PolymersNEET Chemistry · Class 12 · NCERT Chapter 11

Introduction to Polymers

The word "polymer" comes from the Greek words poly (many) and meros (parts). A polymer is a large molecule (macromolecule) made of many repeating smaller units called monomers, joined together by covalent bonds. You can think of a polymer chain as a long necklace, where each bead is one monomer unit.

Polymers are everywhere in your life: the plastic in your water bottle, the fibres in your shirt, the rubber in your shoes, and the DNA in your cells are all polymers. Understanding their structure tells you exactly why each material behaves the way it does.

Key Definitions You Must Know

The molecular weight of a polymer is always given as an average because different chains in the same sample have different lengths. This is different from simple organic compounds, which have a single exact molecular weight.

Why does molecular weight matter? Higher molecular weight means longer chains, stronger intermolecular forces, higher melting point, and higher tensile strength. The degree of polymerisation (n) tells you the chain length directly.

Classification of Polymers

You can classify the same polymer in multiple ways. For NEET, you need to know all four classification systems and apply them to every important polymer.

1. Classification by Source of Origin

2. Classification by Structural Type

3. Classification by Mode of Polymerisation

4. Classification by Thermal Behaviour

5. Classification by Elasticity (Special Category)

Elastomers are polymers with low cross-linking (just enough to allow shape recovery) that can stretch to several times their length and snap back. Natural rubber, Buna-S, Buna-N, and Neoprene are elastomers. The key is: elastic with few cross-links (just enough for memory, not rigid).

Fibres are polymers with high tensile strength and can be drawn into fine threads. They are either linear (no branching) or have a few H-bonds holding chains in parallel. Nylon 6,6, Dacron, and cellulose are fibres.

Plastics sit between elastomers and fibres in rigidity and elasticity. PVC, polyethylene, and polystyrene are plastics.

Addition Polymerization

Addition polymerisation (also called chain-growth polymerisation) occurs when monomers with a carbon-carbon double bond (alkenes) or a strained ring join together without losing any atoms. The repeating unit in the polymer has the same molecular formula as the monomer.

Key requirement: the monomer must have a C=C double bond or a ring that can open (like caprolactam in Nylon 6). No small molecule is lost.

Free Radical Mechanism of Addition Polymerisation

The most common industrial mechanism uses free radicals (species with an unpaired electron). It has three stages: initiation, propagation, and termination.

Step 1 -- Initiation

An initiator (like benzoyl peroxide, ROOR) is heated or exposed to UV light. The weak O-O bond breaks homolytically (each oxygen gets one electron), forming two free radicals:

ROOR → 2RO• (free radical with a dot = unpaired electron)

The radical then attacks the double bond of a monomer (say, ethylene):

RO• + CH₂=CH₂ → RO-CH₂-CH₂• (a new, larger radical)

Step 2 -- Propagation

The chain radical keeps reacting with more monomers. Each step adds one monomer unit and regenerates a radical at the end:

RO-CH₂-CH₂• + CH₂=CH₂ → RO-CH₂-CH₂-CH₂-CH₂•

This continues rapidly, adding thousands of monomer units in a fraction of a second. The chain length (degree of polymerisation) is determined by the ratio of monomer to initiator and the reaction conditions.

Step 3 -- Termination

The growing chain can be stopped in two ways:

(a) Combination: two radicals combine to give one chain without a radical. Chain₁• + •Chain₂ → Chain₁-Chain₂

(b) Disproportionation: one chain transfers a hydrogen to the other; one chain becomes unsaturated and the other becomes saturated.

Important Addition Polymers and Their Monomers

Cationic and Anionic Polymerisation

Free radical is not the only mechanism. In cationic polymerisation, a proton (H⁺) or Lewis acid (like BF₃) generates a carbocation that reacts with monomer. In anionic polymerisation, a carbanion (negative carbon) is the active species (organolithium or sodium naphthalide initiators). The mechanism you need for NEET is the free radical one. The ionic mechanisms apply mainly to specific monomers (e.g., isobutylene uses cationic for butyl rubber).

Condensation Polymerization

Condensation polymerisation (step-growth polymerisation) involves monomers that each have at least two functional groups. Each time two functional groups react to form a bond, a small molecule is eliminated (usually water, H₂O; sometimes HCl or methanol).

Key requirement: every monomer must be bifunctional (two reactive groups) or one monomer must have two different types of groups (like amino acids with -NH₂ and -COOH).

Nylon 6,6 -- The Classic Polyamide

Monomers: hexamethylenediamine (H₂N-(CH₂)₆-NH₂) + adipic acid (HOOC-(CH₂)₄-COOH)

Linkage formed: amide bond (-CO-NH-). Water is eliminated.

Reaction: H₂N-(CH₂)₆-NH₂ + HOOC-(CH₂)₄-COOH → [-NH-(CH₂)₆-NH-CO-(CH₂)₄-CO-]n + n H₂O

The "6,6" in the name refers to the 6 carbons in the diamine and the 6 carbons in the diacid. This is a polyamide (the repeating unit contains amide -CO-NH- linkages).

