Scission polysilanes

Table 2. Quantum yields for oolymer scission and crosslinking for some representative polysilanes (SiR R )n (a) Molecular weights were measured by GPC using polystyrene calibration standards (b) Ban, H Sukegawa, K. J. Polym. Sci., Polym. Chem. Ed., 1988, 26, 521...

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 15 Schematic representation of the mechanism of polysilane chain scission.

In an effort to increase the efficiency of scission, particularly in the solid-state where chain repair is most competitive, a number of external additives were auditioned. Since both the Xmax and the absorptivity of polysilane derivatives are functions of molecular weight, splitting the main polymer chain results in a rapid bleaching of the original absorption. This unusual feature can be used as a qualitative, diagnostic test of scission efficiency which greatly facilitates the testing of additives. 

Polysilanes absorb electromagnetic energy and undergo chain scission. This is an extremely important property of these polymers in terms of applications, Photochemistry is exhibited both in solution and in the solid slate. 

When irradiated with ultraviolet light, most polysilanes undergo chain scission into smaller fragments. When polysilanes (RR Si), are photolyzed with UV light X = 254 nm)... 

When aryl substituent groups are present, more complex photolytic reactions occur, and some cross-linking takes place as well as chain scission. Groups containing C=C double bonds are even more effective at bringing about cross-linking. This leads directly to the topic of cross-linking in polysilanes. 

The polysilanes can be used as photoresists because they undergo scission when exposed to ultraviolet light, as explained in Section 5.10. Both the extinction coefficient and the wavelength of maximum absorption decrease with increasing exposure, so that polysilanes can be bleached photochemically, as illustrated for ( -HexSiMe) in Figure 5.27. Dialkylsilane polymers are generally better as positive photoresists than... 

When polysilanes are used in free radical polymerization, silyl-type radicals formed by chain scission are the primary initiating radicals [61,62], However, in the presence of pyridinium ions these radicals undergo oxidation to yield reactive cations capable of initiating the polymerization of appropriate monomers (Scheme 14). 

The quantum yields for scissioning ( >g) and crosslinking ( ) were determined for some representative polysilane derivatives both in solution and in the solid state (8). In all cases polymer scission is the predominant process and the values ranged from 0.2 to 1.0. For two cases, poly (methyl phenylsilane) and poly(cyclohexyl methylsilane) which were also examined in the solid state, the quantum yields were reduced by at least an order of magnitude from the solution values. 

Effect of Polyhalogenated Additives. We (36) have also observed that the rate of bleaching of solid polysilane films upon irradiation is considerably slower than that observed for solutions at comparable optical densities. Although this result is consistent with the observed decrease in the quantum yields for scission, 4>(s), in going from solution to the solid state (24) (vide infra), this decreased sensitivity is inconvenient for imaging processes. For this reason, a search was made for compatible additives that might influence the bleaching rate of poly silane derivatives in the solid state. 

Similar extrusion reactions have been observed for acyclic polysilanes (55). The isolation of hydrogen-terminated silanes containing fewer silicon atoms than the starting materials was taken as evidence for the intermediacy of silyl radicals, and the importance of chain scission seemed to increase with increasing catenation. 

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. 

In these equations, and are the initial molecular weights, 4>(s) and 4>(x) are the quantum yields for the scissioning and cross-linking reactions, respectively D is absorbed dose and is Avogadro s number. The slopes of the respective plots of I/M and I/M versus dose produce two simultaneous equations, the solution of which yields values for 4>(s) and 4>(x). We (60) have analyzed the data obtained for a number of polysilane derivatives by GPC (gel permeation chromatography) to evaluate the respective molecular weights and distributions (Table III). Polystyrene standards were used for molecular weight calibration. 

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