Polysilanes carbosilanes

The variety of silicon based polymers is completed by high tech ceramics like silicon nitride and carbide. These materials are produced by pyrolysis of appropriate polymeric precursors such as polysilanes, -carbosilanes, and silazanes. Another important approach is realized by sol-gel processing. 

The interaction of polysilanes with X-ray photoelectrons (XPS) has been used for theoretical studies" and for analytical purposes.19 Seven polymers with silane, carbosilane, and siloxane fragments—(-Si(CH3)2-)n (PDMS),... 

The carbosilane monomer, H2MeSiCH2CH2SiH3, contains both a secondary and a primary silicon center. After about 48 h with Cp2TiMe2 or Cp2TiCl2/ BuLi at r.t. the dehydrocoupling reaction occurred at the primary silane center to give a polysilane with a substituent at each silicon center that contained a tether to the secondary silane center. The initial polymer had a Mw of approximately 1000. After another 72 h at r.t., the... 

As shown in Eq. (e), dodecamethylcyclohexasilane polymerizes when heated. However, polymerization is accompanied by a rearrangement to a poly(carbosilane), -E-Si(H)(Me)CH2-9- , which is an intermediate in converting polysilanes to silicon nitride . 

The pyrolysis of polysilanes and polycarbosilanes is usually carried out using inert gas (e.g., argon) as pyrolysis atmosphere. A general problem associated with the pyrolytic formation of carbides is the desired stoichiometry of the calcined products in contrast to nitrides, excess carbon carmot be evaporated during calcining it may therefore contaminate the powders obtained as an elemental impurity and thus influences the physical, especially mechanical and electrical, properties of the sintered ceramic bodies. The volatiles evaporated during pyrolytic treatment of carbosilanes to form a network structure are H2 and CH4, and they depend on the structure of the polycarbosilane used (Fig. 2). 

The thermal treatment of vinylic polysilane SiC precursor polymers involves <300 °C cross-linking and loss of low molecular weight oligomers. Polymer degradation, which is characterized by significant decomposition of the side groups and chain scission, along with conversion of the polysilane skeleton to poly(carbosilane), takes place between 300 and 750 °C [124],... 

The decomposition chemistry of most polysilanes above 300 °C is quite complex and involves restructuring of the polymer backbone as well as substantial cleavage of the side groups. Generdly a poly(alkyl)silane must go through a carbosilane intermediate prior to formation of the SiC network (Kumada rearrangement) [125] ... 

Secondly - at higher temperatures (T > 380 °C) the polysilane-to-poly(carbosilane) transformation becomes the dominating process (Eq. 28). 

Design and synthesis of hybrid polymers and of dendrimers for special applications and as building blocks for carbosilanes, polysilanes catalysis and highly crosslinked coatings. 

Viscosity measurements and spectroscopic data (NMR, IR, UV) have shown that the structure of PCS polymers is roughly planar with three different atomic bonding schemes for the silicon atoms [1] [6] [9j. The respective fraction of each atomic bonding scheme depends upon the PDMS/PCS conversion conditions (Table II). The structure of PCSs resulting from the catalytic thermal rearrangement of PDMS (PC-B type polymers) contains more Si-Si bonds. It could thus be better described as polysilane chains connected through highly branched carbosilane nodes [44]. 

At the present time, most of the work is centered on the development of silicon carbide and silicon-nitride powders. Polysilanes are the basic precursor for the synthesis of silicon carbide from sol-gel. This was first achieved by the conversion of polydimethyl silane through a carbosilane intermediate. However this silane has poor solubility and low char yield and considerable shrinkage occurs during conversion to the final product. Precursors are being developed with a much larger SiC yield notably those based on trifiinctional alkoxides and chlorosilanes. These materials are being evaluated for the production of powders.P JP l... 

Several dendrimers containing heteroatoms have been well studied, especially Si- and P-based. The Si-based dendrimers were the first to contain second-row heteroatoms and can be classified into polysilanes (Si-Si), carbosilanes (Si-C), carbosiloxanes (Si-O-C), siloxanes (Si-O-Si),... 

If the reaction temperature is low, it tends to give cyclic polysilanes (mainly the six membered ring shown in eq. (8.34), and the other products are four membered ring or eight membered ring compounds). The linear polysilanes (eq. (8.53)) are insoluble white solids. If these polysilanes are heated at 400 in the presence of B(OSiCl3)3 in a sealed system, the polymers are rearranged to form poly-carbosilanes ((-SiMe(H)—CH2-) ) as a soluble solid. 

Organosilicon polymers have attracted much research interest due to their potentially useful properties. For example, the preparation, characterization, and applications of poly(siloxanes) (/), poly(carbosilanes) (2), poly(carbosiloxanes) (S-S), and poly(silanes) (9) have been extensively documented in the literature. Recent research has focused on developing methods for modifying these polymers in order to prepare a variety of substituted derivatives with altered properties. Different approaches used to modify organosilicon polymers have included free-radical hydrosilylation of polysilanes (JO), nucleophilic substitution in poly(chlorosilylenemethylenes) (IJ), hydrosilylation of poly(silylenemethylenes) (12), and deprotonation of... 

Poly(carbosilane)s are usually produced by thermal decomposition of polysilane polymers or cyclic oligomers, optionally in an autoclave to prevent loss of volatile material and enhance the yield. In early work, dodecamethylcyclohexasilane (1), produced by dechlorination of dimethyldichloro-silane using lithium, was employed as the precursor to poly(carbosilane). This compound can be converted to poly(carbosilane) by heating in an autoclave at 400 ""C, but useful spinnable polymer is only obtained after complex purification procedures, and the yield is thus relatively low. ... 

The dechlorination of dimethyldichlorosilane can be effected using sodium in xylene to give a linear polysilane. This polymer is better suited for conversion to poly(carbosilane) than is dodecamethylcyclohexasilane since the reaction product of pyrolysis requires a far less complex purification process to give a polymer suitable for spinning. 

Conversion of linear polysilane to poly(carbosilane) can be carried out at atmospheric pressure or at high pressure in an autoclave.At atmospheric pressure, the reported yield is 35%, whereas autoclave pyrolysis increases the yield to 50-60%. The yield at atmospheric pressure can be increased to about 65% by addition of a minor proportion of poly(borodiphenylsiloxane) (2) to the... 

Pannell KH, Kobayashi T, Cervantes-Lee F (2003) Photochemical transformation of a cyclic polysilane to a cyclic carbosilane via ((ti -C5H5)Fe(CO)2)CH2-, FpCH2-, substitution. J Organomet Chem 685 189... 

---