New insights into the polymerization of methyl methacrylate initiated by rare-earth borohydride complexes: A combined experimental and computational approach

作者：Barros Noemi; Schappacher Michele; Dessuge Paul; Maron Laurent^*; Gullaume Sophie M

来源：Chemistry - A European Journal, 2008, 14(6): 1881-1890.

DOI：10.1002/chem.200701438

• boron
• density functional calculations
• lanthanides
• methyl methacrylate
• polymerization

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*=eta-C5Me5) proceeds at ambient temperature to give rather syndiotactic poly(methyl methacrylate) (PMMA) with molar masses M, 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 eta-C2H5 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(Me-2)(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),(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)1(thf)3] or [Sm(BH4)(CP*)2(thf)].

• 出版日期2008