Introduction

Aromatic poly(ether ketone)s, which consist of aromatic, carbonyl and ether units, have drawn considerable attention owing to their outstanding chemical and physical stability.1, 2, 3 General synthetic methods of aromatic poly(ether ketone)s are mainly (i) nucleophilic aromatic substitution polymerization of aromatic difluorides with aromatic diols, (ii) electrophilic aromatic substitution polymerization of highly electron-rich aromatics with aromatic dicarboxylic acids/acid dichlorides and (iii) transition metal-catalyzed aromatic coupling polymerization, such as NiBr2/Zn-mediated aromatic homo-coupling polymerization of aromatic dichlorides.2, 3 Almost all of the reported aromatic poly(ether ketone)s have been synthesized through nucleophilic aromatic substitution polymerization owing to high conversion, easy treatment and wide application. As ether bonds are inevitably formed through this polymerization, they behave as flexible units and lower the thermal stability of the resulting polymers. On the other hand, aromatic carbonyl bonds are formed in electrophilic aromatic substitution polymerization. However, selection of polymerizable and regioselective electron-rich arenes is extremely limited. Although aromatic–aromatic bonds are formed in NiBr2/Zn-mediated aromatic homo-coupling polymerization, strict treatment against oxygen and moisture during polymerization is required for smooth polymerization.

We have reported the synthesis of several aromatic polyketones using these three methods.4, 5, 6, 7, 8, 9, 10, 11 Among these studies, it has been revealed that aromatic polyketones containing twisted aromatic ring-assembly units show both excellent thermal stability and sufficient solubility to typical organic solvents, affording flexible and thermally stable polymer films. In particular, introduction of 1,1′-binaphthyl-6,6′-diyl units to polyketone backbones was effective for the development of aromatic polyketones with both high Tg and sufficient solubility through these three methods.8, 9, 10, 11

The Suzuki–Miyaura cross-coupling reaction is one of the most useful protocols for aromatic–aromatic bond formation reactions and has been applied for the synthesis of a lot of π-conjugated organic materials, including polymers.12, 13, 14, 15 Although it has the advantage of high conversion and easy treatment, it has scarcely been applied to the synthesis of aromatic polyketones. In particular, Suzuki–Miyaura coupling reaction of bis(halobenzoyl)-type monomers requires long reaction times, probably because of lower reactivity based on carbonyl groups.15

Recently, we presented a preliminary report on the successful synthesis of aromatic poly(ether ketone)s through nanosized palladium-catalyzed16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 Suzuki–Miyaura coupling polymerization of 2,2′-bis(iodobenzoyl)-1,1′-binaphthyls with aromatic diboric acid pinacol esters.27 In this article, we would like to discuss the nanosized palladium-catalyzed Suzuki–Miyaura coupling polymerization from the viewpoints of the scope and limitations of monomer structures and detail an investigation of reaction conditions.

Experimental Procedure

Materials

(S)-BINOL (1, [α]D25: −33 °), triphenylphosphine and trimethyl borate were purchased from Kanto Chemical Co. Ltd. (Tokyo, Japan). Pd(OAc)2, potassium carbonate and pinacol were purchased from Wako Pure Chemical Industries (Osaka, Japan). Bu4NOAc and MS4A were purchased from Aldrich (St Louis, MO, USA). These reagents were used as received. 1,4-Dioxane was purchased from Kanto Chemical Co. Ltd. and was used after distillation in the presence of CaH2. Dimethylformamide was purchased from Kanto Chemical Co. Ltd. and was used after distillation in the presence of CaH2 under reduced pressure.

4-Fluoro-4′-iodobenzophenone (2A) and related halobenzophenones 2B–2E were prepared through AlCl3-mediated Friedel–Crafts acylation reaction of fluorobenzene with the corresponding halobenzoyl chloride. Diboric acid pinacol esters 4m and 4p were prepared through esterification of the corresponding diboric acids with pinacol. The diboric acids were prepared through lithiation of the corresponding dibromide followed by transmetallation by trimethyl borate and acidic workup.

