A food packaging company is developing a new biodegradable material to replace single-use plastics. They decide to use condensation polymerisation to produce a polyester from two different monomers: a diol and a dicarboxylic acid. Explain how the structure of these monomers allows condensation polymerisation to occur, and describe the key features of the polyester polymer that is formed.

OCR A-Level Chemistry B: Salters (H433) — 6.2 Nitrogen compounds, polymers and synthesis · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Condensation polymerisation is used to make polyesters from monomers containing two functional groups. A small molecule is lost each time two monomers join together.

Model answer (5 marks)

A diol has two –OH groups, one at each end, and a dicarboxylic acid has two –COOH groups, one at each end. The –OH of the diol reacts with the –COOH of the acid to give an ester bond and water is eliminated:

HO–R–OH + HOOC–R'–COOH → –O–C(=O)–R'–C(=O)–O–R– + H₂O

Because each monomer carries two reactive groups, the new ester linkage is formed at both ends of the growing chain, allowing the chain to extend in both directions and produce a long‑chain polyester. The repeating unit of the polymer is therefore –O–C(=O)–R'–C(=O)–O–, an ester linkage in the backbone.

Key features of the polyester: a linear backbone of ester bonds, high density of –COO– groups, and the ability to be biodegradable due to the ester linkages that can be hydrolysed.

Examiner tips

  • Show the two functional groups on each monomer and explain the reaction step; include the loss of water; mention chain growth in both directions; state the ester linkage in the repeating unit; keep answer concise and use correct terminology.

Common mistakes

  • Forgetting to show that each monomer has two functional groups; confusing the ester linkage with an ether; not mentioning the loss of water; writing the repeating unit incorrectly (e.g. missing one –COO–).

Mark scheme (5 marks)

  1. Each monomer must have two functional groups (one at each end of the molecule)
  2. The two different functional groups (–OH and –COOH) react together to form a bond between monomers
  3. A small molecule (water) is lost / eliminated each time two monomers join
  4. Because each monomer has two functional groups, the chain can extend in both directions to form a long-chain polymer
  5. The resulting polyester contains an ester linkage / –COO– repeating unit in its backbone

Key terms in this question

condensation polymerisation · monomer · polyester

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