When a plurality of commands are received in succession to readwrite data fromto memory cells in accordance with the combination of these commands, a word line for controlling the transfer switches of the memory cells are activated after the reception of one of the commands excluding the first command. 3 to 20 times the olefin partial pressure at the initiation of the polymerization. Moreover, the powders, as prepared, possess good aerosol properties, which are maintained upon storage.