Poly(arylene ether)s with Low Refractive Indices: Poly(biphenylene

Mar 26, 2012 - Byungyong Lee , Sun Dal Kim , Jongmin Park , Taejoon Byun , Seong Jong Kim , Myungeun Seo , Sang Youl Kim. Journal of Polymer Science ...
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Poly(arylene ether)s with Low Refractive Indices: Poly(biphenylene oxide)s with Trifluoromethyl Pendant Groups via a Meta-Activated Nitro Displacement Reaction Sun Dal Kim,† Duyoun Ka,† Im Sik Chung,*,‡ and Sang Youl Kim*,† †

Department of Chemistry, KAIST, Daejeon 305-701, Korea Immunotherapy Research Center, KRIBB, Daejeon 305-806, Korea

‡

S Supporting Information *

ABSTRACT: High-molecular-weight poly(biphenylene oxide)s (PBPO) were prepared from AB-type monomers, 4′(3′)hydroxy-4-nitro-2,6-bis(trifluoromethyl)biphenyl, through a meta-activated nitro displacement reaction. The displacement of nitro leaving group activated by the two trifluoromethyl groups at the meta-position produced high-molecular-weight polymers, which implies that nucleophilic aromatic substitution reaction of the nitro leaving group proceeded very effectively with two activating groups at the meta-position. The obtained polymers have weight-average molecular weight of 20 800−143 300 g/mol and molecular weight distribution of 1.68−2.85. While two homopolymers of 4′- or 3′-hydroxy-4-nitro-2,6-bis(trifluoromethyl)biphenyl, p-PBPO and m-PBPO, showed a semicrystalline morphology, copolymers of the two monomers were amorphous and dissolved in a wide range of organic solvents. The PBPOs possessed a high glass transition temperature (Tg) in the range of 169 to 208 °C depending on their structure and high thermal stability with 10% weight loss temperatures from 486 to 542 °C in nitrogen and from 465 to 516 °C in air. Moreover, PBPOs containing two trifluoromethyl groups showed low refractive indices in the range of 1.4979−1.5052 as well as low birefringence values of 0.0095−0.0148.

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INTRODUCTION Fluorine-containing polymers are of considerable interest for optical and electronic applications owing to their low refractive index, optical loss, dielectric constant, surface energy, moisture absorption, and good thermal and chemical stability.1,2 Although many synthetic methods for fluorine-containing polymers have been reported, a use of the trifluoromethyl group is known as very effective and practical method to incorporate fluorines into the polymer chains.2 Trifluoromethyl groups provide not only several desirable properties, as abovementioned, but also good solubility and good optical transparency.3−10 Poly(arylene ether)s have been widely used as engineering thermoplastics due to their good mechanical properties, high thermal stability, and excellent resistance to hydrolysis and oxidation reaction.11 The known approaches for the synthesis of poly(arylene ether)s include electrophilic aromatic substitution, nucleophilic aromatic substitution, and metal catalyzed coupling reactions.11−13 Among these methods, the nucleophilic aromatic substitution (SNAr) reaction is the most practical method for the preparation of poly(arylene ether)s despite the fact that it requires some structural features, leaving group activated with electron withdrawing groups.14,15 Various heterocyclic rings16 and electron withdrawing groups such as sulfone, ketone, and imide groups have been used as an activating group for SNAr reactions to produce high-molecularweight poly(arylene ether)s.17−19 Trifluoromethyl groups are © 2012 American Chemical Society

also useful for SNAr reactions to activate fluoro or nitro groups for displacement by phenoxides.20−33 The activating groups in SNAr reaction stabilize the negative charge that develops during the transition state largely through conjugation,34−36 and the electron withdrawing group at the meta position of the leaving group is less effective than the same group at the ortho or para position. Therefore, activating groups at the ortho or para position are generally required to obtain high-molecular-weight polymers. Since we first succeeded in making linear poly(arylene ether)s via a meta-activated SNAr reaction,37 some examples of the meta-activated SNAr reaction have been reported to make linear polymers38−42 as well as hyperbranched polymers.43,44 In previous work,45 we found that trifluoromethyl groups and ether linkages are stable in a nitro displacement reaction, even at 190 °C, and that the displacement reaction occurs quantitatively without side reactions that are frequently observed during the nitro displacement reactions at high temperatures due to the reactive nitrite ion byproduct.46−52 In this study, new poly(biphenylene oxide)s containing two trifluoromethyl groups positioned symmetrically on one phenyl ring were prepared from 4′-hydroxy-4-nitro-2,6-bis(trifluoromethyl)biphenyl and 3′-isomer through a metaactivated nucleophilic nitro displacement reaction. The effect Received: December 30, 2011 Revised: March 14, 2012 Published: March 26, 2012 3023

