S/O and vinyl isomerization enables ultrafast cationic ring-opening polymerization toward CO₂-derived polythioester with migrated in-chain C=C substituents

Abstract

Developing high-performance CO₂-based polymers is promising to address the challenges of CO₂ sequestration and the environmental impact of petroleum-based plastics. The δ-lactone 3-ethylidene-6-vinyltetrahydro-2H-pyran-2-one and its derivatives, synthesized from CO₂ with 1,3-butadiene, have emerged as very promising CO₂-derived monomers. However, their general ring-opening polymerizations face challenges with thermodynamics and kinetics, generally resulting in long reaction times, low conversions, and low-molecular-weight polyesters with poor mechanical properties. Herein, we report a dual isomerization-driven cationic ring-opening polymerization (DI-CROP) of a CO₂-derived thionolactone, 3-ethyl-6-vinyltetrahydro-2H-pyran-2-thione, in which the relayed S/O and vinyl isomerizations significantly enhance polymerization activity, enabling the rapid synthesis of high-molecular-weight CO₂-based polythioesters, achieving near-quantitative conversion within just a few minutes. Also, the relayed S/O and vinyl isomerizations in DI-CROP can easily migrate C=C substituents on the ring of thionolactone into its backbone. These features further enable the production of sustainable CO₂-based materials through efficient copolymerization and post-polymerization functionalization. This study enriches the realm of isomerization-driven polymerizations, and provides a new synthetic approach to CO₂-derived polymeric materials.

The ring-opening polymerization of CO2-derived EVP, a cyclic lactone, struggles with poor thermodynamics and kinetics. Here, the authors convert EVP to a thionolactone and demonstrate that S/O and vinyl isomerization enhances the polymerization activity and enables the rapid synthesis of high-molecular-weight CO2-based polythioesters.

Introduction

Developing sustainable polymers by using CO₂ represents a promising solution, addressing the dual challenges of large-scale CO₂ sequestration^1,2 and the environmental impact of widely used petroleum-based plastics^3–5. Recently, CO₂-based aliphatic polycarbonates, polyesters and polyurethanes have been synthesized through copolymerizations of CO₂ with epoxides^6–9, ROPs of CO₂-derived lactone intermediates^10–13, and step-growth polymerizations^14,15. However, other promising sustainable polymers, such as polythioesters^16–19, which exhibit fascinating properties, excellent degradability and recyclability, have been rarely explored through CO₂ polymerization. Additionally, enhancing the activity of CO₂ and CO₂-derived monomers to produce high-performance sustainable polymers that can compete with common plastics remains a challenge²⁰. To this end, there is an urgent need to develop new CO₂-derived compounds and synthetic methods for creating CO₂-based polymers.

3-Ethylidene-6-vinyltetrahydro-2H-pyran-2-one (EVP), synthesized from CO₂ and 1,3-butadiene, has emerged as a highly promising CO₂-based monomer^21–24 for four key features. First, EVP can be synthesized with high selectivity and yield^25,26. Second, 1,3-butadiene, an economical platform chemical, can be sourced from biomass^27,28 and can also be converted from CO₂ through a two-step hydrogenation and dehydrogenation-coupling process. This CO₂-derived butadiene has already found application in rubber production by Toyo Tires²⁹, indicating that EVP can be prepared via a pathway with extremely high CO₂ content. Third, EVP contains CO₂-derived lactone structure and its ROP is expected to produce sustainable polyester materials with degradation and chemical recyclability. Fourth, the retained carbon-carbon double bond group provides a scaffold for functionalization, allowing further tailoring of the properties of EVP polymers (Fig. 1A). Nevertheless, EVP exhibits very low ROP activity due to the inherent low ring strain from its stable six-membered disubstituted structure, making the ROP process become thermodynamically unfavorable^10,30–33. Furthermore, the secondary oxyanion that forms following the cleavage of the acyl-oxygen bond exhibits reduced nucleophilicity, making the ROP process kinetically less favorable. As a consequence, its ROP requires long reaction times at low temperatures, generally leading to low conversion (<45 %) and the production of low-molecular-weight polyesters (M_n < 16 kg/mol)¹⁰, often accompanied by side reactions³³. Tonks³⁰, Lin³⁴, and Ni³⁵ have made significant contributions to modifying the EVP structure to eliminate conjugated double bonds, thereby improving monomer conversion. But ROPs of these EVP derivatives still require prolonged reaction time and cannot reach high molecular weights in a controlled manner. In addition, all resulting polyesters exhibited poor mechanical properties. Attempts to enhance mechanical performance through copolymerization strategies remained unsatisfactory due to the low polymerization activity of EVP and its derivatives^36–39. Therefore, significantly enhancing the ROP activity of EVP and its derivatives to facilitate the simple, rapid, and controllable production of high-molecular-weight CO₂-based sustainable polymers with good performance is highly desired but still a challenge.

