Peroxide-substituted polysilazanes

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. 

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

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. 

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. 

Peroxide initiators are equally effective, but often kinetically faster, than platinum-based crosslinking systems. The crosslinking of silicon-based polymers with organic peroxides is well-known, and in this section several examples will illustrate the translation of this technology to preceramic polymer systems. For many vinyl-substituted precursors, and polysilazanes in particular, free radical-based crosslinking is the cure chemistry of choice. 

Several methods have been developed to crosslink vinyl-substituted preceramic polymers. While simple thermal treatment or heating in the presence of platinum catalysts does produce crosslinked systems, the cure kinetics are often slow. Peroxide initiators promote rapid vinyl crosslinking, especially for polysilazanes with both Si-CH=CH2 and Si-H subtituents. 

---