Abstract: Catalyst compositions suitable for producing ethylene polymers in a low pressure gas phase process are produced by forming a partially activated precursor composition from a magnesium compound, titanium compound, inert carrier material and an organoaluminum activator compound in an electron donor solvent.

Abstract: A high surface area catalyst component or support is produced by swelling a magnesium halide, e.g., MgCl.sub.2, with a secondary or tertiary alcohol, adding a hydrocarbon diluent, and removing some or substantially all of the free or unreacted alcohol. In one preferred embodiment, the alcoholate product is contacted with a halogenated tetravalent titanium compound, e.g., TiCl.sub.4, to form a first catalyst component (A) which can be combined with a cocatalyst component (B) comprising a metallic hydride or an organometallic compound, e.g., an organoaluminum compound to form a catalyst system active for olefin polymerization.