Polymer 54 (2013) 4161e4170. Contents lists available at SciVerse ScienceDirect. Polymer. journal homepage:

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1 Polymer 54 (2013) 4161e4170 Contents lists available at SciVerse ScienceDirect Polymer journal homepage: Synthesis and properties of de-cross-linkable acrylate polymers based on hexaarylbiimidazole Takeru Iwamura a, *, Saori Nakamura b a Department of Chemistry and Energy Engineering, Faculty of Engineering, Tokyo City University, Tamazutsumi, Setagaya-ku, Tokyo , Japan b Institute for Environmental Sciences, University of Shizuoka, 52-1, Yada, Suruga-ku, Shizuoka , Japan article info abstract Article history: Received 8 March 2013 Received in revised form 18 May 2013 Accepted 29 May 2013 Available online 5 June 2013 Keywords: Cross-linking De-cross-linking Triphenylimidazole The radical polymerization of 4-(4,5-diphenyl-1H-imidazol-yl)-phenyl acrylate (DPIPA) was carried out in N,N-dimethylformamide at 60 C for 24 h by using 2,2 0 -azobis(isobutyronitrile) (5 mol%) as an initiator to give a poly(dpipa) (1) as a homopolymer. Poly(DPIPA-co-MMA) (2) was obtained by the radical copolymerization of DPIPA with methyl methacrylate (MMA) in a good yield. Moreover, copolymeriztion parameters of DPIPA were evaluated by the copolymerization of MMA. The cross-linking reaction of the obtained 2 proceeded with K 3 [Fe(CN) 6 ] and KOH. The resulting cross-linked poly(dpipa-co-mma) (3) was de-cross-linked by pressurization or visible light irradiation. Consequently, the thiol-capped decross-linked poly(dpipa-co-mma) (4) was obtained in a good yield. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Since cross-linked polymers, which have a three-dimensional network structure, cannot be melted by heating, it is difficult to reproduce the cross-linked polymer by processing. Consequently, it is difficult to recycle cross-linked polymers. In many cases, the used cross-linked polymers are practically buried or incinerated. However, in recent years, due to increasing environmental concerns, the recycling of cross-linked polymers has been studied [1e5]. Generally, it is difficult to thermoplasticize and to recycle these polymers. However, if the cross-linked polymers that became useless can be recycled as a thermoplastic resin, the moulded items which were derived from the cross-linked polymers could be used by material recycling in a wide range of fields. In addition, the thermoplasticization of the cross-linked polymers having insoluble and infusible properties means that these polymers would be converted into solvent-soluble liner polymers. Such a thermoplasticization technology would promote recycling of the cross-linked polymers. The thermoplasticization or solubilization of cross-linked polymers is disturbed by the covalent bond of a cross-linked moiety. If the crosslinked moiety, which is included in the cross-linked polymers, can be cleaved under external stimulation, these polymers will become * Corresponding author. Fax: þ address: iwamurat@tcu.ac.jp (T. Iwamura). recycled materials. We have proposed a possible approach which consists of the introduction of a de-cross-linking system through the use of weak covalent bonding into cross-linked polymers. Therefore, we paid attention to hexaarylbiimidazole (HABI) as a molecule with a weak covalent bonding. HABI, which is known as photo, thermo, and piezochromic compound, was first synthesized by Hayashi and Maeda in 1960 [6]. HABI has two kinds of 2,4,5-triphenylimidazole dimers which are prepared from 2,4,5-triphenylimidazole with K 3 [Fe(CN) 6 ] under basic conditions. One of them shows photochromic and thermochromic properties with irradiation and heating [7,8]. The other shows a piezochromic property on grinding [9]. The photochromic compound is a HABI dimer and photo, thermo, and piezochromism of the two compounds are due to reversible dissociation of the dimers into a 2,4,5-triphenylimidazolyl radical which forms by a reversible reaction. The photochromic and piezochromic dimers are isomeric dimers which differ in their positions of combination of the 2,4,5-triphenylimidazolyl radicals and are at equilibrium with the radical in solution [10]. HABI derivatives are readily cleaved under mild conditions such as visible light irradiation and pressure. We have reported a novel de-cross-linking system utilizing pressure or visible light irradiation [11]. The cross-linking reaction of a polymer having a 2,4,5-triphenylimidazole moiety which was incorporated into the side chains of the polymers proceeded with K 3 [Fe(CN) 6 ] and KOH, under pressure or visible light. The signal of the 2,4,5-triphenylimidazolyl radicals which indicated that the de /$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.

