Synthesis of recyclable molecular LEGO block polymers utilizing the Diels-Alder reaction

Polymer 101 (2016) 98e106

Contents lists available at ScienceDirect

Polymer journal homepage: www.elsevier.com/locate/polymer

Synthesis of recyclable molecular LEGO block polymers utilizing the Diels-Alder reaction Shunsaku Motoki a, Takeshi Nakano b, Yudai Tokiwa a, Kouhei Saruwatari b, Ikuyoshi Tomita c, Takeru Iwamura a, b, * a

Department of Chemistry and Energy Engineering, Graduate School of Engineering, Tokyo City University, 1-28-1 Tamazutumi, Setagaya-ku, Tokyo 1588857, Japan Department of Chemistry and Energy Engineering, Faculty of Engineering, Tokyo City University, 1-28-1 Tamazutsumi, Setagaya-ku, Tokyo 158-8557, Japan c Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259-G1-9 Nagatsuta, Midori-ku, Yokohama, Japan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 June 2016 Received in revised form 3 August 2016 Accepted 7 August 2016 Available online 9 August 2016

Molecular LEGO blocks having two reversible covalent bond moieties were employed to synthesize corresponding molecular LEGO block polymers. Hydrophilic and hydrophobic molecular LEGO blocks having two furan moieties were synthesized. These molecular LEGO blocks were polymerized with a molecular LEGO block having two maleimide moieties under Diels-Alder conditions. The resulting molecular LEGO block polymers were obtained in good yield. The molecular LEGO block polymers were depolymerized under retro-Diels-Alder conditions. In addition, these polymers proceeded in a scrambling reaction between the molecular LEGO block polymers and other molecular LEGO blocks. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Recyclable polymer Molecular LEGO block Depolymerization

1. Introduction The reduction of greenhouse gas emissions to acceptable levels will almost certainly be the greatest environmental challenge facing humans. This increase in CO2 in the atmosphere is primarily due to huge utilization of fossil fuels and forest destruction [1]. Polymer materials derived from fossil fuels such as plastics are inexpensive, easy to mold, and lightweight. These benefits have become the driving forces that have resulted in many commercial applications. However, due to insufficient recycling, polymer materials derived from fossil fuels have been burned or have been discarded in landfills. For example, the overall U.S. post-consumer plastic waste for 2008 was estimated at 33.6 million tons, 2.2 million tons (6.5%) of which was recycled, 2.6 million tons (7.7%) were burned for energy, and 28.9 million tons, or 85.5%, were discarded in landfills [2].

* Corresponding author. Department of Chemistry and Energy Engineering, Graduate School of Engineering, Tokyo City University, 1-28-1 Tamazutumi, Setagaya-ku, Tokyo 158-8857, Japan. E-mail address: [email protected] (T. Iwamura). http://dx.doi.org/10.1016/j.polymer.2016.08.024 0032-3861/© 2016 Elsevier Ltd. All rights reserved.

As described above, these methods have two fundamental problems. Firstly, CO2 gas, which is generated by the incineration of waste polymer materials, has diffused into the atmosphere. Secondly, landfills of waste polymer materials have not led to the effective utilization of plastic. Therefore, in order to avoid the diffusion of CO2 gas and the useless landfills of waste plastics, the molecular design of polymers is necessary to facilitate the recycling of polymer materials. Recyclable polymers are expected to be important eco-friendly materials for overcoming serious problems such as environmental degradation. We have been conducting research on the synthesis and recycling properties of de-cross-linkable polymers based on hexaarylbiimidazole (HABI) [3,4]. Among a large number of recycling methods used for polymers, chemical recycling is the most important and substantial target. Many researchers have reported on the chemical recycling of polymers, but in general, chemical recycling is difficult [5e9]. However, new forms of chemical recycling should be continuously developed for a wide range of polymers. To overcome these limitations, we have proposed a possible approach which consists of molecular LEGO blocks having moieties with reversible covalent bonds and various element groups.

S. Motoki et al. / Polymer 101 (2016) 98e106

These molecular LEGO blocks might be able to be detached as monomeric units as in the toy LEGO. To realize such a strategy, it is suitable to use the Diels-Alder reaction. The Diels-Alder reaction is one of the most famous reactions in organic chemistry. In the Diels-Alder reaction an alkene adds 1,4 to a conjugated diene, and the resulting product becomes a six-membered ring. Since Wudl et al. first reported an example of using the Diels-Alder reaction for cross-linked material [10], various polymers based on the Diels-Alder reaction have been reported in recent years in the field of polymer chemistry [11e18]. Among these studies, self-healing materials have been intensively investigated [19,20]. In contrast, there are several reports of depolymerization of Diels-Alder polymers based on the retro-Diels-Alder reaction [17,21,22]. As far as we know, a method that changes the properties of original polymers by a scrambling reaction has not been reported. Therefore, we attempted to synthesize recyclable polymers utilizing molecular LEGO blocks. Moreover, we examined upgrading these polymers by scrambling molecular LEGO blocks with characteristics that can be recomposed. 2. Experimental procedure 2.1. Materials N,N-Dimethylformamide (DMF) was dried over CaH2, distilled under nitrogen, and stored under nitrogen. Dichloromethane and 1,2-dichroloethane were dried over P2O5, distilled, and stored under nitrogen. N,N-Dimethyl-4-aminopyridine (DMAP) was recrystallized from n-hexane. 4,40 -Bismaleimidodiphenylmethane (BMI) as a molecular LEGO block was purchased from Tokyo Chemical Industry Co., Ltd. Other solvents and reagents were used as supplied. 2.2. Measurements 1

