Q6(b). Nylons, combustion of polyethylene, polystyrene, Teflon and PVC

Introduction. Polymers are named from their monomers and made by step-growth (condensation, ring-opening) or chain-growth (addition) polymerisation. Their mechanical and chemical behaviour follows from chain structure: the side group, the glass transition temperature T_g and the strength of the bonds in the chain.

(i) Nylon 6 and nylon 6,6

Nylon 6 is made from one monomer, caprolactam, by ring-opening polymerisation: heating at about 250°C with a little water opens some lactam to 6-aminohexanoic acid, whose -NH₂ then attacks further lactam rings. [structure] → (H₂O (trace), about 250°C) -[NH-(CH₂)₅-CO]_(n)⁻ nylon 6 Nylon 6,6 is a condensation polymer of hexamethylenediamine and adipic acid (6 C each). The two are first combined in exact 1: 1 ratio as the “nylon salt”, which is heated (about 280°C) under pressure; water is removed. n H₂N(CH₂)₆NH₂ + n HOOC(CH₂)₄COOH -[NH(CH₂)₆NH-CO(CH₂)₄CO]_(n)⁻ + 2n H₂O

(ii) Moles of CO₂ from 20 g of polyethylene

Given: polyethylene =(CH₂)ₙ; repeat-unit mass M(CH₂) = 12.011 + 2(1.008) = 14.03 g mol⁻¹. Each CH₂ gives one CO₂:

(CH₂)ₙ + 1.5n O₂ n CO₂ + n H₂O

n(CO₂) = n(CH₂) = 20 g14.03 g mol⁻¹ = 1.43 mol

n(CO₂) ≈ 1.43 mol (≈ 62.7 g) The answer does not depend on the chain length, because the end groups are a negligible fraction of the mass.

(iii) Polystyrene brittle, polyethylene flexible

• Polystyrene carries a bulky, rigid phenyl group on every second carbon. It hinders rotation about the backbone C–C bonds, and the usual atactic polymer cannot crystallise. Its T_g is about 100°C, so at room temperature the chains are frozen in a glass: under stress they cannot slide or uncoil, and cracks run through the material (brittle).
• Polyethylene has only H atoms on the chain. Rotation is easy and its T_g is about -120°C; at room temperature the amorphous regions are rubbery, while the crystalline regions give strength. The chains can uncoil and slide, so the polymer is flexible and tough.

(iv) Teflon inert, PVC reactive on heating

• In Teflon, -(CF₂-CF₂)ₙ⁻, the C–F bond is very strong (about 485 kJ mol⁻¹), and the larger F atoms form a tight helical sheath round the carbon chain, shielding it from attack. There are no C–H bonds to abstract and no functional group to attack: Teflon resists acids, alkalis and oxidants and is stable to about 300°C.
• In PVC, -(CH₂-CHCl)ₙ⁻, the C–Cl bond is much weaker (about 330 kJ mol⁻¹), and H and Cl sit on neighbouring carbons. Above about 150°C HCl is lost (dehydrochlorination), starting at defect sites. Each new C=C makes the next Cl allylic and easier to lose, so HCl is lost in a “zipper” sequence, leaving a coloured polyene; the HCl released catalyses further loss.

Conclusion. (i) Nylon 6 by ring-opening of caprolactam; nylon 6,6 by condensation of hexamethylenediamine with adipic acid. (ii) 20 g polyethylene gives 1.43 mol CO₂. (iii) Polystyrene is a glass at room temperature (T_g about 100°C, bulky phenyl); polyethylene has T_g far below. (iv) Strong, shielded C–F bonds make Teflon inert; weak C–Cl bonds allow zipper dehydrochlorination of PVC.

Application. Heat stabilisers (organotin and Ca/Zn soaps) are added to PVC to scavenge HCl; Teflon's inertness is used in non-stick cookware, chemical seals and in PTFE tubing for laboratory work.