A series of zinc benzylalkoxide complexes, [L n Zn(l-OBn)] 2 (L ¼ L 1 H-L 5 H), supported by NNO-tridentate ketiminate ligands with various electron withdrawing-donating subsituents have been synthesized and characterized. X-ray crystal structural studies revealed that complexes 2b and 4b are dinuclear bridging through the benzylalkoxy oxygen atoms with penta-coordinated metal centers. All the metal complexes have acted as efficient initiators for the ring-opening polymerization of L-lactide (within 12 min, 0 C). Remarkably, a molecular weight of PLLA up to 580,000 can be achieved using [(L 5 Zn(l-OBn)] 2 (5b) as an initiator.The kinetic studies for the polymerization of L-lactide with complex 3b at À10 C corresponded to first-order reactions in the monomer. The ring-opening polymerization (ROP) of e-caprolactone, e-decalactone, b-butyrolactone and their copolymer with complex 3b was investigated.

A series of magnesium benzylalkoxide complexes, [LnMg(μ‐OBn)]2 (1–14) supported by NNO‐tridentate pyrazolonate ligands with various electron withdrawing‐donating subsituents have been synthesized and characterized. X‐ray crystal structural studies revealed that Complexes 1–3, 5, 7, 9, and 10 are dinuclear bridging through benzylalkoxy oxygen atoms with penta‐coordinated metal centers. All of these complexes acted as efficient initiators for the ring‐opening polymerization of L‐lactide and rac‐lactide. Based on kinetic studies, the activity of these metal complexes is significantly influenced by the electronic effect of the ancillary ligands with the electron‐donating substituents at the phenyl rings enhancing the polymerization rate. In addition, the “living” and “immortal” character of 6 has paved a way to synthesize as much as 40‐fold polymer chains of polylactides with a very narrow polydispersity index in the presence of a small amount of initiator. Among all of magnesium complexes, Complex 6 exhibits the highest stereoselectivity toward ring‐opening polymerization of rac‐lactide with Pr up to 88% in THF at 0 °C. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Four metal benzylalkoxides, [L 2 M 2 (l-OBn) 2 ] (M ¼ Mg or Zn), based on NNO-tridentate ketiminate ligands are synthesized and characterized. X-ray crystal structural studies of [(L 1 ) 2 Mg 2 (l-OBn) 2 ] (1a) and [(L 1 ) 2 Zn 2 (l-OBn) 2 ] (1b) (L 1 -H ¼ (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 Pr 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.

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