Macromolecular Research

Article

DOI 10.1007/s13233-017-5107-8

www.springer.com/13233 pISSN 1598-5032 eISSN 2092-7673

An Ionic Conjugated Polymer from the Catalyst-Free Polymerization of 2-Ethynylpyridine Using 3,4,5-Trimethoxybenzoyl Chloride Yeong-Soon Gal*,1 Sung-Ho Jin2 Jongwook Park3 Kwon Taek Lim4

1

Department of Fire Safety, Kyungil University, Gyeongsan, Gyeongbuk 38428, Korea Department of Chemistry Education, Pusan National University, Busan 46279, Korea 3 Department of Chemical Engineering, Kyunghee University, Suwon, Gyeonggi 17104, Korea 4 Department of Display Science and Engineering, Pukyong National University, Busan 48513, Korea 2

Received March 16, 2017 / Revised May 11, 2017 / Accepted May 22, 2017 Abstract: An ionic conjugated polymer was synthesized via the catalyst-free polymerization of 2-ethynylpyridine using 3,4,5-trimethoxybenzoyl chloride. The activated triple bond of quaternized monomer, formed at first reaction step, was susceptible to linear polymerization. The chemical structure of resulting polymer was characterized by various instrumental methods to have the conjugated polymer backbone structure with the N-(3,4,5-trimethoxybenzoyl)pyridinium chlorides. The inherent viscosity of the resulting polymer was 0.14 dL/g and the polymer was soluble in such organic solvents as pyridine, DMF, DMSO, NMP, etc. Maximum value of photoluminescence (PL) spectrum was exhibited 570 nm according to the photon energy of 2.18 eV. The band gap of 2.23 eV was calculated by absorption spectrum. Oxidation and reduction of polymer were performed in the range of -1.8 V to 1.50 V using cyclic voltammetry and exhibited overall stable electrochemical property for 30 cycles. Keywords: non-catalyst polymerization, polyacetylene, 2-ethynylpyridine, 3,4,5-trimethoxybenzoyl chloride, photoluminescence, cyclic voltammograms.

1. Introduction Acetylenic molecules contain a triple bond with 2 -bonds, which can be opened to yield conjugated double bonds in the polymer main chain. This unique electronic structure has potential to endow the polymers with such novel properties as semiconductivity, geometrical isomerism, paramagnetism, optical nonlinearity, energy transfer, and chemical reactivity.1-7 Among conjugated organic materials, polyacetylene (PA) is structurally the simplest conjugated polymer,8 which exhibits high electrical conductivity upon chemical doping.9,10 The pioneer works on the discovery of the metallic conductivity of the doped PA films led to the 2000 Nobel Prize in Chemistry awarded to Hideki Shirakawa, Alan MacDiarmid, and Alan Heeger.11-13 Nevertheless, this material has found only very limited applications because PA itself shows the lack of processibility because of its poor solubility in organic solvents and the insufficient stability under atmospheric conditions.1,2 The incorporation of functional pendants to acetylene and a large variety of acetylenic polymerization reactions could yield the functional conjugated Acknowledgments: This work was supported by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea (NRF-2013M3C1A3065522). This work was supported by the research fund of Kyungil University. *Corresponding Author: Yeong-Soon Gal ([email protected]) © The Polymer Society of Korea and Springer 2017

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organic materials with the improved stability and solubility.14-16 Polymers possessing both charges and hydrophobes along or pendant to the polymer backbone are of great interest because they exhibit ionic conductivity in a flexible but solid membrane and one of the most important classes of polyelectrolytes. Ionic conductivity is different from the electronic conductivity of metals and organic conducting polymers, since the current is carried through the movements of ions. During the past three decades, the solid polymer electrolytes have been and continue to be active area of scientific research and commercial growth, which have been mainly derived from their molecular self-organization phenomena in relevance to nanoscopic molecular architecture and to biological macromolecular systems as a basis of material science.17-19 Nitrogen-containing conjugated polymers contain the nitrogen heteroatom either in the conjugated polymer main chains or in the pendant side chains, which are susceptible to the facile quaternization reaction and protonation of the nitrogen sited.20 Simionescu et al. reported the first synthesis of poly(vinyl- and ethynylpyridines)-TCNQ anionic radical salts.21 Quaternized ethynylpyridine homopolymer were easily prepared by the quaterization polymerization of ethynylpyridines using methyl iodide/ethyl iodide and the consecutive mixing of the quaternized polymers with LiTCNQ in acetonitrile yielded the poly (ethynylpyridine)-TCNQ salts in high yield.21 Blumstein et al. also reported that the quaternized acetylenic monomers produced in the first reaction step of ethynylpyridines by alkyl halides Macromol. Res., 25(6), 552-558 (2017)

