Process for the gas-phase polymerization of olefins

A process which comprises polymerizing α-olefins CH2═CHR, where R is hydrogen or a hydrocarbon radical having 1-12 carbon atoms, in the gas phase in two interconnected polymerization zones, a riser and a downcomer, wherein the growing polymer particles flow through the riser under fast fluidization conditions, leave said riser and enter the downcomer through which they flow downward in a densified form, leave said downcomer and are reintroduced into said riser, thus establishing a circulation of polymer between the riser and the downcomer, wherein:

• (a) the gas mixture present in the riser is totally or partially prevented from entering the downcomer by introducing into the upper part of said downcomer a liquid barrier LB having a composition different from the gaseous mixture present in the riser;
• and
• (b) the ratio R between the flow rate Fp of polymer circulated between said downcomer and said riser and the flow rate LB of said liquid barrier is adjusted in a range from 10 to 50.