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Designed for Circularity: Chemically Recyclable and Enzymatically Degradable Biorenewable Schiff Base Polyester-Imines
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Designed for Circularity: Chemically Recyclable and Enzymatically Degradable Biorenewable Schiff Base Polyester-Imines
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  • Sathiyaraj Subramaniyan
    Sathiyaraj Subramaniyan
    Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden
    Wallenberg Wood Science Center (WWSC), KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden
  • Nasim Najjarzadeh
    Nasim Najjarzadeh
    Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden
    Wallenberg Wood Science Center (WWSC), KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden
    Biotechnology and Health, Science for Life Laboratory, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden
  • Sudarsana Reddy Vanga
    Sudarsana Reddy Vanga
    Biotechnology and Health, Science for Life Laboratory, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden
  • Anna Liguori
    Anna Liguori
    Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden
    More by Anna Liguori
  • Per-Olof Syrén
    Per-Olof Syrén
    Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden
    Wallenberg Wood Science Center (WWSC), KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden
    Biotechnology and Health, Science for Life Laboratory, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden
  • Minna Hakkarainen*
    Minna Hakkarainen
    Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden
    Wallenberg Wood Science Center (WWSC), KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden
    *Email: [email protected]
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ACS Sustainable Chemistry & Engineering

Cite this: ACS Sustainable Chem. Eng. 2023, 11, 8, 3451–3465
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https://doi.org/10.1021/acssuschemeng.2c06935
Published February 15, 2023

Copyright © 2023 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Bio-based plastics potentially have several positive impacts on the environment; however, in order to make a real difference, they need to have managed and sustainable end of life. This means they should from the start be designed for chemical, mechanical, and/or organic (biological) recycling. Development of energy-efficient and selective chemical recycling processes is a necessary part in reaching truly circular plastic flows. Polyesters are generally well suited for chemical recycling due to the presence of reversible ester bonds. Utilization of dynamic covalent chemistry to include a second, even more easily reversed bond, such as Schiff base (SB, imine bond), could further facilitate chemical recycling, enabling depolymerization back to monomeric products under mild conditions. Here, we present the synthesis of three vanillin-derived SB monomers SBM1, SBM2, and SBM3 and the corresponding polymers SBP3a–b, SBP4a–b, and SBP5a–b. Three different diamines and two potentially bio-sourced diesters were utilized to yield altogether six different polyester-imines with different aliphatic/aromatic contents. All the obtained SB-based polyesters were thermally stable at ∼290–330 °C and had a high char yield during the pyrolysis, which may indicate inherent flame resistance. All the polyesters were amorphous with glass transition temperatures from 36 to 76 °C. The chemical recyclability and hydrolytic degradation of the synthesized polyesters was evaluated by using real-time 1H NMR spectroscopy. Finally, the susceptibility of the synthesized polyester-imines to enzymatic degradation by PETase was demonstrated. The experimental results were further supported by induced-fit docking experiments to theoretically evaluate the potential productive binding of the produced polyester-imines and intermediates thereof to the active site of PETase.

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Copyright © 2023 The Authors. Published by American Chemical Society

Synopsis

Biobased polyester-imines exhibiting enzymatic degradability and closed-loop chemical recyclability under mild conditions provide a step toward circular bioeconomy.

Introduction

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Most of the plastics available in the market are entirely made from fossil-based resources. (1−3) Fossil-based plastics were originally designed and optimized for the application requirements and linear economy mode. (4,5) This is not sustainable and causes significant pollution to our environment. (6,7) At the same time, plastics have many favorable properties and they have penetrated all areas of our lives from agriculture to packaging, construction, transportation, leisure, health care, and electronics. This has caused plastic production to increase year by year. (8−14) 10.5 billion metric tons of plastic have been produced globally, with an annual production now reaching 380 million metric tons. Consequently, based on the estimation made in 2015 that 6300 million tons of plastic waste had been generated and yearly plastic waste production of 300 million tons, we have now reached >8000 million tons. (15−17) Most of this waste has not been recycled.
Aromatic polyesters, such as poly(ethylene terephthalate) (PET), poly(butylene terephthalate), poly(ethylene naphthalate), and emerging poly(ethylene furanoate), are an important class of commercial materials. (18,19) These polymers have good thermal, mechanical, and barrier properties and are widely used in packaging, resins, fibers, films, and clothing. (18,20) Although these materials can be fully or partly produced from biobased raw materials, a managed and sustainable end of life is still a challenge. (18,21−26) Among synthetic polymers, PET is the most recycled plastic. Mechanical recycling of PET often leads to the so-called downcycling, for which products of lower value than the starting material are generated. Chemical recycling typically requires energy, although PET can be recycled through multiple processes such as glycolysis, (27) methanolysis, (28) aminolysis, (29,30) hydrogenolysis, (31−33) and hydrosilylation. (21,26,34) However, only some of these processes lead to monomers that can be utilized for production of new PET in closed loop. (18,24,35) Due to their aromatic nature, these polymers are not readily (bio)degradable, although there are large research efforts to develop engineered enzymes to degrade and enzymatically recycle PET back to original monomers. (22,25,36−38) Still, significant pre-treatment of PET is required for obtaining reasonable degradation rates. (39) Poly(butylene adipate-co-terephthalate) (PBAT) is an interesting commercial aliphatic/aromatic polyester which, in contrast to the abovementioned fully aromatic polyesters, is readily biodegradable due to the tuned ratio of aliphatic/aromatic units. (40,41) However, this comes with some reduction of properties. (42,43)
Bio-based plastics can have positive impacts on the environment by reducing carbon emission, if produced from sustainable resources by green processes. (3,15,44,45) However, to make a real impact, they need a sustainable and managed end of life. (46−48) They must from the start be designed for chemical, mechanical, and/or organic (biological) recycling. (49,50) Developing energy-efficient and selective chemical recycling processes is a necessary part of achieving truly circular plastic flows. (3,49−54) Polyesters are well suited for chemical recycling due to the presence of reversible ester linkages. (2,3,47,49,53,55,56) The use of dynamic covalent chemistry to include a second, even more easily reversible bond, such as a Schiff base (SB, i.e., imine bond), can further facilitate chemical recycling back to monomers under mild conditions. (3,57−64)
Vanillin-derived SB monomers were earlier utilized for production of chemically and mechanically recyclable thermosets. (60,61) Although linear thermoplastics can be thermally reprocessed without dynamic covalent bonds, repeated thermal processing and service life leads to deterioration of properties. We therefore aimed to produce a series of linear polyester-imines starting partially from biobased monomers and with different aliphatic/aromatic ratios to tailor structural similarity to PET and/or PBAT. Our hypothesis was that the imine bond could provide facile chemical recyclability under mild conditions, while a balanced ratio of aliphatic/aromatic units could provide thermally stable materials with improved susceptibility to enzyme-catalyzed hydrolysis. This would further enable chemical recycling with green catalysts and indicate potential degradability in favorable natural environments or compost. Three diol monomers were first produced from vanillin, ethylene carbonate (EC), and diamines by a simple two-step process, which involved nucleophilic substitution reaction followed by SB formations. The monomers were further polymerized with aliphatic and aromatic diesters to yield altogether six different polyester-imines with different aliphatic/aromatic contents. All products were fully characterized, and the chemical recyclability and enzyme-catalyzed hydrolytic degradation were evaluated. Induced-fit docking experiments were performed to theoretically evaluate the potential binding of the produced polyester-imines and derivatives thereof to the active side of the PETase.

