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Polymer 101 (2016) 98e106

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Polymer

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Synthesis of recyclable molecular LEGO block polymers utilizing theDiels-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 158-8857, Japanb Department of Chemistry and Energy Engineering, Faculty of Engineering, Tokyo City University, 1-28-1 Tamazutsumi, Setagaya-ku, Tokyo 158-8557,Japanc Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259-G1-9 Nagatsuta,Midori-ku, Yokohama, Japan

a r t i c l e i n f o

Article history:Received 7 June 2016Received in revised form3 August 2016Accepted 7 August 2016Available online 9 August 2016

Keywords:Recyclable polymerMolecular LEGO blockDepolymerization

* Corresponding author. Department of ChemistGraduate School of Engineering, Tokyo City UnivSetagaya-ku, Tokyo 158-8857, Japan.

E-mail address: iwamurat@tcu.ac.jp (T. Iwamura).

http://dx.doi.org/10.1016/j.polymer.2016.08.0240032-3861/© 2016 Elsevier Ltd. All rights reserved.

a b s t r a c t

Molecular LEGO blocks having two reversible covalent bond moieties were employed to synthesizecorresponding molecular LEGO block polymers. Hydrophilic and hydrophobic molecular LEGO blockshaving two furan moieties were synthesized. These molecular LEGO blocks were polymerized with amolecular LEGO block having two maleimide moieties under Diels-Alder conditions. The resulting mo-lecular LEGO block polymers were obtained in good yield. The molecular LEGO block polymers weredepolymerized under retro-Diels-Alder conditions. In addition, these polymers proceeded in a scram-bling reaction between the molecular LEGO block polymers and other molecular LEGO blocks.

© 2016 Elsevier Ltd. All rights reserved.

1. Introduction

The reduction of greenhouse gas emissions to acceptablelevels will almost certainly be the greatest environmental chal-lenge facing humans. This increase in CO2 in the atmosphere isprimarily due to huge utilization of fossil fuels and forestdestruction [1]. Polymer materials derived from fossil fuels suchas plastics are inexpensive, easy to mold, and lightweight. Thesebenefits have become the driving forces that have resulted inmany commercial applications. However, due to insufficientrecycling, polymer materials derived from fossil fuels have beenburned or have been discarded in landfills. For example, theoverall U.S. post-consumer plastic waste for 2008 was estimatedat 33.6 million tons, 2.2 million tons (6.5%) of which was recy-cled, 2.6 million tons (7.7%) were burned for energy, and 28.9million tons, or 85.5%, were discarded in landfills [2].

ry and Energy Engineering,ersity, 1-28-1 Tamazutumi,

As described above, these methods have two fundamentalproblems. Firstly, CO2 gas, which is generated by the incinerationof waste polymer materials, has diffused into the atmosphere.Secondly, landfills of waste polymer materials have not led to theeffective utilization of plastic. Therefore, in order to avoid thediffusion of CO2 gas and the useless landfills of waste plastics, themolecular design of polymers is necessary to facilitate the recy-cling of polymer materials. Recyclable polymers are expected tobe important eco-friendly materials for overcoming seriousproblems such as environmental degradation. We have beenconducting research on the synthesis and recycling properties ofde-cross-linkable polymers based on hexaarylbiimidazole (HABI)[3,4]. Among a large number of recycling methods used forpolymers, chemical recycling is the most important and sub-stantial target. Many researchers have reported on the chemicalrecycling of polymers, but in general, chemical recycling isdifficult [5e9]. However, new forms of chemical recycling shouldbe continuously developed for a wide range of polymers. Toovercome these limitations, we have proposed a possibleapproach which consists of molecular LEGO blocks having moi-eties with reversible covalent bonds and various element groups.

S. Motoki et al. / Polymer 101 (2016) 98e106 99

These molecular LEGO blocks might be able to be detached asmonomeric units as in the toy LEGO. To realize such a strategy, itis suitable to use the Diels-Alder reaction. The Diels-Alder reac-tion is one of the most famous reactions in organic chemistry. Inthe Diels-Alder reaction an alkene adds 1,4 to a conjugated diene,and the resulting product becomes a six-membered ring. SinceWudl et al. first reported an example of using the Diels-Alderreaction for cross-linked material [10], various polymers basedon the Diels-Alder reaction have been reported in recent years inthe 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 ofDiels-Alder polymers based on the retro-Diels-Alder reaction[17,21,22]. As far as we know, a method that changes the prop-erties of original polymers by a scrambling reaction has not beenreported. Therefore, we attempted to synthesize recyclablepolymers utilizing molecular LEGO blocks. Moreover, we exam-ined upgrading these polymers by scrambling molecular LEGOblocks with characteristics that can be recomposed.

