A one-pot strategy for the synthesis and functionalization of hyperbranched polytriazoles

ABSTRACT

Herein we report a one-pot strategy for the synthesis and functionalization of hyperbranched polytriazoles by means of the Huisgen 1,3-dipolar cycloaddition of an AB₂-type monomer containing an alkyne and two azide groups. The AB₂ monomer is synthesized starting from dimethyl 5-hydroxyisophthalate in four steps with an overall yield of 58%, and the synthesis and purification are straightforward. The synthesis of end-capped hyperbranched polytriazoles (HBPTs) can be achieved via the Huisgen cycloaddition of the AB₂ monomer using only heat, followed by functionalization with either sulfonic acid (-SO₃H) or pentafluorophenyl (-Ph-F₅) end groups. The resulting functional hyperbranched polytriazoles, HBPT-Ph-SO₃H and HBPT-Ph-F₅, are characterized by ¹H NMR and FT-IR spectroscopies as well as gel-permeation chromatography. The film is fabricated simply by blending the two polymers, and the ion exchange capacity and ion conductivity are measured.

1. Introduction

Hyperbranched polymers (HBPs) are highly branched macromolecules with three-dimensional dendritic architectures [1,2]. Unlike structurally well-defined dendrimers, HBPs have an irregular branching structure and exhibit a broad molecular weight distribution [3,4]. Despite the relatively imperfect structure, HBPs often maintain the advantages of dendrimers, such as low viscosity, good solubility, and high density of functional groups [5,6]. Most importantly, highly functional HBPs can be easily produced by one-pot synthesis that simplifies synthetic procedures and purification steps, which is in sharp contrast to the dendrimers [7–9].

1,2,3-triazole is an attractive functional group because it is chemically stable against oxidation, reduction, and hydrolysis, even at high temperature while exhibiting both antioxidant properties and proton transfer abilities [10–12]. The 1,2,3-triazole functionality particularly serves as a useful building block for the preparation of dendritic polymers as a result of the synthetic accessibility through the copper-catalyzed azide-alkyne cycloaddition (CuAAC), the so-called ‘click’ reaction [13,14]. CuAAC has proven to be a highly effective tool for one-pot synthesis of HBPs [9,15,16]. For example, Gao and co-workers have reported the CuAAC living chain-growth polymerization of AB₂-type monomers to obtain well-defined HBPs in one-pot, in which the CuAAC plays a key role in achieving high molecular weights, low polydispersities, and high degrees of branching [17–19]. However, the CuAAC reaction presents a significant challenge with respect to complete removal of the residual copper that can form complexes with 1,2,3-triazoles having the large dipole moment [20–22]. Indeed, the use of copper to prepare polytriazole polymers limits their practical applications, for example, cation exchange membranes where the proton conductivity is reduced to some extent by residual copper ions [23,24].

The most effective strategy for overcoming this challenge involves a metal-free system for the synthesis of 1,2,3-triazoles, i.e., thermal Huisgen 1,3-dipolar cycloaddition (HC), which is simply induced by heating [25–29]. Despite the limited regioselectivity in the resulting triazoles, the HC reaction provides a facile route for the synthesis of triazole polymers, eliminating the need for copper catalysts. Since Voit and co-workers first reported the use of thermal 1,3-dipolar cycloaddition for the synthesis of HBPs, some further research has been conducted, but there remains a need for further investigation [30–33].

In this study, we developed a one-pot strategy for the synthesis and functionalization of hyperbranched polymers via the thermal azide-alkyne Huisgen 1,3-dipolar cycloaddition. Our approach as a greener alternative to the conventional CuAAC-based methods offers a more environmentally friendly synthetic pathway by avoiding the residual metal contamination. The key structural feature of the resulting polymers reported in this study is a hyperbranched polytriazole (HBPT) scaffold functionalized with either hydrophilic sulfonic acid (-SO₃H) or hydrophobic pentafluorophenyl (-Ph-F₅) end groups. We further investigated the potential application of the HBPT-Ph-SO₃H and HBPT-Ph-F₅ as a proton exchange membrane (PEM), with the membrane fabricated by blending the two polymers.

