Editing Formaldehyde (section)

===Polymerization and hydration===
Monomeric CH<sub>2</sub>O is a gas and is rarely encountered in the laboratory.  Aqueous formaldehyde, unlike some other small aldehydes (which need specific conditions to oligomerize through [[aldol condensation]]) oligomerizes spontaneously at a common state. The trimer 1,3,5-trioxane, {{chem2|(CH2O)3}}, is a typical oligomer. Many cyclic [[oligomer]]s of other sizes have been isolated. Similarly, formaldehyde hydrates to give the geminal diol [[methanediol]], which condenses further to form hydroxy-terminated oligomers HO(CH<sub>2</sub>O)<sub>''n''</sub>H. The polymer is called [[paraformaldehyde]]. The higher concentration of formaldehyde—the more equilibrium shifts towards polymerization. Diluting with water or increasing the solution temperature, as well as adding alcohols (such as methanol or ethanol) lowers that tendency.

Gaseous formaldehyde polymerizes at active sites on vessel walls, but the mechanism of the reaction is unknown.<ref>{{Cite journal |last1=Boyles |first1=James G. |last2=Toby |first2=Sidney |date=June 1966 |title=The mechanism of the polymerization of gaseous formaldehyde |url=https://onlinelibrary.wiley.com/doi/10.1002/pol.1966.110040608 |journal=Journal of Polymer Science Part B: Polymer Letters |volume=4 |issue=6 |pages=411–415 |doi=10.1002/pol.1966.110040608 |bibcode=1966JPoSL...4..411B |access-date=2023-04-20 |archive-date=2023-04-20 |archive-url=https://web.archive.org/web/20230420083849/https://onlinelibrary.wiley.com/doi/10.1002/pol.1966.110040608 |url-status=live|url-access=subscription }}</ref> Small amounts of [[hydrogen chloride]], [[boron trifluoride]], or [[stannic chloride]] present in gaseous formaldehyde provide the catalytic effect and make the polymerization rapid.<ref>{{Cite journal |date=1951-03-07 |title=The catalyzed polymerization of gaseous formaldehyde |url=https://royalsocietypublishing.org/doi/10.1098/rspa.1951.0046 |journal=Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences |language=en |volume=205 |issue=1083 |pages=516–529 |doi=10.1098/rspa.1951.0046 |bibcode=1951RSPSA.205..516B |issn=0080-4630 |last1=Bevington |first1=J. C. |last2=Norrish |first2=R. G. W. |s2cid=95395629 |access-date=2023-04-20 |archive-date=2019-10-25 |archive-url=https://web.archive.org/web/20191025013353/https://royalsocietypublishing.org/doi/10.1098/rspa.1951.0046 |url-status=live|url-access=subscription }}</ref>