Ionic H-bonding organocatalysts for the ring-opening polymerization of cyclic esters and cyclic carbonates

Ring-opening polymerization (ROP) of cyclic monomers is a prevalent and convenient method for the synthesis of well-defined polymers with initiators/catalysts that promote a nucleophilic or electrophilic attack on the monomers. Selective activation of functional groups or linkages of the monomer without those carried out in the polymer chains, especially at high conversion, is one of the challenges faced by ROP catalysts. H-bonding organocatalysts can offer precise selectivity for ROP in a wide range of monomers. The firstly reported neutral H-bonding organocatalysts are characterized by high selectivity but long reaction time and low reactivity. In contrast, ionic H-bonding organocatalysts, which have extensively developed over the last ten years, exhibit fast polymerization rates and high selectivity. Besides, some ionic H-bonding organocatalysts with good thermal stability and high reactivity can be used in a wide range of ROP temperatures (-60 °C to over 200 °C). Furthermore, ionic H-bonding organocatalysts comply with biosafety principles promoted by green chemistry. This review covers the mechanistic insights (monomer activation, initiator/chain-end activation, synergistic activation, and bifunctional activation) of ionic H-bonding organocatalytic ROP, as well as the strategies for monomer and initiator/chain-end activation.