Experimental Section

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Materials

Vanillin (99%), EC (98%), ethylenediamine (EDA, >99%), 1,4-diaminobutane (BDA, 99%), m-xylylenediamine (m-XylDA, 99%), dimethyl succinate (DMS, 98%), dimethyl terephthalate (DMT, 99%), mesitylene (98%), dimethyl sulfoxide (DMSO, 99%), dibutyltin oxide (DBTO, >98%), potassium carbonate (K2CO3, 99%), and sodium sulfate (Na2SO4) were purchased from Sigma-Aldrich. Hydrochloric acid (HCl, 37%), N,N-dimethylformamide (DMF, ACS, Reag. Ph. Eur.), ethyl acetate (EtOAc, ACS, Reag. Ph. Eur.), ethanol, and n-heptane were obtained from VWR Chemicals and xylene (Analytical grade, ACS) from Scharlau. All chemicals and reagents were used as received.

Analytical Methods

Nuclear Magnetic Resonance Spectroscopy

1H and 13C nuclear magnetic resonance spectroscopy (NMR) measurements were performed on an Avance 400 spectrometer at 298 K, and the chemical shifts were reported as δ values (ppm) of the residual DMSO-d6 (2.50 and 39.52 ppm) taken as a reference. The proton frequency was 400.13 MHz, and the carbon frequency was 100.61 MHz.

Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy

Fourier transform infrared (FT-IR) spectroscopy was performed with a PerkinElmer 2000 spectrophotometer equipped with an attenuated total reflection setup. For all the samples, 8 successive scans over the range of 400–4000 cm–1 were performed.

Size Exclusion Chromatography

The molecular weight of the polyesters was determined by size exclusion chromatography (SEC) measurements. The SEC system consisted of three PSS GRAM columns in series (KF-805, 2804, and 2802.5) and an Agilent Technologies 1260 Infinity (PSS, Germany) equipped with a refractive index detector (G1362A 1260 RID Agilent Technologies) and a multi-angle laser light scattering detector (SLD 7000 PSS, Germany). All the measurements were carried out at room temperature, using DMSO with 0.5 wt % LiBr as mobile phase, and the elution rate was 1 mL min–1. The system was calibrated with pullulan standards of narrow polydispersity (PSS, Germany). Polyesters were dissolved in SEC eluent and kept for 12 h. Afterward, the polymer samples were filtered by using a membrane filter and transferred into special SEC vial, and a volume of 100 μL of each sample was injected with the flow rate of 0.5 mL min–1. Finally, the number and weight-average molar mass (Mn and Mw) and dispersity () were calculated with the WinGPC UniChrom GPC/SEC software (PSS, Germany) using the light scattering calibration method.

Thermogravimetric Analysis

Thermogravimetric analysis (TGA) was performed with a Mettler Toledo TGA 1 instrument at a heating rate of 10 °C min–1 under a nitrogen atmosphere with a purge rate of 50 mL min–1 at the temperature range 50–600 °C. The samples were kept isothermally at 150 °C for 15 min to remove solvent residues and then cooled into 50 °C, followed by heating at 600 °C, and the thermal decomposition maxima (Td), 5% weight loss (T5), and char yield were determined. TGA data were analyzed by Mettler Toledo STARe v. 15.00 software.

Differential Scanning Calorimetry

Differential scanning calorimetry (DSC) measurements were performed using a Mettler Toledo DSC 1 instrument. The samples were studied with a heating rate of 10 °C min–1 under nitrogen flow with a purge rate of 50 mL min–1. The sequence consisted of a heating ramp from −30 to 200 °C, followed by a cooling ramp to 0 °C and finally a second heating ramp to 200 °C. The glass transition temperature (Tg) was determined from the second heating cycle. DSC data were analyzed by Mettler Toledo STARe v. 15.00 software.

Synthesis

Synthesis of 4-(2-Hydroxyethoxy)-2-methoxybenzaldehyde (1)

A mixture of vanillin (32.86 mmol, 1.00 equiv), EC (36.15 mmol, 1.10 equiv), and K2CO3 (39.43 mmol, 1.20 equiv) in 50 mL of DMF was added into a 100 mL round-bottomed flask and stirred at 110 °C under a N2 atmosphere for 12 h. The completion of the reaction was monitored by thin-layer chromatography (TLC). The reaction mixture was subsequently cooled to room temperature and extracted with EtOAc (200 mL), after which water (400 mL) was added. The aqueous phase was separated and extracted with EtOAc (2 × 100 mL). The organic phases were combined, washed with water (3 × 200 mL) and brine (200 mL), dried over Na2SO4, and concentrated and recrystallized by using a mixture of 1:9 ratio of ethyl acetate and n-heptane. The product thus obtained was a white solid. (65)
White solid; yield 60%; 1H NMR (400.13 MHz, DMSO-d6): δ ppm 9.84 (s, 1H, −CHO), 7.54 (d, 1H, Ar), 7.40 (s, 1H, Ar), 7.19 (d, 1H, Ar), 4.93 (t, 1H, benzylic −OH), 4.10 (t, 2H, OCH2CH2OH), 3.84 (s, 3H, −OCH3), 3.76 (m, 2H, −OCH2CH2OH). 13C NMR (100.61 MHz, DMSO-d6): δ ppm 191.53, 153.75, 149.28, 129.63, 126.21, 112.13, 109.61, 70.49, 59.42, and 55.52.

General Procedure for Synthesis of Schiff Base Monomers (SBM1–3)

Three different SB monomers SBM1–3 were synthesized by using a 2:1 molar ratio of 4-(2-hydroxyethoxy)-2-methoxybenzaldehyde (1) and three different diamines, EDA (SBM1), BDA (SBM2), and m-xylylenediamine (SBM3), in the presence of EtOH (10 mL) at room temperature under a N2 atmosphere for 4 h. The completion of the reaction was monitored by TLC, and then the precipitate was filtered and washed with EtOH (20 mL). The product was dried with help of a vacuum pump at 60 °C for 12 h.
Monomer SBM1
White solid; yield 88%; mp 149 °C (DSC); 1H NMR (400.13 MHz, DMSO-d6): δ ppm 8.21 (s, 2H, imine), 7.33 (s, 2H, Ar), 7.16 (d, 2H, Ar), 6.97 (d, 2H, Ar), 4.88 (broad s, 2H, benzylic −OH), 3.99 (t, 4H, OCH2CH2OH), 3.80 (t, 4H, imine −CH2), 3.76 (s, 6H, −OCH3) and 3.72 (t, 4H, −OCH2CH2OH). 13C NMR (100.61 MHz, DMSO-d6): δ ppm 161.88, 150.87, 149.46, 129.54, 122.86, 112.62, 109.63, 70.58, 61.41, 59.94 and 55.73. FT-IR ν (cm–1): 3195, 2916, 1631, 1585, 1501, 1445, 1259, 1129, 1027, 794, and 748. LC–MS calculated for C22H28N2O6; M = 416.19, found [M + H]+ (m/z = 417).
Monomer SBM2
White solid; yield 86%; mp 169 °C (DSC); 1H NMR (400.13 MHz, DMSO-d6): δ ppm 8.23 (s, 2H, imine), 7.35 (s, 2H, Ar), 7.19 (d, 2H, Ar), 7.01 (d, 2H, Ar), 4.88 (broad s, 2H, benzylic −OH), 4.01 (t, 4H, −OCH2CH2OH), 3.78 (s, 6H, −OCH3), 3.73 (t, 4H, imine −NCH2), 3.56 (broad s, 4H, −OCH2CH2OH) and 1.65 (t, 4H, imine −NCH2CH2). 13C NMR (100.61 MHz, DMSO-d6): δ ppm 160.53, 150.78, 149.49, 129.62, 122.77, 112.68, 109.63, 70.59, 60.72, 59.93, 55.75 and 28.90. FT-IR ν (cm–1): 3185, 2925, 1631, 1585, 1511, 1268, 1147, 1027, 896, 810, and 729. LC–MS calculated for C24H32N2O6; M = 444.23, found [M + H]+ (m/z = 445).
Monomer SBM3
White solid; yield 91%; mp 143 °C (DSC); 1H NMR (400.13 MHz, DMSO-d6): δ ppm 8.37 (s, 2H, imine), 7.39 (d, 2H, Ar), 7.23 (m, 4H, Ar), 7.02 (d, 2H, Ar), 4.92 (broad s, 2H, benzylic −OH), 4.72 (s, 4H, m-xylylene Ar −CH2), 4.02 (t, 4H, −OCH2CH2OH), 3.80 (t, 4H, imine −CH2) and 3.75 (broad s, 10H, −OCH3 and −OCH2CH2OH). 13C NMR (100.61 MHz, DMSO-d6): δ ppm 161.68, 151.02, 149.51, 140.35, 129.47, 128.87, 128.06, 126.95, 123.14, 112.62, 109.65, 70.62, 64.41, 59.96 and 55.69. FT-IR ν (cm–1): 3390, 2916, 1640, 1594, 1511, 1268, 1120, 1027, 784, and 710. LC–MS calculated for C28H32N2O6; M = 492.23, found [M + H]+ (m/z = 493).

