1 Introduction

Poly(ionic liquid)s (PILs) have attracted significant attention in recent years as they ideally merge the properties of ionic liquids (e.g. high thermal, chemical, electrochemical stabilities, tunable viscosity and solubility, enhanced ionic conductivity…) and those of polymer materials (e.g. viscoelasticity, processability, film forming capacities and broad structural design…) [1,2,3,4,5,6]. They have shown promising performances in a broad range of applications including thermoresponsive polyelectrolytes [7], self-assembled colloids [8,9,10], electrolyte gated transistors [11], dye sensitized solar cells [12], anion exchange membranes for fuel cells [13, 14], batteries and supercapacitors [15,16,17,18], sensors and actuators [19,20,21], electrochromic devices [22], gas separation membranes [23, 24], antimicrobial surfaces [25], and catalysis [26, 27]. Their widespread interest rely on the broad array of cations (e.g. ammonium, pyridinium, imidazolium, pyrrolidinium, phosphonium, piperidinium, thiazolium, triazolium…), anions (e.g. halides, carboxylates, sulfonates, phosphates, inorganic fluorides, perfluorinated sulfonimides…) and synthetic approaches (e.g. different chain growth and step growth polymerizations, or post-polymerization chemical modification techniques…) involved in their synthesis.

1,2,3-Triazolium based poly(ionic liquid)s (TPILs) are a recent addition to the PILs family that merge the best attributes of state-of-the-art polymerization and post-polymerization chemical modification techniques with copper(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC) [28], quantitative N-alkylation of 1,2,3-triazole groups and ion metathesis reactions [2]. So far, a broad variety of side-chain TPILs (e.g. poly(meth)acrylates [29,30,31], poly(N-vinyl triazolium) [32], poly(vinyl ester) [33], poly(aryl ether) [14], polysiloxane [34]) and main-chain TPILs (e.g. ionenes [35,36,37], dendritic structures [38, 39]) have been developed using high precision macromolecular designs. However, up to now there are still important polymer classes that have not been reported as TPIL derivatives. For instance, polyimides are high performance polymer materials that exhibit improved mechanical properties, as well as high chemical stability and heat resistance relevant for many technical applications [40, 41]. Scarce examples of polyimide-based PILs have already been reported so far but none of them included 1,2,3-triazolium cations. Kim and coworkers developed ionic polyimides with side-chain imidazolium groups as well as polyimide networks cross-linked through piperazinium linkages both having enhanced gas separation properties compared to classical polyimides [42,43,44]. Besides, Shaplov and co-workers reported the transformation of polybenzimidazoles into ionic polyimides carrying main-chain benzimidazolium or side-chain quinuclidinium groups and studied their gas separation properties [45].

Herein, we report the synthesis of a series of poly(1,2,3-triazolium imide)s using a three-step synthetic strategy involving: (i) AA + BB CuAAC polyaddition, (ii) N-alkylation of 1,2,3-triazole groups, and (iii) ion metathesis to introduce bis(trifluoromethylsulfonyl)imide (TFSI) counter anions. Their structural and physical properties are discussed based on NMR spectroscopy, size exclusion chromatography (SEC), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and solubility measurements.

2 Experimental Section

2.1 Materials

3,3′,4,4′-Biphenyltetracarboxylic dianhydride (97%), propargylamine (98%), sodium acetate (99%), acetic anhydride (99%), diisopropylethylamine (DIPEA, 99%), iodomethane (CH3I, 99%), copper(I) iodide triethylphosphite (CuP(OEt)3I, 97%), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI, 99.95%), as well as all other reagents and solvents were purchased from Merck and used as received. Dipropargyl monomers 2 and 3 [46], and α,ω-diazido tetraethylene glycol 4 [47] were synthesized as described previously.

