Metathesis Cyclopolymerization of Imidazolium-Functionalized 1,6

Sep 9, 2014 - *(M.X.) Telephone: +86 21 54340058. ... after Its Invention: Revealing a Comprehensive Picture of Cyclopolymerization Using Grubbs Catal...
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Metathesis Cyclopolymerization of Imidazolium-Functionalized 1,6Heptadiyne toward Polyacetylene Ionomer Wei Song, Huijing Han, Xiaojuan Liao, Ruyi Sun, Jianhua Wu, and Meiran Xie* Department of Chemistry, East China Normal University, Shanghai 200241, China S Supporting Information *

ABSTRACT: Metathesis cyclopolymerization (MCP) of ionic 1,6heptadiyne is successfully applied to synthesize polyacetylene (PA) ionomer in different solvents especially in imidazolium-based ionic liquid (IL), and MCP of ionic monomer in the mixture of THF/IL proceeded in a controlled manner by the action of Grubbs second and third generation catalysts (Ru−II, Ru−III). The influence of catalysts and solvents on polymerization behavior and chain microstructure was investigated, and the results revealed that the isolated polymer yield could reach to 97% and 82% by Ru−III and Ru−II, respectively, in 1:2 THF/IL after optimization, and PAs incorporating imidazolium pendent contained ≥95% five-membered-ring structure and almost all trans-double bonds along the backbone. PAs formed in different solvents reflected significant variance in optical absorption properties, and a bathochromic shift effect was observed with the mixture of THF/IL.

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INTRODUCTION Ionomers incorporated with imidazolium-based ionic moiety have become the focus of intense research in the field of high performance materials and ion conductive matrices for their tunable structure, thermal stability, relatively high ionic conductivity, wide electrochemical window, and their amphoteric behavior in solution.1−6 Consequently, the exploitation of chemical modification of the imidazolium-based cation to introduce polymerizable groups is now an active topic. To date, the polymerizable imidazolium cation research has concentrated overwhelmingly on the two common acryloyl or vinyl groups,1,3,6−14 which further undergone atom transfer radical polymerization (ATRP) or acyclic diene metathesis (ADMET) polymerization to result in imidazolium-containing ionomers. Indeed, one of the demerits of ATRP is the presence of residual transition metal in the final polymer, which may limit the application of polymer and cause environmental problem, while polymers generated via ADMET polymerization have broad molecular weight distribution and consumed long time for preparation. Polyacetylene (PA), the first generation of semiconducting polymers,15 has been extensively studied by a great number of chemists, because of its promising properties, such as electrical conductivity,16 optical nonlinearity,17 and photoconductivity.18 Unfortunately, it suffers from extremely low solubility and poor processability.19 The metathesis cyclopolymerization (MCP) of 4-subsititued 1,6-heptadiynes is a well-known solution for obtaining soluble π-conjugated PAs,20 and a breakthrough on MCP was disclosed very recently by Choi et al., who found that Grubbs third generation catalyst (Ru−III) underwent selective α-addition to produce PAs with only five-membered rings.21−24 Particularly, such polymers solely based on five-membered rings © 2014 American Chemical Society

possess more planar structure, thus induced higher effective conjugation length and conductivity,25 making the polymers potential materials for use in organic electronic and optic devices. Although the MCP chemistry has got tremendous success in the construction of various PAs, the application in ionic 1,6-heptadiyne to construct ionomer was still rare.26−29 Currently available ionized PAs are limited to main-chain selfdoped architectures.26 This structure is beneficial for controlling the doping levels, and stabilizing the interface between conjugated polymers with different doping types. Nevertheless, it is subjected to limited freedom to change the ion density and ion species. Therefore, it is of great importance to develop efficient and robust methodologies for synthesis of other doping types of PAs. Aiming to synthesize doping PAs sharing excellent optical properties and higher conductivity, we consider the following aspects in monomer and polymer design: (i) selecting conjugated backbone as pathway to provide the n-channel for facilitating electron/ion transport; (ii) incorporating imidazolium into side-chain to support ionic conductivity, improve the doping density and doping species, and modify solubility and enhance stability of PAs; (iii) using 1,6-heptadiyne as polymerizable group and commercially Grubbs catalysts as initiator to generate PAs solely containing five-membered rings with relatively high ionic conductivity; (iv) homogeneous polymerization in a controlled manner is expectative. Imidazolium-based ionic liquids (ILs) are known to be nonvolatile, nonflammable, and thermally stable solvents. The Received: June 12, 2014 Revised: August 26, 2014 Published: September 9, 2014 6181

