Synthesis and thermal properties of an acetylenic monomer containing boron and silicon

Abstract

To improve the thermo-oxidative stability of acetylenic aromatic compounds, 1,2-bis(4-trimethylsilylethynylphenyl)-carborane (CBTMS) was designed, synthesized and characterized by FT-IR, ¹H-NMR, ¹³C-NMR and mass spectrometry. The analysis of the DSC results showed that the acetylenic monomer had a melting point at 195.5 °C. The cross-linking process of CBTMS included a Diels–Alder cycloaddition reaction confirmed by FT-IR spectroscopy. Nonisothermal DSC studies showed CBTMS has an activation energy similar to that of the phenylethynyl-terminated compound. The thermoset and ceramic derived from the acetylenic monomer exhibited extremely thermo-oxidatively stable properties studied using thermogravimetric analysis (TGA). The thermoset showed a weight gain in air at elevated temperature and char yield of 98.8% at 1000 °C in air, and the ceramic residue had almost no weight loss up to 1000 °C in air. We demonstrated that trimethylsilylethynyl could be used as a crosslinking group for thermosetting polymers.

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Introduction

High-temperature resistant polymers have been developed with the purpose of being used as high-performance matrices in the electronics and aerospace industries.^1,2 Resins with terminal acetylenic groups have received a great deal of attention in the past four decades for the fact that these resins cure via an addition reaction a consequence of which is that no volatile byproducts are evolved during the processing stage.³ Besides maintaining easy processing, they afford cured products that have high thermal stability and good mechanical properties. Hence, much effort has been focused on using these resins as matrices for high-performance, fiber-reinforced composites.^4–8 In addition, their high thermal stability also qualifies them as excellent candidates for carbon matrices in carbon/carbon composites.^7,9

However, acetylenic aromatic monomers have the same disadvantages as carbon based materials, which are susceptible to decomposition at elevated temperatures in oxidative environments.^10–14 Incorporating inorganic elements, such as silicon and boron, into a polymeric material improves the thermo-oxidative stability relative to carbon.^5,6 Protective inorganic oxides are formed acting as a barrier to protect the interior from oxidation and degradation when these materials are pyrolyzed and exposed to an oxidative environment at elevated temperatures.¹⁵ Polymers containing the carborane unit within the backbone exhibit appreciable enhancement in their oxidative stability at elevated temperatures.^16–23 Upon exposure to air at elevated temperatures, boron is oxidized to B₂O₃ that is reported to fill and seal micro-cracks that develop from thermal cycling at elevated temperatures.¹⁵ Compounds with trimethylsilylethynyl groups have better solubility compared with phenylethynyl groups. In addition, trimethylsilyl groups have excellent thermal stability.^24,25 Reactive end groups such as ethynyl, phenylethynyl have been used as thermally reactive end groups for polymer cross-linking.^4,26–29 However, to the best of our knowledge, there has no report about using trimethylsilylethynyl as crosslinking groups for thermosetting polymers in previous literatures. Here we report our studies on acetylenic monomer containing carborane and trimethylsilylethynyl. This monomer has been well characterized, and its curing behaviors and thermal and thermo-oxidative stability were studied.

Experimental

Materials

B₁₀H₁₂(CH₃CN)₂ was synthesized according to literature procedures.³⁰ Decaborane was purchased from Zhengzhou Sigma Chemical Co., Ltd. (China). Trimethylsilylacetylene, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), copper(I) iodide (CuI), palladium(II)bis(triphenylphosphine) dichloride (PdCl₂(Ph₃)₂), 1-bromo-4-iodobenzene were purchased from J&K Chemical Co and used without further purification. Acetonitrile, toluene and triethylamine were purchased from Tianjin Fuyu Fine Chemical Co., Ltd. (China). Acetonitrile was distilled from phosphorus pentoxide prior to use. Triethylamine was distilled from CaH₂ prior to use. Toluene was distilled from sodium and benzophenone prior to use.

