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  Chemistry of Silicones

     Oganosilicon Compounds

	What is Organsilicon compound
	Preparation

     Silicones

	What is Silicones
	Production of Silicone Polymers
	Properties of silicones

Discussion

Organosilicon compounds

What are organosilicon compounds?

Organosilicon compounds are those contain Si-C bonds. The Si-C bonds are as strong as C-C bonds so silicon carbide SiC is extremely hared and stable. Thousands of organosilicon compounds have been made artificially, most since 1950. Many of these are inert and stable to heat (e.g. SiPh₄ can be distilled in air at 428 ^oC.)

By the Rochow 'Direct Process' Alkyl or aryl halides react directly with a fluidized bed of silicon in the presence of large amounts (10%) of a copper catalyst.

Si + 2CH₄Cl à (CH₃)₂SiCl₂

^{(Cu catalyst 553-573K)}

This is the main industrial method for making methyl and phenyl chlorosilanes which are of considerable commercial importance in the production of silicones. The yield is about 70%, with varying amounts of other products: MeSiCl₃ (10%)and Me₃SiCl (5%), and smaller amounts of Me₄Si and SiCl₄. Both Grignard and direct methods yield a mixture of products, and very careful fractionation is important as the boiling points are close.

Silicones

The silicones are a group of organosilicon polymers. They have a wide variety of commercial uses as fluids, oils, rubbers and resins. Annual production is estimated as about 300 000 tones. They are now produced on a larger scale than any other group of organometallic compounds.

The complete hydrolysis of SiCl₄ yields SiO₂, which has a very stable three-dimensional structure. The fundamental research of F.S.Kipping on the hydrolysis of alkyl substituted chlorosilanes led to the discovery of long-chain polymers called silicones.

                     

The starting materials for the manufacture of silicones are alkyl or aryl substitute chlorosilanes. Methyl compounds are mainly used, though some phenyl derivatives are used as well. Hydrolysis of dimethyldichlorosilane (CH₂)₂SiCl₂ gives rise to straight chain polymers and, as an active OH group is left at each end of the chain, polymerization continues and the chain increases in length. (CH₂)₂SiCl₂ is therefore a chain building unit. Normally, high polymers are obtained.

　

                              

Hydrolysis under carefully controlled conditions can produce cyclic structures with rings containing three to six Si atoms.

                             

                                tris cyclo-dmethylsiloxane     Tetrakis cyclo-dimethylsiloxane

Hydrolysis of trimethylmonochlorosilane (CH₃)₃SiCl yields trimethylsilanol (CH₃)₃SiOH, which is a volatile liquid. It can condense and give hexamethyldi-siloxane. It cannot polymerize any more since there is no OH groups.

                                                      

                                                                               Hexamethyldisiloxane

If some (CH₃)₃SiCl is mixed with (CH₃)₂SiCl₂ and hydrolysed, the (CH₃)₃SiCl will block the end of the straight chain produced by (CH₃)₂SiCl₂. Since there is no longer a functional group at this end of the chain, it cannot grow any more at this end. Eventually the other end will be blocked in the similar way. Thus (CH₃)₃SiCl is a chain stopping unit, and the ratio of (CH₃)₃SiCl and (CH₃)₂SiCl₂ in the starting mixture will determine the average chain size.

Silicones are comparably economic while with many desirable features. They were originally developed as electrical insulators, because they are more  stable to heat than organic polymers. Also when they break down, no conducting materials will be released as carbon does. So they are good electrical insulators.

Their water repellency is due to being surrounded by organic side groups, which give them the low surface tension. Silicones may exist as liquids, oils, greases, rubbers or resins. Straight chain polymers of 20 to 500 units form silicone fluids. They make up 63% of total silicones uses. When the length of chains increase, the viscosity and boiling point will increase as well. Then different products can be produced by controlling the chain length.

Silicone rubbers are made of long straight chain polymers between 6000 and 600,000 Si units long. They are useful because they can retain their elasticity from 183K-533K. If cross-links are formed, the polymers will become brittle and cracks, and low molecular weight, cyclic structures are evolved. Strong heating in the absence of air causes silicones to soften and form volatile products.

Silicone resins are rigid polymers. They are extensively cross-linked. About 12% of   silicones produced are resins.

Discussion

Although organosilicon compounds are very useful, they cannot replicate organic compounds because:

	Silicon has little tendency to bond itself, while carbon has a strong tendency to do so.
	Silicon does not form pπ-pπdouble bonds, which forms readily in organic compounds.
	Silcon forms a number of compounds containing pπ-dπdouble bonds in which the silicon atom uses d orbitals.

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