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132.48 (q, J = 31.9 Hz), 126.71 (q, J = 5.7 Hz), 125.63, 121.68 (q, J = 272.9 Hz). Anal. Calcd for C8H2BrF6NO2: C, 28.43; H, 0.60; N, 4.14. Found: C, 28.57; H, 0.76; N, 4.03. 4′-Hydroxy-4-nitro-2,6-bis(trifluoromethyl)biphenyl (5). 4-Methoxyphenylboronic acid (3.37 g, 22.2 mmol) was reacted with 2 (4.98 g, 14.7 mmol) in the presence of Pd(PPh3)4 (1.00 g). The reaction mixture of 40 mL of toluene, 20 mL of ethanol, and 8.18 g of K2CO3 dissolved in 40 mL of water was refluxed for 27 h. The product was extracted with ethyl acetate and passed through a silica column using CH2Cl2/hexane (v/v = 1/4) as an eluent to give pale yellow 4′methoxy-4-nitro-2,6-bis(trifluoromethyl)biphenyl (3) (4.16 g, 77.5% yield): mp 94−95 °C. FTIR (KBr, cm−1): 2847 (−OCH3); 1610 (aromatic CC); 1536, 1331 (NO2); 1124−1183 (C−F). 1H NMR (DMSO-d6, 400 MHz, ppm): 8.76 (s, 2H), 7.22 (d, 2H, J = 8.52 Hz), 7.02 (d, 2H, J = 8.88 Hz), 3.81(s, 3H, −OCH3). 13C NMR (DMSO-d6, 100 MHz, ppm): 159.78, 146.82, 146.07, 131.91(q, J = 30.3 Hz), 130.59, 124.73(q, J = 5.7 Hz), 123.69, 122.16(q, J = 273.5 Hz), 112.73, 55.13. Anal. Calcd for C15H9F6NO3: C, 49.33; H, 2.48; N, 3.84. Found: C, 51.15; H, 2.48; N, 3.72. A solution of 3 (3.00 g, 8.21 mmol) in 60 mL of CH2Cl2 was cooled to −78 °C, and 18 mL of 1 M BBr3 was added slowly. The resulting mixture was stirred at 0 °C for 7 h and quenched by cautiously pouring it into 300 mL of cold water. The mixture was heated until CH2Cl2 evaporated completely. The product was extracted with excess CH2Cl2 and recrystallized from toluene/n-hexane to give pale yellow 4′hydroxy-4-nitro-2,6-bis(trifluoromethyl)biphenyl (5) (2.89 g, 100% yield): mp 139−140 °C. FTIR (KBr, cm−1): 3418 (O−H); 1613 (aromatic CC); 1538, 1335 (NO2); 1142−1189 (C−F). 1H NMR (DMSO-d6, 400 MHz, ppm): 9.80 (s, 1H, −OH), 8.74 (s, 2H), 7.07 (d, 2H, J = 8.33 Hz), 6.82 (d, 2H, J = 8.56 Hz). 13C NMR (DMSO-d6, 100 MHz, ppm): 158.06, 146.68, 146.58, 131.90 (q, J = 30.2 Hz), 130.45, 124.70 (q, J = 5.6 Hz), 122.17 (q, J = 273.6 Hz), 121.98, 114.12. Anal. Calcd for C14H7F6NO3: C, 47.88; H, 2.01; N, 3.99. Found: C, 49.74; H, 1.54; N, 3.83. 3′-Hydroxy-4-nitro-2,6-bis(trifluoromethyl)biphenyl (6). 3-Methoxyphenylboronic acid (3.80 g, 25.0 mmol) was reacted with 2 (6.97 g, 20.6 mmol) in the presence of Pd(PPh3)4 (1.00 g). The reaction mixture of 40 mL of toluene, 40 mL of ethanol, and 8.61 g of K2CO3 dissolved in 40 mL of water was refluxed for 31 h. The product was extracted with ethyl acetate and passed through a silica column using CH2Cl2/hexane (v/v = 1/4) as an eluent to give pale yellow 3′methoxy-4-nitro-2,6-bis(trifluoromethyl)biphenyl (4) (5.94 g, 78.8% yield). 1H NMR (DMSO-d6, 400 MHz, ppm): 8.76 (s, 2H), 7.37(t, 1H, J = 7.96 Hz), 7.06 (m, 1H), 6.86 (s, 1H), 6.85 (d, 1H, J = 12.16 Hz), 3.75 (s, 3H, −OCH3). 13C NMR (DMSO-d6, 100 MHz, ppm): 158.03, 146.89, 145.50, 133.06, 131.58 (q, J = 30.8 Hz), 128.42, 124.71 (q, J = 5.7 Hz), 122.09 (q, J = 273.5 Hz), 121.65, 115.49, 114.38, 55.08. A solution of 4 (3.44 g, 9.42 mmol) in 69 mL of CH2Cl2 was cooled to −78 °C, and 21 mL of 1 M BBr3 was slowly added. The resulting mixture was stirred at 0 °C for 19 h and quenched by cautiously pouring it into 300 mL of cold water. The mixture was heated until the CH2Cl2 evaporated completely. The product was extracted with excess CH2Cl2 and passed through a silica column using ethyl acetate/nhexane (v/v = 1/3) as an eluent to give pale yellow 3′-hydroxy-4-nitro2,6-bis(trifluoromethyl)biphenyl (6) (3.30 g, 100% yield): mp 84−86 °C. FTIR (KBr, cm−1): 3271 (O−H); 1610 (aromatic CC); 1539, 1333 (NO2); 1141−1187 (C−F). 1H NMR (DMSO-d6, 400 MHz, ppm): 9.69 (s, 1H, −OH), 8.76 (s, 2H), 7.24 (t, 1H, J = 8.20 Hz), 6.88 (m, 1H), 6.69 (d, 1H, J = 6.98 Hz), 6.68 (s, 1H). 13C NMR (DMSOd6, 100 MHz, ppm): 156.10, 146.87, 145.69, 132.94, 131.30 (q, J = 30.5 Hz), 128.36, 124.89 (q, J = 5.7 Hz), 122.14 (q, J = 273.6 Hz), 120.09, 116.35, 116.15. Anal. Calcd for C14H7F6NO3: C, 47.88; H, 2.01; N, 3.99. Found: C, 50.29; H, 1.52; N, 3.90. Model Reactions. Model Reaction of 3 with Sodium Phenoxide at Room Temperature. A 25 mL three-necked flask equipped with an N2 inlet was charged with 3 (500 mg, 1.37 mmol), sodium phenoxide trihydrate (300 mg, 1.77 mmol), and 5 mL of NMP. The reaction mixture was stirred at room

of the two trifluoromethyl groups on the reactivity of the metaactivated SNAr reaction and other mechanical and optical properties of the polymers were investigated.

