Microchemical Journal 182 (2022) 107878 Contents lists available at ScienceDirect Microchemical Journal journal homepage: www.elsevier.com/locate/microc Electroinduced crosslinking of triphenylamine-based polybenzoxazines Carolina Gascó, Luis Rodríguez-Santiago, Mariona Sodupe, Rosa María Sebastián *, Gonzalo Guirado * Departament de Química, Universitat Autònoma de Barcelona, 08193- Bellaterra, Barcelona, Spain A R T I C L E I N F O A B S T R A C T Keywords: Polybenzoxazines attract much attention as good phenolic resins due to their interesting and useful properties. Polybenzoxazines Recently, benzoxazine monomers have been employed as crosslinking agents to enhance the properties of Electrochemistry different types of polymers by being used as additives to a secondary cure process. In this paper, we propose a Electroinduced crosslinking new electroinduced approach to increase the crosslinking density of electroactive polybenzoxazines through the design of new monomers that contain triphenylamine groups as electrodimerization points. The use of electro­ chemical techniques means the crosslinking process can be monitored and also the electrochemical reaction process upon polybenzoxazine oxidation either in solution or ionogel matrixes can be disclosed. These studies lay the foundations for the design of smart polybenzoxazines that could undergo dual-cure mechanisms at low electrochemical potential values. 1. Introduction form covalent (reversible or irreversible) bonds between themselves when an electric stimulus is applied, resulting in a crosslinked matrix Crosslinked polymers have been widely investigated due to their [10]. Triphenylamine (1, TPA) is a well-defined electroactive unit with interesting thermal and mechanical properties [1]. However, the highly the ability to generate electro-coupling when a low oxidative potential is crosslinked matrix that is generated limits post-processing actions, such applied. This molecule has been linked to, for instance, polystyrene and as annealing, melting [2] and, consequently, potential molding [3], thus polythiophene, arising in electro-induced crosslinking [11] (Scheme S1, hindering some applications. To overcome this problem, dual-cure supporting information) [12]. polymerization has been developed over the years, which consists of Polybenzoxazines, a new kind of phenolic resins, have recently been starting from a well-designed monomer, and combining two polymeri­ postulated as useful and relevant polymers due to their notable prop­ zation reactions that happen sequentially. Epoxy-acrylate oligomers erties, namely excellent static and dynamic mechanical properties, low cured in two steps (photo and thermal activation) have been described water absorption and flammability, near-zero shrinkage or volumetric to improve its processability and adhesion properties [4]. Another expansion upon curing, high UV and chemical resistance, and consid­ example based on thiol-acrylate networks involves performing an initial erable molecule-design flexibility [13]. Benzoxazines are classically pre-polymerization reaction through Michael addition chemistry fol­ obtained from a Mannich type reaction from phenols, formaldehyde, lowed by a secondary photoinduced free radical polymerization of the and amines. The possibility of selecting the appropriate groups linked to acrylate moieties. This substrate is used in impression materials and phenols and amines leads to an extraordinary benzoxazine and poly­ shape memory polymers [5]. benzoxazine design versatility, generating thermosets that present In dual-curing methodologies, in a first step, the main polymer chain interesting properties (Scheme S2.1.) [14]. is built, and the second polymerization event can be activated by photo Benzoxazines have also been used in the dual-curing polymerization [6], free-radical [7], thermally [8] or electrochemically [9] induced process. Scheme S2.2. is a simple illustration of the three different polymerization. Despite being a clean stimulus, having a localized ac­ strategies found in the literature. The approach described in Scheme tion and being an on-demand technique, the electrochemically induced S2.2.a consists of a first polymerization process (PP) of a functionalized process is the least used. In the example described in the literature, the monomer obtaining a well-known polymeric main chain as poly­ main chain was functionalized with an electroactive molecule able to butadiene [15], polystyrene [16] and polyvinylchloride [17]. This * Corresponding authors. E-mail addresses: rosamaria.sebastian@uab.cat (R.M. Sebastián), gonzalo.guirado@uab.cat (G. Guirado). https://doi.org/10.1016/j.microc.2022.107878 Received 13 May 2022; Received in revised form 10 August 2022; Accepted 11 August 2022 Available online 19 August 2022 0026-265X/© 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by- nc-nd/4.0/). C. Gascó et al. Microchemical Journal 182 (2022) 107878 process is followed by a benzoxazine functionalization of the principal ice bath for 10 min, after which a solution of 4-(diphenylamino)phenol, polymer that can crosslink the network after a thermal ring opening 2, (0.60 g, 2.30 mmol) dissolved in 5 mL of dioxane was added dropwise polymerization (ROP). In another strategy shown in Scheme S2.2.b, the and the mixture was refluxed (the reaction was followed by 1H NMR). benzoxazine is functionalized beforehand