PolymersNEET Chemistry · Class 12 · NCERT Chapter 11

Addition polymerisation (also called chain-growth polymerisation) involves monomers joining one after another without losing any atoms. The monomer must have a double bond (alkene) or a ring that opens. The repeating unit has the same molecular formula as the monomer. No by-product is released. Examples: polyethylene (ethylene), PVC (vinyl chloride), Teflon (tetrafluoroethylene). Condensation polymerisation (step-growth) involves monomers with two different functional groups (like -NH₂ and -COOH) reacting with each other to form a bond and simultaneously release a small molecule (usually water, HCl, or methanol). The monomer must be bifunctional (two reactive groups). Examples: Nylon 6,6 (releases water), Dacron/Terylene (releases water), Bakelite (releases water). The key NEET test: does the polymer release a by-product during formation? If yes, condensation; if no, addition.

Nylon 6,6 is formed from TWO different monomers: hexamethylenediamine (H₂N-(CH₂)₆-NH₂, 6 carbons with amino groups at both ends) and adipic acid (HOOC-(CH₂)₄-COOH, 6 carbons with carboxyl groups at both ends). The two "6"s refer to the 6 carbons in each monomer. It forms by condensation polymerisation releasing water. Nylon 6 is formed from a SINGLE monomer: caprolactam, which is a cyclic amide (lactam) with 6 carbons. It forms by ring-opening polymerisation -- the ring opens and the monomer units add to the growing chain. Although Nylon 6 has only 6 carbons per repeat unit and Nylon 6,6 has 12, both are polyamides (they contain the amide bond -CO-NH-). The NEET distinction: Nylon 6 = ring-opening (caprolactam); Nylon 6,6 = condensation of two 6-carbon monomers.

The answer is in the 3D structure. Thermoplastic polymers like PVC have linear or slightly branched chains with no covalent bonds between them. When you heat them, the weak intermolecular forces (van der Waals) between chains break and the polymer softens and flows. When cooled, it re-solidifies. This can be repeated many times. Thermosetting polymers like Bakelite are cross-linked: covalent bonds form between adjacent polymer chains during curing, creating a rigid 3D network. These covalent cross-links cannot be broken by heating; instead, the material degrades or burns. That is why thermosetting polymers cannot be remoulded once set. PVC chains are held only by intermolecular forces (no cross-links) so they can slide past each other on heating. Bakelite cross-links are permanent covalent bonds so the chains cannot move relative to each other.

Natural rubber is cis-poly(isoprene): the polymer chains have a very irregular, coiled conformation because of the cis double bonds at every repeat unit. When you stretch rubber, these coiled chains straighten out (entropy decreases). When you release, the chains return to their coiled state (entropy increases). This entropic elasticity is what makes rubber stretchy. However, raw natural rubber has two problems: it is sticky in summer (too soft) and brittle in winter (too hard), and the chains can slip past each other irreversibly. Vulcanisation fixes both problems. It involves heating rubber with sulfur (2-8% by weight). Sulfur atoms form disulfide cross-links (-S-S- or -S-Sx-S-) between adjacent polymer chains. These cross-links prevent the chains from slipping past each other permanently, giving rubber its resilience and shape memory. Hard rubber (ebonite) uses 30-40% sulfur for very dense cross-linking.

A homopolymer is made from a single type of monomer. All repeat units are identical. Examples: polyethylene (only ethylene), PVC (only vinyl chloride), Teflon (only tetrafluoroethylene), polystyrene (only styrene). A copolymer is made from two or more different types of monomers. The different monomers may be arranged in different patterns: alternating (ABABAB), random (AABBAAB), block (AAAA-BBBB), or graft. Examples: Buna-S (butadiene and styrene, so called SBR = styrene-butadiene rubber), Buna-N (butadiene and acrylonitrile). For NEET: if the polymer name contains two monomer names or the polymer has two distinct repeat units, it is a copolymer. Both Buna-S and Buna-N are copolymers (synthetic rubbers).

Teflon is poly(tetrafluoroethylene), PTFE. Its backbone is a carbon-carbon chain completely surrounded by fluorine atoms. The C-F bond is one of the strongest bonds in organic chemistry (bond energy ~485 kJ/mol, compared to ~410 kJ/mol for C-H). Fluorine is the most electronegative element, so the C-F bond is very short, very strong, and very polar. The fluorine atoms form a tight sheath around the carbon backbone, shielding it from attack by other chemicals. This is why Teflon does not react with strong acids, strong bases, or oxidising agents. It also has a very low coefficient of friction because the smooth, waxy fluorine surface repels other molecules. Uses: non-stick cookware (the original "non-stick" coating), chemical-resistant pipes and gaskets, electrical insulation. NEET key: Teflon = PTFE = monomer tetrafluoroethylene = addition polymer = chemically inert.

Biodegradable polymers are polymers that can be broken down by microorganisms (bacteria or fungi) under natural conditions into small, non-toxic molecules like CO₂, water, and biomass. Most synthetic polymers (PVC, polyethylene, Nylon) are not biodegradable and accumulate as persistent plastic waste. Two important biodegradable polymers for NEET: (1) PHBV (poly-beta-hydroxybutyrate-co-beta-hydroxyvalerate): a copolymer made by bacteria. It is used in orthopaedic devices and drug-delivery systems. PHBV is formed by condensation polymerisation of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid. (2) Nylon 2-Nylon 6: an alternating copolymer of glycine (Nylon 2, with 2 carbons in the repeat unit) and aminocaproic acid (Nylon 6, with 6 carbons). The amide bonds in these polymers are susceptible to hydrolysis by enzymes in the environment. NEET most commonly asks about PHBV as the key example of a biodegradable polymer.

Polymers chapter contributes 1-2 questions per year in NEET. The most frequently tested topics and polymers are: (1) Monomer identification: given a polymer, identify its monomer (or vice versa). High frequency for Teflon, Nylon 6, Nylon 6,6, Dacron, PVC. (2) Addition vs condensation classification: Nylon 6,6, Dacron, Bakelite are condensation; PVC, Teflon, Polystyrene are addition. (3) Thermoplastic vs thermosetting: Bakelite and melamine-formaldehyde are thermosetting; most others are thermoplastic. (4) Natural rubber structure: cis-polyisoprene, vulcanisation with sulfur. (5) Biodegradable polymers: PHBV is the most tested example. (6) Buna-S and Buna-N composition. Key memory tip: learn each polymer as a four-item card: monomer(s), polymerisation type, thermoplastic/thermosetting, and one key use.