Measurements

All NMR spectra were measured on an Bruker AVANCE III 500 spectrometer (500 MHz). High Performance Liquid Chromatography (HPLC) analyses were performed on a Alliance2695-2696 instrument. Elemental analyses were performed on an elemental analysis Vario EL series. The FT-IR spectra were obtained using a Thermo Nicolet Nexus 470 Fourier transform infrared (FT-IR) spectrometer. The wide-angle X-ray diffraction (WAXD) measurements were undertaken on a D/Max 2400. Thermogravimetric analyses (TGA) of the polymers were performed on a Mettler TGA/SDTA851 thermogravimetric analysis instrument in a nitrogen atmosphere at a heating rate of 20 °C min⁻¹ from 30 to 800 °C. The differential scanning calorimetric (DSC) was measured on Mettler DSC822 DSC under a nitrogen flow (50 mL min⁻¹) at various heating rate of 5 °C min⁻¹, 10 °C min⁻¹, 15 °C min⁻¹, 20 °C min⁻¹, 30 °C min⁻¹ from 25 °C to 450 °C.

Synthesis of compound 1

To 500 mL flask, equipped with a magnetic stirrer and a rubber septum, was charged 1.26 g (1.8 mmol) PdCl₂(PPh₃)₂, 1.14 g (5.85 mmol) CuI, 50.9 g (0.18 mol) 4-iododiphenyl ether. The flask was purged with dry nitrogen. DBU (72 mL), thrimethylsilyethynylene (13.77 mL), distilled water (1.30 mL), acetonitrile (250 mL) was then added by syringe. The reaction flask was covered in aluminum foil and was stirred at 50 °C for 48 h. After cooling, the reaction mixture was poured into 500 mL distilled water. The precipitate was filtered and washed with distilled water. The pure product was obtained by recrystallization from chloroform in a yield of 85% (25.7 g). Purity: 99.13% (HPLC). Mp: 185 °C (from chloroform). Found: C, 49.88%; H, 2.22%. Anal. calcd for C₁₄H₈Br₂: C, 50.04%; H, 2.40%. FT-IR (KBr, cm⁻¹): 1905.5, 1488.2 (C=C), 1070.9, 1002.7, 822.7, 509.1 (C–Br). ¹H NMR (500 MHz, CDCl₃, Me₄Si): δ 7.49–7.48 (4H, d, Ph), 7.39–7.37 (4H, d, Ph). ¹³C NMR (500 MHz, CDCl₃, Me₄Si): δ 132.93, 131.60, 122.70, 121.78, 89.29.

Synthesis of compound 2

20.16 g (60 mmol) of compound 1, 14.56 g (72 mmol) of B₁₀H₁₂(CH₃CN)₂ and 400 mL toluene were added to a 1000 mL three-necked round bottom flask equipped with a magnetic stirrer, a nitrogen inlet and a reflux condensing tube. The reaction mixture was magnetically stirred under nitrogen for 2 h at 100 °C and then stirred for 12 h at 115 °C. When the reaction was completed, the reaction solution was cooled to room temperature and 50 mL methanol was added to resolve unreacted B₁₀H₁₂(CH₃CN)₂. The toluene was evaporated to afford crude product and the crude product was purified by silica gel column chromatography using petroleum ether as eluent to obtain the pure compound 2 as a white powder (13.1 g yield: 48%). Purity: 99.90% (HPLC). Found: C, 37.17%; H, 3.90%. Anal. calcd for C₁₄H₁₈B₁₀Br₂: C, 37.02%; H, 3.99%. FT-IR (KBr, cm⁻¹): 3068.7 (Ar–H), 2592.7 (B–H), 2569.1 (B–H), 2554.4 (B–H), 1586.7 (C=C), 1490.2 (C=C), 1397.1, 1064.9, 1013.6, 829.7. ¹H NMR (500 MHz, CDCl₃, Me₄Si): δ 7.32–7.30 (4H, d, Ph), 7.29–7.27 (4H, d, Ph), 3.5–1.5 (10H, br s, B–H). ¹³C NMR (500 MHz, CDCl₃, Me₄Si): δ 132.05, 131.73, 129.56, 125.38, 83.10. HRMS (EI-TOF) m/z: M⁺ calculated for C₁₄H₁₈B₁₀Br₂ 454.0758; found 454.0792.