2. Method

2.1. Materials

Acetone, tetrahydrofuran (THF), dichloromethane (DCM), N,N-dimethylformamide (DMF), ethyl acetate (EA), n-hexane, magnesium sulfate anhydrous (MgSO₄), sodium chloride (NaCl), concentrated sulfuric acid (H₂SO₄), 35% hydrochloric acid (HCl) were purchased from DUKSAN pure chemicals Co., Ltd. Dimethyl 5-hydroxyisophthalate, phosphorous tribromide (PBr₃), sodium azide (NaN₃) were purchased from Tokyo chemical industry Co., Ltd. Propargyl bromide, phenol, pentafluorophenol were purchased from Thermo Fisher scientific, Inc. Potassium carbonate (K₂CO₃), sodium bicarbonate (NaHCO₃) were purchased from SAMCHUN chemical Co., Ltd. Lithium aluminium hydride was purchased from DAEJUNG chemicals & metals, Inc. For thin layer chromatography (TLC) analysis throughout this work, Merck precoated TLC plates (silica gel 60 F₂₅₄) were used.

2.2. Characterization

¹H-NMR and ¹³C-NMR spectra were measured in CDCl₃ or DMSO-d₆ using a Bruker Avance III HD 400 spectrometer. ¹⁹F-NMR spectrum was measured in DMSO-d₆ using a Spinsolve 80 MHz benchtop NMR. The mass spectrum was acquired using a JMS-T200GC AccuTOF GCx-plus mass spectrometer with a field desorption ion source. Infrared spectra were obtained using a PerkinElmer Spectrum3 Fourier transform infrared (FT-IR) spectrometer. The surface morphology of the membranes was observed using a field emission scanning electron microscope (FE-SEM) SU-70. Thermogravimetric analysis (TGA) was performed on a TGA2 Mettler Toledo.

The molecular weight distributions of the synthesized polymers were analyzed using a Waters Alliance gel-permeation chromatography (GPC) system. The GPC measurements were conducted on a DMF system equipped with a series of three Styragel columns (HR3, HR4 and HR5E). Polystyrene standards were used for calibration, and the mobile phase consisted of DMF containing 0.01 M LiBr as an additive to suppress polymer-column interactions. The GPC analysis was performed at a controlled temperature of 40°C, with a flow rate of 1.0 mL/min. M_n, M_w, and PDI represent the number-average molecular weight, weight-average molecular weight, and polydispersity index, respectively.

The ion exchange capacity (IEC) was measured by Mohr’s method. A dried, acidified membrane was immersed in NaCl solution for 24 h, allowing proton exchange with sodium ions. The resulting solution was titrated with NaOH using phenolphthalein, and the IEC was calculated by using the following equation [34]:

$$\mathit{IEC}\mathit{\hspace{0.17em}}=\mathit{\hspace{0.17em}}\left(C_{\mathit{NaOH}}\mathit{\hspace{0.17em}}\times \mathit{\hspace{0.17em}}{V}_{\mathit{NaOH}}\right)\mathit{\hspace{0.17em}}/\mathit{\hspace{0.17em}}{W}_{\mathit{Dry}}\mathit{\hspace{0.17em}}$$ (1)

where C_NaOH and V_NaOH are the concentration and volume of the NaOH solution, respectively, and W_dry is the weight of the dry membrane.

The proton conductivity of the membrane was assessed using a four-probe electrochemical impedance spectroscopy (EIS) set-up. A Gamry 1010e impedance analyzer was employed in the potentiostat mode with a perturbation amplitude of 10 mV and a frequency ranging from 1 Hz to 15 MHz. The measurements were conducted under fully hydrated conditions at 30°C and 80°C, with the conductivity cell submerged in a deionized water bath. The conductivity (σ) was calculated using the equation:

$$\mathit{\sigma }=\mathit{d}/\left(\mathit{w}\times \mathit{t}\times \mathit{r}\right)$$ (2)

where d represents the inter-electrode distance (1 cm), w and t are the width and thickness of the membranes, respectively, and r (Ω) is the membrane resistance determined from the Nyquist plot.