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EXPERIMENTAL SECTION

Materials. 4-Bromo-3,5-bis(trifluoromethyl)aniline (1, Fluorochem), oxone (Aldrich), 4-methoxyphenylboronic acid (Aldrich), 3methoxyphenylboronic acid (TCI), tetrakis(triphenylphosphine) palladium(0) (Pd(PPh3)4, Aldrich), boron tribromide (BBr3 1 M solution in CH2Cl2, Aldrich) were used as received. Potassium carbonate (K2CO3) was dried in vacuo at 150 °C for 24 h prior to use. N-methylpyrrolidone (NMP) was stirred in the presence of CaH2 overnight and then distilled under reduced pressure. Other commercially available reagent-grade chemicals were used without further purification. General Measurements. The Fourier-transform infrared (FTIR) spectra of the compounds were obtained with a Bruker EQUINOX-55 spectrophotometer using a KBr pellet. The NMR spectra of the synthesized compounds were recorded on a Bruker Fourier Transform Avance 400 or Avance 300 spectrometer. The chemical shift of the NMR was reported in parts per million (ppm) using tetramethylsilane as an internal reference. Splitting patterns were designated as s (singlet), d (doublet), t (triplet), q (quartet), or m (multiplet). Elemental analyses (EA) of the synthesized compounds were carried out with a CE Instrument EA1110-FISONS analyzer. Gel permeation chromatography (GPC) diagrams were obtained with a Viscotek TDA302 equipped with a triple RI detector array and a packing column (PLgel 10 μm MIXED-B) using tetrahydrofuran (THF) as an eluent at 35 °C. The number and weight-average molecular weight of the polymers were calculated relative to linear polystyrene standards. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on a TA Instruments TGA Q500 and a DSC Q100 instrument, respectively. The TGA measurements were conducted at a heating rate of 10 °C/min in N2 and air. The melting points (m.p.) of the synthesized compounds and the Tg and Tm values of the polymers were obtained with DSC instrument at a heating rate of 10 °C/min in N2. The Tm values were taken from the first heating scan ranging from 0 to 450 °C. Tg values were taken from the second heating scan after cooling to 0 °C from 350 °C. The wide-angle X-ray diffraction (WAXD) patterns were obtained with a Rigaku D/MAX III diffractometer. The refractive indices nTE and nTM for the transverse electric (TE) and transverse magnetic (TM) modes of the polymer films were measured with a Sairon SPA-4000 prism coupler with a gadolinium gallium garnet (GGG) prism at the wavelength of 1310 nm at room temperature. The birefringence values (Δn) were calculated as the difference between nTE and nTM. Monomer Syntheses. 1-Bromo-4-nitro-2,6-bis(trifluoromethyl)benzene (2). In a three-neck round-bottom flask fitted with two addition funnels and a pH electrode was placed 50 mL of dichloromethane, 50 mL of acetone, 50 mL of a 0.8 M aqueous solution of sodium phosphate, 170 mg of tetrabutylammonium hydrogen sulfate, and 0.950 g (3.08 mmol) of 1. In one addition funnel was added a solution of 20.0 g (32.5 mmol) of oxone in 150 mL of water, and in the other addition funnel was placed 100 mL of a 2 N aqueous solution of potassium hydroxide. After cooling the mixture to 0 °C, the aqueous solution of oxone was added dropwise over 30 min while maintaining a pH of 7.5−8.5 with the addition of the aqueous solution of potassium hydroxide. The resulting suspension was filtered off, and the filtrate was partitioned. The organic layer was washed with water, dried over anhydrous magnesium sulfate, filtered, and concentrated under a reduced pressure environment. The solid residue was passed through a silica column using CH2Cl2/hexane (v/v = 1/4) as an eluent to give pale yellow 1-bromo-4-nitro-2,6-bis(trifluoromethyl)benzene (0.860 g, 82.6% yield): mp 56−57 °C. FTIR (KBr, cm−1): 1613, 1598 (aromatic CC); 1540, 1332 (NO2); 1125−1197 (C−F). 1H NMR (CDCl3, 400 MHz, ppm): 8.71(s, 2H). 13C NMR (DMSO-d6, 100 MHz, ppm): 146.62, 3024

