Chemically Recyclable Polymers: Tunable Performance Enabled by Precision Ring-Opening (Co)polymerization

Abstract

Chemical recycling to monomers (CRM) stands as a cornerstone for a circular plastics economy by enabling the closed-loop regeneration of high-purity monomers from waste polymers without compromising material properties. However, the key challenge lies in how to reconcile robust performance with efficient depolymerization under mild conditions. This perspective highlights that precise ring-opening (co)­polymerization (PROP), enabled by advanced catalytic strategies to exert exacting control over polymer chain architecture, represents a powerful and promising approach. We systematically illustrate how PROP facilitates the rational design of polymers by tailoring molecular parameters such as molecular weight/dispersity, stereoregularity, and copolymer sequence. This structural precision governs key aggregation state characteristics, including crystallinity, chain packing, and conformational behavior, etc., thereby directly modulating macroscopic material properties and enabling designed recyclability. By summarizing recent PROP-driven advancements, we present a bottom-up synthetic strategy that links polymer microstructure and macroscopic properties. Finally, we outline remaining challenges and future research directions, underscoring the importance of next-generation catalytic systems and rigorous life-cycle assessments to translate these approaches from the laboratory to industrial scale.

1. Introduction

Plastics have become indispensable to modern society, ubiquitous in virtually every facet of daily life and industrial production, thanks to their advantageous combination of lightness, durability, processability, and cost-effectiveness. , However, their massive production and disposal have led to severe resource depletion and the accumulation of hundreds of millions of tons of waste annually, creating a global environmental crisis known as “white pollution”. In response, chemical recycling to monomers (CRM) has emerged as a foundational strategy toward a circular plastics economy. − Unlike mechanical recycling, which typically downgrades properties through contamination and chain degradation, , CRM targets the quantitative depolymerization of end-of-life plastics into high-purity monomers. These monomers can, in principle, be reintroduced into existing manufacturing streams to regenerate polymers with virgin-like performance, thereby achieving circularity at the molecular level.

The feasibility of CRM is fundamentally governed by the thermodynamics and kinetics of reversible polymerization–depolymerization equilibria (Figure ). In this context, the concept of the ceiling temperature (T _c) provides a unifying framework: for a given monomer–polymer pair, T _c defines the temperature above which depolymerization is thermodynamically favored and below which polymerization dominates. For most commodity polymers with inert C–C backbones (e.g., polyolefins, polystyrenes), T _c is exceedingly high, rendering depolymerization endergonic and typically necessitating harsh conditions that often yield complex product mixtures. In contrast, heterochain polymers containing labile linkages, such as esters, carbonates, thioesters, or ethers, can exhibit significantly more accessible T _c values, especially when synthesized from cyclic monomers via ring-opening polymerization (ROP). , This underscores the importance of “moderate ring strain” as depicted in Figure : while excessively high ring strain leads to a very high T _c that inhibits depolymerization, an insufficient thermodynamic driving force (low ring strain) hampers efficient polymerization; thus, moderate strain provides the ideal balance for reversible interconversion. ROP thus represents a particularly attractive platform for CRM, as appropriately designed catalytic systems enable high monomer conversion under mild conditions, avoid stoichiometric byproducts, and facilitate reversible monomer-polymer interconversion with controlled kinetics. , This synergy not only simplifies polymerization and depolymerization but also supports high-yield monomer recovery with minimal side reactions at end-of-life. Over the past decade, these advantages have been demonstrated across various classes of ROP-derived polymers, including polyesters, polycarbonates, polythioesters, and polyamides, underscoring the potential of ROP as a versatile strategy for enabling closed-loop plastic cycles.

1.

Design principles for CRM polymers. RCD: ring-closing depolymerization; ROP: ring-opening polymerization.

Despite these inherent advantages, a core challenge remains: how to achieve efficient depolymerization under mild conditions while maintaining excellent material properties. , Recent advances suggest that the rational control of polymer chain structure through precise ring-opening (co)­polymerization (PROP) offers a promising approach to overcoming this challenge. − By precisely controlling molecular parameters, researchers can directly regulate the polymer aggregation state (e.g., crystallinity, chain stacking, and supramolecular order), thereby decoupling performance from recyclability. Such microstructure-mediated design has enabled significant progress toward high-performance materials that maintain selective and efficient depolymerizability.

This Perspective highlights how PROP serves as a powerful methodology to address this dichotomy. In this context, “precision” is defined by the ability to exert rigorous control over molecular weight, molecular weight distribution, monomer sequence, and stereoregularity. Such molecular-level precision dictates the polymer’s secondary and tertiary structuresincluding crystallinity, chain stacking, and supramolecular orderwhich are the fundamental determinants of macroscopic performance. Through precise control over polymer chain architecture (molecular weight/dispersity, stereoregularity, and copolymer sequence), PROP facilitates the rational manipulation of a polymer’s aggregation state, which in turn governs its macroscopic properties toward tunable material performance with efficient chemical recyclability (Figure ). Finally, the remaining scientific and translational challenges: the need to develop advanced catalytic systems, expand monomer diversity, and conduct comprehensive life-cycle assessments to combine laboratory innovation with a scalable circular plastics economy were pointed out.

2.

Precise chain architecture control in ring-opening (co)­polymerization enables chemically recyclable polymers with tunable performance.