2.3. Synthesis of monomer 4 and terminal groups 5 and 6

2.3.1. Synthesis of 1,3-dimethyl 5-(2-propyn-1-yloxy)-1,3-benzenedicarboxylate (1) [35]

A mixture of dimethyl 5-hydroxyisophthalate (10.0 g, 47.6 mmol), propargyl bro-mide (5.82 mL, 52.3 mmol), and anhydrous K₂CO₃ (13.2 g, 95.5 mmol) in acetone (200 mL) was refluxed at 60°C for 16 h. After the reaction was completed, the solvent was removed by rotary evaporation. The mixture was dissolved in ethyl acetate and washed with distilled water. The organic layer was dried with MgSO₄, filtered and evaporated under reduced pressure to give 1 as a white solid. Yield: 11.6 g (98%). ¹H NMR (400 MHz, DMSO-d₆, δ): 8.12 (t, J = 1.5 Hz, 1 H), 7.76 (d, J = 1.5 Hz, 2 H), 4.99 (d, J = 2.4 Hz, 2 H), 3.89 (s, 6 H), 3.67 (t, J = 2.4 Hz, 1 H).

2.3.2. Synthesis of 5-(2-propyn-1-yloxy)-1,3-benzenedimethanol (2) [36]

To a stirred solution of the ester compound 1 (11.6 g, 46.7 mmol) in THF (230 mL) was added LiAlH₄ (5.33 g, 140 mmol) carefully in 5 portions at 0°C. The reaction mixture was then refluxed at 80°C for 16 h. After the reaction was completed, sodium sulfate decahydrate was added to the flask at 0°C until no further gas evolution was observed. The mixture was filtered and the filtrate was evaporated under reduced pressure. The crude product was recrystallized by using ethyl acetate (150 mL) and n-hexane (190 mL) to give 2 as a white solid. Yield: 6.90 g (78%). ¹H NMR (400 MHz, DMSO-d₆, δ): 6.88 (s, 1 H), 6.78 (s, 2 H), 5.19 (t, J = 5.7 Hz, 2 H), 4.75 (d, J = 2.4 Hz, 2 H), 4.44 (d, J = 5.7 Hz, 4 H), 3.56 (t, J = 2.4 Hz, 1 H).

2.3.3. Synthesis of 1,3-bis(bromomethyl)-5-(2-propyn-1-yloxy)benzene (3)

To a stirred solution of the hydroxyl compound 2 (6.90 g, 35.9 mmol) in DCM (140 mL) was added PBr₃ (13.5 g, 143 mmol) dropwise at −10°C. After 30 min, the external cooling was removed, and the mixture was stirred at room temperature. Upon completion, a saturated sodium bicarbonate solution was added to quench the reaction, and the mixture was extracted with DCM. The combined organic layer was dried with MgSO₄, filtered and evaporated under reduced pressure. The crude product was directly used for the next step without further purification. Yield: 8.94 g (78%)

2.3.4. Synthesis of AB₂-type monomer 4

To a stirred solution of 3 (8.90 g, 28.0 mmol) in DMF (100 mL) was added NaN₃ (14.6 g, 225 mmol). After the mixture was stirred at 25°C for 16 h, water was added and the mixture was extracted with diethyl ether. The organic layer was dried with MgSO₄, filtered and evaporated under reduced pressure to give 4 as a yellow oil. Yield: 6.67 g (98%) ¹H NMR (400 MHz, DMSO-d₆, δ): 6.99 (s, 3 H), 4.83 (d, J = 2.4 Hz, 2 H), 4.45 (s, 4 H), 3.60 (t, J = 2.4 Hz, 1 H). ¹³C NMR (400 MHz, DMSO-d₆, δ): 157.6, 137.7, 121.0, 114.4, 79.0, 78.5, 55.6, 53.3. HRMS(TOF) m/z [M]⁺: C₁₁H₁₀N₆O⁺ calcd. 242.0916, found. 242.0912.

