| http://www.w3.org/ns/prov#value | - U.S. Pat. No. 5,064,799 describes catalyst components obtained from the reaction of a tetravalent titanium halide and an electron-donor compound with a solid obtained by reacting a metal oxide containing surface hydroxyl groups (such as silica or alumina) with an organometallic magnesium compound of the formula MgR2-x Xx, where R is a hydrocarbon radical, X is a halogen or an OR or COX' radical (w
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