Synthesis of Pendant Nitronyl Nitroxide Radical-Containing Poly

Article pubs.acs.org/Macromolecules

Synthesis of Pendant Nitronyl Nitroxide Radical-Containing Poly(norbornene)s as Ambipolar Electrode-Active Materials Takashi Sukegawa, Ayumi Kai, Kenichi Oyaizu, and Hiroyuki Nishide* Department of Applied Chemistry, Waseda University, Tokyo 169-8555, Japan S Supporting Information *

ABSTRACT: Ambipolar redox-active polymers with a reversible charging and discharging capability were synthesized via ringopening metathesis polymerization (ROMP) of nitronyl nitroxide radical (NN) mono- and disubstituted norbornenes which exhibited p- and n-type redox processes (i.e., one-electron oxidation and reduction per NN group, respectively), using Grubbs catalyst to avoid side reactions of the radical moiety allowing over 95% of radicals to survive after ROMP. ROMP of the NN monomers was accomplished with well-controlled molecular weights of the resulting NN polymers which were coincident with theoretical values in the ratio of [monomer]/[catalyst] = 25−200, narrow polydispersity index (ca. 1.2), and high yields even with [monomer]/[catalyst] > 600. The living character for the ROMP of the NN monomers also allowed block copolymerization. NN-containing block copolymers were synthesized through sequential ROMP with benzyl ether-containing norbornene in high yields. The NN polymer/carbon composite electrode exhibited both p- and n-type charging/discharging with plateau potentials near the redox potentials of the polymer at 0.78 and −0.80 V vs Ag/AgCl, respectively. The spin-coated layer electrode of the NN polymer immobilized on a current collector also demonstrated a fast charging/discharging performance in the range of 10−100 C rates and a cycle stability especially for the p-type reaction. These results made the NN polymer accessible as ambipolar electrodeactive materials and also encouraged other organic radicals to be candidates for electroactive polymers.

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INTRODUCTION Robust organic radical-containing polymers have attracted much attention for their superior charge transfer and storage properties due to the reversible redox reactions of radical sites and their fast electron exchange reactions between densely populated radical sites in spite of their nonconjugated backbones.1−5 When these radical polymers are applied as electrode active materials for organic secondary batteries, the electrochemical properties embodied in advantages such as high charging/discharging rate (120 C, i.e., full charging/discharging in 30 s), high charge capacity (>140 mAh g−1), and long cycle stability (over 1000 cycles).6−9 We have proposed nitroxide radicals such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) for cathode-active materials and phenyl nitroxide radicals and a galvinoxyl radical for anode-active materials, since these groups exhibit the reversible one-electron oxidation (p-type) and the reduction (n-type) to give cations and anions from the neutral radicals, respectively.6,10 n-Type compounds including various redox-active sites other than radicals have also been explored recently.11−13 By contrast, ambipolar materials which demonstrated both p- and n-type properties had been limited to a few π-conjugated polymers for inadequate stability of n-doped states.14 Nitronyl nitroxide (NN), known as an organic magnetic material,15,16 has an ambipolar redox property, and hence a NN-containing poly(styrene) was also prepared as an ambipolar electro-active material for use in a totally organic secondary battery consisted of two electrodes both utilizing the © 2013 American Chemical Society

NN polymer, which demonstrated the properties of a pole-less battery.17 Since ambipolar materials have been extensively studied for organic optoelectronic devices18−21 in addition to the organic batteries, NN-containing polymers have also a potential to contribute to advances in various organic devices. In spite of recent advances in controlled radical polymerization, organic radicals terminate the growth of polymeric chains reversibly by making a covalent bond with the living radical chain ends, represented by nitroxide-mediated radical polymerization.22 Therefore, radical polymerizations of organic radical-containing vinyl monomers are not suitable due to their potential side reactivity. We have previously reported the preparation of NN-containing poly(styrene) via free radical polymerization of NN-containing styrene protected by alkylsilyl groups for nitroxide radicals, followed by deprotection and oxidation.17 However, the inherent heterogeneity of these macromolecular reactions leads to a defect of radical units in the resulting polymer chain and results in decrease of the total amount of charging capacity in battery applications. To achieve the polymerization of radical monomers, we have recently focused on coordination polymerizations in which monomers coordinate selectively to organometallic catalyst centers avoiding side reactions of their radical moieties with the Received: November 6, 2012 Revised: January 26, 2013 Published: February 5, 2013 1361

dx.doi.org/10.1021/ma302278h | Macromolecules 2013, 46, 1361−1367

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Scheme 1. Synthesis of Nitronyl Nitroxide-Substituted Polymers P1 and P2