2.2 Characterization Methods

1H NMR (300 or 400 MHz) and 13C NMR (100 MHz) spectra were recorded on Bruker Avance 300 or 400 spectrometers in deuterated dimethylsulfoxide (DMSO-d6) using as references residual hydrogenated DMSO-d6 for 1H NMR, DMSO-d6 signal for 13C NMR, and the signal of trifluoroethanol for 19F NMR. DSC measurements were carried out under helium using a DSC Q200 apparatus (TA instruments) by applying two cycles in the temperatures range of − 50 to 120 °C at a heating rate of 20 °C/min. Tg values were measured during the second heating cycle. TGA measurements were performed under helium using a TGA Q500 apparatus (TA instruments) at a heating rate of 20 °C/min. SEC measurements were carried out at 50 °C on a setup comprising a Viscotek VE 5111 injector valve bracket, a Viscotek T60 A dual detector, a Viscotek VE 3580 RI detector, a Viscotek VE 1122 solvent delivery system, and a PL gel 5 µm Mixed C (300 × 7.5 mm, polystyrene/divinylbenzene) column using a 0.01 M LiTFSI solution in N,N-dimethylformamide (DMF) as eluent. 2 mg/mL solutions of poly(1,2,3-triazolium imide)s 8–10 in DMF with 0.01 M LiTFSI were prepared and filtered through a 0.45 μm pore size PTFE filter prior to injection. Number average molar masses (Mn), weight average molar masses (Mw) and chain dispersities (Ð) were calculated using a refractive index calibration curve obtained from polystyrene (PS) standards.

2.3 Synthesis of dipropargyl monomer 1

3,3′,4,4′-(Biphenyl-tetracarboxylic) dianhydride (6.00 g, 18.6 mmol) was added portion wise to a solution of propargylamine (2.47 g, 41.0 mmol) in N-methyl-2-pyrrolidone (NMP, 50 mL) and the mixture was stirred for 7 h at 50 °C. A solution of sodium acetate (0.08 g, 0.97 mmol) in a 2:1 vol/vol mixture of acetic anhydride and pyridine (6.7 mL) was added and the mixture was further stirred for 10 h at 70 °C. After cooling down to room temperature, the reaction mixture was precipitated in water and filtered. The crude product was recrystallized from DMF/H2O, filtered and dried under reduced pressure to yield dipropargyl monomer 1 as a white powder (4.20 g, 61.3%). 1H NMR (300 MHz, DMSO-d6): δ 8.37 (s, 2H, H-f), 8.33 (dd, J = 7.8 Hz, J = 1.5 Hz, 2H, H-i), 8.05 (d, J = 7.8 Hz, 2H, H-h), 4.41 (d, J = 2.5 Hz, 4H, NCH2C≡CH), 3.29 (t, J = 2.5 Hz, 2H, NCH2C≡CH) ppm. 13C NMR (400 MHz, DMSO-d6): δ 166.20 (4C, NCO), 144.25 (2C, C-g), 133.67 (2C, C-i), 132.39 (2C, C-e), 131.11 (2C, C-j), 123.97 (2C, C-h), 122.28 (2C, C-f), 78.06 (2C, NCH2C≡CH), 73.87 (2C, NCH2C≡CH), 26.77 (2C, NCH2C≡CH) ppm. HRMS (ESI) m/z: [M + Na]+ calculated for C22H12N2O2Na 391.0689; found, 391.0672.

2.4 Synthesis of Poly(1,2,3-triazole imide)s 5–7

2.4.1 General Procedure for CuAAC Polyaddition. Synthesis of 5

A solution of dipropargyl 1 (866 mg, 2.35 mmol), diazide 4 (580 mg, 2.35 mmol), CuP(OEt)3I (17 mg, 0.047 mmol) and DIPEA (300 mg, 2.35 mmol) in NMP (24 mL) was stirred for 48 h at 60 °C. The crude reaction mixture was precipitated twice in diethyl ether (Et2O), centrifuged, and dried under vacuum to obtain poly(1,2,3-triazole imide) 5 as an orange solid (1.20 g, 79.4%). 1HNMR (300 MHz, DMSO-d6): δ 8.27–8.14 (m, 4H, H-f, H-i), 8.04 (s, 2H, NCH2C=CH), 7.90 (s, 2H, H-h), 4.83 (s, 4H, NCH2C=CH), 4.45 (s, 4H, NCH2CH2OCH2CH2O), 3.75 (s, 4H, NCH2CH2OCH2CH2O), 3.43 (s, 4H, NCH2CH2OCH2CH2O), 3.38 (s, 4H, NCH2CH2OCH2CH2O) ppm.

