Synthesis of mono- and tetra-nuclear β-pyridylenolate zinc complexes and their catalysis toward the ring-opening copolymerization of ε-caprolactone and lactide

Abstract

The synthesis of mono-nuclear β-pyridylenolate zinc complexes L12Zn (1) (L1 = (2-C5H4N)–CH=C(CH3)–O–) and L22Zn (2) (L2 = (2-C5H4N)–CH=C (2-CH3C6H5)–O–) were performed by the protonolysis of the corresponding β-pyridylenol bidentate ligands 1-(2-pyridinyl)-2-propanone (L1H) and 1-(2-methylphenyl)-2-(2-pyridinyl)-ethanone (L2H) with diethylzinc in a 2:1 M ratio, respectively. The unanticipated tetra-nuclear β-pyridylenolate zinc complex [L12Zn-μ-O (Et)ZnEt]2 (3) was isolated by the reaction of L1H with an equiv. of ZnEt2 and oxygenation with air. All the complexes were well characterized by single crystal X-ray crystallography. Complexes 1–3 were tested as an initiator for the ring opening polymerization (ROP) of ε-caprolactone and l-lactide. In the same conditions, 1 and 2 are shown inefficient for ROP due to the lack of suitable initiating group for polymerization, however, tetra-nuclear complex 3 is a highly active single-component initiator in the ROP of ε-caprolactone and l-lactide. Complex 3 was also researched for its catalytic activity in block (PCL-b-PLLA) and gradient (LA-grad-CL) copolymerization of ε-caprolactone and l-lactide.