Process for preparing a vanadium-based catalyst suitable for olefin polymerization

A process for preparing an elastomeric copolymer of propylene with ethylene and/or 1-butene having a propylene content by weight of 30 to 72% and obtaining said elastomeric copolymer directly in the form of a non-adhesive powder with a high yield, which process comprises copolymerising in a gas phase reactor propylene with ethylene and/or 1-butene at a temperature from 0.degree. to 60.degree. C. under a total pressure from 0.1 to 5 MPa in the presence of (i) a catalyst based on a vanadium compound precipitated within a liquid hydrocarbon by reduction reaction of vanadium on a spherical support of magnesium dichloride and (ii) an organoaluminum compound cocatalyst, said catalyst being prepared by contacting with said liquid hydrocarbon:

• (a) a vanadium-reducing agent selected from the group consisting of organoaluminum, organomagnesium or organozinc compounds with,
• (b) a vanadium compound soluble in said liquid hydrocarbon and containing halogen atom, X, and alkoxy group, OR, in a molar ratio X:OR, from 0.051 to 20:1, and
• (c) a solid support consisting of spherical particles and containing (i) from 80 to 99.5 mol % of magnesium dichloride substantially free from any product containing a Mg--C bond, and (ii) from 0.5 to 20 mol % of at least one organic electron-donor compound, D, free from labile hydrogen.