Chemistry of amino resins

The methylol products formed by the above reaction are partially or fully etherified with alcohol to confer compatibility with co-resins and improved storage stability. This reaction is carried out under acidic pH conditions. The condensation reaction of two methylol groups (polymerization) competes with an alkylation reaction, which can be controlled by a molar excess of alcohol. The processing conditions are quite different for methylated aminoplasts compared to butylated ones, because for methanol, being water soluble, it is not possible to remove water of the reaction easily, while it is in the case of butanol. Therefore, to achieve the desired level of alkylation, a large excess of methanol, up to 1.6 mole per mole of the methylol group, is used. Primary alcohols react more readily with methylol groups than secondary alcohols, while tertiary alcohols are non- 

---

Melamine. Melamine (cyanurotriamide, 2,4,6-ttiainino-j -triazine) C H N, is a white crystalline soHd, melting at approximately 350°C with vaporization, only slightly soluble in water. The commercial product, recrystallized grade, is at least 99% pure. Melamine was synthesized eady in the development of organic chemistry, but it remained of theoretical interest until it was found to be a usehil constituent of amino resins. Melamine was first made commercially from dicyandiamide [461-58-5] (see Cyanamides), but is now made from urea, a much cheaper starting material (9—12) (see also... 

This, in its very simplest form, is the chemistry of amino coatings. There are, of course, an enormous variety of types and reactivities of both amino compounds and polymers. While it would be useless to try to enumerate the variations possible, some systematization of the reactivity of amino resins is necessary to understand the trends in the coating industry. 

Of these constituents, the resins and essential oil are peculiar to the hop and are responsible for its brewing value. In wort boiling tannins, sugars, amino acids, and proteins derived from the hop will go into solution and react as discussed in Chapter 14 but in general the larger proportion of these constituents in beer will be derived from malt. The chemistry of the resins and essential oil will be discussed in this chapter. During wort boiling, the a-acids are isomerized into iso-a-acids but discussion of this most important reaction is deferred until Chapter 14. 

Vale, C. P., and Taylor, W. G. K., Amino Plastics, London, Iliffe Books, Ltd., 1964. A veiy comprehensive review of amino resin chemistry and technology. 

The chemistry of melamine has been reviewed (63,64). Melamine, although moderately basic, is better considered as the triamide of cyanuric acid than as an aromatic amine (see Cyanuric AND ISOCYANURIC AClDs). Its reactivity is poor in nearly all reactions considered typical for amines. In part, this may be a result of its low solubiUty (see Amino resins and plastics). 

As with the other aminoplastics, the chemistry of resin formation is incompletely understood. It is, however, believed that under acid conditions at aniline-formaldehyde ratios of about 1 1.2, which are similar to those used in practice, the reaction proceeds via p-aminobenzyl alcohol with subsequent condensation between amino and hydroxyl groups (Figure 24.10). 

Moore, S., Spackman, D., and Stein, W. H., 1958. Chromatography of amino acids on sulfonated polystyrene resins. Analytical Chemistry 30 1185-1190. 

Aminoquinolines, 21 185, 199 Amino resins, 16 409. See also Amino resins and plastics coating resins, 7 96-98 formaldehyde in, 12 121 Amino resins and plastics, 2 618-652 chemistry of resin formation,... 

To overcome the chain fragmentation of sensitive peptides that contain /V-alkyl amino acids caused by acids as described vide supra, the following measures are recommended (1) in solution synthesis the exploitation of protecting groups removed by acids should be minimized (e.g., apply Fmoc chemistry, use acid-labile side-chain protection as little as possible) (2) if the peptides must be subjected to acid use low temperatures (—20 °C) for the shortest time possible (monitor the reaction by HPLC ) (3) in SPPS if the peptide does not contain functionalized side chains, prepare the peptide by Fmoc chemistry on Trt resin and remove the peptide from the resin with HFIP (see Section 10.1.1.2.2). If the peptides have functionalized side chains see point (1). 

The side-chain protected peptide was first synthesized on 4-nitrobenzophenone oxime resin utilizing Boc chemistry (see Section 12.4.1.2). Briefly, the first amino acid residue was attached to the resin using DIC activation. Subsequent couplings of residues to the resin-bound peptide were accomplished employing the BOP reagent for carboxy activation of the a-amino acids avoiding preneutralization of the resin-bound peptides for reasons discussed above. A six-fold excess of amino acid is utilized for the third... 

Partially protected peptide thioesters that are prepared from SPPS and purified by RP-HPLC can condense with other peptide segments to form highly homologous peptides which can comprise as many as 100 amino acid residues. 9 Syntheses of peptide thioesters using Boc-SPPS have been quite successful.161214 However, the preparation of thioesters by Fmoc chemistry is difficult because the piperidine used to remove the Fmoc group attacks the carbonyl moiety of the resin-bound thioester to release the peptide from solid support. However, peptide thioesters have been prepared by SPPS using Fmoc chemistry. 9 ... 

The chemistry of melamines and phenolics is quite similar. In both cases formaldehyde [50-00-0] is added to the reactive sites on the parent ring to form methylol phenols (3) or methylol melamines (4) (see PHENOLRESINS Amino resins). There are six reactive sites on the triazine ring of melamine [108-78-1] (1) so it is possible to form hexa.methylolmelamine. However, the most common degree of methylolation is 1.5—2.0. The ortho and para positions of phenol are active thus phenol can be trimethylolated (2). However, as with melamine, lower degrees of methylolation such as 1.2—2.5 are common. 

---