Silicones

Silicones are a group of organosilicon polymers which are also known as siloxanes. Organosilicon compounds are those in which organic group is attached to silicon. preparations properties, types and applications of silicones.
references for study of silicones.

1. TUMKUR UNIVERSITY DEPARTMENT OF STUDIES AND RESEARCH IN CHEMISTRY UNIVERSITY COLLEGE OF SCIENCE, TUMAKURU. 2019-2020 SEMINAR TOPIC SILICONES Presentation by, BHAVANA R I MSc, I Sem

2. CONTENT Introduction Preparation of alkylchlorosilanes Preparation of silicones Linear polymer Cyclic polymer Crosslinked polymer Properties of silicones Applications of slicones References

3. INTRODUCTION Silicon ( Si ), Z = 14 , A = 28.09 Ground state electronic configuration of Si- [Ne] 3s 2 3p X 1 3p y 1 3p z Second most abundant element by weight in the earth’s crust. Ionization energy (KJ/mol) 1 st IE- 786.5 2 nd IE- 1573 3 rd IE- 3232 4 th IE- 4351 Si crystallizes in diamond lattice, corresponding to Si-Si distance of 235.17 pm and covalent atomic radius of 117.59 pm . MP 1420 C and BP nearly 3280 C Electronegativity in Pauling scale is 1.8 Common oxidation state of Si is +4 Other oxidation states of Si is -4, -2, 0, +2

4. Si is more volatile than C and has substantially lower energy of vapourization , H vap = 383 ± 10 KJ/mol thus reflecting smaller Si-Si bond energy . It has distinct shiny blue-grey metallic lustre and is widely used as semiconductor. C has unique ability to form p π -p π bonds , where as Si donot form p π -p π bonds but uses d orbitals in multiple bonding and can form d π -p π bonds . Carbon has a special property of catenation which is mainly the result of very strong C-C bond having bond energy 348KJ/mol where as Silicon exhibits less tendency for catenation due to the less bond energy of Si-Si bond 297KJ/mol . Si forms compounds such as Silicates, Silicon carbides, Silicon oxides, organosilicon compounds.

5. SILICONES Silicones are a group of organosilicon polymers which are also known as siloxanes . Organosilicon compounds are those in which organic group is attached to silicon. The term silicone was coined by analogy to ketone under the mistaken belief that monomeric R 2 Si=O compound could be isolated . Silicon compounds that are formally analogous to carbon compounds are found to have quite different structure, thus CO 2 is a gaseous monomer but SiO 2 is an infinite bonded 3 Dimensional network polymeric solid. In a similar manner gem- diols are unstable relative to ketones and analogous Si compounds are also unstable.

6. PREPARATION OF ALKYL CHLOROSILANES The starting material for manufacture of silicones are alkyl or aryl substituted chlorosilanes . Laboratory method Organochlorosilanes are prepared by grignard reaction.

7. II. Industrial method ( Rochow method/Direct method) Pure silicon is produced by reduction of SiO 2 in an electric furnace. Chlorinated hydrocarbons react with silicon in presence of metal catalyst to produce alkyl chloro silane . Based on the substitution of chloro groups in tetrachlorosilanes by alkyl group , there are 3 classes-

8. PREPARATION OF SILICONE LINEAR POLYMER Dimethyl dicholoro silane on careful hydrolysis gives straight chain polymer. As an active – OH group is left at each end of the chain, polymerization continues and the straight chain is continuously built. (CH 3 ) 2 SiCl 2 is the chain building unit.

9. Hydrolysis of trimethylmonochlorosilane yields trimethylsilanol as a volatile liquid, which can condense giving hexamethyldisiloxane . Since this compound has no OH group, it cannot polymerize any further. If some (CH 3 ) 3 SiCl is mixed with (CH 3 ) 2 SiCl 2 and hydrolysed, the (CH 3 ) 3 SiCl will block the end of straight chain produced by (CH 3 ) 2 SiCl 2 . Since there is no longer a functional OH group at this end of the chain, it cannot grow. Eventually the other end will be blocked in a similar way.

10. Thus (CH 3 ) 3 SiCl is a chain stopping unit and the ratio of (CH 3 ) 3 SiCl and (CH 3 ) 2 SiCl 2 in the starting mixture will determine the average chain size.

11. CYCLIC POLYMER Hydrolysis under careful controlled conditions can produce cyclic structures with rings containing 3- 6 Si atoms. Trimer has a chair structure analogous to cyclohexane and the tetramer has a crown structure analogous to S 8

12. By extension of reaction involved in the formation of cyclopolysilanes , bicyclic and cage permethylpolysilanes are also synthesised. In order to get branching to form cages( bridge head silicon atoms) some methyl trichlorosilane is added to dimethyldichlorosilane in the reaction.

13. CROSS-LINKED POLYMER The hydrolysis and polymerization of CH 3 SiCl 3 gives a complex crosslinked polymer. This group is trifunctional . A small amount of CH 3 SiCl 3 added to a mixture of dimethyldichlorosilane introduces a cross linked polymer.

14. PROPERTIES OF SILICONES Low thermal conductivity. Low chemical reactivity. Low toxicity. Thermal stability (constancy of properties over a wide temperature range of -100 to 250 C) Electrical insulation property. Resistant to heat, oxidation and many chemicals. High gas permeability, at room temperature ( 25 C ) the permeability of silicone rubber for gases such as oxygen is approximately 400 times that of butyl rubber , making silicone useful for medical applications in which increased aeration is desired. Ability to repel water and form water tight seals. Doesnt stick to many substrates but adheres very well to others example glass. Their strength and inertness are related to 2 factors i . Their stable silica like skeleton of Si-O-Si-O-Si . The Si-O bond energy is very high (502 KJ/mol) ii. High strength of Si-C bond .

15. APPLICATIONS OF SILICONES Electrical insulators Silicone Grease High voltage transformers Silicone Rubber Printed circuit board Car tyres moulds Silicone resin Non stick pans Silicone oil Silicone rubber bands

16. REFERENCES Chemistry of the elements- N.N.Greenwood and A.Earnshaw , II edition (1997). Concise inorganic chemistry- J.D.Lee , V edtition (1996). Inorganic chemistry- Keith F.Purcell , John.C.Kotz , Indian edition, Cengage learning publication. Inorganic chemistry, principles of structure and reactivity- James.E.Huheey , Ellen.A.Keiter , Richard.L.Keiter , Okhil.K.Medhi , IV edition, Pearson publication. Polymer Science- V.R.Gowariker , N.V.Viswanathan , Jayadev Sreedhar , II edition, New age international(P) limited publishers.

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