2.4.2 Synthesis of 6

The general procedure for CuAAC polyaddition was applied to dipropargyl 2 (550 mg, 1.39 mmol), diazide 4 (340 mg, 1.39 mmol), CuP(OEt)3I (10 mg, 0.028 mmol) and DIPEA (180 mg, 1.39 mmol) in NMP (10 mL) to obtain poly(1,2,3-triazole imide) 6 as a dark brown solid (600 mg, 64.4%). 1H NMR (300 MHz, DMSO-d6): δ 8.16 (d, J = 6.3 Hz, 2H, H-i), 8.07–8.04 (m, 4H, H–f, H-h, NCH2C=CH), 4.84 (s, 4H, NCH2C=CH), 4.46 (s, 4H, NCH2CH2OCH2CH2O), 3.76 (s, 4H, NCH2CH2OCH2CH2O), 3.45 (s, 4H, NCH2CH2OCH2CH2O), 3.40 (s, 4H, NCH2CH2OCH2CH2O) ppm.

2.4.3 Synthesis of 7

The general procedure for CuAAC polyaddition was applied to dipropargyl 3 (2.67 g, 5.16 mmol), diazide 4 (1.26 g, 5.16 mmol), CuP(OEt)3I (40 mg, 0.11 mmol) and DIPEA (0.66 g, 5.16 mmol) in NMP (10 mL) to obtain poly(1,2,3-triazole imide) 7 as a dark brown solid (3.10 g, 75.8%). 1H NMR (300 MHz, DMSO-d6): δ 8.08–8.04 (m, 4H, H-i, NCH2C=CH), 7.87 (s, 2H, H-h), 7.70 (s, 2H, H-f), 4.83 (s, 4H, NCH2C=CH), 4.45 (s, 4H, NCH2CH2OCH2CH2O), 3.76 (s, 4H, NCH2CH2OCH2CH2O), 3.46 (s, 4H, NCH2CH2OCH2CH2O), 3.38 (s, 4H, NCH2CH2OCH2CH2O) ppm.

2.5 Synthesis of Poly(1,2,3-triazolium imide)s 8–10

2.5.1 General Procedure for N-alkylation and Anion Exchange. Synthesis of 8

A solution of poly(1,2,3-triazole imide) 5 (1.10 g, 1.74 mmol of 1,2,3-triazole groups) and iodomethane (1.00 mL, 17.4 mmol) in DMF (30 mL) was stirred for 72 h at 60 °C. The reaction mixture was precipitated once in ethyl acetate and centrifuged to afford, after drying under reduced pressure the resulting poly(3-methyl-1,2,3-triazolium iodide) as an orange solid (1.30 g, 80.6%). A solution of this iodide-containing poly(1,2,3-triazolium imide) intermediate (1.00 g, 1.07 mmol of 1,2,3-triazolium groups) and LiTFSI (460 mg, 1.62 mmol) was dissolved in DMF (3 mL) and heated at 45 °C for 20 h. The mixture was precipitated in water, dissolved in CH3CN and precipitated twice in methanol (MeOH) to afford, after centrifugation and drying under reduced pressure, 8 as an orange solid (900 mg, 68.4%). 1H NMR (300 MHz, DMSO-d6): δ 9.02 (s, 2H, NCH2C=CH), 8.39-8.32 (m, 4H, H-f, H-i), 8.06 (s, 2H, H-h), 5.18 (s, 4H, NCH2C=CH), 4.79 (s, 4H, NCH2CH2OCH2CH2O), 4.36 (s, 6H, NCH3), 3.91 (s, 4H, NCH2CH2OCH2CH2O), 3.56 (s, 4H, NCH2CH2OCH2CH2O), 3.48 (s, 4H, NCH2CH2OCH2CH2O) ppm. 13C NMR (100 MHz, DMSO-d6): δ 166.69,166.65 (C-d), 144.17 (C-g), 138.48 (C-b), 133.58 (C-i), 132.80 (C-e), 131.50 (C-j), 130.74 (C-a), 124.05 (C-h), 122.12 (C-f), 119.38 (q, J = 320 Hz, C-TFSI), 69.40 (C-n, C-o), 67.21 (C-m), 53.04 (C-l), 38.24 (C-k), 29.95 (C-c) ppm. 19F NMR (282 MHz, DMSO-d6): –79.40 (s, 12F, CF3SO2N) ppm.

