Part 6: Polyesters: repeat units, condensation and hydrolysis

Build correct polyester repeat units from functional groups, recover their monomers and relate the ester links to hydrolysis without overclaiming biodegradability.

Two reacting ends make continued chain growth possible

A condensation polymer forms when monomers join through reactions that eliminate small molecules such as water or HCl. A polyester contains repeated –C(=O)–O– links. A diol and a dicarboxylic acid each have two reacting functional groups, so after one link forms there are still groups available at the ends for chain growth.

A monofunctional alcohol and a monofunctional acid usually give a small ester rather than a long chain. Alternatively, a hydroxycarboxylic acid contains both OH and COOH in the same molecule and can self-condense. Condensation therefore does not always require two different monomer species.

A diacyl chloride can replace the diacid and react with a diol, releasing HCl. The more reactive derivative can give rapid formation under suitable conditions, but its moisture sensitivity and corrosive HCl are important practical differences.

Worked repeat-unit construction

Take HOCH₂CH₂OH and HOOCCH₂CH₂COOH. Keep the carbon skeletons. Remove H from each alcohol OH and OH from each acid COOH at the links, then join alcohol O to carbonyl C. A repeat unit is [–O–CH₂–CH₂–O–C(=O)–CH₂–CH₂–C(=O)–]ₙ.

The bond leaving the right-hand carbonyl carbon continues to the O at the start of the next unit; its valency is then four. Draw brackets through the two continuation bonds and put n outside. Do not cap both ends of a repeat unit with OH or H unless you are drawing a specific finite molecule with end groups.

Check both ester oxygens: every –C(=O)–O– link has a carbonyl oxygen and a single-bonded oxygen. Accidentally writing –C(=O)–CH₂– in place of the oxygen creates a different polymer. The benzene-1,4-dicarboxylic-acid/ethane-1,2-diol polyester similarly has [–OCH₂CH₂O–C(=O)–C₆H₄–C(=O)–]ₙ, with the two ring substituents in para positions.

One monomer can supply both groups

For 2-hydroxypropanoic acid, HOCH(CH₃)COOH, the repeat unit is [–O–CH(CH₃)–C(=O)–]ₙ. The methyl group remains a side chain; it does not disappear into the polymer backbone. The acid and alcohol groups on different molecules form ester links.

To recover the monomer from a repeat unit, locate each ester acyl C–O bond, split at that bond, return OH to the carbonyl carbon and H to the ester oxygen. For a single hydroxy-acid unit these groups reappear on the same molecule.

This reverse-construction method is more secure than guessing from a polymer name. Count carbons before and after: condensation removes small molecules but never removes the carbon skeleton of either monomer.

Count links when a finite molecule is specified

Every ester link made from COOH and OH releases one H₂O. For a single linear molecule made from n diacid molecules and n diol molecules, there are 2n monomer molecules and 2n − 1 links, leaving two terminal functional groups. Thus it releases 2n − 1 water molecules if no cycles or branches form.

For n molecules of a hydroxy acid forming one linear chain, there are n − 1 links and n − 1 waters. A repeat-unit equation often uses an idealised n-based convention that omits end groups; do not apply its water count blindly to a short explicitly drawn chain.

Original worked check: four diacid molecules plus four diols give eight connected monomer units, seven links and seven H₂O molecules for one linear chain. Seven waters contain fourteen H atoms and seven O atoms; these must be missing from the finite product relative to the starting monomers.

An ester backbone provides a chemical route to chain cleavage

Heating a polyester with aqueous acid hydrolyses its ester links to the diacid/diol or hydroxy-acid building blocks. Aqueous alkali gives carboxylate salts plus alcohol groups; acidification then produces COOH. Each cleaved linkage reduces chain length and changes the material’s properties.

The polar ester link provides a reactive site for hydrolysis that a simple C–C addition-polymer backbone lacks. Nevertheless, “contains ester links” does not guarantee rapid biodegradation in soil or seawater. Accessibility of links, water uptake, temperature, crystallinity and enzymes all affect the rate.

For the ethane-1,2-diol/butanedioic-acid polyester above, complete alkaline hydrolysis gives ethane-1,2-diol and butanedioate ions. Each diacid-derived unit ultimately requires two OH⁻ equivalents to form its two carboxylate groups. Identify the specified medium before choosing acid or salt products.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official Edexcel mark allocations.

Sources