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PrepSiliconePolymerC118 - Preparation of a Silicone...

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1 Preparation of a Silicone Polymer (Bouncing Putty) & Polystyrene Objectives – To synthesize and investigate the product of a inorganic polymer reaction To synthesize and investigate the product of an organic polymer reaction Introduction Polymers are macromolecules made by linking together smaller compounds, called monomers , through covalent bonds. Polymers can be classified into two large categories, biopolymers and synthetic polymers. Biopolymers are polymers that occur naturally in organisms, examples of which are DNA, RNA, proteins, and polysaccharides. Synthetic polymers are important commercial products in today’s economy. A very common polymer is polyethylene, which is found as both low-density (e.g. plastic shopping bags) and high-density polyethylene (e.g. milk jugs). Polyethylene is called an addition polymer, formed by the addition of the monomer ethylene, as seen below. In this experiment, one of the polymers you will pursue is the addition polymerization of styrene to make polystyrene. The backbone of polystyrene is similar to polyethylene; however, in polystyrene there is a phenyl (-C 6 H 5 ) side chain that dangles off the side of the chain. This difference in the R— group lends different properties to the polymer. Examples of different polymers you may be familiar with are found below. H 2 C CH R H 2 C CH R CH 2 CH R CH 2 CH R n monomer polyethylene, R = H polypropylene, R = CH 3 polystyrene, R = Phenyl polyvinylchloride, R = Cl Figure 1: Generic reaction to form an organic polymer
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2 You have most likely encountered polystyrene in insulated coffee cups, the windows of mailing envelopes, and as packing “peanuts”. Polystyrene is a good electrical and heating insulator, which is why you have used it as a calorimeter in previous experiments in C117. Polystyrene has found numerous applications as it can easily be extruded, molded and made into foam. In this experiment, you will be treating the monomer styrene with an ionic catalyst, aluminum chloride, to produce polystyrene via a very vigorous reaction. Aluminum chloride is thought to autoionize while in solution although this equilibrium is not very large to the right. The reaction is thought to proceed via activation by the small amount of AlCl 2 + present in solution; since AlCl 2 + is a strong electrophile, the double bond on styrene attacks the AlCl 2 + to produce a carbocation and initiate the reaction. Once the carbocation is formed, the carbocation intermediate reacts with another equivalent of styrene to add another monomer. This reaction continues until a true polymer macromolecule is formed.
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