The present invention relates to novel halogenated polycarbosilanes and to processes for their preparation.
Polycarbosilanes are polymers having a skeletal structure formed of the elements carbon and silicon in which, in general, Si groups and hydrocarbon groups are alternatingly present. The skeletal structure of such polycarbosilanes is composed, for example, of repeating structural units corresponding to the formula ##STR1## wherein R.sup.0 represents, for example, a hydrocarbon substituent. According to known preparation processes, such polycarbosilanes are obtained by converting monosilanes such as, for example, tetramethylsilane, trimethylchlorosilane, dimethyldichlorosilane or methyltrichlorosilane into mixtures of different polycarbosilanes by thermal decomposition. Another known process for preparing such polycarbosilanes starts from polysilanes in which at least one of the two substituents on the silicon atom is a methyl group. These polysilanes are converted pyrolytically into the polycarbosilane at temperatures of 350.degree. to 450.degree. C., methylene groups, which are each inserted between adjacent Si atoms of the polysilane, being formed from a part of the methyl substituents during the thermal conversion, and a hydrogen atom remaining on the silicon atom. Such pyrolyses are radical processes.
U.S. Pat. No. 4,761,458 discloses converting such polycarbosilanes, which carry at least 0.1% by weight of SiH groups, into chlorinated or brominated polycarbosilanes by reaction with chlorinating or brominating reagents in a radical reaction. In this process, SiCl or SiBr groups are formed from the SiH groups. This patent employs, as starting materials for the halogenation reaction, ordinary polycarbosilanes of the above-described type which are known in the prior art and are substituted with lower alkyl groups. These were prepared by pyrolysis, for example, of polydimethylsilane (--(CH.sub.3).sub.2 Si--).sub.n.
Prepolymers which are composed of ceramic forming elements for the preparation of ceramic polymeric materials and in which readily detachable elements were in part replaced by elements detachable with difficulty such as fluorine or fully fluorinated hydrocarbon compounds, are disclosed in German published application No. DE 3,616,378. In this German application, hydrogen is named as a readily detachable element. The illustrative embodiment of this German application likewise starts from an ordinary polycarbosilane of the above-described type which is known in the prior art and which was prepared by pyrolysis of polydimethylsilane (--(CH.sub.3).sub.2 Si--).sub.n. Fluorine is introduced into this polycarbosilane by electrofluorination with tetraethylammonium fluoride or by direct fluorination (by radical means) with elemental fluorine. In this case, in addition to the conversion of SiH groups into SiF groups fluorine atoms are also introduced into the methyl substituents of the silicon atoms and into the methylene bridges of the Si--CH.sub.2 --Si skeleton of the polycarbosilane.
A number of disadvantages exist with regard to the product properties and the preparation process for these halogenated polycarbosilanes known in the prior art, i.e. halogenated polycarbosilanes prepared by radical halogenation methods from pyrolytically obtained polycarbosilane starting materials.
In part, the disadvantageous properties of the known halogenated polycarbosilanes are to be attributed to the unfavorable properties of the pyrolytically obtained unhalogenated polycarbosilanes to the extent that their use as starting materials essentially already predetermines the basic structure and the maximum achievable degree of purity of the halogenated products. Thus, for example, the ability to introduce halogen atoms into these polycarbosilanes depends directly on the SiH groups present in these polycarbosilanes since these SiH groups are converted into the SiHal groups (Hal=halogen) in the known processes for preparing halogenated polycarbosilanes. In this connection, however, it is disadvantageous that the formation of the SiH groups in the pyrolytic preparation of the polycarbosilane starting material can be controlled only with difficulty and this fact consequently directly affects also the properties of the halogenated polycarbosilanes prepared therefrom.
Furthermore, the preparation of the known halogenated polycarbosilanes under radical halogenation conditions also entails disadvantages. Thus, the degree and position of halogenation can be controlled with difficulty. Although the SiH groups first react preferentially to form SiHal groups, side reactions come about to a substantial extent, in particular with long reaction times and under slightly more severe reaction conditions. Thus, in addition to the desired halogenation in the SiH groups, halogenation reactions also occur in the hydrocarbon substituents (for example, formation of CH.sub.2 F--, CHF.sub.2 -- and CF.sub.3 -- from methyl substituents) or in the methylene bridges (for example, formation of Si--CHF--Si or Si--CF.sub.2 --Si) of the polycarbosilane employed. In addition, the radical reaction conditions may also result in cleavage reactions in the Si--C--Si skeleton of the polycarbosilane, as a result of which the polycarbosilane employed and/or the halogenated polycarbosilane is partially decomposed into undesirable fragments or even into more or less readily volatile, low molecular weight compounds.
On the one hand, the halogenated polycarbosilanes known in the prior art consequently are nonuniform products which have irregular Si--C skeletons just like the basic unhalogenated polycarbosilanes and which are accompanied by more or less readily volatile decomposition products due to the manner in which they are prepared. On the other hand, further measures for the purpose of restriction to a product spectrum more favorable for an intended use (for example, purification and/or separation by fractional crystallization or fractional distillation) are labor- and energy-intensive and consequently are also costly.