Achieving simultaneously high selectivity and high rate in the coupling reaction of CO2 with poorly reacting oxetanes remains a major challenge. Here, the selective and nearly quantitative conversion of the coupling reaction of oxetanes with CO2 into six-membered cyclic organic carbonates (COCs) is described, when a binary metal-free system composed of commercially available alkyl borane and onium iodide salts is used under 10 bar CO2 pressure between 90 and 110 °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 CO2/oxetane coupling reaction, these alkyl boranes activate the cyclic ethers as unambiguously confirmed by density functional theory studies. Upon selecting onium salts other than iodide-based ones, in particular those with poor leaving ability, the process is driven toward 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 five-membered COCs from epoxides and CO2.