Stability of siloxane bonds

In situ SAXS investigations of a variety of sol-gel-derived silicates are consistent with the above predictions. For example, silicate species formed by hydrolysis of TEOS at pH 11.5 and H20/Si = 12, conditions in which we expect monomers to be continually produced by dissolution, are dense, uniform particles with well defined interfaces as determined in SAXS experiments by the Porod slope of -4 (non-fractal) (Brinker, C. J., Hurd, A. J. and Ward, K. D., in press). By comparison, silicate polymers formed by hydrolysis at pH 2 and H20/Si = 5, conditions in which we expect reaction-limited cluster-cluster aggregation with an absence of monomer due to the hydrolytic stability of siloxane bonds, are fractal structures characterized by D - 1.9 (Porod slope — -1.9) (29-30). 

Values of constants for silicic acids from known crystalline silicates are given in Table 3.1. It is emphasized that these apply only when Funk and Frydrych s type of reagent is used. The reaction rate decreases more rapidly than the increase in number of siloxane bonds that must be hydrolyzed to depolymerize the polysilicic acid to monomer. This is probably because of the greater stability of the ring structures as compared to corresponding chain polymers. 

The high thermal stability of a polydimethylsiloxane rubber is really a reflection of the good stability of the bonds involved (Si—O, Si—C, C—H) and lack of weak bonds rather than any special outstanding feature. A rather different situation exists with the exceptionally stable m-carborane-siloxane elastomers due to a resonance stabilization effect. 

The hydrolysis and condensation of oligomeric species, for example, dimers, trimers, and cubic octamers (Structures 14-16) have been investigated by several groups. Klemperer et al. [38,39] observed no degradation of the trimer (15) or octamer (16) during hydrolysis and condensation under neutral conditions employing 20 and 8 equivalents of water, respectively (r = 2.5 and 1). However, production of monomer during neutral hydrolysis of the dimer (14) is clear evidence of siloxane bond hydrolysis (reverse of Eq. 10). The increased stability of the trimer and octamer compared to the dimer may be attributable to steric factors. 

An important property of these siloxane phases is their stability under the conditions used in most chromatographic separations the siloxane bonds are attacked only in very acidic (pH < 2) or basic (pH > 9) conditions. A large number of commercial bonded-phase packings are available in particle sizes suitable for HPLC.48... 

---