Poly vinylferrocene

Side-Chain Polymetallocene Homopolymers and Block Copolymers 

Organic Polymers with Metallocene Side Croups 

Synthetic Metal-Contmning Polymers. Ian Manners Copyright 2004 W ey-VCH Verlag GmbH Co. KGaA ISBN 3-527-29463-5 

Considerable effort has been invested in studies of the properties of poly(vinylferrocene), a yellow powder that is stable in air in the solid state. The material is also soluble in most common, moderately polar, organic solvents and the resulting solutions are usually stable to air. Poly (vinylferrocene) has been studied in solution, and THE was found to be a good solvent for this material and 

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F. la-c. Cyclic voltammograms of dissolved and stance confined ferrcx ne in a< tonitrile/0.1 M TBAP. a. 4 X 10 M dissolved ferrocene at Pt. b. 4-ferrocenyl-phenylacetamid monolayer bound to Pt (ref. ). c. Poly-vinylferrocene dip coated on Pt,r = 1 x lO raolcm. Straight arrows indicate diffusional events. Curved arrows electron transfer events (from ref. ). 

Zinc sulfide, with its wide band gap of 3.66 eV, has been considered as an excellent electroluminescent (EL) material. The electroluminescence of ZnS has been used as a probe for unraveling the energetics at the ZnS/electrolyte interface and for possible application to display devices. Fan and Bard [127] examined the effect of temperature on EL of Al-doped self-activated ZnS single crystals in a persulfate-butyronitrile solution, as well as the time-resolved photoluminescence (PL) of the compound. Further [128], they investigated the PL and EL from single-crystal Mn-doped ZnS (ZnS Mn) centered at 580 nm. The PL was quenched by surface modification with U-treated poly(vinylferrocene). The effect of pH and temperature on the EL of ZnS Mn in aqueous and butyronitrile solutions upon reduction of per-oxydisulfate ion was also studied. EL of polycrystalline chemical vapor deposited (CVD) ZnS doped with Al, Cu-Al, and Mn was also observed with peaks at 430, 475, and 565 nm, respectively. High EL efficiency, comparable to that of singlecrystal ZnS, was found for the doped CVD polycrystalline ZnS. In all cases, the EL efficiency was about 0.2-0.3%. 

The electrochemical and spectroscopic data indicates that sites on these polymers can communicate with each other, in the electron transfer sense, on a relatively short time scale and without the formation of stable mixed valence clusters. Electronic tranport via hopping or tunnelling and modulated by means of neighboring molecular group collisions would be consistent with these requirements. The relative molecular nonspecificity of this mechanism suggests that other polymeric materials would show similar effects and this has been seen for thin films of poly — (vinylferrocene) and poly — (nitrostyrene). 

The third approach has been to graft the redox couple by means of a covalent bond to the polyelectrolyte backbond as described early in 1965 in the book of Cassidy and Run [20]. Several of these systems are charged polymers in at least one oxidation state, like poly(viologen), poly(vinylferrocene), and so on. Examples of polyelectrolytes like polyacrylic acid with covalently bound viologen were reported by Fernandez, Katz and coworkers [21], hydroquinone [22] and Anson et al. with bound ferrocene [23]. 

During continuous redox cycling, the first cycle usually differs from the following ones. This effect is referred as break-in. In poly(vinylferrocene), PVF, films this has been related to the incorporation of solvent and ions into the film, decreasing its resistivity [132]. This effect has been observed for several polyelectrolyte and polymer-modified electrodes, for example, polyaniline [155]. 

An enormous number of polymers have been used to prepare chemically modified electrodes. Some examples are given in Table 13.2 Albery and Hillman provide a more extensive list [8]. As indicated in Table 13.2, these polymers can be divided into three general categories—redox polymers, ion-exchange and coordination polymers, and electronically conductive polymers. Redox polymers are polymers that contain electroactive functionalities either within the main polymer chain or in side groups pendant to this chain. The quintessential example is poly(vinylferrocene) (Table 13.2). The ferrocene groups attached to the polymer chain are the electroactive functionality. If fer-... 

