Polymers containing ferrocene units

The synthesis of conjugated polymers containing ferrocene units in their backbones has received much attention in recent years (78-81). Bochmann and co-workers have described the synthesis of ferrocene-based polymers via palladium-catalyzed coupling reactions of dihalide or divinyl functionalized ferrocene monomers with aromatic spacers (37) (78). 

It has also been reported that polymers containing ferrocene units in the main-chain or sidechains may possess liquid crystalline characteristics. " Ferrocene-based liquid crystalline polyesters (3) containing phosphorous groups in their backbones have been reported by Senthd and Kannan. ... 

The aim of our work was the synthesis of tailor-made polymers containing ferrocene units at different positions of the polymer chain. 

Polymerization of organoiron monomers has also resulted in the production of liquid crystalline polymers containing ferrocene units in their sidechains. " Scheme 6 illustrates Deschenaux s free-radical synthesis of ferrocene functionalized thermotropic liquid crystalline polymethacrylates Monomer 31 and its corresponding polymer 32 exhibited enantiotropic smectic A and C phases. 

Another class of coordination-type polymer containing ferrocene units in the backbone is shown in Scheme 42. Reaction of monomer 154 with the bipyridine-based monomer 155 resulted in the isolation of polymer 156. In the solid state, polymer 156 is thermally stable up to 240°C however, depolymerization occurs at about 85°C in toluene. 

We briefly mention here the use of the ferrocene/ferrocenium redox couple to mediate electron transfer on the oxidation (anodic) side, especially in derivatized electrode. This broad area has been reviewed [349]. For instance, polymers and dendrimers containing ferrocene units have been used to derivatize electrodes and mediate electron transfer between a substrate and the anode. Recently, ferrocene dendrimers up to a theoretical number of 243 ferrocene units were synthesized, reversibly oxidized, and shown to make stable derivatized electrodes. Thus, these polyferrocene dendrimers behave as molecular batteries (Scheme 42). These modified electrodes are characterized by the identical potential for the anodic and cathodic peak in cyclic voltammetry and by a linear relationship between the sweep rate and the intensity [134, 135]. Electrodes modified with ferrocene dendrimers were shown to be efficient mediators [357-359]. For the sake of convenience, the redox process of a smaller ferrocene dendrimer is represented below. 

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. 

Fig. 27. The first oligomeric unit (123) of an organometallic polymer containing ferrocene- and [2.2]paracyclophane units (reaction paths j and k) [103,105]...

The hydrosilylation strategy has also been used to prepare well-characterized sidepolymers containing ferrocene [60]. Grafting of vinyl-terminated mesogens onto poly(methylhydrosiloxane-r-dimethylsiloxane) yielded materials with 1,T- (2.27a) and 1,3-disubstituted (2.27b) ferrocene units... 

Ferrocene is both thermally and electrochemically very robust. These properties have motivated attempts to incorporate the ferrocene unit as part of a polymer framework. This section deals with various types of polymers containing ferrocene. 

It was possible to prepare a soluble polymer containing ferrocene groups linked to the backbone via carbonate functions by modification of PVOCCl with ferrocenyl alcohol (run 18). An experiment was conducted with quinine and led to a crosslinked polymer containing 45% of quinine units. Other conditions should be tried to obtain a soluble polymer to increase the degree of substitution with this system. Work to this effect is in progress. 

The same reasons for the interest in incorporating ferrocene units into polymers also provided motivation for the synthesis of dendritic macromolecules of well-defined size and structure containing ferrocenyl units. An important additional rationale for the construction of ferrocenyl dendrimers is provided by the fact that such macromolecules raise the possibility of combining the unique and valuable redox properties associated with the ferrocene nucleus with the highly structured macromolecular chemistry. This may provide access to materials of nanoscopic size possessing unusual symmetrical architectures, as well as specific physical and chemical properties which would be expected to differ from those of the ferrocene-based materials prepared to date. 

In other examples, compounds in which a metal atom is already coordinated in a molecule can be used as a comonomer in an addition polymerization. Two examples involve the ferrocenes discussed in Chapter 6. The vinyl ferrocene molecule is shown in 7.14, and a similar vinyl manganese complex in 7.15.30 An alternative approach involves condensation polymerization. For example, if the R group in the ferrocene unit shown in 7.16 contains a hydroxyl group, it can be copolymerized with a diacid chloride. If it is an acid chloride, it can be copolymerized with a diamine. (This type of polymer is called a heteroannular chain if only one of the rings in the repeat unit is in the backbone, the polymer is called homoannular.)7 Similarly, the titanium complex shown in 7.17 is copolymerized with diacids or diols.30 Numerous other examples involving ferrocenes are discussed in Chapter 6. 

Polyesters 26 represent the first ferrocene-containing liquid crystal polymers [28]. These copolymers were found to be insoluble in THF, toluene, dichloromethane, chloroform, p-chlorophenol, and tetrachloroethane. The terephthaloyl chloride/iso-phthaloyl chloride ratio was maintained constant (7 3), but the content of the ferrocene unit was varied from 0 to 100%. 

ZnCl2/HCI-catalyzed polymerization of (dimethylamino)methylferrocene. However, the resulting material was of low molecular weight (M 2400), and a mixture of 1,2-, and M -disubstituted ferrocene units was found to be present in the main chain 78). Recently, however, the synthesis of well-defined polymers of structure 29 (M 18,000-139,000) containing face to face ferrocene units with naphthalene bridging groups was achieved (79). 

Linear, saturated, polymers obtained by radical initiation of 1,1 -divinylferrocene hive the Mossbauer parameters (6, 0.23 mm s l LEq, 2.29 mm s l) expected for cyclopolymer8 with three-carbon bridged ferrocene units in the main chain. However, cationic initiation yields polymers which exhibit differences from the radical polymers in Mossbauer (, 0.27 mm s 7 Eq, 2.40 ntn s 7) and other spectroscopic studies. The cationic polymers may contain a bicyclic unit or a ladder structure in the chain. 

These results therefore indicate that the soluble, saturated polymers produced by radical initiation of DVF are cyclopolymers containing a three-carbon bridged ferrocene unit (I). However, the Mossbauer spectra obtained for polymers produced by cationic initiation do not show evidence for the three-membered bridge. Certainly PDVF(cationic) contains some acyclic unsaturated units, not necessarily pendant vinyl groups (II), but the low level of unsaturation suggests that cyclic units exist. Thus, for example, a five-carbon bridged bicyclic unit (VI) or a ladder structure (VII) may be found in these polymers, and would be consistent with the Mossbauer parameters obtained. Further polymerisation studies and Mossbauer and other spectroscopic studies of these polymers are in progress. 

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