Ring-opening polymerization of cyclic esters initiated by zirconium, titanium and yttrium complexes

Abstract

A series of dinuclear zirconium [LZr(OiPr)2]2[(μ-OiPr)2] (1–3) as well as mononuclear titanium [LTi(OiPr)2] (4), yttrium [LYCl] (5) and zirconium [LZr(OiPr)2] (6–7) salen-type complexes (L = dianionic [ONNO]-tetradentate ancillary ligand) have been prepared and characterized. Dinuclear zirconium salen complexes [LZr(OiPr)2]2[(μ-OiPr)2] (1–3) have shown effective activity toward the ring opening polymerization of lactides and ε-caprolactone. However, mononuclear titanium, yttrium and zirconium complexes 4–7 are almost inactive for the ROP of L-lactide. Solvent-free bulk polymerization of L-lactide initiated by dinuclear zirconium complex 2 with a conversion >90% can be achieved within 10 min, yielding high molecular weight polylactide with low polydispersity index. Kinetic studies reveal the ROP of L-lactide initiated by dinuclear zirconium complex 3 has a first-order dependency on [LA].