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.
A cationic iron(III) complex was active for the polymerization of various epoxides, whereas the analogous neutral iron(II) complex was inactive. Cyclohexene oxide polymerization could be "switched off" upon in situ reduction of the iron(III) catalyst and "switched on" upon in situ oxidation, which is orthogonal to what was observed previously for lactide polymerization. Conducting copolymerization reactions in the presence of both monomers resulted in block copolymers whose identity can be controlled by the oxidation state of the catalyst: selective lactide polymerization was observed in the iron(II) oxidation state and selective epoxide polymerization was observed in the iron(III) oxidation state. Evidence for the formation of block copolymers was obtained from solubility differences, GPC, and DOSY-NMR studies.
The reactivity of iron-based ring opening polymerization catalysts is compared when the catalyst is in three different oxidation states. Formally iron(i) monoalkoxide complexes 3a (p-methoxyphenoxide) and 3b (neopentoxide) supported by bis(imino)pyridine ligands were synthesized and investigated as catalysts for the ring opening polymerization and copolymerization of various monomers. For most monomers, 3a and 3b were superior catalysts compared to analogous, formally iron(ii) and iron(iii) complexes (1a/1b and 2a/2b, respectively) for the ring opening polymerization of various cyclic ester and cyclic carbonate monomers. Experimental and computational investigation into the electronic structures of 3a and 3b revealed that they are most accurately described as containing a high spin iron(ii) center that is antiferromagnetically coupled to a singly reduced bis(imino)pyridine ligand. This electronic structure leads to increased electron density near the metal center without modulating the apparent metal oxidation state, which results in superior catalytic performance for the more highly reduced 3a and 3b compared to the increasingly more oxidized complexes (i.e.1a/1b and 2a/2b, respectively) in ring opening polymerization reactions. These findings have significant ramifications for the emerging field of redox-switchable polymerization catalysis.
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