General Procedure for Synthesis of Schiff Base Polymers (SBP1a–b to SBP3a–b)

SB diol monomer SBM1–3 (9.60 mmol, 1.00 equiv), a diester [DMS (6a), 1.44 mmol, 1.50 equiv or DMT (6b), 4.46 mmol, 1.10 equiv], DBTO (47 mg, 2 mol % with respect to the corresponding diol monomer), and xylene (1 mL) were added to a three-necked round-bottom flask equipped with a mechanical stirrer. The reaction mixture was heated at reflux for 15 min under a flow of N2, until xylene was distilled off. Afterward, the reaction mixture was heated up to 130 °C. Once the transesterification was completed (approximately after 3 h, as observed by 1H NMR), the flow of N2 was increased and the mixture was heated to 140 °C for 12 h. Afterward, the reaction mixture was cooled to room temperature, dissolved in DMSO (10 mL) and precipitated in EtOH (200 mL). The pale-yellow precipitate was collected by suction filtration, washed with ethanol (100 mL), and dried over vacuum at 60 °C for 24 h, yielding the corresponding polyester. The polyesters synthesized from monomers SBM1 (3), SBM2 (4), and SBM3 (5) were denoted as SBP3a–b, SBP4a–b and SBP5a–b, respectively, where a further indicates polyesters synthesized with aliphatic DMS and b polyesters synthesized with aromatic DMT.
Polyester SBP3a
Pale yellow solid (4.18 g, 84%). SEC Mn = 13,600 g mol–1, Mw = 29,700 g mol–1, PDI = 2.2. 1H NMR (400.13 MHz, DMSO-d6): δH (ppm) 8.22 (br, imine −CH), 7.34 (s, Ar-CH), 7.16 (d, Ar-CH), 6.97 (d, Ar-CH), 4.32 (s, imine −CH2), 4.16 (br, Ar-OCH2), 3.81 (s, Ar-OCH2CH2), 3.76 (s, Ar-OCH3) and 2.59 (s, aliphatic succinic-CH2). 13C NMR (100.61 MHz, DMSO-d6): δC (ppm) 172.37, 161.80, 150.26, 149.52, 130.05, 122.73, 113.13, 109.88, 66.95, 63.18, 61.38, 55.83, and 28.96. FT-IR ν (cm–1): 2916, 1706, 1631, 1585, 1501, 1250, 1120, 1027, 803, and 738.
Polyester SBP3b
Light brown solid (3.40 g, 70%). SEC Mn = 7800 g mol–1, Mw = 18,300 g mol–1, PDI = 2.3. 1H NMR (400.13 MHz, DMSO-d6): δH (ppm) 8.22 (br, imine −CH), 8.05 (d, Ar-CH), 7.34 (s, Ar-CH), 7.16 (d, Ar-CH), 6.97 (d, Ar-CH), 4.62 (s, imine −NCH2), 4.37 (br, Ar-OCH2), 4.00 (s, Ar-OCH2CH2), and 3.88–3.67 (m, aliphatic −CH2 and −OCH3). 13C NMR (100.61 MHz, DMSO-d6): δC (ppm) 165.07, 161.44, 150.47, 149.08, 129.61, 122.41, 113.46, 112.33, 109.79, 109.34, 70.20, 66.77, 63.92, 60.96, 59.53, and 55.37. FT-IR ν (cm–1): 2943, 1724, 1631, 1585, 1501, 1241, 1101, 1017, 803, and 729.
Polyester SBP4a
Light brown solid (3.80 g, 77%). SEC Mn = 19,700 g mol–1, Mw = 47,000 g mol–1, PDI = 2.4. 1H NMR (400.13 MHz, DMSO-d6): δH (ppm) 8.21 (br, imine −CH), 7.35 (s, Ar-CH), 7.17 (d, Ar-CH), 6.99 (d, Ar-CH), 4.32 (s, imine −NCH2), 4.16 (br, Ar-OCH2), 3.77 (s, Ar-OCH2CH2), 3.54 (s, Ar-OCH3), 2.67 (s, aliphatic succinic-CH2) and 1.63 (s, imine −NCH2CH2). 13C NMR (100.61 MHz, DMSO-d6): δC (ppm) 172.37, 160.43, 150.18, 149.55, 130.14, 122.64, 113.16, 109.88, 66.96, 63.18, 60.72, 55.83 and 28.93. FT-IR ν (cm–1): 2934, 1724, 1640, 1576, 1511, 1268, 1120, 1017, 859, 803, and 738.
Polyester SBP4b
Light brown solid (3.80 g, 71%). SEC Mn = 10,600 g mol–1, Mw = 18,000 g mol–1, PDI = 1.7. 1H NMR (400.13 MHz, DMSO-d6): δH (ppm) 8.22 (br, imine −CH), 8.03–7.99 (m, Ar-CH), 7.34 (s, Ar-CH), 7.19 (br, Ar-CH), 7.09 (d, Ar-CH), 4.62 (s, imine −NCH2), 4.38 (br, Ar-OCH2), 3.75 (s, Ar-OCH2CH2), 3.55 (s, Ar-OCH3), and 1.63 (s, imine −NCH2CH2). 13C NMR (100.61 MHz, DMSO-d6): δC (ppm) 165.50, 160.42, 150.25, 130.15, 130.04, 129.98, 122.63, 110.08, 67.23, 64.37, 60.70, 55.88, and 28.86. FT-IR ν (cm–1): 2925, 1715, 1640, 1585, 1501, 1259, 1101, 1010, 859, 803, and 719.
Polyester SBP5a
Pale yellow solid (4.0 g, 83%). SEC Mn = 6800 g mol–1, Mw = 9300 g mol–1, PDI = 1.2. 1H NMR (400.13 MHz, DMSO-d6): δH (ppm) 8.36 (s, imine −CH), 7.37 (s, Ar-CH), 7.24 (broad s, Ar-CH), 7.00 (s, Ar-CH), 4.71 (s, imine −NCH2), 4.33 (br, Ar-OCH2), 4.17 (br, Ar-OCH2CH2), 3.74 (s, Ar-OCH3), and 2.60 (s, aliphatic succinic-CH2). 13C NMR (100.61 MHz, DMSO-d6): δC (ppm) 172.40, 161.59, 150.41, 149.56, 140.33, 129.99, 128.88, 128.09, 126.96, 123.01, 113.14, 109.90, 66.97, 64.40, 63.19, 65.78, and 28.96. FT-IR ν (cm–1): 2925, 1733, 1640, 1576, 1501, 1259, 1138, 1027, 803, and 702.
Polyester SBP5b
White solid (3.8 g, 73%). SEC Mn = 6500 g mol–1, Mw = 9400 g mol–1, PDI = 1.4. 1H NMR (400.13 MHz, DMSO-d6): δH (ppm) 8.38 (s, imine −CH), 8.08–7.99 (m, Ar-CH), 7.37–7.08 (m, Ar-CH), 4.70 (s, imine −NCH2), 4.62 (br, Ar-OCH2), 4.39 (br, Ar-OCH2CH2), and 3.76–3.62 (m, aliphatic −CH2 and −OCH3). 13C NMR (100.61 MHz, DMSO-d6): δC (ppm) 165.40, 161.60, 150.46, 149.74, 140.29, 130.15, 129.98, 126.91, 122.95, 113.49, 110.10, 67.13, 64.33, 59.91, and 55.85. FT-IR ν (cm–1): 2943, 1715, 1640, 1585, 1501, 1259, 1110, 1017, 803, and 729.

