Addition reactions methylol group

Resoles. The advancement and cure of resole resins foUow reaction steps similar to those used for resin preparation the pH is 9 or higher and reaction temperature should not exceed 180°C. Methylol groups condense with other methylols to give dibenzyl ethers and react at the ortho and para positions on the phenol to give diphenyknethylenes. In addition, dibenzyl ethers eliminate formaldehyde to give diphenyknethanes. 

In summary, it is clear that methylolation is a bimolecular, second-order reaction. As methylol groups are added to the ring, the ring undergoes general activation. Addition of o-methylol groups increases the acidity of the phenolic hydroxyl, which could increase reaction rates. However, all methylol groups ap-... 

The basic amino group of the 1-position in semicarbazide or thiosemi-carbazide may be used to react by a substitution reaction with activated halides [52], ethers [51], hydroxy [53], phenoxy [54], and amino groups [55] to yield substituted 1-semicarbazides or thiosemicarbazides. In addition, the amino group of the 1-position may add to electron-deficient double bonds [56]. Formaldehyde and other aldehydes may add to all the available free NH groups to give methylol, alkylol, or polymeric products under basic conditions [57]. Aldehydes or ketenes usually give semicarbazone derivatives, and these in turn are used analytically to identify the purity or structure of a known aldehyde [3]. 

Two different classes of chemical reactions are singled out, namely the reactions of addition of formaldehyde to the aromatic ring, which introduce a methylol group as a substituent, and the reactions of condensation, which produce components with higher molecular weight. In the presence of an alkaline catalyst, the reactions of addition are strongly oriented in the -orto and -para positions of the aromatic ring, whereas the reactions of condensation occur both between two substituted positions... 

In these reactions, a large number of two-ring isomers are formed, depending on the position of attack. In a later stage of reaction, these diphenols can undergo both addition reactions of one more methylol group on a free position of the aromatic... 

In order to estimate the kinetic parameters for the addition and condensation reactions, the procedure proposed in [11, 14] has been used, where the rate constant kc of each reaction at a fixed temperature of 80°C is computed by referring it to the rate constant k° at 80°C of a reference reaction, experimentally obtained. The ratio kc/k°, assumed to be temperature independent, can be computed by applying suitable correction coefficients, which take into account the different reactivity of the -ortho and -para positions of the phenol ring, the different reactivity due to the presence or absence of methylol groups and a frequency factor. In detail, the values in [11] for the resin RT84, obtained in the presence of an alkaline catalyst and with an initial molar ratio phenol/formaldehyde of 1 1.8, have been adopted. Once the rate constants at 80°C and the activation energies are known, it is possible to compute the preexponential factors ko of each reaction using the Arrhenius law (2.2). 

In addition to the two main reactions, methylolation and condensation, there are a number of other reactions that are important for the manufacture and use of amino resins. For example, two methylol groups may combine to produce a dimethylene ether linkage and liberate a molecule of water. 

Novolacs are low-molecular-weight, fusible but insoluble prepolymers (18) prepared by reaction of formaldehyde with molar excess of phenol. Novolacs, unlike resoles, do not contain residual methylol groups. A high-molecular-weight network polymer similar to that of resoles is formed by heating novolac with additional formaldehyde, paraformaldehyde, or hexamethylenetetramine. 

In amine or ammonia-catalyzed reactions the additions and condensations occur almost simultaneously with each other. Methylol groups are still present in the finished resins to the extent of 15-30 groups per 100 phenol residues. The structures are branched and the degree of branching depends upon the amine used. 

The methylol group gets its name because it consists of a methyl group in which one hydrogen atom is replaced by an alcoho/ic hydroxyl. Note particularly that acid conditions give a condensation reaction, while basic conditions give an addition reaction. It is the latter route that interests us at present, because the water-soluble dimethylol urea could at this stage be converted to a water-soluble resin, by a switch to acid conditions ... 

Two reactions between the phenolic resin and the NBR are possible, but both depend on the fact that during cure some methylol groups will be available to react, preferentially, with the NBR rather than with further novolac species. The first is a condensation/addition reaction across the double bond (chroman formation) and the second is a condensation reaction with the a-hydrogen atom on the elastomer. 

The methylol compounds produced by these reactions are relatively stable under neutral or alkaline conditions, but undergo condensation, forming polymeric products under acidic conditions. Consequently, the first step in making an amino plastic is usually carried out under alkaline conditions. The amino compound and formaldehyde are combined and form a stable resin intermediate that may be used as an adhesive or combined with filler to make a molding compound. The second step is the addition of an acidic substance to catalyze the curing reaction, often with the application of heat to cure the amino resin to the solid cross-linked state. In this reaction, the methylol group is probably protonated and a molecule... 

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