Addition Polymers
Mechanism of Addition Polymers
Addition polymerisation occurs when alkene monomers link via opening of their \(\text{C}=\text{C}\;\pi\) bonds into long saturated carbon chains. There is 100% atom economy (no small molecules eliminated).
Non-biodegradability: Addition polymers have non-polar, unreactive \(\text{C}-\text{C}\) backbones that resist biological enzyme and chemical hydrolysis.
| 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
| Property | Addition Polymers | Condensation Polymers |
|---|---|---|
| Monomers | Alkenes with \(\text{C}=\text{C}\) | Diols + diacids, or amino acids |
| By-product | None (100% atom economy) | Small molecule (\(\text{H}_2\text{O}\) or \(\text{HCl}\)) |
| Polymer Linkage | \(\text{C}-\text{C}\) backbone | Ester (\(-\text{COO}-\)) or Amide (\(-\text{CONH}-\)) |
| Biodegradability | Non-biodegradable | Hydrolysable (biodegradable) |
Polyesters and Polyamides
Polyesters (e.g. PET / Dacron)
Diol + Dicarboxylic acid → Polyester + \(\text{H}_2\text{O}\)
Linkage: \(-\text{COO}-\) (ester linkage)
Polyamides (e.g. Nylon-6,6 / Kevlar)
Diamine + Dicarboxylic acid → Polyamide + \(\text{H}_2\text{O}\)
Linkage: \(-\text{CONH}-\) (amide/peptide linkage)
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)
Optical Isomerism and Chirality
Optical Isomerism (Enantiomers):
- Occurs when a molecule contains a chiral (asymmetric) carbon atom bonded to four different groups.
- Forms non-superimposable mirror images (enantiomers) that rotate plane-polarised light in equal and opposite directions.
- A 1:1 equimolar mixture of both enantiomers is optically inactive (a racemic mixture).
Spectroscopy & Structural Analysis
Mass Spectrometry and IR Spectroscopy
Mass Spectrometry (MS)
\(\text{M}^+\) peak: Gives the molecular mass (\(M_{\text{r}}\)).
Fragmentation patterns indicate lost groups (e.g. \(\text{M}-15 = \text{CH}_3^+\), \(\text{M}-29 = \text{C}_2\text{H}_5^+\)).
Infrared Spectroscopy (IR)
Identifies functional groups via bond vibration wavenumbers (\(\text{cm}^{-1}\)).
E.g. Broad \(\text{O}-\text{H}\;3200-3600\text{ cm}^{-1}\), sharp \(\text{C}=\text{O}\;1700-1750\text{ cm}^{-1}\).
High-Resolution 1H NMR Spectroscopy
- Number of peaks: Number of non-equivalent chemical environments for protons (\(^1\text{H}\)).
- Chemical shift (\(\delta\)): Type of proton environment relative to TMS (\(\delta = 0\text{ ppm}\)).
- Peak integration: Relative ratio of hydrogen atoms in each environment.
- Spin-spin splitting (\(n+1\) rule): Number of protons on adjacent carbons (\(n\)) → singlet (\(0\text{H}\)), doublet (\(1\text{H}\)), triplet (\(2\text{H}\)), quartet (\(3\text{H}\)).
TMS (tetramethylsilane) is used as the reference standard (δ = 0 ppm).
Index of Hydrogen Deficiency (IHD)
For a compound with formula \(\text{C}_c\text{H}_h\text{N}_n\text{O}_o\text{X}_x\) (where X = halogen):
\[\mathbf{\text{IHD} = c + 1 - \frac{h}{2} - \frac{x}{2} + \frac{n}{2}}\]- 1 double bond (\(\text{C}=\text{C}\) or \(\text{C}=\text{O}\)) or 1 ring = \(\text{IHD} = 1\).
- 1 triple bond (\(\text{C}\equiv\text{C}\)) = \(\text{IHD} = 2\).
- Benzene ring (aromatic ring) = \(\text{IHD} = 4\) (1 ring + 3 double bonds).
Deducing Aromatic Rings from IHD
For \(\text{C}_6\text{H}_6\), \(\text{IHD} = 6 + 1 - \frac{6}{2} = 4\). An IHD of 4 or more strongly indicates the presence of an aromatic benzene ring.
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