Silanol, coupling

Since these reactions occur at the glass surface, there is a competition between surface silanol-coupling agent condensation, step-growth polymerization and cyclic oligomer formation. 

Coupling to a mineral surface requires the presence of active hydroxyls on the substrate. The coupling reaction is a multi-step process that proceeds from a state of physisorption through hydrogen bond formation to actual covalent bond formation through condensation of surface hydroxyls with silanols ... 

The Si—OH group on the glass surface forms a hydrogen bond with the silanol group derived from the silane coupling agent. 

Prior to the chemical reaction of the silane with the silanol-groups on the sUica surface, the silane molecule has to make contact with the sUica surface by adsorption. Then the chemical reaction of silica with an alkoxy-silyl moiety of the coupling agent takes place in a two-step, endothermic reaction. The primary step is the reaction of alkoxy-groups with silanol-groups on the silica filler surface [4]. Two possible mechanisms are reported ... 

A second industrially important field in which silanols play a vital role is in the preparation of polysiloxanes, often known as silicones. As in the case of silane coupling agents, there is a vast literature available on the topic of polysiloxanes, and only the most general discussion will be given here, concentrated on the reactions that involve silanols. 

All compounds described in this report have been well characterized by means of IR and NMR spectroscopy. Most valuable is the coupling constant J(WSi) which, as has been demonstrated for J(WP) [17], can be correlated with the s-electron density of the main group element transition metal bond (e. g. J(WSi) = 44.7 Hz (21i), 71.1 Hz (22b)). Related studies concerning metallo-silanols and siloxanes with other metals of groups 6 and 8 are in progress. 

Silanes with heterocyclic residues were also found to be reactive (Equation (20)), although the most likely reason is the in situ cleavage of the heterocyclic residue in the presence of hydrated TBAF to form silanols.290-292 Similarly, among various arylgermanes, only those containing 2-furyl groups were reactive in cross-coupling reactions.293... 

---