Condensation of silanols

Unfortunately, because self-condensation of silanols on the same silicone can occur almost spontaneously, the reaction of disdanol or trisilanol compounds with telechelic sdanol polymers to form a three-dimensional network is not feasible. Instead, the telechelic polymers react with cross-linkers containing reactive groups such as alkoxysdanes, acyloxysdanes, silicon hydrides, or methylethyloximesilanes, as in the reactions in equations 18—21 (155). 

During the aqueous hydrolysis of dichlorosilanes there is always a very important side reaction. It is the self-condensation of silanols which are formed initially during the hydrolysis. These reactions also give rise to the formation of cyclic siloxanes together with the linear oligomers or polymers (Reaction Scheme III). The amount of cyclic products usually depends on the hydrolysis conditions and the degree of the self-condensation attained as well as concentration considerations. 

Although organosilanes appear to react slowly (if at all) with water alone, in the presence of acids or bases (e.g., alkali metal hydroxides), reactions to give a silanol and H2 are rapid, with bases being particularly powerful catalysts. The evolution of H2 in this type of reaction may be used as both a qualitative and a quantitative test for Si-H bonds, and the mechanism of the acid and the base hydrolysis has been discussed in detail (30,31). This hydrolytic method is not very common for the preparation of silanols that are to be isolated, because both acids and bases catalyze the condensation of silanols to siloxanes, and therefore, only compounds containing large substituents are conveniently made in this way. If an anhydrous alkali metal salt is used, a metal siloxide may be isolated and subsequently hydrolyzed to give the silanol [Eq. (10)] (32). 

Pearce, E. Kwei, T. K. Lu, S. Hydrogen Bond Interactions and Self-Condensation of Silanol-Containing Polymers in Polymer Blends and Organic-Inorganic Polymeric Hybrids. In Silicones and Silicone-Modified Materials-, Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Eds. ACS Symposium Series 729 American Chemical Society Washington, DC, 2000 pp 419-432. 

Preparation of MCM-41 materials [1] usually includes a high temperature calcination step that in fact opens mesopores by removing structure directing molecules (template) as well as results in the structure shrinkage due to high temperature condensation of silanol groups [2], Functionalization of mesoporous materials with organosilanes is often employed to synthesize materials of desired surface properties for advanced adsorption and catalytic applications [3] as well as to improve their stability [4]. 

Individual examples of monomeric silanols such as triethylsilanol [10] and phenylsilanetriol [II] have been prepared under regimens different from those used for surface treatments and they exhibit extended stability. To date, no monomeric silanetriols have been isolated from aqueous hydrolysates of alkoxysilanes. The kinetics of silanetriol condensation have been studied [12]. The conditions which promote the hydrolysis of alkoxysilanes also promote condensation of silanols the persistence of monomeric silanetriols for more than a few hours in typical solutions is unlikely. However, the persistence of silanols in reaction mixtures containing condensed structures have been observed empirically [ 13] and by JVSi-NMR [ 14,15],... 

Extent of silane condensation (degree of polymerization). In order to provide insight into the extent of condensation of silanols and their phase separation as a function of time, the hydrolysis of propyltrimethoxysilane was... 

Abstract—A review of the literature is presented for the hydrolysis of alkoxysilane esters and for the condensation of silanols in solution or with surfaces. Studies using mono-, di-, and trifunctional silane esters and silanols with different alkyl substituents are used to discuss the steric and electronic effects of alkyl substitution on the reaction rates and kinetics. The influences of acids, bases, pH, solvent, and temperature on the reaction kinetics are examined. Using these rate data. Taft equations and Brensied plots are constructed and then used to discuss the mechanisms for acid and base-catalyzed hydrolysis of silane esters and condensation of silanols. Practical implications for using organofunctional silane esters and silanols in industrial applications are presented. 

Condensation of silanol species is of major industrial importance as the mechanism by which siloxane backbone polymers are formed. Mono-, di-, tri-, and tetrafunctional silanol species are all used extensively in these processes and products. As a result, many studies of condensation mechanisms have been published. Most infer primary chemistry from the condensed products obtained [8, 10, 11, 17, 23, 24, 28. 31, 57, 58], Accurate determination of rate data and mechanistic insights can provide fundamental support for these processes and others, such as sol-gel processes and treatment of mineral fillers and glass fibers with solutions of reacting silanols. 

From these studies of condensation of mono-, di-, and trifunctional silanols in aqueous and aqueous-organic solutions, a kinetic expression describing the rate of silanol disappearance (condensation of silanol to disiloxane) is obtained ... 

