Organosilicon preceramic polymers

We have described new routes to useful preceramic organosilicon polymers and have demonstrated that their design is an exercise in functional group chemistry. Furthermore, we have shown that an organosilicon polymer which seemed quite unpromising as far as application is concerned could, through further chemistry, be incorporated into new polymers whose properties in terms of ceramic yield and elemental composition were quite acceptable for use as precursors for ceramic materials. It is obvious that the chemist can make a significant impact on this area of ceramics. However, it should be stressed that the useful applications of this chemistry can only be developed by close collaboration between the chemist and the ceramist. 

This chapter gives an introduction to the preceramic polymer route to ceramic materials and focuses on the reasons why this new approach was needed and on the chemical considerations important in its implementation, with examples from research on organosilicon polymers. Novel polysilazanes have been prepared by the dehydro-cyclodimerization reaction, a new method for polymerizing suitably substituted cyclooligosilazanes. The living polymer intermediate in this reaction has been used to convert Si-H-containing organosilicon polymers that are not suitable for pyrolytic conversion to ceramics into useful preceramic polymers. 

The first useful organosilicon preceramic polymer, a silicon carbide fiber precursor, was developed by S. Yajima and his coworkers at Tohoku University in Japan [5]. As might be expected on the basis of the 2 C/l Si ratio of the (CH3)2SiCl2 starting material used in this process, the ceramic fibers contain free carbon as well as silicon carbide. A typical analysis [5] showed a composition 1 SiC/0.78 C/0.22 Si02- (The latter is introduced in the oxidative cure step of the polycarbosilane fiber). 

Summary Catalytic crosslinking processes of organosilicon preceramic polymers using transition metal complexes as well as stoichiometric reactions between such polymers and transition metal complexes and powders that lead to new ceramic phases are reviewed. 

The first useful organosilicon preceramic polymer, a silicon carbide fiber precursor, was developed by Yajima and his co-workers at Tohoku University... 

Polymeric organosilicon compounds consist of two kinds of compounds, one of which has a silicon atom in the side chain and the other having a silicon atom in the principal chain as shown in Table 8.6. In these compounds, polysilylmethylene, polysilylphenylene, polysilazane and polymetalosiloxane (e.g.. Si—O—Ti—O) are used as preceramic polymers of SiC, Si3N4 and Si—C—Ti—O [42]. 

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