General Procedure for the Chemical Recyclability Studies

1 g of each polymer (SBP3a–b, SBP4a–b, and SBP5a–b) was placed in a vial with 20 mL of 0.1 M HCl solution of methanol and deionized water (8:2 v/v) at room temperature for 24 h. After 24 h, the solution was poured into 200 mL of deionized water and stirred for 30 min. The precipitate was filtered, washed with water (100 mL), and dried over vacuum at 60 °C for 24 h, yielding the corresponding recycled products RP1 (for a series polymers) and RP2 (for b series polymers), respectively.
Recycled Product RP1
Pale yellow solid (0.78 g, 90%). 1H NMR (400.13 MHz, DMSO-d6): δH (ppm) 9.84 (s, −CHO), 7.54 (dd, Ar-CH), 7.40 (s, Ar-CH), 7.18 (d, Ar-CH), 4.37 (t, Ar-OCH2), 4.28 (t, Ar-OCH2CH2), 3.84 (s, Ar-OCH3), and 2.61 (s, aliphatic succinic-CH2). 13C NMR (100.61 MHz, DMSO-d6): δC (ppm) 191.88, 172.37, 153.48, 149.40, 130.42, 126.33, 112.87, 110.32, 67.17, 63.01, 56.01, and 28.96. FT-IR ν (cm–1): 2934, 2811, 2713, 1730, 1677, 1580, 1517, 1265, 1137, 1031, 729.
Recycled Product RP2
Light brown solid (0.71 g, 88%). 1H NMR (400.13 MHz, DMSO-d6): δH (ppm) 9.84 (s, −CHO), 8.07 (d, Ar-CH), 7.26–7.57 (m, Ar-CH), 4.65 (d, Ar-OCH2), 4.47 (d, Ar-OCH2CH2), and 3.81 (s, Ar-OCH3). 13C NMR (100.61 MHz, DMSO-d6): δC (ppm) 191.91, 165.40, 153.56, 149.84, 133.96, 130.55, 130.07, 126.31, 113.29, 110.50, 67.22, 64.16, and 56.07. FT-IR ν (cm–1): 2934, 2846, 1715, 1677, 1571, 1505, 1260, 1128, 1022, 729.

Mapping Chemical Recycling Process by Real-Time 1H NMR (66)

50 mg of polymers SBP4a, SBP4b, and SBP5b were placed in NMR tubes, which contained 1 mL of 0.1 M HCl solution of deuterated methanol (methanol-d4) and deionized water (8:2, v/v), and the chemical changes (degradation) were analyzed via a real-time 1H NMR spectroscopy at room temperature. 1H NMR spectra were recorded after different time periods from 15 min to 24 h.

Enzymatic Degradation

Enzyme Production and Purification

Escherichia coli C43(DE3) harboring plasmid carrying wild type PETase (PDB ID 6EQE (4)) from Ideonella sakaiensis was prepared as explained previously. (37) Preculture was prepared in 2× YT liquid media (pH 7) containing NaCl (5 g L–1), yeast extract (10 g L–1), tryptone (16 g L–1), and ampicillin (100 μg mL–1). After overnight incubation at 37 °C and 200 rpm (yielding an OD600 = 0.15), the preculture was transferred to a 2 L flask containing the same medium and conditions until the final OD600 reached 0.6. Then 1 mM isopropyl β-d-1-thiogalactopyranoside was added to the medium, which was incubated at 30 °C (160 rpm) for 20 h. The cell pellet was collected by centrifuging (4800g for 15 min), and the proteins were extracted by using B-PER complete lysis buffer for 15 min at 25 °C, 80 rpm. Proteins were purified by ÄKTA Start (GE Healthcare, Sweden) and His MultiTrap FF column and desalted by PD-10 columns (Cytiva) into 10 mM Tris-HCl buffer pH 7.4. (37)

Enzymatic Degradation of the Schiff Base Polymers

The ability of PETase to catalyze the hydrolysis of the SB polymers was evaluated according to the previously reported procedure. (37) SBP3a, SBP4a, and SBP5b were selected as model polymers representing different aliphatic/aromatic contents. 15 mg of each polymer was soaked into a mixture of phosphate buffer (800 μL, 50 mM, pH 7.2), DMSO (200 μL), and PETase (3 μg mL–1). The reaction mixtures and controls (all in triplicate) were incubated at 30 °C with shaking (300 rpm) for 1 and 24 h. In the meantime, the non-enzymatic degradations of the same polymers SBP3a, SBP4a, and SBP5b were carried out as negative control under identical conditions without enzyme. After the incubation, the samples were centrifuged at 14,000g for 10 min and the supernatant was then analyzed by LC–MS (Agilent 1260 Infinity II HPLC system, Agilent Technologies, USA) with an X-bridge C18 column (50 × 3.0 mm, 3.5 u, mobile phase 10–97% MeCN 3 min, 1 mL min–1, A: MeCN B: NH4HCO3).

Molecular Docking

X-ray crystal structures of PETase (PDB ID 6EQE) (67) were obtained from RCSB Protein Data Bank. PETase was prepared for induced-fit docking (IFD) (68−70) using a protein preparation wizard (71) built within Schrödinger Maestro Suite (Schrödinger, LLC; USA). In the protein preparation, crystal water molecules were removed, bond orders were assigned using the CCD database and missing hydrogens were added to the structure. Using Epik, (72,73) het states were generated at a pH of 7.0 ± 2.0 using Prime. (74,75) Subsequently, a brief relaxation was performed on each starting structure using the OPLS4 force field. (76) This is a two-part procedure that consists of optimizing hydroxyl and thiol torsions in the first stage followed by an all-atom constrained minimization in the second stage to relieve clashes. The minimization was terminated when the RMSD reached a maximum value of 0.18 Å.
The polymer substrates SBP3a, SBP4a, and SBP5b and degradation products DP1 and DP4 were prepared and minimized in Maestro (Schrödinger, LLC, New York, NY, 2021) using the LigPrep module with the OPLS4 force field. LigPrep generates all possible ring conformations, ionization states, tautomers, and stereoisomers at predefined pH 7.0 ± 2.0 and assigning bond orders. The structures obtained from the LigPrep were used for the IFD in the PETase. The induced-fit docking protocol embedded in the Schrödinger suite was used. The binding site was defined by generating a grid centered on the catalytic residues: Ser 160, His 237, and Asp 206. The grid box size was increased with ≤35 Å to allow for docking of lengthy substrates. All other parameters were kept at their default values.

