Article pubs.acs.org/IECR
Catalytic Oxidation of 2,7-Dihydroxynaphthalene Ali Bilici,†,§ Fatih Doğan,‡,§ and Iṡ met Kaya*,§ †
Lapseki Vocational School, Ç anakkale Onsekiz Mart University, 17020, Lapseki, Ç anakkale, Turkey Faculty of Education, Secondary Science and Mathematics Education, Ç anakkale Onsekiz Mart University, 17100, Ç anakkale, Turkey § Faculty of Sciences and Arts, Department of Chemistry, Polymer Synthesis and Analysis Laboratory, Ç anakkale Onsekiz Mart University, 17020 Ç anakkale, Turkey ‡
S Supporting Information *
ABSTRACT: Two polymer−metal complexes were used for catalytic oxidative polymerization of 2,7-dihydroxynaphthalene (DHN) with the aim of exploiting new and potent catalysts. The structure of the resulting poly(2,7-dihydroxynaphthalene) (PDHN) was confirmed by various spectral techniques: UV−vis, FT-IR, 1H NMR, 13C NMR, 1H−1H COSY, and 1H−13C HSQC. The 3 and 6 positions of the DHN ring were suggested as main coupling sites. The effect of the reaction conditions on the monomer conversions and the polymer molecular weight was determined: yields in the range 74−99% and number-average molecular weights in the range 1700−83 000 g mol−1 were obtained. Thermal, optical, and electrochemical behaviors of the polymer were also studied. From fluorescence analyses, a case of negative solvatochromism for PDHN was established.
1. INTRODUCTION Polyphenols are usually produced by condensation of phenol derivates with the formaldehyde. However, the usage of toxic formaldehyde in industry is limited. Therefore, alternative polymerization processes for production of polyphenols are highly desired.1−6 Many catalytic systems such as enzymes, Schiff base metal complexes, and transition metal salts have been employed to achieve different polyphenol derivatives.7−12 The catalytic properties of Co(II)−salen type complexes for the oxidative polymerization of dihydroxynaphthalene derivatives were investigated by Habaue et al.13 Fukuoka and co-workers prepared poly(2,6-difluoro-1,4-phenylene oxide) using Fe− salen/H2O2 systems.14 Sasada et al. reported the regiocontrolled synthesis of poly(2,6-dihydroxy-1,5-naphthylene) with high molecular weights.9 The potential usage of this polymer as a photoresist material was investigated by Tsuchiya et al.11 The catalytic oxidative polymerization of various dihydroxynaphthalenes was carried out by Yamaguchi et al.12 Enzymecatalyzed and photocatalyzed oxidative polymerization of the aromatic monomers, however, yielded polymers with low molecular weights. It is therefore necessary to devise new highly efficient systems of catalytically oxidative polymerization.6 Many studies have been reported on the catalytic activities of polymer−metal complexes.15 Schiff base polymer−metal complex compounds may be used in various catalytic processes due to their intrinsic properties. They are polynuclear systems and contain more active centers.16 These compounds are insoluble in common organic solvents, and this feature can be useful for heterogeneous catalytic applications. Because of their polymeric network structure, they have higher thermal stabilities than those of corresponding Schiff base monomer− metal complexes.17 This enables their use at higher temperatures. Very recently, we have achieved the synthesis of poly-2,2′dihydroxybiphenyl by using both Schiff base monomer−Cu(II) © 2013 American Chemical Society
complex and Schiff base polymer−Cu(II) complex as catalysts and hydrogen peroxide as oxidant. Under selected conditions, the catalytic activities of monomer/polymer copper(II) complexes were studied, comparatively.18 In the present study, catalytic oxidative coupling polymerization of 2,7-dihydroxynaphthalene (DHN) is presented by using Schiff base polymer−copper(II) and −cobalt(II) complexes as catalysts and H2O2 as oxidant. The aim of varying the experimental conditions of the reaction was to obtain higher conversion and PDHN with higher molecular weight. For these reasons, the following parameters were studied using poly[2,3-bis[(2-hydroxy-3-methoxyphenyl)methylene]diaminopyridine] (PHMPMDAP)−Cu(II) and PHMPMDAP−Co(II) as representative catalysts: • effect of type and volume of solvent • effect of amount of catalyst • effect of H2O2 concentration • effect of reaction temperature
