Polymers
Large molecules built from repeating monomer units, ranging from natural biopolymers such as proteins and DNA to synthetic plastics.
Polymers are large molecules — macromolecules — made of many repeating subunits called monomers, linked by covalent bonds into long chains. The word comes from Greek for many parts. Polymer properties depend on chain length, structure (linear, branched, cross-linked), and the chemistry of the monomers, so a single monomer family can yield materials as different as flexible bags and rigid pipes. Polymerization is one of the most important reactions of industrial chemistry: global plastics production exceeds 400 million tonnes per year.
Two main polymerization routes exist. Addition polymerization joins monomers — typically alkenes with carbon–carbon double bonds — by opening the double bond and linking the monomers end to end, with no byproduct; polyethylene, polypropylene, and polystyrene are made this way. Condensation polymerization joins monomers with the elimination of a small molecule such as water, and produces polyesters, polyamides (nylon, Kevlar), and polycarbonates. Copolymers combine two or more monomer types in one chain, enabling fine-tuned properties.
Synthetic polymers are classified by behavior on heating: thermoplastics soften and melt reversibly, so they can be molded and recycled (PET bottles, PVC pipes); thermosets cross-link irreversibly and cannot be remelted (epoxy, bakelite). Rubber is an elastomer — lightly cross-linked chains that stretch and recover. The same principles explain natural polymers: cellulose and starch are sugar polymers, proteins are chains of amino acids whose folding makes them work as enzymes and structural materials, and DNA is a polymer of nucleotides whose sequence stores genetic information.
The usefulness of polymers is matched by their environmental cost: most are not biodegradable, degrade slowly into microplastics, and are largely single-use. Their chemistry — the functional groups along the chain and at its ends — determines both their material behavior and their environmental fate, and designing recyclable or biodegradable polymers is an active field of materials science.
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macromolecules organic chemistry plastics polymers
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