Mechanistic Comparison of Cyclic Ester Polymerizations by Novel Iron(III)−Alkoxide Complexes:  Single vs Multiple Site Catalysis

The complexes Fe2(OCHPh2)6 and L2FeOR (R = Et or CHPh2, L = N,N'-bis(trimethylsilyl)benzamidinate) were structurally characterized, and comparative studies of the behavior of those compounds comprising the same alkoxide (Ph2HCO-) in polymerizations of -caprolactone (CL) and D,L-lactide (LA) were performed. Both Fe2(OCHPh2)6 and L2FeOCHPh2 are effective polymerization catalysts, as reflected by molecular weight control, polydispersities, and end group analysis, but the diiron complex generally exhibits greater polymerization control, particularly for CL. Kinetic investigations of the polymerization of CL revealed the same first-order dependence on CL for both catalysts, but different orders in catalyst that signified a distinct contrast in mechanism. Analysis that invoked the presence of a termination-causing impurity at low concentration yielded a first-order dependence on Fe2(OCHPh2)6, but the order in L2FeOCHPh2 was found to be one-half. This fractional dependence was interpreted by using a model of active chain aggregation. Comparison of the derived propagation rate constants (k(prop)) revealed a approximately 50-fold greater value for the diiron complex compared to the single site mononuclear compound. Implications of these findings for understanding cyclic ester polymerization mechanisms and catalyst design are discussed.