Fig. 1. Ring-opening polymerization of CO₂-derived EVP lactone and its derivatives.

A The previously reported anionic ring-opening polymerization (AROP) of CO₂-derived EVP and its derivatives via the cleavage of acyl-oxygen bond. B The new CO₂-derived thionolactone EtVT and its dual isomerization-driven cationic ring-opening polymerization (DI-CROP) via the cleavage of alkyl-oxygen bond.

Here, we report a dual isomerization-driven cationic ring-opening polymerization (DI-CROP) of a new CO₂-derived thionolactone, 3-ethyl-6-vinyltetrahydro-2H-pyran-2-thione (EtVT), in which the relayed S/O and vinyl isomerizations occurred in the polymerization. The ROPs of various thionolactones can selectively proceed via anionic, cationic, or radical-mediated S/O isomerization pathways^40–45. Notably, Hong and coworkers utilized S/O isomerization to transform structurally challenging yet high-value butyrolactones into high-performance polymers^40,41. In our assumption, S/O isomerization promotes the cleavage of the alkyl-oxygen bond rather than the acyl-oxygen bond, while the pendant vinyl group could isomerize into a π-allyl intermediate structure sequentially, further facilitating the occurrence of ring-opening, thereby highly enhancing the ROP activity. This is different from previously reported ROPs of EVP and its derivatives, in which the polymerization occurred through the cleavage of the acyl-oxygen bond, and the vinyl group not only failed to facilitate the polymerization but actually reduced the polymerization activity from both thermodynamic and kinetic perspectives. As a result, this DI-CROP allows the rapid production of high-molecular-weight polythioesters in just a few minutes (Fig. 1B). In addition, the relayed S/O and vinyl isomerizations occurring in the polymerization could easily migrate C=C substituents into the polymer backbone, creating a new polymer with a new structure. Compared to the reported polymerization of CO₂-derived monomers, this DI-CROP of EtVT offers significant advantages of extremely high ROP activity and migrating the C=C substituents of EtVT into the polythioester backbone. These features enable the rapid and controllable production of sustainable materials with excellent mechanical performance through efficient copolymerizations and post-polymerization functionalization. In the review process of this work, an interesting paper by Ni et al. reported a carbenium-based CROP of EVP via the cleavage of the alkyl–oxygen bond, also leading to the partial migration of the pendant double bond into the polymer backbone⁴⁶. This contribution provides a very valuable example that facilitates the challenging ROP of EVP. However, the polymerization still required several days with low conversions and low molecular weights.

Results

Our investigations began by carrying out the one-step thiocarbonylation of 3-ethyl-6-vinyltetrahydro-2H-pyran-2-one (EtVP) by P₂S₅ and hexamethyldisiloxane, producing the corresponding thionolactone monomer, namely 3-ethyl-6-vinyltetrahydro-2H-pyran-2-thione (EtVT). Full characterization of this new CO₂-derived thionolactone compound through ¹H NMR, ¹³C NMR and mass spectroscopy were obtained (Figure S1–S3).

We then tried the CROP of EtVT using the commonly used Lewis acid BF₃•Et₂O as the catalyst with an initial feed ratio of [EtVT]₀/[BF₃•Et₂O]₀ = 100/1 and [EtVT]₀ = 1 M in dichloromethane at room temperature. Surprisingly, the polymerization proceeded very fast, and the EtVT conversion reached 94 % within just 3 min, obtaining a polymer PEtVT with a number-average molecular weight (M_n) of 17.8 kg/mol (entry 1 in Table 1 and Figs. S4, S5). The structure of resulted PEtVT was characterized by ¹H NMR and ¹³C NMR spectroscopy (Fig. 2A, B). Different from the thiocarbonyl carbon signal of pristine EtVT (~220 ppm), the characteristic signal in the PEtVT was at ~201 ppm, which was consistent with that of thioester group, clearly indicating that the exclusive S/O isomerization occurred during the ring opening process. Interestingly, we discovered that the signals corresponding to the carbon-carbon double bond group of the resulted polymer exhibited distinct multiple series of signals in both ¹H NMR and ¹³C NMR spectra, sharply differing from those of a typical terminal double bond on a polymer backbone. Moreover, the integration of carbon-carbon double bond signals in the ¹H NMR spectrum did not match expectations, suggesting the participation of the double bond in the CROP and the presence of more than one type of double bond structure in the product. Only one diffusion coefficient in the DOSY NMR spectroscopy (Figure S6) was further observed, confirming that the resulted product PEtVT comprised homogeneous polymer chains with different double bond structures. By further carefully analyzing the 2D HSQC spectrum (Figure S7), we identified the structure of the produced PEtVT, revealing that the double bonds on part of EtVT monomers were migrated to the main chain during the CROP. The ratio of pendant double bonds to in-chain double bonds is nearly 30/70 according to the ¹H NMR integration, indicating the migration of double bonds is dominant in the CROP of EtVT. We conducted further analysis of the polymer using ATR-IR. This analysis revealed a strong peak at 965 cm⁻¹, corresponding to the bending vibration (C − H) of trans configuration (Figure S8)^47,48. Combined with the ¹³C NMR spectrum, we concluded that the in-chain double bonds predominantly adopt the thermodynamically more stable trans configuration.