H NMR and 13C NMR spectra were recorded on a JEOL JNM-EPC 300 spectrometer. 1H NMR spectra (1H NMR: 60 MHz) were recorded on an Oxford Instruments Pulsar spectrometer. FT-IR spectra were measured on a JASCO FT/IR-4200 spectrometer. Gel permeation chromatography (GPC) measurement of molecular LEGO block polymers 3 (runs 1e4) were performed with a Tosoh Co. HLC-8020 GPC system (Tosoh Co. TSK-GEL G3000XL, THF as eluent, and ultraviolet detector) using polystyrene as the standard. GPC measurements of molecular LEGO block polymers 4 (runs 5e8) were performed with a GPC system (Shimadzu LC-10AD, Hitachi L7400, and Tosoh Co. TSK gel a-3000) by using DMF containing LiBr (5.8 mM) as the eluent at room temperature after calibration with polystyrene as standard samples. The high resolution fast atom bombardment mass spectrum (FAB-MS) was recorded by using a JEOL JMS-700 spectrometer in which a mixture of a sample and mnitrobenzyl alcohol on a standard FAB target was subjected to a beam of xenon atoms. 2.3. Synthesis of molecular LEGO block 1 Methanesulfonyl chloride (2.08 g, 15.1 mmol) was added dropwise to a stirred solution of 1,10-decanediol (1.05 g, 6.05 mmol), pyridine (1.19 g, 15.1 mmol) and DMAP (0.74 g, 6.05 mmol) in anhydrous THF (4.6 mL). After stirring at room temperature for 3 h, the reaction mixture was diluted with dichloromethane and then was washed with water. The organic phase was dried over MgSO4, and evaporated under reduced pressure to leave a solvent. The residue was purified by chromatography on silica gel with chloroform as the eluent to isolate dimesylate. Yield was 73%. IR (KBr): 3034, 3020, 2982, 2962, 2943, 2921, 2853, 1475, 1346,

