BORON-CONTAINING POLYSILYLCARBODI-IMIDES - A NEW CLASS OF MOLECULAR PRECURSORS FOR SI-B-C-N CERAMICS

The synthesis of boron-containing polysilylcarbodi-imides of general t ype {B[C2H4(R)SiNCN](3)}(n) [6a R=CH3, 6b R=H, 6c R=(NCN)(0.5)] by dif ferent reactions and their thermal behaviour are discussed. The title compounds 6a-6c can be obtained by a hydroboration reaction of BH3S(C H3)(2)(5) with the vinyl-substituted polysilylcarbodi-imides [(H2C=CH) (R)SiNCN](n) [3a R=CH3, 3b R=H, 3c R=(NCN)(0.5)], which themselves are accessible from the reaction of the vinyl-substituted chlorosilanes ( H2C=CH)(R)SiCl2 (1a R=CH3, 1b R=H, 1c R=Cl) with stoichiometric amount s of H2N-C-N (2). Furthermore, a method for the synthesis of compounds 6a-6c is given by treatment of compounds 1a-1c with (H3C)(3)SiN=C=NSi (CH3)(3) (4). An alternative reaction sequence, which finally leads to compounds 6a-6c is the hydroboration of the monomeric vinyl-substitut ed chlorosilanes 1a-1c with BH3S(CH3)(2) (5) to yield the chlorosilyl ethylboranes B[C2H4(R)SiCl2](3) (7a R=CH3, 7b R=H, 7c R=Cl) at first. Treatment of compounds 7a-7c with (H3C)(3)SiN=C=NSi(CH3)(3) (4), even with or without solvent, stoichiometrically or with excess 4, produces the hydroborated polysilylcarbodi-imides 6a-6c. The thermogravimetric behaviour of the polysilycarbodi-imides 3a-3c as well as the boron-co ntaining polymers 6a-6c have been determined by simultaneous TGA (20-1 100 degrees C, argon, heating rate 2K min(-1)). The ceramic yields of the polymers are in the range of 53-73%, depending on the structure an d the composition of the polymers applied. (C) 1997 Elsevier Science S .A.