Benzocyclobutene resin with fluorene backbone: a novel thermosetting material with high thermostability and low dielectric constant

Yuanqiang Wang a, Jing Suna, Kaikai Jina, Jiajia Wanga, Chao Yuana, Jiawei Tonga, Shen Diao*ab, Fengkai Hea and Qiang Fang*a
aLaboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, PR China. E-mail: qiangfang@mail.sioc.ac.cn; Fax: +86 21 54925337; Tel: +86 21 54925337
bSchool of Chemistry and Chemical Engineering, Yantai University, Yantai, 264005, PR China. E-mail: diaoshen315@163.com

Received 20th June 2014 , Accepted 18th August 2014

First published on 18th August 2014


Abstract

A fluorene-based monomer (FB) with thermally cross-linkable benzocyclobutene groups is reported here. This monomer showed good solubility in the common organic solvents and had a low melting point (128 °C). When being treated at high temperature (>200 °C), the monomer was converted to a cross-linked network structure (PFB). TGA data exhibited that PFB had high thermostability with a 5% weight loss temperature of 437 °C and 372 °C in N2 and air, respectively. Moreover, PFB showed a char yield of 47.6% at 1000 °C in N2. With regard to the electrical properties, PFB indicated an average of dielectric constants of about 2.7 ranging from 0.15 MHz to 30 MHz. All these results suggest that FB could be used as the varnish for insulating enameled wire in the electrical industry, and as encapsulation resins in the microelectronics industry.


1. Introduction

Fluorene-based π-conjugated polymers and small molecules have been receiving much attention in the past decades because of their excellent optical properties and potential applications in the fields of information and biology.1–7 Among these polymers and small molecules, the 9,9-dialkylfluorene derivatives have been well investigated due to the fact that their properties can be conveniently controlled by converting the substituent groups at the 9th position of the fluorene molecules.8–12 Therefore, there have been a lot of reports regarding to the polymers or small molecules containing a 9,9-dialkyl-2,7-fluorene unit. These fluorene-containing derivatives have been utilized in organic light emitting diodes (OLEDs) and bioscience fields.13–20

In comparison with the above mentioned π-conjugated fluorene-based derivatives, however, the insulating fluorene-based derivatives have attracted less attention. It is noted that fluorene is derived from coal tar and its large output should be consumed. Accordingly, developing the new application of fluorene is very necessary.

Recently, we have paid much attention to develop the new materials with high thermostability and low dielectric constants, which could be used in microelectronic industry.21–24 On the basis of high thermostability and large free volume of fluorene moiety, the introduction of fluorene groups into the backbone of the organic materials may endow the materials with good heat resistance and high insulating capacity. On the other hand, benzocyclobutene-based derivatives have recognized as high performance materials.25–27 Thus, the combination of fluorene and benzocyclobutene in a molecule could obtain a new high performance material. This inspired us to design and synthesize a molecule with a fluorene backbone and cross-linkable benzocyclobutene groups. The chemical structure of the new molecule (FB) is shown in Scheme 1. This compound showed good solubility in common organic solvents and low melting point (128 °C). When treated at high temperature (>200 °C), FB converted to a cross-linked network structure (PFB), which exhibited high thermostability and low dielectric constant. This is a new example of low-k benzocyclobutene (BCB) resin with fluorene backbone.28 Here, we report the details.


image file: c4ra05898d-s1.tif
Scheme 1 Chemical structure of new monomer in this work.

2. Experimental

2.1 Materials

All starting chemicals were purchased from Aldrich and used as received. Benzocyclobutene-4-boronic acid was purchased from Chemtarget Technologies Co., Ltd, China, and purified by column chromatography on SiO2 using a mixture of ethyl acetate and n-hexane (40[thin space (1/6-em)]:[thin space (1/6-em)]1, v/v) as the eluent. The obtained compound showed a purity of 98.7% (HPLC). Dimethylsulfone (DMSO) was dried over CaH2 and distilled under reduced pressure before use.

