Process for the polymerization of ethylene

A process for preparing a broad molecular weight polyethylene by polymerizing ethylene in the presence of a polymerization catalyst, the process comprising the following steps, in any mutual order:

• a) polymerizing ethylene monomer, optionally together with at least one first α-olefinic comonomer having from 3 to 12 carbon atoms, in a first gas-phase reactor in the presence of a first amount of hydrogen, thereby forming an ethylene polymer;
• b) copolymerizing ethylene with at least one second α-olefinic comonomer having from 3 to 12 carbon atoms in a second gas-phase reactor in the presence of a second amount of hydrogen, wherein the second amount of hydrogen is less than the first amount of hydrogen;
• where in at least one of said gas-phase reactors growing polymer particles flow upward through a first polymerization zone (riser) under fast fluidization or transport conditions, leave said riser and enter a second polymerization zone (downcomer) 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 said two polymerization zones,
• wherein the ethylene polymer obtained from step a) represents from 40 to 65% by weight of a total ethylene polymer produced in the overall process.