Chemically Recyclable Se-Containing Plastics from a Cascade Multicomponent Polymerization

Open AccessCCS ChemistryRESEARCH ARTICLES2 Jul 2024Chemically Recyclable Se-Containing Plastics from a Cascade Multicomponent Polymerization Xinchen Yue, Xiaoxian Lu, Chengjian Zhang and Xinghong Zhang Xinchen Yue , Xiaoxian Lu , Chengjian Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] and Xinghong Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Cite this: CCS Chemistry. 2024;0:1–9https://doi.org/10.31635/ccschem.024.202404338 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Researchers are pursuing promising chemically recyclable polymers. Here, we demonstrate a series of recyclable Se-containing polyesters from the first reported cascade multicomponent polymerization of elemental Se, H2O, CO, and diacrylate. The polymerization is considered to be through a cascade mechanism: Se is first reduced by CO to COSe; COSe is then reduced by H2O to H2Se; H2Se is finally coupled with diacrylate to form a polymer. The method uses common organic bases as a catalyst and is performed under mild conditions with high yields. The obtained polymer is thermally stable with a tunable melting point, and can be easily depolymerized to Se and diacrylate by H2O2 under mild conditions. Owing to the versatile synthesis, the structure/properties of the polymer are scalable, such as P4 and P5 with high-density polyethylene-like mechanical performance. Overall, the polymer is a promising material owing to its modular synthesis, practical recyclability, scalable performance, and commercial raw materials. Download figure Download PowerPoint Introduction Polymer research has entered its second century since Staudinger's first polymerization report published in 1920.1 Synthetic polymers have played a tremendous role in all aspects of our lives, from materials for cars, aircrafts, and buildings to clothing, packaging, and biomedical devices.2 However, due to today's growing concerns about the environment and sustainability, the reputation of synthetic polymers is being severely corrupted.3,4 Because of the high durability, 59% of all polymers made (∼8600 Mt) are discarded and end up in landfills or accumulate in the ocean, while only 17% are recycled or incinerated.5–7 In particular, the amount of single-use plastics (such as masks and gloves) has tremendously increased due to the COVID-19 pandemic. In addition, most synthetic polymers are derived from nonrenewable fossil feedstocks,8,9 resulting in unsustainable production when they cannot be recycled/reused. In the current second century of polymer science, we have to accelerate the green revolution in synthetic polymers. The development of chemically recyclable polymers offers a particularly attractive approach to address the challenges of polymer sustainability.10–34 Hydrolysis or alcoholysis of the ester bond can enable the recycling of polyesters, as reported by Mecking,35 Coates,36,37 and other groups.38 We also reported hydrolysis to realize the chemical cycle of sulfur-containing polyesters synthesized from carbonyl sulfide, diacrylates, and H2O/diols.39,40 However, hydrolysis and alcoholysis methods are energy-intensive and use a large amount of strong bases or acids. Furthermore, the polycondensation method used in polyester synthesis usually requires energy-intensive consumption. Here, we develop a series of chemically recyclable selenium (Se)-containing polyesters from the first reported cascade multicomponent polymerization of elemental Se, H2O, carbon monoxide (CO), and diacrylate (Figure 1). This method features the characteristics of commercial raw materials, mild polymerization conditions, a common base as the catalyst, high yield, and versatile synthesis. The produced polymers possess widely tunable structures/properties and Se-giving functionality. The Se-containing polyesters can be easily depolymerized by hydrogen peroxide (H2O2), which is more environmentally friendly than the use of strong bases or acids for polyester degradation. In addition, the incorporation of Se atoms into the polymer to endow new features has attracted widespread attention.41–48 Se-containing polymers have shown unusual prospects as biomaterials, redox responsive materials, optical materials, electrically conducting polymers, and so on.49–52 The advancement of new Se-containing polymers and their practical synthesis methods is highly desired. Figure 1 | Illustration of the multicomponent polymerization route to chemically