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Fascinating Silicone™ Chemistry Corner – Hydrolysis & Condensation

How chlorosilanes are hydrolyzed & condensed

When water is added to dimethyldichlorosilane (the principal chlorosilane), the two react to form disilanol and hydrochloric acid. This occurs because oxygen “likes” silicon more than chlorine; and chlorine “likes” hydrogen more than oxygen (Me = CH3).

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The disilanols are unstable and strongly attracted to one another. Catalyzed by the hydrochloric acid, they condense into polydimethylsiloxanes – molecules containing a backbone of silicon atoms bonded to oxygen atoms.

Hydrolysis and condensation occur spontaneously, simultaneously, and very quickly. These reactions are a veritable “perpetual chemistry machine,” resulting in a mixture of:

  1. Cyclic rings (with 3-6 repeating SiO units)
  2. Linear chains (with 30-50 repeating SiO units)

The cyclic and linear oligomers (mini-polymers) are separated from one another and distilled into cuts based on the number of SiO units in the chain. Some are used as they are (cyclosiloxane and low-molecular-weight silicone fluids, for example). But the majority are further polymerized and finished to create a wide array of materials with an amazing range of performance capabilities.

Did you know ... the ability to form long chains is one of the key differences between silicones and organics. When organic molecules start growing into longer chains, they tend to turn into cyclics. Or if the chains get past 15 units, they turn into solids. Silicones can grow extremely long chains stretching as long as 3,000 units! Learn more about the differences between silicones and organics (carbon-based materials).

Did you know ... the hydrochloric acid produced during chlorosilane hydrolysis is recycled at virtually 100% efficiency, and reused in the conversion of methanol to methyl chloride. In silicone manufacturing, hardly anything goes to waste!


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