1. Introduction

 The presence of conjugated system in the polymer chains endows such unique properties as conductivity, nonlinear optical properties, magnetic properties, image patterning, chemical sensing, gas pemeability, photoluminescent and electroluminescent properties.^[1-3] Conjugated polymers are expected to have a great potentials in the fields of electronic and photonic devices, such as polymer light-emitting diodes (PLEDs), organic photovoltaic cell (OPVs), and organic thin film transistors (OTFTs).^[4]

 Conjugated polyelectrolytes are charged conducting macromolecules containing a large number of ionizable or ionic groups.^[5,6] Various substituted ionic polyacetylenes have been prepared by the linear polymerization of the corresponding acetylene monomers.^[2] Especially, 2-ethynylpyridine was firstly thermally polymerized to give a low yield of polymer with low molecular weight.^[7] For 2- or 3-ethynylpyridine homopolymers with iodomethanee or iodoethane, mixing the quaternarized polymers with lithium tetracyanoquinodimethane (LiTCNQ) or 7,7,8,8-tetracyanoquinodimethane (TCNQ) in acetonitrile and refluxing for 30 min yielded a new type of conjugated polymer.^[8]

 Blumstein et. al prepared the well-defined ionic polyacetylenes through the activated polymerization of ethynylpyridines using alkyl halides.^[9,10] They prepared various ionic conjugated polymers by the reaction of ethynylpyridines with alkyl halides, ethanesulfonic acids, halogens, or halogenic acids. The first self-doped polyaniline was synthesized by heating emeraldine base of polyaniline in DMSO with either propanesultone or butanesultone to produce the corresponding self-doped polyaniline derivative, poly(aniline-N-propylsulfonic acid) or poly(aniline-N-butylsulfonic acid).^[11] Furthermore, self-doped ionic conjugated polymers, poly(2-ethynylpyridinium-N-benzoylsulfonate) and poly[2-ethynyl-N-(4-sulfobutyl)pyridinium betaine], were prepared via the activated polymerization of 2-ethynylpyridine with the ring-opening of 2-sulfobenzoic acid cyclic anhydride and 1,4-butanesultone, respectively.^[5,12,13]

 Pyridine-containing ionic polyacetylenes have been used as an active candidate  materials for the fabrication of intercalatednanocomposite films and nanosheets,^[14,15] efficient fluorescence quencher,^[16] cyclodextrininduced fluorescence enhancement,^[17] hybrid polyacetylene gels,^[18] nanocrystalline CdS polymer sensitizer,^[19,20] and conductive polymer-silver nanoparticles.^[21,22]

 Here, we report the synthesis of a new conjugated polyelectrolyte with ethanesulfonate by the uncatalyzed polymerization of 2-ethynylpyridine using sodium 2-bromoethanesulfonate and the characterization for the chemical structure and physical properties of resulting polymer.

2. Experiment

2-Vinylpyridine (Sigma-Aldrich, 97%), bromine (Sigma-Aldrich, A.C.S. reagent, 99.5+%), and sodium amide (NaNH₂, Sigma-Aldrich, tech., 90%) were used as received. 2-Ethynylpyridine was prepared by the bromination of 2-vinylpyridine and the consecutive dehydrobromination reaction according to the literature method^[23] and vacuum distilled after drying with CaH₂ (85 ^oC/12mmHg). Sodium 2-bromoethanesulfonate (Sigma-Aldrich, 98%) was used as received. The analytical grade solvents were dried with an appropriate drying agent and distilled. 

 Poly[N-(ethylsulfonate sodium)-2-ethynyl pyridinium bromide] (PESEPB), an ionic conjugated polymer with covalently bound ethylsulfonate side group, was prepared by the uncatalyzed polymerization of 2-ethynylpyridine using sodium 2-bromoethanesulfonate without any additional initiator or catalyst. The polymerization procedure is as follows. In a 50 mL three-neck flask equipped with reflux condenser, rubber septum, and purified nitrogen inlet-oulet, 16.3 mL of methanol ([M]₀ = 0.5M), 2-ethynylpyridine (1.0 g, 9.70 mmol), and sodium 2-bromoethanesulfonate (2.05 g, 9.70 mmol) were added into the reaction flask. Then the reaction solution was warmed to 65 ^oC under nitrogen atmosphere and stirring was continued at this temperature for 24 h. During this time, the color of reaction mixture changed fromthe light brown of the initial mixture into dark red. After the polymerization time, the resulting polymer solution was precipitated into an excess amount of ethyl ether, followed by filtration. The collected black powder was dried under vacuum overnight at 40 ^oC to afford PESEPB in 78 % yield.

 FT-IR spectra were obtained with a Bruker EQUINOX 55 spectrometer using a pressed KBr pellet. NMR (¹H and ¹³C) spectra were obtained in DMSO-d₆ solutions at room temperature by using a Varian 500 MHz FT-NMR spectrometer (Model : UnityINOVA) and the chemical shifts are reported in ppm units with tetramethylsilane as an internal standard. The inherent viscosities of polymers were determined at a concentration of 0.5 g/dL in DMF at 30 ^oC.