recyclable Se-containing polyesters. Download figure Download PowerPoint Experimental Methods Materials The used diacrylates, solvents, CO, H2O, catalysts, and H2O2 solution (30 wt % in H2O) were used as received without further purification. Instrumentation Nuclear magnetic resonance (NMR) spectra were performed on a Bruker Advance DMX 400 MHz (Billerica, Massachusetts, United States). Infrared spectra of polymer samples were recorded on Nicolet iS20 attenuated total reflection spectrometer (Waltham, Massachusetts, United States). The weight-average molecular weight and distribution of the polymers were determined with a PL-GPC 220 chromatograph equipped with an HP 1100 pump from Agilent Technologies (Santa Clara, California, United States). Calibration was performed using monodisperse polystyrene (PS) standards. The decomposition temperatures of the polymers were determined by TAQ 50 instrument (New Castle, Delaware, United States). Differential scanning calorimetry (DSC) measurements of polymers were carried out on a TAQ 200 instrument (New Castle, Delaware, United States) with a heating/cooling rate of 10 °C/min. The data reported are from second heating cycles. Gas chromatography mass spectrometry was carried out on a Waters GCT Premier (Milford, Massachusetts, United States). Representative procedure for reactions Taking the synthesis of P1 as an example. A 20 mL high pressure reactor with magnetic stirrer was dried in an oven at 80 °C overnight. Then the reactor was taken into the glove box and cooled in nitrogen atmosphere. In the glove box, 1,4-butylene glycol diacrylate (0.861 g, 3.33 mmol, 1.0 equiv) was added into the reactor first. Selenium powder (0.2896 g, 3.66 mmol, 1.10 equiv) and ultrapure water (69 μL, 3.83 mmol, 1.15 equiv) were then added into the reactor. Then N-methyl-1,5,7-triazabicyclododecene (MTBD; 19.2 μL, 0.14 mmol, 0.04 equiv) was added into the reactor. After that, tetrahydrofuran (THF; 1 mL) was added as solvent. The reactor was then sealed and charged with CO (1.5 MPa). The reaction was carried out at 60 °C for 6 h. After that, the reactor was cooled in a water bath. Next, the crude product was quenched with HCl in ethanol (1 mol/L). The crude product was dissolved with THF and then precipitated in ethanol. The product was collected by decantation and dried in vacuum at 60 °C until a constant weight. Results and Discussion Elemental Se is used as an easily accessible source of Se in our method, which is a byproduct from the metal refinery industry and has low toxicity.53 CO is the essential chemical for industrial manufacturing of abundant organic and inorganic products.54 The polymerization is proposed to undergo a cascade mechanism: under the reduction capacity of CO, elemental Se is first reduced to carbonyl selenide (COSe); then, hydrolysis of COSe generates hydrogen selenide (H2Se) and CO2; finally, H2Se is coupled with the electron-deficient C=C bond of diacrylates to generate a polymer (Figure 2a). The organic base can efficiently catalyze the above cascade reactions under mild conditions, resulting in a metal-free and efficient polymerization method. Additionally, cascade and multicomponent polymerization methods have gained significant attention because of their unique transformation and convenience.55–61 This study has developed a new cascade multicomponent polymerization to facilitate the synthesis of chemically recyclable polymers. Figure 2 | Kinetics and mechanism study of the cascade multicomponent polymerization. (a) Illustration of the cascade mechanism of the polymerization. (b) Time-dependent 2D stack plot of in situ IR spectra, polymerization conditions: [Se]0/[H2O]0/[diacrylate]0/[MTBD] = 110/115/100/4, at 1.5 MPa CO pressure, THF as a solvent, at 60 °C. (c) Time-dependent peak intensity at 1736 cm−1 of in situ Fourier transform infrared profile. (d) Time-dependent 2D stack plot of in situ IR spectra, reaction conditions: [Se]0/[MTBD] = 110/4, at 1.5 MPa CO pressure, THF as a solvent, at 60 °C. (e) Time-dependent 2D stack plot of in situ IR spectra, reaction conditions: [Se]0/[H2O]0/[MTBD] = 110/115/4, at 1.5 MPa CO pressure, THF as a solvent, at 60 °C. Download figure Download PowerPoint We monitored chemical transformations during the polymerization process through an in situ infrared (IR) spectrometer technique (Figure 2b–e and Supporting Information Figures S1–S3). We used 1,10-decanediyl diacrylate and MTBD as the representative diacrylate and organic catalyst, respectively. Figure 2b