The present invention relates to a process for preparing olefin polymers by polymerization or copolymerization of at least one olefin of the formula Ra—CH═CH—Rb, where Ra and Rb are identical or different and are each a hydrogen atom or a hydrocarbon radical having from 1 to 20 carbon atoms, or Ra and Rb together with the atoms connecting them can form a ring, at a temperature of from −60 to 200° C. and a pressure of from 0.5 to 100 bar, in solution, in suspension or in the gas phase in the presence of a catalyst system comprising one or more catalyst constituents, wherein the catalyst system comprises at least one photochromic group in one of the catalyst constituents, the corresponding catalyst systems, a process for preparing the catalyst systems of the invention, specific transition metal coordination compounds having photochromic groups, specific cyclopentadienyl derivatives and precursors thereof, in each case comprising a specific photochromic group, the use of the catalyst systems of the invention for the polymerization of olefins, the use of specific transition metal coordination compounds for preparing polymerization catalysts and polyolefins obtainable by the polymerization process of the invention.