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temperature (ca. 25 °C) for 4 days. The major part of the starting compound (3) remained intact in TLC. Model Reaction of 3 with m-Cresol. A 25 mL three-necked flask equipped with an N2 inlet, a Dean−Stark trap, and a condenser was charged with 3 (500 mg, 1.37 mmol), m-cresol (190 mg, 1.75 mmol), K2CO3 (284 mg, 2.06 mmol), and 3 mL of NMP. The reaction mixture was heated to 90 °C for 31 h. The product was extracted with CH2Cl2 to give colorless product 7. (570 mg, 98.0% yield). The procedure was repeated at 140 °C and at 175 °C. The reaction was complete within 4 and 1 h at 140 and 175 °C, respectively. The isolation yield was almost quantitative in all cases: mp 104−105 °C. FTIR (KBr, cm−1): 1614, 1583 (aromatic CC); 1295, 1259 (C−O− C); 1133−1185 (C−F). 1H NMR (DMSO-d6, 400 MHz, ppm): 7.57 (s, 2H), 7,36 (t, 1H, J = 7.84 Hz), 7.15 (d, 2H, J = 8.42 Hz), 7.09 (d, 1H, J = 7.56 Hz), 7.04 (s, 1H), 6.99 (dd, 1H, J = 8.09 Hz, J = 2.11 Hz), 3.79 (s, 3H), 2.32 (s, 3H). 13C NMR (DMSO-d6, 100 MHz, ppm): 159.19, 156.84, 154.59, 140.52, 133.44, 131.97 (q, J = 29.2 Hz), 131.34, 130.21, 126.03, 124.83, 122.74 (q, J = 273.3 Hz), 120.42, 118.23 (q, J = 5.4 Hz), 116.87, 112.50, 55.00, 20.80. Anal. Calcd for C22H16F6O2: C, 61.98; H, 3.78; N, 0. Found: C, 63.96; H, 3.69; N, 0. Polymerizations. All polymerizations were conducted under a dry nitrogen atmosphere. p-PBPO. A 25 mL three-necked flask equipped with a N2 inlet, a mechanical stirrer, a Dean−Stark trap, and a condenser was charged with 804 mg (2.29 mmol) of 5, 470 mg (3.40 mmol) of K2CO3, 4 mL of NMP, and 2 mL of toluene. The reaction mixture was heated to 135 °C for 3 h, at which the toluene was brought to reflux. The toluene was periodically removed from the Dean−Stark trap, and fresh dry toluene was added to the reaction mixture to ensure dehydration of the system. After the reaction temperature was raised to 175 °C, precipitation occurred within 1 min. The precipitated product was washed with hot water and methanol repeatedly and dried in vacuo at 100 °C for 24 h (654 mg, 93.8% yield).: FTIR (KBr, cm−1): 1603 (aromatic CC); 1295, 1261, 1235 (C−O−C); 1135−1186 (C−F). m-PBPO. The same procedure used for p-PBPO was repeated with 499 mg (1.42 mmol) of 6, 216 mg (1.57 mmol) of K2CO3, and a total of 4.5 mL of NMP. The precipitation also occurred after 1 h at 175 °C. The product was washed with hot water and methanol repeatedly and dried in vacuo at 100 °C for 24 h (498 mg, 99.0% yield). The soluble part in THF was collected for characterization.: FTIR (KBr, cm−1): 1610, 1585 (aromatic CC); 1287, 1268 (C−O−C); 1133−1189 (C−F). 1H NMR (soluble part in THF, THF-d8, 400 MHz, ppm): 7.62 (s, 2H), 7.51(t, 1H, J = 7.94 Hz), 7.26 (d, 1H, J = 8.17 Hz), 7.19 (d, 1H, J = 7.65 Hz), 7.09 (s, 1H). 1p3m-PBPO. A 25 mL three-necked flask equipped with a N2 inlet, a mechanical stirrer, a Dean−Stark trap, and a condenser was charged with 201 mg (0.570 mmol) of 5, 600 mg (1.71 mmol) of 6, 348 mg (2.52 mmol) of K2CO3, and 4 mL of NMP. The reaction mixture was heated to 135 °C for 3 h, at which the toluene was brought to reflux. The toluene was periodically removed from the Dean−Stark trap, and fresh dry toluene was added to the reaction mixture to ensure dehydration of the system. The temperature was raised to 175 °C and the polymerization continued for 14 h. The polymer was precipitated into a 400 mL of vigorously stirred water and then filtered. The precipitated polymer was washed with hot water and methanol repeatedly and dried in vacuo at 100 °C for 24 h. Further purification was carried out by dissolving the polymers in THF and then precipitating it into methanol (575 mg, 83.0% yield). FTIR (KBr, cm−1): 1607, 1583 (aromatic CC); 1293, 1262, 1227 (C−O−C); 1136−1188 (C−F). 1H NMR (THF-d8, 400 MHz, ppm): 7.70 (m, 2H), 7.62 (m, 6H), 7.52 (m, 3H), 7.37 (m, 2H), 7.28 (m, 3H), 7.19 (m, 5H), 7.10 (m, 3H). 1p1m-PBPO. The same procedure used for 1p3m-PBPO was repeated with 400 mg (1.14 mmol) of 5, 402 mg (1.14 mmol) of 6, 348 mg (2.518 mmol) of K2CO3, and 4 mL of NMP. (554 mg, 80.0% yield).: FTIR (KBr, cm−1): 1606, 1583 (aromatic CC); 1294, 1261, 1230 (C−O−C); 1133−1188 (C−F). 1H NMR (THF-d8, 400 MHz, ppm): 7.71(m, 2H), 7.63 (m, 2H), 7.53 (m, 1H), 7.38 (m, 2H), 7.31 (m, 1H), 7.20 (m, 3H), 7.11 (m, 1H).

2p1m-PBPO. The same procedure used for 1p3m-PBPO was repeated with 520 mg (1.48 mmol) of 5, 264 mg (0.750 mmol) of 6, 348 mg (2.52 mmol) of K2CO3, and 4 mL of NMP. (633 mg, 93.2% yield).: FTIR (KBr, cm−1): 1605, 1584 (aromatic CC); 1294, 1261, 1232 (C−O−C); 1134−1187 (C−F). 1H NMR (THF-d8, 400 MHz, ppm): 7.72 (m, 4H), 7.64 (m, 2H), 7.54 (m, 1H), 7.39 (m, 4H), 7.30 (m, 1H), 7.20 (m, 5H), 7.12 (m, 1H). 3p1m-PBPO. The same procedure used for 1p3m-PBPO was repeated with 602 mg (1.71 mmol) of 5, 201 mg (0.750 mmol) of 6, 346 mg (2.52 mmol) of K2CO3, and 4 mL of NMP. (689 mg, 99.4% yield). FTIR (KBr, cm−1): 1605, 1584 (aromatic CC); 1293, 1261, 1233 (C−O−C); 1136−1186 (C−F). 1H NMR (soluble part in THF, THF-d8, 400 MHz, ppm): 7.72 (m, 4H), 7.65 (m, 2H), 7.55 (m, 1H), 7.40 (m, 5H), 7.20 (m, 6H).