shows the time-dependent two-dimensional (2D) stack plot of the in situ IR spectra of the polymerization. The peaks at 1721 and 1736 cm−1 are attributed to the C=O bond in the diacrylate and polymer, respectively. As shown in Figure 2c, the stacked in situ IR profile of the peak at 1736 cm−1 grows over time and approaches saturation nearly at 140 min, while the peak at 1721 cm−1 decreases over time and eventually remains stable. The changes in these peaks clearly reflect the real-time polymerization process. Accordingly, the resulting polymers possess weight-average molecular weights (Mw) of 3.7, 4.2, 12.6, 55.8, and 81.7 kDa at 60, 120, 180, 240, and 360 min, respectively, as determined by gel permeation chromatography (GPC). The peak at 1590 cm−1 in the IR spectrum (Figure 2b) is attributed to the C=O bond in COSe, which first gradually increases and then disappears, consistent with the proposed cascade polymerization mechanism. In the MTBD-catalyzed reaction of CO and elemental Se, we clearly observed the production of COSe through the growth of the peak at 1590 cm−1 (Figure 2d). Additionally, from the MTBD-catalyzed reaction of CO, Se, and H2O, the production and hydrolysis of COSe was determined by the increase and disappearance of the peak at 1590 cm−1 during the reaction process (Figure 2e). By comparing the in situ IR spectra in Figure 2d,e, we proposed that the hydrolysis of COSe promotes the production of COSe from CO and Se. To our knowledge, this study reveals for the first time the Michael addition of H2Se and activated olefins to form a polymer. Furthermore, compared with the direct use of H2Se,62,63 the method of using CO, Se, and H2O has a price advantage. After understanding the cascade polymerization mechanism, we then synthesized various Se-containing polyesters from diverse diacrylates with this method. Diacrylate is a promising raw material for polymer synthesis, which has a wide variety and is industrially produced from the esterification of (biorenewable) diols and acrylic acid ( Supporting Information Figures S4–S8).64 The optimized polymerization conditions include: THF as a solvent, feeding liquids and solids under nitrogen conditions, the feeding ratio of [Se]0/[H2O]0/[diacrylate]0/[catalyst] = 110/115/100/4, at 1.5 MPa CO pressure, at 60 °C for 6 h ( Supporting Information Tables S1–S3). When conducted in air, the polymerization always yields crosslinked and insoluble polymers, which are generated from the free radical polymerization of diacrylates. Trace amounts of oxygen in the air act as free radical initiators. Because the selenium powder is heterogeneous in the polymerization system, uniform stirring to fully disperse the selenium powder is necessary for obtaining a high molecular weight polymer. THF as a solvent can visibly disperse selenium powder better than other solvents, thus yielding the polymer with higher molecular weights ( Supporting Information Table S1). The selenium powder disappears during the polymerization. Probably due to the incomplete reaction of Se and H2O, their small excess is conducive to the synthesis of high-MW polymers ( Supporting Information Table S3). The common organic bases of MTBD and 1,8-diazabicyclo[5.4.0]-7-undecene were determined as effective catalysts for the polymerization ( Supporting Information Table S4). Among the alkaline catalysts used, these two catalysts are relatively tolerant to water, thus affording the polymer with higher molecular weights ( Supporting Information Table S4). Under the optimized polymerization conditions, we used commercial diacrylates (1,4-butanediol diacrylate, 1,5-pentanediyl diacrylate, 1,6-hexanediyl diacrylate, 1,9-nonanediol diacrylate, and 1,10-decanediyl diacrylate) to synthesize P1 to P5 with high yields of 88%–93% calculated by gravimetry (Figure 3a). Such polymers possess high Mw of 81.2–147.2 kDa and dispersities (Đ) of 1.6–2.2, as determined by GPC in THF calibrated with a PS standard ( Supporting Information Table S5 and Figures S9–S22). Also, the GPC with triple detectors (refractive index, intrinsic viscosity, and light scattering detectors) was employed to determine the absolute molecular weight of the polymer. The absolute Mw of P4 is determined as 79.0 kDa (Đ = 2.2, Supporting Information Figure S19), which is comparable to the relative Mw of 81.2 kDa (Đ = 2.2). P1 is weakly crystalline, which exhibits a glass transition temperature (Tg) of −54 °C and a melting temperature (Tm) of 47 °C, as determined by DSC (Figure 3b). P2 