99

1329, 1163, 1086, 991, 976, 941, 847, 753, 742, 720, 541, 528, 516, 468 cm1; 1H NMR (CDCl3, 300 MHz) d: 4.19 (t, J ¼ 6.60 Hz, 4H,eOeCH2eCH2e), 2.98 (s, 6H,eOeSO2eCH3), 1.72 (dt, J ¼ 6.57 Hz, 4H,eOeCH2eCH2e), 1.21e1.50 (m, 12H, eCH2e) ppm; 13C NMR (CDCl3, 75 Hz) d: 70.07, 37.13, 29.03, 28.89, 28.72, 25.16 ppm. To a 100 mL round bottomed flask equipped with a reflux condenser containing an anhydrous THF (36 mL) suspension of NaH (60 wt % in oil) (0.29 g, 7.24 mmol) and n-Bu4NBr (0.22 g, 0.65 mmol), furfulyl alcohol (0.35 g, 3.57 mmol) was added under nitrogen. After stirring at room temperature for 1 h, dimesylate (0.49 g, 1.49 mmol) was added and the mixture was refluxed for 15 h. The resulting mixture was extracted with diethyl ether after the addition of water. The organic phase was dried over MgSO4 and evaporated under reduced pressure to leave a solvent. The residue was purified by chromatography on silica gel with chloroform as the eluent to isolate molecular LEGO block 1. Yield was 75%. IR (KBr): 3119, 2929, 2903, 2871, 2851, 1468, 1359, 1225, 1157, 1145, 1098, 1079, 1023, 980, 914, 881, 833, 766, 757, 740, 726, 603 cm1; 1H NMR (CDCl3, 300 MHz) d: 7.40 (dd, J ¼ 1.65 and 0.81 Hz, 2H, eOeCH]CHe), 6.33 (dd, J ¼ 3.02 Hz, 2H, eOeCH]CHe), 6.30 (d, J ¼ 3.02 and 1.92 Hz, 2H, eOeC(R)¼CHe), 4.43 (s, 4H, C4H4OeCH2eOe), 3.45 (t, J ¼ 6.70 Hz, 4H, eOeCH2eCH2e), 1.57 (dt, J ¼ 6.70 Hz, 4H, eOeCH2eCH2e), 1.16e1.40 (m, 12H, eCH2e) ppm; 13 C NMR (CDCl3, 75 Hz) d: 152.06, 142.54, 110.11, 108.86, 70.34, 64.64, 29.53, 29.40, 29.32, 25.98 ppm; High-resolution FAB-MS [MþH]þ: found, 335.2224; calcd for C20H31O4, 335.2222. 2.4. Synthesis of molecular LEGO block 2 Methanesulfonyl chloride (8.97 g, 78.3 mmol) was added dropwise to a stirred solution of triethylene glycol (3.92 g, 26.1 mmol), pyridine (6.19 g, 78.3 mmol) and DMAP (3.19 g, 26.1 mmol) in anhydrous dichloromethane (20 mL). After stirring at room temperature for 1 h, the reaction mixture was diluted with dichloromethane and then was washed with water. The organic phase was dried over MgSO4 and evaporated under reduced pressure to leave a solvent. The residue was purified by chromatography on silica gel with hexane/ethyl acetate (from 4/6 to 3/7, v/v) as the eluent to isolate dimesylate. Yield was 82%. IR (Neat): 3027, 2939, 2903, 2878, 2754, 1728, 1632, 1455, 1414, 1347, 1249, 1171, 1130, 1015, 974, 917, 803, 732 cm1; 1H NMR (CDCl3, 300 MHz) d: 4.37 (t, J ¼ 4.41 Hz, 4H, eOeCH2eCH2eOeSO2e), 3.77 (t, J ¼ 4.41 Hz, 4H, eOeCH2eCH2eOeSO2e), 3.68 (s, 4H, eCH2eOeCH2CH2eOeSO2e), 3.08 (s, 6H, eOeSO2eCH3) ppm; 13C NMR (CDCl3, 75 Hz) d: 70.02, 68.88, 68.50, 37.13 ppm. To a 200 mL round bottomed flask equipped with a reflux condenser containing an anhydrous THF (104 mL) suspension of NaH (60 wt % in oil) (1.38 g, 34.5 mmol) and n-Bu4NBr (1.09 g, 3.28 mmol), furfulyl alcohol (3.22 g, 32.8 mmol) was added under nitrogen. After stirring at room temperature for 1 h, dimesylate (4.21 g, 13.7 mmol) was added and the mixture was refluxed for 24 h. The resulting mixture was extracted with diethyl ether following the addition of water. The organic phase was dried over MgSO4, and evaporated under reduced pressure to leave a solvent. The residual oil was purified by chromatography on silica gel with n-hexane/ethyl acetate (9/1, v/v) as the eluent to isolate molecular LEGO. Yield was 63%. IR (Neat): 3143, 3118, 2867, 1504, 1461, 1349, 1290, 1247, 1223, 1150, 1094, 1015, 983, 918, 885, 844, 815, 751, 600 cm1; 1H NMR (CDCl3, 300 MHz) d: 7.39 (dd, J ¼ 2.60 and 1.65 Hz, 2H, eOeCH¼CHe), 6.30-6.35 (m, 4H, eOeC(R)¼CHe, eOeCH]CHe), 4.50 (s, 4H, C4H4eOeCH2eOe), 3.58e3.69 (m, 12H, C4H4eOeCH2e OeCH2eCH2eOeCH2e) ppm;13C NMR (CDCl3, 75 Hz) d: 151.63, 142.61,110.12,109.23, 70.46, 70.41, 69.12, 64.90 ppm; High-resolution FAB-MS [MþH]þ: found, 311.1492; calcd for C16H23O6, 311.1492.

100

S. Motoki et al. / Polymer 101 (2016) 98e106

173.73, 140.97, 138.13, 136.69, 134.10, 129.61, 126.48, 81.35, 79.86, 72.04, 64.58, 49.83, 48.25, 41.01, 29.30, 25.87 ppm.

2.5. Synthesis of molecular LEGO block 3 Terephthaloyl chloride (1.24 g, 6.13 mmol) was added dropwise to a stirred solution of furfuryl alcohol (1.20 g, 12.3 mmol) and pyridine (1.21 g, 15.3 mmol) in anhydrous THF (61 mL). After stirring at room temperature for 24 h, the reaction mixture was diluted with diethyl ether and then was washed with water. The organic phase was dried over MgSO4 and evaporated under reduced pressure to leave a solvent. The residue was purified by recrystallization from methanol/H2O. Yield was 66%. IR (KBr): 3416, 3122, 2957, 1947, 1723, 1675, 1632, 1613, 1566, 1503, 1439, 1407, 1390, 1372, 1318, 1277, 1249, 1212, 1155, 1123, 1105, 1081, 1019, 974, 937, 927, 888, 869, 814, 742, 726, 688, 679, 659, 644, 600, 572, 557, 547, 529, 504 cm1; 1H NMR (CDCl3, 300 MHz) d: 8.09 (s, 4H, AreH), 7.45 (t, J ¼ 1.65 Hz, 2H, eOeCH]CHe), 6.50 (d, J ¼ 3.30 Hz, 2H, eCHeCH]CHe), 6.39 (dd, J ¼ 3.30 and 1.65 Hz, 2H, eCH]CHeCH), 5.32 (s, 4H, eCeCH2eOe) ppm; 13C NMR (CDCl3, 75 MHz) d: 165.42, 149.13, 143.48, 133.78, 129.62, 111.14, 110.66, 58.87 ppm; High-resolution FAB-MS [MþH]þ: found, 326.0795; calcd for C18H14O6, 326.0790. 2.6. Polymerization of molecular LEGO block 1 and BMI (typical procedure) Molecular LEGO block 1 (99.9 mg, 0.30 mmol) and BMI (108 mg, 0.30 mmol) were dissolved in 1,2-dichloroethane (2.0 mL) under nitrogen. After stirring at 60  C for 48 h, the mixture was poured into n-hexane/benzene (5/5, v/v) and the isolated molecular LEGO block polymer 4 was dried in vacuo. Yield was 75%. Mn ¼ 7800 (Mw/ Mn ¼ 2.68). IR (KBr): 3469, 2927, 2855, 1776, 1710, 1512, 1382, 1283, 1187, 1150, 1115, 1071, 984, 938, 882, 849, 803, 715 cm1; 1H NMR (CDCl3, 300 MHz) d: 7.14e7.54 (m, 8H, eC6H4e), 6.51e6.76 (m, 4H, eCH] CHe), 5.33e5.47 (m, 2H, eOeCHeCH]CHe), 4.13e4.25 (m, 2H  0.5, eNeCOeCHe endo), 3.83e3.94 (m, 2H  0.5, eNe COeCHeCHe endo), 3.97e4.11 (m, 2H, eC6H4eCH2eC6H4e), 3.40e3.72 (m, 4H, eCH2eOeCH2eCH2e), 3.07e3.16 (m, 2H  0.5, eNeCOeCHe exo), 2.96e3.05 (m, 2H  0.5, eNeCOeCHeCHe exo), 1.52e1.78 (m, 4H, eOeCH2eCH2e(CH2)6e), 1.19e1.51 (m, 12H, eOeCH2eCH2e(CH2)6e) ppm; 13C NMR (CDCl3, 75 Hz) d: 175.15,