2.3.5. Synthesis of (2-propyn-1-yloxy)benzene (5) [37]

A mixture of phenol (5.10 g, 54.2 mmol), propargyl bromide (6.63 mL, 59.6 mmol), and anhydrous K₂CO₃ (9.33 g, 67.5 mmol) in acetone (100 mL) was refluxed at 60°C for 16 h. After the reaction was completed, the solvent was removed by rotary evaporation. The reaction mixture was dissolved in ethyl acetate and washed with distilled water. The organic layer was dried with MgSO₄, filtered and evaporated under reduced pressure to give 5 as a yellow oil. Yield: 6.89 g (97%) ¹H NMR (400 MHz, CDCl₃, δ): 7.31 (m, 2 H), 7.00 (m, 3 H), 4.70 (d, J = 2.4 Hz, 2 H), 2.53 (t, J = 2.4 Hz, 1 H).

2.3.6. Synthesis of 1,2,3,4,5-pentafluoro-6-(2-propyn-1-yloxy)benzene (6) [38]

A mixture of 2,3,4,5,6-pentafluorophenol (4.10 g, 22.3 mmol), propargyl bromide (3.09 mL, 27.8 mmol), and anhydrous K₂CO₃ (3.85 g, 27.9 mmol) in acetone (80 mL) was refluxed at 60°C for 16 h. After the reaction was completed, the solvent was removed by rotary evaporation. The reaction mixture was dissolved in ethyl acetate and washed with distilled water. The organic layer was dried with MgSO₄, filtered and evaporated under reduced pressure to give 6 as a yellow oil. Yield: 4.72 g (95%) ¹H NMR (400 MHz, CDCl₃, δ): 4.83 (d, J = 2.4 Hz, 2 H), 2.56 (t, J = 2.4 Hz, 1 H).

2.4. Synthesis of hyperbranched polytriaozles (HBPTs)

2.4.1. Synthesis of HBPT-Ph

A mixture of AB₂-type monomer 4 (3.00 g, 12.4 mmol) in DMF (6 mL) was stirred at 100°C for 16 h. The polymerization reaction progress was monitored by ¹H NMR spectroscopy. Upon completion, 5 (3.27 g, 24.7 mmol) was added to terminate the polymerization. After stirring at 100°C for 16 h, the mixture was diluted with DCM and precipitated into cold methanol. The obtained precipitate was collected and dried under vacuum. Yield: 4.14 g (89%) ¹H NMR (400 MHz, DMSO-d₆, δ): 8.33–8.05 (br, 1,4-disubstituted 1,2,3-triazole-H), 7.90–7.71 (br, 1,5-disubstituted 1,2,3-triaozle-H), 7.32–7.08 (br, Ar-H), 7.07–6.56 (br, Ar-H), 5.70–5.31 (br, Ar-CH₂-triazole), 5.27–4.79 (br, triazole-CH₂-O-Ar).

2.4.2. Synthesis of HBPT-Ph-SO₃H

For the sulfonation of HBPT-Ph, 4.14 g of HBPT-Ph was dissolved in concentrated H₂SO₄ (82.8 mL) and the resulting 5% (w/v) solution was stirred at 25°C for 20 h. The mixture was precipitated into ethanol (800 mL), and the precipitate was redissolved in an aqueous solution of NaOH (1 M, 10 mL) followed by precipitation into ethanol (200 mL). The precipitated crude product was dissolved in distilled water (40 mL), and 35% aqueous HCl was added until no more precipitate formed. The precipitate was collected and dried under vacuum. Yield: 2.83 g (56%) ¹H NMR (400 MHz, DMSO-d₆, δ): 8.46–8.16 (br, 1,4-disubstituted 1,2,3-triazole-H), 7.93–7.78 (br, 1,5-disubstituted 1,2,3-triaozle-H), 7.60–7.44 (br, Ar-H), 7.25–6.54 (br, Ar-H), 5.75–5.36 (br, Ar-CH₂-triazole), 5.29–4.74 (br, triazole-CH₂-O-Ar).

