Synthesis, characterization and catalytic activity study of zinc aminobis(phenolate) complexes towards ring-opening polymerization of rac-lactide

Abstract

A series of bimetallic and trimetallic zinc complexes of amino-bis(phenolato) ligands was synthesized and characterized. Bimetallic complexes (ZnEt)₂[L1] (1a) and (ZnEt)₂[L2] (1b) (where [L1] = n-propylamine-N,N-bis(2-methylene-4,6-di-t-butylphenolate) and [L2] = n-propylamine-N,N-bis(2-methylene-6-t-butyl-4-methylphenolate) were prepared via reaction of the proligands H₂[L1] and H₂[L2] with ZnEt₂ in pentane. A THF adduct (ZnEt)₂[L1]·THF (1a·THF) was obtained by using THF as solvent. Treatment of 1a with one or two equiv. of isopropyl alcohol gave a trimetallic zinc complex Zn₃(i-PrO)₂[L1]₂ (2). The catalytic activities of these complexes towards ring-opening polymerization (ROP) of rac-lactide were studied in the absence and presence of benzyl alcohol or isopropyl alcohol. These complexes exhibit moderate to good activity for ROP of rac-lactide both in the melt phase and solution. The influence of catalyst and co-initiator loading were studied and thermodynamic activation parameters were determined. Characterization of the polymers by GPC and MALDI-TOF MS showed controlled and living polymerizations for rac-lactide in the presence of alcohol.