2. Experimental procedure

2.1. Materials

N,N-Dimethylformamide (DMF) was dried over CaH2, distilledunder nitrogen, and stored under nitrogen. Dichloromethane and1,2-dichroloethane were dried over P2O5, distilled, and stored un-der nitrogen. N,N-Dimethyl-4-aminopyridine (DMAP) was recrys-tallized from n-hexane. 4,40-Bismaleimidodiphenylmethane (BMI)as a molecular LEGO block was purchased from Tokyo ChemicalIndustry Co., Ltd. Other solvents and reagents were used assupplied.

2.2. Measurements

1H NMR and 13C NMR spectra were recorded on a JEOL JNM-EPC300 spectrometer. 1H NMR spectra (1H NMR: 60 MHz) wererecorded on an Oxford Instruments Pulsar spectrometer. FT-IRspectra were measured on a JASCO FT/IR-4200 spectrometer. Gelpermeation chromatography (GPC) measurement of molecularLEGO block polymers 3 (runs 1e4) were performed with a TosohCo. HLC-8020 GPC system (Tosoh Co. TSK-GEL G3000XL, THF aseluent, and ultraviolet detector) using polystyrene as the standard.GPCmeasurements of molecular LEGO block polymers 4 (runs 5e8)were performed with a GPC system (Shimadzu LC-10AD, Hitachi L-7400, and Tosoh Co. TSK gel a-3000) by using DMF containing LiBr(5.8 mM) as the eluent at room temperature after calibration withpolystyrene as standard samples. The high resolution fast atombombardment mass spectrum (FAB-MS) was recorded by using aJEOL JMS-700 spectrometer in which a mixture of a sample and m-nitrobenzyl alcohol on a standard FAB target was subjected to abeam of xenon atoms.

2.3. Synthesis of molecular LEGO block 1

Methanesulfonyl chloride (2.08 g, 15.1 mmol) was added drop-wise 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) inanhydrous THF (4.6 mL). After stirring at room temperature for 3 h,the reaction mixture was diluted with dichloromethane and thenwas washed with water. The organic phase was dried over MgSO4,and evaporated under reduced pressure to leave a solvent. Theresidue was purified by chromatography on silica gel with chloro-form as the eluent to isolate dimesylate. Yield was 73%.

IR (KBr): 3034, 3020, 2982, 2962, 2943, 2921, 2853, 1475, 1346,

1329, 1163, 1086, 991, 976, 941, 847, 753, 742, 720, 541, 528, 516,468 cm�1; 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 6.57Hz, 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 refluxcondenser containing an anhydrous THF (36mL) 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 undernitrogen. After stirring at room temperature for 1 h, dimesylate(0.49 g, 1.49 mmol) was added and the mixture was refluxed for15 h. The resulting mixture was extracted with diethyl ether afterthe addition of water. The organic phase was dried over MgSO4 andevaporated under reduced pressure to leave a solvent. The residuewas purified by chromatography on silica gel with chloroform asthe 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 cm�1; 1H NMR (CDCl3, 300 MHz) d: 7.40 (dd, J ¼ 1.65 and 0.81Hz, 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, J¼ 6.70 Hz, 4H,eOeCH2eCH2e), 1.57 (dt,J ¼ 6.70 Hz, 4H, eOeCH2eCH2e), 1.16e1.40 (m, 12H, eCH2e) ppm;13C 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 addeddropwise 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,26.1mmol) in anhydrous dichloromethane (20mL). After stirring atroom temperature for 1 h, the reaction mixture was diluted withdichloromethane and then was washed with water. The organicphase was dried over MgSO4 and evaporated under reduced pres-sure to leave a solvent. The residuewas purified by chromatographyon silica gel with hexane/ethyl acetate (from 4/6 to 3/7, v/v) as theeluent 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 cm�1; 1H NMR (CDCl3,300MHz) d: 4.37 (t, J¼ 4.41 Hz, 4H,eOeCH2eCH2eOeSO2e), 3.77 (t,J¼ 4.41 Hz, 4H,eOeCH2eCH2eOeSO2e), 3.68 (s, 4H,eCH2eOeCH2-CH2eOeSO2e), 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 refluxcondenser containing an anhydrous THF (104 mL) suspension ofNaH (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 undernitrogen. After stirring at room temperature for 1 h, dimesylate(4.21 g, 13.7 mmol) was added and the mixture was refluxed for24 h. The resulting mixture was extracted with diethyl etherfollowing the addition of water. The organic phase was dried overMgSO4, and evaporated under reduced pressure to leave a solvent.The residual oil was purified by chromatography on silica gel withn-hexane/ethyl acetate (9/1, v/v) as the eluent to isolate molecularLEGO. Yield was 63%.