Synthesis of compound 3 (CBTMS)

A 250 mL pear-shaped flask equipped with a magnetic stirrer and a reflux condensing tube was charged with 0.336 g (0.48 mmol) of PdCl₂(Ph₃)₂, 0.304 g (1.60 mmol) of CuI and 10.9 g (24 mmol) of compound 2. And then the flask was sealed with a septum, evacuated and refilled with nitrogen three times. 11.3 mL (80 mmol) of trimethylsilylacetylene, 24 mL (173 mmol) of trimethylamine and 80 mL of toluene were added to the flask under the nitrogen purge. After addition, the reaction mixture was stirred for 24 h at 80 °C. Then, the reaction was quenched with 100 mL of 10% HCl aqueous solution, and the mixture was extracted with diethyl ether. The organic layer was washed with brine and dried over anhydrous Na₂SO₄, followed by vacuum evaporation. The yellow crude product was finally passed through silica gel column chromatography using petroleum ether as eluent to give the pure compound as a white powder (yield: 92.3%). Purity: 99.04% (HPLC). Found: C, 58.79%; H, 7.41%. Anal. calcd for C₂₄H₃₆B₁₀Si₂: C, 58.97%; H, 7.42%. FT-IR (KBr, cm⁻¹): 2963.5 (C–H), 2592.4 (B–H), 2565.0 (B–H), 2157.9 (C≡C), 1602.9 (C=C), 1502.0 (C=C), 1405.7, 1249.9 (Si–C), 863.8, 842.7. ¹H NMR (500 MHz, CDCl₃, Me₄Si): δ 7.34–7.33 (4H, d, Ph), 7.22–7.21 (4H, d, Ph), 0.22 (9H, S, Si–CH₃). ¹³C NMR (500 MHz, CDCl₃, Me₄Si): δ 131.90, 130.55, 125.54, 103.44, 97.71, 84.91, 0.00. MS (TOF-LD): m/z 489.2 M⁻.

Thermal cure of compound 3

CBTMS-280 was prepared by curing of CBTMS in air at 280 °C/2 h. CBTMS-350 was prepared by curing of CBTMS in air at 280 °C/2 h and 350 °C/2 h. CBTMS-400 was prepared by curing of CBTMS in air at 280 °C/2 h, 350 °C/2 h and 400 °C/2 h.

Preparation of ceramic

Ceramic residue (CBTMS-1000) was obtained by heating CBTMS-400 to 1000 °C in air at a heating rate of 5 °C min⁻¹ (yield: 95%).

Results and discussion

Preparation and characterization of monomers

The synthesis routes for the desired compound 3 (CBTMS) are illustrated in Scheme 1. Compound 1 was synthesized using Sonogashira coupling reaction according to a reported literature.³¹ Compound 2 was synthesized by combining compound 1 with B₁₀H₁₂(CH₃CN)₂ and refluxing in toluene. Compound 3 was synthesized through a similar synthetic procedure as synthesis of compound 1, except for using trimethylamine rather than DBU as base. Chemical structure of compound 3 was confirmed by FT-IR, ¹H-NMR, ¹³C-NMR and mass spectrometry.

Scheme 1 Synthetic routes of acetylenic monomer CBTMS.

