Polymerization of Lactide by Monomeric Sn(II) Alkoxide Complexes

Monomeric Sn(II) alkoxide complexes of bulky amidinate ligands N,N ‘ -bis(trimethylsilyl)benzamidinate, L SiMe3 -, and N,N ‘ -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 °C. The controlled polymerization of LA by L SiMe2Ph Sn(OCPh 3 ) 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 L SiMe2Ph Sn(OCPh 3 ) 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.