| http://www.w3.org/ns/prov#value | - The method for preparing the catalysts as defined in claim 9, characterized in that catalyst-forming component (a) is mixed with a catalyst-forming component (b) prepared by contacting the titanium or vanadium halide with a magnesium dihalide obtained by decomposition of an organometallic compound of the formula RMgX in which R is a hydrocarbon radical and X is a halogen.
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