External donor systems, catalyst systems and olefin polymerization processes are described herein. The external donor systems generally include a first external donor represented by the general formula SiR.sup.2.sub.m(OR.sup.3).sub.4-m, wherein each R.sup.2 is independently selected from alkyls, cycloalkyls, aryls and vinyls, each R.sup.3 is independently selected from alkyls and m is from 0 to 4. The external donor systems further include a second external donor represented by the general formula SiR.sup.4.sub.m(OR.sup.5).sub.4-m, wherein each R.sup.4 is independently selected from alkyls, cycloalkyls, aryls and vinyls, each R.sup.5 is independently selected from alkyls, m is from 0 to 4 and at least one R.sup.4 is a C.sub.3 or greater alkyl.