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 preparing a neodymium catalyst system which 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. The present invention further discloses a synthetic polyisoprene rubber which is comprised of repeat units that are derived from isoprene, wherein the synthetic polyisoprene rubber has a cis-microstructure content which is within the range of 98.0% to 99.5%, a 3,4-microstructure content which is within the range of 0.5% to 2.0%, and a trans-microstructure content which is within the range of 0.0% to 0.5%.