Investigation of maleate‐fumarate isomerization during esterification with various glycols

Abstract It is known that maleic acid is partially and completely converted to fumaric acid, or its ester, on esterification. This phenomenon has a great practical importance, especially for unsaturated polyesters, because of the properties of the maleates, notably their reactivity during copolymerization, where their behavior is quite different from that of the fumarates. The effect of many factors relating to this phenomenon has not yet been clarified. In this paper is described a study in which the effect of glycols on the isomerization was determined by using glycols of different molecular structures. The following method was used: molten maleic anhydride was condensed with various glycols. In the course of the polycondensation, samples were taken, and the ratio of maleic to fumaric was determined. The determinations were carried out by polarography after saponification in the cold. For the polycondensation the following glycols were used: ( 1 ) ethylene glycol; ( 2 ) 1,2‐propylene glycol; ( 3 ) 1,3‐butylene glycol; ( 4 ) diethylene glycol. It was established that the structure of the glycol and the time of polycondensation, as well as the molecular weight of the polyester when identical glycols were used, exercise a considerable influence on the isomerization. When the degree of isomerization (quantity of fumaric acid, in per cent) is plotted as a function of the molecular weight, one obtains curves which become horizontal after the initial portion. The values of the ordinate at variable concentrations of fumaric acid are always constant; the slope of the curves changes according to the glycol used. A comparison of the number of linkages in the chain of the glycol used (chain length) and the steric configuration with the isomerization effect shows that in this case the polymerization is conditioned by stereochemical factors. The isomerization is greater, the denser the polyester chain. The stereochemical interpretation has also been confirmed by the utilization of compounds containing phenyl groups (phthalic anhydride); the mixed esters were obtained (for example, butylene glycol‐maleate phthalate). Conforming with our studies it was found that with mixed esters containing o ‐phthalate, the isomerization of maleate into fumarate occurred in all cases to a greater extent than was the case with pure maleic acid.