Hydrolysis of Polymers and Comparison

Spec Mapping — OCR A-Level Chemistry A (H432) Module 6.2.5 (c) — Hydrolysis and comparison of polymers, covering: hydrolysis of polyesters (acid hydrolysis — dilute HCl or H₂SO₄, reflux; base hydrolysis — NaOH (aq), reflux) and polyamides (acid hydrolysis — 6 mol dm⁻³ HCl, prolonged reflux; base hydrolysis — NaOH (aq), prolonged reflux) regenerating their original monomers (diacid + diol or diacid salt + diamine); the contrast with addition polymers, whose all-C–C backbones make them essentially non-hydrolysable and environmentally persistent; the economic and environmental implications, including mechanical recycling (clean-stream sorting + remelting; thermal degradation limits the number of cycles), chemical recycling / feedstock recycling (depolymerisation back to monomers, viable for PET, nylon), incineration with energy recovery (CO₂ + H₂O for hydrocarbons; HCl from PVC requires alkaline scrubbing of flue gases), and landfill as a last resort; and biological hydrolysis of natural polyamides (proteins) by proteases at 37 °C and pH 7 as a striking example of enzymatic catalysis (refer to the official OCR H432 specification document for exact wording).

Now that you have seen both types of polymer — addition (from alkene monomers, no by-product, C–C backbone) and condensation (from difunctional monomers, ester/amide linkages, water/HCl by-product) — it is time to compare them quantitatively. The critical difference is how they behave in the environment and how chemists dispose of them. Condensation polymers can be hydrolysed back to their original monomers under acidic, basic or (for proteins) enzymatic conditions, which means they are biodegradable (slowly in nature, fast in industrial chemical-recycling reactors) and recyclable (the recovered monomers can be repurified and re-polymerised into fresh polymer). Addition polymers are not hydrolysable; their all-C–C backbones are kinetically inert under environmental conditions, so they persist in the environment for decades to centuries.

This lesson covers the OCR A-Level Chemistry A (H432) specification point 6.2.5 (c): hydrolysis of polyesters and polyamides, and comparison of biodegradability between condensation and addition polymers. It builds on Lesson 4 (ester hydrolysis), Lesson 8 (amide / peptide hydrolysis) and Lesson 9 (condensation polymerisation) — applying the same chemistry on a polymer scale — and sets up the comparison with addition polymers met in OCR's basic-organic module (poly(ethene), poly(propene), PVC, polystyrene, PTFE). The economic stakes are enormous: global plastic production is over 400 million tonnes per year, less than 10% of which is recycled, with the difference between polymer types fundamentally determining the path forward for a sustainable plastics economy.

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1. Hydrolysis of Polyesters

Because a polyester contains ester linkages, the same hydrolysis conditions that work for a single ester (Lesson 4) will break down the polymer chain. At each ester linkage, water adds across the –CO–O– bond, cutting the chain in two.

1.1 Acid hydrolysis

$-[CO-R-CO-O-R'-O]_n- + 2n H_2O (H^+) \longrightarrow n HOOC-R-COOH + n HO-R'-OH$−[CO−R−CO−O−R′−O]n​−+2nH2​O(H+)⟶nHOOC−R−COOH+nHO−R′−OH

• Conditions: Dilute H₂SO₄ or HCl, prolonged reflux.
• Products: The original dicarboxylic acid and diol (in their free forms).
• Reversible: Use a large excess of water to drive hydrolysis forwards.

For PET:

$-[CO-C_6H_4-CO-O-CH_2CH_2-O]_n- + 2n H_2O \longrightarrow n HOOC-C_6H_4-COOH + n HO-CH_2CH_2-OH$−[CO−C6​H4​−CO−O−CH2​CH2​−O]n​−+2nH2​O⟶nHOOC−C6​H4​−COOH+nHO−CH2​CH2​−OH

1.2 Base hydrolysis

$-[CO-R-CO-O-R'-O]_n- + 2n NaOH \longrightarrow n NaOOC-R-COONa + n HO-R'-OH$−[CO−R−CO−O−R′−O]n​−+2nNaOH⟶nNaOOC−R−COONa+nHO−R′−OH

• Conditions: Aqueous NaOH, reflux.
• Products: The sodium salt of the diacid + the diol.
• Irreversible.

