Cyanurate rings

The cyanurate ring is formed hy the trimerization of the cyanate ester. 

Three triple bonds react to give the aromatic cyanurate ring without any hy-products. This is especially enticing for fiber matrices where void formation will he reduced. 

All the cyanated phenols polymerize by a trimerization mechanism to give cyanurate ring systems ... 

The reaction of aromatic bis-cyanate with BMI has been investigated. The cy-anate trimerizes, giving a cyanurate ring but can also copolymerize with the maleimide double bond giving a pyrimidine ring (Fig. 19) during the cure cycle [70,71]. 

Figure 3.40 Cyanurate monomers polymerize with cyanurate ring formation.

The tria2ine ring-containing product 1,3,5-triglycidyl isocyanurate (6) is synthesized by glycidylation of cyanuric acid with epichlorohydrin. The commercial product is a crystalline powder that exhibits an epoxy equivalent weight of ca 108 and softens in the 85—110°C range (see Cyanuric AND ISOCYANURIC acids). 

Figure 8.25 The planar slruclure of various cyanuric compounds all 6 C-N distances within the ring are equal.

Goi. As noted previously, an a-chlorine atom renders a ring-nitrogen atom very weakly basic. Cyanuric chloride (5) is a very weak base both because s-triazines are of low basicity and because each of the ring-nitrogen atoms is alpha to two chlorine atoms. Hence, this compound should be insensitive to acid catalysis or acid autocatalysis and this has been observed for the displacement of the first chlorine atom with alcohols in alcohol-acetone solution and with water (see, however. Section II,D,2,6). 

Compounds of type 6 containing ZR substituents with a -electron pair on the Z atom bound to the ring carbon (Z = O, S, NH) are formed by displacement of the first chlorine atom of cyanuric chloride by weU-known nucleophiles. Their basicity should be greater than that of cyanuric chloride because of the mesomeric electron release of the ZR... 

It also follows that protonation of the triazine ring makes it more susceptible to attack by nucleophilic reagents unless the reagent itself is also protonated. If the triazine ring remains unprotonated when a nucleophilic base, such as an alkylamine, is present as its acid salt the reaction is slower, of course. Cyanuric chloride itself is a very weak base that becomes protonated only under strongly acidic conditions. Thus step 1 in Scheme 11.2 can be carried out in aqueous solution even at pH 2 without risk of undesirable hydrolysis of cyanuric chloride, water being an extremely weak nucleophile. 

Approximately 75% of fluorescent brighteners belong to the stilbene class. These are almost invariably derived from 4,4, diaminostilbene 2,2, disulphonic acid (1.6 X = NH2), often condensed with cyanuric chloride to take advantage of the further contribution of the s-triazine rings to substantivity for cellulose. 

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