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 SiR2m(OR3)4-m, wherein each R2 is independently selected from alkyls, cycloalkyls, aryls and vinyls, each R3 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 SiR4m(OR5)4-m, wherein each R4 is independently selected from alkyls, cycloalkyls, aryls and vinyls, each R5 is independently selected from alkyls, m is from 0 to 4 and at least one R4 is a C3 or greater alkyl.