2. EXPERIMENTAL SECTION 2.1. Materials. DHN and solvents were purchased from Merck and used without further purification. Schiff base polymer, PHMPMDAP (poly[2,3-bis[(2-hydroxy-3methoxyphenyl)methylene]diaminopyridine]), PHMPMDAP−Cu complex, and PHMPMDAP−Co complex (Scheme 1) were prepared according to previously reported procedures.19 The general procedure is given below: 2,3-Bis[(2-hydroxy-3-methoxyphenyl)methylene]diaminopyridine (HMPMDAP) was prepared by the condensation of o-vanillin and 2,3-diaminopyridine in methanol Received: Revised: Accepted: Published: 104
June 3, 2013 November 20, 2013 December 12, 2013 December 12, 2013 dx.doi.org/10.1021/ie401735j | Ind. Eng. Chem. Res. 2014, 53, 104−109
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Scheme 1. (a) Bidentate and (b) Tetradentate Structures of PHMPMDAP [(M = Cu (II) and Co (II) Ions)]
Scheme 2. Catalytic Oxidative Polymerization of DHN
used to perform cyclic voltammetry (CV) measurements. The electrochemical cell consisted of an Ag wire pseudoreference electrode (RE), Pt wire as counter electrode (CE), and glassy carbon or transparent ITO/glass as working electrode (WE) immersed in 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) as the supporting electrolyte. The experiments were carried out under argon atmosphere. The potentials were calibrated to the ferrocene redox couple (E1/2) (Fc/Fc+) 0.41 V versus Ag/Ag+. All reported potentials were given versus Ag/ Ag+. The fluorescence spectra were recorded by use of a Schimadzu RF-5301PC spectrofluorophotometer. A Keithley 2400 electrometer was used to determine the solid state conductivity of the polymer. For this, a powdery sample was pressed on a hydraulic press and the obtained polymer pellet was doped with saturated iodine vapors at atmospheric pressure in a desiccator at 25 °C. 2.3. Polymerization. DHN (1 mmol) and catalyst (1 mmol %) in acetonitrile (3 mL) were placed in a 25 mL flask. A 100 μL volume of hydrogen peroxide (1 mmol) was added dropwise to the reaction mixture at room temperature, under air (Scheme 2). The reaction mixture was vigorously stirred at room temperature for 3 h. The precipitates formed were removed from the reaction mixtures by filtration, and then the solution was evaporated to obtain a brown precipitate. Both precipitates obtained were combined, washed with chloroform, and then solved in DMSO. After filtration, the black solution was reprecipitated in hexane. The obtained product was dried for the determination of the polymer yield.
achieved by boiling the mixture under reflux for 3 h. The monomer prepared by condensation was oxidized in alkaline medium with NaOCl solution to obtain polymer (PHMPMDAP), and polymer−metal complexes used as catalysts were prepared by the mixture of methanol solutions of Cu(AcO)2·4H2O and Co(AcO)2·4H2O with a tetrahydrofuran (THF) solution of polymer.19 HMPMDAP, FT-IR (cm−1): 3378 (Ar−OH), 1603 (CN), 1460, 1440, 1423 (ArC C), 1244 (C−O). PHMPMDAP, FT-IR (cm−1): 3400 (Ar− OH), 1620 (CN), 1603, 1560, 1463 (Ar−CC), 1245 (C− O). PHMPMDAP−Cu, FT-IR (cm−1): 3361 (Ar−OH), 1603 (CN), 1540, 1473, 1440 (ArCC), 1240 (C−O), 652 (Cu−N), 573 (Cu−O). PHMPMDAP−Co, FT-IR (cm−1): 3339 (Ar−OH), 1604 (CN), 1560, 1542, 1420 (ArC C), 1242 (C−O), 664 (Cu−N), 575 (Cu−O). 2.2. Instrumentation. UV−vis measurements were recorded in DMSO using a Perkin-Elmer Lambda 25. Fourier transform infrared (FT-IR) spectra were recorded on a PerkinElmer FTIR Spectrum One spectrometer. 1H NMR and 13C NMR spectra were recorded in DMSO-d6 by using a Bruker Avance (DPX-400 and 100.6 MHz, respectively) instrument. TMS was used as internal standard. Molecular weights were determined by gel permeation chromatography (GPC; Shimadzu Co. Japan) with a Macherey-Nagel GmbH & Co. (Germany) (100 Å and 7 nm diameter loading material) 7.7 mm i.d. × 300 mm columns, DMF/MeOH eluent (0.4 mL min−1, v/v, 4/1), polystyrene standards, and a refractive index detector. Differential scanning calorimetry (DSC) analyses were performed on Perkin-Elmer Pyris Sapphire with a heating rate of 20 °C min−1 under nitrogen flow. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer Diamond TG-DTA with a heating rate of 20 °C min−1 under nitrogen flow. CH Instruments 660 C electrochemical workstations were