This is how polyester clothing is chemically recycled — industrial processes using methanol (methanolysis) or hot alkaline solutions break PET back into its monomers, which can be repurified and reused to make new PET. This is known as chemical recycling and is a genuinely circular approach compared with mechanical recycling (which just shreds and remelts, degrading the polymer).

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2. Hydrolysis of Polyamides

Polyamides contain amide linkages, which — as you saw in Lesson 8 — are more resistant to hydrolysis than esters because the N lone pair donates into the C=O. Harsher conditions are therefore required.

2.1 Acid hydrolysis

$-[CO-R-CO-NH-R'-NH]_n- + 2n H_2O (H^+) \longrightarrow n HOOC-R-COOH + n ^+H_3N-R'-NH_3^+$−[CO−R−CO−NH−R′−NH]n​−+2nH2​O(H+)⟶nHOOC−R−COOH+n+H3​N−R′−NH3+​

• Conditions: Concentrated HCl (6 mol dm⁻³ or stronger), prolonged reflux (hours).
• Products: Diacid + diamine (as a salt — the diamine is protonated in the acidic medium).

2.2 Base hydrolysis

$-[CO-R-CO-NH-R'-NH]_n- + 2n NaOH \longrightarrow n NaOOC-R-COONa + n H_2N-R'-NH_2$−[CO−R−CO−NH−R′−NH]n​−+2nNaOH⟶nNaOOC−R−COONa+nH2​N−R′−NH2​

• Conditions: Aqueous NaOH, prolonged reflux.
• Products: Sodium salt of the diacid + free diamine.

2.3 Biological hydrolysis

In the body, proteins (which are polyamides) are hydrolysed by protease enzymes in the stomach (pepsin) and small intestine (trypsin, chymotrypsin). These enzymes catalyse hydrolysis at physiological pH and 37 °C — conditions far milder than the concentrated acid or base needed in the lab. This is a striking example of the power of enzyme catalysis.

graph TD
  A[Polyamide or polyester] --> B[Acid hydrolysis: conc acid, reflux]
  A --> C[Base hydrolysis: NaOH, reflux]
  A --> D[Enzymatic hydrolysis: proteases, at 37 deg C and pH 7]
  B --> E[Monomers recovered for recycling]
  C --> E
  D --> F[Amino acids used for metabolism]

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3. Why Can Condensation Polymers Be Hydrolysed, But Not Addition Polymers?

3.1 Addition polymers — all C–C

An addition polymer has a backbone made entirely of C–C single bonds:

$$-CH2-CH2-CH2-CH2-CH2-CH2- ...$$−CH2−CH2−CH2−CH2−CH2−CH2−...

C–C bonds are:

• Non-polar (C and C have the same electronegativity).
• Strong (~347 kJ mol⁻¹).
• Kinetically stable — there is no good way for water or most other nucleophiles to insert into a C–C bond.

Consequently, addition polymers like poly(ethene), poly(propene) and PVC are essentially unreactive under normal environmental conditions. They persist in the environment for centuries and do not break down naturally.

3.2 Condensation polymers — polar C=O next to O or N

A condensation polymer's backbone contains polar –CO–O– or –CO–NH– linkages. At each of these:

• The C of C=O is δ+, so it can be attacked by nucleophiles (OH⁻, H₂O, enzymes).
• The linkage can be hydrolysed back to the original monomers under appropriate conditions.
• Enzymes have evolved to catalyse this hydrolysis in living systems.

The net effect: condensation polymers can be broken down, recycled or digested, while addition polymers cannot.

3.3 Summary comparison table

Feature	Addition polymer	Condensation polymer
Example	Poly(ethene), PVC	PET, nylon-6,6, proteins
Monomers	Alkene (one type)	Two difunctional monomers, or an amino acid
By-product	None	Water or HCl
Backbone bonds	C–C (non-polar)	C–C plus polar C–O and C–N
Backbone reactivity	Unreactive	Susceptible to hydrolysis
Biodegradability	Poor	Much better
Recyclability	Mechanical only (degrades)	Chemical recycling possible
Environmental persistence	Decades to centuries	Faster breakdown

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4. Environmental Implications

