Upcycling Polytetrahydrofuran to Polyester

Open AccessCCS ChemistryRESEARCH ARTICLE21 Jun 2022Upcycling Polytetrahydrofuran to Polyester Xun Zhang, Yue Sun, Chengjian Zhang and Xinghong Zhang Xun Zhang State Key Laboratory of Motor Vehicle Biofuel Technology, International Research Center for X Polymers, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027 , Yue Sun State Key Laboratory of Motor Vehicle Biofuel Technology, International Research Center for X Polymers, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027 , Chengjian Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Motor Vehicle Biofuel Technology, International Research Center for X Polymers, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027 and Xinghong Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] https://doi.org/10.31635/ccschem.022.202202072 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail One major remaining challenge in polymer chemistry is the development of efficient chemical recycling strategies to fully retrieve starting materials. Polytetrahydrofuran (PTHF) is widely used, but its stable ether bonds make it difficult to chemically reuse after disposal. Here, we propose a "polymer A → polymer B" strategy for one-step quantitative upcycling of PTHF to polyesters. The route undergoes a cascade process: PTHF is depolymerized to give tetrahydrofuran (THF) that then alternately copolymerizes with cyclic anhydrides in situ, thereby pushing the chemical equilibrium of "PTHF ⇌ THF" to the right. The protocol demonstrates facile features: the use of common and metal-free Brønsted/Lewis acid as catalyst, a favorable reaction temperature of 100 °C, and no use of solvents. This method also accommodates 18 cyclic anhydrides to give a library of polyesters with alternating sequences, tunable thermal properties, and high-fidelity carboxyl terminals. This is an unprecedented strategy for chemical recycling of waste polyether. Download figure Download PowerPoint Introduction Plastics do manifest unique benefits during the use stage in our current society. However, plastic waste is currently one of the biggest global concerns.1,2 Approximately 250 million metric tons of plastic waste are generated globally every year while less than 10% of the plastics used is recycled.3,4 Not only can the traditional methods of landfill and incineration of plastic waste cause other serious ecological problems, but they also waste resources.5,6 Another mechanical recycling method always results in a significant deterioration in performance.7 The use of biodegradable polymers, such as aliphatic polyesters and polycarbonates, which can be enzymatically or hydrolytically degraded into H2O, CO2, and other nontoxic natural substances in nature, is a strategy newly developed over the past 20 years.8–24 More recently, chemists have explored ways to develop polymers that can complete a "polymer → monomer → polymer" closed-loop cycle,25–37 or convert plastics into valuable products.38–43 The evolution of facile and economical methods to deal with ordinary plastics is at the forefront of sustainable chemistry. Polytetrahydrofuran (PTHF) is a crucial intermediate in the manufacture of thermoplastic polyurethane (PU) and polyetherester elastomers.44 PTHF is commercially produced by acid-catalyzed ring-opening polymerization (ROP) of THF, with annual global production exceeding 1 million tons.45–47 The regular C–O–C bond in the polymer chain of PTHF is extremely stable, giving it excellent low-temperature flexibility, hydrolytic stability, and mildew resistance, but it also causes the problem of refractory degradation and chemical recycling after disposal.48 The traditional method of chemical recovery PTHF is to depolymerize and