The chemical linking of small monomer molecules into long polymer chains, the reaction behind the plastics, rubbers, and resins of modern life. In foods it shows up in oil degradation, where repeatedly heated frying oils polymerize into gums and polar compounds that darken the oil, raise viscosity, and force its disposal. In packaging manufacture, controlled polymerization builds the polyethylene, PET, and other plastics that protect foods, with catalysts and conditions tuned for food-contact purity. The same chemistry underlies protein cross-linking in doughs and gels, where heat and enzymes join protein chains to build texture.