Introduction

Liquid crystalline polymers (LCPs) offer the advantageous combination of the anisotropy of liquid crystals with the attractive bulk properties and processing possibilities of polymers. On the other hand, owing to their efficient photoisomerization and photoinduced anisotropy, polymers with azobenzene moieties have potential utilities in holographic data storage [1, 2], optical switching devices [3, 4], diffractive optical elements [5], and other optical and photonic applications.

In the past few years, “click” reactions, as termed by Sharpless et al. [6], have gained a great deal of attention due to their high specificity, quantitative yields, and near perfect fidelity in the presence of most functional groups. The most popular “click” chemistry reaction is the Huisgen dipolar cycloaddition reaction between an azide and an alkyne, leading to 1,2,3-triazole [7–9]. The great potential of this coupling procedure for the construction of well defined (functional) polymers [10–16], bioconjugated polymers [17, 18], and polymers with complex topologies [19–24] has been quickly recognized and is the subject of intensive research.

In our previous work [25], we described the first synthesis of a side-chain liquid crystalline polymer containing the cholesteryl moiety (Ch-SCLCP) by combining the ring-opening polymerization and “click” chemistry. Recently, the preparation of side-chain liquid crystalline azo-polymers [26–28] by “click” chemistry was also reported. Tang [29] and Senthil [30] reported the synthesis of main-chain liquid crystalline polymers via “click” chemistry. However, to our knowledge, there is no report on the synthesis of main-chain liquid crystalline azo-polymer via “click” chemistry.

We describe herein the first synthesis of a main-chain liquid crystalline azo-polymer via “click” chemistry and the characterization of the resulting polymer.

Experimental

Caution: Because of the high explosive characteristics of compounds bearing azide group, they should be handled in very small amounts, and all reactions with these compounds must be conducted in a well designed hood, using a safety shield or barrier to protect against possible explosion.

Materials

Propargyl alcohol (99 %) was used as received from Acros Organics. Cu(PPh3)3Br was prepared by the reported procedure [31]. Dimethylformamide (DMF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), and dichloromethane were purchased from Tianjin Kewei Ltd. and distilled from CaH2. All other chemicals were purified according to conventional methods or used as received.

Characterization

1H NMR (400 MHz) spectra were recorded at room temperature on a Varian-400 spectrometer. CDCl3 or DMSO-d 6 was used as the solvent. The chemical shifts were referenced relative to the solvent. IR spectra were recorded on a Bio-Rad FTS-135 spectrometer. The spectral resolution was maintained at 2 cm−1. Molecular weights and molecular weight distributions were measured with a Waters 1515 gel permeation chromatograph (GPC) equipped with a Waters 2410 differential refractometer as detector. Three Styragel columns (HR2, HR4, and HR6) were used in series. GPC samples were run at 40 °C with THF as the mobile phase at a flow rate of 1 mL/min, and the columns were calibrated using narrow distribution polystyrene standards obtained from Polymer Laboratories Ltd. UK. The phase transition temperature of the polymer was determined with a NETZSCH DSC204 calorimeter under nitrogen atmosphere, at the heating and cooling rates of 10 °C/min. High purity indium and zinc were used to calibrate the calorimeter. The thermogravimetric analysis (TGA) was performed on a NETZSCH TG209 instrument, using nitrogen as purge gas, operating from 50 to 800 °C with a heating rate of 10 °C/min. Polarized optical microscopy (POM) was performed on an Olympus BX51M microscope fitted with a Linkam TH600 hot stage and a LinkamTMS94 thermal controller. To obtain micrographs, a powder sample was placed between two glass plates. The UV–Vis study of the solution of polymer in chloroform was performed on a Hitachi U-3000 spectrophotometer at room temperature.

