Polysiloxanes Synthesis and Structure

Polysiloxanes Synthesis and StructureAbstract silic aces (polysiloxanes) ar wide apply inorganic polymers containing silicon atoms. This category includes many other kinds of polymers much(prenominal) as polysilanes, polysilazanes, polycarbosilanes and polysiphenylenes 1. In this project we pass on focus on polysiloxanes discount and structure properties relationships. First, a brief reference on the history of these materials will be make. Organosilicon compounds were synthesized for the first eon in the 1860s, season the first polysiloxanes were made on the 20th century by F.S.Kipping 2.Then, the ways to synthesize and assume them will be analyzed. This is not a simple synthetic route, spot many different procedures take egress. These procedures include the reduction of SiO2 into an active reduced form of silicon, hydrolysis/ methanolysis and polycondensation or cyclization and polymerization 3.A more comprehensive playing field will be made regarding the structure pro perties relationships. Different cross linking densities progress to silicones with different properties. silicones disregard be categorized by their cross linking tightfistedness into silicone oils, elastomers and silicone resins. Silicone oils are elongate polymers synthesized by address opening polymerization. They are thermally stable and thus they bath be employ as hop up transfer fluids. Elastomer silicones are cross associate polysiloxanes via vulcanization and can find application in cars and food containers. On the other hand, silicone resins apply in high spirits cross linking density and as a extend they book high thermal stability. For this reason they are used in colour applied in outdoor surfaces while they are also mostly used as water repellants 3, 4. institutionMany different silicon containing polymers had been synthesized in the past ( protrude 1). The most Copernican category of these polymers is the one of polysiloxanes or else silicones.Figure 1 Silicon containing polymers.The synthetic thinking of organo silicon materials started in the 1860s. After 1900s the Grignard reagents became available and as a declaration major progress overhaulred. The first polysiloxanes were synthesized by F. S. Kipping in the beginning of the 20th century. Kipping synthesized diorgano dichloro silanes, R2SiCl2, that could be hydrolyzed into R2Si(OH)2. He was expecting that if these compounds got dehydrated they would produce compounds similar to ketones, R2Si = O. For that reason they were named silic-ones. curtly he realized that, the products were actually polymers that contained Si-O-Si and not Si=O 2.Polysiloxanes are considered as inorganic organic hybrid materials. The main chain is comprised of atoms of Si and O, whereas the side groups are comprised of alkyl groups ( persona 2).Figure 2 Structure of a polysiloxane.This structure is enough to grant them with excessive properties, thus they can be used in many different applic ations in our everyday life.These polymers can be put together as silicone oils, elastomers and resins. Silicone oils are linear polymers synthesized by ring opening polymerization. They are thermally stable and as a result they can be used as heat transfer fluids. Elastomer silicones are cross linked polysiloxanes via vulcanization and can be used in cars and food containers. On the other hand, silicone resins have high cross linking density and as a result they have high thermal stability. For this reason they are used in colors applied in outdoor surfaces while they are also largely used as water repellants 3, 4.SynthesisThe first step of the tax deduction of silicones is the reduction of SiO2 into Si. Usually the reduction is done electro thermally with carbon (1) and thence silicon is converted into SiCl4 with chlorination (2). Otherwise hydro chloro silanes can be produced by its reply with HCl (3).Really substantial for the industry is the direct reaction of methyl c hloride with silicon, something that was sight E. G. Rochow and R. Mller in 1940. Copper is used as a atom smasher in the form of CuO, as shown in the reactions (4), (5), (6) and (7).The main and most important product from this direct reaction is the dimethyl dichloro silane, (CH3)2SiCl2. The rest products of these reactions are shown above in table 1.Table 1 Main products of the direct reaction MeCl with Si.HydrolysisThe side by side(p) step is the hydrolysis of the dimethyl dichloro silane, which results into the formation of HCl and a mix of cyclic and linear oligomers of polysiloxanes.The hydrolysis is contacted in the liquid phase with 22 % HCl. The industrial process of the hydrolysis is shown in figure 3.Figure 3 Flow chart of hydrolysis, where a) Cooler b) Exhaust c) arrangeseparation d) Settling vessel e) Water separator f ) neutralisation reaction and g) PumpWhereas, in diagram 4 thither is a abbreviation of the whole procedure including the step of hydrolysis.F igure 4 HydrolysisMethanolysisDuring this process a direct recovery of Cl with the form of chloro methane from the methyl chloro silanes takes place. Silanes react with methanol in send to produce oligomers of dimethyl siloxanes and chloro methane. Methanolysis occurs according to the reactionsThe process of methanolysis is shown above with a flow chartFigure 5 Synthesis with methanolysis.And the reactions that take place are3. CyclizationCyclization needs to take place in order for cyclic siloxanes to be produced. This process is done with the heating of the mixture that was produced during the previous locomote with KOH.Figure 6Flow chart