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RESULTS AND DISCUSSION Monomer Syntheses. The monomers, 4′-hydroxy-4-nitro2,6-bis(trifluoromethyl)biphenyl (5) and 3′-isomer (6), were Scheme 1

Scheme 2

Chart 1

prepared as shown in Scheme 1 with a good yield. 4-Bromo3,5-bis(trifluoromethyl)aniline (1) was oxidized with oxone.53 The resulting nitro compound (2) was reacted with p- and m(methoxyphenylbronic acid) through a Suzuki coupling reaction 5 4 to produce 4′-methoxy-4-nitro-2,6-bis(trifluoromethyl)biphenyl (3) and its 3′-isomer (4), respectively. The methoxy compounds were converted to the corresponding monomers by demethylation with BBr3. The 3025

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hydroxyl group and a nitro leaving group activated by two trifluoromethyl groups at the meta position was confirmed by spectroscopic data (Experimental Section and Supporting Information, Figures S1 and S2). Model Reactions. To investigate the reactivity of the metaactivated nitro displacement, model reactions were conducted with 3 and m-cresol in NMP at several temperatures. The model compound containing diary ether linkage (7) was obtained as outlined in Scheme 2. The reactions were complete without any detectable side reactions within 31, 4, and 1 h at 90, 140, and 175 °C, respectively, and the yields were quantitative in all cases. The structure of 7 was confirmed by spectroscopic data (Experimental Section and Supporting Information, Figures S3 and S4). On the other hand, the model reaction at room temperature was not effective. The major part of 3 remained intact even after 4 days. In the previous report,42 the reactivity of 2,2′-bis(trifluoromethyl)-4,4′-dinitro-1,1′-biphenyl (8) was higher than that of 4-nitro-2-trifluoromethylbiphenyl (9) due to the electron withdrawing characteristic of the nitro leaving group and the trifluoromethyl group attached to the other aromatic ring of the biphenyl structure (Chart 1). The reaction of 8 with two equivalents of 4-tert-buthylphenol was complete at 175 °C for 4 h, whereas the reaction of 9 with 4-tert-butylphenol was complete at 175 °C for 12 h.37 Interestingly, the reaction of 4′methoxy-4-nitro-2,6-bis(trifluoromethyl)biphenyl (3) with mcresol was complete at 175 °C for 1 h; even at a reaction temperature of 140 °C, the reaction was complete in 4 h. It appears that the higher reactivity of 3 compared to 8 and 9 is caused by the very strong electron withdrawing characteristic of the two trifluoromethyl groups at the meta position of the nitro leaving group in the same phenyl ring. These results imply that activation by the two activating groups at the meta position is a very effective strategy for successful SNAr reactions. Polymerizations. On the basis of the successful results of the model reaction, we carried out the polymerization process at 175 °C. The monomers were polymerized according to the conventional poly(arylene ether) synthesis method with K2CO3 as a base in NMP, as shown in Scheme 3. The initial solid contents were maintained at 20 w/v %, and toluene as an azeotrope was used during the initial stage of polymerization to remove water that formed due to phenoxide generation. Initially, we tried to obtain the homopolymers (p-PBPO and m-PBPO) with 5 and 6 as a single monomer. However, the precipitation occurred in 1 h at 175 °C in both cases. It has been known that the incorporation of trifluoromethyl groups in polymer chains improves the solubility of polymers because bulky trifluoromethyl groups disturb the crystal packing between the polymer chains,55 but the resulting homopolymers, containing numerous trifluoromethyl groups in the main chain, showed limited solubility. It seems that the symmetric incorporation of trifluoromethyl groups cannot effectively hinder the chain packing of poly(arylene ethers). (The details will be discussed later.) To resolve the unexpected solubility problem, copolymerization, which can hinder chain packing around the diaryl ether by introducing irregular para- and meta- connections in the main chain, were also conducted. The mixture of 5 and 6 were polymerized to the corresponding copolymers (1p3m-PBPO, 1p1m-PBPO, 2p1m-PBPO, and 3p1m-PBPO). In italics, each number indicates the feed ratio of the para- and metamonomers. For example, 1p3m corresponds to the 1 to 3 feed ratio of para to meta monomers. When the polymerization

Scheme 3

Table 1. Molecular Weight and Polydispersity Index of the Polymers PBPOs p-PBPOc 3p1m-PBPOd 2p1m-PBPO 1p1m-PBPO 1p3m-PBPO m-PBPOd

feed ratio 5/6 100/0 75/25 67/33 50/50 25/75 0/100

(3/1) (2/1) (1/1) (1/3)

Mna/103

Mwa/103

PDIb

− 9.3 34.9 70.6 24.6 9.3

− 26.4 58.6 143.3 42.0 20.8

− 2.85 1.68 2.03 1.70 2.23

Determined by GPC using THF as an eluent at 35 °C with polystyrene standards. bPolydispersity Index = Mw/Mn. cInsoluble. d Soluble part in THF. a

monomers were purified by chromatography and recrystallization and were dried in vacuo for 24 h prior to polymerization. The structure of the synthesized monomers having both a

Figure 1. FTIR spectra of monomer 5, model compound 7, polymer p-PBPO, and 1p1m-PBPO. 3026

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Figure 2. 1H NMR spectra of PBPOs (THF-d8, 400 MHz, 25 °C).

