Strong-acid-catalyzed silanol

The condensation catalyzed by a strong base is first order with respect to substrate and catalyst (74,75). Because of the high acidity of silanol, all the alkah metal base (MtOH) is usually transformed into the silanolate anion. In the rate-determining step, the sdanolate anion attacks the siHcon atom in the silanol end group (eq. 12 and 13). 

The condensation reactions of silanols are catalyzed by acids [19, 25-27,63—68, 72], Grubb measured the hydrogen chloride catalyzed silanoi condensation reaction of trimethylsilanol in methanol [19]. Lasocki and Michalska studied the effect of acid type on the condensation of dialkylsilanediols in dioxane [68]. Under anhydrous conditions, the rate of acid catalysis by strong acids (such as hydrogen bromide and perchloric acid) was directly related to the acid concentrations. The catalytic effects of weaker acids, such as hydrogen chloride, were not linearly related to the concentration. They postulated that in anhydrous dioxane, the strong acids were completely ionized while the weaker acids were not [68]. When small amounts of water were added to the solvent, all the acids behaved in a similar manner. Lasocki [64-67] extended the studies to examine the effects of alkyl or aryl substitution of silanediols on the condensation rate in aqueous dioxane [64-67]. The rate constants for acid catalyzed condensation of... 

Silanol polycondensation is catalyzed by strong acids or bases, as well as by species as mild as amines or carboxylic acids combined with their quaternary ammonium salts (67). With strong acids or bases, the process is highly reversible, and the equilibrium can be represented as follows ... 

The rate equation with strongly acidic catalysts is also second order in silanol and first order in catalyst (75). The mechanism is proposed to proceed via protonation of silanol, followed by an electrophilic attack of the conjugate acid on nonprotonated silanol. The condensation processes outlined in reactions 16a and 16b for sulfonic acids is also an applicable mechanism for the acid catalysis. The condensation polymerization in emulsion catalyzed by dodecylbenzenesulfonic acid is second order in silanol, but the rate has a complex dependence on sulfonic acid concentration (69). This process was most likely a surface catalysis of the oil-water interface and was complicated by self-associations of the catalyst-surfactant. 

In this sol-gel system, the phase-separation inducer is distributed in the gel phase rather than in the solvent phase, due to strong hydrogen bonding between silanol and ether oxygen of PEO. The presence of abundant meso- and micropores has been evidenced by the nitrogen adsorption measurements. Furthermore, due to acid-catalyzed sol-gel at room temperature, all the Si-H groups of precursor remained unchanged after hydrolysis/polycondensation. 

The above report by Jones and coworkers strongly indicates that it is necessary to keep the amount of silanols constant for the investigation of the cooperative catalysis of both organic acid and base groups. Additionally, it can be said that surface silanol groups possess potential usefulness for the promotion of amine-catalyzed reactions. The detailed role of silanol groups in the cooperative catalysis will be discussed in the next section. 

The above studies [8-10] strongly indicate the participation of silanol groups as acid sites in the amine-catalyzed reactions. The synergistic catalysis of immobilized organic bases and silanol groups has been highlighted by the cyclic carbonate synthesis from carbon dioxide and epoxide. Acid-base dual activation mechanism of the cyclic carbonate synthesis is widely accepted. Sakakura and coworkers [11] reported silica-supported phosphonium salts as highly active catalysts for the cyclic carbonate synthesis. The results for propylene carbonate synthesis are summarized... 

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