CH 104: THE SYNTHESIS OF COMMON POLYMERS

1. CH 104: THE SYNTHESIS OF COMMON POLYMERS • Polymers are long chains of repeating units called monomers. Literally, polymers are many (poly) small units (meros). Monomers are one (mono) small unit (mero). • Examples of natural polymers include DNA and RNA (from nucleic acids), proteins (from amino acids), starch and cellulose (from glucose), and rubber (from isoprene). • Examples of manmade polymers include polyethylene, polypropylene, polyester, polyurethane, polyvinylchloride (PVC), epoxy, nylon, slime, silicone rubber, Dacron, Kevlar, Lucite, Orlon, and Teflon. • In today’s experiment you will make nylon, slime, and silicone rubber.

2. CHEMICAL STRUCTURES OF POLYMERS • In simplest terms, a polymer is made by repeatedly adding 1,000 to 1,000,000 or more monomers to a molecule. • Many A → etc. –A – A – A – A – etc. or (– A –)n • A copolymer has 2 or more alternating monomers. • Many A + Many B → etc. –A – B – A – B – etc. or (– A – B –)n

3. CLASSIFYING POLYMERS BY SYNTHESIS • Addition polymers are made from monomers that link together without losing any atoms. These monomers typically have at least 1 carbon-carbon double bond. Polyethylene is an addition polymer. • Condensation polymers are made from monomers that link together and lose small molecules such as water (H2O), ammonia (NH3), or hydrogen chloride (HCl). These monomers have 2 or more reactive functional groups. Most condensation polymers are copolymers. Dacron is a condensation polymer.

4. CLASSIFYING POLYMERS BY STRUCTURE • Polyethylene is a linear polymer. • Polypropylene is a branched polymer. • Kevlar is a cross-linked polymer. • The polymer chains of Kevlar crystallize in parallel, like dry spaghetti noodles in a box. These parallel chains are cross-linked with hydrogen bonds. As a result, Kevlar is 5 times stronger than steel. It is used in bulletproof vests, helmets, suspension bridge cables, and radial tires.

5. CLASSIFYING POLYMERS BY THERMAL PROPERTIES • Linear and branched polymers are long molecules that are more or less crystalline. They interact in these crystals through weak dipole-dipole forces. These weak dipole-dipole forces allow the crystals to soften when heated. These softened polymers can be molded or extruded into other shapes. Therefore, linear and branched polymers are often called thermoplastics. Polyethylene (an addition polymer) and nylon (a condensation polymer) are thermoplastics. • In contrast, a cross-linked polymer is essentially 1 gigantic molecule. The cross-links are strong bonds that make an irregular 3-dementional structure. Therefore, these thermoset plastics do not soften when heated. In fact, heating can produce more cross-links and make the material even harder. Kevlar is a thermoset plastic.

6. PROBLEMS WITH PLASTICS • PLASTICIZERS AND HEALTH: Plasticizers are often added to synthetic polymers to make the final product soft and pliable. If plasticizers are not added, these products would be hard, brittle, and useless for many purposes. • Similarly, the National Institutes of Health (NIH) reports that PVC flooring in bedrooms is associated with asthma and allergies in children. • The Food and Drug Administration (FDA) has warned that the plasticizer di(2-ethylhexyl)phthalate (DEHP) produces “a range of adverse effects in laboratory animals” and might cause “adverse events in humans”. The FDA has further warned that DEHP is released from polyvinyl chloride (PVC) medical devices, including intravenous (IV) bags, blood storage bags, feeding bags, dialysis bags, catheters, and various tubing.

7. PROBLEMS WITH PLASTICS • DISPOSAL: Plastics do not corrode, do not rust, and last almost indefinitely. Unfortunately, these desirable properties are a problem when plastics are thrown away. They do not biodegrade. • Thermoset plastics cannot be recycled by resoftening and remolding. Therefore, they should not be used for “disposable goods”. • Fortunately, most plastic wastes are recyclable. Plastic wastes are about 85% polyethylene, polypropylene, polystyrene, and polyvinylchloride. These are all thermoplastics and can be resoftened and remolded into new goods.

8. PROBLEMS WITH PLASTICS • FIRE HAZARDS: Toxic gases are sometimes released when plastics burn. For example, hydrogen cyanide (HCN) is released when polyacrylonitriles burn. And hydrogen chloride (HCl) is released when polyvinylchloride (PVC) burns.

9. PROBLEMS WITH PLASTICS • DEPENDENCE ON FOSSIL FUELS: Plastics use fossil fuels as a raw material and as a source of energy for manufacturing. Unfortunately, fossil fuels are nonrenewable and will one day be gone.

10. SAFETY • Give at least 1 safety concern for the following procedures that will be used in today’s experiment. • Using 20 % NaOH, 5 % aqueous hexamethylene diamine, 5 % sebacoly chloride in cyclohexane, 4 % polyvinyl alcohol, 4 % borax solution, sodium silicate (water glass), and 1-propanol. • These are irritants. Wear your goggles at all times. Immediately clean all spills. If you do get either of these in your eye, immediately flush with water. • In addition, many of these reagents are flammable. • Finally, you must wear protective gloves at all times. • Your laboratory manual has an extensive list of safety procedures. Read and understand this section. • Ask your instructor if you ever have any questions about safety.

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