Organic Catalysis for the Polymerization of Lactide and Related Cyclic Diesters

Abstract

Poly(lactic acid)/poly(lactide) (PLA) is an aliphatic polyester nowadays emerging as the most popular and promising bio-based, sustainable, (bio)degradable, biocompatible and ecological/environmentally friendly “plastic” candidate. PLA is a commercialized viable alternative to the traditional petrochemically derived polymeric materials in many short-time commodity and engineering applications. Whereas polycondensation of lactic acid gives PLA, ring-opening polymerization (ROP) of lactide (LA) is the preferred route towards well-defined PLA. Following the traditional metal-catalyzed ROP, the organocatalyzed ROP (OROP) of LA, enables, under mild and highly selective polymerization conditions, to prepare PLA with predictable/tunable molar mass values and extremely narrow dispersities. Organic activators most typically used in the ROP of LA, most commonly based on commercially available molecules, revolve around single organic initiators, Brønsted/Lewis acids or bases featuring different functional groups including amines and pyridine derivatives, amidines and guanidines, N-heterocyclic carbenes (NHCs), as well as phosphines and phosphazenes, and mono- or multi-component dual catalytic systems involving hydrogen-bond interactions. Recent advances in LA OROP developed over the past decade are thus accordingly addressed, emphasizing polymerization mechanism aspects and monomer activation modes, catalyst development, methodologies, stereocontrol/stereoselectivity, and functional group incorporation. The OROP of O-carboxy anhydrides (OCAs), which similarly to LA produce poly(α-hydroxy acid)s, thus enabling the introduction of functional groups along the polyester backbone, is also covered.