Rubber, cellulose, and other polymers are loose aggregates of small molecules rather than giant covalently bonded molecules.

Early chemists had a size problem.

Rubber, cellulose, and similar materials behaved as if their molecules were enormously larger than familiar chemicals. Their solutions could be extremely viscous, and measurements suggested molecular masses that seemed almost unreasonable.

One way to explain that behavior was to assume there were no giant molecules at all. Perhaps many ordinary small molecules were simply clustering together into loose aggregates.

Hermann Staudinger argued for the more radical alternative in the 1920s: the large molecules were real.

He called them macromolecules. In a polymer, many smaller building blocks are joined together by ordinary covalent chemical bonds, producing long chains or networks containing enormous numbers of atoms.

The proposal met strong resistance because molecules of that size did not fit comfortably with prevailing chemical intuition. Evidence from chemical reactions, viscosity measurements, and increasingly reliable molecular-weight measurements eventually supported Staudinger’s model.

Modern polymer chemistry rests on that idea. Rubber, cellulose, many plastics, and numerous biological molecules owe their properties in part to genuinely large covalently bonded structures.

Polymers can still interact with one another through weaker forces, become tangled, crystallize, or form cross-links. Those larger-scale interactions affect material properties, but they do not replace the covalently bonded macromolecules underneath.