Chemically recyclable, fully biobased polyolefins with performance parity to low-density polyethylene

Abstract

Plastics underpin modern society yet impose unsustainable burdens: petroleum consumption and environmental persistence. Developing chemically recyclable, biobased plastics with performance parity to conventional polymers represents a critical advance toward circular materials. Herein, we achieve this through molecular design of a structurally precise macrocyclic lactone derived from starch and castor oil. Homopolymerization affords high-molecular-weight polyolefins with strategically embedded ester linkages. Through a simple backbone hydrogenation strategy, the properties of such polyolefins (which may alternatively be classified as polyesters) can be dramatically tuned, converting them from elastomers into plastics. These linkages—uniformly positioned along the backbone—enable low-density polyethylene (LDPE)-comparable thermomechanical properties while serving as programmed break points. Crucially, these polyolefin plastics with a biomass content of up to 100 wt% could undergo quantitative degradation to telechelic diols/diacids under mild conditions, which were efficiently repolymerized via polycondensation to regenerate virgin materials. Owing to their favorable water vapor permeability and thermomechanical properties, these biobased polyolefins are ideal for use in packaging and agricultural films.

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Green foundation

1. This study presents a solution to the non-renewable origin and end-of-life non-recyclability of low-density polyethylene (LDPE) through the construction of fully bio-based polyolefin plastics.

2. The green chemistry achievement in this work is reflected in the renewable sourcing and recyclability of the resulting polyolefin materials. Crucially, these new polyolefin plastics with a biomass content of up to 100 wt% could undergo quantitative degradation (>95% yield) to valuable diols/diacids under mild conditions. Unlike the green materials presented in this work, conventional polyolefins are primarily derived from fossil fuels and, in contrast, cannot degrade after use, presenting a significant environmental challenge.

3. The homopolymer designing strategy in this work, featuring built-in ester bond sequences proposed here, can be generalized to other polyolefin/polyester systems, enabling further optimization of the recyclable polymer performance.

Introduction

The pervasive integration of synthetic polymer-based materials into contemporary society has rendered them indispensable, yet their predominantly linear utilization paradigm and inadequate post-consumer management strategies have precipitated severe ecological challenges, including resource depletion and environmental accumulation of persistent waste.¹ While polymer recycling represents a crucial mitigation approach, conventional mechanical reprocessing techniques invariably compromise material integrity, yielding inferior products with limited applicability.^2,3 This technological constraint necessitates the development of advanced recycling methodologies capable of preserving material performance characteristics.⁴ Within this context, chemical depolymerization emerges as a particularly promising solution, wherein macromolecular architectures are selectively disassembled into their constituent molecular building blocks (monomers or oligomeric intermediates) for subsequent repolymerization, ultimately regenerating polymeric materials with identical properties indistinguishable from virgin materials.^5–9

Polyethylene, derived from petroleum feedstocks, dominates global polymer production as the most extensively manufactured high-performance commodity plastic, with an annual output exceeding 100 million tonnes.¹⁰ However, the efficient thermodepolymerization of polyethylene under mild conditions remains a formidable challenge due to its high ceiling temperature (T_c).^11,12 To circumvent this limitation, recent strategies have focused on introducing chemically labile linkages into the polyethylene backbone, enabling controlled chain scission during chemical recycling to regenerate monomers or oligomers.^13–18 Among these approaches, ester functionalities have emerged as particularly promising scissile sites due to their susceptibility to cleavage under basic conditions.^19–26

The synthesis of chemically recyclable polyethylene can be broadly classified into three methodologies (Fig. 1): (1) post-polymerization modification, (2) step-growth polymerization, and (3) chain-growth polymerization. Each approach presents distinct advantages and challenges. Notably, the mechanical performance of polyethylene is highly sensitive to the spatial distribution of ester groups along the polymer backbone,^27,28 necessitating precise structural control.²⁹ While post-polymerization functionalization offers versatility, the distribution of modification sites is rarely addressed, and achieving truly uniform ester incorporation remains problematic.³⁰ In contrast, step-growth polymerization of telechelic diols and diacid/diester monomers affords well-defined ester placement,^31–35 yet attaining ultrahigh molecular weights (M_n > 100 kDa), essential for robust mechanical properties via interchain entanglement, is inherently constrained by the equilibrium-driven nature of polycondensation. Although stoichiometric optimization of telechelic macromonomer coupling can enhance the molecular weight,^36–39 this approach introduces synthetic complexity. Alternatively, chain-growth copolymerization enables the preparation of high-molecular-weight polyethylene with in-chain functionalities.^27,40,41 However, this method often requires meticulous control over monomer reactivity ratios to ensure uniform ester distribution along the backbone.^42–45

Fig. 1 Comparative strategies providing ester-functionalized recyclable polyethylene.

