Monomeric Sn(II) alkoxide complexes of bulky amidinate ligands NN'-bis(trimethylsilyl)-benzamidinate, [L-siMe3](-), and NN'-bis(dimethylphenylsilyl)benzamidinate, [L-SiMe2Ph](-), 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 degreesC. The controlled polymerization of LA by (LSn)-Sn-SiMe2Ph(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 [(LSn)-Sn-SiMe2Ph(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.