2.7. Polymerization of molecular LEGO block 2 and BMI (typical procedure) Molecular LEGO block 2 (91.0 mg, 0.29 mmol) and BMI (105 mg, 0.29 mmol) were dissolved in 1,2-dichloroethane (1.9 mL) under nitrogen. After stirring at 60  C for 48 h, the mixture was poured into n-hexane/benzene (5/5, v/v) and the isolated molecular LEGO block polymer 5 was dried in vacuo. Yield was 85%. Mn ¼ 38000 (Mw/Mn ¼ 3.50). IR (KBr): 3467, 2872, 1775, 1708, 1512, 1385, 1285, 1191, 1109, 1074, 1023, 982, 939, 884, 850, 835, 803, 717, 691, 647, 598 cm1; 1H NMR (CDCl3, 300 MHz) d: 7.10e7.32 (m, 8H, eC6H4e), 6.47e6.65 (m, 4H, eCH]CHe), 5.22e5.41 (m, 2H, eOeCHeCH]CHe), 4.19e4.33 (m, 4H, ReCH2eOeCH2e), 3.86e4.06 (m, 2H, eC6H4eCH2eC6H4e), 3.54e3.83 (m, 12H, eCH2eOeCH2eCH2eOeCH2e), 3.0e3.13 (m, 2H, eNeCOeCHe exo), 2.97e3.04 (m, 2H, eNeCOeCHeCHe exo) ppm; 13C NMR (CDCl3, 75 Hz) d: 175.15, 173.79, 141.05, 138.24, 136.72, 134.19, 129.67, 126.56, 81.44, 79.92, 71.13, 70.52, 70.34, 68.47, 49.90, 48.28, 41.07 ppm. 2.8. Depolymerization of molecular LEGO block polymer 4 (typical procedure) Molecular LEGO block polymer 4 (0.147 g) was dissolved in 1,1,2,2-tetrachloroethane (1.0 mL). After stirring at 150  C for 1 h, the mixture was poured into n-hexane, and precipitated oligomer and BMI were separated by using a TOMY LC-131 (3000 rpm, 10 min). The molecular LEGO block 1 was obtained as an insoluble n-hexane part. BMI was purified by column chromatography on silica gel with hexane/ethyl acetate (from 9/1 to 8/2, v/v). Yield was 77%. 2.9. Scrambling reaction between molecular LEGO block polymer 4 and molecular LEGO block 3 (typical procedure) Molecular LEGO block polymer 4 (0.101 g) and molecular LEGO block 3 (46.9 mg, 0.45 mmol) were dissolved in 1,1,2,2-

Scheme 1. Synthesis of molecular LEGO block 1 and 2.

S. Motoki et al. / Polymer 101 (2016) 98e106

101

Scheme 2. Polymerization of 1 and BMI.

Table 1 Polymerization of molecular LEGO block 1 and BMI.a Run

Molecular LEGO block 1 (mg)

BMI (mg)

Solvent (mL)

Time (h)

Yield of 3b (%)

M nc

Mw/Mnc

1 2 3 4

50.0 49.8 49.4 99.9

53.6 53.2 52.7 108.0

1.0 1.0 1.0 2.0

6 12 24 48

17 32 72 75

2900 2500 3500 7800

1.97 1.66 1.97 2.68

a b c

Conditions: solvent: 1,2-dichloroethane; reaction temperature: 60  C. n-Hexane/benzene insoluble part (5/5, v/v). Estimated by GPC, based on polystyrene standards; eluent, THF.

