Organocatalyzed Step-Growth Polymerization through Desymmetrization of Cyclic Anhydrides: Synthesis of Chiral Polyesters

The polymerization of prochiral bis-anhydrides with diols catalyzed by a cinchona alkaloid was shown to provide chiral polyesters in good yields and with high levels of stereocontrol. The structures of the polyesters were determined by H-1 and C-13 NMR analyses, whereas their size was estimated by both size-exclusion chromatography (SEC) and MALDI-TOF mass spectrometry, which indicated that moderate degrees of polymerization were attained through this step-growth polymerization. The enantioselectivity of the process was evaluated by using chiral HPLC analysis of the bis-lactones resulting from a controlled chemoselective degradation of the polyesters. The best stereocontrol was reached for oligomers formed from bis-anhydride and diol monomers bearing rigid aromatic spacers between the reactive functional groups. In this case, average enantioselectivities were comparable to those observed during ring-opening of simple anhydrides with similar alcohols. In contrast, the use of more flexible spacers between reactive entities generally led to lower levels of stereocontrol.