Polygermanes and Polystannanes

The third class of polymers that we will investigate in this chapter is polyynes which are prepared by the involvement of acetylene fimctional groups in the polymerization reaction. These polymers are also called rigid-rod organometallic polymers. In these types of polymers various late transition metal ions alternate with a rigid organic unit. 

Polygermanes and polystannanes are heavier Group 14 congeners of polysilanes (Fig. 8.1). 

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Much of the interest in the polysilanes, polygermanes, and polystannanes involves their sigma delocalization and their sigma-pi delocalization when coupled with arenes or acetylenes. This is not unexpected since silicon exists as a covalent network similar to diamond. In exhibiting electrical conductivity, germanium and tin show more typical metallic bonding. Some polystannanes have been referred to as molecular metals. ... 

The polysilanes can be regarded as one-dimensional analogs to elemental silicon, on which, of course, nearly all of modem electronics is based. The photophysical behavior of polysilanes is not approached by any other materials, save for the less stable and more costly polygermanes and polystannanes. The remarkable properties of polysilanes have led to intense interest, and to numerous proposed high-tech applications. But the great promise of polysilanes as materials has yet to be realized. Their only commercial use at present is as precursors to silicon carbide ceramics, an application which takes no advantage of their optical or electronic properties. 

Currently, there is growing interest in polymers the frameworks of which are constructed from the Group 14 elements silicon, germanium and tin. Several reviews on polysilanes have appeared in the literature . In the first section of this article we focus on recent developments in the chemistry of polygermanes and polystannanes. The chemistry and structure-property relationship of oligostannanes and low molecular weight polystannanes was the subject of earlier reviews . ... 

Figure 31 Band structures for polysilanes, polygermanes, and polystannanes (top, frans-planar conformation below, gauche-helical conformation). (Adapted with permission of Elsevier Ltd. from Takeda, K. and Shikaishi K., Chem. Phys. Lett 1992,195,121.)...

Table 3. Ultraviolet-visible Absorption Data for Some Typical Polygermanes and Polystannanes [ERiRz]n ...

US6S. Both polygermanes and polystannanes may find applications because of their unique optical, electronic, and chemical properties. Some of these potential uses include photoresist layers (44,59,60), third-order nonlinear optical materials (50), charge transport polymers (61,62), photoconductors, microlitho-graphic materials (63), and photoinitiators (59). 

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