Cycloolefin copolymers for use for optical data memories, can be prepared by copolymerization of monomers from the group consisting of cyclic olefins, &agr;-olefins having 2 or more C atoms and optionally conjugated or non-conjugated diolefins, the catalysts employed being metallocene compounds or &pgr; complex compounds of the formula in which CpI and CpII represent carbanions having a cyclopentadienyl-containing structure, &pgr;I and &pgr;II represent charged or electrically neutral &pgr; systems, D represents a donor atom and A represents an acceptor atom, where D and A are linked by a reversible coordinate bond such that the donor group assumes a positive (part) charge and the acceptor group assumes a negative (part) charge, M represents a transition metal of sub-group III, IV, V or VI of the Periodic Table of elements (Mendeleev), including the lanthanides and the actinides, X denotes an anion equivalent and n denotes the number zero, one, two, three or four, depending on the charge of M.