The FT-IR Spectra of CBTMS is showed in Fig. 3. It is obvious that CBTMS exhibits an absorption peak at 2592 cm⁻¹, which is ascribed to the stretching vibration of B–H on carborane cage according to a literature.³² The absorptions observed at 2153 cm⁻¹ and 1249 cm⁻¹ were attributed to the stretching of C≡C and Si–CH₃ groups respectively.^20,33

Fig. 1 shows the ¹H-NMR and ¹³C-NMR results of CBTMS in CDCl₃. All of the protons and carbons in CBTMS were detected as expected, confirming that acetylenic monomer was synthesized successfully. The ¹H-NMR showed a singlet at 0.2 ppm corresponding to the hydrogen atoms of CH₃–Si. The broad peak at δ 1.5–3.5 was assigned to the protons of B–H in o-carborane cage.¹

Fig. 1 ¹H-NMR and ¹³C-NMR spectra of CBTMS in CDCl₃.

Curing behavior of CBTMS

The curing behavior of the CBTMS was investigated by DSC analysis, and the result is shown in Fig. 2. In the DSC thermogram of CBTMS, one endothermic peak centered at 195.5 °C and one strong exothermic peak centered at 385.4 °C were observed. The endothermic peak corresponds to the melting point of CBTMS, while the exothermic peak that starts at 303.0 °C and ends at 416.8 °C with an enthalpy of 197.7 J g⁻¹ is attributed to thermal cross-linking reactions of the reactive C≡C linkages of CBTMS. The results indicate that trimethylsilylethynyl could be cured without any incorporation of cross-linking agents. Due to the steric hindrance of terminal trimethylsilyl groups, the activity of the ethynyl groups in CBTMS is lower than acetylene-terminated compounds reported in the literature, as indicated by a broader exothermic peak at higher temperature and a low exothermic energy.^34,35

Fig. 2 DSC curve for CBTMS at heating rate of 10 °C min⁻¹.

The chemical structures of CBTMS before and after thermal curing at different temperature were determined by FT-IR. The results are shown in Fig. 3. The crosslinking reaction of the trimethylsilylethynyl end group was determined by the disappearance of acetylene group absorption band at 2153 cm⁻¹. The absorption band of the acetylene group could still be observed after curing at 350 °C for 2 h, whereas the absorption band of the triple bond disappeared after curing at 400 °C for 2 h. So the curing processes were determined as 280 °C for 2 h, at 350 °C for 2 h and at 400 °C for 2 h. As shown in Scheme 2, Kuroki et al. reported from ¹³C-NMR and the semiempirical MO method (MOPAC93/PM3) that the naphthalene ring forms by a Diels–Alder reaction between the Ph–C≡C group and the C≡C group in [–Si(Ph)H–C≡C–C₆H₄–C≡C–]_n because the hydrogen transfer reaction occurs easily.³⁶ The activation energy of the hydrogen transfer reaction calculated by Kuroki et al. was smaller than the activation energy of the Diels–Alder reaction. However, CBTMS has no Si–H group for hydrogen transfer reaction. Therefore, we had reason to believe that Diels–Alder cycloaddition reaction is one possible mode of the cross linking reaction of CBTMS. In addition, Keller et al. reported the thermosetting mechanism study of poly(carborane-siloxane-arylacetylene) by solid-state NMR spectroscopy, which has similar crosslinking groups as CBTMS.³⁷ According to Keller's study, Diels–Alder cycloaddition reaction was the pathway for the thermal crosslinking process of [–Si(CH₃)₂–O–Si(CH₃)₂–(m-carborane)–Si(CH₃)₂–O–Si(CH₃)₂–C≡C–Ph–C≡C–]. FT-IR spectra of CBTMS and cured CBTMS possess several characteristic absorption bands (Fig. 3), the bands at 2963 cm⁻¹ and 1250 cm⁻¹ correspond to –CH₃ and Si–CH₃ respectively were all still remaining after crosslinking, while the acetylene group absorption band at 2153 cm⁻¹ became invisible after curing process further confirming the occurrence of the Diels–Alder reaction. Accordingly, we concluded that the intermolecular cross-linking reactions of CBTMS due to the Diels–Alder cycloaddition reaction between the Ph–C≡C group and the C≡C group were the mode of the cross linking reaction and they were self-crosslinking reactions. Scheme 3 shows the thermosetting mechanism of CBTMS. These results suggested that trimethylsilylethynyl could be used as crosslinking groups for thermosetting polymers. The absorptions at 2593 cm⁻¹ due to stretching of B–H remained unchanged during the curing, which suggested the invariance of carborane cages.

