Silanols condensations with

To get further insight into the reactivity of the metallo-silanols, condensation with diverse chlorosilanes has been studied. It offers easy and general access to the ferrio-disiloxanes 15a-d via interaction of 2a or its lithiation product 13 with alkyl-, aryl- or metallo-chlorosilanes in the presence of NEt3 (Eq.(3)). 

The trihalogeno-substituted silanes are very readily hydrolyzed, and the initially formed silanols condense with themselves and with halo-genosilanes with extreme ease. Consequently, few simple intermediate hydrolysis products of this class have been isolated or characterized. Linear halogen-substituted silanols and siloxanes are certainly first... 

The silane ester moiety hydrolyzes because of heat and moisture present on the surface of the silica (RO—Si —> HO—Si) and the then-formed silanol condenses with a silanol group on the silica surface to produce a siloxane bond linking coupling agent to silica (—Si—O—Si—). 

Polymerization and depolymerization of sihcate anions and their interactions with other ions and complexing agents are of great interest in sol—gel and catalyst manufacture, detergency, oil and gas production, waste management, and limnology (45—50). The complex silanol condensation process may be represented empirically by... 

Interesting products may also be produced by introducing boron atoms into the chain. The amount of boron used is usualy small (B Si 1 500 to 1 200) but its presence increases the self-adhesive tack of the rubber, which is desirable where hand-building operations are involved. The products may be obtained by condensing dialkylpolysiloxanes end-blocked with silanol groups with boric acid, or by reacting ethoxyl end-blocked polymers with boron triacetate. 

Newer silicone adhesives having solids levels up to 97% are also commercially available [109]. Instead of using silanol condensation reactions, they rely on addition chemistry between vinyl functional silicone oligomers and silicon hydride terminated silicones. This addition reaction is typically facilitated with platinum derived catalysts. This hydrosilation process can be run at reduced oven temperatures, but the finished products typically do not yield the same balance of properties as seen for condensation cure materials. 

The resulting silanol group of a polymer chain condenses with acetoxy siloxy group of another polymer chain to form a siloxane (Si-O-Si) linkage (Scheme 8). Further similar reactions finally result in a crosslinked elastomer. Acetic acid is... 

Two-part room temperature condensation cure. Silicone can be formulated into two-part systems [3,12,14,33] that prevent the reactive groups from coming into contact before they are needed. The reactions in these systems are based on the condensation of a silanol group with an alkoxy silane group, catalyzed by organo-tin compounds (Scheme 9). 

Apart from the mentioned advantages, the polymeric reagents covalently adsorbed by silica also diminish its inherent non-specific adsorptivity. One of the ways to synthesize a polymeric modifier of this type is a copolymerization of a vinylsilane with a compound of the desired functionality. The segments carrying silyl groups will condense with the surface silanols forming anchors or trains . 

An important reaction of silicon compounds containing electronegative substituents, such as the halogens or a Group 15 or 16 element, is hydrolysis, which often occurs very readily even with atmospheric moisture. The primary reaction in silicone production is the hydrolysis of halosilanes, usually the readily available chlorosilanes, to give silanols, which then undergo acid-catalyzed condensation with loss of water, as shown in Scheme 1. 

The fabrication of colloidal silica and optical glasses by the sol-gel process has attracted a great deal of attention (8). The process relies on the hydrolytic polycondensation reactions of alkoxysilanes, usually (EtO)4Si, in which the reactive silanols (EtO)4 Si(OH)n (n = 1-4) are formed. These then undergo acid- or base-catalyzed condensation with both water and alcohol formation, as shown in Scheme 2. 

Photolytic generation of silene Me3Si(Me)C=SiMe2 in the presence of an excess of D20 or a trace of H20 affords disiloxanes 25, presumably via condensation of the silanol 26 in the case of D20, or by reaction of the silanol 27 with further silene in the case of H20 (Scheme 7) (138, 139). 

Polysilanes bearing triflate groups are very reactive and form an insoluble gel with a trace of moisture. The triflate group is hydrolized to silanol which rapidly condenses with the remaining triflate groups to a siloxane unit, linking intra or intermolecularly silicon atoms ... 

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). 

FIGURE 1.3 Schematic representation of the silanization procedure of borosilicate or fused silica capillary column inner walls, (a) Surface etching under alkaline conditions, (b) attachment of reactive groups by condensation with silanol, (c) chemical linkage of polymer (PS/DVB considered as example) by free radical polymerization. 

The preferentially employed approach for the fabrication of inorganic (silica) monolithic materials is acid-catalyzed sol-gel process, which comprises hydrolysis of alkoxysilanes as well as silanol condensation under release of alcohol or water [84-86], whereas the most commonly used alkoxy-silane precursors are TMOS and tetraethoxysilane (TEOS). Beside these classical silanes, mixtures of polyethoxysiloxane, methyltriethoxysilane, aminopropyltriehtoxysilane, A-octyltriethoxysilane with TMOS and TEOS have been employed for monolith fabrication in various ratios [87]. Comparable to free radical polymerization of vinyl compounds (see Section 1.2.1.5), polycondensation reactions of silanes are exothermic, and the growing polymer species becomes insoluble and precipitates... 

Fig. 23a. Hydration and dehydration reactions of the silica surface and the IR absorption frequencies of the surface species. Chemisorption of water produces surface silanols, which serve as adsorption sites for water, b Reaction of octadecyltrichlorosilane (OTS). Hydrolysis of the chloride group by trace amounts of water in solution to silanol is followed by condensation with surface silanols, resulting in covalent bond formation between the monolayer and the substrate. OTS molecules can also cross-link to form polymeric species during film curing [201]...

The APS molecule is quickly hydrolzyed from the triethoxy ester to the trisilanol in aqueous solution. (See below.) It is well known that the silanols are very reactive and will condense with each other to form oligomerized networks... 

On the other hand, water molecules in traces, as they appear on the hydrophilic silica surface, activate the surface as well as the silanes. This is due to the two-step process of the silanization. When a silane approaches the surface the groups X will react with the water to form silanol SiOH groups, a reaction called hydrolysis. The hydrolysis determines the overall reaction speed and depends on the groups X [425], It decreases in the series Cl > OMe > OEt. After hydrolysis the silanol condensates in a second step with the silanol groups from the silica surface. 

Even when methyl radicals are replaced by silanol units, the surface of the material does not remain hydrophilic (water-wettable) very long. Either the silanol groups condense with other silanol units to restore the siloxane structure, or unmodified chain segments migrate to the surface. In any case, a self-repair mechanisms underlies the recoverability of siloxane surfaces, and this is an important part of their durability. 

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