Linear phenol-formaldehyde oligomers

Calixarenes, coined by C. D. Gutsche, are the cyclic oligomers produced by condensation of phenols and formaldehyde (1). In other words, they are cyclic phenol resins, but their physical and chemical properties are much different from those of linear phenol resins. Many of the calixarenes are crystalline and generally have poor solubilities in either water or in organic solvents. Their melting points are generally high, while ordinary phenol resins, novolaks, soften below 150°C. 

Linear novolac resins prepared by reacting para-alkylphenols with paraformaldehyde are of interest for adhesive tackifiers. As expected for step-growth polymerization, the molecular weights and viscosities of such oligomers prepared in one exemplary study increased as the ratio of formaldehyde to para-nonylphenol was increased from 0.32 to 1.00.21 As is usually the case, however, these reactions were not carried out to full conversion, and the measured Mn of an oligomer prepared with an equimolar phenol-to-formaldehyde ratio was 1400 g/mol. Plots of apparent shear viscosity versus shear rate of these p-nonylphenol novolac resins showed non-Newtonian rheological behavior. 

Tubular reactors are used for some polycondensations. Para-blocked phenols can be reacted with formalin to form linear oligomers. When the same reactor is used with ordinary phenol, plugging will occur if the tube diameter is above a critical size, even though the reaction stoichiometry is outside the region that causes gelation in a batch reactor. Polymer chains at the wall continue to receive formaldehyde by diffusion from the center of the tube and can crosslink. Local stoichiometry is not preserved when the reactants have different diffusion coefficients. See Section 2.8. 

Novolacs are obtained by the reaction of phenol and formaldehyde in acidic conditions. Novolac oligomers are linear or slightly branched addition products linked by methylene bridges (molar masses in the range 500 5000 g mol-1). The reaction is usually carried out using a molar ratio CH20/ PhOH close to 0.8, to avoid gelation in the reactor (see Chapter 3) ... 

Little is known about the overall mechanism of cyclic oligomer formation, although the mechanism of the initial stages of the sequence seems fairly clear. The first chemical event is the reaction of formaldehyde (formed in the Petrolite procedures by depolymerization of paraformaldehyde) with phenol to form 2-hydroxy-methyl- and 2,6-6w(hydroxymethyl)phenols in a base-catalyzed process, as shown in Fig. 3. Such compounds were characterized many years ago50), obtained from the action of aqueous formaldehyde on phenol in basic solution at room temperature. Subsequent condensation between the hydroxymethylphenols and the starting phenol occurs to form linear dimers, trimers, tetramers, etc. via a pathway that might involve o-quinonemethide intermediates which react with phenolate ions in a Michael-like reaction, as portrayed in Fig. 4. The condensation of hydroxymethyl-... 

Fig. 4. Base-catalyzed formation of linear oligomers from phenols and formaldehyde...

The linear bi- and tri-phenolic oligomers formed in the condensation of p-tert-butylphenol and formaldehyde i.e. the precursors to calixarenes) have been shown to form intermolecular associated dimers even at concentrations lower than 10 These have been called hemicalixarenes (see ref. 1, pp. 53-54) and presumably play a significant role in the proclivity of the linear oligomers to form cyclic oligomers. 

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