Properties and uses: Nylon 6,6 is strong, resistant to abrasion and chemicals, and can be drawn into fibres. Uses include ropes, parachutes, toothbrush bristles, and tyre cords.

Nylon 6 -- Ring-Opening Polymerisation

Monomer: caprolactam (epsilon-caprolactam), a 7-membered cyclic amide (lactam) with 6 carbons.

Mechanism: the ring of caprolactam opens (ring-opening polymerisation) and the open-chain units link head-to-tail. No separate small molecule is typically released as with condensation, though the ring opening itself can be considered a type of condensation. The repeat unit is (-NH-(CH₂)₅-CO-)n.

Key NEET distinction: Nylon 6 uses ONE monomer (caprolactam, 6 carbons). Nylon 6,6 uses TWO different monomers (6-carbon diamine + 6-carbon diacid). Both are polyamides, but their origins and properties differ slightly.

Dacron (Terylene) -- The Polyester

Monomers: ethylene glycol (HO-CH₂-CH₂-OH, a diol) + terephthalic acid (HOOC-C₆H₄-COOH, para-phthalic acid, a diacid)

Linkage formed: ester bond (-COO-). Water is eliminated.

Reaction: HO-CH₂-CH₂-OH + HOOC-C₆H₄-COOH → [-O-CH₂-CH₂-O-CO-C₆H₄-CO-]n + n H₂O

Dacron is also called Terylene (UK trade name) or PET (poly(ethylene terephthalate)). Since the linkage is -COO- (ester), Dacron is a polyester. Uses: polyester fibre for clothing, PET bottles (the "PET" in PET bottles), magnetic recording tapes, and mylar films.

Bakelite -- The Thermosetting Phenolic Resin

Monomers: phenol (C₆H₅OH) + formaldehyde (HCHO)

Mechanism: Phenol is ortho/para-activated by the -OH group. Formaldehyde (an aldehyde) reacts at the ortho or para positions of phenol in an electrophilic substitution, with elimination of water.

Stage 1 (Novolac): Linear/slightly branched chain called novolac forms under acidic conditions. Novolac is thermoplastic at this stage and can be moulded.

Stage 2 (Bakelite): Adding more formaldehyde and applying heat causes cross-linking between novolac chains. Methylene (-CH₂-) bridges connect adjacent chains at ortho/para positions. A rigid 3D network forms -- this is Bakelite, a thermosetting polymer.

Uses: electrical switches, plugs, radio cabinets (old-style), billiard balls, pan handles. Bakelite was the first fully synthetic plastic (invented by Leo Baekeland, 1907).

Melamine-Formaldehyde Resin

Similar to Bakelite but uses melamine (a triamine) instead of phenol. Melamine has three -NH₂ groups, making it trifunctional. Reacting with formaldehyde creates an even more heavily cross-linked 3D network. The result is harder, more scratch-resistant, and fire-retardant than Bakelite. Sold as Formica, Melmac. Used in laminate flooring, tableware, and fire-resistant fabrics.

Key Comparison: Nylon vs Dacron vs Bakelite

Important NEET Polymers

This section gives you the complete reference table for all polymers that appear in NEET questions. Learn the monomer(s), polymerisation type, and at least one use for each polymer. The interactive tool below lets you explore each polymer's profile.

Natural and Synthetic Rubber

Natural Rubber

Natural rubber is collected as latex (a milky-white suspension) from the bark of the rubber tree Hevea brasiliensis. Chemically, natural rubber is cis-poly(isoprene), where isoprene is 2-methylbuta-1,3-diene (CH₂=C(CH₃)-CH=CH₂).

The repeating unit of natural rubber is: (-CH₂-C(CH₃)=CH-CH₂-)n

The word "cis" is critical here. In the repeating unit, the methyl group (-CH₃) and the long chain (-CH₂-) are on the same side of the double bond. This cis geometry prevents the chains from aligning closely (unlike the trans form), giving an amorphous, elastic material.

Gutta-percha is the trans isomer of polyisoprene. The trans configuration allows chains to pack closely, giving a hard, non-elastic, brittle material used in golf ball cores and early submarine cables.

Properties of Raw Natural Rubber

Vulcanisation of Rubber

Vulcanisation was discovered by Charles Goodyear in 1839. Raw rubber is mixed with sulfur (2-8% by weight) and heated to 150-180°C.

What happens: Sulfur atoms react with the allylic C-H bonds adjacent to the double bonds in polyisoprene chains. Monosulfide (-S-) or disulfide (-S-S-) bridges form between adjacent polyisoprene chains. These bridges are covalent cross-links.

Effect: Cross-links prevent permanent slipping of chains when stretched. When stretched, the chains extend between cross-links. When released, cross-links pull the chains back. Tensile strength, resilience, and temperature stability all improve.

