Process for manufacturing in gas phase copolymers of propylene using a catalytic system of high activity

This invention relates to a manufacturing process in the gas phase copolymers of propylene with ethylene and / or butene-1, and optionally one or more other alpha-olefins. The method uses a particular catalyst obtained by contacting a support successively with at least one electron donating compound labile hydrogen donor D2, and D3 with an internal electron donor compound chosen from aromatic acid esters and then by two successive contacting the support with titanium tetrachloride separated by a washing with a liquid hydrocarbon, the said support consisting of 80 to 99 mole% of magnesium dichloride and from 1 to 20% mol of an organic electron donor compound, D1, free from labile hydrogen. This process comprises a prepolymerization step which consists in contacting the catalyst with at least one alpha-olefin having of 2 to 12 carbon atoms, a compound of D4 external electron donor and a cocatalyst.