Stereoselective Bulk Polymerization of Racemic Lactide for Stereoblock Poly(racemic lactide) Using an Achiral Aluminum Complex.

The article presents a study which examined stereoselective bulk polymerization of racemic lactide (rac-LA) for stereoblock polyracemic lactide using an achiral aluminum complex. Poly(-L-lactide) (PLLA) is a renewable and environmentally benign polymer with biodegradable and biocompatible features. PLLA is commercially synthesized via the ring-opening polymerization of L-lactide with high enantiomeric purity, because the polymerization of lactide with low optical purity using conventional catalysts affords amorphous polylactide due to its stereoirregularity. The industrial block bulk process for the synthesis of PLLA adopts a high reaction temperature to keep the reaction mixture molten for direct extrusion from the reactor. In this process, the amount of the catalyst must be reduced to a minimum level as a consequence of minimizing the contaminates in the product, therefore, a high catalytic activity is one of the essential components for the development of an industrial polymerization catalyst. In the study, the highly reactive and stereoselective polymerization of rac-LA in bulk at 130-180°C using an achiral aluminum complex to produce the crystalline isotactic poly(rac-LA), in which the T_m values are comparable to that of homochiral PLLA. All manipulations were carried out using standard Schlenk-line techniques. Molecular weights of the polymers were estimated by size-exclusion chromatography using polystyrene standards. For the preparation of the catalysts, a solution of each ligand (0.50 mmol) in toluene (0.50 mL) was added 0.10 M Et₃ Al in toluene (0.50 mL, 0.050 mmol) under N₂ at room temperature. The clear solution was stirred at 70°C for 3h (0.050 M catalyst).