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polystyrene standard. Cyclic voltammetry was carried out with an Autolab PGSTAT12 potentiostat from Eco Chemie coupled to an electrochemical cell with three electrodes. A glassy carbon electrode was used as working electrode, a Pt wire as counter, and Ag/AgCl was used as the reference electrode. 0.1 M Bu4NPF6 of CH3CN solution as the supporting electrolyte. The scan rate is 100 mV s−1 and Fc+/Fc was used as the reference. X-ray diffraction (XRD) analysis was performed on a Rigaku D/MAX2400 diffractometer. Thermal gravimetric analysis (TGA) was performed using a SDTA851e/SF/ 1100 TGA instrument under nitrogen flow at a heating rate of 10 °C/ min from 50 to 800 °C. Differential scanning calorimeter (DSC) was performed on a Netzsch 204F1 in nitrogen atmosphere. An indium standard was used for temperature and enthalpy calibrations. The sample was first heated from 25 to 300 °C, held at this temperature for 3 min to eliminate the thermal history, and then cooled to room temperature and heated again from 25 to 300 °C at a heating or cooling rate of 10 °C/min. Synthesis of 1,6-Heptadiyne-Derived Ionic Monomer (IM). 1Methyl-3-(2-carboxylethyl)-imidazolium hexafluorophosphate [EMImCOOH][PF6] (10.80 g, 36 mmol), 4-(hydroxymethyl)-1,6heptadiyne (3.66 g, 30 mmol), DMAP (0.37 g, 3 mmol), and 80 mL of CH3CN were charged into a 250 mL of round-bottom equipped with a magnetic stirrer under nitrogen atmosphere. The mixture was stirred at 0 °C for 10 min, EDCI·HCl (6.30 g, 33 mmol) was then added to the solution, and stirred for 3 days after the solution warmed to room temperature. After solvent distillation, it was purified by chromatography (SiO2, CH2Cl2/AcOEt = 40/1) to give the ionic monomer IM as a white waxy solid in 85% yield (10.30 g, 25 mmol). 1H NMR ((CD3)2CO), δ (ppm): 9.04 (s, 1 H, imidazolyl−H), 7.78 (s, 1 H, imidazolyl−CHCH), 7.69 (s, 1H, imidazolyl−CHCH), 4.66 (m, 2 H, CHCH2), 4.19 (m, 2 H, NCH2CH2), 4.06 (s, 3 H, NCH3), 3.15 (m, 2H, NCH2CH2), 2.47 (d, 2H, CHC−CH2), 2.36−2.35 (m, 5H, CH2CHCH2). 13C NMR ((CD3)2CO), δ (ppm): 169.1, 137.2, 123.1, 122.5, 80.4, 70.6, 64.9, 44.7, 35.8, 35.31, 33.3, 18.8. 31P NMR ((CD3)2CO), δ (ppm): −143.3 (PF6). HR-ESIMS: calcd For C15H19N2O2 (M− PF6), 259.1358; found, 259.1356. IR (KBr): 3301 (C−H), 3175, 3122 (NC−H), 2957, 2115(CC), 1725(C O), 1579−1617(CN), 1457−1388(CC), 1174(C−N), 1166(C− O), and 847 (PF6) cm−1. Anal. Calcd: C, 44.563; H, 4.737; N, 6.929. Found: C, 44.570; H, 4.717; N, 6.732. General Metathesis Cyclopolymerization Procedures. Typically, MCP was carried out in a Schlenk tube under dry nitrogen atmosphere for a preset time. After polymerization was terminated by addition of a small amount of ethyl vinyl ether and stirring for 1 h, the solution was poured into an excess of methanol. The precipitated solid was redissolved in acetone, and precipitated out once again from methanol. After that, the obtained black solid was dried in a vacuum oven at 60 °C to a constant weight. Metathesis Cyclopolymerization of Ionic Monomer in Organic Solvents. A 10 mL of Schlenk flask was charged with monomer IM (126 mg, 0.3 mmol) dissolved in 0.1 mL of CHCl3. In another 10 mL of flask, Ru−III (2.5 mg, 3 μmol) was dissolved in 0.2 mL of CHCl3. After being degassed with three freeze−vacuum−thaw cycles, the catalyst solution was then injected into the preheated monomer solution via a syringe under vigorous stirring at 45 °C for 3 h. The MCP procedures of IM in DCE and THF were the same as that in CHCl3. Metathesis Cyclopolymerization of Ionic Monomer in [BMIm][PF6] or [BMIm][PF6]/THF. A 10 mL Schlenk flask was charged with IM (126 mg, 0.3 mmol) dissolved in 0.3 mL of [BMIm][PF6] or [BMIm][PF6]/THF. After being degassed with three freeze− vacuum−thaw cycles, the catalyst Ru−III (2.5 mg, 3 μmol) was added directly in solid form to the monomer solution under vigorous stirring at 45 or 0 °C for 3 h. 1H NMR ((CD3)2CO), δ (ppm): 8.96 (s, imidazolyl-H), 7.74 (s, imidazolyl−CHCH), 7.67 (s, imidazolyl− CHCH), 6.83 (s, CHCH on polymer chain), 4.63 (m, NCH2CH2), 4.24−3.91 (m, OCH2CH + NCH3), 2.71−2.73 (s, OCOCH2), 2.79−2.39 (m, CCH2CH). 13C NMR ((CD3)2CO), δ (ppm): 171.1, 138.4, 125.2, 124.2, 72.6, 46.5, 36.1, 35.5, 33.7. IR (KBr): 3288(associated water), 3175, 3130 (NC−H), 2981, 2150,