Table 1.

Results of Dual Isomerization-driven Cationic Ring-Opening Polymerization^a

entry	catalyst	solvent	[M]0/[Cat]0	time (min)	conv. (%)b	Mn,SEC (kg/mol)c	Ðc	m/nd
1	BF3·Et2O	DCM	100/1	3	94	17.8	1.50	30/70
2	TfOMe	DCM	100/1	1	97	9.1	1.44	30/70
3	Tf2NTMS	DCM	100/1	15	96	10.3	1.49	32/68
4	Ph3C+B(C6F5)4−	DCM	100/1	1	99	18.6	1.40	37/63
5	Et3O+B(C6F5)4−	DCM	100/1	1	99	17.7	1.48	37/63
6	BF3·Et2O	toluene	100/1	10	92	12.9	1.49	20/80
7	BF3·Et2O	xylene	100/1	10	92	10.8	1.50	24/76
8	BF3·Et2O	CCl4	100/1	10	93	21.8	1.45	21/79
9	BF3·Et2O	dichloroethane	100/1	1	95	20.8	1.41	31/69

^aPolymerizations were conducted at room temperature, [M]₀ = 1 M. ^bConversions were measured by ¹H NMR of the quenched solution in CDCl₃. ^cDetermined by SEC at 40 °C in THF with a PSt standard. ^dThe ratios of pendant vinyl group (m) and in-chain vinyl group (n) were determined by ¹H NMR.

Fig. 2. CROP of EtVT catalyzed by BF₃•Et₂O.

A ¹H NMR spectrum (400 MHz, CDCl₃) of produced PEtVT. B ¹³C NMR spectrum (101 MHz, CDCl₃) of produced PEtVT. C Overlay of SEC profiles at different [EtVT]₀ /[BF₃•Et₂O]₀ ratios. D Plots of M_n and Đ as a function of [EtVT]₀ /[BF₃•Et₂O]₀ ratio.

This remarkably high polymerization activity encouraged us to investigate the CROP process of EtVT in detail. Excitingly, we observed gradual growth in PEtVT M_ns with increasing monomer conversion and initial monomer-to-catalyst feeding molar ratio from 100/1 to 500/1 using BF₃•Et₂O as the catalyst in dichloromethane at −25 °C, while maintaining relatively narrow and unimodal molecular weight distributions (Fig. 2C). All the polymerizations can achieve over 92% monomer conversions within 20 min even at high monomer-to-initiator ratios (Table S1 and Figure S9–S13). At higher initiator concentrations of 200/1, 100/1, 75/1, and 50/1, the measured M_ns were slightly higher than the theoretical molecular weights, possibly due to the extremely high chain growth rate. The measured M_ns were close to the theoretical values at initial monomer-to-catalyst feeding ratios of 300/1, 400/1, and 500/1 (Fig. 2D). As a result, a high-molar-mass PEtVT (75.4 kg/mol, Đ = 1.35) can be obtained at a monomer to catalyst ratio of 500/1. To identify the chain structure of resulted PEtVT, we performed matrix-assisted laser desorption/ionization-time of flight mass (MALDI-TOF) analysis on a low-molecular-weight sample initiated by BF₃•Et₂O and quenched by H₂O. As shown in Figure S14, only one set of molecular peaks with a mass interval of 170.1 Da is observed, which confirmed the definite structure of PEtVT with an α-end group corresponding to the mass of H atom and an ω-end group corresponding to -OH group. Moreover, the CROP of EtVT conducted across varied initial EtVT concentrations, ranging from 3 M to as low as 0.5 M, maintained high conversions in a short time (Figure S15–S17), demonstrating exceptionally high polymerization activity. In comparison, the reported anionic ROPs of EVP and its derivatives required bulk conditions or high concentrations greater than 5 M, and even at high initiator concentrations ([monomer]/[initiator] = 25/1, 50/1, etc.), they still required long reaction times of 8 h to 7 days^10,30, resulting in low molecular weight polyesters. Notably, changing the feed ratio, concentration and temperature (Figure S18, S19) had little effect on the ratio of pendant double bonds to in-chain double bonds of resulting PEtVT, which remained at nearly 30/70 according to the ¹H NMR integrations.

