Polymers

Concept overview

Polymers are large molecules formed by linking many small monomer units. JEE splits the chapter along two axes: mechanism of polymerisation (addition versus condensation) and origin (natural versus synthetic).

Addition polymerisation, or chain-growth polymerisation, uses monomers with a C=C double bond. A free-radical initiator (peroxide) opens the double bond and the resulting radical adds to another monomer, propagating the chain. Polythene from ethene, PVC from vinyl chloride, polystyrene from styrene, and Teflon (PTFE) from tetrafluoroethene are the workhorse examples.

Condensation polymerisation, or step-growth, uses monomers with two functional groups that react to form a bond while releasing a small molecule (usually water). Nylon-6,6 forms from adipic acid + hexamethylenediamine with loss of water at each amide bond. Terylene (PET, polyester) forms from terephthalic acid + ethylene glycol. Bakelite forms from phenol + formaldehyde, and is a thermosetting polymer — once cast, it can't be reshaped.

Copolymers combine two different monomers in the same chain. Buna-S is styrene + butadiene copolymer (synthetic rubber). Buna-N is acrylonitrile + butadiene. Natural rubber is cis-poly-isoprene; vulcanisation with sulfur cross-links the chains to give the tougher rubber used in tires.

Molecular mass distribution (number average vs weight average, and the polydispersity index) and biodegradable polymers (PHBV, nylon 2-nylon 6 copolymers) round out the chapter as smaller sections that generate direct-recall JEE Main questions.

Key reactions & principles

• Addition polymer example: Polythene: n CH₂=CH₂ → –(CH₂–CH₂)_n–
• Condensation polymer example: Nylon-6,6: adipic acid + hexamethylenediamine → amide + water
• PET / Terylene: Terephthalic acid + ethylene glycol → polyester
• Bakelite: Phenol + formaldehyde → thermosetting cross-linked polymer
• Buna-S: Butadiene + styrene → synthetic rubber (copolymer)
• Vulcanisation: Rubber + S₈ → cross-linked rubber (tougher, less elastic)
• Natural rubber: cis-poly-isoprene

Common mistakes to avoid

• Classifying Bakelite as thermoplastic — it's thermosetting; once cured, it doesn't melt.
• Confusing Nylon-6 (from caprolactam alone) with Nylon-6,6 (from adipic acid + hexamethylenediamine).
• Assuming natural rubber is cis-polybutadiene — it's cis-polyisoprene.
• Predicting condensation polymerisation without asking whether a small molecule is released — that's the defining feature.

JEE Main relevance

JEE Main asks 1 polymer question every paper — usually direct identification (which monomers give a given polymer) or a classification question (addition vs condensation, thermoplastic vs thermosetting). Score with a small memorised fact set.

JEE Advanced relevance

Advanced problems on polymers are rare but tend to focus on mechanism (radical vs cationic vs anionic addition) or on copolymer composition given rate data. Structural questions on Bakelite (a 3D cross-linked network) are also common.

Previous-year questions (PYQ)

Filter the PYQ bank by 'Polymers' and drill 10 direct-recall problems. This is a chapter where a single evening of focused memorisation delivers a guaranteed mark.