with radical or photo-induced Then, after 24 h, the reaction crude was cooled and 30 mL of diethyl polymerizable vinyl groups [18] or methacryloyl groups [19] forming, ether was added, and the resulting solution was washed three times with again, the main chain with benzoxazine moieties that polymerize in a 30 mL of H2O. The resulting organic phase was dried with anhydrous secondary step. In the last one, Scheme S2.2.c, the process starts, as Na2SO4 and the solvent evaporated under reduced pressure. The ben­ before, with a functionalized benzoxazine, but in this case, the first zoxazine was purified by flash silica gel column chromatography (7:3 polymerization step is the ROP of benzoxazine, and the second evolves hexane: ethyl acetate v/v) obtaining the desired benzoxazine as a yellow the coupling between the reactive groups, which are linked using, for viscous liquid 0.41 g, 56 % yield). 1H NMR (CDCl3, 360 MHz) δ (ppm): instance, photo-induced coumarin-coumarin dimerization [20]. The 7.25–7.21 (apparent t, J = 7.2 Hz, 4H), 7.06–7.04 (apparent d, J = 7.1 extension of the applicability and the better mechanical properties, Hz, 4H), 6.98–6.92 (apparent t, J = 7.0 Hz, apparent s, 3H), 6.78–6.74 durability and stability of the final polymer justified the search for (apparent m, 2H), 4.79 (s, 2H), 3.87 (s, 2H), 2.63 (s, 3H). 13C NMR crosslinkable polybenzoxazine matrixes that are already used in anti­ (CDCl3, 90 MHz) δ (ppm): 150.8, 148.2, 140.6, 129.1, 125.7, 125.0, corrosion coatings [21] or electronic devices [22]. The crosslinking 122.9, 121.9, 120.7, 117.2, 83.9, 52.2, 40.0. IR (ATR, cm− 1): 3034, methodologies of the polymeric part that is not related with the ben­ 2943, 1585, 1485, 1448, 1427, 1286, 1273, 1228, 1211, 1191, 1143, zoxazine (ROP) are mainly driven by light induced polymerization [23]. 1110, 926, 748, 694, 631. HR-ESI-MS (m/z): calculated for C21H21N2O, Although electrochemical stimulus shares some advantage with the 317.1648; found [M+]: 317.1648. light-induced one, there is a lack of literature related to enhancement of N-(4-methoxyphenyl)-3-methyl-N-phenyl-3,4-dihydro-2H- the crosslinking density of polybenzoxazine derivatives using electric benzo[e][1,3]oxazin-6-amine (B3): The synthetic procedure and pu­ stimuli due to the low conductivity of this kind of polymer. rification of this benzoxazine is similar to B2, reaction among 3, MeNH2 In the current manuscript, we propose the design of electroactive and CH2O, obtaining 0.53 g of a orange viscous liquid with an 80 % benzoxazine polymers by introducing triphenylamine to their chemical yield. 1H NMR (CDCl3, 360 MHz): 7.19–7.15 (t, J = 7.2 Hz, 2H), structure, which can undergo σ-dimers through a C–C bond formation 7.06–7.03 (br d, J = 7.0 Hz 2H), 6.95–6.92 (d, J = 6.9 Hz, 2H), upon oxidation [12,24,25] to achieve a crosslinked matrix through an 6.90–6.81 (apparent m, 4H), 6.72–6.70 (br d, J = 6.7 Hz, 2H), 4.77 (s, electric stimulus (Scheme S2.2.c). Various possibilities, from triphenyl­ 2H), 3.85 (s, 2H), 3.80 (s, 3H), 2.61 (s, 3H). 13C NMR (CDCl3, 90 MHz): amine derivates to electroactive polybenzoxazine oligomers, have been 155.8, 149.8, 148.9, 141.2, 141.1, 129.1, 126.6, 124.9, 124.1, 121.2, investigated to control and trigger at low oxidation potential values the 120.7, 120.64, 117.2, 114.8, 83.9, 55.6, 52.3, 40.0. IR (ATR, cm− 1): crosslinking process of the functionalized polybenzoxazines in solution 2944, 2897, 2834, 1486, 1236, 1210, 1032, 924, 821, 749, 695. HR-ESI- and in conductive ionogel matrixes. MS (m/z): Calculated for C22H23N2O2, 347.1754; found [M+], 347.1750. Synthesis of compounds 2 and 3. 2. Experimental section 4-(diphenylamino)phenol (2): Compound 2 was synthesized following procedures similar to those as reported [27]. 2.1. Materials and methods 4-((4-methoxyphenyl)(phenyl)amino)phenol (3):[27] In a 25 mL schlenk, 4-bromophenol (1.20 g, 6.94 mmol), 4-methoxy-N-phenylani­ Nuclear magnetic resonance (NMR) spectra were recorded using line (1.22 g, 6.12 mmol) and sodium tert-butoxide (1.60 g, 16.23 mmol) Bruker spectrometers DPX-250, DXP-360 and AVANCE-III 400 (Servei were dissolved in 20 mL of anhydrous toluene. At the same time, in de Ressonància Magnètica Nuclear, UAB): 250 MHz (1H), 62.5 MHz another schlenk, palladium acetate (0.007 g, 0.029 mmol) was dissolved (13C); 360 MHz (1H), 90 MHz (13C); 400 MHz (1H), 100 MHz (13C) in 20 mL of anhydride toluene. Once it had turned into a solution, tri- respectively. 1H and 13C chemical shifts are reported in ppm relative to tert-butylphosphine (0.8 mL, 0.062 g, 0.31 mmol) was added over it. tetramethylsilane, using residual 1H and 13C resonances from solvent as After 5 min, both solutions were mixed using a cannula and the resulting internal standards. Infrared spectra were recorded using a Bruker Tensor mixture was refluxed for 5 h (the reaction was followed by 1H NMR). 27 instrument equipped with an ATR Golden Gate cell and a diamond After this time, the reaction crude was cooled, toluene was evaporated window. Gel permeation chromatography (GPC) analysis was per­ and, then ethyl acetate (30 mL) was added. The mixture was washed formed using Agilent Technologies 1260 Infinity hardware using THF as with deionized water (3 × 30 mL), the organic phase was dried over solvent at 1 mL/min. The GPC instrument was equipped with three gel anhydrous Na2SO4, and ethyl acetate was removed under reduced columns: PLgel 5 μm Guard /50 × 7.5 mm), PLgel 5 μm 10000 Å MW 4 pressure. The pure product was achieved by flash column chromatog­ K-400 K and PL Mixed gel C 5 μm MW 200–3 M. Oligomeric synthetized raphy (silica gel, hexane: ethyl acetate = 7:3 v/v) as a dark blue sticky polybenzoxazine standards were used to calibrate the GPC (similar re­ solid (caramel aspect) (1.70 g, 96 % yield). 