We further investigated the CROP of EtVT using different catalysts and solvents. Besides BF₃•Et₂O, other commonly used cationic catalysts, such as methyl trifluoromethanesulfonate (TfOMe), N-trimethylsilyl bis(trifluoromethanesufonyl)imide (Tf₂NTMS), [Ph₃C]⁺[B(C₆F₅)₄]^– as well as [Et₃O]⁺[B(C₆F₅)₄]^–, which was developed by Hong and coworkers were explored. All these catalysts were efficient for the CROP of EtVT, reaching high conversions in a short time (entry 1−5 in Table 1 and Figure S20–S27). Specifically, when using onium salts [Ph₃C]⁺[B(C₆F₅)₄]^– and [Et₃O]⁺[B(C₆F₅)₄]^– as catalysts, the polymerization proceeded so rapidly that it even caused the solution to boil at room temperature. The EtVT conversion reached nearly 100 % upon addition of the catalyst (<1 min), as these two catalysts readily form loose/free ion pair active species due to the bulky hindrance of the non-coordinating counteranion [B(C₆F₅)₄]^–. Cationic polymerization is typically significantly influenced by solvents, with better polymerization results often observed in low-polarity, non-protic solvents. We then examined a range of solvents, including dichloromethane, dichloroethane, toluene, xylene, tetrachloromethane, and found that CROP of EtVT could also effectively proceed in all of them (entry 6−9 in Table 1 and Figure S32–S39), demonstrating the high efficiency and feasibility of this CROP of EtVT. All the resulting PEtVTs produced by these catalysts and solvents had similar structures, but with differences in the ratios of pendant double bonds to in-chain double bonds (entry 2−9 in Table 1, Figure S28–S31 and Figure S40–S43). The content of in-chain double bonds in the PEtVT can be regulated between 63 % and 80 % through the choice of catalysts and solvents. An exclusive polymerization route to obtain PEtVT with fully in-chain double bonds was not as effective as anticipated, suggesting that the selectivity is primarily influenced by the inherent structural characteristics of EtVT. During the CROP, the vinyl groups only underwent migration to the main chain, and no addition reactions of the double bonds were observed under various conditions such as changing the catalyst, solvent, or feeding ratio. The purified PEtVTs exhibited good solubility in common organic solvents.

To elucidate the roles of thiocarbonylation and the pendant vinyl group in this CROP, we conducted the cationic polymerizations of EtVP, 3,6-diethyl-tetrahydro- 2H-pyran-2-one (DEP) and 3,6-diethyltetrahydro-2H-pyran-2-thione (DET, Figure S44–S46) as control experiments (Fig. 3). First, the CROP of DEP lactone under the same conditions and in bulk showed no conversion even after 24 h (Figure S47). The CROP of DET proceeded smoothly at a high initiator concentration of [DET]₀/[BF₃•Et₂O]₀ = 100/1, achieving a 47 % conversion but with a prolonged time of 8 h (Figure S48). The structure of PDET was confirmed as a polythioester by ¹H NMR and ¹³C NMR spectroscopy, demonstrating that S/O isomerization provides the driving force to open the low-strain ring by cleaving the alkyl-oxygen bond. However, the obtained PDET had an uncontrolled M_n (Figure S49–S51). On the other hand, the CROP of EtVP lactone under the same conditions reached a conversion of 71 % but with a prolonged time of 16 h, obtaining a polymer PEtVP with a very low M_n of 1.8 kg/mol. The resulted polyester also had pendant double bonds and in-chain double bonds (Figure S52–S55). Therefore, compared to the unsuccessful CROP of DEP lactone, the vinyl group in EtVP plays an important role to facilitate the ring opening via alkyl-oxygen bond cleavage. However, compared to the CROP of EtVT, the CROP activities of DET and EtVP were depressed. Based on the sharp activity difference and the observed double bond migration, the vinyl group was identified as participating in and strongly promoting the CROP and cooperation of S/O isomerization and vinyl isomerization contributes to the very high CROP efficiency of EtVT. Therefore, the relayed S/O and vinyl isomerization-driven cationic ring-opening polymerization (DI-CROP) has been successfully established.