Results and Discussion

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Synthesis of Monomers and Polyesters

Six different linear SB polyesters with different aliphatic/aromatic ratios originating from vanillin, aliphatic/aromatic diamine, and aliphatic/aromatic dimethyl esters were successfully synthesized. First, a series of SB diol monomers were synthesized by a simple two-step protocol, which involved SN2 nucleophilic substitution reaction of vanillin with EC, yielding 1. The aldehyde group of vanillin was then subjected to a SB formation with three different diamines at room temperature to furnish the corresponding diol monomers 3–5 (SBM1, SBM2, and SBM3) in a satisfactory yield (≥85%). The structure and purity of the monomers were unambiguously confirmed by 1H and 13C and 2D NMR (COSY, HMBC, HMQC, and DEPT) spectroscopy; see (Figures S1–S17). In the 1H NMR spectra of all the monomers (Figures 1A, S1A, and S3A), the signal of the benzylic −OH group appeared at δ 4.88 ppm, aliphatic −CH2 and aromatic −OCH3 at δ 1.65 to 4.01 ppm, the imine −CH at δ 4.88 ppm, and the aromatic peaks at δ 6.96 to 7.35 ppm. The structure of the monomers was further confirmed by 13C NMR spectroscopy (Figures 2A, S2A, and S4A). For all the monomers, the imine carbonyl peak appeared at δ 162.00 ppm and aromatic O–C carbon peaks were observed at δ 149.46 and 150.87 ppm, respectively. The aromatic carbon peak appeared at δ 129.54 to 109.63 ppm. Aliphatic methylene peaks and aromatic −OCH3 peak were observed at δ 70.58 to 28.90 ppm, respectively.

Figure 1

Figure 1. 1H NMR (400.13 MHz, DMSO-d6) spectra of (A) monomer SBM2 (4) and (B,C) its corresponding polymers SBP4a–b, received after reaction with DMS and DMT, respectively.

Figure 2

Figure 2. 13C NMR (100.61 MHz, DMSO-d6) spectra of (A) monomer SBM2 (4) and (B,C) its corresponding polymers SBP4a–b formed after reaction with DMS and DMT, respectively.

Afterward, the obtained diol monomers were polymerized with potentially bio-sourced aliphatic and aromatic diesters, DMS and DMT respectively. A conventional two-step polycondensation protocol (transesterification and polycondensation) was employed using a DBTO as a metal catalyst. In the first step, transesterification was achieved at 130 °C in the presence of a small amount of xylene (1 mL/1 g of the diol monomers) with slow nitrogen flow for 4 h, and then in the second step, polycondensation was carried out at 140 °C with high nitrogen flow for 12 h to remove condensed MeOH. Once the polymerization reaction was completed, the obtained viscous reaction mixtures were dissolved in DMSO and precipitated in ethanol to yield 70–80% of the SB polyesters shown in Scheme 1.

Scheme 1

Scheme 1. General Reaction Scheme for the Synthesis of Schiff Base Polymersa

aFirst, the synthesis of SB diol monomers SBM1–SBM3 (3–5) starting by reaction between modified vanillin [4-(2-hydroxyethoxy)-2-methoxybenzaldehyde] and EDA (2a), BDA (2b) or m-xylylenediamine (XylDA, 2c), respectively, followed by polymerizations of the diol monomers with aliphatic or aromatic diester, DMS (6a) and DMT (6b), respectively, yielding polyesters (SBP3a–b and SBP5a–b) with different aliphatic/aromatic contents.

After purifications, the structures of the SB-based polyesters were confirmed by using 1H and 13C NMR spectroscopy. The successful polymerizations were shown by the disappearance of the benzylic −OH peak at δ 4.88 present in the 1H NMR spectra of monomers SBM1, SBM2, and SBM3. A new peak was generated at δ 2.60 or 7.99–8.05 corresponding to the DMS −CH2 and aromatic (DMT) −CH peaks, respectively. The other methylenes (−CH2, 2 and 3) were shifted downfield due to electron-withdrawing ester groups (compared to the initial −OH groups in the monomers), and methoxyl (−OCH3) and aromatic −CH signals remained almost unchanged in chemical shifts, but became broadened, which indicated formation of polymers SBP3a–b, SBP4a–b, and SBP5a–b, as shown in Figures 1B,C and S1B,C to S3B,C.
The structures of the SB polyesters were further confirmed by 13C NMR spectroscopy. After the polymerization, all the aromatic, −OCH3, and butylene bridge −CH2 carbon peaks remained exactly the same as for the corresponding monomers, except that ethylene bridge −CH2 (3 and 5) was shifted due to the electron-withdrawing nature of the ester groups. New peaks also appeared at δ 28.96 (a) and 172.37 ppm (b), which correspond to the DMS carbon and ester carbonyl carbon peak of SBP3a (Figure S2B,C). A chemical shift was observed at δ 165.50 ppm, which corresponds to the DMT carbonyl carbon peak of SBP3b. Similar 13C NMR spectroscopy patterns were observed for the monomers SBM2 and SBM3 and their corresponding polyesters SBP4a–b and SBP5a–b, as shown in Figures 2B,C and S4B,C.
The monomers and polymers were further characterized by FT-IR analysis (Figure S5). For the monomers SBM1, SBM2, and SBM3, the benzylic −OH peak was observed at ∼3300 cm–1, aliphatic and aromatic C–H stretching bands appeared at 2800–3000 cm–1, imine peak was observed at ∼1640 cm–1, −C–O stretching was observed at ∼1260 cm–1, and the aliphatic and aromatic C–H bending bands appeared at ∼743 cm–1. After the polymerization, the peak at ∼3300 cm–1 completely disappeared and a new peak was observed at ∼1730 cm–1, which corresponds to the ester carbonyl stretching signal. The remaining peaks in polymers SBP3a–b, SBP4a–b, and SBP5a–b in-line with those observed for the monomers, which further confirmed the structures of the synthesized polymers.
Finally, the number and weight average molar mass (Mn and Mw) and dispersity () of the synthesized polyesters was determined by SEC analysis; see (Table 1). Polyesters SBP3a and SBP4a illustrated reasonably high molar masses with Mn = 19.7 and 13.6 kg mol–1, while all the polyesters containing aromatic m-xylylene or terephthalate units (SBP3b, SBP4b, SBP5a, and SBP5b) had somewhat lower molar masses with Mn = 7.8, 10.6, 6.8, and 6.5 kg mol–1, respectively. The molar mass of polymers containing m-xylylene and terephthalate structures remained relatively low. However, higher reaction temperature could not be used due to instability of the imine bond. The polydispersity of all the polyesters was in the range of Đ = 1.2 to 2.4.
Table 1. Molar Mass and Thermal Properties of the Schiff Base Polyesters SBP3a–b, SBP4a–b, and SBP5a–b and the Three Monomers SBM1 (3), SBM2 (4), and SBM3 (5)a
samplesMn (kg mol–1)Mw (kg mol–1)ĐTg (°C)T5 (°C)Td (°C)CY (%)
SBP3a13.629.72.1851296316, 37434.2
SBP3b7.818.32.3466300309, 35035.3
SBM1 (3)    287307, 37529.0
SBP4a19.747.02.3836289283, 39932.1
SBP4b10.618.01.6957315278, 39033.6
SBM2 (4)    275297, 35731.7
SBP5a6.89.31.2158318330, 39040.1
SBP5b6.59.41.4478330340, 45743.7
SBM3 (5)    307321, 40241.4
a

Mn, Mw, and Đ were measured by SEC in DMSO. Tg values were obtained from the second heating scan of DSC analysis. T5 (temperature for 5% mass loss) and Td (temperature for the maximal decomposition rate) were obtained from TGA analysis.

