2.1 Materials
DL-Camphorquinone (CQ, 98%) and p-toluenesulfonic acid (PTSA, monohydrate, 98.5%) were purchased from Sigma-Aldrich. CQ and PTSA were recrystallized from ethanol and others were used as received. Catechol (> 99%) was obtained from Tokyo Chemical Industry (Japan). Phosphorous pentoxide (P2O5, 98%), toluene (anhydrous, 99.9%), sodium hydroxide (NaOH, 97%), nitric acid (60 ~ 62%), tin(II) chloride dehydrate (97%), dichloromethane (MC, 99.5%), ethylacetate (EA, 99.8%), methanol (99.9%), N, N-Dimethylformamide (DMF, anhydrous, 99.9%), and magnesium sulfite (99%) were purchased from DAEJUNG. Chloroform-d (99.8 atom % D) was purchased from Sigma-Aldrich.
2.2 Synthesis of bio-based monomers and polyimides
2.2.1 Camphor aromatic diacetal (CAD diacetal)
1',7',7'-trimethyldispiro[benzo[1, 3]dioxole-2,2'-bicycle[2.2.1]heptane-3',2''-benzo[1, 3]dioxole] (abbreviated as CAD diacetal) was synthesized by reacting camphorquinone and catechol. A three-neck flask was filled with 10 g of camphorquinone, 2.5 equiv of catechol, 5 mol % of PTSA and, 10 mol % of P2O5 as the catalyst, and 200 ml of toluene as the solvent. The mixture was refluxed for 24 h with a Dean-Stark apparatus. After the reaction, the organic layer was extracted with benzene and washed with brine. The organic layer was dried over MgSO4, and the solvent was evaporated using a rotary evaporator. The crude product was purified by column chromatography (volume ratio of eluents VEA: Vhexane = 1 : 3, Rf = 0.8, silica gel), yielding the product as a white solid. Molecular formula: C22H22O4; molecular weight: 350.41 g∙mol-1; yield: 40%, mp 193–194°C 1H-NMR (600 MHz, CHCl3-d): δ (ppm) 6.78 (m, 4H, CHCH), 6.67 (m, 4H, CHCCH), 2.41 (d, 1H, CHCH2), 2.16 (m, 1H, CCH), 2.04 (m, 1H, CHCH2CH2), 1.79 (m, 1H, CCH2CH2), 1.56 (m, 1H, CHCH2CH2), 1.21 (s, 3H, CCH3), 1.15 (s, 3H, CCH3), 1.08 (s, 3H, CCH3). FT-IR: ν (cm-1) = 3087, 3046, 2963, 2875, 1602, 1494, 1393, 1311, 1273, 1125, 1030, 838.
2.2.2 Camphor aromatic dinitro (CAD dinitro)
To a solution of camphor aromatic diacetal (1.50 g, 4.28 mmol) in dichloromethane (15 ml) was added 12 ml of 60% aqueous HNO3 in dichloromethane (15 ml) at keep the temperature at about 25°C. The layers were separated, and the aqueous layer was extracted with EA. The combined organic layers were washed with brine, dried (MgSO4), and concentrated in vacuo to give CAD dinitro. The crude product was purified by column chromatography (eluent VEA: Vhexane = 1:3, Rf = 0.7, silica gel), yielding the product as a yellow solid. Molecular formula: C22H20N2O8; molecular weight: 440.12 g∙mol-1; yield: 50%. 1H-NMR (600 MHz, CHCl3-d): δ (ppm) 7.73 (m, 2H, CHCCH), 7.66 (m, 1H, CHCHCH), 6.95 (m, 1H, CHCCH), 6.85 (m, 1H, CHCCH), 6.76 (m, 1H, CHCHCH), 2.51 (d, 1H, CHCH2), 2.05–1.90 (m, 3H, CHCH2CH2), 1.65 (m, 1H, CCH2CH2), 1.23 (s, 3H, CCH3), 1.18 (s, 3H, CCH3), 1.10 (s, 3H, CCH3). FT-IR: ν (cm-1) = 3084, 3051, 2962, 2871, 1598, 1527, 1492, 1342, 1283, 1223, 1123, 1068, 1001, 821.
