AbstractA preceramic polymer for SiBNC fiber, polyborosilazane, has been synthesized by one‐step condensation reaction of dichloromethylsilane, BCl3, and hexamethyldisilazane with high yield. The reaction mainly involves the condensation of SiCl and BCl with NSiMe3 followed by SiMe3Cl evaporation and dehydrogenation between NH and SiH. The resulted polymer is a soluble colorless transparent solid with melting point of 70°C and molecular weight of 10,800. The backbone of the polymer is mainly composed of SiNB‐bridge with some borazine rings. The polymer exhibits good processability and flexible polymer fibers with diameter of 15–20 μm were obtained by melt spinning. Pyrolysis of the as‐synthesized polymer to 1000°C under nitrogen atmosphere results in a ceramic yield of 63 wt %, and the obtained SiBNC ceramic remains fully amorphous up to 1700°C, and only small amount of poorly crystallized BN, Si3N4, and SiC phases were observed upon heating at 1850°C. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008
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