Addition Polymers
How Addition Polymerisation Works
Monomers containing a C=C double bond link together by opening their π bonds to form new C−C single bonds. The entire monomer is incorporated → 100% atom economy, no by-products.
Drawing repeat units: Redraw the monomer with C=C changed to C−C, extend bonds from each side, and place inside square brackets with "n".
| Monomer | Polymer | Uses |
|---|---|---|
| Ethene (CH₂=CH₂) | Poly(ethene) | Bags, bottles |
| Chloroethene (CH₂=CHCl) | PVC | Pipes, insulation |
| Propene (CH₂=CHCH₃) | Polypropene | Crates, ropes |
| Tetrafluoroethene (CF₂=CF₂) | PTFE (Teflon) | Non-stick coatings |
| Phenylethene (CH₂=CHC₆H₅) | Polystyrene | Packaging, insulation |
⚠️ Addition polymers are non-biodegradable. Their strong, non-polar C−C backbone resists hydrolysis and biological degradation.
Condensation Polymers
Addition vs Condensation Polymers
| Addition | Condensation | |
|---|---|---|
| Monomer | Alkenes (C=C) | Diols + diacids, amino acids |
| By-product | None | Water (H₂O) |
| Linkage | C−C only | Ester (−COO−) or amide (−CONH−) |
| Atom economy | 100% | < 100% |
| Biodegradable? | No | Usually yes (hydrolysable) |
Polyesters
Diol + diacid → polyester + H₂O
Linkage: −COO− (ester bond)
Polyamides
Diamine + diacid → polyamide + H₂O
Linkage: −CONH− (amide/peptide bond). Proteins are natural polyamides!
Isomerism
Structural Isomers (SL)
Same molecular formula, different structural formula (different connectivity of atoms).
Chain Isomers
Different arrangement of the carbon skeleton (straight vs branched)
e.g. Butane vs methylpropane
Position Isomers
Same functional group at different positions on the chain
e.g. Propan-1-ol vs propan-2-ol
Functional Group Isomers
Different functional group entirely
e.g. Propanal vs propanone (both C₃H₆O)
🔬 HL. Stereoisomers
Same structural formula but different spatial arrangement.
Cis-Trans Isomerism
- Restricted rotation around C=C double bond
- Each C must have two different groups
- Cis = same side; Trans = opposite sides
- E/Z nomenclature not assessed
Optical Isomerism
- Chiral carbon bonded to four different groups → enantiomers
- Non-superimposable mirror images; drawn with wedge-dash notation
- 50:50 mixture = racemic mixture
Spectroscopy & Structural Analysis
Mass Spectrometry (MS)
M⁺ peak = molecular ion → gives molecular mass
Fragmentation pattern helps deduce structural features. Data in the data booklet.
IR Spectroscopy
Identifies bond types present (O−H, N−H, C=O, C−O)
Absorptions measured in wavenumber (cm⁻¹). Key values provided in data booklet.
🔬 HL. ¹H NMR Spectroscopy
Gives detailed information about different proton (H) environments in a molecule.
| Feature | What It Tells You |
|---|---|
| Number of signals | Number of different H environments |
| Chemical shift (δ/ppm) | Type of environment (data booklet) |
| Integration | Ratio of H atoms in each environment |
| Splitting (n+1 rule) | Number of H on adjacent carbons (singlet, doublet, triplet, quartet) |
TMS (tetramethylsilane) is used as the reference standard (δ = 0 ppm).
Index of Hydrogen Deficiency (IHD)
Also called degree of unsaturation. Tells you the number of rings + π bonds.
Formula: IHD = (2C + 2 + N − H − X) / 2
Where C = carbons, N = nitrogens, H = hydrogens, X = halogens. Oxygen doesn't count!
IHD = 1 → one double bond or one ring. IHD = 4 → likely a benzene ring.
Think About It
Why are condensation polymers typically biodegradable, but addition polymers are not?
Condensation polymers contain ester or amide linkages that can be hydrolysed (broken by water/enzymes). Addition polymers have only strong C−C bonds along the backbone, which are very resistant to chemical attack and biological degradation.