regenerate THF monomers due to the low ring tension of THF.49–51 However, the depolymerization process must result in a mixture of PTHF and THF because of the existing chemical equilibrium of "PTHF ⇌ THF," causing separation difficulties. Additionally, to obtain a high yield (>90%) of THF, a high reaction temperature (≥160 °C) is required to facilitate the shift of chemical equilibrium to THF, which inevitably increases production costs and difficulties. Furthermore, concentrated sulfuric acid is often used as a catalyst for PTHF depolymerization, but it is highly corrosive to equipment and cannot be reused, resulting in the discharge of strong acid wastewater. Enthaler and coworkers reported on FeCl3-catalyzed PTHF depolymerization with 92% THF yield at 180 °C.51 Due to the inevitable existence of a small amount of water in the PTHF raw material, FeCl3 is easily hydrolyzed during the high-temperature process, which not only causes the deactivation of the catalyst but also introduces the problem of HCl emissions. Hence, the development of an atom-economy and environmentally friendly method to dispose of PTHF waste remains challenging. In this work, we hypothesize the introduction of cyclic anhydrides into the PTHF depolymerization system, which can copolymerize with in situ-generated THF in the presence of Brønsted/Lewis acid as catalyst and push the "PTHF ⇌ THF" chemical equilibrium to the right. Based on this idea, we designed a one-step method to convert PTHF into alternating polyesters using cyclic anhydride as raw material (Figure 1). Cyclic anhydride is an inexpensive and readily available industrial chemical, which is prepared by condensation of dicarboxylic acids.52 Owing to the rich variety of cyclic anhydrides and the robust manner of this strategy, we obtain a family of polyesters with diverse structures. Our approach proposes a "polymer A → polymer B" mode of the chemical recycling of polymers, eliminating purification and repolymerization steps of monomers. This method also develops PTHF as a new feedstock for polyester synthesis. Figure 1 | Upcycling PTHF to alternating polyesters using cyclic anhydrides. Download figure Download PowerPoint Experimental Methods Materials PTHF-250, PTHF-650, PTHF-1000, PTHF-2000, and PTHF-2900 were purchased from Macklin (Shanghai) and used as received. Cyclic anhydrides of succinic anhydride (SA), maleic anhydride (MA), 3-methylsuccinic anhydride (MSA), 3,3-dimethylsuccinicanhydride (3,3-DMSA), n-octylsuccinic anhydride (2-OctSA), diglycolic anhydride (DGA), 3-methylglutaric anhydride (MGA), 3,3-dimethylglutaric anhydride (3,3-DMGA), 2,2-dimethylglutaric anhydride (2,2-DMGA), hexahydrophthalic anhydride (HHPA), phthalic anhydride (PA), 4-methylphthalic anhydride (MPA), 4-chlorophthalic anhydride (ClPA), phenylsuccinic anhydride (PSA), 3-isobutyl-glutaric anhydride (BuGA), 1,1-cyclohexanediacetic anhydride (CHDA), and 1,1-cyclopentanediacetic anhydride (CPDA) were purchased from Aladdin Reagent Company (Shanghai) and sublimated twice before use. Acetyl chloride, CF3SO3H, CF3(CF2)3SO3H, MeOTf, (CF3SO2)2O, BF3•Et2O, B(C6F5)3, and Bu2BOTf were purchased from Sigma-Aldrich (Shanghai) and used as received. Instrumentation 1H, 13C, and Heteronuclear Single Quantum Coherence (HSQC) NMR spectra were recorded on a Bruker Advance DMX 400 MHz spectrometer. Chemical shift values were referenced to CHCl3 as internal standard at 7.26 ppm or tetramethylsilane as internal standard at 0 ppm for 1H NMR (400 MHz) and against CDCl3 at 77.16 ppm for 13C NMR (100 MHz). Molecular weights and molecular weight distributions of polymers were determined with a PL-GPC220 chromatograph (Polymer Laboratories Ltd.) equipped with an HP 1100 pump from Agilent Technologies. The gel permeation chromatography (GPC) columns were eluted with THF at 1.0 mL/min at 40 °C. The sample concentration was 0.5 wt %, and the injection volume was 20 μL. Calibration was performed using monodisperse polystyrene standards covering the molecular weight range from 580 to 460,000 Da. Differential scanning calorimetry (DSC) was taken on a DSCQ200 equipped with a liquid nitrogen cooling system. Approximately 3–5 mg of samples were placed in aluminum pans. The cooling and heating rates were 10 °C/min. