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🧪 Polymers

Spec 5.2.2.5 📙 Higher
📖 In-Depth Theory

What Are Polymers?

POLYMERS are very large molecules made from many small repeating units called MONOMERS joined together in long chains.
The process of joining monomers is called POLYMERISATION.
Key terminology:
MONOMER — small molecule that can join with others to form a polymer.
POLYMER — the long chain molecule formed from thousands of monomers.
The repeating unit in a polymer is typically written in brackets with an n outside: [—CH₂—CH₂—]ₙ
Examples:
Poly(ethene) — monomer: ethene (CH₂=CH₂) → polymer: long chain of —CH₂—CH₂— units.
Poly(propene) — monomer: propene.
PVC (polyvinyl chloride) — monomer: chloroethene.
Nylon — forms from two different monomers (a diamine and a dicarboxylic acid).
Protein — polymer of amino acid monomers.
DNA — polymer of nucleotide monomers.
Starch and cellulose — polymers of glucose monomers.

Properties of Polymers

Polymers are SIMPLE MOLECULAR structures — they consist of individual polymer chains (very large molecules) held together by INTERMOLECULAR FORCES.
Properties depend on the CHAIN LENGTH and STRENGTH OF INTERMOLECULAR FORCES:
LONG CHAIN LENGTH:
Longer chains have more surface area → stronger intermolecular forces → higher melting point.
Very long polymers tend to be solid at room temperature.
TYPE OF POLYMER (affects intermolecular force strength):
Poly(ethene) — weak forces → flexible, low melting point → used in plastic bags.
PVC — stronger intermolecular forces (polar C-Cl bonds) → stiffer, higher melting point.
Nylon — hydrogen bonds between chains → strong, high melting point.
GENERAL POLYMER PROPERTIES:
Solid at room temperature (long chains, reasonable intermolecular forces).
Insulators — do not conduct electricity (no free electrons or ions).
Flexible (unless cross-linked or crystalline).
Low density compared to metals.
Can be moulded when heated — THERMOPLASTIC polymers soften when heated.
Some polymers are THERMOSETTING — they harden permanently when heated (cross-links form).

Addition Polymerisation

ADDITION POLYMERISATION occurs when monomers containing DOUBLE BONDS (C=C) join together.
The double bond opens up — one bond breaks and the freed electrons form a new bond with the next monomer.
Example — poly(ethene):
Monomer: ethene CH₂=CH₂
Polymerisation: n CH₂=CH₂ → [—CH₂—CH₂—]ₙ
The product is a SATURATED polymer (no double bonds remain in the chain).
Other addition polymers:
Poly(propene): monomer propene (CH₂=CHCH₃)
PVC: monomer chloroethene (CH₂=CHCl)
Poly(styrene): monomer styrene (CH₂=CHC₆H₅)
Notice: in addition polymerisation, ALL atoms of the monomer appear in the polymer — no atoms are lost or expelled.
⚠️ Common Mistake

Polymers are LARGE MOLECULES — they are NOT giant covalent structures. Each polymer chain is one large molecule held to others by intermolecular forces (which are weak). Giant covalent structures (like diamond) have covalent bonds extending throughout the entire structure — very different. This is why polymers have much lower melting points than diamond.

📐 Key Equations
n CH₂=CH₂ → [—CH₂—CH₂—]ₙ (poly(ethene))
📌 Key Note

Polymer: long chain of repeating monomer units. Addition polymerisation: monomers with C=C double bonds join together. Properties depend on chain length and intermolecular forces. Solid at room temperature, flexible, insulators. Thermoplastic: soften on heating. Thermosetting: permanently hard.

🎯 Matching Activity — Match the Polymer to its Monomer

Match each polymer to its monomer and a use. — drag the symbols on the right to match the component names on the left.

Poly(ethene)
Drop here
PVC
Drop here
Nylon
Drop here
Protein
Drop here
Starch
Drop here
Monomer: diamine + dicarboxylic acid — used in clothing and rope
Monomer: amino acids — biological polymer in all living things
Monomer: chloroethene (CH₂=CHCl) — used in pipes and window frames
Monomer: glucose — energy storage polymer in plants
Monomer: ethene (CH₂=CH₂) — used in plastic bags and bottles
🎯 Test Yourself
Question 1 of 2
1. What is the monomer used to make poly(ethene)?
2. Why do polymers have lower melting points than giant covalent structures like diamond?
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