Polycarbosilane polymers preparation

Many examples of inorganic polymers with metallocene-containing side-groups also exist. Most of the polymers prepared have been based on main chains of polyphosphazenes, polysilanes, polysiloxanes, and polycarbosilanes. These materials are surveyed in the following sections. 

Polymetallocarbosilanes. PolymetaHocarbosilanes having a number-average molecular weight of 700—100,000 can be prepared by reaction of polycarbosilane, /2 2 fx where R is H, or lower alkyl, with a tetraalkyl titanate, to give a mono-, di-, tri-, or tetrafunctional polymer... 

The cyclotrisilazane (R = Me) produced in reaction (14) is recycled at 650°C [by reaction with MeNHo) the reverse of reaction (14)] to increase the yield of processible polymer. Physicochemical characterization of this material shows it to have a softening point at 190°C and a C Si ratio of 1 1.18. Filaments 5-18 pm in diameter can be spun at 315°C. The precursor fiber is then rendered infusible by exposure to air and transformed into a ceramic fiber by heating to 1200°C under N2- The ceramic yield is on the order of 54% although, the composition of the resulting amorphous product is not reported. The approach used by Verbeek is quite similar to that employed by Yajima et al. (13) in the pyrolytic preparation of polycarbosilane and its transformation into SiC fibers. 

Other organosilicon polymer precursors for ceramics have either been prepared or improved by means of transition metal complex-catalyzed chemistry. For instance, the Nicalon silicon carbide-based ceramic fibers are fabricated from a polycarbosilane that is produced by thermal rearrangement of poly(dimethylsilylene) [18]. The CH3(H)SiCH2 group is the major constituent of this 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,... 

A hyperbranched polycarbosilane (hb-P22) was prepared by platinum-catalyzed polyhydrosilylation of methyldiethynylsilane 22 (Scheme 11) [43]. The tacky, highly soluble and stable polymer underwent thermo- and photo-induced cross-linking reactions through the peripheral ethynyl groups. 

Preparative organosilicon chemistry offers manifold possibilities for the synthesis of precursors for nonoxide ceramics (Scheme 18.1). The focus has been on the synthesis of polymers such as polysilanes A, polysilazanes B, polycarbosilanes... 

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]. 

SiC fibers were produced using polycarbosilanes by Yajima et al. in 1975 [1,2]. Besides SiC fibers, Si-Ti-C-O fibers prepared from a polytitanocarbosi-lane have been obtained by adding a titanium tetrabutoxide to polycarbosilane or polysilane [3]. SiC fibers (Nicalon) and Si-Ti-C-O fibers (Tyranno) are manufactured on an industrial scale. Colorless silicon oxynitride fibers and silicon nitride fibers [4] have been obtained by the nitridation of polycarbosilanes in the author s laboratory. Polymers used for ceramic precursor and the resulting ceramic fibers are listed in Table 1. 

Polycarbosilane is a general term for organosilicon polymers with -Si-C- bonds in the main chain. Many kinds of polycarbosilanes have been synthesized using various methods. Figure 1 illustrates some polycarbosilanes that have been prepared in the author s laboratory. 

By the way, although a dominant majority of papers concerning the formation of amorphous SiC layers describes appUcations of CVD or PVD techniques, there have also been some attempts to use the polymer route for preparing SiC films. Starting from solutions of various polysilanes or polycarbosilanes, frequently films are formed by spin-coating and pyrolyzed under inert atmosphere [215-218]. Of course, such a procedure does not form a part of this section SiC layers via gas phase reactions . However, in this connection it should be mentioned that polysilanes are also applied to form films via evaporation, not only with the aim to build amorphous and/or crystalline SiC films, but also to use special properties of the polysilane films themselves, i.e. without a subsequent pyrolysis of these films. Such amorphous films are characterized by non-linear optical effects [219, 220] and their properties may be controlled by the uniformity of the orientation of polysilane chains which is susceptible to epitaxial influences [221-223]. 

Some other inorganic polymers may be mentioned [8,10] polyoxothiazenes 18 (prepared by thermal condensation of sulfon imidates, R0(0)S(R)=NH) highly conducting polythiazyl (poly(sulfur nitride)) 19 (prepared from S2CI2 and NH3 via a cyclic alternating nitride tetramer (SN)4) polycarboranes and polycarbosilanes (e.g. (-CB oH oC-Si(CH3)2-0-) ) containing a /w-carborane moiety. 

Indeed, many Si-containing ceramic systems have been already prepared from polymers of different composition such as polysiloxanes (-Rj R2Si-0-) to give SiOC [18,19], polysilanes (-R]R2-Si-) [20,21], and polycarbosilanes (-R2Si-CH2-) ... 

This work describes recent advances in the preparation of hydrogen bond acidic polycarbosilanes and their application as chemical sensor coatings. We have prepared hydrogen bond acidic polymers based on polycarbosilanes with the goal of improving upon the chemical and thermal properties of this class of functionalized polymer (3-5). Results pertaining to the preparation of selected model compounds are also described. 

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