2. Evolution of Chemically Recyclable ROP Systems: From Concept to Precision Ring-Opening (Co)Polymerization

2.1. Rational Monomer Design

CRM via ROP has become a cornerstone strategy for advancing the circular plastics economy. Over the past decade, both conceptual and synthetic advances have significantly broadened the scope of ROP-based CRM systems, extending their applicability to a wide range of polymer classes, including polyesters, polycarbonates, polyamides, and sulfur-containing polymers. ,, The successful implementation of these chemically recyclable polymers fundamentally relies on the rational design of cyclic monomers with tailored thermodynamic properties, particularly those possessing optimal ring strain. As illustrated in Table , the diversity of monomers now accessible for ROP-based CRM underscores the expanding structural toolkit enabling closed-loop polymer life cycles.

1. Representative Cyclic Monomers for Chemically Recyclable Polymers via ROP.

To date, three primary strategies have been developed for modulating monomer thermodynamics: the fused/bridged-ring strategy, the substituent-based strategy, and the heteroatom-substitution strategy.

The fused/bridged-ring strategy introduces conformational constraints that enhance ring strain, thereby enabling polymerization under relatively mild conditions. This approach has redefined the thermodynamic boundaries of aliphatic polyesters. For instance, γ-butyrolactone (γBL), long considered “non-polymerizable” due to its low ring strain, was successfully polymerized by employing low-temperature conditions to shift the equilibrium toward polymer formation, opening a route to fully recyclable poly­(γ-butyrolactone) (PγBL). To address the thermal limitations of such low-strain systems, rigid bridged bicyclic lactones have been developed. These moderate-strain monomers yield polymers with higher glass transition temperatures and improved mechanical robustness, while maintaining efficient recyclability. ,

The substituent-based strategy modulates thermodynamic stability through steric and electronic effects. For example, systematic variation of substituent size and position on ε-caprolactone (CL) has been shown to lower its T _c, thereby enabling energy-efficient polymerization–depolymerization cycling. This strategy also plays a crucial role in the design of recyclable polyamides, whose strong intermolecular hydrogen bonding traditionally impedes depolymerization. Through judicious substituent engineering, next-generation polyamides can be programmed to undergo facile and selective depolymerization under mild conditions. ,

The heteroatom-substitution strategy has recently emerged as a versatile method for fine-tuning polymerization thermodynamics. Notably, replacing oxygen with sulfur in cyclic monomers effectively reduces T _c while preserving controlled polymerization behavior. This approach has led to the development of polythioesters from thiolactones, which exhibit favorable thermodynamics for chemical recycling. ,− Beyond their closed-loop recyclability, these materials often combine robust thermal and mechanical performance with distinctive functionalities such as high refractive indices.

Despite these advances, a fundamental trade-off persists between polymer performance and recyclability. Polymers with low T _c for facile depolymerization often lack the mechanical or thermal robustness required for practical applications. To overcome this limitation, we propose that the field should increasingly embrace PROP. By exerting precise control over chain architecture, including molecular weight/dispersity, stereoregularity, and copolymer sequence, PROP enables the manipulation of polymer aggregation states, thereby allowing the tailored adjustment of macroscopic properties. This approach offers a promising pathway to reconcile high performance with efficient chemical recyclability in next-generation sustainable polymers.

2.2. Precise Molecular Weight/Dispersity Control

Molecular weight (M _n), representing the average chain length, and dispersity (Đ = M _w/M _n), indicating the breadth of the chain length distribution, are fundamental parameters in polymer science. Precise control over molecular weight and dispersity is essential for designing chemically recyclable polymers that meet both performance and circularity goals. Due to the hierarchical structure of polymers, chain length and dispersity critically affect key properties, including crystallization behavior, entanglement density, processing windows, and mechanical performance. Furthermore, these molecular parameters strongly influence depolymerization kinetics and the purity of recovered monomers, thus determining the feasibility of CRM. To achieve such control, current strategies focus primarily on modulating the polymerization process itself, especially through advances in catalytic systems and polymerization reaction engineering. By establishing a well-defined catalytic system, living or controlled polymerization can be achieved, thereby enabling precise control of chain growth, predictable molecular weight, and narrow dispersity, while maintaining the thermodynamic reversibility required for efficient CRM (Figure ).

3.

ROP systems for chemically recyclable polymers with precise molecular weight and dispersity control.

Poly­(γ-butyrolactone) (pγBL) represents a cornerstone in the development of sustainable aliphatic polyesters, yet its monomer, γ-butyrolactone (γBL), has long been considered effectively nonpolymerizable under practical conditions owing to its low ring strain. Hong and Chen overcame this barrier by demonstrating that operating significantly below the T _c (−40  °C) enables controlled, lanthanide-catalyzed coordination ROP of γBL (Figure a). This approach yields polymers with tunable molar masses (up to 30 kg mol^–1) and relatively narrow dispersities. Notably, cyclic pγBL topologies exhibit enhanced thermal stability (T _d,5%  =   273 °C) compared to their linear counterparts, while still permitting quantitative depolymerization back to γBL. However, this work also reveals a key limitation of the “near-equilibrium” γBL system: its polymerization capability depends on strict low-temperature control to make the polymerization reaction thermodynamically more favorable. Therefore, subsequent research has turned to the structural redesign of γBL-derived monomers to broaden the polymerization window and enhance material properties, while preserving closed-loop recyclability. For instance, a bicyclic lactone derived from trans-fused cyclohexyl γBL (4,5-trans-cyclohexyl-fused γBL) can undergo living coordination ROP at room temperature to obtain polymers with high thermal stability (T _d,5% >  300 °C) and quantitative monomer recovery via catalyzed depolymerization (Figure b). Similarly, a 3,4-trans-cyclohexyl-fused γBL derivative supports rapid, solvent-free polymerization at ambient temperature while maintaining narrow dispersity (M _n = 5.88 × 10⁵ Da, Đ = 1.02); stereocomplexes of these polymers exhibit tensile strengths up to 54.7 MPa and enable complete closed-loop monomer recovery (Figure c). Furthermore, by combining the ε-caprolactone unit with a high T _c with the γBL motif with a low T _c in a bridged bicyclic structure, Chen’s team also established a hybrid monomer design strategy. This method balances efficient polymerization and selective depolymerization, resulting in a recyclable platform with a T _g as high as 135 °C and a T _m as high as 263 °C (Figure d).