2.5.2 Synthesis of 9

The general procedure for N-alkylation was applied to a mixture of 6 (380 mg, 0.56 mmol of 1,2,3-triazole groups) and iodomethane (0.35 mL, 5.66 mmol) in DMF (7 mL). The general procedure for anion exchange was applied to the poly(3-methyl-1,2,3-triazolium iodide) intermediate (130 mg, 0.136 mmol of 1,2,3-triazolium groups) and LiTFSI (60 mg, 0.21 mmol) in DMF (2 mL) to yield 9 as a dark brown solid (110 mg, 64.1%). 1H NMR (300 MHz, DMSO-d6): δ 9.01 (s, 2H, NCH2C=CH), 8.16-8.13 (m, 6H, H-f, H-h, H-i), 5.19 (s, 4H, NCH2C=CH), 4.80 (s, 4H, NCH2CH2OCH2CH2O), 4.35 (s, 6H, NCH3), 3.92 (s, 4H, NCH2CH2OCH2CH2O), 3.55 (s, 4H, NCH2CH2OCH2CH2O), 3.49 (s, 4H, NCH2CH2OCH2CH2O) ppm. 13C NMR (100 MHz, DMSO-d6): δ 193.16 (C-p), 166.36 (C-d), 141.49 (C-g), 138.27 (C-b), 135.75 (C-i), 134.77 (C-j), 132.01 (C-e), 130.89 (C-a), 123.75,123.62 (C-h, C-f), 119.40 (q, J = 320 Hz, C-TFSI), 69.43 (C-n, C-o), 67.25 (C-m), 53.07 (C-l), 38.30 (C-k), 30.00 (C-c) ppm. 19F NMR (282 MHz, DMSO-d6): –79.40 (s, 12F, CF3SO2N) ppm.

2.5.3 Synthesis of 10

The general procedure for N-alkylation was applied to a mixture of 7 (2.60 g, 3.20 mmol of 1,2,3-triazole groups) and iodomethane (2.00 mL, 32.0 mmol) in DMF (3 mL). The general procedure for anion exchange was applied to the poly(3-methyl-1,2,3-triazolium iodide) intermediate (2.50 g, 2.32 mmol of 1,2,3-triazolium groups) and LiTFSI (1.00 g, 3.48 mmol) in DMF (4 mL) to yield 10 as a yellow solid (2.00 g, 62.5%). 1H NMR (300 MHz, DMSO-d6): δ 8.99 (s, 2H, NCH2C=CH), 8,14 (d, J = 7.6 Hz, 2H, H-i), 7.89 (d, J = 6.8 Hz, 2H, H-h), 7.69 (s, 2H, H-f), 5.17 (s, 4H, NCH2C=CH), 4.80 (s,4H, NCH2CH2OCH2CH2O), 4.35 (s, 6H, NCH3), 3.91 (s, 4H, NCH2CH2OCH2CH2O), 3.55 (s, 4H, NCH2CH2OCH2CH2O), 3.47 (s, 4H, NCH2CH2OCH2CH2O) ppm. 13C NMR (100 MHz, DMSO-d6): δ 166.12,166.02 (C-d), 138.20 (C-b), 137.22 (C-g), 135.84 (C-h), 133.03 (C-j), 132.62 (C-e), 130.91 (C-a), 124.31 (C-i), 123.34 (C-f), 121.79 (C-q), 119.40 (q, J = 320 Hz, C-TFSI), 69.63,69.47,69.16 (C-n, C-o), 67.27 (C-m), 64.49 (t, J = 26 Hz, C-p), 53.13 (C-l), 38.36 (C-k), 30.08 (C-c) ppm. 19F NMR (282 MHz, DMSO-d6): –63.56 (s, 6F, CF3C), –79.40 (s, 12F, CF3SO2N) ppm.