2 4162 T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e4170 cross-linking reaction had proceeded, was detected by ESR spectroscopy. However, because the reaction conditions of de-crosslinking could not be sufficiently examined, the yield of the decross-linking polymer could not be clarified. Therefore, we aimed to further investigate the de-cross-linking reaction of the crosslinked polymers. The properties of HABI derived from triphenylimidazole may be applied in not only recycling materials, but also adhesion field, and printing field. In this article, a new acrylate monomer having a 2,4,5-triphenylimidazole moiety was synthesized. To examine the radical polymerization behaviour of this monomer in detail, it was polymerized. In addition, de-crosslinking properties of the obtained de-cross-linkable polymers are also described. 2. Experimental procedure 2.1. Materials 1-Butylimidazolium tetrafluoroborate ([Hbim]BF 4 ) was prepared by a reported procedure [12]. 4-(4,5-Diphenyl-1H-imidazol- 2-yl)-phenol was prepared by Siddiqui s reported procedure [13]. Anhydrous N,N-dimethylformamide (DMF) and anhydrous ethanol were purchased from Wako Pure Chemical Industries, Ltd. 2,2 0 - Azobis(isobutyronitrile) (AIBN) was recrystallized from ethanol. Methyl methacrylate (MMA) was distilled and stored under nitrogen. Triethylamine was dried over CaH 2, distilled, and stored under nitrogen. Other solvents and reagents were used as supplied Measurements 1 H NMR and 13 C NMR spectra were recorded on a JEOL ECA-500 spectrometer. IR spectra were measured on a JASCO FT/IR-4200 spectrometer. Gel permeation chromatographic analyses were carried out on a Tosoh Co. HLC-8020 GPC system (TSK-GEL G3000XL, DMF containing LiBr (5.8 mm) as an eluent, and an ultraviolet (UV) detector) using polystyrene as a standard. Melting points were determined on a MEL-270 Shibata melting point apparatus and are uncorrected Synthesis of 4-(4,5-diphenyl-1H-imidazol-yl)-phenyl acrylate (DPIPA) Acryloyl chloride (0.145 g, 1.60 mmol) was added dropwise to a stirred solution of 4-(4,5-diphenyl-1H-imidazol-2-yl)-phenol (0.441 g, 1.41 mmol) and triethylamine (0.171 g, 1.69 mmol) in anhydrous THF (8 ml). After being stirred at room temperature for 15 h, the reaction mixture was diluted with diethyl ether and then was washed with water. The organic phase was dried over MgSO 4, and evaporated under reduced pressure to leave a solvent. The residue was purified by chromatography on silica gel with chloroform as an eluent to isolate DPIPA. Yield: 87%. Mp. 203e205 C; IR (KBr): 3087, 3060, 3034, 1739, 1633, 1605, 1492, 1450, 1248, 1205, 1169, 1149, 847, 767, 697 cm 1 ; 1 H NMR (CDCl 3, 500 MHz) d: 6.03 (dd, J ¼ and 1.15 Hz, 1H, CH 2 ] CHe cis), 6.30 (dd, J ¼ and Hz, 1H, CH 2 ]CHe), 6.60 (dd, J ¼ and 1.15 Hz, 1H, CH 2 ]CHe trans), 7.11 (d, J ¼ 8.88 Hz, 2H, e COOeC 6 H 4 e), 7.20e7.40 (m, 6H, PheH), 7.40e7.60 (m, 4H, PheH), 7.80 (d, J ¼ 8.88 Hz, 2H, ec 6 H 4 ec 3 N 2 He) ppm; 13 C NMR (CDCl 3 / DMSO-d 6, 125 MHz) d: , , , , , , , , ppm; Anal. calcd for C 24 H 18 N 2 O 2 :C,78.67%;H, 4.95%; N, 7.65%. Found: C, 78.69%; H, 4.85%; N, 7.65% Radical homopolymerization of DPIPA (typical procedure) DPIPA (0.304 g, 0.83 mmol) and AIBN (7.1 mg, 5 mol%) were dissolved into DMF (0.83 ml) in a test tube, which was then degassed and