and P3 are amorphous and exhibit Tg values of −58 and −60 °C, respectively (Figure 3b). Such afforded polymers are colorless or yellowish. The chemicals, including elemental Se, CO, H2O, diacrylates, and catalysts, were used directly as purchased without any purification. Figure 3 | Synthesis and properties of the recyclable Se-containing polyesters. (a) Obtained polymers with Mw (Đ) determined by GPC in THF and calibrated with PS standards. (b) DSC curves of P1 to P6. (c) Stress–strain curves of HDPE, P4, and P5. (d) P5 membrane formed by hot pressing for packaging materials. (e) DSC curves of P7 to P9. (f) Stress–strain curves of P7 to P9. Download figure Download PowerPoint Owing to the incorporation of long carbon chains,65,66 P4 and P5 are crystalline and exhibit Tms of 43 and 47 °C, respectively. Via further increases in the number of carbon atoms to 22, P6 exhibits a higher Tm of 69 °C. Because of the relatively low solubility in THF during the polymerization, P6 possesses a lower Mw of 39.4 kDa (Đ of 1.4, Supporting Information Figures S23–S25). P4 and P5 demonstrate excellent melt processing and high-density polyethylene (HDPE)-like mechanical properties. By stress–strain experiments, the specimens of P4 and P5 prepared by melt processing show the ultimate tensile strength (σB) of 25.4 ± 1.9 and 27.1 ± 2.4 MPa, an elongation at break (ϵB) of 441 ± 28% and 557 ± 34%, and calculated Young's modulus (E) of 117 ± 16 and 126 ± 5 MPa (Figure 3c). Figure 3d demonstrates the potential application of P5 in packaging materials. Due to the low molecular weight, the specimens of P6 exhibit relatively weak toughness with a σB of 15.6 ± 0.5 MPa, ϵB of 58 ± 4%, and E of 80 ± 3 MPa. To further improve the Tm of the Se-containing polyesters, we then synthesized P7 using commercially available hexadecafluoro-1,10-decanediol as a raw material ( Supporting Information Figures S26–S28). Because of the incorporation of a large amount of fluorine atoms that have strong negative charge and small volume, P7 is highly crystalline and exhibits a high Tm of 113 °C (Figure 3e). Due to the relatively low solubility in THF during the polymerization, P7 possesses a low Mw of 34.5 kDa (Đ of 1.4). The specimens of P7 are brittle and exhibit σB of 15.7 ± 0.6 MPa, ϵB of 17 ± 1%, and E of 108 ± 4 MPa (Figure 3f). To improve the mechanical properties of the polymer, we then added 1 mol % trimethylolpropane triacrylate to the polymerization system to yield the slightly cross-linked P8 with Mw of 40.2 kDa and Đ of 2.9 ( Supporting Information Figures S29–S31). P8 shows a high Tm of 113 °C (Figure 3e), good melt processing, and enhanced mechanical properties with σB of 23.5 ± 4.6 MPa, ϵB of 115 ± 20%, and E of 134 ± 5 MPa (Figure 3f). Furthermore, the optimized combination of 2 mol % trimethylolpropane triacrylate, 48 mol % hexadecafluoro-1,10-decanediol diacrylate, and 50 mol % 1,10-decanediol diacrylate yielded P9 with high Mw of 86.1 kDa and Đ of 2.4 ( Supporting Information Figures S32–S34). P9 demonstrates a Tm of 76 °C (Figure 3e), good solubility in THF, good melt processing, and improved mechanical properties with σB of 16.9 ± 2.1 MPa, ϵB of 416 ± 104%, and E of 70 ± 12 MPa (Figure 3f and Supporting Information Figures S35–S40). Notably, fluorinated polymers have shown interesting properties, such as chemical and thermal stability, low dielectric constant and refractive index, and low surface tension.67,68 This study provides a facile synthesis of fluorinated polymers. Both the obtained polymers ( P1 to P9) are thermally stable, which exhibit high decomposition temperatures (Td, at polymer decomposition of 5% in mass fraction) of 228–276 °C, as determined by thermogravimetric analysis ( Supporting Information Figures S41–S49). The polymers also exhibit high chemical stability in air. As an example, the Td of P5 in air (265 °C, Supporting Information Figure S50) is similar to that (264 °C) in a N2 atmosphere. In addition, after 536 days of storage, the Mw of P5 decreased slightly from 80.6 to 79.3 kDa ( Supporting Information Figure S51). Also, the appearance and color of the polymer have hardly changed. We also examined the oxidation-triggered depolymerization of the Se-containing polyesters. Xu and coworkers69 reported the oxidation-elimination process of a β-selenylated carbonyl compound by H2O2. The proposed decomposition mechanism is demonstrated in Figure 4a: it is first oxidized to a selenoxide structure that undergoes a syn elimination to form an acrylate