tetrachloroethane (1.0 mL). After stirring at 110  C for 1 h, the reaction mixture was stirred at 60  C for 48 h, the reaction mixture was poured into n-hexane/benzene (from 5/5, v/v), and the polymer was precipitated. The obtained polymer was dried in vacuo. Yield was 93%. Mn ¼ 2200 (Mw/Mn ¼ 1.94). 1 H NMR (CDCl3, 300 MHz) d: 7.12e7.32 (m, 8H, eC6H4e), 6.47e6.68 (m, 4H, eCH]CHe), 5.19e5.48 (m, 2H, eOeCHeCH] CHe, ReCH2eOe), 4.13e4.24 (m, 2H  0.5, eNeCOeCHe endo), 3.83e3.93 (m, 2H  0.5, eNeCOeCHeCHe endo), 3.95e4.08 (m, 2H, eC6H4eCH2eC6H4e), 3.39e3.66 (m, 4H, eCH2eOeCH2eCH2e), 3.15e3.21 (m, 2H  0.5, eNeCOeCHe exo), 2.97e3.03 (m, 2H  0.5, eNeCOeCHeCHe exo), 1.49e1.83 (m, 4H, eOeCH2 eCH2e(CH2)6e), 1.18e1.39 (m, 12H, eOeCH2eCH2e(CH2)6e) ppm; 13 C NMR (CDCl3, 75 MHz) d: 175.30, 173.88, 165.16, 141.24, 138.25, 137.92, 136.83, 134.22, 129.72, 128.35, 126.60, 81.45, 79.97, 72.16, 67.96, 62.25, 49.97, 48.36, 41.11, 29.48, 29.44, 29.41, 25.97 ppm.

were well characterized by IR, 1H NMR, and 13C NMR spectroscopy. Their high-resolution FAB-MS spectra conformed to their structures.

3.2. Polymerization of molecular LEGO block 1 and BMI The polymerization of molecular LEGO block 1 and BMI, which is another molecular LEGO block, was carried out in 1,2dichloroethane at 60  C (Scheme 2). The results of the polymerization are summarized in Table 1. Molecular LEGO block polymer 4

2.10. Contact angle measurements Distilled water as solvent was prepared with an EYELA automatic water distillation apparatus SA-2100A. Cast films were prepared by a casting technique in which polymer solution (10 mg/mL dichloromethane solution) was cast onto a glass plate. The cast film was dried at 60  C for 12 h. After the solvent completely evaporated, the contact angle of a droplet was measured at 20  C. Contact angles reported are the average of at least three measurements. 3. Results and discussion 3.1. Synthesis of molecular LEGO blocks 1, 2 and 3 Molecular LEGO block 1 was prepared by a two-step reaction. First, decanediol reacted with methanesulfonyl chloride to yield the corresponding dimesylate (Scheme 1, Eq. (1)). Then, molecular LEGO block 1 was prepared from dimesylate and furfulyl alcohol by means of Williamson ether synthesis (Scheme 1, Eq. (2)). Molecular LEGO block 2 was also prepared in the same manner described in Scheme 1, and Eqs. (3) and (4). Molecular LEGO blocks 1 and 2

Fig. 1. 1H NMR spectrum of molecular LEGO block polymer 4.

102

S. Motoki et al. / Polymer 101 (2016) 98e106

Fig. 2.

13

C NMR spectrum of molecular LEGO block polymer 4.

was successfully isolated by reprecipitation from a 1,2dichloroethane solution into n-hexane/benzene (5/5, v/v). When the polymerization of 1 was carried out at 60  C for 6 h, 4 was obtained in 17% yield (run 1). To increase polymer yield, polymerization was carried out for a longer period (i.e. 48 h) (run 4). As a result, 4 was obtained in 75% yield (Run 4). Moreover, the number-

average molecular weight (Mn) of the obtained polymers increased with an increase in reaction time. The 1H and 13C NMR spectra of 4 are shown in Figs. 1 and 2, respectively. In the 1H NMR spectrum, the signals resulting from the aromatic protons were observed at d 7.14e7.54 (a and b) ppm. The peaks resulting from Diels-Alder cycloaddition were observed at d 6.51e6.76 (c and d), and 5.33e5.47 (e) ppm, respectively. The endo/exo ratio of the resulting polymers was determined by consulting the reported literature [23e26]. The peaks attributable to the protons due to endo-cycloadducts were observed at d 4.13e4.25 and 3.83e3.94 (fendo and hendo) ppm, respectively. Additionally, the peaks attributable to the protons due to exocycloadducts were observed at d 3.07e3.16 and 2.96e3.05 (fexo and hexo) ppm, respectively. The endo/exo ratio was determined from the integral ratio between the methyne proton (fendo or hendo) and the methyne proton (fexo or hexo). The endo/exo ratio was estimated to be 5:5 on the basis of the integral ratio of fendo/fexo or hendo/hexo signals in Fig. 1. The peaks attributable to the methylene adjacent to the phenyl group were observed at d 3.97e4.11 (g) ppm. The peaks due to methylene protons were observed at d 3.40e3.72, 1.52e1.78, and 1.19e1.51 (i, j, and k) ppm. This result supports the fact that the polymerization of 1 and BMI involved a Diels-Alder reaction process. The 13C NMR spectrum of 4 also supported the polymerization of 1 and BMI (Fig. 2). The peaks due to carbonyl carbons were observed at d 175.15 and 173.73 (a and b) ppm, respectively. The peaks due to aromatic carbons were observed at d 140.97, 134.10, 129.61, and 126.48 (c, f, g, and h) ppm, respectively. The peaks resulting from Diels-Alder cycloaddition were observed at d 138.13, 136.69, 81.35, and 79.86 (d, e, i, and j) ppm, respectively. Especially, the peaks attributable to the carbons including the cycloaddition ring were observed at d 49.83 and 48.25 (m and n) ppm, respectively. The peaks attributable to the methylenes adjacent to the oxygen atom were observed at d 72.04 and 64.58 (k and l) ppm, respectively. The peak due to methylene carbons was observed at d 29.30 (p, q, and r) ppm. The peak attributable to the methylenes adjacent to the phenyl group was observed at d 41.01 (o) ppm. In Fig. 3, the FT-IR spectrum of 4 showed an absorption band at 1710 cm1 based on the ester carbonyl group (nC]O). Moreover, the absorption bands of the ether observed at 1115 cm1 and 1071 cm1 were assigned to the CeOeC stretching vibration, respectively. 3.3. Polymerization of molecular LEGO block 2 and BMI