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometric (MALDI-TOF MS) measurements were performed on a Bruker Ultraflex MALDI TOF mass spectrometer, equipped with a nitrogen laser delivering 3 ns laser pulses at 337 nm. trans-2-[3-(4-tert-Butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB, 99%, Alfa, Shanghai, China) was used as the matrix. CH3COONa (≥98%, Aladdin, Shanghai, China) was added for ion formation. Infrared spectra were recorded by using a Bruker Vector 22 Fourier transform infrared spectrophotometer. Powder X-ray diffraction (XRD) experiments were performed by using an Ultima IV instrument (Rigaku Corp.) with Cu Kα radiation (λ = 1.542 Å) at room temperature. The 2θ range for wide-angle X-ray diffraction (WAXD) ranged from 5° to 60°, and the scanning step was 0.02°. Thermogravimetric analysis (TGA) was performed on a Q600-SDT thermogravimetric analyzer (TA Instruments Co., Ltd., New Castle, DE, United States). Representative procedure for reactions All reactions were carried out in a glovebox under an Ar atmosphere unless otherwise specified. A 10 mL vial with a magnetic stirrer was dried in an oven at 110 °C overnight and then immediately placed into the glovebox. The copolymerization of PTHF and anhydrides described below is taken from entry 1 in Table 1 as an example. PTHF-650 (2.77 mmol), glutaric anhydride (GA; 4.16 mmol), and CF3SO3H (0.0555 mmol) were added into the reactor. Then, the vial was sealed with a Teflon-lined cap and removed from the glovebox. The reaction mixture was stirred at 100 °C for 3 h. An aliquot portion was then taken from the crude product and quenched for determining the composition of the crude products by 1H NMR. The crude product was dissolved in dichloromethane and then precipitated from cooled methanol or deionized water. Finally, the obtained polymer was dried in a vacuum. Table 1 | Optimization of PTHF Upcycling Conditionsa Entry Polymer Catalyst [Anhydride]/[Catalyst] T (°C) t (h) AD (%)b Yield (%)c Mn (kDa)d Ðd Catalyst screening 1 PTHF-650 CF3SO3H 75/1 100 3 >99 >99 4.8 1.4 2 PTHF-650 CF3SO3H 150/1 100 3 >99 >99 4.7 1.3 3 PTHF-650 CF3(CF2)3SO3H 100/1 100 3 >99 >99 4.4 1.4 4 PTHF-650 MeOTf 100/1 100 3 97 >99 4.3 1.5 5 PTHF-650 (CF3SO2)2O 100/1 100 3 >99 >99 3.8 1.5 6 PTHF-650 BF3•Et2O 30/1 120 12 97 >99 7.7 1.4 7 PTHF-650 B(C6F5)3 30/1 120 12 65 >99 5.5 1.3 8 PTHF-650 Bu2BOTf 30/1 120 12 >99 92 1.3 1.6 Sort of PTHF 9e PTHF-650 CF3SO3H 100/1 100 3 >99 >99 3.2 1.3 10 PTHF-250 CF3SO3H 100/1 100 3 >99 >99 4.1 1.4 11 PTHF-1000 CF3SO3H 100/1 100 3 >99 >99 4.7 1.5 12 PTHF-2000 CF3SO3H 100/1 100 3 >99 >99 5.4 1.6 13 PTHF-2900 CF3SO3H 100/1 100 3 >99 >99 4.6 1.6 14 PTHF-8800 CF3SO3H 100/1 100 3 >99 >99 5.0 1.5 15 PTHF-32000 CF3SO3H 100/1 100 3 >99 >99 5.3 1.6 16 PTHF-32000 CF3SO3H 100/1 100 0.1 63 >99 4.8 1.7 17 PU CF3SO3H 100/1 120 24 80 >99 6.8 1.6 aReactions were performed in the absence of any solvent, and the molar ratio of THF repeating unit to GA was set as 1∶1.5. bAD of the polyester, AD = x/(x + y), determined by 1H NMR spectroscopy. cYield = (x + y)/(x + y + z), determined by 1H NMR spectroscopy. dDetermined by GPC in THF, calibrated with polystyrene standards. eCapped PTHF-650 with acetate terminals ( Supporting Information). Results