1H NMR (acetone‑d6, 360 sults were obtained with commercial polystyrene standards). Bruker MHz) δ (ppm): 8.30 (s, 1H), 7.15 (dd, J = 7.1 Hz, 2H), 7.01 (d, J = 7.0 MicroFLEX or Bruker AutoFLEX III TOF/TOF as MALDI Mass spectra, Hz, 2H), 6.95 (d, J = 6.9 Hz 2H), 6.88 (d, J = 6.9 Hz, 2H) 6.84–6.80 (m, Dithranol (DIT) as matrix, and high-resolution mass spectra, using a 5H), 3.77 (s, 3H). 13C NMR (acetone‑d6, 100 MHz) δ (ppm): 156.7, direct inlet system (ESI). Differential scanning calorimetry (DSC) sam­ 154.7, 149.9, 141.9, 140.6, 129.6, 127.9, 127.1, 120.8, 120.8, 116.9, ples were analyzed with a TA Instrument Q20 using TzeroTM pans and 115.4, 55.6. IR (ATR, cm− 1): 3356 (O–H), 3036, 2835, 1593, 1500, lids calibrated with indium (Tm = 429.75 K, ΔHm = 3267 kJ/mol), 1489, 1460, 1441, 1232, 1178, 1108, 1029, 825, 750, 664. HR-ESI-MS scanning rate: 15 ◦ C/min. Thermal gravimetric analysis (TGA) samples (m/z): calculated for C19H17NO2, 291.1254; found [M+]: 291.1251. were recorded with a Perkin Elmer TGA 8000TM. Synthesis of compounds 4 and 5. Both compounds 4 and 5 were synthetized following the same 2.1.1. Synthesis of benzoxazine monomers method, whereby the derivative 4-(diphenylamino)phenol (2 for 4 and 3 6-methoxy-3-methyl-3,4-dihydro-2H-benzo[e][1,3]oxazine for 5) and benzoxazine B1 were mixed and heated at 80 ◦ C under (B1): Benzoxazine monomer B1 was synthesized following the proced­ continuous stirring forming a black viscous crude. After 24 h, the ure described in the literature [26]. mixture was cooled and purified by flash silica gel column chromatog­ 3-methyl-N,N-diphenyl-3,4-dihydro-2H-benzo[e][1,3]oxazin-6- raphy (hexane: ethyl acetate 7:3 v/v) obtaining a pale orange solid amine (B2): To a round flask containing 4 mL of dioxane, formalin (37 product (yield = 4: 53 %; 5: 38 %). % H2O, 10 % MeOH) (0.40 mL, 4.60 mmol) and methylamine (33 % 4: Melting point: 114–120 ◦ C. 1H NMR (CDCl3, 400 MHz) δ (ppm): EtOH) (0.30 mL, 2.30 mmol) were added with continuous stirring in an 7.20 (t, J = 7.1 Hz, 4H), 7.03 (d, J = 8.4 Hz, 4H), 6.97–6.91 (m, 3H), 2 C. Gascó et al. Microchemical Journal 182 (2022) 107878 6.82 (s, 4H), 6.77 (d, J = 8.4 Hz, 1H), 3.88 (s, 3H), 3.68 (m, 4H), 2.26 (s, isolated in 15 % yield. Melting point: 100 – 120 ◦ C. 1H NMR (acetona-d6, 3H). 13C NMR (CDCl3, 100 MHz) δ (ppm): 154.6, 148.2, 146.6, 144.7, 400 MHz) δ (ppm): 8.25 (s, 2H), 7.42 (d, J = 8.7 Hz, 4H), 7.05 (d, J = 138.9, 128.9, 126.9, 123.3, 123.2, 122.7, 122.49, 122.45, 121.4, 119.3, 8.6 Hz, 4H), 6.98 (d, J = 8.8 Hz, 4H), 6.90 (d, J = 8.7 Hz, 4H), 6.88 (d, J 116.8, 110.1, 60.1, 56.1, 55.9, 41.0. IR (ATR, cm− 1): 3278 (O–H), = 8.6 Hz, 4H), 6.82 (d, J = 8.8 Hz, 4H), 3.78 (s, 6H). 13C NMR (ace­ 2948, 2833, 1592, 1490, 1239, 1034, 821, 748, 695. HR-ESI-MS (m/z): tone‑d6, 100 MHz) δ (ppm): 156.9, 154.9, 148.7, 141.8, 140.6, 133.2, calculated for C28H29N2O3, 441.2173; found [M+], 441.2159. 128.0, 127.4, 127.2, 121.0, 117.0, 115.5, 55.7. IR (ATR, cm− 1): 3315 5: Melting point: 140–143 ◦ C. 1H NMR (CDCl3, 360 MHz) δ (ppm): (O–H), 2921, 2852, 1603, 1490, 1460, 1314, 1235, 1177, 1031, 818. 7.15 (t, J = 7.17, 2H), 7.02 (d, J = 7.03, 2H), 6.92 – 6.70 (m, 10H), 6.64 HR-ESI-MS (m/z): calculated for C38H32N2O4, 580.2357; found [M+], (br. s, 1H), 3.79 (s, 3H), 3.74 (s, 3H), 3.66 (s, 2H), 3.62 (s, 2H), 2.25 (s, 580.2336. 3H). 13C NMR (CDCl3, 90 MHz) δ (ppm): 155.7, 152.9, 152.7, 150.1, 148.9, 141.3, 140.0, 129.0, 127.5, 126.4, 126.45, 123.5, 120.9, 120.5, 2.1.4. Spectro-electrochemical experiments 116.6, 116.1, 114.7, 113.9, 59.2, 55.9, 55.6, 41.1. IR (ATR, cm− 1): 3254 In a range of 3–5 mM solutions of a mixture of acetonitrile and (O–H), 2949, 2833, 2359, 1592, 1490, 1460, 1432, 1239, 1149, 1107, toluene with TBAPF6 0.1 M were analyzed in a 0.33 mm thin-layered 1034, 822, 748, 694. HR-ESI-MS (m/z): calculated for C29H31N2O4, quartz glass cell using platinum gauze and platinum wire as working 471.2278; found [M+], 471.2287. and counter electrodes, respectively. A saturated calomel electrode Preparation of polymers 6 and 7 (SCE) was employed as a reference electrode. A PC-controlled VSP- Both polymers were obtained by heating (50–100 mg) of B2 or B3 in Potentiostat synchronized with a monolithic miniature spectrometer an open vial at 200 ◦ C for 2 h in a sand bath. (MMS) UV − vis high speed diode array spectrometer with a bandwidth of 300–1100 nm and a deuterium/ tungsten light source coupled to an 2.1.2. Cyclic voltammetry experiments. optical fiber was employed to register the spectroelectrochemical mea­ The cyclic voltammetry set-up consisted of a three-electrode system. surements. BioKine32 software was used for data acquisition and The working electrode (WE) was a glassy carbon disk of a diameter of treatment. 