Heteroleptic β-Ketoiminate Magnesium Catalysts for the Ring- Opening Polymerization of Lactide

A series of heteroleptic beta-ketoimine magnesium complexes (LL)-L-1'(1) Mg2Cp*(1), (L2Mg2Cp2)-Mg-2 (2), (L2Mg2)-Mg-1(OAr)(2) (3 and 4), and (L2Mg2)-Mg-2(OAr)(2) (5 and 6) (L'(1) = Me2NC2H4NC(Me)CHC(Me)O, L-'1 = Me2NC2H4NC(CH2)CHC(Me)O, L-2 = Me2NC3H6NC(Me)CHC(Me)O; Ar = Ph and 2,6-t-Bu-2-C6H3) with different steric and electronic properties were synthesized. 1 and 3-6 adopt binuclear structures in the solid state and in solution, whereas 2 forms a monomer/dimer equilibrium in solution. Complexes 1 and 2 showed very poor activity in the ring-opening polymerization (ROP) of racemic-lactide (rac-LA), whereas heteroleptic phenoxide complexes 3-6 are active polymerization catalysts at variable temperature in solution in the absence of any co-initiator, yielding isotactic-enriched polylactide (PLA). The catalytic activity and stereoselectivity is controlled by the electronic and steric properties of the phenoxide substituent and reaction temperature. Kinetic studies with catalyst 3 and 5 proved the first-order dependence on monomer concentration, and mechanistic studies showed that the polymerization reactions follow the coordination insertion mechanism (CIM).