pylene in the presence of a catalyst system comprising (a) a solid component consisting essentially of magnesium, titanium, silicon, a halogen atom or a compound comprising a halogen and dialkyl phthalate, (b) triisobutylaluminum, (c) an aromatic compound containing alkoxy group represented by the following general formula (I): ##STR3## wherein R1 is an alkyl group having 1 to 20 carbon atoms, R2