Plastics and polymers

1. Polymers

• Polymers are macromolecules made of repeating monomer units.
• Types of polymers include:
  • Natural polymers (e.g., proteins, cellulose, DNA).
  • Synthetic polymers (plastics, nylon, polyester).

2. Synthetic Polymers: Plastics

• Contain carbon in their structure.
• Lightweight due to low density.
• Longer polymer chains = stronger plastics (due to stronger intermolecular forces).
• Non-conductors of heat and electricity.
• Mouldable and can be used as foam or solid materials.

Types of Plastics

Thermoplastics	Thermoset Plastics
Can be softened and reshaped by heating.	Cannot be remoulded once set.
Weak intermolecular forces.	Strong cross-links between molecules.
Example: Polyethene (plastic bags, bottles).	Example: Melamine (plates, cups).

3. Properties of Plastics, Nylons, and Polyesters

Material	Properties	Examples
Plastics	Flexible, low melting point, non-biodegradable.	Polyethene (packaging), PVC (pipes).
Nylons	Strong fibres, high tensile strength.	Ropes, carpets, clothes.
Polyesters	Varying strength, used in textiles.	Clothing, plastic bottles.

4. Polymerisation

• Polymerisation is the chemical process where monomers join to form polymers.
• Two types:
  1. Addition Polymerisation
  2. Condensation Polymerisation

4.1 Addition Polymerisation

• Monomers join without eliminating any atoms.

• Example: Ethene → Polyethene n$$C_2H_4 \rightarrow (-C_2H_4-)_n

• Used in: Plastic bags, bottles.

4.2 Condensation Polymerisation

• Monomers join with the elimination of a small molecule (e.g., H₂O).
• Example: Polyester formation.
• Used in: Fibres (e.g., Kevlar, Mylar).

5. Types of Polyethene (PE)

Type	Properties	Uses
Low-Density Polyethene (LDPE)	Weak, many branches, flexible.	Freezer bags, soft bottles.
High-Density Polyethene (HDPE)	Strong, few branches, rigid.	Shopping bags, plastic crates.

6. Condensation Polymerisation: Polyester Formation

• Polyester is formed when an alcohol reacts with a carboxylic acid, eliminating $H₂O$.
• Example: Kevlar and Mylar (used in bulletproof vests and insulation).

7. Key Takeaways

✅ Plastics are synthetic polymers that are lightweight and durable.

✅ Thermoplastics can be reshaped, while thermoset plastics cannot.

✅ Polymerisation creates long-chain molecules from monomers.

✅ Addition polymerisation forms polyethene, while condensation polymerisation forms polyesters.