Both are polyethylene (-CH2CH2-)n but differ dramatically due to branching.

LDPE (Low Density Polyethylene): Made at high pressure (1000-2000 atm), high temperature (200 degC), free radical initiator (peroxide). Chains are highly branched → cannot pack closely → low density (0.91-0.94 g/cm3), low crystallinity, flexible, transparent. Uses: plastic bags, squeeze bottles, cling wrap, packaging films.

HDPE (High Density Polyethylene): Made at low pressure (1-50 atm), low temperature (50-100 degC), Ziegler-Natta catalyst. Chains are linear with minimal branching → close packing → high density (0.94-0.97 g/cm3), high crystallinity, rigid, opaque. Uses: pipes, bottles, containers, buckets, crates.

The Ziegler-Natta catalyst (TiCl4 + Al(C2H5)3) was revolutionary — it allows control over polymer stereochemistry (isotactic, syndiotactic, atactic) and chain linearity. Karl Ziegler and Giulio Natta shared the 1963 Nobel Prize.