Synthesis and chemical properties of polyphenols with oligoaniline pendant groups

Materials and measurements

Chemicals were purchased and used without further purification. Solvents were purified by distillation and stored under nitrogen atmosphere.

Microanalysis of C, H, and N was carried out with a Yanagimoto Type MT-5 CHN autocorder. Gel permeation chromatography (GPC) analyses were carried out by a Toso HLC 8020 with polystyrene gel columns (TSKgel G2000H_HR and TSKgel GMH_HR-M) using a DMF solution of LiBr (0.006 mol L⁻¹) as an eluent with RI and UV detectors and a Jasco 830 refractometer with polystyrene gel columns (K-803 and K-804) using chloroform as an eluent with a RI detector. Infrared and NMR spectra were recorded on a JASCO FT/IR-410 PLUS spectrophotometer and a JEOL AL-400 spectrometer, respectively. UV-vis spectra were obtained on a JASCO V-560 spectrometer. Cyclic voltammetry (CV) of the polymers was conducted in a dimethyl sulfoxide (DMSO) solution containing 0.10 mol L⁻¹ [Et₄N]BF₄ with a BAS 100B.

Synthesis of monomer-1

A tetrahydrofuran (THF) solution (50 mL) of hydroquinone (0.66 g, 6.0 mmol) and titanium tetraisopropoxide (4.26 g, 10 mmol) was added to a THF solution (40 mL) of 4-aminophenylaniline (0.92 g, 5.0 mmol). After the reaction solution was stirred at 70 °C for 30 h, precipitate from the solution was filtered off and the solvent in filtrate was removed in vacuo. The resulting solid was purified by silica gel column chromatography (eluent = ethyl acetate/hexane (v/v = 1/3)). The solvents were removed in vacuo and resulting solid was dried in vacuo to give monomer-1 as a blue powder (0.21 g, 15%). ¹H-NMR (400 MHz, DMSO-d ₆): δ 8.86 (s, 1H), 7.69 (s, 1H), 7.43 (s, 1H), 7.11 (t, J = 8.4 Hz, 2H), 6.93 (d, J = 8.8 Hz, 2H), 6.85 (t, J = 8.0 Hz, 6H), 6.64 (d, J = 8.4 Hz, 3H). Anal calcd. for C₁₈H₁₆N₂O: C, 78.24; H, 5.84; N, 10.14. Found: C, 78.15; H, 5.56; N, 10.25.

Synthesis of monomer-2

An N,N-dimethylformamide (DMF) solution (10 mL) of 4-aminobiphenylamine (2.3 g, 13 mmol) and 4-hydroxydiphenylamine (2.3 g, 13 mmol) was added to a mixture of DMF (20 mL), hydrochloric acid (16 mL), and water (155 mL). After ammonium persulfate (2.9 g, 13 mmol) was added to the solution at −10 to −5 °C, the reaction solution was stirred at the temperature for 5 h. The precipitate from the reaction mixture was collected by filtration and dried in vacuo to give blue solid, which was added to hydrazine monohydrate (5 mL). After the reaction mixture was stirred at room temperature for 12 h, hydrazine was removed in vacuo to give blue solid, which was dissolved in DMF (1 mL). The DMF solution was poured into water (300 mL) and the resulting precipitate was collected by filtration and dried in vacuo to give monomer-2 as a blue powder (2.2 g, 49%). ¹H-NMR (400 MHz, DMSO-d₆ ): δ 8.80 (s, 1H), 7.70 (s, 1H), 7.49 (s, 1H), 7.33 (s, 1H), 7.12 (t, J = 8.0 Hz, 2H), 6.95 (d, J = 8.8 Hz, 2H), 6.86–6.89 (m, 14H), 6.83 (d, J = 7.6 Hz, 2H), 6.64 (d, J = 8.4 Hz, 3H). Anal calcd. for C₂₄H₂₁N₃O: C, 78.45; H, 5.76; N, 11.44. Found: C, 78.35; H, 5.89; N, 11.26.

