Achieving simultaneously high selectivity and high rate in the coupling reaction of CO 2 with poorly reacting oxetanes remains a major challenge. Here is described the selective and nearly quantitative conversion of the coupling reaction of oxetanes with CO 2 into six-membered cyclic organic carbonates (COCs) when a binary metal-free system composed of commercially available alkyl borane and onium iodide salts is used under 10 bar CO 2 pressure between 90 and 110 o C. Kinetic investigations provide quantitatively the enthalpy and entropy of activation [ΔH ‡ = 6.7 ± 1.2 kcal/mol and ΔS ‡ = -57 ± 4 cal/(mol•K)] of the back-biting, cyclic formation reaction. In addition to forming borate complexes with the anions responsible for the CO 2 /oxetane coupling reaction, these alkyl boranes activate the cyclic ethers as unambiguously confirmed by DFT studies. Upon selecting onium salts other than iodide-based ones, in particular those with poor leaving ability, the process is driven towards chain growth and the formation of linear polycarbonates. This metal-free system also exhibits both versatility and an activity comparable to that of metal catalysts (turnover frequency values of 14 -124 h -1 ) for the synthesis of various fivemembered COCs from epoxides and CO 2 .