32.4 mmol) were added to anhydrous DMF (5.4 mL) under a nitrogen atmosphere and stirred for 12 h at 75 °C. The reaction mixture was partitioned between CHCl3 and water, and organic layer was dried with MgSO4. After evaporation, the crude product was purified by silica gel column chromatography with hexane/ethyl acetate (7/1 in v/v), followed by distillation to give 1 as a colorless liquid (2.20 g, 11.1 mmol). Yield: 69%. 1H NMR (CDCl3, 500 MHz, ppm): δ = 9.97 (s, 1H, CHO), 7.81 (d, 2H, Ph), 7.43 (d, 2H, Ph), 6.27 (q, 1H, CCH), 6.20 (q, 1H, CCH), 3.01 (s, 1H, CH), 3.00 (s, 1H, CH), 2.79 (q, 1H, CH2), 1.73 (m, 2H, CH2), 1.56 (d, 1H, CH2), 1.48(m, 1H, CH2). 13C NMR (CDCl3, 500 MHz, ppm): δ = 192.0, 153.9, 137.6, 137.0, 134.2, 129.8, 128.2, 48.0, 45.8, 44.2, 42.4, 33.9. FAB-MS (m/z): calcd for M+ 198.1; found 199.1. IR (film, cm−1): 2923 (νC−H), 1697 (νCO). Anal. Calcd for C14H14O: C, 84.81; H, 7.12; N, 0.00. Found: C, 84.44; H, 7.39; N, 0.65. 2-(4-(Bicyclo[2.2.1]hept-5-en-2-yl)phenyl)-4,4,5,5-tetramethylimidazolidine-1,3-diol (2). 1 (2.20 g, 11.1 mmol), N,N′-(2,3dimethylbutane-2,3-diyl)dihydroxylammonium sulfate (3.00 g, 12.2 mmol), and sodium acetate (1.15 g, 13.3 mmol) were added to methanol/H2O (28 mL/4.4 mL) mixed solvent and stirred for 24 h at room temperature under a nitrogen atmosphere. After evaporation, the crude product was filtrated and washed with CHCl3 and water adequately to give 2 as a white solid (1.74 g, 53.0 mmol). Yield: 48%. 1 H NMR((CD3)2SO, 500 MHz, ppm): δ = 7.73 (s, 2H, OH), 7.43 (d, 2H, Ph), 7.28 (d, 2H, Ph), 7.26 (s, 1H, CH), 6.33 (q, 1H, CH), 6.23 (q, 1H, CH), 4.52 (s, 1H, CH), 3.00 (s, 1H, CH), 2.86 (s, 1H, CH), 2.70 (q, 1H, CH), 1.75 (m, 1H, CH2), 1.60 (m, 1H, CH2), 1.56 (d, 1H, CH2), 1.38 (d, 1H, CH2), 1.12 (s, 6H, CH3), 1.09 (s, 6H, CH3). 13 C NMR((CD3)2SO, 500 MHz, ppm): δ = 144.4, 139.2, 137.2, 137.0, 128.4, 126.6, 90.4, 89.7, 66.0, 48.2, 45.3, 43.0, 42.9, 41.8, 33.0, 24.4, 24.3, 17.1. FAB-MS (m/z): calcd for M+ 328.2; found 329.2. IR (KBr, cm−1): 3247 (νO−H), 2973 (νC−H). Anal. Calcd for C20H28N2O2: C, 73.14; H, 8.59; N, 8.53. Found: C, 72.55; H, 8.64; N, 8.36. 2-(4-(Bicyclo[2.2.1]hept-5-en-2-yl)phenyl)-4,4,5,5-tetramethylimidazolidine-3-oxide-1-oxyl (3). 2 (0.50 g, 1.52 mmol) and MnO2 were added in anhydrous CH2Cl2 and stirred for 1 h at room temperature. The resultant deep blue solution was evaporated and purified via aluminum oxide column chromatography with CH2Cl2 followed by recrystallization in hexane/CH2Cl2 to give 5 as a needle-like blue solid (0.43 g, 1.32 mmol). Yield: 87%. FAB-MS (m/z): calcd for M+ 325.2; found 326.1. IR (film, cm−1): 2923 (νC−H). Anal. Calcd for C20H25N2O2: C, 73.82; H, 7.74; N, 8.61. Found: C, 73.76; H, 7.61; N, 8.59. 3,3′-(Bicyclo[2.2.1]hept-5-ene-2,3-diyl)dibenzaldehyde (4). 3-Iodobenzaldehyde (3.32 g, 14.3 mmol), 3-formylphenylboronic acid (2.35 g, 15.7 mmol), triphenylphosphine (0.23 g, 0.86 mmol), potassium carbonate (5.91 g, 42.8 mmol), and 2,5-norbornadiene (5.22 mL, 51.6 mmol) were dissolved in THF/H2O (36 mL/36 mL) under a nitrogen atmosphere, and palladium(II) acetate was added to the mixture. The mixture was stirred for 12 h at 60 °C. The reaction

