers, particularly in efforts to develop polyolefin-like materials with enhanced sustainability.

A straightforward strategy to incorporate ester bonds into polymer chains is through the polycondensation of α-ω dicarboxylic acids or diesters with α-ω diols. For example, Mecking and colleagues synthesized a biorefinery-sourced linear diester and diol containing octadecyl units, and obtained an ester-containing polyethylene-like polymer (PE-18,18) via polycondensation of stoichiometric amounts of the diester and diol catalyzed by titanium-alkoxide (Figure a). Wide-angle X-ray scattering (WAXS) diffractograms of the obtained material closely resembled those of commercial high-density polyethylene (HDPE), indicating a similar crystallization behavior. Although the melting point of PE-18,18 (100 °C) was lower than that of HDPE (130 °C) due to the incorporation of in-chain functional groups within the crystalline lamellae, its thermal stability and mechanical properties remained comparable to commercial HDPE. Importantly, PE-18,18 can be efficiently depolymerized back to the original α-ω difunctional monomers by heating in methanol in the presence of a basic catalyst, achieving >99% conversion. These monomers can then be directly repolymerized to regenerate PE-18,18, without any loss in mechanical performance, demonstrating a fully closed-loop recyclable system.

2.

Methodologies to incorporate ester bonds in polymer chains. (a) Direct polycondensation of α-ω diesters with α-ω diols. (b) Olefin metathesis of unsaturated polymers with 2-hydroxyethyl acrylate (HEA). (c) Ring-opening-insertion metathesis polymerization (ROIMP) of cyclooctene (COE) was performed with diacrylates. (d) Copolymerization of linear diacrylates with carbonyl sulfide (COS).

Another intriguing route to obtain α-ω difunctional monomers for constructing ester-embedded polymers was reported by Coates and colleagues (Figure b). They initially employed bridged biphenyl-phenoxide-hafnium catalysts to copolymerize propylene with butadiene, producing isotactic copolymers with high molecular weights. Unsaturated double bonds were precisely incorporated into the backbone through the preferential 1,4-insertion of butadiene. The creative placement of double bonds was crucial, as they served as reactive handles for subsequent functionalization through olefin metathesis with 2-hydroxyethyl acrylate (HEA). After olefin metathesis, PP-backbone telechelic diesters were prepared. Remaining double bonds were fully saturated via hydrogenation. Subsequent polymerization of the alcohol-terminated telechelic macromonomers using Ti­(O ⁿ Bu)₄ as a catalyst produced a PP-like polymer with thermal (T _m = 97.4 °C) and mechanical properties (yield points at approximately 7 MPa) comparable to those of linear low-density polyethylene (LLDPE). Importantly, this ester-containing PP-like polymer also exhibited good recyclability.

Ring-opening-insertion metathesis polymerization (ROIMP) is another powerful strategy for incorporating renewable linkages into polymer chains. Chen and colleagues developed a subclass of this approach, termed cyclic–acyclic monomers metathesis polymerization (CAMMP), and systematically explored its potential for accessing recyclable polymers (Figure c). In this method, diene monomers bearing various cleavable groups were copolymerized with cyclic olefins, allowing the precise incorporation of degradable units into the polymer backbone. By tuning of the chemical structure of the diene comonomers, the backbone properties and degradability can be systematically controlled. For instance, CAMMP of diacrylates with cyclooctene (COE), followed by hydrogenation of the resulting copolymers, yielded ester-incorporated, recyclable PE-like polymers. The authors highlighted the versatility of ROIMP/CAMMP as a general platform for designing polymers with tailored structures and built-in recyclability.

A more straightforward method to ester-embedded PE-like polymers was reported by Zhang and colleagues. They carried out a cascade polymerization of linear diacrylates with carbonyl sulfide (COS) in the presence of water, successfully producing a series of polymers containing in-chain thioether and ester groups (Figure d). When the spacer between the two acrylate groups was sufficiently long (e.g., x = 7), the resulting polymer exhibited mechanical properties comparable to HDPE, with a tensile strength of roughly 27 MPa and elongation at break around 480%. However, its melting point was relatively low due to the incorporation of in-chain polar groups, consistent with observations by Mecking and co-workers as described above. The recyclability of these polymers was confirmed through base-catalyzed methanolysis and Ti­(O ⁿ Bu)₄-catalyzed repolymerization.

Although the well-established strategy of polycondensation of diester or dicarboxylic acid with diol provides a convenient approach to recyclable polyolefin-like polymers, it faces a fundamental stoichiometry problem. Traditional step-growth polymerizations require precise matching of complementary functional groups to achieve high molecular weights, and even minor deviations can significantly reduce molecular weights, compromising the thermal and mechanical properties of the resulting polymers. Addressing the stoichiometric issue is therefore crucial for developing high-performance recyclable polymers with high molecular weights.

To overcome this limitation, Miyake and colleagues employed a ruthenium-catalyzed symmetrical condensation reaction between hydroxyl groups, eliminating the need for precise stoichiometry (Figure ). Telechelic diol-functionalized hard and soft oligomeric blocks were prepared by Ring-opening metathesis polymerization (ROMP) of COE or 3-hexylcyclooctene, respectively, using an unsaturated long-chain diolcis-hexadec-6-ene-1,16-diolas a chain-transfer agent (CTA). The hydroxyl-terminated oligomers were subsequently hydrogenated to generate diol macromonomers. By using Ru-catalyzed dehydrogenation polymerization, hydroxyl groups at the chain ends condense with each other to form ester linkages. A series of block copolymers with varying fractions of soft blocks were obtained, with molecular weights ranging from 62.7 to 90.4 kDa, which is comparable to commercial HDPE and LLDPE. The resulting multiblock copolymers closely resembled commercial olefin block copolymers (OBC). It was found that the copolymer composed entirely of hard blocks displayed thermal and mechanical properties similar to HDPE, while copolymers polymerized at the feed ratio of 80% hard blocks exhibited thermal and mechanical properties similar to those of LLDPE. Importantly, all multiblock polymers could be depolymerized using the same Ru-catalyst applied in the polycondensation step, achieving a 91.7% isolated yield of monomers. The recovered macromonomers were successfully repolymerized into multiblock polymers for three cycles without a noticeable loss in mechanical performance. In a related study, the same group demonstrated a biomass-derived version of these diol-functionalized macromonomers, with recycling catalyzed by a manganese complex, offering a more cost-effective alternative to ruthenium-based catalysis.

3.

Synthesis and chemical recycling of polyolefin-like multiblock polymers. Reproduced with permission from ref . Copyright 2023, AAAS.

Hillmyer and colleagues reported a macromonomer synthetic method designed to ensure the stoichiometric equivalence between ester and hydroxyl groups (Figure a). They employed a telechelic CTA bearing a hydroxyl and ester group at each end. After ROMP with COE and subsequent hydrogenation, a mixture of diols, diesters, and AB-type telechelic macromonomers was obtained. In this approach, the feed ratio of hydroxyl to ester groups was exactly 1:1, as both functionalities were derived from the same precursor. High-molecular-weight PE-like polyesters (M_n around 100 kg·mol^–1) were then synthesized through multiple steps of transesterification. Their thermal and mechanical properties were competitive with those of HDPE. Additionally, these PE-like polyesters could be depolymerized back to the telechelic macromonomers and repolymerized into high-molecular-weight polyesters (M_n around 100 kg·mol^–1), demonstrating the excellent recyclability of this strategy.

4.

Preparation of recyclable polyolefin-like polymers without stoichiometric concerns. (a) Stoichiometric equivalence ensured by employing an AB-type telechelic CTA. (b) Stoichiometric equivalence was ensured by AB-type telechelic macromonomers. (c) Stoichiometric equivalence was ensured by ring-opening metathesis copolymerization of COE with unsaturated lactones.

Another effective strategy to circumvent the stoichiometric issue is the use of AB-type telechelic macromonomers, in which the ratio of functional groups is inherently fixed at 1:1. Tang and colleagues reported the synthesis of ester-linked OBC-like copolymers with relatively high molecular weights (M_w around 100 kg·mol^–1) (Figure b). In their system, coordination copolymerization of ethylene and α-octene was first performed using specially designed chain-transfer agents (R₂Zn) and quenching agents (R′Cl) for the generation of AB telechelic macromonomers with both ester and hydroxyl terminal groups. Recyclable olefin copolymers were synthesized by Ti­(O ⁿ Bu)₄-catalyzed polycondensation of the resulting AB telechelic macromonomers. The hardness of the macromonomers was tuned by varying the content of α-octene. And it was found that, with a 3.1% incorporation of α-octene, the resulting polyolefin-like polymers exhibited a Young’s modulus (214.2 MPa) nearly identical to that of commercial LLDPE (215.4 MPa). More notably, the OBC-like copolymers, consisting of hard blocks (with a lower α-octene content) and soft blocks (with a higher α-octene content), displayed enhanced hardness and tensile strength.

