Patent ID: 7524903

Claim:
A process for preparing heterophasic propylene copolymers by polymerizing propylene in the presence of a polymerization catalyst and hydrogen as a molecular weight regulator, the process comprising the following steps: a) polymerizing propylene in a gas- or liquid-phase reactor, optionally together with at least one α-olefin comonomer, to prepare a crystalline polymer fraction; and b) copolymerizing ethylene with at least one of propylene and 1-butene, and optionally at least one C 5 -C 12 alpha-olefin comonomer, in a gas-phase reactor having interconnected polymerization zones, wherein growing polymer particles flow upward through a first polymerization zone (riser) under fast fluidization or transport conditions, wherein a first ethylene/propylene rubber component is formed, leave said riser and enter a second polymerization zone (downcomer) through which they flow downward under the action of gravity, wherein a second ethylene/propylene rubber component is formed, leave said downcomer and are reintroduced into the riser, thus establishing a circulation of polymer between said two polymerization zones, wherein the first and second reactors are serially connected, and the crystalline polymer fraction of step a) from the first reactor is transferred to the second gas-phase reactor, and the first ethylene/propylene rubber component and second ethylene/propylene rubber component are distinct fractions of elastomeric polymer.