The mechanism of the initiation step of styrene polymerization promoted by single-component ansa-lanthanidocene catalysts [{Cp'XMe(2)Flu'}Ln(R)(ether)(n)] (Cp' = C(5)H(4); Flu' = 9-C(13)H(8); X = C or Si; R = allyl = CH(2)CHCH(2) or alkyl = CH(2)SiMe(3); ether = THF or Et(2)O, n = 0,1) has been studied from a combined experimental/theoretical perspective. First, detailed (13)C NMR studies conducted on syndiotactic oligostyrenes prepared by chain-growth polymerization with [{C(5)H(4)CMe(2)Flu'}Nd(eta(3)-1,3-C(3)H(3)(SiMe(3))(2))]/Mg(nBu)(2) (1:50) have shown that the insertion of styrene in these lanthanidocene systems proceeds selectively in a secondary (2,1) fashion, both at the initiation and propagation steps. Next, DFT studies of styrene insertion have been carried out on three model compounds, [{Cp'CMe(2)Flu'}Eu(eta(3)-allyl)(thf)] (I), [{Cp'SiMe(2)Flu'}Eu(eta(3)-allyl)(thf)] (II), and [{Cp'CMe(2)Flu'}Eu(eta(1)-CH(2)SiMe(3))(thf)] (III), in order to rationalize the influence of the ansa bridge (CMe(2) vs. SiMe(2)) and that of the "active" R ligand (eta(3)-allyl vs. eta(1)-CH(2)SiMe(3)), previously noticed in styrene polymerization experiments. Dissociation of the THF molecule in precursor I appears not to be a rate-limiting step and proceeds readily. The steric hindrance between the phenyl ring of the incoming styrene monomer and the ancillary ligands (Cp', Flu'), induced by the change of either the bridge or the "active" R ligand, is proposed to control the reactivity of the complexes. In all cases, orientation of the styrene phenyl ring toward the most sterically opened Cp' ligand (as compared to Flu') and 2,1-insertion of styrene have been found to be the most thermodynamically and kinetically favorable pathway. Also, in all cases, insertion of the first styrene unit proceeds directly in the eta(3)-coordinated allyl group. The lack of activity of the ansa-dimethylsilylene allyl complex and of the ansa-isopropylidene alkyl complex appears to be mainly due to the thermodynamics of the insertion reaction rather than the height of the activation barrier.