2.2 Measurement

1H NMR and 13C NMR spectra were recorded on a Bruker 400 spectrometer. FT-IR spectra were run on a Nicolet spectrometer with KBr pellets. Differential scanning calorimetry (DSC) was determined with TA Instrument DSC (Q200) at a heating rate of 10 K min−1 under nitrogen flow with a rate of 50 mL min−1. The DSC measurement was performed for 3 times for each sample. For each test, the Tzero aluminum pans were used and the filled amount of the sample was 4–7 mg. Thermogravimetric analysis (TGA) was performed on a TG209F1 apparatus at a heating rate of 10 K min−1 in a nitrogen atmosphere or air with a flow rate of 30 mL min−1. The TGA measurement was performed for 3 times for each sample. For each experiment, platinum boats were used, and grinded powder-like samples with a filled amount of near 10 mg were employed.

The dielectric constant (k) of the cured samples was measured ranging from 0.15 MHz to 30 MHz on cured cylindric samples (average diameters were 10 mm and thicknesses were 2–3 mm, respectively) at room temperature using a 4294A Precision Impedance Analyzer (Agilent). Before each measurement the samples were thoroughly dried under vacuum.

2.3 Synthesis

Synthesis of 2,7-dibromo-9,9-bis(4-hydroxyphenyl)fluorene (1)29. A solution of 2,7-dibromofluoren-9-one (14.0 g, 41.0 mmol), phenol (23.8 g, 246 mmol), and methansulfonic acid (50 mL) was stirred at room temperature for overnight. Then the mixture was poured into water, and the obtained solid was filtered and washed with water. The crude 1 thus prepared. To purify 1, a solution of crude 1 in ethyl acetate was added drop wise to n-hexane to obtain a precipitate. The precipitate was filtrated and dried under reduced pressure to give pure 1 as a white powder with a yield of 90%. 1H NMR (400 MHz, acetone-d6, ppm) δ 8.33 (s, 2H), 7.85 (d, 2H), 7.56 (d, 4H), 7.02 (d, 4H), 6.76 (d, 4H). 13C NMR (100 MHz, acetone-d6, ppm) δ 156.53, 154.34, 137.99, 135.36, 130.68, 129.05, 128.98, 122.21, 121.29, 115.24, 64.46. EI-MS (m/z): M+ = 508 (100%).
Synthesis of 2,7-dibromo-9,9-bis(4-hexyloxyphenyl)fluorene (2)29. A mixture of 1 (10.16 g, 20.0 mmol), K2CO3 (6.22 g, 45.0 mmol), and DMSO (50 mL) was stirred at 75 °C for 0.5 h under argon atmosphere. To the obtained solution was added 1-bromohexane (6.00 g, 36.3 mmol) through syringe. The mixture was stirred at 75 °C for overnight. After being cooled to room temperature, the solution was diluted by ethyl acetate (100 mL). The separated organic phase was washed with water (3 × 60 mL) and dried over anhydrous Na2SO4. After removal of the solvent, the crude product was obtained, which was purified by a flash column chromatography on SiO2 using a mixture of petroleum ether and CH2Cl2 as the eluent (10[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v). Pure 2 was obtained as a colorless oil with a yield of 84%. 1H NMR (400 MHz, CDCl3, ppm) δ 7.56 (d, 2H), 7.45 (d, 4H), 7.04 (d, 4H), 6.76 (d, 4H), 3.90 (s, 4H), 1.73 (d, 4H), 1.37 (d, 12H), 0.91 (d, 6H). 13C NMR (100 MHz, CDCl3, ppm) δ 158.28, 153.81, 137.88, 136.28, 130.76, 129.32, 129.03, 121.86, 121.55, 114.39, 67.95, 64.44, 31.62, 29.28, 25.78, 22.66, 14.11. EI-MS (m/z): M+ = 676.1 (100%).
Synthesis of 2,7-dibenzocyclobutene-9,9-bis(4-hexyloxyphe-nyl)fluorene (FB). To a mixture of 2 (0.68 g, 1 mmol), benzocyclobutene-4-boronic acid (0.44 g, 3 mmol), tetrakis-(triphenylphosphine)palladium (Pd(PPh3)4) (0.0058 g, 0.05 mmol) and toluene (10 mL) was added aq. K2CO3 solution (5 mL, 2 M) under argon. The mixture was heated to 75 °C and maintained at the temperature for 12 h. After being cooled to room temperature, the mixture was diluted with ethyl acetate (30 mL), and filtered. The separated organic phase was washed with water (3 × 60 mL), and dried over anhydrous Na2SO4, filtered, and concentrated. The obtained residue was purified by flash column chromatography on SiO2 using a mixture solvent of petroleum ether and CH2Cl2 as the eluent (20[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v) to give FB as a white solid with a yield of 85% and a purity of 99.3% (HPLC). 1H NMR (400 MHz, acetone-d6, ppm) δ 7.94 (d, 2H), 7.62 (d, 4H), 7.42 (d, 2H), 7.27 (s, 2H), 7.21 (d, 4H), 7.09 (d, 2H), 6.81 (d, 4H), 3.91 (t, 4H), 3.17 (s, 8H), 1.75–1.63 (m, 4H), 1.45–1.27 (m, 12H), 0.87 (t, 6H). 13C NMR (100 MHz, CDCl3, ppm) δ 157.88, 152.74, 146.22, 144.88, 141.75, 140.58, 138.58, 137.97, 129.26, 126.64, 126.16, 125.04, 122.73, 121.59, 120.30, 114.13, 67.90, 64.36, 31.61, 29.49, 29.43, 29.29, 25.77, 22.64, 14.08. EI-MS (m/z): M+ = 722.4 (100%).