and a selenenic acid; then the unstable selenenic acid undergoes another syn elimination, affording another acrylate along with elemental Se and H2O. Owing to the incorporation of unique β-selenylated carbonyl unit in main chain, our polymers are readily depolymerized through the oxidation of H2O2 ( Supporting Information Figures S52–S72). During the preparation of this study, Xu and coworkers70 reported an unprecedented β-selenocarbonyl-containing polyurethane can be degraded into valuable selenium fertilizers under mild conditions. H2O2 is a valuable oxidation reagent required for many industrial uses and can decompose into nontoxic O2 and H2O. Figure 4 | Oxidation-enabled depolymerization of the selenoether-containing polyester. (a) Illustration of the depolymerization mechanism. (b) 1H NMR spectra of the initial diacrylate, P5, and regenerated diacrylate. (c) GPC curves of the original and regenerated P5. (d) Images for monitoring the depolymerization process of P5 specimens mixed with common plastics. Download figure Download PowerPoint Taking the chemical recycle of P5 as an example, 0.75 g P5 was first completely depolymerized at 60 °C overnight in the presence of 2.17 mL 30% H2O2 (10 equiv relative to the amount of P5 units) and 80 mL THF as a solvent. Then, by the simple distillation in vacuum of the depolymerization mixture, 73 mL pure THF was recovered in 91% yield, while the mixture of 1,5-pentanediyl diacrylate and Se powder is left in the bottle. Finally, by filtration of the mixture, the diacrylate was recovered in 71% yield (0.41 g) with >99% purity determined by gas chromatography mass spectrometry ( Supporting Information Figure S62) and 1H NMR spectrum (Figure 4b), while elemental Se was recovered in 72% yield (0.12 g). The recovered selenium was in a red amorphous form that was converted to the original black form by heating at 100 °C for several hours. We also used X-ray photoelectron spectroscopy (XPS) to determine the valence state of the recovered Se ( Supporting Information Figure S73). The major binding energies of the recovered Se were measured at 55 eV, which was attributed to Se (0).70 A small peak at 59 eV that is attributed to Se (IV) in the form of selenenic acid. By comparing the XPS spectra of purchased Se powder and recovered Se, the recovered Se possesses more selenenic acid. For the repolymerization process, with [Se]0/[H2O]0/[diacrylate]0/[MTBD] = 110/115/100/4, at 1.5 MPa CO, at 60 °C for 6 h, the recovered Se and diacrylate combined with CO and H2O yielded the regenerated P5 with Mw of 77.4 kDa (Đ of 1.5) that is comparable to that of the original P5 (Figure 4c). We also demonstrated the H2O2-enabled depolymerization of P5 mixed with various common plastics including polyethylene (PE), polypropylene, PS, polyvinyl chloride, polyamide, and polyethylene terephthalate. Because of the high durability toward H2O2, the common plastics can be easily separated from the depolymerization mixture, leading to the easy-to-handle depolymerization process (Figure 4d and Supporting Information Figure S72). Conclusion We have demonstrated diverse recyclable Se-containing polyesters from the cascade multicomponent polymerization of Se, H2O, CO, and diacrylate. Owing to the versatile method and the wide variety of diacrylates, the structure and properties of the polymer have a lot of room for expansion, such as P4 and P5 with HDPE-like mechanical properties and P8 and P9 with a high Tm of 113 °C. The polymerization method is carried out under mild conditions, uses common organic bases as a catalyst, and affords polymers with high yields and high Mw of up to 147.2 kDa. The depolymerization of the polymer is easy to handle through the oxidation of H2O2, resulting in the recovery of diacrylates and elemental Se with high yields. By the modular polymer synthesis method, the ongoing work will pursue more Se-containing polymers with chemical recyclability, desired functionality, and high performance. Supporting Information Supporting Information is available and includes Tables S1–S5 and Figures S1–S73. Conflict of Interest The authors declare no competing interests. Funding Information We gratefully acknowledge the financial support of the National Natural Science Foundation of China (grant nos. 52373014, U23A2083, and 52203129). References 1. Staudinger H.Über Polymerisation.Ber. Dtsch. Chem. Ges.1920, 53, 1073–1085. Google Scholar 2. Patrick J. F.; Robb M. J.; Sottos N. 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