2.6 Membrane Preparation

Poly(1,2,3-triazolium imide) membranes were hot pressed at ca. Tg + 20 °C using first a 250 µm thick hollow spacer and then without any spacer directly between two Teflon sheets using 20 bars of pressure. The resulting membranes were ca. 25 µm thick.

3 Results and Discussions

3.1 Synthesis of Dipropargyl Monomers 1–3

While dipropargyl monomers 2 and 3 have been previously reported, dipropargyl monomer 1 was synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and propargylamine adapting the same experimental conditions [46]. After reaction of these two compounds in NMP for 7 h at 50 °C, the cyclodehydration of the amide-acid intermediate was promoted by the addition of a sodium acetate solution in a 2:1 mixture of acetic anhydride/pyridine and further heating at 70 °C for 10 h (Scheme 1). Dipropargyl monomer 1 was then obtained as a white powder in 61.3% yield after purification by recrystallization. Its structure and purity were confirmed by 1H and 13C NMR spectroscopy (Figs. S1 and S2) as well as by high-resolution electrospray ionization mass spectrometry.

Scheme 1
scheme 1

Synthesis of dipropargyl monomer 1

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3.2 Synthesis of Poly(1,2,3-triazole imide)s 5–7

PTIs 5–7 were synthesized by AA + BB CuAAC polyaddition between stoichiometric amounts of dipropargyl monomers 1–3 and α,ω-diazido tetraethylene glycol 4 (Scheme 2). Polyaddition reactions were carried out during 48 h at 60 °C in NMP (6 wt% of AA + BB monomers) using CuIP(OEt)3 and DIPEA as catalytic system (respectively 0.02 and 1.0 equiv. according to alkyne and azide functionalities) [48]. Polymerization media remained homogeneous throughout the reaction and poly(1,2,3-triazole imide)s (PTIs) 5–7 were obtained in yields ranging from 64 to 79% after two precipitations in Et2O and drying under reduced pressure. At room temperature PTIs 5–7 are soluble at 1 mg/mL in polar aprotic solvents such as DMF, DMSO and NMP, while insoluble in heptane (nC7H16), MeOH, Et2O and ethyl acetate (EtOAc) (Table 1). However, while PTI 7 is soluble at 1 mg/mL in acetone, tetrahydrofuran (THF) and dichloromethane (CH2Cl2), and soluble at 10 mg/mL in chloroform (CHCl3) and acetonitrile (CH3CN), PTIs 5 and 6 are insoluble in these solvents. The broader solubility of 7 most probably results from the presence of the hexafluoroisopropylidene groups.

Scheme 2
scheme 2

Synthesis of poly(1,2,3-triazolium imide)s 8–10

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Table 1 Solubility of poly(1,2,3-triazole imide)s 5–7 and poly(1,2,3-triazolium imide)s 8–10
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1H NMR spectroscopy of PTIs 5–7 was thus carried out in DMSO-d6 (Fig. 1). Occurrence of the CuAAC polyaddition reaction was confirmed by the appearance of 1,2,3-triazole signals at ca. 8.05 ppm. This was also corroborated by the significant downfield shifts of the methylene proton signals neighboring the 1,2,3-triazole groups after polyaddition (see as example Fig. S3 for the stack of 1H NMR spectra of monomers 1, 4 and PTI 5). While 1H NMR spectra of PTIs 6 and 7 did not show any chain-end signals, propargyl chain-end signals could be observed at 4.39 and 3.27 in the 1H NMR spectrum of PTI 5. The number average polymerization degree (Xn) of PTI 5 could thus be calculated by comparing the signal of the propargyl chain-ends at 4.39 ppm and the signals of e.g. the N-1 methylene groups at 4.82 ppm. The obtained value (Xn = 31) corresponds to a number average molar mass (Mn) of ca. 9.8 kg/mol. Due to the absence of chain-end signals, the Mn values of PTIs 6 and 7 should thus be significantly higher than the value calculated for 5.

