an ionic metallocene catalyst for olefin polymerization which comprises : a cyclopentadienyl - type ligand , a group ivb transition metal , and alkyl , aryl , or hydride substituents , as a cation , and a weakly coordinating anion comprising boron substituted with halogenated , such as tetra fluoro , aryl substituents preferably containing silylalkyl substitution , such as para - silyl t - butyldimethyl .