2.2.3 Camphor aromatic diamine (CAD diamine)
Dinitro CAD (2.00 g, 4.54 mmol) in ethyl acetate (40 ml) was added to tin(II) chloride dehydrate (10.00 g, 0.04 mol) at 50°C. The pH was adjusted to 2.0 by adding 5% of aqueous HCI solution. The combined organic layers were washed with brine and dried with MgSO4. The solvent was evaporated using a rotary evaporator. Crude product was purified with column chromatography (eluent VMC: Vhexane = 1:1, Rf = 0.5, silica gel). Molecular formula: C22H24N2O4; molecular weight: 380.44 g∙mol-1; yield: 50%, mp 214–215°C 1H-NMR (600 MHz, CHCl3-d): δ (ppm) 6.78 (m, 2H, CHCCH), 6.70 (m, 2H, CHCHCH), 6.57 (m, 2H, CHCCH), 6.19 (m, 2H, CHCCH), 6.01 (m, 2H, CHCHCH), 2.37 (d, 1H, CHCH2), 2.15-2.00 (m, 2H, CHCH2CH2), 1.80–1.50 (m, 2H, CCH2CH2), 1.19 (s, 3H, CCH3), 1.13 (s, 3H, CCH3), 1.07 (s, 3H, CCH3). FT-IR: ν (cm-1) = 3459, 3379, 3030, 2958, 2872, 1631, 1602, 1507, 1393, 1306, 1270, 1211, 1080, 1031, 839.
2.2.4 Homopolyimide as a reference
In a 50 ml three-necked flask, 2 mmol 2,2'-bis(trifluoromethyl) benzidine (TFMB) was added in approximately 9.14 ml of N, N-dimethylformamide (DMF), stirred until TFMB was dissolved completely. Then the same molar amount of 4,4′-(hexafluoroisopropylidene) phthalic anhydride (6FDA) was added and the mixture was stirred for 48 h at ambient temperature under nitrogen atmosphere. 4 mmol pyridine (P) and 8 mmol acetic anhydride (A) were added into the above polyamicacid solution, the mixture reacted for 24 h to yield homogeneous solution. Then the solution was poured slowly into methanol for precipitation. The solid was washed thoroughly with methanol, and then redissolved in DMF and precipitated in methanol again to obtain a purified sample. Finally, it was imidized and dried in a vacuum tube at 80°C. 1H-NMR (600 MHz, CHCl3-d): δ (ppm) 8.12 (d, 1H, CHCH), 8.00 (s, 1H, CHCH), 7.94 (s, 2H, CHCCH), 7.73 (d, 1H, CHCH), 7.51 (d, 1H, CHCH). FTIR: ν (cm-1) = 1785, 1731 (C = O imide), 1367 (C-N imide), 1256 (C-F stretching)
2.2.5 Degradable copolyimides
In a 50 ml three-necked flask, 2 mmol TFMB was added in approximately 9.14 ml of N, Ndimethylformamide (DMF), and stirried with a CAD monomer until the given compositions were dissolved completely. Afterward, the same molar amount of 6FDA was added and the mixture was stirred for 48 h at ambient temperature under nitrogen atmosphere. 4 mmol pyridine (P) and 8 mmol acetic anhydride (A) were added for 24 h to yield a homogeneous solution. The solution was poured slowly into methanol for precipitation. The solid was washed thoroughly with methanol, and then redissolved in DMF and precipitated in methanol again to obtain a purified sample. Finally, it was imidized and dried in a vacuum tube at 80°C. 1H-NMR (600 MHz, CHCl3-d): δ (ppm) 8.12 (d, x·1H, CHCH), 8.00 (s, x·1H, CHCH), 7.94 (s, x·2H, CHCCH), 7.73 (d, x 1H, CHCH), 7.51 (d, x·1H, CHCH), 6.89 (m, y·2H, CHCCH), 6.82 (m, y·2H, CHCHCH), 6.75 (m, y·2H, CHCCH), 2.44 (d, y·1H, CHCH2), 1.80 (m, y·2H, CHCH2CH2), 1.51 (m, y·2H, CCH2CH2), 1.22 (broad s, y·3H, CCH3), 1.18 (broad s, y·3H, CCH3), 1.10 (broad s, 3H, CCH3). FT-IR: ν (cm-1) = 1785, 1731 (C = O imide), 1367 (CN imide), 1256 (C-F stretching)