Solution studies polysilanes

The solid polysilanes are far more complicated than polysilane solutions. Frequently, many different phases can be identified for a single polymer, and the transformations between them may be slow, so that the polysilane conformation and spectroscopic behavior often depends on the thermal history of the sample. The compensating fact is that some additional techniques for studying polysilanes become available in the solid phase, including differential scanning calorimetry (DSC), and especially X-ray scattering. Nevertheless, the conformations and structures of polysilanes as solids are poorly understood. 

The recent interest in substituted silane polymers has resulted in a number of theoretical (15-19) and spectroscopic (19-21) studies. Most of the theoretical studies have assumed an all-trans planar zig-zag backbone conformation for computational simplicity. However, early PES studies of a number of short chain silicon catenates strongly suggested that the electronic properties may also depend on the conformation of the silicon backbone (22). This was recently confirmed by spectroscopic studies of poly(di-n-hexylsilane) in the solid state (23-26). Complementary studies in solution have suggested that conformational changes in the polysilane backbone may also be responsible for the unusual thermochromic behavior of many derivatives (27,28). In order to avoid the additional complexities associated with this thermochromism and possible aggregation effects at low temperatures, we have limited this report to polymer solutions at room temperature. 

The electronic structures of polysilane radical ions have also been studied by pulse radiolysis of the liquid solution [35-40]. However, due to short lifetime of the radical ions, the measurement is limited to electronic absorption spectroscopy. 

A second means of enhancing sensitivity is to increase the concentration of the sample in solution. Thus, 2D-INADEQUATE studies of 29 Si-29 Si correlations on polysilanes at natural 29Si abundance at high concentrations (1 -2 M) was reported12. Sophisticated solid-state NMR studies (see Section V) have also been performed, including two-dimensional... 

On the other hand, the polysilanes are not strictly infinite one-dimensional chains. Several experimental studies (15.45.47) in solution have suggested that the polysilanes are trans or nearly trans planar only over 20-35 silicon atoms, restricted... 

With this new information in mind, an interpretation of the thermochromic behavior of polysilanes in solution can be outlined. We can begin with two calibration points (1) The polymers ( -butyl2Si) and ( -pentyl2Si) absorb at 315 nm both in the solid state and in solution both polymers as solids are known to have an all-D conformation, with co 154°. (2) The well-studied polymer ( -hexyl2 Si) , in its low-temperature phase, absorbs at 375 nm and has a (nearly) all-A conformation. Values of A,nax between 315 and 375 nm should correspond to intermediate values of the average torsional angle, ox... 

Polysilanes with several oxygen atoms in the side chains have been studied by Cleij and coworkers.83 84 These polysilanes may become water soluble, as is the case for [MeSi(CH2)3(OCH2CH2)3OCFl3] . In THF solution at 20 °C this polymer absorbs at 305 nm, but in water the UV band is shifted to 281 nm. The water may cause tighter coiling of the polymer chains, perhaps due to micelle formation. Between 46 and 70 °C, the water solution undergoes phase separation. The UV band shifts to 320 nm, consistent with partial straightening of the polymer chain after phase transition. These polymers also display ionochromism. 

Polysilane derivatives constitute a new class of radiation-sensitive materials with interesting physical and electronic properties. The photochemical decomposition of polymers containing disilanyl units seems adequately explained by silicon-silicon bond homolysis and subsequent radical reactions. The solution photochemistry of longer silicon catenates results in the extrusion of substituted monomeric silylenes, as well as the formation of silyl radicals produced by chain homolysis. Recent studies indicating that the... 

The helical structure of polychloral was proposed by Vogl in 198028 and was demonstrated by Ute, Hatada, and Vogl via a detailed conformational analysis of chloral oligomers.29 As an example of a helical polymer with an inorganic backbone, polysilanes bearing a chiral side chain were synthesized and their conformational aspects were studied. A helical conformation with an excess screw sense for this class of polymers in solution was found in 1994 independently by Fujiki30a and by Moller.300 Matyjaszewski had pointed out such a conformation for chiral polysilanes in the solid state in 1992.30c... 

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