Fig. 3 FT-IR spectra of CBTMS before and after curing at different temperature: (a) uncured, (b) curing at 280 °C for 2 h, (c) curing at 280 °C for 2 h and at 350 °C for 2 h, (d) curing at 280 °C for 2 h, at 350 °C for 2 h and at 400 °C for 2 h.

Scheme 2 Thermosetting mechanism of poly[phenylsilylene]ethynylene-1,3-phenyleneethynylene] (MSP) based on solid-state NMR results: hydrosilylation reaction between Si–H and C≡C; Diels–Alder reaction between Ph–C≡C and C≡C.³⁶

Scheme 3 Thermosetting mechanism of CBTMS by Diels–Alder reaction between the Ph–C≡C group and C≡C group.

Non-isothermal cure kinetics of CBTMS

In order to better understand the thermal curing reaction and cure kinetics of CBTMS, non-isothermal DSC tests at different heating rates were conducted. The experimental results are shown in Table 1.

Table 1 DSC data of CBTMS at different heating rates^a

Sample	Heating rate	Characteristic temperatures
	β (°C min^−1)	Ti (°C)	Tp (°C)	Tt (°C)
a Ti: initial curing tempetature, Tp: peak temperature, Tt: terminal temperature, β: heating rate.
CBTMS	5	285.6	370.3	406.1
	10	303.0	385.4	416.8
	15	313.3	394.6	430.0
	20	323.1	404.1	440.5
	30	334.7	412.0	449.9

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DSC curves of CBTMS at different heating rates of 5, 10, 15, 20, 30 °C min⁻¹ respectively were shown in Fig. 4. It can be seen that the curing peaks become sharper and the cure characteristic temperatures (T_i, T_p and T_t) shift to higher values with increasing heating rates. This can be attributed to the enhanced thermal effect and larger temperature difference at higher heating rates.³² As a result, the endothermic peak shifts to a higher temperature.

Fig. 4 DSC curves of CBTMS at different heating rates.

The characteristic temperatures (T_i, T_p and T_t) of CBTMS were plotted as the function of heating rates (shown in Fig. 5.). It is obvious that all the characteristic temperatures increase linearly with increasing heating rates. The characteristic temperatures at a heating rate of 0 °C min⁻¹, corresponding to the pre-curing temperatu re (T_i0), constant-curing temperature (T_p0) and post-curing temperature (T_t0) respectively, were obtained by extrapolation of the plots of characteristic temperatures as a function of heating rates (β). According to this method, the characteristic temperatures at a heating rate of 0 °C min⁻¹ were obtained and they were 281.4 °C (T_i0), 367.2 °C (T_p0) and 400 °C (T_p0) respectively. These parameters are very important for optimizing the thermal curing process of CBTMS.

Fig. 5 Linear relationship between the heating rates and characteristic temperatures of CBTMS.

The analysis of curing kinetic for CBTMS was performed with Kissinger's and Ozawa's methods.

According to Kissinger equation (eqn (1)), the activation energy E can be obtained from the slope of the plots of ln(β/T_p²) versus 1/T_p.

where β is the heating rate, T_p is the exothermic peak temperature, E_a is the apparent activation energy and R is the gas constant. Plot of ln(β/T_p²) against 1/T_p was shown in Fig. 6. The Pearson's linear correlation coefficient was calculated and given near the line. The activation energy E_a of CBTMS was calculated to be 146.73 kJ mol⁻¹ according to Kissinger's method, which was similar to that of cure reaction of phenylethynyl-terminated compound DPADPE (Table 2).³⁵ The higher activation energy of CBTMS compared with DADPE means that its cure reaction takes place more difficultly, implying that the steric factors from trimethylsilylethynyl moieties linked to the acetylenic groups depress the reactivity of triple bonds.

