Heteroleptic β-Ketoiminate Magnesium Catalysts for the Ring-Opening Polymerization of Lactide
Swarup Ghosh, Eduard Glöckler, Christoph Wölper, Alexander Tjaberings, André H. Gröschel, Stephan Schulz
Abstract
A series of heteroleptic β-ketoimine magnesium complexes L1L′1 Mg2Cp*(1), L22Mg2Cp2 (2), L12Mg2(OAr)2 (3 and 4), and L22Mg2(OAr)2 (5 and 6) (L1 = Me2NC2H4NC(Me)CHC(Me)O, L′1 = Me2NC2H4NC(CH2)CHC(Me)O, L2 = Me2NC3H6NC(Me)CHC(Me)O; Ar = Ph and 2,6-t-Bu2-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).