Tough and Thermally Recyclable Semiaromatic Polyesters by Ring-Opening Polymerization of Benzo-thia-caprolactones

Heteroatom Substitution Strategy Modulates Thermodynamics Towards Chemically Recyclable Polyesters and Monomeric Unit Sequence by Temperature Switching

In this paper, we proposed a heteroatom substitution strategy (HSS) in the δ-valerolactone (VL) system to modulate thermodynamics toward chemically recyclable polyesters. Three VL-based monomers containing different heteroatoms (M1 (N), M2 (S), and M3 (O)), instead of C-5 carbon, were designed and synthesized to verify our proposed HSS. All three monomers undergo organocatalytic living/controlled ROP and controllable depolymerization. Impressively, the resulting P(M1) achieved over 99 % monomer recovery under both mild solution depolymerization and high vacuum pyrolysis conditions without any side reactions, and the recycled monomers can be polymerized again forming new polymers. The systematic study of the relationship between heteroatom substitution and recyclability shows that introducing heteroatoms does change the thermodynamics of the monomers (ΔHp o, ΔSp o and Tc values), thereby adjusting the polymerizability and depolymerizability. DFT calculations found that the introduction of heteroatoms adjusts the ring strain by changing the angular strain of the monomers, and the order of their angular strain (M2>M1>M3) is consistent with the order of the experimentally obtained enthalpy change. Notably, the one-pot/one-step copolymerization of two of each of the three monomers enables the synthesis of sequence-controlled copolymers from gradient to random to block structures, by simply switching the copolymerization temperature.

Recyclable and (Bio)degradable Polyesters in a Circular Plastics Economy

Polyesters carrying polar main-chain ester linkages exhibit distinct material properties for diverse applications and thus play an important role in today's plastics economy. It is anticipated that they will play an even greater role in tomorrow's circular plastics economy that focuses on sustainability, thanks to the abundant availability of their biosourced building blocks and the presence of the main-chain ester bonds that can be chemically or biologically cleaved on demand by multiple methods and thus bring about more desired end-of-life plastic waste management options. Because of this potential and promise, there have been intense research activities directed at addressing recycling, upcycling or biodegradation of existing legacy polyesters, designing their biorenewable alternatives, and redesigning future polyesters with intrinsic chemical recyclability and tailored performance that can rival today's commodity plastics that are either petroleum based and/or hard to recycle. This review captures these exciting recent developments and outlines future challenges and opportunities. Case studies on the legacy polyesters, poly(lactic acid), poly(3-hydroxyalkanoate)s, poly(ethylene terephthalate), poly(butylene succinate), and poly(butylene-adipate terephthalate), are presented, and emerging chemically recyclable polyesters are comprehensively reviewed.

Torsional Strain Enabled Ring-Opening Polymerization towards Axially Chiral Semiaromatic Polyesters with Chemical Recyclability

The development of new chemically recyclable polymers via monomer design would provide a transformative strategy to address the energy crisis and plastic pollution problem. Biaryl-fused cyclic esters were targeted to generate axially chiral polymers, which would impart new material performance. To overcome the non-polymerizability of the biaryl-fused monomer DBO, a cyclic ester Me-DBO installed with dimethyl substitution was prepared to enable its polymerizability via enhancing torsional strain. Impressively, Me-DBO readily went through well-controlled ring-opening polymerization, producing polymer P(Me-DBO) with high glass transition temperature (Tg >100 °C). Intriguingly, mixing these complementary enantiopure polymers containing axial chirality promoted a transformation from amorphous to crystalline material, affording a semicrystalline stereocomplex with a melting transition temperature more than 300 °C. P(Me-DBO) were capable of depolymerizing back to Me-DBO in high efficiency, highlighting an excellent recyclability.

