Polymer

Polymers are large molecules composed of repeating units, called monomers, linked by covalent bonds to form macromolecules with high molecular weight. The repeating units may be identical or different, and the overall properties depend on the chemistry of the monomer, the way units are arranged, and the length of the chain.

Polymers occur naturally in forms such as cellulose, proteins, DNA, starch, and natural rubber, and are also synthesized industrially, including polyethylene, polypropylene, polystyrene, polyvinyl chloride, nylon, and polyurethanes.

Polymerization describes the chemical process by which monomers join to form polymers. Addition (chain-growth) polymerization builds chains one unit at a time, often via free-radical, anionic, or cationic mechanisms. Condensation (step-growth) polymerization links monomers with the loss of small molecules such as water or methanol, forming bonds between functional groups.

Polymer structures vary: linear chains, branched chains, and crosslinked networks. Tacticity (arrangement of side groups) leads to isotactic, syndiotactic, or atactic polymers. Crystallinity and molecular weight influence properties such as strength, toughness, transparency, and thermal resistance.

Classification by response to heat: thermoplastics soften upon heating and can be reshaped; thermosets form rigid networks and do not remelt; elastomers are flexible. Applications span packaging, textiles, electronics, automotive parts, biomedical devices, and more.

Environment and sustainability: some polymers are recycled or mechanically processed; biobased and biodegradable polymers such as polylactic acid (PLA) are developed to reduce dependence on fossil resources.