Feng Renab,
Zhuangzhuang Liangab,
Yifan Jiaab,
Bokun Liab,
Zhiqiang Suna,
Chenyang Hu*a,
Xuan Pang
*ab and
Xuesi Chen
ab
aState Key Laboratory of Polymer Science and Technology, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China. E-mail: cyhu@ciac.ac.cn; xpang@ciac.ac.cn
bSchool of Applied Chemistry and Engineering, University of Science and Technology of China, Hefei 230026, China
First published on 28th August 2025
Poly(glycolic acid) (PGA) is one of the most widely used biodegradable polyesters, but its efficient valorization presents a long-standing challenge. Herein, we report the first facile PGA valorization strategy by utilizing epoxides to upcycle PGA into fused lactones under mild conditions (<100 °C), and subsequent copolymerization to produce copolyesters with wide potential tunability and enhanced performance. In the presence of epoxides and a chromium-based catalyst, PGA was efficiently transformed into fused lactones with a wide range of potential structural adjustability. Subsequently, via copolymerization of the obtained lactones and ε-caprolactone (ε-CL), random copolyesters with tunable compositions and high molecular weights (MWs) were obtained. Notably, the copolyesters show a broad range of thermal and mechanical properties, which also overcomes the trade-off in tensile strength and ductility commonly observed for poly(ε-caprolactone) (PCL) or binary copolymers based on ε-CL/other lactones. For example, high MW copolyesters with optimal compositions (P(6-MDO)6-co-PCL94 and P(6-MDO)8-co-PCL92) show both superior tensile strength (45.4–46.2 MPa) and ductility (1938–2186%). Apart from excellent mechanical properties and thermal stability, all copolyesters possess good chemical recyclability (>87%), establishing a closed-loop life cycle for a sustainable circular economy. This study offers the first efficient, cost-effective and versatile upcycling route for PGA.
Among biodegradable polymers, aliphatic polyesters hold the largest market share and have attracted considerable attention as sustainable substitutes for petroleum-based polymers.8–10 As a representative, PGA has exceptional mechanical robustness, gas barrier properties, and biodegradability, making it broadly applicable in biomedicine, food packaging, agricultural films, and other fields.11–15 Driven by the increasing demand across various industries, PGA's market is projected to reach USD 172 billion by 2025 and USD 954.9 billion by 2033.16
Nevertheless, akin to common biodegradable polymers, most PGAs undergo biodegradation to form glycolic acid and eventually lead to CO2 and H2O products under specific environmental conditions.17–20 In contrast, although PGA can be mechanically recycled, this process is usually accompanied by deterioration of polymer properties.12,21 Therefore, chemical upcycling is one of the most economically efficient and viable methods for PGA at the end of life, which is of particular significance considering the relatively high cost of glycolic acid and glycolide (GA).22–24 However, compared with other polyesters such as poly(lactic acid) (PLA) and PCL, PGA possesses higher crystallinity and solvent resistance, making its chemical (up)cycling much more difficult.25–27 As a result, attempts of PGA chemical (up)cycling have thus far met with limited success, as facile and versatile strategies are yet to be developed. Specifically, to our knowledge, the (up)cycling of PGA was only realized by depolymerizing PGA to GA under rigorous conditions (>200 °C),28,29 or via transesterification, in which γ-butyrolactone (BL) was used as both solvent and comonomer to treat PGA to produce upcycled copolymer of poly(GA-co-BL) that can be further recycled to glycolic acid and BL, but the low MW of the obtained poly(GA-co-BL) might limit further applications.30 However, disadvantages of existing PGA (up)cycling routes, including energy-intensiveness and restricted type of products, render this approach uneconomical and lacking in adaptability.
In a prior work, we disclosed that GA can be coupled with epoxides to form six-membered lactones facilitated by the catalytic system of Salen-Cr(III) and bis(triphenylphosphine)-iminium chloride (PPNCl) at 80 °C.31 During the coupling reaction, GA first underwent ring-opening polymerization to form PGA oligomers, whose chain end was subsequently activated to attack the epoxides to form alkoxy species; and the alkoxy species backbite on the adjacent carbonyl group to eventually form the fused lactones of glycolic acid and epoxides (Scheme S1). Inspired by this work, we envisioned that PGA may be upcycled with epoxides under mild conditions to generate the fused lactones via a similar mechanism, which would open up exciting new possibilities for the facile and versatile upcycling of PGA. Furthermore, while the homopolymerization of this new type of lactones was preliminarily studied in our prior report, its modest polymerizability was discovered to result in homopolymers with limited molecular weight (<10 kDa) and inferior material performance. In contrast, these potential PGA-derived monomers may be uniquely suited as comonomers to be polymerized with common lactones and prepare copolyesters with versatile compositions and unique performance.
