Polymers

A polymer is a large molecule composed of repeating structural units called monomers. Polymers can be natural, such as DNA, cellulose, proteins, and rubber, or synthetic, such as polyethylene, polystyrene, and nylon. The properties of a polymer depend on the identity of the monomer, the way units are linked, and the overall architecture of the chain.

Polymers are formed by polymerization, a chemical reaction that links many monomers into long chains. Two broad classes are chain-growth polymerization and step-growth (condensation) polymerization. Chain-growth methods include free-radical, cationic, and anionic polymerization, while step-growth relies on reactions between functional groups to build molecules gradually.

Polymer structure ranges from linear to branched to crosslinked networks. The arrangement of stereochemistry (tacticity), along with molecular weight and its distribution, influences properties such as strength, toughness, and thermal behavior. Materials are often categorized as thermoplastics (melt and remold), thermosets (permanent networks), or elastomers (high elasticity).

Common synthetic polymers include polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and nylon. Natural polymers include DNA, proteins, cellulose, and starch. Biodegradable polymers such as polylactic acid (PLA) and polyhydroxyalkanoates (PHA) are developed for sustainable applications.

Applications span packaging, electronics, textiles, automotive, and medicine. Processing techniques include extrusion, injection molding, fiber spinning, casting, and additive manufacturing. Recycling, biodegradation, and life-cycle assessment are important considerations for reducing environmental impact.