Natural rubber: cis-1,4-polyisoprene. Monomer = isoprene (2-methyl-1,3-butadiene, CH2=C(CH3)-CH=CH2). The cis configuration gives a kinked/coiled chain → elastic properties. Drawbacks: too soft, sticky when warm, brittle when cold, attacked by solvents.

Gutta-percha: trans-1,4-polyisoprene. The all-trans configuration gives straight chains → crystalline, hard, non-elastic. Used in golf ball cores and dental fillings.

Vulcanization (Charles Goodyear, 1839): Natural rubber + sulfur (2-5%) heated at 150 degC. Sulfur forms -S-S- cross-links between chains (at allylic positions). Result: improved elasticity (chains stretch but cross-links pull them back), higher tensile strength, resistance to temperature extremes and solvents. More sulfur (>30%) → ebonite (hard rubber, used in battery cases).

Synthetic rubbers: Buna-S (styrene-butadiene rubber, SBR — 75% butadiene + 25% styrene; used in car tires). Buna-N (nitrile rubber, NBR — butadiene + acrylonitrile; oil-resistant, used in gaskets). Neoprene (polychloroprene — resistant to oil, heat, and weathering).