A process for polymerizing an epoxide monomer, preferably ethylene oxide, comprising carrying out the process in the presence of a catalytically active amount of a catalyst obtainable by a preparation process comprising admixing at least one alkaline earth metal, liquid ammonia, an alkylene oxide, which is optionally substituted by aromatic radicals, and an organic nitrile having at least one acidic hydrogen atom to prepare a slurry of modified alkaline earth hexammine in liquid ammonia; continuously transferring the slurry of modified alkaline earth hexammine in liquid ammonia into a stripper vessel and continuously evaporating ammonia, thereby accumulating the modified catalyst in the stripper vessel; and upon complete transfer of the slurry of modified alkaline earth hexammine into the stripper vessel, aging the modified catalyst to obtain the final polymerization catalyst; and a polymer obtainable by this process.