Ring-Opening Polymerization of Epoxides Catalyzed by Uranyl Complexes: An Experimental and Theoretical Study of the Reaction Mechanism

摘要

A comprehensive computational study on thenring-opening polymerization of propylene oxide catalyzed bynuranyl chloride [UO2Cl2(THF)3] and the uranyl aryloxiden[UO2(OAr)2(THF)2] (Ar = 2,6-tBu2C6H3) is reported. Theninitiation and propagation steps have been probed andnsignificant differences between the two catalysts discovered.nThe initiation step involving uranyl chloride is an intermolecularnprocess because the orientation of the lone pair on theninitiating chloride nucleophile is optimally oriented toward thenempty σ*-antibonding orbital of the epoxide, which lowers the activation barrier by 22 kcal mol−1. Thus, initiation is orbitallyncontrolled. Propagation occurs through a dimeric species, and low-temperature fluorescence spectroscopy has been used to probenthis experimentally. In contrast the initiation step for the uranyl aryloxide catalyzed mechanism is intramolecular because of thensteric constraints imposed by the bulky substituents on the aryl ring and the fact that the lone pair on the nucleophile is able tonapproach the propylene oxide coordinated to the same uranium center. Thus, initiation is principally sterically controlled.nPropagation is, however, intermolecular, and this can be traced to steric effects. Experimental evidence in the form ofnfluorescence spectroscopy and diffusion NMR has been used to explore the propagation process in solution.