Hard rubber (ebonite): When 30-40% sulfur is used, the degree of cross-linking is very high, giving a rigid, hard material (ebonite or vulcanite) used in bowling balls and electrical insulators.

Synthetic Rubbers

Synthetic rubbers were developed because of insufficient natural rubber supplies (especially during World War II) and because some applications need properties that natural rubber lacks (oil resistance, heat stability, weather resistance).

Naming Trick for NEET: "Buna"

"Bu" = butadiene (one monomer is always butadiene). "na" = natrium = sodium (Na), which was used as the polymerisation catalyst. The letter after the hyphen identifies the second monomer: "S" = styrene, "N" = nitrile (acrylonitrile). So Buna-S contains styrene, Buna-N contains acrylonitrile.

Biodegradable Polymers

Biodegradable polymers can be broken down by microorganisms (bacteria, fungi) or by natural chemical processes (hydrolysis, photodegradation) into CO₂, water, and biomass. They help reduce the plastic pollution caused by persistent synthetic polymers.

Most synthetic polymers are not biodegradable because: (a) microorganisms have not evolved enzymes to break C-C single bonds in the polymer backbone, and (b) the hydrophobic surface of most plastics prevents water-based microbial attack.

Biodegradable polymers often contain ester or amide bonds in their backbone, which can be hydrolysed by microbial enzymes (esterases, amidases) or by simple water.

PHBV

Full name: Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

Monomers: 3-hydroxybutanoic acid (3-hydroxybutyrate) and 3-hydroxypentanoic acid (3-hydroxyvalerate)

How it forms: Condensation polymerisation. The -OH of one monomer reacts with the -COOH of the next monomer, releasing water. The resulting linkage is an ester bond (-COO-), making PHBV a polyester.

Who makes it? PHBV is produced naturally by certain bacteria as an energy storage material (like fat in animals). The bacterium Ralstonia eutropha accumulates PHBV granules inside its cells.

Biodegradation: Microbial enzymes (PHB depolymerases) hydrolyse the ester bonds in PHBV to give CO₂ and water. PHBV degrades fully in soil within weeks to months.

Uses: Orthopaedic devices (screws, pins), packaging, drug-delivery microspheres where controlled release and biodegradation are needed.

Limitation: Currently expensive (produced biologically), so not yet widely used in everyday packaging.

Nylon 2-Nylon 6

This is an alternating copolymer of two amino acids:

Nylon 2 repeat unit: comes from glycine (H₂N-CH₂-COOH), 2 carbons in the repeat unit (-NH-CH₂-CO-)

Nylon 6 repeat unit: comes from aminocaproic acid (H₂N-(CH₂)₅-COOH), 6 carbons in the repeat unit (-NH-(CH₂)₅-CO-)

The copolymer alternates between these two units: (-NH-CH₂-CO-NH-(CH₂)₅-CO-)n. This is a polyamide. The amide bonds can be hydrolysed by amidase enzymes, making it biodegradable.

Why is it called "Nylon 2-Nylon 6"? The name tells you the carbon count of each repeat unit: 2 carbons (from glycine) and 6 carbons (from aminocaproic acid).

Other Biodegradable Polymers (For Awareness)

Biodegradable vs Non-Biodegradable: Key NEET Point

For NEET, you need to know:

Biodegradable: PHBV, Nylon 2-Nylon 6, PLA, natural rubber, starch, cellulose, proteins.

NOT biodegradable (persistent): PVC, polyethylene, polystyrene, Nylon 6,6, Teflon, Bakelite. These cause plastic pollution because microorganisms cannot break their C-C backbone bonds.

Worked NEET Problems

Work through these two NEET-style problems step by step before checking the solution.

Summary Cheat Sheet

Review this table before your NEET exam. For each polymer, you need to know the monomer(s), polymerisation type, and one key fact.

Quick Rules for NEET Questions

Rule 1: If the monomer has a C=C double bond and there is no by-product, it is addition polymerisation.

Rule 2: If two different functional groups react and water (or HCl) is released, it is condensation polymerisation.

Rule 3: If you can remould it by heating, it is thermoplastic (linear/ branched chains). If it burns or chars on heating and cannot be remoulded, it is thermosetting (3D cross-linked network).

Rule 4: All natural rubber = cis-polyisoprene. The trans form = gutta-percha (hard, non-elastic).

Rule 5: Biodegradable = PHBV (polyester, from bacteria) and Nylon 2-Nylon 6 (polyamide). Neither PVC nor polyethylene is biodegradable.

Rule 6: Nylon 6 = ONE monomer (caprolactam, ring-opening). Nylon 6,6 = TWO monomers (6-carbon diamine + 6-carbon diacid). Both are polyamides.

Rule 7: Dacron = polyester (ester bond -COO-). Nylon = polyamide (amide bond -CO-NH-). Bakelite = phenol-formaldehyde resin (thermosetting).