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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 (>10⁷), 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 …