Synthesis and characterization of [60]fullerene-poly(3-azidomethyl-3-methyl oxetane) and its thermal decomposition

Abstract

A new functionalized fullerene derivative, [60]fullerene-poly(3-azidomethyl-3-methyl oxetane) (C₆₀-PAMMO), was synthesized for the first time using a modified Bingel reaction with [60]fullerene (C₆₀) and bromomalonic acid poly(3-azidomethyl-3-methyl oxetane) ester (BM-PAMMO). The product was characterized by Fourier transform infrared (FTIR), ultraviolet-visible (UV-vis), and nuclear magnetic resonance (NMR) spectroscopy analyses. The results confirmed the successful preparation of C₆₀-PAMMO. Moreover, the thermal decomposition of C₆₀-PAMMO was analyzed by differential scanning calorimetry (DSC), thermogravimetric analysis coupled with infrared spectroscopy (TG-IR), and in situ FTIR spectroscopy. The decomposition of C₆₀-PAMMO showed a three-step thermal process. The first step at approximately 150 °C was related to the cycloaddition of the azido groups (–N₃) with [60]fullerene. The second step was ascribed to the decomposition of the remaining PAMMO main chain at approximately 320 °C. The final step was attributed to the burning decomposition of amorphous carbon, the main chain, N-heterocyclic components and the carbon cage around 510 °C.

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Introduction

Given the attractive chemical and physical properties of C₆₀, since its macroscale synthesis,¹ extensive studies on its application have been carried out, which show promising applications of C₆₀ in medicine and materials science. C₆₀ has good features, such as having thermal stability, antioxidation properties, etching resistance, good compatibility with propellant components, and being beneficial for preventing aging.^2–5 Thus, C₆₀ can replace carbon black as the solid rocket fuel additive that can increase the burning rate and combustion catalytic efficiency and decrease the amount of NO_x in the exhaust gas.^6–11 If some energetic groups, such as the nitro group (–NO₂) and azido group (–N₃), are incorporated in the [60]fullerene cage, then a better fuel additive may be obtained.^12–26

Usually, azide energetic materials have a higher thermal stability^27–33 and the thermal decomposition of the azido group is ahead of the main chain and independent, which can help increase the energy and accelerate the decomposition of the propellant. Poly(3-azidomethyl-3-methyl oxetane) (PAMMO), a azido polymer, has attracted researchers’ attention and has been used as an energetic binder in propellant and explosive formulations because of its higher positive heat of formation, higher density, higher nitrogen content, and lower mechanical sensitivity.^34–39 Moreover, the terminal hydroxyl group of PAMMO can be easily modified through various reactions, such as esterification, acetalization, etherification, etc.^40,41 In this paper, PAMMO was further functionalized via esterification with malonyl dichloride and a subsequent brominate reaction to afford bromomalonic acid PAMMO ester (BM-PAMMO). BM-PAMMO easily reacts with C₆₀ through a modified Bingel reaction⁴² to afford an energetic fullerene derivative C₆₀-PAMMO. The thermal decomposition performance and decomposition mechanism of C₆₀-PAMMO were also examined in detail.

Experimental section

Chemicals and apparatus

[60]Fullerene was purchased from Henan Tianan Company. HMMO was purchased from Xiya Reagent. The 300–400 mesh silica gel was purchased from Qingdao Hailang. All the other chemicals were purchased from Chengdu Kelong Chemical Reagents Company. Dichloromethane was dried over P₂O₅ and redistilled under vacuum before use. N,N-Dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were dried over anhydrous MgSO₄ and distilled under vacuum.