promising application of ILs is widespread in polymer preparation through various polymerization techniques,29−34 but is still a slightly explored field for metathesis polymerizations. For example, Dixneuf and Buchmeiser found that ringopening metathesis polymerization (ROMP) can proceed successfully in such unconventional ILs of 1-butyl-2,3-dimethyl imidazolium ([BDMIm][X]), but no polymers formed in the most commonly ILs of 1-methyl-3-butyl imidazolium ([MBIm][X]).35,36 Rooney supplied a different research result on ROMP which was performed well in [BMIm][BF4], while a little polymer was available in [BMIm][PF6].27 On the basis of our earlier reports,30,37 the commonly ILs whatever the hydrophobic [MBIm][PF6] or the hydrophilic [BMIm][BF4] proved to be the suitable solvents for ROMP, facilitating the polymerization with good control over the molecular weight and molecular weight distribution, enhancing the solubility of polymers, or affording the stable activity of catalyst for polymerization. Except for these pioneering works, there is seldom report dealing with MCP in ILs by Buchmeiser et al.,29 they found that MCP in [BDMIm][X] and [MBIm][X] shared the same results as those of ROMP. It should be deduced from the aforementioned results that [MBIm][X] is a suitable medium for metathesis polymerizations, [BDMIm][X] is not only absolutely unnecessary but also unsuitable because of its high viscosity at room temperature and costliness. In view of these points, herein, an investigation on MCP of ionic 1,6-heptadiyne derivative in both organic solvents and IL [MBIm][PF6] was performed, affording the imidazolium-based PA ionomer, and some interesting results in this IL have been achieved. The ability of MCP to produce materials that combine ion conductivity with optical properties may ultimately open the door for the development of novel photoelectric device.

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EXPERIMENTAL SECTION

Materials. Tricyclohexylphosphine[1,3-bis(2,4,6-trimethylphenyl)4,5-dihydroimidazol-2-ylidene]benzylidene ruthenium dichloride (Grubbs second generation catalyst, Ru−II), N-methylimidazole, 3chloropropionic acid and KPF6 were purchased from Aldrich, Fluka, or Lancaster and used without further purification unless otherwise noted. [1,3-Bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2ylidene][3-bromopyridine] 2 benzylidene ruthenium dichloride (Grubbs third generation catalyst, Ru−III),38 4-(hydroxymethyl)1,6-heptadiyne,39 and 1-butyl-3-methyl imidazolium hexafluorophosphate ([BMIm][PF6])30 were prepared according to the synthetic procedures in literatures. 4-Dimethylaminopyridine (DMAP) (98%) and 1-ethyl-3-(3-dimethyllaminopropyl) carbodiimide hydrochloride (EDCI·HCl) were purchased from Shanghai Chemical Reagents Company. Solvents of acetonitrile (CH3CN), chloroform (CHCl3), and 1,2-dichloroethane (DCE) were distilled over calcium hydride under nitrogen prior to use. Characterization. NMR spectra were recorded on a Bruker DPX500 spectrometer using tetramethylsilane or 85% phosphoric acid as an internal standard in (CD3)2CO. Melting point was determined by a micro melting point apparatus (Yanoco). FT-IR spectra were recorded on a Nicolet Nexus 670 in the region of 4000−400 cm−1 using KBr pellets. Elemental analysis was conducted with an Elementar vario EL. The HR-ESIMS was measured by a Bruker QTOF micromass spectrometer. UV−vis absorption spectra were measured on a UV-1800 spectrometer. Gel permeation chromatography (GPC) was used to calculate relative molecular weight and molecular weight distribution equipped with a Waters 1515 Isocratic HPLC pump, a Waters 2414 refractive index detector, and a set of Waters Styragel columns (7.8 × 300 mm, 5 mm bead size; 103, 104, and 105 Å pore size). GPC measurements were carried out at 65 °C using DMF as the eluent with a flow rate of 1.0 mL/min. The system was calibrated with 6182

