Dicyanates

A variety of CEs with tailorable physico-chemical and thermo-mechanical properties have been synthesized by appropriate selection of the precursor phenol [39,40]. The physical characteristics like melting point and processing window, dielectric characteristics, environmental stability, and thermo-mechanical characteristics largely depend on the backbone structure. Several cyanate ester systems bearing elements such as P, S, F, Br, etc. have been reported [39-41,45-47]. Mainly three approaches can be seen. While dicyanate esters are based on simple diphenols, cyanate telechelics are derived from phenol telechelic polymers whose basic properties are dictated by the backbone structure. The terminal cyanate groups serve as crosslinking sites. The polycyanate esters are obtained by cyanation of polyhydric polymers which, in turn, are synthesized by suitable synthesis protocols. Thus, in addition to the bisphenol-based CEs, other types like cyanate esters of novolacs [37,48], polystyrene [49], resorcinol [36], tert-butyl, and cyano substituted phenols [50], poly cyanate esters with hydrophobic cycloaliphatic backbone [51], and allyl-functionalized cyanate esters [52] have been reported. 

Although a large variety of such experimental cyanate esters have been reported, few have achieved commercial success. Some of these have been listed in Table 2 along with their key properties. The properties are very much dependent on the backbone. Thus, the experimental CE resin XU.71787 developed by Dow 

Structure of cyanate ester Trade name/Sup- Melt. Tg H2o Dk (1 

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In order to make these oxidative reactions of 1,3-dienes catalytic, several reoxidants are used. In general, a stoichiometric amount of benzoquinone is used. Furthermore, Fe-phthalocyanine complex or Co-salen complex is used to reoxidize hydroquinone to benzoquinone. Also, it was found that the reaction is faster and stereoselectivity is higher when (phenylsulflnyl)benzoquinone (383) is used owing to coordination of the sulfinyl group to Pd, Thus the reaction can be carried out using catalytic amounts of PdfOAcji and (arylsulfinyl)benzoquinone in the presence of the Fe or Co complex under an oxygen atmosphere[320]. Oxidative dicyanation of butadiene takes place to give l,4-dicyano-2-butene(384) (40%) and l,2-dicyano-3-butene (385)[32l]. 

Fig. 11. Proposed copolymerization mechanism for bismaleimides with dicyanates.

Chemical Designations - Synonyms Ethanedenitrite Dicyan Oxalic Acid Dinitrile Oxalonitrile Dicyanogen Chemical Formula (CbOj. 

A new polymer type which emerged as important materials for circuit hoards are polycyanurates. The simplest monomer is the dicyanate ester of hisphenol A. When polymerized, it forms three-dimensional, densly cross-linked structures through three-way cyanuric acid (2,4,6-triazinetriol) ... 

Vitamin-A-saure-nitril 1 -Cyan-naphthalin 3,4-Dicyan-thiophcn... 

Obwohl Triphenyl-phosphan Oxalsaurc-bis-nitriloxid quantitativzu Dicyan reduziert (s. Bd. X/3, S. 868), ist diesc Rcaktion nicht auf andere Nitriloxide iibertragbar. 

Methyl-4-athyl-3,5-dicyan- 89 2-Methyl-4-athyl-3,5-dicyan-1,2- (bzw. 1,4)-dihy-dro- 89... 

Only after pure, aromatic dicyanates had become available (45) a patent by Schminke et al. (40) described the synthesis of poly-(iminocarbonates) with molecular weights of about 50,000 by the solution polymerization of a diphenol and a dicyanate (Scheme 2). Bulk polymerization was also claimed to be possible. 

Contrary to a literature report (40), we found that the bulk polymerization of diphenols with dicyanates is unsuitable for the... 

Illustrative Procedure 2 Poly(iminocarbonates) by Solution Polymerization (46) Under argon, 1 g of a diphenol and an exact stoichiometric equivalent of a dicyanate were dissolved in 5 ml of freshly distilled THF. 1 mol% of potassium tert-butoxide was added, and the reaction was stirred for 4 hr at room temperature. Thereafter, the poly(iminocarbonate) was precipitated as a gumUke material by the addition of acetone. The crude poly(iminocarbonate) can be purified by extensive washings with an excess of acetone. The molecular weight (in chloroform, relative to polystyrene standards by GPC) is typically in the range of 50,000-80,000. 

In contrast, during the interfacial preparation of poly(iminocar-bonates), the hydrolysis of the dicyanate component regenerates the diphenol component, which is a necessary reactant. Consequently, it is possible to obtain poly(iminocarbonates) simply by the controlled hydrolysis of a dicyanate under phase transfer conditions. 

This feature of the interfacial preparation of poIy(iminocarbon-ates) has an important consequence for the synthesis of copolymers if the dicyanate component is structurally different from the diphenol, partial hydrolysis of the dicyanate will lead to the presence of two structurally different diphenol components that will compete for the reaction with the remaining dicyanate. The interfacial copolymerization will therefore result in a random copolymer. On the other hand, during solution polymerization no hydrolysis can occur. Since the dicyanates can only react with diphenols and vice versa, solution polymerization results in the formation of a strictly alternating copolymer. 

Synthesis of Dicyanates. All dicyanates were prepared according to previously published procedures (2). For IR spectral data see Kohn and Langer (2), for elemental analyses, see reference 10. A specific procedure for the preparation of Z-Tyr-Tyr-Hex-dicyanate has been published (75). 

Preparation of Polyiminocarbonates. Solution polymerizations of equimolar mixtures of diphenols and dicyanates were carried out in THF using potassium tert-... 

The hydrolysis leads to 4-aminoimidazole-5-carboxamide, which under certain conditions can react with various partners (e.g., HCN, dicyan or formamidine) to give purines (i.e., adenine, guanine, hypoxanthine and diaminopurine). 

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