The catalyst structure may be described as metallocene (or bent metallocene in the case of bis-cyclopentadienyl compounds) with ancillary anionic ligands or hydrocarbyl groups, such as metallocenes of the formula Zt (??5 -R'n Hm C5)S MX4-S, where R' is a carbon or a carbon and heteroatom (N, O, P, B, Si and the like) containing C1 to C6 alkyl, C3 to C12 cycloalkyl or a C6 to C14 aryl group.