Condensation polymersAQA A-Level Chemistry: Revision notes

In condensation polymerisation monomers join by reaction between two different functional groups, and a small molecule (usually water) is lost each time a link forms. Each monomer needs two functional groups so that the chain can grow at both ends.

Three combinations are in the specification:

• dicarboxylic acid + diol gives a polyester
• dicarboxylic acid + diamine gives a polyamide
• amino acid (one molecule has both –NH₂ and –COOH) gives a polyamide

The repeating unit contains atoms from both monomers but not the small molecule that is lost.

A polyester has ester links, –COO–, formed when a –COOH group reacts with an –OH group.

Terylene (poly(ethylene terephthalate)) forms from benzene-1,4-dicarboxylic acid and ethane-1,2-diol:

n HOOC–C₆H₄–COOH + n HOCH₂CH₂OH → [–OC–C₆H₄–CO–OCH₂CH₂O–]ₙ + 2n H₂O

Typical uses: clothing fibres, sailcloth, and plastic drinks bottles.

A polyamide has amide links, –CONH–, formed when a –COOH group reacts with an –NH₂ group.

• Nylon-6,6 forms from hexanedioic acid, HOOC(CH₂)₄COOH, and 1,6-diaminohexane, H₂N(CH₂)₆NH₂ (each has six carbons). Used in ropes, carpets, clothing and gears.
• Kevlar forms from benzene-1,4-dicarboxylic acid and benzene-1,4-diamine. Used in body armour, helmets and tyre reinforcement.

n HOOC(CH₂)₄COOH + n H₂N(CH₂)₆NH₂ → [–OC(CH₂)₄CO–NH(CH₂)₆NH–]ₙ + 2n H₂O

An amino acid, H₂N–CHR–COOH, contains both functional groups. The –NH₂ of one molecule reacts with the –COOH of the next, losing water and forming an amide (peptide) link.

Repeating unit: –NH–CHR–CO–.

Example: 6-aminohexanoic acid, H₂N(CH₂)₅COOH, gives nylon-6 with repeating unit –NH(CH₂)₅CO–. The repeating unit has the formula of the monomer minus H₂O.

From monomers: write the two monomers, remove OH from each –COOH and H from each –OH or –NH₂, and join the groups. Draw the unit in brackets with bonds extending through the links at each end.

From a section of chain: find the ester or amide links. Break the C–O (ester) or C–N (amide) bond, then add –OH to the carbonyl carbon and –H to the O or N.

Worked example: –NH(CH₂)₆NH–CO(CH₂)₄CO–NH(CH₂)₆NH–CO(CH₂)₄CO– breaks at each C–N to give H₂N(CH₂)₆NH₂ and HOOC(CH₂)₄COOH.

Polyamides (nylon-6,6, Kevlar): the N–H group is polar, and so is C=O. Hydrogen bonds form between the N–H hydrogen of one chain and the lone pair on the C=O oxygen of a neighbouring chain, as well as London forces.

Polyesters (Terylene): the chain contains C=O but no N–H or O–H, so there is no hydrogen bonding between chains. They have permanent dipole–dipole forces (Cδ+ to Oδ–) and London forces.

Kevlar is exceptionally strong because its chains are rigid and straight (benzene rings) and line up closely, so many hydrogen bonds form.