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