Polymerization of methyl methacrylate (MMA) initiated by the rare-earth borohydride complexes [Ln(BH4)3(thf)3] (Ln=Nd, Sm) or [Sm(BH4)(Cp*)2(thf)] (Cp*=-C5Me5) proceeds at ambient temperature to give rather syndiotactic poly(methyl methacrylate) (PMMA) with molar masses n higher than expected and quite broad molar mass distributions, which is consistent with a poor initiation efficiency. The polymerization of MMA was investigated by performing density functional theory (DFT) calculations on an -C5H5 model metallocene and showed that in the reaction of [Eu(BH4)(Cp)2] with MMA the borate [Eu(Cp)2{(OBH3)(OMe)CC(Me)2}] (e-2) complex, which forms via the enolate [Eu(Cp)2{O(OMe)CC(Me)2}] (e), is calculated to be exergonic and is the most likely of all of the possible products. This product is favored because the reaction that leads to the formation of carboxylate [Eu(Cp)2{OOCC(Me)(CH2)}] (f) is thermodynamically favorable, but kinetically disfavored, and both of the potential products from a Markovnikov [Eu(Cp)2{O(OMe)CCH(Me)(CH2BH3)}] (g) or anti-Markovnikov [Eu(Cp)2{O(OMe)CC(Me2)(BH3)}] (h) hydroboration reaction are also kinetically inaccessible. Similar computational results were obtained for the reaction of [Eu(BH4)3] and MMA with all of the products showing extra stabilization. The DFT calculations performed by using [Eu(Cp)2(H)] to model the mechanism previously reported for the polymerization of MMA initiated by [Sm(Cp*)2(H)]2 confirmed the favorable exergonic formation of the intermediate [Eu(Cp)2{O(OMe)CC(Me)2}] (e) as the kinetic product, this enolate species ultimately leads to the formation of PMMA as experimentally observed. Replacing H by BH4 thus prevents the 1,4-addition of the [Eu(BH4)(Cp)2] borohydride ligand to the first incoming MMA molecule and instead favors the formation of the borate complex e-2. This intermediate is the somewhat active species in the polymerization of MMA initiated by the borohydride precursors [Ln(BH4)3(thf)3] or [Sm(BH4)(Cp*)2(thf)].< Réduire