sealed in vacuo. After being stirred at 60 C for 24 h, the reaction mixture was evaporated under reduced pressure to remove DMF and the residue was dissolved in THF. The resulting THF solution was poured into distilled water, and the precipitated polymer was separated by using a TOMY LC-131 (3000 rpm, 20 min). The obtained poly(dpipa) (1) was dried in vacuo. Yield: 97%. GPC (eluent: THF): Peak 1: M n ¼ 32,600 (M w /M n ¼ 3.89), 94.4%; Peak 2: M n ¼ 2600 (M w /M n ¼ 1.05), 5.6%; IR (KBr): 3059, 2957, 2933, 2809, 1753, 1659, 1605, 1539, 1491, 1449, 1387, 1203, 1168, 1140, 969, 915, 842, 768, 697 cm 1 ; 1 H NMR (DMSO-d 6, 500 MHz) d: 1.54e2.61 (m, ech 2 ech(coor)e, 2H), 2.83e3.38 (m, ech 2 ech(coor)e, 1H), 6.80e8.28 (m, PheH, 14H) ppm; 13 C NMR Scheme 1. Synthesis of DPIPA.

3 T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e Scheme 2. Radical homopolymerization of DPIPA. 60 C, the reaction mixture was evaporated under reduced pressure to remove DMF and the residue was dissolved in THF. The resulting THF solution was poured into distilled water, and the precipitated polymer was separated by centrifuging at 3000 rpm for 20 min. The obtained poly(dpipa-co-mma) (2) was dried in vacuo. Yield: 92%. Peak 1: M n ¼ 34,000 (M w /M n ¼ 3.28), 94.4%; Peak 2: M n ¼ 2700 (M w /M n ¼ 1.08), 6.5%; IR (KBr): 3059, 2987, 2951, 2853, 1751, 1731, 1657, 1605, 1541, 1491, 1450, 1387, 1203, 1167, 1136, 970, 915, 843, 768, 697 cm 1 ; 1 H NMR (DMSO-d 6, 500 MHz) d: 0.62e3.05 (m, e CH 2 ec(ch 3 )(COOR), 5H 0.52, ech 2 ech(coor)e, 3H 0.48), 3.49 (s, eoch 3,3H 0.52), 6.80e7.70 (m, PheH,12H 0.48), 8.10 (s, PheH, 2H 0.48) ppm; 13 C NMR (DMSO-d 6, 125 MHz) d: 16.62, 18.10, 20.07, 21.66, 24.2, 26.3, 35.79, 36.88, 37.34, 41.03, 43.90, 44.43, 44.93, 46.15, 46.2, 47.9, 51.75, 53.5, 54.7, , , , , , , , , , , , , , , , , ppm. Table 1 Radical homopolymerization of DPIPA. Run Initiator Temp ( C) Yield (%) Peak 1 Peak 2 M n M w /M n Peak area (%) M n M w /M n Peak area (%) 1 AIBN , BPO DTBPO (DMSO-d 6, 125 MHz) d: 35.09, 41.50, , , , , , , , , , , , ppm Radical copolymerization of DPIPA and MMA (typical procedure) DPIPA (0.916 g, 2.50 mmol), MMA (0.259 g, 2.59 mmol) and AIBN (42.6 mg, 5 mol%) were dissolved in DMF (5.0 ml) in a test tube, which was then degassed and sealed in vacuo. After 24 h at 2.6. Determination of monomer reactivity ratios The copolymerization of DPIPA and MMA was performed under various feed ratios. This copolymerization was carried out in the same manner described in Section 2.5 (polymerization time ¼ 60 min). The copolymer compositions were determined by their 1 H NMR spectra Cross-linking reaction of 2 A cold 1% aqueous solution of potassium ferricyanide (50 ml) was added slowly to a solution of 2 (0.280 g) dissolved in a mixed solvent of ethanol (11 ml) and THF (14 ml) containing potassium hydroxide (137.6 mg). During addition, the reaction mixture was maintained at 5e10 C and vigorously stirred. After 6 h at 5e10 C, the reaction mixture was neutralized by HCl. The precipitated cross-linked polymer was separated by centrifuging at 3000 rpm for 20 min. The obtained precipitated polymer was dried in vacuo. Inorganic reagents were removed by centrifugation/washing (3 times) with distilled water. Then, the obtained suspension, including cross- Fig H NMR (DMSO-d 6 ) spectrum of 1.

