Ring-opening polymerization of ε-caprolactone, β-butyrolactone and lactides by β-ketiminate pyrazolonate zinc complexes: Preparation and characterization

•Zinc complexes supported by N,O-bidentate β-ketiminate pyrazolonate ligands•Complex 4 catalyzes the ROP of rac-lactide and ε-caprolactone in well performance.•Equilibrium of mononuclear–dinuclear structure has been confirmed by VT-NMR analysis.•The mononuclear form is suggested to be the active species during polymerization.

Four bidentated iminopyrazolones (L1H–L4H) were prepared by the reaction of 3-methyl-4-(4-methylbenzoyl)-1-phenyl-1H-pyrazol-5(4H)-one with POCl3 followed by the addition of the corresponding anilines. The reaction of less steric bulky ligands, L1H–L3H, with ZnEt2 yields homoleptic zinc complexes [L2Zn] (1–3), respectively. However, the most steric bulky ligand, L4H reacts with ZnEt2 producing heteroleptic complex [L4ZnEt] which further reacts with benzyl alcohol (BnOH) giving [L4Zn(OBn)]2 (4). All of these complexes were characterized by elemental analyses, NMR spectroscopic study as well as X-ray diffraction. Experimental results indicate that complexes 1–3 are inactive toward the ring opening polymerization (ROP) of rac-lactide. On the other hand, complex 4 showed excellent activity in the ROP of cyclic esters such as rac-lactide, ε-caprolactone and β-butyrolactone.

Graphical abstractZinc complexes derived from N,O-bidentate β-ketiminate pyrazolonate ligands were synthesized and characterized. 4, a zinc benzylalkoxide complex, is an active catalyst for the ring-opening polymerization of rac-lactide and ε-caprolactone giving polymers with the expected molecular weights and narrow PDIs and with effectively suppressing transesterification approved by MALDI-TOF analysis.Figure optionsDownload full-size imageDownload as PowerPoint slide