Researchers have found a new family of organocatalysts that promote rapid and highly selective ring-opening polymerizations of lactones and carbonates. Previous organocatalysts for ring-opening polymerization have been either fast or selective, but not both, according to Robert M. Waymouth and coworkers at Stanford University and IBM’s Almaden Research Center, who developed the new reaction (Nat. Chem. 2016, DOI: 10.1038/nchem.2574). The anionic thioimidate catalysts are generated by using an alkoxide to deprotonate a thiourea. Waymouth and coworkers note that the catalysts are bifunctional—they simultaneously activate the lactone or carbonate monomer and the growing polymer chain end. This mechanism accounts for the reaction’s high selectivity for chain formation, which leads to well-defined polymer structures with high melting points and narrow molecular weight distributions. Waymouth says the team is using the catalysts to generate oligomers for biomedical applications, and he speculates that similar bifunctional anionic catalysts might prove useful for other types of reactions.