¹H and ¹³C NMR spectra were recorded on a Bruker Avance DRX 400 MHz instrument with CDCl₃ as the solvent and tetramethylsilane as the internal reference. FTIR spectra were obtained on a Nicolet 6700 FT-IR spectrometer with a resolution of 4 cm⁻¹ from 400 cm⁻¹ to 4000 cm⁻¹. UV-vis spectra were recorded on a UNICON UV-2102 PCS spectrometer with CH₂Cl₂ as the solvent. Differential scanning calorimetry (DSC) curves were recorded on a SDT Q600 TGA-DSC instrument in flowing air at a heating rate of 10 °C min⁻¹. Thermogravimetry with infrared spectroscopy (TG-IR) was performed using a Q500 thermogravimetric analysis (TGA) instrument in air at a heating rate of 10 °C min⁻¹. The molecular weights and the polydispersity index were obtained with gel permeation chromatography (GPC; LC-20A, Shimadzu) and the mobile phase was tetrahydrofuran (1.0 mL min⁻¹). The hydroxyl value (OHV) of PAMMO was determined by the acetic anhydride acetylation method.⁴³

Synthesis

[60]Fullerene-poly(3-azidomethyl-3-methyl oxetane) (C₆₀-PAMMO) was synthesized in six steps, as shown in Scheme 1. First, Ts protected 3-hydroxymethyl-3-methyl oxetane (TMMO) was obtained via the reaction of 3-hydroxymethyl-3-methyl oxetane and tosyl chloride. With the use of BF₃·OEt₂ as a catalyst and ethanol as an initiator, TMMO was polymerized to produce monoethyl-terminated PTMMO. The azide substitution of PTMMO resulted in the monoethyl-terminated PAMMO. BM-PAMMO was synthesized by esterifying and then brominating PAMMO and malonyl dichloride. Finally, BM-PAMMO was allowed to react with C₆₀ through a modified Bingel reaction to get C₆₀-PAMMO.

Scheme 1 Synthesis route for C₆₀-PAMMO.

Synthesis of TMMO

20.97 g of tosyl chloride (0.11 mol) dissolved in 40 mL of pyridine was added slowly into a flask containing 10.20 g of 3-hydroxymethyl-3-methyl oxetane (0.10 mol). The mixture was left to react at −5 °C for 0.5 h under a nitrogen atmosphere and then at room temperature for 12 h. The mixture was filtered and the filtrate was added slowly into a vigorously stirred mixture of deionized water (200 mL) and crushed ice, which was stirred for another 2 h. A white precipitate appeared and was collected, washed with cold water three times, and dried under vacuum to obtain a white powder of product (18.20 g, corresponding to a yield of 71.1%). Elemental analysis (EA): found (%): C 58.54; H 5.85; S 12.48. Calculated (%): C 58.47; H 5.88; S 12.54. UV-vis (CH₂Cl₂) λ (nm): 205, 211, 237, 262, 273. IR (KBr) ν (cm⁻¹): 2875 (ν_s CH₂), 1598 (ν_as C–C aromatic ring), 1463 (δ CH₂), 1360 (ν_as S(=O)₂), 1180 (ν_s S(=O)₂), 1096 (δ_s C–H aromatic ring), 977 (ν_as C–O–C oxetane, ν_as S–O–C), 837 (ν_s S–O–C), 665 (ω C–H aromatic ring). ¹H-NMR (CDCl₃, 400 MHz) δ (ppm): 7.83–7.38 (4H, m, aromatic ring), 4.39–4.35 (4H, dd, J = 16.9, 6.2 Hz, CH₂O oxetanic ring), 4.12 (2H, s, CH₂OTs), 2.47 (3H, s, CH₃ aromatic ring), 1.32 (3H, s, CH₃).