 Electrochemical measurements were carried out with a Potentionstat/Galvanostat Model 273A (Princeton Applied Research).The polymer solution was prepared and the electrochemical measurements were performed in DMF solution of 0.1 M trabutylammonium tetrafluoroborate (TBAT). ITO, Ag/AgNO3 and platinum wire were used as a working, reference and counter electrode, espectively. The optical absorption spectra were measured by a HP 8453 UV-visible Spectrophotometer. The photoluminescence spectra were obtained by Perkin Elmer Luminescence Spectrometer LS55 (Xenon flash tube) utilizing a lock-in amplifier system with a chopping frequency of 150 Hz.

3. Results and Discussion

 Acetylenic pyridine compounds such as ethynylpyridines, dipyridylacetylene and dipyridyldiacetylene were known to be polymerized spontaneously by an alkyl halide treatment without giving the simple N-alkyl product.^[8-10] Here, we prepared a new ionic polyacetylene (3, PESEPB) from the uncatalyzed polymerization of2-ethynylpyridine (1) using sodium 2-bromoethanesulfonate (2) (Scheme 1).

Scheme 1. Synthesis of PESEPB

 Although the monomeric salt, N-(ethylsulfonate sodium)-2-ethynylpyridinium bromide, formed at the first quaternarization process, has a highly bulky substituent, the present polymerization proceeded easily in a homogeneous manner to give relatively high yield of polymer at 65 ^oC (polymer yield: 78%). The polymerization proceeded well in regards to such polar organic solvents as methanol, DMSO, and NMP. The color of the reaction solution changed from the initial light brown of 2-ethynylpyridine and 2-bromoethane sulfonate into light red after 1 h, and finally into dark red. The polymer yield calculated from the weight of precipitate with respect to the polymerization time reveals that this polymerization proceeded fast at the initial 3 h, reached about 52% polymer yield, and then a final 78% after 24 h. The absorption spectra of the polymerizationsolution with respect to the polymerization time were also investigated. The UV-visible spectrum of the initial mixture of monomeric solution does not show any absorbance over 300 nm. However, an absorption band at visible region over 400 nm was newly observed after 1 hr of polymerization time.

 The polymerization behaviors are very similar to those of 2-ethynylpyridine with propargyl bromide^[24] and 5-[(5-bromopenthoxy) methyl- 2-norborene].^[25] These polymerizations have been known to involve the first quaternarization of 2-ethynylpyridine by bromoalkyl compounds.^[10,26] The polymerization mechanism involve the first activation of the acetylenic bond followed by rapid, spontaneous polymerization without any additional catalyst or initiator. The polymerization was assumed to proceed via an anionic mechanism. The initiation step of polymerization involves a nucleophilic attack by the nitrogen atom of unreacted 2-ethynylpyridineand/or the bromide anion on the activated acetylenic groups of N-substituted-2-ethynylpyridinium bromide. The activated acetylenic bond of monomeric species formed at the initial reaction times is susceptible to the linear polymerization, followed by an identical propagation step that contains the produced macroanion and the quaternarized monomeric species. Finally, this reaction is terminated by a reaction of macroanioic species with sodium 2-bromoethanesulfonate and/or other components.

 In general, the present polymerization proceeded spontaneously without any additional catalyst or initiator to give highly charged conjugated polymer. Thus this method originally prevents the contamination of polymer samples by the catalyst or initiator residues.

Various instrumental methods such as infrared, NMR, and UV-visible spectroscopies were used for thecharacterization of polymer structure. Figure 1 shows the FT-IR spectrum of PESEPB, which was measured in KBr pellet. FT-IRspectrum of PESEPB did not show the acetylenic C≡C bond stretching (2110 cm^-1) and acetylenic ≡C-H bond stretching (3293 cm^-1) bands of 2-ethynylpyridine monomer. This showed characteristic absorption bands of the pyridyl ring (660, 765 cm^-1) and an intense band of sulfonate moieties (1200 cm^-1). The aromatic =C-H stretching vibrations of pyridine and conjugated backbone were found at around 3020 cm^-1. 

Figure 1. FT-IR spectrum of PESEPB in KBr pellet.

The ¹H-NMR spectrum of PESEPB showed the aromatic protons of pyridyl moieties and the vinyl protons of the conjugated polymer backbone at 6.4 -9.5 ppm. The methylene proton peaks of  N-ethylsulfonate group were also observed at 3.3-4.3 ppm. The ¹³C-NMR spectrum of PESEPB did not show the acetylenic carbon peaks. Instead, the polymer spectrum showed the multiple and complicated peaks at the region of 108-155 ppm, which are originated from the aromatic carbons of pyridyl moieties and the vinyl carbons of conjugated polymer backbone. The methylene carbon peaks were observed at 47-56 ppm. The UV-visible spectrum of polymer showed an absorption band at visible region (up to 750 nm) due to the π→π^* interband transition of the polymer main chain, which is a characteristic peak of the conjugated polyene backbone system. These spectra indicate that the present PESEPB have an ionic conjugated polymer backbone system bearing the designed functional groups.

