The syndiotactic-specific polymerization of propylene can be promoted by single-site catalysts based on vanadium, nickel and group IV metals. The polymerization mechanisms are reasonably well established for nickel and group IV metals. The polymerization mechanisms are reasonably well established for nickel and group IV metals. For vanadium catalysts the polymerization mechanisms reported in the literature are still debatable, since the stereochemistry of the monomer insertion is well known, but the active species have never been isolated. In this paper the mechanisms concerning Ni and group IV metal catalysts are briefly summarized and proposed mechanisms concerning vanadium are compared within one another.