Polycondensation of silicon

The hydrolytic polycondensation of silicon alkoxides of general formula Si(OR)4 or R/ Si(OR)4 , where the non-reactive organofunc-tional group R acts as a network modifier, is carried out in the presence of dopant molecules resulting in the formation of highly porous, reactive organosilicates whose applications span many traditional domains of chemistry. 

Silica gels may also be produced by the hydrolysis and polycondensation of silicon alkoxides, eg, tetraethylorthosilicate, Si(OC2H5)4. This is often... 

FIGURE 2. Glassy silica needles (2 mm x 30 gm) produced in copious amount by a marine sponge. Each needle contains an occluded axial filament comprised of silicateins, enzyme-like proteins that catalyze and spatially direct the polycondensation of silicon alkoxides and silicic acid at neutral pjj41,42 Reprinted from Reference 3, copyright (1999), with permission from Elsevier Science... 

The time-of-flight secondary ion mass spectrum of a thick film prepared from Si(OEt)4 on a hydrophilic silicon substrate (Fig. 1) reveals a distribution of masses up to 1200 amu. The observed formation of cationized oligomers with a distribution shown in Fig. 1 can be explained by bond cleavage within the uppermost monolayer of the polycondensate of TEOS as a result of primary Ar+ ion impact. 

In the second, complementary, approach the polycondensation of silica polymer is followed by the formation of an organic network made by cross-linking reaction of monomers covalently bound to silicon compounds (Scheme 4.1) resulting in polymeric materials with outstanding protective abilities, including thermal, mechanical and corrosion resistance. 

It is well known that silicones can be prepared from dihalogenosilanes that undergo hydrolysis to generate disilanol. These latter intermediates produce cyclic or linear dialkylsiloxanes. The cyclic dialkylsiloxanes can be separated and ringopening polymerized whereas the linear ones can be condensed to yield silicones. However, it is still difficult to purify cyclic and linear precursors. Thus, in order to avoid cyclization, our objective concerned the polycondensation of fluorohybrid silanes. 

Generally, two common methods, the Stober method and the reverse microemulsion method are used for synthesis of silica nanoparticles. As derivatives of a sol-gel process, both methods involve hydrolysis of a silicon alkoxide precursor to form a hydroxysilicate followed by polycondensation of the hydroxysilicate to form a silica nanoparticle [44]. 

In addition, polymers containing an odd number of silicon atoms in the siloxanylene segment can be prepared by this approach. However, reaction conditions employed in condensation of bis-silanols, III, with diaminosilanes (41) or siloxazanes (42) led to the cleavage of the carbonate group. Consequently, the recently reported silanol-acetoxysilane polycondensation reaction (43) was investigated for the polycondensation of bis-silanols, III, with diacetoxysilanes or diacetoxydisiloxanes (reaction 3). 

The rate of the hydrolytic polycondensation of alkyltrichlorosilane markedly depends on the length and branching of the alkyl group at the silicon atom and decreases with X varying in the following order ... 

Silicon is the most abundant metal-like element in the earth s crust. It is seldom present in pure elemental form, but is rather found in a large number of polymers based largely on the polycondensation of the orthosilicate anion, SiO as illustrated in the following equations ... 

The formation of a sol-gel porous material is through a hydrolysis-polycondensation reaction. An example is given in equation 1 with the methoxide of silicon (tetramethyl-orthosilicate, TMOS), but many other alkoxides, aryl oxides and acyl oxides can be used, as well as Si—N and Si—Cl compounds. 

Wang et al. [73-76] performed a study of the catalytic activity of a silicon-based polymer obtained by polycondensation of 4-bis[(3-dimethyl-ethoxy-... 

Optically active poly(silyl ether)s were also synthesized by asymmetric induction on the silicon atom from chiral diols or a chiral Rh catalyst [178]. In the dehydrogenative polycondensations of bis(silane)s and several chiral diols, the highest ee value of the silicon atom was 13.7%. In the polymerization of a bis(silane) and an achiral diol, however, the use of only 5 mol% of RhCl[(R)-BINAP] resulted in induction of 39.8% ee of the silicon atom (Scheme 45). 

At present industry manufactures a wide range of branched, cyclolinear and ladder polyorganosiloxanes. which differ by the type of organic groups of radicals at the silicon atom. The production of such polyorganosiloxanes is based on hydrolytic condensation or cocondensation of organochlorosi-lanes or alkoxyorganosilanes and subsequent polycondensation of the products. 

Water acts as a nucleophile and displaces the OR leaving group from silicon the most common case is the synthesis of silica by hydrolytic polycondensation of tetraalkoxysi-lanes as shown in equations 1, 2 and 3. The condensation leading to Si—O—Si bonds apparently occurs by hydrolysis, hetero-condensation and homo-condensation3 4. Interest in silicon is also based on the fact that very useful and common materials like silicates... 

The formation of compounds (polysiloxanes or silicones) containing Si—O—Si linkages is an important industrial process, a fundamental step of which is the hydrolytic polycondensation of monomeric silicon containing precursors (usually chlorosilanes). The details of siloxane polymer formation will not be dealt with here but can be found in... 

In the silylative coupling reactions of olefins and dienes with vinylsubsti-tuted silanes, ruthenium catalysts, containing initially or generating in situ Ru-H/Ru-Si bonds, catalyze polycondensation of divinylsubstituted silicon compounds to yield unsaturated silylene (siloxylene, silazanylene)-vinyl-ene-alkenylene (arylene) products (Eq. 112). For recent results see Refs. [177, 178] and for reviews see Refs. [6,7,117,118]. 

The effect of substituting agents at silicon atom in difunctional organocyclotetrasiloxanes on reac-tivity of haloid and hydroxyl groups interacting with a,ra-dichloro(dihydroxy)-dimethylsiloxanes was studied. Polycondensation of dichlorohexaorganocyclotetrasiloxanes were performed at room temperature in 70% solution of anhydrous toluene or benzene both with acceptor and without it. 

Abstract This paper proposes new ways of preparation of hybrid silicones, i.e. an alternated multiblock seqnence of silicone and alkyl spacers, via a polycondensation process catalyzed by the tris(pentaflnorophenyl)borane, a water-tolerant Lewis acid, between methoxy and hydrogeno fnnctionalized silanes and siloxanes at room temperature and in the open air. The protocol was first developed with model molecules which led to polydimethylsiloxane (PDMS) chains, in order to seize the best experimental conditions. Several factors were studied such as the contents of each reactants, the nature of the solvent or the rate of addition. The best conditions were then adapted to the synthesis of hybrid silicones, condensing alkylated oligo-carbosiloxanes with methoxy or hydrogeno chain-ends and complementary small molecules. A systematic limitation in final molar masses of hybrid silicones was observed and explained by the formation of macrocycles, which cannot redistribnte or condense further while formed. 

Both approaches as well as the Yajima route require the creation of silicon-silicon bonds involving the polycondensation of (di)chlorosilanes by refluxing in toluene or xylene in the presence of sodium. Since metals different from the alkali ones are not reactive or require the use of special conditions, it was decided to investigate a simple and practical electrochemical way, involving the use of an undivided cell, a sacrificial anode, and a constant current density [5]. 

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