The syndioselective copolymerization of styrene with ethylene (in bulk or in aliphatic hydrocarbon solutions, (nBu)(2)Mg as scavenger, T-polym = 60-140 degrees C) was achieved in the presence of a series of ansa-lanthanidocenes of the type {R2C(C5H4)(R'R'Flu)}Ln(1,3-C3H3(SiMe3)(2))(THF)(x) (1-Nd-K-allyl, 27-Nd, and 2-Sc,La,Sm,Pr). While precursors based on small ionic radius metals (2-Sc) or bearing bulky substituents in 3,6-positions of the fluorenyl moieties (3-Nd and 5-Nd) were poorly or not active under standard polymerization conditions (60 degrees C), 2-La,Pr,Nd,Sm which bear 2,7-tBu(2) substituents on the Flu ligand produced efficiently sPSE materials (productivity 300-400(+) kg mol(Ln)(-1) h(1), [r](5) = 71%; controlled amount of ethylene inserted in the range 115 mol %). Under harsher conditions (T-polym = 100-140 degrees C, [St](0)/[Nd](0) = 40 000-147 000 equiv), 2-Nd produced similar sPSE materials with a productivity increased by 1 order of magnitude (up to 5430 kg mol(Nd)(-1) h(1)). Theoretical DFT investigations including the solvent model, performed on the first three insertion steps for the benchmark catalysts {(Me2C(C5H4)(Flu)}Nd(C3H5)(THF) (I), the putative 1-Nd, and the most effective 2-Nd, allowed to corroborate the nature of the obtained copolymers, with ethylene units randomly distributed within long sPS sequences. These studies established also that the presence of bulky substituents on the fluorenyl ligands is crucial in the activity, but the nature of the substituents on the allyl group has no effect on the chemistry and the nature of the resulting styreneethylene copolymer but only influences the initiation step in which the first ethylene insertion will be more or less favored with respect to the styrene insertion.