Degradation of polysilanes

The degradation of polysilanes in the presence of sodium indicates that the polymer is a kinetic product and is easily degraded to... 

Scheme 30 Metal-induced chemical degradation of polysilanes.

Sanji T, Nakatsuka Y, Ohnishi S, Sakurai H. Preparation of nanometer-sized hollow particles by photochemical degradation of polysilane shell cross-linked micelles and reversible encapsulation of guest molecules. Macromolecules 2000 33 8524-8526. 

Because the coupling reaction is usually carried out in excess sodium ([Na]o/[Si-Cl]o=1.2), degradation of the polymer is possible. We had prepared polysilanes and tested this possibility by their reaction with... 

In summary, the production of substituted silylenes and silyl radicals upon exhaustive irradiation at 254 nm of polysilane high polymers suggested that the polymer photochemistry resembled that previously reported for short chain acyclic and cyclic oligomers (39). More recent experiments, however, have suggested that the photochemical mechanism for the degradation of the high polymers is more complex than first envisioned (vide infra) (48). 

Figure 8.1 shows the GPC analysis of polysilane 2 and its radical-based degradation [12]. Line A shows a typical bimodal distribution... 

To resolve this dilema, we propose that the polymer is interacting with the additive in the excited state, (27) perhaps via electron transfer, and that this interaction leads to the irreversible degradation of the polymer. The direct interaction of photoexcited monomeric polysilanes with halogen derivatives resulting in the cleavage of Si-Si bonds had been reported (28). In a similar fashion, we must conclude either that this interaction does not occur with the alkyl silane polymers or that it does not result in rapid polymer degradation. 

Polysilanes are chemically inert to air and water at ordinary temperature, but their reactivity increases in solvent. In a solvent such as tetrahydrofuran, degradation of the Si—Si backbone by strong bases is quite rapid. Strong oxidizing agents like /77-chloroperbenzoic acid insert oxygen atoms between the silicons to produce Si—O—Si linkages in the backbone (120). 

Yet we know now that polysilane polymers are stable to heat up to almost 300 °C, are inert to oxygen at ordinary temperatures, and are only mildly susceptible to hydrolysis. The principal weakness of polysilanes, as materials, is not any of these properties it is that they become degraded when exposed to ultraviolet light. 

The various photoprocesses are important for several potential applications of polysilanes. Because of their intense fluorescence they have been suggested as scintillation materials for radiation detection. The chain-breaking reactions mean that polysilanes can be degraded by UV light it is this property that makes them useful as photoresist materials. 

To study the structural sensitivity of poly silanes to ionizing radiation, a number of samples were irradiated with a calibrated Co source, and the degraded materials were analyzed by GPC in a manner similar to that described for the determination of photochemical quantum yields (59). In radiation processes, the slopes of the plots of molecular weight versus absorbed dose yield the G values for scissioning, G(s), and cross-linking, G(x), rather than the respective quantum yields. These values, which represent the number of chain breaks or cross-links per 100 eV of absorbed dose, are indicative of the relative radiation sensitivity of the material. The data for a number of polysilanes are given in Table IV. Also included in Table IV for comparison is the value for a commercial sample of poly(methyl methacrylate) run under the same conditions. The G(s) value of this sample compares favorably with that reported in the literature (83). 

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