Preceramic polycarbosilane

SiC is an excellent nonoxide ceramic with high-temperature stability and suitable mechanical properties. Since silicon-containing polymers are generally used for preparing nonoxide ceramics, various polymeric precursors with different structures have been designed. Preceramic polycarbosilane (PCS), used for preparing commercial Nicalon fiber,... 

In conclusion, the lesson learned from the research carried out to date on the subject of polycarbosilanes is that the general rule that linear, noncrosslinked polymers are not suitable preceramic polymers applies here as well. Crosslinked network-type polymers are needed. Such structures can be generated in more than one way, but in the case of the polycarbosilanes they have, to date, been obtained mainly by thermolytic routes thermal treatment (with or without other chemical additives) in the case of the Yajima polycarbosilanes and the thermolysis of tetramethylsilane in the case of the Bayer process-derived polycarbosilane. 

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). 

The chemistry which is involved in the "graft" and "in situ" procedures and the structures of the hybrid polymers which are formed remain to be elucidated. However, there is no doubt that these procedures are useful ones. We have used them also to form new and useful hybrid preceramic polymers from the Yajima polycarbosilane which contains a plurality of [CH3Si(H)CH2l units [14]. 

The spectrum of silicon based polymers is enriched by high tech ceramics like silicon nitride and carbide, respectively. These materials are produced by pyrolysis of appropriate polymeric precursors such as polysilanes, polycarbosilanes and polysilazanes (preceramics). These synthetic ceramics display a certain analogy to silicates, having SiC, SiN, or Si(C,N) as structural subunits instead ofSiO. 

The above thermal analysis studies demonstrated the enhanced thermal stability of POSS materials, and suggested that there is potential to improve the flammability properties of polymers when compounded with these macromers. In a typical example of their application as flame retardants, a U.S. patent39 described the use of preceramic materials, namely, polycarbosilanes (PCS), polysilanes (PS), polysilsesquioxane (PSS) resins, and POSS (structures are shown in Figure 8.6) to improve the flammability properties of thermoplastic polymers such as, polypropylene and thermoplastic elastomers such as Kraton (polystyrene-polybutadiene-polystyrene, SBS) and Pebax (polyether block-polyamide copolymer). 

Keywords Chlorinated Polycarbosilanes / Hydrido-Polycarbosilanes / Dehydrogenation / Preceramic Materials / Ceramic Materials... 

Summary The stepwise synthesis of the polycarbosilanes (Cl2SiCH2CH2) (5) and (H2SiCH2CH2)n (6) are described. On addition of catalytical amounts of transition metal complexes to polymer 6 dehydrogenation occurs and a further crosslinked carbosilane (8) is obtained by formation of new silicon-silicon bonds. Pyrolysis of carbosilane 8 produces a black ceramic material, containing P-SiC together with carbon. The ceramic yield after pyrolysis of 8 is approximately four times the yield obtained when 6 is employed as the starting material. From polymeric 8 preceramic fibers are accessible subsequent pyrolysis yields ceramic fibers. Moreover, the carbosilane 8 can be utilized as a binder for ceramic powders. 

We here describe the synthesis of the polycarbosilane (H2SiCH2CH2) and its use as a preceramic compound for the preparation of silicon carbide in form of a powder or fibers. 

In the past few years, similar reactions using numerous dichlorosilane derivatives were investigated to yield preceramic polymers. Some routes use several different types of chlorosilanes as starting materials (e.g., Refs. 33-35) Analogous to the preparation of polysilanes, polycarbosilanes can also be synthesized using a similar route. Alkyl chloroalkyl chlorosilanes are used as starting materials for this purpose [2]. 

Using bifunctional organic molecules (e.g., 1,3-butadiene) and hydrosilanes, the silicon bridging units in the polymer can be varied (Scheme 15). Because of their low ceramic yield and the low thermal stability, polycarbosilanes containing longer hydrocarbon bridges are not suitable for preceramic... 

S. Aravindi, P. R., Novel Polysiloxane and Polycarbosilane Aerogels via Hydro-silylation of Preceramic Polymers. J. Mater. Chem. 2012,22, 7676-7680. 

As usual, there are some limitations in attachment of various organic groups to siloxane backbone. In some instances, direct hydrosilylation of an olefin derivative of the intended functional group can lead to serious side reactions it may give poor yield or may simply call for unusual and difficult to prepare intermediates. In any case, hydrosilylation of polymers with Si—H is a good source of preceramic polymeric materials (4,18). A variety of linear polysilanes and polycarbosilanes have been reported, but because they tend to depolymerize on heating and afford little ceramic jdeld, they are not useful as SiC precursors. To increase the... 

Bond coat materials consisted predominantly of allyl-hydrido-polycarbosilane (aHPCS, Starfire Corp.), a preceramic polymer. A portion of the preceramic polymer was partially pyrolysed to convert it from liquid form to solid. The partially pyrolysed material was blended... 

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