Contrasting Roles of Counterions in Anionic Ring-Opening Polymerization Mediated by Heterocycle Organocatalysts

A library of structurally related heterocycles containing N-H motifs are explored as ring-opening polymerization (ROP) precatalysts. Upon deprotonation of these heterocycles with appropriate bases, the corresponding salts are formed, which catalyze the ROP of various lactones and cyclic carbonates, affording polymers with dispersity values ranging from 1.01 to 1.12. These catalysts exhibit a wide range of catalytic activities, spanning over 7 orders of magnitude (>10(7)), with their relative rates generally correlating to the pK(a) of the N-H group in the neutral heterocycle. Despite apparent structural and electronic similarities, these heterocycle catalysts display markedly different kinetic behaviors, regarding the identity of different cations. Kinetic and NMR studies have revealed two distinct sets of mechanisms: small alkali-metal cations such as Li+ and Na+ reduce the activity of imidazol-(in)-e-derived catalysts due to their tendency to associate with the alkoxide chain end, thus inhibiting its propagation; conversely, these cations form robust cation-pi assemblies with indolocarbazole anions, simultaneously binding and activating monomer carbonyls toward the nucleophilic attack, resulting in a significant rate enhancement. This distinctive activation motif of the indolocarbazole sets it apart from other catalysts by utilizing cations as a potent handle for modulating the polymerization reactivity. Coupled with its high availability, good solubility, high activity, moderate basicity, and high selectivity, the indolocarbazole heterocycle emerges as one of the most versatile organocatalysts for ring-opening polymerization.