Polymers are macromolecules built from repeating monomer units and classified by source (natural, synthetic, semi-synthetic), structure (linear, branched, cross-linked), polymerization type (addition vs condensation), and thermal behavior (thermoplastic vs thermosetting). HDPE is produced by Ziegler-Natta catalysis at low pressure giving linear chains with high density and strength, while LDPE is produced by free radical polymerization at high pressure giving branched, flexible chains — this distinction is the most tested NEET trap in polymer chemistry. Nylon-6,6 is a polyamide from two 6-carbon monomers (hexamethylenediamine + adipic acid), while Nylon-6 is also a polyamide but from a single monomer (caprolactam) by ring-opening polymerization. Dacron (PET) is a polyester from ethylene glycol and terephthalic acid, used in clothing and bottles. Bakelite is a thermosetting phenol-formaldehyde resin with extensive covalent C–C cross-links that cannot be remolded. Natural rubber (cis-1,4-polyisoprene) is elastic due to its coiled chain structure, while gutta-percha (trans-1,4-polyisoprene) is hard and non-elastic, and vulcanization with sulfur creates S–S cross-links making rubber harder yet more consistently elastic. Normal rain has pH 5.6 from dissolved CO2, and acid rain (pH < 5.6) from SO2/NOx causes marble corrosion, lake acidification, and soil damage. CFCs release Cl• radicals in the stratosphere that catalytically destroy ozone (one Cl• destroys ~100,000 O3 molecules) — Cl• is regenerated and not consumed. Mercury causes Minamata disease (neurological), cadmium causes Itai-Itai disease $\frac{bone}{kidney}$, and high BOD (> 17 ppm) indicates heavily polluted water with insufficient dissolved oxygen for aquatic life. Green chemistry principles prioritize waste prevention, atom economy, and cleaner reagents like H2O2 (only byproduct: H2O) over hazardous alternatives.