By strategically incorporating ester linkages into programmed monomers followed by homopolymerization, it becomes possible to precisely position labile bonds along the polymer backbone, ensuring reproducible and controlled recyclability.^46–50 This approach offers a pathway toward next-generation sustainable materials designed with inherent chemical circularity. The ideal future of sustainable polymer design lies in tailorable materials derived from a single bio-based monomer, combining chemical recyclability with potential biodegradability.⁵¹ Such a paradigm offers three key advantages (Fig. 1): (1) Simplified synthesis: the use of a single monomer source minimizes design complexity without the need for matching the reactivity ratio of comonomers. (2) Enhanced circularity: streamlined monomer recovery improves the recycling efficiency and reduces waste. (3) Reduced environmental impact: biogenic carbon incorporation lowers the carbon footprint of material production.

In this work, we report the design of a structurally well-defined, fully bio-based macrocyclic unsaturated lactone monomer, synthesized from starch-derived isomannide and castor oil-derived undecenoic acid. Through ring-opening metathesis homopolymerization (ROMP) of this cyclic olefin followed by exhaustive hydrogenation, we produced high-molecular-weight polyolefin materials with precisely positioned ester functionalities along the backbone. The resulting polymers exhibited properties comparable to low-density polyethylene (LDPE), demonstrating the feasibility of achieving performance parity with conventional polyolefins while incorporating chemically recyclable linkages.

Results and discussion

Synthesis and characterization

While degradable high-density polyethylene (HDPE) mimics have been extensively documented, reports on chemically recyclable LDPE analogues remain distinctly less explored.¹⁰ We aimed to design a class of biobased polyolefin materials that replicate the characteristic properties of LDPE. Unlike HDPE, LDPE features abundant chain branching, which reduces the polyethylene crystallinity and melting temperature while enhancing ductility. Diverging from the conventional LDPE branching strategies, we propose the strategic incorporation of non-planar aliphatic moieties into the polyethylene backbone. This molecular design perturbs the crystalline architecture of polyethylene,⁵² thereby mimicking the performance profile of LDPE. As shown in Fig. 2a, a bio-based α,ω-difunctional diene, with a biogenic source up to 100 wt%, was synthesized from commercially available biomass derivatives—starch-derived isomannide and castor oil-derived 10-undecenoic acid—via esterification (98% isolated yield). This telechelic precursor underwent quantitative ring-closing metathesis to afford a macrocyclic unsaturated lactone (97% yield). Subsequent ROMP of this monomer using a Grubbs second-generation catalyst (G2) at ambient temperature yielded high-molecular-weight polyolefin P₀ (M_n = 227.8 kDa) within 30 minutes at near-quantitative conversion (96%). Successful ring opening of the lactone was confirmed by ¹H NMR spectroscopy (Fig. 2b), where the monomer's olefinic resonance at δ 5.35 ppm shifted downfield to δ 5.37 ppm.

Fig. 2 (a) Reaction scheme of bio-based polyolefin synthesis. (b) ¹H NMR spectra (400 M, CDCl₃) of cyclic lactone, P₀ and HP_101k. The asterisk denotes signals from residual dichloromethane and ethyl acetate. (c) Conversion kinetics for P₀ hydrogenation quantified by ¹H NMR. (d) Evolution of M_n with hydrogenation time. The gray zone corresponds to the temperature ramp (ambient to 120 °C).

