Novolacs
The phenolic resins that form in acid-catalyzed condensations of phenols with formaldehyde are different from resols. At pH below seven protonation of the carbonyl group of formaldehyde takes place first and is followed by electrophilic aromatic substitution at the ortho and para positions of the phenol. The initial steps of the reactions also take place in water. Here, however, a molar excess of phenol (1.25:1) must be used, because reactions on equimolar basis under acidic conditions form cross-linked resins. At a ratio of eight moles of formaldehyde to ten moles of phenol, novolacs of approximate molecular weight of 850 form. When the ratio of formaldehyde to phenol is 9:10, a molecular weight of approximately 1,000 is reached. This appears to be near the limit, beyond which cross-linking results. The reaction is as follows:
Because the reaction is taking place at pH below 7, the above shown p- and o-methylolphenols are transitory and are present in small concentrations only. Hydrogen ions convert them to benzylic carbocations that react rapidly with free phenol. This can be illustrated as follows:
The ortho substituted methylolphenols react in the same manner. Further metholylation of dihydroxy diphenyl methanes takes place until all the formaldehyde is used up. Methylol groups react with each other quickly and form methylene bridges. The para position is more reactive than ortho at pH below 3. The opposite is true, however, at pH 5–6, where the ortho position is more reactive. Typical novolacs formed in these reactions are not very high in molecular weight and contain no more than six to ten benzene rings. If divalent metal salts, like zinc acetate in acetic acid, are used to catalyze the reaction, then the ortho positions become considerably more reactive . As a result, ortho-methylene bridges predominate in the products:
It is possible to form high molecular weight novolacs by carrying out the reactions of alkyl phenyl ethers with formaldehyde in acetic acid in the presence of perchloric acid [149]:
Novolacs are cross-linked by additions of more formaldehyde to the soluble, thermoplastic materials. The additional formaldehyde can be in the form of paraform, an oligomer of formaldehyde that decomposes to formaldehyde upon heating. It can also come from hexamethylenetetramine, a condensation product of formaldehyde with ammonia:
Hexamethylenetetramine decomposes to formaldehyde and ammonia upon heating. Some of the ammonia is picked up by the novolacs with the result that there are some benzylamine bridges in the product:
The mechanism of this reaction is discussed in the next section.
