$word_{count}$: 200

Same monomer (ethylene), radically different products — illustrating how polymerization conditions control polymer properties. LDPE: High pressure (1000-2000 atm), high temperature (200 degrees C), free radical initiator (benzoyl peroxide). Mechanism: free radical → chain transfer creates branches → highly branched chains → poor packing → low density (0.91-0.94), low crystallinity, low mp (~115 degrees C), flexible, transparent. Uses: plastic bags, cling wrap, squeeze bottles. HDPE: Low pressure (1-50 atm), moderate temperature (50-100 degrees C), Ziegler-Natta catalyst $\frac{TiCl4}{AlEt3}$. Mechanism: coordination → no chain transfer → linear chains → close packing → high density (0.94-0.97), high crystallinity, higher mp (~135 degrees C), rigid, opaque. Uses: pipes, containers, milk bottles. The Ziegler-Natta catalyst coordinates the monomer at Ti, controlling both linearity (no branching) and stereochemistry (isotactic). For polypropylene, this is even more critical — atactic PP is useless, but isotactic PP (Ziegler-Natta) is a major commercial polymer. JEE tests: process conditions, property comparisons, and the catalyst name.