Acid-Cleavable Aromatic Polymers for the Fabrication of Closed-Loop Recyclable Plastics with High Mechanical Strength and Excellent Chemical Resistance

Although closed-loop recycling of dynamic covalent bond-based plastics does not require catalysts, their mechanical strength and chemical stability remain a major concern. In this study, closed-loop recyclable poly(aryl imine) (PAI) plastics with high mechanical strength and excellent chemical resistance are fabricated by copolymerizing aromatic amines and aromatic aldehydes through dynamic imine bonds. The resulting PAI plastic with a tensile strength of 58.2 MPa exhibits excellent chemical resistance and mechanical stability in acidic and basic aqueous solutions and various organic solvents. The PAI plastics can be depolymerized in a mixed solvent of tetrahydrofuran (THF)/HCl aqueous solution through the dissociation of imine bonds, and the monomers can be facilely recovered with high purity and isolated yields due to the solubility difference between the aromatic amines and aromatic aldehydes in selective solvents. The efficient closed-loop recycling of the PAI plastic can also be realized through monomer conversion because the hydrolysis of the aromatic aldehydes generates aromatic amines. The recovered monomers can be used to re-fabricate original PAI plastics. This PAI plastic can be selectively recovered from complicated mixed polymer waste streams due to the mild depolymerization conditions of the PAI plastic and its high stability in most organic solvents.

Tetrabutylammonium Halides as Selectively Bifunctional Catalysts Enabling the Syntheses of Recyclable High Molecular Weight Salicylic Acid-Based Copolyesters

To synthesize high molecular weight poly(phenolic ester) via a living ring-opening polymerization (ROP) of cyclic phenolic ester monomers remains a critical challenge due to serious transesterification and back-biting reactions. Both phenolic ester bonds in monomer and polymer chains are highly active, and it is difficult so far to distinguish them. In this work, an unprecedented selectively bifunctional catalytic system of tetra-n-butylammonium chloride (TBACl) was discovered to mediate the syntheses of high molecular weight salicylic acid-based copolyesters via a living ROP of salicylate cyclic esters (for poly(salicylic methyl glycolide) (PSMG), Mn =361.8 kg/mol, Ð<1.30). Compared to previous catalysis systems, the side reactions were suppressed remarkably in this catalysis system because phenolic ester bond in monomer can be selectively cleaved over that in polymer chains during ROP progress. Mechanistic studies reveal that the halide anion and alkyl-quaternaryammonium cation work synergistically, where the alkyl-quaternaryammonium cation moiety interacts with the carbonyl group of substrates via non-classical hydrogen bonding. Moreover, these salicylic acid-based copolyesters can be recycled to dimeric monomer under solution condition, and can be recycled to original monomeric monomers without catalyst under sublimation condition.

Chemical Recycling of Commercial Poly(l-lactic acid) to l-Lactide Using a High-Performance Sn(II)/Alcohol Catalyst System

Poly(l-lactic acid) (PLLA) is a leading commercial polymer produced from biomass, showing useful properties for plastics and fiber applications; after use, it is compostable. One area for improvement is postconsumer waste PLLA chemical recycling to monomer (CRM), i.e., the formation of l-lactide (l-LA) from waste plastic. This process is currently feasible at high reaction temperatures and shows low catalytic activity accompanied, in some cases, by side reactions, including epimerization. Here, a commercial Sn(II) catalyst, applied with nonvolatile commercial alcohol, enables highly efficient CRM of PLLA to yield l-LA in excellent yield and purity (92% yield, >99% l-LA from theoretical max.). The depolymerization is performed using neat polymer films at low temperatures (160 °C) under a nitrogen flow or vacuum. The chemical recycling operates with outstanding activity, achieving turnover frequencies which are up to 3000× higher than previously excellent catalysts and applied at loadings up to 6000× lower than previously leading catalysts. The catalyst system achieves a TOF = 3000 h-1 at 0.01 mol % or 1:10,000 catalyst:PLLA loading. The depolymerization of waste PLLA plastic packaging (coffee cup lids) produces pure l-LA in excellent yield and selectivity. The new catalyst system (Sn + alcohol) can itself be recycled four times in different PLLA "batch degradations" and maintains its high catalytic productivity, activity, and selectivity.