In the realm of synthesizing copolyesters, research on the copolymerization of ε-CL with other lactones is gathering ever-increasing interest among the scientific community. ε-CL is a popular lactone with ready availability and high polymerizability,32–34 and PCL can be obtained by the ring-opening polymerization (ROP) of ε-CL as a semicrystalline polymeric material. Owing to its advantages of excellent biodegradability and biocompatibility, PCL also holds promise in biomedical applications.35–41 However, as there is a trade-off in the mechanical strength and ductility commonly observed for PCL-based materials, their use in many applications is precluded. While copolymerization of ε-CL and other lactones can be a viable strategy to tackle this dilemma, synthesizing copolyesters simultaneously showing high mechanical strength and high ductility via binary copolymerization of ε-CL/other lactones represents a longstanding challenge. For example, PLA-co-PCL shows modest tensile strength (σB ∼17.6 MPa) and stretchability (εB ∼877%),42 whereas poly(3-hydroxyburtyrate-co-ε-caprolactone) (P3HB-co-PCL) displays better tensile strength (σB ∼20.5 MPa) at the expense of stretchability (εB ∼106%).43 For PGA-co-PCL, both tensile strength and stretchability are significantly undermined (σB ∼6 MPa, εB ∼2.7%).44 In addition, while poly(p-dioxanone-co-ε-caprolactone) (PDO-co-PCL) exhibits good ductility (εB ∼1700%), it shows much lower strength (σB ∼10 MPa).45 Therefore, the discovery of new comonomers that are capable of producing copolyesters together with ε-CL to achieve both enhanced mechanical strength and ductility would be a significant advance.
In this work, we report the first facile PGA upcycling to synthesize fused lactones with epoxides under mild conditions and their copolymerization with ε-CL (Scheme 1). This approach enables versatile modulation of the monomer substitutions by choosing different epoxides. The obtained monomers can be efficiently copolymerized with ε-CL, and a series of high molecular weight copolymers with different compositions can be obtained. The influence of monomer incorporation and monomer substituents on the crystallinity, thermal and mechanical properties of the copolymers was investigated. Besides reporting the first facile and versatile upcycling route of PGA, this work also documents a rare example of novel PCL-based copolymers with both enhanced mechanical strength and ductility, as well as chemical recyclability.
Entry | Monomer | [I]![]() ![]() ![]() ![]() ![]() ![]() |
T (°C) | t (h) | Conv.b (%) (M/ε-CL) | Mincorp.c (mol%) | Mn,GPCd (kDa) | Đd |
---|---|---|---|---|---|---|---|---|
a Conditions: initiator (I) = BDM, bulk copolymerization.b Conversion of monomers and ε-CL measured by 1H NMR spectra.c Measured by the 1H NMR spectra of the purified copolymer.d Determined by GPC analysis (CH2Cl2) and calibrated against the polystyrene standard. | ||||||||
1 | 6-MDO | 1![]() ![]() ![]() ![]() ![]() ![]() |
80 | 32 | 49/94 | 19 | 51.7 | 1.36 |
2 | 6-MDO | 1![]() ![]() ![]() ![]() ![]() ![]() |
100 | 8 | 42/95 | 18 | 41.1 | 1.39 |
3 | 6-MDO | 1![]() ![]() ![]() ![]() ![]() ![]() |
120 | 4 | 40/98 | 18 | 48.5 | 1.34 |
4 | 6-MDO | 1![]() ![]() ![]() ![]() ![]() ![]() |
140 | 1 | 29/98 | 15 | 36.2 | 1.43 |
5 | 6-MDO | 1![]() ![]() ![]() ![]() ![]() ![]() |
120 | 2 | 28/95 | 22 | 23.3 | 1.34 |
6 | 6-MDO | 1![]() ![]() ![]() ![]() ![]() ![]() |
120 | 4 | 21/95 | 28 | 19.4 | 1.39 |
7 | 6-EDO | 1![]() ![]() ![]() ![]() ![]() ![]() |
120 | 4 | 24/97 | 10 | 39.3 | 1.34 |
8 | 6-AMDO | 1![]() ![]() ![]() ![]() ![]() ![]() |
