State that PET can be converted back into monomers and re-polymerised

Organic Chemistry – Polymers (Cambridge IGCSE 0620)

Learning objective

State that polyethylene terephthalate (PET) can be chemically broken down into its original monomers and re‑polymerised to give new PET, and understand the wider context of polymer chemistry required for the syllabus.

1. Core polymer concepts (Core – 11.8)

1.1 Key definitions

• Monomer: a small molecule that can join with other identical or different molecules to form a polymer.
• Polymer: a large (macromolecular) substance formed by the repeated linking of monomer units.
• Addition polymerisation: monomers that contain a carbon–carbon double bond (e.g., ethene) join together without loss of atoms, giving a polymer such as poly‑ethene.
• Condensation polymerisation: each step joins two monomers while eliminating a small molecule (usually water, methanol or HCl). PET, nylon 6,6 and many polyesters are examples.
• Repeat unit: the smallest structural fragment that repeats along the polymer chain.
• Structural isomerism in polymers: branching, head‑to‑head, head‑to‑tail or random arrangements of the repeat unit give polymers with different physical properties.

1.2 Typical families of polymers (core level)

1.3 Comparison of addition and condensation polymerisation

1.4 Environmental issues (core)

• Land‑fill accumulation: non‑recycled PET bottles can persist for centuries.
• Marine litter: PET fragments enter oceans, causing harm to wildlife.
• Incineration: releases CO₂ and, if incomplete, toxic gases (CO, VOCs).
• Resource use: virgin PET requires petroleum‑derived ethylene glycol and terephthalic acid.

Re‑using or recycling PET reduces these impacts and conserves raw materials.

2. Chemical recycling of PET (Supplement – 11.8)

2.1 Depolymerisation routes

All three methods break the ester linkages in PET and give either the original monomers or close intermediates that can be converted back to them.

1. Hydrolysis (water‑based)
   PET + 2 H₂O → ethylene glycol + terephthalic acid
   Note: terephthalic acid is the acid form of the monomer used in PET production; after purification it is directly re‑polymerised.
2. Glycolysis (excess ethylene glycol)
   PET + excess ethylene glycol → bis‑hydroxyethyl terephthalate (BHET) + ethylene glycol
   BHET is a direct PET precursor; it can be polycondensed without further chemical conversion.
3. Methanolysis (methanol‑based)
   PET + 2 CH₃OH → dimethyl terephthalate (DMT) + ethylene glycol
   DMT is an industrial intermediate; it is trans‑esterified with ethylene glycol to give BHET, then polycondensed.

2.2 Comparison of chemical recycling methods

2.3 Re‑polymerisation of the recovered monomers

• From terephthalic acid (hydrolysis route)
  Ethylene glycol + terephthalic acid →[Δ] PET + H₂O
• From BHET (glycolysis route)
  BHET →[Δ] PET + ethylene glycol
• From DMT (methanolysis route)
  DMT + ethylene glycol →[trans‑esterification] BHET →[Δ] PET + CH₃OH

The newly formed PET is chemically identical to virgin PET and can be used for the same applications (bottles, fibres, food‑grade containers).

3. Other recycling approaches (Supplement)

3.1 Mechanical recycling

• PET is collected, sorted, washed, shredded, and melt‑extruded into pellets.
• Advantages: low energy compared with chemical routes; suitable for large‑scale use.
• Disadvantages: polymer chains are shortened (lower molecular weight), colour and contaminant build‑up limit the number of re‑use cycles.

3.2 Enzymatic (biological) recycling

• Specific PET‑hydrolase enzymes (e.g., PETase) cleave the ester bonds to give terephthalic acid and ethylene glycol under mild conditions.
• Advantages: low temperature, low‑energy, potentially high selectivity.
• Disadvantages: current enzymes work slowly; industrial scale‑up is still under research.

3.3 Economic and energy considerations

• Mechanical recycling uses ~30 % of the energy required for virgin PET production, but the quality of the recycled product degrades with each cycle.
• Chemical recycling (hydrolysis, glycolysis, methanolysis) needs higher temperatures (150–250 °C) but yields monomers of near‑virgin quality, allowing a true “closed‑loop”.
• Enzymatic routes promise the lowest energy input, but capital costs for enzyme production and reactor design are currently high.

4. Summary checklist (exam revision)

• Polymer = large molecule built from repeating monomer units.
• Monomer = the small building‑block molecule.
• Addition polymerisation: monomers contain C=C; example – ethene → poly‑ethene (repeat unit –CH₂–CH₂–).
• Condensation polymerisation: monomers contain two functional groups; example – PET from ethylene glycol + terephthalic acid (repeat unit –O–CH₂–CH₂–O–CO–C₆H₄–CO–).
• Structural isomerism (branching, head‑to‑tail) influences physical properties.
• Environmental problems of plastics: landfill, marine litter, incineration, resource consumption.
• PET can be chemically recycled by hydrolysis, glycolysis or methanolysis to give its original monomers (or direct precursors) which are then re‑polymerised to identical PET.
• Mechanical and enzymatic recycling are alternative routes; each has distinct advantages and limitations.
• Closed‑loop chemical recycling reduces waste, saves raw material and, despite higher temperature, can be more energy‑efficient than producing virgin PET from petroleum.