Synthetic procedures

Synthesis of 6-chlorohexyl-4-methylbenzenesulfonate

A suspension of sodium hydroxide (6.0 g, 0.15 mol) and 6-chlorohexan-1-ol (10.0 g, 73.2 mmol) in water (50 mL) was cooled with ice/water bath, and a solution of p-toluenesulfonyl chloride (15.0 g, 78.7 mmol) in THF (50 mL) was added slowly over 2 h. After being stirred for additional 2 h, the mixture was extracted twice with dichloromethane, washed with water, and dried over anhydrous MgSO4. After removal of MgSO4 and solvent, 15.0 g of colorless liquid product was obtained (yield, 84.2 %).

1H NMR (DMSO-d 6): 7.75–7.78 (d, 2H, ArH), 7.44–7.47 (d,2H,ArH), 3.97–4.01 (t, 2H, O-CH2-), 3.53–3.57 (t, 2H, Cl-CH2-), 2.41 (s, 3H, Ar-CH3), 1.52–1.64 (m, 4H, -CH2-), 1.18–1.30 (m, 4H,-CH2-).

Synthesis of 1-chloro-6-(propargyloxy)hexane

To a cold solution of propargyl alcohol (3.8 g, 67.8 mmol) in DMF (30 mL), NaH (2.4 g, 0.10 mol) was added under argon atmosphere. The mixture was stirred for 1.5 h, and then the solution of 6-chlorohexyl-4-methylbenzenesulfonate (CHOTs) (30.0 g, 0.10 mol) in DMF (20 mL) was added slowly over 3 h. The reaction mixture was stirred for additional 24 h and quenched with H2O (300 mL). The mixture was extracted twice with dichloromethane, washed with water, and dried over Na2SO4. The resulting solution was concentrated under vacuum and then purified by column chromatography eluting with mixture of hexane and ethyl acetate (v/v = 20:1) to give 5.0 g of 1-chloro-6-(propargyloxy)hexane (CHOP) (yield: 42.2 %).

1H NMR (CDCl3): 4.12–4.13 (d, 2H, Cl-CH2-), 3.49–3.55(m, 4H, -CH2-O-CH2-), 2.43–2.44 (t, 1H, -CCH), 1.74–1.81 (m, 2H, -CH2-), 1.57–1.62 (m, 2H, -CH2-), 1.37–1.48 (m, 4H, -CH2-).

Synthesis of 4,4′-bis((6-(propargyloxy)hexyl)oxy) biphenyl

A solution of CHOP (1.17 g, 6.7 mmol), 4,4′-biphenol (0.50 g, 2.7 mmol) and anhydrous potassium carbonate (1.12 g, 8.1 mmol) in DMF (30 mL) was stirred at 80 °C for 24 h. The reaction mixture was then poured into 100 mL of water. The orange precipitate was separated and purified by twice recrystallization from ethanol, giving 0.65 g of 4,4′-bis((6-(propargyloxy)hexyl)oxy) biphenyl (BPH) (yield, 52.4 %; mp, 105 °C).

1H NMR (CDCl3): 7.44–7.46 (d, 4H, Ar-H), 6.92–6.94 (d, 4H, Ar-H), 4.13–4.14 (d, 4H, -O-CH2-CCH), 3.97–4.00 (t, 4H, Ar-O-CH2-), 3.51–3.55 (t, 4H, -O-CH2-), 2.41–2.42 (t, 1H, -CCH), 1.77–1.84 (m, 4H, -CH2-), 1.62–1.61 (m, 4H, -CH2-), 1.46–1.51 (m, 8H, -CH2-). IR (KBr, cm–1): 3,287 (≡C-H); 2,109 (C≡C).

