A process for preparing hyperpure propylene (co)polymers comprising polymerizing the monomer(s) in liquid monomer in the presence of a catalyst obtained by reacting an Al-trialkyl compound with a solid catalyst component comprising magnesium dihalide in active form, having supported thereon a titanium chloride or chloroalcoholate, and an ether having the general formula: ##STR1## where R and R" are the same or different and are C.sub.2-18 alkyl, C.sub.3-18 cycloalkyl, C.sub.6-18 aryl, C.sub.7-18 arylalkyl or alkylaryl radicals.