A comprehensive computational study on the ring-opening polymerization of propylene oxide catalyzed by uranyl chloride [UO2Cl2(THF)(3)] and the uranyl aryloxide [UO2(OAr)(2)(THF)(2)] (Ar = 2,6-Bu2C6H3) is reported. The initiation and propagation steps have been probed and significant differences between the two catalysts discovered. The initiation step involving uranyl chloride is an intermolecular process because the orientation of the lone pair on the initiating chloride nucleophile is optimally oriented toward the empty sigma*-antibonding orbital of the epoxide, which lowers the activation barrier by 22 kcal mol(-1). Thus, initiation is orbitally controlled. Propagation occurs through a dimeric species, and low-temperature fluorescence spectroscopy has been used to probe this experimentally. In contrast the initiation step for the uranyl aryloxide catalyzed mechanism is intramolecular because of the steric constraints imposed by the bulky substituents on the aryl ring and the fact that the lone pair on the nucleophile is able to approach the propylene oxide coordinated to the same uranium center. Thus, initiation is principally sterically controlled. Propagation is, however, intermolecular, and this can be traced to steric effects. Experimental evidence in the form of fluorescence spectroscopy and diffusion NMR has been used to explore the propagation process in solution.