1
Preparation of Bouncing Putty
Kevin F. Dunn
Department of Chemistry & Physics, Georgia College and State University,
Milledgeville, GA31061
September 9, 2008
Inorganic Laboratory Fall 2008
Abstract
This experiment involved the preparation of a silicone polymer via the formation of –Si–O–B linkages. In particular, it was prepared from the hydrolysis of dichlorodimethylsilane and cross-linking the residual hydroxyl groups with boric acid, resulting in a clear gum-like substance. The silicone polymer was then characterized through a series of physical tests.
Introduction
In 1857, the German chemist Friedrich Wöhler, known for disproving the theory of organic vitalism with the synthesis of urea, coined the term “silicone.”Wöhler used this term to describe any chemicals that contained an empirical formula of R2SiO, an analogy to the formula of ketones, R2CO.1Silicon does not show a strong tendency to form pπ-pπ double bonds with itself and other elements. In fact this type of bonding is generally unstable. On the other hand, silicon does show a tendency to form pπ-dπ bonds becauseof access to low-lying d orbitals.2As a result silicon forms extremely stable single bonds to oxygen and it is this type of linkage, Si–O, that is the basis of silicone polymers.
Silicones or organopolysiloxanes are polymeric materials that contain silicon, oxygen, and organic groups. There are three types of available linkages, trifunctional, difunctional, and monofunctional.1The length of the polymer chain is determined by the ratio of difunctional to monofunctional groups, while trifunctional units are used in the formation of silicone resins through cross-linking.
Figure 1: Functionality of Si–O linkage.
Methyl silicones are regarded as the most important member of this class of materials.This polymer is usually formed from dichlorodimethylsilane (CH3)2SiCl2, which is a useful starting material because Si–Cl bonds are easily hydrolyzed, making the compound extremely reactive. Also the compound is bifunctional so the chain can propagate in two directions, which results in high molecular weight chains.
Figure 1: Reaction sequence showing the hydrolysis of (CH3)2SiCl2and how silanol condenses to yield siloxane polymers.1
These siloxane polymers are extremely stable due to the –Si–O–Si sequence that comprises the main chain. This sequence makes silicones colorless, stable at high temperatures, poor conductors of electricity, and water/chemical resistant. As a result, silicone polymers play an important role in the medical field, where they incase implants as well as aide in the lubrication of hypodermic needles. Also they are found heavily in the industrial field where they can be used as water repellants, gaskets, and formed into bouncing putty.3
Experimental1
Approximately 1.0 mL of dichlorodimethylsilane was added to 2.0 mL of diethyl ether and attached to a microscale water condensor. Then 2.0 mL of water was added dropwise to the reaction mixture and allowed to stir for ten minutes. Note that this produces hydrogen chloride gas and therefore the addition of water should be made slowly at the beginning of the reaction. The lower aqueous layer was removed and 1.0 mL of 10% soldium bicarbonate solution was added dropwise. Once the bubbling had stopped, the lower aqueous phase was removed and tested with litmus paper, indicating that the layer was basic. The organic layer was washed with 2.0 mL of water and the aqueous layer was discarded. The ether layer was transferred to a Pasteur filter pipet column containing cotton, silica gel, and anhydrous sodium sulfate (~ ¾ in. of each). The dried eluate was collected in a 5.0 mL conical vial and then warmed under a slow stream of nitrogen. Boric acid was added to the silicone fluid in the amount of 5.0% of the weight of the fluid, 1.068 g. The resulting mixture was heated to 170-180°C until a stiff silicone gum was obtained.
Results/Discussion
After the preparation of the silicone polymerwas completed, it was then characterized through a series of physical tests. The polymer was rolled into a ball and dropped on a hard surface giving a lively bounce. Pulling the polymer sharply caused cleavage of the gum. On the other hand, pulling slowly resulted in stretching. When placed on a hard surface, the polymer flowed into a flat plate. Finally, when the polymer was placed on newspaper, the gum revealed a mirror image of the paper. The reason this silicone polymer recorded a positive response for each test preformed can be attributed to the bonds within the polymer. The bond angles and bond lengths of polysiloxanes are relatively large when compared to polymers with a C–C backbone. This results in a very flexible polymer. Also, silicon has the ability to access its d orbital resulting in a very stable compound. This explains why the prepared silicone polymer can be molded and then broken either by cleavage of it or by stretching it slowly.The positive responses to each physical test confirmed that silicone polymer had been prepared successfully.
Acknowledgments
I would like to acknowledge my laboratory partner Claudia Ramirez for assisting with the experiment. Many thanks are also due to the Department of Chemistry here at Georgia College and State University for providing the resources necessary to carry out the experiment.
References
1) Pike, R. M.; Singh M. M.; Szafran Z. Microscale Inorganic Chemistry: A Comprehensive Laboratory Experience;John Wiley & Sons: New York, 1991; 176-180.
2) Miessler, G. L.; Tarr, D. A. Inorganic Chemistry: Third Edition; Pearson Prentice Hall: New Jersey, 2004; 271.
3)Atkins, P.; Overton, T.; Rourke, J.; Weller, M.; Armstrong, F. Shriver & Atkins Inorganic Chemistry: Fourth Edition; W. H. Freeman and Company: New York, 2006; 343.