Polymers

A polymer is an enormous molecule built by linking thousands of small repeating units called monomers. Polymers surround us: the plastic in a water bottle, the nylon in a sports kit, the starch in a potato, and the DNA in every cell are all polymers. Their properties depend entirely on the monomer used and the way the monomers link together.

Key Definitions¶

A polymer is a macromolecule formed by joining 50 or more small monomer units together. The monomers are linked by covalent bonds to form a long chain.

A monomer is the small, repeating structural unit from which a polymer is built. Monomers must have at least one reactive site — either a double bond (for addition polymerisation) or two functional groups (for condensation polymerisation).

Polymerisation is the chemical process of joining monomers to form a polymer.

Addition Polymerisation¶

In addition polymerisation, alkene monomers join together by the opening of their C=C double bonds. No atoms are lost — every atom in every monomer ends up in the polymer. The monomers simply add to a growing chain.

General pattern (using ethene as an example):

$n(\text{CH}_2\text{=CH}_2) \rightarrow (-\text{CH}_2\text{–CH}_2-)_n$

The repeat unit is written inside brackets with a subscript $n$ to show that the pattern repeats thousands of times. A short section of the chain is usually enough to represent the polymer.

Common Addition Polymers¶

To write the repeat unit: remove the double bond and show the two carbons now joined by a single bond, with brackets and subscript $n$. Any substituents remain attached to their respective carbons.

Condensation Polymerisation¶

In condensation polymerisation, monomers join together with the simultaneous elimination of a small molecule — usually water (H₂O) or hydrogen chloride (HCl). Unlike addition polymerisation, monomers for condensation polymers must have two functional groups (one at each end of the molecule) so that each monomer can link to two neighbours, forming a long chain.

Polyamides (Nylon)¶

Nylon is a polyamide formed by condensation polymerisation between a diamine (two –NH₂ groups) and a dicarboxylic acid (two –COOH groups). Water is eliminated as the amide link –CO–NH– forms:

$\text{H}_2\text{N–(CH}_2)_6\text{–NH}_2 + \text{HOOC–(CH}_2)_4\text{–COOH} \rightarrow \text{nylon-6,6} + n\text{H}_2\text{O}$

The amide (–CO–NH–) linkage gives nylons their characteristic strength and toughness.

Uses of nylon: clothing fabrics, stockings, toothbrush bristles, ropes, parachutes, gears and bearings.

Polyesters (Terylene / PET)¶

Polyesters are formed by condensation polymerisation between a diol (two –OH groups) and a dicarboxylic acid (two –COOH groups). Water is eliminated as the ester link –COO– forms.

PET (polyethylene terephthalate, Terylene) is the most common polyester:

Uses of polyester: clothing fabrics, soft-drink bottles (PET), food trays, film for tapes and capacitors.

Natural Polymers¶

Many essential biological molecules are natural polymers formed by condensation polymerisation:

Uses of Polymers¶

Environmental Issues with Plastics¶

Advantages of Synthetic Plastics¶

• Durable and long-lasting
• Lightweight
• Chemically resistant and corrosion-proof
• Inexpensive to produce at scale
• Versatile — can be moulded into any shape

Disadvantages¶

• Non-biodegradable: most synthetic plastics are not broken down by microorganisms. They persist in the environment for hundreds of years, accumulating in oceans, rivers, and landfill.
• Pollution: plastic waste fragments into microplastics that enter food chains and harm marine life.
• Difficult disposal: burning plastics can release toxic gases (e.g. HCl from PVC). Recycling requires sorting by polymer type.
• Fossil fuel dependence: most synthetic polymers are made from petrochemicals — a non-renewable resource.

Biodegradable Polymers¶

Research into biodegradable polymers — polymers that can be broken down naturally by microorganisms — aims to reduce plastic pollution. These include polylactic acid (PLA), made from plant sugars, which is used in compostable packaging and medical sutures.