Polymers

The process. Alkene molecules (with C=C double bonds) join together. The double bonds break, and new single bonds form between molecules. NO atoms are lost — every atom from the monomers ends up in the polymer.

Worked: poly(ethene). $n\,\text{CH}_{2}=\text{CH}_{2} \to (\text{-CH}_{2}\text{-CH}_{2}\text{-})_{n}.$

The double bond in ethene breaks; each ethene becomes a CH₂-CH₂ unit; thousands link end-to-end into a long chain. Conditions: high T, high P, catalyst.

Drawing the polymer. Take the alkene, replace the double bond with single bonds, attach two single bonds to neighbours, and put the unit in brackets with $n$:

   ...— C — C — C — C —...
        |   |   |   |
        H   H   H   H
        |   |   |   |
        H   H   H   H

Common addition polymers.

Polymer	Monomer	Use
Poly(ethene) (PE)	Ethene	Plastic bags, bottles
Poly(propene) (PP)	Propene	Yogurt pots, ropes
Poly(chloroethene) / PVC	Chloroethene	Pipes, window frames, vinyl
Poly(tetrafluoroethene) / PTFE / Teflon	Tetrafluoroethene	Non-stick coatings

Worked. Draw the repeating unit of poly(propene), CH₂=CHCH₃ as monomer.

• The C=C breaks; each propene unit becomes -CH₂-CH(CH₃)-.
• Repeating unit: $\text{(-CH}_{2}\text{-CH(CH}_{3}\text{)-)}_{n}$.

Cambridge tip. Cambridge often gives a monomer's structure and asks for the polymer's repeating unit. Method:

1. Identify the C=C.
2. Break the double bond; replace with single bond.
3. Add ONE bond on each side (to attach to neighbours).
4. Wrap in brackets with subscript $n$.

The process. Monomers must have TWO reactive functional groups (one at each end). When two monomers join, a small molecule (usually H₂O or HCl) is released.

Examples.

1. Nylon (an amide polymer). Monomers: a diamine + a dicarboxylic acid → polyamide + water. $\text{HOOC-(...)-COOH} + \text{H}_{2}\text{N-(...)-NH}_{2} \to (\text{-OC-(...)-CO-NH-(...)-NH-})_{n} + n\text{H}_{2}\text{O}.$

2. Polyester (e.g. terylene/PET). Monomers: a diol + a dicarboxylic acid → polyester + water. $\text{HOOC-(...)-COOH} + \text{HO-(...)-OH} \to (\text{-OC-(...)-CO-O-(...)-O-})_{n} + n\text{H}_{2}\text{O}.$

3. Proteins (biological condensation polymers). Monomers: amino acids → peptide bond + water (similar to nylon mechanism).

Cambridge tip. Cambridge IGCSE typically asks you to:

• IDENTIFY whether a process is addition or condensation.
• Identify the small molecule lost in condensation (usually water).
• Identify the linkage in the polymer (amide -CO-NH- or ester -CO-O-).

Worked qualitative. What's the difference between addition and condensation polymerisation? Addition: alkene monomers join; no by-product; needs C=C. Condensation: monomers with TWO reactive groups join; small molecule released (water); produces amides, esters, etc.

The problem. Most addition polymers (PE, PP, PVC, PET) are:

• Strong (good — useful) AND
• Non-biodegradable (bad — accumulates in landfill / oceans).

Plastic in landfills lasts for hundreds of years. In oceans, it breaks into MICROPLASTICS that enter food chains and cause harm.

Solutions.

1. Recycling. Sort by polymer type (the recycling code 1-7 you see on packaging). Melt down, re-form into new products. Limitations: not all plastics recyclable; quality degrades over cycles; mixed-plastic items can't be recycled cleanly.

2. Biodegradable plastics. Made from cornstarch or similar; broken down by bacteria. Used for compostable bags, food packaging.

3. Replacing single-use plastics. Use paper, glass, metal, or reusable items where possible.

4. Reducing waste in the first place. Refill stations, less packaging.

Combustion of plastics. Burning plastics releases:

• CO₂ (greenhouse gas).
• Toxic gases — esp. PVC: HCl gas (acid).
• Particulates (soot).

So burning plastic is generally NOT a good solution unless done in specialised, scrubbed waste-to-energy plants.

Worked qualitative. Why is incinerating PVC a problem? PVC contains chlorine atoms. Burning releases HCl (corrosive, toxic) and other chlorinated compounds (some highly toxic, e.g. dioxins).