Effects of Ferrocenyl Proximity and Monomer Presence during Oxidation for the Redox-Switchable Polymerization of l‑Lactide

A titanium­(IV) salfen catalyst bearing a redox-active ferrocenyl center in close proximity to the active metal site was examined for the redox-switchable polymerization of l-lactide. The catalyst displayed an atypical β-cis geometry in both the solid- and solution-states, and placement of the redox-active moiety in close proximity to the active metal site was shown to provide an enhancement in catalytic “on–off–on” switching behavior when the redox events were performed in the presence of lactide monomer. More importantly, when comparing oxidized and reduced catalytic species, completely contrasting trends in polymerization behavior were observed, depending on whether the catalyst was oxidized in the absence or in the presence of lactide monomer. On the basis of NMR spectroscopic analysis, we propose that this unusual behavior is a result of a rapid switch in ligand coordination geometry that is facilitated by monomer coordination prior to any redox reactions. Additionally, when redox reactions were conducted in the absence of monomer, a trend in activity that contradicted those of all previously reported Ti-based redox-active catalysts was observed.