Polymers, carbosilane silicone

The first type of polycarbosilane synthesized by using ADMET methodology was a poly[carbo(dimethyl)silane].14c Linear poly(carbosilanes) are an important class of silicon-containing polymers due to their thermal, electronic, and optical properties.41 They are also ceramic precursors to silicon carbide after pyrolysis. ADMET opens up a new route to synthesize poly(carbosilanes), one that avoids many of the limitations found in earlier synthetic methods.41... 

In one approach, six-membered silicon-containing rings are pyrolized giving mixed car-bosilane preceramic polymers through heating to 400°C, and subsequently forming silicon carbides or poly(carbosilanes) at 800°C. 

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... 

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. 

Summary The stepwise synthesis of the polycarbosilanes (Cl2SiCH2CH2) (5) and (H2SiCH2CH2)n (6) are described. On addition of catalytical amounts of transition metal complexes to polymer 6 dehydrogenation occurs and a further crosslinked carbosilane (8) is obtained by formation of new silicon-silicon bonds. Pyrolysis of carbosilane 8 produces a black ceramic material, containing P-SiC together with carbon. The ceramic yield after pyrolysis of 8 is approximately four times the yield obtained when 6 is employed as the starting material. From polymeric 8 preceramic fibers are accessible subsequent pyrolysis yields ceramic fibers. Moreover, the carbosilane 8 can be utilized as a binder for ceramic powders. 

Hydrosilylation of unsaturated organosilicon compounds has also found several applications in molecular and polymer organosilicon chemistry. In particular, the addition of polyfunctional silicon hydrides to poly(vinyl)organosiloxane, catalyzed exclusively by Pt compounds and providing an activated cure for silicon rubber [10], has been of great practical importance. Hydrosilylation of the vinyl group at silicon seems to be effective synthetic method for preparation of oligomers and polymers with a linear or cyclolinear stmcture (polyhydrosilylation), and can occur either via the addition of dihydro-carbosilanes and -siloxanes to divinyl-silanes and -siloxanes [25, 26] or by intermolecular hydrosilylation [4] (eq. (1)). 

But CMCs will be commercially successful only when they are produced cost-effectively. Polymer-derived ceramic (PDC) technology is one of the most promising low cost fabrication methods for ceramic matrix composites, particularly for large, complex shapes. In PDC technology, a silicon-based polymer (siloxane, carbosilane, silazane, etc) with fiber or particle reinforcement is shaped and cured in the polymer condition and then pyrolyzed in a controlled atmosphere to form a stable silicon-based ceramic, such as silicon carbide, sihcon nitride, silicon oxycarbide, or silicon oxynitride. 

Yajima [76] was the first to study the preparation of silicon carbide fibers from carbosilanes. These and other SiC-containing polymers were used to produce SiC powders with a crystallite size as small as several nanometers [77, 78]. The advantage ofthe production route from liquid to solid to produce SiC has also attracted attention for SiC film production in microelectronics or as protection layers. In this way, amorphous, polycrystalline films of high purity produced by the dip-coating of substrates in PCS solutions and subsequent pyrolysis in an inert gas atmosphere, have been prepared [115]. 

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]. 

Matsumoto K, Matsuoka H (2005) Synthesis of core-crosslinked carbosilane block copolymer micelles and their thermal transformation to silicon-based ceramics nanoparticles. J Polym Sci A Polym Chem 43(17) 3778-3787... 

By using a difunctional dihydrosiloxane and/or divinylsiloxane as hydrosily-lation substrates, polycarbosiloxanes can be obtained that differ from their poly-carbosilane analogues in the thermal and mechanical properties (359-362). The organosilicon polymers of this type, containing both Si—O and Si—C bonds in their backbones, are also called hybrid silicones. Because polycarbosiloxanes have properties intermediate between those of linear polydimethysiloxanes and saturated hydrocarbons, they have been synthesized recently to improve thermal and fuel resistance properties of classical silicones (Scheme 39). 

Molecular design and precision synthesis of silicon-containing polymers are described. Polymerizations of substituted silacyclobutanes by phenyllithium and platinum complexes gave poly(carbosilane)s of head to tail regular structure. However, extensive chain transfer seems to have occurred in the polymerization by platinum complexes. 

Silicon-containing polymers, such as poly(siloxane) (3), poly(carbosilane) (4,5), and poly(silylene) (6) have found practical importance in many areas of applications, but they have been insufficiently studied in relation to their primary structure, stereoregularity, molecular weight, and higher order structure. 

Conclusion. Approaches to molecular design of primary structure of silicon-containing polymers were illustrated for poly(carbosilane)s with controlled molecular weight and for stereoregular poly(carbosilane) and poly(carbosiloxane). 

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