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Linear Condensation

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.

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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.