Effect of the nature of saturated aliphatic acids on the rate of isomerization of maleates to fumarates in the synthesis of unsaturated polyesters

Abstract

~I~ATJl~IC anhydride is commonly used as the unsaturated reactant in the preparation of unsaturated polyesters. However, the synthesis of unsaturated polyesters based on maleic anhydride (or maleic acid) by polycondensation in the melt involves the partial or complete isomerization of maleates to fumarates [1, 2]. The properties of the polyesters and of cured products based on them are largely determined by the degree of cis-trans-isomerization. I t is known that polyesters containing mainly fumarate uni ts have higher viscosity, and increased reactivity and shrinkage in the curing process, while in the cured state the polyesters in question have increased hardness and heat resistance. The isomerization of maleic acid to fumaric acid involves the transition of the labile isomer to an energetically more stable state, i.e. to the trans-form. The degree of isomerizat ion of maleic acid is determined by the esterification conditions, i.e. by the time and temperature of the reaction [3], the acidity of the system [4], the nature of the starting reactants, the presence of catalysts, etc. Much detailed information on the effect of glycol structures on isomerization processes during polyesterification reactions is given in refs. [5-7]. The isomerization of maleates was first discovered a long time ago, and has frequently been the subject of investigations, though some aspects of the isomerization processes have not yet been satisfactorily analysed. For instance, we have insufficient data on the degree and rate of isomerization of maleie acid, during polyesterification reactions, in relation to the composition and structure of modifying aliphatic dibasic acids with methylene chains from C~ to C8. t The only information given in the literature in this connection concerns the effect of succinie (C2), adipie (C4) and sebacie (C8) acids [6]. This paper deals with isomerization during the synthesis of polyesters based on diethylene glycol, maleic anhydride and modifying acids with from 2 to 8 CH~ groups in the molecule; the ratio of the glycol and acid reactants was equimolar, and there was a 50 mole ~ content of the modifying reactant in the mixture of acids.