Process for the (co)polymerization of ethylene

A process for preparing ethylene polymers carried out in the presence of a catalyst system comprising (i) a solid catalyst component comprising Mg, Ti, halogen, and optionally an internal electron donor compound (ID), and (ii) an Al-alkyl compound;

• said process comprising at least two steps of polymerization (a) and (b), in which:
• in a first step (a) ethylene is polymerized in the presence of a molecular weight regulator in order to produce an ethylene (co)polymer;
• and
• in a further step (b), which is carried out in the presence of an external electron donor compound (OD) selected from aliphatic ethers, esters and alkoxysilanes, added to this polymerization step as a fresh reactant, ethylene is copolymerized with an alpha olefin comonomer of formula CH2═CHR, in which R is a C1-C20 hydrocarbon group, to produce an ethylene copolymer having a molecular weight higher than that of the ethylene (co)polymer produced in step (a),
• wherein the polymerization is carried out in the gas phase, and the ethylene polymers comprise an MIF/MIP ratio from 10 to 50.