catalysts. For example, TEMPO- and PROXYL-containing polyethers were synthesized using ZnEt2-H2O, which yielded the polymers characterized by the high molecular weight (Mn > 1.0 × 105) and high unpaired electron densities (1.0 spin/ repeating unit).8 Moreover, poly(phenylacetylene) bearing the NN group was produced with the high radical density (0.92 spin/repeating unit) via the Rh-catalyzed polymerization of NN-containing phenylacetylene apart from its relatively poor electrode performances.23 Ring-opening metathesis polymerization (ROMP) is one of the coordination polymerization method and especially a strong tool for synthesis of functional polymers. In particular, Grubbs catalysts have both high activity and broad functional group tolerance.24−27 On the basis of this properties, we successfully prepared a TEMPO-containing poly(norbornene) via ROMP using Grubbs second-generation catalyst (G2) without side reactions.28 Herein, we report the synthesis of a further reactive NN-containing poly(norbornene)s (PNNs) including block copolymers (Scheme 1) via ROMP using G2 and 3bromopyridine-ligated third-generation catalyst (G3) and their electrochemical properties as ambipolar materials. We anticipated that Grubbs catalyst should work even in the presence of the ambipolar redox-active NN moiety, leading to high radical densities and that poly(norbornene) backbone would improve electrode performance with its higher swelling property than that of the polyacetylene backbone.23 The excellent properties of the NN norbornenes as the monomers in ROMP were also suggested by successful block copolymerization, which indicated the possibility of various well controlled polymerization using these monomers.

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EXPERIMENTAL SECTION

Materials. All solvents were purified by distillation, prior to use. A vapor-grown carbon fiber (VGCF) was obtained from Showa Denko Co. A binder powder, poly(vinylidene fluoride) (PVdF) resin (KF polymer), was purchased from Kureha Chemical Co. All starting organic reagents were obtained from Sigma-Aldrich Japan, Tokyo Chemical Industry Co., or Kanto Chemical Co. and were used without further purification. Synthesis of Nitronyl Nitroxide Monomers. Both NNmonosubstituted norbornene 3 and disubstituted norbornene 6 were synthesized through three steps from 2,5-norbornadiene (Scheme 1). 4-(Bicyclo[2.2.1]hept-5-en-2-yl)benzaldehyde (1). 4-Bromobenzaldehyde (3.00 g, 16.2 mmol), bis(triphenylphosphine)palladium(II) dichloride (0.48 g, 0.68 mmol), 2,5-norbornadiene (5.91 mL, 58.3 mmol), triethylamine (6.9 mL, 49.2 mmol), and formic acid (1.22 mL, 1362


