Anionic Ring-Opening Polymerization of Lactide

Anionic ROP of lactide has been much less investigated than the coordination insertion approach. Although higher activities might be anticipated for anionic promoters that typically display strong nucleophilic and/or basic character, the deleterious contribution of transesterification and racemization reactions might be expected to be significantly more important when naked or loosely bonded anionic species are involved [48]. 

Kricheldorf et al. [49] demonstrated the anionic ROP ofL-lactide in solution at room temperature with potassium ieri-butoxide and butyllithium. They found that the monomer conversion did not exceed 80%, and the anionic initiator and 

Kasperczyk et al. [54, 55] demonstrated that the use of lithium terf-butoxide as a promoter is also interesting for the stereocontrolled polymerization of rac-lactide, via chain-end control, leading to highly heterotactic microstructures with stereoselectivities of 90% at room temperature and 94% at - 20 °C. However, these results were obtained at only 35% monomer conversion, and due to transesterification reactions, the regular microstructure of the chain tended to be significantly reduced when the monomer conversion was increased. 

---

Figure 3.6 Proposed mechanism for anionic ring-opening polymerization of lactide by Bredereck-type reagents [59]...

---