Subsequently, P₀ underwent exhaustive hydrogenation to saturate backbone carbon–carbon double bonds. Conventional hydrogenation of unsaturated polyolefins containing ester linkages typically requires high-pressure H₂ and noble metal catalysts. To circumvent these limitations, we employed p-toluenesulfonyl hydrazide (TSH) as an alternative hydrogen donor. Despite the extensive optimization of the initial molecular weight, solvent type, reaction time, temperature, TSH/alkenyl ratio, and base type (Table S1), cleavage of backbone ester bonds during catalytic hydrogenation proved unavoidable, leading to polymer degradation. Screening identified N,N-diisopropylethylamine (DIPEA) as the optimal organic base for neutralizing the acid generated during catalytic hydrogenation.⁴² Subsequently, high hydrogenation efficiency was achieved under conditions of 120 °C in toluene for 40 min with a [TSH]/[internal olefin] ratio of 3.3, while avoiding significant chain scission. Kinetic studies revealed complete hydrogenation within ∼40 min while maintaining a high molecular weight (HP_101k, M_n = 101 kDa; Fig. 2c), a M_n value challenging to attain via step-growth polycondensation. Quantitative saturation of HP_101k was confirmed by complete disappearance of olefinic signals in the ¹H NMR spectra (Fig. 2b). Remarkably, despite comprising four steps, the entire synthesis route from the starting material to the target saturated polyolefin proceeded in a near-quantitative overall yield (up to 89%), underscoring its exceptional efficiency. By varying the hydrogenation time (Fig. 2d), we obtained saturated polyolefins with tunable M_n values from a single P₀ precursor, molecular weight data are summarized in Table 1. As depicted by the FTIR spectra in Fig. 3a, the bio-based polymers maintained consistent chemical structures and structural integrity across a range of molecular weights (15–101 kDa), enabling systematic molecular weight–property studies.

Fig. 3 (a) FTIR spectra of polyolefins. (b) Second heating cycle DSC thermograms of polyolefins. (c) X-ray diffraction (XRD) patterns of polyolefins. (d) Polarized optical micrograph (POM) of HP_42k film; inset: the photographic image of LDPE and bulk HP_42k. (e) Water contact angle of polyolefin films. (f) TGA degradation profiles of polyolefins.

Table 1 Structure and properties of polyolefins

	M n (kDa)	Đ	T d,5% (°C)	T m (°C)	ΔHm^c (J g^−1)	E (MPa)	σ (MPa)	ε (%)
a Determined by GPC in THF with calibration against polystyrene standards. b Determined by TGA with the decomposition temperature at 5% weight loss. c Determined by DSC via the second heating scan. d Determined by a tensile test at a strain rate of 10 mm min^−1.
LDPE	17.9	5.25	429.1	110.5	129.0	82.7	9.7 ± 1.7	979 ± 48
P0	227.8	2.28	416.4	40.2	21.2	2.1	22.7 ± 1.7	1361 ± 320
HP101k	101.2	2.53	389.9	82.0	42.3	32.7	10.8 ± 1.3	708 ± 90
HP68k	68.0	2.10	385.1	83.5	46.3	82.3	14.9 ± 1.3	455 ± 85
HP42k	42.3	2.76	389.7	83.5	50.7	111.9	15.4 ± 0.9	356 ± 25
HP25k	25.3	2.18	348.0	82.5	52.6	86.8	13.3 ± 1.9	539 ± 129
HP15k	15.1	2.20	281.6	81.5	58.7	68.6	7.6 ± 2.0	62 ± 10
rHP44k	44.4	3.81	385.6	83.6	49.2	90.4	16.7 ± 1.7	390 ± 32

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Thermal and surface properties

