Synthesis, Structure And Properties Of Polysilanes With Inorganic-Substituents

Polysilanes consist of a linear Si-Si catenation in the main chain and two substituents on each silicon atom. The σ-electrons in the main chain of polysilanes are strongly delocalized due to the availability of the 3d vacant orbitals of Si atoms. In recent years, polysilanes have been of interest due to their unique properties and potential applications as functional materials such as semi-conductors, photoconductors, nonlinear optical materials and so on. The substituents of polysilanes are usually alkyl or aryl groups in the reported researches. When the substituents of polysilanes are replaced by some inorganic groups, the σ-delocalization will be increased and functional properties will be improved. These new polymers will show even wider potential applications as a new class of functional materials. The molecular design and calculation demonstrate that the electronic energy gap can be lowered when the inorganic groups are introduced. Some small organosilicon compounds containing silicon-metal bonds were reported in published papers, but no study about the polysilane with Si-metal bond was reported.High reactive polymers, polymethylsilane (PMS) and polychloromethylsilane (PCMS), were synthesized in this study, which were used as mother polymers to prepare three kinds of inorganic-substituted polysilanes: silicon-substituted polysilane (SPS), antimony-substituted polysilane (APS) and ferrum-substituted polysilane (FPS). The polymer composition, the molecular structure, the molecular weight, the stability and the other properties of the polysilanes were characterized by FT-IR, element analysis, 'H-NMR, 29Si CP/MAS NMR, GPC, UV, and TGA. In addition, the new polysilanes were doped with iodine and their semi-conductor properties were examined.PMS is of interest due to its plenty of reactive Si-H bonds, but the reported researches were focused on the application as preceramic polymers to transform into SiC ceramics instead of applying to prepare new functional polysilanes. PMS was synthesized by Wurtz coupling reaction from methyldichlorosilane with sodium in this work. The chemicalcomposition of the PMS is [(MeHSi)o.73(MeSi)o.27]n- PCMS was obtained by chlorination of PMS with carbon tetrachloride for a long time in mild reaction condition. In the reaction, above 90% of the hydreogen in the Si-H bonds were selectively replaced by chlorine.SPS can be synthesized by modification Watanabe method starting from chlorodisilanes, but the reaction products were found to be too complicated. In this work, SPS was successfully prepared by PCMS reacting with trimethylchlorosilane in magnesium/THF suspension. The resulted SPS was characterized as the structure [(MeHSi)0.37.(MeSi)o.3o(MeSiSiMe3)0.33]n, with molecular weight 980. The UV max of SPSincreases from 280 of PMS to 310nm and the conductivity to 10-6S/cm when doped with iodine due to the possibly wider a-delocalized system caused by the Si substituents. The SPS is more stable in air than PMS after thermal crosslinking reaction. Much higher ceramic yield (44%) than PMS indicates that SPS is a potential preceramic polymer.The synthesis of APS was realized from PMS and SbCl3 catalyzed by pyridine in THF at room temperature in N2, with the yield of 75.2%. The APS was characterized as chemical composition of [(MeHSi)o.33(MeSi)o.56(MeSiSb)0.11]n, molecular weight of 1600 and a good stability in air. The Sb atom was found to be catenative directly to the Si of the main chain, and Sb plays a role as a crosslinker due to its tri-functional structure. The UV max of APS is 330nm, the intrinsic conductivity is in the range of 10-8-10-7S/cm, the doped conductivity with iodine reaches to 10-5S/cm and the dielectric constant is as high as 6.2. When the APS blended with some commodity resins to form thin coating (0.35~0.41mm), it shows microwave absorbent properties in wide frequency range (2 to 18GHz), with the minimum reflectance of-6.9dB at 13GHz.Since the reaction of PMS with Fe(CO)s gives a very low yield, FPS was synthesized from PCMS and Na2Fe(CO)4 in n-hexane at room temperature...