Thermoplastics: Linear or branched chains held together by intermolecular forces (van der Waals, H-bonds). Heating weakens these forces → polymer softens and can be remolded. Cooling restores rigidity. This cycle can be repeated many times → recyclable. Examples: PE, PP, PVC, PS, nylon, polyester, PMMA.

Thermosetting polymers: Extensive covalent cross-links form a 3D network during molding (curing). Once set, heating cannot soften them — further heating causes decomposition. Irreversible → NOT recyclable by simple remelting. Examples: Bakelite, melamine-formaldehyde, urea-formaldehyde, epoxy resins, vulcanized rubber.

Elastomers: A special category — lightly cross-linked polymers that can stretch and return to original shape. Natural rubber, Buna-S, neoprene. Cross-links are few enough to allow chain extension but sufficient to provide memory/recovery.

Fibers: Very high tensile strength due to strong intermolecular forces (H-bonding in nylon/polyester). Can be drawn into long threads. High crystallinity. Examples: nylon-6,6, Terylene, polyacrylonitrile (Orlon).