Synthesis of PTMMO

1.85 g of BF₃·OEt₂ (13.02 mmol), 1.20 g of absolute ethyl alcohol (26.00 mmol) and 25 mL of dry methylene chloride were introduced into a 100 mL three-necked flask equipped with a mechanical stirrer and thermometer. This mixture was stirred for 0.5 h under argon at 0–5 °C. A solution of 10 g of TMMO (0.04 mol) in 20 mL of methylene chloride was added dropwise to this reaction mixture. The reaction mixture was stirred at 0–5 °C for 18 h, and then the mixture was allowed to reach 35 °C and this temperature was maintained for 25 h. Thereafter, the reaction was completed by adding 100 mL of saturated NaCl solution. The organic layer was washed with water (200 mL × 3) and a mixture of water/methanol, dried over Na₂SO₄, filtered and evaporated in vacuum to produce 9.70 g of PTMMO, with a yield of 87%. UV-vis (CH₂Cl₂) λ (nm): 236, 262, 273. IR (KBr) ν (cm⁻¹): 2878 (ν_s CH₂), 1598 (ν_as C–C aromatic ring), 1460 (δ CH₂), 1358 (ν_as S(=O)₂), 1176 (ν_s S(=O)₂), 1098 (δ_s C–H aromatic ring), 966 (ν_as C–O–C, ν_as S–O–C), 833 (ν_s S–O–C), 666 (ω C–H aromatic ring). ¹H NMR (CDCl₃, 400 MHz) δ (ppm): 7.76–7.34 (4H, dd, J = 12.8, 7.8 Hz, aromatic ring), 3.79 (2H, s, CH₂OTs), 3.43–3.11 (2H, q, J = 10.0, 6.3 Hz, CH₂ ethyl), 3.07 (2H, s, CH₂O main chain), 2.43 (3H, s, CH₃ aromatic ring), 1.09–1.06 (3H, t, J = 4.8 Hz, CH₃ ethyl), 0.75 (3H, s, CH₃). Molecular weight (by GPC): M_w = 1534, M_n = 1225, M_w/M_n = 1.25.

Synthesis of PAMMO

6.0 g of NaN₃, 18.0 g of PTMMO and 30 mL of DMSO were introduced into a 100 mL three-necked flask equipped with a mechanical stirrer and thermometer. The reaction mixture was stirred at 100 °C for 120 h. Thereafter, the reaction solution was poured into 200 mL of distilled water and extracted with dichloromethane (50 mL × 3). The organic layer was washed with distilled water (200 mL × 3), dried over Na₂SO₄, filtered and evaporated in vacuum to produce 6.6 g of PAMMO, with a yield of 83%. UV-vis (CH₂Cl₂) λ (nm): 236, 284. IR (KBr) ν (cm⁻¹): 2972 (ν_as CH₂), 2876 (ν_s CH₂), 2102 (ν_as N₃), 1282 (ν_s N₃), 1455 (ν CH₂), 1378 (δ_s CH₃), 1355 (ν CH₂), 1111 (ν_as C–O–C). ¹H-NMR (CDCl₃, 400 MHz) δ (ppm): 0.94 (3H, s, CH₃ side chain), 1.14–1.22 (3H, t, J = 7.0 Hz, CH₃ ethyl), 3.21 (4H, m, CH₂O), 3.26 (2H, s, CH₂N₃), 3.69–3.33 (2H, q, J = 18.1 Hz, CH₂ ethyl). ¹³C-NMR (CDCl₃, 100 MHz) δ (ppm): 75.68 (–CH₂O–), 68.36 (HOCH), 67.07 (–CH₂CH₃), 55.53 (N₃CH₂CCH₃), 40.80 (N₃CH₂CCH₃), 18.06 (–CH₂N₃), 14.99 (–CH₃). Molecular weight (by GPC): M_w = 1027, M_n = 718, M_w/M_n = 1.43. The hydroxyl equivalent weight was 80.07 mg g⁻¹.