A complementary approach to AB-type telechelic macromonomers was developed by Jian and co-workers (Figure c). They employed the ring-opening metathesis copolymerization of COE with unsaturated lactones to generate ester-containing PE-like parent copolymers. A 16-membered diester ring (Ester-16), synthesized via dimerization ring-closing metathesis (RCM) of ester-functionalized 1,9-dienes, was selected as the comonomer due to its well-matched ring strain with COE, facilitating efficient ROMP. Subsequent hydrogenation and methanolysis of the copolymers produced AB-type telechelic macromonomers of distributed chain lengths. Polycondensation of these macromonomers yielded high-molecular-weight, recyclable PE-like polymers. Interestingly, these ester-linked copolymers exhibited higher melting points (133 °C) than previously reported ester-containing polyolefins, , likely due to differences in the distribution of ester linkages. In addition, they showed excellent mechanical performance, with tensile strengths around 32 MPa and elongations at break of approximately 1900%, comparable to HDPE.

Besides ester groups, many other renewable linkages, such as amide, acetal, and disulfide, can also undergo reversible bond disconnection and reconnection to enable closed-loop recycling. Compared with esters, these alternative linkages are less frequently adopted, largely due to trade-offs between stability and reactivity. Amides provide robust thermal and mechanical stability but typically require harsher conditions for depolymerization. Acetals and disulfides, on the other hand, offer more facile reversibility but may compromise long-term durability. However, these emerging linkages highlight the broader chemical toolbox available for designing recyclable polymers beyond esters.

The acetal bond represents a promising candidate for constructing recyclable polymers, as it can be readily cleaved under acidic or high-temperature conditions to regenerate the corresponding aldehydes and alcohols. Zhang and co-workers demonstrated that alternating copolymerization of aldehydes with cyclic anhydrides affords acetal-containing polyesters (Figure a). By varying the substituents on the aldehyde monomers, they synthesized around 60 distinct polyesters. Mechanical testing of selected examples revealed a tensile strength of approximately 3.4 MPa. Importantly, these polymers could be thermally depolymerized back to their starting monomers, completing a closed-loop cycle. Building on this approach, the same group later reported water-degradable acetal-based polymers via copolymerization of formaldehyde with cyclic anhydrides. More recently, Bowman and co-workers introduced an alternative route for incorporating acetal linkages, by copolymerizing diols with dichloromethane (DCM)a common industrial chemicalunder strongly basic conditions (Figure a). The resulting linear polymers were confirmed to be chemically recyclable, although their mechanical properties have not yet been investigated. More practically, the cationic ring-opening polymerization (CROP) of cyclic acetal monomers provides a straight route to the approach of degradable polyacetals. Coates and co-workers rationally selected an initiator and Lewis acid catalyst system to achieve precise molecular weight control over the CROP of 1,3-dioxolane, with M_n ranging from 30 to 1700 kDa. , In these works, the mechanical properties of the resulting polyacetals were systematically evaluated and found to be comparable to those of polyethylene.

5.

(a) Acetal-containing recyclable polymers. (b) Disulfide-containing recyclable polymers.

Disulfide bonds, which form via oxidation of two thiol groups and can be cleaved back to thiols under reductive conditions, , represent another class of renewable linkages for recyclable polymers. Johnson and co-workers recently exploited this reversible chemistry to synthesize a disulfide-containing, recyclable PS-like polymer (Figure b). Their strategy began with the preparation of a thioester-containing copolymer of styrene, obtained by copolymerization of styrene with the thionolactone, dibenzo­[c,e]-oxepine-5­(7H)-thione (DOT). , Innovatively, the copolymer was then subjected to the aminolysis of thioester groups with cysteamine, yielding dithiol-terminated PS oligomers. Subsequent oxidative coupling of the thiols furnished disulfide-linked PS-like polymers, whose thermal properties were tunable by the DOT content. For instance, with 2.5% DOT incorporation, the material exhibited thermal characteristics comparable to commercial PS, with a glass transition temperature, T _g = 103 °C, and a thermal decomposition temperature at 5% mass loss, T _d,5% = 360 °C (versus T _g = 107 °C and T _d,5% = 408 °C for PS). Importantly, the disulfide linkages enabled chemical recycling: the PS-like polymer could be depolymerized back into the original dithiol oligomers via redox cleavage.

These studies illustrate the tremendous potential of precise chemical design in the development of recyclable materials. By carefully selecting and positioning cleavable linkages, researchers have produced recyclable polymers with mechanical and thermal properties comparable to those of commercial plastics while allowing efficient closed-loop recycling.

2.2. Redesigning Monomers with Proper Depolymerizability

In a typical polymerization reaction, there exists a thermodynamic equilibrium between addition and detachment of monomers. The direction of this equilibrium can be predicted from the Gibbs free energy, ΔG, expressed as

$$\mathrm{\Delta }G=\mathrm{\Delta }H-T\mathrm{\Delta }S$$

where ΔH, ΔS, and T represent enthalpy change, entropy change, and reaction temperature, respectively. Most polymerization reactions are exothermic (ΔH < 0), as new covalent bonds are formed during chain growth. At the same time, polymerization decreases the entropy of the system (ΔS < 0), since free monomers are organized into a more ordered macromolecular structure. Consequently, a characteristic ceiling temperature (T _c) exists. When the reaction temperature is below T _c, ΔG < 0, and polymerization is thermodynamically favored, leading to polymer formation. Conversely, when the temperature exceeds T _c, ΔG > 0 and depolymerization becomes favored, driving polymers back to monomers. Importantly, T _c is highly related to the chemical composition and structure of the monomers. For example, the ceiling temperature of radical polymerization of α-methylstyrene is approximately 60 °C, , whereas that of styrene is 395 °C. From the perspective of chemical recycling, it is therefore crucial to design monomer structures with appropriately tuned T _c values, enabling polymerization and depolymerization under practicable conditions. In a comprehensive review, Chen et al. summarized four general design principles from the perspectives of thermodynamics and kinetics, the trade-offs between recyclability and performance, predictive modeling, and techno-economic analysis. The present review highlights more recent advances in molecular design strategies that achieve recyclability through precise control of the T _c of cyclic monomers.

2.2.1. Reversible Ring-Opening Polymerization (ROP)

ROP is a powerful chain-growth technique commonly applied to small cyclic monomers (Figure ). For small cyclic monomers containing three to eight numbers, the driving force for the monomer-polymer equilibrium arises from the ring strain stored in twisted cyclic structures. By precisely redesigning the structures of cyclic monomers, their intrinsic ring strain can be tuned to achieve an optimal balance between polymerization via ROP and depolymerization via ring-closing depolymerization (RCD). In this section, we describe recent advances in monomer design in the order of increasing ring strain.

6.

Illustration of reversible ring-opening polymerization. T _c: ceiling temperature.

For cyclic monomers with very low or negligible ring strain, such as γ-butyrolactone (γBL), the ΔH of polymerization is insufficient to overcome the relatively large negative ΔS. As a result, ROP of γBL is extremely challenging, with a calculated T _c of −136 °C at an initial monomer concentration of 1.0 M (if not specified, T _c described in this review is calculated at the concentration of 1.0 M), as reported by Chen and colleagues. This low polymerizability poses a significant obstacle to using such monomers as recyclable polymers. A well-established strategy to address this limitation is to raise T _c to a practical range by increasing the enthalpic contribution, which can be achieved by increasing the ring strains of γBL. Several successful redesigns of γBL as a circular monomer have been reported (Table ).

1. γBL and Its Recyclable Analogues.

In 2016, in addition to their work on polymerizing γBL at −40 °C at a high concentration of 10 M, Chen and colleagues explored increasing the polymerization temperature by introducing a methylene group into γBL, producing α-methylene-γ-butyrolactone (MBL). This modification slightly raised T _c, allowing polymerization to proceed at ambient temperature, a result later confirmed by Li and co-workers. Polymers of MBL could be fully depolymerized back to the monomer at 160 °C using a La-complex catalyst.

