A polyether derived from aldehydes (R=CH₂ or CHR′) or ketones (R=C(R′R′)) and containing –O–R–O– groups in the main chain. Of the many possible polyacetals, the most common is a polymer or copolymer of formaldehyde, polyoxymethylene (–O–CH₂–)_n. While the substance paraformaldehyde contains oligomers or low-molecular-weight polyoxymethylenes (n very small), high-molecular-weight, crystalline polyoxymethylenes constitute an important class of engineering plastics that, in commerce, is often simply referred to as polyacetal. Cellulose and its derivatives also have a polyacetal structure. For discussion of other polyethers and polymers containing acetal side groups.