4.1 The problem with addition polymers

Addition polymers dominate global plastic production — poly(ethene) (PE), poly(propene) (PP), polystyrene (PS), polyvinyl chloride (PVC) and poly(ethylene terephthalate) (PET, which is actually a condensation polymer) together account for 80%+ of all plastic made. The majority are addition polymers, and because they cannot be hydrolysed:

• They persist in the environment. A plastic bottle in the ocean may take 400+ years to break down.
• Microplastics accumulate. UV light and mechanical abrasion eventually fragment plastics into particles that enter the food chain.
• Landfill mass builds up. Plastic accounts for ~12% of municipal solid waste by mass.
• Recycling is limited. Thermoplastics can be remelted, but mechanical recycling degrades the polymer (shortens chains), limiting how many cycles are possible.

4.2 Advantages of condensation polymers

Condensation polymers offer several routes to a more sustainable plastic economy:

• Chemical recycling. PET can be hydrolysed back to terephthalic acid and ethylene glycol, which can be repurified and reused. This is genuinely circular.
• Biodegradability. Polyesters like poly(lactic acid) (PLA), made from renewable starch-derived lactic acid, degrade in industrial composters.
• Natural integration. Proteins and cellulose (a natural polysaccharide) are condensation polymers that biodegrade completely into CO₂ and water under normal environmental conditions.

4.3 Where things stand in 2026

Bioplastics (PLA, PHA, PBAT) are a growing fraction of the plastic market and are now used in packaging, single-use cutlery and medical sutures. But the bulk of plastic production is still addition polymers because they are:

• Cheaper to make from oil.
• Stronger, clearer and more versatile.
• Better at food-contact barrier properties.

The trend is towards a mix: addition polymers for durable applications (pipes, insulation, car parts) and condensation polymers / bioplastics for single-use items that will enter the waste stream. Chemical recycling of PET is now happening at industrial scale in several countries.

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5. A Worked Example: Recovering Terephthalic Acid from PET Bottles

A waste PET bottle is ground to small flakes and treated with aqueous NaOH at 200 °C. The ester linkages hydrolyse to give a solution of:

• Disodium terephthalate (NaOOC–C₆H₄–COONa): soluble in the alkaline solution.
• Ethane-1,2-diol (HO–CH₂CH₂–OH): also soluble.

After cooling and filtration, the solution is acidified with HCl. Terephthalic acid crystallises out as a white solid (insoluble in water) and is filtered off. The ethylene glycol stays in solution and can be recovered by distillation. Both monomers can now be re-polymerised into fresh PET.

This kind of process is called depolymerisation or chemical recycling, and it is exactly the reverse of the condensation polymerisation you saw in Lesson 9. It is possible only because of the hydrolysable ester linkages.

5.1 The four disposal routes — a comparative view

OCR expects you to discuss four disposal options for polymers, with environmental and economic trade-offs:

1. Mechanical recycling. Polymer is shredded, washed, melted and re-extruded. Cheap but degrades the polymer (chain scission shortens average molecular weight) — typically 3-5 cycles before mechanical properties decline below useful thresholds. Works for both addition and condensation polymers if the waste stream is clean. PET drinks bottles are a successful example (~50% recovery rate in the UK as of 2026).
2. Chemical recycling (feedstock recycling). Polymer is depolymerised back to its monomers (or to small petrochemical-feedstock molecules), repurified, and re-polymerised. Genuinely circular: the output is indistinguishable from virgin material. Economically viable for PET, nylon, polyurethanes; tough for polyolefins (which are addition polymers). Requires the polymer to have hydrolysable linkages — hence applies to condensation polymers only.
3. Incineration with energy recovery. Polymer is burned at 800-1000 °C in an energy-from-waste (EfW) plant. Hydrocarbons (PE, PP, PS) give CO₂ + H₂O. PVC additionally gives HCl, which must be removed by alkaline scrubbing of flue gases (NaOH or Ca(OH)₂ neutralises HCl to NaCl or CaCl₂ + H₂O). Heat is recovered as electricity or district heating. Wastes the carbon (it ends up as CO₂) but does extract some energy value.
4. Landfill. Polymer is buried. The cheapest option and the most environmentally damaging — the carbon is "stranded" out of the carbon cycle, the polymer may leach plasticisers and additives into groundwater, and addition polymers persist for centuries. The UK Government's Resources and Waste Strategy aims to eliminate landfill of plastic waste by 2042.