A more common strategy to increase T _c involves fusing an additional cyclic bridge on the γBL molecule. The resulting bicyclic structures twist the five-membered lactone ring away from its stable conformation, increasing ring strain and promoting polymerizability (Table ). For instance, six-membered rings were tried fused to γBL to form bicyclo[4.3.0]-based (C4γBL, TriL-H₂, 3.4-T6γBL, TriL) and bicyclo[3.2.1]-based (BiL, BiL⁼ ) compounds. Due to variations in intrinsic ring strain, the five-membered lactone rings became more strained, leading to elevated T _c values. Thus, ROP of these monomers could be conducted at moderate temperatures, producing polymers with reasonable molecular weights. In the meantime, their reversibility of ROPs was preserved, and the efficient monomer recovery (>80%) was realized under certain conditions, except for 4,5-T6γBL, of which ROP did not provide high molecular weight (10.9 kg·mol^–1), and the recyclability was not evaluated.

Even more constrained configurations were explored as early as 1971 by Hall and co-workers, who introduced a single-carbon bridge to form a bicyclo[2.1.1]-based γBL derivative, effectively increasing T _c. However, the recyclability of these early polymers was not demonstrated. More recently, Tang and colleagues designed two new monomers bearing either a phenyl (4Ph-BiL) or n-butyl group (4Bu-BiL), and systematically evaluated the thermal and mechanical properties of the resulting polymers. Poly­(4Ph-BiL) exhibited thermal and mechanical performance comparable to syndiotactic polystyrene, while poly­(4Bu-BiL) was competitive with LDPE. Importantly, nearly complete depolymerization to the original monomers (>98%) was achieved using La-complex catalysts at 250 and 260 °C for 4Ph-BiL and 4Bu-BiL, respectively, demonstrating excellent recyclability.

Similarly, nonstrained five-membered thiolactones and lactams , have been redesigned following principles analogous to those applied to γBL (Figure a). By introduction of cyclic bridges into the monomers, the ring strain was increased, enabling ring-opening polymerizations under mild conditions. As a result, recyclable polymers that were previously difficult to access could be efficiently prepared.

7.

Categories of small cyclic monomers with recyclability. (a, b) Five-membered rings. (c–g) Six-membered rings. (h–j) Seven-membered rings.

In 2022, Hong and colleagues reported a strategy termed isomerization-driven ROP (IROP), in which an S/O isomerization occurs concomitantly with the polymerization process (Figure b). Poly­(γ-thiobutyrolactone) (PTBL) was successfully synthesized by anionic IROP of γ-thionobutyrolactone (TnBL) in the presence of the phosphazene superbase ^t BU-P₄ as a catalyst and BnOH as an initiator. Direct ROP of the nonstrained γ-thiobutyrolactone (TBL) had previously proven challenging. Notably, the resulting PTBL could be completely depolymerized back to TBL within 15 s under TBD catalysis, demonstrating excellent recyclability. Building on this work, in 2023, the same group developed a cationic IROP approach using [Et₃O]⁺[B­(C₆F₅)₄]⁻ as initiator, successfully polymerizing γ-methyl-γ-thiobutyrolactonewhere one of the γ-position hydrogen of TnBL was replaced with a methyl groupthereby further expanding the library of sustainable, recyclable polymers.

For cyclic monomers with moderate ring strain, the enthalpic driving force of the ROP becomes comparable to the entropic penalty that disfavors polymerization. In such a case, fine-tuning either ΔH or ΔS can establish a favorable monomer–polymer equilibrium, effectively expanding the toolbox for monomer redesign and enabling access to recyclable polymers under practical conditions.

The six-membered lactone δ-valerolactone (δVL) possesses ring strain greater than that of γBL, making its ROP thermodynamically more favorable. T _c for δVL was calculated to be 298 °C, meaning that poly­(δVL) was difficult to be depolymerized under practical conditions. In contrast to γBL, structural modifications are needed to decrease T _c and enable effective depolymerization.

The first strategy involves introducing alkyl substituents on the rings (Figure c). Hillmyer and colleagues examined the effects of n-alkyl substituents on δ-position and found that the substituent location strongly influences the entropy change during ROP. In their opinion, alkyl groups at the δ-position restrict segmental rotation along the polymer chain, significantly decreasing the system entropy. The resulting larger entropic loss lowers the T _c, enabling depolymerization under moderate conditions. Recent designs of recyclable δVL analogues have confirmed this effect. −

The Second strategy is substitution at the β-position with heteroatoms, such as O, N, and S atoms − (Figure d). According to Zhang and co-workers, replacing carbon atoms at the β-position of δVL with S, N, or O reduced T _c to 176.2 °C, 71.5 and 42.9 °C, respectively. DFT calculations of energy-minimized ground-state conformations, using s-cis methyl acetate as a strain-free reference, showed that β-heteroatom substitution decreased angular strain and thus reduced the enthalpy contribution, while the entropy change remained largely unaffected, resulting in lower ceiling temperatures.

The third strategy involves adjusting bond angles to reduce the ring strain (Figure e). Li and colleagues demonstrated that introducing a methylene group at the α-position of δVL effectively decreased T _c to 93 °C. , The introduction of the methylene group is likely to optimize bond angles in the six-membered ring. Additionally, Chen and co-workers reported that geminal substitution at the α-position with alkyl groups further lowered T _c. Depending on the chain length of the alkyl substituents, gem-substituted δVL derivatives exhibited T _c values ranging from 67 to 115 °C, in sharp contrast to the much higher T _c of unsubstituted δVL (298 °C).

The reversibility of ROP of six-membered δ-thiovalerolactone (δTVL) was examined early in 1968, and it was found that, unlike its lactone analogue δVL, δTVL disfavors polymerization, reaching only 21% monomer conversion at 155 °C, indicative of a very low T _c. To enable δTVL-based recyclable polymers, the key challenge lies in making polymerization thermodynamically favorable and increasing the T _c. Gutekunst and co-workers systematically investigated this issue, calculating T _c values of various thiolactones and confirming that δTVL indeed has a very low T _c of −66 °C. They systematically explored the effects of sulfur substitution and methylation on the monomer–polymer equilibrium. Substitution of the β-carbon with a sulfur atom dramatically raised T _c from −66 to 34 °C, as the sulfur atom forced the six-membered ring into a boat conformation and increased ring straina result also corroborated by Wang’s group. Geminal substitution with two methyl groups at the α-position further intensified the ring strain in the boat conformation, increasing T _c to 58 °C. However, in Gutekunst’s work, recyclability was evaluated through van’t Hoff’s theoretical analysis rather than depolymerization experiments. Around the same time, Zhu and colleagues experimentally verified the recyclability of similar sulfur-substituted δTVL compounds bearing monosubstituents on the α-position. Bulk depolymerizations conducted at 180 °C yielded complete monomer recovery across all substitution patterns, as confirmed by ¹H NMR. More recently, Wang and co-workers reported that replacing the β-carbon of δTVL with an oxygen atom increased T _c only modestly to −50 °C. DFT optimizations suggested that the energy-minimized conformation of the oxygen-substituted δTVL is a twisted-boat conformation, which does not introduce sufficient ring strain compared to the sulfur-substituted analogue (Figure f).

The recyclability of several analogues of δTVL has also been systematically investigated by different research groups (Figure g). In 2021, Tao and co-workers reported that the cyclization of the six-membered thiolactide is thermodynamically favored. T _c of thiolactide monomer was calculated to be −67 and 197 °C for initial monomer concentrations of 1.0 and 5.0 M, respectively, indicating that polymerization can proceed at elevated temperatures with a concentrated condition, while depolymerization is facilitated under dilute conditions at moderate temperatures. Indeed, complete recovery of thiolactide monomer was achieved within 2 min at 25 °C via ring-closing depolymerization. Remarkably, the depolymerization exhibited high stereoselectivity, yielding 98% rac-thiolactides. This stereochemical fidelity of depolymerization contrasts with the behavior of polylactide and was attributed to the intrinsic thermodynamic stability of rac-thiolactide configuration. In a subsequent work, Tao’s group proposed a single-atom O-to-S substitution strategy that could reduce the ring strain of six-membered thiolactide. The resulting cyclic monothiol-lactide monomer exhibited good recyclability and a moderate T _c of 164.7 °C. The single sulfur substitution was also demonstrated to be an efficient strategy to enhance the recyclability of six-membered cyclic carbonate, as reported by Zhang and co-workers. In their study, replacing one oxygen atom with sulfur effectively reduced the ring stability and lowered the ceiling temperature, thereby enabling more favorable depolymerization under moderate conditions. T _c of single sulfur-substitution carbonate was calculated as 165 °C, while that of the unsubstituted cyclic carbonate was 212 °C. These findings highlight the potential of sulfur incorporation to fine-tune the thermodynamic polymerizability of cyclic monomers for sustainable polymers.

