Ferromagnetic organic polymers

In 1987 Ovchinnikov et al. announced the discovery of the first ferromagnetic organic polymer based on polydiacetylene chain with dangling nitroxyl radicals [8,9]. The compound was obtained by bulk polymerization of stable paramagnetic biradical BlPO initiated by light. The chemical structure of this compound is shown below. 

Finally, synthetic metals made of polymeric organic molecules may also show the property of ferromagnetism. Organic materials of this kind were first demonstrated in 1987 by Ovchinnikov and his co-workers at the Institute of Chemical Physics in Moscow. The polymer they used was based on a polydiacetylene backbone, which contains alternating double-single and triple-single bonds between the carbon atoms of the molecule (10.2). 

In organic polymers ferromagnetism occurs when impurities are present. These may completely mask the true susceptibility of the substance to be investigated. In the following we shall only consider substances free from ferromagnetism, since polymers normally are non-ferromagnetic. 

Organic polymers are also generally believed to manifest ferromagnetism only when impurities (especially transition metals) are present, or when transition metals are deliberately incorporated into them. There is, however, some evidence [10] of ferromagnetism in organic polymers containing some specialized types of structural and electronic features but not containing appreciable amounts of transition metal atoms. 

The discussion of Class I polyradicals is focused on conjugated polymers 2 in which m-phenylene groups are employed to attain through-bond ferromagnetic coupling (Fig. 2). They are by far the most important of all organic polymers studied for their magnetic properties. 

Super-paramagnetic composites have been developed by Tamura [22]. The super-paramagnetic composite material consists of ultra-fine particles of ferromagnetic Fe oxide smaller than sizes of single domain structures dispersed as substantially separated in an organic polymer, and prepared by dispersing the... 

As mentioned in the beginning of this chapter, there have been several reports that claim to have found ferromagnetism in organic polymers. Some of the results are summarized in Table 12.3, where the spin concentrations (per monomer unit) estimated from the reported saturation magnetization, using g = 2 and S =, are shown together with the transition temperatures. Some of them were prepared with moderate oxidation by iodine or oxygen, but most materials were prepared by heat treatment, pyrolysis, or explosion. 

Y. Miura, K. Inui, F. Yamaguchi, M. Inoue, Y. Teki, T. Takui, and K. Itoh, Molecular design, synthesis, and magnetic characterization of poly(phenyl-acetylene) with pi-toporegulated pendant nitronyl nitroxide radicals as models for organic superpara- and ferromagnets, J. Polym. Sci. Polym. Chem. Ed., 30, 959 (1992). 

H. Tanaka, K. Tokuyama, T. Sato, and T. Ota, Preparation and properties of organic polymer ferromagnet with indigo unit, Chem. Lett., 1813 (1990). 

This highly conjugated molecule was stabilised with nitroxyl biradical side chains. The resulting material had sufficient ferromagnetism that a usable compass needle could be made from it. Despite the success of this demonstration, organic ferromagnetism remains a curiosity. Such polymers are not likely to replace conventional ferromagnetic metals in any application within the foreseeable future. 

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