Mechanistic features of the isoselective polymerization of styrene promoted by racemic allyl ansa-lanthanidocene complexes rac-{CMe2(Ind)(2)}Ln(allyl)(THF)(n) (Ind =5-indenyl; allyl = C3H5, n = 1, Ln = Y (1), Nd (2); allyl=1,3-(SiMe3)(2)C3H3, n = 0, Ln = Y (3), Nd (4)), which act as unique single-component catalysts, have been investigated by experimental and theoretical methods. Combined H-1 NMR and MALDI-ToF-MS analyses conducted on low molecular weights isotactic polystyrene (iPS) produced by compounds 1 or 2 established the presence of allyl end-groups and therefore that polymerization is initiated by insertion of styrene into Ln-C(allyl). H-1, D-2, and C-13 NMR analyses performed on H(D)-iPS-nBu polymers, selectively produced by coordinative chain transfer polymerization (CCTP) of styrene with the binary system 3/Mg(nBu)(2). (1:10), evidenced that the polymerization is highly regioselective and proceeds in a secondary insertion mode, both in the initiation and propagation steps. Microstructural analyses of the backbone of crude (unfractionated) polymers produced with 1-4 evidenced a virtually perfect isoselective (mmmmmm > 0.99) polymerization up to 120 degrees C. Stereoerrors observed in the (13)c{H-1} NMR spectra of the iPS produced at 130 degrees C with 1-4 are consistent with an enantiomorphic site control operative during the polymerization. DFT computations of the first and second insertions of styrene in 3 confirmed the above features and the overall mechanistic scenario.