Recent Advances in Metal-Mediated Stereoselective Ring-Opening Polymerization of Functional Cyclic Esters towards Well-Defined Poly(hydroxy acid)s: From Stereoselectivity to Sequence-Control.

Poly(hydroxy acid)s are a family of biocompatible and (bio)degradable polyesters with various outcomes in different domains of application. To date, poly(hydroxy acid)s are best prepared by ring-opening polymerization (ROP) of the corresponding cyclic esters. Using racemic chiral monomers featuring side-chain groups enables to access, providing a stereoselective catalyst/initiator system is implemented, stereoregular functional polymers, thereby improving their physico-chemical properties, and ultimately, widening their range of uses. Here, we highlight a few important advances in metal-mediated stereoselective ROP of cyclic esters towards the synthesis of (functional) stereoregular poly(hydroxy acid)s that have recently been disclosed, emphasizing on (functional) ß- and γ-lactones, diolide and O-carboxyanhydride (OCA) monomers and yttrium-based catalysis. Fine-tuning of the substituents flanked on the catalyst ligand enables reaching poly(hydroxy acid)s with syndiotactic and also isotactic microstructures. The stereocontrol mechanisms at work and their probable origin, relying on steric but also electronic factors imparted in particular by the ligand substituents, are discussed. Taking advantage of such stereoselective ROPs, original copoly(hydroxy acid)s with gradient or alternated patterns then become accessible from the use of mixtures of chemically different, oppositely configured enantiopure monomers.