The rare-earth metallocene chloride complexes (C5Me5)(2)LnCl(2)Li(OEt2)(2) (Ln = Nd, Sm, or Y) combined with an excess of dialkyl-magnesium compounds (butyl-ethyl-magnesium) afford active species for the polymerization of ethylene in alkanes or aromatic solvents at atmospheric pressure. A dynamic equilibrium between dormant species and a low concentration of catalytically active species is suggested to explain the living character of the polymerization process observed at temperatures up to 80 degrees C. A fluxional behavior of the polynuclear structure of the dormant species accounts for the fast and reversible exchange/transfer reaction of alkyl chains between lanthanide and magnesium metallic centers. The resulting mixture contains mainly di(polyethylenyl)magnesium compounds and can be directly used either for block copolymerisation with polar monomers or for any classical Grignard-like reaction for the synthesis of functionalized polyethylenes. (c) 2007 Elsevier Ltd. All rights reserved.