3. RESULTS AND DISCUSSION The oxidative polymerization of DHN was performed using PHMPMDAP−Cu(II) and PHMPMDAP−Co(II) complexes and hydrogen peroxide as the catalysts and oxidizing agent, 105
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Table 1. Preparation of PDHN in Various Solventsa run 1 2 3 4 5 6
catalystb PHMPMDAP−Cu(II) PHMPMDAP−Co(II) PHMPMDAP−Cu(II) PHMPMDAP−Co(II) PHMPMDAP−Cu(II) PHMPMDAP−Co(II)
solvent acetonitrile acetonitrile dioxane dioxane THF THF
d
time (h)
yield (%)
Mnc (×104)
Mw/Mnc
3 3 3 3 3 3
92 87 74 58 66 60
4.9 2.7 2.9 1.7 3.6 2.1
2.3 2.4 3.1 2.8 2.7 2.4
a
The reaction was carried out with 1 mmol of DHN and 1 mmol of H2O2 (35%) in 3 mL of solvent at room temperature under air atmosphere. The amount of catalysts for DHN was 5 mol %. cDetermined by GPC using polystyrene standards (DMF). dA blank test for run 1 (without catalyst) was performed, and the molecular weight of product obtained was found to be 180 g/mol. b
respectively. In the absence of catalysts, low molecular weight products were obtained. In the presence of catalysts, monomer conversions and the molecular weights of the polymers were increased. Three different solvents, viz. THF, dioxane, and acetonitrile, were used to find the best solvent for the oxidative polymerization reaction of DHN, and the results are given in Table 1. The maximum percentage conversion of DHN obtained with various solvents was given as follows: for PHMPMDAP−Cu(II), 66% with THF, 74% with dioxane, and 92% with acetonitrile; for PHMPMDAP−Co(II), 60% with THF, 58% with dioxane, and 87% with acetonitrile. PDHN with the highest molecular weight was obtained in acetonitrile (runs 1 and 2). Thus, the oxidative polymerization of DHN was conducted in acetonitrile under air at room temperature. On the other hand, 3 mL of acetonitrile (MeCN) was found to be the best solvent amount to conduct the polymerization reaction. Increasing the amount of solvent caused a poor performance, and a decreased solvent amount (2 mL) was found to be insufficient to dissolve the monomer. Thus, polymerization reactions were carried out in 3 mL of MeCN. To evaluate to the catalytic performances of PHMPMDAP− Cu(II) and PHMPMDAP−Co(II), the polymerization of DHN was carried out with different catalyst concentrations under similar reaction conditions. These results are given in Table 2. The conversion of DHN increased with increasing PHMPMDAP−Cu(II) concentrations and reached its highest value (92%) at a catalyst amount of 5%. However, the conversion was almost constant with further amounts of catalyst. PDHN with the highest molecular weight (49 000 g mol−1) was obtained in the presence of 5.0% PHMPMDAP− Cu(II) (run 11). Higher catalyst concentration did not increase the molecular weight of PDHN. However, 3.0% PHMPMDAP−Co(II) seemed to be the most appropriate amount to obtain the polymer with the highest molecular weight (29 000 g mol−1; run 10). As seen in Table 2, the maximum conversion of DHN was accomplished when PHMPMDAP−Cu(II) was used. The polymerization was also performed at lower amounts of PHMPMDAP−Cu(II) (0.3%) (not given in Table 2), and PDHN having a molecular weight of 16 000 g mol−1 was obtained. To evaluate the effect of H2O2 concentration on the polymerization of DHN, various molar ratios of H2O2 to DHN (from 0.016 to 0.064 M) were studied. The percentage conversion of DHN increased with the increasing molar ratio of hydrogen peroxide (from 0.016 to 0.032 M) at constant molar ratios of DHN and catalyst. However, further increasing the molar ratio of H2O2 led to decrease the polymer yield for each catalyst. This may be attributed to formation of cross-linkings
Table 2. Effect of Catalyst Concentration on Polymerizationa run 7 8 9 10 11 12 13 14 15 16
catalystb PHMPMDAP−Cu(II) (1) PHMPMDAP−Co(II) (1) PHMPMDAP−Cu(II) (3) PHMPMDAP−Co(II) (3) PHMPMDAP−Cu(II) (5) PHMPMDAP−Co(II) (5) PHMPMDAP−Cu(II) (7) PHMPMDAP−Co(II) (7) PHMPMDAP−Cu(II) (10) PHMPMDAP−Co(II) (10)
time (h)
yield (%)
Mnc (×104)
Mw/Mnc
3
87
3.1
2.4
3
79
2.3
2.1
3
90
3.8
2.2
3
83
2.9
2.5
3
92
4.9
2.3
3
87
2.7
2.4
3
90
4.1
3.1
3
82
2.7
2.8
3
90
3.8
2.4
3
78
2.4
2.7
a
The reaction was carried out with 1 mmol of DHN and 1 mmol H2O2 (35%) in 3 mL of acetonitrile at room temperature under air atmosphere. bThe amount of catalysts for DHN (mol %) is in parentheses. cDetermined by GPC using polystyrene standards (DMF).
