Nickel complexes [(X,O)NiR(PPh3)] (X = N or P), designed for the polymerization of ethylene, were found to be efficient for the homo-and copolymerization of butylacrylate (BuA), methylmethacrylate (MMA), and styrene. Their role as a radical initiator was demonstrated from the calculation of the copolymerization reactivity ratios. It was shown that the efficiency of the radical initiation is improved by the addition of PPh3 to the nickel complexes as well as by increasing the temperature. The dual role of a nickel complex as radical initiator and catalyst was exploited to succeed in the copolymerization of ethylene with BuA and MMA. Multiblock copolymers containing sequences of both ethylene and polar monomers were thus prepared for the first time by a dual radical/catalytic mechanism.