Organometallic ferrocene polymers

The disulfide links in the organometallic ferrocene polymers can be reductively cleaved, as in other disulfide-connected structures such as proteins, and the iron centers can be oxidized electrochemically to give ferrocenium units [273] both these redox processes are reversible in principle. 

The landmark discovery of ferrocene by Kealy and Paulson in 1951 marked the beginning of modern organometallic chemistry. The first organometallic addition polymer was polyvinyl-ferrocene synthesized by Arimoto and Haven in 1955. While polyvinylferrocene (structure 11.32) had been synthesized it was about another decade until the work of Pittman, Hayes, and George, and Baldwin and Johnson allowed a launch of ferrocene-containing polymers. 

One previous synthesis of ferrocene-containing condensation polymers via interfacial methods at room temperature has been reported by Knobloch and Rauscher, who formed low molecular weight polyamides and polyesters by reacting l,l -bis(chloro-formyl)ferrocene with various diamines and diols. Further, Carraher and co-workers have utilized interfacial techniques in the formation of other types of organometallic polymers. 

The advantage of these polymers over poly(vinylferrocenes) or related species with pendent organometallic units is that the condensation polymers have ferrocene units in the main chain where they can exert their maximum influence on polymer thermal stability. The disadvantage of the condensation products is that, except in the last example, the molecular weights are too low to favor fiber or flexible film formation. Nevertheless, this work indicated the potential usefulness of polymers with metallocene units in the main chain. 

Redox polymer — Redox polymers contain electrostatically and spatially localized redox sites, which can be oxidized or reduced. The redox polymers can be divided into subclasses 1) polymers that contain covalently attached redox sites, either built in the chain or as a pendant group. The redox centers may be organic molecules (e.g., tetrathiafulvalene, tetra-cyanoquinodimethane), organometallic molecules (e.g., - ferrocene), or co or dinative ly attached redox couples (e.g., polymerized metal bipyridine(bpy) complexes) 2) ion-exchange polymeric systems, where the redox-active ions are held by electrostatic binding (e.g., Fe(CN)g- 4- in protonated poly(vinylpiridine) or Ru(bpy)j+/2+ in -> Nafion [i]. 

A number of organometallic polymers containing metallocenes and metallocene analogues has been well-known for some time2. Because of the features of high-temperature stability and radiation resistance of the ferrocene nucleus, ferrocene-containing polymers are of special interest. Basically, these polymers may be divided into two classes the metallocene moiety is either located in a pendant group or in a backbone of the polymer chain. The former polymers have been synthesized by vinyl polymerization of vinyl metallocene monomers such as vinylferrocene. The latter polymers have been prepared by polycondensation of l,l -disubstituted metallocenes or metallocene dihalides with a.w-disubstituted monomers, and fell into two main types, (A) and (B). 

Except for the structure containing 100% of ferrocene unit, which decomposed before melting, all the organometallic copolymers exhibited birefringent melts. Nematic textures were identified by means of polarized optical microscopy and, in one case, by X-ray diffraction studies. For comparison purposes, a polymer without ferrocene unit was prepared, but showed no mesomorphism. The authors deduced that the ferrocene framework was contributing to the liquid crystallinity of the ferrocene-containing polymers. 

Organometallic polymers, in contrast to coordination polymers, have metal-to-carbon bonds. They are the topic of this chapter. Three seminal events provided the foundation for the field of organometallic polymers to develop. The landmark discovery of ferrocene by Kealy and Pauson in 1951 marked the first organometallic compound.11 This was quickly followed by the full elucidation of its structure and an understanding of its reactivity by Wilkinson, Rosenblum, Whiting, and Woodward.12,13 This history was celebrated in a 2001 feature article in Chemical and Engineering News 4 Finally, the first polymerization of an organometallic compound was reported by Arimoto and Haven at Dupont Co. in 1955.15,16 Vinylferrocene 1... 

The electropolymerization of metallocene-functionalized pyrrole and thiophene has been utilized for the creation of organometallic polypyrroles and polythiophenes.181 241 242 243 Zotti et al. reported the electrochemical homo- and copolymerization of ferrocene-functionalized pyrrole, 177.242 Polymer 179 was prepared via the copolymerization of monomer 177 with pyrrole (Scheme 2.48). This polymer possessed an electrical conductivity of 1.5 X 10-2 S/cm. 

Ferrocene is an attractive and convenient organometallic species to incorporate into conjugated polymers—it is stable, easy to functionalize, has reversible electrochemistry, and its derivatives (both monomeric and polymeric) can be easily characterized [e.g., by nuclear magnetic resonance (NMR) spectroscopy]. Ferrocene has been incorporated into the backbone and side chains of polythiophene 45 In 1999, both Higgins and Wolf independently reported the synthesis of polythiophenes with ferrocene in the backbone.46,47 Electropolymerization of ferrocene substituted... 

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