The neodymium catalyst system prepared by the technique of this invention can be used in the polymerization of isoprene monomer into synthetic polyisoprene rubber having an extremely high cis-microstructure content and high stereo regularity. This polyisoprene rubber will crystallize under strain and can be compounded into rubber formulations in a manner similar to natural rubber. This invention more specifically discloses a process for the synthesis of polyisoprene rubber which comprises polymerizing isoprene monomer in the presence of a neodymium catalyst system, wherein the neodymium catalyst system is prepared by a process that comprises (1) reacting a neodymium carboxylate with an organoaluminum compound in an organic solvent to produce neodymium-aluminum catalyst component, and (2) subsequently reacting the neodymium-aluminum catalyst component with an elemental halogen to produce the neodymium catalyst system, wherein the neodymium catalyst system is void of nickel-containing compounds.