Ring-opening polymerization of rac-lactide and α-methyltrimethylene carbonate catalyzed by magnesium and zinc complexes derived from binaphthyl-based iminophenolate ligands

Abstract

A series of racemic 2-[(2′-(dimethylamino)binaphthyl-2-ylimino)methyl]-4-R²-6-R¹-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 [(L^4–6)MgN(SiMe₃)₂] (4a: R¹ = R² = cumyl; 5a: R¹ = CPh₃, R² = Me; 6a: R¹ = CPh₃, R² = ^tBu) and one heteroleptic zinc silylamido complex [(L⁴)ZnN(SiMe₃)₂] (4b: R¹ = R² = cumyl). The molecular structures of complexes 4a, 4b and a homolepic zinc complex (L⁵)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 (P_r = 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 (X_reg = 0.98).