IR (Neat): 3143, 3118, 2867, 1504, 1461, 1349, 1290, 1247, 1223,1150, 1094, 1015, 983, 918, 885, 844, 815, 751, 600 cm�1; 1H NMR(CDCl3, 300MHz) d: 7.39 (dd, J¼2.60and1.65Hz,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, 64.90ppm;High-resolutionFAB-MS [MþH]þ: found, 311.1492; calcd for C16H23O6, 311.1492.

S. Motoki et al. / Polymer 101 (2016) 98e106100

2.5. Synthesis of molecular LEGO block 3

Terephthaloyl chloride (1.24 g, 6.13 mmol) was added dropwiseto a stirred solution of furfuryl alcohol (1.20 g, 12.3 mmol) andpyridine (1.21 g, 15.3 mmol) in anhydrous THF (61 mL). After stir-ring at room temperature for 24 h, the reactionmixturewas dilutedwith diethyl ether and then was washed with water. The organicphase was dried over MgSO4 and evaporated under reduced pres-sure to leave a solvent. The residue was purified by recrystallizationfrom 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 cm�1; 1H NMR (CDCl3, 300MHz) d: 8.09(s, 4H, AreH), 7.45 (t, J ¼ 1.65 Hz, 2H, eOeCH]CHe), 6.50 (d,J¼ 3.30 Hz, 2H,eCHeCH]CHe), 6.39 (dd, J¼ 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 (typicalprocedure)

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) undernitrogen. After stirring at 60 �C for 48 h, the mixture was pouredinto n-hexane/benzene (5/5, v/v) and the isolated molecular LEGOblock polymer 4was 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 cm�1; 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, eNeCOeCHeCHe 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, eNeCOeCHeCHeexo), 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,

Scheme 1. Synthesis of molec

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.

2.7. Polymerization of molecular LEGO block 2 and BMI (typicalprocedure)

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) undernitrogen. After stirring at 60 �C for 48 h, the mixture was pouredinto n-hexane/benzene (5/5, v/v) and the isolated molecular LEGOblock 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 cm�1; 1HNMR (CDCl3, 300MHz) 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 (typicalprocedure)

Molecular LEGO block polymer 4 (0.147 g) was dissolved in1,1,2,2-tetrachloroethane (1.0 mL). After stirring at 150 �C for 1 h,the mixture was poured into n-hexane, and precipitated oligomerand BMI were separated by using a TOMY LC-131 (3000 rpm,10 min). The molecular LEGO block 1 was obtained as an insolublen-hexane part. BMI was purified by column chromatography onsilica gel with hexane/ethyl acetate (from 9/1 to 8/2, v/v). Yield was77%.

2.9. Scrambling reaction between molecular LEGO block polymer 4and molecular LEGO block 3 (typical procedure)

Molecular LEGO block polymer 4 (0.101 g) and molecular LEGOblock 3 (46.9 mg, 0.45 mmol) were dissolved in 1,1,2,2-

ular LEGO block 1 and 2.

Scheme 2. Polymerization of 1 and BMI.

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

Run Molecular LEGO block 1 (mg) BMI (mg) Solvent (mL) Time (h) Yield of 3b (%) Mnc Mw/Mn

c

1 50.0 53.6 1.0 6 17 2900 1.972 49.8 53.2 1.0 12 32 2500 1.663 49.4 52.7 1.0 24 72 3500 1.974 99.9 108.0 2.0 48 75 7800 2.68

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

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

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

tetrachloroethane (1.0 mL). After stirring at 110 �C for 1 h, the re-action mixture was stirred at 60 �C for 48 h, the reaction mixturewas poured into n-hexane/benzene (from 5/5, v/v), and the poly-mer was precipitated. The obtained polymer was dried in vacuo.Yield was 93%. Mn ¼ 2200 (Mw/Mn ¼ 1.94).

1H 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, eOeCH2eCH2e(CH2)6e), 1.18e1.39 (m, 12H, eOeCH2eCH2e(CH2)6e) ppm;13C 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.

