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 L 1 2 Zn ( 1 ) (L 1 = (2-C 5 H 4 N)–CH=C(CH 3 )–O–) and L 2 2 Zn ( 2 ) (L 2 = (2-C 5 H 4 N)–CH=C (2-CH 3 C 6 H 5 )–O–) were performed by the protonolysis of the corresponding β-pyridylenol bidentate ligands 1-(2-pyridinyl)-2-propanone (L 1 H) and 1-(2-methylphenyl)-2-(2-pyridinyl)-ethanone (L 2 H) with diethylzinc in a 2:1 M ratio, respectively. The unanticipated tetra-nuclear β-pyridylenolate zinc complex [L 1 2 Zn- μ -O (Et)ZnEt] 2 ( 3 ) was isolated by the reaction of L 1 H with an equiv. of ZnEt 2 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 e-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 e-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 e-caprolactone and l -lactide.