Instruments

1H-NMR spectra were recorded on a JEOL ECX-400 (JEOL, Tokyo, Japan, 400 MHz) and a JEOL AL-300 (300 MHz). Chemical shifts are expressed in p.p.m. relative to the internal standard of Me4Si (δ=0.00). 13C-NMR spectra were recorded on a JEOL ECX-400 (100 MHz) and a JEOL AL-300 (75 MHz). Chemical shifts are expressed in p.p.m. relative to the internal standard of CDCl3 (δ=77.0). Infrared (IR) measurement was recorded on a JEOL FTIR-4100 and a Horiba FT-210 (Horiba, Kyoto, Japan). Molar rotations ([Φ]25D) were estimated on the basis of the measurement in a CHCl3 solution (1 g dl−1) using a JASCO DIP-1000 digital polarimeter (JASCO, Tokyo, Japan). Circular dichroism measurement was performed in a CHCl3 solution (6.6 × 10−5 mol l−1) using a JASCO J-720WI. Gel permeation chromatography measurements were carried out at a flow rate of 1.0 ml min−1 at 40 °C using CHCl3 as an eluent on a JASCO PU-2080 equipped with a Ultraviolet detector (254 nm) and a Shodex K-804L column (Shodex, Tokyo, Japan). Glass transition temperatures (Tg) were estimated on the basis of differential scanning calorimetry thermograms. The differential scanning calorimetry thermograms were recorded on a PerkinElmer DSC-4000 differential scanning calorimeter with a heating rate of 10 K min−1 (Waltham, MA, USA). Thermal degradation temperatures (Td) were estimated on the basis of thermogravimetric analysis thermograms. The thermogravimetric analysis thermograms were recorded on a PerkinElmer TGA 4000 thermogravimeter with a heating rate of 10 K min−1.

Synthesis of diiodide 3P(I), 3M(I) 3O(I), dibromide 3P(Br) and dichloride 3P(Cl)

To a 50-ml two-necked flask, (S)-BINOL (1, 1.14 g, 4.0 mmol), 4-fluoro-4′-iodobenzophenone (2A, 4.54 g, 14 mmol), anhydrous potassium carbonate (1.66 g, 12 mmol) and freshly distilled dimethylformamide (5 ml) were added. The mixture was stirred at 150 °C for 24 h. The reaction mixture was poured into water in a beaker. The precipitate was collected by suction filtration and extracted with CHCl3 three times. The combined extracts were dried over anhydrous MgSO4. After removal of the drying reagent, CHCl3 was removed under reduced pressure. The crude product was purified by silica gel column chromatograph (hexane:chloroform=1:1) to yield diiodide 3P(I) in 71% yield (2.55 g) as a white powder. A series of dihalides 3 were also prepared in the same manner.

Diiodide 3P(I): Yield: 71%. 1H-NMR δ (400 MHz, CDCl3): 6.79 (4H, d, J=8 Hz), 7.27–7.33 (6H, m), 7.36 (4H, d, J=8 Hz), 7.45 (2H, t, J=8 Hz), 7.52 (4H, d, J=8 Hz), 7.78 (4H, d, J= 8 Hz), 7.91 (2H, d, J=8 Hz), 7.95 (2H, d, J=8 Hz) p.p.m. 13C-NMR δ (100 MHz, CDCl3): 99.7, 117.4, 120.0, 122.9, 125.5, 126.0, 127.1, 128.3, 130.4, 131.0, 131.2, 131.3, 132.0, 134.2, 137.2, 137.6, 151.1, 161.6 and 194.2 p.p.m. IR ν (KBr): 1648, 1587, 1500 and 1240 cm−1. HRMS-FAB (m/z) For C46H28I2O4: Calcd [M+]: 898.0082. Found: 898.0096. [α]D25: −24 °. [Φ]D25: −216 °.

