The combination of the neodymium tert-butoxide Nd 3(μ 3-OBu t )2(μ 2-OBu t ) 3(OBu t ) 4(THF) 2 ( 1), resulting from the metathetic exchange between NdCl 3 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 (0 °C, 1 atm) in the presence of PhSiH 3 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.