The relationship between the stereospecificity of active sites and hydrogen effects on propylene polymerization was investigated for MgCl2‐supported TiCl3 catalysts at ultra‐low Ti contents. Hydrogen had no effect on the catalyst activities for isospecific sites and for aspecific sites. The efficiency of hydrogen as a chain‐transfer agent was found to depend on the stereospecificity of the active sites. Hydrogen was effective for isospecific sites but was not effective for aspecific sites. From the viewpoint of hydrogen dissociation, isospecific sites should have hydrogen dissociation sites in their surroundings. Therefore, isospecific sites may be in the island to be affected by hydrogen and isolated sites may be aspecific or have low isospecificity.

Summary: Hydrogen effects for propylene polymerization were investigated with ultra low TiCl 3 loading MgCl 2 -supported catalysts in which the electric states of Ti species can be almost uniform. Hydrogen did not affect the catalyst activity, while the efficiency of hydrogen as a chain transfer agent was found to depend on the Ti content of the catalyst and the stereospecificity of the polymerization sites: Hydrogen was effective for isospecific sites independent of Ti contents, but inert for aspecific sites only at the extremely low Ti content. These results were explained within the island model, where isospecific sites may be located in the islands with other Ti species in their surroundings acting, as a steric hindrance for isospecific polymerization and as hydrogen dissociation sites after deactivation. Most of the aspecific sites should be isolated only at the extremely low Ti content. These isolated sites have no other Ti species in their surroundings, i.e. no hydrogen dissociation sites, and are inert to hydrogen.