How polymers are built: the chemistry of polymerization

One reaction, repeated thousands of times

Every polymer starts as a small molecule — a monomer — carrying reactive groups that let it link to its neighbors. Run that linking reaction thousands of times and the monomer becomes a repeat unit strung into a long chain. Ethylene (CH₂=CH₂) becomes polyethylene, —CH₂—CH₂— repeated; the two loose ends of the chain are capped by whatever started and stopped it, called end groups.

A real batch of plastic is never one length. It is a mixture of chains — some short, some enormous — so a polymer is described by an average chain length (the degree of polymerization) and by how wide the spread is (the dispersity). That distribution is not a nuisance detail: it shifts melting behavior, melt viscosity, and toughness, which is why the same chemistry can be sold as a thin film or a structural molding.

Two ways to join a chain

There are two fundamentally different ways to build a chain, and the difference governs everything downstream. In chain-growth (addition) polymerization, one reactive site — a radical, ion, or metal center — sits at the end of a growing chain and swallows monomers one at a time, so a full-length chain forms almost instantly while unreacted monomer still floats nearby. In step-growth (condensation) polymerization, any two molecules with matching reactive ends can join — monomer to monomer, short chain to short chain — so the average length creeps up slowly.

That distinction has a hard consequence engineers cannot ignore: a step-growth polymer only reaches useful, high molecular weight when the reaction is pushed to near-complete conversion — around 99%. Stop at 95% and you have brittle goo. Chain-growth polymers reach high molecular weight early, so they behave more forgivingly. Many step-growth reactions also release a small molecule (water, methanol) as each bond forms, which the process has to carry away.

Chain-growth: four mechanisms, four personalities

Radical polymerization is the workhorse. An initiator splits into radicals, each adds monomer after monomer down the chain (propagation), and growth ends when two radicals meet or a hydrogen is stolen from a neighbor (termination and chain transfer). It is cheap and tolerant, and it makes polyethylene, polystyrene, PVC, PMMA (acrylic), and polyacrylonitrile — the precursor to carbon fiber.

The ionic routes trade robustness for control. Anionic polymerization can be living — chains keep their active end and stop growing only when starved of monomer, so feeding a second monomer next produces a clean block copolymer. Cationic polymerization suits electron-rich monomers like isobutylene (butyl rubber). Coordination polymerization — Ziegler–Natta and metallocene catalysts — is the one that reshaped industry: a metal center places each monomer with a defined orientation, giving linear high-density polyethylene and stereoregular (isotactic) polypropylene that ordinary radicals could never make.

Step-growth families: from bottle to glue

Pair a molecule with two acid groups and one with two alcohols and you get a polyester — polyethylene terephthalate (PET), the plastic of drink bottles and polyester fabric. Swap the alcohols for amines and hydrogen bonds lock the chains together into a polyamide: nylon-6,6 (from two different monomers) and nylon-6 (from one), tough and abrasion-resistant. These bifunctional (two-ended) monomers give linear, meltable thermoplastics.

Give a monomer three or more reactive groups and the chains stop being strands and become a single crosslinked network — a thermoset that cannot remelt. Phenol plus formaldehyde makes Bakelite, the first fully synthetic plastic; urea– and melamine–formaldehyde make hard tableware and laminate surfaces; epoxies cure into the stiff, chemically resistant network that glues carbon-fiber laminates; and isocyanates plus alcohols make polyurethanes, which span soft foams to rigid coatings depending on the mix. The cure-kinetics simulation on this site models exactly this network forming over time.

Architecture: same atoms, arranged differently

Two plastics with identical chemistry can behave nothing alike because of how the chains are arranged. Chains can be linear, branched, or crosslinked; branching alone is the difference between soft low-density polyethylene (LDPE, cling film) and stiff high-density polyethylene (HDPE, milk jugs). Tacticity — whether side groups sit on the same side, alternate, or scatter randomly along the backbone — decides whether chains can pack into crystals at all, which is why isotactic polypropylene is a rigid engineering plastic and its atactic twin is a gum.

Mixing two monomers multiplies the options. Random and alternating copolymers blend properties; block copolymers join long runs of each into one chain and can self-assemble into tough, rubbery materials; graft copolymers hang one polymer off the backbone of another (the toughened plastic ABS is built this way); and ladder polymers run two parallel linked backbones for extreme heat resistance. Architecture, not just composition, is a design lever.

From chain to solid: why structure sets properties

A chain on paper becomes a material because of the forces between neighboring chains. Weak van der Waals attraction is enough to hold polyethylene together; nylon adds strong hydrogen bonds between chains, which is why it is tougher, higher-melting, and more solvent-resistant. Where regular chains line up, they crystallize into stiff, dense, opaque regions; the disordered regions in between stay flexible.

Those two ingredients — chain stiffness and packing — set the two temperatures that define a plastic's working range: the glass transition Tg, below which the amorphous regions freeze glassy, and the melting point Tm, above which any crystals let go. Read against the Tg and Tm on a material's database card, the whole chain of reasoning closes: monomer choice → chain structure → intermolecular forces → crystallinity → the temperatures where the part keeps its stiffness. The viscoelasticity model on this site picks up the story of how those chains flow under load over time.

Nature got there first

The same rules build the polymers of life. Proteins are step-growth-like chains of amino acids whose sequence folds them into enzymes and into structural fibers such as collagen, the cable in tendon and skin. Cellulose is a crystalline chain of sugar units — the stiff fiber in wood and cotton — and DNA is an information-carrying polymer. Natural rubber is a chain-growth-style polymer of isoprene that engineers vulcanize (crosslink with sulfur) into a durable elastomer.

Seeing synthetic and natural polymers as one subject is the point: monomer, chain, architecture, and the forces between chains explain a nylon gear and a strand of collagen with the same handful of ideas.

Go deeper (open resources)

For precise definitions and nomenclature, the IUPAC Gold Book and IUPAC's polymer recommendations are the authoritative, freely available reference. For a friendly guided tour of how each polymer is made and used, the Polymer Science Learning Center's Macrogalleria (pslc.ws) is free and excellent, and the open Chemistry LibreTexts covers polymerization mechanisms in depth. Everything above is written to complement those — start there when you want the full derivations.