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). | Monomers contain two different functional groups (e.g. -COOH, -OH, -NH2, -COCl). |
| 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
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).
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
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-