PREFORMED CATALYST SYSTEM COMPRISING A METALLOCENE RARE EARTH

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A catalytic system based at least on a preformation monomer selected from the group consisting of 1, 3-dienes, ethylene, α-olefins and their mixtures, on a metallocene of formula {P(Cp)(Flu)LnG} and on an organometallic compound as cocatalyst is provided. In the formula, Ln denotes a metal atom which is a rare earth metal, G denotes a group comprising the borohydride BH4 unit or denotes a halogen atom X selected from the group consisting of chlorine, fluorine, bromine and iodine, Cp denotes a cyclopentadienyl group of formula C5H4, Flu denotes a fluorenyl group of formula C13H8, P being a group bridging the two Cp and Flu groups and comprising a silicon or carbon atom. Such a catalytic system exhibits an improved stability of the catalytic activity over time, in particular on storage.