Thermosetting copolymers, silicon carbide-based fiber and processes for producing same

A process for producing a thermosetting copolymer, comprising reacting a perhydropolysilazane or polyborosilazane, a thermoplastic silicon-containing polymer and metal compound, said perhydropolysilazane comprising a main repeating unit represented by the formula:

• ##STR17## and having a number average molecular weight of 100 to 50,000, said polyborosilazane having a main cross linkage represented by any one of the following formulae (i), (ii), (iii) or (iv):
• ##STR18## where R.sup.5 stands for a hydrogen atom, halogen atom, alkyl having 1 to 20 carbon atom, alkenyl, cycloalkenyl, aryl, alkoxy, alkylamino, hydroxy or amino, and R.sup.6 stands for a hydrogen atom, halogen atom and alkyl having 1 to 20 carbon atoms, and in the formula (iv), at least two of the three nitrogen atoms and three boron atoms are used for the cross linkage and the remaining atoms are bound to R.sup.5
• and having an atomic ratio of B/Si of 0.01 to 3 and a number average molecular weight of 200 to 50,000, said metal compound being represented by the formula MX.sub.n where M stands for at least one metal element of B, Al, Ti, Zr and Hf, X may be the same or different and stands for a hydrogen atom, halogen atom, hydroxide, carbonyl, or 1 to 20 carbon atoms-containing alkoxy, phenoxy, acetylacetoxy, alkyl, alkenyl, cycloalkyl, alkylamino or amino, and n is a valence of the metal element M, an atomic ratio of (total Si)/M of said perhydropolysilazane or polyborosilazane, thermoplastic silicon-containing polymer and metal compound being not more than 500.

A silicon carbide-based fiber comprising a thermosetting copolymer comprising a perhydropolysilazane or polyborosilazane block A and a thermoplastic silicon-containing polymer block B, said perhydropolysilazane block comprising a main repeating unit represented by the formula:

• ##STR19## and having a number average molecular weight of 100 to 50,000, said polyborosilazane block having a main cross linkage represented by any one of the following formulae (i), (ii), (iii), or (iv);
• ##STR20## where R.sup.1 stand for hydrogen atom, halogen atom, alkyl having 1 to 20 carbon atoms, alkenyl, cycloalkyl, aryl, alkoxy, alkylamino, hydroxy or amino, and R.sup.2 stands for a hydrogen atom, halogen atom and alkyl having 1 to 20 carbon atoms, and in the formula (iv), at least two of the three nitrogen atoms and three boron atoms are used for the cross linkage and the remaining atoms are bound to R.sup.1, and having an atomic ratio of B/Si of 0.01 to 3 and a number average molecular weight of 200 to 50,000, said fiber comprising silicon, nitrogen and carbon as essential components and oxygen, hydrogen and metal elements of Al, B, Ti, Zr and Hf as optional components;
• having atomic ratios of N/Si of 0.01 to 1, C/Si of 0.1 to 1.5, O/Si of not more than 0.3, M/Si of 0 to 0.5 where M stands for B, Al, Ti, Zr and hf, and H/Si of not more than 0.1;
• being substantially comprised of an amorphous phase of silicon, nitrogen and carbon, or an assembly or mixture of an amorphous phase of silicon, nitrogen and carbon and .beta.-SiC microcrystalline phases having a crystallite size of not more than 50nm;
• and having ratios of an X-ray small angle scattering intensity thereof to that of air at 1.degree. and 0.5.degree. of 1 to 20 respectively.