 In the X-ray diffractogram of PESEPB powder, because the peak in the diffraction pattern was broad and the ratio of the half-height width to diffraction angle (2/2) is greater than 0.35, this polymer was amorphous.^[1,2] The inherent viscosities of the resulting polymers were in the range of 0.12-0.15 dL/g.

 The optical absorption and luminescence properties of PESEPB were characterized by UV-visible absorption and photoluminescence (PL) spectroscopies. Figure 2 shows the result of UV-visible and PL spectra for PESEPB (DMF solution, 1.3 x 10^-4 M for UV-visible, 2.6 x 10^-4 M for PL). PESEPB showed UV-visible maximum absorption value of 507 nm and yellowish orange PL maximum value at 593 nm, which is corresponding to a photon energy of 2.09 eV.

Figure 2. Optical absorption and photoluminescence spectra of PESEPB polymer solution.

 In our previous study,^[27] we reported the UV-visible and PL properties of poly[2-ethynyl- N-(3-thienylmethyleneoxy)-hexylpyridinium bromide] (PETHPB). PETHPB has same polymer main chain and longer methylene unit of hexyl-oxy group between pyridinium and thiophene groups than PESEPB. Longer side chain moiety of PETHPB than PESEPB exhibited UV-visible and PL maximum values of 469 and 510 nm. It is explained by that the small sized side group affects the molecular planar arrangement of main chain backbone such as conjugated polyene. Considering this difference, it is thought that the present PESEPB shows red-shifted UV-visible and PL spectra compared to that of PETHPB.

 To evaluate the electrochemical kinetic properties of PESEPB for electro-active application, cyclic voltammetry (CV) was measured. As shown in Figure 3, we performed the cyclic voltammograms (CVs) of PESEPB solution with consecutive scans (a) as well as various scan rates, 30 mV/s ～ 150 mV/s (b) to determine the electrochemical kinetic behavior of PESEPB. It was observed that PESEPB shows very stable cyclovoltammetric behavior for the consecutive scans (up to 30 cycles), which means that this material has a stable redox process under -1.80 to 1.50 V range. This result suggests that the electrochemical process of PESEPB is reproducible in the potential range of -1.80 to 1.50 V vs Ag/AgNO₃.

Figure 3. Cyclic voltammograms of PESEPB [0.1M TBAT/DMF] with 30 consecutive scans at 100 mV/s (a), and various scan rates of 30 mV/sec ～120 mV/sec (b).

 Additionally, the oxidation of PESEPB starts at 0.41 V in the scan and this oxidation happens at the vinylene unit of the conjugated polymer backbone. The related oxidation and reduction peaks were separated. This means that the redox process was irreversible. Interestingly, as the scan rate of CV has increased, the redox current substantially increased.

The relationship between the redox peak current and the scan rate can be expressed as a power law as follows.^[28,29] 

 where i_p,a = oxidation peak current density, v = scan rate, k = proportiona lconstant, and x = exponent of scan rate.

 Based on these empirical equations, the exponent value of scan rate, x was calculated as shown in Figure 4. From the empirical data values, if x = 1, it means that the electrochemical system is controlled by electron transfer process. On the other hand, when x = 0.5, the system is defined by the reactant diffusion process.^[28]  The exponent of the scan rate of PESEPB was found to be 0.886, which suggests the redox kinetics is close to an electron transfer process.^[28-30] For comparison, PETHPB exhibited a diffusion control process (x = 0.54).^[27]

Figure 4. Plot of log i_p,a vs log v for PESEPB.

4. Conclusions

 The uncatalyzed polymerization method was used for the synthesis of a new conjugated ionic polyacetylene with N-(ethylsulfonate sodium)pyridinium bromide substituent. This polymerization of 2-ethynylpyridine using 2-bromoethanesulfonate proceed well to give high yield of polymer. The resulting polymer powder was black and was soluble in organic solvents such as DMF, DMSO, and DMA. The inherent viscosities were in the range of 0.12-0.15 dL/g. Instrumental analyses such as IR, NMR, and UV-visible spectroscopies indicated that PESEPB has a conjugated polymer backbone system having the designed substituents. PESEPB showed characteristic wide UV-visible absorption band, centered at 507 nm and yellowish orange PL maximum value at 593 nm, which is corresponding photon energy of 2.09 eV. The oxidation of PESEPB was started at 0.41 V, where the vinylene unit of the conjugated polymer backbone could be oxidized in the scan and the redox process was irreversible.

Acknowledgement

 This work was supported by a grant (M2009010025) from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy (MKE), Korea and a grant (No. 2012001846) of the National Research Foundation of Korea (NRF) funded by the Korea government (MEST).