Synthesis and pyrolysis of oligo(methylsilylene)-ethynylene polymer to near-stoichiometric SiC ceram

Available online at www.sciencedirect.comCHINESEScienceDirectC HEMICALL .ETTERSELSEVIERChinese Chemical Letters 21 (2010) 1299- 1302www.elsevier.com/locate/ccletSynthesis and pyrolysis of oligo(methylsilylene )-ethynylenepolymer to near-stoichiometric SiC ceramicWei Jian Hana,b, Ji Dong Hu", Li Yea,b, Song Shir, Xue Yu Tao a,b, Tong Zhao a,*a Laboratory of Advanced Polymer Materials, Center for Molecular Science, Institute of Chemistry; Chinese Academy of Sciences,Beijing 100190, Chinab Graduate University of the Chinese Academy of Sciences, Beijing 100190, China。National Key Laboratory of Advanced Functional Composite Materials, Aerospace Research Institute of Materials & Processing Technology,Beijing 1076, ChinaReceived 15 March 2010AbstractOligo(methylsilylene )-ethynylene polymer with crosslinkable ethynyl unit in the main chain was synthesized by condensationof a,w-dicholorooligo(methylsilane) with dilithioacetylene. The as- synthesized polymer was characterized by GPC, FT-IR andNMR. The results showed ethynyl group was succesfully introduced to the polymer main chain. The ceramic yield wasdramatically increased by the cross-polymerization of ethynyl group, and the excess silicon of PMS was compensated. Near-stoichiometric (C/Si = 1.17) and partial crystallization β- SiC monoliths were obtained by pressureless pyrolysing PSPA at 1200 °Cin a high yield (73.6 wt%).C 2010 Tong Zhao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.Keywords: Polymethylsilane; Silicon carbide; Precursor; CrosslinkableSince the synthesis of polymethylsilane (PMS) by Schilling and Williams in the mid- 1980s, [1] many works wereconducted with use of it as a precursor for silicon carbide (SiC) precursors [2-5]. The major interest to PMS is itspotential for the fabrication of SiC with reduced amount of excess carbon. However, PMS suffers from a low ceramicyield of 20- -60 wt% according to the cross-linking density [1]. Introduction of partially cross-linked structure intoPMS was explored for enhancing the ceramic yield. Group 4 metallocene compounds and boron compounds werereported to improve the ceramic yields at the same time increased the viscosity of PMS [2- -4]. The presence ofpotentially reactive functionalities such as vinylene and ethynylene is another route that balances the processabilityand ceramic yield [6]. Vinylic poly(methylsilanes) (VPS) was prepared by dehalocoupling of MexSiCl4- -x andMe(CH-=CH)SiCl2, with ceramic yield of 17-44 wt% [2]. The ethynyl derivatives may be more specifically suit forhigh yield SiC precursor with sufficient cross-linking density. But ethylnyl modified PMS cannot be prepared by thetradition dehalocoupling reaction or substitution reaction, due to the lack of commercial available ethynyl containingchlorosilane, and the alkynylation (such as, -C=CH, C=C(CH2)2CH3) substitution of side groups of PMS usually* Corresponding author.E-mail address: tzhao@ iccas.ac.cn (T. Zhao).中国煤化工1001-8417/$ - see front matter◎2010 Tong Zhao. Published by Elsevier B.V. on beh;MHCNMH G,M_tyl rights reserved.doi: 10.1016/.cclet.2010.06.0081300W.J. Han et al./ Chinese Chemical Ltters 21 (2010) 1299- 1302given a low oxidation potential results, which is related to the highly delocalized σ backbone of polysilanes [7]. In thispaper, a novel ethynyl-modified PMS (PSPA) was synthesized for the first time by the condensation reaction of Q,w-dichlorooligo(methylsilane) and dilithioacetylene.1. ExperimentalAll experiments were carried out under a nitrogen atmosphere. Solvents were distilled from sodium/benzophenoneprior to use. All other chemicals were purchased from Aldrich or Acros Co. and used as received. 