Titanium complexes of pyrrolylaldiminate ligands and their exploitation for the ring-opening polymerization of cyclic esters

• Published in: Dalton transactions : an international journal of inorganic chemistry 2021-08, Vol.5 (31), p.1964-1981
• Main Authors: Upitak, Kanokon, Wattanathana, Worawat, Nanok, Tanin, Chuawong, Pitak, Hormnirun, Pimpa
• Format: Article
• Language: English
• Subjects: Coordination compounds; Crystallography; Esters; Ligands; Monomers; Nitrogen; Nitrogen atoms; NMR; Nuclear magnetic resonance; Polymerization; Ring opening polymerization; Titanium; Toluene
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d1dt01470f

A series of six-coordinate titanium complexes 1-6 supported by pyrrolylaldiminate ligands were prepared via the reaction of 2 equivalents of ligands and Ti(O i Pr) 4 in toluene at 70 °C. The X-ray structure of 2 revealed that the two ligands were κ 2 -coordinated to the titanium center with the two pyrrole nitrogen atoms in trans positions and the two imine nitrogen atoms in cis positions. All complexes were active initiators for the ring-opening polymerization (ROP) of rac -lactide ( rac -LA), -caprolactone ( -CL), and three substituted -caprolactones (γ-methyl- -caprolactone (γMeCL), γ-ethyl- -caprolactone (γEtCL), and γ-phenyl- -caprolactone (γPhCL)). Polymerizations of all monomers were well controlled, affording predetermined molar masses and narrow dispersity values. Complex 5 exhibited the highest polymerization activities with rac -LA and -CL and its performance was comparable to other highly active six-coordinate titanium complexes reported thus far. Kinetic results revealed a first-order dependency on the monomer concentration, and the rate of polymerization was greatly influenced by the substituent on the imine nitrogen. End-group analysis of the isolated PLA and PCL suggested a coordination-insertion mechanism. The first report on the use of six-coordinate titanium pyrrolylaldiminate complexes for the ROP of rac -lactide and substituted -caprolactones.