Exploring Oxidation State-Dependent Selectivity in Polymerization of Cyclic Esters and Carbonates with Zinc(II) Complexes

Abstract

SummaryNeutral zinc alkoxide complexes show high activity toward the ring-opening polymerization of cyclic esters and carbonates, to generate biodegradable plastics applicable in several areas. Herein, we use a ferrocene-chelating heteroscorpionate complex in redox-switchable polymerization reactions, and we show that it is a moderately active catalyst for the ring-opening polymerization of L-lactide, ɛ-caprolactone, trimethylene carbonate, and δ-valerolactone. Uniquely for this type of catalyst, the oxidized complex has a similar polymerization activity as the corresponding reduced compound, but displays significantly different rates of reaction in the case of trimethylene carbonate and δ-valerolactone. Investigations of the oxidized compound suggest the presence of an organic radical rather than an Fe(III) complex. Electronic structure and density functional theory (DFT) calculations were performed to support the proposed electronic states of the catalytic complex and to help explain the observed reactivity differences. The catalyst was also compared with a monomeric phenoxide complex to show the influence of the phosphine-zinc interaction on catalytic properties.

Coordination Ring-Opening Polymerization of Cyclic Esters: A Critical Overview of DFT Modeling and Visualization of the Reaction Mechanisms

“…As demonstrated in a number of publications, heteroleptic metal alkoxides form stable binuclear complexes [69,94,97,99,140,147,148,149] that are able to catalyze ROP of cyclic esters by both binuclear and mononuclear mechanisms. DFT modeling has been shown to be an effective instrument of choice of the preferable reaction pathway.…”

“…In conclusion, we should like to emphasize that some aspects of the molecular structure of ROP precatalysts have been refined by DFT modeling [148,152,153,154,155,156,157]. However, the results of such modeling do not reflect fully the catalytic behavior of these complexes and therefore were irrelevant for our review.…”

Redox-Switchable Ring-Opening Polymerization with Ferrocene Derivatives

“…On the other hand, the interactions between the metal center and the carbonyl group increase the electron density around the metal, which facilities the nucleophilic attack of the propagation. As mentioned above in section and observed with other classes of metal complexes, ,,,, the coordination mode of the tetradentate ferrocene ligand to the metal center can also affect the activity strongly. Since during the oxidation/reduction reaction a change in the coordination mode of the supporting ligand is possible, small changes in the central metal and the ligand will affect the redox-switchable ROP significantly.…”

“…A consideration of possible mechanistic issues brings an understanding of the origin of this selectivity. However, many other aspects remain relatively unexplored, especially those related to the prediction of catalysts capable of showing both high activity and high selectivity and of new designs for compounds capable of performing other types of redox-switchable reactions. ,− Five years ago there were no examples of switchable catalysts that can carry out selective copolymerizations of cyclic esters/ethers or combine any two orthogonal reactions; at the present, in addition to the two types of redox-switchable systems discussed above, a chemically switchable system can be used for the synthesis of up to pentablock copolymers, , and a light-switchable system can be used for the synthesis of diblock copolymers . Despite these achievements, a lot of the past work was based on chemical intuition and trial and error.…”

“…Our group has studied the use of ferrocene as a redox switch (Figure ) for several years. − The main application of such systems is the formation of biodegrable copolymers. Aliphatic polyesters are a major category of biodegradable polymers that can be prepared from a variety of natural compounds, allowing the use of renewable feedstocks as well as an assortment of various monomers. , By the introduction of redox-switchable catalysts into this field, various copolymers of aliphatic esters could be prepared readily.…”