Titanium complexes bearing 2,6-Bis(o-hydroxyalkyl)pyridine ligands in the ring-opening polymerization of L-Lactide and ε-caprolactone

Abstract

A series of titanium (Ti) compounds bearing 2,6-bis(o-hydroxyalkyl)pyridine ligands were synthesized and investigated as catalysts for the ring-opening polymerization of ε-caprolactone (CL) and L-lactide (LA). The Ti complexes with electron-withdrawing groups in pyridine rings or steric bulky groups in hydroxymethyl groups reduced the catalytic activity in CL and LA polymerizations, and MeLPh-TiOPr2 exhibited the highest catalytic activity ([CL] = 0.5 M, [CL]:[Cat] = 100:8, 60 °C, 7.5 h, conversion = 93%; [LA] = 0.5 M, [LA]:[Cat] = 100:8, 60 °C, 3.5 h, conversion = 98%). The density functional theory calculation results revealed that CL interacted with the ligand through weak hydrogen bonds, and then approached the Ti center. After being attacked by isopropyl oxide, the carbonyl group of CL bonded to the Ti center. After the rearrangement and ring opening of CL ring, CL was converted to poly-ε-caprolactone.