Polymers: Classification, Polymerization and Engineering

Polymers are giant molecules made by joining thousands of small repeating units called monomers, like a very long chain built from identical links. The name comes from Greek — poly (many) + meros (parts). Plastics, rubber, nylon, starch and proteins are all polymers; they are in our clothes, bottles, tyres and even our own muscles.

Polymers matter because their properties are tunable: the same long-chain idea yields a soft carry bag, a bulletproof vest and a heat-proof pan coating. Choosing the right polymer and polymerization method decides flexibility, heat behaviour and lifespan — which is why classification and synthesis are core engineering chemistry.

Key Concepts

• Monomer: the small repeating molecule (ethene, vinyl chloride, glucose).
• Degree of polymerization (DP): the number of monomer units in one chain; DP = number-average molecular weight ÷ monomer molar mass.
• By source: natural (starch, cellulose, natural rubber), synthetic (polythene, PVC, nylon), semi-synthetic (rayon, cellulose acetate).
• By structure: linear (HDPE — strong, high density), branched (LDPE — flexible, low density), cross-linked or network (bakelite, vulcanised rubber — rigid, infusible).
• By heat behaviour: thermoplastics soften on heating and can be remoulded (polythene, PVC); thermosetting plastics harden permanently on first heating (bakelite, melamine); elastomers stretch and snap back (rubber).

How It Works

Addition (chain-growth) polymerization — monomers with double bonds join with no by-product:

1. Initiation: an initiator such as benzoyl peroxide splits into free radicals; a radical attacks the double bond of an ethene molecule, starting a chain.
2. Propagation: the new radical attacks the next monomer, again and again — the chain grows by one unit per step, thousands of times per second.
3. Termination: two radicals meet and combine, ending growth. The polymer has the same composition as the monomer (polythene, PVC, Teflon).

Condensation (step-growth) polymerization — monomers with two functional groups join with a small molecule eliminated:

1. Hexamethylenediamine and adipic acid meet end to end.
2. One water molecule is eliminated at each new link, step by step; any two growing chains can also join.
3. Growth is slow and by-products must be removed. Result: nylon-6,6 plus water (also terylene, bakelite).

Worked Example

A polythene sample has a number-average molecular weight (Mn) of 56,000 g/mol. Find its average degree of polymerization.

1. Monomer is ethene, C₂H₄. Molar mass = 2×12 + 4×1 = 28 g/mol.
2. DP = Mn ÷ monomer molar mass = 56,000 ÷ 28 = 2,000.

So an average chain here holds about 2,000 ethene units — a molecule 2,000 links long.

Common Misconceptions

• "All plastics are the same." No — thermoplastics melt and remould, while thermosetting plastics char and cannot be reshaped once set.
• "Polymers are always man-made." Starch, cellulose, proteins, silk and natural rubber are natural polymers, made by plants and animals.
• "Biodegradable plastic vanishes in days." It needs the right microbes, moisture and temperature; many such plastics only break down in industrial composting plants.

Real-World Applications

• Packaging: LDPE films, HDPE bottles and PET containers protect food and goods worldwide.
• Construction: PVC pipes, window frames and insulation exploit chemical resistance and durability.
• Textiles: nylon ropes and parachutes, polyester fabrics and acrylic wool substitutes.
• Engineering plastics: polycarbonate in helmets and CDs, PTFE (Teflon) non-stick coatings, ABS in car dashboards — chosen for strength and heat resistance.
• Medicine: dissolvable sutures, implants and drug-delivery capsules use biocompatible polymers.

Polymers are long monomer chains whose structure, source and heat behaviour decide their properties. Addition polymerization grows chains radically with no by-product; condensation eliminates small molecules at each link. Matching the polymer to the job — from carry bags to bulletproof vests — is what makes modern materials work.