Q7(d). Natural rubber, polystyrene, Teflon; terylene and PVC
Write structures of repeating units of natural rubber, polystyrene and teflon. How will you prepare terylene and PVC?
Answer in brief
Repeat units: natural rubber -[CH₂C(CH₃)=CHCH₂]⁻ (cis), polystyrene -[CH₂CH(C₆H₅)]⁻, Teflon -[CF₂CF₂]⁻. Terylene: dimethyl terephthalate/terephthalic acid + ethylene glycol, condensation; PVC: radical polymerisation of vinyl chloride.
Key points
- Dimethyl terephthalate is heated with excess ethylene glycol (zinc or manganese acetate catalyst): transesterification gives bis(2-hydroxyethyl) terephthalate and methanol.
- Heating under vacuum with Sb₂O₃ at about 270°C removes ethylene glycol: polycondensation to high-molar-mass PET, melt-spun into terylene fibre.
- Vinyl chloride (CH₂=CHCl, from ethene by chlorination and HCl elimination) is polymerised by radical chain addition with a peroxide or azo initiator, usually as a suspension in water at about 50–60°C.
- Initiation (R• adds to CH₂=), propagation (head-to-tail growth) and termination give -[CH₂-CHCl]_(n)⁻.
The full model answer
Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.
Introduction. Natural rubber, polystyrene, Teflon and PVC are addition (chain-growth) polymers; terylene (Dacron, PET) is a condensation (step-growth) polyester.
| Polymer | Repeat unit | Monomer |
| Natural rubber | -[CH₂-C(CH₃)=CH-CH₂]_(n)⁻, cis-1,4-polyisoprene | isoprene (2-methylbuta-1,3-diene) |
| Polystyrene | -[CH₂-CH(C₆H₅)]_(n)⁻ | styrene |
| Teflon | -[CF₂-CF₂]_(n)⁻ | tetrafluoroethene |
Terylene (polyethylene terephthalate)
Scheme: Esterification (or ester interchange), then polycondensation with removal of water or methanol.
- Dimethyl terephthalate is heated with excess ethylene glycol (zinc or manganese acetate catalyst): transesterification gives bis(2-hydroxyethyl) terephthalate and methanol.
- Heating under vacuum with Sb₂O₃ at about 270°C removes ethylene glycol: polycondensation to high-molar-mass PET, melt-spun into terylene fibre.
PVC
- Vinyl chloride (CH₂=CHCl, from ethene by chlorination and HCl elimination) is polymerised by radical chain addition with a peroxide or azo initiator, usually as a suspension in water at about 50–60°C.
- Initiation (R• adds to CH₂=), propagation (head-to-tail growth) and termination give -[CH₂-CHCl]_(n)⁻.
Conclusion. Repeat units: natural rubber -[CH₂C(CH₃)=CHCH₂]⁻ (cis), polystyrene -[CH₂CH(C₆H₅)]⁻, Teflon -[CF₂CF₂]⁻. Terylene: dimethyl terephthalate/terephthalic acid + ethylene glycol, condensation; PVC: radical polymerisation of vinyl chloride.
Application. PET is used for bottles and polyester fabric and is recycled by glycolysis back to its monomers; PVC is used for pipes and cable insulation, with plasticisers for flexible grades.
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All parts of Question 7
- 7(a) Paternò–Büchi reaction; NMR of bromopropanes; IR of α-chloro ketones15 marks
- 7(b) Kishi dihydroxylation; ketal protection; Na/NH₃ on a terpene alcohol15 marks
- 7(c) UV spectra of cis- and trans-stilbene10 marks
- 7(d) Natural rubber, polystyrene, Teflon; terylene and PVC10 marks