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

TLDR

Tin(II) β‑diketiminate complexes have been studied as initiators for rac‑lactide polymerization. The propagating species was modeled by synthesizing (MeBDIDIPP)SnOCH(Me)COOiPr and computationally showing that the first lactide insertion is facile while subsequent insertions are slower, accounting for the induction period. Kinetic studies reveal first‑order lactide dependence with a significant induction period, and the polymerizations are well‑controlled and living, yielding narrow MWDs; less hindered and halogenated ligands accelerate propagation, and all initiators produce heterotactic polylactide via a chain‑end control mechanism dominated by the Sn 5s² lone pair.

Abstract

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.