4 4164 T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e4170 Fig C NMR (DMSO-d 6 ) spectrum of 1. linked poly(dpipa-co-mma) (3), was freeze-dried in vacuo. Yield: 61%. IR (KBr): 3133, 3095, 3067, 3020, 2989, 2953, 2907, 2841, 1731, 1606, 1502, 1448, 1394, 1235, 1174, 1140, 848, 754, 696 cm De-cross-linking of 3 under visible light irradiation 3 (20.0 mg) was dispersed with DMF (1.0 ml) in a Pyrex test tube. The suspension of 3 was irradiated under visible light irradiation using a desk lamp (HITACHI RS3205, 34W). After visible light irradiation, ethanethiol (25 ml, 4.0 eq.) was added to a stirred solution of the reaction mixture. After stirring, the reaction mixture was filtered using a membrane filter (Millipore, Millex Ò -LH, 0.45 ml). The filtrate was poured into distilled water (10 ml), and the precipitated polymer was separated by centrifuging at 3000 rpm for 15 min. The obtained thiol-capped de-cross-linked polymer (4) was dried in vacuo. Yield: 99%. GPC (eluent: THF): Fig. 3. FT-IR spectrum (KBr) of 1. M n ¼ 5000 (M w /M n ¼ 3.38); IR (KBr): 3093, 3070, 3028, 2925, 2855, 1720, 1678, 1600, 1557, 1494, 1408, 1363, 1293, 1238, 1179, 1136, 846, 756, 696 cm 1 ; 1 H NMR (DMSO-d 6, 500 MHz) d: 0.60e1.40 (m, e CH 2 ec(ch 3 )(COOR), 3H 0.52, CH 3 ech 2 ese, 1.01H [¼3H 0.34]), 1.40e2.47 (m, ech 2 ec(ch 3 )(COOR), 2H 0.52, ech 2 ech(coor)e, 3H 0.48), 3.48e3.70 (m, eoch 3,3H 0.52), 3.68e4.35 (m, CH 3 e CH 2 ese, 0.68H [¼2H 0.34]), 6.82e8.22 (m, PheH, 14H 0.48) ppm; 13 C NMR (DMSO-d 6, 125 MHz) d: 13.72, 13.90, 16.55, 17.79, 17.96, 19.88, 21.76, 22.18, 23.10, 23.40, 24.79, 25.48, 27.44, 29.06, 30.47, 31.42, 33.26, 36.40, 42.58, 43.13, 43.88, 44.24, 46.44, 46.74, 47.73, 50.63, 51.70, 53.66, , , , , , , , , , , , , , , , , , , , , , , , ppm De-cross-linking of 3 under pressure 3 (13.1 mg) was pressurized by an agate mortar and pestle. After being pressurized for 5 min, the pressurized solid was dispersed in DMF (1.0 ml), and then ethanethiol (25 ml, 4.0 eq.) was added to a stirred solution of the pressurized solid in DMF. After stirring, the reaction mixture was filtered using a membrane filter (Millipore, Millex Ò -LH, 0.45 ml). The filtrate was poured into distilled water (10 ml), and the precipitated polymer was separated by centrifuging at 3000 rpm for 15 min. The obtained thiol-capped de-crosslinked polymer (4) was dried in vacuo. Yield: 80%. GPC (eluent: THF): M n ¼ 7000 (M w /M n ¼ 3.27); IR (KBr): 2981, 2949, 2925, 1728, 1655, 1608, 1554, 1519, 1492, 1447, 1385, 1094, 1034, 987, 918, 847, 749, 698 cm 1 ; 1 H NMR (DMSO-d 6, 500 MHz) d: 0.60e1.40 (m, e CH 2 ec(ch 3 )(COOR), 3H 0.52, CH 3 ech 2 ese, 0.83H [¼3H 0.28]), 1.40e2.47 (m, ech 2 ec(ch 3 )(COOR), 2H 0.52, ech 2 ech(coor)e, 3H 0.48), 3.48e3.70 (m, eoch 3,3H 0.52), 3.68e4.35 (m, CH 3 e CH 2 ese, 0.56H [¼2H 0.28]), 6.82e8.22 (m, PheH, 14H 0.48) ppm; 13 C NMR (DMSO-d 6, 125 MHz) d: 13.71, 13.89, 17.72, 17.99, 18.54, 19.69, 19.90, 21.69, 22.11, 23.11, 25.37, 25.66, 28.71, 29.01,