Fig. 1
figure 1

1H NMR spectra (25 °C, DMSO-d6) of PTIs 5–7 (*peaks assigned to propargyl chain-ends)

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3.3 Synthesis of Poly(1,2,3-triazolium imide)s 8–10

PTIs 5–7 were then submitted to N-alkylation reaction using iodomethane and subsequent anion exchange reaction using LiTFSI to afford the corresponding TFSI-containing poly(1,2,3-triazolium imide)s (TPTIs) 8–10 (Scheme 2). As generally observed [2], TPTIs 8–10 are soluble in a broader range of solvents compared to their neutral PTI precursors 5–7 due to the presence of highly delocalized 1,2,3-triazolium/TFSI ion pairs. Indeed, while still insoluble in heptane and EtOAc, and still soluble at 1 mg/mL in DMF, DMSO and NMP such as PTIs 5–7, TPTIs 8–10 are also soluble at 1 mg/mL in CH3CN (Table 1). Solubility in other tested solvents is highly structure-dependent and TPTI 10, most probably due to the presence of hexafluoroisopropylidene groups, exhibit the broadest solubility as it is soluble at 1 mg/mL in acetone, THF and CH2Cl2 while soluble at 10 mg/mL in MeOH and CHCl3.

Solubility of TPTIs 8–10 in DMF allowed their characterization by SEC using 0.01 M LiTFSI in DMF as eluent and calibration against PS standards (Fig. 2). TPTIs 8–10 exhibit monomodal traces with Mn values ranging from 15.4 to 46.9 kg/mol and chain dispersities (Ð) ranging from 1.47 to 1.95 (Table 2). However, Mn values are biased by the use of PS calibration as the distinct chemical structures of dipropargyl monomers most probably induce different solubility, chain stiffness and radius of giration of the chains in the SEC eluent. For instance, TPTI 8 exhibit an intermediate Mn value of 22.6 kg/mol although it is the only sample of the series where propargyl chain-end signals could be detected by 1H NMR. Its comparably low Ð value most probably stems from the fractionation of polymer chains during precipitations but in this case higher Mn values should be obtained. There is however a fair agreement between the Mn values obtained from 1H NMR of 5 and SEC of 8 (Mn = 18.6 and 22.6 kg/mol, respectively). TPTI 9 exhibit the highest Mn value (Mn = 46.9 kg/mol) and a Ð value which is the closest to the theoretical value of 2.0 (Ð = 1.95) expected for a step growth polymerization process. However, whereas no chain-ends could be observed by 1H NMR, TPTI 10 exhibit the smallest Mn value (Mn = 15.4 g/mol). Therefore, whereas SEC provides reliable chain dispersity values, the Mn values should be addressed carefully and only the Mn value calculated by 1H NMR of TPTI 8 should be considered. It can thus be concluded that TPTIs 9 and 10 should have comparable chain lengths much higher than for TPTI 8 due to the absence of noticeable chain-end signals in their 1H NMR spectra.

Fig. 2
figure 2

SEC traces (25 °C, DMF 0.01 M LiTFSI) of TPTIs 8 (solid line), 9 (dashed line) and 10 (dotted line)

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Table 2 Physical properties of PTIs 5–7 and TPTIs 8–10
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1H NMR spectroscopy of TPTIs 8–10 was carried out in DMSO-d6 (Fig. 3). The quantitative downfield shifts of the chemical displacement of the 1,2,3-triazolium protons (from ca. 8.05 ppm for PTIs 5–7 to ca. 9.01 ppm for TPTIs 8–10) and the appearance of the N-3 methyl signal at ca. 4.36 ppm with a 1:3 ratio attested the completion of the N-alkylation reaction. Additionally, the chemical displacements of most proton signals of the polymer backbones were shifted downfield as a result of the different electron delocalization of the 1,2,3-triazolium groups compared to the parent 1,2,3-triazole ones (see as example Fig. S4 for the stack of 1H NMR spectra of PTI 5 and TPTI 8). The structures of TPTIs 8–10 were further corroborated by 13C and 19F NMR spectroscopy (Figs. S5 and S6).