Macromolecular Research undergos catalyst-free, rapid polymerization giving extensive conjugated, charged organic polyelectrolytes.22-24 In our previous works, we have reported the synthesis of various ionic conjugated polymers via the linear polymerization of substituted ionic acetylenes using transition metal catalysts and the catalyst-free polymerization of ethynylpyridines using functional alkyl or carbonyl halides.25-30 This quaternization polymerization can essentially prevent the contamination of polymer sample, which may be originated by the catalyst or initiator used in other polymerization methods. The pyridine-based conjugated polymers have been shown to be as material candidates for amphiphilic ultrathin films,31 organic electroluminescent devices,32 hybrid polymer gels,33 nanostructured cationic polyacetylene-silica hybrids,34 cyclodextrin-induced fluorescence enhancement,35 SERS (surface-enhanced Raman spectroscopy) active conjugated polymer-silver nanocomposites,36 unipolar write-once-read-many-times (WORM) memory devices,37 and an interfacial dipole layer of polymer: fullerene solar cells.38 In this article, we report the synthesis of a novel ionic polyacetylene with the bulky N-(3,4,5-trimethoxybenzoyl)pyridinium chloride substituents by the quaternization polymerization of 2-ethynylpyridine using the corresponding benzoyl chloride. The polymer structure and properties were characterized by using various instrumental methods.

2. Experimental The bromination of 2-vinylpyridine and the consecutive dehydrobromination reaction were used for the synthesis of 2-ethynylpyridine according to the literature method25 and the 2-ethynylpyridine was distilled at vacuum (85 oC/12 mmHg) after drying with CaH2. 3,4,5-Trimethoxybenzoyl chloride (Aldrich Chemicals, 98%, mp 81-84 oC) was used as received. The analytical grade solvents were dried with an appropriate drying agent and distilled. Poly[2-ethynyl-N-(3,4,5-trimethoxybenzoyl)pyridinium chloride] (PETPC) was prepared by the catalyst-free polymerization of 2-ethynylpyridine using 3,4,5-trimethoxybenzoyl chloride without any additional catalyst. Typical polymerization procedure is as follows. The equimolar mixture of 2-ethynylpyridine (2.00 g, 19.39 mmol) and 3,4,5-trimethoxybenzoyl chloride (4.47 g, 19.39 mmol) in N,N-dimethylformamide (20 mL, [M]0: 0.73 M) was stirred for 24 h at 80 oC under nitrogen atmosphere. As the reaction time passed, the color of 3,4,5-trimethoxybenzoyl chloride and 2-ethynylpyridine mixture was continuously changed from the light brown of the initial mixture into dark brown and the solution viscosity was also increased continuously. The reaction solution diluted with 10 mL N,N-dimethylformamide was precipitated into a large excess amount of ethyl ether. The filtered polymer was dried under vacuuo at 40 oC for 24 h. The dark-brown powder of PETPC was obtained in 60% yield. Infrared spectra were recorded on a Bruker EQUINOX 55 FT-IR spectrophotometer using a KBr plates in the scanning range of 400-4000 cm-1. The Varian 500 MHz FT-NMR spectrometer (Model: Unity INOVA) in dimethyl sulfoxide-d6 using tetramethylsilane as an internal standard was used to measure 1H- and 13CMacromol. Res., 25(6), 552-558 (2017)