4.

(a) Lanthanide-catalyzed coordination ROP of γBL at −40 °C for the synthesis of linear and cyclic PγBL, along with their CRM. Adapted with permission from ref . Copyright 2015 Springer Nature. (b–d) Examples of structurally redesigned γBL-derived monomers enabling ROP at ambient temperature and their respective CRM pathways: (b) 4,5-trans-cyclohexyl-fused γBL. Adapted with permission from ref . Copyright 2018 Wiley. (c) 3,4-trans-cyclohexyl-fused γBL. Adapted with permission from ref . Copyright 2018 The American Association for the Advancement of Science. (d) ε-caprolactone/γBL hybrid monomer. Adapted with permission from ref . Copyright 2021 Elsevier Inc.

Building upon pγBL, a diverse range of chemically recyclable polyesters have been developed through rational monomer design and advanced catalytic systems, achieving efficient synthesis with controlled molecular weights and narrow dispersity. Representative examples include the gem-disubstituted δ-valerolactone (VL^R₂ ) platform, which via living polymerization affords high molar masses (M _n up to 333 kg mol^–1) and low dispersity (Đ ∼ 1.16), while maintaining high melting temperatures (∼140 °C) and tensile strength (44 MPa) alongside near-quantitative monomer recovery (Figure a). Similarly, entropy-modulated spiro-valerolactone (SL) achieves molar masses exceeding 1000 kg mol^–1 with excellent barrier properties (oxygen permeability 0.03 Barrer) and tensile strength (43 MPa), all while supporting quantitative recycling. For biobased polyesters, a norcamphor-derived bicyclic lactone has been reported to achieve complete closed-loop recycling, yielding polymers with M _n up to 164 kg mol^–1, a T _g of −9 °C, and quantitative monomer recovery through thermolysis. A series of sugar-derived glucurono-1,6-lactones undergo living polymerization to pseudopolysaccharide architectures, offering tunable T _g values (−36 to 89 °C) and >90% monomer recovery via catalyzed depolymerization. To incorporate functionality, a biorenewable bifunctional monomer, α-methylene-δ-valerolactone (MVL), was synthesized and polymerized chemoselectively (suppressing vinyl addition) to produce side chain functionalizable yet fully recyclable polyesters, affording tensile strength up to 29.9 MPa and 451% elongation at break with >95% monomer recovery. This strategy complements CO₂/butadiene-derived δ-lactone systems, which similarly achieve controlled ROP with low dispersity (Đ < 1.6), tunable T _g =  –38.8 to −42 °C, and efficient catalytic depolymerization with up to 84% monomer recovery. ,

5.

(a) A circular polyester platform based on simple gem-disubstituted valerolactones. Adapted with permission from ref . Copyright 2022 Springer Nature. (b) Switching from controlled ring-opening polymerization (cROP) to controlled ring-closing depolymerization (cRCDP) by adjusting the reaction parameters that determine the T _c. Adapted with permission from ref . Copyright 2016 American Chemical Society. (c) 4-Hydroxyproline-derived sustainable polythioesters: controlled ring-opening polymerization, complete recyclability, and facile functionalization. Adapted with permission from ref . Copyright 2019 American Chemical Society. (d) Redesigned hybrid nylons with optical clarity and chemical recyclability. Adapted with permission from ref . Copyright 2022 American Chemical Society.

In the field of chemically recyclable polycarbonates, an early conceptual milestone was established through precise modulation of the T _c (77–234 °C tunable by solvent/concentration), enabling tunable control over the monomer–polymer equilibrium. This principle was exemplified by the fully reversible conversion of 2-allyloxymethyl-2-ethyl-trimethylene carbonate (AOMEC) to its corresponding polymer and back to monomer within 10 h (Figure b). Subsequent advances have significantly broadened the accessible monomer library, with several representative systems emerging as follows: a two-step route employing CO₂ and bioderived diols delivers recyclable six-membered cyclic carbonates suitable for controlled ROP, affording tunable glass-transition temperatures (−25 to 3 °C), thermal stability (T _d,5%, 209–245 °C), and 99% monomer recovery. A scalable “polycondensation–depolymerization–ROP” strategy further enables the synthesis of high-molar-mass aliphatic polycarbonates (M _n >   100  kDa) with melting temperatures up to 217 °C, tensile strengths of 70 MPa, and dual closed-loop recycling. Performance-oriented designs show that high strength need not compromise circularity, for example, controlled ROP of 5,5-dimethyl-1,3-dioxan-2-one (DTC) affords low Đ (<1.30), tensile strength up to 80.0 MPa, and 99.3% high-purity monomer recovery. Additionally, fused-ring cyclic carbonates exhibit ultrafast polymerization kinetics (3–10 s), yielding polymers with M _n up to 673 kg mol^–1 and excellent thermal stability (T _d,5% > 300 °C) while enabling reversible depolymerization to dimers and repolymerization. In parallel, functional ring-fused epoxides enable fully alternating CO₂-based polycarbonates with tunable glass transition temperatures (46 °C), and high thermal stability (T _d,5% ∼ 297 °C) that remain recyclable while preserving low Đ and offering postpolymer modification handles.

