Polymers are substances composed of macromolecules with recurring structural units.
Key knowledge
A polymer is a substance composed of macromolecules whose structures contain repeated units derived actually or conceptually from smaller molecules. Natural and synthetic polymers span diverse properties; control of branching and stereochemical arrangement can change material behaviour.
Conditions and scope
A polymer sample can contain a distribution of molecular sizes and structures.
A repeating-unit description does not imply that all polymer chains are identical or formed by one reaction pathway.
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In chain-growth polymerization, monomer addition regenerates a growing active centre. Radical, cationic and anionic routes have different initiation and chain-carrying chemistry, so monomer substitution and conditions affect which route is suitable. Termination consumes an active centre and ends that chain; combination of two radical chains is one termination mechanism.
Step-growth polymerization joins compatible functional groups on monomers and oligomers rather than requiring all growth at one persistent chain centre. Difunctional acid/amine or ester/alcohol chemistry can form polyamides or polyesters. Diol/isocyanate addition gives a polyurethane linkage, showing that step growth need not release a small-molecule condensation product.
A semicrystalline polymer contains ordered crystallites and amorphous material. Melting disrupts the crystalline regions, whereas the glass transition concerns the changing mobility and response of amorphous chain segments. Chain architecture, packing, permanent crosslinks, temperature and observation timescale influence mechanical behavior. Permanent network junctions restrict long-range chain flow; an uncrosslinked melt can instead flow as chains rearrange. Crystallinity and chain orientation can alter stiffness and directional strength without uniquely fixing all material properties.