Mechanism of Anionic Polymerization of Siloxanes


Mechanism of Anionic Polymerization of Siloxanes


Silioxane polymerization follows an ionic process. Basic catalyst such as alkali metal hydroxide or alkoxide, cleave Si-0 skeletal bonds to yield linear species which can function as chain propagation sites. Cyclic molecules are formed due to O- K+ionic bond.

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A sketch of the reactor in which methyl. chloride and silicon were contacted to prepare silicone is shown in Fig.7.11.  A “superheater” heats the methyl chloride before it reaches the reactor. The silicon is fed into a reactor that is stirred or “fluidized” by passage of hot gas through the bed of copper-containing silicon. A “heat transfer coil” removes excess heat by transferring it to a cooler, circulating fluid. The engineers stop silicon powder from being carried through the reactor by collecting it in two different types of filters. A “condenser” cools the gases from the reactor to a liquid that contains all the products. But a chemical reactor is not sufficient equipment to isolate a usable chemical product in the purified state from all other substances. A complex distilling plant is used to isolate the products formed.

Dimethylsiloxane structure forms the Dasic basis of most silicone polymers; other substituents groups have also been introduced. These include vinyl, ethyl trifluoropropyl, p-cynoethyl, and phenyl and biphenyl groups. The introduction of specific group improves the oil resistance strength and toughness, flame resistance or compatibility of the polymers.

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