A Level Chemistry: Condensation Polymerisation — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Condensation Polymerisation revision notes

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Questions

1. State the essential difference between addition and condensation polymerisation. [2]

2. Explain why each monomer in a condensation polymerisation must have two functional groups. [2]

3. A polyester is made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol.

(a) Name the linkage formed and the small molecule eliminated. [2] (b) Draw or describe the repeat unit. [2]

4. Nylon-6,6 is made from hexanedioic acid and 1,6-diaminohexane.

(a) Name the linkage formed. [1] (b) Explain what the numbers 6,6 refer to. [1] (c) Explain why polyamides are stronger than polyesters. [3]

5. Kevlar, made from benzene-1,4-dicarboxylic acid and 1,4-diaminobenzene, has an exceptional strength-to-weight ratio.

(a) Name the type of linkage formed between the monomers. [1] (b) Explain, in structural terms, why it is so strong. [3]

6. Give the products of hydrolysing a polyamide under (a) acidic and (b) alkaline conditions. [4]

7. Explain why condensation polymers are biodegradable but addition polymers are not. [4]

8. Compare addition and condensation polymerisation on atom economy, explaining the difference. [3]

9. A biodegradable surgical suture is made by the self-condensation of a single monomer, 6-aminohexanoic acid, H₂N–(CH₂)₅–COOH.

(a) Explain why this single monomer, unlike the diol/diacid or diamine/diacid pairs seen so far, is able to undergo condensation polymerisation on its own. [2] (b) Deduce the repeat unit of the resulting polymer, and name the linkage formed. [3]

10. Methyl 2-methylpropenoate, CH₂=C(CH₃)–COOCH₃, is the monomer used to make Perspex. The molecule already contains an ester group as well as a C=C double bond.

(a) State, with a reason, whether this monomer polymerises by addition or condensation. [2] (b) Explain why the ester group already present in the monomer takes no part in the polymerisation reaction. [2]

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Answers

1. In addition polymerisation the monomers join with no other product [1]; in condensation polymerisation a small molecule such as water or HCl is eliminated at each linkage [1].

2. The chain must be able to grow at both ends [1]; a monomer with only one reactive group would terminate the chain [1].

3. (a) An ester linkage (–COO–) [1]; water is eliminated [1]. (b) –[OOC–C₆H₄–COO–CH₂CH₂]– with bonds extending through the brackets and n outside [1] [1].

4. (a) An amide linkage (–CONH–) [1]. (b) Each monomer contains six carbon atoms [1]. (c) Amide groups form hydrogen bonds between adjacent chains [1], because N–H and C=O are both present [1]. Ester groups can only form weaker permanent dipole–dipole attractions, since there is no N–H [1].

5. (a) An amide linkage (–CONH–) [1]. (b) The rigid benzene rings hold the chains straight and allow them to align closely [1]. Extensive hydrogen bonding between the amide groups on adjacent chains locks them together [1], and the regular aligned structure distributes stress efficiently along the chains [1].

6. (a) Acid: a carboxylic acid [1] and an ammonium salt (the protonated amine) [1]. (b) Alkali: a carboxylate salt [1] and a free amine [1].

7. Condensation polymers contain polar C–O or C–N bonds in the linkage [1], which can be hydrolysed by water and attacked by enzymes [1]. Addition polymers have a non-polar C–C backbone [1] with no bonds that water or enzymes can attack [1].

8. Addition polymerisation has 100% atom economy because all the atoms of the monomer end up in the polymer [1]. Condensation is below 100% [1] because a small molecule is eliminated as a second product, so not all the reactant mass becomes polymer [1].

9. (a) The single monomer carries two different reactive functional groups, an amine and a carboxylic acid [1], so the –NH₂ end of one molecule can react with the –COOH end of the next, and the reaction can repeat at both ends of the growing chain without needing a second monomer [1]. (b) The repeat unit is –[NH–(CH₂)₅–CO]– with bonds extending through the brackets and n outside [2]; the linkage formed is an amide linkage, so the product is a form of nylon (nylon-6) [1].

10. (a) Addition [1]. A C=C double bond is present, and any monomer containing a C=C bond polymerises by addition across that bond, regardless of what other functional groups are also present [1]. (b) Addition polymerisation only involves the breaking of the C=C π bond, which links the monomers directly to each other [1]; the ester group is not itself a site of reaction in addition polymerisation, so it survives unchanged as a pendant side group hanging off the finished polymer backbone [1].

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Where marks are usually lost

• Forgetting to show the eliminated small molecule.
• Saying polyesters hydrogen bond between chains.
• Giving the wrong hydrolysis products for the stated conditions.
• Explaining Kevlar’s strength without both chain alignment and hydrogen bonding.
• Assuming a single-monomer polymer such as nylon-6 must be an addition polymer because only one type of molecule is involved — condensation only requires two reactive groups, not two different monomers.
• Missing that the repeat unit of a self-condensed amino acid still loses water at every linkage, exactly as a diol/diacid pair does.
• Classifying methyl 2-methylpropenoate as a condensation monomer purely because it contains an ester group, rather than checking for a C=C bond first — the presence of C=C always signals addition polymerisation, whatever else the molecule contains.
• Forgetting that a functional group not directly involved in the polymerisation reaction (such as the ester in methyl 2-methylpropenoate) survives unreacted as a pendant group on the finished chain, rather than being eliminated.

Deducing monomers from a repeat unit, or predicting the type of polymerisation from a monomer’s structure, both come down to the same check: look for a C=C bond first (addition, whatever else is present), and if there isn’t one, look for the two reactive functional groups at each end of the repeating section (condensation, whether those two ends came from one monomer or two). For the underlying reasoning in full, see the Condensation Polymerisation revision notes and the Condensation Polymerisation and Polymer Degradability study guide.