Synthesis polysilazanes

Synthesis. Polysilazanes, [-SiRR NR"-] , can be prepared by a variety of techniques. A typical preparative reaction is the ammonolysis of methyldichlorosilane (10) ... 

West (p. 6), Miller (p. 43), Zeigler (10), and Sawan (p. 112) outline the synthesis of a wide variety of soluble, processable polydiorganosilanes, a class of polymers which not long ago was thought to be intractable. Matyjaszewski (p. 78) has found significant improvements in the synthetic method for polydiorganosilane synthesis as well as new synthetic routes to unusual substituted polydiorganosilanes. Seyferth (p. 21, 143) reports synthetic routes to a number of new polycarbosilanes and polysilazanes which may be used as precursors to ceramic materials. 

Organometallic polymer precursors offer the potential to manufacture shaped forms of advanced ceramic materials using low temperature processing. Polysilazanes, compounds containing Si-N bonds in the polymer backbone, can be used as precursors to silicon nitride containing ceramic materials. This chapter provides an overview of the general synthetic approaches to polysilazanes with particular emphasis on the synthesis of preceramic polysilazanes. 

With the exception of reaction (3), the utility of all of the above reactions for the synthesis of oligo- and polysilazanes has been examined in varying detail. In the following paragraphs, we attempt to examine the pertinent studies in each area especially as it relates to the synthesis of preceramic polymers. 

The simplicity of the ammonolysls/amlnolysis of dlhalohalosHanes, reactions (6) and (7), made them the original method of choice for the synthesis of oligo- and polysilazanes, especially because of the analogy to the hydrolytic synthesis of polysiloxanes. The ammonolysls of dihalosilanes, reaction (6), has been found to be extremely sensitive to steric factors (6). 

Catalytic dehydrocoupling, as shown in reaction (3), was pioneered by Fink (6-10) and later studied by Andrianov et al (43). Although Fink described the synthesis of the first polysilazanes, (9) reaction (23), this route to oligo- and polysilazanes remained unexplored... 

The following reactions were performed to demonstrate the utility of transition metal dehydrocoupling for the synthesis of different types of polysilazanes and a novel polysiloxazane ... 

We have also explored the use of the dehydrocoupling reaction for the synthesis of preceramlc polysilazanes starting from the precursor MeNH-[H2SlNMe]x-H. As described above, MeNH-[H2SlNMe]X H Is produced by aminolysis of H2SiCl2 with MeNH2 under conditions where x 18-20 (Mn - 1150). 

Introductory Remarks. In contrast with the popularity and usefulness of the polysiloxane chains, which constitute the structural backbone of silicones, the knowledge of polymers based on the silazane unit is still limited. In the sixties there was still some hope of the possibility of producing long chain polysilazane molecules and a number of laboratories were active in seeking convenient methods for their synthesis (e.g. see review by Aylett (12)). 

Figure 4. shows the route from the high boiling residue of the direct synthesis to silicon carbo-nitride fibers. Methylchlorodisilanes and trichlorosilanes as additives are mixed in a specific ratio and react with methylamine and a small amount of ammonia to form an aminodisilane/oligosilazane. The subsequent polycondensation reaction of this mixture by heating to 250 °C yields a soluble and melt spinnable polysilazane. In comparision with the polysilane the properties of the polysilazane depend on the ratios of the disilanes/silanes and methylamine/ammonia and also on the reaction conditions. 

Although Si—N—B—O materials containing trace amounts of nitrogen (2-3 wt%) have been prepared solely via a processing approach (ammonolysis of a borosilicate gel), complete conversion to Si—N—B was not achieved86. However, there are several literature examples of routes to Si—N—B materials that couple synthesis with a specific processing approach. These include early work on Si—N—B precursors by Seyferth and Plenio, who reacted the now ubiquitous — [MeHSiNH] — polysilazane with H3B SMe287 as shown in equation 26. 

A modified procedure for the synthesis of compounds 1-3 is the hydroboration of vinyl-substituted polysilazanes with borane dimethylsulfide. 

A further method for the synthesis of the title compounds with only hydrogen as byproduct is the base-catalyzed dehydrogenative coupling (index D) of ammonia and tris(hydridosilylethyl)boranes, B[C2H4Si(R)H2]3 (R = H, CH3). Initially, the strong base, e.g. n-butyl lithium, deprotonates ammonia. The highly nucleophilic amide replaces a silicon-bonded hydride to form a silylamine and lithium hydride, which then deprotonates ammonia, resuming the catalytic cycle. Under the conditions used, silylamines are not stable and by elimination of ammonia, polysilazane frameworks form. In addition, compounds B[C2l-L Si(R)H2]3 can be obtained from vinylsilanes, H2C=CHSi(R)H2 (R - H, CH3), and borane dimethylsulfide. 

