Redox Control of a Ring-Opening Polymerization Catalyst

Abstract

The activity of an yttrium alkoxide complex supported by a ferrocene-based ligand was controlled using redox reagents during the ring-opening polymerization of L-lactide. The oxidized complex was characterized by X-ray crystallography and (1)H NMR, XANES, and Mössbauer spectroscopy. Switching in situ between the oxidized and reduced yttrium complexes resulted in a change in the rate of polymerization of L-lactide. Synthesized polymers were analyzed by gel permeation chromatography. Polymerization of trimethylene carbonate was also performed with the reduced and oxidized forms of an indium alkoxide complex. The indium system showed the opposite behavior to that of yttrium, revealing a metal-based dependency on the rate of polymerization.

Reactivity and Properties of Metal Complexes Enabled by Flexible and Redox-Active Ligands with a Ferrocene Backbone

“…43,[59][60][61] We first introduced a Schiff base derivative, salfen, to cerium and yttrium 78 and later developed the phosfen ligand that could be cleanly oxidized by a ferrocenium oxidant to enable redox controlled catalysis. 61 The salfan, thiolfan and thiolfan* ligands were found to fit group 4 metals well and have been exploited in redox-switchable copolymerization processes. 43 …”

“…[59][60][61]78 Due to the high acidity of the phenolic proton of H 2 (OEEO fc ), the installation of the metal fragment was facile; the choice of synthetic routes was mainly based on the desired composition of the pre-catalysts used for polymerization and the ease of availability of a metal precursor. Group 4 metal complexes supported by salfan, thiolfan, and thiolfan* ligands were prepared in a similar way using Zr(OtBu) 4 or Ti(OiPr) 4 as metal precursors.…”

“…59 Therefore, we achieved control of lactide polymerization by chemically oxidizing or reducing the catalyst at a remote site of catalysis, the ferrocene backbone of the phosfen ligand (Figure 2). 61 The difference in reactivity between the oxidized and reduced forms of the catalyst was attributed to changes in their electronic profiles. We hypothesized that the oxidized ferrocenium ligand would make the rare-earth metal center so electron deficient that once the substrate binds to the metal it cannot leave, thus interrupting the catalytic cycle.…”

High activity of an indium alkoxide complex toward ring opening polymerization of cyclic esters

“…[3][4][5][6][7][8][9][10][11][12][13] Lactide polymerizations have been particularly well examined, but many of the precatalysts [14][15][16][17][18][19][20] often require initiators. β-butyrolactone, 8,21 ε-caprolactone, 20 and trimethylene carbonate 22 polymerizations have also been achieved by indium catalysts. Although less studied, low catalyst loadings 21 and the potential range of lactone and carbonate monomers 23 make their polymerization a promising area toward finding new biodegradable polymers.…”

“…24 Phosphinimine analogues with yttrium and indium also demonstrated good activity in lactide and trimethylene carbonate polymerizations. 22 Therefore, we decided to combine indium's activity with the Schiff base, ferrocene-derived ligand in order to study the activity of the resulting complexes toward a broad range of cyclic ester polymerizations. Additional motivation was found from recent studies that point to the biocompatibility, robustness, and high activity of indium catalysts in the ring opening polymerization of cyclic esters.…”