Silanation-condensation reactions

Even silicone-based materials cannot fulfill all requirements in diverse applications. One property of silicone materials is their high gas and moisture permeability, which is disadvantageous in, e.g., edge sealing of insulation gls s, where a barrier for water and gas has to be provided. Hydrocarbon polymer-based materials like poly-isobutene possess these desirable properties. However, they lack the ready curability and adhesion of RTV silicone materials. Preferably the advantages of the hydrocarbon base polymer and the silane condensation reaction should be combined. 

The mechanism of chemical adhesion is probably best studied and demonstrated by the use of silanes as adhesion promoters. However, it must be emphasized that the formation of chemical bonds may not be the sole mechanism leading to adhesion. Details of the chemical bonding theory along with other more complex theories that particularly apply to silanes have been reviewed [48,63]. These are the Deformable Layer Hypothesis where the interfacial region allows stress relaxation to occur, the Restrained Layer Hypothesis in which an interphase of intermediate modulus is required for stress transfer, the Reversible Hydrolytic Bonding mechanism which combines the chemical bonding concept with stress relaxation through reversible hydrolysis and condensation reactions. 

At present, the only synthetic method leading to high molecular weight polysilanes is based on the Wurtz condensation reaction between dichlorodisubstituted silanes and alkali metals (low molecular weight oligomers were also prepared by dehydrogenation of secondary... 

The catalysts are prepared as described previously. [6] The templating solution consists of a solution of n-dodecylamine (5.09 g) in aqueous ethanol (53 ml water and 46 ml ethanol). To this is added a total of 0.1 mol silane. The reaction is allowed to proceed for 18 h at room temperature. After filtration and extraction of template with ethanol, the material is filtered and dried. The filtrate from the preparation is generally free of unreacted silanes, indicating that all the silanes are condensed, a fact borne out by the excellent agreement between theoretical and experimental composition. The template and the ethanol can both be recovered pure (the template with 99% efficiency). Both can be reused, as can the templating solution. This means that the process is essentially waste-free[7],... 

In this review, we will briefly discuss selected literature on hydrolysis and condensation reactions at silicon which are relevant to organofunctional silanes. Some studies will involve simpler (lower functionality) models some may involve related chemical reaction classes illustrating a mechanistic point. We will then discuss selected references focused on silanes of the types used as coupling agents and crosslinkers. Finally, some reactions at the interface will be mentioned. 

Hydrolysis and condensation reactions of silanes may be considered in the broad category of nucleophilic substitutions at silicon. The common nomenclature for these reactions is SN.V-Si, where A represents the kinetic order or molecularity, Si indicates that silicon is the reaction center, and SN indicates that the reaction is a nucleophilic substitution. Nucleophilic reactions at silicon have been reviewed thoroughly and have been the subject of fundamental studies by several laboratories over the last three decades [33]. The literature is not as voluminous as the literature on the corresponding reactions at carbon. A general mechanistic view of these reactions has, however, emerged. There are many parallels to carbon-centered reaction mechanisms. One distinction from carbon-centered reactions is clearly apparent. Silicon is able to form relatively stable higher coordinated (pentavalent) intermediates carbon is not [33]. 

Under optimum conditions, hydrolysis is rapid, and precedes slower condensation reactions. However, the silanol groups formed in hydrolysis are very reactive, and condensation can be a significant factor even in dilute aqueous solutions. Understanding the kinetics of the hydrolysis and condensation reactions for a specific silane in solution is important because its effectiveness as an adhesion promoter is influenced by the extent of condensation, which, in turn, affects the structure of the interphase in composites. 

The effect of the concentration of water on the hydrolysis and condensation reactions of silanes has been investigated by Kang et al. [12] in conjunction with recent studies of the adsorption of silane coupling agents on silica surfaces from acetone-water mixtures as a function of the solvent composition [14]. Adsorption isotherms were determined by infrared (IR) spectroscopy and hydrolysis was followed by NMR. 

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