For chemically recyclable polythioesters (PTEs), achieving precise molecular-weight and dispersity control fundamentally requires suppressing polymerization side reactions such as transthioesterification and backbiting. Proline-derived thiolactones were designed and enabled controlled ROP by organocatalysis, which yields polymers with molar masses up to 259 kg mol^–1, tunable glass-transition temperatures (32–67 °C), and rapid base-triggered unzipping to pristine monomer (Figure c). Bridged bicyclic thiolactones further broaden the polymerization window, providing access to high-performance, tactic yet intrinsically crystalline PTEs with melting temperatures of 166–213 °C, tensile strengths of 36.3–49.1 MPa, and elongations exceeding 200%, all while retaining selective depolymerization to monomer. Single-atom oxygen-to-sulfur substitution in dilactone scaffolds (monothiodilactones) similarly balances polymerizability with cyclizability, yielding tough PTEs that exhibit tensile strengths of 21.2 MPa, elongations at break of 466.9%, and a melting temperature of 77.6 °C, while depolymerizing rapidly with >95% monomer recovery and purity. Moreover, an interfacial organocatalytic ROP of thioglycolide (TGA) overcomes long-standing side-reaction challenges, producing high-molecular-weight poly­(thioglycolic acid) (PTGA) as a chemically recyclable, high-performance alternative to isotactic polypropylene (iPP), with comparable thermal and mechanical properties alongside enhanced barrier performance.

It is essential to explicitly distinguish between thermodynamic feasibility, dictated by the T _c and equilibrium monomer-polymer interconversion, and kinetic accessibility, which is governed by activation barriers along the depolymerization pathway. In many widely studied chemically recyclable systems, including polyesters, polycarbonates, and polythioesters, the absence of strong intermolecular interactions results in relatively low kinetic barriers, allowing depolymerization to proceed under conditions approaching thermodynamic control. In stark contrast, high-performance polyamides exemplify a fundamental kinetic bottleneck: the dense, cohesive networks of interchain hydrogen bonding that confer exceptional tensile strength and thermal stability also severely restrict chain mobility and catalyst access. These morphological constraints elevate activation barriers and impede depolymerization under mild conditions, even when the monomer-polymer equilibrium is thermodynamically favorable. , Consequently, achieving efficient chemical recycling in such systems necessitates not only favorable thermodynamics but also deliberate catalytic and monomer design strategies to selectively lower kinetic barriers, by disrupting cohesive packing, facilitating catalyst penetration. For chemically recyclable polyamides, the strong interchain hydrogen bonds that give them high mechanical properties also impose a kinetic barrier on depolymerization. Monomer redesign, combined with tailored catalytic systems, is therefore essential to achieve precise molecular weight and dispersity control while ensuring efficient chemical recyclability. Chen and co-workers introduced a hybrid lactam (5/7-membered ring lactam, 5/7-LM) that integrates a polymerizable high T _c segment with a recyclable low T _c motif, enabling rapid anionic ROP to high molar mass (M _n ≈ 48.7 kDa, Đ = 1.24) (Figure d). The resulting polyamides exhibit high thermal stability (T _d,5% = 357  °C), optical clarity, a tensile strength of 54 MPa, and 93%–98% monomer recovery over cycles. Bridged bicyclic lactams (5/6-LM) introduce conformational locking, yielding crystalline polyamides (T _d,5% = 283 °C, M _n up to 75.7  kDa) that undergo clean polymerization and depolymerization back to monomer with >75% yield. For commodity-relevant systems, Marks and Chen systematically employed gem-dimethyl (Thorpe–Ingold) substitution of ε-caprolactam to restore living-like control (e.g., M _n = 68.1  kDa, Đ = 1.12). The resulting γ-substituted variants display amorphous transparency, a T _g elevated by 30 °C, a tensile strength of 60.5 MPa, 70.2% elongation, and 93% monomer recovery at 180  °C. Related studies on dimethyl-substituted caprolactams further achieve M _n = 90.5  kDa, T _d,5% = 344 °C, and 91% monomer regeneration. , Bioderived seven-membered lactams (from lignocellulose and CO₂) afford polyamides with a T _g of 129  °C, thermal decomposition at 335  °C, and 72% monomer recovery via acid-catalyzed depolymerization, while benzo-fused caprolactams expand to semiaromatic regimes with T _d,5% = 341–368 °C, < 3% water absorption, tensile strength 68 MPa, 82%–91% optical transmittance, and 72%–80% monomer recovery at 100–150 °C. ,

In parallel, polyacetals have also demonstrated controlled ROP through the design of adaptive catalytic systems, enabling both precise synthesis and efficient monomer recovery. Coates and his co-workers reported a reversible-deactivation cationic ROP of 1,3-dioxolane (DXL), which allows predictable molecular weight targeting (M _n up to 220  kDa, Đ = 1.51–1.74) based on the monomer-to-initiator ratio. The resulting polyacetals achieve a tensile strength of 40 MPa and an elongation at break of 720%, matching the mechanical performance of conventional polyolefins, while depolymerization via direct distillation yields 98% pure DXL suitable for repolymerization (Figure a). Subsequently, Coates and co-workers extended this system into the ultrahigh molecular weight regime (M _n up to 2031  kDa, Đ = 1.4–2.1), significantly enhancing mechanical properties to a tensile strength of 105 MPa and an elongation at break of 1035%, exceeding the stress performance of ultrahigh molecular weight polyethylenewhile maintaining practical recyclability through simple thermal distillation that recovers DXL in 93% yield (Figure b). This acetal platform exemplifies an optimal scenario in which controlled polymerization and high-fidelity monomer recovery operate synergistically.

