Renewable and recyclable covalent adaptable networks based on bio-derived lipoic acid

The modern materials economy is inefficient since most products are principally derived from non-renewable feedstocks and largely single-use in nature. Conventional thermoset materials are often inherently unreprocessable due to their irreversible covalent crosslinks and hence are challenging to recycle and/or reprocess. Covalent adaptable networks (CAN)s, which incorporate reversible or dynamic covalent bonding, have emerged as an efficient means to afford reprocessable crosslinked materials and increasing the feedstock sustainability of CANs is a developing aim. In this study, the biomass-derived lipoic acid, which possesses a dynamic cyclic disulfide moiety, was transformed into a series of bifunctional monomers via a one-step esterification or amidation reaction and reacted with a commercially available multi-valent thiol in the presence of an organobase catalyst to afford dynamically crosslinked networks. Large differences in material properties, such as storage modulus and glass transition temperature, were observed when the ratio of the lipoic acid-based monomer to thiol (from 1 : 1 to 16 : 1) and the composition of the monomer were changed to modify the network architecture. The thermomechanical properties of an optimised formulation were investigated more thoroughly to reveal a moderately strong rubber (ultimate tensile strength = 1.8 ± 0.4 MPa) possessing a large rubbery plateau (from 0 to 150 °C) which provides an adaptable material with a wide operational temperature range. Finally, the chemical recycling, or depolymerisation, of the optimised network was also demonstrated by simply solvating the material in the presence of an organobase catalyst.


Mass Spectrometry. High Resolution Electrospray Ionization Mass Spectrometry was performed in the
School of Chemistry at University of Birmingham on a Waters Xevo G2-XS QTof Quadrupole Time-of-Flight mass spectrometer.
Differential Scanning Calorimetry (DSC). The thermal characteristics of the polymers were determined using differential scanning calorimetry (STARe system DSC3, Mettler Toledo) from −80 to 180 °C at a heating rate of 10 °C min −1 for two heating/cooling cycles unless otherwise specified. The glass transition temperature (Tg) was determined from the inflection point in the second heating cycle of DSC.
Thermogravimetric Analysis (TGA). TGA thermograms were obtained using a TGA/DSC 1 -Thermogravimetric Analyzer (Mettler Toledo). Thermograms were recorded under an N2 atmosphere at a heating rate of 10 °C min −1 , from 10 − 600 °C, with an average sample weight of ca. 10 mg. Aluminium pans were used for all samples. Decomposition temperatures were reported as the 5% weight loss temperature (Td 5%).

Fourier-transform infrared (FTIR) spectroscopy. FTIR spectra were collected out using an Agilent
Technologies Cary 630 FTIR spectrometer. 16 Scans from 600 to 4000 cm -1 were taken at a resolution of 4 cm −1 , and the spectra were corrected for background transmittance.
Rheology. Rheological measurements were performed on an Anton Paar MCR 302 using Anton Paar PP8 parallel-plate, a diameter of 8 mm. Temperature was controlled with a P-PTD 200/AIR Peltier and a P-PTD 200 hood. Gelation time was monitored at 10% strain, frequency of 1 Hz and 0 N of normal force. Frequency sweeps were performed at 1% strain from 0.01 to 10 Hz (0.06 to 62 rad/s). Amplitude sweeps were performed at 1 Hz from 0.01 to 500%. Stress relaxation tests were performed at 2% strain at 25 °C, 50 °C, and 100 °C. Sample thickness was approximately 1 mm.
Dynamic Mechanical Analysis (DMA). Dynamic mechanical thermal analysis (DMTA) data were obtained using a Mettler Toledo DMA 1 star system and analyzed using the software package STARe V13.00a (build 6917). Thermal sweeps were conducted using films (L x W x thickness = 15.08 mm x 6.16 mm x 0.30 mm) cooled to −80°C and held isothermally for ca. 5 minutes. Storage and loss moduli, as well as the loss factor (ratio of E" and E', tan δ) were probed as the temperature was swept from −80 to 180 °C, 5 °C min −1 , 1 Hz.
Thermomechanical behavior was determined from three samples in this way.
Uniaxial Tensile Testing. Dumbbell-shaped samples were cut directly from the synthesized films using a custom ASTM Die D-638 Type V. Tensile tests at different stretching speed were carried out using a Testometric M350-5CT universal mechanical testing instrument fitted with a load cell of 5 kN at room temperature (22 ± 1 °C). The gauge length was set as 7.1 mm and the crosshead speed was set 10 mm min -1 . The dimensions of the neck of the specimens were 7.1 mm in length, 1.6 mm in width and 0.2 mm in thickness. The reported results are average values from at least three individual measurements (n ≥ 3).