The saturation of the polyolefin backbone through hydrogenation, while constituting only a minor structural change, results in a dramatic alteration of the polymer's properties. The melting point (T_m) of saturated polyolefins increases significantly from 40 °C to ∼80 °C (Fig. 3b). This substantial T_m elevation indicates thickening of crystalline lamellae due to the elimination of carbon–carbon double bonds along the polymer backbone. The phenomenon is attributed to the formation of longer –CH₂CH₂– units in saturated chains,⁴² which exhibit enhanced structural regularity and facilitate chain folding. Concurrently, the melting enthalpy (ΔH_m) also increases, corresponding to increased crystallinity. Such significant enhancement in crystallinity enables a dramatic shift in the polyolefin's properties from elastomeric to plastic, thereby opening up entirely new application scenarios. Nevertheless, the ΔH_m values remain lower than those of LDPE, indicating relatively low crystallinity in these saturated polyolefins. This observation aligns with the X-ray diffraction (XRD) results (Fig. 3c), where all molecular weight variants exhibit broad diffraction peaks lacking sharp features. These saturated polyolefins exhibit diffraction peaks at 21.7° and 23.9° (albeit broad) in Fig. 3c, characteristic of the orthorhombic crystal structure of polyethylene (110 and 200 lattice planes). Significantly, the presence of bulky isomannide groups along the polymer backbone generates prominent low-angle diffraction peaks, indicating the inclusion of isomannide moieties into crystalline domains.⁴⁸ This structural integration expands the interlamellar spacing relative to conventional polyethylene. Notably, the melting behavior of saturated polyolefins shows minor variations across molecular weights. For instance, HP_101k displays a single melting peak, suggesting a monomorphic crystalline structure. Intriguingly, HP_68k exhibits a slightly higher T_m than HP_101k, implying that an increased molecular weight leads to thinner lamellae. This trend is rationalized by enhanced chain entanglement at higher molecular weights, which elevates the melt viscosity and ultimately suppresses crystallinity. When the molecular weight decreases below 42.3 kDa, melt-recrystallization occurs during heating. This effect becomes increasingly pronounced with further reduction in molecular weight, resulting from structural reorganization of both original and recrystallized lamellae during thermal processing. Similar behavior has been documented about multicomponent semicrystalline copolyester systems.⁵³Fig. 3d displays the polarized optical microscopy (POM) images of the representative sample HP_42k. This sample exhibits homogeneously distributed microcrystalline domains at the micrometer scale, accounting for its excellent macroscopic transparency. The density of the bio-based polyolefin (ρ = 0.912 g cm⁻³) is also close to LDPE (ρ = 0.923 g cm⁻³).

These bio-based polyolefins exhibit enhanced surface properties due to the presence of polar in-chain ester groups, as evidenced by water contact angle measurements (Fig. 3e). Although these polymers share an identical chemical structure, their contact angles still differ. At lower molecular weights, the exposed hydroxyl end groups increase the surface hydrophilicity of the polymer. Thus, the contact angle is expected to gradually increase with increasing molecular weight until eventually plateauing. However, experimentally, the contact angle displays an interesting non-monotonic trend—first increasing and then decreasing with increasing molecular weight. We attribute this behavior to variations in crystallinity. As the molecular weight increases, the degree of crystallinity decreases, resulting in a greater proportion of hydrophilic groups being situated in the amorphous regions, thereby enhancing the surface hydrophilicity. Consequently, the water contact angle of HP_101k shows a notable difference—as large as 34°—compared to that of LDPE (Fig. 3e).

The thermal stability of these polyolefins was studied by thermogravimetric analysis (TGA). Hydrogenation resulted in a slight decrease in T_d,max (Fig. 3f). This decrease is attributed to the difference in bond dissociation energy between the carbon–carbon double bonds present in the precursor (P₀) and the resulting single bonds. Nevertheless, the saturated polyolefins still exhibit exceptional thermal stability, with T_d,max values maintained at approximately 450 °C across all molecular weights, comparable to that of LDPE (T_d,max = 472 °C). However, the 5% weight-loss temperature (T_d,5%) showed a significant molecular weight dependence. When the M_n falls below 42.3 kDa, T_d,5% progressively decreases with diminishing M_n, demonstrating a pronounced chain-end effect. Conversely, at M_n > 42.3 kDa, this end-group influence becomes negligible, and T_d,5% plateaus between 385 and 390 °C.

Mechanical properties

The mechanical properties of these polymers were evaluated by uniaxial tensile testing. As shown in Fig. 4a, all saturated polyolefins exhibited ductile and tough behavior except for HP_15k, which lacked sufficient chain entanglements.⁵⁴ Overall, the strain at break increased progressively with molecular weight. Unlike LDPE—which maintained constant stress during cold drawing—the saturated polyolefins in this study displayed pronounced strain hardening after yielding, ultimately achieving tensile strengths (up to 15.4 MPa) exceeding that of LDPE. To elucidate the mechanism of strain hardening, we examined HP_101k (the sample with the highest strain at break). DSC analysis of post-fracture specimens revealed an increase in melting enthalpy (Fig. 4b), indicative of strain-induced crystallization.^48,55 Furthermore, multiple melting peaks emerged during heating. After stretching, the melting enthalpy associated with the first peak (40 °C)—attributed to thin lamellar crystals—decreased significantly. This suggests that polymer chains within these crystalline regions were stretched and melted under stress, subsequently recrystallizing into new domains.⁵⁶ Notably, stretched samples exhibited enhanced melt-recrystallization, implying improved chain mobility and slippage during reorganization. This further confirms that stretching promotes chain orientation. Complementary XRD measurements of HP_101k (strained to 400%) showed attenuation of the amorphous peak with increasing strain (Fig. S21), corroborating strain-induced crystallization. In summary, the high level of alignment of chain segments and associated strain-induced crystallization drive the observed strain hardening after large deformations, ultimately enhancing mechanical strength.⁵⁷

