Synthesis of amine-bridged bis(phenolate) rare-earth metal aryloxides and their catalytic performances for the ring-opening polymerization of l-lactic acid O-carboxyanhydride and l-lactide

Abstract

A series of rare-earth metal complexes stabilized by different amine bridged bis(phenolate) ligands were synthesized and well characterized. Their catalytic performances for the ring-opening polymerization of L-lactic acid O-carboxyanhydride (L-lacOCA) and L-lactide were explored. Sequential reactions of amine bridged bis(phenol) with (C₅H₅)₃RE(THF) in a 1 : 1 molar ratio, followed by 1 equivalent of p-cresol gave the amine bridged bis(phenolate) rare-earth metal aryloxides {La[NNOO-R](OC₆H₄-4-CH₃)(THF)}₂ {[NNOO-R] = Me₂NCH₂CH₂N}{CH₂-(2-O-C₆H₂-R₂-3,5)}₂, R = Me (1), La[NNOO-R](OC₆H₄-4-CH₃)(THF)₂ [R = ^tBu (2); R = Cumyl (3)], Y[NNOO-R](OC₆H₄-4-CH₃)(THF) [R = ^tBu (4)] and {Y[^THFONOO-^tBu](OC₆H₄-4-CH₃)}₂ {^THFONOO-^tBu = (CH2)₃OCHCH₂N[CH₂-(2-O-C₆H₂-3,5-^tBu₂)]₂} (5) in good isolated yields. Single-crystal structure determination revealed that complexes 1 and 5 have dimeric structures, whereas complexes 2 and 3 have monomeric structures in the solid state. It was found that these complexes are efficient initiators for the ring-opening polymerization of L-lacOCA, and the overall coordination environments around the metal centers and the ionic radii of the metal centers have an obvious influence on the catalytic performance. A comparative study on the polymerization kinetics of L-lacOCA and L-lactide initiated by complex 5 was carried out for the first time. The results obtained demonstrated that the polymerization of L-lacOCA and L-lactide is first order for the catalyst and monomer concentration in dichloromethane, respectively, and the Gibbs energy of activation of L-lacOCA and L-lactide is found to be essentially the same. Mechanistic studies revealed that L-lacOCA polymerization initiated by these rare-earth metal complexes proceeded via a coordination–insertion mechanism.