2.4 Sample preparation

FB (5.0 g) was placed in a flat-bottomed glass tube with a diameter of 10 mm and a highness of 85 mm under argon atmosphere. The tube was heated to 130 °C and kept at the temperature for 30 min so that a transparent melting liquid was obtained. The temperature was then elevated and kept at 150 °C for 1 h, 170 °C for 1 h, 210 °C for 5 h, 240 °C for 5 h, and 280 °C for 5 h respectively. Thus, a completely cured sample was obtained.

3. Results and discussion

3.1 Synthesis and characterization

The procedure for the synthesis of FB is shown in Scheme 2. As shown in Scheme 2, the monomer FB was prepared in a yield of 85% by using the standard Suzuki coupling reaction between 2,7-dibromo-9,9-bis(4-hexyloxyphenyl)fluorene (2) and benzo-cyclobutene-4-boronic acid. The obtained monomer showed a purity of 99.3% (HPLC), implying that it can meet the requirement of the polymerization.
image file: c4ra05898d-s2.tif
Scheme 2 Synthetic route of FB.

The chemical structure of FB was characterized by its 1H and 13C NMR spectra (Fig. 1 and 2). As can be seen from Fig. 1, the peaks at 6.81–7.94 ppm are attributed to the protons at the aromatic ring. The peak at 3.17 ppm is attributed to the hydrogen of –CH2 group at cyclobutene. The peaks at 3.91, 1.63–1.75, 1.27–1.45, and 0.87 ppm derive from the protons of alkyl chains (–C6H13). Moreover, as can be seen from 13C NMR spectrum (Fig. 2), the peaks of the carbon atoms in the aromatic ring appear at 157.88, 152.74, 146.22, 144.88, 141.75, 140.58, 138.58, 137.97, 129.26, 126.64, 126.16, 125.04, 122.73, 121.59, 120.30, 114.13 ppm. The peaks of the carbon atoms of cyclobutene locate at 29.49 and 29.43 ppm, respectively. For the peak appearing at 67.90 ppm is ascribed to the carbon atom at 9th position of the fluorene. The peaks at 29.43–29.49 ppm are attributed to the carbon atoms of cyclobutene. The signals of the carbon atoms in the alkyl chain (–C6H13) appear at 64.36, 31.61, 29.29, 25.77, 22.64, 14.08 ppm. Thus, all data are consistent with the proposed, exhibiting that the chemical structure of FB has been confirmed.


image file: c4ra05898d-f1.tif
Fig. 1 1H NMR spectrum of FB (400 MHz, acetone-d6).

image file: c4ra05898d-f2.tif
Fig. 2 13C NMR spectrum of FB (100 MHz, CDCl3).