6.

(a) Chemically recyclable polyacetals synthesized via reversible-deactivation cationic ROP of cyclic acetals. Adapted with permission from ref . Copyright 2021 The American Association for the Advancement of Science. (b) Ultrahigh molecular weight poly­(dioxolane) demonstrating enhanced mechanical performance while maintaining chemical recyclability. Adapted with permission from ref . Copyright 2023 American Chemical Society.

Above all, precise molecular weight and dispersity control can be systematically achieved when the catalytic system promotes active or controllable chain growth and the monomers are designed within a practical thermodynamic range. Besides serving as a synthetic baseline, narrow dispersity and predictable molecular weight enhance depolymerization kinetics, simplify monomer purification, and ensure that recovered monomers meet the purity requirements for high-fidelity repolymerization. In summary, these advances demonstrate that polymers with tunable properties and reliable ring-closure recyclability can be prepared by tailoring catalysts to suppress side reactions and designing monomers within specific maximum T _c.

2.3. Precise Copolymer Sequence Control

Sequence refers to the specific constitutional arrangement of different comonomer units along the polymer backbone. In the design of chemically recyclable polymers, sequence has emerged as a decisive structural parameter that directly governs material performance and recyclability. By dictating the spatial arrangement of comonomer unitssuch as statistical, alternating, gradient, and block sequences, researchers can strategically influence crystallization behavior, thermal transitions, mechanical properties, and depolymerization pathways, often using an identical set of monomers. Through advances in ring-opening (co)­polymerization and related catalytic techniques, precise sequence regulation now enables the rational design of polymers whose microstructure not only meets functional requirements but also ensures selective and efficient chemical recycling, forming a critical bridge between molecular structure and macroscopic circularity (Figure ). ,

7.

ROP systems for chemically recyclable polymers with precise copolymer sequence control.

While γ-butyrolactone (γBL) can undergo ROP, its homopolymers have some inherent limitations, such as a low glass transition temperature (T _g ≈ −45 °C) and relatively high permeability to oxygen and water vapor, which restrict its thermomechanical and barrier properties. Therefore, γBL has been more widely used as a comonomer, and its performance range can be broadened by controlling its sequence distribution without affecting its recycling performance. In pioneering work by the Chen group, γBL was copolymerized with ε-caprolactone (εCL) or δ-valerolactone (δVL) across a wide composition range (up to 84 mol % γBL) (Figure a). Reactivity-ratio analysis revealed a predominantly statistical microstructure with a marked alternating-dyad bias (e.g., r _(CL) r _(γBL) ≈ 0.11 and r _(VL) r _(γBL) ≈ 0.14). This local sequence preference directly modulates phase behavior and crystallization: γBL/εCL copolyesters display isodimorphic cocrystallization with a eutectic point near 11  °C, enabling composition-dependent tuning of thermal transitions and degradation profiles around ambient conditions. Sequence effects extend to biodegradable polyhydroxyalkanoate (PHA) analogs, where γBL serves as a 4-hydroxybutyrate (4HB) precursor. Stereoselective copolymerization of rac-β-butyrolactone derivatives with γBL yields random P­(3HB-co-4HB) copolymers with low 4HB incorporation (∼3.7–4.6 mol %), which exhibit single-step thermal degradation around 222 °C illustrating how controlled γBL inclusion can define thermal decomposition pathways. Beyond statistical copolymers, sequence and architecture design in the same study produced a “hard-and-tough” P3HB-b-PCL block copolymer with a modulus of ∼1.45 GPa and elongation at break of ∼106%, markedly outperforming brittle isotactic P3HB (modulus ∼3 GPa, elongation ∼3%). This result directly links microstructural control to mechanical enhancement (Figure b). In packaging-oriented systems, random copolymers of trans-hexahydrophthalide (T6HP) with γBL combine high ductility (ε _b ≈ 436%), useful strength (∼28.8 ± 2.4 MPa), and improved barrier properties (e.g., O₂ permeability ≈ 0.45 ± 0.04 Barrer; water-vapor permeability ≈ 1.83 ± 0.03 g mm m^–2 day^–1) while retaining full chemical recyclability via ZnCl₂-catalyzed chemolysis at 120 °C, which regenerates both cyclic monomers (Figure c). In summary, γBL-based systems demonstrate that intentional, often subtle, adjustments to sequence statistics and chain structure can profoundly influence crystallization behavior, thermal transitions, mechanical properties, and end-of-life outcomes, transforming thermodynamically marginal monomers into a multifunctional, performance-dependent platform for designing cyclic polymers.

8.

(a) “Non-strained” γ-butyrolactone-based copolyesters: copolymerization behavior and composition-dependent properties. Adapted with permission from ref . Copyright 2017 American Chemical Society. (b) Toughening of biodegradable isotactic poly­(3-hydroxybutyrate) through stereoselective copolymerization of a diolide and lactones. Adapted with permission from ref . Copyright 2021 American Chemical Society. (c) Random copolymers of trans-hexahydrophthalide (T6HP) with γBL as packaging-oriented systems. Adapted from ref under the terms of the Creative Commons CC BY license. Copyright 2019 The Author(s).