mixtures reacted for 14 h at 175 °C, precipitation did not occur, contrary to homopolymerization. After the polymerization process, the brown polymers were precipitated in a water/ methanol (v/v = 1/1) mixture and washed with hot water and

methanol repeatedly, after which they were dried in vacuo. Further purification was carried out by dissolving the polymers in THF and then precipitating this mixture into methanol. The molecular weight and polydispersity indices as determined by GPC are listed in Table 1 (Supporting Information, Figure S7). The weight-average molecular weights of the soluble PBPOs (1p3m-PBPO, 1p1m-PBPO, and 2p1m-PBPO) and the partially soluble PBPOs (the soluble part in THF of 3p1mPBPO and m-PBPO) were in the range 20800−143000 g/mol based on the polystyrene standard, indicating high-molecularweight polymers were obtained. The structures of the synthesized polymers were confirmed by spectroscopic data, which indicated the formation of ether linkages without degradation of their trifluoromethyl groups. The representative FTIR spectra are shown in Figure 1. All of the FTIR spectra of monomer (5), model compound (7), and p-PBPO and 1p1m-PBPO show absorption bands at 1130− 1190 cm−1 corresponding to trifluoromethyl (C−F) stretching. While the FTIR spectrum of monomer (5) shows the absorption bands at 3418 cm−1 and at 1538 and 1335 cm−1 corresponding to hydroxyl (O−H) and nitro (N−O) stretching, respectively, the FTIR spectra of model compound (7) and the polymers show a new absorption band around 1250 cm−1 corresponding to the diaryl ether (Ph−O−Ph) stretching generated by successful meta-activated SNAr reactions without any trace of nitro or hydroxyl stretching. FTIR spectra of all PBPOs showed similar patterns (Supporting Information, Figure S6). The 1H NMR spectra of the PBPOs are shown in Figure 2. The lack of traces of −OH (9−10 ppm) or −NO2 (8−9 ppm) end groups in the soluble polymers supports the formation of high-molecular-weight polymers. The

Table 2. Solubility of PBPOsa solvents

pPBPO

3p1mPBPO

2p1mPBPO

1p1mPBPO

1p3mPBPO

mPBPO

NMP DMF DMAc DMPU DMSO THF 1,4-dioxane cyclohexanone acetone ODCB chloroform ethyl acetate anisole toluene n-hexane acetonitrile methanol

− − − − − − − − − − − − − − − − −

+− +− − +− − +− +− − − +− − − − − − − −

++ ++ +− ++ +− ++ ++ ++ +− ++ ++ +− ++ +− − − −

++ ++ ++ ++ +− ++ ++ ++ ++ +− +− ++ +− − − − −

++ ++ ++ ++ +− ++ ++ ++ ++ ++ ++ ++ +− ++ − − −

+− +− +− +− +− +− +− +− − +− +− − +− − − − −

a Solubility: ++, soluble at room temperature; +−, partially soluble; −, insoluble. Abbreviations: THF, tetrahydrofuran; DMF, N,N-dimethylformamide; DMAc, N,N-dimethylacetamide; DMPU, 1,3-dimethyltetrahydropyrimidin-2(1H)-one; DMSO, dimethyl sulfoxide; ODCB, 1,2-dichlorobenzene.

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Figure 4. TGA curves of PBPOs in (a) nitrogen and (b) air at a heating rate of 10 °C/min.

Figure 3. (a) WAXD patterns and (b) DSC curves (the first scan ranging from 0 to 450 °C, Heating rate: 10 °C/min, in N2) of pPBPO, m-PBPO, 3p1m-PBPO, and 1p3m-PBPO.

2 to 1 integral ratio of para to meta moieties, indicating that a 2 to 1 ratio is a critical point to obtain good solubility. Solubility of the Polymer. The solubility of the PBPOs in various solvents is summarized in Table 2. While p-PBPO and

Table 3. Thermal Properties of the Polymers Td5 (°C)a

Td10 (°C)a

PBPOs

in N2

in air

in N2

in air

Tgb (°C)

Tmc (°C)

p-PBPO 3p1m-PBPO 2p1m-PBPO 1p1m-PBPO 1p3m-PBPO m-PBPO

430 500 432 505 463 394

398 457 416 475 399 377

486 542 522 534 521 516

465 511 499 516 480 474

d 208 192 187 176 169

405 429 (403) d d d 337

a

5% and 10% weight loss temperature measured by TGA at a heating rate of 10 °C/min. bGlass transition temperature measured by DSC (the second scan) in N2 at a heating rate of 10 °C/min. cCrystalline melting temperature measured by DSC (the first scan) in N2 at a heating rate of 10 °C/min. dNot detected.

peaks at 7.7 and 7.6 ppm in the spectra indicate the representative repeating unit of para- and meta-moieties in the PBPOs, respectively. The integral ratio of the peaks between 7.7 and 7.6 ppm are well matched with the feed ratio of the para- and meta-monomers, implying that the copolymers contain consistent amounts of the monomers as regards the feed ratio. However, the soluble part of 3p1m-PBPO showed a

Figure 5. DSC curves (the second scan ranging from 0 to 350 °C, heating rate: 10 °C/min, in N2) of PBPOs. 3028