Synthesis of M-PAMMO

0.25 mL of malonyl chloride (2.63 mmol) in 10 mL of dry CH₂Cl₂ was added dropwise to a stirred solution of 2.80 g of PAMMO (4.0 mmol hydroxyl group) and 0.60 mL DMF (7.78 mmol) in 25 mL of dry CH₂Cl₂ under an Ar atmosphere in an ice bath. The duration of this process was more than 30 min. The reaction mixture was stirred at room temperature for 11 h and then was washed repeatedly with deionized water to neutralise. The organic layer was dried with anhydrous MgSO₄, filtered, and evaporated in vacuum to afford 2.0 g of brown M-PAMMO, with a yield of 66%. UV-vis (CH₂Cl₂) λ (nm): 235, 335. IR (KBr) ν (cm⁻¹): 2974 (ν_as CH₂), 2877 (ν_s CH₂), 2102 (ν_as N₃), 1740 (ν C=O), 1279 (ν_s N₃), 1456 (ν CH₂), 1377 (δ_s CH₃), 1353 (ν CH₂), 1112 (ν_as C–O–C). ¹H-NMR (CDCl₃, 400 MHz) δ (ppm): 0.94 (3H, s, CH₃ side chain), 1.14–1.22 (3H, t, J = 7.0 Hz, CH₃ ethyl), 3.21 (4H, m, CH₂O), 3.26 (2H, s, CH₂N₃), 3.69–3.33 (2H, q, J = 5.3 Hz, CH₂ ethyl), 3.33 (2H, s, COCH₂CO). ¹³C-NMR (CDCl₃, 100 MHz) δ (ppm): 166.17 (–CH₂COCH₂–), 76.03 (HOCH–), 68.85 (–CCH₂CH₂–), 66.82 (–CH₂CH₃), 55.56 (–CH₂N₃), 41.58 (N₃CH₂CCH₃), 41.44 (–COCH₂CO–), 21.62 (–CCH₃), 15.01 (–CH₃). Molecular weight (by GPC): M_w = 2082, M_n = 1520, M_w/M_n = 1.37.

Synthesis of BM-PAMMO

0.13 mL of Br₂ (2.54 mmol) in 10 mL of dry CH₂Cl₂ was added dropwise to a stirred solution of 3.1 g of M-PAMMO in 45 mL of dry CH₂Cl₂ at room temperature. Then the reaction mixture was stirred at room temperature for 6 h. The reaction was terminated by adding saturated NaBr solution. The mixture was washed with water until the pH was neutral to obtain 2.9 g of BM-PAMMO, with a yield of 85%. UV-vis (CH₂Cl₂) λ (nm) (log ε): 235 (4.89), 339 (3.98). IR (KBr) ν (cm⁻¹): 2973 (ν_as CH₂), 2877 (ν_s CH₂), 2102 (ν_as N₃), 1740 (ν C=O), 1279 (ν_s N₃), 1455 (ν CH₂), 1377 (δ_s CH₃), 1112 (ν_as C–O–C), 604 (ν C–Br). ¹H-NMR (CDCl₃, 400 MHz) δ (ppm): 0.94 (3H, s, CH₃ side chain), 1.14–1.22 (3H, t, J = 7.0 Hz, CH₃ ethyl), 3.21 (4H, m, CH₂O), 3.26 (2H, s, CH₂N₃), 3.61–3.33 (2H, q, J = 6.4 Hz, CH₂ ethyl), 5.54 (1H, s, CHBr). ¹³C-NMR (CDCl₃, 100 MHz) δ (ppm): 166.17 (–CH₂COCH₂–), 76.03 (HOCH–), 68.85 (–CCH₂CH₂–), 66.82 (–CH₂CH₃), 55.56 (–CH₂N₃), 41.83 (–COCHBrCO–), 41.58 (N₃CH₂CCH₃), 21.62 (–CCH₃), 15.01 (–CH₃). Molecular weight (by GPC): M_w = 2157, M_n = 1586, M_w/M_n = 1.36.