The sequence control in copolymerization serves as a pivotal strategy for designing chemically recyclable polymers with tailored properties while utilizing commercially available monomers, thereby addressing the scalability and performance requirements for practical applications. In representative work, strong base/urea binary catalysts enable controlled ROP of biosourced δ-caprolactone (δCL), yielding recyclable polyesters that depolymerize quantitatively via thermolysis. Sequential copolymerization of δCL with l-lactide further produces well-defined PLLA-b-PδCL-b-PLLA triblock copolymers, which exhibit thermoplastic elastomeric behavior, combining excellent elastic recovery, tensile strength, and elongation, and can be chemically upcycled to recover ethyl lactate and δCL (Figure a). Similarly, rapid ROP of alkyl-substituted δ-lactones catalyzed by organophosphazene/urea systems affords closed-loop recyclable polyesters, whose polymerization kinetics and material properties are modulated by alkyl-chain length. These monomers can be copolymerized with l-lactide to form triblock architectures that function as high-performance, tackifier-free pressure-sensitive adhesives, with full monomer recovery achieved through alcoholysis and distillation. Moreover, bulk ROP of δ-caprolactone and trans-hexahydro-(4,5)-benzofuranone using organobase/urea catalysts yields composition-tunable copolyesters that depolymerize under mild conditions. With suitable compositions, these materials directly serve as pressure-sensitive adhesives exhibiting peel strength comparable to commercial tapes, and when blended with plasticizers, they function as removable adhesive notes. Furthermore, one-pot sequential copolymerization of biorenewable β-methyl-δ-valerolactone (βMVL) and δ-valerolactone (δVL) yields well-defined PδVL-b-PβMVL-b-PδVL triblock copolymers that exhibit superior tensile strength and toughness, along with elasticity comparable to conventional polyester-based and commercial styrenic thermoplastic elastomers. Notably, these materials can be quantitatively depolymerized to recover pristine monomers under reduced pressure in the presence of Sn­(Oct)₂, and the recovered monomers can be directly repolymerized to regenerate TPEs with performance comparable to the originals.

9.

(a) Controlled ROP of biosourced δ-caprolactone toward fully recyclable polyesters and thermoplastic elastomers. Adapted with permission from ref . Copyright 2022 Wiley-VCH GmbH. (b) Thermodynamic modulation via heteroatom substitution toward chemically recyclable polyesters and temperature-switched monomer sequence control. Adapted with permission from ref . Copyright 2024 Wiley-VCH GmbH. (c) High-elasticity, chemically recyclable thermoplastics derived from biobased monomers: carbon dioxide, limonene oxide, and ε-decalactone. Adapted with permission from ref . Copyright 2020 The Royal Society of Chemistry.

In parallel, Wu and colleagues demonstrated a one-pot sequential copolymerization of γ-BL and p-dioxanone (PDO) using a urea/alkoxide catalytic system, which affords well-defined PγBL-b-PPDO diblock copolymers with tunable thermal and crystalline characteristics. These block copolymers efficiently depolymerize via low-temperature pyrolysis (>95% yield), and the resulting monomers are selectively separated (≈ 99% purity) and repolymerized to regenerate materials with nearly identical structures, establishing a robust closed-loop cycle for sequence-controlled copolymers. , Wang, Zhang, and co-workers introduced a heteroatom substitution strategy that modulates the thermodynamics of δ-valerolactone-derived monomers by incorporating nitrogen, sulfur, or oxygen at the C-5 position, achieving tunable T _c ranging from 42.9 to 176.2 °C, and enabling controlled ROP with near-quantitative depolymerization. Temperature-regulated one-pot/one-step copolymerization of two of each of the three monomers further yields sequence-controlled copolymers with architectures ranging from gradient to random to block structures, providing a straightforward and efficient route to tailor both copolymer sequence and polymer properties (Figure b).

For chemically recyclable polycarbonates, the strategy of controlling the copolymer sequence through catalytic copolymerization with complementary monomers can also effectively regulate the physical properties of polycarbonates while maintaining their recyclability. For example, Williams and colleagues employed controlled ring-opening copolymerization to synthesize ABA triblock copolymers comprising rigid poly­(limonene carbonate) end-blocks and a soft poly­(ε-decalactone) midblock. This architecture overcomes the inherent brittleness of the parent polycarbonate, enhancing elongation at break to 400% and tensile toughness by 20-fold while retaining a tensile strength of 21.2 MPa and a Young’s modulus of 321 MPa. The resulting phase-separated structure provides dual glass-transition temperatures (−50 and 50 °C) and a broad processing window (>140  °C), and catalyzed depolymerization enables the selective recovery of limonene oxide, CO₂, and the polyester segment, thereby preserving a closed-loop lifecycle (Figure c). Similarly, switchable organoboron-mediated catalysis enables the sequential copolymerization of terminal epoxides/o-phthalaldehyde and internal epoxides/CO₂, yielding well-defined polyacetal-b-polycarbonate block copolymers with tailored thermal properties (dual glass transition temperatures of 63 and 107 °C, two-step thermal decomposition at 288 and 343 °C) and supports selective recyclability (>95% o-phthalaldehyde and 99% cyclohexene oxide recovery) alongside programmed degradability via orthogonal acid/base hydrolysis. Furthermore, systematic studies on ABA copolymers derived from lactide, ε-decalactone, and substituted cyclic carbonates demonstrate that the chemical identity of the soft midblock critically governs both mechanical and depolymerization behavior. Replacing a polyester midblock (PεDL) with a polycarbonate one (PTMC) elevates Young’s modulus from 14 MPa to 200 MPa and increases elongation at break by over 600%. Leveraging thermodynamic differences between blocks, selective ring-closing depolymerization (conversion rates of 87%–88% for end blocks) or nonselective overall monomer recovery (total conversion of 97%) was realized, thereby facilitating efficient recovery and recycling of monomers from each block (Figure ).