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appear in crystalline polymers. The WAXD results are consistent with the DSC results. While 1p3m-PBPO shows a glass transition temperature (Tg) at 172 °C, p-PBPO, mPBPO, and 3p1m-PBPO show crystalline melting temperatures (Tm) at 405, 337, and 429 (403) °C, respectively. These WAXD and DSC results show that p-PBPO, m-PBPO, and 3p1m-PBPO have crystallinity, leading to its limited solubility. Thermal Properties. The thermal properties of the polymers were evaluated by TGA and DSC, and the results are summarized in Table 3. They have high thermal stability, as expected for fluoroalkyl-containing poly(arylene ether)s. The TGA and DSC thermograms are shown in Figures 4 and 5, respectively. The dynamic TGA result shows that a 10% weight loss occurred for PBPOs in the range of 486−542 °C in nitrogen and 465−516 °C in air. Although p-PBPO and mPBPO, precipitated during the polymerization process, showed relatively low thermal stability due to the remaining end groups, the main decomposition of p-PBPO and m-PBPO starts at above 500 °C in nitrogen. As mentioned above, p-PBPO, mPBPO, and 3p1m-PBPO have Tm values. In the second scan of the DSC measurement in the range of 0 and 350 °C, all of the polymers except p-PBPO showed a high glass transition temperature (Tg) in the range of 169−208 °C. When increasing the quantity of the para-linkage in the main chain, the Tg value of PBPOs increased gradually from 169 to 208 °C. The Tg value of 3p1m-PBPO is comparable to that of PPOTM (Tg = 210 °C)56 despite the fact that 3p1m-PBPO does not have any pendent group near the ether linkage. 3p1m-PBPO did not show Tm in the second heating scan of DSC from 0 to 350 °C, although they showed a melting peak in the first heating cycle up to 450 °C while the melting peak of m-PBPO was observed at 337 °C. The first heating, cooling, and second heating traces of m-PBPO are shown in Figure 6. In the first cooling and the second heating, the trace of Tc and Tm is observed with Tg, respectively. These results imply that a thermal treatment may change the crystalline state of the polymer to an amorphous state in the cases of 3p1m-PBPO and m-PBPO. The Refractive Indices of the Polymers. The refractive indices and birefringence of the polymers were measured using a prism coupling method with the laser beam having 1330 nm wavelength. The results are summarized in Table 4. As expected, all of the soluble PBPOs which have two CF3 groups per biphenyl unit showed very low refractive indices (nav) in the range of 1.4979 to 1.5052 due to the high fluorine content. The refractive indices (nav) of CF3-substituted poly(arylene ether)s, with one CF3 group per biphenyl unit of their polymer chains, were reported in the range of 1.5992 to 1.6223.37,42,45 These values are higher than those of the PBPOs synthesized in this study. The low refractive indices of the PBPOs likely originated from the low molecular polarizability and density caused by the two CF3 groups of polymeric repeating unit of PBPOs5,57−61 Moreover, the polymers showed low birefringence (Δn) of less than 0.015, indicating that the linear polarizability and segmental orientation of the amorphous PBPOs are nearly isotropic. The dielectric constant (ε) can be estimated from the refractive index n according to Maxwell’s equation, ε ≈ n2. The ε value at 1 MHz was determined to be ε ≈ 1.10 nav2, including an additional contribution of approximately 10% due to infrared absorption.62 The ε values of the polymer films estimated from the average refractive indices were in the range of 2.47−2.49. The low dielectric constants may be also attributed to the existence of trifluoromethyl groups in the main chain.

Figure 6. DSC curves (the second scan ranging from 0 to 350 °C, heating rate: 10 °C/min, in N2) of m-PBPO.

Table 4. Refractive Indices of the Synthesized Polymersa PBPOs

nTEb

nTMc

navd

Δne

εf

dg (μm)

2p1m-PBPO 1p1m-PBPO 1p3m-PBPO

1.5084 1.5028 1.5052

1.4988 1.4880 1.4957

1.5052 1.4979 1.5020

0.0096 0.0148 0.0095

2.49 2.47 2.48

4.15 6.50 4.51

a

Measured at a wavelength of 1310 nm at room temperature. bnTE: the in-plane refractive index. cnTM: the out-of plane refractive index. dnav: the average refractive index (nav = (2nTE + nTM)/3). eΔn: birefringence (nTE − nTM). fDielectric constant estimated from the refractive index: ε ≈ 1.10nav2 gFilm thickness for the refractive index measured.

m-PBPO have limited solubility, the copolymers were soluble in various organic solvents, including NMP, DMF, DMPU, THF, 1,4-dioxane, and cyclohexanone. Generally, the solubility of the polymers increases as the amount of the meta-connected moiety in the polymer backbone increases. The solubility of 1p1m-PBPO, which was soluble in ethyl acetate and acetone, was better than that of 2p1m-PBPO presumably due to its higher meta-linkage content. In addition, 1p3m-PBPO was soluble even in toluene. In contrast, 3p1m-PBPO showed poor solubility due to its higher content of the para-moiety. All of the polymers were insoluble in n-hexane, acetonitrile, and methanol. It is known that trifluoromethyl pendent groups in rigid polymer chains improve the solubility of polymers because bulky trifluoromethyl groups disturb the crystal packing between the polymer chains.55 While the PBPOs having one trifluoromethyl group on their repeating unit showed good solubility,37 three polymers (p-PBPO, m-PBPO, and 3p1mPBPO) have limited solubility. p-PBPO is insoluble in any organic solvents, and less than 10% of m-PBPO and 3p1mPBPO are soluble in THF. The unexpected limited solubility behavior was caused by their crystalline characteristics, which indicates that the increased symmetry in the polymer structure overrides the bulkiness of trifluoromethyl groups in their crystal packing structure. The wide-angle X-ray diffraction (WAXD) patterns and the DSC curves of p-PBPO, m-PBPO, 3p1mPBPO, and 1p3m-PBPO are shown in Figure 3, parts a and b, respectively. While the WAXD pattern of 1p3m-PBPO is broad without any obvious peak features, the patterns of p-PBPO, mPBPO, and 3p1m-PBPO show relatively sharp peaks that often 3029