1.02 mm and the counter electrode (CE) was a glassy carbon disk (d = 3 mm). All the potentials were recorded using a CHI600E, vs saturated 2.1.5. Computational methods calomel electrode (SCE), isolated from the WE by a salt bridge. Tetra­ Quantum mechanical calculations were performed using the B3LYP- butylammonium hexafluorophosphate (TBAPF6) 0.1 M was used as the D3 method, that is, the B3LYP functional [28–30] plus Grimme’s supporting electrolyte at room temperature. correction (D3) for dispersion forces [31] and the 6-31G(d,p) basis set Solutions were prepared using acetonitrile and a mixture of aceto­ with the Gaussian 16 package [32]. Solvent effects (toluene) were taken nitrile and toluene 1:1 in anhydrous conditions obtained using molec­ into account using the SMD [33] implicit solvation model. ular sieves 3 Ǻ as a drying agent (24 h in activated molecular sieves). NPT molecular dynamics (MD) simulations at 300 K were performed with the Amber 16 package [34] using the GAFF force field [35]. Point 2.1.3. Controlled-potential electrolysis charges for the solute (polybenzoxazines) and solvent (toluene) mole­ The set-up for electrosynthesis is a three-electrode system including a cules were obtained through RESP [36,37] calculations at the optimized carbon graphite rod as WE, a platinum rod as CE separated from the B3LYP-D3/6-31G(d,p) minima in implicit toluene using Gaussian 16. solution by a salt bridge ended with a frit and a saturated calomel The B3LYP-D3/6-31G(d,p) structures optimized in toluene were also electrode as a reference. Electrolysis experiments at controlled poten­ used as starting points for the MD simulations. The solute molecules tials were undertaken with an EG&G Princeton Applied Research (PAR) were solvated in a cubic box of size L = 50 Å (L = 60 Å in the case of the 273A potentiostat and a conical electrochemical cell equipped with an hexamers), which was treated under periodic boundary conditions while argon bubbling source. All these electrochemical measurements were ensuring that the model reproduced the density of toluene at ambient performed in dry acetonitrile solution containing 0.1 M of TBAPF6 as a conditions. The size of the box guarantees a minimum of 24 Å buffer supporting electrolyte and at room temperature. Crude obtained after between two solvent molecules. The following equilibration scheme was electrosynthesis was directly analyzed by high resolution liquid used: (i) initial minimization in two steps. 5000 points with the steepest chromatography-mass spectrometry (LC-MS). The chemicals obtained descent method fixing the solute and then 5000 more points, 2500 with during electrolysis were isolated by flash silica gel column chromatog­ steepest descent and 2500 with conjugated gradient, relaxing the whole raphy and characterized. system; (ii) 200 ps of NVT dynamics raising the temperature from 0 to Synthesis and characterization of dimers obtained by 300 K keeping the solute fixed with weak restraints (force constant of controlled-potential electrolysis 500 kcal mol− 1 Å− 2 on all atoms of the solute); (iii) 2 ns of NPT dynamics Compound 9: at 300 K with no constraints; (iv) 400 ns of NPT production time To 25 mM of 2 in anhydrous acetonitrile in TBAPF6 0.1 M under simulation at 300 K with a time step of 1 fs, the Andersen temperature continuous flow of N2, was applied a controlled-potential of 0.94 V vs coupling scheme [38] and the Berendsen barostat [39]. SCE until 1F (enough charge to oxidise completely 2). The electrolysis To analyze the results, 2000 equally spaced frames along the simu­ mixture was evaporated, and the residue was flash chromatografied in lations were extracted. Based on the RMSD values, the 2000 structures gel of silica using toluene as eluent. Product 9 was isolated in 26 % yield. were grouped into conformationally related families using the ensemble Melting point: 100–120C. 1H NMR (acetone‑d6, 400 MHz) δ (ppm): 8.33 cluster scheme [40] implemented in the Chimera package.[41] Only the (s, 2H), 7.50 (d, J = 8.7 Hz, 4H), 7.25 (t, J = 7.8 Hz, 4H), 7.02 (m, 12H), carbon and nitrogen atoms of the polybenzoxazine moiety were 6.96 (t, J = 7.4 Hz, 2H), 6.86 (d, J = 8.7 Hz, 4H). 13C NMR (acetone‑d6, considered for the calculation of the RMSD. Finally, the method also 100 MHz) δ (ppm): 154.6, 148.2, 147.2, 139.3, 133.6, 129.1, 127.9, identified the most representative structure of each family or cluster. 126.9, 122.6, 122.3, 121.8, 116.3. IR (ATR, cm− 1): 3316 (O–H), 3029, 2919, 1590, 1486, 1222, 819, 721, 693. HR-ESI-MS (m/z): calculated for 3. Results and discussion C36H28N2O2, 520.2145; found [M+], 520.2119. Compound 10: In this work we will disclose the electroinduced crosslinking mech­ To 25 mM de OMeTPAOH in anhydrous acetronitrile in TBAPF6 0.1 anism of redox polybenzoxazines, which contain triphenylamine moi­ M under continuous flow of N2, was applied a controlled-potential of eties in their skeleton. However, prior to analyze these complex systems, 0.83 V vs SCE until 1F (enough charge to oxidise completely 3). The model molecules are previously studied being most of them prepared electrolysis mixture was evaporated, and the residue was flash chro­ and investigated for the first time. matografied in gel of silica using toluene as eluent. Product 10 was 3 C. Gascó et al. Microchemical Journal 182 (2022) 107878 3.1. Synthesis of triphenylamine derivatives tetraphenylbenzidine (8). To confirm these results, compound 8 was prepared following a previously described synthetic procedure [46]. A In this work we were interested in studing polybenzoxazines con­ cyclic voltammogram of a pure sample of 8 under the same previously taining in their