Diiodide 3M(I): Yield: 61%. 1H-NMR δ (400 MHz, CDCl3): 6.81(4H, d, J=8 Hz), 7.17(2H, t, J=8 Hz), 7.27–7.35(6H, m), 7.45(2H, t, J=8 Hz), 7.55(4H, d, J=8 Hz), 7.58(2H, d, J=8 Hz), 7.85(2H, d, J=8 Hz), 7.91(2H, d, J=8 Hz), 7.96(2H, d, J=8 Hz), 7.97(2H, s) p.p.m. 13C-NMR δ (100 MHz, CDCl3): 94.2, 117.4, 120.1, 123.0, 125.5, 126.0, 127.1, 128.3, 129.0, 130.0, 130.4, 131.0, 131.1, 132.2, 134.1, 138.4, 140.0, 140.9, 151.1, 161.7 and 193.6 p.p.m. IR ν (KBr): 1654, 1587, 1500 and 1243 cm−1. HRMS-FAB (m/z) for C46H28I2O4: Calcd [M+]: 898.0082. Found: 898.0096. [α]D25: −38 ° [Φ]D25: −341 °.

Diiodide 3O(I): Yield: 83%. 1H-NMR δ (400 MHz, CDCl3): 6.77 (4H, d, J=8 Hz), 7.13 (2H, t, J=8 Hz), 7.20–7.34 (6H, m), 7.38–7.47 (6H, m), 7.54 (4H, d, J=8 Hz), 7.85–7.96 (6H, m) p.p.m. 13C-NMR δ (100 MHz, CDCl3): 92.3, 117.7, 119.9, 122.8, 125.5, 125.9, 127.1, 127.9, 128.3, 128.4, 130.0, 130.3, 131.0, 132.5, 134.1, 139.7, 144.2, 151.8, 162.2 and 195.9 p.p.m. IR ν (KBr): 1664, 1585, 1500 and 1243 cm−1. [α]D25: −32 °. [Φ]D25: −287 °.

Dibromide 3P(Br): Yield: 83%. 1H-NMR δ (400 MHz, CDCl3): 6.68 (4H, d, J=8 Hz), 7.30 (4H, d, J=8 Hz), 7.32 (2H, t, J=8 Hz), 7.46 (2H, t, J=8 Hz), 7.49–7.57 (12H, m), 7.91 (2H, d, J= 8 Hz), 7.95(2H, d, J=8 Hz) p.p.m. 13C-NMR δ (100 MHz, CDCl3): 117.4, 120.0, 122.9, 125.5, 126.0, 127.1, 127.2, 128.3, 130.4, 131.0, 131.2, 131.4, 131.6, 132.0, 134.1, 136.6, 151.1, 161.6 and 194.2 p.p.m. IR ν(KBr): 1654, 1587, 1500, 1245 and 1068 cm−1. HRMS-FAB (m/z) for C46H28Br2O4: Calcd [M+]: 802.0356. Found: 802.0362. [α]D25: −49 °. [Φ]D25: −393 °.

Diiodide 3P(Cl): Yield: 90%. 1H-NMR δ (300 MHz, CDCl3): 6.80 (4H, d, J=8.7 Hz), 7.29–7.35 (6H, m), 7.39–7.48 (6H, m), 7.53 (4H, d, J=8.7 Hz), 7.59 (4H, d, J=8.4 Hz), 7.92 (2H, d, J=8.4 Hz), 7.96 (2H, d, J= 8.7 Hz) p.p.m. 13C-NMR δ (75 MHz, CDCl3): 117.3, 119.8, 122.8, 125.3, 125.8, 126.9, 128.2, 128.5, 130.2, 130.9, 131.1, 131.2, 131.9, 134.0, 136.1, 138.5, 151.0, 161.4 and 194.0 p.p.m. IR ν(KBr): 1656, 1590, 1243 and 1090 cm−1. HRMS-FAB (m/z) for C46H28Cl2O4: Calcd [M+]: 714.1366. Found: 714.1379. [α]D25: −4.1 °. [Φ]D25: −29 °.

