Reactions of Cyanate Esters

CEs are known to react with groups like epoxy, bismaleimide, etc. These reactions are described in detail in Sect. 8 devoted to blends. Although reaction with triple bonded compounds has been postulated, evidences for this remain to be furnished. 

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Figure 2. Reaction of cyanate ester with an epoxy to form an oxazoline ring.

Cyanate esters (CEs) are formed in excellent yields by the reaction of the corresponding phenols with cyanogen halide [34]. The reaction scheme is shown in Scheme la. The reaction is usually carried out in solution, in the presence of a tertiary amine as the acid scavenger at very low temperatures. Since the trimer-ization reactions of cyanate esters are highly prone to catalysis by spurious impurities, the most difficult aspect of cyanate ester synthesis is their scrupulous purification. Low molar mass esters are purified by distillation or recrystallization. Polymeric cyanates are purified by repeated precipitation in non-solvents such as water, isopropanol, etc. While distillation and recrystallization lead to pure materials, the precipitation method for polymeric cyanates is not always conducive to obtaining pure materials. A recent patent application claims a purification method for polymeric cyanates based on treatment with cation and anion exchangers [35]. 

Researchers have conducted a number of studies highlighting the synthesis of CEs, reaction conditions including solvents and catalysis [36-38], etc.,but most of the details are covered by patents. The solvents selected include acetone [39], methyl isobutyl ketone [40], dichloromethane [41], etc., and acid scavengers chosen mainly include triethyl amine and pyridine [39, 41]. The use of tri-alkylamines often leads to the formation of dialkyl cyanamides as side products, which are known to catalyze the reactions of cyanate esters, reducing the yield. The formation of these impurities depends on the mode and rate of addition of reagents, reaction temperature, etc. Factors like stirring rate, dropping rate, and... 

Despite their many attractive features, the search for further improvements in performance and reduction in cost of cyanate esters is never ending and a large number of studies have been devoted to matrix modification through blends, and co-curing of these systems. Reactions of cyanate esters with a variety of functional groups like amines [169], hydroxyl [34, 66], epoxy [170-172], phenols, etc., have been reported, among which the most studied are reactions with epoxy [173,174]. Epoxy-cyanate blends are common and found in many commercial and patented resin formulations. 

Figure 2.41 Reaction of cyanate ester resin with water to form carbamate and...

The reaction rate of cyanate ester resins can be increased by using catalysts such as carboxylate salts or chelates of transition metal ions. The role of transition metal ions in the polymerization reaction consists of facilitating the cycli-zation reaction of three cyanate monomer functionalities by the formation of... 

Tris aryl cyannrate ( Tris aiyloxy s-triazine) Scheme 1. Synthesis of cyanate ester and its reaction with water and phenol... 

Scheme 14. Reaction mechanism for the polymerisation of cyanate ester proposed by Lous-talot et al.

Transition metal organometallic complexes like dicarbonyl cyclopentadienyl iron [128], tricarbonyl cyclopentadienyl manganese [129] and iron-arene complexes [130,131] have also been reported as photoinitiators for photochemical crosslinking of cyanate esters. Photosubstitution of carbonyl groups by -OCN during irradiation initiates the reaction in the former case whereas photochemical dissociation of arene triggers it in the latter system. 

The reactions of amino esters 97 (R =H, CH2Ph) with potassium cyanate afforded c/.y-cyclopenta[c/]pyrimidine-2,4-dione 98, while those with potassium thiocyanate led to cw-2-thioxocyclopenta[[Pg.291]

Densely cross-linked PCN networks, synthesized by polymerization, via cy-clotrimerization reaction, of dicyanic ester of bisphenol A [159-161], consist of rigid triazine heterocycles as junctions connected with bisphenol A remainders (see a scheme in Fig. 26). At the total conversion of cyanate groups (Xcn pcn 1). PCN exhibits high Tg 300°C and rigidity, good adhesion to different substrates and chemical resistance, as well as low values of dielectric constant and water uptake. This allowed in particular its application as the matrix for glass- and aramid fiber-reinforced plastics in the electronics and aerospace industries. 

Su WFA (1993) Thermoplastic and thermoset main-chain liquid-crystal polymers prepared from biphenyl mesogen. J Polym Sci Polym Chem 31 3251-3256 Su WFA, Chuang CM (2002) Effects of chemical structure changes on curing reactions and thermal properties of cyanate ester-cured rigid-rod epoxy resins. J Appl Polym Sci 85 2419-2422... 

Some organosilicon compounds undergo transmetallation. The allylic cyanide 461 was prepared by the reaction of an allylic carbonate with trimethylsi-lyl cyanide[298]. The oriho esters and acetals of the o. d-unsaturated carbonyl compounds 462 undergo cyanation with trimefhylsilyl cyanide[95]. 

Acetylation of the hydroxymethyl imidazole 63 affords the corresponding ester (64), nitration (65) followed by hydrolysis gives intermediate 66, and reaction of this alcohol with potassium cyanate in hydrogen fluoride gives the carbamate ronidazole (67).16... 

Cyanogen bromide can be used to activate hydroxyl groups on particles to create reactive cyanate esters, which then can be coupled to amine-containing ligands to form an isourea bond (Figure 14.17). CNBr activation also can produce cyclic imidocarbonate groups, which are less reactive than the cyanate ester, but can form imidocarbonate bonds. The exact reactive species formed by the reaction is dependent on the structure of the hydroxylic support being activated (Kohn and Wilchek, 1982). 

The curing reaction can be carried out thermally or with the addition of a catalyst. The thermal cure is strongly influenced by impurities associated with the synthesis. The greater the degree of monomer purity, the more slowly the thermal cure proceeds. If the monomer is sufficiently purified, the cure rate can be predictably controlled by the addition of catalysts. As with the aromatic cyanate esters, the fluoromethylene cyanate esters can be cured by the addition of active hydrogen compounds and transition metal complexes. Addition of 1.5 wt% of the fluorinated diol precursor serves as a suitable catalyst.9 The acetylacetonate transition metal salts, which work well for the aromatic cyanate esters,1 are also good catalysts. 

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