The solubility of FB was also investigated. As depicted in Table 1, FB showed good solubility in common organic solvents.

Table 1 Solubility of FB in organic solventsa
Toluene DMF DMSO CH2Cl2 Acetone Ethyl acetate Ethanol CH3CN Cyclohexanone
a FB (20 mg) in organic solvent (1 g), + soluble at room temperature, ♀ soluble at heating.
+ + +


3.2 Curing behavior

Upon heating, the four-membered-ring on benzocyclobutene opens to produce a highly reactive o-quinodimethane intermediate.22,25–27,30–32 This active intermediate has a tendency to form a product with eight-membered-ring via self-coupling or produce a copolymer through Diels–Alder reaction.22,25–27,30–32 To well understand the thermally cross-linking (or curing) of FB, Scheme 3 describes the polymerization reaction.
image file: c4ra05898d-s3.tif
Scheme 3 Thermal polymerization reaction of FB.

The curing behavior of FB was characterized by DSC, and the results are shown in Fig. 3. As depicted in Fig. 3, the endothermic peak indicating the melting point of FB is observed at 128 °C. The curing onset temperature of FB is at about 204 °C and the curing peak temperature locates at 260 °C. The curing of FB is attributed to the ring opening and polymerization of benzocyclobutene groups in FB, similar to the curing behaviors of the benzocyclobutene containing compounds.22,25–27,30,31


image file: c4ra05898d-f3.tif
Fig. 3 DSC traces of FB and PFB at a heating rate of 10 K min−1.

DSC trace of cured FB (called as PFB) showed a smooth line (Fig. 3, red line), suggesting that FB has completely cured under the curing conditions, shown in the Experimental section. Moreover, there is no evidence of a glass transition in a range of temperatures from 50 to 350 °C, indicating that PFB has high thermostability.

The curing degree of FB was also monitored by DSC. When FB was heated at 204 °C for 1 h and 3 h, respectively, an obvious exothermal peak was observed, indicating that FB was partly cured at the onset temperature. Further elevating the temperature to the curing peak temperature (260 °C, see Fig. 3) resulted in weakening of the exothermal peak. After FB was heated to 280 °C and kept at the temperature for 5 h, the DSC trace exhibited that FB was fully cured (Fig. 3, red line).

The curing reaction of FB was also detected by FT-IR spectroscopy. Fig. 4 shows FT-IR spectra for FB and PFB. The absorption peaks at 922 and 877 cm−1 belonging to the in-plane ring stretching vibration of C–H in the four-membered ring of benzocyclobutene group disappear after the curing reaction, suggesting that FB has been fully converted to PFB.22,32


image file: c4ra05898d-f4.tif
Fig. 4 FT-IR spectra of FB and PFB.

3.3 Thermostability

The thermostability of PFB was evaluated by thermogravimetry (TGA). The results are shown in Fig. 5. As depicted in Fig. 5, PFB shows 5% weight loss temperature (T5) in N2 and air at 437 °C and 372 °C, respectively. Such results imply that PFB has high thermostability. Furthermore, PFB shows a residual weight (R) of 47.6% at 1000 °C in N2. This high thermostability is attributed to the rigid biphenyl structure of fluorene backbone.
image file: c4ra05898d-f5.tif
Fig. 5 TGA curves of PFB, measured in N2 and air with a heating rate of 10 K min−1. T5 is the temperature when weight loss is 5%. R is residual weight of the sample at 1000 °C in N2.