Fig. 3. FT-IR spectrum of molecular LEGO block polymer 4.

The polymerization of molecular LEGO block 2 and BMI was carried out in 1,2-dichloroethane at 60  C (Scheme 3). The results of the polymerization are summarized in Table 2. Molecular LEGO block polymer 5 was successfully isolated by reprecipitation from a 1,2-dichloroethane solution into n-hexane/benzene

Scheme 3. Polymerization of 2 and BMI.

S. Motoki et al. / Polymer 101 (2016) 98e106

103

Table 2 Polymerization of molecular LEGO block 2 and BMI.a Run

Molecular LEGO block 2 (mg)

BMI (mg)

Solvent (mL)

Time (h)

Yield of 4b (%)

Mnc

Mw/Mnc

1 2 3 4

61.6 26.6 46.7 107.0

71.6 28.5 54.4 124.0

1.0 1.0 1.0 2.0

6 12 24 48

54 57 79 85

18000 20000 36000 38000

5.81 5.23 4.03 3.50

a b c

Conditions: solvent: 1,2-dichloroethane; reaction temperature: 60  C. n-Hexane/benzene insoluble part (5/5, v/v). Estimated by GPC, based on polystyrene standards; eluent, DMF containing LiBr (5.8 mM).

Fig. 4. 1H NMR spectrum of molecular LEGO block polymer 5.

Fig. 5.

13

C NMR spectrum of molecular LEGO block polymer 5.

(5/5, v/v). When the polymerization of 2 was carried out at 60  C for 6 h, 5 was obtained in 54% yield (run 1). To increase polymer yield, polymerization was carried out for a longer period (i.e. 48 h) (run 4). As a result, 5 was obtained in 85% yield (run 4). Moreover, Mn of the obtained polymers increased with an increase in reaction time. The 1H and 13C NMR spectra of 5 are shown in Figs. 4 and 5, respectively. These NMR spectra support the fact that the polymerization of 2 and BMI involved a Diels-Alder reaction process. In Fig. 6, the FT-IR spectrum of 5 showed an absorption band at 1708 cm1 based on the ester carbonyl group (nC]O). Moreover, the absorption bands of ether observed at 1109 cm1 and 1074 cm1 were assigned to CeOeC stretching vibration. 3.4. Depolymerization of molecular LEGO block polymer We attempted the depolymerization of molecular LEGO block polymer 4 using retro-Diels-Alder reaction conditions (Scheme 4). The results of the depolymerization of 4 are summarized in Table 3. Molecular LEGO block polymer 4 dissolved in 1,1,2,2tetrachloroethane as solvent. After predetermined reaction times, the mixture was poured into n-hexane, and precipitated oligomer and BMI were separated by centrifugation at 3000 rpm for 10 min. BMI was purified by column chromatography. Molecular LEGO block 1 was obtained as a n-hexane-soluble part (Figs. S7 and S8). On the other hand, BMI was obtained as a n-hexane-insoluble part

Fig. 6. FT-IR spectrum of molecular LEGO block polymer 5.

(Figs. S9 and S10). These compounds were determined by TLC analyses, FT-IR spectra and 1H NMR spectra. In the case of run 3, the corresponding molecular LEGO block 1 and BMI were obtained in good yields. The GPC analysis of the depolymerization of 4 was carried out in

104

S. Motoki et al. / Polymer 101 (2016) 98e106

Scheme 4. Depolymerization of molecular LEGO block polymer 4.

Table 3 Depolymerization of molecular LEGO block polymer 4.a Run

3 (mg)

Solvent (mL)

Temp. ( C)

Time (h)

Yield of 1 (%)

Yield of BMI (%)

1 2 3

198 148 148

1.5 1.0 1.0

110 130 150

1 1 1

43 64 77

48 54 73

a

Conditions: solvent: 1,1,2,2-tetrachloroethane.

Fig. 8. GPC profiles of scrambled molecular LEGO block polymer 6 and authentic samples.

Fig. 7. GPC profiles of before and after depolymerization of 4.

