Abstract: Preparation of a block copolymer comprising a polyethylene block and a block formed of the polymerization of one or more vinyl monomers, at least 50 mol % of which being selected from the list consisting of ethylene, vinyl esters, non-conjugated N-vinyl monomers, acrylonitrile, (meth)acrylates and (meth)acrylamides, at least one of said one or more vinyl monomers not being ethylene, the process comprising: polymerizing ethylene or the one or more vinyl monomers in presence of an organic cobalt complex, thereby forming a macroinitiator; contacting the macroinitiator with either; the one or more vinyl monomers, if ethylene was polymerized in step a, or; ethylene, if the one or more vinyl monomers were polymerized in step a, thereby forming a second polymer block, and thereby forming the block copolymer, wherein forming the polyethylene block is performed under a pressure above 1 bar.