Certain new poly(silyl ether)s with reactive pendant chloromethyl groups were synthesized by the polyaddition of bis(epoxide)s with dichlorosilanes or bis(chlorosilane)s. The polyaddition proceeded very smoothly and regioselectively in toluene catalyzed by quaternary ammonium or phosphonium salts such as tetrabutylammonium chloride (TBAC) and tetrabutylphosphonium chloride (TBPC) under mild reaction conditions to give the corresponding poly(silyl ether)s having high molecular weights, although the reaction did not occur without a catalyst nor with 4-(dimethylamino)pyridine (DMAP) as the catalyst under the same reaction conditions. The polyaddition also proceeded using either triphenylphosphine (TPP) or a complex of 18-crown-6 with potassium chloride as catalysts to afford the poly(silyl ether), but with low molecular weights. That is; the catalytic activity-of the above quaternary onium salts such as TBAC and TBPC was higher than that of DMAP, TPP, or the crown ether complex. Furthermore, it was found that the resulting poly(silyl ether)s with reactive pendant chloromethyl groups exhibited excellent solubility in common organic solvents.