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Preparation of a Silicone Polymer: Bouncing Putty Muhammad Hazim Tarar Department of Chemistry and Chemical Engineering, Syed Babar Ali School of Science and Engineering, Lahore University of Management Sciences (LUMS), Sector U, DHA Lahore 54792 Email: [email protected]

Abstract: In this experiment, polydimethylsiloxane, a type of silicone polymer, was synthesized by hydrolyzing dichlorodimethylsilane. The resulting polymer solution was then crosslinked with the addition of boric acid, causing it to solidify into a pliable and flexible gellike material.

Introduction: The development of silicone polymers began and grew in the early 1900s when their properties were being tested in order to help with the war effort. Since then, silicone has been used in countless applications. Important applications of silicone polymers include lubricating oils, heat resistant materials, medical supplies, and many more. Silicones can be prepared having a wide range of viscosities, lubricating properties and reactivities. They see extensive use in industrial chemistry in automobile polishes, cosmetics, water repellants, high-temperature electrical insulation, release agents, gaskets, antifoam agents, high-temperature paints, glass cloth laminates, elastomers (rubbers), greases and other general polymers. Silicones, or Polyorganosiloxanes, are an interesting class of inorganic polymeric materials that consist of chains of alternating silicon and oxygen atoms, with organic groups bonded to silicon. The methyl silicones, also called polydimethylsiloxanes, are perhaps the most important members of this class of materials, as well as the most common.

Fig 1.1 Structure of Silicone Oil The most common starting material for the formation of the methyl silicone polymers is dichlorodimethylsilane, (CH3)2SiCl2. The aforementioned compound is, in turn, made in large scale industrial quantities by the action of CH3Cl on silicon powder in the presence of a copper catalyst at 250- 300 °C (the Rochow Process). The resulting mixture of compounds includes SiCl4, CH3SiCl3, (CH3)2SiCl2, (CH3)3SiCl and other less abundant products. Dichlorodimethylsilane is a useful starting material for two reasons: (1) The Si-Cl bonds are

easily hydrolyzed, making the compound very reactive. (2) The compound is bifunctional, since there are two chlorines. The chain can therefore propagate in two directions, resulting in high molecular weight polymers.

Fig 1.2 Hydrolysis and Condensation The polyorganosiloxanes are prepared by the hydrolysis of the selected chlorosilane. Thus, hydrolysis of (CH3)2SiCl2 gives rise to the corresponding silanol, (CH3)2Si(OH)2 and hydrogen chloride (HCl). The outstanding characteristic of the Silanols is the ease in which they condense to yield siloxane polymers as shown in the above reaction sequence. In this experiment, the chemistry of silicones was investigated by preparing “bouncing putty”, a silicone polymer, via the hydrolysis of dichlorodimethylsilane. The silicone, containing residual hydroxyl groups was cross-linked using Boric acid. This trifunctional acid, B(OH)3, which also contains hydroxyl groups, forms -Si-O-B- linkages between the siloxane chains, resulting in a peculiar type of gum.

Experimental section Chemicals, Dichlorodimethylsilane, Diethyl ether, M sodium carbonate solution, Anhydrous sodium sulfate, Distilled Water, Boric acid Glassware/Equipment, Pipette 10 mL, 5 mL, Hot plate, Erlenmeyer flask 100-mL, pH paper, Separatory funnel, Filter paper, Volumetric flask 100 mL, Round bottom flask 100 mL, Beakers 100 mL (2), Glass funnel. Procedure, 5 mL of dichlorodimethylsilane was measured using a dry graduated cylinder or volumetric pipette. The reagent was then transferred into a dry 100-mL Erlenmeyer flask. Furthermore, 10 mL diethyl ether was added to the reagent. The silane, was then hydrolyzed by dropwise addition of water (10 mL). HCl gas is evolved in this hydrolysis step, therefore as a safety precaution, the hood sash was kept as low as possible and the glass doors were closed. The addition of the water was done slowly at first, because the reaction was very vigorous. An ice bath was kept in close proximity in case the reaction became too exothermic. The addition of the first 3 mL of water was the most vigorous, and after this quantity had been added, we increased the addition rate. The product had a very pungent odor; therefore, everything was kept in the hood. After the hydrolysis reaction, the ether layer was isolated by pouring the mixture into a separatory funnel. The ether layer, was washed 3 times with 1.0 M Na2CO3 (3 x 25 mL) to neutralize any residual acid remaining in the wet either. The evolution of CO2 was observed at this stage; whilst, the separatory funnel, was kept open, as the gas was being evolved. 2.5 mL of diethyl ether, was then added to the separatory funnel after each wash as diethyl ether was lost due to the heat of reaction. The final wash was performed with distilled 25 mL water. A pH paper test was done, to ensure its neutrality. The ether layer, was