Chemically Recyclable Dithioacetal Polymers via Reversible Entropy-Driven Ring-Opening Polymerization

In a sustainable circular economy, polymers capable of chemical recycling to monomers are highly desirable. We report an efficient monomer-polymer recycling of polydithioacetal (PDTA). Pristine PDTAs were readily synthesized from 3,4,5-trimethoxybenzaldehyde and alkyl dithiols. They then exhibited depolymerizability via ring-closing depolymerization into macrocycles, followed by entropy-driven ring-opening polymerization (ED-ROP) to reform the virgin polymers. High conversions were obtained for both the forward and reverse reactions. Once crosslinked, the network exhibited thermal reprocessability enabled by acid-catalyzed dithioacetal exchange. The network retained the recyclability into macrocyclic monomers in solvent which can repolymerize to regenerate the crosslinked network. These results demonstrated PDTA as a new molecular platform for the design of recyclable polymers and the advantages of ED-ROP for which polymerization is favored at higher temperatures.

Insights into substitution strategy towards thermodynamic and property regulation of chemically recyclable polymers

The development of chemically recyclable polymers serves as an attractive approach to address the global plastic pollution crisis. Monomer design principle is the key to achieving chemical recycling to monomer. Herein, we provide a systematic investigation to evaluate a range of substitution effects and structure-property relationships in the ɛ-caprolactone (CL) system. Thermodynamic and recyclability studies reveal that the substituent size and position could regulate their ceiling temperatures (T_c). Impressively, M4 equipped with a tert-butyl group displays a T_c of 241 °C. A series of spirocyclic acetal-functionalized CLs prepared by a facile two-step reaction undergo efficient ring-opening polymerization and subsequent depolymerization. The resulting polymers demonstrate various thermal properties and a transformation of the mechanical performance from brittleness to ductility. Notably, the toughness and ductility of P(M13) is comparable to the commodity plastic isotactic polypropylene. This comprehensive study is aimed to provide a guideline to the future monomer design towards chemically recyclable polymers.

Endowing Polythioester Vitrimer with Intrinsic Crystallinity and Chemical Recyclability

Technologically important thermosets face a long-standing end-of-life (EoL) problem of non-reprocessability, a more sustainable solution of which has resolved to nascent vitrimers that can merge the robust material properties of thermosets and the reprocessability of thermoplastics. However, the lifecycle of vitrimers is still finite, as they often suffer from significant deterioration of mechanical performance following multiple reprocessing cycles, analogous to mechanical recycling, and they often show undesired creep under working conditions. To address these two key limitations, we have developed a cross-linked semi-crystalline polythioester with both dynamic covalent bonds and intrinsic crystallinity and chemical recyclability, affording a vitrimeric system that exhibits not only reprocessability and crystallinity-restricted creep but also complete chemical recyclability to initial monomer by catalyzed depolymerization in solution or bulk. Therefore, reported herein is an "infinite" vitrimer system that is empowered with a facile closed-loop EoL option once serial reprocessing deteriorates performance and the material can no longer meet the application requirements. Specifically, the polythioester vitrimer was constructed by copolymerization of a bicyclic thioester with a bis-dithiolane, producing dynamically cross-linked polythioesters with excellent property tunability, from amorphous to semi-crystalline states and melting transition temperatures from 91 to 178 °C.

Converting Non-strained γ-Valerolactone and Derivatives into Sustainable Polythioesters via Isomerization-driven Cationic Ring-Opening Polymerization of Thionolactone Intermediate

This contribution reports the efficient conversion of γ-valerolactone and its derivatives, abundant but unexplored renewable feedstocks, into sustainable and degradable polythioesters via the establishment of the first isomerization-driven ring-opening polymerizations (IROPs) of corresponding thionolactone intermediates. The key to this success relies on the development of a new simple and robust [Et₃ O]⁺ [B(C₆ F₅ )₄ ]^- cationic initiator which possesses high activity, exclusive selectivity, living nature, and broad scope of thionolactones. A complete inversion of configuration during IROP of enantiopure γ-thionovalerolactone is also disclosed, affording isotactic semicrystalline polythioesters (T_m =87.0 °C) with mechanical property compared well to the representative commodity polyolefins. The formation of a highly crystalline supramolecular stereocomplex with enhanced thermal property (T_m =117.6 °C) has also been revealed.

