Cross-linked polymers are generally stiffer and less flexible than linear polymers made of similar repeating units. What structural feature accounts for this rigidity?

Core Concept

Flexibility in plastics comes mainly from chains slipping, uncoiling, and rearranging when stress is applied. Cross-links are covalent bonds between backbones, so chains lose their independence: bending or stretching must distort real chemical bonds instead of merely overcoming weak interchain attractions. Linear chains held together by dispersion and dipole-dipole forces creep past one another and recover. Vulcanizing rubber with sulfur cross-links turns sticky raw gum into a firm, elastic product.