New Insights into the Polymerization of Methyl Methacrylate Initiated by Rare-Earth Borohydride Complexes: A Combined Experimental and Computational Approach

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*=η‐C5Me5) proceeds at ambient temperature to give rather syndiotactic poly(methyl methacrylate) (PMMA) with molar masses M̄n 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 η‐C5H5 model metallocene and showed that in the reaction of [Eu(BH4)(Cp)2] with MMA the borate [Eu(Cp)2{(OBH3)(OMe)CC(Me)2}] (e‐2) complex, which forms via the enolate [Eu(Cp)2{O(OMe)CC(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{OOCC(Me)(CH2)}] (f) is thermodynamically favorable, but kinetically disfavored, and both of the potential products from a Markovnikov [Eu(Cp)2{O(OMe)CCH(Me)(CH2BH3)}] (g) or anti‐Markovnikov [Eu(Cp)2{O(OMe)CC(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)CC(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)]. Rare‐earth initiators: The polymerization of MMA initiated by [Ln(BH4)3(thf)3] (Ln=Nd, Sm) or [Sm(BH4)(Cp*)2(thf)] has been studied and the experimental results correlated to the mechanism investigated by DFT (see figure).