Ammonolysis, polysilazanes

To produce a ceramic material containing only Si3N4, the white solid polysilazane derived from DHCD of the oil obtained by ammonolysis of 6 1 CH3SiHCl2/CH3SiCl3 was pyrolyzed in a stream of ammonia (to 1000°C). A white ceramic residue containing only 0.36% by weight C resulted. 

Polysilazanes that can serve as precursors for silicon carbonitride have been prepared using a Ru3(CO)i2-catalyzed Si-H/N-H dehydrogenative coupling process by workers at SRI [21]. Thus the ammonolysis product of CH3SiHCl2, whose composition approximates [CH3Si(H)NH]n, could be crosslinked by heating at 40 °C with a catalytic quantity of Ru3(CO)i2. Other polysilazanes were prepared by this procedure ... 

Precursors that fall in this category are generically called polysilazanes or polysilsesquiazanes. Synthetic routes to these materials have been reviewed63,24 thus, we provide an overview of typical methods. The most common route to polysilazanes is via ammonolysis/aminolysis of chlorosilanes25-27. 

Polysilazanes can also be synthesized by catalytic dehydrocoupling (equation 4)28,29. The reaction works best with monosubstituted silanes (MeSiH3 is flammable) however, as with the ammonolysis reactions, molecular weights are normally less than 2000 Da. Hence, as-formed oligomers are not useful precursors. However, combinations of ammonolysis followed by catalytic dehydrocouphng provide access to useful precursors as discussed in the SiCN section. 

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. 

It is also possible to form novel polysilazanes by dehydrocyclodimerization of the ammonolysis product of CH3SiHCl2, or coammonolysis products of this silane with other chlorosilanes.33 Ring compounds that contain the desired Si and N atoms may also be used as starting materials, followed by hydrolysis, thermolysis, polymerization, or copolymerization to convert them into silazane-type materials. Examples are cyclic bis (silyl) imidates, /V-acylcyclosilazoxanes, /V-arylcyclosilazoxanes, and A,A,-diarylcyclosilazanes.34... 

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

Preparation of Preceramic Polysilazanes. At MIT (Massachusetts Institute of Technology), our initial research on silicon-based preceramic polymers was aimed at developing a precursor for silicon nitride. To this end, we studied the ammonolysis of dichlorosilane, H2SiCl2 (ii). This reaction had already been carried out on a millimolar scale in the gas phase and in benzene solution by Stock and Somieski in 1921 (12). We found that this reaction gave a much better yield of soluble ammonolysis product when it was carried out in more polar solvents such as dichloromethane or diethyl ether (ii). 

Seyferth and co-workers (12) used the ammonolysis of dichlorosilane to prepare carbon-free polysilazanes that could be converted into silicon nitride (equation 8),... 

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. ... 

Ammonolysis of dichlorosilanes yields oligo- or polysilazanes with alternating Si-N backbones. In general, product mixtures of cyclic trimers or tetramers (six- and eight-membered rings, respectively) and low molecular weight chain molecules with M < 2(X)0 are obtained. 

Depolymerization reactions can also be avoided when attaching reactive groups to the polymer backbone, which increase the cross-linking density of the polymeric precursor at ambient temperature. In this regard, the attachment of vinyl groups HC=CH2 was studied in much detail (see below). According to Scheme 18.17, vinyl-substituted polysilazanes are best synthesized by ammonolysis of chlorovinylsilanes (H2C=CH)Si(R)Cl2 (R = H, CH3). 

Liu and Harrod ° reported on dehydrocoupling of ammonia and silanes catalyzed by dimethyltitanocene. Tertiary silanes, for example, Ph2SiMeH, were transformed into disilazanes. Yet no polysilazanes could be obtained when reacting PhSiH3 under similar conditions. Homodehydrocoupling (which proceeds with Si-Si linking compare Scheme 18.3) effectively competed with amination reactions, indicating that the products obtained by ammonolysis of PhSiHj are polyaminosilanes [SiPh(NH2)] rather than polysilazanes [SiPhH-NH] . 

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). 

Seyferth synthesized soluble polysilazanes in high yield by ammonolysis of SiH2Cl2 in polar solvents [24], such as dimethyl ether or dichloromethane. The ammonolysis of CH3SiHCl2 also yields silazane oligomers, which can be poly-... 

A soluble polysilazane was obtained by the ammonolysis of an adduct between SiH2Cl2 and pyridine [25]. A formula for the polymer is (SiH2NH)o.46(SiH2)o.36(SiH3)o.l8, and a structural model is shown in Fig. 3. This polymer is produced as the precursor for the Si3N4 fiber and Si-N coating by Tonen Corporation. 

When compared to SiC, less work has been reported on the production of Si3N4 by the polymer pyrolysis route. Most efforts have focused on polymer precursors based on polysilazanes, a class of polymers having Si-N bonds in the main chain (58-61). The reactions to produce the Si-N bond in the chain backbone are based on the ammonolysis of methylchlorosilanes. A preceramic polymer can be prepared by the ammonolyis of methyldichlorosilane, followed by the polymerization of the silazane product catalyzed by potassium hydride (69) ... 

There are a number of published reviews on the synthesis of poly-silazanes. - " The present discussion is limited to a brief history and development of oligo- and polysilazane synthesis. The methods of synthesizing polysilazanes include (1) polymerization by aminolysis (or ammonolysis) of chlorosilanes, (2) ringopening polymerization of oligomeric cyclosilazanes, (3) deamination and condensation of bis- and tra-aminosilanes, (4) redistribution polymerization of chlorosilanes and aminosilanes, and (5) catalyzed dehydrocoupling of diamines and organosilanes. 

Likewise, vinyl-substituted polysilazanes may be thermally crosslinked at elevated temperatures. A liquid oligovinylsilazane, (CH2=CHSiHNH)x, prepared by the ammonolysis of vinyldichlorosilane, thermoset to an infusible, insoluble solid after heating for 2 hours at 110°C (23,24). This solid had a char yield of over 85 wt% when pyrolyzed to 1200°C. For this system, infrared analysis showed that hydrosilylation, and not vinyl polymerization, was the predominant crosslinking mechanism. 

---