The(salen)Co(II)complex((1R,2R)-(-)-1,2-cyclohexanediamino-N,N'-bis(3,5-di-tert-butyl-salicylidene)cobalt(II)) in the presence of an anion initiator, e.g. bromide, has been shown to be a very effective catalytic system for the coupling of oxetane and carbon dioxide, to provide the corresponding polycarbonate with minimal amount of ether linkages. The mechanism of the coupling of oxetane and carbon dioxide has been studied by in situ infrared spectroscopy, where the first formed product is trimethylene carbonate (TMC). TMC is formed by a backbiting mechanism following ring-opening of oxetane by the anion initiator, subsequent to CO2 insertion into the cobalt-oxygen bond. The formation of the copolymer is shown to proceed mostly by way of the anionic ring-opening polymerization of preformed trimethylene carbonate in the presence of an anion in solution. Anions that are good leaving groups, i.e., bromide and iodide, are most effective at affording copolymer via this route. In the presence of greater than 2 equiv of anions the overall rate of copolymer production is decreased, presumably due to inhibition of oxetane monomer binding to the cobalt center. However, under these conditions copolymer formation through ROP of TMC is enhanced, with mass spectral evidence found for the formation of a dimer of TMC.