120 | 4 | 52/96 | 21 | 31.1 | 1.29 |
9 | 6-PhDO | 1![]() ![]() ![]() ![]() ![]() ![]() |
120 | 4 | 61/96 | 28 | 39.8 | 1.31 |
10 | 6-MDO | 1![]() ![]() ![]() ![]() ![]() ![]() |
120 | 8 | 31/98 | 6 | 101.3 | 1.34 |
11 | 6-MDO | 1![]() ![]() ![]() ![]() ![]() ![]() |
120 | 8 | 41/97 | 8 | 103.9 | 1.31 |
12 | 6-MDO | 1![]() ![]() ![]() ![]() ![]() ![]() |
120 | 8 | 35/96 | 10 | 91.7 | 1.35 |
13 | 6-MDO | 1![]() ![]() ![]() ![]() ![]() ![]() |
120 | 8 | 48/97 | 14 | 95.4 | 1.41 |
14 | 6-MDO | 1![]() ![]() ![]() ![]() ![]() ![]() |
120 | 10 | 46/98 | 17 | 87.6 | 1.31 |
15 | 6-EDO | 1![]() ![]() ![]() ![]() ![]() ![]() |
120 | 12 | 27/98 | 9 | 84.8 | 1.32 |
16 | 6-AMDO | 1![]() ![]() ![]() ![]() ![]() ![]() |
120 | 24 | 68/95 | 11 | 92.4 | 1.36 |
17 | 6-PhDO | 1![]() ![]() ![]() ![]() ![]() ![]() |
120 | 24 | 71/98 | 10 | 93.1 | 1.40 |
18 | ε-CL | 1![]() ![]() ![]() ![]() ![]() ![]() |
120 | 4 | -/89 | — | 102.7 | 1.22 |
To further obtain high-MW copolyesters with tunable compositions, we conducted copolymerizations under the established polymerization protocol with higher monomer molar ratios ([6-MDO]/[ε-CL] ranging from 100:
1000 to 500
:
1000). And we successfully produced high-MW P(6-MDO)-co-PCL (Mn ∼87.6–103.9 kDa, Đ ∼1.31–1.41) with variable 6-MDO incorporation ratios ranging from 6 mol% to 17 mol% (Table 1, entries 10–14). Similarly, the copolymerization of other monomers with ε-CL also successfully produced high-MW copolyesters with tunable incorporation ratios. By fine-tuning the feeding ratio, we were able to synthesize P(6-EDO)-co-PCL, P(6-AMDO)-co-PCL, P(6-PhDO)-co-PCL with similar high MW, dispersity (Mn ∼84.8–93.1 kDa, Đ ∼1.32–1.40) and comonomer content (10 mol%) (Table 1, entries 15–17).
To illuminate the copolymerization process in depth, we conducted a kinetic study on the copolymerization of ε-CL and 6-MDO. The crude mixture was probed by 1H NMR analysis sequentially, and the conversion of each comonomer was plotted against reaction time. During the whole period of copolymerization, ε-CL and 6-MDO were converted simultaneously. Meanwhile, the conversion rate for ε-CL was apparently faster than 6-MDO: for example, after 1 h, the conversion of ε-CL was 82% whereas the conversion of 6-MDO was 28%. Upon further extending the reaction time to 3.5 h, the conversion of ε-CL was 97% and the ultimate conversion of 6-MDO reached 39% to produce the final random copolymer (Fig. 1A)
![]() | ||
Fig. 1 (A) Monomer conversion as a function of time for the copolymerization of 6-MDO and ε-CL. (B) 1H NMR spectra of P(6-MDO)-co-PCL (Table 1, entry 3). (C) A typical DOSY NMR spectrum of P(6-MDO)-co-PCL (Table 1, entry 3). |
The structure of copolyester was further verified by the 1H NMR spectrum (Fig. 1B and S11–S17). Within 4.15–3.95 ppm, the peaks belonging to ε-CL units (the methylene group proximal to the oxygen atom of ε-CL units) appeared as two sets of triplet signals, corresponding to CL-6-MDO heterosequence and CL–CL homosequence. Whereas only one set of singlet signal was observed for 6-MDO units (the methylene protons near the carbonyl group of 6-MDO units). This result further confirms that the copolyesters are mainly composed of repeating CL–CL units on the backbone with randomly incorporated 6-MDO units. Furthermore, the DOSY NMR spectrum of P(6-MDO)-co-PCL (Fig. 1C) gave only a single diffusion coefficient, confirming that the polymer is solely comprised of random copolyester rather than a physical blend of PCL and P(6-MDO). The 1H and DOSY NMR spectra of all other copolymers also showed similar structural evidence of copolyesters (Fig. S12, S14, S16 and S18–S20).
