Hydrolysis of Polymers and Comparison

Now that you have seen both types of polymer — addition and condensation — it is time to compare them. 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, which means they are biodegradable (slowly) and recyclable. Addition polymers are not hydrolysable and persist in the environment for 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.

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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- ...

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