Substituted polysilazanes

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

Burns and co-workers (i 7) prepared a series of alkyl-, aryl-, and arylalkyl-substituted polysilazane polymers (equation 12), and a mechanistic study of pyrolysis was carried out to determine the effect of substituents on char yield, char composition, and stability of the resulting ceramic powders. 

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). 

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). 

Figure 5 TGA profile of a phenyl-substituted polysilazane (jc) compared with hexyl groups containing polymers.

A number of processible nitrogen-containing ceramic precursors are also discussed. These include polyalazanes (Jensen, Chapter 32), cross-linkable vinyl substituted polysilazanes (Schwark, Chapter 5), mixed sol-gels fi om aminolysis reactions (Gonsalves, Chapter 16), and polyborazines (Sneddon and Paine, Chapter 27 and Kimura, Chapter 28) which serve as precursors to AIN, Si3N4, BN/AJN composites, and BN, respectively. Arylene and alkylene bridged polysilsesquioxanes (Loy, Chapter 11) and carborane-polysiloxanes (Keller, Chapter 31) have bIso been employed to make modified silicas. 

This chapter discusses the development of thermosetting preceramic polymers, emphasizing peroxide-curable polysilazanes. These polymers are excellent precursors for both silicon nitride and silicon carbide. Vinyl-substituted polysilazanes may be readily thermoset with peroxide initiators. In addition, a new class of polysilazanes which contain peroxide substituents directly boxmd to the polymer has been developed The utility of peroxide-cured polysilazane precursors for the formation of silicon nitride articles has been demonstrated. 

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. 

Polysilazanes with Vinyl Substituents. Little fundamental work has been described on the peroxide crosslinking of polysilazanes. In 1984, it was reported that simple organooligocyclosilazanes, prepared by the polycondensation of methylvinyl-cyclosilazanes, could be crosslinked by heating to 220°C with 0.5-2.5 wt% of the silylperoxide (MeSiOOfBuO)x (28), Even for the highest levels of peroxide, only about 60% of the liquid silazane could be converted to a gel. Still, diis work demonstrated the potential for crosslinking vinyl-substituted polysilazanes using a free radical approach. 

A further extension of the concept of thermosetting polysilazanes with peroxide initiators has been the preparation of new polysilazanes with peroxide groups bound directly to the polymer 20). Potential advantages of a peroxide-substituted polysilazane over systems in which the peroxide is simply admixed with the polymer include 1) segregation of the peroxide upon storage cannot occur, 2) dissolution or dispersion of a peroxide in the polysilazane is not necessary, and 3) homogeneous distribution of e peroxide in solid, as well as liquid, polysilazanes is possible. We have prepared a new class of peroxide-substituted polysilazanes by the reaction of a hydroperoxide with a poly(methylvinyl)silazane. The liquid polymers may be thermoset, even with extremely low levels of peroxide substitution. This chemistry provides access to a class of polysilazanes previously unknown as ceramic precursors. 

No examples, however, were reported in which a lower hydroperoxide/Si-N stoichiometry was used. Such a ratio would be expected to produce silylperoxides containing silylamine moieties. We adopted this strategy for the preparation of peroxide-substituted polysilazanes. 

The reaction mixture which contained 6 had no other products by si NMR spectroscopy. This peroxide-substituted silylamine was successfully prepared by control of the reaction stoichiometry to supply the hydroperoxide as Ae limiting reagent, which prevented complete conversion to an alkyl silylperoxide as in equation 3. This approach was then used to prepare a peroxide-substituted polysilazane. 

The level of silylperoxide substitution required to cure 4 and produce a solid with a good char yield was investigated. The data in Table I show the maximum exotherm reached during cure and TGA results obtained from the vinyl-substituted polysilazane 4 reacted in hexane at room temperature with the indicated weight percentage of BuOOH. Accurate dispensing of the hydroperoxide was possible because it was supplied as a 3.0 M solution in isooctane. In each reaction, a 1-2°C exotherm and gas evolution (ammonia) accompanied the hydroperoxide addition. Following in vacuo removal of the hexane, each liquid polymer was thermoset in a... 

Table I. Peroxide-Substituted Polysilazane Cure/TGA Results ...

Despite the expectation that the silylperoxide substituents might be inefficient initiators, the vinyl-substituted polysilazanes containing such moieties thermoset readily producing highly crosslinked solids which had good pyrolysis yields. 

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