Fig. 3.

Comparison of CROP activities among EtVT, DET, EtVP, and DEP under identical reaction conditions.

Based on above study, the DI-CROP mechanism of EtVT is proposed. A nucleophilic attack of the thiocarbonyl sulfur of an EtVT monomer to the propagating cationic center affords carbenium cation species before this EtVT undergoes ring opening. The subsequent cleavage of the alkyl-oxygen bond to open the ring is driven by S/O isomerization, similar to that observed in the CROP of DET and some reported thionolactones, and is further accelerated by the vinyl isomerization, where the neighboring vinyl group contributes to the stabilization of the carbocation, facilitating the ring opening to form a π-allyl cation. As for the selectivity and predominant in-chain C=C bonds, we speculate that the monomer is more inclined to attack the less substituted side of the π-allyl cation, resulting in the thermodynamically stable trans in-chain C=C bonds configuration being dominant. This process is similar to the early cationic polymerization of butadiene⁴⁹, where a majority of 1,4-addition products were formed and the double bonds were exclusively in the trans configuration. The cooperation of S/O isomerization and vinyl isomerization provides sufficient driving force for the CROP of EtVT, achieving near-quantitative conversion within minutes. This efficiency is significantly higher than that of DET and EtVP, which is driven solely by S/O isomerization and vinyl isomerization, respectively (Figure S56). During the polymerization, the formed carbocation active species after ring opening was not stable enough, which typically leads to side reactions such as chain transfer. However, the controllability and ability to achieve high molecular weights in the DI-CROP of EtVT here benefit from the much stronger coordination ability of the >C = S group in EtVT monomer to allyl carbocation active species than thioester groups in the polymer chain and other impurities, ensuring a preference for chain propagation over side reactions. In contrast, the previously discussed CROP of EtVP showed poor controllability (Figure S57), achieving only a M_n of 1.8 kg/mol due to significant side reactions affecting the carbocation active species.

The thermal properties of produced PEtVTs were investigated with thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA results exhibit a one-step decomposition profile, with 5% weight loss temperatures ranging from 236.9 °C to 247.7 °C for PEtVTs with varying molecular weights and in-chain double bond contents, indicating moderate thermal stability (Figure S58–S61). The impact of in-chain double bond content on thermal stability is negligible, whereas increasing molecular weight slightly improves the thermal stability of PEtVTs. The DSC curve gives glass transition temperature of −32 °C − −27 °C for PEtVTs with different contents of in-chain double bonds (Figure S62–S64). All the produced PEtVTs exhibited behavior as viscous amorphous liquids.

The high CROP activity, fast polymerization rate and high conversion of EtVT and the formation of in-chain double bonds encouraged us to further explore the copolymerization and post-polymerization functionalization strategies to prepare high-performance CO₂-based sustainable polymers. High affinity of the carbocationic species for > C = S group and very high activity of this DI-CROP would facilitate the copolymerization of EtVT with other thionolactones, thereby enabling the regulation of the properties of CO₂-based copolymer. To the best of our knowledge, there are currently no reported studies on the cationic copolymerization between different thionolactones. Hence, we attempted the CROP of EtVT with a biaryl-fused thionolactone, dibenzo[c,e]-oxepine-5(7H)-thione (DOT). DOT is a well-studied thionolactone known for preparing degradable vinyl polymers through radical polymerizations with vinyl monomers^43,50. The homopolymer of DOT is rigid with a high T_g of 98 °C^51,52, making it very suitable for copolymerization with EtVT (Fig. 4A). Successful cationic copolymerization at varying initial molar ratios of EtVT to DOT using BF₃•Et₂O as the catalyst achieved nearly quantitative conversions. By adjusting the initial feed ratio, we could precisely control the copolymer compositions and properties, leading to a series of copolythioesters, which were thoroughly characterized through ¹H NMR, DOSY NMR and SEC analysis (Fig. 4B, C and Figure S65–S69). The mechanical performance tests further confirmed that the combination of soft PEtVT units and rigid DOT units significantly enhances the copolymers’ mechanical properties, particularly at a [EtVT]/[DOT] molar ratio of 1/0.8, characterized by an ultimate tensile strength of 15.2 MPa, and 338 % elongation at break (Fig. 4D). The tensile strength and elongation are comparable to those of widely used low-density polyethylene, highlighting that high-activity CROP facilitates the copolymerization for producing high-performance, CO₂-based sustainable copolymers.

