Polymers covers how small repeating monomers join into giant high-mass molecules, the three ways polymers are classified (source, structure and molecular force), and the two main routes that build them: addition (chain-growth) and condensation (step-growth) polymerisation. It also deals with named polymers and their monomers, natural and synthetic rubber with vulcanisation, biodegradable polymers, and average molecular mass. For NEET it is a high-recall Organic Chemistry chapter where most marks come from matching a polymer to its monomer and its type.
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What Is a Polymer?
A polymer is a giant high-mass molecule built by linking many small repeating units called monomers.
Polymer molar mass is huge (≈104–107 g/mol); the monomer is always the small unit.
- Polymerisation = the joining reaction; n = number of repeat units
- Monomer is small; only the polymer has high molecular mass
- Identify the monomer first — that is what NEET asks
Three Ways to Tag a Polymer
Every polymer is labelled three independent ways: by source, by structure, and by molecular force.
LDPE is branched, HDPE is linear — the most common trap.
- Source: natural / semi-synthetic / synthetic
- Structure: linear / branched / cross-linked (network)
- Force: elastomer / fibre / thermoplastic / thermosetting
Thermoplastic vs Thermosetting
Thermoplastics soften and remould on heating; thermosetting plastics cross-link and set permanently.
Bakelite is thermosetting (cannot remelt); polythene/PVC are thermoplastic.
- Thermoplastic = re-mouldable again and again (polythene, PVC)
- Thermosetting = cross-links on first heating, never remelts (Bakelite)
- Most assertion-reason traps sit on this difference
Addition (Chain-Growth) Polymerisation
Unsaturated C=C monomers add one after another with no small molecule lost.
If H2O or HCl is released, it is NOT addition.
- Needs an unsaturated monomer (C=C), e.g. ethene CH2=CH2
- Examples: ethene→polythene, vinyl chloride→PVC, CF2=CF2→Teflon
- Acrylonitrile→PAN/Orlon; styrene→polystyrene
Free-Radical Mechanism
Chain-growth addition runs in three steps started by a peroxide radical.
Initiator = benzoyl peroxide (not sulphur, not sodium).
- Initiation: peroxide gives a radical that opens a C=C bond
- Propagation: radical attacks the next monomer, chain grows fast
- Termination: two radical ends pair up and stop growth
Condensation (Step-Growth) Polymerisation
Bi/poly-functional monomers join end-to-end, eliminating a small molecule at each link.
Polymer formula differs from the monomers because something leaves.
- Monomers carry two groups: –OH, –COOH or –NH2
- Amide (–CONH–) link → polyamide (nylon); ester (–COO–) link → polyester
- Nylon-6,6 = hexamethylenediamine + adipic acid (both 6 C)
Nylons, Polyesters & Resins
The high-yield condensation polymers come from named monomer pairs.
Glyptal uses phthalic (ortho) acid; Terylene uses terephthalic (para) acid.
- Nylon-6,6 = diamine + diacid (2 monomers); Nylon-6 = caprolactam (1)
- Bakelite = phenol + formaldehyde, cross-linked; Novolac = linear
- Melamine–formaldehyde → unbreakable crockery
Rubber, Copolymers & Vulcanisation
Natural rubber is cis-1,4-polyisoprene; sulphur cross-links it during vulcanisation.
Buna-S = butadiene + styrene; Buna-N = butadiene + acrylonitrile.
- Monomer of natural rubber = isoprene (2-methyl-1,3-butadiene), cis geometry
- Sulphur forms cross-links (≈3–5% tyres, ≈30% ebonite) — not an initiator
- Neoprene = polychloroprene (homopolymer); Buna types are copolymers
Biodegradable Polymers
Some polymers are built so microbes and enzymes can break them down, unlike polythene or PVC.
PVC, polythene and Teflon are non-biodegradable.
- PHBV: copolyester used in packaging and orthopaedics
- PGA, PLA, Dextron: dissolvable surgical sutures
- Nylon-2–nylon-6: alternating biodegradable polyamide
Average Molecular Mass & PDI
Chains differ in length, so polymers are described by two average molar masses.
M̄_w ≥ M̄ₙ always; PDI = 1 only for a monodisperse sample.
- M̄ₙ counts every chain equally; M̄_w weights heavier chains more
- PDI ≥ 1; natural polymers (e.g. proteins) are nearly monodisperse
- Don’t swap the two formulas — divide M̄ₙ by ΣNᵢ
📝 Practice Polymers — 10 NEET PYQs
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Frequently Asked Questions
A polymer is a very large molecule built by linking many small repeating units, and each small unit is called a monomer. The reaction that joins them is polymerisation, and the number of repeat units is the degree of polymerisation, so only the polymer has a high molecular mass while the monomer stays small.
Addition (chain-growth) polymerisation links unsaturated C=C monomers one after another with nothing eliminated, so the polymer has the same empirical formula as the monomer, for example ethene giving polythene. Condensation (step-growth) polymerisation joins bi or poly-functional monomers and eliminates a small molecule such as water at each link, for example nylon-6,6 from hexamethylenediamine and adipic acid.
Thermoplastics have no cross-links, so they soften on heating and can be remoulded again and again, like polythene and PVC. Thermosetting polymers cross-link permanently on first heating and cannot be remelted, like Bakelite, which is one of the most common assertion-reason traps in NEET.
Vulcanisation is heating natural rubber (cis-1,4-polyisoprene) with sulphur, which forms cross-links between the chains and makes the rubber tougher, more elastic and heat resistant. Tyre rubber uses about 3 to 5 percent sulphur while hard ebonite uses around 30 percent, and sulphur acts as a cross-linker, not an initiator.
Yes, Polymers is part of the NEET Class 12 Chemistry syllabus and is high-recall, usually giving one direct question. Most marks come from matching a polymer to its monomer and its type, such as Buna-S being a butadiene and styrene copolymer and Nylon-6 coming from a single monomer caprolactam.