Imidazolium-Based Sulfonating Agent to Control the Degree of Sulfonation of Aromatic Polymers and Enable Plastics-to-Electronics Upgrading

The accumulation of plastic waste in the environment is a growing environmental, economic, and societal challenge. Plastic upgrading, the conversion of low-value polymers to high-value materials, could address this challenge. Among upgrading strategies, the sulfonation of aromatic polymers is a powerful approach to access high-value materials for a range of applications, such as ion-exchange resins and membranes, electronic materials, and pharmaceuticals. While many sulfonation methods have been reported, achieving high degrees of sulfonation while minimizing side reactions that lead to defects in the polymer chains remains challenging. Additionally, sulfonating agents are most often used in large excess, which prevents precise control over the degree of sulfonation of aromatic polymers and their functionality. Herein, we address these challenges using 1,3-disulfonic acid imidazolium chloride ([Dsim]Cl), a sulfonic acid-based ionic liquid, to sulfonate aromatic polymers and upgrade plastic waste to electronic materials. We show that stoichiometric [Dsim]Cl can effectively sulfonate model polystyrene up to 92% in high yields, with minimal defects and high regioselectivity for the para position. Owing to its high reactivity, the use of substoichiometric [Dsim]Cl uniquely allows for precise control over the degree of sulfonation of polystyrene. This approach is also applicable to a wide range of aromatic polymers, including waste plastic. To prove the utility of our approach, samples of poly(styrene sulfonate) (PSS), obtained from either partially sulfonated polystyrene or expanded polystyrene waste, are used as scaffolds for poly(3,4-ethylenedioxythiophene) (PEDOT) to form the ubiquitous conductive material PEDOT:PSS. PEDOT:PSS from plastic waste is subsequently integrated into organic electrochemical transistors (OECTs) or as a hole transport layer (HTL) in a hybrid solar cell and shows the same performance as commercial PEDOT:PSS. This imidazolium-mediated approach to precisely sulfonating aromatic polymers provides a pathway toward upgrading postconsumer plastic waste to high-value electronic materials.


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by a gold counter electrode and a silver pseudoreference electrode (-0.133V vs. Ag/AgCl).The connection tracks are made of silver.Interdigitated electrodes (IDE) (catalog NO.G-IDEAU5) were also purchased from Metrohm, with two 5-µm-wide interdigitated gold electrodes with two gold connection tracks.
Molecular weights and dispersity of PS were determined in reference to PS standards (purchased from Polymer Laboratories) using SEC with tetrahydrofuran (1.0 mL min −1 ) as the eluent, using a HLC-8320 GPC EcoSEC equipped with TSKgel GMHhr-N column (5μm, 7.8 × 300 mm) in series with a RI-8320 refractive index (RI) detector.

Nuclear magnetic resonance (NMR)
1 H NMR spectra were collected on a Bruker 400 MHz spectrometer.The polymers were dissolved in D2O or DMSO-d5.The regioselectivity experiments were done by collecting solution phase 13 C NMR spectra using a Bruker UltraShield 500 MHz spectrometer ( 13 C NMR = 125 MHz).Chemical shifts for 13 C spectra were referenced using internal solvent resonances and are reported relative to tetramethylsilane (TMS).Quantitative 13 C NMR spectra were acquired with inverse gated decoupling; the zgig30 pulse sequence was employed with a delay of 8 s and an average of at least 15000 scans was used.Spectra were then processed using Mnova by Mestrelab Research.Phase correction followed by baseline correction (Bernstein polynomial) was performed prior to integration of the peaks.

