Ring‐opening polymerization of lactides initiated by magnesium and zinc complexes based on NNO‐tridentate ketiminate ligands: Activity and stereoselectivity studies

Four metal benzylalkoxides, [L 2 M 2 (μ‐OBn) 2 ] (M = Mg or Zn), based on NNO‐tridentate ketiminate ligands are synthesized and characterized. X‐ray crystal structural studies of [(L 1 ) 2 Mg 2 (μ‐OBn) 2 ] ( 1a ) and [(L 1 ) 2 Zn 2 (μ‐OBn) 2 ] ( 1b ) (L 1 ‐H = (Z)‐4‐((2‐(dimethylamino)ethylamino)(phenyl)methylene)‐3‐methyl‐1‐phenyl‐pyrazol‐5‐one) reveal that both complexes 1a and 1b are dinuclear species whereas the geometry around the metal center is penta‐coordinated bridging through the benzylalkoxy oxygen atoms in the solid structure. The activities and stereoselectivities of these four complexes toward the ring‐opening polymerization of L ‐lactide and rac ‐lactide are investigated. Polymerization of L ‐lactide initiated by these four metal benzyloxides proceeds rapidly with good molecular weight control and yields polymer with a very narrow molecular weight distribution. The kinetic studies for the polymerization of L ‐lactide with compound 1a show first order in both compound 1a and lactide concentrations with the polymerization rate constant, k , of 6.94 M/min. Besides, experimental results demonstrate that among these metal benzylalkoxides, complex 1a exhibits the highest stereoselectivity with a P r up to 87% and complex 1b possesses the highest activity indicating that the terminal group of NNO‐tridentate ketimine ligands exerts a significant influence on both the reactivity and stereoselectivity of these complexes.