and Discussion Optimization of the upcycling process We initially optimized reaction conditions using raw materials of bioderived GA and commercially available PTHF diol with a molar mass (Mn) of 650 Da (PTHF-650). To achieve the complete conversion of PTHF into polyesters, the amount of GA added was optimized to 1.5 times the number of PTHF repeating units ( Supporting Information Table S1). We were pleased to find that a variety of common metal-free Brønsted/Lewis acids were effective catalysts for this process without using solvents (entries 1–8 in Table 1). As an example, using the organic super Brønsted acid of CF3SO3H ([GA]∶[CF3SO3H] = 75∶1 or 150∶1), at the industrially advantageous temperature of 100 °C for 3 h, PTHF-650 was quantitatively converted to an alternating polyester of poly(THF-alt-GA) without the observation of THF (entries 1 and 2 in Table 1). The primary structure of the polyester was revealed by the 1H, 13C, and 1H–13C HSQC NMR spectra ( Supporting Information Figure S1). The 1H NMR spectrum shows proton signals at 2.36 attributed to COCH2C ( 1), at 1.93 attributed to CCH2C ( 2), at 4.09 attributed to OCH2C ( 3), and at 1.69 ppm attributed to CCH2C ( 4). The 13C NMR spectrum shows carbon signals at 172.94 ( a), 63.92 ( d), 33.25 ( b), 25.27 ( e), and 20.10 ppm ( c), respectively. The 1H and 13C NMR analysis was further verified by the 1H–13C HSQC NMR spectrum. According to the NMR spectra, no PTHF units were observed in the copolymer chain, indicating the fully alternating structure (i.e., alternating degree of >99%) of the prepared polyester. Other organic acids, including CF3(CF2)3SO3H, MeOTf, and (CF3SO2)2O, have also achieved PTHF quantitatively upcycled to poly(THF-alt-GA) under mild conditions (at 100 °C, for 3 h, with the feed ratio [GA]∶[catalyst] = 100∶1), affording polyesters with high alternating degree (AD) values of 97 to >99% (entries 3–5 in Table 1). In contrast, moderately acidic Lewis acids, such as BF3•Et2O, B(C6F5)3, and Bu2BOTf, showed relatively low activity for the process, which required a higher reaction temperature of 120 °C, more catalyst loading ([GA]∶[catalyst] = 30∶1), and a longer reaction time of 12 h, resulting in copolymer yields of 92 to >99% with AD of 65 to >99% (entries 6–8 in Table 1). Chain microstructures of carboxyl telechelic poly(THF-alt-GA) The produced polyesters possess high-fidelity carboxyl terminals as revealed by MALDI-TOF MS (Figure 2a), which can be attributed to the overfeeding of cyclic anhydride and full conversion of PTHF. As shown in Figure 2a, the main series peak [HO+(GA+THF)n+GA+H+Na+] belongs to poly(THF-alt-GA) with dual carboxyl terminals and perfect alternating sequences. It is worth noting that carboxyl-terminated polyester has been widely used in the preparation of powder coatings.53,54 Traditionally, carboxyl-terminated polyesters are produced by reacting the corresponding hydroxyl-terminated polyester with carboxylic acids or cyclic anhydrides and is thus a time- and energy-consuming process.54 This strategy enables a one-step synthetic method for carboxyl-terminated polyesters. Figure 2 | MALDI-TOF MS of P1, P14, P17, and P18 produced from CF3SO3H-catalyzed upcycle of PTHF-650. Download figure Download PowerPoint Mechanism analysis of the upcycling process Using this method, the acetate-capped PTHF-650 (entry 9 in Table 1 and Supporting Information Figure S2) demonstrated similar reaction results to the dihydroxy-terminated PTHF-650, suggesting that PTHF is mainly cleaved from within the polymer chain during the process. To further understand the mechanism, we subsequently monitored the upcycling process of PTHF-650 by 1H NMR spectra ( Supporting Information Figure S3). The content of PTHF segments