Synthetic, Structural, Mechanistic, and Computational Studies on Single-Site β-Diketiminate Tin(II) Initiators for the Polymerization of <i>r</i><i>ac</i>-Lactide

A family of tin(II) complexes supported by β-diketiminate ligands has been investigated as initiators for the polymerization of rac-lactide. Kinetic studies reveal a first-order dependence on [lactide], but with a significant induction period. Linear plots of Mn versus conversion and [M]o/[I]o versus conversion, along with narrow molecular weight distributions (typically 1.07−1.10), are indicative of well-controlled, “living” polymerizations. Less sterically hindered derivatives promote faster propagation than their bulky analogues, in accord with a more accessible active site. Enhanced rates of polymerization are observed for ligands bearing halogenated N-aryl substituents, a consequence of the more Lewis acidic nature of the Sn(II) centers. All of the initiators exhibit a similar bias toward heterotactic polylactide, which is attributed to a chain-end control mechanism influenced predominantly by the presence of the Sn 5s2 lone pair of electrons rather than the steric or electronic properties of the β-diketiminate ligand. The tin(II) isopropyl-(S)-lactate complex, (MeBDIDIPP)SnOCH(Me)COOiPr (14), has been synthesized as a model compound for the propagating species by treatment of (MeBDIDIPP)Sn(NMe2) with isopropyl-(S)-lactate. An X-ray structure determination showed that the lactate ligand forms a five-membered chelate ring with a weak donor bond from the carbonyl oxygen atom to the tin center. A B3LYP density functional computational study indicates that insertion of the first lactide monomer into the tin(II) alkoxide bond is facile, with the induction period arising from a slower insertion of the second (and possibly third and fourth) monomer units.