Synthesis of aromatic polyketones 5

To a dried 30-ml two-necked flask equipped with a reflux condenser, palladium acetate (4.5 mg, 20 μmol) and triphenylphosphine (10.5 mg, 40 μmol) were added. Tetrabutylammonium acetate (603 mg, 2.0 mmol), pottasium carbonate (276 mg, 2.0 mmol), molecular sieves 4A (70 mg), diiodide 3P(I) (180 mg, 0.2 mmol), boric acid pinacol ester 4m (77.9 mg, 0.2 mmol) and 1,4-dioxane (1.0 ml) were added to this flask. After stirring at 100 °C for 1 h, the reaction mixture was poured into 150 ml of MeOH/aqueous HCl(v/v=9/1) in a beaker. The precipitant was washed with water and acetone. The solid was dissolved in CHCl3 and filtrated by celite. After removal of CHCl3 under reduced pressure, polyketone 5P m was obtained in 90% yield (140.4 mg). Other polyketones were synthesized in the same manner.

Polyketone 5P m: 1H-NMR δ (400 MHz, CDCl3): 3.84 (6H, s), 6.60 (1H, s), 6.83 (4H, d, J=8 Hz), 7.22–7.35 (10H, m), 7.38–7.51 (3H, m), 7.51–7.74 (8H, m), 7.84–8.00 (4H, m) p.p.m. 13C-NMR δ (100 MHz, CDCl3): 56.0, 96.2, 117.5, 120.0, 122.3, 122.9, 125.4, 126.0, 127.0, 128.3, 129.3, 129.9, 130.4, 131.0, 132.0, 132.2, 132.8, 134.2, 135.9, 142.1, 151.4, 157.7, 161.3 and 195.1 p.p.m. IR ν (KBr): 1654, 1600, 1498, 1234 and 1203 cm−1. [α]D25: −5.8 ° [Φ]D25: −45 °.

Polyketone 5Pp: Yield 91%. 1H-NMR δ (400 MHz, CDCl3): 3.78(6H, s), 6.85(4H, d, J=8 Hz), 6.98(2H, s), 7.26–7.36(10H, m), 7.40–7.48(2H, m), 7.60-7.70(4H, m), 7.73(4H, d, J=8 Hz), 7.86–7.99(4H, m) p.p.m. 13C-NMR δ (100 MHz, CDCl3): 56.5, 114.7, 117.5, 120.0, 122.9, 125.4, 126.0, 127.0, 128.3, 129.4, 129.8, 130.0, 130.3, 131.0, 131.9, 132.2, 134.2, 136.5, 142.2, 150.9, 151.4, 161.3 and 195.0 p.p.m. IR ν (KBr): 1654, 1600, 1498, 1236, 1211 cm−1. [α]D25: −5.9°. [Φ]D25: −46°.

Polyketone 5Mm: Yield 49%. 1H-NMR δ (400 MHz, CDCl3): 3.75(6H, s), 6.57(1H, s), 6.81(4H, d, J=8 Hz), 7.19–7.44(6H, m), 7.50–7.68(11H, m), 7.78–7.93(8H, m) p.p.m. 13C-NMR δ (100 MHz, CDCl3): 55.9, 96.0, 117.5, 120.0, 122.3, 122.9, 125.4, 125.9, 127.0, 128.1, 128.3, 130.3, 131.0, 131.9, 132.3, 132.7, 133.1, 134.1, 137.7, 138.0, 151.4, 157.2, 161.3 and 195.4 p.p.m. IR ν (KBr): 1654, 1585, 1500, 1232 and 1203 cm−1. [α]D25: −6.6 °. [Φ]D25: −51 °.

