A polymer is a substance composed of macromolecules: large molecules containing many connected structural units derived, actually or conceptually, from smaller molecules. These building blocks are associated with monomers, which can participate in reactions that form polymer molecules. Polymers include natural substances, such as cellulose and proteins, and synthetic materials used in plastics, fibers, coatings, and adhesives. Strictly, polymer denotes the substance, whereas macromolecule denotes an individual molecule within it. Their large molecular size and varied architectures distinguish polymers from substances composed of small molecules. (goldbook.iupac.org)
Molecular structure and classification
In conventional polymers, covalent bonds connect units into chains or networks. A chain’s main sequence of connected atoms is its backbone; attached groups and branches provide additional structural variation. Polymers need not be exclusively carbon-based: their structures may contain organic, inorganic, or organometallic groups. The backbone, side groups, and connections between chains help determine the resulting material’s behavior. (iupac.org)
A homopolymer derives from one monomer species, whereas a copolymer derives from more than one. Copolymers may contain alternating sequences, statistically distributed units, long blocks, or branches of one composition attached to a backbone of another. Molecular architectures include linear, branched, star-shaped, and network structures. Cross-linking connects chains, potentially producing a three-dimensional network rather than separate, freely moving molecules. (iupac.org)
The degree of polymerization expresses the number of monomeric units in a chain. Synthetic polymer samples generally contain molecules of different lengths, so they are characterized by averages and distributions rather than one molecular mass. Number-average molar mass gives each molecule equal statistical weight; mass-average molar mass gives greater weight to heavier molecules. Their ratio, called dispersity, describes the breadth of the molar-mass distribution. These distinctions matter because materials with the same chemical composition can differ substantially in processing and performance. (iupac.org)
Formation and synthesis
Polymerization converts monomers into polymers. Two major mechanistic classes are chain polymerization and step polymerization, also historically called chain-growth and step-growth polymerization. In chain polymerization, monomers react with active sites on growing chains, regenerating those sites during successive growth reactions. In step polymerization, monomers, short oligomers, and longer molecules can react with one another. The distinction concerns how molecular growth occurs, not simply whether a small-molecule by-product forms. (iupac.org)
Step polymerization includes polyaddition, without elimination of small molecules during growth, and polycondensation, with such elimination. Chain polymerization likewise has additive and condensative subclasses. Reaction conditions, monomer composition, and competing reactions influence the final architecture and chain-length distribution. Consequently, a polymer’s identity involves both its constituent units and the way those units have been assembled. (iupac.org)
Physical properties
Polymer properties depend on molecular composition, chain length, branching, intermolecular interactions, and morphology. For example, high-density and low-density polyethylene share the same basic chemical unit but differ in branching and packing. Their resulting properties suit them to different products: relatively rigid containers in the former case and flexible films in the latter. Additives further modify characteristics such as color, flexibility, and resistance to degradation. (nist.gov)
Many solid polymers contain both ordered crystalline regions and disordered amorphous regions. The glass transition concerns the amorphous material: on cooling, cooperative segmental motions become sufficiently slow that the material behaves as a glass. This is distinct from melting crystalline regions. Thermal history and molecular mobility therefore influence whether a polymer behaves as a rigid glass, a rubber-like material, or a flowing melt at a particular temperature. (media.iupac.org)
Thermoplastics can soften or melt on heating and solidify on cooling, allowing reshaping when degradation does not intervene. Thermosetting polymers develop networks during curing and ordinarily cannot be remelted into a processable liquid. Polymer materials can also display viscoelasticity, combining elastic response with time-dependent deformation. Mechanical measurements must therefore account for temperature and loading timescale, rather than treating stiffness as an invariant property. (acs.org)
Natural polymers and historical development
Natural polymers include cellulose, a major structural component of plants, and biological macromolecules such as proteins and DNA. Proteins contain amino-acid residues, while DNA contains nucleotide residues. Their varied sequences illustrate that a polymer need not consist of one identical unit repeated in a perfectly regular arrangement. Natural polymeric materials also supplied fibers and other useful substances long before synthetic polymer production. (acs.org)
The introduction of Bakelite in 1907 marked an important development in fully synthetic plastics. In the early 1920s, Hermann Staudinger argued that polymers were genuinely large molecules composed of connected chains, rather than merely aggregates of small molecules. His discoveries in macromolecular chemistry earned the 1953 Nobel Prize in Chemistry and helped establish the molecular basis of polymer science. (acs.org)
Characterization, applications, and disposal
Size-exclusion chromatography separates dissolved molecules according to their effective size and, with suitable calibration or detectors, provides information about molar-mass distributions and branching. Infrared spectroscopy helps identify polymer chemistry through characteristic spectral patterns. Thermal analysis measures transitions and decomposition, while mechanical testing assesses deformation and strength. Together, these methods connect molecular structure with material performance. (nist.gov)
Polymers serve in packaging, textiles, electrical components, coatings, and numerous other products. A plastic is typically a formulated polymer-based material containing additives, rather than a chemically pure polymer. Recycling is complicated by incompatible polymer mixtures, contamination, additives, and changes caused by processing. Biological origin and biodegradability are separate attributes: a bio-based polymer is not necessarily biodegradable, and degradation depends on environmental conditions. Compostability in one setting therefore does not establish rapid degradation in another. (acs.org)