Seven-membered rings represent another important class of cyclic monomers with moderate ring strain. In 2016, Shaver and co-workers identified a promising family of recyclable monomers by fusing seven-membered lactones with benzene rings. , Since then, a variety of benzene-fused bicyclic monomers with diverse substitution patterns have been reported, all demonstrating good recyclability − (Figure h). Building on this, Zhu and co-workers developed stereoregular recyclable block copolymers via stereo- and sequence-controlled ROP. Extending this strategy to seven-membered cyclic carbonates, Zhu’s group systematically investigated bicyclic carbonates with different cis–trans configurations , (Figure i). They found that polymers derived from both cis- and trans-isomers could undergo selective depolymerization back to their corresponding dimers by using CH₃OK or ^t BuOK as catalysts. Furthermore, the dimers could repolymerize to regenerate the original polymers, demonstrating closed-loop recyclability. A similar mechanism was reported by Odelius and co-workers, who studied cyclic carbonates ranging in size from 7 to 13 members.

Beyond carbonates, ε-caprolactam (CL) is the classic seven-membered lactam whose ROP product, polyamide 6 (PA6), has been intensively investigated for chemical recycling due to its widespread use and associated environmental concerns. − However, direct depolymerization of PA6 requires high temperatures (>220 °C), due to the high melting point (220 °C). To address this limitation, Zhu et al. and Chen et al. independently redesigned CL by introducing geminal dimethyl substitutions (Figure j). They both focused on the effects of gem-substitution of two methyl groups on the recyclability of CL. In Zhu’s work, ROP of both α- and γ-gem-dimethyl substituted CL using potassium caprolactamate (CL-K) as the catalyst afforded polymers in good yields (>50%). Notably, the γ-gem-dimethyl substituted CL enabled efficient recyclability, achieving 91% monomer recovery within 4 h at 200 °C, whereas the α-substituted analogue showed only 35% recovery at 240 °C for a period of 5.5 h. Chen and co-workers expanded the scope by investigating the polymerizability of α-, β-, γ-, δ-, and ε-gem-dimethyl substituted CL using ^t BU-P₄ as the catalyst. Among these, only the γ- and δ-isomers underwent successful polymerization. Consistent with Zhu’s findings, the γ-isomer exhibited excellent recyclability (93% recovery at 180 °C in 6 h with ^t BuOK), while the δ-isomer was less efficient (81% recovery at 250 °C in 18 h). These results highlight the Thorpe–Ingold effect, wherein geminal dimethyl substitution at the γ-position adjusts the ring strain of CL to a favorable value, yielding practical ceiling temperatures of 27 and 13 °C, as calculated by Zhu and Chen, respectively.

Most recently, Zhu and co-workers adapted the benzene-fused strategy to redesign CL-based lactams (Figure j). This modification allowed the ceiling temperature of seven-membered lactams to be tuned into more practical ranges, for instance, T _c = 165 °C when a hexyl substituent was introduced. In addition, the resulting polymers not only demonstrated enhanced recyclability but also exhibited comparable thermal stability as well as significantly improved water resistance and ductility relative to conventional PA6.

Unlike low- or moderately strained cyclic monomers, where the challenge in balancing the monomer–polymer equilibrium lies in promoting polymerization, highly strained small rings require the opposite strategy: reducing excessive ring strain to enable depolymerization and monomer recycling. In cyclic monomers with inherently high ring strain, such as three- or four-membered rings, the heat evolved during ROP is substantial, resulting in a highly negative ΔH. This large enthalpic driving force easily overwhelms the unfavorable entropic penalty (TΔS), even at elevated temperatures. As a result, these monomers strongly favor ROP, and the resulting polymers are resistant to depolymerization back to their highly strained parent rings. Instead, degradation often produces larger, less strained cyclic multimers. For example, poly­(3-hydroxybutyrate) (P3HB), the extensively studied polylactone, was found to degrade into cyclic trimers rather than regenerating the original four-membered lactone , (Figure a). The strategy to achieve monomer recycling in a highly strained system relies on reducing ring strain and promoting ring closure. One effective approach is taking advantage of the Thorpe–Ingold effect, where geminal disubstitution on cyclic rings facilitates cyclization. For instance, Chen and co-workers reported that polymers derived from cyclic α-gem-substituted 3-hydroxybutyrate could be depolymerized back to the original four-membered lactone with a 60% yield at 210 °C in the presence of 5 wt % NaOH, owing to the enhanced tendency for ring closure imparted by the gem-disubstituent (Figure b). More recently, other successful designs of recyclable four-membered lactones and thiolactones based on the same strategy have been reported. − In addition, Chen’s group reported a chain-end controlled depolymerization of polymers of α-gem-substituted propionate (Figure c). In this case, carboxyl-terminated polymers predominantly degraded to cyclic trimers (up to 70% yield), whereas ester-terminated polymers favored depolymerization to regenerate the original cyclic propionate (up to a 58% yield). Importantly, both recycled products could be repolymerized to their corresponding polymers, demonstrating the feasibility of a dual closed-loop recycling pathway.

8.

Recyclability of four-membered cyclic lactones and thiolactones. (a) Recycling of poly­(3-hydroxybutyrate) (P3HB). (b) Recycling to monomer of gem-substituted propiolactones and β-thiolactones. (c) Chain-end controlled depolymerization of α-gem-substituted propiolactone.

The recycling behaviors discussed above were achieved by tuning the ring strain of cyclic monomers to an appropriate level or, more precisely, by adjusting the enthalpic contribution to obtain a suitable T _c. This T _c-manipulation principle applies well to small cyclic monomers, where both ΔH and ΔS of polymerization are negative. By contrast, macrocyclic monomers (typically ≥ 14 atoms) possess minimal or negligible ring strain, as bond angles and bond lengths can closely approach their ideal values within larger rings. As a result, the enthalpic contribution of polymerization is slightly negative or even positive. On the other hand, upon ring opening, the freedom of rotation and vibration of the linear repeating unit increases from the cyclic form, leading to a positive ΔS after ROP. The positive ΔS drives the ROP of macrocycles at elevated temperatures, in sharp contrast to the polymerization behavior of small rings. In such cases, the feasibility of depolymerization and recyclability is dictated more by monomer concentration than by temperature. Building on this understanding, the focus shifts from enthalpy-driven polymerization of small rings to entropically controlled polymerization of macrocycles, where the monomer concentration and chain mobility become key levers for designing recyclable polymers.

Recently, Li and colleagues reported a closed-loop recyclable poly­(ketal-ester), prepared via the entropy-driven ROP of a bioderived macrocyclic diester (Figure a). Both the enthalpy change (2.8 kJ·mol^–1) and entropy change (94.9 ± 6.7 J·mol^–1·K^–1) of ROP were positive. Consequently, the floor temperature (T _f) exists, above which polymerization is favored, opposite to the concept of the ceiling temperature. T _f of the macrocyclic diester was calculated to be 115 °C. The polymerization was performed at 170 °C, above T _f, in the presence of a Ti-catalyst. Importantly, the resulting polymer could be completely depolymerized back to the macrocyclic monomers at room temperature using a base catalyst ( ^t BuOK), demonstrating T _f-regulated circularity. Another example of entropy-driven ROP was reported by Odelius and co-workers (Figure b). They constructed a library of macrocyclic carbonates, which were efficiently polymerized using tert-butoxide with conversions higher than 97% within 3–10 s. The resulting polymers could be regenerated to the original cyclic monomers with high yields and 95–99% selectivity. This entropy-driven, closed-loop strategy demonstrates that macrocyclic monomers, especially those consisting of long alkyl chains, can achieve effective and highly selective recyclability.

9.

Recyclability of macro rings regulated by floor temperature. (a) Macrocyclic ketal-ester. (b) Macrocyclic carbonates. (c) Macrocyclic polar olefin.