Ring-Opening Polymerization for the Goal of Chemically Recyclable Polymers

A crucial modern dilemma relates to the ecological crisis created by excess plastic waste production. An emerging technology for reducing plastic waste is the production of "chemically recyclable" polymers. These polymers can be efficiently synthesized through ring-opening polymerization (ROP/ROMP) and later recycled to pristine monomer by ring-closing depolymerization, in an efficient circular-type system. This Perspective aims to explore the chemistry involved in the preparation of these monomer/polymer systems, while also providing an overview of the challenges involved, including future directions.

Rapid Ring-Opening Polymerization of γ-Butyrolactone toward High-Molecular-Weight Poly (γ-butyrolactone) by an Organophosphazene Base and Bisurea Binary Catalyst

The low temperature condition, long reaction time and associated high energy inputs involved in the polymerization process still hampered the scalable production of poly(γ-butyrolactone) (PγBL) via ring-opening polymerization (ROP) of low strained γBL due to its unfavorable thermodynamics. In this contribution, we presented the rapid ROP of γBL using a bisurea in combination with an organophosphazene base as the binary catalyst. Well-defined PγBL samples with various terminal groups were prepared by using different alcohol initiators. The bisurea as a co-catalyst exhibited much higher catalytic activity even compared to the most active monourea in previous report as supported by the kinetic experiments. A moderate monomer conversion of 61% was achieved within 10 mins, producing high-molecular-weight PγBL with M_n up to 37.5 kDa and good mechanical properties. The short polymerization time considerably reduced the energy cost for the ROP of γBL conducted at low temperature condition. This study may clear away obstacles for the scalable production and practical applications for PγBL.

Polyarylates containing phthalazinone moieties with excellent thermal resistance

Polyarylates containing phthalazinone moieties are synthesized by interfacial polymerization of 2,4-(4-hydroxyphenyl)-2,3-phthalazin-1-one with isophthaloyl dichloride (IPC) and terephthaloyl dichloride (TPC). The effects of organic solvents and phase transfer catalysts (PTC) on the intrinsic viscosity ( η int) are systematically investigated for polymers with η int up to 1.52 dL g−1. The polyarylate has a high η int with 1,2-dichloroethane and cetyltrimethylammonium bromide used as the organic phase solvent and PTC. It is found that polyarylates prepared from BPPZ with IPC and TPC have excellent thermal resistance, with glass transition temperatures of 292 and 337°C, respectively. The polyarylates exhibit excellent thermal stability with 5% mass-loss temperature above 469°C in both N2 and air, and residual mass ratios at 800°C in N2 and air above 54.1% and 4.0%, respectively.

Advances in the Synthesis of Chemically Recyclable Polymers

The development of modern society is closely related to polymer materials. However, the accumulation of polymer materials and their evolution in the environment causes not only serious environmental problems, but also waste of resources. Although physical processing can be used to reuse polymers, the properties of the resulting polymers are significantly degraded. Chemically recyclable polymers, a type of polymer that degrades into monomers, can be an effective solution to the degradation of polymer properties caused by physical recycling of polymers. The ideal chemical recycling of polymers, i. e., quantitative conversion of the polymer to monomers at low energy consumption and repolymerization of the formed monomers into polymers with comparable properties to the original, is an attractive research goal. In recent years, significant progress has been made in the design of recyclable polymers, enabling the regulation of the "polymerization-depolymerization" equilibrium and closed-loop recycling under mild conditions. This review will focus on the following aspects of closed-loop recycling of poly(sulfur) esters, polycarbonates, polyacetals, polyolefins, and poly(disulfide) polymer, illustrate the challenges in this area, and provide an outlook on future directions.