To further analyze the microstructure of copolymers, 2D heteronuclear multiple bond correlation (HMBC) NMR was conducted for the obtained copolyesters (Fig. 2A and S21–24). The 3J long–range correlation between the methylene proton signal at 4.08–4.03 ppm [–CH2OC(O)–, H6] and the carbonyl carbon at 173.4 ppm [–CH2OC(O)CH2CH2–, C1] corresponded to CL–CL homosequences. And the 3J long–range correlation between the methylene proton at 4.16–4.12 ppm [–CH2OC(O)–, H6′] and the carbonyl carbon at 170.1 ppm [–CH2OC(O)CH2O–, C7] corresponded to the CL-6-MDO heterosequences. Besides, the 3J long–range correlation between the methine proton at 5.14–5.06 ppm [–CH2CH(CH3)OC(O)CH2CH2–, H10] and the carbonyl carbon at 172.9 ppm [–CH2CH(CH3)OC(O)CH2CH2–, C1′] corresponded to 6-MDO-CL heterosequences. The weak 3J long–range correlation signal of the methine proton at 5.20–5.15 ppm [–CH2CH(CH3)OC(O)CH2CH2–, H10′] with carbonyl carbon at 169.7 ppm [–CH2CH(CH3)OC(O)CH2O–, C7′] was attributed to 6-MDO-6-MDO homosequences, indicating the presence of trace consecutive 6-MDO units along the polymer chains. Therefore, the copolymer microstructure was inferred to consist of PCL chain segments containing randomly distributed and discrete monomer units, as well as trace repeating fused monomer segments.
![]() | ||
Fig. 2 (A) Selected regions from the 2D HMBC NMR spectra of P(6-MDO)-co-PCL (Table 1, entry 3). (B) Schematic representations of the relative correlations observed in P(6-MDO)-co-PCL (Table 1, entry 3). |
To confirm the well-defined structure of the obtained copolyesters, we further conducted MALDI-TOF mass spectrometry on a low-MW copolyester (Fig. S26). The m/z values can be rationalized as a function of x and y: m/z = 114.14x+130.14y+138 (BDM) + 23 (Na+), where x and y are the degrees of polymerization of ε-CL and 6-EDO, respectively. Therefore, the peaks in the spectrum can be assigned as a series of P(6-EDO)-co-PCL with different amounts of 6-EDO and ε-CL repeating units together with the BDM initiator.
As revealed by the curves of DSC (Fig. 3A and S43–45), all the copolymers displayed a melting peak in the second DSC heating run, indicating their semicrystalline nature. By increasing the content of 6-MDO from 6 mol% to 17 mol%, the melting point (Tm) of the copolymer decreased from 51.8 °C to 36.1 °C, validating that the disorder of polymer chains increased with the increase of 6-MDO incorporation. It is interesting to note that there was a linear correlation between Tm and the amount of ε-CL (mol%) (Fig. 3B), implying the precise tunability of the copolymer's primary structure over thermal properties. Meanwhile, the Tms of P(6-MDO)10-co-PCL90, P(6-EDO)9-co-PCL91 and P(6-AMDO)11-co-PCL89 were similar (between 43.1 and 44.8 °C), which was attributed to the flexibility of the side groups that had little effect on the crystallization properties of the copolymers. However, the Tm of P(6-PhDO)10-co-PCL90 dropped to 35.6 °C, possibly because the bulky side phenyl group hampered the packing and neat arrangement of polymer chains and retarded the crystallization of the copolymer.
According to the TGA results (Fig. 3C), compared with pure PCL (decomposition temperature at 5% weight loss (Td,5%) = 333.6 °C), the incorporation of monomers resulted in a slight increase in Td,5%s for all copolymers: copolymers with increased amount of 6-MDO (from 6 mol% to 17 mol%) showed a slight decrease of Td,5% from 345.2 to 333.9 °C. Besides, all copolymers showed good thermal stability with a Td,5% of 333.8–345.2 °C (Fig. S52–S54) except for P(6-PhDO)10-co-PCL90, which show a decrease in Td,5% to 316.7 °C.