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NMR spectra for polymers and the chemical shifts are reported in ppm units. The optical UV-Vis absorption spectrum was measured using a Lambda 1050 UV/Vis/NIR spectrometer (PerkinElmer) in solution state (concentration: 1.0×10-4 M, solvent: N,N-dimethylformamide). A Perkin-Elmer luminescence spectrometer LS55 (Xenon flash tube) was used to obtain PL spectroscopy. It was measured by a lock-in amplifier system using a chopping 150 Hz frequency. The inherent viscosities of PETPC were determined at a concentration of 0.5 g/dL in N,N-dimethylformamide at 30 oC. The Hitachi JEOL system (S-4200) was used for the energy dispersive X-ray (EDX) analyses of polymers. The polymer powder morphology was characterized by an electronic microscope. The JEOL JSM- 6360 LV Scanning Electron Microscope was used to obtain the images under 10 kV. Thermogravimetric analyses of polymers were performed using a DuPont 2200 thermogravimetric analyzer at a heating rate of 10 oC/min under a nitrogen atmosphere. DSC thermograms of polymers were taken on a DuPont 910 differential scanning calorimeter at a heating rate of 10 oC/min under a nitrogen atmosphere. Electrochemical property was measured with a Potentionstat/Galvanostat Model 273A (Princeton Applied Research) cyclic voltammetry (CV) at a scan rate of 100 mV/s. All electrochemical measurements were carried out in air condition at room temperature. 0.1 M tetra-n-butylammonium tetrafluoroborate (TBAT) in anhydrous acetonitrile was used as electrolyte and polymer solution was prepared in N,N-dimethylformamide. A platinum wire and Ag/AgNO3 were used as the counter electrode and reference electrode, respectively.

3. Results and discussion Pyridine-containing ionic polyacetylenes have been prepared by the catalyst-free polymerization of ethynylpyridines by using iodoalkanes, methyl trifluoromethanesulfonate, alkyl halides, metal salts without further catalyst or initiator.21-24 The intermolecular hydrogen bonding is expected in the pyridine ring of ethynylpyridines because the terminal acetylene functional groups have both proton-donating and proton-accepting properties.39 However, the quaternization of pyridine ring having terminal acetylenic groups results in loss of the intermolecular hydrogen bonding. The polymerizable terminal acetylenic groups of quaternized N-substituted-2-ethynylpyridinium monomers were found to be susceptible to the consecutive linear polymerization, yielding the ionic conjugated polymers with the designed substituents.22-24,27-29 This method is very interesting synthetic pathway for the ionic functional polyacetylenes with long sequences of conjugated double bonds in the polymer main chain. Additionally, this catalyst-free polymerization method have advantage because it can originally exclude the contamination of polymer sample by catalyst residues. Here, we synthesized an ionic conjugated polymer with the N-(3,4,5-trimethoxybenzoyl)pyridinium chloride via the catalyst-free polymerization of 2-ethynylpyridine by using 3,4,5trimethoxybenzoyl chloride without any additional catalyst (Scheme 1). The polymerization reaction of 2-ethynylpyridine using 3,4,5trimethoxybenzoyl chloride as the activating agent was per© The Polymer Society of Korea and Springer 2017

Macromolecular Research

Scheme 1. Catalyst-free polymerization of 2-ethynylpyridine using 3,4,5-trimethoxybenzoyl chloride.