A rather unusual Fe(III) species for catalysis is [Cp2Fe]+, ferrocenium. A polymer-bound ferrocenium catalyst was obtained by oxidizing a poly(vinylferrocene-folock-isoprene)copolymer with AgOTf. The activity of this catalyst was tested with the reaction of P-oxo ester 24a and MVK (41a) (cf. Scheme 8.27) [93]. 

Ferrocene was one of the earliest mediators used [10] but is somewhat hydrophobic so derivatives of the molecule are often employed [39-43]. Ferricyanide can also be used, and the use of MWCNT with this mediator was shown to enhance its effectiveness [33]. Other groups have studied a wide diversity of novel mediator systems such as poly(vinylferrocene-co-acrylamide) dispersed within an alumina nanoparticle membrane [34], ruthenium [35] and osmium [36,37] complexes, and the phenazine pigment pyocyanin, which is produced by the bacteria Pseudomonas aeruginosa [38]. 

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. 

Polythiophenes, with ferrocene groups, 12, 305 Polyurea microcapsules, Pd-containing, 12, 713-714 Poly(vinylcymantrene), synthesis, 12, 311 Poly(vinylferrocene), preparation and properties, 12, 301 Polyynes... 

Glucose Sensors. Siloxane polymers are known to be extremely flexible. This flexibility will, of course, be sensitive to the amount of side-chain substitution present along the polymer backbone. For instance, in the homopolymer used in these studies (polymer A), the presence of a ferrocenylethyl moiety bound to each silicon subunit should provide an additional degree of steric hindrance, and thus a barrier to rotation about the siloxane backbone, in comparison with the copolymers, which have ferrocene relays attached to only a fraction of the Si atoms. Because these siloxane polymers are insoluble in water, their flexibility is an important factor in their ability to facilitate electron transfer from the reduced enzyme. Relays contained within more rigid redox polymers, such as poly(vinylferrocene), cannot achieve close contact with the enzyme s redox centers and are thus less effective as electron transfer mediators (25,34). The importance of this feature can be seen quite clearly by comparing the mediating ability of the homopolymer A with that of copolymers B-D, as shown in Figures 4 and 5. 

An alternative approach is to compare the frequency data with electrochemical (coulometric) data. This is exemplified for PVF and a copolymer with vinylpyrrolidone, poly(vinylferrocene-co-vinylpyrrolidone) (20 80 PVF-co-PVP). Table I shows in and ex situ frequency data for an electrode before and after coating with 20 80 PVF-co-PVP. 

Scheme 13.3. Electrochemically driven precipitation of poly(vinylferrocene).

Scheme 13.4. Poly(vinylferrocene) film redox chemistry.

Fig. 13.9. EQCM data obtained during the "break-in" of a poly(vinylferrocene) film (T = 23.7 nmol cm 2). Frames a and b, respectively, correspond to the current- and mass change-potential curves for the first two redox cycles in aqueous 0.1 mol dm-- NaCI04 immediately following deposition from CH.CI ,. In frame a, the current (electron flux) was converted to the equivalent mass flux of counter ions, for subsequent correlation with the total (observed) gravimetric response of frame b. Potential scan rate 5mVs l. The star symbol denotes the first anodic scan responses. (Adapted from Ref. [37] with permission.)...

Fig. 13.13. Poly(vinylferrocene) break-in first oxidation half cycle, third step (solvation). Reconfiguration is assumed faster than solvation in the upper cube.

Fig. 13.14. Poly(vinylferrocene) break-in completion of first oxidation half cycle, by a second reconfiguration followed by a second solvation step, to form the most stable reduction product (doubly solvated, doubly reconfigured O ).

Fig. 13.16. Redox cycling of a broken-in poly(vinylferrocene) film on short time scales. Reconfiguration is assumed faster than (de-)soivation in the lower cube.

During deposition, poly(vinylferrocene) films are non-rigid. Analysis of crystal impedance data yields shear moduli that are typical of a rubbery material this contrasts with the rigid film characteristics observed in aqueous media. 

Redox cycling of poly(vinylferrocene) films involves not only coupled electron/anion transfer, but also solvation and polymer configuration changes. The situation is further complicated by both reversible and irreversible elements of solvation and configuration changes, commonly referred to as break-in", immediately following deposition. Here, a dou-... 

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