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Electrochemical Measurements. Electrochemical analyses were carried out in a conventional cell under nitrogen. The auxiliary electrode was a coiled platinum wire. The reference electrode was a commercial Ag/AgCl immersed in a solution of 0.1 M TBAClO4 in CH3CN. The formal potential of the ferrocene/ferrocenium couple was 0.45 V vs this Ag/AgCl electrode. An ALS 660C electrochemical analyzer was employed to obtain the cyclic voltammograms and chronopotentiograms. Heterogeneous electron-transfer rate constants k0 for the electrode reactions were determined from the variations of a peak- to-peak separation ΔEp with a potential sweep rate v in cyclic voltammetry according to k0 = ψ{πDFv/(RT)}1/2 (Nicholson’s method), where D is the diffusion coefficient of the reactant determined from the slope of the peak current−v1/2 plot29 and ψ was given by the data linking to ΔEp.30 The macroscopic diameters of the glassy carbon disk microelectrodes used for the electrochemical measurements (φ = 1.0 and 3.0 mm) were confirmed from the magnitude of the current for the electrolysis of ferrocene as the internal standard. Measurements. 1H and 13C NMR spectra were recorded on a JEOL ECX-500 spectrometer with chemical shifts downfield from tetramethylsilane as the internal standard. Infrared spectra were obtained using a JASCO FT-IR 410 spectrometer with sodium chloride plates. Molecular weight measurements were done by gel permeation chromatography using a Shimadzu Prominence UFLC instrument with 0.1 M lithium chloride/DMF solution as the eluent. Calibration was done with polystyrene standards. Elemental analyses were performed using a PerkinElmer PE-2400 II and a Metrohm 645 multi-DOSIMAT. Two parallel analyses were performed for each sample. Mass spectra were obtained using a JMS-SX102A or a Shimadzu GCMS-QP5050 spectrometer. ESR spectra were recorded using a JEOL JES-TE200 spectrometer with a 100 kHz field modulation frequency and a 0.1 mT width. The magnetization and the magnetic susceptibility of the powdery polymer samples were measured by a Quantum Design MPMS-7 SQUID magnetometer. The magnetic susceptibility was measured from 10.0 to 300 K in a 0.5 T field.

mixture was partitioned between CHCl3 and water, and organic layer was dried with MgSO4. After evaporation, the crude product was purified by silica gel column chromatography with chloroform/ethyl acetate (19/1 in v/v), followed by recrystallization to give 4 as a colorless solid (3.00 g, 9.92 mmol). Yield: 69%. 1H NMR (CDCl3, 500 MHz, ppm): δ = 9.79 (s, 2H, CHO), 7.43 (d, 4H, Ph), 7.12 (d, 4H, Ph), 6.48 (2, 2H, HCCH), 3.35 (s, 2H, CH), 3.12 (s, 2H, CH), 2.32 (d, 1H, CH2), 1.77 (d, 1H, CH2). 13C NMR (CDCl3, 500 MHz, ppm): δ = 193.2, 150.3, 140.1, 134.1, 129.9, 129.2, 49.8, 47.2, 46.4. FAB-MS (m/z): calcd for M+ 302.1; found 303.1. IR (film, cm−1): 2974 (νC−H), 1698 (νCO). Anal. Calcd for C21H18O2: C, 83.42; H, 6.00; N, 0.00. Found: C, 83.12; H, 6.04; N, 0.01. 2,2′-(Bicyclo[2.2.1]hept-5-ene-2,3-diylbis(3,1-phenylene))bis(4,4,5,5-tetramethylimidazolidine-1,3-diol) (5). Followed the same route for 2. Yield: 65%. 1H NMR ((CD3)2SO, 500 MHz, ppm): δ = 7.63 (s, 2H, OH), 7.59 (d, 2H, 0H), 6.97 (m, H, Ph), 7.46 (s, 2H, CHCH), 4.21 (2, 2H, CH), 3.16 (s, 2H, CH), 3.04 (s, 2H, CH), 2.22 (d, 1H, CH), 1.64 (d, 1H, CH2), 1.02 (d, 24H, CH3). 13C NMR ((CD3)2SO, 500 MHz, ppm): δ = 142.1, 140.7, 139.7, 130.6, 128.1, 127.7, 125.5, 91.2, 79.7, 66.7, 48.9, 47.7, 45.7, 24.7, 17.6. FAB-MS (m/ z): calcd for M+ 562.35; found 562.37. IR (KBr, cm−1): 3448 (νO−H), 2969 (νC−H). 2,2′-(Bicyclo[2.2.1]hept-5-ene-2,3-diylbis(3,1-phenylene))bis(4,4,5,5-tetramethylimidazolidine-3-oxide-1-oxyl) (6). Followed the same route for 3. Yield: 92%. FAB-MS (m/z): calcd for M+ 556.30; found 556.54. IR (film, cm−1): 2986 (νC−H). Anal. Calcd for C33H49N4O4: C, 71.20; H, 7.24; N, 10.06. Found: C, 71.23; H, 7.21; N, 10.06. Determination of Radical Content. The polymers P1 (Mn = 7.6 × 103, PDI = 1.21) and P2 (Mn = 5.2 × 104, PDI = 1.29) were characterized by g values of the electron spin resonance (ESR) signal at 2.0067 and 2.0069, respectively. These g values were close to those of the corresponding monomers 3 (2.0068) and 6 (2.0069), respectively. The radical concentrations were determined by means of superconducting quantum interference device (SQUID) using the Curie plots and the values for saturated magnetization. General Procedure of Ring-Opening Metathesis Polymerization of NN Monomers. A solution of G3 (10.8 mg, 0.012 mmol) in CH2Cl2 (0.2 mL) and a solution of 3 (101 mg, 0.31 mmol) in CH2Cl2 (0.9 mL) were prepared separately. Each solution was thoroughly degassed by three times freeze−thaw cycles. The catalyst solution was added rapidly to the monomer solution, and the resultant solution was stirred vigorously under a nitrogen atmosphere. After 5 min, ethyl vinyl ether (0.74 mL, 7.75 mmol) was added to the reaction mixture and stirred for further 30 min. The polymer was then precipitated form excess diethyl ether/hexane (1/1 in v/v) mixed solvent twice to yield a blue powder. General Procedure of Block Copolymerization of 3 and 7. A solution of G3 (2.83 mg, 3.2 μmol) in CH2Cl2 (0.2 mL), a solution of 3 (51.1 mg, 0.16 mmol) in CH2Cl2 (0.4 mL), and a solution of 7 (50.6 mg, 0.15 mmol) in CH2Cl2 (0.5 mL) were prepared separately. Each solution was thoroughly degassed by three times freeze−thaw cycles. The catalyst solution was added rapidly to the solution of 7, and the resultant solution was stirred vigorously under a nitrogen atmosphere. After 5 min, the solution of 3 was added to the reaction mixture and stirred for 1 h. Ethyl vinyl ether (0.74 mL, 7.75 mmol) was added to the reaction mixture and stirred for further 30 min. The polymer was then precipitated form excess diethyl ether/hexane (1/1 in v/v) mixed solvent twice to yield a blue powder. Preparation of Thin Layer Electrodes of P1. The P1 (Mn = 7.1 × 105, PDI = 1.52, 10.8 mg) was dissolved in THF (0.56 mL). The P1 solution was spread on the glassy carbon electrode and then coated to give a thin layer by a spin-coater (MIKASA, MS-A100, 5000 rpm, 30 s). The P1 was dried in a vacuum dryer for 2 h. Preparation of P1/Carbon Composite Electrodes. The P1 (Mn = 7.1 × 105, PDI = 1.52, 5.0 mg) was mixed with VGCF (40.0 mg) and PVdF (5.0 mg) in N-methyl-2-pyrrolidone (NMP). The mixture was pasted on the glassy carbon electrode and dried under vacuum at room temperature for 12 h to give the composite electrode with a composition of P1/VGCF/PVdF = 1/8/1 (w/w/w).

