The combination of the neodymium tert-butoxide Nd-3(mu(3)-OBut)2(mu(2)-OBut)(3)(OBut)(4)(THF)(2) (1), resulting from the metathetic exchange between NdCl3 and Bu-t ONa in THF, with 1 equiv. of a dialkylmagnesium reagent affords a moderately active cyclopentadienyl-free lanthanide catalyst for the pseudo-living polymerization of ethylene. Polymerizations performed under mild conditions (0degreesC, 1 atm) in the presence of PhSiH3 or H-2 as a transfer agent led to low molecular weight linear polyethylenes with activities in the range 5-20 kg PE (mol Nd)(-1) atm(-1) h(-1). In the absence of such a co-reagent, highly linear and crystalline polyethylenes of high molecular weight are produced with ca. halved activity. Under those reaction conditions, a polymer-like precipitate (2) forms during the polymerization course. Most interestingly, solid 2 proved to be an equivalent catalyst to the initial in situ system for the slurry polymerization of ethylene. Solid 2, which is assumed to be the Nd-polyethylenyl intermediate, shows also significant ethylene polymerization activity under solid-gas conditions and enables the efficient preparation of ethylene-methyl methacrylate diblock copolymers. (C) 2002 Elsevier Science B.V. All rights reserved.