A polymerization process includes contacting an olefin or a mixture of the olefin and one or more copolymerizable comonomers under polymerization conditions with a catalyst composition and forming a polymer with a total ash content of less than 15 ppm. The catalyst composition includes one or more polymerization catalysts; and a mixed external electron donor comprising a selectivity control agent comprising at least one silicon-containing compound containing at least one C1-C10 alkoxy group bonded to a silicon atom.