Polymers

Q1. (4 marks)

• Addition polymer: formed by repeated addition of monomers (usually unsaturated, containing C=C) without loss of any small molecule. (1.5) Example: polythene (from ethene). (0.5)
• Condensation polymer: formed by repeated condensation between bi-/poly-functional monomers, usually with elimination of a small molecule like $H_2O$ or $HCl$. (1.5) Example: nylon-6,6. (0.5) Why: the defining criterion is loss/no-loss of a small molecule.

Q2. (4 marks)

• Thermoplastic: softens on heating and hardens on cooling; can be remoulded; has linear/branched chains held by weak forces. (1.5) Example: polythene / PVC. (0.5)
• Thermosetting: undergoes permanent hardening (cross-linking) on heating; cannot be remelted/remoulded. (1.5) Example: bakelite / melamine. (0.5)

Q3. (4 marks, 1 each) (a) PVC → vinyl chloride, $CH_2{=}CHCl$ (b) Teflon → tetrafluoroethene, $CF_2{=}CF_2$ (c) Nylon-6,6 → adipic acid + hexamethylenediamine (d) Terylene → ethylene glycol + terephthalic acid

Q4. (4 marks, 1 each) (a) Coordination (Ziegler–Natta) polymerization (b) Step-growth (condensation) polymerization (c) Free-radical polymerization (d) Cationic polymerization Why: $BF_3$ generates carbocation; peroxide generates free radicals; diacid+diamine = step growth.

Q5. (5 marks)

• $\overline{M}_n = \dfrac{\sum N_i M_i}{\sum N_i}$ — averaged over number of molecules. (2)
• $\overline{M}_w = \dfrac{\sum N_i M_i^2}{\sum N_i M_i}$ — averaged weighting by mass. (2)
• $\text{PDI} = \dfrac{\overline{M}_w}{\overline{M}_n}$; for a monodisperse polymer $\text{PDI}=1$. (1)

Q6. (6 marks) Data: $N_1=3,\ M_1=10000$; $N_2=2,\ M_2=20000$.

$\overline{M}_n = \frac{3(10000)+2(20000)}{3+2}=\frac{30000+40000}{5}=\frac{70000}{5}=14000\ \text{g/mol}$ (2)

$\overline{M}_w = \frac{3(10000)^2+2(20000)^2}{3(10000)+2(20000)}=\frac{3(10^8)+2(4\times10^8)}{70000}=\frac{11\times10^8}{70000}=15714.3\ \text{g/mol}$ (3)

$\text{PDI}=\frac{15714.3}{14000}=1.122$ (1)

Q7. (4 marks, 1 each) (a) Kevlar → body armour / bullet-proof vests (also parachutes, tyres) (b) Nomex → fire-resistant clothing / thermal protection (c) Epoxy resin → adhesives / composite matrix (CFRP/GFRP) (d) Polyimide → high-temperature insulation / aerospace components

Q8. (4 marks)

• Biodegradable polymers: polymers that are decomposed by microorganisms/enzymes into simpler substances (e.g. $CO_2, H_2O$) in a reasonable time. (2)
• Examples: PHBV and nylon-2-nylon-6. (1)
• PHBV monomers: 3-hydroxybutanoic acid and 3-hydroxypentanoic (valeric) acid. (1)

Q9. (3 marks, 1 each) (a) Cellulose — natural (b) Nylon-6 — synthetic (c) Rubber (natural) — natural

Q10. (2 marks)

• Bakelite is heavily cross-linked (3-D network from phenol + formaldehyde) so it cannot be softened/remoulded on heating → thermosetting. (1)
• Polyethene has linear chains with only weak van der Waals forces, which loosen on heating allowing softening/remoulding → thermoplastic. (1)

[
  {"claim":"Mn = 14000 g/mol","code":"N=[3,2]; M=[10000,20000]; Mn=sum(N[i]*M[i] for i in range(2))/sum(N); result = Mn==14000"},
  {"claim":"Mw = 15714.2857... g/mol","code":"N=[3,2]; M=[10000,20000]; Mw=sum(N[i]*M[i]**2 for i in range(2))/sum(N[i]*M[i] for i in range(2)); result = abs(Mw-15714.2857142857)<1e-3"},
  {"claim":"PDI ~ 1.122","code":"N=[3,2]; M=[10000,20000]; Mn=sum(N[i]*M[i] for i in range(2))/sum(N); Mw=sum(N[i]*M[i]**2 for i in range(2))/sum(N[i]*M[i] for i in range(2)); PDI=Mw/Mn; result = abs(PDI-1.1224489795918)<1e-4"},
  {"claim":"PDI of monodisperse = 1 when Mw=Mn","code":"Mn=14000; Mw=14000; result = (Mw/Mn)==1"}
]