5 T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e Scheme 3. Radical copolymerization of DPIPA and MMA. Fig H NMR (DMSO-d 6 ) spectrum of , 31.21, 32.16, 32.91, 36.33, 36.76, 36.95, 42.30, 42.61, 43.00, 43.86, 44.26, 47.40, 50.57, 51.66, 52.35, , , , , , , , , , , , , , , , , , , , , , , , , , , , , ppm. 3. Results and discussion 3.1. Synthesis of DPIPA DPIPA was prepared by a two-step reaction. First, 4-(4,5- diphenyl-1h-imidazol-2-yl) phenol was synthesized by means of Siddiqui s protocol (Scheme 1, eq. 1) [13]. Then, 4-(4,5-diphenyl- 1H-imidazol-2-yl) phenol reacted with acryloyl chloride to yield DPIPA (Scheme 1, eq. 2). It was well characterized by 1 H NMR, 13 C NMR, and FT-IR spectroscopy. DPIPA was also characterized by elemental analysis Radical homopolymerization of DPIPA The radical homopolymerization of DPIPA was carried out at 60 C for 24 h using a radical polymerization initiator (5 mol%) such as AIBN (Scheme 2). The results of the polymerization are summarized in Table 1. 1 was obtained as a water-insoluble part. In the case of the AIBN initiation system, 1 was obtained in a good yield. However, in the case of the benzoyl peroxide (BPO) and di-tert-butyl peroxide (DTBPO) initiation system, the corresponding polymers were obtained in low yields. Otsu et al. reported the relative

6 4166 T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e4170 Fig C NMR (DMSO-d 6 ) spectra of 2. reactivity of MMA toward primary radicals [14]. The relative reactivity of MMA toward 2-cyano-2-propyl radical ((CH 3 ) 2 (CN)C$) derived from AIBN was reported as being They also reported that the relative reactivity values of MMA derived from BPO or Fig. 6. FT-IR spectrum of 2. DTBPO were low (0.12 and 0.06, respectively). DPIPA is a kind of acrylic monomer similar to MMA. Therefore, these relative reactivities might be similar. Since the initiator efficiencies of BPO and DTBPO were low, the yields of the corresponding polymer might be low. In the gel permeation chromatography (GPC) analysis, the obtained homopolymers showed bimodal peaks. The reason for these peaks is probably the chain-transfer reaction to initiator which may be taking place in the growing polymer chain. The 1 H and 13 C NMR spectra of 1 are shown in Figs. 1 and 2, respectively. In the 1 H NMR spectrum, the signals resulting from the aromatic protons (a) in the triphenylimidazole moieties were observed at d 6.80e8.28 ppm. Additionally, the peaks due to the methine and methylene protons (b and c) were observed at d 2.83e 3.38 and 1.54e2.61 ppm, respectively (Fig. 1). This result supported the fact that the polymerization of DPIPA involved a radical homopolymerization process. The 13 C NMR spectrum of 1 also supported radical homopolymerization (Fig. 2). The peaks due to methine and methylene carbons (f and g) were observed at d 41.5 and 35.1 ppm, respectively. In Fig. 3, the FT-IR spectrum of 1 showed an absorption band at 1753 cm 1 based on the ester carbonyl group (n C ] O ). Moreover, the absorption bands of the imidazole ring observed at 1659 cm 1 and

