Abstract
The three main reactions that occur during the commercial preparation of unsaturated polyester resins were studied. These were the addition of a glycol to an acid anhydride, the condensation esterification of an alcohol with an acid, and the isomerisation of maleate to fumarate groups. The addition of the glycol to the anhydride was found to be catalysed by the reaction product and the rate depended on the concentration of alcohol groups, anhydride molecules, and hydrogen ions. Substitution of one end of the glycol molecule reduced the reactivity of the other hydroxyl group and hence made the kinetics very complex. The condensation esterification reaction was studied mainly in sealed tubes in the temperature range 50-170°. The results were consistent with order reaction where the rate depended on the concentration of alcohol groups, carboxylic groups, and hydrogen ions, the two latter leading to a dependence on acid concentration to the power 3/2. The isomerisation reaction was studied using equilibrium esterified mixtures and low molecular weight polymers. In all cases, the rate was 1st order with respect to maleate concentration. The reaction had an activation energy greater than that of the esterification reactions. The rate of isomerisation of partially esterified materials decreased with increase in the degree of estrification. The very high initial rate of isomeration that was detected was attributed to the presence of a glycol mono(hydrogen maleate), and was dependent on the structure of the glycol used. The nature of possible intermediates in this facile isomerisation is discussed. Although the isomerisation was shown to be an equilibrium process most of the maleate was converted into fumarate at the reaction temperature.