Abstract: In a process for preparing polymers based on monomers having a C═C double bond by homopolymerization or copolymerization of these monomers in the presence of a catalyst system comprising a metallocene complex A) and a compound B) capable of forming metallocenium ions and, if desired, an organometallic compound of main group I, II or III of the Periodic Table of the Elements C), the metallocene complex A) used is a compound of the formula (I) where the substituents and indices have the following meanings: R1 to R11 are hydrogen, C1-C10-alkyl, 5- to 7-membered cycloalkyl which may in turn bear C1-C6-alkyl groups as substituents, C6-C15-aryl or arylalkyl, where two adjacent radicals R1 to R8 may together form a cyclic group having from 4 to 15 carbon atoms, or Si(R12)3, where R12 is C1-C10-alkyl, C6-C15-aryl or C3-C10-cycloalkyl, M is a metal of transition groups III to VI of the Periodic Table of the Elements or a metal of the lanthanide series, X are identical or different and are hy