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2850, 1734 CO), 1583(CN), 1170(C−N), 836 (PF6), 739 (trans-double bonds) cm−1.

spectrum (Figure S2A) showed both the acetylenic hydrogen stretching (3301 cm−l) and the carbon−carbon triple-bond stretching (2115 cm−l). Further HR-ESIMS analysis also suggested that IM has the right molecular weight. Apparently, all these points confirmed the successful preparation of IM with the expected structure. Metathesis Cyclopolymerization of ImidazoliumFunctionalized 1,6-Heptadiyne toward Polyacetylene Ionomer. Effect of Solvents on Metathesis Cyclopolymerization. The Ru-based Grubbs third generation catalyst (Ru−III) would be an effective catalyst for MCP of neutral 1,6heptadiyne derivatives for selective preparation of conjugated polymers with five-membered ring backbones and narrow PDIs.21,22 The conversion of ionic monomer IM showed extremely strong dependence on reaction temperature, solvents, and kind of catalysts, and the data were collected in Table 1. Initially, the MCP of IM was performed by Ru−III at 45 °C in DCE. Although the monomer IM and the catalyst Ru−III were fully soluble in DCE, the resultant polymer, poly(IM), precipitated out from DCE as a powdered solid after polymerization within 2 min. Consequently, only low polymer yields in the range of 31−40% were found even prolonged reaction time to 12 h (runs 1−3). The MCP of IM proceeded in CHCl3 yielded sticky precipitation in moderate yield of 46% (run 4), which is slightly higher than that reported in literature.29 Usually, the precipitated polymer enwrapped the growing species, which limited the access of the monomer to the catalytic living center in DCE and CHCl3, leading to the reduced polymer yields.33 What is more important, the common noncoordinating solvents of DCE and CHCl3 for MCP reaction should be responsible for the lower polymer yields, because of the instability of the propagating species.41 To improve polymer yield, THF, a weak coordinating solvent, was screened. As illustrated by Choi,21 using a weakly coordinating solvent greatly improved the catalyst lifetime by stabilizing the propagating species through solvent coordination. When MCP of IM was performed in THF (run 5) in the same conditions as those in DCE and CHCl3, a little enhancement of the isolated polymer yield up to 56% achieved. Identically, poly(IM) precipitated out from the solution because of its low solubility in THF. Effect of Reaction Temperature on Metathesis Cyclopolymerization. Similar to the previous report that using CH2Cl2 as solvent,26 MCP triggered by Ru−III at higher

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RESULTS AND DISCUSSION Synthesis of Ionic Monomer (IM). The ionic monomer IM was simply synthesized by esterification reaction of [EMImCOOH][PF6], which was prepared according to the literature procedure (Supporting Information, Scheme S1),40 with 4-(hydroxymethyl)-1,6-heptadiyne as shown in Scheme 1. Scheme 1. Syntheses of IM and Poly(IM)

The structure of IM was evidenced by 1H NMR, 13C NMR, P NMR, IR, and HR-ESIMS. In the 1H NMR spectrum (Figure 1A), δ = 9.04, 7.78, and 7.69 ppm can be assigned to imidazolyl protons, 2.48 ppm is the characteristic peak of the terminal acetylenic proton, and the chemical shifts of the methylene protons N−CH2CH2CO are 4.67 and 3.16 ppm. The 13C NMR spectrum is shown in Figure 1C, the peak at 64.9 ppm is attributed to acetylenic carbon, 169.7 ppm is assigned to the carbonyl carbon, and the imidazolyl carbons appear at 136.7, 123.4, and 122.3 ppm. The 31P NMR spectrum of IM (Supporting Information, Figure S1) showed the resonance signals of PF6 group at −143.3 ppm. Infrared 31

Figure 1. 1H NMR spectra of (A) IM, (B) poly(IM), and 13C NMR spectra of (C) IM, (D) poly(IM) (run 2; asterisk denotes solvent). 6183

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Table 1. GPC and UV Data for Poly(IM) Prepared under Various Conditionsa run

cat.