Fig. 3
figure 3

1H NMR spectra (25 °C, DMSO-d6) of TPTIs 8–10 (*peak assigned to propargyl chain-ends)

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3.4 Physical Properties of Poly(1,2,3-triazole imide)s 5–7 and Poly(1,2,3-triazolium imide)s 8–10

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to investigate the thermal properties of PTIs 5–7 and TPTIs 8–10 (Table 2). All samples are amorphous materials as their DSC traces exhibit a single transition corresponding to their glass transition temperature (Tg). Due to the presence of flexible tetraethylene glycol segments Tg values are rather low for polyimide materials as they range from – 19 to 97 °C for PTIs 5–7 and from 27 to 44 °C for TPTIs 8–10. The differences in Tg values for each series are mainly related to the flexibility of the dipropargyl monomer. As generally observed [2], thermal stability of TPTIs 8–10 is lower than their PTIs 5–7 neutral precursors (Fig. 4). After a weight loss at 200 °C that could not be precisely attributed, all samples exhibit a steep weight loss at ca. 400 °C for PTIs 5–7 and ca. 350 °C for TPTIs 8–10. Temperatures for 20% weight loss values (Td20) show that the main difference in thermal stability for both series is due to the lower stability of 1,2,3-triazolium groups compared to 1,2,3-triazole ones. For both series Td20 values decreases according to the following order Td20(5 and 8) > Td20(6 and 9) > Td20(7 and 10). The generally observed enhanced thermal stability characteristic of polyimide materials is thus significantly altered by the non-aromatic structure of the triethylene glycol segments as well as the weaker thermal stability of 1,2,3-triazole/1,2,3-triazolium units compared to imide ones.

Fig. 4
figure 4

TGA traces of (top) PTIs 5–7 and (bottom) TPTIs 8–10

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3.5 Elaboration of TPTI 8–10 Self-Standing Membranes by Hot Press

TPTIs 8–10 were pressed between two Teflon sheets at temperatures ca. 20 °C above their Tgs. Although ca. 25 µm thick self-standing membranes could be obtained from TPTIs 8 and 9, TPTI 10 yielded brittle membranes that broke into pieces after trying to separate the membrane from the Teflon sheets (Fig. 5). This is probably due to the lower Tg value of 10 although the differences in Mn, chain stiffness and entanglement might also impact the processability and mechanical properties of the different membranes. Nevertheless, even if self-standing membranes could be obtained from TPTIs 8 and 9, their mechanical properties were insufficient to mount them on a permeation cell in order to perform the characterization of their gas separation properties which was the initial motivation of this study.

Fig. 5
figure 5

Optical images of ca. 25 µm thick membranes of TPTIs 8 (left), 9 (middle) and 10 (right)

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4 Conclusion

We have developed a series of TPTIs by a three-step synthetic approach combining CuAAC polyaddition, N-alkylation of the resulting PTIs and anion exchange to introduce TFSI counter anions. The structures of neutral PTIs 5–7 were confirmed by 1H NMR spectroscopy while those of TPTIs 8–10 were characterized by SEC as well as 1H, 13C and 19F NMR spectroscopy. However, although having bisphtalimide segments, the presence of triethylene glycol segments and 1,2,3-triazole/1,2,3-triazolium units afforded materials with Tg values and thermal stabilities significantly lower than other class of neutral and ionic polyimides. These moderate properties impeded the preparation of robust polymer membranes required for the study of their gas permeation properties. Further studies regarding the synthesis and gas permeation properties of cross-linked TPTI membranes are currently undergoing.