3.4 Dielectric properties

Fig. 6 shows the dielectric constant (k) of PFB depending on the frequencies. As can be seen from Fig. 6, PFB shows an average dielectric constant (k) of about 2.70 in a range of frequencies from 0.15 MHz to 30 MHz. Such a result is attributed to the large free volume of bulky fluorene. It is noted that the k value of PFB is also comparable to the values of organic low-k materials such as polyimides (3.1–3.4),33,34 SiLK resins (2.65)35 and polycyanate esters (2.61–3.12).36 The average dissipation factors of PFB were also measured, which was about 0.028 ranging the frequencies from 0.15 MHz to 30 MHz.
image file: c4ra05898d-f6.tif
Fig. 6 Dielectric constant (k) of PFB at different frequencies.

4. Conclusions

A new monomer (FB) consisting of fluorene and benzocyclobutene units was successfully synthesized. After being heated at a high temperature (>200 °C), FB converted to a cross-linked network structure (PFB), which showed an average dielectric constant (k) of about 2.70 in the range of frequencies from 0.15 MHz to 30 MHz. Such a low k value was comparable to the commercially polymeric low-k materials such as polyimides (PIs), SiLK resins, polycyanate esters and most of benzocyclobutane-containing polymers. PFB also showed high thermostability with a 5% weight loss temperature (T5) of 437 °C and 372 °C in N2 and air, respectively. Moreover, PFB had a residual weight (R) of 47.6% at 1000 °C under N2. All these results suggest that FB could be used as the varnish for insulating enameled wire in electrical industry, and as encapsulation resins in microelectronic industry.

Acknowledgements

Financial supports from Ministry of Science and Technology of China (2011ZX02703) and the Natural Science Foundation of China (NSFC, no. 21374131) are gratefully acknowledged.