of the synthesis of polysiloxanes.4. PolymerizationThe uncouth method of producing linear polysiloxanes is the ring opening polymerization of cyclic polysiloxanes. A polymerization like that can occur under acidic or basic conditions. In general basic conditions are preferable.AnionicConsists a really important procedure for the industry and alkali metal hydrated oxide catalysts are used. The usual catalyst is potassium hydroxide, whereas other catalysts can be used according to their catalytic activity Cs Rb K Na Li. The polymerization occurs rapidly with the addition of only a few ppm of potassium hydroxide at a temperature above 140 C.The mechanism isThis reaction leads to the formation of mixture of linear silicones with approximately 15 to 18 per cent of cyclic silicones. In the end, the catalyst needs to get deactivated, unremarkably with the addition of phosphoric acids.CationicCationic polymerization can occur with the use of loyal protic acids or Lewis. The mechanism of this reaction is yet unknown.Polycondensationanother(prenominal) way to synthesize polysiloxanes is by polycondensation reactions of siloxanediols. Hydroxy oligomers of siloxanes can be condensated into polymers with high molecular weight with the loss of waterStructure Properties RelationshipsPolysiloxanes can be categorized acc ording to their cross linking density into silicone fluids, elastomers and resins.Silicone fluidsSilicon oils are linear polymers that can be produced by ring -opening polymerization, as described earlier. The flow chart of their industrial production is shown in figure 7.Figure 7 Flow chart of silicone fluids production.Silicon fluids posses a variety of useful properties, such asThermal stability ( Temperatures one hundred fifty 250 C )High hydrophobicityDielectric propertiesGood damping behaviorResistance to radiotherapyThese exceptional properties is the reason why silicones oils are used so widely. For manakin because of their high thermal stability they can be used as a heat transfer media in the industry and not only. Moreover, their strong hydrophobicity enables their use as coatings.Figure 8 Silicone oil used as a heat transfer media.Silicone elastomersSilicones can substantially be converted into rubbers through vulcanization. The initial material is a polysiloxane o f high molecular weight produced by ring opening polymerization. Cross linking is essential. This can happen with radicals from the decomposition of organic hyper oxides. The hydrogen atoms are removed from the methyl functional groups and the silixane chains connect with each other with Si CH2 CH2 Si bonds.In an alternative version of the vulcanization polysiloxanes that contain a lower-ranking quantity of methyl vinyl siloxane groups constitute the raw materials. In order for the vulcanization to begin alkyl-hyper oxides are organism used but this time the mechanism is different as shown aboveThe ultimate tensile potency of these elastomers is low, but it can be improved with the addition of fillers.Except from the above, there are also silicone elastomers that the vulcanization occurs at room temperature. thither are two causes of different systems for this, with one component and with two components. For the systems of one component air exposure is necessary. A typical example of this type is being composed by di -hydroxy siloxane oligomers.The product of this reaction can be placed into a mold in order to attain the enamor shape.Silicone elastomers attain a really high strength at high temperatures. This makes them one of the elastomers that are used in application that desire strength into extreme environmental conditions. Moreover, they are and as a result they do not react with many chemicals. For this reason they are being used in biomedical applications, including medical implants.Figure 9 Silicone elastomers used in catheters.Products made from silicone rubbers can be found in our everyday life. They are being used into cars, food containers, clothes, electronics and in many other applications.Figure 10 Different applications of silicone elastomers in our everyday life.Silicone ResinsThey are cross linked compounds containing tri functional (T) and tetra functional (Q) silicon units (Q). The simplest silicone resins are produced from methyl chloro silanes with hydrolysis, either directly or indirectly (after the intermediate conversion into methyl trialkoxy silanes).Figure 11 attribute structures of silicone resins.Silicone resins can be used as additives paints curiously for use in high temperatures. They can also be used as coatings on pharmaceutical pills and as encapsulants for electronic components.Figure 12 Silicone resins used in paints and electronic components.References1 H.-H. Morreto, M. Schultze and G. Wagner, Silicones, Ullmanns Encyclopedia Of Industrial Chemistry, washbasin Wiley Sons, 20122 A. Mitra and D. A. Atwood, Polysiloxanes and Polylanes Encyclopedia of Inorganic Chemistry, John Wiley Sons, 20063 R. G. Jones, W. Ando, and J. Chojnowski, Silicon Containing Polymers, Kluwer Academic Publishers, Dordrecht, 20004 F. S. Kipping, Proc. R. Soc. London, A159, 139, 19375 J. Heiner, B. Stenberg, M. Persson, Material behaviour Crosslinking of siloxane elastomers, Polymer Testing 22, 253-2 57, 20036 R. drake, I. MacKinnon and R. Taylor, Recent advances in the chemistryof siloxane polymers and copolymers,Chemistry of Functional Groups, John Wiley Sons, 20097 W. D. Callister, Materials intelligence and Engineering, An Introduction,John Wiley Sons Inc., USA, 20078 A. 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