Polysilanes photolysis

Silicon centered radicals can be generated by transfer to silanes and by photolysis of polysilanes. Rate constants for addition to monomer are several orders of magnitude higher than similar carbon centered radicals.453,43 The radicals have nucleophilic character. 

The coordinated silylenes in both the iron and the chromium compounds can be photolytically activated Photolysis of the complexes in the presence of triphenylphosphine gives the trans-silylene-phosphine complex, which in a second step is transformed into the trnns-bisphosphine compound by excess phosphine. If the silylenes are not trapped, polysilanes are isolated in almost quantitative... 

Another family of silenes, those derived from the photolysis of acyldi- or polysilanes at A > 360 nm, also show somewhat unusual behavior compared to simpler silenes. These silenes exhibit great stability which in some cases has allowed isolation of solid silenes, and which has allowed acquisition of much physical data relating to silicon-carbon double bonds as mentioned earlier. 

The cut-and-paste technique has two steps (i) M (or P) screw-sense-selective photolysis at the longer kmax region in CC14 to form P (or M) screw sense telomers with Si-Cl termini, and (ii) Na-mediated recondensation of the telomers in hot toluene. Formation of an optically active helical polysilane with an almost single screw sense was evidenced by significant changes in the UV and CD spectra after the cut-and-paste technique, as shown in Figure 4.6b. 

Carboxylic acids were introduced by the reaction of the polysilane(II) with carboxylic acid anhydrides in the presence of an amine. Table 2 shows the results. In order to examine the effect of unsaturated bonds and halides in the side chain on photolysis, double bonds or chloride groups were introduced with the carboxylic acid. Polysilanes(III) were also soluble in polar solvents and basic aqueous solution. The solubility in tetramethyl ammonium hydroxide aqueous solution(TMAH) depended strongly... 

During photolysis, the double bond content of the polysilane(P-l)(15mol% in this experiment) decreased to 10mol%, as measured by 1H-NMR spectroscopy. However, the ratio, quantum yield of scission(Q(S))/quantum yield of crosslinking(Q(X)), was not affected by the reaction of the double bond. West and his coworkers have reported that poly((2-(3-cyclohexenyl)-ethyl)methylsilane-co-methylphenylsilane) crosslinked upon irradiation(55). The difference between our results and West s may lie in the amount of the double bond and inhibitation of the radical closslinking by the phenol moiety. Polysilane with a halogen moiety, P-8, photodecomposed rapidly, compared with P-1 or P-3. The introduction of a chloride moiety was effective for the sensitization of the photodegradation. Similar results has already been reported(55). 

Figure 10. Solubility of some functionalized polysilanes in TMAH aqueous solutions during photolysis.

Scheme 38 Photolysis of ladder polysilane and trapping products of extruded cyclic disilene (substituents /-Pr, except as noted)308...

The photolysis of cyclic polysilanes results in ring contraction with concomitant extrusion of a silylene fragment. Although the formation of two reactive intermediates potentially complicates mechanisms for product formation, it has provided a useful method for the synthesis of both unstable and stable disilenes... 

In an effort to quantify the effect of photolysis, polysilanes 2 and 8 were selected as typical models. For this experiment, samples of 2 were fractionated by repeated precipitation from toluene using isopropanol. No additional... 

Photoinitiation. Since photolysis of polysilanes generates silyl radicals, which can add to carbon—carbon double bonds, these polymers have been used for the free-radical polymerization of unsaturated oiganic monomers (135,136). Though about one-tenth as efficient as other oiganic photoinitiators, polysilanes are nevertheless quite insensitive to oxygen effects, which somewhat compensates for their lower efficiency. 

TABLE 3. Silenes synthesized by photolysis or thermolysis of acyl di- or polysilanes... 

Recently, several studies have been made of the photolysis of disilanes or polysilanes in the presence of an electron-deficient alkene using a photosensitizer (such as phenanthrene) and acetonitrile as solvent. These conditions result in the addition of silyl groups to one end of the alkene double bond and hydrogen to the other end (equation 18) and evidently involve the reaction of the radical anions of the electron-deficient silene with silyl radicals67 (see also Section VIII.A). 

Understanding of the behavior of tri-, tetra- or polysilanes during photolysis has been actively pursued in recent years, in part because of the potential application of the polysilanes as photoresists, and for other industrial applications. In addition to the reactions mentioned above, other reactions have now been recognized as occurring in the course of photolysis. These are summarized below. 

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