- What Are Silicones?
- Silicones Uses
- Sustainability & Environment
- Health & Safety
The linear and cyclic oligomers obtained by hydrolysis of the dimethyldichlorosilane have too short a chain for most applications.
They need to be condensed (linears) or polymerised (cyclics) to give macromolecules of sufficient length. This section looks at the linears.
Click on the image above to zoom in
This reaction is catalysed by many acids or bases to give long chains by intermolecular condensation of terminals SiOH. A distribution of chain length is obtained and longer chains are favoured when working under vacuum and/or at elevated temperatures to reduce the residual water concentration. Acid catalysts are more efficient when the organosilanol carries electron-donating groups, base catalysts when it carries electron-withdrawing groups.
Some catalysts can induce redistribution by attacking the polymer chain with the formation of cyclics. This is important when condensing a mixture of linear oligomers such as dimethyl- and methylphenylpolysiloxanes. A sequential polymer will be obtained in the absence of redistribution, while a random polymer will result if a catalyst capable of opening the main chain is used.