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(BH 4 ) 3 (thf) 3 ] (Ln=Nd, Sm) or [Sm(BH 4 )(Cp*) 2 (thf)] (Cp*=η‐C 5 Me 5 ) proceeds at ambient temperature to give rather syndiotactic poly(methyl methacrylate) (PMMA) with molar masses M̄ 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 η‐C 5 H 5 model metallocene and showed that in the reaction of [Eu(BH 4 )(Cp) 2 ] with MMA the borate [Eu(Cp) 2 {(OBH 3 )(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)(CH 2 )}] ( f ) is thermodynamically favorable, but kinetically disfavored, and both of the potential products from a Markovnikov [Eu(Cp) 2 {O(OMe)CCH(Me)(CH 2 BH 3 )}] ( g ) or anti‐Markovnikov [Eu(Cp) 2 {O(OMe)CC(Me 2 )(BH 3 )}] ( h ) hydroboration reaction are also kinetically inaccessible. Similar computational results were obtained for the reaction of [Eu(BH 4 ) 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 BH 4 thus prevents the 1,4‐addition of the [Eu(BH 4 )(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(BH 4 ) 3 (thf) 3 ] or [Sm(BH 4 )(Cp*) 2 (thf)].