The hierarchy (top = best, bottom = worst) is roughly: chemical recycling > mechanical recycling > incineration with energy recovery > landfill. Different polymers map onto different points in this hierarchy based on their chemistry.

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6. Enzyme-Catalysed Hydrolysis — the Biological Solution

In the body, proteins (polyamides) are hydrolysed by protease enzymes in the stomach (pepsin) and small intestine (trypsin, chymotrypsin), and pancreatic lipase hydrolyses dietary fats (esters). These enzymes catalyse hydrolysis at physiological pH 7 and 37 °C — conditions far milder than the concentrated acid or base needed in the lab.

The rate enhancement is extraordinary: without an enzyme, peptide-bond hydrolysis at pH 7 and 37 °C has a half-life of ~500 years; a protease reduces this to milliseconds, an acceleration of $10^{11}$1011 to $10^{12}$1012. The mechanism positions a water molecule (or serine nucleophile) precisely against the peptide carbonyl and stabilises the tetrahedral transition state via the "oxyanion hole" hydrogen-bonding network.

Biotechnology exploits this for plastic recycling: PET-degrading enzymes (PETase + engineered LCC-ICCG variants) hydrolyse polyester at 50-70 °C, below the 200 °C of alkaline hydrolysis. The same chemistry that digests proteins now digests plastic waste.

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Synoptic Links

Synoptic Links — Connects to:

• ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / esters-esterification-hydrolysis (Lesson 4 — single ester hydrolysis is the molecular-scale version of polyester hydrolysis here; same mechanism, polymer-chain context).
• ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / peptides-amides (Lesson 8 — single amide / peptide hydrolysis is the molecular-scale version of polyamide hydrolysis here; the resistance argument is the same N-lone-pair-into-C=O delocalisation).
• ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / condensation-polymers (Lesson 9 — these are the polymers being hydrolysed; the products of hydrolysis are precisely the difunctional monomers that condensation polymerisation joined together).
• ocr-alevel-chemistry-basic-organic / addition-polymerisation-polymer-disposal (OCR basic organic Lesson 10 — addition polymers and their disposal options; this lesson extends the disposal discussion to polymer chemistry as a whole).
• ocr-alevel-chemistry-acids-redox-bonding / acids-and-bases (the Brønsted–Lowry framework explains why protonated diamines and deprotonated dicarboxylates appear as products depending on acidic or basic conditions).
• ocr-alevel-chemistry-enthalpy-rates-equilibrium (the kinetic argument — enzymes catalyse a thermodynamically downhill but kinetically slow reaction — is rate-equilibrium territory).
• ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / chromatography (Lesson 12 — TLC and HPLC are routine analytical techniques for monitoring the progress of polymer hydrolysis).

Practical Activity Group anchor: PAG 6 (synthesis) + PAG 7 (qualitative analysis). A PAG 6 + PAG 7 combined activity might hydrolyse a sample of nylon-6,6 by refluxing with 6 M HCl for several hours, neutralise to pH 6, and then use TLC + ninhydrin staining to identify the hydrolysis products (hexanedioic acid and hexane-1,6-diamine; the diamine gives a purple-blue ninhydrin spot, the diacid does not). The same protocol applied to a sample of PET would give a paper hydrolysate from which terephthalic acid (white solid, recrystallises) and ethylene glycol (in solution) can be recovered — a school-bench analogue of industrial chemical recycling.

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Specimen question modelled on the OCR H432 paper format

Question (9 marks): PET (the polyester from benzene-1,4-dicarboxylic acid + ethane-1,2-diol) and poly(ethene) (the addition polymer from ethene) are two of the most widely used plastics in the world.

(a) Write equations for the acid hydrolysis of PET, identifying the conditions and products. Explain in terms of the polymer backbone why PET can be hydrolysed at all. (3 marks)

(b) Explain why poly(ethene) is essentially non-hydrolysable, despite being thermodynamically less stable than PET. Use this to predict the relative environmental persistence of the two polymers. (3 marks)

(c) Discuss the four main disposal routes for waste plastic — mechanical recycling, chemical (feedstock) recycling, incineration with energy recovery, and landfill — and which polymer types each route is most suited to. (3 marks)

AO breakdown