2.4.3. Synthesis of HBPT-Ph-F₅

A mixture of AB₂-type monomer 4 (3.00 g, 12.4 mmol) in DMF (6 mL) was stirred at 100°C for 16 h. The polymerization reaction progress was monitored by ¹H NMR spectroscopy. Upon completion, 6 (5.50 g, 24.8 mmol) was added to terminate the polymerization. After stirring at 100°C for 16 h, the mixture was diluted with DCM and precipitated into cold methanol. The obtained precipitate was collected and dried under vacuum. Yield: 5.11 g (89%) ¹H NMR (400 MHz, DMSO-d₆, δ): 8.33–8.06 (br, 1,5-disubstituted 1,2,3-triaozle-H), 7.95–7.90 (br, 1,5-disubstituted 1,2,3-triaozle-H), 7.03–6.56 (br, Ar-H), 5.70–5.43 (br, Ar-H), 5.39–4.81 (br, Ar-CH₂-triazole and triazole-CH₂-O-Ar). ¹⁹F NMR (80 MHz, DMSO-d₆, δ): −153.41 – −154.35 (d, J = 21.6 Hz, 2F), −160.84 – −162.85 (m, 3F)

2.5. Fabrication of the blended membrane

The HBPT-Ph-SO₃H/HBPT-Ph-F₅ at the required weight ratio of 8:2 was dissolved in DMF to form a 20% (w/v) solution. The resulting solution was cast onto clean Petri dishes and subjected to a temperature-control vacuum oven. The cast film was initially dried in an oven at 80°C overnight. Subsequently, a stepwise heating process was employed, involving heating at 90°C, 100°C, and 110°C for 2 h each, followed by a final heating step at 120°C for 1 h. After cooling, the glass plate was submerged in deionized water for detachment of the membrane. The membrane was then immersed in water for over 24 h, accompanied by multiple washes to ensure the complete removal of any residual DMF. The final membrane was dried under vacuum at 120°C for 24 h. The prepared sample was stored in deionized water or under dry conditions until further use.

3. Results and discussion

3.1. Synthesis and characterization of hyperbranched polymers

The synthetic route for HBPT-Ph-SO₃H and HBPT-Ph-F₅ is described in Scheme 1. AB₂-type monomer 4 with an alkyne and two azide groups is designed to form triazole polymers, in which multiple peripheral azide groups can be further reacted with alkyne-functionalized chain-terminators such as 5 and 6 allowing for the end group modification. The polymerization and subsequent functionalization can be achieved in one-pot simply by heating the reaction mixtures. The synthesis of 1–3, 5 and 6 have been reported before [35–38].

Scheme 1.

Synthesis of (a) AB₂ monomer 4 and chain-terminators 5 and 6, and (b) hyperbranched polytriazoles HBPT-Ph-SO₃H and HBPT-Ph-F₅.

Firstly, dimethyl 5-hydroxyisophthalate was O-propargylated with propargyl bromide to produce 1, which was then reduced using LiAlH₄ to yield 2 containing two hydroxyl groups. The bromination of 2 was carried out with PBr₃ to obtain 3 that was used for the next step without further purification. The final AB₂ monomer 4 was prepared by conversion of the two bromo groups in 3 to azides, and characterized by the ¹H-NMR spectrum of 4 which displays the terminal alkyne proton at δ 3.06 ppm and α-protons of the azide groups at δ 4.45 ppm (Figure 1(a)). The synthesis and purification were straightforward as shown by an overall yield of 58% for the four steps. Note that AB₂ monomer 4 needs to be kept at −20°C to avoid unwanted polymerization observed at room temperature.

Figure 1.

¹H-NMR spectra of (a) AB₂ monomer 4, (b) crude HBPT-N₃, (c) HBPT-Ph, (d) HBPT-Ph-SO₃H, and (e) HBPT-Ph-F₅.