3.5. Scrambling reaction between molecular LEGO block polymer 4 and molecular LEGO block 3 THF using polystyrene as the standard. The GPC profiles before and after depolymerization confirmed the depolymerization of 4 (Fig. 7). In the case of depolymerization at 150  C, the depolymerization reaction of 4 was almost completed, and polymer and oligomer were not detected by GPC measurement. In conclusion, the depolymerization of 4 was accomplished via retro-Diels-Alder reaction conditions.

The scrambling reaction between molecular LEGO block polymer 4 and molecular LEGO block 3 was carried out in 1,1,2,2tetrachloroethane (Scheme 5). Molecular LEGO block polymer 4 and molecular LEGO block 3 were dissolved in 1,1,2,2tetrachloroethane. After stirring at 110  C for 1 h, the reaction mixture was stirred at 60  C for 48 h, the corresponding scrambled

Scheme 5. Scrambling reaction between molecular LEGO block polymer 4 and 3.

S. Motoki et al. / Polymer 101 (2016) 98e106

Fig. 9. 1H NMR spectrum of scrambled molecular LEGO block polymer 6.

105

reaction, Mn and Mw/Mn of molecular LEGO block polymer 4 were estimated at 7800 and 2.68, respectively. On the other hand, molecular LEGO block polymer as an authentic sample was prepared from 3 and BMI in the same manner described in Sections 2.6 and 2.7, with 73% yield. Mn and Mw/Mn of an authentic sample were measured and shown to be 1500 and 1.31, respectively. From these results, it was clarified that the scrambled polymer 6 has an intermediate molecular weight between 4 and the authentic sample. These results also indicated that the scrambling reaction between 4 and 3 proceeded. In the 1H NMR spectrum of 6, the signals resulting from the aromatic protons (a) were observed at d 7.99e8.15 ppm (Fig. 9). The unit ratio was determined from the integral ratio between the methylene protons (l) derived from molecular LEGO block 1, aromatic protons (a) derived from molecular LEGO block 3 and methylene protons (i) derived from BMI. As a result, the unit ratio was determined to be 1/3/BMI ¼ 25/32/43 from the integral ratio between l, a and i. The 13C NMR spectrum of 6 also supported the scrambling reaction (Fig. 10). The peaks due to aromatic carbons (h and j) were observed at d 126.60 and 129.72 ppm, respectively. These results support the scrambling reaction between molecular LEGO block polymer 4 and 3. The surface characteristics of cast films containing molecular LEGO block polymer 4 or 5 were observed by the contact angle of a water droplet. Molecular LEGO block polymer films containing 4 or 5 were cast from a dichloromethane solution onto clean glass slides. The contact angles of 4 and 5 were 79.9 ± 2.07 and 66.9 ± 0.83 , respectively. This indicates that the surface properties of the polymers are changed by replacing the molecular LEGO blocks (Fig. 11). 4. Conclusions

Fig. 10.

13

C NMR spectrum of scrambled molecular LEGO block polymer 6.

molecular LEGO block polymer was purified by reprecipitation from its 1,1,2,2-tetrachloroethane solution into n-hexane/benzene and dried in vacuo. The scrambled polymer 6 was obtained in good yield (93%). From the GPC analysis, Mn and Mw/Mn of 6 were estimated at 2200 and 1.94, respectively (Fig. 8). Before the scrambling

Molecular LEGO blocks having two reversible covalent bond moieties such as furan were synthesized. These molecular LEGO blocks were polymerized with a molecular LEGO block having two maleimide moieties under Diels-Alder conditions. The resulting molecular LEGO block polymers, which were obtained in good yield, were depolymerized under retro-Diels-Alder conditions. As a result, the corresponding molecular LEGO block 1 and BMI were obtained in good yield. Additionally, these polymers proceeded in the scrambling reaction between molecular LEGO block polymers and other molecular LEGO blocks. The contact angle of cast films containing molecular LEGO block polymer 4 or 5 were observed to be 79.9 ± 2.07 and 66.9 ± 0.83 , respectively. This indicates that the surface properties of the polymers change by replacing the molecular LEGO blocks. In the future, chemical recycling of polymer materials may become possible by this molecular LEGO block

Fig. 11. The contact angle of a water droplet on the surface of 4 and 5.

106

S. Motoki et al. / Polymer 101 (2016) 98e106

recycling system. Acknowledgment

[11] [12]

This work was supported by a Grant-in-Aid for Scientific Research (C) (Grant Number JP 16K00659) from Japan Society for the Promotion of Science.

[13] [14] [15]

Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2016.08.024.

