PET: a polyester template

Protonation, attack of the glycol OH, proton transfers, loss of water and deprotonation form each ester link of poly(ethylene terephthalate), [–OCH₂CH₂O–CO–C₆H₄–CO–]n.

Mechanism step by step

Step growth by acid-catalysed esterification of terephthalic acid with ethylene glycol

1. Protonation – Acid activates the carbonyl
2. Addition – The glycol OH attacks the carbonyl carbon
3. Proton transfer 1 – Water takes H⁺ from the new O
4. Proton transfer 2 – An OH takes H⁺: water becomes a leaving group
5. Loss of water – The OH lone pair pushes out H₂O
6. Deprotonation – The ester link; H⁺ is regenerated

Key points

• Every step is reversible: water (or ethylene glycol) must be removed under vacuum to reach high molar mass.
• Industrially PET is made by transesterification of dimethyl terephthalate or by direct esterification, then polycondensation over Sb₂O₃.
• The rigid para-phenylene rings make PET stiff and crystallisable (Tg ≈ 75 °C, Tm ≈ 260 °C): bottles and polyester fibre.
• Like nylon-6,6, chain length follows the Carothers equation and needs an exact 1:1 balance of functional groups.

Drawn with 130 atoms, 19 arrows, 14 shapes – every one editable in MolDraw.

Sources

• Odian, Principles of Polymerization, 4th ed., Wiley 2004, ch. 2 Textbook
• Young & Lovell, Introduction to Polymers, 3rd ed., CRC Press 2011 Textbook