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Cyclic Polymerisation

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 polymerisation of the cyclics.

Opening and polymerising cyclics, (R2SiO)m, to form long linear chains is catalysed by many acid or base compounds and gives at equilibrium a mixture of cyclic oligomers plus a distribution of polymers. The proportion of cyclics will depend on the substituents along the chain, the temperature and the presence of a solvent. Polymer chain length will depend on the presence of substances capable of giving chain ends. For example, in the polymerisation of (Me2SiO)4 with KOH, the average length of the polymer chains will depend on the KOH concentration:

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A stable and -OH terminated polymer, HO(Me2SiO)zH, can be isolated after neutralizing and stripping the above mixture, under vacuum, of the remaining cyclics. In fact, distribution of chains with different lengths is achieved. The reaction can also be made in the presence of Me3SiOSiMe3 which will act as a chain endblocker according to:

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The Me3SiOK formed will attack another chain to reduce the average molecular weight the linear polymer formed.

The copolymerisation of (Me2SiO)4 in the presence of Me3SiOSiMe3 displays a surprising viscosity change over time. First, a peak or viscosity maximum is observed at the beginning of the reaction. With such a base catalyst, the presence of two oxygen atoms on each silicon in the cyclics makes them more susceptible to a nucleophilic attack by the catalyst than the silicon of the endblocker, which is substituted by one oxygen atom.

The cyclics are polymerised first in very long, viscous chains that are subsequently reduced in length by the addition of terminal groups provided by the endblocker, which is slower to react. This reaction can be described as follows:

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The ratio between D and M units will define the average molecular weight of the polymer formed. Catalyst removal (or neutralization) is always an important step in silicone preparation. Most catalysts used to prepare silicones can also catalyse the depolymerisation (attack along the chain), particularly in the presence of water traces at elevated temperatures:

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It is therefore essential to remove all remaining traces of the catalyst to benefit as much as possible from the silicone's thermal stability. Labile catalysts have been developed. These decompose or are volatilised above the optimum polymerisation temperature and so can be eliminated by a brief overheating; in this way, catalyst neutralization or filtration can be avoided.

The cyclic trimer, (Me2SiO)3, is characterized by an internal ring tension and can be polymerised without re-equilibration of the resulting polymers. With this cyclic, polymers with narrow molecular weight distribution and polymers carrying only one terminal reactive function (living polymerisation) can be prepared. Starting from a mixture of different "tense" cyclics also allows the preparation of block or sequential polymers.

For more information visit www.cyclosiloxanes.org