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Engineering LibreTexts

Silicon-Carbon-Oxygen molecules

Silicon-carbon-oxygen bonded molecules do not occur naturally, are completely synthetic and are generically known as silicones or coupling agents. They are the reaction product of organo-chlorosilanes ( see also the module on silicon-carbon-chlorine bonded molecules) and di-hydroxy organics ( e.g., glycols and organic di-acids).  Because of the ability to customize organo-chlorosilane intermediates, the variety of silicone molecules available is almost infinite. An additional degree of variation is in the degree of cross-linking of such smaller chain silicones into macro-molecules with molecular weights into the tens of thousands.

Unusual/Noteworthy Properties

The properties of Si-C-O molecules are characterized by several aspects: the base organo-chlorosilane; the di-hydroxy  chain continuing compounds; the chain-ending compound; and the cross-linking compound.

Once the above recipe options are selected, the only additional aspects are that of its curing, or final reaction.  Typically the smaller chain silicone molecules are solvated by a volatile organic compound that will have some function in the final polymer, such as acetic acid. As an example of the above, consider the following recipe:

  • Di-methyl dichlorosilane is the organo-chlorosilane
  • Ethylene glycol

React and remove most of the by-product HCl by distillation while maintaining some mild agitation, and using acetic acid as a solvent to prevent over-heating. The following polymerization reaction occurs:



Main Applications/Uses

The main commercial and residential applications of longer chain silicones are in manufacture of synthetic oils, waxes, coatings, and caulking. Secondarily, small-to medium chain silicones can act as a bridge between organic and inorganic functional groups. In this application, they are known as coupling agents. Lastly they can serve to prevent water from wetting surfaces, and therefore act as concrete curing agents, and water-proofing agents.