in polymer chains due to overreaction. In addition, the high hydrogen peroxide concentration may be also responsible for the decreased catalytic activities of the polymer complexes. As seen in Table 3, 1:1 mole ratio (0.032 M) was used to obtain the maximum degree of conversion and a product with a maximum molecular weight (runs 21 and 22). The reaction temperatures also influence the conversion of DHN and molecular weight of PDHN as seen in Table 4. Four different reaction temperatures (50, 60, 70, and 75 °C) were used while other reaction parameters were fixed. The molecular weight of PDHN increased from 55 000 to 83 000 g mol−1 as the temperature was increased from 40 to 75 °C for PHMPMDAP−Cu(II). This is may be due to the fast decomposition of H2O2 by increasing temperature. The highest molecular weight obtained was about 52 000 g mol−1 at 75 °C as PHMPMDAP−Co(II) was used (run 36). However, when the reaction temperature was increased from 25 to 75 °C, the color of the obtained polymer turned from light brown into black. As reported by Oguchi et al., the black tarry material is probably due to hydroxylation of the polymer.20 These results are consistent with our previous study.18 106
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Table 3. Effect of H2O2 Concentration on Polymerizationa run
catalystb
H2O2 (mol L−1)
yield (%)
Mnc (×104)
Mw/Mnc
19 20 21 22 23 24 25 26
PHMPMDAP−Cu(II) PHMPMDAP−Co(II) PHMPMDAP−Cu(II) PHMPMDAP−Co(II) PHMPMDAP−Cu(II) PHMPMDAP−Co(II) PHMPMDAP−Cu(II) PHMPMDAP−Co(II)
0.016 0.016 0.032 0.032 0.048 0.048 0.064 0.064
88 78 92 87 81 81 83 74
3.6 2.3 4.9 2.7 4.0 2.0 3.2 1.7
2.7 2.4 2.3 2.4 2.8 3.1 2.6 2.7
further structural characterization, 13C NMR data are evaluated. The 13C NMR peak assignments of DHN and PDHN are given in Figure S3 in the Supporting Information. Before analyses, it is noted that the hydroxyl group is a ortho and para director. Therefore, ortho-coupled DHN units (via C1, C8 and C3, C6 carbons) are mainly expected because of the substituted structure of the para positions.23 It is also noted that, in the case of a phenyl group linked to another by C−C couplings, the 13C NMR signals of o- and m-carbons shift to a higher field and a lower field, respectively.24−26 Upon polymerization, C3 and C6 carbon signals of DHN (115.28 ppm) were shifted to low fields of spectra along with the appearance of a broadened peak (120.52 and 122.51 ppm). This indicates that the polymerization takes place probably on these carbon atoms. If polymerization proceeds via C3−C6 carbon atoms, as suggested, the C1, C8, and C10 carbons of DHN are situated in the meta positions and should shift to low field. After the polymerization, C1−C8 signals of DHN are shifted from 107.29 to 115.41 ppm and the C10 signal is shifted from 122.59 to 135.87 ppm. On the other hand, C4 and C5 atoms of DHN are located in ortho positions of the coupling carbons. Therefore, these carbon atom signals shifted from 129.14 to 129.04 ppm as seen in Figure S3 in the Supporting Information.26 These data confirmed the presence of 3,6-linked monomer units in the polymer chains. Considering the existence of the oxynaphthalene units in the polymer chains (from 1H NMR and FT-IR data), it is conceivable to suggest a polymer structure consisting of a mixture of unit A, unit B, unit C, and unit D with the random sequences as shown in Scheme 2. To further confirm the suggested polymer structure, twodimensional (2D) NMR analyses were performed. Figure S4 in the Supporting Information shows 2D 1H−1H COSY spectra of DHN and PDHN recorded in DMSO-d6. The cross peaks at 6.88 and 7.59 ppm indicated the correlation between Hb and Ha in the DHN structure as expected (Supporting Information, Figure S4a). However, the 1 H−1H correlation spectroscopy (COSY) of PDHN exhibited cross peaks centered at 6.89 and 7.75 ppm (Supporting Information, Figure S4b). They should be attributed to a correlation between Hb and Ha which is caused by unit B, unit C (from ring B), and unit D (from ring A), in the polymer backbone (see also Scheme 2). Figure S5 in the Supporting Information belongs to 2D 1H−13C heteronuclear single quantum coherence (HSQC) spectra of DHN and PDHN recorded in DMSO-d6.