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

formed in DMF solvent, and kept for 24 h in heated oil bath (80 oC). As the polymerization of quaternized monomeric salts proceeded, the solution viscosity of reaction mixture was gradually increased and the color of reaction mixture was also changed from the light brown of the initial reaction mixture into dark brown. Although the quaternized monomer, 2-ethynyl-N-(3,4,5-trimethoxybenzoyl)pyridinium chloride formed at the first reaction step, has a highly bulky ionic substituent, this polymerization proceeded homogeneously to give a moderate yield of polymer (polymer yield: 60%). In our previous work,40 the similar polymerization of 2-ethynylpyridine using simple benzoyl chloride proceeded well at relatively mild reaction condition (reaction temperature: 60 oC), as was the case in the analogous polymerizations of ethynylpyridines using simple alkyl halides27,41,42 without additional catalysts. The similar catalyst-free polymerization of 2-ethynylpyridine using benzoyl chloride proceeded well to give a high yield (89%) of polymer. However, the present polymerization of 2-ethynylpyridine using more bulky benzoyl chloride with the electrondonating three methoxy groups showed relatively lower reactivity than that of the similar polymerization using the simple benzoyl chloride. The reason of relatively poor reactivity may be originated to the increased electron density of carbonyl carbon by the electron-donating three methoxy groups and/or the increase of bulkiness of substituents. In our previous work,27 the catalyst-free polymerization of 2ethynylpyridine using 3-bromo-1-propyne (mole ratio=1:1) gave only the poly(2-ethynylpyridine) with pendant acetylenic function groups. This result means that the acetylenic functional groups of quaternized monomeric salts only participate in the polymerization reaction of quaternized N-substituted 2-ethynylpyridinium monomers.27 The quaternized ethynylpyridinium salts from the reactions of 3-bromo-1-propyne and 3-chloro-1-propyne with pyridine itself were isolated by Katritzky et al.43 It was proposed that the addition of nucleophiles such as pyridines, tertiary amines, and halide anions can initiate the polymerization of the quaternized pyridinium monomers with acetylenic functional group. These polymerizations seem to proceed in anionic mecahnism. The quarternization reaction of 2-ethyn-

ylpyridine by 3,4,5-trimethoxybenzoyl chloride occurs in the first step. The initiation step of polymerization involves a nucleophilic attack by the nonbonding electron pairs at the nitrogen atom of 2-ethynylpyridine monomer and/or the chloride ion on the terminal acetylenic bond of monomeric 2-ethynyl-N(3,4,5-trimethoxybenzoyl)pyridinium chloride. The activated triple bond of monomeric salt, 2-ethynyl-N-(3,4,5-trimethoxybenzoyl)pyridinium chlorides, formed at the first reaction step, was susceptible to the initiation reaction of polymerization, followed by the consecutive propagation steps, which yields the macroanions with quaternized repeating units. Finally the polymerization reaction is terminated by reaction of quaternized macroanioic species with the 3,4,5-trimethoxybenzoyl chloride and/ or other components. Such instrumental methods as NMR, infrared, and UV-visible spectroscopies were used to characterize the molecular structure of PETPC. The FT-IR spectra of PETPC is given in Figure 1. The acetylenic CC bond stretching peak at 2110 cm-1 and acetylenic C-H bond stretching (3293 cm-1) peak of 2-ethynylpyridine were not observed in the infrared spectrum of PETPC as well as in that of poly(N-benzoyl-2-ethynylpyridinium chloride).40 The aromatic =C-H stretching peaks of pyridyl and benzoyl substituents of PETPC and poly(N-benzoyl-2-ethynylpyridinium chloride) are observed at 3114 cm-1. The peaks at around 1600 cm-1 are due to the C=C stretching frequencies of aromatic substituents (pyridyl and 3,4,5-trimethoxybenzoyl) and conjugated polymer backbone. The CH out-of-plane deformation peak of pyridyl ring substituents was also observed at 757 cm-1. Figure 2 shows the 1H NMR spectrum of PETPC measured in dimethylsulfoxide-d6. The 1H NMR spectrum of a similar polymer having the simple N-benzoyl substituents, poly(N-benzoyl-2ethynylpyridinium chloride), was also included for comparison in Figure 2. The 1H NMR spectrum of PETPC is free of the signal of acetylenic hydrogens, which means the absence of 2-ethynylpyridine itself as well as its monomeric salts in samples. Instead, the 1H NMR spectrum of PETPC shows a partly resolved multiplet of the vinyl protons of conjugated polymer backbone and the aromatic protons of pyridyl and substituted benzoyl substituents in the range of 5.6-10.2 ppm, whereas the broad peaks of

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Macromolecular Research

Figure 2. 1H NMR spectrum of PETPC in DMSO-d6 (inner: 1H NMR spectrum of poly(N-benzoyl-2-ethynylpyridinium chloride)).

