Controlled and stereoselective polymerization of lactide: Kinetics, selectivity, and microstructures
Chiral aluminum isopropoxides based on enantiopure or racemic cyclohexylsalen ligand (Jacobsen ligand) have been prepared and employed for stereoelective/stereoselective ring-opening polymerization of lactide in toluene at 70 degreesC. The kinetics, selectivity of the catalysts, and microstructure of the obtained polylactides, using different combinations of lactide enantiomers and catalysts, were determined. In all cases, polylactides of controlled molecular weight, low polydispersity, and defined end groups were obtained. The polymerizations are first-order in both monomer(s) and catalyst. (R,R)-CyclohexylsalenAlO(i)Pr [(R,R)-1] polymerizes L-lactide significantly faster than D-lactide with a rate constant ratio k(L)/k(D) of similar to14. The polymerization of rac-lactide using (R,R)-1 yields crystalline polymers, for which a selectivity factor of similar to5.5 could be calculated up to 50% conversion based on the optical purity of the isolated polymers. The polymerization of a L-lactide/D-lactide (molar ratio: 80/20) mixture by (R,R)-1 furnishes an isotactic-atactic block copolylactide, which is highly crystalline with a T-m = similar to155 degreesC. Polymerization of rac-lactide applying rac-cyclohexylsalenAlO(i)Pr [rac-1] yields isotactic stereoblock polylactides with a high T-m = similar to185 degreesC and a high degree of crystallinity.