kept decreasing for 3 h until it disappeared, affording the polyester with strict alternating sequences ( Supporting Information Figure S4). During the process, we clearly observed that THF started to be produced (up to 14 mol % within 2 min) and eventually disappeared. We thus proposed that the PTHF chain was cleaved by H+ or a Lewis acid randomly at the ether bond (C–O–C), yielding a tetrahydrofuranium ion as an active species at the chain end, as illustrated in Figure 3. The active polymer chains were depolymerized sequentially to THF by ring closure. Monomers of THF and cyclic anhydrides were then continuously inserted into the active polymer chain, producing alternating copolymers by a typical active chain end mechanism.55 As a control experiment, in the absence of cyclic anhydrides, the depolymerization of PTHF only yielded 73 mol % THF in the crude products under the same reaction conditions ( Supporting Information Figure S5). Thereby we reason that in the upcycling process of PTHF, the copolymerization of THF with cyclic anhydrides keeps pushing "PTHF⇌THF" to the right, resulting in a complete conversion of PTHF to polyesters. Figure 3 | Proposed mechanism for conversion of PTHF to polyesters. PTHF is depolymerized to form THF that then copolymerizes with cyclic anhydrides in situ to give alternating polyesters. Download figure Download PowerPoint Expansion of this method to various PTHF and PTHF-based PU We then extended this method for upcycling PTHF with different molar masses. In the presence of GA and CF3SO3H ([GA]∶[CF3SO3H] = 100∶1), at 100 °C, for 3 h, commercially available PTHF-250, PTHF-1000, PTHF-2000, and PTHF-2900 diols (entries 10–13 in Table 1) were quantitatively converted to poly(THF-alt-GA) with AD values ranging from 95% to >99%. We also synthesized high-molar mass PTHF-8800 and PTHF-32000 (entries 14 and 15 in Table 1) from the ROP of THF to test the upcycle process. Under the same reaction conditions as other PTHF, PTHF-8800, and PTHF-32000 were completely converted to poly(THF-alt-GA) with fully alternating sequences. In addition, PTHF-32000 at conversion of 63% had a low Mn of 4.8 kDa, which is consistent with the proposed mechanism that the PTHF chain was cleaved randomly (entry 16 in Table 1). Due to the fast chain transfer reaction of cationic polymerizations,56,57 all of these obtained polyesters in Table 1 possess low molar masses (Mn = 1.3–7.7 kDa) with relatively narrow polydispersities (Р= 1.3–1.6). Of interest, we also achieved the direct upcycle of linear PTHF-based PU elastomer to polyester by this method (entry 17 in Table 1, Supporting Information Figure S6). PU was purchased from Badische Anilin-und-Soda-Fabrik Company Limited and used directly without any purification. There are many carbamate groups in the PU polymer chain that can potentially interact with the acid catalyst, thus leading to a low activity of the upcycling process. At 120 °C and for 24 h, we obtained a polyester with a yield of >99%, AD of 80%, Mn of 6.8 kDa, and Ð of 1.6. Hence this method is expected to be widely used in the direct recycling of PTHF-based commercial materials. Expansion of this method to various cyclic anhydrides We next extended this strategy to 17 other commercially available cyclic anhydrides (SA, MA, MSA, 3,3-DMSA, 2-OctSA, DGA, MGA, 3,3-DMGA, 2,2-DMGA, HHPA, PA, MPA, ClPA, PSA, BuGA, CHDA, and CPDA). These reactions proceeded at 100 °C within 3–24 h, affording a library of THF-based polyesters with Mn of 0.8 to 6.1 kDa, Ð of 1.1 to 1.5, high AD of 96% to 99%, and high yields of 93% to 99% ( P2– P18 in Figure 4 and Supporting Information Table S2 and Figures S7–S23). These polyesters also possess precious carboxyl terminals according to the representative