Polymers and environmental chemistry represent the closing chapter of organic chemistry in the NEET syllabus — a factual-heavy topic that reliably delivers 1–2 questions per year. Success depends on precise memorization of monomer-polymer pairs, polymerization conditions, and a handful of key numerical values in environmental chemistry.
Polymer Chemistry
Polymers are macromolecules formed by the repeated joining of small monomer units through covalent bonds. They are classified along four independent axes that NEET frequently tests in combination. By source: natural polymers (cellulose, starch, natural rubber, proteins — found in organisms), synthetic polymers (polythene, nylon, PVC, Bakelite, Teflon — made in laboratories), and semi-synthetic polymers (rayon/cellulose acetate, vulcanized rubber — derived from natural sources but chemically modified). By structure: linear polymers (HDPE, nylon-6,6 — long unbranched chains that pack closely, giving high density and tensile strength), branched polymers (LDPE, amylopectin — irregular side chains reducing crystallinity and density), and cross-linked/network polymers (Bakelite, melamine-formaldehyde — extensive 3D covalent connections making them rigid and infusible). By polymerization mechanism: addition polymers (unsaturated C=C monomers join without any byproduct; same empirical formula in product and monomer; chain-growth mechanism) and condensation polymers (bifunctional monomers react with loss of a small molecule like H2O; step-growth mechanism). By thermal behavior: thermoplastics (polythene, PVC, nylon — soften on heating due to only intermolecular forces, can be remolded repeatedly) and thermosets (Bakelite, melamine — set permanently due to covalent cross-links, char on further heating).
Addition Polymers — Critical Details: HDPE uses the Ziegler-Natta catalyst (TiCl4 + Al(C2H5)3) at LOW pressure — coordination mechanism → linear, unbranched chains → high density (0.94–0.96 g/), high crystallinity (80–90%), high tensile strength. LDPE uses a free radical initiator at HIGH pressure (1000–2000 atm, 350–570 K) → branched chains → low density (0.91–0.93 g/), lower crystallinity (50–60%), flexible. The most frequently tested NEET trap is reversing these conditions. Other key addition polymers: PVC (vinyl chloride, C=CCl, free radical → pipes, cables), Teflon/PTFE (tetrafluoroethene, FC(F)=C(F)F, free radical → chemically inert, non-stick), polystyrene (styrene, C=Cc1ccccc1 → packaging, insulation), PAN/Orlon (acrylonitrile, C=CC#N → synthetic wool fibers).
Condensation Polymers — Critical Details: Nylon-6,6 has TWO monomers — hexamethylenediamine (H2N–(CH2)6–NH2, NCCCCCCN) and adipic acid (HOOC–(CH2)4–COOH, OC(=O)CCCCC(=O)O) — each with 6 carbons, hence "6,6." Linkage: amide (–CO–NH–). Nylon-6 has ONE monomer — caprolactam (O=C1CCCCCN1) — ring-opening polymerization. Also amide linkage. Dacron/PET/Terylene: ethylene glycol (OCCO) + terephthalic acid (OC(=O)c1ccc(C(=O)O)cc1) → polyester (–COO– linkage) — used in clothing and bottles. Bakelite: phenol + formaldehyde → first novolac (linear, thermoplastic intermediate), then with excess HCHO and heat → extensive C–C cross-linked network (thermosetting). Copolymers: Buna-S (butadiene + styrene, tire rubber), Buna-N (butadiene + acrylonitrile, oil-resistant rubber).
Rubber: Natural rubber = cis-1,4-polyisoprene (monomer: isoprene, C=CC(=C)C) — the cis configuration causes chain coiling → elasticity. Gutta-percha = trans-1,4-polyisoprene — chains are straighter and pack closely → hard, non-elastic. Vulcanization: heating natural rubber with 2–3% sulfur → S–S cross-links between chains → product is harder, less sticky, more consistently elastic, better temperature resistance. Biodegradable polymers: PHBV (poly-beta-hydroxybutyrate-co-beta-hydroxyvalerate) — bacterial polyester used in packaging; nylon-2-nylon-6 — biodegradable polyamide.
Environmental Chemistry
Air Pollution: London smog (classical/reducing): cool, humid conditions; pollutants SO2 + particulates/smoke from coal combustion; reducing in nature; causes respiratory problems and poor visibility. Photochemical smog (oxidizing): warm, sunny conditions; NOx + VOCs from automobiles + UV light → O3 + PAN (peroxyacetyl nitrate) + secondary pollutants; oxidizing in nature; causes eye irritation and plant damage.
Acid rain: pH < 5.6. Normal rain has pH = 5.6 (CO2 dissolves in rainwater → H2CO3, a weak acid). Acid rain contains H2SO4 (from SO2) and HNO3 (from NOx) which are strong acids. Effects: marble/limestone corrosion (CaCO3 + H2SO4 → CaSO4 + H2O + CO2), lake acidification killing aquatic life, soil acidification, metal corrosion.
Ozone Depletion: CFCs (chlorofluorocarbons) are stable in the troposphere but decompose in the stratosphere under high-energy UV radiation, releasing Cl• radicals. Catalytic cycle: Cl• + O3 → ClO• + O2; ClO• + O• → Cl• + O2. Net: O3 + O• → 2O2. Cl• is NOT consumed — it is regenerated catalytically, allowing one Cl• radical to destroy approximately 100,000 O3 molecules. The Montreal Protocol (1987) successfully initiated global CFC phase-out; ozone layer is slowly recovering.
Greenhouse Effect: Greenhouse gases (CO2, CH4, N2O, CFCs, H2O vapor) trap outgoing infrared radiation from Earth's surface. CO2 is the most important by total radiative forcing (highest volume from fossil fuels), though CFCs have a much higher GWP per molecule. This enhanced greenhouse effect causes global warming and climate change.
Water Pollution: BOD (Biochemical Oxygen Demand) measures how much dissolved O2 bacteria need to decompose organic matter — clean water < 5 ppm; heavily polluted/sewage > 17 ppm. Eutrophication: excess N and P from agriculture/sewage → algal bloom → algae die → bacteria consume O2 decomposing dead algae → BOD increases → O2 depletion → fish die. Heavy metal toxicity: Hg (methylmercury, lipid-soluble, crosses blood-brain barrier) → Minamata disease (neurological); Cd (replaces Ca in bones) → Itai-Itai disease (painful bone fractures, kidney failure); Pb → neurological damage (especially children). DDT: non-biodegradable, lipid-soluble → biomagnification through food chain.
Green Chemistry: Prevention over cure; atom economy (maximize atoms from reactants incorporated into product); H2O2 as clean oxidant (only byproduct = H2O); water as solvent replacing toxic organic solvents.