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RESULTS AND DISCUSSION Synthesis and Characterization of NN Mono- and Disubstituted Norbornenes. Since monomer structures should be as compact as possible to maximize unpaired electron density (i.e., number of unpaired electron/molecular weight) for higher battery capacity, monomers were designed in which the phenyl group of NN was directly bound to norbornene without spacers such as ester and ether bonds and alkyl chains. Pd-catalyzed coupling reactions were selected to bind the phenyl group to 2,5-norbornadiene via direct C−C bonding (Scheme 1). Both 3 and 6 were prepared in three steps: (i) reductive Heck reaction to link norbornene with benzaldehyde, (ii) condensation of aldehyde with bis(hydroxylamine), and then (iii) oxidation to form NN. In the case of a NN-disubstituted monomer in which the NN group is p-positioned from the norbornene unit, the condensation reaction remained in low reactivity (65%). This difference originates from the cis, exo insertion of phenyl groups as mentioned in a previous report.31 In the preparation of the p-substituted NN monomer, two benzaldehydes were inserted into the cis, exo moiety so that the p-positioned aldehydes aligned parallel to each other and interfered with the condensation reaction due to its steric crowding. On the other hand, the m-positioned aldehydes of 4 were spatially separated with each other and therefore condensed more efficiently in the step ii. These cis, exo insertions were supported by two-dimensional NMR (Figure S1) which indicated that 1H located in the phenyl 1363