Polyketone 5Mp: Yield 63%. 1H-NMR δ (400 MHz, CDCl3): 3.71(6H, s), 6.84(4H, d, J=8 Hz), 6.94(2H, s), 7.22–7.32(4H, m), 7.34–7.43(4H, m), 7.46(2H, t, J=8 Hz), 7.61(2H, d, J=8 Hz), 7.67(4H, d, J=8 Hz), 7.72(2H, d, J=8 Hz), 7.82–7.95(6H, m) p.p.m. 13C-NMR δ (100 MHz, CDCl3): 56.5, 114.6, 117.4, 120.0, 122.9, 125.4, 125.9, 127.0, 128.1, 128.3, 128.7, 129.8, 130.3, 130.8, 130.9, 131.8, 132.3, 133.2, 134.2, 137.8, 138.3, 150.7, 151.3, 161.4 and 195.3 p.p.m. IR ν (KBr): 1654, 1587, 1500, 1234 and 1211 cm−1. [α]D25: −4.5 °. [Φ]D25: −35 °.

Results and Discussion

Preparation of monomers 3

According to our previous papers,4, 9 three regioisomeric diiodides 3M(I), 3P(I) and 3O(I) were prepared via nucleophilic aromatic substitution reaction of (S)-BINOL (1) with 4-fluoroiodobenzophenones 2A-C, which are prepared through Friedel–Crafts acylation of fluorobenzene with iodobenzoyl chloride in the presence of AlCl3, in moderate yields (Scheme 1). The corresponding dibromide 2D and dichloride 2E were also prepared in the same procedure.

Optimization of reaction conditions—polymerization between monomer 3P(I) and 4m

Polymerization of monomer 3P(I) with diboric acid pinacol ester 4m was conducted under the various reaction conditions. The results are shown in Table 1. When the nanosized palladium catalyst,16 which was prepared in situ from Pd(OAc)2, PPh3 and Bu4NOAc in 1,4-dioxane, was employed, smooth and rapid polymerization proceeded to give poly(ether ketone) 5Pm. As shown in runs 1–3, 10 mol% Pd(OAc)2 (=20 μmol) against the monomers (0.2 mmol, respectively) were the most suitable to obtain high-molecular-weight polymers. When 20 mol% catalyst was employed, the molecular weights of the resulting polymer slightly decreased, probably owing to aggregation of nanosized palladium under concentrated conditions (run 3). Even during polymerization in the presence of 10 mol% catalyst, it seems that active nanosized palladium species are gradually deactivated by slow aggregation. Consequently, prolongation of reaction time gave no effects on increase in molecular weights. When Bu4NOAc, was not added, that is, under general palladium catalyst system (conditions B), only low-molecular-weight polymer was obtained even after stirring for 6 h (run 4). Use of a more polar solvent, that is, dimethylformamide was also ineffective under the general palladium catalyst system (run 5). On the other hand, addition of Bu4NBr was also effective to enable smooth polymerization (run 6). When Pd(PPh3)4 was employed instead of Pd(OAc)2/2PPh3, the moderate-molecular-weight polymer was obtained. Consequently, the Pd(OAc)2/PPh3/Bu4NOAc system (run 2) was determined as the optimized reaction conditions.

Table 1 Nanosized palladium-catalyzed Suzuki–Miyaura coupling polymerization of diiodide 3P(I) with diboric acid pinacol ester 4m

Synthesis of aromatic poly(ether ketone)s 5

At first, polymerization of the corresponding dibromide 3P(Br) and dichloride 3P(Cl) with diboric acid pinacol ester 4m was carried out under the above optimized conditions. Although the corresponding poly(ether ketone) 5Pm was obtained, the molecular weights of the polymers derived from 3P(Br) and 3P(Cl) were lower than those derived from 3P(I). In the case of dibromide 3P(Br) and dichloride 3P(Cl), the oxidation addition step proceeds slower as imagined, affording lower-molecular-weight polymer. However, it could be evaluated that the nanosized palladium-catalyzed polymerization of dichloride 3P(Cl) proceeded moderately (Table 2).