Tough while Recyclable Plastics Enabled by Monothiodilactone Monomers

The current scale of plastics production and the attendant waste disposal issues represent an underexplored opportunity for chemically recyclable polymers. Typical recyclable polymers are subject to the trade-off between the monomer's polymerizability and the polymer's depolymerizability as well as insufficient performance for practical applications. Herein, we demonstrate that a single atom oxygen-by-sulfur substitution of relatively highly strained dilactone is an effective and robust strategy for converting the "non-recyclable" polyester into a chemically recyclable polymer by lowering the ring strain energy in the monomer (from 16.0 kcal mol^-1 in dilactone to 9.1 kcal mol^-1 in monothiodilactone). These monothio-modification monomers enable both high/selective polymerizability and recyclability, otherwise conflicting features in a typical monomer, as evidenced by regioselective ring-opening, minimal transthioesterifications, and quantitative recovery of the pristine monomer. Computational and experimental studies demonstrate that an n→π* interaction between the adjacent ester and thioester in the polymer backbone has been implicated in the high selectivity for propagation over transthioesterification. The resulting polymer demonstrates high performance with its mechanical properties being comparable to some commodity polyolefins. Thio-modification is a powerful strategy for enabling conversion of six-membered dilactones into chemically recyclable and tough thermoplastics that exhibit promise as next-generation sustainable polymers.

Ring-Opening Polymerization of a Bicyclic Lactone: Polyesters Derived from Norcamphor with Complete Chemical Recyclability

Chemical recycling of polymers is an elegant approach to achieve a circular economy and address the sustainability and end-of-life issues of plastics. Herein, we report the ring-opening polymerization of a bicyclic lactone that is easily accessible from norcamphor. High molecular weight polyesters (M_n up to 164 kg mol^-1) are obtained using ZnEt₂ as catalyst, while the polymerizability of the monomer is good even at high temperatures. More importantly, the polymers can be completely depolymerized under thermolysis conditions to selectively recover the pristine monomer. Thus, the monomer design strategy of using ring-fused/hybridized lactones enables an innovative monomer-polymer system that shows both high polymerizability and high depolymerizability.

Functionalizable and Chemically Recyclable Thermoplastics from Chemoselective Ring-Opening Polymerization of Bio-renewable Bifunctional α-Methylene-δ-valerolactone

It is a highly attractive strategy to develop chemically recyclable polymers to establish a circular plastic economy. Despite the recent advancements, chemically recyclable polymers still face challenges including high energy cost for polymer preparation or recycling, poor monomer recovery selectivity and efficiency as well as undesired material performance. In this contribution, we present the chemoselective controlled ring-opening polymerization of bio-renewable bifunctional α-methylene-δ-valerolactone (MVL) to produce exclusive functionalizable polyester using strong base/urea binary catalysts. The obtained polyester with high molar mass exhibits good tensile strength comparable to that of some commodity plastics. Remarkably, the obtained polyester can be depolymerized to recover pristine monomer with a 96 % yield by thermolysis, thus successfully establishing a closed-loop life cycle.

Modulating Polymerization Thermodynamics of Thiolactones Through Substituent and Heteroatom Incorporation

A central challenge in the development of next-generation sustainable materials is to design polymers that can easily revert back to their monomeric starting material through chemical recycling to monomer (CRM). An emerging monomer class that displays efficient CRM are thiolactones, which exhibit rapid rates of polymerization and depolymerization. This report details the polymerization thermodynamics for a series of thiolactone monomers through systematic changes to substitution patterns and sulfur heteroatom incorporation. Additionally, computational studies highlight the importance of conformation in modulating the enthalpy of polymerization, leading to monomers that display high conversions to polymer at near-ambient temperatures, while maintaining low ceiling temperatures (T_c). Specifically, the combination of a highly negative enthalpy (-19.3 kJ/mol) and entropy (-58.4 J/(mol·K)) of polymerization allows for a monomer whose equilibrium polymerization conversion is very sensitive to temperature.