a
The reaction was carried out with 1 mmol of DHN in 3 mL of acetonitrile at room temperature in air for 3 h. bThe amount of catalyst for DHN was 5 mol %. cDetermined by GPC using polystyrene standards (DMF).
PDHN prepared by oxidative polymerization is constituted of naphthalene (Nh) and oxynaphthalene (On) units, as shown in Scheme 2. The chemical structure of PDHN was confirmed by means of UV−vis, FT-IR, and NMR spectroscopies. DHN has sharp absorption bands (λmax at 295 and 340 nm). After polymerization, these bands became broad. The band edge moved to longer wavelengths, indicating the longer conjugation network formed. The FT-IR spectra of both DHN and PDHN are presented in Figure S1 in the Supporting Information. Comparing the FT-IR spectra of monomer and polymer, significant changes were observed. All peaks became significantly broad as a result of polymerization.12,21 A new absorption was appeared at 1720 cm−1 due to the formation of carbonyl groups in the polymer backbone (Pummerer’s ketone). The peaks at 1208 and 1170 cm−1 were ascribed to the asymmetric vibrations of the C−O−C/C−OH groups. This finding indicates that the resulting polymer is a mixture of naphthalene (Nh) and oxynaphthalene (On) units.12 1H NMR spectra of DHN and PDHN recorded in DMSO-d6 are given in Figure S2 in the Supporting Information. The 1H NMR spectrum of PDHN showed multiple peaks in the 6.5−8.5 ppm range which were assigned to the aromatic protons. The broad proton line from 8.50 to 10.50 ppm was attributed to the phenolic −OH protons. A significant downfield shift (from 10.50 to 12.60 ppm) of the phenolic proton was probably due to hydrogen bonding interaction.21 The ratio of Nh/On in the polymer was determined by a conventional titration method22,23 and found to be 60%. For Table 4. Effect of Reaction Temperatures on Polymerizationa run
catalystb
temp (°C)
time (h)
yield (%)
Mnc (×104)
Mw/Mnc
27 28 29 30 31 32 33 34 35 36
PHMPMDAP−Cu(II) PHMPMDAP−Co(II) PHMPMDAP−Cu(II) PHMPMDAP−Co(II) PHMPMDAP−Cu(II) PHMPMDAP−Co(II) PHMPMDAP−Cu(II) PHMPMDAP−Co(II) PHMPMDAP−Cu(II)d PHMPMDAP−Co(II)
40 40 50 50 60 60 70 70 75 75
3 3 3 3 3 3 3 3 3 3
96 90 99 96 99 95 97 94 99 95
5.5 3.4 6.4 4.0 7.8 4.8 7.9 5.2 8.3 5.0
2.4 2.7 2.9 2.3 3.1 2.8 2.4 2.7 2.3 2.5
a
The reaction was carried out with 1 mmol DHN and 1 mmol H2O2 (35%) in 3 mL of acetonitrile under air atmosphere. bThe amount of catalysts for DHN was 5 mol %. cDetermined by GPC using polystyrene standards (DMF). dA blank test for run 35 (without catalyst) was performed, and the molecular weight of product obtained was found to be 500 g/mol. 107
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groups in the polymer chains. In the following anodic scan, the intensity of the anodic current is highly diminished. According to Yamaguchi and Yamamoto, this situation is explained as follows: H+ generated by the oxidation process (from hydroxyl units) cannot be captured from the oxidized species (quinonoid groups) and be diffused in the solution. This restricts the reformation of the dihydroxynaphthalene unit in the polymer chains. In the reduction region, two coupled peaks were observed as expected. They should be attributed to n-doping of polynaphthaquinone units. Fluorescence measurements of PDHN were performed in different solvents (100 mg/L), and spectra are given in Figure S9 in the Supporting Information. PDHN exhibited an emission maximum at 493 nm in