We report the use of an organic catalyst system capable of switching between active and dormant propagating states during the ROP of cyclic monomers. While the ROP of both 蔚-caprolactone and trimethylene carbonate proceeds under nitrogen, the simple addition of CO2 results in a dormant 鈥渙ff鈥?state. Cycling between atmospheres provides the ability to regulate the molecular weights of the resulting polymers without appreciable loss of catalytic activity for several 鈥渙n/off鈥?cycles.