Takafumi Imai

Preparation of substituted network polysilanes by a disproportionation reaction of alkoxydisilanes in the presence of alkoxysilanes

Network methylethoxypolysilanes containing various substituents such as hexyl, phenyl, ethylene, hexamethylene, phenylene, and thiophene groups were prepared by a disproportionation reaction of 1,1,2,2-tetraethoxy-1,2-dimethyldisilane in the presence of the corresponding substituted alkoxysilanes. The reaction was considered to proceed via a silyl anion mechanism. The silyl anion produced from a disilane or a higher homologue attacked the alkoxysilane, and that caused the alkoxysilane moiety to be introduced into the polysilane. The amount of incorporated substituents varied from 3 to 29% of all organic substituents, depending on which alkoxysilane was reacted. The electrical conductivities of some polysilanes were measured, and the polymer with thiophene groups was found to show relatively higher conductivity of 10 -5 Scm -1 after iodine doping.

Photoreaction of alkoxypolysilanes

Abstract The photoreaction of network ethoxypolysilanes ((MeSi)n[Me(EtO)Si]m) using low pressure mercury lamps was investigated in the presence and in the absence of oxygen, and was compared with that of poly(hexylsilyne) and poly(dihexylsilylene). The photoreaction of the ethoxypolysilanes in the presence of oxygen led to their photooxidation, and resulted in a decreased molecular weight and the disappearance of ultraviolet absorption over 300 nm. The molecular weight of the ethoxypolysilanes was more slowly reduced than that of poly(hexylsilyne). On the other hand, photolysis of the ethoxypolysilanes under an inert atmosphere caused an increase in molecular weight, though similar photolysis of the alkylpolysilanes showed a molecular weight decrease. These results could be explained by the influence of the ethoxysilyl groups in the polysilanes. The decomposition of polymer chains would be depressed by the reaction of the alkoxysilyl groups with silanols produced from photooxidation and the insertion reaction of silylenes into SiOEt bonds.

Effect of backbone structures of polysilanes on the photooxidation reaction

Photooxidation reactions of linear polysilanes [poly(cyclohexylmethylsilylene) and poly(methylphenylsilylene)], a network polysilane [poly(hexylsilyne)] and branched polysilanes [poly(cyclohexylmethylsilylene-co-phenylsilyne) and poly(cyclohexylmethylsilylene-co-hexylsilyne)] were investigated both in solution and in the solid state in the presence of oxygen. The reactions were monitored by u.v. and i.r. spectroscopies and size exclusion chromatography. Polysilanes containing a greater number of silyne units, i.e. highly branched polysilanes, required a greater dose of u.v. exposure for the completion of the photooxidation, which was determined by a decrease in the intensity of the polysilanes characteristic u.v. absorption and an increase in the intensity of the siloxanes i.r. absorption. The molecular weight change of the polymers also depended on the backbone structure. The degradation of the network and branched polysilanes was slower and smaller than that of linear polymers during the reactions.

Preparation of substituted network polysilanes by a disproportionation reaction of ethoxydisilane initiated by a small amount of organolithium reagents

Network methylethoxypolysilanes containing various substituents such as phenyl, butyl, phenylene, thiophene, and anthracene groups were prepared by a disproportionation reaction of 1,1,2,2-tetraethoxy-1,2-dimethyldisilane initiated by addition of a small amount of organolithium reagents that have the corresponding substituents. The reaction was considered to be catalyzed by lithium ethoxide, which was formed by the substitution reaction of the ethoxydisilane with the lithium reagents. Both the substituted and pristine disilanes participated in the disproportionation reaction to yield the network polysilanes. The amount of the substituents and the molecular weight of the polysilanes varied, depending on what and how much of the lithium reagent were used. The electrical conductivity of some polysilanes was measured, and polymers with thiophene or anthracene groups were found to show relatively higher conductivity of 10−4 Scm−1 after iodine doping.

Photochemical conversion of electrically conductive polysilane films into insulators

Abstract Electrically conductive polysilane films doped with iodine can be converted into insulators to semiconductors by a photooxidation reaction. The conductivity can be controlled in the range 10 −15 –10 −4 S cm −1 by regulation of the extent of the photooxidation reaction. Since the non-irradiated area of the film keeps its doping susceptibility, it is possible to prepare a conducting pattern on the film by a photolithographic technique. Network polysilanes containing various substituents, prepared by the disproportionation reaction of alkoxydisilanes, were primarily examined, in addition to polysilanes prepared by the Wurtz coupling reaction. UV and IR analyses indicated that the Si-Si bonds in the polysilanes were changed into Si-O-Si bonds and Si-OH bonds by the photooxidation.