Most recently, Qu and co-workers extended the scope of entropy-driven ROP to dynamic polar olefin bonds. They developed a closed-loop recycling system based on bioderived macrocycles featuring dynamic polar olefin bonds (Figure c). The key innovation involves a metal-free, TBD-catalyzed Knoevenagel-type polar olefin metathesis under solvent-free melting conditions, yielding high-molecular-weight polymers with a cyclic topology. Thermodynamic analysis confirmed the entropy-driven nature (ΔS = 32.09 J·mol^–1·K^–1 and T _f = −19.18 °C under bulk conditions). Efficient depolymerization was achieved under dilute conditions (100 mM in CHCl₃ at 50 °C), affording 63% monomer recovery and demonstrating practical closed-loop recyclability. This work establishes polar olefin chemistry as a versatile, metal-free platform for recyclable polymers.

2.2.2. Reversible Ring-Opening Metathesis Polymerization (ROMP)

Besides ROP, ring-opening metathesis polymerization (ROMP) represents another attractive reversible strategy for producing circular polymers. Like ROP, the monomer–polymer equilibrium in ROMP is largely governed by the intrinsic ring strain of the cyclic monomers, allowing recyclability to be tuned through structural design. For example, Wang and co-workers fused a four-membered ring onto cyclooctene using a two-step strategy, reducing the overall ring strain to a level comparable to cyclopentene (Figure ). As expected, the resulting polymers from these bicyclic monomers displayed excellent recyclability. Upon heating at 50 °C in the presence of a Grubbs second-generation catalyst (G2) for 3 min, gel permeation chromatography (GPC) analysis showed no residual polymer peak. Most oligomers reverted to the monomer with equilibrium fractions of 93% monomer and 7% oligomers. Notably, the polymer derived from the phenyl-containing bicyclic monomer retained mechanical properties comparable to those of polystyrene in tensile tests.

10.

Synthesis schemes of recyclable cyclooctene; cyclic olefins with different ring strain energies and reversibility.

In another study from Wang’s group, several bicyclic cyclooctenes were designed by fusing cyclooctene with cyclobutane, cyclopentane, or other five-membered cyclic acetals. Their investigation revealed that cyclooctenes fused with cyclobutane exhibited the best recyclability among the series, highlighting the influence of the fused-ring size and structure on depolymerization efficiency.

In 2022, Chen and co-workers designed an intriguing bicyclic monomer, BiL=, which can be regarded as a fusion of γ-butyrolactone with cyclohexene, while the synthetic route started from 3-cyclohexene-1-carboxylic acid50 (Figure ). As described earlier in the section on the recyclability of γBL-based cyclic lactones, BiL⁼ underwent reversible ROP. Remarkably, it can also undergo reversible ROMP in the presence of a G2 catalyst. The twisted bicyclic configuration simultaneously strains both the five- and six-membered rings, raising their ceiling temperatures to suitable values: 118 °C for ROP and 66 °C for ROMP. Poly­(BiL⁼)_ROPobtained via ROP of BiL⁼can be depolymerized with ZnCl₂ at 40 °C, regenerating 94% of the monomer after 12 h, whereas Poly­(BiL⁼)_ROMPobtained via ROMP of BiL⁼can be recovered in 93% yield after refluxing in dichloromethane with G2 for 24 h. The two polymers exhibited distinct thermal properties: Poly­(BiL⁼)_ROMP displayed a relatively high decomposition temperature of 361 °C, characteristic of poly­(cyclic olefin)-like backbones, whereas Poly­(BiL⁼)_ROP decomposed at a lower temperature of 278 °C, reflecting its polyester nature.

11.

Reversible ROP and ROMP of the olefin/lactone bifunctional monomer (BiL⁼).

Similar to macrocyclic lactones, macrocyclic olefins lack significant ring strain and therefore undergo entropically driven ROMP. In these systems, the equilibrium between macrocycles and linear polymers is highly concentration-dependent, and ring-closing depolymerization is favored under diluted concentrations. Recently, Jia and co-workers reported recyclable thermoplastic elastomers (TPEs) capable of closed-loop recycling through three distinct pathways, including an entropy-driven reversible ROMP route (Cycle 3) (Figure ). They synthesized an α-ω-diene monomer by coupling biomass-derived rigid isomannide with flexible undecanoic acid. The diene monomer was polymerized into oligomeric TPEs via acyclic diene metathesis polymerization (ADMET). The TPEs could then be hydrolyzed into isomannide and dicarboxylic acid, of which the isomannide could react with undecanoic acid to regenerate the original diene monomers (Cycle 1). Alternatively, the hydrolysis productsisomannide and dicarboxylic acidcould be directly repolymerized into TPEs via polycondensation (Cycle 2). In Cycle 3, ring-closing metathesis depolymerization of the TPEs proceeded efficiently with >99% conversion, producing macrocycles of various sizes as confirmed by GPC and high-resolution mass spectrometry. In this entropy-driven process, dilution strongly favored monomer formation: at a concentration of 1 mM of repeat-units, the monomer yield exceeded 90%, while yields were ∼50 and ∼20% at 10 and 100 mM, respectively. Conversely, ROMP of the recycled macrocyclic monomer was favored under concentrated conditions, proceeding quantitatively (>95% conversion) at a relatively high monomer concentration of 1.0 M and yielding TPEs with high molecular weights. Notably, the mechanical properties of the optimized TPEs were comparable to commercial TPEs such as SBS (styrene-butadiene-styrene) and SEBS (styrene-ethylene/1-butene-styrene) block copolymers, making them promising recyclable alternatives.

12.

Chemical recycling of TPEs through three cycles. Reproduced with permission from ref . Copyright 2024, The Author(s).

In summary, the balance of polymerization and depolymerization can be rearranged by tuning the ring strain and molecular architecture of monomers, allowing efficient closed-loop recycling across a wide range of cyclic monomers. Reversible ROP and ROMP provide versatile platforms to access polymers with tailored thermal and mechanical properties, from small strained lactones to macrocyclic olefins. The design principles are strongly dependent on the ring size of the cyclic monomers, as the ring size dictates whether polymerization is governed primarily by enthalpy or entropy.

For small- to medium-sized rings, polymerization is predominantly enthalpy-driven, where ΔH is closely associated with intrinsic ring strain. Therefore, the design principle is to tune the ring strain to an appropriate level. Based on their intrinsic ring strains, these strategies can be broadly categorized into three scenarios:

• 1.For monomers with little ring strain, such as γBL, polymerization is thermodynamically disfavored due to insufficient enthalpic driving force. The key strategy is therefore to increase ring strain, typically by introducing fused-ring structures.
• 2.For monomers with moderate to relatively high ring strain, such as δVL, polymerization is strongly favored, and depolymerization becomes difficult. In this case, the principle is to moderate or reduce ring strain, for example, by introducing alkyl substituents, incorporating heteroatoms to adjust bond angles to decrease the enthalpic driving force.
• 3.For monomers with extremely high ring strain, such as four-membered lactones, the enthalpic driving force is so large that depolymerization to the parent monomer is disfavored. Here, geminal substitution is often employed to invoke the Thorpe–Ingold effect, which promotes ring closure and effectively lowers the energetic barrier for monomer regeneration.

In contrast, for large macrocyclic monomers, ring strain becomes negligible, and polymerization is primarily entropy-driven. The equilibrium between polymerization and depolymerization is highly concentration-dependent. Rather than structural redesign to tune ring strain, controlling monomer concentration and reaction conditions becomes the primary lever for regulating reversibility.

2.3. Structural Transformation of Polymers into Recyclable Ones

Besides the precise design of monomers that enables direct polymerization–depolymerization between monomers and their corresponding polymers, another important strategy involves intelligent structural modifications at the polymer level to facilitate efficient or selective recycling.