Synthesis of polymer-1(HRP)

Monomer-1 (0.17 g, 0.60 mmol) and HRP (2.3 mg, 1000 units) purchased from Sigma Chemical Co. in a mixture of dimethyl sulfoxide (35 mL) and an aqueous phosphate buffer solution (pH 6.9, 15 mL) were placed in a round-bottomed flask. To the mixture, 500 µL of 35% hydrogen peroxide was added dropwise (less than 0.5 mmol H₂O₂ at each time) over 3 h. After the reaction mixture was stirred at room temperature for 24 h, the solvent was removed in vacuo. The resulting solid was washed with water, dried in vacuo, and reprecipitated from ether. The precipitate was collected by filtration and added to hydrazine monohydrate (5 mL). After the reaction mixture was stirred at room temperature for 12 h, hydrazine was removed in vacuo to give polymer-1(HRP) as a dark brown solid (0.078 g, 47%). ¹H-NMR (400 MHz, DMSO-d ₆): δ 6.79-7.25 (br). Anal calcd. for (C₁₈H₁₄N₂O)_n: C, 78.81; H, 5.14; N, 10.21. Found: C, 78.57; H, 4.91; N, 9.88.

Synthesis of polymer-2(HRP)

Polymer-2(HRP) was synthesized in an analogous method to polymer-1(HRP). ¹H-NMR (400 MHz, DMSO-d ₆): δ 6.68–7.12 (br). Anal calcd. for (C₂₄H₁₉N₃O)_n: C, 78.88; H, 5.24; N, 11.50. Found: C, 78.60; H, 5.26; N, 10.23.

Synthesis of polymer-1(Fe)

Monomer-1 (0.28 g, 1.0 mmol) and N,N′-bis(salicylidene)ethylenediamineiron (III) (0.036 g, 0.11 mmol) in DMF (5 mL) were placed in a round-bottomed flask. To the mixture, 500 µL of 35% hydrogen peroxide was added dropwise (less than 0.5 mmol H₂O₂ at each time) over 3 h. After the reaction mixture was stirred at room temperature for 24 h, the solvent was removed in vacuo. The resulting solid was washed with water, dried in vacuo, and added to hydrazine monohydrate (5 mL). After the reaction mixture was stirred at room temperature for 12 h, hydrazine was removed in vacuo to give polymer-1(Fe) as a dark brown solid (0.26 g, 94%). ¹H-NMR (400 MHz, DMSO-d ₆): δ 6.70–7.25 (br). Anal calcd. for (C₁₈H₁₄N₂O)_n: C, 78.81; H, 5.14; N, 10.21. Found: C, 78.59; H, 5,09; N, 0.89.

Synthesis of polymer-2(Fe)

Polymer-2(HRP) was synthesized in an analogous method to polymer-1(Fe). ¹H-NMR (400 MHz, DMSO-d ₆): δ 6.69–7.14 (br). Anal calcd. for (C₂₄H₁₉N₃O)_n: C, 78.88; H, 5.24; N, 11.50. Found: C, 79.21; H, 4.86; N, 11.13.

Synthesis of polymer-1OAc

Acetate anhydrate (0.5 mL) was added to a pyridine (1 mL) solution of polymer-1(HRP) (0.15 g). The reaction solution was stirred at 100 °C for 1 h and reprecipitated from ether (200 mL). The precipitate was collected by filtration and dried in vacuo to give polymer-1OAc as a black powder (0.13 g, 85%). ¹H NMR (400 MHz, DMSO-d ₆): δ 7.00-7.74 (br, 13H), 1.78 (s, 0.50H).

Synthesis of polymer-2OAc

Polymer-2OAc was synthesized in an analogous method to polymer-1OAc. ¹H-NMR (400 MHz, DMSO-d ₆): δ 6.72-7.30 (br, 18H), 1.75 (s, 0.49H).

Synthesis of monomers and polymers

The reactions of 4-aminophenylaniline with p-hydroquinone and 4-hydroxyphenylaniline yielded monomer-1 and monomer-2, respectively (Scheme 1).

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Scheme 1.

Synthesis of monomers.

The HRP-catalyzed polymerization of monomer-1 and monomer-2 using hydrogen peroxide as an oxidizing reagent in a mixture of 1,4-dioxane and phosphate buffer (pH = 7.4 or 6.9) resulted in 47 and 94% yields of polymer-1(HRP) and polymer-2(HRP), respectively (Scheme 2). The N,N′-bis(salicylidene)ethylenediamineiron(III)-catalyzed polymerization of the same monomers yielded polymer-1(Fe) and polymer-2(Fe) in 94 and 79% yields, respectively (Scheme 2b). The synthesis results are summarized in Table 1.

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Scheme 2.

Synthesis of polymers.

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Table 1.

Synthesis results.