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group of NN spatially closed to 1H located on the C-bridge of norbornene.32 The unpaired electrons of 3 and 6 were characterized by ESR spectroscopy and SQUID measurements. The ESR spectrum of 3 indicated five lines in the ratio of 1:2:3:2:1 in intensity due to the coupling of two identical nitrogens and a g value (2.0068) which were unique to NN (Figure S2a). The ESR spectrum of 6 indicated nine lines, but the g value (2.0069) was attributed to NN, suggesting that the nine lines arose from the spin−spin interaction between the neighboring two NN sites of the monomer based on their spatially closed positions (Figure S2b). The unpaired electron densities of 3 and 6 were estimated by SQUID for 1.85 × 1021 unpaired electrons/g (1.0 radicals per monomer unit) and 2.17 × 1021 unpaired electrons/g (1.0 radicals per monomer unit), respectively. Cyclic voltammograms of the monomers 3 and 6 are given in Figure 1. Two redox potentials of the monomer at +0.75 and

transfer rate constants (k0) of 3 for both p- and n-type redox reactions using the Nicolson plot (Figures S4 and S5). The diffusion coefficient (D) was estimated from the slope of the plots between anodic peak currents and the square root of the scan rates, and then k0 was calculated using the equation mentioned in the Experimental Section. The linearity of the plots revealed the electrochemically reversible properties of both of the p- and the n-type reactions for NN. The k0 for pand n-type reactions were 7.1 × 10−2 and 2.4 × 10−2 cm s−1, respectively, which were as fast as that of TEMPO as the electrode-active materials.33 Synthesis and Characterization of NN Mono- and Disubstituted Poly(norbornene)s. NN-containing poly(norbornene)s were synthesized by ROMP using G2 and G3 as polymerization catalyst. The characteristics of NN polymers are shown in Table 1. Three was polymerized by G2 in more than 70% yield of P1 (with [M]/[C] = 25, 50, and 100), which became insoluble in organic solvents after purification even in GPC eluents such as chloroform, THF, and DMF, despite their homogeneous solutions at the end of the polymerization. Although low molecular weight P1 might dissolve readily in the solvents due to the potential solubility of NN, the viscosity of polymerization solutions obviously increased after few minutes from the injection of G2 and then decreased to be like homogeneous state at the end. The initial increase of viscosity indicated that the molecular weights of P1 were much higher than the theoretical values and the polydispersity indexes (PDIs) were relatively wide for each ROMP as in the case of [M]/[C] = 25. This result was suspected to originate from side reactions. However, the high radical density of P1 estimated by SQUID (1.76 × 1021 unpaired electrons/g, i.e., 0.95 radicals per monomer unit) indicated that NN was as stable as TEMPO against G2 and that the side reactions of NN were unlikely during the polymerization (Figure S6). Furthermore, the obtained P1 was always insoluble for several kinds of polymerization conditions such as solvent, reaction time, and temperature. Such properties are rather ascribed to the reactivity of G2 which is characterized by relatively slow rate of the initiation reaction compared to the propagation, which prompted us to explore more efficient catalysts for the polymerization of the NN-containing monomers. To apply P1 for organic electrode-active materials, molecular weight should be high and narrow in dispersion to immobilize

Figure 1. Cyclic voltammograms obtained for 1.0 mM CH3CN solutions of (a) 3 and (b) 6 with 0.1 M (n-C4H9)4NClO4. Scan rate: 100 mV/s.

−0.88 V for 3 and at +0.77 and −0.85 V (vs Ag/AgCl) for 6 were ascribed to NN. Differential pulse voltammograms of 3 and 6 guarantee their reversible redox reactions at both positive and negative potentials (Figure S3). Typically, NN exhibited one-electron redox for the n-type reaction. However, two oxidation current peaks were observed near −0.69 and −0.81 V (Figure 1b). Considering the spin−spin interaction between the NN sites of 6 observed in ESR measurement, the two-step oxidation is ascribed to the electronic interaction between the neighboring NN sites in 6 that especially occurred in the reduced state. We examined the heterogeneous electron Table 1. ROMP of NN-Containing Norbornene Monomersa P1

P2

a

catalyst

monomer

[M]/[C]

yield [%]

Mn,theo [× 103]

