Polymers & Environmental Chemistry

Polymers and environmental chemistry form the final organic chemistry session — a factual-recall-heavy topic that consistently yields 1-2 NEET questions through monomer identification, polymer classification, and pollution type matching.

Polymer Classification: By source — natural (rubber, cellulose, proteins), synthetic (polythene, nylon, polyester, bakelite), semi-synthetic (cellulose acetate, rayon, vulcanized rubber). By structure — linear (HDPE, nylon-6,6: long chains, high tensile strength, high density), branched (LDPE, amylopectin: irregular packing, low density), cross-linked/network (bakelite, melamine-formaldehyde: extensive covalent cross-links, hard, infusible, thermosetting). By polymerization — addition (chain-growth: unsaturated monomers, no byproduct) vs condensation (step-growth: bifunctional monomers, loss of H₂O). Thermoplastic polymers (polythene, PVC, nylon) soften on heating and can be remolded; thermosetting (bakelite, melamine) set permanently and cannot be remolded.

Addition Polymers: HDPE uses Ziegler-Natta catalyst (TiCl₄ + Al(C₂H₅)₃) at low pressure — linear chains, high density, strong. LDPE uses high pressure (1000-2000 atm, 350-570 K) with a free radical initiator — branched, flexible, low density. PVC monomer: CH₂=CHCl (C=CCl). Polystyrene monomer: C₆H₅CH=CH₂ (). Teflon (PTFE) monomer: CF₂=CF₂ () — chemically inert, non-stick. Polyacrylonitrile (Orlon) monomer: CH₂=CHCN (C=CC#N).

Condensation Polymers: Nylon-6,6 has TWO monomers: hexamethylenediamine () + adipic acid (), each with 6 carbons — the "6,6" refers to 6C in each monomer (polyamide linkage). Nylon-6 has ONE monomer: caprolactam (, ring-opening polymerization). Dacron/PET/Terylene: ethylene glycol (OCCO) + terephthalic acid () — polyester. Bakelite: phenol () + formaldehyde (C=O) — initial novolac (linear) then cross-linked with excess HCHO under base catalysis; thermosetting.

Copolymers: Buna-S (SBR): butadiene (C=CC=C) + styrene () — used in tires. Buna-N: butadiene + acrylonitrile (C=CC#N) — oil-resistant rubber.

Natural Rubber is cis-1,4-polyisoprene (monomer: isoprene ); the cis configuration gives elasticity. Gutta-percha is trans-1,4-polyisoprene (hard, non-elastic). Vulcanization heats rubber with 2-3% sulfur, forming S-S cross-links between chains — the product is harder, less sticky, more elastic, and has better temperature resistance. PHBV (biodegradable) is used in packaging; nylon-2-nylon-6 is a biodegradable polyamide.

Environmental Chemistry — Tropospheric Pollution: London smog (classical smog) — cool, humid, smoke + fog, SO₂ + particulates, reducing nature. Photochemical smog — warm, sunny, NO₂ + hydrocarbons + O₃ + PAN (peroxyacetyl nitrate), oxidizing nature, causes eye irritation. Acid rain: pH < 5.6 (normal rain pH = 5.6 due to dissolved CO₂ forming carbonic acid); caused by SO₂ + NOx → H₂SO₄/HNO₃; damages marble (CaCO₃), corrodes metals, acidifies lakes.

Stratospheric Ozone Depletion: CFCs release Cl. radicals under UV: Cl. + O₃ → ClO. + O₂; ClO. + O → Cl. + O₂. Cl. is regenerated catalytically — one Cl. can destroy ~100,000 O₃ molecules.

Greenhouse Effect: CO₂, CH₄, N₂O, CFCs, and water vapor trap IR radiation, causing global warming. Water Pollution: BOD (Biochemical Oxygen Demand) — higher BOD = more polluted (clean water < 5 ppm, sewage > 17 ppm). Eutrophication: excess nutrients → algal bloom → O₂ depletion. Heavy metals: Hg (Minamata disease), Pb (lead poisoning), Cd (Itai-Itai disease). Green Chemistry: prevention over cure, atom economy, H₂O₂ as clean oxidant (byproduct: water only).

The key testable concept is the distinction between HDPE (Ziegler-Natta catalyst, linear, dense) and LDPE (high pressure, free radical, branched, flexible), and that normal rain pH is 5.6 (not 7.0) with acid rain defined as pH < 5.6.