structures triphenylamine moieties. However, to analyse described experimental conditions confirmed the formation of 8 after these complex systems, model molecules were previously studied, and the oxidation of 1 (Fig. 1b). The electrochemical oxidation mechanism some of them were prepared for the first time. Triphenylamine, 1 (TPA), of 1 in an acetonitrile: toluene media was fully elucidated by spec­ was commercially available, whereas -compounds 2 and 3 were syn­ troelectrochemical measurements. thetized following a Pd-catalyzed N-arylation reaction (Scheme 1a). Spectroelectrochemical experiments were performed to record the Compounds 4 and 5, model molecules of polybenzoxazine structures, UV–visible absorption spectrum profile of the electrochemically gener­ were obtained by heating in bulk using the described triphenylamines ated transient intermediates. A platinum gauze electrode is utilized to (2, 3) [27] and 6-methoxy-3-methyl-3,4-dihydro-2H-benzo[e][1,3] transmit a light beam as an OTE working electrode in a 0.05 cm thick oxazine (B1) as synthetized previously in our group [26]. Here, an quartz cell. The UV–visible spectra were recorded while a controlled- intrinsic ring opening of oxazine favors the attack of the orto-activated potential electrolysis at 1.03 V vs SCE was occurring for 100 s phenolic ring, obtaining both compounds (Scheme 1a). Column chro­ (Fig. 1c). It can clearly be seen that new absorption bands at 650 and matography purifications were carried out to eliminate side polymeri­ 482 nm developed in the spectrum. In accordance with previous studies zation reactions of the starting reagents. New compounds were [12] these bands can be attributed to 1⋅þ and 82þ, respectively. Fig. 1d characterized as indicated in the experimental section. shows the spectroelectrochemical response of a pure 8 sample, whereby Scheme 1b describes the synthesis of polybenzoxazines from tri­ the reversible appearance of the absorption band at 482 nm upon phenylamine monomeric benzoxazines. First, compounds 2 and 3 oxidation (1.03 V vs SCE – 150 s) and reduction (0.00 V vs SCE – 200 s) reacted with methylamine and formaldehyde through a Mannich type confirmed the presence of 82þ in the electrochemical oxidation mech­ reaction [42] yielding benzoxazines B2 and B3. Later, these two ben­ anism of 1. zoxazines were heated in bulk, and after a ring-opening polymerization Once the electrochemical behavior of 1 was determined, the elec­ (ROP) process polybenzoxazines 6 and 7 were obtained without any loss trochemical behaviour of electroactive units 2 and 3 was fully disclosed of weight after 2 h at 200 ◦ C. Similar N-Ph B2 benzoxazine have been using cyclic voltammetry, controlled potential electrolysis and spec­ synthesized from aniline and thermal polymerized to be studied as troelectrochemistry. Fig. 1e (solid line) shows a reversible oxidation electrochromic material [43]. wave, the anodic peak potential value being located at 0.81 V vs SCE. In Finally, polymeric mixtures were identified and characterized using the corresponding counter cathodic scan, a reversible reduction peak proton nuclear magnetic resonance (1H NMR). The characteristic signals could be detected at 0.73 V. This cathodic peak was assigned to the related with the methylenes belonging to the oxazine ring of monomer reduction of 2⋅þ which could generate a dimer compound, as was pre­ disappear when polymerization occurs, so it is easy to follow the ROP viously observed for the electrochemical oxidation process of 1. To mechanism through NMR experiments. Fourier Transform Infrared verify that hypothesis, a controlled potential electrolysis of 2 at 0.94 V Spectroscopy (FT-IR) is a great tool to determine the benzoxazine for­ was performed, after the passage of 1F dimer 9 was isolated from the mation, its structure and the polymerization process [44,45]. Peaks reaction crude in a 26 % yield (Scheme S3). The redox behavior of a pure assigned to benzoxazine at 1230 cm− 1 (C–O–C antisymmetric sample of 9 is depicted in Fig. 1e (dotted line) where two successive stretching mode of the oxazine ring) and 910–940 cm− 1 (benzene ring oxidation reversible waves were measured at 0.74 and 0.86 V vs SCE. mode with oxazine ring) clearly decreased during polymerization pro­ Finally, when spectroelectrochemical experiments were performed, a cess, indicating the ring-opening reaction of oxazine ring occurred. New new absorption band appeared at 578 nm after applying a 1.00 V con­ bands appeared in the interval of 3300–3500 cm− 1 corresponding to stant potential for 100 s. In this case, the only intermediate detected at new generated hydrogen-bonded hydroxyls of open oxazine ring spe­ this time scale was 2⋅þ (Fig. 1f) related to the maximum at 578 nm. cies. An increase in the aliphatic CH at 2927 cm− 1 that joined the Hence, the presence of a hydroxy group, and an electrodonating group phenolic derivatives to each other was also observed. Thermogravimetry by resonance, not only diminishes the oxidation potential value of the analysis (TGA) and differential scanning calorimetry (DSC) enable triphenylamine but also stabilizes the radical cation formed upon determination of the behavior of the intrinsic compounds against a heat oxidation. flow. Gel permeation chromatography (GPC) was used to determine the In this regard, in 4-((4-methoxyphenyl)(phenyl)amino)phenol (3) distribution of weight