Dehydrocoupling of tris(hydridosilylethyl)boranes and cyanamide: a novel access to boron-containing polysilylcarbodiimides

The synthesis by a novel reaction pathway and the polymer-to-ceramic conversion of boron-containing polysilylcarbodiimides are reported. The title compounds are accessible by reaction of tris(hydridosilylethyl)boranes, B[C2H4Si(R)H2]3 (R=H, CH3; C2H4=CH2CH2, CHCH3) with different amounts of cyanamide, H2NCN, in tetrahydrofuran solution. Polymerization occurs by dehydrocoupling without the formation of solid by-products and without the use of catalysts. Overall compositions range from C6.5H20BNSi3 to C9H15BN6Si3 in the case of the polymers obtained from B(C2H4SiH3)3 and from C9.5H26BNSi3 to C12H21BN6Si3 in the case of the polymers obtained from B[C2H4Si(CH3)H2]3, depending on the B[C2H4Si(R)H2]3: H2NCN ratio used. Molecular structure and chemical composition were investigated by spectroscopic methods (solid state MAS-NMR, FT-IR) and elemental analysis. Thermolysis in an Ar atmosphere delivers SiBCN ceramics in 65–85% yield, depending on the elemental composition and the molecular structure, i.e. the cross-linking of the polymers.