Synthesis of C₆₀-PAMMO

0.89 g of C₆₀ (1.24 mmol) and 0.556 g of glycine (7.41 mmol) were dissolved in 250 mL of chlorobenzene and 100 mL of DMSO by using an ultrasonic irradiation method. Thereafter, 1.21 g of BM-PAMMO was added to the above solution. The mixture was stirred at room temperature for 12 h, and then the resulting solution was repeatedly washed with distilled water to remove the DMSO and glycine. Chlorobenzene was then removed under vacuum. The residue was separated on a silica gel column using carbon disulfide and CH₂Cl₂/ethyl acetate (1 : 2) as the eluent to obtain 44.30 mg of unreacted C₆₀ and 1.58 g of C₆₀-PAMMO, with a yield of 87% (based on the consumption of C₆₀). UV-vis (CH₂Cl₂) λ (nm) (log ε): 235 (4.85), 258 (4.86), 330 (4.35), 435 (3.22). IR (KBr) ν (cm⁻¹): 2965 (ν_as CH₂), 2856 (ν_s CH₂), 2096 (ν_as N₃), 1736 (ν C=O), 1262 (ν_s N₃), 1456 (ν CH₂), 1384 (δ_s CH₃), 1171 (C₆₀ specific absorbing peak), 1103 (ν_as C–O–C), 526 (C₆₀ specific absorbing peak). ¹H-NMR (CDCl₃, 400 MHz) δ (ppm): 0.87 (3H, s, CH₃ side chain), 1.16–1.08 (3H, t, CH₃ ethyl), 3.15 (4H, m, CH₂O), 3.19 (2H, s, CH₂N₃), 3.61–3.33 (2H, q, CH₂ ethyl). Molecular weight (by GPC): M_w = 3255, M_n = 2276, M_w/M_n = 1.43.

Results and discussion

Characterization of C₆₀-PAMMO

The structure of C₆₀-PAMMO was confirmed by FT-IR, UV-vis, ¹H NMR, and ¹³C NMR spectral analyses. The UV-vis spectra of BM-PAMMO and C₆₀-PAMMO are presented in Fig. 1. As shown in Fig. 1, the UV-vis spectrum of BM-PAMMO shows an absorption peak at 235 nm, which is ascribed to the π → π* transition of the –N₃ group. The UV-vis spectrum of C₆₀-PAMMO also shows a sharp absorption peak at approximately 235 nm, which implies the presence of the –N₃ group in C₆₀-PAMMO. Moreover, compared with BM-PAMMO, three new absorption peaks at 258, 330 and 435 nm are observed in the UV-vis spectrum of C₆₀-PAMMO. The peaks at 258 and 330 nm are the characteristic absorption peaks for the skeleton structure of C₆₀, and the weak absorption band at 435 nm is typical of monoadducts at the closed 6,6-junction of C₆₀.^44,45

Fig. 1 UV-vis absorption spectra of BM-PAMMO and C₆₀-PAMMO. (a) C₆₀-PAMMO and (b) BM-PAMMO.

Fig. 2 shows the FTIR spectra of BM-PAMMO and C₆₀-PAMMO. As shown in Fig. 2(b), in the FTIR spectrum of BM-PAMMO, the peak at 1740 cm⁻¹ can be assigned to the C=O stretching vibration, and the peaks at 1112 cm⁻¹ and 1279 cm⁻¹ correspond to the C–O–C symmetric and antisymmetric stretching vibrations. The strong absorption peak at 2102 cm⁻¹ corresponds to the –N₃ group of PAMMO, and the peaks at 2973, 2934 and 2877 cm⁻¹ are ascribed to the C–H symmetric and antisymmetric stretching vibrations of PAMMO, respectively.³⁹ The peak at 604 cm⁻¹ is due to the C–Br stretching vibration. Compared with BM-PAMMO (Fig. 2(b)), the C–Br stretching vibration peak has disappeared in Fig. 2(a). In addition, the new peaks in Fig. 2(a) at 526 cm⁻¹ and 1171 cm⁻¹ are the characteristic absorption peaks of the C₆₀ cage.⁴⁶

Fig. 2 ATR-FTIR absorption spectra of BM-PAMMO and C₆₀-PAMMO. (a) C₆₀-PAMMO and (b) BM-PAMMO.