Polymer	Yield (%)	M n a	M W a	Unit A′ b	Unit B′ b
Polymer-1(HRP)	47	1400	1900
Polymer-2(HRP)	72	4600	6100
Polymer-1(Fe)	94	3200	4000
Polymer-2(Fe)	79	5100	5400
Polymer-1(HRP)OAc	85			0.81	0.19
Polymer-2(HRP)OAc	90			0.85	0.15
Polymer-1(Fe)OAc	77			0.80	0.20
Polymer-2(Fe)OAc	82			0.78	0.22

^a Determined by GPC (eluent = DMF containing 0.006 mol L⁻¹ LiBr).

^b Estimated from ¹H-NMR spectra.

The molar ratios of the hydroxyphenylene (unit A) and oxyphenylene (unit B) units in the obtained polymers were determined using ¹H-NMR analysis. Accordingly, O-acetylated polymers, i.e. polymer-1(HRP)OAc, polymer-2(HRP)OAc, polymer-1(Fe)OAc, and polymer-2(Fe)OAc, were synthesized by the reactions of polymer-1(HRP), polymer-2(HRP), polymer-1(Fe), and polymer-2(Fe) with acetic acid anhydride, respectively (Scheme 3).

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Scheme 3.

Synthesis of O-acetylated polymers.

The polymers obtained in this study were soluble in DMF, DMSO, and N-methyl-2-pyrrolidone (NMP). The M _n and M _w values of the obtained polymers determined by GPC measurements are summarized in Table 1. The M _n values of polymer-1(Fe) and polymer-2(Fe) were slightly higher than those of polymer-1(HRP) and polymer-2(HRP). This is in agreement with previous reports that N,N′-bis(salicylidene)ethylenediamineiron(III)-catalyzed polymerization of phenols yielded PPs with higher molecular weights than those synthesized by HRP-catalyzed polymerization. ¹⁹ The structures of obtained compounds were determined by ¹ H- and ¹³ C-NMR spectroscopy and elemental analysis.

¹H-NMR spectra

Figure 1 shows the ¹H-NMR spectra of the monomers and the polymers in DMSO-d ₆. Peak assignments are shown in the figure. The OH protons of monomer-1 and monomer-2 were observed at δ 8.86 and 8.80, respectively. Monomer-1 exhibited two peaks corresponding to NH protons at δ 7.69 and 7.43, whereas monomer-2 exhibited three signals corresponding to NH protons at δ 7.70, 7.94, and 7.33. The peak corresponding to the protons adjacent to the hydroxyl group of monomer-1 and monomer-2 was observed at δ 6.64. This peak decreased in the ¹H-NMR spectrum of polymer-1(HRP) and disappeared in the ¹H-NMR spectrum of polymer-2(HRP), which suggests that oxidative coupling occurred at the 2-positions of the hydroxyphenyl group. This result is consistent with the fact that HRP-catalyzed polymerization of phenols proceeds predominantly at this 2-position. ^{1 –4} The phenylene protons of polymer-1(HRP) and polymer-2(HRP) were observed as broad peaks in the range of δ 6.9–7.7. The peaks corresponding to the CH₃ groups of polymer-1(HRP)OAc and polymer-2(HRP)OAc were observed at δ 1.78 and 1.75, respectively. The relative integrals of the peaks corresponding to the CH₃ and the phenylene protons suggest that the molar ratios of unit A′ and unit B′ in polymer-1(HRP)OAc and polymer-2(HRP)OAc were 81 : 19 and 85 : 15, respectively. The molar ratios of unit A′ and unit B′ in polymer-1(Fe)OAc and polymer-2(Fe)OAc were also estimated from the ¹H-NMR spectra to be 80 : 20 and 78 : 22, respectively. These values are in good agreement with the fact that HRP-catalyzed polymerization of phenol derivatives yields polymers with a higher content of the hydroxyphenylene unit. ^{1 –4}

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Figure 1.

¹H-NMR spectra of monomer-1, monomer-2, polymer-1(HRP), polymer-2(HRP), and polymer-1(HRP)OAc in DMSO-d ₆.