Thermal Properties of the Schiff Base-Based Polyesters

Thermal properties of the SB polyesters were investigated by TGA and DSC analyses. The thermal stability of the polyesters was evaluated by TGA, as shown in Figure 3 and Table 1. All the synthesized polyesters showed two thermal decomposition maxima (Td) between ∼278 and 460 °C. (77) The lower temperature maximum is likely connected to the decomposition of imine bonds. This is supported by this maximum being the dominant decomposition step for the monomers, while the higher temperature maximum was the main decomposition step for the polymers, with the exception of SBP5b.

Figure 3

Figure 3. TGA curves showing the weight loss (A,C,E) and first derivative weight loss curves (B,D,F) of monomers SBM1 (3), SBM2 (4), and SBM3 (5) and polyesters SBP3a–b, SBP4a–b, and SBP5a–b.

All the polyesters SBP3a–b, SBP4a–b, and SBP5a–b showed thermal decomposition (T5, determined at 5% weight loss) at temperatures above ∼285 °C. In comparison with the corresponding monomers SBM1, SBM2, and SBM3, all the synthesized polymers were thermally more stable compared to corresponding diol monomers as determined by T5 and the polyester with the highest aromatic content SBP5b, obtained from monomer SBM3, showed the highest thermal decomposition temperatures (T5) at 330 °C. Furthermore, the char yield was evaluated after thermal degradation of the polyesters at 600 °C. For all the polyesters, high char yields (∼31 to ∼44%) were observed, which correspond to the high content of aromatic structures and could indicate a certain level of inherent flame resistance. (78−80)
Thermal transitions of the polyesters SBP3a–b, SBP4a–b, and SBP5a–b were analyzed by DSC, and the data are shown in Figure 4 and Table 1. Tg of the polyesters was easily observable and was in the temperature range from 36 to 78 °C. No melting transitions were observable, indicating a completely amorphous nature. There was a clear correlation between the chemical structure and observed Tg. Polyesters SBP5a–b exhibited higher Tg values of 58 and 78 °C, compared with the other two series of polyesters, which is attributed to the aromatic bulky diamine, m-xylylenediamine. Furthermore, the Tg values of SBP3a–b (51 and 66 °C) with a shorter aliphatic diamine were higher than those of SBP4a–b (36 and 57 °C) with longer and more flexible aliphatic diamine.

Figure 4

Figure 4. DSC thermograms from the second heating of polyesters SBP3a–b, SBP4a–b, and SBP5a–b.

In a similar manner, SBP3b, SBP4b, and SBP5b with aromatic terephthalate units exhibited higher Tg compared to the corresponding a-series of polyesters, SBP3a, SBP4a, and SBP5a with aliphatic succinate units. For all series, the Tg values, thus, clearly decreased upon increasing flexibility originating from aliphatic units and/or lower aromatic content, while the opposite was seen as a function of increased rigidity and aromatic content.

Chemical Recyclability of the Synthesized Polyesters

Dynamic covalent bonds such as SB exhibit potential for chemical recyclability under mild conditions, even at room temperature. The chemical recyclability and degradation behavior of the synthesized polyesters SBP3a–b, SBP4a–b, and SBP5a–b were therefore evaluated. Thanks to the imine bond, the synthesized SB polyesters could be chemically recycled back to monomeric dialdehydes and diamines by placing them under aqueous acidic (0.1 M HCl, pH 1) conditions at room temperature for 24 h. The degradation rate increased as a function of the acidity of the solution and temperature. The conditions tested included 0.01, 0.1, and 1 M HCl (pH 0–2), room temperature or 50 °C. After initial testing, the degradation/recycling experiments were performed in 0.1 M HCl at room temperature to obtain a moderate degradation rate. After 24 h, SBP3a–5a and SBP3b–5b had been recycled to back monomeric products including dialdehydes, which were denoted RP1 and RP2, and the original diamines with ≥90% yield (Figure S18). RP1 corresponds to the recycled dialdehyde product originating from a-series of polyesters containing succinate units (DMS 6a), while RP2 corresponds to the recycled dialdehyde product from b-series with terephthalate units (DMT 6b). In addition, depending on the polymer, EDA, BDA, or m-XylDA is released when the imine bonds are reversed, but these water-soluble products remained in aqueous phase and were not further characterized. The structure of the recycled dialdehyde products, RP1 and RP2, was confirmed by 1D and 2D NMR spectroscopy (Figures 5A–D and S19 to S22).

Figure 5

Figure 5. 1H (400.13 MHz, DMSO-d6) and 13C NMR (100.61 MHz, DMSO-d6) spectra of the recycled products RP1 (A,C) and RP2 (B,D) respectively.

In the 1H NMR spectrum of RP1, the aromatic aldehyde peak appeared at δ 9.84 ppm, aromatic −CH peaks appeared at δ 7.17–7.55 ppm, while the aromatic −OCH3 and aliphatic −OCH2 peaks were observed at δ 3.84 and 2.61–4.38 ppm, respectively. In the spectra of RP2, the aromatic aldehyde peak was observed at δ 9.84 ppm and aromatic −CH and terephthalic −CH peaks appeared at δ 7.26–7.57 and 8.07 ppm, respectively. The aromatic −OCH3 and aliphatic −OCH2 peaks were observed at δ 3.81 and 4.48–4.67 ppm, respectively. Furthermore, in the 13C NMR spectrum of RP1 and RP2, the aromatic aldehyde carbonyl peaks were observed at ∼δ 191 ppm and ester carbonyl peaks were observed at δ 172.37 and 165.39 ppm, and aromatic and aliphatic −CH peaks appeared at ∼δ 110.32–153.55 ppm and ∼δ 56.01–67.22 ppm, respectively. The chemical structure RP1 and RP2 was also analyzed by using FT-IR spectroscopy (Figure S23). The aliphatic and aromatic C–H stretching bands appeared at ∼2700–3000 cm–1, ester and aldehyde carbonyl (−C═O) band were observed at ∼1670 and 1730 cm–1, −C–O stretching was observed at ∼1260 cm–1, and the aliphatic and aromatic C–H bending bands appeared at ∼730 cm–1. After the recycling process, the imine peak at ∼1640 cm–1 completely disappeared and a new peak was observed at ∼1730 cm–1, which correspond to the aldehyde carbonyl stretching signal. The remaining peaks overlap with those of the corresponding polymers, SBP3a–b, SBP4a–b, and SBP5a–b, which further confirmed the structures of the recycled products RP1 and RP2.
Finally, we followed the recycling (degradation) process of SBP4a, SBP4b, and SBP5b by the real-time NMR spectroscopic technique. 50 mg of each polyester was placed in an NMR sample tube with 0.1 M HCl solution (methanol-d4/H2O = 8:2, v/v). The solution was monitored and analyzed by 1H NMR after different time intervals up to 24 h. The 1H NMR spectra are shown in Figures 6 and S24–S26. In the original polymer 1H NMR spectra, we observed the imine (−C═N) peaks at δ 8.54 ppm. After 30 min, a new peak was generated at δ 9.47 ppm corresponding to the aldehyde (−CHO) formed when the imine bond was reversed (opened). Over time, the intensity of the imine (−C═N) peak at δ 8.54 ppm continuously decreased, while the aldehyde (−CHO) peak became clearer at δ 9.47 ppm. The complete hydrolysis of the SB bonds demonstrates that the developed SB polyesters can be chemically and selectively recycled to simple monomeric products under mild conditions.