Fig. 6 Plots of ln(β/T_p²) as a function of 1/T_p for CBTMS to calculate E_a according to Kissinger's method.

Table 2 Apparent activation energy Ea for the cure reaction of various compounds

Structure	Ea (kJ mol^−1)
	152.0
	108.6
	146.7

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Ozawa's method assumes that the degree of conversion at peak temperatures for different heating rates is constant. According to Ozawa's equation:

where C is constant, plot of ln β against 1/T_p is prepared and shown as a straight line in Fig. 7. The activation energy of the curing reaction of CBTMS was calculated to be 150.05 kJ mol⁻¹. Very close E_a values were obtained through Kissinger's and Ozawa's methods, which verified the nonisothermal DSC studies of the curing kinetics of CBTMS.

Fig. 7 Plots of ln β as a function of 1/T_p for CBTMS to calculate E_a according to Ozawa's method.

Thermal stability of cured CBTMS

The thermal stability of cured CBTMS was characterized by TGA under nitrogen and air atmosphere at a heating rate of 20 °C min⁻¹, and the thermograms were shown in Fig. 8. The weight loss temperatures of 5% and 10% (T₅ and T₁₀) for cured CBTMS and the char yield at 1000 °C were summarized in Table 3. As shown in Fig. 8, thermoset exhibited excellent heat resistant property in nitrogen. When heating rate was 20 °C min⁻¹, there was no weight loss up to 510 °C. Then the weight loss process was very slow with resulting char yields of 92.0% at 1000 °C in nitrogen. In contrast to the TGA curve under nitrogen atmosphere, there was a weight gain from 400 °C and peak at 573 °C in the TGA curve under air atmosphere, which was attributed to oxidation of boron to B₂O₃ and of silicon to SiO₂ upon exposure to oxidizing atmospheres at elevated temperatures.³⁸ The CBTMS-400 began to lose weight when the temperature was above 573 °C under air atmosphere, indicating that decomposition reaction was predominant at elevated temperature. The char obtained from CBTMS-400 showed weight retention of 98.8% at 1000 °C in air. However, most acetylene-substituted aromatic thermosets are usually observed to undergo catastrophic weight losses in the 500–600 °C temperature range upon exposure to air. The outstanding thermal and oxidative stability are attributed to the incorporation of both boron and silicon units into the thermoset.

Fig. 8 TGA curves of CBTMS-400 under nitrogen and air.

Table 3 TGA data of CBTMS-400^a

CBTMS-400	Td5 (°C)	Td10 (°C)	Char yield at 1000 °C
a Td5: temperature for 5% weight loss; Td10: temperature for 10% weight loss.
In N2	669.5	>1000	92.0
In air	>1000	>1000	98.8

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XRD studies

CBTMS-400 was hard solid with amber color. The WAXD pattern of CBTMS-400 was performed in the region of 2θ = 10–80° at room temperature. As shown in Fig. 9, WAXD pattern for CBTMS-400 consists of a broad peak, which reveals its non-crystalline character.

Fig. 9 Wide angle X-ray diffraction patterns for (a) CBTMS-400 and (b) CBTMS-1000.

Preparation and characterization of ceramic

Ceramic residue (CBTMS-1000) was obtained by heating CBTMS-400 to 1000 °C in air at a heating rate of 5 °C min⁻¹. After pyrolysis, the thermo-oxidative stability of CBTMS-1000 in air and nitrogen were determined by TGA (Fig. S4†). We found that the ceramic possessed excellent oxidative stability. When heated in air to 1000 °C, there was only 0.33% weight loss was observed. In nitrogen, however, CBTMS-1000 showed a weight loss from 540 °C and char yield was 96.2%.

