Process for preparing a multiblock polymer, which comprises the statistical copolymerization of a monomer mixture containing ethylene and a 1,3-diene, followed by the subsequent polymerization of ethylene in the presence of a catalytic system based on at least one metallocene of formula (Ia) or (Ib) and an organomagnesium reagent of formula (Ila) or (IIb) {P(Cp 1 )(Cp 2 )Y} (Ia) Cp 3 Cp 4 Y (Ib) Y denoting a group including a rare-earth metal atom, Cp 1 and Cp 2 , which are identical or different, being chosen from the group consisting of fluorenyl groups, cyclopentadienyl groups and indenyl groups, the groups being substituted or unsubstituted, P being a group bridging the two groups Cp 1 and Cp 2 and comprising a silicon or carbon atom, Cp 3 and Cp 4 , which are identical or different, are chosen from the group consisting of fluorenyl groups, cyclopentadienyl groups and indenyl groups, the groups being substituted or unsubstituted, R B -(Mg-R A ) m -Mg-R B (IIa) X-Mg-R A -Mg-X (IIb) R A being a divalent aliphatic hydrocarbon-based chain, interrupted or not with one or more oxygen or sulfur atoms or with one or more arylene groups, R B comprising a benzene nucleus substituted with the magnesium atom, one of the carbon atoms of the benzene nucleus ortho to the magnesium being substituted with a methyl, an ethyl, an isopropyl or forming a ring with the carbon atom which is its closest neighbour and which is meta to the magnesium, the other carbon atom of the benzene nucleus ortho to the magnesium being substituted with a methyl, an ethyl or an isopropyl, X being a halogen atom, m being a number greater than or equal to 1 and preferably equal to 1. Y denoting a group including a rare-earth metal atom, Cp 1 and Cp 2 , which are identical or different, being chosen from the group consisting of fluorenyl groups, cyclopentadienyl groups and indenyl groups, the groups being substituted or unsubstituted, P being a group bridging the two groups Cp 1 and Cp 2 and comprising a silicon or carbon atom, Cp 3 and Cp 4 , which are identical or different, are chosen from the group consisting of fluorenyl groups, cyclopentadienyl groups and indenyl groups, the groups being substituted or unsubstituted, R B -(Mg-R A ) m -Mg-R B (IIa) X-Mg-R A -Mg-X (IIb) R A being a divalent aliphatic hydrocarbon-based chain, interrupted or not with one or more oxygen or sulfur atoms or with one or more arylene groups, R B comprising a benzene nucleus substituted with the magnesium atom, one of the carbon atoms of the benzene nucleus ortho to the magnesium being substituted with a methyl, an ethyl, an isopropyl or forming a ring with the carbon atom which is its closest neighbour and which is meta to the magnesium, the other carbon atom of the benzene nucleus ortho to the magnesium being substituted with a methyl, an ethyl or an isopropyl, X being a halogen atom, m being a number greater than or equal to 1 and preferably equal to 1. R A being a divalent aliphatic hydrocarbon-based chain, interrupted or not with one or more oxygen or sulfur atoms or with one or more arylene groups, R B comprising a benzene nucleus substituted with the magnesium atom, one of the carbon atoms of the benzene nucleus ortho to the magnesium being substituted with a methyl, an ethyl, an isopropyl or forming a ring with the carbon atom which is its closest neighbour and which is meta to the magnesium, the other carbon atom of the benzene nucleus ortho to the magnesium being substituted with a methyl, an ethyl or an isopropyl, X being a halogen atom, m being a number greater than or equal to 1 and preferably equal to 1. <EOS>