Figure 3. 13C NMR spectrum of PETPC in DMSO-d6 (inner: 13C NMR spectrum of poly(N-benzoyl-2-ethynylpyridinium chloride)).

three methoxy protons are observed in the range of 3.0-4.3 ppm. On the other hand, the 1H NMR spectrum of poly(N-benzoyl-2ethynylpyridinium chloride) shows the pyridyl and benzoyl protons and the vinyl protons of conjugated polymer main chain broadly at 5.8-9.9 ppm. As can be seen, the spectrum of conjugated polymers having especially pyridyl substituents showed generally low resolution because of the line-broadening effect and/or the absorbed moisture and organic residues, which were originated from the pendant cationic pyridyl substituents.40 Macromol. Res., 25(6), 552-558 (2017)

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The 13C NMR spectrum of PETPC measured in dimethylsulfoxide-d6 is given in Figure 3. The 13C NMR spectrum of poly(Nbenzoyl-2-ethynylpyridinium chloride) was also included for comparison. The 13C NMR spectrum of polymer showed more complicated peaks at the region of 102-158 ppm, which are originated from the aromatic carbons of pyridyl and 3,4,5-trimethoxybenzoyl moieties and the olefinic carbons of conjugated polymer main chain. The two methoxy carbon peaks at 3,5-position and one methoxy carbon paeks at 4-position of 3,4,5-tri© The Polymer Society of Korea and Springer 2017

Macromolecular Research

Figure 5. SEM image of PETPC powder.

Figure 6. X-ray diffractogram of PETPC powder.

Figure 4. EDX spectrum of PETPC powder.

methoxybenzoyl substituents are observed at 56.1 ppm and 60.0 ppm, respectively, whereas the carbonyl carbon peak of trimethoxybenzoyl substituents is observed at 162.3 ppm. Figure 4 shows the energy dispersive X-ray (EDX) spectrum of PETPC powder. The EDX spectrum of PETPC confirmed the presence of O and Cl atoms. Only one peak of chloride atom is observed in EDX spectrum of PETPC. Whereas the EDX spectra of similar conjugated polymer having iodide atom as counter ion showed the multiple iodide peaks, which means that the iodide atoms have many different forms and/or different environments.44 The UV-visible spectrum of PETPC taken from N,N-dimethylformamide solution show the almost continuous decrease in optical absorption from blue to the red region without any distinct band. Distinct shoulders are seen at around 331 nm and 395 nm. The absorption peaks in the visible regions are due to the electronic transitions in the conjugated main chains. This means that the present PETPC have an extended -conjugation system along the polymer main chain. From these analytical results, we conclude that the present PETPC has an ionic conjugated polymer backbone bearing the designed N-(3,4,5trimethoxybenzoyl)pyridinium chloride as substituent. Images of PETPC powder morphology were obtained from scanning electron microscope (SEM). Figure 5 shows SEM micrograph of typical PETPC granules. Their shapes are globular and their surfaces are rough, apparently as the result of a binding together of numerous very small agglomerates to form much larger agglomerates, or granules. XRD data was collected for powdery PETPC sample (Figure 6). X-ray diffraction patterns of PETPC powder indicate that its © The Polymer Society of Korea and Springer 2017

morphology is amorphous because the ratio of the half-height width to diffraction angle (2/2) of PETPC powder is greater than 0.35.1,2,29 This may be due to the presence of bulky N-(3,4,5trimethoxybenzoyl)pyridinium chloride and/or the non-selective geometric structure of the polymer main chain. Thermal analyses of PETPC were performed under nitrogen atmosphere at a heating rate of 10 oC/min. Figure 7 shows the TGA thermogram of PETPC. As seen in Figure 7, TGA thermogram of PETPC showed a slight weight loss around 100 oC, which may be due to the moisture, which can be contained in process because of the hygroscopic properties of ionic polymers. An abrupt weight loss was observed in the temperature ranges of 170-400 oC, which was originated by the decomposition of conjugated polymer sys-

Figure 7. TGA thermogram of PETPC under nitrogen atmosphere at a heating rate of 10 oC/min.

