Polymerization catalyst obtained by reaction of a rare earth borohydride, a boron activator i.e. an

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The present invention relates to a polymerization catalyst obtained by reacting (a) borohydride rare earth ; (b) a boron activator which is an ammonium borate, a borate or a borane carbenium carrier inductive effect withdrawing groups; and (c) a trialkyl aluminum. The invention also relates to the use of the catalyst in the polymerization of ethylenically unsaturated monomers, in particular for the preparation of polyisoprene having a stereochemistry 1, 4-cis majority.