compound, (c) an aromatic compound containing alkoxy group represented by the following general formula (I): ##STR1## wherein R1 is an alkyl group having 1 to 20 carbon atoms, R2 is an hydrocarbon group having 1 to 10 carbon atoms, a hydroxyl group or a nitro group, m is an integer of 1 to 6 and n is an integer of 0 to (6-m), and (d) an electron donating compound used according to necessity.