In the section on ROP of six-membered lactones, we noted their poor polymerizability due to the nearly strainless cyclic configuration and summarized strategies developed to enhance the polymerizability. Although these approaches effectively promoted polymerization, they generally relied on monomer redesign and consequently yielded new polymer backbones rather than preserving the desirable parent structures. To address this limitation, in 2024, Li and co-workers proposed a novel chaperone-assisted strategy for the ROP of an α,β-conjugated δ-valerolactone, 5,6-dihydro-2H-pyran-2-one (DPO) (Figure a). Because of its highly stable six-membered ring, DPO is unreactive toward both ROP and ROMP under practical conditions. To enhance the polymerizability, cyclopentadiene was employed as a diene donor to undergo a Diels–Alder (DA) reaction with DPO, affording a tricyclic δ-valerolactone with 90% endoselectivity. The resulting strained tricyclic δ-valerolactone readily polymerized via ROP to produce polymers with M_n values up to 24.0 kDa. The retro-DA reaction was triggered at 190 °C in the presence of excess maleic anhydride, removing the cyclopentadiene chaperone and converting poly­(HPO) to poly­(DPO) with M_n = 12.9 kDa (vs. 19.2 kDa for the original poly­(HPO)). The reduction in molecular weights may be attributed to partial thermal degradation. Interestingly, despite the low thermodynamic favorability of DPO for ROP, poly­(DPO) exhibited excellent thermal stability, which the authors attributed to kinetic trapping: DPO monomers contained cis-CC bonds, while poly­(DPO) was locked in the trans-configuration, impeding its depolymerization. Attempts at in situ photoisomerization and TBD-catalyzed depolymerization yielded only a 10% monomer recovery after 60 h. By contrast, ring-closing metathesis with a G2 catalyst efficiently depolymerized poly­(DPO), recovering DPO in 96% yield with 99% selectivity after 6 days.

13.

Chemical recycling of DPO with a cyclopentadiene (a) and a cycloheptene-derived ketone with vicinal diols (b).

In the previous case, a chaperone-assisted strategy was employed to promote the polymerization of poor polymerizable monomers, whereas here, the chaperone-assisted strategy was taken to promote the depolymerization. Sun and co-workers recently designed a circular recycling loop for a seven-membered cycloheptene-derived ketone (Figure b), which exhibited a relatively high RSE of 7.21 kcal·mol^–1. As discussed for cyclooctene, high ring strain favors ROMP, and accordingly, the cycloheptene-derived ketone proved difficult to recover from its polymer. RCM of the ketone-embedded unsaturated polymers with a G2 catalyst afforded only 67% monomer recovery. Calculations revealed that pinacol modification of the cyclic ketone to form a cyclic ketal reduced the RSE to 3.85 kcal·mol^–1, close to the reported value of recyclable cyclobutane-fused cyclooctene (4.9 kcal·mol^–1), suggesting improved recyclability. Based on this insight, the authors developed a closed-loop recycling route: the cyclic ketone monomer was polymerized via ROMP under neat conditions; the resulting ketone-containing polymers were converted into ketal-embedded polymers by ketalization with pinacol using p-toluenesulfonic acid (p-TsOH) as a catalyst; and the ketal-embedded polymers were depolymerized to the cycloheptene-derived ketal in 96% yield. Finally, the original cycloheptene-derived ketone was then regenerated by simple acid hydrolysis of the cycloheptene-derived ketal.

Apart from strategies centered on imparting recyclability to polymers, another complementary approach focuses on enhancing the stability of degradable polymers. Usually, degradability is introduced through cleavable functionalities such as esters, ketals, and disulfides, which can compromise mechanical and physical stability during storage. To address this issue, Wang and Craig simultaneously proposed a “locked degradability” strategy , (Figure ). In this design, the cleavable groups were shielded from the polymer backbone by mechanophores, typically cyclobutane units. Under mechanical stress, these mechanophores are activated, for example, through force-induced ring opening of the cyclobutane moieties, thereby exposing the previously protected cleavable functionalities along the main chain. Only after mechanical activations can the cleavage of exposed linkages lead to polymer degradation. In other words, degradation is gated by a dual requirement: activation of the mechanophores followed by scission of the revealed cleavable groups. Because mechanophores are stable under thermal, radiative, acidic, and basic conditions, they effectively “lock” degradability, allowing polymers to retain stability while remaining chemically recyclable.

14.

Illustration of “locked degradability.”

For instance, in Wang’s work (Figure a), a tricyclic cyclooctene bearing a cyclobutyl-gated lactone was synthesized via [2 + 2] cycloaddition between cyclooctadiene and maleic anhydride, followed by LiAlH₄ reduction. Subsequent ROMP yielded polymers with locked degradability, in which cyclobutane served as the mechanophore and lactone served as the cleavable functionality. The stability of the nonmechanically activated polymers was confirmed by direct methanolysis, which caused no change in molecular weight, indicating that the degradability remained locked. Upon ultrasonication, the cyclobutane rings were activated and opened in a manner dependent on the molecular weight and sonication duration. A maximum ring-opening ratio of 48.2% was observed for polymers with M_n = 105 kDa after 240 min of ultrasonication. Once the mechanophores were activated, the exposed ester groups enabled subsequent methanolysis, leading to chain scission and a decrease in the M_n value to 15 kDa. Overall, this work elegantly reconciled the backbone stability and degradabilitytwo typically conflicting propertiesthrough deliberate molecular design. Other designs have further demonstrated the potential of locked degradability for developing sustainable polymeric materials by incorporating various cleavable functionalities such as ketals, enol ethers, and imides (Figure b–d).

15.

(a) Schemes of synthesis and degradation of the degradability-locked polymer with cyclobutane as the mechanophore and lactone as the cleavable functionality. (b–d) Degradability-locked polymers with different degradable functionalities.

However, the degradation of these systems generally requires external stimuli, such as nucleophiles or acids, to unlock the degradable linkages. To overcome this limitation, Craig and co-workers introduced an ingenious self-immolative design that enables backbone cleavage without external catalysts or reactants (Figure ). They synthesized polymers containing a tetrasubstituted [4.2.0]­bicyclooctene (BCOE) group. Under mechanical stress, BCOE underwent a 4π-electron ring-opening reaction to form a less strained 1,3-cyclooctadiene conjugated intermediate, which subsequently triggered spontaneous intramolecular lactonization, leading to backbone scission and release of bis-lactone products. The in situ intramolecular lactonization proceeded rapidly at the initial stage and plateaued within 100 h. After storage under ambient conditions for 17 days, polymers with molecular weights around 2 kDa were detected, confirming delayed degradation. This delayed scission mechanism allows multiple mechanical activation events along a single chain, which provides a promising strategy for enhancing mechanically induced degradation.

16.

Polymer bearing BCOE mechanophores undergoes mechanically induced forbidden ring opening and subsequent lactonization rearrangement, leading to enhanced degradation. Reproduced with permission from ref . Copyright 2020, the Author(s).

In addition, Wang and co-workers recently reported a particularly intriguing design that exemplifies the concept of locked degradability while showcasing the potential of polymer mechanochemistry to develop sustainable polymer materials (Figure ). They synthesized a ten-membered cyclic olefin fused with a cyclobutane mechanophore and a latent tetrahydrofuran (THF) ring. The strained cyclobutane unit rendered the monomer susceptible to ROMP, producing polymers with embedded mechanophores. Upon ultrasonication, the cyclobutane rings opened, generating poly­(2,5-dihydrofuran) (poly­(DHF)) in situ. Notably, 2,5-dihydrofuran (DHF) features a nearly strainless five-membered ring (RSE ≈ 3.4 kcal·mol^–1) and therefore favors depolymerization rather than polymerization. Leveraging this property, we successfully depolymerized the mechanically activated polymers into DHF under dilute conditions in the presence of a G2 catalyst. Although incomplete mechanophore activation left some residual polymers after 6 h, this elegant structural design effectively linked locked degradability with low-T _c polymer behavior, providing a new mechanochemical pathway toward sustainable, fully recyclable polymers. The concept of locked degradability provides an elegant balance between polymer stability and controlled degradability, resolving a long-standing trade-off in sustainable materials design. By integrating mechanochemical activation with latent cleavable functionalities, these systems remain robust under normal conditions yet become degradable upon specific mechanical or chemical triggers.

17.

Combination of locked degradability and RCM depolymerization.