7 T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e Table 2 Radical copolymerization of DPIPA and MMA. Run 1/MMA (mol ratio) Yield (%) x/y Peak 1 Peak 2 M n M w /M n Peak area (%) M n M w /M n Peak area (%) / / , / / , / / , / / , / / , / / , / / , Table 3 Monomer reactivity ration of the copolymerization of DPIPA with MMA. M 1 M 2 r 1 r 2 r 1 r 2 Q 1 e 1 1 MMA cm 1 were assigned to the C]N stretching frequency and Ne H in-plane deformation vibration, respectively Radical copolymerization of DPIPA and MMA The radical copolymerization of DPIPA and MMA was carried out at 60 C for 24 h using AIBN as an initiator under various feed ratios (Scheme 3). The corresponding copolymers were obtained in good yields (90e100%). The copolymer composition was determined by its 1 H NMR spectra (Fig. 4). The unit ratio was determined from the integral ratio between the aromatic protons (a) and methyl protons (f). The 13 C NMR spectrum of 2 is shown in Fig. 5. The signals ascribable to the aryl imidazole moiety (x unit) were observed at d 120.7e151.0 (c) ppm. The signals ascribable to the MMA moiety (y unit) were observed at 174.8e178.8 (a), 51.8 (d), and 15.3e25.2 (i) ppm. The 13 C NMR spectrum of 2 also supported the fact that DPIPA undertook radical copolymerization. The FT-IR spectrum of 2 showed an absorption band at 1731 cm 1 based on the ester carbonyl group (n C ] O )infig. 6. Moreover, the absorption bands of the imidazole ring observed at 1657 cm 1 and 1541 cm 1 were assigned to the C]N stretching frequency and NeH in-plane deformation vibration, respectively. GPC analysis of the copolymer from DPIPA and MMA was carried out in THF using polystyrene as the standard. In the GPC profiles, the obtained polymers showed bimodal peaks (Table 2). The peak area decreased on the high-molecular-weight side of the peak and increased on the low-molecular-weight side of the peak as the amount of MMA increased. Moreover, M n, which was calculated from the high-molecular-weight side of the peak, decreased as the amount of MMA increased. Monomer reactivity ratios were evaluated by using the Mayoe Lewis equation [15]. The calculated values, r 1, r 2 and r 1 r 2, and AlfreyePrice Qee values are shown in Table 3 [16]. Q and e values indicate that DPIPA has a large resonance effect and electronwithdrawing character due to the ester and triphenylimidazole groups, playing an important role in alternating copolymerizability with MMA Cross-linking reaction of 2 The cross-linking reactions of 2 were carried out by a previously reported procedure [11]. To the solution of 2 in a solution of KOH in a mixed solvent of ethanol and THF, a cold ethanol 1% K 3 [Fe(CN) 6 ] solution was added with vigorous stirring at 5e10 C for 6 h (Scheme 4). The precipitated cross-linked polymer was repeatedly washed with distilled water and then freeze-dried in water. Consequently, 3 was obtained in 61% yield. The IR spectra of 3 are shown in Fig. 7. In the FT-IR spectrum of 3, the signals ascribable to the ester group were 1731 cm 1. In the region of 1800e1400 cm 1, the FT-IR spectra of 2 and 3 were compared. In Scheme 4. Cross-linking reaction of 2.