'H, 13C, and 29Sinuclear magnetic resonance spectra (NMR) were obtained on a Bruker DMS 300 MHz spectrometer using CDCl3 assolvent. Fourier transform infrared spectra (FT-IR) were recorded on a Bruker 27 spectrometer. The molecular weightof the polymer samples were determined by gel permeation chromatography (GPC) with tetrahydrofuran as the eluentat 35。C. Sample detection was taken with a Waters 24 10 refractive index detector. Thermogravimetric analysis (TGA)was carried out on a Netzsch STA 409PC instrument in nitrogen atmosphere with a ramping rate of 10。C/min fromroom temperature (RT) to 1000。C. X-ray powder diffraction (XRD) measurements were obtained using Cu Karadiation (40 kV, 200 mA, λ = 0.154056) with a Rigaku D/MAX 2400 diffractometer. Silicon content was determinedby optical emission spectrometry with inductively coupled plasma excitation (OES-ICP, Thermo iCAP 6300). Carbonand oxygen contents were analyzed by carrier gas heat extraction (CGHE, LECO C, S-analyzer 244 and N,O-determinator TC 600).The PSPA precursor as a new acetylene containing PMS was synthesized by a three-step reaction as shown inScheme 1. Firstly, a,w-dichlorooligo(methylsilane) 1 was prepared by the Wurtz-type reductive dehalogenationreaction of excess MeHSiCl2 and sodium [1]. Dilithioacetylene 2 was prepared by the reaction of n-BuLi andtrichloroethylene. Then PSPA precursor was synthesized by the condensation reaction of 1 and 2.a,w-Dichlorooligo(methylsilane) 1: dry toluene (80 mL) and sodium metal (0.32 mol, 7.36g) were added to a500 ml three neck fask, and heated to 110 °C, then stirred rapidly for 10 min. The system was cooled down to 90 °C,and MeHSiCl2 (0.20 mol, 20.81 mL) was added dropwise over 40 min, then the solution turned blue and furtherrefuxed for 4 h.Dilithioacetylene (LiC=CLi) 2 was prepared by the procedure described in the literature [8PSPA precursor: 0.04 mol 2 was added drop wise to 1 in 30 min at - 10 °C. After addition, the mixture wasgradually warmed to room temperature and further reacted for 12 h. The obtained slurry was filtered under nitrogenatmosphere, and an orange solution was obtained. After removing the solvent under vacuum, 5.2 g red viscous liquidpolymer was obtained with a yield of 59%. FT-IR (KBr, cm~ ): v 3286 (C=C- H), 2954, 2889 (- _CH3), 2168, 2082 (-Si- H), 2107 (C=C), 1411 (C-H), 1253 (- -Si- CH3), 1086 (Si- _O- Si), 933 (Si-H), 868 (C-H), 798, 687 (Si- _C). HNMR (300 MHz, CDCl3): 83.444.35 (m, 0.68H), 0.43 (s, 1.14H), 0.28 (s, 1.86H). C NMR (75.47 MHz, CDCl3): 80(- Si- -CH3), 110(-C=C-). 29si NMR (59.63 MHz, CDCl3):δ - 61 (SiC(H)- C= EC-), -63 (SiC(H)Si2). The weightaverage molecular and polydispersity index determined by GPC were 1668 and 2.44. PSPA is air sensitive, but it is notpyrophoric like PMS. Addition of 0.5 wt% of 2,6-di-tert-butyl-4-methylphenol (BHT) to PSPA inhibited its airoxidation for weeks [3].PMS was prepared by the procedure described in the literature [1].HsHs CHH3 CHy-ctoluene, 90°CHtf-+.mCILi一-c= =c一 -LiTHF, -78C中国煤化工MYHCNMH G .Scheme 1. Reaction sequence of PSPA.W.J. Han et al./Chinese Chemical Letters 21 (2010) 1299- 130213012. Results and discussionThe chemical structure of PSPA was confirmed by FT-IR and NMR spectroscopy. The function groups of theprecursor were identified by FT-IR. The Si-C stretching at 687 cm-' and 798 cm , and the Si-CH3 bending at1253 cm~ ' indicate the Si- C backbone. The vibration adsorption bands at 2168 cm 1 and 2082 cm“1 are assigned toSi-H group [3]. The strong vibration band of C=C at 2107 cm-1 and weak vibration band at 3286 cm-' ofC= EC-H[9], confirm the ethynyl group was introduced