Boron-modified polysilylcarbodi-imides as precursors for Si–B–C–N ceramics: Synthesis, plastic-forming and high-temperature behavior

Abstract

The synthesis, by two different reaction pathways, of boron-modified polysilylcarbodi-imides of general type {B[C 2 H 4 Si(R)NCN] 3 } n (R 14=­singly bonded organic ligand) and the plastic-forming and the thermal behavior of these polymers are described. Compounds {B[C 2 H 4 Si(R)NCN] 3 } n [ 2a , R 14= 14H; 2b , R 14= 14CH 3 ; 2c, R 14= 14(NCN) 0.5 ] can be obtained by treatment of the vinyl-substituted polysilylcarbodi-imides [(H 2 C[bouble bond]CH)(R)SiNCN] n [ 1a , R 14= 14H; 1b , R 14= 14CH 3 ; 1c , R 14= 14(NCN) 0.5 ] with borane dimethylsulfide BH 3 ·S(CH 3 ) 2 . The polysilylcarbodi-imides 1a – 1c themselves are accessible via the reaction of vinyl-substituted chlorosilanes (H 2 C=CH)-­(R)SiCl 2 with cyanamide H 2 N-C≡N in the presence of pyridine or by a non-oxide sol–gel process of vinylated chlorosilanes and bis(trimethylsilyl)carbodi-imide, (H 3 C) 3 SiN[bouble bond]C[bouble bond]NSi(CH 3 ) 3 . In the second method for the synthesis of 2a – 2c , hydroboration of vinyl-substituted chlorosilanes (H 2 C[bouble bond]CH)(R)SiCl 2 with borane dimethylsulfide, borane trimethylamide or borane triethylamide to yield the tris[(chlorosilyl)ethyl]boranes B[C 2 H 4 Si(R)Cl 2 ] 3 ( 3a , R 14= 14H; 3b , R 14= 14CH 3 ; 3c , R 14= 14Cl) is followed by treatment of the as-obtained compounds with bis(trimethylsilyl)carbodi-imide, which results in the formation of the hydroborated polysilylcarbodi-imides 2a – 2c . The thermogravimetric behavior of the polymers 1a – 1c and 2a – 2c up to 2300°C is reported. It is shown that boron-modified polysilylcarbodi-imides are suitable precursors for the preparation of dense bulk ceramics. Therefore, the preparation of green bodies of the hydroborated polysilylcarbodi-imides 2a – 2c by plastic forming (PF) is described. A series of experiments points to the fact that the microstructure of the as-obtained ceramic monoliths obtained by subsequent thermolysis of the plastic-formed green bodies is strongly influenced by the conditions during plastic forming. © 1998 John Wiley & Sons, Ltd.