Polysilanes 7-irradiation

As these experiments indicate, polysilanes can in some cases be converted to silicon carbide directly, without the necessity for formation of polycarbosilane, fractionation, or oxidation. For example, polysilastyrene copolymers can be formed into films or fibers and then crosslinked by irradiation with UV light. The crosslinked polysilane forms silicon carbide when heated to 1100°C in vacuum. (1U This method can be used in a "printing" mode, if a film of polysilane is cast onto a ceramic or metal substrate, then... 

Thin films of this polymer generate third harmonic radiation upon irradiation at 1064 nm, believed to be due to a three-photon resonance. Thus polysilane polymers may eventually find use in laser technology. 

These results indicate that the reactions triggered by the irradiation of polysilanes are rather complex and that the present understanding of these processes is clearly still quite limited. 

Figure 5.2. Gel permeation chromatogram (GPC) of the liquid silicon precursor for Si film formation for (a) cyclopentasilane (CPS) and (b) UV-irradiated CPS, both of which were diluted with toluene (1 vol.%) before GPC measurements. The UV-irradiation conditions were 405 nm, 100mW/cm2, and a 10-min irradiation for 1cm3 of CPS. The broad peak around Mw = 2600 corresponds to polysilanes of various molecular weights, as a result of the photo-induced polymerization of CPS. [Reproduced with permission from Ref. 10. Copyright 2006 Nature Publishing Group.]...

Photochemical transformations of cyclic and short chain polysilane oligomers have been intensively investigated (39). Irradiation of these materials in the presence of trapping reagents, such as silanes or alcohols, has suggested that substituted silylenes and silyl radicals are primary reactive intermediates. The former have been... 

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). 

Table I. Trapping. products from the exhaustive irradiation of some polysilane derivatives [ (SiR R2 ) ] at 254 nm in the presence of triethylsilane (a) yields were less than 2% ...

Regarding this proposal, it should be noted that while 1,1-eliminations on Si-Si-C units to generate silylenes are well known thermal processes (54) the photochemical variant seems not to have been described. The rearrangement of silylsilylenes (4) to disilenes is known to be rapid (55), and silyl radical addition at the least hindered site would produce the observed persistent radical. Preliminary evidence for the operation of 1,1-photoelimination processes in the polysilane high polymers has been obtained, in that the exhaustive irradiation at 248 nm of poly(cyclohexylmethylsilane) (PCHMS) produces —10-15% volatile products which contain trialkylsilyl terminal groups. For example, the following products were produced and identified by GC— MS (R=cyclohexyl,R = methyl) H(RR Si)2H (49%), H(RR Si)3H (19%), R2R SiH (2%), R 2RSiRR SiH (5%) and R2R SiRR SiH (7%). 

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). 

The ladder polysilanes also show interesting photochemistry on irradiation with a high-pressure mercury lamp, the tricyclic ladder extruded the transient intermediate four-membered cyclic disilene which could be trapped using methanol, 1,3-butadiene, and anthracene, as shown in Scheme 38. 

The photoreaction of polysilanes with Ceo has also been investigated [35]. Reaction (8.16) shows an example in which the irradiation in benzene with a low-pressure mercury-arc lamp afforded a product that contains 14wt% of Ceo into the polysilane chain. The incorporation of Ceo into the polysilane backbone has not been observed upon irradiation with X > 300 nm, when the cleavage of Si—Si bond does not take place. The adduct obtained from Reaction (8.16) has a lower oxidation potential than C6o( + 0-77 vs + 1.21 V) and a lower reduction potential than polysilane (—1.24 vs — 2 V). 

Figure 12 Observed (—) and simulated (—) ESR spectra of disilane and polysilane radical cations at 77 K in -y-irradiated freon matrices. The simulation for polymers was carried out under the assumption that an unpaired electron is localized on a part of polymer skeleton composed of about six Si atoms.

The key intermediates in the process are the silylenes and the radical fragments, both of which should be amenable to trapping experiments. With this in mind, two representative, polysilane derivatives 6 and 9, were irradiated (254 nm) in the presence of excess triethylsilane, and the results of this experiment are shown in Figure 6. Since the polysilane 9 was not soluble in the trapping reagent, it was dissolved in cyclohexane which contained a 100 fold molar excess of triethylsilane. In each case, the major isolable volatile products... 

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