[16]

References

[17]

[1] M.T. Ravanchi, S. Sahebdelfar, F.T. Zangeneh, Carbon dioxide sequestration in petrochemical industries with the aim of reduction in greenhouse gas emissions, Front. Chem. Sci. Eng. 5 (2011) 173e178. [2] Weigel, M. Journalist's Resource. http://journalistsresource.org/studies/ environment/energy/energy-non-recycled-plastics-landfill/(accessed 07.06.16). [3] T. Iwamura, M. Sakaguchi, A novel decrosslinking system from crosslinked polymer to linear polymer utilizing pressure or visible light irradiation, Macromolecules 41 (2008) 8995e8999. [4] T. Iwamura, S. Nakamura, Synthesis and properties of de-cross-linkable acrylate polymers based on hexaarylbiimidazole, Polymer 54 (2013) 4161e4170. [5] J. Blazevska-Gilev, D. Spaseska, Chemical recycling of poly(vinyl chloride): alkaline dechlorination in organic solvents and plasticizer leaching in caustic solution, J. Univ. Chem. Technol. Met. 42 (2007) 29e34. [6] H. Kanazawa, M. Higuchi, K. Yamamoto, Synthesis and chemical degradation of thermostable polyamide with imine bond for chemical recycling, Macromolecules 39 (2006) 138e144. [7] D. Paszum, T. Spychaj, Chemical recycling of poly(ethylene terephthalate), Ind. Eng. Chem. Res. 36 (1997) 1373e1383. [8] M. Kacperski, T. Spychaj, Chemical recycling of waste saturated polyesters and urethane polymers to yield raw materials for the production of polyurethanes, Prog. Rubber Plast. Technol. 16 (2000) 61e68. [9] K. Yoshida, F. Sanda, T. Endo, Synthesis and cationic ring-opening polymerization of mono- and bifunctional spiro orthoesters containing ester groups and depolymerization of the obtained polymers: an approach to chemical recycling for polyesters as a model system, J. Polym. Sci. Part A Polym. Chem. 25 (1999) 2551e2558. [10] X. Chen, M.A. Dam, K. Ono, A. Mal, H. Shen, S.R. Nutt, K. Sheran, F. Wudl,

[18]

[19] [20] [21]

[22]

[23]

[24]

[25]

[26]

A thermally re-mendable cross-linked polymeric material, Science 295 (2002) 1698e1702. A. Gandini, The furan/maleimide Diels-Alder reaction: a versatile click-unclick tool in macromolecular synthesis, Prog. Polym. Sci. 38 (2013) 1e29. N. Yoshie, Diels-alder Polymers, in: Encyclopedia of Polymer Science and Technology, fourth ed., vol. 4, 2014, pp. 493e510. S.D. Bergman, F. Wudl, Mendable polymers, J. Mater. Chem. 18 (2008) 41e62. K.A. Williams, D.R. Dreyer, C.W. Bielawski, The underlying chemistry of selfhealing materials, MRS Bull. 33 (2008) 759e765. J. Zhou, N.K. Guimard, A.J. Inglis, M. Namazian, C.Y. Lin, M.L. Coote, E. Spyrou, S. Hilf, F.G. Scmidt, C. Barner-Kowollik, Thermally reversible Diels-Alder-based polymerization: an experimental and theoretical assessment, Polym. Chem. 3 (2012) 628e639. A. Gandini, A. Silvestre, D. Coelho, Reversible click chemistry at the service of macromolecular materials. Part 4: diles-Alder non-linear polycondensations involving polyfunctional furan and maleimide monomers, Polym. Chem. 4 (2013) 1364e1371. H. Satoh, A. Mineshima, T. Nakamura, N. Teramoto, M. Shibata, Thermoreversible Diels-Alder polymerization of difurfurylidene diglycerol and bismaleimide, React. Funct. Polym. 76 (2014) 49e56. T.M. Lecerda, A.J.F. Carvalho, A. Gandini, A minimalist furan-maleimide ABtype monomer and its thermally reversible Diels-Alder polymerization, RSC Adv. 6 (2016) 45696e45700. A. Gandini, A.J.D. Silvestre, D. Coelho, Reversible click chemistry at the service of macromolecular materials, Polym. Chem. 2 (2011) 1713e1719. Y.-L. Liu, T.-W. Chuo, Self-healing polymers based on thermally reversible Diels-Alder chemistry, Polym. Chem. 4 (2013) 2194e2205. N. Kuramoto, K. Hayashi, K. Nagai, Thermoreversible reaction of Diels-Alder polymer composed of difurfuryladipate with bismaleimidodiphenylmethane, J. Polym. Sci. Part A Polym. Chem. 32 (1994) 2501e2504. M. Watanabe, N. Yoshie, Synthesis and properties of readily recyclable polymers from bisfuranic terminated poly(ethylene adipate) and multi-maleimide linkers, Polymer 47 (2006) 4946e4952. C. Gousse, A. Gandini, H. Hodge, Application of the DielsAlder reaction to polymers bearing furan moieties. 2. DielsAlder and retro-DielsAlder reactions involving furan rings in some styrene copolymers, Macromolecules 31 (1998) 314e321. C. Jegat, N. Mignard, Effect of the polymer matrix on the thermal behavior of a furan-maleimide type adduct in the molten state, Polym. Bull. 60 (2008) 799e808. E. Dolci, G. Michaud, F. Simon, B. Boutevin, S. Fouquay, S. Caillol, Remendable thermosetting polymers for isocyanate-free adhesives: a preliminary study, Polym. Chem. 6 (2015) 7851e7861. V. Froidevaux, M. Borne, E. Laborbe, R. Auvergne, A. Gandini, B. Boutevin, Study of the Diels-Alder and retro-Diels-Alder reaction between furan derivatives and maleimide for the creation of new materials, RSC Adv. 5 (2015) 37742e37754.