acetonitrile. An increase in solvent polarity led to blue-shifted emission maxima (about 30 nm). This negative solvatochromism behavior was reported in the literature for other phenolbased compounds such as 2,6-di-tert-butyl-4-(1-pyridinio)phenolate, 4-(4′-hydroxystyryl)-N-methylpyridinium iodide, and 4-[N-methyl-4-pyridinio]-phenolate.27 However, to best of our knowledge, this phenemenon has not been reported for the phenolic polymers, up to the present. The origin of this phenomenon is explained by the differential solvation of the ground and first excited states of the light-absorbing molecule. In other words, if solvent polarity increases, the ground state of the molecule is better stabilized by solvation than the molecule in the excited state and negative solvatochromism will result as reported by Reichardt.27 The solid state conductivity of the polymer pellet was also determined by the four-point probe technique after doping with iodine vapor for 1 week and the conductivity value was found to be 2 × 10−5 S/cm.
The spectrum of the monomer exhibited three C−H correlation peaks with almost the same intensity (Supporting Information, Figure S5a). Peak A (δH = 7.57 ppm, δC = 129.19 ppm), peak B (δH = 6.96 ppm, δC = 115.24 ppm), and peak C (δH = 6.93 ppm, δC = 107.15 ppm) should be originated from the C4,5−Ha correlation, the C3,6−Hb correlation, and the C1,8−Hc correlation, respectively. However, the 1H−13C HSQC spectrum of PHDN exhibited the three cross peaks with different intensities (Supporting Information, Figure S5b): Peak A (δH = 7.77 ppm, δC = 129.73 ppm) and peak B (δH = 6.91 ppm, δC = 115.26 ppm) should be originated from the C4,5−Ha correlation and the C1,8−Hc correlation, respectively. The intensity of cross peaks belonging to C3,6−Hb (peak C) was highly diminished and shifted to the high field of the spectrum after polymerization. Peak C should be caused from unit B (from ring A and ring B), unit C (from ring B), and unit D (from ring A) in polymer chains because they are nonconjugated structures (see Scheme 2). The intensities of peak A, peak B, and peak C belonging to DHN and PDHN are also given for comparison on a three-dimensional (3D) scale (Supporting Information, Figure S6). This figure also confirms that polymerization takes place mainly at the 3,6 carbons of DHN. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to evaluate the thermal behaviors of PDHN. TG/DTG/DTA curves of the polymer are given in Figure S7 in the Supporting Information. The initial, 20% weight loss, and 50% weight loss decomposition temperatures of PDHN were found to be 222, 248, and 432 °C, respectively. A gradual weight loss of PDHN was observed below 150 °C. This can be attributed to the evaporation of low molecular weight compounds. A high amount of residue (36.5%) was formed at 1000 °C. According to DSC measurement, the glassy transition temperature (Tg) of PDHN was found to be 178 °C. The high Tg value was assigned to the cross-linked polymer structure. In the DSC curve, three endothermic peaks were observed at 268, 314, and 394 °C due to the decomposition of PDHN. The endothermic peaks at 268 and 314 °C should be attributed to main decomposition stages of the polymer. DSC peaks harmonize with those of DTG. The last peak observed at 394 °C may be due to a phase transition. In addition, the broad DSC curve is due to the decomposition of the polymer with different chemical surroundings (unit A, unit B, unit C, and unit D). The wide peaks in the DSC curve also suggest that PHDN is highly amorphous. The cyclic voltammograms of PDHN were examined in an acetonitrile solution of 0.1 M Bu4NPF6 at a scanning rate of 100 mV s−1. As shown in Figure S8 in the Supporting Information, PDHN was electrochemically active in both the oxidation and reduction regions. At the first