The ¹H NMR spectrum also confirmed the successful introduction of PAMMO to the C₆₀ cage. Fig. 3 shows the ¹H NMR spectrum of C₆₀-PAMMO. As shown in Fig. 3, the peaks at 3.10 and 3.60 ppm are attributed to the methylene protons of the main chain (–CH₂O– protons) (denoted a and d) and the methylene protons of the side chain (–CH₂N₃ protons) (denoted c). The methyl protons of the CCH₃ (denoted b) and CH₃CH₂O– (denoted e) groups appear at 0.75–1.20 ppm.

Fig. 3 ¹H NMR spectrum of C₆₀-PAMMO.

The ¹³C NMR spectrum of C₆₀-PAMMO is presented in Fig. 4. The signals of the carbonyl carbon (–C=O, denoted d) and the quaternary carbon bound to C₆₀ (denoted c) appear at δ = 162.20–163.00 ppm and δ = 50.20 ppm, respectively.⁴⁷ The signals at 74.00 and 71.20 ppm are attributed to the methylene carbon atoms of the main chain (the –CH₂O– group) (denoted f, k and i). The signals from 54.22 ppm to 52.34 ppm are attributed to the azidomethyl carbon of the side chain (the –CH₂N₃ group) (denoted h). The quaternary carbon (denoted e) of the main chain appears at 39.52 ppm. The methyl carbon atoms of the side chain (denoted g) and terminal CH₃CH₂O– group (denoted j) appear at 16.68 ppm and 20.89 ppm, respectively. The two sp³-carbon atoms of the C₆₀ cage (denoted b) appear at 68.34 ppm, and the 58 sp²-carbon atoms of the fullerene cage (denoted a) overlap and appear from 150.00 ppm to 135.40 ppm as a broad peak.⁴⁸

Fig. 4 ¹³C NMR spectrum of C₆₀-PAMMO.

Thermal analysis

The thermal stability of energetic materials has an important effect on the preparation, storage, processing, and application scope. Thus, DSC and TGA methods were adopted to evaluate the decomposition behavior of C₆₀-PAMMO.

The DSC curve of C₆₀-PAMMO under an air atmosphere is shown in Fig. 5. Three exothermic peaks were observed from 100 °C to 600 °C. The first exothermic peak at 157 °C is probably caused by the initial intramolecular or intermolecular reaction of the –N₃ group with the fullerene carbon cage.⁴⁹ The second exothermic peak at 324 °C is due to the decomposition of the PAMMO main chain. The third peak at 513 °C can be attributed to the decomposition of the C₆₀ skeleton. In addition, the decomposition enthalpy values of the three exothermic peaks are 39 J g⁻¹, 2858 J g⁻¹ and 1005 J g⁻¹, respectively.

Fig. 5 DSC curve of C₆₀-PAMMO under an air atmosphere.

In order to understand the thermal decomposition mechanism of C₆₀-PAMMO, thermogravimetric analysis coupled with infrared spectroscopy (TGA-IR) was used to rapidly identify the constituents of the thermal decomposition gas (Fig. 6). As shown in Fig. 6(a), the TGA and DTG (derivative thermogravimetric analysis) curves show the three-step thermal degradation of C₆₀-PAMMO under an air atmosphere. The first thermal degradation appears at 148 °C, with around 10.36% weight loss. Thereafter, two thermal degradations appear at 323 °C and 512 °C with approximately 35.99% and 51.91% weight losses, respectively. Fig. 6(b) shows the IR spectra of the thermal decomposition gas of C₆₀-PAMMO at 323 °C and 512 °C. As depicted in Fig. 6(b), the decomposed products of C₆₀-PAMMO at 323 °C under an air atmosphere are mainly CO₂ (2310 and 2367 cm⁻¹), CO (2185 and 2110 cm⁻¹) and C=O (1749 cm⁻¹), and the decomposed products at 512 °C under an air atmosphere are mainly CO₂ (2302 and 2364 cm⁻¹), CO (2109 and 2177 cm⁻¹), N₂O (2247 cm⁻¹) and H₂O (3543 cm⁻¹). Obviously, these results are different from the previously reported work, where the thermal degradation of an azide polymer appears around 200 °C, and the main gas products are CO, HCN, HCHO and NH₃.²⁹ Thus, these results indicate that the first thermal degradation of C₆₀-PAMMO at 148 °C is probably due to the decomposition of the –N₃ group, and the decomposition mechanism is probably through the initial intramolecular or intermolecular cycloaddition reaction of –N₃ with the fullerene carbon cage, and subsequent decomposition to release nitrogen (Scheme 2).