into polymer main chain. The weak peak assigned to Si- 0 bonds(1040 cm ~) is observed in polymer, which was due to the oxidation of Si-H bonds in the polymer, during itsmanipulation and measurement [10].The 'H NMR spectra of PSPA and PMS (Fig. 1) showed complex multiplets and broadened peaks, indicating highlybranched structure. The broaden peak at δ 0.28 is assigned to methyl group in segment3 [3]. The peak at δ 0.43 isconsistent with the methyl group in segment 4, and confirmed the ethynyl group [11]. The ratio of 3 and 4 wascalculated by the integration results. The two peaks of δ 3.44 and 4.35 are consistent with the protons attach to thesilicon atom in 3 and 4 [3]. Two weak peaks at δ 4.01 and 4.72 are assigned to -C=CH and Si-OH, which wereintroduced by the hydrolysis reactionof -C=C-Li or - Si- Cl terminated polymer. The multiplets weak peaks at 82 areconsistent to the methylene protons of - -SiOCH2CH2CH2CH2- segments derived from the side reaction of MeHSiCl2and THF solvent [12]. The integrations of 'H NMR were available to calculate the approximate formula[(MeHSi)o.68(MeSi0.32(C= EC)o.19]m. The 13C NMR spectrum of PSPA showed characteristic signals of- Si- CH3 groupat δ 0 and -Me(H)Si-C三C-Me(H)Si- group at 8110 [9]. In the 29si NMR spectrum of PSPA, there were two broadpeaks, indicating the complex chemical environments of silicon. The signals around δ - 61 are assigned to - SiC(H)-C=C, and the peaks around 8 -63 are due to the absorption of -SiC(H)Si- [11,13]. The data from the 'C and 29siNMR spectrum are consistent with the assignments from the H NMR, and also prove the existence of ethynyl group inthe polymer main chain.The polymer-to-ceramic conversion of PSPA was monitored by thermogravimetric analysis (Fig. 2). The residue ofas-synthesized PSPA was 73.6 wt%, and after cured at 250 °C, the residue increased to 77.8 wt%. Comparing with theceramic yield of PMS (29.4%), the ceramic yield of PSPA was effectively increased by the introduction of ethynylgroup. X-ray powder diffraction revealed a set of broad peaks at 36° (1 1 1), 60° (220) and 72° (3 1 1), which areattributed to B-SiC. Analysis of ceramic derived from PSPA has C, 31.2 wt%; Si, 61.6 wt%; and O, 4.6 wt%, with theempirical formula of Sin 0oC1.17Oo.13 that is close to the near-stoichiometric. On the contract, the ceramic derived fromabd34，cPSPA___儿a,bPMS86中国煤化工ppmYHCNMH GFig. 1. 'H NMR spectra of PSPA and PMS (CDCl3).1302W.J. Han et al./Chinese Chemical Letters 21 (2010) 1299- -1302100-o-60-40-PMS20-0一2004060801000Temperature (°C)Fig. 2. TGA curves of PSPA and PMS.PMS has C, 22.9 wt%; Si, 73.0 wt%; and O, 4.1 wt% with the empirical formula of Si 00Co.73Oo.10. These resultsindicate the carbon content of ceramic derived from PSPA was increased by the introduction of ethynyl group.In this paper, oligo(methylsilylene )-ethynylene polymer with crosslinkable ethynyl unit in the main chain wassynthesized by condensation of a ,c-dicholorooligo(methylsilane) with dilithioacetylene. Near-stoichiometric (1.17)SiC ceramic was obtained by pyrolyzed PSPA in a satisfactory ceramic yield (73.6 wt%).AcknowledgmentsThe authors gratefully acknowledge the financial support of the National Science Foundation of China (No.50903086) and the Chinese Academy of Sciences.References[1] C.L. Schilling, TC. Williams, Abstr. Paper Am. Chem. Soc. 187 (1984) 1.[2] R.D. Miller, J. Michl, Chem. Rev. 89 (1989) 1359.[3] D. Seyferth, T. Wood, H. Tracy, J. Am. Ceram. Soc. 75 (1992) 1300.[4] Z.F Zhang, C.S. Scotto, R.M. Laine, J. Mater. Chem.8 (1998) 2715.[5] T Iseki, M. Narisawa, Y. Katase, Chem. Mater. 13 (200) 4163.[6] R. Corriu, P. Gerbier, C. 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