Synthesis of 4-carboxy-4′-hydroxy-azobenzene

4-carboxy-4′-hydroxy-azobenzene (CHA) was prepared according to the previous literature [32] as follows: Cold NaNO2(3.8 g, 55.1 mmol) aqueous solution (30 mL water) was added to the mixture of 4-aminobenzoic acid (6.9 g, 50.3 mmol) in hydrochloric acid (15 mL, wt 36 %) at 0 °C and stirred for 20 min. Then, the solution of phenol (4.8 g, 50.6 mmol) and NaOH (7.2 g, 180.0 mmol) in water (80 mL) was added over 30 min at 0 °C. After being stirred for 2 h with ice/water bath, the pH value of the solution was adjusted to about 3 with diluted hydrochloric acid (1 M). The orange precipitate was separated and purified by twice recrystallization from saturated NaHCO3 aqueous solution, giving 6.5 g of CHA (yield, 53 %).

1H NMR (DMSO-d 6): 8.00–8.02 (d, 2H, Ar-H), 7.76–7.79 (d, 2H, Ar-H), 7.70–7.72 (d, 2H, Ar-H), 6.94–6.97 (d, 2H, Ar-H).

Synthesis of 4-(6-chlorohexyloxy)carbonyl-4′-(6′-chlorohexyloxy)azobenzene

A solution of CHA (2.7 g, 11.0 mmol), CHOTs (6.4 g, 22.0 mmol) and anhydrous potassium carbonate (3.0 g, 21.7 mmol) in DMF (100 mL) was stirred at 50 °C for 18 h. Then the mixture was poured into 100 mL of water. The orange precipitate was separated and purified by twice recrystallization from ethanol to give 4.3 g of 4-(6-chlorohexyloxy)carbonyl-4′-(6′-chlorohexyloxy)azobenzene (CHCHA) (yield, 81 %; mp, 64–65 °C).

1H NMR (CDCl3): 8.15–8.17 (d, 2H, Ar-H), 7.89–7.95 (m, 4H, Ar-H), 7.00–7.02 (d, 2 h, Ar-H), 4.34–4.37 (t, 2H, ArCOO-CH2-), 4.04–4.07 (t, 2H, Ar-O-CH2-), 3.54–3.58 (m, 4H, Cl-CH2-), 1.78–1.83 (m, 8H, -CH2-), 1.50–1.59 (m, 8H,-CH2-).

Synthesis of 4-(6-azidohexyloxy)carbonyl-4′-(6′-azidohexyloxy)azobenzene

A solution of CHCHA (2.0 g, 3.9 mmol) and NaN3 (0.60 g, 9.2 mmol) in DMSO (100 mL) was stirred at 65 °C for 24 h. Then the mixture was poured into 100 mL of water. The orange precipitate was separated and purified by twice recrystallization from ethanol to give 1.6 g of 4-(6-azidohexyloxy)carbonyl-4′-(6′-azidohexyloxy)azobenzene (AHCHA) (yield, 78 %; mp, 39–40 °C).

1H NMR (CDCl3): 8.15–8.17 (d, 2H, Ar-H), 7.89–7.95 (m, 4H, Ar-H), 7.00–7.02 (d, 2H, Ar-H), 4.34–4.37 (t, 2H, ArCOO-CH2-), 4.04–4.07 (t, 2H, Ar-O-CH2-), 3.27–3.31 (m, 4H, N3-CH2-), 1.80–1.86 (m, 4H, -CH2-), 1.64–1.67 (m, 4H, -CH2-), 1.49–1.52 (m, 8H, -CH2-). IR (KBr, cm–1): 2100(−N3), 1711(−C=O).

Synthesis of azo-polymer (BPHCHA) via “click” reaction

A solution of BPH (0.10 g, 0.21 mmol), AHCHA (0.11 g, 0.22 mmol), Cu(PPh3)3Br (4.10 mg, 4.40 × 10−3 mmol), and N,N-Diisopropylethylamine (0.17 g, 1.31 mmol) in CH2Cl2 (5 mL) was stirred at 30 °C for 24 h under argon atmosphere. The resulting solution was precipitated into methanol, giving 0.19 g of BPHCHA with 90 % yield. GPC: Mn=17,446 g/mol; Mw/Mn=1.31.