We further studied the crystallinity of copolyesters with X-ray diffraction (XRD) analysis. It could be observed that the position of the diffraction peak for the copolymers remained unchanged in the case of copolymers with increased 6-MDO content (Fig. 3D) or copolymers composed of ε-CL and other comonomers (Fig. S56–S59), which were basically the same as the diffraction peak position of PCL. It is therefore inferred that all copolymer samples crystallize as an orthorhombic lattice, the same as PCL.
Entry | Sample | Mn (kDa) | Đ | σy (MPa) | σB (MPa) | εB (%) | E (MPa) |
---|---|---|---|---|---|---|---|
1 | P(6-MDO)6-co-PCL94 | 101.3 | 1.34 | 12.6 ± 0.2 | 46.2 ± 2.8 | 1938 ± 51 | 211 ± 8 |
2 | P(6-MDO)8-co-PCL92 | 103.9 | 1.31 | 10.0 ± 0.1 | 45.4 ± 0.3 | 2186 ± 91 | 162 ± 2 |
3 | P(6-MDO)10-co-PCL90 | 91.7 | 1.35 | 7.8 ± 0.1 | 33.5 ± 0.8 | 2209 ± 75 | 86.3 ± 8.9 |
4 | P(6-MDO)14-co-PCL86 | 95.4 | 1.41 | 5.7 ± 0.4 | 31.0 ± 0.6 | 2661 ± 58 | 54.9 ± 2.2 |
5 | P(6-MDO)17-co-PCL83 | 87.6 | 1.31 | 5.6 ± 0.1 | 27.1 ± 0.3 | 2803 ± 26 | 65.4 ± 2.9 |
6 | P(6-EDO)9-co-PCL91 | 84.8 | 1.32 | 7.9 ± 0.3 | 36.3 ± 0.1 | 2434 ± 36 | 98.9 ± 2.6 |
7 | P(6-AMDO)11-co-PCL89 | 92.4 | 1.36 | 6.9 ± 0.3 | 32.3 ± 1.7 | 2097 ± 77 | 93.9 ± 7.9 |
8 | P(6-PhDO)10-co-PCL90 | 93.1 | 1.40 | 4.2 ± 0.2 | 21.8 ± 1.0 | 2049 ± 47 | 73.7 ± 3.9 |
9 | PCL | 102.7 | 1.22 | 17.3 ± 0.4 | 36.7 ± 1.2 | 1464 ± 13 | 250 ± 13 |
Compared to PCL, copolyesters with <10 mol% incorporated 6-MDO showed significantly improved tensile strength and ductility: P(6-MDO)6-co-PCL94 and P(6-MDO)8-co-PCL92 exhibited higher ultimate breaking tensile strength (σB) of up to 46.2 MPa with elongation at break (εB) of 1938–2186%. As a reference, the homopolymer of PCL displayed a σB of 36.7 ± 1.2 MPa and εB of 1464 ± 13% (Table 2, entry 9). Moreover, the mechanical performance of P(6-MDO)6-co-PCL94 and P(6-MDO)8-co-PCL92 was also superior to low-density polyethylene (LDPE, melt-flow index = 7.5) in terms of ultimate strength and ductility (σB = 12.0 ± 0.8 MPa and εB = 385 ± 21%).50 Further increasing the content of 6-MDO in the random copolymer contributed to a reduction in tensile strength but a dramatic enhancement in ductility. P(6-MDO)10-co-PCL90 displayed reduced tensile strength (σB = 33.5 ± 0.8 MPa) but enhanced ductility (εB = 2209 ± 75%). And further increasing the 6-MDO content to 14 mol% led to a boost in elongation to 2661 ± 58% for P(6-MDO)14-co-PCL86 with a decreased σB of 31.0 ± 0.6 MPa. The copolymer with the maximum 6-MDO incorporation (17 mol%, P(6-MDO)17-co-PCL83) possessed the lowest σB of 27.1 ± 0.3 MPa but the highest stretchability of 2803 ± 26%. In addition, the modulus decreased gradually from 211 MPa to 65 MPa as the content of 6-MDO increased. While homo-PCL generally exhibited unvaried crystallinity during orientation due to the lack of chain mobility, PCL incorporated with a minor amount of 6-MDO may have higher segmental mobility of polymer chains, so as to foster the strain-induced crystallization of copolyesters during the tensile test. In this way, both tensile strength and stretchability would be dramatically enhanced. As the content of 6-MDO exceeded 10 mol%, the CL segment lengths in the polymer chains would be reduced. This in turn resulted in disrupted crystallization of copolymers and decreased tensile strength but continuously improved ductility. Notably, P(6-MDO)14-co-PCL86 exhibited the characteristics of an elastomer. To further evaluate the properties of elastomers, P(6-MDO)14-co-PCL86 was subjected to 10 reciprocating tensile tests where the sample was stretched to 100% strain and relaxed. Obvious plastic deformation and strain softening were observed in the first cycle, probably due to stress-induced polymer chain disentanglement. However, the plastic deformation produced in the first cycle was overcome, and the almost identical elastic behavior was maintained in the subsequent 2–10 cycles, with elastic recovery of 74–96% (Fig. 4B).
