Patent ID: 8178632

Claim:
A gas-phase polymerization process carried out in at least two interconnected polymerization zones for preparing a propylene polymer comprising a total content of units derived from at least one linear or branched alpha-olefin comprising 2 to 8 carbon atoms other than propylene ranging from 4.7 to 5.5 wt %, and a melting temperature Tm (measured by DSC on the as-reactor polymer) ranging from 150° C. to 158° C.; said process comprising feeding propylene and ethylene to said interconnected polymerization zones in presence of a highly stereospecific heterogeneous Ziegler-Natta catalyst system under reaction conditions, and collecting a polymer product from said interconnected polymerization zones, wherein growing polymer particles flow upward through one of said interconnected polymerization zones, a riser, under fast fluidization conditions, leave said riser and enter another polymerization zone, a downcomer, through which the growing polymer particles flow downward under the action of gravity, leave said downcomer and are reintroduced into the riser, establishing a circulation of the growing polymer particles between the riser and the downcomer, whereby means are provided which are capable of totally or partially preventing a gas mixture present in the riser from entering the downcomer and a gas and/or liquid mixture comprising a composition different from the gas mixture present in the riser is introduced into the downcomer, the process further comprising: a monomer feed ratio C x − /(C x − +C 3 − ) to the riser ranging from 0.055 to 0.70 mol/mol, wherein C x − is at least one alpha-olefin comprising 2 to 8 carbon atoms other than propylene used as comonomer for preparing the propylene polymers, and C 3 − is propylene.