2.10. Contact angle measurements

Distilled water as solvent was prepared with an EYELA auto-matic water distillation apparatus SA-2100A. Cast films were pre-pared by a casting technique in which polymer solution (10 mg/mLdichloromethane solution) was cast onto a glass plate. The cast filmwas dried at 60 �C for 12 h. After the solvent completely evaporated,the contact angle of a droplet was measured at 20 �C. Contact an-gles 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 yieldthe corresponding dimesylate (Scheme 1, Eq. (1)). Then, molecularLEGO block 1was prepared from dimesylate and furfulyl alcohol bymeans of Williamson ether synthesis (Scheme 1, Eq. (2)). Molec-ular LEGO block 2was also prepared in the samemanner describedin Scheme 1, and Eqs. (3) and (4). Molecular LEGO blocks 1 and 2

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

3.2. Polymerization of molecular LEGO block 1 and BMI

The polymerization of molecular LEGO block 1 and BMI, which isanother molecular LEGO block, was carried out in 1,2-dichloroethane at 60 �C (Scheme 2). The results of the polymeri-zation are summarized in Table 1. Molecular LEGO block polymer 4

Fig. 2. 13C NMR spectrum of molecular LEGO block polymer 4.

S. Motoki et al. / Polymer 101 (2016) 98e106102

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

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

Scheme 3. Polymeriza

average molecular weight (Mn) of the obtained polymers increasedwith 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 fromthe aromatic protons were observed at d 7.14e7.54 (a and b) ppm.The peaks resulting from Diels-Alder cycloaddition were observedat d 6.51e6.76 (c and d), and 5.33e5.47 (e) ppm, respectively. Theendo/exo ratio of the resulting polymers was determined byconsulting the reported literature [23e26]. The peaks attributableto the protons due to endo-cycloadducts were observed atd 4.13e4.25 and 3.83e3.94 (fendo and hendo) ppm, respectively.Additionally, the peaks attributable to the protons due to exo-cycloadducts were observed at d 3.07e3.16 and 2.96e3.05 (fexo andhexo) ppm, respectively. The endo/exo ratio was determined fromthe integral ratio between the methyne proton (fendo or hendo) andthe methyne proton (fexo or hexo). The endo/exo ratio was estimatedto be 5:5 on the basis of the integral ratio of fendo/fexo or hendo/hexo

signals in Fig. 1. The peaks attributable to themethylene adjacent tothe phenyl group were observed at d 3.97e4.11 (g) ppm. The peaksdue 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 thepolymerization of 1 and BMI involved a Diels-Alder reactionprocess.

The 13C NMR spectrum of 4 also supported the polymerizationof 1 and BMI (Fig. 2). The peaks due to carbonyl carbons wereobserved at d 175.15 and 173.73 (a and b) ppm, respectively. Thepeaks 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 peaksresulting from Diels-Alder cycloaddition were observed atd 138.13, 136.69, 81.35, and 79.86 (d, e, i, and j) ppm, respectively.Especially, the peaks attributable to the carbons including thecycloaddition ring were observed at d 49.83 and 48.25 (m and n)ppm, respectively. The peaks attributable to the methylenesadjacent to the oxygen atomwere observed at d 72.04 and 64.58 (kand l) ppm, respectively. The peak due to methylene carbons wasobserved at d 29.30 (p, q, and r) ppm. The peak attributable to themethylenes 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 at1710 cm�1 based on the ester carbonyl group (nC]O). Moreover, theabsorption bands of the ether observed at 1115 cm�1 and1071 cm�1 were assigned to the CeOeC stretching vibration,respectively.

3.3. Polymerization of molecular LEGO block 2 and BMI

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

tion of 2 and BMI.

Table 2Polymerization 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/Mn

c

1 61.6 71.6 1.0 6 54 18000 5.812 26.6 28.5 1.0 12 57 20000 5.233 46.7 54.4 1.0 24 79 36000 4.034 107.0 124.0 2.0 48 85 38000 3.50

a Conditions: solvent: 1,2-dichloroethane; reaction temperature: 60 �C.b n-Hexane/benzene insoluble part (5/5, v/v).c 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. 13C NMR spectrum of molecular LEGO block polymer 5.

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

S. Motoki et al. / Polymer 101 (2016) 98e106 103

(5/5, v/v). When the polymerization of 2 was carried out at 60 �Cfor 6 h, 5 was obtained in 54% yield (run 1). To increase polymeryield, 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 in-crease 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 poly-merization of 2 and BMI involved a Diels-Alder reaction process.

In Fig. 6, the FT-IR spectrum of 5 showed an absorption band at1708 cm�1 based on the ester carbonyl group (nC]O). Moreover, theabsorption bands of ether observed at 1109 cm�1 and 1074 cm�1

were assigned to CeOeC stretching vibration.

