Monomeric Sn(II) alkoxide complexes of bulky amidinate ligands N,N′-bis(trimethylsilyl)-benzamidinate, [LSiMe3]-, and N,N′-bis(dimethylphenylsilyl)benzamidinate, [LSiMe2Ph]-, and the corresponding bis(amidinate) compounds were synthesized and structurally characterized by single-crystal X-ray diffraction. The monomeric nature of these complexes in the solid state was retained in solution as evinced by NMR spectroscopy. Both alkoxide complexes were active for DL-lactide (LA) polymerization in toluene solution at 80°C. The controlled polymerization of LA by LSiMe2PhSn(OCPh3) was observed in the presence of 1.0 equiv of an exogenous alcohol. Under these conditions, the polymerization reaction was found to be first order in [LA] and about one-third order in [LSiMe2PhSn(OCPh3)]0. The polymerization kinetics were further analyzed by applying a model that invokes aggregation of the active species, with one inactive, aggregated form and one active, unaggregated form. The importance of aggregation in understanding catalyst reactivity is highlighted.