Fig. 6 (a) TGA and DTG curves of C₆₀-PAMMO under an air atmosphere; (b) IR spectra of the thermal decomposition gas of C₆₀-PAMMO at 323 °C and 512 °C under an air atmosphere; (c) thermal decomposition gas intensity at different temperatures.

Scheme 2 The decomposition mechanism of C₆₀-PAMMO.

In order to demonstrate this decomposition mechanism, in situ FTIR was used to rapidly identify the constituents of the decomposition condensed-phase residue. Fig. 7(a) shows the infrared spectra of the C₆₀-PAMMO decomposition condensed-phase residue at 27 °C, 127 °C, 152 °C, 172 °C, 207 °C and 262 °C under an air atmosphere. Fig. 7(b) shows the infrared absorption intensity ratios of –N₃ (ν = 2096 cm⁻¹) and C=O (ν = 1736 cm⁻¹), and C₆₀ (ν = 526 cm⁻¹) and C=O (ν = 1736 cm⁻¹) in the C₆₀-PAMMO decomposition condensed-phase FTIR spectra at specified temperatures. As shown in Fig. 7, both the infrared absorption intensity ratio of –N₃ (ν = 2096 cm⁻¹) and C=O (ν = 1736 cm⁻¹) (A_–N3/A_C=O) and the infrared absorption intensity ratio of C₆₀ (ν = 526 cm⁻¹) and C=O (ν = 1736 cm⁻¹) (A_C60/A_C=O) rapidly decreased at 148 °C. The decrease of A_–N3/A_C=O shows that the –N₃ group started to rapidly decompose at 148 °C, and the reduction of A_C60/A_C=O at 148 °C indicates that the decomposition of the –N₃ group is accompanied by damage to the fullerene conjugated structure. Thus, the in situ FTIR analytical results strictly corroborate the TGA-IR analysis that C₆₀-PAMMO decomposition at 148 °C is through the initial intramolecular or intermolecular reaction of –N₃ with the fullerene carbon cage.

Fig. 7 (a) IR spectra of the remaining solid after thermal decomposition of C₆₀-PAMMO at 27, 127, 152, 172, 207 and 262 °C; (b) ratios of the IR absorption peaks of –N₃ (ν = 2096 cm⁻¹)/–COO– (ν = 1736 cm⁻¹) and C₆₀ (ν = 526 cm⁻¹)/C=O (ν = 1736 cm⁻¹) at different temperatures.

Conclusions

A new energetic polymer, [60]fullerene-poly(3-azidomethyl-3-methyl oxetane), has been successfully synthesized through the Bingel reaction of BM-PAMMO and C₆₀. Its structure was systematically characterized by FTIR, UV-vis, ¹H NMR and ¹³C NMR spectroscopy analyses. The thermal stability and thermal decomposition mechanism of [60]fullerene-poly(3-azidomethyl-3-methyl oxetane) was analyzed by DSC, TGA-FTIR, and in situ FTIR. The results showed that the initial decomposition of [60]fullerene-poly(3-azidomethyl-3-methyl oxetane) at 148 °C could be ascribed to the intramolecular or intermolecular reaction of the –N₃ group with the fullerene carbon cage.

Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (Project No. 21301142, 51372211), National Defense Fundamental Research Projects (Project No. A3120133002), Applied Basic Research Program of Sichuan Province (2014JY0170) and Southwest University of Science and Technology Researching Project (13zx9107, 13zxfk09, 14tdfk05).

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