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

摘要

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)(3)RE(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](OC6H4-4CH(3))(THF)}(2) {[NNOO-R] = Me2NCH2CH2N}{CH2-(2-O-C6H2-R-2-3,5)}(2), R = Me (1), La[NNOO-R] (OC6H4-4-CH3)(THF)(2) [R = Bu-t (2); R = Cumyl (3)], Y[NNOO-R](OC6H4-4-CH3)(THF) [R = Bu-t (4)] and {Y[THFONOO-Bu-t](OC6H4-4-CH3)}(2) {THFONOO-Bu-t = (CH2)(3)OCHCH2N[CH2-(2-O-C6H2-3,5-Bu-t(2))](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.