Table 2 Synthesis of aromatic poly(ether ketone)s 5a

Next, polymerization of m-substituted and o-substituted diiodides, 3M(I) and 3O(I), were also performed. When m-substituted diiodide 3M(I) was used, poly(ether ketone)s 5Mm and 5Mp with lower molecular weights were obtained. When o-substituted diiodide 3O(I) was used, no polymerization proceeded and only diiodide 3O(I) was recovered. These reactions demonstrate that steric hindrance around reactive halogens influences the reactivity in the transmetallation step and the o-substitution structure is more disadvantageous.

All of the poly(ether ketone)s 5Pm, 5Pp, 5Mm and 5Mp were soluble in typical solvents as shown in Table 3. Among these polymers, poly(ether ketone)s 5Mm and 5Mp, which are derived from m-substiutited diiodide 3M(I), were more soluble than 5Pm and 5Pp. The difference of the solubility is probably because of not only the decrease of molecular weights but also the bent structures based on m-substitution compared with linear ones based on p-substitution.

Table 3 Solubility of aromatic poly(ether ketone)s 5a

Thermal properties

Thermogravimetric analysis of poly (ether ketone)s 5 proved that they have excellent thermal stability. No loss of the weights in the temperature ranging up to ca. 450 °C under N2 flow was observed. These thermal behaviors are similar to the polyketones bearing methoxy groups, which indicates that the cleavage of O-CH3 bonds occurs firstly during thermal degradation.

Glass transition temperatures (Tg) of poly(ether ketone)s 5 are in the range of 188–224 °C, which are higher than those of PEEK (143 °C) and almost all of the reported aromatic poly(ether ketone)s including our previous works.4, 5, 6, 7, 8, 9, 10 The excellent thermal stability and sufficient solubility depend on non-coplanar structures of 1,1′-binaphthyl-2,2′-diaryoxy units, which give suitable suppression of the free rotation and the π,π-stacking to polymer main chains. In addition, the exclusion of flexible ether units from the main chains is another factor to increase Tg.

Optical properties

Optical rotation of poly (ether ketone)s 5 was measured in a 1-g dl−1 CHCl3 solution. Specific rotation [α] and molar rotation [Φ] of poly (ether ketone)s 5 are shown in Table 4. There is only negligible difference among these values. Figure 1 shows circular dichroism spectra of monomers 3P(I) and 3M(I) derived from (S)-1, and poly(ether ketone)s 5Pm and 5Mm derived from the (S)-monomers. Although larger cotton effects in the spectra of 3M(I) and 5Mm were observed than those of 3P(I) and 5Pm, there are no differences between the diiodides and the corresponding polymers. These results probably indicate that the poly(ether ketone)s 5 hold no specific regular secondary structures.

Table 4 Thermal and optical properties of aromatic poly(ether ketone)s 5
Figure 1
figure 1

Circular dichroism spectra of monomers 3 and poly(ether ketone)s 5.

Conclusions

Nanosized palladium species, which are easily prepared in situ from Pd(OAc)2, PPh3 and Bu4NOAc in 1,4-dioxane, were suitable for Suzuki–Miyaura cross-coupling polymerization of diiodides 3 and aromatic diboric acid pinacol esters 4 to synthesize 1,1′-binaphthyl-2,2′-diaryloxy-bearing aromatic poly(ether ketone)s 5. This catalyst was able to be applied to polymerization of aromatic dibromide and even aromatic dichloride. Addition of ammonium salt, that is, Bu4NOAc and Bu4NBr, was requisite for smooth polymerization. The resulting poly(ether ketone)s 5 are soluble in typical organic solvents, such as CHCl3, tetrahydrofuran and dimethylformamide. Poly(ether ketone)s 5 are optically active on the basis of optically active 1,1′-binaphthyl-2,2′-oxy units. Glass transition temperatures (Tg′s) of poly(ether ketone)s 5 are in the range of 188–224 °C. Temperatures where 10% weight losses of poly(ether ketone)s 5 occur under N2 flow are in the range of 445–480 °C.

scheme 1

Suzuki–Miyaura coupling polymerization of dihalides 3 with diboric acid pinacol esters 4m and 4p.