+ HCl
The sulfonating agent, [Dsim]Cl, was synthesized under strictly anhydrous conditions and characterized by adapting a previously reported procedure. 1,2To a round-bottomed flask (100 mL) containing imidazole (1.02 g, 15 mmol) in dry dichloromethane (DCM) (100 mL), was added chlorosulfonic acid (2.1 mL, 3.6 g, 30.8 mmol, d = 1.75 g/mL) dropwise over a period of 20 min at room temperature.After the addition was completed, the reaction mixture was stirred for 12 h.The progress of the reaction was monitored by Fourier-transform infrared spectroscopy (FTIR).
The residual oil was washed with anhydrous dichloromethane (3×50 mL) and dried under vacuum for overnight to give [Dsim]Cl as a viscous pale-yellow oil in 95% yield (3.55 g).The oil was characterized by Fourier transform infrared (FTIR) spectroscopy (Figure S1a) and proton nuclear magnetic resonance spectroscopy ( 1 H NMR, Figure S1b).The FTIR spectrum showed a characteristic broad peak at 3192 cm −1 indicating the presence of two OH groups of a SO3H moiety in the ionic liquid.The bands at 1626 and 1588 cm −1 were assigned to C=C and C=N stretching vibrations, respectively, whereas the stretching band of the SO3H group appeared at 1219 cm −1 .
The other peaks at 1179, 1050, and 936 cm −1 were assigned to S−O symmetric stretching, S−O antisymmetric stretching, and N−S stretching vibrations, respectively, whereas the band centered at 576 cm −1 was attributed to the bending vibration of the SO3H group.The 1 H NMR spectra (Figure S1b) is consistent with [Dsim]Cl, however it also showed 11% of the mono-sulfonated and protonated imidazolium, [Sim]Cl, as a side product.Our best efforts to limit this side product (e.g., by quenching excess HCl), were unsuccessful.We therefore proceeded with the sulfonation of aromatic polymers by accounting for this impurity, present in ~10% in each batch of Polystyrene (PS) (0.5 g, 4.8 mmol of styrene repeat units) was dissolved in 150 mL of dichloromethane (DCM).The sulfonating agent, [Dsim]Cl (90% purity, 1.4 g, n([Dsim]Cl) = 5.3 mmol), was added dropwise to the mixture at room temperature in a glovebox.The reaction was left to react for 4 h at 70 °C, during which time the sulfonated polymer precipitated.The reaction was stopped by adding 50 mL of DI water to dissolve the precipitate.The sulfonated polymer was extracted from the DCM layer with DI water (3×10 mL) using a separatory funnel.The combined water-soluble PSS fractions were purified by dialyzing for 2 days against DI water using a tubing with a molecular weight cut-off (MWCO) of 3500, then stirred over 100 mL of an acidic resin (Dowex Marathon C hydrogen form) for 60 min to remove excess imidazole.The polymers were then dried under vacuum.

General procedure for the sulfonation of PS by Vink's approach.
Polystyrene (PS) (1.0 g) was dissolved in 100 mL of cyclohexane, and incrementally added to a mixture containing 6 g of P2O5 and 28 mL of H2SO4 maintained at a temperature of 50 °C.This mixture was continuously stirred for four hours and then left undisturbed for an hour.The mixture was then cooled in an ice bath, and 17 g of ice-cold water were added.This procedure resulted in the formation of a yellowish-white, sticky substance.The resultant product was repeatedly rinsed with cooled distilled water, then dissolved in 300 mL of water, and finally was purified by dialyzing for 2 days against DI water using a tubing with a MWCO of 3500.The polymer was then dried under vacuum.

General procedure for the determination of the degree of sulfonation by titration.
We first used UV-Vis spectroscopy to accurately measure the concentration of the PSS solution for the titration.A calibration was used to determine the mass extinction coefficient of the materials (1.85 mL/mg at 262 nm) using the commercial PSS samples purchased from sigma. 3,4Once the concentration was precisely determined, sulfonated polystyrene (0.1 to 0.  a Determined by quantitative 13 C NMR in D2O (Figure S2).

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Determination of the regioselectivity by quantitative 13 C NMR.

Recovery and regeneration of [Dsim]Cl.
To recover imidazole and regenerate [Dsim]Cl after the sulfonation of PS (0.5 g) with the [Dsim]Cl (90% purity, 1.45 g, n([Dsim]Cl) = 5.48 mmol), PSS was extracted from the reaction mixture in water and purified by dialysis (MWCO 3500) as described in the general procedure above.During the dialysis, the water-soluble small molecule by-product, identified as a protonated imidazolium salt (Figure S3), leached out of the dialysis tube.We note that the chemical shifts of the imidazolium protons shift with water content in the DMSO solvent as seen in the 1 H NMR below.This imidazolium was concentrated in vacuo then converted to imidazole by the addition of 50 mL 0.1M NaOH, and subsequently extracted in ethyl acetate (3 times with 50 mL).After evaporation of the solvent, pure imidazole (0.26 g, 3.8 mmol) (Figure S4) was obtained which can be re-used for the synthesis of [Dsim]Cl (90% yield but contained 17 mol% [Sim]Cl, Figure S5).
Overall, we were able to recover and regenerate 66% of the [Dsim]Cl sulfonating agent.