Rapid and Controlled Polymerization of Bio-sourced δ-Caprolactone toward Fully Recyclable Polyesters and Thermoplastic Elastomers

The development of chemically recyclable polymers presents the most appealing solution to address the plastics' end-of-use problem. Despite the recent advancements, it is highly desirable to develop chemically recyclable polymers from commercially available monomers to avoid the costly and time-consuming commercialization. In this contribution, we achieve the controlled ring-opening polymerization (ROP) of bio-sourced δ-caprolactone (δCL) using strong base/urea binary catalysts. The obtained PδCL is capable of chemical recycling to δCL in an almost quantitative yield by thermolysis. Sequential ROP of δCL and l-lactide (l-LA) affords well-defined PLLA-b-PδCL-b-PLLA triblock copolymers, which behave as thermoplastic elastomers with excellent elastic recovery, tensile strength and ultimate elongation. The upcycling of PLLA-b-PδCL-b-PLLA to recover ethyl lactate and δCL with high yields is achieved by refluxing with ethanol and then distillation under reduced pressure.

Advancing the Development of Recyclable Aromatic Polyesters by Functionalization and Stereocomplexation

The development of innovative synthetic polymer systems to overcome the trade-offs between the polymer's depolymerizability and performance properties is in high demand for advanced material applications and sustainable development. In this contribution, we prepared a class of aromatic cyclic esters (M1-M5) from thiosalicylic acid and epoxides by facile one-pot synthesis. Ring-opening polymerization of Ms afforded aromatic polyesters P(M)s with high molecular weights and narrow dispersities. The physical and mechanical properties of P(M)s can be modulated by stereocomplexation and regulation of the side-chain flexibility of the polymers, ultimately achieving high-performance properties such as high thermal stability and crystallinity (T_m up to 209 °C), as well as polyolefin-like high mechanical strength, ductility, and toughness. Furthermore, the functionalizable moieties of P(M)s have driven a wide array of post-polymerization modifications toward access to value-added materials. More importantly, the P(M)s were able to selectively depolymerize into monomers in excellent yields, thus establishing its circular life cycle.

Understanding the Structure-Polymerization Thermodynamics Relationships of Fused-Ring Cyclooctenes for Developing Chemically Recyclable Polymers

Polymers that can be chemically recycled to their constituent monomers offer a promising solution to address the challenges in plastics sustainability through a circular use of materials. The design and development of monomers for next-generation chemically recyclable polymers require an understanding of the relationships between the structure of the monomers/polymers and the thermodynamics of polymerization/depolymerization. Here we investigate the structure-polymerization thermodynamics relationships of a series of cyclooctene monomers that contain an additional ring fused at the 5,6-positions, including trans-cyclobutane, trans-cyclopentane, and trans-five-membered cyclic acetals. The four- and five-membered rings trans-fused to cyclooctene reduce the ring strain energies of the monomer, and the enthalpy changes of polymerizations are found to be in the range of -2.1 to -3.3 kcal mol^-1. Despite the narrow range of enthalpy changes, the ceiling temperatures at 1.0 M span from 330 to 680 °C, due to the low entropy changes, ranging from -2.7 to -5.0 cal mol^-1 K^-1. Importantly, geminal substituents on the trans-five-membered cyclic acetal fused cyclooctenes are found to reduce the ceiling temperature by ∼300 °C, although they are not directly attached to the cyclooctene. The remote gem-disubstituent effect demonstrated here can be leveraged to promote depolymerization of the corresponding polymers and to tune their thermomechanical properties.

Biobased High-Performance Aromatic-Aliphatic Polyesters with Complete Recyclability

The development of high-performance recyclable polymers represents a circular plastics economy to address the urgent issues of plastic sustainability. Herein, we design a series of biobased seven-membered-ring esters containing aromatic and aliphatic moieties. Ring-opening polymerization studies showed that they readily polymerize with excellent activity (TOF up to 2.1 × 10⁵ h^-1) at room temperature and produce polymers with high molecular weight (M_n up to 438 kg/mol). The variety of functionalities allows us to investigate the substitution effect on polymerizability/recyclability of monomers and properties of polymers (such as T_gs from -1 to 79 °C). Remarkably, a stereocomplexed P(M2) exhibited significantly increased T_m and crystallization rate. More importantly, product P(M)s were capable of depolymerizing into their monomers in solution or bulk with high efficiency, thus establishing their circular life cycle.