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.