8 4168 T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e4170 Table 4 De-cross-linking reaction of 3. Run External stimulation Solvent Time (min) Yield (%) 1 hn DMF Pressure None a Determined by 1 H NMR spectrum. Modification ratio (%) a Fig. 7. FT-IR spectra of 2 and 3. the case of 2, the C]N stretching vibrations ascribable to the imidazole moiety were observed at 1605 and 1583 cm 1. Moreover, the C]C stretching vibration and C]NeC]C symmetric stretching vibration were observed at 1491 and 1450 cm 1, respectively. On the other hand, the HABI moiety as a crosslinking point was included in 3. White et al. and Tanino et al. reported the IR spectra of HABI isomers. Therefore, 3 was identified by these reported data. In these reported data, the 1,2 0 - isomer of HABI absorbed at 1604 (m), 1554 (m), and 1480 (m) cm 1. 3 absorbed at in the same region at 1606 (m), 1557 (m), and 1481 (m) cm 1. White et al. reported that the isomer of HABI has an IR band at 1605 (m), 1562 (s), and 1489 (s) cm 1 [10]. On the other hand, Abe et al. pointing out that the 2,2 0 -isomer was more appropriate than the 4,4 0 -isomers that White et al. proposed [17]. In the FT-IR spectrum of 3, these peaks, which might be buried in other peaks, could not be clearly confirmed. Additionally, the strong band at 1610 cm 1 due to the 1,4 0 -isomer and the very strong band at 1613 cm 1 due to the 2,4 0 -isomer were not observed in this spectrum. Therefore, the 1,2 0 -isomer might be main constituent of the cross-linking point (Scheme 5). Fig. 8. FT-IR spectra of 2, 4 prepared under visible light (Table 4, Run 1), and 4 prepared under pressure (Table 4, Run 2) De-cross-linking of cross-linked polymer 3 We attempted the de-cross-linking reaction of 3 under pressure or visible light irradiation. In the case of the de-cross-linking reaction of 3 under pressure, 3 was ground manually in an agate mortar with a pestle at room temperature for 5 min. On the other hand, in the case of the de-cross-linking reaction of 3 under visible light irradiation, 3 was compulsorily dispersed in DMF. The suspension of 3 was irradiated under visible light irradiation using a desk lamp for 60 min. In our previous work, the ESR peak of cleaved imidazolyl radicals in the side chain of the cross-linked polymer exhibited a single peak with a width of 7.5 G and a g Scheme 5. De-cross-linking reaction of cross-linked polymer 4.

9 T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e Fig H NMR (DMSO-d 6 ) spectrum of 4 prepared under visible light (Table 4, Run 1). value of after irradiation for 23 min. The intensity of the ESR peak was spins/mg under visible light irradiation for 23 min. Under visible light irradiation for 40 min, the intensity of the ESR peak increased with time ( spins/mg). After stopping the irradiation with visible light, the peak of the cleaved imidazolyl radicals gradually decreased with time in the dark [11]. Therefore, to isolate the de-cross-linked polymer, ethanethiol was added to reaction mixture after the de-cross-linking reaction. The corresponding 4 was obtained as a water-insoluble part in a good yield. The yield of the de-cross-linked polymer, which was unclear in our previous research, was clarified in this study. The results of the de-cross-linking reaction of 3 are summarized in Table 4. The IR spectra of 2 and 4 are shown in Fig. 8. Especially, the FT-IR spectra of 2 and 4 were compared in the region of 1800e1000 cm 1. The peaks aei of2 are peaks which can almost all be found in 4 although one of these peaks has been buried in other peaks. This result shows that the structure of 4 is mostly maintained in 2. Fig C NMR (DMSO-d 6 ) spectrum of 4 prepared under visible light (Table 4, Run 1). Fig. 11. GPC profiles of before cross-linking polymer (2), after de-cross-linking polymer (4) prepared under visible light (Table 4, Run 1), and after de-cross-linking polymer (4) prepared under pressure (Table 4, Run 2).

