A series of conformationally flexible bis(thio)urea H-bond donors plus base cocatalyst were applied to the ring-opening polymerization (ROP) of lactones. The rate of the ROP displays a strong dependence on the length and identity of the tether, where a circa five methylene-unit long tether exhibits the fastest ROP. Any constriction to conformational freedom is deleterious to catalysis. For the ROP of delta-valerolactone (VL) and epsilon-caprolactone (CL), the bisurea H-bond donors are more effective, but for lactide, the bisthioureas are more active catalysts. The ROP reactions are rapid and controlled across a wide range of reaction conditions, including solvent-free conditions, exhibiting excellent weight control from low M-n to high polymers. The active mechanism is highly dependent on the identity of the base cocatalyst, and a mechanistic rationale for the observations is discussed. Implications for the design of future generation catalysts are discussed.