Polysilastyrene polymers

A nearly fully ordered polysilane copolymer has been synthesized by careful condensation of a 1,3-dibromotrisilane, yielding the AABAAB-type copolymer shown in equation (30).118 The best method for obtaining ordered polysilane copolymers is however by the anionic polymerization of masked disilenes (Section 5.3.3). Formation of an ABAB-type copolymer is illustrated in equation (31).119 These polymers show only two narrow lines in their 29Si NMR spectra, consistent with their ordered structure. In a similar reaction, the Sakurai group synthesized an exactly alternating polysilastyrene polymer, (PhMeSi-SiMe2) .120... 

Research in polysilane polymers grew slowly at first after this reawakening. But within the past few years, both the unusual scientific interest and the technological possibilities of the polysilanes have been recognized, and activity in this field has increased sharply. Commercial manufacture of both poly dimethylsilylene) and "polysilastyrene" is now being carried out in Japan, so that these two polymers are readily available in quantity. 

Soluble polysilane polymers can also be used as precursors to silicon carbide. The first such application, using (PhMeSi)n-(Me2Si)m copolymers ("Polysilastyrene"), was to strengthen silicon nitride ceramics. The Si3N4 ceramic body was soaked in polysilane and refired, leading to the formation of silicon carbide whiskers in the pore spaces and a consequent increase in strength. (U)... 

West and co-workers [9,10] prepared polysilastyrene, a soluble and fusible polymer, by a Wurtz-coupling of dimethyldichlorosilane and methylphenyldichlorosilane (Eq. 4). Thermolysis of this polymer at 450°C results in a polycarbosilastyrene which can be cured by irradiation with UV-light in air at room temperature [11],... 

The silicon-branched polymer also became semiconducting upon exposure to iodine vapor it is, therefore, closely related to polysilastyrene , for which West et al. (2) used highly toxic pentafluoroarsine as dopant. Polymer 1 is originally insulating, but an iodine-doped sample, a black solid, showed a conductivity (a) of I0" /fl-cm. The black solid was stable in air for several days and then gradually softened. The stability of the doped sample remains to be improved further. 

Thermogravimetric analysis of the polymer was carried out under very nearly the same conditions as previously reported for permethylpolysilane and polysilastyrene (17, 18) (Figure 5). Unfractionated polymer 2 lost 35% of its weight upon heating under nitrogen at a 10 °C/min rise in temperature, up to 800 C. The theoretical loss according to equation 4 or 5 is 27 or 40%, respectively, and the results indicate that practically no volatile silicon compounds have been driven off. 

This synthetic strategy has allowed us to synthesize a real head-to-tail polysilastyiene. In 1981 West et al. reported the first synthesis of a soluble polysilane copolymer from methylphenyldichlorosilane and dimethyldichlorosilane [10]. The copolymer was called polysilastyrene, based on its structural similarity to carbon-based polymer, i.e., polystyrene. 

However, the structure of the copolymer was blocky with runs of the respective monomer units, and hence the polymer was not the real polysilastyrene [11]. Phenylation of the chloro-substituted polysilane 3 with phenyllittuum gave poly(l,l,2-trimethyl-2-phenyldisilene) in good yield. The structure of the polymer was confirmed with the SEC and NMR spectra. We have now succeeded in the preparation of the real head-to-tail polysilastyrene (4, R = Ph) about 20 years after the first reported synthesis of soluble polysilastyrene. This procedure is a simple method for preparing polysilanes of special structure with functional groups which are otherwise very difficult to prepare. 

This new polymer, called polysilastyrene , is fusible at a low temperature, soluble in numerous solvents, and can be processed into fibers and coatings. 

Further testing [535] was confirmed the procedure for producing polysilastyrene , and has established that the polymers are stable in the open air to about 300 C. They are insoluble in acetone, methanol, acetonitrile and hexane. Their solubility in toluene, tetrachloromethane and chloroform has been measured as a function of the molecular weight of the polymer. 

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