Most recently, Gu and co-workers reported an innovative strategy that uses conformational preorganization to control the polymer degradation through the self-deconstruction of polyphosphate. Self-deconstruction of polyphosphate has been demonstrated as an effective method for polymer degradation in several works. − In these studies, polyphosphate units bearing pendant free hydroxyl groups were incorporated into the polymer backbone. The neighboring hydroxyl groups can intramolecularly attack the phosphotriester moieties, thereby triggering backbone cleavage and leading to polymer degradation. In Gu’s work, the rate of self-deconstruction was significantly accelerated by conformational preorganization: a 1,2-cis scaffold was introduced to position the hydroxyl group and the polyphosphate unit in close proximity, facilitating intramolecular attack. Importantly, ortho-nitrobenzyl groups were employed to temporarily protect the hydroxyl functionalities, effectively locking the degradability of the polymer until photodeprotection. This design provides a compelling strategy to balance stability during use and ease of degradation. (Figure a) They strategically designed a seven-membered cyclic olefin with a phosphotriester group embedded, along with a 1,2-cis arranged ring and an ortho-nitrobenzyl. The well-designed alkene monomers copolymerized effectively with COE through G2-catalyzed ROMP, producing polymers with M_n of approximately 51.7 kDa. Before UV exposure, the resultant polymer remained stable and resistant to degradation. Upon exposure to UV light, the ortho-nitrobenzyl groups were cleaved, unlocking the polymer’s degradability by revealing free hydroxyl groups. The preorganized 1,2-cis scaffold, which locks the phosphotriester group in gauche conformation, ensures that the hydroxyl group and phosphotriester group are brought into close proximity, promoting the self-deconstruction process. (Figure b) The deprotected polymer showed a sharp decrease in M_n from 51.7 to 3.82 kDa after just 2 days in ambient THF, while polymers lacking the preorganized conformation required weeks for similar degradation. This work establishes conformational control as a powerful strategy for fine-tuning polymer deconstruction, offering a novel method to balance stability and ease of degradation.

18.

(a) ROMP of preorganized phosphotriester-containing monomers with COE, and degradation of the resultant polymers. (b) Preorganized conformation of the 1,2-cis scaffold.

Overall, these studies highlight the growing importance of polymer-level structural engineering as a complement to the monomer precise design in chemical recycling. Through the introduction of auxiliary structureswhether as temporary chaperones to assist (de)­polymerization or as mechanochemical gates to regulate degradabilityresearchers have achieved fine control over both the kinetics and thermodynamics of polymer transformation. Such intelligent molecular architectures not only reconcile the traditional trade-off between stability and recyclability but also expand the conceptual landscape of sustainable polymer design.

3. Precise Structural Design for Chemically Recyclable Cross-Linked Polymers

Cross-linked polymers represent a vast and significant class of polymeric materials, including vulcanized rubber and phenol formaldehyde resin; the former is the most widely used cross-linked polymer material, and the latter is the first fully synthetic polymer. Owing to their outstanding mechanical strength and exceptional chemical stability attributed to the three-dimensional architectures, cross-linked polymers play indispensable roles across virtually all areas of modern life. However, in the meantime, the very three-dimensional architectures also render them “immortal” in the environment, leading to serious challenges in end-of-life management. Over the past decades, extensive efforts have been devoted to developing recyclable cross-linked polymers, and substantial progress has been achieved.

3.1. Recyclable Cross-Linked Polymers via Strategies for Linear Polymers

As discussed above, numerous precise structural designs have been developed to enable chemical recyclability in linear polymers. Taking one step further, recyclable cross-linked polymers can be constructed by the similar methodologies. ,,,,,,− In other words, the recyclability can also be imparted to cross-linked polymers by the incorporation of chemically cleavable linkages, the use of monomers with appropriate depolymerizability, or the structural transformation of network chains (Figure ).

19.

Strategies for constructing recyclable cross-linked polymers. (a) Incorporating renewable linkages in networks. (b) Using monomers with appropriate depolymerizability. (c) Transforming structures of network chains.

For example, in the work of catalytic chemical recycling of polyethylene by Coates and co-workers in 2022, they prepared a recyclable cross-linked polyester with HDPE-like mechanical properties using HDPE as recycling materials (Figure a). Telechelic diol monomers were prepared in the way that resembles another work described in Figure b. As the linear repolymerized products of the diol monomers exhibited poor mechanical properties, they used diethanolamine to convert part of the diol monomers into cross-linkers. The copolymerization of diol monomers with the obtained cross-linkers provided a degradable cross-linked polymer with mechanical properties comparable to those of HDPE.

Well-designed depolymerizable cyclic monomers are also useful in constructing recyclable cross-linked polymers. In the Section , we described the work of Wang and co-workers, who designed a recyclable cyclooctene with a four-membered ring fused on it. In the same work, they strategically designed and synthesized a cross-linker with two depolymerizable fused cyclooctenes and polymerized it with depolymerizable fused cyclooctenes (Figure b). The obtained recyclable cross-linked polymer can be recovered to the origin monomers and cross-linkers in bulk with a conversion of 94%.

In the work of combining mechanically locked-degradability and reversible ROMP of DHF reported by Wang’s group, the concept was also extended to degradable polymer networks (Figure c). In this study, the deliberately designed monomers were copolymerized with a bis-cyclooctene cross-linker to form mechanically responsive cross-linked films. The resulting networks were subjected to ball milling for 10 min to induce mechanophore activation, followed by RCM depolymerization catalyzed by a G2 catalyst. Based on ¹H NMR integration, the mechanophore activation reached 17%, corresponding to a 6 mol % recovery of DHF.

Overall, the construction of recyclable cross-linked polymers can be readily achieved by similar strategies used for preparing recyclable linear polymers. However, these recycling strategies are more of the means to destroy the network structures completely and recycle low molecules, which arises from another problem that rebuilding cross-linked polymer materials with the origin mechanical properties can be really difficult. Cross-linking the obtained recycling products usually offers materials that fail to meet expectations.

3.2. Recyclable Cross-Linked Polymers via Incorporation of Dynamic Covalent Bonds

Another appealing chemical strategy for constructing recyclable cross-linked polymers is to endow polymer networks with plasticity through the incorporation of dynamic covalent bonds, giving rise to covalent adaptable networks (CANs). Owing to their covalently cross-linked nature, CANs exhibit robust stability and excellent mechanical properties comparable to traditional thermosets. Meanwhile, the reversible exchangeability of dynamic covalent bonds imparts CANs with reprocessability similar to that of thermoplastics. During reprocessing, the network topology remains intact rather than being destroyed and rebuilt, thereby preserving the intrinsic mechanical performance of the original cross-linked material to the greatest extent.

CANs are generally classified into two groups based on the mechanism of strand rearrangement (Figure ). The first group undergoes a dissociative exchange mechanism in which chemical bonds are temporarily broken and then reformed with new partners. Although this process reduces the instantaneous cross-link density, the network does not fully disassemble. The transient loss of cross-links leads to a sharp drop in viscosity, allowing for rapid topological rearrangement. Once the bonds are reformed, the cross-link density is restored, preserving the original mechanical properties. The second group follows an associative exchange mechanism in which dynamic covalent bonds are reconstructed by nucleophilic attack from unbound groups. Here, network strands are broken only when new bonds form, maintaining a constant cross-link density during reprocessing and nearly fully preserving mechanical properties. In 2011, Leibler and co-workers demonstrated this concept with polyester-linked epoxy networks in the presence of unreacted hydroxyl groups and zinc acetate, which acted as a transesterification catalyst. Afterward, they termed these materials vitrimers, as they exhibited a gradual decrease in viscosity upon heating, similar to the behavior of vitreous silica.

20.

Two types of CANs: the dissociative and associative types.

The ingenious feature of CANs (imparting recyclability while preserving mechanical properties) has inspired numerous efforts to combine dynamic covalent bonds for preparing associative CANs. Choices of dynamic covalent bonds include carboxylic esters, ,− urethanes, − siloxanes, − imines, , olefins, , disulfides, − dithioacetals, , boroxines, − and more recently, CC/CN compounds. , Many studies have advanced the utility of these bonds in constructing CANs (Figure ). Comprehensive reviews on CANs have been published by Bowman and Prez. − In this section, we typically focus on works designed strategically to promote the recyclability of CANs.

21.

Various dynamic covalent bonds applied in CANs.

Recently, Xie and co-workers reported a recyclable dithioacetal-based system for circular 3D printing. Usually, dithioacetals are typically known as dynamic covalent bonds, as listed in Figure a. They dissociate upon heating and revert to the associated state upon cooling. In this work, the dissociated states were strategically stabilized by neutralizing the dissociated residue to facilitate the next round of photopolymerization (Figure b). The authors selected a photoresponsive benzyl aldehyde, vanillin, as the aldehyde component, where the para-hydroxyl group enhanced storage stability while facilitating bond dissociation. Using a stereolithographic 3D-printing setup, they successfully fabricated geometrically complex objects from formulations containing vanillin-derived dithioacetals with dithiol and tetra-thiol cross-linkers. The resulting cross-linked polymers could be thermally degraded at 80 °C in THF into soluble raw resin, and the dissociative–associative equilibrium was quenched by neutralization with NaHCO₃. Importantly, the recycled resin could be reprinted into new 3D objects in the presence of a fresh photoacid generator (PAG), demonstrating truly circular recyclability (Figure b). This recyclable resin system was further showcased in a practical application for the fabrication of dental braces.