G2 G2 G2 G3 G3 G3 G3 G3 G3 G3 G3 G3 G3 G3

3 3 3 3 3 3 3 3 3 3 6 6 6 6

25 50 100 25 50 100 150 200 615 1843 25 50 100 150

100 81 72 98 98 95 94 94 94 40 97 97 99 99

8.1 16.3 32.5 8.1 16.3 32.5 48.8 65.1 200 600 13.9 27.8 55.7 83.5

Mn,obs [× 103]b 127

6.8 14.2 34.5 44.8 62.4 246 705 11.1 24.1 35.8 52.4

Mw,obs [× 103]b

PDIb

322

2.53

8.3 17.4 42.9 56.4 73.0 382 1070 14.4 31.7 54.7 67.9

1.21 1.23 1.25 1.26 1.16 1.55 1.52 1.28 1.31 1.52 1.29

Polymerization in CH2Cl2, rt, 5 min. bEstimated by GPC with 0.1 M LiCl/DMF eluent. 1364


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the polymer on a current collector despite their good solubility into electrolyte solutions. To accomplish high molecular weight and narrow PDI, we have selected G3, which allows controlled ROMP for various kinds of functional monomers due to the much faster rate of the initiation reaction than that of the propagation reaction.34 The polymerization results are shown in Figure 2. G3 demonstrated its high reactivity to 3 as well as

Considering the well-controlled ROMP of NN-containing norbornenes, we examined block copolymerization of the NN monomer via living ROMP (Scheme S1). Copolymerization was carried out with benzyl ether-containing norbornene 7, often used to test the living system in ROMP of other monomers.35,36 The results of living ROMP are summarized in Table 2. Experimental molecular weights of the block copolymers were slightly less than theoretical values with narrow PDIs; moreover, GPC traces of ROMP of the first monomer shifted to higher molecular weights (Figure S8), and block copolymers were obtained in over 95% yield regardless of the NN monomer 3 as first or second monomer. These results strongly supported the living character of ROMP of the NNcontaining monomers. Performance as Electrode-Active Materials. The charging/discharging property of P1 was determined using chronopotentiometry of the P1/VGCF (carbon nanofiber)/ PVdF (binder) composite electrode on a glassy carbon current collector in a three-electrode cell. The charging/discharging curves for the p-type and the n-type reactions appeared at 10 C rate in the ranges of 0.55 to 1.05 and −0.60 to −1.10 V, respectively (Figure 3). The plateau potential well agreed with

Figure 2. Molecular weight and PDI dependence on [M]/[C] for (a) P1 and (b) P2. Open and closed circles represent Mn and PDI, respectively. Solid line represents the theoretical Mn assuming a living nature of the polymerization.

other monomers, so that reaction time before the termination with ethyl vinyl ether was only 5 min for each polymerization which was enough for P1 to be obtained in high yield (>94%) with [M]/[C] = 615 (i.e., Mn > 200 kDa). Contrary to the gelation of the polymerization using G2, the polymerization solutions kept homogeneous state through ROMP even when [M]/[C] was more than 200. The experimental molecular weight agreed with the theoretical values with sufficiently narrow PDI of 1.16 ([M]/[I] = 200) and indicated a good linear relationship to [M]/[C] ratio in the range of 25−200. In addition to the well-controlled molecular weight, the radical density of P1 was 1.80 × 1021 unpaired electrons/g (i.e., 0.97 radicals per monomer unit). These results strongly supported that NN was compatible with G3 and does not interfere with ROMP. To prepare more densely populated NN-containing polymer, ROMP of the NN-disubstituted monomer 6 was also examined, the theoretical capacity of which was up to 96 mAh g−1 (17% increased from P1). Under the same condition of ROMP for 3, the polymerization underwent in more than 97% yield with relatively narrow PDI of 1.28 ([M]/[C] = 25). The linear relationship between Mn and [M]/[C] was also observed as in the case of P1; however, there existed small differences between theoretical and experimental values in molecular weight. The reason for these incompatible results is undetermined at the present but expected to be the excluded volume effects of P2 in GPC based on the increased bulkiness of the repeating unit. Even in these results of ROMP, radical density of P2 was as high as P1 (95%) (Figure S7), indicating that the multi NNfunctionalized norbornenes were stable against the Grubbs catalysts as multi TEMPO-functionalized norbornene.28

Figure 3. Charging/discharging curves of P1 composite electrode for the (a) p-type and (b) n-type processes at 10 C rate, obtained with 0.1 M (n-C4H9)4NClO4 and 0.01 M (n-C4H9)4NOH (for n-type)/ CH3CN as a bathing solution. Inset: cyclic voltammograms recorded for the two types of the electrode process scanned at 5 mV/s.

