Synthesis, Structure, and Catalytic Studies of Mixed Lithium−Magnesium and Sodium−Magnesium Complexes: Highly Isospecific Initiators for Polymerization of Methyl Methacrylate

The unusual lithium n-butylmagnesium complex [(EDBP)Mg(μ2-nBu)Li(Et2O)]2 (2) and sodium
n-butylmagnesium complex [(EDBP)Na(Et2O)MgnBu]2 (5), lithium−magnesium enolate, {(EDBP)Mg[μ2-OC(Mes)CH2]Li(Et2O)}2 (3), and the sodium aggregate, [(EDBP)Na2]4 (4), have been synthesized
and structurally characterized. Among them, 2, 3, and 5 have been used as initiators for the polymerization
of methyl methacrylate. The reaction of 2,2‘-ethylidenebis(4,6-di-tert-butylphenol) (EDBP-H2) with nBuLi in diethyl ether gives [(EDBP-H)Li(Et2O)3] (1). The dimeric lithium n-butylmagnesium complex 2
was obtained from the reaction of 1 with a stoichiometric amount of MgnBu2. Alternatively, 2 can also
be prepared by the reaction of [(EDBP)Mg(Et2O)]2 (A) with a stoichiometric amount of nBuLi in Et2O.
Furthermore, the reaction of 2 with 2‘,4‘,6‘-trimethylacetophenone (MesC(O)CH3, Mes = 2,4,6-Me3C6H2)
produces 3. Reaction of EDBP-H2 with excess sodium metal in Et2O furnishes compound 4. In the presence
of a stoichiometric amount of MgnBu2 in Et2O, 4 can be converted to the dimeric sodium−magnesium
mixed-metal complex [(EDBP)Na(Et2O)MgnBu]2 (5). Experimental results show that 2, 3, and 5 efficiently
initiate the anionic polymerization of methyl methacrylate.