Redox-Controlled Polymerization of Lactide Catalyzed by Bis(imino)pyridine Iron Bis(alkoxide) Complexes

Bis­(imino)­pyridine iron bis­(alkoxide)
complexes have been synthesized
and utilized in the polymerization of (rac)-lactide.
The activities of the catalysts were particularly sensitive to the
identity of the initiating alkoxide with more electron-donating alkoxides
resulting in faster polymerization rates. The reaction displayed characteristics
of a living polymerization with production of polymers that exhibited
low molecular weight distributions, linear relationships between molecular
weight and conversion, and polymer growth observed for up to fifteen
sequential additions of lactide monomer to the polymerization reaction.
Mechanistic experiments revealed that iron bis­(aryloxide) catalysts
initiate polymerization with one alkoxide ligand, while iron bis­(alkylalkoxide)
catalysts initiate polymerization with both alkoxide ligands. Oxidation
of an iron­(II) catalyst precursor lead to a cationic iron­(III) bis-alkoxide
complex that was completely inactive toward lactide polymerization.
When redox reactions were carried out during lactide polymerization,
catalysis could be switched off and turned back on upon oxidation
and reduction of the iron catalyst, respectively.