Fig. 4 (a) Stress–strain curves of polyolefins. (b) First DSC heating scan of HP_101k before and after stretching. (c) Stress–molecular weight dependence of polyolefins. (d) Metling enthalpy–number average molecular weight dependence of polyolefins.

Notably, the tensile strength exhibited a nonmonotonic dependence on molecular weight across the HP_15k–HP_101k series, reaching a maximum value (σ_b = 15.4 MPa) at 42.3 kDa (Fig. 4c). Beyond this peak, tensile strength decreased with further increases in molecular weight. This trend largely correlates with the observed crystallinity values (Fig. 4d). The mechanical strength aligns with the Young's modulus values shown in Table 1, which also reflect the crystallinity. While higher molecular weights typically enhance the chain entanglement density, leading to reduced crystallinity, a divergence between the influences of crystallinity and molecular weight on mechanical strength was observed within the 15.1–42.3 kDa range. This suggests that a competitive balance exists between the reinforcing effects of chain entanglement and crystallinity. However, once the molecular weight exceeds a critical threshold, specifically, 42.3 kDa in this study (well above the entanglement molecular weight of polyethylene),⁵⁴ crystallinity becomes the predominant factor governing the plastic's mechanical strength, surpassing the contribution from chain entanglement.

The comprehensive property characterization reveals that the HP_42k polyolefin exhibits a performance comparable to commercial LDPE in key properties—including transparency, mechanical strength, elongation at break, modulus, and thermal stability—suggesting its potential as a viable alternative to LDPE in practical applications. To further validate its applicability, we evaluated the water vapor permeability of an HP_42k film. Permeability tests conducted at 60 °C showed close values between HP_42k and LDPE, with water vapor transmission rates (WVTR) of 5.6 and 5.1 g (m² day)⁻¹, respectively (Fig. S22a), indicating that HP_42k also meets the requirements for use in agricultural films. Moreover, in a practical packaging demonstration, an HP_42k bag successfully stored water for one day without leakage or significant loss, performing on par with its LDPE counterpart (Fig. S22b). These findings collectively confirm the practical suitability of biobased polyolefins in packaging and related film applications.

Recycling studies

The in-chain ester groups can also serve as break points, enabling chemical recycling of these polyolefins via hydrolysis (Fig. 5a). Gentle heating (80 °C) of either single-component polyolefins (e.g., HP_42k) or multi-component mixtures (saturated polyolefins with different molecular weights) in alkaline aqueous solution for several hours completely degraded the polymers. Following simple purification, we recovered the original isomannide and industrially valuable telechelic diacids in near-quantitative yields (>95%; Fig. 5b) with high purity (Fig. 5c). To demonstrate the sustainability of mixed-plastics recycling, a blend of HP_42k, commercial low-density polyethylene (LDPE), and commercial isotactic polypropylene (iPP) was treated with alkaline aqueous solution at 80 °C for several hours (Fig. S23). Under these conditions, HP_42k dissolved completely, while both iPP and LDPE remained unchanged. The undissolved iPP and LDPE were readily removed by filtration. In contrast, the bio-based polyolefin (HP_42k) dissolved in the hot alkaline medium and was degraded into isomannide and a diacid, which were efficiently isolated and recycled in quantitative yield. This straightforward separation underscores the value of the bio-based polyolefin as a readily recyclable material in a circular economy.