Subsequently, we investigated the mechanical properties of copolymers derived from other comonomers and ε-CL with similar high-MW and incorporation ratios (∼10 mol%). Among the copolymers, P(6-EDO)9-co-PCL91 possessed the highest σB of 36.3 ± 0.1 MPa and the highest εB of 2434 ± 36%. Furthermore, P(6-MDO)10-co-PCL90 showed slightly lower tensile strength (σB = 33.5 ± 0.8 MPa) and ductility (εB = 2209 ± 75%). In addition, P(6-AMDO)11-co-PCL89 and P(6-PhDO)10-co-PCL90 showed comparable stretchability of 2097 ± 77% and 2049 ± 47%, but the tensile strength (σB = 32.3 ± 1.7 MPa) of P(6-AMDO)11-co-PCL89 was superior to that of P(6-PhDO)10-co-PCL90 (σB = 21.8 ± 1.0 MPa). Compared with poly(butylene adipate-co-terephthalate) (PBAT, Biocosafe 2003, Tm = 120 °C, σB = 16.4 ± 0.8 MPa, εB = 503 ± 47%, E = 64.2 ± 3.1 MPa),51 the four copolymers exhibited higher tensile strength, ductility and modulus. For copolymers synthesized from 6-EDO, the ethyl side group can promote the regular arrangement and dense stacking of the polymer chains, due to its low conformational degree of freedom.52 Meanwhile, it would also increase the flexibility of copolymers by breaking the integrity of crystallization. In this way, P(6-EDO)9-co-PCL91 displays better tensile strength and ductility than P(6-MDO)10-co-PCL90. Besides, because allyloxymethyl has larger steric hindrance than that of methyl and ethyl but is more flexible than the phenyl group at the same time, the polymer chains of P(6-AMDO)11-co-PCL89 are more prone to regular stacking than P(6-PhDO)10-co-PCL90, resulting in higher tensile strength for P(6-AMDO)11-co-PCL89 than P(6-PhDO)10-co-PCL90.
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Fig. 5 Overlay of the 1H NMR spectrum of P(6-MDO)-co-PCL, recycled ε-CL and 6-MDO, and started monomer 6-MDO and ε-CL for comparison. |
When the incorporation amount of 6-MDO was less than 10 mol%, the tensile strength (45.4–46.2 MPa) and ductility (1938–2186%) of the copolymers were significantly improved in comparison to PCL. Further increasing the content of 6-MDO in the copolymer contributed to a dramatic enhancement in the material ductility but reduced tensile strength. Among them, P(6-MDO)14-co-PCL86 exhibited the features of an elastomer, with elastic recovery of 74–96%. When comparing the mechanical properties of copolymers with the incorporation of different substituent monomers of around 10 mol%, among the four copolymers, P(6-EDO)9-co-PCL91, owing to the low conformational degree of freedom of ethyl, displayed the highest σB of up to 36.3 ± 0.1 MPa, with the highest stretchability of 2434 ± 36%. The largest steric hindrance of phenyl caused a decrease in tensile strength (σB = 21.8 ± 1.0 MPa) of P(6-PhDO)10-co-PCL90 while maintaining high ductility (εB = 2049 ± 47%). All copolymers demonstrated a wider range of thermal properties and mechanical performances depending on their compositions and monomer substituents. And these copolymers could be efficiently closed-loop depolymerized into initial monomers, establishing a closed-loop life cycle for a sustainable circular polymer economy.
Supplementary information: experimental procedures and detailed characterization of monomers and copolyesters. See DOI: https://doi.org/10.1039/d5sc05815e.
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