Fig. 4. Copolymerizations of EtVT with DOT and mechanical properties of the produced copolythioesters.

A The structure of DOT and EtVT. B ¹H NMR spectrum (400 MHz, CDCl₃) of produced copolythioester of EtVT with DOT ([EtVT]/[DOT] = 1/0.6). C SEC curve of produced copolythioester of EtVT with DOT ([EtVT]/[DOT] = 1/0.6). D Tensile stress-strain curve of the copolythioester produced through random copolymerization. E Tensile stress-strain curve and F Cyclic tensile test of the copolythioester produced through sequential addition copolymerization.

We conducted a kinetic study on the copolymerization of EtVT with DOT. Starting with a 1:1 molar ratio of EtVT to DOT, EtVT achieved a high conversion of 87 % at 0.5 min of copolymerization, followed by increasing DOT conversion (Figure S70). This distinct conversion behavior in the copolymerization stems from EtVT’s exceptionally high polymerization activity, which led us to further explore the preparation of a thermoplastic elastomer using a sequential feeding method. It is worth noting that the most widely used commercial thermoplastic elastomers are styrenic block copolymers, which are derived from finite petroleum resources and are non-degradable⁵³. Therefore, developing CO₂-based degradable thermoplastic elastomers represents a highly promising and sustainable alternative. We conducted the CROP of DOT using [Ph₃C]⁺[B(C₆F₅)₄]^– as the catalyst with an initial feed ratio of [DOT]₀/[Ph₃C]⁺[B(C₆F₅)₄]^–₀ = 100/1 and [DOT]₀ = 1 M in dichloromethane at −5 °C. After achieving an approximate 40 % conversion of DOT, as determined by ¹H NMR (Figure S71), a solution of EtVT was added to the system at an initial molar ratio of DOT/EtVT equal to 1/2. Due to EtVT’s significantly higher polymerization activity compared to DOT, when EtVT was added to the solution, it polymerized rapidly, reaching 91 % conversion in just 0.5 min. Meanwhile, the conversion of DOT only increased to 50 %, revealing that only 10 % of DOT structural units were present in the EtVT segment (Figure S72). Then, the homopolymerization of DOT continued until its conversion also reached 97 % after 1 h (Figure S73). The ¹H NMR analysis of the resulted copolymer showed that it contained 35 % DOT-derived units and 65 % EtVT-derived units, aligning with the initial feed ratio (Figure S74). The SEC curve exhibited a well-controlled M_n of 40.4 kg/mol and a narrow unimodal distribution of 1.36 (Figure S75), and the DOSY NMR measurement also displayed a consistent diffusion coefficient (Figure S76). Benefiting from much stronger coordination ability of > C = S group in EtVT monomer to allyl carbocation active species, the CROP of EtVT has satisfactory control. Although chain transfer remained inevitable, the triblock-like copolymer structure predominated. Excitingly, the tensile testing of the copolythioester showed an elongation at break above 1000 % and an ultimate tensile strength at break of 6.2 MPa (Fig. 4E). The subsequent cyclic tensile test also showed good elastic recovery of 90 %, indicative of good elastomeric characteristics (Fig. 4F). Therefore, benefiting from the DI-CROP of EtVT, we can easily regulate the material properties from plastic to elastomer.

In addition to its ultrahigh polymerization activity, another advantage of this DI-CROP is the controllable formation of predominantly in-chain double bonds. These in-chain double bonds have different reactivity compared to pendant double bonds, offering the potential for selectively tailoring PEtVT’s properties. The post-polymerization modifications were then conducted by thiol-ene click reaction and electrophilic addition reaction with dithiophosphoric acid (Fig. 5)⁵⁴. A PEtVT with the ratio of pendant double bonds to in-chain double bonds at 30/70 was chosen as a model. A thiol-ene click reaction was performed by using an equimolar amount of benzyl mercaptan to the pendant double bonds. After irradiation with low-power UV light for 4 h, the ¹H NMR signals corresponding to the pendant double bonds of PEtVT almost completely disappeared, while the in-chain double bonds remained largely unchanged. This indicates the preferential reactivity of the pendant double bonds in the click reaction and demonstrates the distinct selectivity of the unique in-chain double bonds formed during this CROP. Then, dithiophosphoric acid was added to the reaction and heated to 100 °C for 6 h. Up to 72 % of the in-chain double bonds underwent electrophilic addition with dithiophosphoric acid, resulting in a dual post-polymerization modification product with selective click reaction and electrophilic addition (Figure S77).