corresponding to each polybenzoxazine. For both the presence of a methoxy group would also increase the stability of its oligomers, 6 and 7, three major distributions were obtained in tetra­ radical cation (3⋅þ) as well as reducing its oxidation potential value. hydrofuran (see supporting information). The deconvolution of the GPC Cyclic voltammetry experiments confirmed that the compound showed distribution for the synthesis of 6 revealed a c.a. 1 % of opened monomer a single one-electron reversible wave at 0.71 V vs SCE (Fig. 1g, solid derivative (6a), 29 % of trimer (6b) and 70 % of oligomer of five units line). Note that at this time scale no presence of its corresponding dimer (6c); whereas GPC distribution for 7 showed 8 % of opened monomer 10 is observed since no reduction peaks were detected in the cathodic (7a), 39 % of trimer (7b) and 53 % of oligomer of five units (7c) in counter scan. The formation of 3⋅þ was also confirmed by spectroelec­ oligomer mixtures, respectively (Scheme 1c). trochemistry, whereby an absorption band appeared at 525 nm when the sample was electrolyzed at 0.90 V vs SCE (Fig. 1h). Finally, when 3.2. Electrochemical behavior of triphenylamine derivatives long time scale electrochemical experiments were performed, such as controlled potential electrolysis at 0.83 V vs SCE, the corresponding Fig. 1a shows a typical cyclic voltammogram of commercial triphe­ dimer 10 was isolated in a 15 % yield (Scheme 2 supporting informa­ nylamine (1), using an acetonitrile: toluene solution containing 0.1 M of tion). Fig. 2g (dotted line) shows the cyclic voltammogram of a pure supporting electrolyte (TBAPF6) in an inert atmosphere. A one-electron sample of 10, revealing two reversible one electron waves correspond­ wave was observed on the first anodic scan at 1.06 V vs SCE, whereas ing to the successive formation of 10⋅þ and 102þ at 0.47 and 0.71 V vs two reduction waves at 0.90 V and 0.73 V vs SCE were detected in the SCE, respectively. corresponding cathodic counter scan. In a second scan (Fig. 1a, dotted At this point, it is possible to conclude that compounds 2 and 3 follow line), it was possible to establish the reversibility of at least this second the same electrochemical oxidation mechanism as previously disclosed reduction peak (0.73 V vs SCE). According to results published previ­ for triphenylamine (1), and it was depicted in Scheme S1. The overall ously in similar solvents [12,25], the new peak that appeared in the process is summarized in Scheme S2 supporting information. second cycle after the first oxidation of 1 corresponds to dimerization of triphenylamine radical cation (1⋅þ) leading to N, N, N’, N’- 4 C. Gascó et al. Microchemical Journal 182 (2022) 107878 Scheme 1. a) Synthetic route of compounds 2, 3, 4 and 5b) Synthetic route to obtain electroactive benzoxazines, B2 and B3, and polybenzoxazines 6 and 7c) Oligomer distribution of 6 and 7. 5 C. Gascó et al. Microchemical Journal 182 (2022) 107878 Fig. 1. a) Cyclic voltammograms of 5 mM solution of 1 and (b) 3 mM of 8 acetonitrile:toluene (0.1 M TBAPF6; scan rate = 0.5 V s− 1). c) Spectroelectrochemistry of 2 mM 1 and (d) 8 acetonitrile:toluene (0.1 M TBAPF6) (e) Cyclic voltammograms of 2 (6 mM) and 9 (2 mM) in acetonitrile:toluene (0.1 M TBAPF6; scan rate = 0.5 V s− 1); (f) UV–vis of initial compound 2 (2 mM) and its spectrum after applying 1.0 V vs SCE to acetonitrile:toluene (0.1 M TBAPF6) for 100 s; (g) Cyclic voltam­ mograms of 3 (3 mM) and 10 (2 mM) in acetonitrile:toluene (0.1 M TBAPF6; scan rate = 0.5 V s− 1); (h) UV–vis initial compound 3 (2 mM) (t = 0 s) and its spectrum after applying 1.0 V vs SCE to acetonitrile:toluene (0.1 M TBAPF6) for 100 s. 6 C. Gascó et al. Microchemical Journal 182 (2022) 107878 Fig. 2. Cyclic voltammograms of 3 mM (a) 4 and (b) 5 in acetonitrile:toluene (0.1 M TBAPF6; scan rate = 0.5 V s− 1). UV–vis spectrum of 2 mM of (c) 4 and (d) 5 at initial state (to = 0 s) and after applying 1.0 V in acetonitrile:toluene (0.1 M TBAPF6) (e) Proposed electroinduced coupling for compounds 4 and 5 to generate compound 11 (R1 = H) and 12 (R2 = OMe). 7 C. Gascó et al. Microchemical Journal 182 (2022) 107878 3.3. Electrochemical behavior of triphenylamine-polybenzoxazine resonance (1H NMR), differential scanning calorimetry (DSC) and derivatives thermogravimetric analysis (TGA). Although no significant changes were observed in 1H NMR spectrum before and after oxidation of 6 Prior to studying the complex electroactive polybenzoxazines, sim­ (Fig. 3e), DSC and TGA analysis revealed significant changes. The DSC ple models 4 and 5 (Scheme 1) were analyzed to determine whether the experiments depicted in Fig. 3b revealed an increase in the Tg from the radical cation of those compounds followed a dimerization mechanism. electrolyzed sample (Tg (6) = 75.6 ◦ C; Tg (E6) = 109.6 and 162.7 ◦ C, Compounds 4 and 5 showed a very similar redox and optical behavior Fig. 3f). This phenomenon is attributed to a more crosslinked poly­ (Fig. 2). In both cases, in the first anodic scan two oxidation peaks were benzoxazine that is supported by TGA experiments (Fig. 3c and Fig. 3h), observed, which led to the formation of their respective cation radical in which the degradation temperature observed for 5 and 10 % of weight and dication. Note that in both cases the first oxidation peak at 0.97 loss was higher in the electrolyzed sample; char yield was also increased (Fig. 2a) and 0.80 V (Fig. 2b) corresponds to a pseudoreversible peak, its in more than 10 % (Fig. 3g,h) [47]. Hence, our experiments indicate that current function being slightly higher than one electron. Spectroelec­ crosslinking of the material through electrodimerization of electroactive trochemistry experiments show an absorption band at 584 (Fig. 2c) and moieties incorporated in polybenzoxazine 6 was achieved. 