E1/2 of 0.78 and −0.80 V for the two types of the reactions that were observed in cyclic voltammograms (Figure 3, inset). The charging/discharging capacity for the p-type reactions was 67/ 59 mAh g−1 (82 and 71% with respect to theoretical capacity, respectively), and those for the n-type reaction was 52/44 mAh g−1 (63 and 54%, respectively). These capacities were higher than those of the NN-containing poly(phenylacetylene) in terms of both absolute capacity and ratio between the actual capacity and the theoretical one (ca. 30% for poly(phenylacetylene)).23 The improved charging/discharging capacity indicates that poly(norbornene) backbone swell up with electrolyte solutions better than the rigid polyacetylene backbone which inhibited the diffusion of counterions so that the mobility of counterions increase to access redox sites easily.

Table 2. Block Copolymerization of 3 and 7a Mn [× 103]

Mn [× 103] b

catalyst

M1

M2

[M1]/[M2]/[C]

M1, theor

M1, obsd

PDIMI

block, theor

block, obsdb

PDIblockb

yield [%]

radical densityc [%]

G3 G3

3 7

7 3

50/50/1 50/50/1

16.3 16.6

11.9 14.2

1.19 1.17

32.9 32.9

29.9 29.3

1.23 1.22

96 99

46 49

a

Polymerization in CH2Cl2, rt for 5 and 30 min for each monomer. bEstimated by GPC with 0.1 M LiCl/DMF eluent. cRadical density = [radical unit]/[3 + 7 unit], estimated by ESR. 1365


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The electrode performance of P1 thin layer (ca. 150 nm thickness) for the p-type reaction was also examined, taking advantage of an electrochromic behavior of the p-type redox process from blue (radical) to red (cation) layers. P1 electrode demonstrated a high rate performance keeping a constant discharging capacity at 10−100 C rate (i.e., 360−36 s discharging), stability of cycle life maintaining 70% of first cycle capacity, and reversible color switches (Figure 4 and

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Grants-in-Aid for Scientific Research (Nos. 21550120, 24108739, and 24225003) from MEXT, Japan. We thank Dr. Shigeyuki Iwasa and Dr. Kentaro Nakahara of NEC Co. for technical discussions.

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REFERENCES

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Figure 4. Discharging rate performance of a P1 thin layer electrode (ca. 150 nm in thickness) for the p-type process recorded after charging at 10 C. The discharging rate was changed in the range of 10−100 C. The electrolyte was a 0.1 M (n-C4H9)4NClO4/CH3CN solution. Inset: cyclic performance at 40 C rate for charging and discharging.

Figure S9). Electrochemical properties of P2 solution exhibited both the p- and n-type redox reactions at 0.77 and −1.0 V, respectively, as well as the corresponding monomer 6. Although the electrode performance of P2 was inferior to P1 because of their low molecular weight and solubility, we anticipate that these problems could be overcome by cross-linking using poly(norbornene) backbone28,37 and by improvement of composite techniques, which is the topic of our continuous investigations.

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CONCLUSION Nitronyl nitroxide radical-containing poly(norbornene)s, P1 and P2, were successfully prepared via ROMP of nitronyl nitroxide radical monomers using Grubbs catalysts with wellcontrolled molecular weight, narrow PDI, high yield, and high radical density (ca. 100%). ROMP of nitronyl nitroxide radical monomers is expected to be a versatile method for the preparation of various radical polymers such as the verdazyl radical derivatives which also exhibit the similar both p- and ntype redox reactions. P1 electrode demonstrated both p- and ntype charging/discharging capabilities and capacities of which were higher than those of the poly(phenylacetylene) derivatives. Living polymerization was realized in ROMP of the nitronyl nitroxide monomers using G3 catalyst, which opened the door for the synthesis of well-defined nanostructured and multifunctional radical block copolymers which have been regarded as essential in exploring the electric properties of various radical polymer solids.38

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Article

ASSOCIATED CONTENT

S Supporting Information *

Additional figures, characterizations, and calculations. This material is available free of charge via the Internet at http:// pubs.acs.org. 1366


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Synthesis of Pendant Nitronyl Nitroxide Radical-Containing Poly

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