[M]0/[Cat]0b

T (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Ru−III Ru−III Ru−III Ru−III Ru−III Ru−III Ru−III Ru−III Ru−III Ru−III Ru−III Ru−III Ru−III Ru−III Ru−III Ru−III Ru−II Ru−II/5% H+ f Ru−II/10%H+ f Ru−II/15%H+ f Ru−II/20%H+ f Ru−II/10%H+ f Ru−II/10%H+ f Ru−II/10%H+ f

100 100 100 100 100 100 100 100 150 100 100 100 100 50 150 200 100 100 100 100 100 50 150 200

45 45 45 45 45 0 0 0 0 45 0 0 0 0 0 0 0 0 0 0 0 0 0 0

sol. DCE DCE DCE CHCl3 THF DCE CHCl3 THF THF THF/IL THF/IL THF/IL THF/IL THF/IL THF/IL THF/IL THF/IL THF/IL THF/IL THF/IL THF/IL THF/IL THF/IL THF/IL

0:1 1:1 1:2 1:3 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2

t (h)

Mnc (kDa)

PDI

λmaxd (nm)

Ne

yield (%)

1 3 12 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3

17.6 25.3 25.5 22.2 20.8 28.7 25.4 22.7 23.0 13.9 14.2 17.5 18.2 7.8 24.2 31.0 13.2 13.5 14.4 14.2 12.7 6.7 19.5 26.5

1.01 1.03 1.05 1.05 1.05 1.06 1.10 1.10 1.01 1.01 1.01 1.01 1.01 1.01 1.03 1.05 1.02 1.02 1.03 1.03 1.03 1.01 1.04 1.10

528 535 535 521 521 537 529 521 521 520 522 529 529 513 539 536 486 487 489 489 482 462 518 528

88 126 128 111 104 143 127 113 115 69 71 88 91 40 121 154 66 68 71 72 63 34 97 131

31 38 40 46 56 82 54 87 79 35 75 92 72 97 78 43 60 78 82 76 40 90 72 43

a

[M] = 1 mol/L, IL = [BMIm][PF6], DCE = 1,2-dichloroethane. bThe molar ratio of monomer to catalyst. cDetermined by GPC in DMF relative to monodispersed polystyrene standards. dThe maximum absorption wavelength measured in acetone with concentration of 10−2 mg/mL. eNumber of conjugated double bonds in the backbone, N = 2 × (number of monomer units) + 1. fH+ = H3PO4.

low yield polymer (35%) available, it may be attributed to both the side backbiting reaction at this higher temperature and higher viscosity of neat IL, which should be improved by down the reaction temperature and the solvent viscosity. Taking into consideration of the high viscosity of neat IL at 0 °C, THF is therefore used in conjunction with IL to form a homogeneous system, and MCP reaction was then conducted by Ru−III in the mixture of THF and IL. The solution polymerization proceeded smoothly, and the THF/IL volume ratio of 1:2 allowed for the maximum isolated yield (92%) among the three cases with the THF/IL volume ratios from 1:1 to 1:3 (runs 11−13). MCP of IM with various [M]/[Cat] molar ratios from 50:1 to 200:1 produced the polymers in isolated yields from 97% to 43%, and the corresponding Mn values of polymer increased in the range of 7.8−31.0 kDa with narrow PDIs of 1.01−1.05 (runs 14−16). Alternatively, homogeneous MCP of IM by Ru−II in 1:2 THF/IL at 0 °C was conducted to generate the polymer in moderate yield of 60%, which is lower obviously than that by Ru−III under the same conditions (run 17 vs run 12). To further enhance the isolated polymer yield, the phosphoric acid/Ru−II system was selected for catalyzing MCP in the same conditions, the acid was added before the reaction and the concentration of phosphoric acid varied from 5% to 20% with respect to monomer IM, and resulted in polymers in different yields as the variation of phosphoric acid. When the catalyst was introduced to an acidic solution of monomer (5 mol % acid), the polymer yield (78%) increased obviously (run 18); more precisely, the 10 mol % acid induced a maximum isolated polymer yield of 82% (run 19), but higher concentrations of acid (15−20 mol %) accelerated the catalyst death,3 as indicated by the sharply decreased isolated yields in 76−40%