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Macromolecular Research

Figure 8. DSC thermogram of PETPC under nitrogen atmosphere at a heating rate of 10 oC/min.

Figure 10. Cyclic voltammograms of PETPC in 4×10-4 M TBAT/DMF solution: consecutive scans up to 30 cycles.

tammetry (CV). Figure 10 showed oxidation and reduction current of PETPC increased from 5 cycles to 30 cycles in consecutive scan mode. Oxidation and reduction were performed in the range of -1.8 V to 1.50 V. Starting point of oxidation and reduction was 0.5 V and -0.9 V, respectively and exhibited overall stable electrochemical property for 30 cycles. Oxidation current was maintained up to the number of 30 cycles but reduction current was gradually increased by repeated electrochemical reaction. This is not clearly explained, but it might be due to the anion penetration from electrolyte into polymer during reduction cycles.45 In addition, the reduction current had larger amount than oxidation. Further studies are underway to clarify this issue. Figure 9. Optical absorption and photoluminescence spectra of PETPC (excitation wavelength: 500 nm, solvent: DMF).

4. Conclusions

tem losing the bulky pendant groups. The present PETPC polymer retained 97.3% of its original weight at 100 oC, 92.5% at 200 oC, 73.2% at 300 oC , 51.5% at 400 oC, and 42.3% at 700 oC. Figure 8 shows the DSC thermogram of PETPC measured under nitrogen atmosphere. The DSC curve of PETPC gave no sign of any thermal events on crystallinity before decomposition. This suggests an armorphous structure of conjugated polysalts. PETPC was mostly soluble in such organic solvents as pyridine, N,Ndimethylformamide, dimethylsulfoxide, N-methylpyrrolidine, etc. And the inherent viscosity of PETPC was 0.14 g/dL. This value is very similar with those of poly(N-benzoyl-2-ethynylpyridinium chloride)s prepared by the same method.40 In solution state, UV-Visible (UV-Vis.) and photoluminescence (PL) spectra of PETPC were measured in order to obtain the electro-optical properties of PETPC (Figure 9). The absorption spectrum showed broad pattern in the range of 300 nm to 800 nm which is visible wavelength region. It came from →* interband transition of conjugated polymer. Maximum values of absorption spectrum were appeared at 331 and 395 nm. The band gap of 2.23 eV was calculated by absorption spectrum. When PETPC was excited at 500 nm and cut off at 515 nm due to excitation light, maximum value of PL spectrum was exhibited 570 nm. It corresponds to the photon energy of 2.18 eV. The electrochemical property was checked using cyclic vol-

In this article, we reported the synthesis of a new conjugated ionic polyacetylene with N-(3,4,5-trimethoxybenzoyl)pyridinium chloride via the non-catalyst polymerization in a moderate yield. The polymerization mechanism included that the activated terminal acetylenic groups of 2-ethynyl-N-(3,4,5-trimethoxybenzoyl)pyridinium chloride are susceptible to the consecutive linear polymerization. The relative low activity of 2-ethynyl-N(3,4,5-trimethoxybenzoyl)pyridinium chloride compared to that of N-(benzoyl)-2-ethynylpyridinium chloride was attributed to the increase of electron density at carbonyl carbons of benzoyl groups by the three elelctron-donating methoxy substituents. The resulting polymer was completely soluble in such organic solvents as pyridine, N,N-dimethylformamide, dimethyl sulfoxide, N-methylpyrrolidine, and was found to be mostly amorphous by X-ray diffraction analysis. The results of spectral analyses on polymer structure indicated that the polymer have a polyacetylene backbone system with the designed N-(3,4,5-trimethoxybenzoyl)pyridinium chloride as substituent. UV-Vis absorption of PETPC was mainly showed at 331 and 395 nm and maximum value of PL was exhibited 570 nm according to the photon energy of 2.18 eV. When oxidation and reduction of PETPC were performed in the range of -1.8 V to 1.50 V, current of PETPC were maintained similar shape under consecutive scans up to 30 cycles and had stable electrochemical property.

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