10.

Systematic investigation of ABA copolymers derived from lactide, ε-decalactone, and substituted cyclic carbonates reveals that the chemical identity of the soft midblock critically governs both mechanical performance and depolymerization behavior. Adapted from ref under the terms of the Creative Commons CC BY 4.0 license. Copyright 2023 The Author(s).

In summary, by strategically selecting comonomers and customizing catalytic copolymerization strategies, the sequence of copolymers can be precisely controlled, thereby directly achieving the synergistic design of physical properties and selective depolymerization, ultimately chemically transforming recycled monomers into applicable sustainable materials.

2.4. Precise Stereoregularity Control

Stereoregularity (or tacticity) describes the spatial arrangement of chiral centers within the polymer repeat units. It is a crucial factor determining the aggregated structure of polymer chains, crystallization behavior, and other characteristics, directly impacting the polymer’s mechanical strength, thermal stability, and depolymerization efficiency. For chemically recyclable polymers, precise control of stereoregularity allows for the regulation of aggregate morphology and degradation pathways without altering monomer composition. In ROP, stereoregularity is primarily regulated using chiral catalysts or stereoselective initiators. These catalysts or initiators impart specific stereoregularity (isotactic, syndiotactic) to the polymer during chain growth, providing a direct synthetic method for controlling material properties and achieving closed-loop recyclability (Figure ).

11.

ROP systems for chemically recyclable polymers with precise stereoregularity control. (ESC: Enantiomorphic Site Control and CEC: Chain End Control).

In recent years, notable advances have been achieved in controlling stereoregularity through ROP, establishing a crucial methodological foundation for the precise design of chemically recyclable polymers with tailored properties. For instance, Coates and co-workers reported the synthesis of methylated polyhydroxybutyrates (PHMBs) via stereoregulated ROP of cis/trans-configured dimethyl-substituted lactones derived from industrial C4 feedstocks. The cis-PHMB homopolymers exhibited tacticity-independent crystallinity and high melting points, which allows for the discovery of high-melting, thermally stable and mechanically tough copolymers, and a full range of polyolefin-like properties can be further achieved by tailoring the cis/trans ratio of the repeating units. These polymers can be chemically recycled or upcycled at their end of life. Chen and co-workers developed a catalyst-controlled stereodivergent strategy to access all di-isotactic polyhydroxyalkanoate (PHA) diastereomers from a single chiral source. Metal-catalyzed coordination–insertion ROP produced threo-(R,R)-di-isotactic PHAs with stereoretention, while organocatalytic anionic ROP generated erythro-(R,S)-di-isotactic PHAs via stereoinversion. The stereoregular architecture governed chain packing and crystallization, leading to distinct thermal and mechanical behaviors: erythro-configured PHAs displayed enhanced ductility (elongation >300%) and lower melting points compared to the threo-configured analogues. Furthermore, this approach enabled the stereodivergent synthesis of all four stereoisomers of α,α-dialkylated PHAs, which underwent depolymerization to chiral α,α-dialkylated-β-butyrolactones with high stereoselectivity.

Other representative examples are as follows: Zhu and Lu groups have independently reported pioneering advances in stereoregularity control through dynamic kinetic resolution polymerization (DKRP) to overcome the inherent 50% yield limitation of conventional kinetic-resolution processes. Both teams employed orthogonal catalytic systems, comprising a Lewis acid–base pair in combination with a chiral salen-aluminum complex, to enable stereoconvergent ROP of racemic β-propiothiolactones. This approach achieves full monomer conversion, yielding isotactic polythioesters with enantiomeric excess up to 96% and isotacticity (P _m) nearing 0.99. By integrating rapid in situ racemization with stereoselective propagation, these works establish DKRP as a generalizable strategy for synthesizing enantiopure polymers with precise stereochemical control from racemic monomer feeds. ,

A landmark study by Chen, Beckham, Xu, and co-workers demonstrates how precise stereoregularity control transforms biorenewable poly­(3-hydroxybutyrate) (P3HB) into high-performance sustainable adhesives. While stereoperfect microbial P3HB lacks adhesion, syndio-rich (sr-) P3HB, engineered via chemocatalytic design, exhibits exceptional adhesion strength on diverse substrates, surpassing petroleum-based adhesives. This sr-microstructure imparts optimal viscoelastic and thermomechanical properties, enabling strong, reprocessable adhesion independent of molar mass. In contrast, isotactic or syndiotactic variants show negligible adhesion, confirming that adhesion depends on specific stereosequence distributions. The biodegradable, chemically recyclable sr-P3HB adhesives establish stereoregularity as a key parameter for balancing functionality, processability, and circularity (Figure a).

12.

(a) Biodegradable adhesives regulated by stereomicrostructure design. Adapted with permission from ref . Copyright 2025 The American Association for the Advancement of Science. (b) Performance-advantaged stereoregular recyclable plastics enabled by aluminum-catalytic ROP of dithiolactone. Adapted with permission from ref . Copyright 2023 Wiley-VCH GmbH. (c) Stereoselective ROP of lactones with a fused ring leading to semicrystalline polyesters. Adapted with permission from ref . Copyright 2022 American Chemical Society. (d) High-performance recyclable polymers achieved through stereo- and sequence-controlled polymerization. Adapted with permission from ref . Copyright 2025 Springer Nature.

