Ladder structures, polysilanes

The structures of the radical anions were confirmed by the following experiment (Scheme 9). The reduction of the ladder polysilanes was monitored by UY-visible-NIR spectroscopy. When the absorption of the ladder polysilanes was completely replaced by the absorption of the radical anions, the sealed tube was opened. The radical anions were immediately oxidized, and the starting ladder polysilanes were recovered in high isolated yields. It is reasonable to conclude that the radical anions of the ladder polysilanes retain the ladder structure, and the Si-Si bond cleavage or skeletal rearrangement does not occur. 

The structural constraints imposed by the fused cyclic structure of the catenates and the ability to cleanly separate product mixtures greatly facilitates crystallization, and single crystal X-ray structures have been obtained for a number of ladder polysilanes.3 8 For the all-anti molecules, the non-planarity of the SI rings induces a unidirectional helical twist to the ladder structure, as is evident in Figure 63. 

In this review, we explain the synthesis, structures, and properties of ladder polysilanes.18 The ladder polysilanes have the unique structure consisting of catenated and fused cyclotetrasilane rings. This system has remarkable features ... 

The structures of ladder polysilanes were determined by X-ray crystallography.41,42 In Fig. 6, molecular structures of the ladder polysilanes with the anti structure are shown. A remarkable feature of the anti ladder polysilanes is a helical structure. These molecules can be regarded as silicon double helices, in which two polysilane main chains twist in one way and are bridged by Si-Si bonds. The twist angles between the terminal rungs are... 

On the other hand, the ladder polysilanes containing a syn moiety have more strained structures (Fig. 8). The Si-Si bond lengths of syn-2... 

Fundamental knowledge on the structures and properties of the ladder polysilanes has accumulated in our research for the past 15 years. Some results were unpredictable, including the silicon double helix structure, the domino oxidation, the formation of persistent radical anions, the Diels-Alder reactions at the 1,4-positions of anthracene, etc. These results let us recognize that the construction of novel structures will open the new chemistry. 

The twist angles between the terminal rungs of 141, 142 and 143 are 44.0, 63.0, and 80.3°, respectively. The double helix structures crystallize with equal amounts of left- and right-handed screw sense forms, as expected in the absence of any chiral field. Some ladder polysilanes crystallize with both helical forms in the same centrosymmetric unit cell for others, the two forms can crystallize separately. It will be interesting to carry out the crystallization in a chiral field. 

Figure 63 Single crystal X-ray structures of all-anti ladder polysilanes 141, 142, and 143.308 Reprinted with permission from Kyushin, S. Matsumoto, H. Adv. Organomet. Chem. 2003, 49, 133-166, 2003 Elsevier. 

The polymers evaluated are shown in Fig. 1. These twelve polymers cover typical types of organosilicon polymers, such as polysiloxane, ladder type polysiloxane, polysilane, polymethacrylate, polysilyl-styrene, and novolak. Polymers and were supplied by Shin-etsu Chemical Co., Ltd. Actually, polymer" 2 evaluated contains very small amount of vinylsiloxane for crosslinking by deep UV irradiation after coating, because the pure polysiloxane is a fluid at room temperature and is impossible to measure the etching depth. These polymers have Si-0 bonds in the main chain structure. Polymer was purchased from Shin Nisso Kako Co., Ltd. and was purified in house. Polymers 4 to 12 were synthesized in house. Polymers 1J and 12 have Si-0 bonds in side chain structure. Thermal Si02 1J was evaluated as a reference. ... 

Two types of new silicon-branched organosilicon polymers, linear and ladder polysilane structures, were produced from dihalo- and tetrahalodisilane, respectively, via alkali-metal-mediated reactions. Further investigations disclosed that the polymers may he useful as photoresists, semiconductors, ceramic precursors, and composite materials in high-technology fields. 

Cyclic ladder polysilanes with staircase structure 03MI75. 

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