Q.Explain, in terms of chain structure (branching), why LDPE is a soft, flexible plastic used for bags while HDPE is a rigid, tough plastic used for buckets and pipes.

LDPE is produced by a free-radical mechanism in which the growing chain occasionally 'backbites' on itself, introducing short and long branches at random points along the backbone. These branches act like obstacles that keep neighbouring chains from lying close and parallel to one another, so LDPE has lower crystallinity, weaker net intermolecular (dispersion) attraction between chains, and a lower density -- properties consistent with a soft, flexible plastic suited to bags and film, which need to deform easily. HDPE, made by the Ziegler-Natta coordination mechanism, adds monomer units in an essentially unbranched, linear fashion; these straight chains can pack closely and regularly together, maximising crystallinity and the total surface area over which dispersion forces act between …

Link the manufacturing mechanism (free-radical vs Ziegler-Natta) to the resulting chain shape (branched vs linear), then link chain shape to packing/crys …

Explaining the property difference by molecular weight instead of branching/packing, when both forms can have comparable molecular weights; no …