Addition Vs Condensation Polymerization

Polymerisation is the process of joining many small monomer molecules into a giant polymer chain, and it happens by two fundamentally different routes. In addition polymerisation, monomers with carbon–carbon double bonds simply add to each other one after another, while in condensation polymerisation, monomers with two functional groups join by repeatedly eliminating a small molecule such as water.

This distinction matters because it decides the polymer's structure, its properties and how it is manufactured — polythene bags come from addition polymerisation, while nylon ropes and terylene fabric come from condensation. Examiners love asking you to classify a polymer and write its formation equation.

Key Concepts

• Addition polymerisation: chain-growth process; monomers must contain a C=C double bond; no byproduct is lost; usually proceeds by a free-radical mechanism.
• Condensation polymerisation: step-growth process; monomers must be bifunctional (two −OH, two −COOH, or one of each); a small molecule (H₂O, HCl, CH₃OH) is eliminated at each step.
• Repeat unit: the structural unit that repeats in the polymer; its mass equals the monomer mass minus any eliminated small molecules.
• Examples: addition — polyethene, PVC, Teflon, polystyrene; condensation — nylon-6,6, terylene (Dacron/PET), bakelite.

In addition polymerisation an initiator creates a free radical that attacks the double bond of ethene, and the chain grows rapidly: nCH₂=CH₂ → −(CH₂−CH₂)ₙ−. In condensation polymerisation, hexamethylenediamine (H₂N−(CH₂)₆−NH₂) and adipic acid (HOOC−(CH₂)₄−COOH) react −NH₂ with −COOH groups step by step to give nylon-6,6, losing one water molecule per amide bond. Terylene forms similarly from ethylene glycol and terephthalic acid with water eliminated, and bakelite from phenol and formaldehyde.

Formulas

• Addition: n(monomer) → polymer, with M(repeat unit) = M(monomer). No atoms are lost; e.g. ethene (28) gives a 28 g mol⁻¹ repeat unit.
• Condensation: M(repeat unit) = M(monomer A) + M(monomer B) − M(eliminated molecule). For amide formation, subtract 18 g mol⁻¹ (H₂O) per bond.
• Nylon-6,6 repeat unit: −CO−(CH₂)₄−CO−NH−(CH₂)₆−NH−, formed from one diamine and one diacid with 2 H₂O eliminated per repeat unit.

Worked Example

Calculate the molar mass of the nylon-6,6 repeat unit. Step 1: molar mass of hexamethylenediamine, H₂N−(CH₂)₆−NH₂ = 116 g mol⁻¹. Step 2: molar mass of adipic acid, HOOC−(CH₂)₄−COOH = 146 g mol⁻¹. Step 3: one repeat unit forms two amide bonds, eliminating 2 × 18 = 36 g mol⁻¹ of water. Step 4: repeat-unit mass = 116 + 146 − 36 = 226 g mol⁻¹. So each nylon-6,6 repeat unit weighs 226 g per mole.

Common Misconceptions

• "Addition polymers lose water too." No — only condensation polymerisation eliminates small molecules; addition simply opens the double bond.
• "PVC is a condensation polymer because it contains chlorine." Wrong — vinyl chloride (CH₂=CHCl) has a C=C bond and polymerises by addition; the chlorine is just a substituent on the monomer.
• "Condensation polymers always come from two different monomers." Not necessarily — nylon-6 forms from a single monomer (caprolactam) by ring opening, and a single amino acid can self-condense.

Real-World Applications

• Addition polymers: polyethene (carry bags, bottles), PVC (pipes, cables), Teflon (non-stick coatings), polystyrene (packaging foam).
• Condensation polymers: nylon-6,6 (ropes, parachutes, textiles), terylene/PET (clothes, beverage bottles), bakelite (electrical switches, handles).
• Industry: the mechanism chosen decides the reactor design — addition needs initiators and heat control, condensation needs continuous removal of water to drive the equilibrium.

Addition polymerisation adds double-bond monomers with nothing lost; condensation polymerisation joins bifunctional monomers with a small molecule eliminated at every step. Spot the monomer type, write the balanced formation equation, subtract the eliminated mass for the repeat unit — and every polymer question falls into place.