Lanthanum complexes stabilized by a pentadentate Schiff-base ligand: synthesis, characterization, and reactivity in statistical copolymerization of ε-caprolactone and l-lactide.

ABSTRACT

Developing catalysts that are capable of catalyzing the copolymerization of ε-caprolactone and l-lactide to give random CL/LA copolymers is of great importance. One-pot reaction of La[N(SiMe3)2]3 with 1 equiv. of LH2 (LH2 = NH(CH2CH2N[double bond, length as m-dash]CHC6H2-3,5-tBu2-2-OH)2) in THF at room temperature, followed by protolysis with one equivalent amount of ROH (R = C6H2-2,6-tBu2-4-CH3, tBu, iPr, Bn, and Et) at 60 °C gave the mono-Schiff-base-ligated lanthanum aryloxide complex LLa(OC6H2-2,6-tBu2-4-CH3)(THF) (1), and lanthanum alkyloxide complexes LLaOtBu(THF) (2), [LLaOiPr]2 (3), [LLaOBn]2 (4), and [LLaOEt]2 (5) in 59-69% isolated yields. These lanthanum complexes were capable of initiating the homopolymerization of l-lactide and rac-lactide with extremely high activity, and the copolymerization of ε-caprolactone (ε-CL) and l-lactide (l-LA) to give statistical CL/LA copolymers via a transesterification reaction.