An interesting and commercially important class of polymers can be synthesised by the hydrolysis of suitably functionalized silicon derivatives. Note that leaving groups (halides, alkoxides, carboxylates, or amines) attached to silicon react readily with water, and hydrolyze to silanols. These silicon-analogs of alcohols are also unstable, and in most cases quickly dimerize to produce siloxanes. Difunctional variations lead to linear polymers, as shown below for the polymerization of dimethyl dimethoxy silane.
The polymer shown above is polydimethylsiloxane (PDMS), better known as silicone oil. Note that there are no carbon atoms in the backbone; PDMS has an inorganic chain. The glass transition of PDMS is the lowest of all polymers, presumably because of the extremely flexible nature of the Si-O linkage. The linear polymer can also be made by ring-opening polymerization.
Crosslinked versions of PDMS are important rubbery sealants, caulkings, coatings, and tubing materials. Because of their very low Tg, these materials remain flexible even when extremely cold. The above chemistry can be the basis for crosslinking, so that exposure to atmospheric moisture cures the material. Carboxylate is a common leaving group in silicone bathtub caulking that you can purchase in hardware-stores. As curing proceeds, one can smell acetic acid being liberated from the material.
The name "silicone" is an artifact of early chemists, who wrongly presumed based on the empirical formula that these componds contained a silicone-oxygen double bond (R-Si=O-R'). It was thought that these compounds were the silicon analog of ketones, therefore the name silicone. We now know that silicone-oxygen double bonds are unstable, and compounds containing Si=C are extremely difficult to prepare.
If the silicone atom is substituted with four leaving groups, the hydrolysis-condensation reaction will eventually lead to materials with the general formula SiO2. This famous chemistry is known as the sol-gel reaction, and is a method for producing thin films of glass.
The reaction mixture has to be heated strongly to drive the product all the way to SiO2, and lots of by-product (methanol in the above example) is driven off. Therefore, the sol-gel reaction is best performed in thin films which have a lot of surface area to allow evaporation of the by-product. The system shrinks a lot as it cures, making it difficult to produce thick parts with dimensional precision.
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