C21 11/08/2013 16:8:37 Page 311 CHAPTER 21 POLYMERS: MACROMOLECULES SOLUTIONS TO REVIEW QUESTIONS 1. An addition polymer is one that is produced by the successive addition of repeating monomer molecules. A condensation polymer is one that is formed from monomer molecules in a reaction which splits out water or some other simple molecule. Condensation polymerization usually involves two different monomers. 2. Those polymers which soften on reheating are thermoplastic polymers; those which set to an infusible solid and do not soften on reheating are thermosetting polymers. 3. Low-density polyethylene is used in packing material, molded articles, plastic films, garbage bags, flexible bottles, containers, and toys. High-density polyethylene is used in plastic grocery bags; bottles for milk, juice, water and laundry products; and medical products. 4. Unique problems of plastic recycling that distinguish it from aluminum or glass recycling are as follows: (a) Recycled plastic is not as good as new plastic. While plastic can typically be recycled once, glass and aluminum may be recycled many times; Sorting of plastics during recycling is much more difficult than sorting glass or aluminum; (c) In contrast to glass and aluminum recycling, it is much cheaper to create new plastic rather than recycle. 5. The word plastic means capable of being shaped. Many (but not all) organic polymers have this capability. 6. A propagation step always reproduces a reactive compound that can continue the chain reaction. A termination step destroys the reactive compounds that are responsible for the polymerization. 7. The big problem of recycling is with the separating the different types of plastics. 8. Copolymers contain more than one monomer. Polyethylene is made from only one monomer, ethylene. - 311 -
C21 11/08/2013 16:8:37 Page 312 SOLUTIONS TO EXERCISES 1. Each propylene unit is composed of CH 2 CH and has a molar mass of 42 g/mole of propylene. Thus, ð20;000 g=mole of polymerþ=ð42:0 g=mole of propyleneþ ¼ 476 moles of propylene=mole of polymer There are about 476 propylene units per polymer. 2. The monomer that makes up teflon is tetrafluoroethylene, CF 2 CF 2, 100.0 g/mole of monomer unit. There are 3500 monomer units/teflon polymer so there are 3500 moles of monomer units/mole of teflon. ð3500 moles of monomer units=mole of teflonþð100:0 g=mole of monomer unitþ ¼ 350;000 g=mole of teflon The molar mass of the teflon polymer is 350,000 g/mole. ( ) n 3. (a) CH 2 CH (c) ( CF 2 CF 2 ) n OC O ( CH 2 C ) n (d) ( CH 2 C ) n C O O 4. (a) ( CH 2 C 2 ) (c) n ( CH 2 CH ) n ( CH 2 CH ) n (d) ( CH 2 CH ) n - 312 -
C21 11/08/2013 16:8:37 Page 313 5. Free radical polymerization starts with an initiation step that forms the following free radical: RO This free radical then attacks one vinyl chloride to start polymerization: ROCH 2 CH A second vinyl chloride then reacts to give the following free radical: ROCH 2 CH CH 2 CH 6. Free radical polymerization starts with an initiation step that forms the following free radical: RO This free radical then attacks one propylene to start polymerization: ROCH 2 CH A second propylene then reacts to give the following free radical: ROCH 2 CH CH 2 CH 7. Formulas of the polymers that can be formed from: (a) (c) - 313 -
C21 11/08/2013 16:8:37 Page 314 8. Formulas of the polymers that can be formed from: (a) (c) 1-butene a a CH 2 CH b b n CH 2 9. Two possible ways in which vinyl chloride can polymerize to form polyvinyl chloride: a a CH 2 CH CH 2 CH CH 2 CH CH 2 CH b b n a a CH 2 C H C 2 CH 2 CHCH (other possibilities also) l C H C 2 b b n CHCH l 10. Two possible ways in which acrylonitrile can polymerize to form Orlon. a a CH 2 CH CH 2 CH CH 2 CH CH 2 CH b b n a a CH 2 C C 2 CH 2 C C 2 b b n CH CHCH CH CHCH (other possibilities also) N N N N 11. The polymer is low-density polyethylene. It is made from ethylene, CH 2 ¼CH 2. 12. The polymer is polypropylene. It is made from propylene, CH 2 CH 13. ( CH 2 CH 2 CH 2 CH ) n 14. ( CH 2 CH CH 2 C ) n - 314 -
C21 11/08/2013 16:8:38 Page 315 15. Natural rubber (all cis) n 16. Gutta percha (all trans) 17. Yes, smog in the atmosphere contains ozone and ozone attacks natural rubber at the site of the double bond causing age hardening and cracking. 18. Yes, styrene-butadiene rubber contains carbon-carbon double bonds and is attacked by the ozone in smog causing age hardening and cracking. 19. Chemical structures for the monomers of: n (a) 20. Chemical structures for the monomers of: (a) neoprene rubber CH 2 C CH ¼ CH 2 21. Syndiotactic polypropylene - 315 -
C21 11/08/2013 16:8:38 Page 316 22. (a) 23. (a) none 24. (a) one, CH 2 CH CHCH 2 none 25. (a) trans cis (c) trans (d) trans 26. (a) trans cis (c) cis 27. Polystyrene has the highest mass percent carbon. 28. - 316 -
C21 11/08/2013 16:8:38 Page 317 29. Cyanoacrylate ester polymer 30. (a) If produced by the free-radical mechanism, a random mixture of cis and trans connections are made. It is possible, using catalysts, for the reaction to proceed by an ionic mechanism which will give a stereochemically controlled polymer. 31. It is easier to recycle a thermoplastic polymer because they are linear and can easily be reformed on heating. On the other hand, the cross linkages in thermosetting polymers are not easily broken or reformed. 32. CH 2 CH 2 ethylene CF 2 CF 2 tetrafluoroethylene 33. A representative structure for the polymer of methylmethacrylate follows: This plastic goes by the trade names Lucite and Plexiglas. It is a hard, transparent plastic that is used, for example, in contact lenses, in medical enclosures such as those used for premature babies, in furniture, etc. - 317 -
C21 11/08/2013 16:8:38 Page 318 34. During vulcanization, sulfur atoms cross-link rubber polymers together. In an analogous fashion, artificial polymers cross-link to form a thermosetting plastic. Both rubber and thermosetting plastic gain strength from this cross-linking. 35. Disagree. HDPE is thermoplastic and can be reused by melting and reforming without breaking down the polymer to monomers. - 318 -