Ring-opening polymerization of rac-lactide and α-methyltrimethylene carbonate catalyzed by magnesium and zinc...

A series of racemic 2-[(2′-(dimethylamino)binaphthyl-2-ylimino)methyl]-4-R2-6-R(1)-phenols (L1H–L6H) were reacted with 1 equiv. of Mg[N(SiMe3)2]2 or Zn[N(SiMe3)2]2 to provide three heteroleptic magnesium silylamido complexes [(L4–(6))MgN(SiMe3)2] (4a: R(1) = R(2) = cumyl; 5a: R(1) = CPh3, R(2) = Me; 6a: R(1) = CPh3, R(2) = (t)Bu) and one heteroleptic zinc silylamido complex [(L(4))ZnN(SiMe3)2] (4b: R(1) = R(2) = cumyl). The molecular structures of complexes 4a, 4b and a homolepic zinc complex (L(5))2Zn (5b′) were further confirmed by X-ray diffraction studies. In the solid state, the heteroleptic magnesium complex 4a has a four-coordinated metal core chelated by three donor atoms of the ligand and one bis(trimethylsilyl)amido group; however, the heteroleptic zinc complex 4b has a three-coordinated metal core chelated by two donor atoms of the ligand and one bis(trimethylsilyl)amido group, where the NMe2 group of the iminophenolate ligand is not coordinated to the zinc center. Magnesium complexes 4a–6a efficiently initiated the ring-opening polymerization (ROP) of rac-lactide in the presence/absence of 2-propanol, affording atactic polylactides in toluene and heterotactic bias polylactides in THF. Zinc complex 4b exhibited the highest stereoselectivity for the ROP of rac-lactide, affording substantially heterotactic polylactides in both solvents (Pr = 0.80–0.84). The kinetic studies of the polymerization revealed that the rate of polymerization is first-order in both monomer and catalyst concentrations. All these complexes were also very active in the polymerization of racemic α-methyltrimethylene carbonate (α-MeTMC). Detailed analyses using 1H and 13C NMR spectroscopy indicated the preferential ring-opening of α-MeTMC at the most hindered oxygen–acyl bond and zinc complex 4b gave the highest regioselectivity (Xreg = 0.98).