Epoxide-anhydride-proton donor reaction

Table 1. Kinetic parameters for non-catalyzed epoxide-anhydride-proton donor reactions"...

Epoxide Anhydride Proton donor Equa- tion Overall reaction order Wi X T rc) Ea kJ/mol Ref. 

Our attention was concentrated mainly on the kinetics and mechanisms of copolymerization, the effect of epoxide and anhydride structure in copolymerization, the effect of type and structure of initiatior on the rate, and the course of copolymerization. The probable mechanisms are discussed. The copolymerization in the presence of proton-donor compounds as well as the effect of proton-donors are also considered. For a better understanding of the processes, data and theoretical views on the non-catalyzed reaction are included. 

In the system epoxide (epoxy resin) — anhydride, we can thus expect the presence of anhydride, epoxy- and proton donor groups. In their study of the reaction mechanism, Fisch and Hofmann 20 22-24) proposed a sequence of reactions leading to the crosslinking of epoxy resins or to the formation of linear polyesters. The first step is the reaction of the anhydride with hydroxyl groups giving a monoester (Eq.(l))... 

Tanaka and Kakiuchi 35,36) proposed a new mechanism of non-catalyzed copoly-merization of epoxides with anhydrides. In the presence of proton donors, they expected the formation of a transition ternary complex composed of all three components of the reaction system. The proposed mechanism (Eqs. (8-10)) is similar to the reaction of phenol with epoxide catalyzed by phenolate 38). 

In contrast to the copolymerization of cyclic carbonates, the molecular weights are lower in the epoxide copolymerization 6,41 43). We assume that this is due to the presence of proton donors in the reaction mixture. They occur as impurities in epoxides 19,20) or anhydrides, moisture in all components of the copolymerization system, or their presence is a consequence of the high rate of hydrolysis of cyclic anhydrides 21). Proton donors added to the monomer feed remarkably decrease the molecular weight42 even in the copolymerization of ethylene glycol carbonate at 200 °C. Under these conditions, when recyclization of phthalic acid takes place 64) and the released C02 can tear off moisture to the gas phase, the molecular weight Mv decreases without proton donors from 45200 to 7100 in the presence of 5% phthalic acid or ethylene glycol or to 9300 in the presence of 15% water42,54. ... 

If the proton donor is an alcohol or a phenol, the active centre is formed directly in reactions (56)—(58) without epoxide isomerization. The propagation steps are the same as in the previous mechanism (Eqs. (59) and (60)). Antoon and Koenig61) proposed a copolymerization scheme for the curing of epoxy resins by anhydrides as a refinement of the mechanisms suggested by Tanaka52) and Lustoft 45 74). However, copolymerization again occurs in the presence of proton donors. The complete sequence proposed by Antoon and Koenig 67) is as follows ... 

The Feltzin mechanism 73) takes account of the presence of proton donors at the beginning of copolymerization. However, initiation probably proceeds in two ways 74) and depends on the type of the proton donor and its concentration in the copolymerization mixture. If HA in Eq. (45) is alcohol, phenol or moisture, initiation occurs according to Eq. (46), i.e. through interaction with the anhydride yielding an ammonium salt of the monoester. The formation of monoesters as primary active centres accounts here for the lower cocatalytic effect of phenols as compared with alcohols. If the proton donor is a carboxylic acid, activation of the tertiary amine (Eq. (63)) is followed by reaction with the epoxide according to Eq. (76)74. ... 

Second-order kinetics with respect to the amine and epoxide was also found by Antipova et al. 65) for the curing of epoxy resins with hexahydrophthalic anhydride, by Sorokin et al.321 for the reaction of phenylglycidyl ether with phthalic anhydride in the presence of butanol, Luston and Manasek 45 74) for the copolymerization of 2-hydroxy-4-(2,3-epoxypropoxy)benzophenone with phthalic anhydride in the absence or in the presence of proton donors, and Kudyakov et al. 98) for the curing of epoxy resins with maleic anhydride. 

At present, we can say that copolymerization initiated by various salts proceeds by an anionic mechanism, after dissociation of the initiators in the reaction medium. The primary step is the addition of the initiator anion to the epoxide. In the initiation by Lewis bases, i.e. by tertiary amines, initiation involves formation of a primary active centre of an anionic character. This active centre is probably generated by interaction of the tertiary amine with the anhydride and an allyl alcohol. The allyl alcohol can be formed by a base-catalyzed isomerization of the epoxide. In the presence of a proton donor, the formation of active centres is possible through interaction of tertiary amine, anhydride and proton donor without epoxide isomerization. Another way of initiation consists in a direct reaction of epoxide with tertiary amine yielding an anionic primary active centre. We believe that in both kinds of initiation in the strict absence of proton donors, the growing chain end has the character of a living polymer. The presence of proton donors, however, gives rise to transfer reactions. 

In spite of a great number of studies devoted to curing with cyclic anhydrides, there existed a number of contraversial views concerning the reaction mechanism. Recent studies have revealedthat in the presence of tertiary amines the reaction can proceed also in the absence of proton donors. The tertiary amine reacts first with epoxide most probably via a zwitterion The simplified mechanism is as follows... 

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