Process for the gas-phase polymerization olefins

A process for the gas-phase polymerization of at least one alpha-olefin in the presence of a catalyst system, the process comprising:

• a) contacting in a continuous way at least one of said alpha-olefins with said catalyst system in a prepolymerization reactor, wherein a reaction comprising an average residence time is carried out in a liquid medium at a temperature ranging from 23° to 50° C., thereby forming a prepolymer;
• b) feeding in continuously the prepolymer into a gas-phase reactor having interconnected polymerization zones, where the growing polymer particles flow upward through a first polymerization zone (riser) comprising a gas mixture, a bottom portion, and an upper part, under fast fluidization or transport conditions, leave said riser and enter a second polymerization zone (downcomer) comprising an upper part through which they flow downward under the action of gravity, leave said downcomer and are reintroduced into the riser, thus establishing a circulation of polymer between the riser and the downcomer,
• wherein the prepolymer is continuously fed at a section of said riser comprising an upwardly gas velocity higher than 3.0 m/s.