Chemistry

1. Chemistry Addition Polymers Kull Chem101 F08

2. Chlorine-substituted ethenes form the basis of many plastics and solvents. • One of the simplest halogen substituted ethenes is chloroethene, also known as vinyl chloride: • Vinyl chloride is the starting material for PVC or polyvinylchloride, one of the most common plastics. Kull Chem101 F08

3. Organic Molecules Containing Halogen Atoms • Substitution of three of the hydrogens in ethene results in trichloroethene (TCE): • It is a commonly used solvent which used to be used for a variety of processes such as dry cleaning, grease stripping, paint stripping, and coffee decaffeination. It has been replaced for many of these uses for environmental reasons. • Trichloroethene is very stable and is now a common pollutant of groundwater due to its extensive prior use. Kull Chem101 F08

4. Organic Molecules Containing Halogen Atoms • Substitution of all four of the hydrogens in ethene results in perchloroethene (PCE): • It is also a commonly used solvent . • Perchloroethene is very stable and is also a common pollutant of groundwater due to its extensive use. Kull Chem101 F08

5. Addition Polymers • Polymer: Very long chains of atoms with the same structural unit repeating • Plastics: materials that can be molded. • Advantages: Do not rust, corrode or break easily; Can be permanently colored • Thermoplastics: lose rigidity at high temperatures; do not decompose; recyclable • Polyethylene is the simplest organic polymer. • Packs tightlyhigh density (HDPE: high-density-polyethylene) • Opaque; possesses considerable structural strength, toughness and rigidity. Kull Chem101 F08

6. Addition Polymers • This can be thought of as the polymer: • where −M− stands for: These polymers are known as addition polymers since they consist of intact individual monomers that combine together. In the above case, the monomer would be: which is the repeating unit of the polymer. • Note that the double bonds in the monomer become single bonds in the polymer. Kull Chem101 F08

7. 2. Addition Polymers • The class of addition polymers derived from C=C units are called polyolefins. One of the most useful of these is PVC or polyvinylchloride. This polymer is built from vinyl chloride monomers • PVC plastics are rigid and strong. They are used in floor tiles, pipes, siding, and can also be made flexible enough to use in hoses and clothing. • The flexibility in PVC is obtained by the addition of a plasticizer: a liquid that blends with the polymer and acts as a lubricant between the polymer molecules. • Plasticizers eventually leak out of the plastics which then become brittle and break. • Plasticizer leakage also results in environmental pollution. Kull Chem101 F08

8. Addition Polymers • In polypropylene, a hydrogen atoms on the carbon chain are replaced by methyl groups. The monomer for this type of plastic is propylene: CH2=CHCH3. • The properties of polypropylene are similar to those of polyethylene, however it can be heated to a higher temperature before softening. It is used where materials must be sterilized, such as medical syringes or dishwasher save plastics. Kull Chem101 F08

9. Addition Polymers • Synthetic fibers made from polymers such a polypropylene are formed by squeezing molten polymer or polymer dissolved in a liquid through a small hole in a spinneret. • The fibers then solidify and are stretched to make them strong. • Some HDPE fibers have a tensile strength 20 times that of steel. Kull Chem101 F08

10. Polystyrene is a polymer in which a hydrogen atom on every other carbon in the polymer chain is replaced by a benzene ring. • It is a transparent, hard, brittle polymer and can be molded into many shapes. • It is often encountered as a foam such as in coffee cups, egg cartons, packing materials, and “peanuts”. • Research is underway to replace this and other non-degrading polymers with new polymers that break down under the action of sunlight or water. Kull Chem101 F08

11. Polyesters • The OCR example here is terylene, a polymer of benzene-1,4-dicarboxylic acid and ethane-1,2-diol. • The ester linkage is formed between the monomers

12. Addition Polymers • Polyvinylidene Chloride is produced from the monomer vinylident chloride. • This polymer resists penetration by oxygen but is too rigid to be useful. Kull Chem101 F08

13. Addition Polymers • To retain the oxygen penetration resistance and at the same time make the plastic more flexible, a copolymer between vinylidene chloride and vinyl chloride is produced: • This copolymer can be made into a flexible film which is very impervious to oxygen and so will prevent food from spoiling. One brand name of this type of plastic is Saran. Kull Chem101 F08

14. Addition Polymers • Polymerization of monomers having the formula: CX2=CX2 produce polymers having the structure: …-CX2-CX2-… • The most important of these is Teflon: …-CF2-CF2-…, which has a very high resistance to chemical attack and thermal decomposition. • It is most familiar as the non-stick surface on cooking pots and utensils. • Teflon does decompose above 400o C so frying pans should not be heated above this temperature. Kull Chem101 F08

15. Addition Polymers • Teflon fibers are used to make Gore-Tex brand clothing. This fabric repels liquid water but allows water molecules (from sweat) to readily pass through. Kull Chem101 F08

16. Rubber is a polymer chain that contains double bonds. • Natural rubber becomes sticky and loses its natural resilience when heated. • In 1839 Charles Goodyear discovered that the addition of elemental sulfur to natural rubber produced a substance that retained its desirable properties when heated. Kull Chem101 F08

17. 4. The Recycling of Plastics • The recycling of plastic is a controversial issue. • The plastics industry argues that “virgin” plastic is a low cost material made from relatively low cost precursors, natural gas and petroleum. • The cost of collection, cleaning and converting plastics back into monomer for reuse is substantial compared to the current cost of oil. • They argue for the collection and burning of the plastics to utilize the heat energy provided. • Environmentalists argue that environmental impacts have not been included in the cost analysis and the burning of some plastics releases toxic gases into the atmosphere. Kull Chem101 F08

18. 4. The Recycling of Plastics • There are several alternative ways to recycle plastics. • There are four ways to recycle plastics: • Reprocess by remelting or reshaping • Production of plastic trash cans from recycled HDPE • Depolymerize back into monomers. • Difficult. Plasticizers and other substances must be removed. Yields of monomers are often low • Transform into a lower-quality substance from which other materials can be made. • React with oxygen and steam to produce synthesis gas. • Heat to high temperature to “crack” the polymer molecules and produce synthetic crude oil. • Burn to obtain energy Kull Chem101 F08