Lead-silicon polymers preparation

Organosilicon polymers. Silicon resembles carbon in certain respects and attempts have been made to prepare polymers combining carbon and silicon units in the molecule with the object of increasing the heat resistance of polymers. It has been found that the hydrolysis of a dialkyl-dichlorosilicane or an alkyltrichlorosilicane, or a mixture of the two, leads to polymers (Silicones), both solid and liquid, which possess great thermal stability. Thus dimethyldichlorosilicane (I) is rapidly converted by water into the silicol (II), which immediately loses water to give a silicone oil of the type (III) ... 

Uses Coolant and refrigerant herbicide and fumigant organic synthesis-methylating agent manufacturing of silicone polymers, pharmaceuticals, tetramethyl lead, synthetic rubber, methyl cellulose, agricultural chemicals and nonflammable films preparation of methylene chloride, carbon tetrachloride, chloroform low temperature solvent and extractant catalytic carrier for butyl rubber polymerization topical anesthetic fluid for thermometric and thermostatic equipment. 

However, catalysts such as the potassium slloxanolate catalyst are not transient. Non-transient catalysts must be neutralized or removed by some other method In order to give a thermally stable polymer. If the catalyst Is not removed. It will cause depolymerization at high temperatures. For example, a silicone gum prepared by reacting D4 with O.OlZ KOH has been reported to lose over 99% of Its weight at 2S0 C In 24 hours (11). Non-transient catalysts can often be used at much higher temperatures than the transient catalysts, leading, of course, to faster rates of reaction. 

The hydrosilylation reaction, leading to the formation of an alkylsilane by addition of a hydrosilane unit (Si-H) to a double bond, is widely used in the production of silicon polymers, paper release coatings and pressure-sensitive adhesives. Many homogeneous organometalhc complexes based on Co, Ni, Pd, Rh or Pt have been used to catalyze this reaction, but strong evidence has proved that the really active species were metal colloids [95-97]. Moreover, it has been demonstrated that Pt colloids were the most effective catalysts for SiH addition to terminal olefins (98). However, Lewis et al. [99] have reported Rh colloids-catalyzed hydrosilylation (Scheme 11.16). The particles were prepared by the reduction of rhodium chloride by the silane reagents. The authors have shown that Rh colloids present an interesting activity in the addition of di- and trihydrides to olefins, compared to platinum colloids. 

When a thin liquid film with a thickness of approximately 2 pm prepared by spin coating of a 15% benzene solution of polymer 1 was irradiated with a 500-W Xe-Hg lamp for 300 s in air, a transparent solid film was obtained. The UV spectrum of this solid film shows that an absorption at 235 nm due to phenyldisilanyl units vanishes after UV-irradiation (Figure 1). This clearly indicates that photolytic cleavage of silicon-silicon bonds leading to the cross-linking occurred. Similar photolysis of the thin liquid films under a nitrogen atmosphere again afforded transparent solid films whose UV spectra show no absorption at 235 nm due to phenyldisilanyl units. 

Tetravalent silicon is the only structural feature in all silicon sources in nature, e.g. the silicates and silica even elemental silicon exhibits tetravalency. Tetravalent silicon is considered to be an ana-logon to its group 14 homologue carbon and in fact there are a lot of similarities in the chemistry of both elements. Furthermore, silicon is tetravalent in all industrially used compounds, e.g. silanes, polymers, ceramics, and fumed silica. Also the reactions of subvalent and / or low coordinated silicon compounds normally lead back to tetravalent silicon species. It is therefore not surprising that more than 90% of the relevant literature deals with tetravalent silicon. The following examples illustrate why "ordinary" tetravalent silicon is still an attractive field for research activities Simple and small tetravalent silicon compounds - sometimes very difficult to synthesize - are used by theoreticians and preparative chemists as model compounds for a deeper insight into structural features and the study of the reactivity influenced by different substituents on the silicon center. As an example for industrial applications, the chemical vapor decomposition (CVD) of appropriate silicon precursors to produce thin ceramic coatings on various substrates may be mentioned. 

Research and development in the field are still continuing at a fast pace, particulady in the area of absorption and emission characteristics of the polymers. Several reasons account for this interest. First, the intractable poly dime thylsilane [30107-43-8] was found to be a precursor to the important ceramic, silicon carbide (86—89). Secondly, a number of soluble polysilanes were prepared, which allowed these polymers to be studied in detail (90—93). As a result of studies with soluble polymers it became clear that polysilanes are unusual in their backbone CT-conjugation, which leads to some very interesting electronic properties. 

In the following sections some examples are given of the ways in which these principles have been utilized. The first example is the use of these techniques for the low temperature preparation of oxide ceramics such as silica. This process can also be used to produce alumina, titanium oxide, or other metal oxides. The second example describes the conversion of organic polymers to carbon fiber, a process that was probably the inspiration for the later development of routes to a range of non-oxide ceramics. Following this are brief reviews of processes that lead to the formation of silicon carbide, silicon nitride, boron nitride, and aluminum nitride, plus an introduction to the synthesis of other ceramics such as phosphorus nitride, nitrogen-phosphorus-boron materials, and an example of a transition metal-containing ceramic material. 

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