Preparation of network π-conjugated copolymers with Ullmann type polycondensation

Available online at www.ilcpa.pl International Letters of Chemistry, Physics and Astronomy 6 (2014) 33-38 ISSN 2299-3843 Preparation of network -conjugated copolymers with Ullmann type polycondensation Kuniharu Nakajima, Hiromasa Goto* Division of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan *E-mail address: ABSTRACT Ullmann type polycondensations in the presence of CuI and a base were carried out to afford network type -conjugated copolymers. Infrared absorption spectroscopy measurements and surface observation using a scanning electron microscopy are carried out. Electron spin resonance spectroscopy measurements revealed that the cross-linked copolymers thus obtained contain small amount of copper. This polymerization conveniently allows production of network -conjugated polymers. The polymer can be expected to have thermo-resistance. Keywords: copolymer; cross-link; polyaniline; Ullmann reaction 1. .INTRODUCTION Polyaniline (PANI) [1] is one of the most promising conducting polymers because it has relatively high conductivity, and processability. PANI has been applied for sensors [2], anti-corrosion agent, and buffer layer for O-LED, electrochromics, and anti-static materials. Ullmann condensation reaction is widely known as a coupling reaction by using copper. Especially, this is quite useful for condensation reaction between amine and halogen to synthesize -conjugated compounds. To obtain polyaniline (PANI), oxidative polycondensation with ammonium persulfate (APS) in the water has been employed. This method allows production of PANI in the form of doped state (emeraldine salt). Ullmann polycondensation [3] of m-chloro-aniline and m-dibromobenzene allows poly(m-aniline) [4]. Buchwald-Hartwig reaction with an aid of Pd catalyst allows reaction of aniline and halogen substituted aromatic group [5,6]. Palladium-catalyzed polycondensation of aryl dibromides with aryl primary diamines has been performed to afford linear polymers [7]. Classic Ullmann reaction has advantages for condensation reaction because low cost and the reaction can be conducted in the air, although high reaction temperature is required. The Ullmann polycondensation reaction can afford tertiary amines to give network polymers. Natural network polymers such as lignin have high strength. Phenol resin as a synthetic network polymers has been widely applied in industry. Therefore, network type -conjugated International Letters of Chemistry, Physics and Astronomy 6 (2014) 33-38 polymers can be expected for mechanical strength, durability, and electronic property derived from the -electrons. In this research, we perform Ullmann type polycondensation to obtain network poly(arylene-co-polyaniline)s. Infrared absorption (IR) spectroscopy measurements, surface observation using a scanning electron (SEM) microscopy, electron spin resonance (ESR), and UV-VIS optical absorption spectroscopy measurements are carried out for the polymers thus obtained. 2. .EXPERIMENT O O N S O Br O S O S S Br O N O O S N O Poly1 O N Br CuI, K2CO3 NH2 NH2 N N Br N N N Nitrobenzene N N N N Poly2 Br S N S Br S S N S N Poly3 Poly4 Scheme 1. Synthesis of network polymers. 34 International Letters of Chemistry, Physics and Astronomy 6 (2014) 33-38 Poly1, quantity used: EDOTBr2 (0.1 g, 0.335 mmol), p-phenylenediamine (0.36 g, 0.335 mmol), nitrobenzene (1 mL), CuI (6.5 mg, 3.4 x 10 -5 mol), K2CO3 (46 mg, 3.3 x 10-4 mol). Y = 0.081 g. Poly2, quantity used: 2,5-Dibromopyridine (0.5 g, 2.1 mmol), p-phenylenediamine (0.23 g, 2.1 mmol), nitrobenzene (5 mL), CuI (0.02 g, 1.05 x 10 -4 mol), K2CO3 (0.3 g,2.1 mmol). Y = 0.385 g. Poly3, quantity used: 2,5-Dibromothiophene (3 g, 0.012 mol), p-phenylenediamine (1.34 g, 0.012 mol), nitrobenzene (20 mL), CuI (0.2 g, 1.05 x 10-3 mol), K2CO3 (1.66 g, 0.012 mmol). Y = 0.920 g. Poly4, quantity used: 2,5-Dibromothiophene (3 g, 0.012 mol), p-phenylenediamine (1.34 g, 0.012 mol), nitrobenzene (20 mL), CuI (0.02 g, 1.05 x 10-4 mol), K2CO3 (1.66 g, 0.012 mmol). Y = 0.958 g. Polymerization was conducted between dibromoarylene, such as dibromo-3,4-ethylenediioxythiophene (EDOTBr2), 2,5-dibromothiophene, or 2,5-dibromopyridine), and p-phenylenediamine as shown in Scheme 1. A solution of the dibromoalyrene, dibromobenzene, and K2CO3 in nitrobenzene was stirred for 24 h at 140 ºC. Then, the reaction mixture was poured into a large volume of methanol. After filtration, the product was washed with a large volume of distilled water, and a large volume of methanol again. Filtration followed by drying in vacuum yielded network copolymers. The copolymers thus obtained are abbreviated as Poly1 (poly(EDOT-aniline), Poly2 (pyridine-aniline), Poly3 (thiophene-aniline). The copolymers are infusible because of formation of polymer network by cross-link. 3. .RESULTS AND DISCUSSION 3. 1. IR IR absorption spectroscopy measurements were carried out with the KBr method (Figure 1). Poly1 shows absorption bands at 1628 cm-1 (C=C stretching, quinonoid) [8], 1518 cm-1 (C=C stretching, benzenoid), and 1089 cm-1 (C-O-C, stretching, benzenoid). An absorption bands at around 3300 cm-1 due to N-H stretching was not observed. Wide range absorption from 4000-2000 cm-1 indicates that the polymer is doped state. Poly4 shows the same absorption bands as Poly3. Absorbance/arb. units Poly1 Poly2 Poly3 Poly4 4000 3500 3000 2500 2000 1500 Wavenumber/cm-1 1000 Figure 1. Infrared absorption spectra of the polymers. 35 500 International Letters of Chemistry, Physics and Astronomy 6 (2014) 33-38 HBr yielded during the reaction functioned to be a dopant to the resultant polymer (K2CO3 in the reaction may not neutralize it). Poly1 shows electrical conductivity of 9.0 × 10-3 S/cm. Poly2 and Poly3 display absorption bands at long wavenumber due to N-H stretching, absorption at 3233, 3319, 3184. Poly2 (IR, cm-1): 3233 (N-H stretching), 1579 (C=C stretching, quinonoid), 1505 (C=C stretching, benzenoid), 1420, 1260 (aromatic ring stretching). Poly3 (IR, cm-1): 3319 and 3184 (N-H stretching), 1616, 1505, 1303 cm-1 (aromatic ring stretching). 3. 2. SEM Figures 2,3 show surface structure of the polymers. These polymers exhibit pebbles like structure [9,10]. Poly3 partly displays bubble-like structure. 3. 3. ESR Electron spin resonance (ESR) measurements were carried out for the powder of the polymers [11]. Polymers show the ESR signals at around g = 2.08. Spin numbers of the polymers are to be in the range of 7-9 × 10-6 spins/g. However, an ESR signal due to paramagnetic copper was overlapped on the polymers signal. This result indicates that removal of the trace amount of copper as a catalyst from the resultant polymer seems to be difficult. Figure 2. Scanning electron microscopy (SEM) images of Poly2 (1,000×). 36 International Letters of Chemistry, Physics and Astronomy 6 (2014) 33-38 Figure 3. SEM images of Poly1 (l (...truncated)