To further understand the chemical structure of the ceramic, FT-IR (Fig. S5†) and XRD (Fig. 9) were employed to analyze the chemical structure of the ceramic. According to previous studies, a carbon-based polymer with protective layer composed of boron and silicon could provide oxidation protection to 1000 °C.³⁹ The outermost carbon burns away under high-temperature oxidizing environments, while silicon and boron formed SiO₂ and B₂O₃, respectively. Liquid B₂O₃ melts above 450 °C⁴⁰ and can flow into cracks in the SiO₂ overlayer, sealing them and hindering oxygen access to the active oxidation sites on carbon. Therefore, combination silicon and boron oxide layers provide oxidation protection to 1000 °C.³⁹ Here we believe that the structures of B–H in carborane and Si–CH₃ in silane units of CBTMS-400 are completely oxidized into boron oxide and silicon oxide in the surface of the material during the ceramic process during the ceramic process, respectively. While boron oxide was very hygroscopic,⁴¹ the presence of moisture in the residue resulted in the formation of B–OH bonds. So two small, sharp peaks at 2θ = 15° and 28° were observed in XRD pattern, which was a typical feature of B(OH)₃.^42,43 In addition, absorptions at 1463 cm⁻¹ and 1195 cm⁻¹ in FT-IR spectrum can be attributed to B–O stretching and B–OH deformation vibrations, respectively.⁴³ Additionally, peak at 3240 cm⁻¹ assigned to –OH stretching mode was recorded.⁴⁴ Huang⁴⁵ has reported several carbon materials prepared by oxidative pyrolysis of carborane-incorporated poly(dimethylsilylene-ethynylenephenyleneethynylene) (CB-PSEPE), a similar polymer precursor as CBTMS, were mainly composed of carbon. According to Huang's reports, ceramic residues prepared by oxidative pyrolysis of CB-PSEPE at 1000 °C for 1 h under static air were carbon materials with silicon and boron elements confirmed by elemental analyses.⁴⁵ In this work, we are inclined to believe that the bulk composition of CBTMS-1000 is carbon material with silicon and boron elements too. In the XRD pattern of ceramic residues, two broad diffraction reflections located at 2θ = 25.8 and 42.3° (corresponding to (002) and (001) planes of graphite structure)^42,46,47 were presented. The peak at 797 cm⁻¹ was assigned to Si–C deformation vibrations. In addition, there was no sign of absorptions at 1120–1020 cm⁻¹ (ν Si–O) and 460–400 cm⁻¹ (δ Si–O–Si)⁴⁸ in the FT-IR spectrum confirming that CBTMS-1000 was not composed of silicon oxide. The ceramic we obtained showed absence of SiC or B₄C crystals based on the analysis of FT-IR and XRD spectra, which suggested 1000 °C may be insufficient for the formation of these crystalline structures.⁴⁹

Conclusion

A novel carborane-containing acetylenic monomer was synthesized and confirmed by ¹H-NMR, ¹³C-NMR, FT-IR spectroscopy and mass spectrometry. Because of the high steric hindrance of trimethylsilyl groups, the endothermic peak for CBTMS appeared at a higher temperature than traditional ethynyl-terminated compounds. Thermosetting mechanism of CBTMS was Diels–Alder reaction between the Ph–C≡C group and the C≡C group confirmed by FT-IR spectroscopy. The thermal curing kinetics studies show that cure reaction of CBTMS has an activation energy similar to that of phenylethynyl-terminated compounds. The excellent thermo-stability of thermoset derived from CBTMS was confirmed by TGA under air with 5% weight loss temperature above 1000 °C and a high char yield of 98.8% at 1000 °C, which was attributed to the incorporation of inorganic elements boron and silicon. In summary, trimethylsilylethynyl could be used as crosslinking groups for thermosetting polymers and thermosets derived from it exhibit excellent thermal stability. We believe CBTMS has potential utility of as matrix materials for advanced composites and precursors to shaped ceramics due to the oxidative stability, high ceramic yield.

Acknowledgements

We thank the National High Technology Research and Development Program (**863** Program) of China (No. 2015AA033802) and the National Science Foundation of China (21074017 and 51273029).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19410a

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