Experiment 1. The Synthesis of Silly Putty, a Cross-linked Silicone Polymer Silicones are polymeric materials consisting of silicon, oxygen, and organic functionalities. The term “silicone” was first used by Friedrich Wöhler, better known for his contributions to organic chemistry, in 1857. It was used to describe a class of compounds of empirical formula R2SiO, by analogy with organic ketones, R2CO. Today we know these compounds to be molecular cyclic rings consisting of -O-Si-O-Si- linkages. The [Me2SiO]4 tetramer is shown below. Me Me Si O O Me Me Si Si Me Me O O Si Me Me Linear and branched silicone polymers consist of three levels of functionality defined by the number of oxygen links to the polymer chain. Most commonly, the difunctional silicon unit consisting of R2Si(O-)2 groups, with each oxygen bridging R R R O Si O O Si O R Si O O R R to another silicon, define the repeating unit of a linear polymer. The polymer chain may be capped by a terminal R3SiO- group. Hard resinous polymeric materials may be formed using a RSi(O-)3 center to cross-link polymeric chains. The most common starting material for the formation of methyl silicone polymers is dichlorodimethylsilane, (CH3)2SiCl2. It is useful because the Si-Cl bonds are easily hydrolyzed to form strong Si-O bonds, and the presence of two chlorine substituents permits polymer propagation in two directions to give a high molecular weight linear polymer with –OH terminal groups at each end. These terminal –OH groups may be capped with Me3SiCl, branched with MeSiCl3 to give a resin, or attached to another substituent or surface, illustrating the versatility of the linear polymer. In this experiment, trifunctional boric acid, B(OH)3, will be used as a cross- linking agent to give a gummy material known as “silly putty”. Once the synthetic procedure has been carried out we will investigate some of the unique properties of this resinous material. Experimental Procedure Materials - Dichlorodimethylsilane, boric acid, diethyl ether, anhydrous sodium sulfate, 10% sodium bicarbonate solution. Dichlorodimethylsilane is a volatile compound (bp 70° C) that undergoes hydrolysis exothermically to produce hydrogen chloride. It is important to avoid contact with (CH3)2SiCl2, and to work in a well-ventilated hood. Equipment – magnetic stirring hot plate, 20 mL round-bottom flask, magnetic stirring bar, 20 mL beaker, 5 mL syringe, water-cooled condenser, Pasteur pipet, litmus paper. Procedure Place 8 mL of diethyl ether in a 20 mL round-bottom flask containing a magnetic stirring bar. Using a volumetric syringe add 4 mL of dichlorodimethylsilane to the ether solvent and attach a water condenser to the flask. Arrange the assembly on a magnetic stirring hot plate, and, with rapid stirring, carefully add 8 mL of water dropwise through the top of the condenser. Initially, there will be vigorous evolution of HCl gas, and heat generated by the reaction will cause the ether to reflux (hence the need for the water condenser). Once all the water has been added, allow the mixture to stir for an additional 10 min at room temperature. Remove the condenser, and, using a Pasteur pipet, remove and discard most of the lower aqueous layer. Reattach the condenser and carefully add 4 mL of 10% sodium bicarbonate solution dropwise through the top of the condenser, with stirring. Evolution of CO2 gas may be observed as residual HCl in the wet ether solution is neutralized. Once gas evolution has stopped, remove the condenser and, with the pipette used earlier, remove most of the lower aqueous solution. Repeat the procedure with the Na(HCO3) solution until the lower aqueous layer no longer gives an acidic indication with litmus paper. Finally, wash the ether solution with 10 mL of water. Separate and discard the aqueous layer. Transfer the ether solution containing the silicone polymer to a small beaker and dry the solution by adding anhydrous sodium sulfate (roughly 2 gram). Filter the solution to remove the Na2(SO4), tare the beaker to obtain the mass of the silicone polymer, return the dry ether solution to the beaker, and concentrate the solution by evaporating the ether on a warm hot plate. Once the ether has evaporated, determine the weight of the clear residual silicone fluid. Add boric acid to the silicone fluid in an amount of 5% the weight of the fluid, stirring constantly with a spatula. Continue stirring for five minutes. Heat the resulting mixture to ~180° C on the hot plate with stirring until a stiff silicone gum is obtained (~20 min). Allow the product to cool to room temperature, and roll it into a ball. If the gum is somewhat brittle, continued kneading will produce the desired gum-like texture. Perform the following tests and record the results in your lab notebook. 1. It should give a lively bounce on a hard surface. 2. Pulling sharply causes cleavage of the gum. 3. Pulling slowly results in stretching reminiscent of chewing gum. 4. Placed on a hard surface it will flow into a flate plate. 5. If test 4 is done on newspaper, careful removal of the flat gum will reveal the mirror image of the print. Questions 1. The repeating unit in the dimethyl silicone polymer is (CH3)2SiO. Dimethyl ketone, (CH3)2CO, is a gas at room temperature. Why is there a difference in the physical states of these compounds? A table of standard bond enthalpies for group 4 (14) elements is given below. 2. From the bond enthalpies given in the table, account for the extreme exothermicity of the reaction between (CH3)2SiCl2 and water. What is the sign of the entropy change for the reaction? 3. How is the structure of the B(OH)3 crosslinking agent different from the structure of the link that might be formed from (CH3)Si(OH)3? This difference turns out to be quite significant; the material formed using MeSiCl3 to connect polymeric strands is an intractable solid.
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