604 nm (Fig. 2d), which corroborates the existence of the intermediates To shed light on the structure of the functionalized poly­ 4⋅þ and 5⋅þ, respectively, upon controlled potential oxidation at 1.0 V. benzoxazines, we selected trimers 6b and 7b as models and performed However, a closer look at the cathodic scan of cyclic voltammograms atomistic molecular dynamics (MD) simulations and DFT calculations. (Fig. 2a and 2b) reveals that the current values associated to those Figure S1 shows the initial quantum mechanics (QM) optimized linear cathodic peaks are higher than those previously observed in the first structures for both systems. These structures were used as the starting anodic scan. These results can be rationalized considering how the point for the molecular dynamics simulations. RMSD values along MD oxidized forms of dimers 11⋅þ and 12⋅þ, which are formed during simulation are shown in Figure S2. The results of the simulations show oxidation of 4 and 5, are reduced at the same potential value as 4⋅þ and that the preferred arrangement for the trimers involves a central core 5⋅þ. The current function of the cathodic peaks is increased in the with the phenol and amine groups of the polybenzoxazine moiety reverse scan. In Fig. 2e, the proposed electroinduced dimerization pro­ interacting through a network of hydrogen bonds and the diphenyl­ cess is shown for compounds 4 and 5, presenting the redox in­ amine groups pointing to the solvent. Analysis of the intramolecular H- termediates described above. bonds throughout the trajectory shows that they are labile, and the main Those results were confirmed by controlled-potential electrolysis interactions include O–H⋅⋅⋅OH and O–H⋅⋅⋅N contacts. Values presented (Scheme S4). The analysis of the high-performance liquid chromatog­ in table S1 indicate that the former are more frequent, probably due to raphy of the reaction crudes coupled with high resolution mass spec­ entropic factors because OH⋅⋅⋅N ones imply more folded structures, as trometry confirmed the formation of the corresponding σ-dimers 11 and shown in previous studies for other polybenzoxazines [48]. 12 formed upon oxidation of 4 and 5 after the passage of 1F at 1.20 V Cluster analysis of the trajectories was carried out to obtain repre­ and 1.00 V, respectively. sentative structures of the systems. Figure S3 shows the population of Having demonstrated that two-unit triphenylamine-poly­ each cluster for both 6b and 7b MD simulations, and Table 1 (first row) benzoxazine models (4 and 5) undergo an electroinduced dimerization shows the representative structure of the most populated cluster for each process, triphenylamine-polybenzoxazine oligomers 6 and 7 were syn­ system, with the network of H-bond highlighted. As commented above, thetized from B2 and B3, respectively, in bulk under heating through a diphenylamine groups are exposed to the solvent and are thus available ring-opening polymerization (Scheme 1c). The above-mentioned prod­ for crosslinking with neighbor oligomers, thus favoring polymerization. uct distributions were well correlated by cyclic voltammetry experi­ It should also be noted that diphenylamine groups belonging to the same ments (Fig. 3a and Fig. 3b). In both cases, three different oxidation peaks oligomer are far apart, thus preventing intramolecular crosslinking. at 0.89 V, 0.99 V and 1.14 for 6 and 0.80 V, 0.87 V and 0.94 V for 7 were Representative structures for other clusters with significant populations observed. As expected, the presence of a methoxy electrodonating group are also shown in Figures S4 and S5. in the diphenylamine structure of the polymer makes it easier to oxidize To determine the most prone position/s for polymerization through the polybenzoxazine derivative. The total number of electrons involved σ-dimerization after oxidation of the sample, oxidized structures of the in this first electron transfer process were 1.67 and 1.87 for mixtures 6 trimers were also computed. These structures were obtained by and 7, respectively. These results seem to indicate that there is a C–C oxidizing the corresponding representative structure of the most popu­ coupling process and an electron transfer at the same potential, as it has lated cluster of 6b and 7b at the QM level (see Figures S6 and S7) and been previously noted for two-units of triphenylamine-polybenzoxazine reoptimizing at the same level of theory. The results show that the ge­ derivatives. Spectroelectrochemistry of polybenzoxazine 6 (Fig. 3c) ometries do not change significantly after oxidation. The spin density is shows the formation of 6⋅þ at 650 nm as well as an intense band at 458 completely delocalized among the aromatic rings, computed values nm that could be associated with the oxidized dimers 62þ. The same being around 0.1 a.u. in each of the nine aromatic rings. Within the results were obtained when control potential spectroelectrochemical aromatic ring, the largest value (~0.05 