Redox-Switchable Ring-Opening Polymerization with Ferrocene Derivatives

Abstract

CONSPECTUS: Switchable catalysts incorporate stimuli-responsive features and allow synthetic tasks that are difficult or impossible to accomplish in other ways. They mimic biological processes in that they can provide both spatial and temporal control, unlike most reactions promoted by human-made catalysts that usually occur according to carefully optimized conditions. In the area of switchable catalysis, redox-switchable ring-opening polymerization (ROP) has attracted much attention, emerging as a powerful strategy for the development of environmentally friendly biodegradable copolymers, especially those containing blocks with complementary properties. Controlling the sequence and regularity of each copolymeric building block can affect the material properties significantly since they are directly related to the respective microstructures. Such control can be exerted with a well-designed redox-switchable catalyst by timing the oxidation and reduction events. In highly selective systems, one form of the catalyst reacts with a monomer until the redox state of the catalyst is altered, at which point the altered state of the catalyst reacts with another monomer. The reaction time may be varied from one cycle to another to generate various designer multiblock copolymers. The first instance of redox-mediated ROP was described by N. Long and co-workers in 2006. This example, as well as many early reported redox-switchable catalysts, could only achieve an on/off switch of activity toward a single monomer or substrate. However, our efforts brought on a general strategy for designing redox-switchable metal complexes that can catalyze different reactions in two oxidation states. In recent years, our contributions to this research field led to the synthesis of several multiblock copolymers prepared from biorenewable resources. This Account provides an overview of reported redox-switchable polymerization catalysts that allow for complementary reactivity in different oxidation states and highlights the potential of this strategy in preparing biodegradable materials. First, we define the field of redox-switchable catalysis and illustrate the design and significance of our ferrocene-chelating ligands, in which the oxidation state of iron in ferrocene can control the reactivity of the resulting metal complexes remotely. Next, we illustrate recent advances in the synthesis of new biodegradable copolymers including (1) how to tune the activity of the ROP catalysts by exploring various metal centers and ferrocene-based ligand combinations; (2) how to synthesize new multiblock copolymers of cyclic esters, epoxides, and carbonates by redox-switchable ROP; and (3) how to understand the mechanism of these reactions by discussing both experimental and theoretical investigations. By the application and development of redoxswitchable strategies, various novel materials and reactions can be expected in the future.

2‐[(4‐Aminobutyl)ferrocenylmethylidene]‐5,6‐dimethoxy‐1‐indanone derivatives: Synthesis, characterization, and investigation of electro‐optical properties

“…Compounds in this study compete with each other without any electron acceptor group. With E g values, HOMO and LUMO energy levels can be estimated using onset oxidation potentials for the sample (onset E ox ) in cyclic voltammograms using equations (2) and (3). [62] E HOMO = − E ox + 4:8 −E ferrocene ½ ð 2Þ…”

“…Since their discovery ferrocene (Fc) and its derivatives have been proved to be the excellent chemical reagents in organic and organometallic synthesis, electrochemistry, catalysts, and design of new chiral materials . Nowadays, they have wide applications, for example, medicine, electro‐optical devices, modified electrodes and capacitors, biosensors, and protective coating materials …”

“…Since their discovery ferrocene (Fc) and its derivatives have been proved to be the excellent chemical reagents in organic and organometallic synthesis, electrochemistry, catalysts, and design of new chiral materials. [1][2][3][4][5] Nowadays, they have wide applications, for example, medicine, [6,7] electro-optical devices, [8,9] modified electrodes and capacitors, [10,11] biosensors, [12,13] and protective coating materials. [14] Cyclic voltammetry (CV) is perhaps a more versatile electrochemical technique in modern analytical chemistry that can be used for characterizing electroactive compounds.…”

Controlling the Ring‐Opening Polymerization Process Using External Stimuli

“…Ideally, a carefully chosen stimulus could enable site‐specific transformation of the catalytic center through allosteric, redox, acid/base, electrochemical, light, mechanochemical modulation, and so on . Recently, tremendous advances in the burgeoning field of the switchable polymerization have been reported such as controlled radical polymerization, cationic polymerization, ring‐opening metathesis polymerization, ring‐opening polymerization (ROP) and olefin polymerization . Among the above‐mentioned various polymerizations, the switchable ROP has attracted much attention, emerging as a powerful strategy for the development of eco‐friendly biodegradable polymers, which can potentially serve as alternatives to petroleum‐based polymers.…”

Switchable Ring‐Opening Polymerization by a Ferrocene Supported Aluminum Complex

“…We– and others have approached the design of switchable ring‐opening polymerization catalysts by employing redox switches. In recent years, we showed that, by using metal complexes supported by a ferrocene‐based ligand, orthogonal activity could be achieved between the reduced and oxidized state of the metal complex, enabling the synthesis of block copolymers.…”