Epoxide-anhydride-initiator copolymerization

The first of these relates to the differences between reactions catalyzed by these materials in the presence and absence of hydroxyl compound initiators. Polymerization with added amounts of such initiators proceeds slower than without them at the same catalyst concentration. This is in contrast to polymerization of epoxides and to copolymerization of epoxides and anhydrides using potassium hydroxide as the catalyst. In the latter cases, little or no reaction occurs in the absence of an initiator. Hydroxyl compounds also retard the activation of the catalyst that is, the induction periods are longer in their presence. Finally, the molecular-weight distribution of the polymer prepared without the initiators is much broader than that of the polymers made with the initiators. 

To understand and comparev the mechanisms and rates of polyester formation in catalyzed copolymerization, processes taking place in the system epoxide-anhydride without any initiator are described. In this review, copolymerization in the absence of compounds that do not occur in the initial reaction mixture is regarded as a non-catalyzed reaction. This means that the presence of alcohols, phenols or acids is not excluded. These compounds may be considered as copolymerization catalysts however, because of their possible occurrence in the polymerization system they are not regarded as initiators. The presence of OH groups in epoxy compounds, especially in resins where they occur as chlorohydrines (I), monoethers (II), and diethers of glycerol (III)... 

In contrast to the non-catalyzed reaction, the base-initiated copolymerization was found to be a specific reaction 35,36,39 -45) and the consumption of both monomers, epoxide and anhydride, is the same. The initiator not only affects the rate of copolymerization but also suppresses the undesirable homopolymerization of the epoxide. At equimolar ratio, epoxide and anhydride are strictly bifunctional. 

Hilt et al. 42,541 found by conductivity measurements that the copolymerization of epoxides with cyclic anhydrides initiated by alkali salts is of ionic character. Luston and Manasek 36,57), who used ammonium salts, came to the same conclusion. The rate of copolymerization increases linearly with rising initiator concentration (Fig. 1, the slope of the curve log k vs log c,n is unity) but only up to a limit which depends on the solubility of the initiator in the reaction system54). A rise in the copolymerization rate is accompanied by an increase in the conductivity of the reaction... 

Hamann and co-workers 41,42 54) applied this mechanism to the copolymerization of epoxides with cyclic anhydrides initiated by organic and inorganic salts. They suggested individual stages of copolymerization according to Eqs. (16—23).1 Initiator dissociation ... 

Therefore, a similarity between the mechanisms of copolymerization of epoxides with anhydrides initiated either by phosphonium salts or alkali metal or ammonium salts can be expected and copolymerization then proceeds according to Hamann s mechanism illustrated by Eqs. (16M23). 

Only few data are avalaible on the copolymerization of epoxides with cyclic anhydrides initiated by inorganic and organic salts. 

Several mechanism have been proposed for the copolymerization of epoxides with cyclic anhydrides, initiated by tertiary amines. In one of the first papers on copolymerization mechanisms Fischer 39,40) suggested the following scheme ... 

Tsubokawa, N. Yamada, A. Sone, Y. (1983). Grafting of Polyester onto Carbon Black. 4. Copolymerization of Epoxide with Phthalic Anhydride Initiated by COOK Groups on Carbon Black Surface. Polymer Bulletin, 10, 63-69... 

Tsubokawa N, Kogure A and Sone Y, Grafting of polyesters from ultrafine inorganic particles copolymerization of epoxides with cyclic acid anhydrides initiated by COOK groups introduced onto the surface , J Polym Sci Polym Chem 1990 28 1923-33. 

In this paper, we try to review results obtained from anionic copolymerization of cyclic ethers with cyclic anhydrides. For a better understanding data and theoretical views on non-catalyzed copolymerizations are also included. We concentrate mainly on the kinetics and mechanism of copolymerization and the effect of the type and character of the initiator used. The influence of the epoxide and anhydride structure on copolymerization, of proton donors on the rate and course of copolymerization, and on the molecular weight of the resulting polyesters are also discussed. 

Anionic initiators for copolymerization of epoxides can be divided into two groups. The first group comprises salts of inorganic and organic acids and the second group Lewis bases. Among them, tertiary amines are the most often used. Both types of initiators for the polyreaction of epoxides with anhydrides have some properties in common ... 

Equation (31) allows the determination of the ratio of propagation rate constants k3/k2 by correlation of the experimental results obtained in copolymerization using equimolar ratios of monomers with the results obtained at non-equimolar monomer ratio and excess of anhydride (an excess of epoxide should not be used because of homopolymerization of the monomers at high degree of conversion). A value of k3,/k2 equal to 0.2 0.1 was found for the system 2-hydroxy-4-(2,3-epoxypropoxy) benzophenone-phthalic anhydride in nitrobenzene initiated by hexadecyltrimethyl-ammonium bromide 56). 

Data on molecular weights of polyesters can provide information on the mechanism and termination and transfer reactions. As follows from section 3.2.3 the copolymerization of epoxides with cyclic anhydrides should proceed stoichiometrically without transfer or termination reactions, and the average degrees of copolymerization should only depend on the molar ratio of monomers to the initiator. Polymers with a narrow molecular Weight distribution should be obtained. 

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). 

The question of initiation, structure and character of the active centre is fundamental for the copolymerization of epoxides with cyclic anhydrides. We therefore analyze arguments supporting individual mechanisms. Since in most mechanisms HA compounds are involved, the question arises whether proton donors are necessary for the formation of an active centre. 

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. ... 

All authors accept the alternating incorporation of epoxide and anhydride into the macromolecular chain 36 39.40.45 52.73-74). However, the mechanisms of termination and chain transfer have not yet been elucidated. Although the lability of the nitrogen atom is obvious 39 40 44> and its salts or associates are readily thermally decomposed 89), Fischer 39 detected its presence in precipitated polyesters by elemental analysis. A simple calculation confirms the presence of the nitrogen atom in almost every tenth macromolecule. In this case, the isolated polyester might be a living polymer and, on the addition of monomers, it might initiate another copolymerization. Similar experiments have not been reported so far. 

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. 

Table 2.25 Anhydride-epoxide copolymerization initiated by tertiary amines... 

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