a. u.) corresponds to the phenyl experiments were performed after the first oxidation wave for 7, carbon atoms in para with respect to the substituted carbon, of external whereupon the formation of radical cations 7⋅þand dimers 72þ at 604 aromatic rings. Therefore, one would expect dimerization to occur nm and 465 nm, respectively, were also detected (Fig. 3d). Thus, an through these carbon atoms. Consequently, as a model for the polymer electroinduced crosslinking process may occur through the formation of we used a hexamer structure obtained by joining two molecules of 7b σ-dimers in 6 and 7 polymeric mixtures. through this carbon atom of the N-terminal monomers. The geometry of In order to confirm electroinduced crosslinking via a triphenylamine the resulting hexamer was optimized at the DFT level and used as radical cation coupling process, we decided to use compound 6 since it starting points for the MD simulations as a model case. has a larger number of free coupling positions in its structure. Thus, a Table 1 (second row) shows the representative structure of the most controlled potential electrolysis of 6 in an acetonitrile: toluene (1:4) populated cluster. The system behaves as two independent trimers (0.1 M of TBAPF6) solution was performed at 1.20 V vs SCE. For poly­ connected by a biphenyl bridge at the para position. That is, each trimer meric mixture 6 after the passage of 3F, the crude of electrosynthesis subunit is arranged around a central core where the OH and the amino was evaporated and digested in acetonitrile for 24 h to eliminate groups interact through hydrogen bonds. However, it should be noted TBAPF6, and filtered. The solid obtained was suspended in clean that the hydrogen bonds are more labile than in the previous case, and acetonitrile and centrifugated for 3 min at 3000 rpm; this process was most of the structures do not show three hydrogen bonds per unit. The repeated three times to remove completely the soluble salt. The terminal aromatic rings that are not involved in the bridge between remaining solid was dried and characterized by proton nuclear magnetic subunits are exposed to the solvent or, in some cases, aligned with the 8 C. Gascó et al. Microchemical Journal 182 (2022) 107878 Fig. 3. Cyclic voltammograms of 1.3 mM (a) 6 and 2.1 mM of (b) 7 in acetonitrile:toluene (0.1 M TBAPF6; scan rate = 0.5 V s− 1). UV–vis spectrum of 10 mM of (c) 6 and (d) 7 at initial state and after applying 1.2 V to acetonitrile:toluene 1:4 (0.1 M TBAPF6) (e) 1H NMR spectrum, (f) DSC thermogram, and (g) TGA thermograms of compound 6 before and after the electrosynthesis process (E6) (h) Table with glass transition and degradation temperatures at 5 % and 10 % loss of weight. 9 C. Gascó et al. Microchemical Journal 182 (2022) 107878 Table 1 Representative structure of the most populated cluster of DFT simulations of 6b, 7b and representative structure of the most populated cluster of the MD simulations of the hexamer resulting from the crosslink of two 7b trimer molecules. Structure 6b Structure 7b Trimer of Structure 7b Fig. 4. Redox polybenzoxazine system in polymeric gel electrolytes. 10 C. Gascó et al. Microchemical Journal 182 (2022) 107878 other subunit. In this sense, several structures show small distances Data availability between aromatic rings of both subunits (for example in clusters 1 and 2), which is also in good agreement with the possibility of an intra­ Data will be made available on request. molecular crosslink in a subsequent oxidation. Acknowledgements 3.4. Electroinduced crosslinking process of triphenylamine- The authors are grateful for financial support from projects PID2019- polybenzoxazine oligomers in ionogel matrixes 106171RB-I00 and PID2020-112715 GB-I00 of the Ministerio de Ciencia e Innovación (MICINN). C.G. and also to the Spanish Ministry of Science, As a proof of concept to work in a solid state, we decided to introduce Innovation and Universities for the award of a research studentship via the redox polybenzoxazine systems to polymeric gel electrolytes, ion­ the FPI program (BES-2016-078179) and to Spain’s MEC (CTQ2016- ogels (IGs), based on poly(vinylidenefluoride-co-hexafluoropropylene (P 81797-REDC and RED2018-102387-T), the ORFEO CINQA Network and (VDF-co-HFP)) and ionic liquids (ILs). Those IGs would act as a solid for funding from DURSI-Generalitat de Catalunya (2017- SGR465). matrix as well as electrolytes introducing flexibility, elasticity, and high ionic conductivity [49] to improve the brittle and isolating poly­ Appendix A. Supplementary data benzoxazine materials. Based on previous results obtained in our research group [50,51], for this study an IG composed of P(VDF-co-HFP) Supplementary data to this article can be found online at https://doi. - [BMIM][TFSI] at a weight ratio of 1:5 was selected since it shows org/10.1016/j.microc.2022.107878. enough ionic conductivity, transparency, and flexibility. A mixture of P (VDF-co-HFP) - [BMIM][TFSI] and polybenzoxazine at a weight ratio of References 1:5:0.5 was solved in acetone and dried at room temperature. Hence, the resulting triphenylamine-polybenzoxazine IGs was stuck onto screen- [1] D. Kuckling, A. Doering, F. Krahl, K.-F. Arndt, 8.15 - Stimuli-Responsive Polymer Systems, Editor(s): Krzysztof Matyjaszewski, Martin Möller, Polymer Science: A printed electrodes. 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