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 following, the synthesis of different types of organosilicon polymers as potential precursors for ceramics is highlighted topologically, starting from polysilanes with Si-Si linkages, followed by polycarbosilanes. Subsequently polysilazanes with Si-N building blocks and polymers with multi-element sequences, such as PSCs (Si-N=C=N), will be discussed. 

There are several structurally different types or polymers that are suitable precursors for ternary Si-C-N ceramics. By far the most investigated precursors are polysilazanes of the general type [Si(R )(R°)N(R°)] (R, R°, R° = H, alkyl, aryl, alkenyl, etc.). In contrast to the limited number of starting compounds, H SiCl(4 ) (x = 0-3) as the silicon source and NH3 or H2N-NH2 as the nitrogen source for synthesis of polysilazanes as precursors for binary Si-N ceramics, the chemistry of polycarbosilazanes, that is, carbon-containing or modified polysilazanes, is very multifaceted. The attachment of various organic groups to the silicon atoms allows adjustment of their physicochemical properties, to control their thermolysis chemistry, and also to influence materials properties. The first... 

H3C)3Si-NH-Si(CH3)3 (HMDS). It is commercially available and synthesized in large quantities by ammonolysis of trimethylchlorosilane. Costs per kilogram are less than US 10. HMDS cannot be directly used as a precursor for ceramics because it is a nonprocessable volatile liquid (boiling point 125°C). Nevertheless, it is an important source for the synthesis of polysilazanes by transamination reactions (see below) and also a valuable single-source precursor for the preparation of Si-C-N coatings by CVD processes. ... 

Previously it was shown that ROP of 1,3-disilacyclobutane is an efhcient method for the synthesis of polycarbosilanes (Scheme 18.9). Likewise, ROP of 1,3-disiladiazetanes, cyclo-(SiR2-NR )2, delivers linear high-molecular weight polysilazanes. Synthesis of 1,3-disiladiazetanes from bis(amido)silanes [R2Si(NRLi)2] and dichlorosilanes [R2SiCl2] was reviewed in detail by Fink in 1966.9 ... 

Four-membered ring systems, which have higher tension, react faster and more readily than cyclotrisilazanes, independent whether basic or acidic conditions are applied. Polysilazanes with molecular weights of up to 18,000 using acidic conditions and up to 100,000 using basic conditions together with narrow molar mass distribution could be obtained by this method. ROP allows for the synthesis of copolymers, if starting from cyclodisilazanes with differently substituted silicon atoms. 

A cheaper approach to PSCs by means of a redistribution reaction of bis(tri-alkylsilyl)carbodiimides and dichlorosilanes or trichlorosilanes was described soon after in a patent by Klebe and Mnrray. °" Similar to the synthesis of polysilazanes by transsilylation of dichlorosilanes or trichlorosilanes with HMDS (Equation 18.12), the procednre takes advantage of cheap and commercially available starting compounds, simple processing (i.e., no solid by-product formation), and high polymer yields. 

SCHEME 18.27 Synthesis of C-B-C bridged polysilazane T(2)l as precursor to high-temperature Si-B-C-N ceramics by ammonolysis of tris(dichloromethylsilyl-ethyl-enefborane. ... 

SCHEME 18.28 Synthesis of boron-modified polysilazanes by ammonolysis of tris(chlorosilylethylene)boranes (M, monomer route) and by hydroboration of vinyl-substituted polysilazanes (P, polymer route). 

Borazine and its derivatives are also possible educts to synthesize precursors for Si-B-N-C ceramics. Sneddon and co-workers prepared Si-B-N-C preceramic precursors via the thermal dehydrocoupling of polysilazanes and borazines [7]. A further synthesis route is the hydroboration of borazines. The work group of Sneddon found that definite transition metal reagents catalyze hydroboration reactions with olefins and alkynes to give 5-substituted borazines [8]. Recently, Jeon et al. reported the synthesis of polymer-derived Si-B-N-C ceramics even by uncatalyzed hydroboration reactions from borazines and dimethyldivinylsilane [9]. 

Summary Precursor-derived quaternary Si-B-C-N ceramics frequently possess an enhanced thermal stability compared to SiC, SisN4 or Si-C-N ceramics. The stability of the materials towards crystallization and/or decomposition is directly coimected to the molecular structure and the elemental composition of the polymeric precursors. This paper highlights recent investigations on the synthesis of boron-modified polysilazanes and polysilylcarbodiimides. Hydroboration of polyvinylsilazanes and dehydrocoupling reactions of boron-modified silanes with ammonia or amines as well as cyanamide are described. It is shown that simple organosilicon chemistry provides a means to efficiently optimize ceramic yields and tune elemental composition as well as thermal properties of the polymer-derived ceramics. 

An alternative access to boron-modified polysilazanes is a dehydrocoupling of ammonia or alkyl amines with tris(hydridosilylethyl)boranes, B(C2H4Si(R)H2)3 (R = H, CH3 C2H4 = CH2CH2, CHCH3 Scheme 2) [6]. This avoids the formation of solid by-products and allows for the synthesis... 

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