temperature accelerates the formation of dimer (Scheme S2) via a side backbiting process, which finally resulted in lower polymer yield.26,41,42 In view of recent work by Choi et al.,21 the extent of backbiting could be alleviated by running the MCP at a lower temperature. The MCP of IM was thus performed in DCE or CHCl3 at 0 °C, and the isolated polymer yields increased to 82 and 54%, respectively (runs 6, 7). The MCP of IM was also performed in THF at 0 °C, and the isolated polymer yields differed from 87% to 79% with monomer to catalyst ratio ([M]/[Cat]) varying from 100:1 to 150:1 (runs 8, 9). It is worth noting that in THF, polymers were readily precipitated out from the reaction mixture after the beginning of the process. This allows running such polymerization in the form of precipitation polymerization. The precipitated solid can be readily dissolved in polar solvent, such as acetone, acetonitrile, N,N-dimethylformamide (DMF), N-methyl-2pyrrolidone (NMP), and dimethyl sulfoxide (DMSO). Although all the polymers have narrow polydispersity index (PDI = 1.01−1.05), the molecular weights (Mn,GPC) of polymers were lower than the theoretical values (Mn,th), and they also were unproportionate to the feed ratio of monomer to catalyst, which revealed that these polymerizations were performed in an ill-controlled manner. Homogenous Metathesis Cyclopolymerization in THF/[MBIm][PF6]. In order to direct the MCP reaction under homogeneous condition, an effective medium which can serve as the polymerization solvent and be capable of dissolving the polymer, should be picked up. On the basis of the above illustration, the conventional IL [MBIm][PF6] was filtered to be the proper solvent. As expected, homogeneous MCP of IM was realized smoothly by Ru−III at 45 °C in this IL (run 10), affording poly(IM) without any solid precipitated while just 6184

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(runs 20−21). This result is consistent with previous reports, which illustrates ROMP with Ru−II could be completely inhibited with catalytic amounts of N-alkyl imidazole, addition of catalytic quantities of Brönsted acids can protonate the inhibitor and increase the rate of catalyst initiation and turnover frequency by enhancing tricyclohexyl phosphine dissociation.43,44 Under other [M]/[Cat] ratios varying from 50:1 to 200:1, this catalytic system produced polymers in 90% to 43% isolated yield, and the corresponding Mn and PDI values were in the range of 6.7−26.5 kDa and 1.01−1.10, respectively (runs 22−24). A plot of Mn vs the ratio of monomer to initiator is shown in Figure 2, and interestingly to find that Mn increased

Figure 3. 1H NMR spectra of initial and propagating carbene (left) and the conjugated CHCH regions (right) in THF-d8/IL (1:2).

Microstructures of Imidazolium-Functionalized Conjugated Polyacetylene. The NMR spectroscopy was an efficient method to characterize the microstructure of polymers, and a representative 1H NMR spectrum of poly(IM) is shown in Figure 1B. After MCP, the acetylenic proton peak of monomer at 2.48 ppm disappeared, and the multipeaks appeared at 6.5− 7.0 ppm may be contributed to the protons on the conjugated double bonds or terminal group. To fully understand the assignment of these protons, the 500 MHz 1H,1H COSY and NOESY spectra were recorded for poly(IM) (Figures S6 and S7). The single peak at 6.85 ppm was assigned to the E-olefin proton.42 Obviously, the cross peak coupling between 6.85 (A) and 7.0 ppm (B) in the COSY was observed, confirming that 7.0 ppm was also an E-olefin proton, but located in a different environment from A. Meanwhile, there are another two couplings between 6.62 and 7.55 ppm as well as between 7.32 and 7.55 ppm, and the integration values of peaks at 6.62, 7.32, and 7.55 ppm were nearly 1:2:2, illustrating the three peaks belong to the protons on terminal phenyl group, while not from the protons of cis double bonds. The NOESY analysis further confirmed this result. Consequently, we can see from Figure 4 that MCP by Ru−III in three solvents produced

Figure 2. Plot of Mn vs the ratio of monomer to catalyst.

