Zinc complexes coordinated by bipyridine-phenolate ligands as an efficient initiator for ring-opening polymerization of cyclic esters

A series of zinc complexes coordinated by different bipyridine-phenolate (BpyPh) ligands, 2-([2,2′-bipyridin]-6-yl)-4,6-di-tert-butylphenol (BpyPh2,4-tBu-H, 3) and 2-([2,2′-bipyridin]-6-yl)-4-(tert-butyl) phenol (BpyPh4-tBu-H, 4) have been synthesized. The reaction of BpyPh2,4-tBu-H with ZnEt2 gave a six-coordinated complex of [(BpyPh2,4-tBu)2Zn] (5) with the distorted octahedral geometry. However, the dinuclear complexes of [(BpyPh)Zn(μ-OBn)]2 (6 and 7) were formed when benzyl alcohol was added and then can be converted to mononuclear zinc benzylalkoxide species by the increased temperature. The ε-CL and L-LA polymerizations initiated by complex 6 showed the living characteristics of narrow molecular weight distribution (PDIs < 1.15) and the capability of block copolymer synthesis demonstrated by the formation of PCL-b-PVL and PCL-b-PHB. The ring-opening polymerizations of cyclic esters initiated by complex 5, however, was not effective possibly due to the highly steric hindered metal center.