Results and discussion

The synthetic route for the preparation of the main-chain liquid crystalline azo-polymer is shown in Scheme 1.

Scheme 1
scheme 1

Reaction scheme for the preparation of 4,4′-bis((6-(propargyloxy)hexyl)oxy) biphenyl (BPH), 4-(6-azidohexyloxy)carbonyl-4′-(6′-azidohexyloxy)azobenzene (AHCHA), and the subsequent copper(I)-catalyzed “click” reaction

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CHOTs was synthesized through tosylation of 6-chlorohexan-1-ol in THF/H2O with NaOH as the base. CHOTs reacted then with propargyl alcohol in DMF to yield CHOP. BPH was obtained by alkylation of 4,4′-biphenol under the Williamson aryl-alkyl ether synthesis using CHOP by the nucleophilic displacement reaction in DMF. The structure of BPH was confirmed by 1H NMR (Fig. 1) with the C≡CH signal at 2.4 ppm and FT-IR analysis (Fig. 2) with the alkyne C-H stretching frequency at 3,287 cm−1, C≡C signal at 2,109 cm−1. To prepare the azobenzene-functionalized diazides, CHA was synthesized according to the reported procedure [32]. CHA reacted then with CHOTs to yield CHCHA. The expected azobenzene-functionalized diazides, namely AHCHA was prepared through azidation of CHCHA in DMSO at 65 °C for 24 h. FT-IR spectrum of AHCHA is shown in Fig. 2. The azide and carbonyl group showed absorption band at 2,100 and 1,711 cm−1, respectively.

Fig. 1
figure 1

1HNMR spectra of AHCHA, BPH, and the azo-polymer BPHCHA

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Fig. 2
figure 2

FT-IR spectra of AHCHA, BPH, and the azo-polymer BPHCHA

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The “click” reaction between BPH and AHCHA was conducted in CH2Cl2 at 30 °C, with (PPh3)3CuBr as the catalyst, in the presence of N,N-diisopropylethylamine. As evaluated with GPC, the number average molecular weight and the molecular weight distribution of the resulting azo-polymer BPHCHA were 17,446 g/mol and 1.31, respectively. 1H NMR spectrum of BPHCHA is shown in Fig. 1. The peaks at 7.5 and 4.6 ppm were assigned to the proton of triazole ring and its neighboring methylene protons, respectively. Moreover, the disappearance of the C≡CH signal at 2.4 ppm was a clear indication of the “click” reaction. The “click” reaction was further confirmed by FT-IR analysis (Fig. 2) with the disappearance of the alkyne C-H stretching frequency at 3,287 cm−1, C≡C signal at 2,109 cm−1. It can be seen from Fig. 2, weak absorption band at 2,100 cm−1 still exhibited, which should be ascribed to the polymer chain end group, i.e., azide group, resulting from the stoichiometry excess of AHCHA in the “click” reaction.

The phase transition behavior of BPH, AHCHA, and the resulting azo-polymer BPHCHA was investigated by differential scanning calorimetry (DSC) and polarized optical microscopy (POM) with heating/cooling rate of 10 °C/min.

In Fig. 3, DSC thermograms of BPH of the fist cooling and second heating scans are shown. Three exothermic peaks at 100.1, 66.3, and 40.6 °C were observed in the thermogram of the fist cooling scan. The microscopic observation suggested that these three peaks corresponded to liquid–crystal, crystal–crystal and crystal–crystal transitions, respectively. During the second heating scan, endothermic peak at 45.5 °C should be the crystal–crystal transition, and the other at 106.2 °C corresponds to the melting of BPH.