linearly with the number of monomer to initiator ratio, which demonstrated that MCP of IM in the mixture of THF/IL proceeded in a controlled manner by the action of initiators Ru−II and Ru−III. It should be noted that the GPC results (Figure S3, S4) were tested in DMF with monodispersed polystyrene as standard, and therefore, the values of Mn,GPC for poly(IM) deviated greatly from Mn,th.29 Even so, the changing trend should still have rationality. The MCP of IM initiated by Ru−III at [M]/[Cat] ratio of 20:1 was traced via NMR spectroscopy. Initially, the detection of MCP in CDCl3 was unsuccessful, as the resulting poly(IM) is no longer soluble in CDCl3 (Figure S5A), and thus no signals of the newly formed propagating carbene were observed. The reaction condition was subsequently optimized by changing the solvent, MCP in THF-d8/IL (1:2) proceeded homogeneously (Figure S5B), and the processes were monitored successfully by NMR spectroscopy as shown in Figure 3. As MCP proceeded, the signal (18.52 ppm) attributed to initial benzylidene moiety of Ru−III disappeared, a relatively sharp peak of the newly formed propagating carbene began to appear at 19.14 ppm, and the signal intensity was essentially the same without any changing of the concentration throughout the polymerization process, which meant the stability of the living propagating chain over time. The peaks of the new conjugated protons between 6.5−6.9 ppm changed from broad to sharp peak as the reaction proceeded. This result indicated that MCP occurred successfully in THF-d8/IL in a living manner, which is similar to the case of ROMP of imidazolium-functionalized cyclooctene in CDCl3/IL,25,29 and also proved that the presence of the most commonly medium of IL [MBIm][X] without a methyl group in the 2-position of imidazolium-ring did not result in the poisoning of the Grubbs’ catalyst and had no bad influence on the activity of the catalyst.

Figure 4. 1H NMR spectra of the conjugated polymer backbone obtained in DCE (top), THF/IL (middle), and CHCl3 (bottom).

poly(IM)s with almost all trans double bonds along the backbone. Particularly, such polymers solely based on trans rings possess more planar structure, thus should induce higher effective conjugation length and higher conductivity. The 1H NMR spectrum of poly(IM) prepared by Ru−II in THF/IL (Figure S8) is similar to that obtained by Ru−III in CHCl3. 6185

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display solvatochromic behavior, and the values of λmax (0−0) changed in different solvents as the following order: 575 nm (acetone) < 585 nm (DMF) < 588 nm (NMP) < 599 nm (DMSO). Interestingly, the absorption intensity of 0−0 band at 585 nm weakened as the addition of water to the DMF solution of poly(IM) obtained by Ru−III, and finally disappeared when DMF/H2O = 1:4 as shown in Figure 6. To further explain this

Figure 1D shows the 13C NMR spectrum of poly(IM), the characteristic peaks belonging to the 1,6-heptadiyne moiety, which should appear at 70.6 (Cb) and 64.9 (Ca) ppm, are not seen in the spectrum of poly(IM). The carbon peaks of the conjugated double bond appear between 140−120 ppm, which were overlapped with those of carbons on imidazolium moiety. The carbon peaks of Cd appear at 34.74 ppm in acetone-d6 and 33.28 ppm in DMSO-d6, no matter what catalysts and solvents were applied, indicating that (I) poly(IM) contains ≥95% fivemembered rings and (II) solvents and the type of the two catalysts used seem to play a negligible role in determining the ratio of five- to six-membered rings (Figure 1D and Figure S9). This observation is in accordance with other polymerization data for diethyl dipropargylmalonate with MoCl5-based initiators, which typically produced ca. 70% five-membered rings.45 The incorporation of imidazolium moiety as pendent to PA generates trans-double bonds along the backbone. The NMR results indicated that poly(IM)s initiated by both Ru−II and Ru−III have almost wholly five-membered repeating units and trans double bonds along the backbone. UV−Vis Analysis and Solvatochromic Behavior. Except for NMR technique, the microstructure of the fully conjugated polymer could be verified via UV−vis analysis, which was obtained immediately after MCP, and the results are listed in Table 1. The maximum absorption wavelength (λmax) (0−1 transition) of the polymers, which have different number of conjugated double bonds in the backbone, differed from each other. In principle, λmax values of poly(IM) initiated by Ru−III and Ru−II range from 513 to 539 nm and from 462 to 528 nm, respectively. This result indicates that the former owns a comparable strong coplanarity of polymer backbone. As can be deduced from the representative UV−vis spectra of poly(IM) formed in THF/IL by Ru−III depicted in Figure 5,

Figure 6. UV−vis spectra of poly(IM)s in mixture solvent at 10−2 mg/ mL (Run 12).

