Phenols reactivity

The study of PF polymerization is far more difficult than that of methylolation due to the increased complexity of the reactions, the intractability of the material, and a resulting lack of adequate analytical methods. When dealing with methylolation, we saw that every reactive ring position had its own reaction rate with formaldehyde that varied with the extent of prior reaction of the ring. Despite this rate sensitivity and complexity, all reactions kinetics were second-order overall, first-order in phenol reactive sites and first-order in formaldehyde. This is not the case with the condensation reactions. 

The reactivity of phenols depends on several structural factors, as well as on the conditions of oxidation (solvent, temperature). Let us discuss the main factors that determine phenol reactivity. This analysis will be performed within the scope of IPM (see Chapter 6). 

The addition of phenol-formaldehyde precondensate to lignin or methylolated lignin has been known as a way to introduce phenolic reactivity into lignin, and this is usually applied to the preparation of lignin-based... 

The fulvic/humic acid separation appears to be a meaningful, valid, and useful separation of stream humic substances. Stream humic acids are always higher in nitrogen content, aromatic carbons, methoxyl and phenolic reactive groups, higher in molecular weight, and more intense in color per carbon atom than stream fulvic acids. 

Ehrenson, Brownlee, and Taft have reported that their a/t values do not give results with phenol which are comparable to the results obtained with other systems. We have examined a number of set of substituted phenol ionization constants in water and other protic solvents and one set in a dipolar aprotic solvent. The results are fairly comparable with those obtained for NH " ", NMe2H", and NHZa as active groups. We would suggest that the a/ values can be applied successfully to phenolic reactivities and that the differences observed by Ehrenson, Brownlee, and Taft may be due in part to the data examined. Best results are obtained when the data have all been obtained in the same laboratory or alternatively when only those values are considered for which the error is known and is not more than 0.05 pX units. The one set of pX data for 4-substituted benzene thiols in water also gives excellent results. We conclude then, that the ionization constants of active sites of the type NHa" ", NMeCH", NHZa, CHZa, OH, and SH and probably to other... 

Model reactions showed that the co-condensation reaction of melamine with phenolic resin occurs only by the reaction of phenolic methylol groups and the unsubstituted amino group at slightly acidic conditions. In contrast, a strong acidic or alkaline environment leads only to the formation of phenol methanal homocondensation products. The phenol reactivity in the reaction with methanal is low in comparison to the melamine reactivity in neutral and mildly acidic environments. An alkaline environment reverses this relative reactivity [99,100]. CiDsslinking of the products obtained by the reaction of 4-methylphenol and melamine with methanal appears to produce an interpenetrating polymer network [101]. 

Bifunctional monomers, such as A-A, B-B and A-B, yield linear polymers. Branched and crosslinked polymers are obtained from polyfunctional monomers. For example, polymerization of formaldehyde with phenol may lead to complex architectures. Formaldehyde is commercialized as an aqueous solution in which it is present as methylene glycol, which may react with the trifunctional phenol (reactive at its two ortho and one para positions). The type of polymer architecture depends on the reaction conditions. Polymerization imder basic conditions (pH = 9-11) and with an excess of formaldehyde yields a highly branched polymer (resols. Figure 1.8). In this case, the polymerization is stopped when the polymer is still liquid or soluble. The formation of the final network (curing) is achieved during application (e.g., in foundry as binders to make cores or molds for castings of steel, iron and non-ferrous metals). Under acidic conditions (pH = 2-3) and with an excess of phenol, linear polymers with httle branching are produced (novolacs). 

The classic study on methylolation of phenol is a 1953 report by Freeman and Lewis [125]. In their work. Freeman and Lewis used paper chromatography to separate the various methylolated species [126,127]. Prior to their work, only an overall rate relationship in phenol and formaldehyde had been reported. They were able to provide a rate constant for formation of each species that we have shown in Scheme 2. Their work was done in concentrated solutions in the presence of sodium hydroxide catalyst. Reaction mixtures contained a 1 1 molar ratio of base to phenol and a 3 1 molar ratio of formaldehyde to phenol. Where a methylolphenol was used as starting material, they employed a molar ratio of 1 1 formaldehyde to available phenolic reactive site. Formalin solutions were of 37% concentration and probably contained methanol, though they do not mention it. Their experiments were done at 30°C. They followed the reactions for up to 1000 h. The samples collected were diluted 10 I with 75% methanol before analysis. 

With reference to the addition reaction and the subsequent condensation reaction, phenol is trifunctional (two ortho and one para positions) and formaldehyde is bifunctional. Therefore, to obtain non-cross-linked products, the molecular weight has to be kept low, which can be achieved by using less than the theoretical amount of formaldehyde. It has been found empirically that a formaldehyde/phenol molar ratio of 0.75 1 must not be exceeded in order to avoid the formation of a cross-linked product. This molar ratio corresponds to an equivalent ratio of the functional groups of r = (2 X 0.75) (3 x 1) = 0.5. Of course, theoretically, the equivalent ratio should be 0.5 in the absence of intramolecular reactions and with quantitative reaction at all the phenol reactive sites. Yet this agreement is fortuitous, since theory cannot correspond exactly with experiment because of the occurrence of intramolecular reactions, the nonequivalence of o- and p-phenol sites in these irreversible reactions, and the formation of open-chain formal structures. Correspondingly, the so-called pseudonovolaks, the o- and p-alkyl phenols, are only bifunctional. They do not give a cross-linked product with an excess of formaldehyde and therefore cannot be cured. 

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