Aluminum and titanium complexes bearing 2,6-Bis(2-hydroxyphenyl)pyridines for ring-opening polymerization of lactide, ε-caprolactone and their copolymerization: Effect of a ligand on coordination chemistry and reactivity

Abstract

In ring-opening polymerization (ROP) of cyclic esters, the rational design of the catalyst is generally applied to solve the task of providing polyester biocompatibility (non-toxicity) and the problem of copolymer homogeneity when using comonomers of different nature (e. g. lactones and lactides). In the present work, we report the synthesis of aluminum and titanium complexes, based on substituted 2,6-bis(2-hydroxyphenyl)pyridine pro-ligands 1 and 2. It was found that the structure of the pro-ligand drastically affect the structure of the reaction product. Pro-ligand 2 with a bulky tert‑butyl group leads to the monomeric complexes 6 (LAlMe) and 7 (LTi(OiPr)2), while the ligands with less bulky groups lead to the oligomeric Al-containing compounds 3 and 4, and in the case of Ti bis-ligand complex 5 is formed. Complexes 6 (in presence of BnOH) and 7 turned out to be active in the ROP of ε-caprolactone and l-lactide and gave PCL and PLA with high molecular masses. Compound 6 was an effective initiator of copolymerization of ε-caprolactone and l-lactide which led to statistical copolymer poly(LA-stat-CL) with comonomer subunits ratio of 1:1.