phenomenon, pyridine and acetic acid were added to the DMF solution, respectively. The addition of pyridine do not affect the absorption, while acetic acid induced absorption wavelength moving 10 nm to short-wave; that is, a blue shift occurred. This result is completely different from the report in the literature,29 in which water and acetic acid can solvate the polymer chain via hydrogen bond formation, leading to the formation of new bands. This implied the effective conjugated length shortened, suggesting that distortion of the backbone possibly occurred when water was added to the DMF solution of poly(IM). Compared to the UV spectrum of poly(IM) formed in IL/ THF by Ru−III (N = 121, run 15), which displayed two absorption maxima at 541 nm (0−1) and 580 nm (0−0), the UV spectra of poly(IM) formed in CHCl3 by Ru−III (N = 127, run 7) and in IL/THF by Ru−II (N = 131, run 24) show decreased intensity of 0−0 transition at 580 nm (Figure 7). The decrease of the intensity for the 0−0 band indicates less coplanar conformation of the conjugated polymers.49 Cyclic voltammetry of poly(IM) was tested on glassy carbon electrode. The polymer showed only irreversible processes, the oxidation Epa was 700 mV, and the highest occupied molecular orbital (HOMO) of poly(IM) is −5.2 eV50 (Figure S11). The much lower HOMO is indicative of the improved stability of poly(IM) in air, allowing for all of the characterizations in the ambient atmosphere. Because this polymer was too rapidly soluble away from the electrode surface, making it difficult to reproducibly obtain cyclic voltammograms. The XRD pattern of poly(IM) (Figure S12) shows a diffusing peak ranging in 2θ = 15° to 35° region with d = 0.40 nm, implying that there is strong π−π stacking interaction of PA segments, which is facilitate the electron/ion transport. DSC thermogram for poly(IM) shows no glass−rubber and melting transition up to 250 °C, near the onset of thermal decomposition. The thermal stability of the polymer was then investigated by means of thermogravimetric analysis (TGA). As shown in Figure 8, poly(IM) starts to decompose at 245 °C,

Figure 5. UV−vis spectra of poly(IM) in various solvents at 10−2 mg/ mL (run 12).

two well-resolved absorption maxima at 541−564 nm (0−1) and 575−599 nm (0−0) indicated regular PA with defined and uniform backbone structures. According to the report in literatures,23,46,47 only poly(1,6-heptadiyne)s with five-membered ring units lead to λmax (0−0) in the range of 580−590 nm. Furthermore, the absorption for the film of poly(IM) extended to 775 nm (Figure S10) with bandgap narrowed to 1.6 eV, which was in perfect agreement with the bandgap for trans-PA.48 Therefore, MCP catalyzed by Ru−III resulted in poly(IM)s primary based on trans alternative five-membered ring repeat units along the backbone. Besides, polymers would 6186

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forming ability of polymers should be further improved for fulfilling the measurement for ionic and electrical conductivity of the polymer film, and it is currently in progress.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures, reaction schemes, IR, UV−vis and NMR spectra, CV curve, XRD pattern, and pictures of the polymers formed. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author Figure 7. UV−vis spectra of poly(IM)s in acetone at 1.2 × 10 mL.

−2

*(M.X.) Telephone: +86 21 54340058. Fax: +86 21 54340058. E-mail: [email protected].

mg/

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (No.21374030, No.21074036) and the Large Instruments Open Foundation of East China Normal University (No. 2014-23) for financial supports of this work.

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REFERENCES

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Figure 8. TGA curve of poly(IM).

and 5 wt % loss temperature is up to 303 °C, this is nearly 100 °C higher than neutral PAs (∼200 °C), indicating the stability contribution of imidazolium moiety to the conjugated polymer backbone.

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CONCLUSIONS A new imidazolium-contained PA ionomer, poly(IM), was synthesized via MCP of IM by Grubbs catalysts in different solvents. The most relevant findings were as follows: (i) among the solvents and catalysts, DCE, THF/[MBIm][PF6], and Ru− III are preferable choice for producing poly(IM)s with more coplanar conformation. After intensive optimization, poly(IM) was obtained in 97% isolated yield in 1:2 THF/[MBIm][PF6] solvent; (ii) the 1H NMR results indicated that poly(IM)s initiated by both Ru−II and Ru−III have ≥95% five-membered repeating units and all trans double bonds along the backbone; (iii) PAs generated in different solvents reflected significant variance in optical absorption properties, and λmax of poly(IM) showed strong bathochromic shift effect with the mixture of THF/IL; (iv) the presence of imidazolium pendent has positive effect on the thermal stability of poly(IM)s. As far as we know, the obtained poly(IM) is the first conjugated polymer based on imidazolium-doped PA using MCP technique. A big potential advantage for this strategy is the catalysts’ tolerance to various functional groups, which offer the versatile possibility of exchanging the associated counteranion of imidazolium cation to further tailor the solubility and conductivity. The film6187

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