Ring-Opening Polymerization of rac- and meso-Lactide Initiated by Indium Bis(phenolate) Isopropoxy Complexes

Ring-opening polymerization of rac- and meso-lactide initiated by indium bis(phenolate) isopropoxides {1,4-dithiabutanediylbis(4,6-di-tert-butylphenolate)}(isopropoxy)indium (1) and {1,4-dithiabutanediylbis(4,6-di(2-phenyl-2-propyl)phenolate)}(isopropoxy)indium (2) is found to follow first-order kinetics for monomer conversion. Activation parameters H-double dagger and S-double dagger suggest an ordered transition state. Initiators 1 and 2 polymerize meso-lactide faster than rac-lactide. In general, compound 2 with the more bulky cumyl ortho-substituents in the phenolate moiety shows higher polymerization activity than 1 with tert-butyl substituents. meso-Lactide is polymerized to syndiotactic poly(meso-lactides) in THF, while polymerization of rac-lactide in THF gives atactic poly(rac-lactides) with solvent-dependent preferences for heterotactic (THF) or isotactic (CH2Cl2) sequences. Indium bis(phenolate) compound rac-(1,2-cyclohexanedithio-2,2-bis{4,6-di(2-phenyl-2-propyl)phenolato}(isopropoxy)indium (3) polymerizes meso-lactide to give syndiotactic poly(meso-lactide) with narrow molecular weight distributions and rac-lactide in THF to give heterotactically enriched poly(rac-lactides). (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4983-4991