transferred to an Erlenmeyer flask and a scoop full of anhydrous sodium sulfate, was added to the flask; a stopper was placed in the flask to avoid evaporation of the ether. The anhydrous sodium sulfate removed any remaining water from the ether solution. The latter step was allowed to proceed for about half an hour. The ether solution, was filtered through filter paper and a glass funnel collecting the solvent into a pre-weighed 100- mL round bottom flask. The diethyl ether was evaporated off. The yield of silicone oil at this stage, was noted before proceeding to the next step. About 5% (by weight) of boric acid (i.e., 0.5 g for 10 g of oil), was added while stirring continuously with a spatula during the addition and for a few minutes after the addition. At this point, the oil, became more viscous. The mixture, was heated to 170-180°C directly on a hot plate, while the temperature was increased, until a stiff gum was obtained. The product, was removed from the flask and the material, was rolled into a ball with a gloved hand.

Results and discussion: Silicon is bigger and less electronegative than carbon, allowing it to better contribute its own electrons into making strong bonds. Because of its size, silicon bond angles are also wider and less rigid than those of carbon, allowing for more movement. Silicon can form alternating bonds with oxygen to create one of the strongest single bonds known.

Fig 1.3 Silicon-Oxygen Bonds The framework of all the polymers is the very stable -Si-O-Si sequence. This gives silicones good thermal stability at high temperatures and flexibility at low temperatures. The organic groups are hydrophobic and, thus, so is the polymer. The strong Si-O bond gives rise to the stability (chemical inertness) of silicone polymers while the hydrophobicity of the R groups allows them to be water resistant. The synthesis of polydimethylsiloxane depends on the susceptibility of dimethyldichlorosilane towards hydrolysis. Silyl chlorides hydrolyze much more readily and completely than carbon chlorides such as (CH3)2CCl2 because of the size difference between carbon and silicon. Because silicon is bigger, there is more room between the Si-Cl bonds, allowing for attack by water. The hydrolysis of dimethyldichlorosilane, produces HCl as a side product. Therefore, addition of dilute sodium carbonate was necessary to neutralize the polymer solution. After the washings were no longer acidic, signifying that the HCl was no longer present, the solution was heated to evaporate off the ether and isolate the polymer solution. A significant step between the hydrolysis of dimethyldichlorosilane and the final silicone polymer, is the production of silanol, Si(CH3)2(OH)2. As the solution stirred, condensation occurred which caused the loss of most, but not all, of the hydroxyl groups. At this point, the silicone actually exists in the form of a

mixture of cyclic and open-chained oligomers, which are chains consisting of a several monomers, but not enough to be considered a full polymer. The addition of boric acid (H3BO3) then causes the formation of Si-O-B linkages via the remaining OH groups. Because boric acid is trifunctional, it can crosslink together three chains. This process continues until all of the chains are connected into what is actually one single molecule. The material, while somewhat pliable, was prone to breaking into many small pieces when pulled apart quickly. Because, the Polydimethylsiloxane synthesized did not contain any such additives, its structure was not completely like that of the characteristic bouncy putty. Conclusion: The aim of this experiment was to synthesize and characterize the silicone polymer polydimethylsiloxane. The synthesis was done by hydrolyzing dimethyldichlorosilane to produce the PDMS and then boric acid was added to crosslink the polymer into a flexible, gumlike material

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