Figure 6

Figure 6. Real-time 1H NMR spectra of (A) original SBP4a and (B) SBP4a after 30 min in acidic solution. (C) SBP4a after 24 h in acidic solution illustrating the chemical recycling of polyester SBP4a in 0.1 M HCl solution (methanol-d4 and water = 8:2 ratio v/v) at room temperature during 24 h. Reference spectra for (D) BDA and (E) recycled dialdehyde product (RP1).

First Evaluation of Susceptibility to Enzymatic Degradation of SB Polyesters

Both the chemical recyclability and biodegradability of the materials generated herein would benefit from susceptibility to enzyme-catalyzed degradation, raising the question whether naturally occurring enzymes can hydrolyze the ester linkages or potentially even the imine linkages in these polyester-imines. SBP3a and SBP4a synthesized from aliphatic diamines and aliphatic dimethyl ester and SBP5b synthesized from aromatic diamine and aromatic dimethyl ester were subjected to PETase for 1 and 24 h at 30 °C to evaluate their susceptibility to enzymatic degradation. Control experiments were performed under the same conditions but without enzymes to differentiate between non-catalyzed and enzymatically catalyzed hydrolysis. The formed water-soluble degradation products were then monitored by performing LC–MS analysis on the aqueous phases. A comparison of the samples treated with and without enzymes shows that the amount of degradation products significantly increases in the samples containing enzymes compared to the control samples without enzymes especially after the longer exposure time of 24 h.
Figure 7A presents possible shorter degradation products (DP) that can be formed by opening of the imine bond and ester hydrolysis. The only structural difference between SBP3a and SBP4a is the length of the aliphatic diamine. As the aliphatic diamines have no UV absorbance, they were not detected in the chromatograms (even if they are formed). Thus, the expected degradation products observable in the HPLC chromatograms from these two polymers are the same, the dialdehyde (DP1 with the same chemical structure as RP1) and two vanillin derivatives formed by the opening of the ester bond in this dialdehyde (DP3 and DP4). Concerning the polymer SBP5b, the potential degradation products are m-xylylenediamine, dialdehyde (DP2 with same chemical structure than RP2), and its hydrolysis products (DP3 and DP5). Interestingly, DP3 can be formed from all the three polymers and gives a good possibility for comparative evaluation of the degradation rate.

Figure 7

Figure 7. (A) Chemical structures and molecular weights of the expected aldehydes formed from SBP3a, SBP4a, and SBP5b by opening of the imine bonds and hydrolysis of ester bonds. The degradation products DP1 and DP2 formed by opening of the imine bond have same chemical structure than the recycled products RP1 and RP2. Comparative analysis of (B) degradation product DP3 released from all three polymers and (C) degradation product DP4 released from SBP3a and SBP4a and DP5 released from SBP5b after 1 and 24 h with or without (control) enzymes.

Figure 7B presents the relative amount of DP3 formed from the three polymers after 1 and 24 h exposure to PETase compared to control experiments performed without the presence of enzymes. After 1 h exposure, a small amount of DP3 was formed without enzymes and a slightly higher amount in the sample containing enzymes. When the exposure time was increased to 24 h, the difference was clear: the amount of DP3 in the samples incubated with enzymes was up to 6 times higher, while the amount of DP3 in control samples remained very similar to the amount detected after 1 h. Comparison of the amounts formed from the three polymers indicates that the aliphatic ester bond in SBP3a and SBP4a was more easily cleaved compared to the aromatic ester bonds of SBP5b. From a chemical reactivity perspective, aliphatic ester bonds are more easily cleaved than aromatic ones; however, PETase is known to be able to hydrolyze aromatic PET (at least after pre-treatment) and some activity toward aromatic ester bonds was clearly shown even here. These results further illustrate the importance of an appropriate balance between aromatic and aliphatic units in the scissile chain for enhancing bioactivity. Furthermore, the longer aliphatic diamine also seems to have a small positive effect on the hydrolysis rate. It is possible that the longer aliphatic chain increases substrate flexibility, facilitating productive binding to the active site, while SBP5b contains a stiffer aromatic unit adjacent to the scissile bond. It should be noted that the molecular weight of the studied polyester-imines was relatively low and the molecular weight effect should be further investigated in future studies. However, as discussed above and shown in Figure 7B, the chemical structure also plays an important role in the enzymatic degradation process. In fact, the polyester-imine SBP4a with a relatively high Mn of 19,700 g mol–1 had the highest susceptibility to enzymatic degradation, while SBP5b with the highest aromatic content and lowest molecular weight (Mn = 6500 g mol–1) had the lowest susceptibility to enzymatic degradation.
Figure 7C compares the amount of DP4 formed from SBP3a and SBP4a and DP5 formed from SBP5b. These results further support a relatively similar degradation rate for SBP3a and SBP4a with a slightly higher rate for SBP4a. The relative peak areas of DP5 were higher compared to DP4, but this likely depends on the three aromatic groups in DP5 increasing the UV-absorbance and subsequent peak area and should not be taken as indication of higher concentration. This is supported by larger peak areas measured also for the control samples.
The dialdehydes, DP1 and especially DP2 with the aromatic ester group, have limited water solubility, which influences their detection. A small amount of DP1 was detected in the chromatograms of SBP3a and SBP4a, while DP2 was in most cases not detected. DP1 and DP2 are also susceptible to further hydrolysis to DP3, DP4, and DP5, which might contribute to the low amounts detected. In addition, two minor products were sometimes detected. These products are similar to products DP4 (SBP3a and SBP4a) and DP5 (SBP5b) and could be formed by oxidation of aldehyde group to carboxyl group.
To estimate the amount of degradation products formed, a calibration curve was prepared for DP3 (Figure S29), which was the only degradation product formed from all three polymers. Based on this calibration curve, approximately 0.03, 0.04, and 0.01 g of DP3 was formed per gram of polymer due to enzymatic hydrolysis of SBP3a, SBP4a, and SBP5b, respectively. If assumed that the same amount of mol of DP4 is formed in the most degraded materials, SBP4a, approximately 10% of original material was hydrolyzed to DP3 and DP4 during only 24 h (n(DP3) = weight(DP3)/molecular weight(DP3) = 0.04 g/196 g mol–1 = 0.0002 mol; weight(DP4) = 0.0002 mol × 296 g mol–1 = 0.06 g; total weight(DP3+DP4) = 0.1 g). This gives promising indication of enzymatic degradability apt for further engineering, which could be additionally tuned by the chemical structure and utilized for production of enzymatically recyclable or possibly biodegradable thermoplastic materials.
To support the degradation pathway, as shown in Figure 7A, the intermediate degradation products DP1 and DP2 were docked into the active site of PETase to identify the binding pose. Both degradation products show productive binding with the ester (−C═O) as scissile bond, as shown in (Figure 8). The aromatic rings in enzyme and substrate show either parallel or T-shaped π-stacking interactions, stabilizing the intermediate in a productive conformation ready for catalysis by nucleophilic attack of the catalytic Ser (S160).

Figure 8

Figure 8. Binding mode of the degradation products, DP1 (panel A; green) and DP4 (panel B; blue), are shown. Key residues are represented in cyan color lines.