Thus, to summarize the condensation data, the condensation of silanols in... 

Rate constants for the hydrogen chloride catalyzed condensation of silanols in aqueous dioxane at 25°C... 

The condensation of silanols in solution or with surfaces has not been as extensively studied and therefore is less well understood. The limitation until recently has been the lack of suitable analytical methods necessary to monitor in real time the many condensation products that form when di- or trifunctional silanols are used as substrates. With the advent of high-field wSi-NMR techniques, this limitation has been overcome and recent studies have provided insights into the effects of silanol structure, catalysts, solvent, pH, and temperature on the reaction rates and mechanisms. Analysis of the available data has indicated that the base catalyzed condensation of silanols proceeds by a rapid deprotonation of the silanol, followed by slow attack of the resulting silanolate on another silanol molecule. By analogy with the base catalyzed hydrolysis mechanism, this probably occurs by an SN2 -Si or SN2 -Si type mechanism with a pentavalent intermediate. The acid catalyzed condensation of silanols most likely proceeds by rapid protonation of the silanol followed by slow attack on a neutral molecule by an SN2-Si type mechanism. 

Figure 7 shows the time dependence of the Si—O—C, Si—OH, Si—O—Si, and methanol bands for a 4% (w/w) solution of SiQAC under three sequentially adjusted pH conditions. A solution of SiQAC has a pH of approximately 5.5. At this pH, little hydrolysis of the methoxy groups occurs even in several hours, as confirmed by the presence of the CH 0— rock band at 1193 cm-1 and the Si—O—C band at 1082 cm"1. In a matter of minutes after adjusting the pH to 2.7, extensive hydrolysis has occurred, as shown by the decrease in the intensity of the Si—O—C band, and the appearance of the methanol band at 1030 cm"1 and the Si—OH band at 920 cm-. There is no evidence of significant formation of siloxane bands even if the solution is permitted to stand for several hours. Adjustment of the pH to 7.0 initiates rapid and extensive condensation of silanol groups to siloxane bonds, as shown by the decrease in the Si—OH band and the appearance of a Si—O—Si band at 1100 cm -1. 

Grubb considered this problem in detail and concluded (24) that under these conditions the reaction of silanols with methanol is rapid compared to self-condensation. Even under milder conditions the self-condensation of silanols is not complete when much alcohol is present, but an equilibrium is established in which most of the silanol is present as alkoxysilane. Grubb discussed the equilibria involved ... 

After supplying silanol, the heating of apparatus 17 is switched on and the solvent is distilled, sending air through the bubble at the speed of 3 m3/h at 150 °C. The solvent vapours are condensed in cooler 11 and collected in receptacle 18. The distillation of the solvent occurs simultaneously with the condensation of silanol. During the distillation the temperature in apparatus 17 is increased to 170-200 °C, and the speed of air supply is raised to 30 m3/h. The mixture is periodically sampled to determine the viscosity and polymerisation time of the polymer. After the distillation air supply is stopped, the reactor receives a necessaiy amount of the solvent from batch box 10 and the mixture is cooled to 60 °C. The solution (var-... 

Qzm has been treated to decrease the surface hydrophilicity. Three different hydrophobization procedures have been followed (i) the condensation of silanols into siloxane bridges by outgassing the powder at 1073 K, largely irreversible by re-exposure to water (ii) the functionalisation of silanols with tryethylsilyl groups and (iii) treatment with diluted HF. This last treatment dissolves the external disordered layers of particles containing impurities. In Figure 4, the amount of BSA adsorbed on the three treated quartz dusts is compared with untreated samples of pure quartz. All treatments decreased the... 

In the absence of L H, the condensation of silanol groups is slow and inefficient, since after three days the molar masses slightly shifted (note that the reactions qnoted in Table 1 were carried out only during 24h, unless otherwise stated). In the presence of L H, the reaction is much faster and larger molar masses are reached, which seems to indicate that the SiH/SiOH condensation is competitive with the SiH/SiOMe one. Snch result may explain the double distribution found in some rnns of Table 1, where both fast L H addition and fair content of water present the best conditions for this co-condensation reaction to occur. 

It is generally agreed that both processes, namely addition polymerization (the nature of active species still of much debate) and acidolysis/ condensation reaction, occur simultaneously in the cationic ring-opening polymerization [247,248], although the contribution of both mechanisms is still a matter of discussion. Kinetics of the acid-catalyzed condensation of silanol groups was studied in detail [249,250]. 

Condensation Cure. The condensation of silanol groups to form siloxanes is an extremely important industrial reaction and may be represented in its simplest form as follows ... 

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