Resoles ortho substitution

Crosslinking resoles in the presence of sodium carbonate or potassium carbonate lead to preferential formation of ortho-ortho methylene linkages.63 Resole networks crosslinked under basic conditions showed that crosslink density depends on the degree of hydroxymethyl substitution, which is affected by the formaldehyde-to-phenol ratio, the reaction time, and the type and concentration of catalyst (uncatalyzed, with 2% NaOH, with 5% NaOH).64 As expected, NaOH accelerated the rates of both hydroxymethyl substitution and methylene ether formation. Significant rate increases were observed for ortho substitutions as die amount of NaOH increased. The para substitution, which does not occur in the absence of the catalyst, formed only in small amounts in the presence of NaOH. 

Alkaline Catalysts, Resoles. Resole-type phenoHc resins are produced with a molar ratio of formaldehyde to phenol of 1.2 1 to 3.0 1. For substituted phenols, the ratio is usually 1.2 1 to 1.8 1. Common alkaline catalysts are NaOH, Ca(OH)2, and Ba(OH)2. Whereas novolak resins and strong acid catalysis result in a limited number of stmctures and properties, resoles cover a much wider spectmm. Resoles may be soHds or Hquids, water-soluble or -insoluble, alkaline or neutral, slowly curing or highly reactive. In the first step, the phenolate anion is formed by delocali2ation of the negative charge to the ortho and para positions. 

Resole syntheses entail substitution of formaldehyde (or formaldehyde derivatives) on phenolic ortho and para positions followed by methylol condensation reactions which form dimers and oligomers. Under basic conditions, pheno-late rings are the reactive species for electrophilic aromatic substitution reactions. A simplified mechanism is generally used to depict the formaldehyde substitution on the phenol rings (Fig. 7.21). It should be noted that this mechanism does not account for pH effects, the type of catalyst, or the formation of hemiformals. Mixtures of mono-, di-, and trihydroxymethyl-substituted phenols are produced. 

Resoles. Resoles are phenolic resins produced under alkaline conditions with a molar excess of formaldehyde, HCHO, over phenol in the reaction mixture. The initial reaction is the substitution of phenol with methylol (-CH2OH) groups, as shown in Figure 1, both at the ortho (I) and para (II) positions. Furthermore, because more than 1 mol of formaldehyde is used for each mole of phenol, products carrying two or three methylol groups (HI, IV) are also formed. The ortho para substitution ratio depends on the type of catalyst and pH, and decreases from 1.1 at a pH of 8.7 to 0.38 at a... 

Phenolic resins, introduced in 1908, are formed by either base- or acid-catalyzed addition of formaldehyde to phenol to give ortho- and para-substituted products. The nature of these products depends largely on the type of catalyst and the mole ratio of formaldehyde to phenol. In resole formation, excess formaldehyde is reacted with phenol under basic conditions. The initial reaction products are ortho- and para-substituted mono-, di-, and trimethylolphenols ... 

The phenolic resins that form in acid-catalyzed condensations of phenols with formaldehyde are different from resols. At pH below 7, 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 crosslinked resins. At a ratio of 8 moles of formaldehyde to 10 moles of phenol, novolacs of approximate molecular weight 850 form. When the ratio of formaldehyde to phenol is 9 10, a molecular weight of approximately 1000 is reached. This appears to be near the limit, beyond which crosslinking results. The reaction is as follows ... 

The pH value used for prepolymer synthesis is an important parameter in order to obtain novolacs or resoles. Novolacs are formed if the reaction between phenol and methanai occurs in an acidic medium. Typical catalysts used are oxalic acid, sulfuric acid, and 4-methylbenzene sulfonic acid. The use of slight acidic conditions and bivalent metal salts (e.g. Ca, Zn, Mg, Pb, Co, Cu, Ni) to catalyze the phenol methanai reaction results in ortho, ortho -novolacs. Furthermore, no catalyst is necessary for the reaction of a substituted phenol with paraformaldehyde at temperatures between 150-200 C for the manufacture of novolacs. Resoles are formed when phenol reacts with an excess of methanai atpH values larger than 5. This process is catalyzed by inorganic salts, ammonia, and 1,3,5,7-tetraazatricyclo-... 

Unlike resoles, which show a definite preference for methylolation and condensation at the para position, ring positions in novolacs are less differentiated. The normal ratio of o,p-, and /j,p-linkages in a novolac will be 1 2 1. This may be affected by the choice of catalyst, and much work has been done to control this aspect of novolac synthesis, with the emphasis on producing highly or/Zio-Iinked resins. In some cases, the judicious choice of protic acid may lead to the desired result. More commonly, a Lewis acid salt is chosen as the catalyst. These are usually divalent metal salts of acetates or similar small carboxylates. Zinc acetate is probably the most common example. Often resins made using these salts cannot be cleanly characterized as resole or novolac. They may have a resole molar ratio and a novolac pH or they may be made near neutral conditions. As mentioned before, commercial phenolic polymers showing 85% ortho linkage are available. Solvent choices may also be important to determination of substitution patterns. 

Model compounds of high ortho-ortho resoles have been prepared by Kammerer et al from the condensation of p-chloro-substituted phenols followed by subsequent dehalogenation. They also prepared a series of p-methyl-substituted benzylic ether-containing compounds and analyzed their structures by IR and UV. 

Isomeric composition of resolic oligomers is determined by the nature of the catalyst, pH value, and the composition of the reaction medium. Thus, substitution for NaOH by hydroxides or oxides of alkaline-earth metals increases the ortho derivative content of the reaction product. The same modification is obtained by reducing pH value to as low as 6-8 or carrying out the process in a medium with a low water content. As a rule, resolic oligomers have an extremely heterogeneous composition, which represents one of the main causes of structural faults found in resite. These faults determine the alteration of the physicomechanical performance of resite [8]. 

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