High-molecular-weight polysilanes

At present, the only synthetic method leading to high molecular weight polysilanes is based on the Wurtz condensation reaction between dichlorodisubstituted silanes and alkali metals (low molecular weight oligomers were also prepared by dehydrogenation of secondary... 

The discovery that soluble high molecular weight polysilanes may be prepared by the reductive coupling of dichlorodialkylsilanes by alkali metals (1,2) has led to considerable work on the properties of this interesting class of polymers (3,4,5). The preparation of the polymers leaves much to be desired as frequently the high polymer is only a minor product. Mechanistic studies of the reaction with a view to improving the relevant yields have been few (6). The major ones by Zeigler (7,8,9) showed that a silylene diradical was not involved in the reaction, and stressed the importance of polymer solvent interactions. 

Figure 5. Mechanistic hypothesis for the photodegradation of high molecular weight polysilane derivatives.

Typically, polygermane-polysilane copolymers have similar UV properties as compared with polygermane and polysilane homopolymers . The thermochromic behaviour of the copolymers depends on their monomer-to-monomer ratios in the polymer chain and on the identity of the alkyl substituents. In principle, the fine tuning of the absorption maximum can be achieved by a controlled copolymerization. Unfortunately, the copolymers reported so far often have relatively low molecular weights, which in turn results in the kmax values being even lower than those observed for high molecular weight polysilanes (Table 4). 

The formation of the Si-Si bond occupies a special position in organosilicon chemistry. Silicon is one of the few elements, which can form stable catenated compounds. Besides high-molecular weight polysilanes, oligomeric polysilanes are a very important class of compounds that are related to silylenes,... 

The reactions that limit chain growth and initiate polymerization have not been defined. Is polymerization terminated by impurities Does solvent participate in transfer Does polymerization start by the reaction with a monomer or with impurities in the system These questions must be answered. For example, we noticed that successful formation of the monomodal high-molecular-weight polysilane requires the addition of a few drops of monomer to sonicated sodium dispersion prior to the addition of the main part of disubstituted dichlorosilane. 

Polysilane Complexes. The photolability of the Si-Si bonds in the Fp complexes just described suggests that the incorporation of a metal center into high-molecular-weight polysilanes might affect the photodepolymerization of these photoresist materials (30, 31). Incorporation of the metals into the polymers was expected to permit new redox chemistry. Polysilanes... 

Despite the considerable interest in the radiation sensitivity of high-molecular-weight polysilanes, relatively few detailed studies on the nature of the... 

Reactive Intermediates in Photolysis. Silylenes and Silyl Radicals, The results of exhaustive irradiation of substituted high-molecular-weight polysilanes in solution suggest that at least two reactive intermediates (i.e., silylenes and silyl radicals) are produced. On the basis of this information, a tentative reaction scheme (Scheme V) is proposed. 

Quantum Yields of Photolysis. These photochemical studies on high-molecular-weight polysilanes suggest that these polymers constitute a new class of photosensitive materials. The bleaching studies indicate that significant molecular weight reduction occurs upon irradiation in air or in vacuo, which in turn implies that the polymers are predominantly of the scissioning type. 

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