Sonochemical Synthesis of a Poly(methylsilane), a Precursor for Near-Stoichiometric SiC

We report a sonochemical process for a single-pot synthesis of high molecular weight poly(methylsilane) (PMS) of composition [(CH 3 SiH) x (CH 3 Si) y (CH 3 SiH 2 ) z ] n, where x + y + z = 1 and 0.65 ≥ ( x + z ) ≥ 0.23 by the sodium polycondensation reaction of CH 3 SiHCl 2 in a solvent mixture of hexane or toluene and tetrahydrofuran (THF) using ultrasonic activation (47 kHz, 130 W). This polycondensation reaction takes place in two stages: (1) 10−25 h of reaction in which an oligo(methylsilane) (OMS) oil (0.91 ≥ ( x + z ) ≥ 0.83) is obtained that gives low ceramic residue yields of silicon-rich SiC on pyrolysis in argon and (2) 25−40 h of reaction during which is obtained a PMS solid that gives high ceramic residue yields of near stoichiometric SiC on pyrolysis. PMS also may be prepared by branching of the oligo(methylsilane) with sodium and ultrasonication for ∼5 h in a toluene and THF solvent mixture. PMS is soluble in hexane and toluene and its pyrolysis in argon gives a ceramic residue yield up to 90%. In addition to the main product of the condensation reaction, a high molecular weight PMS may be extracted from large NaCl crystals (the other product of the CH 3 SiHCl 2 /Na reaction). A comparison of sonochemical and conventional (reflux) activation also is provided. Ceramic fibers, films, and solid monoliths were prepared from PMS without requiring a curing step.