Fig. 5 (a) Reaction scheme for recycling of polyolefins. (b) Alkaline degradation protocol of HP_42k. (c) ¹H NMR spectra (400 M, CDCl₃) of HP_42k, recovered difunctional monomers, and rHP_44k. (d) GPC elution traces of HP_42k and rHP_44k. (e) Mechanical properties of HP_42k and rHP_44k.

Subsequently, the two hydrolyzed products also have the potential to undergo polycondensation to recover the virgin polymer with similar thermomechanical properties.⁵⁸ However, the low reactivity of isomannide during the polycondensation reaction will restrict the molecular weight of the resulting product.⁵⁹ As indicated in Table S2, the polycondensation of isomannide, along with its optical isomers isosorbide and isoidide, with linear primary diacids has been the subject of extensive research. However, all these conventional methods have only yielded oligomers with molecular weights ranging from several kDa. These low molecular weights result in fragile polymers that are unsuitable for most thermoplastic applications. Although the length of diacids in this study is significantly longer, it still proves challenging to achieve high molecular weight polymers despite our considerable efforts on this reaction (Table S3). Inspired by Gruter's work, the inherently low reactivity of secondary diols in polyester synthesis can be overcome by the addition of an aryl alcohol to diol and diacid monomers. This leads to the in situ formation of reactive aryl esters during esterification, which promotes chain growth in the polycondensation stage, thereby enabling the production of high molecular weight polyolefins containing embedded ester bonds.⁵⁹ By optimizing the reaction conditions, polycondensation at 260 °C for 5 h using butyltin hydroxide oxide (BuSnOOH) as a catalyst and p-cresol as an aryl alcohol yielded a polyolefin with M_n = 44.4 kDa (Fig. 5d), which well match the molecular weight requirement with the desired property performance. The regenerated polyolefin, rHP_44k, exhibits an identical chemical structure to HP_42k, as confirmed by ¹H NMR spectroscopy (Fig. 5c), while maintaining a mechanical performance comparable to its precursor HP_42k (Fig. 5e).

Conclusions

In summary, we have demonstrated a homopolymerization strategy for synthesizing fully biobased, chemically recyclable, high-molecular-weight polyolefins with well-defined structures, balanced thermomechanical properties, and controlled degradability. This approach utilizes ring-opening metathesis polymerization (ROMP) of a uniquely designed 26-membered unsaturated lactone monomer derived from renewable resources (starch and castor oil), followed by exhaustive hydrogenation. Crucially, the incorporation of non-planar aliphatic defects, specifically, bulky isomannide motifs, disrupts the polyethylene-like crystallinity, endowing the resulting chemically recyclable polyolefin with desirable thermomechanical properties comparable to those of LDPE, despite the absence of side chains. Furthermore, the uniform distribution of strategically placed ester linkages along the polymer backbone enables the quantitative degradation and recovery of the polyolefin into well-defined diol and diacid monomers under mild basic conditions. These monomers can be directly repolymerized via polycondensation to regenerate virgin-quality polyolefins. Through structure–property relationship studies, we confirmed that the saturated polyolefin specimen with M_n = 42 kDa exhibited an optimal performance. Given the commercial availability of the recycled products, i.e., isomannide and eicosanedioic acid, the methodology established herein enabled the identification of a chemically recyclable candidate viable for scale-up production via polycondensation as an LDPE alternative. Thus, this work demonstrates a chemical recycling pathway for high-performance polyolefin materials that rival commodity plastics in performance while decoupling production from fossil feedstocks. Looking forward, the homopolymer strategy, featuring built-in ester bond sequences proposed here, can be generalized to other polyolefin/polyester systems, enabling further optimization of the recyclable polymer performance.⁶⁰

Author contributions

Y. S. and P. J. contributed to the conceptualization of the project. W. S. contributed the methodology for polymer design and chemical recycling. Y. P. contributed the methodology for monomer and polymer characterization. Y. H. contributed towards analysing the data. W. S. wrote the original draft. All authors contributed towards writing the final draft and editing. Y. S. and P. J. supervised the research. Z. L. and Y. Z. carried out project administration. All authors commented on the final manuscript.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5gc05344g.

Acknowledgements

We thank the Fundamental Research Funds for the Central Non-profit Research Institution of CAF (CAFYBB2025QH007), the National Natural Science Foundation of China (22375099 and 32471815), and the Natural Science Foundation of Jiangsu Province of China (BK20241745).

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