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Procedure for the sulfonation of SEBS at room temperature.
Styrene-ethylene-butylene-styrene (SEBS) (1.58 g, 20% of styrene, total includes 0.5 g of styrene ) was dissolved in 150 mL of DCM.The sulfonating agent, [Dsim]Cl (69%, purity, 1.45 g, n([Dsim]Cl) = 5.5 mmol, Figure S8), was added dropwise to the mixture at room temperature and left to react for 48 h at room temperature.The reaction was stopped by adding 50 mL of DI water and was filtered via a 10 µm PTE filter to collect the solids.The purification process and characterization were identical to the ones above for insoluble aromatic polymers.The XPS of s-SEBS showed a DS of 98% (Figure S9).Procedure for the sulfonation of EPS at room temperature.
Expanded polystyrene (EPS) (0.5 g, 4.8 mmol of styrene repeat units) was dissolved in 150 mL of DCM.The sulfonating agent, [Dsim]Cl (69% purity, 1.45 g, n([Dsim]Cl) = 5.5 mmol, Figure S8), was added dropwise to the mixture at room temperature.The reaction was left to react for 48 h at room temperature, during which time, the sulfonated polymer precipitated.Then, 50 mL of DI water was added to dissolve the precipitate.The sulfonated polymer was then separated from the dichloromethane (DCM) layer by extracting it into DI water (three times with 10 mL each) using a separatory funnel.The combined aqueous phases containing the PSS were then purified through dialysis for two days against DI water, employing tubing with a MWCO of 3500.Subsequently, the solution was mixed with 100 mL of an acidic resin (Dowex Marathon C in hydrogen form) for 60 minutes.The polymer was dried under a vacuum to afford a DS of 92%.Electronic performance of PEDOT:PSS from upgraded EPS.
To determine the volumetric capacitance ( * ), we performed cyclic voltammetry (CV) on the PEDOT:PSS samples at various film thicknesses (Figure S13).The OECTs were fabricated similarly to our previously reported work on screen printed electrodes.

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Table S2.Dimensions and figures of merit for the OECT devices.Hybrid photovoltaic devices fabrication and characterization. 13type silicon (100)-textured substrates (doped with phosphorous, CZ) with back surface field (BSF) were fabricated at Arizona State University (Solar Power Lab, ASU, Tempe, AZ, USA).

Sample Name
These wafers were 145 µm thick with 1-5 Ω-cm resistivity.Random texturization was performed on both sides using potassium hydroxide (2% KOH yielding pyramid sizes of about 3-5 µm base size) alkaline etching.To create BSF n + layer on the n-type wafers, phosphorous oxychloride diffusion was performed at 820 °C, 15 min with a POCl3 carrier gas flow rate of 1500 sccm (standard cubic centimeters per minute) for phosphosilicate glass (PSG) growth and dopant drivein.Finally, a 10-min buffered oxide etch was used to remove the PSG.The sheet resistance value of the BSF side was 55 Ω/square.
In these studies, we used 7 wt% ethylene glycol (EG) as the co-solvent and 0.25 wt% Capstone FS-30 as the surfactant added to PEDOT:PSS.All wafers were cleaned using a Piranha etch (H2SO4:H2O2 = 4:1) for five minutes, followed by a 5-min DI water rise and a two-minute immersion in hydrofluoric acid (HF, 2 wt%).After cleaning, the substrates were blow-dried with nitrogen.The PEDOT:PSS dispersions with EG and Capstone FS-30 were then spin-coated on the front of the wafers (2250 rpm for 300 s) on a Headway Research spin coater, then immediately baked on a hot plate at 135 °C for 15 min.For complete hybrid solar cell devices (Figure S14), metal contacts were deposited using electron beam physical vapor deposition on a dual electronbeam evaporator (Wilmington, MA, USA).Aluminum (2µm) was used as the back contact and deposited on the BSF-treated side of the wafer, while silver (500nm) was used as the front contact and was deposited with the help of a finger patterned shadow mask directly on the PEDOT:PSS films.

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The devices were tested using illuminated current density-voltage measurements (J-V) (Figure S15).JV response was measured by means of a DC source meter (Keithley 2400 sourcemeter, USA) both in the dark and light, under air mass 1.5G standard illumination.
2 g) and 3 drops of phenolphthalein solution as a pH indicator were added to a vial (20 mL) with DI water (5 mL).A 0.1 M solution of NaOH(aq) was then slowly added at room temperature.The color change of phenolphthalein indicated the end-point of the titration.The degree of sulfonation is defined as the mole percentage of sulfonated styrene units, which can be expressed by the following equation: 4 S6    = � *   − 81 *  *  104 � * 100.whereN (mol/L) and V (L) are the concentration and the titration volume of sodium hydroxide solution, respectively.W (g) is the weight of sulfonated polystyrene sample, 104 and 81 are the molar mass of the styrene unit and -SO3H groups respectively.

Figure S3. 1 HFigure S4. 1 H
Figure S3.1 H NMR of the protonated imidazolium by-product obtained after the reaction and recovered from outside the dialysis tube, containing high (top) or low (bottom) levels of water in DMSO-d 6 .

Figure S8. 1 H
Figure S8. 1 H NMR spectrum in DMSO-d 6 of the sulfonating agent used in the room temperature sulfonation of SEBS and EPS.

Figure S10 .Figure S11 .
Figure S10.SEC selected PSS samples obtained by sulfonation of EPS with [Dsim]Cl at 70 °C for 4 h or room temperature for 48 h.

Figure S15 .
Figure S15.J-V performance of hybrid PV devices with a PEDOT:PSS HTL.