MALDI-TOF MS. The main series peaks [HO+(CIPA+THF)n+CIPA+H+Na+] (Figure 2b), [HO+(CHDA+THF)n+CHDA+H+Na+] (Figure 2c), and [HO+(CPDA+THF)n+CPDA+H+Na+] (Figure 2d) belong to poly(THF-alt-CIPA) ( P14), poly(THF-alt-CHDA) ( P17), and poly(THF-alt-CPDA) ( P18) with dual carboxyl terminals, respectively, demonstrating a promising application prospect as reactive polymeric precursors. Figure 4 | Obtained polyesters using various cyclic anhydrides and representative GPC curves. Download figure Download PowerPoint Widely tunable thermal properties of the obtained polyesters The series of polyesters exhibited a wide range of glass transition temperatures (Tg) ranging from −59 °C ( P9) to 17 °C ( P13), as determined by DSC (Figure 5a, Supporting Information Figures S24–S41). These polyesters also possessed high thermal stability with Td, 5% (the temperature at polymer decomposition of 5% in mass fraction) from 241 to 350 °C, as determined by TGA ( Supporting Information Table S2). Of special interest, P1 and P2 were semicrystalline according to DSC (Figure 5b) and XRD ( Supporting Information Figure S42). P1 exhibited a Tg of −55 °C along with a melting temperature (Tm) of 30 °C and an enthalpy change (ΔHm) of 4.7 J g−1, while P2 had a much higher Tm of 106 °C and a ΔHm of 94.4 J g−1. According to the WAXD, the degrees of crystallinity (XcWAXD) of P1 and P2 were calculated to be 19% and 46%, respectively. Notably, crystallization is an important factor affecting the performance of polymers, which can increase their yield stress, strength, modulus, and hardness.58,59 Figure 5 | (a) Tg values of produced polyesters and (b) DSC curves and wafers by the hot press of P1 and P2. Download figure Download PowerPoint Conclusion We have demonstrated a new and facile method for one-step upcycling of chemically inert PTHF to polyesters. In the presence of commercially available cyclic anhydride as feedstock and common Brønsted/Lewis acid as catalyst, PTHF was initially depolymerized to form THF that then alternately copolymerized with cyclic anhydrides, thereby leading to a direct upcycle of PTHF into polyesters. The process was performed at 100 °C in the absence of any solvents, affording polyesters with nearly quantitative yields, alternating sequences, and carboxyl terminals. This method demonstrates the excellent applicability of a library of cyclic anhydrides, affording 18 functional polyesters that exhibit thermodynamic properties with a large regulatory space. We have also developed a versatile method for polyester synthesis and disclosed PTHF as a new feedstock. Ongoing work will focus on understanding the mechanism in detail and investigating applications of these polyesters as reactive precursors in the preparation of elastomers and powder coatings. Supporting Information Supporting Information is available and includes Tables S1 and S2 and Figures S1–S42. Conflict of Interest The authors declare no competing interests. Funding Information X.Z. gratefully acknowledges the financial support of the National Science Foundation of China (grant no. 51973190) and the Zhejiang Provincial Department of Science and Technology (grant no. 2020R52006). References 1. Jambeck J. R.; Geyer R.; Wilcox C.; Siegler T. R.; Perryman M.; Andrady A.; Narayan R.; Law K. L.Plastic Waste Inputs from Land into the Ocean.Science2015, 347, 768–771. Google Scholar 2. Geyer R.; Jambeck J. R.; Law K. L.Production, Use, and Fate of All Plastics Ever Made.Sci. Adv.2017, 3, e1700782. Google Scholar 3. Kunwar B.; Cheng H. N.; Chandrashekaran S. R.; Sharma B. K.Plastics to Fuel: A Review.Renew. Sustain. Energy Rev.2016, 54, 421–428. Google Scholar 4. Martin A. J.; Mondelli C.; Jaydev S. 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