IR spectra

Figure 2 shows the IR spectra of monomer-1, polymer-1(HRP), polymer-1(HRP)OAc, monomer-2, and polymer-2(HRP). An absorption due to N–H stretching vibrations of monomer-1 and monomer-2 was observed both as a sharp peak at 3371 cm⁻¹. In contrast, the absorption due to the N–H stretching vibrations of polymer-1(HRP) and polymer-2(HRP) was observed both as a broad peak at 3365 cm⁻¹. The observation of absorptions corresponding to N–H stretching vibrations in the IR spectra of the polymers suggests that the NH groups remain in the polymers. The stronger absorptions due to the N–H stretching vibrations of monomer-2 and polymer-2(HRP) than those of monomer-1 and polymer-2(HRP) correspond to the greater numbers of NH groups in monomer-2 and polymer-2(HRP). A broad absorption corresponding to the O–H stretching vibrations of polymer-1(HRP) was observed around 3500 cm⁻¹. Additionally, the absorption corresponding to the O–H stretching vibration disappeared and that corresponding to the C=O stretching vibrations of the acetyl group appeared at 1762 cm⁻¹ in the IR spectrum of polymer-1(HRP)OAc. These IR data suggest that O-acetylation proceeded to completion. The IR spectra of polymer-1(Fe) and polymer-2(Fe) were almost the same as those of polymer-1(HRP) and polymer-2(HRP).

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Figure 2.

IR spectra of monomer-1, polymer-1(HRP), polymer-1(HRP)OAc, monomer-2, and polymer-2(HRP).

UV-vis spectra

Figure 3 shows the UV-vis spectra of DMSO solutions of monomer-2 and polymer-2(HRP) in the absence and presence of hydrazine monohydrate. The UV-vis spectra of DMSO solutions of monomer-1 and monomer-2 each exhibited an absorption maximum (λ _max) at 310 nm corresponding to the π–π* transition of the benzonoid structures in the OANs of the monomers. Additionally, an absorption corresponding to the quinoid structures, which were generated by the oxidation of the benzonoid structures by air, appeared at 600 nm. The DMSO solutions of polymer-1(HRP) and polymer-2(HRP), which were partly oxidized in air, exhibited absorptions corresponding to the benzonoid and quinoid structures in the OAN groups at 300 and 600 nm, respectively. It has been reported that luecoemeraldine-type polyanilines are partly oxidized in air, resulting in the formation of quinoid structures. ^20,21 The addition of hydrazine monohydrate to the DMSO solutions of the partly oxidized monomers and polymers reduced their quinoid structures to form benzonoid structures. This conversion was confirmed by the disappearance of the absorption at 600 nm after treatment with hydrazine monohydrate, as shown in Figure 3. These spectral changes suggest that the NH groups remained in the polymers. This is also supported by the abovementioned IR results.

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Figure 3.

UV-vis spectra of DMSO solutions of monomer-2 and polymer-2(HRP) in the absence (thick lines) and presence (thin lines) of hydrazine monohydrate.

Cyclic voltammograms

Figure 4 shows cyclic voltammograms (CVs) of monomer-1, monomer-2, polymer-1(HRP), and polymer-2(HRP). The CVs of polymer-1(HRP) and monomer-1 exhibited two peaks corresponding to the electrochemical oxidation of the NH groups in the side chain at 0.45 and 0.62 V and 0.50 and 0.58 V (vs Ag⁺/Ag), respectively (Figure 4(a)). The fact that the difference between the two oxidation peak potentials of monomer-1 is smaller than that of polymer-1(HRP) is attributed to facile electron exchange between the nitrogen atoms of one-electron oxidized monomer-1, as shown in Scheme 4.

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Figure 4.

Cyclic voltammograms of monomer-1, monomer-2, polymer-1(HRP), and polymer-2(HRP) in a DMSO solution of [Et₄N]BF₄ (0.1 mol L⁻¹). Sweep rate was 50 mVs⁻¹.

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Scheme 4.

Electrochemical oxidation of monomer-1.

In contrast, such electron exchange is considered to be difficult in oxidized polymer-1(HRP) because of the presence of units A and B in the polymer. Monomer-2 exhibited three peaks corresponding to the electrochemical oxidation of the NH groups in the side chain at 0.18, 0.50, and 0.76 V (vs Ag⁺/Ag) (Figure 4(b)), whereas polymer-2(HRP) exhibited a clear peak at 0.19 V and two broad peaks at 0.48 and 0.68 V. However, the corresponding reduction (p-dedoping) peak does not appear in the CVs of the polymers and monomers; this is probably because of the formation of a stable adduct between the electrochemically oxidized polymer and BF₄ ⁻. Electrochemically oxidized π-conjugated polymers have been reported to form stable adducts with BF₄ ⁻ during cyclic voltammetry measurements. ²²