We were then interested to investigate whether SBP3a, SBP4a, and SBP5b could also bind. To simulate a longer polymer chain representing SBP3a, SBP4a, and SBP5b, a pool of possible binding orientations were obtained after induced-fit docking calculations (Table S1). The docking results clearly indicate that SBP3a, SBP4a, and SBP5b could bind into the active site of PETase and docking poses could be classified into three main categories: pose A, pose B (with Asp 206–His 237–Ser 160 interactions preserved), and others (unproductive), as shown in Table S1.
In SBP3a, a total of 196 binding poses were predicted. However, 48% of these binding poses were either unproductive or did not have good interactions that can result in cleavage of the polymer. About 70 poses in SBP3a show the imine bond in closest proximity to the catalytic Ser, and the remaining 31 poses show the ether bond in the active site, as shown in Figure 9 panels A and D, respectively. For the latter, it is clear that minor structural repositioning is needed to reach the pose shown in Figure 8 with the carbonyl juxtaposed to S160, which can explain the low but detectable degradation rate. In SBP4a and SBP5b, the unproductive docking poses amounted to more than 50%. Interestingly, SBP4a only shows a higher number of docking poses with the ether unit and thus the reactive carbonyl in close proximity of the catalytic triad (as shown in Figure 9 panel E). This result is aligned with the slightly higher formation of DP3 from SBP4a. The docking shows favorable interactions between the polymer and enzyme. In all of the substrates, Tyr 87, Trp 159, and/or Trp 185 forms π–π stacking interactions with the aromatic rings of the substrates, as shown in Figure 9.

Figure 9

Figure 9. Binding mode of polymers SBP3a (panel A,D; magenta), SBP4a (panel B,E; yellow), and SBP5b (panel C,F; orange) obtained in the induced-fit docking calculations of PETase (PDB ID: 6EQE) in poses A and B, respectively. Key residues are represented in cyan color lines.

Conclusions

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The synthesis of linear polyester-imines from potentially bio-sourced SB monomers was successfully demonstrated. The monomers and polymers were synthesized by simple nucleophilic substitution, followed by SB formation and then conventional two-step bulk polycondensation protocol (transesterification and polycondensation) with dibutyltinoxide (DBTO) as a metal catalyst. All the monomers and polymers were thermally stable and had adequate glass transition temperatures. The polyesters were easily chemically recycled under aqueous acidic (0.1 M, HCl) conditions at room temperature. Finally, the synthesized polyester-imines illustrated susceptibility to enzymatic degradation by PETase, resulting in release of monomeric degradation products. These results contribute to design of chemically recyclable and enzymatically degradable polymers from the sustainable resources.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acssuschemeng.2c06935.

  • 1D and 2D NMR and FT-IR spectra of monomers [SBM1 (3), SBM2 (4), and SBM3 (5)] and their corresponding polymers (SBP3a–b and SBP5a–b), the recycled products (RP1 and RP2), real-time 1H NMR spectra of SBP4a, SBP4b and SBP5b, SDS-PAGE of purified PETase after expression in E. coli, LC–MS calibration curve of DP3, and a table presenting the total number of binding poses obtained in each category (PDF)

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Author Information

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  • Corresponding Author
    • Minna Hakkarainen - Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, SwedenWallenberg Wood Science Center (WWSC), KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, SwedenOrcidhttps://orcid.org/0000-0002-7790-8987 Email: [email protected]
  • Authors
    • Sathiyaraj Subramaniyan - Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, SwedenWallenberg Wood Science Center (WWSC), KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden
    • Nasim Najjarzadeh - Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, SwedenWallenberg Wood Science Center (WWSC), KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, SwedenBiotechnology and Health, Science for Life Laboratory, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden
    • Sudarsana Reddy Vanga - Biotechnology and Health, Science for Life Laboratory, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden
    • Anna Liguori - Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden
    • Per-Olof Syrén - Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, SwedenWallenberg Wood Science Center (WWSC), KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, SwedenBiotechnology and Health, Science for Life Laboratory, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, SwedenOrcidhttps://orcid.org/0000-0002-4066-2776
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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Authors are grateful for the financial support from the Wallenberg Wood Science Center (WWSC) financed by the Knut and Alice Wallenberg Foundation. Authors also acknowledge the financial support from the Kamprad Family Foundation (grant no. 20200076) and the Novo Nordisk Foundation (grant no. NNF20OC0064972). We thank Luyao Zhao for helping with the SDS-page gel.

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Cite this: ACS Sustainable Chem. Eng. 2023, 11, 8, 3451–3465
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  • Abstract

    Figure 1

    Figure 1. 1H NMR (400.13 MHz, DMSO-d6) spectra of (A) monomer SBM2 (4) and (B,C) its corresponding polymers SBP4a–b, received after reaction with DMS and DMT, respectively.

    Figure 2

    Figure 2. 13C NMR (100.61 MHz, DMSO-d6) spectra of (A) monomer SBM2 (4) and (B,C) its corresponding polymers SBP4a–b formed after reaction with DMS and DMT, respectively.

    Scheme 1

    Scheme 1. General Reaction Scheme for the Synthesis of Schiff Base Polymersa

    aFirst, the synthesis of SB diol monomers SBM1–SBM3 (3–5) starting by reaction between modified vanillin [4-(2-hydroxyethoxy)-2-methoxybenzaldehyde] and EDA (2a), BDA (2b) or m-xylylenediamine (XylDA, 2c), respectively, followed by polymerizations of the diol monomers with aliphatic or aromatic diester, DMS (6a) and DMT (6b), respectively, yielding polyesters (SBP3a–b and SBP5a–b) with different aliphatic/aromatic contents.

    Figure 3

    Figure 3. TGA curves showing the weight loss (A,C,E) and first derivative weight loss curves (B,D,F) of monomers SBM1 (3), SBM2 (4), and SBM3 (5) and polyesters SBP3a–b, SBP4a–b, and SBP5a–b.

    Figure 4

    Figure 4. DSC thermograms from the second heating of polyesters SBP3a–b, SBP4a–b, and SBP5a–b.

    Figure 5

    Figure 5. 1H (400.13 MHz, DMSO-d6) and 13C NMR (100.61 MHz, DMSO-d6) spectra of the recycled products RP1 (A,C) and RP2 (B,D) respectively.

    Figure 6

    Figure 6. Real-time 1H NMR spectra of (A) original SBP4a and (B) SBP4a after 30 min in acidic solution. (C) SBP4a after 24 h in acidic solution illustrating the chemical recycling of polyester SBP4a in 0.1 M HCl solution (methanol-d4 and water = 8:2 ratio v/v) at room temperature during 24 h. Reference spectra for (D) BDA and (E) recycled dialdehyde product (RP1).

    Figure 7

    Figure 7. (A) Chemical structures and molecular weights of the expected aldehydes formed from SBP3a, SBP4a, and SBP5b by opening of the imine bonds and hydrolysis of ester bonds. The degradation products DP1 and DP2 formed by opening of the imine bond have same chemical structure than the recycled products RP1 and RP2. Comparative analysis of (B) degradation product DP3 released from all three polymers and (C) degradation product DP4 released from SBP3a and SBP4a and DP5 released from SBP5b after 1 and 24 h with or without (control) enzymes.

    Figure 8

    Figure 8. Binding mode of the degradation products, DP1 (panel A; green) and DP4 (panel B; blue), are shown. Key residues are represented in cyan color lines.

    Figure 9

    Figure 9. Binding mode of polymers SBP3a (panel A,D; magenta), SBP4a (panel B,E; yellow), and SBP5b (panel C,F; orange) obtained in the induced-fit docking calculations of PETase (PDB ID: 6EQE) in poses A and B, respectively. Key residues are represented in cyan color lines.

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  • Supporting Information

    Supporting Information


    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acssuschemeng.2c06935.

    • 1D and 2D NMR and FT-IR spectra of monomers [SBM1 (3), SBM2 (4), and SBM3 (5)] and their corresponding polymers (SBP3a–b and SBP5a–b), the recycled products (RP1 and RP2), real-time 1H NMR spectra of SBP4a, SBP4b and SBP5b, SDS-PAGE of purified PETase after expression in E. coli, LC–MS calibration curve of DP3, and a table presenting the total number of binding poses obtained in each category (PDF)


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