S3
Chemical recycling studies. The network film was manually cut into small pieces, placed into 20 mL scintillation vial equipped with a stirrer bar and then diluted with DCM containing DBU. For reactions using 0.01 M DBU solution, 200 mg of sample was diluted with 5 mL of solution. For reactions using 0.05 M DBU solution, 150 mg of sample was diluted with 3.75 mL of solution. The reaction mixture was then stirred at ambient temperature (22 ± 1 °C) and the degradation of the network was monitored using video recording until the solution was homogeneous.

C6A Synthesis
Lipoic acid (10 g, 2 equiv., 48.4 mmol), 1,6hexanediamine (2.81 g, 1 equiv., 24.2 mmol), DMAP (5.91 g, 1 equiv., 48.4 mmol) were placed in an oven-dried 250 mL 2-neck round-bottom flask and back-filled with N2. DCM (100 mL) was added to the flask and the mixture was stirred for ca. 10 min. Note: the DCM did not have to be rigorously dried and reagent grade solvent was adequate for the reaction. The reaction mixture was then cooled to 0 °C in an ice-water bath and EDC‧HCl (9.28 g, 1 equiv., 48.4 mmol) was added portion-wise over 5 min. During the addition, white precipitate formed and after the addition was complete, the reaction was stirred at 0 °C, slowly warming in the bath overnight (c.a. 16 h). The reaction mixture was transferred to a 1 L separatory funnel and diluted with ~ 500 mL CHCl3 (since solubility in DCM is poorer) to adequately dissolve the amide product. The organic layer was washed with 1 M HCl (3 x 200

S5
were placed in an oven-dried 250 mL 2-neck round-bottom flask and back-filled with N2. DCM (100 mL) was added to the flask and the mixture was stirred for ca. 10 min. Note: the DCM did not have to be rigorously dried and reagent grade solvent was adequate for the reaction. The reaction mixture was then cooled to 0 °C in an ice-water bath and EDC‧HCl (9.28 g, 1 equiv., 48.4 mmol) was added portion-wise over 5 min. During the addition, white precipitate formed and after the addition was complete, the reaction was stirred at 0 °C, slowly warming in the bath overnight (c.a. 16 h). The reaction mixture was transferred to a 1 L separatory funnel and diluted with ~ 500 mL CHCl3 (since solubility in DCM is poorer) to adequately dissolve the amide product. The organic layer was washed with 1 M HCl (3 x 200 94, 77.48, 77.16, 76.84, 70.33, 70.03, 56.54, 40.35, 39.24, 38.57, 36.48, 34.73, 28.99, 25.46 C22H40N2O4S4 H 525.1949; found 525.1960.
The film was left overnight (c.a. 16 h) to ensure solvent removal and then peeled off the substrate for analysis. In order to obtain thicker films (c.a. 1 mm thickness), the reaction was proportionally scaled up 3fold (i.e. 3.0 mL of disulfide stock solution was used), mixed and left in the 20 mL scintillation vial. The vial was covered with an evaporating dish to ensure slower evaporation of the solvent (for more homogeneous film formation) and left overnight (c.a. 16 h) to dry.
Representative synthesis of C6E-3T 8:1 using DBU Trimethylolpropane tris(3-mercaptopropionate) (0.033 g, 1.00 molar equiv., 0.083 mmol) was weighed into a 20 mL scintillation vial. A 1.0 mL stock solution of disulfide monomer (1.0 mL, 12.00 molar equiv., 1.00 mmol) was added to the vial and the mixture was lightly mixed. A 100 mg·mL -1 stock solution of DBU (1.26 µL, 0.01 molar equiv., 0.00083 mmol) was added in one portion, the mixture was vigorously shaken for 5-10 s and then poured onto a glass slide (L × W = 75 mm × 25 mm). The film was left overnight (ca. 16 h) and then peeled off the substrate for analysis.