scanning, a large peak was observed and then became very small in the following scans. The rapid decrease in the peak is attributed to the oxidation of the terminal hydroxyl group of PDHN to quinone groups. CV data are in good agreement with those previously reported for dihydroxynaphthalene polymers.12 Yamaguchi and Yamamoto12 reported the enzymatic synthesis of poly(1,5-dihydroxynaphthalene) and poly(2,6-dihydroxynaphthalene). They also reported the electrochemical nature of these polymer films. A very similar cyclic voltammogram was also obtained for PDHN film probably due to the existence of similar repeating structural units. The broad peak centered at 0.85 eV was observed at first scan. This should be attributed to the oxidation of hydroxyl
4. CONCLUSIONS The oxidative polymerization of 2,7-dihydroxynaphthalene (DHN) was accomplished by using two polymer−metal complexes as catalysts. The structure of the obtained polymer was confirmed by different spectroscopic techniques including UV−vis, FT-IR, 1H and 13C NMR, 1H−1H COSY, and 1H−13C HSQC, and the 3,6 positions of DHN were determined as coupling sites. Optimum polymerization conditions were established. We believe that these kind of macromolecule ligand complexes will be promising catalysts for many catalytic reactions. Further studies on macromolecular−metal complex catalyzed polymerization of phenolic monomers are under way.
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ASSOCIATED CONTENT
S Supporting Information *
FT-IR, 1H NMR, and 13C NMR spectra for 2,7-dihydroxynaphthalene (DHN) and poly(2,7-dihydroxynaphthalene) (PDHN); 1H−1H COSY and 1H−13C HSQC spectra of DHN and PDHN; 3D view of 1H−13C HSQC spectra of DHN and PDHN; TGA, DTG, and DTA curves of PDHN; fluorescence spectra of PDHN in (1) EtOH, (2) DMSO, (3) THF, (4) MeCN solutions (100 mg/L); cyclic voltammograms of PDHN film in an acetonitrile solution of 0.1 M Bu4NPF6 at a scanning rate of 100 mV s−1. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +90 286 218 00 18. Fax: +90 286 218 05 33. E-mail:
[email protected]. 108
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Notes
synthesis, characterization, thermal, and conductivity properties. Polym. Adv. Technol. 2008, 19, 1154. (20) Oguchi, T.; Tawaki, S.; Uyama, H.; Kobayashi, S. Soluble polyphenol. Macromol. Rapid Commun. 1999, 20, 401. (21) Saito, K.; Sun, G.; Nishide, H. Green synthesis of soluble polyphenol: oxidative polymerization of phenol in water. Green Chem. Lett. Rev. 2007, 1, 47. (22) Tonami, H.; Uyama, H.; Kobayashi, S.; Kubota, M. Peroxidasecatalyzed oxidative polymerization of m-substituted phenol derivatives. Macromol. Chem. Phys. 1999, 200, 2365. (23) Alva, K. S.; Samuelson, L.; Kumar, J.; Tripathy, S.; Cholli, A. L. Enzyme-catalyzed polymerization of 8-hydroxyquinoline-5-sulfonate by in situ nuclear magnetic resonance spectroscopy. J. Appl. Polym. Sci. 1998, 70, 1257. (24) Bilici, A.; Kaya, I.̇ ; Yıldırım, M. Biosynthesis and characterization of organosoluble conjugated poly(2-aminofluorene) with the pyrazine bridged. Biomacromolecules 2010, 11, 2593. (25) Fukuda, M.; Sawada, K.; Yoshino, K. Synthesis of fusible and soluble conducting polyfluorene derivatives and their characteristics. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 2465. (26) Ayyagari, M. S.; Marx, K. A.; Tripathy, S. K.; Akkara, J. A.; Kaplan, D. L. Controlled Free-Radical Polymerization of Phenol Derivatives by Enzyme-Catalyzed Reactions in Organic Solvents. Macromolecules 1995, 28 (15), 5192. (27) Reichardt, C. Solvatochromic Dyes as Solvent Polarity Indicators. Chem. Rev. 1994, 94, 2319.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Ç anakkale Onsekiz Mart University Research Fund for financial support (Project No. 2009/39). REFERENCES
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