Epoxy reactions with cyclic anhydride

High molecular weight thermoplastics called phenoxy resins are formed by the hydrolysis of the epoxy resins so that no epoxy groups are present. These transparent resins can be further reacted, forming cross-linked material through reaction of the hydroxyl pendant groups with diisocyanates or cyclic anhydrides. 

This article summarizes and analyzes the results obtained for the anionic copolymerization of cyclic ethers with cyclic anhydrides. This reaction is of great practical importance, especially as curing reaction of epoxy resins and is also used for the preparation of linear polyesters with special functional pendant groups. 

For the copolymerization of epoxides with cyclic anhydrides and curing of epoxy resins, Lewis bases such as tertiary amines are most frequently used as initiators. In this case, terminal epoxides react with cyclic anhydrides at equimolar ratios. The time dependence of the consumption of epoxide and anhydride is almost the same for curing 35-36> and for model copolymerizations 39,40,45). The reaction is specific 39,40) to at least 99 %. In contrast, the copolymerization with non-terminal epoxides does not exhibit this high specificity, probably because of steric hindrances. The copolymerization of vinylcyclohexene oxide or cyclohexene oxide is specific only to 75-80 % and internal epoxides such as alkylepoxy stearates react with anhydrides only to 60-65 %. On the other hand, in the reaction of epoxy resins with maleic anhydride the consumption of anhydride is faster 65the products are discoloured and the gel is formed at a low anhydride conversion 39). Fischer 39) assumes that the other resonance form of maleic anhydride is involved in the reaction according to Eq. (33). 

A reaction order of 1/2 was determined by Malavasic et al.91) for the curing of epoxy resins with cyclic anhydrides over the conversion range 18-79 %. At 86-98.5 % conversion, the authors established a first-order curing reaction. Booss and Hau-schildt90) regard copolymerization and curing as a zero-order process with respect... 

After the amines, acid anhydrides constitute the next most commonly used reagents for curing epoxy monomers. The epoxy-acid reaction proceeds through a stepwise mechanism (Sec. 2.2.4) while the reaction of epoxides with cyclic anhydrides, initiated by Lewis bases, proceeds through a chain-wise polymerization, comprising initiation, propagation, and termination or chain transfer steps. Some of the postulated reactions are shown in Table 2.25 (Matejka et al., 1985b Mauri et al., 1997). 

Curing of epoxy resins by polycarboxylic acids and cyclic anhydrides is also important in applications, but it is much less understood due to more complex reaction mechanism. Also, the statistical treatment is less developed and partly requires a revision. In this section, the statistics of curing of epoxy resins with polycarboxylic acids and cyclic anhydrides is discussed. 

For the copolymerization of epoxides with cyclic anhydrides and curing of epoxy resins, Lewis bases such as tertiary amines are most frequently used as initiators. In this case, terminal epoxides react with cyclic anhydrides at equimolar ratios. The time dependence of the consumption of epoxide and anhydride is almost the same for curing copolymerizations jjjg reaction is specific... 

Another usual family of hardeners employed to cure epoxy monomers are cyclic anhydrides, with the reaction initiated by tertiary amines or ammonium salts. The reaction proceeds through an alternating chainwise copolymerization, as shown in Figure 28.3. 

The reaction of an alkoxide group with a cyclic anhydride is much faster than the reaction of a carboxylate group with an epoxy ring. 

Cyclic anhydrides are the second most important class of comonomers for cure of epoxy resins (Fig. 3.34). Whereas amine cure usually leaves linear segments and hydrophilic -OH groups, anhydride cure can also react with the -OH groups to produce many more cross-links, thus increasing molecular rigidity and water resistance. These cure reactions generally require heat and catalysis. 

Two resin types were investigated, including epoxy esters from stepwise reaction of sucrose-partial drying oil ester with cyclic anhydride and diepoxide, and polyurethanes from sucrose-partial drying oil ester and diisocyanate. Details of synthesis of the resins and their evaluation as coatings are discussed below. 

The curing reaction between an epoxy resin and an acid anhydride with or without catalyst has been studied by Tanaka and Kazinschi [78], Fisch and coworkers [80,81], Wegler and coworkers [82], Dearborn and coworkers [83,84], Fischer [85], and Schechter and Wynstra [79J. Anhydride curing agents are widely used with epoxy resins. The majority of these are cyclic anhydrides. 

Cyclic anhydrides of carboxylic acid are also used as cross-linkers for epoxy resins. Curing is initiated by reaction of a hydroxyl with an anhydride to yield a half ester and a carboxylic acid group. The newly formed carboxylic acid will in turn react with the epoxy to generate an ester and a new hydroxyl group. These complementary reactions result in a cross-linked network. Etherification reactions... 

A cyclic anhydride, such as phthalic anhydride or maleic anhydride, will form a nearly 1 1 alternating copolymer with alkylene oxides. This reaction is the same one used in the curing (hardening) of epoxy resins. A large number of initiators can be used for this reaction, including tertiary-amines, various Lewis acids, and bases including alkoxides. The uncatalyzed reaction invokes initiation with a hydroxyl group (125,126). This reaction can be used to introduce a polyester unit into a polyether polymer chain. With phthalic anhydride, the reaction proceeds as follows ... 

The isolation of the non-cyclic amino(aryl)carbenes and amino(alkyl)car-benes demonstrated that singlet carbene centers can be sufficiently stabilized by only one a-nitrogen atom. In 2005, Bertrand and co vorkers succeeded in preparing the first cyclic alkyl(amino)carbenes (CAACs, Scheme 1.15). The precursor for CAAC 123 was obtained from an imine by deprotonation with LDA (LDA = lithium diisopropylamide) and subsequent reaction with l,2-epoxy-2-methylpropane to give 121, which was converted into cyclic aldirain-ium salt 122 by reaction with trifluoromethanesulfonic acid anhydride. Deprotonation of 122 with LDA afforded CAAC 123 as a colorless solid (Scheme 1.15). 

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