10 4170 T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e4170 However, in the case of 4, the peaks not observed in 2 were observed at 1440 (region of A) and 1167 (region of B) cm 1. Harrison et al. and Krishnakumar et al. reported the IR spectra of (alkylthio) benzimidazole. From their reported data, these peaks were assigned to the S-alkyl group of the imidazole ring [18]. The 1 H NMR spectrum of 4 is shown in Fig. 9. A peak due to the methyl group of the S-alkyl group was observed at around d 1.23 ppm (g). Additionally, in the 13 C NMR spectrum of 4, a peak due to the methyl group of the S-alkyl group was observed at around d 13.8 ppm in Fig. 10. The GPC analysis of 4 was carried out in DMF containing LiBr as an eluent. The GPC profiles before cross-linking and after de-cross-linking of 3 were accomplished by pressure or visible light (Fig. 11). 4. Conclusions The radical polymerization of DPIPA prepared by the reaction of 4-(4,5-diphenyl-1H-imidazole-2-yl)-phenol with acryloyl chloride, was carried out at 60 C for 24 h in DMF using AIBN as an initiator. The corresponding 1 and 2 were obtained in good yields. Copolymerization parameters of DPIPA were evaluated by the copolymerization with MMA. The monomer reactivity ratios were found to be r 1 ¼ and r 2 ¼ from which the Qee values of DPIPA were estimated as Q ¼ and e ¼ The crosslinking reaction of 2 proceeded with K 3 [Fe(CN) 6 ] under basic conditions. The de-cross-linking reaction of 3 proceeded by pressure or visible light irradiation. The corresponding 4 was obtained in a good yield. From the view point of environmental science, it is significant that the cross-linked polymer can decross-link to a soluble polymer. The polymer having HABI moiety is interesting as an environmental material. Moreover, this polymer can be expected in many fields such as adhesion field, and printing field. Acknowledgement This work was supported by a Grant-in-Aid for Scientific Research (C) (No ) from Japan Society for the Promotion of Science. Appendix A. Supplementary data Supplementary data related to this article can be found at dx.doi.org/ /j.polymer References [1] Goussé C, Gandini A, Hodge P. Macromolecules 1998;31:314. [2] Oku T, Furusho Y, Takata T. Angew Chem Int Ed 2004;43:966. [3] Nakamura T, Ochiai B, Endo T. Macromolecules 2005;38:4065. [4] Zhang Y, Broekhuis AA, Picchioni F. Macromolecules 2009;42:1906. [5] Yonekawa M, Furusho Y, Endo T. Macromolecules 2012;45:6640. [6] Hayashi T, Maeda K. Bull Chem Soc Jpn 1960;33:565. [7] Hayashi T, Maeda K, Morinaga M. Bull Chem Soc Jpn 1964;37:1563. [8] Hayashi T, Maeda K, Kanaji T. Bull Chem Soc Jpn 1965;38:857. [9] Hayashi T, Maeda K. Bull Chem Soc Jpn 1965;38:685. [10] White DM, Sonnenberg J. J Am Chem Soc 1966;88:3825. [11] Iwamura T, Sakaguchi M. Macromolecules 2008;41:8995. [12] Hasegawa M, Ogata T, Sato M. Powder Technol 1995;85:269. [13] Siddiqui SS, Narkhede UC, Palimkar SS, Daniel T, Lahoti RJ, Srinivasan KV. Tetrahedron 2005;61:3539. [14] Otsu T, Matsumoto A. Kobunshi 1986;35:1050. [15] Mayo FR, Lewis FM. J Am Chem Soc 1944;66:1594. [16] Alfrey T, Price CC. J Polym Sci 1947;2:101. [17] Abe J, Sano T, Shirai Y. Fine Chem 1999;28:60. [18] Harrison D, Ralph JT. J Chem Soc B 1967;1:14.