Fig. 3
figure 3

DSC thermograms of BPH in the first cooling and second heating mode (10 °C/min)

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Figure 4 shows the POM images of AHCHA. Upon heating, the sample melted at 39 °C. Above this point, we observed a liquid crystalline texture [Fig. 4a], which was transformed into the dark-field view at 69 °C. Upon cooling, the birefringence appeared at 60 °C, and focal-conic texture [Fig. 4b] was seen in the crystals. To support this finding, AHCHA was investigated by means of DSC. Upon the first cooling (Fig. 5), exothermic peak at 65.7 °C represented transition from isotropic phase to liquid crystalline phase, and the second exothermic peak at 14.3 °C corresponded to the crystallization of AHCHA. In the second heating scan, two endothermic peaks at 39.2 and 69.7 °C were associated with melting and isotropization transitions, respectively.

Fig. 4
figure 4

POM images of AHCHA. a At 60.0 °C on the first heating scan. b At 30.7 °C on the first cooling scan

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Fig. 5
figure 5

DSC thermograms of AHCHA in the first cooling and second heating mode (10 °C/min)

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Figure 6 show DSC thermograms of the resulting azo-polymer BPHCHA of the first cooling and second heating scans. In the cooling scan, two exothermic transitions were exhibited at 125.2 and 110.2 °C, which should ascribed to the isotropic to liquid crystal and liquid crystal to crystal transitions, respectively. During the second heating scan, only one endothermic peak at 149.9 °C corresponding to the melting transition showed on the DSC thermogram. The phase transitions were confirmed by the POM. Sample was heated from room temperature to 165 °C, and the isotropic melting temperature was observed to be 150 °C. When it cooled to 130 °C and was annealed for 15 min, Schlieren texture was observed (Fig. 7). The DSC and POM studies showed that the resulting azo-polymer BPHCHA exhibited monotropic mesomorphic behavior, which is similar to the work, reported by Macknight [33] and Chiellini [34].

Fig. 6
figure 6

DSC thermograms of BPHCHA in the first cooling and the second heating mode (10 °C/min)

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Fig. 7
figure 7

POM image of BPHCHA at 130 °C on the first cooling scan

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The UV–Vis absorption spectra of the solution of azo-polymer BPHCHA (0.048 mg/mL) in chloroform are given in Fig. 8. Two absorption bands centered around 365 and 455 nm related, respectively, to the π–π* and n–π* electronic transitions of the azobenzene group. Upon irradiation with 365 nm light, azobenzene group undergoes a trans-to-cis photoisomerization process. Correspondingly, the absorption band at 365 nm decreased obviously, while the absorption band at 455 nm slightly increased.

Fig. 8
figure 8

UV-visible absorption spectra of the solution of BPHCHA in chloroform at room temperature a before UV irradiation and b after 120 seconds irradiation with 365 nm light

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The TGA curve of azo-polymer BPHCHA is shown in Fig. 9. The weight loss of 5.0 % was at 354.7 °C. The fairly high thermal stability for the azo-polymer BPHCHA might be attributed to the introduction of rigid groups, namely biphenyl groups and triazole rings onto the polymer chain [35].

Fig. 9
figure 9

Thermogravimetric curve of BPHCHA

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Conclusion

Alkyne-terminated and azide-terminated compounds, BPH and AHCHA, were prepared through the copper-catalyzed “click” reaction between the alkyne groups on BHP and the azide groups on AHCHA, a main-chain liquid crystalline azo-polymer BPHCHA was obtained. The “click” reaction was confirmed by FT-IR, 1H NMR, and GPC studies. The phase transition behavior of BPH, AHCHA, and the resulting azo-polymer BPHCHA was investigated by DSC and POM. A liquid crystalline phase from 39 to 69 °C for AHCHA upon heating was observed under POM. Schlieren texture was observed at 130 °C for BPHCHA on cooling from 150 °C and annealed at 130 °C for 15 min. DSC and POM studies showed that BPHCHA exhibited monotropic mesomorphic behavior. The UV–Vis analysis showed trans-to-cis photoisomerization of azobenzene group when the solution of azo-polymer BPHCHA in chloroform was irradiated with 365 nm light. Moreover, the azo-polymer BPHCHA exhibited fairly high thermal stability as investigated with TGA.