Notes and references

  1. L. Xie, C. Yin, W. Lai, Q. Fan and W. Huang, Prog. Polym. Sci., 2012, 37, 1192 CrossRef CAS PubMed.
  2. A. D. Schluter, J. Polym. Sci., Part A: Polym. Chem., 2001, 39, 1533 CrossRef CAS PubMed.
  3. H. Zhong, H. Lai and Q. Fang, J. Phys. Chem. C, 2011, 115, 2423 CAS.
  4. J. Sun, H. Lai, H. Zhong and Q. Fang, Thin Solid Films, 2011, 519, 7772 CrossRef CAS PubMed.
  5. M. Stork, B. S. Gaylord, A. J. Heeger and G. C. Bazan, Adv. Mater., 2002, 14, 3613 CrossRef.
  6. M. Svensson, F. L. Zhang and S. C. Veenstra, Adv. Mater., 2003, 15, 988 CrossRef CAS PubMed.
  7. T. Cliferson, L. Aurore and B. Kevin, Macromolecules, 2011, 44, 4012 CrossRef PubMed.
  8. C. Huang, C. Tsai, C. Liu, T. Jen, N. Yang and S. Chen, Macromolecules, 2012, 45, 1281 CrossRef CAS.
  9. F. He, M. Yu, S. Wang, Y. Li and D. Zhu, Adv. Funct. Mater., 2006, 16, 91 CrossRef CAS PubMed.
  10. F. Feng, Y. Tang, F. He, M. Yu, X. Duan, S. Wang, Y. Li and D. Zhu, Adv. Mater., 2007, 19, 3490 CrossRef CAS PubMed.
  11. M. Stork, B. S. Gaylord, A. J. Heeger and G. C.Bazan, Adv Mater., 2002, 14, 361 CrossRef CAS.
  12. C. Xing, M. Yu, S. Wang, Z. Shi, Y. L. Li and D. Zhu, Macromol. Rapid Commun., 2007, 28, 241 CrossRef CAS PubMed.
  13. J. Pei, X. Liu and W. Huang, Macromolecules, 2003, 36, 323 CrossRef CAS.
  14. B. Liu, W. Yu and W. Huang, Macromolecules, 2002, 35, 4975 CrossRef CAS.
  15. S. Marco and J. Andres, J. Am. Chem. Soc., 2010, 132, 8372 CrossRef PubMed.
  16. H. Wang, Z. Li, P. Shao, J. Qin and Z. Huang, J. Phys. Chem. B, 2010, 114, 22 CrossRef CAS PubMed.
  17. X. Ouyang, Y. Huo, L. Lu, Z. Ge and W. Ji, Appl. Phys. A, 2011, 105, 891 CrossRef CAS.
  18. S. Wang, W. H. Janice and C. B. Guillermo, Org. Lett., 2005, 7, 1907 CrossRef CAS PubMed.
  19. C. Xing, Q. Xu, H. Tang, L. Liu and S. Wang, J. Am. Chem. Soc., 2009, 131, 13117 CrossRef CAS PubMed.
  20. F. Feng, F. He, L. An, S. Wang, Y. Li and D. Zhu, Adv. Mater., 2008, 20, 2959 CrossRef CAS PubMed.
  21. C. Yuan, K. Jin, K. Li, S. Diao, J. Tong and Q. Fang, Adv. Mater., 2013, 25, 4875 CrossRef CAS PubMed.
  22. F. He, C. Yuan, K. Li, S. Diao, K. Jin, J. Wang, J. Tong, J. Ma and Q. Fang, RSC Adv., 2013, 3, 23128 RSC.
  23. X. Chen, K. Li, S. Zheng and Q. Fang, RSC Adv., 2012, 2, 6504 RSC.
  24. J. Wu, H. Lai, S. Diao, K. Jin, C. Yuan and Q. Fang, Chin. J. Org. Chem., 2013, 33, 1042 CrossRef CAS.
  25. X. Zuo, R. Yu, S. Shi, Z. Feng, Z. Li, S. Yang and L. Fan, J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 6246 CrossRef CAS PubMed.
  26. J. Yang, S. Liu, F. Zhu, Y. Huang, B. Li and L. Zhang, J. Polym. Sci., Part A: Polym. Chem., 2011, 49, 381 CrossRef CAS PubMed.
  27. M. F. Farona, Prog. Polym. Sci., 1996, 21, 505 CrossRef CAS.
  28. A report regarding to benzocyclobutene resin with the fluorene groups as the side chain has been found in a patent see D. J. Brennan, J. E. White, D. M. Scheck, R. A. Kirchoff and C. Z. Holtz, US Pat. 5391650, 1995.
  29. R. Grisorio, G. Allegretta, P. Mastrorilli and G. P. Suranna, Macromolecules, 2011, 44, 7977 CrossRef CAS.
  30. R. J. Hohlfelder, D. A. Maidenberg, R. H. Dauskardt, Y. Wei and J. W. Hutchinson, J. Mater. Res., 2001, 16, 243 CrossRef CAS.
  31. F. Niklaus, R. J. Kumar, J. J. McMahon, J. Yu, J. Q. Lu, T. S. Cale and R. J. Gutmann, J. Electrochem. Soc., 2006, 153, 291 CrossRef PubMed.
  32. J. Yang, Y. Cheng, Y. Jin, D. Deng and F. Xiao, Eur. Polym. J., 2013, 49, 1642 CrossRef CAS PubMed.
  33. G. Maier, Prog. Polym. Sci., 2001, 26, 3 CrossRef CAS.
  34. W. Yasufumi, S. Yuji and A. Shinji, Polym. J., 2006, 38, 79 CrossRef.
  35. S. J. Martin, J. P. Godschalx, M. E. Mills, E. O. Shaffer and P. H. Townsend, Adv. Mater., 2000, 12, 1769 CrossRef CAS.
  36. T. Fang and D. A. Shimp, Prog. Polym. Sci., 1995, 20, 61 CrossRef CAS.

Footnote

These authors contributed equally.

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