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Natural rubber = cis-1,4-polyisoprene from Hevea brasiliensis. The CIS configuration creates kinked/coiled chains → elastic. But unvulcanized rubber is too soft (warm), too brittle (cold), and dissolves in solvents. Gutta-percha = trans-1,4-polyisoprene → extended chains → hard, crystalline, non-elastic. Same atoms, different geometry → opposite properties. Vulcanization (Goodyear, 1839): Heat rubber + 2-5% S at 150 degrees C → sulfur cross-links at allylic C-H positions → improved elasticity (chains stretch but snap back), tensile strength, temperature resistance, solvent resistance. More S → harder: 30%+ S → ebonite (hard rubber). Synthetic rubbers solve specific problems: Buna-S (SBR, butadiene + styrene) — general purpose, tires (75% of synthetic rubber market). Buna-N (NBR, butadiene + acrylonitrile) — oil-resistant (polar CN groups repel non-polar oils), used in fuel hoses and O-rings. Neoprene (polychloroprene) — heat/oil/weathering resistant, used in wetsuits and automotive belts. JEE favorites: cis vs trans configuration, vulcanization mechanism and purpose, identifying monomers of synthetic rubbers, and classifying rubber as an elastomer (weak forces + slight cross-linking).