The precursor polymer conversion method is based on the concept of molecular structure design. The molecular precursor composed of the target ceramic elements is synthesized by the monomer containing silicon or boron. After polymerization, the precursor polymer with good spinnability and high ceramic yield is obtained. Then the organic fiber is prepared by spinning, and then it is crosslinked into a three-dimensional network structure "thermosetting" fiber by chemical reaction Finally, the target ceramic fibers were obtained by high temperature cracking and high temperature ceramic treatment. The structure design, synthesis and performance control of precursor polymer are the most important in the precursor polymer conversion process. For the synthesis of high performance ceramic fiber, the ideal precursor polymer should meet the following requirements:

1) there are few non target elements in the composition, the product is pure and the ceramic conversion rate is high.

2) the polymer has stable structure or can be transformed into stable structure before thermal decomposition.

3) there are active groups in the molecular structure of the polymer, and the stable structure or cross-linking structure can be obtained through the reaction.

By borohydride of vinylsilalkane with BH3 · s (CH3) 2, a single source precursor containing si-c-b connection was synthesized, and then polymerized with methylamine to form a si-c-b connection ceramic fiber precursor. The sibn (c) fiber can be prepared by spinning the PBZ in the melt spinning equipment made in the laboratory, then hot crosslinking the precursor in the mixed atmosphere of nitrogen and ammonia, and finally ceramic in nitrogen at 1400 ℃, the diameter of the fiber can reach 8-10 μ m, and the tensile strength is 1.3-1.5gpa.