Part 2: Polymer hydrolysis and identifying monomers

Use the reaction medium to predict the correct acid, carboxylate, amine or ammonium products and evaluate degradability carefully.

A polymer link reacts like the corresponding small molecule

Ester and amide links can be hydrolysed under suitable conditions. Hydrolysis breaks the functional-group connections in the backbone and reduces chain length. Complete hydrolysis recovers the corresponding monomer-derived molecules, in acid–base forms determined by the medium.

For polyesters, acidic hydrolysis gives carboxylic acids and alcohols. Alkaline hydrolysis gives carboxylate salts and alcohols. A hydroxycarboxylic-acid polyester gives one monomer type rather than a separate diacid and diol.

Polyamide hydrolysis requires suitable heating with acid or alkali. Acidic conditions give carboxylic acid groups and protonated amine groups; alkaline conditions give carboxylate groups and neutral amine groups. Do not write a free amine as the only species in strongly acidic solution.

Worked reversal: mark the link before cutting the chain

Consider [–NH–CH₂–CH₂–NH–C(=O)–CH₂–C(=O)–]ₙ. The two amide links per repeat connect a diamine-derived fragment to a dicarboxylic-acid-derived fragment. Complete hydrolysis gives the skeletons H₂NCH₂CH₂NH₂ and HOOCCH₂COOH.

In hot aqueous HCl, the diamine is predominantly ⁺H₃NCH₂CH₂NH₃⁺ with chloride counterions, while the acid product is HOOCCH₂COOH. In hot NaOH, the products are H₂NCH₂CH₂NH₂ and NaOOCCH₂COONa.

The CH₂ within the diacid fragment remains between two carbonyl carbons. Cutting a neighbouring C–C bond instead of the amide link changes the carbon skeleton and cannot be justified by ordinary amide hydrolysis. Check that recombining the proposed monomers reconstructs the original repeat.

A hydrolysable bond is an opportunity, not a promise of rapid decay

Polyesters and polyamides contain bonds that can be broken by hydrolysis, unlike the all-carbon backbone of a simple polyalkene. This gives a chemical basis for discussing their potential degradation.

Actual biodegradation also depends on polymer structure, accessibility of the bonds, crystallinity, temperature, water, microorganisms and time. It is too broad to claim that every hydrolysable polymer will biodegrade rapidly in any environment. Some materials require particular industrial-composting conditions.

In an exam comparison, first identify the relevant bond and the possible chemical process. Then use any supplied evidence about the environment or rate. Distinguish “can undergo hydrolysis under suitable conditions” from “will quickly disappear when discarded”.

A repeat-unit decision process for unfamiliar polymers

First look for C(=O)–O or C(=O)–N in the backbone. These identify ester or amide links; a pendant ester group attached to an all-carbon chain does not automatically make the backbone a polyester.

Next identify the fragments between links and restore the appropriate terminal groups to obtain possible monomers. Decide whether one bifunctional monomer or a complementary pair is needed. Finally check the specified acid/base medium and assign product charges.

For a requested addition-polymer monomer, find the two-carbon backbone segment and restore a C=C while retaining side groups. Do not apply the condensation “cut at carbonyl” procedure to an addition polymer merely because a side chain contains oxygen.

Quick checks

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

Sources