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(12) Rogers, M. E.; Long, T. E. Synthesis of Poly(arylene ether)s. In Synthetic Methods In Step-Growth Polymers; John Wiley & Sons, Inc.; Hoboken, NJ, 2003. (13) Johnson, H. N.; Farnham, A. G.; Clendinning, R. A.; Hale, W. F.; Merriam, C. N. J. Polym. Sci., Part A: Polym. Chem. 1967, 5, 2375. (14) Theil, F. Angew. Chem., Int. Ed. 1999, 38, 2435. (15) Pitsinos, E. N.; Vidali, V. P.; Couladouros, E. A. Eur. J. Org. Chem. 2011, 1207. (16) Labadie, J. W.; Hedrick, J. L.; Ueda, M. Step growth polymers for high-performance materials: New synthetic method; ACS Symposium Series 624; American Chemical Society: Washington, DC, 1996; pp 210−25. (17) Kricheldorf, H. R.; Handbook of polymer synthesis; CRC Press: New York, 1992; pp 545−615. (18) Mati, S.; Mandal, B. Prog. Polym. Sci. 1986, 12, 111. (19) Chung, I. S.; Eom, H. J.; Kim, S. Y. Polym. Bull. 1998, 41, 631. (20) Labadie, J. W.; Hedrick, J. L. Macromolecules 1990, 23, 5371. (21) Kim, S. Y.; Labadie, J. W. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1991, 32 (1), 164. (22) Banerjee, S.; Maier, G.; Burger, M. Macromolecules 1999, 32, 4279. (23) Banerjee, S.; Maier, G. Chem. Mater. 1999, 11, 2179. (24) Maier, G. Prog. Polym. Sci. 2001, 26, 3. (25) Lee, M. S.; Kim, S. Y. Macromol. Rapid Commun. 2005, 26, 52. (26) Banerjee, S.; Komber, H.; Häussler, L.; Voit, B. Macromol. Chem. Phys. 2009, 210, 1272. (27) Ghosh, A.; Banerjee, S.; Komber, H.; Lederer, A.; Häussler, L.; Voit, B. Macromolecules 2010, 43, 2846. (28) Carter, K. R.; Kim, S. Y.; Labadie, J. W. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1993, 34 (1), 415. (29) Park, S. K.; Kim, S. Y. Macromolecules 1998, 31, 3385. (30) Lee, H. S.; Kim, S. Y. Macromol. Rapid Commun. 2002, 23, 665. (31) Kim, Y. J.; Chung, I. S.; Kim, S. Y. Macromolecules 2003, 36, 3809. (32) Lee, M. S.; Kim, S. Y. Macromolecules 2005, 38, 5844. (33) Kim, Y. J.; Kakimoto, M.; Kim, S. Y. Macromolecules 2006, 39, 7190. (34) Bunnett, J. F.; Zahler, R. E. Chem. Rev. 1951, 49, 273. (35) Bartoli, G.; Todesco, P. E. Acc. Chem. Res. 1977, 10, 125. (36) Terrier, F. Nucleophilic aromatic displacement: the influence of the nitro group; VCH Publishers: New York, 1991. (37) Chung, I. S.; Kim, S. Y. J. Am. Chem. Soc. 2001, 123, 11071. (38) Kaiti, S.; Himmelberg, P.; Williams, J.; Abdellatif, M.; Fossum, E. Macromolecules 2006, 39, 7909. (39) Beek, D. V.; Fossum, E. Macromolecules 2009, 42, 4016. (40) Tienda, K.; Yu, Z.; Constandinidis, F.; Fortney, A.; Feld, W. A.; Fossum, E. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 2908. (41) Rusanov, A. L.; Komarova, L. G. High Perf. Polym. 2009, 21, 535. (42) Chung, I. S.; Kim, K. H.; Lee, Y. S.; Kim, S. Y. Polymer 2010, 51, 4477. (43) Hawker, C. J.; Chu, F. Macromolecules 1996, 29, 4370. (44) Himmelberg, P.; Fossum, E. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 3178. (45) Chung, I. S.; Kim, S. Y. Macromolecules 2000, 33, 9474. (46) Williams, F. J.; Donahue, P. E. J. Org. Chem. 1977, 42, 3414. (47) Markezich, R. L.; Zamek, O. S.; Donahue, P. E.; Williams, F. J. J. Org. Chem. 1977, 42, 3435. (48) Takekoshi, T.; Wirth, J. G.; Heath, D. R.; Kochanowski, J. E.; Manello, J. S.; Webber, M. J. J. Polym. Sci., Part A: Poym. Chem. 1980, 18, 3069. (49) White, D. M.; Takekoshi, T.; Williams, F. J.; Relles, H. M.; Donahue, P. E.; Klopfer, H. J.; et al. J. Polym. Sci., Part A: Poym. Chem. 1981, 19, 1635. (50) Takekoshi, T. Polym. J. 1987, 19, 191. (51) In, I. S.; Eom, H. J.; Kim, S. Y. Polymer (Korea) 1998, 22, 544. (52) In, I.; Kim, S. Y. Polymer 2006, 47, 4549. (53) Zabrowski, D. L.; Moormann, A. E.; Beck, K. R. Tetrahedron Lett. 1988, 29, 4501. (54) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.

CONCLUSION In this study, we prepared new aromatic polymers with low refractive indices as well as high temperature resistances. The polymers, poly(biphenylene oxide)s containing two pendent trifluoromethyl groups, were synthesized from 4′-hydroxy-4nitro-2,6-bis(trifluoromethyl)biphenyl and 3′-isomer through a meta-activated nucleophilic aromatic substitution (SNAr) reaction. The nitro leaving group activated by the two trifluoromethyl groups at the meta-position was effectively displaced, resulting in high-molecular-weight polymers. This implies that activation by two activating groups at the metaposition is an effective strategy for successful SNAr reactions. While both the para- and meta-connected homopolymers were crystalline, the copolymers of the para- and meta-connected monomers were amorphous and dissolved in a wide range of organic solvents. The polymers were thermally stable and showed very low refractive indices in the range of 1.488−1.508 with low birefringence.

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ASSOCIATED CONTENT

S Supporting Information *

1

H and 13C NMR spectra of the monomers and the model compound, FTIR of polymers, and GPC curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (I.S.C.); [email protected] (S.Y.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Korean Government (Ministry of Education, Science, and Technology (MEST)), the Basic Science Research Program (2010-07592) and the Nano/Bio Science and Technology Program (200501321). This work was also supported by a NRF grant funded by MEST through the NRL (R0A-2008-000-20121-0) and the ERC (R11-2007-050-00000-0) program.

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