Oligomers as Particles

The corresponding siloxane (HSiOj.,)3 was obtained as a crystalline compound (m.p. 250°C) by Muller, Kbhne, and Sliwinski (101). If this were oxidized in dilute solution it might yield the pure octasilicic acid. 

When R is a bulky alkyl group, the condensation of RSi(OH)j is forced into the smallest possible three-dimensional unit, polycyclic tetramer, (RSiOi.s) (102). However, it is unlikely that this tetrahedral structure involving only four silicon atoms can be formed by polymerization of monomer in water, since it was not formed from RSifOH), unless the R groups were bulky. 

However, in water, condensation of Si(OH)4, dimer, and trimer with cyclic species, for example, tetramer, must occur randomly. Thus it is not likely that any regular three-dimensional fully internally condensed species such as cubic octasilicic acid, (HOSiO,. ), will be formed in appreciable amount. Instead, less regular, incompletely condensed three-dimensional oligomers must appear. Later, after higher molecular weight polymers equivalent to very small particles have appeared, the more soluble oligomers depolymerize or dissolve and the dissolved silica is added to the growing particles. 

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