1. Condensation vs Addition Polymerisation
Polymers are giant macromolecules formed by linking thousands of small monomer units together:
| Feature | Addition Polymerisation (AS Unit 2) | Condensation Polymerisation (A2 Unit 4) |
|---|---|---|
| Monomer Requirements | Monomers contain a carbon-carbon double bond (C=C alkenes). | Each monomer has two functional groups that react: either two monomers each with two identical groups (e.g. a diacid with a diol or a diamine), or one monomer with two different groups (e.g. an amino acid). |
| Byproducts Formed | Zero byproducts: 100% atom economy. | A small molecule is eliminated at each linkage (usually H₂O or HCl). |
| Polymer Backbone | Non-polar, inert carbon-carbon chain (-C-C-C-C-). | Polar chain containing ester (-COO-) or amide (-CONH-) linkages. |
| Biodegradability | Non-biodegradable (chemically inert C-C bonds resist bacterial and enzymatic attack). | Biodegradable (polar linkages can be slowly hydrolysed by water and environmental enzymes). |
2. Polyesters: Terylene (PET)
Polyesters are formed when dicarboxylic acids react with diols (or from hydroxycarboxylic acids containing both groups):
n HOOC-C₆H₄-COOH + n HO-CH₂CH₂-OH → -[CO-C₆H₄-CO-O-CH₂CH₂-O]ₙ- + 2n H₂O
Applications: Synthetic textile clothing fibres (Terylene/Dacron), plastic beverage bottles, and food packaging trays.
3. Condensation Polymer Repeating Units Diagram
Each repeating unit has an open bond at each end. The ester or amide link is boxed. To find the monomers, break the link and add back -OH to the C=O and -H to the O or N.
4. Polyamides: Nylon 6,6 and Kevlar
Polyamides are formed when dicarboxylic acids (or acyl dichlorides) react with diamines (or from amino acids):
1. Nylon 6,6
Formed from hexanedioic acid (6 carbons) and hexane-1,6-diamine (6 carbons):
n HOOC-(CH₂)₄-COOH + n H₂N-(CH₂)₆-NH₂ → -[CO-(CH₂)₄-CO-NH-(CH₂)₆-NH]ₙ- + 2n H₂O
The amide links (-CONH-) form extensive intermolecular hydrogen bonds between adjacent polymer chains, giving Nylon high tensile strength, elasticity, and abrasion resistance (ropes, textiles, carpets).
2. Kevlar (High-Performance Aromatic Polyamide)
Synthesised from benzene-1,4-dicarboxylic acid and benzene-1,4-diamine:
n HOOC-C₆H₄-COOH + n H₂N-C₆H₄-NH₂ → -[CO-C₆H₄-CO-NH-C₆H₄-NH]ₙ- + 2n H₂O
Why Kevlar is Exceptionally Strong
- The planar benzene rings make the polymer chains extremely rigid and linear.
- These straight chains pack together tightly in parallel sheets.
- Immense numbers of regular intermolecular hydrogen bonds form between C=O and N-H groups on neighbouring chains.
- This produces an extraordinarily high tensile strength-to-weight ratio: Kevlar is 5 times stronger than steel on an equal-weight basis (used in bulletproof vests, aerospace composites, and puncture-resistant tyres).
Forces Between Chains
- Polyamides (nylon 6,6 and Kevlar): hydrogen bonds form between the N-H of one chain and the C=O of a neighbouring chain.
- Polyesters (Terylene): there is no N-H or O-H in the chain, so there is no hydrogen bonding between chains. The polar C=O groups give permanent dipole-dipole forces between chains.
Uses
| Polymer | Uses |
|---|---|
| Terylene | Clothing fibres, and bottles (as PET) |
| Nylon | Ropes and clothing |
| Kevlar | Bulletproof vests and helmets. Its strength comes from strong hydrogen bonds between the chains. |
5. Polymer Hydrolysis & Disposal
Hydrolysis of Condensation Polymers
Unlike addition polymers, polyesters and polyamides can be chemically cleaved by hydrolysis under reflux:
- Acid Hydrolysis (hot aqueous HCl):
Polyester → original dicarboxylic acid + diol
Polyamide → original dicarboxylic acid + diamine salt (e.g. Cl⁻ ⁺H₃N-(CH₂)₆-NH₃⁺ Cl⁻) - Alkaline Hydrolysis (hot aqueous NaOH):
Polyester → dicarboxylate salt (e.g. Na⁺⁻OOC-R-COO⁻Na⁺) + diol
Polyamide → dicarboxylate salt + diamine
Biodegradability
- Polyesters and polyamides can be hydrolysed at the ester or amide link. So they are biodegradable.
- Polyalkenes (addition polymers such as poly(ethene)) have only non-polar C-C and C-H bonds. There is nothing for water or nucleophiles to attack. So they are not biodegradable.
Disposal of Polymers
| Method | Advantages | Disadvantages |
|---|---|---|
| Landfill | Cheap. | Uses land. Non-biodegradable polymers last for centuries. |
| Incineration | Reduces the volume of waste. Energy from burning can be recovered. | Makes CO₂. Toxic gases (for example HCl from PVC) must be removed. |
| Recycling | Saves raw materials and energy. | Sorting is costly. The recycled polymer can be lower quality. |
6. Worked Repeating Unit Calculations
-[CO-(CH₂)₈-CO-NH-(CH₂)₄-NH]ₙ-
Step 1: Locate the amide linkages:
Break the -CO-NH- bonds to restore the monomer functional groups.
Step 2: Add -OH to carbonyl groups to reform carboxylic acids:
HO-CO-(CH₂)₈-CO-OH = decanedioic acid (10 carbons).
Step 3: Add -H to nitrogen atoms to reform amines:
H-NH-(CH₂)₄-NH-H = butane-1,4-diamine (4 carbons).
Draw the repeating unit of PLA.
Step 1: Identify reacting functional groups:
The single monomer contains both a carboxyl group (-COOH) and an alcohol group (-OH).
Step 2: Eliminate H₂O:
Loss of -OH from -COOH and -H from the alcohol group links monomers via ester bonds.
Step 3: Draw repeating unit with trailing bonds:
-[O-CH(CH₃)-CO]ₙ-
Go deeper
These UK AQA pages cover the same ideas in more detail. They may include content that is not in 9620.
7. Practice Questions
Show Answer & Mark Scheme
Answer: -[O-CH₂CH₂-O-CO-(CH₂)₄-CO]ₙ-
- Ester links correct, with the C=O drawn (1).
- Correct repeating unit with an open bond at each end (1).
Show Answer & Mark Scheme
Answer:
- (a) Hydrogen bonds, between N-H and C=O on neighbouring chains (1).
- (b) Permanent dipole-dipole forces (1).
Show Answer & Mark Scheme
Answer:
- Nylon 6,6 has amide links, which can be hydrolysed (1).
- Poly(ethene) has only non-polar C-C and C-H bonds (1).
- There is nothing for water or nucleophiles to attack (1).
Show Answer & Mark Scheme
Answer:
- Advantage: reduces the volume of waste, or energy can be recovered (1).
- Disadvantage: makes CO₂, or toxic gases such as HCl (from PVC) must be removed (1).