Hydrolytic Condensation of Alkoxysilanes

In this section we take a brief look at inorganic polymer networks produced by the add-catalyzed hydrolytic condensation of tetraalkoxysilanes e.g., tetraethoxysilane (TEOS). The hydrolytic condensation may be represented by the following reactions  

TEOS is a tetrafunctional monomer that homopolymerizes, forming Si-O-Si bonds by a stepwise mechanism. According to the statistics of stepwise polymerizations discussed in Sec. 3.1, the theoretical gel conversion 

However, Si nuclear magnetic resonance shows experimental gel conversions. 

To provide a rough estimation of the gel conversion arising from such a mechanism, let us assume a very simple model of network formation in which a first stage involves the hydrolytic condensation of the monomer to produce octahedra as shown in Fig. 3.23. In the formation of octahedra. 

75% of the total SiOH groups were condensed. In a second stage, it may be assumed that these octahedra are randomly interconnected. For the stepwise homopolymerization of an Ag(8-functional monomer), gelation is attained when 1/7 of the initial free functional groups are reacted. Then, the model predicts an overall gel conversion. 

---

In the rest of the chapter we will consider separately stepwise and chainwise polymerizations. A small separate section will be devoted to the hydrolytic condensation of alkoxysilanes. This system exhibits such a large departure from the usual assumptions involved in the description of network formation that it merits particular consideration. 

More specifically, tetrafluoroborate anion showed an outstanding catalytic activity in hydrolysis [23]. [BF4] anion is known to undergo some hydrolysis in the presence of water, even at room temperature, and releases some HF, which is known as a very powerful catalyst for hydrolytic condensation of alkoxysilanes. 

In addition to the unconventional methods of the formation of siloxane bonds such as those discussed earlier, the non-hydrolytic reactions of chlorosilanes in the presence of dimethyl sulfoxide, 7 and the reaction of dichlorosilanes with metal oxides,111 a new method has recently been discovered which involves the condensation of alkoxysilanes with organohydrosilanes with the release of hydrocarbon.112... 

As expected, octasilacubane possesses high reactivity toward electrophiles. As shown in Scheme 3, mCPBA oxidation of silyl-substituted 1 resulted in a hi -yield formation of octasilsesquioxane (3) (Tg). Silsesquioxanes are well studied, but were previously only prepared by the hydrolysis-dehydration of halo- or alkoxysilanes. When the substituents are bulky, the reaction proceeds no further than the silanol stage, and silsesquioxanes are not obtained. For example, hydrolytic condensation of t-butyldimethylsilyltrichlorosilane gave only partly hydrolyzed silanols. Thus, 3 is the octasilsesquioxane with the bulkiest substituents, and the only silyl-substituted one ever reported. The structure was determined by X-ray ciystallography [7], and the bond lengths and angles are similar to those of known octasilsesquioxanes. 

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. 

---