With the AB₂ monomer containing an alkyne and two azide groups in hand, the formation of highly branched polytriazoles was investigated using thermal Huisgen-1,3-dipolar cycloaddition (HC) in the absence of copper catalysts and any other additives. In our initial attempt to obtain the triazole polymers, an appropriate polymerization condition was determined by investigating the effect of concentration and reaction time on the polymer properties such as solubility and molecular weight distribution. The triazole polymers prepared at concentrations above 0.5 g/mL were not soluble in common organic solvents while no further changes in the molecular weight distribution were observed from 16 to 40 hours. Thus, in this study, the HC-based polymerization of 4 was conducted at 100°C for 16 hours using a concentration of 0.5 g/mL in DMF, affording multiple peripheral azide-functionalized triazole polymer HBPT-N₃. Subsequently, the crude HBPT-N₃ was reacted with alkyne-functionalized chain-terminators 5 and 6 to yield HBPT-Ph and HBPT-Ph-F₅, respectively. For the end group functionalization, the subsequent HC reaction continued at 100°C for additional 16 hours. It is noteworthy that the post-polymerization modification via the thermal HC could be achieved in one-pot and the resulting hyperbranched polytriazoles could be readily purified by precipitation in methanol. Furthermore, the phenyl groups in the HBPT-Ph were sulfonated to obtain HBPT-Ph-SO₃H decorated with hydrophilic end groups. The successful synthesis of both hydrophilic HBPT-Ph-SO₃H and hydrophobic HBPT-Ph-F₅ demonstrates the versatility of our synthetic approach, enabling the facile incorporation of diverse functionalities into hyperbranched polymers.

The one-pot polymerization and end group modification starting from AB₂ monomer 4 were successfully monitored by comparing the ¹H-NMR spectra of 4 and HBPTs as shown in Figure 1. For example, the HC-based polymerization of 4 could be confirmed by the loss of the terminal alkyne proton at δ 3.06 ppm in the ¹H-NMR spectrum of the crude HBPT-N₃ (Figure 1(b)). This spectrum, taken from an unpurified aliquot of the thermal HC, displays the characteristic broad signals of a polymer. Key signals include the newly formed triazole protons (δ 8.37–7.69 ppm) and the methylene protons adjacent to the terminal azide groups (δ 4.45 ppm), confirming the formation of the desired azide-terminated intermediate. The successful functionalization of HBPT-N₃ was also verified by the complete loss of the α-protons of the azide groups at δ 4.45 ppm in the ¹H-NMR spectra of HBPT-Ph and HBPT-Ph-F₅ (Figure 1(c,e)). Furthermore, for HBPT-Ph-F₅, the successful incorporation of the pentafluorophenyl groups was confirmed by ^{1 9} F NMR spectroscopy (Figure S10). Note that the formation of 1,4- and 1,5-disubstituted triazole regioisomers was observed in the thermal HC reactions as shown in the ¹H-NMR spectra which revealed the two main signals corresponding to the regioisomeric triazoles at δ 8.37–8.12 and δ 7.87–7.69 ppm (Figure 1(b,c,e)). The integration of the respective peaks showed a ratio of approximately 1 : 0.6 for all synthesized polymers (HBPT-N₃, HBPT-Ph, and HBPT-Ph-F₅), indicating a preference for the formation of the 1,4-disubstituted triazole isomer. This observation provides valuable insight into the regioselectivity of the polymerization under our one-pot thermal conditions. Finally, complete sulfonation was confirmed by the ¹H-NMR spectrum of HBPT-Ph-SO₃H showing the characteristic downfield shift of the signal (c) from δ 7.31–7.10 to δ 7.64–7.40 ppm (Figure 1(d)). In addition, GPC analyses of all the hyperbranched polymers were carried out, and the number-average molecular weights (M_n) and polydispersity indices (PDIs) were found to be 6.7 kDa and 3.45 for HBPT-Ph, 39.6 kDa and 2.80 for HBPT-Ph-SO₃H, and 8.3 kDa and 2.96 for HBPT-Ph-F₅ (Figure 2).

Figure 2.

GPC traces of polymers HBPT-Ph, HBPT-Ph-SO₃H, and HBPT-Ph-F₅.

