n used in the present invention is represented by the following general formula ##STR22## wherein X represents a direct bond, a lower alkylene group having from 1 to 4 carbon atoms, ##STR23## --SO.sub.2 --, --O-- or --S-- whereineach R independently represents a hydrogen atom or a lower alkyl group having from 1 to 4 carbon atoms, and n is an integer of 1 or over.