For chemically recyclable polymers, methods for controlling polymer physical properties and maintaining closed-loop recyclability through precise stereoregularity have been initially established. For example, Tao and co-workers employed an aluminum-catalyzed stereoretentive ROP of a dithiolactone to produce highly isotactic polythioesters (P _m up to 0.92) that form stereocomplexes with a melting temperature of ∼94.5 °C and exhibit mechanical properties comparable to low-density polyethylene (tensile strength 9.6 MPa, elongation 354.7%), while the same catalyst mediates quantitative depolymerization back to enantiopure monomer (ee >96%) (Figure b). In parallel, fused-ring seven-membered lactones undergo stereoselective ROP (P _r up to 92%) to yield semicrystalline polyesters with melting transitions around 101 °C and thermal stability up to 292 °C, which can be regenerated to monomer via hydrolysis-lactonization cycles (Figure c). Furthermore, stereo- and sequence-controlled polymerization of benzodioxepinone-based monomers with two stereogenic centers affords isoenriched block and triblock architectures (e.g., P­(cis-M)-b-P­(trans-M)) that display tunable toughness and ductility matching isotactic polypropylene or low-density polyethylene, with full depolymerization back to the original monomers (Figure d). Enantiopure bicyclic ether-ester monomers derived from poly­(3-hydroxybutyrate) undergo living ROP to give stereoregular polyesters with melting points up to 176 °C, exceeding many commodity polyolefins, while maintaining efficient monomer recovery. Finally, camphor-derived lactones (CamL) polymerize via tacticity-modulated ROP to produce intrinsically crystalline polyesters with high melting temperatures (161–225 °C), yield stresses of 24–39 MPa, and tunable ductility (6–218% elongation), all of which can be chemically recycled to monomer and repolymerized without property loss.

In summary, these works illustrate that precise stereoregularity control achieved through customized catalyst design and monomer engineering provides a multifunctional pathway for controlling crystallization, thermomechanical behavior, and depolymerization pathways, thereby facilitating the development of high-performance, chemically recyclable polymers.

3. Challenges and Outlook

Despite significant advancements in PROP for designing chemically recyclable polymers, several scientific and transformational challenges remain to be overcome to achieve scalable recycling applications.

First, the catalytic precision required to simultaneously control molecular weight, sequence, and stereoregularity is currently largely limited to customized monomer-catalyst combinations. At this foundational stage, most studies emphasize mechanistic proof-of-concept under idealized conditions. Nevertheless, the practical viability of these systems critically hinges on catalyst recovery and reuse over multiple cycles of ROP and subsequent RCD. To date, research addressing catalyst tolerance toward real-world waste contaminants (e.g., pigments and additives) and efficient separation from the polymer matrix or monomer pool remains nascent. , Developing broadly applicable and robust catalytic systems capable of maintaining high selectivity across multiple monomer families, especially under industrially relevant conditions, remains an ongoing challenge.

Second, the thermodynamic and kinetic trade-offs between material properties and depolymerization capabilities continue to constrain design scope. While PROP can achieve customized upper limits to polymerization/depolymerization equilibrium, further improvements in monomer thermodynamics and catalyst-mediated reaction pathway control are needed to simultaneously achieve high thermomechanical stability and low-energy monomer recovery under mild conditions.

Third, monomer accessibility and scalability must be prioritized as a central translational challenge. Many advanced chemically recyclable systems depend on structurally engineered monomers that currently require multistep synthesis, potentially offsetting their environmental benefits through increased energy and carbon footprints. While the field remains at a foundational stage focused on elucidating structure–property-recyclability relationships, future efforts must progressively shift toward the development of synthetically streamlined, atom-economical routes. Accessible synthetic protocols leveraging biobased or waste-derived feedstocks, coupled with efficient monomer purification and separation strategies, will be essential for establishing economic viability and enabling practical deployment.

Fourth, holistic analysis is needed for end-of-life system integration. Research on the selective depolymerization of complex or mixed polymer streams simulating real-world waste remains insufficient. Furthermore, life-cycle assessments (LCAs) quantifying the environmental and energy impacts of closed-loop polymerization systems are still in their early stages, but are crucial for guiding sustainable development.

Looking ahead, interdisciplinary efforts combining catalyst innovation, monomer feedstock engineering, and system-level recycling design will be key. Advances in in situ characterization and machine learning-guided discovery technologies promise to accelerate the identification of robust catalytic systems and optimal monomer structures. In addition, integrating PROP-based polymers into existing recycling infrastructure and establishing standardized recycling metrics will help bridge the gap between laboratory innovation and industrial applications.

4. Conclusions

PROP has become a powerful platform for designing chemically recyclable polymer molecular structures. PROP enables precise control over chain structure, including molecular weight/dispersibility, copolymer sequence, and stereoregularity, providing a rational strategy for decoupling polymer properties from CRM. This paper focuses on how customized catalytic systems and engineered monomer structures jointly determine polymerization thermodynamics, solid-state morphology, thermomechanical properties, and selective CRM pathways, paving the way for closed-loop plastic lifecycles.

The progress summarized in this paper demonstrates that high-performance, ready-to-use materials, from high-toughness thermoplastics and elastomers to barrier packaging and adhesives, can be designed to be fully chemically recyclable without sacrificing functionality. However, the transition from academic innovation to scalable recycling technologies requires ongoing research into robust catalytic systems, sustainable monomer supply chains, and systematic lifecycle management. With advancements in this field, PROP is expected to play a central role in the development of a circular polymer economy, providing a precise, versatile, and environmentally conscious approach to addressing global challenges to plastics sustainability.

The authors declare no competing financial interest.

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