One of the earliest, and still one of the most important, polymers is the material known as Bakelite, which is a condensation polymer of phenol, $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH},$ and formaldehyde, $\mathrm{H}_{2} \mathrm{C}=\mathrm{O} .$ Formaldehyde will react with phenol to produce the following compounds when the ratio is 1: 1 :
These species, which can be taken to be monomers, on being heated condense with each other and themselves, eliminating water (formed from the $\mathrm{OH}$ groups on $\mathrm{CH}_{2} \mathrm{OH}$ and ring hydrogen atoms) and linking benzene rings by $\mathrm{CH}_{2}$ groups. If the phenol:formaldehyde ratio is $1: 1,$ a linear polymer forms. If the ratio is $1: 2,$ two $\mathrm{H}_{2} \mathrm{C}=\mathrm{O}$ molecules react with each ring, and the chains cross-link at every benzene ring, forming the infusible, insoluble, hard, brittle solid we know as Bakelite. Sketch the linear chain polymer and the cross-linked solid.