Fig. 5.

Selective post-polymerization dual modification of PEtVT via thiol-ene click reaction and electrophilic addition with dithiophosphoric acid.

Lastly, after establishing PEtVT as a new CO₂-derived polythioester material, its degradation behavior was subsequently investigated. PEtVT was dissolved in a mixed solution of THF and potassium hydroxide aqueous solution, and then the solution was heated to 95 °C and stirred for 3 days. After acidification with hydrochloric acid and separation treatment, PEtVT was efficiently degraded into corresponding 2-ethyl-5-mercaptohept-6-enoic acid and 2-ethyl-7-mercaptohept-5-enoic acid with an overall yield of 85%. (Figure S78). These degradation products could potentially serve as new valuable building blocks for the synthesis of sustainable materials. Further utilizing the in-chain double bonds of PEtVT generated by the DI-CROP as well as the use of recycled products to construct high-performance CO₂-based polymeric materials is still under investigation.

Discussion

In summary, a CO₂-derived thionolatone (EtVT) was designed and polymerized via dual isomerization-driven cationic ring-opening polymerization (DI-CROP), achieving nearly quantitative conversion within minutes and easily migrating C=C substituents on the ring of thionolactone into polythioesters backbone. This DI-CROP yielded a CO₂-based polythioester containing migrated in-chain C=C bonds with a number-average molecular weight of up to 75.4 kg/mol. The high efficiency of DI-CROP is attributable to a new ROP mechanism through the cleavage of alkyl-oxygen bond triggered by relayed S/O isomerization and vinyl isomerization, providing sufficient driving force for ring-opening. The content of in-chain double bonds in the PEtVT can be effectively regulated between 63 % and 80 % through the choice of catalysts and solvents. EtVT’s high activity further facilitates efficient copolymerization and selective post-polymerization dual modification, creating versatile CO₂-based materials ranging from plastic to elastomer. This study enriches the realm of isomerization-driven polymerizations, and provides a powerful synthetic approach to CO₂-derived sustainable polymeric materials.

Methods

Materials and reagents

The solvents used for polymerizations were degassed and dried over CaH₂ for 24 h. Unless otherwise specified, other reagents were used as received. Carbon dioxide (99.9%) and 1,3-butadiene (99.9%) were purchased from Nanjing Special Gas Company. Tris(2-methoxyphenyl) phosphine (97%), bis(dibenzylideneacetone) palladium (Pd > 18.5%), hexamethylphosphoramide (HMPA, 99%), trichlorosilane (99.9%), 4-methylbenzenesulfonhydrazide (97%), hexamethyldisiloxane (99%), di-tert-butyl decarbonate (98%), triphenylmethylium tetrakis(perfluorophenyl) borate (98%), potassium tetrakis(pentafluorophenyl) borate (97%), TfOMe (97%), Tf₂NTMS (95%), O,O-diethyl dithiophosphate (95%), calcium hydride (CaH₂, 97%) and triethyloxonium tetrafluoroborate (95%) were purchased from Energy Chemical. Benzoin methyl ether (98%) was purchased from Tokyo Chemical Industry. BF₃•Et₂O (98%) and 2,2′-biphenyldicarboxylic acid (98%) were purchased from Aladdin Chemical. Benzyl mercaptan (98%) and P₂S₅ (99%) were purchased from Macklin Chemical.

