Bakelite: Phenol-Formaldehyde Resin

Bakelite, the first fully synthetic polymer ever commercialised (developed by Leo Baekeland in 1907), is a condensation polymer of phenol, $\text{C}_6\text{H}_5\text{OH}$, and formaldehyde, $\text{HCHO}$ (methanal), and is the syllabus's representative example of a thermosetting polymer -- one that, once fully cured/cross-linked, cannot be softened and remoulded by reheating (unlike a thermoplastic such as polythene or nylon, which does soften on heating).

Formation. Under mildly basic or acidic catalysis, formaldehyde first adds to phenol at the ring positions ortho and para to the $-\text{OH}$ group (these positions being activated by the strongly electron-donating $-\text{OH}$ group) to give hydroxymethylphenols, e.g. $o$-hydroxymethylphenol. Under controlled, limited-formaldehyde conditions, these units condense with further phenol molecules, eliminating water at each step, to build up a linear, low-molecular-weight prepolymer -- called novolac if made with excess phenol (acid catalysis), which remains fusible and soluble, or resole if made with excess formaldehyde (base catalysis), which is only partially cross-linked. On further heating (and, for novolac, the addition of more formaldehyde as a cross-linking agent, often together with a hardener), the remaining reactive $-\text{CH}_2\text{OH}$ groups and open ring positions condense extensively with neighbouring chains, forming numerous methylene ($-\text{CH}_2-$) and methylene-ether ($-\text{CH}_2\text{--O--CH}_2-$) bridges that link many phenol rings together in every direction, not just end-to-end. This builds a rigid, three-dimensional, cross-linked network rather than a simple linear chain -- once this network has fully formed ('cured'), the material is permanently hard and infusible, because softening it would require breaking many strong covalent cross-links throughout the structure, not just overcoming weak intermolecular forces between separate chains as happens when a thermoplastic is reheated. …