Q.How is polythene manufactured ? Give their properties and uses.
Step 1 (LDP manufacture). Ethylene, CH2=CH2, is polymerized under very high pressure (1000-2000 atm) and temperature (350-570 K), using traces of oxygen or a peroxide as free-radical initiator; the mechanism involves free-radical addition together with intramolecular H-atom abstraction, which branches the growing chain.
Step 2 (LDP properties/uses). Because its branched chains pack loosely, LDP has low density; it is extremely flexible, tough, chemically inert, moisture-resistant, a poor electrical conductor, and melts around 110 degC. It is used for agricultural/irrigation pipes, domestic water lines, submarine cable insulation, extruded films/sheets for packaging, and household items like squeeze bottles.
Step 3 (HDP manufacture). Ethylene is instead polymerized using a Ziegler-Natta catalyst (triethyl aluminium + titanium tetrachloride) at a much milder temperature (333-343 K) and pressure (6-7 atm), giving an essentially linear, unbranched chain.
Step 4 (HDP properties/uses). Because its unbranched chains pack closely, HDP is crystalline and denser, with a higher melting point (144-150 degC), greater tensile strength, hardness and chemical resistance than LDP. It is used for toys, buckets, dustbins, bottles, pipes, and laboratory ware needing high strength and stiffness.
Polythene, -[CH2-CH2]n-, is made as LDP (high pressure/temperature, O2/peroxide initiator -> branched, flexible, low-density: pipes, cable insulation, packaging films) or HDP (Ziegler-Natta catalyst, mild conditions -> linear, stiff, high-density: buckets, bottles, laboratory ware).
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