3.4. Depolymerization of molecular LEGO block polymer

We attempted the depolymerization of molecular LEGO blockpolymer 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 1,1,2,2-tetrachloroethane as solvent. After predetermined reaction times,the mixture was poured into n-hexane, and precipitated oligomerand BMI were separated by centrifugation at 3000 rpm for 10 min.BMI was purified by column chromatography. Molecular LEGOblock 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

(Figs. S9 and S10). These compounds were determined by TLC an-alyses, FT-IR spectra and 1H NMR spectra. In the case of run 3, thecorresponding molecular LEGO block 1 and BMI were obtained ingood yields.

The GPC analysis of the depolymerization of 4was carried out in

Scheme 4. Depolymerization of molecular LEGO block polymer 4.

Table 3Depolymerization of molecular LEGO block polymer 4.a

Run 3 (mg) Solvent (mL) Temp. (�C) Time (h) Yield of 1 (%) Yield of BMI (%)

1 198 1.5 110 1 43 482 148 1.0 130 1 64 543 148 1.0 150 1 77 73

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

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

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

S. Motoki et al. / Polymer 101 (2016) 98e106104

THF using polystyrene as the standard. The GPC profiles before andafter depolymerization confirmed the depolymerization of 4(Fig. 7). In the case of depolymerization at 150 �C, the depolymer-ization reaction of 4 was almost completed, and polymer andoligomer were not detected by GPC measurement. In conclusion,the depolymerization of 4 was accomplished via retro-Diels-Alderreaction conditions.

Scheme 5. Scrambling reaction between m

3.5. Scrambling reaction between molecular LEGO block polymer 4and molecular LEGO block 3

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

olecular LEGO block polymer 4 and 3.

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

Fig. 10. 13C NMR spectrum of scrambled molecular LEGO block polymer 6.

S. Motoki et al. / Polymer 101 (2016) 98e106 105

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

Fig. 11. The contact angle of a water d

reaction, Mn and Mw/Mn of molecular LEGO block polymer 4 wereestimated at 7800 and 2.68, respectively. On the other hand, mo-lecular LEGO block polymer as an authentic sample was preparedfrom 3 and BMI in the same manner described in Sections 2.6 and2.7, with 73% yield. Mn and Mw/Mn of an authentic sample weremeasured and shown to be 1500 and 1.31, respectively. From theseresults, it was clarified that the scrambled polymer 6 has an in-termediate molecular weight between 4 and the authentic sample.These results also indicated that the scrambling reaction between 4and 3 proceeded.

In the 1H NMR spectrum of 6, the signals resulting from thearomatic protons (a) were observed at d 7.99e8.15 ppm (Fig. 9). Theunit ratio was determined from the integral ratio between themethylene protons (l) derived from molecular LEGO block 1, aro-matic protons (a) derived from molecular LEGO block 3 andmethylene protons (i) derived from BMI. As a result, the unit ratiowas determined to be 1/3/BMI ¼ 25/32/43 from the integral ratiobetween l, a and i. The 13C NMR spectrum of 6 also supported thescrambling reaction (Fig. 10). The peaks due to aromatic carbons (hand j) were observed at d 126.60 and 129.72 ppm, respectively.These results support the scrambling reaction between molecularLEGO block polymer 4 and 3.

The surface characteristics of cast films containing molecularLEGO block polymer 4 or 5 were observed by the contact angle of awater droplet. Molecular LEGO block polymer films containing 4 or5 were cast from a dichloromethane solution onto clean glassslides. The contact angles of 4 and 5 were 79.9 ± 2.07� and66.9 ± 0.83�, respectively. This indicates that the surface propertiesof the polymers are changed by replacing the molecular LEGOblocks (Fig. 11).

4. Conclusions

Molecular LEGO blocks having two reversible covalent bondmoieties such as furan were synthesized. These molecular LEGOblocks were polymerized with a molecular LEGO block having twomaleimide moieties under Diels-Alder conditions. The resultingmolecular LEGO block polymers, which were obtained in goodyield, were depolymerized under retro-Diels-Alder conditions. As aresult, the corresponding molecular LEGO block 1 and BMI wereobtained in good yield. Additionally, these polymers proceeded inthe scrambling reaction between molecular LEGO block polymersand other molecular LEGO blocks. The contact angle of cast filmscontaining molecular LEGO block polymer 4 or 5 were observed tobe 79.9 ± 2.07� and 66.9 ± 0.83�, respectively. This indicates thatthe surface properties of the polymers change by replacing themolecular LEGO blocks. In the future, chemical recycling of polymermaterials may become possible by this molecular LEGO block

roplet on the surface of 4 and 5.

S. Motoki et al. / Polymer 101 (2016) 98e106106

recycling system.

Acknowledgment

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

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.

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