3.2. Fabrication of membranes

The membrane was fabricated by blending HBPT-Ph-SO₃H and HBPT-Ph-F₅ at a weight ratio of 8:2. The ratio was determined to provide the optimal conductivity while maintaining thc chemical stability and avoiding the phase separation. Interestingly, increasing the proportion of HBPT-Ph-F₅ in the blend led to phase separation during solvent removal process, for example, at a 7:3 ratio. The blending process could be monitored by comparing the FT-IR spectra of polymers (HBPT-Ph-SO₃H and HBPT-Ph-F₅) and their blended membrane as shown in Figure 3. For both polymers, the absorption bands corresponding to the triazole and alkyl aryl ether (R-O-Ar) were observed at 1465 and 1296 cm⁻¹, respectively [39,40]. In the IR spectrum of HBPT-Ph-SO₃H, the characteristic peaks were observed at 1496 cm⁻¹ for the aromatic C = C stretching vibration, and at 1052 and 1026 cm⁻¹ for the sulfonic acid (-SO₃H), along with a broad band around 3400 cm⁻¹ corresponding to the water molecules surrounding the sulfonic acid groups (Figure 3(a)) [12]. On the other hand, the aromatic C = C and C-F stretching vibrations of HBPT-Ph-F₅ appeared at 1511 and 994 cm⁻¹, respectively (Figure 3(b)) [41]. As shown in Figure 3(c), the IR spectrum of the blended membrane clearly showed the presence of the characteristic signals for the aromatic C = C stretching vibrations at 1496 and 1511 cm^{− 1}, consistent with those observed in HBPT-Ph-SO₃H and HBPT-Ph-F₅, respectively. This observation proved the successful blending of the hydrophilic HBPT-Ph-SO₃H and hydrophobic HBPT-Ph-F₅, consistent with the surface morphology observed by scanning electron microscope (SEM) (Figure S12). The thermal stability of the blended polymers was also evaluated by thermogravimetric analysis (TGA), as shown in the Supporting Information (Figure S13).

Figure 3.

FT-IR spectra of (a) HBPT-Ph-SO₃H, (b) HBPT-Ph-F₅, and (c) blended HBPT-Ph-SO₃H/HBPT-Ph-F₅ (8/2) membrane.

3.3. Ion exchange capacity (IEC) and ionic conductivity of membranes

The ion exchange capacity (IEC) is a measure of the concentration of ionizable groups such as sulfonic acid present in the fabricated membrane. The IEC value correlates with the proton transfer ability and its increase may improve the water absorption property and ionic conductivity of the membrane. The IEC of the HBPT-Ph-SO₃H/HBPT-Ph-F₅ (8/2) was measured to be 1.38 meq/g. In addition, the electrochemical impedance spectroscopy (EIS) measurements of the membrane in the fully hydrated state were conducted at 30 and 80°C and the Nyquist plots were obtained. The average thickness of the membranes used for these measurements was approximately 140 μm. The proton conductivities calculated by Equation (2) were found to be 3.02 and 4.69 mS/cm at 30 and 80°C, respectively.

4. Conclusion

In this study, we successfully demonstrated a one-pot strategy for the synthesis and functionalization of hyperbranched polytriazoles via thermal Huisgen 1,3-dipolar cycloaddition using an AB₂ monomer which has one terminal alkyne and two azide groups. This environmentally friendly approach offers a greener alternative to traditional CuAAC-based methods by eliminating the need for copper catalysts. The versatility of this method was proved by synthesizing two distinct hyperbranched polymers; HBPT-Ph-SO₃H and HBPT-Ph-F₅, functionalized with hydrophilic sulfonic acid groups and hydrophobic pentafluorophenyl moieties, respectively. Furthermore, we were able to fabricate a blended membrane composed of HBPT-Ph-SO₃H and HBPT-Ph-F₅ with completely different hydrophilic/hydrophobic properties. The properties of the membrane were confirmed by investigating its IEC and ionic conductivity. This study provides a valuable synthetic platform for the development of functional hyperbranched polymers with potential applications in various fields.

Funding Statement

The author(s) reported there is no funding associated with the work featured in this article.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials.

Supplementary Information

Supplemental data for this article can be accessed online at https://doi.org/10.1080/15685551.2025.2547342