General analytical information

All NMR spectra were recorded on a Bruker NMR spectrometer (resonance frequency of 400 MHz for ¹H and 101 MHz for ¹³C) operated in the Fourier transform mode. The samples were dissolved in chloroform-d with tetramethylsilane (TMS) as an internal reference. ESI-MS analysis was performed on an LTQ-Orbitrap XL electrostatic field orbital trap mass spectrometer (positive mode). Samples were dissolved in THF. MALDI-TOF MS analysis was performed on an Autoflex Speed TOF/TOF instrument (positive mode). A Cyano-4-hydroxycinnamic acid was used as matrix with NaI added as the cation source. Samples were dissolved in THF. Molecular weights and dispersity (Ð) were measured by using a Waters 150 C size exclusion chromatography (SEC) equipped with microstyragel columns and an RI-201H detector at 40 °C. THF solution with a flow rate of 1.0 mL min⁻¹ was used as eluent. The molecular weights were calibrated against monodispersed polystyrene standards. Differential scanning calorimetry thermogram was measured on a TA Q2000 differential scanning calorimeter instrument in an aluminum pan with a heating or cooling rate of 10 °C min⁻¹ under a flowing nitrogen atmosphere. The T_g values were obtained from the second scan after removing the thermal history. Thermogravimetric analysis was carried out using a TA Instruments Q5000 Thermogravimetric Analyzer. Characterization was performed under nitrogen using aluminum pans with a heating rate of 10 °C/min from room temperature to 600 °C. Infrared spectrum was collected by a Nicolet 380 spectrometer using Attenuated Total Reflectance in the range of wavenumbers of 4000–400 cm⁻¹. A standard test method, ASTM 638, was followed to measure the tensile properties of the copolymer samples. The dog-bone-shaped tensile test specimens were obtained by heat pressing method, which showed a 25 mm gauge length, 2 mm width, and thickness of 0.5 mm. The stress-strain experiments were performed at 20 mm min⁻¹ or 50 mm min⁻¹ using a Universal Test Machine (UTM2502) at room temperature. At least three specimens of each polymer were tested.

General polymerization of EtVT

The polymerization of EtVT was performed in a 2 mL oven-dried vial containing a magnetic stir bar. The vial was first charged with EtVT (85 µL, 0.5 mmol, 100 eq) and DCM (390 µL, [EtVT]₀ = 1.0 M) in the glove box. Then the vial was sealed with a septum, taken out of the glovebox, and placed in low temperature and constant temperature reaction bath. After equilibration at the desired polymerization temperature (−25 °C), the polymerization was initiated by rapid addition of BF₃•Et₂O (25 µL, 0.005 mmol, 1.0 equiv., 0.20 M in DCM) via a gastight syringe. After the desired reaction time, 3 drops of methanol were added to quench the polymerization. A crude aliquot was taken from the reaction mixture and monitored by ¹H NMR spectroscopy to determine monomer conversion. The quenched mixture was then precipitated into cold methanol three times and dried in a vacuum oven to a constant weight.

General copolymerization of EtVT with DOT

Taking the 1/1 ratio copolymerization of two monomers as an example. The copolymerization of EtVT with DOT was performed in a 2 mL oven-dried vial containing a magnetic stir bar. The vial was firstly charged with EtVT (43 µL, 0.25 mmol, 200 eq), DOT (57 mg, 0.25 mmol, 200 eq), and added DCM to a volume of 0.5 mL ([EtVT]₀/[DOT]₀ = 1/1) in the glove box. Then the vial was sealed with a septum, taken out of the glovebox, and placed in low temperature and constant temperature reaction bath. After equilibration at the desired polymerization temperature (0 °C), the polymerization was initiated by rapid addition of [Ph₃C]⁺[B(C₆F₅)₄]^– (12.5 µL, 0.00125 mmol, 1.0 equiv., 0.10 M in DCM) via a gastight syringe. After the desired reaction time, 3 drops of methanol were added to quench the polymerization. A crude aliquot was taken from the reaction mixture and monitored by ¹H NMR spectroscopy to determine monomer conversion. The quenched mixture was then precipitated into cold methanol three times and dried in a vacuum oven to a constant weight.

General degradation experiments of PEtVT

PEtVT (1.36 g 8 mmol) was dissolved in 40 mL THF, and 20 mL of sodium hydroxide solution (4 M) was added. The resulting solution was stirred and heated to 95 °C for 3 days. After cooling, the phases were separated. The aqueous phase was acidified with conc. HCl to pH 2–3, and then extracted with DCM. The combined organic phases were dried over anhydrous Na₂SO₄ and concentrated in vacuo. The residue was purified by column chromatography on silica gel (petroleum ether/ethyl acetate, 2/1) to yield the corresponding 2-ethyl-5-mercaptohept-6-enoic acid and 2-ethyl-7-mercaptohept-5-enoic acid with an overall yield of 85%.

Author contributions

Y.Z.Y. and Z.Z. designed and performed the conceptualization and methodology; Z.B.L., S.R.P., Z.W., Y.X., X.N., and G.C. performed the investigation; Y.Z.Y. and Z.Z. supervised all the experiments; Y.Z.Y., Z.Z., C.Y.H. and L.X. wrote the original manuscript; and Y.Z.Y., Z.Z., L.X., and C.Y.H. revised the final manuscript.

Peer review

Peer review information

Nature Communications thanks Jingsong Yuan and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

Data supporting the findings of this study are available within the article (and its Supplementary information files). All other data are available from the corresponding author upon request.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-025-64559-9.