Monomodal polysilylenes with relatively narrow molecular weight distributions (M(w)/M(n) < 1.5) have been prepared by the reductive coupling of methylphenyldichlorosilane and di-n-hexyldichlorosilane with alkali metals in toluene in the presence of ultrasound. The optimum conditions for the sonochemical synthesis of well-defined polysilylenes in the presence of an immersion-type pe probe were studied. Sonochemical synthesis is accompanied by selective degradation which decreases the molecular weight to the limiting value of M(n) approximate to 50000 and also reduces polydispersities (M(w)/M(n) < 1.2). Polymerization depends strongly on the substituents on the silicon atoms, solvents, alkali metals, and temperature. For example, the polymerization of methylphenyldichlorosilane in toluene with potassium, in contrast to that with sodium, does not produce a polysilylene because the reactivity of potassium is too high, leading to side reaction.