Ring-opening polymerization of L-lactide with rare earth aryloxides substituted by various alkyl groups

Rare earth aryloxides substituted by various alkyl groups [Ln(OAr)3] such as methyl, isopropyl and tertbutyl, were used as single component catalysts to affect ring-opening polymerization of L-lactide (LLA). The catalytic activity, polymerization characteristics, polymerization kinetics and the mechanism were studied. It was found that the catalytic activity of rare earth aryloxides is influenced by both the structure and the number of alkyl groups on the phenyl ring. The stronger the electron-donation ability of the alkyl group, the higher the catalytic activity will be. An increase in the number of the substitute group will result in a higher catalytic activity. Lanthanum tris(2,4,6-tri-tert-butylphenolate) [La(OTTBP)3] exhibits the highest activity among all lanthanum aryloxides. According to the 1H-NMR data, it was proposed that the LLA polymerization proceeded via a coordination-insertion mechanism involving cleavage of acyl-oxygen bond of the lactide.