22.

(a) Reversible reaction between the vanillin and thiols. (b) Circular 3D-printing process. Reproduced with permission from ref . Copyright 2025 AAAS.

It is noteworthy that most CANs require external stimuli to initiate a network topology exchange, which can limit their practical applications. Various strategies have been explored to enable reprocessing under mild conditions, including weakening dynamic bonds via steric or electronic effects, and using external or internal catalysts. Among these, internal catalysis is particularly attractive as it avoids issues of catalyst leaching or instability. A systematic review on internal catalysis in CANs has been reported by Du Prez and Winne.

In 2024, You and co-workers reported a polyurethane CAN incorporating internal catalysis (Figure ). A tetrafunctional diaminoglyoxime (AMG) was reacted with diols and diisocyanates to form a polyurethane ionogel. In this network, urea bonds and oxime–urethane bonds are positioned in close proximity, forming four-arm cross-linking units. The nucleophilic nitrogen atoms of the urea bonds facilitate the reversible dissociation of oxime–urethane bonds via neighboring group participation, enabling bond exchange at room temperature. Consequently, the ionogels exhibit excellent mechanical self-healing, outperforming most room-temperature self-healing ionogels and demonstrating efficient recyclability.

23.

Neighboring group participated in an internal catalyzed oxime–urethane exchange for constructing polyurethane CANs.

In addition, our group demonstrated an alternative strategy to promote the dynamic exchange of covalent bonds by incorporating mechanical bonds to construct mechanically interlocked vitrimers (MIVs) (Figure a). By integrating slidable [2]­rotaxane motifs with vinylogous urethane bonds, the MIVs achieved catalyst-free network reconfiguration. The slidable mechanical bonds accelerated the transamination exchange, reducing the activation energy to 79.5 kJ·mol^–1, compared to 86.5 kJ·mol^–1of CANs without mechanical bonds. Moreover, the precise design enabled full chemical recyclability, where the rotaxane monomers could be recycled in a modified form using n-butylamine and subsequently reprocessed (Figure b). There are also a number of works proving the synergistic effects of mechanical bonds and covalent adaptable networks. −

24.

(a) Composition and structure of the mechanically interlocked vitrimers (MIVs). (b) Recycling loop of MIVs.

Overall, significant progress has been made toward recyclable cross-linked polymers through two complementary strategies. First, cross-linked networks can be constructed from chemically recyclable linear polymers. Second, covalent adaptable networks (CANs) leverage dynamic covalent bonds to impart reprocessability and recyclability without sacrificing mechanical performance. Collectively, these approaches demonstrate that careful molecular designthrough both monomer selection and network engineeringcan reconcile the inherent stability of cross-linked polymers with circularity, providing a versatile framework for sustainable polymer materials.

4. Conclusion and Perspectives

In conclusion, there are plenty of methodologies to embed recyclability into polymer materials at the molecular level, such as introducing cleavable linkages, enhancing reversibility of monomers, transforming polymers to recyclable ones, and utilizing dynamic covalent bonds. Precise chemical design provides a powerful tool for polymer recycling, enabling plastics to perform not as terminal-use commodities but as materials circulating in a sustainable economy. Precision chemistry may unlock a new materials philosophy: polymers that are not only high-performing in their use phase but also perpetually recoverable, feeding back into a circular materials ecosystem. For the moment, designing polymers with recyclability is no longer a problem, but creating innovative recyclable materials with low-cost, high-performance, and wide versatility remains a long-term challenge.

4.1. Cost and Practical Considerations

Even though delightful achievements have been made for recycling commodity polymer materials, the practical recycling attempts are very small in the scale of production. The hugely accumulated plastic waste over mountains and oceans is an extremely urgent issue waiting to be solved, and should not be the elephant in the room ever. The recycling approach for polymer wastes must be both low-cost and efficient. This means that the synthetic routes to the recyclable polymers should not be very complicated, and the energy cost in recycling must be controlled. For example, recyclable polymers rely on sophisticated monomers, or recycling at supremely high temperatures may not be acceptable. Constructing recyclable polymer material with low-cost biomass − is one of the next frontiers. In addition, some efforts have been made to permit a more effective recycling process. We envision a future where precision chemistry is not confined to laboratory-scale demonstrations but embedded within global production chains, driving a new era of accessible circular plastics.

4.2. Property Optimization

One of the most compelling strengths of precise chemical design is its capacity to balance two historically conflicting demands: robust performance and facile recyclability. In innate bias, materials with excellent mechanical properties are difficult to be reformed or recycled, and chemically recyclable polymers usually possess vulnerable linkages. Emerging strategies, such as locked degradability and dynamic covalent chemistry, show that recyclability can coexist with mechanical strength, thermal stability, and chemical resistance. However, we are still struggling to maintain the chemical properties comparable to those of commodity polymer materials if considering the cost. The vision ahead is not just about recycling polymers into feedstocks but to reprogram, reset, and even upgrade their properties via precise chemical designs.

4.3. Versatility Expansion

For the world’s most produced polymer materials, PE, PP, PVC, and PS, the efficient way for recycling is still absent. Despite the identical all-carbon backbones, the methodologies are suite separated from each other. Some approaches were taken to synthesize recyclable polymers with a similar structure and comparable properties. ,, Some approaches were taken using of metal catalyst. ,,, Some approaches were taken using coupcycling, especially in the case of PVC. , It would be more practical and efficient if there existed a universal approach to recycle polymers with some similarities.

In addition, the horizon of recyclable polymers extends far beyond low-value packaging. Precisely designed materials hold the promise of enabling recyclable polymers to be useful in both structural and functional materials. For example, recyclable and reprocessable elastomers, engineering plastics, and polymer composites used in vehicles, infrastructures, and constructions could save millions of dollars and eliminate terrible environment damage. On the functional side, the incorporation of recyclability into advanced materials for electronics, energy storage, and biomedical devices could reshape high-tech industries with sustainability at their core.

Glossary

Abbreviations

Mt

million tonnes

PE

polyethylene

PP

polypropylene

PVC

polyvinyl chloride

PS

polystyrene

PET

polyethylene terephthalate

WAXS

wide-angle X-ray scattering

HDPE

high-density polyethylene

HEA

2-hydroxyethyl acrylate

LLDPE

linear low-density polyethylene

ROIMP

ring-opening-insertion metathesis polymerization

CAMMP

cyclic–acyclic monomers metathesis polymerization

COE

cyclooctene

COS

carbonyl sulfide

ROMP

ring-opening metathesis polymerization

CTA

chain-transfer agent

OBC

olefin block copolymers

RCM

ring-closing metathesis

DCM

dichloromethane

DOT

dibenzo­[c,e]-oxepine-5­(7H)-thione

ROP

ring-opening polymerization

RCD

ring-closing depolymerization

γBL

γ-butyrolactone

MBL

α-methylene-γ-butyrolactone

IROP

isomerization-driven ROP

PTBL

poly­(γ-thiobutyrolactone)

TBL

γ-thiobutyrolactone

TBD

1,5,7-triazabicyclo­[4.4.0]­dec-5-ene

DFT

density functional theory

¹H NMR

proton nuclear magnetic resonance spectroscopy

CL

ε-caprolactam

PA 6

polyamide 6

ROMP

ring-opening metathesis polymerization

GPC

gel permeation chromatography

ADMET

acyclic diene metathesis polymerization

SBS

styrene-butadiene-styrene block copolymer

SEBS

styrene-ethylene/1-butene-styrene block copolymer

DPO

5,6-dihydro-2H-pyran-2-one

DA

Diels–Alder reaction

RSE

ring strain energy

BCOE

[4.2.0]­bicyclooctene

THF

tetrahydrofuran

DHF

2,5-dihydrofuran

UV

ultraviolet

PAG

photoacid generator

CANs

covalent adaptable networks

AMG

diaminoglyoxime

MIVs

mechanical interlocked vitrimers

§.

X.X. and J.D. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Published as part of Precision Chemistry special issue “Precision Chemistry for Polymer Recycling”.