Why can polyethylene and polypropylene stretch so much without breaking?

Polyethylene and polypropylene are able to stretch significantly—often between 100 and 1,000 percent—because they are semicrystalline polymers that exist in a rubbery state at room temperature.1

These plastics are composed of crystalline regions embedded within an amorphous, rubbery matrix. Because their glass transition temperatures ($T_g$) are well below room temperature, the amorphous regions remain flexible and mobile, allowing the material to deform without immediate failure. This unique morphology provides a "leathery toughness" that enables the polymer chains to slide and rearrange under stress rather than snapping.12

Furthermore, the ability to stretch is a fundamental characteristic of their molecular structure. In the plastics industry, this property is utilized in a process called drawing, where the polymer is intentionally stretched to align its molecules. This alignment increases the material's crystallinity and enhances its overall strength, which is why drawn fibers can achieve such high tensile properties.23