The catalytic synthesis of polycarbosilane in the presence of boron tributoxide

Abstract

Synthesis of substantially polycrystalline silicon carbide fibre, which has excellent mechanical properties and heat resistance, has been reported. Such a silicon carbide fibre can be prepared from preceramic polymers [1, 2]. Usually, the synthesis of precursor polycarbosilane (PC) was at high temperature and under high pressure. Then, in the synthesis process, polyborodiphenylsiloxane (PBSO) was used as a reaction catalyst [3-5], which is prepared from the slow, complex reaction between boric acid and diphenyldichlorosilane. In our experiment, polycarbosilane precursor containing titanium was obtained by the reaction between titanium tetrabutoxide [Ti(OBu)4] and liquid polysilane (LPS), which was derived from destructive distillation of polysilane (PS) in nitrogen gas [6]. Therefore, a similar reaction between boron tributoxide [B(OBu)3 ] and LPS was carried out in order to prepare polycarbosilane containing boron. The experimental results show that it seems to be difficult to introduce B(OBu)3 into the molecular structure of polycarbosilane, but the increase of molecular weight of polycarbosilane is accelerated remarkably in the presence of B(OBu)3. Evidently, the troublesome preparation process of PBSO can be eliminated by replacement of PBSO with B(OBu)3 as a catalyst. In the present work, the synthesis of polycarbosilane from LPS and B(OBu)3 according to different weight ratios is reported. The effects of B(OBu)3 on the structure and properties of polycarbosilane products are discussed. B(OBu)3 and LPS derived from destructive distillation in nitrogen gas up to 420 °C were mixed in certain weight ratios in a three-necked flask equipped with a quartz reactor tube, a recycling system and a thermometer. The mixtures were heated slowly in a nitrogen gas flow to a given temperature and the reaction proceeded at this temperature for 1 h, then the products were dissolved in toluene. After filtering and distillation, PC as a yellow solid was obtained. The infrared (IR) spectra of products were measured using a Hitachi 270-30 infrared spectrophotometer. The solid samples were measured by the KBr pellet method and the samples obtained at different t e m p e ~ e s during the reaction process were measured with a fixed-thickness (0.2 ram) cell of KBr in CC14 solution. The munber average molecular weights (Mn) and molecular weight distributions were measured using a Watters-244 liquid chromatograph with Ultrastyragel as the gel permeation chromatography (GPC) column in toluene solution. The melting points (Tm) were obtained with a melting point apparatus (model HMK, Germany). LPS can be regarded as a mixture of cyclic and linear low molecular weight polysilane, and its structure is assumed to be [6]: